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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
  • 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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  • C21D1/00—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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  • 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
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties 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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  • 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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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  • 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties 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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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C—ALLOYS
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
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  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • 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
  • C23C2/04—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
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  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/22—Electroplating: Baths therefor from solutions of zinc
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  • C21D2201/00—Treatment for obtaining particular effects
  • C21D2201/05—Grain orientation
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a high strength steel sheet excellent in tensile strength and delayed fracture resistance, and to a method for manufacturing the same.
• the high strength steel sheet of the present invention may be suitably used as structural members, such as automobile parts.
• Steel sheets for automobiles are being increased in strength to reduce CO 2 emissions by weight reduction of vehicles and to enhance crashworthiness by weight reduction of automobile bodies at the same time, with introduction of new laws and regulations one after another.
• high strength steel sheets having a tensile strength (TS) of 1320 MPa or higher class are increasingly applied to principal structural parts of automobiles.
• YR yield strength YS/tensile strength TS
• automobile frame parts such as bumpers, are required to exhibit excellent impact absorption at the time of collision.
• steel sheets that have excellent YR correlated with impact absorption are favorably used.
• Automobile frame parts have many end faces formed by shearing.
• the morphology of a sheared end face depends on the shear clearance.
• the morphology of a sheared end face affects delayed fracture resistance.
• delayed fracture is a phenomenon in which, when a formed part is placed in a hydrogen penetration environment, hydrogen penetrates into the steel sheet constituting the part to cause a decrease in interatomic bonding force or to cause local deformation, thus giving rise to microcracks that grow to fracture.
• High strength steel sheets used for automobiles are required to have a wide range of appropriate clearances not leading to delayed fracture.
• Patent Literature 1 provides a high strength steel sheet having a tensile strength of 980 MPa or more and excellent bending formability, and a method for manufacturing the same.
• the technique described in Patent Literature 1 does not consider YR or the range of appropriate clearances not leading to delayed fracture.
• the steel sheets described in Patent Literature 1 do not achieve YR ⁇ 85%.
• Patent Literature 2 provides a high strength steel sheet having a tensile strength of 1320 MPa or more and excellent delayed fracture resistance at sheared end faces, and a method for manufacturing the same.
• the technique described in Patent Literature 2 does not consider the range of appropriate clearances not leading to delayed fracture.
• Patent Literature 3 provides a high strength steel sheet having a tensile strength of 1100 MPa or more and being excellent in YR, surface quality, and weldability, and a method for manufacturing the same.
• the technique described in Patent Literature 3 does not consider the range of appropriate clearances not leading to delayed fracture.
• the present invention has been developed in view of the circumstances discussed above. Objects of the present invention are therefore to provide a high strength steel sheet having a TS of 1320 MPa or more and a YR of 85% or more and having a wide range of appropriate clearances not leading to delayed fracture; and to provide a method for manufacturing the same.
• a high strength steel sheet can be obtained that has a TS of 1320 MPa or more and a YR of 85% or more and has a wide range of appropriate clearances not leading to delayed fracture.
• the high strength steel sheet of the present invention may be applied to automobile structural members to reduce the weight of automobile bodies and thereby to enhance fuel efficiency.
• the present invention is highly valuable in industry.
• Carbon is one of the important basic components of steel, and, particularly in the present invention, is an important element that affects TS. If the C content is less than 0.15%, it is difficult to achieve 1320 MPa or higher TS. Thus, the C content is limited to 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 still 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 still more preferably 0.30% or less.
• the C content is most preferably 0.26% or less.
• Si 0.10% or more and 2.00% or less
• Silicon is one of the important basic components of steel, and, particularly in the present invention, is an important element that affects TS and retained austenite. If the Si content is less than 0.10%, 1320 MPa or higher TS is hardly achieved. Thus, the Si content is limited to 0.10% or more. The Si content is preferably 0.15% or more. The Si content is more preferably 0.20% or more. The Si content is still more preferably 0.30% or more. The Si content is most preferably 0.40% or more. However, if the Si content is more than 2.00%, the amount of retained austenite excessively increases to make it difficult to achieve 85% or higher YR. Thus, the Si content is limited to 2.00% or less. The Si content is preferably 1.80% or less. The Si content is more preferably 1.60% or less. The Si content is still more preferably 1.50% or less. The Si content is most preferably 1.20% or less.
• Mn 0.5% or more and 3.5% or less
• Manganese is one of the important basic components of steel, and, particularly in the present invention, is an important element that affects the fraction of ferrite and the fraction of bainite. If the Mn content is less than 0.5%, the fraction of ferrite and the fraction of bainite are increased to make it difficult to achieve 1320 MPa or higher TS and to achieve 85% or higher YR. Thus, the Mn content is limited to 0.5% or more.
