Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
• • 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
  • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
  • B32B15/043—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material of metal
• • 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
  • B32B15/18—Layered products comprising a layer of metal comprising iron or steel
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • 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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/34—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
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12785—Group IIB metal-base component
  • Y10T428/12792—Zn-base component
  • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

• This disclosure relates to a high strength galvanized steel sheet having excellent bendability and weldability with tensile strength (TS) of 980 MPa or larger, which steel sheet is suited for use as an automobile component to be subjected to severe bending process or the like, and a method of producing the same.
• TS tensile strength
• galvanized steel sheet collectively refers to a so-called “galvannealed” steel sheet that has been subjected to alloy annealing treatment subsequent to hot-dip galvanization.
• a high strength galvanized steel sheet for use as an automobile component or the like is required to have excellent workability as well as high strength, due to the nature of its application.
• vehicle body processing involves an assembly process subsequent to press forming and it is necessary to perform resistance spot welding in the assembly process, excellent weldability is also required along with workability.
• JP-A 2004-232011, JP-A 2002-256386, JP-A 2002-317245, JP-A 2005-105367, JP-B 3263143, JP-B 3596316, and JP-A 2001-011538 each propose a method of obtaining a high strength galvanized steel sheet with high workability by specifying steel components and structures or optimizing hot rolling condi-tions and annealing conditions.
• JP-A 02-175839, JP-A 05-195149, JP-A 10-130782, JP-A 2005-273002, and JP-A 2002-161336 each disclose a technology of obtaining a cold-rolled steel sheet excellent in bendability
• JP-A 2006-161064 discloses a technology of obtaining a high tensile strength galvanized steel sheet excellent in bendability
• JP-A 2008-280608 discloses a technology of providing a galvanized steel sheet excellent in workability and weldability.
• JP-A 2004-232011 describes a steel satisfying a tensile strength of 980 MPa or so and having a high content of C and Si.
• JP-A 2004-232011 gives no consideration to improvement in stretch flangeability and bendability.
• JP-A 2002-256386, JP-A 2002-317245 and JP-A 2005-105367 each disclose a steel member that utilizes Cr, but gives no consideration to improvement in stretch flangeability and bendability.
• JP-B 3263143, JP-B 3596316, and JP-A 2001-011538 each refer to ductility ratio X, which is one of the indexes for evaluating stretch flangeability.
• TS tensile strengths
• JP-A 02-175839, JP-A 05-195149, JP-A 10-130782, JP-A 2005-273002 each disclose a technology involving bendability of steel sheets by softening the steel sheets in regions measured from surfaces thereof in the thickness direction to a depth of at least 10 vol % or at least 10 ⁇ m.
• the soft layer in the steel sheet surface layer is so thick that fatigue strength deteriorates.
• JP-A 2002-161336 describes that a steel sheet can be improved in bendability when a soft layer is formed in the steel sheet surface to a depth up to 10 ⁇ m.
• JP-A 2002-161336 fails to specify the steel sheet structure and thus inevitably faces a problem that fatigue strength of the steel sheet decreases as a whole.
• JP-A 2006-161064 although it discloses improving bendability of a steel sheet by setting the area ratio of a ferrite phase to be 80% or more in vicinities of the top surfaces (a depth of 1 to 10 ⁇ m measured from the surface layer) of the steel sheet, fails to make any reference to the internal structure of the steel sheet. Further, JP-A 2006-161064 has no description of weldability and plane-bending fatigue properties of the steel sheet, thereby still having a problem in terms of weldability and plane-bending fatigue properties.
• JP-A 2008-280608 discloses a high strength galvanized steel sheet excellent in workability and weldability, as well as bendability to attain a critical bend radius not larger than 1.5t (hereinafter, t represents sheet thickness of the steel sheet) at 90° V-bending. JP-A 2008-280608 has actually achieved a critical bend radius as small as 0.36t. However, to further expand the scope of application of the high strength steel sheet to a vehicle body, bendability needs to be further increased, that is, critical bend radius needs to be further reduced. Specifically, critical bend radius not larger than 0.3t is required.
• high strength means tensile strength of 980 MPa or larger.
• the expression that a steel sheet “has excellent bendability” means that a critical bend radius at 90° V-bending is not larger than 0.3t.
• the expression that a steel sheet “is excellent in weldability” means that a base material fracture occurs when a nugget diameter is 4t 1/2 (mm) or larger.
• the thickness of the steel sheet surface layer formed mainly of a ferrite phase is set not to exceed 10 ⁇ m or so, the aforemetioned adverse effect on fatigue resistance property can be suppressed while achieving significant improvement in bendability.
• a steel sheet structure on the inner side than the steel sheet surface layer needs to include a ferrite phase to a certain extent, specifically, at least about 20% by a volume fraction to ensure bendability.
• a ferrite phase of the inner structure exceeds 70%, the fatigue resistance property deteriorates, making it difficult to ensure the strength of 980 MPa or larger.
