Classifications

• • 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
  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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
• • 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
• • 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
  • C22C38/16—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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
  • C22C38/40—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
  • 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

Definitions

• the present invention relates to a steel sheet, a member, and methods for manufacturing them, the steel sheet being suitable for press-formed products having complex shapes, which are formed through a press-forming process and used for, for example, automobiles and home appliances, and having excellent phosphatability.
• transformation-induced plasticity (TRIP) steels in which retained austenite (retained ⁇ ) is dispersed in microstructures of steel sheets, have been developed as one technique to improve the ductility of steel sheets.
• Processes for manufacturing TRIP steels include austempering treatment including isothermal holding in the bainite transformation temperature range after soaking, and a heat treatment process known as quenching and partitioning (Q&P) (quenching and carbon partitioning from martensite to austenite), in which the steel is cooled once during a cooling process to a temperature range between the martensite start temperature (Ms temperature) and the martensite finish temperature (Mf temperature), and then reheated and held to stabilize the retained austenite.
• Q&P quenching and partitioning
• Q&P is a heat treatment process in which part of the microstructure is transformed into martensite during the cooling process, and then the martensite microstructure is tempered by reheating and holding, thereby reducing the difference in hardness between different phases in the microstructure and improving not only ductility but also flangeability.
• Patent Literature 1 discloses a steel sheet containing retained austenite with a volume fraction of 5% to 15%, achieving both a tensile strength (TS) of 980 MPa or higher and ductility, as indicated by an elongation of 17%, and having a hole expansion ratio of 50%, which indicates excellent flangeability, and discloses a method for manufacturing the steel sheet, the method including holding a cold rolled steel sheet containing 0.6% to 2.5% of Si at a first soaking temperature of 750°C or higher, subsequently cooling it to a cooling stop temperature in the temperature range of 150°C to 350°C, and then reheating it to a temperature range of 350°C to 500°C.
• TS tensile strength
• Patent Literature 2 discloses a method for improving phosphatability by adding Ni to inhibit the enrichment of Si on the surface of a steel sheet.
• Patent Literature 3 discloses a method for improving phosphatability by appropriately controlling the amount of Mn, which is enriched on the surface together with Si, in such a manner that the Si/Mn ratio is 0.40 or less, thereby forming a Mn-Si complex oxide on the surface.
• Patent Literature 4 discloses a method for improving phosphatability by directly removing Si-based oxides through pickling or brushing after annealing.
• Patent Literatures 2 and 4 are effective as methods for improving the phosphatability of steels with a high Si content.
• the alloying elements to be incorporated, the annealing conditions, and so forth are adjusted.
• Patent Literature 3 The inventors' investigations revealed that even in the method disclosed in Patent Literature 3, good phosphatability is not necessarily ensured.
• the present invention has been accomplished in consideration of the above circumstances. It is an object of the present invention to provide a steel sheet, a member, and methods for manufacturing them, the steel sheet having excellent ductility, flangeability, and phosphatability, and a tensile strength of 780 MPa or more.
• the tensile strength refers to the tensile strength (TS) obtained in accordance with JIS Z2241 (2011).
• Excellent phosphatability indicates that after degreasing (treatment temperature: 40°C, treatment time: 120 seconds, spray degreasing, degreasing agent: FC-E2011 manufactured by Nihon Parkerizing Co., Ltd.), surface conditioning (pH: 9.5, treatment temperature: room temperature, treatment time: 20 seconds, surface conditioner: PL-X manufactured by Nihon Parkerizing Co., Ltd.), and then zinc phosphate treatment (temperature of zinc phosphate treatment solution: 35°C, treatment time: 120 seconds, zinc phosphate treatment solution: Palbond PB-L3065 manufactured by Nihon Parkerizing Co., Ltd.) using a zinc phosphate treatment solution, the area where a steel substrate is exposed is less than 10% of the total area. Solution to Problem
• the inventors have conducted intensive studies on the influence of steel components, heat treatment conditions, and microstructures on ductility and phosphatability y for various thin steel sheets having a tensile strength of 780 MPa or more.
• a high-strength cold rolled steel sheet having excellent ductility, flangeability, and phosphatability can be obtained by having a chemical composition containing, in mass%, C: 0.05% to 0.25%, Si: 0.30% to 1.50%, Mn: 1.5% to 4.5%, P: 0.005% to 0.050%, S: 0.01% or less, sol.
• the present invention has been made on the basis of these findings, and the gist thereof is described below.
• a steel sheet and a member having a high tensile strength TS of 780 MPa or more, excellent ductility, flangeability, and phosphatability.
• Fig. 1 is a graph for explaining the maximum concentration of P [Pm] of the present invention.
• a steel sheet of the present invention has a high tensile strength TS of 780 MPa or more, excellent ductility, flangeability, and phosphatability, has a chemical composition containing, in mass%, C: 0.05% to 0.25%, Si: 0.30% to 1.50%, Mn: 1.5% to 4.5%, P: 0.005% to 0.050%, S: 0.01% or less, sol.
