WO2025075094A1 - Steel sheet, member, and production methods thereof - Google Patents

Abstract

Provided is a steel sheet having a TS of 780 MPa or more and less than 1,180 MPa and having high YS and excellent press formability of the inside of the steel sheet and at an edge of the steel sheet.　This steel sheet has a base steel sheet having a predetermined composition, and a surface soft layer having a Vickers hardness of 84% or less with respect to the 1/4 position of the sheet thickness from the surface of the base steel sheet and satisfying formula (1), wherein the surface soft layer has a structure within a specific range, the structure at the 1/4 position of the sheet thickness of the base steel sheet is within a specific range, and the steel sheet has a tensile strength of 780 MPa or more and less than 1,180 MPa.　Formula (1): 20≤X≤120-3800×[Sb]-1900×[Sn], provided that in formula (1), X is the thickness (μm) of the surface soft layer; and [Sb] and [Sn] are the content (mass%) of Sb and Sn in the steel, respectively.

Description

Steel plates, components and their manufacturing methods

The present invention relates to steel plates, components made from the steel plates, and methods for manufacturing them.

Improving the fuel efficiency of automobiles is an important issue from the standpoint of protecting the global environment. As a result, there has been a growing movement to reduce the weight of automobile bodies by increasing the strength and thinning of the steel plates that are the raw material for automobile parts.

In addition, social demands for improved collision safety of automobiles are becoming ever higher. Therefore, there is a demand for the development of steel sheets that not only have high strength but also have excellent impact resistance properties when an automobile collides while traveling (hereinafter simply referred to as impact resistance properties). In particular, from the perspective of the rust prevention performance of the vehicle body, steel sheets that are used as the raw material for automobile parts are often zinc-plated. Therefore, there is a demand for the development of zinc-plated steel sheets that not only have high strength but also have excellent impact resistance properties.

As an example of a steel sheet that can be used as a material for such automobile parts, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a thickness of 0.6 to 5.0 mm and a plating layer on the surface of the steel sheet, the steel sheet structure includes a ferrite phase with a volume fraction of 40 to 90% and a retained austenite phase with a volume fraction of 3 to 25%, the retained austenite phase has a solute carbon content of 0.70 to 1.00%, an average particle size of 2.0 μm or less, an average distance between particles of 0.1 to 5.0 μm, a decarburized layer thickness in the steel sheet surface layer of 0.01 to 10.0 μm, an average particle size of oxides contained in the steel sheet surface layer of 30 to 120 nm, and an average density of 1.0×10 ¹² particles/m The present invention discloses a high- ^strength hot-dip galvanized steel sheet having excellent mechanical cutting properties, characterized in that the steel sheet has a maximum tensile strength of 900 MPa or more, a work-hardening coefficient (n value) of 0.080 or more on average at the time of 3 to 7% plastic deformation, and has high ductility while maintaining a high strength of 900 MPa or more.

Patent Document 2 also discloses a high-strength hot-dip galvanized steel sheet with excellent delayed fracture resistance, which has a volume fraction of 40 to 90% ferrite phase and 5% or less retained austenite phase, the volume fraction of unrecrystallized ferrite in the entire ferrite phase is 50% or less, the grain size ratio, which is the value obtained by dividing the average grain size in the rolling direction of the ferrite phase by the average grain size in the sheet width direction, is 0.75 to 1.33, the length ratio, which is the value obtained by dividing the average length in the rolling direction of the hard structure dispersed in island shapes by the average length in the sheet width direction, is 0.75 to 1.33, and the average aspect ratio of inclusions is 5.0 or less.

Patent Document 3 discloses a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having good elongation characteristics and bendability, the hot-dip galvanized steel sheet comprising a steel sheet and a hot-dip galvanized layer, the steel sheet including a base material and a decarburized ferrite layer, a structure at a depth position of ¼ of the sheet thickness of the steel sheet containing 5.0 vol% or more of tempered martensite and 0.5 vol% or more and less than 7.0 vol% of retained austenite, the balance mainly consisting of 4 to 70 vol% of ferrite and bainite, a part or all of the tempered martensite and the retained austenite forming M-A, the decarburized ferrite layer containing 120% or more of ferrite with respect to the content of ferrite at a depth of ¼ of the sheet thickness, an average ferrite grain size of 20 μm or less, a thickness of ⁵ μm or more and 200 μm or less, 1.0 vol% or more of tempered martensite, and a number density of 0.01 particles/μm2 or more, and a method for manufacturing the same.

Patent Document 4 also discloses a high-strength hot-dip galvanized steel sheet with excellent workability and a high TS-El balance, excellent stretch flangeability, and low YR, characterized by a composition containing, by mass%, 0.05-0.3% C, 0.01-2.5% Si, 0.5-3.5% Mn, 0.003-0.100% P, 0.02% or less S, 0.010-1.5% Al, 0.007% or less N, with the balance being Fe and unavoidable impurities, and a microstructure containing, by area ratio, 20-87% ferrite, 3-10% martensite and retained austenite in total, and 10-60% tempered martensite, and a method for manufacturing the same.

Patent No. 5354135 Patent No. 5352793 Patent No. 6536294 Patent No. 5256689

In recent years, the use of steel plates with a tensile strength (TS) of 780 MPa or higher has been increasingly used for automobile impact energy absorbing components, such as front and rear side members.

In other words, improving the yield stress YS (hereinafter also referred to as YS) is effective in increasing the energy absorbed during impact (hereinafter also referred to as impact absorption energy) so as to obtain excellent fracture resistance characteristics during a vehicle collision. However, increasing the TS and YS of a steel plate generally reduces press formability, particularly properties such as ductility, hole expandability, and bendability. Therefore, when it is assumed that such a steel plate with increased TS and YS is used for the above-mentioned automobile impact energy absorbing components, press forming becomes difficult, and variation during forming reduces the yield. In particular, a decrease in press formability at the ends of the steel plate leads to the occurrence of end cracks in the actual component.

Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by the generation of retained austenite inside the steel sheet and mechanical cuttability is improved by the formation of a decarburized layer on the surface of the steel sheet. However, no consideration is given at all to the improvement in bendability and fracture resistance during vehicle collision due to the formation of a surface soft layer (decarburized layer), and the press formability of the steel sheet end portion.
Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which the main structure inside the steel sheet is soft ferrite and unrecrystallized ferrite is limited to a small amount to improve ductility, and delayed fracture resistance and its anisotropy are improved by forming a decarburized layer on the surface of the steel sheet. However, no consideration is given at all to the improvement in bendability and fracture resistance during vehicle collision due to the formation of a surface soft layer (decarburized layer), and the press formability of the steel sheet end portions.
Patent Document 3 discloses a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet in which ductility is improved by the generation of M-A inside the steel sheet and bendability is improved by the formation of a soft layer (decarburized ferrite layer) on the surface layer of the steel sheet, but gives no consideration at all to the press formability of the end portions of the steel sheet.
Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet having improved ductility, which is the press formability inside the steel sheet, and stretch flangeability, which is the press formability at the ends of the steel sheet, but does not take into consideration at all the improvement of bendability by forming a soft surface layer (decarburized layer) or the improvement of fracture resistance during a vehicle collision.
From the above, it cannot be said that the steel plates disclosed in Patent Documents 1 to 4 have a TS of 780 MPa or more, a high YS, excellent press formability inside the steel plate (bendability and bulging formability of the steel plate), excellent press formability of the steel plate end (bendability of the steel plate end (shear cross section)), and fracture resistance properties during collision (bending fracture properties and axial crush properties).

The present invention has been developed in view of the above-mentioned current situation, and aims to provide a steel plate having a tensile strength TS of 780 MPa or more and less than 1180 MPa, and also having a high yield stress YS, excellent press formability inside the steel plate (bendability and stretch formability of the steel plate), and excellent press formability of the steel plate end (bendability of the steel plate end (shear cross section)), and a manufacturing method thereof.
Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.

The steel sheet referred to here includes zinc-plated steel sheet, which can be hot-dip galvanized steel sheet (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA).

Here, the tensile strength TS is measured by a tensile test in accordance with JIS Z 2241 (2011).
In addition, having a high yield stress YS, excellent press formability inside the steel plate (bendability and bulging formability of the steel plate), and excellent press formability of the steel plate end (bendability of the steel plate end (shear cross section)) means satisfying the following.
A high yield stress YS means that the YS measured by a tensile test in accordance with JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured by the tensile test.
(A) When 780 MPa ≦ TS < 980 MPa, 550 MPa ≦ YS
(B) 980 MPa ≦ TS < 1180 MPa, 700 MPa ≦ YS

In addition, the bendability of a steel plate is excellent when a 90-degree V-bend test with a bending radius of 0.5 mm is performed in accordance with JIS Z 2248 (2022), and the length of a crack that propagates along a bending ridge formed other than the end of the bending ridge (crack length other than the V-bend end surface) is 200 μm or less;
A close contact bending test is performed, and the spacer plate thickness at the crack limit where cracks of 0.5 mm or more do not occur along the bending ridge is 3.0 mm or less;
This refers to a condition in which a contact bending test with a 3.0 mm spacer is performed, the crack depth (contact bending internal crack depth) that progresses in the plate thickness direction at the bending ridge subjected to compressive stress is 200 μm or less, and when a contact bending + orthogonal 90 degree V bending test is performed, the bending radius at which cracks of 0.5 mm or more do not occur along the bending ridge is defined as the crack limit bending radius (handkerchief bending boundary bending radius), and the crack limit bending radius is 5.0 mm or less.
Detailed measurement methods for the 90 degree V-bend test with a bending radius of 0.5 mm, the close bending test, and the close bending + orthogonal 90 degree V-bend test are as described in the examples below.

In addition, excellent internal stretch formability of the steel sheet means excellent ductility, and refers to a state in which the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured in the tensile test.
(A) When 780 MPa ≦ TS < 980 MPa, 17.0% ≦ El
(B) 980 MPa ≦ TS < 1180 MPa, 11.0% ≦ El

In addition, excellent bendability at the end of the steel plate (shear cross section) means that in a 90-degree V-bend test with a bending radius of 0.5 mm in accordance with JIS Z 2248 (2022), the length of the crack that propagates from the end of the bent ridge in the ridge direction (V-bend end face crack length) is 200 μm or less.

