EP2719787A1 - Article moulé par pressage à chaud, procédé pour produire celui-ci, et tôle d'acier mince pour moulage à la presse à chaud - Google Patents

Article moulé par pressage à chaud, procédé pour produire celui-ci, et tôle d'acier mince pour moulage à la presse à chaud Download PDF

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Publication number
EP2719787A1
EP2719787A1 EP12796568.9A EP12796568A EP2719787A1 EP 2719787 A1 EP2719787 A1 EP 2719787A1 EP 12796568 A EP12796568 A EP 12796568A EP 2719787 A1 EP2719787 A1 EP 2719787A1
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Prior art keywords
steel sheet
forming
hot press
temperature
area
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EP12796568.9A
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German (de)
English (en)
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EP2719787A4 (fr
EP2719787B1 (fr
Inventor
Junya Naitou
Toshio Murakami
Shushi Ikeda
Keisuke Okita
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
    • B21D22/22—Deep-drawing with devices for holding the edge of the blanks
    • 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
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C22C38/28—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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a hot press-formed product required to have high strength, such as used for structural members of automobile parts, a process for producing the same, and a thin steel sheet for hot press forming.
  • the present invention relates to a hot press-formed product that can be provided with a prescribed shape and at the same time heat treated to have prescribed strength when a preheated steel sheet (blank) is formed into the prescribed shape, a process for producing such a hot press-formed product, and a thin steel sheet for hot press forming.
  • a hot press-forming method for production of parts, in which method a steel sheet is heated to a prescribed temperature (e.g., a temperature for change in austenite phase) to lower its strength (i.e., make it easily formable) and then formed with a press tool at a temperature (e.g., room temperature) lower than that of the thin steel sheet, whereby the steel sheet is provided with a shape and at the same time heat treated by rapid cooling (quenching), which makes use of a temperature difference between both, to secure its strength after forming.
  • a prescribed temperature e.g., a temperature for change in austenite phase
  • a press tool at a temperature (e.g., room temperature) lower than that of the thin steel sheet
  • a steel sheet is formed in a state of low strength, and therefore, the steel sheet has decreased springback (favorable shape fixability).
  • Such a hot press-forming method has been called with various names, in addition to a hot press method, such as a hot forming method, a hot stamping method, a hot stamp method, and a die quench method.
  • Fig. 1 is a schematic explanatory view showing the structure of a press tool for carrying out hot press forming as described above (hereinafter represented sometimes by "hot stamp").
  • reference numerals 1, 2, 3, and 4 represent a punch, a die, a blank holder, and a steel sheet (blank), respectively
  • abbreviations BHF, rp, rd, and CL represent a blank holding force, a punch shoulder radius, a die shoulder radius, and a clearance between the punch and the die, respectively.
  • punch 1 and die 2 have passage la and passage 2a, respectively, formed in the inside thereof, through which passages a cooling medium (e.g., water) can be allowed to pass, and the press tool is made to have a structure so that these members can be cooled by allowing the cooling medium to pass through these passages.
  • a cooling medium e.g., water
  • the forming is started in a state where steel sheet (blank) 4 is softened by heating to a temperature within two-phase region, which is from Ac 1 transformation point to Ac 3 transformation point, or a temperature within single-phase region, which is not lower than Ac 3 transformation point. More specifically, steel sheet 4 is pushed into a cavity of die 2 (between the parts indicated by reference numerals 2 and 2 in Fig. 1 ) by punch 1 with steel sheet 4 in high-temperature state being sandwiched between die 2 and blank holder 3, thereby forming steel sheet 4 into a shape corresponding to the outer shape of punch 1 while reducing the outer diameter of steel sheet 4.
  • hot stamp e.g., hot deep drawing
  • steel sheets for hot stamp which have widely been used at present, there are known steel sheets based on 22MnB5 steel. These steel sheets have tensile strengths of 1500 MPa and elongations of about 6% to 8%, and have been applied to impact-resistant members (members neither deformed nor fractured as much as possible at the time of impact). In addition, some developments have also proceeded for C content increase and further highly strengthening (in 1500 to 1800 MPa class) based on 22MnB5 steel.
