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  • 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
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  • C21D1/18—Hardening; Quenching with or without subsequent tempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/78—Combined heat-treatments not provided for above
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
  • C21D7/10—Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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  • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0242—Flattening; Dressing; Flexing
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0278—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment 
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C21D2211/00—Microstructure comprising significant phases
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a high-strength steel sheet with excellent tensile strength, elongation, and delayed fracture resistance, and a method for manufacturing the same.
• the high-strength steel sheet of the present invention can be suitably used as structural members such as automobile parts.
• sheared edge In addition, there are many end faces formed by shearing in automobile frame parts.
• the morphology of the sheared edge depends on the shear clearance. In the process of molding the part, it is required that the sheared edge face does not crack due to hole-expanding deformation. Cracking that occurs due to hole expansion deformation after shearing depends on the shape of the sheared end surface, that is, the shearing clearance, and an appropriate clearance range that does not cause cracking is required to be wide.
• Shear clearance also affects delayed fracture resistance.
• delayed fracture means that when a molded part is placed in an environment where hydrogen penetrates, hydrogen penetrates into the steel sheet that composes the part, reducing the interatomic bonding force and causing local deformation. This is a phenomenon in which microcracks are generated as a result, and breakage occurs as the microcracks propagate.
• High-strength steel sheets used in automobiles are also required to have a wide appropriate clearance range against delayed fracture.
• Patent Document 1 provides a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent bending workability, and a method for manufacturing the same.
• the technique described in Patent Literature 1 does not consider the appropriate clearance range for hole expansion deformation and the appropriate clearance range for delayed fracture.
• Patent Document 2 provides a high-strength steel sheet having a tensile strength of 1320 MPa or more and excellent delayed fracture resistance of sheared edges, and a method for manufacturing the same.
• the technique described in Patent Literature 2 does not consider the appropriate clearance range for hole expansion deformation and the appropriate clearance range for delayed fracture.
• Patent Document 3 provides a high-strength steel sheet having a tensile strength of 1100 MPa or more and excellent YR, surface properties and weldability, and a method for manufacturing the same.
• the technique described in Patent Document 3 does not consider the appropriate clearance range for hole expansion deformation and the appropriate clearance range for delayed fracture.
• the invention was developed in view of such circumstances, and provides a high-strength steel sheet having a TS of 1320 MPa or more, El ⁇ 8%, and an excellent appropriate clearance range for hole expansion deformation and an appropriate clearance range for delayed fracture, and a method for producing the same. intended to
• the present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows. [1] % by mass, C: 0.15% or more and 0.45% or less, Si: 0.50% or more and 2.00% or less, Mn: 1.50% or more and 3.50% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 1.000% or less, N: 0.0100% or less, H: 0.0020% or less, a component composition with the balance being Fe and unavoidable impurities; Tempered martensite has an area fraction of 80% or more, Retained austenite is 5% or more and 15% or less in volume fraction, The total area fraction of ferrite and bainitic ferrite is 10% or less, The carbon concentration in the retained austenite is 0.50% or more, A high-strength steel sheet having a structure that satisfies the following formulas (1) and (2).
• KAM(S) is the KAM (Kernel Average Misorientation) value of the surface layer of the steel sheet
• KAM(C) is the KAM value of the center of the steel sheet.
• Hv(Q) indicates the hardness of the 1/4 portion of the plate thickness
• Hv(S) indicates the hardness of the surface layer of the steel plate.
• a component composition in mass%, Ti: 0.100% or less, B: 0.0100% or less, Nb: 0.100% or less, Cu: 1.00% or less, Cr: 1.00% or less, V: 0.100% or less, Mo: 0.500% or less, Ni: 0.50% or less, Sb: 0.200% or less, Sn: 0.200% or less, As: 0.100% or less, Ta: 0.100% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, REM: The high-strength steel sheet according to [1], containing one or more elements selected from 0.0200% or less.
• [3] The high-strength steel sheet according to [1] or [2], which has a plating layer on the surface of the steel sheet.
• [4] A method for producing a high-strength steel sheet according to [1] or [2] above, Cold-rolled steel sheets produced by subjecting steel slabs to hot rolling, pickling and cold rolling, The temperature T1 is 850° C. or higher and 1000° C. or lower, After annealing under the condition that the holding time t1 at T1 is 10 seconds or more and 1000 seconds or less, After cooling to a temperature T2 of 100° C. or higher and 300° C. or lower, temperature T3 is T2 or more and 450° C.
• Reheating is performed under the condition that the holding time t3 at the temperature T3 is 1.0 seconds or more and 1000.0 seconds or less, Cool to 100° C. or less, Starting processing within an elapsed time t4 of 1000 seconds or less after reaching 100° C., The processing has a processing start temperature T4 of 80° C. or less, The processing is performed under the condition that the equivalent plastic strain is 0.10% or more and 5.00% or less, The temperature T5 is 100° C. or higher and 400° C.
