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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • 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
  • C22C38/002—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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/12—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with 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
  • C22C38/40—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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • 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
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
• • 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
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/08—Iron or steel
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/22—Electroplating: Baths therefor from solutions of zinc
• • 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
  • C21D2211/001—Austenite
• • 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
• • 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
  • C25D5/48—After-treatment of electroplated surfaces
  • C25D5/50—After-treatment of electroplated surfaces by heat-treatment

Definitions

• the present invention relates to steel plates, members, and methods of manufacturing them. More specifically, the present invention relates to steel plates and members having a tensile strength (TS) of 980 MPa or more and excellent formability and material stability, and methods for manufacturing them.
• the steel plate of the present invention is suitable as a material for automobile frame members.
• TRIP steel sheets have been developed in which retained austenite is dispersed in the structure.
• steel containing C: 0.04 to 0.12%, Si: 0.8 to 2.5%, and Mn: 0.5 to 2.0% is annealed at 300 to 500°C.
• Austempering carbon distribution accompanying bainite transformation held for 10 to 900 seconds generates 2 to 10% residual ⁇ , resulting in high ductility of TS ⁇ El ⁇ 21000MPa ⁇ % and high stretch flange formability of 70% or more. It is disclosed that a steel plate having the following properties can be obtained.
• Patent Document 2 during the cooling process, the temperature is once cooled to a temperature range between the martensitic transformation start temperature (Ms point) and the martensitic transformation completion temperature (Mf point), and then, the residual austenite is stabilized by reheating and holding.
• a technique has been disclosed for increasing the ductility of a steel sheet by utilizing the principle of so-called Q&P; Quenching & Partitioning (quenching and distribution of carbon from martensite to austenite).
• a cold rolled steel sheet having a predetermined chemical composition is held at a first soaking temperature of 750°C or higher, then cooled to a cooling stop temperature in a temperature range of 150 to 350°C, and then heated to a temperature range of 350 to 500°C.
• a retained austenite volume fraction of 5% to 15% is achieved, which achieves both TS of 980 MPa or more and ductility of 17% or more, and an excellent hole expansion rate of 50% or more.
• the present invention discloses a steel plate having good hole expandability and a method for manufacturing the same.
• DP steel (Dual Phase steel) has been developed as a steel plate that has a low yield ratio that is effective in reducing springback.
• General DP steel is a multi-phase steel in which martensite is dispersed in the ferrite structure as the main phase, and has a high TS, low yield ratio, and excellent ductility.
• DP steel has the disadvantage of poor stretch flange formability because cracks are likely to occur due to stress concentration at the interface between ferrite and martensite. Examples of techniques for improving the stretch flange formability of DP steel include Patent Document 3 and Patent Document 4.
• the space factor of ferrite is controlled to be 50% or more and the space factor of martensite is controlled to 3 to 30% with respect to the entire structure, and the average crystal grain size of ferrite is controlled to be 10 ⁇ m or less, and the average crystal grain of martensite is controlled to be 10 ⁇ m or less.
• a technique is disclosed in which deterioration of stretch flange formability is suppressed by setting the diameter to 5 ⁇ m or less.
• the space factor of ferrite is controlled to 5 to 30% and the space factor of martensite to the entire structure is controlled to 50 to 95%, and fine ferrite particles with an average grain size of 3 ⁇ m or less in equivalent circle diameter are formed. It is disclosed that ductility and stretch flange formability can be improved by controlling the average grain size to martensite having a circular equivalent diameter of 6 ⁇ m or less.
• Patent Document 1 and Patent Document 4 mentioned above disclose a method for manufacturing a steel plate with excellent ductility and stretch-flange formability, it is necessary to form a large amount of soft phase ferrite, so for example, a high temperature of 780 MPa or more is required. Strengthening is difficult.
• Patent Document 2 has excellent ductility and stretch flange formability, since the steel plate has a YR of 0.8 or more, dimensional accuracy may be impaired due to springback during press forming.
• Patent Document 3 discloses a method for manufacturing a DP steel sheet that has low YR and excellent stretch flange formability, but since it has a DP structure, ductility is not necessarily sufficient.
• the present invention provides a steel plate and member having a tensile strength (TS) of 980 MPa or more, excellent press formability, ductility and stretch flange formability, and excellent material stability in the width direction.
• TS tensile strength
• the tensile strength refers to tensile strength (TS) obtained in accordance with JIS Z2241 (2011).
• TS tensile strength
• Excellent press formability means that the yield ratio YR obtained according to JIS Z2241 (2011) is 0.8 or less.
• Excellent ductility means that the total elongation EL obtained according to JIS Z2241 (2011) satisfies either (A) or (B) below.
• the positions in the board width direction are W/24, 2W/24, 3W/24, 4W/24, 5W/24, 6W/24, 7W/24, 8W/24.
• measurement positions X are defined as measurement positions X.
• the present inventors investigated various factors affecting press formability, ductility, stretch-flange formability, and material stability for various thin steel sheets having a tensile strength of 980 MPa or more, and investigated the chemical composition of the steel sheets.
