Galvanized steel sheet and member, and method for producing same

Abstract

Provided is a galvanized steel sheet having both high yield strength and excellent durability of a spot weld zone. The base steel sheet has a predetermined component composition, wherein the total area ratio of tempered martensite and lath-like bainite is 45% to 80%, the area ratio of massive bainite is 1% to 15%, LMn / HMn is 0.20 or more, Lv / Hv is 0.2 to 2.5, and the amount of diffusible hydrogen in the base steel sheet is 0.60 mass ppm or less.

Description

Galvanized steel sheets and components, and methods for manufacturing them.

[0001] This invention relates to galvanized steel sheets and components, as well as methods for manufacturing them.

[0002] From the perspective of preserving the global environment, carbon dioxide (CO2) ２ A reduction in emissions from ) is desired. For this reason, the automotive industry, for example, desires to improve the fuel efficiency of automobiles.

[0003] Reducing the weight of a vehicle is an effective way to improve its fuel efficiency. However, since a certain level of strength is required for the vehicle body, it is necessary to reduce the weight while maintaining that required strength. For example, if the steel plates used as vehicle components are made stronger and the structure of the vehicle body is simplified to reduce the number of components, the weight of the vehicle body can be reduced.

[0004] On the other hand, to increase the strength of steel plates, a large amount of alloying elements are usually added to the steel plates. When such steel plates are welded together, for example, in spot welds obtained by spot welding, the toughness of the heat-affected zone around the nugget (molten and solidified area) decreases, which can reduce the joint strength. Spot welding is often used in the manufacture of car bodies.

[0005] In this regard, for example, Patent Documents 1 to 3 disclose technologies relating to steel plates that are high in strength and have excellent joint strength at spot welds.

[0006] Patent No. 7151948, Patent No. 4577100, Patent No. 7120461

[0007] Incidentally, in recent years, there has been a demand for further improvement in the crash performance of automobiles from the perspective of ensuring occupant safety. As will be discussed later, improving the crash performance of automobiles is effectively achieved by simultaneously improving strength, especially yield strength, and the durability of spot welds.

[0008] However, the technologies described in Patent Documents 1 to 3 do not take into consideration the durability of the spot welds. Therefore, there is a current demand for the development of galvanized steel sheets that combine high yield strength with excellent spot weld durability.

[0009] The present invention was developed in view of the above-mentioned circumstances, and aims to provide a galvanized steel sheet that combines high yield strength and excellent durability of spot welds, along with an advantageous manufacturing method thereof. The present invention also aims to provide a component made from the above-mentioned galvanized steel sheet and a method for manufacturing the same. In this disclosure, any numerical range expressed using "~" means a range that includes the numerical values ​​written before and after "~" as the lower limit and upper limit, respectively.

[0010] The inventors, after careful consideration, have found that the intended objective can be achieved by simultaneously satisfying the following configurations. In other words, the essential configuration of the present invention is as follows.

[0011] 1. A galvanized steel sheet having a base steel sheet and a zinc plating layer on the surface of the base steel sheet, wherein the base steel sheet has a composition in mass%, C: 0.150% to 0.450%, Si: 0.50% to 3.00%, Mn: 1.50% to 4.00%, 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 remainder being Fe and unavoidable impurities, and the total area ratio of tempered martensite and lath-like bainite from the 1 / 8 thickness position to the 3 / 8 thickness position of the base steel sheet is 45% to 80%, the area ratio of massive bainite is 1% to 15%, and LMn / HMn is 0.20 or more. A galvanized steel sheet having a steel structure in which LMn and HMn are the area percentage (%) of the region where the Mn concentration is 3.0 mass% or less and the area percentage (%) of the region where the Mn concentration is greater than 4.0 mass%, respectively, with Lv / Hv: 0.2 to 2.5, where Lv and Hv are the points in the nanohardness distribution at the 1 / 4 thickness position of the base steel sheet where the nanohardness is 3.0 GPa or less and the nanohardness is 4.5 GPa or more, respectively, and the diffusible hydrogen content of the base steel sheet is 0.60 mass ppm or less.

[0012] 2. The composition of the base steel sheet is further, in mass%, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less, Mo: 1.00% or less, Cr: 1.00% or less, Sb: 0.200% or less, Sn: 0.200% or less, Zr: 0.1000% or less, Te: 0.100% or less, Cu: 1.000% or less, Ni: 1.000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Co: 0.500% or less, Ta: 0.10% or less, Hf: 0.10% or less. The zinc-plated steel sheet according to claim 1, containing at least one selected from Bi: 0.200% or less, As: 0.100% or less, Pb: 0.100% or less, and Zn: 0.100% or less.

[0013] 3. The galvanized steel sheet according to 1 or 2, wherein the structural composition of the base steel sheet has a retained austenite area ratio of 5% to 30% and prior austenite grain size of 3 μm to 12 μm, and the n value at 2% tensile strain is 0.15 or more.

[0014] 4. The galvanized steel sheet according to any one of 1 to 3, wherein the base steel sheet has a soft surface layer, the soft surface layer is a region from the surface of the base steel sheet to a position 1 / 4 of the thickness of the base steel sheet, where the hardness is 90% or less of the hardness at the 1 / 4 position of the thickness of the base steel sheet, and the thickness of one side of the soft surface layer is 10 μm to 150 μm.

[0015] 5. A component made using a galvanized steel sheet as described in any of items 1 to 4 above.

[0016] 6. A method for producing a galvanized steel sheet according to any one of items 1 to 4 above, the method comprising: a hot rolling step of hot rolling a steel slab having the component composition described in item 1 or 2 above to obtain a hot-rolled steel sheet; a cold rolling step of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; a primary heat treatment step of heating and cooling the cold-rolled steel sheet; a zinc plating step of zinc plating the cold-rolled steel sheet to obtain a galvanized steel sheet; and a secondary heat treatment step of cooling and reheating the galvanized steel sheet, wherein in the primary heat treatment step, the intermediate heating temperature T1: 800°C to Ac ３ Point (℃), Maximum heating temperature T2: Ac ３ A method for manufacturing a galvanized steel sheet, wherein the temperature range is from 1°C to 950°C, the average heating rate V1 from T1 to T2 is 6°C / sec or more, the heat-affected index E defined by the following equation (1) is 150 to 1500, the primary cooling stop temperature T3 is (Ms point + 50)°C to 650°C and the average cooling rate V2 from T2 to T3 is 4°C / sec or more, and in the secondary heat treatment step, the secondary cooling stop temperature T4 is 50°C to 300°C, the reheating temperature T5 is greater than 300°C and less than or equal to 450°C, and the holding time t2 in the temperature range of greater than 300°C and less than or equal to 450°C during reheating is 1 sec to 150 sec. E = T2 × (4 × log(T2 - T1) + 10) / V1 ... (1)

[0017] 7. The method for manufacturing a galvanized steel sheet according to 6, wherein the final reduction ratio in the finish rolling of the hot rolling is 25% to 60%.

[0018] 8. A method for manufacturing a galvanized steel sheet according to 6 or 7, wherein the dew point in the primary heat treatment step is -35°C or higher.

[0019] 9. A method for manufacturing a component, comprising the step of forming or joining a galvanized steel sheet according to any one of items 1 to 4 above to form a component.

[0020] According to the present invention, a galvanized steel sheet is obtained that combines high yield strength and excellent durability of spot welds. The galvanized steel sheet of the present invention can be used particularly suitably as a material for structural members of automobiles, which are important components for improving the collision performance of automobiles, for example, CO ２ It is also extremely advantageous from the standpoint of reducing emissions. Furthermore, the galvanized steel sheet of the present invention can be suitably used as a component of transport equipment other than automobiles, for example.

[0021] The present invention will be described based on the following embodiments. First, the component composition of the base steel sheet for a galvanized steel sheet according to one embodiment of the present invention will be described. All units in the component composition are "mass%", and hereafter, unless otherwise specified, will be simply referred to as "%".

[0022] C: 0.150% to 0.450% C generates tempered martensite and lath-like bainite, increasing the yield strength. If the amount of C is too low, the total area ratio of tempered martensite and lath-like bainite decreases, and the yield strength decreases. For this reason, the amount of C is 0.150% or more, preferably 0.180% or more, and more preferably 0.200% or more. On the other hand, if the amount of C is too high, the hardness increases excessively. As a result, Lv / Hv falls below the appropriate range, and the durability of the spot weld decreases. For this reason, the amount of C is 0.450% or less, preferably 0.430% or less, and more preferably 0.400% or less.