The present inventors have conducted extensive research in order to achieve the above object.
As a result, the composition of the base steel sheet of the steel sheet is appropriately adjusted, and the base steel sheet of the steel sheet has a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1/4 position of the sheet thickness, and the surface soft layer satisfies the following formula (1):
The structure in the superficial soft layer is as follows:
Area ratio of ferrite: 50.0% or more and 100.0% or less,
Among structures other than ferrite, the area ratio of fresh martensite divided by the total area ratio of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less,
The structure at 1/4 of the thickness of the base steel sheet is as follows:
an area ratio of ferrite: 76.5% or less (including 0.0%); a total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less; a volume ratio of retained austenite: 3.5% or more and 10.0% or less; an area ratio of fresh martensite: 10.0% or less (including 0.0%);
It has been found that a steel plate having a tensile strength of 780 MPa or more and less than 1,180 MPa, high yield stress YS, excellent press formability inside the steel plate (bendability and stretch formability of the steel plate), and excellent press formability of the steel plate end (bendability of the steel plate end (shear cross section)) can be obtained.
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the surface soft layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

The present invention was completed based on the above findings and through further investigation.

That is, the gist and configuration of the present invention are as follows.
[1] In mass%,
C: 0.050% or more and 0.400% or less,
Si: 0.20% or more and 3.00% or less,
Mn: 1.00% or more and less than 3.50%;
P: 0.001% or more and 0.100% or less,
S: 0.0001% or more and 0.0200% or less,
Al: 0.005% or more and 2.000% or less,
N: 0.0100% or less,
Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%)
and the balance being Fe and unavoidable impurities,
The steel sheet has a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1/4 sheet thickness position from the surface of the steel sheet,
The surface soft layer satisfies the following formula (1):
The structure of the surface soft layer is
The area ratio of ferrite is 50.0% or more and 100.0% or less,
When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less;
The structure at 1/4 of the sheet thickness of the base steel sheet is as follows:
The area ratio of ferrite is 76.5% or less (including 0.0%),
The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less,
The area ratio of retained austenite is 3.5% or more and 10.0% or less,
The area fraction of fresh martensite is 10.0% or less (including 0.0%),
A steel plate having a tensile strength of 780 MPa or more and less than 1180 MPa.
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.
[2] The composition further includes, in mass%,
Nb: 0.200% or less,
Ti: 0.200% or less,
V: 0.200% or less,
B: 0.0100% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 1.000% or less,
Cu: 1.000% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.1000% or less,
Ca: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0500% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The steel sheet according to the above [1], containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.
[3] The steel sheet according to [1] or [2], having a plating layer on one or both sides of the base steel sheet, the plating layer being a hot-dip galvanized layer.
[4] The steel sheet according to [1] or [2], having a plating layer on one or both sides of the base steel sheet, the plating layer being a galvannealed layer.
[5] A member made using the steel plate according to any one of [1] to [4] above.
[6] A hot rolling process in which a steel slab having the composition according to [1] or [2] is hot rolled to obtain a hot rolled steel sheet;
After the hot rolling step, a pickling step of pickling the hot-rolled steel sheet;
a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction rate of 20% or more and 80% or less;
An annealing process in which the steel sheet after the cold rolling process is heated and annealed under conditions satisfying formulas (2) and (3) at an annealing temperature of Ac1 (°C) or more and 900°C or less, an annealing time of 20 seconds or more, and a dew point of -10°C or more in an atmosphere;
A cooling process in which the steel sheet after the annealing process is cooled to a cooling stop temperature of 100°C or more and 300°C or less;
A first holding step of reheating the steel sheet after the cooling step to a reheat holding temperature range of 370 ° C. or more and 460 ° C. or less and holding the temperature for 10 seconds or more;
A surface strain introducing step of applying a tension of 2.0 kgf/mm2 or ^more to the steel sheet after the first holding step in the reheating holding temperature range;
A second holding step of holding the steel sheet after the surface strain introduction step at 300 ° C. or more and 460 ° C. or less for 10 seconds or more.
Manufacturing method of steel plate.
2400≦Y≦20000...Formula (2)
Y=[{(T-Ac1)×t1}/2}]+{(T-Ac1)×t2}...(3)
In the formula (3), T is the annealing temperature (° C.), t1 is the time (s) from 650° C. to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (° C.).
[7] The method for producing a steel sheet according to [6], further comprising a hot-dip galvanizing step of subjecting the steel sheet to a hot-dip galvanizing treatment after the annealing step to form a hot-dip galvanized layer.
[8] The method for producing the steel sheet according to [6], further comprising a galvannealed hot-dip galvanizing step of subjecting the steel sheet to a galvannealed hot-dip galvanizing treatment after the annealing step to form a galvannealed layer.
[9] A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of [1] to [4] to at least one of forming and joining to form a component.

According to the present invention, a steel plate having a tensile strength TS of 780 MPa or more and less than 1180, high yield stress YS, excellent press formability inside the steel plate (bendability and stretch formability of the steel plate), and excellent press formability of the steel plate end (bendability of the steel plate end (shear cross section)) can be obtained.
Furthermore, members made from the steel plate of the present invention have high strength and can be used extremely advantageously as impact energy absorbing members for automobiles.

FIG. 1 is an example of a tissue image taken by SEM used for identifying the tissue. FIG. 2(a) is a schematic diagram of the sample after being bent at 90 degrees in a V-shape, and FIG. 2(b) is a diagram of the sample shown in FIG. 2(a) as viewed in the Z direction (negative direction). 3-1(a) and (b) are schematic diagrams for explaining end surface cracks caused by 90-degree V-bending. Figure 3-2(a) is a schematic diagram for explaining a method for measuring the crack length of an end face crack that occurs in a 90-degree V-bend, and Figure 3-2(b) is a diagram showing an example of a profile waveform used to measure the crack length. Figure 4-1(a) is a diagram for explaining a method for measuring the spacer plate thickness at the crack limit when performing a close contact bending test, and Figure 4-1(b) is a diagram for explaining a method for measuring the crack depth that progresses in the plate thickness direction at the bending ridge portion subjected to compressive stress when performing a close contact bending test. Figure 4-2(c) is a diagram for explaining a method of cutting out an observation cross-section for measuring the crack depth propagating in the plate thickness direction at the bend ridge subjected to compressive stress when a contact bending test is performed, and Figure 4-2(d) is a diagram for explaining a method of measuring the crack depth propagating in the plate thickness direction at the bend ridge subjected to compressive stress in the above observation cross-section.

The present invention will be described based on the following embodiment.

[1. Steel plate]
The steel sheet of the present invention has a base steel sheet having a component composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%), with the balance being Fe and unavoidable impurities;
The steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet,
The surface soft layer satisfies the following formula (1):
The structure in the superficial soft layer is as follows:
The area ratio of ferrite is 50.0% or more and 100.0% or less,
When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less;
The structure at 1/4 of the thickness of the base steel sheet is as follows:
The area ratio of ferrite is 76.5% or less (including 0.0%),
The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less,
The area ratio of retained austenite is 3.5% or more and 10.0% or less,
The area fraction of fresh martensite is 10.0% or less (including 0.0%),
The tensile strength is 780 MPa or more and less than 1180 MPa, and the steel sheet has a high yield stress YS, excellent press formability inside the steel sheet (bendability and stretch formability of the steel sheet), and excellent press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)).
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.

First, the composition of the steel sheet according to the embodiment of the present invention will be described. Note that the unit of the composition is always "mass%", but hereinafter, unless otherwise specified, it will be simply shown as "%".

C: 0.050% or more and 0.400% or less C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite and retained austenite to ensure a TS of 780 MPa or more and less than 1180 MPa and a high YS. Here, if the C content is less than 0.050%, the area ratio of ferrite increases, making it difficult to achieve a TS of 780 MPa or more. In addition, this also leads to a decrease in YS.
On the other hand, when the C content exceeds 0.400%, the area ratio of fresh martensite increases excessively, making it difficult to make TS less than 1180 MPa. In addition, fresh martensite becomes the starting point of void generation during 90 degree V-bend test, close bending test, and close bending + orthogonal 90 degree V-bend test, so that the desired bendability of the steel sheet and the bendability of the sheared end surface cannot be achieved. Furthermore, the area ratio of retained austenite and the amount of solute C in the retained austenite increase excessively. And, when subjected to shear processing, the hardness of fresh martensite generated by the processing-induced transformation of retained austenite increases significantly, which promotes the subsequent generation of voids and crack growth, making it more difficult to achieve the desired bendability of the sheared end surface.
Therefore, the C content is set to 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. The C content is preferably 0.300% or less. The C content is more preferably 0.200% or less.

Si: 0.20% or more and 3.00% or less Si suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite. That is, Si is an element that affects the area ratio of retained austenite. Here, if the Si content is less than 0.20%, the area ratio of retained austenite decreases, and ductility decreases.
On the other hand, if the Si content exceeds 3.00%, the area ratio of ferrite increases excessively, and the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired bendability at the sheared edge.
Therefore, the Si content is set to 0.20% or more and 3.00% or less, preferably 2.00% or less, and more preferably 1.50% or less.
The Si content is preferably 0.50% or more.

Mn: 1.00% or more and less than 3.50% Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, etc. Here, if the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. In addition, this also leads to a decrease in YS.
On the other hand, when the Mn content is 3.50% or more, the martensite transformation start temperature Ms (hereinafter also referred to simply as Ms point or Ms) decreases, and the martensite generated in the cooling process decreases. As a result, the martensite generated during final cooling increases, and the martensite generated at that time is not sufficiently tempered, and the area ratio of hard fresh martensite increases. Fresh martensite becomes the starting point of void generation during a 90 degree V-bend test, a close bending test, and a close bending + orthogonal 90 degree V-bend test, and when the area ratio of fresh martensite exceeds 10.0%, the desired bendability of the steel plate and the bendability of the sheared end face cannot be achieved.
Therefore, the Mn content is set to 1.00% or more and less than 3.50%, preferably 2.00% or more, and preferably 3.00% or less.

P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the TS and YS of a steel sheet. To obtain such an effect, the P content is set to 0.001% or more.
On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries, so that after the steel sheet is subjected to shearing, the amount of voids generated increases and the desired bendability of the sheared edge cannot be achieved.
Therefore, the P content is set to 0.001% or more and 0.100% or less, and preferably 0.003% or more.
The P content is preferably 0.030% or less. The P content is preferably 0.010% or less, and more preferably 0.005% or less.

S: 0.0001% or more and 0.0200% or less S exists as sulfides in steel. In particular, if the S content exceeds 0.0200%, the amount of voids generated increases after the steel sheet is subjected to shear processing, and the desired bendability of the sheared edge cannot be achieved.
Therefore, the S content is set to 0.0200% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.
Due to restrictions on production technology, the S content is set to 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0005% or more.

