US20160222483A1 - Method for manufacturing press-molded article, and press-molded article - Google Patents

Method for manufacturing press-molded article, and press-molded article Download PDF

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US20160222483A1
US20160222483A1 US14/917,845 US201314917845A US2016222483A1 US 20160222483 A1 US20160222483 A1 US 20160222483A1 US 201314917845 A US201314917845 A US 201314917845A US 2016222483 A1 US2016222483 A1 US 2016222483A1
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mass
steel sheet
amount
press
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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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Assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) reassignment KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: IKEDA, SHUSHI, MURAKAMI, TOSHIO, NAITOU, JUNYA, OKITA, KEISUKE
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    • 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
    • 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
    • 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
    • C22C38/26—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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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

Definitions

  • the present invention relates to a press-formed article to be used when manufacturing an automotive structural component, and a method for manufacturing such a press-formed article. More specifically, the present invention relates to a press-formed article manufactured by applying, when forming a previously heated steel sheet (blank) into a predetermined shape, a press forming method of imparting a shape together with applying a heat treatment to obtain a predetermined strength, and a method useful for the manufacture of such a press-formed article.
  • blade previously heated steel sheet
  • 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 1 a and 2 a allowing for passing of a cooling medium (e.g., water) are formed in respective insides and the members 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 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).
  • Patent Documents 1 to 4 As the steel sheet for hot-pressing which exerts good elongation, 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 1,270 MPa or more is about 12.7% at the maximum, and further improvement is demanded.
  • Non-Patent Document 1 An automotive component needs to be joined mainly by spot welding, but in a hot-stamped formed article having, a microstructure mainly including martensite, 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).
  • Patent Document 1 JP-A-2010-65292
  • Patent Document 2 JP-A-2010-65293
  • Patent Document 3 JP-A-2010-65294
  • Patent Document 4 JP-A-2010-65295
  • 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 method useful for manufacturing a press-formed article which is capable of achieving a high-level balance between high strength and elongation and has good anti-softening property in HAZ; and a press-formed article which exerts the above properties.
  • a steel sheet for hot-pressing is heated at 900° C. or more and 1,100° C. or less, the steel sheet for hot-pressing including:
  • N from 0.001 to 0.01%, with the remainder being iron and unavoidable impurities, in which 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 steel sheet is 6 nm or less, and a precipitated Ti amount and a total Ti amount in a steel satisfy the following formula (1),
  • the steel sheet is cooled to a temperature equal to or less than a temperature 100° C. below a bainite transformation starting temperature Bs and equal to or more than a martensite transformation starting temperature Ms, while ensuring an average cooling rate of 20° C./sec or more in a mold during forming and after the completion of forming, and thereafter the steel sheet is cooled to 200° C. or less at an average cooling rate of less than 20° C./sec.
  • the “equivalent-circle diameter” is the diameter of a circle having the same area as the size (area) of a Ti-containing precipitate (e.g., TiC) when the precipitate is converted to a circle (“the average equivalent-circle diameter” is the average value thereof).
  • the steel sheet for hot-pressing to be used in the manufacturing method 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 metal microstructure of the press-formed article includes bainitic ferrite: from 60 to 97 area %, martensite: 37 area % or less, retained austenite: from 3 to 20 area %, and remainder microstructure: 5 area % or less, the average equivalent-circle diameter of 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, and a relationship of the formula (1) is satisfied, and thus, a high-level balance between high strength and elongation can be achieved as uniform properties in the formed article.
  • a steel sheet where the chemical component composition is strictly specified, the size of the Ti-containing precipitate is controlled and the precipitation rate of Ti not forming TiN is controlled is used, so that by hot-pressing the steel sheet under predetermined conditions, the strength-elongation balance of the formed article can be made to be a high-level balance and 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 press-formed article 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.
