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US20170029914A1 - Hot formed steel sheet component and method for producing the same as well as steel sheet for hot forming - Google Patents

Hot formed steel sheet component and method for producing the same as well as steel sheet for hot forming Download PDF

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Publication number
US20170029914A1
US20170029914A1 US15/039,339 US201415039339A US2017029914A1 US 20170029914 A1 US20170029914 A1 US 20170029914A1 US 201415039339 A US201415039339 A US 201415039339A US 2017029914 A1 US2017029914 A1 US 2017029914A1
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steel sheet
hot
martensite
ferrite
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Inventor
Koutarou Hayashi
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Publication of US20170029914A1 publication Critical patent/US20170029914A1/en
Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • B21D35/00—Combined processes according to or processes combined with methods covered by groups B21D1/00 - B21D31/00
    • B21D35/002—Processes combined with methods covered by groups B21D1/00 - B21D31/00
    • B21D35/005—Processes combined with methods covered by groups B21D1/00 - B21D31/00 characterized by the material of the blank or the workpiece
    • 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 formed steel sheet component to be used, for example, in a machine structural component such as an automobile body structural component, and to a method for producing the same as well as to a steel sheet for hot forming.
  • the invention relates to a hot formed steel sheet component having superior ductility and bendability together with a high tensile strength, and to a method for producing the same as well as to a steel sheet for hot forming for yielding the same.
  • a hot pressing by which a heated steel sheet is press-formed as disclosed in Patent Document 1, a component having a complicated shape can be formed with high dimensional accuracy, because a steel sheet is soft and highly-ductile at a high temperature. Further, when a steel sheet is heated up to an austenite single phase region and then quenched (hardened) in a mold, enhancement of the strength of a component can be achieved at the same time due to a martensitic transformation. Therefore, such a hot pressing method is a superior forming method, which can attain a high strength of a component and superior formability of a steel sheet simultaneously.
  • Patent Document 2 discloses a pre-press quenching method, by which a steel sheet is formed in advance to a predetermined shape at room temperature, heated up to an austenite region, and then quenched in a mold to achieve a higher strength of a component. Since such a pre-press quenching method, which is an embodiment of hot pressing, can suppress deformation to be caused by a thermal strain by restraining the component with a mold, it is a superior forming method that secures a higher strength of a component and high dimensional accuracy at the same time.
  • Patent Document 1 Japanese Patent Document 2
  • Patent Document 2 in which the steel structure is substantially a martensite single phase, confronts a problem that such a requirement cannot be satisfied.
  • a hot pressed steel sheet component that is allegedly superior in terms of high strength and ductility owing to the two phase structure of ferrite and martensite, for which a steel sheet is heated to a two phase temperature range of ferrite and austenite, pressed while keeping the two phase structure, and quenched in a mold, is disclosed in Patent Document 3.
  • a steel structure is apt to become nonuniform, and the bendability and the toughness of a hot pressed steel sheet component may be deteriorated and the impact absorption characteristic may be impaired extremely.
  • Patent Document 4 discloses a hot pressed steel sheet component, which is yielded by heating a steel sheet having a steel structure with 80 volume-% or more of martensite or bainite at an Ac 1 transformation point or higher, and then quenching it in a mold to have a structure containing from 3 to 20 volume-% of retained austenite, from 30 to 97 volume-% of tempered martensite or tempered bainite, and from 0 to 67 volume-% of martensite, and is allegedly superior in terms of high strength and ductility.
  • Patent Document 5 discloses a high strength pressed component satisfying that the area rate of martensite with respect to the entire steel sheet structure is from 10% to 85%, 25% or more of the martensite is tempered martensite, the content of retained austenite is from 5% to 40%, the area rate of bainitic ferrite in bainite with respect to the entire steel sheet structure is 5% or more, and a total of the area rate of martensite, the area rate of retained austenite, and the area rate of bainitic ferrite in the bainite with respect to the entire steel sheet structure is 65% or more.
  • Patent Document 6 discloses a steel sheet for hot pressing in which a total fraction of bainite and martensite is 80% by area or more.
  • Patent Document 7 discloses a steel sheet for hot pressing in which the fraction of ferrite is 30% by area or more.
  • a specific object of the invention is to provide a hot pressed steel sheet component, which has a high tensile strength and is superior in ductility and bendability after hot pressing, not available according to conventional art as described above, and a method for producing the same as well as a steel sheet for hot pressing for yielding the same.