• mass % C: 0.05 to 0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol.
• the area ratio of polygonal ferrite is 10% to 57%, and the total area of upper bainite, tempered martensite, and lower bainite
• the aspect ratio is 40% or more and 80% or less, the area ratio of retained austenite (residual ⁇ ) is 3% or more and 15% or less, and the area ratio of quenched martensite is 12% or less (including 0%).
• a C-enriched region (S C ⁇ 0 By making the steel structure have an area ratio of 15% or less to the entire structure of .5 ), it has excellent press formability, ductility and stretch flange formability, and also has excellent material stability in the sheet width direction ( It was discovered that high-strength cold-rolled steel sheets with small material variations can be obtained.
• the present invention has been made based on the above findings, and the gist thereof is as follows. [1] In mass%, C: 0.05-0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol.
• the component composition further includes, in mass%, Ti: 0.1% or less, B: 0.01% or less, The steel plate according to [1], containing one or two selected from among the above.
• the component composition further includes, in mass%, Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, Nb: 0.1% or less, The steel plate according to [1] or [2], containing one or more selected from the following.
• the component composition further includes, in mass%, Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, REM: 0.0050% or less,
• [6] A member using the steel plate according to any one of [1] to [5].
• the obtained cold rolled steel plate is A method of manufacturing a steel plate that undergoes annealing,
• the annealing is A holding step of heating the cold rolled steel plate to an annealing temperature of 750 to 880°C and holding at the annealing temperature for 10 to 500 seconds;
• a method for manufacturing steel plates comprising the step of subjecting the steel plate according to any one of [1] to [5] to at least one of forming and bonding to produce a member.
• a steel plate can be obtained that has a high tensile strength TS of 980 MPa or more, has excellent press formability, ductility, and stretch flange formability, and has excellent material stability in the width direction.
• the steel sheet of the present invention can be applied to, for example, automobile structural members with complex shapes, thereby reducing the weight of the automobile body, and also reducing environmental load by improving yield during manufacturing.
• the steel plate of the present invention contains, by mass%, C: 0.05 to 0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: 0.005 to 0.50%, N: less than 0.015%, with the balance being iron and unavoidable impurities.
• the steel plate of the present invention has a composition comprising: C: 0.05 to 0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol.
• the steel has a volume fraction of austenite: 3% or more and 15% or less, an area fraction of quenched martensite: 12% or less (including 0%), and a steel structure consisting of a remaining structure, in which the total area fraction of the quenched martensite and retained austenite having an aspect ratio of 3 or less and a circle equivalent diameter of 1.6 ⁇ m or more is 20% or less relative to the total area fraction of the quenched martensite and retained austenite, and the area fraction of C-enriched regions (S C ⁇ 0.5 ) having a C concentration of 0.5 mass% or more relative to the entire structure is 15% or less.
• C is contained from the viewpoint of securing a predetermined strength through transformation strengthening, and from the viewpoint of securing a predetermined amount of retained austenite (residual ⁇ ) to improve ductility. If the C content is less than 0.05%, these effects cannot be sufficiently ensured. On the other hand, when the C content exceeds 0.20%, the martensitic transformation start temperature (Ms point) decreases. As a result, in the third cooling process in which cooling is performed in the temperature range from the reheating temperature to 50°C at a third average cooling rate: 0.05 to 1.0°C/s, martensitic transformation and subsequent tempering of martensite occur. is not done enough.
• the C content is set to 0.05% or more and 0.20% or less.
• the C content is preferably 0.08% or more. Further, the C content is preferably 0.18% or less.
• Si is contained from the viewpoint of strengthening the ferrite and increasing its strength, and from the viewpoint of suppressing the formation of carbides in martensite and bainite to ensure a predetermined amount of residual ⁇ and improving ductility. If the Si content is less than 0.40%, these effects cannot be sufficiently ensured. On the other hand, when the Si content exceeds 1.50%, carbon distribution to untransformed austenite is excessively promoted, and the formation of a C-enriched region of 0.5 mass% or more ( SC ⁇ 0.5 ) is promoted. , stretch flange formability and material stability in the plate width direction decrease. Therefore, the Si content is set to 0.40% or more and 1.50% or less. The Si content is preferably 0.60% or more. Further, the Si content is preferably 1.20% or less.
• Mn improves the hardenability of steel sheets, suppresses excessive transformation of ferrite, and promotes high strength through transformation strengthening, and similarly to Si, suppresses the formation of carbides in bainite and contributes to ductility. It is included from the viewpoint of further promoting the formation of retained austenite and further improving ductility. In order to obtain these effects, the Mn content needs to be 1.9% or more. On the other hand, when the Mn content exceeds 3.5%, bainite transformation is delayed, a predetermined amount of retained austenite cannot be secured, and ductility decreases.
• the Mn content is set to 1.9% or more and 3.5% or less.
• the Mn content is preferably 2.1% or more. Further, the Mn content is preferably 3.3% or less, more preferably 3.0% or less.
