WO2024203266A1 - Steel sheet and method for manufacturing same - Google Patents

Abstract

Provided are a steel sheet and a method for manufacturing same. The steel sheet has a predetermined chemical composition and has a metallic microstructure containing, in area%, 60.0-85.0% of martensite, 10.0-30.0% of granular bainite, in which the maximum misorientation is 3.5° or less at 0.1 μm intervals and the in-grain misorientation is at least 10° within a grain surrounded by grain boundaries having a misorientation of at least 15°, and 20.0% or less of ferrite, wherein the average spacing between grains of the granular bainite is 50.0 μm or less.

Description

Steel plate and its manufacturing method

The present invention relates to a steel plate and a manufacturing method thereof.

In recent years, in order to address environmental issues, there has been a demand for weight reduction in automotive parts in order to reduce _CO2 gas emissions and improve fuel efficiency. At the same time, social demands for improved collision safety are also increasing. In order to achieve both weight reduction and improved collision safety, increasing the strength of steel materials is an effective means. However, increasing the strength of steel materials usually reduces their workability, so there is a need for steel materials that simultaneously improve both strength and workability.

In relation to the improvement of strength and workability, for example, Patent Document 1 describes a high-strength hot-rolled steel sheet that has a specified chemical composition, has a bainite phase with an area ratio of more than 95% throughout the thickness direction, and has an average grain size of 5 μm or less in a thickness section parallel to the rolling direction and 4 μm or less in a thickness section perpendicular to the rolling direction in a region from the surface to 1/4 of the thickness in the thickness direction, and further has a structure in which there are 7 or less crystal grains with an aspect ratio of 5 or more extending in the rolling direction in a region whose width in the thickness direction is 1/10 of the thickness centered on the center position of the thickness, and has a tensile strength TS of 780 MPa or more. Patent Document 1 also teaches that the above configuration makes it possible to easily and inexpensively manufacture a high-strength hot-rolled steel sheet with tensile strength TS of 780 MPa or more, with significantly improved punching workability, and excellent punching workability.

Patent Document 2 describes a cold-rolled annealed steel sheet having a predetermined chemical composition and a microstructure consisting of martensite and/or lower bainite in a surface abundance ratio, the martensite including fresh martensite and/or self-tempered martensite, the total surface abundance ratio being in the range of 60 to 95% for martensite and lower bainite, 4 to 35% for low-carbide-containing bainite, 0 to 5% for ferrite, and less than 5% for island-form retained austenite. Patent Document 2 also teaches that with the above configuration, a tensile strength in the range of 1180 to 1320 MPa, an elongation at break of at least 5%, and a hole expansion ratio Ac% of 30% or more can be achieved, along with a yield strength in the range of 800 to 970 MPa before the skin pass operation.

Patent Document 3 describes a hot-rolled steel sheet having a predetermined chemical composition, a total area ratio of martensite phase and lower bainite structure at the 1/4 plate thickness position of 85% or more, an average grain size of 20 μm or less surrounded by a boundary with a crystal orientation difference of 15° or more, crystal grains with an aspect ratio of 0.30 or less accounting for 50% or less in area ratio, and an average value of X-ray random intensity ratio of {100}<011> to {211}<011> orientation group at the plate thickness center position of 6.0 or less and a maximum value of 8.0 or less. Patent Document 3 also teaches that the above configuration allows for the stable manufacture of high-strength hot-rolled steel sheet that has high strength and excellent hole expandability and low-temperature toughness.

Patent Document 4 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, a steel structure in which the main phases are martensite and bainite with a total area ratio of 80-100%, the total area ratio of martensite in the bainite being 2-20%, and the area ratio of martensite in the bainite having an orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one of the bainite adjacent to the martensite being 50% or more relative to the total martensite. Patent Document 4 also teaches that the above configuration can provide a high-strength hot-rolled steel sheet with excellent ductility, end cracking resistance, and hole expansion property, suitable as a material for automotive parts.

Patent Document 5 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, a steel structure in which the main phases are martensite and bainite with a total area ratio of 80 to 100%, the total area ratio of martensite in the bainite being 2 to 20%, the area ratio of martensite in the bainite having an orientation difference of 15° or more between the crystal orientation of the martensite and the crystal orientation of at least one of the bainite adjacent to the martensite being more than 50% of the total martensite, and the average aspect ratio of the crystal grains present in a region from the surface of the steel sheet to a depth of 5 μm being 2.0 or less when the region surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more is regarded as a crystal grain. Patent Document 5 also teaches that the above configuration can provide a high-strength hot-rolled steel sheet with excellent ductility and bending and unbending properties suitable as a material for automobile parts.

JP 2012-062562 A Special Publication No. 2017-507241 JP 2017-057472 A International Publication No. 2022/244706 International Publication No. 2022/244707

As mentioned above, it is known that the workability of steel declines with increasing strength, and properties such as hole expandability as described in Patent Documents 2 to 4 decline. If the hole expandability declines, it may not be possible to process the steel into the desired shape, for example, in automobile suspension parts. For this reason, in the development of high-strength steel sheets such as high-strength hot-rolled steel sheets, it is important to increase strength while ensuring a certain level of properties according to the application, such as the above-mentioned hole expandability as well as uniform elongation. For example, in automobile suspension parts with complex shapes such as lower arms and trailing arms, the decline in workability associated with increasing strength can cause necking in the formed parts and reduce their functionality.

In addition, for components that require impact resistance, plastic deformation occurs when they receive an impact that exceeds their yield strength. Therefore, from the perspective of ensuring the crash safety of automobiles, it is necessary to improve not only the tensile strength but also the yield strength, and therefore there is a demand to increase the yield ratio, which is the ratio of yield strength to tensile strength.

The present invention was made in consideration of these circumstances, and its purpose is to provide a steel sheet with a new structure that has high strength, high uniform elongation, hole expansion property, and yield ratio, and that can suppress the occurrence of necking during forming, as well as a manufacturing method thereof.

In order to achieve the above object, the inventors conducted a study focusing on the metal structure of steel sheet, particularly hot-rolled steel sheet. As a result, the inventors discovered that by forming the metal structure of a hot-rolled steel sheet having a specified chemical composition with a structure mainly composed of martensite but controlled within a specified range, it is possible to achieve high strength and improved uniform elongation, that by including a specified amount of specific granular bainite in the metal structure, it is possible to improve the yield ratio and hole expandability while significantly suppressing the occurrence of necking during forming, and that by utilizing precipitation strengthening by adding Ti, it is possible to further increase the yield ratio and reduce the hardness difference between each phase in the metal structure, and that by combining such a reduction in hardness difference with the improvement in hole expandability due to the specific granular bainite, it is possible to more significantly improve the hole expandability, and thus completed the present invention.

The present invention which has achieved the above object is as follows.
(1) Chemical composition, in mass%,
C: 0.060-0.200%,
Si: 0.30-2.00%,
Mn: 1.20-2.70%,
P: 0.100% or less,
S: 0.0300% or less,
sol. Al: 0.001-0.500%,
Nb: 0.001-1.000%,
O: 0.0100% or less,
N: 0.0070% or less,
Ti: 0.070-0.200%,
B: 0 to 0.0030%,
Cr: 0-0.90%,
Mo: 0 to 0.12%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
V: 0-0.300%,
Sn: 0 to 0.040%,
As: 0 to 0.100%,
Zr: 0 to 0.050%,
Ca: 0-0.0010%,
Mg: 0 to 0.0010%,
Bi: 0 to 0.010%,
Co: 0 to 0.010%,
W: 0-0.100%,
Zn: 0 to 0.010%,
REM: 0 to 0.0100%, and the balance: Fe and impurities;
The metal structure is, in area percent,
Martensite: 60.0 to 85.0%,
Granular bainite: 10.0 to 30.0%; and ferrite: 20.0% or less, the maximum misorientation at 0.1 μm intervals being 3.5° or less within grains surrounded by grain boundaries with a misorientation of 15° or more, and the intragranular misorientation being 10° or more.
A steel plate characterized in that the average spacing of granular bainite grains is 50.0 μm or less.
(2) The chemical composition is, in mass%,
B: 0.0001 to 0.0030%,
Cr: 0.001-0.90%,
Mo: 0.001-0.12%,
Cu: 0.001-0.40%,
Ni: 0.001 to 0.30%,
V: 0.001-0.300%,
Sn: 0.001-0.040%,
As: 0.001 to 0.100%,
Zr: 0.001 to 0.050%,
Ca: 0.0001 to 0.0010%,
Mg: 0.0001 to 0.0010%,
Bi: 0.001-0.010%,
Co: 0.001 to 0.010%,
W: 0.001-0.100%,
Zn: 0.001 to 0.010%, and REM: 0.0001 to 0.0100%
The steel sheet according to the above (1), characterized in that it contains at least one of the following:
(3) The steel plate according to (1) or (2) above, characterized in that the metal structure further contains, in area percentage, at least one of bainite, pearlite and retained austenite: a total content of 20.0% or less.
(4) The steel sheet according to any one of (1) to (3) above, characterized in that the average grain size of the granular bainite grains is 5.0 to 30.0 μm.
(5) A part, comprising the steel sheet according to any one of (1) to (4) above.
(6) A heating step including heating a slab having the chemical composition described in (1) or (2) above and holding it at a temperature of 1180 to 1320° C. for 6000 seconds or more;
A hot rolling process including finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, the hot rolling process satisfying the following conditions (a) to (c); and (a) the rolling temperature in each of the rolling passes immediately preceding the last two rolling passes is 960 to 1080°C, and the rolling reduction in each of the rolling passes is 30 to 40%,
(b) cooling the rolled material to 910°C or less at an average cooling rate of 400°C/sec or more within 0.20 seconds after the two rolling passes immediately preceding the latter two stages, and (c) the rolling reduction in each rolling pass of the latter two stages is 20 to 30%. A method for producing a steel plate, comprising a cooling step including: water-cooling the finish-rolled steel plate, cooling to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, then air-cooling in said temperature range for 2.0 to 6.0 seconds, and water-cooling the steel plate to 50°C or less within 13 seconds after air-cooling.