Al: 0.005% or more and 2.000% or less Al suppresses the formation of carbides during cooling after annealing and promotes the formation of retained austenite. In other words, Al is an element that affects the area ratio of retained austenite. To obtain such effects, the Al content is set to 0.005% or more.
On the other hand, when the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. It also leads to a decrease in YS. In addition, the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired bendability of the sheared edge.
Therefore, the Al content is set to 0.005% or more and 2.000% or less, and preferably, the Al content is 0.010% or more.
The Al content is preferably 1.000% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

N: 0.0100% or less N exists as nitrides in steel. In particular, if the N content exceeds 0.0100%, the amount of voids generated increases after the steel sheet is subjected to shearing, and the desired bendability of the sheared edge cannot be achieved.
Therefore, the N content is set to 0.0100% or less, and preferably to 0.0050% or less.
Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more, more preferably 0.0010% or more, and further preferably 0.0020% or more.

Sb: 0.200% or less (including 0%)
Sb is a useful element that can improve plating and chemical conversion treatment properties by segregating on the steel sheet surface during annealing. Therefore, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. The Sb content is more preferably 0.007% or more, and further preferably 0.009% or more.
On the other hand, if the Sb content exceeds 0.200%, the effect of improving the plating property and the chemical conversion property is saturated, and there is a possibility that the press formability (bendability of the steel sheet) and the crack propagation resistance inside the steel sheet are deteriorated. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is more preferably 0.020% or less. Further preferably, it is 0.018% or less. The Sb content is more preferably 0.016% or less, and further preferably 0.014% or less.

Sn: 0.200% or less (including 0%)
Like Sb, Sn is a useful element that can improve plating and chemical conversion treatment properties by segregating on the steel sheet surface during annealing. Therefore, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. The Sn content is more preferably 0.007% or more, and further preferably 0.009% or more.
On the other hand, if the Sn content exceeds 0.200%, the effect of improving the plating property and the chemical conversion property will be saturated, and there is a possibility that the press formability (bendability inside the steel sheet) and the crack propagation resistance inside the steel sheet will be deteriorated. Therefore, when Sn is contained, it is necessary to make the Sn content 0.200% or less. The Sn content is more preferably 0.020% or less, and even more preferably 0.016% or less. The Sn content is more preferably 0.014% or less, and even more preferably 0.012% or less.

The above describes the basic composition of the base steel sheet of the steel sheet according to one embodiment of the present invention, but the base steel sheet of the steel sheet according to one embodiment of the present invention has a composition that contains the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of the steel sheet according to one embodiment of the present invention has a composition that contains the above basic components, with the balance consisting of Fe and unavoidable impurities.

In addition to the basic components described above, the base steel sheet of the steel sheet according to one embodiment of the present invention may contain at least one selected from the optional components shown below. Note that the effects of the present invention can be obtained so long as the optional components shown below are contained in amounts below the upper limit amounts, so no lower limit is set. Note that when the optional elements listed below are contained in amounts below the preferred lower limit values described below, the elements are considered to be included as unavoidable impurities.

Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less , Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.100 At least one selected from the following: 0% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less

Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. In order to obtain such an effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. The Nb content is more preferably 0.010% or more, and further preferably 0.020% or more.
On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test, and the desired bendability of the steel sheet and the bendability of the shear end surface may not be achieved. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

Ti: 0.200% or less Like Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In order to obtain such an effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. The Ti content is more preferably 0.010% or more.
On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test, and the desired bendability of the steel sheet and the bendability of the shear end surface may not be achieved. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less. The Ti content is more preferably 0.050% or less, and even more preferably 0.030% or less.

V: 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. In order to obtain such an effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is further preferably 0.010% or more, and even more preferably 0.020% or more.
On the other hand, if the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions may become the starting point of cracks during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test, and there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end surface may not be achieved. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

B: 0.0100% or less B is an element that enhances hardenability by segregating at the austenite grain boundaries. Also, B is an element that suppresses the formation and grain growth of ferrite during cooling after annealing. In order to obtain such an effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is further preferably 0.0005% or more, and even more preferably 0.0007% or more.
On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. In addition, after the steel sheet is subjected to shearing, the amount of voids generated increases, and the desired bendability of the sheared end surface may not be achieved.
Therefore, when B is contained, the B content is preferably 0.0100% or less, more preferably 0.0050% or less, and more preferably 0.0020% or less.

Cr: 1.000% or less Cr is an element that enhances hardenability, so the addition of Cr produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. In order to obtain such an effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. Cr is more preferably 0.030% or more, and even more preferably 0.040% or more.
On the other hand, when the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, and the fresh martensite becomes the origin of void generation in the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end face cannot be achieved. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. Moreover, the Cr content is more preferably 0.800% or less, and even more preferably 0.700% or less. The Cr content is more preferably 0.100% or less, and even more preferably 0.080% or less.

Ni: 1.000% or less Ni is an element that enhances hardenability, so the addition of Ni produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. In order to obtain such an effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is even more preferably 0.040% or more, and even more preferably 0.060% or more.
On the other hand, when the Ni content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and fresh martensite becomes the origin of void generation in the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end face cannot be achieved. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.
The Ni content is more preferably 0.600% or less, even more preferably 0.400% or less, and more preferably 0.200% or less.

Mo: 1.000% or less Mo is an element that enhances hardenability, so the addition of Mo produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To obtain such effects, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more.
On the other hand, when the Mo content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and fresh martensite becomes the origin of void generation in the 90 degree V bending test, the close bending test, and the close bending + orthogonal 90 degree V bending test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end face cannot be achieved. Therefore, when Mo is contained, it is preferable that the Mo content is 1.000% or less. The Mo content is more preferably 0.500% or less, further preferably 0.450% or less, and further preferably 0.400% or less. The Mo content is more preferably 0.350% or less, and further more preferably 0.300% or less. The Mo content is more preferably 0.100% or less, and further preferably 0.080% or less.

Cu: 1.000% or less Cu is an element that enhances hardenability, so the addition of Cu produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. In order to obtain such an effect, it is preferable to set the Cu content to 0.005% or more. The Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more. The Cu content is more preferably 0.020% or more. The Cu content is more preferably 0.050% or more, and even more preferably 0.100% or more.
On the other hand, when the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and a large amount of coarse precipitates and inclusions may be generated. In such a case, the fresh martensite and the coarse precipitates and inclusions may become the starting point of void generation during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test, and there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end surface cannot be achieved. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

Ta: 0.100% or less Ta, like Ti, Nb and V, increases TS and YS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In addition, Ta partially dissolves in Nb carbides or Nb carbonitrides to generate composite precipitates such as (Nb, Ta) (C, N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS. In order to obtain such effects, it is preferable that the Ta content is 0.001% or more. It is more preferable that the Ta content is 0.002% or more, and even more preferable that it is 0.004% or more.
On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the coarse precipitates and inclusions become the starting points for void generation during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end surface cannot be achieved. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less.
The Ta content is more preferably 0.090% or less, even more preferably 0.080% or less, more preferably 0.050% or less, and even more preferably 0.020% or less.

W: 0.500% or less W is an element that enhances hardenability, so the addition of W produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS. To obtain such effects, the W content is preferably 0.001% or more. The W content is more preferably 0.020% or more.
On the other hand, when the W content exceeds 0.500%, the area ratio of hard fresh martensite increases excessively, and fresh martensite becomes the origin of void generation in the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end face cannot be achieved. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less. It is even more preferable that the W content is 0.300% or less. The W content is more preferably 0.100% or less, and even more preferably 0.050% or less.

Mg: 0.0200% or less Mg is an element that is effective in making the shape of inclusions such as sulfides and oxides spherical and improving the bendability of the sheared end surface. In order to obtain such an effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. The Mg content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
On the other hand, if the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions become the starting point of void generation during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end face cannot be achieved. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Mg content is more preferably 0.0100% or less, and even more preferably 0.0080% or less.

Zn: 0.0200% or less Zn is an element that is effective in making the shape of inclusions spherical and improving the bendability of the sheared end surface. In order to obtain such an effect, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
On the other hand, if the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions become the starting point of void generation during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end face cannot be achieved. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Zn content is more preferably 0.0100% or less, and even more preferably 0.0080% or less.

Co: 0.0200% or less Co, like Zn, is an element that is effective in making the shape of inclusions spherical and improving the bendability of the sheared end surface. In order to obtain such an effect, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more. The Co content is more preferably 0.0050% or more.
On the other hand, if the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the coarse precipitates and inclusions become the starting point of void generation during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end surface cannot be achieved. Therefore, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.

Zr: 0.1000% or less Zr is an element that is effective in making the shape of inclusions spherical and improving the bendability of the sheared end surface, similar to Zn and Co. In order to obtain such an effect, the Zr content is preferably 0.0010% or more.
On the other hand, if the Zr content exceeds 0.1000%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the coarse precipitates and inclusions become the starting points for void generation during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end surface cannot be achieved. Therefore, when Zr is contained, the Zr content is preferably 0.1000% or less.
The Zr content is more preferably 0.0300% or less, further preferably 0.0100% or less, and more preferably 0.0050% or less.

Ca: 0.0200% or less Ca exists as inclusions in steel. Here, if the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated. In such cases, the coarse precipitates and inclusions become the starting points for void generation during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end surface cannot be achieved. Therefore, when Ca is contained, it is preferable that the Ca content is 0.0200% or less.
The Ca content is preferably 0.0020% or less. The lower limit of the Ca content is not particularly limited, but the Ca content is preferably 0.0005% or more. In addition, due to restrictions on production technology, the Ca content is more preferably 0.0010% or more.

Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, REM: 0.0200% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all effective elements for improving the bendability of the sheared end surface. In order to obtain such an effect, it is preferable that the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each 0.0001% or more.
On the other hand, when the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM exceed 0.0200% and/or the content of As exceeds 0.0500%, coarse precipitates and inclusions may be generated in large quantities. In such cases, the coarse precipitates and inclusions may become the starting point of void generation during the 90 degree V-bend test, the close bending test and the close bending + orthogonal 90 degree V-bend test, and the desired bendability of the steel sheet and the bendability of the sheared end face may not be achieved. Therefore, when at least one of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM is contained, it is preferable that the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM are each 0.0200% or less. In addition, in the case where As is contained, the As content is preferably 0.0500% or less.

The Se content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Se content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Se content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Te content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Te content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Te content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Ge content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Ge content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Ge content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The As content is more preferably 0.0010% or more, and even more preferably 0.0015% or more. The As content is more preferably 0.0100% or more, and even more preferably 0.0150% or more. The As content is more preferably 0.0400% or less, and even more preferably 0.0300% or less.
The Sr content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Sr content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Sr content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Cs content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Cs content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Cs content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Hf content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Hf content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Hf content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Pb content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Pb content is more preferably 0.0010% or more, and even more preferably 0.0050% or more. The Pb content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
The Bi content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The Bi content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The Bi content is more preferably 0.0100% or less, and even more preferably 0.0050% or less.
The REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0030% or more. The REM content is more preferably 0.0180% or less, and even more preferably 0.0150% or less. The REM content is more preferably 0.0100% or less.
In the present invention, REM refers to scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanoids ranging from lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs.
The REM is not particularly limited, but is preferably at least one of Sc, Y, Ce and La.

