Hot-dip galvanized steel sheet, production method for same, member and production method for same, and automobile skeleton structure component or automobile reinforcing component comprising member

Abstract

The purpose of the present invention is to provide: a hot-dip galvanized steel sheet that has a high YR, has high stretch-flangeability and bendability immediately after production, and has improved trim edge quality; a production method for the hot-dip galvanized steel sheet; and a member.　A hot-dip galvanized steel sheet according to the present invention comprises a parent material steel sheet and a hot-dip galvanized layer that is at the surface of the parent material steel sheet. The parent material steel sheet has a prescribed component composition. At 1 / 4 of the sheet thickness thereof, the parent material steel sheet has a microstructure that has an area percentage of martensite of at least 30%, an area percentage of ferrite of no more than 70%, and a volume percentage of retained austenite of no more than 20.0%. The amount of hydrogen diffusible in a low-temperature range, which is the amount of hydrogen released from the parent material steel sheet when the parent material steel sheet is heated from room temperature to 50°C at 24 hours after the production of the hot-dip galvanized steel sheet, is no more than 0.015 mass ppm. The density of cracks that pass through the hot-dip galvanized layer is at least 30 / mm. The full width at half maximum of a δ1 phase of the hot-dip galvanized layer is at least 0.100.

Description

Hot-dip galvanized steel sheet and its manufacturing method, component and its manufacturing method, and automotive frame structural component or automotive reinforcing component made of said component

[0001] The present invention relates to a hot-dip galvanized steel sheet and a method for manufacturing the same, a member and a method for manufacturing the same, and an automobile frame structural part or automobile reinforcing part made of the member.

[0002] CO2 emissions from lighter vehicles ２ With the aim of achieving both reduced emissions and improved crashworthiness through vehicle weight reduction, efforts are underway to increase the strength of automotive steel sheets. Furthermore, new legal regulations are being introduced one after another. Therefore, in order to increase the strength of the vehicle body, there has been an increase in the number of cases where high-strength steel sheets are used for the main structural and reinforcing parts that form the framework of the automobile cabin (hereinafter also referred to as automotive framework structural parts, etc.). In particular, there has been an increase in the number of cases where high-strength steel sheets with a tensile strength (hereinafter also simply referred to as TS) of 980 MPa or more are used.

[0003] Furthermore, high-strength steel sheets used in automotive structural components and the like are required to have high part strength when formed into automotive structural components and the like. For example, increasing the yield ratio (= YS / TS × 100, hereinafter also referred to simply as YR) of the steel sheet is effective in increasing part strength. This increases the impact absorption energy (hereinafter also referred to simply as impact absorption energy) during a vehicle collision.

[0004] Furthermore, among the structural components of automobiles, for example, crash boxes have punched end faces and bent portions. Therefore, from the viewpoint of formability, it is preferable to apply steel sheets having high stretch-flange formability and bendability to such components. Note that, since the stretch-flange formability and bendability are inspected immediately after production, it is necessary to guarantee the stretch-flange formability and bendability immediately after production.

[0005] In addition, from the viewpoint of corrosion prevention performance of the vehicle body, hot-dip galvanized steel sheets obtained by hot-dip galvanizing are sometimes used as steel sheets for the frame structural parts of automobiles.

[0006] As a technique relating to such hot-dip galvanized steel sheets, for example, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, and having a tensile strength of 780 MPa or more, wherein the base steel sheet contains, in mass%, C: 0.050% or more and 0.200% or less, Si: 0.10% or more and 0.90% or less, Mn: 2.00% or more and 3.50% or less, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, Ca: 0.0200% or less, and Cr: 0.300% or less, and the relationship [%Mn] / [%Si] is 2.9 or more and 11.7 or less, "A high-strength hot-dip galvanized steel sheet having a component composition consisting of 5% or more and 85% or less of bainite and ferrite, with the balance being Fe and unavoidable impurities, and having a steel structure in which one or two types selected from the group consisting of bainite and ferrite account for 5% or more and 85% or less in total area ratio, an area ratio of tempered martensite is 65% or less, an area ratio of quenched martensite is 5% or more and 40% or less, and an area ratio of retained austenite is 5.0% or less, a ratio of Si concentration to Mn concentration in a surface layer of the base steel sheet is 0.7 or more and 1.3 or less, and the amount of diffusible hydrogen in the base steel sheet is 0.80 ppm by mass or less, where [%Mn] and [%Si] respectively represent the Mn and Si contents (% by mass) in the steel."

[0007] Patent No. 6777267

[0008] In continuous annealing lines, the coil edges are trimmed at the exit of the production line, and a stable trimmed edge is also required. However, the hot-dip galvanized steel sheet described in Patent Document 1 does not take into consideration the quality of the trimmed edge. Furthermore, in the production of hot-dip galvanized steel sheets, immediately after production, i.e., when the steel sheet is wound into a coil at the exit of the continuous annealing line, a large amount of diffusible hydrogen is contained in the steel sheet, and the amount of diffusible hydrogen decreases over time at room temperature. Therefore, from immediately after production until 24 hours have passed, the amount of diffusible hydrogen in the steel sheet is large, making it difficult to obtain high stretch flangeability and bendability, and a stable trimmed edge may not be obtained.

[0009] Therefore, from the viewpoint of increasing the application ratio of hot-dip galvanized steel sheets, in particular, to automotive frame structural parts and the like, there is a demand for the development of hot-dip galvanized steel sheets that have high YR, stretch flangeability immediately after production, bendability immediately after production, and improved trim edge quality.

[0010] The present invention has been developed in view of the above-described current situation, and has an object to provide a hot-dip galvanized steel sheet having a high YR, high stretch flangeability immediately after production, and high bendability immediately after production, and having improved trim edge quality.

[0011] Another object of the present invention is to provide a method for manufacturing the above-mentioned hot-dip galvanized steel sheet, and a member made using the above-mentioned hot-dip galvanized steel sheet.

[0012] Here, "high YR" means that the YR is 55% or more. The YR is calculated by the following formula (1): YR = YS / TS × 100 (1) TS and YS are each measured in accordance with JIS Z 2241. "High stretch flangeability immediately after production" means that the hole expansion ratio (hereinafter simply referred to as λ) measured in accordance with JIS Z 2256 is 30% or more when measured 24 hours after production.

[0013] "High bendability immediately after manufacture" means that the crack length at the ridgeline of the bend apex of all samples after a bending test conducted in accordance with JIS Z 2248 (see the Examples section below for details) is 200 μm or less when measured 24 hours after manufacture. This is determined by conducting a bending test using the V-block method with a bending angle of 90 degrees. Here, five samples are subjected to a bending test at an R where the value R / t, obtained by dividing the bending radius (R) by the plate thickness (t), is approximately 4.5, i.e., 4.3 to 4.7. Next, the crack length at the ridgeline of the bend apex of all five samples is evaluated, and if the crack length of all samples is 200 μm or less, the sample is deemed to have high bendability immediately after manufacture.

[0014] "High quality of trimmed edge" means that no cracks are observed in the trimmed edge described in the examples below.

[0015] The inventors conducted extensive research to achieve the above-mentioned object. As a result, they discovered the following: (1) A base steel sheet has a predetermined chemical composition and a steel structure containing a certain amount of martensite (quenched martensite, tempered martensite, and bainite). This results in a high YR. (2) By setting the crack density penetrating the coating layer to 30 cracks / mm or more and setting the full width at half maximum of the δ1 phase of the coating layer to 0.100 degrees or more, it is possible to reduce the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production. This results in high stretch flangeability and bendability, and improves the quality of the trim edge.