[0023] Si: 0.50% to 3.00% Si increases yield strength through solid solution strengthening. For this reason, the Si content is 0.50% or more, preferably 0.70% or more, and more preferably 0.80% or more. On the other hand, if the Si content is too high, the toughness of the spot weld decreases, and the durability of the spot weld also decreases. For this reason, the Si content is 3.00% or less, preferably 2.60% or less, and more preferably 2.40% or less.

[0024] Mn: 1.50% to 4.00% Mn increases yield strength through solid solution strengthening. For this reason, the amount of Mn is 1.50% or more, preferably 1.80% or more, and more preferably 2.00% or more. On the other hand, if the amount of Mn is excessive, the hardness increases excessively. As a result, Lv / Hv falls below the appropriate range, and the durability of the spot weld decreases. For this reason, the amount of Mn is 4.00% or less, preferably 3.80% or less, and more preferably 3.50% or less.

[0025] P: 0.100% or less. P segregates at grain boundaries. This reduces the toughness of the spot weld and also reduces the durability of the spot weld. For this reason, the amount of P is 0.100% or less, preferably 0.030% or less, and more preferably 0.010% or less. The lower limit of the amount of P is not particularly limited and may be 0%. Also, P is a solid solution strengthening element and increases the yield strength. For this reason, the amount of P is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

[0026] S: 0.0200% or less. S combines with Mn to form coarse MnS. This reduces the toughness of the spot weld and also reduces the durability of the spot weld. For this reason, the amount of S is 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0020% or less. The lower limit of the amount of S is not particularly limited and may be 0%. Also, due to production technology constraints, the amount of S is preferably 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0003% or more.

[0027] Al: 0.100% or less. Al acts as a deoxidizing agent. However, if the amount of Al is excessive, oxides and nitrides will aggregate and become coarser. This reduces the toughness of the spot weld and also reduces the durability of the spot weld. For this reason, the amount of Al is 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less. The lower limit of the amount of Al is not particularly limited and may be 0%. Furthermore, from the viewpoint of obtaining the effect of adding Al, the amount of Al is preferably 0.010% or more, and more preferably 0.020% or more.

[0028] N: 0.0100% or less. N combines with Ti and other elements to form nitrides. If the amount of N is excessive, the amount of nitrides formed will also be excessive. This reduces the toughness of the spot weld and the durability of the spot weld. For this reason, the amount of N is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less. The lower limit of the amount of N is not particularly limited and may be 0%. Also, due to production technology constraints, the amount of N is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0029] O: 0.0100% or less. O forms oxides. This reduces the toughness of the spot weld and also reduces the durability of the spot weld. For this reason, the amount of O is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020% or less. The lower limit of the amount of O is not particularly limited and may be 0%. Also, due to production technology constraints, the amount of O is preferably 0.0001% or more, and more preferably 0.0003% or more.

[0030] The basic elements (hereinafter also referred to as basic component elements) that make up the component composition of the base steel sheet for a galvanized steel sheet according to one embodiment of the present invention have been described above. In addition to the basic component elements described above, the component composition of the base steel sheet for a galvanized steel sheet according to one embodiment of the present invention may also include at least one of the following as an optional additive element.

[0031] B: 0.0100% or less. B improves hardenability by segregating at austenite grain boundaries. This increases the area ratio of tempered martensite and lath-like bainite. However, if the amount of B is excessive, Fe ２３ (CB) ６ This forms a decrease in the toughness of the spot weld and the durability of the spot weld. For this reason, when B is included, the amount of B is preferably 0.0100% or less, more preferably 0.0050% or less, even more preferably 0.0040% or less, and even more preferably 0.0030% or less. The lower limit of the amount of B is not particularly limited. Also, from the viewpoint of obtaining the effect of adding B, the amount of B is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0032] Ti: 0.200% or less. Ti increases the strength by forming fine carbides, nitrides or carbonitrides during hot rolling and heat treatment. However, when the amount of Ti is excessive, it combines with N to form coarse nitrides. As a result, the toughness of the spot weld decreases, and the durability of the spot weld also decreases. Therefore, when Ti is contained, the amount of Ti is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The lower limit of the amount of Ti is not particularly limited. Also, from the viewpoint of obtaining the addition effect of Ti, the amount of Ti is preferably 0.005% or more, more preferably 0.010% or more.

[0033] Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less. Nb, V and W increase the strength by forming fine carbides, nitrides or carbonitrides during hot rolling and heat treatment. However, when the amounts of these elements are excessive, coarse carbides are formed and remain undissolved even during heating of the steel slab. Such coarse carbides reduce the toughness of the spot weld and also reduce the durability of the spot weld. Therefore, when Nb is contained, the amount of Nb is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The lower limit of the amount of Nb is not particularly limited. Also, from the viewpoint of obtaining the addition effect of Nb, the amount of Nb is preferably 0.005% or more, more preferably 0.010% or more. When V is contained, the amount of V is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The lower limit of the amount of V is not particularly limited. Also, from the viewpoint of obtaining the addition effect of V, the amount of V is preferably 0.005% or more, more preferably 0.010% or more. When W is contained, the amount of W is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less. The lower limit of the amount of W is not particularly limited. Also, from the viewpoint of obtaining the addition effect of W, the amount of W is preferably 0.010% or more, more preferably 0.020% or more.

[0034] Mo: 1.00% or less, Cr: 1.00% or less Mo and Cr increase yield strength by improving hardenability. However, if the amount of these elements is excessive, hard martensite will be formed. This reduces the toughness of the spot weld and also reduces the durability of the spot weld. For this reason, when Mo is included, the amount of Mo is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less. There is no particular lower limit to the amount of Mo. Also, from the viewpoint of obtaining the effect of adding Mo, the amount of Mo is preferably 0.01% or more, and more preferably 0.02% or more. When Cr is included, the amount of Cr is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less. There is no particular lower limit to the amount of Cr. Also, from the viewpoint of obtaining the effect of adding Cr, the amount of Cr is preferably 0.01% or more, and even more preferably 0.02% or more.

[0035] Sb: 0.200% or less, Sn: 0.200% or less. Sb and Sn increase the yield strength by suppressing an excessive increase in the thickness of the soft surface layer of the base steel sheet. However, if the amount of these elements is excessive, it may lead to an increase in the amount of diffusible hydrogen in the base steel sheet. For this reason, when Sb is included, the amount of Sb is preferably 0.200% or less, more preferably 0.080% or less, and even more preferably 0.040% or less. There is no particular lower limit to the amount of Sb. Also, from the viewpoint of obtaining the effect of adding Sb, the amount of Sb is preferably 0.001% or more, and more preferably 0.002% or more. When Sn is included, the amount of Sn is preferably 0.200% or less, more preferably 0.080% or less, and even more preferably 0.040% or less. There is no particular lower limit to the amount of Sn. Furthermore, from the viewpoint of obtaining the effect of adding Sn, the amount of Sn is preferably 0.001% or more, and more preferably 0.002% or more.

[0036] Zr: 0.1000% or less, Te: 0.100% or less. Zr and Te spheroidize the shape of nitrides and sulfides, thereby improving the toughness of the spot weld. However, when the amounts of these elements are excessive, the amount of coarse precipitates remaining undissolved during the heating of the hot-rolled steel slab increases. As a result, the toughness of the spot weld decreases and the durability of the spot weld also decreases. Therefore, when Zr is contained, the amount of Zr is preferably 0.1000% or less, more preferably 0.0800% or less, and even more preferably 0.0500% or less. The lower limit of the amount of Zr is not particularly limited. Also, from the viewpoint of obtaining the addition effect of Zr, the amount of Zr is preferably 0.0050% or more, more preferably 0.0100% or more. When Te is contained, the amount of Te is preferably 0.100% or less, more preferably 0.080% or less. The lower limit of the amount of Te is not particularly limited. Also, from the viewpoint of obtaining the addition effect of Te, the amount of Te is preferably 0.005% or more, more preferably 0.010% or more.

[0037] Cu: 1.000% or less. Cu increases the yield strength by enhancing hardenability. However, when the amount of Cu is excessive, the inclusions of Cu increase. As a result, the toughness of the spot weld decreases and the durability of the spot weld also decreases. Therefore, when Cu is contained, the amount of Cu is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less. The lower limit of the amount of Cu is not particularly limited. Also, from the viewpoint of obtaining the addition effect of Cu, the amount of Cu is preferably 0.010% or more, more preferably 0.020% or more.