• a high strength galvanized steel sheet having excellent bendability and weldability comprising by mass %: C: equal to or more than 0.05% and less than 0.12%; P: 0.001 to 0.040%; and S: equal to or less than 0.0050%, wherein a steel sheet surface layer, constituting a portion of the steel sheet up to a depth of 10 ⁇ m measured from each surface of the steel sheet, has a structure containing more than 70% of ferrite phase by a volume fraction, a steel sheet inner layer portion, on the inner side than the depth of 10 ⁇ m measured from each surface, has a structure containing 20 to 70% by a volume fraction of ferrite phase with an average crystal grain size equal to or smaller than 5 ⁇ m, the steel sheet has a tensile strength equal to or larger than 980 MPa, and the steel sheet has a galvanized layer on a surface thereof.
• the high strength galvanized steel sheet having excellent bendability and weldability according to the above-mentioned item 1, wherein the structure of the steel sheet inner layer portion contains: 20 to 70% of ferrite phase by a volume fraction, with an average crystal grain size equal to or smaller than 5 ⁇ m; 30 to 80% of bainite phase and/or martensitic phase by a volume fraction, with an average crystal grain size of equal to or smaller than 5 ⁇ m; and residual austenite phase and/or pearlite phase equal to or less than 5% (inclusive of 0%) by a volume fraction as the remainder. 3.
• the high strength galvanized steel sheet having excellent bendability and weldability according to the above-mentioned item 1 or 2, wherein the steel sheet, further comprising by mass %: C: equal to or more than 0.05% and less than 0.12%; P: 0.001 to 0.040%; S: equal to or less than 0.0050%; Si: 0.01 to 1.6%; Mn: 2.0 to 3.5%; Al: 0.005 to 0.1%; N: equal to or less than 0.0060%; and the remainder as Fe and incidental impurities. 4.
• the high strength galvanized steel sheet having excellent bendability and weldability according to the above-mentioned item 3 or 4 further comprising by mass %: at least one element selected from the group consisting of Ti: 0.010 to 0.080% and Nb: 0.010 to 0.080%; and the remainder as Fe and incidental impurities. 6.
• the high strength galvanized steel sheet having excellent bendability and weldability comprising by mass %: C: equal to or more than 0.05% and less than 0.12%; P: 0.001 to 0.040%; S: equal to or less than 0.0050%; Si: 0.01 to 1.6%; Mn: 2.0 to 3.5%; Al: 0.005 to 0.1%; N: equal to or less than 0.0060%; Cr: more than 0.5% and equal to or less than 2.0%; Mo: 0.01 to 0.50%; Ti: 0.010 to 0.080%; Nb: 0.010 to 0.080%; B: 0.0001 to 0.0030%; and the remainder as Fe and incidental impurities.
• the steel sheet described in the above-mentioned item 6 is the high strength galvanized steel sheet having excellent bendability and weldability according to the above-mentioned item 1 or 2, characteristically containing by mass %, C: equal to or more than 0.05% and less than 0.12%, P: 0.001 to 0.040%, S: equal to or less than 0.0050%, Si: 0.01 to 1.6%, Mn: 2.0 to 3.5%, Al: 0.005 to 0.1%, N: equal to or less than 0.0060%, Cr: more than 0.5% and equal to or less than 2.0%, Mo: 0.01 to 0.50%, Ti: 0.010 to 0.080%, Nb: 0.010 to 0.080%, and B: 0.0001 to 0.0030%, and the remainder as Fe and incidental impurities.
• a method of manufacturing a galvanized steel sheet having excellent bendability and weldability by: subjecting a steel slab having a composition according to any one of items 1 to 6 above to heating and hot rolling; winding up the hot-rolled steel into a coil; subjecting the steel to pickling and cold rolling; and galvanizing the resulting steel sheet, the method comprising the steps of: subjecting the steel slab to heating at a temperature in the range of 1150 to 1300° C.
• hot rolling at a hot finishing rolling temperature in the range of 850 to 950° C.; cooling the steel from the hot finishing rolling temperature to (the hot finishing rolling temperature minus 100° C.) at an average cooling rate of 5 to 200° C./second; winding up the steel into a coil at a temperature of 400 to 650° C.; subjecting the steel to the pickling and the cold rolling; and subjecting the cold-rolled steel sheet to annealing including two-stage temperature raising processes, wherein the annealing step includes: primary heating of the steel sheet from 200° C. to an intermediate temperature in the range of 500 to 800° C.
• a high strength galvanized steel sheet which is effectively improved in bendability and weldability can be obtained.
• This high strength galvanized steel can satisfy both the strength and the workability necessitated by an automobile component and thus can be suitably used for an automobile component which is to be press formed into a shape requiring severe bending.
• compositions of the components of the steel sheet to the above-mentioned ranges. “%” regarding a content of each component of the steel sheet represents “mass %” unless otherwise specified.
