Galvanized steel sheets, components, and methods for manufacturing the 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

[Technical Field]

[0001] This invention relates to galvanized steel sheets, components, and methods for manufacturing the same, which are high-strength and have excellent impact characteristics. The galvanized steel sheets of this invention are suitable for use mainly as automotive steel sheets. [Background technology]

[0002] From the perspective of protecting the global environment, reducing CO2 emissions by reducing the weight of automobile bodies while maintaining their strength and improving fuel efficiency has always been a crucial challenge for the automotive industry. To reduce the weight of automobile bodies while maintaining their strength, it is effective to make the steel sheets used as materials for automobile parts thinner by increasing their strength. On the other hand, automobile parts made from steel sheets must ensure the safety of people inside the vehicle in the event of a collision. Therefore, high-strength galvanized steel sheets used as materials for automobile parts are required to have not only the desired strength but also excellent collision characteristics.

[0003] In recent years, the application of high-strength galvanized steel sheets with a tensile strength (TS) of 780 MPa or higher has been expanding in automobile bodies. From the perspective of collision characteristics, automobile parts are broadly classified into non-deformable members such as pillars and bumpers, and energy-absorbing members such as side members, and each requires specific collision characteristics to ensure the safety of occupants in the event of a collision while the vehicle is in motion. In non-deformable members, the strength has been increased, and high-strength galvanized steel sheets with a tensile strength (hereinafter simply referred to as TS) of 780 MPa or higher have already been put into practical use. However, when applied to energy-absorbing members, high-strength galvanized steel sheets with a tensile strength of 780 MPa or higher are prone to fracture at the point where they underwent primary processing during forming during a collision. Therefore, there is a challenge in that they cannot stably exhibit collision energy absorption capacity, and materials with a tensile strength of 590 MPa or lower are mainly used. Consequently, in the development of energy-absorbing members, there is still room to suppress member fracture during collisions, ensure safety during collisions by stably exhibiting high absorbed energy, and contribute to environmental protection through weight reduction. Therefore, it is necessary to use high-strength galvanized steel sheets with a TS of 780 MPa or higher, which have excellent collision characteristics, as the energy absorbing members.

[0004] In response to such requirements, for example, Patent Document 1 discloses technology relating to a high-strength galvanized steel sheet with a maximum tensile strength of 780 MPa or more, applicable to impact absorbing members during collisions. Furthermore, Patent Document 2 discloses a patent relating to a galvanized steel sheet with a tensile strength of 980 MPa or more, good fracture resistance, and excellent impact characteristics with good energy absorption. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2015-175061 [Patent Document 2] Japanese Patent Publication No. 2022-34015 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, while Patent Document 1 evaluates fracture resistance by performing axial crushing tests on hat-shaped materials to assess impact characteristics, it does not evaluate the process from crack initiation during crushing to fracture, which is crucial for impact characteristics, because the crack determination is performed after crushing. The reason for this is that if cracks occur early in the crushing process, even minor cracks that do not penetrate the plate thickness may reduce the absorbed energy, but it is difficult to determine such minor cracks after crushing. Furthermore, if cracks occur later in the crushing process, even large cracks that penetrate the plate thickness may have little effect on the absorbed energy. Therefore, it is considered that evaluating fracture resistance solely based on crack determination after crushing is insufficient.

[0007] Furthermore, Patent Document 2 evaluates the impact characteristics by measuring the void number density in the cross section within the 0-50 μm region from the surface of the compressed steel plate when it is bent 90° at a stroke speed of 20 mm / min, and by measuring the stroke at the time of maximum load in the stroke-load curve during the orthogonal bending test when a bending-orthogonal bending test is performed at a stroke speed of 20 mm / min, and the absorbed energy is evaluated by an axial crush test of a hat-shaped member. However, the fracture resistance characteristics required for energy absorption members in automobiles are those that deform at high speeds. Therefore, evaluation of fracture resistance characteristics by bending tests and bending-orthogonal bending tests at a stroke speed of 20 mm / min is insufficient for evaluating the fracture resistance characteristics of materials for energy absorption members in automobiles that deform at high speeds. In other words, the fracture resistance characteristics were not necessarily sufficient for materials for energy absorption members in automobiles.

[0008] Thus, there was a strong demand for the establishment of new technologies for galvanized steel sheets that exhibited excellent tensile properties, including tensile strength (TS), and also superior impact resistance.

[0009] The present invention has been made in view of such circumstances, and an object thereof is to provide a galvanized steel sheet, a member, and a method for manufacturing the same, which are suitable for an energy absorption member of an automobile and have excellent tensile characteristics and collision characteristics.

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

[0011]

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[0012] Good fracture resistance characteristics mean that U-bending processing (high-speed U-bending processing) and a close contact bending test (high-speed close contact bending test) under specific conditions are performed on the galvanized steel sheet, and the length of cracks formed on the outer side (tensile side) of the bent portion is less than 0.5 mm. Here, the above U-bending processing is performed with a bending radius R = 4 mm and a stroke speed of 1500 mm / min. In addition, for the adhesion bending test, the spacer sandwiched between the galvanized steel sheets has a plate thickness of 5 mm, and the test is carried out at a stroke speed of 1500 mm / min, a pressing load of 10 tons, and a pressing time of 3 seconds. [Means for Solving the Problems]

[0013] As a result of intensive research to solve the above problems, the present inventors have found the following. The galvanized steel sheet has a steel structure in which, in terms of area ratio, ferrite is less than 30.0% (including 0.0%), bainite is more than 40.0% and 80.0% or less, 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 is provided with a galvanized layer formed on the surface of the steel sheet. Further, in the above galvanized steel sheet, when the ferrite is more than 0.0% in terms of area ratio, 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 with respect to the total area ratio of ferrite, and with respect to the total area ratio of the hard second phase composed of retained austenite and fresh martensite, the area ratio of the hard second phase in which 30% or more of the circumference is in contact with bainite is 30% or more. When U-bending with a bending radius R = 4 mm is performed on the above galvanized steel sheet at a stroke speed of 1500 mm / min, a spacer with a thickness of 5 mm is installed inside (compression side) the bent portion of the galvanized steel sheet, and the two surfaces of the galvanized steel sheet existing at positions facing each of the upper and lower surfaces in the thickness direction of the spacer are brought into close contact with the spacer. When the bending of the bent portion is carried out at a stroke speed of 1500 mm / min, a pressing load of 10 tons, and a pressing time of 3 seconds, it is ensured that no crack of 0.5 mm or more occurs on the outside (tensile side) of the bent portion. From these, it has been found that a steel sheet excellent in tensile properties and impact properties can be obtained.