• the Mn content is preferably 0.7% or more.
• the Mn content is more preferably 1.0% or more.
• the Mn content is still more preferably 1.1% or more.
• the Mn content is most preferably 1.5% or more.
• the Mn content is more than 3.5%, manganese macro-segregation occurs to lower the ultimate deformability of the steel and thereby to narrow the range of appropriate clearances not leading to delayed fracture.
• the Mn content is limited to 3.5% or less.
• the Mn content is preferably 3.3% or less.
• the Mn content is more preferably 3.1% or less.
• the Mn content is still more preferably 3.0% or less.
• the Mn content is most preferably 2.8% or less.
• the P content is more than 0.100%, phosphorus is segregated at grain boundaries to make the steel sheet brittle and to narrow the range of appropriate clearances not leading to delayed fracture.
• the P content is limited to 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 but is preferably 0.001% or more due to production technology limitations.
• the S content is more than 0.0200%, sulfides are formed making the ultimate deformability of the steel lower and thereby narrow the range of appropriate clearances not leading to delayed fracture.
• the S content is limited to 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 but is preferably 0.0001% or more due to production technology limitations.
• Al 0.010% or more and 1.000% or less
• the Al content needs to be 0.010% or more.
• the Al content is preferably 0.012% or more.
• the Al content is more preferably 0.015% or more.
• the Al content is still more preferably 0.020% or more.
• the Al content is preferably 0.500% or less.
• the Al content is more preferably 0.100% or less.
• the N content is more than 0.0100%, the cast slab becomes brittle and is easily cracked to cause a significant decrease in productivity.
• the N content is limited to 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 still 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 but is preferably 0.0010% or more due to production technology limitations.
• the H content exceeds not more than 0.0020%, the ultimate deformability of the steel is lowered and the range of appropriate clearances not leading to delayed fracture is narrowed.
• the H content is limited to 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 lower the H content, the wider the range of appropriate clearances not leading to delayed fracture.
• the high strength steel sheet of the present invention preferably further contains one, or two or more elements selected from, by 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, and REM: 0.0200% or less.
• the Ti content is more than 0.100%, the cast slab becomes brittle and is easily cracked to cause a significant decrease in productivity.
• the content thereof is limited to 0.100% or less.
• the Ti content is preferably 0.075% or less.
• the Ti content is more preferably 0.050% or less.
• the Ti content is still more preferably less than 0.050%.
• the addition of titanium increases the strength of the steel sheet and facilitates achieving 1320 MPa or higher TS.
• the Ti content is preferably 0.001% or more.
• the Ti content is more preferably 0.005% or more.
• the Ti content is still more preferably 0.010% or more.
• the B content is more than 0.0100%, the cast slab becomes brittle and is easily cracked to cause a significant decrease in productivity.
• the content thereof is limited to 0.0100% or less.
• the B content is preferably 0.0080% or less.
• the B content is more preferably 0.0050% or less.
• the addition of boron increases the strength of the steel sheet and facilitates achieving 1320 MPa or higher TS.
• the B content is preferably 0.0001% or more.
• the B content is more preferably 0.0002% or more.
• the Nb content is more than 0.100%, the cast slab becomes brittle and is easily cracked to cause a significant decrease in productivity.
• the content thereof is limited to 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 still more preferably 0.030% or less.
• the addition of niobium increases the strength of the steel sheet and facilitates achieving 1320 MPa or higher TS.
• the Nb content is preferably 0.001% or more.
• the Nb content is more preferably 0.002% or more.
• the Cu content is more than 1.00%, the cast slab becomes brittle and is easily cracked to cause a significant decrease in productivity.
• the Cu content is limited to 1.00% or less.
• the Cu content is preferably 0.50% or less.
• copper suppresses the penetration of hydrogen into the steel sheet and improves the range of appropriate clearances not leading to delayed fracture.
• the Cu content is preferably 0.01% or more.
• the Cu content is preferably 0.03% or more.
• the Cu content is more preferably 0.10% or more.
• the Cr content is more than 1.00%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• the content thereof is limited to 1.00% or less.
• the Cr content is preferably 0.70% or less.
• the Cr content is more preferably 0.50% or less.
• chromium not only serves as a solid solution strengthening element but also can stabilize austenite and suppress ferrite formation in the cooling process during continuous annealing, thus 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.
• the V content is more than 0.100%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• vanadium when added, the content thereof is limited to 0.100% or less.
• the V content is preferably 0.060% or less.
• vanadium increases the strength of the steel sheet.
• the V content is preferably 0.001% or more.
• the V content is more preferably 0.005% or more.
