EP1365037B1 - Hochfestes stahlblech mit hervorragender formbarkeit und verfahren zu seiner herstellung - Google Patents

Hochfestes stahlblech mit hervorragender formbarkeit und verfahren zu seiner herstellung Download PDF

Info

Publication number
EP1365037B1
EP1365037B1 EP02710418A EP02710418A EP1365037B1 EP 1365037 B1 EP1365037 B1 EP 1365037B1 EP 02710418 A EP02710418 A EP 02710418A EP 02710418 A EP02710418 A EP 02710418A EP 1365037 B1 EP1365037 B1 EP 1365037B1
Authority
EP
European Patent Office
Prior art keywords
temperature
steel sheet
point
continuous annealing
hot rolling
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP02710418A
Other languages
English (en)
French (fr)
Other versions
EP1365037A1 (de
EP1365037A4 (de
Inventor
Takahiro Kakogawa Works K.K.K SEIKO SHO KASHIMA
Shunichi c/o CBMM Asia Co. Ltd. Hashimoto
Shushi K.C. Res.Lab. K.K.K.Kobe seiko sho IKEDA
Hiroshi K.C.R. Lab. k.K.K Kobe Seiko Sho AKAMIZU
Koh-Ichi Sugimoto
Akihiko Nagasaka
Akinobu Kanda
Ryo Kikuchi
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Kobe Steel Ltd
Original Assignee
Kobe Steel Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from JP2001264175A external-priority patent/JP2003073773A/ja
Priority claimed from JP2001300503A external-priority patent/JP4188582B2/ja
Priority claimed from JP2001300502A external-priority patent/JP4188581B2/ja
Application filed by Kobe Steel Ltd filed Critical Kobe Steel Ltd
Publication of EP1365037A1 publication Critical patent/EP1365037A1/de
Publication of EP1365037A4 publication Critical patent/EP1365037A4/de
Application granted granted Critical
Publication of EP1365037B1 publication Critical patent/EP1365037B1/de
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • 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
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22—Martempering
    • 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
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—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
    • 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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a high strength steel sheet having excellent formability (stretch flange formability and total elongation). More particularly, the present invention is concerned with a high strength steel sheet having both high strength of the order of 500 to 1400 MPa and excellent formability in an ultra-high strength region, further, a high strength steel sheet also superior in fatigue characteristic, and further a high strength steel sheet also superior in bake hardening property [hardening property after baking finish, may be referred to hereinafter also as "BH (Bake Hardening)" property] which can ensure a high strength by baking finish.
  • BH Bike Hardening
  • a composite ferrite-martensite steel sheet [dual phase (DP) steel sheet] comprising ferrite as a base and a low temperature transformation structure contained therein which structure is constituted mainly by marternsite (see, for example, JP-A No. 122820/1980 ).
  • This steel sheet is not only superior in ductility but also characteristic in that yield elongation does not appear due to a large quantity of free dislocation introduced into a martensite producing region, and yield stress becomes lower, and that therefore a shape freezing characteristic in working is satisfactory.
  • TS tensile strength
  • El superior in elongation
  • JP-A No. 43425/1985 discloses a steel sheet which is high in strength and extremely superior in ductility and which is produced by controlling the structure of a composite phase steel sheet into a structure having 10% or more of ferrite and 10% or more of ⁇ R in terms of volume fraction, with the balance being bainite or martensite or a mixture thereof.
  • JP-A No. 104947/1997 discloses a steel sheet having a three-phase microstructure of ferrite, bainite and ⁇ R and with a ferrite occupancy rate/ferrite grain size ratio and ⁇ R occupancy rate being controlled to predetermined ranges.
  • BH property bake hardening property
  • EP0997548 describes a hot rolled high strength steel sheet having a satisfactory strength - elongation balance and an stretch frangibility and with excellent formability comprising a steel, which contains: C : 0.05 - 0.15%, Si : 0.5 - 2.0%, Mn : 0.5 - 2.0%, P : 0.05% or less, S : 0.010% or less, Al : 0.005 - less than 0.10%, and the balance of Fe and inevitable impurities, and in which the metal structure mainly comprises three phases of ferrite, retained austenite and bainite, the amount of the ferrite [V(F)] is 60 - 95 area%, the amount of the retained austenite [V( ⁇ )] is 3 - 15 vol%, the amount of the bainite [V(B)] in the area ratio is 1.5 times or more of the amount of the retained austenite [V( ⁇ )], and the average hardness of the bainite is 240 - 400 Hv.
  • the present invention has been accomplished in view of the above-mentioned circumstances and it is a first object of the invention to provide a high strength steel sheet superior in formability (stretch flange formability and total elongation) and a method which can produce such a steel sheet efficiently. It is a second object of the present invention to provide a high strength steel sheet superior not only in the aforesaid formability but also in fatigue characteristic, i.e., a high strength steel sheet having well-balanced stretch flange formability, total elongation and fatigue characteristic, and a method which can produce such a steel sheet efficiently. It is a third object of the present invention to provide a high strength steel sheet superior not only in the aforesaid formability but also in bake hardening property, and a method which can produce such a steel sheet efficiently.
  • (1) contains the following chemical components in mass %:
  • (2) has a structure comprising:
  • Methods for producing known high strength steel sheets involve the following methods according to the following structures (A) and (B) (methods not according to the invention):
  • a method for producing high strength steel sheet according to the invention involves the following methods according to the following structures (A) and (B) (methods according to the invention):
  • a method for producing the foregoing high strength steel which has a hardening property (BH) after baking finish which property satisfies: BH 2 % ⁇ 70 ⁇ MPa and BH 10 % ⁇ BH 2 % / 2 involves the following methods according to the following structures (A) and (B):
  • the present inventors found out that the desired object could be achieved by using as a base phase structure 1 tempered martensite or 2 tempered bainite, which is a soft lath structure low in dislocation density, or 3 a mixed structure of the tempered martensite and ferrite, or 4 a mixed structure of the tempered bainite and ferrite, and by making control to, as a second phase structure, a structure having ⁇ R phase with a C concentration (C ⁇ R ) in retained austenite ( ⁇ R ) of not lower than 0.8%.
  • a base phase structure 1 tempered martensite or 2 tempered bainite which is a soft lath structure low in dislocation density
  • 3 a mixed structure of the tempered martensite and ferrite
  • 4 a mixed structure of the tempered bainite and ferrite
  • a conventional retained austenite steel sheet has a demerit such that with progress of deformation of a soft phase (base phase) around a hard phase, voids are apt to occur in the interface with the soft phase, resulting in stretch flange formability being deteriorated.
  • tempered martensite or tempered bainite, or a mixed structure of tempered martensite and ferrite, or a mixed structure of tempered bainite and ferrite, which will be described later
  • the formation of voids has been suppressed and stretch flange formability improved.
  • by controlling the form of lath ⁇ R so as to give a predetermined axial ratio, it has become possible to improve elongation and stretch flange formability as compared with the conventional ⁇ R .
  • Tempoered martensite used in the present invention has the following features.
  • tempered martensite in the present invention means a soft and lath structure low in dislocation density.
  • martensite is a hard structure high in dislocation density and is different in this point from the tempered martensite. Both can be distinguished from each other, for example, by observation under a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • a conventional ⁇ R steel sheet has a soft block-like ferrite structure low in dislocation density and is also different in this point from the steel sheet of the present invention which uses the tempered martensite as a base phase structure.
  • the tempered martensite has a tendency that its Vickers hardness (Hv) is generally high as compared with polygonal ferrite in the same component system (a system common in point of basic components of C, Si, and Mn).
  • Fig. 1 is a graph comparing between the hardness of tempered martensite (axis of ordinate) and that of polygonal ferrite (axis of abscissa) in steels of the same components (C: 0.1 to 0.3%, Mn: 1.0 to 2.0%, Si: 1.0 to 2.0%).
  • Vickers hardness there was made observation through an optical microscope for Lepera etching and Vickers hardness (Hv) of a base phase (gray) portion was measured (load: 1g).
  • Fig. 2 the data of Fig. 1 are arranged for each of the cases of C being 0.1%, 0.2%, and 0.3%, showing the influence of the amount of C on the hardness of tempered martensite and that of polygonal ferrite. From Fig. 2 it is seen that, in the same amount of C, the hardness of tempered martensite tends to be higher than that of polygonal ferrite and that this tendency becomes outstanding as the amount of C becomes higher.
  • An upper limit in hardness of tempered martensite can vary depending on a component composition for example, but it is recommended that the said upper limit be approximately 500[C]+30[Si]+3[Mn]+200, preferably 500[C]+30[Si]+3[Mn]+150.
  • tempered martensite having such a characteristic is obtained by providing martensite which has been quenched from a temperature of not lower than A 3 point ( ⁇ region) and annealing the martensite at a temperature of not lower than A 1 point (about 700°C or higher) and not higher than A 3 point.
  • the tempered martensite be present not less than 50% (preferably not less than 60%) in terms of a space factor relative to the whole structure.
  • the amount of the tempered martensite is determined in consideration of its balance with ⁇ R . It is recommended for control to be made appropriately so that a desired characteristic can be exhibited.
  • the details of tempered martensite is as described above in 1.
  • the tempered martensite in order for the tempered martensite to function effectively, it is necessary that the tempered martensite be present not less than 15% (preferably not less than 20%) in terms of a space factor relative to the whole structure.
  • the amount of the tempered martensite is determined, taking into account the balance of ferrite and ⁇ R which will be described later. It is recommended for control to be made appropriately so that a desired characteristic can be exhibited.
  • ferrite as referred to herein means polygonal ferrite, i.e., ferrite low in dislocation density.
  • the ferrite is superior in elongation characteristic but is inferior in stretch flange formability.
  • a steel sheet according to the present invention having the foregoing mixed structure of ferrite and tempered martensite is improved in stretch flange formability while retaining an excellent elongation characteristic.
  • the steel sheet of the present invention is different from the conventional TRIP steel sheet.
  • ferrite be present not less than 5% (preferably not less than 10%) in terms of a space factor relative to the whole structure.
  • the content of ferrite exceeds 60%, it will become difficult to ensure a required strength; besides, like the conventional TRIP steel sheet, there will occur many voids from the interface between ferrite and a second phase, with consequent deterioration of the stretch flange formability.
  • the upper limit of ferrite content be set at 60%. Controlling the upper limit to less than 30% is very preferable because the ferrite-second phase ( ⁇ R , martensite) interface will diminish to suppress the formation of voids, thus leading to improvement of the stretch flange formability.
  • Tempoered bainite used in the present invention has the following features.
  • tempered bainite in the present invention means a soft and lath structure low in dislocation density.
  • bainite is a hard structure high in dislocation density and is different in this point from the tempered bainite. Both can be distinguished from each other, for example, by observation under a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • a conventional ⁇ R steel sheet has a soft block-like soft structure low in dislocation density and is also different in this point from the steel sheet of the present invention which uses the tempered bainite as a base phase structure.
  • tempered bainite has a tendency that its Vickers hardness (Hv) is generally high as compared with polygonal ferrite in the same component system (a system common in point of basic components of C, Si, and Mn).
  • Fig. 1 is a graph comparing the hardness of tempered bainite and that of tempered martensite (axis of ordinate) with the hardness of polygonal ferrite (axis of abscissa) in steels of the same components (C: 0.1 to 0.3%, Mn: 1.0 to 2.0%, Si: 1.0 to 2.0%).
  • Vickers hardness there was made observation through an optical microscope for Lepera etching and Vickers hardness of a base phase (gray) portion was measured (load: 1g).
  • Fig. 2 the data of Fig. 1 are arranged for each of the cases of C being 0.1%, 0.2%, and 0.3%, showing the influence of the amount of C on the hardness of tempered bainite, tempered martensite, and polygonal ferrite. From Fig. 2 it is seen that, in the same amount of C, the hardness of tempered bainite tends to be higher than that of polygonal ferrite and that this tendency becomes more outstanding as the amount of C increases.
  • An upper limit in hardness of tempered bainite can vary depending on a component composition for example, but it is recommended that the said upper limit be approximately 500[C]+30[Si]+3[Mn]+200, preferably 500[C]+30[Si]+3[Mn]+150.
  • tempered bainite having such a characteristic is obtained by providing bainite which has been quenched from a temperature of not lower than A3 point ( ⁇ region) to a temperature of not lower than Ms point and not higher than Bs point and by annealing the bainite at a temperature of not lower than A 1 point (about 700°C or higher) and not higher than A 3 point.
  • the tempered bainite be present not less than 50% (preferably not less than 60%) in terms of a space factor relative to the whole structure.
  • the amount of the tempered bainite is determined in consideration of its balance with ⁇ R which will be described later. It is recommended for control to be made appropriately so that a desired characteristic can be exhibited.
  • the tempered bainite in order for the tempered bainite to function effectively, it is necessary that the tempered bainite be present not less than 15% (preferably not less than 20%) in terms of a space factor relative to the whole structure.
  • the amount of the tempered bainite is determined, taking into account the balance of ferrite and ⁇ R which will be described later. It is recommended for control to be made appropriately so that a desired characteristic can be exhibited.
  • ⁇ R is effective in improving the fatigue characteristic and in order for this function to be exhibited effectively it is necessary that ⁇ R be present 3% (preferably 5% or more) in terms of a space factor relative to the whole structure.
  • ⁇ R be present 5% or more (more preferably 7% or more). If ⁇ R is present in a large amount, the stretch flange formability will be deteriorated. Therefore, we have determined an upper limit of the ⁇ R content to be 30%.
  • a base phase structure is a single phase structure of tempered martensite or tempered bainite, it is recommended that the upper limit be controlled to 20% (more preferably 15%).
  • a base phase structure is a mixed structure of tempered martensite and ferrite or a mixed structure of tempered bainite and ferrite, it is recommended to set the upper limit at 25%.
  • the concentration of C (C ⁇ R ) in the ⁇ R be not less than 0.8%.
  • the C ⁇ R exerts a great influence on the characteristic of TRIP (transformation induced plasticity), and controlling the C ⁇ R to 0.8% or more will be effective particularly in improving elongation, etc.
  • the C ⁇ R is not less than 1%, more preferably not less than 1.2%.
  • an adjustable upper limit in practical operation is considered to be approximately 1.6%
  • ⁇ R of random orientation is present in a pre-austenite grain boundary, while, in the present invention, ⁇ R having the same orientation along for example a block boundary within the same packet is apt to be present.
  • a feature of the ⁇ R in the present invention is illustrated schematically in Fig. 3 .
  • the numeral 1 denotes a pre-austenite grain boundary
  • numeral 2 denotes a packet grain boundary
  • numeral 3 denotes a block grain boundary
  • numeral 4 denotes martensite lath.
  • FIGs. 4 and 5 illustrate results obtained using EBSP photographs (color maps: magnification 1000 times) of sections in sheet thickness direction of a steel sheet according to the present invention (No. 3 in Table 2 to be described later) and a conventional ⁇ R steel sheet (No. 16 in Table 3 to be described later).
  • the EBSP stands for Electron Back Scatter Diffraction Pattern, and as an EBSP analyzer there was used an analyzer manufactured by TexSEM Laboratories.
  • ⁇ R in the sheet thickness direction of different crystal orientations can be identified on the basis of a color tone difference. That is, if ⁇ R is checked by a crystal orientation observing method using EBSP different from the ordinary structure observation, a large number of ⁇ R of random orientations are found to be present in a pre-austenite grain boundary in the conventional steel sheet ( Fig. 5 ), while in the steel sheet according to the present invention ( Fig. 4 ) it can be seen that a large number of ⁇ R having the same orientation are present within a certain region, though both of the steel sheet have almost the same structure in appearance. It is presumed that, in the steel sheet of the present invention, ⁇ R having the same orientation is produced along a block boundary for example. In this point the ⁇ R in the steel of the present invention has a different form from the that in the conventional steel sheet.
  • the ⁇ R in the present invention be in lath form.
  • lath form is meant an average axial ratio (major axis/minor axis) of 2 or more (preferably 4 or more, a preferred upper limit being 30 or less).
  • the ⁇ R in lath form not only affords the same TRIP effect as in the prior art but also affords improved elongation and a more outstanding improvement in stretch flange formability.
  • Bainite and/or Martensite (including 0%)
  • the second phase structure may further contain bainite and/or martensite as other structures insofar as the operation of the present invention is not impaired. These structures may remain inevitably in the manufacturing process of the present invention, but the smaller their content, the better.
  • martensite in the second high strength steel sheet according to the present invention, which will be described later, mention may be made mainly of martensite as another structure.
  • C is an element essential for ensuring a high strength and for ensuring ⁇ R . More particularly, C is an element important for ensuring a sufficient amount of C in ⁇ phase and for allowing a desired ⁇ phase to remain even at room temperature. However, if C is added in an amount exceeding 0.25%, the weldability will be deteriorated and cementite will become coarse in a tempering process which will be described later, leading finally to coarsening of the second phase structure.
  • Si and Al are elements which effectively prevent the formation of carbide by decomposition of ⁇ R .
  • Si is useful also as a solid solution hardening element.
  • Si and Al be added a total of 0.5% or more, preferably 0.7% or more, more preferably 1% or more. But even if both elements are added in an amount exceeding 3% in total, the aforesaid effect will become saturated, which is wasteful from the economic standpoint; besides, the addition thereof in a large amount will cause hot shortness. For this reason, an upper limit thereof is set at 3%, preferably 2.5% or less, more preferably 2% or less.
  • Mn is an element necessary for stabilizing ⁇ and for obtaining a desired ⁇ R .
  • Mn is an element necessary for stabilizing ⁇ and for obtaining a desired ⁇ R .
  • Mn is added in an amount of 0.5% or more, preferably 0.7% or more, more preferably 1% or more.
  • Mn is added in an amount of not larger than 2.5%, more preferably not larger than 2%.
  • P is an element effective for ensuring a desired ⁇ R .
  • P is an element effective for ensuring a desired ⁇ R .
  • P is added in an amount of 0.03% or more (more preferably 0.05% or more).
  • the amount of P added exceeds 0.1%, secondary formability will be deteriorated. More preferably, P is added in an amount of not larger than 0.1%. S : 0.02% or less (including 0%)
  • S is an element which forms a sulfide inclusion such as MnS and acts as an origin of cracking, with consequent deterioration of formability.
  • the content of S is preferably not more than 0.02%, more preferably not more than 0.015%.
  • the steel of the present invention basically contains the above components, with the balance being substantially iron and impurities, but the following components may be added insofar as they do not impair the operation of the present invention:
  • These elements are not only useful as steel strengthening elements but also effective in stabilizing ⁇ R and ensuring a predetermined amount thereof.
  • these elements be added in such amounts as Mo: 0.05% or more (more preferably 0.1% or more), Ni: 0.05% or more (more preferably 0.1% or more), Cu: 0.05% or more (more preferably 0.1% or more), and Cr: 0.05% or more (more preferably 0.1% or more).
  • Mo 0.05% or more
  • Ni 0.05% or more
  • Cu 0.05% or more
  • Cr 0.05% or more
  • these elements are added in such amounts as Mo: 0.8% or less, Ni: 0.4% or less, Cu: 0.4% or less, and Cr: 0.8% or less. At least one of Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), V: 0.1% or less (not including 0%)
  • These elements have a precipitation strengthening and microstructurization effect and are useful for the attainment of a high strength.
