Zinc-plated steel sheet, member, and method for manufacturing same

Abstract

The purpose of the present invention is to provide: a zinc-plated steel sheet and a member that are excellent in tensile characteristics and collision characteristics; and a method for manufacturing the zinc-plated steel sheet and the member.　The zinc-plated steel sheet comprises a steel sheet having a prescribed steel structure, and a zinc plating layer on a surface of the steel sheet, wherein: after the zinc-plated steel sheet is subjected to U-bending for a bending radius of R=4 mm at a stroke speed of 1,500 mm / min, a spacer having a thickness of 5 mm is installed on the inner side (compression side) of the bent portion of the zinc-plated steel sheet; when the bent portion has been subjected to bending until the two surfaces of the steel sheet that are present at locations each facing toward an individual one of the upper and lower surfaces of the spacer in the thickness direction comes into close contact with the spacer, no cracks are produced on the outer side (tension side) of the bent portion; the tensile strength is at least 780 MPa and less than 1,180 MPa; and the yield ratio is 65-95%.

Description

Galvanized steel sheets, components, and their manufacturing methods

[0001] The present invention relates to a galvanized steel sheet and a member having high strength and excellent crashworthiness, and a method for producing the same. The galvanized steel sheet of the present invention can be suitably used mainly as a steel sheet for automobiles.

[0002] From the perspective of global environmental conservation, CO ２ In order to reduce emissions, reducing the weight of automobile bodies while maintaining their strength and improving automobile fuel efficiency have always been important challenges in the automotive industry. To achieve this while maintaining the strength of automobile bodies, it is effective to increase the strength of steel sheets used as raw materials for automobile parts, thereby reducing their thickness. On the other hand, the premise of automobile parts made from steel sheets is to ensure the safety of people inside the vehicle in the event of a collision. Therefore, high-strength galvanized steel sheets used as raw materials for automobile parts are required to have not only the desired strength but also excellent crashworthiness.

[0003] In recent years, the use of high-strength galvanized steel sheets with a tensile strength (TS) of 780 MPa or more has been expanding in automobile bodies. From the perspective of crashworthiness, automotive components are broadly divided into non-deformable components such as pillars and bumpers and energy-absorbing components such as side members. Each component is required to have the necessary crashworthiness to ensure the safety of occupants in the event of a collision while the vehicle is in motion. Progress has been made in increasing the strength of non-deformable components, and high-strength galvanized steel sheets with a tensile strength (TS) of 780 MPa or more have already been put into practical use. However, when applied to energy-absorbing components, high-strength galvanized steel sheets with a tensile strength of 780 MPa or more are prone to component fracture during a collision, originating from the area subjected to primary processing (forming). Therefore, they have the problem of being unable to stably absorb collision energy, and materials with a tensile strength of 590 MPa or less are mainly used. Therefore, there is still room for the development of energy-absorbing components that can suppress component fracture during a collision and stably absorb high energy, thereby ensuring safety during a collision while contributing to environmental conservation through weight reduction. For these reasons, it is necessary to use high-strength galvanized steel sheets with excellent collision resistance and a TS of 780 MPa or more for the energy absorbing members.

[0004] In response to such demands, for example, Patent Document 1 discloses a technology relating to a high-strength galvanized steel sheet having a maximum tensile strength of 780 MPa or more and applicable to an impact absorbing member during a collision. Also, Patent Document 2 discloses a patent relating to a galvanized steel sheet having a tensile strength of 980 MPa or more, good fracture resistance, and good energy absorption, which has excellent collision characteristics.

[0005] JP 2015-175061 A JP 2022-34015 A

[0006] However, in Patent Document 1, regarding crash characteristics, crack determination is performed on the hat material in an axial crush test to evaluate fracture resistance characteristics. However, because the crack determination is performed after crushing, it is not possible to evaluate the process from the occurrence of cracks during crushing to fracture, which is important for crash characteristics. The reason for this is that if a crack occurs early in the crushing process, even a minor crack that does not penetrate the plate thickness may reduce the absorbed energy, but it is difficult to determine such a minor crack after crushing. Furthermore, if a crack occurs late in the crushing process, even a large crack that penetrates the plate thickness may have little effect on the absorbed energy. Therefore, it is considered that only determining cracks after crushing is insufficient for evaluating fracture resistance characteristics.

[0007] Furthermore, in Patent Document 2, the fracture resistance of a steel sheet is evaluated for crashworthiness by measuring the void number density in a cross section within a 0-50 μm region from the surface of the steel sheet on the compression side when the steel sheet is bent 90° at a stroke speed of 20 mm / min, and by measuring the stroke at maximum load on the stroke-load curve when the steel sheet is bent at an orthogonal bending speed of 20 mm / min. The absorbed energy is evaluated by an axial crushing test of a hat-shaped member. However, the fracture resistance required for an automotive energy absorption component is the fracture resistance when the component is deformed at high speed. Therefore, evaluation of fracture resistance by a bending test and a bending-orthogonal bending test at a stroke speed of 20 mm / min is insufficient for evaluating the fracture resistance of a material for an automotive energy absorption component that deforms at high speed. In other words, the fracture resistance is not necessarily sufficient for a material for an automotive energy absorption component.

[0008] Thus, there has been a demand for the establishment of a new technology for producing galvanized steel sheets that are excellent in tensile properties including tensile strength TS, and also excellent in collision properties.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a galvanized steel sheet, a member, and a method for manufacturing the same, which are suitable for use as an energy absorbing member for an automobile and have excellent tensile properties and crashworthiness.

[0010] Here, "excellent tensile properties" refers to a tensile test conducted in accordance with JIS Z2241 (2011) where the required tensile strength TS is 780 MPa or more but less than 1180 MPa and the elongation El is 11% or more. Furthermore, "excellent crash properties" refers to a high load-bearing capacity, good energy absorption capacity, and good fracture resistance. "High load-bearing capacity" refers to a tensile test conducted in accordance with JIS Z2241 (2011) where the required yield ratio YR is 65% or more but less than 95%. Good energy absorption capacity means that a tensile test is conducted in accordance with the provisions of JIS Z2241 (2011), and the value B (hereinafter, see formula (2)) obtained by dividing the area surrounded by the stress P (MPa)-strain Q (%) curve in the strain Q range of 0 to 4%, the line of stress P = 0, and strain Q = 4 by the tensile strength TS is 3.15 (%) or more.

[0011] In the formula, P: stress (MPa), Q: strain (%), TS: tensile strength (MPa).

[0012] "Good fracture resistance" means that when a galvanized steel sheet is subjected to a U-bending process (high-speed U-bending process) and a contact bending test (high-speed contact bending test) under specific conditions, the length of the crack formed on the outer side (tensile side) of the bent portion is less than 0.5 mm. Here, the U-bending process is performed with a bending radius R of 4 mm and a stroke speed of 1500 mm / min. The contact bending test is performed with a spacer sandwiched between the galvanized steel sheets having a thickness of 5 mm, a stroke speed of 1500 mm / min, a pressing load of 10 tons, and a pressing time of 3 seconds.

[0013] The present inventors conducted extensive research to solve the above-mentioned problems and found the following: A galvanized steel sheet includes a steel sheet having a steel structure in which, by area percentage, ferrite is less than 30.0% (including 0.0%), bainite is more than 40.0% and not more than 80.0%, the total of tempered martensite and bainite is more than 60.0%, retained austenite is 1.5% or more and 20% or less, and fresh martensite is 20.0% or less, and a galvanized layer is formed on the surface of the steel sheet. In addition, in the above-mentioned galvanized steel sheet, when the area ratio of the ferrite is more than 0.0%, the area ratio of high Mn ferrite (HF) in which the Mn concentration is more than 0.80 times the Mn concentration of the steel sheet is 30% or more relative to the total area ratio of ferrite, the area ratio of the hard second phase consisting of retained austenite and fresh martensite, the hard second phase being in contact with bainite over 30% or more of its circumferential length is 30% or more relative to the total area ratio of the hard second phase consisting of retained austenite and fresh martensite, and the above-mentioned galvanized steel sheet is subjected to U-bending with a bending radius R of 4 mm by straw bending. After bending at a stroke speed of 1500 mm / min, a 5 mm thick spacer was placed on the inside (compression side) of the bent portion of the galvanized steel sheet, and the bent portion was bent at a stroke speed of 1500 mm / min, a pressing load of 10 ton, and a pressing time of 3 seconds until the two surfaces of the galvanized steel sheet located opposite the upper and lower surfaces in the thickness direction of the spacer came into close contact with the spacer. When this bending was performed, no cracks of 0.5 mm or more were generated on the outside (tension side) of the bent portion. It was found that this resulted in a steel sheet with excellent tensile properties and crashworthiness.

