WO2017183348A1 - Plaque d'acier, plaque d'acier plaquée et procédé pour les produire - Google Patents

Plaque d'acier, plaque d'acier plaquée et procédé pour les produire Download PDF

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WO2017183348A1
WO2017183348A1 PCT/JP2017/009300 JP2017009300W WO2017183348A1 WO 2017183348 A1 WO2017183348 A1 WO 2017183348A1 JP 2017009300 W JP2017009300 W JP 2017009300W WO 2017183348 A1 WO2017183348 A1 WO 2017183348A1
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amount
retained austenite
steel sheet
martensite
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PCT/JP2017/009300
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English (en)
Japanese (ja)
Inventor
孝子 山下
由康 川崎
崇 小林
植野　雅康
長滝　康伸
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Ｊｆｅスチール株式会社
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Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to US16/090,883 priority Critical patent/US20190276907A1/en
Priority to CN201780018499.0A priority patent/CN108779536B/zh
Priority to JP2017533038A priority patent/JP6210183B1/ja
Priority to MX2018012659A priority patent/MX2018012659A/es
Priority to KR1020187033203A priority patent/KR102128838B1/ko
Priority to EP17785691.1A priority patent/EP3447159B1/fr
Publication of WO2017183348A1 publication Critical patent/WO2017183348A1/fr

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    • C—CHEMISTRY; METALLURGY
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    • 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
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    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/12—Aluminium or alloys based thereon
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    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • 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 steel sheet, a hot dip galvanized steel sheet, a hot dip galvanized steel sheet, an electrogalvanized steel sheet, and a method for producing them, and particularly suitable for use as a member used in industrial fields such as automobiles and electricity.
  • the present invention relates to a steel sheet having excellent hole expandability and a high yield ratio.
  • Patent Document 1 proposes a high-strength steel sheet having an extremely high ductility utilizing a work-induced transformation of retained austenite having a tensile strength of 1000 MPa or more and a total elongation (EL) of 30% or more.
  • Patent Document 2 proposes a high-strength steel plate having a high balance between strength and ductility by performing heat treatment in a two-phase region of ferrite and austenite using high-Mn steel.
  • high Mn steel the structure after hot rolling is a structure containing bainite and martensite, and further, after forming fine retained austenite by annealing and tempering, tempered bainite or A high-strength steel sheet that has improved local ductility by using a structure containing tempered martensite has been proposed.
  • the steel sheet described in Patent Document 1 is manufactured by performing a so-called austempering process in which a steel sheet containing C, Si and Mn as basic components is austenitized, and then quenched into a bainite transformation temperature range and held isothermally. Is done. And when this austemper process is performed, a retained austenite is produced
  • Patent Document 1 is mainly intended to improve ductility, and no consideration is given to hole expandability, bendability, and yield ratio.
  • Patent Documents 2 and 3 from the viewpoint of formability, although improvement of the ductility of the steel sheet is described, consideration is not given to its bendability, yield ratio, and hole expandability.
  • the present invention has been made paying attention to the above-mentioned problems, and the object thereof is a steel sheet, hot-dip galvanized steel having a TS of 590 MPa or more, excellent YR of 68% or more, and excellent formability and hole expansibility.
  • An object of the present invention is to provide a steel plate, a hot dip galvanized steel plate, an electrogalvanized steel plate, and a production method thereof.
  • the steel component is in a range of Mn: 2.60 mass% to 4.20 mass%, and after appropriately adjusting the addition amount of other alloy elements such as Ti, hot rolling is performed to perform hot rolling. And Next, after removing the scale from the hot-rolled sheet by pickling, the hot-rolled sheet is maintained in the temperature range of Ac 1 transformation point + 20 ° C. or higher and Ac 1 transformation point + 120 ° C. or lower in the range of 600 s to 21600 s, and further annealed after hot rolling. As it is, or cold rolling is performed at a reduction rate of less than 30% to obtain a cold-rolled sheet. Further, this hot-rolled sheet or cold-rolled sheet is cooled after being held for 20 to 900 s in a temperature range of Ac 1 transformation point + 10 ° C. or higher and Ac 1 transformation point + 100 ° C. or lower.
  • the hot-rolled sheet or cold-rolled sheet has an area ratio of polygonal ferrite of 20% or more and 65% or less, non-recrystallized ferrite of 8% or more, and martensite of 5% or more and 25% or less.
  • the volume ratio of the retained austenite is 8% or more, and the average aspect ratio of the crystal grains of each phase (polygonal ferrite, martensite, retained austenite) is 2.0 or more and 15.0 or less, respectively.
  • the polygonal ferrite has an average crystal grain size of 6 ⁇ m or less
  • the martensite has an average crystal grain size of 3 ⁇ m or less
  • the retained austenite has an average crystal grain size of 3 ⁇ m or less.
  • the value obtained by dividing the amount of Mn (mass%) in the retained austenite by the amount of Mn (mass%) in the polygonal ferrite is controlled to be 2.0 or more. 8% or more of retained austenite stabilized with can be secured.
  • the gist configuration of the present invention is as follows. 1. In mass%, C: 0.030% to 0.250%, Si: 0.01% to 3.00%, Mn: 2.60% to 4.20%, P: 0.001% or more 0.100% or less, S: 0.0200% or less, N: 0.0100% or less and Ti: 0.005% or more and 0.200% or less, Furthermore, by mass%, Al: 0.01% to 2.00%, Nb: 0.005% to 0.200%, B: 0.0003% to 0.0050%, Ni: 0.005 %: 1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and 1.000% or less, Cu: 0 0.005% to 1.000%, Sn: 0.002% to 0.200%, Sb: 0.002% to 0.200%, Ta: 0.001% to 0.010%, Ca : 0.0005% or more and 0.0050% or less, Mg: 0.0005% or
  • the amount of C in the retained austenite is related to the amount of Mn in the retained austenite by the following formula: 0.09 ⁇ [Mn amount] ⁇ 0.026 ⁇ 0.150 ⁇ [C amount] ⁇ 0.09 ⁇ [ Amount of Mn] ⁇ 0.026 + 0.150 [C amount]: C amount (% by mass) in retained austenite [Mn amount]: Mn amount (% by mass) in retained austenite 2.
