Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
• • 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
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips
• • 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
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F17/00—Multi-step processes for surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by subclass C21D or C22F or class C25
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a steel sheet capable of obtaining an excellent crashworthiness suitable for an automobile member.
• the steel sheet for automobile is required to have excellent formability, a high strength after paint baking, and an excellent crashworthiness.
• a steel sheet used for an automobile conventionally, a dual phase (DP) steel sheet having a dual phase structure of ferrite and martensite and a transformation induced plasticity (TRIP) steel sheet have been cited.
• the DP steel sheet and the TRIP steel sheet have a problem that their mechanical properties after paint baking sometimes vary in a member. That is, in molding of the steel sheet, strain is applied according to the shape of a member to be obtained, and thus the molded steel sheet includes a portion with strain strongly applied thereto and a portion with less strain applied thereto. Then, as the applied strain is larger in the portion, the amount of strain age hardening by paint baking is larger and the hardness increases in the portion. As a result, the difference in yield strength after the paint baking is sometimes large between the portion with strain applied thereto and the portion with less strain applied thereto by molding. In this case, the portion with less strain applied thereto is soft and a break occurs in this portion, to thus fail to obtain a sufficient reaction force characteristic and crashworthiness.
• Patent Literature 1 Japanese Laid-open Patent Publication No. 2009-185355
• Patent Literature 2 Japanese Laid-open Patent Publication No. 2011-111672
• Patent Literature 3 Japanese Laid-open Patent Publication No. 2012-251239
• Patent Literature 4 Japanese Laid-open Patent Publication No. 11-080878
• Patent Literature 5 Japanese Laid-open Patent Publication No. 11-080879
• Patent Literature 6 International Publication Pamphlet No. WO2013/047821
• Patent Literature 7 Japanese Laid-open Patent Publication No. 2008-144233
• Patent Literature 8 International Publication Pamphlet No. WO2012/070271
• An object of the present invention is to provide a steel sheet capable of obtaining a stable yield strength after paint baking while obtaining good formability.
• the present inventors conducted earnest examinations in order to solve the above-described problems. As a result, they found out that in the case where a dislocation density in ferrite and a dislocation density in bainite are high, the yield strength improves by aging accompanying paint baking even in the portion with less strain applied thereto at the time of molding. They also found out that in the case of an average grain diameter of the ferrite and the bainite being small, the yield strength further improves by the aging.
• a steel sheet includes:
• V and Ta 0.0% to 0.3% in total
• an average dislocation density in the ferrite and an average dislocation density in the bainite are both 3 ⁇ 10 12 m/m 3 to 1 ⁇ 10 14 m/m 3 , and
• an average grain diameter of the ferrite and the bainite is 5 ⁇ m or less.
• the steel sheet according to (1) in which the steel structure contains, in area fraction, ferrite and bainite: 2% to 60% in total and martensite: 10% to 90%,
• an area fraction of retained austenite in the steel structure is 15% or less
• a ratio of an area fraction of the ferrite to an area fraction of the martensite is 0.03 to 1.00.
• V and Ta 0.01% to 0.3% in total is established.
• La 0.001% to 0.005%
• the present invention it is possible to obtain a stable yield strength even after paint baking because the average dislocation density in ferrite, the average dislocation density in bainite, and the like are appropriate.
• the steel sheet according to the embodiment of the present invention is manufactured by going through hot rolling, cold rolling, annealing, temper rolling, and so on of the steel.
• the chemical compositions of the steel sheet and the steel consider not only properties of the steel sheet, but also these treatments.
• “%” being the unit of the content of each element contained in the steel sheet means “mass %” unless otherwise noted.
• the steel sheet according to this embodiment has a chemical composition represented by, in mass %, C: 0.05% to 0.40%, Si: 0.05% to 3.0%, Mn: 1.5% to 4.0%, Al: 1.5% or less, N: 0.02% or less, P: 0.2% or less, S: 0.01% or less, Nb and Ti: 0.005% to 0.2% in total, V and Ta: 0.0% to 0.3% in total, Cr, Mo, Ni, Cu, and Sn: 0.0% to 1.0% in total, B: 0.00% to 0.01%, Ca: 0.000% to 0.005%, Ce: 0.000% to 0.005%, La: 0.000% to 0.005%, and the balance: Fe and impurities.
