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

Definitions

• the present invention relates to zinc-based plated steel sheets and their manufacturing method.
• delayed fracture refers to a phenomenon in which, when a formed part is placed in a hydrogen penetration environment, hydrogen penetrates into the steel plate that constitutes the part, reducing the interatomic bonding strength or causing localized deformation, resulting in microcracks, which then propagate and lead to the destruction of the steel plate.
• Patent Document 1 describes a plated steel sheet that has excellent mechanical properties, reduces the amount of hydrogen that penetrates during manufacturing, and has excellent hydrogen embrittlement resistance and plating adhesion, and a manufacturing method thereof.
• Patent Document 2 describes an ultra-high strength thin steel sheet that has excellent hydrogen embrittlement resistance by controlling the chemical composition and residual austenite of the steel sheet, and a manufacturing method thereof.
• the present invention aims to provide a zinc-based plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0% or more, a hole expansion ratio ( ⁇ ) of 25% or more, and excellent delayed fracture resistance in stretch flanged areas, and a manufacturing method thereof.
• TS tensile strength
• El elongation
• ⁇ hole expansion ratio
• the present inventors have obtained the following findings. (1) At a position 1/4 of the thickness of the base steel sheet, the total area ratio of tempered martensite and fresh martensite is 70.0% or more, the area ratio of bainite is 10.0% or less, the thickness of the carbon concentration reduction portion in the surface layer of the base steel sheet is 150.0 ⁇ m or less, and the total coverage rate of retained austenite and fresh martensite at the prior austenite grain boundary directly below the carbon concentration reduction portion is 30.0% or more, thereby achieving a TS of 1,470 MPa or more.
• a steel slab having a predetermined component composition is used, and in the holding steps or each cooling step after the annealing step and the plating step, the holding time or cooling time and cooling rate are controlled, whereby a zinc-based plated steel sheet having a structure that satisfies the above (1) to (3) can be obtained.
• a zinc-based plated steel sheet according to any one of [1] to [3] above, in which the zinc-based plated layer is an electrolytic zinc-plated layer, a hot-dip zinc-plated layer, or an alloyed hot-dip zinc-plated layer.
• a zinc-based plated steel sheet according to an embodiment of the present invention has a base steel sheet and a zinc-based plated layer formed on the surface of the base steel sheet.
• the base steel sheet has a composition containing, in mass%, C: 0.180% to 0.250%, Si: 0.800% to 1.550%, Mn: 2.400% to 3.200%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities.
• % representing the content of a component element of the base steel sheet means “mass%” unless otherwise specified.
• C is one of the important basic components of the base steel sheet, and in particular in the present invention, it is an important element that affects the total area ratio of tempered martensite and fresh martensite and the area ratio of bainite. If the C content is less than 0.180%, the total area ratio of tempered martensite and fresh martensite decreases, the area ratio of bainite increases, and it becomes difficult to achieve a TS of 1470 MPa or more. Therefore, the C content is 0.180% or more, preferably 0.200% or more, and more preferably 0.210% or more.
• Si is one of the important basic components of the base steel sheet and is an important element that affects the TS and the volume fraction of the retained austenite. If the Si content is less than 0.800%, the strength of the tempered martensite and the fresh martensite decreases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the Si content is 0.800% or more, preferably 0.850% or more, and more preferably 0.900% or more. On the other hand, if the Si content exceeds 1.550%, the retained austenite increases excessively and the hole expandability decreases. Therefore, the Si content is 1.550% or less, preferably 1.500% or less, and more preferably 1.400% or less.
• Mn is one of the important basic components of the base steel sheet, and is an important element that affects the total area ratio of tempered martensite and fresh martensite, and the area ratio of bainite. If the Mn content is less than 2.400%, the total area ratio of tempered martensite and fresh martensite decreases, the area ratio of bainite increases, and it becomes difficult to achieve a TS of 1470 MPa or more. Therefore, the Mn content is 2.400% or more, preferably 2.500% or more, and more preferably 2.600% or more.
• the Mn content is 3.200% or less, preferably 3.100% or less, and more preferably 3.000% or less.
• the P content is set to 0.100% or less, and preferably 0.070% or less.
• the P content is preferably set to 0.001% or more.