[0014] The present invention has been made based on such findings, and the gist thereof is as follows. [1] A galvanized steel sheet comprising a steel sheet and a galvanized layer formed on the surface of the steel sheet, The aforementioned steel plate has an area ratio at the position where the plate thickness is 1 / 8 to 3 / 8, Ferrite: Less than 30.0% (including 0.0%) Baynight: Over 40.0% and under 80.0% Total tempered martensite and bainite: over 60.0% Residual austenite: 1.5% to 20% Fresh martensite: Having a steel structure with 20.0% or less (including 0.0%), If the area ratio of the ferrite is greater than 0.0%, then the area ratio of high-Mn ferrite, where the Mn concentration is greater than 0.80 times that of the steel plate, is 30% or more of the total area ratio of the ferrite. With respect to the total area ratio of the hard second phase consisting of the retained austenite and the fresh martensite, the area ratio of the hard second phase in which 30% or more of the perimeter is in contact with bainite is 30% or more. When a U-bend with a bending radius R=4mm is performed on the galvanized steel sheet at a stroke speed of 1500mm / min, a 5mm thick spacer is placed inside the bent portion of the galvanized steel sheet, and the bending of the bent portion is performed at a stroke speed of 1500mm / min, a pressing load of 10 tons, and a pressing time of 3 seconds until two surfaces of the galvanized steel sheet, located opposite each other in the thickness direction of the spacer, are in close contact with the spacer, the crack generated on the outside of the bent portion is less than 0.5mm. The tensile strength is 780 MPa or more and less than 1180 MPa. Galvanized steel sheet with a yield ratio of 65% to 95%. [2] The steel plate is, by mass %, C: 0.05~0.20%, Si: 0.10~2.00%, Mn: 2.0~3.5%, P: 0.050% or less, S: 0.050% or less, sol.Al:0.005~2.000% and N: Contains 0.010% or less, The zinc-plated steel sheet according to [1], having a component composition in which the remainder consists of Fe and unavoidable impurities. [3] The above component composition is further expressed in mass% as follows: 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, 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 The galvanized steel sheet according to [2] above, containing at least one selected from REM:0.0200% or less. [4] The zinc-plated steel sheet according to any one of [1] to [3], wherein the zinc-plated layer is an electro-zinc-plated layer, a hot-dip zinc-plated layer, or an alloyed hot-dip zinc-plated layer. [5] A component obtained by forming and welding a galvanized steel sheet as described in any of [1] to [4] above. [6] A hot rolling process in which a steel slab having the component composition described in [2] or [3] above is subjected to hot rolling at a finish rolling temperature of 850 to 950°C and wound at a winding temperature of 600°C or less, A cold rolling process is performed on the hot-rolled steel sheet after the hot-rolling process, with a reduction ratio of more than 20%. The cold-rolled steel sheet after the cold-rolling process is heated to an annealing temperature of 750°C or higher under conditions where A, represented by the following formula (1), is between 4 and 70, and is held at the annealing temperature for 30 seconds or more. After the annealing process, the steel plate is cooled to a temperature range of 300-600°C, held at that temperature range for 10-300 seconds, and then subjected to a zinc plating process. After the plating process, the material is cooled to a cooling stop temperature of (Ms-300℃) to (Ms-50℃), followed by a quenching and tempering process in which it is held at a tempering temperature of 250 to 500℃ for 20 to 500 seconds. A method for manufacturing a galvanized steel sheet, comprising a cooling step after the quenching and tempering step, in which the steel is cooled from the tempering temperature to 50°C at an average cooling rate of 20°C / s or more.

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[0015] According to the present invention, a galvanized steel sheet with excellent tensile and impact properties can be obtained. A component obtained by forming or welding the galvanized steel sheet of the present invention can be suitably used as an energy absorbing component in the automotive field. [Modes for carrying out the invention]

[0016] The details of the present invention are described below.

[0017] <Galvanized steel sheet> The galvanized steel sheet of the present invention comprises a steel sheet having a structure in which, at positions 1 / 8 to 3 / 8 of the sheet thickness, the area ratio is as follows: ferrite: less than 30.0% (including 0.0%), bainite: more than 40.0% and 80.0% or less, total of tempered martensite and bainite: more than 60.0%, retained austenite: 1.5% to 20%, and fresh martensite: 20.0% or less (including 0.0%), and a zinc plating layer on the surface of the steel sheet. When ferrite accounts for more than 0.0% by area percentage, the area percentage of high-Mn ferrite (HF), where the Mn concentration is more than 0.80 times that of the steel plate, is 30% or more of the total area percentage of ferrite. In the hard second phase, which consists of retained austenite and fresh martensite, the area ratio of the hard second phase where 30% or more of the circumference is in contact with bainite is 30% or more of the total area ratio of the hard second phase. After performing a U-bend with a bending radius R=4mm on a galvanized steel sheet at a stroke speed of 1500mm / min, When a 5mm thick spacer is placed on the inside (compression side) of the bent portion of a galvanized steel sheet, and the bending process is performed at a stroke speed of 1500mm / min, a pressing load of 10 tons, and a pressing time of 3 seconds until two surfaces of the galvanized steel sheet, located opposite each other in the thickness direction of the spacer, are in close contact with the spacer, the crack generated on the outside (tensile side) of the bent portion is less than 0.5mm, the tensile strength is 780MPa or more and less than 1180MPa, and the yield ratio is 65% or more and 95% or less.

[0018] Ferrite area ratio: Less than 30.0% (including 0.0%) While ferrite is effective in improving elongation, it reduces yield strength (YS) and tensile strength TS (hereinafter also simply referred to as TS). Furthermore, the increased hardness difference region between the soft ferrite and the adjacent tissue makes it easier for fracture to occur due to void formation and connection during high-speed impacts, thus reducing fracture resistance during high-speed impacts. As a result, when the area ratio of ferrite is 30.0% or more, it becomes difficult to satisfy all the requirements for a yield ratio YR (hereinafter also simply referred to as YR) of 65% or more, a TS of 780 MPa or more, and fracture resistance during high-speed impacts. Therefore, the area ratio of ferrite is less than 30.0%, preferably 20.0% or less, and more preferably less than 10.0%. The lower limit is not particularly limited, but the area ratio of ferrite is preferably 1.0% or more, and more preferably 3.0% or more.

[0019] Area percentage of bainite: over 40.0% and under 80.0% Bainite is an intermediate hardness phase, and its formation with an area ratio exceeding 40.0% reduces the hardness difference with adjacent martensite and ferrite, suppressing void formation during high-speed impacts and improving high-speed impact characteristics. Furthermore, bainite improves elongation. If the area ratio of bainite is 40.0% or less, these effects cannot be fully obtained, making it difficult to satisfy all the requirements of a total stress point (TS) of 780 MPa or higher, elongation of 11% or higher, and high-speed impact characteristics. Therefore, the area ratio of bainite is greater than 40.0%, preferably greater than 45.0%, and more preferably greater than 50.0%. Furthermore, if the bainite area ratio exceeds 80.0%, it is difficult to obtain the desired tensile strength. Therefore, the bainite area ratio should be 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: over 60.0% Tempered martensite and bainite are effective in improving strength while simultaneously enhancing high-speed impact characteristics by suppressing member fracture during impact deformation. If the total area ratio of tempered martensite and bainite is 60.0% or less, these effects cannot be fully obtained, making it difficult to satisfy both a total stress point (TS) of 780 MPa or higher and high-speed impact characteristics. Therefore, the total area ratio of these materials is greater than 60.0%, preferably greater than 65.0%, and more preferably greater than 82.0%. Furthermore, while 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 consideration the balance with other organizations.

