US20100330392A1 - Galvanized steel sheet excellent in uniformity and method for producing the same - Google Patents

Galvanized steel sheet excellent in uniformity and method for producing the same Download PDF

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Publication number
US20100330392A1
US20100330392A1 US12/667,707 US66770708A US2010330392A1 US 20100330392 A1 US20100330392 A1 US 20100330392A1 US 66770708 A US66770708 A US 66770708A US 2010330392 A1 US2010330392 A1 US 2010330392A1
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steel sheet
galvanized steel
phase
steel
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Yoshihiko Ono
Hideyuki Kimura
Kaneharu Okuda
Takeshi Fujita
Michitaka Sakurai
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KIMURA, HIDEYUKI, OKUDA, KANEHARU, ONO, YOSHIHIKO, SAKURAI, MICHITAKA, FUJITA, TAKESHI
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • 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
    • C21D8/0226—Hot 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
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    • 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
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    • 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
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    • C—CHEMISTRY; METALLURGY
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    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C23C2/29—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12771—Transition metal-base component
    • Y10T428/12785—Group IIB metal-base component
    • Y10T428/12792—Zn-base component
    • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

  • This disclosure relates to a high-strength galvanized steel sheet for press forming which is used for automobiles, home electric appliances, and the like through a press forming process, and to a method for manufacturing the steel sheet.
  • BH steel sheets with 340 MPa grade in tensile strength (bake-hardenable steel sheets, simply referred to as “340BH” hereinafter) and IF steel sheets with 270 MPa grade in tensile strength (Interstitial Free steel sheets, simply referred to as“270IF” hereinafter), which is ultra-low-carbon steel containing carbide/nitride-forming elements such as Nb and Ti to control the amount of dissolved C, have been applied to automotive outer panels, such as hoods, doors, trunk lids, back doors, and fenders, which require sufficient dent resistance.
  • 270IF Interstitial Free steel sheets
  • a solution-hardening element such as Mn, P, or the like is further added to 340BH with a yield strength (YP) of 230 MPa or 270IF with a YP of 180 MPa to strengthen and thin a steel sheet
  • surface distortion represents micro wrinkles or wavy patterns produced in a press-formed surface due to an increase in YP.
  • the occurrence of surface distortion impairs the design or design property of a door, a trunk lid, or the like. Therefore, the steel sheet for such application is desired wherein YP after press forming and baking finish treatment is increased over the YP of conventional steel sheets while maintaining extremely low YP before press forming.
  • Japanese Examined Patent Application Publication No. 62-40405 discloses a method for producing a steel sheet having low YP, high work-hardenability WH, and high BH by appropriately controlling the cooling rate after annealing of steel to form a dual phase mainly composed of ferrite and martensite, the steel containing 0.005 to 0.15% of C, 0.3 to 2.0% of Mn, and 0.023 to 0.8% of Cr.
  • 2004-307992 discloses a method for satisfying both surface distortion resistance and anti-cracking property of steel containing 0.005 to 0.05% of C and 3% or less of Mn by adjusting the average grain diameter of martensite to 1.5 ⁇ m or less, the ratio of martensite in a second phase to 60% or more, and the ratio of the number of martensite grains to 0.7 to 2.4 relative to the number of ferrite grains.
  • 2001-207237 discloses that a steel sheet with high ductility and low yield ratio YR is produced by appropriately controlling a cooling rate after annealing of steel containing 0.010 to 0.06% of C, 0.5 to 2.0% of Mn, and 1% or less of Cr and increasing the ratio of martensite in a second phase to 80% or more. Further, Japanese Unexamined Patent Application Publication No. 2001-303184 discloses that a low-YP dual phase steel sheet composed of ferrite and martensite is produced by decreasing the C content to 0.02 to 0.033% in steel containing 1.5 to 2.5% of Mn and 0.03 to 0.5% of Cr.