  • these elements be added in such amounts as Ti: 0.01% or more (preferably 0.02% or more), Nb: 0.01% or more (more preferably 0.02% or more), and V: 0.01% or more (more preferably 0.02% or more).
  • Ti 0.01% or more
  • Nb 0.01% or more
  • V 0.01% or more
  • an amount exceeding 0.1% will result in saturation of the above effects, which is wasteful from the economic standpoint.
  • these elements are added in such amounts as Ti: 0.08% or less, Nb: 0.08% or less, and V: 0.08% or less.
  • Ca and REM rare earth elements function to control the form of sulfide in steel and are effective in improving formability.
  • rare earth elements employable in the present invention are mentioned Sc, Y, and lanthanoid.
  • Sc, Y, and lanthanoid examples of rare earth elements employable in the present invention.
  • these elements be each added in an amount of 0.0003% or more (more preferably 0.0005% or more).
  • an amount thereof exceeding 0.003% would result in saturation of the above effect, which is wasteful from the economic standpoint. It is more preferable that they each be added in an amount of 0.0025% or less.
  • This method produces a desired steel sheet through 1 a hot rolling process or 2 a continuous annealing process or a plating process.
  • the hot rolling process 1 is illustrated in Fig. 6 (in case of a base phase structure being tempered martensite) and Fig. 7 (in case of a base phase structure being quenched bainite), and the continuous annealing process or plating process 2 is illustrated in Fig. 8 .
  • the hot rolling process comprises a step of terminating finish rolling at a temperature of not lower than (A r3 -50) °C and a step of cooling the rolled steel sheet to a temperature of not higher than Ms point (in case of a base phase structure being tempered martensite) or a temperature of not lower than Ms point and not higher than Bs point (in case of a base phase structure being tempered bainite) at an average cooling rate of not lower than 20°C /s and winding up the steel sheet.
  • the hot rolling conditions have been established for obtaining a desired base phase structure (quenched martensite or quenched bainite).
  • a hot rolling finish temperature FDT
  • FDT hot rolling finish temperature
  • the cooling which follows the hot rolling process, be carried out to a temperature of not higher than Ms point at an average cooling rate of not lower than 20°C/s while avoiding ferrite transformation and pearlite transformation.
  • This enables a desired quenched martensite or quenched bainite to be obtained without formation of polygonal ferrite, etc.
  • the average cooling rate after the hot rolling also exerts an influence on the final form of ⁇ R . If the average cooling rate is high, there will be obtained a lath form.
  • An upper limit of the average cooling rate is not specially limited, and the higher, the better. But in relation to the actual operation level it is recommended to make control appropriately.
  • each of the foregoing steps be controlled appropriately in order to obtain a desired quenched martensite or quenched bainite.
  • other conditions including the heating temperature, there may be selected conventional conditions (e. g., about 1000 to 1300°C) suitably.
  • the above hot rolling process 1 is followed by continuous annealing or plating.
  • a cooling process after the hot rolling 1 and before the continuous annealing or plating 2.
  • the cold rolling rate be set at 1 to 30%. This is because, if cold rolling is carried out at a cold rolling rate exceeding 30%, the rolling load will increase and it will become difficult to effect cold rolling.
  • the continuous annealing or plating process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in the said temperature range for 1 second or more.
  • These conditions have been established for tempering the base phase structure (quenched martensite or quenched bainite) produced in the hot rolling process to afford not only a desired tempered martensite but also a fine second phase.
  • the holding time is not shorter than 10 seconds, tempering will be insufficient and there will not be obtained the desired base phase structure (tempered martensite or tempered bainite).
  • the holding time is not shorter than 20 seconds, more preferably not shorter than 30 seconds. If the holding time exceeds 600 seconds, it will become impossible to maintain the lath structure which is a feature of tempered martensite or tempered bainite, with consequent deterioration of mechanical characteristics.
  • the holding time is not longer than 500 seconds, more preferably not longer than 400 seconds.
  • cooling is made to a temperature (bainite transformation: T4 in Fig. 8 ) of not lower than 300°C (preferably not lower than 350°C) and not higher than 480°C (preferably not higher than 450°C) while controlling an average cooling rate (CR) to a temperature of not lower than 3°C/s (preferably not lower than 5°C/s) and while avoiding pearlite transformation, and the steel sheet is held in this temperature range for 1 second or more (preferably 5 seconds or more: t4 in Fig. 8 ), whereby the concentration of C to ⁇ R can be attained in a large quantity and in an extremely short time.
  • a temperature bainite transformation: T4 in Fig. 8
  • the average cooling speed is lower than the above range, the desired structure will not be obtained, with formation of pearlite.
  • An upper limit of the average cooling rate is not specially limited and the higher, the better. However, in relation to the actual operation level it is recommended that control be made appropriately.
  • a two-step cooling method comprising 1 a step of cooling the steel sheet to a temperature (Tq) of (A 1 point to 600°C) at an average cooling rate of not higher than 15°C/s and 2 a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 20°C/s.
  • the above cooling is followed by austempering.
  • the austempering temperature (T4) is important for ensuring a desired structure and allowing the present invention to fulfill its operation. If the austempering temperature is controlled to a temperature in the foregoing range, there will be obtained ⁇ R stably in a large quantity, whereby TRIP effect based on ⁇ R is exhibited. In contrast therewith, if the austempering temperature is lower than 300°C, martensite phase will exist, while if it exceeds 480°C, the amount of bainite phase will increase to a great extent.
  • An upper limit of the holding time (t4) is not specially limited, but if the time taken for transformation of austenite into bainite is taken into account, it is recommended to control the upper limit to a time of not longer than 3000 seconds, preferably not longer than 2000 seconds.
  • bainite structure may be produced insofar as it does not impair the operation of the present invention, in addition to the desired base phase structure (tempered martensite or tempered bainite) and martensite. Further, plating and alloying may be performed insofar as the desired structure is not decomposed markedly nor does the application of plating and alloying impair the operation of the present invention.
  • the coarse Zn-Fe alloy crystal grains present on the alloyed, hot dip galvanized layer surface mean Zn-Fe alloy crystal grains each having a major side twice as long as a minor side, or less, and having an average grain diameter of 4 ⁇ m or more. By Fe pre-plating it is possible to decrease the number of such coarse crystal grains to five or less (preferably three or less) /70 ⁇ m ⁇ 50 ⁇ m.
  • the average grain diameter of the Zn-Fe alloy crystal grains is determined by observing the alloyed layer surface through an SEM (scanning electron microscope) (1500 ⁇ ) and calculating an average length between a length measured in a largest length direction of the crystal grains present in a visual field of 70 ⁇ m ⁇ 50 ⁇ m and a length in a direction orthogonal thereto.
  • the pre-plating step (a) is carried out under conditions which satisfy the following relation (1): 0.06 ⁇ W ⁇ X where W stands for the amount of hot dip Zn plating deposited (g/m 2 ) and X stands for the amount of Fe pre-plating deposited (g/m 2 ).
  • the amount (X) of Fe pre-plating is controlled to a value of not smaller than 0.06W in relation to the amount (W) of hot dip Zn plating deposited. This is because, if X is less than 0.06W, Si concentrates on the steel sheet surface as alloying proceeds, causing the formation of coarse Zn-Fe alloy crystal grains which exert a bad influence on the sliding property of the plated surface.
  • X is 0.08W or more, more preferably 0.10W or more.
  • An upper limit of W is not specially limited from the standpoint of improving the sliding property of the plated surface, but if X is too much, an increase of cost and deterioration of productivity will result. Therefore, it is recommended to control the upper limit to 0.30W, preferably 0.28W or less, more preferably 0.25W or less.
  • a plating bath composition to FeSO 4 ⁇ 7H 2 O: 300 to 450 g/L
  • a plating bath pH to 1.7 to 2.6
  • a plating liquid temperature to 40 to 70°C
  • a current density to 10 to 250A/dm 2
  • control the electrolysis time appropriately in accordance with a desired amount of plating to be deposited.
  • the Fe pre-plating vanishes in the plated surface layer portion, but at the interface between the steel sheet and the alloyed, hot dip galvanized layer there may remain the Fe pre-plating layer insofar as it does not impair the operation of the present invention.
  • the Fe plating is followed by annealing and subsequent hot dip galvanizing referred to in the above 2. The detailed thereof are as described in the above 2.
  • an effective Al concentration in the plating bath be controlled to a value in the range of 0.08 to 0.12 mass % and the plating bath temperature to a temperature in the range of 445° to 500 °C. This is because alloying is accelerated and powdering resistance is improved remarkably thereby.
  • an effective Al concentration in the plating bath be controlled to 0.08 to 0.12%.
  • the effective Al concentration in the plating bath is controlled to a value in the range of about 0.08 to 0.14%.
  • the alloying temperature is set low for the purpose of obtaining a desired ⁇ R , which will be described later. Therefore, alloying no longer takes place as the Al concentration becomes higher.
  • the upper limit of Al concentration is controlled preferably to 0.12% (more preferably 0.11%).
  • the Al concentration is not lower than 0.09%.
  • the plating bath temperature be controlled to a temperature in the range of 445° to 500°C.
  • a general plating bath temperature is 430° to 500°C, but in the present invention, since Si which suppresses alloying is added in a large amount, the plating bath temperature range is set to the above range for the purpose of accelerating alloying and enhancing the powdering resistance. If the plating bath temperature is lower than 445°C, there will remain an ⁇ layer (pure zinc). More preferably, the plating bath temperature is not lower than 450°C. On the other hand, a plating bath temperature exceeding 500°C will result in a lowering of powdering resistance. More preferably, the plating bath temperature is not higher than 490°C.
  • alloying be carried out at a temperature of 400° to 470°C for 5 to 100 seconds. If the alloying temperature is lower, the alloying will slow down, with consequent deterioration of productivity. On the other hand, if the alloying temperature is higher, ⁇ R once produced will vanish. If the alloying time is shorter, alloying does not take place and there will remain an ⁇ layer (pure zinc) on the surface. Conversely, a longer alloying time will lead to a lowering of productivity.
  • the Fe pre-plating is applicable not only to the production of an alloyed hot dip galvanized steel sheet but also to the production of a ht dip galvanized steel sheet. More specifically, in producing a hot dip galvanized steel sheet, if the foregoing (a) Fe pre-plating and (b) hot dip galvanizing are performed, an Fe plated layer not affected by surface concentration of Si is formed on the steel sheet surface, so that not only there is efficiently obtained ⁇ R which is effective in obtaining a high elongation characteristic, but also the occurrence of disadvantages caused by the addition of a large amount of Si can be prevented. Thus, the application of the plating steps in question is extremely useful.
  • This method produces a desired steel sheet through a hot rolling process, a cooling process, a first continuous annealing process, and a second annealing process or a plating process.
  • the first continuous annealing process which features this method is illustrated in Fig. 9 (in case of a base phase structure being quenched martensite) and Fig. 10 (in case of a base phase structure being quenched bainite).
  • the hot rolling process and the cooling process are carried out. Conditions for these processes are not specially limited, but there may be selected suitable working conditions. This is because in this method (2) it is not that a desired structure is ensured through the hot rolling process and the cooling process, but this method is characteristic in that the desired structure is obtained by controlling the subsequent first continuous annealing process and second continuous annealing process or plating process.
  • the hot rolling process there may be adopted for example conditions such that after the end of hot rolling at a temperature of not lower than Ar 3 point, cooling is performed at an average cooling rate of about 30°C/s, followed by winding at a temperature of about 500° to 600°C.
  • cooling it is recommended that cold rolling be carried out at a cooling rate of about 30% to 70%.
  • no limitation is made thereto.
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 3 point and a step of cooling the steel sheet to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point at an average cooling rate of 10°C/s.
  • cooling is performed to a temperature of not higher than Ms point (T2 in Fig. 9 ) or a temperature of not lower than Ms point and not higher than Bs point (T2 in Fig. 10 ) while controlling an average cooling rate (CR) to a 20°C/s or higher (preferably 30°C/s or higher), whereby a desired quenched martensite or quenched bainite is obtained while avoiding ferrite transformation or pearlite transformation.
  • CR average cooling rate
  • the average cooling rate (CR) is lower than the above cooling rate, there will be produced ferrite and pearlite and the desired structure will not be obtained.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. However, it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in the said temperature range from 1 second or more.
  • This process is the same as the continuous annealing process or plating process 2 described in the foregoing method (1).
  • This process has been established for tempering the base phase structure (quenched martensite or quenched bainite) produced in the first continuous annealing process 3 to obtain not only a desired tempered martensite but also a fine, second phase structure.
  • This method produces a desired steel sheet through 1 a hot rolling process and 2 a continuous annealing process or a plating process.
  • the hot rolling process 1 is illustrated in Fig. 6 in case of a base phase structure comprising quenched martensite and ferrite and in Fig. 7 in case of a base phase structure comprising quenched bainite and ferrite.
  • the continuous annealing process or plating process 2 is illustrated in Fig. 8 .
  • the hot rolling process comprises a step of terminating finish rolling at a temperature of not lower than (A r3 -50) °C and a step of cooling the rolled steel sheet to a temperature of not higher than Ms point (in case of a base phase structure comprising quenched martensite and ferrite) or a temperature of not lower than Ms point and not higher than Bs point (in case of a base phase structure comprising quenched bainite and ferrite) at an average cooling rate of not lower than 10°C/s and winding up the steel sheet
  • Ms point in case of a base phase structure comprising quenched martensite and ferrite
  • Bs point in case of a base phase structure comprising quenched bainite and ferrite
  • the hot rolling finish is followed by cooling.
  • ferrite is partially produced during cooling to provide a two phase region ( ⁇ + ⁇ ), followed by cooling to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point, whereby it is possible to obtain a desired mixed structure.
  • the average cooling rate after hot rolling exerts an influence on not only the formation of ferrite but also the final form of ⁇ R . If the average cooling rate is high (preferably 50°C/s or higher), there will be obtained a lath form.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. However, in relation to the actual operation level it is recommended to control the upper limit appropriately.
  • a two-step cooling method which comprises 1 a step of cooling the steel sheet to a temperature in the range of 700 ⁇ 100°C (preferably 700 ⁇ 50°C) at an average cooling rate (CR1) of not lower than 30°C/s, 2 a step of cooling the steel sheet with air in the said temperature range for 1 to 30 seconds, and 3 a subsequent step of cooling the steel sheet to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point at an average cooling rate (CR2) of not lower than 30°C/s and winding up the steel sheet.
  • stepwise cooling polygonal ferrite low in dislocation density can be produced in a more positive manner.
  • cooling be done at an average cooling rate of not lower than 30°C/s, preferably not lower than 40°C/s.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • the air cooling time is not longer than 20 seconds.
  • the winding temperature (CT) is as described in the foregoing (1) -1.
  • the hot rolling process 1 there is performed continuous annealing or plating. But if the shape after hot rolling is unsatisfactory, then for the purpose of correcting the shape there may be performed cooling after the hot rolling 1 and before the continuous annealing or plating 2. It is recommended that the cooling rate be set in the range of 1% to 30%.
  • This continuous annealing or plating process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in the said temperature range for 1 second or more.
  • These conditions have been established for tempering the base phase structure produced in the hot rolling process to afford not only a desired mixed structure (tempered martensite and ferrite, or tempered bainite and ferrite) but also a desired second phase structure. The details thereof are as described above in the continuous annealing process or plating process 2 in connection with the foregoing method (1).
  • This method (4) produces a desired steel sheet through a hot rolling process, a cooling process, a first continuous annealing process, and a second continuous annealing process or a plating process.
  • the first continuous annealing process which features the method (4) is illustrated in Fig. 11 in case of a base phase structure comprising quenched marternsite and ferrite and in Fig. 12 in case of a base phase structure comprising quenched bainite and ferrite.
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point and a step of cooling the steel sheet to a temperature of not higher than Ms point (in case of a base phase structure comprising quenched martensite and ferrite) or a temperature of not lower than Ms point and not higher than Bs point (in case of a base phase structure comprising quenched bainite and ferrite) at an average cooling rate of not lower than 10°C/s.
  • soaking is performed to a temperature of not lower than A 1 point and not higher than A 3 point (T1 in Figs. 11 and 12 ) (preferably 1300° or lower). Ferrite is produced partially during soaking if the soaking temperature is in the range of A 1 to A 3 or during cooling if the soaking temperature is not lower than A 3 point to provide two phases of [ferrite ( ⁇ ) + ⁇ ], followed by cooling to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point to obtain desired ( ⁇ + quenched martensite) or ( ⁇ + quenched bainite).
  • an average cooling rate is controlled to 10°C/s or higher (preferably 20°C/s or higher) and cooling is allowed to proceed to a temperature of not higher than Ms point (T2 in Fig. 11 ) or a temperature of not lower than Ms point and not higher than Bs point (T2 in Fig. 12 ) to obtain a desired mixed structure (quenched martensite and ferrite, or quenched bainite and ferrite) while avoiding pearlite transformation.
  • the content of ferrite be controlled to a value of not less than 5% and less than 30%.
  • the average cooling rate be controlled to 30°C/s or higher.
  • the average cooling rate exerts an influence not only on the formation of ferrite but also on the final form of ⁇ R , and a high average cooling rate (preferably 50°C/s or higher) will result in a lath form.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in the said temperature range for 1 second or more.
  • This process is the same as the second continuous annealing process or plating process 4 in the foregoing method (2) and has been established for tempering the base phase structure produced in the first continuous annealing process 3 to afford not only a desired structure but also a desired second phase structure.
  • the second phase structure it is necessary for the second phase structure to satisfy the structure described above and further satisfy the following expression (1): S ⁇ 1 / S ⁇ 100 ⁇ 20 where S stands for a total area of the second phase structure and S1 stands for a total area of coarse second phase structure crystal grains (Sb) present in the second phase structure, the Sb occupying three times or more of an average crystal grain area (Sm) of the second phase structure.
  • the above expression (1) means that the ratio of coarse crystal grains [those three times or more as large as an average crystal grain area (Sm) of the second phase structure] to the whole of the second phase structure which contains retained austenite is to be suppressed to 20% or less in terms of an area ratio.
  • Sm average crystal grain area
  • it is intended to improve fatigue characteristic.
  • the lowering in fatigue characteristic of the TRIP steel sheet is attributable to the formation of coarse ⁇ R and the fatigue characteristic is improved if the coarse ⁇ R is diminished and that, for example, such a tempering process as will be described later [allowing a carbide (cementite) to be precipitated between laths of the base phase structure] is effective for that purpose.