  • a plated steel sheet wherein the steel sheet according to 1 or 2 further comprises one selected from a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum plated layer, and an electrogalvanized layer.
  • the manufacturing method of the steel plate of said 4 which performs a process.
  • a high-yield ratio type high-strength steel sheet excellent in formability and hole-expandability with TS of 590 MPa or more and YR of 68% or more can be obtained.
  • the present invention will be specifically described.
  • the reason why the component composition of steel is limited to the scope of the present invention in the present invention will be described.
  • the% display concerning the component composition of the following steel or slab means the mass%.
  • the balance of the component composition of steel or slab is Fe and inevitable impurities.
  • C 0.030% or more and 0.250% or less C is an element necessary for generating a low-temperature transformation phase such as martensite and increasing the strength. It is also an element effective in improving the stability of retained austenite and improving the ductility of steel. If the amount of C is less than 0.030%, it is difficult to secure a desired martensite area ratio, and a desired strength cannot be obtained. Moreover, it is difficult to ensure a sufficient volume ratio of retained austenite, and good ductility cannot be obtained. On the other hand, if C is added in excess of 0.250%, the area ratio of hard martensite becomes excessive, and the microvoids at the grain boundaries of martensite increase, and the bending test and the hole expansion test.
  • the C amount is set to be 0.030% or more and 0.250% or less. Preferably, it is 0.080% or more. Preferably, it is 0.200% or less.
  • Si 0.01% or more and 3.00% or less Si is an element effective for ensuring good ductility in order to improve the work hardening ability of ferrite. If the amount of Si is less than 0.01%, the effect of addition becomes poor, so the lower limit is made 0.01%. On the other hand, excessive addition of Si exceeding 3.00% not only causes embrittlement of steel but also causes deterioration of surface properties due to the occurrence of red scale and the like. For this reason, Si amount is taken as 0.01% or more and 3.00% or less of range. Preferably, it is 0.20% or more. Preferably, it is 2.00% or less.
  • Mn 2.60% or more and 4.20% or less Mn is an extremely important element in the present invention.
  • Mn is an element that stabilizes retained austenite and is effective in securing good ductility.
  • Mn is an element that can raise TS of steel by solid solution strengthening. Such an effect is recognized when the Mn content of the steel is 2.60% or more.
  • an excessive addition of Mn exceeding 4.20% causes an increase in cost.
  • the amount of Mn is set in the range of 2.60% to 4.20%. Preferably, it is the range of 3.00% or more and 4.20% or less.
  • P 0.001% or more and 0.100% or less
  • P is an element that has a solid solution strengthening action and can be added according to a desired TS. It is also an element that promotes ferrite transformation and is effective in the formation of a composite structure of steel sheets. In order to acquire such an effect, it is necessary to make P amount in a steel plate 0.001% or more. On the other hand, if the amount of P exceeds 0.100%, weldability is deteriorated, and when galvanizing is alloyed, the alloying speed is lowered and the quality of galvanizing is impaired. Therefore, the P amount is in the range of 0.001% to 0.100%. Preferably it is 0.005% or more. Preferably it is 0.050% or less.
  • the S content is 0.0200% or less, preferably 0.0100% or less, more preferably 0.0050% or less.
  • the amount of S is preferably 0.0001% or more because of restrictions on production technology. Therefore, the S amount is preferably in the range of 0.0001% to 0.0200%. More preferably, it is 0.0001% or more and 0.0100% or less, and still more preferably 0.0001% or more and 0.0050% or less.
  • N 0.0100% or less
  • N is an element that deteriorates the aging resistance of steel.
  • the amount of N is preferably set to 0.0005% or more because of restrictions on production technology.
  • the N amount is preferably in the range of 0.0005% to 0.0100%. More preferably, it is 0.0010% or more. More preferably, it is 0.0070% or less.
  • Ti 0.005% or more and 0.200% or less Ti is an extremely important additive element in the present invention. Ti is effective for the precipitation strengthening of steel, can secure the desired area ratio of non-recrystallized ferrite, and contributes to a high yield ratio of the steel sheet. In addition, by utilizing relatively hard non-recrystallized ferrite, the hardness difference from the hard second phase (martensite or retained austenite) can be reduced, which contributes to the improvement of stretch flangeability. And these effects are acquired by addition of Ti amount 0.005% or more.
  • the amount of Ti added is in the range of 0.005% to 0.200%. Preferably it is 0.010% or more. Preferably it is 0.100% or less.
  • the basic components of the present invention have been described above.
  • the balance other than the above components is Fe and inevitable impurities, but in addition, the following elements can be appropriately contained as required.
  • Al 0.01% to 2.00%, Nb: 0.005% to 0.200%, B: 0.0003% to 0.0050%, Ni: 0.005% to 1.000%
  • Cr 0.005% to 1.000%
  • V 0.005% to 0.500%
  • Mo 0.005% to 1.000%
  • Cu 0.005% to 1. 000% or less
  • Sn 0.002% or more and 0.200% or less
  • Sb 0.002% or more and 0.200% or less
  • Ta 0.001% or more and 0.010% or less
  • Ca 0.0005% or more Contains at least one element selected from 0.0050% or less
  • Mg 0.0005% or more and 0.0050% or less
  • REM 0.0005% or more and 0.0050% or less.
  • Al is composed of ferrite and austenite Expanded biphasic area This is an element effective in reducing the annealing temperature dependency, that is, the material stability. Further, Al acts as a deoxidizer and is also an effective element for maintaining the cleanliness of steel. However, if the Al content is less than 0.01%, the effect of addition is poor, so the lower limit is made 0.01%. On the other hand, a large amount of addition exceeding 2.00% increases the risk of steel piece cracking during continuous casting, and decreases productivity. From such a viewpoint, the Al amount when added is in the range of 0.01% to 2.00%. Preferably, it is 0.20% or more. Preferably, it is 1.20% or less.
