EP3045553A1 - Stahlplatte zur heisspressung, pressgeformter artikel und verfahren zur herstellung des pressgeformten artikels - Google Patents

Stahlplatte zur heisspressung, pressgeformter artikel und verfahren zur herstellung des pressgeformten artikels Download PDF

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EP3045553A1
EP3045553A1 EP13893228.0A EP13893228A EP3045553A1 EP 3045553 A1 EP3045553 A1 EP 3045553A1 EP 13893228 A EP13893228 A EP 13893228A EP 3045553 A1 EP3045553 A1 EP 3045553A1
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Prior art keywords
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steel sheet
press
amount
area
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EP13893228.0A
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English (en)
French (fr)
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EP3045553A4 (de
Inventor
Toshio Murakami
Junya Naitou
Keisuke Okita
Shushi Ikeda
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
    • B21D22/208—Deep-drawing by heating the blank or deep-drawing associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
    • B21D22/28—Deep-drawing of cylindrical articles using consecutive dies
    • B21D22/286—Deep-drawing of cylindrical articles using consecutive dies with lubricating or cooling means
    • 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
    • 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
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—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
    • C22C38/14—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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/18—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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • 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

Definitions

  • the present invention relates to a steel sheet for hot-pressing to be used for an automotive structural component and suitable for hot-press forming, a press-formed article obtained from the steel sheet for hot-pressing, and a method for manufacturing a press-formed article. More specifically, the present invention relates to a steel sheet for hot-pressing which is useful, when forming a previously heated steel sheet (blank) into a predetermined shape, for the application to a hot-press forming method of imparting a shape, and applying a heat treatment to obtain a predetermined strength, a press-formed article, and a method useful for the manufacture of such a press-formed article.
  • a steel sheet for hot-pressing which is useful, when forming a previously heated steel sheet (blank) into a predetermined shape, for the application to a hot-press forming method of imparting a shape, and applying a heat treatment to obtain a predetermined strength, a press-formed article, and a method useful for the manufacture of such a press-formed article
  • a component is manufactured by employing a hot-press forming method where a steel sheet is heated to a given temperature (e.g., a temperature for forming an austenite phase) to lower the strength and then formed with a mold at a temperature (e.g., room temperature) lower than that of the steel sheet to impart a shape and, perform rapid-cooling heat treatment (quenching) by making use of a temperature difference therebetween so as to ensure the strength after forming.
  • a hot-press forming method is referred to by various names such as hot forming method, hot stamping method, hot stamp method and die quenching method, in addition to hot-pressing method.
  • FIG. 1 is a schematic explanatory view showing the mold configuration for carrying out the above-described hot-press forming.
  • 1 is a punch
  • 2 is a die
  • 3 is a blank holder
  • 4 is a steel sheet (blank)
  • BHF is a blank holding force
  • rp is a punch shoulder radius
  • rd is a die shoulder radius
  • CL is a punch-to-die clearance.
  • the punch 1 and the die 2 are configured such that passages 1a and 2a allowing for passing of a cooling medium (e.g., water) are formed in respective insides and the parts are cooled by passing a cooling medium through the passage.
  • a cooling medium e.g., water
  • the forming is started in a state where the steel sheet (blank) 4 is softened by heating at a two-phase zone temperature of (Ac 1 transformation point to Ac 3 transformation point) or a single-phase zone temperature equal to or more than Ac 3 transformation point. More specifically, in the state of the steel sheet 4 at a high temperature being sandwiched between the die 2 and the blank holder 3, the steel sheet 4 is pushed into a hole of the die 2 (between 2 and 2 in FIG. 1 ) by the punch 1 and formed into a shape corresponding to the outer shape of the punch 1 while reducing the outer diameter of the steel sheet 4.
  • a two-phase zone temperature of (Ac 1 transformation point to Ac 3 transformation point) or a single-phase zone temperature equal to or more than Ac 3 transformation point More specifically, in the state of the steel sheet 4 at a high temperature being sandwiched between the die 2 and the blank holder 3, the steel sheet 4 is pushed into a hole of the die 2 (between 2 and 2 in FIG. 1 ) by the punch 1 and formed into a
  • a steel sheet using 22MnB5 steel as the material As the steel sheet for hot-pressing which is widely used at present, a steel sheet using 22MnB5 steel as the material is known.
  • This steel sheet has a tensile strength of 1,500 MPa and an elongation of approximately from 6 to 8% and is applied to an impact-resistant member (a member that undergoes as little a deformation as possible at the time of collision and is not fractured).
  • an impact-resistant member a member that undergoes as little a deformation as possible at the time of collision and is not fractured.
  • its application to a component requiring a deformation, such as energy-absorbing member is difficult because of low elongation (ductility).
  • the techniques of, for example, Patent Documents 1 to 4 have also been proposed.
