Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath

Definitions

• the present invention relates to galvanized steel sheets and components with high strength and excellent crashworthiness, as well as methods for manufacturing them.
• the galvanized steel sheets of the present invention are suitable for use primarily as automotive steel sheets.
• high-strength galvanized steel sheets with a tensile strength of 780 MPa or more are prone to fracture during a collision, originating from areas subjected to primary processing (forming). This poses the challenge of inability to consistently absorb impact energy, and materials with a tensile strength of 590 MPa or less are primarily used. Therefore, there is still room for improvement in the development of energy-absorbing components that can suppress component fracture during impact and consistently absorb high energy, thereby ensuring crash safety, 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 energy absorption components.
• Patent Document 1 discloses technology relating to high-strength galvanized steel sheet with a maximum tensile strength of 780 MPa or more, which can be used as an impact absorbing component during a collision.
• 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 good energy absorption, resulting in excellent collision characteristics.
• Patent Document 1 while crash performance is evaluated by judging cracks in an axial crush test on the hat material, the fracture resistance characteristics are evaluated.
• the crack judgment is performed after the crush, it is not possible to evaluate the process from the occurrence of cracks during the crush to fracture, which is important for crash performance. 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 can reduce absorbed energy, but it is difficult to judge such minor cracks after the crushing.
• a crack occurs late in the crushing process, even a large crack that penetrates the plate thickness can have almost no effect on absorbed energy. Therefore, it is considered that judging cracks after the crushing alone is insufficient for evaluating fracture resistance characteristics.
• crashworthiness is evaluated by measuring the void density in a cross section within a 0-50 ⁇ m region from the surface of the steel plate on the compressed side when bending the steel plate 90° at a stroke speed of 20 mm/min, and by measuring the stroke at maximum load on the stroke-load curve when bending-orthogonal bending tests are performed at a stroke speed of 20 mm/min. Absorbed energy is evaluated by an axial crushing test on a hat-shaped member.
• the fracture resistance required for automotive energy absorption components is the fracture resistance when deformed at high speed.
• the present invention was made in light of these circumstances, and aims to provide zinc-plated steel sheets and components with excellent tensile and crash properties, suitable for use in automotive energy absorption components, as well as methods for manufacturing these.
• excellent tensile properties means 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. Furthermore, excellent crashworthiness means high load resistance, good energy absorption capacity, and good breaking resistance.
• high load-bearing capacity 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 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 outside of the bent portion (tensile side) is less than 0.5 mm.
• the U-bending process is performed with a bending radius R of 4 mm and a stroke speed of 1500 mm/min.
• the adhesive bending test is performed with a spacer sandwiched between galvanized steel sheets having a thickness of 5 mm, a stroke speed of 1500 mm/min, a pressing load of 10 ton, and a pressing time of 3 seconds.
• the galvanized steel sheet comprises a steel sheet having a steel structure in which, in area percentages, ferrite is less than 30.0% (including 0.0%), bainite is more than 40.0% and not more than 80.0%, the sum of tempered martensite and bainite is more than 60.0%, retained austenite is 1.5% to 20%, and fresh martensite is 20.0% or less, and a galvanized layer is formed on the surface of the steel sheet.
• the area percentage of high-Mn ferrite (HF), in which the Mn concentration is more than 0.80 times the Mn concentration of the steel sheet, relative to the total area percentage of ferrite is 30% or more
• the area percentage of the hard second phase, which is composed of retained austenite and fresh martensite and is in contact with bainite over 30% or more of its circumferential length is 30% or more relative to the total area percentage of the hard second phase, which is composed of retained austenite and fresh martensite, and the galvanized steel sheet is subjected to a U-bending process with a bending radius R of 4 mm by a straw bending.