• C is contained from the viewpoints of ensuring a predetermined strength through transformation strengthening and also ensuring a predetermined amount of retained austenite (hereinafter, also referred to as retained ⁇ ) to improve ductility.
• retained ⁇ retained austenite
• the upper limit of the C content is set to 0.25% in consideration of flangeability, which is important for press formability, and weldability, which is important for spot welding or laser welding when formed automobile members are assembled into automobile bodies.
• the C content is set to 0.05% to 0.25%.
• the C content is preferably 0.08% or more, more preferably 0.10% or more.
• the C content is preferably 0.22% or less, more preferably 0.20% or less.
• Si is contained from the viewpoint of strengthening ferrite to increase strength, and of inhibiting the formation of carbides in martensite and bainite and ensuring a predetermined amount of retained ⁇ to improve ductility.
• Si content is less than 0.30%, these effects cannot be sufficiently provided.
• the Si content is set to 0.30% to 1.50%.
• the Si content is preferably 0.35% or more, more preferably 0.40% or more.
• the Si content is preferably 1.20% or less, more preferably 1.00% or less.
• Mn is contained from the viewpoint of improving the hardenability of a steel sheet to promote an increase in strength through transformation strengthening, and from the viewpoint of inhibiting the formation of carbides in bainite, similar to Si, to promote the formation of retained austenite that contributes to ductility, thereby improving ductility.
• the Mn content needs to be 1.5% or more.
• the Mn content is set to 1.5% or more and 4.5% or less.
• the Mn content is preferably 1.8% or more, more preferably 2.0% or more.
• the Mn content is preferably 3.5% or less, more preferably 3.0% or less.
• P is an element that strengthens steel.
• P is an element that can form a P-rich surface portion on the surface of the steel sheet after annealing by appropriately controlling the P content, thereby improving the phosphatability. From this point of view, the P content is set to 0.005% or more.
• the P content is set to 0.050% or less.
• the P content is set to 0.005% to 0.050%.
• the P content is preferably 0.007% or more, more preferably 0.009% or more.
• the P content is preferably 0.040% or less, more preferably 0.030% or less.
• S has the effect of improving descalability in hot rolling and the effect of inhibiting nitriding during annealing, but is an element that adversely affects spot weldability, bendability, and flangeability.
• the S content is set to 0.01% or less at most, and preferably 0.0050% or less.
• the S content is preferably 0.0001% or more.
• the S content is more preferably 0.0005% or more, even more preferably 0.0010% or more.
• Al is contained for the purpose of deoxidization or obtaining retained ⁇ .
• the lower limit of the sol. Al is not particularly specified, in order to perform stable deoxidation, the sol. Al content is preferably 0.005% or more.
• the sol. Al content is 1.0% or more, the number of coarse Al-based inclusions increases significantly, thereby leading to a deterioration in stretch flange formability (flangeability).
• Al is an element that degrades the phosphatability of steel sheets.
• the sol. Al content is 1.0% or more, good phosphatability cannot be ensured even in the present invention. For this reason, the sol. Al content is set to less than 1.0%.
• the sol. Al content is preferably 0.80% or less, more preferably 0.06% or less.
• N is an element that forms nitrides, such as BN, AlN, and TiN, in steel and reduces stretch flange formability (flangeability), and the N content needs to be limited.
• the N content is set to less than 0.015%.
• the N content is preferably 0.010% or less, more preferably 0.006% or less.
• the N content need not be contained. However, reducing the N content to less than 0.0001% entails high costs. Thus, 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 0.001% or more. Si / Mn ⁇ 0.35
• [Si] is the Si content (mass%)
• [Mn] is the Mn content (mass%)
• [Si]/[Mn] determines the component ratio of Si to Mn in the surface oxide formed during annealing.
• [Si]/[Mn] ratio is more than 0.35, good phosphatability cannot be ensured.
• [Si]/[Mn] is set to 0.35 or less.
• [Si]/ [Mn] is preferably 0.32 or less, more preferably 0.30 or less.
• the lower limit is not specifically limited; however, [Si]/[Mn] is preferably 0.10 or more, more preferably 0.15 or more.
• the chemical composition of the steel sheet in the present invention contains the above-mentioned component elements as basic components, with the balance containing iron (Fe) and incidental impurities.
• the steel sheet in the present invention preferably has a chemical composition with the balance being Fe and incidental impurities.
• the chemical composition of the steel sheet of the present invention may contain, in addition to the above-mentioned components, one or two or more elements selected from the following as optional elements (selected elements).
• Ti has the effect of fixing N in steel in the form of TiN to improve hot ductility and provides the effect of improving the hardenability effect of B.
• the precipitation of TiC has the effect of refining the microstructure.
• the Ti content is preferably 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.
• a Ti content of more than 0.1% leads to an increase in rolling load and a decrease in ductility due to the increased amount of precipitation strengthening.
• the Ti content is set to 0.1% or less.
• the Ti content is preferably 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 facilitating the formation of tempered martensite and/or bainite with a specified area fraction.
• the B content is preferably 0.0005% or more.
