High-strength zinc-coated steel sheet, member, automobile component, method for manufacturing high-strength zinc-coated steel sheet, and method for manufacturing member

Abstract

The present invention contains 0.090-0.390% of C, 0.25-2.00% of Si, 2.00-3.70% of Mn, 0.100% or less of P, 0.0200% or less of S, 1.000% or less of Al, 0.0100% or less of N, and 0.0100% or less of O. At a position 1 / 4 through the sheet thickness, tempered martensite (M) is included at a proportion of 80% or more, fresh martensite (FM) is included at a proportion of less than 8%, ferrite (F) and bainite (B) are included at a total proportion of 15% or less, and retained austenite is included at a proportion of 15% or less. At a position located at the interface with respect to a zinc plating layer, tempered M and FM are included at a total proportion of 40% or less, F and B are included at a total proportion of 60% or more, and oxide particles (average aspect ratio of 1.2-6.0 and average long diameter of 0.10-0.70 µm) are included at a proportion of 2-18%. As a result, a high-strength zinc-coated steel sheet having excellent impact resistance, stretch flange properties, bendability, and shear disturbance stability for delayed fracture resistance can be obtained.

Description

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

[0001] The present invention relates to a high-strength galvanized steel sheet, a member, an automobile part, a method for manufacturing a high-strength galvanized steel sheet, and a method for manufacturing a member.

[0002] Improved fuel efficiency through lighter vehicle body (CO ２ In order to simultaneously achieve both a reduction in carbon dioxide emissions and improved crashworthiness, steel sheets for automobiles have been made stronger, and new regulations have been 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 used in major structural parts of automobiles in order to increase the strength of the vehicle body.

[0003] International Publication No. 2019 / 187090

[0004] For example, high-strength galvanized steel sheets used in automobile parts (particularly automobile frame structural parts or automobile reinforcing parts) are required to have excellent crash resistance, i.e., a high yield ratio YR (= 100 × yield strength YS / tensile strength TS).

[0005] High-strength galvanized steel sheets are subjected to punching and bending when used as automobile parts such as crash boxes, etc. Therefore, the high-strength galvanized steel sheets used for these parts are required to have excellent stretch flangeability and bendability.

[0006] Furthermore, in components 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 component suddenly breaks) may occur due to hydrogen penetration in an atmospheric corrosive environment. Delayed fracture is highly likely to occur at the sheared edge caused by shearing. Even when shearing is performed under a single condition (e.g., shear angle, clearance), the conditions may vary due to disturbances such as the thickness and warpage of the steel sheet. Furthermore, the sheared edge condition (e.g., damage, residual stress) varies significantly depending on the shear angle and clearance. Therefore, high-strength galvanized steel sheets are also required to have excellent delayed fracture resistance at the sheared edge when shearing is performed under various conditions (e.g., shear angle, clearance) (hereinafter referred to as "shear disturbance stability of delayed fracture resistance").

[0007] The present invention has been made in view of the above points, and an object of the present invention is to provide a high-strength galvanized steel sheet that is excellent in crash resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance.

[0008] As a result of extensive investigations, 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 sheet and a zinc-plated layer disposed on a surface of the steel sheet, wherein the steel sheet has a chemical composition, in mass %, of C: 0.090% to 0.390%, Si: 0.25% to 2.00%, Mn: 2.00% to 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, with the balance being Fe and unavoidable impurities, and wherein an area ratio of tempered martensite is 80% or more and a ratio of fresh martensite is 100% or less at a quarter-thickness position of the steel sheet. 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 [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 the above [1] or [2]. [4] An automotive part made of the member according to the above [3].[5] A method for producing the 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, the temperature range T is 650°C or higher and the finish rolling delivery temperature FT or lower. Ａ The average cooling rate v Ａ is 15 ° C. / s or more, and the temperature range T Ｂ Residence time t Ｂ The hot-rolled sheet is then subjected to pickling and cold rolling to obtain a cold-rolled sheet, and the cold-rolled sheet is subjected to heating H1, followed by a galvanizing treatment at a cooling stop temperature T of 150°C or less. Ｃ Cooling C2 is performed to 600°C or more (A Ｃ１ +20) ° C. or less, the average heating rate v1 in the temperature range T1 is 8 ° C. / s or more, Ｃ１ +20)℃ or higher A Ｃ３ The average heating rate v2 in the temperature range T2 of 1.0°C / s or more and less than 8.0°C / s, the average dew point DP2 in the temperature range T2 of -20°C or more and 15°C or less, and A ｃ３a 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) where the unit of the temperature X is °C, and the unit of the holding time Y is s. [6] A method for manufacturing a member, comprising: subjecting the high-strength galvanized steel sheet according to [1] or [2] above to at least one of forming and joining to obtain a member.

[0009] According to the present invention, a high-strength galvanized steel sheet having excellent crash resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance can be provided.

[0010] [High-strength galvanized steel sheet] The high-strength galvanized steel sheet of this embodiment comprises 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 component composition and microstructure described below. As a result, 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. 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 crash resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance will be described later.

[0011] <Steel Sheet> 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 sheet is not particularly limited, and is, for example, 0.3 mm or more and 2.8 mm or less.