• C is an indispensable element in terms of strengthening steel by utilizing a hard phase such as a martensitic phase or a bainite phase. 0.05% or more of C is required to obtain a tensile strength (which will be referred to as “TS” hereinafter) of 980 MPa or more. As the C content increases, TS increases accordingly. In a case where the C content is 0.12% or more, not only spot weldability significantly deteriorates, but also workability such as bendability tends to significantly deteriorate due to hardening caused by increase in the hard phase. The C content is thus to be limited to 0.05% or more and less than 0.12%. The C content is more preferably 0.105% or less. In view of stably ensuring TS of 980 MPa or larger, the C content is preferably 0.08% or more.
• the element P is an element that contributes to improving strength and the content thereof is 0.001% or more.
• the element P deteriorates weldability. Weldability improves when a steel sheet surface layer portion ranging from a surface of a steel sheet up to the 10 ⁇ m depth of the steel sheet, i.e., from an interface between the steel sheet and an zinc plating up to the 10 ⁇ m depth of the steel sheet (which surface layer portion will be referred to simply as a “steel sheet surface layer portion” hereinafter) contains ferrite by a volume fraction of 70% or more.
• the P content exceeds 0.040%, P significantly causes an adverse effect of deteriorating weldability.
• the P content is therefore to be limited to 0.001 to 0.040%, preferably 0.001 to 0.025%, and more preferably 0.001 to 0.015%.
• the S content is preferably as low as possible, although it may be tolerated up to 0.0050%.
• the S content is preferably 0.0030% or less.
• the lower limit of the S content is preferably about 0.0001%.
• C, P, and S Contents of C, P, and S are specified as described above to attain excellent weldability. It is effective to include, for example, Si, Mn, or Al in the steel sheet to obtain desired characteristics, in particular, to improve strength and weldability. Further, to improve the quench hardenability, it is also effective to add one or more elements selected from among Cr, Mo, and B in a predetermined content. Yet further, one or two elements selected from Ti and Nb can be added in a predetermined content as an element capable of facilitating precipitation, to further improve bendability.
• N 0.0060% or less because an increase in content of N significantly affects ductility of the steel sheet as described below.
• Si is an element that contributes to improving strength and bendability of the steel sheet through solid solution strengthening.
• the Si content needs to be 0.01% or more to cause a good effect of Si addition.
• Si is concentrated as an oxide on the steel sheet surface, causing plating failure.
• the Si content is therefore preferably 0.01 to 1.6%.
• the Si content is more preferably 0.8% or less, further more preferably less than 0.35%, and most preferably 0.20% or less to avoid plating failure.
• Mn 2.0 to 3.5%
• the structure suffers from localized variation in transformation point due to segregation of Mn or the like.
• the steel sheet has a non-uniform structure in which a ferrite phase and a martensitic phase exist in the form of bands, resulting in poor bendability.
• Mn is concentrated as an oxide on the steel sheet surface, causing plating failure.
• the Mn content is thus 2.0 to 3.5%, preferably 2.2 to 2.8%.
• Al is an element which is effective as a deoxidizing agent in the steelmaking process and also useful for separating a non-metal inclusion which would deteriorate bendability away into slag. Further, Al has a function of suppressing generation of Mn/Si-oxides, which would adversely affect plating properties during annealing and thus improve appearance of a plating surface.
• the Al content needs to be 0.005% or more to obtain such good effects. In a case where the Al content thus added exceeds 0.1%, not only the steelmaking cost is increased but also weldability deteriorates.
• the Al content is therefore 0.005 to 0.1%, and preferably 0.01 to 0.06%.
• the N content is preferably as low as possible.
• the upper limit of the N content is to be 0.0060%.
• the lower limit of the N content is preferably 0.0001% or so in view of the refining cost.
• the N content is to be 0.0060% or less, and preferably in the range of 0.0001 to 0.0060%.
• Cr is an element which is effective in facilitating quench hardening of steel. Cr also improves quench hardenability of an austenite phase, makes a hard phase extend uniformly such that the hard phase is finely dispersed, and effectively contributes to the improvement in stretch flangeability and bendability.
• the Cr content needs to be more than 0.5% to cause a good effect of Cr addition. However, in a case where the Cr content exceeds 2.0%, this good effect reaches a plateau and the surface quality rather deteriorates.
• the Cr content is therefore more than 0.5% and 2.0% or less, and preferably more than 0.5% and 1.0% or less.
• Mo is an element which is effective in facilitating quench hardening of steel. A very small amount of Mo added to a low-carbon steel component system readily ensures good strength of steel. Further, Mo causes an effect of improving weldability and bendability.
• the Mo content needs to be 0.01% or more to cause the good effect of Mo addition. In a case where the Mo content exeeds 0.50%, the good effect reaches a plateau and the production cost may rise up.
• the Mo content is thus 0.01 to 0.50% and preferably 0.01 to 0.35%.
• Ti reacts with C or N in steel to form a fine carbide or a fine nitride, to thereby grain-refine a hot rolled sheet structure and a steel sheet structure after annealing to improve bend-ability, and also effectively facilitate precipitation.
