EP3495522A1 - Hochfeste stahlplatte und herstellungsverfahren dafür - Google Patents

Hochfeste stahlplatte und herstellungsverfahren dafür Download PDF

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Publication number
EP3495522A1
EP3495522A1 EP17836780.1A EP17836780A EP3495522A1 EP 3495522 A1 EP3495522 A1 EP 3495522A1 EP 17836780 A EP17836780 A EP 17836780A EP 3495522 A1 EP3495522 A1 EP 3495522A1
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less
mass
temperature
retained austenite
amount
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EP17836780.1A
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English (en)
French (fr)
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EP3495522B1 (de
EP3495522A4 (de
Inventor
Elijah KAKIUCHI
Toshio Murakami
Shigeo Otani
Yuichi Futamura
Tadao Murata
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • C21D1/34—Methods of heating
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—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
    • 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
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to a high-strength sheet that can be used in various applications including automobile parts.
  • steel sheets used for automobile parts and the like are required to achieve both improvement in strength and improvement in impact resistance properties.
  • Patent Document 1 discloses a high-strength steel sheet in which an attempt is made to improve impact resistance properties by heating a slab to 1,210°C or higher and controlling the hot-rolling conditions to form fine TiN particles having a size of 0.5 ⁇ m or less, thereby suppressing the formation of AlN particles having a particle size of 1 ⁇ m or more that act as a starting point of low temperature fracture.
  • Patent Document 2 discloses a high-strength sheet in which an attempt is made to improve collision resistance properties by forming a network structure in which 50% or more of a ferrite grain size is in contact with a hard phase while defining the C amount to more than 0.45% and 0.77% or less, the Mn amount to 0.1% or more and 0.5% or less and the Si amount to 0.5% or less, and defining each addition amount of Cr, Al, N and O.
  • Patent Document 3 discloses a high-strength sheet in which an attempt is made to improve collision resistance properties by adding 3.5 to 10% of Mn, thereby adjusting the amount of retained austenite to 10% or more and an average interval of retained austenite to 1.5 ⁇ m or less.
  • Patent Document 4 discloses a high-strength sheet that has a tensile strength of 980 to 1,180 MPa and also exhibits satisfactory deep drawability.
  • steel sheets used for automobile parts are required to have sufficient strength and impact resistance properties while being made thinner.
  • steel sheets having higher tensile strength and excellent impact properties are required.
  • steel sheets are required to have not only high tensile strength and impact properties, but also excellent strength-ductility balance, high yield ratio, excellent deep drawability and excellent hole expansion ratio.
  • the followings are required for each of the tensile strength, the strength-ductility balance, the yield ratio, the deep drawing properties and the hole expansion ratio.
  • the tensile strength is required to be 980 MPa or higher.
  • high yield strength (YS), in addition to high tensile strength (TS).
  • TS high tensile strength
  • a joint strength of the spot welded portion is also required as basic performances of the steel sheet for automobiles.
  • a cross tensile strength of the spot welded portion is required to be 6 kN or higher.
  • the product (TS ⁇ EL) of TS and total elongation (EL) is required to be 20,000 MPa% or higher.
  • LDR showing deep drawability is 2.05 or more and the hole expansion ratio ⁇ showing expansion properties is 20% or more.
  • Patent Documents 1 to 4 it is difficult for the high-strength sheets disclosed in Patent Documents 1 to 4 to satisfy all of these requirements, and there has been required a high-strength steel sheet that can satisfy all of these requirements.
  • the embodiment of the present invention has been made to respond to these requirements, and it is an object thereof to provide a high-strength sheet in which all of tensile strength (TS), yield ratio (YR), the product (TS ⁇ EL) of (TS) and total elongation (EL), LDR, hole expansion ratio ( ⁇ ), thickness reduction ratio (RA) of the fracture portion during a tensile test, and cross tensile strength (SW cross tension) of the spot welded portion are at a high level, and a manufacturing method thereof.
