US20090014098A1 - High-Strength Hot-Dip Galvanized Steel Sheet Excellent in Formability and Method for Producing Same - Google Patents

High-Strength Hot-Dip Galvanized Steel Sheet Excellent in Formability and Method for Producing Same Download PDF

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US20090014098A1
US20090014098A1 US11/886,622 US88662206A US2009014098A1 US 20090014098 A1 US20090014098 A1 US 20090014098A1 US 88662206 A US88662206 A US 88662206A US 2009014098 A1 US2009014098 A1 US 2009014098A1
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steel sheet
dip galvanized
less
formability
galvanized steel
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Hiroshi Matsuda
Tatsuya Nakagaito
Takayuki Futatsuka
Shusaku Takagi
Yasunobu Nagataki
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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: FUTATSUKA, TAKAYUKI, MATSUDA, HIROSHI, NAGATAKI, YASUNOBU, NAKAGAITO, TATSUYA, TAKAGI, SHUSAKU
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    • C—CHEMISTRY; METALLURGY
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    • 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
    • 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
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    • 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
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
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    • C—CHEMISTRY; METALLURGY
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • 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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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • 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
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    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • 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
    • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F17/00—Multi-step processes for surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by subclass C21D or C22F or class C25
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite

Definitions

  • the present invention relates to a high-strength hot-dip galvanized steel sheet having excellent formability and used in industrial fields such as automobiles and electrics, and relates to a method for producing the high-strength hot-dip galvanized steel sheet.
  • Patent Document 1 discloses a steel sheet excellent in press formability by controlling the chemical composition and the amount of retained austenite in a steel sheet.
  • Patent Document 2 discloses a method for producing such a steel sheet.
  • Patent Document 3 discloses a steel sheet containing 5% or more retained austenite and having excellent formability (in particular, local ductility).
  • Patent Document 4 discloses a steel sheet containing 3% or more retained austenite, having an average axial ratio of 3 to 20, and having an average hardness of a matrix of 270 HV or less and thus having a balance between stretch and stretch-flange formability.
  • Patent Documents 5 and 6 each disclose a steel sheet containing 3% or more retained austenite and either 50% or more tempered martensite or 50% tempered bainite and thus having a balance between high ductility and high stretch-flange formability.
  • Patent Document 7 discloses a steel sheet having an appropriate volume fraction of retained austenite, an appropriate content of carbon, and an appropriate aspect ratio in a ferrite phase and thus having excellent formability after preworking, and a method for producing the same.
  • Patent Document 8 discloses a high-tensile-strength hot-dip galvanized steel sheet having a sufficient strength-elongation balance and excellent fatigue properties and having a content of retained austenite of 3% or more, wherein 70% or more of grains of retained austenite has a ratio of the major axis to the minor axis of 0.2 to 0.4, i.e., an aspect ratio of 2.5 to 5.
  • Patent Document 9 discloses a steel sheet also having excellent hole expansibility obtained by adjusting the steel sheet disclosed in Patent Document 8 in such a manner that the proportion of martensite in a low-temperature transformation phase is 20% or less and that the ratio of the hardness of bainite in low-temperature transformation phase to the hardness of ferrite as a main phase is 2.6 or less.
  • the steel sheet disclosed in Patent Document 7 needs to have an appropriate volume fraction of retained austenite, an appropriate content of carbon, and an appropriate aspect ratio in a ferrite phase.
  • austempering in which the steel sheet is held for a relatively prolonged period of time in a bainite-transformation-temperature range.
  • it is necessary to modify the process e.g., a reduction in line speed, thereby significantly reducing productivity.
  • a structure before final annealing needs to be a structure including a low-temperature transformation phase such as bainite or martensite.
  • a structure needs to be formed during a hot-rolling step or by repeating an annealing step twice. Providing such a step restricts a production line and increases production costs, as described above.