• the Ti content needs to be 0.010% or more to cause a good effect. In a case where the Ti content exceeds 0.080%, this good effect reaches a plateau and an excessive amount of precipitates is generated in ferrite, which deteriorates ductility of ferrite.
• the Ti content is 0.010 to 0.080%, and preferably 0.010 to 0.060%.
• Nb 0.010 to 0.080%
• Nb is an element that contributes to improving strength of a steel sheet by facilitating solid solution or precipitation. Nb also has an effect of strengthening the ferrite phase to reduce the difference in hardness between the ferrite and martensitic phases, to thereby effectively contribute to improving stretch flangeability. Further, Nb contributes to grain-refining of ferrite grains and crystal grains in the bainite/martensitic region, thereby improving bendability.
• the Nb content needs to be 0.010% or more to cause a good effect of Nb addition. In a case where the Nb content exceeds 0.08%, the hot-rolled sheet is hardened, which results in an increase in rolling load during hot rolling or cold rolling. Further, in this case, ductility of ferrite deteriorates and workability is impaired.
• the Nb content is thus 0.010 to 0.080%. In view of achieving good strength and workability, the Nb content is preferably 0.030 to 0.070%.
• the B content needs to be 0.0001% or more to cause a good effect. In a case where the B content exceeds 0.0030%, the good effect reaches a plateau.
• the B content is therefore 0.0001 to 0.0030% and preferably 0.0005 to 0.0020%.
• an element composition is as follows:
• the element composition may further optionally contain following elements in an appropriate manner.
• Ca causes an effect of improving ductility by shape control of a sulfide such as MnS. If the Ca content increases too much, the effect tends to reach a plateau. In view of this, when Ca is contained, the content thereof is 0.0001 to 0.0050% and more preferably 0.0001 to 0.0020%.
• Vanadium has an effect of strengthening a ferrite phase through formation of carbide.
• the V content is therefore preferably 0.001% or more and less than 0.05% and more preferably 0.001% or more and less than 0.005%.
• the element composition includes REM capable of controlling sulfide inclusion morphology without significantly changing plating properties to thereby effectively contribute to improving workability, or Sb capable of rendering crystal grains of a steel sheet surface layer uniform by a content of 0.0001 to 0.1%, respectively.
• Contents of other elements such as Zr, Mg which would form precipitates are preferably as low as possible. Therefore, it is not necessary to intentionally add these elements.
• the contents of these other elements are tolerated up to less than 0.0200% and preferably less than 0.0002%.
• Cu and Ni are elements that adversely affect weldability and appearance of a galvanized surface, respectively.
• the Cu and Ni contents are tolerated up to less than 0.4% and preferably less than 0.04%.
• the remainder other than the aforementioned component elements is preferably constituted of Fe and incidental impurities.
• Structure having a steel sheet surface layer portion containing more than 70% by volume fraction of ferrite phase
• Bendability of a steel sheet can be improved by designing a structure of a steel sheet surface layer portion as a structure mainly constituted of a ferrite phase. This effect cannot be obtained when the volume fraction of the ferrite phase is 70% or less. Accordingly, the volume fraction of the ferrite phase in the steel sheet surface layer portion is to be more than 70%, and preferably equal to or more than 85%.
• Observation of a structure of the steel sheet is carried out by: photographing a surface parallel to the rolling direction of the steel sheet by using a scanning electron micrograph (SEM) at an appropriate magnification of about 1,000 to 3.000-fold magnification; selecting three arbitrary sites in a steel sheet surface layer portion in the photography thus obtained; and calculating a volume fraction of ferrite or the like in these sites.
• SEM scanning electron micrograph
• an inner layer portion of the steel sheet located deeper than 10 ⁇ m measured from the steel sheet surface, has a structure containing at least 20 to 70% volume fraction of a ferrite phase with an average crystal grain size of 5 ⁇ m or less.
• the structures at both positions are observed, and in a case where a ferrite phase with the average crystal grain size of 5 ⁇ m or less exists at 20 to 70% volume fraction at both of the positions, it is determined that the steel sheet inner layer portion satisfies the above-mentioned structure.
• SEM images of the structures of a region located from the 10 ⁇ m depth to the 50 ⁇ m depth measured from the steel sheet surface or the steel sheet/zinc plating interface toward the center of the steel sheet (a region extending by 40 ⁇ m in the depth direction and by 20 ⁇ m in the rolling direction); and a region located at a position of the 1 ⁇ 4 sheet thickness (a region extending by 40 ⁇ m in the depth direction and by 20 ⁇ m in the rolling direction) are taken, respectively, at an appropriate magnification of about 1,000 to 3.000-fold magnification, for a surface in the rolling direction of the steel sheet, and three arbitrary points are observed for analysis.
• a ferrite phase is a soft phase and contributes to ductility of a steel sheet.
• the structure of the steel sheet inner layer needs to contain 20% or more of a ferrite phase by a volume fraction.
• the volume fraction of the ferrite phase exceeds 70%, the steel sheet is excessively softened, making it difficult to ensure sufficient strength and good plane-bending fatigue property of the steel sheet.