• a B content of more than 0.001% causes the enrichment of B in an oxide during annealing to promote the coarsening of the oxide, thereby resulting in a deterioration in phosphatability.
• the B content is set to 0.001% or less.
• Cu improves the corrosion resistance in the environment in which the automobile is used.
• the corrosion products of Cu have the effect of covering the surface of the steel sheet to inhibit hydrogen ingress into the steel sheet.
• Cu is an element that is mixed in when scrap is utilized as a raw material. When Cu is allowed to be mixed in, recycled materials can be used as raw materials to reduce manufacturing costs. From this point of view, the Cu content is preferably 0.005% or more. Furthermore, from the viewpoint of improving delayed fracture resistance, the Cu content is more preferably 0.05% or more.
• the Cu content is even more preferably 0.10% or more.
• the Cu content is even more preferably 0.25% or more, even further more preferably 0.50% or more.
• the Cu content is set to 1% or less.
• Ni like Cu, is an element that acts to improve corrosion resistance. Ni also inhibits the formation of surface defects that are likely to occur when Cu is contained. For this reason, Ni is desirably contained in an amount of 0.01% or more. The Ni content is more preferably 0.04% or more, even more preferably 0.06% or more.
• Ni content is set to 1% or less.
• the Ni content is preferably 0.5% or less, more preferably 0.3% or less.
• the Cr content is preferably 0.01% or more.
• the Cr content is more preferably 0.03% or more, even more preferably 0.06% or more.
• the Cr content is set to 1% or less.
• the Cr content is preferably 0.3% or less, more preferably 0.1% or less.
• Mo can be added because of its effect of improving the hardenability of steel and its effect of inhibiting the formation of carbides in martensite, upper bainite, and lower bainite.
• the Mo content is preferably 0.01% or more.
• the Mo content is more preferably 0.03% or more, even more preferably 0.06% or more.
• the Mo content is even more preferably 0.1% or more, even further more preferably 0.2% or more.
• Mo significantly degrades the phosphatability of the cold rolled steel sheet.
• the Mo content is set to 0.5% or less.
• V can be added because of its effects of improving the hardenability of steel, inhibiting the formation of carbides in martensite, upper bainite, and lower bainite, refining the microstructure, and precipitating carbides to improve delayed fracture resistance.
• the V content is preferably 0.003% or more.
• the V content is more preferably 0.005% or more, even more preferably 0.010% or more.
• the V content is even more preferably 0.020% or more, even further more preferably 0.050% or more.
• the V content is set to 0.5% or less.
• the V content is preferably 0.3% or less, more preferably 0.2% or less.
• the V content is preferably 0.2% or less, more preferably 0.1% or less.
• Nb can be added because it has the effects of refining the steel microstructure to increase strength, promoting bainite transformation through grain refinement, improving bendability, and improving delayed fracture resistance.
• the Nb content is preferably 0.010% or more.
• the Nb content is preferably 0.015% or more, more preferably 0.020% or more.
• the Nb content is set to 0.1% or less.
• the Nb content is preferably 0.08% or less, more preferably 0.05% or less.
• Mg fixes O in the form of MgO to contribute to improving formability, such as bendability.
• the Mg content is preferably 0.0002% or more.
• the Mg content is preferably 0.0010% or more, more preferably 0.0015% or more.
• the Mg content is set to 0.0050% or less.
• the Mg content is preferably 0.0040% or less.
• Ca fixes S in the form of CaS to contribute 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, even more preferably 0.0010% or more.
• the Ca content is set to 0.0050% or less.
• the Ca content is preferably 0.0040% or less.
• the Sn content is preferably 0.003% or more.
• the Sn content is more preferably 0.010% or more, even more preferably 0.015% or more.
• the Sn content is preferably 0.020% or more, more preferably 0.030% or more.
• the Sn content is more than 0.1%, the castability deteriorates. Sn segregates at the prior ⁇ grain boundaries to degrade the delayed fracture resistance. Thus, when Sn is contained, the Sn content is set to 0.1% or less.
• the Sb content is preferably 0.002% or more.
• the Sb content is more preferably 0.004% or more, even more preferably 0.006% or more.
• the Sb content is more preferably 0.008% or more, even more preferably, 0.010% or more.
• the Sb content is preferably 0.015% or more, more preferably 0.030% or more.
• the Sb content is more than 0.1%, the castability deteriorates. Furthermore, Sb segregates at prior ⁇ grain boundaries to degrade the delayed fracture resistance. Thus, when Sb is contained, the Sb content is set to 0.1% or less.
• the REM is an element that makes sulfides spherical in shape to inhibit the adverse effects of sulfides on stretch flange formability, thereby improving the stretch flange formability.
• the REM content is preferably 0.0005% or more.
• the REM content is more preferably 0.0010% or more, even more preferably 0.0020% or more.
• a REM content of more than 0.0050% results in the saturation of the effect of improving the stretch flange formability.
• the REM content is set to 0.0050% or less.
• REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoid elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
• the REM concentration in the present invention is the total content of one or two or more elements selected from the above-mentioned REM elements.
• the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention.
• the optional elements are considered to be contained as incidental impurities.