[0012] <<Composition>> First, the composition of the steel sheet (base steel sheet) will be described. The unit "%" in the composition means "mass %" unless otherwise specified.

[0013] (C: 0.090% or more and 0.390% or less) C is one of the important basic components of steel. In this embodiment, it affects the area fraction of tempered martensite and the area fraction of fresh martensite at the 1 / 4 thickness position. If the C content is too low, the area fraction of tempered martensite at the 1 / 4 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, 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 becomes significantly hard fresh martensite during shearing, thereby reducing stretch flangeability. In addition, delayed fracture crack propagation is promoted, and the shear disturbance stability of delayed fracture resistance properties is reduced. Furthermore, if the area fraction of retained austenite increases, stress-induced martensitic transformation reduces the yield ratio (YR) and crash resistance properties. Therefore, the C content is 0.390% or less, preferably 0.375% or less, and more preferably 0.360% or less.

[0014] (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 oxide particles is reduced, and the shear disturbance stability of delayed fracture resistance properties 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 oxide particles is increased, and the shear disturbance stability of delayed fracture resistance properties is reduced. Therefore, the Si content is 2.00% or less, preferably 1.75% or less, and more preferably 1.50% or less.

[0015] (Mn: 2.00% or more and 3.70% or less) Mn is an important hardenability element and affects the area fraction of tempered martensite and the area fraction of ferrite. Furthermore, Mn is an austenite-stabilizing element and affects the shear stress stability of delayed fracture resistance. If the Mn content is too low, the area fraction of tempered martensite decreases and the area fraction 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 increases excessively, and the hardness of martensite generated from the retained austenite during shear stress increases significantly. As a result, the shear stress stability of delayed fracture resistance decreases. Therefore, the Mn content is 3.70% or less, preferably 3.50% or less, and more preferably 3.30% or less.

[0016] (P: 0.100% or less) P segregates at prior austenite grain boundaries and embrittles 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. The lower limit of the P content is not particularly limited. However, 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 may be 0.003% or more.

[0017] (S: 0.0200% or less) S exists as sulfide and reduces the ultimate deformability of the steel sheet, thereby reducing 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. However, due to constraints in production technology, the S content may be 0.0001% or more.

[0018] (Al: 1.000% or less) Al performs sufficient deoxidation and reduces inclusions in steel. If the Al content is too high, a large amount of ferrite is formed, and TS decreases. Therefore, 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 perform stable deoxidation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.

[0019] (N: 0.0100% or less) N exists as a nitride and reduces the ultimate deformability of the steel sheet, thereby reducing bendability. Therefore, the N content is 0.0100% or less, preferably 0.0060% or less, and more preferably 0.0050% or less. The lower limit of the N content is not particularly limited. However, due to constraints on production technology, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0020] (O: 0.0100% or less) O ​​exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing bendability. Therefore, the O content is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020% or less. The lower limit of the O content is not particularly limited. However, due to constraints in production technology, the O content may be 0.0001% or more.

[0021] (Optional Elements) The steel sheet may further contain the elements described below as part of its chemical composition.

[0022] ((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, and do not reduce the ultimate deformability of the steel sheet, so that bendability does not decrease. 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. There are no particular restrictions on the lower limits of the contents of Ti, Nb, and V. 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.

[0023] ((Ta and W)) Unless the content of Ta and W is excessive, they do not form large amounts of coarse precipitates or inclusions, and do not reduce the ultimate deformability of the steel sheet, so that bendability does not decrease. Therefore, when these elements are contained, the contents of Ta and W are each preferably 0.10% or less, and more preferably 0.08% or less. There are no particular restrictions on the lower limits of the contents of Ta and W. 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.

[0024] ((B)) Unless the content of B is excessive, cracks do not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet is not reduced, so that 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. The lower limit of the B content is not particularly limited. However, since B is an element that segregates to 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.

[0025] ((Cr, Mo, and Ni)) As long as the contents of Cr, Mo, and Ni are not excessive, they do not increase coarse precipitates or inclusions, and do not reduce the ultimate deformability of the steel sheet, so that bendability does not decrease. 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. There are no particular restrictions on the lower limits of the contents of Cr, Mo, and Ni. 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.

[0026] ((Co)) As long as the Co content is not excessive, the 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. 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.

[0027] ((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.

[0028] ((Sn)) As long as the Sn content is not excessive, cracks will not form inside the steel sheet during casting or hot rolling, and the ultimate deformability of the steel sheet will not be reduced, so that bendability will not be 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.

[0029] ((Sb)) As long as the Sb 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 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.

[0030] ((Ca, Mg, and REM)) As long as the content of Ca, Mg, and REM (rare earth metals) is not excessive, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so bendability is not 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. There are no particular restrictions on the lower limits of the contents of Ca, Mg, and REM. However, these elements spheroidize the shape of nitrides and sulfides, improving 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.

[0031] ((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 bendability is not reduced. Therefore, the contents of Zr and 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 shape 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.

[0032] ((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.

[0033] ((Bi)) As long as the Bi 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 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.

[0034] 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), when the content of each is less than the above-mentioned preferable lower limit, it is treated as an unavoidable impurity.