[0016] The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows: [1] A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, wherein the base steel sheet contains, by mass%, C: 0.030% to 0.500%, Si: 0.01% to 2.50%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; and an area ratio of martensite at a quarter-thickness position of the base steel sheet is 30% or more, a microstructure in which an area fraction of ferrite is 70% or less and a volume fraction of retained austenite is 20.0% or less, wherein an amount of diffusible hydrogen in a low temperature range, which is the amount of hydrogen released from the base steel sheet when the base steel sheet is heated from room temperature to 50°C 24 hours after production of the hot-dip galvanized steel sheet, is 0.015 mass ppm or less, and wherein the hot-dip galvanized steel sheet has a number density of 30 cracks / mm or more penetrating the hot-dip galvanized layer and a full width at half maximum of a δ1 phase in the hot-dip galvanized layer of 0.100 degrees or more. [2] The hot-dip galvanized steel sheet according to [1], wherein the chemical composition further contains, in mass%, at least one element selected from 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, Co: 0.010% or less, Ni: 1.00% 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 Bi: 0.200% or less. [3] The hot-dip galvanized steel sheet according to [1] or [2], wherein the hot-dip galvanized layer is a galvannealed layer. [4] A member made using the hot-dip galvanized steel sheet according to any one of [1] to [3]. [5] An automobile frame structural part or an automobile reinforcing part made of the member according to [4].[6] A method for producing a hot-dip galvanized steel sheet, comprising: hot-rolling a steel slab having the chemical composition according to [1] or [2] to obtain a hot-rolled steel sheet; pickling the hot-rolled steel sheet to obtain a pickled sheet; cold-rolling the pickled sheet at a cumulative reduction of 20% to 75% to obtain a cold-rolled steel sheet; annealing the cold-rolled steel sheet at an annealing temperature of 780°C or higher; hot-dip galvanizing the cold-rolled steel sheet to obtain a plated steel sheet; cooling the plated steel sheet at an average cooling rate of 1.0°C / s or higher in a temperature range of 250°C to 400°C; and processing the plated steel sheet to impart a uniaxial tensile strain of 0.1% or higher to a surface layer. [7] The method for producing a hot-dip galvanized steel sheet according to [6], wherein, during cooling of the plated steel sheet, the plated steel sheet is maintained at a temperature range of 100°C to 450°C for 5 seconds or more, then cooled, and then subjected to the processing. [8] The method for producing a hot-dip galvanized steel sheet according to [6], wherein, during cooling of the plated steel sheet, cooling is stopped at a cooling stop point of 250°C or less, and then the plated steel sheet is reheated to a temperature range of (cooling stop temperature + 50°C) to 450°C, maintained at this temperature range for 5 seconds or more, then cooled, and then subjected to the processing. [9] The method for producing a hot-dip galvanized steel sheet according to [6], wherein, during cooling of the plated steel sheet, cooling is stopped at a cooling stop point of 250°C or less, then the plated steel sheet is subjected to the processing, then reheated to a temperature range of (cooling stop temperature + 50°C) to 450°C, maintained at this temperature range for 5 seconds or more, and then cooled.

[10] The method for producing a hot-dip galvanized steel sheet according to any of [6] to [9], wherein the steel sheet after the hot-dip galvanizing treatment is subjected to an alloying treatment.

[11] A method for producing a hot-dip galvanized steel sheet according to any one of [6] to

[10] , wherein the hot-dip galvanized steel sheet is cold-rolled at a final pass with a threading speed of 50 mpm or more.

[12] A method for producing a member, comprising a step of forming or joining the hot-dip galvanized steel sheet according to any one of [1] to [3] to produce a member.

[0017] According to the present invention, a hot-dip galvanized steel sheet having a high YR, high stretch flangeability and bendability, and improved trim edge quality can be obtained. This high YR ensures high component strength when applied to automotive frame structural parts.

[0018] In particular, the hot-dip galvanized steel sheet of the present invention has various excellent properties and can be applied to automotive frame structural parts of various sizes and shapes, which can reduce the weight of the vehicle body and improve fuel efficiency, making it extremely valuable in industry.

[0019] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0020] Regarding the base steel sheet: First, the chemical composition of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described. Note that although the unit of chemical composition is always "% by mass," hereinafter, unless otherwise specified, it will be simply expressed as "%."

[0021] [C: 0.030% or more and 0.500% or less] C is one of the important basic components of steel. In particular, in the present disclosure, C is an important element that affects the area fractions of martensite and ferrite, the volume fraction of retained austenite, and the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production. If the C content is less than 0.030%, the area fraction of martensite decreases and the area fraction of ferrite increases, making it difficult to achieve the desired YR. On the other hand, if the C content exceeds 0.500%, the martensite fraction increases, and the fraction of quenched martensite in martensite also increases, resulting in an increase in the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production. As a result, λ and bendability decrease, and the quality of the trim edge also deteriorates. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably set to 0.080% or more. The C content is preferably 0.400% or less, more preferably 0.110% or more, and more preferably 0.350% or less.

[0022] [Si: 0.01% or More and 2.50% or Less] Si is one of the important basic components of steel. In particular, in the present disclosure, Si suppresses carbide formation during annealing and promotes the formation of retained austenite, thereby affecting the volume fraction of retained austenite. Furthermore, Si forms an internal oxide layer in the surface layer of a steel sheet and induces cracks in the coating when strain is applied to the surface layer of a plated steel sheet. Therefore, Si is an important element that affects the number density of cracks penetrating the coating layer. To achieve this effect, the Si content is set to 0.01% or more. On the other hand, if the Si content exceeds 2.50%, the increase in the volume fraction of retained austenite increases the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production, thereby reducing λ and bendability and further degrading the quality of the trim edge. Therefore, the Si content is set to 0.01% or more and 2.50% or less. The Si content is preferably set to 0.20% or more. The Si content is preferably set to 2.00% or less. The Si content is more preferably 0.25% or more, and more preferably 1.50% or less.

[0023] [Mn: 0.10% or More and 5.00% or Less] Mn is one of the important basic components of steel. In particular, in the present disclosure, Mn is an important element that affects the area fractions of martensite and ferrite, and the volume fraction of retained austenite. Furthermore, Mn forms an internal oxide layer on the surface of a steel sheet and induces cracks in the coating when strain is applied to the coating layer of a plated steel sheet. Therefore, Mn is an important element that affects the number density of cracks penetrating the coating layer. To achieve this effect, the Mn content is set to 0.10% or more. On the other hand, if the Mn content exceeds 5.00%, the fraction of martensite increases, and furthermore, the fraction of quenched martensite in the martensite increases, resulting in an increase in the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production. As a result, λ and bendability decrease, and further, the quality of the trim edge deteriorates. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably set to 1.00% or more. The Mn content is preferably 4.00% or less, more preferably 2.00% or more, and more preferably 3.50% or less.

[0024] [P: 0.100% or less] Excessive P segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, resulting in reduced λ and bendability. Therefore, the P content must be 0.100% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.001% or more. The P content is preferably 0.070% or less.

[0025] [S: 0.0200% or less] S exists as sulfides and reduces the ultimate deformability of steel, thereby reducing λ and bendability. Therefore, the S content needs to be 0.0200% or less. Although there is no particular lower limit for the S content, due to constraints on production technology, the S content is preferably 0.0001% or more. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0001% or more. The S content is preferably 0.0050% or less.

[0026] [Al: 0.100% or less] When Al is excessive, A ３ The transformation point rises and a large amount of ferrite is contained in the microstructure, making it difficult to achieve the desired YR. Therefore, the Al content needs to be 0.100% or less. Although there is no particular lower limit for the Al content, the Al content is preferably 0.001% or more because it suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite. Therefore, the Al content is set to 0.100% or less. The Al content is preferably set to 0.001% or more. The Al content is preferably set to 0.050% or less.

[0027] [N: 0.0100% or less] N exists as nitride and reduces the ultimate deformability of the steel sheet, thereby reducing λ and bendability. Therefore, the N content needs to be 0.0100% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more. Therefore, the N content is 0.0100% or less. The N content is preferably 0.0005% or more. The N content is preferably 0.0050% or less.

[0028] [O: 0.0100% or less] O exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing λ and bendability. Therefore, the O content needs to be 0.0100% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0001% or more. The O content is preferably 0.0050% or less.

[0029] The base steel sheet of a hot-dip galvanized steel sheet according to one embodiment of the present invention has a composition containing the above elements with the balance including Fe and unavoidable impurities. Preferably, the base steel sheet according to one embodiment of the present invention has a composition containing the above elements with the balance consisting of Fe and unavoidable impurities. Examples of the unavoidable impurities include Zn, Pb, and As. The inclusion of these impurities is permitted as long as the total content is 0.100% or less.

[0030] The composition of the base steel sheet of the hot-dip galvanized steel sheet of the present disclosure, in addition to the above-mentioned essential components, 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, and At least one element selected from the group consisting of 0.00% 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 Bi: 0.200% or less may be contained alone or in combination.

[0031] [Ti: 0.200% or less] Ti generates large amounts of coarse precipitates and inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the Ti content exceeds 0.200%, λ and bendability decrease. Therefore, the Ti content is set to 0.200% or less. Although there is no particular lower limit for the Ti content, by setting the Ti content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing. This increases the strength of the steel sheet and enables the YR to be controlled within a desired range. For this reason, the Ti content is preferably set to 0.001% or more. Therefore, when added, the Ti content is set to 0.200% or less. The Ti content is preferably set to 0.001% or more. The Ti content is preferably set to 0.100% or less.

[0032] [Nb: 0.200% or less] Nb generates large amounts of coarse precipitates and inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the Nb content exceeds 0.200%, λ and bendability decrease. Therefore, the Nb content is set to 0.200% or less. Although there is no particular lower limit for the Nb content, by setting the Nb content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing. This increases the strength of the steel sheet and enables the YR to be controlled within a desired range. For this reason, the Nb content is preferably set to 0.001% or more. Therefore, when Nb is added, the Nb content is set to 0.200% or less. The Nb content is preferably set to 0.001% or more. The Nb content is preferably set to 0.100% or less.