  • the invention is also applicable to hot forming provided with a means for cooling a steel sheet simultaneously during or immediately after forming as in the case of hot pressing. Therefore, a specific object of the invention is to provide a hot formed steel sheet component superior in ductility and bendability while having a high tensile strength after hot forming, and a method for producing the same as well as a steel sheet for hot forming for yielding the same.
  • a steel sheet for hot forming including a chemical composition in which Si is actively added to specific amounts of C and Mn, and also including a steel structure containing ferrite and at least one of martensite or bainite is to be utilized. Further, a heat treatment condition for hot forming optimum to the steel sheet for hot forming is to be applied.
  • the steel structure can be made to include a dual phase, which contains no, or not more than 5% of, retained austenite in terms of area rate, and contains ferrite, at least one of tempered martensite or tempered bainite, and martensite at predetermined area rates.
  • a hot formed steel sheet component comprising:
  • a chemical composition comprising in terms of mass % C at from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder consisting of Fe and impurities; and
  • a steel structure comprising ferrite, at least one of tempered martensite or tempered bainite, and martensite, wherein an area rate of ferrite is from 5% to 50%, a total area rate of tempered martensite and tempered bainite is from 20% to 70%, an area rate of martensite is from 25% to 75%, a total area rate of ferrite, tempered martensite, tempered bainite and martensite is 90% or more, and an area rate of retained austenite is from 0% to 5%.
  • the chemical composition comprises B at 0.0025% or less, in terms of mass % instead of a part of the Fe.
  • a chemical composition comprising in terms of mass % C at from 0.100% to 0.340%, Si at from 0.50% to 2.00%, Mn at from 1.00% to 3.00%, P at 0.050% or less, S at 0.0100% or less, sol. Al at from 0.001% to 1.000%, and N at 0.0100% or less, with a remainder consisting of Fe and impurities; and
  • a steel structure comprising ferrite with an aspect ratio of 2.0 or less, and at least one of martensite or bainite, wherein an area rate of ferrite is from 5% to 50%, a total area rate of martensite and bainite is from 45% to 90%, and a total area rate of ferrite, martensite, and bainite is 90% or more.
  • a method for producing a hot formed steel sheet component comprising: heating the steel sheet for hot forming according to any one of Paragraph [6] to Paragraph [10] to a temperature range of 720° C. or higher but lower than an Ac 3 point; performing hot forming within a time period of from 3 sec to 20 sec, during which the steel sheet is exposed to air cooling from the end of the heating until the initiation of the hot forming; and cooling to a temperature range not above an M S point at an average cooling rate of from 10° C./sec to 500° C./sec.
  • a hot formed steel sheet component according to the invention exhibits extremely superior collision characteristics, such that it can absorb an impact through a bending deformation even at a collision causing most severe plastic deformation. Therefore, a hot formed steel sheet component according to the invention is especially suitable for producing a structural component of an automobile body, however it is naturally applicable to another use such as a machine structural component.
  • FIG. 1 is a photograph showing an example of a steel structure according to the invention.
  • a numerical range expressed by “x to y” includes the values of x and y in the range as the minimum and maximum values, respectively.
  • the C content is a very important element, which enhances the hardenability of a steel and predominantly decides the strength after hot pressing (after quenching).
  • the C content should be 0.100% or more, and is preferably 0.120% or more.
  • the C content should be 0.340% or less. From the viewpoint of weldability, the C content is preferably 0.300% or less and still more preferably 0.280% or less.
  • the Si is a very effective element for enhancing the ductility of a steel heated into a two phase temperature range of ferrite and austenite, and securing a stable strength after hot pressing (after quenching).
  • the Si content should be 0.50% or more.
  • the Si content is preferably 0.70% or more, and still more preferably 1.10% or more.
  • the Si content should be 2.00% or less.
  • the Si content is preferably 1.80% or less, and still more preferably 1.50% or less.
  • Mn is a very effective element for improving the hardenability of a steel and securing a strength after hot pressing (after quenching).
  • the Mn content should be 1.00% or more.
  • the Mn content is preferably 1.10% or more, and still more preferably 1.20% or more.
  • the Mn content when the Mn content is above 3.00%, a steel structure after hot pressing (after quenching) shows an obvious band caused by Mn segregation, which compromises the toughness and remarkably deteriorates the collision characteristics. Therefore, the Mn content should be 3.00% or less. From the viewpoint of the productivity of hot rolling and cold rolling, the Mn content is preferably 2.50% or less and still more preferably 2.40% or less.