  • TS tensile strength
  • YiR yield ratio
  • TS ⁇ EL product
  • EL total elongation
  • LDR hole expansion ratio
  • ⁇ thickness reduction ratio
  • SW cross tension cross tensile strength
  • TS tensile strength
  • YiR yield ratio
  • TS ⁇ EL product of (TS)
  • EL total elongation
  • LDR hole expansion ratio
  • RA thickness reduction ratio
  • SW cross tension cross tensile strength
  • Fig. 1 is a diagram explaining a method for manufacturing a high-strength sheet according to the embodiment of the present invention, especially a heat treatment.
  • TS tensile strength
  • YiR yield ratio
  • TS ⁇ EL product of (TS) and total elongation
  • LDR hole expansion ratio
  • ⁇ thickness reduction ratio
  • SW cross tension cross tensile strength
  • a ferrite fraction is 5% or less
  • the total fraction of tempered martensite and tempered bainite is 60% or more
  • the amount of retained austenite ( ⁇ ) is 10% or more
  • MA has an average size of 1.0 ⁇ m or less
  • retained austenite has an average size of 1.0 ⁇ m or less
  • Ferrite generally has excellent workability but has a problem such as low strength. As a result, a large amount of ferrite leads to a decrease in yield ratio. Therefore, a ferrite fraction was set at 5% or less (5 volume % or less).
  • the ferrite fraction is preferably 3% or less, and more preferably 1% or less.
  • the ferrite fraction can be determined by observing with an optical microscope and measuring the white region by the point counting method. By such a method, it is possible to determine the ferrite fraction by an area ratio (area %). The value obtained by the area ratio may be directly used as the value of the volume ratio (volume %).
  • the total fraction of tempered martensite and tempered bainite is preferably 70% or more.
  • the retained austenite causes the TRIP phenomenon of being transformed into martensite due to strain induced transformation during working such as press working, thus making it possible to obtain large elongation. Martensite thus formed has high hardness. Therefore, excellent strength-ductility balance can be obtained.
  • the amount of retained austenite is set at 10% or more (10 volume % or more), it is possible to realize TS ⁇ EL of 20,000 MPa% or more and excellent strength-ductility balance.
  • the amount of retained austenite is preferably 15% or more.
  • MA is abbreviation of a martensite-austenite constituent and is a composite (complex structure) of martensite and austenite.
  • ferrite including tempered martensite and untempered martensite in X-ray diffraction
  • austenite by X-ray diffraction
  • Co-K ⁇ ray can be used as an X-ray source.
  • MA is a hard phase and the vicinity of matrix/hard phase interface acts as a void forming site during deformation.
  • the average size of MA is preferably 0.8 ⁇ m or less.
  • the inventors of the present invention have found that a high work hardening rate is maintained during deformation by setting the average size of retained austenite at 1.0 ⁇ m and setting the ratio (volume ratio) of the amount of retained austenite having a size of 1.5 ⁇ m or more to the total amount of retained austenite at 2% or more, thus making it possible to obtain excellent deep drawability (LDR).
  • LDR deep drawability
  • the TRIP phenomenon occurs and high elongation can be obtained.
  • the martensitic structure formed by strain induced transformation is hard and acts as a starting point of fracture. Larger martensite structure easily acts as the starting point of fracture. It is also possible to obtain the effect of suppressing fracture by setting the average size of retained austenite at 1.0 ⁇ m or less to reduce the size of martensite formed by strain induced transformation.
  • each austenite phase is obtained from the obtained Phase map and a circle equivalent diameter (diameter) of each austenite phase is obtained from the area, and then an average of the obtained diameter is taken as the average size of retained austenite.
  • EBSD electron back scatter diffraction patterns
  • the ratio of retained austenite having a size of 1.5 ⁇ m or more to the entire austenite is the area ratio and is equivalent to the volume ratio.
  • X-ray small angle scattering means that the size distribution of fine particles (e.g., cementite particles dispersed in a steel sheet) contained in the steel sheet can be obtained by irradiating the steel sheet with X-rays and measuring scattering of X-rays transmitted through the steel sheet.
  • fine particles e.g., cementite particles dispersed in a steel sheet
  • X-ray small angle scattering it is possible to analyze the size and the fraction of cementite particles using the q value and the scattering intensity.