• the impurities include ones contained in raw materials such as ore and scrap and ones contained in manufacturing steps.
• the C contributes to an improvement in tensile strength.
• the C content is less than 0.05%, it is impossible to obtain a sufficient tensile strength, for example, a tensile strength of 980 MPa or more.
• the C content is 0.05% or more.
• the C content is preferably 0.08% or more so as to obtain a higher tensile strength.
• the C content is greater than 0.40%, it is impossible to obtain a sufficient density dislocation in ferrite, and it is difficult to obtain a preferred steel structure.
• the C content is 0.40% or less. From the weldability viewpoint, the C content is preferably 0.35% or less.
• Si affects formation of iron carbides and age hardening accompanying this.
• the Si content is 0.05% or more.
• the Si content is preferably 0.10% or more so as to further increase the yield strength.
• the Si content is set to 3.0% or less. From the viewpoints of suppressing season cracking of a slab and suppressing edge cracking during hot rolling, the Si content is preferably 2.5% or less and more preferably 2.0% or less.
• Mn suppresses transformation from austenite to ferrite, and contributes to an improvement in tensile strength.
• the Mn content is less than 1.5%, it is impossible to obtain a sufficient tensile strength, for example, a tensile strength of 980 MPa or more.
• the Mn content is 1.5% or more.
• the Mn content is preferably 2.0% or more so as to obtain a higher tensile strength.
• the Mn content is greater than 4.0%, it is impossible to obtain sufficient formability.
• the Mn content is 4.0% or less.
• the Mn content is preferably 3.5% or less so as to obtain more excellent formability.
• Al is not an essential element, but is used for deoxidation intended for reducing inclusions, for example, and is able to remain in the steel.
• the Al content is greater than 1.5%, it is impossible to obtain ferrite or bainite having an average dislocation density in a later-described range.
• the Al content is 1.5% or less. Reducing the Al content is expensive, and thus, when the Al content is tried to be reduced down to less than 0.002%, its cost increases significantly. Therefore, the Al content may be set to 0.002% or more. After sufficient deoxidation is performed, Al, which is 0.01% or more, sometimes remains.
• N is not an essential element, and is contained in the steel as an impurity, for example.
• the N content is greater than 0.02%, nitrides in large amounts precipitate to fail to obtain sufficient formability.
• the N content is 0.02% or less. Reducing the N content is expensive, and thus, when the N content is tried to be reduced down to less than 0.001%, its cost increases significantly. Therefore, the N content may be set to 0.001% or more.
• P is not an essential element, and is contained in the steel as an impurity, for example.
• the P content is greater than 0.2%, P compounds in large amounts precipitate to fail to obtain sufficient formability.
• the P content is 0.2% or less.
• the P content is preferably 0.07% or less. Reducing the P content is expensive, and thus, when the P content is tried to be reduced down to less than 0.001%, its cost increases significantly. Therefore, the P content may be set to 0.001% or more.
• S is not an essential element, and is contained in the steel as an impurity, for example.
• the S content is greater than 0.01%, sulfides in large amounts precipitate to fail to obtain sufficient formability.
• the S content is 0.01% or less.
• the S content is preferably 0.003% or less so as to more suppress the decrease in formability. Reducing the S content is expensive, and thus, when the S content is tried to be reduced down to less than 0.0002%, its cost increases significantly. Therefore, the S content may be set to 0.0002% or more.
• Nb and Ti contribute to making crystal grains of ferrite or bainite fine and precipitation strengthening of ferrite or bainite.
• Nb and Ti form (Ti, Nb) carbonitrides, and thus, according to the contents of Nb and Ti, the content of solid-solution C and the content of solid-solution N after annealing change.
• the total content of Nb and Ti is less than 0.005%, it is impossible to obtain ferrite or bainite having an average grain diameter in a later-described range, and the yield strength does not increase sufficiently even by aging accompanying paint baking.
• the total content of Nb and Ti is 0.005% or more.
• the total content of Nb and Ti is preferably 0.010% or more so as to sufficiently increase the yield strength by the aging.
• Nb and Ti when the total content of Nb and Ti is greater than 0.2%, (Ti, Nb) carbonitrides in large amounts precipitate to fail to obtain sufficient formability.
• the total content of Nb and Ti is 0.2% or less.
• the total content of Nb and Ti is preferably 0.1% or less.