[0033] [V: 0.200% or less] V generates large amounts of coarse precipitates and inclusions, reducing the ultimate deformability of the steel sheet. Therefore, if the V content exceeds 0.200%, λ and bendability decrease. Therefore, the V content is set to 0.200% or less. Although there is no particular lower limit for the V content, by setting the V content to 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing. This increases the strength of the steel sheet and enables the YR to be controlled within a desired range. For this reason, the V content is preferably set to 0.001% or more. Therefore, when added, the V content is set to 0.200% or less. The V content is preferably set to 0.001% or more. The V content is preferably set to 0.100% or less.

[0034] [Ta: 0.10% or less, W: 0.10% or less] If the Ta and W contents exceed 0.10%, large amounts of coarse precipitates and inclusions are formed, reducing the ultimate deformability of the steel sheet and thereby reducing λ and bendability. Therefore, the Ta and W contents are each set to 0.10% or less. Although there are no particular lower limits for the Ta and W contents, they form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. This increases the strength of the steel sheet, so the Ta and W contents are preferably set to 0.01% or more. Therefore, when added, the Ta and W contents are each set to 0.10% or less. The Ta and W contents are preferably set to 0.01% or more. The Ta and W contents are preferably set to 0.08% or less.

[0035] [B: 0.0100% or less] If B is 0.0100% or less, 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 λ and bendability will not be reduced. Therefore, the B content is set to 0.0100% or less. Although there is no particular lower limit for the B content, since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is more preferably 0.0003% or more. Therefore, when B is contained, its content is set to 0.0100% or less. The B content is more preferably 0.0003% or more. The B content is even more preferably 0.0080% or less.

[0036] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less] If the Cr, Mo, and Ni contents exceed 1.00%, the amount of coarse precipitates and inclusions increases, reducing the ultimate deformability of the steel sheet and thereby reducing λ and bendability. Therefore, the Cr, Mo, and Ni contents are each set to 1.00% or less. Although there are no particular lower limits for the Cr, Mo, and Ni contents, these elements improve hardenability, so the Cr, Mo, and Ni contents are preferably set to 0.01% or more. Therefore, when added, the Cr, Mo, and Ni contents are each set to 1.00% or less. The Cr, Mo, and Ni contents are preferably set to 0.01% or more. The Cr, Mo, and Ni contents are preferably set to 0.80% or less.

[0037] [Co: 0.010% or less] If the Co content exceeds 0.010%, the amount of coarse precipitates and inclusions increases, reducing the ultimate deformability of the steel sheet, and therefore reducing λ and bendability. Therefore, the Co content is set to 0.010% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, the Co content is preferably set to 0.001% or more. Therefore, when Co is added, the Co content is set to 0.010% or less. The Co content is preferably set to 0.001% or more. The Co content is preferably set to 0.008% or less.

[0038] [Cu: 1.00% or less] If the Cu content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet, thereby reducing λ and bendability. Therefore, the Cu content is set to 1.00% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is preferably set to 0.01% or more. Therefore, when added, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.01% or more. The Cu content is preferably set to 0.80% or less.

[0039] [Sn: 0.200% or less] If the Sn content exceeds 0.200%, cracks are generated inside the steel sheet during casting or hot rolling, reducing the ultimate deformability of the steel sheet, resulting in reduced λ and bendability. Therefore, the Sn content is set to 0.200% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability, the Sn content is preferably set to 0.001% or more. Therefore, when Sn is added, the Sn content is set to 0.200% or less. The Sn content is preferably set to 0.001% or more. The Sn content is preferably set to 0.100% or less.

[0040] [Sb: 0.200% or less] If the Sb content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet, thereby reducing λ and bendability. Therefore, the Sb content is set to 0.200% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is preferably set to 0.001% or more. Therefore, when Sb is added, the Sb content is set to 0.200% or less. The Sb content is preferably set to 0.001% or more. The Sb content is preferably set to 0.100% or less.

[0041] [Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less] If the Ca, Mg, and REM contents exceed 0.0100%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet and thereby reducing λ and bendability. Therefore, the Ca, Mg, and REM contents are each set to 0.0100% or less. Although there are no particular lower limits for the Ca, Mg, and REM contents, these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, when added, the Ca, Mg, and REM contents are each set to 0.0100% or less. The Ca, Mg, and REM contents are preferably set to 0.0005% or more. The contents of Ca, Mg, and REM are preferably 0.0050% or less. Note that REM (rare earth elements) is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.

[0042] [Zr: 0.100% or less, Te: 0.100% or less] If the Zr and Te contents exceed 0.100%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet, resulting in reduced λ and bendability. Therefore, the Zr and Te contents must each be 0.100% or less. Although there are no particular lower limits for the Zr and Te contents, the Zr and Te contents are preferably 0.001% or more because they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, when added, the Zr and Te contents are set to 0.100% or less. The Zr and Te contents are preferably 0.001% or more. The Zr and Te contents are preferably 0.080% or less.

[0043] [Hf: 0.10% or less] If the Hf content exceeds 0.10%, coarse precipitates and inclusions increase, reducing the ultimate deformability of the steel sheet, and therefore reducing λ and bendability. Therefore, the Hf content is set to 0.10% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is preferably set to 0.01% or more. Therefore, when Hf is added, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.01% or more. The Hf content is preferably set to 0.08% or less.

[0044] [Bi: 0.200% or less] If the Bi content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, reducing the ultimate deformability of the steel sheet, and therefore reducing λ and bendability. Therefore, the Bi content is set to 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is preferably set to 0.001% or more. Therefore, when Bi is added, the Bi content is set to 0.200% or less. The Bi content is preferably set to 0.001% or more. The Bi content is preferably set to 0.100% or less.

[0045] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferred lower limit, the effect of the present invention is not impaired, and therefore these elements are included as inevitable impurities.

[0046] Next, the microstructure of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0047] [Area Fraction of Martensite at 1 / 4 Thickness Position of Base Steel Plate: 30% or More] The microstructure of the base steel plate contains a certain amount of martensite. Specifically, by setting the area fraction of martensite at the 1 / 4 thickness position of the base steel plate to 30% or more, it is possible to achieve a desired YR, high λ, and high bendability. Therefore, the area fraction of martensite at the 1 / 4 thickness position of the base steel plate is set to 30% or more. The area fraction of martensite at the 1 / 4 thickness position of the base steel plate is preferably 35% or more, more preferably 40% or more, and even more preferably 45% or more. Note that there is no particular limitation on the upper limit of the area fraction of martensite at the 1 / 4 thickness position of the base steel plate. However, from the viewpoint of obtaining a high YR, high λ, and high bendability, the area fraction of martensite at the 1 / 4 thickness position of the base steel plate is preferably 99% or less, more preferably 98% or less, and even more preferably 97% or less. The martensite referred to here includes not only quenched martensite (fresh martensite) but also tempered martensite and bainite.

[0048] [Area fraction of ferrite at 1 / 4 thickness position of base steel plate: 70% or less] By setting the area fraction of ferrite at 1 / 4 thickness position of the base steel plate to 70% or less, the YR increases. Furthermore, λ increases and bendability improves. Therefore, the area fraction of ferrite at 1 / 4 thickness position of the base steel plate is set to 70% or less. The area fraction of ferrite at 1 / 4 thickness position of the base steel plate is preferably 60% or less. Note that the area fraction of ferrite at 1 / 4 thickness position of the base steel plate may be 0%. However, from the viewpoint of reducing the amount of low-temperature diffusible hydrogen in the base steel plate 24 hours after production and improving the quality of the trim edge, the area fraction of ferrite at 1 / 4 thickness position of the base steel plate is preferably 1% or more, more preferably 2% or more. Note that ferrite here may also be defined as bainitic ferrite.

[0049] Here, the method for measuring the area ratio of martensite (quenched martensite, tempered martensite, and bainite) and ferrite (bainitic ferrite) at the 1 / 4 position in the plate thickness of the base steel plate is as follows.

[0050] After cutting out a sample so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate is the observation surface, the observation surface is mirror-polished using diamond paste, and then etched with 3 vol.% nital to reveal the structure. Under the condition of an acceleration voltage of 15 kV, using an SEM (Scanning Electron Microscope), the observation position is set to 1 / 4 of the plate thickness of the base steel plate, and at a magnification of 5000 times, three fields of view are observed in a field of view of 17 μm × 23 μm. The obtained structure image is processed as follows using Adobe Photoshop from Adobe Systems. That is, the area ratio of each constituent structure (ferrite (bainitic ferrite), martensite (tempered martensite, bainite, and quenched martensite)) divided by the measured area is calculated for three fields of view, and these values ​​are averaged to determine the area ratio of each structure. In the above structure image, ferrite (bainitic ferrite) is a recessed structure that is a flat structure that does not contain carbides. Furthermore, tempered martensite and bainite are recessed structures that contain fine carbides, while quenched martensite is a protruding structure with fine irregularities inside the structure, and they are distinguishable from each other. Note that tempered martensite and bainite do not need to be distinguishable from each other, as the total area ratio is calculated as the area ratio of martensite.