  • Nb is effective for precipitation strengthening of steel, and the effect of addition is obtained at 0.005% or more. Further, similarly to the effect of adding Ti, it is possible to ensure the desired area ratio of non-recrystallized ferrite, which contributes to a high yield ratio of the steel sheet. In addition, by utilizing relatively hard non-recrystallized ferrite, the hardness difference from the hard second phase (martensite or retained austenite) can be reduced, which contributes to the improvement of stretch flangeability. On the other hand, if the amount of Nb exceeds 0.200%, the area ratio of hard martensite becomes excessive and the number of microvoids at the grain boundaries of martensite increases, and crack propagation occurs during bending and hole expansion tests. Becomes easier to progress.
  • Nb when adding Nb, it is set as 0.005% or more and 0.200% or less. Preferably it is 0.010% or more. Preferably it is 0.100% or less.
  • B has the effect of suppressing the formation and growth of ferrite from the austenite grain boundaries, and can be flexibly controlled in the structure, so it can be added as necessary.
  • the effect of addition is obtained at 0.0003% or more.
  • the amount of B exceeds 0.0050%, the formability of the steel sheet is lowered. Therefore, when adding B, it is set as 0.0003% or more and 0.0050% or less of range. Preferably it is 0.0005% or more. Preferably it is 0.0030% or less.
  • Ni is an element that stabilizes retained austenite, and is effective in ensuring good ductility. Further, Ni is an element that raises steel TS by solid solution strengthening. The effect of addition is obtained at 0.005% or more. On the other hand, if added over 1.000%, the area ratio of hard martensite becomes excessive, the number of microvoids at the grain boundary of martensite increases, and crack propagation propagates during the bending test and the hole expansion test. Easy to progress. As a result, the bendability and stretch flangeability of the steel sheet are deteriorated. In addition, the cost increases. Therefore, when adding Ni, it is set as 0.005% or more and 1.000% or less.
  • Cr, V, and Mo are elements that can be added as necessary because they have an effect of improving the balance between TS and ductility.
  • the addition effect is obtained when Cr: 0.005% or more, V: 0.005% or more, and Mo: 0.005% or more.
  • Cr when Cr is added in excess of 1.000%, V: 0.500%, and Mo: 1.000%, the area ratio of hard martensite becomes excessive, and martensite crystal grains. The number of microvoids at the boundary increases, and crack propagation easily progresses during a bending test and a hole expansion test. As a result, the bendability and stretch flangeability of the steel sheet are deteriorated. In addition, the cost increases. Therefore, when these elements are added, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, and Mo: 0.005% or more and 1.000%, respectively. The following range.
  • Cu is an element effective for strengthening steel, and may be used for strengthening steel as long as it is within the range specified in the present invention.
  • the effect of addition is obtained at 0.005% or more.
  • the area ratio of hard martensite becomes excessive, the number of microvoids at the grain boundary of martensite increases, and crack propagation propagates during the bending test and the hole expansion test. Easy to progress. As a result, the bendability and stretch flangeability of the steel sheet are deteriorated. Therefore, when adding Cu, it is set as 0.005% or more and 1.000% or less.
  • Sn and Sb are added as necessary from the viewpoint of suppressing decarburization in a thickness region of about several tens of ⁇ m of the steel sheet surface layer caused by nitriding and oxidation of the steel sheet surface.
  • it is effective to prevent the martensite area ratio on the steel sheet surface from decreasing and to secure TS and material stability.
  • excessive addition over 0.200% causes a reduction in toughness. Therefore, when adding Sn and Sb, the range is from 0.002% to 0.200%.
  • Ta like Ti and Nb, generates alloy carbides and alloy carbonitrides and contributes to increasing the strength of steel.
  • Nb carbide or Nb carbonitride by partially dissolving in Nb carbide or Nb carbonitride and generating a composite precipitate such as (Nb, Ta) (C, N), the coarsening of the precipitate is effectively suppressed, It is considered that there is an effect of stabilizing the contribution to the strength improvement of the steel sheet by precipitation strengthening. For this reason, in this invention, it is preferable to contain Ta.
  • the effect of adding Ta is obtained by setting the content of Ta to 0.001% or more.
  • Ta even if Ta is added excessively, the addition effect is saturated and the alloy cost is also increased. Therefore, when Ta is added, the range is 0.001% or more and 0.010% or less.
  • Ca, Mg, and REM are effective elements for spheroidizing the shape of the sulfide and improving the adverse effect of the sulfide on the hole expandability (stretch flangeability). In order to obtain this effect, 0.0005% or more must be added. On the other hand, excessive addition exceeding 0.0050% causes an increase in inclusions and the like, and causes the surface and internal defects of the steel sheet. Therefore, when adding Ca, Mg, and REM, it is set as 0.0005% or more and 0.0050% or less, respectively.
  • microstructure In order to ensure sufficient ductility in the steel sheet, it is only necessary to promote the formation of polygonal ferrite in the structure, which causes a decrease in tensile strength and yield strength. It also changes depending on the martensite area ratio, and the ductility is greatly influenced by the amount of retained austenite. Therefore, controlling the amount (area ratio, volume ratio) of these phases (structures) is effective in building the mechanical properties of high-strength steel sheets.
  • the inventors have studied from such a viewpoint and have newly found that the area ratio of polygonal ferrite and non-recrystallized ferrite can be controlled by the rolling reduction ratio of cold rolling.
  • the area ratio of martensite and the volume ratio of retained austenite are largely determined by the amount of Mn added.
  • the area ratio of polygonal ferrite is reduced (relative to the entire structure) by not performing cold rolling or preventing the rolling reduction of cold rolling from exceeding 30% (can be controlled within an appropriate range).
  • the final product has a large change in the shape of the structure, resulting in a steel sheet having crystal grains with a large aspect ratio.
  • the value of the hole expansibility ⁇ was improved. That is, the microstructure of a steel sheet having high ductility and good hole expansibility is as follows.
  • Polygonal ferrite area ratio 20% or more and 65% or less
  • the area ratio of polygonal ferrite needs to be 20% or more.
  • the area ratio is 30% or more.
  • the area ratio is 55% or less.
  • the polygonal ferrite in the present invention is a ferrite that is relatively soft and rich in ductility.
