Galvanized steel sheet and method for manufacturing the same

Abstract

Provided are galvanized steel sheets that have high strength, excellent bake hardenability, and excellent impact resistance and crack resistance, and a method for producing the same. A galvanized steel sheet containing, by mass%, C: 0.150-0.450%, Si: 0.50-3.00%, Mn: 1.50-4.00%, 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, wherein the amount of diffusible hydrogen in the steel is 0.60 mass ppm or less, the difference in yield strength before and after forming and baking is 300 MPa or more, and the microstructure in the range of 1 / 8 of the sheet thickness to 3 / 8 of the sheet thickness is, by area%, (i) the sum of tempered martensite and bainite: 55-90%, (ii) fresh martensite with an aspect ratio of 4 or more: 30% or less, (iii) retained austenite: 5-30%, and (iv) the ratio of the area ratio of retained austenite after sub-zero treatment to that before sub-zero treatment is 0.95 or less.

Description

[Technical Field]

[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. [Background technology]

[0002] In recent years, from the viewpoint of protecting the global environment, it has been desired to improve the fuel efficiency of automobiles and reduce the amount of carbon dioxide (CO2) emitted from such automobiles. Reducing the weight of the vehicle body is a very effective way to improve 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 the automobile parts and simplifying the structure of the vehicle body to reduce the number of parts (see Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6777274 [Patent Document 2] Patent No. 7160199 [Patent Document 3] Patent No. 7120461 Summary of the Invention [Problem to be solved by the invention]

[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 part, 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. [Means for solving the problem]

[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 having a chemical composition 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 being 0.60 ppm by mass or less, and the difference in yield strength before and after forming and baking being 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 a 1 / 4 position from the surface of the steel sheet, is, by area%, (i) Tempered martensite and bainite combined: 55-90%; (ii) Fresh martensite with an aspect ratio of 4 or more: 30% or less; (iii) Retained austenite: 5-30%; (iv) A galvanized steel sheet characterized in that the ratio of the area fraction of retained austenite after sub-zero treatment in which the steel is kept 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] Further, 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 Bottom, 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 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] Furthermore, in the zinc-plated steel sheet of the present invention, when the soft layer is a region of the steel sheet having 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 above-mentioned galvanized steel sheet, comprising the steps of hot-rolling a slab of steel having the above-mentioned chemical composition to obtain a hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, then subjecting the cold-rolled steel sheet to a first heating treatment, a first cooling treatment, a second cooling treatment and a galvanizing treatment to obtain a galvanized steel sheet, and subjecting the galvanized steel sheet to a tertiary cooling treatment, a second heating treatment, a third heating treatment and a fourth cooling treatment to produce the galvanized steel sheet, (a) In the first heat treatment, the cold-rolled steel sheet is heated 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, the cold-rolled steel sheet is cooled 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 V1 of 4 ° C. / sec or higher, (c) In the secondary cooling treatment, the cold-rolled steel sheet is cooled in a temperature range T3 of Ms point or higher and (Ms point + 200) °C or lower at an average cooling rate of V20 = 2.0 to 5.0 °C / sec. (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 heat treatment, the galvanized steel sheet is held at a heating temperature T5 of 250 to 400 °C for 1 to 150 sec. (f) In the tertiary heat treatment, the galvanized steel sheet is reheated from the heating temperature T5 to a reheating temperature T6 where T5 < T6 < T5 + 50 °C at an average heating rate V3 of 0.2 to 10.0 °C / sec, and the heat influence index E from the heating temperature T5 to the reheating temperature T6 represented by the following formula (1) satisfies 4000 to 11000. (g) In the quaternary cooling treatment, the galvanized steel sheet is recooled without holding at the reheating temperature T6. A method for manufacturing a galvanized steel sheet, characterized by the above. recording E = T6 × (6 × log((T6 - T5) / V3) + 15) ··· (1)

Effects of the Invention

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

Embodiments for Carrying Out the Invention

[0011] In the galvanized steel sheet according to the present invention, the base steel sheet has the following component composition and microstructure. The galvanized steel sheet according to the present invention has high strength, high bake hardenability, excellent collision resistance and crack resistance characteristics, and sufficient strength against collisions. Therefore, the galvanized steel sheet according to the present invention can be suitably used as a component for transportation equipment 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 a main component, and includes pure galvanized steel sheets in which no metal elements or the like are added to the zinc plating layer, hot-dip galvanized steel sheets (GI) in which alloying elements such as iron, nickel, magnesium, aluminum, etc. are added to zinc, alloyed hot-dip galvanized steel sheets (GA), as well as hot-dip zinc-aluminum alloy-plated steel sheets, hot-dip zinc-aluminum-silicon alloy-plated steel sheets, hot-dip zinc-aluminum-magnesium alloy-plated steel sheets, and the like.