The present invention provides a steel sheet, particularly a hot-rolled steel sheet, that has high strength, high uniform elongation, hole expandability, and yield ratio, and that can suppress the occurrence of necking during forming, and a manufacturing method thereof.

<Steel Plate>
The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition, in mass%,
C: 0.060-0.200%,
Si: 0.30-2.00%,
Mn: 1.20-2.70%,
P: 0.100% or less,
S: 0.0300% or less,
sol. Al: 0.001-0.500%,
Nb: 0.001-1.000%,
O: 0.0100% or less,
N: 0.0070% or less,
Ti: 0.070-0.200%,
B: 0 to 0.0030%,
Cr: 0-0.90%,
Mo: 0 to 0.12%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
V: 0-0.300%,
Sn: 0 to 0.040%,
As: 0 to 0.100%,
Zr: 0 to 0.050%,
Ca: 0-0.0010%,
Mg: 0 to 0.0010%,
Bi: 0 to 0.010%,
Co: 0 to 0.010%,
W: 0-0.100%,
Zn: 0 to 0.010%,
REM: 0 to 0.0100%, and the balance: Fe and impurities;
The metal structure is, in area percent,
Martensite: 60.0 to 85.0%,
Granular bainite: 10.0 to 30.0%; and ferrite: 20.0% or less, the maximum misorientation at 0.1 μm intervals being 3.5° or less within grains surrounded by grain boundaries with a misorientation of 15° or more, and the intragranular misorientation being 10° or more.
The granular bainite grains are characterized in that the average spacing between grains is 50.0 μm or less.

As mentioned above, it is known that the properties such as hole expandability decrease with increasing strength of steel material. For example, in order to manufacture parts with complex shapes such as lower arms and trailing arms among the suspension parts of automobiles, a steel sheet with excellent hole expandability while maintaining high strength, for example, a tensile strength of 1180 MPa or more that enables weight reduction, is required. From the viewpoint of increasing strength, it is preferable that the metal structure of the steel sheet is composed mainly of martensite. However, although martensitic steel has excellent strength, it generally has a problem of low workability because excessive inclusion of martensitic steel reduces properties such as uniform elongation. In addition, in parts with complex shapes such as lower arms and trailing arms, the workability decreases with increasing strength, which may cause necking in the formed parts and reduce their function. Therefore, there is a demand for a high-strength steel sheet that can improve properties such as hole expandability and uniform elongation, suppress the occurrence of necking even when forming parts with complex shapes, and further has a high yield ratio from the viewpoint of automobile collision safety, etc.

The inventors therefore conducted research focusing on the metal structure of the hot-rolled steel sheet, in addition to making the chemical composition of the steel sheet, particularly the hot-rolled steel sheet, appropriate. First, the inventors discovered that by configuring the metal structure of a hot-rolled steel sheet having a specified chemical composition to a structure mainly composed of hard martensite, more specifically, a structure containing 60.0 to 85.0% martensite by area percentage, it is possible to achieve high strength, for example a tensile strength of 1180 MPa or more, while significantly improving the uniform elongation of the resulting hot-rolled steel sheet.

Next, the inventors discovered that by including a predetermined amount of specific granular bainite in the metal structure, more specifically, by including 10.0 to 30.0% by area of granular bainite in which the maximum orientation mismatch at 0.1 μm intervals is 3.5° or less within grains surrounded by grain boundaries with an orientation mismatch of 15° or more, the intragranular orientation mismatch is 10° or more, and the average spacing between adjacent grains is 50.0 μm or less, the yield ratio and hole expandability can be improved while significantly suppressing the occurrence of necking during forming. Although it is not intended to be bound by any particular theory, it is believed that the characteristic orientation change of granular bainite particularly contributes to suppressing the occurrence of necking. More specifically, the characteristic that "the maximum orientation mismatch at 0.1 μm intervals within a grain surrounded by grain boundaries with an orientation mismatch of 15° or more is 3.5° or less, and the orientation mismatch within a grain is 10° or more" means that although the orientation change within the grain of granular bainite is relatively gentle and continuous, the orientation mismatch within the entire grain is relatively large. For example, bainite has many different interfaces within the grain, which causes discontinuous and steep orientation changes. On the other hand, ferrite has the characteristic that the orientation change within the grain is relatively small, and therefore continuous, but the orientation mismatch within the entire grain is also relatively small. Therefore, granular bainite can be considered to have characteristics between bainite and ferrite in terms of orientation change. Although necking is likely to occur in the bainite structure due to discontinuous orientation changes, granular bainite, unlike bainite and martensite, which also have many interfaces within the crystal grains, shows a continuous orientation change as described above, even though it shows a relatively large orientation difference within the entire crystal grain. Therefore, it is believed that it is possible to significantly suppress the occurrence of necking during forming due to such a characteristic orientation change of granular bainite. In addition, the inventors have found that the hole expandability can also be improved by including 10.0% or more of granular bainite by area %. Although it is not intended to be bound by any particular theory, it is believed that the presence of a certain amount of granular bainite, which has properties intermediate between martensite or bainite and ferrite, in steel suppresses the generation of voids from the interface between different phases during hole expansion processing, thereby improving hole expandability.

However, as mentioned above, granular bainite has characteristics similar to those of ferrite. For this reason, if the amount of granular bainite becomes too large in a metal structure mainly composed of martensite, it is thought that the metal structure will be similar to so-called DP steel (dual phase steel) composed of martensite and ferrite, which will lead to a decrease in the yield ratio. Even if the amount of granular bainite is appropriate, if the amount of ferrite becomes excessively large or the amount of martensite becomes small so that the total amount of granular bainite and ferrite becomes relatively large, the metal structure will similarly be similar to DP steel, which will lead to a decrease in the yield ratio. Therefore, from the viewpoint of maintaining a high yield ratio while sufficiently suppressing the occurrence of necking, it is necessary to have an appropriate amount of granular bainite present in the metal structure, while maintaining the area ratio of martensite at 60.0% or more to control the total amount of granular bainite and ferrite within an appropriate range. In addition, the inventors conducted further studies and found that, although the reason is not entirely clear, arranging the granular bainite grains at an appropriate interval, more specifically, controlling the average interval of the granular bainite grains to 50.0 μm or less, can improve the hole expandability of the steel sheet, and further, that controlling the average interval of the granular bainite grains in this manner is also important in suppressing the occurrence of necking during forming. Based on these findings, according to the steel sheet according to the embodiment of the present invention, by containing 10.0 to 30.0% by area of granular bainite in the metal structure, in which the maximum orientation mismatch at 0.1 μm intervals is 3.5° or less within grains surrounded by grain boundaries with an orientation mismatch of 15° or more, and the intragranular orientation mismatch is 10° or more, and by controlling the average interval of the granular bainite grains to 50.0 μm or less, it is possible to significantly suppress the occurrence of necking during forming while improving the yield ratio and hole expandability.

In addition, the inventors have found that the yield ratio can be further increased by utilizing precipitation strengthening by adding Ti, and that the hole expandability can be improved more significantly by combining it with the improvement in hole expandability caused by the specific granular bainite. Without intending to be bound by any particular theory, it is believed that such improvement in hole expandability due to precipitation strengthening is due to a reduction in the hardness difference between each phase in the metal structure. To explain in more detail, in the steel plate according to the embodiment of the present invention, as described above, the metal structure is composed mainly of martensite, but also contains other structures softer than martensite, for example, ferrite, which is a soft structure, may be contained up to 20.0% by area. In this case, the hardness difference between each phase in the metal structure increases, and the hole expandability decreases. However, in the steel plate according to the embodiment of the present invention, by controlling the Ti content in the steel to 0.070 mass% or more, the Ti precipitates strengthen soft structures such as ferrite, thereby reducing the hardness difference between the phases in the metal structure, and it is believed that the combination of this reduction in hardness difference and the improvement in hole expandability due to the specific granular bainite can more significantly improve the hole expandability.