  • a steel sheet of 490-590 MPa in TS grade is required to decrease temperature and time of the baking finish process while reducing the thickness of the sheet. But, such a high-strength steel sheet has the problem of extremely large variation in mechanical properties.
  • the steel sheets described in Japanese Examined Patent Application Publication No. 62-40405 and Japanese Unexamined Patent Application Publication Nos. 2004-307992, 2001-207237 and 2001-303184 show 440 MPa in TS and 210 to 260 MPa in YP, and thus YP is suppressed to a low level as compared with the conventional YP level of 320 MPa in solid-solution hardening 440 MPa grade IF steel.
  • the surface distortion in these steel sheets is improved compared with conventional 440 MPa grade IF steel with YP of 320 MPa.
  • a larger surface distortion still arises as compared to 340BH. Therefore, it is also desired to decrease the absolute value of YP.
  • a high-strength galvanized steel sheet including steel having a composition which contains, by % by mass, 0.01 to 0.12% of C, 0.2% or less of Si, less than 2% of Mn, 0.04% or less of P, 0.02% or less of S, 0.3% or less of sol.
  • the high-strength galvanized steel sheet preferably satisfies 2.2 ⁇ [Mneq] ⁇ 2.9 and 0.34 ⁇ [% Cr]/[% Mn].
  • B 0.005% by mass or less.
  • at least one of 0.15% by mass or less of Mo and 0.2% by mass or less of V is preferably contained.
  • at least one of less than 0.014% by mass of Ti, less than 0.01% by mass of Nb, 0.3% by mass or less of Ni, and 0.3% by mass or less of Cu is preferably contained.
  • the high-strength galvanized steel sheet can be produced by a method for producing a high-strength galvanized steel sheet, the method including hot-rolling and cold-rolling a steel slab having the above-described composition, heating at an average heating rate of less than 3° C./sec in a temperature range of 680° C. to 740° C. in CGL, annealing at an annealing temperature of over 740° C. to less than 820° C., cooling from the annealing temperature at an average cooling rate of 3 to 20° C./sec, dipping in a galvanization bath or dipping in the galvanization bath and further alloying the coating, and then cooling at an average cooling rate of 7 to 100° C./sec.
  • heating is preferably performed at an average heating rate of less than 2° C./sec in a temperature range of 680° C. to 740° C. in CGL.
  • hot rolling is preferably performed by starting cooling within 3 seconds after hot rolling, cooling to 600° C. or less at an average cooling rate of 40° C./sec or more, and coiling at a coiling temperature of 400° C. to 600° C., and then cold rolling is preferably performed with a rolling reduction of 70 to 85%.
  • a high-strength galvanized steel sheet with low YP excellent in uniformity can be produced.
  • the high-strength galvanized steel sheet is excellent in resistance to surface distortion and is thus suitable for strengthening and thinning automotive parts.
  • FIG. 1 is a graph showing a relation between YP and the area ratio of grains with a grain diameter of less than 0.8 ⁇ m in a second phase.
  • FIG. 2 is a graph showing a relation between YP and the heating rate in annealing.
  • C is an element necessary for securing a predetermined area ratio of a second phase.
  • the C content is excessively low, the second phase cannot be secured at a sufficient area ratio, and low YP cannot be achieved. Further, sufficient BH cannot be secured, and the anti-aging property is degraded.
  • the C content is required to be 0.01% or more to secure a sufficient area ratio of the second phase.
  • the C content exceeds 0.12%, the area ratio of the second phase is excessively increased to increase YP, and ⁇ YP with the annealing temperature is also increased. In addition, weldability is also degraded. Therefore, the C content is 0.12% or less.
  • the C content is preferably less than 0.08% for achieving lower YP and more preferably less than 0.06% for achieving further lower YP.