[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) Composition of steel sheets The chemical composition of the steel sheet that serves as the base material for 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:0.150~0.450%> C is a component that generates martensite and increases the strength of the steel sheet. If the C content is too low, the total area ratio of tempered martensite and bainite decreases, resulting in a decrease in collision resistance. Therefore, 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 is the starting point for void generation, increases, resulting in a decrease in crack resistance. Therefore, 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 - 3.00%> Si is an element that suppresses the formation of carbides during heat treatment and affects the stability of retained austenite. From the perspective of ensuring an appropriate amount of retained austenite in the sub-zero treated retained austenite, the Si content should be 0.50% or more, preferably 0.70% or more, more preferably 0.80% or more. On the other hand, if the Si content is too high, the solid solution carbon concentration in the retained austenite will increase excessively, and the area ratio of the retained austenite will become too high. Therefore, the upper limit of the Si content should be 3.00% or less, preferably 2.60% or less, more preferably 2.40% or less.

[0017] <Mn: 1.50 - 4.00%> Mn affects the area ratios of tempered martensite and bainite. From the perspective of obtaining good impact resistance, the Mn content should be 1.50% or more, preferably 1.80% or more, 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 origin of void generation, will increase, resulting in a decrease in crack resistance properties. Therefore, the upper limit of the Mn content should be 4.00% or less, preferably 3.80% or less, more preferably 3.50% or less.

[0018] <P: up to 0.100%> P segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and has an adverse effect on the crack resistance properties during impact. Therefore, the P content should be 0.100% or less, preferably 0.030% or less, more preferably 0.010% or less. On the other hand, the lower limit of the P content is not particularly limited, but preferably 0.001%, more preferably 0.002%, and even more preferably 0.003% from the perspective of production technology constraints.

[0019] <S: up to 0.0200%> S combines with Mn to form coarse MnS, which serves as the origin of void generation and thus has an adverse effect on the crack resistance property. Therefore, the S content should be 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 production technology constraints, it is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0020] <Al: 0.100% or less> Al raises the A3 transformation point. As a result, the ferrite increases, and the area ratios of tempered martensite and bainite decrease. Therefore, the Al content should be 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 in order to suppress the formation of carbides and promote the formation of retained austenite during heat treatment, it is, for example, 0.010%, preferably 0.020%.

[0021] <O: 0.0100% or less> O forms oxides, which serve as the origin of void generation and thus have an adverse effect on the crack resistance property. Therefore, the O content should be 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 serves as the origin of void generation and thus has an adverse effect on the crack resistance property. Therefore, the N content should be 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 production technology constraints, it is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

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

[0024] <B: 0.0100% or less> B can improve the hardenability of the steel sheet by segregating at the austenite grain boundaries. Therefore, B is preferably added because it increases the area ratios of tempered martensite and bainite. However, if the amount of B is too large, it forms (CB)6 and becomes a void generation origin, thus having an adverse effect on the crack resistance property. For this reason, the amount of B is preferably 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0040% or less, and particularly preferably 0.0030% or less. , , The lower limit of the amount of B is not particularly limited, but from the viewpoint of obtaining the addition effect of B, it is, for example, 0.0005% and preferably 0.0010%.

[0025] <Ti: 0.200% or less> Ti preferably is 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 amount of Ti is too large, it combines with N to form coarse nitrides and becomes a void generation origin, thus having an adverse effect on the crack resistance property. For this reason, the amount of Ti is preferably 0.200% or less, more preferably 0.100% or less, still more preferably 0.050% or less. The lower limit of the amount of Ti is not particularly limited, but from the viewpoint of obtaining the addition effect of Ti, it is, for example, 0.005% and preferably 0.010%.

