EP1338667B1 - Tole d'acier laminee a froid presentant une resistance elevee a la traction du type structure composite - Google Patents

Tole d'acier laminee a froid presentant une resistance elevee a la traction du type structure composite Download PDF

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Publication number
EP1338667B1
EP1338667B1 EP01998666A EP01998666A EP1338667B1 EP 1338667 B1 EP1338667 B1 EP 1338667B1 EP 01998666 A EP01998666 A EP 01998666A EP 01998666 A EP01998666 A EP 01998666A EP 1338667 B1 EP1338667 B1 EP 1338667B1
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Prior art keywords
mass
phase
steel sheet
cold rolled
rolled steel
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EP01998666A
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German (de)
English (en)
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EP1338667A1 (fr
EP1338667A4 (fr
Inventor
Saiji Kawasaki Steel Corporation Matsuoka
Kazuhiro Kawasaki Steel Corporation HANAZAWA
Tetsuo c/o Kawaski Steel Corporation SHIMIZU
Kei Kawasaki Steel Corporation Sakata
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JFE Steel Corp
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JFE Steel Corp
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Priority claimed from JP2001312688A external-priority patent/JP4010132B2/ja
Priority claimed from JP2001312687A external-priority patent/JP4010131B2/ja
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Publication of EP1338667A1 publication Critical patent/EP1338667A1/fr
Publication of EP1338667A4 publication Critical patent/EP1338667A4/fr
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • C22C38/001—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
    • C22C38/04—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
    • 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
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • 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
    • 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 invention relates to a high-strength dual-phase steel sheet having an excellent deep drawability, and particularly to a high-strength dual-phase cold rolled steel sheet having an excellent deep drawability and a high strength dual phase galvanized steel sheet having an excellent deep drawability which have a tensile strength of 440 MPa or more and are suitable for use in steel sheets for vehicles as well as a method of producing the same.
  • the high-strength steel sheet used is required to have an excellent press formability.
  • r-value Lankford value
  • EI high ductility
  • YS low yield stress
  • a high corrosion resistance is required according to a position of the vehicle part to be applied, so that various surface-treated steel sheets having an excellent corrosion resistance are used as a steel sheet for the vehicle parts up to now.
  • a galvanized steel sheet is manufactured in a continuous galvanizing equipment conducting recrystallization annealing and galvanizing at the same line, so that the provision of an excellent corrosion resistance and a cheap production are possible.
  • an alloyed galvanized steel sheet obtained by subjecting to a heat treatment after the galvanization is excellent in the weldability and press formability in addition to the excellent corrosion resistance. Therefore, they are widely used.
  • the high-strength steel sheet having a good press formability is mentioned a dual-phase steel sheet having a dual-phase microstructure of a soft ferrite phase and a hard martensite phase.
  • the dual-phase steel sheet produced by cooling with a gas jet after the continuous annealing is low in the yield stress and possesses a high ductility and an excellent baking hardenability.
  • the above dual-phase steel sheet is generally good in the workability, but has a drawback that the workability under severer condition is poor and particularly, the r-value is low and the deep drawability is bad.
  • the continuous galvanizing line is general to set up the annealing equipment and the plating equipment continuously.
  • the cooling after the annealing is constrained by a plating temperature and can not drop down to a temperature lower than the plating temperature at once and hence the cooling is interrupted.
  • an average cooling rate necessarily becomes smaller. Therefore, when the galvanized steel sheet is produced in the continuous galvanizing line, it is difficult to generate martensite phase produced under a cooling condition of a large cooling rate into the steel sheet after the galvanization. To this end, it is generally difficult to produce the high-strength galvanized steel sheet having a dual-phase microstructure of a ferrite phase and a martensite phase through the continuous galvanizing line.
  • JP-B-55-10650 discloses a technique that a box annealing is carried out at a temperature ranging from a recrystallization temperature to A c3 transformation point after the cold rolling and thereafter the continuous annealing inclusive of quenching and tempering is carried out after the heating to 700-800°C in order to obtain the mixed microstructure.
  • the quenching and tempering are carried out during the continuous annealing, so that the yield stress is high and hence a low yield ratio can not be obtained.
  • the steel sheet having such a high yield stress is not suitable for the press formability and has a drawback that the shapability in the pressed parts is bad.
  • JP-A-55-100934 a method for lowering the high yield stress is disclosed in JP-A-55-100934 .
  • the box annealing is first carried out in order to obtain a high r-value, wherein the temperature in the box annealing is made to a two-phase region of ferrite ( ⁇ )-austenite ( ⁇ ) and Mn is enriched from ⁇ phase to ⁇ phase during the soaking.
  • Mn enriched phase preferentially becomes ⁇ phase during the continuous annealing, the dual-phase microstructure is obtained even at a cooling rate as in the gas jet cooling, and further the yield stress becomes low.
  • the high cooling rate of 100°C/s is difficult to attain in the gas jet cooling usually used in the continuous annealing line or continuous galvanizing line after the cold rolling, and is required to use an equipment for water-quenching, and also a problem becomes actual in the surface treatment of the water-quenched steel sheet, so that there are problems in the production equipment and the materials.
  • the high-strength dual-phase galvanized steel sheet is generally used as the method of producing the high-strength dual-phase galvanized steel sheet as the method of producing the high-strength dual-phase galvanized steel sheet is generally used a method wherein the formation of low-temperature transformation phase is facilitated by using a steel added with a large amount of an alloying element such as Cr or Mo for enhancing a hardenability.
  • an alloying element such as Cr or Mo
  • the addition of the large amount of the alloying element undesirably brings about the rise of the production cost.
  • JP-B-62-40405 there is proposed a method of producing the high-strength dual-phase galvanized steel sheet by defining the cooling rate after the annealing or the plating in the continuous galvanizing line.
  • this method is not actual from the constraint on the equipment for the continuous galvanizing line and also the steel sheet obtained by this method is not said to have a sufficient ductility.
  • EP 0 969 112 discloses a dual-phase high strength steel having excellent dynamic deformation properties and a method of production thereof.