  • both functions as an impact-resistant portion and an energy-absorbing portion may sometimes be provided in parts such as B pillars or rear side members.
  • To produce such members there has mainly been used so far, for example, a method in which ultra-high tensile strength steel sheets having high strength of 980 MPa class and high tensile strength steel sheets having elongation of 440 MPa class are laser welded (to prepare a tailor welded blank, abbreviated as TWB) and then cold press formed.
  • TWB tailor welded blank
  • Non-patent Document 1 has proposed a method of laser welding 22MnB5 steel for hot stamp and a material that does not have high strength even if quenched with a press tool (to prepare a tailor welded blank, abbreviated as TWB), followed by hot stamp, in which method different strengths are provided so that tensile strength at a high strength side (i.e., impact-resistant portion side) becomes 1500 MPa (and elongation becomes 6% to 8%) and tensile strength at a low strength side (i.e., energy-absorbing portion side) becomes 440 MPa (and elongation becomes 12%).
  • some techniques have also been proposed, such as disclosed in Non-patent Documents 2 to 4.
  • Non-patent Documents 1 and 2 provide a tensile strength of not higher than 600 MPa and an elongation of about 12% to 18% at an energy-absorbing portion side, in which techniques, however, laser welding (to prepare a tailor welded blank, abbreviated as TWB) is needed previously, thereby increasing the number of steps and resulting in high cost. In addition, it results in the heating of energy-absorbing portions, which need not to be hardened originally. Therefore, these techniques are not preferred from the viewpoint of energy consumption.
  • Non-patent Document 3 is based on 22MnB5 steel, in which boron addition, however, adversely affects the robustness of strength after quenching against heating to a temperature within two-phase region, making difficult the control of strength at an energy-absorbing portion side, and further making it possible to obtain only an elongation as low as 15%.
  • Non-patent Document 4 is based on 22MnB5 steel, and therefore, this technique is not economic in that control is made in such a manner that 22MnB5, which originally has excellent hardenability, is not hardened (control of press tool cooling).
  • the present invention has been made in view of the above-described circumstances, and its object is to provide a hot press-formed product in which balance between strength and elongation can be controlled in a proper range and high ductility can be achieved, a process useful for producing such a hot press-formed product, and a thin steel sheet for hot press forming.
  • the hot press-formed product of the present invention which can achieve the above object, is a hot press-formed product, characterized by comprising a thin steel sheet formed by a hot press-forming method, and having a metallic structure that contains ferrite at 30% to 80% by area, bainitic ferrite at lower than 30% by area (not including 0% by area), martensite at 30% by area or lower (not including 0% by area), and retained austenite at 3% to 20% by area.
  • the chemical element composition thereof is not particularly limited, typical examples of which may include the following chemical element composition: C at 0.1% to 0.3% (where "%” means “% by mass", and the same applies to the below with respect to the chemical element composition); Si at 0.5% to 3%; Mn at 0.5% to 2%; P at 0.05% or lower (not including 0%); S at 0.05% or lower (not including 0%); Al at 0.01% to 0.1%; and N at 0.001% to 0.01%, and the remainder consisting of iron and unavoidable impurities.
  • the hot press-formed product of the present invention it is also useful to allow additional elements to be contained, when needed; for example, (a) B at 0.01% or lower (not including 0%) and Ti at 0.1% or lower (not including 0%); (b) one or more selected from the group consisting of Cu, Ni, Cr, and Mo at 1% or lower (not including 0%) in total; and (c) V and/or Nb at 0.1% or lower (not including 0%) in total.
  • the hot press-formed product may have further improved characteristics.
  • the following steps may be used, i.e., heating a hot-rolled steel sheet having a metallic structure that contains ferrite at 50% by area or higher, or a cold-rolled steel sheet at a reduction of 30% or higher, to a temperature not lower than Ac 1 transformation point and not higher than (Ac 1 transformation point x 0.3 + Ac 3 transformation point x 0.7); and then starting the forming of the hot-rolled steel sheet or the cold-rolled steel sheet with a press tool to produce the hot press-formed product, during which forming an average cooling rate of 20°C/sec or higher is kept in the press tool, and which forming is finished at a temperature not higher than (bainite transformation starting temperature Bs - 100°C).