  • Si has the effect of delaying scale formation in hot rolling and improves surface appearance quality when added in a small amount, the effect of appropriately delaying an alloying reaction between ferrite and zinc in a galvanization bath or galvannealing treatment, and the effect of further homogenizing and coarsening the microstructure of a steel sheet. Therefore, Si can be added from this viewpoint. However, when Si is added in an amount exceeding 0.2%, the surface appearance quality is impaired and cause difficulty in application to outer panels, and YP is increased. Therefore, the Si content is 0.2% or less.
  • Mn enhances hardenability, suppresses the formation of pearlite and bainite in cooling and in allying treatment after annealing, and decreases the amount of dissolved C in ferrite. Therefore, from the viewpoint of decreasing YP, Mn is added. However, when the Mn content is excessively high, the second phase is made fine and heterogeneous, and ⁇ YP with respect to the annealing temperature is increased.
  • the Mn content is excessively increased, the recrystallization temperature is decreased, and ⁇ grains are finely and nonuniformly produced in fine ferrite grain boundaries immediately after recrystallization or boundaries of recovered grains during recrystallization, thereby increasing the area ratio of the second phase grains with a grain diameter of less than 0.8 ⁇ m, which will be described below, in the structure after annealing. As a result, reduction in YP and ⁇ YP is inhibited.
  • the Mn content is necessary to be less than 2% to decrease YP and ⁇ YP with respect to the annealing temperature. From the viewpoint of more decreasing ⁇ YP and YP, the Mn content is preferably less than 1.8%.
  • the Mn content is preferably less than 1.6%.
  • the Mn content preferably exceeds 0.1% because the Mn content of 0.1% or less causes red shortness due to MnS precipitation and easily causes surface defects.
  • P has the effect of appropriately delaying an alloying reaction between ferrite and zinc in a galvanization bath or galvannealing treatment and the effect of further coarsening the microstructure of a steel sheet. From this viewpoint, P can be added. However, P has a large solution hardening ability and thus significantly increases YP when excessively added. Therefore, the P content is 0.04% or less which has a small adverse effect on an increase in YP.
  • S precipitates as MnS in steel but decreases the ductility of a steel sheet and decreases press formability when added in a large amount.
  • hot ductility is decreased in hot rolling of a slab, and thus surface defects easily occur. Therefore, the S content is 0.02% or less but is preferably as low as possible.
  • Al is used as a deoxidizing element or an element for improving the anti-aging property by fixing N as AlN.
  • Al forms fine AlN during coiling or annealing after hot rolling to suppress the growth of ferrite grains and slightly inhibit reduction in YP.
  • Al is preferably added in an amount of 0.02% or more.
  • the ferrite grain growth property is improved by increasing the coiling temperature to 620° C. or more, but the amount of fine AlN is preferably as small as possible. Therefore, preferably, the sol.
  • Al content is 0.15% or more, and AlN is coarsely precipitated during coiling. However, since the cost is increased when the sol.
  • the sol. Al content exceeds 0.3%, the sol. Al content is 0.3% or less.
  • the sol. Al content exceeds 0.1%, castabiliy is impaired to cause deterioration of the surface appearance quality. Therefore, the sol. Al content is preferably 0.1% or less for application to exterior panels which are required to be strictly controlled in surface appearance quality.
  • the N content is 0.01% or less but is preferably as low as possible.
  • an increase in the N content causes deterioration of the anti-aging property.
  • the N content is preferably less than 0.008% and more preferably less than 0.005%.
  • Cr is the most important element in our steels. Since Cr has a small amount of solution hardening and the small effect of making fine martensite and can impart high hardenability, Cr is an element effective in decreasing the absolute value of YP and decreasing ⁇ YP with respect to the annealing temperature. As described below, the Cr content is required to be controlled according to the Mn content so that [Mneq] and [% Cr]/[% Mn] are in the above-described respective ranges, but the Cr content is necessary to exceed at least 0.3%.
  • the Cr content is required to exceed at least 0.5%.
  • the Cr content exceeds 2%, the cost is increased, and the surface appearance quality of a galvanized steel sheet is degraded. Therefore, the Cr content is 2% or less.