  • an object of the invention to solve the aforementioned problems and to provide high-strength dual-phase cold rolled steel sheets having an excellent deep drawability and high-strength dual-phase galvanized steel sheets having an excellent deep drawability as well as a method of producing the same.
  • galvanized steel sheet used herein means to include a galvanized steel sheet obtained by subjecting to a galvanization containing aluminum or the like in addition to zinc and an alloyed galvanized steel sheet obtained by subjecting to a heat (alloying) treatment for diffusing iron of the matrix steel sheet into the plated layer after the galvanization.
  • the inventors have made various studies with respect to an influence of the alloying element upon the microstructure and the recrystallization texture in the steel sheet.
  • the experiments are performed with respect to a high-strength dual-phase cold rolled steel sheet of TS: 590 MPa grade and a high-strength dual-phase cold rolled steel sheet of TS: 780 MPa grade.
  • the hot rolled steel sheet after the finish rolling is subjected to a temperature holding treatment of 650°C ⁇ 1 hour as a coiling treatment. Subsequently, the sheet is subjected to a cold rolling at a rolling reduction of 70% to obtain a cold rolled steel sheet having a thickness of 1.2 mm. Next, the cold rolled steel sheet is subjected to a recrystallization annealing at 850°C for 60 seconds and cooled at a cooling rate of 30°C/s.
  • the sheet after the finish rolling is subjected to a temperature holding treatment of 650°C ⁇ 1 hour as a coiling treatment. Subsequently, the sheet is subjected to a cold rolling at a rolling reduction of 70% to obtain a cold rolled steel sheet having a thickness of 1.2 mm. Next, the cold rolled steel sheet is subjected to a recrystallization annealing at 850°C for 60 seconds and cooled at a cooling rate of 30°C/s.
  • the tensile test is carried out by using JIS No. 5 tensile test piece.
  • an abscissa in FIGS. 1a and 1b is an atomic ratio ((V/51)/(C/12)) of V content to C content, and an ordinate is r-value in FIG. 1a and yield ratio (YR) in FIG. 1b .
  • a high r-value and a low yield ratio are obtained by limiting V content in the steel slab to a range of 0.5-3.0 as the atomic ratio to C content and it is possible to produce high-strength dual-phase cold rolled steel sheet having an excellent deep drawability.
  • the inventors found that a high r-value is obtained because solid-solute C and N are less and ⁇ 111 ⁇ recrystallization texture is strongly developed before the recrystallization annealing. And also, the inventors found that by annealing at ⁇ - ⁇ two-phase region is dissolved V carbide and the solid-solute C is enriched into austenite phase in large quantity and the austenite can be easily transformed into martensite in the subsequent cooling process to obtain a dual-phase microstructure of ferrite and martensite.
  • V having a solubility of carbide higher than those of Ti and Nb for effectively obtaining the solid-solute C by annealing at a higher temperature region. That is, it is found that since V carbide easily dissolves as compared with Ti carbide and Nb carbide in the annealing at a high temperature, a sufficient amount of solid-solute C for transforming austenite to martensite is obtained by annealing at the ⁇ - ⁇ two-phase region. In addition, it is clear that this phenomenon is most remarkably generated by V, but the similar result is obtained by adding Nb and Ti together.
  • FIGS. 2a and 2b show an influence of V, Nb and Ti contents in the steel slab upon tensile strength (TS) and Lankford value (r-value) of a cold rolled steel sheet in the cold rolled steel sheets of TS: 590 MPa grade and TS: 780 MPa grade produced by using the V, Nb and Ti containing steel slab. Moreover, an abscissa in FIGS.
  • 2a and 2b is an atomic ratio (2 ⁇ Nb/93+2 ⁇ Ti/48)/(V/51) of Nb and Ti contents to V content, and an ordinate is tensile strength (TS) in FIG. 2a and r-value in FIG. 2b .
  • the r-value is considerably improved when the value of (2 ⁇ Nb/93+2 ⁇ Ti/48)/(V/51) is a range of not less than 1.5.
  • the invention is accomplished by further examining based on the above knowledge.
  • the summary of the invention is as follows.
  • a high-strength dual-phase cold rolled steel sheet having an excellent deep drawability wherein the steel sheet has a composition comprising further not more than 0.3 mass% in total of one or tow of Nb: more than 0 mass% but not more than 0.3 mass% and Ti: more than 0 mass% but not more than 0.3 mass% optionally not more than 2.0 mass% in total of one or two of Cr and Mo; provided that V, Nb, Ti and C satisfy a relationship represented by the following equation (ii) instead of the equation (i): 0.5 ⁇ C / 12 ⁇ V / 51 + 2 ⁇ Nb / 93 + 2 ⁇ Ti / 48 ⁇ 3 ⁇ C / 12 and the remainder being Fe and inevitable impurities.
  • Nb 0.001-0.3 mass%
  • Ti 0.001-0.3 mass% is not more than 0.3 mass% in total.
  • a high-strength dual-phase cold rolled steel sheet having an excellent deep drawability wherein the steel sheet comprises C: 0.03-0.08 mass%, Si: 0.1-2.0 mass%, Mn: 1.0-3.0 mass%, P: not more than 0.05 mass% and S: not more than 0.01 mass% and V, Nb and Ti satisfy a relationship of 1.5 ⁇ (2 ⁇ Nb/93+2 ⁇ Ti/48)/(V/51) ⁇ 15.
  • a method of producing a high-strength dual-phase cold rolled steel sheet having an excellent deep drawability according to the item (5), wherein the steel sheet has a composition comprising further not more than 0.3 mass% in total of one or tow of Nb: more than 0 mass% but not more than 0.3 mass% and Ti: more than 0 mass% but not more than 0.3 mass% optionally not more than 2.0 mass% in total of one or two of Cr and Mo; optionally provided that V, Nb, Ti and C satisfy a relationship represented by the following equation (iv) instead of the equation (iii): 0.5 ⁇ C / 12 ⁇ V / 51 + 2 ⁇ Nb / 93 + 2 ⁇ Ti / 48 ⁇ 3 ⁇ C / 12 and the remainder being Fe and inevitable impurities.