  • the steel structure of a steel sheet for hot forming can be made to a steel structure with a dual phase containing ferrite and at least one of martensite or bainite. Further, by defining the heating conditions during hot pressing according to the invention, the steel structure of a hot formed steel sheet component can be made to a desired steel structure with a dual phase.
  • P is generally an impurity contained in a steel, since it has an action to enhance the strength of a steel sheet by solid solution strengthening, it may be added actively. However, when the P content is above 0.050%, deterioration of the weldability becomes conspicuous. Therefore, the P content should be 0.050% or less. The P content is preferably 0.018% or less. For securing an effect of the action, the P content is preferably 0.003% or more.
  • S is an impurity contained in a steel, and the content is preferably as low as possible from a viewpoint of weldability.
  • the S content should be 0.0100% or less.
  • the S content is preferably 0.0030% or less, and still more preferably 0.0015% or less. From the viewpoint of desulfurization costs, the S content is preferably 0.0006% or more.
  • Al is an element having an action to make the quality of a steel robust through deoxidation.
  • the sol. Al content should be 0.001% or more, and is preferably 0.015% or more.
  • the sol. Al content should be 1.000% or less, and is preferably 0.080% or less.
  • sol. Al means acid-soluble Al, which is not in a form of an oxide such as Al 2 O 3 , and soluble in an acid.
  • N is an impurity contained in a steel, and the content is preferably as low as possible from a viewpoint of weldability.
  • the N content should be 0.0100% or less, and is preferably 0.0060% or less. From a viewpoint of denitrification cost, the N content is preferably 0.0020% or more.
  • Impurities refer to ingredients contained in a source material or ingredients entered during a process of production, which are ingredients that are not intentionally added to a steel sheet component or a steel sheet for hot forming.
  • compositions of a steel sheet component and a steel sheet for hot forming according to the invention may contain further at least one kind of the elements described below.
  • Each of the elements is an element having an effect of securing a stably high strength after hot pressing (after quenching). Therefore, one kind or two or more kinds of the elements may be added.
  • Ti, Nb and V when any of them is contained beyond 0.200%, not only hot rolling and cold rolling may become difficult, but also securement of a stably high strength may become difficult. Therefore, preferably the Ti content, the Nb content, and the V content are respectively 0.200% or less.
  • Cr when the content exceeds 1.000%, securement of a stably high strength may become difficult. Therefore, the Cr content is preferably 1.000% or less.
  • Mo when the content exceeds 1.000%, hot rolling and cold rolling may become difficult.
  • the Mo content is preferably 1.000% or less.
  • the Cu content, and the Ni content are respectively 1.000% or less.
  • the lower limit of the Ti content is preferably 0.003%.
  • the lower limit of the Nb content is preferably 0.003%.
  • the lower limit of the V content is preferably 0.003%.
  • the lower limit of the Cr content is preferably 0.005%.
  • the lower limit of the Mo content is preferably 0.005%.
  • the lower limit of the Cu content is preferably 0.005%.
  • the lower limit of the Ni content is preferably 0.005%.
  • B is an element having an action to enhance the toughness of a steel. Therefore, B may be added. However, when B is added in an amount exceeding 0.0025%, it may become difficult for the steel structure in a steel sheet for hot forming to contain ferrite, and the ductility and the bendability of a hot formed steel sheet component may be deteriorated. Therefore, the B content should be preferably 0.0025% or less. Further, for securing an effect of the action, the B content is preferably 0.0003% or more.
  • any of these elements is an element having an action to enhance the toughness through contribution to control of inclusion, especially to microdispersion of inclusions. Therefore, one kind or two or more kinds of the elements may be added. However, when the content of any element exceeds 0.0100%, deterioration of a surface condition may become conspicuous. Therefore, the content of each element is preferably 0.0100% or less. For obtaining an effect of the action more securely, the content of at least one of the elements should be preferably 0.0003% or more. Namely, preferably the lower limits of the Ca content, the Mg content, the REM content, and the Zr content are respectively 0.0003%.
  • REM represents at least one kind of a total of 17 elements including Sc, Y, and lanthanoids.
  • the above REM content means a total content of at least one kind of the elements.
  • a lanthanoid is industrially added in a form of a misch metal.
  • Bi is an element having an action to make the structure uniform and enhance the bendability. Therefore, Bi may be contained. However, when Bi is added more than 0.0100%, the hot working property may be deteriorated so that hot rolling may become difficult. Therefore, the Bi content should be preferably 0.0100% or less. For obtaining an effect of the action more securely, the Bi content should be preferably 0.0003% or more.