• the volume fraction of a ferrite phase is therefore to be 20 to 70%, preferably in the range of 30 to 50%.
• Grain refining of crystal grains contributes to improvement in ductility, stretch flangeability, and bendability of a steel sheet.
• the average crystal grain size of a ferrite phase in a multi phase structure is limited to 5 ⁇ m or less, to thereby improve bendability. In a case where the average crystal grain size of the ferrite phase exceeds 5 ⁇ m, a desired plane-bending fatigue cannot be ensured.
• the steel sheet is deformed non-uniformly, leading to degradation in bendability.
• the steel sheet can be uniformly deformed during processing.
• the average crystal grain size of a ferrite phase is therefore preferably as small as possible.
• the average crystal grain size of a ferrite phase is preferably in the range of 1 to 3.5 ⁇ m to well suppress deterioration of bendability of a steel sheet.
• volume fraction of bainite phase and/or martensitic phase 30 to 80%
• a bainite phase and/or a martensitic phase are hard phases, respectively, and each have a function of increasing strength of the steel sheet through structure reinforcement by transformation. Contents of the bainite phase and/or the martensitic phase are preferably 30% or more by a volume fraction to attain TS of 980 MPa or larger. The volume fraction of these phases is preferably 80% or less to obtain a desired bendability.
• Average crystal grain size of bainite phase and/or martensitic phase 5 ⁇ m or less
• Grain refining of a bainite phase and/or a martensitic phase improves hole expansion properties, bendability, and plane-bending fatigue of the steel plate.
• desired properties can be attained by setting the average crystal grain size of a bainite phase and a martensitic phase in a multi phase structure at 5 ⁇ m or less, preferably 3 ⁇ m or less, in particular.
• Possible structural aspects other than the aforementioned ferrite phase, martensitic phase and bainite phase in the steel sheet inner layer portion are residual austenite phase or a pearlite phase.
• the residual austenite phase and/or the pearlite phase do not adversely affect as long as the content thereof is 5% or less (including 0%) by a volume fraction.
• a slab is manufactured through, for example, a continuous casting process or an ingot-making and blooming process from molten steel prepared to have an element composition. Then, the slab thus obtained is subjected to a series of processes in which the slab is either cooled, reheated and hot rolled or hot rolled immediately after the casting process skipping a heating process.
• the slab heating temperature is 1,150 to 1,300° C. to attain uniform structure of the hot rolled sheet and the finishing rolling temperature in hot rolling 850 to 950° C. to improve workability such as ductility and stretch flangeability, so that formation of a structure in which a ferrite phase and a pearlite phase exist in two bands-like configuration is suppressed.
• the hot rolled steel is cooled from the hot finishing rolling temperature by 100° C. at an average cooling rate of 5 to 200° C./second, and a coiling temperature at which the hot rolled steel is wound into a coil is adjusted to 400 to 650° C. to improve the surface quality and easiness of cold rolling of the steel sheet, to thereby complete hot rolling.
• the steel cold rolled, after pickling, into a desired thickness is pre-ferably 30% or more to facilitate recrystallization of a ferrite phase to thereby improve ductility.
• the steel sheet is subjected to annealing that undergoes two-stage temperature raising processes. Structures of the surface layer portion and the inner layer portion of the steel sheet are controlled, respectively, by this annealing including two-stage temperature raising processes. Specifically, an average rate of primary heating from 200° C. to an intermediate temperature in the range of 500 to 800° C. is to be 5 to 50° C./second, and an average rate of secondary heating from the intermediate temperature to the annealing temperature in the range of 730 to 900° C. is to be 0.1 to 10° C./second.
• the steel sheet is then maintained at the annealing temperature in the aforementioned range for 10 to 500 seconds and cooled to a cooling stop temperature in the range of 450 to 550° C. at an average cooling rate of 1 to 30° C./second.
• the structure of the steel sheet surface layer portion is adjusted in a state where an excess air ratio in the annealing furnace during the primary heating is 1.10 to 1.20.
• An excess air ratio in the annealing furnace during the secondary heating is to be less than 1.10.
• the steel sheet is dipped in a molten zinc bath.
• the coating weight of zing plating is then controlled by gas wiping or the like.
• the steel sheet is then optionally heated for alloying treatment and cooled to a room temperature.
• a high strength galvanized steel sheet can be obtained as described above.
• the galvanized steel sheet may further optioanally be subjected to skinpass rolling.
• Precipitates existing at the stage of heating a steel slab result in coarse precipitates in a steel sheet as a final product. These coarse precipitates do not contribute to improving strength, but may rather make it difficult to achieve a uniform structure of the hot rolled sheet. Therefore, it is necessary to remelt the precipitates deposited during casting.
• the heating at a temperature of 1,150° C. or more can solve even Ti/Nb-precipitates, for example.
• reheating the steel sheet to 1,150° C. or more is useful in view of scaling off defects such as blow holes and segregation in the slab surface layer and reducing cracking and irregularities at a surface of the steel sheet surface, to obtain a smooth steel sheet surface.