  • Nb 0.001-0.3 mass%
  • Ti 0.001-0.3 mass% is not more than 0.3 mass% in total.
  • a method of producing a high-strength dual-phase cold rolled steel sheet having an excellent deep drawability according to the item (6), wherein the steel slab comprises C: 0.03-0.08 mass%, Si: 0.1-2.0 mass%, Mn: 1.0-3.0 mass%, P: not more than 0.05 mass% and S: not more than 0.01 mass% and V, Nb and Ti satisfy a relationship of 1.5 ⁇ (2 ⁇ Nb/93+2 ⁇ Ti/48)/(V/51) ⁇ 15.
  • a high-strength dual-phase galvanized steel sheet having an excellent deep drawability comprising a galvanized coating on the steel sheet disclosed in any one of the items above.
  • a method of producing a high-strength dual-phase galvanized steel sheet having an excellent deep drawability wherein a galvanization is carried out after the continuous annealing at a temperature range from a A C1 transformation point to a A C3 transformation point in the production method described in any one of the items (5)-(7).
  • a method of producing a high-strength dual-phase galvanized steel sheet having an excellent deep drawability according to the item (10), which further comprising a continuous annealing step between the cold rolling step and the continuous annealing step at a temperature range from a A C1 transformation point to a A C3 transformation point.
  • the cold rolled steel sheet and the galvanized steel sheet according to the invention are high-strength dual-phase steel sheets having a tensile strength (TS) of not less than 440 MPa and an excellent deep drawability.
  • TS tensile strength
  • C is an element for increasing the strength of the steel sheet and further promoting the formation of a dual-phase microstructure of ferrite and martensite, and is necessary to contain not less than 0.015% from a viewpoint of the formation of the dual-phase microstructure in the invention. Moreover, if it is intended to increase the strength to TS: not less than 540 MPa and TS: not less than 780 MPa, the C content is not less than 0.015% and not less than 0.03%, respectively. On the other hand, when the C content exceeds 0.08%, the development of ⁇ 111 ⁇ recrystallization texture is obstructed to degrade the deep drawability. Therefore, the invention limits the C content to 0.015-0.08%. When it is particularly required to increase the strength of the steel sheet, it is preferable to be 0.03-0.08%. Moreover, it is preferable to be not more than 0.05% from a viewpoint of the deep drawability.
  • Si is a useful reinforcing element capable of increasing the strength of the steel sheet without remarkably lowering the ductility of the steel sheet, if the content exceeds 2.0%, the deterioration of the deep drawability is caused, but also the surface properties are degraded. Therefore, Si is limited to not more than 2.0%. Moreover, if it is intended to increase the strength to TS: not less than 780 MPa, it is preferable to be not less than 0.1% for ensuring the required strength. And also, it is preferable to be not less than 0.01% for increasing the strength to TS: not less than 440 MPa which is a main object of the invention.
  • Mn has an action reinforcing the steel and further has an action of lessening a critical cooling rate for the obtention of the dual-phase microstructure of ferrite and martensite to promote the formation of the dual-phase microstructure of ferrite and martensite, so that it is preferable to contain a content in accordance with the cooling rate after the recrystallization annealing.
  • Mn is an effective element preventing the hot tearing through S, so that it is preferable to contain an appropriate content in accordance with S content.
  • the Mn content exceeds 3.0%, the deep drawability and weldability are degraded. In the invention, therefore, the Mn content is limited to not more than 3.0%.
  • the Mn content is not less than 0.5% for remarkably developing the above effect, and particularly it is preferable to be not less than 1.0% for increasing the strength to TS: not less than 780 MPa. And also, it is preferable to be not less than 0.1% for increasing the strength to TS: not less than 440 MPa which is a main object of the invention.
  • the P has an action reinforcing the steel and can be contained in a required amount in accordance with the desired strength.
  • the P content exceeds 0.10%, the press formability is degraded. Therefore, the P content is limited to not more than 0.10%.
  • the P content is preferable to be not more than 0.08%.
  • the P content is preferable to be not more than 0.05% in order to prevent the degradation of the weldability.
  • it is intended to increase the strength to TS: not less than 440 MPa it is preferable to be not less than 0.001%.
  • S is existent as an inclusion in the steel sheet and is an element bringing about the degradation of the ductility and the formability of the steel sheet, particularly the stretch-flanging property. Therefore, it is preferable to be decreased as far as possible, and when it is decreased to not more than 0.02%, S does not exert a bad influence, so that the S content is 0.02% as an upper limit in the invention.
  • the S content is preferable to be not more than 0.01 %, more preferably not more than 0.005%.
  • the S content is preferable to be not less than 0.0001% considering a cost for the removal of S in the steelmaking process.
  • the Al is added to the steel as a deoxidizing element and is a useful element for improving the cleanliness of the steel, but the addition effect is not obtained at less than 0.005%.
  • the addition effect is not obtained at less than 0.005%.
  • the invention does not exclude a steelmaking method through deoxidization other than the Al deoxidization.
  • Ti deoxidization or Si deoxidization may be conducted.
  • the steel sheets made by these deoxidizing methods are included within a scope of the invention. In this case, even if Ca, REM and the like are added to the molten steel, the characteristics of the steel sheet according to the invention are not obstructed, so that the steel sheet including Ca, REM and the like is naturally included within the scope of the invention.
  • N is an element increasing the strength of the steel sheet by the solid-solution hardening and the strain ageing hardening, but when N content exceeds 0.02%, the nitride is increased in the steel sheet to remarkably degrade the deep drawability of the steel sheet. Therefore, the N content is limited to not more than 0.02%. Moreover, in case of requiring the more improvement of the press formability, the N content is preferable to be not more than 0.01 %, more preferably not more than 0.004%. In this case, considering the cost for denitrification in the steelmaking process, the N content is preferable to be not less than 0.0001%.
  • V 0.01-0.5% and 0,5 ⁇ C/12 ⁇ V/51 ⁇ 3 ⁇ C/12
  • V is a most important element in the invention.
  • the solid-solute C is precipitated and fixed as V carbide to develop the ⁇ 111 ⁇ recrystallization texture, whereby a high r-value can be obtained.