[0035] (Balance: Fe and unavoidable impurities) The steel sheet contains the above-mentioned elements as its component composition, with the balance consisting of Fe and unavoidable impurities. It is preferable that the steel sheet contains only the above-mentioned elements and the balance, with 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.

[0036] <Microstructure at 1 / 4 position of plate thickness> Next, the microstructure at 1 / 4 position of plate thickness among the microstructures of the steel plate will be described.

[0037] (Area Fraction 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 fraction 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 fraction of tempered martensite may be 100%. Tempered martensite is a structure formed by precipitation of carbides in martensite generated 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) carbide, eta (η) carbide, and chi (χ) carbide. Tempered martensite also includes lower bainite generated at or below the Ms point.

[0038] (Area fraction of fresh martensite: less than 8%) If there is too much fresh martensite, the yield ratio (YR) decreases due to an increase in mobile dislocations, and crashworthiness decreases. In addition, the difference in hardness between structures increases, and stretch flangeability decreases. Therefore, the area fraction of fresh martensite is 8% or less, preferably 7% or less, and more preferably 6% or less. There is no particular lower limit, and the area fraction of fresh martensite may be 0%. Fresh martensite is martensite in which no carbides are observed in SEM images obtained by the measurement method described below.

[0039] (Total area fraction 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 structures increases, resulting in reduced stretch flangeability. In addition, these structures become the starting point for plastic deformation, reducing the yield ratio (YR) and reducing crashworthiness. Therefore, the total area fraction of ferrite and bainite is 15% or less, preferably 13% or less, and more preferably 10% or less. There is no particular lower limit, and the total area fraction of ferrite and bainite may be 0%. Ferrite is a soft BCC iron formed at high temperatures and includes allotriomorph ferrite and idiomorph ferrite. Bainite is an angular BCC iron containing fine carbides formed at temperatures higher than the Ms point.

[0040] (Area fraction of retained austenite: 15% or less) If there is too much retained austenite, it will turn into extremely hard fresh martensite during shearing, thereby reducing 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), reducing 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%.

[0041] (Method for Measuring Area Ratio) The method for measuring the area ratios of tempered martensite, fresh martensite, ferrite, and bainite at the 1 / 4 sheet thickness position 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 volume % nital to reveal the structure. Next, the 1 / 4 sheet thickness position of the steel sheet on the observation surface of the sample is observed at a magnification of 3000x using a scanning electron microscope (SEM) under an acceleration voltage of 10 kV, and SEM images of three fields of view are obtained. From the obtained SEM images, 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 ratios of each structure are calculated for three fields of view, and their average value 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.

[0042] The method for measuring the area fraction of retained austenite is as follows. First, a steel sheet is ground so that the measurement surface is at a 1 / 4 position in the sheet thickness direction (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the surface of the steel sheet), and then the steel sheet is 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 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.

[0043] (Remaining structure) The steel sheet 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 in the sheet thickness. Examples of the remaining structure include pearlite; alloy carbonitrides precipitated in ferrite; and other structures known as the structure of steel sheet. The area ratio of the remaining structure is preferably 5% or less.

[0044] Next, the microstructure at the interface between the coating layer and the steel sheet (hereinafter simply referred to as the "interface") will be described. The interface is the polished surface (sheet surface) of the steel sheet from which the coating layer has been removed by polishing.

[0045] (Total area ratio of tempered martensite and fresh martensite: 40% or less) If there is too much tempered martensite and fresh martensite at the interface, the number of initiation points for bending cracks increases, resulting in poor bendability. Furthermore, if there is too much tempered martensite and fresh martensite at the interface, 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%.

[0046] (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. In addition, the propagation of delayed fracture cracks generated at the shear edge is suppressed, improving the shear disturbance stability of the delayed fracture resistance. The upper limit is not particularly limited, and may be 100%.

[0047] (Method for Measuring Area Fractions) The method for measuring the area fractions of tempered martensite, fresh martensite, ferrite, and bainite at the interface is as follows. First, a sample is cut out from a galvanized steel sheet so that the sheet surface (surface) perpendicular to the rolling direction serves as the observation surface. The galvanized 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, etching is performed using 1 volume % 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 3000x magnification using a scanning electron microscope (SEM) at an acceleration voltage of 10 kV, and SEM images of three fields of view are obtained. From the obtained SEM images, the area fraction 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 fraction of each structure. The area ratio of each structure is calculated for three fields of view, and the average value 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.

[0048] (Area ratio of oxide particles: 2% or more and 18% or less) The oxide particles contain, for example, at least one element of 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 exhibits excellent shear disturbance stability. Changes in the shear angle and clearance conditions change the damage and residual stress of the steel sheet at the sheared edge, changing the initiation point and propagation direction of delayed fracture microcracks. The microcracks reach the surface of the steel sheet (base steel sheet) (i.e., the interface) and then propagate from the interface in the thickness direction, resulting in macroscopic delayed fracture cracks. At this time, the presence of oxide particles at a certain area ratio at the interface suppresses microcracks initiated at the sheared edge from reaching the interface and further propagating 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.