  • a steel sheet is subjected to Lepera etching and is then observed through an optical microscope ( ⁇ 1000) to provide two pictures of steel sheet structure. Then, an area of 50 ⁇ m ⁇ 50 ⁇ m is selected and cut out arbitrarily from each of the photographs. With respect to the two pictures thus cut out there are determined a total area of the second phase structure ( ⁇ R , martensite as necessary) relative to the total area of the two pictures (50 ⁇ m ⁇ 50 ⁇ m ⁇ 2), as well as an average crystal grain area (Sm) of the second phase structure.
  • the following methods (5) and (6) are mentioned as typical methods for producing the second steel sheet. These methods are substantially the same as the method (1) and (2) described previously in connection with the first steel sheet. A difference resides in that in the following methods (5) and (6) there is provided a predetermined tempering process between the hot rolling process and the continuous annealing process or the plating process or between the first continuous annealing process and the second continuous annealing process or the plating process.
  • This method produces a desired steel sheet through 1 hot rolling process, 2 tempering process, and 3 continuous annealing process or plating process.
  • the annealing process 1 is illustrated in Fig. 6 (in case of a base phase structure being quenched martensite) and Fig. 7 (in case of a base structure being quenched bainite), and the continuous annealing or plating process 3 is illustrated in Fig. 8 .
  • the hot rolling process comprises a step of terminating finish rolling at a temperature of not lower than (A r3 -50)°C and a step of cooling the rolled steel sheet to a temperature of not higher than Ms point (in case of a base phase structure being tempered martensite) or a temperature of not lower than Ms point and not higher than Bs point (in case of the base phase structure being tempered bainite) at an average cooling rate of not lower than 20°C/s and winding up the steel sheet.
  • These hot rolling conditions are established for obtaining a desired base phase structure (quenched martensite or quenched bainite).
  • a hot rolling finish temperature be set at a temperature of not lower than (A r3 -50)°C, preferably not lower than A r3 point. This is for obtaining a desired quenched martensite or quenched bainite in cooperation with subsequent "cooling to a temperature of not higher than Ms point” or "cooling to a temperature of not lower than Ms point and not higher than Bs point.”
  • cooling conditions it is recommended that cooling be performed to a temperature of not higher than Ms point while avoiding ferrite transformation and pearlite transformation at an average cooling rate of not lower than 20°C/s (preferably not lower than 30°C/s). This permits to obtain a desired quenched martensite or quenched bainite without formation of polygonal ferrite.
  • the average cooling rate after the hot rolling also exerts an influence on the final form of ⁇ R , and if the average cooling rate is high, a lath form will result.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • the above hot rolling process 1 is followed by a tempering process.
  • cooling may be performed after the hot rolling 1 and before the tempering 2. In this case, it is recommended to set the cooling rate at 1 to 30%. This is because, if cold rolling is performed at a cooling rate exceeding 30%, the rolling load will increase, making it difficult to carry out cold rolling.
  • the annealing process comprises carrying out tempering at a temperature of not lower than 400°C and not higher than A c1 point for a period of time of not shorter than 10 minutes and shorter than 2 hours.
  • This tempering process has been established for obtaining a desired ⁇ R (fine ⁇ R ) which is effective in improving the fatigue characteristic.
  • cementite is precipitated in the lath boundary of the base phase structure (quenched martensite or quenched bainite), and in the subsequent continuous annealing process or plating process 2 there is formed a fine ⁇ R with the cementite as nucleus, so that it becomes possible to diminish coarse ⁇ R produced in the pre-austenite grain boundary and block boundary.
  • tempering is carried out at a temperature of not lower than 400°C and not higher than A c1 point (about 700°C) for a period of time of not shorter than 10 minutes and shorter than 2 hours.
  • the tempering temperature is not higher than 650°C.
  • the lower limit of the tempering temperature has been determined so as to permit cementite to be precipitate as short a time as possible, taking productivity into account.
  • the lower limit is 450°C.
  • the tempering time is also important for obtaining a desired structure, and if it is shorter than 10 minutes, the precipitation of cementite will be insufficient.
  • the tempering time is 15 minutes or longer.
  • the tempering time is 2 hours or longer, cementite will become coarse to a remarkable extent, not affording the effect of microstructurization of ⁇ R .
  • the tempering time is not longer than 1 hour.
  • the tempering process 2 is not needed. This is because the foregoing hot rolling process is the same as this tempering process 2. In this case, therefore, the hot rolling process may be immediately followed by continuous annealing or plating 3 which will be described below.
  • This continuous annealing or plating process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in this temperature range for 1 second or more.
  • These conditions have been established for tempering the base phase structure (quenched martensite or quenched bainite) produced in the hot rolling process to obtain not only a desired tempered martensite but also a fine, second phase.
  • soaking is performed at a temperature of not lower than A 1 point and not higher than A 3 point (T3 in Fig. 8 ) for 10 to 600 seconds (t3 in Fig. 8 ) to produce a desired structure (tempered martensite and ⁇ R , or tempered bainite and ⁇ R ) (annealing in two phase region).
  • a desired structure tempered martensite and ⁇ R , or tempered bainite and ⁇ R
  • the holding time is not less than 20 seconds, more preferably not less than 30 seconds. If the holding time exceeds 600 seconds, it becomes impossible to retain the lath structure which is a feature of tempered martensite or tempered bainite, with consequent deterioration of mechanical characteristics.
  • the holding time is not more than 500 seconds, more preferably not more than 400 seconds.
  • an average cooling rate is controlled to a rate of not lower than 3°C/s (preferably not lower than 5°C/s) and cooling is made to a temperature of not lower than 300°C (preferably not lower than 350°C) while avoiding pearlite transformation, followed by holding in this temperature range for 1 second or more (preferably 5 seconds or more: t4 in Fig. 8 ) (austempering), whereby the concentration of C to ⁇ R can be done in a large quantity and in an extremely short time.
  • CR average cooling rate
  • the austempering temperature (T4) is important for ensuring the desired structure and allowing the operation of the present invention to be exhibited. If the austempering temperature is controlled to the above temperature range, ⁇ R will be obtained stably in a large quantity, whereby there is exhibited TRIP effect based on ⁇ R . An austempering temperature of lower than 300°C will lead to the presence of martensite phase, while an austempering temperature exceeding 480°C will result in a largely increased amount of bainite phase.
  • An upper limit of the holding time (t4) is not specially limited, but when the time taken for transformation of austenite into bainite is considered, it is recommended to control the holding time to a time of not longer than 3000 seconds, preferably not longer than 2000 seconds.
  • bainite structure insofar as it does not impair the operation of the present invention.
  • plating and alloying may be conducted insofar as the desired structure is not decomposed remarkably nor does the application of plating and alloying impair the operation of the present invention.
  • This method produces a desired steel sheet through a hot rolling process, a cold rolling process, a first continuous annealing process, a tempering process, and a second annealing process or a plating process.
  • the first annealing process which features this method is illustrated in Fig. 9 (in case of a base phase structure being quenched martensite) and Fig. 10 (in case of a base phase structure being quenched bainite).
  • the hot rolling process and the cooling process are carried out. These processes are not specially limited, but conventional conditions may be selected and adopted suitably. This is because in this method (6) the hot rolling process and the cooling process are not for ensuring a desired structure, but a feature of this method resides in controlling the subsequent first continuous annealing process, tempering process, and second continuous annealing process or plating process to obtain a desired structure.
  • conditions for the hot rolling process there may be adopted such conditions as cooling at an average cooling rate of about 30°C/s after the end of hot rolling conducted at a temperature of not lower than A r3 point and winding at a temperature of about 500° to 600°C.
  • cooling at an average cooling rate of about 30°C/s after the end of hot rolling conducted at a temperature of not lower than A r3 point and winding at a temperature of about 500° to 600°C.
  • the cooling process it is recommended to perform cold rolling at a cooling rate of about 30% to 70%. It goes without saying that no limitation is made thereto.
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 3 point and a step of cooling the steel sheet to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point at an average cooling rate of not lower than 10°C/s.
  • soaking is performed at a temperature of not lower than A 3 point (T1 in Figs. 9 and 10 ) (preferably not higher than 1300°C), then an average cooling rate (CR) is controlled to a temperature of not lower than 20°C/s (preferably not lower than 30°C/s) and cooling is made to a temperature of not higher than Ms point (T2 in Fig. 9 ) or a temperature of not lower than Ms point and not higher than Bs point (T2 in Fig. 10 ), whereby a desired quenched martensite or quenched bainite is obtained while avoiding ferrite transformation and pearlite transformation.
  • T1 in Figs. 9 and 10 preferably not higher than 1300°C
  • CR average cooling rate
  • the average cooling rate (CR) is lower than the above range, there will be produced ferrite and pearlite and it will be impossible to obtain the desired structure.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • This process is the same as the tempering process 2 in the foregoing method (5) and has been established for forming a desired fine ⁇ R .
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in this temperature range for 1 second or more.
  • This process is the same as the continuous annealing process or plating process 3 in the foregoing method 5 and has been established for tempering the base phase structure (quenched martensite or quenched bainite) produced in the first continuous annealing process 4 to obtain not only a desired tempered martensite but also a desired fine, second phase structure.
  • the base phase structure quenched martensite or quenched bainite
  • the following methods (7) and (8) are mentioned as typical methods for producing the steel sheet according to the present invention. These methods are substantially the same as the foregoing methods (3) and (4) described in connection with the known steel sheet. A difference resides in that in these methods a predetermined tempering process is provided between the hot rolling process and the continuous annealing process or the plating process or between the first continuous annealing process and the second continuous annealing process or the plating process in the methods (3) and (4).
  • This method produces a desired steel sheet through 1 a hot rolling process, 2 a tempering process, and 3 a continuous annealing process or a plating process.
  • the hot rolling process 1 is illustrated in Fig. 6 in case of a base phase structure comprising quenched martensite and ferrite and in Fig. 7 in case of a base phase structure comprising quenched bainite and ferrite, and the continuous annealing or plating process 3 is illustrated in Fig. 8 .
  • the hot rolling process comprises a step of terminating finish rolling at a temperature of not lower than (A r3 -50)°C and a step of cooling the rolled steel sheet to a temperature of not higher than Ms point (in case of a base phase structure comprising quenched martensite and ferrite) or a temperature of not lower than Ms point and not higher than Bs point (in case of a base phase structure comprising quenched bainite and ferrite) at an average cooling rate of not lower than 10°C/s and winding up the steel sheet.
  • These hot rolling conditions have been established for obtaining a desired base phase structure (a mixed structure of quenched martensite and ferrite or quenched bainite and ferrite).
  • the hot rolling finish condition is as described in the hot rolling process 1 in connection with the foregoing method (5).
  • Cooling is performed after the above hot rolling finish. According to the present invention, by controlling the cooling rate (CR), ferrite is partially produced during cooling to provide a two phase region of ( ⁇ + ⁇ ), and by cooling to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point there can be obtained a desired mixed structure.
  • CR cooling rate
  • an average cooling rate of not lower than 10°C/s there is made cooling to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point while avoiding pearlite transformation.
  • the average cooling rate by controlling the average cooling rate appropriately it is possible to obtain a desired mixed structure (quenched martensite + ferrite, or quenched bainite + ferrite).
  • the average cooling rate after hot rolling exerts an influence not only on the formation of ferrite but also on the final form of ⁇ R , and if the average cooling rate is high (preferably 50°C/s or higher), a lath form will result.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • a two-step cooling method which comprises 1 a step of cooling the steel sheet to a temperature in the range of 700 ⁇ 100°C (preferably 700 ⁇ 50°C) at an average cooling rate (CR1) of not lower than 30°C/s, 2 a step of conducting air cooling in the said temperature range for 1 to 30 seconds, and 3 a step of subsequently cooling the steel sheet to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point at an average cooling rate (CR2) of not lower than 30°C/s and winding up the steel sheet.
  • CR1 average cooling rate
  • cooling be done at an average cooling rate of not lower than 30°C/s, preferably not lower than 40°C/s.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • the temperature range 2 it is preferable that air cooling be done for 1 second or more, more preferably 3 seconds or more, whereby a predetermined ferrite quantity is attained efficiently.
  • the air cooling time exceeds 30 seconds, ferrite will be produced in an amount exceeding the preferred range, with the result that a desired strength is not attained and the stretch flange formability is deteriorated.
  • the air cooling time is not longer than 20 seconds.
  • the winding temperature (CT) is as described in the hot rolling process 1 in connection with the foregoing method (5).
  • the hot rolling 1 described above is followed by tempering. However, if the shape after the hot rolling is unsatisfactory, then for the purpose of correcting the shape there may be performed cooling after the hot rolling 1 and before the tempering 2. In this case, it is recommended to set the cold rolling rate at 1 to 30%.
  • the tempering process has been established for obtaining a desired fine ⁇ R and the details thereof are as described in the tempering process 2 in connection with the foregoing method (5).
  • the tempering process 2 becomes unnecessary. This is because the above hot rolling process is the same as this tempering process 2. In this case, the above hot rolling process maybe immediately followed by 3 continuous annealing or plating which will be described later.
  • the above tempering process 2 is followed by continuous annealing or plating.
  • the continuous annealing or plating process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in this temperature range for 1 second or more.
  • These conditions have been established for tempering the base phase structure produced in the hot rolling process to obtain not only a desired mixed structure (tempered martensite + ferrite, or tempered bainite + ferrite) but also a fine, second phase structure.
  • the details thereof are as described in the continuous annealing process or plating process 3 in connection with the foregoing method (5).
  • a two-step cooling method comprising 1 a step of cooling the steel sheet to a temperature (Tq) of (A 1 point to 600°C) at an average cooling rate of not higher than 15°C/s and 2 a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 20°C/s.
  • ferrite is the first to be produced and C contained in the ferrite is concentrated to ⁇ . If cooling is subsequently performed to the above temperature range 2 at an average cooling rate of not lower than 20°C/s (preferably not lower than 30°C/s, more preferably not lower than 40°C/s), the transformation of ⁇ into pearlite is suppressed and ⁇ remains even at a low temperature, thus affording the desired ⁇ R structure.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • This method (8) produces a desired steel sheet through a hot rolling process, a cooling process, a first continuous annealing process, a tempering process, and a second continuous annealing process or a plating process.
  • the first continuous annealing process which features the method (8) is illustrated in Fig. 11 in case of a base phase structure comprising quenched martensite and ferrite and in Fig. 12 in case of a base phase structure comprising quenched bainite and ferrite.
  • the hot rolling process and the cooling process are executed. These processes are not specially limited. Usually, suitable working conditions may be selected and adopted, the details of which are as described in the foregoing method (6).
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point and a step of cooling the steel sheet to a temperature of not higher than Ms point (in case of a base phase structure comprising quenched martensite and ferrite) or a temperature of not lower than Ms point and not higher than Bs point (in case of a base phase structure comprising quenched bainite and ferrite) at an average cooling rate of not lower than 10°C/s.
  • soaking is performed at a temperature of not lower than A 1 point and not higher than A 3 point (T1 in Figs. 11 and 12 ) (preferably 1300°C or higher). If soaking is conducted at a temperature of A1 to A3, ferrite is partially produced during soaking, while if soaking is conducted at a temperature of not lower than A 3 point, ferrite is partially produced during cooling, to provide two phases of [ferrite ( ⁇ ) + ⁇ ], followed by cooling to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point to obtain desired ( ⁇ + quenched martensite) or ( ⁇ + quenched bainite).
  • an average cooling rate is controlled to a rate of not lower than 10°C/s (preferably not lower than 20°C/s) and cooling is performed to a temperature of not higher than Ms point (T2 in Fig. 11 ) or a temperature of not lower than Ms point and not higher than Bs point (T2 in Fig. 12 ) to afford a desired mixed structure (quenched martensite + ferrite, or quenched bainite + ferrite) while avoiding pearlite transformation.
  • the average cooling rate exerts not only on the formation of ferrite but also on the final form of ⁇ R , and if the average cooling rate is high (preferably 50°C/s or higher), a lath form will result.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • the tempering process 5 becomes unnecessary. This is because the foregoing first continuous annealing process is the same as the tempering process 5. In this case, the first continuous annealing process may be immediately followed by the second continuous annealing or plating process 6 which will be described below.
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in this temperature range for 1 second or more.
  • This process is the same as the second continuous annealing or plating process 6 in the foregoing method (6) and has been established for tempering the base phase structure produced in the foregoing first continuous annealing process 4 to obtain not only a desired structure but also a fine, second phase structure.
  • BH hardening property
  • BH property is obtained by an interaction (fixing of dislocation by C) between dislocation and solid solution C, but particularly in a large strain area there occurs a phenomenon that solid solution C is insufficient, although sufficient dislocation is obtained.
  • the retained austenite as the second phase which is a supply source of solid solution C
  • the diffusion distance up to dislocation becomes shorter, so that a decrease of BH quantity due to the lack of solid solution C can be prevented. This is presumed to be the reason why an extremely excellent BH property is exhibited.
  • BH (2%) quantity As referred to herein.
  • a tensile test piece usually a JIS No. 5 test piece
  • a deformation stress ⁇ 1 is measured, then after the removal of load, the test piece is held at 170°C for 20 minutes, then tensile test is again conducted and an upper yield stress ⁇ 2 (a stress corresponding to 0.2% proof stress in the case where a yield point does not appear) is measured.
  • the BH (2%) quantity in question is represented by the difference between ⁇ 1 and ⁇ 2. (In the working Examples to be described later it will be referred to as BH2.)
  • BH (10%) quantity is measured in the same way as is the case with the above BH (2%) quantity except that in the above measurement of BH (2%) quantity a tensile test piece (usually a JIS No. 5 test piece) is pulled up to 10% in terms of a nominal strain and the resulting deformation stress is measured. In the working Examples to be described later it will be referred to as BH10.
  • the BH (2%) quantity defines BH property in an ordinary strain region, while the BH (10%) defines BH property in a large strain region.
  • the steel sheet according to the present invention satisfies the condition that the BH (2%) quantity should be not less than 70 MPa (preferably not less than 80 MPa, more preferably not less than 90 MPa) and that the BH (10%) quantity should be not less than half of the BH (2%) quantity, (not less than 35 MPa), preferably not less than 40 MPa, more preferably not less than 45 MPa.