  • a hot formed steel sheet component according to the invention includes a steel structure containing ferrite, at least one of tempered martensite or tempered bainite, and martensite at the following predetermined area rates.
  • the steel structure may contain one of tempered martensite or tempered bainite, or contain both of the same. Further, the steel structure contains no retained austenite, or contains the same only not more than 5% by area rate.
  • FIG. 1 shows an example of a steel structure according to the invention.
  • the steel structure shown in FIG. 1 is a steel structure containing ferrite, tempered martensite, and martensite, but not containing retained austenite.
  • the area rate of ferrite When the area rate of ferrite is less than 5%, the ductility and the bendability decline. Therefore, the area rate of ferrite should be 5% or more, and is preferably 15% or more. Meanwhile, when the area rate of ferrite is above 50%, the bendability declines. Therefore, the area rate of ferrite should be 50% or less, and is preferably 40% or less,
  • the aspect ratio of ferrite is preferably 2.0 or less from a viewpoint of suppression of decline in bendability.
  • the aspect ratio of ferrite exceeds 2.0, the anisotropy of ferrite (crystal grain of ferrite) increases, and such ferrite may constitute an origin of stress concentration, and the bendability may decline. Therefore, the aspect ratio of ferrite is preferably 2.0 or less, and more preferably 1.8 or less.
  • the lower limit of the aspect ratio of ferrite is preferably 1.0.
  • the lower limit of the aspect ratio of ferrite is preferably 1.2.
  • An aspect ratio of ferrite is a value measured by the method described precisely in Example presented below.
  • the total area rate of tempered martensite and tempered bainite When the total area rate of tempered martensite and tempered bainite is less than 20%, the bendability declines. Therefore, the total area rate of tempered martensite and tempered bainite should be 20% or more, and is preferably 30% or more. Meanwhile, when the total area rate of tempered martensite and tempered martensite is above 70%, the ductility declines. Therefore, the total area rate of tempered martensite and tempered bainite should be 70% or less, and is preferably 50% or less.
  • the strength after hot pressing (after quenching) can be enhanced.
  • the area rate of martensite is less than 25%, it becomes difficult to secure a high tensile strength (for example, tensile strength of 980 MPa or more) after hot pressing (after quenching). Therefore, the area rate of martensite should be 25% or more. Meanwhile, when the area rate of martensite is above 75%, the ductility declines. Therefore, the area rate of martensite should be 75% or less, and is preferably 50% or less.
  • martensite means both of as-quenched martensite, and martensite after age hardening formed by age-hardening as-quenched martensite.
  • area rate of martensite means the total area rate of as-quenched martensite, and martensite after age hardening formed by age-hardening as-quenched martensite.
  • a hot formed steel sheet component according to the invention has a structure containing ferrite, tempered martensite, tempered bainite, and martensite.
  • a phase or a structure other than the above one kind or two or more kinds of bainite, retained austenite, cementite, or pearlite may be mixed in.
  • the percentage of such a phase or a structure other than ferrite, tempered martensite, tempered bainite, and martensite exceeds 10%, an intended characteristic may not be obtained due to an influence of the phase or structure.
  • the mixture of a phase or a structure other than ferrite, tempered martensite, tempered bainite, and martensite should be 10% or less, and is preferably 5% or less.
  • the total area rate of ferrite, tempered martensite, tempered bainite, and martensite should be 90% or more, and preferably 95% or more.
  • the upper limit of the total area rate of ferrite, tempered martensite, tempered bainite, and martensite is 100%.
  • retained austenite should not be contained, or even if it should be contained, the area rate of retained austenite should be 5% or less, and is preferably 3% or less.
  • the area rate of retained austenite is most preferably 0%.
  • the area rate of each phase and structure in the steel structure of a hot formed steel sheet component is a value measured by the method described precisely in Example presented below.
  • a steel sheet component according to the invention means a component hot formed from a steel sheet, and includes, for example, a steel sheet component formed by hot pressing.
  • a door guard bar to be used as an automobile body structural component.
  • a bumper reinforcement As a machine structural component, there is a hot formed steel pipe for a building structure produced from a steel sheet as a source material.
  • a hot formed steel sheet component according to the invention has a tensile strength (TS) of 980 MPa or more, which is adequate to contribute to weight reduction of an automobile.
