Classifications

• • 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
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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
  • 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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  • 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
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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
  • C21D8/0221—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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  • 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 plate capable of obtaining excellent collision characteristics suitable for automobile members.
• steel plates used in automobiles include dual-phase (DP) steel plates having a two-phase structure of ferrite and martensite, and transformation-induced plasticity (TRIP) steel plates.
• DP dual-phase
• TRIP transformation-induced plasticity
• the DP steel plate and the TRIP steel plate have a problem in that the mechanical properties after painting and baking may vary within the member. That is, in the forming of the steel sheet, strain is added according to the shape of the member to be obtained, and therefore the formed steel sheet includes a portion where the strain is strongly added and a portion where the strain is hardly added. And as the added strain becomes larger, the amount of strain age hardening by paint baking is larger and the hardness increases. As a result, there may be a large difference in the yield strength after baking the paint between the portion that is strained by molding and the portion that is hardly strained. In this case, a portion to which almost no strain is applied is soft, and bending occurs at this portion, so that sufficient reaction force characteristics and collision characteristics cannot be obtained.
• An object of the present invention is to provide a steel sheet that can obtain a stable yield strength after baking after painting while obtaining good formability.
• the present inventors have intensively studied to solve the above problems. As a result, it was found that when the dislocation density in ferrite and the dislocation density in bainite are high, the yield strength is improved by aging accompanying paint baking even in a portion where strain is hardly applied during molding. It has also been found that the yield strength is further improved by aging when the average grain sizes of ferrite and bainite are small.
• the inventor of the present application has come up with the following aspects of the invention as a result of further intensive studies based on such knowledge.
• the steel structure includes, in terms of area fraction, ferrite and bainite: 2% to 60% in total, and martensite: 10% to 90%,
• the area fraction of retained austenite in the steel structure is 15% or less,
• the average dislocation density in ferrite and the average dislocation density in bainite are appropriate, stable yield strength can be obtained even after baking.
• the chemical composition of the steel plate used for the embodiment of the present invention and the steel used for the manufacture will be described. Although details will be described later, the steel sheet according to the embodiment of the present invention is manufactured through hot rolling, cold rolling, annealing, temper rolling, and the like of the steel. Therefore, the chemical composition of the steel sheet and steel takes into account these treatments as well as the characteristics of the steel sheet.
• “%”, which is a unit of the content of each element contained in the steel sheet, means “mass%” unless otherwise specified.
• the steel sheet according to the present embodiment is, 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: 0.0. It has a chemical composition represented by 000% to 0.005%, Ce: 0.000% to 0.005%, La: 0.000% to 0.005%, and the balance: Fe and impurities. Examples of the impurities include those contained in raw materials such as ore and scrap and those contained in the manufacturing process.
• C (C: 0.05% to 0.40%) C contributes to an improvement in tensile strength.
• the C content is less than 0.05%, a sufficient tensile strength, for example, a tensile strength of 980 MPa or more cannot be obtained. Accordingly, the C content is 0.05% or more.
• the C content is preferably 0.08% or more.
• the C content is 0.40% or less. From the viewpoint of weldability, the C content is preferably 0.35% or less.
• Si 0.05% to 3.0%
• Si affects the formation of iron carbide and the age hardening associated therewith. If the Si content is less than 0.05%, a sufficient amount of solute C cannot be obtained, and the yield strength does not sufficiently increase due to aging accompanying paint baking. Accordingly, the Si content is 0.05% or more. In order to further increase the yield strength, the Si content is preferably 0.10% or more. On the other hand, when the Si content exceeds 3.0%, dislocations having a sufficient density cannot be obtained in the ferrite, and it is difficult to obtain a preferable steel structure. Therefore, the Si content is 3.0% or less. From the viewpoint of suppression of slab cracking and suppression of end cracks during hot rolling, the Si content is preferably 2.5% or less, more preferably 2.0% or less.
• Mn suppresses transformation from austenite to ferrite and contributes to improvement of tensile strength.