[0049] For these reasons, the area ratio of oxide particles at the interface 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 concentrates on the oxide particles themselves, which become the starting point of delayed fracture, and the shear disturbance stability of delayed fracture resistance decreases. Therefore, the area ratio of oxide particles is 18% or less, preferably 16% or less, and more preferably 14% or less.

[0050] (Remaining structure) 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 sheet. The area ratio of the remaining structure is preferably 5% or less.

[0051] In order to obtain excellent shear disturbance stability with delayed fracture resistance, it is necessary to set not only the area ratio of oxide particles but also the average aspect ratio and average major axis of the oxide particles within specific ranges, as will be described below.

[0052] (Average aspect ratio of oxide particles: 1.2 or more and 6.0 or less) If the average aspect ratio of oxide particles is too low, the detouring effect of delayed fracture cracks at grain boundaries at the interface position becomes insufficient, and the shear disturbance stability of delayed fracture resistance properties decreases. Therefore, the average aspect ratio of 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 oxide particles is too high, stress tends to concentrate on the oxide particles themselves, which can become the starting point of delayed fracture, and the shear disturbance stability of delayed fracture resistance properties decreases. Therefore, the average aspect ratio of oxide particles is 6.0 or less, preferably 5.5 or less, and more preferably 4.5 or less.

[0053] (Average major axis of oxide particles: 0.08 μm or more and 0.70 μm or less) If the average major axis of oxide particles is too short, the detouring effect of delayed fracture cracks at grain boundaries at the interface position becomes insufficient, and the shear disturbance stability of delayed fracture resistance properties decreases. Therefore, the average major axis of 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 oxide particles is too long, stress tends to concentrate on the oxide particles themselves, which become the starting point of delayed fracture, and the shear disturbance stability of delayed fracture resistance properties decreases. Therefore, the average major axis of oxide particles is 0.70 μm or less, preferably 0.65 μm or less, and more preferably 0.60 μm or less.

[0054] (Method for measuring oxide particles) The area ratio, average aspect ratio, and average major axis of 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-plated layer (plating layer) corresponding to 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 because their contrast is clearly different when observed using an optical microscope. Next, the observation surface of the sample is observed at a magnification of 5000 times 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 portions using an electron probe microanalyzer (EPMA) or an energy dispersive X-ray analyzer (SEM-EDX) attached to an 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 portions using EPMA or SEM-EDX.

[0055] 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 ratios are calculated for three fields of view, and their average value is used as the area ratio of the oxide particles. Furthermore, from the obtained binarized image, each oxide particle is approximated to an ellipse, and the value obtained by dividing the major axis of the particle by the minor axis is defined as the aspect ratio. The aspect ratios of the oxide particles contained in the binarized images for the three fields of view are calculated, and their average value is used as the average aspect ratio of the oxide particles. Furthermore, the average value of the major axes of all the calculated oxide particles is used as the average major axis of the oxide particles. Note that 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 the oxide particles. For example, SEM images obtained by observing a sheet thickness cross section (L cross section) parallel to the rolling direction are insufficient.

[0056] <Zinc Plated Layer> Next, the zinc plated layer (plated layer) will be described. The plated layer is formed by a zinc plating process described below. The zinc plated 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 plated layer may contain elements such as Al and Mg. The composition of the plated layer is not particularly limited and may be a common composition. A typical composition includes Fe: 20 mass% or less, Al: 0.001 to 1.0 mass%, and 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 in a total amount of 0 mass% to 3.5 mass%, with the balance consisting of Zn and unavoidable impurities. The coating weight of the plated layer per side is, for example, 20 g / m. ２ or more, and ２ The Fe content in the plating layer may be, for example, less than 7 mass%. When the plating layer is a galvannealed layer, the Fe content in the plating layer is preferably 7 mass% or more. In this case, the Fe content in the plating layer is preferably 20 mass% or less, more preferably 15 mass% or less.

[0057] [Manufacturing Method of High-Strength Galvanized Steel Sheet] Next, a method of manufacturing the high-strength galvanized steel sheet of the present embodiment described above will be described. Briefly, first, a steel slab having the above-described chemical composition is hot-rolled to obtain a hot-rolled sheet. The obtained hot-rolled sheet is then subjected to cooling C1 and coiling. Next, the hot-rolled sheet is pickled and cold-rolled to obtain a cold-rolled sheet. Thereafter, the obtained cold-rolled sheet is subjected to heating H1, cooling C2, and reheating H2. During cooling C2, the cold-rolled sheet is subjected to a galvanizing treatment.

[0058] <Steel Slab> First, a steel slab (also simply referred to as "slab") having the above-described composition is prepared. The method for producing molten steel to become the steel slab is not particularly limited, and known methods using a converter, electric furnace, or the like can be adopted. In order to prevent macrosegregation, the steel slab is preferably produced using a continuous casting method, but it can also be produced by other methods such as an ingot casting method or a thin slab casting method.

[0059] <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 strip without being cooled to room temperature, or may be briefly held at room temperature and then immediately rough-rolled.

[0060] Rough rolling: A roughly rolled sheet is obtained by rough rolling a steel slab. The slab heating temperature when rough rolling 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.