• V, Ta, Cr, Mo, Ni, Cu, Sn, B, Ca, Ce, and La are not an essential element, but are an arbitrary element that may be appropriately contained, up to a predetermined amount as a limit, in the steel sheet and the steel.
• V and Ta contribute to an improvement in strength by formation of carbides, nitrides, or carbonitrides and grain refining of ferrite and bainite.
• V or Ta or the both of these may be contained.
• the total content of V and Ta is greater than 0.3%, carbonitrides in large amounts precipitate and ductility decreases.
• the total content of V and Ta is 0.3% or less.
• the total content of V and Ta is preferably 0.1% or less.
• the total content of V and Ta is preferably 0.01% or more so as to securely obtain an effect by the above-described functions.
• Cr, Mo, Ni, Cu, and Sn are used in order to suppress transformation from austenite to ferrite, similarly to Mn.
• Cr, Mo, Ni, Cu, or Sn, or an arbitrary combination of these may be contained.
• the total content of Cr, Mo, Ni, Cu, and Sn is greater than 1.0%, workability deteriorates significantly and elongation decreases.
• the total content of Cr, Mo, Ni, Cu, and Sn is 1.0% or less.
• the total content of Cr, Mo, Ni, Cu, and Sn is preferably 0.5% or less.
• the content of Cr, Mo, Ni, Cu, and Sn is preferably 0.1% or more so as to securely obtain an effect by the above-described functions.
• B increases hardenability of the steel sheet, suppresses formation of ferrite, and promotes formation of martensite.
• B may be contained.
• the B content is greater than 0.01% in total, boride in large amounts precipitates to fail to obtain sufficient formability.
• the B content is 0.01% or less.
• the B content is preferably 0.003% or less in total so as to more suppress the decrease in ductility.
• the B content is preferably 0.0003% or more so as to securely obtain an effect by the above-described functions.
• Ca, Ce, and La make oxides and sulfides in the steel sheet fine and change properties of oxides and sulfides, to thereby suppress the decrease in workability, particularly, elongation.
• Ca, Ce, or La, or an arbitrary combination of these may be contained.
• the Ca content, the Ce content, and the La content each are 0.005% or less.
• the Ca content, the Ce content, and the La content each are preferably 0.003% or less so as to more suppress the decrease in formability.
• the Ca content, the Ce content, and the La content each are preferably 0.001% or more so as to securely obtain an effect by the above-described functions. That is, “Ca: 0.001% to 0.005%,” “Ce: 0.001% to 0.005%,” or “La: 0.001% to 0.005%,” or an arbitrary combination of these is preferably satisfied.
• the steel structure of the steel sheet according to the embodiment of the present invention contains, in area fraction, 2% or more in total of ferrite and bainite.
• the average dislocation density in the ferrite and the average dislocation density in the bainite are both 3 ⁇ 10 12 m/m 3 to 1 ⁇ 10 14 m/m 3 , and the average grain diameter of the ferrite and the bainite is 5 ⁇ m or less.
• the present inventors revealed that in the case of the dislocation density in the ferrite and the dislocation density in the bainite being high, the yield strength improves by the aging accompanying paint baking even in a portion with less strain applied thereto at the time of molding.
• the average dislocation density in the ferrite, or the average dislocation density in the bainite, or the both of these are less than 3 ⁇ 10 12 m/m 3 , the yield strength in the portion with less strain applied thereto at the time of molding does not improve sufficiently by the aging to fail to obtain a sufficient crashworthiness.
• the average dislocation density in the ferrite and the average dislocation density in the bainite are both 3 ⁇ 10 12 m/m 3 or more.
• the average dislocation density in the ferrite and the average dislocation density in the bainite are both preferably 6 ⁇ 10 12 m/m 3 or more so as to obtain a more excellent crashworthiness.
• the average dislocation density in the ferrite, or the average dislocation density in the bainite, or the both of these are greater than 1 ⁇ 10 14 m/m 3 , the formability decreases and the yield strength in the portion with less strain applied thereto at the time of molding does not improve sufficiently by the aging, to fail to obtain a sufficient crashworthiness in some cases.
• the average dislocation density in the ferrite and the average dislocation density in the bainite are both 1 ⁇ 10 14 m/m 3 or less.
• the average dislocation density in the ferrite and the average dislocation density in the bainite are both preferably 8 ⁇ 10 13 m/m 3 or less so as to obtain more excellent formability and crashworthiness.
