US20150027602A1 - Steel sheet for hot pressing use, press-formed product, and method for manufacturing press-formed product - Google Patents

Steel sheet for hot pressing use, press-formed product, and method for manufacturing press-formed product Download PDF

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US20150027602A1
US20150027602A1 US14/382,158 US201314382158A US2015027602A1 US 20150027602 A1 US20150027602 A1 US 20150027602A1 US 201314382158 A US201314382158 A US 201314382158A US 2015027602 A1 US2015027602 A1 US 2015027602A1
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steel sheet
martensite
temperature
area
formed product
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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
    • 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
    • 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
    • B21D37/00—Tools as parts of machines covered by this subclass
    • B21D37/16—Heating or cooling
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • C22C38/02—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
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    • 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
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    • 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
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—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
    • C22C38/60—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
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D2221/00—Treating localised areas of an article
    • 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
    • C21D2221/00—Treating localised areas of an article
    • C21D2221/10—Differential treatment of inner with respect to outer regions, e.g. core and periphery, respectively

Definitions

  • the present invention relates to a steel sheet for hot pressing use used in manufacturing structural components of an automobile and suitable for hot press forming, a press-formed product obtained from such a steel sheet for hot pressing use, and a method for manufacturing the press-formed product, and relates more specifically to a steel sheet for hot pressing use that is useful in being applied to a hot press forming method securing a predetermined strength by being subjected to heat treatment simultaneously with impartation of the shape in forming a pre-heated steel sheet (blank) into a predetermined shape, a press-formed product, and a useful method for manufacturing such a press-formed product.
  • a hot press forming method has been employed for manufacturing components in which a steel sheet is heated to a predetermined temperature (for example, a temperature at which a state of an austenitic phase is achieved), the strength is lowered, the steel sheet is thereafter formed using a tool of a temperature (room temperature for example) lower than the steel sheet, thereby impartation of a shape and rapid heat treatment (quenching) utilizing the temperature difference of the both are executed simultaneously, and the strength after forming is secured.
  • a hot-press forming method is referred to by various names such as a hot forming method, hot stamping method, hot stamp method, die quench method, and the like in addition to the hot press method.
  • FIG. 1 is a schematic explanatory drawing showing a tool configuration for executing hot press forming described above, 1 in the drawing 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 punch shoulder radius, rd is die shoulder radius, and CL is punch/die clearance respectively. Also, out of these components, in the punch 1 and the die 2 , passages 1 a , 2 a through which a cooling medium (water for example) can pass are formed inside of each, and it is configured that these members are cooled by making the cooling medium pass through these passages.
  • a cooling medium water for example
  • forming is started in a state the steel sheet (blank) 4 is heated to a two-phase zone temperature (between Ac 1 transformation point and Ac 3 transformation point) or a single-phase zone temperature of Ac 3 transformation point or above and is softened. That is, in a state the steel sheet 4 in a high temperature state is sandwiched between the die 2 and the blank holder 3 , the steel sheet 4 is pressed in to the inside of a hole of the die 2 by the punch 1 , and is formed into a shape corresponding to the shape of the outer shape of the punch 1 while reducing the outside diameter of the steel sheet 4 .
  • the steel sheet As a steel sheet for hot pressing use widely used at present, one using 22Mn—B5 steel as a raw material is known.
  • the steel sheet has the tensile strength of approximately 1,500 MPa and the elongation of approximately 6-8%, and is applied to a shock resistant member (a member not causing deformation as much as possible and not causing breakage in collision).
  • a shock resistant member a member not causing deformation as much as possible and not causing breakage in collision.
  • application to a component requiring deformation such as an energy absorption member is difficult because elongation (ductility) is low.
  • the tensile strength: 1,500 MPa class is achieved on the high strength side (shock resistant portion side) according to these technologies, the maximum tensile strength is 700 MPa and the elongation EL is approximately 17% on the low strength side (energy absorption portion side), and achievement of higher strength and higher ductility are required in order to further improve the energy absorption properties.