  • the q value is an index of the size of particles (e.g., cementite particles) in the steel sheet.
  • the "q value of 1 nm -1 " corresponds to cementite particles having a particle size of about 1 nm.
  • the scattering intensity is an index of the volume fraction of particles (e.g., cementite particles) in the steel sheet. The larger the scattering intensity, the larger the volume fraction of cementite becomes.
  • the scattering intensity at a certain q value semi-quantitatively indicates the volume fraction of cementite particles of the size corresponding to the q value.
  • the scattering intensity at the q value of 1 nm -1 semi-quantitatively indicates the volume fraction of fine cementite particles having a size of about 1 nm.
  • large scattering intensity at the q value of 1 nm -1 indicates large volume fraction of fine cementite particles having a size of about 1 nm.
  • the scattering intensity at the q value of 1 nm -1 is 1.0 cm -1 or less
  • the volume fraction of fine cementite particles having a size of about 1 nm existing in the steel sheet is a predetermined value (the value corresponding to the scattering intensity of 1.0 cm -1 ) or less.
  • the steel sheet in which "the scattering intensity at the q value of 1 nm -1 is 1.0 cm -1 or less" is excellent in collision resistance properties since the volume fraction of cementite having a size of about 1 nm is suppressed to a low value.
  • the steel sheet according to the embodiment of the present invention by suppressing the volume fraction of fine cementite to a low value, more specifically, by setting the scattering intensity at the q value of 1 nm -1 at 1 cm -1 or less, fine carbide formed in laths of tempered martensite is reduced to enhance the deformability in martensite. Thus, fracture of the steel sheet upon collision is suppressed to improve collision resistance properties of the steel sheet.
  • X-ray small angle scattering was measured using a Nano-viewer, Mo tube manufactured by Rigaku Corporation. A 3 mm ⁇ disk-shaped sample was cut out from the steel sheet and samples having a thickness of 20 ⁇ m were cut out from the vicinity of the thickness of 1/4 and then used. Data at the q value of 0.1 to 10 nm -1 were collected. Among them, absolute intensity was determined for the q value of 1 nm -1 .
  • steel structures other than the above-mentioned ferrite, tempered martensite, tempered bainite retained austenite and cementite are not specifically defined.
  • pearlite, untempered bainite, untempered martensite and the like may exist, in addition to the steel structures such as ferrite.
  • the steel structure such as ferrite satisfies the above-mentioned structure conditions, the effects of the present invention are exhibited even if pearlite or the like exists in the steel.
  • composition of the high-strength sheet according to the embodiment of the present invention will be described below. Main elements C, Si, Al, Mn, P and S will be described. Note that all percentages as unit with respect to the composition are by mass.
  • Carbon (C) is an element indispensable for ensuring properties such as high strength-ductility balance (TS ⁇ EL balance) by increasing the amount of desired structure, especially retained ⁇ . In order to effectively exhibit such effect, there is a need to add C in the amount of 0.15% or more. However, the amount of more than 0.35% is not suitable for welding. The amount is preferably 0.18% or more, and more preferably 0.20% or more. The amount is preferably 0.30% or less. If the C amount is 0.25% or less, welding can be easily performed.
  • Si and Al each have the effect of suppressing the precipitation of cementite, thus remaining retained austenite. In order to effectively exhibit such effect, there is a need to add Si and Al in the total amount of 0.5% or more. If the total amount of Si and Al exceeds 3.0%, the deformability of the steel is degraded, thus degrading TS ⁇ EL.
  • the total amount is preferably 0.7% or more, and more preferably 1.0% or more.
  • the total amount is preferably 2.5% or less.
  • Al may be added in the amount enough to function as an deoxidizing element, i.e., less than 0.10% by mass.
  • Al may be added in a larger amount of 0.7% by mass or more.
  • Mn suppresses the formation of ferrite. In order to effectively exhibit such effect, there is a need to add Mn in the amount of 1.0% or more. If the amount exceeds 4.0%, MA becomes coarse, thus degrading hole expansion properties.
  • the amount is preferably 1.5% or more, and more preferably 2.0% or more. The amount is preferably 3.5% or less.
