JP7754375B1 - High-strength galvanized steel sheet, components, automotive parts, manufacturing method for high-strength galvanized steel sheet and manufacturing method for components - Google Patents

Abstract

Contains C: 0.090-0.390%, Si: 0.25-2.00%, Mn: 2.00-3.70%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less. At the 1/4 thickness position, tempered martensite (M) is 80% or more, fresh martensite (FM) is less than 8%, the sum of ferrite (F) and bainite (B) is 15% or less, and retained austenite is 15% or less. At the interface with the zinc-plated layer, the sum of tempered M and FM is 40% or less, the sum of F and B is 60% or more, and oxide particles (average aspect ratio: 1.2-6.0, average major axis: 0.10-0.70 μm) are 2-18%. This makes it possible to obtain a high-strength galvanized steel sheet that is excellent in collision resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance.

Description

The present invention relates to high-strength galvanized steel sheets, components, automotive parts, methods for manufacturing high-strength galvanized steel sheets, and methods for manufacturing components.

With the aim of improving fuel efficiency (reducing _CO2 emissions) by reducing the weight of the vehicle body and improving crashworthiness at the same time, efforts are being made to increase the strength of thin steel sheets for automobiles, and new legal regulations are being introduced one after another.
In recent years, there has been an increasing number of cases in which high-strength galvanized steel sheets having a tensile strength (TS) of 1180 MPa or more (see, for example, Patent Document 1) are applied to major structural parts of automobiles in order to increase the strength of the vehicle body.

International Publication No. 2019/187090

For example, high-strength galvanized steel sheets used in automobile parts (especially automobile frame structural parts or automobile reinforcing parts) are required to have excellent crash resistance. In other words, they require a high yield ratio YR (= 100 x yield strength YS / tensile strength TS).

When high-strength galvanized steel sheets are used for automotive parts such as crash boxes, they are subjected to punching and bending processes. For this reason, the high-strength galvanized steel sheets used for these parts are required to have excellent stretch flangeability and bendability.

Furthermore, in members obtained by processing high-strength galvanized steel sheets having a tensile strength (TS) of 1180 MPa or more, delayed fracture (a phenomenon in which a member suddenly breaks) may occur due to hydrogen penetration in an atmospheric corrosive environment.
There is a high risk of delayed fracture occurring at the sheared edge caused by shearing.
Even if shearing is performed under a single set of conditions (shear angle, clearance, etc.), in reality, the conditions may vary due to disturbances such as the thickness and warpage of the steel plate. Furthermore, the sheared edge condition (damage, residual stress, etc.) varies greatly depending on the shear angle and clearance.
For this reason, high-strength galvanized steel sheets are also required to have excellent delayed fracture resistance at the sheared end surface when shearing is performed under various conditions (shear angle, clearance) (hereinafter referred to as "shear disturbance stability of delayed fracture resistance").

The present invention has been made in consideration of the above points, and aims to provide a high-strength galvanized steel sheet that has excellent shear disturbance stability in terms of impact resistance, stretch flangeability, bendability, and delayed fracture resistance.

As a result of extensive research, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.
That is, the present invention provides the following [1] to [6].
[1] A steel sheet comprising: a steel plate; and a zinc-plated layer disposed on a surface of the steel plate, wherein the steel plate has a composition, in mass%, of C: 0.090% or more and 0.390% or less, Si: 0.25% or more and 2.00% or less, Mn: 2.00% or more and 3.70% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; and wherein an area ratio of tempered martensite is 80% or more and an area ratio of fresh martensite is 1.00% or more at a 1/4 position of the thickness of the steel plate. and an area ratio of oxide particles of 2% to 18% at an interface between the steel sheet and the zinc-plated layer.
[2] The above-mentioned composition further includes, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% The high-strength galvanized steel sheet according to the above-mentioned [1], containing at least one element selected from the group consisting of Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.
[3] A member made using the high-strength galvanized steel sheet according to [1] or [2] above.
[4] An automobile part made of the member described in [3] above.
[5] A method for producing a high-strength galvanized steel sheet according to the above [1] or [2], wherein a steel slab having the chemical composition according to the above [1] or [2] is subjected to hot rolling, including rough rolling and finish rolling, to obtain a hot-rolled sheet, and then cooling C1 and coiling are performed, and in the cooling C1, an average cooling rate _vA in a temperature range T _A of 650°C or higher and a finish-rolling delivery temperature FT or lower is 15°C/s or higher, and a residence time _tB in a temperature range T _B of 550°C or higher and 600°C or lower is 2.0 s or lower, and the hot-rolled sheet is then pickled and cold-rolled to obtain a cold-rolled sheet, and the cold-rolled sheet is subjected to heating H1, and then cooling C2, including a galvanizing treatment, to a cooling stop temperature T _C of 150°C or lower, and then reheating H2, and in the heating H1, a temperature of 600°C or higher (A _C1 The average heating rate v1 in the temperature range T1 of (A _{C1 +20) ° C or less is 8 ° C/s or more, the average heating rate v2 in the temperature range T2 of (A C1} +20) ° C or more and A _C3 ° C or less is 1.0 ° C/s or more and less than 8.0 ° C/s, the average dew point DP2 in the temperature range T2 is -20 ° C or more and 15 ° C or less, and A _c3 a residence time t3 in a temperature range T3 of 500°C or higher and 1000°C or lower is 30 seconds or higher and 500 seconds or lower, an average dew point DP3 in the temperature range T3 is higher than DP2 and 20°C or lower, in the cooling C2, an average cooling rate v4 in a temperature range T4 of 500°C or higher and 750°C or lower is 3°C/s or higher and 35°C/s or lower, and a residence time t5 in a temperature range T5 of 400°C or higher and 500°C or lower is 10 seconds or higher and 250 seconds or lower, the galvanization treatment is carried out during or after residence in the temperature range T5, an average cooling rate v6 in a temperature range T6 of 150°C or higher and 400°C or lower is higher than 10.0°C/s, and in the reheating H2, a temperature X which is a maximum temperature reached and a holding time Y at (X-10)°C or higher satisfy the following formula (1):
7800≦(273+X)×(20+log(Y/3600))≦11800 (1)
[6] A method for manufacturing a component, comprising subjecting the high-strength galvanized steel sheet according to [1] or [2] to at least one of forming and joining to obtain the component, wherein the temperature X is expressed in °C and the holding time Y is expressed in seconds.

The present invention provides high-strength galvanized steel sheets that have excellent shear disturbance stability in terms of impact resistance, stretch flangeability, bendability, and delayed fracture resistance.

[High-strength galvanized steel sheet]
The high-strength galvanized steel sheet of this embodiment includes a steel sheet (base steel sheet) and a galvanized layer (galvanized layer) disposed on the surface of the steel sheet, and the steel sheet satisfies the below-described chemical composition and microstructure. As a result, the high-strength galvanized steel sheet of this embodiment has excellent crash resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance.
Here, "high strength" means that the tensile strength (TS) determined by the tensile test described below is 1180 MPa or more.
The criteria for determining whether a steel sheet is excellent in shear disturbance stability in terms of impact resistance, stretch flangeability, bendability, and delayed fracture resistance will be described later.

<Steel plate>
First, the steel sheet (base steel sheet) included in the high-strength galvanized steel sheet of this embodiment will be described.
The thickness of the steel plate is not particularly limited, and is, for example, 0.3 mm or more and 2.8 mm or less.

《Component composition》
First, the chemical composition of the steel sheet (base steel sheet) will be described.
The unit "%" in the composition of a component means "% by mass" unless otherwise specified.