  • the forming finishing temperature may preferably be controlled in a temperature range of not higher than (bainite transformation starting temperature Bs - 100°C) and not lower than martensite transformation starting temperature Ms point, in which temperature range the steel sheet may preferably be retained for 10 seconds or longer, followed by the forming.
  • the following method may be adopted as the other method.
  • the thin steel sheet When a thin steel sheet is press formed with a press tool, the thin steel sheet may be heated to a temperature not lower than Ac 3 transformation point and not higher than 1000°C, and then cooled to a temperature not higher than 700°C and not lower than 500°C at an average cooling rate of 10°C/sec or lower, and then the forming of the thin steel sheet may be started, during which forming an average cooling rate of 20°C/sec or higher may be kept in the press tool, and which forming may be finished at a temperature not higher than (bainite transformation starting temperature Bs - 100°C).
  • the forming finishing temperature may preferably be controlled in a temperature range of not higher than (bainite transformation starting temperature Bs - 100°C) and not lower than martensite transformation starting temperature Ms point, in which temperature range the steel sheet may preferably be retained for 10 seconds or longer, followed by the forming.
  • the present invention further includes a thin steel sheet for hot press forming, which is intended for producing a hot press-formed product as described above, and this thin steel sheet is characterized by being a hot-rolled steel sheet having a metallic structure that contains ferrite at 50% by area or higher, or a cold-rolled steel sheet at reduction of 30% or higher.
  • the present invention makes it possible that: retained austenite can be allowed to exist at a proper fraction to adjust the metallic structure of a hot press-formed product by properly controlling the conditions of a hot press-forming method; a hot press-formed product having more enhanced ductility (retained ductility) inherent to the formed product as compared with the case where conventional 22MnB5 steel is used; and strength and elongation can be controlled by a combination of heat treatment conditions and pre-forming steel sheet structure (initial structure).
  • the control of heating temperature within two-phase region makes it possible to provide different strengths and elongations freely.
  • Fig. 1 is a schematic explanatory view showing the structure of a press tool for carrying out hot press forming.
  • the present inventors have studied from various angles to realize a hot press-formed product having prescribed strength and further exhibiting excellent ductility (elongation) after forming when a steel sheet is heated to a prescribed temperature and then hot press formed to produce the formed product.
  • the present inventors have found that a hot press-formed product having excellent balance between strength and ductility can be achieved when the type of a steel sheet, heating temperature, and forming conditions are properly controlled so that its structure is controlled to contain retained austenite at 3% to 20% by area in the hot press forming of a steel sheet with a press tool, thereby completing the present invention.
  • High ductility of a hot press-formed product can be achieved by making its structure composed mainly of fine and high-ductility ferrite.
  • the area fraction of ferrite should be controlled to 30% by area or higher. However, when this area fraction is higher than 80% by area, prescribed strength becomes not secured.
  • the fraction of ferrite may preferably be not lower than 40% by area as the preferred lower limit (more preferably not lower than 45% by area) and not higher than 70% by area as the preferred upper limit (more preferably not higher than 65% by area).
  • the fraction of bainitic ferrite should be controlled to lower than 30% by area as the upper limit.
  • the fraction of bainitic ferrite may preferably be not lower than 5% by area as the preferred lower limit (more preferably not lower than 10% by area) and not higher than 25% by area as the preferred upper limit (more preferably not higher than 20% by area).
  • the fraction of martensite should be controlled to not higher than 30% by area as the upper limit.
  • the fraction of martensite may preferably be not lower than 5% by area as the preferred lower limit (more preferably not lower than 10% by area) and not higher than 25% by area as the preferred upper limit (more preferably not higher than 20% by area).