  • V dissolves the V carbide in the annealing at ⁇ - ⁇ two-phase region to enrich a large quantity of the solid-solute C in austenite phase, which is easily transformed into martensite at the subsequent cooling process, whereby the dual-phase steel sheet having a dual-phase microstructure of ferrite and martensite can be obtained.
  • V content is not less than 0.01%, more preferably not less than 0.02% and satisfies 0.5 ⁇ C/12 ⁇ V/51 in relation to the C content.
  • V content exceeds 0.5% or when it is V/51 > 3 ⁇ C/12 in relation to the C content, the dissolution of the V carbide at the ⁇ - ⁇ two-phase region hardly occurs and the dual-phase microstructure of ferrite and martensite is hardly obtained. Therefore, the V content is limited to 0.01-0.5% and to 0.5 ⁇ C/12 ⁇ V/51 ⁇ 3 ⁇ C/12.
  • V/51 ⁇ 2 ⁇ C/12 is preferable for obtaining the dual-phase microstructure of ferrite and martensite.
  • Nb not more than 0.3% in total of one or tow of Nb: more than 0% but not more than 0.3% and Ti: more than 0% but not more than 0.3%
  • V, Nb, Ti and C satisfy 0.5 ⁇ C/12 ⁇ (V/51+2 ⁇ Nb/93+2 ⁇ Ti/48) ⁇ 3 ⁇ C/12
  • Nb and Ti are carbide forming elements likewise V and have the same action as V mentioned above. That is, a high r-value can be obtained by precipitating and fixing the solid-solute C as Nb and Ti carbides before the recrystallization to develop the ⁇ 111 ⁇ recrystallization texture, and also a dual-phase steel sheet having a dual-phase microstructure of ferrite and martensite can be obtained by dissolving the Nb and Ti carbides in the annealing at the ⁇ - ⁇ two-phase region to enrich a large quantity of the solid-solute C in austenite phase and transforming into martensite in the subsequent cooling process. Moreover, as the above effect of Nb and Ti is considerably small as compared with that of V, when only Nb and Ti are added to the steel slab without adding V, the deep drawability aiming at the invention can not be enhanced sufficiently.
  • Nb and Ti of more than 0%. More preferably, each of the Nb and Ti contents is not less than 0.001%. In this case, it is preferable to satisfy 0.5 ⁇ C/12 ⁇ (V/51+2 ⁇ Nb/93+2 ⁇ Ti/48) in relation to the C and V contents for developing the above effect. On the other hand, when each of Nb and Ti contents or both in total thereof exceeds 0.3%, or when the Nb and Ti contents satisfy (V/51+2 ⁇ Nb/93+2 ⁇ Ti/48) > 3 ⁇ C/12 in relation to the C and V contents, the dissolution of the carbide at the ⁇ - ⁇ two-phase region hardly occurs and hence the dual-phase microstructure of ferrite and martensite is hardly obtained.
  • each of the Nb content and the Ti content is within a range of more than 0% but not more than 0.3%, and when both of Nb and Ti are added together, the Nb and Ti contents are not more than 0.3% in total and satisfy 0.5 ⁇ C/12 ⁇ (V/51+2 ⁇ Nb/93+2 ⁇ Ti/48) ⁇ 3 ⁇ C/12 in relation to the V and C contents.
  • the deep drawability is apt to be easily degraded by the addition of large quantities of solid-solution strengthening elements such as C, Mn and the like.
  • the V, Nb and Ti contents are further desirable to be a range of 1.5 ⁇ (2 ⁇ Nb/93+2 ⁇ Ti/48)/ (V/51) ⁇ 15.
  • (2 ⁇ Nb/93+2 ⁇ Ti/48)/ (V/51) is limited to not less than 1.5 is considered due to the fact that although the detail of the cause is not clear, the formation of carbide after the hot rolling is promoted to decrease the solid-solute C by adding large quantities of Nb and Ti as compared with V and hence the ⁇ 111 ⁇ recrystallization texture is easily developed.
  • (2 ⁇ Nb/93+2 ⁇ Ti/48)/ (V/51) is desirable to be a range of not more than 15.
  • the steel according to the invention may further comprise one or two of not more than 2.0% in total of one or two of Cr and Mo; not more than 2.0% in total of one or two of Cr and Mo
  • All of Cr and Mo in the A-group have an action of decreasing the critical cooling rate for providing the dual-phase microstructure of ferrite and martensite to promote the formation of the dual-phase microstructure of ferrite and martensite likewise Mn and can be included, if necessary.
  • the lower limits of the Cr content and Mo content preferable for obtaining the above effect are Cr: 0.05%, Mn: 0.05%.
  • Cr and Mo exceed 2.0% in total, the deep drawability is degraded.
  • one or more of Cr and Mo in the are limited to not more than 2.0% in total.
  • the reminder other than the above elements is Fe and inevitable impurities.
  • the inevitable impurity are mentioned, for example, Sb, Sn, Zn, Co and the like.
  • acceptable ranges of their contents are Sb: not more than 0.01%, Sn: not more than 0.1%, Zn: not more than 0.01% and Co: not more than 0.1%.
  • the cold rolled steel sheet according to the invention has a microstructure consisting of ferrite phase as a primary phase and a secondary phase including not less than 1% of martensite phase at an area ratio with respect to a whole of the microstructure.
  • the microstructure of the steel sheet according to the invention In order to provide the cold rolled steel sheet having a low yield stress (YS), a high ductility (El) and an excellent deep drawability, it is required to render the microstructure of the steel sheet according to the invention into a dual-phase microstructure consisting of a ferrite phase as a primary phase and a secondary phase including a martensite phase. It is preferable that the ferrite phase as a primary phase is not less than 80% at an area ratio and hence the secondary phase is not more than 20%. When the area ratio of the ferrite phase is less than 80%, it is difficult to ensure the high ductility and the press formability tends to lower.
  • the ferrite phase is not less than 85% at the area ratio and hence the secondary phase is not more than 15%. Moreover, in order to utilize the advantage of the dual-phase microstructure, the ferrite phase is required to be not more than 99%.