• the average dislocation density in the ferrite and the average dislocation density in the bainite can be obtained by using a transmission electron microscopy (TEM) photograph, for example. That is, a TEM photograph of a thin film sample is prepared, an arbitrary line is drawn on this TEM photograph, and in the case of trying to obtain the average dislocation density in the ferrite, the place where this line intersects with a dislocation line in the ferrite is counted.
• TEM transmission electron microscopy
• the dislocation density in the ferrite in the thin film sample is expressed as “2N/(Lt).”
• TEM photographs taken at plural places of the thin film sample are used, and an average value of dislocation densities obtained from these plural TEM photographs is obtained as the average dislocation density in the ferrite.
• an actual measured value may be used, or 0.1 ⁇ m may be used simply.
• the average dislocation density in the bainite can be obtained by a method similar to the method of obtaining the average dislocation density in the ferrite as long as an intersecting place is counted in the bainite and the length of a line in the bainite is used.
• the present inventors revealed that in the case of the grain diameter of the ferrite and the bainite being small, the yield strength further improves by the aging.
• the average grain diameter of the ferrite and the bainite is greater than 5 ⁇ m, the yield strength of the portion with less strain applied thereto at the time of molding does not improve sufficiently by the aging to fail to obtain a sufficient crashworthiness.
• the average grain diameter of the ferrite and the bainite is 5 ⁇ m or more.
• the average grain diameter of the ferrite and the bainite is preferably 3 ⁇ m or less so as to obtain a more excellent crashworthiness.
• the total area fraction of the ferrite and the bainite is less than 2%, it is impossible to obtain sufficient formability and it is impossible to obtain sufficient collision performance even though the average dislocation density in the ferrite and the average dislocation density in the bainite are both 3 ⁇ 10 12 m/m 3 to 1 ⁇ 10 14 m/m 3 and the average grain diameter of the ferrite and the bainite is 5 ⁇ m or less.
• the total area fraction of the ferrite and the bainite is 2% or more.
• the total area fraction of the ferrite and the bainite is preferably 5% or more so as to obtain more excellent formability and collision performance.
• the ferrite includes polygonal ferrite ( ⁇ p), quasi-polygonal ferrite ( ⁇ q), and granular bainitic ferrite ( ⁇ B), and the bainite includes lower bainite, upper bainite, and bainitic ferrite ( ⁇ ° B).
• the granular bainitic ferrite has a recovered dislocation substructure containing no laths, and the bainitic ferrite has a structure having no precipitation of carbides and containing bundles of laths, and prior ⁇ grain boundaries remain as they are (see Reference: “Atlas for Bainitic Microstructures-1” The Iron and Steel Institute of Japan (1992) p. 4). This reference includes the description “Granular bainitic ferrite structure; dislocated substructure but fairly recovered like lath-less” and the description “sheaf-like with laths but no carbide; conserving the prior austenite grain boundary.”
• the ferrite and the bainite contribute also to the improvement in formability of the steel sheet.
• the total area fraction of the ferrite and the bainite is greater than 60%, it is sometimes impossible to obtain a sufficient crashworthiness.
• the total area fraction of the ferrite and the bainite is preferably 60% or less.
• the total area fraction of the ferrite and the bainite is further preferably 40% or less so as to obtain a more excellent crashworthiness.
• Martensite contributes to securing of the tensile strength.
• an area fraction of the martensite is less than 10%, it is impossible to obtain a sufficient tensile strength, for example, a tensile strength of 980 MPa or more, and the average dislocation density in the ferrite is brought to less than 3 ⁇ 10 12 m/m 3 in some cases.
• the area fraction of the martensite is preferably 10% or more.
• the area fraction of the martensite is further preferably 15% or more so as to obtain a more excellent tensile strength and crashworthiness.
• the area fraction of the martensite is greater than 90%, the average dislocation density in the ferrite, or the average dislocation density in the bainite, or the both of these are brought to greater than 1 ⁇ 10 14 m/m 3 and it becomes impossible to obtain sufficient ductility in some cases.
• the area fraction of the martensite is preferably 90% or less.
• the area fraction of the martensite is further preferably 85% or less so as to obtain more excellent collision performance and ductility.
• the martensite includes as-quenched martensite and tempered martensite, and 80 area % or more to the whole martensite is desired to be the tempered martensite.