  • the carbon content in the steel sheet is set in various ranges to adjust the fundamental strength class of respective steel sheets, and the elongation is enhanced by introducing a ferrite having high deformability and reducing the average particle diameters of ferrite and martensite.
  • the techniques above are effective in enhancing the elongation but in view of elongation enhancement according to the strength of the steel sheet, it is still insufficient.
  • the elongation EL of a steel sheet having a tensile strength TS of 1,470 MPa or more is about 10.2% at the maximum, and further improvement is demanded.
  • a formed article of a low strength class as compared with hot-stamp formed articles which have been heretofore studied for example, a formed article having a tensile strength TS of 980 MPa class or 1,180 MPa class, also has a problem with the forming accuracy in the cold pressing, and as an improvement measure thereof, there is a need for low-strength hot pressing. In this case, the energy absorption properties in a formed article must be greatly improved.
  • a tensile strength of 1,500 MPa class is achieved on the high strength side (impact resistant site-side), but the low strength side (energy absorption site-side) stays at a maximum tensile strength of 700 MPa and an elongation EL of about 17% and in order to further improve the energy absorption properties, it is required to realize higher strength and higher ductility.
  • Non-Patent Document 1 An automotive component needs to be joined mainly by spot welding, but it is known that strength in the weld heat affected zone (HAZ) is reduced significantly and the welded joint is subject to a strength reduction (softening) (for example, Non-Patent Document 1).
  • Non-Patent Document 1 Hirosue et al. "Nippon Steel Technical Report", No. 378, pp. 15-20 (2003 )
  • the present invention has been made under these circumstances, and an object thereof is to provide a steel sheet for hot-pressing which makes it possible to easily conduct forming or working before hot pressing, obtain a press-formed article capable of achieving a high-level balance between high strength and elongation when uniform properties are required in a formed article, achieve a high-level balance between high strength and elongation according to respective regions when regions corresponding to an impact resistant site and an energy absorption site are required in a single formed article, and moreover, improve the anti-softening property in HAZ; a press-formed article exerting the above-described properties; and a method useful for manufacturing such a press-formed article.
  • the steel sheet for hot-pressing in the present invention which can attain the object above, contains:
  • the steel sheet for hot-pressing in the present invention it is also useful to contain, as the other element(s), at least one of the following (a) to (c), if desired.
  • the properties of the press-formed article are further improved according to the kind of the element that is contained according to need.
  • the steel sheet for hot-pressing in the present invention is heated at a temperature equal to or more than Ac 1 transformation point+20°C and equal to or less than Ac 3 transformation point-20°C, then press forming of the steel sheet is started, and the steel sheet is cooled to a temperature equal to or less than a temperature 100°C below a bainite transformation starting temperature Bs while ensuring an average cooling rate of 20°C/sec or more in a mold during forming and after a completion of forming.
  • the metal microstructure of the press-formed article includes retained austenite: from 3 to 20 area%, ferrite: from 30 to 80 area%, bainitic ferrite: less than 30 area% (exclusive of 0 area%), and martensite: 31 area% or less (exclusive of 0 area%), and an average equivalent-circle diameter of a Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the press-formed article is 10 nm or less, a precipitated Ti amount and a total Ti amount in a steel satisfy the relationship of the following formula (1), and a high-level balance between high strength and elongation can be achieved as uniform properties in the formed article.
  • Precipitated Ti amount mass % ⁇ 3.4 N ⁇ 0.5 ⁇ total Ti amount mass % ⁇ 3.4 N indicates the content (mass%) of N in the steel).
  • the above steel sheet for hot-pressing is used, a heating region of the steel sheet is divided into at least two regions, one region of them is heated at a temperature of Ac 3 transformation point or more and 950°C or less, another region of them is heated at a temperature equal to or more than Ac 1 transformation point+20°C and equal to or less than Ac 3 transformation point-20°C, then press forming of both regions is started, and the steel sheet is cooled to a temperature equal to or less than a martensite transformation starting temperature Ms while ensuring an average cooling rate of 20°C/sec or more in a mold in both of the regions during forming and after a completion of forming.
  • Another press-formed article in the present invention is a press-formed article of a steel sheet having the chemical component composition above, and the press-formed article has a first region having a metal microstructure including retained austenite: from 3 to 20 area% and martensite: 80 area% or more and a second region having a metal microstructure including retained austenite: from 3 to 20 area%, ferrite: from 30 to 80 area%, bainitic ferrite: less than 30 area% (exclusive of 0 area%), and martensite: 31 area% or less (exclusive of 0 area%), and an average equivalent-circle diameter of a Ti-containing precipitate having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in a steel of the second region is 10 nm or less, and a precipitated Ti amount and a total Ti amount in the steel satisfy the relationship of the following formula (1).