  • the content of P is set at 0.05% or less (including 0%).
  • the content is 0.03% or less (including 0%).
  • S inevitably exists as an impurity element. If more than 0.01% of S exists, sulfide-based inclusions such as MnS are formed and act as a starting point of cracking, thus degrading ⁇ . Therefore, the content of S is set at 0.01% or less (including 0%). The content is preferably 0.005% or less (including 0%).
  • the balance is composed of iron and inevitable impurities. It is permitted to mix, as inevitable impurities, trace elements (e.g., As, Sb, Sn, etc.) incorporated according to the conditions of raw materials, materials, manufacturing facilities and the like. There are elements whose content is preferably as small as possible, like P and S, that are therefore inevitable impurities in which the composition range is separately defined as mentioned above. Therefore, "inevitable impurities" constituting the balance as used herein means the concept excluding elements whose composition range is separately defined.
  • trace elements e.g., As, Sb, Sn, etc.
  • any other element may be further included.
  • the high-strength sheet has TS of 980 MPa or higher.
  • TS is 1,180 MPa or higher. If TS is lower than 980 MPa, excellent fracture properties can be more surely obtained, but it is not preferable since withstand load upon collision decreases.
  • the high-strength sheet has an yield ratio of 0.75 or more. This makes it possible to realize a high yield strength combined with the above-mentioned high tensile strength and to use the final product obtained by working such as deep drawing under high stress.
  • the high-strength sheet has a yield ratio of 0.80 or more.
  • TS ⁇ EL is 20,000 MPa% or more. By having TS ⁇ EL of 20,000 MPa% or more, it is possible to obtain high-level strength-ductility balance that has both high strength and high ductility. Preferably, TS ⁇ EL is 23,000 MPa% or more.
  • LDR is an index used for evaluation of the deep drawability.
  • D/d is referred to as a limiting drawing ratio (LDR), where d denotes a diameter of a cylinder obtained in cylindrical drawing and D denotes a maximum diameter of a disk-shaped steel sheet (blank) capable of obtaining a cylinder without causing fracture by one deep drawing process.
  • LDR limiting drawing ratio
  • a disk-shaped sample having a thickness of 1.4 mm and various diameters is subjected to cylindrical deep drawing using a die having a punch diameter of 50 mm, a punch angle radius of 6 mm, a die diameter of 55.2 mm and a die angle radius of 8 mm. It is possible to determine LDR by finding a maximum sample diameter (maximum diameter D) among the sample diameters of the disc-shaped sample that was completely deep-drawn without causing fracture.
  • the high-strength sheet according to the embodiment of the present invention has LDR of 2.05 or more, and preferably 2.10 or more, and has excellent deep drawability.
  • a hole expansion ratio ⁇ is determined in accordance with Japan Iron and Steel Federation Standard JFS T1001.
  • JFS T1001 Japan Iron and Steel Federation Standard JFS T1001.
  • ⁇ % d ⁇ d 0 / d 0 ⁇ 100
  • the high-strength sheet according to the embodiment of the present invention has a hole expansion ratio ⁇ of 20% or more, and preferably 30% or more. This makes it possible to obtain excellent workability such as press formability.
  • a tensile test was performed at a deformation rate of 10 mm/min and the test piece was fractured. Then, the fracture surface was observed and the value (t 1 /t 0 ) obtained by dividing a thickness t 1 in a thickness direction of the fracture surface by an original thickness t 0 was taken as a thickness reduction ratio.
  • the thickness reduction rate in this test is 50% or more, preferably 52% or more, and more preferably 55% or more. This makes it possible to obtain a steel sheet having excellent impact resistance properties since the steel sheet is hardly fractured even if it deforms greatly upon collision.
  • Cross tensile strength of spot welding was evaluated in accordance with JIS Z 3137. Two 1.4 mm-thick steel sheets laid one upon another were used as the conditions of spot welding. Using a dome radius type electrode, spot welding was performed under a welding pressure of 4 kN by increasing a current by 0.5 kA in a range from 6 kA to 12 kA, and the current value (minimum current value) at which dust was generated during welding was examined. A cross joint spot-welded at a current that is 0.5 kA lower than the minimum current value was used as a test piece for measurement of a cross tensile strength. Samples having a cross tensile strength of 6 kN or higher were rated "Good". The cross tensile strength is preferably 8 kN or higher, and more preferably 10 kN or higher.