[0021] Area percentage of retained austenite: 1.5% to 20% Retained austenite is effective in delaying crack initiation during impact and improving impact properties. Although the mechanism is not clear, it is thought to be as follows: Retained austenite work hardens during impact deformation, increasing the radius of curvature during bending deformation and thus distributing the strain in the bent area. This strain distribution reduces stress concentration in the void formation area due to primary processing, resulting in improved impact properties. This effect may not be obtained if the area ratio of retained austenite is less than 1.5%. Also, elongation decreases when the area ratio of retained austenite is less than 1.5%. Therefore, the area ratio of retained austenite should be 1.5% or more, preferably 3% or more, and more preferably 5% or more. On the other hand, if the area ratio of retained austenite exceeds 20%, the fresh martensite generated by processing-induced transformation may reduce the fracture resistance during impact. Therefore, the area ratio of retained austenite should be 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, it is prone to void formation at grain boundaries with the soft phase, and if the area ratio of fresh martensite exceeds 20.0%, it may reduce impact properties. Therefore, the area ratio 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) where the Mn concentration is greater than 0.80 times that of steel plate: 30% or more of the total ferrite area (including 100.0%) Low-Mn ferrite (LF), where the Mn concentration is 0.80 times or less than that of the steel plate, has a lower dislocation density and is relatively softer compared to high-Mn ferrite (HF), where the Mn concentration is greater than 0.80 times that of the steel plate. To suppress fracture during high-speed collisions, it is necessary to reduce the amount of the high-hardness difference region and the hardness difference. If the area ratio of relatively soft low-Mn ferrite (LF) is 70% or less of the total ferrite area, that is, if the area ratio of high-Mn ferrite (HF) is 30% or more, a TS of 780 MPa or more and high high-speed collision characteristics can be obtained. Therefore, when the area ratio of ferrite is greater than 0.0%, the area ratio of high-Mn ferrite (LF) should be 30% or more (including 100.0%) of the total ferrite area ratio. The area ratio of high-Mn ferrite is preferably 50% or more of the total ferrite area ratio, and more preferably 70% or more. The area ratio of low-Mn ferrite (LF) shall be 70% or less of the total ferrite area ratio (including 0.0%). The area ratio of low-Mn ferrite relative to the total ferrite area ratio is preferably 50% or less, and more preferably 30% or less. Here, the low-Mn ferrite (LF) may include recrystallized ferrite (RF) formed during the heating process, and may consist of this recrystallized ferrite. Furthermore, high-Mn ferrite (HF) may include cooling-generated ferrite (TF) formed during cooling, and may consist of this cooling-generated ferrite.

[0024] Area percentage of hard secondary phase where more than 30% of the circumference is in contact with bainite: More than 30% of the total area percentage of hard secondary phase Among the island-like regions that constitute the hard second phase, which consists of retained austenite and fresh martensite, those in which more than 30% of the circumference is in contact with bainite have a high solid solution carbon concentration and high stability of retained austenite. Therefore, these island-like regions in which more than 30% of the circumference is in contact with bainite play an important role in ensuring good work hardening ability and ductility. In other words, by generating bainite under appropriate conditions during cooling after annealing, solid-solution carbon diffuses from the bainite into the surrounding untransformed austenite, making it possible to locally increase the amount of solid-solution carbon only in the region of the untransformed austenite that is in contact with the bainite. Subsequently, by performing a reheating treatment under appropriate conditions in this state, a hard second phase with a high concentration of solid-solution carbon is generated around or inside the bainite. Therefore, the retained austenite contained in the hard second phase with a high concentration of solid-solution carbon around or inside the bainite has high stability and plays an important role in ensuring good work hardening ability and ductility. In this invention, ensuring ductility means that the elongation (El) is 11% or more. Based on the above, the total area ratio of the hard second phase in which 30% or more of the circumference is in contact with bainite is 30% or more of the total area ratio of the hard second phase, preferably 40% or more, and more preferably 60% or more. There is no particular upper limit, and it may be 100%.

[0025] The area ratio of each tissue refers to the proportion of the area of ​​each phase to the observed area. The area ratio of each tissue is measured as follows: After polishing the thickness cross section of a steel plate cut perpendicular to the rolling direction, it is etched with 3 volume% nital, and three fields of view are taken at a position 1 / 4 of the plate thickness using a scanning electron microscope (SEM) at 1500x magnification with a field of view of 85 μm × 64 μm. From the obtained image data, the area ratio of each tissue is determined using Image-Pro from Media Cybernetics. The average of the area ratios of the three fields of view is taken as the area ratio of each tissue in this invention. In the image data, ferrite, bainite, retained austenite, fresh martensite, and tempered martensite are distinguished as follows. Ferrite: Black regions within the crystal grains that do not contain retained austenite or fresh martensite with an aspect ratio of 2.0 or higher. Bainite: A black region containing one or more structures within the crystal grains, consisting of retained austenite and fresh martensite with an aspect ratio of 2.0 or higher. Tempered martensite: A light gray region containing one or more structures of retained austenite and fresh martensite with an aspect ratio of 2.0 or higher, and containing carbides. Residual austenite and fresh martensite can be distinguished as white regions.

[0026] Here, it is difficult to distinguish between fresh martensite and retained austenite in SEM images. Therefore, the area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite, which is obtained by the method described later, from the total area ratio of fresh martensite and retained austenite.

[0027] In this invention, the area fraction of retained austenite is determined by measuring the X-ray diffraction intensity to find 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 by the ratio of the X-ray diffraction integral intensity of the (200), (220), and (311) planes of fcc iron to the X-ray diffraction integral intensity of the (200), (211), and (220) planes of bcc iron on a 1 / 4 thickness plane.

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

[0029] Cracks occurring on the outer side (tensile side) of the bent section after high-speed U-bending + high-speed tight-fitting bending test: Less than 0.5 mm In the galvanized steel sheet of the present invention, after performing a U-bend (high-speed U-bend) with a bending radius R=4mm at a stroke speed of 1500mm / min, a close-contact bending test (high-speed close-contact bending test) is performed. A 5mm thick spacer is placed on the inside (compression side) of the bent portion of the galvanized steel sheet, and the bending process (stroke speed: 1500mm / min, pressing load: 10tn, pressing time: 3 seconds) is performed on the bent portion until two surfaces of the galvanized steel sheet, located opposite each other in the thickness direction of the spacer, are in close contact with the spacer. In this case, no cracks of 0.5mm or more occur on the outside (tensile side) of the bent portion. This results in high impact characteristics. If cracks of 0.5mm or more occur, the fracture resistance is insufficient, and the impact characteristics are insufficient due to the occurrence and propagation of cracks during impact. At this point, the U-bending process can be terminated and the spacer placed inside the bent portion of the galvanized steel sheet when the angle formed by the opposing steel sheet surfaces becomes 0 to 20° (= 10 ± 10°). Furthermore, stroke speed can be measured, for example, by measuring the strokes per unit of time using a scale and converting this to the amount of movement per 60 seconds. The amount of movement can be defined as the relative movement of one end to the other when the steel plate being bent is viewed in a U-shape. The stroke can be measured at any position on the bending test machine that moves in conjunction with the pressing mechanism that presses against the steel plate. The time can be adjusted within a range that allows for accurate measurement; for example, a 5-second stroke may 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, a yield ratio of 65% or more and 95% or less, and exhibits high load-bearing capacity. When the yield ratio exceeds 95%, the fracture resistance decreases.

[0031] Furthermore, the desired impact characteristics can be obtained by controlling the heating rate and temperature before annealing and the cooling rate after annealing, as described later, and by performing a plating treatment before the quenching and tempering processes. By controlling the heating rate and temperature before annealing and holding, it is possible to control the amount of low-Mn ferrite (LF), high-Mn ferrite (HF), and bainite generated during the holding process before quenching. This allows for the formation of high-Mn ferrite (HF), which is relatively harder than low-Mn ferrite (LF), and generates bainite during the holding process before quenching, which is then softened during the plating and tempering processes. This reduces the hardness difference between ferrite and bainite, suppressing the formation of voids at the interface between ferrite and bainite. As a result, it becomes possible to suppress the crack length to less than 0.5 mm (including 0 μm). Furthermore, if ferrite is absent, the formation of voids caused by differences in the hardness of the structure is suppressed, improving impact characteristics; therefore, the presence of ferrite is not necessary. Furthermore, the bainite generated in the tempering process is prevented from softening due to tempering during cooling by increasing the cooling rate after tempering, and further softening due to tempering during the plating process is prevented by performing a plating treatment before the tempering process, thereby suppressing void formation at the interface with hard fresh martensite. As a result, it is possible to suppress the crack length to less than 0.5 mm (including 0 μm).