  • Area ratio of non-recrystallized ferrite 8% or more It is extremely important in the present invention that the area ratio of non-recrystallized ferrite is 8% or more.
  • non-recrystallized ferrite is effective in increasing the strength of the steel sheet, it causes a significant decrease in the ductility of the steel sheet, and is therefore generally reduced.
  • polygonal ferrite and retained austenite it is possible to ensure good ductility and to actively utilize relatively hard non-recrystallized ferrite, such as an area ratio exceeding 25%. It is possible to secure the TS of the intended steel sheet without requiring a large amount of martensite.
  • the yield strength (YP) and YR of the steel sheet can be increased.
  • the area ratio of non-recrystallized ferrite needs to be 8% or more. Preferably, it is 10% or more.
  • the non-recrystallized ferrite in the present invention is a ferrite having a crystal orientation difference of less than 15 ° in the grains, and is harder than the above-described polygonal ferrite rich in ductility.
  • the upper limit of the area ratio of non-recrystallized ferrite is not particularly limited, but is preferably about 45% because material anisotropy in the plane of the steel sheet may be increased.
  • the martensite area ratio 5% or more and 25% or less
  • the area ratio of ferrite (polygonal ferrite and non-recrystallized ferrite) and martensite can be determined as follows. That is, after polishing a plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate, it corrodes with 3 vol% nital and corresponds to a plate thickness 1/4 position (1/4 of the plate thickness in the depth direction from the steel plate surface).
  • Position is observed using a SEM (scanning electron microscope) at a magnification of 2000 times for about 10 fields of view to obtain a tissue image.
  • SEM scanning electron microscope
  • the area ratio of each structure can be calculated for 10 visual fields using Image-Pro of Media Cybernetics, and the area ratio can be obtained by averaging. it can.
  • polygonal ferrite and non-recrystallized ferrite are identified by showing a gray structure (underground structure) and martensite by showing a white structure.
  • the area ratio of polygonal ferrite and non-recrystallized ferrite can be obtained as follows. That is, by using EBSD (Electron Backscatter Diffraction), a low-angle grain boundary having a crystal orientation difference of 2 ° to less than 15 ° and a large-angle grain boundary having a crystal orientation difference of 15 ° or more are identified. . Then, IQ Map is created by using ferrite containing low-angle grain boundaries in the grains as non-recrystallized ferrite.
  • EBSD Electro Backscatter Diffraction
  • the areas of polygonal ferrite and unrecrystallized ferrite are calculated by obtaining the areas of the low-angle and large-angle grain boundaries in the 10 fields of view respectively. Then, the area ratio of polygonal ferrite and unrecrystallized ferrite for 10 fields of view is obtained. Then, the area ratios of the polygonal ferrite and the non-recrystallized ferrite are obtained by averaging those area ratios.
  • volume ratio of retained austenite 8% or more
  • the volume ratio of retained austenite needs to be 8% or more in order to ensure sufficient ductility. Preferably it is 10% or more.
  • the upper limit of the volume fraction of retained austenite is not particularly limited. However, since the concentration of the retained austenite is small and unstable, such as C or Mn, which has a small effect on improving ductility, the amount of retained austenite increases. % Is preferable.
  • the volume ratio of retained austenite is determined by polishing the steel sheet to a 1 ⁇ 4 surface in the plate thickness direction (a surface corresponding to 1 ⁇ 4 of the plate thickness in the depth direction from the steel plate surface). It is determined by measuring the diffracted X-ray intensity. MoK ⁇ rays are used as incident X-rays, and ⁇ 111 ⁇ , ⁇ 200 ⁇ , ⁇ 220 ⁇ , ⁇ 311 ⁇ planes of the retained austenite have peak integrated intensities of ferrite ⁇ 110 ⁇ , ⁇ 200 ⁇ , ⁇ 211 ⁇ . The intensity ratios of all 12 combinations with respect to the integrated intensity of the peak of the surface are obtained, and the average value thereof is taken as the volume ratio of retained austenite.
  • Polygonal ferrite average crystal grain size 6 ⁇ m or less Refinement of polygonal ferrite crystal grains contributes to improvement of YP and TS. Therefore, in order to ensure high YP and high YR and the desired TS, the average crystal grain size of polygonal ferrite needs to be 6 ⁇ m or less. Preferably, it is 5 ⁇ m or less. In the present invention, the lower limit of the average crystal grain size of polygonal ferrite is not particularly limited, but is preferably about 0.3 ⁇ m industrially.
  • Martensite average crystal grain size 3 ⁇ m or less
  • Refinement of martensite crystal grains contributes to the improvement of bendability and stretch flangeability (hole expandability). Therefore, in order to ensure high bendability and high stretch flangeability (high hole expansibility), it is necessary to suppress the average crystal grain size of martensite to 3 ⁇ m or less. Preferably, it is 2.5 ⁇ m or less.
  • the lower limit of the average grain size of martensite is not particularly limited, but industrially, it is preferably about 0.1 ⁇ m.
  • Average crystal grain size of retained austenite 3 ⁇ m or less
  • the refinement of crystal grains of retained austenite contributes to the improvement of ductility and the improvement of bendability and stretch flangeability (hole expandability). Therefore, in order to ensure good ductility, bendability, stretch flangeability (hole expandability), the average crystal grain size of retained austenite needs to be 3 ⁇ m or less. Preferably, it is 2.5 ⁇ m or less.
  • the lower limit of the average crystal grain size of retained austenite is not particularly limited, but industrially, it is preferably about 0.1 ⁇ m.
  • the average crystal grain size of polygonal ferrite, martensite and retained austenite was determined by calculating the area of each of the polygonal ferrite grains, martensite grains and retained austenite grains using the above-mentioned Image-Pro. Calculate and average the values. Polygonal ferrite, non-recrystallized ferrite, martensite, and retained austenite are separated by EBSD, and martensite and retained austenite are identified by Phase Map of EBSD. In the present invention, when the average crystal grain size is determined, those having a grain size of 0.01 ⁇ m or more are measured. This is because a thickness of less than 0.01 ⁇ m does not affect the present invention.