[0051] [Volume Fraction of Retained Austenite at 1 / 4 Thickness of Base Steel Plate: 20.0% or Less] If the volume fraction of retained austenite in the microstructure of the base steel plate is 20.0% or less, the amount of low-temperature diffusible hydrogen in the base steel plate 24 hours after production can be reduced. Therefore, the volume fraction of retained austenite is set to 20.0% or less. Furthermore, if the volume fraction of retained austenite is 15.0% or less, the carbon concentration in the retained austenite increases, suppressing stress-induced transformation of the retained austenite during tensile deformation, thereby achieving a high YR. Therefore, the volume fraction of retained austenite is preferably 15.0% or less, more preferably 10.0% or less, and even more preferably 5.0% or less. Note that there is no particular lower limit for the volume fraction of retained austenite; desired properties can be obtained even if the volume fraction is 0%.

[0052] Here, the volume fraction of retained austenite at the 1 / 4 position in the plate thickness direction of the base steel plate is measured as follows.

[0053] After grinding, the base steel sheet is further polished by 0.1 mm by chemical polishing so that the observation surface is located at a position 1 / 4 of the sheet thickness from the surface layer (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the surface of the base steel sheet). For this surface, an X-ray diffractometer is used 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 using a Co Kα radiation source. The volume fraction of austenite is then calculated from the intensity ratio of the integrated reflection intensity from each plane of fcc iron (austenite) to the integrated reflection intensity from each plane of bcc iron, and this is defined as the volume fraction of retained austenite.

[0054] Furthermore, at a quarter-thickness position of the base steel plate, the area ratio of the remaining structure other than martensite, ferrite, and retained austenite is preferably 5% or less. Examples of the remaining structure include other known structures of steel plates, such as pearlite, cementite, and carbides such as metastable carbides (epsilon (ε) carbide, eta (η) carbide, chi (χ) carbide, etc.). The remaining structure can be identified, for example, by observation using a SEM.

[0055] The area fraction of the remaining structure is calculated by the following formula. Note that the volume fraction of retained austenite is considered to be approximately equal to the area fraction, and is defined by the following formula: [Area fraction of remaining structure (%)] = 100 - [Area fraction of martensite (%)] - [Area fraction of ferrite (%)] - [Area fraction of retained austenite (%)]. In addition, in a hot-dip galvanized steel sheet according to one embodiment of the present invention, it is important to appropriately control the amount of diffusible hydrogen in the low temperature region of the base steel sheet.

[0056] [Amount of diffusible hydrogen in low temperature range of base steel sheet 24 hours after production: 0.015 mass ppm or less] The amount of diffusible hydrogen in low temperature range of base steel sheet 24 hours after production is an extremely important requirement. That is, the inventors of the present invention have conducted extensive research to obtain a hot-dip galvanized steel sheet having a high YR, high λ and bendability immediately after production, and improved trim edge quality. As a result, they have found that the amount of diffusible hydrogen in low temperature range of base steel sheet 24 hours after production, i.e., the amount of hydrogen released from the base steel sheet when the base steel sheet 24 hours after production is heated from room temperature to 50°C, affects the above-mentioned properties. In particular, they have found that the amount of hydrogen significantly affects the quality of the trimmed edge at the exit of a continuous annealing line. That is, the inventors have found that the quality of the trim edge is largely dependent on the amount of hydrogen released from the base steel sheet in the low temperature range, specifically, the temperature range from room temperature to 50°C, rather than the amount of hydrogen released from the base steel sheet in the high temperature range when the base steel sheet is heated. Furthermore, the inventors have found the following in order to significantly improve the quality of the trim edge while maintaining a high YR, λ, and bendability. That is, it is essential to reduce the amount of diffusible hydrogen in the low temperature range of the base steel sheet 24 hours after production, particularly to make it 0.015 mass ppm or less. This finding led to the development of the present invention. Therefore, the low temperature diffusible hydrogen amount of the base steel sheet 24 hours after production is set to 0.015 mass ppm or less. The lower the amount of diffusible hydrogen in the base steel sheet 24 hours after production, the better, and it is preferably 0.010 mass ppm or less, and more preferably 0.006 mass ppm or less. The lower limit of the low-temperature diffusible hydrogen content of the base steel sheet 24 hours after production is not particularly limited and may be 0 ppm by mass. However, due to constraints on production technology, the low-temperature diffusible hydrogen content of the base steel sheet 24 hours after production is preferably 0.001 ppm by mass or more.

[0057] Here, the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production is measured as follows.

[0058] Specifically, a large plate sample measuring 500 mm in length is taken from a steel strip produced on a hot-dip galvanizing line. 24 hours after production, a test piece measuring 30 mm in length and 5 mm in width is taken from the center of the large plate sample by shearing. Immediately after taking, the test piece is immersed in liquid nitrogen. The hot-dip galvanized layer of the test piece is then alkali-removed while controlling the temperature of the treatment solution so that the surface temperature of the test piece is below room temperature. The amount of hydrogen released from the test piece when heated is measured using thermal desorption analysis. Specifically, the test piece is heated from room temperature to a temperature of 300°C at a heating rate of 200°C / hr, and then cooled to room temperature. The cumulative amount of hydrogen released from the test piece in the temperature range from room temperature to 50°C during heating (hereinafter also referred to as the cumulative released hydrogen amount) is measured. The low-temperature diffusible hydrogen content of the base steel sheet is then calculated using the following formula: To determine whether the hot dip galvanized layer of the test piece has been completely peeled off, the test piece is heated from room temperature, and when it is confirmed that the amount of released hydrogen is zero by TDA analysis at 200-210°C, the coating layer is considered to have been completely peeled off. [Amount of diffusible hydrogen in the low temperature range of the base steel sheet (ppm by mass)] = [Cumulative amount of released hydrogen (g)] ÷ [Mass of test piece (g)] x 10 ６ Here, the point in time 24 hours after production refers to the point in time 24 hours±2 hours from the point in time when all the processes in the hot-dip galvanizing line (the annealing process to the reheating process in Table 2) are completed and the coiling into a coil is completed on the outlet side of the continuous annealing line.

[0059] Furthermore, the room temperature does not have a significant effect on the measurement of the amount of diffusible hydrogen in the low temperature region of the base steel sheet, so long as it is within the range of 10 to 25° C. However, if the room temperature is outside the range of 10 to 25° C., the cumulative amount of released hydrogen from the test piece may be measured in the temperature range from 25° C. to 50° C., with 25° C. being the representative room temperature.

[0060] The low-temperature diffusible hydrogen content may be measured in the same manner for hot-dip galvanized steel sheets (members) that have been subjected to cold working such as punching, stretch flange forming, and bending, and for members manufactured by further welding the hot-dip galvanized steel sheets (members) after the above-mentioned working. Furthermore, the low-temperature diffusible hydrogen content of the base steel sheet portion may be measured in the same manner as above for automotive frame structural parts or reinforcing parts made of such members.

[0061] The thickness of the base steel sheet of the hot-dip galvanized steel sheet according to one embodiment of the present invention is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less.

[0062] Regarding the hot-dip galvanized layer: Next, the hot-dip galvanized layer of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described. Note that the hot-dip galvanized layer referred to here also includes an alloyed hot-dip galvanized layer obtained by performing an alloying treatment on hot-dip galvanization. The hot-dip galvanized layer is formed on both surfaces of the base steel sheet.

[0063] The composition of the hot-dip galvanized layer is not particularly limited and may be any common composition. In one example, the hot-dip galvanized layer contains 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al. Furthermore, the hot-dip galvanized layer contains one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass or more and 3.5% by mass or less. The balance consists of Zn and unavoidable impurities. In the case of an unalloyed hot-dip galvanized layer, in one example, the Fe content in the coated layer is less than 7% by mass. In the case of an alloyed hot-dip galvanized layer, in one example, the Fe content in the coated layer is 7% by mass or more and 15% by mass or less, more preferably 8% by mass or more and 13% by mass or less. However, the present invention is not limited thereto.

[0064] The plating amount per side is not particularly limited, but is preferably 20 to 80 g / m ２ is preferred.

[0065] Additionally, in the hot-dip galvanized steel sheet according to one embodiment of the present invention, it is essential to appropriately control the cracks in the hot-dip galvanized layer and the full width at half maximum of the δ1 phase.

[0066] [Number density of cracks penetrating the hot-dip galvanized layer: 30 cracks / mm or more] This is an extremely important inventive constituent element of the present disclosure. Increasing the number density of cracks penetrating the galvanized layer can reduce the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production. As a result, the quality of the trim edge can be improved while maintaining high λ and bendability. To achieve this effect, the number density of cracks penetrating the galvanized layer needs to be 30 cracks / mm or more. There is no particular upper limit to the number density of cracks penetrating the galvanized layer, but it is preferably 100 cracks / mm or less because it reduces plating quality. Therefore, the number density of cracks penetrating the galvanized layer is set to 30 cracks / mm or more. The number density of cracks penetrating the galvanized layer is preferably 40 cracks / mm or more. The number density of cracks penetrating the galvanized layer is preferably 100 cracks / mm or less.