Generally, steel sheets for automobiles are often processed into the desired part shape by press forming. Since press forming is usually performed in multiple steps, there are relatively many locations where, for example, a steel sheet undergoes a primary deformation, and then undergoes another deformation while strain is accumulated inside the steel sheet. However, when strain is introduced into a steel sheet, the steel sheet undergoes work hardening and becomes stronger, so that the workability in subsequent processes generally decreases, and necking may occur in the formed part. In the current study by the present inventors, it was found that improving the bendability after pre-strain is effective in suppressing the occurrence of such necking in the formed part during forming, and more specifically, it was found that the occurrence of necking can be reproduced by applying a 10% pre-strain to a steel sheet test piece by uniaxial tension in a certain direction, and then performing a 90° bending test in a direction intersecting that direction. In particular, the steel sheet has poor ductility in the C direction (direction perpendicular to the rolling direction), and in relation to this, a tensile test was performed in the C direction, followed by a bending test in the L direction (rolling direction), and it was found that, when necking does not occur in the bending test piece, necking can be improved in the forming of actual parts. According to the steel sheet according to the embodiment of the present invention, the occurrence of necking can be reliably suppressed even in such a bending test after prestrain by including the above-mentioned specific granular bainite in the metal structure at 10.0 to 30.0% by area. The fact that the occurrence of necking in the forming of actual parts can be reproduced by such a bending test after prestrain, and further the fact that the occurrence of necking in the bending test after prestrain can be significantly suppressed by including 10.0% or more by area of granular bainite showing the above-mentioned characteristic orientation change, were not known in the past, and were clarified for the first time by the present inventors. Therefore, according to the embodiment of the present invention, despite the high strength of, for example, tensile strength of 1180 MPa or more, the steel sheet has high uniform elongation, hole expandability and yield ratio, and can reliably suppress the occurrence of necking even in the molding of actual parts, so that the steel sheet according to the embodiment of the present invention is particularly useful for use in the automotive field.

Below, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values written before and after it are included as the lower and upper limits, unless otherwise specified.

[C:0.060-0.200%]
C is an element effective in increasing the strength of steel plate. In addition, C forms carbides and/or carbonitrides with Nb in steel, and refines the structure due to the pinning effect of the precipitates formed. In order to fully obtain these effects, the C content is set to 0.060% or more. The C content is set to 0.070% or more, 0.080% or more, 0.100% or more, or 0 . 120% or more. On the other hand, if the C content is excessive, the hole expandability may decrease. Therefore, the C content is set to 0.200% or less. The C content is set to 0. It may be 180% or less, 0.160% or less, 0.150% or less, or 0.140% or less.

[Si:0.30-2.00%]
Silicon is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. In order to fully obtain such effects, the silicon content is set to 0.30% or more. Silicon Content may be 0.40% or more, 0.50% or more, 0.60% or more, 0.70% or more, 0.85% or more, 1.00% or more, or 1.20% or more. However, if the Si content is excessive, the ferrite fraction becomes high, which may reduce the hole expansion property. In addition, the high ferrite fraction increases the total amount of granular bainite and ferrite, which is insufficient for DP steel. The metal structure becomes similar to that of the steel, so the yield ratio may decrease. Therefore, the Si content is set to 2.00% or less. The Si content is set to 1.80% or less, 1.60% or less, and 1.50% or less. It may be 1.40% or less.

[Mn: 1.20-2.70%]
Mn is an element that is effective in increasing strength as an element for hardenability and solid solution strengthening. In order to fully obtain these effects, the Mn content is set to 1.20% or more. The Mn content is set to 1.30%. On the other hand, if the Mn content is excessive, the fraction of granular bainite decreases, and holes are formed. The spreadability decreases and the occurrence of necking during forming may not be sufficiently suppressed. Therefore, the Mn content is set to 2.70% or less. The Mn content is set to 2.60% or less, and 2.50% or less. , 2.40% or less, 2.30% or less, or 2.20% or less.

[P: 0.100% or less]
If P is contained in excess, it may cause a decrease in workability due to grain boundary segregation, etc. Therefore, the P content is set to 0.100% or less. The P content is set to 0.050% or less, 0.030% or less The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction of the P content leads to an increase in costs. The content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[S: 0.0300% or less]
If S is contained in excess, a large amount of sulfides such as MnS is generated, which may reduce workability. Therefore, the S content is set to 0.0300% or less. The S content is set to 0.0200% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. The amount may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[sol. Al: 0.001-0.500%]
Sol. Al is an element that acts as a deoxidizer for molten steel. Sol. Al is also an element that is effective in increasing the fraction of granular bainite. In order to obtain these effects, sol. Al-containing The sol. Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, if sol.Al is contained excessively, the ferrite fraction becomes high and the hole expansion property may decrease. Also, the total amount of granular bainite and ferrite becomes large due to the high ferrite fraction. The metal structure becomes similar to that of DP steel, and the yield ratio may decrease. Therefore, the sol. Al content is set to 0.500% or less. The sol. Al content is set to 0.400% or less, 0 .300% or less, or 0.200% or less. Al means acid-soluble Al, and indicates solute Al that is present in the steel in a solid solution state.

[Nb: 0.001 to 1.000%]
Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to the refinement of prior austenite grains through a pinning effect, thereby contributing to the high strength of the steel sheet. Nb is also an effective element for increasing the fraction of bainite and controlling its morphology. To fully obtain these effects, the Nb content is set to 0.001% or more. The Nb content is set to 0.005%. On the other hand, if Nb is excessively contained in the steel, Coarse carbides may be generated, which may reduce the workability of the steel sheet. Therefore, the Nb content is set to 1.000% or less. The Nb content is set to 0.800% or less, 0.600% or less, 0.700% or less, 0.800% or less, 0.900% or less, 0.100% or less, and 0.200% or less. It may be 0.500% or less or 0.400% or less.

[O: 0.0100% or less]
O is an element that is mixed in during the manufacturing process. If O is contained in excess, coarse inclusions may form, which may reduce the workability of the steel sheet. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but 0. In order to reduce the O content to less than 0.0001%, refining takes time, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.

[N: 0.0070% or less]
If N is contained excessively, it may form coarse nitrides and cause slab cracking during hot rolling. Therefore, the N content is set to 0.0070% or less. The N content is set to 0.0050%. The lower limit of the N content is not particularly limited and may be 0%; however, excessive reduction of the N content leads to an increase in costs. The N content may be 0.0001% or more, or 0.0005% or more.

[Ti: 0.070-0.200%]
Ti is an element that precipitates in steel as Ti carbides such as TiC, strengthens soft structures such as ferrite through precipitation strengthening, and contributes to improving strength and yield ratio. Since the hardness difference between the phases in the metal structure can be reduced, it is also effective in improving the hole expandability. In order to fully obtain these effects, the Ti content is set to 0.070% or more. The Ti content may be 0.080% or more, 0.090% or more, 0.100% or more, or 0.120% or more. On the other hand, if Ti is excessively contained, coarse carbides are formed in the steel. etc., may occur, causing slab cracking during hot rolling and reducing the workability of the steel sheet. Therefore, the Ti content is set to 0.200% or less. The Ti content is set to 0.180 % or less, 0.170% or less, 0.160% or less, or 0.150% or less.

The basic chemical composition of the steel plate according to the embodiment of the present invention is as described above. Furthermore, the steel plate may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[B: 0 to 0.0030%]
B is an element that improves the hardenability of steel and contributes to improving strength. The B content may be 0%, but in order to obtain such an effect, the B content should be 0.0001% or less. % or more. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, even if B is excessively contained, the effect becomes saturated. However, this may lead to an increase in manufacturing costs. Therefore, the B content is preferably 0.0030% or less. The B content is preferably 0.0025% or less, 0.0020% or less, 0.0015% or less, or It may be 0.0010% or less.

[Cr: 0-0.90%]
Cr is an element that enhances the hardenability of steel and contributes to improving strength and/or corrosion resistance. The Cr content may be 0%, but in order to obtain these effects, the Cr content is The content of Cr is preferably 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, even if Cr is excessively contained, the effect is saturated, There is a risk of an increase in manufacturing costs. Therefore, the Cr content is preferably 0.90% or less, more preferably 0.70% or less, 0.50% or less, 0.40% or less, or 0.30% or less. may be also possible.

[Mo: 0 to 0.12%]
Mo is an element that improves the hardenability of steel and contributes to improving strength. The Mo content may be 0%, but in order to obtain such an effect, the Mo content should be 0.001% or less. % or more. The Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, if Mo is contained excessively, the Mo content may be increased during hot working. The deformation resistance increases, and the load on the equipment may become large. Therefore, the Mo content is preferably 0.12% or less. The Mo content is preferably 0.10% or less, 0.08% or less, 0. It may be 0.06% or less or 0.05% or less.

[Cu: 0-0.40%]
Cu is an element that contributes to improving strength by precipitation strengthening or solid solution strengthening. The Cu content may be 0%, but in order to obtain such an effect, the Cu content should be 0.001% or less. % or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive inclusion of these elements may not be effective. The Cu content is preferably 0.40% or less. The Cu content is preferably 0.30% or less, 0.20% or less, 0.10% or less. It may be 0.08% or less.