• N 0.0100% or less
• the N content is set to 0.0100% or less, and preferably 0.0050% or less.
• the N content is 0.0001% or more due to constraints on production technology.
• the O content is set to 0.0100% or less, and preferably 0.0050% or less.
• the O content be 0.0001% or more due to constraints on production technology.
• the base steel sheet further contains, by mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.
• Sn 0.200% or less
• Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flange processing portion will not deteriorate. Therefore, when Sn is contained, its content is 0.200% or less, and preferably 0.100% or less.
• the Sn content is preferably 0.001% or more.
• Sb 0.200% or less
• the Sb content is 0.200% or less, the amount of coarse precipitates or inclusions will not increase, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, when Sb is contained, its content is set to 0.200% or less, and preferably 0.100% or less.
• the Sb content is preferably set to 0.001% or more.
• the lower limit of the content of Zr, Zn, Pb, and Te is not particularly specified, these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the base steel sheet, so that the content of Zr, Zn, Pb, and Te is preferably 0.001% or more.
• Hf 0.10% or less
• the Hf content is 0.10% or less, the amount of coarse precipitates or inclusions will not increase, and the base steel sheet will not be embrittled, so that the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, if Hf is contained, its content should be 0.10% or less, and preferably 0.08% or less.
• the Hf content is preferably 0.01% or more.
• Bi 0.200% or less
• the Bi content is 0.200% or less, the amount of coarse precipitates or inclusions will not increase, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, when Bi is contained, its content is set to 0.200% or less, and preferably 0.100% or less.
• the Bi content is preferably set to 0.001% or more.
• the base steel sheet according to one embodiment of the present invention contains the above basic components, with the balance consisting of Fe (iron) and unavoidable impurities.
• the base steel sheet according to one embodiment of the present invention contains only the above basic components and the balance, with the balance consisting of Fe (iron) and unavoidable impurities.
• the base steel sheet has a structure in which, at a depth from the surface of the base steel sheet that is 1 ⁇ 4 of the sheet thickness, the sum of the area ratios of tempered martensite and fresh martensite is 70.0% to 94.0%, the volume ratio of retained austenite is 6.0% to 15.0%, the area ratio of bainite is 10.0% or less, and the area ratio of the remaining structure is 10.0% or less.
• the thickness of the carbon concentration reduction portion in the surface layer of the base steel plate is 1.0 ⁇ m or more and 150.0 ⁇ m or less
• the thickness of the bainite precipitation region immediately below the carbon concentration reduction portion is 10 ⁇ m or more
• the total coverage rate of the retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction portion is 30.0% or more and 60.0% or less
• the carbon concentration of the retained austenite covering the prior austenite grain boundary immediately below the carbon concentration reduction portion is less than 0.60 mass%.
• the total area ratio of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less.
• the total area ratio of tempered martensite and fresh martensite is set to 70.0% or more and 94.0% or less.
• the area ratio of tempered martensite is 80.0% or more, since TS can be suitably obtained.
• the upper limit of the area ratio of tempered martensite is not particularly limited, but the area ratio of tempered martensite is generally 94.0% or less.
• the area ratio of fresh martensite is 10.0% or less, since ⁇ can be suitably obtained.
• the lower limit of the area ratio of fresh martensite is not particularly limited, but the area ratio of fresh martensite is generally 1.0% or more.
• the total area ratio of tempered martensite and fresh martensite can be obtained as follows. After polishing the L-section of the base steel sheet, it is corroded with 3 vol. % nital, and 10 fields of view are observed at 1/4 of the sheet thickness (a position corresponding to 1/4 of the sheet thickness in the depth direction from the steel sheet surface) using a SEM at a magnification of 3000 times.
• the area ratio of fresh martensite can be obtained by subtracting the volume ratio of retained austenite obtained by the method described below from the area ratio of the structure having a smooth surface in the observed image. Note that fresh martensite is a convex part with a width of 50 nm or more.
• Tempered martensite has a substructure (lath boundary, block boundary) and is a structure in which carbides are precipitated with multiple variants. Note that the volume ratio of retained austenite is almost equal to the area ratio, so in this invention it is treated as being equivalent to the area ratio.
• volume fraction of retained austenite 6.0% or more and 15.0% or less
• the volume fraction of the retained austenite is set to 6.0% or more, preferably 6.5% or more, and more preferably 7.0% or more.