(C: 0.090% or more and 0.390% or less)
C is one of the important basic components of steel, and in this embodiment, it affects the area ratio of tempered martensite and the area ratio of fresh martensite at the 1/4 position of the plate thickness.
If the C content is too low, the area ratio of tempered martensite at the quarter thickness position decreases, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the C content is 0.090% or more. The C content is preferably 0.115% or more, and more preferably 0.140% or more.
On the other hand, if the C content is too high, the area fraction of retained austenite at the 1/4 thickness position increases, which transforms into significantly hard fresh martensite during shearing, thereby reducing stretch flangeability. Furthermore, delayed fracture crack propagation is promoted, reducing the shear disturbance stability of delayed fracture resistance. Furthermore, an increase in the area fraction of retained austenite reduces the yield ratio (YR) due to stress-induced martensitic transformation, resulting in reduced crash resistance. Therefore, the C content is 0.390% or less. The C content is preferably 0.375% or less, more preferably 0.360% or less.

(Si: 0.25% or more and 2.00% or less)
Si affects the area ratio of oxide particles.
If the Si content is too low, the generation of Si-containing oxide particles is suppressed, the area ratio of the oxide particles is reduced, and the shear disturbance stability of the delayed fracture resistance property is reduced. Therefore, the Si content is 0.25% or more, preferably 0.30% or more, and more preferably 0.35% or more.
On the other hand, if the Si content is too high, the generation of Si-containing oxide particles is significantly promoted, the area ratio of the oxide particles increases, and the shear disturbance stability of the delayed fracture resistance property decreases. Therefore, the Si content is 2.00% or less, preferably 1.75% or less, and more preferably 1.50% or less.

(Mn: 2.00% or more and 3.70% or less)
Mn is an important hardenability element that affects the area ratio of tempered martensite and the area ratio of ferrite. Mn is also an austenite stabilizing element that affects the shear stress stability of delayed fracture resistance.
If the Mn content is too low, the area ratio of tempered martensite decreases and the area ratio of ferrite increases, resulting in a decrease in TS. Therefore, the Mn content is 2.00% or more, preferably 2.20% or more, and more preferably 2.40% or more.
On the other hand, if the Mn content is too high, austenite is stabilized, the amount of retained austenite at the 1/4 thickness position is excessively increased, and the hardness of martensite formed from the retained austenite during shearing is significantly increased. As a result, the shear disturbance stability of delayed fracture resistance is reduced. Therefore, the Mn content is 3.70% or less, preferably 3.50% or less, and more preferably 3.30% or less.

(P: 0.100% or less)
P segregates at prior austenite grain boundaries to embrittle the grain boundaries, thereby reducing the ultimate deformability of the steel sheet and therefore reducing bendability. Therefore, the P content is 0.100% or less, preferably 0.070% or less, and more preferably 0.040% or less.
There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, the P content may be 0.001% or more, or 0.003% or more.

(S: 0.0200% or less)
S exists as sulfide and reduces the ultimate deformability of the steel sheet, resulting in reduced bendability. Therefore, the S content is 0.0200% or less, preferably 0.0050% or less, and more preferably 0.0030% or less.
The lower limit of the S content is not particularly limited, but due to constraints on production technology, the S content may be 0.0001% or more.

(Al: 1.000% or less)
Al provides sufficient deoxidation and reduces inclusions in the steel.
If the Al content is too high, a large amount of ferrite is formed, resulting in a decrease in TS, so the Al content is 1.000% or less, preferably 0.500% or less, and more preferably 0.100% or less.
On the other hand, in order to stably perform deoxidation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.

(N: 0.0100% or less)
N exists as nitrides and reduces the ultimate deformability of the steel sheet, resulting in reduced bendability. Therefore, the N content is 0.0100% or less, preferably 0.0060% or less, and more preferably 0.0050% or less.
There is no particular lower limit for the N content, although due to constraints on production technology, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

(O: 0.0100% or less)
O exists as an oxide and reduces the ultimate deformability of the steel sheet, resulting in reduced bendability. Therefore, the O content is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020% or less.
There is no particular lower limit for the O content, although the O content may be 0.0001% or more due to constraints in production technology.

(arbitrary element)
The steel sheet may further contain the elements described below as its chemical composition.

((Ti, Nb and V))
As long as the contents of Ti, Nb, and V are not excessive, they do not form large amounts of coarse precipitates or inclusions, do not reduce the ultimate deformability of the steel sheet, and therefore do not reduce bendability. Therefore, when these elements are contained, the contents of Ti, Nb, and V are each preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less.
The lower limits of the contents of Ti, Nb, and V are not particularly limited. However, these elements increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heating H1, which will be described later. Therefore, the contents of Ti, Nb, and V are each preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

((Ta and W))
Unless the contents of Ta and W are excessive, they do not form large amounts of coarse precipitates or inclusions, do not reduce the ultimate deformability of the steel sheet, and therefore do not reduce bendability. Therefore, when these elements are contained, the contents of Ta and W are preferably 0.10% or less, and more preferably 0.08% or less, respectively.
The lower limits of the contents of Ta and W are not particularly limited. However, these elements increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heating H1, which will be described later. Therefore, the contents of Ta and W are each preferably 0.01% or more, and more preferably 0.03% or more.

((B))
As long as the B content is not excessive, cracks are not generated inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet is not reduced, so that the bendability is not reduced. Therefore, the B content is preferably 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less.
There is no particular lower limit for the B content. However, since B is an element that segregates at austenite grain boundaries during heating H1 described below and improves hardenability, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more.

((Cr, Mo and Ni))
As long as the contents of Cr, Mo, and Ni are not excessive, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so that the bendability is not reduced. Therefore, the contents of Cr, Mo, and Ni are each preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.60% or less.
The lower limits of the contents of Cr, Mo, and Ni are not particularly limited. However, since these elements improve hardenability, the contents of Cr, Mo, and Ni are each preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

((Co))
Co content does not increase coarse precipitates or inclusions and does not reduce the ultimate deformability of the steel sheet, so that bendability does not decrease unless the Co content is excessive. Therefore, the Co content is preferably 0.010% or less, and more preferably 0.008% or less.
The lower limit of the Co content is not particularly limited. However, since Co is an element that improves hardenability, the Co content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

((Cu))
Unless the Cu content is excessive, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so that bendability is not reduced. Therefore, the Cu content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.60% or less.
The lower limit of the Cu content is not particularly limited. However, since Cu is an element that improves hardenability, the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.08% or more.

((Sn))
As long as the Sn content is not excessive, cracks are not generated inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet is not reduced, so that the bendability is not reduced. Therefore, the Sn content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.060% or less.
The lower limit of the Sn content is not particularly limited. However, since Sn is an element that improves hardenability, the Sn content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

((Sb))
As long as the Sb content is not excessive, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so that the bendability is not reduced. Therefore, the Sb content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less.
The lower limit of the Sb content is not particularly limited. However, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

((Ca, Mg and REM))
As long as the contents of Ca, Mg, and REM (rare earth metals) are not excessive, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that the bendability will not be reduced. Therefore, the contents of Ca, Mg, and REM are each preferably 0.0100% or less, more preferably 0.0085% or less, and even more preferably 0.0050% or less.
The lower limits of the contents of Ca, Mg, and REM are not particularly limited. However, these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the contents of Ca, Mg, and REM are each preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.

((Zr and Te))
As long as the content of Zr and Te is not excessive, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so that the bendability is not reduced. Therefore, the content of Zr and the content of Te are each preferably 0.100% or less, more preferably 0.090% or less, and even more preferably 0.080% or less.
The lower limits of the contents of Zr and Te are not particularly limited. However, these elements spheroidize the shapes of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the contents of Zr and Te are each preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

((Hf))
As long as the Hf content is not excessive, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so that bendability is not reduced. Therefore, the Hf content is preferably 0.10% or less, more preferably 0.09% or less, and even more preferably 0.08% or less.
The lower limit of the Hf content is not particularly limited. However, Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet. Therefore, the Hf content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

((Bi))
As long as the Bi content is not excessive, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so that the bendability is not reduced. Therefore, the Bi content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less.
The lower limit of the Bi content is not particularly limited. However, since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

With regard to these optional elements (Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf and Bi), if the content of each is less than the preferred lower limit value mentioned above, it will be treated as an unavoidable impurity.