  • a steel sheet where the chemical component composition is strictly specified and the size of the Ti-containing precipitate is controlled, and where the precipitation rate of Ti not forming TiN is controlled, and as to the metal microstructure, the ratio of ferrite is adjusted, is used, so that by hot-pressing the steel sheet under predetermined conditions, the strength-elongation balance of the press-formed article can be made to be a high-level balance.
  • hot-pressing is performed under different conditions among a plurality of regions, an impact resistant site and an energy absorption site can be formed in a single formed article, and a high-level balance between high strength and elongation can be achieved in respective sites, and moreover, the anti-softening property in HAZ is improved.
  • FIG. 1 A schematic explanatory view showing the mold configuration for carrying out hot-press forming.
  • the present inventors have made studies from various aspects to realize a steel sheet for hot-pressing which ensures that, in the manufacture of a press-formed article by heating a steel sheet at a predetermined temperature and then hot-press forming the steel sheet, a press-formed article exhibiting good ductility (elongation) is obtained while assuring high strength after press forming.
  • the chemical component composition needs to be strictly specified, and the reason for limiting the range of each chemical component is as follows.
  • C is an important element in achieving a high-level balance between high strength and elongation when uniform properties are required in a press-formed article, or in ensuring retained austenite particularly in the low strength/high ductility site when regions corresponding to an impact resistant site and an energy absorption site are required in a single formed article.
  • C is enriched into austenite during heating in the hot press forming, so that retained austenite can be formed after quenching.
  • this element contributes to increasing the amount of martensite and increases the strength. In order to exert such effects, the C content must be 0.15% or more.
  • the C content is too large and exceeds 0.5%, the two-phase zone heating region becomes narrow, and when uniform properties are required in a formed article, the balance between high strength and elongation is not achieved at a high level, or when regions corresponding to an impact resistant site and an energy absorption site are required in a single formed article, adjustment to a metal microstructure (microstructure where predetermined amounts of ferrite, bainitic ferrite and martensite are ensured) targeted particularly in the low strength/high ductility site is difficult.
  • the lower limit of the C content is preferably 0.17% or more (more preferably 0.20% or more), and the upper limit is preferably 0.45% or less (more preferably 0.40% or less).
  • Si exerts an effect of forming retained austenite by preventing martensite from being tempered during cooling of mold quenching to form cementite or by suppressing decomposition of untransformed austenite.
  • the Si content must be 0.2% or more. If the Si content is too large and exceeds 3%, ferrite transformation is promoted during cooling after hot rolling, and TiC in the resulting ferrite is likely to be coarsely formed, and as a result, the anti-softening property in HAZ is not obtained.
  • the lower limit of the Si content is preferably 0.5% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).
  • Mn is an element effective in enhancing the hardenability during quenching and suppressing the formation of a microstructure (e.g., ferrite, pearlite, bainite) other than martensite and retained austenite during cooling of mold quenching.
  • Mn is an element capable of stabilizing austenite and is an element contributing to an increase in the retained austenite amount. In order to exert such effects, Mn must be contained in an amount of 0.5% or more.
  • the Mn content is preferably larger, but since the cost of alloying addition rises, the upper limit is set to 3% or less.
  • the lower limit of the Mn content is preferably 0.7% or more (more preferably 1.0% or more), and the upper limit is preferably 2.5% or less (more preferably 2.0% or less).
  • the P content is preferably reduced as much as possible.
  • the upper limit is set to 0.05% or less (exclusive of 0%).
  • the upper limit of the P content is preferably 0.045% or less (more preferably 0.040% or less).
  • the S content is preferably reduced as much as possible.
  • the upper limit is set to 0.05% or less (exclusive of 0%).
  • the upper limit of the S content is preferably 0.045% or less (more preferably 0.040% or less).
  • Al is useful as a deoxidizing element and allows the solute N present in the steel to be fixed as AIN, which is useful in enhancing the ductility.
  • the Al content In order to effectively exert such an effect, the Al content must be 0.01% or more. However, if the Al content is too large and exceeds 1 %, Al 2 O 3 is excessively produced to deteriorate the ductility.
  • the lower limit of the Al content is preferably 0.02% or more (more preferably 0.03% or more), and the upper limit is preferably 0.8% or less (more preferably 0.6% or less).
  • B is an element having an action of suppressing ferrite transformation, pearlite transformation and bainite transformation on the high strength site-side and therefore, contributes to preventing the formation of ferrite, pearlite and bainite during cooling after heating at a two-phase zone temperature of (Ac 1 transformation point to Ac 3 transformation point), and ensuring retained austenite.
  • B In order to exert such effects, B must be contained in an amount of 0.0002% or more, but even when this element is contained excessively over 0.01%, the effects are saturated.
  • the lower limit of the B content is preferably 0.0003% or more (more preferably 0.0005% or more), and the upper limit is preferably 0.008% or less (more preferably 0.005% or less).