  • TS tensile strength
  • the steel structure after hot pressing should better, as described above, not be a martensite single phase, but rather be a dual phase structure, in which the area rate of ferrite is from 5% to 50%, the total area rate of tempered martensite, and tempered bainite is from 20% to 70%, the area rate of martensite is from 25% to 75%, the total area rate of ferrite, tempered martensite, tempered bainite, and martensite is 90% or more, as well as the area rate of retained austenite is from 0% to 5%.
  • a steel sheet which is a source material for hot forming, a steel sheet including the above chemical composition, and a steel structure (dual phase structure) containing ferrite with an aspect ratio of 2.0 or less, and at least one of martensite or bainite, in which structure the area rate of ferrite is from 5% to 50%, the total area rate of martensite and bainite is from 45% to 90%, the total area rate of ferrite, martensite, and bainite is 90% or more, is preferably used. Then, preferably the steel sheet (steel sheet for hot forming) is heated into a temperature range of 720° C.
  • a hot formed steel sheet component having a desired steel structure after hot pressing with a high tensile strength (for example, tensile strength of 980 MPa or more), and superior in ductility and bendability can be obtained.
  • a high tensile strength for example, tensile strength of 980 MPa or more
  • the aspect ratio of ferrite in a steel structure of a steel sheet component after hot pressing may also exceed 2.0, and further the ferrite area rate in a steel sheet component after hot pressing may fall below 5%, because ferrite is transformed excessively to austenite during heating.
  • the aspect ratio of ferrite of steel sheet component exceeds 2.0, the anisotropy of ferrite (crystal grain of ferrite) increases and constitutes an origin of stress concentration, so that the bendability may decline. Therefore, the aspect ratio of ferrite should be 2.0 or less, and is preferably 1.8 or less.
  • the lower limit of the aspect ratio of ferrite is preferably 1.0.
  • the lower limit of the aspect ratio of ferrite is preferably 1.2.
  • An aspect ratio of ferrite is a value measured by the method described precisely in Example presented below.
  • the area rate of ferrite When the area rate of ferrite is less than 5%, the area rate of ferrite in a steel structure of a steel sheet component after hot pressing may also become less than 5%. Therefore, the area rate of ferrite should be 5% or more, and is preferably 15% or more. Similarly, when the area rate of ferrite is above 50%, the area rate of ferrite in a steel structure of a steel sheet component after hot pressing may also exceed 50%. Therefore, the area rate of ferrite should be 50% or less, and is preferably 45% or less.
  • the total area rate of martensite and bainite When the total area rate of martensite and bainite is less than 45%, the total area rate of tempered martensite and tempered bainite in the steel structure of a steel sheet component after hot pressing may become less than 20%. Further, the area rate of martensite in the steel structure of a steel sheet component after hot pressing may become less than 25%. Therefore, the total area rate of martensite and bainite should be 45% or more, and is preferably 50% or more. Similarly, when the total area rate of martensite and bainite is above 90%, the total area rate of tempered martensite and tempered bainite in the steel structure of a steel sheet component after hot pressing may also exceed 70%. Further, the area rate of martensite in the steel structure of a steel sheet component after hot pressing may exceed 75%. Therefore, the total area rate of martensite and bainite should be 90% or less, and is preferably 80% or less.
  • the total area rate of ferrite, martensite, and bainite is less than 90%, mixture of a phase or a structure other than ferrite, tempered martensite, tempered bainite, and martensite in the steel structure of a steel sheet component after hot pressing may exceed 10%. Especially, the area rate of retained austenite may exceed 5%. Therefore, the total area rate of ferrite, martensite, and bainite should be 90% or more, and is preferably 93% or more. The upper limit of the total area rate of ferrite, martensite, and bainite is 100%.
  • An area rate of each phase and structure in the steel structure of a steel sheet for hot forming is a value measured by the method described precisely in Example presented below.
  • a steel sheet for hot forming may be any of a hot-rolled steel sheet, a cold-rolled steel sheet, and a coated steel sheet.
  • a coated steel sheet include aluminum coated steel sheet, and zinc coated steel sheet.
  • a hot-rolled steel sheet having the above steel structure can be produced in a hot rolling step, by defining C, Si and Mn within the ranges in terms of the chemical composition, and completing finish-rolling at from 850° C. to 930° C., retaining the product in process in a range from 740° C. to 660° C. for 3 sec or more, and winding the same in a temperature range of 450° C. or less.
  • a cold-rolled steel sheet having the above steel structure can be produced, after cold rolling, by heating the product in process at from 780° C. to 900° C., and then cooling the same at an average cooling rate of 10° C./sec or more in an annealing step.