[0032] In this invention, the crack length refers to the length of the crack that occurs on the outside (tensile side) of the bent portion of a galvanized steel sheet after performing a U-bend with a bending radius R=4mm at a stroke speed of 1500mm / min, placing a 5mm thick spacer on the inside (compression side) of the bent portion, and continuing the bending process until two surfaces of the galvanized steel sheet, located opposite each other in the thickness direction of the spacer, are in close contact with the spacer. The method for measuring crack length is as follows: For a galvanized steel sheet, a high-speed U-bend with a bending radius R=4mm is performed at a stroke speed of 1500mm / min. Then, as a close-contact bending test (high-speed close-contact bending test), a 5mm thick spacer is placed on the inside (compression side) of the bent portion of the galvanized steel sheet. The bending of the bent portion is performed at a stroke speed of 1500mm / min, a pressing load of 10 tons, and a pressing time of 3 seconds until two surfaces of the steel sheet, located opposite each other in the thickness direction of the spacer, are in close contact with the spacer. The length of the crack that occurs on the outside (tensile side) of the bent portion is measured by visual inspection of its appearance.

[0033] The zinc-plated steel sheet of the present invention has a zinc plating layer on the surface of the steel sheet. The zinc plating layer is, for example, an electro-zinc plated layer, a hot-dip zinc plated layer, or an alloyed hot-dip zinc plated layer.

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

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

[0036] The galvanized steel sheet of the present invention exhibits excellent impact characteristics. In this invention, excellent impact characteristics mean high load-bearing capacity, good fracture resistance, and good energy absorption. In this invention, high load-bearing capacity means that the yield ratio YR is 65% or more and 95% or less. The method for measuring the yield ratio YR is as described in the examples. In this invention, "good fracture resistance" means that the crack length when performing the U-bending + tight bending described in the examples is less than 0.5 mm. In this invention, "good impact characteristics" means that when performing the tensile test described in the examples, the value B obtained by dividing the area enclosed by the stress P (MPa) - strain Q (%) curve during the tensile test, the curve in the range of strain Q from 0 to 4%, the straight line at stress P=0, and the line at strain Q=4, by the tensile strength (TS) is 3.15 (%) or more.

[0037] (Composition of steel plates) Next, we will describe the preferred component composition of the steel sheet that makes up the galvanized steel sheet. Unless otherwise specified, the "%" indicating the content of component elements means "mass percent".

[0038] C: 0.05~0.20% Carbon (C) facilitates the formation of phases other than ferrite and forms alloy compounds with elements such as Nb and Ti, making it an essential element for improving strength. If the C content is less than 0.05%, the desired strength may not be achieved even with optimized manufacturing conditions. Therefore, the C content is preferably 0.05% or more, and more preferably 0.07% or more. On the other hand, if the carbon content exceeds 0.20%, the strength of the martensite increases excessively, and even if the manufacturing conditions are optimized, the impact characteristics of the present invention may not be obtained. Therefore, the carbon content is preferably 0.20% or less, and more preferably 0.18% or less.

[0039] Si: 0.10~2.00% Si contributes to the stabilization of retained austenite by suppressing carbide formation. Furthermore, Si is a solid solution strengthening element, contributing to an improved balance between strength and ductility. To obtain this effect, the Si content is preferably 0.10% or more, and more preferably 0.50% or more. On the other hand, if the Si content exceeds 2.00%, the tempering of the martensite is suppressed, increasing the hardness difference between the tempered martensite and the ferrite. As a result, even if the microstructure of the present invention is met, the desired yield ratio may not be obtained. Therefore, the Si content is preferably 2.00% or less, and more preferably 1.50% or less.

[0040] Mn: 2.0~3.5% Mn is a building block element of martensite and also a solid solution strengthening element. Furthermore, Mn contributes to the stabilization of retained austenite. To obtain these effects, the Mn content is preferably 2.0% or more. More preferably, the Mn content is 2.5% or more. On the other hand, if the Mn content exceeds 3.5%, the residual austenite fraction may increase excessively, potentially degrading the collision properties. Therefore, the Mn content is preferably 3.5% or less, and more preferably 3.3% or less.

[0041] P:0.050% or less P is an effective element for strengthening steel. However, if the P content exceeds 0.050%, it can significantly slow down the alloying rate. Furthermore, if the P content exceeds 0.050%, it can cause embrittlement due to grain boundary segregation, and even if the steel structure of the present invention is met, it may degrade the fracture resistance properties during impact. Therefore, the P content is preferably 0.050% or less, and more preferably 0.010% or less. While there is no specific lower limit for P content, the currently industrially feasible lower limit is around 0.002%, meaning that in practice, it will be higher than that.

[0042] S: 0.050% or less S, acting as inclusions such as MnS, can cause cracking along the metal flow in the weld, potentially reducing impact characteristics even if the steel structure of the present invention is met. Therefore, the amount of S should be as low as possible, but from the standpoint of manufacturing costs, the S content is preferably 0.050% or less. More preferably, the S content is 0.010% or less. There is no particular lower limit for the S content, but the currently industrially feasible lower limit is around 0.0002%, and it is practically higher than that.

[0043] sol.Al: 0.005~2.000% Al acts as a deoxidizing agent 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 the 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%, it degrades the slab quality during steelmaking. Furthermore, if the sol.Al content exceeds 2,000%, even if the steel structure of the present invention is satisfied, the fracture resistance may decrease. Therefore, the sol.Al content is preferably 2,000% or less, and more preferably 1,000% or less.

[0044] N: 0.010% or less Since nitrogen (N) forms coarse nitrides, it can act as a starting point for void formation during impact deformation, potentially degrading impact characteristics. Therefore, it is preferable to keep the amount of N as low as possible, but from a manufacturing cost perspective, the N content is preferably 0.010% or less, and more preferably 0.006% or less. There is no particular lower limit to the N content, but the lower limit currently industrially feasible is around 0.0003%, and in practice, it will be more than that.

[0045] The composition of the steel sheet according to the present invention preferably contains the above-mentioned elemental components as basic components, with the remainder consisting of iron (Fe) and unavoidable impurities.

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

[0047] Nb: 0.200% or less Nb increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To obtain this effect, it is preferable to have an Nb content of 0.001% or more. More preferably, the Nb content is 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, these coarse precipitates and inclusions can become the starting points for voids and cracks during high-speed bending tests, which may prevent the acquisition of good fracture resistance. Therefore, when Nb is included, the Nb content is preferably 0.200% or less. More preferably, the Nb content is 0.060% or less.

[0048] Ti: 0.200% or less Similar to Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To obtain this effect, it is preferable to have a Ti content of 0.001% or more. More preferably, the Ti content is 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, these coarse precipitates and inclusions can become the initiation points for voids and cracks during high-speed bending tests, which may prevent the acquisition of good fracture resistance. Therefore, when Ti is included, the Ti content is preferably 0.200% or less. More preferably, the Ti content is 0.060% or less.