[0061] <<Finish 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 rough-rolled sheet using a bar heater or the like before finish rolling in order to prevent problems during hot rolling. Finish rolling may be performed continuously by joining the rough-rolled sheets together. The rough-rolled sheet may be temporarily coiled before finish rolling. The finish-rolling outlet temperature FT is preferably 700°C or higher. This reduces the rolling load. Furthermore, the reduction ratio in the unrecrystallized austenite state is reduced, suppressing the development of abnormal structures elongated in the rolling direction, and improving workability. In order to reduce the rolling load, some or all of the finish rolling may be performed as lubricated rolling. Lubricated rolling is also preferable from the viewpoint of uniforming the shape and material properties of the steel sheet. The friction coefficient during lubricated rolling is preferably 0.10 or higher and 0.25 or lower.

[0062] <Cooling C1> The hot-rolled sheet obtained by hot rolling is subjected to cooling C1 under the following conditions.

[0063] 《Temperature range T Ａ The average cooling rate v Ａ : 15 ° C. / s or more》 Temperature range T of 650 ° C. or more and finish rolling delivery temperature FT or less Ａ In this temperature range T Ａ The average cooling rate v Ａ If the average cooling rate v is slow, oxides containing Si and Mn are generated on the steel sheet surface. These oxides are removed by pickling, which will be described later, and the Si and Mn in the steel sheet surface layer are reduced. 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. For this reason, Ａ is 15°C / s or more, and preferably 20°C / s or more. The upper limit is not particularly limited, but from the viewpoint of equipment capacity, the average cooling rate v Ａ is preferably 200° C. / s or less, more preferably 170° C. / s or less, and even more preferably 130° C. / s or less.

[0064] 《Temperature range T Ｂ Residence time t Ｂ:2.0s or less》Temperature range T of 550℃ or higher and 600℃ or lower Ｂ In this temperature range T Ｂ Residence time t Ｂ If the residence time t is long, oxide particles generated by hot rolling grow excessively during heating H1 described later, and the average major axis of the oxide particles becomes long. Ｂ The residence time t is 2.0 seconds or less, and preferably 1.5 seconds or less. Ｂ is preferably 0.1 s or more, more preferably 0.3 s or more.

[0065] <Coiling> The hot-rolled sheet that has been subjected to cooling C1 is coiled. The coiling temperature is preferably 400°C or higher and 550°C or lower, from the viewpoint of improving sheet passability during cold rolling, which will be described later.

[0066] <Pickling> Next, the coiled hot-rolled sheet is subjected to pickling before being subjected to cold rolling. Pickling removes oxides from the surface of the hot-rolled sheet, resulting in a high-quality coating layer in the final product, a high-strength galvanized steel sheet. Pickling may be performed once or multiple times.

[0067] <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 not particularly limited.

[0068] <Heating H1> Next, the obtained cold-rolled sheet is subjected to heating H1 under the following conditions: Heating H1 is generally a treatment in which the cold-rolled sheet is heated to a temperature range T3 described later and retained therein.

[0069] Average heating rate v1 in temperature range T1: 8°C / s or more 600°C or more (A Ｃ１If the average heating rate v1 in the temperature range T1 of +20°C or less 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. For this reason, the average heating rate v1 is 8°C / s or more, and preferably 9°C / s or more. There is no particular upper limit, 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.

[0070] A Ｃ１ The temperature (unit: °C) is calculated based on the following formula: 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.

[0071] Average heating rate v2 in temperature range T2: 1.0 ° C. / s or more and less than 8.0 ° C. / s Ｃ１ +20)℃ or higher A Ｃ３ The temperature region T2 of 8.0°C or less 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 large, the generation of oxide particles with a high aspect ratio is suppressed, and the average aspect ratio becomes small. 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.

[0072] A Ｃ３ The temperature (unit: °C) is calculated based on the following formula: 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 plate, and is set to 0 when element X is not contained.

[0073] <<Average dew point DP2 in temperature region 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 is reduced. 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 is increased, and the total area ratio of ferrite and bainite is reduced. Therefore, 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 is increased. Therefore, the average dew point DP2 is 15°C or lower, preferably 14°C or lower, and more preferably 13°C or lower.

[0074] Residence time t3 in temperature range T3: 30 seconds or more and 500 seconds or less A Ｃ３In the temperature region T3 of 200°C or higher and 1000°C or lower, generation and grain growth of oxide particles, and crystal grain growth of austenite grains in which reverse transformation is completed at the 1 / 4 position in the plate thickness occur. If the residence time t3 in the temperature region T3 is too short, the average major axis of the oxide particles becomes shorter. Furthermore, the austenite grain size becomes finer at the 1 / 4 position in the plate 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 longer, preferably 35 seconds or longer, and more preferably 40 seconds or longer. On the other hand, if the residence time t3 is too long, the average major axis of the oxide particles becomes longer. Therefore, the residence time t3 is 500 seconds or shorter, preferably 450 seconds or shorter, and more preferably 400 seconds or shorter.

[0075] <<Average dew point DP3 in temperature range T3: more than DP2 and not more than 20°C>> As described above, generation and grain growth of oxide particles occur in the temperature range T3. If the average dew point DP3 in the temperature range T3 is too low, grain growth of the oxide particles becomes difficult, and the average major axis of the oxide particles becomes shorter. In order to increase the average major axis of the oxide particles, an 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, excessive generation of oxide particles occurs, and the area ratio of the oxide particles increases. Therefore, the average dew point DP3 is 20°C or less.