[0038] Ni: 1.000% or less. Ni increases the yield strength by enhancing hardenability. However, when the amount of Ni is excessive, the amount of hard martensite increases. As a result, the toughness of the spot weld decreases and the durability of the spot weld also decreases. Therefore, when Ni is contained, the amount of Ni is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less. The lower limit of the amount of Ni is not particularly limited. Also, from the viewpoint of obtaining the addition effect of Ni, the amount of Ni is preferably 0.010% or more, more preferably 0.020% or more.

[0039] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. Ca, Mg, and REM (Rare Earth Metal) spheroidize the shape of precipitates such as sulfides and oxides. This improves the toughness of the spot weld. However, if the amount of these elements is excessive, the sulfides become coarser. This reduces the toughness of the spot weld and also reduces the durability of the spot weld. For this reason, when Ca is included, the amount of Ca is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0040% or less. There is no particular lower limit to the amount of Ca. Also, from the viewpoint of obtaining the effect of Ca addition, the amount of Ca is preferably 0.0005% or more, and more preferably 0.0010% or more. When Mg is included, the amount of Mg is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0040% or less. There is no particular lower limit to the amount of Mg. Also, from the viewpoint of obtaining the effect of adding Mg, the amount of Mg is preferably 0.0005% or more, and more preferably 0.0010% or more. When REM is included, the amount of REM is preferably 0.0100% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. There is no particular lower limit to the amount of REM. Also, from the viewpoint of obtaining the effect of adding REM, the amount of REM is preferably 0.0005% or more, and even more preferably 0.0010% or more. Here, REM is a collective term for Sc, Y, and elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The amount of REM is the total content of these elements.

[0040] Co: 0.500% or less, Ta: 0.10% or less, Hf: 0.10% or less, Bi: 0.200% or less Co, Ta, Hf, and Bi spheroidize the shape of the precipitates. This improves the toughness of the spot weld. However, if the amount of these elements is excessive, the precipitates become coarser. This reduces the toughness of the spot weld and also reduces the durability of the spot weld. For this reason, when Co is included, the amount of Co is preferably 0.500% or less, more preferably 0.008% or less, and even more preferably 0.007% or less. The lower limit of the amount of Co is not particularly limited. Also, from the viewpoint of obtaining the effect of adding Co, the amount of Co is preferably 0.001% or more, and more preferably 0.002% or more. When Ta is included, the amount of Ta is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.07% or less. The lower limit of the amount of Ta is not particularly limited. Furthermore, from the viewpoint of obtaining the effect of adding Ta, the amount of Ta is preferably 0.01% or more, and more preferably 0.02% or more. When Hf is included, the amount of Hf is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.07% or less. The lower limit of the amount of Hf is not particularly limited. Furthermore, from the viewpoint of obtaining the effect of adding Hf, the amount of Hf is preferably 0.01% or more, and more preferably 0.02% or more. When Bi is included, the amount of Bi is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.080% or less. The lower limit of the amount of Bi is not particularly limited. Furthermore, from the viewpoint of obtaining the effect of adding Bi, the amount of Bi is preferably 0.001% or more, and even more preferably 0.005% or more.

[0041] As: 0.100% or less, Pb: 0.100% or less, Zn: 0.100% or less. As, Pb, and Zn spheroidize the shape of the precipitates. This improves the toughness of the spot weld. However, if the amount of these elements is excessive, the precipitates become coarser. This reduces the toughness of the spot weld and also reduces the durability of the spot weld. For this reason, when As is included, the amount of As is preferably 0.100% or less, more preferably 0.050% or less, and even more preferably 0.010% or less. There is no particular lower limit to the amount of As. Also, from the viewpoint of obtaining the effect of adding As, the amount of As is preferably 0.001% or more, and more preferably 0.002% or more. When Pb is included, the amount of Pb is preferably 0.100% or less, more preferably 0.050% or less, and even more preferably 0.010% or less. There is no particular lower limit to the amount of Pb. Furthermore, from the viewpoint of obtaining the effect of adding Pb, the amount of Pb is preferably 0.001% or more, and more preferably 0.002% or more. When Zn is included, the amount of Zn is preferably 0.100% or less, more preferably 0.050% or less, and even more preferably 0.010% or less. There is no particular lower limit to the amount of Zn. Furthermore, from the viewpoint of obtaining the effect of adding Zn, the amount of Zn is preferably 0.001% or more, and more preferably 0.002% or more.

[0042] The remainder of the mixture, other than the elements listed above, consists of Fe and unavoidable impurities. Note that any of the optional additives may be present at 0%. Furthermore, if the content of any of the optional additives is below the preferred lower limit, that element can be considered to be present as an unavoidable impurity.

[0043] Next, the structural integrity of a base steel sheet for a galvanized steel sheet according to one embodiment of the present invention will be described. Here, the area ratio, LMn, HMn, and prior austenite grain size of each phase measured at the 1 / 4 thickness position of the base steel sheet, or measured with the 1 / 4 thickness position of the base steel sheet as the center in the thickness direction, will be considered as the area ratio, LMn, HMn, and prior austenite grain size of each phase at the 1 / 8 thickness position to the 3 / 8 thickness position of the base steel sheet.

[0044] The total area ratio of one or two types of tempered martensite and lath-like bainite (hereinafter also referred to as the tempered M + lath-like B area ratio): 45% to 80% From the viewpoint of stably ensuring the desired yield strength, the tempered M + lath-like B area ratio is 45% or more, preferably 48% or more, and more preferably 50% or more. On the other hand, if the tempered M + lath-like B area ratio is excessive, the fracture limit deformation of the spot weld and, consequently, the durability of the spot weld will decrease. For this reason, the tempered M + lath-like B area ratio is 80% or less, preferably 78% or less, and more preferably 75% or less. Here, the fracture limit deformation of the spot weld is the amount of deformation that the spot weld can withstand without breaking. Furthermore, the fracture limit deformation of the spot weld corresponds to the crosshead displacement at the maximum load in the cross tensile test and shear test related to the evaluation of the durability of the spot weld described later. The larger the fracture limit deformation of the spot weld, the more uniformly the stress load can be distributed, and the more the durability of the spot weld improves.

[0045] Tempered martensite and lath-like bainite are phases composed of lath-like regions, within which cementite is formed. Since tempered martensite and lath-like bainite have similar properties for achieving the desired effect, it is not necessary to specify the area ratio of each.

[0046] Area ratio of granular bainite (hereinafter also referred to as granular B area ratio): 1% to 15%. Granular bainite is effective in improving the durability of spot welds. That is, granular bainite is flexible and ductile with respect to deformation. For this reason, granular bainite effectively contributes to improving the fracture limit deformation of spot welds and, consequently, the durability of spot welds. Therefore, the granular B area ratio is 1% or more, preferably 3% or more, and more preferably 5% or more. On the other hand, if the granular B area ratio is too large, it leads to a decrease in yield strength. For this reason, the granular B area ratio is 15% or less, preferably 13% or less, and more preferably 11% or less.

[0047] Here, massive bainite is bainite with an aspect ratio of 4 or less. In other words, massive bainite can be said to be composed of massive regions with an aspect ratio of 4 or less. The aspect ratio of a massive region can be expressed as a / b, where a is the major axis (maximum length) of the region and b is the minor axis (maximum length when crossing the region in a direction perpendicular to the direction of the maximum length).

[0048] Area ratio of retained austenite (hereinafter also referred to as retained γ area ratio): 5% to 30%. Retained austenite improves ductility. In addition, martensite that transforms from retained austenite during processing suppresses the propagation of liquid metal embrittlement cracking (hereinafter also referred to as LME cracking). For this reason, a retained γ area ratio of 5% or more is preferable, 6% or more is more preferable, and 8% or more is even preferable. On the other hand, if the retained γ area ratio is too high, the toughness of the spot weld may decrease, and the durability of the spot weld may also decrease. For this reason, a retained γ area ratio of 30% or less is preferable, 25% or less is more preferable, and 20% or less is even preferable.