  • [Mneq] it is necessary to control [Mneq] to 2.1 or more to suppress the formation of pearlite and bainite in cooling and alloying treatment after annealing to decrease YP and ⁇ YP with respect to the annealing temperature. Further, from the viewpoint of decreasing the pearlite formation to decrease YP, [Mneq] preferably exceeds 2.2. To substantially disappear pearlite and bainite in the thermal history of CGL to decrease YP, [Mneq] more preferably exceeds 2.3. On the other hand, when [Mneq] is excessively increased, the surface appearance quality of the coating is degraded, and the cost is increased by adding large amounts of alloy elements. Therefore, [Mneq] is 3 or less and preferably less than 2.9.
  • the microstructure can be homogenized and coarsened by controlling [% Cr]/[% Mn] to the predetermined range, and thus a strength change with a change in area ratio of the second phase due to a change in the annealing temperature can be suppressed to a low level.
  • [% Cr]/[% Mn] ⁇ 0.34 is preferred, and a more preferred range is [% Cr]/[% Mn] ⁇ 0.44.
  • the balance includes iron and inevitable impurities, but the elements below may be contained at predetermined contents.
  • B can be utilized as an element for enhancing hardenability.
  • B has the function to fix N as BN to improve the grain growth property.
  • the effect of improving the ferrite grain growth property can be sufficiently exhibited by adding over 0.001% of B, thereby achieving extremely low YP. Therefore, B is preferably added in an amount of over 0.001%.
  • the B content is preferably 0.005% or less.
  • Mo is an element for enhancing hardenability and can be added for the purpose of improving hardenability or the purpose of improving the surface appearance quality of a galvanized steel sheet.
  • Mo is preferably added in the range of 0.15% or less which has the small influence on an increase in YP and ⁇ YP with the annealing temperature. From the viewpoint of further decreasing YP and ⁇ YP, the Mo content is preferably less than 0.02% (not added).
  • V is an element for enhancing hardenability and can be added for the purpose of improving the surface appearance quality of a galvanized steel sheet.
  • V is preferably added in the range of 0.2% or less.
  • Ti has the effect of improving the anti-aging property by fixing N and the effect of improving castability.
  • Ti forms fine precipitates of TiN, TiC, Ti(C, N), and the like in steel to inhibit the grain growth property. Therefore, from the viewpoint of decreasing YP, the Ti content is preferably less than 0.014%.
  • Nb has the effect of delaying recrystallization in hot rolling to control the texture and decrease YP in a direction at 45 degrees with the rolling direction.
  • Nb forms fine NbC and Nb(C, N) in steel to significantly degrade the grain growth property. Therefore, Nb is preferably added in the range of less than 0.01% which has the small influence on an increase in YP.
  • Cu is an element mixed when craps or the like are positively utilized and a recycled material can be used as a raw material when Cu is allowed to be mixed, thereby decreasing the production cost.
  • Cu has a small influence on the material quality, but mixing of excessive Cu causes surface flaws. Therefore, the Cu content is preferably 0.3% or less.
  • Ni also has a small influence on the material quality of a steel sheet, but Ni can be added from the viewpoint of decreasing surface flaws when Cu is added. However, when Ni is excessively added, surface defecting due to heterogeneity of scales is promoted. Therefore, the Ni content is preferably 0.3% or less.
  • the steel sheet is mainly composed of ferrite, martensite, pearlite, and bainite and contains small amounts of retained ⁇ and carbides. The method of measuring the morphology of the microstructures is first described.
  • the area ratio of the second phase was determined by observing a L section (vertical section parallel to the rolling direction) of the steel sheet, which was prepared by polishing and etching with natal, in 12 fields of view with SEM with a magnification of 4000 times power and then image processing of structure photographs.