[Ni: 0-0.30%]
Ni is an element that contributes to improving strength by precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but in order to obtain such an effect, the Ni content should be 0.001% or less. % or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive inclusion of these elements does not have an effect. The Ni content is saturated, which may lead to an increase in manufacturing costs. Therefore, the Ni content is preferably 0.30% or less. The Ni content is preferably 0.20% or less, 0.15% or less, 0.10% or less. It may be 0.08% or less.

[V: 0-0.300%]
V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but in order to obtain such an effect, the V content must be 0.001% or more. The V content may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if V is excessively contained, the effect is saturated and the manufacturing cost is increased. Therefore, the V content is preferably 0.300% or less. The V content may be 0.200% or less, 0.100% or less, or 0.080% or less. good.

[Sn: 0-0.040%, As: 0-0.100%, Zr: 0-0.050%, Ca: 0-0.0010%, Mg: 0-0.0010%, Bi: 0- 0.010%, Co: 0 to 0.010%, W: 0 to 0.100%, Zn: 0 to 0.010%, and REM: 0 to 0.0100%]
Sn, As, Zr, Ca, Mg, Bi, Co, W, Zn, and REM may be contained in the steel sheet as optional elements, or may be present in the steel sheet as tramp elements. The contents of the elements are: Sn: 0 to 0.040% or 0.020%, As: 0 to 0.100% or 0.050%, Zr: 0 to 0.050% or 0.030%, Ca : 0-0.0010% or 0.0008%, Mg: 0-0.0010% or 0.0008%, Bi: 0-0.010%, Co: 0-0.010%, W: 0-0 . 100% or 0.050%, Zn: 0 to 0.010%, and REM: 0 to 0.0100% or 0.0050%. Regarding the lower limit of these elements, for example, the Sn, As, Zr, Bi, Co, W and Zn contents may be 0.001% or more, 0.005% or more, or 0.008% or more, respectively. Similarly, the Ca, Mg and REM contents may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

In the steel plate according to the embodiment of the present invention, the remainder other than the above elements consists of Fe and impurities. Impurities are, for example, components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel plate is industrially manufactured. It is permissible for them to be included within a range that does not affect the effects of the present invention.

The chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel plate may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[Metal structure]
[Martensite: 60.0 to 85.0%]
The metal structure of the steel plate according to the embodiment of the present invention includes, in terms of area%, 60.0 to 85.0% martensite. By configuring the metal structure of the steel plate with a structure containing hard martensite within such a range, it is possible to achieve high strength, for example, high strength with a tensile strength of 1180 MPa or more, while significantly improving the uniform elongation of the resulting steel plate. From the viewpoint of further increasing the strength, the higher the area ratio of martensite, the more preferable, and for example, it may be 62.0% or more, 65.0% or more, 68.0% or more, or 70.0% or more. From the viewpoint of further improving the uniform elongation, the lower the area ratio of martensite, the more preferable, and for example, it may be 82.0% or less, 80.0% or less, 78.0% or less, or 75.0% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[Granular bainite in which the maximum misorientation at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more is 3.5° or less and the intragranular misorientation is 10° or more: 10.0 to 30.0%]
The metal structure of the steel plate according to the embodiment of the present invention contains, by area%, 10.0 to 30.0% granular bainite in which the maximum misorientation at 0.1 μm intervals is 3.5° or less within grains surrounded by grain boundaries with a misorientation of 15° or more, and the intragranular misorientation is 10° or more. Here, the structure called granular bainite in the prior art does not necessarily have the characteristic of "the maximum misorientation at 0.1 μm intervals is 3.5° or less within grains surrounded by grain boundaries with a misorientation of 15° or more, and the intragranular misorientation is 10° or more." The structure called granular bainite in the prior art is often not fully defined, and therefore it is not recognized that the mere term granular bainite is the same as the granular bainite according to the embodiment of the present invention. In an embodiment of the present invention, it is extremely important that the metal structure of the steel sheet contains 10.0 to 30.0% by area of specific granular bainite having the above characteristics, in other words, granular bainite having the characteristic that "the orientation change in the crystal grain is relatively gentle and continuous, but the orientation difference in the entire crystal grain is relatively large", and such technical matters and the effects obtained thereby were discovered for the first time by the inventors. As described above, by containing 10.0% or more by area of granular bainite having the characteristic that the orientation change in the crystal grain is relatively gentle and continuous, but the orientation difference in the entire crystal grain is relatively large, it is possible to significantly suppress the occurrence of necking during forming due to such a characteristic orientation change. In addition, as described above, by containing 10.0% or more by area of the granular bainite, it is thought that the occurrence of voids from the interface between different phases during hole expansion processing is suppressed, and as a result, it is possible to improve the hole expansion property. From the viewpoint of further suppressing the occurrence of necking and/or further improving the hole expandability, the higher the area ratio of granular bainite, the more preferable, for example, 12.0% or more, 15.0% or more, or 18.0% or more. On the other hand, as described above, since granular bainite has characteristics similar to ferrite, if the area ratio of granular bainite becomes too high in a metal structure mainly composed of martensite, the metal structure becomes similar to so-called DP steel, resulting in a decrease in the yield ratio. Therefore, from the viewpoint of maintaining a higher yield ratio, the lower the area ratio of granular bainite, the more preferable, for example, 28.0% or less, 25.0% or less, or 22.0% or less.

　In the current study, the inventors applied a 10% prestrain to the steel plate in the C direction (direction perpendicular to the rolling direction) by uniaxial tension, and then performed a 90° bending test in the L direction (rolling direction). As a result, it was found that if necking does not occur in the bending test piece, it is possible to improve necking in the actual forming of parts. According to the steel plate according to the embodiment of the present invention, by including 10.0 to 30.0% of the above-mentioned specific granular bainite in the metal structure by area%, it is possible to reliably suppress the occurrence of necking even in such bending tests after prestrain. Therefore, according to the embodiment of the present invention, it is possible to reliably suppress the occurrence of necking even in the later deformation stage of the forming operation, which is performed in multiple steps such as actual press forming of steel plate for automobiles, and therefore the steel plate according to the embodiment of the present invention is particularly useful for use in the automobile field.

[Ferrite: 20.0% or less]
The metal structure of the steel plate according to the embodiment of the present invention includes ferrite: 20.0% or less in area %. If the soft structure of ferrite can be limited to 20.0% or less in area %, the hardness difference between each phase in the metal structure can be sufficiently reduced by precipitation strengthening the soft structure containing the ferrite with Ti precipitates. Therefore, the hole expandability can be improved more significantly by combining such a reduction in hardness difference with the improvement of the hole expandability due to the control of the average spacing of granular bainite, which will be described later. When the area ratio of ferrite exceeds 20.0%, the hole expandability may not be sufficiently improved even if the precipitation strengthening by Ti precipitates and the control of the average spacing of granular bainite are combined. In addition, when the area ratio of ferrite exceeds 20.0%, the total amount of granular bainite and ferrite increases, resulting in a metal structure similar to that of DP steel, and the yield ratio may decrease. From the viewpoint of increasing the hole expandability and/or the yield ratio, the lower the area ratio of ferrite, the more preferable, and may be, for example, 18.0% or less, 15.0% or less, 12.0% or less, 10.0% or less, 8.0% or less, 5.0% or less, or 3.0% or less. The lower limit of the area ratio of ferrite is not particularly limited and may be 0%, or may be, for example, 0.5% or more or 1.0% or more.

[Remainder structure]
The remaining structure other than martensite, the specific granular bainite, and ferrite may be 0% in area percent, but when the remaining structure is present, the remaining structure may include at least one of bainite, pearlite, and retained austenite: a total of 20.0% or less in area percent. If the total area ratio of at least one of bainite, pearlite, and retained austenite exceeds 20.0%, uniform elongation may decrease, or other structures such as martensite and granular bainite may not be controlled within a desired range. Therefore, the smaller the area ratio of the remaining structure, the more preferable it is. For example, the total area ratio of at least one of bainite, pearlite, and retained austenite may be 15.0% or less, 10.0% or less, 8.0% or less, 5.0% or less, or 3.0% or less. On the other hand, the lower limit is not particularly limited, and the total area ratio of at least one of bainite, pearlite, and retained austenite may be 0%, or may be, for example, 0.1% or more, 0.5% or more, or 1.0% or more.

[Average spacing of granular bainite grains: 50.0 μm or less]
In the metal structure of the steel sheet according to the embodiment of the present invention, the average spacing of the granular bainite grains is controlled to 50.0 μm or less. Here, the granular bainite grains refer to grains (crystal grains) of granular bainite in which the maximum orientation difference at 0.1 μm intervals is 3.5° or less within a grain surrounded by a grain boundary with an orientation difference of 15° or more, and the intragranular orientation difference is 10° or more. By controlling the average spacing between the grains of granular bainite exhibiting the above-mentioned characteristic orientation change to 50.0 μm or less, it is possible to significantly improve the hole expandability of the steel sheet by combining with the precipitation strengthening by ferrite and Ti precipitates of 20 area % or less as described above. In addition, the average spacing of the granular bainite grains is also a factor that determines the arrangement of the granular bainite structure, and therefore, if there is a bias in the spacing of the granular bainite grains, even if the granular bainite exhibiting the above-mentioned characteristic orientation change is contained in an area % of 10.0% or more, it may not be possible to reliably suppress the occurrence of necking during forming. From the viewpoint of further improving the hole expandability and further reliably suppressing the occurrence of necking, the smaller the average interval of the granular bainite grains, the more preferable, and may be, for example, 35.0 μm or less, 30.0 μm or less, 28.0 μm or less, 25.0 μm or less, or 23.0 μm or less. Although the lower limit is not particularly limited, for example, the average interval of the granular bainite grains may be 5.0 μm or more, 7.0 μm or more, 10.0 μm or more, or 15.0 μm or more.