  • the present invention has been developed in view of such circumstances as described above, and its object is to provide a steel sheet for hot pressing use capable of obtaining a hot press-formed product that can achieve the balance of high strength and elongation with a high level when uniform property is required within a formed product and is useful in obtaining a press-formed product that can achieve the balance of high strength and elongation with a high level according to each region when regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product, a press-formed product exerting the properties described above, and a useful method for manufacturing such a hot press-formed product.
  • the steel sheet for hot pressing use of the present invention which could achieve the object described above contains:
  • the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by formula (1) shown below, and the sum total of the fraction of bainite and the fraction of martensite in the metal microstructure is 80 area % or more.
  • “equivalent circle diameter” is the diameter of an imaginary circle having an area same to the size (area) of Ti containing precipitates (TiC for example) (“the average equivalent circle diameter” is the average value thereof).
  • the steel sheet for hot pressing use of the present invention it is also useful to contain, as other elements, (a) at least one element selected from the group consisting of V, Nb and Zr by 0.1% or less (exclusive of 0%) in total, (b) at least one element selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of 0%) in total, (c) at least one element selected from the group consisting of Mg, Ca and REM by 0.01% or less (exclusive of 0%) in total, and the like, and the properties of the press-formed product is improved further according to the kind of the elements contained.
  • the method for manufacturing a press-formed product of the present invention which could achieve the object described above includes the steps of using such a steel sheet for hot pressing use of the present invention as described above, heating the steel sheet to a temperature of Ac 1 transformation point+20° C. or above and Ac 3 transformation point ⁇ 20° C. or below, thereafter starting press forming, and executing cooling to a temperature or below, the temperature being lower than the bainite transformation starting temperature Bs by 100° C., while securing the average cooling rate of 20° C./s or more within a tool during forming and after completion of forming.
  • the metal microstructure includes retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, the amount of carbon in the retained austenite is 0.60% or more, and the balance of high strength and elongation can be achieved with a high level and as a uniform property within the formed product.
  • the area ratio of annealed martensite and/or annealed bainite means the total area ratio of both microstructures when both microstructures are included, and means, when either one microstructure is included, the area ratio of the microstructure.
  • another method for manufacturing a press-formed product of the present invention which could achieve the object described above includes the steps of using such a steel sheet for hot pressing use of the present invention as described above, dividing a heating region of the steel sheet into two regions, heating one region thereof to a temperature of Ac 3 transformation point or above and 950° C. or below, heating the other region to a temperature of Ac 1 transformation point+20° C. or above and Ac 3 transformation point ⁇ 20° C. or below, thereafter starting press forming, and executing cooling to a temperature of martensite transformation starting temperature Ms or below while securing the average cooling rate of 20° C./s or more within a tool during forming and after completion of forming.
  • a first region whose metal microstructure includes retained austenite: 3-20 area % and martensite: 80 area % or more and a second region whose metal microstructure includes retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area % with the amount of carbon in the retained austenite being 0.60% or more are included, the balance of high strength and elongation can be achieved with a high level according to each region, and regions corresponding to a shock resistant portion and an energy absorption portion are present within a single formed product.
  • the size of Ti-containing precipitates is controlled, the precipitation rate is controlled for Ti that does not form TiN, and the ratio of tempered hard phase (martensitic phase, bainitic phase and the like), hard phase (as-quenched martensite phase) and retained austenite phase is adjusted with respect to the metal microstructure, by hot-pressing the steel sheet under a predetermined condition, high strength-elongation balance of the press-formed product can be made a high level. Also, when hot-pressing is executed under different conditions in plural regions, the shock resistant portion and the energy absorption portion can be formed within a single formed product, the balance of high strength and elongation can be achieved with a high level for each portion.
  • FIG. 1 is a schematic explanatory drawing showing a tool configuration for executing hot press forming.
  • the present inventors carried out studies from various aspects in order to achieve such a steel sheet for hot pressing use that can obtain a press-formed product exhibiting excellent ductility (elongation) also while securing high strength after press-forming in manufacturing the press-formed product by heating a steel sheet to a predetermined temperature and thereafter executing hot press forming.