• a galvanized steel sheet comprising a steel sheet and a galvanized layer on the surface of the steel sheet,
• the steel plate has an area ratio at a position of 1/8 to 3/8 of the plate thickness, Ferrite: less than 30.0% (including 0.0%) Bainite: more than 40.0% and not more than 80.0%; Sum of tempered martensite and bainite: more than 60.0%; Retained austenite: 1.5% or more and 20% or less, Fresh martensite: Having a steel structure in which the content is 20.0% or less (including 0.0%),
• the ferrite exceeds 0.0% in area ratio, the area ratio of high Mn ferrite having a Mn concentration exceeding 0.80 times the Mn concentration of the steel plate is 30% or more relative to the total area ratio of the ferrite, an 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
• the steel plate comprises, in mass%, 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, Contains sol. Al: 0.005 to 2.000% and N: 0.010% or less,
• the component 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, 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,
• 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, followed by a plating step of subjecting the steel sheet surface to a zinc plating treatment.
• a quenching and tempering process is performed in which 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; 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 steps.
• T Ac1 Temperature at Ac1 point (°C)
• T RT annealing temperature (°C)
• t RT the time (seconds) required from the start of heat treatment in the annealing step to reach the annealing temperature T RT
• T(t) Temperature (°C) t seconds after the start of heat treatment in the annealing step.
• the present invention it is possible to obtain a galvanized steel sheet with excellent tensile properties and crashworthiness.
• Components obtained by subjecting the galvanized steel sheet of the present invention to forming, welding, etc. can be suitably used as energy absorbing components in the automotive field.
• 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, the steel structure comprising, by area percentage, 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% or more and 20% or less, and fresh martensite: 20.0% or less (inclusive 0.0%); and a galvanized layer on a surface of the steel sheet,
• the area ratio of 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 of the total area ratio of ferrite
• the area ratio of the hard second phase which is composed of retained austenite and fresh martensite and has 30% or more of its circumferential length in contact with bainite, is
• Ferrite area ratio less than 30.0% (including 0.0%) Although ferrite is effective in improving elongation, 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 requirements of 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.
• 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 difference in hardness with adjacent martensite and ferrite, suppresses the generation of voids during high-speed collisions, and improves high-speed collision properties. Furthermore, bainite improves elongation. If the bainite area fraction is 40.0% or less, these effects cannot be fully achieved, making it difficult to achieve a TS of 780 MPa or more, an elongation of 11% or more, and high-speed collision properties. Therefore, the bainite area fraction is more than 40.0%, preferably more than 45.0%, and more preferably more than 50.0%.
• the area fraction of bainite exceeds 80.0%, it is difficult to obtain the desired tensile strength. Therefore, the area fraction of bainite is 80.0% or less, preferably 70.0% or less, and more preferably 65.0% or less.
• Total area ratio of tempered martensite and bainite more 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 sufficiently obtained, making it difficult to achieve both a TS of 780 MPa or more and high-speed impact properties. Therefore, the total area ratio of these is more than 60.0%, preferably more than 65.0%, and more preferably more than 82.0%. There is no upper limit to the total area ratio, but in consideration of the balance with other structures, the total area ratio is preferably 95.0% or less.
• 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 portions 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.
• the area fraction of retained austenite is 1.5% or more, preferably 3% or more, and more preferably 5% or more.
• 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 fraction of retained austenite is 20% or less, preferably 15% or less, and more preferably 10% or less.
• 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%, it may degrade the impact properties. 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.
• High Mn ferrite (HF) in which the Mn concentration is more than 0.80 times the Mn concentration of the steel sheet: 30% or more (including 100.0%) of the total ferrite area ratio Low-Mn ferrite (LF), whose Mn concentration is 0.80 times or less that of the steel sheet, has a lower dislocation density and is relatively softer than high-Mn ferrite (HF), whose Mn concentration is more than 0.80 times that of the steel sheet.
• LF total ferrite area ratio
• HF high-Mn ferrite
• 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 high high-speed impact properties can be obtained. Therefore, when the area fraction of ferrite is more than 0.0%, the area fraction of high-Mn ferrite (LF) is set to 30% or more (including 100.0%) of the area fraction of the total ferrite.
• the area fraction of high-Mn ferrite is preferably 50% or more, more preferably 70% or more, of the area fraction of the total ferrite.
• the area ratio of low Mn ferrite (LF) is 70% or less (including 0.0%) of the area ratio of total ferrite.
• the area ratio of low Mn ferrite to the area ratio of total ferrite is preferably 50% or less, more preferably 30% or less.