[0014] The present invention was made based on these findings, and the gist of the present invention is as follows. [1] A galvanized steel sheet comprising a steel sheet and a galvanized layer on a surface of the steel sheet, wherein the steel sheet has a steel structure having, in area ratios at a 1 / 8 to 3 / 8 position of the sheet thickness, ferrite: less than 30.0% (inclusive 0.0%), bainite: more than 40.0% and not more than 80.0%, total of tempered martensite and bainite: more than 60.0%, retained austenite: 1.5% or more and not more than 20%, and fresh martensite: 20.0% or less (inclusive 0.0%), and when the ferrite is more than 0.0% in area ratio, the area ratio of high-Mn ferrite having an Mn concentration more than 0.80 times the Mn concentration of the steel sheet is 30% or more relative to the total area ratio of the ferrite, and the area ratio of the hard second phase, which is in contact with bainite over 30% or more of its circumferential length, is 30% or more relative to the total area ratio of the hard second phase consisting of the retained austenite and the fresh martensite, the zinc-plated steel sheet is subjected to a U-bending process with a bending radius R of 4 mm at a stroke speed of 1500 mm / min, and then a spacer having a thickness of 5 mm is placed inside the bent portion of the zinc-plated steel sheet, and the bent portion is bent at a stroke speed of 1500 mm / min, a pressing load of 10 ton, and a pressing time of 3 seconds until two surfaces of the zinc-plated steel sheet that are located opposite each other in the thickness direction of the spacer come into close contact with the spacer; the zinc-plated steel sheet has a crack of less than 0.5 mm on the outside of the bent portion, a tensile strength of 780 MPa or more and less than 1180 MPa, and a yield ratio of 65% or more and 95% or less. [2] The galvanized steel sheet according to [1], wherein the steel sheet has a chemical composition containing, by mass%, C: 0.05 to 0.20%, Si: 0.10 to 2.00%, Mn: 2.0 to 3.5%, P: 0.050% or less, S: 0.050% or less, sol. Al: 0.005 to 2.000%, and N: 0.010% or less, with the balance being Fe and unavoidable impurities.[3] The composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, The galvanized steel sheet according to [2] above, containing at least one selected from Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. [4] The galvanized steel sheet according to any one of [1] to [3] above, wherein the galvanized layer is an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed hot-dip galvanized layer. [5] A member obtained by subjecting the galvanized steel sheet according to any one of [1] to [4] above to at least one of forming and welding.[6] A hot rolling process in which a steel slab having the chemical composition according to [2] or [3] is hot rolled at a finish rolling temperature of 850 to 950°C and coiled at a coiling temperature of 600°C or less; a cold rolling process in which the hot rolled steel sheet after the hot rolling process is cold rolled at a reduction rate of more than 20%; an annealing process in which the cold rolled steel sheet after the cold rolling process is heated to an annealing temperature of 750°C or more under conditions where A represented by the following formula (1) satisfies 4 to 70 and held at the annealing temperature for 30 seconds or more; and a plating process in which, after the annealing process, the steel sheet is cooled to a temperature range of 300 to 600°C and held at the temperature range for 10 to 300 seconds, and then zinc plating is carried out on the steel sheet surface. The method for producing a galvanized steel sheet includes: a quenching and tempering step of cooling the steel sheet to a cooling stop temperature of (Ms-300°C) to (Ms-50°C) after the plating step, and then holding the steel sheet at a tempering temperature of 250 to 500°C for 20 to 500 seconds; and a cooling step of cooling the steel sheet from the tempering temperature to 50°C at an average cooling rate of 20°C / s or more after the quenching and tempering step. In the formula, T Ａｃ1 : Temperature at Ac1 point (°C), T ＲＴ : Annealing temperature (℃), t ＲＴ : annealing temperature T ＲＴ Time (seconds) required to reach t Ａｃ1 : Temperature T at Ac1 point after starting heat treatment in the annealing process Ａｃ1 where T(t) is the time (seconds) required for the temperature to reach t, ​​and T(t) is the temperature (°C) t seconds after the start of the heat treatment in the annealing step. [7] The method for producing a galvanized steel sheet according to [6] above, wherein the galvanizing treatment is a treatment of subjecting a steel sheet surface to electrogalvanizing, hot-dip galvanizing, or galvannealed hot-dip galvanizing. [8] A method for producing a component, comprising subjecting the galvanized steel sheet according to any one of [1] to [4] above to at least one of forming and welding.

[0015] According to the present invention, a galvanized steel sheet having excellent tensile properties and crashworthiness can be obtained. A member obtained by subjecting the galvanized steel sheet of the present invention to forming, welding, or the like can be suitably used as an energy absorbing member in the automotive field.

[0016] The present invention will be described in detail below.

[0017] <Galvanized steel sheet> The galvanized steel sheet of the present invention comprises a steel sheet having a steel structure at a 1 / 8 to 3 / 8 position of the sheet thickness, in area percentages, of ferrite: less than 30.0% (inclusive 0.0%), bainite: more than 40.0% and not more than 80.0%, the total of tempered martensite and bainite: more than 60.0%, retained austenite: 1.5% to 20% and fresh martensite: 20.0% or less (inclusive 0.0%), and a galvanized layer on a surface of the steel sheet, wherein when the area percentage of ferrite exceeds 0.0%, the area percentage of high-Mn ferrite (HF) having a Mn concentration exceeding 0.80 times the Mn concentration of the steel sheet is 30% or more of the total area percentage of ferrite, and among hard second phases consisting of retained austenite and fresh martensite, the area percentage of the hard second phase which is in contact with bainite over 30% or more of its circumferential length is 30% or more of the total area percentage of the hard second phase, A galvanized steel sheet is subjected to U-bending with a bending radius R of 4 mm at a stroke speed of 1500 mm / min, and then a spacer having a thickness of 5 mm is placed on the inside (compression side) of the bent part of the galvanized steel sheet. The bent part is then bent at a stroke speed of 1500 mm / min, a pressing load of 10 ton, and a pressing time of 3 seconds until the two surfaces of the galvanized steel sheet that face the upper and lower surfaces in the thickness direction of the spacer come into close contact with the spacer. When this is done, cracks of less than 0.5 mm are generated on the outside (tension side) of the bent part, the tensile strength is 780 MPa or more and less than 1180 MPa, and the yield ratio is 65% or more and 95% or less.

[0018] Ferrite area ratio: less than 30.0% (including 0.0%). Ferrite is effective in improving elongation, but it reduces yield strength (YS) and tensile strength (TS) (hereinafter simply referred to as TS). Furthermore, the increase in the hardness difference region between the soft ferrite and the adjacent structure makes it easier for voids to form and connect during high-speed impact, thereby reducing the fracture resistance during high-speed impact. As a result, if the ferrite area ratio is 30.0% or more, it becomes difficult to satisfy all of the following requirements: a yield ratio (YR) of 65% or more, a TS of 780 MPa or more, and fracture resistance during high-speed impact. Therefore, the ferrite area ratio is less than 30.0%, preferably 20.0% or less, and more preferably less than 10.0%. Although there is no particular lower limit, the ferrite area ratio is preferably 1.0% or more, more preferably 3.0% or more.

[0019] Bainite Area Fraction: More than 40.0% and Not More than 80.0% Bainite is an intermediate hardness phase, and when formed at an area fraction of more than 40.0%, it reduces the hardness difference with adjacent martensite and ferrite, suppresses void formation during high-speed impact, and improves high-speed impact properties. Furthermore, bainite improves elongation. If the bainite area fraction is 40.0% or less, these effects are not fully achieved, making it difficult to achieve a TS of 780 MPa or more, an elongation of 11% or more, and high-speed impact properties. Therefore, the bainite area fraction is more than 40.0%, preferably more than 45.0%, and more preferably more than 50.0%. Furthermore, if the bainite area fraction is more than 80.0%, it is difficult to obtain the desired tensile strength. Therefore, the bainite area fraction is 80.0% or less, preferably 70.0% or less, and more preferably 65.0% or less.