  • the secondary phase is required to include the martensite phase at the area ratio of not less than 1% with respect to the whole of the microstructure.
  • the martensite is less than 1% at the area ratio, the low yield stress (YS) and the high ductility (El) can not be satisfied simultaneously. More preferably, the martensite phase is not less than 3% but not more than 20% at the area ratio. In case of requiring a good ductility, the martensite phase is preferable to be not more than 15% at the area ratio.
  • the secondary phase may be constituted by only the martensite phase at the area ratio of not less than 1% or by mixed phases of the martensite phase at the area ratio of not less than 1% and any of a pearlite phase, a bainite phase and a retained austenite as an additional phase and is not especially limited.
  • the pearlite phase, the bainite phase and the retained austenite are preferable to be not more than 50% in total at the area ratio with respect to the microstructure of the secondary phase in order to more effectively develop the effect of the martensite phase.
  • the cold rolled steel sheet and the galvanized steel sheet having the above microstructure are steel sheets having a low yield stress, a high ductility and an excellent deep drawability.
  • composition of the steel slab used in the production method of the invention is the same as the compositions of the aforementioned cold rolled steel sheet and the galvanized steel sheet, so that the explanation on the reason of the limitation in the steel slab is omitted.
  • the cold rolled steel sheet according to the invention is produced by using a steel slab having a composition of the above range as a starting material and successively subjecting this starting material to a hot rolling step of subjecting to a hot rolling to obtain a hot rolled steel sheet, a pickling step of pickling the hot rolled steel sheet, a cold rolling step of subjecting the hot rolled steel sheet to a cold rolling to obtain a cold rolled steel sheet, and a recrystallization annealing step of subjecting the cold rolled steel sheet to a recrystallization annealing to obtain a cold rolled annealed steel sheet.
  • the galvanized steel sheet according to the invention is produced by using a steel slab having a composition of the above range as a starting material and successively subjecting this starting material to a hot rolling step of subjecting to a hot rolling to obtain a hot rolled steel sheet, a pickling step of pickling the hot rolled steel sheet, a cold rolling step of subjecting the hot rolled steel sheet to a cold rolling to obtain a cold rolled steel sheet, and a continuous galvanization step of subjecting the cold rolled steel sheet to a recrystallization annealing and a galvanizing to obtain a galvanized steel sheet. Furthermore, it is produced by subjecting the cold rolled steel sheet to a step of annealing and pickling before the continuous galvanization step, if necessary.
  • the steel slab used is preferable to be produced by a continuous casting process in order to prevent the macro-segregation of the components, but may be produced by an ingot casting process or a thin slab casting process. Furthermore, in addition to the conventional process of cooling to a room temperature once after the production of the steel slab and again heating, energy-saving processes such as a process for inserting a hot steel slab into a heating furnace without cooling, a process for direct sending rolling or direct rolling immediately after slight heat-holding and the like can be applied without problems.
  • the above starting material (steel slab) is subjected to the hot rolling step of forming the hot rolled steel sheet by heating and hot rolling.
  • the hot rolling step there is particularly no problem even in the use of usual rolling conditions as .long as the hot rolled steel sheet having a desired thickness can be produced.
  • preferable hot rolling conditions are mentioned below for the reference.
  • the slab heating temperature is desirable to be made lower as far as possible in order to improve the deep drawability by coarsening the precipitate to develop the ⁇ 111 ⁇ recrystallization texture.
  • the slab heating temperature is lower than 900°C, the rolling load increases and the risk of causing troubles in the hot rolling increases.
  • the slab heating temperature is preferable to be not lower than 900°C.
  • the upper limit of the slab heating temperature is more preferable to be 1300°C in terms of the lowering of the yield resulted from the increase of scale loss accompanied with the increase of the oxide weight.
  • the utilization of a so-called sheet bar heater of heating the sheet bar in the hot rolling is an effective process from a viewpoint that the slab heating temperature is lowered and the troubles in the hot rolling are prevented.
  • Finisher delivery temperature not lower than 700°C
  • the finisher delivery temperature (FDT) is preferable to be not lower than 700°C in order to obtain a uniform microstructure of the hot rolled parent sheet for providing an excellent deep drawability after the cold rolling and the recrystallization annealing. That is, when the finish deformation temperature is lower than 700°C, not only the microstructure of the hot rolled parent sheet becomes nonuniform, but also the rolling load in the hot rolling becomes higher and the risk of causing the trouble in the hot rolling is increased.
  • Coiling temperature not more than 800°C
  • the coiling temperature is preferable to be not higher than 800°C. That is, when the coiling temperature exceeds 800°C, the scale increases and the yield tends to lower due to the scale loss. And also, when the coiling temperature is lower than 200°C, the shape of the steel sheet remarkably is disordered and the risk of causing problems in the actual use increases, so that the lower limit of the coiling temperature is more preferable to be 200°C.
  • the steel slab is heated above 900°C, subjected to the hot rolling at the finish deformation temperature of not lower than 700°C, and coiled at the coiling temperature of not higher than 800°C.
  • a lubrication rolling may be conducted in a part of the finish rolling or between passes thereof in order to reduce the rolling load in the hot rolling.
  • the application of the lubrication rolling is effective from a viewpoint of the uniformization of the steel sheet shape and the homogenization of the material.
  • the coefficient of friction in the lubrication rolling is preferable to be within a range of 0.10-0.25.
  • the hot rolling step is preferable to be a continuous rolling process wherein the sheet bars located in front and rear are joined to each other and continuously subjected to the finish rolling.
  • the application of the continuous rolling process is desirable from a viewpoint of the operating stability in the hot rolling.
  • the pickling step is sufficient according to the usual manner and it is preferable to use a treating solution such as hydrochloric acid, sulfuric acid or the like as a pickling solution.
  • the cold rolled steel sheet is formed by subjecting the hot rolled steel sheet to the cold rolling.
  • the cold rolling conditions are not especially limited as long as the cold rolled steel sheet having desired size and shape can be obtained, but it is preferable that a rolling reduction in the cold rolling is not less than 40%. When the rolling reduction is less than 40%, the ⁇ 111 ⁇ recrystallization texture is not developed and the excellent deep drawability can not be obtained.