[0049] The area percentage of the remaining tissue other than tempered martensite, lath-like bainite, massive bainite, and retained austenite is preferably 15% or less, more preferably 14% or less, and even more preferably 13% or less. The area percentage of the remaining tissue may also be 0%. The area percentage of the remaining tissue is calculated as follows: [Area percentage of remaining tissue (%)] = 100 - [Area percentage of tempered M + lath-like B (%)] - [Area percentage of massive B (%)] - [Area percentage of retained γ (%)]

[0050] The type of residual structure is not particularly limited. Examples of residual structures include fresh martensite, pearlite, ferrite, iron-based carbonitrides, alloy carbonitrides, and MnS and Al. ２ O ３ Examples of known tissues include inclusions such as the above.

[0051] The area ratio of tempered M+ lath-like B and the area ratio of agglomerate B in the base steel sheet can be measured using the point counting method with the microstructure image of the base steel sheet obtained by SEM (Scanning Electron Microscope) observation. The area ratio of residual γ in the base steel sheet can be measured using X-ray diffraction.

[0052] LMn / HMn: 0.20 or higher LMn and HMn are the area percentage (%) of the region where the Mn concentration is 3.0 mass% or less and the area percentage (%) of the region where the Mn concentration is greater than 4.0 mass%, respectively. Hereinafter, the region where the Mn concentration is 3.0 mass% or less will also be called the "low Mn region," and the region where the Mn concentration is greater than 4.0 mass% will also be called the "high Mn region." The microstructure that makes up the high Mn region is hard and has the effect of increasing yield strength. The microstructure that makes up the low Mn region is mainly composed of massive bainite and effectively contributes to improving the fracture limit deformation of the spot weld and, consequently, the durability of the spot weld. Therefore, by appropriately controlling LMn / HMn, it is possible to achieve both high yield strength and excellent durability of the spot weld. Accordingly, LMn / HMn is 0.20 or higher, preferably 0.30 or higher, and more preferably 0.40 or higher. On the other hand, if the LMn / HMn ratio becomes too high, it may become difficult to obtain the desired yield strength. Therefore, the LMn / HMn ratio is preferably 1.20 or less, more preferably 1.10 or less, and even more preferably 1.00 or less.

[0053] LMn and HMn can be measured using an electron probe microanalyzer (EPMA).

[0054] Prior austenite grain size (hereinafter also referred to as prior γ grain size): 3 μm to 12 μm. LME cracking propagates along the prior austenite grains. As the prior γ grain size decreases, the propagation of LME cracking is suppressed. Also, dislocations are more easily introduced with smaller prior γ grain sizes, thus increasing the yield strength. For this reason, the prior γ grain size is preferably 12 μm or less, more preferably 11 μm or less, and even more preferably 10 μm or less. Furthermore, the prior γ grain size is preferably 3 μm or more, more preferably 4 μm or more, and even more preferably 5 μm or more.

[0055] The prior γ grain size can be measured using a sectioning method in accordance with JIS G 0551:2020, based on the microstructure image of the underlying steel plate obtained by SEM (Scanning Electron Microscope) observation.

[0056] Next, the Lv / Hv, diffusible hydrogen content, and thickness per side of the surface soft layer of the base steel sheet for the galvanized steel sheet according to one embodiment of the present invention will be described.

[0057] Lv / Hv: 0.2 to 2.5 Lv and Hv are the points in the nanohardness distribution at 1 / 4 of the thickness of the base steel plate where the nanohardness is 3.0 GPa or less and 4.5 GPa or more, respectively. Hereinafter, points where the nanohardness is 3.0 GPa or less are also called "low hardness areas," and points where the nanohardness is 4.5 GPa or more are also called "high hardness areas." The microstructure that makes up the high hardness areas is fine and has the effect of increasing yield strength. In addition, the microstructure of the low hardness areas effectively contributes to improving the fracture limit deformation amount of the spot weld and, consequently, the durability of the spot weld. For this reason, by appropriately controlling Lv / Hv, it is possible to achieve both high yield strength and excellent durability of the spot weld. Here, if Lv / Hv falls below 0.2, the fracture limit deformation amount of the spot weld and, consequently, the durability of the spot weld decreases. Therefore, Lv / Hv is 0.2 or higher, preferably 0.5 or higher, and more preferably 1.0 or higher. On the other hand, if Lv / Hv is too high, it becomes difficult to obtain the desired yield strength. For this reason, Lv / Hv is 2.5 or lower, preferably 2.3 or lower, and more preferably 2.1 or lower.

[0058] Nanohardness measurement (acquisition of nanohardness distribution) can be performed by nanoindentation testing in accordance with ISO 14577-1. Note that other hardness testing methods, such as Vickers hardness testing, cannot obtain plastic deformation resistance in localized areas at the submicron level.

[0059] Diffusible hydrogen content of the base steel plate: 0.60 ppm by mass or less. If the diffusible hydrogen content of the base steel plate is too high, delayed fracture will reduce the joint strength of the spot weld and decrease the durability of the spot weld. For this reason, the diffusible hydrogen content of the base steel plate is 0.60 ppm by mass or less, preferably 0.50 ppm by mass or less, and more preferably 0.40 ppm by mass or less. The lower limit of the diffusible hydrogen content of the base steel plate is not particularly limited and may be 0 ppm by mass.

[0060] The amount of diffusible hydrogen in the base steel plate can be measured by measuring the amount of hydrogen released from the base steel plate using a temperature-controlled desorption analysis method.

[0061] Thickness of the surface soft layer per side (hereinafter also referred to as the thickness of the surface soft layer): 10 μm to 150 μm By forming a surface soft layer near the surface of the base steel plate, the fracture limit deformation is further increased, and the durability of the spot weld is further improved. For this reason, a thickness of 10 μm or more is preferable. However, if the thickness of the surface soft layer exceeds 150 μm, the yield strength may decrease. For this reason, a thickness of 150 μm or less is preferable.

[0062] Here, the soft surface layer is the region from the surface of the base steel plate (which can also be called the interface between the base steel plate and the galvanized layer) to the point where the base steel plate is 1 / 4 of its thickness, where the hardness is 90% or less of the hardness at the 1 / 4 point of the base steel plate's thickness. The hardness at each point on the base steel plate can be measured by a Vickers hardness test in accordance with JIS Z 2244-1:2024. The soft surface layer may be present on only one surface of the base steel plate, or on both surfaces.

[0063] Next, the zinc plating layer of a zinc-plated steel sheet according to one embodiment of the present invention will be described. The zinc plating layer may be provided on only one surface of the base steel sheet, or on both surfaces. The zinc plating layer refers to a plating layer whose main component is Zn (Zn content of 50.0% by mass or more). Examples of zinc plating layers include hot-dip galvanized layers and alloyed hot-dip galvanized layers. When the zinc plating layer is a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, the zinc-plated steel sheet can also be called a hot-dip galvanized steel sheet (GI) and an alloyed hot-dip galvanized steel sheet (GA), respectively.

[0064] Next, the target properties and preferred properties of a galvanized steel sheet according to one embodiment of the present invention will be described.

[0065] Yield strength (hereinafter also referred to as YS): 900 MPa or more. The YS of a galvanized steel sheet according to one embodiment of the present invention is 900 MPa or more. The upper limit of the YS of a galvanized steel sheet according to one embodiment of the present invention is not particularly limited. For example, the YS of a galvanized steel sheet according to one embodiment of the present invention is preferably 1300 MPa or less.

[0066] Here, the measurement of YS should be performed in accordance with JIS Z 2241:2022.

[0067] n-value (work hardening index, hereinafter also simply referred to as n-value) at 2% tensile strain: 0.15 or higher The n-value of a galvanized steel sheet according to one embodiment of the present invention is preferably 0.15 or higher, more preferably 0.20 or higher, and even more preferably 0.25 or higher. A higher n-value is advantageous for improving LME crack resistance. There is no particular upper limit to the n-value of a galvanized steel sheet according to one embodiment of the present invention. For example, the n-value of a galvanized steel sheet according to one embodiment of the present invention is preferably 0.45 or lower.

[0068] Here, the measurement of the n value should be carried out in accordance with JIS Z 2253:2020.

[0069] Excellent spot weld durability means that a spot welded joint made of galvanized steel sheet satisfies both of the following conditions (A) and (B) simultaneously: (A) The product of the maximum load and the crosshead displacement at the maximum load, as measured by a cross tensile test in accordance with JIS Z 3137:1999: 10 kN・mm or more, preferably 30 kN・mm or more. (B) The product of the maximum load and the crosshead displacement at the maximum load, as measured by a shear test in accordance with JIS Z 3136:1999: 10 kN・mm or more, preferably 20 kN・mm or more.