• Mn content is less than 1.5%, sufficient tensile strength, for example, tensile strength of 980 MPa or more cannot be obtained. Therefore, the Mn content is 1.5% or more.
• the Mn content is preferably 2.0% or more.
• the Mn content exceeds 4.0%, sufficient moldability cannot be obtained. Therefore, the Mn content is 4.0% or less. In order to obtain better moldability, the Mn content is preferably 3.5% or less.
• Al 1.5% or less
• Al is not an essential element, but can be used in deoxidation for reducing inclusions and remain in steel, for example. If the Al content exceeds 1.5%, ferrite or bainite having an average dislocation density in the range described later cannot be obtained. Therefore, the Al content is 1.5% or less. Reduction of the Al content is costly, and if it is attempted to reduce it to less than 0.002%, the cost increases remarkably. For this reason, the Al content may be 0.002% or more. If sufficient deoxidation is performed, 0.01% or more of Al may remain.
• N (N: 0.02% or less) N is not an essential element but is contained as an impurity in steel, for example. If the N content exceeds 0.02%, a large amount of nitride precipitates and sufficient moldability cannot be obtained. Accordingly, the N content is 0.02% or less. Reduction of the N content is costly, and if it is attempted to reduce it to less than 0.001%, the cost increases remarkably. For this reason, N content is good also as 0.001% or more.
• P 0.2% or less
• P is not an essential element but is contained as an impurity in steel, for example.
• the P content exceeds 0.2%, a large amount of the P compound is precipitated and sufficient moldability cannot be obtained. Therefore, the P content is 0.2% or less.
• the P content is preferably 0.07% or less. Reduction of the P content is costly, and if it is attempted to reduce it to less than 0.001%, the cost increases remarkably. For this reason, the P content may be 0.001% or more.
• S (S: 0.01% or less) S is not an essential element but is contained as an impurity in steel, for example. If the S content exceeds 0.01%, a large amount of sulfide precipitates and sufficient moldability cannot be obtained. Accordingly, the S content is 0.01% or less. In order to further suppress a decrease in moldability, the S content is preferably 0.003% or less. Reduction of the S content takes a cost, and if it is attempted to reduce it to less than 0.0002%, the cost increases remarkably. For this reason, S content is good also as 0.0002% or more.
• Nb and Ti contribute to refinement and precipitation strengthening of ferrite or bainite crystal grains. Since Nb and Ti form (Ti, Nb) carbonitride, the amount of solute C and the amount of solute N after annealing change depending on the contents of Nb and Ti. If the total content of Nb and Ti is less than 0.005%, ferrite or bainite having an average particle size in the range described later cannot be obtained, and the yield strength is not sufficiently increased even by aging accompanying paint baking. Therefore, the total content of Nb and Ti is 0.005% or more. In order to sufficiently increase the yield strength by aging, the contents of Nb and Ti are preferably 0.010% or more in total.
• Nb and Ti exceeds 0.2% in total, (Ti, Nb) carbonitride precipitates in large amounts and sufficient formability cannot be obtained. Therefore, 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 essential elements, but are arbitrary elements that may be appropriately contained in steel plates and steels up to a predetermined amount.
• V and Ta contribute to the improvement of strength by forming carbides, nitrides, or carbonitrides and by making ferrite and bainite finer. Therefore, V or Ta or both of them may be contained. However, if the total content of V and Ta exceeds 0.3%, a large amount of carbonitride precipitates and ductility decreases. Therefore, the total content of V and Ta is 0.3% or less. From the viewpoint of suppressing cracks in the slab and end cracks during hot rolling, the V and Ta contents are preferably 0.1% or less in total. In order to surely obtain the effect by the above action, the contents of V and Ta are preferably 0.01% or more in total.
• Cr, Mo, Ni, Cu and Sn 0.0% to 1.0% in total
• Cr, Mo, Ni, Cu, and Sn are used to suppress transformation from austenite to ferrite, like Mn. Therefore, Cr, Mo, Ni, Cu or Sn or any combination thereof may be contained.