• the volume fraction of the retained austenite is set to 15.0% or less, preferably 13.0% or less, and more preferably 11.0% or less.
• the volume fraction of retained austenite can be determined as follows.
• the base steel sheet is polished to a position 0.1 mm thicker than the 1/4 position of the sheet thickness.
• the surface is polished further 0.1 mm by chemical polishing to the 1/4 position of the sheet thickness, and the integrated intensity ratios of the diffraction peaks of the ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ planes of fcc iron and the ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ planes of bcc iron are measured using CoK ⁇ radiation in an X-ray diffraction device.
• the volume fraction of retained austenite can be determined by averaging the nine integrated intensity ratios obtained.
• the area ratio of bainite can be determined as follows. After polishing the L-section of the base steel sheet, it is corroded with 3 vol. % nital, and a position at 1/4 of the sheet thickness is observed in 10 fields of view at 3000x magnification using an SEM. In the observed structural images, bainite is a concave structure with a flat interior. The area ratio of bainite is determined in each field of view, and the average of these values is taken as the area ratio of bainite.
• the steel structure of the present invention may contain carbides such as pearlite and cementite or other known structures of steel sheets as the remaining structure. If the area ratio of the remaining structure is 10.0% or less, the effect of the present invention is not impaired. Therefore, the area ratio of the remaining structure is set to 10.0% or less. On the other hand, the lower limit of the area ratio of the remaining structure is not particularly limited, and the area ratio of the remaining structure may be 0.0%.
• the area ratio of the remaining structure can be determined as follows. After polishing the L-section of the base steel sheet, it is corroded with 3 vol. % nital, and a position at 1/4 of the sheet thickness is observed in 10 fields of view at a magnification of 3000 times using an SEM. In the observed structural image, the area ratio of the remaining structure is determined as 100.0% minus the area ratios of tempered martensite, fresh martensite, ferrite, and bainitic ferrite, and the volume ratio of retained austenite. The area ratio of the remaining structure is determined in each field of view, and the average of these values is taken as the area ratio of the remaining structure.
• the thickness of the carbon concentration reduction portion in surface layer is 1.0 ⁇ m or more and 150.0 ⁇ m or less. If the thickness of the carbon concentration reduction portion in the surface layer is less than 1.0 ⁇ m, it is not possible to provide a region with a high bainite precipitation start temperature in the surface layer, and it is not possible to control the area ratio of bainite and the thickness of the bainite precipitation region described later, making it difficult to realize excellent delayed fracture resistance properties of the stretch flange processing portion. Therefore, the thickness of the carbon concentration reduction portion in the surface layer is 1.0 ⁇ m or more, and preferably 3.0 ⁇ m or more.
• the thickness of the carbon concentration reduction portion in the surface layer exceeds 150.0 ⁇ m, it is difficult to realize a TS of 1470 MPa or more. Therefore, the thickness of the carbon concentration reduction portion in the surface layer is 150.0 ⁇ m or less, and preferably 50.0 ⁇ m or less.
• the thickness of the carbon concentration reduced portion in the surface layer can be determined as follows.
• a test piece having a length of 20 mm and a width of 20 mm is taken from a zinc-based plated steel sheet.
• the C intensity of the test piece is measured in the depth direction from the surface of the steel sheet by glow discharge spectroscopy (GDS). From the measurement results, a region where the C intensity is equal to or less than (C intensity when the measured value in GDS becomes a constant value ⁇ 0.8) is defined as the carbon concentration reduced portion in the surface layer, and the thickness of this region is defined as the thickness of the carbon concentration reduced portion.
• the GDS device used is a GD-PROFILER 2 manufactured by Horiba, Ltd., and the measurement conditions can be determined as follows. Power supply: High frequency Anode diameter: 4 mm Measurement mode: constant power 35W Carrier gas: Argon Discharge gas pressure: 300 Pa Pulse frequency: 100Hz ⁇ Ducty cycle: 50% Capture interval: 100 ms
• the thickness of the bainite precipitation region immediately below carbon concentration reduction portion is set to 10 ⁇ m or more.
• the thickness is generally 100 ⁇ m or less.