(balance: Fe and unavoidable impurities)
The steel sheet contains the above-mentioned elements as a chemical composition, with the balance being Fe and unavoidable impurities. It is preferable that the steel sheet contains only the above-mentioned elements and the balance being Fe and unavoidable impurities.
Examples of the unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. The total content of the unavoidable impurities is preferably 0.100% or less.

<Microstructure at 1/4 of the plate thickness>
Next, the microstructure of the steel sheet at the 1/4 position in the sheet thickness will be described.

(Area ratio of tempered martensite: 80% or more)
When the steel sheet contains tempered martensite, a TS of 1180 MPa or more can be obtained. Therefore, the area ratio of tempered martensite is 80% or more, preferably 82% or more, and more preferably 84% or more.
The upper limit is not particularly limited, and the area ratio of tempered martensite may be 100%.
Tempered martensite is a structure formed by precipitation of carbides in martensite formed during cooling C2 and subsequent reheating H2.
Tempered martensite is martensite in which carbides are observed in SEM images obtained by the measurement method described below. These carbides include cementite (θ), epsilon (ε), eta (η), and chi (χ) carbides. Tempered martensite also includes lower bainite, which forms below the Ms point.

(Area ratio of fresh martensite: less than 8%)
If the fresh martensite content is too high, the yield ratio (YR) decreases due to an increase in mobile dislocations, resulting in a decrease in impact resistance. Furthermore, the difference in hardness between the microstructures increases, resulting in a decrease in stretch flangeability. Therefore, the area fraction of fresh martensite is 8% or less, preferably 7% or less, and more preferably 6% or less.
The lower limit is not particularly limited, and the area ratio of fresh martensite may be 0%.
Fresh martensite is martensite in which no carbides are observed in an SEM image obtained by the measurement method described below.

(Total area ratio of ferrite and bainite: 15% or less)
If the amount of ferrite and bainite is too large, it becomes difficult to obtain a tensile strength (TS) of 1180 MPa or more. Furthermore, the difference in hardness between the microstructures increases, resulting in poor stretch flangeability. Furthermore, these structures become the starting point for plastic deformation, resulting in a low yield ratio (YR) and poor crashworthiness.
Therefore, the total area ratio of ferrite and bainite is 15% or less, preferably 13% or less, and more preferably 10% or less.
The lower limit is not particularly limited, and the total area ratio of ferrite and bainite may be 0%.
Ferrite is a soft BCC iron that forms at high temperatures and includes allotriomorphic and idiomorphic ferrite. Bainite is a prismatic BCC iron containing fine carbides that forms above the Ms point.

(Area ratio of retained austenite: 15% or less)
If there is too much retained austenite, it will turn into extremely hard fresh martensite during shearing, resulting in a decrease in stretch flangeability. Furthermore, delayed fracture crack propagation will be accelerated, reducing the shear disturbance stability of delayed fracture resistance. Furthermore, if the area fraction of retained austenite increases, stress-induced martensitic transformation will reduce the yield ratio (YR), resulting in a decrease in crash resistance. Therefore, the area fraction of retained austenite is 15% or less, preferably 13% or less, and more preferably 10% or less. There is no particular lower limit, and the area fraction of retained austenite may be 0%.

(Method for measuring area ratio)
The method for measuring the area ratios of tempered martensite, fresh martensite, ferrite, and bainite at the 1/4 position in the plate thickness is as follows.
First, a sample is cut out from a galvanized steel sheet so that the sheet thickness cross section (L cross section) parallel to the rolling direction serves as the observation surface. The observation surface of the sample is mirror-polished using diamond paste, then finish-polished using colloidal silica, and further etched using 1% by volume of nital to reveal the structure.
Next, a position at 1/4 of the sheet thickness of the steel sheet on the observation surface of the sample is observed at a magnification of 3000 times using a scanning electron microscope (SEM) under conditions of an acceleration voltage of 10 kV, and SEM images of three fields of view are obtained.
From the obtained SEM image, the area ratio of each structure is calculated using Adobe Photoshop (registered trademark) (manufactured by Adobe Systems). Specifically, the value obtained by dividing the area of each structure by the measured area is taken as the area ratio of each structure. The area ratio of each structure is calculated for three fields of view, and the average value of these is taken as the area ratio of each structure.
In SEM images, tempered martensite has a hierarchical structure with fine internal irregularities and is a structure in which carbides are dispersed. Fresh martensite has a hierarchical structure with fine internal irregularities and is a structure in which carbides are not dispersed in SEM images. Ferrite is a gray, flat structure region that does not contain carbides. Bainite is a mixed structure region composed of gray, angular bainitic ferrite and iron-based carbides that contrast white. Therefore, tempered martensite, fresh martensite, ferrite, and bainite can be distinguished from each other. The same distinction is also made at the interface positions described below.

The method for measuring the area ratio of retained austenite is as follows.
First, the steel plate is ground so that the measurement surface is at 1/4 of the plate thickness (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate), and then further polished by 0.1 mm by chemical polishing to obtain a sample. For the measurement surface of the sample, an X-ray diffractometer is used with a Co Kα radiation source to measure the integrated reflection intensities of the (200), (220), and (311) planes of fcc iron (austenite), and the (200), (211), and (220) planes of bcc iron. The intensity ratio of the integrated reflection intensity of each plane of fcc iron to the integrated reflection intensity of each plane of bcc iron is calculated. The average value of the nine intensity ratios is taken as the volume fraction of retained austenite. The volume fraction of retained austenite is considered to be the area fraction of retained austenite.

(remaining tissue)
The steel plate may have a structure (remaining structure) other than the above-mentioned tempered martensite, fresh martensite, ferrite, bainite, and retained austenite at the 1/4 position of the plate thickness.
Examples of the remaining structure include pearlite, alloy carbonitrides precipitated in ferrite, and other structures known as the structure of steel sheets.
The area ratio of the remaining structure is preferably 5% or less.

<Microstructure at the interface>
Next, the microstructure of the steel sheet at the interface between the coating layer and the steel sheet (hereinafter simply referred to as the "interface position") will be described.
The interface position is the polished surface (sheet surface) of the steel sheet from which the plating layer has been removed by polishing.

(Total area ratio of tempered martensite and fresh martensite: 40% or less)
If the tempered martensite and fresh martensite at the interface are too abundant, the number of bending crack initiation sites increases, resulting in poor bendability. Furthermore, if the tempered martensite and fresh martensite at the interface are too abundant, the difference in hardness between the microstructures increases, accelerating the propagation of delayed fracture cracks generated at the shear edge, and reducing the shear disturbance stability of the delayed fracture resistance. Therefore, the total area ratio of tempered martensite and fresh martensite at the interface is 40% or less, preferably 38% or less, and more preferably 35% or less. The lower limit is not particularly limited and may be 0%.

(Total area ratio of ferrite and bainite: 60% or more)
The total area ratio of ferrite and bainite at the interface is 60% or more, preferably 63% or more, and more preferably 65% or more. This reduces the number of bending crack initiation points, improving bendability. Furthermore, the propagation of delayed fracture cracks generated at the shear edge is suppressed, improving the shear disturbance stability of delayed fracture resistance. The upper limit is not particularly limited, and may be 100%.