  • cross tensile strength is 6 kN or higher, it is possible to obtain parts having high bonding strength during welding when automobile parts and the like are manufactured from the steel sheet.
  • the inventors of the present application have found that the above-mentioned desired steel structure is attained by subjecting a rolled material with predetermined composition to a heat treatment (multi-step austempering treatment) mentioned later, thus obtaining a high-strength steel sheet having the above-mentioned desired properties.
  • Fig. 1 is a diagram explaining a method for manufacturing a high-strength sheet according to the embodiment of the present invention, especially a heat treatment.
  • the rolled material to be subjected to the heat treatment is usually produced by cold-rolling after subjecting to hot-rolling.
  • the process is not limited thereto, and the rolled material may be produced by any one of hot-rolling and cold-rolling.
  • the conditions of hot-rolling and cold-rolling are not particularly limited.
  • a rolled material is heated to a temperature of an Ac 3 point or higher and heated for a predetermined heating time, thereby austenitizing the rolled material.
  • the heating time at this heating temperature is, for example, 1 to 1,800 seconds.
  • the upper limit of the heating temperature is preferably the Ac 3 point or higher and the Ac 3 point + 100°C or lower. This is because grain coarsening can be suppressed by setting at the temperature of the Ac 3 point + 100°C or lower.
  • the heating temperature is more preferably the Ac 3 point + 10°C or higher and the Ac 3 point + 90°C or lower, and still more preferably the Ac 3 point + 20°C or higher and the Ac 3 point + 80°C or lower. This is because the formation of ferrite can be more completely suppressed and grain coarsening can be more surely suppressed by more complete austenitizing.
  • Heating during austenitizing shown in [1] of FIG. 1 may be performed at an arbitrary heating rate, and the average heating rate is preferably 1°C/sec or more, and more preferably 20°C/sec.
  • Cooling is performed at an average cooling rate of 15°C/sec or more and less than 200°C/sec between at least 650°C and 500°C. This is because the formation of ferrite during cooling is suppressed by setting the average cooling rate at 15°C/sec or more. It is also possible to prevent the occurrence of excessive thermal strain due to rapid cooling by setting the cooling rate at less than 200°C/sec.
  • Preferred example of such cooling includes cooling to a rapid cooling starting temperature of 650°C or higher at relatively low average cooling rate of 0.1°C/sec or more and 10°C/sec or less, as shown in [3] of Fig. 1 , followed by cooling from the rapid cooling starting temperature to a retention starting temperature of 500°C or lower at an average cooling rate of 20°C/sec or more and less than 200°C/sec, as shown in [4] of Fig. 1 .
  • Retention is performed at a temperature in a range of 300°C to 500°C at a cooling rate of 10°C/sec or less for 10 seconds or more.
  • the material is left to stand at a temperature in a range of 300°C to 500°C in a state where the cooling rate is 10°C/sec or less for 10 seconds or more.
  • the state where the cooling rate is 10°C/sec or less also includes the case of holding at a substantially constant temperature (i.e., cooling rate is 0°C/sec), as shown in [5] of Fig. 1 .
  • bainite has solid solubility limit of carbon that is lower than that of austenite, carbon exceeding the solid solubility limit is discharged from bainite, thus forming a region of austenite in which carbon is concentrated around bainite.
  • this region becomes somewhat coarse retained austenite.
  • this somewhat coarse retained austenite it is possible to enhance the deep drawability as mentioned above.
  • the retention temperature is higher than 500°C, since the carbon-concentrated region excessively increases, not only retained austenite but also MA becomes coarse, thus decreasing the hole spreading ratio. Meanwhile, if the retention temperature is lower than 300°C, the carbon-concentrated region decreases and the amount of coarse retained austenite becomes insufficient, thus degrading the deep drawability.