[0026] <Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less> Nb, V and W preferably are added because they increase the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or heat treatment. However, if the addition amounts of these elements are excessively high, they will remain as coarse carbides without dissolving during the heating of the slab. Such coarse carbides serve as the generation origin of voids, resulting in a decrease in crack resistance properties.

[0027] Therefore, the Nb amount 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 Nb amount is not particularly limited, but from the perspective of obtaining the addition effect of Nb, it is, for example, 0.005%, and preferably 0.010%.

[0028] Also, the V amount 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 V amount is not particularly limited, but from the perspective of obtaining the addition effect of V, it is, for example, 0.005%, and preferably 0.010%.

[0029] Also, the W amount 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 W amount is not particularly limited, but from the perspective of obtaining the addition effect of 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 enhance the hardenability of the steel sheet and increase the strength of the steel sheet by suppressing decarburization on the surface of the steel sheet. However, if the amounts of these elements are excessively high, the amount of fresh martensite with an aspect ratio of 4 or more, which serves as the generation origin of voids, increases, resulting in a decrease in crack resistance properties. 〔

[0031] For this reason, the Mo amount 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 Mo amount is not particularly limited, but from the perspective of obtaining the addition effect of 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 still more 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%, preferably 0.020%.

[0033] <Sb: 0.200% or less, Sn: 0.200% or less> Sb and Sn are preferably added because they can suppress decarburization in the surface layer region of the steel sheet caused by nitridation 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 later will be insufficient, resulting in a decrease in crack resistance characteristics.

[0034] Therefore, the Sb content is preferably 0.200% or less, more preferably 0.080% or less, and still more 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%, preferably 0.002%.

[0035] Also, the Sn content is preferably 0.200% or less, more preferably 0.080% or less, and still 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%, preferably 0.002%.

[0036] <Zr: 0.1000% or less, Te: 0.100% or less> Zr and Te can sphericalize the shape of nitrides and sulfides, reduce the generation sites of voids, and improve crack resistance characteristics. However, when the amounts of these elements are excessively large, during heating by hot rolling, coarse precipitates remain in the slab in an undissolved state, thereby deteriorating crack resistance characteristics.

[0037] Therefore, the Zr content is preferably 0.1000% or less, more preferably 0.0800% or less, and still more 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] Also, the Te content is preferably 0.100% or less, more preferably 0.080% or less, and still more preferably 0.050% or less. The lower limit of the Te content 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 can be added preferably because it can increase the strength of the steel sheet by enhancing the hardenability of the steel sheet. However, when the Cu content is excessively high, the occurrence starting points of voids increase due to the increase in Cu inclusions, resulting in deterioration of the crack resistance properties. Therefore, the Cu content is preferably 1.000% or less, more preferably 0.800% or less, and still 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%, and preferably 0.020%.

[0040] <Ni: 1.000% or less> Ni can be added preferably because it can increase the strength of the steel sheet by enhancing the hardenability of the steel sheet. However, when the Ni content is excessively high, the amount of fresh martensite with an aspect ratio of 4 or more, which becomes the occurrence starting point of voids, increases, resulting in deterioration of the crack resistance properties. Therefore, the Ni content is preferably 1.000% or less, more preferably 0.800% or less, and still 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%, and 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 Metal) are preferably added because they can spheroidize the shape of precipitates such as sulfides and oxides, reduce the generation sites of voids, and improve the crack resistance properties. However, when the amounts of these elements are excessively large, the sulfides coarsen, become the generation sites of voids, and the crack resistance properties deteriorate.

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

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

[0044] Similarly, the amount of REM is preferably 0.0100% or less, more preferably 0.0040% or less, and still more preferably 0.0030% or less. The lower limit of the amount of REM is not particularly limited, but from the viewpoint of obtaining the addition effect of 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 can spheroidize the shape of precipitates, reduce the generation sites of voids, and improve the crack resistance properties. However, when the amounts of these elements are excessively large, the coarsened precipitates become the generation sites of voids, and the crack resistance properties deteriorate.

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

[0047] Also, the Ta content 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 Ta content is not particularly limited, but from the perspective of obtaining the addition effect of Ta, it is, for example, 0.01%, preferably 0.02%.

[0048] Also, the Hf content 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 Hf content is not particularly limited, but from the perspective of obtaining the addition effect of Hf, it is, for example, 0.01%, preferably 0.02%.