  • Average aspect ratio of polygonal ferrite, martensite and retained austenite crystal grains 2.0 to 15.0 or less
  • the average aspect ratio of polygonal ferrite, martensite and retained austenite crystal grains is 2.0 or more. This is extremely important in the present invention.
  • the fact that the aspect ratio of the crystal grains is small means that during the holding in the heat treatment after cold rolling (cold rolled sheet annealing), the grains grow after the ferrite and austenite have recovered and recrystallized, resulting in equiaxed grains. This means that near crystal grains were formed.
  • the ferrite produced here is soft.
  • the aspect ratio of the crystal grain here is a value obtained by dividing the major axis length of the crystal grain by the minor axis length, and the average aspect ratio of each crystal grain can be obtained as follows. It can. That is, using the above-mentioned Image-Pro, the major axis length and minor axis length of 30 crystal grains are calculated for each of the polygonal ferrite grains, martensite grains, and retained austenite grains, and the major axis lengths are calculated. Can be obtained by dividing the value by the minor axis length and averaging the values.
  • the amount of Mn (mass%) in retained austenite is the amount of Mn in polygonal ferrite ( It is extremely important in the present invention that the value divided by (mass%) is 2.0 or more. This is because in order to ensure good ductility, it is necessary to increase stable retained austenite enriched in Mn.
  • the upper limit of the value obtained by dividing the amount of Mn (mass%) in retained austenite by the amount of Mn (mass%) in polygonal ferrite is not limited, but from the viewpoint of securing stretch flangeability, 16 About 0.0 is preferable.
  • the amount of Mn (mass%) in the retained austenite and the amount of Mn (mass%) in the polygonal ferrite can be obtained as follows. That is, using an EPMA (Electron Probe Micro Analyzer; electronic probe microanalyzer), the distribution state of Mn to each phase of the cross section in the rolling direction at the 1 ⁇ 4 thickness position is quantified. Subsequently, the amount of Mn of 30 retained austenite grains and 30 ferrite grains is analyzed. And the amount of Mn calculated
  • EPMA Electrical Probe Micro Analyzer
  • the microstructure of the present invention in addition to the above-described polygonal ferrite and martensite, etc., it is usually found in steel sheets such as granular ferrite, acicular ferrite, bainitic ferrite, tempered martensite, pearlite and cementite. Carbides (except for cementite in pearlite) may be included. If these structures are within a range of 10% or less in terms of area ratio, the effects of the present invention are not impaired even if they are included.
  • the inventors diligently investigated the steel sheet structure when press forming and processing were performed on the steel sheet.
  • martensite transformation occurs immediately, and it remains as retained austenite until the amount of processing increases, and finally martensite transformation occurs to cause a TRIP phenomenon (processing induced transformation phenomenon). It was found that there is something that produces. It was found that good elongation can be obtained particularly effectively when the amount of retained austenite that undergoes martensitic transformation after the amount of processing increases.
  • the degree of work means a tensile test using a JIS No. 5 test piece obtained by taking a sample so that the tensile direction is perpendicular to the rolling direction of the steel sheet, and means the elongation at that time.
  • FIG. 1 it can be seen that a sample with good elongation has a gradual way of reducing retained austenite when the degree of processing is increased.
  • the residual austenite amount of a sample having a TS of 780 MPa class and a tensile processing of 10% in elongation value was measured, and the effect of the ratio of this value and the residual austenite amount before processing on the total elongation of the steel sheet. investigated.
  • the result is shown in FIG.
  • the value obtained by dividing the residual volume ratio of retained austenite when tensile processing of 10% is given by the elongation value by the residual austenite volume ratio before processing is 0.3 or more. It can be seen that a high elongation is obtained, and those that fall outside this range have a low elongation.
  • the volume ratio of the retained austenite remaining in the steel after the tensile processing of 10% in terms of the elongation value is set to 0.3 or more by the value divided by the residual austenite volume ratio before the tensile processing. It is preferable. This is because retained austenite that undergoes martensitic transformation after the processing amount becomes large can be secured.
  • the TRIP phenomenon requires that residual austenite be present before press molding or processing.
  • the determined Ms point (martensitic transformation start point) needs to be as low as about 15 ° C. or less.
  • the step of applying a tensile process of 10% with an elongation value according to the present invention will be specifically described.
  • a JIS No. 5 test piece in which a sample was taken so that the tensile direction was a direction perpendicular to the rolling direction of the steel sheet.
  • a tensile test is performed, and when the elongation rate is 10%, the test is interrupted to give the test piece a tensile process of 10% in terms of elongation value.
  • the volume ratio of retained austenite can be determined by the method described above.
  • the TRIP phenomenon which is the main factor for improving ductility, can be intermittently expressed until the end of the processing of the steel sheet, and so-called stable retained austenite can be achieved.
  • the amount of C (% by mass) in the retained austenite can be determined by the following procedure. That is, using the above-mentioned EPMA, the distribution state of C to each phase of the cross section in the rolling direction at the 1 ⁇ 4 thickness position is quantified. Next, the amount of C in 30 residual austenite grains is analyzed. And the C amount calculated
  • Heating temperature of steel slab 1100 ° C or higher and 1300 ° C or lower
  • the heating temperature of the steel slab is less than 1100 ° C., it is difficult to sufficiently dissolve the carbide, and problems such as an increased risk of trouble during hot rolling due to an increase in rolling load occur. Therefore, the heating temperature of the steel slab is preferably 1100 ° C. or higher. In addition, the heating temperature of the steel slab should be 1100 ° C.
  • the heating temperature of the steel slab exceeds 1300 ° C., the scale loss increases as the oxidation amount increases. Therefore, the heating temperature of the steel slab is preferably 1300 ° C. or lower. Therefore, the heating temperature of the slab is preferably 1100 ° C. or higher and 1300 ° C. or lower. More preferably, it is 1150 degreeC or more. More preferably, it is 1250 degrees C or less.