[0070] Furthermore, a galvanized steel sheet according to one embodiment of the present invention preferably has excellent LME crack resistance. Excellent LME crack resistance means that, in a limit strain evaluation test for a spot welded joint obtained using a galvanized steel sheet as the joined material, the limit strain amount at a strain application temperature of 800°C (the upper limit of strain at which LME cracking does not occur) is 0.4% or more. Preferably, the limit strain amount at a strain application temperature of 800°C is 0.8% or more, and the n value is 0.15 or more.

[0071] Here, the limit strain evaluation test should be carried out in accordance with Japanese Patent No. 7304476.

[0072] More specific procedures for each of the measurements described above are as shown in the examples described later. Furthermore, since the steel structure of the base steel sheet is usually roughly symmetrical in the thickness direction, each measurement related to the steel structure of the base steel sheet, Lv / Hv, and the thickness of the soft surface layer can be performed using any one surface of the base steel sheet (front and back) as the reference (any one surface of the base steel sheet is designated as the zero thickness position).

[0073] Furthermore, the thickness of the galvanized steel sheet according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more and 3.0 mm or less.

[0074] [2] Member Next, a member according to an embodiment of the present invention will be described. A member according to an embodiment of the present invention is a member (used as a material) made of the above galvanized steel sheet. For example, at least one of forming and joining is performed on the galvanized steel sheet as a material to form a member. Here, the above galvanized steel sheet has both high yield strength and excellent durability of the spot weld portion. Therefore, a member according to an embodiment of the present invention is particularly suitable for application to members used in the automotive field.

[0075] [3] Manufacturing method of galvanized steel sheet Next, a manufacturing method of a galvanized steel sheet according to an embodiment of the present invention will be described. In the manufacturing method of a galvanized steel sheet according to an embodiment of the present invention, a hot rolling process, a cold rolling process, a primary heat treatment process, a galvanizing process, and a secondary heat treatment process are performed in this order. Hereinafter, each process will be described. In addition, each temperature mentioned here means the surface temperature of the steel slab and the steel sheet unless otherwise specified.

[0076] [Hot rolling process] In the hot rolling process, hot rolling is performed on the steel slab to obtain a hot rolled steel sheet. In the hot rolling process, for example, rough rolling and finish rolling are performed in this order. In finish rolling, it is preferable to satisfy the following conditions.

[0077] Final reduction ratio in finish rolling (reduction ratio of the final pass in finish rolling): 25% to 60% When the final reduction ratio in finish rolling is less than 25%, recrystallization may not proceed sufficiently and a suitable prior γ grain size may not be obtained. There is also a risk of reducing the n value. That is, from the viewpoint of obtaining excellent LME crack resistance, the reduction ratio of the final pass is preferably 25% or more, more preferably 30% or more, and even more preferably 35% or more. On the other hand, when the final reduction ratio in finish rolling exceeds 60%, it causes an increase in equipment load. Therefore, the final reduction ratio in finish rolling is preferably 60% or less, more preferably 55% or less, and even more preferably 50% or less. The final reduction ratio of finish rolling can be obtained by the following formula. [Final reduction ratio of finish rolling (%)] = (1 - x / x ０ ) × 100 In the formula, x is the thickness (mm) of the rolled material on the exit side of the final pass of finish rolling, and x ０This is the thickness (mm) of the rolled material at the entry side of the final pass during finish rolling.

[0078] Other than the conditions mentioned above, there are no specific limitations; you may follow the usual law.

[0079] For example, a steel slab having the above-mentioned component composition is prepared as follows. First, molten steel is produced. The production method is not particularly limited, and known production methods such as converter production and electric furnace production can be used. Next, a steel slab is obtained from the molten steel. The method for obtaining a steel slab from molten steel is not particularly limited. For example, continuous casting, ingot-part rolling, or thin slab casting can be used. Among these, continuous casting is preferred.

[0080] Next, for example, the obtained steel slab is cooled to room temperature, reheated in a heating furnace, and then hot-rolled. Direct rolling may also be applied to the obtained steel slab. Direct rolling is a method in which the obtained steel slab is charged into a heating furnace while still hot, without being cooled to room temperature, and then hot-rolled. Furthermore, from the viewpoint of dissolving carbides in the steel slab and reducing the rolling load, the slab heating temperature is preferably 1100°C or higher, and more preferably 1150°C or higher. On the other hand, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower, and more preferably 1280°C or lower. The finish rolling completion temperature is preferably 700°C or higher, and more preferably 800°C or higher. Furthermore, the finish rolling completion temperature is preferably 1100°C or lower, and more preferably 1000°C or lower. The coiling temperature is preferably 450°C or higher, and more preferably 470°C or higher. Furthermore, the winding temperature is preferably 600°C or lower, and more preferably 580°C or lower.

[0081] [Cold Rolling Process] In the cold rolling process, a hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The conditions for the cold rolling process are not particularly limited and can be carried out according to conventional methods. For example, the rolling ratio of the cold rolling process is preferably 30% or more, and more preferably 35% or more. Furthermore, the rolling ratio of the cold rolling process is preferably 70% or less, and more preferably 65% ​​or less.

[0082] [Primary Heat Treatment Process] In the primary heat treatment process, the cold-rolled steel sheet is heated and cooled under the following conditions.

[0083] Intermediate heating temperature T1: 800℃~Ac３ Point (°C) In the primary heat treatment process, the cold-rolled steel sheet is first heated to T1. As T1 decreases, the area ratio of tempered M + lath-like B decreases. This reduces the yield strength. For this reason, T1 is 800°C or higher, preferably 810°C or higher, and more preferably 820°C or higher. On the other hand, if T1 is Ac ３ Beyond this point, the austenite single-phase region begins. Therefore, LMn / HMn becomes less than 0.20, and excellent spot weld durability cannot be obtained. Furthermore, the increase in hydrogen partial pressure increases the amount of hydrogen penetrating the cold-rolled steel sheet (base steel sheet), increasing the diffusible hydrogen content of the base steel sheet. Therefore, T1 is... ３ It is below [amount].

[0084] Ac ３ The point (in °C) can be calculated using the following formula: Ac ３ Point=910-203×[%C] １／２ -15.2 × [%Ni] + 44.7 × [%Si] + 104 × [%V] + 31.5 × [%Mo] + 13.1 × [%W] In the formula, [% element] represents the content of each element in the above component composition (unit: mass%). Note that if element M (M: C, Ni, Si, V, Mo, and W) is not included, the [% element] for that element M is calculated as 0.

[0085] Furthermore, the holding time t1 at T1 is not particularly limited. For example, t1 is preferably 10s or more, more preferably 50s or more, even more preferably 80s or more, and even more preferably 120s or more. For example, t1 is preferably 500s or less, more preferably 300s or less, and even more preferably 200s or less.

[0086] Maximum heating temperature T2: Ac ３ 0°C to 950°C After heating the cold-rolled steel sheet to T1, the cold-rolled steel sheet is further heated to T2. T2 is Ac ３If the temperature falls below 120°C, the cold-rolled steel sheet will remain in the ferrite-austenite two-phase region for a long time. As a result, the steel structure of the final base steel sheet will contain a large amount of ferrite, and the area ratio of tempered M + lath-like B will decrease. This will reduce the yield strength. On the other hand, if T2 exceeds 950°C, the amount of hydrogen penetrating the cold-rolled steel sheet (base steel sheet) will increase due to the increase in hydrogen partial pressure, and the amount of diffusible hydrogen in the base steel sheet will increase. For this reason, T2 should be 950°C or lower, preferably 930°C or lower, and more preferably 910°C or lower. Furthermore, T1 < T2 and T1 + 120°C < T2 are preferred.

[0087] The average heating rate V1 between T1 and T2 is 6°C / sec or higher. If V1 is too slow, the cold-rolled steel sheet will remain in the ferrite and austenite two-phase region for a long time. As a result, the steel structure of the base steel sheet of the final product will contain a large amount of ferrite, and the area ratio of tempered M + lath-like B will decrease. This will reduce the yield strength. Therefore, V1 is 6°C / sec or higher, preferably 10°C / sec or higher, more preferably 15°C / sec or higher, even more preferably 20°C / sec or higher, and even more preferably 30°C / sec or higher. The upper limit of V1 is not particularly limited. For example, V1 is preferably 200°C / sec or lower, more preferably 150°C / sec or lower, and even more preferably 100°C / sec or lower.