  • the cold rolled steel sheet according to the invention is subjected to a recrystallization annealing in the subsequent recrystallization annealing step to obtain a cold rolled annealed steel sheet.
  • the recrystallization annealing is carried out in a continuous annealing line.
  • the galvanized steel sheet according to the invention is produced by subjecting the cold rolled steel sheet to recrystallization annealing and galvanizing in the continuous galvanization line after the cold rolling.
  • the annealing temperature in the recrystallization annealing is required to be conducted at a ( ⁇ + ⁇ ) two-phase region within a temperature range from A C1 transformation point to A C3 transformation point.
  • the annealing is carried out at ( ⁇ + ⁇ ) two-phase region to dissolve the carbides of V, Ti and Nb to thereby distribute an amount of solid-solute C sufficient to transform austenite to martensite into the austenite phase.
  • the annealing temperature is lower than the A C1 transformation point, the microstructure is rendered into the ferrite single phase and the martensite can not be generated, while when it is higher than the A C3 transformation point, the crystal grains are coarsened and the microstructure is rendered into the austenite single phase and the ⁇ 111 ⁇ recrystallization texture is not developed and hence the deep drawability is deteriorated remarkably.
  • the cooling in the recrystallization annealing is preferable to be conducted at a cooling rate of not less than 5°C/s in order to produce the martensite phase to obtain the dual-phase microstructure of ferrite and martensite.
  • the galvanized steel sheet according to the invention it is preferable to quench to a temperature region of 380-530°C after the above recrystallization annealing.
  • a stop temperature of the quenching is lower than 380°C, the defective plating easily occurs, while when it exceeds 530°C, the unevenness easily occurs on the plated surface.
  • the cooling rate is preferable to be not less than 5°C/s in order to produce the martensite phase to obtain the dual-phase microstructure of ferrite and martensite.
  • the galvanization is carried out by dipping in a galvanizing bath.
  • Al concentration in the galvanizing bath is preferable to be within a range of 0.12-0.145 mass%.
  • the Al concentration in the galvanizing bath is less than 0.12 mass%, the alloying excessively advances and the plating adhesion (resistance to powdering) tends to be deteriorated, while when it exceeds 0.145 mass%, the defective plating easily occurs.
  • the plated layer may be subjected to an alloying treatment after the galvanization.
  • the alloying treatment is preferable to be conducted so that Fe content in the plated layer is 9-12%.
  • the alloying treatment it is preferable to conduct the alloying of the galvanized layer by reheating up to a temperature region of 450-550°C. After the alloying treatment, it is preferable to cool at a cooling rate of not less than 5°C/s to 300°C.
  • the alloying at a high temperature is difficult to form the martensite phase and there is caused a fear of degrading the ductility of the steel sheet, while when the alloying temperature is lower than 450°C, the progress of the alloying is slow and the productivity tends to lower.
  • the cooling rate after the alloying treatment is extremely small, the formation of the martensite becomes difficult.
  • the cooling rate at a temperature region from after the alloying treatment to 300°C is preferable to be not less than 5°C/s.
  • the annealing is separately conducted in the continuous annealing line and subsequently an enriched layer of components in the steel produced on the surface of the steel sheet is removed by pickling and thereafter the above treatment is conducted in the continuous galvanization line.
  • the pickling may be carried out in the pickling line or in the pickling bath arranged in the continuous galvanization line.
  • the atmosphere in the continuous annealing line is preferable to be a reducing atmosphere with respect to the steel sheet in order to prevent the formation of the scale, and it is generally sufficient to use a nitrogen gas containing several % of H 2 .
  • the annealing is preferable to be conducted under a condition that a temperature of the steel sheet reaching in the continuous annealing line is not lower than the A C1 transformation point decided by the components in the steel. Because it is required to promote the enrichment of the alloying element on the surface of the steel sheet and to enrich the alloying element in the secondary phase by once forming the dual-phase microstructure in the continuous annealing line. In the steel sheet after the annealing in the continuous annealing line, there is a tendency that P among the components in the steel is diffused to segregate on the surface of the steel sheet and Si, Mn, Cr and the like enrich as an oxide, so that it is preferable to remove the enriched layer formed on the surface of the steel sheet by the pickling.
  • the same annealing as in the above is performed in the continuous galvanization line.
  • the annealing in the continuous galvanization line is preferable to be performed at ( ⁇ + ⁇ ) two-phase region within a temperature range of from the A C1 transformation point to the A C3 transformation point.
  • the reason why the annealing is performed at not lower than the A C1 transformation point in both the continuous annealing line and the continuous galvanization line is due to the fact that the dual-phase microstructure is formed as mentioned above.
  • the dual-phase microstructure as a final microstructure in the continuous annealing line
  • the alloying element is further enriched in the secondary phase or ⁇ -phase and hence the ⁇ -phase easily transforms into the martensite phase during the cooling process.
  • alloying element means a substitutional alloying element such as Mn, Mo or the like, which makes a situation that diffusion hardly occurs and enrichment easily occurs at the temperature in the annealing step in order to lower the yield ratio.
  • the cold rolled steel sheet after the recrystallization annealing process and the galvanized steel sheet after the plating process or after the alloying process may be subjected to a temper rolling at a rolling reduction of not more than 10% for correcting the shape and adjusting the surface roughness and the like.
  • the cold rolled steel sheet according to the invention can be applied as not only a cold rolled steel sheet for the working but also a blank of a surface treated steel sheet for the working.
  • the surface treated steel sheet for the working are mentioned tin-plated steel sheets, porcelain enamels and so on in addition to the aforementioned galvanized steel sheets (including alloyed sheets).
  • the galvanized steel sheet according to the invention may be subjected to a special treatment after the galvanization in order to improve the chemical conversion property, weldability, press formability, corrosion resistance and the like.
  • Each of molten steels having compositions shown in Tables 1-4 is made in a converter and subjected to a continuous casting process to obtain a slab.