  • C is an important element in achieving the balance of high strength and elongation of a case uniform properties are required within a formed product with a high level or in securing retained austenite particularly in the low strength/high ductility portion of a case the regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product. Also, by concentration of C to austenite in heating of hot press forming, retained austenite can be formed after quenching. Also, C contributes to increase of the amount of martensite, and increases the strength. In order to exert such effects, C content should be 0.15% or more.
  • C content becomes excessive and exceeds 0.5%, two phase zone heating range becomes narrow, and the balance of high strength and elongation of a case uniform properties are required within a formed product is not achieved with a high level, or it becomes hard to adjust the metal microstructure to that targeted particularly in the low strength/high ductility portion (a microstructure in which a predetermined amount of annealed martensite and/or annealed bainite is secured) of a case the regions corresponding to a shock resistant portion and an energy absorption portion are required within a single formed product.
  • Preferable lower limit of C content is 0.17% or more (more preferably 0.20% or more), and more preferable upper limit is 0.45% or less (further more preferably 0.40% or less).
  • Si exerts an effect of forming retained austenite by suppressing that martensite is tempered during cooling of tool-quenching and cementite is formed, or that untransformed austenite is disintegrated.
  • Si content should be 0.2% or more.
  • ferrite is liable to be formed, formation of single-phase microstructure becomes hard in heating, and required fractions of bainite and martensite cannot be secured in a steel sheet for hot pressing use.
  • Preferable lower limit of Si content is 0.5% or more (more preferably 1.0% or more), and preferable upper limit is 2.5% or less (more preferably 2.0% or less).
  • Mn is an element effective in enhancing quenchability and suppressing formation of a microstructure (ferrite, pearlite, bainite and the like) other than martensite and retained austenite during cooling of tool-quenching. Also, Mn is an element stabilizing austenite, and is an element contributing to increase of retained austenite amount. In order to exert such effects, Mn should be contained by 0.5% or more. Although Mn content is preferable to be as much as possible when only properties are considered, because the cost of adding alloy increases, Mn content is made 3% or less. Preferable lower limit of Mn content is 0.7% or more (more preferably 1.0% or more), and preferable upper limit is 2.5% or less (more preferably 2.0% or less).
  • P content is made 0.05% or less (exclusive of 0%).
  • Preferable upper limit of P content is 0.045% or less (more preferably 0.040% or less).
  • S is also an element inevitably included in steel, S deteriorates ductility, and therefore S is preferable to be reduced as much as possible.
  • S content is made 0.05% or less (exclusive of 0%).
  • Preferable upper limit of S content is 0.045% or less (more preferably 0.040% or less).
  • Al is useful as a deoxidizing element, fixes solid-solution N present in steel as AlN, and is useful in improving ductility.
  • Al content should be 0.01% or more.
  • preferable lower limit of Al content is 0.02% or more (more preferably 0.03% or more), and preferable upper limit is 0.8% or less (more preferably 0.6% or less).
  • B is an element contributing to prevention of formation of ferrite, pearlite and bainite during cooling after heating to a two-phase zone temperature of (Ac 1 transformation point-Ac 3 transformation point) because B has an action of suppressing ferrite transformation, pearlite transformation and bainite transformation on the high strength portion side, and to secure retained austenite.
  • B should be contained by 0.0002% or more, however, even when B is contained excessively exceeding 0.01%, the effects saturate.
  • Preferable lower limit of B content is 0.0003% or more (more preferably 0.0005% or more), and preferable upper limit is 0.008% or less (more preferably 0.005% or less).
  • Ti develops improvement effect of quenchability by fixing N and holding B in a solid solution state.
  • Ti content becomes excessive to be more than 3.4[N]+0.1% Ti-containing precipitates formed are finely dispersed and impede the growth of martensite during cooling after heating to the two phase zone temperature, a lath (lath-like martensite) with a small aspect ratio is formed, discharging of carbon (C) to retained austenite between the laths becomes slow, and the carbon amount in the retained austenite reduces.
  • Preferable lower limit of Ti content is 3.4[N]+0.02% or more (more preferably 3.4[N]+0.05% or more), and preferable upper limit is 3.4[N]+0.09% or less (more preferably 3.4[N]+0.08% or less).