[0049] V:0.200% or less Like Nb and Ti, V increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To obtain such an effect, it is preferable that the V content be 0.001% or more. More preferably, the V content is 0.003% or more. Even more preferably, the V content is 0.005% or more. On the other hand, if the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, these coarse precipitates and inclusions can become the initiation points for voids and cracks during high-speed bending tests, which may prevent the acquisition of good fracture resistance. Therefore, when V is included, the V content is preferably 0.200% or less. More preferably, the V content is 0.060% or less.

[0050] B: 0.0100% or less B is an element that enhances hardenability by segregating at austenite grain boundaries. Furthermore, B controls ferrite formation and grain growth during cooling after annealing. To obtain these effects, it is preferable to have a B content of 0.0001% or more. More preferably, the B content is 0.0002% or more. Even more preferably, the B content is 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, void formation and crack propagation may occur starting from the martensite, potentially preventing the acquisition of good fracture resistance. Therefore, when B is included, it is preferable to keep the B content at 0.0100% or less. More preferably, the B content is 0.0050% or less.

[0051] Cr:1.000% or less Since Cr is an element that enhances hardenability, the addition of Cr leads to the formation of an appropriate amount of martensite, thereby increasing TS and YS. To obtain such an effect, it is preferable that the Cr content be 0.0005% or more. More preferably, the Cr content is 0.0010% or more. Even more preferably, the Cr content is 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, void formation and crack propagation may occur starting from the martensite, potentially preventing the acquisition of desirable fracture resistance. Therefore, when Cr is included, it is preferable to keep the Cr content at 1.000% or less. Furthermore, the Cr content is more preferably 0.250% or less, and even more preferably 0.100% or less.

[0052] Ni: 1.000% or less Since Ni is an element that enhances hardenability, the addition of Ni generates a large amount of tempered martensite, thereby increasing TS and YS. To obtain such an effect, it is preferable to have a Ni content of 0.005% or more. More preferably, the Ni content is 0.010% or more. Even more preferably, the Ni content is 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, void formation and crack propagation may occur starting from the martensite, potentially preventing the acquisition of desirable fracture resistance. Therefore, when Ni is included, it is preferable to keep the Ni content at 1.000% or less. More preferably, the Ni content is 0.800% or less.

[0053] Mo: 1.000% or less Since Mo is an element that enhances hardenability, the addition of Mo generates a large amount of tempered martensite, thereby increasing TS and YS. To obtain this effect, it is preferable to have a Mo content of 0.010% or more. More preferably, the Mo content is 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, void formation and crack propagation may occur starting from the martensite, potentially resulting in poor fracture resistance. Therefore, when Mo is included, it is preferable to keep the Mo content at 1.000% or less. More preferably, the Mo content is 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 from the viewpoint of suppressing nitriding and oxidation of the steel sheet surface and decarburization in the region near the steel sheet surface. By suppressing such nitriding and oxidation, it is possible to prevent a decrease in the amount of martensite formed on the steel sheet surface and improve the impact properties. However, if the Sb content exceeds 0.200%, the impact properties may decrease due to grain boundary embrittlement. Therefore, when Sb is included, it is preferable that the Sb content be 0.200% or less. The effects of the present invention can be obtained even with a low Sb content, so there is no particular lower limit to each content. In order to more effectively obtain the effect of improving impact properties, it is preferable that the Sb content be 0.003% or more. It is even more preferable that the Sb content be 0.005% or more.

[0055] Sn: 0.200% or less Similar to Sb, Sn can be added as needed to suppress nitriding and oxidation of the steel sheet surface and decarburization in 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 characteristics. However, if the Sn content exceeds 0.200%, impact characteristics may decrease due to grain boundary embrittlement. Therefore, when Sn is included, it is preferable that the Sn content be 0.200% or less. The effects of the present invention can be obtained even with a low Sn content, so there is no particular lower limit to the respective content. To more effectively obtain the effect of improving impact characteristics, it is preferable that the Sn content be 0.003% or more. It is even more preferable that the Sn content be 0.005% or more.

[0056] Cu:1.000% or less Since Cu is an element that enhances hardenability, the addition of Cu generates a large amount of martensite, thereby increasing TS and YS. To obtain such an effect, it is preferable to have a Cu content of 0.005% or more. More preferably, the Cu content is 0.010% or more. Even more preferably, the Cu content is 0.020% or more. On the other hand, if the Cu content exceeds 1.000%, the area ratio of martensite may increase excessively. In addition, a large amount 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 starting from the martensite during high-speed bending tests, potentially resulting in poor fracture resistance. Therefore, when Cu is included, it is preferable to keep the Cu content at 1.000% or less. More preferably, the Cu content is 0.200% or less.

[0057] Ta:0.100% or less Like Ti, Nb, and V, Ta increases TS and YS during hot rolling and annealing by forming fine carbides, nitrides, or carbonitrides. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides, generating composite precipitates such as (Nb,Ta)(C,N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. To obtain these effects, it is preferable to have a Ta content of 0.001% or more. More preferably, the Ta content is 0.002% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be generated. In such cases, during high-speed bending tests, void formation and crack propagation may occur starting from the martensite, potentially resulting in poor fracture resistance. Therefore, when Ta is included, a Ta content of 0.100% or less is preferable.

[0058] W: 0.500% or less Since W is an element that enhances hardenability, the addition of W generates a large amount of martensite, thereby increasing TS and YS. To obtain such an effect, it is preferable that the W content be 0.001% or more. More preferably, the W content is 0.010% or more. Even more preferably, the W content is 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, void formation and crack propagation may occur starting from the martensite, potentially resulting in poor fracture resistance. Therefore, when W is included, it is preferable to keep the W content at 0.500% or less. More preferably, the W content is 0.450% or less, and even more preferably 0.400% or less.

[0059] Mg: 0.0200% or less Mg is an effective element for increasing the fracture resistance of steel sheets by spheroidizing the shape of inclusions such as sulfides and oxides. To obtain this effect, it is preferable to have a Mg content of 0.0001% or more. More preferably, the Mg content is 0.0002% or more. On the other hand, if the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the martensite during high-speed bending tests, potentially resulting in poor fracture resistance. Therefore, when Mg is included, it is preferable to keep the Mg content below 0.0200%.

[0060] Zn: 0.0200% or less Zn is an effective element for spheroidizing the shape of inclusions and increasing the fracture resistance of steel sheets. To obtain such an effect, the Zn content is preferably 0.0005% or more. More preferably, the Zn content is 0.0010% or more. On the other hand, if the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the martensite during high-speed bending tests, potentially resulting in poor fracture resistance. Therefore, when Zn is included, it is preferable to keep the Zn content below 0.0200%.

[0061] Co:0.0200% or less Co, like Zn, is an effective element for spheroidizing inclusions and increasing the fracture resistance of steel sheets. To obtain this effect, it is preferable that the Co content be 0.0010% or higher. On the other hand, if the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the martensite during high-speed bending tests, potentially resulting in poor fracture resistance. Therefore, when Co is included, it is preferable to keep the Co content below 0.0200%.

[0062] Zr: 0.1000% or less Zr, like Zn and Co, is an effective element for spheroidizing the shape of inclusions and increasing the fracture resistance of steel sheets. To obtain such an effect, the Zr content is preferably 0.0005% or more. More preferably, the Zr content is 0.0010% or more. On the other hand, if the Zr content exceeds 0.1000%, in such cases, excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the martensite during high-speed bending tests, potentially resulting in poor fracture resistance. Therefore, when Zr is included, it is preferable to keep the Zr content below 0.1000%.