• the present invention it is possible to obtain a high-strength steel sheet having a TS of 1320 MPa or more, an El of 8% or more, and an appropriate clearance range for hole-expanding deformation and an appropriate clearance range for delayed fracture. Further, by applying the high-strength steel sheet of the present invention to automobile structural members, for example, it is possible to improve fuel consumption by reducing the weight of the vehicle body. Therefore, the industrial utility value is extremely large.
• C 0.15% or more and 0.45% or less C is one of the important basic components of steel, and particularly in the present invention, it is an important element that affects TS. If the C content is less than 0.15%, it becomes difficult to achieve a TS of 1320 MPa or more. Therefore, the C content should be 0.15% or more.
• the C content is preferably 0.16% or more.
• the C content is more preferably 0.17% or more.
• the C content is more preferably 0.18% or more.
• the C content is most preferably 0.19% or more.
• the C content is preferably 0.40% or less.
• the C content is more preferably 0.35% or less.
• the C content is more preferably 0.30% or less.
• the C content is most preferably 0.26% or less.
• Si 0.50% or more and 2.00% or less Si is one of the important basic components of steel. is. If the Si content is less than 0.50%, a large amount of carbide precipitates during the reheating treatment and tempering treatment, the retained austenite fraction and the carbon concentration in the retained austenite decrease, and an El of 8.0% or more is difficult to achieve, and the appropriate clearance range for hole expansion deformation decreases. Therefore, the Si content should be 0.50% or more.
• the Si content is preferably 0.60% or more.
• the Si content is more preferably 0.70% or more.
• the Si content should be 2.00% or less.
• the Si content is preferably 1.95% or less, and more preferably 1.80% or less.
• the Si content is more preferably 1.50% or less.
• Mn 1.50% or more and 3.50% or less
• Mn is one of the important basic components of steel, and particularly in the present invention, it is an important element that affects the ferrite fraction and the bainite fraction. If the Mn content is less than 1.50%, the ferrite fraction and bainite fraction increase, and the proper clearance range for hole-expanding deformation decreases. Therefore, the Mn content should be 1.50% or more.
• the Mn content is preferably 1.60% or more.
• the Mn content is more preferably 1.80% or more.
• the Mn content is more preferably 2.00% or more.
• the Mn content should be 3.50% or less.
• the Mn content is preferably 3.30% or less.
• the Mn content is more preferably 3.20% or less.
• the Mn content is more preferably 3.00% or less.
• the P content should be 0.100% or less.
• the P content is preferably 0.080% or less.
• the P content is more preferably 0.060% or less.
• the lower limit of the P content is not particularly limited, it is preferably 0.001% or more due to restrictions on production technology.
• the S content should be 0.0200% or less.
• the S content is preferably 0.0100% or less.
• the S content is more preferably 0.0050% or less.
• the lower limit of the S content is not particularly limited, it is preferably 0.0001% or more due to production technology restrictions.
• Al 0.010% or more and 1.000% or less
• the Al content must be 0.010% or more. Therefore, the Al content should be 0.010% or more.
• the Al content is preferably 0.012% or more.
• Al content is more preferably 0.015% or more.
• the Al content is more preferably 0.020% or more.
• the Al content should be 1.000% or less.
• the Al content is preferably 0.500% or less.
• Al content is more preferably 0.100% or less.
• the N content should be 0.0100% or less.
• the N content is preferably 0.0080% or less.
• the N content is more preferably 0.0070% or less.
• the N content is more preferably 0.0060% or less.
• the N content is most preferably 0.0050% or less.
• the lower limit of the N content is not particularly limited, it is preferably 0.0010% or more due to production technology restrictions.
• the H content should be 0.0020% or less.
• the H content is preferably 0.0015% or less.
• the H content is more preferably 0.0010% or less.
• the lower limit of the H content is not particularly limited, the H content may be 0% because the smaller the H content, the better the appropriate clearance range against delayed fracture.
• the high-strength steel sheet of the present invention further has, in mass%, Ti: 0.100% or less, B: 0.0100% or less, Nb: 0.100% or less, Cu: 1.00% or less, Cr: 1.00% or less, V: 0.100% or less, Mo: 0.500% or less Ni: 0.50% or less Sb: 0.200% or less Sn: 0.200% or less As: 0.100% or less Ta: 0.100% or less Ca: 0.100% or less 0200% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, REM: 0.0200% or less Alternatively, two or more elements are preferably contained.
• Ti 0.100% or less
• the Ti content is preferably 0.090% or less.
• the Ti content is more preferably 0.075% or less.
• the Ti content is more preferably 0.050% or less.
• the Ti content is most preferably less than 0.050%.
• the inclusion of Ti increases the strength of the steel sheet, making it easier to achieve a TS of 1320 MPa or more.
• the Ti content is preferably 0.001% or more.
• the Ti content is more preferably 0.005% or more.
• the Ti content is more preferably 0.010% or more.
• B 0.0100% or less
• the B content is preferably 0.0080% or less.
• the B content is more preferably 0.0050% or less.
• the inclusion of B increases the strength of the steel sheet, making it easier to achieve a TS of 1320 MPa or more.
• the B content is preferably 0.0001% or more.