• the low Mn ferrite (LF) may include recrystallized ferrite (RF) formed during the temperature rise in the heat treatment, or may consist of this recrystallized ferrite.
• the high Mn ferrite (HF) may contain ferrite formed during cooling (TF) formed during cooling, or may consist of this ferrite formed during cooling.
• Area fraction of hard second phases that contact bainite over 30% or more of their circumferential length 30% or more of the area fraction of the entire hard second phase
• those that contact bainite over 30% or more of their circumferential length have a high concentration of solute C and high stability of the retained austenite. Therefore, the island regions that contact bainite over 30% or more of their circumferential length play an important role in ensuring good work hardenability and ductility.
• solute C diffuses from the bainite to the surrounding untransformed austenite, and only the region of the untransformed austenite in contact with the 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 within the bainite is highly stable and plays an important role in ensuring good work hardening ability and ductility.
• ensuring ductility means that the elongation (El) is 11% or more.
• the total area ratio of the hard second phases, the hard second phases of which 30% or more of the circumferential length is in contact with bainite is 30% or more, preferably 40% or more, and more preferably 60% or more of the total area ratio of all the hard second phases.
• the upper limit is not particularly limited and may be 100%.
• 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 ⁇ m ⁇ 64 ⁇ m. 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.
• ferrite, bainite, retained austenite, fresh martensite, and tempered martensite are distinguished as follows.
• Ferrite A black region that does not contain retained austenite or fresh martensite structures with an aspect ratio of 2.0 or more inside the crystal grains.
• Bainite A black region containing one or more structures of retained austenite and fresh martensite with an aspect ratio of 2.0 or more inside the crystal grains.
• 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 more, and containing carbides;
• Retained austenite and fresh martensite can be distinguished as white areas.
• 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.
• 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 intensity of the (200), (220), and (311) planes of fcc iron to the integrated X-ray diffraction intensity of the (200), (211), and (220) planes of bcc iron in the 1/4 plane of the plate thickness.
• the average Mn concentration of the steel sheet is calculated from the 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 ratio of high Mn ferrite is determined by subtracting the area ratio of the obtained low Mn ferrite from the total area ratio of ferrite.
• the low-Mn ferrite region is formed by the distribution of Mn from ferrite to austenite during the annealing process (heating process and annealing holding process), and the Mn concentrations of bainite, martensite, tempered martensite, and austenite formed from austenite during the subsequent cooling process are always more than 0.80 times the Mn concentration of the base material.
• 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.
• the stroke speed can be measured, for example, by measuring the stroke per certain time using a scale and converting it into the amount of movement per 60 seconds.
• the amount of movement can be the relative movement of one end of the steel plate to one end when the steel plate to be bent is viewed as a U.
• the stroke can be measured at any position on the bending tester that moves integrally with the metal fitting that presses against the steel plate.
• the time may be adjusted within a range that allows normal measurement, and for example, a stroke may be measured for 5 seconds.
• 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.
• Desired collision 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 amounts of low-Mn ferrite (LF), high-Mn ferrite (HF), and bainite produced during the pre-quenching process. This allows high-Mn ferrite (HF), which is harder than low-Mn ferrite (LF), to be formed. Bainite is produced during the pre-quenching process, and softened during the plating and tempering processes.
• LF low-Mn ferrite
• HF high-Mn ferrite
• Bainite is produced during the pre-quenching process, and softened during the plating and tempering processes.
• the crack length can be suppressed to less than 0.5 mm (including 0 ⁇ m).
• the absence of ferrite does not necessarily mean that ferrite is present, since the generation of voids due to differences in hardness of the structure is suppressed and the impact characteristics are improved.
• 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 ⁇ m).
• the crack length referred to in the present invention is the length of a crack that occurs on the outside (tensile side) of a bent portion when a U-bending process with a bending radius R of 4 mm is performed at a stroke speed of 1500 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 bent until the two surfaces of the galvanized steel sheet that are positioned opposite the upper and lower surfaces in the thickness direction of the spacer, respectively, come into close contact with the spacer.