[0067] Here, the number density of cracks penetrating the plating layer is measured as follows.

[0068] A sample was cut out so that the thickness cross section (L cross section) of the hot-dip galvanized steel sheet parallel to the rolling direction served as the observation surface, and then the observation surface was mirror-polished using diamond paste. Using an SEM under conditions of an acceleration voltage of 15 kV, the hot-dip galvanized layer was set as the observation position, and a 2 mm area in the rolling direction was observed at a magnification of 2000x, with a field of view of 140 μm in the rolling direction and 44 μm in the thickness direction per sheet. In the above structure image, the number of cracks penetrating the hot-dip galvanized layer was counted and divided by 2 mm to calculate the crack density penetrating the plating layer.

[0069] [Full Width at Half Maximum of δ1 Phase in Hot-Dip Galvanized Layer: 0.100° or More] This is an extremely important inventive feature of the present disclosure. Increasing the full width at half maximum of the δ1 phase in the coated layer can reduce the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production, resulting in improved trim edge quality while maintaining high λ and bendability. To achieve this effect, the full width at half maximum of the δ1 phase in the coated layer must be 0.100° or more. While there is no particular upper limit for the full width at half maximum of the δ1 phase in the coated layer, it is preferably 0.150° or less because this may result in a decrease in coating quality. Therefore, the full width at half maximum of the δ1 phase in the coated layer is set to 0.100° or more. The full width at half maximum of the δ1 phase in the coated layer is preferably 0.110° or more. The full width at half maximum of the δ1 phase in the coated layer is preferably 0.150° or less.

[0070] Here, the full width at half maximum of the δ1 phase of the plating layer is measured as follows.

[0071] After cutting out a sample so that the surface of the hot-dip galvanized steel sheet serves as the observation surface, the surface is measured using an X-ray diffractometer with a Cu tube, a sampling angle step of 0.006°, and 0.36 seconds per point. Furthermore, to subtract the full width at half maximum (FWHM) derived from the X-ray diffractometer and the X-ray source itself, heat-treated Si and LaB6 are measured as strain-free standard samples at angle steps of 0.004° and 0.002°, respectively. From the results, the change in FWHM is determined using a method similar to that described in "N.L. Okamoto, K. Tanaka, A. Yasuhara and H. Inui: Acta Cryst., B70 (2014), 275."

[0072] The thickness of the hot-dip galvanized steel sheet according to one embodiment of the present invention is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less.

[0073] Regarding the manufacturing method of hot-dip galvanized steel sheet: Next, a manufacturing method of hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0074] First, a steel material having the above-described composition is melted to produce a steel slab. In the present disclosure, the method for melting the steel material is not particularly limited, and any known melting method, such as a converter or electric furnace, is suitable. Furthermore, the steel slab is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method or a thin slab casting method. After the steel slab is produced, in addition to the conventional method of cooling it to room temperature and then reheating it, it can also be charged into a heating furnace as a hot slab without cooling it to room temperature. Alternatively, energy-saving processes such as direct rolling, in which the slab is immediately rolled after a short period of heat retention, can also be applied without any problems.

[0075] Next, the produced steel slab is subjected to hot rolling consisting of rough rolling and finish rolling to produce a hot-rolled sheet. In one example, the steel slab produced as described above is once cooled to room temperature, then slab heated and then rolled. The slab heating temperature is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. Furthermore, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the temperature of the slab surface during heating.

[0076] Next, the steel slab is roughly rolled under normal conditions to form a sheet bar. When the slab heating temperature is set low, it is preferable to heat the sheet bar using a bar heater or the like before the finish rolling in order to prevent trouble during rolling. The finish rolling temperature is set at Ar ３ The temperature is preferably equal to or higher than the transformation point. If the finish rolling temperature is excessively lowered, the rolling load increases and the reduction ratio increases in the non-recrystallized state of austenite. This may cause the development of an abnormal structure elongated in the rolling direction, which may result in a decrease in the workability of the steel sheet obtained after annealing. ３ The transformation point is calculated using the following formula: Ar ３ (°C) = 868 - 396 x [%C] + 24.6 x [%Si] - 68.1 x [%Mn] - 36.1 x [%Ni] - 20.7 x [%Cu] - 24.8 x [%Cr]. In the above formula, the [% element symbol] represents the content (mass%) of the corresponding element in the above composition, and is set to 0 if the element is not contained.

[0077] In addition, the rough-rolled sheets may be joined together during hot rolling and continuous finish rolling may be performed. Furthermore, the rough-rolled sheet (sheet bar) may be temporarily wound before finish rolling. Furthermore, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape of the steel sheet and the material quality. The friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0078] Next, the hot-rolled sheet is subjected to pickling. Pickling can remove oxides from the steel sheet surface, and is therefore important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet product. Pickling may be performed once or multiple times.

[0079] Next, the hot-rolled sheet after pickling or the hot-rolled sheet (hot-rolled annealed sheet) that has been optionally heat-treated after pickling is cold-rolled to obtain a cold-rolled steel sheet. Since strain is introduced uniformly and efficiently and a uniform structure is obtained, it is preferable to perform cold rolling by multi-pass rolling that requires two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0080] In this case, it is an extremely important constituent requirement of the present invention that the cumulative rolling reduction is set to 20% or more and 75% or less.

[0081] [Cold Rolling Cumulative Reduction: 20% or More and 75% or Less] Increasing the cold rolling cumulative reduction increases the area fraction of martensite and decreases the area fraction of ferrite, thereby achieving a high YR, a high λ, and high bendability. To achieve these effects, the cold rolling cumulative reduction is set to 20% or more. On the other hand, if the cold rolling cumulative reduction exceeds 75%, the grain size of austenite generated during annealing becomes finer, and the amount of retained austenite in the annealed sheet increases. As a result, the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production increases, resulting in decreased λ and bendability, and further decreased trim edge quality. Therefore, the cold rolling cumulative reduction is set to 20% or more and 75% or less. The cold rolling cumulative reduction is preferably set to 25% or more. The cold rolling cumulative reduction is preferably set to 70% or less. The cold rolling cumulative reduction is more preferably set to 27% or more. The cumulative reduction rate of the cold rolling is more preferably 60% or less.

[0082] [Strip threading speed in the final pass of cold rolling: 50 mpm or more (preferred condition)] Increasing the tripping speed in the final pass of cold rolling can introduce strain into the surface layer of the base steel sheet and increase the number density of cracks penetrating the coating layer. This is particularly effective when the steel sheet is further alloyed after hot-dip galvanizing. As a result, the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production can be reduced, resulting in improved trim edge quality while maintaining high λ and bendability. To achieve this effect, the tripping speed in the final pass of cold rolling is set to 50 mpm or more. While there is no particular upper limit for the tripping speed in the final pass of cold rolling, it is preferable that it be 300 mpm or less due to production technology constraints. Therefore, the tripping speed in the final pass of cold rolling is set to 50 mpm or more. The tripping speed in the final pass of cold rolling is preferably set to 70 mpm or more. The sheet threading speed in the final pass of cold rolling is preferably 300 mpm or less.

[0083] The cold-rolled steel sheet obtained as described above is subjected to an annealing process under the following annealing conditions.

[0084] [Heating temperature: 780°C or higher] If the heating temperature (annealing temperature) is lower than 780°C, the annealing treatment will be performed in the two-phase region of ferrite and austenite, and a large amount of ferrite will be contained after annealing, making it difficult to achieve the desired YR. While there is no particular upper limit for the heating temperature, an increase in the heating temperature increases the thickness of the softened surface layer after annealing, reduces TS, coarsens the prior austenite grain size, and reduces YR. Therefore, the heating temperature is preferably 1000°C or lower. Therefore, the heating temperature is set to 780°C or higher. Preferably, it is set to 820°C or higher. The heating temperature is preferably set to 1000°C or lower. The heating temperature is more preferably set to 830°C or higher. The heating temperature is more preferably set to 980°C or lower. The heating temperature is measured based on the temperature of the steel sheet surface. The heat retention time at the heating temperature is not particularly limited, but is preferably set to 10 seconds or higher and 600 seconds or lower.

[0085] After the annealing, the cold-rolled steel sheet is cooled before the hot-dip galvanizing treatment. The conditions for this cooling are not particularly limited, and may be conventional. For example, the average cooling rate in the temperature range of 500°C or more below the heating temperature is not particularly limited, but is preferably 5°C / s or more and 50°C / s or less from the viewpoint of controlling the area ratio of martensite and ferrite.