  • a coated steel sheet having the above steel structure can be produced, after production of the hot-rolled steel sheet or the cold-rolled steel sheet, by performing a well known plating treatment on a surface of the hot-rolled steel sheet or the cold-rolled steel sheet.
  • Heating of Steel Sheet for Hot Forming Heating to Temperature Range of 720° C. or Higher but Lower than an Ac 3 Point
  • Heating of a steel sheet for hot forming is performed up to a temperature of 720° C. or higher but lower than an Ac 3 point.
  • the Ac 3 point (° C.) is a temperature defined by the following empirical Formula (i), which is lower than the Ac 3 point (° C.) of an austenite single phase.
  • a heating temperature should be 720° C. or higher, and is preferably 750° C. or higher.
  • a heating temperature should be not higher than an Ac 3 point, and is preferably not higher than an Ac 3 point ⁇ 30° C.
  • a heating rate up to 720° C. and a heating time for retention in the temperature range are preferably in the following ranges respectively.
  • the average heating rate in heating up to 720° C. is preferably from 0.2° C./sec to 100° C./sec.
  • the average heating rate is 0.2° C./sec or more, high productivity can be secured. Further, when the average heating rate is 100° C./sec or less, the heating temperature can be regulated easily, even in a case in which heating is conducted in an ordinary furnace.
  • the heating time in a temperature range of 720° C. or higher and lower than an Ac 3 point is preferably from 2 min to 10 min.
  • a heating time is a time period from a time point, when the temperature of a steel sheet reaches 720° C., to a time point of completion of heating.
  • the time point of completion of heating means, in the case of furnace heating, a time point, when a steel sheet is taken out of a heating furnace, and in the case of Joule heating or induction heating, a time point, when the power supply is cut off.
  • the heating time is 2 min or more, the strength after hot pressing (after quenching) can be made more stable.
  • the retention time is 10 min or less, the structure of a steel sheet component can be micronized further, so that the toughness of a steel sheet component can be further improved.
  • a steel sheet for hot forming is transported after heating in a heating furnace to a hot press.
  • a steel sheet may be partly exposed to air cooling. Since ferrite is newly formed or grown during such air cooling, the time duration of air cooling has influence on tensile strength. Therefore, for securing stably a high strength after hot pressing (after quenching), the time duration of air cooling should be preferably short.
  • the time period from completion of heating to initiation of hot pressing, during which a steel sheet is exposed to air cooling is above 20 sec, the tensile strength of a steel sheet component after hot pressing (after quenching) decreases, or even when a high tensile strength (for example, tensile strength of 980 MPa or more) is secured, carbon concentration in austenite becomes conspicuous and martensite transformed region is apt to crack, so that the bendability declines. Therefore, the time period from completion of heating to initiation of hot pressing, during which a steel sheet is exposed to air cooling, should be 20 sec or less, and is preferably 16 sec or less. Meanwhile, austenite formed during heating has precipitated in an acicular form.
  • the time period from completion of heating to initiation of hot pressing, during which a steel sheet is exposed to air cooling should be 3 sec or more, and is preferably 7 sec or more, more preferably 10 sec or more.
  • the time period allowing exposure to air cooling can be regulated by regulating a transportation time from extraction out of a heating furnace to a press mold, which is ordinarily exposed to air cooling.
  • M S point starting temperature of martensitic transformation
  • M S point starting temperature of martensitic transformation
  • the average cooling rate is less than 10° C./sec
  • bainitic transformation advances excessively.
  • pearlitic transformation occurs, so that the area rate of martensite, which is a reinforcing phase, cannot be secured, and a high tensile strength (for example, tensile strength of 980 MPa or more) after hot pressing (after quenching) is difficult to secure.
  • austenite is stabilized, so that the bendability declines.
  • the average cooling rate in the temperature range should be 10° C./sec or more, and is preferably 30° C./sec or more. Meanwhile, when the average cooling rate is over 500° C./sec, it becomes extremely difficult to maintain the soaking of a steel sheet component, and the strength becomes unstable. Therefore, the average cooling rate should be 500° C./sec or less, and is preferably 200° C./sec or less.
  • an average cooling rate is a value obtained by dividing the deference between a temperature for performing hot pressing (° C.) and an Ms point (° C.) by a time period required from the temperature for performing hot pressing (° C.) to the Ms point (° C.).
  • cooling from 400° C. to an M S point is required to be performed more strongly than cooling down to 400° C., and preferably performed as specified below.