  • Patent Document 1 Japanese Patent No. 2660644
  • Patent Document 2 Japanese Patent No. 2704350
  • Patent Document 3 Japanese Patent No. 3317303
  • Patent Document 4 Japanese Unexamined Patent Application Publication No. 2000-54072
  • Patent Document 5 Japanese Unexamined Patent Application Publication No. 2002-302734
  • Patent Document 6 Japanese Unexamined Patent Application Publication No. 2002-309334
  • Patent Document 7 Japanese Unexamined Patent Application Publication No. 2001-254138
  • Patent Document 8 Japanese Unexamined Patent Application Publication No. 2004-256836
  • Patent Document 9 Japanese Unexamined Patent Application Publication No. 2004-292891
  • the present invention has been made. It is an object of the present invention to provide a high-strength hot-dip galvanized steel sheet having excellent formability, the steel sheet eliminating special pre-structure control and capable of being produced by using a hot-dip galvanized steel-sheet production line that is not capable of sufficiently ensuring an austempering time after annealing, and to provide a method for producing the high-strength hot-dip galvanized steel sheet.
  • the inventors have conducted studies on factors affecting mechanical properties of a high-strength hot-dip galvanized steel sheet. Specifically, the inventors have investigated the relationship among chemical compositions, austempering conditions, and structures formed (states of retained austenite) in detail. Furthermore, the inventors have clarified the relationship between the structures formed and the mechanical properties. Therefore, the inventors have found that the incorporation of Cr in an appropriate amount (0.1% to 0.5%) exhibits characteristics different from Cr-free steel and Cr-rich steel; and the active utilization of the characteristics results in a steel sheet excellent in mechanical properties different from those in the known art.
  • the present invention provides items (1) to (6) described below.
  • a high-strength hot-dip galvanized steel sheet excellent in formability contains, on the basis of mass percent, 0.05-0.3% C, 1.4% or less (including 0%) Si, 0.08%-3% Mn, 0.003-0.1% P, 0.07% or less S, 0.1-2.5% Al, 0.1-0.5% Cr, and 0.007% or less N, Si+Al ⁇ 0.5%, and the balance being Fe and incidental impurities,
  • the steel sheet has a retained austenite content of 3% or more by volume fraction, and wherein the average aspect ratio of retained austenite grains is 2.5 or less.
  • the high-strength hot-dip galvanized steel sheet excellent in formability according to item (1) further contains, on the basis of mass percent, at least one element selected from 0.005-2% V and 0.005-2% Mo.
  • the high-strength hot-dip galvanized steel sheet excellent in formability according to item (1) or (2) further contains, on the basis of mass percent, at least one element selected from 0.01-0.5% Ti, 0.01-0.1% Nb, 0.0003-0.005% B, 0.005-2.0% Ni, and 0.005-2.0% Cu.
  • a method for producing a high-strength hot-dip galvanized steel sheet excellent in formability includes annealing a steel sheet in a first temperature region having a temperature of 700° C. to 900° C. for 15 to 600 seconds, the steel sheet containing, on the basis of mass percent, 0.05-0.3% C, 1.4% or less (including 0%) Si, 0.08%-3% Mn, 0.003-0.1% P, 0.07% or less S, 0.1-2.5% Al, 0.1-0.5% Cr, and 0.007% or less N, Si+Al ⁇ 0.5%, and the balance being Fe and incidental impurities; and cooling the steel sheet to a second temperature region having a temperature of 360° C. to 490° C. at a cooling rate of 5° C./s or more, wherein a retention time in the second temperature region is controlled on the basis of Formula (1):
  • t represents the total retention time (second) in the temperature region having a temperature of 360° C. to 490° C.
  • T represents an average temperature (° C.) when the steel sheet is retained for the total retention time in the temperature region having a temperature of 360° C. to 490° C.
  • the steel sheet further contains, on the basis of mass percent, at least one element selected from 0.01-0.5% Ti, 0.01-0.1% Nb, 0.0003-0.005% B, 0.005-2.0% Ni, and 0.005-2.0% Cu.