  • the C is an important element in lowering the bainite transformation starting temperature Bs to refine bainitic ferrite produced in the cooling process, and increasing the dislocation density in bainitic ferrite to enhance the strength. In addition, the amount of fine retained austenite formed between bainitic ferrite laths is increased, and a high-level balance between high strength and elongation can be ensured. If the C content is less than 0.15%, the bainite transformation starting temperature Bs elevates to bring about coarsening of bainitic ferrite and reduction in the dislocation density, and the strength of a hot press-formed article cannot be ensured. If the C content is too large and exceeds 0.5%, the strength is excessively high, and good ductility is not obtained.
  • the lower limit of the C content is preferably 0.18% 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 suppressing cementite formation due to decomposition of retained austenite formed between bainitic ferrite laths during cooling of mold quenching, and forming retained austenite thereby.
  • the Si content must be 0.2% or more. If the Si content is too large and exceeds 3%, ferrite is readily formed, making it difficult to produce a single phase of austenite during heating, and the fraction of a microstructure other than bainitic ferrite and retained austenite in the steel sheet for hot-pressing exceeds 5 area %.
  • 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 quenchability and suppressing the formation of a soft microstructure such as ferrite and pearlite during cooling of mold quenching.
  • this is an important element in lowering the bainite transformation starting temperature Bs to refine bainitic ferrite produced in the cooling process and increasing the dislocation density in bainitic ferrite to enhance the strength.
  • this is an element capable of stabilizing austenite and is an element contributing to an increase in the retained austenite amount.
  • 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 content 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.
  • an extreme reduction causes an increase in the steelmaking cost, and it is difficult in Willis of manufacture to reduce the content to 0%.
  • the content thereof 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.
  • an extreme reduction causes an increase in the steelmaking cost, and it is difficult in terms of manufacture to reduce the content to 0%.
  • the content thereof 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 AlN, 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 and pearlite transformation, 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 quenchability by fixing N and maintaining B in a solid solution state. In order to exert such an effect, it is important to contain this element in an amount larger than the stoichiometric ratio of Ti and N (3.4 times the N content) by 0.01% or more.
  • 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 more preferably 3.4[N]+0.02% or more (further preferably 3.4[N]+0.05% or more), and the upper limit is more preferably 3.4[N]+0.09% or less (further preferably 3.4[N]+0.08% or less).
  • N decrease the improvement effect of the hardenability during quenching by fixing B as BN, and thus, 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 to be used 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 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 and pearlite transformation, and therefore, effectively act to prevent the formation of ferrite and perlite 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.
  • Ti-containing precipitates needs to be finely dispersed and to this end, 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 size (average equivalent-circle diameter) of the Ti-containing precipitate is preferably 5 nm or less, more preferably 3 nm or less.
  • Examples of 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 nm 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 ⁇ 3.4[N]
  • the “precipitated Ti amount ⁇ 3.4[N]” is preferably 0.4 ⁇ [(total Ti amount) ⁇ 3.4[N]] or less, more preferably 0.3 ⁇ [(total Ti amount) ⁇ 3.4[N]] or less.
  • 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 1,000° C. or less (preferably 950° 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 500° C. or less (preferably 450° C. or less) and after that, it may be wound at a temperature of 350° C. or more (preferably 380° C. or more) and 450° C. or less (preferably 430° C. or less).
  • the steel sheet for hot-pressing which has the above-described chemical component composition 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 10 to 80% (preferably from 20 to 70%) after pickling and then used for the manufacture by hot pressing.
  • the steel sheet for hot-pressing or a cold rolled material thereof may be subjected to a heat treatment including heating at 830° C. or more (preferably 850° C. or more and 900° C. or less), then rapid cooling at a cooling rate of 20° C./sec or more (preferably 30° C./sec or more) until 500° C. or less (preferably 450° C. or less), and then holding at 500° C.
  • the surface of the steel sheet for hot-pressing (the surface of the base steel sheet) in the present invention may be subjected to plating containing one or more kinds of Al, Zn, Mg and Si.
