Galvanized steel sheet and method for producing galvanized steel sheet

Abstract

The present invention provides: a galvanized steel sheet which has excellent high strength, excellent high bake hardening, excellent collision resistance, and excellent crack resistance; and a method for producing the same. This galvanized steel sheet contains, in mass%, 0.150-0.450% of C, 0.50-3.00% of Si, 1.50-4.00% of Mn, 0.100% or less of P, 0.0200% or less of S, 0.100% or less of Al, 0.0100% or less of O, and 0.0100% or less of N, wherein: the amount of diffusible hydrogen in the steel is 0.60 ppm by mass or less; the difference in yield strength before and after processing and baking is 300 MPa or more; and the microstructure in the range from 1 / 8-3 / 8 the sheet thickness satisfies, in area%, (i) the total of tempered martensite and bainite is 55-90%, (ii) fresh martensite having an aspect ratio of 4 or more is 30% or less, (iii) retained austenite is 5-30%, and (iv) the ratio of the area ratio of retained austenite after a sub-zero treatment to the area ratio of retained austenite before the sub-zero treatment is 0.95 or less.

Description

Galvanized steel sheet and method for manufacturing the same

[0001] The present invention relates to a galvanized steel sheet having high strength, high bake hardenability, excellent impact strength and crack resistance, and a method for producing the same.

[0002] In recent years, from the viewpoint of global environmental conservation, it has become necessary to improve the fuel efficiency of automobiles and reduce the amount of carbon dioxide (CO ２ It is desirable to reduce the amount of steel used. Reducing the weight of the vehicle body is extremely effective in improving the fuel efficiency of automobiles. For this reason, studies have been conducted to reduce the weight of the vehicle body while maintaining its strength by increasing the strength of the steel sheets that make up automobile parts and simplifying the structure of the vehicle body to reduce the number of parts (see Patent Documents 1 to 3).

[0003] Patent No. 6777274 Patent No. 7160199 Patent No. 7120461

[0004] Although Patent Documents 1 to 3 examine the crashworthiness of steel sheets, they all examine the crashworthiness of steel sheets that have not been bent or baked. These patent documents do not examine the crashworthiness of steel sheets that have been bent or baked. In this regard, for example, in a non-deformable part such as a pillar, if a crack occurs in the bent portion, the crashworthiness of the entire part may be reduced. Therefore, when evaluating the crashworthiness of steel sheets, it is considered insufficient to examine only steel sheets that have not been bent or baked.

[0005] The present invention was developed in consideration of the above points, and focuses on the bent portion of a non-deformable part that has been bent and baked with paint, and aims to suppress voids that are generated in the bent portion due to external forces during a collision. Therefore, an object of the present invention is to provide a galvanized steel sheet that has improved crash resistance by improving the crack resistance and yield strength of the steel sheet, and in particular, a galvanized steel sheet that has high strength and excellent bake hardenability, and a method for manufacturing the same.

[0006] The inventors have conducted extensive research to solve the above-mentioned technical problems of the prior art, and as a result have found that a zinc-plated steel sheet having the following configuration is effective and can achieve the above-mentioned object, leading to the development of the present invention. That is, the present invention provides a galvanized steel sheet having a galvanized layer on at least one surface of a steel sheet serving as a base material, the steel sheet containing, by 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, O: 0.0100% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities, the amount of diffusible hydrogen in the steel is 0.60 mass ppm or less, and the difference in yield strength before and after working and baking is 300 MPa or more, and the microstructure of the steel sheet in the range of 1 / 8 to 3 / 8 of the sheet thickness, centered at the 1 / 4 position from the surface of the steel sheet, is, by area %, (i) a sum of tempered martensite and bainite: 55 to 90%, (ii) fresh martensite having an aspect ratio of 4 or more: 30% or less; (iii) retained austenite: 5 to 30%; and (iv) the ratio of the area fraction of retained austenite after sub-zero treatment in which the steel sheet is held in liquid nitrogen at -196°C for 2 hours to the area fraction of retained austenite before the sub-zero treatment is 0.95 or less.

[0007] In addition, in the galvanized steel sheet of the present invention, the steel sheet contains, 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: 1.000% or less, Cr: 1.000% 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: 0.0 A preferred solution is to contain at least one element selected from the group consisting of: Cr: 100% 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, Bi: 0.200% or less, As: 0.100% or less, Pb: 0.100% or less, and Zn: 0.100% or less.

[0008] In addition, in the zinc-plated steel sheet of the present invention, when a region of the steel sheet has a hardness of 90% or less of the hardness at a position of 1 / 4 of the sheet thickness from the interface between the steel sheet and the zinc-plated layer on the steel sheet side, the soft layer is preferably present in a range of one-side thickness of 10 μm or more and 150 μm or less from the interface on the steel sheet side.

[0009] The present invention also provides a method for producing the galvanized steel sheet, comprising: hot rolling a slab of steel having the above-mentioned chemical composition to obtain a hot-rolled steel sheet; cold rolling the resulting hot-rolled steel sheet; subjecting the cold-rolled steel sheet to a primary heating treatment, a primary cooling treatment, a secondary cooling treatment, and a galvanizing treatment to obtain a galvanized steel sheet; and subjecting the galvanized steel sheet to a tertiary cooling treatment, a secondary heating treatment, a tertiary heating treatment, and a quaternary cooling treatment to produce the galvanized steel sheet, the method comprising the steps of: (a) in the primary heating treatment, heating the cold-rolled steel sheet at a heating temperature T1 of 800 to 950°C in an atmosphere having a dew point of -35°C or higher; (b) in the primary cooling treatment, cooling the cold-rolled steel sheet from the heating temperature T1 to a temperature T2 of (Ms point + 50)°C or higher and 650°C or lower at an average cooling rate V １ (c) In the secondary cooling treatment, the cold-rolled steel sheet is cooled at an average cooling rate V in a temperature range T3 of not less than the Ms point and not more than (Ms point + 200) ° C. ２ (d) in the tertiary cooling treatment, the galvanized steel sheet is cooled to a cooling stop temperature T4 of 50 to 300°C; (e) in the secondary heating treatment, the galvanized steel sheet is held at a heating temperature T5 of 250 to 400°C for 1 to 150 seconds; (f) in the tertiary heating treatment, the galvanized steel sheet is heated from the heating temperature T5 to a reheating temperature T6 of T5<T6<T5+50°C at an average heating rate V ３ and (g) in the fourth cooling treatment, the steel sheet is recooled without being held at the reheating temperature T6. The method for producing a galvanized steel sheet is characterized in that: E=T6×(6×log((T6−T5) / V))３ ) + 15) ... (1)

[0010] According to the present invention having the above-described summary configuration, it is possible to provide a galvanized steel sheet having high strength, high bake hardenability, and excellent crash strength and crack resistance, and a method for manufacturing the same.