[Average grain size of granular bainite grains is 5.0 to 30.0 μm]
In the metal structure of the steel plate according to the embodiment of the present invention, the average grain size of the granular bainite grains is preferably 5.0 to 30.0 μm. By controlling the average grain size of the granular bainite grains within the range of 5.0 to 30.0 μm, a fine and uniform granular bainite structure can be obtained, and the bendability after pre-strain can be further improved. For example, the average grain size of the granular bainite grains may be 6.0 μm or more, 8.0 μm or more, or 10.0 μm or more. Similarly, the average grain size of the granular bainite grains may be 25.0 μm or less, 22.0 μm or less, 20.0 μm or less, or 18.0 μm or less.

[Identification of martensite, bainite, pearlite and retained austenite and calculation of area ratio]
Identification of martensite, bainite, pearlite and retained austenite and calculation of the area ratio are performed by optical microscope observation after corrosion using a Nital reagent or Lepera solution and X-ray diffraction method. The structural observation by optical microscope is performed on the plate thickness cross section in the direction perpendicular to the plate surface. The plate thickness cross section is preferably parallel to the rolling direction. Specifically, first, a sample is taken from the steel plate, and the observation surface of the sample is etched with Nital. Next, image analysis is performed on a structural photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 μm × 300 μm using an optical microscope, thereby calculating the total area ratio of martensite and bainite, and the area ratio of pearlite. Next, using a sample whose observation surface has been Lepera-etched, image analysis is similarly performed on a structural photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 μm × 300 μm using an optical microscope, thereby calculating the total area ratio of martensite and retained austenite. Next, using a sample that has been milled from the normal direction of the rolled surface to a depth of 1/4 of the plate thickness, the volume fraction of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of retained austenite is equivalent to the area fraction, this is taken as the area fraction of retained austenite. The area fraction of martensite is calculated by subtracting the obtained area fraction of retained austenite from the total area fraction of martensite and retained austenite calculated previously. Finally, the area fraction of bainite is calculated by subtracting the obtained area fraction of martensite from the total area fraction of martensite and bainite calculated previously.

[Identification of ferrite and calculation of area ratio]
The identification of ferrite and the calculation of the area ratio are performed by electron backscattered diffraction (EBSD) as follows. Specifically, first, a sample is taken from the steel sheet so that the plate thickness cross section perpendicular to the plate surface is the observation surface. The plate thickness cross section is preferably parallel to the rolling direction. Next, EBSD analysis is performed at a measurement interval of 0.2 μm on a rectangular area of 200 μm in the plate thickness direction and 400 μm in the direction perpendicular to the plate thickness direction, centered on the 1/4 position of the plate thickness from the steel sheet surface, to obtain crystal orientation information of this rectangular area. The EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, from the crystal orientation information of this rectangular region, the grain average misorientation (GAM value) is calculated using software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. Finally, the region with a GAM value of 0.5° or less is identified as ferrite, and its area ratio is calculated. Here, the "GAM value" is the average value of the misorientation between adjacent pixels in a region surrounded by grain boundaries with a misorientation of 15° or more.

[Identification of granular bainite and calculation of area ratio]
The identification of granular bainite and the calculation of the area ratio are performed by EBSD as follows. Specifically, first, a sample is taken from the steel plate so that the plate thickness cross section perpendicular to the plate surface is the observation surface. The plate thickness cross section is preferably parallel to the rolling direction. Next, EBSD analysis is performed at measurement intervals of 0.1 μm on a rectangular region of 200 μm in the plate thickness direction and 400 μm in the direction perpendicular to the plate thickness direction, centered at a 1/4 position of the plate thickness from the steel plate surface, to obtain crystal orientation information of this rectangular region. The EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, using the crystal orientation information of this rectangular region, the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to define the region surrounded by grain boundaries with an orientation misorientation of 15° or more as a crystal grain, calculate the intragranular orientation misorientation of the crystal grain, and identify crystal grains with a maximum orientation misorientation of 3.5° or less at 0.1 μm intervals and an intragranular orientation misorientation, more specifically, a maximum intragranular orientation misorientation of 10° or more as granular bainite, and calculate its area fraction. The average of the area fractions obtained for any three intragranular lines is determined as the area fraction of the granular bainite. The "maximum intragranular orientation misorientation" for granular bainite is obtained by "Grain Reference Orientation Deviation (GROD)". The maximum orientation difference within a grain is determined as the misorientation with other pixels within the grain, based on the orientation of the pixel with the minimum KAM value (Kernel Average Misorientation) within the same crystal grain. In the embodiment of the present invention, the reference crystal orientation is the orientation with the minimum KAM value within the same crystal grain. The GROD and KAM values can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" provided with the EBSD analyzer.

[Method of determining average spacing and average grain size of granular bainite grains]
The average spacing of granular bainite grains is determined by measuring the distance between the center of gravity of the granular bainite grains identified in EBSD and the center of gravity of the nearest granular bainite grain, and averaging the distances measured at 100 or more points as the average spacing of granular bainite grains. Also, the average of the circle equivalent diameters of all granular bainite grains measured at 100 or more points is determined as the average grain size of granular bainite grains.

[Thickness]
The steel sheet according to the embodiment of the present invention generally has a sheet thickness of 1.0 to 8.0 mm, although it is not particularly limited thereto. For example, the sheet thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

The steel plate according to the embodiment of the present invention can suppress the occurrence of necking even in the molding of parts having a complex shape, and therefore can reliably achieve a high level of compatibility between the contradictory properties of high strength and excellent workability. Therefore, the steel plate according to the embodiment of the present invention is useful for use in parts in technical fields where compatibility between these properties is required, and is particularly useful for use in parts in the automotive field. For this reason, in a preferred embodiment, an automobile part, particularly an automobile suspension part, containing the steel plate according to the embodiment of the present invention is provided. Examples of automobile suspension parts include lower arms and trailing arms. These automobile parts, particularly automobile suspension parts, only need to contain the steel plate according to the embodiment of the present invention in at least a part of these parts, and therefore at least a part of these parts will satisfy the above-mentioned chemical composition and metal structure characteristics. In a part of a steel plate that has been relatively lightly processed in molding such as press molding, the characteristics of the steel plate do not change particularly before and after molding. A part of a steel plate that has been relatively lightly processed is judged by characteristics such as a smooth shape that has not been subjected to deformation such as bending, and a small rate of increase or decrease in plate thickness. In parts with complex shapes, such as lower arms and trailing arms, multiple forming operations can cause necking or constriction, reducing the rigidity of certain parts. For this reason, these parts cannot be manufactured from a single steel plate, and certain parts may become separate, resulting in increased part costs. However, with the steel plate according to the embodiment of the present invention, even in parts with complex shapes, such as lower arms and trailing arms, it is possible to perform multiple forming operations from a single steel plate without causing necking, which is economically advantageous.

[Mechanical properties]
[Tensile strength (TS) and uniform elongation (u-El)]
According to the steel sheet having the above chemical composition and metal structure, particularly the hot-rolled steel sheet, a high tensile strength, specifically a tensile strength of 1180 MPa or more can be achieved. The tensile strength is preferably 1200 MPa or more, 1220 MPa or more, or 1240 MPa or more. According to the steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, the occurrence of necking during forming can be significantly suppressed while improving uniform elongation and hole expandability by the specific combination of the chemical composition and metal structure described above. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel sheet may be 1780 MPa or less, 1470 MPa or less, or 1400 MPa or less. In addition, according to the steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, a high uniform elongation can be achieved, specifically a uniform elongation of 5.0% or more can be achieved. The uniform elongation is preferably 5.2% or more, 5.5% or more, 5.8% or more, or 6.0% or more. The upper limit of the uniform elongation is not particularly limited, but for example, the uniform elongation of the steel sheet may be 15.0% or less, 10.0% or less, or 8.0% or less. The tensile strength and uniform elongation are measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011. For example, when it is difficult to take a JIS No. 5 test piece due to dimensional constraints, other test pieces described in JIS Z 2241:2011 can be used. However, when the sheet thickness is less than 0.5 mm, the lower limit is set to 0.5 mm in order to perform an appropriate evaluation. For example, when it is difficult to obtain a JIS No. 5 test piece due to dimensional constraints and it is also difficult to use other test pieces described in JIS Z 2241:2011, a micro Vickers test in accordance with JIS Z 2244-1:2020 can be performed, and the value obtained by converting the hardness (HV) into tensile strength can be used. A sample to be subjected to the micro Vickers test can be prepared as follows. First, a sample is cut out from an arbitrary position 50 mm or more away from the end face of the steel plate (if a sample cannot be obtained from this position, a position avoiding the end) so that a plate thickness cross section perpendicular to the plate surface can be observed. The plate thickness cross section is preferably parallel to the rolling direction. The size of the sample depends on the measuring device, but is set to a size that allows observation of about 10 mm in the direction perpendicular to the plate thickness direction. The cross section of the sample is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the observation surface is finished by electrolytic polishing. The micro Vickers test is performed by measuring 30 points at 1/4 of the plate thickness with a load of 500 gf, and the average value of the measurements may be used. Conversion can be performed using the following formula.
Tensile strength [MPa] = 3.12 x Vickers hardness [HV] + 16