• the steel sheet of the present invention has a tensile strength (TS) of 780 MPa or more.
• TS tensile strength
• the upper limit of the tensile strength is not particularly limited. However, from the viewpoint of compatibility with other properties, the tensile strength is preferably 1,300 MPa or less.
• the steel sheet of the present invention has excellent ductility, i.e., a total elongation EL of EL: 16.0% or more for TS: 780 MPa or more and less than 980 MPa, EL: 14.0% or more for TS: 980 MPa or more and less than 1,180 MPa, and EL: 12.0% or more for TS: 1,180 MPa or more are ensured.
• a hole expansion ratio ⁇ of 45% or more is ensured. This significantly improves the stability of press forming.
• a steel sheet with a tensile strength of 780 MPa or more is defined as a high strength steel sheet.
• a steel sheet having a total elongation EL of 16.0% or more for TS: 780 MPa or more and less than 980 MPa, 14.0% or more for TS: 980 MPa or more and less than 1,180 MPa, or 12.0% or more for TS: 1,180 MPa or more is defined as having excellent ductility.
• the area fraction of polygonal ferrite is set to 10% or more.
• the area fraction of polygonal ferrite is preferably set to 20% or more.
• a polygonal ferrite fraction of more than 70% can lead to failure to provide desired strength.
• the area fraction of polygonal ferrite is set to 70% or less, preferably 65% or less, and more preferably 60%.
• the total area fraction of upper bainite, tempered martensite, and lower bainite is set to 20% or more. To obtain even higher strength, the total area fraction is preferably set to 25% or more.
• a total area fraction of upper bainite, tempered martensite, and lower bainite of more than 80% results in a reduction in ductility due to excessively high strength.
• the area fraction is set to 80% or less.
• the area fraction is more preferably 75% or less, even more preferably 70% or less.
• a volume fraction of retained austenite of less than 5% can result in failure to ensure the desired ductility.
• a volume fraction of retained austenite of less than 5% can result in failure to ensure the desired strength.
• a volume fraction of retained austenite of less than 5% can result in failure to ensure the desired flangeability.
• the volume fraction of retained austenite is set to 5% or more, preferably 7% or more.
• retained austenite When retained austenite is more than 20%, stretch flange formability (flangeability) is degraded. Thus, retained austenite is set to 20% or less. Retained austenite is preferably 15% or less, more preferably 13% or less.
• the hard quenched martensite microstructure reduces ⁇ .
• the area fraction of quenched martensite needs to be controlled.
• the area fraction of quenched martensite is set to 13% or less.
• the area fraction of quenched martensite is preferably 11% or less, more preferably 9% or less.
• the area fraction of quenched martensite may be 0% or may be 5% or more.
• the steel microstructure other than the above includes the remaining microstructure.
• the area fraction of the remaining microstructure is preferably 5% or less.
• the remaining microstructure may be non-recrystallized ferrite, carbide, and pearlite. These microstructures may be determined by SEM observation as described below.
• [P] (which can also be referred to as [Pi]) is the P content (mass%).
• the maximum concentration of P in the surface layer is locally high relative to the steel components.
• this maximum concentration of P is insufficient, the shape of zinc phosphate crystals after zinc phosphate treatment is scaly. From this, the local enrichment of P on the surface is considered to provide the effect of inhibiting the formation of Si-based oxides and Si-Mn-based oxides on the surface, which have an adverse effect on phosphatability.
• [Pm] is preferably 0.030 mass% or more, more preferably 0.035 mass% or more.
• the upper limit is not particularly limited. However, [Pm] is preferably 0.100 mass% or less, more preferably 0.090 mass% or less.
• [Pm]/[Pi] is preferably 1.7 or more, more preferably 1.9 or more.
• the upper limit is not particularly limited.
• [Pm]/[Pi] is preferably 10.0 or less, more preferably 9.0 or less.
• Polygonal ferrite refers to a relatively equiaxed ferrite with almost no carbides inside. It appears as the darkest region under the SEM.
• Upper bainite is a ferrite microstructure inside of which carbides or retained austenite, which appear white under the SEM, are formed.
• the area fractions are calculated by classifying the ferrite region having an aspect ratio ⁇ 2.0 as polygonal ferrite and the region having an aspect ratio > 2.0 as upper bainite.
• the aspect ratio is determined as follows: A grain length with the greatest grain length is defined as a long-axis length a. A grain length with the greatest grain length in a direction perpendicular to the long axis is defined as a short-axis length, b.
• the aspect ratio is defined as a/b.
• Tempered martensite and lower bainite are regions that contain a lath-shaped submicrostructure and carbide precipitates therein under the SEM.
• Quenched martensite fresh martensite is a massive region that appears white under the SEM with no internal submicrostructure visible.
• the remaining microstructure refers to a microstructure containing at least one of non-recrystallized ferrite, carbides, and pearlite.
• Each can be identified with the SEM, where non-recrystallized ferrite can be observed as ferrite with dark contrast, the ferrite containing deformed microstructures introduced by a rolling process, while carbides and pearlite can be observed as microstructures with bright contrast.