• the crack length was measured as follows: A galvanized steel sheet was 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 contact bending test (high-speed contact bending test) was performed in which a spacer having a thickness of 5 mm was placed on the inside (compression side) of the bent part of the galvanized steel sheet, and the bent part 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 steel sheet located opposite the upper and lower surfaces in the thickness direction of the spacer came into close contact with the spacer, and the length of a crack that occurred on the outside (tensile side) of the bent part was measured by visual observation of the appearance.
• 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 an alloyed hot-dip galvanized layer.
• the galvanized steel sheet of the present invention has excellent tensile properties.
• 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.
• High strength in the present invention means that the tensile strength TS is 780 MPa or more and less than 1180 MPa.
• excellent ductility means that the elongation El is 11% or more.
• the methods for measuring the tensile strength TS and elongation El are as described in the Examples.
• the thickness of the galvanized steel sheet of the present invention is preferably 0.2 mm or more and 3.2 mm or less.
• the galvanized steel sheet of the present invention has excellent impact properties.
• excellent impact properties means high load capacity, good fracture resistance, and good energy absorption.
• high load 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.
• Good fracture resistance as used herein means that the crack length is less than 0.5 mm when subjected to U-bending and close bending as described in the examples.
• composition of steel plate (Composition of steel plate) 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.
• C 0.05-0.20% C is an element necessary for improving strength because it facilitates the formation of phases other than ferrite and also forms alloy compounds with Nb, Ti, etc. If the C content is less than 0.05%, the desired strength may not be ensured even if the manufacturing conditions are optimized. Therefore, the C content is preferably 0.05% or more, 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 obtained even if the manufacturing conditions are optimized. Therefore, the C content is preferably 0.20% or less, and more preferably 0.18% or less.
• Si 0.10-2.00%
• Si suppresses carbide formation and contributes to stabilizing retained austenite.
• Si is also a solid solution strengthening element, contributing to improving the balance between strength and ductility.
• the Si content is preferably 0.10% or more, and more preferably 0.50% or more.
• the Si content is preferably 2.00% or less, and more preferably 1.50% or less.
• Mn 2.0-3.5%
• 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, and more preferably 2.5% or more. On the other hand, if the Mn content exceeds 3.5%, the fraction of retained austenite increases excessively, which may result in a deterioration in impact properties. Therefore, the Mn content is preferably 3.5% or less, and more preferably 3.3% or less.
• P 0.050% or less
• the P content is preferably 0.050% or less, and more preferably 0.010% or less.
• the lower limit that is currently industrially feasible is about 0.002%, and it is substantially higher than that.
• 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.
• Sol. 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 satisfied. Therefore, the sol. Al content is preferably 0.005% or more. On the other hand, if the sol. Al content exceeds 2.000%, the quality of the slab during steelmaking deteriorates. Furthermore, if the sol. Al content exceeds 2.000%, the fracture resistance may deteriorate even if the steel structure of the present invention is satisfied. Therefore, the sol. Al content is preferably 2.000% or less, and more preferably 1.000% or less.
• 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.
• composition of the steel sheet according to the present invention preferably contains the above-mentioned elemental components as the basic components, with the remainder consisting of iron (Fe) and unavoidable impurities.
• the steel sheet according to the present invention may contain the following components (optional elements) as appropriate, depending on the desired properties.
• Nb 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS.
• the Nb content is preferably 0.001% or more, and more preferably 0.005% or more.
• 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 result in poor 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.
• Ti 0.200% or less Like Nb, Ti forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. 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%, a large amount 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 result in poor 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.
• V 0.200% or less
• V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS.
• the V content is preferably 0.001% or more.
• the V content is more preferably 0.003% or more.
• the V content is further preferably 0.005% or more.
• the V content exceeds 0.200%, a large amount 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 result in poor 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.
• B 0.0100% or less
• B is an element that segregates at austenite grain boundaries to improve hardenability.
• B is also an element that controls the formation and grain growth of ferrite during cooling after annealing.
• the B content is preferably 0.0001% or more.
• the B content is more preferably 0.0002% or more.
• the B content is further preferably 0.0003% or more.
• the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling.
• voids may be generated and cracks may propagate from the martensite, which may result in poor 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.