  • a region with a light black contrast was regarded as ferrite
  • regions including lamellar or dot-sequential carbides formed therein were regarded as pearlite and bainite
  • grains with a white contrast were regarded as martensite or retained ⁇ .
  • dot-like fine grains with a diameter of 0.4 ⁇ m or less which were observed in a SEM photograph were mainly composed of carbides according to TEM observation.
  • the area ratio and average grain diameter were determined for a structure containing white-contrast grains mainly composed of martensite and lamellar or dot sequential carbides composed of pearlite and bainite.
  • the area ratio of the second phase shows a total area ratio of these microstructures.
  • the diameters thereof were used for the average grain diameter, while for elliptical grains on a SEM screen, long axis a and single axis b perpendicular to the long axis were measured to determine (a ⁇ b) 0.5 as an equivalent grain diameter.
  • Grains having a slightly rectangular shape were handled in the same manner as elliptical grains, and the long axis and the single axis were measured to determine a grain diameter according to the above equation.
  • the phases in contact with the same width as a grain boundary were separately counted, while the phases in contact with a larger width than a grain boundary, i.e., in contact with a certain width, were counted as one grain.
  • the steel sheet has a structure mainly composed of ferrite and containing as the second phase martensite, pearlite, bainite, a small amount of retained ⁇ , and carbides.
  • the area ratio of the carbides is as small as less than 1%.
  • the ferrite grain diameter is preferably 4 to 15 ⁇ m.
  • the area ratio of the second phase is 2% or more to decrease YPEl of the steel sheet to sufficiently decrease YP. This can impart functions required for exterior panels, such as high WH, high BH, and excellent anti-aging property.
  • the area ratio of the second phase exceeds 25%, sufficiently low YP cannot be achieved, and ⁇ YP with the annealing temperature is increased. Therefore, the area ratio of the second phase is in the range of 2 to 25%.
  • the steel sheet has a structure composed of ferrite, martensite, pearlite, bainite, and retained ⁇ , but mostly composed of ferrite and martensite.
  • YP is increased.
  • TEM observation indicated that many dislocations imparted by quenching are introduced in the periphery of martensite, and when martensite is finely and nonuniformly dispersed, regions around martensite where dislocations are introduced overlap each other. Such dislocations around martensite have already been entangled, and it is thus considered that the dislocations hardly contribute to initial deformation at low stress.
  • the grains in the second phase are as large as possible in diameter and dispersed as uniformly as possible. It is necessary that the average grain diameter of the second phase is at least 0.9 ⁇ m or more to sufficiently decrease YP and ⁇ YP with the annealing temperature of the high-[Mneq] steel sheet. On the other hand, when the grain diameter of the second phase exceeds 7 ⁇ m, it is necessary to significantly coarsen ferrite grains, and surface roughness may occur in press forming. Therefore, the grain diameter of the second phase is 7 ⁇ m or less.
  • the structure can be frozen by rapid cooling from near 700° C., and the second phase can be relatively coarsely dispersed.
  • slow cooling at an appropriate cooling rate is required after annealing, and thus ⁇ transformation proceeds during slow cooling in the temperature range of 700° C. to 500° C. with the result of a fine second phase.
  • the second phase can be coarsely dispersed even in the thermal history of CGL by appropriately controlling the Mn equivalent, the Cr and Mn composition ranges, and the heating rate in annealing.
  • the area ratio of pearlite or bainite in the second phase is 0% to less than 20%, sufficiently low YP can be achieved with the result that decreasing ⁇ YP. Further, the area ratio is preferably 0 to 10%.
  • the area ratio of pearlite or bainite in the second phase represents the area ratio of pearlite or bainite relative to the area ratio of 100 of the second phase.
  • FIG. 1 shows a relation between YP and the area ratio of grains with a diameter of less than 0.8 ⁇ m in the second phase.
  • the area ratio of grains with a diameter of less than 0.8 ⁇ m in the second phase is less than 15%, YP is decreased to 210 MPa or less, while when the area ratio is less than 12%, YP is decreased to 205 MPa or less. Further, as a result of measurement of ⁇ YP by changing the annealing temperature from 760° C.