  • Ti exerts an effect of improving the hardenability during quenching by fixing N and maintaining B in a solid solution state.
  • the strength reduction in HAZ can be suppressed by virtue of precipitation strengthening due to formation, as TiC, of Ti dissolved in solid during welding of the hot-stamp formed article or by virtue of an effect such as delaying increase of the dislocation density due to the dislocation movement-preventing effect of TiC.
  • the Ti-containing precipitate e.g., TiN
  • the lower limit of the Ti content is preferably 3.4[N]+0.02% or more (more preferably 3.4[N]+0.05% or more), and the upper limit is preferably 3.4[N]+0.09% or less (more preferably 3.4[N]+0.08% or less).
  • N is an element unavoidably mixed in, and the content thereof is preferably reduced as much as possible, but the reduction in an actual process is limited and therefore, the lower limit is set to 0.001%. If the N content is too large, the Ti-containing precipitate (e.g., TiN) formed is coarsened, and this precipitate works as a fracture origin to deteriorate the ductility of the steel sheet. For this reason, the upper limit is set to 0.01%.
  • the upper limit of the N content is preferably 0.008% or less (more preferably 0.006% or less).
  • the basic chemical components in the steel sheet for hot-pressing in the present invention are as described above, and the remainder is iron and unavoidable impurities (e.g., O, H) other than P, S and N.
  • the properties of the steel sheet for hot-pressing i.e., press-formed article) are further improved according to the kind of the element that is contained according to need. In the case of containing such an element, the preferable range and the reason for limitation on the range are as follows.
  • V, Nb and Zr have an effect of forming fine carbide and refining the microstructure by a pinning effect.
  • these elements are preferably contained in an amount of 0.001 % or more in total.
  • the content of these elements is preferably 0.1% or less in total.
  • the lower limit of the content of these elements is more preferably 0.005% or more (still more preferably 0.008% or more) in total, and the upper limit is more preferably 0.08% or less (still more preferably 0.06% or less) in total.
  • Cu, Ni, Cr and Mo suppress ferrite transformation, pearlite transformation and bainite transformation and therefore, effectively act to prevent the formation of ferrite, perlite and bainite during cooling after heating and ensure retained austenite.
  • these are preferably contained in an amount of 0.01 % or more in total.
  • the content is preferably larger, but since the cost of alloying addition rises, the content is preferably 1 % or less in total.
  • these elements have an action of greatly increasing the strength of austenite and put a large load on hot rolling, making it difficult to manufacture a steel sheet. Therefore, also from the standpoint of manufacturability, the content is preferably 1% or less.
  • the lower limit of the content of these elements is more preferably 0.05% or more (still more preferably 0.06% or more) in total, and the upper limit is more preferably 0.5% or less (still more preferably 0.3% or less) in total.
  • these elements refine the inclusion and therefore, effectively act to enhance the ductility.
  • these elements are preferably contained in an amount of 0.0001% or more in total.
  • the content is preferably larger, but since the effect is saturated, the content is preferably 0.01% or less in total.
  • the lower limit of the content of these elements is more preferably 0.0002% or more (still more preferably 0.0005% or more) in total, and the upper limit is more preferably 0.005% or less (still more preferably 0.003% or less) in total.
  • the Ti-containing precipitate and formula (1) is controlled for preventing softening of HAZ and such a control is originally a control required of a formed article, but these values are little changed between before and after hot-press forming. Therefore, the control needs to be already done at the stage before forming (the steel sheet for hot-pressing).
  • the Ti-containing precipitate can be maintained in a solid solution state or refined state during heating of hot pressing.
  • the amount of Ti precipitated in the press-formed article can be controlled to not more than a predetermined amount, and softening in HAZ can be prevented, whereby the joint properties can be improved.
  • the average equivalent-circle diameter of Ti-containing precipitates having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet must be 6 nm or less (requirement of (A) above).
  • the equivalent-circle diameter of the target Ti-containing precipitate is specified to be 30 nm or less, because it is necessary to control Ti-containing precipitates excluding TiN that is formed coarsely in the melting stage and thereafter does not affect the microstructural change or properties.
  • the size (average equivalent-circle diameter) of the Ti-containing precipitate is preferably 5 nm or less, more preferably 3 nm or less.
  • the Ti-containing precipitate targeted in the present invention include TiC, TiN and other Ti-containing precipitates such as TiVC, TiNbC, TiVCN and TiNbCN.
  • the average equivalent-circle diameter of Ti-containing precipitates in the press-formed article is specified to be 10 nm or less, whereas that before forming (steel sheet for hot-pressing) is specified to be 6 nm or less.
  • the reason why the size of the precipitate is specified to be larger in the formed article than in the steel sheet is that Ti is present as a fine precipitate or in a solid solution state in the steel sheet and when heated at near 800°C for 15 minutes or more, the Ti-containing precipitate is slightly coarsened.