  • N is an element inevitably mixed in and is preferable to be reduced, however, because there is a limit in reducing N in an actual process, 0.001% is made the lower limit. Also, when N content becomes excessive, the ductility deteriorates because of time aging, N precipitates as BN, the quenchability improvement effect by solid-dissolved B is deteriorated, and therefore the upper limit is made 0.01%. Preferable upper limit of N content is 0.008% or less (more preferably 0.006% or less).
  • the basic chemical composition in the steel sheet for hot pressing use of the present invention is as described above, and the remainder is iron and inevitable impurities other than P, S (0, H and the like for example). Further, in the steel sheet for hot pressing use of the present invention, according to the necessity, it is also useful to further contain (a) at least one element selected from the group consisting of V, Nb and Zr by 0.1% or less (exclusive of 0%) in total, (b) at least one element selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of 0%) in total, (c) at least one element selected from the group consisting of Mg, Ca and REM (rare earth elements) by 0.01% or less (exclusive of 0%) in total, and the like, and the properties of the steel sheet for hot pressing use are improved further according to the kind of the element contained. Preferable range when these elements are contained and reasons for limiting the range are as follows.
  • V, Nb and Zr have effects of forming fine carbide and miniaturizing the microstructure by a pinning effect. In order to exert such effects, it is preferable to contain them by 0.001% or more in total. However, when the content of these elements becomes excessive, coarse carbide is formed and becomes a start point of breakage, and ductility is deteriorated adversely. Therefore, it is preferable to contain these elements by 0.1% or less in total. More preferable lower limit of the content of these elements in total is 0.005% or more (further more preferably 0.008% or more), and more preferable upper limit in total is 0.08% or less (further more preferably 0.06% or less).
  • Cu, Ni, Cr and Mo suppress ferrite transformation, pearlite transformation and bainite transformation, therefore prevent formation of ferrite, pearlite and bainite during cooling after heating, and act effectively in securing retained austenite.
  • the content is preferable to be as much as possible when only the properties are considered, because the cost for adding alloys increases, 1% or less in total is preferable.
  • More preferable lower limit of these elements in total is 0.05% or more (further more preferably 0.06% or more), and more preferable upper limit in total is 0.5% or less (further more preferably 0.3% or less).
  • these elements miniaturize inclusions, they act effectively in improving ductility. In order to exert such effects, it is preferable to contain them by 0.0001% or more in total. Although the content is preferable to be as much as possible when only the properties are considered, because the effects saturate, 0.01% or less in total is preferable. More preferable lower limit of these elements in total is 0.0002% or more (further more preferably 0.0005% or more), and more preferable upper limit in total is 0.005% or less (further more preferably 0.003% or less).
  • Ti-containing precipitates should be dispersed coarsely, and, for that purpose, it is necessary that some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 3 nm or more (the requirement of (A) described above). Also, the reason the equivalent circle diameter of the Ti-containing precipitates of the object is stipulated to be 30 nm or less is that it is necessary to control the Ti-containing precipitates and excluding TiN formed coarsely in the melting stage that does not affect microstructure change and properties thereafter.
  • the size of the Ti-containing precipitates (the average equivalent circle diameter of the Ti-containing precipitates whose equivalent circle diameter is 30 nm or less) is preferably 5 nm or more, more preferably 10 nm or more.
  • the Ti-containing precipitates of the object of the present invention also include precipitates containing Ti such as TiVC, TiNbC, TiVCN, TiNbCN and the like in addition to TiC and TiN.
  • the Ti amount present as the precipitates other than TiN is more than 0.5 times of the balance obtained by deducting Ti that forms TiN from total Ti (that is, more than 0.5 ⁇ [total Ti amount (mass %) ⁇ 3.4[N]]) (the requirement of (B) described above).
  • Precipitated Ti amount (mass %) ⁇ 3.4[N] is preferably 0.6 ⁇ [total Ti amount (mass %) ⁇ 3.4[N]] or more, more preferably 0.7 ⁇ [total Ti amount (mass %) ⁇ 3.4[N]] or more.