[0086] [Plating Treatment Step] Next, the cold-rolled steel sheet is plated to obtain a plated steel sheet. Examples of the plating treatment include hot-dip galvanizing treatment. Furthermore, after the hot-dip galvanizing treatment, an alloying treatment may be performed. In addition, annealing, cooling, and plating treatment may be performed continuously in one line (CGL (Continuous Galvanizing Line)). For example, after annealing, the cold-rolled steel sheet is cooled to a temperature range of about 500°C. Next, the cold-rolled steel sheet is passed through the steel strip outlet side of the cooling zone, and further cooled while being moved into the hot-dip galvanizing bath via a snout whose leading end is immersed in the hot-dip galvanizing bath. The time from the end of cooling of the cold-rolled steel sheet until the cold-rolled steel sheet enters the hot-dip galvanizing bath is not particularly limited, but is preferably 1 s or more and 300 s or less from the viewpoint of controlling the area ratios of martensite and ferrite. A roll is provided immediately before the connection between the cooling zone and the snout to change the direction of travel of the cold-rolled steel sheet and cause it to enter the snout, and the cold-rolled steel sheet passes through the roll before entering the snout. The cold-rolled steel sheet is then guided to the hot-dip galvanizing bath via the snout and immersed in the hot-dip galvanizing bath to be subjected to hot-dip galvanizing treatment, thereby producing a plated steel sheet.

[0087] In the hot-dip galvanizing treatment, for example, the cold-rolled steel sheet is immersed in a hot-dip galvanizing bath at 440° C. or more and 500° C. or less. It is preferable to use a hot-dip galvanizing bath having an Al content of 0.10 mass % or more and 0.23 mass % or less, with the balance being Zn and unavoidable impurities.

[0088] Furthermore, after the above-described hot-dip galvanizing treatment, an alloying treatment may be performed at a temperature range of 460°C or higher and 600°C or lower. If the alloying treatment temperature is lower than 460°C, the Zn—Fe alloying rate becomes excessively slow, resulting in reduced productivity. On the other hand, if the alloying treatment temperature exceeds 600°C, untransformed austenite may transform into pearlite, making it impossible to obtain the desired area ratio of martensite. Therefore, the alloying treatment temperature is preferably 460°C or higher and 600°C or lower. The alloying treatment temperature is preferably 470°C or higher. Furthermore, the alloying treatment temperature is preferably 560°C or lower.

[0089] The plating weight is 20 to 80 g / m per side. ２ (Double-sided plating) is preferred. The coating weight can be adjusted by performing gas wiping or the like after the hot-dip galvanizing treatment.

[0090] [Optional Cooling Step] After the annealing step, the plated steel sheet is optionally cooled (cooling step). The average cooling rate from below 500°C to above 400°C is not particularly limited, but is preferably 5°C / s or more and 30°C / s or less. In addition, in a temperature range below the heating temperature and above 400°C, the high-strength steel sheet may be cooled once and the steel sheet temperature may be raised again.

[0091] [Average Cooling Rate in the Temperature Range of 250°C to 400°C: 1.0°C / s or More (Cooling Step)] Increasing the average cooling rate in the temperature range of 250°C to 400°C can reduce the area ratio of ferrite and increase the area ratio of martensite. After the hot-dip galvanized layer solidifies, the base steel sheet undergoes martensite transformation, thereby increasing the number density of cracks penetrating the galvanized layer. As a result, the amount of diffusible hydrogen in the low-temperature range of the base steel sheet 24 hours after production can be reduced, resulting in improved trim edge quality while maintaining high λ and bendability. To achieve this effect, the average cooling rate in the temperature range of 250°C to 400°C is set to 1.0°C / s or more. The average cooling rate in the temperature range of 250°C to 400°C is preferably 2.0°C / s or more, more preferably 3.0°C / s or more, and even more preferably 4.0°C / s or more. Although there is no particular upper limit for the average cooling rate in the temperature range of 250°C or higher and 400°C or lower, due to constraints on production technology, it is preferably 100.0°C / s or lower, and more preferably 80.0°C / s or lower. When the cooling stop temperature exceeds 250°C, the average cooling rate is the value in the temperature range of the cooling stop temperature or higher and 400°C or lower. The average cooling rate is measured based on the temperature of the steel sheet surface.

[0092] As a cooling method in the temperature range of 250° C. or higher and 400° C. or lower, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be applied.

[0093] [Uniaxial tensile strain in surface layer: 0.1% or more] By subjecting the hot-dip galvanized steel sheet that has undergone the above-described annealing and cooling to a processing step that sets the uniaxial tensile strain in the surface layer to a certain level or more, the number density of cracks penetrating the coating layer and the full width at half maximum of the δ1 phase in the coating layer can be increased. As a result, the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production can be reduced, resulting in improved trim edge quality while maintaining high λ and bendability. To achieve this effect, the uniaxial tensile strain in the surface layer needs to be 0.1% or more. The uniaxial tensile strain in the surface layer is preferably 0.5% or more, more preferably 1.0% or more. While there is no particular upper limit for the uniaxial tensile strain in the surface layer, it is preferable that the uniaxial tensile strain in the surface layer be 10.0% or less due to production technology constraints. The uniaxial tensile strain in the surface layer is more preferably 7.5% or less.

[0094] Here, the surface layer means the coating layer of the hot-dip galvanized steel sheet, and the amount of uniaxial tensile strain in the surface layer is calculated from the displacement of a marker in the sheet-threading direction after strain is applied by applying a marker in the sheet-threading direction to the surface of the hot-dip galvanized steel sheet that has been annealed and cooled. Note that the strain application methods described here include temper rolling, a tension leveler, and processing methods such as repeated bending with rolls.

[0095] [Heat retention temperature in cooling step: 100°C or higher and 450°C or lower (preferred conditions)] In the cooling step described above, it is preferable to retain heat for 5 seconds or longer at a heat retention temperature in the temperature range of 100°C or higher and 450°C or lower. By retaining the heat within the above range, the area ratio of bainitic ferrite is further reduced by subjecting the hot-dip galvanized steel sheet to heat retention, thereby making it possible to bring the YR, λ, and bendability within more preferred ranges. The heat retention temperature in the cooling step is more preferably 150°C or higher, and even more preferably 200°C or higher. The heat retention temperature in the cooling step is more preferably 400°C or lower, and even more preferably 350°C or lower. The temperature in the cooling step is based on the surface temperature of the steel sheet. After retaining heat at this temperature, the steel sheet is cooled, and the above-mentioned strain-imparting processing is performed to provide a uniaxial tensile strain of 0.1% or higher in the surface layer. The cooling conditions after this heat retention are not particularly limited, and may be performed according to conventional methods.

[0096] [Heat retention time in cooling step: 5 seconds or more (preferred condition)] By retaining the heat at the heat retention temperature in the cooling step, the YR, λ, and bendability can be kept within a more preferable range. To achieve this effect, the heat retention time at the heat retention temperature in the cooling step is preferably 5 seconds or more, more preferably 10 seconds or more, and even more preferably 15 seconds or more. There is no particular upper limit to the heat retention time at the heat retention temperature in the cooling step, but in order to keep the TS within a more preferable range, the heat retention time at the heat retention temperature in the cooling step is preferably 500 seconds or less, more preferably 250 seconds or less.

[0097] [Cooling stop temperature: 250°C or less (preferred conditions)] In the cooling step described above, the cooling stop temperature is preferably 250°C or less, more preferably 200°C or less. If the cooling stop temperature is 250°C or less, it is possible to prevent a large amount of retained austenite from being generated after annealing, and to further improve the YR, λ, and bendability. There is no particular restriction on the lower limit of the cooling stop temperature, but from the viewpoint of productivity, it is preferably room temperature or higher. The cooling stop rate is measured based on the temperature of the steel sheet surface.

[0098] Although the average cooling rate to the cooling stop temperature below 250° C. is not particularly specified, in order to further improve the YR, the average cooling rate to the cooling stop temperature below 250° C. is preferably 1° C. / s or more, and more preferably 2° C. / s or more. On the other hand, due to constraints on production technology, the average cooling rate to the cooling stop temperature below 250° C. is preferably 1000° C. / s or less, and more preferably 150° C. / s or less.

[0099] The cold-rolled steel sheet may be cooled from the cooling stop temperature to room temperature. The average cooling rate from the cooling stop temperature to room temperature is not particularly limited, and the steel sheet may be cooled to room temperature by any method. Examples of the cooling method that can be used include gas jet cooling, mist cooling, water cooling, and air cooling.

[0100] The hot-dip galvanized steel sheet annealed as described above may be cooled to the cooling stop temperature and then rolled. The elongation rate in rolling is preferably 0.05% or more, more preferably 0.10% or more. By setting the elongation rate in rolling performed after cooling to the cooling stop temperature to 0.05% or more, it is possible to control the YR within a desired range. Furthermore, the elongation rate in rolling is preferably 2.00% or less, more preferably 1.00% or less. By setting the elongation rate in rolling after cooling to the cooling stop temperature to 2.00% or less, it is possible to set the volume fraction of retained austenite within a more suitable range, and to set the bendability and the degree of damage at the sheared edge in a corrosive environment within more suitable ranges.