[0020] Total area ratio of tempered martensite and bainite: greater than 60.0% Tempered martensite and bainite are effective in improving strength while improving high-speed impact properties by suppressing component fracture during impact deformation. If the total area ratio of tempered martensite and bainite is 60.0% or less, this effect cannot be fully achieved, making it difficult to achieve both a TS of 780 MPa or more and high-speed impact properties. Therefore, the total area ratio is greater than 60.0%, preferably greater than 65.0%, and more preferably greater than 82.0%. Furthermore, although there is no upper limit to the total area ratio, it is preferable that the total area ratio be 95.0% or less, taking into account the balance with other structures.

[0021] Area Fraction of Retained Austenite: 1.5% or More and 20% or Less Retained austenite is effective in delaying the occurrence of cracks during a collision and improving collision characteristics. Although the mechanism is unclear, it is thought to be as follows: Retained austenite work-hardens during collision deformation, increasing the radius of curvature during bending deformation, thereby dispersing strain in the bent portion. Dispersing strain alleviates stress concentration in void-generated areas due to primary processing, resulting in improved collision characteristics. If the area fraction of retained austenite is less than 1.5%, this effect may not be achieved. Furthermore, if the area fraction of retained austenite is less than 1.5%, elongation decreases. Therefore, the area fraction of retained austenite is 1.5% or more, preferably 3% or more, and more preferably 5% or more. On the other hand, if the area fraction of retained austenite exceeds 20%, fresh martensite formed by stress-induced transformation may reduce fracture resistance during a collision. Therefore, the area ratio of retained austenite is 20% or less, preferably 15% or less, and more preferably 10% or less.

[0022] Fresh martensite: 20.0% or less (including 0.0%) Fresh martensite is effective for increasing strength. However, voids are likely to occur at the grain boundaries with the soft phase, and if the area fraction of fresh martensite exceeds 20.0%, impact properties may be reduced. Therefore, the area fraction of fresh martensite is 20.0% or less (including 0.0%), preferably 15.0% or less, and more preferably 10.0% or less.

[0023] High Mn ferrite (HF) having a Mn concentration greater than 0.80 times that of the steel sheet: 30% or more (including 100.0%) of the total ferrite area fraction. Low Mn ferrite (LF) having a Mn concentration less than 0.80 times that of the steel sheet has a lower dislocation density and is relatively softer than high Mn ferrite (HF) having a Mn concentration greater than 0.80 times that of the steel sheet. To suppress fracture during high-speed collisions, it is necessary to reduce the amount of high hardness difference regions and the hardness difference. If the area fraction of relatively soft low Mn ferrite (LF) is 70% or less of the total ferrite area, i.e., if the area fraction of high Mn ferrite (HF) is 30% or more, a TS of 780 MPa or more and excellent high-speed collision properties can be obtained. Therefore, when the area fraction of ferrite exceeds 0.0%, the area fraction of high Mn ferrite (LF) is set to 30% or more (including 100.0%) of the area fraction of total ferrite. The area fraction of high Mn ferrite is preferably 50% or more, more preferably 70% or more, of the area fraction of total ferrite. The area fraction of low Mn ferrite (LF) is set to 70% or less (including 0.0%) of the area fraction of total ferrite. The area fraction of low Mn ferrite relative to the area fraction of total ferrite is preferably 50% or less, more preferably 30% or less. Here, low Mn ferrite (LF) may include recrystallized ferrite (RF) formed during temperature rise in heat treatment, and may consist of this recrystallized ferrite. Furthermore, high Mn ferrite (HF) may include cooling-formed ferrite (TF) formed during cooling, and may consist of this cooling-formed ferrite.

[0024] Area fraction of the hard second phase in contact with bainite over 30% or more of its perimeter: 30% or more of the area fraction of the total hard second phase. Among the island regions constituting the hard second phase consisting of retained austenite and fresh martensite, the island regions in contact with bainite over 30% or more of its perimeter have a high solute C concentration and high stability of the retained austenite. Therefore, the island regions in contact with bainite over 30% or more of its perimeter play an important role in ensuring good work hardening ability and ductility. In other words, if bainite is formed under appropriate conditions during cooling after annealing, solute C diffuses from the bainite to the surrounding untransformed austenite, and only the region of the untransformed austenite in contact with bainite can be locally enriched in solute C. Then, by performing reheating under appropriate conditions in this state, a hard second phase with a high solute C concentration is formed around or within the bainite. Therefore, the retained austenite contained in the hard second phase with a high solute C concentration around or inside the bainite is highly stable and plays an important role in ensuring good work hardening ability and ductility. In the present invention, ensuring ductility means that the elongation (El) is 11% or more. From the above, the total area fraction of the hard second phases, which are in contact with bainite over 30% or more of their circumferential length, is 30% or more, preferably 40% or more, and more preferably 60% or more of the total area fraction of all the hard second phases. The upper limit is not particularly limited and may be 100%.

[0025] The area ratio of each structure refers to the ratio of the area of ​​each phase to the observed area. The area ratio of each structure is measured as follows. A cross section of a steel plate cut perpendicular to the rolling direction is polished and then corroded with 3 volume % nital. Three fields of view at a position 1 / 4 of the plate thickness are photographed with an SEM (scanning electron microscope) at a magnification of 1500 times and with a field of view of 85 μm × 64 μm. The area ratio of each structure is determined from the obtained image data using Image-Pro manufactured by Media Cybernetics. The average value of the area ratios of the three fields of view is defined as the area ratio of each structure in the present invention. In the image data, ferrite, bainite, retained austenite, fresh martensite, and tempered martensite are distinguished as follows. Ferrite: Black region that does not contain retained austenite or fresh martensite structures with an aspect ratio of 2.0 or more within the crystal grains; Bainite: Black region that contains one or more of retained austenite and fresh martensite structures with an aspect ratio of 2.0 or more within the crystal grains; Tempered martensite: Light gray region that contains one or more of retained austenite and fresh martensite structures with an aspect ratio of 2.0 or more and also contains carbides; Retained austenite and fresh martensite: White region.

[0026] Here, it is difficult to distinguish between fresh martensite and retained austenite in an SEM image, so the area fraction of fresh martensite is determined by subtracting the area fraction of retained austenite, determined by the method described below, from the total area fraction of fresh martensite and retained austenite.

[0027] In the present invention, the area fraction of retained austenite is determined by measuring the X-ray diffraction intensity to determine the volume fraction of retained austenite, and this volume fraction is considered to be the area fraction of retained austenite. The volume fraction of retained austenite is determined as the ratio of the integrated X-ray diffraction intensities of the (200), (220), and (311) planes of fcc iron to the integrated X-ray diffraction intensities of the (200), (211), and (220) planes of bcc iron in the quarter-thickness plane.

[0028] In the present invention, the average Mn concentration of the steel sheet is calculated from numerical data of the Mn concentration obtained by mapping analysis using FE-EPMA, and the area fraction of low-Mn ferrite is determined by calculating the area fraction of the Mn concentration that is 0.80 times or less of the average Mn concentration of the steel sheet. The area fraction of high-Mn ferrite is determined by subtracting the obtained area fraction of low-Mn ferrite from the total area fraction of ferrite. Note that the low-Mn ferrite region is formed by the distribution of Mn from ferrite to austenite during the annealing process (heating process and annealing and holding process), and the Mn concentrations of bainite, martensite, tempered martensite, and austenite formed from austenite during the subsequent cooling process always exceed 0.80 times the Mn concentration of the base material.

[0029] Cracks occurring on the outer side (tensile side) of the bent portion after high-speed U-bending + high-speed contact bending test: less than 0.5 mm. The zinc-plated steel sheet of the present invention was subjected to a U-bending (high-speed U-bending) with a bending radius R of 4 mm at a stroke speed of 1500 mm / min, followed by a contact bending test (high-speed contact bending test) in which a 5 mm-thick spacer was placed on the inner side (compression side) of the bent portion of the zinc-plated steel sheet, and the bent portion was bent (stroke speed: 1500 mm / min, pressing load: 10 tons, pressing time: 3 seconds) until the two surfaces of the zinc-plated steel sheet located opposite the upper and lower surfaces in the thickness direction of the spacer were in close contact with the spacer. When this test was performed, no cracks of 0.5 mm or more occurred on the outer side (tensile side) of the bent portion. This resulted in high crashworthiness. If cracks of 0.5 mm or more occurred, the fracture resistance was insufficient, resulting in insufficient crashworthiness due to the initiation and propagation of cracks during impact. Here, the U-bending process can be completed and the spacer placed inside the bent portion of the galvanized steel sheet when the angle formed by the opposing steel sheet surfaces is 0 to 20° (= 10 ± 10°). The stroke speed can be measured, for example, by measuring the stroke per certain time using a scale and converting it into the movement amount per 60 seconds. The movement amount can be the relative movement amount of one end of the steel sheet to the other end when viewing the steel sheet to be bent as a U-shape. The stroke measurement can be performed at any position on the bending tester that moves integrally with the press fitting pressed against the steel sheet. The time can be adjusted within a range that allows normal measurement; for example, a stroke for 5 seconds can be measured.