• the B content is more preferably 0.0002% or more.
• Nb 0.100% or less
• the Nb content is preferably 0.090% or less.
• the Nb content is more preferably 0.050% or less.
• the Nb content is more preferably 0.030% or less.
• the inclusion of Nb increases the strength of the steel sheet, making it easier to achieve a TS of 1320 MPa or more.
• the Nb content is preferably 0.001% or more.
• the Nb content is more preferably 0.002% or more.
• the Cu content should be 1.00% or less.
• the Cu content is preferably 0.50% or less.
• Cu content is more preferably 0.30% or less.
• containing Cu suppresses penetration of hydrogen into the steel sheet and improves the appropriate clearance range for delayed fracture. In order to obtain this effect, the Cu content is preferably 0.01% or more.
• Cu content is more preferably 0.03% or more.
• Cr 1.00% or less
• the Cr content is preferably 0.70% or less.
• the Cr content is more preferably 0.50% or less.
• Cr not only plays a role as a solid-solution strengthening element, but also stabilizes austenite and suppresses formation of ferrite in the cooling process during continuous annealing, thereby increasing the strength of the steel sheet.
• the Cr content is preferably 0.01% or more.
• the Cr content is more preferably 0.02% or more.
• V 0.100% or less
• the V content is preferably 0.060% or less.
• V increases the strength of the steel sheet. In order to obtain such effects, the V content is preferably 0.001% or more.
• the V content is more preferably 0.005% or more.
• the V content is more preferably 0.010% or more.
• Mo 0.500% or less
• the Mo content is preferably 0.450% or less, more preferably 0.350% or less.
• Mo not only plays a role as a solid-solution strengthening element, but also stabilizes austenite and suppresses the formation of ferrite in the cooling process during continuous annealing, thereby increasing the strength of the steel sheet.
• the Mo content is preferably 0.010% or more.
• Mo content is more preferably 0.020% or more.
• Ni 0.50% or less
• the Ni content is preferably 0.45% or less.
• the Ni content is more preferably 0.30% or less.
• Ni stabilizes austenite and suppresses the formation of ferrite in the cooling process during continuous annealing, thereby increasing the strength of the steel sheet.
• the Ni content is preferably 0.01% or more.
• the Ni content is more preferably 0.02% or more.
• Sb 0.200% or less
• the Sb content is preferably 0.100% or less.
• the Sb content is more preferably 0.050% or less.
• Sb suppresses the formation of surface layer softening and increases the strength of the steel sheet. In order to obtain such effects, the Sb content is preferably 0.001% or more.
• the Sb content is more preferably 0.005% or more.
• Sn 0.200% or less
• the Sn content is preferably 0.100% or less.
• the Sn content is more preferably 0.050% or less.
• Sn suppresses the formation of surface layer softening and increases the strength of the steel sheet. In order to obtain such effects, the Sn content is preferably 0.001% or more. More preferably, it is 0.005% or more.
• As 0.100% or less
• As content is preferably 0.060% or less.
• the As content is more preferably 0.010% or less.
• the As content is preferably 0.001% or more.
• the As content is more preferably 0.005% or more.
• Ta 0.100% or less
• the Ta content is preferably 0.050% or less.
• the Ta content is more preferably 0.010% or less.
• Ta increases the strength of the steel sheet. In order to obtain such effects, the Ta content is preferably 0.001% or more. Ta content is more preferably 0.005% or more.
• Ca 0.0200% or less
• the Ca content is preferably 0.0100% or less.
• Ca is an element used for deoxidation, and is an element effective for making the shape of sulfides spherical, improving the ultimate deformability of the steel sheet, and improving the appropriate clearance range for delayed fracture. In order to obtain such effects, the Ca content is preferably 0.0001% or more.
• Mg 0.0200% or less
• Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are formed, which reduces the ultimate deformability of the steel. Lower range. Therefore, when Mg is added, its content should be 0.0200% or less.
• Mg is an element used for deoxidation, and is also an effective element for making sulfides spherical, improving the ultimate deformability of the steel sheet, and improving the appropriate clearance range for delayed fracture. In order to obtain such effects, the Mg content is preferably 0.0001% or more.
• Zn 0.020% or less
• Co 0.020% or less
• Zr 0.020% or less
• the contents of Zn, Co and Zr each exceed 0.020%, large amounts of coarse precipitates and inclusions are produced. Since it forms in the steel and lowers the ultimate deformability of the steel, the appropriate clearance range for hole expansion deformation decreases. Therefore, when Zn, Co and Zr are added, their contents should be 0.020% or less.
• Zn, Co and Zr are all effective elements for making inclusions spherical, improving the ultimate deformability of the steel sheet, and improving the appropriate clearance range for delayed fracture. In order to obtain such effects, it is preferable that the contents of Zn, Co and Zr each be 0.0001% or more.
• REM 0.0200% or less
• the REM content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Lower range. Therefore, when REM is added, its content should be 0.0200% or less.