(Method for measuring area ratio)
The area ratios of tempered martensite, fresh martensite, ferrite, and bainite at the interface were measured as follows.
First, a sample is cut out from a zinc-plated steel sheet so that the sheet surface (surface) perpendicular to the rolling direction serves as the observation surface. The zinc plating layer (plating layer) corresponding to the sheet surface of the sample is polished using diamond paste. Polishing is stopped when the plating layer disappears and the steel sheet (base steel sheet) is exposed. Further, the sample is etched using 1% by volume of nital to reveal the structure at the interface between the plating layer and the steel sheet.
Next, the observation surface of the sample is observed at a magnification of 3000 times using a scanning electron microscope (SEM) under the condition of an acceleration voltage of 10 kV, and SEM images of three fields of view are obtained.
From the obtained SEM image, the area ratio of each structure is calculated using Adobe Photoshop (registered trademark) (manufactured by Adobe Systems). Specifically, the value obtained by dividing the area of each structure by the measured area is taken as the area ratio of each structure. The area ratio of each structure is calculated for three fields of view, and the average value of these is taken as the area ratio of each structure.
In the SEM image, each structure is identified in the same way as at the 1/4 position of the plate thickness.

(area ratio of oxide particles: 2% or more and 18% or less)
The oxide particles contain, for example, at least one of the elements Si and Mn, but may contain other elements such as Cr and Al in addition to (or instead of) these elements.
When such oxide particles are dispersed at a certain area ratio at the interface, the delayed fracture resistance property has excellent shear disturbance stability.
Changes in the shear angle and clearance conditions change the damage and residual stress in the steel sheet at the sheared edge, which in turn changes the initiation point and propagation direction of microcracks in delayed fracture. Microcracks reach the surface of the steel sheet (base steel sheet) (i.e., the interface position) and then propagate from the interface in the thickness direction, resulting in macroscopic delayed fracture cracks. At this time, the presence of a certain area ratio of oxide particles at the interface position prevents microcracks initiated at the sheared edge from reaching the interface and propagating further in the thickness direction.
The mechanism is unclear, but two possible effects are thought to be that the oxide particles make hydrogen non-diffusible, and that the presence of a hard oxide phase diverts crack propagation at grain boundaries.

From the above, the area ratio of oxide particles at the interface position is 2% or more, preferably 3% or more, and more preferably 4% or more.
On the other hand, if the area ratio of oxide particles is too high, stress is concentrated on the oxide particles themselves, which become the starting points of delayed fracture, and the shear disturbance stability of delayed fracture resistance is reduced. Therefore, the area ratio of oxide particles is 18% or less, preferably 16% or less, and more preferably 14% or less.

(remaining tissue)
The steel sheet may have a structure (remaining structure) other than the above-mentioned tempered martensite, fresh martensite, ferrite, bainite, and oxide particles at the interface position.
Examples of the remaining structure include pearlite, alloy carbonitrides precipitated in ferrite, and other structures known as the structure of steel sheets.
The area ratio of the remaining structure is preferably 5% or less.

In order to achieve excellent shear stress stability with delayed fracture resistance, it is necessary to keep not only the area ratio of oxide particles but also the average aspect ratio and average long diameter of the oxide particles within specific ranges, as explained below.

(Average aspect ratio of oxide particles: 1.2 or more and 6.0 or less)
If the average aspect ratio of the oxide particles is too low, the effect of detouring delayed fracture cracks at the grain boundaries at the interface position becomes insufficient, and the shear disturbance stability of delayed fracture resistance deteriorates. Therefore, the average aspect ratio of the oxide particles is 1.2 or more, preferably 1.3 or more, and more preferably 1.4 or more.
On the other hand, if the average aspect ratio of the oxide particles is too high, stress tends to concentrate on the oxide particles themselves, which can become the starting point of delayed fracture and reduce the shear disturbance stability of the delayed fracture resistance. Therefore, the average aspect ratio of the oxide particles is 6.0 or less, preferably 5.5 or less, and more preferably 4.5 or less.

(Average major axis of oxide particles: 0.08 μm or more and 0.70 μm or less)
If the average major axis of the oxide particles is too short, the effect of detouring delayed fracture cracks at the grain boundaries at the interface position becomes insufficient, and the shear disturbance stability of the delayed fracture resistance property decreases. Therefore, the average major axis of the oxide particles is 0.08 μm or more, preferably 0.10 μm or more, and more preferably 0.12 μm or more.
On the other hand, if the average major axis of the oxide particles is too long, stress tends to concentrate on the oxide particles themselves, which can become the starting point of delayed fracture, thereby reducing the shear disturbance stability of the delayed fracture resistance. Therefore, the average major axis of the oxide particles is 0.70 μm or less, preferably 0.65 μm or less, and more preferably 0.60 μm or less.

(Method for measuring oxide particles)
The area ratio, average aspect ratio and average major axis of the oxide particles at the interface position are measured as follows.
First, a sample is cut out from a zinc-plated steel sheet so that the sheet surface (surface) perpendicular to the rolling direction serves as the observation surface. The zinc plating layer (plating layer) on the sheet surface of the sample is polished using diamond paste, and polishing is stopped when the plating layer disappears and the steel sheet (base steel sheet) is exposed. Note that the plating layer and the steel sheet can be distinguished by their clearly different contrast when observed using an optical microscope.
Next, the observation surface of the sample is observed at a magnification of 5000x using a scanning electron microscope (SEM) at an acceleration voltage of 10 kV, and SEM images of three fields of view are obtained. A backscattered electron detector is used to capture the SEM images. In the SEM images, the black contrast represents oxide particles, and the gray contrast represents other parts (structure) of the steel sheet. The presence of oxygen elements (i.e., oxide particles) can be confirmed by analyzing the black contrast parts using an electron probe microanalyzer (EPMA) or an energy dispersive X-ray analyzer (SEM-EDX) attached to the SEM. Similarly, the presence of elements contained in the oxide particles (e.g., at least one of Si and Mn) can be confirmed by analyzing the black contrast parts using EPMA or SEM-EDX.

The obtained SEM image is subjected to binarization processing using image processing software (e.g., Image J) to separate the oxide particles from other parts (structures). From the obtained binarized image, the area of the oxide particles is divided by the measured area to calculate the area ratio. The area ratio is calculated for three fields of view, and the average value is taken as the area ratio of the oxide particles.
Furthermore, from the obtained binary image, each oxide particle is approximated as an ellipse, and the value obtained by dividing the long axis of the particle by the short axis is defined as the aspect ratio. The aspect ratios of the oxide particles contained in the binary images for three fields of view are calculated, and the average value thereof is defined as the average aspect ratio of the oxide particles.
The average value of the major axis diameters of all the oxide particles thus determined is defined as the average major axis diameter of the oxide particles.
It is important to observe the steel sheet from the sheet surface direction and measure each parameter (area ratio, average aspect ratio, and average major axis) of oxide particles. For example, SEM images obtained by observing a sheet thickness cross section (L cross section) parallel to the rolling direction are insufficient.

<Zinc plating layer>
Next, the zinc plating layer (plating layer) will be described. The plating layer is formed by a zinc plating process described later.
The galvanized layer is not particularly limited, and may be a hot-dip galvanized layer, an alloyed hot-dip galvanized layer (alloyed hot-dip galvanized layer), or an electrogalvanized layer.
The plating layer may contain elements such as Al and Mg.
The composition of the plating layer is not particularly limited and may be a general composition, which generally contains 20 mass % or less of Fe, 0.001 to 1.0 mass % of Al, and a total of 0 mass % to 3.5 mass % of at least one element selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the remainder being Zn and unavoidable impurities.
The coating weight of the plating layer per side may be, for example, 20 g/m ² or more and 80 g/m ² or less.
The Fe content in the plating layer is, for example, less than 7 mass %.
When the plating layer is a galvannealed layer, the Fe content in the plating layer is preferably 7% by mass or more, and more preferably 20% by mass or less, and more preferably 15% by mass or less.