That is, the base steel sheet of the steel sheet of the present invention has, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%), and optionally Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less It has a composition containing at least one selected from the following: Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less, with the balance being Fe and unavoidable impurities.

Steel structure (structure at 1/4 of the plate thickness of the base steel plate)
Next, the steel structure of the steel plate according to one embodiment of the present invention will be described.
In the structure at the 1/4 position of the plate thickness of the base steel plate, the area ratio of ferrite is 76.5% or less (including 0.0%), the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less, the area ratio of retained austenite is 3.5% or more and 10.0% or less, and the area ratio of fresh martensite is 10.0% or less (including 0.0%).
The reasons for each limitation will be explained below.

Ferrite area ratio: 76.5% or less (including 0.0%)
Soft ferrite is a phase that improves ductility. However, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. It also leads to a decrease in YS. In addition, the C concentration in austenite during annealing increases excessively, making it impossible to achieve the desired bendability of the sheared edge. Therefore, the area ratio of ferrite is set to 76.5% or less. The area ratio of ferrite is preferably 60.0% or less.
The lower limit of the area ratio of ferrite is not particularly limited and may be 0.0%. The area ratio of ferrite may be 5.0% or more, or 10.0% or more.

Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less Bainitic ferrite and tempered martensite have intermediate hardness between soft ferrite and hard fresh martensite, and are important phases for ensuring good bending properties of steel sheets and bending properties of sheared end faces. Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Tempered martensite is effective for improving TS. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more. Preferably, it is 30.0% or more.
On the other hand, if the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) increases excessively, ductility decreases. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is set to 90.0% or less. The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 87.0% or less. It is more preferable that the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 80.0% or less.
Bainitic ferrite is upper bainite with little carbide that is formed in a relatively high temperature range.

Area fraction of retained austenite: 3.5% or more and 10.0% or less In order to obtain good ductility, the area fraction of retained austenite is set to 3.5% or more. The area fraction of retained austenite is preferably more than 3.5%.
On the other hand, if the volume fraction of the retained austenite is excessively increased, the fresh martensite generated by the processing-induced transformation during shear processing becomes the starting point for void generation, and the desired bendability of the sheared edge cannot be achieved. Therefore, the area fraction of the retained austenite is set to 10.0% or less. The area fraction of the retained austenite is preferably 9.0% or less, and more preferably 8.0% or less.

Area ratio of fresh martensite: 10.0% or less (including 0.0%)
If the area ratio of fresh martensite increases excessively, the fresh martensite may become the starting point for void generation in the 90 degree V-bend test, the contact bending test, and the contact bending + orthogonal 90 degree V-bend test, and the desired bendability of the steel plate and the bendability of the sheared end face may not be achieved.
From the viewpoint of ensuring good bendability of the steel sheet and bendability of the sheared end surface, the area ratio of fresh martensite is set to 10.0% or less, and preferably 5.0% or less.
The lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0.0%.
It should be noted that fresh martensite is martensite that has not been quenched (i.e., has not been tempered).

The area ratio of the remaining structure other than the above is preferably 10.0% or less. The area ratio of the remaining structure is more preferably 7.0% or less, and further preferably 5.0% or less. The area ratio of the remaining structure may be 0.0%.
The remaining structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, and cementite. The type of the remaining structure can be confirmed by observation using, for example, a scanning electron microscope (SEM).

Soft surface layer In the base steel sheet of the steel sheet according to one embodiment of the present invention, it is preferable that the base steel sheet has a soft surface layer on its surface. The soft surface layer contributes to suppressing the progression of bending cracks during press forming, thereby further improving the bendability of the steel sheet. The soft surface layer means a decarburized layer, and is a surface region having a Vickers hardness of 84% or less of the Vickers hardness of the cross section at the 1/4 position of the sheet thickness.
In order to obtain the effect of improving the bendability of the steel sheet, the thickness of the surface soft layer is set to 20 μm or more. Also, the thickness of the surface soft layer is set to 120 μm or less.
The Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.

Surface soft layer thickness (X): 20≦X≦120−3800×[Sb]−1900×[Sn] ... (1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.
The soft surface layer in the present invention refers to a region in which the Vickers hardness is 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet. The thickness (X) of the soft surface layer must satisfy the formula (1).
If the thickness (X) of the surface soft layer is less than 20 μm, the desired bendability intended by the present invention may not be obtained.
On the other hand, if the surface soft layer thickness (X) exceeds (120-3800×[Sb]-1900×[Sn]) μm, it is not possible to achieve both high strength and excellent press formability as intended by the present invention. Therefore, the surface soft layer thickness (X) is specified to be 20 μm or more and (120-3800×[Sb]-1900×[Sn]) μm or less.
In the present invention, as described above, Sb and Sn are added as necessary to improve plating property and chemical conversion property, but when Sb and Sn are added, the allowable upper limit of the soft surface layer thickness (X) that affects bending cracks is lowered due to the surface segregation of these elements as described above. For this reason, the upper limit of the soft surface layer that provides good bending property is (120-3800 x [Sb]-1900 x [Sn]) μm.
The thickness of the surface soft layer is preferably 25 μm or more, and more preferably 30 μm or more.
The thickness of the surface soft layer is preferably 100 μm or less, and more preferably 90 μm or less.

Steel structure in the soft surface layer Ferrite area ratio: 50.0% to 100.0% When subjected to bending, the surface layer is more deformed than the inside. Therefore, voids are more likely to form in the surface layer. In the present invention, by controlling the amount of ferrite in the soft surface layer to 50.0% or more, voids that serve as crack starting points are less likely to form in the surface layer, and the progress of cracks is suppressed. The area ratio of ferrite in the soft surface layer is preferably 60.0% or more.
The area ratio of ferrite may be 100.0%. The area ratio of ferrite may be 99.9% or less, 95.0% or less, or 90.0% or less.

Value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite): 0.5 or less If the area ratio of fresh martensite in the soft surface layer increases excessively, the fresh martensite becomes the origin of void generation in the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test, and the desired bendability of the steel sheet cannot be achieved. From the viewpoint of ensuring good bendability of the steel sheet and bendability of the sheared end face, when the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of martensite in the soft surface layer by the area ratio of hard phases other than ferrite is set to 0.5 or less.
Here, the hard phase other than ferrite refers to bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite).
The lower limit of the value obtained by dividing the area ratio of martensite in the soft surface layer by the area ratio of the hard phase other than ferrite is not particularly limited, and may be 0.00.

For example, by controlling the tension during the surface strain introduction step in the manufacturing method described later, the area ratio of fresh martensite in the surface soft layer divided by the area ratio of hard phases other than ferrite can be suppressed to 0.5 or less. By applying a tension of 2.0 kgf/ ^mm2 or more once or more after the first holding step, untransformed austenite undergoes processing-induced transformation to become fresh martensite, which is then tempered during the subsequent second holding step, and finally becomes tempered martensite.

Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard phase (hard second phase (retained austenite + fresh martensite)) in the 1/4 position of the sheet thickness of the base steel sheet and in the soft surface layer are measured as follows. The structure of the soft surface layer is measured at a position of 1/2 of the thickness of the soft surface layer.
A sample is cut out from the base steel sheet so that the plate thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet becomes the observation surface. The observation surface of the sample is then mirror-polished using diamond paste. The observation surface of the sample is then finish-polished using colloidal silica, and etched with 3 vol. % nital to reveal the structure.
Then, three fields of view of 25.6 μm×17.6 μm were photographed at the outermost layer position (half the thickness of the soft surface layer) and within a range of ±100 μm at the 1/4 position of the plate thickness on the observation surface of the sample under conditions of an acceleration voltage of 15 kV and a magnification of 5000 times using a SEM (Scanning Electron Microscope). At the outermost layer position, the zinc plating layer was excluded and the internal oxide layer was included in the photograph.
From the obtained structural image (see FIG. 1), ferrite, bainitic ferrite, tempered martensite and other hard phases (hard second phase (retained austenite + fresh martensite)) are identified as follows.

Ferrite: A black region with a blocky shape. It contains almost no iron-based carbides. However, if it does contain iron-based carbides, the area of the ferrite includes the area of the iron-based carbides. The same applies to bainitic ferrite and tempered martensite, which will be described later.
Bainitic ferrite: This is a region that is black to dark gray in color and has a massive or amorphous shape. It also contains no iron carbides or contains a relatively small amount of them.
Tempered martensite: This is a gray area with an amorphous morphology. It also contains a relatively large number of iron-based carbides.
Hard second phase (retained austenite + fresh martensite): This is a region that is white to light gray in color and has an amorphous form. It does not contain iron-based carbides. If the size is relatively large, the color gradually darkens as it moves away from the interface with other structures, and the interior may be dark gray.
Carbides: These are white areas that are dot-like or linear in shape and are included in tempered martensite, bainitic ferrite, and ferrite.
Remaining structure: The above-mentioned lower bainite, pearlite, inner oxides, etc. are included, and the forms thereof are as known in the art.

Then, the area of each phase identified in the structural image is calculated using the following method. An equally spaced 20 x 20 grid is placed on an area of 25.6 μm x 19.2 μm in actual length on the 5000x magnification SEM image, and the area ratios of ferrite, bainitic ferrite, tempered martensite, and other hard phases (hard second phases) are investigated using the point counting method, which counts the number of points on each phase. The area ratio is the average value of three area ratios determined on separate 5000x magnification SEM images.

The area ratio of retained austenite is measured as follows.
The base steel sheet is mechanically ground in the thickness direction (depth direction) to a position of 1/4 of the thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. MoKα rays are used as the incident X-rays, and the ratio of the diffraction intensity of each of the (200), (220) and (311) faces of fcc iron (austenite) to the diffraction intensity of each of the (200), (211) and (220) faces of bcc iron is obtained, and the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensity of each face. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.

The area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite from the area ratio of the hard second phase determined as described above.
[Area ratio of fresh martensite (%)] = [Area ratio of hard second phase (%)] - [Area ratio of retained austenite (%)]

The area ratio of the remaining structure is determined by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of other hard phases (hard second phases) determined as described above from 100.0%.
[Area ratio of remaining structure (%)] = 100.0 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard second phase (%)]

Next, we will explain the mechanical properties of a steel plate according to one embodiment of the present invention.