[0011] In the galvanized steel sheet according to the present invention, the basic base steel sheet is characterized by having the following chemical composition and microstructure. The galvanized steel sheet according to the present invention has high strength and high bake hardenability, and is excellent in crashworthiness and crack resistance, providing sufficient crash strength. Therefore, the galvanized steel sheet according to the present invention can be suitably used, for example, as parts for transportation vehicles such as automobiles. Here, in the present invention, the galvanized steel sheet is not particularly limited as long as it is a plated steel sheet having a plating layer containing zinc as the main component, and includes pure galvanized steel sheets in which no metal elements are added to the zinc plating layer, hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA) in which alloy elements such as iron, nickel, magnesium, and aluminum are added to zinc, as well as hot-dip zinc-aluminum alloy-plated steel sheets, hot-dip zinc-aluminum-silicon alloy-plated steel sheets, and hot-dip zinc-aluminum-magnesium alloy-plated steel sheets.

[0012] In addition, in the method for producing the galvanized steel sheet according to the present invention, general processing methods such as press working can be used without any restrictions. In addition, as a method for welding the galvanized steel sheet, general welding methods such as spot welding and arc welding can be used.

[0013] In the following description, the "galvanized steel sheet" may be simply referred to as the "steel sheet." The thickness of the steel sheet is not particularly limited, but is, for example, 0.5 mm or more and 3.0 mm or less.

[0014] (1) Regarding the chemical composition of the steel sheet The chemical composition of the steel sheet that serves as the base material of the galvanized steel sheet according to the present invention will be described below. "%" in the chemical composition according to the present invention means "% by mass" unless otherwise specified.

[0015] <C: 0.150 to 0.450%> C is a component that generates martensite and increases the strength of the steel plate. If the C content is too low, the total area ratio of tempered martensite and bainite decreases, resulting in a decrease in collision resistance. For this reason, the C content is set to 0.150% or more, preferably 0.180% or more, and more preferably 0.200% or more. On the other hand, if the C content is too high, the amount of fresh martensite with an aspect ratio of 4 or more, which serves as the starting point for void generation, increases, resulting in a decrease in crack resistance. For this reason, the upper limit of the C content is set to 0.450% or less, preferably 0.430% or less, and more preferably 0.400% or less.

[0016] <Si: 0.50 to 3.00%> Si is an element that suppresses the formation of carbides during heat treatment and affects the stability of retained austenite. From the viewpoint of ensuring an appropriate amount of retained austenite in the retained austenite that has been subjected to subzero treatment, the Si content is set to 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 solute carbon concentration in the retained austenite increases excessively, and the area fraction of the retained austenite becomes too high. Therefore, the upper limit of the Si content is set to 3.00% or less, preferably 2.60% or less, and more preferably 2.40% or less.

[0017] <Mn: 1.50 to 4.00%> Mn affects the area ratios of tempered martensite and bainite. From the viewpoint of obtaining good crashworthiness, the Mn content is set to 1.50% or more, preferably 1.80% or more, and more preferably 2.00% or more. On the other hand, if the Mn content is too high, the amount of fresh martensite with an aspect ratio of 4 or more, which serves as the starting point for void generation, increases, thereby deteriorating crack resistance. Therefore, the upper limit of the Mn content is set to 4.00% or less, preferably 3.80% or less, and more preferably 3.50% or less.

[0018] <P: 0.100% or less> P segregates at prior austenite grain boundaries, embrittling the grain boundaries and adversely affecting crack resistance during impact. Therefore, the P content is set to 0.100% or less, preferably 0.030% or less, and more preferably 0.010% or less. While the lower limit of the P content is not particularly limited, it is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%, due to constraints on production technology.

[0019] <S: 0.0200% or less> S combines with Mn to form coarse MnS, which becomes the starting point for void generation and therefore has an adverse effect on crack resistance. Therefore, the S content is set to 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0020% or less. The lower limit of the S content is not particularly limited, but due to constraints on production technology, it is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0020] <Al: 0.100% or less> Al is A ３ The transformation point is increased. This increases the amount of ferrite and decreases the area ratios of tempered martensite and bainite. Therefore, the Al content is set to 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less. The lower limit of the Al content is not particularly limited, but is, for example, 0.010%, preferably 0.020%, because Al suppresses the formation of carbides during heat treatment and promotes the formation of retained austenite.

[0021] <O: 0.0100% or less> O forms oxides that become the starting points for void generation and have an adverse effect on crack resistance. Therefore, the O content is set to 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020% or less.

[0022] <N: 0.0100% or less> N combines with Ti to form TiN, which becomes the starting point for void generation and therefore has an adverse effect on crack resistance. Therefore, the N content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less. The lower limit of the N content is not particularly limited, but due to constraints on production technology, it is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0023] The composition of the steel sheet according to the present invention may further contain, in mass %, at least one element selected from the group consisting of the elements described below in addition to the above basic components.

[0024] <B: 0.0100% or less> B can improve the hardenability of the steel sheet by segregating at the austenite grain boundaries. Therefore, B increases the area ratio of tempered martensite and bainite, so it is preferable to add B. However, if the amount of B is too large, Fe ２３ (CB) ６ These form voids, which act as starting points for void generation and have an adverse effect on crack resistance. Therefore, the B content is preferably 0.0100% or less, more preferably 0.0050% or less, even more preferably 0.0040% or less, and particularly preferably 0.0030% or less. The lower limit of the B content is not particularly limited, but from the viewpoint of obtaining the effect of adding B, it is, for example, 0.0005%, and preferably 0.0010%.

[0025] <Ti: 0.200% or less> Ti is preferably added because it increases the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment. However, if the Ti content is too high, it bonds with N to form coarse nitrides, which become the starting points for void generation and have an adverse effect on crack resistance. For this reason, the Ti content 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 Ti content is not particularly limited, but from the viewpoint of obtaining the effect of adding Ti, it is, for example, 0.005%, preferably 0.010%.

[0026] <Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less> Nb, V, and W are preferably added because they form fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment, thereby increasing the strength of the steel sheet. However, if these elements are added in excessively large amounts, they will not dissolve during slab heating and will remain as coarse carbides. These coarse carbides will act as starting points for voids, thereby reducing crack resistance.

[0027] Therefore, the Nb content is preferably 0.200% or less, more preferably 0.100% or less, and further preferably 0.050% or less. The lower limit of the Nb content is not particularly limited, but from the viewpoint of obtaining the effect of adding Nb, it is, for example, 0.005%, and preferably 0.010%.

[0028] The V content is preferably 0.200% or less, more preferably 0.100% or less, and further preferably 0.050% or less. The lower limit of the V content is not particularly limited, but from the viewpoint of obtaining the effect of adding V, it is, for example, 0.005%, and preferably 0.010%.

[0029] The W content is preferably 0.100% or less, more preferably 0.080% or less, and further preferably 0.050% or less. The lower limit of the W content is not particularly limited, but from the viewpoint of obtaining the effect of adding W, it is, for example, 0.010%, and preferably 0.020%.