  • the present invention provides a high-strength hot-dip galvanized steel sheet having excellent formability, the steel sheet eliminating special pre-structure control and capable of being produced by using a hot-dip galvanized steel-sheet production line that is not capable of sufficiently ensuring an austempering time after annealing, and to provide a method for producing the high-strength hot-dip galvanized steel sheet.
  • FIG. 1 is a graph showing the relationship between the austempering time and the TS ⁇ T. El balance of each of 0.3%-Cr steel and Cr-free steel.
  • FIG. 2 is a graph showing the maximum hole-expanding ratio of each of 0.3%-Cr steel and Cr-free steel.
  • FIG. 3 is a graph showing the relationship between the aspect ratio and the TS ⁇ T. El balance of retained-austenite grains.
  • FIG. 4 is a graph showing the relationship between the aspect ratio and the maximum hole-expanding ratio of retained-austenite grains.
  • FIG. 5 is a graph showing the relationship between the Cr content and the TS ⁇ T. El balance.
  • FIG. 6 is a graph showing the relationship between the Cr content and the hole-expanding ratio.
  • FIG. 7 is a graph showing the relationship between the average retention temperature in a second temperature range and the retention time in the second temperature range.
  • a high-strength hot-dip galvanized steel sheet excellent in formability according to the present invention will be described in detail below.
  • FIG. 1 is a graph showing the relationship between the austempering time and the TS ⁇ T. El balance.
  • Steel A is a steel having a Cr content of 0.3%
  • Steel B is Cr-free steel.
  • Steel A has satisfactory mechanical properties even when subjected to austempering for a short time, compared with Steel B.
  • Steel A having satisfactory properties is maintained even when subjected to austempering for a long time, whereas in Steel B, mechanical properties are improved with increasing austempering time but are degraded with further increasing austempering time. That is, Steel B has a narrow range in which satisfactory properties are obtained.
  • the fact that the satisfactory properties can be ensured by austempering for a short time shows that the steel sheet can be produced using a CGL line that is not capable of performing austempering for a long time without a reduction in line speed, which is advantageous in view of mass productivity (productivity).
  • the line speed may be changed in response to the thickness of the sheet even when the same type of steel is used.
  • the fact that mechanical properties are largely unchanged with the austempering time is advantageous from the viewpoint that the stability of the mechanical properties of the steel sheet is ensured in mass production.
  • FIG. 2 shows the evaluation results of stretch-flange formability in terms of the maximum hole-expanding ratio ⁇ (%) of each of sheets of Steel A subjected to heat treatment under conditions X1 and X2 and sheets of Steel B subjected to heat treatment under conditions Y1 and Y2. This figure demonstrates that although these steel sheets are comparable in TS ⁇ T. El balance, Steel A containing Cr has stretch-flange formability superior to that of Steel B not containing Cr.
  • the inventors have conducted detailed investigation of causes for such difference based on the absence or presence of Cr, and found as follows: Hitherto, to obtain high ductility in TRIP steel, the promotion of an increase in carbon content in retained austenite by bainite transformation is believed to result in higher ductility. In contrast, when an appropriate amount of Cr is incorporated, sufficient properties are obtained even in the case of retained austenite having a shape relatively close to a block due to insufficient bainite transformation.
  • FIG. 3 shows the relationship between the aspect ratio and the TS ⁇ T. El balance of retained-austenite grains.
  • FIG. 4 shows the relationship between the aspect ratio and the maximum hole-expanding ratio ⁇ of retained-austenite grains.
  • Cr-free steel a low aspect ratio results in a high hole-expanding ratio, satisfactory stretch-flange formability, and a low TS ⁇ T. El balance.
  • a high aspect ratio improves the TS ⁇ T.
  • El balance and degrades stretch-flange formability In the case where an appropriate amount of Cr is incorporated (Cr: 0.1% to 0.5%), a high aspect ratio exhibits the same tendency as that of Cr-free steel.
  • FIG. 5 shows the Cr content and the TS ⁇ T. El balance.