[0030] Tensile strength: 780 MPa or more and less than 1180 MPa Yield ratio: 65% or more and 95% or less The galvanized steel sheet of the present invention has a tensile strength of 780 MPa or more and less than 1180 MPa and a yield ratio of 65% or more and 95% or less, and exhibits high load-bearing capacity. If the yield ratio exceeds 95%, the fracture resistance property will decrease.

[0031] Desired impact properties can be obtained by controlling the heating rate and temperature before annealing and the cooling rate after annealing, as described below, and by performing plating before the quenching and tempering processes. Controlling the heating rate and temperature before annealing makes it possible to control the amount of low-Mn ferrite (LF), high-Mn ferrite (HF), and bainite produced during pre-quenching. This allows high-Mn ferrite (HF), which is harder than low-Mn ferrite (LF), to be formed. Bainite is produced during pre-quenching and softened during the plating and tempering processes, reducing the hardness difference between ferrite and bainite and suppressing the generation of voids at the interface between ferrite and bainite. As a result, the crack length can be suppressed to less than 0.5 mm (including 0 μm). The absence of ferrite suppresses the generation of voids due to differences in the hardness of the structure, improving impact properties, so ferrite is not necessary. Furthermore, by increasing the cooling rate after tempering, the bainite formed in the tempering process is prevented from softening due to tempering during cooling, and by performing plating before the tempering process, the softening due to tempering during plating is prevented, thereby preventing void formation at the interface with the hard fresh martensite. As a result, it is possible to suppress the crack length to less than 0.5 mm (including 0 μm).

[0032] The crack length in the present invention refers to the length of a crack that occurs on the outside (tensile side) of the bent portion when a U-bend with a bending radius R of 4 mm is performed at a stroke speed of 1,500 mm / min, a spacer with a thickness of 5 mm is placed on the inside (compression side) of the bent portion of the galvanized steel sheet, and the bent portion is continued until the two surfaces of the galvanized steel sheet that are located at positions opposite each other on the upper and lower surfaces in the thickness direction of the spacer come into close contact with the spacer. The crack length is measured as follows. A galvanized steel sheet is subjected to high-speed U-bending with a bending radius R of 4 mm at a stroke speed of 1500 mm / min, and then a close contact bending test (high-speed close contact bending test) is carried out in which a spacer having a thickness of 5 mm is placed on the inside (compression side) of the bent part of the galvanized steel sheet, and the bent part is bent at a stroke speed of 1500 mm / min, a pressing load of 10 ton, and a pressing time of 3 seconds until the two surfaces of the steel sheet that are located opposite the upper and lower surfaces in the thickness direction of the spacer come into close contact with the spacer. The length of any cracks that occur on the outside (tensile side) of the bent part is measured by visual observation of the appearance.

[0033] The galvanized steel sheet of the present invention has a galvanized layer on the surface of the steel sheet. The galvanized layer is, for example, an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed layer.

[0034] The galvanized steel sheet of the present invention has excellent tensile properties. In the present invention, "excellent tensile properties" refers to high strength and excellent ductility. The tensile strength TS of the galvanized steel sheet of the present invention is 780 MPa or more and less than 1180 MPa. In the present invention, "high strength" refers to a tensile strength TS of 780 MPa or more and less than 1180 MPa. In the present invention, "excellent ductility" refers to an elongation El of 11% or more. The methods for measuring tensile strength TS and elongation El are as described in the examples.

[0035] The thickness of the galvanized steel sheet of the present invention is preferably 0.2 mm or more and 3.2 mm or less, from the viewpoint of effectively obtaining the effects of the present invention.

[0036] The galvanized steel sheet of the present invention has excellent crashworthiness. In the present invention, "excellent crashworthiness" means high load-bearing capacity, good fracture resistance, and good energy absorption. In the present invention, "high load-bearing capacity" means a yield ratio YR of 65% or more and 95% or less. The method for measuring the yield ratio YR is as described in the Examples. In the present invention, "good fracture resistance" means that the crack length when subjected to U-bending and close bending as described in the Examples is less than 0.5 mm. In the present invention, "good crashworthiness" means that, when a tensile test as described in the Examples is conducted, the area enclosed by the stress P (MPa)-strain Q (%) curve in the strain Q range of 0 to 4%, the line at stress P = 0, and the line at strain Q = 4 during the tensile test is divided by the tensile strength (TS), and the value B is 3.15% or more.

[0037] (Composition of Steel Sheet) Next, a preferred composition of the steel sheet constituting the galvanized steel sheet will be described. Note that "%" representing the content of component elements means "mass %" unless otherwise specified.

[0038] C: 0.05 to 0.20% C is an element necessary for improving strength because it facilitates the formation of phases other than ferrite and forms alloy compounds with Nb, Ti, and the like. If the C content is less than 0.05%, the desired strength may not be achieved even when the manufacturing conditions are optimized. Therefore, the C content is preferably 0.05% or more, and more preferably 0.07% or more. On the other hand, if the C content exceeds 0.20%, the strength of martensite increases excessively, and the impact properties of the present invention may not be achieved even when the manufacturing conditions are optimized. Therefore, the C content is preferably 0.20% or less, and more preferably 0.18% or less.

[0039] Si: 0.10 to 2.00% Si suppresses carbide formation, thereby contributing to the stabilization of retained austenite. Si is also a solid-solution strengthening element, contributing to an improved balance between strength and ductility. To achieve this effect, the Si content is preferably 0.10% or more, more preferably 0.50% or more. On the other hand, if the Si content exceeds 2.00%, the tempering of martensite is suppressed, increasing the difference in hardness between the tempered martensite and ferrite. Even if the microstructure of the present invention is satisfied, the desired yield ratio may not be obtained. Therefore, the Si content is preferably 2.00% or less, more preferably 1.50% or less.

[0040] Mn: 2.0 to 3.5% Mn is a martensite-forming element and also a solution strengthening element. Mn also contributes to stabilizing retained austenite. To achieve these effects, the Mn content is preferably 2.0% or more. The Mn content is more preferably 2.5% or more. On the other hand, if the Mn content exceeds 3.5%, the retained austenite fraction increases excessively, which may result in a deterioration in impact properties. Therefore, the Mn content is preferably 3.5% or less, more preferably 3.3% or less.

[0041] P: 0.050% or less P is an element effective in strengthening steel. However, if the P content exceeds 0.050%, the alloying rate may be significantly delayed. Furthermore, if the P content exceeds 0.050%, embrittlement may occur due to grain boundary segregation, and even if the steel structure of the present invention is satisfied, fracture resistance during collision may be deteriorated. Therefore, the P content is preferably 0.050% or less, more preferably 0.010% or less. There is no particular lower limit for the P content, but the lower limit currently industrially feasible is about 0.002%, which is substantially higher.

[0042] S: 0.050% or less S forms inclusions such as MnS, which can cause cracks along the metal flow of the weld, and even if the steel structure of the present invention is met, collision properties may be reduced. Therefore, the S content should be as low as possible, but from the perspective of production costs, the S content is preferably 0.050% or less. The S content is more preferably 0.010% or less. There is no particular lower limit for the S content, but the lower limit currently industrially feasible is about 0.0002%, which is substantially higher.

[0043] Sol. Al: 0.005 to 2.000% Al acts as a deoxidizer and is also a solid-solution strengthening element. If the sol. Al content is less than 0.005%, these effects may not be obtained, and strength may decrease even if the steel structure of the present invention is met. Therefore, the sol. Al content is preferably 0.005% or more. On the other hand, if the sol. Al content exceeds 2.000%, the slab quality during steelmaking may deteriorate. Furthermore, if the sol. Al content exceeds 2.000%, fracture resistance may decrease even if the steel structure of the present invention is met. Therefore, the sol. Al content is preferably 2.000% or less, and more preferably 1.000% or less.