[0030] <Mo: 1.000% or less, Cr: 1.000% or less> Mo and Cr are preferably added because they suppress decarburization of the surface of the steel sheet, thereby improving the hardenability of the steel sheet and increasing its strength. However, if the amounts of these elements are excessively large, the amount of fresh martensite with an aspect ratio of 4 or more, which serves as the starting point for void generation, increases, thereby reducing the crack resistance.

[0031] Therefore, the Mo content is preferably 1.000% or less, more preferably 0.800% or less, and further preferably 0.500% or less. The lower limit of the Mo content is not particularly limited, but from the viewpoint of obtaining the effect of adding Mo, it is, for example, 0.010%, and preferably 0.020%.

[0032] Similarly, the Cr content is preferably 1.000% or less, more preferably 0.800% or less, and further preferably 0.500% or less. The lower limit of the Cr content is not particularly limited, but from the viewpoint of obtaining the effect of adding Cr, it is, for example, 0.010%, and preferably 0.020%.

[0033] <Sb: 0.200% or less, Sn: 0.200% or less> Sb and Sn are preferably added because they suppress decarburization of the surface region of the steel sheet caused by nitriding or oxidation of the surface of the steel sheet and suppress the formation of an excessive soft layer. However, if the amounts of these elements are excessively large, the thickness of the soft layer described below becomes insufficient, resulting in a decrease in crack resistance.

[0034] Therefore, the Sb content is preferably 0.200% or less, more preferably 0.080% or less, and further preferably 0.040% or less. The lower limit of the Sb content is not particularly limited, but from the viewpoint of obtaining the effect of adding Sb, it is, for example, 0.001%, and preferably 0.002%.

[0035] The Sn content is preferably 0.200% or less, more preferably 0.080% or less, and even more preferably 0.040% or less. The lower limit of the Sn content is not particularly limited, but from the viewpoint of obtaining the effect of adding Sn, it is, for example, 0.001%, and preferably 0.002%.

[0036] <Zr: 0.1000% or less, Te: 0.100% or less> Zr and Te can spheroidize the shape of nitrides and sulfides, reduce the origin of voids, and improve crack resistance. However, if the amounts of these elements are excessively large, coarse precipitates remain in an undissolved state in the slab during heating in hot rolling, which deteriorates crack resistance.

[0037] Therefore, the Zr content is preferably 0.1000% or less, more preferably 0.0800% or less, and further preferably 0.0500% or less. The lower limit of the Zr content is not particularly limited, but from the viewpoint of obtaining the effect of adding Zr, it is, for example, 0.0050%, and preferably 0.0100%.

[0038] The amount of Te is preferably 0.100% or less, more preferably 0.080% or less, and further preferably 0.050% or less. The lower limit of the amount of Te is not particularly limited, but from the viewpoint of obtaining the effect of adding Te, it is, for example, 0.005%, and preferably 0.010%.

[0039] <Cu: 1.000% or Less> Cu is preferably added because it can increase the strength of the steel sheet by improving the hardenability of the steel sheet. However, if the Cu content is excessively high, the number of void initiation points increases due to an increase in Cu inclusions, thereby deteriorating crack resistance. Therefore, the Cu content 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 Cu content is not particularly limited, but from the viewpoint of obtaining the effect of adding Cu, it is, for example, 0.010%, preferably 0.020%.

[0040] <Ni: 1.000% or less> Ni is preferably added because it can increase the strength of the steel sheet by improving the hardenability of the steel sheet. However, if the Ni content is excessively high, the amount of fresh martensite with an aspect ratio of 4 or more, which is the starting point for void generation, increases, thereby deteriorating the crack resistance. For this reason, the Ni content 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 Ni content is not particularly limited, but from the viewpoint of obtaining the effect of adding Ni, it is, for example, 0.010%, preferably 0.020%.

[0041] <Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less> Ca, Mg, and REM (Rare Earth Metals) are preferably added because they can spheroidize precipitates such as sulfides and oxides, thereby reducing the number of void initiation points and improving crack resistance. However, if the amounts of these elements are excessively large, the sulfides will coarsen and become the initiation points of voids, deteriorating crack resistance.

[0042] Therefore, the Ca content is preferably 0.0100% or less, more preferably 0.0050% or less, and further preferably 0.0040% or less. The lower limit of the Ca content is not particularly limited, but from the viewpoint of obtaining the effect of adding Ca, it is, for example, 0.0005%, and preferably 0.0010%.

[0043] The Mg content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0040% or less. The lower limit of the Mg content is not particularly limited, but from the viewpoint of obtaining the effect of adding Mg, it is, for example, 0.0005%, and preferably 0.0010%.

[0044] Similarly, the REM content is preferably 0.0100% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. The lower limit of the REM content is not particularly limited, but from the viewpoint of obtaining the effect of adding REM, it is, for example, 0.0005%, and preferably 0.0010%.

[0045] <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 are preferably added because they spheroidize the shape of precipitates, reduce the number of void initiation points, and improve crack resistance. However, if the amounts of these elements are excessively large, coarse precipitates become the initiation points of void initiation, deteriorating crack resistance.

[0046] Therefore, the Co content is preferably 0.500% or less, more preferably 0.400% or less, and further preferably 0.300% or less. The lower limit of the Co content is not particularly limited, but from the viewpoint of obtaining the effect of adding Co, it is, for example, 0.001%, and preferably 0.002%.

[0047] The Ta content is preferably 0.10% or less, more preferably 0.08% or less, and further preferably 0.07% or less. The lower limit of the Ta content is not particularly limited, but from the viewpoint of obtaining the effect of adding Ta, it is, for example, 0.01%, and preferably 0.02%.

[0048] The Hf content is preferably 0.10% or less, more preferably 0.08% or less, and further preferably 0.07% or less. The lower limit of the Hf content is not particularly limited, but from the viewpoint of obtaining the effect of adding Hf, it is, for example, 0.01%, and preferably 0.02%.

[0049] The Bi content 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 Bi content is not particularly limited, but from the viewpoint of obtaining the effect of adding REM, it is, for example, 0.001%, and preferably 0.005%.

[0050] <As: 0.100% or less, Pb: 0.100% or less, Zn: 0.100% or less> As, Pb, and Zn can make nitrides and sulfides more spherical, reducing the number of void initiation sites and improving crack resistance. However, if the amounts of these elements are excessively large, coarse precipitates remain in an undissolved state in the slab during heating in hot rolling, which deteriorates crack resistance.

[0051] Therefore, the As content is preferably 0.100% or less, more preferably 0.080% or less, and further preferably 0.050% or less. The lower limit of the As content is not particularly limited, but from the viewpoint of obtaining the effect of adding As, it is, for example, 0.004%, and preferably 0.010%.

[0052] The Pb content 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 Pb content is not particularly limited, but from the viewpoint of obtaining the effect of adding Pb, it is, for example, 0.001%, and preferably 0.004%.