  • ⁇ YP is 24 MPa, while.in a sample (heating rate: less than 3° C./sec) containing of grains with a diameter of less than 0.8 ⁇ m at an area ratio of less than 15% in the second phase, ⁇ YP is decreased to 15 MPa.
  • a steel sheet with low YP and low ⁇ YP with the annealing temperature can be produced by decreasing the amount of grains of less than 0.8 ⁇ m produced. Therefore, the area ratio of grains with a diameter of less than 0.8 ⁇ m in the second phase is less than 15%. From the viewpoint of further decreasing YP and ⁇ YP with the annealing temperature, the area ratio is preferably less than 12%. Like in the above-mentioned measuring method, the area ratio of grains with a diameter of less than 0.8 ⁇ m in the second phase represents the area ratio of grains with a diameter of less than 0.8 ⁇ m relative to the area ratio of 100 of the second phase.
  • the steel sheet can be produced by the method including hot-rolling and cold-rolling a steel slab having the above-described composition, heating at an average heating rate of less than 3° C./sec in a temperature range of 680° C. to 740° C. in CGL, annealing at an annealing temperature of over 740° C. to less than 820° C., cooling from the annealing temperature at an average cooling rate of 3 to 20° C./sec, dipping in a galvanization bath or dipping in the galvanization bath and further alloying the coating, and then cooling at an average cooling rate of 7 to 100° C./sec.
  • the slab can be hot-rolled by a method of rolling the slab after heating, a method of directly rolling the slab without heating after continuous casting, or a method of rolling the slab by heating for a short time after continuous casting.
  • the hot rolling may be performed according to a general method, for example, at a slab heating temperature of 1100° C. to 1300° C., a finish rolling temperature of Ar 3 transformation point or more, an average cooling rate after finish rolling of 10 to 200° C./sec, and a coiling temperature of 400° C. to 720° C.
  • the slab heating temperature is 1200° C. or less
  • the finish rolling temperature is 840° C. or less.
  • descaling is preferably sufficiently performed for removing primary and secondary scales formed on the surface of the steel sheet.
  • the coiling temperature is preferably as high as possible and 640° C. or more.
  • Mn and Cr can be sufficiently concentrated in the second phase in the state of the hot-rolled sheet, and stability of y in the subsequent annealing step is improved, contributing to a decrease in YP.
  • cooling in hot rolling is started within 3 seconds after finish rolling and performed to 600° C. or less at an average cooling rate of 40° C./sec or more, followed by coiling at a coiling temperature of 400° C. to 600° C.
  • a fine low-temperature transformed phase mainly composed of bainite can be produced at an area ratio of 30% or more, and the development of a texture in which YP in the direction at 45° is relatively suppressed can be promoted.
  • YP (YP D ) in the direction at 45° generally tends to be 5 to 15 MPa higher than YP (YP L ) in the rolling direction and YP (YP C ) in a direction perpendicular to the rolling direction.
  • the above-described hot-rolling conditions can suppress to the range of ⁇ 10 ⁇ YP D ⁇ YP C ⁇ 5 MPa.
  • the rolling reduction of cold rolling may be 50% to 85%.
  • YP C is decreased by decreasing the rolling rate to 50% to 65%.
  • YP in the direction at 45° is relatively increased by decreasing the rolling rate, thereby increasing anisotropy. Therefore, for a steel sheet for application such as a door knob, the rolling rate is preferably 70% to 85%.
  • the steel sheet after cold rolling is annealed and galvanized in CGL.
  • FIG. 2 shows a relation between YP and the heating rate in the temperature region of 680° C. to 740° C. during annealing.
  • the results of FIG. 2 were obtained by arranging data of an experiment conducted for leading to the results shown in FIG. 1 .