• the total content of Cr, Mo, Ni, Cu and Sn exceeds 1.0%, the workability is remarkably deteriorated and the elongation is reduced. Therefore, the total content of Cr, Mo, Ni, Cu and Sn is 1.0% or less.
• the contents of Cr, Mo, Ni, Cu and Sn are preferably 0.5% or less in total. In order to surely obtain the effect by the above action, the content of Cr, Mo, Ni, Cu and Sn is preferably 0.1% or more.
• B (B: 0.00% to 0.01%) B enhances the hardenability of the steel sheet, suppresses the formation of ferrite, and promotes the formation of martensite. Therefore, B may be contained. However, when the B content exceeds 0.01% in total, a large amount of boride precipitates and sufficient moldability cannot be obtained. Therefore, the B content is 0.01% or less. In order to further suppress the decrease in ductility, the B content is preferably 0.003% or less in total. In order to surely obtain the effect by the above action, the B content is preferably 0.0003% or more.
• Ca, Ce, and La suppress the deterioration of workability, particularly elongation, by making the oxides and sulfides in the steel sheet fine or changing the characteristics of the oxides and sulfides. Therefore, Ca, Ce or La or any combination thereof may be contained. However, if any of the Ca content, Ce content, and La content exceeds 0.005%, the effect of the above action is saturated, the cost is increased, and the moldability is lowered. Therefore, the Ca content, Ce content, and La content are all 0.005% or less.
• the Ca content, Ce content, and La content are each preferably 0.003% or less.
• the Ca content, Ce content, and La content are preferably 0.001% or more. That is, “Ca: 0.001% to 0.005%”, “Ce: 0.001% to 0.005%”, “La: 0.001% to 0.005%”, or any combination thereof. Preferably it is satisfied.
• the steel structure of the steel sheet according to the embodiment of the present invention includes ferrite and bainite in an area fraction of 2% or more in total.
• the average dislocation density in ferrite and the average dislocation density in bainite are both 3 ⁇ 10 12 m / m 3 to 1 ⁇ 10 14 m / m 3 , and the average particle diameter of ferrite and bainite is 5 ⁇ m or less.
• the present inventors have improved the yield strength by aging accompanying paint baking even in places where distortion is hardly applied during molding. It became clear to do.
• the average dislocation density in ferrite, the average dislocation density in bainite, or both of these are less than 3 ⁇ 10 12 m / m 3 , the yield strength of the portion to which almost no strain is added during forming cannot be sufficiently improved by aging. A sufficient impact characteristic cannot be obtained. Therefore, the average dislocation density in ferrite and the average dislocation density in bainite are both 3 ⁇ 10 12 m / m 3 or more.
• the average dislocation density in ferrite and the average dislocation density in bainite are both preferably 6 ⁇ 10 12 m / m 3 or more. If the average dislocation density in ferrite, the average dislocation density in bainite, or both of these exceeds 1 ⁇ 10 14 m / m 3 , the yield strength of the portion where the strain is hardly added or the strain is hardly added at the time of forming is low. It does not improve sufficiently due to aging, and sufficient impact characteristics cannot be obtained. Therefore, the average dislocation density in ferrite and the average dislocation density in bainite are both 1 ⁇ 10 14 m / m 3 or less. In order to obtain better formability and impact characteristics, the average dislocation density in ferrite and the average dislocation density in bainite are both preferably 8 ⁇ 10 13 m / m 3 or less.
• the average dislocation density in ferrite and the average dislocation density in bainite can be obtained, for example, using a transmission electron microscope (TEM) photograph. That is, when a TEM photograph of a thin film sample is prepared and a line is arbitrarily drawn on the TEM photograph to obtain an average dislocation density in the ferrite, the number of places where the line intersects with the dislocation line in the ferrite is counted. . Then, assuming that the length of the wire in the ferrite is L, the number of points where the line and the dislocation line intersect in the ferrite is N, and the thickness of the sample is t, the dislocation density in the ferrite in the thin film sample is “2N / (Lt) ".