  • Retained austenite is transformed into martensite during plastic deformation, thereby having the effect of increasing work hardening rate (transformation-inducing plasticity) to improve the ductility of a formed product.
  • the fraction of retained austenite should be controlled to 3% by area or higher. When the fraction of retained austenite is higher, ductility becomes more excellent.
  • retained austenite that can be secured is limited, of which upper limit becomes about 20% by area.
  • the fraction of retained austenite may preferably be not lower than 5% by area as the preferred lower limit (more preferably not lower than 7% by area) and not higher than 15% by area as the preferred upper limit (more preferably not higher than 10% by area).
  • a hot-rolled steel sheet having a metallic structure that contains ferrite at 50% by area or higher, or a cold-rolled steel sheet at a reduction of 30% or higher may be used, and when the hot-rolled steel sheet or the cold-rolled steel sheet is press formed with a pres tool, the hot-rolled steel sheet or the cold-rolled steel sheet may be heated to a temperature not lower than Ac 1 transformation point and not higher than (Ac 1 transformation point x 0.3 + Ac 3 transformation point x 0.7), and then the forming of the hot-rolled steel sheet or the cold-rolled steel sheet may be started, during which forming an average cooling rate of 20°C/sec or higher may be kept in the press tool, and which forming may be finished at a temperature not higher than (bainite transformation starting temperature Bs - 100°C).
  • the reasons for defining the respective requirements in this process are as follows:
  • the type of a steel sheet should properly be selected.
  • the hot-rolled steel sheet to be used may preferably have a metallic structure that contains ferrite at 50% by area or higher.
  • the fraction of ferrite may preferably be not lower than 60% by area as the preferred lower limit (more preferably not lower than 70% by area).
  • the fraction of ferrite in the hot-rolled steel sheet may preferably be not higher than 95% by area, more preferably not higher than 90% by area.
  • a cold-rolled steel sheet it becomes an important requirement that recrystallization occurs during heating to form dislocation-free ferrite, and therefore, rolling (cold rolling) should be carried out at a prescribed reduction or higher so that recrystallization occurs.
  • a cold-rolled steel sheet it may have any structure. From this viewpoint, when a cold-rolled steel sheet is used, it is preferable to use a cold-rolled steel sheet at a reduction of 30% or higher. The reduction may preferably be 40% or higher, more preferably 50% or higher.
  • the heating temperature should be controlled in a prescribed range.
  • the proper control of the heating temperature makes it possible to cause transformation into retained austenite or martensite in the subsequent cooling step to provide the final hot press-formed product with a desired structure.
  • the heating temperature of the steel sheet is lower than Ac 1 transformation point, a sufficient fraction of austenite cannot be obtained during heating, and therefore, a prescribed fraction of retained austenite cannot be secured in the final structure (the structure of a formed product).
  • the average cooling rate during forming and the forming finishing temperature should properly be controlled.
  • the average cooling rate during forming should be controlled to 20°C/sec or higher, and the forming finishing temperature should be controlled to a temperature not higher than (bainite transformation starting temperature Bs point - 100°C, sometimes abbreviated as "Bs - 100°C").
  • the average cooling rate during forming may preferably be 30°C/sec or higher (more preferably 40°C/sec or higher).
  • the forming may be finished, while cooling to room temperature at an average cooling temperature as described above.
  • the cooling is stopped after the cooling to a temperature not higher than Bs - 100°C, and then the forming may be finished.
  • the control of the average cooling rate during forming can be achieved by a means of, for example, (a) controlling the temperature of a press tool (using a cooling medium shown in Fig. 1 above) or (b) controlling the thermal conductivity of a press tool (the same applies to the cooling in the method described below).
  • the thin steel sheet when a steel sheet is press formed with a press tool, the thin steel sheet may be heated to a temperature not lower than Ac 3 transformation point and not higher than 1000°C, and then the thin steel sheet is cooled to a temperature not higher than 700°C and not lower than 500°C at an average cooling temperature of 10°C/sec or lower, and then the forming of the thin steel sheet may be started, during which forming an average cooling rate of 20°C/sec or higher may be kept in the press tool, and which forming may be finished at a temperature not higher than (bainite transformation starting temperature Bs - 100°C).