  • FIG. 6 shows the Cr content and the hole-expanding ratio.
  • FIGS. 5 and 6 demonstrate that a Cr content in the range of the present invention, i.e., a Cr content of 0.1% to 0.5%, results in high ductility and high stretch-flange formability.
  • the present invention provides a steel sheet having a balance between high ductility and high stretch-flange formability achieved by incorporating an appropriate amount of Cr even in the presence of retained austenite having a low aspect ratio of crystal grains due to insufficient bainite transformation.
  • the chemical composition of a steel sheet of the present invention will be described below.
  • the term “%” used in the composition of the steel sheet refers to percent by mass.
  • C is an element which stabilizes austenite, which is required to ensure the amount of martensite, and which allows austenite to remain at room temperature.
  • a carbon content of less than 0.05% it is difficult to ensure the strength of a steel sheet and the amount of retained austenite to provide predetermined properties even when manufacturing conditions are optimized.
  • a carbon content exceeding 0.3% significantly hardens a weld zone and a heat-affected zone, thus degrading weldability. From the viewpoint, the carbon content is in the range of 0.05% to 0.3% and preferably 0.05% to 0.2%.
  • Si 1.4% or less (including 0%)
  • Si is an element effective in strengthening steel.
  • Si is an element that forms ferrite.
  • Si suppresses an increase in the carbon content of austenite and suppresses the formation of carbides, thus promoting the formation of retained austenite.
  • Si is often incorporated in dual-phase steel and TRIP steel.
  • the Si content is set in the range of 1.4% or less (including 0%).
  • Mn is an element which is effective in strengthening steel, which stabilizes austenite, and which is required to increase in the volume of martensite and retained austenite. The effect is exerted at a Mn content of 0.08% or more.
  • P is an element effective in strengthening steel. This effect is exerted at a P content of 0.003% or more. An excessive amount of P incorporated, i.e., a P content exceeding 0.1%, causes embrittlement due to grain boundary segregation, thereby degrading impact resistance. Therefore, the P content is set in the range of 0.003% to 0.1%.
  • S is formed into an inclusion, such as MnS, that causes a deterioration in impact resistance and causes cracks along flow of a metal in a weld zone.
  • the S content is preferably minimized. From the viewpoint of production costs, the S content is set at 0.07% or less.
  • Al is an element that forms ferrite. Al suppresses an increase in the carbon content of austenite and suppresses the formation of carbides, thus promoting the formation of retained austenite. Al has the effect of suppressing the degradation of plating properties and a surface state of a plating film due to Si. The effect is exerted at an Al content of 0.1% or more. A large amount of Al is incorporated in dual-phase steel and TRIP steel, in some cases. Excessive incorporation causes embrittlement of ferrite, thereby degrading the strength-ductility balance of the material. An Al content exceeding 2.5% increases the number of inclusions in steel sheet, thus degrading ductility. Therefore, the Al content is set in the range of 0.1% to 0.5%.
  • Cr is an element that forms ferrite. Cr suppresses an increase in the carbon content of austenite and suppresses the formation of carbides, thus promoting the formation of retained austenite. An appropriate amount of Cr incorporated results in a satisfactory strength-ductility balance even in the case of retained austenite having a shape relatively close to a block, thereby resulting in a balance between high ductility and high stretch-flange formability. The effect is exerted at a Cr content of 0.1% to 0.5%. Therefore, the Cr content is set in the range of 0.1% to 0.5%.
  • N is an element that most degrades the aging resistance of steel.
  • the N content is preferably minimized.
  • a N content exceeding 0.007% causes significant degradation in aging resistance. Therefore, the N content is set at 0.007% or less.
  • each of Si and Al is an element that forms ferrite and has the effect of promoting the formation of retained austenite.
  • the content of Si+Al is required to be 0.5% or more. Therefore, the content of Si+Al is set at 0.5% or more.
  • At least one element selected from V and Mo may be incorporated as an optional component.