• P is an element that strengthens steel, but if its content is large, it deteriorates spot weldability. Therefore, the P content is 0.02% or less, preferably 0.01% or less. Note that although it is not necessary to contain P, it is preferable that the P content is 0.001% or more because reducing it to less than 0.001% requires a great deal of cost. The P content is more preferably 0.002% or more, and still more preferably 0.005% or more.
• S has the effect of improving scale peelability during hot rolling and suppressing nitridation during annealing, but is an element that has an adverse effect on spot weldability, bendability, and hole expandability.
• the S content is at least 0.01% or less, preferably 0.0020% or less.
• the S content is preferably 0.0001% or more from the viewpoint of manufacturing costs.
• the S content is more preferably 0.0005% or more, and still more preferably 0.0015% or more.
• sol. Al 0.005-0.50%> Al is contained for the purpose of deoxidizing or obtaining residual ⁇ .
• Al content shall be 0.005% or more.
• N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces stretch flange formability, so it is necessary to limit its content. Therefore, the N content should be less than 0.015%. Note that although it is not necessary to contain N, reducing the N content to less than 0.0001% requires a great deal of cost, so the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The N content is more preferably 0.0005% or more, and even more preferably 0.0015% or more.
• the component composition of the steel sheet in the present invention contains the above-mentioned component elements as basic components, and the remainder includes iron (Fe) and inevitable impurities.
• the component composition of the steel plate in the present invention has a component composition in which the balance consists of Fe and unavoidable impurities.
• the composition of the steel sheet of the present invention can appropriately contain one or more optional elements selected from the following (A) to (C).
• Ti fixes N in steel as TiN, and has the effect of improving hot ductility and the effect of B on improving hardenability. Further, the precipitation of TiC has the effect of making the structure finer. In order to obtain these effects, it is desirable that the Ti content be 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. The Ti content is more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.1%, the rolling load will increase and the ductility will decrease due to an increase in the amount of precipitation strengthening, so if Ti is contained, the Ti content should be 0.1% or less. Preferably, the Ti content is 0.05% or less, more preferably 0.03% or less.
• B is an element that improves the hardenability of steel, and has the advantage of easily producing tempered martensite and/or bainite with a predetermined area ratio. Therefore, it is preferable that the B content is 0.0005% or more. Further, the B content is more preferably 0.0010% or more. On the other hand, when the B content exceeds 0.01%, the effect not only becomes saturated, but also causes a significant decrease in hot ductility and causes surface defects. Therefore, when B is contained, the B content is set to 0.01% or less. Preferably, the B content is 0.005% or less, more preferably 0.003% or less.
• Cu improves corrosion resistance in the automotive environment. Further, the corrosion products of Cu coat the surface of the steel sheet, which has the effect of suppressing hydrogen intrusion into the steel sheet.
• Cu is an element that is mixed in when scrap is used as a raw material, and by allowing Cu to be mixed in, recycled materials can be used as raw materials and manufacturing costs can be reduced. From this viewpoint, it is preferable to contain Cu in an amount of 0.005% or more, and from the viewpoint of improving delayed fracture resistance, it is more desirable to contain Cu in an amount of 0.05% or more. More preferably, it is 0.10% or more. However, if the Cu content becomes too large, surface defects will occur, so when Cu is contained, the Cu content is set to 1% or less.
• Ni is also an element that has the effect of improving corrosion resistance. Further, Ni has the effect of suppressing the occurrence of surface defects that are likely to occur when Cu is included. For this reason, it is desirable to contain Ni in an amount of 0.01% or more.
• the Ni content is more preferably 0.04% or more, still more preferably 0.06% or more.
• the Ni content is set to 1% or less.
• the Ni content is 0.5% or less, more preferably 0.3% or less.
• ⁇ Cr 1% or less> Cr can be contained because of its effect of improving the hardenability of steel and suppressing the formation of carbides in martensite and upper/lower bainite.
• the Cr content is preferably 0.01% or more.
• the Cr content is more preferably 0.03% or more, and still more preferably 0.06% or more.
• the Cr content is set to 1% or less.
• Mo can be contained because it has the effect of improving the hardenability of steel and suppressing the formation of carbides in martensite and upper/lower bainite.
• the Mo content is preferably 0.01% or more.
• the content is more preferably 0.03% or more, and even more preferably 0.06% or more. More preferably, the Mo content is 0.1% or more, even more preferably 0.2% or more.
• the Mo content is set to 0.5% or less.
• V 0.5% or less> V is included because it has the effect of improving the hardenability of steel, suppressing the formation of carbides in martensite and upper/lower bainite, refining the structure, and precipitating carbides to improve delayed fracture resistance. be able to.
• the V content is preferably 0.003% or more.
• the V content is more preferably 0.05% or more, and still more preferably 0.015% or more. Even more preferably, the V content is 0.02% or more, even more preferably 0.05% or more.
• the V content is preferably 0.15% or more, more preferably 0.25% or more. However, if a large amount of V is contained, the castability will be significantly deteriorated, so when V is contained, the V content should be 0.5% or less.
• the V content is 0.4% or less, more preferably 0.3% or less.