  • the reasons for defining the respective requirements in this process are as follows (the same as described above applies to the cooling finishing temperature):
  • the heating temperature should be controlled in a prescribed range.
  • the proper control of the heating temperature makes it possible to cause transformation into a structure composed mainly of ferrite while securing a prescribed fraction of retained austenite in the subsequent cooling step to provide the final hot press-formed product with a desired structure.
  • the heating temperature of the thin steel sheet is lower than Ac 3 transformation point, a sufficient fraction of austenite cannot be obtained during heating, and therefore, a prescribed fraction of retained austenite cannot be secured in the final structure (the structure of a formed product).
  • the heating temperature of the thin steel sheet is higher than 1000°C, the grain size of austenite becomes increased during heating, and therefore, ferrite cannot be formed in the subsequent cooling.
  • This cooling step is an important step for forming ferrite during cooling.
  • the average cooling rate in this cooling step becomes higher than 10°C/sec, a prescribed fraction of ferrite cannot be secured.
  • the average cooling rate may preferably be 7°C/sec or lower, more preferably 5°C/sec or lower.
  • the cooling stopping temperature in this cooling step should be controlled to not higher than 700°C and not lower than 500°C.
  • the cooling stopping temperature may preferably be not higher than 680°C as the preferred upper limit (more preferably not higher than 660°C) and not lower than 520°C as the preferred lower limit (more preferably not lower than 550°C).
  • the forming finishing temperature should be controlled to not higher than (Bs - 100°C), but may preferably be controlled in a temperature range of not lower than martensite transformation starting temperature Ms (a temperature in this range may sometimes be referred to as the "cooling temperature changing temperature), in which temperature range retention may preferably be carried out for 10 seconds or longer.
  • the bainite transformation can proceed from super-cooled austenite to form a structure composed mainly of ferrite by retention in the above temperature range for 10 seconds or longer.
  • the retention time may preferably be 50 seconds or longer (more preferably 100 seconds or longer). When the retention time becomes too long, austenite starts to decompose, so that the fraction of retained austenite cannot become secured. Therefore, the retention time may preferably be 1000 seconds or shorter (more preferably 800 seconds or shorter).
  • Retention as described above may be any of isothermal retention, monotonic cooling, and re-heating step, so long as it is in the above temperature range.
  • retention as described above may be added at the stage when forming is finished.
  • a retention step may be added within the above temperature range during the finish of forming. After forming is finished in such a manner, the steel sheet may be left as it is for cooling or cooled at a proper cooling rate to room temperature (25°C).
  • the process for producing the hot press-formed product of the present invention can be applied, not only to the case where a hot press-formed product having a simple shape as shown in Fig. 1 above is produced (i.e., direct method), but also to the case where a formed product having a relatively complicated shape is produced, even if any of the methods described above is adopted.
  • direct method a method of cold press forming in a step prior to hot press forming
  • This method includes previously forming a difficult-to-form portion into an approximate shape by cold processing and then hot press forming the other portions.
  • a method is used to produce, for example, a formed product having three projections (profile peaks) by forming, two projections are formed by cold press forming and the third projection is then formed by hot press forming.
  • the present invention is intended for a hot press-formed product made of a high-strength steel sheet, the steel grade of which is acceptable, if it has an ordinary chemical element composition as a high-strength steel sheet, in which, however, C, Si, Mn, P, S, Al, and N contents may preferably be controlled in their respective proper ranges.
  • C, Si, Mn, P, S, Al, and N contents may preferably be controlled in their respective proper ranges.
  • C is an important element for securing retained austenite.
  • concentration of austenite during heating at a temperature within two-phase region or at a temperature within single-phase region which is not lower than Ac 3 transformation point, allows the formation of retained austenite after quenching. It further contributes to an increase of martensite fraction.
  • C content is lower than 0.1%, a prescribed fraction of retained austenite cannot be secured, making it impossible to obtain excellent ductility.
  • C content becomes higher than 0.3%, it results in that strength becomes too high.