• 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.
• the Cr content is preferably 0.0005% or more.
• the Cr content is more preferably 0.0010% or more.
• the Cr content is further preferably 0.010% or more.
• the Cr content exceeds 1.000%, the area ratio of martensite increases, and during a high-speed bending test, voids may be generated and cracks may propagate from the martensite, making it difficult to obtain 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.
• 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.
• the Ni content is preferably 0.005% or more.
• the Ni content is more preferably 0.010% or more.
• the Ni content is further preferably 0.020% or more.
• the Ni content exceeds 1.000%, the area ratio of martensite increases, and during a high-speed bending test, voids are generated and cracks propagate from the martensite, which may prevent the desired fracture resistance from being obtained. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less.
• the Ni content is more preferably 0.800% or less.
• Mo 1.000% or less
• Mo is an element that improves hardenability, and the addition of Mo increases the formation of a large amount of tempered martensite, thereby increasing TS and YS.
• the Mo content is preferably 0.010% or more.
• the Mo content is more preferably 0.030% or more.
• the Mo content exceeds 1.000%, the area ratio of martensite increases, and during a high-speed bending test, voids may be generated and cracks may propagate from the martensite, possibly 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.
• 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.
• 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.
• Sn 0.200% or less
• 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.
• 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.
• 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.
• the Cu content is preferably 0.005% or more.
• the Cu content is more preferably 0.010% or more.
• the Cu content is further preferably 0.020% or more.
• the area ratio of martensite may increase excessively.
• a large amount of coarse precipitates and inclusions may be formed. In such cases, the excessively formed martensite and the coarse precipitates and inclusions may cause void formation and crack propagation starting from the martensite during high-speed bending tests, which may result in poor fracture resistance. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less.
• the Cu content is more preferably 0.200% or less.
• 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. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses the 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%, a large amount of coarse precipitates and inclusions may be generated. In such cases, during a high-speed bending test, voids may be generated and cracks may grow from the martensite, and good fracture resistance may not be obtained. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less.
• 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.
• the W content is preferably 0.001% or more.
• the W content is more preferably 0.010% or more.
• the W content is further preferably 0.030% or more.
• the W content exceeds 0.500%, the area ratio of martensite increases, and during a high-speed bending test, voids may be generated and cracks may propagate from the martensite, making it difficult to obtain good fracture resistance. Therefore, when W is contained, 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.
• Mg 0.0200% or less
• Mg is an element that is effective in spheroidizing inclusions such as sulfides and oxides and improving the fracture resistance of steel sheets.
• the Mg content is preferably 0.0001% or more.
• the Mg content is more preferably 0.0002% or more.
• the Mg 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 voids to form and cracks to propagate from the martensite during a high-speed bending test, which may result in poor fracture resistance. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less.
• Zn 0.0200% or less
• Zn is an element that is effective in spheroidizing the shape of inclusions and improving the fracture resistance of the steel sheet.
• the Zn content is preferably 0.0005% or more.
• the Zn content is more preferably 0.0010% or more.
• the Zn 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 starting from the martensite during a high-speed bending test, which may result in poor fracture resistance. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.
• Co 0.0200% or less
• Co is an element that is effective in making inclusions spheroidal and improving the fracture resistance of the steel sheet.
• the Co content is preferably 0.0010% or more.
• 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 starting from the martensite during a high-speed bending test, which may result in poor fracture resistance. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.
• Zr 0.1000% or less
• Zr is an element that is effective in spheroidizing the shape of inclusions and improving the fracture resistance of the steel sheet.
• the Zr content is preferably 0.0005% or more.
• the Zr content is more preferably 0.0010% or more.
• the Zr content is preferably 0.1000% or less.
• Ca 0.0200% or less Ca exists as inclusions in steel.
• the Ca content is preferably 0.0200% or less.
• the Ca content is preferably 0.0020% or less.
• the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more due to constraints on production technology.
• the Se content is more preferably 0.0002% or more, and further preferably 0.0003% or more.
• the Se content is more preferably 0.0190% or less, and further preferably 0.0180% or less.