[0076] <Cooling C2> Next, the cold-rolled sheet subjected to heating H1 is subjected to cooling C2 under the following conditions. In cooling C2, the cold-rolled sheet subjected to heating H1 is cooled to a temperature of 150°C or less (cooling stop temperature T Ｃ ) during which the cold-rolled sheet is allowed to stay in a temperature range T5 or is subjected to a galvanizing treatment.

[0077] <<Average Cooling Rate v4 in Temperature Region T4: 3°C / s or More and 35°C / s or Less>> The temperature region T4 of 500°C or more and 750°C or less is the temperature region in which ferrite transformation occurs. If the average cooling rate v4 in the temperature region T4 is too low, excessive ferrite transformation occurs at the 1 / 4 thickness position of the steel plate, increasing the area fraction of ferrite at the 1 / 4 thickness position of the steel plate and decreasing the area fraction of tempered martensite. Therefore, the average cooling rate v4 is 3°C / s or more, preferably 4°C / s or more, and more preferably 5°C / s or more. On the other hand, if the average cooling rate v4 is too high, ferrite transformation becomes difficult to occur at the interface position, and the area fraction of ferrite at the interface position 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.

[0078] <<Dwell Time t5 in Temperature Region T5 is 10 Seconds or More and 250 Seconds or Less>> The temperature region T5 of 400°C or more and 500°C or less is a temperature region in which bainite transformation occurs. If the dwell time t5 in the temperature region T5 is too short, bainite transformation is less likely to occur at the interface, and the area fraction of bainite at the interface is reduced. Therefore, the dwell time t5 is 10 seconds or more, preferably 13 seconds or more, and more preferably 15 seconds or more. On the other hand, if the dwell time t5 is too long, excessive bainite transformation occurs at the 1 / 4 thickness position of the steel plate, increasing the area fraction of bainite at the 1 / 4 thickness position of the steel plate and reducing the area fraction of tempered martensite. Therefore, the dwell time t5 is 250 seconds or less, preferably 180 seconds or less, and more preferably 130 seconds or less.

[0079] <Galvanizing Treatment> The steel sheet (cold-rolled sheet) is subjected to a galvanizing treatment while retained in the temperature range T5 or after retaining in the temperature range T5. The galvanizing treatment is, for example, a hot-dip galvanizing treatment. After the hot-dip galvanizing treatment, a hot-dip galvanizing treatment in which an alloying treatment is performed 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 or the like. 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 galvanizing bath is preferably 0.10% by mass or higher and 0.23% by mass or lower. When an alloying treatment is performed, the temperature of the alloying treatment is preferably 470°C or higher to improve the Zn—Fe alloying rate and productivity. In order to suitably prevent untransformed austenite from transforming into pearlite and to make TS more suitable, the alloying temperature is preferably 600°C or less, and more preferably 560°C or less.

[0080] When a certain temperature T (e.g., the bath temperature of the zinc plating bath) among the various temperatures in the zinc plating process overlaps with a temperature range T5 (400°C or higher and 500°C or lower), a 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 retention in the temperature range T5, and the time during which the process P is carried out is included in the retention time t5 in the temperature range T5.

[0081] <<Average Cooling Rate v6 in Temperature Region T6: More than 10.0°C / s>> The temperature region T6 of 150°C or higher and 400°C or lower is a temperature region 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 region 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. In the subsequent reheating H2, the austenite is further stabilized, increasing the area fraction of retained austenite. Therefore, the average cooling rate v6 is greater than 10.0°C / s, preferably 11.0°C or higher, and more preferably 12.0°C / s or higher. The upper limit of the average cooling rate v6 is not particularly limited, and the average cooling rate v6 may be, for example, 180°C / s or lower, or may be 150°C / s or lower, or may be 120°C / s or lower.

[0082] 《Cooling stop temperature T Ｃ :150℃ or less》 Cooling stop temperature T Ｃ If the cooling stop temperature T is too high, the martensitic transformation will not be completed, and in the subsequent reheating H2, tempering will not occur, resulting in too much fresh martensite. 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.

[0083] <Reheating H2> Next, the cooling stop temperature T Ｃ The cold-rolled sheet after the galvanization treatment is cooled to a temperature of 1000 K. The cold-rolled sheet after the galvanization treatment is then subjected to reheating H2. In reheating H2, the cold-rolled sheet after the galvanization treatment 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.

[0084] <<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, the cooling stop temperature T ＣWhen a cold-rolled sheet cooled to 0°C is appropriately heated again, carbide precipitation occurs in 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 decomposes into ferrite, making it difficult to secure the desired amount of tempered martensite, resulting in a decrease in tensile strength (TS). Therefore, the value of the variable part is 11800 or less, preferably 11500 or less, and more preferably 11300 or less.

[0085] The heat treatment in the above-mentioned manufacturing method is not particularly limited in other conditions as long as the above-mentioned thermal history is satisfied, and the equipment for carrying out the heat treatment is also not particularly limited. Manufacturing conditions other than the above-mentioned conditions are the same as usual.