• Nb can be contained because it has the effect of refining the steel structure and increasing its strength, promoting bainite transformation through grain refinement, improving bendability, and improving delayed fracture resistance.
• the Nb content is preferably 0.002% or more.
• the Nb content is more preferably 0.004% or more, and still more preferably 0.010% or more.
• the Nb content is set to 0.1% or less.
• the Nb content is 0.07% or less, more preferably 0.05% or less.
• Mg fixes O as MgO and contributes to improving formability such as bendability. Therefore, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0004% or more, and still more preferably 0.0006% or more. On the other hand, if a large amount of Mg is added, the surface quality and bendability will deteriorate, so when Mg is included, the Mg content should be 0.0050% or less. Preferably, the Mg content is 0.0030% or less.
• Ca fixes S as CaS and contributes to improving bendability and delayed fracture resistance.
• the Ca content is preferably 0.0002% or more.
• the Ca content is more preferably 0.0005% or more, and still more preferably 0.0010% or more.
• the Ca content should be 0.0050% or less.
• the Ca content is 0.0040% or less.
• Sn suppresses oxidation and nitridation of the surface layer of the steel sheet, and thereby suppresses a reduction in the content of C and B in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing its strength and improving its fatigue resistance.
• the Sn content is preferably 0.002% or more.
• the Sn content is more preferably 0.004% or more, and still more preferably 0.006% or more. More preferably, the Sn content is 0.01% or more, even more preferably 0.05% or more.
• the Sn content exceeds 0.1%, castability deteriorates. Furthermore, Sn is segregated at the prior ⁇ grain boundaries, deteriorating the delayed fracture resistance. Therefore, when Sn is contained, the Sn content is 0.1% or less.
• Sb suppresses oxidation and nitridation of the surface layer of the steel sheet, and thereby suppresses a reduction in the content of C and B in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing its strength and improving its fatigue resistance.
• the Sb content is preferably 0.002% or more.
• the Sb content is more preferably 0.004% or more, and still more preferably 0.006% or more. More preferably, the Sb content is 0.01% or more, even more preferably 0.05% or more.
• the Sb content exceeds 0.1%, castability deteriorates, and Sb segregates at prior ⁇ grain boundaries, degrading delayed fracture resistance. Therefore, when Sb is contained, the Sb content is set to 0.1% or less.
• REM is an element that suppresses the adverse effects of sulfide on stretch flange formability and improves stretch flange formability by making the shape of sulfide spheroidal.
• the REM content is preferably 0.0005% or more.
• the REM content is more preferably 0.0010% or more, and still more preferably 0.0020% or more.
• the REM content exceeds 0.0050%, the effect of improving stretch flange formability will be saturated, so when REM is contained, the REM content should be 0.0050% or less.
• REM as used in the present invention refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
• Y yttrium
• La lanthanum
• Lu lutetium
• the REM concentration in the present invention is the total content of one or more elements selected from the above-mentioned REMs.
• the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. Therefore, when the above-mentioned arbitrary element is included in an amount less than the lower limit value, the above-mentioned arbitrary element is included as an unavoidable impurity.
• the steel plate of the present invention has a tensile strength (TS) of 980 MPa or more.
• TS tensile strength
• the upper limit of the tensile strength is not particularly limited, from the viewpoint of coexistence with other properties, the tensile strength is preferably 1300 MPa or less.
• the stability of press forming is significantly improved by ensuring the total elongation EL is 14.0% or more at TS: 980 MPa or more, and 12.0% or more at TS: 1180 MPa or more.
• ⁇ is set to 40% or more.
• the measurement position contact points for each width are W/24, 2W/24, 3W/24, 4W/24, 5W/24, 6W/24, 7W/24, 8W/24, 9W/24, 10W/24, 11W.
• the region A has a length in the sheet width direction of 80% or more of the total sheet width.
• the deviation of EL in the sheet width direction is 10% or less with respect to the measured value at the sheet width center position, and the deviation of ⁇ in the sheet width direction is less than 10% with respect to the measured value at the sheet width center position.
• the area where the thickness is 10% or less shall be 80% or more of the entire board width area.
• the range of the unsteady portion is allowed to be up to 20% in total at both ends in the width direction. Because the end of the steel plate comes into contact with other structures during transportation and work processes, the end is not used to ensure quality. Therefore, the usable effective plate width does not reach 100%. Therefore, the effective plate width is preferably less than 100%.
• the area where the deviation of EL in the sheet width direction is 10% or less of the measured value at the center of the sheet width and the deviation of ⁇ is 10% or less to 80% or more of the entire sheet width, yields are significantly improved. Therefore, in the present invention, the area where the deviation of EL in the board width direction is 10% or less of the measured value at the center of the board width, and the deviation of ⁇ is 10% or less is 80% or more of the entire board width region. . Preferably it is 85% or more.
• a steel plate having a tensile strength of 980 MPa or more is defined as a high-strength steel plate.
• a steel plate having a yield ratio YR of 0.8 or less is a steel plate having excellent press formability.
• a steel plate with excellent ductility has a total elongation EL of 14.0% or more when TS: 980 MPa or more, and 12.0% or more when TS: 1180 MPa or more.