• Carbides have a microstructure with a grain size of 1 ⁇ m or less.
• Pearlite has a lamellar (layer) microstructure. Thus, they can be distinguished from each other.
• the volume fraction of retained austenite is determined by chemically polishing a portion from a surface layer to the 1/4 thickness position, and then subjecting the portion to X-ray diffraction.
• a Co-K ⁇ radiation source is used for the incident X-rays.
• the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.
• the retained austenite is randomly distributed.
• the volume fraction of the retained austenite determined by X-ray diffraction can be taken as the area fraction of the retained austenite.
• sputtering analysis in the depth direction is performed with a GDS (manufactured by Shimadzu Corporation) under the conditions of Ar gas pressure: 600 Pa, high-frequency output: 35 W, measurement time interval: 0.1 seconds, and measurement time: 150 seconds to measure the amount of surface enrichment of P.
• the highest P intensity value measured during the measurement time of 150 seconds is converted into a value in units of mass% using the calibration curve, and the resulting value is defined as the maximum concentration ([Pm]).
• the conversion method to mass% is as follows: In the data obtained by measuring under the same conditions using standard materials with known amounts of P, the correlation between the intensity of the P element obtained with the GDS and the amount of P is determined. The intensity of P measured in the example is converted to a concentration.
• Pi is the P content (mass%) in the steel sheet.
• a method for manufacturing a steel sheet according to a first embodiment of the present invention includes subjecting a steel slab having the chemical composition described above to hot rolling, pickling, and cold rolling, and then subjecting the resulting cold rolled steel sheet to annealing, in which the annealing includes a soaking step of, in a furnace atmosphere having a dew point of -40°C or lower, subjecting the cold rolled steel sheet to heating to a soaking temperature that is an A c1 point + 20°C or higher and an A c3 point or lower and that is higher than or equal to Tc calculated from formula (3) and holding at the soaking temperature for 30 to 500 seconds, a first cooling step of performing cooling to a first cooling stop temperature of 350°C to 550°C at a first average cooling rate: 2 to 50 °C/s in a temperature range from the soaking temperature to the first cooling stop temperature, a second cooling step of, after stopping cooling at the first cooling stop temperature, performing holding in a temperature range of 350°C to 550°C for 10 to 60
• Examples of a method for hot-rolling a steel slab include a method in which a slab is heated and then rolled, a method in which a slab is rolled immediately after continuous casting without being heated, and a method in which a slab is subjected to a short-term heat treatment after continuous casting and then rolled.
• the hot rolling may be performed in the usual manner.
• the slab heating temperature may be 1,100°C or higher.
• the slab heating temperature may be set to 1,300°C or lower.
• the soaking temperature may be set to 20 minutes or more.
• the soaking temperature may be set to 300 minutes or less.
• the finish rolling temperature may be set to an A r3 transformation point or higher.
• the finish rolling temperature may be set to the A r3 transformation point + 200°C or lower.
• the coiling temperature may be set to 400°C or higher.
• the coiling temperature may be set to 720°C or lower.
• the coiling temperature is preferably controlled from the viewpoints of inhibiting thickness fluctuation to stably ensure high strength.
• the coiling temperature is preferably 430°C or higher.
• the coiling temperature is preferably 530°C or lower.
• the A r3 transformation point can be calculated from the components of the steel sheet and the following empirical formula (A):
• a r3 point (°C) 910 - 310 ⁇ [C] - 80 ⁇ [Mn] - 20 ⁇ [Cu] - 15 ⁇ [Cr] - 55 ⁇ [Ni] - 80 ⁇ [Mo] (where in the above formula, each [M] is the amount of element M contained (mass%) in the steel slab, and the value of an element that is not contained is zero (0)).
• the pickling may be performed in the usual manner.
• Cold rolling may be performed in the usual manner.
• the rolling reduction ratio (cumulative rolling reduction ratio) may be 30% or more.
• the rolling reduction ratio (cumulative rolling reduction ratio) may be 85% or less.
• the rolling reduction ratio is preferably controlled from the viewpoints of stably ensuring high strength and reducing anisotropy. Specifically, the rolling ratio is preferably 35% or more.
• softening annealing treatment can be performed at 450°C to 730°C in a continuous annealing line (CAL) or box annealing furnace (BAF).
• CAL continuous annealing line
• BAF box annealing furnace
• a cold rolled steel sheet (steel sheet subjected to cold rolling) produced in the usual manner is annealed under the following conditions.
• the annealing facility is not particularly limited. However, from the viewpoints of productivity and ensuring desired heating and cooling rates, annealing is preferably performed in a continuous annealing line (CAL).
• CAL continuous annealing line
• the dew point affects the formation of oxides on the surface of the steel sheet during annealing.
• a dew point higher than -40°C results in an excessive increase in the amount of oxides formed on the surface of the steel sheet, thereby leading to a deterioration in phosphatability. For this reason, the dew point is set to -40°C or lower.
• the lower limit is not particularly limited.
• the dew point is preferably -70°C or higher, more preferably -60°C or higher.
• the steel sheet obtained according to the present invention contains a soft ferrite microstructure, thereby improving ductility.