  • the steel slab is preferably manufactured by a continuous casting method in order to prevent macro segregation, but can also be manufactured by an ingot-making method or a thin slab casting method. Moreover, in this invention, after manufacturing a steel slab, after cooling to room temperature once, the conventional method of heating again can be used. Furthermore, in the present invention, energy-saving processes such as direct feed rolling and direct rolling that are not cooled to room temperature, are charged in a heating furnace as they are, but are heated immediately after performing slight heat retention, can be applied without any problem. be able to. Steel slabs are made into sheet bars by rough rolling under normal conditions. However, if the heating temperature is low, a bar heater or the like is used before finish rolling from the viewpoint of preventing problems during hot rolling. It is preferred to use and further heat the sheet bar.
  • Finishing rolling exit temperature of hot rolling 750 ° C. or higher and 1000 ° C. or lower
  • the heated steel slab is hot rolled by rough rolling and finish rolling to become a hot rolled sheet.
  • the finish rolling exit temperature exceeds 1000 ° C.
  • the amount of oxide (scale) generated increases rapidly, the interface between the base iron and the oxide becomes rough, and pickling and cold rolling are performed.
  • the surface quality of the steel sheet tends to deteriorate.
  • a part of the hot-rolled scale remains after pickling, it adversely affects the ductility and stretch flangeability of the steel sheet.
  • the crystal grain size becomes excessively large, and the surface of the pressed product may be roughened during processing.
  • the finish rolling exit temperature is less than 750 ° C.
  • the rolling load increases, and the reduction ratio of the austenite in an unrecrystallized state increases.
  • an abnormal texture develops in the steel sheet, the in-plane anisotropy in the final product becomes remarkable, and not only the material uniformity (material stability) is impaired, but also the ductility of the steel sheet itself decreases.
  • the finish rolling outlet temperature of hot rolling is less than 750 ° C. or more than 1000 ° C.
  • a structure having a volume ratio of 8% or more of retained austenite cannot be obtained. Therefore, in the present invention, it is necessary to set the finish rolling exit temperature of hot rolling to 750 ° C. or more and 1000 ° C. or less.
  • it is 800 degreeC or more.
  • it is 950 degrees C or less.
  • Average coiling temperature after hot rolling 300 ° C. or more and 750 ° C. or less
  • the average coiling temperature after hot rolling exceeds 750 ° C.
  • the ferrite crystal grain size of the hot rolled sheet structure becomes large, and the final annealed sheet It is difficult to secure the desired strength.
  • the average crystal grain size of polygonal ferrite is 6 ⁇ m or less
  • the average crystal grain size of martensite is 3 ⁇ m or less
  • the average crystal grain size of retained austenite is 3 ⁇ m. The following organization cannot be obtained.
  • the average winding temperature after hot rolling needs to be 300 ° C. or higher and 750 ° C. or lower. Preferably it is 400 degreeC or more. Preferably it is 650 degrees C or less.
  • rough rolling sheets may be joined together to perform finish rolling continuously. Moreover, you may wind up a rough rolling board once. Furthermore, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable that the friction coefficient at the time of lubrication rolling shall be 0.10 or more and 0.25 or less.
  • the pickling is performed on the hot-rolled sheet manufactured through this process. Since pickling can remove oxides on the surface of the steel sheet, it is important for ensuring good chemical conversion properties and plating quality of the high-strength steel sheet as the final product.
  • the pickling may be performed once or may be performed in a plurality of times.
  • Hot-rolled sheet annealing (first heat treatment): Ac 1 transformation point + 20 ° C. or higher, Ac 1 transformation point + 120 ° C. or lower, maintained at 600 s or more and 21600 s or lower Ac 1 transformation point + 20 ° C. or higher, Ac 1 transformation point + 120 ° C. or lower It is very important in the present invention to maintain the temperature in the temperature range of 600 s to 21600 s.
  • the annealing temperature of hot-rolled sheet annealing is less than Ac 1 transformation point + 20 ° C., more than Ac 1 transformation point + 120 ° C., and when the holding time is less than 600 s, all of Mn concentrates in austenite.
  • the hot-rolled sheet annealing (first heat treatment) of the present invention is held for a period of 600 s to 21600 s in a temperature range of Ac 1 transformation point + 20 ° C. or higher and Ac 1 transformation point + 120 ° C. or lower.
  • the annealing method may be any annealing method such as continuous annealing or batch annealing.
  • it is cooled to room temperature, but the cooling method and cooling rate are not particularly specified, and any cooling such as furnace cooling in batch annealing, gas jet cooling in air annealing and continuous annealing, mist cooling, water cooling, etc. I do not care.
  • the pickling may be performed according to a conventional method.
  • Annealing (second heat treatment): Ac 1 transformation point + 10 ° C. or higher, and Ac 1 transformation point + 100 ° C. or lower and held for 20 to 900 s Ac 1 transformation point + 10 ° C. or higher, Ac 1 transformation point + 100 ° C. or lower Holding for 20 to 900 s is extremely important in the present invention.
  • the annealing temperature is less than Ac 1 transformation point + 10 ° C., or more than Ac 1 transformation point + 100 ° C., and when the holding time is less than 20 s, none of the Mn concentration in the austenite proceeds sufficiently. It is difficult to ensure a sufficient volume ratio of retained austenite, and ductility is reduced.
  • divided the amount of Mn (mass%) in retained austenite by the amount of Mn (polygonal ferrite) in polygonal ferrite is 2.0 or more cannot be obtained.
  • the area ratio of unrecrystallized ferrite decreases, the amount of heterophase interface between ferrite and hard second phase (martensite and retained austenite) increases, and YP decreases. , YR also decreases.
  • a structure in which the average crystal grain size of martensite is 3 ⁇ m or less and the average crystal grain size of retained austenite is 3 ⁇ m or less cannot be obtained.
  • Cold rolling reduction less than 30%
  • Cold rolling may be performed after the hot-rolled sheet annealing and before annealing (second heat treatment). In that case, it is essential that the rolling reduction is less than 30%.
  • polygonal ferrite formed by recrystallization after heat treatment does not form, and a structure stretched in the rolling direction remains, and finally This is because polygonal ferrite, retained austenite and martensite having a high aspect ratio are obtained, and not only the strength-ductility balance is improved but also stretch flangeability (hole expandability) is improved.