[0101] The rolling after cooling to the cooling stop temperature may be performed on an apparatus continuous with the above-mentioned continuous annealing apparatus (online), or on an apparatus discontinuous with the above-mentioned continuous annealing apparatus (offline). The target elongation may be achieved in a single rolling pass, or a total elongation of 0.05% to 2.00% may be achieved by performing multiple rolling passes. The rolling described here generally refers to temper rolling, but any other processing method, such as repeated bending using a tension leveler or rolls, may also be used as long as it can impart an elongation equivalent to that achieved by temper rolling.

[0102] [Reheating Step] [Reheating Temperature: (Cooling Stop Temperature + 50°C) or More and 450°C or Less (Preferred Condition)] After cooling to the cooling stop temperature, or after further rolling after cooling to the cooling stop temperature, the hot-dip galvanized steel sheet may be reheated (reheating step). Reheating the hot-dip galvanized steel sheet can bring the YR and bendability within more preferred ranges. To achieve this effect, the reheating temperature is preferably (Cooling Stop Temperature + 50°C) or more, more preferably (Cooling Stop Temperature + 100°C) or more, and even more preferably (Cooling Stop Temperature + 150°C) or more. On the other hand, as the reheating temperature increases, tempering of martensite progresses and TS decreases. Therefore, the reheating temperature is preferably 450°C or less, more preferably 400°C or less, and even more preferably 380°C or less. Note that the reheating temperature is based on the surface temperature of the steel sheet. After reheating and maintaining at this temperature, the steel sheet is cooled. This reheating may be carried out after or before the above-mentioned strain-imparting processing in which the uniaxial tensile strain amount in the surface layer is 0.1% or more. The cooling conditions after this reheating and heat retention are not particularly limited, and may be in accordance with a conventional method.

[0103] [Heat retention time at reheating temperature: 5 seconds or more (preferred condition)] By retaining the heat at the reheating temperature, the YR and bendability can be kept within a more preferable range. To achieve this effect, the heat retention time at the reheating temperature is preferably 5 seconds or more, more preferably 10 seconds or more, and even more preferably 15 seconds or more. While there is no particular upper limit to the heat retention time at the reheating temperature, in order to keep the TS within a more preferable range, the heat retention time at the reheating temperature is preferably 500 seconds or less, more preferably 250 seconds or less. The cooling rate from the reheating temperature to room temperature is not particularly limited, and cooling to room temperature can be performed by any method. Cooling methods that can be used include gas jet cooling, mist cooling, water cooling, and air cooling.

[0104] When the hot-dip galvanized steel sheet is to be traded, it is usually cooled to room temperature before being traded. The manufacturing conditions other than those described above can be the same as those of the conventional method.

[0105] Regarding Components: Next, components according to one embodiment of the present invention will be described.

[0106] A member according to one embodiment of the present invention is a member made using the hot-dip galvanized steel sheet according to one embodiment of the present invention described above. The member according to one embodiment of the present invention is, for example, a member obtained by forming the hot-dip galvanized steel sheet according to one embodiment of the present invention described above into a desired shape by cold pressing or the like. Therefore, even after being formed into a member, the member retains the microstructure and low-temperature diffusible hydrogen content of the hot-dip galvanized steel sheet, as well as the properties of the hot-dip galvanized layer. The member according to one embodiment of the present invention is preferably used for automotive frame structural parts or automotive reinforcing parts, and in one embodiment, these are made of the member of the present invention.

[0107] The hot-dip galvanized steel sheet according to the embodiment of the present invention is a hot-dip galvanized steel sheet having a high YR, stretch flangeability, bendability, and improved trim edge quality. Therefore, the member according to the embodiment of the present invention can contribute to weight reduction of the vehicle body, and can be suitably used in general for automotive frame structural parts or automotive reinforcing parts.

[0108] A steel slab (steel material) having the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities, was melted in a converter and obtained by continuous casting. The obtained steel slab was heated to 1250°C and rough rolled to obtain a sheet bar. The obtained sheet bar was then finish rolled at a finish rolling temperature of 900°C and coiled at a coiling temperature of 500°C. It was then cooled to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was pickled and then cold rolled under the conditions shown in Table 2 to obtain a cold-rolled steel sheet with a thickness of 1.4 mm.

[0109] The obtained cold-rolled steel sheets were then annealed under the conditions shown in Table 2. The cold-rolled steel sheets were then subjected to the types of plating treatments shown in Table 2 to obtain plated steel sheets having hot-dip galvanized layers on both sides. In the types of plating treatments in Table 2, GI means that only hot-dip galvanizing treatment was performed (hot-dip galvanized steel sheets without alloying treatment), and GA means that hot-dip galvanizing treatment was performed in addition to alloying treatment (hot-dip galvannealed steel sheets).

[0110]

[0111]

[0112] For GI, a hot-dip galvanizing bath containing 0.20 mass% Al, with the balance consisting of Zn and unavoidable impurities, was used. For GA, a hot-dip galvanizing bath containing 0.14 mass% Al, with the balance consisting of Zn and unavoidable impurities, was used. The bath temperature was 470°C in both cases. The coating weight for GI was 45 to 72 g / m per side. ２ (Both sides plating) and for GA, 45 g / m per side ２ In the case of GA, the alloying treatment temperature was set to about 550°C.

[0113] The composition of the hot-dip galvanized layer of GI was 0.1 to 1.0 mass% Fe, 0.2 to 1.0 mass% Al, and the balance was Zn and unavoidable impurities.The composition of the alloyed hot-dip galvanized layer of GA was 7 to 15 mass% Fe, 0.1 to 1.0 mass% Al, and the balance was Zn and unavoidable impurities.

[0114] The obtained hot-dip galvanized steel sheets were then cooled and reheated under the conditions shown in Table 2. Conditions not specified were those according to conventional methods.

[0115] The steel sheets thus obtained were subjected to microstructural identification at a 1 / 4 position in the plate thickness direction of the base steel sheet using the method described above. Furthermore, the amount of diffusible hydrogen in the low-temperature region, the number density of cracks penetrating the coating layer, and the full width at half maximum of the δ1 phase in the coating layer were measured 24 hours after production. The results are shown in Table 3. The composition of the base steel sheets of the obtained steel sheets was substantially the same as the composition at the steel slab stage, and all of the conforming steels were within the range of the composition according to the embodiment described above, while all of the comparative steels were outside the range of the composition according to the embodiment described above.

[0116] The obtained steel sheets were also evaluated for tensile properties, stretch flangeability immediately after production, bendability immediately after production, and trim edge quality according to the following test methods. The results are also shown in Table 3. In the examples, "immediately after production" means the time point 24 hours after production.

[0117] [Tensile Test] The tensile test was carried out in accordance with JIS Z 2241:2022. JIS No. 5 test pieces were taken from the obtained steel sheets in a direction perpendicular to the rolling direction of the steel sheets, and the test pieces were rolled at a crosshead speed of 1.67 × 10 －１ Tensile tests were conducted at a speed of 1000 kJ / s, and the YS and TS were measured. In the present invention, a yield ratio (YR) of 55% or more was determined to be high strength. The YR was calculated using the formula (1) above.

[0118] [Hole Expanding Test] The hole expanding test was carried out in accordance with JIS Z 2256. 24 hours after production, the steel plate was sheared to 100 mm x 100 mm, and then a hole with a diameter of 10 mm was punched into the sheared steel plate with a clearance of 12.5%. Next, the steel plate was held down using a die with an inner diameter of 75 mm with a blank holding force of 9 ton (88.26 kN), and in this state, a conical punch with an apex angle of 60° was pressed into the hole to measure the hole diameter at the crack initiation limit. Then, the limit hole expanding ratio: λ (%) was calculated using the following formula. Limit hole expanding ratio: λ (%) = {(D ｆ -D ０ ) / D ０}×100 where D ｆ is the hole diameter (mm) when the crack occurs, D ０ is the initial hole diameter (mm). When the limiting hole expanding ratio: λ is 30% or more, it was determined that the stretch flangeability immediately after production was excellent.

[0119] [Bending Test] The bending test was conducted in accordance with JIS Z 2248:2022. A rectangular test piece with a width of 30 mm and a length of 100 mm was taken from the steel plate 24 hours after production, so that the axial direction of the bending test was parallel to the rolling direction of the steel plate. A 90° V-bending test was then conducted under conditions of an indentation load of 100 kN and a pressing hold time of 5 seconds. In the present disclosure, bending tests were conducted on five samples at an R where the value R / t, obtained by dividing the bending radius (R) by the plate thickness (t), was approximately 4.5, i.e., 4.3 to 4.7. Next, the crack length at the ridgeline of the bending apex of all five samples was evaluated, and samples with a crack length of 200 μm or less were determined to have excellent bendability immediately after production. Here, the crack length was evaluated by measuring the ridge line at the apex of the bend using a digital microscope (RH-2000: manufactured by Hirox Co., Ltd.) at a magnification of 40 to 160 times.