  • C content may more preferably be not lower than 0.15% as the more preferred lower limit (still more preferably not lower than 0.20%) and not higher than 0.27% as the more preferred upper limit (still more preferably not higher than 0.25%).
  • Si suppresses austenite after heating at a temperature within two-phase region or at a temperature within single-phase region, which is not lower than Ac 3 transformation point, from being formed into cementite, and exhibits the action of increasing the fraction of retained austenite. It further exhibits the action of enhancing strength by solid solution enhancement without deteriorating ductility too much.
  • Si content is lower than 0.5%, retained austenite cannot be secured at a prescribed fraction, making it impossible to obtain excellent ductility.
  • Si content becomes higher than 3%, the degree of solid solution enhancement becomes too high, resulting in the drastic deterioration of ductility.
  • Si content may more preferably be not lower than 1.15% as the more preferred lower limit (still more preferably not lower than 1.20%) and not higher than 2.7% as the more preferred upper limit (still more preferably not higher than 2.5%).
  • Mn is an element to stabilize austenite, and it contributes to an increase of retained austenite. To make such an effect exhibited, Mn may preferably be contained at 0.5% or higher. However, when Mn content becomes excessive, the formation of ferrite is prevented, thereby making it impossible to secure a prescribed fraction of ferrite, and therefore, Mn content may preferably be 2% or lower. In addition, a considerable improvement of austenite strength increases a hot rolling load, thereby making it difficult to produce steel sheets, and therefore, even from the viewpoint of productivity, it is not preferable that Mn is contained at higher than 2%. Mn content may more preferably be not lower than 0.7% as the more preferred lower limit (still more preferably not lower than 0.9%) and not higher than 1.8% as the more preferred higher limit (still more preferably not higher than 1.6%).
  • P is an element unavoidably contained in steel and deteriorates ductility. Therefore, P content may preferably be reduced as low as possible. However, extreme reduction causes an increase of steel production cost, and reduction to 0% is difficult in the actual production. Therefore, P content may more preferably be controlled to 0.05% or lower (not including 0%). P content may more preferably be not higher than 0.045% as the more preferred upper limit (still more preferably not higher than 0.040%).
  • S is also an element unavoidably contained in steel and deteriorates ductility, similarly to P. Therefore, S content may preferably be reduced as low as possible. However, extreme reduction causes an increase of steel production cost, and reduction to 0% is difficult in the actual production. Therefore, S content may preferably be controlled to 0.05% or lower (not including 0%). S content may more preferably be not higher than 0.045% as the more preferred upper limit (still more preferably not higher than 0.040%).
  • Al is useful as a deoxidizing element and further useful for fixation of dissolved N in steel as AlN to improve ductility.
  • Al content may preferably be controlled to 0.01% or higher. However, when Al content becomes higher than 0.1%, it results in the excessive formation of Al 2 O 3 to deteriorate ductility.
  • Al content may more preferably be not lower than 0.013% as the more preferred lower limit (still more preferably not lower than 0.015%) and not higher than 0.08% as the more preferred upper limit (still more preferably not higher than 0.06%).
  • N is an element unavoidably incorporated in steel, and a reduction of
  • N content may be preferred, which has, however, a limitation in actual process. Therefore, the lower limit of N content was set to 0.001%.
  • the upper limit of N content was set to 0.01%.
  • N content may more preferably be not higher than 0.008% as the more preferred upper limit (still more preferably not higher than 0.006%).
  • the basic chemical components in the press-formed product of the present invention are as described above, and the remainder consists essentially of iron.
  • the wording "consists essentially of iron” means that the press-formed product of the present invention can contain, in addition to iron, minor components (e.g., besides Mg, Ca, Sr, and Ba, REM such as La, and carbide-forming elements such as Zr, Hf, Ta, W, and Mo) in such a level that these minor components do not inhibit the characteristics of the steel sheet of the present invention, and can further contain unavoidable impurities (e.g., O, H) other than P, S, and N.