[0049] Also, 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 perspective of obtaining the addition effect of REM, it is, for example, 0.001%, preferably 0.005%.

[0050] <As: 0.100% or less, Pb: 0.100% or less, Zn: 0.100% or less> As, Pb, and Zn can spheroidize the shape of nitrides and sulfides, reduce the generation origin of voids, and improve the crack resistance characteristics. However, when the amounts of these elements are excessively large, coarse precipitates remain in the slab in an undissolved state during heating by hot rolling, thereby deteriorating the crack resistance characteristics.

[0051] Therefore, the As 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 As content is not particularly limited, but from the perspective of obtaining the addition effect of As, it is, for example, 0.004%, 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 even more 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] <Remainder: Fe and unavoidable impurities> The balance in the composition of the present invention is Fe and unavoidable 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 unavoidable impurities. The total content of Ge, Sr, and Cs contained in the unavoidable 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 present microstructure") will be described. To obtain the galvanized steel sheet according to the present invention, it is not enough to simply satisfy the component composition (1) above. The present 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 following configurations (i) to (iv). Hereinafter, the area ratio is 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 will decrease. Therefore, this total area ratio is set to 90% or less, preferably 88% or less, and more preferably 85% or less.

[0057] (ii) Area ratio 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 ratio of fresh martensite with an aspect ratio of 4 or more is an important constituent element of the present invention in order to improve the crack resistance. Fresh martensite with an aspect ratio of 4 or more is easily fractured by bending, and voids are generated inside the fresh martensite and at the grain boundaries with the soft layer described below, which reduces the crack resistance. Therefore, the area ratio 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 ratio 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 ratio 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 ratio of retained austenite before the sub-zero treatment is 0.95 or less. Unstable retained austenite is an important constituent element of the present invention. The term "unstable retained austenite" refers to retained austenite reduced by subzero treatment, in which the steel is held in liquid nitrogen at -196°C for two hours. This unstable retained austenite transforms into martensite when bending is performed, thereby exhibiting high work hardening capacity. 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 bending and baking to be 300 MPa or more.

[0060] Therefore, the ratio of the area fraction of retained austenite after subzero treatment (holding in liquid nitrogen at −196° C. for 2 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] Such a microstructure may contain, as a structure (remaining structure) other than tempered martensite, bainite, fresh martensite, and retained austenite, known structures such as pearlite, ferrite, iron-based carbonitrides, alloy carbonitrides, and inclusions such as MnS and Al2O3. The area ratio of the remaining structure is preferably 10% or less, more preferably 8% or less, and even more preferably 5% or less. 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 one side of the soft layer present on the steel sheet side from the interface between the base steel sheet and the zinc plating layer: 10 to 150 μm The crack resistance can be improved by providing a soft layer on the steel sheet side from the interface between the base steel sheet and the zinc-plated layer. The soft layer is a region from the interface between the base steel sheet and the zinc-plated layer to the steel sheet side, which has 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 has a thickness of 10 to 150 μm on one side. The soft layer 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 thickness on one side is less than 10 μm, the crack resistance deteriorates. On the other hand, if the soft layer 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) Diffusible hydrogen content in steel: 0.60 mass ppm or less If the amount of diffusible hydrogen in steel is too high, delayed fracture occurs and the crashworthiness decreases. Therefore, the amount of diffusible hydrogen in steel 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 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, working and baking refers to applying a 2% prestrain to a JIS No. 5 test piece and then heat treating it at 170°C for 20 minutes. The yield strength after working and baking is greater than the yield strength before working 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 working 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 converters, electric furnaces, etc. can be used. It is preferable to obtain slabs from molten steel by continuous casting. Slabs may also be obtained by other methods, such as ingot casting and blooming, thin slab continuous casting, etc.

[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, while 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 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 cold-rolled 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 speaking, 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 heating will occur in the two-phase region of ferrite and austenite, resulting in the final microstructure containing ferrite and 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 t1) 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. On the other hand, the upper limit of the heating time t1 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 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. If the dew point of the atmosphere 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 dew point of the atmosphere, but in order to keep the steel sheet strength within a suitable range, the dew point of the atmosphere is preferably 25° C. or lower, more preferably 20° C. or lower.