• the thickness of the bainite precipitation region immediately below the carbon concentration reduction area can be determined as follows. After polishing the L-section of a zinc-based plated steel sheet, it is corroded with 3 vol. % nital, and the above-mentioned GDS analysis is performed on the steel sheet surface using an SEM. The area after the carbon concentration reduction area determined by the GDS analysis, where bainite precipitation is observed, is observed in 10 fields of view at 3000x magnification. In each observed field of view, the bainite is a concave structure with a flat structure inside. If the bainite does not fit in one field of view, photographs of consecutive fields of view are taken.
• the area ratio of bainite is determined in each field of view, and the area where the area ratio of bainite is 20% to 60% is defined as the bainite precipitation region.
• the thickness of the bainite precipitation region in the sheet thickness direction is determined in each field of view, and the average value is defined as the thickness of the bainite precipitation region.
• the total coverage rate of retained austenite and fresh martensite at the prior austenite grain boundary directly below the carbon concentration reduction portion is 30.0% or more and 60.0% or less]
• a TS of 1470 MPa or more can be achieved. Therefore, the total coverage of the retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction portion is set to 30.0% or more, preferably 40.0% or more.
• the total coverage of the retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction portion is set to 60.0% or less, preferably 55.0% or less.
• the total coverage of the retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction portion can be determined as follows. After polishing the L-section of the zinc-based plated steel sheet, it is etched with 3 vol. % nital, and the above-mentioned GDS analysis is performed on the steel sheet surface using an SEM. Ten prior austenite grain boundaries that are the area following the carbon concentration reduction portion determined by the GDS analysis and are closest to the carbon concentration reduction portion are randomly selected. On the prior austenite grain boundary selected, a convex portion that has a width of 50 nm or more in the direction perpendicular to the prior austenite grain boundary and has a smooth surface is defined as the portion covered by the retained austenite and fresh martensite.
• the total circumferential length of the portion covered by the retained austenite and fresh martensite is divided by the circumferential length of the prior austenite grain boundary selected.
• the average value obtained at each location is calculated to be the total coverage of the retained austenite and fresh martensite at the prior austenite grain boundary.
• Carbon concentration in retained austenite covering prior austenite grain boundaries directly below carbon concentration reduced portion is less than 0.60 mass%
• the carbon concentration of the retained austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction portion is 0.60 mass% or more, a second phase having a hardness significantly different from that of the parent phase is present, and the second phase becomes a stress concentration portion during processing, deteriorating the delayed fracture resistance. Therefore, the carbon concentration of the retained austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction portion is less than 0.60 mass%, and preferably 0.55 mass% or less.
• the lower limit of the carbon concentration of the retained austenite covering the prior austenite grain boundaries immediately below the carbon concentration reduction portion is not particularly limited, and the carbon concentration is generally 0.30 mass% or more.
• yield strength (YS) is 1000 MPa or more (preferred condition)
• the zinc-based plated steel sheet preferably has a yield strength of 1000 MPa or more.
• the yield strength of the zinc-based plated steel sheet is preferably 1350 MPa or less.
• the yield strength can be determined by a tensile test in the same manner as the tensile strength.
• the zinc-based plated steel sheet has a hole expansion ratio ( ⁇ ) of 25% or more.
• the upper limit of the hole expansion ratio of the zinc-based plated steel sheet is not particularly limited, but is generally 50% or less.
• the hole expansion ratio ( ⁇ ) can be obtained as follows. A hole expansion test is performed in accordance with JIS Z 2256. After the test material is sheared to 100 mm x 100 mm, a hole with a diameter of 10 mm is punched with a clearance of 12.5%.
• the second cooling stop temperature T2 is less than T-20 (°C), and preferably T-30 (°C) or lower.
• the second cooling stop temperature T2 is Ms (°C) or higher, when the alloying treatment described later is performed, the precipitation of martensite before alloying is suitably prevented, and excessive tempering is not performed during alloying, so that a TS is suitably obtained. Therefore, the second cooling stop temperature T2 is preferably Ms (°C) or higher.