  • minor components e.g., besides Mg, Ca, Sr, and Ba, REM such as La
  • carbide-forming elements such as Zr, Hf, Ta, W, and Mo
  • press-formed product of the present invention may contain additional elements, when needed; for example, (a) B at 0.01% or lower (not including 0%) and Ti at 0.1% or lower (not including 0%); (b) one or more selected from the group consisting of Cu, Ni, Cr, and Mo at 1% or lower (not including 0%) in total; and (c) V and/or Nb at 0.1% or lower (not including 0%) in total.
  • the press-formed product may have further improved characteristics depending on the kinds of elements contained. When these elements are contained, their preferred ranges and grounds for limitation of their ranges are as follows:
  • B is an element to prevent the formation of cementite during cooling after heating, thereby contributing to the securement of retained austenite.
  • B may preferably be contained at 0.0001% or higher, but even if B is contained beyond 0.01%, the effect is saturated.
  • B content may more preferably be not lower than 0.0002% as the more preferred lower limit (still more preferably not lower than 0.0005%) and not higher than 0.008% as the more preferred upper limit (still more preferably not higher than 0.005%).
  • Ti fixes N and maintains B in solid solution state, thereby exhibiting the effect of improving hardenability.
  • Ti may preferably be contained at least 4 times higher than N content.
  • Ti content may more preferably be not lower than 0.05% as the more preferred lower limit (still more preferably not lower than 0.06%) and not higher than 0.09% as the more preferred higher limit (still more preferably not higher than 0.08%).
  • these elements may preferably be contained at 0.01% or higher in total. Taking only characteristics into consideration, their content may be preferable when it is higher, but may preferably be controlled to 1% or lower in total because of a cost increase by alloy element addition. In addition, these elements have the action of considerably enhancing the strength of austenite, thereby increasing a hot rolling load so that the production of steel sheets becomes difficult. Therefore, even from the viewpoint of productivity, their content may preferably be controlled to 1% or lower. These elements' content may more preferably be not lower than 0.05% as the more preferred lower limit (still more preferably not lower than 0.06%) in total and not higher than 0.9% as the more preferred upper limit (still more preferably not higher than 0.8%) in total.
  • V and Nb have the effect of forming fine carbide and make structure fine by pinning effect.
  • these elements may preferably be contained at 0.001% or higher in total.
  • these elements' content may preferably be controlled to 0.1% or lower in total.
  • These elements' content may more preferably be not lower than 0.005% as the more preferred lower limit (still more preferably not lower than 0.008%) in total and not higher than 0.08% as the more preferred upper limit (still more preferably not higher than 0.06%) in total.
  • the thin steel sheet for hot press forming of the present invention may be either a non-plated steel sheet or a plated steel sheet.
  • the type of plating may be either ordinary galvanization or aluminium coating.
  • the method of plating may be either hot-dip plating or electroplating. After the plating, alloying heat treatment may be carried out, or additional plating may be carried out as multilayer plating.
  • the characteristics of formed products can be controlled by properly adjusting press forming conditions (heating temperature and cooling rate), and in addition, hot press-formed products having high ductility (retained ductility) can be obtained, so that they can be applied even to parts (e.g., energy-absorbing members), to which conventional hot press-formed products have hardly been applied; therefore, the present invention is extremely useful for extending the application range of hot press-formed products.
  • the formed products, which can be obtained in the present invention have further enhanced residual ductility as compared with formed products, of which structure was adjusted by ordinary annealing after cold press forming.
  • Table 1 further shows the calculated values of (Ac 1 transformation point x 0.3 + Ac 3 transformation point x 0.7) (these calculated values may hereinafter be referred to as "A values").
  • Ac 1 transformation point °C 723 + 29.1 x Si - 10.7 x Mn + 16.9 x Cr - 16.9 x Ni
  • the steel sheets thus obtained were heated under the respective conditions shown in Table 2 below, and then subjected to forming and cooling treatment using a high speed heat treatment testing system for steel sheets (CAS series, available from ULVAC-RIKO, Inc.), which can control an average cooling rate.