• REM is an element effective in making the shape of inclusions spherical, improving the ultimate deformability of the steel sheet, and improving the appropriate clearance range for delayed fracture. In order to obtain such effects, the REM content is preferably 0.0001% or more.
• the balance other than the above components is Fe and unavoidable impurities. If the content of the above optional components is less than the lower limit, the effect of the present invention is not impaired. Therefore, if the content of these optional elements is less than the lower limit, these optional elements are included as unavoidable impurities.
• Tempered martensite 80% or more in terms of area fraction
• this is an extremely important invention constituent feature.
• martensite As the main phase, it is possible to achieve a TS of 1320 MPa or more.
• the area fraction of tempered martensite must be 80% or more. Therefore, the area fraction of tempered martensite is set to 80% or more.
• the area fraction of tempered martensite is preferably 85% or more.
• the area fraction of tempered martensite is more preferably 87% or more.
• the upper limit is not particularly limited, the area fraction of tempered martensite is preferably 95% or less from the viewpoint of securing the amount of retained austenite.
• tempered martensite is a structure in which the inside of the structure has fine irregularities and which has carbide inside.
• the tempered martensite fraction can be determined from the average of these values.
• Retained austenite 5% or more and 15% or less in volume fraction
• this is an extremely important invention constituent feature. If the volume fraction of retained austenite is less than 5%, it becomes difficult to achieve El of 8.0% or more. Therefore, the retained austenite should be 5% or more in volume fraction.
• the volume fraction of retained austenite is preferably 6% or more.
• the volume fraction of retained austenite is more preferably 7% or more.
• the retained austenite should be 15% or less in volume fraction.
• the volume fraction of retained austenite is preferably 14% or less.
• the volume fraction of retained austenite is more preferably 12% or less.
• the volume fraction of retained austenite is preferably 10% or less.
• the method for measuring retained austenite is as follows.
• the retained austenite fraction is obtained by polishing the steel plate from 1/4 part of the plate thickness to a surface of 0.1 mm, and then chemically polishing the surface to 0.1 mm.
• ⁇ 200 ⁇ , ⁇ 220 ⁇ , ⁇ 311 ⁇ planes and ⁇ 200 ⁇ , ⁇ 211 ⁇ , ⁇ 220 ⁇ planes of bcc iron were measured for each of the integrated intensity ratios of the diffraction peaks, and the nine integrated intensity ratios obtained was obtained by averaging
• Total of ferrite and bainitic ferrite 10% or less in terms of area fraction
• the total area fraction of ferrite and bainitic ferrite is preferably 8% or less.
• the total area fraction of ferrite and bainitic ferrite is more preferably 5% or less.
• the lower limit of the total content of ferrite and bainitic ferrite is not particularly limited, the lower the content, the better, and the lower limit of the total content of ferrite and bainitic ferrite may be 0%.
• the method for measuring the sum of ferrite and bainitic ferrite is as follows. After polishing the L cross section of the steel plate, 3 vol. % nital, and 1/4 part of the plate thickness (position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface) is observed in 10 fields of view at a magnification of 2000 using SEM. In the above tissue images, ferrite and bainitic ferrite are recessed structures with a flat interior. The sum of the ferrite fraction and the bainitic ferrite fraction can be obtained from the average of these values.
• Carbon concentration in retained austenite 0.50% or more In the present invention, this is an extremely important invention constituent feature. If the carbon concentration in the retained austenite is less than 0.50%, the stability of the retained austenite is low and transforms into hard martensite at the initial stage of deformation, resulting in a decrease in the appropriate clearance range for hole-expanding deformation. Therefore, the carbon concentration in retained austenite is set to 0.50% or more.
• the carbon concentration in retained austenite is preferably 0.60% or more.
• the upper limit is preferably 1.00% or less due to restrictions on production technology.
• the method for measuring the carbon concentration C ⁇ in the retained austenite is as follows.
• the carbon concentration in retained austenite is calculated by first calculating the lattice constant of retained austenite from the diffraction peak shift amount of the ⁇ 220 ⁇ plane of austenite by formula (3), and substituting the obtained lattice constant of retained austenite into formula (4).
• a 3.578 + 0.00095 [Mn] + 0.022 [N] + 0.0006 [Cr] + 0.0031 [Mo] + 0.0051 [Nb] + 0.0039 [Ti] + 0.0056 [Al] + 0.033 [ C] (4)
• a is the lattice constant ( ⁇ ) of retained austenite
• ⁇ is the value obtained by dividing the diffraction peak angle of the ⁇ 220 ⁇ plane by 2 (rad)
• [M] is the mass % of the element M in the retained austenite.
• the mass % of the element M other than C in the retained austenite is defined as the mass % of the entire steel.
• KAM(S) is the KAM (Kernel Average Misorientation) value of the surface layer of the steel sheet
• KAM(C) is the KAM value of the center of the steel sheet.
• the steel plate surface layer portion is a position moved 100 ⁇ m from the steel plate surface toward the plate thickness central portion side.
• the central portion of the steel plate is the position of 1/2 of the plate thickness.
• KAM(S)/KAM(C) should be less than 1.00.