[0086] The high-strength galvanized steel sheet of the present 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. In this case, 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 rolling reduction may be obtained in one rolling run, or the rolling may be performed in several runs.

[0087] From the viewpoint of productivity, it is preferable that a series of treatments including the zinc plating treatment be carried out in a CGL (Continuous Galvanizing Line).

[0088] [Members and Automobile Parts] Next, a member according to the present embodiment will be described. The following description also serves as a description of an automobile part made of the member according to the present embodiment. The member according to the present embodiment is a member at least partially made using the high-strength galvanized steel sheet according to the present embodiment described above. For example, the high-strength galvanized steel sheet according to the present embodiment is formed into a desired shape by forming or joining. The member according to the present embodiment is preferably a member for an automobile part. Examples of automobile parts include automobile frame structural parts and automobile reinforcement parts. As described above, the high-strength galvanized steel sheet according to the present embodiment has excellent crash resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance. Therefore, the member according to the present embodiment is suitable as a member generally used for automobile parts (particularly automobile frame structural parts or automobile reinforcement parts).

[0089] [Method for manufacturing member] 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 forming process is not particularly limited, and examples thereof include press working. The joining process is not particularly limited, and examples thereof include general welding such as spot welding and arc welding; caulking joining using rivets, etc.

[0090] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0091] <Production of Galvanized Steel Sheet> A steel slab having the chemical composition shown in Table 1 below (the balance consisting of Fe and unavoidable impurities) was obtained by continuous casting using molten steel produced in a converter. The obtained steel slab was hot-rolled to obtain a hot-rolled sheet. More specifically, in the hot-rolling, the steel slab was heated to 1250°C and rough-rolled, and then finish-rolled 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 ratio of 50%. In this way, a cold-rolled sheet having a thickness of 1.4 mm was obtained. The cold-rolled sheet was then subjected to heating H1, cooling C2, and reheating H2 under the conditions shown in Table 2 below. During the cooling C2, the cold-rolled steel sheet was subjected to a galvanizing treatment under the conditions described below to obtain a galvanized steel sheet.

[0092] <Galvanizing Treatment> The cold-rolled sheet was subjected to hot-dip galvanizing treatment to form hot-dip galvanized layers on both sides. That is, a hot-dip galvanized steel sheet (GI) was obtained. A galvanizing bath (bath temperature: 470°C) containing 0.20 mass% Al, with the remainder consisting of Zn and unavoidable impurities, was used. The coating weight of the hot-dip galvanized layer per side was 45 to 72 g / m. ２ The composition of the formed hot-dip galvanized layer contained 0.1 to 1.0 mass % of Fe, 0.2 to 1.0 mass % of Al, and the remainder being Fe and unavoidable impurities.

[0093] Some of the cold-rolled sheets were subjected to a galvannealed hot-dip galvanizing treatment to form a galvannealed layer on both sides. That is, a galvannealed hot-dip galvanized steel sheet (GA) was obtained. In this case, a galvannealed bath (bath temperature: 470°C) containing 0.14 mass% Al, with the balance being Zn and unavoidable impurities, was used. The alloying treatment was performed at 550°C. The coating weight of the galvannealed layer per side was 45 g / m. ２The composition of the formed alloyed hot-dip galvanized layer contained 7 to 15 mass % of Fe, 0.1 to 1.0 mass % of Al, and the balance being Fe and unavoidable impurities.

[0094] In the "Type of plating" column of Table 2 below, "GI" is entered when a hot-dip galvanized layer was formed, and "GA" is entered when an alloyed hot-dip galvanized layer was formed.

[0095] <Observation of Microstructure> The microstructure of the obtained galvanized steel sheets was observed. That is, 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 sheet thickness position were measured according to the method described above. In addition, 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 according to the method described above. The results are shown in Table 3 below.

[0096] In the measurement of oxide particles, the black contrast areas (oxide particles) in the obtained SEM image were analyzed using EPMA, and the presence of oxygen element and at least one of Si and Mn elements was confirmed.

[0097] <Evaluation> The obtained galvanized steel sheets were subjected to the tests described below to evaluate various properties. The results are shown in Table 3 below.

[0098] Tensile Test The tensile test was carried out in accordance with JIS Z 2241:2022. Specifically, a JIS No. 5 test piece was taken from the obtained galvanized steel sheet so that the longitudinal direction was perpendicular to the rolling direction of the steel sheet. The taken test piece was used for the tensile test at a crosshead speed of 1.67 × 10 －１A tensile test was carried out under the condition of 100 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). When the tensile strength (TS) was 1180 MPa or more, it was judged to have high strength. When the yield ratio (YR) was 68% or more, it was judged to have excellent crash resistance properties.

[0099] <Hole Expanding Test> The hole expanding test was performed 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 expanding ratio λ [%] was calculated using the following formula (2), where D0 [mm] is the initial hole diameter. λ = {(Df - D0) / D0} × 100 (2) A hole expanding ratio λ of 35% or more was determined to have excellent stretch flangeability.