[Method of manufacturing high-strength galvanized steel sheet]
Next, a method for producing the high-strength galvanized steel sheet of the present embodiment will be described.
In general, a steel slab having the above-described chemical composition is first hot-rolled to obtain a hot-rolled sheet. The obtained hot-rolled sheet is then cooled (C1) and coiled. Next, the hot-rolled sheet is pickled and cold-rolled to obtain a cold-rolled sheet. The obtained cold-rolled sheet is then heated (H1), cooled (C2), and reheated (H2). During cooling (C2), the cold-rolled sheet is galvanized.

<Steel slab>
First, a steel slab (also simply referred to as a "slab") having the above-mentioned component composition is prepared.
The method for producing molten steel to be used as a steel slab is not particularly limited, and known methods using a converter, an electric furnace, or the like can be employed.
Steel slabs are preferably produced using a continuous casting method to prevent macrosegregation, but can also be produced by other methods such as ingot casting and thin slab casting.

<Hot rolling>
The steel slab is then subjected to hot rolling, including rough rolling and finish rolling, to obtain a hot-rolled sheet. In one example, the steel slab is cooled to room temperature, and then reheated and hot-rolled (rough rolling and finish rolling). The produced steel slab may be charged into a heating furnace as a hot piece without being cooled to room temperature, or may be briefly held at room temperature and then immediately rough-rolled.

《Rough rolling》
A rough-rolled plate is obtained by rough rolling a steel slab.
The slab heating temperature during rough rolling of a steel slab is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. On the other hand, the slab heating temperature is preferably 1300°C or lower to prevent an increase in scale loss.

Finishing rolling
Next, the rough rolled sheet is subjected to finish rolling to obtain a hot rolled sheet.
When the slab heating temperature is set low, it is preferable to heat the roughly rolled sheet using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.
The finish rolling may be carried out continuously by joining the rough rolled sheets together, or the rough rolled sheet may be temporarily wound up before the finish rolling is carried out.
The finish rolling delivery temperature FT is preferably 700°C or higher. This reduces the rolling load. Furthermore, the rolling reduction in the non-recrystallized state of austenite is reduced, suppressing the development of abnormal structures elongated in the rolling direction, and improving workability.
In order to reduce the rolling load, part or all of the finish rolling may be performed as lubricated rolling. Lubricated rolling is also preferred from the viewpoint of uniforming the shape and material properties of the steel sheet. The coefficient of friction during lubricated rolling is preferably 0.10 or more and 0.25 or less.

<Cooling C1>
The hot-rolled sheet obtained by hot rolling is subjected to cooling C1 under the following conditions.

Average cooling rate v _A in temperature range T _A : 15° C./s or more
In a temperature range _TA of 650°C or higher and equal to or lower than the finish rolling delivery temperature FT, oxidation occurs on the steel sheet surface. If the average cooling rate _vA in this temperature range _TA is slow, oxides containing Si and Mn are formed on the steel sheet surface. These oxides are removed by pickling, which will be described later, thereby reducing the Si and Mn in the steel sheet surface layer. As a result, during heating H1, which will be described later, oxides are less likely to be formed, and the area ratio of oxide particles decreases.
Therefore, the average cooling rate _vA is 15°C/s or more, and preferably 20°C/s or more. Although there is no particular upper limit, from the viewpoint of facility capacity, the average cooling rate _vA is preferably 200°C/s or less, more preferably 170°C/s or less, and even more preferably 130°C/s or less.

Residence time t _B in temperature range _TB : 2.0 seconds or less
In a temperature range T _B of 550° C. or higher and 600° C. or lower, Si and Mn are oxidized at grain boundaries in the surface layer of the steel sheet. If the residence time t _B in this temperature range T _B is long, oxide particles generated by hot rolling grow excessively during heating H1, which will be described later, and the average major axis of the oxide particles becomes longer.
Therefore, the residence time _tB is 2.0 seconds or less, and preferably 1.5 seconds or less.
Although there is no particular lower limit, from the viewpoint of facility capacity, the residence time _tB is preferably 0.1 s or more, and more preferably 0.3 s or more.

<Winding>
The hot-rolled sheet that has been subjected to the cooling C1 is then coiled.
The coiling temperature is preferably 400°C or higher and 550°C or lower from the viewpoint of improving the sheet passing properties during cold rolling, which will be described later.

<Pickling>
The coiled hot-rolled sheet is then pickled before being subjected to cold rolling.
Pickling removes oxides from the surface of the hot-rolled steel sheet, resulting in a high-quality coating layer in the final high-strength galvanized steel sheet. Pickling may be performed once or multiple times.

<Cold rolling>
The hot-rolled sheet after pickling is optionally subjected to a softening heat treatment and then cold-rolled to obtain a cold-rolled sheet. The cold-rolling conditions are not particularly limited, but the cumulative reduction in cold rolling is preferably 20 to 75%. The number of rolling passes and the reduction in each pass are also not particularly limited.

<Heating H1>
Next, the obtained cold-rolled sheet is subjected to heating H1 under the following conditions.
In general, the heating H1 is a process of heating the cold-rolled sheet to a temperature range T3 described later and retaining the sheet therein.

Average heating rate v1 in temperature range T1: 8°C/s or more
If the average heating rate v1 in the temperature range T1 of 600°C or higher and (A _C1 + 20)°C or lower is too small, oxide particles are generated in the unrecrystallized structure formed by cold rolling. Then, due to the subsequent reverse transformation and grain growth, the oxide particles are left behind in the austenite grains, and the aspect ratio of the oxide particles decreases.
Therefore, the average heating rate v1 is 8° C./s or more, and preferably 9° C./s or more.
The upper limit is not particularly limited, but from the viewpoint of operability and damage to the furnace body, the average heating rate v1 is preferably 100°C/s or less, more preferably 80°C/s or less, and even more preferably 50°C/s or less.

A _C1 point (unit: °C) is calculated based on the following formula.
A _C1 =720+29×[%Si]-21×[%Mn]+17×[%Cr]
In the above formula, [% X] represents the content (unit: mass %) of element X in the chemical composition of the steel sheet, and is set to 0 when element X is not contained.

Average heating rate v2 in temperature range T2: 1.0°C/s or more and less than 8.0°C/s
The temperature region T2 from (A _C1 + 20) ° C. to A _C3 ° C. is a two-phase region of ferrite and austenite, and oxide particles with a high aspect ratio are generated at the interface between ferrite and austenite. If the average heating rate v2 in the temperature region T2 is too small, the generation of oxide particles with a high aspect ratio is promoted, and the average aspect ratio becomes excessively large. Therefore, the average heating rate v2 is 1.0 ° C./s or more, preferably 1.5 ° C./s or more, and more preferably 2.0 ° C./s or more.
On the other hand, if the average heating rate v2 is too high, the generation of oxide particles with a high aspect ratio is suppressed, resulting in a small average aspect ratio. Therefore, the average heating rate v2 is less than 8.0°C/s, preferably less than 7.5°C/s, and more preferably less than 7.0°C/s.

The A _C3 point (unit: °C) is calculated based on the following formula.
A _C3 =910-203×[%C]1/2+44.7×[%Si]-30×[%Mn]+700×[%P]+400×[%Al]+400×[% Ti]+104×[%V]+13.1×[%W]-11×[%Cr]+31.5×[%Mo]-15.2×[%Ni]-20×[%Cu]
In the above formula, [% X] represents the content (unit: mass %) of element X in the chemical composition of the steel sheet, and is set to 0 when element X is not contained.