[0120] [Trim Edge Quality] The quality of the trim edge was measured by trimming the coil edge on the CGL output side so that the actual clearance was 15-20%, and then cutting the trimmed edge perpendicular to the observation surface to include the trim edge. The observation surface was then mirror-polished using diamond paste. The trim edge was observed at a magnification of 30x using an SEM at an acceleration voltage of 15 kV, with the trim edge as the observation position. The trim edge was determined to have excellent quality if no cracks were observed in the obtained image. Here, "cracks" refers to cracks with a length of 100 μm or more.

[0121] According to the above-mentioned method, the area ratio of martensite and ferrite, the volume ratio of retained austenite, the amount of low-temperature diffusible hydrogen in the base steel sheet 24 hours after production, the number density of cracks penetrating the coating layer, and the full width at half maximum of the δ1 phase in the coating layer were determined. Furthermore, the remaining structure was observed using the method described below. A sample was cut out so that the observation surface was a thickness cross section (L cross section) parallel to the rolling direction of the steel sheet, and the observation surface was mirror-polished using diamond paste. The observation surface was then etched with 3 vol.% nital to reveal the structure. Using an SEM, under conditions of an acceleration voltage of 15 kV, the observation position was set at ¼ of the steel sheet thickness, and observation was performed at 5000x magnification in three fields of view with a field of view of 17 μm × 23 μm. Carbides were identified as the remaining structure from the obtained structural images.

[0122] As shown in Table 3, the examples of the present invention are excellent in YR, λ, bendability, and trim edge quality, while the comparative examples are inferior in one or more of YR, λ, bendability, and trim edge quality.

[0123]

[0124] Although the embodiments of the present invention have been described above, the present invention is not limited to the descriptions of the present embodiments, which form part of the disclosure of the present invention. In other words, other embodiments, examples, and operational techniques made by those skilled in the art based on the present embodiments are all included in the scope of the present invention. For example, in the series of heat treatments in the above-described manufacturing method, there are no particular limitations on the equipment used to perform the heat treatment on the steel sheet as long as the thermal history conditions are met.

[0125] According to the present invention, it is possible to obtain a hot-dip galvanized steel sheet having excellent stretch flangeability, bendability and trim edge quality, as well as high part strength.

[0126] In particular, the hot-dip galvanized steel sheet of the present invention has excellent trim edge quality, and therefore can be applied to automotive frame structural parts of various sizes and shapes while achieving high part strength, thereby enabling improvement in fuel efficiency through reduction in vehicle body weight and making the steel sheet extremely valuable in industry.

Claims

1. A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, The base steel plate comprises, in mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less; O: 0.0100% or less; The balance is composed of Fe and unavoidable impurities; At a 1 / 4 position of the plate thickness of the base steel plate, The area ratio of martensite is 30% or more, The area ratio of ferrite is 70% or less, and A microstructure having a volume fraction of retained austenite of 20.0% or less; a low-temperature region diffusible hydrogen amount, which is the amount of hydrogen released from the base steel sheet when the base steel sheet is heated from room temperature to 50° C. 24 hours after the production of the hot-dip galvanized steel sheet, is 0.015 mass ppm or less; In the hot-dip galvanized layer, The density of cracks penetrating the hot-dip galvanized layer is 30 cracks / mm or more; The hot-dip galvanized steel sheet, wherein the full width at half maximum of the δ1 phase of the hot-dip galvanized layer is 0.100 degrees or more.

2. The 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, Co: 0.010% or less, Ni: 1.00% 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, Bi: 0.200% or less, The hot-dip galvanized steel sheet according to claim 1, further comprising at least one element selected from the group consisting of:

3. The hot-dip galvanized steel sheet according to claim 1 , wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

4. The hot-dip galvanized steel sheet as described in claim 2, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

5. A member made using the hot-dip galvanized steel sheet according to any one of claims 1 to 4.

6. A frame structural part of an automobile or a reinforcing part of an automobile, comprising the member according to claim 5.

7. A steel slab having the composition according to claim 1, Hot rolling is performed to obtain hot rolled steel sheet, Next, the hot-rolled steel sheet is subjected to pickling to obtain a pickled sheet, Next, the pickled steel sheet is cold-rolled at a cumulative rolling reduction of 20% to 75% to obtain a cold-rolled steel sheet; Next, the cold-rolled steel sheet is annealed under the condition of a heating temperature of 780° C. or more, Next, the cold rolled steel sheet is subjected to a hot-dip galvanizing treatment to obtain a plated steel sheet, Next, the plated steel sheet is cooled under the condition that the average cooling rate in the temperature range of 250° C. or more and 400° C. or less is 1.0° C. / s or more, The method for producing a hot-dip galvanized steel sheet further comprises processing the plated steel sheet so as to impart a uniaxial tensile strain of 0.1% or more to a surface layer of the plated steel sheet.

8. A steel slab having the composition according to claim 2, Hot rolling is performed to obtain hot rolled steel sheet, Next, the hot-rolled steel sheet is subjected to pickling to obtain a pickled sheet, Next, the pickled steel sheet is cold-rolled at a cumulative rolling reduction of 20% to 75% to obtain a cold-rolled steel sheet; Next, the cold-rolled steel sheet is annealed under the condition of a heating temperature of 780° C. or more, Next, the cold rolled steel sheet is subjected to a hot-dip galvanizing treatment to obtain a plated steel sheet, Next, the plated steel sheet is cooled under the condition that the average cooling rate in the temperature range of 250° C. or more and 400° C. or less is 1.0° C. / s or more, The method for producing a hot-dip galvanized steel sheet further comprises processing the plated steel sheet so as to impart a uniaxial tensile strain of 0.1% or more to a surface layer of the plated steel sheet.

9. The method for producing a hot-dip galvanized steel sheet according to claim 7, wherein during cooling of the plated steel sheet, the plated steel sheet is kept at a temperature range of 100°C or more and 450°C or less for 5s or more, then cooled, and then subjected to the processing.

10. A method for manufacturing a hot-dip galvanized steel sheet as described in claim 8, wherein during cooling of the plated steel sheet, the plated steel sheet is kept at a temperature range of 100°C or higher and 450°C or lower for 5s or more, then cooled, and then the processing is performed.

11. 8. The method for producing a hot-dip galvanized steel sheet according to claim 7, wherein, during cooling of the galvanized steel sheet, cooling is stopped at a cooling stop point of 250°C or less, and then the galvanized steel sheet is reheated to a temperature range of (cooling stop temperature + 50°C) or more and 450°C or less, and the heat is maintained in this temperature range for 5 seconds or more, and then cooled, and then the processing is performed.

12. A method for manufacturing a hot-dip galvanized steel sheet as described in claim 8, wherein during cooling of the plated steel sheet, cooling is stopped at a cooling stop point below 250°C, and then the plated steel sheet is reheated to a temperature range of (cooling stop temperature + 50°C) or higher and 450°C or lower, maintained at this temperature range for 5 s or more, then cooled, and then subjected to the processing.

13. 8. The method for producing a hot-dip galvanized steel sheet according to claim 7, wherein during cooling of the galvanized steel sheet, cooling is stopped at a cooling stop point of 250°C or less, the galvanized steel sheet is then subjected to the processing, and then reheated to a temperature range of (cooling stop temperature + 50°C) or more and 450°C or less, the heat is maintained in this temperature range for 5 seconds or more, and then cooled.

14. A method for manufacturing a hot-dip galvanized steel sheet as described in claim 8, wherein, during cooling of the plated steel sheet, cooling is stopped at a cooling stop point below 250°C, the processing is then performed, the plated steel sheet is then reheated to a temperature range of (cooling stop temperature + 50°C) or higher and 450°C or lower, the heat is maintained in this temperature range for 5 s or more, and then the plated steel sheet is cooled.

15. The method for producing a hot-dip galvanized steel sheet according to any one of claims 7 to 14, further comprising subjecting the steel sheet after the hot-dip galvanizing treatment to an alloying treatment.

16. The method for producing a hot-dip galvanized steel sheet according to any one of claims 7 to 14, wherein, when cold rolling is performed, the sheet threading speed in a final pass is 50 mpm or more.

17. A method for manufacturing a hot-dip galvanized steel sheet as described in claim 15, wherein when cold rolling is performed, the sheet passing speed in the final pass is 50 mpm or more.

18. A method for producing a component, comprising a step of subjecting the hot-dip galvanized steel sheet according to any one of claims 1 to 4 to at least one of forming and joining to form a component.