[Hole expansion ratio (λ)]
According to the steel plate having the above chemical composition and metal structure, high hole expandability, specifically, a hole expansion ratio of 40% or more can be achieved. The hole expansion ratio may be preferably 42% or more, more preferably 45% or more or 50% or more. The upper limit of the hole expansion ratio is not particularly limited, but for example, the hole expansion ratio may be 150% or less, 100% or less, or 70% or less. The hole expansion ratio is determined as follows. First, a test piece having a width of 100 mm and a length of 100 mm is taken from the steel plate, and a punch hole (initial hole: hole diameter d0 = 10 mm) is made using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, the initial hole is expanded with a conical punch with an apex angle of 60° until a crack penetrating the plate thickness occurs, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) of each test piece is calculated by the following formula. This hole expanding test is carried out three times, and the average value thereof is determined as the hole expanding ratio λ.
λ=100×{(d1-d0)/d0}

[Yield ratio (YR)]
According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the yield ratio can be increased, and more specifically, a yield ratio of 80% or more can be achieved. The yield ratio is preferably 82% or more, more preferably 85% or more. The upper limit is not particularly limited, but for example, the yield ratio may be 95% or less or 92% or less. The yield ratio is determined by the following formula based on the tensile strength and 0.2% proof stress measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the steel sheet and performing a tensile test in accordance with JIS Z 2241:2011.
Yield ratio YR = 0.2% yield strength / tensile strength TS x 100

<Method of manufacturing steel sheet>
Next, a preferred method for manufacturing the steel sheet according to the embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing the steel sheet according to the embodiment of the present invention, particularly a steel sheet having preferred properties, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, the following will specifically show the manufacture of a hot-rolled steel sheet, but the steel sheet according to the embodiment of the present invention includes any steel sheet having the above-described chemical composition and metal structure, i.e., not only a hot-rolled steel sheet but also a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description merely describes one example of a preferred manufacturing method when the steel sheet according to the embodiment of the present invention is a hot-rolled steel sheet.

The method for producing a steel sheet according to an embodiment of the present invention includes:
A heating step comprising heating a slab having the chemical composition described above in relation to the steel plate and holding it at a temperature of 1180-1320°C for at least 6000 seconds;
A hot rolling process including finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, the hot rolling process satisfying the following conditions (a) to (c); and (a) the rolling temperature in each of the rolling passes immediately preceding the last two rolling passes is 960 to 1080°C, and the rolling reduction in each of the rolling passes is 30 to 40%,
(b) cooling the rolled material to 910°C or less at an average cooling rate of 400°C/sec or more within 0.20 seconds after the rolling passes of the two stages immediately preceding the latter two stages, and (c) the rolling reduction rate in each rolling pass of the latter two stages is 20 to 30%. The method is characterized by including a cooling step in which the finish-rolled steel plate is water-cooled, cooled to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, then air-cooled in the temperature range for 2.0 to 6.0 seconds, and water-cooled the steel plate to 50°C or less within 13 seconds after air cooling. In the above manufacturing method, the temperatures described for the slab and steel plate refer to the surface temperature of the slab and the surface temperature of the steel plate, respectively. Each step will be described in detail below.

[Heating process]
First, a slab having the chemical composition described above in relation to the steel plate is heated and held at a temperature range of 1180 to 1320°C for 6000 seconds or more. From the viewpoint of productivity, it is preferable to use a slab obtained by continuous casting, but a slab obtained by casting and blooming can also be used, and if necessary, a slab obtained by hot working or cold working may be used. In this manufacturing method, holding at a temperature range of 1180 to 1320°C includes not only the case where the temperature of the slab is held at a constant temperature within the range of 1180 to 1320°C, but also the case where the temperature of the slab is held fluctuating within the range of 1180 to 1320°C. By holding the slab at a temperature range of 1180 to 1320°C for 6000 seconds or more, the coarse carbides present in the structure can be completely solid-dissolved, and the starting point of cracks can be eliminated. If the holding temperature is less than 1180°C or the holding time is less than 6000 seconds, the solid-dissolution of the coarse carbides is incomplete. If the solid solution of the coarse carbides is incomplete, the area ratio of martensite may become less than 60.0% due to the occurrence of ferrite or bainite transformation originating from such carbides in the cooling process described below, and as a result, the desired strength may not be obtained. The upper limit of the heating temperature of the slab is set to 1320°C or less from the viewpoint of the capacity and productivity of the heating equipment. The upper limit of the holding time in the temperature range of 1180 to 1320°C is preferably 10000 seconds or less.

[Hot rolling process]
[Rough rolling]
In the present manufacturing method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[Finish rolling]
[(a) Rolling temperature in each rolling pass of the two stages immediately preceding the last two stages: 960 to 1080° C., and rolling reduction in each rolling pass of the two stages immediately preceding the last two stages: 30 to 40%]
The heated slab or the slab that has been rough-rolled as required is then subjected to finish rolling. In this manufacturing method, the finish rolling is performed using a tandem rolling mill consisting of four or more rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, it is necessary to appropriately control the rolling temperature and the rolling reduction in each rolling pass of the two stages immediately preceding the last two stages, specifically, the rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is controlled to 960 to 1080°C, and similarly, the rolling reduction in each rolling pass of the two stages immediately preceding the last two stages is controlled to 30 to 40%. By performing rolling under relatively high pressure under relatively high temperature conditions in each rolling pass of the two stages immediately preceding the last two stages, recrystallization can be promoted to refine the austenite grains. In relation to this, it is possible to reduce the average spacing of granular bainite to within a desired range in the metal structure finally obtained.

On the other hand, if the rolling temperature in each of the rolling passes immediately before the last two stages is less than 960°C and/or the reduction ratio in each rolling pass is less than 30%, recrystallization is not sufficiently promoted, and the average spacing of granular bainite cannot be reduced to within the desired range in the metal structure of the steel sheet obtained at the end. On the other hand, if the rolling reduction ratio in each of the rolling passes immediately before the last two stages exceeds 40%, flat austenite grains are formed due to the introduction of excessive strain, and similarly the average spacing of granular bainite cannot be reduced to within the desired range in the metal structure obtained at the end. In addition, if the rolling temperature in each of the rolling passes immediately before the last two stages exceeds 1080°C, the austenite grains become coarse, and the desired structure fraction cannot be obtained even by subsequent rolling and cooling control, or in addition, the average spacing and/or average grain size of granular bainite cannot be controlled to within the desired range.

[(b) Cool to 910° C. or less at an average cooling rate of 400° C./s or more within 0.20 seconds after the rolling passes of the two stages immediately preceding the last two stages]
In this manufacturing method, the rolled material is cooled to 910°C or less at an average cooling rate of 400°C/s or more within 0.20 seconds after the two rolling passes immediately preceding the last two stages. By cooling the rolled material to 910°C or less relatively quickly in this manner after the two rolling passes immediately preceding the last two stages, grain growth after recrystallization can be suppressed, and the average spacing of granular bainite can be reduced to a desired range in the finally obtained metal structure. If the time taken to cool to 910°C or less after the two rolling passes immediately preceding the last two stages exceeds 0.20 seconds, grain growth after recrystallization cannot be sufficiently suppressed, and even if appropriate cooling is performed in the subsequent cooling step, the average spacing and/or average grain size of granular bainite cannot be controlled to a desired range.

The average cooling rate between the last two rolling passes and the two immediately preceding rolling passes is very important in generating granular bainite having the desired morphology within a specified range. More specifically, if the average cooling rate during this period is less than 400°C/s, the maximum orientation mismatch at 0.1 μm intervals within grains surrounded by grain boundaries with an orientation mismatch of 15° or more may exceed 3.5°, and therefore it becomes impossible to generate 10.0% or more of granular bainite having a maximum orientation mismatch of 3.5° or less and an intragranular orientation mismatch of 10° or more. The average cooling rate between the last two rolling passes and the two immediately preceding rolling passes is preferably 500°C/s or more. Similarly, if the cooling stop temperature is higher than 910°C, it may not be possible to generate 10.0% or more of granular bainite, in which the maximum orientation mismatch at 0.1 μm intervals is 3.5° or less and the intragranular orientation mismatch is 10° or more, within grains surrounded by grain boundaries with an orientation mismatch of 15° or more.