[0078] (2) Primary cooling treatment Next, a primary cooling process 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 by the following formula (2). Ms=550-350×[C]-40×[Mn]-35×[V]-20×[Cr]-17×[Ni]-10×[Cu]-10×[Mo]-5×[W]+15×[Co]+30×[Al]...(2) In the above formula (2), [M] is the content (unit: mass %) of element M in the above-mentioned component composition (main component composition).

[0079] <Average cooling rate V1: 4℃ / sec or more> In the first cooling treatment, if the average cooling rate V1 from the heating temperature T1 to the temperature T2 is too slow, ferrite transformation occurs during cooling, reducing the total area ratio of tempered martensite and bainite. Therefore, the average cooling rate V1 is set to 4°C / sec or more, preferably 5°C / sec or more, and more preferably 6°C / sec or more. The upper limit of the average cooling rate V1 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, a 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 being subjected to a galvanizing treatment described later.

[0081] <Average cooling rate V2: 0.2~5.0℃ / sec> The secondary cooling treatment removes hydrogen introduced into the cold-rolled steel sheet during the primary heat treatment. At this time, slow cooling of the cold-rolled steel sheet in the temperature range T3, which is equal to or higher than the Ms point and equal to or lower than (Ms point + 200)°C, causes bainite transformation, allowing the total area ratio of tempered martensite and bainite to be controlled. If the cooling rate is too slow, the total area ratio of tempered martensite and bainite will be too high. Therefore, the average cooling rate V2 in the temperature range T3 is set to 0.2°C / sec or higher, preferably 0.3°C / sec or higher, and more preferably 0.4°C / sec or higher.

[0082] On the other hand, if the average cooling rate V2 in the temperature region T3 is too fast, the hydrogen introduced into the steel during the primary heat treatment will be difficult to escape, and the amount of diffusible hydrogen in the steel will increase. Therefore, the upper limit of the average cooling rate V2 is set to 5.0°C / sec or less, preferably 4.5°C / sec or less, and more preferably 4.0°C / sec or less.

[0083] (4) Zinc plating 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 coating 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%, with 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 be transformed 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) Third cooling process Next, the galvanized steel sheet is subjected to a tertiary cooling treatment. <Cooling stop temperature T4: 50~300℃> If the cooling stop temperature T4 is too low, the total area ratio of tempered martensite and bainite increases, resulting in a decrease in crack resistance. 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 with 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, a 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: 250~400℃, heating time: 1~150sec> When 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. Therefore, the secondary heating temperature T5 is set to 250°C or higher, preferably 280°C or higher, more preferably 300°C or higher. Also, from the viewpoint of reducing the amount of diffusible hydrogen in the steel, the heating time is set to 1 sec or more.

[0091] On the other hand, when the heating temperature T5 is high and the heating time is too long, the area ratios of tempered martensite and bainite increase excessively, and the crack resistance property deteriorates. Therefore, the heating temperature T5 is set to 400°C or lower, preferably 380°C or lower, more preferably 350°C or lower. Also, the heating time is set to 150 sec or less.

[0092] (7) Tertiary heat treatment Next, the tertiary heat treatment is carried out. Specifically, the galvanized steel sheet is reheated from the heating temperature T5 at an average heating rate V3 of 0.2 to 10.0°C / sec to a reheating temperature T6 where T5 < T6 < T5 + 50.

[0093] <Average heating rate V3: 0.2 to 10.0°C / sec> If the average heating rate V3 is too slow, the area ratios of tempered martensite and bainite increase excessively, and the crack resistance property deteriorates. Therefore, the average heating rate V3 is set to 0.2°C / sec or more, preferably 0.5°C / sec or more, more preferably 1.0°C / sec or more. On the other hand, if the average heating rate V3 is too fast, the amount of diffusible hydrogen in the steel becomes difficult to escape. Therefore, the average heating rate V3 is set to 10.0°C / sec or less, preferably 9.0°C / sec or less, more preferably 8.0°C / sec or less.

[0094] <Heating temperature T6: T5 < T6 < T5 + 50> When the galvanized steel sheet is reheated to the temperature range of T5 < T6 < T5 + 50, the diffusion of carbon is promoted, and the amount of retained austenite with non-uniform carbon concentration increases. Therefore, it is possible to create retained austenite (unstable retained austenite) that decreases by sub-zero treatment of holding in liquid nitrogen at -196°C for 2 hours.