[0053] The Zn content is preferably 0.100% or less, more preferably 0.080% or less, and further preferably 0.050% or less. The lower limit of the Zn content is not particularly limited, but from the viewpoint of obtaining the effect of adding Zn, it is, for example, 0.001%, and preferably 0.004%.

[0054] <Balance: Fe and inevitable impurities> The balance in the composition of the present invention is composed of Fe and inevitable impurities. Here, it is preferable that a steel sheet according to one embodiment of the present invention contains only the above basic components and the balance, with the balance being Fe and inevitable impurities. Note that the total content of Ge, Sr, and Cs contained in the inevitable impurities is 0.10% or less.

[0055] (2) Microstructure Next, the microstructure that the galvanized steel sheet according to the present invention should have (hereinafter, for convenience, also referred to as "the microstructure") will be described. To obtain the galvanized steel sheet according to the present invention, it is not sufficient to merely satisfy the component composition described in (1) above. The microstructure in the range of 1 / 8 to 3 / 8 of the sheet thickness, centered at the 1 / 4 position from the surface of the base steel sheet, must satisfy the configurations (i) to (iv) described below. Hereinafter, the area ratio refers to the area ratio relative to the entire microstructure. The area ratio of each structure is determined by the method described in the Examples below.

[0056] (i) Total area ratio of tempered martensite and bainite: 55 to 90%. From the viewpoint of stably ensuring good crashworthiness and steel sheet strength, the total area ratio of tempered martensite and bainite is 55% or more, preferably 58% or more, and more preferably 60% or more. On the other hand, if the total area ratio of tempered martensite and bainite is too high, crack resistance decreases. Therefore, this total area ratio is set to 90% or less, preferably 88% or less, and more preferably 85% or less.

[0057] (ii) Area fraction of fresh martensite with an aspect ratio of 4 or more: 30% or less In the galvanized steel sheet of the present invention, the area fraction of fresh martensite with an aspect ratio of 4 or more is an important constituent element of the present invention in order to improve crack resistance. Fresh martensite with an aspect ratio of 4 or more is easily fractured by bending, and therefore voids are generated inside the fresh martensite and at the grain boundaries with the soft layer described below, resulting in a decrease in crack resistance. Therefore, the area fraction of fresh martensite with an aspect ratio of 4 or more is set to 30% or less, preferably 25% or less, and more preferably 20% or less.

[0058] (iii) Area Fraction of Retained Austenite: 5 to 30% Retained austenite is effective in delaying the generation of voids during a collision and improving crack resistance. Therefore, the area fraction of retained austenite is set to 5% or more, preferably 6% or more, and more preferably 8% or more. On the other hand, if the area fraction of retained austenite is too high, the amount of retained austenite that transforms into martensite increases when bending is performed, increasing the number of void generation starting points and deteriorating crack resistance. Therefore, the area fraction of retained austenite is set to 30% or less, preferably 25% or less, and more preferably 20% or less.

[0059] (iv) The ratio of the area fraction of retained austenite after subzero treatment in which the steel is held in liquid nitrogen at -196°C for two hours to the area fraction of retained austenite before the subzero treatment is 0.95 or less. Unstable retained austenite is an important constituent element of the present invention. The unstable retained austenite is the retained austenite reduced by the subzero treatment in which the steel is held in liquid nitrogen at -196°C for two hours. This unstable retained austenite transforms to martensite when bending is performed, thereby exhibiting high work hardenability. Furthermore, when baking is performed, interstitial elements (C and N) fix dislocations, improving bake hardenability and enabling the difference in yield strength before and after the bending and baking to be 300 MPa or more.

[0060] Therefore, the ratio of the area fraction of retained austenite after the subzero treatment in which the steel is held in liquid nitrogen at −196° C. for two hours to the area fraction of retained austenite before the subzero treatment is set to 0.95 or less, preferably 0.93 or less, and more preferably 0.91 or less. The lower limit of the area fraction ratio is not particularly limited, but from the viewpoint of delaying the generation of voids during collision, it is, for example, 0.40, and preferably 0.45.

[0061] In addition, such microstructures may contain, as structures (residual structures) other than tempered martensite, bainite, fresh martensite, and retained austenite, for example, pearlite, ferrite, iron-based carbonitrides, alloy carbonitrides, MnS, Al ２ O ３The area ratio of the remaining structure is preferably 10% or less, more preferably 8% or less, and even more preferably 5% or less, and as long as the area ratio of the remaining structure is within this range, the effects of the present invention are not impaired.

[0062] Furthermore, the galvanized steel sheet of the present invention can exhibit even more preferable effects by satisfying the following requirements.

[0063] (3) Thickness of a soft layer on one side of the steel sheet from the interface between the base steel sheet and the zinc-plated layer: 10 to 150 μm. Having a soft layer on the steel sheet side from the interface between the base steel sheet and the zinc-plated layer can improve crack resistance. The soft layer is a region from the interface between the base steel sheet and the zinc-plated layer to the steel sheet side, having a hardness of 90% or less of the hardness at the 1 / 4 position in the sheet thickness. This soft layer preferably contains 5% or more ferrite. The soft layer's thickness on one side is 10 to 150 μm. The soft layer's thickness on one side refers to the thickness of the soft layer on either surface of the steel sheet from the interface with the zinc-plated layer to the steel sheet side. If the soft layer's thickness on one side is less than 10 μm, the crack resistance deteriorates. On the other hand, if the soft layer's thickness on one side is more than 150 μm, the strength of the steel sheet decreases and the difference in yield strength before and after forming and baking decreases.

[0064] (4) Amount of diffusible hydrogen in steel: 0.60 mass ppm or less If the amount of diffusible hydrogen in steel is too high, delayed fracture occurs and the collision resistance decreases. Therefore, the amount of diffusible hydrogen in steel is 0.60 mass ppm or less, preferably 0.50 mass ppm or less, and more preferably 0.40 mass ppm or less. The amount of diffusible hydrogen in steel can be determined by the method described in the examples below.

[0065] (5) Difference in yield strength before and after processing and baking: 300 MPa or more In the present invention, processing and baking refers to applying a 2% pre-strain to a JIS No. 5 test piece and then heat treating it at 170°C for 20 minutes. The yield strength after processing and baking is greater than the yield strength before processing and baking, and if the difference is less than 300 MPa, the impact resistance will decrease. Therefore, in the present invention, the difference in yield strength before and after processing and baking is set to 300 MPa or more.

[0066] The galvanized steel sheet according to the present invention preferably has a hot-dip galvanized layer on at least one surface of the steel sheet from the viewpoint of improving corrosion resistance, etc. The hot-dip galvanized layer is formed by a galvanizing treatment and may be a galvannealed layer.

[0067] Next, a method for manufacturing a galvanized steel sheet according to this embodiment (hereinafter also referred to as "the present manufacturing method") will be described. The present manufacturing method is a method for manufacturing the above-described galvanized steel sheet. Note that the temperatures when heating or cooling a slab, steel sheet, etc., described below refer to the surface temperatures of the slab, steel sheet, etc., unless otherwise specified.