• the cooling rate in the second cooling step is 1°C/s or more, excessive nucleation and growth of bainite during cooling is effectively prevented, the total coverage of retained austenite and fresh martensite at the old austenite grain boundaries is effectively obtained, and TS is effectively obtained. Therefore, the cooling rate in the second cooling step is preferably 1°C/s or more, and more preferably 2°C/s or more. On the other hand, when the cooling rate in the second cooling step is 20°C/s or less, the cooling stop temperature can be effectively controlled. Therefore, the cooling rate in the second cooling step is preferably 20°C/s or less.
• the cold-rolled steel sheet is subjected to a zinc-based plating treatment to obtain a plated steel sheet.
• the zinc-based plating treatment include electrolytic galvanizing treatment, hot-dip galvanizing treatment, or hot-dip galvanizing treatment followed by alloying treatment.
• Electrolytic plating such as Zn-Ni electric alloy plating may be performed, or hot-dip zinc-aluminum-magnesium alloy plating may be performed.
• the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment
• the coating weight is preferably adjusted to 20 to 80 g/m 2 per side (double-sided plating).
• the hot-dip galvanizing treatment is preferably performed using a galvanizing bath having an Al content of 0.10 mass% to 0.23 mass%.
• the hot-dip galvanizing treatment is preferably performed by retaining the cold-rolled steel sheet in a temperature range of Ms (°C) or more and 700°C or less, defined by the following formula (3).
• [% X] indicates the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X.
• the alloying temperature is set to 470°C or higher to prevent the Zn-Fe alloying rate from becoming excessively slow, and favorable productivity can be obtained. Therefore, the alloying temperature is preferably 470°C or higher.
• the alloying temperature is preferably 600°C or lower, and more preferably 560°C or lower.
• the Fe concentration in the plating layer of alloyed hot-dip galvanized steel sheet (GA) is set to 7 to 15 mass% by performing the alloying treatment.
• the series of processes from the annealing process to the plating process is not particularly limited, but from the viewpoint of productivity, it is preferable to carry out the processes in a continuous galvanizing line (CGL), which is a hot-dip galvanizing line.
• CGL continuous galvanizing line
• Cooling step cooling to a temperature T3 of Ms-200 (°C) or more and Ms-80 (°C) or less at a cooling rate of 5°C/s or more]
• the plated steel sheet is subjected to a third cooling process. If the cooling rate in the third cooling process is less than 5°C/s, the total area ratio of tempered martensite and fresh martensite decreases, and the area ratio of bainite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the cooling rate in the third cooling process is set to 5°C/s or more. On the other hand, if the third cooling rate is 20°C/s or less, the cooling stop temperature can be suitably controlled. Therefore, the cooling rate in the third cooling process is preferably 20°C/s or less.
• T3 of the third cooling step is less than Ms-200 (°C)
• the volume fraction of untransformed austenite decreases, the amount of retained austenite in the final structure decreases, and it becomes difficult to achieve excellent ductility.
• the carbon concentration of the retained austenite covering the prior austenite grain boundaries directly below the carbon concentration reduction area becomes 0.60 mass% or more, and the delayed fracture resistance properties of the stretch flange processing area become inferior. Therefore, T3 is set to Ms-200 (°C) or more.
• T3 exceeds Ms-80 (°C)
• the total area fraction of tempered martensite and fresh martensite decreases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, T3 is set to Ms-80 (°C) or less.
• the total residence time during the period from the plating process to the third cooling process in which the temperature of the plated steel sheet is in the temperature range of Bf (°C) or more and T-20 (°C) or less is set to 30 s or less.
• the lower limit of the total residence time during the period from the plating process to the third cooling process in which the temperature of the plated steel sheet is in the temperature range of Bf (°C) or more and T-20 (°C) or less is not particularly limited, but the residence time is generally 20 s or more.
• [% X] indicates the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X.
• the plated steel sheet is subjected to a fourth cooling step. If the cooling rate in the fourth cooling step exceeds 3.0°C/s, the diffusion of carbon into untransformed austenite does not proceed sufficiently during the fourth cooling step, the volume fraction of retained austenite in the final structure decreases, and it becomes difficult to realize excellent ductility and excellent delayed fracture resistance of the stretch flanged portion. Therefore, the cooling rate in the fourth cooling step is 3.0°C/s or less, preferably 2.0°C/s or less, and more preferably 1.5°C/s or less. On the other hand, if the fourth cooling rate is 0.5°C/s or more, it is preferable because cooling can be performed without heating in a furnace. Therefore, the cooling rate in the fourth cooling step is preferably 0.5°C/s or more.