• the V content is still more preferably 0.010% or more.
• the Mo content is more than 0.500%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• the content thereof is limited to 0.500% or less.
• the Mo content is preferably 0.450% or less.
• the Mo content is more preferably 0.400% or less.
• molybdenum not only serves as a solid solution strengthening element but also can stabilize austenite and suppress ferrite formation in the cooling process during continuous annealing, thus increasing the strength of the steel sheet. To obtain these effects, the Mo content is preferably 0.010% or more.
• the Mo content is more preferably 0.020% or more.
• the Ni content is more than 0.50%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• the content thereof is limited to 0.50% or less.
• the Ni content is preferably 0.45% or less.
• the Ni content is more preferably 0.30% or less.
• nickel can stabilize austenite and suppress ferrite formation in the cooling process during continuous annealing, thus 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.
• the Sb content is more than 0.200%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• antimony when added, the content thereof is limited to 0.200% or less.
• the Sb content is preferably 0.100% or less.
• the Sb content is more preferably 0.050% or less.
• antimony suppresses the formation of a soft superficial layer and increases the strength of the steel sheet. To obtain these effects, the Sb content is preferably 0.001% or more.
• the Sb content is more preferably 0.005% or more.
• the Sn content is more than 0.200%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• the content thereof is limited to 0.200% or less.
• the Sn content is preferably 0.100% or less.
• the Sn content is more preferably 0.050% or less.
• tin suppresses the formation of a soft superficial layer and increases the strength of the steel sheet. To obtain these effects, the Sn content is preferably 0.001% or more.
• the Sn content is more preferably 0.005% or more.
• the As content is more than 0.100%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• arsenic when added, the content thereof is limited to 0.100% or less.
• the As content is preferably 0.060% or less.
• the As content is more preferably 0.010% or less.
• Arsenic increases the strength of the steel sheet. To obtain this effect, the As content is preferably 0.001% or more.
• the As content is more preferably 0.005% or more.
• the Ta content is more than 0.100%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• the content thereof is limited to 0.100% or less.
• the Ta content is preferably 0.050% or less.
• the Ta content is more preferably 0.010% or less.
• tantalum increases the strength of the steel sheet. To obtain this effect, the Ta content is preferably 0.001% or more.
• the Ta content is more preferably 0.005% or more.
• the Ca content is more than 0.0200%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• the content thereof is limited to 0.0200% or less.
• the Ca content is preferably 0.0100% or less.
• calcium is an element used for deoxidation, and furthermore this element is effective for controlling the shape of sulfides to spherical, enhancing the ultimate deformability of the steel sheet, and enhancing the range of appropriate clearances not leading to delayed fracture.
• the Ca content is preferably 0.0001% or more.
• the Mg content is more than 0.0200%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• the content thereof is limited to 0.0200% or less.
• magnesium is an element used for deoxidation, and furthermore this element is effective for controlling the shape of sulfides to spherical, enhancing the ultimate deformability of the steel sheet, and enhancing the range of appropriate clearances not leading to delayed fracture.
• the Mg content is preferably 0.0001% or more.
• zinc, cobalt, and zirconium are each more than 0.020%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• zinc, cobalt, and zirconium are added, the contents thereof are each limited to 0.020% or less.
• zinc, cobalt, and zirconium are elements effective for controlling the shape of inclusions to spherical, enhancing the ultimate deformability of the steel sheet, and enhancing the range of appropriate clearances not leading to delayed fracture.
• the contents of zinc, cobalt, and zirconium are preferably each 0.0001% or more.
• the REM content is more than 0.0200%, large amounts of coarse precipitates and inclusions are formed to lower the ultimate deformability of the steel, thus narrowing the range of appropriate clearances for hole expanding deformation.
• the content thereof is limited to 0.0200% or less.
• rare earth metals are elements effective for controlling the shape of inclusions to spherical, enhancing the ultimate deformability of the steel sheet, and enhancing the range of appropriate clearances not leading to delayed fracture.
• the REM content is preferably 0.0001% or more.
• the balance of the composition is Fe and incidental impurities.
• the content of any of the above optional elements is below the lower limit, the element does not impair the advantageous effects of the present invention.
• an optional element below the lower limit content is regarded as an incidental impurity.
• Tempered martensite 85% or more in terms of area fraction
• 1320 MPa or higher TS may be achieved by making martensite as the main phase.
• the area fraction of tempered martensite needs to be 85% or more.
• the area fraction of tempered martensite is limited to 85% or more.
• the area fraction of tempered martensite is preferably 90% or more.
• the area fraction of tempered martensite is more preferably 92% or more and is further preferably 95% or more.
• the upper limit of the area fraction of tempered martensite is not particularly limited and may be 100%.