• the soaking temperature is the A c1 point + 20°C or higher and the A c3 point or lower, at which ferrite is formed.
• the soaking temperature is the A c1 point + 20°C or higher and the A c3 point or lower, and is Tc (°C) or higher.
• the holding time (soaking time) at the soaking temperature is less than 30 seconds, the formation of austenite at the soaking temperature is insufficient, polygonal ferrite increases, and the desired total area fraction of upper bainite, tempered martensite, and lower bainite cannot be obtained; thus, the desired strength may fail to be obtained. In addition, retained austenite may fail to be sufficiently obtained; thus, the desired ductility may fail to be ensured.
• the holding time (soaking time) at the above-mentioned soaking temperature is more than 500 seconds, the microstructure coarsens significantly, thereby failing to ensure the desired strength.
• the holding time (soaking time) at the annealing temperature is set to 30 to 500 seconds.
• the holding time (soaking time) at the soaking temperature is preferably 60 seconds or more, more preferably 100 seconds or more.
• the holding time (soaking time) at the soaking temperature is preferably 400 seconds or less, more preferably 300 seconds or less.
• a c1 and A c3 described above may be obtained from empirical formulae (4) and (5) below:
• a c1 723 + 22 ⁇ [C] - 18 ⁇ [Si] + 17 ⁇ [Cr] + 4.5 ⁇ [Mo] + 16 ⁇ [V]
• a c3 910 - 203 ⁇ ([C]) 1/2 + 44.7 ⁇ [Si] - 30 ⁇ [Mn] + 700 ⁇ [P] + 400 ⁇ [sol. Al] - 20 ⁇ [Cu] + 31.5 ⁇ [Mo] + 104 ⁇ [V] + 400 ⁇ [Ti] where [M] is the mass percentage of each element.
• cooling is performed at the first average cooling rate of 2 to 50 °C/s in the temperature range from the soaking temperature to the first cooling stop temperature of 350°C to 550°C.
• 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 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 "(soaking temperature (°C) - first cooling stop temperature (°C))/cooling time (seconds) from the soaking temperature to the first cooling stop temperature”.
• Upper bainite is formed at the first cooling stop temperature or lower and in the temperature range (residence temperature) of 350°C to 550°C.
• a large amount of retained ⁇ can be obtained, as compared with a manufacturing method that does not include performing holding at that temperature for 10 seconds to 60 seconds.
• This enables an improvement in ductility.
• whether the second cooling step (1), which is residence at a residence temperature of 350°C to 550°C for 10 seconds to 60 seconds, is performed or not may be determined by considering the desired characteristics.
• the bainite transformation has an incubation period. To obtain a desired amount of bainite, holding must be performed at the temperature for a certain period of time.
• the residence time is more than 60 seconds, the enrichment of C from bainite to the massive non-transformed ⁇ proceeds to lead to an increase in the amount of remaining massive microstructure, possibly resulting in a decrease in ⁇ .
• the residence time is set to 10 seconds or more and 60 seconds or less.
• the residence time is preferably 20 seconds or more.
• the residence time is preferably 50 seconds or less.
• the average cooling rate (second average cooling rate) in the temperature range from the residence end temperature to the second cooling stop temperature of 100°C or higher and 300°C or lower is less than 2 °C/s
• the bainite transformation may proceed excessively to excessively increase retained austenite and also to fail to ensure the desired amount of quenched martensite, thereby leading to a reduction in strength.
• the second average cooling rate is less than 2 °C/s, the desired ductility and flangeability may fail to be obtained.
• the second average cooling rate in the temperature range from the residence end temperature to the second cooling stop temperature of 100°C or higher and 300°C or lower is set to 2 °C/s or more.
• the second average cooling rate is preferably 5 °C/s or more, more preferably 8 °C/s or more.
• An excessively high cooling rate in this temperature range results in the deterioration of the sheet shape.
• the cooling rate (second average cooling rate) in this temperature range is set to 50 °C/s or less, preferably 40 °C/s or less.
• the predetermined area fraction of tempered martensite or lower bainite is not obtained to thereby increase the area fraction of quenched martensite after annealing, leading to a deterioration in flangeability.
• the second cooling stop temperature is set to 300°C or lower.
• the second cooling stop temperature is preferably 280°C or lower.
• a second cooling stop temperature lower than 100°C results in excessive martensite transformation to sometimes fail to obtain a predetermined amount of retained ⁇ and so forth, leading to a deterioration in ductility.
• the second cooling stop temperature is set to 100°C or higher.
• the second cooling stop temperature is preferably 220°C or higher.
• the second average cooling rate is "(residence end temperature (°C) - second cooling stop temperature (°C))/cooling time (seconds) from residence end temperature to second cooling stop temperature".
• the steel sheet is heated from the second cooling stop temperature to the reheating temperature that is the second cooling stop temperature + 50°C or higher and 450°C or lower.
• a reheating temperature lower than the second cooling stop temperature + 50°C results in failure to provide the effect of the partitioning of C from martensite to austenite, thereby failing to obtain the desired volume fraction of retained austenite.