• the lower limit of KAM(S)/KAM(C) is not particularly limited, it is preferably 0.80 or more due to production technology restrictions.
• the method for measuring the KAM value is as follows. First, a test piece for structure observation was taken from the cold-rolled steel sheet. Next, the sampled test piece was polished by colloidal silica vibration polishing so that the cross section in the rolling direction (L cross section) was the observation surface. The observation surface was a mirror surface. Electron backscatter diffraction (EBSD) measurements were then performed to obtain local crystallographic orientation data. At this time, the SEM magnification was 3000 times, the step size was 0.05 ⁇ m, the measurement area was 20 ⁇ m square, and the WD was 15 mm. Analysis software: OIM Analysis 7 was used to analyze the obtained local orientation data. The analysis was performed for each of 10 fields of view for the target plate thickness, and the average value was used.
• EBSD Electron backscatter diffraction
• Hv(Q)-Hv(S) ⁇ 8 Hv(Q) is the hardness of 1/4 part of the sheet thickness
• Hv(S) is the hardness of the surface layer of the steel sheet.
• the steel plate surface layer portion is a position moved 100 ⁇ m from the steel plate surface toward the plate thickness central portion side.
• Hv(Q)-Hv(S) should be 8 or more.
• Hv(Q)-Hv(S) is preferably 9 or more.
• Hv(Q)-Hv(S) is more preferably 10 or more.
• the upper limit of Hv(Q)-Hv(S) is not particularly limited, it is preferably 30 or less due to production technology restrictions.
• the preferred ranges of Hv(Q) and Hv(S) are 400-600 and 400-600, respectively.
• the hardness measurement method is as follows. First, a test piece for structure observation was taken from the cold-rolled steel sheet. Next, the sampled test piece was polished so that the cross section in the rolling direction (L cross section) was the observation surface. The observation surface was a mirror surface. Then, the hardness was determined using a Vickers tester with a load of 1 kg. The hardness was measured at 10 points at intervals of 20 ⁇ m with respect to the target plate thickness, and the average value of the 8 points was used, excluding the maximum hardness and the minimum hardness.
• the method of melting the steel material is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable.
• Steel slabs are preferably produced by continuous casting to prevent macro-segregation.
• the slab heating temperature, slab soaking holding time and coiling temperature in hot rolling are not particularly limited.
• Methods for hot rolling steel slabs include a method of rolling after heating the slab, a method of directly rolling the slab after continuous casting without heating, and a method of subjecting the slab after continuous casting to heat treatment for a short period of time before rolling. etc.
• the slab heating temperature, slab soaking holding time, finish rolling temperature and coiling temperature in hot rolling are not particularly limited, but the slab heating temperature is preferably 1100° C. or higher.
• the slab heating temperature is preferably 1300° C. or less.
• the slab soaking holding time is preferably 30 minutes or more.
• the slab soaking holding time is preferably 250 minutes or less.
• the finish rolling temperature is preferably the Ar 3 transformation point or higher.
• the winding temperature is preferably 350°C or higher.
• the winding temperature is preferably 650°C or lower.
• the hot-rolled steel sheet manufactured in this way is pickled. Since pickling can remove oxides from the surface of the steel sheet, it is important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet. Also, the pickling may be performed once, or may be divided into a plurality of times. Further, the hot-rolled pickling-treated sheet may be cold-rolled, or the cold-rolled sheet may be heat-treated and then cold-rolled.
• the rolling reduction in cold rolling and the sheet thickness after rolling are not particularly limited, the rolling reduction in cold rolling is preferably 30% or more. It is preferable that the rolling reduction in the rolling is 80% or less.
• the number of rolling passes and the rolling reduction of each pass are not particularly limited, and the effects of the present invention can be obtained.
• the cold-rolled steel sheet obtained as described above is annealed. Annealing conditions are as follows.
• Annealing temperature T1 850° C. or higher and 1000° C. or lower
• this is an extremely important invention constituent feature.
• the annealing temperature T1 is set to 850° C. or higher.
• Annealing temperature T1 is preferably 860° C. or higher.
• the annealing temperature T1 is set to 1000° C. or lower.
• Annealing temperature T1 is preferably 970° C. or lower.
• Annealing temperature T1 is more preferably 950° C. or lower.
• the annealing temperature T1 is more preferably 900° C. or less.
• Holding time t1 at annealing temperature T1 10 seconds or more and 1000 seconds or less If the holding time t1 at annealing temperature T1 is less than 10 seconds, austenitization is insufficient, and the total area fraction of ferrite and bainitic ferrite is It exceeds 10%, and the appropriate clearance range for hole widening deformation decreases. Therefore, the holding time t1 at the annealing temperature T1 is set to 10 seconds or longer.
• the holding time t1 at the annealing temperature T1 is preferably 30 seconds or longer. t1 is more preferably 45 seconds or longer. t1 is more preferably 60 seconds or longer. t1 is most preferably 100 seconds or more.
• the holding time t1 at the annealing temperature T1 is set to 1000 seconds or less.
• the holding time t1 at the annealing temperature T1 is preferably 800 seconds or less.