• tempered martensite is measured as follows. A longitudinal cross section of the steel sheet is polished and is subjected to etching in 3 vol% Nital solution. A portion at 1/4 sheet thickness (a location corresponding to 1/4 of the sheet thickness in the depth direction from the steel sheet surface) is observed using SEM in 10 fields of view at a magnification of ⁇ 2000. In the microstructure images, tempered martensite is structures that have fine irregularities inside the structures and contain carbides within the structures. The values thus obtained are averaged to determine tempered martensite.
• volume fraction of retained austenite is 5% or more, it is difficult to achieve 85% or higher YR.
• the lowering in YR is ascribed to the fact that the amount of retained austenite is so large that strain induced transformation of retained austenite results in low YS.
• retained austenite is limited to less than 5% and is preferably 4% or less.
• the lower limit of retained austenite is not particularly limited. A lower fraction of retained austenite is more preferable, and the fraction may be 0%.
• retained austenite is measured as follows.
• the steel sheet was polished to expose a face 0.1 mm below 1/4 sheet thickness and was thereafter further chemically polished to expose a face 0.1 mm below the face exposed above.
• the face was analyzed with an X-ray diffractometer using CoK ⁇ radiation to determine the integral intensity ratios of the diffraction peaks of ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ planes of fcc iron and ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ planes of bcc iron.
• Nine integral intensity ratios thus obtained were averaged to determine retained austenite.
• the total of ferrite and bainitic ferrite is more than 10%, it is difficult to achieve 1320 MPa or higher TS and to achieve 85% or higher YR.
• the lowering in YR is ascribed to the fact that ferrite and bainitic ferrite are soft microstructures and hasten the occurrence of yielding.
• the total of ferrite and bainitic ferrite is limited to 10% or less.
• the total is preferably 8% or less and is more preferably 5% or less.
• the lower limit of the total of ferrite and bainitic ferrite is not particularly limited. A smaller fraction is more preferable.
• the lower limit of the total of ferrite and bainitic ferrite may be 0%.
• the total of ferrite and bainitic ferrite is measured as follows. A longitudinal cross section of the steel sheet is polished and is subjected to etching in 3 vol% Nital solution. A portion at 1/4 sheet thickness (a location corresponding to 1/4 of the sheet thickness in the depth direction from the steel sheet surface) is observed using SEM in 10 fields of view at a magnification of ⁇ 2000. In the microstructure images, ferrite and bainitic ferrite are recessed structures with a flat interior. The values thus obtained are averaged to determine the total of ferrite and bainitic ferrite.
• KAM S / KAM C KAM (Kernel average misorientation) value of a superficial portion of the steel sheet
• KAM (C) KAM value of a central portion of the steel sheet
• the superficial portion of the steel sheet is located 100 um below the steel sheet surface toward the center of the sheet thickness.
• the central portion of the steel sheet is located at 1/2 of the sheet thickness.
• KAM (S)/KAM (C) of less than 1.00 is effective for improving the YR and the range of appropriate clearances not leading to delayed fracture.
• KAM (S)/KAM (C) is limited to less than 1.00.
• the lower limit of KAM (S)/KAM (C) is not particularly limited but is preferably 0.80 or more due to production technology limitations.
• the KAM values are measured as follows. First, a test specimen for microstructure observation was sampled from the cold rolled steel sheet. Next, the sampled test specimen was polished by vibration polishing with colloidal silica to expose a cross section in the rolling direction (a longitudinal cross section) for use as observation surface. The observation surface was specular. Next, electron backscatter diffraction (EBSD) measurement was performed. Local crystal orientation data was thus obtained. Here, the SEM magnification was ⁇ 3000, the step size was 0.05 um, the measured region was 20 um square, and the WD was 15 mm. The local orientation data obtained was analyzed with analysis software: OIM Analysis 7. The analysis was performed with respect to 10 fields of view of the portion at the target sheet thickness, and the results were averaged.
• EBSD electron backscatter diffraction
• the superficial portion of the steel sheet is located 100 um below the steel sheet surface toward the center of the sheet thickness.
• Hv (Q) - Hv (S) of 8 or more is effective for improving the YR and the range of appropriate clearances not leading to delayed fracture.
• Hv (Q) - Hv (S) is limited to 8 or more.
• the upper limit of Hv (Q) - Hv (S) is not particularly limited but is preferably 30 or less due to production technology limitations.
• Preferred ranges of Hv (Q) and Hv (S) are 400 to 600 and 400 to 600, respectively.
• the hardness is measured as follows.
• a test specimen for microstructure observation was sampled from the cold rolled steel sheet.