[0088] Here, V1 can be calculated using the following formula: V1 (°C / sec) = (T2 (°C) - T1 (°C)) ÷ [Time (sec) from the end of holding at T1 to the arrival of T2]

[0089] The heat-affected material index E: 150 to 1500 E is defined by the following equation (1). The amount of heat introduced in the temperature range from T1 to T2, in other words, E, can control the LMn / HMn ratio. For this reason, E is 1500 or less, preferably 1200 or less, and more preferably 1000 or less. However, if E is less than 150, there will be insufficient austenite in the structure of the cold-rolled steel sheet during heating, and the area ratio of tempered M + lath-like B in the structure of the base steel sheet of the final product will decrease. For this reason, E is 150 or more, preferably 180 or more, and more preferably 200 or more. E = T2 × (4 × log(T2 - T1) + 10) / V1 ... (1) In the equation, log represents the common logarithm.

[0090] Primary cooling stop temperature T3: (Ms point + 50)°C to 650°C. T3 is the cooling stop temperature in the cooling process after reaching T2. In order to obtain the desired steel structure in the base steel sheet of the final product, T3 is set to (Ms point + 50)°C to 650°C.

[0091] The Ms point (unit: °C) can be calculated using the following formula: Ms point = 550 - 350 × [%C] - 40 × [%Mn] - 35 × [%V] - 20 × [%Cr] - 17 × [%Ni] - 10 × [%Cu] - 10 × [%Mo] - 5 × [%W] + 15 × [%Co] + 30 × [%Al] In the formula, [% element] is the content of each element in the above component composition (unit: mass%). Note that if element M (M: C, Mn, V, Cr, Ni, Cu, Mo, W, Co, and Al) is not included, the [% element] for that element M is calculated as 0.

[0092] Average cooling rate V2 between T2 and T3: 4°C / sec or higher. If V2 is too slow during cooling in the temperature range of T2 to T3, ferrite transformation occurs during cooling, and the area ratio of tempered M + lath-like B decreases. For this reason, V2 is 4°C / sec or higher, preferably 5°C / sec or higher, and more preferably 6°C / sec or higher. The upper limit of V2 is not particularly limited. For example, V2 is preferably 30°C / sec or lower, more preferably 25°C / sec or lower, and even more preferably 20°C / sec or lower.

[0093] Here, V2 can be calculated using the following formula: V2 (°C / sec) = (T2 (°C) - T3 (°C)) ÷ [Time (sec) from the point of reaching T2 to the point of reaching T3]

[0094] Dew point of the atmosphere: -35°C or higher. From the viewpoint of forming a soft surface layer of a predetermined thickness, the dew point of the atmosphere is preferably -35°C or higher, more preferably -20°C or higher, and even more preferably -10°C or higher. There is no particular upper limit to the dew point of the atmosphere. From the viewpoint of obtaining the desired yield strength, the dew point of the atmosphere is preferably, for example, 25°C or lower, and more preferably 20°C or lower.

[0095] Other than the conditions mentioned above, there are no specific limitations; you may follow the usual law.

[0096] [Zinc Plating Process] In the zinc plating process, cold-rolled steel sheets are subjected to zinc plating to obtain zinc-plated steel sheets. Examples of zinc plating processes include hot-dip galvanizing and alloyed hot-dip galvanizing. The processing conditions should follow conventional methods.

[0097] For example, in the case of hot-dip galvanizing, it is preferable to immerse the cold-rolled steel sheet in a zinc plating bath at 440°C to 500°C, and then adjust the amount of plating by gas wiping or the like. As the zinc plating bath, it is preferable to use a plating bath with a composition in which the Al content is 0.10% by mass or more and 0.23% by mass or less, with the remainder being Zn and unavoidable impurities.

[0098] Furthermore, in the case of alloying hot-dip galvanizing, it is preferable to perform the alloying treatment in a temperature range of 450°C to 600°C after performing the hot-dip galvanizing treatment in the manner described above. If the alloying temperature is below 450°C, the Zn-Fe alloying rate may become excessively slow, making alloying difficult. For this reason, the alloying temperature is preferably 450°C or higher, and more preferably 470°C or higher. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite may transform into pearlite. For this reason, the alloying temperature is preferably 600°C or lower, more preferably 550°C or lower, and even more preferably 530°C or lower.

[0099] Furthermore, the amount of plating deposited is 20 g / m² per side. ２80g / m or more ２ The following is preferable. Note that the amount of plating can be adjusted by gas wiping or the like.

[0100] [Secondary heat treatment process] In the secondary heat treatment process, the galvanized steel sheet is cooled and reheated.

[0101] Secondary cooling stop temperature T4: 50°C to 300°C. T4 is the cooling stop temperature during cooling after zinc plating. If T4 is excessively low, the area ratio of tempered metal (M) + lath-like metal (B) becomes excessive, reducing the durability of the spot weld. For this reason, T4 is 50°C or higher, preferably 100°C or higher, and more preferably 120°C or higher. On the other hand, if T4 is excessively high, the area ratio of tempered metal (M) + lath-like metal (B) becomes insufficient, making it impossible to stably secure the desired yield strength. For this reason, T4 is 300°C or lower, preferably 280°C or lower, and more preferably 260°C or lower.

[0102] Reheating temperature T5: greater than 300°C and less than or equal to 450°C After the above cooling, the galvanized steel sheet is reheated. T5 is the highest temperature reached during this reheating. If T5 is excessively low, the area ratio of the lumpy B becomes too small, and the durability of the spot weld decreases. For this reason, T5 is greater than 300°C, preferably 320°C or higher, and more preferably 330°C or higher. On the other hand, if T5 is excessively high, the area ratio of the tempered M + lath-like B becomes too large, and the durability of the spot weld decreases. For this reason, T5 is 450°C or lower, preferably 440°C or lower, and more preferably 430°C or lower. Note that T4 < T5, and T4 + 220°C < T5 are preferred.

[0103] The residence time t2 in the temperature range between 300°C and 450°C during reheating is 1 sec to 150 sec. If t2 is too short, the area ratio of the lumpy B becomes insufficient, reducing the durability of the spot weld. Therefore, t2 should be 1 sec or longer. On the other hand, if t2 is too long, the area ratio of tempered M + lath-like B becomes excessive, reducing the durability of the spot weld. Therefore, t2 should be 150 sec or less.

[0104] Other than the conditions mentioned above, there are no particular limitations, and conventional methods should be followed. Furthermore, the holding temperature, cooling rate, and heating rate in each of the above processes may or may not be constant.

[0105] Furthermore, the equipment used to perform each of the above processes is not particularly limited.

[0106] According to the method for manufacturing a galvanized steel sheet in one embodiment of the present invention described above, it is possible to manufacture a galvanized steel sheet that has both high yield strength and excellent durability of spot welds.

[0107] [4] Method for Manufacturing a Member Next, a method for manufacturing a member according to one embodiment of the present invention will be described. The method for manufacturing a member according to one embodiment of the present invention includes the step of forming a member by subjecting the above-mentioned galvanized steel sheet to at least one of forming and joining processes. Here, the forming method is not particularly limited, and for example, a general processing method such as press forming can be used. Similarly, the joining method is not particularly limited, and for example, a general welding method such as spot welding, laser welding and arc welding, rivet joining, and crimping joining can be used. The forming conditions and joining conditions are not particularly limited and can be followed according to conventional methods.

[0108] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0109] Molten steel having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted in a converter, and steel slabs were obtained by continuous casting. Then, under the conditions shown in Table 2, the steel slabs were subjected to hot rolling consisting of rough rolling and finish rolling to obtain hot-rolled steel sheets. Next, the obtained hot-rolled steel sheets were subjected to pickling and cold rolling to obtain cold-rolled steel sheets with a thickness of 1.4 mm. The reduction ratio in cold rolling was 50% in all cases. Next, the obtained cold-rolled steel sheets were subjected to primary heat treatment, zinc plating treatment and secondary heat treatment under the conditions shown in Table 2 to obtain galvanized steel sheets. Conditions not specified were followed according to conventional methods.

[0110] In the zinc plating process, hot-dip galvanizing or alloyed hot-dip galvanizing was performed to obtain hot-dip galvanized steel sheets (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheets (hereinafter also referred to as GA). In Table 2, the types of zinc plating processes are simply described as "GI" (hot-dip galvanizing) and "GA" (alloyed hot-dip galvanizing).