• the holding time t1 at the annealing temperature T1 is more preferably 500 seconds or less.
• the holding time t1 at the annealing temperature T1 is more preferably 300 seconds or less.
• Cooling stop temperature T2 100° C. or more and 300° C. or less This is an extremely important invention constituent feature in the present invention. If the cooling stop temperature T2 is less than 100° C., martensitic transformation proceeds excessively, the retained austenite becomes less than 5%, and it becomes difficult to achieve El of 8% or more. Therefore, the cooling stop temperature T2 is set to 100° C. or higher. Cooling stop temperature T2 is preferably 150° C. or higher. The cooling stop temperature T2 is more preferably 180° C. or higher. On the other hand, if the cooling stop temperature T2 exceeds 300° C., the martensitic transformation becomes insufficient, the retained austenite exceeds 15%, and the appropriate clearance range for hole expansion deformation decreases. Therefore, the cooling stop temperature T2 is set to 300° C. or lower. Cooling stop temperature T2 is preferably 250° C. or lower.
• Reheating temperature T3 not less than T2 and not more than 450° C. In the present invention, this is an extremely important invention constituent feature. After cooling is stopped, the temperature is maintained as it is, or the temperature is reheated and maintained at a temperature of 450° C. or less to stabilize the retained austenite. At temperatures below T2, the desired retained austenite cannot be obtained. Therefore, the reheating temperature T3 is set to T2 or higher.
• the reheating temperature T3 is preferably 300° C. or higher. If the reheating temperature T3 exceeds 450°C, bainite transformation proceeds excessively, the total area fraction of ferrite and bainitic ferrite exceeds 10%, and the appropriate clearance range for hole-expanding deformation decreases. Therefore, the reheating temperature T3 is set to 450° C. or lower.
• the reheating temperature T3 is preferably 420° C. or lower.
• the reheating temperature T3 is more preferably 400° C. or lower.
• Holding time t3 at reheating temperature T3 1.0 seconds or more and 1000.0 seconds or less This is a very important invention constituent feature in the present invention. After cooling is stopped, the temperature is maintained as it is, or the temperature is reheated and maintained at a temperature of 450° C. or less to stabilize the retained austenite. When the holding time t3 at the reheating temperature T3 is less than 1.0 second, the stabilization of retained austenite becomes insufficient, the retained austenite decreases, and it becomes difficult to achieve El of 8% or more. Therefore, the retention time t3 at the reheating temperature T3 is set to 1.0 seconds or longer. The holding time t3 at the reheating temperature T3 is preferably 5.0 seconds or longer.
• the retention time t3 at the reheating temperature T3 is more preferably 100.0 seconds or longer.
• the holding time t3 at the reheating temperature T3 is more preferably 150.0 seconds or longer.
• the holding time t3 at the reheating temperature T3 exceeds 1000.0 seconds, the bainite transformation proceeds excessively, the total of ferrite and bainitic ferrite exceeds 10%, and the appropriate clearance range for hole expanding deformation is reduced. descend. Therefore, reheating, therefore, the holding time t3 at the reheating temperature T3 is set to 1000.0 seconds or less.
• the holding time t3 at the reheating temperature T3 is preferably 500.0 seconds or less.
• the holding time t3 at the reheating temperature T3 is preferably 300.0 seconds or less.
• Cooling down to 100° C. or lower after reheating In the cooling process down to 100° C. or lower, austenite is transformed into martensite. In order to obtain tempered martensite of 80% or more, it is necessary to cool down to 100°C or less after reheating. Therefore, after reheating, it is cooled to 100° C. or less.
• the cooling stop temperature after reheating is preferably 0° C. or higher due to production technology restrictions.
• Elapsed time t4 from the time the temperature reaches 100° C. to the start of processing 1000 seconds or less This is a very important invention constituent feature in the present invention. If the elapsed time t4 from the time when the temperature reaches 100° C. to the start of working exceeds 1000 seconds, aging of the martensitic structure proceeds excessively, and the amount of strain introduced into the steel sheet surface layer and the steel sheet center due to working changes. Therefore, KAM(S)/KAM(C) becomes 1.00 or more, and the appropriate clearance range for hole expansion deformation and the appropriate clearance range for delayed fracture are lowered. Therefore, the elapsed time t4 from the time when the temperature reaches 100° C. to the start of processing is set to 1000 seconds or less.
• the elapsed time t4 from the time when the temperature reaches 100° C. to the start of processing is preferably 900 seconds or less.
• the elapsed time t4 from the time when the temperature reaches 100° C. to the start of processing is more preferably 800 seconds or less.
• the lower limit is not particularly limited, it is preferable that the elapsed time t4 from the time when the temperature reaches 100° C. to the start of processing is 5 seconds or more due to production technology restrictions. As a result of investigation by the inventor, it was found that the elapsed time from the time when the temperature reaches 100° C. to the end of working does not affect the amount of strain introduced into the steel sheet surface layer portion and the steel plate center portion due to working.
• Processing start temperature T4 is 80° C. or less This is an extremely important invention constituent feature in the present invention.