[0044] N: 0.010% or less N forms coarse nitrides, which can become the starting point for void formation during impact deformation and may reduce impact properties. Therefore, the N content should be as small as possible, but from the viewpoint of production costs, the N content is preferably 0.010% or less, more preferably 0.006% or less. The lower limit of the N content is not particularly limited, but the lower limit currently industrially feasible is about 0.0003%, which is substantially higher.

[0045] The steel sheet according to the present invention preferably has a composition containing the above-mentioned constituent elements as basic components, with the balance being iron (Fe) and unavoidable impurities.

[0046] The steel sheet according to the present invention may contain the following components (optional elements) as appropriate depending on the desired properties.

[0047] Nb: 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during high-speed bending tests, which may prevent good fracture resistance. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0048] Ti: 0.200% or less Like Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during high-speed bending tests, which may prevent good fracture resistance. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0049] V: 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling and annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.003% or more. The V content is even more preferably 0.005% or more. On the other hand, if the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during high-speed bending tests, which may prevent good fracture resistance. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0050] B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. Furthermore, B controls the formation and grain growth of ferrite during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. The B content is even more preferably 0.0003% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. Furthermore, during high-speed bending tests, voids may form and cracks may grow from the martensite, potentially preventing good fracture resistance. Therefore, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

[0051] Cr: 1.000% or less Cr is an element that improves hardenability, and the addition of Cr generates an appropriate amount of martensite, thereby increasing TS and YS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.0010% or more. The Cr content is even more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.000%, the area ratio of martensite increases, and during high-speed bending tests, voids may form and cracks may grow from the martensite, potentially preventing the desired fracture resistance. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.250% or less, and even more preferably 0.100% or less.

[0052] Ni: 1.000% or less Ni is an element that improves hardenability, and the addition of Ni results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.010% or more. The Ni content is even more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.000%, the area ratio of martensite increases, and during high-speed bending tests, voids may form and cracks may grow from the martensite, potentially preventing the desired fracture resistance. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.

[0053] Mo: 1.000% or less Mo is an element that improves hardenability, and the addition of Mo results in the formation of a large amount of tempered martensite, thereby increasing TS and YS. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.000%, the area ratio of martensite increases, and during high-speed bending tests, voids may form and cracks may grow from the martensite, potentially preventing good fracture resistance. Therefore, when Mo is contained, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, and even more preferably 0.400% or less.

[0054] Sb: 0.200% or less Sb can be added as needed to suppress nitriding and oxidation of the steel sheet surface and decarburization of the region near the steel sheet surface. Suppressing such nitriding and oxidation prevents a decrease in the amount of martensite formed on the steel sheet surface, thereby improving impact performance. However, if Sb exceeds 0.200%, impact performance may be reduced due to grain boundary embrittlement. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. Since the effects of the present invention can be obtained even with a low Sb content, the lower limits of each content are not particularly limited. To more effectively obtain the effect of improving impact performance, the Sb content is preferably 0.003% or more. The Sb content is more preferably 0.005% or more.

[0055] Sn: 0.200% or less Like Sb, Sn can be added as needed to suppress nitriding and oxidation of the steel sheet surface and decarburization of the region near the steel sheet surface. Suppressing such nitriding and oxidation prevents a decrease in the amount of martensite formed on the steel sheet surface, thereby improving impact performance. However, if Sn exceeds 0.200%, impact performance may be reduced due to grain boundary embrittlement. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. Since the effects of the present invention can be obtained even with a low Sn content, the lower limits of each content are not particularly limited. To more effectively obtain the effect of improving impact performance, the Sn content is preferably 0.003% or more. The Sn content is more preferably 0.005% or more.

[0056] Cu: 1.000% or less Cu is an element that improves hardenability, and the addition of Cu results in the formation of a large amount of martensite, thereby increasing TS and YS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.010% or more. The Cu content is even more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.000%, the area ratio of martensite may increase excessively. Furthermore, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively formed martensite and coarse precipitates and inclusions may cause void formation and crack propagation from the martensite during high-speed bending tests, potentially preventing good fracture resistance. Therefore, when Cu is added, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

[0057] Ta: 0.100% or less Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. To achieve this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more. On the other hand, if the Ta content exceeds 0.100%, large amounts of coarse precipitates and inclusions may be formed. In such cases, voids may form and cracks may propagate from the martensite during high-speed bending tests, potentially hindering good fracture resistance. Therefore, when Ta is added, the Ta content is preferably 0.100% or less.

[0058] W: 0.500% or less W is an element that improves hardenability, and the addition of W results in the formation of a large amount of martensite, thereby increasing TS and YS. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.010% or more. The W content is even more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, the area ratio of martensite increases, and during high-speed bending tests, voids may form and cracks may grow from the martensite, potentially preventing good fracture resistance. Therefore, when W is added, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less.

[0059] Mg: 0.0200% or less Mg is an element that effectively spheroidizes the shape of inclusions such as sulfides and oxides and improves the fracture resistance of steel sheets. To achieve this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0002% or more. On the other hand, if the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may cause void formation and crack propagation from the martensite during high-speed bending tests, potentially preventing good fracture resistance. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less.

[0060] Zn: 0.0200% or less Zn is an element effective in spheroidizing the shape of inclusions and improving the fracture resistance of steel sheets. To achieve this effect, the Zn content is preferably 0.0005% or more. The Zn content is more preferably 0.0010% or more. On the other hand, if the Zn content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may cause void formation and crack propagation from the martensite during high-speed bending tests, potentially preventing good fracture resistance. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.

[0061] Co: 0.0200% or less Like Zn, Co is an effective element for spheroidizing inclusions and improving the fracture resistance of steel sheets. To achieve this effect, the Co content is preferably 0.0010% or more. On the other hand, if the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation from the martensite during high-speed bending tests, which may result in poor fracture resistance. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.

[0062] Zr: 0.1000% or less Like Zn and Co, Zr is an element that is effective in spheroidizing the shape of inclusions and improving the fracture resistance of steel sheets. To achieve this effect, the Zr content is preferably 0.0005% or more. The Zr content is more preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, excessively coarse precipitates and inclusions may cause void formation and crack propagation from the martensite during high-speed bending tests, making it difficult to obtain good fracture resistance. Therefore, when Zr is contained, the Zr content is preferably 0.1000% or less.

[0063] Ca: 0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, excessively coarse precipitates and inclusions may cause void formation and crack propagation from the martensite during high-speed bending tests, potentially preventing good fracture resistance. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. While the lower limit of the Ca content is not particularly limited, a Ca content of 0.0005% or more is preferred. Due to production technology constraints, a Ca content of 0.0010% or more is more preferred.

[0064] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all effective elements for increasing the fracture resistance of steel sheet. To achieve such effects, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.0200% or if the content of As exceeds 0.0500%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation from the martensite during high-speed bending tests, which may result in poor fracture resistance. Therefore, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, it is preferable that the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are each 0.0200% or less, and the content of As is 0.0500% or less.

[0065] The Se content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The Se content is more preferably 0.0190% or less, and even more preferably 0.0180% or less. The Te content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The Te content is more preferably 0.0190% or less, and even more preferably 0.0180% or less. The Ge content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The Ge content is more preferably 0.0190% or less, and even more preferably 0.0180% or less. The As content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The As content is more preferably 0.0490% or less, and even more preferably 0.0480% or less. The Sr content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The Sr content is more preferably 0.0190% or less, and even more preferably 0.0180% or less. The Cs content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The Cs content is more preferably 0.0190% or less, and even more preferably 0.0180% or less. The Hf content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The Hf content is more preferably 0.0190% or less, and even more preferably 0.0180% or less. The Pb content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The Pb content is more preferably 0.0190% or less, and even more preferably 0.0180% or less. The Bi content is more preferably 0.0002% or more, and further preferably 0.0003% or more. The Bi content is more preferably 0.0190% or less, and further preferably 0.0180% or less.The REM content is more preferably 0.0002% or more, and even more preferably 0.0003% or more. The REM content is more preferably 0.0190% or less, and even more preferably 0.0180% or less.

[0066] In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content in the present invention refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably La and / or Ce.