  • each of the slabs having the compositions shown in Tables 1 and 2 is prepared for the purpose of experiments with respect to the cold rolled steel sheet
  • each of the slabs having the compositions shown in Tables 3 and 4 is prepared for the purpose of experiments with respect to the galvanized steel sheet.
  • the slabs shown in Tables 2 and 4 are prepared for the purpose of obtaining the cold rolled steel sheet and galvanized steel sheet of TS: not less than 780 MPa, respectively.
  • the steel slab is heated to 1150°C and subjected to a hot rolling under conditions of a finish deformation temperature: 900°C and a coiling temperature: 650°C at a hot rolling step to obtain a hot rolled steel strip having a thickness of 4.0 mm.
  • the hot rolled steel strip is pickled and subjected to a cold rolling at a rolling reduction of 70% at a cold rolling step to obtain a cold rolled steel strip or a cold rolled sheet having a thickness of 1.2 mm.
  • each of the cold rolled steel sheets in Tables 1 and 2 is subjected to a recrystallization annealing at an annealing temperature shown in Tables 5 and 6 in a continuous annealing line.
  • each of the cold rolled sheets in Tables 3 and 4 is subjected to a recrystallization annealing at an annealing temperature shown in Tables 7 and 8 and further to a galvanizing in a galvanizing bath having an Al concentration of 0.13% in a continuous galvanization line.
  • a part of steel sheets (Steel sheet Nos.
  • the steel sheet after the cold rolling is subjected to an annealing at 830°C in a continuous annealing line and then pickled and annealed and galvanized at a galvanizing bath temperature of 480°C under an Al concentration in the bath of 0.13% in a continuous galvanization line and further the thus obtained steel strip (galvanized steel sheet) is subjected to a temper rolling at a rolling reduction of 0.8%.
  • the steel sheets 75 and 77 in Table 7 they are subjected to an alloying treatment at an alloying temperature of 520°C after the galvanization.
  • a test piece is cut out from the obtained steel strip and a microstructure thereof with respect to a section (C section) perpendicular to the rolling direction is imaged by using an optical microscope or a scanning electron microscope to measure a structure ratio of ferrite phase as a primary phase and a kind and a structure ratio of a secondary phase by using an image analysis device.
  • a specimen for observing the microstructure is subjected to a mirror-like polishing and an etching with an alcohol solution containing 2% HNO 3 and then used for the observation.
  • the cold rolled steel sheets in all invention examples have a low yield stress (YS), a high elongation (El) and a low yield ratio (YR) and further indicate a high r-value and are excellent in the deep drawability, and have a tensile strength (TS) of not less than 440 MPa.
  • the yield stress (YS) is high, the elongation (El) is low, or the r-value is low.
  • the somewhat lowering of the r-value accompanied with the high-strengthening is observed in the high-strength steel sheets of TS: not less than 780 MPa shown in Table 6, for example, the steel sheet No. 28 produced by using the steel No. 2-A containing V and no Nb and Ti and the steel sheet No. 38 produced by using the steel No. 2-I containing V, Nb and Ti and satisfying a relationship of 0.5xC/12 ⁇ (V/51+2 ⁇ Nb/93+2 ⁇ Ti/48) ⁇ 3 ⁇ C/12 but satisfying a relationship of (2 ⁇ Nb/93+2 ⁇ Ti/48)/(V/51) ⁇ 0.5.
  • the r-value is improved in the steel sheet Nos.
  • the invention develops an industrially remarkable effect that the high-strength cold rolled steel sheet and galvanized steel sheet having an excellent deep drawability can be produced stably.
  • the cold rolled steel sheet and the galvanized steel sheet according to the invention are applied to vehicle parts, there are effects that the press forming is easy and they can sufficiently contribute to reduce the weight of the vehicle body.

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Abstract

L'invention concerne une tôle d'acier laminée à froid présentant une résistance élevée du type structure composite possédant une composition chimique qui contient, en pourcentage en masse : 0,01à 0,08 % de C, au plus de 2 % de Si, au plus 3 % de Mn, au plus 0,1 % de P, au plus 0,02 % de S, entre 0,005 et 0,2 % d'Al, au plus 0,02 % de N, entre 0,01 et 0,5 % de V et le reste étant constitué essentiellement de Fe et d'impuretés inévitables, à condition que les teneurs en V et C satifassent le rapport suivant : 0,5 x C/12 ≤ V/51 ≤ 3 x C/12. Une tôle d'acier possède une structure comprenant une phase de ferrite en tant que phase primaire et une seconde phase contenant une phase de martensite dans un pourcentage en superficie d'au moins 1 par rapport à la structure totale. L'invention concerne également une tôle galvanisée en acier présentant une résistance élevée du type structure composite recouverte de la tôle d'acier et d'un placage zingué par immersion à chaud, et un procédé de production de la tôle d'acier. La tôle de l'invention présente une excellente capacité d'emboutissage.

Claims (11)

  1. Tôle d'acier laminée à froid à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage, caractérisée en ce que la tôle d'acier a une composition comprenant C : 0,015 à 0,08 % en masse, Si : pas plus de 2,0 % en masse, Mn : 0,5 à 3,0 % en masse, P : pas plus de 0,10 % en masse, S : pas plus de 0,02 % en masse, Al : 0,005 à 0,20 % en masse, N : pas plus de 0,02 % en masse, V : 0,01 à 0,5 % en masse, éventuellement pas plus de 0,3 % en masse au total d'un ou deux parmi Nb : plus de 0 % en masse mais pas plus de 0,3 % en masse, et Ti : plus de 0 % en masse mais pas plus de 0,3 % en masse, et éventuellement pas plus de 2,0 % en masse au total d'un ou deux parmi Cr et Mo, étant entendu que V, Nb, Ti et C - s'ils sont présents - satisfont à une relation de 0,5xC/12 ≤ (V/51 + 2xNb/93 + 2xTi/48) ≤ 3xC/12, et le reste étant Fe et des impuretés inévitables, et a une microstructure consistant en une phase de ferrite en tant que phase primaire et une phase secondaire incluant une phase de martensite à une proportion de surface non inférieure à 1 % par rapport à la totalité de la microstructure.