• TEM transmission electron microscope
• an average value of dislocation densities obtained from the plurality of TEM photographs is obtained as an average dislocation density in the ferrite.
• An actual measurement value may be used as the thickness t of the sample, or 0.1 ⁇ m may be simply used.
• the average dislocation density in bainite can be obtained by a method similar to the method of obtaining the average dislocation density in ferrite by counting the number of intersecting points in bainite and using the length of the line in bainite.
• the present inventors have clarified that the yield strength is further improved by aging when the ferrite and bainite grain sizes are small. If the average grain size of ferrite and bainite exceeds 5 ⁇ m, the yield strength of the portion where strain is hardly added during molding is not sufficiently improved by aging, and sufficient impact characteristics cannot be obtained. Therefore, the average particle size of ferrite and bainite is 5 ⁇ m or more. In order to obtain better collision characteristics, the average particle diameter of ferrite and bainite is preferably 3 ⁇ m or less.
• the average dislocation density in ferrite and the average dislocation density in bainite are both 3 ⁇ 10 12 m / m 3 to 1 ⁇ 10 14 m / m 3 , and the average grain size of ferrite and bainite is 5 ⁇ m or less.
• the area fractions of ferrite and bainite are less than 2% in total, sufficient formability cannot be obtained or sufficient impact performance cannot be obtained. Therefore, the area fraction of ferrite and bainite is 2% or more in total. In order to obtain better formability and impact performance, the area fractions of ferrite and bainite are preferably 5% or more in total.
• the ferrite includes polygonal ferrite ( ⁇ p), pseudopolygonal ferrite ( ⁇ q), and granular bainitic ferrite ( ⁇ B), and bainite includes lower bainite, upper bainite, and bainitic ferrite ( ⁇ ° B) is included.
• Granular bainitic ferrite has a recovered dislocation substructure without lath
• bainitic ferrite has a structure in which lath without carbide precipitation is bundled, and the old ⁇ grain boundary remains as it is (references) : “Steel Bainite Photobook-1”, Japan Iron and Steel Institute (1992), p.4).
• This reference includes the description “Granular bainitic ferrite structure; dislocated substructure but fairly recovered like lath-less” and “sheaf-like with laths but no carbide; conserving the prior austenite grain boundary”.
• Ferrite and bainite also contribute to improving the formability of the steel sheet. However, if the total area fraction of ferrite and bainite exceeds 60%, sufficient impact characteristics may not be obtained. Therefore, the area fractions of ferrite and bainite are preferably 60% or less in total. In order to obtain better collision characteristics, the area fractions of ferrite and bainite are more preferably 40% or less in total.
• Martensite contributes to securing tensile strength.
• the area fraction of martensite is less than 10%, sufficient tensile strength, for example, tensile strength of 980 MPa or more cannot be obtained, or the average dislocation density in ferrite is less than 3 ⁇ 10 12 m / m 3.
• the area fraction of martensite is preferably 10% or more.
• the martensite area fraction is more preferably 15% or more.
• the area fraction of martensite exceeds 90%, the average dislocation density in ferrite, the average dislocation density in bainite, or both of them exceeds 1 ⁇ 10 14 m / m 3 , or sufficient ductility is obtained.
• the area fraction of martensite is preferably 90% or less. In order to obtain better collision performance and ductility, the martensite area fraction is more preferably 85% or less.
• the martensite includes martensite and tempered martensite as quenched, and 80% by area or more of the entire martensite is preferably tempered martensite.
• the ratio (f F / f M ) of the ferrite area fraction f F to the martensite area fraction f M is less than 0.03, the average dislocation density in the ferrite is more than 1 ⁇ 10 14 m / m 3. Or sufficient ductility may not be obtained. Therefore, the ratio (f F / f M ) is preferably 0.03 or more. In order to obtain better collision performance and ductility, the ratio (f F / f M ) is more preferably 0.05 or more. On the other hand, when the ratio (f F / f M ) exceeds 1.00, the average dislocation density in the ferrite may be less than 3 ⁇ 10 12 m / m 3 . Therefore, the ratio (f F / f M ) is preferably 1.00 or less. In order to obtain better collision performance, the ratio (f F / f M ) is more preferably 0.80 or less.