• d 0 is the initial hole diameter (mm)
• d is the hole diameter at the time of crack occurrence (mm)
• the hole expansion rate ⁇ (%) ⁇ (d - d 0 )/d 0 ⁇ 100
• the plate width of the steel plate in the present invention is preferably 600 mm or more. Moreover, the plate width of the steel plate in the present invention is preferably 1700 mm or less.
• the area ratio of polygonal ferrite is 10% or more, and in order to obtain higher ductility, it is preferably 20% or more.
• the area ratio of the polygonal ferrite is 57% or less, preferably 55% or less. More preferably it is 50% or less.
• Total area ratio of upper bainite, tempered martensite, and lower bainite 40% or more and 80% or less>
• the total area ratio of upper bainite, tempered martensite, and lower bainite is set to 40% or more, and in order to obtain higher strength, it is preferably set to 45% or more.
• the area ratio is set to 80% or less. More preferably, it is 75% or less.
• volume fraction of retained austenite (retained ⁇ ): 3% or more and 15% or less>
• the volume fraction of retained austenite is 3% or more, preferably 5% or more.
• the retained austenite is set to 15% or less. More preferably it is 13% or less.
• ⁇ Quenched martensite 12% or less (including 0%)> Since the hard quenched martensitic structure lowers ⁇ , it is necessary to suppress its area ratio. In order to obtain the desired ⁇ , the area ratio of hardened martensite is set to 12% or less. In order to obtain ⁇ more stably, the area ratio of hardened martensite is preferably 10% or less.
• the steel structure other than the above, it consists of the remainder structure.
• the area ratio of the remaining tissue is preferably 5% or less.
• the remaining structure may be carbide or pearlite. These tissues may be determined by SEM observation as described later.
• the retained austenite becomes a hard martensitic structure due to the TRIP effect during press molding, tensile processing, etc. Therefore, in the present invention, from the viewpoint of stretch flangeability, quenched martensite and retained austenite are controlled together.
• hardened martensite or retained austenite with a circular equivalent diameter of 1.6 ⁇ m or more is formed, voids are formed at stress concentration areas at the interface with other structures, making it impossible to obtain the desired stretch flange formability.
• the total area ratio of hardened martensite and retained austenite is set to 20% or less. Preferably it is 18% or less.
• the hardness of quenched martensite is determined by the amount of C dissolved in the quenched martensite.
• the structures in which a large amount of solid solute C exists are quenched martensite and retained austenite.
• Retained austenite is a structure that contributes to high ductility, and the C concentration is 0.5 mass% or more, but the area ratio of the structure with a C concentration of 0.5 mass% or more is 15% or less of all constituent structures.
• the space factor of the C-enriched region (SC ⁇ 0.5 ) where the C concentration is 0.5 mass% or more is 15% or less.
• the C concentration is 0.5 mass% or more
• it is 12% or less, more preferably 10% or less.
• it is preferably 6% or more, more preferably 8% or more.
• polygonal ferrite To measure the area ratio of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and hardened martensite (fresh martensite), cut out a cross section parallel to the rolling direction, mirror polish it, and then use 1 vol% nital. Corroded, 10 fields of view were observed at 1/4 thickness using SEM at 5000x magnification, and the photographed tissue photographs were quantified by image analysis.
• Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. This is the area that appears blackest in the SEM.
• Upper bainite is a ferritic structure with the formation of carbides or retained austenite that appear white under SEM.
• the area of ferrite with an aspect ratio ⁇ 2.0 is classified as polygonal ferrite, and the area with an aspect ratio >2.0 is classified as upper bainite, and the area ratio is calculated.
• the aspect ratio is determined by determining the major axis length a where the particle length is the longest, and setting the particle length that crosses the particle longest in the direction perpendicular to it to be the minor axis length b, and a/b is the aspect ratio. Take the ratio.
• the tempered martensite and lower bainite are regions with a lath-like substructure and carbide precipitation in the SEM.
• Quenched martensite (fresh martensite) is a massive region that appears white with no underlying structure visible in the SEM.
• the remaining structure is a carbide and/or pearlite structure, which can be confirmed by white contrast in SEM, but the carbide is a structure with a particle size of 1 ⁇ m or less, and the pearlite is a lamellar (layer) structure. It is possible to distinguish it from the fact that it has a similar structure.
• the quantitative evaluation of the structure described above and the measurement of the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be performed using image analysis software such as Image J (Fiji).
• image analysis software such as Image J (Fiji).
• a cross-section of the plate parallel to the rolling direction was cut out, polished to a mirror surface, corroded with 1 vol% nital, and observed at 1/4 thickness position with an SEM at 5000x magnification for 10 fields of view, and machine learning using Image J (Fiji) was performed.
• Each tissue can be identified and quantitatively evaluated using the Trainable Weka segmentation method that allows area identification.
• the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be measured using a particle analysis program that is also a function of Image J, and only the quenched martensite and retained austenite identified as above can be extracted and measured. do.
• the volume fraction of retained austenite is determined by chemically polishing a 1/4 thickness position from the surface layer and using X-ray diffraction.