  • FIG. 2 indicates that at the heating rate of less than 3° C./sec, YP of 210 MPa or less can be obtained, while at the heating rate of less than 2° C./sec, YP of 205 MPa or less can be obtained.
  • the heating rate is less than 3° C./sec, the formation of ⁇ grains in ferrite grain boundaries which remain unrecrystallized can be suppressed, and the formation of the fine second phase can be suppressed, thereby decreasing YP.
  • the heating rate is less than 2° C./sec, nucleation of ⁇ from unrecrystallized ferrite can be suppressed, and recrystallized ferrite grains can be sufficiently grown. Therefore, the structure is further homogenized and coarsened, thereby further decreasing YP and ⁇ YP.
  • the annealing temperature is over 740° C. to less than 820° C.
  • the area ratio of the second phase cannot be secured because of insufficient solid solution of carbides.
  • the ⁇ ratio is excessively increased in annealing, and elements such as Mn, C, and the like are not sufficiently concentrated in ⁇ grains, thereby failing to achieve sufficiently low YP. This is possibly because when elements are not sufficiently concentrated in ⁇ grains, strain is not sufficiently applied to the periphery of martensite, and pearlite and bainite transformation easily occurs in the cooling step.
  • the holding time during annealing is preferably 20 seconds or more in the temperature range of over 740° C.
  • cooling is performed at an average cooling rate of 3 to 20° C./sec from the annealing temperature to the temperature of the galvanization bath generally kept at 450° C. to 500° C.
  • the cooling rate is lower 3° C./sec, large amounts of pearlite and bainite are formed in the second phase because of the passage through the pearlite nose in the temperature region of 550° C.
  • the steel sheet is dipped in the galvanization bath, and if required, alloying treatment can be also performed by keeping the sheet in the temperature region of 500° C. to 650° C. for 30 seconds or less.
  • alloying treatment can be also performed by keeping the sheet in the temperature region of 500° C. to 650° C. for 30 seconds or less.
  • [Mneq] is not appropriately controlled
  • the mechanical properties are significantly degraded by the galvannealing treatment.
  • YP is slightly increased, and good mechanical properties can be obtained.
  • cooling is performed at an average cooling rate of 7 to 100° C./sec. At the cooling rate of lower than 7° C./sec, pearlite is produced near 550° C., and bainite is produced in the temperature region of 400° C.
  • the resultant galvanized steel sheet has YPEl of less than 0.5% and sufficiently decreased YP in a galvanized state and thus can be used directly as a steel sheet for press forming as long as the area ratio of the second phase, the average grain diameter of the second phase, the area ratio of grains with a grain diameter of less than 0.8 ⁇ m in the second phase, and the area ratio of pearlite and bainite are controlled.
  • skin-pass rolling is generally performed.
  • the elongation is preferably 0.3% to 0.5%.
  • the resultant hot-rolled sheet was cold-rolled with a rolling reduction of 67% to form a cold-rolled sheet of 0.75 mm in thickness.
  • the resultant cold-rolled sheet was annealed in CGL at the average heating rate in the temperature range of 680° C. to 740° C., the annealing temperature AT, and the cooling rate shown in Tables 2 and 3 and galvanized in a cooling step. Cooling from the annealing temperature AT to the galvanization bath temperature of 460° C. is primary cooling, and cooling from the galvanization bath temperature or the alloying temperature when alloying was performed is second cooling. Tables 2 and 3 show the average cooling rate of each of the primary cooling and second cooling. Alloying treatment was performed by heating to 510° C. to 530° C.
  • the obtained sample was examined with respect to the area ratio of the second phase, the average grain diameter of the second phase, the area ratio of pearlite or bainite in the second phase, and the area ratio of grains with a grain diameter of less than 0.8 ⁇ m in the second phase. Further, a JIS No. 5 test piece was collected in the rolling direction and the perpendicular direction and subjected to a tensile test (according to JISZ2241) to evaluate YP and TS. In addition, the annealing temperature for the steel sheet with each of the compositions was changed in the range of 760° C. to 810° C. to measure the maximum and minimum of YP and determine variation ⁇ YP of YP.