• Residual austenite is effective in improving moldability and impact energy absorption characteristics. Residual austenite also contributes to an improvement in the strain age hardening during baking of the paint. However, if the area fraction of retained austenite exceeds 15%, the average dislocation density in the ferrite may exceed 1 ⁇ 10 14 m / m 3 , or the steel sheet may become brittle after forming. Accordingly, the area fraction of retained austenite is preferably 15% or less. In order to obtain better impact properties and toughness, the area fraction of retained austenite is more preferably 12% or less. When the area fraction of retained austenite is 2% or more, an effect of improving the strain age hardening can be expected.
• Perlite is an example of what is contained in the steel structure other than ferrite, bainite, martensite and retained austenite.
• the area fraction of pearlite is preferably 2% or less.
• the area ratio of ferrite, bainite, martensite, and pearlite can be measured by a point count method or image analysis using, for example, a photograph of a steel structure taken with an optical microscope or a scanning electron microscope (SEM). Discrimination between granular bainitic ferrite ( ⁇ B) and bainitic ferrite ( ⁇ ° B) can be performed based on the description of the reference by observing the structure with an SEM and a transmission electron microscope (TEM).
• SEM scanning electron microscope
• the area fraction of residual austenite can be measured by, for example, an electron backscatter diffraction (EBSD) method or an X-ray diffraction method.
• EBSD electron backscatter diffraction
• X-ray diffraction method the diffraction intensity of the ferrite (111) plane ( ⁇ (111)) and the diffraction intensity of the retained austenite (200) plane ( ⁇ (200)) using Mo—K ⁇ rays.
• the diffraction intensity ( ⁇ (211)) of the (211) plane of ferrite and the diffraction intensity ( ⁇ (311)) of the (311) plane of retained austenite are measured, and the area fraction of residual austenite (f A ) can be calculated.
• f A (2/3) ⁇ 100 / (0.7 ⁇ ⁇ (111) / ⁇ (200) +1) ⁇ + (1/3) ⁇ 100 / (0.78 ⁇ ⁇ (211) / ⁇ (311) +1) ⁇
• the steel plate according to the present embodiment preferably has a tensile strength of 980 MPa or more. This is because if the tensile strength is less than 980 MPa, it is difficult to obtain the advantage of weight reduction by increasing the strength of the member.
• the collision characteristics after forming and baking the steel sheet can be evaluated by the parameter P 1 represented by (Equation 1).
• "YS BH5” is the yield strength (MPa) after aging when 5% tensile pre-strain is added
• "YS BH0” is the yield strength (MPa) after aging when no tensile pre-strain is added.
• TS is the maximum tensile strength (MPa).
• the aging temperature is 170 ° C. and the time is 2 hours.
• the parameter P 1 is the ratio of the difference between the yield strength YS BH5 after paint baking of the portion with pre-strain applied and the yield strength YS BH0 after paint baking of the portion without pre-strain with respect to the maximum tensile strength TS.
• the value of the parameter P 1 is preferably 0.27 or less. In order to obtain better collision performance, the value of the parameter P 1 is more preferably 0.18 or less.
• Formability of the steel sheet can be evaluated by the parameter P 2 represented by formula (2).
• “UEl” is a uniform elongation (%) obtained by a tensile test, and correlates with stretch formability, stretch flange formability and draw formability. Is less than the value of the parameter P 2 is 7000, often cracking caused by molding or collision, hardly contribute to the weight reduction of automobile parts. Therefore, the value of the parameter P 2 is preferably 7000 or more. In order to obtain more excellent formability, the value of the parameter P 2 is still more preferably 8000 or more.
• P 2 TS ⁇ uEl (Formula 2)
• the average dislocation density in ferrite and the average dislocation density in bainite can be controlled by adjusting the elongation of temper rolling and the line load / tension ratio in temper rolling. Therefore, in this production method, hot rolling, cold rolling, annealing, temper rolling and the like of the steel having the above chemical composition are performed.