[0068] The method for producing molten steel to be used as a slab (steel material) is not particularly limited, and known methods using a converter, electric furnace, etc. can be used. It is preferable to obtain slabs from molten steel by continuous casting. Alternatively, slabs may be obtained by other methods such as ingot making-blooming and thin slab continuous casting.

[0069] In the manufacturing method of the present invention, first, a slab having the above-described component composition is hot-rolled to obtain a hot-rolled steel sheet. When hot-rolling, the slab may be reheated in a heating furnace before rolling. If the slab is maintained at a temperature equal to or higher than a predetermined temperature, the slab may be directly rolled without being heated. In the hot-rolling, the slab is subjected to rough rolling and finish rolling. Before rough rolling, it is preferable to heat the slab to dissolve carbides in the slab.

[0070] The temperature at which carbides are dissolved or the slab is heated (slab heating temperature) is preferably 1100° C. or higher, more preferably 1150° C. or higher, from the viewpoint of preventing an increase in rolling load. On the other hand, the slab heating temperature is preferably 1300° C. or lower, more preferably 1280° C. or lower, from the viewpoint of preventing an increase in scale loss.

[0071] As described above, if the slab before rough rolling is maintained at a temperature equal to or higher than a predetermined temperature and the carbides in the slab are melted, heating of the slab before rough rolling can be omitted. The conditions for rough rolling and finish rolling are not particularly limited, but for example, the finish rolling end temperature is preferably 700 to 1100°C, more preferably 800 to 1000°C.

[0072] Next, the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The rolling reduction ratio of the cold rolling is preferably 30% or more, more preferably 35% or more. The upper limit is not particularly limited, and is, for example, 70% or less, preferably 65% ​​or less.

[0073] Next, in the heat treatment of the steel sheet, roughly, a cold-rolled steel sheet is subjected to (1) a primary heating treatment, (2) a primary cooling treatment, (3) a secondary cooling treatment, and (4) a zinc plating treatment in this order to obtain a zinc-plated steel sheet. The zinc-plated steel sheet is then subjected to (5) a tertiary cooling treatment, (6) a secondary heating treatment, (7) a tertiary heating treatment, and (8) a quaternary cooling treatment in this order. The zinc-plated steel sheet that has undergone the quaternary cooling treatment corresponds to the zinc-plated steel sheet of the present invention described above.

[0074] (1) Primary Heat Treatment First, the cold-rolled steel sheet is subjected to a primary heat treatment at a heating temperature T1. If the heating temperature T1 is too low, the steel sheet will be heated in the two-phase region of ferrite and austenite, and the final microstructure will contain ferrite, resulting in a decrease in the total area ratio of tempered martensite and bainite. Therefore, the heating temperature T1 is set to 800°C or higher, preferably 820°C or higher, and more preferably 850°C or higher.

[0075] On the other hand, if the heating temperature T1 is too high, the amount of hydrogen that penetrates into the steel increases due to an increase in hydrogen partial pressure, which increases the amount of diffusible hydrogen in the steel, as well as the area ratio of martensite, increasing the amount of fresh martensite with an asbestos ratio of 4 or more. For this reason, the primary heating temperature T1 is set to 950°C or lower, preferably 930°C or lower, and more preferably 900°C or lower.

[0076] The time for which the cold-rolled steel sheet is held at the heating temperature T1 (heating time t １) is not particularly limited, but the lower limit is, for example, 10 seconds or more, preferably 50 seconds or more, and more preferably 80 seconds or more. １ The upper limit is, for example, 500 seconds or less, preferably 300 seconds or less, and more preferably 200 seconds or less.

[0077] <Atmospheric dew point: −35° C. or higher> In the primary heat treatment, a soft layer of a desired thickness is formed from the surface of the steel sheet in the sheet thickness direction. From the viewpoint of obtaining excellent crack resistance, the atmospheric dew point is preferably −35° C. or higher, more preferably −20° C. or higher, and even more preferably −10° C. or higher. If the atmospheric dew point is lower than −35° C., it becomes difficult to form a soft layer of a desired thickness. Note that there is no particular upper limit to the atmospheric dew point, but in order to keep the steel sheet strength within a suitable range, the atmospheric dew point is preferably 25° C. or lower, more preferably 20° C. or lower.

[0078] (2) Primary Cooling Treatment Next, a primary cooling treatment is carried out. Specifically, the cold-rolled steel sheet heated at a heating temperature T1 is cooled to a temperature T2 of (Ms point + 50)°C or higher and 650°C or lower. The Ms point (unit: °C) is the temperature at which transformation from austenite to martensite begins, and can be calculated using the following formula (2): Ms = 550 - 350 x [C] - 40 x [Mn] - 35 x [V] - 20 x [Cr] - 17 x [Ni] - 10 x [Cu] - 10 x [Mo] - 5 x [W] + 15 x [Co] + 30 x [Al] (2) In the above formula (2), [M] is the content (unit: mass%) of element M in the above-mentioned composition (present composition).

[0079] <Average cooling rate V １ : 4 ° C. / sec or more > In the primary cooling treatment, the average cooling rate V from the heating temperature T1 to the temperature T2 １ If the average cooling rate V is too slow, ferrite transformation occurs during cooling, and the total area ratio of tempered martensite and bainite decreases. １ The average cooling rate V is 4°C / sec or more, preferably 5°C / sec or more, and more preferably 6°C / sec or more. １The upper limit of the heating rate is not particularly limited, but is, for example, 30° C. / sec, preferably 25° C. / sec, and more preferably 20° C. / sec.

[0080] (3) Secondary Cooling Treatment Next, secondary cooling treatment is carried out. Specifically, the cold-rolled steel sheet cooled to the temperature T2 is cooled to at least the Ms point before carrying out the galvanizing treatment described later.

[0081] <Average cooling rate V ２ : 0.2 to 5.0°C / sec> The secondary cooling treatment removes hydrogen introduced into the cold-rolled steel sheet by the above-mentioned primary heating treatment. At this time, in the temperature range T3 of not less than the Ms point and not more than (Ms point + 200)°C, the cold-rolled steel sheet is slowly cooled to cause bainite transformation, and the total area ratio of tempered martensite and bainite can be controlled. If the cooling rate is too slow, the total area ratio of tempered martensite and bainite becomes too high. For this reason, the average cooling rate V in the temperature range T3 ２ is 0.2°C / sec or more, preferably 0.3°C / sec or more, and more preferably 0.4°C / sec or more.

[0082] On the other hand, the average cooling rate V in the temperature range T3 ２ If the average cooling rate V is too fast, the hydrogen in the steel that entered during the primary heat treatment will not be able to escape, and the amount of diffusible hydrogen in the steel will increase. ２ The upper limit of the heating rate is 5.0° C. / sec or less, preferably 4.5° C. / sec or less, and more preferably 4.0° C. / sec or less.