• the working start temperature T4 exceeds 80 ° C., the steel sheet is soft, so the amount of strain introduced into the steel sheet surface layer and the steel sheet center due to working changes, and KAM (S) / KAM (C) is 1.00 or more.
• the processing start temperature T4 is set to 80° C. or lower.
• the processing start temperature T4 is preferably 60° C. or lower.
• the processing start temperature T4 is more preferably 50° C. or lower.
• the lower limit is not particularly limited, it is preferably 0° C. or higher due to production technology restrictions.
• Equivalent plastic strain 0.10% or more and 5.00% or less
• this is a very important invention constituent feature.
• the equivalent plastic strain is less than 0.10%, the amount of processing is insufficient, KAM (S) / KAM (C) is 1.00 or more, and the carbon concentration in retained austenite is less than 0.50%, The appropriate clearance range for hole expansion deformation and the appropriate clearance range for delayed fracture are lowered. Therefore, the equivalent plastic strain should be 0.10% or more.
• the equivalent plastic strain is preferably 0.15% or more.
• the equivalent plastic strain is more preferably 0.30% or more.
• the equivalent plastic strain exceeds 5.00%, the retained austenite is less than 5%, making it difficult to achieve El of 8% or more. Therefore, the equivalent plastic strain should be 5.00% or less.
• the equivalent plastic strain is preferably 3.00% or less.
• the equivalent plastic strain is more preferably 1.00% or less.
• strain is applied by processing in two or more steps, and the total equivalent plastic strain of each processing is 0.10% or more.
• the equivalent plastic strain in the first processing is less than 0.10%, if the total equivalent plastic strain becomes 0.10% or more in the second and subsequent processing, KAM (S) / KAM (C) It becomes less than 1.00, and the appropriate clearance range for hole expansion deformation and the appropriate clearance range for delayed fracture are improved. Therefore, in the working process before tempering, the strain may be imparted by working twice or more, and the total equivalent plastic strain of each working should be 0.10% or more. If the total equivalent plastic strain of each working exceeds 5.00%, the retained austenite becomes less than 5%, making it difficult to achieve El of 8% or more.
• the strain may be imparted by working twice or more, and the total equivalent plastic strain of each working should be 5.00% or less.
• the upper limit of the number of times of processing is not particularly limited, it is preferably 30 times or less due to production technology restrictions.
• the time from the time when the temperature reaches 100° C. to the time when the second and subsequent processing is started is not particularly limited. This is because the mobility of dislocations in martensite decreases due to the first working.
• the representative processing methods for the above processing include temper rolling and a tension leveler.
• the equivalent plastic strain in temper rolling is the elongation rate of the steel sheet, and can be obtained from the change in length of the steel sheet before and after working.
• the method of calculating the equivalent plastic strain of the steel sheet during leveling was calculated by the method of Reference 1 below. The following data inputs were used in the calculations, and the work hardening behavior of the material was assumed to be linear hardening elastoplastic, ignoring Bausinger hardening and tension reduction due to bend loss. Misaka's formula was used as the processing curvature formula.
• Tempering temperature T5 100° C. or higher and 400° C. or lower This is an extremely important invention constituent feature in the present invention.
• tempering temperature T5 shall be 100 degreeC or more.
• Tempering temperature T5 is preferably 150° C. or higher.
• the tempering temperature T5 exceeds 400° C., the tempering of martensite progresses, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the tempering temperature T5 is set to 400° C. or lower. Tempering temperature T5 is preferably 350° C. or lower. Tempering temperature T5 is more preferably 300° C. or lower.
• Holding time t5 at tempering temperature T5 1.0 seconds or more and 1000.0 seconds or less This is a very important invention constituent feature in the present invention.
• the holding time t5 at the tempering temperature T5 is set to 1.0 seconds or longer.
• the holding time t5 at the tempering temperature T5 is preferably 5.0 seconds or longer.
• the holding time t5 at the tempering temperature T5 is more preferably 100.0 seconds or longer.
• the holding time t5 at the tempering temperature T5 exceeds 1000.0 seconds, the tempering of martensite progresses, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the holding time t5 at the tempering temperature T5 is set to 1000.0 seconds or less.
• the holding time t5 at the tempering temperature T5 is preferably 800.0 seconds or less.
• the holding time t5 at the tempering temperature T5 is more preferably 600.0 seconds or less.
• Cooling rate .theta.1 from tempering temperature T5 to 80.degree When the cooling rate ⁇ 1 from the tempering temperature T5 to 80° C. exceeds 100° C./sec, the diffusion distance of carbon is short, so the hardness of the steel plate surface and the steel plate interior becomes small, and Hv(Q) ⁇ Hv(S) becomes It becomes less than 8, and the appropriate clearance range for hole expansion deformation and the appropriate clearance range for delayed fracture are lowered. Therefore, the cooling rate ⁇ 1 from the tempering temperature T5 to 80°C is set to 100°C/sec or less. Preferably, the cooling rate ⁇ 1 from the tempering temperature T5 to 80°C is 50°C/sec or less. Although the lower limit of the cooling rate ⁇ 1 from the tempering temperature T5 to 80° C. is not particularly limited, it is preferably set at 10° C./second or more due to production technology restrictions.