  • the average equivalent-circle diameter of Ti-containing precipitates must be 10 nm or less, and for realizing this precipitation state in a hot-stamp formed article, it is necessary that in the state of the steel sheet for hot-stamping, the average equivalent-circle diameter of fine precipitates of 30 ⁇ m or less is adjusted to 6 nm or less and many of Ti is caused to be present in a solid solution state.
  • the majority of Ti except for Ti to be used for precipitating and fixing N must be caused to be present in a solid solution state or refined state.
  • the amount of Ti present as a precipitate other than TiN i.e., precipitated Ti amount (mass%)-3.4[N]
  • the amount of Ti present as a precipitate other than TiN needs to be an amount smaller than 0.5 times the remainder after deduction of Ti that forms TiN from total Ti (i.e., 0.5 ⁇ [(total Ti amount (mass%))-3.4[N]]) (requirement of (B) above).
  • the "precipitated Ti amount (mass%)-3.4[N]” is preferably 0.4 ⁇ [(total Ti amount (mass%))-3.4[N]] or less, more preferably 0.3 ⁇ [(total Ti amount (mass%))-3.4[N]] or less.
  • the steel material must be necessarily processed before hot stamping and is sometimes subjected to press forming, and in such a case, a predetermined amount of ferrite as soft microstructure needs to be ensured.
  • the ferrite fraction in the steel sheet for hot-pressing must be 30 area% or more (requirement of (C) above).
  • the ferrite fraction is preferably 50 area% or more, more preferably 70 area% or more.
  • the remainder of the metal microstructure is not particularly limited but includes, for example, at least any one of pearlite, bainite, martensite and retained austenite.
  • a slab prepared by melting a steel material having the above-described chemical component composition may be hot-rolled at a heating temperature: 1,100°C or more (preferably 1,150°C or more) and 1,300°C or less (preferably 1,250°C or less) and a finish rolling temperature of 850°C or more (preferably 900°C or more) and 1,050°C or less (preferably 1,000°C or less), and immediately after that, it may be cooled (rapid cooling) at an average cooling rate of 20°C/sec or more (preferably 30°C/sec or more) until 650°C or less (preferably 625°C or less) and after that, it may be cooled at 10°C/sec or less (preferably 5°C/sec or less) from 620°C to 580°C, and then, it may be cooled at an average cooling rate of 10°C/sec or more, and thereafter, it may be wound at a temperature of 350°C or more (preferably 1,150°C or more) and 1,300°C or less (preferably 1,250°C or
  • the steel sheet for hot-pressing which has the above-described chemical component composition, metal microstructure and Ti-precipitation state may be directly used for the manufacture by hot pressing or may be subjected to cold rolling at a rolling reduction of 60% or less (preferably 40% or less) after pickling and then used for the manufacture by hot pressing.
  • the microstructure thereof may be produced during heat treatment of a hot rolling material in a continuous annealing furnace or in a continuous hot-dip galvanizing line. In short, as long as the required properties such as metal microstructure and Ti precipitation state are satisfied, the steel sheet is included in the steel sheet for hot-pressing in the present invention.
  • the steel sheet is heated at a temperature equal to or more than Ac 1 transformation point+20°C (Ac 1 +20°C) and equal to or less than Ac 3 transformation point-20°C (Ac 3 -20°C) and after starting press forming, the steel sheet is cooled to a temperature equal to or less than a temperature 100°C below the bainite transformation starting temperature Bs (Bs-100°C) while ensuring an average cooling rate of 20°C/sec or more in a mold during forming as well as after the completion of forming, whereby an optimal microstructure as a formed article with low strength and high ductility can be produced in a press-formed article having a single property (hereinafter, sometimes referred to as "single-region formed article").
  • the reason for specifying each requirement in this forming method is as follows.
  • the heating temperature In a steel sheet containing a predetermined amount of ferrite, in order to cause a partial transformation to austenite while allowing part of the ferrite to remain, the heating temperature must be controlled to a predetermined range. If the heating temperature of the steel sheet is less than Ac 1 transformation point+20°C, a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of retained austenite cannot be ensured in the final microstructure (microstructure of a formed article). If the heating temperature of the steel sheet exceeds Ac 3 transformation point-20°C, the transformation amount to austenite is excessively increased during heating, and a predetermined amount of ferrite cannot be ensured in the final microstructure (microstructure of a formed article).
  • the average cooling rate during forming as well as after forming and the cooling finishing temperature must be appropriately controlled. From such a standpoint, it is necessary that the average cooling rate during forming is 20°C/sec or more and the cooling finishing temperature is equal to or less than a temperature 100°C below the bainite transformation starting temperature Bs.
  • the average cooling rate during forming is preferably 30°C/sec or more (more preferably 40°C/sec or more).