  • This method produces a desired steel sheet through 1 a hot rolling process and 2 a continuous annealing process or a plating process.
  • the hot rolling process 1 is illustrated in Fig. 6 (in case of a base phase structure being quenched martensite) and in Fig. 7 (in case of a base phase structure being quenched bainite) and the continuous annealing process or plating process 2 is illustrated in Fig. 8 .
  • This process comprises a step of controlling a heating temperature (SRT) before hot rolling to a temperature of 950° to 1100°C and terminating finish rolling at a temperature of not lower than (A r3 -50)°C and a step of cooling the resulting steel sheet to a temperature of not higher than Ms point (in case of a base phase structure being tempered martensite) or a temperature of not lower than Ms point and not higher than Bs point (in case of a base phase structure being tempered bainite) at an average cooling rate of not lower than 20°C/s and winding up the steel sheet.
  • SRT heating temperature
  • the heating temperature (SRT) before hot rolling is controlled to a temperature of 950° to 1100°C and a hot rolling finish temperature (FDT) is set at a temperature of not lower than (A r3 -50)°C.
  • the control of the heating temperature (SRT) before hot rolling is extremely important for obtaining a desired second phase structure (finely dispersed ⁇ R ) and it is not until controlling the heating temperature to a temperature in the range of 950° to 1100°C that the above structure can be obtained.
  • a heating temperature lower than 950°C substantially overlaps the hot rolling finish temperature (FDT) which will be described later.
  • FDT hot rolling finish temperature
  • the heating temperature exceeds 1100°C, it will become impossible to obtain the desired BH property [especially BH (10%)].
  • the heating temperature is not lower than 975°C and not higher than 1075°C.
  • the SRT is controlled lower than that in the conventional TRIP steel sheet.
  • the SRT is controlled generally to the range of 1100°C exclusive to 1300°C inclusive.
  • Controlling the hot rolling finish temperature is important for obtaining a desired quenched martensite or quenched bainite in cooperation with "cooling to a temperature of not higher than Ms point” or “cooling to a temperature of not lower than Ms point and not higher than Bs point” which follows the finish rolling. It is recommended to control the FDT to a temperature of not lower than (A r3 -50)°C, preferably not lower than A r3 point.
  • the FDT plays an important role also for obtaining a desired second structure, so in addition to the foregoing control of SRT, if FDT is controlled to a temperature of not lower than (A r3 -50) °C and not higher than A r3 point, a desired second phase can be obtained more efficiently. That is, by controlling both SRT and FDT to lower values than those for the conventional steel sheet it is possible to ensure an extremely superior BH property.
  • the above hot rolling process is followed by cooling. It is recommended that cooling be performed at an average cooling rate (CR) of not lower than 20°C/s (preferably not lower than 30°C/s) to a temperature of not higher than Ms point while avoiding ferrite transformation and pearlite transformation. With this cooling, a desired quenched martensite or quenched bainite can be obtained.
  • the average cooling rate after hot rolling exerts an influence also on the final form of ⁇ R , and if the average cooling rate is high, a lath form will result.
  • An upper limit of the average cooling rate is not specially limited, and the higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • the above hot rolling process 1 is followed by continuous annealing or plating.
  • cooling may be done after the hot rolling 1 and before the continuous annealing or plating. In this case it is recommended to set the cold rolling rate at 1 to 30%. This is because if cold rolling is carried out at a cooling rate exceeding 30%, the rolling load will increase, making it difficult to effect cold rolling.
  • the continuous annealing process or plating process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s and a step of holding the steel sheet in the temperature range for 1 second or more.
  • These conditions have been established for tempering the base phase structure (quenched martensite or quenched bainite) produced in the hot rolling process to afford not only a desired tempered martensite but also a fine, second phase ( ⁇ R ).
  • soaking is performed at a temperature of not lower than A 1 point and not higher than A 3 point (T3 in Fig. 8 ) for 10 to 600 seconds (t3 in Fig. 8 ) to produce a desired structure (tempered martensite and ⁇ R , or tempered bainite and ⁇ R ) (annealing in two phase region).
  • a desired structure tempered martensite and ⁇ R , or tempered bainite and ⁇ R
  • the holding time is 20 seconds or longer, more preferably 30 seconds or longer. If the holding time exceeds 600 seconds, it becomes impossible to retain the lath structure which is a feature of tempered martensite or tempered bainite, and mechanical properties are deteriorated.
  • the holding time is not longer than 500 seconds, more preferably not longer than 400 seconds.
  • the average cooling rate (CR) is controlled to a rate of not lower than 3°C/s (preferably not lower than 5°C/s) and cooling is performed to a temperature (bainite transformation: T4 in Fig. 4 ) of not lower than 300°C (preferably not lower than 350°C) and not higher than 480°C (preferably not higher than 450°C) while avoiding pearlite transformation, followed by holding in this temperature range for 1 second or more (preferably 5 seconds or more: t4 in Fig. 8 ) (austempering), whereby the concentration of C to ⁇ R can be obtained in a large quantity and in an extremely short time.
  • a temperature bainite transformation: T4 in Fig. 4
  • T4 in Fig. 4 a temperature of not lower than 300°C (preferably not lower than 350°C) and not higher than 480°C (preferably not higher than 450°C) while avoiding pearlite transformation, followed by holding in this temperature range for 1 second or more (preferably 5 seconds or more: t4 in Fig. 8
  • the average cooling rate is lower than the above range, the desired structure will not be obtained, with formation of pearlite, etc.
  • An upper limit of the average cooling rate is not specially limited, and the higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • the above cooling step be carried out by a two-step cooling method which comprises 1 a step of cooling the steel sheet up to a temperature (Tq) of (A 1 point to 600°C) at an average cooling rate of not higher than 15°C/s and 2 a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 20°C/s.
  • Tq temperature of (A 1 point to 600°C) at an average cooling rate of not higher than 15°C/s
  • 2 a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 20°C/s.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • the cooling described above is followed by austempering.
  • the austempering temperature (T4) is important for ensuring the desired structure and allowing the operation of the present invention to be exhibited. If control is made to the foregoing temperature range, a stable and large amount of ⁇ R will be obtained, whereby TRIP effect based on ⁇ R is exhibited. If the austempering temperature is lower than 300°C, martensite phase will exist, while if it exceeds 480°C, bainite phase will increase in a large amount.
  • An upper limit of the holding time (t4) is not specially limited, but when the time taken for transformation of austenite into bainite is considered, it is recommended to control the holding time to a time of not longer than 3000 seconds, preferably not longer than 2000 seconds.
  • bainite structure insofar as the operation of the present invention is not impaired. Further, plating and alloying may be performed insofar as the desired structure is not decomposed markedly nor is impaired the operation of the present invention.
  • This method (10) produces a desired steel sheet through a hot rolling process, a cold rolling process, a first continuous annealing process, and a second continuous annealing process or plating process.
  • the first continuous annealing process which features this method is illustrated in Fig. 9 (in case of a base phase structure being quenched martensite) and in Fig. 10 (in case of a base phase structure being quenched bainite).
  • the hot rolling process and the cold rolling process are carried out.
  • controlling the heating temperature (SRT) before hot rolling is extremely important for obtaining a desired second phase structure (finely dispersed ⁇ R ). It is not until controlling the heating temperature to a temperature in the range of 950° to 1100°C that the desired structure can be obtained. If the heating temperature is lower than 950°C, it substantially overlaps a hot rolling finish temperature (FDT) which will be described later, while if it exceeds 1100°C, a desired BH property [especially BH (10%)] is not attained.
  • the heating temperature in question is not lower than 975°C and not higher than 1075°C.
  • the SRT is controlled to a lower temperature than in the conventional TRIP sheet.
  • the SRT is controlled generally to a temperature in the range of 1100°C exclusive to 1300°C inclusive.
  • a desired, finely dispersed, retained austenite phase is not obtained and that it is impossible to ensure an excellent bake hardening property particularly in a large strain region (see the working Examples to be described later).
  • hot rolling and cold rolling conditions are not specially limited, but there may be adopted conventional conditions.
  • cold rolling process it is recommended to carry out cold rolling at a cold rolling rate of about 30% to 70%.
  • no limitation is made thereto.
  • This process comprises a step of holding the steel sheet at a temperature of not lower than A 3 point and a step of cooling the steel sheet to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point at an average cooling rate of not lower than 10°C/s.
  • soaking is performed at a temperature of not lower than A 3 point (T1 in Figs. 9 and 10 ) (preferably not higher than 1300°C), then an average cooling rate (CR) is controlled to a rate of not lower than 20°C/s (preferably not lower than 30°C/s) and cooling is performed to a temperature of not higher than Ms point (T2 in Fig. 9 ) or a temperature of not lower than Ms point and not higher than Bs point (T2 in Fig. 10 ), whereby desired quenched martensite or quenched bainite is obtained while avoiding ferrite transformation and pearlite transformation.
  • CR average cooling rate
  • the average cooling rate (CR) is lower than the above range, ferrite and pearlite will be produced and the desired structure will not be obtained.
  • An upper limit of the average cooling rate (CR) is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • This process comprises a step of holding the steel sheet at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C, and a step of holding the steel sheet in this temperature range for 1 second or more.
  • This process is the same as the continuous annealing process or plating process 3 in the foregoing method (9) and has been established for tempering the base phase structure (quenched martensite or quenched bainite) produced in the first continuous annealing process 4 to obtain not only a desired tempered martensite but also a fine, second phase structure.
  • the base phase structure quenched martensite or quenched bainite
  • This method produces a desired steel sheet through 1 a hot rolling process and 2 a continuous annealing process or a plating process.
  • the hot rolling process 1 is illustrated in Fig. 6 in case of a base phase structure comprising quenched martensite and ferrite and in Fig. 7 in case of a base phase structure being quenched bainite and ferrite.
  • the continuous annealing process or plating process 2 is illustrated in Fig. 8 .
  • the hot rolling process comprises a step of controlling the heating temperature (SRT) before hot rolling to a temperature in the range of 950° to 1100°C, a step of terminating finish rolling at a temperature of not lower than (A r3 -50)°C, and a step of cooling the sheet to a temperature of not higher than Ms point (in case of a base phase structure comprising quenched martensite and ferrite) or a temperature of not lower than Ms point and not higher than Bs point (in case of a base phase structure comprising quenched bainite and ferrite) at an average cooling rate of not lower than 10°C/s and winding up the steel sheet.
  • SRT heating temperature
  • hot rolling conditions have been established for obtaining a desired base phase structure (quenched martensite and ferrite, or quenched bainite and ferrite) and a second phase structure.
  • hot rolling start and finish conditions are as described in the hot rolling process 1 in the foregoing method (9).
  • the hot rolling finish step is followed by cooling.
  • this method by controlling the cooling rate (CR), ferrite is partially produced during cooling to provide a two-phase region ( ⁇ + ⁇ ), and by cooling to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point it is possible to obtain a desired mixed structure.
  • an average cooling rate of not lower than 10°C/s (preferably not lower than 20°C/s) to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point while avoiding pearlite transformation.
  • the average cooling rate it is recommended to control the ferrite content to not less than 5% and less than 30% in terms of a space factor relative to the whole structure. In this case it is preferable to control the average cooling rate to a rate of not lower than 30°C/s.
  • the average cooling rate after hot rolling exerts an influence not only on the formation of ferrite but also on the final form of ⁇ R , and if the average cooling rate is high (preferably 50°C/s or higher), a lath form will result.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • a two-step cooling method which comprises 1 a step of cooling the steel sheet to a temperature in the range of 700 ⁇ 100°C (preferably 700 ⁇ 50°C) at an average cooling rate (CR1) of not lower than 30°C/s, 2 a step of cooling the steel sheet with air in the said temperature range for 1 to 30 seconds, and 3 a step of subsequently cooling the steel sheet to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point at an average cooling rate (CR2) of not lower than 30°C/s and winding up the steel sheet.
  • the temperature range in the above step 2 it is preferable that air cooling be done for 1 second or more, preferably 3 seconds or more, whereby a predetermined ferrite quantity is obtained efficiently.
  • the air cooling time exceeds 30 seconds, the ferrite quantity will exceed a preferred range, making it impossible to attain a desired strength and leading to deterioration of the stretch flange formability.
  • the air cooling time is not longer than 20 seconds.
  • the winding temperature (CT) is as described in the rolling process 1 in the foregoing method (9).
  • the above hot rolling process 1 is followed by continuous annealing or plating. But if the shape after hot rolling is unsatisfactory, then for the purpose of correcting the shape, cooling may be performed after the hot rolling 1 and before the continuous annealing or plating 2. In this case, it is recommended that the cooling be done at a cold rolling rate of 1 to 30%.
  • the continuous annealing or plating process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in the said temperature range for 1 second or more.
  • These conditions have been established for tempering the base phase structure produced in the hot rolling process to afford not only a desired mixed structure (tempered martensite + ferrite, or tempered bainite + ferrite) but also a fine, second phase structure.
  • the details thereof are as described in the continuous annealing process or plating process 3 in connection with the foregoing method (1).
  • the above cooling step be carried out by a two-step cooling method which comprises 1 a step of cooling the steel sheet to a temperature (Tq) of (A 1 point to 600°C) at an average cooling rate of not higher than 15°C/s and 2 a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 20°C/s.
  • Tq temperature of (A 1 point to 600°C) at an average cooling rate of not higher than 15°C/s
  • 2 a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 20°C/s.
  • cooling is made to the temperature range in the above step 1 at an average cooling rate of not higher than 15°C/s (preferable not higher than 10°C/s), first ferrite is produced and C contained in the ferrite is concentrated into ⁇ . Subsequently, if cooling is conducted to the temperature range in the above step 2 at an average cooling rate of not lower than 20°C/s (preferably not lower than 30°C/s, more preferably not lower than 40°C/s), the transformation of ⁇ into pearlite is suppressed and ⁇ remains even at a low temperature, resulting in that the desired ⁇ R structure is obtained.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • This method (12) produces a desired steel sheet through a hot rolling process, a cold rolling process, a first continuous annealing process, a tempering process, and a second annealing process or a plating process.
  • the first continuous annealing process which features this method (12) is illustrated in Fig. 11 in case of a base phase structure comprising quenched martensite and ferrite and in Fig. 12 in case of a base phase structure comprising quenched bainite and ferrite.
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point and a step of cooling the steel sheet to a temperature of not higher than Ms point (in case of a base phase structure comprising quenched martensite and ferrite) or a temperature of not lower than Ms point and not higher than Bs point (in case of a base phase structure comprising quenched bainite and ferrite) at an average cooling rate of not lower than 10°C/s.
  • soaking is performed at a temperature of not lower than A 1 point and not higher than A 3 point (T1 in Figs. 11 and 12 ) (preferably 1300°C or lower). Ferrite is partially produced during soaking when soaking is done at a temperature of A 1 to A 3 or during cooling when soaking is done at a temperature of not lower than A 3 point, to provide two phases of [ferrite ( ⁇ ) + ⁇ ], followed by cooling to a temperature of not higher than Ms point or a temperature of not lower than Ms point and not higher than Bs point, whereby there is obtained desired ( ⁇ + quenched martensite) or ( ⁇ + quenched bainite).
  • an average cooling rate is controlled to a rate of not lower than 10°C/s (preferably not lower than 20°C/s) and cooling is performed to a temperature of not higher than Ms point (T2 in Fig. 11 ) or a temperature of not lower than Ms point and not higher than Bs point (T2 in Fig. 12 ), whereby a desired mixed structure (quenched martensite + ferrite, or quenched bainite + ferrite) while avoiding pearlite transformation.
  • the average cooling rate exerts an influence not only on the formation of ferrite but also on the final form of ⁇ R , and if the average cooling rate is high (preferably 50°C/s or higher), a lath form will result.
  • An upper limit of the average cooling rate is not specially limited. The higher, the better. But it is recommended to control the upper limit appropriately in relation to the actual operation level.
  • This process comprises a step of holding the steel sheet in a heated state at a temperature of not lower than A 1 point and not higher than A 3 point for 10 to 600 seconds, a step of cooling the steel sheet to a temperature of not lower than 300°C and not higher than 480°C at an average cooling rate of not lower than 3°C/s, and a step of holding the steel sheet in this temperature range for 1 second or more.
  • This process is the same as the second continuous annealing process or plating process 6 in the foregoing method (2) and has been established for tempering the base phase structure produced in the first continuous annealing process 3 to afford not only a desired structure but also a fine, second phase structure.
  • Example 1 A study (part 1) of components compositions in the known high strength steel sheet (base phase structure: tempered martensite)
  • each of the slabs was heated at 1150°C for 30 minutes, then the finish temperature (FDT) was set at 900°C and cooling was performed to room temperature at an average cooling rate of 50°C/s (hot rolling process), followed by annealing in two phase region for 120 seconds, subsequent cooling to 400°C at an average cooling rate of 30°C/s, and holding at this temperature for 30 seconds (austempering). These conditions were used as basic conditions.
  • TS tensile strength
  • El elongation
  • YP yield strength
  • a stretch flange formability test there was used a disc-like test piece having a diameter of 100 mm and a thickness of 2.0mm. More specifically, a hole 10 mm in diameter was formed by punching and was subjected to a hole expanding work on burr with a 60° conical punch, then a hole expanding rate ( ⁇ ) upon crack penetration was measured (Japan Steel Federation JFST 1001).
  • each steel sheet was subjected to Lepera etching, then the structure thereof was identified by observation under a transmission electron microscope (TEM; magnification 15000X), and thereafter a space factor of the structure was measured by observation through an optical microscope (magnification 1000X).
  • the space factor of ⁇ R and the concentration of C in ⁇ R were measured by an X-ray diffraction method after chemical polishing, following grinding the steel sheet to a quarter thickness thereof ( ISIJ Int. Vol.33.(1933), No.7, P.776 ).
  • a TEM photograph (magnification: 15000X) of a steel sheet (No. 3) according to the present invention is shown in Fig. 13 . From this photograph it is seen that the steel sheet according to the present invention has tempered martensite of a clear lath structure.
  • No.1 which is an example of a small amount of C, is low in both TS and El because desired tempered martensite and ⁇ R are not obtained.
  • No. 1 is a DP steel sheet of ferrite and martensite using No. 2 steel sample shown in Table 1
  • No. 2 is a TRIP steel sheet using No. 3 steel sample in Table 1 and with polygonal ferrite as a base phase
  • No. 3 is a two phase steel sheet of ferrite and bainite, using No. 2 steel sample shown in Table 1.
  • Example 2 A study (part 2) of components' compositions in the known high strength steel sheet (base phase structure: tempered martensite)