[0067] The aforementioned inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are allowed to be included to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include H, O, etc.

[0068] <Method for manufacturing galvanized steel sheet> Hereinafter, one embodiment of the method for manufacturing a galvanized steel sheet of the present invention will be described in detail. Note that the temperatures when heating or cooling a steel slab (steel material), steel sheet, etc. described below refer to the surface temperatures of the steel slab (steel material), steel sheet, etc. unless otherwise specified.

[0069] The method for producing a galvanized steel sheet of the present invention includes a hot rolling step in which a steel slab having the above-mentioned chemical composition is hot-rolled at a finish rolling temperature of 850 to 950°C and coiled at a coiling temperature of 600°C or less, a cold rolling step in which the hot-rolled steel sheet after the hot rolling step is cold-rolled at a reduction rate of more than 20%, and an annealing step in which the cold-rolled steel sheet after the cold rolling step is heated to an annealing temperature of 750°C or more under conditions where A, represented by the following formula (1), satisfies 4 to 70, and is held at the annealing temperature for 30 seconds or more. and a plating process in which, after the annealing process, the steel sheet is cooled to a temperature range of 300 to 600°C and held at that temperature range for 10 to 300 seconds, and then zinc plating is performed on the steel sheet surface. After the plating process, the steel sheet is cooled to a cooling stop temperature of (Ms-300°C) to (Ms-50°C) and then held at a tempering temperature of 250 to 500°C for 20 to 500 seconds. Furthermore, after the quenching and tempering process, the steel sheet is cooled from the tempering temperature to 50°C at an average cooling rate of 20°C / s or more.

[0070] In the formula, T Ａｃ1 : Temperature at Ac1 point (°C), T ＲＴ : Annealing temperature (℃), t ＲＴ : annealing temperature T ＲＴ Time (seconds) required to reach t Ａｃ1 : Temperature T at Ac1 point after starting heat treatment in the annealing process Ａｃ1 T(t): the temperature (°C) t seconds after the start of the heat treatment in the annealing step.

[0071] (Hot Rolling Step) First, the conditions of the hot rolling step will be described.

[0072] Finish rolling temperature: 850 to 950°C If the finish rolling temperature is less than 850°C, ferrite transformation occurs during rolling, resulting in localized strength reduction, and strength may not be obtained even if the structure of the present invention is met. Therefore, the finish rolling temperature is 850°C or higher, preferably 880°C or higher. On the other hand, if the finish rolling temperature exceeds 950°C, crystal grains become coarse, and strength may not be obtained even if the structure of the present invention is met. Therefore, the finish rolling temperature is 950°C or lower, preferably 930°C or lower.

[0073] Coiling temperature: 600°C or less If the coiling temperature exceeds 600°C, the carbides in the hot-rolled steel sheet will coarsen, and since these coarsened carbides do not completely dissolve during soaking during annealing, it may not be possible to obtain the required collision properties. Therefore, the coiling temperature is 600°C or less, and preferably 580°C or less. There are no particular restrictions on the lower limit of the coiling temperature, but from the viewpoint of making it difficult for shape defects to occur in the steel sheet and preventing the steel sheet from becoming excessively hard, it is preferable to set the coiling temperature to 400°C or more.

[0074] (Cold Rolling Step) The hot-rolled steel sheet obtained in the hot rolling step is subjected to pre-treatment such as pickling and degreasing by a commonly known method, and then cold rolling is performed. The conditions of the cold rolling step when cold rolling is performed will be described.

[0075] Cold rolling reduction: More than 20% If the cold rolling reduction (cumulative reduction) is 20% or less, ferrite recrystallization is not promoted, and unrecrystallized ferrite remains, which may result in the steel structure of the present invention not being obtained or the steel structure becoming coarse or non-uniform during the annealing process, resulting in a risk of reduced TS and bendability in the final product. Furthermore, if the cold rolling reduction is 20% or less, the fracture resistance may be reduced. Therefore, the cold rolling reduction is more than 20%, preferably 30% or more. While there is no particular upper limit, it is preferable that the cold rolling reduction be 95% or less.

[0076] (Annealing Step) Next, the conditions of the annealing step when annealing the cold-rolled steel sheet obtained in the cold rolling step will be described.

[0077] Heating to the annealing temperature under the condition that A represented by formula (1) is 4 or more and 70 or less. In the formula, T Ａｃ1 : Temperature at Ac1 point (°C), T ＲＴ : Annealing temperature (℃), t ＲＴ : annealing temperature T ＲＴ Time (seconds) required to reach t Ａｃ1 : Temperature T at Ac1 point after starting heat treatment in the annealing process Ａｃ1T(t): the time (seconds) required for the temperature to reach t, ​​T(t): the temperature (°C) t seconds after the start of heat treatment in the annealing step. Regarding the heating conditions up to the annealing temperature, if A defined by formula (1) is less than 4, the generation of austenite becomes insufficient, resulting in the generation of excessive ferrite, and the steel structure of the present invention cannot be obtained. Therefore, the fracture resistance during high-speed bending deteriorates. Therefore, A is set to 4 or more, preferably 5 or more, and more preferably 8 or more. On the other hand, regarding the heating conditions up to the annealing temperature, if A defined by formula (1) exceeds 70, the diffusion of elements progresses and the elements in the austenite during annealing are uniformly distributed. This suppresses the generation of bainite in the subsequent holding step and tempering step before quenching, and the steel structure of the present invention cannot be obtained. Therefore, the fracture resistance during high-speed bending deteriorates. Therefore, A is set to 70 or less, preferably 65 or less, and more preferably 60 or less. Furthermore, if T Ａｃ1 can be calculated by the following formula (3): Ａｃ1 = 723 + 29 × [Si%] - 21 × [Mn%] - 17 × [Ni%] + 17 × [Cr%] Formula (3) Here, in formula (3), [element symbol %] is the content (mass %) of each element contained in the steel sheet, and elements that are not contained are represented as 0 (zero).

[0078] Annealing temperature: 750°C or higher, holding time: 30 seconds or higher If the annealing temperature is lower than 750°C, the generation of austenite will be insufficient, resulting in the generation of excessive ferrite, and the steel structure of the present invention will not be obtained. Therefore, the annealing temperature is 750°C or higher. On the other hand, the upper limit of the annealing temperature is not particularly limited, but from the viewpoint of manufacturability, it is preferably 900°C or lower. Furthermore, if the holding time at the annealing temperature is less than 30 seconds, the generation of austenite will be insufficient, resulting in the generation of excessive ferrite, and the steel structure of the present invention will not be obtained. Therefore, the holding time at the annealing temperature is 30 seconds or higher, preferably 60 seconds or higher. Although the upper limit of the holding time is not particularly limited, it is preferable to set the holding time to 600 seconds or lower so as not to impair productivity.

[0079] (Plating Step) Next, the conditions for the plating step will be described. In the plating step, after the annealing step, the steel sheet is cooled to a temperature range of 300 to 600°C, and is held at that temperature range for 10 to 300 seconds, and then the steel sheet surface is subjected to a zinc plating treatment.

[0080] Holding time in the temperature range of 300 to 600°C: 10 to 300 seconds. Cooling to a temperature range of 300 to 600°C after the annealing process and holding at the temperature range of 300 to 600°C for 10 to 300 seconds is effective for obtaining bainite. Furthermore, the formation of bainite concentrates C in the untransformed austenite, resulting in the production of a large amount of retained austenite. Holding times in the temperature range of 300 to 600°C of less than 10 seconds may not achieve these effects. Holding times in the temperature range of 300 to 600°C of less than 10 seconds may also result in the failure to obtain the desired yield ratio. Therefore, the holding time is 10 seconds or longer. On the other hand, holding times in the temperature range of 300 to 600°C of more than 300 seconds may result in the formation of excessive bainite, excessive concentration of C in the untransformed austenite, the formation of pearlite, and the failure to obtain the desired amount of retained austenite. Therefore, the holding time is 300 seconds or less, preferably 100 seconds or less.