[0083] (4) Galvanizing Treatment Next, the cold-rolled steel sheet cooled to at least the Ms point is subjected to a galvanizing treatment. This results in a galvanized steel sheet having a galvanized layer formed on at least one surface of the steel sheet. The galvanizing treatment is preferably a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

[0084] When hot-dip galvanizing is performed, it is preferable to perform the hot-dip galvanizing treatment by, for example, immersing the steel sheet in a zinc bath having a bath temperature of 440 to 500°C, and then adjusting the coating weight of the plating layer by gas wiping, etc. It is preferable to use a zinc bath having a component composition with an Al content of 0.10 to 0.23 mass%, the balance being Zn and unavoidable impurities.

[0085] Furthermore, a galvannealed layer is formed by performing a galvannealed hot-dip plating treatment. If the alloying temperature is too low, the Zn—Fe alloying rate may be excessively slow, making alloying extremely difficult. On the other hand, if the alloying temperature is too high, untransformed austenite may transform into pearlite. Therefore, the alloying temperature is preferably 450 to 600°C, more preferably 470 to 550°C, and even more preferably 470 to 530°C.

[0086] The coating weight of the hot-dip galvanized layer on hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA) is 20 to 80 g / m per side. ２ (Double-sided plating) is preferred.

[0087] (5) Tertiary Cooling Treatment Next, the galvanized steel sheet is subjected to a tertiary cooling treatment. <Cooling Stop Temperature T4: 50 to 300°C> If the cooling stop temperature T4 is too low, the total area ratio of tempered martensite and bainite increases, and crack resistance deteriorates. Therefore, the cooling stop temperature T4 is set to 50°C or higher, preferably 100°C or higher, and more preferably 120°C or higher.

[0088] On the other hand, if the cooling stop temperature T4 is too high, the total area ratio of tempered martensite and bainite decreases, and fresh martensite having an aspect ratio of 4 or more increases. Therefore, the cooling stop temperature T4 is 300°C or less, preferably 280°C or less, and more preferably 260°C or less.

[0089] (6) Secondary Heat Treatment Next, the secondary heat treatment is carried out. Specifically, the galvanized steel sheet is heated to a heating temperature T5 and held at that temperature for 1 to 150 seconds.

[0090] <Heating temperature T5: 250 to 400°C, heating time: 1 to 150 seconds> If the heating temperature T5 is low and the heating time is short, the amount of fresh martensite with an aspect ratio of 4 or more increases. For this reason, the secondary heating temperature T5 is set to 250°C or higher, preferably 280°C or higher, and more preferably 300°C or higher. Furthermore, from the viewpoint of reducing the amount of diffusible hydrogen in the steel, the heating time is set to 1 second or longer.

[0091] On the other hand, if the heating temperature T5 is high and the heating time is too long, the area ratio of tempered martensite and bainite increases excessively, and the crack resistance deteriorates. Therefore, the heating temperature T5 is set to 400°C or less, preferably 380°C or less, and more preferably 350°C or less. The heating time is set to 150 seconds or less.

[0092] (7) Third Heat Treatment Next, the third heat treatment is carried out. Specifically, the galvanized steel sheet is heated from the heating temperature T5 at an average heating rate V of 0.2 to 10.0°C / sec. ３ Then, the material is reheated to a reheating temperature T6, where T5<T6<T5+50.

[0093] <Average heating rate V ３ : 0.2 to 10.0°C / sec> Average heating rate V ３ If the average heating rate V is too slow, the area ratio of tempered martensite and bainite increases excessively, and the crack resistance decreases. ３ is 0.2°C / sec or more, preferably 0.5°C / sec or more, and more preferably 1.0°C / sec or more. ３ If the average heating rate V is too fast, the amount of diffusible hydrogen in the steel will be difficult to remove. ３ is set to 10.0° C. / sec or less, preferably 9.0° C. / sec or less, and more preferably 8.0° C. / sec or less.

[0094] <Heating temperature T6: T5<T6<T5+50> When galvanized steel sheet is reheated to a temperature range of T5<T6<T5+50, carbon diffusion is promoted and the amount of retained austenite with a non-uniform carbon concentration increases. This allows for the creation of reduced retained austenite (unstable retained austenite) through sub-zero treatment, in which the steel sheet is held in liquid nitrogen at -196°C for two hours.

[0095] <Thermal effect index E from heating temperature T5 to reheating temperature T6: 4000 to 11000> The thermal effect index E from heating temperature T5 to reheating temperature T6 can be expressed by the following formula (1): E=T6×(6×log((T6−T5) / V ３ ) + 15) ... (1)

[0096] The heat effect index E expressed by the above formula (1) is an extremely important constituent element of the present invention. The area fractions of unstable retained austenite and fresh martensite having an aspect ratio of 4 or more can be controlled by the amount of heat (heat effect index E) introduced in the temperature range from the heating temperature T5 to the reheating temperature T6.

[0097] If the heat effect index E is less than 4000, the solute carbon will not be sufficiently diffused into the untransformed austenite layer, and the amount of retained austenite reduced by the sub-zero treatment of holding in liquid nitrogen at -196°C for two hours will be insufficient. Furthermore, the martensite will not be sufficiently tempered, and the total area ratio of tempered martensite and bainite will be less than 55%. Therefore, the heat effect index E is set to 4000 or more, preferably 4500 or more, and more preferably 5000 or more.

[0098] On the other hand, if the heat effect index E exceeds 11,000, the area ratio of fresh martensite having an aspect ratio of 4 or more exceeds 30%. Therefore, the upper limit of the heat effect index E is set to 11,000 or less, preferably 10,000 or less, and more preferably 9,500 or less.

[0099] (8) Fourth Cooling Treatment Next, a fourth cooling treatment is carried out in which the material is re-cooled without being held at the reheating temperature T6.

[0100] In this manufacturing method, for example, the holding temperature such as the heating temperature or reheating temperature does not have to be constant as long as it is within the above-mentioned temperature range. The cooling rate may be changed during cooling as long as it is within the above-mentioned rate range. As long as the above-mentioned temperature range and other conditions are met, the heat treatment may be performed in any equipment.

[0101] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0102] Molten steel having the chemical compositions shown in Tables 1-1 and 1-2, with the balance being Fe and unavoidable impurities, was produced in a converter and subjected to continuous casting to obtain slabs No. 1 to No. 49. Note that the underlined values ​​in Tables 1-1 and 1-2 indicate values ​​outside the scope of the present invention (the same applies to Tables 2 and 3 described below).

[0103]

[0104]

[0105] The obtained slabs were hot-rolled under the conditions shown in Tables 2-1 and 2-2 to obtain hot-rolled steel sheets. Specifically, the slabs were heated to 1250°C, and the hot-rolled steel sheets were finish-hot-rolled at 890°C and coiled at 500°C. The hot-rolled steel sheets were then pickled and cold-rolled at a rolling reduction ratio of 50% to obtain cold-rolled steel sheets (sheet thickness: 1.4 mm).