  • a TEM photograph (magnification: 15000X) of a steel sheet (No. 3) is shown in Fig. 14 . From this photograph it is seen that the steel sheet has tempered martensite of a clear lath structure.
  • No. 1 and 2 are low in El because their C contents, which are 0.15% and 0.20%, are smaller than in the other examples (all being not less than 0.4% in the amount of C).
  • No. 7 which is an example of a small amount of Mn and a small total amount of (Si+Al), is as low as 20% in El because a desired ⁇ R is not obtained.
  • No. 16 is an example of having produced a large amount of pearlite structure as a second phase structure due to adoption of a somewhat low cooling rate, in which both El and ⁇ are low.
  • Table 6 shows the results of having evaluated characteristics of a conventional TRIP steel sheet using No. 3 steel sample shown in Table 1 and with polygonal ferrite as a base phase.
  • Example 3 A study of manufacturing conditions for the known high strength steel sheet (base phase structure: tempered martensite)
  • Example 2 the structure of each of the steel sheets was checked in the same way as in Example 1. The results obtained are also shown in Tables 7 and 8.
  • the steels used in this Example are different in only the amount of C (C of No. 3 in Table 1 is 0.15% and that of No. 4 in Table 4 is 0.48%) but are substantially the same in the contents of other components, so that all of the structures obtained were the same.
  • No. 1 to 24 in Table 7 were produced by the foregoing method (1). More specifically, No. 1 to 23 were subjected to hot rolling ⁇ continuous annealing and No. 24 was subjected to hot rolling ⁇ plating (further, alloying).
  • Plating bath FeSO 4 ⁇ 7H 2 O (400 g/L) Liquid pH: 2.0 Liquid temp.: 60°C Current density: 50A/dm 2
  • the alloyed, hot dip galvanized steel sheet thus Fe pre-plated afforded a satisfactory structure and was extremely superior in plating characteristics (not shown in the table) such as excellent sliding property and powdering resistance of the plated surface without the lack of plating.
  • No. 2 is an example of a low hot rolling finish temperature (FDT), in which a desired structure was not obtained, but ferrite structure was produced.
  • FDT low hot rolling finish temperature
  • No. 4 is an example of a low average cooling rate (CR) in hot rolling, in which ferrite and pearlite were produced.
  • No. 5 is an example of a high winding temperature (CT) in hot rolling, in which bainite was produced in a large quantity.
  • CT high winding temperature
  • No. 7 is an example of using a conventional TRIP steel (with a base phase being polygonal ferrite), in which a desired structure was not obtained.
  • No. 8 is an example of a high two phase region temperature (T3) in continuous annealing, in which a desired structure was not obtained, but bainite structure was obtained as a base phase structure.
  • T3 high two phase region temperature
  • No. 12 is an example of a low T3, in which ⁇ R structure was not obtained.
  • No. 15 is an example of a short holding time (t3) at a two phase region temperature in continuous annealing, in which tempering was insufficient and a desired tempered martensite was not obtained.
  • No. 17 is an example of a low average cooling rate (CR) in continuous annealing, in which pearlite was produced.
  • No. 20 and 21 are examples low in austempering temperature (T4) (i.e., austempering is not performed), in which a desired structure was not obtained, but martensite was produced.
  • T4 austempering temperature
  • No. 25 to 27 in Table 7 are examples in which cold rolling was performed in the foregoing method (1). More specifically, No. 25 and 26 are examples of having gone through hot rolling ⁇ cold rolling ⁇ continuous annealing and No. 27 is an example having gone through hot rolling ⁇ cold rolling ⁇ plating (further, alloying).
  • No. 28 to 52 in Table 8 followed the foregoing method (2). More specifically, No. 28 to 51 have gone through hot rolling ⁇ cold rolling ⁇ first continuous annealing ⁇ second continuous annealing, while No. 52 has gone through hot rolling ⁇ cold rolling ⁇ first continuous annealing ⁇ plating (further, alloying).
  • No. 29 and 30 are examples of low ⁇ region temperatures (T1) in the first continuous annealing process, in which ferrite was produced.
  • No. 33 is an example of a low average cooling rate (CR) in the first continuous annealing process, in which polygonal ferrite and pearlite were produced.
  • CR average cooling rate
  • No. 35 is an example of a high two phase region temperature (T3) in the second continuous annealing process, in which bainite structure was obtained as a base phase structure.
  • No. 39 is an example of a low T3, in which a desired ⁇ R structure was not obtained.
  • No. 40 is an example of a long holding time (t3) in a two phase temperature region in the second continuous annealing process, in which ferrite structure was obtained as a base phase structure.
  • No. 43 is an example of a short t3, in which tempering was insufficient and a desired tempered martensite was not obtained.
  • No. 45 is an example of a low average cooling rate (CR) in the second continuous annealing process, in which pearlite was produced.
  • No. 48 and 49 are examples low in austempering temperature (T4) (i.e., austempering is not performed), in which martensite was produced and a desired structure was not obtained.
  • T4 austempering temperature
  • Example 4 A study (part 1) of components' compositions in the known high strength steel (base phase structure: tempered bainite)
  • Example 2 a check was made about the influence of varying components' compositions on mechanical properties mainly with respect to low C steels having a C content of 0.25% or less [steels high in strength (TS) x stretch flange formability ( ⁇ ) and taking weldability into account]. More specifically, steel samples comprising components' compositions shown in Table 1 (unit in the table is mass %) were vacuum-melted into slabs for experiment, followed by the same procedure as in Example 1 in accordance with the foregoing method (1) (hot rolling ⁇ continuous annealing) to afford hot rolled steel sheets each having a thickness of 2.0 mm.
  • TS high in strength
  • ⁇ stretch flange formability
  • Example 2 the steel sheets thus obtained were measured for tensile strength (TS), elongation [total elongation (El)], yield strength (YP), and stretch flange formability (hole expanding property: ⁇ ), and in each of the steel sheets there were measured an area fraction of structure, a space factor of ⁇ R , and the concentration of C in ⁇ R .
  • TS tensile strength
  • El total elongation
  • YP yield strength
  • ⁇ stretch flange formability
  • a TEM photograph (magnification: 15000 ⁇ ) of a steel sheet (No. 3) according to the present invention is shown in Fig. 15 . From this picture it is seen that the steel sheet according to the present invention has tempered bainite of a clear lath structure.
  • No. 1 is an example of a small amount of C, in which TS and El are low because desired tempered bainite and ⁇ R are not obtained.
  • No. 6 is an example of a small total amount of (Si+Al) and a small amount of Mn, in which El is as low as 10% because a desired ⁇ R is not obtained.
  • Example 5 A study (part 2) of components' compositions in the known high strength steel (base phase structure: tempered bainite)
  • a TEM photograph (magnification: 15000X) of a steel sheet (No. 3) according to the present invention is shown in Fig. 16 . From this photograph it is seen that the steel sheet according to the present invention has tempered bainite of a clear lath structure.
  • No. 1 and 2 are low in El because their C quantities are smaller than in the other examples (all being not less than 0.4% in the amount of C).
  • No. 7 is an example of a small amount of Mn and a small total amount of (Si+Al), in which El is as low as 12% because a desired ⁇ R is not obtained.
  • No. 16 is an example of having produced a large amount of pearlite structure as a second phase structure due to adoption of a somewhat low cooling rate, in which both El and ⁇ are low.
  • Table 11 shows the results of having evaluated characteristics of a conventional TRIP steel sheet using No. 3 steel sample shown in Table 1 and with polygonal ferrite as a base phase.
  • Example 6 A study of manufacturing conditions for the known high strength steel sheet (base phase structure: tempered bainite)
  • Example 2 the structure of each of the steel sheets was checked in the same way as in Example 1.
  • the results obtained are as shown in Tables 12 and 13.
  • the steels used in this Example are different in only the amount of C (C of No. 3 in Table 1 is 0.15% and that of No. 3 in Table 4 is 0.41%) but are substantially the same in the contents of other components, so that all of the structures obtained were the same.
  • No. 1 to 23 were produced by the foregoing method (1). More specifically, No. 1 to 22 were subjected to hot rolling ⁇ continuous annealing and No. 23 was subjected to hot rolling ⁇ plating (further, alloying).
  • No. 1, 3, 8 to 10, 12, 13, 15, 17, 18, and 21 to 23 are examples of production carried out using known conditions, in which desired structures were obtained.
  • the alloyed, hot dip galvanized steel sheet thus Fe pre-plated afforded a satisfactory structure and was extremely superior in plating characteristics (not shown in the table) such as excellent sliding property and powdering resistance of the plated surface without the lack of plating.
  • No. 2 is an example of a low hot rolling finish temperature (FDT), in which a desired structure was not obtained, but ferrite structure was produced.
  • FDT low hot rolling finish temperature
  • No. 4 is an example of a low average cooling rate (CR) in hot rolling, in which ferrite and pearlite were produced.
  • No. 5 is an example of a low winding temperature (CT) in hot rolling, in which tempered martensite was produced.
  • CT low winding temperature
  • No. 6 is an example of a high CT, in which a desired structure was not obtained, but there was obtained the same structure as that of a conventional TRIP steel (with a base phase being polygonal ferrite).
  • No. 7 is an example of a high two phase region temperature (T3) in continuous annealing, in which a desired structure was not obtained, but bainite structure was obtained as a base phase structure.
  • T3 high two phase region temperature
  • No. 11 is an example of a low T3, in which a retained austenite ( ⁇ R ) structure was not obtained.
  • No. 14 is an example of a short holding time (t3) at a two phase region temperature in continuous annealing, in which tempering was insufficient and a desired tempered bainite was not obtained.
  • No. 16 is an example of a low average cooling rate (CR) in continuous annealing, in which pearlite was produced.
  • No. 19 and 20 are examples low in austempering temperature (T4) (i.e., austempering is not performed), in which a desired structure was not obtained, but martensite was produced.
  • T4 austempering temperature
  • No. 24 to 26 in Table 12 are examples in which cold rolling was performed in the foregoing method (1). More specifically, No. 24 and 25 are examples of having gone through hot rolling ⁇ cold rolling ⁇ continuous annealing and No. 26 is an example having gone through hot rolling ⁇ cold rolling ⁇ plating (further, alloying).
  • No. 27 to 51 in Table 13 followed the foregoing method (2). More specifically, No. 27 to 50 have gone through hot rolling ⁇ cold rolling ⁇ first continuous annealing ⁇ second continuous annealing, and No. 51 have gone through hot rolling ⁇ cold rolling ⁇ first continuous annealing ⁇ plating (further, alloying).
  • No. 28 and 29 are examples of low ⁇ region temperatures (T1) in the first continuous annealing process, in which ferrite was produced.
  • No. 32 is an example of a low average cooling rate (CR) in the first continuous annealing process, in which polygonal ferrite and pearlite were produced.
  • CR average cooling rate
  • No. 34 is an example of a high two phase region temperature (T3) in the second continuous annealing process, in which all of the structure obtained was not a tempered bainite structure, but was an ordinary bainite structure.
  • No. 38 is an example of a low T3, in which a desired ⁇ R was not obtained.
  • No. 39 is an example of a long holding time t3) in a two phase temperature region in the second continuous annealing process, in which ferrite structure was obtained as a base phase structure.
  • No. 42 is an example of a short t3, in which tempering was insufficient and a desired tempered bainite was not obtained.
  • No. 44 is an example of a low average cooling rate (CR) in the second continuous annealing process, in which pearlite was produced.
  • CR average cooling rate
  • No. 47 and 48 are examples low in austempering temperature (T4) (i.e., austempering is not performed), in which martensite was produced and a desired structure was not obtained.
  • T4 austempering temperature
  • Example 7 A study (part 1) of components' compositions in the known high strength steel sheet (base phase structure: a mixed structure of tempered martensite and ferrite)
  • Example 2 a check was made about the influence of varying components' compositions on mechanical properties mainly with respect to low C steels having a C content of 0.25% or less [steels high in strength (TS) x stretch flange formability ( ⁇ ) and taking weldability into account]. More specifically, steel samples comprising components' compositions shown in Table 1 (unit in the table is mass %) were vacuum-melted into slabs for experiment and thereafter the procedure of Example 1 was repeated in accordance with the foregoing method (3) (hot rolling ⁇ continuous annealing) to afford hot rolled steel sheets having a thickness of 2.0 mm.
  • TS high in strength
  • ⁇ stretch flange formability
  • Example 2 the steel sheets thus obtained were measured for tensile strength (TS), elongation [total elongation (El)], yield strength (YP), and stretch flange formability (hole expanding property: ⁇ ), and also there were measured an area fraction of structure in each of the steel sheets, a space factor of ⁇ R , and the concentration of C in ⁇ R .
  • TS tensile strength
  • El total elongation
  • YP yield strength
  • ⁇ stretch flange formability
  • an optical microphotograph (magnification: 1000X) of a steel sheet (No. 3) according to the present invention is shown in Fig. 17 . From this photograph it is seen that the steel sheet according to the present invention has tempered martensite of a lath structure.
  • No. 1 is an example of a small amount of C, in which ⁇ R was not obtained and it was impossible to ensure a desired El.
  • No. 2 is an example of a C ⁇ R content of less than 0.8%, in which it was impossible to ensure a desired El.
  • No. 7 is an example of a small amount of Mn and a small total amount of (Si+Al), in which a desired ⁇ R was not obtained and hence El was low.
  • No. 19 is an example of having adopted a somewhat low cooling rate and a consequent large proportion of pearlite structure, in which a predetermined tempered martensite was not obtained and both El and ⁇ were deteriorated.
  • Example 8 A study (part 2) of components' compositions in the known high strength steel sheet (base phase structure: a mixed structure of tempered martensite and ferrite)
  • Fig. 18 an optical microphotograph (magnification: 1000X) of a steel sheet (No. 3) according to the present invention is shown in Fig. 18 . From this photograph it is seen that the steel sheet according to the present invention has tempered martensite of a lath structure.
  • No. 1 is smaller in the amount of C, which is 0.15%, than in the other examples (C: 0.4% or more) and is low in El.
  • No. 2 is also small in the amount of C, which is 0.15%, and is less than 0.8% in the amount of C ⁇ R , low in El.
  • No. 3 is less than 0.8% in the amount of C ⁇ R and it was impossible to ensure a desired El.
  • No. 8 is an example of a small amount of Mn and a small total amount of (Si+Al), in which El is low because a desired ⁇ R is not obtained.
  • No. 20 is an example of a large proportion of pearlite structure because of adoption of a somewhat low cooling rate, in which a predetermined tempered martensite was not obtained and both El and ⁇ were deteriorated.
  • No. 22 is a DP steel plate of ferrite and martensite using the steel sample of No. 3 in Table 1
  • No. 23 is a conventional TRIP steel sheet using the steel sample of No. 3 in Table 1 and with a base phase being polygonal ferrite
  • No. 24 is a conventional two phase steel sheet of ferrite and bainite using the steel sample of No. 3 in Table 1.
  • Example 9 A study of manufacturing conditions for the known high strength steel sheet (base phase structure: a mixed structure of tempered martensite and ferrite)
  • Example 2 the structure of each of the steel sheets was checked in the same manner as in Example 1. The results obtained are also shown in Tables 18 and 19.
  • the steels used in this Example are different in only the amount of C (C of No. 4 in Table 1 is 0.20% and that of No. 4 in Table 15 is 0.48%) but are substantially the same in the contents of other components, so that all of the structural constructions (types of second phase) obtained were the same.
  • No. 1 to 25 in Table 18 were produced by the foregoing method (3). More specifically, No. 1 to 23 were subjected to hot rolling ⁇ continuous annealing. In No. 5 to 7 and No. 25 there was conducted one-step cooling in the hot rolling process, while in the other runs there was conducted two-step cooling in the same process. No. 24 and 25 were subjected to hot rolling ⁇ plating (further, alloying), of which No. 24 is an example of having conducted two-step cooling in the hot rolling process and No.25 is an example of having conducted one-step cooling in the same process.
  • No. 1, 3 to 4, 7, 9 to 11, 13 to 14, 16, 18 to 19, and 22 to 25 are example of production carried out using known conditions, in which desired structures were obtained.
  • the alloyed, hot dip galvanized steel sheet having been subjected to Fe pre-plating proved to have a good structure equal to that obtained without going through pre-plating, and also proved to have extremely superior plating characteristics (not shown in the table) such as superior sliding property and powdering resistance of the plated surface without the lack of plating.
  • No. 2 is an example of a high winding temperature (CT) in hot rolling, in which ferrite and tempered bainite were produced.
  • CT high winding temperature
  • No. 5 is an example of a high CT, in which the same structure as in a conventional TRIP steel (TRIP steel with a base phase being polygonal ferrite) was obtained, but a desired structure was not obtained.
  • TRIP steel TRIP steel with a base phase being polygonal ferrite
  • No. 6 is an example of a low average cooling rate (CR1) in hot rolling, in which, due to the absence of tempered martensite in the as-hot-rolled structure, a desired structure was not obtained and a conventional TRIP steel structure was produced.
  • CR1 low average cooling rate
  • No. 8 is an example of a high two phase region temperature (T3) in continuous annealing, in which a desired texture was not obtained and a conventional TRIP steel structure was produced.
  • T3 high two phase region temperature
  • No. 12 is an example of a low T3, in which desired ⁇ R was not obtained.
  • No. 15 is an example of a short holding time (t3) at a two phase region temperature in continuous annealing, in which tempering was insufficient and desired tempered martensite was not obtained.
  • No. 17 is an example of a low average cooling rate (CR) in continuous annealing, in which pearlite was produced.
  • No. 20 and 21 are examples of a low austempering temperature (T4) (i.e., austempering was not performed), in which desired structure was not obtained and martensite was produced.
  • T4 low austempering temperature
  • No. 26 to 30 in Table 19 are example of having performed cold rolling in the foregoing method (3). More specifically, No. 26 to 28 are example of having gone through hot rolling ⁇ cold rolling ⁇ continuous annealing and No. 29 to 30 are examples of having gone through hot rolling ⁇ cold rolling ⁇ plating (further, alloying), of which No. 28 and 30 are examples in which one-step cooling was performed in the hot rolling process, and the other examples adopted two-step cooling.
  • No. 26 and 28 to 30 are examples using known conditions, in which desired structures were obtained.
  • No. 27 is an example of a high cold rolling rate, in which pre-structure was destroyed by cold rolling and a desired tempered martensite was not obtained.
  • No. 31 to 57 in Table 19 followed the foregoing method (4). More specifically, No. 31 to 56 have gone through hot rolling ⁇ cold rolling ⁇ first continuous annealing ⁇ second continuous annealing, while No. 57 has undergone hot rolling ⁇ cold rolling ⁇ first continuous annealing ⁇ plating (further, alloying).
  • No. 31 to 34, 36, 37, 39, 41 to 43, 46 to 47, 49, 51 to 52, and 55 to 57 adopted known conditions, in which desired structures were obtained.
  • No. 35 is an example of a low T1, in which a desired ⁇ R was not obtained.
  • No. 38 is an example of a low average cooling rate (CR) in the first continuous annealing process, in which polygonal ferrite and pearlite were produced.
  • CR average cooling rate
  • No. 40 is an example of a high two phase region temperature (T3) in the second continuous annealing process, in which a conventional TRIP steel structure was obtained.
  • No. 44 is an example of a low T3, in which a desired ⁇ R was not obtained.
  • No. 45 is an example of a long holding time (t3) in two phase region in the second continuous annealing process, in which ferrite structure was produced as a base phase and tempered martensite vanished.
  • No. 48 is an example of a short t3, in which tempering was insufficient and desired tempered martensite was not obtained.
  • No. 50 is an example of a low average cooling rate (CR) in the second continuous annealing process, in which pearlite was produced.
  • No. 53 and 54 are examples low in austempering temperature (T4) (i.e., austempering is not performed), in which martensite was produced and a desired structure was not obtained.
  • T4 austempering temperature
  • Example 10 A study (part 1) of components' compositions in the known high strength steel plate (base phase structure: a mixed structure of tempered bainite and ferrite)