[0081] After the above-mentioned holding, the steel sheet is subjected to a galvanizing treatment. The galvanizing treatment is, for example, a treatment of subjecting the steel sheet surface to electrogalvanizing, hot-dip galvanizing, or galvannealed hot-dip galvanizing. When hot-dip galvanizing is performed on the steel sheet surface, it is preferable to immerse the steel sheet obtained as described above in a galvanizing bath at a temperature of 440°C to 500°C to form a hot-dip galvanized layer on the steel sheet surface. Here, after the plating treatment, it is preferable to adjust the coating weight by gas wiping or the like. The steel sheet after the hot-dip galvanizing treatment may be subjected to alloying. When alloying the hot-dip galvanized steel sheet, it is preferable to hold the hot-dip galvanized steel sheet at a temperature of 450°C to 580°C for 1 second to 60 seconds. When electrogalvanizing the steel sheet surface, the treatment conditions for the electrogalvanizing treatment are not particularly limited, and may be in accordance with conventional methods.

[0082] (Quenching and Tempering Process) Next, the conditions for the quenching and tempering process performed after the plating process will be described.

[0083] Cooling stop temperature: (Ms - 300°C) to (Ms - 50°C) If the cooling stop temperature exceeds (Ms - 50°C), the formation of tempered martensite may be insufficient or fresh martensite may be excessively formed, and the steel structure of the present invention may not be obtained. On the other hand, if the cooling stop temperature is less than (Ms - 300°C), the formation of tempered martensite may be excessive and the formation of retained austenite may be insufficient. Therefore, the cooling stop temperature is (Ms - 300°C) to (Ms - 50°C). The cooling stop temperature is preferably (Ms - 280°C) or higher. Furthermore, the cooling stop temperature is preferably (Ms - 100°C) or lower. Note that the cooling rate up to the cooling stop temperature is not limited in the present invention.

[0084] Note that Ms is the martensitic transformation start temperature, and can be calculated using the following formula (4): Ms (°C) = 550 - 361 x [C%] - 39 x [Mn%] - 10 x [Cu%] - 17 x [Ni%] - 20 x [Cr%] - 5 x [Mo%] - 35 x [V%] + 30 x [Al%] - 5 x [W%] + 15 x [Co%] Formula (4) In formula (4), the [element symbol %] represents the content (mass%) of each element contained in the steel sheet, and elements that are not contained are represented as 0.

[0085] Tempering temperature: 250 to 500°C, holding time: 20 to 500 seconds. If the tempering temperature is less than 250°C, the martensite will not be tempered sufficiently, which is likely to cause voids to form at the interface between the tempered martensite and ferrite during primary processing, resulting in a deterioration in impact properties. Therefore, the tempering temperature is 250°C or higher, preferably 300°C or higher. On the other hand, if the tempering temperature exceeds 500°C, the martensite and bainite will be excessively tempered, which is likely to cause voids to form at the interface between the fresh martensite and the tempered martensite and bainite during primary processing, resulting in a deterioration in impact properties. Therefore, the tempering temperature is 500°C or lower, preferably 450°C or lower. Furthermore, if the holding time at the tempering temperature is less than 20 seconds, the martensite will not be tempered sufficiently, which is likely to cause a deterioration in impact properties. Therefore, the holding time at the tempering temperature is 20 seconds or higher, preferably 30 seconds or higher. Furthermore, if the holding time at the tempering temperature exceeds 500 seconds, the proportion of retained austenite may decrease. Therefore, the upper limit of the holding time at the tempering temperature is 500 seconds or less, and preferably 450 seconds or less.

[0086] (Cooling Step) The conditions for the cooling step performed after the quenching and tempering steps will be described.

[0087] Average cooling rate from the tempering temperature to 50°C: 20°C / s or more If the average cooling rate from the tempering temperature to 50°C is less than 20°C / s, the impact properties of the present invention cannot be obtained. The reason for this is unclear, but is thought to be as follows: In order to suppress void formation in the primary processed portion and improve impact properties, it is necessary to reduce the hardness difference between the soft phase (ferrite) and the hard phase (fresh martensite) by using an intermediate hardness phase (tempered martensite, bainite). The hardness difference between the soft phase and the intermediate hardness phase is reduced by softening the bainite formed before the plating treatment and the martensite formed during quenching in the tempering process, thereby suppressing void formation. The hardness difference between the hard phase and the intermediate hardness phase is reduced by forming relatively hard bainite in the tempering process, thereby suppressing void formation. When bainite formed in the tempering process softens, the difference in hardness between it and the hard phase increases. Therefore, a plating process involving exposure to high temperatures is performed before the tempering process, which produces bainite. Furthermore, by increasing the cooling rate after the tempering process, tempering of the bainite during cooling is suppressed, thereby reducing the difference in hardness between the soft phase and the hard phase and suppressing the formation of voids. Therefore, if the average cooling rate to room temperature after the tempering process is less than 20°C / s, the bainite will temper during cooling, increasing the difference in hardness between the bainite and the hard phase. This is thought to facilitate the formation of voids at the interface during primary processing and degrade impact properties. The average cooling rate is preferably 25°C / s or higher. While there is no particular upper limit for the average cooling rate, a rate of 70°C / s or lower is preferred from the perspective of energy conservation in cooling equipment. The average cooling rate (°C / s) is calculated by the formula: "(tempering temperature (°C) - 50) / cooling time (s) from the tempering temperature to 50°C."

[0088] The galvanized steel sheet of the present invention can be subjected to temper rolling for the purpose of correcting the shape, adjusting the surface roughness, etc. However, if the temper rolling rate exceeds 0.5%, the bendability may deteriorate due to surface hardening, so the temper rolling rate is preferably 0.5% or less, and more preferably 0.3% or less. In addition, various painting treatments such as resin or oil coating can also be applied.

[0089] The conditions for other manufacturing methods are not particularly limited, but are preferably as follows. To prevent macrosegregation, slabs are preferably manufactured by continuous casting, but can also be manufactured by ingot casting or thin slab casting. To hot-roll a slab, the slab may be cooled to room temperature and then reheated before hot-rolling. Alternatively, the slab may be loaded into a heating furnace without being cooled to room temperature and then hot-rolled. An energy-saving process in which the slab is immediately hot-rolled after a short period of heat retention can also be applied. When heating a slab, it is preferable to heat it to 1100°C or higher to prevent an increase in rolling load and to dissolve carbides. Furthermore, the heating temperature of the slab is preferably 1300°C or lower to prevent an increase in scale loss.

[0090] When hot rolling a slab, the rough bar after rough rolling can be heated to prevent problems during rolling when the slab heating temperature is low. Alternatively, a so-called continuous rolling process can be applied in which rough bars are joined together and finish rolling is performed continuously. Furthermore, in order to reduce the rolling load and to uniformize the shape and material, it is preferable to perform lubricated rolling with a friction coefficient of 0.10 to 0.25 in all or some passes of finish rolling.

[0091] After coiling, the steel sheet may be subjected to pickling to remove scale, etc. After pickling, the steel sheet is subjected to cold rolling, annealing, and galvanization under the above conditions.

[0092] <Member and manufacturing method of member> Next, the member of the present invention and its manufacturing method will be described. The member of the present invention is obtained by subjecting the zinc-plated steel sheet of the present invention to at least one of forming and welding. Furthermore, the manufacturing method of the member of the present invention includes a step of subjecting the zinc-plated steel sheet manufactured by the manufacturing method of the zinc-plated steel sheet of the present invention to at least one of forming and welding.

[0093] The galvanized steel sheet of the present invention has excellent tensile properties and crashworthiness. Therefore, a member obtained using the galvanized steel sheet of the present invention also has excellent tensile properties and crashworthiness, and is less likely to break during deformation due to collision. Therefore, the member of the present invention can be suitably used as an energy absorbing member in an automobile part.

[0094] The forming process can be performed by any common processing method such as press working, etc., without any restrictions. The welding process can be performed by any common welding method such as spot welding or arc welding, without any restrictions.

[0095] The present invention will be specifically described with reference to examples, but the scope of the present invention is not limited to the following examples.

[0096] Steels having the chemical compositions shown in Table 1 were melted in a vacuum melting furnace and bloomed to form steel slabs. These steel slabs were heated and subjected to hot rolling, cold rolling, annealing, plating, quenching and tempering, and cooling under the conditions shown in Table 2 to produce galvanized steel sheets. In the plating treatment, an electrogalvanized layer (EG), a hot-dip galvanized layer (GI), or a galvannealed layer (GA) was formed on the surface of the steel sheet. In the electrogalvanizing treatment, an electric current was passed through the steel sheet while immersed in a zinc solution, and a coating weight of 10 to 100 g / m was obtained. ２ In the hot dip galvanizing treatment, the steel sheet was immersed in a galvanizing bath and a coating weight of 10 to 100 g / m was formed. ２ In the galvannealing process, a hot-dip galvanized layer was formed on the steel sheet, and then an alloying treatment was carried out to form a hot-dip galvannealed layer (GA). The final thickness of each galvanized steel sheet was 1.2 mm.