  • a hot pressing method cooling is achieved ordinarily by a steel-made mold at normal temperature or several tens of degrees Celsius. Therefore, for changing a cooling rate, a mold dimension may be changed so as to change the heat capacity. Further, the cooling rate can be changed also by changing a mold material to a different metal (for example, copper).
  • a water-cooled mold may be used to change the cooling rate by changing the cooling water amount. Further, using a mold, on which several grooves are cut in advance, the cooling rate may be changed by flowing water in the groves during pressing, or the cooling rate may be also changed by lifting a press interrupting pressing and flowing water in between. Furthermore, the cooling rate may be also changed by changing a mold clearance so as to change the contact area with a steel sheet.
  • a means for changing the cooling rate for example, above and below 400° C., the following means are conceivable.
  • a component is transferred to a mold having a different heat capacity or a mold at a room temperature condition, to change a cooling rate.
  • a component is transferred to a mold having a different heat capacity or a mold at a room temperature condition, to change a cooling rate.
  • the water flow rate in the mold is changed to change a cooling rate.
  • water is flown between a mold and a component and the water flow rate is changed to change a cooling rate.
  • a form of forming by a hot pressing method according to the invention includes bending, draw forming, stretch-expand forming, bore expansion forming, and flange forming. Suitable one may be selected appropriately according to a kind of an intended hot formed steel sheet component.
  • Typical examples of a hot formed steel sheet component include a door guard bar and a bumper reinforcement, which are automobile reinforcing components, as described above.
  • a hot formed steel sheet component according to the invention is characterized in that it is superior in ductility and bendability.
  • ductility to withstand practical use total elongation in a tensile test of 12% or more is preferable, and total elongation of 14% or more is still more preferable.
  • bendability a limit bending radius of 5 t or less in a V-bend test with a tip angle of 90° is preferable.
  • a hot formed steel sheet component after hot pressing may be subjected to a shotblasting treatment in order to remove a scale.
  • the shotblasting treatment has an effect of introducing a compression stress in a surface, and therefore offers an advantage of suppressing a delayed fracture and also enhancing the fatigue strength.
  • hot forming has been described taking hot pressing, which is its specific embodiment, as an example, the invention is applicable similarly to hot pressing also to hot forming provided with a means for cooling a steel sheet simultaneously with or immediately after forming, for example to roll forming.
  • Steel sheets having chemical compositions set forth in Table 1 were used as test materials. Each of the steel sheets was prepared by heating a slab ingoted in a laboratory at 1,250° C. for 30 min, then, except test materials No. 6 and No. 22, subjected to hot rolling, such that finish-rolling is completed in a range of from 880° C. to 910° C., and the material is retained in a range of from 720° C. to 680° C. for 5 sec, to yield a 2.6 mm-thick hot-rolled steel sheet. After hot rolling, the sheet was cooled by water spraying down to 420° C. or lower, and then cooled slowly at 20° C./hour to room temperature, simulating a step for winding a hot rolled sheet in a temperature range of 420° C. or lower.
  • a thus obtained hot-rolled steel sheet had a complex structure of ferrite and martensite, or of ferrite and bainite.
  • test material No. 6 simulated a step for winding a hot-rolled sheet at room temperature by retaining the sheet in a range of from 740° C. to 660° C. for 2 sec, and cooling the same by water spraying to room temperature.
  • Test material No. 22 simulated a step for winding a hot-rolled sheet at 670° C., by cooling the sheet by water spraying to 670° C., and then cooling the same slowly at 20° C./hour to room temperature.
  • a part of a hot-rolled steel sheet obtained as above was freed from a scale by pickling, then subjected to cold rolling to a sheet thickness of 1.6 mm, heated at from 780° C. to 900° C., and annealed under condition of cooling at an average cooling rate of 30° C./sec.
  • test material No. 27 was heated at 920° C., and annealed under condition of cooling at an average cooling rate of 30° C./sec.
  • EBSP Electro Back Scatter Pattern
  • the respective area rates of ferrite, martensite, and bainite were determined as an average value of area rates measured respectively based on respective IQ images of EBSP for both cross-sections in the rolling direction and in the direction vertical to the rolling direction.
  • acceleration voltage 25 kV
  • working distance 15 mm
  • scan step 0.2 ⁇ m.