  • the cooling finishing temperature is equal to or less than a temperature 100°C below the bainite transformation starting temperature Bs, austenite present during heating is transformed to bainite or martensite while impeding production of a microstructure such as ferrite or pearlite, whereby fine austenite is caused to remain between bainite or martensite laths and a predetermined amount of retained austenite is assured while ensuring bainite and martensite.
  • the cooling finishing temperature exceeds the temperature 100°C below the bainite transformation starting temperature Bs or the average cooling rate is less than 20°C/sec, a microstructure such as ferrite and pearlite is formed, and a predetermined amount of retained austenite cannot be ensured, resulting in deterioration of the elongation (ductility) in a formed article.
  • the cooling finishing temperature is not particularly limited as long as it is equal to or less than a temperature 100°C below Bs, and the cooling finishing temperature may be, for example, equal to or less than the martensite transformation starting temperature Ms.
  • the average cooling rate need not be controlled, but the steel sheet may be cooled to room temperature at an average cooling rate of, for example, 1°C/sec or more and 100°C/sec or less.
  • Control of the average cooling rate during forming as well as after the completion of forming can be achieved by a technique of, for example, (a) controlling the temperature of the forming mold (the cooling medium shown in FIG. 1 ), or (b) controlling the thermal conductivity of the mold.
  • the metal microstructure includes retained austenite: from 3 to 20 area%, ferrite: from 30 to 80 area%: bainitic ferrite: less than 30 area% (exclusive of 0 area%), and martensite: 31 area% or less (exclusive of 0 area%), and a high-level balance between high strength and elongation can be achieved as a uniform property in a formed article.
  • the reason for setting the range of each requirement (basic microstructure) in such a hot press-formed article is as follows.
  • Retained austenite has an effect of increasing the work hardening ratio (transformation induced plasticity) and enhancing the ductility of the press-formed article by undergoing transformation to martensite during plastic deformation.
  • the retained austenite fraction In order to exert such an effect, the retained austenite fraction must be 3 area% or more. The ductility is more improved as the retained austenite fraction is higher.
  • the assurable retained austenite In the composition to be used for an automotive steel sheet, the assurable retained austenite is limited, and the upper limit is about 20 area%.
  • the lower limit of the retained austenite is preferably 5 area% or more (more preferably 7 area% or more).
  • the ferrite fraction is 30 area% or more. However, if this fraction exceeds 80 area%, the strength of a formed article cannot be ensured.
  • the lower limit of the ferrite fraction is preferably 35 area% or more (more preferably 40 area% or more), and the upper limit is preferably 75 area% or less (more preferably 70 area% or less).
  • the bainitic ferrite is a microstructure effective in enhancing the strength of a formed article but is a structure slightly lacking in ductility and therefore when present in a large amount, it deteriorates the elongation. From such a standpoint, the bainitic ferrite fraction is less than 30 area%.
  • the upper limit of the bainitic ferrite fraction is preferably 25 area% or less (more preferably 20 area% or less).
  • the martensite (as-quenched martensite) is a microstructure effective in enhancing the strength of a formed article but is a structure lacking in ductility and therefore when present in a large amount, it deteriorates the elongation. From such a standpoint, the martensite fraction is 31 area% or less.
  • the upper limit of the martensite fraction is preferably 25 area% or less (more preferably 20 area% or less).
  • microstructure other than those described above is not particularly limited, and pearlite, etc. may be contained as a remainder microstructure, but such a microstructure is inferior to other microstructures in terms of contribution to strength or contribution to ductility, and it is fundamentally preferable not to contain such a microstructure (may be even 0 area%).
  • the average equivalent-circle diameter of Ti-containing precipitates having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the press-formed article (i.e., in the steel sheet constituting the press-formed article) is 10 nm or less.
  • the average equivalent-circle diameter of the Ti-containing precipitate is preferably 8 nm or less, more preferably 6 nm or less.
  • the amount of Ti present as a precipitate other than TiN is smaller than 0.5 times the remainder Ti after deduction of Ti that forms TiN from total Ti (i.e., smaller than 0.5 ⁇ [total Ti amount (%)-3.4[N]]).
  • Ti dissolved in solid during welding is finely precipitated in HAZ or the existing fine Ti-containing precipitate suppresses recovery, etc. of the dislocation, and as a result, softening in HAZ is prevented, and the weldability is improved.
  • the "precipitated Ti amount-3.4[N]” is preferably 0.4 ⁇ [total Ti amount (mass%))-3.4[N]] or less, more preferably 0.3 ⁇ [total Ti amount (mass%))-3.4[N]] or less.
  • the properties such as strength and elongation of a press-formed article can be controlled by appropriately adjusting the press-forming conditions (heating temperature and cooling rate) and moreover, a press-formed article having high ductility (residual ductility) is obtained, making its application possible to a site (e.g., energy absorption member) to which the conventional press-formed article can be hardly applied. This is very useful in expanding the application range of a press-formed article.