• the ratio (f F /f M ) is preferably 0.03 or more.
• the ratio (f F /f M ) is further preferably 0.05 or more so as to obtain more excellent collision performance and ductility.
• the ratio (f F /f M ) is preferably 1.00 or less.
• the ratio (f F /f M ) is further preferably 0.80 or less so as to obtain more excellent collision performance.
• Retained austenite is effective for an improvement in formability and an improvement in impact energy absorption characteristic.
• the retained austenite contributes also to an increase in amount of strain age hardening to occur at the time of paint baking.
• an area fraction of the retained austenite is greater than 15%, the average dislocation density in the ferrite is brought to greater than 1 ⁇ 10 14 m/m 3 and the steel sheet becomes brittle after molding in some cases.
• the area fraction of the retained austenite is preferably 15% or less.
• the area fraction of the retained austenite is further preferably 12% or less so as to obtain a more excellent crashworthiness and toughness.
• the area fraction of the retained austenite is 2% or more, it is possible to expect an effect of the increase in amount of strain age hardening.
• pearlite As an example to be contained in the steel structure other than the ferrite, the bainite, the martensite, and the retained austenite, pearlite can be cited.
• An area fraction of the pearlite is preferably 2% or less.
• Area ratios of ferrite, bainite, martensite, and pearlite can be measured by a point counting method or an image analysis while using a steel structure photograph taken by an optical microscope or a scanning electron microscopy (SEM), for example. Distinction between the granular bainitic ferrite ( ⁇ B) and the bainitic ferrite ( ⁇ ° B) can be performed based on the descriptions of the reference after a structure is observed by a SEM and a transmission electron microscope (TEM).
• SEM scanning electron microscopy
• the area fraction of the retained austenite can be measured by an electron backscatter diffraction (EBSD) method or an X-ray diffractometry, for example.
• EBSD electron backscatter diffraction
• X-ray diffractometry it is possible to calculate the area fraction (f A ) of the retained austenite from the following expression after measuring a diffraction intensity ( ⁇ (111)) of the (111) plane of ferrite, a diffraction intensity ( ⁇ (200)) of the (200) plane of retained austenite, a diffraction intensity ( ⁇ (211)) of the (211) plane of ferrite, and a diffraction intensity ( ⁇ (311)) of the (311) plane of retained austenite by using a Mo-K ⁇ line.
• the steel sheet according to this embodiment preferably has a tensile strength of 980 MPa or more. This is because in the case of the tensile strength being less than 980 MPa, it is difficult to obtain an advantage of a reduction in weight achieved by the strength of a member being increased.
• the crashworthiness after molding and paint baking of the steel sheet can be evaluated by a parameter P 1 expressed by (Expression 1).
• “YS BH5 ” is a yield strength (MPa) after aging in the case of a 5%-tensile prestrain being applied
• “YS BH0 ” is a yield strength (MPa) after aging in the case of no tensile prestrain being applied
• “TS” is the maximum tensile strength (MPa).
• the temperature of the aging is 170° C., and its time period is two hours.
• the parameter P 1 is equivalent to the ratio of the difference between the yield strength YS BH5 after paint baking in a portion with the prestrain applied thereto and the yield strength YS BH0 after paint baking in a portion with no prestrain applied thereto to the maximum tensile strength TS.
• the smaller value of the parameter P 1 means that the difference in yield strength in a member obtained through molding and paint baking is small.
• the reason why the magnitude of the tensile prestrain is set to 5% is because it is considered that a molding strain of 5% or more is generally introduced into a bending portion and a drawing portion in the manufacture of an automobile frame member.
• the value of the parameter P 1 is preferably 0.27 or less.
• the value of the parameter P 1 is further preferably 0.18 or less so as to obtain more excellent collision performance.
• the formability of the steel sheet can be evaluated by a parameter P 2 expressed by (Expression 2).
• “uEl” is uniform elongation (%) obtained by a tensile test, and correlates with stretch formability (stretchability), stretch f Siemensability, and drawability.
• the value of the parameter P 2 is less than 7000, cracking often occurs by molding or collision, which is difficult to contribute to a reduction in weight of an automobile member. Therefore, the value of the parameter P 2 is preferably 7000 or more.
• the value of the parameter P 2 is further preferably 8000 or more so as to obtain more excellent formability.