• T4 is set to 100°C or higher, preferably 110°C or higher, and more preferably 120°C or higher. Since the fourth cooling step is performed following the third cooling step, T4 will be less than T3 (°C). From the viewpoint of reducing fresh martensite, T4 is preferably Ms-100 (°C) or lower.
• the plated steel sheet is subjected to a tempering step.
• the plated steel sheet is reheated and tempered to stabilize the untransformed austenite. If the tempering temperature is T4 (°C) or lower, the desired amount of retained austenite is not obtained, making it difficult to achieve excellent ductility and excellent delayed fracture resistance of the stretch flanged portion. Therefore, the tempering temperature is set to be higher than T4 (°C), and preferably T4+50 (°C) or higher.
• the tempering temperature is set to be 350°C or lower, and preferably 340°C or lower.
• the holding time at the tempering temperature in the tempering process is less than 10 seconds, the austenite will not be stabilized sufficiently, will transform into martensite during final cooling, and the volume fraction of the retained austenite in the final structure will decrease, making it difficult to achieve excellent ductility. Furthermore, since the tempering of the martensite is insufficient, it will be difficult to achieve excellent hole expandability. Therefore, the holding time at the tempering temperature should be 10 seconds or more, and 40 seconds or more is preferable. On the other hand, if the holding time at the tempering temperature exceeds 1000 seconds, the tempering will proceed excessively, the strength will decrease, and the retained austenite will decompose, making it difficult to achieve an El of 9% or more. Therefore, the holding time at the tempering temperature should be 1000 seconds or less, and 800 seconds or less is preferable.
• a tension with an average load tension of 2.0 kgf/ mm2 or more is applied, and bending and unbending are performed one or more times (preferred conditions)]
• it is preferable to apply a tension of 2.0 kgf/mm2 or more with an average load tension and perform bending and bending back at least once 6 s or more after the start of holding at the tempering temperature and 10 s or more before the end of holding at the tempering temperature.
• tempering by applying tension and bending and bending back 10 s or more before the end of holding, tempering is suitably performed after the untransformed austenite is transformed into fresh martensite, and the yield strength (YS) and hole expansion ratio ( ⁇ ) are suitably obtained.
• the tension applied in the tempering process when the tension applied when performing the above-mentioned one or more bending and bending back is 2.0 kgf/ mm2 or more, the yield strength and hole expansion ratio are preferably obtained by processing-induced transformation of unstable retained austenite during tempering. Therefore, it is preferable that the tension applied in the tempering process is 2.0 kgf/ mm2 or more.
• the tension applied in the tempering process when the tension applied in the tempering process is 5.0 kgf/mm2 or less , the untransformed austenite during tempering is preferably prevented from decreasing, and the strain remaining in the steel sheet due to the application of tension is preferably prevented, so that the product characteristics are preferably obtained. Therefore, it is preferable that the tension applied in the tempering process is 5.0 kgf/ mm2 or less.
• a zinc-based plated steel sheet having a sheet thickness of 0.8 to 2.4 mm was obtained.
• the plating treatments shown in Table 2 are represented as "GI” for hot-dip galvanizing treatment, "GA” for alloyed hot-dip galvanizing treatment, and "EG” for electrolytic galvanizing treatment.
• the total area ratio of tempered martensite and fresh martensite, the volume ratio of retained austenite, the area ratio of bainite, and the area ratio of the remaining structure were each determined by the above-mentioned method. Furthermore, at a position 1/4 of the sheet thickness of the test material, the thickness of the carbon concentration reduction area in the surface layer of the base steel sheet, the thickness of the bainite precipitation area immediately below the carbon concentration reduction area, the total coverage of the retained austenite and fresh martensite at the prior austenite grain boundary immediately below the carbon concentration reduction area, and the carbon concentration in the retained austenite covering the prior austenite grain boundary were each determined. The measurement results are shown in Table 3.
• the present invention provides a zinc-based plated steel sheet and its manufacturing method that has a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0% or more, a hole expansion ratio ( ⁇ ) of 25% or more, and excellent delayed fracture resistance in the stretch flange processed portion.
• TS tensile strength
• El elongation
• ⁇ hole expansion ratio

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