[0106] The obtained cold-rolled steel sheet was subjected to a first heating treatment, a first cooling treatment, a second cooling treatment, and a hot-dip galvanizing treatment under the conditions shown in Tables 2-1 and 2-2 to obtain a hot-dip galvanized steel sheet. In the first heating treatment, the holding time at the heating temperature T1 (heating time t １ ) was set to 120 seconds.

[0107] In the hot-dip galvanizing treatment, both surfaces of a cold-rolled steel sheet were subjected to a hot-dip galvanizing treatment or a galvannealed hot-dip galvanizing treatment to obtain a hot-dip galvanized steel sheet (GI) or a galvannealed hot-dip galvanized steel sheet (GA). When producing GI, a zinc bath containing 0.20 mass% Al, with the balance consisting of Zn and unavoidable impurities, was used as the hot-dip galvanizing bath. When producing GA, a zinc bath containing 0.14 mass% Al, with the balance consisting of Zn and unavoidable impurities was used as the hot-dip galvanizing bath. The bath temperature was 470°C in both the production of GI and GA.

[0108] When manufacturing GI, the coating weight of the hot-dip galvanized layer is 45 to 72 g / m per side. ２When manufacturing GA, the thickness is 45 g / m per side. ２ The alloying temperature for producing GA was set to 530°C.

[0109] The hot-dip galvanized layer of GI contained 0.1 to 1.0% Fe, 0.2 to 1.0% Al, and the balance was Fe and unavoidable impurities. The hot-dip galvanized layer of GA contained 7 to 15% Fe, 0.1 to 1.0% Al, and the balance was Fe and unavoidable impurities.

[0110] The obtained hot-dip galvanized steel sheet (GI or GA) was then subjected to a tertiary cooling treatment, a secondary heating treatment, and a tertiary heat treatment under the conditions shown in Tables 2-1 and 2-2. The hot-dip galvanized steel sheet (GI or GA) that had undergone such heat treatments (specifically, had undergone the tertiary heating treatment) and then had undergone a fourth cooling treatment will hereinafter also be simply referred to as a "steel sheet."

[0111]

[0112]

[0113] The microstructure of the obtained steel sheets was observed as follows, and the observation results are shown in Tables 3-1 and 3-2.

[0114] The microstructure was the steel structure in the range of 1 / 8 to 3 / 8 of the plate thickness, with the center being the position 1 / 4 of the plate thickness from the surface of the steel plate.

[0115] <Total Area Ratio of Tempered Martensite and Bainite> The obtained steel sheet was polished so that a cross section (L cross section) parallel to the rolling direction at a position corresponding to 1 / 4 of the sheet thickness was used as the observation surface. The observation surface was corroded with 1 volume % nital and then observed at 3000x magnification using a scanning electron microscope (SEM). Ten fields of view of the observation surface were observed, and the obtained SEM images were analyzed to determine the total area ratio (unit: %) of tempered martensite and bainite.

[0116] More specifically, structures in which numerous carbides and corrosion marks were observed within the crystal grains in the obtained SEM images were determined to be tempered martensite and bainite, and their area ratios (average area ratios over 10 fields of view) were calculated. Image-Pro (manufactured by Media Cybernetics) was used as analysis software for analyzing the SEM images.

[0117] <Area Fraction of Fresh Martensite with an Aspect Ratio of 4 or More> The obtained steel sheet was polished so that the cross section (L cross section) parallel to the rolling direction at a position corresponding to 1 / 4 of the sheet thickness was the observation surface. The observation surface was corroded with 1 vol% nital and then observed at 3000x magnification using a scanning electron microscope (SEM). From the SEM image, areas where no carbides were observed within the grains and were white or light gray were determined to be fresh martensite and retained austenite, and retained austenite having a face-centered cubic (fcc) structure was excluded. Specifically, EBSD (electron backscatter diffraction) data was obtained from the same field of view, and based on this data, the structure having a face-centered cubic (fcc) structure was excluded, and the remaining structure was determined to be fresh martensite.

[0118] For fresh martensite, the longest particle length was defined as the major axis length a, and the particle length perpendicular to this, which was the longest across the particle, was defined as the minor axis length b, with a / b being the aspect ratio. When multiple particles were in contact with each other, they were divided approximately equally and considered as individual particles. The area of ​​fresh martensite with an aspect ratio (a / b) of 4 or more was determined. The average value of 10 visual fields was taken as the area ratio (unit: %) of fresh martensite with an aspect ratio of 4 or more in each steel sheet.

[0119] <Area Fraction of Retained Austenite> The observation surface of the obtained steel sheet (a cross section at 1 / 4 of the sheet thickness and parallel to the rolling direction) was analyzed using an X-ray diffraction (XRD) device with CoKα as an X-ray source to determine the diffraction intensities of the following crystal planes. Specifically, the ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron were determined. The average value of the nine ratios determined was used as the volume fraction of retained austenite. The determined volume fraction was considered to be the area fraction of retained austenite (unit: %).

[0120] <Ratio of Area Fraction of Retained Austenite After Subzero Treatment by Holding in Liquid Nitrogen at −196°C for 2 Hours to Area Fraction of Retained Austenite Before Subzero Treatment> Ten test specimens for X-ray diffraction were taken parallel to the plate surface from a 1 / 4 position of the plate thickness of each steel plate obtained, and the area fraction of retained austenite was measured for each test specimen. Each test specimen was subjected to subzero treatment by holding in liquid nitrogen at −196°C for 2 hours, and the area fraction of retained austenite after the subzero treatment was determined. The ratio of this area fraction to the area fraction of the retained austenite of the test specimen before the subzero treatment was calculated. The average of the area fractions calculated for each of the 10 test specimens was used as the ratio of the area fraction of retained austenite after subzero treatment by holding in liquid nitrogen at −196°C for 2 hours to the area fraction of retained austenite of each steel plate before the subzero treatment.

[0121] <Measurement of the Soft Layer> A sample was taken from the obtained steel sheet, with the thickness cross section parallel to the rolling direction as the observation surface. The observation surface was then corroded with 1 vol% nital, and an SEM image was taken from the steel sheet surface in the thickness direction at 3000x magnification using a scanning electron microscope (SEM). In this example, the soft layer was defined as the region having 5% or more ferrite from the interface between the cold-rolled steel sheet and the hot-dip galvanized layer toward the cold-rolled steel sheet. From the SEM image, a structure that was black and had no visible corrosion marks or carbides within the grains was determined to be ferrite, and the ferrite fraction was determined using a cutting method in accordance with ASTM E112-10. From the ferrite fraction, the distance (depth) from the surface of the steel sheet to the deepest depth position containing 5% or more ferrite was measured, and this measurement value was defined as the thickness of the soft layer.