  2. Tôle d'acier laminée à froid à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage selon la revendication 1, dans laquelle la tôle d'acier comprend en outre pas plus de 0,3 % en masse au total d'un ou deux parmi Nb : plus de 0 % en masse mais pas plus de 0,3 % en masse et Ti : plus de 0 % en masse mais pas plus de 0,3 % en masse.
  3. Tôle d'acier laminée à froid à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage selon la revendication 2, dans laquelle la tôle d'acier comprend pas plus de 0,3 % en masse au total d'un ou deux parmi Nb : 0,001 à 0,3 % en masse et Ti : 0,001 à 0,3 % en masse.
  4. Tôle d'acier laminée à froid à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage selon la revendication 2, dans laquelle la tôle d'acier comprend C : 0,03 à 0,08 % en masse, Si : 0,1 à 2,0 % en masse, Mn : 1,0 à 3,0 % en masse, P : pas plus de 0,05 % en masse et S : pas plus de 0,01 % en masse, étant entendu que V, Nb et Ti satisfont à une relation de 1,5 ≤ (2xNb/93 + 2xTi/48) / (V/51) ≤ 15.
  5. Procédé de fabrication d'une tôle d'acier laminée à froid à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage, qui comprend un laminage à chaud d'une brame d'acier ayant une composition comprenant : C : 0,015 à 0,08 % en masse, Si : pas plus de 2,0 % en masse, Mn : 0,5 à 3,0 % en masse, P : pas plus de 0,10 % en masse, S : pas plus de 0,02 % en masse, Al : 0,005 à 0,20 % en masse, N : pas plus de 0,02 % en masse, V : 0,01 à 0,5 % en masse, éventuellement pas plus de 0,3 % en masse au total d'un ou deux parmi Nb : plus de 0 % en masse mais pas plus de 0,3 % en masse, et Ti : plus de 0 % en masse mais pas plus de 0,3 % en masse, et éventuellement pas plus de 2,0 % en masse au total d'un ou deux parmi Cr et Mo, étant entendu que V, Nb, Ti et C - s'ils sont présents - satisfont à une relation de 0,5C/12 ≤ (V/51 + 2xNb/93 + 2xTi/48) ≤ 3xC/12, et le reste étant Fe et des impuretés inévitables, un décapage, un laminage à froid, et puis une soumission à un recuit continu à gamme de températures depuis un point de transformation AC1 jusqu'à un point de transformation AC3.
  6. Procédé de fabrication d'une tôle d'acier laminée à froid à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage selon la revendication 5, dans lequel la brame d'acier comprend en outre pas plus de 0,3 % en masse au total d'un ou deux parmi Nb : plus de 0 % en masse mais pas plus de 0,3 % en masse et Ti : plus de 0 % en masse mais pas plus de 0,3 % en masse.
  7. Procédé de fabrication d'une tôle d'acier laminée à froid à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage selon la revendication 6, dans lequel la brame d'acier comprend pas plus de 0,3 % en masse au total d'un ou deux parmi Nb : 0,001 à 0,3 % en masse et Ti : 0,001 à 0,3 % en masse.
  8. Procédé de fabrication d'une tôle d'acier laminée à froid à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage selon la revendication 6, dans lequel la brame d'acier comprend C : 0,03 à 0,08 % en masse, Si : 0,1 à 2,0 % en masse, Mn : 1,0 à 3,0 % en masse, P : pas plus de 0,05 % en masse et S : pas plus de 0,01 % en masse, étant entendu que V, Nb et Ti satisfont à une relation de 1,5 ≤ (2xNb/93 + 2xTi/48) / (V/51) ≤ 15.
  9. Tôle d'acier galvanisée à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage, comprenant un revêtement galvanisé sur la tôle d'acier telle que revendiquée dans l'une quelconque des revendications 1 à 4.
  10. Procédé de fabrication d'une tôle d'acier galvanisée à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage, caractérisé par le fait de soumettre à une galvanisation après le recuit continu à une gamme de températures depuis un point de transformation AC1 jusqu'à un point de transformation AC3 dans le procédé dans l'une quelconque des revendications 5 à 8.
  11. Procédé de fabrication d'une tôle d'acier galvanisée à deux phases à haute résistance ayant une excellente aptitude à l'emboutissage selon la revendication 10, caractérisé en ce qu'il comprend en outre une étape de recuit continu entre l'étape de laminage à froid et l'étape de recuit continu à une gamme de températures depuis un point de transformation AC1 jusqu'à un point de transformation AC3.
EP01998666A 2000-11-28 2001-11-27 Tole d'acier laminee a froid presentant une resistance elevee a la traction du type structure composite Expired - Lifetime EP1338667B1 (fr)

Applications Claiming Priority (9)

Application Number Priority Date Filing Date Title
JP2000361273 2000-11-28
JP2000361274 2000-11-28
JP2000361273 2000-11-28
JP2000361274 2000-11-28
JP2001312688A JP4010132B2 (ja) 2000-11-28 2001-10-10 深絞り性に優れた複合組織型高張力溶融亜鉛めっき鋼板およびその製造方法
JP2001312687A JP4010131B2 (ja) 2000-11-28 2001-10-10 深絞り性に優れた複合組織型高張力冷延鋼板およびその製造方法
JP2001312687 2001-10-10
JP2001312688 2001-10-10
PCT/JP2001/010340 WO2002044434A1 (fr) 2000-11-28 2001-11-27 Tole d'acier laminee a froid presentant une resistance elevee a la traction du type structure composite

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CA2398126A1 (fr) 2002-06-06
KR20020073564A (ko) 2002-09-27
EP1338667A1 (fr) 2003-08-27
WO2002044434A1 (fr) 2002-06-06
US20030129444A1 (en) 2003-07-10
AU2411802A (en) 2002-06-11
DE60143907D1 (de) 2011-03-03
CN1193110C (zh) 2005-03-16
EP1338667A4 (fr) 2005-08-17
TW520398B (en) 2003-02-11
CN1419607A (zh) 2003-05-21
AU776043B2 (en) 2004-08-26

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