  • the metal microstructure cannot be controlled only by the hot pressing condition, and it is necessary to control the microstructure of the raw material steel thereof (the steel sheet for hot pressing use) beforehand.
  • the metal microstructure In order to secure the proper amount of annealed martensite and annealed bainite which are fine and largely contributing to ductility in the press forming steel sheet, it is necessary to make the sum total of the fraction of bainite and the fraction of martensite in the steel sheet 80 area % or more.
  • the sum total of the fraction of bainite and the fraction of martensite is less than 80 area %, the fraction of annealed martensite and/or annealed bainite targeted is hardly secured, and the amount of other microstructure (ferrite for example) increases to deteriorate the strength-elongation balance.
  • the sum total of the fraction of bainite and the fraction of martensite is preferably 90 area % or more, more preferably 95 area % or more.
  • the remainder of the metal microstructure is not particularly limited, at least any of ferrite, pearlite or retained austenite can be cited for example.
  • the steel sheet (the steel sheet for hot pressing use) of the present invention as described above can be manufactured by that a billet obtained by melting steel having the chemical component composition as described above is subjected to hot rolling with the heating temperature: 1,100° C. or above (preferably 1,150° C. or above) and 1,300° C. or below (preferably 1,250° C. or below) and the finish rolling temperature of 750° C. or above (preferably 780° C. or above) and 850° C. or below (preferably 830° C. or below), cooling thereafter (slow cooling: intermediate cooling) so as to stay for 10 s or more (preferably 50 s or more) between 700-750° C. (preferably 720-740° C.), cooling (rapid cooling) thereafter to 450° C.
  • the heating temperature 1,100° C. or above (preferably 1,150° C. or above) and 1,300° C. or below (preferably 1,250° C. or below) and the finish rolling temperature of 750° C. or above (preferably 780° C. or above) and 850° C
  • the method described above is for executing control so that (1) rolling is finished at a temperature range where dislocation introduced by hot rolling remains within austenite, (2) Ti-containing precipitates such as TiC and the like are formed finely on the dislocation by rapid cooling immediately thereafter, and (3) bainite transformation or martensite transformation is caused by rapid cooling and winding thereafter.
  • the steel sheet for hot pressing use having the chemical component composition, metal microstructure and Ti-precipitation state as described above may be used as it is for manufacturing by hot press forming, and may be subjected to cold rolling with the draft: 10-80% (preferably 20-70%) after pickling. Further, the steel sheet for hot pressing use or the material obtained by cold rolling thereof may be subjected to such heat treatment of heating to such a temperature range where TiC is not dissolved by 100% (1,000° C. or below: for example 870-900° C.), rapidly cooling thereafter to 450° C. or below (preferably 400° C. or below) at a cooling rate of 20° C./s or more (preferably 30° C./s or more), and holding thereafter at 450° C.
  • the steel sheet for hot pressing use of the present invention may be subjected to plating containing at least one element out of Al, Zn, Mg and Si on the surface thereof (the surface of the base steel sheet).
  • the press formed product having a single property (may be hereinafter referred to as “single region formed product”) can have an optimum microstructure of low strength and high ductility.
  • the reasons for stipulating each requirement in this forming method are as described below.
  • the heating temperature should be controlled to a predetermined range.
  • the heating temperature of the steel sheet is below Ac 1 transformation point+20° C., sufficient amount of austenite cannot be secured in heating, and a predetermined amount of retained austenite cannot be secured in the final microstructure (the microstructure of the formed product).
  • the transformation amount to austenite increases excessively in heating, and a predetermined amount of annealed martensite and annealed bainite cannot be secured in the final microstructure (the microstructure of the formed product).
  • the average cooling rate during forming is preferably 30° C./s or more (more preferably 40° C./s or more).
  • the cooling finishing temperature becomes higher than the temperature that is lower than the bainite transformation starting temperature Bs by 100° C. and the average cooling rate is less than 20° C./s, the microstructure such as ferrite, pearlite and the like is formed, a predetermined amount of retained austenite cannot be secured, and elongation (ductility) in the formed product deteriorates.