• the sampled test specimen was polished to expose a cross section in the rolling direction (a longitudinal cross section) for use as observation surface.
• the observation surface was specular.
• the hardness was determined using a Vickers tester with a load of 1 kg. The hardness was measured with respect to 10 points at 20 um intervals at the target sheet thickness. The values of 8 points excluding the maximum hardness and the minimum hardness were averaged.
• a steel material may be obtained by any known steelmaking method without limitation, such as a converter or an electric arc furnace.
• the steel slab (the slab) is preferably produced by a continuous casting method.
• the slab heating temperature, the slab soaking holding time, and the coiling temperature in hot rolling are not particularly limited.
• the steel slab may be hot rolled in such a manner that the slab is heated and is then rolled, that the slab is subjected to hot direct rolling after continuous casting without being heated, or that the slab is subjected to a short heat treatment after continuous casting and is then rolled.
• the slab heating temperature, the slab soaking holding time, the finish rolling temperature, and the coiling temperature in hot rolling are not particularly limited.
• the slab heating temperature is preferably 1100°C or above.
• the slab heating temperature is preferably 1300°C or below.
• 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 Ar 3 transformation temperature or above.
• the coiling temperature is preferably 350°C or above.
• the coiling temperature is preferably 650°C or below.
• the hot rolled steel sheet thus produced is pickled.
• Pickling can remove oxides on the steel sheet surface and is thus important to ensure good chemical convertibility and a high quality of coating in the final high strength steel sheet.
• Pickling may be performed at a time or several.
• the hot rolled sheet that has been pickled may be cold rolled directly or may be subjected to heat treatment before cold rolling.
• 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.
• the rolling reduction in cold rolling is preferably 80% or less.
• the advantageous effects of the present invention may be obtained without limitations on the number of rolling passes and the rolling reduction in each pass.
• the cold rolled steel sheet obtained as described above is annealed. Annealing conditions are as follows.
• Annealing temperature T1 850°C or above and 1000°C or below
• the annealing temperature T1 is below 850°C, the area fraction of the total of ferrite and bainitic ferrite exceeds 10% to make it difficult to achieve 1320 MPa or higher TS and to achieve 85% or higher YR. Thus, the annealing temperature T1 is limited to 850°C or above. T1 is preferably 860°C or above. T1 is more preferably 870°C or above. However, if the annealing temperature T1 is higher than 1000°C, the prior-austenite grain size excessively increases and the range of appropriate clearances not leading to delayed fracture is narrowed. Thus, the annealing temperature T1 is limited to 1000°C or below. The annealing temperature T1 is preferably 970°C or below. T1 is more preferably 950°C or below.
• Holding time t1 at the annealing temperature T1 10 seconds or more and 1000 seconds or less
• the holding time t1 at the annealing temperature T1 is less than 10 seconds, austenitization is insufficient with the result that the area fraction of the total of ferrite and bainitic ferrite exceeds 10% to make it difficult to achieve 1320 MPa or higher TS and to achieve 85% or higher YR.
• the holding time t1 at the annealing temperature T1 is limited to 10 seconds or more.
• the holding time t1 at the annealing temperature T1 is preferably 30 seconds or more.
• t1 is more preferably 45 seconds or more.
• t1 is still more preferably 60 seconds or more.
• t1 is most preferably 100 seconds or more.
• the holding time t1 at the annealing temperature T1 is limited to 1000 seconds or less.
• the holding time t1 at the annealing temperature T1 is preferably 800 seconds or less.
• t1 is more preferably 500 seconds or less.
• the annealed steel sheet In the step of cooling to 100°C or below, austenite is transformed into martensite. To obtain 85% or more martensite, the annealed steel sheet needs to be cooled to 100°C or below. Thus, cooling after annealing is effected to 100°C or below.
• the lower limit of the cooling complete temperature is not particularly limited but is preferably 0°C or above due to production technology limitations.
• the elapsed time t2 from the time when the temperature reaches 100°C until the start of working is preferably 900 seconds or less. t2 is more preferably 800 seconds or less.
• the lower limit of the elapsed time t2 from the time when the temperature reaches 100°C until the start of working is not particularly limited but is preferably 5 seconds or more due to production technology limitations. Studies by the present inventors have shown that the elapsed time from the time when the temperature reaches 100°C until the end of working does not affect the amounts of strains introduced by working into the superficial portion of the steel sheet and the central portion of the steel sheet.
• the working start temperature T2 is higher than 80°C, the steel sheet is soft and working introduces varied amounts of strains into the superficial portion of the steel sheet and the central portion of the steel sheet with the result that KAM (S)/KAM (C) becomes 1.00 or more. As a result, the YR is lowered and the range of appropriate clearances not leading to delayed fracture is narrowed.