[0063] Ca:0.0200% or less Ca exists as an inclusion in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the martensite during high-speed bending tests, potentially resulting in poor fracture resistance. Therefore, when Ca is included, it is preferable to keep the Ca content below 0.0200%. Preferably, the Ca content is below 0.0020%. While there is no particular lower limit to the Ca content, a Ca content of 0.0005% or higher is preferred. Furthermore, due to production technology constraints, a Ca content of 0.0010% or higher 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 sheets. To obtain such an effect, it is preferable that the content of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM be 0.0001% or more each. On the other hand, if the content of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceeds 0.0200% each, or if the content of As exceeds 0.0500% each, a large amount of coarse precipitates and inclusions may be generated. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the martensite during high-speed bending tests, potentially preventing the acquisition of good fracture resistance. Therefore, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is included, it is preferable that the content of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM be 0.0200% or less each, and the content of As be 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 even more preferably 0.0003% or more. The Bi content is more preferably 0.0190% or less, and even more 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 this invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this invention, REM content refers to the total content of one or more elements selected from the above-mentioned REMs. The REM is not particularly limited, but La and / or Ce are preferred.

[0067] The aforementioned unavoidable impurities are impurities that are inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included to the extent that they do not hinder the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap. Examples of impurities include H and O.

[0068] <Method for manufacturing galvanized steel sheets> Hereinafter, one embodiment of the method for manufacturing galvanized steel sheets according to the present invention will be described in detail. Unless otherwise specified, the temperatures used when heating or cooling steel slabs (steel materials), steel sheets, etc., as described below refer to the surface temperatures of the steel slabs (steel materials), steel sheets, etc.

[0069] The present invention provides a method for manufacturing a galvanized steel sheet, comprising: a hot rolling step in which a steel slab having the above-mentioned component composition is hot-rolled at a finishing rolling temperature of 850 to 950°C and wound at a winding temperature of 600°C or lower; a cold rolling step in which the hot-rolled steel sheet after the hot-rolling step is cold-rolled with a reduction ratio 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 higher under conditions in which A, represented by the following formula (1), is between 4 and 70, and held at the annealing temperature for 30 seconds or more. The process includes: a plating process in which, after the annealing process, the material is cooled to a temperature range of 300-600°C and held at that temperature range for 10-300 seconds, and then zinc-plated on the surface of the steel plate; a quenching and tempering process in which, after the plating process, the material is cooled to a cooling stop temperature of (Ms-300°C) to (Ms-50°C) and then held at a tempering temperature of 250-500°C for 20-500 seconds; and a cooling process in which, after the quenching and tempering process, the material is cooled from the tempering temperature down to 50°C at an average cooling rate of 20°C / s or more.

[0070]

Number

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

[0072] Finishing rolling temperature: 850 - 950 °C When the finishing rolling temperature is less than 850 °C, ferrite transformation occurs during rolling, and the strength locally decreases. Therefore, even if the structure of the present invention is satisfied, the strength may not be obtained. Thus, the finishing rolling temperature is 850 °C or higher, preferably 880 °C or higher. On the other hand, when the finishing rolling temperature exceeds 950 °C, the crystal grains coarsen, and even if the structure of the present invention is satisfied, the strength may not be obtained. Therefore, the finishing rolling temperature is 950 °C or lower, preferably 930 °C or lower.

[0073] Coiling temperature: 600 °C or lower When the coiling temperature exceeds 600 °C, the carbides in the hot-rolled steel sheet coarsen, and such coarsened carbides may not completely dissolve during soaking at annealing, so the required impact properties may not be obtained. Therefore, the coiling temperature is 600 °C or lower, preferably 580 °C or lower. The lower limit of the coiling temperature is not particularly limited, but from the viewpoint of making it difficult to generate shape defects in the steel sheet and preventing the steel sheet from being overly hardened, it is preferable to set the coiling temperature to 400 °C or higher.

[0074] (Cold rolling process) After the hot-rolled steel sheet obtained by the hot-rolling process is subjected to pretreatment such as pickling and degreasing using commonly known methods, it is then cold-rolled. The conditions for the cold-rolling process are described below.

[0075] Cold rolling reduction ratio: over 20% If the reduction ratio (cumulative reduction ratio) during cold rolling is 20% or less, ferrite recrystallization is not promoted, unrecrystallized ferrite remains, and the steel structure of the present invention cannot be obtained. Alternatively, coarsening and non-uniformity of the steel structure are likely to occur during the annealing process, which may lead to a decrease in TS and bendability in the final product. Furthermore, if the reduction ratio in cold rolling is 20% or less, the fracture resistance may decrease. Therefore, the reduction ratio in cold rolling is greater than 20%, preferably 30% or more. There is no particular upper limit, but it is preferable that the reduction ratio for cold rolling be 95% or less.

[0076] (Annealing process) Next, we will explain the conditions for the annealing process when annealing the cold-rolled steel sheet obtained by the cold rolling process.

[0077] Heating up to the annealing temperature under conditions where A, represented by equation (1), is between 4 and 70.

number

[0078] Annealing temperature: 750°C or higher, holding time: 30 seconds or higher If the annealing temperature is below 750°C, austenite formation will be insufficient, and excess ferrite will be generated, preventing the steel structure of the present invention from being obtained. Therefore, the annealing temperature should be 750°C or higher. On the other hand, while there is no particular upper limit to the annealing temperature, it is preferable that it be 900°C or lower from the viewpoint of manufacturability. Furthermore, if the holding time at the annealing temperature is less than 30 seconds, austenite formation will be insufficient, and excess ferrite will be generated, preventing the steel structure of the present invention from being obtained. Therefore, the holding time at the annealing temperature is 30 seconds or more, preferably 60 seconds or more. There is no particular upper limit to the holding time, but in order to avoid impairing productivity, it is preferable to keep the holding time at 600 seconds or less.

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

[0080] Holding time in the temperature range of 300-600°C: 10-300 seconds Cooling to a temperature range of 300-600°C after the annealing process and holding it at this temperature range for 10-300 seconds is effective in obtaining bainite. Furthermore, the formation of bainite leads to increased carbon concentration in the untransformed austenite, resulting in a large amount of retained austenite. These effects may not be obtained if the holding time in the 300-600°C range is less than 10 seconds. Also, the desired yield ratio may not be obtained if the holding time in the 300-600°C range is less than 10 seconds. Therefore, the holding time should be 10 seconds or longer. On the other hand, if the holding time in the temperature range of 300-600°C exceeds 300 seconds, excessive bainite formation occurs, leading to excessive carbon concentration in the untransformed austenite, pearlite formation, and potentially preventing the desired amount of retained austenite from being obtained. Therefore, the holding time is 300 seconds or less, preferably 100 seconds or less.

[0081] Furthermore, after the above holding process, the steel plate is subjected to zinc plating. Zinc plating is, for example, a process of applying electro-zinc plating, hot-dip zinc plating, or alloyed hot-dip zinc plating to the surface of the steel plate. When applying hot-dip zinc plating to the surface of the steel plate, it is preferable to immerse the steel plate obtained above in a zinc plating bath at 440°C to 500°C to form a hot-dip zinc plating layer on the surface of the steel plate. Here, it is preferable to adjust the amount of plating adhesion by gas wiping or the like after the plating process. Alloying may also be performed on the steel plate after hot-dip zinc plating. When alloying hot-dip zinc plating, it is preferable to hold it at a temperature range of 450°C to 580°C for 1 second to 60 seconds to perform the alloying. When applying electro-zinc plating to the surface of the steel plate, the processing conditions for the electro-zinc plating are not particularly limited and can be followed according to conventional methods.

[0082] (Quenching and tempering process) Next, we will explain the conditions for the quenching and tempering processes that are performed after the plating process.