  • Example 2 a check was made about the influence of varying components' compositions on mechanical properties mainly with respect to low C steels having a C content of 0.25% or less [steels high in strength (TS) ⁇ stretch flange formability ( ⁇ ) and taking weldability into account]. More specifically, steel samples comprising components' compositions shown in Table 1 (unit in the table is mass %) were vacuum-melted into slabs for experiment and thereafter the procedure of Example 1 was repeated in accordance with the foregoing method (3) (hot rolling ⁇ continuous annealing) to afford hot rolled steel sheets having a thickness of 2.0 mm.
  • TS high in strength
  • ⁇ stretch flange formability
  • Example 2 the steel sheets thus obtained were measured for tensile strength (TS), elongation [total elongation (El)], yield strength (YP), and stretch flange formability (hole expanding property: ⁇ ), and also there were measured an area fraction of structure in each of the steel sheets, a space factor of ⁇ R , and the concentration of C in ⁇ R .
  • TS tensile strength
  • El total elongation
  • YP yield strength
  • ⁇ stretch flange formability
  • Fig. 19 an optical microphotograph (magnification: 1000X) of a steel sheet (No. 3) according to the present invention is shown in Fig. 19 . From this photograph it is seen that the steel sheet according to the present invention has tempered bainite of a lath structure and ferrite
  • No. 1 is an example of small amount C, in which it was impossible to attain a desired El.
  • No. 2 is an example of a C ⁇ R quantity of less than 0.8%, in which it was impossible to attain a desired El.
  • No. 7 is an example of a small amount of Mn and a small total amount of (Si+Al), in which a desired ⁇ R was not obtained and therefore El was low.
  • No. 19 is an example of having adopted a low cooling rate and a consequent large proportion of pearlite structure, in which a predetermined tempered bainite was not obtained and both El and ⁇ were deteriorated.
  • Example 11 A study (part 2) of components' compositions in the known high strength steel sheet (base phase structure: a mixed structure of tempered bainite and ferrite)
  • Fig. 20 For reference, an optical microphotograph (magnification: 1000X) of a steel sheet (No. 3) according to the present invention is shown in Fig. 20 . From this photograph it is seen that the steel sheet according to the present invention has tempered bainite of a lath structure and ferrite.
  • No. 1 is smaller in the amount of C, which is 0. 15%, than the other examples (C: 0.4% or more) and is therefore low in El.
  • No. 2 is also as low as 0.20% in the amount of C and has a C ⁇ R content of less than 0.8%, in which El is low.
  • No. 7 is an example of a small amount of Mn and a small total amount of (Si+Al), in which a desired ⁇ R was not obtained and hence El was low.
  • No. 19 is an example of having adopted a somewhat low cooling rate and a consequent large proportion of pearlite structure, in which a predetermined tempered bainite was not obtained and both El and ⁇ were deteriorated.
  • two types of steel sheets (a conventional TRIP steel sheet using polygonal ferrite as a base phase and a conventional two phase steel sheet of ferrite and bainite) were produced by using steel samples No. 2 and No. 3 shown in Table 1 and by suitably adjusting heat treatment conditions and were then evaluated for various characteristics, the results of which are set out in table 22.
  • Table 22 shows that the conventional TRIP steel sheet using No. 3 in Table 1 is high in El but low in ⁇ and that the conventional ferrite-bainite two phase steel sheet using No. 2 in Table 1 is low in El.
  • Example 12 A study of manufacturing conditions for the known high strength steel sheet (base phase structure: a mixed structure of tempered bainite and ferrite)
  • Example 2 the structure of each of the steel sheets was checked in the same way as in Example 1.
  • the results obtained are also set out in Tables 23 and 24.
  • the steels used in this Example are different in only the amount of C (C of No. 3 in Table 1 is 0.20% and that of No. 4 in Table 15 is 0.48%) but are substantially the same in the contents of other components, so that all of the structures obtained were the same.
  • No. 1 to 25 in Table 23 were produced by the foregoing method (3). More specifically, No. 1 to 23 were subjected to hot rolling ⁇ continuous annealing, of which No. 5 to 7 and No. 25 adopted one-step cooling in the hot rolling process and the others adopted two-step cooling. Further, No. 24 and 25 are examples of having been subjected to hot rolling ⁇ plating (further, alloying), of which No. 24 is an example of having adopted two-step cooling in the hot rolling process and No. 25 is an example of having adopted one-step cooling. No. 1 to 3, 7, 9 to 11, 13, 14, 16, 18, 19, and 22 to 25 are examples of production carried out using known conditions, in which desired structures were obtained.
  • the alloyed, hot dip galvanized steel sheet thus Fe pre-plated afforded a satisfactory structure and was extremely superior in plating characteristics (not shown in the table) such as excellent sliding property and powdering resistance of the plated surface without the lack of plating.
  • No. 4 is an example of a low winding temperature (CT) in hot rolling, in which ferrite and tempered martensite were produced.
  • CT low winding temperature
  • No. 5 is an example of a high CT, in which there was obtained the same structure as that of a conventional TRIP steel (with a base phase being polygonal ferrite) and a desired structure was not obtained.
  • No. 6 is an example of a low average cooling rate (CR) in hot rolling, in which a desired structure was not obtained because of absence of tempered bainite in the as-hot-rolled structure, and a conventional TRIP steel structure was produced.
  • CR average cooling rate
  • No. 8 is an example of a high two phase region temperature (T3) in continuous annealing, in which a desired structure was not obtained, but a conventional TRIP steel structure was produced.
  • T3 high two phase region temperature
  • No. 12 is an example of a low T3, in which ⁇ R structure was not obtained.
  • No. 15 is an example of a short holding time (t3) at a two phase region temperature in continuous annealing, in which tempering was insufficient and desired tempered bainite was not obtained.
  • No. 17 is an example of a low average cooling rate (CR) in continuous annealing, in which pearlite was produced.
  • No. 20 and 21 are examples low in austempering temperature (T4) (i.e., austempering is not performed), in which a desired structure was not obtained, but martensite was produced.
  • T4 austempering temperature
  • No. 26 to 30 in Table 23 are examples in which cold rolling was performed in the foregoing method (3). More specifically, No. 26 to 28 are examples which were subjected to hot rolling ⁇ cold rolling ⁇ continuous annealing and No. 29 and 30 are examples which were subjected to hot rolling ⁇ cold rolling ⁇ plating (further, alloying). In No. 28 and 30 there was adopted one-step cooling in the hot rolling process, while in the other examples there was adopted two-step cooling.
  • No. 27 is an example of a high cold rolling rate, in which a desired tempered bainite was not obtained.
  • No. 31 to 57 in Table 24 followed the foregoing method (4). More specifically, No. 31 to 56 have gone through hot rolling ⁇ cold rolling ⁇ first continuous annealing ⁇ second continuous annealing, while No. 57 has gone through hot rolling ⁇ cold rolling ⁇ first continuous annealing ⁇ plating (further, alloying).
  • No. 32 to 34, 36, 37, 39, 41 to 43, 46 to 47, 49, 51 to 52, and 55 to 57 are examples of production carried out under known conditions, in which desired structures were obtained.
  • No. 31 is an example of a high ⁇ region temperature (T1) in the continuous annealing process, in which not tempered bainite but ferrite and tempered martensite were produced.
  • No. 35 is an example of a low T1, in which a desired ⁇ R structure was not obtained.
  • No. 38 is an example of a low average cooling rate (CR) in the first continuous annealing process, in which polygonal ferrite and pearlite were produced.
  • CR average cooling rate
  • No. 40 is an example of a high two phase region temperature (T3) in the second continuous annealing process, in which a conventional TRIP steel structure was obtained.
  • No. 44 is an example of a low T3, in which a desired ⁇ R was not obtained.
  • No. 45 is an example of a long holding time (t3) in a two phase temperature region in the second continuous annealing process, in which ferrite structure was obtained as a base phase structure, and tempered bainite vanished.
  • No. 48 is an example of a short t3, in which tempering was insufficient and a desired tempered bainite was not obtained.
  • No. 50 is an example of a low average cooling rate (CR) in the second continuous annealing process, in which pearlite was produced.
  • No. 53 and 54 are examples low in austempering temperature (T4) (i.e., austempering is not performed), in which martensite was produced and a desired structure was not obtained.
  • T4 austempering temperature
  • Example 13 A study of components' compositions in the high strength steel sheet according to the invention.
  • a space factor of the structure in each of the steel sheets was measured and an area ratio [(S1/S) ⁇ 100] of a coarse second phase structure was calculated.
  • the amount of ⁇ R and the concentration of C in ⁇ R were measured by X-ray diffractometry after grinding to a quarter depth of each steel sheet and after subsequent chemical polishing ( ISIJ Int. Vol.33 (1933), No.7, P.776 ).
  • No. 1 is an example of a low content of C, in which it was impossible to ensure a desired El, provided its fatigue characteristic is satisfactory because the second phase structure ( ⁇ R /martensite) defined in the present invention was not produced.
  • No. 2 is an example of omission of a predetermined tempering treatment, in which it was impossible to ensure a desired El and the fatigue characteristic was deteriorated.
  • No. 6 is an example of a small total amount of (Si+Al), in which a desired El was not obtained.
  • No. 15 is an example of a low cooling rate and consequent production of a large amount of pearlite structure, in which El and ⁇ were deteriorated.
  • No. 20 is a DP steel sheet of ferrite and martensite using No. 2 steel sample in Table 1
  • No. 21 is a conventional TRIP steel sheet using No. 2 steel sample in Table 1 and with polygonal ferrite as a base phase
  • No. 22 is a conventional two phase steel sheet of ferrite and bainite using No. 2 steel sample in Table 1.
  • No. 20 (conventional DP steel sheet) is inferior in all of elongation, stretch flange formability, and fatigue characteristic.
  • No. 21 (conventional TRIP steel sheet) contains a large proportion of a coarse second phase structure and is inferior in both stretch flange formability and fatigue characteristic.
  • No. 22 (conventional two phase steel sheet) is superior in fatigue characteristic but inferior in elongation because of absence of the second phase structure defined in the present invention.
  • Example 14 A study (part 1) of manufacturing conditions for the high strength steel sheet according to the invention.
  • 1, 2, and 5 are examples in which one-step cooling was conducted in the hot rolling process, while in the other examples there was conducted two-step cooling (after cooling to 700°C at an average cooling rate of 40°C/s, air-cooling was performed in this temperature range for 10 seconds, followed by cooling to 200°C or 450°C at an average cooling rate of 40°C/s).
  • the results obtained are shown in Table 30.
  • No. 2 is an example according to the present invention in which a desired base phase structure of tempered martensite was obtained through predetermined steps of hot rolling ⁇ tempering ⁇ continuous annealing
  • No. 4 is an example according to the present invention in which a desired mixed base phase structure of (temperedmartensite + ferrite) was obtained through predetermined hot rolling ⁇ tempering ⁇ continuous annealing
  • No. 5 is an example according to the present invention in which a desired base phase structure of tempered bainite was obtained through predetermined steps of hot rolling (tempering can be omitted because a winding process is performed at a CT of 450°C for 1 hour) ⁇ continuous annealing
  • a desired mixed base phase structure of (tempered bainite + martensite) was obtained through predetermined steps of hot rolling (tempering can be omitted because a winding process is performed at a CT of 450°C for 1 hour) ⁇ continuous annealing.
  • All of these examples due to formation of fine second phase structures, are 10% or more higher in stretch flange formability ( ⁇ ) and fatigue characteristic ( ⁇ w /YP) than in case of steel of the same components having been heat-treated without going through a predetermined tempering treatment (note: even when a tempering treatment is not performed, if a predetermined heat treatment capable of being regarded as equal to the tempering treatment is applied, it is regarded that tempering has been conducted).
  • No. 1 and 3 in Table 30 are examples of production carried out without going through tempering, which are low in fatigue characteristic or in both fatigue characteristic and stretch flange formability due to a large proportion of a coarse second phase structure.
  • Example 15 A study (part 2) of manufacturing conditions for the high strength steel sheet according to the invention.
  • No. 4, 7 to 9, 13, 16, 20, 22, 24, 26, 28, 30, 32, and 34 in Table 32 are examples of production carried out under conditions defined in the present invention, which are 10% or more higher in stretch flange formability ( ⁇ ) and fatigue characteristic ( ⁇ w /YP) than in case of steel of the same components having been heat-treated without going through a predetermined tempering treatment (note: even when a tempering treatment is not performed, if a predetermined heat treatment capable of being regarded as equal to the tempering treatment is applied, it is regarded that tempering has been conducted).
  • No. 1 and 2 in Table 32 are examples of production using steel 1 (low C steel) shown in Table 1, in which a predetermined base phase structure was obtained, but due to a small amount of C there was not obtained a desired ⁇ R and TS ⁇ El were low.
  • No. 3, 5, 11 to 12, 14 to 15, 19, 21, 23, 25, 27, 29, 31, and 33, in Table 32, as well as No. 1, 3 to 4, and 6 in Table 34, are all examples of production performed without going through tempering, in which fatigue characteristic or both fatigue characteristic and stretch flange formability were deteriorated due to a large proportion of a coarse second phase structure.
  • No. 6 in Table 32 is an example of a low tempering temperature, in which stretch flange formability and fatigue characteristic were deteriorated.
  • No. 10 in Table 32 is an example of a long-time treatment conducted at a high tempering temperature, in which stretch flange formability and fatigue characteristic were deteriorated.
  • No. 17 and 18 in Table 32 are examples of production using steel 5 [steel having a small total amount of (Si+Al)] in Table 25, in which a desired ⁇ R was not produced and elongation was deteriorated.
  • a photograph (magnification: 4000X) taken through an SEM (scanning electron microscope) of a steel sheet according to the present invention (No. 13 in Table 32) and that of a comparative steel sheet (No. 12 in Table 32) are shown in Figs. 21 and 22 , respectively. From these photographs it is seen that the steel sheet according to the present invention afforded a desired structure [a base phase structure (tempered martensite) of a lath form and a fine second phase structure] because it was treated under conditions specified in the present invention, but that the comparative steel sheet of Fig. 22 cannot afford a desired structure (a coarse second phase structure was formed) due to omission of a predetermined tempering treatment.
  • the high strength steel sheet according to the present invention is superior in the balance of stretch flange formability, total elongation, and fatigue characteristic in a high strength and ultra-high strength region of the order of about 500 to 1400 MPa.
  • Example 16 A study of components' compositions and heating temperature (SRT) before hot rolling in the high strength steel sheet which has a hardening property (BH) after baking finish which property satisfies: BH 2 % ⁇ 70 ⁇ MPa and BH 10 % ⁇ BH 2 % / 2
  • No. 1 is an example of a small amount of C, in which it was impossible to obtain desired BH characteristics.
  • No. 5 is an example of a small total amount of (Si+Al), in which a desired El is not obtained and BH characteristics are also deteriorated markedly.
  • No. 14 is an example of a low cooling rate and a consequent formation of a large amount of pearlite structure as a second phase structure, in which El and ⁇ are low and BH characteristics are also inferior.
  • No. 1 is a DP steel sheet of ferrite and martensite produced using No. 2 steel sample in Table 35
  • No. 2 is a conventional steel sheet using No. 3 steel sample in Table 35 and with polygonal ferrite as a base phase
  • No. 3 is a conventional two phase steel sheet of ferrite and bainite produced using No. 2 steel sample in Table 35.
  • No.1 (conventional DP steel sheet) is low in all of elongation, stretch flange formability, and BH characteristics
  • No. 2 (conventional TRIP steel sheet) is low in both stretch flange formability and BH characteristics
  • No. 3 (conventional two phase steel sheet) is low in both elongation and BH characteristics.
  • Example 17 A study (part 1) of manufacturing conditions in the high strength steel sheet which has a hardening property (BH) after baking finish which property satisfies: BH 2 % ⁇ 70 ⁇ MPa and BH 10 % ⁇ BH 2 % / 2.
  • BH hardening property
  • 2, 4, 6, and 8 are examples in which two-step cooling (cooling to 700°C at an average cooling rate of 40°C/s is followed by air cooling in this temperature range for 10 seconds and subsequent cooling to 200°C or 450°C at an average cooling rate of 40°C/s) was adopted.
  • No. 1 is an example of the present invention in which a desired base phase structure of tempered martensite was obtained through predetermined steps of hot rolling ⁇ continuous annealing
  • No. 2 is an example of the present invention in which a desired mixed base phase structure of (tempered martensite + ferrite) was obtained through predetermined steps of hot rolling ⁇ continuous annealing
  • No. 3 is an example of the present invention in which a desired base phase structure of tempered bainite was obtained through predetermined steps of hot rolling (winding at a CT of 450°C for 1 hour) ⁇ continuous annealing
  • No. 1 is an example of the present invention in which a desired base phase structure of tempered martensite was obtained through predetermined steps of hot rolling ⁇ continuous annealing
  • No. 2 is an example of the present invention in which a desired mixed base phase structure of (tempered martensite + ferrite) was obtained through predetermined steps of hot rolling ⁇ continuous annealing
  • No. 3 is an example of the present invention in which a desired base
  • a desired mixed base phase structure of (tempered bainite + ferrite) was obtained through predetermined steps of hot rolling (winding at a CT of 450°C for 1 hour). All of them are superior in stretch flange formability; besides, in all of them, a desired fine second phase structure is dispersed uniformly in pre-austenite grain boundaries and block and packet boundaries, and thus their BH characteristics also meet the conditions defined in the present invention.
  • No. 5 to 8 in Table 42 are examples of production carried out at a heating temperature (SRT) (before hot rolling) exceeding the range defined in the present invention, in which a desired fine second phase structure is not obtained and therefore BH (10%) does not satisfy the condition defined in the present invention although BH (2%) is satisfactory.
  • SRT heating temperature
  • Example 18 A study (part 2) of manufacturing conditions in the high strength steel sheet which has a hardening property (BH) after baking finish which property satisfies: BH 2 % ⁇ 70 ⁇ MPa and BH 10 % ⁇ BH 2 % / 2.
  • BH hardening property
  • No. 2 to 7 and 9 to 20 in Table 44 and No. 1 to 4 in Table 46 are examples of production carried out under the conditions defined in the present invention, which are superior not only in tensile strength (TS), elongation (EL), and stretch flange formability ( ⁇ ), but also in both BH (2%) and BH (10%).
  • No. 1 in Table 44 is an example of production using steel 1 (low C steel) shown in Table 35, in which desired BH characteristics were not obtained because of a small amount of C although a predetermined base phase structure was produced.
  • No. 8 in Table 44 is an example of production using steel 5 ⁇ steel having a small total amount of (Si+Al) ⁇ shown in Table 35, in which desired BH characteristics were not obtained.
  • the third high strength steel according to the present invention is superior in the balance of stretch flange formability, total elongation, and bake hardening property in a high strength and ultra-high strength region of the order of about 500 to 1400 MPa and that above all, in a large strain region, it exhibits an excellent bake hardening property.
  • the present invention it is possible to provide a high strength steel sheet superior in formability (stretch flange formability and total elongation), a high strength steel sheet also having an excellent fatigue characteristic, further, a high strength steel sheet further having a satisfactory bake hardening property, as well as a method which can produce those steel sheets efficiently.
  • the present invention is extremely useful. Table 1 No.
  • Base phase TS E1 ⁇ BH2 BH10 structure (MPa) (%) (MPa) (MPa) 1 TM 755 40 44 100 55 2 F+TM 755 40 45 85 45 3 TB 755 40 69 80 45 4 F+TB 755 40 64 80 45 5 TM 750 40 44 95 30 6 F+TM 750 40 45 80 25 7 TB 750 40 69 80 20 8 F+TB 750 40 64 75 15
  • TM tempered martensite
  • T8 tempered bainite
  • F ferrite Table 43 No. Steel No. .
  • Hot rolling Cold rolling Continuous annealing Continuous annealing or plating SRT FDT CR CT Cold rolling rate T1 CR T2 T3 t3 Tq CR T4 t4 Zn ⁇ GA °C °C °C/s °C % °C °C/s °C °C sec °C °C/s °C sec °C Hot rolling ⁇ Cold rolling ⁇ First continuous annealing ⁇ Second continuous annealing 1 1 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100 - 2 2 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100 - 3 3 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100 - 4 3 1150 850 40 550 50 850 20 450 800 60 700 10 400 100 - 5 3 1150 850 40 550 50 900 20 RT 800 60 700 10 400 100 - 6 3 1150 850 40 550 50 900 20 RT 800 60 700 10 400