  • V suppresses the formation of pearlite during cooling from an annealing temperature and thus may be incorporated, according to need.
  • Mo is effective for delayed fracture resistance and the like and may be incorporated, according to need.
  • the effect is exerted at a Mo content of 0.005% or more.
  • a Mo content exceeding 2% degrades formability. Therefore, when Mo is incorporated, the Mo content is set in the range of 0.005% to 2%.
  • At least one element selected from Ti, Nb, B, Ni, and Cu may be incorporated as an optional component.
  • Ti 0.01% to 0.5%
  • Nb 0.01% to 0.1% Ti and Nb are effective for precipitation strengthening and thus may be incorporated, according to need.
  • the effect is exerted when the Ti content is 0.01% or more or when the Nb content is 0.01% or more.
  • the effect may be utilized to strengthen steel as long as each of the contents is within the range specified in the present invention.
  • a Ti content exceeding 0.5% or a Nb content exceeding 0.1% formability and shape fixability are degraded. Therefore, when Ti is incorporated, the Ti content is set in the range of 0.01% to 0.5%.
  • the Nb content is set in the range of 0.01% to 0.1%.
  • B 0.0003% to 0.005%
  • B has the effect of suppressing the formation of ferrite from austenite grain boundaries and thus may be incorporated, according to need. The effect is exerted at a B content of 0.0003%. However, a B content exceeding 0.005% results in an excessively small amount of ferrite, thus degrading formability. Therefore, when B is incorporated, the B content is set in the range of 0.0003% to 0.005%.
  • Ni 0.005% to 2.0%
  • Cu 0.005% to 2.0%
  • Ni and Cu are each an element that stabilizes austenite.
  • Ni and Cu each have the effect of retaining austenite and increasing strength. The effect is exerted when the Ni content is 0.0005% or more or when the Cu content is 0.0005% or more.
  • the Ni content is set in the range of 0.005% to 2.0%.
  • the Cu content is set in the range of 0.005% to 2.0%.
  • the volume fraction of retained austenite and the average aspect ratio of retained austenite grains are specified below.
  • the retained austenite content is set at 3% or more by volume fraction.
  • Average aspect ratio of retained austenite grains 2.5 or less
  • the average aspect ratio of retained austenite grains is set at 2.5 or less.
  • a steel sheet having the above-described composition is annealed for 15 to 600 seconds in a first temperature region having a temperature of 700° C. to 900° C., specifically, in an austenite single-phase region or a two-phase region including an austenite phase and a ferrite phase.
  • the annealing temperature is less than 700° C. or when the annealing time is less than 15 seconds, in some cases, carbides in the steel sheet do not sufficiently dissolve, and the recrystallization of ferrite is not completed, thereby not obtaining target properties.
  • An annealing temperature exceeding 900° C. causes significant growth of austenite grains. This may reduce the number of nucleation sites for ferrite formed from a second phase during subsequent cooling.
  • An annealing time exceeding 600 seconds consumes a lot of energy, thus disadvantageously increasing costs.
  • the steel sheet After annealing, the steel sheet is cooled to a second temperature region having a temperature of 350° C. to 600° C. at a cooling rate of 5° C./s or more and is then retained in this temperature region for 5 to 200 seconds.
  • a cooling rate of less than 5° C./s results in the precipitation of pearlite and a significant reduction in the content of carbon dissolved in untransformed austenite.
  • a target structure is not obtained, in some cases.
  • the retention time is less than 5 seconds in this temperature region, the stabilization of untransformed austenite does not proceed. As a result, a retained austenite content of 3% or more is not obtained; hence, sufficient ductility is not ensured, in some cases.
  • the inventors have conducted studies on heat treatment conditions such that a steel sheet having satisfactory properties is produced more stably, and have found that with respect to heat treatment of the steel sheet after cooling, specifying the second temperature region so as to have a narrower temperature range of 360° C. to 490° C. and controlling the retention time at this temperature region on the basis of Formula (1) stably ensures a retained austenite content of 3% or more and an average aspect ratio of retained austenite of 2.5 or less.