[0083] Cooling stop temperature: (Ms-300℃)~(Ms-50℃) If the cooling stop temperature exceeds (Ms-50℃), the formation of tempered martensite may be insufficient, or fresh martensite may be excessively formed, resulting in the failure to obtain the steel structure of the present invention. On the other hand, if the cooling stop temperature is below (Ms-300°C), there may be an excess of tempered martensite, resulting in insufficient formation of retained austenite. Therefore, the cooling stop temperature is between (Ms-300°C) and (Ms-50°C). Preferably, the cooling stop temperature is (Ms-280°C) or higher. Also, preferably, the cooling stop temperature is (Ms-100°C) or lower. In this invention, the cooling rate up to the cooling stop temperature is not limited.

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

[0085] Tempering temperature: 250~500℃, holding time: 20~500 seconds If the tempering temperature is below 250°C, the martensite will not be sufficiently tempered, making it easier for voids to form at the interface between the tempered martensite and ferrite during primary processing, which is thought to reduce the impact properties. Therefore, the tempering temperature should be 250°C or higher, preferably 300°C or higher. On the other hand, if the tempering temperature exceeds 500°C, the tempering of martensite and bainite becomes excessive, making it easier for voids to form at the interface between fresh martensite and tempered martensite and bainite during primary processing, which is thought to reduce impact properties. Therefore, the tempering temperature should be 500°C or lower, preferably 450°C or lower. Furthermore, if the holding time at the tempering temperature is less than 20 seconds, the tempering of the martensite will be insufficient, and the impact properties are likely to decrease. Therefore, the holding time at the tempering temperature should be 20 seconds or more, preferably 30 seconds or more. Also, 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 should be 500 seconds or less, preferably 450 seconds or less.

[0086] (cooling process) The conditions for the cooling process performed after the quenching and tempering processes will be explained.

[0087] Average cooling rate from the above tempering temperature to 50°C: 20°C / s or higher The impact characteristics of the present invention cannot be obtained if the average cooling rate from the tempering temperature to 50°C is less than 20°C / s. The reason for this is not clear, but it is thought to be as follows: In order to suppress void formation in the primary processed area and improve impact characteristics, it is necessary to reduce the hardness difference between the soft phase (ferrite) and the hard phase (fresh martensite) with 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 process 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 generating relatively hard bainite in the tempering process, thereby suppressing void formation. When bainite generated during the tempering process softens, the hardness difference between it and the hard phase increases. Therefore, a plating treatment that exposes the material to high temperatures is performed before the tempering process in which bainite is generated. Furthermore, by increasing the cooling rate after the tempering process, the tempering of bainite during cooling is suppressed, thereby reducing the hardness difference with the soft phase and suppressing void formation. Consequently, if the average cooling rate to room temperature after the tempering process is less than 20°C / s, the bainite will be tempered during cooling, increasing the hardness difference with the hard phase. This makes it easier for voids to form at the interface during primary processing, and is thought to reduce impact characteristics. The average cooling rate is preferably 25°C / s or higher. There is no particular upper limit to the average cooling rate, but from the viewpoint of energy saving of the cooling equipment, 70°C / s or lower is preferred. The average cooling rate (°C / s) mentioned above is calculated using the formula: "(Tempering temperature (°C) - 50) / Cooling time from tempering temperature to 50°C (s)".

[0088] The galvanized steel sheet of the present invention can be subjected to temper rolling for purposes such as shape correction and surface roughness adjustment. However, if the pressure adjustment ratio exceeds 0.5%, the bendability may deteriorate due to surface hardening, so it is preferable to keep the pressure adjustment ratio at 0.5% or less. More preferably, it is 0.3% or less. In addition, various coating treatments such as resin or oil coatings can be applied.

[0089] Other conditions for the manufacturing method are not particularly limited, but it is preferable to carry them out under the following conditions. Slabs are preferably manufactured by continuous casting to prevent macrosegregation, but they can also be manufactured by ingot casting or thin slab casting. To hot roll the slabs, they may be cooled to room temperature and then reheated before hot rolling. Alternatively, the slabs can be charged into a heating furnace and hot rolled without cooling to room temperature. An energy-saving process in which the slabs are hot rolled immediately after a short period of heat retention can also be applied. When heating the slabs, it is preferable to heat them to 1100°C or higher to prevent an increase in rolling load and to dissolve carbides. Furthermore, to prevent an increase in scale loss, it is preferable to keep the heating temperature of the slabs below 1300°C.

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

[0091] After winding, the steel sheet may have the scale removed by pickling or other methods. After pickling, under the above conditions The process involves cold rolling, annealing, and zinc plating.

[0092] <Components and methods for manufacturing components> Next, the component of the present invention and its manufacturing method will be described. The component of the present invention is obtained by subjecting a galvanized steel sheet of the present invention to at least one of forming and welding. Furthermore, the method for manufacturing the component of the present invention includes the step of subjecting a galvanized steel sheet manufactured by the method for manufacturing a galvanized steel sheet of the present invention to at least one of forming and welding.

[0093] The zinc-plated steel sheet of the present invention exhibits excellent tensile and impact properties. Therefore, components obtained using the zinc-plated steel sheet of the present invention also exhibit excellent tensile and impact properties, and are less prone to fracture during impact deformation. Accordingly, the components of the present invention can be suitably used as energy-absorbing components in automobile parts.

[0094] Forming processes can utilize common methods such as press working without restriction. Similarly, welding processes can utilize common welding methods such as spot welding and arc welding without restriction. [Examples]

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

[0096] Steel with the component composition shown in Table 1 was melted in a vacuum melting furnace and rolled into 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 process, an electro-galvanized layer (EG), a hot-dip galvanized layer (GI), or an alloyed hot-dip galvanized layer (GA) was formed on the surface of the steel sheet. In the electro-galvanized process, the steel sheet was immersed in a zinc solution while an electric current was applied, resulting in a plating adhesion of 10 to 100 g / m². 2 An electro-galvanized layer (EG) was formed. In addition, in the hot-dip galvanizing process, the steel sheet was immersed in the plating bath, and the plating adhesion amount was 10 to 100 g / m². 2 A hot-dip galvanized layer (GI) was formed. In addition, in alloyed hot-dip galvanizing, an alloying treatment was performed after forming a hot-dip galvanized layer on the steel sheet to form an alloyed hot-dip galvanized layer (GA). The final thickness of each galvanized steel sheet was 1.2 mm.

[0097] In Table 2, A is represented by the following equation (1).

number

[0098] [Table 1]

[0099] [Table 2]

[0100] After the obtained galvanized steel sheet was subjected to temper rolling with a reduction ratio of 0.2%, the area ratios of ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and retained austenite (RA) were determined according to the method described above. Furthermore, following the method described above, the area ratio of low-Mn ferrite (LF) and high-Mn ferrite (HF) relative to the total ferrite, and the area ratio of the hard second phase, where 30% or more of the perimeter is in contact with bainite, relative to the total area ratio of the hard second phase consisting of retained austenite and fresh martensite, were determined.

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

[0102] <Tensile Test> From each of the obtained galvanized steel sheets, a JIS No. 5 tensile test specimen (JIS Z2201) was taken perpendicular to the rolling direction, and the strain rate was 10 -3Tensile tests were conducted in accordance with the provisions of JIS Z2241 (2011) with a value of / s, and the tensile strength (TS), yield ratio (YR), which is the value obtained by dividing the yield strength by the tensile strength, elongation (El), and the value B (see equation (2) below), which is the area enclosed by the stress P (MPa)-strain Q (%) curve in the range of strain Q from 0 to 4%, the straight line at stress P=0, and strain Q=4, divided by the tensile strength (TS). Tensile properties were evaluated as good when TS was 780 MPa or more and less than 1180 MPa, El was 11% or more, high load-bearing capacity was evaluated when YR was 65% or more and 95% or less, and good energy absorption capacity was evaluated when B was 3.15 (%) or more.