Tensile strength (TS): 780 MPa or more and less than 1180 MPa The tensile strength TS of a steel plate according to one embodiment of the present invention is 780 MPa or more and less than 1180 MPa.
The specified yield stress (YS), yield ratio (YR), internal stretch formability (total elongation (El)), bendability of the steel plate, and bendability of the sheared end surface of the steel plate according to one embodiment of the present invention are as described above.
It is preferable that the ratio YR (yield ratio) of the yield stress YS to the tensile strength TS satisfies 0.70≦YR.

Furthermore, the tensile strength (TS), yield ratio (YR), yield stress (YS), and total elongation (El) are measured by a tensile test conforming to JIS Z 2241 (2011) described later in the Examples. The bendability of the steel plate is measured by a close bending test and a close bending + orthogonal 90 degree V-bend test described later in the Examples. The bendability of the sheared end surface is measured by a 90 degree V-bend test described later in the Examples.

Coating layer (hot-dip galvanized layer, galvannealed hot-dip galvanized layer)
A steel sheet according to one embodiment of the present invention may have a plating layer formed on the base steel sheet (on the surface of the base steel sheet), and this plating layer may be provided on only one surface of the base steel sheet, or on both surfaces.

The plating layer (zinc plating layer) referred to here refers to a plating layer whose main component is Zn (Zn content is 50.0% or more), and examples of this include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.

Here, it is preferable that the hot-dip galvanized layer is composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.

The galvannealed layer is preferably composed of, for example, 20% by mass or less Fe and 0.001% by mass or more and 1.0% by mass or less Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of 0% by mass or more and 3.5% by mass or less. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

In addition, the coating weight of the plating layer (zinc plating layer) per side is not particularly limited, but is preferably 20 g/ ^m2 or more. Also, the coating weight of the plating layer (zinc plating layer) per side is preferably 80 g/ ^m2 or less.

The plating weight of the plating layer (zinc plating layer) is measured as follows.
That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% aqueous hydrochloric acid solution. Next, a test steel sheet (galvanized steel sheet) is immersed in the treatment solution to dissolve the plating layer (galvanized layer). The mass loss of the test material before and after dissolution is then measured, and the value is divided by the surface area of the base steel sheet (the surface area of the part that was covered with plating) to calculate the plating coverage (g/ ^m2 ).

The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more, more preferably 0.6 mm or more, and even more preferably 0.8 mm or more. The thickness of the steel plate is preferably 2.3 mm or less, more preferably 1.6 mm or less, and even more preferably 1.2 mm or less.

[2. Manufacturing method of steel sheet]
Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described.

A method for producing a steel sheet according to an embodiment of the present invention includes a hot rolling step of hot rolling a steel slab having the above-mentioned composition to obtain a hot-rolled steel sheet;
After the hot rolling process, a pickling process of pickling the hot-rolled steel sheet;
a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction rate of 20% or more and 80% or less;
An annealing process in which the steel sheet after the cold rolling process is heated and annealed under conditions satisfying formula (2) and formula (3) in an atmosphere having an annealing temperature Ac ₁ point (°C) or more and 900°C or less, an annealing time of 20 seconds or more, and a dew point of -10°C or more;
A cooling process in which the steel sheet after the annealing process is cooled to a cooling stop temperature of 100°C or more and 300°C or less;
A first holding step of reheating the steel sheet after the cooling step to a reheat holding temperature range of 370 ° C. or more and 460 ° C. or less and holding the temperature for 10 seconds or more;
a surface layer strain introducing step of applying a tension of 2.0 kgf/ ^mm2 or more to the steel sheet after the first holding step in a reheating first holding temperature range;
and a second holding step of holding the steel sheet after the surface strain introducing step at 300° C. or more and 460° C. or less for 10 seconds or more.
2400≦Y≦20000...Formula (2)
Y=[{(T-Ac1)×t1}/2}]+{(T-Ac1)×t2}...(3)
In the formula (3), T is the annealing temperature (° C.), t1 is the time (s) from 650° C. to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (° C.).
Ac1 (°C): 727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn], where [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).
Unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and the steel plate.

First, a steel slab having the above-mentioned composition is prepared. For example, a steel material is melted to obtain molten steel having the above-mentioned composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. The obtained molten steel is then solidified to obtain a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited, and for example, a continuous casting method, an ingot casting method, a thin slab casting method, etc. can be used. From the viewpoint of preventing macrosegregation, a continuous casting method is preferred.

[Hot rolling process]
The steel slab is then hot rolled to produce a hot rolled steel sheet.
The hot rolling may be performed by applying an energy-saving process, such as direct rolling (a method in which a steel slab is not cooled to room temperature, but is charged as a hot piece into a heating furnace and hot rolled) or direct rolling (a method in which a steel slab is briefly kept at a certain temperature and then immediately rolled).

The hot rolling conditions are not particularly limited, and the hot rolling can be performed, for example, under the following conditions.
That is, the steel slab is once cooled to room temperature, and then reheated and rolled. The slab heating temperature (reheating temperature) is preferably 1100°C or higher from the viewpoints of dissolving carbides and reducing the rolling load. In addition, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the temperature of the steel slab surface.

Then, the steel slab is subjected to rough rolling according to the usual method to obtain a rough rolled plate (hereinafter also referred to as a sheet bar). The sheet bar is then subjected to finish rolling to obtain a hot rolled steel plate. If the slab heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before the finish rolling in order to prevent problems during the finish rolling. The finish rolling temperature is preferably 800°C or higher in order to reduce the rolling load. In addition, if the reduction rate in the unrecrystallized state of austenite becomes high, an abnormal structure elongated in the rolling direction may develop, which may reduce the workability of the annealed sheet. Furthermore, by setting the finish rolling temperature to 800°C or higher, the steel structure at the hot rolled steel plate stage and, by extension, the steel structure of the final product are likely to become uniform. If the steel structure becomes non-uniform, the bendability tends to decrease. On the other hand, if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) generated increases. As a result, the interface between the base steel and the oxide may become rough, which may deteriorate the surface quality of the steel sheet after pickling and cold rolling. In addition, the crystal grains may become coarse, which may cause a decrease in the strength and bendability of the steel sheet. For these reasons, it is preferable that the finish rolling temperature be in the range of 800°C or higher. It is also preferable that the finish rolling temperature be in the range of 950°C or lower.

After finish rolling, the hot-rolled steel sheet is coiled. The coiling temperature is preferably 450°C or higher. It is also preferable that the coiling temperature is 750°C or lower.

In addition, the sheet bars may be joined together during hot rolling and continuous finish rolling may be performed. Also, the sheet bar may be wound once before the finish rolling. In order to reduce the rolling load during hot rolling, a part or all of the finish rolling may be lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape of the steel sheet and the material. In addition, the friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25.
In the hot rolling process (hot rolling process) including rough rolling and finish rolling, generally, steel slabs are made into sheet bars by rough rolling and then made into hot rolled steel sheets by finish rolling. However, depending on the mill capacity, etc., such division is not important and it is not a problem as long as the specified size is achieved.

[Pickling process]
After the hot rolling process, the hot-rolled steel sheet is pickled. By pickling, oxides on the surface of the steel sheet can be removed, and good chemical conversion treatment properties and plating quality are ensured. Pickling may be performed once or multiple times. There are no particular limitations on the pickling conditions, and the usual methods may be followed.

[Cold rolling process]
The cold rolling is carried out by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.
The reduction ratio (cumulative reduction ratio) of the cold rolling is not particularly limited, but is set to 20% or more and 80% or less. If the reduction ratio of the cold rolling is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, and there is a risk that the TS and bendability of the final product will decrease.
On the other hand, if the reduction rate in cold rolling exceeds 80%, the steel sheet is likely to have defective shape and the coating weight may become non-uniform.
Furthermore, the cold-rolled steel sheet obtained after cold rolling may be optionally subjected to pickling.

[Annealing process]
Next, in one embodiment of the present invention, after the cold rolling step, the steel sheet obtained as described above is heated and annealed in an atmosphere having an annealing temperature of Ac1 (° C.) or more and 900° C. or less, an annealing time of 20 seconds or more, and a dew point (annealing dew point) of −10° C. or more. The number of annealing steps may be two or more, but one annealing step is preferable from the viewpoint of energy efficiency.

Annealing temperature: Ac1 (°C) to 900°C When the annealing temperature is lower than Ac1 (°C), the ratio of austenite generated during heating in the two-phase region of ferrite and austenite becomes insufficient, and the area ratio of ferrite increases excessively after annealing, lowering TS and YS.
On the other hand, if the annealing temperature exceeds 900°C, excessive grain growth of austenite occurs, the Ms point increases, and a large amount of tempered martensite containing carbides is formed, making it difficult to obtain a retained austenite area ratio of 3.5% or more, and reducing ductility.
Therefore, the annealing temperature is set to be equal to or higher than the Ac1 point (° C.) and equal to or lower than 900° C. The annealing temperature is preferably equal to or lower than 880° C. The annealing temperature is the maximum temperature (soaking temperature) reached in the annealing process.

Ac1 (°C) is calculated according to the following formula:
Ac1(°C)=727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn]
Here, [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).

Annealing time (soaking time): 20 seconds or more If the annealing time is less than 20 seconds, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and TS and YS decrease. In addition, the C concentration in austenite during annealing increases excessively, and the desired bendability of the sheared end surface cannot be achieved. Furthermore, a surface soft layer thickness of 20 μm or more cannot be formed during annealing, and the desired bendability of the steel sheet cannot be achieved. Therefore, the annealing time is 20 seconds or more. The annealing time is preferably 40 seconds or more.
The annealing time (soaking time) is a holding time in a temperature range of (annealing temperature -40°C) or more and (annealing temperature) or less. In other words, the annealing time includes not only the holding time at the annealing temperature, but also the residence time in a temperature range of (annealing temperature -40°C) or more and (annealing temperature) or less during heating and cooling before and after reaching the annealing temperature.

2400≦Y≦20000...Formula (2)
Y=[{(T-Ac1)×t1}/2}]+{(T-Ac1)×t2}...(3)
In the formula (3), T is the annealing temperature (° C.), t1 is the time (s) from 650° C. to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (° C.).
In the present invention, it is necessary to perform annealing under annealing conditions that satisfy formula (2) and formula (3). When Y in formula (3) is less than 2400, the soft surface layer defined in the present invention is less than 20 μm. On the other hand, when Y exceeds 20000, the soft surface layer defined in the present invention is more than (120-3800×[Sb]-1900×[Sn]) μm. Therefore, Y in formula (3) is set to be 2400 or more and 20000 or less.
It is preferable that t1 is 30 s or more. Also, it is preferable that t1 is 80 s or less.