• a reheating temperature higher than 450°C may result in excessive tempering of martensite and therefore failure to achieve the desired TS. Furthermore, the decomposition reaction of austenite occurs, thus failing to obtain the desired volume fraction of retained austenite.
• the reheating temperature is set to be the second cooling stop temperature + 50°C or higher and 450°C or lower.
• the average heating rate is set to 2.0 °C/s or more.
• the average heating rate is preferably 4.0 °C/s or more, more preferably 6.0 °C/s or more.
• the average heating rate is preferably 50.0 °C/s or less, more preferably 35.0 °C/s or less.
• the holding at the reheating temperature that is the second cooling stop temperature + 50°C or higher and 450°C or lower is performed from the viewpoints of promoting the enrichment of C to retained ⁇ and adjusting strength by tempering the formed martensite.
• a holding time of less than 60 seconds at the reheating temperature leads to insufficient tempering, resulting in the formation of high-strength martensite.
• bainite transformation does not sufficiently occur, and the enrichment of C to retained ⁇ is inhibited, thereby decreasing retained ⁇ and increasing quenched martensite.
• the desired ductility, flangeability, or both are not ensured, in some cases.
• a holding time of more than 3,000 seconds at the reheating temperature results in the occurrence of the decomposition reaction of retained austenite.
• the desired volume fraction of retained austenite cannot be obtained, failing to ensure ductility.
• the holding time at the reheating temperature is set to 60 seconds or more and 3,000 seconds or less.
• the holding time at the reheating temperature is preferably 100 seconds or more, more preferably 150 seconds or more.
• the holding time at the reheating temperature is preferably 2,500 seconds or less, more preferably 2,000 seconds or less.
• a method for manufacturing a steel sheet according to a second embodiment of the present invention includes subjecting a steel slab having the chemical composition described above to hot rolling, pickling, and cold rolling, and then subjecting a resulting cold rolled steel sheet to annealing, in which the annealing includes a soaking step of, in a furnace atmosphere having a dew point of -40°C or lower, subjecting the cold rolled steel sheet to heating to a soaking temperature that is an A c1 point + 20°C or higher and an A c3 point or lower and that is higher than or equal to Tc calculated from formula (3) and holding at the soaking temperature for 30 to 500 seconds, a cooling step of performing cooling to a cooling stop temperature of 100°C to 300°C at an average cooling rate: 2 to 50 °C/s in the temperature range from the soaking temperature to the cooling stop temperature, a reheating and holding step of performing heating from the cooling stop temperature to a reheating temperature that is the cooling stop temperature + 50°C or higher and 450°C or lower at
• the treatments in the hot rolling, the pickling, the cold rolling, and the soaking step of the annealing can be performed under the same conditions as in the first embodiment.
• the treatment in the first cooling step in the annealing in the first embodiment can be omitted.
• the cooling step in the annealing corresponds to the second cooling step in the annealing in the first embodiment.
• residence treatment holding for 10 to 60 seconds in a temperature range of 350°C to 550°C
• the second cooling step in the first embodiment can be omitted.
• the reheating and holding step in the annealing of the second embodiment can be performed under substantially the same conditions as the reheating and holding step in the annealing of the first embodiment, except that the second cooling stop temperature is the cooling stop temperature.
• the cooling step in the annealing will be mainly described below.
• the average cooling rate in the temperature range from the soaking temperature to the cooling stop temperature that is 100°C or higher and 300°C or lower is set to 2 °C/s or more.
• the average cooling rate is preferably 5 °C/s or more, more preferably 8 °C/s or more.
• the cooling rate (average cooling rate) in this temperature range is set to 50 °C/s or less, preferably 40 °C/s or less.
• a cooling stop temperature of higher than 300°C results in failure to provide the predetermined area fraction of tempered martensite or lower bainite, thereby increasing the area fraction of quenched martensite after annealing. This can result in failure to ensure retained ⁇ , possibly leading to a deterioration in flangeability.
• a cooling stop temperature of higher than 300°C can result in failure to obtain the desired flangeability.
• the cooling stop temperature is set to 300°C or lower.
• the cooling stop temperature is preferably 280°C or lower.
• a cooling stop temperature of lower than 100°C results in excessive formation of martensite transformation.
• the predetermined amount of retained austenite cannot be obtained, possibly leading to a deterioration in ductility.
• the cooling stop temperature is set to 100°C or higher.
• the cooling stop temperature is preferably 120°C or higher.
• the average cooling rate is "(soaking temperature (°C) - cooling stop temperature (°C))/cooling time (seconds) from soaking temperature to cooling stop temperature”.
• the steel sheet of the present invention obtained as described above preferably has a thickness of 0.5 mm or more.
• the thickness is preferably 3.0 mm or less.
• a member of the present invention and a method for manufacturing the member will be described below.
• the member of the present invention is obtained by subjecting the steel sheet of the present invention to at least one of forming or joining.
• the method for manufacturing a member of the present invention includes a step of subjecting the steel sheet of the present invention to at least one of forming or joining to provide a member.