[0111] In this hot-dip galvanizing process, the plating bath contained 0.20% by mass of Al, with the remainder being Zn and unavoidable impurities. The plating bath temperature was 470°C. The plating adhesion amount was 45 to 72 g / m² per side. ２ (Double-sided plating) was used. The final composition of the zinc plating layer of the GI was Fe: 0.1 to 1.0 mass%, Al: 0.2 to 1.0 mass%, with the remainder consisting of Zn and unavoidable impurities.

[0112] Furthermore, in the alloyed hot-dip galvanizing process, the plating bath had a composition containing Al: 0.14% by mass, with the remainder consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The plating adhesion amount was 45 g / m² per side. ２ (Double-sided plating) was used. The alloying temperature was set to 530°C. The final zinc plating layer of GA consisted of Fe: 7-15% by mass, Al: 0.1-1.0% by mass, with the remainder being Zn and unavoidable impurities.

[0113] Using the galvanized steel sheets thus obtained, the following measurements were taken of the tempered M+ lath-like B area ratio, lump-like B area ratio, residual γ area ratio, LMn / HMn, prior γ particle size, Lv / Hv, diffusible hydrogen content, and thickness of the surface soft layer of the base steel sheet. The measurement results are shown in Table 3.

[0114] - Area ratio of tempered M + lath-like B and area ratio of massive B The sample was cut from a galvanized steel sheet so that the cross section (hereinafter also referred to as the L section) parallel to the rolling direction and thickness direction of the base steel sheet of the galvanized steel sheet would be the observation surface. Next, the observation surface of the sample was polished. Next, the observation surface of the sample was etched with 1 volume% nital to reveal the microstructure. Then, using SEM at a magnification of 3000x, 10 fields of view of a 40 μm × 40 μm area were observed on the observation surface of the sample so that the position 1 / 4 of the thickness of the base steel sheet was the center in the thickness direction, and microstructure images were obtained for each field of view. Next, the obtained microstructure images were analyzed using Image-Pro from Media Cybernetics, and based on shape and color, tempered martensite and lath-like bainite, massive bainite, and other microstructures were defined. In the microstructure images, tempered martensite and lath-like bainite are observed as gray regions. Cementite is also observed within these regions. Massive bainite is observed as black regions with an aspect ratio (a / b) of 4 or less. Here, the aspect ratio was calculated by defining a as the maximum length of the black region and b as the maximum length when crossing the region perpendicular to the direction of the maximum length. Furthermore, when multiple particles were in contact with each other and it was difficult to distinguish their interfaces, they were considered as a single region. The area ratio of tempered M + lath-like B and massive B was then calculated using the point counting method. Specifically, in a region of actual length: 40 μm × 40 μm in each SEM image, 7 × 7 grid points were placed at 5 μm intervals. Then, the number of grid points on tempered martensite, lath-like bainite, and massive bainite was counted. Next, the area ratio of tempered martensite + lath-like bainite and the area ratio of massive bainite were calculated by dividing the number of lattice points on tempered martensite, lath-like bainite, and massive bainite by the total number of lattice points and multiplying by 100.

[0115] - A sample was taken from a galvanized steel sheet with a residual γ area ratio. The sample was then polished in the thickness direction (depth direction) of the base steel sheet so that the observation surface was at the 1 / 4 position of the sheet thickness of the base steel sheet. Next, the observation surface of the sample was observed by X-ray diffraction. CoKα rays were used as the incident X-rays, and the ratio of the diffraction intensity of the (200), (211), and (220) faces of fcc iron (austenite) to the diffraction intensity of the (200), (220), and (311) faces of bcc iron was determined. Next, the volume fraction of retained austenite was calculated from the ratio of the diffraction intensity of each face. Then, the calculated volume fraction of retained austenite was considered as the area ratio and was defined as the residual γ area ratio.

[0116] - LMn / HMn A sample was taken from a galvanized steel sheet. The sample was then polished in the thickness direction (depth direction) of the base steel sheet so that the analysis surface was at the 1 / 4 position of the sheet thickness of the base steel sheet. Next, the Mn concentration of a 100 μm × 100 μm area of ​​the analysis surface of the sample was measured using an electron probe microanalyzer (EPMA). Then, LMn (area percentage (%) of the area where the Mn concentration is 3.0 mass% or less) and HMn (area percentage (%) of the area where the Mn concentration is greater than 4.0 mass%) were determined, and LMn / HMn was calculated by dividing LMn by HMn.

[0117] - The prior γ grain size and prior austenite grain boundaries become easier to identify due to P segregation. Therefore, a galvanized steel sheet was heat-treated. Here, the galvanized steel sheet was heated to 600°C, held for 10 min, and then cooled. Next, the sample was prepared and observed by SEM in the same manner as the measurement of the tempered M + lath-like B area ratio and the lump-like B area ratio of the base steel sheet, and a microstructure image was obtained. Using the obtained microstructure image, the prior γ grain size was determined by the sectioning method in accordance with JIS G 0551:2020.

[0118] - Lv / Hv A sample was cut from a galvanized steel sheet so that the L-shaped cross-section of the base steel sheet would serve as the observation surface. Next, the observation surface of the sample was mirror-polished using diamond paste, and then finished polished using colloidal silica. Then, the nanohardness of the base steel sheet was measured by a nanoindentation test in accordance with ISO 14577-1, and the nanohardness distribution at the 1 / 4 thickness position of the base steel sheet was obtained. Here, nanohardness was measured at 225 points at the 1 / 4 thickness position of the base steel sheet so that the distance between indentations in the rolling direction was 2 μm or more. The test was performed under load control conditions, with the loading speed and unloading speed set to 50 μN / s, the maximum load to 500 μN, and the data acquisition pitch to 5 msec. Then, the number of measurement points with a nanohardness of 3.0 GPa or less and the number of measurement points with a nanohardness of 4.5 GPa or more were defined as Lv and Hv, respectively, and Lv / Hv was calculated by dividing Lv by Hv.

[0119] - Diffusible hydrogen content of the base steel sheet A test piece measuring 30 mm in length and 5 mm in width was taken from a galvanized steel sheet, and the galvanized layer was removed using a router (precision grinder). Then, the amount of hydrogen released from the test piece was measured by a temperature rise desorption analysis method. Here, the test piece was continuously heated from 25°C (room temperature) to 210°C at a heating rate of 200°C / hr, and then cooled to room temperature. During this continuous heating, the amount of hydrogen released from the test piece (cumulative hydrogen content) was measured in the temperature range from 25°C to 210°C. The measured amount of hydrogen was then divided by the mass of the test piece (test piece after removal of the galvanized layer and before continuous heating), and the value converted to mass ppm was defined as the diffusible hydrogen content of the base steel sheet.

[0120] - Thickness of the soft surface layer A sample was cut from a galvanized steel sheet so that the L-shaped cross-section of the base steel sheet of the galvanized steel sheet would be the evaluation surface. Next, the evaluation surface of the sample was mirror-polished with diamond paste. Then, a Vickers hardness test was performed in accordance with JIS Z 2244-1:2024. Here, a square pyramidal Vickers indenter with a vertex angle of 136° was pressed with a load of 10 gf at a pitch of 10 μm in the thickness direction of the base steel sheet, starting from a depth of 10 μm from the surface of the base steel sheet to a position 1 / 4 of the thickness of the base steel sheet. The deepest measurement point where the Vickers hardness at the 1 / 4 thickness position of the base steel sheet was 90% or less was identified, and the distance from the surface of the base steel sheet to this deepest point was defined as the thickness of the soft surface layer of the base steel sheet.

[0121] Furthermore, the yield strength and n-value were measured using the obtained galvanized steel sheets according to the following procedure. The results are shown in Table 3.

[0122] - Yield strength and n-value The yield strength and n-value were measured in accordance with JIS Z 2241:2022 and JIS Z 2253:2020. Specifically, a No. 5 test specimen according to JIS Z 2201:1998 was taken from the obtained galvanized steel sheet so that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Tensile tests were performed on the taken test specimen at a crosshead speed of 10 mm / min to measure the yield strength and n-value. The n-value was calculated in accordance with JIS Z 2253:2020 using a simplified method to determine the n-value (work hardening index) at 2% tensile strain from two points on the nominal stress-nominal strain curve where the nominal strain is 1% and 3%. Tensile tests were performed five times each, and the average value of the yield strength and n-value measured in each test was taken as the yield strength and n-value of the galvanized steel sheet.