  • the steel sheets to be subjected to cooling treatment had a size of 190 mm x 70 mm (and a sheet thickness of 1.4 mm).
  • Test Nos. 1 to 14, 17 to 19, and 21 to 25 were the cases where hot-rolled steel sheets were used as steel sheets for forming.
  • Test Nos. 15, 16, and 20 were the cases where cold-rolled steel sheets were used steel sheets for forming.
  • the term "cooling 1" shown in Table 2 indicates cooling from a heating temperature to a temperature of 700°C to 500°C.
  • cooling 2 indicates cooling from then to a temperature range of [(Bs - 100°C) to Ms point] (In Test Nos. 19 to 23, forming was started at this stage).
  • the steel sheet was subjected to hot-dip galvanization to obtain a hot-dip galvanized steel sheet (Test No. 25).
  • JIS No. 5 specimens were used for tensile tests to measure tensile strength (TS) and elongation (EL). At that time, strain rate in the tensile tests was set to 10 mm/sec. In the present invention, the specimens were evaluated as "passing" when fulfilling any of the conditions that: (a) tensile strength (TS) is from 780 to 979 MPa and elongation (EL) is 25% or higher; and (b) tensile strength (TS) is from 980 to 1179 MPa and elongation (EL) is 15% or higher.
  • Test Nos. 1 to 10, 13, 15, 19 to 21, and 25 are Examples fulfilling the requirements defined in the present invention, thereby indicating that parts having satisfactory balance between strength and ductility were obtained.
  • Test Nos. 11 to 12, 14, 16 to 18, and 22 to 24 are Comparative Examples not fulfilling any of the requirements defined in the present invention, thereby deteriorating any of the characteristics. More specifically, Test No. 11 was the case where steel having insufficient C content (steel grade K shown in Table 1) was used, so that retained austenite was not secured, thereby obtaining only low elongation (EL). Test No. 12 was the case where steel having insufficient Si content (steel grade L shown in Table 1), so that retained austenite was not secured, thereby obtaining only low elongation (EL).
  • Test No. 14 was intended for conventional 2MnB5 equivalent steel (steel grade N shown in Table 1), so that retained austenite was not secured, thereby obtaining only low elongation (EL), although high strength was obtained.
  • Test No. 16 was the case where cold-rolled steel sheet having low reduction was used, so that the formed product had a structure containing ferrite at 25% by area, thereby lowering elongation (EL).
  • Test No. 17 was the case where the heating temperature was lower than Ac 1 transformation point, so that the formed product had a structure containing ferrite at 81% by area (the remainder was martensite and cementite) and retained austenite was not secured, thereby lowering elongation (EL) and tensile strength.
  • Test No. 18 was the case where the heating temperature was higher than A value, so that ferrite and bainitic ferrite were not secured by excessive formation of martensite, thereby lowering elongation (EL).
  • Test No. 22 was the case where the average cooling rate in cooling 1 was high, so that ferrite was not secured by the formation of bainitic ferrite, thereby lowering elongation (EL).
  • Test No. 23 was the case where the average cooling rate in cooling 1 was low and the cooling rate changing temperature was low, so that the formed product had a structure containing ferrite at 83% by area (the remainder was bainitic ferrite) and retained austenite was not secured, thereby lowering elongation (EL).
  • Test No. 24 was the case where the forming finishing temperature was high, so that pearlite was formed in the structure of the formed product and retained austenite was not secured, thereby lowing elongation (EL).
  • the present invention makes it possible to provide a hot press-formed product, including a steel sheet formed by a hot press-forming method, and having a metallic structure that contains ferrite at 30% to 80% by area, bainitic ferrite at lower than 30% by area (not including 0%), martensite at 30% by area or lower (not including 0%), and retained austenite at 3% to 20% by area, whereby balance between strength and elongation can be controlled in a proper range and high ductility can be achieved.

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EP3327152A1 (fr) * 2016-11-29 2018-05-30 Tata Steel UK Ltd Procédé de formage à chaud d'une ébauche d'acier

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JP5883350B2 (ja) 2016-03-15
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