[0095] <Thermal effect index E from heating temperature T5 to reheating temperature T6: 4000~11000> The thermal effect index E from the heating temperature T5 to the reheating temperature T6 can be expressed by the following formula (1). E=T6×(6×log((T6-T5) / V3)+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 with 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 the steel 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 process is carried out in which the material is cooled again 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. [Example]

[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 then continuously cast 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] [Table 1-1]

[0104] [Table 1-2]

[0105] The obtained slab was hot-rolled under the conditions shown in Tables 2-1 and 2-2 to obtain a hot-rolled steel sheet. Specifically, the slab was heated to 1250°C, and the hot-rolled steel sheet was finish-hot-rolled at 890°C and coiled at 500°C. The hot-rolled steel sheet was then pickled and cold-rolled at a rolling reduction ratio of 50% to obtain a cold-rolled steel sheet (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 t1) was 120 seconds.

[0107] In the hot-dip galvanizing process, both sides of a cold-rolled steel sheet were subjected to hot-dip galvanizing or galvannealed hot-dip galvanizing 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 remainder 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 remainder 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] The coating weight of the hot-dip galvanized layer is 45 to 72 g / m per side when manufacturing GI. 2 When 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 0.1-1.0% Fe, 0.2-1.0% Al, and the balance was Fe and unavoidable impurities. The hot-dip galvanized layer of GA contained 7-15% Fe, 0.1-1.0% 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] [Table 2-1]

[0112] [Table 2-2]

[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 from the surface of the steel plate to 1 / 4 of the plate thickness.

[0115] <Total area ratio of tempered martensite and bainite> The obtained steel sheet was polished so that the cross section (L cross section) parallel to the rolling direction at a position 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). Ten fields of view were observed for the observation surface, and the obtained SEM images were analyzed to determine the total area ratio (unit: %) of tempered martensite and bainite.

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

[0117] <Area ratio 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 1 / 4 of the sheet thickness was the observation surface. The observation surface was then corroded with 1 vol% nital and 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 with 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 with a face-centered cubic (fcc) structure was excluded, and the remaining structure was determined to be fresh martensite.

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

[0119] <Area ratio of retained austenite> The observation surface of the obtained steel sheet (a cross section parallel to the rolling direction at a position 1 / 4 of the sheet thickness) was analyzed using an X-ray diffraction (XRD) device with CoKα as the 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 the area fraction of retained austenite after sub-zero treatment (holding in liquid nitrogen at -196°C for 2 hours) to the area fraction of retained austenite before sub-zero treatment> Ten test pieces for X-ray diffraction were taken parallel to the plate surface from the 1 / 4 position of the plate thickness of the obtained steel plate, and the area fraction of retained austenite was measured for each test piece. Each test piece was subjected to subzero treatment by holding it in liquid nitrogen at -196°C for two 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 retained austenite in the test piece before the subzero treatment was calculated. The average of the area fractions calculated for each of the 10 test pieces was used as the ratio of the area fraction of retained austenite after subzero treatment by holding it in liquid nitrogen at -196°C for two hours to the area fraction of retained austenite in each steel plate before the subzero treatment.

[0121] <Measurement of the soft layer> A specimen 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% by volume of 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 containing 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 methods, and the results are shown in Tables 3-1 and 3-2. <Yield strength evaluation test before and after processing and baking (tensile test)> Ten No. 5 test pieces according to JIS Z 2201 were taken from the obtained steel plate, with the longitudinal direction (tensile direction) at an angle of 90° to the rolling direction. Tensile tests in accordance with JIS Z 2241 were carried out on the five taken test pieces, and the yield strength (YS1) was calculated from the average value of the 0.2% proof stresses of the five pieces. In addition, a 2% pre-strain was applied to the remaining five test pieces (JIS No. 5 test pieces), followed by heat treatment at 170°C for 20 minutes, after which a tensile test was conducted again. If an upper yield point appeared, this was taken as the yield strength (YS2). If no upper yield point appeared, the 0.2% proof stress was taken as the yield strength (YS2). The increase in yield strength, ΔYS (YS2 - YS1), was evaluated as the difference in yield strength before and after processing and baking.