Average dew point DP2 in temperature range T2: -20°C or higher and 15°C or lower
As described above, the temperature region T2 is a two-phase region of ferrite and austenite, and oxide particles with a high aspect ratio are generated at the interface between ferrite and austenite. If the average dew point DP2 in the temperature region T2 is too low, the generation of oxide particles is suppressed, and the area ratio of oxide particles decreases. Furthermore, decarburization in the surface layer of the steel sheet is suppressed, and the total area ratio of tempered martensite and fresh martensite at the interface position increases, while the total area ratio of ferrite and bainite decreases. For this reason, the average dew point DP2 is −20°C or higher, preferably −18°C or higher, and more preferably −15°C or higher.
On the other hand, if the average dew point DP2 is too high, the generation of oxide particles is promoted and the area ratio of oxide particles increases. Therefore, the average dew point DP2 is 15°C or less, preferably 14°C or less, and more preferably 13°C or less.

Residence time t3 in temperature range T3: 30 seconds or more and 500 seconds or less
In the temperature region T3 between A _C3 °C and 1000 °C, oxide particle generation and grain growth, and crystal grain growth of austenite grains in which reverse transformation is completed at the 1/4 position in the sheet thickness occur. If the residence time t3 in the temperature region T3 is too short, the average major axis of the oxide particles becomes short. Furthermore, the austenite grain size becomes fine at the 1/4 position in the sheet thickness, and ferrite transformation and bainite transformation are promoted during cooling C2, which will be described later, and the area ratio of tempered martensite decreases. Therefore, the residence time t3 is 30 seconds or more, preferably 35 seconds or more, and more preferably 40 seconds or more.
On the other hand, if the residence time t3 is too long, the average major axis of the oxide particles becomes long, so the residence time t3 is 500 seconds or less, preferably 450 seconds or less, and more preferably 400 seconds or less.

Average dew point DP3 in temperature range T3: above DP2 and below 20°C
As described above, in the temperature range T3, oxide particles are generated and grow. If the average dew point DP3 in the temperature range T3 is too low, the oxide particles do not grow easily, and the average major axis of the oxide particles becomes shorter. In order to increase the average major axis of the oxide particles, the increase in the average major axis can be promoted by making DP3 > DP2. Therefore, the average dew point DP3 is greater than DP2.
On the other hand, if the average dew point DP3 is too high, the generation of oxide particles will be excessive, and the area ratio of oxide particles will become high. Therefore, the average dew point DP3 is 20° C. or less.

<Cooling C2>
Next, the cold-rolled sheet subjected to the heating H1 is subjected to the cooling C2 under the following conditions.
Cooling C2 is a process of cooling the cold-rolled sheet that has been subjected to heating H1 to a temperature of 150°C or less (cooling stop temperature T _C ), and during this process, the cold-rolled sheet is retained in a temperature range T5 or subjected to a galvanizing process.

<<Average cooling rate v4 in temperature range T4: 3°C/s or more and 35°C/s or less>>
The temperature range T4 of 500°C or higher and 750°C or lower is a temperature range in which ferrite transformation occurs. If the average cooling rate v4 in the temperature range T4 is too low, excessive ferrite transformation occurs at the 1/4 position in the steel plate thickness, resulting in a high area fraction of ferrite and a low area fraction of tempered martensite at the 1/4 position in the steel plate thickness. Therefore, the average cooling rate v4 is 3°C/s or higher, preferably 4°C/s or higher, and more preferably 5°C/s or higher.
On the other hand, if the average cooling rate v4 is too high, ferrite transformation at the interface becomes difficult to occur, and the area ratio of ferrite at the interface decreases. Therefore, the average cooling rate v4 is 35° C./s or less, preferably 33° C./s or less, and more preferably 30° C./s or less.

<<Residence time t5 in temperature range T5 is 10 seconds or more and 250 seconds or less>>
The temperature range T5 of 400°C or higher and 500°C or lower is a temperature range in which bainite transformation occurs. If the residence time t5 in the temperature range T5 is too short, bainite transformation becomes difficult to occur at the interface position, and the area ratio of bainite at the interface position decreases. Therefore, the residence time t5 is 10 seconds or longer, preferably 13 seconds or longer, and more preferably 15 seconds or longer.
On the other hand, if the residence time t5 is too long, excessive bainite transformation occurs at the 1/4 position in the plate thickness of the steel plate, the area ratio of bainite at the 1/4 position in the plate thickness of the steel plate increases, and the area ratio of tempered martensite decreases. Therefore, the residence time t5 is 250 seconds or less, preferably 180 seconds or less, and more preferably 130 seconds or less.

<Zinc plating treatment>
The steel sheet (cold-rolled sheet) is subjected to a galvanizing treatment while it is retained in the temperature range T5 or after it has been retained in the temperature range T5. The galvanizing treatment is, for example, a hot-dip galvanizing treatment. After the hot-dip galvanizing treatment, an alloying hot-dip galvanizing treatment may be performed.
In the hot-dip galvanizing treatment, it is preferable to immerse the steel sheet in a galvanizing bath and then adjust the coating weight of the coating layer by gas wiping, etc. The bath temperature of the galvanizing bath is not particularly limited, but is preferably 440°C or higher and 500°C or lower.
The Al content of the zinc plating bath is preferably 0.10 mass % or more and 0.23 mass % or less.
When alloying treatment is performed, the temperature of the alloying treatment is preferably 470°C or higher in order to improve the Zn-Fe alloying rate and improve productivity. Furthermore, in order to effectively prevent untransformed austenite from transforming into pearlite and improve TS, the temperature of the alloying treatment is preferably 600°C or lower, more preferably 560°C or lower.

If, among the various temperatures in the zinc plating process, a certain temperature T (e.g., the bath temperature of the zinc plating bath) overlaps with the temperature range T5 (400°C or higher and 500°C or lower), the process P (e.g., immersion of the steel sheet in the zinc plating bath) at that temperature T in the zinc plating process is carried out during the residence time in the temperature range T5, and the time during which process P is carried out is included in the residence time t5 in the temperature range T5.

Average cooling rate v6 in temperature region T6: greater than 10.0°C/s
The temperature range T6 of 150°C or higher and 400°C or lower is a temperature range in which martensitic transformation and partitioning of C from the generated martensite to untransformed austenite occur. If the average cooling rate v6 in the temperature range T6 is slow, partitioning of C from the generated martensite to the untransformed austenite occurs, stabilizing the austenite. When cooling C2 is stopped, the amount of untransformed austenite increases, and in the subsequent reheating H2, the austenite is further stabilized, increasing the area fraction of retained austenite. For this reason, the average cooling rate v6 is greater than 10.0°C/s, preferably 11.0°C/s or higher, and more preferably 12.0°C/s or higher.
There is no particular upper limit, and the average cooling rate v6 is, for example, 180° C./s or less, may be 150° C./s or less, or may be 120° C./s or less.

《Cooling stop temperature T _C : 150℃ or less》
If the cooling stop temperature T _C is too high, the martensitic transformation will not be completed, and the subsequent reheating H2 will not cause tempering, resulting in too much fresh martensite. Therefore, the cooling stop temperature T _C is 150°C or less, preferably 130°C or less, and more preferably 100°C or less.
The lower limit is not particularly limited, and the cooling stop temperature T _C is, for example, 2° C. or higher, and may be 4° C. or higher.

<Reheating H2>
Next, the galvanized cold-rolled sheet cooled to the cooling stop temperature T _C is subjected to reheating H2. In reheating H2, the galvanized cold-rolled sheet is heated and held at the highest temperature (temperature X), and then cooled to room temperature, for example. This provides the high-strength galvanized steel sheet of the present embodiment.

《Formula (1)》
The reheating H2 is carried out under the condition that the temperature X (unit: ° C.) which is the maximum temperature reached and the holding time Y (unit: s) at (X-10) ° C. or higher satisfy the following formula (1):
7800≦(273+X)×(20+log(Y/3600))≦11800 (1)
As described above, when the cold-rolled sheet cooled to the cooling stop temperature T _C is appropriately heated again, carbides are precipitated in the martensite, and tempered martensite is formed.
In this case, if the value of the variable part of formula (1), "(273 + X) × (20 + log (Y/3600))", is too small, carbide precipitation in martensite becomes insufficient, and fresh martensite increases. Therefore, the value of the variable part is 7800 or more, preferably 8000 or more, and more preferably 8200 or more.
On the other hand, if the value of the variable part is too large, tempered martensite is decomposed into ferrite, making it difficult to secure the desired amount of tempered martensite, and therefore the tensile strength (TS) decreases. Therefore, the value of the variable part is 11,800 or less, preferably 11,500 or less, and more preferably 11,300 or less.