• the Te content is more preferably 0.0002% or more, and further preferably 0.0003% or more.
• the Te content is more preferably 0.0190% or less, and further preferably 0.0180% or less.
• the Ge content is more preferably 0.0002% or more, and further preferably 0.0003% or more.
• the Ge content is more preferably 0.0190% or less, and further preferably 0.0180% or less.
• the As content is more preferably 0.0002% or more, and further preferably 0.0003% or more.
• the As content is more preferably 0.0490% or less, and further preferably 0.0480% or less.
• the Sr content is more preferably 0.0002% or more, and further preferably 0.0003% or more.
• the Sr content is more preferably 0.0190% or less, and further preferably 0.0180% or less.
• the Cs content is more preferably 0.0002% or more, and further preferably 0.0003% or more.
• the Cs content is more preferably 0.0190% or less, and further preferably 0.0180% or less.
• the Hf content is more preferably 0.0002% or more, and further preferably 0.0003% or more.
• the Hf content is more preferably 0.0190% or less, and further preferably 0.0180% or less.
• the Pb content is more preferably 0.0002% or more, and further preferably 0.0003% or more.
• the Pb content is more preferably 0.0190% or less, and further 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 further preferably 0.0003% or more.
• the REM content is more preferably 0.0190% or less, and further preferably 0.0180% or less.
• REM refers to scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides from lanthanum (La), which has atomic number 57, to lutetium (Lu), which has 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. There are no particular limitations on the REM, but La and/or Ce are preferred.
• the aforementioned inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, or manufacturing equipment, and are permitted to be included to the extent that they do not impair the objectives of the present invention.
• Raw materials include iron ore, reduced iron, scrap, etc.
• Impurities include, for example, H, O, etc.
• the method for manufacturing galvanized steel sheet of the present invention includes a hot rolling process in which a steel slab having the above-mentioned chemical composition is hot-rolled at a finishing 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%; and 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 is held at the annealing temperature for 30 seconds or more.
• the steel sheet is then cooled to a temperature range of 300 to 600°C after the annealing process, held at that temperature range for 10 to 300 seconds, and then zinc-plated on the surface.
• 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.
• the steel sheet is cooled from the tempering temperature to 50°C at an average cooling rate of 20°C/s or more.
• T Ac1 Temperature at Ac1 point (°C)
• T RT annealing temperature (°C)
• t RT the time (seconds) required from the start of heat treatment in the annealing step to reach the annealing temperature T RT
• T(t) Temperature (°C) t seconds after the start of heat treatment in the annealing step.
• 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 in some cases, strength may not be obtained even if the structure of the present invention is satisfied. Therefore, the finish rolling temperature is 850°C or higher, and preferably 880°C or higher. On the other hand, if the finish rolling temperature exceeds 950°C, the crystal grains become coarse, and even if the structure of the present invention is satisfied, strength may not be obtained. Therefore, the finish rolling temperature is 950°C or lower, and preferably 930°C or lower.
• 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.
• Cold rolling process The hot-rolled steel sheet obtained by the hot rolling process is subjected to pre-treatment such as pickling and degreasing by a commonly known method, and then cold rolling is carried out.
• pre-treatment such as pickling and degreasing by a commonly known method
• cold rolling is carried out.
• the conditions of the cold rolling process when cold rolling is carried out will be described below.
• Cold rolling reduction rate More than 20% If the cold rolling reduction rate (cumulative reduction rate) is 20% or less, the recrystallization of ferrite 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 being prone to coarsening and non-uniformity in the annealing step, which may result in reduced TS and bendability in the final product. Furthermore, if the reduction rate of cold rolling is 20% or less, the fracture resistance may decrease. Therefore, the reduction ratio of the cold rolling is more than 20%, and preferably 30% or more. Although there is no particular upper limit, the reduction rate of the cold rolling is preferably 95% or less.
• T Ac1 Temperature at Ac1 point (°C)
• T RT annealing temperature (°C)
• t RT the time (seconds) required from the start of heat treatment in the annealing step to reach the annealing temperature T RT
• T(t) Temperature (°C) t seconds after the start of heat treatment in the annealing step.