[0122] The obtained steel sheets were evaluated by the following method. The results are shown in Tables 3-1 and 3-2. <Evaluation test (tensile test) of yield strength before and after working and baking> Ten No. 5 test pieces as specified in JIS Z 2201 were taken from the obtained steel sheets, with the longitudinal direction (tensile direction) at an angle of 90° to the rolling direction. A tensile test in accordance with JIS Z 2241 was carried out on five of the taken test pieces, and the yield strength (YS １ The remaining five test pieces (JIS No. 5 test pieces) were subjected to a 2% pre-strain, then heat treated at 170°C for 20 minutes, and the tensile test was carried out again. When an upper yield point appeared, the upper yield point was used as the yield strength (YS ２ When no upper yield point appears, the 0.2% yield strength is taken as the yield strength (YS ２ ) The increase in yield strength ΔYS (YS ２ -YS １ ) was evaluated as the difference in yield strength before and after processing and baking.

[0123] <Measurement of the amount of diffusible hydrogen in steel> The hot-dip galvanized layer was removed from the obtained steel sheet using a router (precision grinder), and test pieces 30 mm long and 5 mm wide were taken. The amount of diffusible hydrogen in the steel was measured for the taken test pieces by thermal desorption analysis. The heating rate was 200°C / hr. The cumulative value of the amount of hydrogen detected in the temperature range from room temperature (25°C) to less than 210°C was taken as the amount of diffusible hydrogen in the steel (unit: ppm by mass).

[0124] <Evaluation test for collision strength and crack resistance (V-bending - baking treatment (170°C x 20 min) - orthogonal VDA bending test)> The end surfaces of the obtained steel plates were subjected to grinding to obtain test specimens measuring 1.4 mm in thickness x 60 mm in width x 60 mm in length. First, the obtained test specimens were subjected to V (90°) bending by bending in the L direction (C-axis bending) with a bending radius of R = 5 mm under the conditions of a load of 10 tons, a stroke speed of 30 mm / min, and a holding time of 5 seconds.

[0125] Next, the V-bent test specimens were heat-treated at 170°C for 20 minutes and then rotated horizontally by 90°. A VDA bending test was performed on the test specimens using a bending tester at a stroke speed of 20 mm / min, bending them in the C-direction (L-axis bending) so that the crest of the V-bend faced the punch, and the maximum load F was measured. The bending tester used had a roller distance of 2 x plate thickness + 0.5 mm and a punch tip curvature radius R = 0.4 mm. Note that if F ≥ 7000 N, it was marked with "◎", if F ≥ 5000 N, it was marked with "○", and if F was less than 5000 N, it was marked with "×". A rating of "◎" or "○" indicated excellent crashworthiness.

[0126] To evaluate crack resistance, the maximum load point was read from the results of the VDA bending test, and the bending test was stopped when the maximum load point was reached. The ridgeline at the apex of the bending of the test piece stopped at the maximum load point was then observed at 20x magnification using a digital microscope (RH-2000, manufactured by Hirox Corporation), and the length of the crack was measured.

[0127] When the total maximum length of the cracks was 6000 μm or less, it was marked with "◎", when the total maximum length of the cracks was more than 6000 μm but less than 9000 μm, it was marked with "◯", and when the total maximum length of the cracks was 9000 μm or more, it was marked with "×". If it was marked with "◎" or "◯", it was evaluated as having excellent crack resistance.

[0128] As can be seen from these results, all of the steel plates satisfying the above-mentioned conditions of the present invention exhibited a maximum load F of 5000 N or more, and the sum of the maximum lengths of cracks at the bending apex when bent to the maximum load point was less than 9000 μm, and thus had both excellent collision resistance and crack resistance properties.

[0129]

[0130]

Claims

1. A galvanized steel sheet having a galvanized layer on at least one surface of a base steel sheet, the steel sheet containing, by 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, O: 0.0100% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities, the amount of diffusible hydrogen in the steel is 0.60 mass ppm or less, and the difference in yield strength before and after forming and baking is 300 MPa or more, and the microstructure of the steel sheet in the range of 1 / 8 to 3 / 8 of the sheet thickness, centered at the 1 / 4 position from the surface to the sheet thickness, is, by area %, (i) a total of tempered martensite and bainite: 55 to 90%; (ii) fresh martensite having an aspect ratio of 4 or more: 30% or less; (iii) retained austenite: 5 to 30%; and (iv) a ratio of the area fraction of retained austenite after sub-zero treatment in which the steel is held in liquid nitrogen at -196°C for 2 hours to the area fraction of retained austenite before the sub-zero treatment is 0.95 or less.

2. The steel plate contains, 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: 1.000% or less, Cr: 1.000% 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, 2. The galvanized steel sheet according to claim 1, further comprising at least one element selected from the group consisting of 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, characterized in that when the soft layer is defined as a region having a hardness of 90% or less of the hardness at a position 1 / 4 of the sheet thickness from the interface between the steel sheet and the galvanized layer on the steel sheet side, the soft layer exists in a range of 10 μm or more and 150 μm or less on one side from the interface on the steel sheet side.

4. The galvanized steel sheet according to claim 2, characterized in that when the soft layer is defined as a region having a hardness of 90% or less of the hardness at a position 1 / 4 of the sheet thickness from the interface between the steel sheet and the galvanized layer on the steel sheet side, the soft layer exists in a range of 10 μm or more and 150 μm or less on one side from the interface on the steel sheet side.

5. A method for producing a galvanized steel sheet according to any one of claims 1 to 4, comprising: hot-rolling a slab of steel having the chemical composition according to claim 1 or 2 to obtain a hot-rolled steel sheet, cold-rolling the resulting hot-rolled steel sheet, subjecting the cold-rolled steel sheet to a primary heating treatment, a primary cooling treatment, a secondary cooling treatment and galvanizing treatment to obtain a galvanized steel sheet, and subjecting the galvanized steel sheet to a tertiary cooling treatment, a secondary heating treatment, a tertiary heating treatment and a quaternary cooling treatment to produce a galvanized steel sheet, the method comprising the steps of: (a) in the primary heating treatment, heating the cold-rolled steel sheet at a heating temperature T1 of 800 to 950°C in an atmosphere having a dew point of -35°C or higher; (b) in the primary cooling treatment, cooling the cold-rolled steel sheet from the heating temperature T1 to a temperature T2 of (Ms point + 50)°C or higher and 650°C or lower at an average cooling rate V １ (c) In the secondary cooling treatment, the cold-rolled steel sheet is cooled at an average cooling rate V in a temperature range T3 of not less than the Ms point and not more than (Ms point + 200) ° C. ２ (d) in the tertiary cooling treatment, the galvanized steel sheet is cooled to a cooling stop temperature T4 of 50 to 300°C; (e) in the secondary heating treatment, the galvanized steel sheet is held at a heating temperature T5 of 250 to 400°C for 1 to 150 seconds; (f) in the tertiary heating treatment, the galvanized steel sheet is heated from the heating temperature T5 to a reheating temperature T6 of T5<T6<T5+50°C at an average heating rate V ３ and (g) in the fourth cooling treatment, the steel sheet is recooled without being held at the reheating temperature T6. The method for producing a galvanized steel sheet is characterized in that: E=T6×(6×log((T6−T5) / V)) ３ ) + 15) ... (1)

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