As long as the heat treatment in the above-described manufacturing method satisfies the above-described thermal history, other conditions are not particularly limited, and the equipment for carrying out the heat treatment is also not particularly limited.
The manufacturing conditions other than those mentioned above are the same as those in the ordinary method.

The high-strength galvanized steel sheet of this embodiment obtained by the above-described manufacturing method may be subjected to temper rolling or the like using a skin pass mill, a tension leveler, or the like.
At this time, the rolling reduction is preferably 0.01% or more from the viewpoint of stabilizing the shape. Although there is no particular upper limit, the rolling reduction is preferably 1.50% or less from the viewpoint of productivity.
In temper rolling, the target reduction may be obtained in one rolling operation, or rolling may be carried out in several separate operations.

From the standpoint of productivity, it is preferable to carry out a series of processes including zinc plating using a CGL (Continuous Galvanizing Line).

[Materials and Automotive Parts]
Next, the members of this embodiment will be described. The following description also includes a description of an automobile part made of the members of this embodiment.
The member of this embodiment is a member that uses the high-strength galvanized steel sheet of this embodiment described above as at least a part thereof, and is, for example, formed into a desired shape by forming or joining the high-strength galvanized steel sheet of this embodiment.
The member of this embodiment is preferably a member for an automobile part, such as an automobile frame structural part or an automobile reinforcing part.
As described above, the high-strength galvanized steel sheet of this embodiment is excellent in crash resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance. Therefore, the member of this embodiment is suitable for use in general as an automobile part (particularly, an automobile frame structural part or an automobile reinforcing part).

[Member manufacturing method]
Next, a method for manufacturing the member of this embodiment will be described.
The member of this embodiment can be obtained, for example, by subjecting the high-strength galvanized steel sheet of this embodiment to at least one of forming and joining.
The molding process is not particularly limited, and examples thereof include press working.
The joining process is not particularly limited, and examples thereof include common welding such as spot welding and arc welding; and caulking using rivets or the like.

The present invention will be specifically explained below using examples. However, the present invention is not limited to the examples described below.

<Manufacturing of galvanized steel sheets>
Using the molten steel produced in the converter, steel slabs having the chemical compositions shown in Table 1 below (the balance being Fe and unavoidable impurities) were obtained by continuous casting.
The obtained steel slab was subjected to hot rolling to obtain a hot-rolled sheet. More specifically, in the hot rolling, the steel slab was heated to 1250°C, subjected to rough rolling, and then subjected to finish rolling at a finish rolling outlet temperature FT of 900°C.
The hot-rolled sheet was subjected to cooling C1 under the conditions shown in Table 2 below, and then coiled at a coiling temperature of 500°C. The hot-rolled sheet was then cooled to room temperature, pickled, subjected to softening heat treatment at 500°C, and then cold-rolled at a rolling reduction of 50%. In this way, a cold-rolled sheet having a thickness of 1.4 mm was obtained.
Thereafter, the cold-rolled sheet was subjected to heating H1, cooling C2, and reheating H2 under the conditions shown in Table 2 below. During cooling C2, the cold-rolled sheet was subjected to a galvanizing treatment under the conditions described below. In this way, a galvanized steel sheet was obtained.

<Zinc plating treatment>
The cold-rolled sheet was subjected to a hot-dip galvanizing treatment to form hot-dip galvanized layers on both sides thereof, that is, a hot-dip galvanized steel sheet (GI) was obtained.
A galvanizing bath (bath temperature: 470°C) containing 0.20% by mass of Al, with the balance being Zn and unavoidable impurities, was used. The coating weight of the hot-dip galvanized layer per side was 45 to 72 g/ ^m2 . The composition of the formed hot-dip galvanized layer contained 0.1 to 1.0% by mass of Fe and 0.2 to 1.0% by mass of Al, with the balance being Fe and unavoidable impurities.

Some of the cold-rolled sheets were subjected to a galvannealed treatment to form galvannealed layers on both sides, thereby obtaining galvannealed steel sheets (GA).
In this case, a galvannealing bath (bath temperature: 470°C) containing 0.14 mass% Al, with the balance being Zn and unavoidable impurities, was used. The alloying treatment was carried out at 550°C. The coating weight of the galvannealed layer per side was approximately 45 g/m2. The composition of the formed galvannealed layer contained ⁷ to 15 mass% Fe and 0.1 to 1.0 mass% Al, with the balance being Fe and unavoidable impurities.

If a hot-dip galvanized layer was formed, "GI" was entered, and if an alloyed hot-dip galvanized layer was formed, "GA" was entered in the "Plating Type" column in Table 2 below.

Observation of microstructure
The microstructure of the obtained zinc-plated steel sheet was observed.
That is, according to the method described above, the area ratios of tempered martensite (tempered M), fresh martensite (FM), ferrite (F), bainite (B), retained austenite (retained γ), and the remaining structure at the 1/4 position in the plate thickness were measured.
Furthermore, according to the above-mentioned method, the area ratios of tempered martensite (tempered M), fresh martensite (FM), ferrite (F), bainite (B), oxide particles and the remaining structure at the interface position, as well as the average aspect ratio and average major axis of the oxide particles were measured.
The results are shown in Table 3 below.

In addition, when measuring the oxide particles, the black contrast areas (oxide particles) in the obtained SEM image were analyzed using EPMA, and the presence of oxygen and at least one of the elements Si and Mn was confirmed.

<evaluation>
The obtained galvanized steel sheets were subjected to the tests described below to evaluate various properties, and the results are shown in Table 3 below.

Tensile test
The tensile test was carried out in accordance with JIS Z 2241:2022.
Specifically, JIS No. 5 test pieces were taken from the obtained galvanized steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the steel sheets. Using the taken test pieces, a tensile test was carried out at a crosshead speed of 1.67 × 10 ^-1 mm/s to measure the yield strength (YS) [MPa] and tensile strength (TS) [MPa]. Furthermore, the yield ratio (YR) (= 100 × YS/TS) [%] was calculated from the measured yield strength (YS) and tensile strength (TS).
A tensile strength (TS) of 1180 MPa or more was determined to be high strength.
When the yield ratio (YR) was 68% or more, it was determined that the crash resistance characteristics were excellent.

<Hole expansion test>
The hole expansion test was carried out in accordance with JIS Z 2256.
Specifically, the obtained galvanized steel sheet was sheared to obtain test pieces measuring 100 mm x 100 mm. A 10 mm diameter hole was punched into the obtained test piece with a 12.5% clearance. Then, using a die with an inner diameter of 75 mm and a blank holding force of 9 ton (88.26 kN), a conical punch with an apex angle of 60° was pressed into the hole, and the hole diameter Df [mm] at the crack initiation limit was measured. The hole expansion ratio λ [%] was calculated using the following formula (2), where the initial hole diameter was D0 [mm].
λ={(Df-D0)/D0}×100 (2)
When the hole expanding ratio λ was 35% or more, it was determined that the stretch flangeability was excellent.