  • the manufacture may be performed by diving a heating region of the steel sheet into at least two regions, heating one region (hereinafter, referred to as first region) at a temperature of Ac 3 transformation point or more and 950°C or less, heating another region (hereinafter, referred to as second region) at a temperature equal to or more than Ac 1 transformation point+20°C and equal to or less than Ac 3 transformation point-20°C, then starting press forming of both the first and second regions, and cooling the steel sheet to a temperature equal to or less than the martensite transformation starting temperature Ms while ensuring an average cooling rate of 20°C/sec or more in a mold in both of the first and second regions during forming as well as after the completion of forming.
  • first region heating one region at a temperature of Ac 3 transformation point or more and 950°C or less
  • second region heating another region at a temperature equal to or more than Ac 1 transformation point+20°C and equal to or less than Ac 3 transformation point-20°C
  • a heating region of the steel sheet is divided into at least two regions (high strength-side region and low strength-side region), and the manufacturing conditions are controlled according to respective regions, whereby a press-formed article exerting a strength-ductility balance depending on respective regions is obtained.
  • the second region corresponds to the low strength-side region, and the manufacturing conditions, microstructure and properties in this region are basically the same as those of the above-described single-region formed article.
  • the manufacturing conditions for forming the first region (corresponding to the high strength-side region) are described.
  • regions different in the heating temperature need to be formed in a single steel sheet, but the temperature can be controlled while keeping a temperature boundary portion of 50 mm or less, by using an existing heating furnace (e.g., far infrared furnace, electric furnace+shield).
  • an existing heating furnace e.g., far infrared furnace, electric furnace+shield.
  • the heating temperature In order to appropriately adjust the microstructure of the press-formed article, the heating temperature must be controlled to a predetermined range. By appropriately controlling this heating temperature, in the subsequent cooling process, transformation to a microstructure mainly including martensite can be caused to occur while ensuring a predetermined amount of retained austenite, and a desired microstructure can be produced in the region of a final hot press-formed article. If the steel sheet heating temperature in this region is less than the Ac 3 transformation point, a sufficient amount of austenite cannot be obtained during heating, and a predetermined amount of retained austenite cannot be ensured in the final microstructure (the microstructure of a formed article).
  • the heating temperature of the steel sheet is preferably Ac 3 transformation point+50°C or more and 930°C or less.
  • the average cooling rate during forming as well as after forming and the cooling finishing temperature must be appropriately controlled. From such a standpoint, the average cooling rate during forming needs to be 20°C/sec or more, and the cooling finishing temperature needs to be equal to or less than the martensite transformation starting temperature (Ms point).
  • the average cooling rate during forming is preferably 30°C/sec or more (more preferably 40°C/sec or more).
  • the cooling finishing temperature is equal to or less than the martensite transformation starting temperature (Ms point), austenite present during heating is transformed to martensite while impeding production of a microstructure such as ferrite or pearlite, whereby martensite is ensured.
  • the cooling finishing temperature is 400°C or less, preferably 300°C or less.
  • the metal microstructure, precipitate, etc. are different between the first region and the second region.
  • the metal microstructure includes retained austenite: from 3 to 20 area% (the action and effect of retained austenite are the same as above), and martensite: 80 area% or more.
  • the second region satisfies the metal microstructure and Ti state (e.g., the average equivalent-circle diameter of Ti-containing precipitates, the value of "precipitated Ti amount (mass%)-3.4[N]") which are the same as in the above-described single-region formed article.
  • the area fraction of martensite needs to be 80 area% or more.
  • the martensite fraction is preferably 85 area% or more (more preferably 90 area% or more).
  • the first region may partially contain ferrite, pearlite, bainite, etc. as a remainder microstructure.
  • Steel materials (Steel Nos. 1 to 16 and 18 to 32) having the chemical component composition shown in Tables 1 and 2 below were melted in vacuum to make an experimental slab, then hot-rolled to prepare a steel sheet, followed by cooling and subjecting to a treatment simulating the winding (sheet thickness: 1.6 mm or 3.0 mm).
  • a treatment simulating the winding sheet thickness: 1.6 mm or 3.0 mm.
  • the sample was cooled to a winding temperature, and put in a furnace heated at the winding temperature, followed by holding for 30 minutes and then cooling in the furnace.
  • Tables 3 and 4 The manufacturing conditions of the steel sheets are shown in Tables 3 and 4 below.
  • Treatment (1) The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm), then heated at 800°C for simulating continuous annealing in a heat treatment simulator, held for 90 seconds, cooled to 500°C at an average cooling rate of 20°C/sec, and held for 300 seconds.
  • Treatment (2) The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 mm), then heated at 860°C for simulating a continuous hot-dip galvanizing line in a heat treatment simulator, cooled to 400°C at an average cooling rate of 30°C/sec, held, further held under the conditions of 500°C ⁇ 10 seconds for simulating immersion in a plating bath and alloying treatment, and thereafter cooled to room temperature at an average cooling rate of 20°C/sec.