• a Co-K ⁇ ray source is used for incident X-rays, and the volume of retained austenite is determined from the intensity ratio of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite. Calculate the rate.
• the volume fraction of retained austenite determined by X-ray diffraction can be taken as the area fraction of retained austenite.
• the area ratio of the C-enriched region where the C concentration is 0.5 mass% or more is measured using a JEOL field emission electron A line microanalyzer (FE-EPMA) JXA-8500F is used. Then, the C concentration distribution is measured by mapping analysis using an accelerating voltage of 6 kV, an irradiation current of 7 ⁇ 10 ⁇ 8 A, and a beam diameter of the minimum, and an area ratio at which the C concentration is 0.5 mass% or more is calculated. However, in order to eliminate the influence of contamination, background components are subtracted so that the average value of C obtained in the analysis is equal to the carbon content of the base material.
• FE-EPMA JEOL field emission electron A line microanalyzer
• the increased amount is considered to be contamination, and the true value at each location is calculated by uniformly subtracting that increased amount from the analysis value at each location. Let the amount of C be .
• the method for producing a steel plate of the present invention includes hot rolling, pickling, and cold rolling a steel slab having a chemical composition, and then annealing the obtained cold rolled steel plate.
• the annealing includes a holding step in which the cold rolled steel sheet is heated to an annealing temperature of 750 to 880°C and held at the annealing temperature for 10 to 500 seconds, and a holding step at 350 to 550°C from the annealing temperature.
• a first cooling step in which the temperature range up to the cooling stop temperature is cooled to the first cooling stop temperature with a first average cooling rate of 2 to 50°C/s, and a residence temperature of 350 to 550°C for 10 seconds to 60 seconds. After that, a second cooling step of cooling to a second cooling stop temperature of 100 to 300 ° C.
• a second average cooling rate 3 to 50 ° C / s
• Hot rolling steel slabs include rolling the slab after heating, directly rolling the slab after continuous casting without heating it, and rolling after subjecting the slab after continuous casting to a short heat treatment. and so on.
• Hot rolling may be carried out according to a conventional method, for example, the slab heating temperature is 1100 to 1300°C, the soaking temperature is 20 to 300 min, the finish rolling temperature is Ar 3 transformation point to Ar 3 transformation point + 200°C, and rolling The temperature may be 400 to 720°C.
• the winding temperature is preferably 430 to 530° C. from the viewpoint of suppressing plate thickness variations and stably ensuring high strength.
• the Ar 3 transformation point can be calculated from the composition of the steel plate and the following empirical formula (A).
• ⁇ Acid washing> Pickling may be carried out according to a conventional method.
• Cold rolling may be carried out according to a conventional method, and the cumulative rolling ratio may be 30 to 85%. From the viewpoint of stably securing high strength and reducing anisotropy, the rolling ratio is preferably 35 to 85%. Note that when the rolling load is high, it is possible to perform softening annealing treatment at 450 to 730° C. in a CAL (continuous annealing line) or BAF (box annealing furnace).
• CAL continuous annealing line
• BAF box annealing furnace
• a cold rolled steel plate (cold rolled steel plate) manufactured according to a conventional method is annealed under the following conditions.
• the annealing equipment is not particularly limited, it is preferable to use a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL) from the viewpoint of productivity and ensuring desired heating and cooling rates.
• CAL continuous annealing line
• CGL continuous hot-dip galvanizing line
• the annealing temperature (soaking temperature) is set to 750°C or higher.
• the annealing temperature (soaking temperature) exceeds 880°C, the temperature becomes an austenite single phase temperature, the desired polygonal ferrite cannot be obtained, and the YR increases and the ductility decreases. Therefore, the annealing temperature (soaking temperature) is set to 880° C. or lower.
• the annealing temperature (soaking temperature) is preferably 850°C or lower, more preferably 830°C or lower.
• the time for holding at the above annealing temperature is less than 10 seconds, austenite will not be formed sufficiently at the above annealing temperature (soaking temperature), and polygonal ferrite will become excessive, resulting in a specified amount of Since upper bainite, tempered martensite, and lower bainite cannot be obtained, not only the desired strength cannot be obtained, but also sufficient residual austenite cannot be obtained, and the desired ductility cannot be secured.
• the time for holding at the above annealing temperature (soaking time) exceeds 500 seconds, the structure will significantly coarsen, making it impossible to secure the desired strength. Therefore, the time for holding at the above annealing temperature (soaking time) is set to 10 to 500 seconds.
• the time for holding at the annealing temperature is preferably 80 seconds or more, more preferably 100 seconds or more. Further, the time for holding at the annealing temperature (soaking time) is preferably 400 seconds or less, more preferably 300 seconds or less.
• First cooling step cooling the temperature range from the annealing temperature to the first cooling stop temperature of 350 to 550°C to the first cooling stop temperature at a first average cooling rate of 2 to 50°C/s]
• the temperature range from the above annealing temperature to the first cooling stop temperature of 350 to 550°C is set at a first average cooling rate of 2 to 50°C/s. Cool it down. If the cooling rate is less than 2°C/s, operability will deteriorate, so the first average cooling rate is set to 2°C/s or more.