<Bending test>
The bending test was carried out in accordance with JIS Z 2248:2022.
Specifically, rectangular test pieces 30 mm wide and 100 mm long were cut from the resulting galvanized steel sheets so that the axial direction of the bending test was parallel to the rolling direction of the steel sheets. The longitudinal end faces of the test pieces were ground. Using the cut test pieces, a 90° V-bend test was performed under the conditions of an indentation load of 100 kN and a holding time of 5 seconds.
A bending test was carried out on five test pieces at an appropriate bending radius R. Then, the presence or absence of cracks was confirmed at the ridge line of the bending apex.
The occurrence of cracks was confirmed by observing the ridge line at the apex of bending using a digital microscope (RH-2000, manufactured by Hirox Co., Ltd.) at a magnification of 40 times.
The minimum bending radius R at which no cracks occurred in any of the five test pieces was determined, and the value (R/t) obtained by dividing this by the thickness t of the steel sheet (cold-rolled sheet) was taken as the critical bending radius. When the critical bending radius (R/t) was 4.0 or less, it was determined that the bendability was excellent.

<Delayed fracture resistance test to evaluate the shear load stability of delayed fracture resistance properties>
A delayed fracture resistance test for evaluating the shear disturbance stability of delayed fracture resistance properties was carried out by applying stress to samples having various shear edge conditions by four-point bending, and then immersing them in hydrochloric acid.
Specifically, first, a strip-shaped test piece having a width of 70 mm and a length of 18 mm was taken from the obtained galvanized steel sheet by shearing so that the sheared end surface was perpendicular to the rolling direction of the steel sheet.
At this time, shearing was performed under a total of six conditions by setting the shear angle to 0° or 0.5° and the clearance to 10%, 15%, or 20%, and six test pieces with different sheared end surface conditions were obtained from one galvanized steel sheet.
The collected test specimens were subjected to bending deformation by four-point bending so that a stress equivalent to TS was applied to the center of the specimen. The bent and deformed test specimens were then subjected to a heat treatment at 150°C for 20 minutes (a heat treatment simulating a baking finish). The heat-treated test specimens were then immersed in a hydrochloric acid solution (pH: 2.0) and removed from the hydrochloric acid solution after 48 hours.
Thereafter, the sheared end surfaces of the test specimens were observed visually and with a digital microscope (RH-2000, manufactured by Hirox Corporation) at a magnification of 20x. If, as a result of the observation, cracks were confirmed in any of the six test specimens, a "C" was entered in the "Delayed fracture resistance" column of Table 3 below. On the other hand, if cracks were not confirmed in any of the test specimens, a "B" was entered in the "Delayed fracture resistance" column of Table 3 below.
When no cracks were found in any of the test pieces, all the test pieces were immediately immersed again in an aqueous hydrochloric acid solution (pH: 2.0), and after 48 hours, were taken out from the aqueous hydrochloric acid solution.
Thereafter, the test specimens were observed in the same manner. If cracks were found in any of the test specimens, the test specimen was left as "B," whereas if cracks were not found in any of the test specimens, the test specimen was given an "A" in the "Delayed fracture resistance" column in Table 3 below.
When the rating was "A" or "B", it was determined that the delayed fracture resistance property had excellent shear disturbance stability.
When the rating was "A", it was determined that the delayed fracture resistance property had better shear disturbance stability.

<Summary of evaluation results>
As is clear from the results shown in Tables 1 to 3 above, it was found that the galvanized steel sheets of Nos. 1 to 2, 5, 9, 11, 13, 15, 23, 25, 27, 29, 31 to 36 and 43 to 63 were all excellent in crash resistance, stretch flangeability, bendability and shear disturbance stability of delayed fracture resistance.
In contrast, the galvanized steel sheets of Nos. 3 to 4, 6 to 8, 10, 12, 14, 16 to 22, 24, 26, 28, 30, and 37 to 42 were insufficient in at least one of the crash resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance.

Claims (6)

A steel plate and a zinc plating layer disposed on a surface of the steel plate,
The composition of the steel plate is, in mass%,
C: 0.090% or more and 0.390% or less,
Si: 0.25% or more and 2.00% or less,
Mn: 2.00% or more and 3.70% or less,
P: 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0100% or less, and
O: 0.0100% or less, the balance being Fe and unavoidable impurities;
At a 1/4 position of the plate thickness of the steel plate,
The area ratio of tempered martensite is 80% or more,
The area ratio of fresh martensite is less than 8%.
The total area ratio of ferrite and bainite is 15% or less,
The area ratio of retained austenite is 15% or less,
At the interface between the steel sheet and the zinc plating layer,
The total area ratio of tempered martensite and fresh martensite is 40% or less,
The total area ratio of ferrite and bainite is 60% or more,
The area ratio of oxide particles is 2% or more and 18% or less,
the average aspect ratio of the oxide particles is 1.2 or more and 6.0 or less;
A high-strength galvanized steel sheet, wherein the oxide particles have an average major axis of 0.10 μm or more and 0.70 μm or less. The component composition further includes, in mass %,
Ti: 0.200% or less,
Nb: 0.200% or less,
V: 0.200% or less,
Ta: 0.10% or less,
W: 0.10% or less,
B: 0.0100% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
Ni: 1.00% or less,
Co: 0.010% or less,
Cu: 1.00% or less,
Sn: 0.200% or less,
Sb: 0.200% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
REM: 0.0100% or less,
Zr: 0.100% or less,
Te: 0.100% or less,
Hf: 0.10% or less, and
The high-strength galvanized steel sheet according to claim 1, further comprising at least one element selected from the group consisting of Bi: 0.200% or less. A component made using the high-strength galvanized steel sheet described in claim 1 or 2. An automotive part made from the member described in claim 3. A method for producing the high-strength galvanized steel sheet according to claim 1 or 2,
A hot-rolled sheet is obtained by subjecting a steel slab having the component composition according to claim 1 or 2 to hot rolling including rough rolling and finish rolling, and then cooling C1 and coiling are performed;
In the cooling C1,
The average cooling rate v _A in the temperature range T _A of 650 ° C. or higher and the finish rolling delivery temperature FT or lower is 15 ° C./s or higher,
The residence time _tB in the temperature range _TB of 550°C or higher and 600°C or lower is 2.0 seconds or less,
Next, the hot-rolled sheet is subjected to pickling and cold rolling to obtain a cold-rolled sheet,
The cold-rolled sheet is subjected to heating H1, then subjected to cooling C2 including galvanizing treatment to a cooling stop temperature T _C of 150 ° C or less, and then subjected to reheating H2;
In the heating H1,
The average heating rate v1 in the temperature range T1 of 600°C or higher and (A _C1 + 20)°C or lower is 8°C/s or higher,
the average heating rate v2 in the temperature region T2 of (A _C1 + 20) ° C. or more and A _C3 ° C. or less is 1.0 ° C./s or more and less than 8.0 ° C./s;
The average dew point DP2 in the temperature range T2 is −20° C. or higher and 15° C. or lower,
A residence time t3 in a temperature range T3 of not less than A _c3 ° C. and not more than 1000 ° C. is not less than 30 seconds and not more than 500 seconds,
The average dew point DP3 in the temperature region T3 is higher than DP2 and 20°C or lower,
In the cooling C2,
The average cooling rate v4 in the temperature region T4 of 500° C. or more and 750° C. or less is 3° C./s or more and 35° C./s or less,
The residence time t5 in the temperature range T5 of 400°C or more and 500°C or less is 10 seconds or more and 250 seconds or less,
The galvanizing treatment is carried out during or after the retention in the temperature zone T5,
The average cooling rate v6 in the temperature region T6 of 150°C or higher and 400°C or lower is greater than 10.0°C/s,
In the reheating H2,
A method for producing a high-strength galvanized steel sheet, wherein a temperature X that is a maximum temperature reached and a holding time Y at (X-10) ° C. or higher satisfy the following formula (1):
7800≦(273+X)×(20+log(Y/3600))≦11800 (1)
However, the unit of the temperature X is ° C., and the unit of the holding time Y is s. A method for manufacturing a component, comprising subjecting the high-strength galvanized steel sheet according to claim 1 or 2 to at least one of forming and joining processes to obtain the component.