  • control of the average cooling rate basically becomes unnecessary at the stage the temperature becomes equal to or below the temperature lower than the bainite transformation starting temperature Bs by 100° C.
  • cooling may be executed to the room temperature with the average cooling rate of 1° C./s or more and 100° C./s or less for example.
  • control of the average cooling rate during forming and after completion of forming can be achieved by means such as (a) to control the temperature of the forming tool (the cooling medium shown in FIG. 1 above), and (b) to control the thermal conductivity of the tool.
  • the metal microstructure is formed of retained austenite: 3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %, the carbon amount in the retained austenite is 0.60% or more, and the balance of high strength and elongation can be achieved with a high level and as a uniform property within the formed product.
  • the reasons for setting the range of each requirement (the basic microstructure and the carbon amount in the retained austenite) in such a hot press-formed product are as described below.
  • Retained austenite has an effect of increasing the work hardening ratio (transformation induced plasticity) and improving ductility of the press-formed product by being transformed to martensite during plastic deformation.
  • the fraction of retained austenite should be made 3 area % or more. Ductility becomes more excellent as the fraction of retained austenite is higher.
  • retained austenite that can be secured is limited, and approximately 20 area % becomes the upper limit.
  • Preferable lower limit of retained austenite is 5 area % or more (more preferably 7 area % or more).
  • the main microstructure annealed martensite and/or annealed bainite which is fine and has low dislocation density
  • ductility (elongation) of the press-formed product can be enhanced while securing a predetermined strength.
  • the fraction of annealed martensite and/or annealed bainite is made 30 area % or more. However, when this fraction exceeds 87 area %, the fraction of retained austenite becomes insufficient, and ductility (residual ductility) deteriorates.
  • Preferable lower limit of annealed martensite and/or annealed bainite is 40 area % or more (more preferably 50 area % or more), and preferable upper limit is less than 80 area % (more preferably less than 70 area %).
  • the fraction of martensite as quenched is made 10 area % or more.
  • the fraction of martensite as quenched increases excessively, strength increases excessively and elongation becomes insufficient, and therefore the fraction thereof should be 67 area % or less.
  • Preferable lower limit of the fraction of martensite as quenched is 20 area % or more (more preferably 30 area % or more), and preferable upper limit is 60 area % or less (more preferably 50 area % or less).
  • ferrite, pearlite, bainite and the like may be included as the remainder microstructure, however, these microstructures are inferior in contribution to strength and contribution to ductility compared to other microstructures, and it is basically preferable not to be contained (it may also be 0 area %). However, up to 20 area % is allowable.
  • the remainder microstructure is preferably 10 area % or less, more preferably 5 area % or less.
  • the carbon amount in retained austenite affects the timing of work induced transformation of retained austenite to martensite at the time of deformation such as the tensile test and the like, and enhances the transformation induced plasticity (TRIP) effect by causing the work induced transformation at a higher strain zone as the carbon amount is higher.
  • TRIP transformation induced plasticity
  • carbon is discharged during cooling from the martensite lath formed to surrounding austenite. At that time, if Ti-carbide or carbonitride dispersed in steel is dispersed coarsely, growth of the martensite lath in the longitudinal direction proceeds without being impeded, and therefore the martensite lath narrow in the width, long, and having a large aspect ratio is obtained.
  • the carbon amount in retained austenite in steel was stipulated to be 0.60% or more. Further, although the carbon amount in retained austenite can be concentrated to approximately 0.70%, approximately 1.0% is the limit.
  • the press forming condition heat forming condition
  • the properties such as strength, elongation and the like of the press-formed product can be controlled, the press-formed product with high ductility (residual ductility) is obtained, and therefore application to a portion (energy absorption member for example) to which it has been difficult to apply conventional press-formed products becomes also possible which is very useful in expanding the application range of the press-formed product.
  • a press-formed product exerting strength-ductility balance according to each region (may be hereinafter referred to as “plural region formed product”) is obtained when the heating temperature and the condition of each region in forming are properly controlled and the microstructure of each region is adjusted in manufacturing the press-formed product by press forming of a steel sheet using a press-forming tool.