[0097] In Table 2, A is represented by the following formula (1). In the formula, T Ａｃ1 : Temperature at Ac1 point (°C), T ＲＴ : Annealing temperature (℃), t ＲＴ : annealing temperature T ＲＴ Time (seconds) required to reach t Ａｃ1 : Temperature T at Ac1 point after starting heat treatment in the annealing process Ａｃ1 T(t): the temperature (°C) t seconds after the start of the heat treatment in the annealing step.

[0098]

[0099]

[0100] The obtained galvanized steel sheet was subjected to temper rolling at a rolling reduction of 0.2%, and then the area fractions of ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and retained austenite (RA) were determined according to the above-mentioned method. In addition, according to the above-mentioned method, the area fractions of low-Mn ferrite (LF) and high-Mn ferrite (HF) relative to the total area fraction of ferrite, and the area fraction of the hard second phase consisting of retained austenite and fresh martensite, which is in contact with bainite over 30% or more of its circumferential length, relative to the total area fraction of the hard second phase consisting of retained austenite and fresh martensite, were determined.

[0101] Further, the tensile properties (tensile strength TS, elongation El) and impact properties (load resistance (yield ratio YR), break resistance, energy absorption capacity (B)) were evaluated according to the following test methods.

[0102] <Tensile test> A JIS No. 5 tensile test piece (JIS Z2201) was taken from each of the obtained zinc-plated steel sheets in the direction perpendicular to the rolling direction. －３ A tensile test was conducted in accordance with the provisions of JIS Z2241 (2011) where tensile strength (TS), yield ratio (YR) obtained by dividing yield strength by tensile strength, elongation (El), and the value B (see formula (2) below) obtained by dividing the area enclosed by the stress P (MPa)-strain Q (%) curve in the range of strain Q from 0 to 4%, the line where stress P = 0, and strain Q = 4 by tensile strength (TS). Note that when TS was 780 MPa or more and less than 1180 MPa and El was 11% or more, the tensile properties were evaluated as good; when YR was 65% or more and 95% or less, the load capacity was evaluated as high; and when B was 3.15(%) or more, the energy absorption capacity was evaluated as good.

[0103] In the formula, P: stress (MPa), Q: strain (%), TS: tensile strength (MPa).

[0104] <High-Speed ​​U-Bending + High-Speed ​​Contact Bending Test> Using the obtained galvanized steel sheet, test pieces with a thickness of 1.2 mm, width of 60 mm, and length of 30 mm were prepared by grinding the end faces of both sides. U-bending (high-speed U-bending) was performed using a hydraulic bending tester with a punch bending radius of R = 4 mm, a stroke speed of 1500 mm / min, and longitudinal bending (bending ridge length: 30 mmL). Next, a contact bending test (high-speed contact bending test) was performed using a hydraulic bending tester with a spacer thickness of 5 mm sandwiched between the galvanized steel sheets, a stroke speed of 1500 mm / min (high speed), a pressing load of 10 ton, and a pressing time of 3 seconds. The fracture resistance was evaluated as good if no cracks of 0.5 mm or more occurred on the outer side (tensile side) of the bent portion. In the table, good fracture resistance was indicated by ○, and poor fracture resistance was indicated by ×.

[0105]

[0106] The galvanized steel sheets of the invention examples were excellent in tensile properties and impact resistance, while the galvanized steel sheets of the comparative examples were poor in at least one of the tensile properties and impact resistance.

[0107] According to the present invention, a galvanized steel sheet having excellent tensile properties and crashworthiness can be obtained. If a member obtained from the galvanized steel sheet of the present invention is used as an automobile part, it can contribute to reducing the weight of the automobile and greatly contribute to improving the performance of the automobile body.

Claims

1. A galvanized steel sheet comprising a steel sheet and a galvanized layer on a surface of the steel sheet, wherein the steel sheet has a steel structure with the following area ratios at a position 1 / 8 to 3 / 8 of the sheet thickness: ferrite: less than 30.0% (including 0.0%), bainite: more than 40.0% and not more than 80.0%, total of tempered martensite and bainite: more than 60.0%, retained austenite: 1.5% to 20% and fresh martensite: not more than 20.0% (including 0.0%), and when the ferrite is more than 0.0% in area ratio, the area ratio of high-Mn ferrite having an Mn concentration more than 0.80 times the Mn concentration of the steel sheet is 30% or more relative to the total area ratio of the ferrite, and the area ratio of the hard second phase, consisting of the retained austenite and the fresh martensite, which is in contact with bainite over 30% or more of its circumferential length is 30% or more relative to the total area ratio of the hard second phase, the zinc-plated steel sheet is subjected to a U-bending process with a bending radius R of 4 mm at a stroke speed of 1500 mm / min, and then a spacer having a thickness of 5 mm is placed inside the bent portion of the zinc-plated steel sheet, and the bent portion is bent at a stroke speed of 1500 mm / min, a pressing load of 10 ton, and a pressing time of 3 seconds until two surfaces of the zinc-plated steel sheet that are located opposite each other in the thickness direction of the spacer come into close contact with the spacer; the zinc-plated steel sheet has a crack of less than 0.5 mm on the outside of the bent portion, a tensile strength of 780 MPa or more and less than 1180 MPa, and a yield ratio of 65% or more and 95% or less.

2. The galvanized steel sheet according to claim 1, wherein the steel sheet has a chemical composition containing, by mass%, C: 0.05 to 0.20%, Si: 0.10 to 2.00%, Mn: 2.0 to 3.5%, P: 0.050% or less, S: 0.050% or less, sol. Al: 0.005 to 2.000%, and N: 0.010% or less, with the balance consisting of Fe and unavoidable impurities.

3. The chemical composition further includes, in mass%, Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, 3. The galvanized steel sheet according to claim 2, containing at least one selected from Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

4. The galvanized steel sheet according to any one of claims 1 to 3, wherein the galvanized layer is an electrogalvanized layer, a hot-dip galvanized layer, or a galvannealed hot-dip galvanized layer.

5. A member obtained by subjecting the galvanized steel sheet according to any one of claims 1 to 4 to at least one of forming and welding.

6. A hot rolling process in which a steel slab having the chemical composition according to claim 2 or 3 is hot rolled at a finish rolling temperature of 850 to 950°C and coiled at a coiling temperature of 600°C or less; a cold rolling process in which the hot rolled steel sheet after the hot rolling process is cold rolled at a reduction of more than 20%; an annealing process in which the cold rolled steel sheet after the cold rolling process is heated to an annealing temperature of 750°C or more under conditions where A, represented by the following formula (1), is between 4 and 70, and the steel sheet is held at said annealing temperature for 30 seconds or more; and a plating process in which, after the annealing process, the steel sheet is cooled to a temperature range of 300 to 600°C, held at said temperature range for 10 to 300 seconds, and then zinc plating is applied to the steel sheet surface. The method for producing a galvanized steel sheet includes: a quenching and tempering step of cooling the steel sheet to a cooling stop temperature of (Ms-300°C) to (Ms-50°C) after the plating step, and then holding the steel sheet at a tempering temperature of 250 to 500°C for 20 to 500 seconds; and a cooling step of cooling the steel sheet from the tempering temperature to 50°C at an average cooling rate of 20°C / s or more after the quenching and tempering step. In the formula, T Ａｃ1 : Temperature at Ac1 point (°C), T ＲＴ : Annealing temperature (℃), t ＲＴ : annealing temperature T ＲＴ Time (seconds) required to reach t Ａｃ1 : Temperature T at Ac1 point after starting heat treatment in the annealing process Ａｃ1 T(t): the temperature (°C) t seconds after the start of the heat treatment in the annealing step.

7. The method for producing a galvanized steel sheet according to claim 6, wherein the galvanizing treatment is a treatment of applying electrogalvanizing, hot-dip galvanizing or alloyed hot-dip galvanizing to the surface of the steel sheet.

8. A method for manufacturing a component, comprising subjecting the galvanized steel sheet according to any one of claims 1 to 4 to at least one of forming and welding to form the component.

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