[(c) Reduction rate in each rolling pass in the latter two stages: 20 to 30%]
In this manufacturing method, the reduction ratio in each rolling pass of the latter two stages of finish rolling is controlled to 20 to 30%. By introducing strain at such an appropriate reduction ratio in each rolling pass of the latter two stages, it is possible to increase the number of nucleation sites for generating granular bainite in the subsequent cooling process. If the reduction ratio in each rolling pass of the latter two stages is less than 20%, nucleation sites for generating granular bainite cannot be sufficiently formed, and the desired area ratio of granular bainite cannot be obtained in the finally obtained metal structure. On the other hand, if the reduction ratio in each rolling pass of the latter two stages exceeds 30%, flat austenite grains are formed due to the introduction of excessive strain, and the average spacing of granular bainite cannot be reduced to within the desired range in the finally obtained metal structure.

[Cooling process]
[Cool to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, then air cool for 2.0 to 6.0 seconds]
The finish-rolled steel sheet is water-cooled in the next cooling step, cooled to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, and then air-cooled in this temperature range for 2.0 to 6.0 seconds. First, by cooling to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, it is possible to reliably suppress the generation of pearlite, and therefore it is possible to achieve a desired area fraction of the metal structure in the finally obtained steel sheet. In contrast, if the time from the start of water cooling to the temperature range of 500 to 650°C exceeds 4.0 seconds, a relatively large amount of pearlite is generated, and it becomes impossible to obtain a desired amount of martensite and/or granular bainite in the metal structure of the finally obtained steel sheet.

Furthermore, by air-cooling for 2.0 to 6.0 seconds in the temperature range of 500 to 650°C after water cooling, the transformation to granular bainite can be promoted and Ti precipitates can be properly precipitated. Therefore, the air-cooling operation for 2.0 to 6.0 seconds in the temperature range of 500 to 650°C after water cooling is extremely important not only from the viewpoint of the effect of suppressing the occurrence of necking due to granular bainite, but also from the viewpoint of the effect of improving hole expandability and the like due to precipitation strengthening caused by Ti precipitates. For example, if the air-cooling temperature is less than 500°C, the transformation to granular bainite cannot be sufficiently promoted, while a relatively large amount of bainite may be generated. In such a case, not only is the effect of suppressing the occurrence of necking reduced, but the generation of a large amount of bainite reduces the uniform elongation, and further, the generation of martensite in conjunction with the generation of bainite reduces the generation of sufficient strength, which may result in insufficient strength being obtained.

Also, when the air cooling temperature exceeds 650 ° C, the transformation to granular bainite cannot be sufficiently promoted, while the ferrite transformation is promoted, and a relatively large amount of ferrite may be generated. In addition, Ti precipitates cannot be sufficiently precipitated. In such a case, in addition to the effect of suppressing the occurrence of necking being reduced, the hole expandability and yield ratio of the obtained steel sheet are reduced due to the relatively large generation of ferrite and the insufficient precipitation strengthening by Ti precipitates. In addition, when the air cooling time is less than 2.0 seconds, the transformation to granular bainite cannot be sufficiently promoted, and furthermore, a relatively large amount of martensite may be generated by subsequent cooling. In such a case, in addition to the hole expandability being reduced and/or the effect of suppressing the occurrence of necking being reduced, the uniform elongation is reduced due to the excessive generation of martensite. On the other hand, when the air cooling time exceeds 6.0 seconds, a relatively large amount of granular bainite may be generated. In such a case, the amount of martensite is reduced and the total amount of granular bainite and ferrite is relatively high. As mentioned above, granular bainite has characteristics similar to ferrite, so when the total area ratio of granular bainite and ferrite in a metal structure mainly composed of martensite becomes relatively high, the metal structure becomes similar to that of so-called DP steel, resulting in a decrease in the yield ratio. The air cooling temperature is preferably 525 to 625°C, and the air cooling time is preferably 3.0 to 5.0 seconds.

[Water-cool to 50°C or less within 13 seconds after air-cooling]
After air cooling for 2.0 to 6.0 seconds in the temperature range of 500 to 650 ° C., the steel sheet is water-cooled to 50 ° C. or less within 13 seconds. By performing such rapid cooling, martensite can be generated within the desired area ratio range. If the water cooling to 50 ° C. or less exceeds 13 seconds or the cooling stop temperature is higher than 50 ° C., it may not be possible to achieve a martensite area ratio of 60.0% or more. In such a case, it becomes impossible to achieve the desired steel sheet strength. The lower limit of the water cooling time is not particularly limited, but for example, the water cooling time to 50 ° C. or less after air cooling may be 4 seconds or more or 5 seconds or more. In addition, the lower limit of the water cooling stop temperature is not particularly limited, but for example, the water cooling stop temperature may be 20 ° C. or more or 25 ° C. or more. The water-cooled steel sheet can finally be wound into the form of a hot-rolled coil. The winding conditions are not particularly limited, and can be performed under any appropriate temperature conditions.

　With the steel sheet manufactured by the above manufacturing method, by configuring the metal structure to include 60.0-85.0% martensite by area percent, it is possible to achieve high strength, for example tensile strength of 1180 MPa or more, while significantly improving uniform elongation. Furthermore, by including 10.0-30.0% by area percent granular bainite in the metal structure, in which the maximum orientation mismatch at 0.1 μm intervals is 3.5° or less within grains surrounded by grain boundaries with an orientation mismatch of 15° or more, and the intragranular orientation mismatch is 10° or more, and by controlling the average spacing of the granular bainite grains to 50.0 μm or less, it is possible to significantly suppress the occurrence of necking during forming while improving the yield ratio and hole expandability. In addition, by controlling the Ti content in the steel to 0.070 mass% or more, the Ti precipitates strengthen soft structures such as ferrite, thereby reducing the hardness difference between the phases in the metal structure, and the combination of the reduction in hardness difference and the improvement in hole expandability caused by the specific granular bainite makes it possible to more significantly improve the hole expandability. Therefore, the steel sheet manufactured by the above manufacturing method can suppress the occurrence of necking even in the molding of parts having complex shapes, and therefore can reliably achieve a high level of the contradictory properties of high strength and excellent workability, making it particularly useful in the automotive field where both properties are required.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

In the following examples, steel sheets according to embodiments of the present invention, particularly hot-rolled steel sheets, were manufactured under various conditions, and the tensile strength (TS), yield ratio (YR), uniform elongation (u-El), hole expansion ratio (λ), and occurrence of necking in bending tests after pre-straining of the resulting steel sheets were investigated.

First, molten steel was cast by continuous casting to form slabs having various chemical compositions shown in Tables 1 and 2. These slabs were heated to a temperature of 1180 to 1320°C and held for a time of 6000 to 10000 seconds, and then hot-rolled. Hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and the finish rolling was performed under the conditions shown in Table 3 using a tandem rolling mill consisting of five rolling stands. Next, the finish-rolled steel plate was water-cooled, air-cooled, and water-cooled under the conditions shown in Table 3, and then coiled to obtain a steel plate having a plate thickness of 2.4 to 3.4 mm.

The properties of the resulting steel plates were measured and evaluated using the following methods.

[Tensile strength (TS) and uniform elongation (u-El)]
The tensile strength (TS) and uniform elongation (u-El) were measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the rolling direction of the steel plate (C direction) and performing a tensile test in accordance with JIS Z 2241:2011.

[Hole expansion ratio (λ)]
The hole expansion ratio (λ) was determined as follows. First, a test piece having a width of 100 mm and a length of 100 mm was taken from the steel plate, and a punching tool having a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%) was used to make a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the burr was placed on the die side, and the initial hole was expanded with a conical punch having an apex angle of 60 ° until a crack penetrating the plate thickness occurred, and the hole diameter d1 mm at the time of the crack occurrence was measured, and the hole expansion ratio λ (%) of each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ.
λ=100×{(d1-d0)/d0}

[Yield ratio (YR)]
The yield ratio (YR) was determined by the following formula based on the tensile strength (TS) and 0.2% proof stress measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the rolling direction of the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241:2011.
Yield ratio YR = 0.2% yield strength / tensile strength TS x 100

[Whether or not necking occurs during bending tests after prestrain]
First, a tensile test piece taken from the steel plate, with a parallel part width of 36 mm, a parallel part length of 86 mm, R36 mm, a gripping part width of 50 mm, and a total length of 372 mm, was subjected to uniaxial tension in the C direction and prestrained by 10%. Next, a test piece of 60 mm [C direction] x 30 mm [L direction] was taken from the center of the tensile test piece, and a 90° bending test was performed in the L direction to check for the occurrence of necking. The test piece was evaluated as pass if no necking was observed, and as fail if necking was observed.

Steel sheets with a tensile strength (TS) of 1180 MPa or more, a uniform elongation (u-El) of 5.0% or more, a hole expansion ratio (λ) of 40% or more, a yield ratio (YR) of 80% or more, and no necking observed in a bending test after pre-strain were evaluated as having high strength, high uniform elongation, hole expansion property, and yield ratio, and capable of suppressing the occurrence of necking during forming. The results are shown in Table 4. "GB grains" in Table 4 refer to granular bainite grains.