• a defined by formula (1) As for the heating conditions up to the annealing temperature, if A defined by formula (1) is less than 4, the formation of austenite becomes insufficient and excessive ferrite is formed, failing to obtain the steel structure of the present invention. As a result, the fracture resistance during high-speed bending deteriorates. Therefore, A should be 4 or more, preferably 5 or more, and more preferably 8 or more. On the other hand, if the heating condition up to the annealing temperature is such that A defined by formula (1) exceeds 70, element diffusion proceeds and the elements in the austenite during annealing are uniformly distributed, suppressing the formation of bainite in the subsequent holding step before quenching and the tempering step, and the steel structure of the present invention cannot be obtained.
• T Ac1 can be calculated by the following formula (3).
• T Ac1 723+29 ⁇ [Si%]-21 ⁇ [Mn%]-17 ⁇ [Ni%]+17 ⁇ [Cr%]...Formula (3)
• [element symbol %] represents the content (mass %) of each element contained in the steel sheet, and elements that are not contained are represented as 0 (zero).
• Annealing temperature 750°C or higher, holding time: 30 seconds or longer If the annealing temperature is less than 750°C, the formation of austenite will be insufficient and excessive ferrite will be formed, making it impossible to obtain the steel structure of the present invention. Therefore, the annealing temperature is 750°C or higher.
• the upper limit of the annealing temperature is not particularly limited, but from the viewpoint of manufacturability, it is preferably 900°C or less.
• 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 not to impair productivity, it is preferable to set the holding time to 600 seconds or less.
• the conditions for the plating process will be described.
• 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, after which the steel sheet surface is subjected to a zinc plating treatment.
• 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 causes C to concentrate in the untransformed austenite, resulting in the production of a large amount of retained austenite. If the holding time in the temperature range of 300 to 600°C is less than 10 seconds, these effects may not be obtained. Furthermore, if the holding time in the temperature range of 300 to 600°C is less than 10 seconds, the desired yield ratio may not be obtained. Therefore, the holding time is 10 seconds or more.
• the retention time is 300 seconds or less, preferably 100 seconds or less.
• Galvanizing treatment is, for example, a treatment in which the steel sheet surface is subjected to electrogalvanizing, hot-dip galvanizing, or alloyed hot-dip galvanizing.
• hot-dip galvanizing 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.
• the steel sheet after the hot-dip galvanizing treatment may also be alloyed.
• the hot-dip galvanized steel sheet When alloying hot-dip galvanizing, it is preferable to hold the hot-dip galvanized steel sheet at a temperature of 450°C to 580°C for 1 to 60 seconds.
• the treatment conditions for the electrogalvanizing treatment are not particularly limited and may be conventional.
• Cooling stop temperature (Ms-300°C) ⁇ (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 formed in excess, making it impossible to obtain the steel structure of the present invention. On the other hand, if the cooling stop temperature is less than (Ms-300°C), the tempered martensite becomes excessive and the generation 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. In addition, the cooling stop temperature is preferably (Ms-100°C) or lower. In the present invention, the cooling rate up to the cooling stop temperature is not limited.
• Ms is the martensitic transformation start temperature, and can be calculated by the following formula (4).
• Ms (°C) 550-361 ⁇ [C%]-39 ⁇ [Mn%]-10 ⁇ [Cu%]-17 ⁇ [Ni%]-20 ⁇ [Cr%]-5 ⁇ [Mo%]-35 ⁇ [V%]+30 ⁇ [Al%]-5 ⁇ [W%]+15 ⁇ [Co%]...Formula (4)
• the symbol "%" represents the content (mass %) of each element contained in the steel sheet, and elements that are not contained are set to 0.
• 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, and 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 thought to make it easier for 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.
• the tempering temperature is 500°C or lower, and preferably 450°C or lower. Furthermore, if the holding time at the tempering temperature is less than 20 seconds, the tempering of martensite will be insufficient, and it is believed that the crashworthiness will be reduced. Therefore, the holding time at the tempering temperature is 20 seconds or more, preferably 30 seconds or more. 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, preferably 450 seconds or less.
• cooling process The conditions for the cooling process performed after the quenching and tempering processes will be described.