• a slab having the above-described chemical composition is manufactured to be subjected to hot rolling.
• the slab to be subjected to hot rolling can be manufactured by a continuous casting method, a blooming method, a thin slab caster, or the like, for example.
• Such a process as continuous casting-direct rolling in which hot rolling is performed immediately after casting may be employed.
• the slab heating temperature is set to 1100° C. or more.
• rough rolling and finish rolling are performed.
• the condition of the rough rolling is not limited in particular, and the rough rolling can be performed by a conventional method, for example.
• a reduction ratio, a time period between passes, and a rolling temperature in the finish rolling are not limited in particular, but the finish rolling temperature is preferably set to an Ar 3 point or more.
• the condition of descaling is also not limited in particular, and the descaling can be performed by a conventional method, for example.
• the steel sheet After the finish rolling, the steel sheet is cooled to be coiled.
• a coiling temperature is greater than 680° C., it is impossible to bring the average grain diameter of the ferrite and the bainite to 5 ⁇ m or less, and the yield strength does not increase sufficiently even by the aging accompanying paint baking in some cases.
• the coiling temperature is set to 680° C. or less.
• the steel sheet is cooled to be subjected to pickling and cold rolling.
• Annealing may be performed between the pickling and the cold rolling.
• the temperature of this annealing is greater than 680° C., it is impossible to bring the average grain diameter of the ferrite and the bainite to 5 ⁇ m or less, and the yield strength does not increase sufficiently even by the aging accompanying paint baking in some cases.
• the temperature is set to 680° C. or less.
• a continuous annealing furnace or a batch annealing furnace can be used.
• the number of rolling passes of the cold rolling is not limited in particular, and is set to the same as that in a conventional method.
• a reduction ratio of the cold rolling is less than 30%, it is impossible to bring the average grain diameter of the ferrite and the bainite to 5 ⁇ m or less, and the yield strength does not increase sufficiently even by the aging accompanying paint baking in some cases.
• the reduction ratio of the cold rolling is set to 30% or more.
• the maximum temperature of this annealing is set to (Ac 3 ⁇ 60)° C. or more.
• the maximum temperature is preferably set to (Ac 3 ⁇ 40)° C. or more in order to obtain a more excellent crashworthiness.
• the maximum temperature is set to 900° C. or less.
• the maximum temperature is preferably set to 870° C. or less in order to obtain a more excellent crashworthiness.
• a holding time period at the maximum temperature is preferably set to three seconds to 200 seconds in order to bring the average grain diameter of the ferrite and the bainite to 5 ⁇ m or less.
• the holding time period is preferably set to 10 seconds or more and 180 seconds or less.
• an average cooling rate between 700° C. and 550° C. is set to 4° C./s to 50° C./s.
• this average cooling rate is less than 4° C./s, the average dislocation density in the bainite is brought to less than 3 ⁇ 10 12 m/m 3 .
• this average cooling rate is greater than 50° C./s, the average dislocation density in the bainite is brought to greater than 1 ⁇ 10 14 m/m 3 .
• this average cooling rate is set to 4° C./s to 50° C./s.
• temper rolling of the steel sheet is performed.
• the temper rolling is performed under the condition that a parameter P 3 expressed by (Expression 3) is 2 or more and an elongation ratio is 0.10% to 0.8%.
• “A” is a line load (N/m) and “B” is a tension (N/m 2 ) to be applied to the steel sheet.
• the parameter P 3 affects the uniformity of dislocation densities in the steel sheet.
• the parameter P 3 is set to 2 or more.
• the parameter P 3 is preferably set to 10 or more in order to obtain a more excellent crashworthiness.
• the elongation ratio of the temper rolling is set to 0.10% or more.
• the elongation ratio is preferably set to 0.20% or more in order to obtain a more excellent crashworthiness.
• the elongation ratio is set to 0.8% or less.
• the elongation ratio is preferably set to 0.6% or less in order to obtain more excellent formability.
• a plating treatment may be performed on the steel sheet.
• the plating treatment may be performed in a plating line provided in a continuous annealing line, or performed in a line exclusive to plating, which is different from the continuous annealing line, for example.
• the composition of plating is not limited in particular.
• a hot-dip plating treatment, an alloying hot-dip plating treatment, or an electroplating treatment can be performed.
• the average dislocation density in the ferrite and the average dislocation density in the bainite, and the like are appropriate, it is possible to obtain a stable yield strength after paint baking.