• a slab having the above chemical composition is manufactured and hot-rolled.
• the slab to be subjected to hot rolling can be produced by, for example, a continuous casting method, a block method, or a thin slab caster.
• a process such 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 higher.
• rough rolling and finish rolling are performed.
• the conditions for rough rolling are not particularly limited, and can be performed by, for example, a conventional method.
• the rolling reduction, time between passes, and rolling temperature in finish rolling are not particularly limited, but the finish rolling temperature is preferably Ar 3 points or more.
• the descaling conditions are not particularly limited, and can be performed by, for example, a conventional method.
• the steel sheet After finish rolling, the steel sheet is cooled and wound.
• the coiling temperature is higher than 680 ° C., the average grain size of ferrite and bainite cannot be made 5 ⁇ m or less, and the yield strength may not be sufficiently increased by aging accompanying paint baking. Accordingly, the winding temperature is set to 680 ° C. or lower.
• the steel sheet After winding, the steel sheet is cooled, and pickling and cold rolling are performed. Annealing may be performed between pickling and cold rolling. If the annealing temperature exceeds 680 ° C., the average particle size of ferrite and bainite cannot be made 5 ⁇ m or less, and the yield strength may not be sufficiently increased due to aging accompanying paint baking. Therefore, when performing annealing between pickling and cold rolling, the temperature shall be 680 degrees C or less. For this annealing, for example, a continuous annealing furnace or a batch annealing furnace can be used.
• the number of cold rolling rolling passes is not particularly limited, and is the same as that in the ordinary method. If the rolling reduction of cold rolling is less than 30%, the average particle size of ferrite and bainite cannot be made 5 ⁇ m or less, and the yield strength may not be sufficiently increased due to aging accompanying paint baking. Therefore, the rolling reduction of cold rolling is 30% or more.
• Annealing is performed after cold rolling. Is less than the maximum temperature reached (Ac 3 -60) °C of this annealing, an insufficient amount of solute C and N, is not sufficiently increased yield strength by aging due to baking, also a preferred steel structure Hard to get. Accordingly, the maximum temperature reached is (Ac 3 -60) ° C. or higher. In order to obtain better impact characteristics, the maximum temperature reached is preferably (Ac 3 -40) ° C. or higher. On the other hand, if the maximum temperature reached is over 900 ° C., the average grain size of ferrite and bainite cannot be made 5 ⁇ m or less, and the yield strength may not be sufficiently increased due to aging accompanying paint baking. Therefore, the highest temperature reached 900 ° C.
• the maximum temperature reached is preferably 870 ° C. or lower.
• the holding time at the highest temperature is preferably 3 seconds to 200 seconds. In particular, the holding time is preferably 10 seconds or more, and preferably 180 seconds or less.
• the average cooling rate between 700 ° C. and 550 ° C. is 4 ° C./s to 50 ° C./s.
• the average dislocation density in bainite is less than 3 ⁇ 10 12 m / m 3 .
• the average cooling rate exceeds 50 ° C./s, the average dislocation density in bainite exceeds 1 ⁇ 10 14 m / m 3 . Therefore, the average cooling rate is 4 ° C./s to 50 ° C./s.
• Temper rolling (Equation 3) represented by the parameter P 3 is 2 or more, elongation carried out at 0.10% 0.8% conditions.
• A is a line load (N / m)
• B is a tension (N / m 2 ) applied to the steel sheet.
• P 3 B / A (Formula 3)
• Parameter P 3 affects the uniformity of the dislocation density in the steel sheet.
• the parameter P 3 is less than 2, not introduced sufficient dislocations ferrite thickness center portion of the steel sheet, the yield strength by aging due to baking may not be sufficiently increased. Accordingly, the parameter P 3 is two or more. In order to obtain more excellent crashworthiness, parameter P 3 is preferably 10 or more.