  • an aspect ratio of ferrite in a steel sheet to be subjected to hot pressing was measured as follows. Specifically, cross-sections both in the rolling direction and in the direction vertical to the rolling direction were sliced out from a steel sheet to be subjected to hot pressing. The sliced out cross-sections were subjected to polishing and nital etching. Next, using a scanning electron microscope (SEM) equipped with an EBSP detector (trade name QUANTA 200, produced by FEI Company), an IQ image (image quality map: magnification 2000 ⁇ ) of EBSP was obtained for each sliced out cross-section by an EBSP analysis.
  • SEM scanning electron microscope
  • EBSP detector trade name QUANTA 200, produced by FEI Company
  • the aspect ratio of ferrite was determined as an average value of aspect ratios of each 50 ferrite crystal grains measured based on each IQ image of EBSP for cross-sections both in the rolling direction and in the direction vertical to the rolling direction.
  • acceleration voltage 25 kV
  • working distance 15 mm
  • scan step 0.2 ⁇ m.
  • the obtained steel sheets were heated in a gas furnace at an air fuel ratio of 0.85 and under the conditions set forth in Table 3. Then the heated steel sheets were taken out of the heating furnace, and after an air cooling time until hot pressing (time period from extraction of a sheet out of the furnace to placement of the same into a mold, namely time period in which a steel sheet is exposed to air cooling between the completion of heating and the initiation of hot forming) regulated to a time as set forth in Table 3, subjected to hot pressing using a flat plate steel-made mold.
  • hot pressing time period from extraction of a sheet out of the furnace to placement of the same into a mold, namely time period in which a steel sheet is exposed to air cooling between the completion of heating and the initiation of hot forming
  • hot-pressed steel sheet After hot pressing, the steel sheets were cooled at an average cooling rate set forth in Table 3 down to 150° C., which was not higher than an M S point, while keeping the steel sheets in contact with the mold, and thereafter taken out from the mold and left standing to allow cooling, thereby completing various test steel sheets (such a test steel sheet is hereinafter referred to as “hot-pressed steel sheet”).
  • Cooling was performed 1) after cooling the periphery of a mold with cooling water, 2) after cooling in a mold, which had been at normal temperature, or 3) after cooling in a heated mold, by cooling the periphery of a mold with cooling water.
  • An average cooling rate down to 150° C. was determined by attaching a thermocouple to an edge of a steel sheet to be subjected to hot pressing, and reading the temperature.
  • a heating time means a time period from a time point when a steel sheet reaches 720° C. after placement of the same in a furnace, until the same is taken out from the furnace.
  • various test steel sheets were prepared by conducting gas cooling at a predetermined cooling rate after air cooling for a predetermined time period, for simulating a hot pressing condition, under which a cooling rate is changed using a mold with grooves,
  • the area rates of ferrite, tempered martensite, tempered bainite, and martensite of a hot pressed steel sheet were measured identically with the respective area rates of ferrite, martensite, and bainite of a steel sheet to be subjected to hot pressing, applying an EBSP (Electron Back Scatter Pattern) method.
  • the results are shown in Table 4.
  • the aspect ratio of ferrite of a hot pressed steel sheet was measured identically with the aspect ratio of ferrite of a steel sheet to be subjected to hot pressing.
  • JIS No. 5 test piece for tensile test was sampled from each steel sheet in the direction normal to the rolling direction, and a tensile test was carried out to measure TS (tensile strength) and El (total elongation).
  • a rectangular sample was cut from each steel sheet allowing a bending ridge line to be directed normal to the rolling direction, and a surface thereof was machined to prepare a bending test piece with a thickness of 1 mm, a width of 30 mm, and a length of 60 mm.
  • the test piece was subjected to a V-bend test with a tip angle of 90°, and tip radii of 5 mm, 4 mm, and 3 mm for evaluating the bendability.
  • the machined surface constituted an inner surface of a bend.
  • the surface of a bend after the test was examined visually, and rated according to the following rating criteria.
  • A After a V-bend test with a tip radius of 4 mm, a crack is not recognized.
  • An underlined value in Table 1 to Table 4 means that a content, a condition, or a mechanical property expressed by the value is outside the scope of the invention.
  • test materials No. 1, 3, 5, 6, 9, 10, 11, 13, 15, 17, 19, 21, 22, 24, 27, 28, 29, 31, and 33 as Inventive Examples in Table 4 are steel sheet components of Inventive Examples, namely hot pressed steel sheet components, satisfying all the requirements according to the invention.
  • Any of the hot pressed steel sheet components of Inventive Examples as hot-formed has a tensile strength as high as 980 MPa or more, and superior in ductility as well as bendability.

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