• Hot-rolled steel sheets obtained by the hot rolling were cooled to be coiled at 550° C. to 700° C. Then, pickling of the hot-rolled steel sheets was performed to remove scales. Thereafter, cold rolling was performed at a reduction ratio of 25% to 70%, to thereby obtain cold-rolled steel sheets each having a thickness of 1.2 mm. Between the pickling and the cold rolling, annealing at 550° C. was performed on some of the hot-rolled steel sheets.
• annealing was performed. In this annealing, its temperature was set to 780° C. to 900° C. and its time period was set to 60 seconds, and cooling at an average cooling rate of 20° C./s between 700° C. and 550° C. was performed. Then, temper rolling was performed under the condition of the elongation ratio being 0.3% and the parameter P 3 being 80.
• a hot-dip galvanizing treatment or an alloying hot-dip galvanizing treatment was performed during continuous annealing or after continuous annealing, and on another of the steel sheets, an electrogalvanizing treatment was performed after continuous annealing.
• Steel types corresponding to the plating treatments are illustrated in Table 2.
• Table 2 “GI” indicates a hot-dip galvanized steel sheet obtained after the hot-dip galvanizing treatment was performed, “GA” indicates an alloyed hot-dip galvanized steel sheet obtained after the alloying hot-dip galvanizingtreatment was performed, “EG” indicates an electrogalvanized steel sheet obtained after the electrogalvanizing treatment was performed, and “CR” indicates the cold-rolled steel sheet that was not subjected to a plating treatment.
• each 1 ⁇ 4 thickness portion of the steel sheets was analyzed by a point counting method or an image analysis using a structure photograph taken by a SEM or a TEM, or an X-ray diffractometry.
• a region surrounded by a grain boundary with an inclination of 15° or more was set as one crystal grain and the average of nominal grain diameters of 50 or more crystal grains of each of the ferrite and the bainite was set as an average grain diameter d.
• a total area fraction f F+B of the ferrite and the bainite, an area fraction f F of the ferrite, an area fraction f M of the martensite, an area fraction f A of the retained austenite, and a ratio (f F /f M ) of the area fractions are illustrated in Table 2.
• Each underline in Table 2 indicates that a corresponding numerical value is outside the range of the present invention.
• the average dislocation density was obtained from (Expression 4) using a TEM photograph.
• Each thin film sample for a TEM observation was taken from a portion at the 1 ⁇ 4 thickness from the surface of the steel sheet.
• As a thickness t of the thin film sample 0.1 ⁇ m was used simply.
• As for each of the ferrite and the bainite a TEM photograph was taken at five or more places of each of the thin film samples and an average value of dislocation densities obtained from these TEM photographs was set as an average dislocation density of the thin film sample.
• An average dislocation density ⁇ F in the ferrite and an average dislocation density ⁇ B in the bainite are also illustrated in Table 2. Each underline in Table 2 indicates that a corresponding numerical value is outside the range of the present invention.
• each of the samples was subjected to a tensile test in conformity with JIS Z 2241.
• this tensile test each tensile test piece in conformity with JIS Z 2201 with its sheet width direction (direction perpendicular to the rolling direction) set to a longitudinal direction was used.
• the maximum tensile strength TS, a yield strength YS, a uniform elongation uEl, a yield strength YS BH5 after aging in the case of a 5%-tensile prestrain being applied, and a yield strength YS BH0 after aging in the case of no tensile prestrain being applied were measured.
• the parameter P 1 regarding the yield strength expressed by (Expression 1) and the parameter P 2 regarding the formability expressed by (Expression 2) were calculated.
• Samples No. 1, No. 2, No. 10 to No. 13, No. 20 to No. 23, and No. 25 to No. 27 each being an invention example, exhibited an excellent crashworthiness and formability because of including the requirements of the present invention.
• Samples No. 1, No. 2, No. 12, No. 13, No. 21 to No. 23, No. 26, and No. 27 in which the total area fraction of the ferrite and the bainite, the area fraction of the martensite, the area fraction of the retained austenite, and the ratio of the area fraction of the ferrite to the area fraction of the martensite each are within a preferred range, the parameter P 2 was 8000 or more and the formability was particularly excellent.
• the present invention can be utilized for the industries relating to a steel sheet suitable for an automotive vehicle body, for example.

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