  • the steel sheet is heated at a temperature of 900° C. or more and 1,100° C. or less, and after press forming is started, 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.) and equal to or more than the martensite transformation starting temperature Ms, 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, and then cooled to 200° C.
  • Bs bainite transformation starting temperature
  • the heating temperature of the steel sheet is less than 900° C., a sufficient amount of austenite cannot be obtained during heating, and the martensite fraction is too large in the final microstructure (microstructure of a formed article). If the heating temperature of the steel sheet exceeds 1,100° C., the austenite grain size grows during heating, the martensite transformation starting temperature Ms and martensite transformation finishing temperature Mf are elevated, retained austenite cannot be ensured during quenching, and good formability is not achieved.
  • the heating temperature is preferably 950 or more and 1,050° C. or less.
  • the heating time is preferably shorter.
  • the heating time is preferably 3,600 seconds or less, and more preferably 20 seconds 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, 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 and equal to or more than martensite transformation starting temperature Ms.
  • the average cooling rate during forming is preferably 30° C./sec or more (more preferably 40° C./sec or more).
  • 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 is performed to a temperature less than the martensite transformation starting temperature Ms, the production amount of martensite is increased and the elongation (ductility) of the formed article is deteriorated.
  • 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, from 1° C./sec or more and 100° C./sec or less.
  • the control of the average cooling rate during press 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 bainitic ferrite: from 60 to 97 area %, martensite: 37 area % or less, retained austenite: from 3 to 20 area %, and remainder microstructure: 5 area % or less, and the amount of carbon in the retained austenite is 0.50% or more, so that 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 (the amount of carbon in basic microstructure and retained austenite) in this hot press-formed article is as follows.
  • the area fraction of bainitic ferrite must be 60 area % or more. However, if this fraction exceeds 97 area %, the retained austenite fraction is insufficient, and the ductility (residual ductility) is reduced.
  • the lower limit of the bainitic ferrite fraction is preferably 65 area % or more (more preferably 70 area % or more), and the upper limit is preferably 95 area % or less (more preferably 90 area % or less).
  • the strength of a hot press-formed article can be increased by partially incorporating high-strength martensite, but if the amount thereof is large, the ductility (residual ductility) is reduced. From such a standpoint, the area fraction of martensite must be 37 area % or less.
  • the lower limit of the martensite fraction is preferably 5 area % or more (more preferably 10 area % or more), and the upper limit is preferably 30 area % or less (more preferably 25 area % or less).
  • 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 he 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).
  • ferrite, pearlite, and the like 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 %). However, an area fraction up to 5 area % is acceptable.
  • the area fraction of the remainder microstructure is preferably 4 area % or less, more preferably 3 area % or less.
  • 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 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]]).
  • the “precipitated Ti amount ⁇ 3.4[N]” is preferably 0.4 ⁇ [total Ti amount) ⁇ 3.4[N]] or less, more preferably 0.3 ⁇ [total Ti amount) ⁇ 3.4[N]] or less.
  • the properties such as strength and elongation of a 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 hot press-formed article can be hardly applied.
  • a site e.g., energy absorption member
  • Ms point (° C.) 550-361 ⁇ [C] ⁇ 39 ⁇ [Mn] ⁇ 10 ⁇ [Cu] ⁇ 17 ⁇ [Ni] ⁇ 20 ⁇ [Cr] ⁇ 5 ⁇ [Mo]+30 ⁇ [Al] (3)
  • Treatment (1) The hot-rolled steel sheet was cold-rolled (sheet thickness: 1.6 nun), 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).
  • the “precipitated Ti amount ⁇ 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 ⁇ 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 mm ⁇ 200 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, heating time average cooling rate, and rapid cooling finishing temperature during press forming) are shown in Table 4 below.
  • 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 1,180 MPa or more and an elongation (EL) of 12.0% or more were satisfied and the strength-elongation balance (TS ⁇ EL) was 16,000 (MPa. %) or more.
  • TS tensile strength
  • EL elongation
  • 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.
  • a steel sheet for hot-pressing which has a predetermined chemical component composition, where the 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 and the precipitated Ti amount and the total Ti amount in the steel satisfy a predetermined relationship, is heated at a temperature of 900° C. or more and 1,100° C. or less, and after press forming is started, the steel sheet is cooled to a temperature equal to or less than a temperature 100° C.

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