Dew point of the atmosphere (annealing atmosphere) in the annealing process (annealing dew point): -10°C or higher In one embodiment of the present invention, the dew point of the atmosphere (annealing atmosphere) in the annealing process is preferably -10°C or higher. By performing annealing with the dew point of the annealing atmosphere in the annealing process set to -10°C or higher, the decarburization reaction is promoted and the surface soft layer can be formed deeper. The dew point of the annealing atmosphere in the annealing process is preferably -5°C or higher, more preferably 0°C or higher, and even more preferably +10°C or higher. There is no particular upper limit for the dew point of the annealing atmosphere in the annealing process, but due to production technology constraints, the dew point of the annealing atmosphere in the annealing process is preferably 30°C or lower.

[Isothermal holding process (preferred requirements)]
After the annealing step, during cooling from the annealing temperature, an isothermal holding step may be performed, if necessary, at 400° C. or more and 600° C. or less (hereinafter also referred to as an isothermal holding temperature range) for less than 80 seconds in order to promote bainite transformation.
In the isothermal holding step, bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite, thereby ensuring a predetermined area ratio of retained austenite and improving elongation.

Isothermal holding temperature range: 400°C or higher and 600°C or lower If the isothermal holding temperature is less than 400°C, lower bainite and martensite containing a large amount of carbides are generated, and the diffusion of C into the untransformed austenite is suppressed, so that it may not be possible to ensure the area ratio of the specified amount of retained austenite.
On the other hand, if the isothermal holding temperature exceeds 600° C., untransformed austenite may be transformed into pearlite, and TS and ductility may not be ensured. Therefore, the isothermal holding temperature is preferably set to 400° C. or higher and 600° C. or lower.

Holding time in isothermal holding temperature range: less than 80 seconds If the holding time in the isothermal holding temperature range is 80 seconds or more, the area ratio of bainitic ferrite increases excessively, the C concentration in the untransformed austenite increases excessively, and there is a risk that the desired bendability of the sheared edge cannot be achieved. Therefore, it is preferable that the holding time in the isothermal holding temperature range is less than 80 seconds.

[Cooling process (first cooling process)]
Next, in the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of 100° C. or more and 300° C. or less. The average cooling rate is preferably 10° C./s or more and 50° C./s or less, and the dew point of the atmosphere is preferably −20° C. or less.

Cooling stop temperature: 100°C or higher and 300°C or lower Average cooling rate: 10°C/s or higher and 50°C/s or lower Atmospheric dew point: -20°C or lower (preferable conditions)
In the cooling step, the steel sheet after the annealing step is cooled to a cooling stop temperature of 100°C or higher and 300°C or lower.
At this time, the cooling start temperature can be Ac1 (° C.) or more and 900° C. or less, and can be 400° C. or more and 600° C. or less when an isothermal holding step is performed.
The cooling step is a step necessary for controlling the area ratio of tempered martensite and the area ratio of retained austenite generated in the subsequent first holding step (reheating and holding step) within a predetermined range. If the cooling stop temperature is less than 100°C, the untransformed austenite present in the steel in the cooling step is almost entirely transformed into martensite. As a result, the area ratio of tempered martensite ultimately increases excessively, making it difficult to obtain an area ratio of retained austenite of 3.5% or more, and reducing ductility.
On the other hand, when the cooling stop temperature exceeds 300°C, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases. As a result, fresh martensite becomes the origin of void generation in the 90° V-bend test, the close bending test, and the close bending + orthogonal 90° V-bend test, and the desired bendability of the steel plate and the bendability of the sheared end surface cannot be achieved. Therefore, the cooling stop temperature is 100°C or more and 300°C or less. The cooling stop temperature is preferably 120°C or more. In addition, the cooling stop temperature is preferably 280°C or less.
The average cooling rate during this cooling step is preferably 10° C./s or more. The average cooling rate during this cooling step is preferably 50° C./s or less. The metal phase defined in the present invention can be obtained by this cooling step. Here, if the average cooling rate is less than 10° C./s, the amount of untransformed austenite that is entirely transformed into martensite during the cooling step increases, making it difficult to finally obtain retained austenite at an area ratio of 3.5% or more, and ductility may decrease. On the other hand, if the average cooling rate exceeds 50° C./s, self-relaxation during martensite transformation may be suppressed, and the plate shape may deteriorate.
In addition, the dew point of the atmosphere in this cooling step is preferably -20°C or less. If the dew point of the atmosphere exceeds -20°C, the thickness of the soft surface layer in the in-plane direction of the steel sheet becomes more uneven, and the tensile strength specified in the present invention may not be obtained. For these reasons, the dew point of the atmosphere in this cooling step is preferably -20°C or less.
The above average cooling rate (° C./s) is obtained by dividing the difference between the cooling start temperature (° C.) and the cooling end temperature (° C.) in the cooling step by the cooling time (s).

[First holding process (first reheating holding process)]
Next, in the first holding step (first reheating holding step), the steel sheet is reheated to a temperature range of 370°C or more and 460°C or less (also referred to as reheating holding temperature range, but hereinafter also referred to as first reheating holding temperature range in order to distinguish it from the reheating holding temperature range of the second holding step), and held for 10 seconds or more.

Reheating holding temperature (first reheating holding temperature): 370°C or higher and 460°C or lower In the reheating holding step, C is concentrated in the austenite remaining after the cooling step, thereby reducing the area ratio of fresh martensite in the final structure while ensuring the area ratio of a predetermined amount of retained austenite.
If the reheating holding temperature (first reheating holding temperature) is less than 370°C, the C concentration in the austenite remaining after the cooling step is insufficient, making it difficult to obtain retained austenite at an area ratio of 3.5% or more, and ductility is reduced.
On the other hand, if the reheating temperature (first reheating temperature) exceeds 460°C, C is excessively concentrated in the untransformed austenite, and the untransformed austenite in the surface layer does not undergo processing-induced transformation in the surface strain introduction step described below, and becomes retained austenite or fresh martensite. In the soft surface layer, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite exceeds 0.5. Therefore, the reheating temperature (first reheating temperature) is set to 370°C or higher and 460°C or lower.

Holding time in the reheating holding temperature range (first reheating holding temperature range): 10 seconds or more If the holding time in the reheating holding temperature range is less than 10 seconds, the C concentration in the austenite remaining after the cooling step is insufficient, making it difficult to obtain retained austenite of 3.5% or more in area ratio, and ductility may decrease. Therefore, the holding time in the first reheating holding temperature range is set to 10 seconds or more.

[Surface strain introduction process]
In the surface layer strain introduction step, a tension of 2.0 kgf/ ^mm2 or more is applied between the first holding step (reheating and holding step) and the second holding step, thereby introducing strain into the surface layer.
By applying a tension of 2.0 kgf/ ^mm2 or more once or more, the untransformed austenite in the surface layer structure of the steel sheet undergoes a deformation-induced transformation to martensite, which then becomes tempered martensite in the subsequent second holding step. As a result, the desired bendability of the steel sheet is obtained.
Here, the tension is calculated by dividing the sum of the loads (kgf) of the load cells on the left and right of the roll through which the steel plate passes while in contact with it by the cross-sectional area of the steel plate (=plate thickness (mm) × plate width (mm)) ( ^mm2 ). The load cells must be arranged parallel to the tension direction.
The load cells are preferably disposed at positions 200 mm from both ends of the roll, and the body length of the roll used is preferably 1500 to 2500 mm.
Moreover, this tension is preferably 2.2 kgf/ ^mm2 or more, and more preferably 2.4 kgf/ ^mm2 or more. Moreover, this tension is preferably 15.0 kgf/mm2 or ^less , and more preferably 10.0 kgf/ ^mm2 or less. Here, the unit of tension is 1 kgf/ ^mm2 = 9.8 N/ ^mm2 , and it can be converted from kgf/ ^mm2 to N/ ^mm2 .

[Second holding step]
Next, in the second holding step, the steel sheet is held at 300° C. or more and 460° C. or less for 10 seconds or more.
The term "holding" as used herein also includes cooling (slow cooling) within a range of 300° C. to 460° C. for 10 seconds or more.

Second holding temperature (reheating holding temperature range (second reheating holding temperature range)): 300° C. or higher and 460° C. or lower In the second holding step, the martensite formed in the surface layer in the surface strain introduction step is tempered. As a result, the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) becomes 0.5 or less, and the desired bendability of the steel sheet is obtained.
If the second holding temperature is less than 300°C, the martensite formed in the surface layer in the surface strain introduction process is not tempered, and the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite exceeds 0.5.
On the other hand, if the second holding temperature exceeds 460°C, the retained austenite inside the steel sheet decomposes, and the desired El cannot be obtained.
Therefore, the second holding temperature (second reheating holding temperature range) is set to 300°C or higher and 460°C or lower.

Holding time at second holding temperature (second reheating holding temperature range): 10 seconds or more If the holding time at the second holding temperature (reheating holding temperature range: 300°C or more and 460°C or less) is less than 10 seconds, the martensite generated in the surface layer in the surface strain introduction step will not be tempered sufficiently, and the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite will exceed 0.5. Therefore, the holding time in the reheating holding temperature range is set to 10 seconds or more.

[Plating process (hot-dip galvanizing process, galvannealed hot-dip galvanizing process)]
Next, in the plating step, the steel sheet is subjected to a galvanizing treatment to obtain a galvanized steel sheet. Examples of the galvanizing treatment include a hot-dip galvanizing treatment and a galvannealing treatment.
The galvanizing treatment in the plating step is performed after the annealing step. The galvanizing treatment may be performed, for example, during the cooling step, during the first holding step, after the first holding step and before the surface strain introducing step, after the surface strain introducing step and before the second holding step, during the second holding step, or after the second holding step.

In the case of hot-dip galvanizing, it is preferable to immerse the steel sheet in a zinc plating bath (hot-dip galvanizing bath) at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like. There are no particular limitations on the hot-dip galvanizing bath as long as it has the composition of the zinc plating layer described above, but it is preferable to use, for example, a plating bath with an Al content of 0.10 mass% to 0.23 mass%, with the balance consisting of Zn and unavoidable impurities.

In the case of alloying hot-dip galvanizing treatment, it is preferable to carry out the hot-dip galvanizing treatment as described above, and then to carry out alloying treatment by heating the hot-dip galvanized steel sheet to an alloying temperature of 450° C. or more and 600° C. or less.
If the alloying temperature is less than 450° C., the Zn—Fe alloying rate becomes slow, and alloying may become difficult.
On the other hand, when the alloying temperature exceeds 600° C., untransformed austenite is transformed into pearlite, and ductility is reduced. The alloying temperature is more preferably 480° C. or higher. The alloying temperature is more preferably 550° C. or lower.

In addition, the coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is preferably 20 g/ ^m2 or more per side. In addition, the coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is preferably 80 g/ ^m2 or less per side. The coating weight can be adjusted by gas wiping or the like.

[Second cooling step (preferable condition)]
The steel sheet is then preferably cooled to a second cooling stop temperature of 50° C. or lower.