• the elongation of the temper rolling is less than 0.10%, sufficient dislocations are not introduced into the ferrite, and the yield strength may not be sufficiently increased due to aging associated with paint baking. Therefore, the elongation is set to 0.10% or more. In order to obtain more excellent collision characteristics, the elongation is preferably 0.20% or more. On the other hand, if the elongation exceeds 0.8%, sufficient moldability may not be obtained. Therefore, the elongation is set to 0.8% or less. In order to obtain better moldability, the elongation is preferably 0.6% or less.
• the steel sheet according to the embodiment of the present invention can be manufactured.
• the steel sheet may be plated between the annealing after cold rolling and the temper rolling.
• the plating treatment may be performed by a plating facility provided in a continuous annealing facility, or may be performed by a dedicated plating facility different from the continuous annealing facility.
• the composition of the plating is not particularly limited.
• As the plating process for example, a hot dipping process, an alloying hot dipping process, or an electroplating process can be performed.
• the average dislocation density in ferrite, the average dislocation density in bainite, and the like are appropriate, stable yield strength can be obtained after paint baking.
• the hot-rolled steel sheet obtained by hot rolling was cooled and wound at 550 ° C to 700 ° C. Next, the hot rolled steel sheet was pickled to remove the scale. Thereafter, cold rolling was performed at a rolling reduction of 25% to 70% to obtain a cold rolled steel sheet having a thickness of 1.2 mm. Some hot-rolled steel sheets were annealed at 550 ° C. between pickling and cold rolling.
• Annealing was performed after cold rolling.
• the temperature was 780 ° C. to 900 ° C.
• the time was 60 seconds
• cooling was performed so that the average cooling rate between 700 ° C. and 550 ° C. was 20 ° C./s.
• 0.3% elongation rate were temper rolling under the condition of the parameter P 3 is 80.
• Table 2 shows the steel types corresponding to the plating treatment.
• “GI” in Table 2 indicates a hot dip galvanized steel sheet that has been subjected to hot dip galvanizing treatment
• GA indicates an galvannealed steel sheet that has been subjected to alloy hot dip galvanizing treatment
• “EG” An electrogalvanized steel sheet that has been electrogalvanized is shown
• “CR” is a cold-rolled steel sheet that has not been plated.
• the area fraction of ferrite, bainite, martensite and retained austenite and the average particle diameter of ferrite and bainite were measured.
• a quarter thickness portion of the steel sheet was analyzed by a point count method using a photograph of a structure taken by SEM or TEM, an analysis by image analysis, or an analysis by an X-ray diffraction method.
• a region surrounded by a grain boundary having an inclination of 15 ° or more was defined as one crystal grain, and an average nominal grain size of 50 or more crystal grains was defined as an average grain size d.
• the average dislocation density was determined from (Equation 4) using a TEM photograph.
• a thin film sample for TEM observation was collected from a 1/4 thickness portion from the surface of the steel plate.
• As the thickness t of the thin film sample 0.1 ⁇ m was simply used.
• TEM photographs were taken at five or more locations for each thin film sample, and the average dislocation density obtained from these TEM photographs was taken as the average dislocation density in the thin film sample.
• Table 2 also shows the average dislocation density ⁇ F in ferrite and the average dislocation density ⁇ B in bainite. The underline in Table 2 indicates that the numerical value is out of the scope of the present invention.
• ⁇ 2N / (Lt) (Formula 4)
• sample No. 1 no. 2, No. 10-No. 13, no. 20-No. 23, no. 25-No. Since No. 27 had the requirements of the present invention, it exhibited excellent collision characteristics and moldability.
• Sample No. In No. 28 since the C content was too small, sufficient tensile strength could not be obtained.
• Sample No. In 32 since the Mn content was too small, sufficient tensile strength could not be obtained.
• Sample No. In No. 33 since the Mn content was excessive, the average dislocation density ⁇ F and the average dislocation density ⁇ B were excessive, and sufficient formability was not obtained.
• sample No. 1 was subjected to temper rolling in a preferred range. 43-No. 46, no. In 50, the steel plate which satisfy
• the present invention can be used, for example, in industries related to steel sheets suitable for automobile bodies.

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