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Claims (19)

  1. Hochfestes Stahlblech mit überlegener Formbarkeit, das
    (1) die folgenden chemischen Komponenten in Masse-% enthält:
    C : 0,06 bis 0,25%
    Si+Al : 0,5 bis 3%
    Mn : 0,5 bis 3%
    P : 0,15% oder weniger (nicht einschließlich 0%)
    S : 0,02% oder weniger (nicht einschließlich 0%), und bei Bedarf ferner wenigstens eine der folgenden Komponenten in Masse-% enthält:
    Mo : 1% oder weniger (nicht einschließlich 0%)
    Ni : 0,5% oder weniger (nicht einschließlich 0%)
    Cu : 0,5% oder weniger (nicht einschließlich 0%)
    Cr : 1% oder weniger (nicht einschließlich 0%)
    Ti : 0,1% oder weniger (nicht einschließlich 0%)
    Nb : 0,1% oder weniger (nicht einschließlich 0%)
    V : 0,1% oder weniger (nicht einschließlich 0%)
    Ca : 0,003% oder weniger (nicht einschließlich 0%), und/oder
    REM: 0,003% oder weniger (nicht einschließlich 0%)
    (2) eine Struktur hat, die Folgendes umfasst:
    (2-1) eine Grundphasenstruktur, wobei die Grundphasenstruktur getempertes Martensit oder getempertes Bainit ist und 50% oder mehr im Hinblick auf einen Raumfaktor relativ zur Gesamtstruktur ausmacht, oder wobei die Grundstruktur getempertes Martensit oder getempertes Bainit umfasst, das 15% oder mehr im Hinblick auf einen Raumfaktor relativ zur Gesamtstruktur ausmacht und ferner Ferrit umfasst,
    wobei das getemperte Martensit oder das getemperte Bainit eine Härte gemäß der folgenden Beziehung hat: Vickers - Härte Hv ≥ 500 C + 30 Si + 3 Mn + 50 ,
    Figure imgb0034
    wobei [ ] den Gehalt (Masse-%) jedes Elementes repräsentiert; und
    (2-2) eine Zweitphasenstruktur, die Abschreckaustenit umfasst, das 3 bis 30% im Hinblick auf einen Raumfaktor relativ zur Gesamtstruktur ausmacht und bei Bedarf ferner Bainit und/oder Martensit umfasst, wobei das Abschreckaustenit eine C-Konzentration (CyR) von 0,8% oder mehr hat,
    wobei die Zweitphasenstruktur den folgenden Ausdruck (1) zum Verbessern der Ermüdungseigenschaft erfüllt: S ⁢ 1 / S × 100 ≤ 20
    Figure imgb0035
    wobei S eine Gesamtfläche der Zweitphasenstruktur und S1 eine Gesamtfläche der groben Zweitphasen-Kristallkörner (Sb) bedeutet, die in der Zweitphasenstruktur enthalten sind, wobei Sb drei- oder mehrmal so groß ist wie eine durchschnittliche Kristallkornfläche (Sm) der Zweitphasenstruktur.
  2. Hochfestes Stahlblech nach Anspruch 1 mit solchen Brennhärtungs-(BH)-Eigenschaften nach dem Fertigbrennen, dass die folgenden Ausdrücke erfüllt werden: BH 2 % ≥ 70 MPa ,
    Figure imgb0036
     und BH 10 % ≥ BH 2 % / 2.
    Figure imgb0037
  3. Hochfestes Stahlblech nach Anspruch 1 oder 2, wobei das Abschreckaustenit in Lanzettenform vorliegt.
  4. Hochfestes Stahlblech nach einem der vorherigen Ansprüche, wobei der Ferritgehalt 5 bis 60% im Hinblick auf einen Raumfaktor relativ zur Gesamtstruktur ausmacht.
  5. Hochfestes Stahlblech nach Anspruch 4, wobei der Ferritgehalt 5 bis 30% im Hinblick auf einen Raumfaktor relativ zur Gesamtstruktur ausmacht.
  6. Verfahren zum Erzeugen des hochfesten Stahlblechs nach Anspruch 1, wobei die Grundphasenstruktur getempertes Martensit oder getempertes Bainit ist, wobei das Verfahren einen Heißwalzprozess, einen Temperprozess und einen Durchlaufglühprozess oder einen Galvanisierprozess beinhaltet,
    wobei der Heißwalzprozess einen Schritt des Beendens des Fertigwalzens bei einer Temperatur von mindestens (Ar3-50) °C und einen Schritt des Kühlens des resultierenden Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 20°C/s beinhaltet,
    wobei der Temperprozess einen Schritt des Temperns des Stahlblechs bei einer Temperatur von mindestens 400°C und höchstens dem Ac1-Punkt für eine Zeitperiode von mindestens 10 Minuten und weniger als 2 Stunden beinhaltet,
    wobei der Durchlaufglühprozess oder der Galvanisierprozess einen Schritt des Haltens des Stahlblechs auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 3°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich von 1 Sekunde oder mehr beinhaltet.
  7. Verfahren zum Erzeugen des hochfesten Stahlblechs nach Anspruch 1, wobei die Grundphasenstruktur getempertes Martensit oder getempertes Bainit ist, wobei das Verfahren einen Heißwalzprozess, einen Kaltwalzprozess, einen ersten Durchlaufglühprozess, einen Temperprozess und einen zweiten Durchlaufglühprozess oder einen Galvanisierprozess beinhaltet,
    wobei der erste Durchlaufglühprozess einen Schritt des Haltens des resultierenden Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A3-Punkt und einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 20°C/s beinhaltet,
    wobei der Temperprozess einen Schritt des Temperns des Stahlblechs bei einer Temperatur von mindestens 400°C und höchstens dem Ac1-Punkt für eine Zeitperiode von mindestens 10 Minuten und kürzer als 2 Stunden beinhaltet,
    wobei der zweite Durchlaufglühprozess oder der Galvanisierprozess einen Schritt des Haltens des Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 3°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder länger beinhaltet.
  8. Verfahren zum Erzeugen des hochfesten Stahlblechs nach Anspruch 1, wobei die Grundphasenstruktur getempertes Martensit und Ferrit oder getempertes Bainit und Ferrit umfasst, wobei das Verfahren einen Heißwalzprozess, einen Temperprozess und einen Durchlaufglühprozess oder einen Galvanisierprozess beinhaltet,
    wobei der Heißwalzprozess einen Schritt des Beendens des Fertigwalzens bei einer Temperatur von mindestens (Ar3-50)°C und einen Schritt des Kühlens des resultierenden Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 10°C/s und des Aufwickelns des Stahlblechs beinhaltet,
    wobei der Temperprozess einen Schritt des Temperns des Stahlblechs bei einer Temperatur von mindestens 400°C und höchstens dem Ac1-Punkt für eine Zeitperiode von mindestens 10 Minuten und weniger als 2 Stunden beinhaltet,
    wobei der Durchlaufglühprozess oder der Galvanisierprozess einen Schritt des Haltens des Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 3°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
  9. Verfahren nach Anspruch 8, wobei der Heißwalzprozess einen Schritt des Beendens des Fertigwalzens bei einer Temperatur von mindestens (Ar3-50)°C, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur im Bereich von 700±100°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 30°C/s, einen Schritt des Kühlens des Stahlblechs mit Luft in dem genannten Temperaturbereich für 1 bis 30 Sekunden und einen Schritt des nachfolgenden Kühlens des Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 30°C/s und des Aufwickelns des Stahlblechs beinhaltet.
  10. Verfahren nach Anspruch 8, wobei der Durchlaufglühprozess einen Schritt des Haltens des Stahlblechs auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von (A1-Punkt bis 600°C) mit einer durchschnittlichen Kühlgeschwindigkeit von höchstens 15°C/s, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 20°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
  11. Verfahren zum Erzeugen des hochfesten Stahls nach Anspruch 1, wobei die Grundphasenstruktur getempertes Martensit und Ferrit oder getempertes Bainit und Ferrit umfasst, wobei das Verfahren einen Heißwalzprozess, einen Kaltwalzprozess, einen ersten Durchlaufglühprozess, einen Temperprozess und einen zweiten Durchlaufglühprozess oder einen Galvanisierprozess beinhaltet,
    wobei der erste Durchlaufglühprozess einen Schritt des Haltens des resultierenden Stahlblechs auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt und einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 10°C/s beinhaltet,
    wobei der Temperprozess einen Schritt des Temperns des Stahlblechs bei einer Temperatur von mindestens 400°C und höchstens dem Ac1-Punkt für eine Zeitperiode von mindestens 10 Minuten und kürzer als 2 Stunden beinhaltet,
    wobei der zweite Durchlaufglühprozess oder der Galvanisierprozess einen Schritt des Haltens des Stahlblechs auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 3°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder länger beinhaltet.
  12. Verfahren nach Anspruch 11, wobei der zweite Durchlaufglühprozess einen Schritt des Haltens des Stahlblechs auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von (A1-Punkt bis 600°C) mit einer durchschnittlichen Kühlgeschwindigkeit von höchstens 15°C/s, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 20°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
  13. Verfahren zum Erzeugen des hochfesten Stahlblechs nach Anspruch 2, wobei die Grundphasenstruktur getempertes Martensit oder getempertes Bainit ist, wobei das Verfahren einen Heißwalzprozess und einen Durchlaufglühprozess oder einen Galvanisierprozess beinhaltet,
    wobei der Heißwalzprozess einen Schritt des Regelns einer Wärmebehandlungstemperatur vor dem Heißwalzen auf eine Temperatur im Bereich von 950° bis 1100°C, einen Schritt des Beendens des Fertigwalzens bei einer Temperatur von mindestens (Ar3-50)°C und einen Schritt des Kühlens des resultierenden Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 20°C/s und des Aufwickelns des Stahlblechs beinhaltet,
    wobei der Durchlaufglühprozess oder der Galvanisierprozess einen Schritt des Haltens des Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 3°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
  14. Verfahren zum Erzeugen des hochfesten Stahlblechs nach Anspruch 2, wobei die Grundphasenstruktur getempertes Martensit oder getempertes Bainit ist, wobei das Verfahren einen Heißwalzprozess, einen Kaltwalzprozess, einen ersten Glühprozess und einen zweiten Glühprozess oder einen Galvanisierprozess beinhaltet,
    wobei der Heißwalzprozess einen Schritt des Regelns einer Wärmebehandlungstemperatur vor dem Heißwalzen auf eine Temperatur im Bereich von 950° bis 1100°C beinhaltet,
    wobei der erste Durchlaufglühprozess einen Schritt des Haltens des resultierenden Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A3-Punkt und einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 20°C/s beinhaltet,
    wobei der zweite Durchlaufglühschritt oder der Galvanisierschritt einen Schritt des Haltens des Stahlblechs auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 3°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
  15. Verfahren zum Erzeugen des hochfesten Stahlblechs nach Anspruch 2, wobei die Grundphasenstruktur getempertes Martensit und Ferrit oder getempertes Bainit und Ferrit umfasst, wobei das Verfahren einen Heißwalzprozess und einen Durchlaufglühprozess oder einen Galvanisierprozess beinhaltet,
    wobei der Heißwalzprozess einen Schritt des Regelns einer Wärmebehandlungstemperatur vor dem Heißwalzen auf eine Temperatur im Bereich von 950° bis 1100°C, einen Schritt des Beendens des Fertigwalzens bei einer Temperatur von mindestens (Ar3-50) °C und einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 10°C/s und des Aufwickelns des Stahlblechs beinhaltet,
    wobei der Durchlaufglühprozess oder der Galvanisierprozess einen Schritt des Haltens des Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 3°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
  16. Verfahren nach Anspruch 15, wobei der Heißwalzprozess einen Schritt des Regelns einer Wärmebehandlungstemperatur vor dem Heißwalzen auf eine Temperatur im Bereich von 950° bis 1100°C, einen Schritt des Beendens des Fertigwalzens bei einer Temperatur von mindestens (Ar3-50)°C, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur im Bereich von 700±100°C mit einer durchschnittlichen Kühlgeschwindigkeit von 30°C/s, einen Schritt des Kühlens des Stahlblechs mit Luft in dem genannten Temperaturbereich für 1 bis 30 Sekunden und einen Schritt des nachfolgenden Kühlens des Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 30°C/s und des Aufwickelns des Stahlblechs beinhaltet.
  17. Verfahren nach Anspruch 15, wobei der Durchlaufglühprozess einen Schritt des Haltens des Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von (A1-Punkt bis 600°C) mit einer durchschnittlichen Kühlgeschwindigkeit von höchstens 15°C/s, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 20°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
  18. Verfahren zum Erzeugen des hochfesten Blechs nach Anspruch 2, wobei die Grundphasenstruktur getempertes Martensit und Ferrit oder getempertes Bainit und Ferrit umfasst, wobei das Verfahren einen Heißwalzprozess, einen Kaltwalzprozess, einen ersten Durchlaufglühprozess und einen zweiten Durchlaufglühprozess oder einen Galvanisierprozess beinhaltet,
    wobei der Heißwalzprozess einen Schritt des Regelns einer Wärmebehandlungstemperatur vor dem Heißwalzen auf eine Temperatur im Bereich von 950° bis 1100°C beinhaltet,
    wobei der erste Durchlaufglühprozess einen Schritt des Haltens des resultierenden Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt und einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von höchstens dem Ms-Punkt oder eine Temperatur von mindestens dem Ms-Punkt und höchstens dem Bs-Punkt mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 10°C/s beinhaltet,
    wobei der zweite Durchlaufglühprozess oder der Galvanisierprozess einen Schritt des Haltens des Stahlblechs auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf einer Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 3°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
  19. Verfahren nach Anspruch 18, wobei der zweite Durchlaufglühprozess einen Schritt des Haltens des Stahlblechs in einem erhitzten Zustand auf einer Temperatur von mindestens dem A1-Punkt und höchstens dem A3-Punkt für 10 bis 600 Sekunden, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von (A1-Punkt bis 600°C) mit einer durchschnittlichen Kühlgeschwindigkeit von höchstens 15°C/s, einen Schritt des Kühlens des Stahlblechs auf eine Temperatur von mindestens 300°C und höchstens 480°C mit einer durchschnittlichen Kühlgeschwindigkeit von mindestens 20°C/s und einen Schritt des Haltens des Stahlblechs in dem genannten Temperaturbereich für 1 Sekunde oder mehr beinhaltet.
EP02710418A 2001-01-31 2002-01-31 Hochfestes stahlblech mit hervorragender formbarkeit und verfahren zu seiner herstellung Expired - Lifetime EP1365037B1 (de)