Second cooling end temperature: 50° C. or less The cooling conditions in the final cooling step are not particularly limited and may be in accordance with a conventional method. For example, gas jet cooling, mist cooling, roll cooling, water cooling, air cooling, etc. may be used as the cooling method.
From the viewpoint of preventing oxidation of the surface, it is preferable to cool to 50° C. or less, and more preferably to about room temperature. The average cooling rate is preferably, for example, 1° C./sec or more and 50° C./sec or less.

The steel sheet obtained as described above may be further subjected to temper rolling. If the reduction rate of temper rolling exceeds 2.00%, the yield stress increases, and the dimensional accuracy when the steel sheet is formed into a component may decrease. Therefore, the reduction rate of temper rolling is preferably 2.00% or less. The lower limit of the reduction rate of temper rolling is not particularly limited, but it is preferably 0.05% or more from the viewpoint of productivity. Temper rolling may be performed on a device connected to the annealing device for performing each of the above-mentioned processes (online), or on a device not connected to the annealing device for performing each of the processes (offline). The number of rolling times of temper rolling may be one or more than two. As long as the same elongation rate as that of temper rolling can be imparted, rolling using a leveler or the like may be used.

Conditions other than those mentioned above are not particularly limited and may be in accordance with conventional methods. From the viewpoint of productivity, it is preferable to carry out a series of processes such as the above-mentioned annealing, hot-dip galvanizing, and alloying treatment of the zinc plating in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping is possible to adjust the coating weight. Note that plating conditions other than those mentioned above may be in accordance with conventional methods for hot-dip galvanizing.

[3. Components]
Next, a member according to one embodiment of the present invention will be described.
A member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material). For example, the raw material steel plate is subjected to at least one of forming and joining to form a member.
Here, the above steel plate has a TS of 780 MPa or more and less than 1180 MPa, and has a high YS, excellent press formability inside the steel plate (bendability and stretch formability of the steel plate), and excellent press formability at the steel plate end (bendability of the steel plate end (shear cross section)). Therefore, the member according to one embodiment of the present invention has high strength and excellent press formability. Therefore, the member according to one embodiment of the present invention is particularly preferably applied to an impact energy absorbing member used in the automotive field.

[4. Manufacturing method of member]
Next, a method for manufacturing a member according to one embodiment of the present invention will be described.
A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate (e.g., a steel plate manufactured by the above-mentioned steel plate manufacturing method) to at least one of forming and joining to form a component.
Here, the molding method is not particularly limited, and for example, a general processing method such as press processing can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, crimp joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.

Steel materials having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) were melted in a converter and made into steel slabs by a continuous casting method. In Table 1, "-" indicates the content of unavoidable impurities and is treated as 0 (zero).
The calculated transformation point Ac1 point (°C) shown in Table 1 is calculated by the following formula:
Ac1 point (°C) = 727.0-32.7×[%C]+14.9×[%Si]+2.0×[%Mn]
Here, [%C] is the C content of the steel plate (steel slab), [%Si] is the Si content of the steel plate (steel slab), and [%Mn] is the Mn content of the steel plate (steel slab).

The obtained steel slab was heated to 1200 ° C., and after heating, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling at a finish rolling temperature of 900 ° C. to obtain a hot-rolled steel sheet. Next, the obtained hot-rolled steel sheet was subjected to pickling and cold rolling (reduction rate: 50%) to obtain a cold-rolled steel sheet having a sheet thickness shown in Table 3. Next, the obtained cold-rolled steel sheet was subjected to treatments in the annealing step, isothermal holding step, cooling step, first holding step (reheating holding step), surface layer strain introduction step, and second holding step under the conditions shown in Table 2, and if necessary, treatment was performed in the plating step (hot-dip galvanizing step or alloyed hot-dip galvanizing step) to obtain a steel sheet.
The plating step was performed after the first holding step and before the surface strain introduction step in Nos. 1, 5 to 9, 11, 14 to 16, 21, 23 to 31, 33 to 36, 38, 40, 44, 45, 47, 51 to 56, 59, 61, 63, 64, 66, and 67, and during the cooling step in Nos. 3, 4, 10, 12, 13, 17 to 20, 22, 32, 37, 39, 41 to 43, 46, 48 to 50, 57, 58, 60, 62, 65, and 68.

Here, in the plating process, hot-dip galvanizing or alloyed hot-dip galvanizing was performed to obtain hot-dip galvanized steel sheet (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA). In Table 2, the type of plating process is also indicated as "GI" or "GA". Steel sheets that have not been subjected to either hot-dip galvanizing or alloyed hot-dip galvanizing are indicated as "CR". In Table 2, in the case of CR steel sheet or GI steel sheet, the alloying temperature is indicated as - because no alloying process is performed.

The galvanizing bath temperature was 470° C. for both GI and GA production.
The zinc plating coverage ^was 45 to 72 g/m2 per side when producing GI, and 45 g/ ^m2 per side when producing GA.
The composition of the finally obtained plating layer (zinc plating layer) of the steel sheet was as follows: GI: 0.1-1.0 mass% Fe, 0.2-0.33 mass% Al, and the balance being Zn and unavoidable impurities, whereas GA: 8.0-12.0 mass% Fe, 0.1-0.23 mass% Al, and the balance being Zn and unavoidable impurities.
In addition, the plating layers (zinc plating layers) were formed on both sides of the base steel sheets in each case.

The steel structure of the base steel plate was identified using the obtained steel plate in the manner described above. The measurement results are shown in Table 3. As shown in Figure 1, F is ferrite, BF is bainitic ferrite, TM is tempered martensite, RA is retained austenite, and FM is fresh martensite. Also, in Table 3, LB is lower bainite, and θ is carbide.

The method for measuring the surface soft layer is as follows. After smoothing the thickness cross section (L cross section) parallel to the rolling direction of the steel sheet by wet polishing, measurements were performed at 1 μm intervals using a Vickers hardness tester with a load of 10 gf (9.8×10 ⁻² N) from a position 1 μm from the steel sheet surface in the thickness direction to a position 100 μm in the thickness direction. Thereafter, measurements were performed at 20 μm intervals to the center of the sheet thickness. The region where the Vickers hardness is reduced to 84% or less compared to the hardness at the 1/4 position of the sheet thickness is defined as the soft layer (surface soft layer), and the thickness of the region in the thickness direction is defined as the thickness of the soft layer.

The structure of the soft surface layer was identified at a position halfway through the thickness of the soft surface layer using a method similar to that used to identify the steel structure of the base steel plate.

Furthermore, tensile tests, 90 degree V-bend tests, contact bending tests, and contact bending + orthogonal 90 degree V-bend tests were conducted according to the following procedures, and the tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), bendability of the steel plate, and bendability of the sheared edge were evaluated according to the following criteria.

・TS (tensile strength)
◯ (Pass): 780 MPa or more and less than 1180 MPa × (Fail): Less than 780 MPa or 1180 MPa or more

・YS (yield stress)
〇 (Pass):
(A) When 780 MPa ≦ TS < 980 MPa, 550 MPa ≦ YS
(B) 980 MPa ≦ TS < 1180 MPa, 700 MPa ≦ YS
× (Fail):
(A) When 780 MPa ≦ TS < 980 MPa, 550 MPa > YS
(B) 980 MPa ≦ TS < 1180 MPa, 700 MPa > YS

・El (stretch formability inside the steel sheet)
〇 (Pass):
(A) When 780 MPa ≦ TS < 980 MPa, 17.0% ≦ El
(B) 980 MPa ≦ TS < 1180 MPa, 11.0% ≦ El
× (Fail):
(A) When 780 MPa ≦ TS < 980 MPa, 17.0% > El
(B) 980 MPa ≦ TS < 1180 MPa, 11.0% > El

A 90-degree V-bend test with a bending radius of 0.5 mm was performed, and the crack length that developed along the bending ridge formed other than the end of the bending ridge (crack length other than the V-bend end surface) (bendability of steel plate)
◯ (Pass): The crack length is 200 μm or less except for the V-bend end surface. × (Fail): The crack length is more than 200 μm except for the V-bend end surface.

- A 90-degree V-bend test with a bending radius of 0.5 mm was performed, and the crack length that propagated from the end of the bending ridge in the ridge direction (V-bend end crack length) (bendability of the steel plate end (shear cross section))
◯ (Pass): The length of the crack on the V-bend end surface is 200 μm or less. × (Fail): The length of the crack on the V-bend end surface is more than 200 μm.

- Conduct a close contact bending test and determine the spacer plate thickness at which cracks of 0.5 mm or more do not occur along the bending ridgeline (close contact bending boundary spacer plate thickness) (bendability of steel plate)
○ (Pass): The thickness of the spacer at the boundary between the close-fitting bends is 3.0 mm or less. × (Fail): The thickness of the spacer at the boundary between the close-fitting bends is more than 3.0 mm.

A close contact bending test was performed with a spacer of 3.0 mm, and the crack depth that progressed in the plate thickness direction at the bending ridge line subjected to compressive stress (close contact bending internal crack depth) (bendability of steel plate)
○ (Pass): The internal crack depth of the tightly bent specimen is 200 μm or less. × (Fail): The internal crack depth of the tightly bent specimen is more than 200 μm.

- Conduct a close bending + perpendicular 90 degree V bending test, and determine the bending radius at which cracks of 0.5 mm or more do not occur along the bending ridge (handkerchief bending boundary bending radius) (bendability of steel plate)
◯ (Pass): Handkerchief bending boundary bending radius is 5.0 mm or less × (Fail): Handkerchief bending boundary bending radius is more than 5.0 mm

(1) Tensile Test The tensile test was performed in accordance with JIS Z 2241 (2011). That is, a JIS No. 5 test piece was taken from the obtained steel sheet at 1/4 of the coil width position so that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Using the taken test piece, a tensile test was performed under the condition of a crosshead speed of 10 mm/min, and TS, YS, YR and El were measured. The results are shown in Table 4.

(2) 90 degree V bending test A strip test piece of 100 mmC (C direction: direction perpendicular to the rolling direction of the steel sheet) x 30 mmL (L direction: direction along the rolling direction) was taken from the coil width 1/4 position of the obtained steel sheet. The cut-out processing of the end face of the 100 mm length was shear processing, and bending processing was performed in the shear processing state (without performing mechanical processing to remove burrs) so that the burrs were on the bending outer periphery side. The clearance of the shear processing was 15%, and the rake angle was 0 degrees. The V bending processing was performed using an autograph of Shimadzu Corporation, with the punch bending radius: R = 0.5 mm, punch bending angle: 90 degrees, punch stroke speed: 30 mm / min, pressing load: 10 ton, pressing time: 5 seconds, and L direction bending (bending ridge length: 30 mmL).