• heating at a temperature higher than 1,300° C. causes coarse growth of austenite grains in the steel sheet structure, which results in a coarse structure of the final product and poor ductility thereof. Accordingly, the slab heating temperature is 1,150 to 1,300° C.
• Finishing rolling temperature 850 to 950° C.
• Bendability can be significantly improved by setting the finishing rolling temperature during in hot rolling at 850° C. or higher. In a case where the temperature falls short of 850° C., crystals which have been subjected to hot rolling have processed or wrought structures, whereby ductility of the steel sheet deteriorates.
• Mn which acts as an element for stabilizing austenite
• the transformation point of Ar 3 in the region is lowered and the austenite range expands to a relatively low temperature, whereby (in the case where finishing rolling temperature is below 850° C.) the unrecrystallized temperature range and the rolling end temperature fall within the same temperature range and unrecrystallized austenite eventually remains during hot rolling. Uniform deformation of the material during processing is inhibited and it is difficult to obtain excellent bendability in such a non-uniform structure.
• the finishing rolling temperature exceeds 950° C.
• the production of oxide (scale) rapidly increases, whereby the interface between the base metal and the oxide is roughened and the surface quality after pickling and cold rolling tends to deteriorate.
• the residue will adversely affect weldability in resistance spot welding.
• the crystal grain size may grow excessively coarse, thereby causing surface roughness in a pressed product during a forming process.
• the finishing rolling temperature is 850 to 950° C., and preferably 900 to 930° C.
• the average cooling rate in a range of (finishing temperature to (finishing temperature minus 100° C.)) is therefore to be equal to or higher than 5° C./sec.
• the average cooling rate in the range of (finishing temperature to (finishing temperature minus 100° C.)) exceeds 200° C./second, the effect of the cooling reaches a plateau and rather causes a disadvantage in cost such as a need for a cooling device exclusively for this cooling process.
• the average cooling rate in a range of (finishing temperature to (finishing temperature minus 100° C.)) is 5 to 200° C./second, and preferably 20 to 100° C./second.
• Winding temperature 400 to 650° C.
• the coiling temperature when the steel sheet thus rolled is wound exceeds 650° C.
• hot rolling scale thickness increases, whereby a steel surface after cold rolling is roughened with irregularities formed thereon and ferrite crystal grain size grow coarse, resulting in poor bendability of the steel sheet.
• the residual adversely affects weldability in resistant spot welding.
• the coiling temperature is lower than 400° C.
• strength of the hot rolled sheet is increased, whereby rolling road to be exerted on the steel sheet in the cold rolling process increases and productivity deteriorates.
• the coiling temperature is 400 to 650° C., and preferably 450 to 600° C.
• the primary average heating rate during annealing prior to the start of galvanization is lower than 5° C./second, the steel sheet experiences coarse growth of crystal grains, which leads to deterioration in ductility and bendability.
• the primary average heating rate is 5 to 50° C./second, preferably 10 to 50° C./second, and more preferably 15 to 30° C./second.
• the intermediate temperature between the primary heating and the secondary heating exceeds 800° C.
• the intermediate temperature is lower than 500° C.
• the effect caused by the primary heating reaches a plateau, eliminating difference in volume fraction of a ferrite phase between the steel sheet surface layer portion and the steel sheet inner layer of a finally obtained product.
• the intermediate temperature is 500 to 800° C.
• the intermediate temperature is preferably to be a temperature lower than annealing temperature by about 200° C.
• An excess air ratio in an annealing furnace during the above-mentioned primary heating is generally set at 1.00 or less. However, the excess air ratio in the furnace during the primary heating is 1.10 to 1.20. By setting the excess air ratio in the furnace during the primary heating to be 1.10 to 1.20 as described above, the structure of the steel sheet surface layer portion can successfully contain more than 70% of a ferrite phase, which is the most critical feature.
• the excess air ratio in the furnace during the primary heating exceeds 1.20, the volume fraction of the ferrite phase also increases in the steel sheet inner layer portion, which results in degradation of the fatigue property of the steel sheet.
• the excess air ratio is less than 1.10, the volume fraction of ferrite in the steel sheet surface layer portion cannot exceed 70% as in the above-mentioned case of the conventional excess air ratio, whereby bendability fails to improve.
• the excess air ratio is preferably to fall to 1.12 to 1.17.
• An excess air ratio represents a ratio of an amount of air in an annealing furnace with respect to a minimum amount of air required for complete combustion of combustible element, which minimum amount of air is obtained from chemical reactions associated with the complete combustion. Accordingly, the excess air ratio 1.00 means that the atmosphere in the furnace contains an amount of air equivalent to the amount of theoretical air. An excess air ratio exceeding 1.00 represents that the atmosphere in the furnace contains an amount of air excessive for complete combustion of the combustible element. An excess air ratio of less than 1.00 means that the atmosphere in the furnace cannot cause complete combustion of the combustible element.