  • the rolling reduction is 30% or more, the average aspect ratio of the crystal structure of polygonal ferrite, martensite, and retained austenite is 2. A structure of 0 or more and 15.0 or less cannot be obtained.
  • Hot dip galvanizing treatment when the hot dip galvanizing treatment is performed, the steel sheet subjected to the annealing (second heat treatment) is immersed in a galvanizing bath at 440 ° C. or higher and 500 ° C. or lower to obtain hot dip galvanizing. Apply processing. Thereafter, the plating adhesion amount on the steel sheet surface is adjusted by gas wiping or the like. In addition, it is preferable to use the zinc plating bath whose amount of Al is 0.10 mass% or more and 0.22 mass% or less for hot dip galvanization.
  • the alloying process of galvanization can be performed in the temperature range of 450 degreeC or more and 600 degrees C or less after the said hot dip galvanization process.
  • the alloying treatment is performed at a temperature exceeding 600 ° C., untransformed austenite is transformed into pearlite, and a desired volume ratio of retained austenite cannot be ensured, and ductility is lowered.
  • the alloying treatment temperature is less than 450 ° C., alloying does not proceed and it is difficult to produce an alloy layer. Therefore, when the alloying treatment of galvanization is performed, the treatment is performed in a temperature range of 450 ° C. or more and 600 ° C. or less.
  • the conditions of other manufacturing methods are not particularly limited, but from the viewpoint of productivity, the series of treatments such as annealing, hot dip galvanization, galvanizing alloying treatment, etc. are performed by CGL (Continuous Galvanizing) which is a hot dip galvanizing line. Line).
  • CGL Continuous Galvanizing
  • the steel plate subjected to the annealing treatment is immersed in an aluminum plating bath at 660 to 730 ° C. to perform the hot dip aluminum plating treatment. Thereafter, the plating adhesion amount is adjusted by gas wiping or the like.
  • a steel plate having a composition suitable for an aluminum plating bath temperature range of Ac 1 transformation point + 10 ° C. or higher and Ac 1 transformation point + 100 ° C. or lower produces finer and more stable retained austenite by hot-dip aluminum plating treatment. Therefore, it is preferable because the ductility can be further improved.
  • the steel plate after the heat treatment may be subjected to an electrogalvanizing treatment.
  • the electrogalvanizing treatment at that time is not particularly limited, but it is preferable to adjust the electrogalvanizing treatment conditions so that the film thickness is in the range of 5 ⁇ m to 15 ⁇ m.
  • skin pass rolling can be performed on the above steel plate, hot dip galvanized steel plate, hot dip galvanized steel plate and electrogalvanized steel plate for the purpose of shape correction, adjustment of surface roughness, and the like.
  • the rolling reduction of the skin pass rolling is preferably in the range of 0.1% to 2.0%. If the rolling reduction of skin pass rolling is less than 0.1%, the effect of skin pass rolling is small and control is difficult, so 0.1% is the lower limit of the preferred range. On the other hand, if the rolling reduction ratio of the skin pass rolling exceeds 2.0%, the productivity of the steel sheet is remarkably lowered, so 2.0% is made the upper limit of the preferred range.
  • the skin pass rolling may be performed online or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps. Furthermore, the steel sheet according to the present invention, the hot dip galvanized steel sheet, the hot dip galvanized steel sheet, and the electrogalvanized steel sheet can be subjected to various coating treatments such as coating using resin or oil.
  • a steel having the composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in a converter and made into a slab by a continuous casting method.
  • the obtained slab was made into the following various steel plates under the conditions shown in Table 2. That is, after hot rolling, annealing is performed at Ac 1 transformation point + 20 ° C. or more, Ac 1 transformation point + 120 ° C. or less, and after cold rolling (in some cases, cold rolling is not performed), Ac 1 transformation point + 10 ° C. or more. And Ac 1 transformation point + 100 ° C. or lower.
  • a cold-rolled steel sheet (CR) is obtained, and further subjected to a plating treatment, and a hot-dip galvanized steel sheet (GI), an alloyed hot-dip galvanized steel sheet (GA), a hot-dip aluminum-plated steel sheet (Al), and an electrogalvanized steel sheet ( EG).
  • a hot-dip galvanized steel sheet (GI)
  • an alloyed hot-dip galvanized steel sheet (GA)
  • Al hot-dip aluminum-plated steel sheet
  • EG electrogalvanized steel sheet
  • a hot dip galvanizing bath a zinc bath containing Al: 0.19% by mass in a hot dip galvanized steel plate (GI), and a zinc containing Al: 0.14% by mass in a galvannealed steel plate (GA).
  • a bath was used. In both cases, the bath temperature was 465 ° C., and the amount of plating adhered was 45 g / m 2 per side (double-sided plating). Furthermore, with GA, the Fe concentration in the plating layer was adjusted to 9 mass% or more and 12 mass% or less.
  • the bath temperature of the hot-dip aluminum plating bath for hot-dip aluminum-plated steel sheets was 700 ° C. The steel sheet thus obtained was examined for the cross-sectional microstructure, tensile properties, hole expansibility, bendability, etc., and the results are shown in Tables 3 to 5.
  • the Ac 1 transformation point was determined using the following equation.
  • Ac 1 transformation point (° C) 751-16 ⁇ (% C) + 11 ⁇ (% Si) ⁇ 28 ⁇ (% Mn) ⁇ 5.5 ⁇ (% Cu) ⁇ 16 ⁇ (% Ni) + 13 ⁇ (% Cr) + 3.4 ⁇ (% Mo)
  • (% C), (% Si), (% Mn), (% Ni), (% Cu), (% Cr) and (% Mo) are the contents of each element in steel (mass% ).
  • the tensile test is performed in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece sampled so that the tensile direction is perpendicular to the rolling direction of the steel sheet, and YP, YR, TS and EL was measured.
  • YR is a value expressed by percentage by dividing YP by TS.
  • YR ⁇ 68% and TS ⁇ EL ⁇ 24000 MPa ⁇ % was judged as good.
  • EL ⁇ 34% for TS: 590 MPa class EL ⁇ 30% for TS: 780 MPa class
  • EL ⁇ 24% for TS: 980 MPa class respectively.