• the working start temperature T2 is limited to 80°C or below.
• the working start temperature T2 is preferably 60°C or below. T2 is more preferably 50°C or below.
• the lower limit of the working start temperature T2 is not particularly limited but is preferably 0°C or above due to production technology limitations.
• the equivalent plastic strain is less than 0.10%, the amount of working is small and KAM (S)/KAM (C) becomes 1.00 or more. As a result, the YR is lowered and the range of appropriate clearances not leading to delayed fracture is narrowed. Thus, the equivalent plastic strain is limited to 0.10% or more.
• the plastic equivalent strain is preferably 0.15% or more.
• the plastic equivalent strain is more preferably 0.20% or more. If the equivalent plastic strain is more than 5.00%, the influences by working are equal between the superficial portion of the steel sheet and the central portion of the steel sheet with the result that KAM (S)/KAM (C) becomes 1.00 or more.
• the upper limit of the equivalent plastic strain is 5.00% or less due to production technology limitations.
• the equivalent plastic strain is limited to 5.00% or less.
• the equivalent plastic strain is preferably 4.00% or less.
• the equivalent plastic strain is more preferably 2.00% or less.
• the equivalent plastic strain is still more preferably 1.00% or less.
• the working step before tempering is preferably performed under conditions where strain is applied by two or more separate working operations, and the total of the equivalent plastic strains applied in the working operations is 0.10% or more.
• the working step before tempering may apply strain by two or more separate working operations as long as the total of the equivalent plastic strains applied in the working operations is 0.10% or more.
• the elapsed time from when the temperature reaches 100°C until the start of the second and subsequent working operations, because the mobility of dislocations in martensite has been lowered by the first working operation.
• the working process may be typically temper rolling or tension leveling.
• the equivalent plastic strain in temper rolling is the ratio by which the steel sheet is elongated and may be determined from the change in the length of the steel sheet before and after the working.
• the equivalent plastic strain of the steel sheet in leveler processing was calculated by the method of Reference 1 below. The data inputs described below were used in the calculation.
• the material was assumed to be a linear hardening elastoplastic material. Bausinger hardening and the decrease in tension due to bend loss were ignored. Misaka's formula was used as the formula of bending curvature.
• Tempering temperature T3 100°C or above and 400°C or below
• the tempering temperature T3 is lower than 100°C, the carbon diffusion distance is so short that the hardness of the steel sheet surface and the inside of the steel sheet is lowered and Hv (Q) - Hv (S) becomes less than 8. As a result, the YR is lowered and the range of appropriate clearances not leading to delayed fracture is narrowed.
• the tempering temperature T3 is limited to 100°C or above.
• the tempering temperature T3 is preferably 150°C or above.
• T3 is more preferably 170°C or above.
• T3 is still more preferably 200°C or above.
• the tempering temperature T3 is higher than 400°C, tempering of martensite proceeds to make it difficult to achieve 1320 MPa or higher TS.
• the tempering temperature T3 is limited to 400°C or below.
• the tempering temperature T3 is preferably 350°C or below.
• T3 is more preferably 300°C or below.
• T3 is still more preferably 280°C or below.
• Holding time t3 at the tempering temperature T3 1.0 second or more and 1000.0 seconds or less
• the holding time t3 at the tempering temperature T3 is less than 1.0 second, the carbon diffusion distance is so short that the hardness of the steel sheet surface and the inside of the steel sheet is lowered and Hv (Q) - Hv (S) becomes less than 8 with the result that the YR is lowered and the range of appropriate clearances not leading to delayed fracture is narrowed.
• the holding time t3 at the tempering temperature T3 is limited to 1.0 second or more.
• the holding time t3 at the tempering temperature T3 is preferably 5.0 seconds or more.
• t3 is more preferably 50.0 seconds or more.
• t3 is still more preferably 100.0 seconds or more.
• the holding time t3 at the tempering temperature T3 is limited to 1000.0 seconds or less.
• the holding time t3 at the tempering temperature T3 is preferably 800.0 seconds or less.
• t3 is more preferably 600.0 seconds or less.
• t3 is still more preferably 500.0 seconds or less.
• the cooling rate ⁇ 1 from the tempering temperature T3 to 80°C is higher than 100°C/sec, the carbon diffusion distance is so short that the hardness of the steel sheet surface and the inside of the steel sheet is lowered and Hv (Q) - Hv (S) becomes less than 8 with the result that the YR is lowered and the range of appropriate clearances not leading to delayed fracture is narrowed.
• the cooling rate ⁇ 1 from the tempering temperature T3 to 80°C is limited to 100°C/sec or less.
• the cooling rate ⁇ 1 from the tempering temperature T3 to 80°C is preferably 50°C/sec or less.