Referring to Tables 1 to 4, in Comparative Example 4, the rolling temperature in each of the two rolling passes immediately preceding the last two passes in the hot rolling process was low, so it is believed that recrystallization was not sufficiently promoted. As a result, the average spacing of the granular bainite grains in the final metal structure exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 5, the rolling temperature in each of the two rolling passes immediately preceding the last two passes was high, so it is believed that the austenite grains became coarse. As a result, the area ratio of granular bainite was less than 10.0%, the average spacing of the granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 6, the reduction rate in the second rolling pass of the two rolling passes immediately preceding the last two passes was low, so it is believed that recrystallization was not sufficiently promoted. As a result, the average spacing of the granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 7, it is considered that the rolling reduction rate in the first rolling pass of the two rolling passes immediately before the last two rolling passes was high, resulting in the formation of flat austenite grains due to the introduction of excessive strain. As a result, the average spacing of the granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 8, it is considered that the time required for cooling to 910° C. or less after the rolling passes of the two rolling passes immediately before the last two rolling passes was more than 0.20 seconds, and therefore grain growth after recrystallization could not be sufficiently suppressed. As a result, the average spacing of the granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 9, the average cooling rate between the last two rolling passes and the two immediately preceding rolling passes was slow, so the area ratio of granular bainite showing the specified orientation change was less than 10.0%, and the area ratio of martensite was related to this and higher than 85.0%. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 10, the cooling stop temperature in the cooling between the last two rolling passes and the two immediately preceding rolling passes was high, so the area ratio of granular bainite showing the specified orientation change was less than 10.0%, and the area ratio of martensite was related to this and higher than 85.0%. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-strain.

In Comparative Examples 11 and 12, the reduction ratios of the first and second rolling passes of the latter two stages were low, so it is believed that nucleation sites for generating granular bainite could not be sufficiently formed. As a result, the area ratio of granular bainite was less than 10.0%, and in connection with this, the area ratio of martensite was higher than 85.0%, so u-El and λ decreased and necking occurred in the bending test after pre-strain. In Comparative Examples 13 and 14, the reduction ratios of the first and second rolling passes of the latter two stages were high, so it is believed that flat austenite grains were formed due to the introduction of excessive strain. As a result, the average spacing of granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 15, the water cooling time until air cooling in the cooling process was long, so a relatively large amount of pearlite was generated. As a result, the area ratio of granular bainite was less than 10.0%, λ decreased, and necking occurred in the bending test after prestrain. In Comparative Example 16, the air-cooling temperature was low, so the transformation to granular bainite could not be sufficiently promoted, and a relatively large amount of bainite was generated in relation to this. As a result, u-El and λ decreased, and necking occurred in the bending test after prestrain. In Comparative Example 17, the air-cooling temperature was high, so the transformation to granular bainite could not be sufficiently promoted, and a large amount of ferrite was generated in relation to this. In addition, it is considered that Ti precipitates could not be sufficiently precipitated. As a result, λ and YR decreased, and necking occurred in the bending test after prestrain. In Comparative Example 18, the air-cooling time was short, so the transformation to granular bainite could not be sufficiently promoted, and furthermore, a large amount of martensite was generated by subsequent cooling. As a result, u-El and λ decreased, and necking occurred in the bending test after prestrain. In Comparative Example 19, the air-cooling time was long, so a relatively large amount of granular bainite was formed, which resulted in less martensite being formed and a relatively large total amount of granular bainite and ferrite. As a result, TS and YR were reduced. In Comparative Example 20, the water-cooling time to 50°C or less after air-cooling was long, so the area ratio of martensite was less than 60.0%, and TS was reduced.

In Comparative Example 46, TS was reduced due to the low C content. In Comparative Example 47, λ was reduced due to the high C content. In Comparative Example 48, u-El was reduced due to the low Si content. In Comparative Example 49, a large amount of ferrite was generated due to the high Si content, and the total amount of granular bainite and ferrite was also high in relation to this. As a result, λ and YR were reduced. In Comparative Example 50, hardenability was reduced due to the low Mn content, and as a result, the area ratio of martensite was low, and the total amount of granular bainite and ferrite was relatively high in relation to this. As a result, TS and YR were reduced. In Comparative Example 51, the area ratio of granular bainite was low due to the high Mn content, λ was reduced, and necking occurred in the bending test after pre-strain. In Comparative Example 52, a large amount of ferrite was generated due to the high sol. Al content, and the total amount of granular bainite and ferrite was also high in relation to this. As a result, λ and YR were reduced. In Comparative Example 53, it is believed that coarse carbides, etc. were formed due to the high Nb content. As a result, the workability of the steel sheet was reduced, u-El and λ were reduced, and necking occurred in the bending test after pre-strain. In Comparative Example 54, it is believed that the Ti content was low, so precipitation strengthening by Ti precipitates could not be fully achieved. As a result, TS and λ were reduced. In Comparative Example 55, it is believed that coarse carbides, etc. were formed due to the high Ti content. As a result, the workability of the steel sheet was reduced, and λ was reduced.

In contrast, in all of the steel sheets according to the invention examples, by appropriately controlling each condition in the manufacturing method and having a predetermined chemical composition, it was possible to obtain a steel sheet in which the metal structure contains, in area percentages, 60.0 to 85.0% martensite, 10.0 to 30.0% granular bainite with a maximum orientation mismatch of 3.5° or less at 0.1 μm intervals within grains surrounded by grain boundaries with an orientation mismatch of 15° or more and an intragranular orientation mismatch of 10° or more, and 20.0% or less ferrite, and the average spacing of the granular bainite grains is 50.0 μm or less. As a result, despite the high tensile strength of 1180 MPa or more, the steel sheet has high uniform elongation, hole expandability, and yield ratio, and the occurrence of necking can be reliably suppressed even in bending tests after pre-straining.

Claims (6)

The chemical composition, in mass%, is
C: 0.060-0.200%,
Si: 0.30-2.00%,
Mn: 1.20-2.70%,
P: 0.100% or less,
S: 0.0300% or less,
sol. Al: 0.001-0.500%,
Nb: 0.001-1.000%,
O: 0.0100% or less,
N: 0.0070% or less,
Ti: 0.070-0.200%,
B: 0 to 0.0030%,
Cr: 0-0.90%,
Mo: 0 to 0.12%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
V: 0-0.300%,
Sn: 0 to 0.040%,
As: 0 to 0.100%,
Zr: 0 to 0.050%,
Ca: 0-0.0010%,
Mg: 0 to 0.0010%,
Bi: 0 to 0.010%,
Co: 0 to 0.010%,
W: 0-0.100%,
Zn: 0 to 0.010%,
REM: 0 to 0.0100%, and the balance: Fe and impurities;
The metal structure is, in area percent,
Martensite: 60.0 to 85.0%,
Granular bainite: 10.0 to 30.0%; and ferrite: 20.0% or less, the maximum misorientation at 0.1 μm intervals being 3.5° or less within grains surrounded by grain boundaries with a misorientation of 15° or more, and the intragranular misorientation being 10° or more.
A steel plate characterized in that the average spacing of granular bainite grains is 50.0 μm or less. The chemical composition, in mass%,
B: 0.0001 to 0.0030%,
Cr: 0.001-0.90%,
Mo: 0.001-0.12%,
Cu: 0.001-0.40%,
Ni: 0.001 to 0.30%,
V: 0.001-0.300%,
Sn: 0.001 to 0.040%,
As: 0.001 to 0.100%,
Zr: 0.001 to 0.050%,
Ca: 0.0001 to 0.0010%,
Mg: 0.0001 to 0.0010%,
Bi: 0.001 to 0.010%,
Co: 0.001 to 0.010%,
W: 0.001-0.100%,
Zn: 0.001 to 0.010%, and REM: 0.0001 to 0.0100%
The steel sheet according to claim 1, characterized in that it contains at least one of the following: The steel plate according to claim 1 or 2, characterized in that the metal structure further contains, by area percentage, at least one of bainite, pearlite and retained austenite: a total of 20.0% or less. The steel sheet according to any one of claims 1 to 3, characterized in that the average grain size of the granular bainite grains is 5.0 to 30.0 μm. A part comprising the steel plate according to any one of claims 1 to 4. A heating step comprising heating a slab having the chemical composition according to claim 1 or 2 and holding it at a temperature of 1180 to 1320°C for 6000 seconds or more;
A hot rolling process including finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, the hot rolling process satisfying the following conditions (a) to (c); and (a) the rolling temperature in each of the rolling passes immediately preceding the last two rolling passes is 960 to 1080°C, and the rolling reduction in each of the rolling passes is 30 to 40%,
(b) cooling the rolled material to 910°C or less at an average cooling rate of 400°C/sec or more within 0.20 seconds after the two rolling passes immediately preceding the latter two stages, and (c) the rolling reduction in each rolling pass of the latter two stages is 20 to 30%. A method for producing a steel plate, comprising a cooling step including: water-cooling the finish-rolled steel plate, cooling to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, then air-cooling in said temperature range for 2.0 to 6.0 seconds, and water-cooling the steel plate to 50°C or less within 13 seconds after air-cooling.