[0123] <Measurement of diffusible hydrogen content in steel> The hot-dip galvanized layer was removed from the obtained steel sheets 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 using thermal desorption analysis. The heating rate was 200°C / hr. The cumulative 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] <Collision resistance and crack resistance evaluation test (V-bend - baking treatment (170°C x 20 min) - orthogonal VDA bending test)> The end surfaces of the obtained steel plates were ground to obtain test pieces measuring 1.4 mm thick x 60 mm wide x 60 mm long. First, the test pieces were subjected to a V (90°) bending process using an L-direction bending (C-axis bending) with a bending radius of R = 5 mm under the following conditions: load 10 ton, stroke speed 30 mm / min, and holding time 5 seconds.

[0125] Next, the V-bent test piece was heat-treated at 170°C for 20 minutes and then rotated horizontally by 90°. A VDA bending test was performed on the test piece using a bending tester at a stroke speed of 20 mm / min, bending in the C-direction (L-axis bending) with the peak of the V-bend facing the punch, and the maximum load F was measured. The bending tester used had a roller distance of 2 × plate thickness + 0.5 mm and a punch tip curvature radius of R = 0.4 mm. In addition, when F≧7000N, it was rated as "◎", when F≧5000N, it was rated as "○", and when F was less than 5000N, it was rated as "×". When it was rated as "◎" or "○", it was evaluated as having 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 ridge line 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 crack length was measured.

[0127] If the total maximum length of the cracks was 6000 μm or less, it was marked with "◎", if the total maximum length of the cracks was more than 6000 μm but less than 9000 μm, it was marked with "○", and if 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 strength and crack resistance.

[0129] [Table 3-1]

[0130] [Table 3-2]

Claims

1. A galvanized steel sheet having a galvanized layer on at least one surface of a base steel sheet, The steel plate contains, in mass %, C: 0.150-0.450%, Si: 0.50-3.00%, Mn: 1.50-4.00%, 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 processing and baking is 300 MPa or more, And, the microstructure in the range of 1 / 8 to 3 / 8 of the plate thickness, centered on the 1 / 4 position from the surface of the steel plate, is, in area%, (i) 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%; (iv) The ratio of the area fraction of retained austenite after sub-zero treatment in which the steel is kept 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. A galvanized steel sheet characterized by:

2. The steel plate comprises, 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, Bi: 0.200% or less, As: 0.100% or less, Pb: 0.100% or less and 2. The galvanized steel sheet according to claim 1, further comprising at least one element selected from the group consisting of Zn: 0.100% or less.

3. 2. The galvanized steel sheet according to claim 1, wherein, when a region of the steel sheet having 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 galvanized layer on the steel sheet side is defined as a soft layer, the soft layer is present in a range of 10 μm or more and 150 μm or less in thickness on one side from the interface on the steel sheet side.

4. 3. The galvanized steel sheet according to claim 2, wherein, when a region of the steel sheet having 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 galvanized layer on the steel sheet side is defined as a soft layer, the soft layer is present in a range of 10 μm or more and 150 μm or less in thickness on one side from the interface on the steel sheet side.

5. A method for producing the galvanized steel sheet according to any one of claims 1 to 4, A method for producing a galvanized steel sheet by hot-rolling a slab of steel having the chemical composition according to claim 1 or 2, cold-rolling the resulting hot-rolled steel sheet to obtain a cold-rolled steel sheet, subjecting the cold-rolled steel sheet to a first heating treatment, a first cooling treatment, a second cooling treatment and a galvanizing treatment to obtain a galvanized steel sheet, and subjecting the galvanized steel sheet to a tertiary cooling treatment, a second heating treatment, a third heating treatment and a fourth cooling treatment, (a) In the primary heat treatment, the cold-rolled steel sheet is heated 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, the cold-rolled steel sheet is cooled 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 of 4 ° C. / sec or higher. １ Cool at (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 Cooling at 0.2 to 5.0°C / sec, (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 heat 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 third heating treatment, the galvanized steel sheet is heated from the heating temperature T5 to a reheating temperature T6 in the range of T5<T6<T5+50°C at an average heating rate V ３ and reheating the material so that the thermal effect index E from the heating temperature T5 to the reheating temperature T6, which is expressed by the following formula (1), satisfies 4000 to 11000; (g) in the fourth cooling treatment, re-cooling is performed without holding at the reheating temperature T6; A method for manufacturing a galvanized steel sheet, characterized by: Note E = T6× (6×log) (T6-T5) / V ３ )+15)・・・(1)

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