• the lower limit of the cooling rate ⁇ 1 from the tempering temperature T3 to 80°C is not particularly limited but is preferably 10°C/sec or more due to production technology limitations.
• cooling is not particularly limited and the steel sheet may be cooled to a desired temperature in an appropriate manner.
• the desired temperature is preferably about room temperature.
• the high strength steel sheet described above may be worked again under conditions where the amount of equivalent plastic strain is 0.10% or more and 5.00% or less.
• the target amount of equivalent plastic strain may be applied at a time or several.
• the steel sheet is usually traded after being cooled to room temperature.
• the high strength steel sheet may be subjected to coating treatment during annealing or after annealing.
• the phrase "during annealing” means a period from the end of the holding time t1 at the annealing temperature T1 until when the steel sheet that has been held for t3 at the tempering temperature T3 is cooled to room temperature.
• the phrase "after annealing” means a period after the steel sheet is cooled to room temperature.
• the coating treatment during annealing may be hot-dip galvanizing treatment and alloying treatment following the hot-dip galvanizing treatment which are performed when the steel sheet that has been held at the annealing temperature T1 is being cooled to 100°C or below.
• the coating treatment after annealing may be Zn-Ni electrical alloying coating treatment or pure Zn electroplated coating treatment performed after the steel sheet that has been held for t3 at the tempering temperature T3 is cooled to room temperature.
• a coated layer may be formed by electroplated coating, or hot-dip zinc-aluminum-magnesium alloy coating may be applied.
• the types of coating metals, such as Zn coating and Al coating are not particularly limited. Other conditions in the manufacturing method are not particularly limited.
• the series of treatments including annealing, hot-dip galvanizing, and alloying treatment of the coated zinc layer is preferably performed on hot-dip galvanizing line, that is CGL (continuous galvanizing line).
• hot-dip galvanizing treatment may be followed by wiping.
• Conditions for operations, such as coating, other than those conditions described above may be determined in accordance with the usual hot-dip galvanizing technique.
• the steel sheet may be worked again under conditions where the amount of equivalent plastic strain is 0.10% or more and 5.00 or less.
• the target amount of equivalent plastic strain may be applied at a time or several.
• the high strength cold rolled steel sheets obtained as described above were used as test steels. Tensile characteristics and delayed fracture resistance were evaluated in accordance with the following test methods.
• the area fraction of tempered martensite, the volume fraction of retained austenite, and the total of the area fraction of ferrite and the area fraction of bainitic ferrite were determined in accordance with the methods described hereinabove.
• the KAM value of a superficial portion of the steel sheet and the KAM value of a central portion of the steel sheet were determined in accordance with the method described hereinabove.
• the hardness of a portion at 1/4 sheet thickness and the hardness of a superficial portion of the steel sheet were determined in accordance with the method described hereinabove.
• a JIS No. 5 test specimen (gauge length: 50 mm, width of parallel portion: 25 mm) was sampled so that the longitudinal direction of the test specimen would be perpendicular to the rolling direction.
• a tensile test was performed in accordance with JIS Z 2241 under conditions where the crosshead speed was 1.67 ⁇ 10 -1 mm/sec. YS and TS were thus measured. In the present invention, 1320 MPa or higher TS was judged to be acceptable, and 85% or higher yield ratio (YR) was judged to be acceptable.
• Test specimens having a size of 16 mm ⁇ 75 mm were prepared by shearing in such a manner that the longitudinal direction would be perpendicular to the rolling direction.
• the rake angle in the shearing process was constant at 0°, and the shear clearance was changed from 5 to 10, 15, 20, 25, 30, and 35%.
• the test specimens were four-point loaded in accordance with ASTM (G39-99) so that 1000 MPa stress was applied to the bend apex.
• the loaded test specimens were immersed in pH 3 hydrochloric acid at 25°C for 100 hours. The rating was " ⁇ " when the shear clearances that did not cause cracking ranged below 10%.
• the rating was “o” when the shear clearances ranged to 10% or above but below 15%.
• the rating was “ ⁇ ” when the shear clearances that did not cause cracking ranged to 15% or above.
• the range of appropriate clearances not leading to delayed fracture was evaluated as excellent when the shear clearances that did not cause cracking ranged to 10% or above.
• INVENTIVE EXAMPLES achieved 1320 MPa or higher TS, 85% or higher YR, and an excellent range of appropriate clearances not leading to delayed fracture.
• COMPARATIVE EXAMPLES were unsatisfactory in one or more of TS, YR, and the range of appropriate clearances not leading to delayed fracture.
• Blanks indicate that the element was not added intentionally.