• Cooling below 80°C does not need to be specified, and may be cooled to the desired temperature by any method. It should be noted that the desired temperature is desirably about room temperature.
• the above high-strength steel sheet may be worked again under the condition that the equivalent plastic strain amount is 0.10% or more and 5.00% or less. Further, the processing to achieve the target equivalent plastic strain amount may be performed at once, or may be performed in several steps.
• the high-strength steel sheet may be plated between annealing and working.
• the period from annealing to working is the period from the end of holding t1 at the annealing temperature T1 until reaching the working start temperature T4.
• Examples of the plating treatment during annealing include hot-dip galvanizing treatment during cooling to 300° C. or less after holding at the annealing temperature T1, and treatment in which alloying is performed after hot-dip galvanizing.
• the plating treatment between annealing and working for example, Zn—Ni electro-alloy plating treatment after tempering or pure Zn electroplating treatment can be exemplified.
• a plating layer may be formed by electroplating, or hot-dip zinc-aluminum-magnesium alloy plating may be applied.
• the type of plating metal such as Zn plating and Al plating is not particularly limited.
• Other manufacturing method conditions are not particularly limited, but from the viewpoint of productivity, a series of treatments such as the above-mentioned annealing, hot-dip galvanizing, galvanizing treatment, etc. Line). After hot-dip galvanization, wiping is possible in order to adjust the basis weight of the plating.
• the conditions of plating etc. other than the above-mentioned conditions can be based on the usual method of hot-dip galvanization.
• working may be performed again under the condition that the equivalent plastic strain amount is 0.10% or more and 5.00 or less. Further, the processing to achieve the target equivalent plastic strain amount may be performed at once, or may be performed in several steps.
• Example No. Tests 77, 82, 85, 88, and 91 were discontinued due to slab fracture during the casting process.
• the tensile properties and delayed fracture resistance properties were evaluated according to the following test methods.
• KAM value The KAM value of the surface layer of the steel sheet and the KAM value of the center of the steel sheet were obtained according to the method described above.
• Test test For the tensile test, a JIS No. 5 test piece (gauge length: 50 mm, width of parallel part: 25 mm) was sampled so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and the test was performed according to JIS Z 2241. A tensile test was performed at a crosshead speed of 1.67 ⁇ 10 ⁇ 1 mm/sec to measure TS and El. In the present invention, a TS of 1320 MPa or more was judged to be acceptable. An El of 8% or more was judged to be acceptable.
• the appropriate clearance range for hole expansion deformation was obtained by the following method. Each obtained steel plate was cut into a size of 100 mm ⁇ 100 mm to prepare a test piece. A 10 mm diameter hole was punched in the center of the specimen. The clearance during punching was changed to 5, 10, 15, 20, 25, 30 and 35%. Using a die with an inner diameter of 75 mm, a wrinkle pressing force of 9 tons (88.26 kN) was used, and a conical punch with an apex angle of 60° was pushed into the hole until cracks were confirmed, and the hole expansion ratio was obtained.
• Hole expansion ratio: ⁇ (%) ⁇ (D f1 - D 0 )/D 0 ⁇ x 100
• Df1 is the hole diameter (mm) at the time of crack confirmation
• D0 is the initial hole diameter (mm).
• Those with a shear clearance range of 20% or more and less than 10% were evaluated as "x”
• those with a shear clearance range of 10% or more and less than 15% were evaluated as " ⁇ ”
• those with 15% or more were evaluated as " ⁇ ”
• ⁇ was 20. % or more and the shear clearance range of 10% or more was judged to be excellent in the appropriate clearance range for hole expansion deformation.
• the appropriate clearance range for delayed fracture was obtained by the following method.
• a test piece of 16 mm ⁇ 75 mm was prepared by shearing the longitudinal direction perpendicular to the rolling direction. The rake angle during shearing was uniform at 0°, and the shearing clearance was changed to 5, 10, 15, 20, 25, 30, and 35%.
• Four-point bending was performed according to ASTM (G39-99), and a stress of 1000 MPa was applied to the bending vertex.
• the stress-applied test piece was immersed in hydrochloric acid of pH 3 at 25° C. for 100 hours.
• a shear clearance range of less than 10% where cracking does not occur is evaluated as “ ⁇ ”, a range of 10% or more and less than 15% is evaluated as “ ⁇ ”, and a shear clearance range where cracking does not occur is 15% or more is evaluated as “ ⁇ ”.
• a specimen having a shear clearance range of 10% or more in which cracking does not occur was judged to have an excellent appropriate clearance range for delayed fracture.
• TS is 1320 MPa or more, El ⁇ 8%, and the proper clearance range for hole expansion deformation and the proper clearance range for delayed fracture Excellent for
• the comparative example one or more of the appropriate clearance range for TS, El, hole expansion deformation, or the appropriate clearance range for delayed fracture is inferior.

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