  • An extraction replica sample was prepared, and a transmission electron microscope image (magnifications: 100,000 times) of Ti-containing precipitates was photographed using a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • the Ti-containing precipitate (those having an equivalent-circle diameter of 30 nm or less) was identified by the composition analysis of precipitates by means of an energy dispersive X-ray spectrometer (EDX).
  • EDX energy dispersive X-ray spectrometer
  • At least 100 pieces of Ti-containing precipitates were measured for the area by image analysis, the equivalent-circle diameter was determined therefrom, and the average value thereof was defined as the precipitate size (average equivalent-circle diameter of Ti-containing precipitates).
  • precipitated Ti amount (mass%)-3.4[N] the amount of Ti present as a precipitate
  • extraction residue analysis was performed using a mesh having a mesh size of 0.1 ⁇ m (during extraction treatment, a fine precipitate resulting from aggregation of precipitates could also be measured), and the "precipitated Ti amount (mass%)-3.4[N]" (in Tables 5 and 6, indicated as “Precipitated Ti Amount-3.4[N]”) was determined.
  • the Ti-containing precipitate partially contained V or Nb, the contents of these precipitates were also measured.
  • Each of the steel sheets above (1.6 mm t ⁇ 150 mmx200 mm) (the thickness t of those other than the treatment (1) and (2) was adjusted to 1.6 mm by hot rolling) was heated at a predetermined temperature in a heating furnace, followed by subjecting to press forming and cooling treatment using a hat-shaped mold ( FIG. 1 ) to obtain a formed article.
  • the press forming conditions (heating temperature, average cooling rate, and rapid cooling finishing temperature during press forming) are shown in Table 7 below. [Table 7] Steel No.
  • the tensile strength (TS), elongation (total elongation EL), observation of metal microstructure (fraction of each microstructure), and hardness reduction amount after heat treatment were measured by the following methods, and the Ti precipitation state was analyzed by the method described above.
  • a tensile test was performed using a JIS No. 5 test piece, and the tensile strength (TS) and elongation (EL) were measured. At this time, the strain rate in the tensile test was set to 10 mm/sec. In the present invention, the test piece was rated "passed" when a tensile strength (TS) of 980 MPa or more and an elongation (EL) of 16% or more were satisfied and the strength-elongation balance (TS ⁇ EL) was 16,000 (MPa ⁇ %) or more.
  • TS tensile strength
  • EL elongation
  • TS ⁇ EL strength-elongation balance
  • the hardness reduction amount ( ⁇ Hv) relative to the original hardness (Vickers hardness) was measured after heating to 700°C at an average heating rate of 50°C/sec in a heat treatment simulator and then cooling at an average cooling rate of 50°C/sec.
  • the anti-softening property in HAZ was judged as good when the hardness reduction amount ( ⁇ Hv) was 50 Hv or less.
  • Each of the steel sheets above (3.0 mm t ⁇ 150 mmx200 mm) was heated at a predetermined temperature in a heating furnace and then subjected to press forming and cooling treatment in a hat-shaped mold ( FIG. 1 ) to obtain a formed article.
  • the steel sheet was placed in an infrared furnace, and a temperature difference was created by applying an infrared ray directly to a portion intended to have high strength (the steel sheet portion corresponding to the first region) so that the portion could be heated at a high temperature, and by putting a cover on a portion intended to have low strength (the steel sheet portion corresponding to the first region) so that the portion could be heated at a low temperature by blocking part of the infrared ray.
  • the formed article has regions differing in the strength in a single component.
  • the press forming conditions (heating temperature, average cooling rate, and rapid cooling finishing temperature of each region during press forming) are shown in Table 14 below.
  • Table 14 Steel No. Region Press Forming Conditions Heating Temperature (°C) Average Cooling Rate (°C/sec) Rapid Cooling Finishing Temperature (°C) 33 low strength side 810 40 300 high strength side 920 40 300 34 low strength side 800 40 300 high strength side 920 40 300 35 low strength side 820 40 300 high strength side 920 40 300 36 low strength side 800 40 300 high strength side 920 40 300 37 low strength side 800 40 300 high strength side 850 40 300
  • TS tensile strength
  • elongation total elongation EL
  • observation of metal microstructure fraction of each microstructure
  • ⁇ Hv hardness reduction amount
  • the steel sheet has a predetermined chemical component composition
  • the average equivalent-circle diameter of Ti-containing precipitates having an equivalent-circle diameter of 30 nm or less among Ti-containing precipitates contained in the steel sheet is 6 nm or less
  • the precipitated Ti amount and the total Ti amount in the steel satisfy a predetermined relationship
  • the ferrite fraction in the metal microstructure is 30 area% or more

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