  • t represents the total retention time (second) in the temperature region having a temperature of 360° C. to 490° C.
  • T represents an average temperature (° C.) when the steel sheet is retained for the total retention time in the temperature region having a temperature of 360° C. to 490° C.
  • FIG. 7 shows the relationship among the temperature and the retention time in the second temperature region and the aspect ratio.
  • hot-dip galvanizing is performed.
  • the temperature of a plating bath may be in a normal range of 450° C. to 500° C.
  • treatment is preferably performed at 600° C. or lower. The reason for this is as follows: When the temperature of the plating bath exceeds 600° C., carbides are precipitated from untransformed austenite, as described above. As a result, stable retained austenite is not obtained, thereby degrading ductility.
  • the retention temperature need not be a constant as long as it is within the specified range. Even when the cooling rate varies during cooling, there is no problem as long as the cooling rate is within the specified range.
  • the steel sheet may be subjected to heat treatment with any equipment as long as the heat history is satisfied.
  • the steel sheet of the present invention may be subjected to skin pass rolling for shape correction after heat treatment.
  • the steel sheet is preferably produced through common steps, i.e., steelmaking, casting, and hot rolling. Alternatively, for example, part or the entirety of the hot rolling step may be omitted by employing thin casting or the like.
  • a cast slab obtained by refining steel having a chemical composition shown in Table 1 was subjected to hot rolling, pickling, and cold rolling to form a cold-rolled steel sheet having a thickness of 1.2 mm.
  • the resulting steel sheet was subjected to skin pass rolling at a reduction of 0.3%.
  • the N content of steel was 0.0020 to 0.0060 percent by mass.
  • the structure of the section (plane parallel to the rolling direction) of the steel sheet was observed with a scanning electron microscope (SEM) at a magnification of ⁇ 2,000 from 10 fields of view.
  • the aspect ratio (major axis/minor axis) of each of retained austenite grain was observed, the average value of the resulting aspect ratio values was defined as the average aspect ratio.
  • a sample used for SEM observation was subjected to heat treatment at 200° C. for 2 hours (in order to be formed into an observable sample by separating martensite from retained austenite), mirror polishing, and natal etching. Then the sample was tested.
  • the resulting SEM image was subjected to image processing to determine the content of retained austenite.
  • the steel sheet was processed into a JIS No. 5 specimen and was subjected to a tensile test.
  • Tensile strength (TS) and total elongation (T. El) were measured to determine the value of a strength-elongation balance expressed by multiplying strength by total elongation (TS ⁇ T. El).
  • TS ⁇ T. El a strength-elongation balance expressed by multiplying strength by total elongation
  • Stretch-flange formability was evaluated as follows: The resulting steel sheet was cut into a piece having a size of 100 mm ⁇ 100 mm. A hole having a diameter of 10 mm was made in the piece by punching at a clearance of 12%. A cone punch with a 60° apex was forced into the hole while the piece was fixed with a die having an inner diameter of 75 mm at a blank-holding pressure of 9 ton. The diameter of the hole was measured when a crack was initiated. The maximum hole-expanding ratio ⁇ (%) was determined with Formula (2). Stretch-flange formability was evaluated on the basis of the maximum hole-expanding ratio. In the present invention, when ⁇ 50%, the maximum hole-expanding ratio was determined to be satisfactory.
  • D f represents the hole diameter (mm) when a crack was initiation; and D 0 represents an initial hole diameter (mm).
  • Tables 2 and 3 also summarize the test results.
  • the results demonstrate that the steel sheet satisfying the requirements specified in the present invention has an excellent balance between strength and elongation and between strength and stretch-flange formability, and target properties are obtained. Furthermore, the results demonstrate that the production of the steel sheet under the conditions satisfying the requirements specified in the present invention stably results in the target properties.
  • the present invention can be widely applied to lightweight, high-strength steel sheets having excellent formability for vehicles such as automobiles.

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