• the first average cooling rate is preferably 5°C/s or more.
• the first average cooling rate becomes too high, the plate shape will deteriorate, so it is set to 50° C./s or less.
• the first average cooling rate is preferably 40°C/s or less, more preferably less than 30°C/s.
• the first average cooling rate is "(annealing temperature (°C) - first cooling stop temperature (°C))/cooling time (seconds) from the annealing temperature to the first cooling stop temperature.”
• a temperature range (retention temperature) below the first cooling stop temperature and from 350° C. to 550° C. upper bainite is formed, a predetermined retained austenite can be obtained, and desired ductility can be obtained.
• Bainite transformation has an incubation period, and must be allowed to stay in a residence temperature range that includes a residence start temperature and a residence end temperature for a certain period of time.
• the residence temperature range is less than 350°C or more than 550°C, bainite transformation is suppressed, resulting in suppressed formation of retained austenite, and desired ductility cannot be obtained.
• the residence temperature range is less than 350° C.
• martensitic transformation occurs, which may unnecessarily increase YR and reduce press formability. Therefore, the residence temperature range is 350 to 550°C.
• the residence time is less than 10 seconds, the desired amount of bainite cannot be obtained, and as a result of suppressing the formation of retained austenite, the desired ductility cannot be obtained.
• the residence time exceeds 60 seconds, the concentration of C from bainite to lumpy untransformed ⁇ progresses, leading to an increase in coarse quenched martensite with a high C concentration, resulting in the desired stretch flange formability and plate width direction. material stability cannot be obtained. Therefore, the residence time is set to 10 seconds or more and 60 seconds or less.
• the second average cooling rate is preferably 5°C/s.
• the second average cooling rate is set to 50°C/s or less.
• the second cooling stop temperature exceeds 300° C., a predetermined tempered martensite cannot be obtained, and as a result, coarse quenched martensite increases, and desired stretch flange formability cannot be obtained. Therefore, the second cooling stop temperature is set to 300°C or less.
• the second cooling stop temperature is preferably 290°C or lower.
• the cooling stop temperature is set to 100°C or higher.
• the second average cooling rate is "retention end temperature (°C) - second cooling stop temperature (°C)/cooling time (seconds) from the residence end temperature to the second cooling stop temperature".
• the reheating temperature is set to a cooling stop temperature +50°C or more and 340°C or less.
• the average heating rate is less than 2.0° C./s, carbide precipitation is promoted more than carbon distribution, and as a result, the desired retained austenite cannot be obtained.
• the temperature range from the cooling stop temperature to 340°C or less is set to an average heating rate of 2.0°C/s or more.
• the average heating rate is "reheating temperature (°C) - second cooling stop temperature (°C)/heating time from the second cooling stop temperature to the reheating temperature (seconds)".
• the cooling rate is set to 0.05°C/s or more.
• the third average cooling rate is "reheating temperature (°C) - 50°C/cooling time (seconds) from reheating temperature (°C) to 50°C".
• the surface of the steel sheet may be galvanized to obtain a steel sheet having a galvanized layer on the surface.
• the type of plating treatment is not particularly limited, and may be either hot-dip galvanizing or electrogalvanizing.
• the alloying hot-dip galvanizing treatment a plating treatment in which alloying is performed after hot-dip galvanizing may be performed. Hot-dip galvanizing is used for automobile steel sheets and the like.
• the steel sheet When applying hot-dip galvanizing, the steel sheet is immersed in a hot-dip galvanizing bath in a continuous annealing furnace at the front stage of the continuous hot-dip galvanizing line after the above-mentioned annealing holding step and first cooling step to form a hot-dip galvanized layer on the surface of the steel sheet. It is sufficient to form an alloyed galvanized steel sheet by subsequently performing an alloying treatment.
• hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment can be performed on the surface of the steel sheet.
• the soaking and cooling steps and the plating step described above may be performed in separate lines.
• electrogalvanizing can be performed after annealing, that is, after the third cooling step.
• the thickness of the steel plate of the present invention obtained as described above is preferably 0.5 mm or more. Further, the thickness of the steel plate of the present invention is preferably 2.0 mm or less. Further, the plate width is preferably 600 mm or more. Moreover, it is preferable that the plate width of the steel plate of the present invention is 1700 mm or less.
• the member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and bonding. Furthermore, the method for manufacturing the member of the present invention includes the step of subjecting the steel plate of the present invention to at least one of forming and joining to produce a member.
• the steel sheet of the present invention has a tensile strength of 980 MPa or more, excellent press formability, ductility, and stretch flange formability, and excellent material stability in the sheet width direction. Therefore, members obtained using the steel sheet of the present invention also have high strength, excellent press formability, ductility, and stretch flange formability, and excellent material stability in the sheet width direction. Furthermore, by using the member of the present invention, it is possible to reduce the weight. Therefore, the member of the present invention can be suitably used for, for example, vehicle body frame parts.
• the members of the invention also include welded joints.
• general processing methods such as press working can be used without restriction.