• the secondary average heating rate during annealing prior to the start of galvanization is higher than 10° C./second, generation of an austenite phase is delayed, which increases the volume fraction of an eventually obtained ferrite phase, making it difficult to ensure sufficient strength of the steel sheet.
• the secondary average heating rate is lower than 0.1° C./second, crystal grain size grows coarse, resulting in deterioration of ductility and bendability.
• the secondary average heating rate is 0.1 to 10° C./second, and preferably 0.5 to 5° C./second.
• the excess air ratio in the furnace during the secondary heating is less than 1.10.
• the volume fraction of a ferrite phase exceeds 70% in the steel sheet inner layer portion located deeper than the 10 ⁇ m depth measured from the steel sheet surface layer, as well, thereby deteriorating fatigue property of the steel sheet.
• the excess air ratio in a furnace during the secondary heating can be set to be equal to or less than 1.00, which is an ordinary excess air ratio.
• the subsequent annealing process can be performed under an excess air ratio falling within this ordinary range, preferably 0.60 to 0.95.
• Annealing temperature 730 to 900° C.
• Retention time 10 to 500 seconds
• the annealing temperature prior to the start of galvanization is lower than 730° C.
• a sufficient amount of austenite is not generated during annealing, whereby the steel sheet cannot reliably have sufficient strength.
• the annealing temperature is higher than 900° C.
• the austenite phase grow coarse during heating, which leads to decrease in amount of ferrite generated in the subsequent cooling process and deterioration of bendability of the steel sheet.
• the crystal grain size of a steel sheet structure eventually obtained tends to be large, as a whole, thereby deteriorating both ductility and bendability.
• the annealing temperature is therefore 730 to 900° C., preferably 750 to 850° C.
• the retention time is less than 10 seconds in the above-mentioned annealing temperature range
• the amount of austenite phase generated during annealing is insufficient, making it difficult to ensure sufficient strength of a steel sheet as a final product.
• crystal grains of a steel sheet structure tend to grow coarse.
• the retention time exceeds 500 seconds
• crystal grain sizes of an austenite phase and a ferrite phase during thermal annealing are excessively gross, whereby bendability of a steel sheet structure obtained after the thermal processing deteriorates.
• Such gross or coarse austenitic grain size is not preferable because a steel sheet surface may be roughened during the press forming process. Yet further, such coarse austenitic and ferrite grain sizes lead to a decreased amount of ferrite phase generated in the cooling process to the cooling stop temperature, deteriorating ductility, as well.
• the retention time is 10 to 500 seconds, and preferably 20 to 200 seconds.
• the average cooling rate when the steel sheet is cooled to the cooling stop temperature plays an important role to controllably adjust an abundance ratio of a relatively soft ferrite phase with respect to a relatively hard bainite phase and/or martensitic phase so that the steel sheet has strength TS in a class of at least 980 MPa while retaining good workability.
• the average cooling rate exceeds 30° C./second, generation of a ferrite phase during cooling is suppressed while bainite and martensitic phases are excessively generated, which ensures easily attaining TS in a class of 980 MPa, but causes deterioration of bendability.
• the average cooling rate when the steel sheet is cooled to the cooling stop temperature is 1 to 30° C./second, preferably 5 to 20° C./second, and more preferably 10 to 20° C./second.
• the method of cooling the steel sheet is preferably the conventional gas cooling method.
• Other examples of the cooling method include any conventional method such as a furnace-cooled method, a mist cooling method, a roll cooking method, a water cooling method and the like, which may also appropriately combined.
• the steel sheet structure is transformed from an austenite phase to a pearlite phase or a bainite phase which are softer than a martensitic phase, whereby it is difficult to ensure a TS value in a class of 980 MPa.
• a hard residual austenite phase may be generated, but this cannot be a help because stretch flangeability is then deteriorated.
• the cooling stop temperature is less than 450° C.
• a residual austenite phase is increased due to progress of bainite transformation, making it difficult to ensure TS in a class of 980 MPa and deteriorating bendability of the steel sheet.
• the steel sheet is subjected to a hot-dip galvanizing treatment, whereby a galvanized steel sheet is obtained.
• the galvanized steel sheet is further optionally subjected to an alloying treatment of reheating it by using an induction heating device or the like, to thereby obtain a galvannealed steel sheet.
• Conditions for the hot-dip galvanizing treatment and the alloying treatment for the steel sheet are not specifically limited and may be the conventionally-known conditions.
• a coating weight of hot-dip zinc plating is preferably about 20 to 150 g/m 2 per one surface. In a case where the coating weight is less than 20 g/m 2 , it is difficult to ensure sufficient corrosion resistance of a product. In a case where the coating weight exceeds 150 g/m 2 , the corrosion resisting effect by zinc plating reaches a plateau and rather the production cost disadvantageously increases.
• the coating weight of zinc plating is thus preferably 30 to 70 g/m 2 .
• the galvannealed steel sheet as a final product may be subjected to skinpass rolling for the purpose of shape correction and surface roughness adjustment.