  • the plural region formed product can be manufactured as described above using the steel sheet for hot pressing use of the present invention by dividing a heating region of the steel sheet into at least two regions, heating one region thereof (hereinafter referred to as the first region) to a temperature of Ac 3 transformation point or above and 950° C. or below, heating another region (hereinafter referred to as the second region) to a temperature of Ac 1 transformation point+20° C. or above and Ac 3 transformation point ⁇ 20° C. or below, thereafter starting press forming of both of the first and second regions, and executing cooling to a temperature of martensite transformation starting temperature Ms or below while securing the average cooling rate of 20° C./s or more within a tool in both of the first and second regions during forming and after forming.
  • the heating region of the steel sheet into at least two regions (high strength side region and low strength side region) and controlling the manufacturing condition according to each region, such a press-formed product that strength-ductility balance according to each region is exerted is obtained.
  • the second region out of two regions corresponds to the low strength side region, and the manufacturing condition, microstructure and properties in this region is basically same to those of the single region formed product described above.
  • the manufacturing condition for forming the other first region (corresponding to the high strength side region) will be described.
  • this manufacturing method it is required to form regions with different heating temperature by a single steel sheet, however, by using an existing heating furnace (for example, far infrared furnace, electric furnace+shield), controlling while making the boundary section of the temperature 50 mm or less is possible.
  • an existing heating furnace for example, far infrared furnace, electric furnace+shield
  • the heating temperature In order to properly adjust the microstructure of the press-formed product, it is necessary to control the heating temperature to a predetermined range. By properly controlling this heating temperature, transformation to a microstructure mainly of martensite is caused while securing a predetermined amount of retained austenite in the cooling step after heating, and a desired microstructure can be achieved within the range of the final hot press-formed product.
  • the steel sheet heating temperature in this region is below Ac 3 transformation point, a sufficient amount of austenite cannot be obtained in heating, and a predetermined amount of retained austenite cannot be secured in the final microstructure (the microstructure of the formed product).
  • the heating temperature of the steel sheet is preferably Ac 3 transformation point+50° C. or above and 900° C. or below.
  • the average cooling rate during forming should be 20° C./s or more and the cooling finishing temperature should be martensite transformation starting temperature (Ms point) or below.
  • the average cooling rate during forming is preferably 30° C./s or more (more preferably 40° C./s or more).
  • the cooling finishing temperature is 400° C. or below, preferably 300° C. or below.
  • the metal microstructure, precipitates and the like are different between the first region and the second region.
  • the metal microstructure is of retained austenite: 3-20 area % (the action and effect of retained austenite are same to the above), and martensite: 80 area % or more.
  • the metal microstructure same to that of the single region formed product described above and 0.60% or more of the carbon amount in retained austenite are fulfilled.
  • Steel (steel Nos. 1-32) having the chemical component composition shown in Table 1 below was molten in vacuum, was made a slab for experiment, was thereafter made a steel sheet by hot rolling, was thereafter cooled, and was subjected to a treatment that simulates winding (sheet thickness: 3.0 mm).
  • the winding simulated treatment method included cooling to the winding temperature, putting the sample thereafter into a furnace heated to the winding temperature, holding for 30 min, and cooling in the furnace.
  • the manufacturing condition for the steel sheet at that time is shown in Table 2 below.
  • Ac 1 transformation point, Ac 3 transformation point, Ms point, and Bs point in Table 1 were obtained using the formula (2)-formula (5) below (refer to “The physical Metallurgy of Steels”, Leslie, Maruzen Company, Limited (1985) for example). Also, the treatments (1)-(3) shown in the remarks column in Table 2 express that each treatment (rolling, cooling, alloying) shown below was executed.
  • Ms point (° C.) 550 ⁇ 361 ⁇ [C] ⁇ 39 ⁇ [Mn] ⁇ 10 ⁇ [Cu] ⁇ 17 ⁇ [Ni] ⁇ 20 ⁇ [Cr] ⁇ 5 ⁇ [Mo]+30 ⁇ [Al] (4)
  • Treatment (2) The hot-rolled steel sheet was cold-rolled, was heated thereafter to 860° C. simulating continuous annealing, was cooled thereafter to 400° C. with the average cooling rate of 30° C./s, and was held.