• Table 2-1 No. Steels Sheet thickness (mm) Annealing temp. T1 (°C) Holding time t1 (sec) Elapsed time t2 from when the temp. reached 100°C until start of working (sec) Working start temp. T2 (°C) Equivalent plastic strain (%) Working operations (times) Tempering temp. T3 (°C) Holding time t3 (sec) Cooling rate ⁇ 1 from tempering temp. T3 to 80°C (°C/sec) Type of product (*) Remarks 1 A 1.4 875 105 729 44 0.44 1 237 160.5 29 CR INV. EX. 2 B 1.4 870 151 653 25 0.55 1 250 62.0 34 CR INV.
• T1 (°C) Holding time t1 (sec) Elapsed time t2 from when the temp. reached 100°C until start of working (sec) Working start temp. T2 (°C) Equivalent plastic strain (%) Working operations (times) Tempering temp. T3 (°C) Holding time t3 (sec) Cooling rate ⁇ 1 from tempering temp. T3 to 80°C (°C/sec) Type of product (*) Remarks 36 B 1.4 872 58 706 29 0.52 1 204 228.1 125 CR COMP. EX. 37 C 0.8 880 65 621 46 0.42 1 187 198.8 41 CR INV. EX. 38 D 2.0 864 133 772 30 0.48 1 255 245.6 29 CR INV. EX.
• EX. 46 E 1.4 877 54 720 11 0.53 1 171 140.1 37 CR INV.
• EX. 47 E 1.4 864 86 663 79 0.33 1 287 161.4 43 CR INV.
• EX. 48 E 1.4 860 190 635 43 0.13 1 228 114.9 28 CR INV.
• EX. 49 E 1.4 867 70 651 47 4.22 1 172 169.5 33 CR INV.
• EX. 50 E 1.4 877 117 755 49 0.51 1 105 245.2 28 CR INV.
• EX. 51 E 1.4 866 58 650 50 0.38 1 381 158.2 38 CR INV.
• EX. 52 E 1.4 870 84 641 50 0.40 1 228 4.6 50 CR INV.
• EX. 2 B 1.4 99 1 0 0.501 0.538 0.931 508 491 17 1418 1538 92 ⁇ INV.
• EX. 3 B 1.4 91 1 8 0.508 0.535 0.949 473 460 13 1235 1440 86 ⁇ INV.
• EX. 4 B 1.4 83 3 14 0.499 0.531 0.940 414 395 19 1024 1240 83 ⁇ COMP.
• EX. 5 B 1.4 99 1 0 0.514 0.541 0.950 548 534 14 1503 1674 90 ⁇ INV.
• EX. 6 B 1.4 99 1 0 0.502 0.534 0.940 511 491 20 1455 1538 95 ⁇ INV.
• EX. 43 E 1.4 99 1 0 0.499 0.537 0.930 498 476 22 1438 1495 96 O INV.
• EX. 44 E 1.4 99 1 0 0.487 0.540 0.901 507 482 25 1489 1510 99 O INV.
• EX. 45 E 1.4 98 2 0 0.529 0.538 0.984 488 478 10 1291 1497 86 ⁇ INV.
• EX. 46 E 1.4 98 2 0 0.482 0.535 0.900 565 538 27 1669 1686 99 O INV.
• EX. 47 E 1.4 99 1 0 0.525 0.534 0.982 485 477 8 1323 1498 88 ⁇ INV.
• EX. 48 E 1.4 99 1 0 0.529 0.539 0.982 511 502 9 1383 1571 88 ⁇ INV.
• EX. 49 E 1.4 99 1 0 0.485 0.538 0.901 564 538 26 1676 1686 99 O INV.
• EX. 50 E 1.4 99 1 0 0.502 0.534 0.940 525 516 9 1414 1614 88 ⁇ INV.
• EX. 51 E 1.4 99 1 0 0.509 0.536 0.950 453 444 9 1192 1390 86 ⁇ INV.
• EX. 52 E 1.4 100 0 0 0.518 0.539 0.960 525 516 9 1394 1616 86 ⁇ INV.
• EX. 53 E 1.4 99 1 0 0.502 0.533 0.940 526 517 9 1425 1620 88 ⁇ INV.
• EX. 54 E 1.4 98 2 0 0.508 0.535 0.949 530 520 10 1405 1633 86 ⁇ INV.
• EX. 55 E 1.4 100 0 0 0.505 0.537 0.939 454 431 23 1311 1352 97 O INV.
• EX. 56 E 1.4 100 0 0 0.518 0.540 0.960 546 522 24 1601 1637 98 O INV.
• EX. 57 E 1.4 98 2 0 0.504 0.536 0.941 486 478 8 1304 1500 87 ⁇ INV.

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