• the steel sheet of the present invention has a tensile strength of 780 MPa or more, excellent ductility, flangeability, and phosphatability.
• the member obtained using the steel sheet of the present invention also has a tensile strength of 780 MPa or more, excellent ductility, flangeability, and phosphatability.
• the use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used for, for example, automobile body frame parts.
• the forming can be performed by any common processing method, such as press forming without limitation.
• the joining can be performed by common welding, such as spot welding or arc welding, or, for example, riveting or caulking without limitation.
• Slabs having chemical compositions given in Table 1 were produced by continuous casting. Each of the slabs was subjected to a hot rolling process in which the slab was heated to 1,200°C, the soaking time was 200 minutes, the finish rolling temperature was 860°C or higher, and the coiling temperature was 550°C. Then cold rolling was performed at a rolling reduction ratio of 50% to produce cold rolled steel sheet having a thickness of 1.4 mm. The cold rolled steel sheet was treated under the annealing conditions given in Table 2 to provide steel sheets of the present invention and steel sheets of comparative examples. [Table 1] Steel grade Chemical composition (mass%) [Si]/[Mn] Remarks C Si Mn P S sol.
• the steel microstructures were measured by the following method. The measurement results are presented in Table 3.
• Polygonal ferrite refers to a relatively equiaxed ferrite with almost no carbides inside. It appears as the darkest region under the SEM.
• Upper bainite is a ferrite microstructure inside of which carbides or retained austenite, which appear white under the SEM, are formed.
• the area fractions were calculated by classifying the ferrite region having an aspect ratio ⁇ 2.0 as polygonal ferrite and the region having an aspect ratio > 2.0 as upper bainite.
• the aspect ratio is determined as follows: A grain length with the greatest grain length is defined as a long-axis length a. A grain length with the greatest grain length in a direction perpendicular to the long axis is defined as a short-axis length, b. Then a/b was determined as the aspect ratio.
• Tempered martensite and lower bainite are regions that contain a lath-shaped submicrostructure and carbide precipitates therein under the SEM.
• Quenched martensite fresh martensite is a massive region that appears white under the SEM with no internal submicrostructure visible.
• the remaining microstructure refers to carbides and/or pearlite microstructures, which can be observed as microstructures with bright contrast under the SEM.
• Carbides have microstructure with a grain size of 1 ⁇ m or less.
• Pearlite has a lamellar (layer) microstructure. Thus, they can be distinguished from each other.
• the volume fraction of retained austenite is determined by chemically polishing a portion from a surface layer to the 1/4 thickness position, and then subjecting the portion to X-ray diffraction.
• a Co-K ⁇ radiation source was used for the incident X-rays.
• the volume fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.
• sputtering analysis in the depth direction was performed with a GDS (manufactured by Shimadzu Corporation) under the conditions of Ar gas pressure: 600 Pa, highfrequency output: 35 W, measurement time interval: 0.1 seconds, and measurement time: 150 seconds to measure the maximum concentration of P in the vicinity of a surface layer (within 1 ⁇ m in the thickness direction from the surface of the steel sheet).
• a calibration curve for P was obtained using standard materials having various P contents ranging from 0.005 to 0.020 mass%.
• the annealed steel sheets were subjected to degreasing and surface conditioning, and then subjected to zinc phosphate treatment using a zinc phosphate treatment solution.
• the zinc phosphate treatment was performed under the following conditions: degreasing step: treatment temperature: 40°C, treatment time: 120 seconds, and spray degreasing; surface conditioning step: pH: 9.5, treatment temperature: room temperature, and treatment time: 20 seconds; and zinc phosphate treatment step: zinc phosphate treatment solution temperature: 35°C, and treatment time: 120 seconds.
• the treatment agents used in the degreasing step, the surface conditioning step, and the zinc phosphate treatment step were an FC-E2011 degreasing agent, a PL-X surface conditioning agent, and a Palbond PB-L3065 zinc phosphate treatment solution, respectively, all manufactured by Nihon Parkerizing Co., Ltd.
• the surface zinc phosphate treatment microstructure was observed by SEM observation in five fields of view (area of 50,000 ⁇ m 2 or more) at a magnification of 1,000 ⁇ . When the area where the steel substrate was exposed was less than 10% of the total area, the surface zinc phosphate treatment microstructure was evaluated as ⁇ , and when the area was 10% or more, the surface zinc phosphate treatment microstructure was evaluated as ⁇ .
• Table 3 The results are presented in Table 3.
• Slabs having chemical compositions given in Table 1 were produced by continuous casting. Each of the slabs was subjected to a hot rolling process in which the slab was heated to 1,200°C, the soaking time was 200 minutes, the finish rolling temperature was 860°C or higher, and the coiling temperature was 550°C. Then cold rolling was performed at a rolling reduction ratio of 50% to produce cold rolled steel sheet having a thickness of 1.4 mm. The cold rolled steel sheet was treated under the annealing conditions given in Table 4 to provide steel sheets of the present invention and steel sheets of comparative examples. The same evaluations as in Example 1 were performed. The results are presented in Table 5. [Table 4] No.

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