  • TS: 590 MPa class is a steel sheet having a TS of 590 MPa or more and less than 780 MPa
  • TS: 780 MPa class is a steel sheet having a TS of 780 MPa or more and less than 980 MPa
  • TS: 980 MPa class is a TS of 980 MPa.
  • the steel sheet is less than 1180 MPa.
  • the bending test was performed based on the V-block method of JIS Z2248 (1996). With respect to the outside of the bent portion, the presence or absence of a crack was determined with a stereomicroscope, and the minimum bending radius at which no crack was generated was defined as a limit bending radius R. In the present invention, when the limit bending R / t ⁇ 1.5 (t: plate thickness of the steel plate) at 90 ° V bending is satisfied, the bendability of the steel plate is determined to be good.
  • the hole expandability was performed in accordance with JIS Z 2256 (2010). Each obtained steel plate was cut into 100 mm ⁇ 100 mm, and a hole with a diameter of 10 mm was punched out with a clearance of 12% ⁇ 1%. Next, a 60 ° conical punch was pushed into the hole with a crease holding force of 9 ton (88.26 kN) using a die having an inner diameter of 75 mm, and the hole diameter at the crack initiation limit was measured. Furthermore, the critical hole expansion rate ⁇ (%) was obtained from the following formula, and the hole expansion property was evaluated from the value of the critical hole expansion rate.
  • Limit hole expansion ratio ⁇ (%) ⁇ (D f ⁇ D 0 ) / D 0 ⁇ ⁇ 100
  • D f hole diameter at crack initiation (mm) D 0 is the initial hole diameter (mm).
  • ⁇ ⁇ 34% for the TS: 590 MPa class, ⁇ ⁇ 30% for the TS: 780 MPa class, and ⁇ ⁇ 25% for the TS: 980 MPa class was determined to be good.
  • the determination of the plateability of hot rolling when the finishing temperature of hot rolling is low, the reduction rate of austenite is high in the non-recrystallized state, or rolling is performed in a two-phase region of austenite and ferrite For example, it was assumed that the risk of troubles such as defective plate shape during hot rolling due to an increase in rolling load increased, and this case was judged as defective.
  • the determination of the plateability of the cold rolling when the coiling temperature of the hot rolling is low and the steel structure of the hot rolled sheet is mainly composed of a low-temperature transformation phase such as bainite or martensite, rolling is performed. This case was judged to be defective, assuming that the risk of troubles such as defective plate shape during cold rolling due to an increase in load would increase.
  • the surface properties of the final annealed plate include a case where the amount of oxide (scale) generated increases sharply, the interface between the base iron and the oxide becomes rough, and the surface quality after pickling and cold rolling deteriorates. It was also judged as defective when there was a part of the hot rolled scale remaining after washing.
  • the amount of C (mass%) in the retained austenite and the amount of Mn (mass%) in the retained austenite were measured according to the method described above. The measurement results are also shown in Table 4.
  • a high-strength steel sheet having a TS of 590 MPa or more, excellent formability with a YR of 68% or more, and having a high yield ratio and hole expansibility is obtained.
  • at least one characteristic among YR, TS, EL, ⁇ , and R / t is inferior.
  • TS of 590 MPa or more, YR of 68% or more, excellent formability of TS ⁇ EL ⁇ 24000 MPa ⁇ %, high yield ratio and hole expandability Steel plate can be manufactured.
  • fuel efficiency can be improved by reducing the weight of the vehicle body, and the industrial utility value is extremely large.

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Abstract

La présente invention concerne une plaque d'acier à haute résistance qui a une composition en composants prédéterminée, une résistance à la traction (TS) de 590 MPa ou plus, d'excellentes propriétés de moulage à un rapport limite élastique sur résistance à la traction (YR) de 68 % ou plus, un rapport limite élastique sur résistance à la traction élevé et une capacité d'expansion de trou élevée, ladite plaque d'acier étant obtenue en étant conçue de la manière suivante : en comprenant de 20 à 65 % de ferrite polygonale, 8 % ou plus de ferrite non recristallisée et de 5 à 25 % de martensite en termes de rapport de surface, de telle sorte que la plaque d'acier comprend 8 % ou plus d'austénite résiduelle en fraction volumique, et les rapports de forme moyen des grains cristallins dans les phases individuelles (ferrite polygonale, martensite et austénite résiduelle) sont compris entre 2,0 et 15,0 ; et en ajustant la taille moyenne des grains cristallins de ferrite polygonale à 6 µm ou moins, la taille moyenne des grains cristallins de martensite étant inférieure ou égale à 3 µm et la taille moyenne des grains cristallins de l'austénite résiduelle à 3 µm ou moins, de telle sorte que la valeur obtenue en divisant la quantité de Mn (% en masse) dans l'austénite résiduelle par la quantité de Mn (% en masse) à l'intérieur de la ferrite polygonale est supérieure ou égale à 2,0.
PCT/JP2017/009300 2016-04-19 2017-03-08 Plaque d'acier, plaque d'acier plaquée et procédé pour les produire WO2017183348A1 (fr)

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US16/090,883 US20190276907A1 (en) 2016-04-19 2017-03-08 Steel sheet, coated steel sheet, and methods for manufacturing same
CN201780018499.0A CN108779536B (zh) 2016-04-19 2017-03-08 钢板、镀覆钢板和它们的制造方法
JP2017533038A JP6210183B1 (ja) 2016-04-19 2017-03-08 鋼板、めっき鋼板、およびそれらの製造方法
MX2018012659A MX2018012659A (es) 2016-04-19 2017-03-08 Lamina de acero, lamina de acero recubierta, y metodos para la fabricacion de las mismas.
KR1020187033203A KR102128838B1 (ko) 2016-04-19 2017-03-08 강판, 도금 강판 및, 그들의 제조 방법
EP17785691.1A EP3447159B1 (fr) 2016-04-19 2017-03-08 Plaque d'acier, plaque d'acier plaquée et procédé pour les produire

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KR102128838B1 (ko) 2020-07-02
KR20180132889A (ko) 2018-12-12
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