• general welding such as spot welding and arc welding, rivet joining, caulking joining, etc. can be used without limitation.
• a slab manufactured by continuous casting having the composition shown in Table 1 was heated to 1200°C, and the soaking time was 200 min.
• Table 2 shows a cold-rolled steel sheet with a thickness of 1.4 mm manufactured by cold rolling at a rolling ratio of 50% after a hot rolling process with a finish rolling temperature of 900°C and a coiling temperature of 550°C.
• a steel plate of the present invention and a steel plate of a comparative example were manufactured by processing under the annealing conditions shown in . The width of all the obtained steel plates was 1500 mm.
• a steel plate was immersed in a galvanizing bath at a temperature of 440° C. or higher and 550° C. or lower to perform hot-dip galvanizing treatment, and then the amount of plating deposited was adjusted by gas wiping or the like.
• a galvanizing bath having an Al content of 0.10% or more and 0.22% or less was used for the hot-dip galvanizing.
• hot-dip galvanized steel sheets were subjected to alloying treatment after the hot-dip galvanizing treatment to obtain alloyed hot-dip galvanized steel sheets (GA).
• alloying treatment was performed in a temperature range of 460° C. or higher and 550° C. or lower.
• steel plates cold rolled steel plates: CR
• EG electrogalvanized steel plates
• the steel structure was measured using the following method. The measurement results are shown in Table 3. To measure the area ratio of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and hardened martensite (fresh martensite), cut out a cross section parallel to the rolling direction, mirror polish it, and then use 1 vol% nital. Corroded, 10 fields of view were observed at 1/4 thickness using SEM at 5000x magnification, and the photographed tissue photographs were quantified by image analysis. Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. This is the area that appears blackest in the SEM. Upper bainite is a ferritic structure with the formation of carbides or retained austenite that appear white under SEM.
• the area of ferrite with an aspect ratio ⁇ 2.0 is classified as polygonal ferrite, and the area with an aspect ratio >2.0 is classified as upper bainite, and the area ratio is calculated.
• the aspect ratio is determined by determining the major axis length a where the particle length is the longest, and setting the particle length that crosses the particle longest in the direction perpendicular to it to be the minor axis length b, and a/b is the aspect ratio. With ratio.
• the tempered martensite and lower bainite are regions with a lath-like substructure and carbide precipitation in the SEM.
• Quenched martensite (fresh martensite) is a massive region that appears white with no underlying structure visible in the SEM.
• the residual structure is a carbide and/or pearlite structure, and is a structure that can be confirmed by white contrast in SEM.
• Carbide has a structure with a particle size of 1 ⁇ m or less, and pearlite has a lamellar structure, so they can be distinguished from each other.
• the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be measured using a particle analysis program that is also a function of Image J, and only the quenched martensite and retained austenite identified as above can be extracted and measured. did.
• the volume fraction of retained austenite was determined by X-ray diffraction after chemically polishing a 1/4 thickness position from the surface layer.
• a Co-K ⁇ ray source is used for incident X-rays, and the volume of retained austenite is determined from the intensity ratio of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite. calculated the rate.
• the area ratio of the C-enriched region where the C concentration is 0.5 mass% or more is measured using a JEOL field emission electron A line microanalyzer (FE-EPMA) JXA-8500F was used. Then, the C concentration distribution was measured by mapping analysis using an accelerating voltage of 6 kV, an irradiation current of 7 ⁇ 10 ⁇ 8 A, and a minimum beam diameter, and an area ratio at which the C concentration was 0.5 mass% or more was calculated. However, in order to eliminate the influence of contamination, background components were subtracted so that the average value of C obtained in the analysis was equal to the carbon content of the base material.
• the increased amount is considered to be contamination, and the true value at each location is calculated by uniformly subtracting that increased amount from the analysis value at each location.
• the amount of C was set to .
• d 0 is the initial hole diameter (mm)
• d is the hole diameter at the time of crack occurrence (mm)
• the hole expansion rate ⁇ (%) ⁇ (d-d 0 )/d 0 ⁇ 100 is calculated.
• the average value of the three points was evaluated as ⁇ . Steels having a ⁇ of 40% or more were judged to have excellent hole expandability and stretch flangeability.
• the material stability evaluation in the board width direction 23 points were evaluated from both board width directions at intervals of 100 mm or less from the board width center position (12W/24 position (W: board width)). (including the width center position), and determine EL and ⁇ at each position (measurement position X). Then, the material stability in the board width direction was evaluated by determining the ratio of the difference between the measured values at the board width center position and each position relative to the measured value at the center position. Using EL and ⁇ at the center of the board width as a reference, consecutive measurement groups where the difference in EL and ⁇ is 10% or less are defined as areas where the difference in EL and ⁇ is 10% or less, and this area is defined for the entire board width.
• members obtained by forming, joining, and forming and joining the steel sheets of the invention examples have a high quality. It has high strength, excellent press formability, ductility, stretch flange formability, and material stability in the sheet width direction. It was found that it has excellent stretch flange formability and material stability in the width direction of the plate.

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• 2023
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