• 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.
• 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 to form 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.
• the average cooling rate to room temperature after the tempering process is less than 20°C/s, the bainite is tempered 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 more. There is no particular upper limit to the average cooling rate, but from the perspective of energy conservation in cooling equipment, 70°C/s or less is preferred.
• the average cooling rate (°C/s) is calculated by "(tempering temperature (°C) - 50)/cooling time (s) from the tempering temperature to 50°C".
• the galvanized steel sheet of the present invention can be subjected to temper rolling for purposes such as correcting the shape and adjusting the surface roughness.
• temper rolling rate exceeds 0.5%, the surface may harden and deteriorate the bendability, so the temper rolling rate is preferably 0.5% or less, and more preferably 0.3% or less.
• Various painting treatments such as resin or oil coatings can also be applied.
• Slabs are preferably produced by continuous casting to prevent macrosegregation, but can also be produced by ingot casting or thin slab casting.
• the slab may be cooled to room temperature and then reheated before hot-rolling.
• the slab may be charged into a heating furnace without being cooled to room temperature and then hot-rolled.
• An energy-saving process can also be applied in which the slab is hot-rolled immediately after a short period of heat retention.
• 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.
• the heating temperature of the slab is preferably 1300°C or lower to prevent an increase in scale loss.
• the rough bar after rough rolling can be heated to prevent problems during rolling when the slab heating temperature is low. It is also possible to join the rough bars together and perform continuous finish rolling, a process known as continuous rolling. Furthermore, to reduce the rolling load and ensure uniform shape and material quality, it is preferable to perform lubricated rolling with a friction coefficient of 0.10 to 0.25 for all or some of the finish rolling passes.
• the steel sheet After coiling, the steel sheet may be subjected to scale removal by pickling, etc. After pickling, the steel sheet is subjected to cold rolling, annealing, and galvanization under the above-mentioned conditions.
• the member of the present invention is obtained by subjecting the galvanized steel sheet of the present invention to at least one of forming and welding. Also, the method for manufacturing a member of the present invention includes a step of subjecting the 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.
• the galvanized steel sheet of the present invention has excellent tensile properties and impact resistance. Therefore, components made using the galvanized steel sheet of the present invention also have excellent tensile properties and impact resistance, and are less likely to break during impact deformation. Therefore, the components of the present invention can be suitably used as energy-absorbing components in automotive parts.
• general processing methods such as press working can be used without restrictions.
• general welding methods such as spot welding and arc welding can be used without restrictions.
• the steel sheet was immersed in a plating bath to form a hot-dip galvanized layer (GI) with a coating weight of 10 to 100 g/ m2 .
• a hot-dip galvanized layer was formed on the steel sheet, followed by an alloying process to form a galvannealed layer (GA).
• the final thickness of each galvanized steel sheet was 1.2 mm.
• T Ac1 Temperature at Ac1 point (°C)
• T RT annealing temperature (°C)
• t RT the time (seconds) required from the start of heat treatment in the annealing step to reach the annealing temperature T RT
• T(t) Temperature (°C) t seconds after the start of heat treatment in the annealing step.
• the obtained galvanized steel sheets were subjected to temper rolling at a rolling reduction of 0.2%, and then the area ratios of ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and retained austenite (RA) were determined according to the above-mentioned method. Furthermore, according to the above-mentioned method, the area ratios of low Mn ferrite (LF) and high Mn ferrite (HF) to the total ferrite, and the area ratio 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, to the total area ratio of the hard second phase consisting of retained austenite and fresh martensite, were determined.
• LF low Mn ferrite
• HF high Mn ferrite
• tensile properties tensile strength TS, elongation El
• impact properties load capacity (yield ratio YR), break resistance, energy absorption capacity (B)) were evaluated according to the following test methods.
• a contact bending test (high-speed contact bending test) was performed using a hydraulic bending tester with a spacer between the galvanized steel sheets having a thickness of 5 mm, 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 ⁇ .
• the zinc-plated steel sheets of the invention examples had excellent tensile properties and impact resistance.
• the zinc-plated steel sheets of the comparative examples had poor tensile properties or impact resistance.

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