[0103]

number

[0104] <High-speed U-bending + high-speed tight-fitting bending test> Using the obtained galvanized steel sheet, a test piece measuring 1.2 mm thick x 60 mm wide x 30 mm long was prepared, with both sides and edges finished by grinding. A U-bend (high-speed U-bend) was performed using a hydraulic bending test machine with a punch bending radius of R=4 mm and a stroke speed of 1500 mm / min, bending in the longitudinal direction (bend ridge length: 30 mmL). Next, as a close-contact bending test (high-speed close-contact bending test), a hydraulic bending tester was used, with a spacer thickness of 5 mm sandwiched between galvanized steel sheets, a stroke speed of 1500 mm / min (high speed), a pressing load of 10 tons, and a pressing time of 3 seconds. The fracture resistance was evaluated as good if no crack of 0.5 mm or more occurred on the outside (tensile side) of the bent section. In the table, ○ indicates good fracture resistance and × indicates poor fracture resistance.

[0105] [Table 3]

[0106] The galvanized steel sheet of the inventive example exhibited excellent tensile and impact properties. On the other hand, the galvanized steel sheet of the comparative example had poor tensile properties and at least one of the impact properties. [Industrial applicability]

[0107] According to the present invention, a galvanized steel sheet with excellent tensile and impact properties can be obtained. If a component obtained from the galvanized steel sheet of the present invention is used as an automobile part, it can contribute to the weight reduction of automobiles and significantly contribute to the performance of automobile bodies.

Claims

1. A galvanized steel sheet comprising a steel sheet and a zinc plating layer on the surface of the steel sheet, The aforementioned steel plate is In mass percent, C: 0.05-0.20%, Si: 0.10-2.00%, Mn: 2.0 to 3.5%, P: 0.050% or less, S: 0.050% or less, Sol. Al: 0.005-2.000% and N: Contains 0.010% or less, The composition consists of Fe and unavoidable impurities as the remainder. In terms of area ratio at positions where the plate thickness is 1 / 8 to 3 / 8, Ferrite: Less than 30.0% (including 0.0%) Baynite: Over 40.0% and under 80.0% Total of tempered martensite and bainite: over 60.0% Residual austenite: 1.5% to 20% Fresh martensite: Having a steel structure with 20.0% or less (including 0.0%), If the area ratio of the ferrite is greater than 0.0%, then the area ratio of high-Mn ferrite, where the Mn concentration is greater than 0.80 times that of the steel plate, is 30% or more of the total area ratio of the ferrite. With respect to the total area ratio of the hard second phase consisting of the retained austenite and the fresh martensite, the area ratio of the hard second phase in which 30% or more of the perimeter is in contact with bainite is 30% or more. When a U-bend with a bending radius R = 4 mm is performed on the galvanized steel sheet at a stroke speed of 1500 mm / min, a 5 mm thick spacer is placed inside the bent portion of the galvanized steel sheet, and the bending of the bent portion is performed at a stroke speed of 1500 mm / min, a pressing load of 10 tons, and a pressing time of 3 seconds until two surfaces of the galvanized steel sheet, located opposite each other in the thickness direction of the spacer, are in close contact with the spacer, the crack generated on the outside of the bent portion is less than 0.5 mm. The tensile strength is 780 MPa or more and less than 1180 MPa. Galvanized steel sheet with a yield ratio of 65% to 95%.

2. The aforementioned component composition is further expressed 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, 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 The zinc-plated steel sheet according to claim 1, comprising at least one selected from REM: 0.0200% or less.

3. The zinc-plated steel sheet according to claim 1, wherein the zinc plating layer is an electro-zinc plated layer, a hot-dip zinc plated layer, or an alloyed hot-dip zinc plated layer.

4. The zinc-plated steel sheet according to claim 2, wherein the zinc plating layer is an electro-zinc plated layer, a hot-dip zinc plated layer, or an alloyed hot-dip zinc plated layer.

5. A component obtained by forming and welding a galvanized steel sheet according to any one of claims 1 to 4.

6. A hot rolling process is performed on a steel slab having the component composition described in claim 1 or 2, with a finish rolling temperature of 850 to 950°C, and then winding at a winding temperature of 600°C or lower. A cold rolling process is performed on the hot-rolled steel sheet after the hot-rolling process, with a reduction ratio of more than 20%. The cold-rolled steel sheet after the cold-rolling process is heated to an annealing temperature of 750°C or higher under conditions where A, represented by the following formula (1), is between 4 and 70, and is held at the annealing temperature for 30 seconds or more. After the annealing process, the steel plate is cooled to a temperature range of 300 to 600°C, held at that temperature range for 10 to 300 seconds, and then subjected to a zinc plating process. After the plating process, the material is cooled to a cooling stop temperature of (Ms-300°C) to (Ms-50°C), followed by a quenching and tempering process in which it is held at a tempering temperature of 250 to 500°C for 20 to 500 seconds. The process includes a cooling step after the quenching and tempering step, in which the temperature is cooled from the tempering temperature down to 50°C at an average cooling rate of 20°C / s or more. In terms of area ratio at positions where the plate thickness is 1 / 8 to 3 / 8, Ferrite: Less than 30.0% (including 0.0%) Baynite: Over 40.0% and under 80.0% Total of tempered martensite and bainite: over 60.0% Residual austenite: 1.5% to 20% Fresh martensite: Having a steel structure with 20.0% or less (including 0.0%), If the area ratio of the ferrite is greater than 0.0%, then the area ratio of high-Mn ferrite, where the Mn concentration is greater than 0.80 times that of the steel plate, is 30% or more of the total area ratio of the ferrite. With respect to the total area ratio of the hard second phase consisting of the retained austenite and the fresh martensite, the area ratio of the hard second phase in which 30% or more of the perimeter is in contact with bainite is 30% or more. When a U-bend with a bending radius R = 4 mm is performed on the galvanized steel sheet at a stroke speed of 1500 mm / min, a 5 mm thick spacer is placed inside the bent portion of the galvanized steel sheet, and the bending of the bent portion is performed at a stroke speed of 1500 mm / min, a pressing load of 10 tons, and a pressing time of 3 seconds until two surfaces of the galvanized steel sheet, located opposite each other in the thickness direction of the spacer, are in close contact with the spacer, the crack generated on the outside of the bent portion is less than 0.5 mm. The tensile strength is 780 MPa or more and less than 1180 MPa. A method for manufacturing galvanized steel sheets having a yield ratio of 65% or more and 95% or less. [Math 1] During the ceremony, T Ａｃ1 : Temperature at point Ac1 (°C), T ＲＴ : Baking temperature (℃), t ＲＴ : From the start of heat treatment in the annealing process until the annealing temperature T ＲＴ Time required to reach (in seconds), t Ａｃ1 : Temperature T at point Ac1 after the start of heat treatment in the annealing process Ａｃ1 Time required to reach (in seconds), T(t): This is the temperature (°C) t seconds after the start of the heat treatment in the annealing process.

7. The method for manufacturing a galvanized steel sheet according to claim 6, wherein the galvanizing treatment is a treatment in which electro-galvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing is applied to the surface of the steel sheet.

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

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