[0100] Bending Test The bending test was conducted in accordance with JIS Z 2248:2022. Specifically, strip-shaped test pieces 30 mm wide and 100 mm long were taken from the obtained galvanized steel sheets so that the axial direction of the bending test was parallel to the rolling direction of the steel sheet. The longitudinal end faces of the test pieces were ground end faces. Using the taken test pieces, a 90° V-bending test was conducted under conditions of an indentation load of 100 kN and a holding time of 5 seconds. Bending tests were conducted on five test pieces with an appropriate bending radius R. Next, the presence or absence of cracks at the ridgeline of the bending apex was confirmed. The presence or absence of cracks was confirmed by observing the ridgeline of the bending apex at 40x magnification using a digital microscope (RH-2000, manufactured by Hirox Corporation). 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.

[0101] Delayed fracture resistance test for evaluating the shear stress stability of delayed fracture resistance properties. The delayed fracture resistance test for evaluating the shear stress stability of delayed fracture resistance properties was performed by four-point bending samples with various shear edge conditions, followed by immersion in hydrochloric acid. Specifically, first, strip-shaped test specimens with a width of 70 mm and a length of 18 mm were cut from the galvanized steel sheet obtained by shearing, so that the shear edge was perpendicular to the rolling direction of the steel sheet. Shearing was performed under six conditions: a shear angle of 0° or 0.5°, and a clearance of 10%, 15%, or 20%. Six test specimens with different shear edge conditions were cut from one galvanized steel sheet. The cut test specimens were subjected to bending deformation by four-point bending so that a stress equivalent to TS was applied to the center of each specimen. Next, the bent specimens were subjected to a heat treatment (heat treatment simulating baked finish) at 150°C for 20 minutes. The heat-treated specimens were then immersed in a hydrochloric acid solution (pH: 2.0) and removed from the solution after 48 hours. The sheared end surfaces of the specimens were then observed visually and at 20x magnification using a digital microscope (RH-2000, manufactured by Hirox Corporation). If cracks were observed in any of the six specimens as a result of the observation, a "C" was recorded in the "Delayed Fracture Resistance" column of Table 3 below. On the other hand, if cracks were not observed in any of the specimens, a "B" was recorded in the "Delayed Fracture Resistance" column of Table 3 below. If cracks were not observed in any of the specimens, all specimens were promptly immersed again in a hydrochloric acid solution (pH: 2.0) and removed from the solution after 48 hours. The specimens were then 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 entered as "A" in the "Delayed fracture resistance" column in Table 3 below. If the test specimen was rated as "A" or "B," it was determined that the delayed fracture resistance had excellent shear disturbance stability. If the test specimen was rated as "A," it was determined that the delayed fracture resistance had even better shear disturbance stability.

[0102]

[0103]

[0104]

[0105] <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 excellent in all of 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 crash resistance, stretch flangeability, bendability, and shear disturbance stability of delayed fracture resistance.

Claims

1. A steel sheet comprising a steel sheet and a zinc-plated layer disposed on a surface of the steel sheet, wherein the steel sheet 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 at a 1 / 4 position in the thickness direction of the steel sheet, an area ratio of tempered martensite is 80% or more, an area ratio of fresh martensite is less than 8%, a total area ratio of ferrite and bainite is 15% or less, and an area ratio of retained austenite is 15% or less, and at an interface position between the steel sheet and the zinc-plated layer, A high-strength galvanized steel sheet, comprising: a total area ratio of tempered martensite and fresh martensite of 40% or less; a total area ratio of ferrite and bainite of 60% or more; an area ratio of oxide particles of 2% or more and 18% or less; an average aspect ratio of the oxide particles of 1.2 or more and 6.0 or less; and an average major axis of the oxide particles of 0.10 μm or more and 0.70 μm or less.

2. The chemical 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.

3. A member made using the high-strength galvanized steel sheet according to claim 1 or 2.

4. An automobile part comprising the member according to claim 3.

5. A method for producing a high-strength galvanized steel sheet according to claim 1 or 2, comprising: subjecting a steel slab having the chemical composition according to claim 1 or 2 to hot rolling, including rough rolling and finish rolling, to obtain a hot-rolled sheet; and then performing cooling C1 and coiling; and in the cooling C1, the steel slab is cooled in a temperature range T of 650°C or higher and not exceeding the finish rolling delivery temperature FT. Ａ The average cooling rate v Ａ is 15 ° C. / s or more, and the temperature range T is 550 ° C. or more and 600 ° C. or less Ｂ Residence time t Ｂ The hot-rolled sheet is then subjected to pickling and cold rolling to obtain a cold-rolled sheet, and the cold-rolled sheet is then subjected to heating H1, followed by a galvanizing treatment at a cooling stop temperature T of 150°C or less. Ｃ Cooling C2 is performed to 600°C or more (A Ｃ１ +20) ° C. or less, the average heating rate v1 in the temperature range T1 is 8 ° C. / s or more, Ｃ１ +20)℃ or higher A Ｃ３ the average heating rate v2 in the temperature range T2 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, 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, 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 the 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 attainable temperature and a holding time Y at or above (X-10)°C satisfy the following formula (1): 7800≦(273+X)×(20+log(Y / 3600))≦11800 (1) where the unit of the temperature X is ° C. and the unit of the retention time Y is s.

6. 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.