[0123] Furthermore, the durability and LME crack resistance of the spot welds were evaluated according to the following procedure. The results are shown in Table 3.

[0124] - Durability of Spot Welds The durability of spot welds was evaluated by cross tensile tests in accordance with JIS Z 3137:1999 and shear tests in accordance with JIS Z 3136:1999. Here, steel plates for cross tensile tests (length in the short direction: 50 mm, length in the long direction: 150 mm) and steel plates for shear tests (length in the short direction: 45 mm, length in the long direction: 125 mm) were taken from galvanized steel sheets. Then, spot welding was performed using the steel plates for cross tensile tests and shear tests as the joined materials to produce cross tensile tests and shear tests. For spot welding, the electrode was DR6mm-40R (tip diameter: 6 mm, tip radius of curvature R: 40 mm), the applied pressure was 4000 N, and the energizing time was 20 cycles, and the current value was adjusted so that the nugget diameter was 5.5 mm. Cross tensile tests and shear tests were performed using the fabricated cross tensile and shear test specimens. The durability of the spot welds was then evaluated according to the following criteria: A (Particularly excellent): The product of the maximum load measured by the cross tensile test and the crosshead displacement at the maximum load is 30 kN·mm or more, and the product of the maximum load measured by the shear test and the crosshead displacement at the maximum load is 20 kN·mm or more. B (Excellent): The product of the maximum load measured by the cross tensile test and the crosshead displacement at the maximum load is 10 kN·mm or more, and the product of the maximum load measured by the shear test and the crosshead displacement at the maximum load is 10 kN·mm or more (excluding A). F (Poor): The product of the maximum load measured by the cross tensile test and the crosshead displacement at the maximum load is less than 10 kN·mm, and / or the product of the maximum load measured by the shear test and the crosshead displacement at the maximum load is less than 10 kN·mm.

[0125] - LME crack resistance The LME crack resistance was evaluated by the critical strain and the n value measured as described above. The critical strain was measured as follows: A steel sheet to be joined was taken from a galvanized steel sheet. The taken steel sheets were then stacked, and two spot-welded joints were made by spot welding the stacked steel sheets. For spot welding, the electrode was DR6mm-40R (tip diameter: 6mm, tip radius of curvature R: 40mm), applied force: 4000N, skewing time: 5 / 50s, energizing time: 12 / 50s, and holding time: 5 / 50s. The current value was adjusted so that the nugget diameter was 4√t (t: thickness of each steel sheet to be joined (mm)). During the cooling process after spot welding, when the temperature of the heat-affected zone (strain application temperature) at a position 0.5 mm away from the nugget melting line (the nugget end of the joint surface of the steel plates) in a direction perpendicular to the thickness direction of the steel plates reached 800°C, a bending strain of 0.4% was applied to one spot welded joint and a bending strain of 0.8% to the other spot welded joint using a bending strain application device. The above temperatures were determined using the general-purpose temperature distribution analysis software SORPAS2D. Next, the spot welded joints were cut, the cross-sections were embedded and polished, and then etched. Then, the cross-sections of the spot welded joints were observed under a microscope to check for cracks. If no cracks were observed in either the spot welded joints with 0.8% and 0.4% bending strain applied, the limit strain amount was set to 0.8% or higher. If cracking was observed in a spot-welded joint subjected to a 0.8% bending strain, but not in a spot-welded joint subjected to a 0.4% bending strain, the critical strain was defined as 0.4% or more and less than 0.8%. If cracking was observed in a spot-welded joint subjected to a 0.4% bending strain, the critical strain was defined as less than 0.4%. The LME crack resistance characteristics were then evaluated according to the following criteria: A (Particularly excellent): Critical strain at a strain application temperature of 800°C is 0.8% or more, and the n value is 0.15 or more. B (Excellent): Critical strain at a strain application temperature of 800°C is 0.4% or more (excluding A). F (Poor): Critical strain at a strain application temperature of 800°C is less than 0.4%.

[0126]

[0127]

[0128]

[0129] As shown in Table 3, the inventive example achieved a yield strength of 900 MPa or higher and excellent spot weld durability. On the other hand, in the comparative example, at least one of the yield strength and spot weld durability was insufficient.

Claims

1. A galvanized steel sheet having a base steel sheet and a zinc plating layer on the surface of the base steel sheet, wherein the base steel sheet has a composition in mass%, C: 0.150% to 0.450%, Si: 0.50% to 3.00%, Mn: 1.50% to 4.00%, 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 remainder being Fe and unavoidable impurities, and the total area ratio of tempered martensite and lath-like bainite from the 1 / 8 thickness position to the 3 / 8 thickness position of the base steel sheet is 45% to 80%, the area ratio of massive bainite is 1% to 15%, and LMn / HMn is 0.20 or more. A galvanized steel sheet having a steel structure in which LMn and HMn are the area percentage (%) of the region where the Mn concentration is 3.0 mass% or less and the area percentage (%) of the region where the Mn concentration is greater than 4.0 mass%, respectively, with Lv / Hv: 0.2 to 2.5, where Lv and Hv are the points in the nanohardness distribution at the 1 / 4 thickness position of the base steel sheet where the nanohardness is 3.0 GPa or less and the nanohardness is 4.5 GPa or more, respectively, and the diffusible hydrogen content of the base steel sheet is 0.60 mass ppm or less.

2. The composition of the base steel sheet is further, in mass%, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less, Mo: 1.00% or less, Cr: 1.00% or less, Sb: 0.200% or less, Sn: 0.200% or less, Zr: 0.1000% or less, Te: 0.100% or less, Cu: 1.000% or less, Ni: 1.000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Co: 0.500% or less, Ta: 0.10% or less, Hf: 0.10% or less. The zinc-plated steel sheet according to claim 1, containing at least one selected from Bi: 0.200% or less, As: 0.100% or less, Pb: 0.100% or less, and Zn: 0.100% or less.

3. The galvanized steel sheet according to claim 1 or 2, wherein the structural composition of the base steel sheet has a retained austenite area ratio of 5% to 30% and prior austenite grain size of 3 μm to 12 μm, and the n value at 2% tensile strain is 0.15 or more.

4. The galvanized steel sheet according to any one of claims 1 to 3, wherein the base steel sheet has a soft surface layer, the soft surface layer is a region from the surface of the base steel sheet to a position 1 / 4 of the thickness of the base steel sheet, where the hardness is 90% or less of the hardness at the 1 / 4 position of the thickness of the base steel sheet, and the thickness of one side of the soft surface layer is 10 μm to 150 μm.

5. A member made using a galvanized steel sheet as described in any one of claims 1 to 4.

6. A method for producing a galvanized steel sheet according to any one of claims 1 to 4, the method comprising: a hot rolling step of hot rolling a steel slab having the component composition according to claim 1 or 2 to obtain a hot-rolled steel sheet; a cold rolling step of cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; a primary heat treatment step of heating and cooling the cold-rolled steel sheet; a zinc plating step of zinc plating the cold-rolled steel sheet to obtain a galvanized steel sheet; and a secondary heat treatment step of cooling and reheating the galvanized steel sheet, wherein in the primary heat treatment step, the intermediate heating temperature T1: 800°C to Ac ３ Point (℃), Maximum heating temperature T2: Ac ３ A method for manufacturing a galvanized steel sheet, wherein the temperature range is from 1°C to 950°C, the average heating rate V1 from T1 to T2 is 6°C / sec or more, the heat-affected index E defined by the following equation (1) is 150 to 1500, the primary cooling stop temperature T3 is (Ms point + 50)°C to 650°C and the average cooling rate V2 from T2 to T3 is 4°C / sec or more, and in the secondary heat treatment step, the secondary cooling stop temperature T4 is 50°C to 300°C, the reheating temperature T5 is greater than 300°C and less than or equal to 450°C, and the holding time t2 in the temperature range of greater than 300°C and less than or equal to 450°C during reheating is 1 sec to 150 sec. E = T2 × (4 × log(T2 - T1) + 10) / V1 ... (1) 7. The method for manufacturing a galvanized steel sheet according to claim 6, wherein the final reduction ratio in the finish rolling of the hot rolling is 25% to 60%.

8. The method for manufacturing a galvanized steel sheet according to claim 6 or 7, wherein the dew point in the primary heat treatment step is -35°C or higher.

9. A method for manufacturing a component, comprising the step of forming or joining a galvanized steel sheet according to any one of claims 1 to 4 to form a component.

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