  • Our steel sheets exhibit small ⁇ YP as compared with a material in the same TS level.
  • Our steel sheets also have YP which is equivalent to or lower than YP of conventional steel, i.e., low YR.
  • YP is equivalent to or lower than YP of conventional steel, i.e., low YR.
  • ⁇ YP is suppressed to 15 MPa or less
  • YP is also as low as 206 MPa.
  • ⁇ YP is suppressed to 20 MPa
  • ⁇ YP is suppressed to 32 MPa.
  • YP and ⁇ YP are in the range of 182 to 198 MPa and the range of 5 to 9 MPa, respectively, and very low under the conditions including a heating rate of 1.5° C./sec, an annealing temperature of 775° C. to 805° C., and a primary cooling rate of 4 to 5° C./sec. Further, when [Mneq] is constant, ⁇ YP decreases as [% Cr]/[% Mn] increases.
  • a change in YP due to the alloying treatment is significantly suppressed.
  • a change in YP due to alloying treatment is as low as 2 MPa, and an increase in YP by alloying treatment is suppressed.
  • our steel sheets exhibit good mechanical properties even after galvannealing treatment and are suitable for applications in which alloying treatment is performed.
  • an increase in YP due to an increase in C is extremely small, and in Steel Nos. H and I, YP is suppressed to 219 MPa or less even when C is increased to 0.051%.
  • YP is suppressed to 262 MPa, and a steel sheet with low YR can be stably obtained.
  • Steel Nos. F, BB, and CC containing over 0.001% of B ferrite grains and the second phase are coarsened, and YP (or YR) and ⁇ YP are suppressed to low levels.
  • [% Cr]/[% Mn] is substantially the same, but YP and ⁇ YP of BB containing B are lower than AA in spite of low [Mneq].
  • a steel sheet in which the heating rate and cooling rate in annealing are not appropriately controlled to produce a large amount of fine grains with a diameter of less than 0.8 ⁇ m in the second phase and a steel sheet in which pearlite and bainite are produced in large amounts have large ⁇ YP and large YP absolute value as compared with our steel sheets in the same strength level.
  • a steel sheet in which the heating rate and cooling rate in annealing are not appropriately controlled to produce a large amount of fine grains with a diameter of less than 0.8 ⁇ m in the second phase and a steel sheet in which pearlite and bainite are produced in large amounts have large ⁇ YP and large YP absolute value as compared with our steel sheets in the same strength level.
  • Steel Nos. P and W having low [Mneq] large amounts of pearlite and bainite are produced, and YP and ⁇ YP are large as compared with the steel sheet of an example of the present invention in the same strength level.
  • the slab having the composition of Steel No. C shown in Table 1 was heated to 1200° C., hot-rolled at a finish rolling temperature of 830° C., maintained for various times shown in Table 4 to control the cooling start time, cooled to 600 ° C. at various cooling rates shown in Table 4, and coiled at the coiling temperature CT shown in Table 4.
  • Each of the resultant hot-rolled band was cold-rolled with a rolling rate of 77% and, in CGL, heated at a heating rate of 1.5° C./sec, annealed at 775° C., cooled at an average primary cooling rate of 4° C./sec, galvanized, alloyed at 520° C. for 20 seconds, and then cooled at an average secondary cooling rate of 30° C./sec.
  • JIS No. 5 tensile test pieces were collected from the result steel sheet in the rolling direction, the perpendicular direction (C direction), and the direction at 45° with the rolling direction and subject to a tensile test.
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EP2169091B1 (en) 2018-11-28
JP5272547B2 (ja) 2013-08-28
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CA2693963A1 (en) 2009-01-15
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KR101147273B1 (ko) 2012-05-16
CN101688279B (zh) 2012-08-01

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