• skinpass rolling introduces too much deformation to the steel sheet, resulting in a roll-processed structure including wrought crystal grains and thus poor ductility.
• the rolling reduction rate in the skinpass rolling is therefore preferably about 0.1 to 1.5%.
• the material tests and an evaluation method of material properties are as follows.
• SEM images of structures of a steel sheet surface layer portion, a steel sheet inner layer portion (a region located from the 10 depth to the 50 ⁇ m depth measured from the steel sheet surface) and another steel sheet inner layer portion (a region located at the 1 ⁇ 4 sheet thickness position) are photographed, respectively, at a magnification in the range of about 1,000 to 3.000-fold magnification.
• a volume fraction of the steel sheet structure is determined by using these SEM images thus obtained.
• the structures of the steel sheet inner layer portions are determined by observing: a structure of a region located from the 10 ⁇ m depth to the 50 ⁇ m depth measured from the steel sheet surface or the steel sheet/zinc plating interface toward the center of the steel sheet (a region extending by 40 ⁇ m in the depth direction and by 20 ⁇ m in the rolling direction); and a region located at a position of the 1 ⁇ 4 sheet thickness (a region extending by 40 ⁇ m in the depth direction and by 20 ⁇ m in the rolling direction), respectively.
• volume ratios of a ferrite phase and a pearlite phase are acquired by: visually specifying a ferrite phase and a pearlite phase by using a photographic image of a sectional structure of a steel sheet at a 1.000-fold magnification; obtaining area values occupied by the ferrite phase and the pearlite phase through image analysis, respectively; and dividing the area values thus obtained by a value of the analyzed area (the area of the sectional structure image), respectively, to thereby calculate volume fractions of the ferrite phase and the pearlite phase.
• An amount of residual austenite is calculated by: grinding a steel sheet to be analyzed to the 1 ⁇ 4 sheet thickness position thereof; polishing by 0.1 mm the steel sheet thus ground, by chemical polishing to obtain a polished surface; analyzing the polished surface by Mo K ⁇ beam from an X-ray diffractometer to measure integrated intensities of a (200) face, a (220) face, and a (311) face of fcc iron and a (200) face, a (211) face, and a (220) face of bcc iron, respectively; and obtaining a fraction (i.e., a volume fraction) of the residual austenite from these measured values.
• the rest of the structure other than the ferrite phase, the austenite phase, and the pearlite phase, is formed of a bainite phase and/or a martensitic phase. Therefore, the total amount of the bainite phase and the martensitic phase is regarded as the remaining portion other than the ferrite phase, the austenite phase, and the pearlite phase.
• the average grain size of a bainite phase and a martensitic phase is acquired by: specifying a bainite phase and a martensite phase by using an SEM image of a 3.000-fold magnification; assuming each single continuous region as a grain and measuring sizes of grains according to a method specified in JIS G 0552: 1998; and calculating the average grain size of the bainite phase and the martensite phase, respectively, by converting the obtained grain sized thereto.
• a tensile test according to JIS Z 2241 is then performed by using the No. 5 type test piece for evaluation.
• Bendability is evaluated according to the V-block method as specified in JIS Z 2248. In the evaluation, generation of cracks at the outer side of a bent portion is visually observed and the minimum bending radius which allows bending without generating a crack is regarded as the limit bending radius. When the limit bending radius is equal to or smaller than 0.3t, it is judged that the bendability is good. Note that Table 4 shows values of the limit bending radius/t, as well.
• spot welding is performed under following conditions:
• Test current is changed at 0.2 kA intervals from 4.6 to 10 kA and at 0.5 kA intervals from 10.5 kA to the welding, respectively, for the steel sheet having the same sample number.
• each test piece is subjected to a cross-tension test and measurement of nugget diameter at a welded portion.
• the cross-tension test on the resistance spot welded joint is carried out according to JIS Z 3137.
• the measurement of the nugget diameter is carried out according to JIS Z 3139.
• a half of the plug portion in a symmetrical circular shape after resistance spot welding is cut along a section perpendicular to the steel sheet surface and substantially including the center of the welding point.
• the cut face is ground, etched and then subjected to optical microscopic observation of a sectional structure, to thereby measure a nugget diameter.
• the maximum diameter of a molten region from which a corona bond has been excluded is assumed as the nugget diameter.
• the welded material having a nugget diameter of 4t 1/2 (mm) or more is subjected to cross-tension test.
• the weldability is evaluated as good.
• the plane-bending fatigue test is carried out according to JIS Z 2275 under a condition of fully alternating stress (stress ratio: 1) at a frequency of 20 Hz. In a case where a durability ratio expressed as fatigue limit/TS is 0.35 or larger, plane-bending fatigue properties are evaluated as good.
• the steel sample of No. 27 having the cooling stop temperature beyond our range exhibits TS lower than 980 MPa.
• Our high strength galvanized steel sheets have excellent bendability and weldability, as well as a high tensile strength, and therefore can be suitably used, to cause a good effect, for applications requiring strict dimensional accuracy and bendability such as automobile components, architecture and household appliances.

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