An example of a sample after the 90-degree V-bend test with a bending radius of 0.5 mm is shown in Figure 2. Figure 2(b) is an overhead view of the sample viewed from the Z direction shown in Figure 2(a). If the section from the bend apex along the steel plate surface with a total width of 5 mm in the C direction (2.5 mm on both sides from the bend apex) is defined as the bend ridgeline, then the section (area o) with a width of 5 mm in the L direction from the very end of the bend ridgeline is defined as the bend ridgeline end. The crack length Y1 that propagates from the bend ridgeline end in the ridgeline direction (L direction) and the crack length Y2 that propagates in the L direction along the bend ridgeline formed other than the bend ridgeline end are each measured using the following methods.

After the above 90 degree V-bend test with a bending radius of 0.5 mm, the length of the crack propagating from the end of the bent ridge in the ridge direction was measured as follows.
The cracks at the end of the bend ridgeline of the sample after the V-bend test are shown in FIG. 3-1(a). When measuring the length of the crack at the center of the bend ridgeline, it is common to observe the plate surface (b-side) from the Z direction. Since the sample after the actual V-bend test has a saddle shape as shown in FIG. 3-1(b), the b-side is significantly deformed, which reduces the measurement accuracy of the crack length and may prevent accurate evaluation of the bendability of the shear end surface. In the present invention, the following measurement method is used to enable accurate measurement. The symbol y shown in FIG. 3-1(a) corresponds to the symbol Y1 (crack length Y1) shown in FIG. 2(b).
The shear surface a of the bent sample after a 90-degree V-bend test with a bending radius of 0.5 mm was placed on top, and the end of the bend ridgeline was photographed at a magnification of 40 times using a one-shot 3D shape measuring machine (Keyence Corporation, VR6000 series or newer models). The obtained height data was analyzed using the analysis software attached to the one-shot 3D shape measuring machine. As shown in Figure 3-2 (a), an arc-shaped measurement line i was drawn as close as possible to the outside of the bend that was subjected to tensile stress in accordance with the bend ridgeline. An example of the obtained profile waveform j is as shown in Figure 3-2 (b), and the length of each crack (y1 + y2) / 2 was obtained using a measurement tool in the software, and the length of the longest crack was taken as the crack length that propagated from the end of the bend ridgeline in the ridgeline direction after a 90-degree V-bend test with a bending radius of 0.5 mm.

After the 90-degree V-bend test with a bending radius of 0.5 mm, the length of the cracks that propagate along the bend ridge formed other than at the end of the bend ridge was measured by visual observation at 25x magnification using a stereo microscope.

(3) Adhesion bending test A test piece of 60 mmC x 30 mmL was taken from the coil width 1/4 position of the obtained steel plate. After finishing both end faces of 60 mm length by grinding, a primary bending process (U-bending process) was performed to prepare a test piece for adhesion bending. The U-bending process was performed using a hydraulic bending tester with a punch bending radius of R = 5.0 mm, stroke speed: 10 mm/s, and C-direction bending (bending ridge length: 30 mmL) at which no cracks occurred in any of the test materials. Next, an adhesion bending process was performed on the test piece after U-bending. The adhesion bending process was performed using a hydraulic bending tester. As shown in FIG. 4-1 (a), a spacer q (plate thickness Z1) was sandwiched between the test pieces as necessary, the stroke speed was 10 mm/min, the pressing load was 10 ton, and the pressing time was 3 seconds, and the bending ridge of the test piece after U-bending was perpendicular to the pressing direction.
In samples subjected to a tight bending test, if the area of width Z2 (Z2 = (2t + Z1) × π/2, where t is the sample thickness) from the bend apex along the steel plate surface on both sides in the circumferential direction (C direction) is defined as the outside of the bend ridgeline, the minimum spacer plate thickness at which the L-direction length Y3 of a crack extending in the L direction outside the bend ridgeline is less than 0.5 mm (no cracks of 0.5 mm or more occur) was defined as the spacer plate thickness at the crack limit. The crack length on the outside of the bend ridgeline was measured by visual observation with a stereo microscope at 25x magnification.

In addition, after conducting a 3.0 mm spacer adhesion bending test (a 3.0 mm spacer thickness bending test), when the portion (see region s) of width Z3 (9 mm) on both sides in the circumferential direction (direction C) from the bend apex along the steel plate surface on the side subjected to compressive stress (the inner surface side of the sample) was set to be the inside of the bending ridge as shown in FIG. 4-1 (b), the crack depth propagating in the plate thickness direction on the inside of the bending ridge was measured as follows.
As shown in FIG. 4-2(c), a specimen was cut out from the sample after the above-mentioned close bending so that a cross section k at a 1/2 position in the L direction was the observation surface.
Next, the observation surface of the sample was mirror-polished using diamond paste. Then, a scanning electron microscope (SEM) was used to photograph a field of view of 2560.0 μm×1920.0 μm (m in FIG. 4-2(d)) at the position m in FIG. 4-2(d) which is the bending apex of the observation surface of the sample, under conditions of an acceleration voltage of 15 kV and a magnification of 50 times, and the entire crack was observed. In the obtained image of the crack, the distance X between the start point and the end point of the crack was taken as the crack depth. This X
A close contact bending test was carried out using a 3.0 mm spacer, and the crack depth was evaluated as the depth of cracks that progressed in the plate thickness direction at the bent ridge line subjected to compressive stress.

(4) Close bending + perpendicular 90 degree V bending test Using the sample after the close bending test with the spacer of 3.0 mm, a V bending test in the L direction (perpendicular 90 degree V bending test) was performed. When the r region (Z2 region in the figure) in FIG. 4-1 (a) after V bending is the bending ridgeline after handkerchief bending, the bending radius of the crack limit at which no cracks with a length of 0.5 mm or more in the L direction are generated in the cracks formed by extending in the L direction on the bending ridgeline was defined as the bending radius of the crack limit (handkerchief bending boundary bending radius). The crack length of the bending ridgeline was measured by visual observation at a magnification of 25 times using a stereomicroscope.

 

 

In Tables 1 to 4, the underlined parts indicate values outside the appropriate range of the present invention.
As shown in Table 4, in all of the inventive examples, the tensile strength (TS), yield stress (YS), and total elongation (El) were acceptable, and the crack length other than at the V-bend end face, the crack length at the V-bend end face, the thickness of the spacer for tight bending, the internal crack depth of tight bending, and the bending radius at the handkerchief bending boundary were all within the specified range.
On the other hand, in the comparative examples, at least one of the tensile strength (TS), yield stress (YS), total elongation (El), crack length other than at the V-bend end face, crack length at the V-bend end face, tight bending spacer thickness, tight bending internal crack depth, and handkerchief bending boundary bending radius was insufficient.

F Ferrite FM Fresh martensite RA Retained austenite BF Bainitic ferrite TM Tempered martensite

Claims (9)

In mass percent,
C: 0.050% or more and 0.400% or less,
Si: 0.20% or more and 3.00% or less,
Mn: 1.00% or more and less than 3.50%;
P: 0.001% or more and 0.100% or less,
S: 0.0001% or more and 0.0200% or less,
Al: 0.005% or more and 2.000% or less,
N: 0.0100% or less,
Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%)
and the balance being Fe and unavoidable impurities,
The steel sheet has a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1/4 sheet thickness position from the surface of the steel sheet,
The surface soft layer satisfies the following formula (1):
The structure of the surface soft layer is
The area ratio of ferrite is 50.0% or more and 100.0% or less,
When the area ratio of ferrite is less than 100.0%, the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less;
The structure at 1/4 of the sheet thickness of the base steel sheet is as follows:
The area ratio of ferrite is 76.5% or less (including 0.0%),
The total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less,
The area ratio of retained austenite is 3.5% or more and 10.0% or less,
The area fraction of fresh martensite is 10.0% or less (including 0.0%),
A steel plate having a tensile strength of 780 MPa or more and less than 1180 MPa.
20≦X≦120-3800×[Sb]-1900×[Sn]...(1)
In the formula (1), X is the thickness (μm) of the soft surface layer, and [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively. The composition further includes, in mass%,
Nb: 0.200% or less,
Ti: 0.200% or less,
V: 0.200% or less,
B: 0.0100% or less,
Cr: 1.000% or less,
Ni: 1.000% or less,
Mo: 1.000% or less,
Cu: 1.000% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
Zr: 0.1000% or less,
Ca: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0500% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The steel plate according to claim 1, containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less. The steel sheet according to claim 1 or 2, having a plating layer on one or both sides of the base steel sheet, the plating layer being a hot-dip galvanized layer. The steel sheet according to claim 1 or 2, having a plating layer on one or both sides of the base steel sheet, the plating layer being a galvannealed layer. A member made using the steel plate according to any one of claims 1 to 4. A hot rolling process in which a steel slab having the composition according to claim 1 or 2 is hot rolled to obtain a hot rolled steel sheet;
After the hot rolling step, a pickling step of pickling the hot-rolled steel sheet;
a cold rolling step of cold rolling the steel sheet after the pickling step at a rolling reduction rate of 20% or more and 80% or less;
An annealing process in which the steel sheet after the cold rolling process is heated and annealed under conditions satisfying formulas (2) and (3) at an annealing temperature of Ac1 (°C) or more and 900°C or less, an annealing time of 20 seconds or more, and a dew point of -10°C or more in an atmosphere;
A cooling process in which the steel sheet after the annealing process is cooled to a cooling stop temperature of 100°C or more and 300°C or less;
A first holding step of reheating the steel sheet after the cooling step to a reheat holding temperature range of 370 ° C. or more and 460 ° C. or less and holding the temperature for 10 seconds or more;
A surface strain introducing step of applying a tension of 2.0 kgf/mm2 or ^more to the steel sheet after the first holding step in the reheating holding temperature range;
A second holding step of holding the steel sheet after the surface strain introduction step at 300 ° C. or more and 460 ° C. or less for 10 seconds or more.
Manufacturing method of steel plate.
2400≦Y≦20000...(2)
Y=[{(T-Ac1)×t1}/2}]+{(T-Ac1)×t2}...(3)
In the formula (3), T is the annealing temperature (° C.), t1 is the time (s) from 650° C. to the annealing temperature T during the temperature rise in the annealing process, t2 is the annealing time (s), and Ac1 is Ac1 (° C.). The method for manufacturing the steel sheet according to claim 6 further includes a hot-dip galvanizing process in which the steel sheet is subjected to a hot-dip galvanizing process after the annealing process to form a hot-dip galvanized layer. The method for manufacturing the steel sheet according to claim 6 further includes a galvannealing process in which the steel sheet is subjected to a galvannealing process after the annealing process to form a galvannealed layer. A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.