Applications Claiming Priority (20)

Application Number Priority Date Filing Date Title
JP2001023402 2001-01-31
JP2001002340 2001-01-31
JP2001023401 2001-01-31
JP2001034335 2001-02-09
JP2001034336 2001-02-09
JP2001003433 2001-02-09
JP2001005563 2001-02-28
JP2001055642 2001-02-28
JP2001055640 2001-02-28
JP2001055641 2001-02-28
JP2001005564 2001-02-28
JP2001055639 2001-02-28
JP2001264175A JP2003073773A (ja) 2001-08-31 2001-08-31 加工性及び疲労特性に優れた高強度鋼板およびその製造方法
JP2001002641 2001-08-31
JP2001300503A JP4188582B2 (ja) 2001-02-09 2001-09-28 加工性に優れた高強度鋼板およびその製造方法
JP2001300505 2001-09-28
JP2001300502A JP4188581B2 (ja) 2001-01-31 2001-09-28 加工性に優れた高強度鋼板およびその製造方法
JP2001300504 2001-09-28
JP2001003005 2001-09-28
PCT/JP2002/000744 WO2002061161A1 (fr) 2001-01-31 2002-01-31 Feuillard en acier a haute resistance ayant une excellente formabilite, et son procede de production

Publications (3)

Publication Number Publication Date
EP1365037A1 EP1365037A1 (de) 2003-11-26
EP1365037A4 EP1365037A4 (de) 2005-02-02
EP1365037B1 true EP1365037B1 (de) 2008-04-02

Family

ID=27584373

Family Applications (1)

Application Number Title Priority Date Filing Date
EP02710418A Expired - Lifetime EP1365037B1 (de) 2001-01-31 2002-01-31 Hochfestes stahlblech mit hervorragender formbarkeit und verfahren zu seiner herstellung

Country Status (2)

Country Link
EP (1) EP1365037B1 (de)
WO (1) WO2002061161A1 (de)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10995386B2 (en) 2014-05-20 2021-05-04 Arcelormittal Double annealed steel sheet having high mechanical strength and ductility characteristics, method of manufacture and use of such sheets
RU2749413C2 (ru) * 2016-05-10 2021-06-09 Юнайтид Стейтс Стил Корпорэйшн Изделия из высокопрочной стали и способы их изготовления

Families Citing this family (23)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7314532B2 (en) 2003-03-26 2008-01-01 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) High-strength forged parts having high reduction of area and method for producing same
US7591977B2 (en) 2004-01-28 2009-09-22 Kabuhsiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) High strength and low yield ratio cold rolled steel sheet and method of manufacturing the same
KR100990772B1 (ko) * 2005-12-28 2010-10-29 가부시키가이샤 고베 세이코쇼 초고강도 박강판
KR101082680B1 (ko) * 2006-07-14 2011-11-15 가부시키가이샤 고베 세이코쇼 고강도 강판 및 그 제조 방법
JP5223360B2 (ja) * 2007-03-22 2013-06-26 Ｊｆｅスチール株式会社 成形性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法
EP2202327B1 (de) * 2007-10-25 2020-12-02 JFE Steel Corporation Herstellungsverfahren für hochfestes galvanisiertes stahlblech mit hervorragender formbarkeit
JP5369663B2 (ja) * 2008-01-31 2013-12-18 Ｊｆｅスチール株式会社 加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法
JP5418047B2 (ja) * 2008-09-10 2014-02-19 Ｊｆｅスチール株式会社 高強度鋼板およびその製造方法
JP5323563B2 (ja) 2009-03-31 2013-10-23 株式会社神戸製鋼所 加工性および形状凍結性に優れた高強度冷延鋼板
CN102337480B (zh) * 2010-07-15 2013-03-13 宝山钢铁股份有限公司 抗环境脆性及疲劳性能优良的超高强度钢板及其制造方法
KR101253885B1 (ko) * 2010-12-27 2013-04-16 주식회사 포스코 연성이 우수한 성형 부재용 강판, 성형 부재 및 그 제조방법
KR101716625B1 (ko) * 2012-04-23 2017-03-14 가부시키가이샤 고베 세이코쇼 핫 스탬핑용 아연도금 강판의 제조 방법, 핫 스탬핑용 합금화 용융 아연도금 강판과 그의 제조 방법, 및 핫 스탬핑 부품
US10253389B2 (en) 2014-03-31 2019-04-09 Jfe Steel Corporation High-yield-ratio, high-strength cold-rolled steel sheet and production method therefor
EP3162908B8 (de) 2014-07-14 2019-07-31 Nippon Steel Corporation Warmgewalztes stahlblech
CN104513927B (zh) * 2014-12-19 2017-04-05 宝山钢铁股份有限公司 一种抗拉强度800MPa级高强度高韧性钢板及其制造方法
EP3378958B1 (de) * 2015-09-22 2021-08-04 Hyundai Steel Company Plattierte stahlplatte und herstellungsverfahren dafür
WO2017109540A1 (en) 2015-12-21 2017-06-29 Arcelormittal Method for producing a high strength steel sheet having improved ductility and formability, and obtained steel sheet
US11560606B2 (en) 2016-05-10 2023-01-24 United States Steel Corporation Methods of producing continuously cast hot rolled high strength steel sheet products
US11993823B2 (en) 2016-05-10 2024-05-28 United States Steel Corporation High strength annealed steel products and annealing processes for making the same
MX2020010313A (es) * 2018-03-30 2020-10-22 Jfe Steel Corp Lamina de acero de alta resistencia y metodo para su produccion.
JP6705562B2 (ja) * 2018-03-30 2020-06-03 Ｊｆｅスチール株式会社 高強度鋼板およびその製造方法
JP6705560B2 (ja) * 2018-03-30 2020-06-03 Ｊｆｅスチール株式会社 高強度鋼板およびその製造方法
WO2020245627A1 (en) * 2019-06-03 2020-12-10 Arcelormittal Cold rolled and coated steel sheet and a method of manufacturing thereof

Family Cites Families (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH06108152A (ja) * 1992-09-30 1994-04-19 Kobe Steel Ltd 曲げ加工性に優れた高強度溶融亜鉛めっき鋼板の製造方法
JPH07252592A (ja) * 1994-03-15 1995-10-03 Nippon Steel Corp 成形性、低温靭性及び疲労特性に優れた熱延高強度鋼板
JP3374644B2 (ja) * 1996-03-28 2003-02-10 株式会社神戸製鋼所 耐孔明き腐食性および加工性に優れた高強度熱延鋼板、および高強度亜鉛系めっき鋼板並びにそれらの製造方法
BE1011149A3 (fr) * 1997-05-12 1999-05-04 Cockerill Rech & Dev Acier ductile a haute limite elastique et procede de fabrication de cet acier.
JP3172505B2 (ja) * 1998-03-12 2001-06-04 株式会社神戸製鋼所 成形性に優れた高強度熱延鋼板
JPH11350064A (ja) * 1998-06-08 1999-12-21 Kobe Steel Ltd 形状凍結性と耐衝撃特性に優れる高強度鋼板及びその製造方法
JP2000144311A (ja) * 1998-11-13 2000-05-26 Sumitomo Metal Ind Ltd 高炭素薄鋼板
JP3587126B2 (ja) * 1999-04-21 2004-11-10 Ｊｆｅスチール株式会社 延性に優れる高張力溶融亜鉛めっき鋼板およびその製造方法
FR2796966B1 (fr) * 1999-07-30 2001-09-21 Ugine Sa Procede de fabrication de bandes minces en acier de type "trip" et bandes minces ainsi obtenues

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10995386B2 (en) 2014-05-20 2021-05-04 Arcelormittal Double annealed steel sheet having high mechanical strength and ductility characteristics, method of manufacture and use of such sheets
RU2749413C2 (ru) * 2016-05-10 2021-06-09 Юнайтид Стейтс Стил Корпорэйшн Изделия из высокопрочной стали и способы их изготовления

Also Published As

Publication number Publication date
EP1365037A1 (de) 2003-11-26
WO2002061161A1 (fr) 2002-08-08
EP1365037A4 (de) 2005-02-02

Similar Documents

Publication Publication Date Title
EP1365037B1 (de) Hochfestes stahlblech mit hervorragender formbarkeit und verfahren zu seiner herstellung
US7090731B2 (en) High strength steel sheet having excellent formability and method for production thereof
EP3309273B1 (de) Galvanisiertes stahlblech und verfahren zu dessen herstellung
EP3287539B1 (de) Plattiertes stahlblech
EP3611285B1 (de) Stahlblech und herstellungsverfahren dafür
EP3128027B1 (de) Hochfestes kaltgewalztes stahlblech mit hohem streckgrenzenverhältnis und herstellungsverfahren dafür
EP2246456B9 (de) Hochfestes stahlblech und herstellungsverfahren dafür
EP3358033B1 (de) Hochfestes heissgewalztes stahlblech und verfahren zur herstellung davon
EP3272892B1 (de) Hochfestes kaltgewalztes stahlblech und verfahren zur herstellung davon
EP2169091B1 (de) Hochfestes feuerverzinktes stahlblech mit niedriger streckfestigkeit und geringerer materialqualitätsschwankung sowie herstellungsverfahren dafür
EP3550047A1 (de) Hochfestes stahlblech
EP2586886B1 (de) Heissgewalztes hochspannungs-stahlblech von hervorragender verarbeitbarkeit sowie verfahren zu seiner herstellung
EP2634281A1 (de) Hochfestes feuerverzinktes stahlblech mit hervorragender gleichmässiger dehnung und hervorragenden plattierungseigenschaften sowie herstellungsverfahren dafür
EP4310205A1 (de) Stahlblech, element, verfahren zur herstellung eines stahlblechs und verfahren zur herstellung eines elements
WO2013046476A1 (ja) 高強度鋼板およびその製造方法
EP3733898B1 (de) Hochfestes kaltgewalztes stahlblech und verfahren zur herstellung davon
EP3255163B1 (de) Hochfestes stahlblech und herstellungsverfahren dafür
EP2465961A1 (de) Hochfeste Stahlbleche und Verfahren zur deren Herstellung
EP3255164A1 (de) Hochfestes stahlblech und herstellungsverfahren dafür
EP3613868B1 (de) Hochfestes feuerverzinktes stahlblech und produktionsverfahren dafür
EP3572536A1 (de) Heissgestanzter formkörper und verfahren zur herstellung davon
EP3733897B1 (de) Hochfestes kaltgewalztes stahlblech und verfahren zur herstellung davon
EP3346019A1 (de) Hochfestes dünnes stahlblech und verfahren zur herstellung davon
EP3889283B1 (de) Hochfestes stahlblech und verfahren zur herstellung desselben
WO2021140663A1 (ja) 高強度亜鉛めっき鋼板およびその製造方法

Legal Events

Date Code Title Description
PUAI Public reference made under article 153(3) epc to a published international application that has entered the european phase

Free format text: ORIGINAL CODE: 0009012

17P Request for examination filed

Effective date: 20030821

AK Designated contracting states

Kind code of ref document: A1

Designated state(s): FR GB

RIN1 Information on inventor provided before grant (corrected)

Inventor name: KIKUCHI, RYO

Inventor name: KANDA, AKINOBU

Inventor name: IKEDA, SHUSHIK.C. RES.LAB. K.K.K.KOBE SEIKO SHO

Inventor name: KASHIMA, TAKAHIROKAKOGAWA WORKS K.K.K SEIKO SHO

Inventor name: SUGIMOTO, KOH-ICHI

Inventor name: HASHIMOTO, SHUNICHIC/O CBMM ASIA CO., LTD.

Inventor name: AKAMIZU, HIROSHIK.C.R. LAB. K.K.K KOBE SEIKO SHO

Inventor name: NAGASAKA, AKIHIKO

RIN1 Information on inventor provided before grant (corrected)

Inventor name: IKEDA, SHUSHIK.C. RES.LAB. K.K.K.KOBE SEIKO SHO

Inventor name: KASHIMA, TAKAHIROKAKOGAWA WORKS K.K.K SEIKO SHO

Inventor name: AKAMIZU, HIROSHIK.C.R. LAB. K.K.K KOBE SEIKO SHO

Inventor name: SUGIMOTO, KOH-ICHI

Inventor name: KANDA, AKINOBU

Inventor name: NAGASAKA, AKIHIKO

Inventor name: KIKUCHI, RYO

Inventor name: HASHIMOTO, SHUNICHIC/O CBMM ASIA CO., LTD.

A4 Supplementary search report drawn up and despatched

Effective date: 20041221

17Q First examination report despatched

Effective date: 20050302

17Q First examination report despatched

Effective date: 20050302

17Q First examination report despatched

Effective date: 20050302

GRAC Information related to communication of intention to grant a patent modified

Free format text: ORIGINAL CODE: EPIDOSCIGR1

GRAP Despatch of communication of intention to grant a patent

Free format text: ORIGINAL CODE: EPIDOSNIGR1

GRAS Grant fee paid

Free format text: ORIGINAL CODE: EPIDOSNIGR3

GRAA (expected) grant

Free format text: ORIGINAL CODE: 0009210

AK Designated contracting states

Kind code of ref document: B1

Designated state(s): FR GB

REG Reference to a national code

Ref country code: GB

Ref legal event code: FG4D

ET Fr: translation filed
PLBE No opposition filed within time limit

Free format text: ORIGINAL CODE: 0009261

STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: NO OPPOSITION FILED WITHIN TIME LIMIT

26N No opposition filed

Effective date: 20090106

PGFP Annual fee paid to national office [announced via postgrant information from national office to epo]

Ref country code: GB

Payment date: 20140129

Year of fee payment: 13

GBPC Gb: european patent ceased through non-payment of renewal fee

Effective date: 20150131

PG25 Lapsed in a contracting state [announced via postgrant information from national office to epo]

Ref country code: GB

Free format text: LAPSE BECAUSE OF NON-PAYMENT OF DUE FEES

Effective date: 20150131

REG Reference to a national code

Ref country code: FR

Ref legal event code: PLFP

Year of fee payment: 15

REG Reference to a national code

Ref country code: FR

Ref legal event code: PLFP

Year of fee payment: 16

REG Reference to a national code

Ref country code: FR

Ref legal event code: PLFP

Year of fee payment: 17

PGFP Annual fee paid to national office [announced via postgrant information from national office to epo]

Ref country code: FR

Payment date: 20201210

Year of fee payment: 20