  • Treatment (3) The hot-rolled steel sheet was cold-rolled, was heated thereafter to 860° C. for simulating continuous hot dip galvanizing line, was cooled thereafter to 400° C. with the average cooling rate of 30° C./s, was held, was thereafter heated further by (500° C. ⁇ 10 s), and was cooled thereafter.
  • 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
  • EDX energy dispersion type X-ray spectrometer
  • the area of the Ti-containing precipitates of at least 100 pieces was measured by image analysis, those having the equivalent circle diameter of 30 nm or less were extracted, and the average value thereof was made the size of the precipitates. Also, in the table, the size is shown as “average equivalent circle diameter of Ti-containing precipitates”.
  • precipitated Ti amount (mass %) ⁇ 3.4[N] the Ti amount present as the precipitates
  • extraction residue analysis in extraction treatment, the precipitates coagulate, and fine precipitates also can be measured
  • precipitated Ti amount (mass %) ⁇ 3.4[N] expressed as “precipitated Ti amount-3.4[N]” in Table 3
  • the Ti-containing precipitates partly contained V and Nb, the contents of these precipitates were also measured.
  • Each steel sheet described above (1.6 mm t ⁇ 150 mm ⁇ 200 mm) (with respect to those other than the treatments of (1)-(3) described above, the thickness was adjusted to 1.6 mm by hot rolling) was heated to a predetermined temperature in a heating furnace, and was thereafter subjected to press forming and cooling treatment using the tool ( FIG. 1 above) of a hat shape to obtain the press-formed product.
  • the press forming conditions (heating temperature, average cooling rate, and rapid cooling finishing temperature in press forming) are shown in Table 4 below.
  • TS tensile strength
  • elongation total elongation
  • observation of the metal microstructure the fraction of each microstructure
  • those of the steel Nos. 3, 6-10, 14, 18, 22 are the comparative examples not fulfilling any of the requirements stipulated in the present invention, and any of the properties is deteriorated. That is, in that of the steel No. 3, a steel sheet with low Si content is used, the fraction of retained austenite in the formed product is not secured, the carbon amount in the retained austenite drops, and the elongation is not enough. In that of the steel No. 6, the heating temperature in forming is high, only low elongation EL is obtained, and the strength-elongation balance (TS ⁇ EL) also deteriorates.
  • TS ⁇ EL strength-elongation balance
  • the average cooling rate in press forming is slow, pearlite and ferrite are formed, the fraction of martensite as quenched cannot be secured, and the strength-elongation balance (TS ⁇ EL) is deteriorated.
  • the rapid cooling finishing temperature is high, pearlite and ferrite are formed, the fraction of martensite as quenched cannot be secured, only low elongation is obtained, and the strength-elongation balance (TS ⁇ EL) is also deteriorated.
  • Each steel sheet described above (3.0 mm t ⁇ 150 mm ⁇ 200 mm) was heated to a predetermined temperature in a heating furnace, and was subjected thereafter to press forming and cooling treatment using the tool ( FIG. 1 above) of a hat shape to obtain the formed product.
  • TS tensile strength
  • elongation total elongation EL
  • observation of the metal microstructure the fraction of each microstructure
  • carbon amount in retained austenite in each region were obtained similarly to Example 1.
  • those of the steel Nos. 34, 36 are the comparative examples not fulfilling any of the requirements stipulated in the present invention, and any of the properties is deteriorated. That is, in that of the steel No. 34, the heating temperature in press forming is low, and the strength on the high strength side drops. In that of the steel No. 36, a steel sheet with small size of Ti-containing precipitates is used, only low strength is obtained on the high strength side, and the strength-elongation balance (TS ⁇ EL) is deteriorated on the low strength side.
  • TS ⁇ EL strength-elongation balance
  • JP-A-No. 2012-053844 Japanese Patent Application
  • the present invention is suitable to a steel sheet for hot pressing use that is used in manufacturing structural components of an automobile.

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