  • the retention time is less than 10 seconds, the area of the carbon-concentrated region decreases and the amount of coarse retained austenite becomes insufficient, thus degrading the deep drawability. Meanwhile, if the retention time is 300 seconds or more, since the carbon-concentrated region excessively increases, not only retained austenite but also MA becomes coarse, thus decreasing the hole expansion ratio.
  • retention is performed at a temperature in a range of 300°C to 500°C at a cooling rate of 10°C/sec or less for 10 seconds or more.
  • Retention is preferably performed at a temperature in a range of 320 to 480°C at a cooling rate of 8°C/sec or less for 10 seconds or more and, during the retention, holding is preferably performed at a constant temperature for 3 to 80 seconds.
  • Retention is more preferably performed at a temperature in a range of 340 to 460°C at a cooling rate of 3°C/sec or less for 10 seconds or more and, during the retention, holding is performed a constant temperature for 5 to 60 seconds.
  • cooling is performed from a second cooling starting temperature of 300°C or higher to a cooling stopping temperature of 100°C or higher and lower than 300°C at an average cooling rate of 10°C/sec or more.
  • the above-mentioned retention end temperature e.g., holding temperature shown in [5] of Fig. 1
  • This cooling causes martensitic transformation while leaving the above-mentioned carbon-concentrated region as austenite.
  • the cooling stopping temperature at a temperature in a range of 100°C or higher and lower than 300°C, final amount of retained austenite is controlled by adjusting the amount of austenite remaining without being transformed into martensite.
  • the cooling rate is less than 10°C/sec, the carbon-concentrated region expands more than necessarily during cooling and MA becomes coarse, thus decreasing the hole spreading ratio. If the cooling stopping temperature is lower than 100°C, the amount of retained austenite becomes insufficient. As a result, TS increases but EL decreases, leading to insufficient TS ⁇ EL balance.
  • the cooling stopping temperature is 300°C or higher, coarse unmodified austenite increases and remains even after the subsequent cooling. Finally, the size of MA becomes coarse, thus decreasing the hole expansion ratio ⁇ .
  • the cooling rate is preferably 15°C/°C or higher, and the cooling stopping temperature is preferably 120°C or higher and 280°C or lower.
  • the cooling rate is more preferably 20°C/sec or more, and the cooling stopping temperature more preferably 140°C or higher and 260°C or lower.
  • holding may be performed at the cooling stopping temperature.
  • the holding time is preferably 1 to 600 seconds. Even if the holding time increases, there is almost no influence on properties. However, the holding time of more than 600 seconds degrades the productivity.
  • heating is performed from the above cooling stopping temperature to a reheating temperature in a range of 300°C to 500°C at a reheating rate of 30°C/sec or more. Rapid heating enables a decrease in retention time in a temperature range where precipitation and growth of carbide are promoted, thus making it possible to suppress the formation of fine carbide.
  • the reheating rate is preferably 60°C/sec or more, and more preferably 70°C/sec.
  • Such rapid heating can be achieved by a method such as high-frequency heating or electric heating.
  • a tempering parameter P represented by the following equation (1) is set at 10,000 or more and 14,500 or less and the holding time is set at 1 to 150 seconds.
  • the tempering parameter P When the tempering parameter P is less than 10,000, carbon diffusion from martensite to austenite does not sufficiently occur and austenite becomes unstable, thus failing to ensure the amount of retained austenite, leading to insufficient TS ⁇ EL balance. If the tempering parameter P is more than 14,500, the formation of carbide cannot be prevented even by a short-time treatment, thus failing to ensure the amount of retained austenite, leading to degradation of TS ⁇ EL balance. Even if the tempering parameter is appropriate, carbide is formed in martensitic laths if the heating rate is too low and heating time is too long, so that crack propagation easily occurs during collision deformation, thus degrading collision resistance properties. The amount of carbide in martensite laths can be determined from the scattering intensity of X-ray small angle scattering.
  • the reheating temperature is lower than 300°C, diffusion of carbon becomes insufficient, thus failing to obtain sufficient amount of retained austenite, leading to degradation of TS ⁇ EL. If the reheating temperature is higher than 500°C, retained austenite is decomposed into cementite and ferrite, thus failing to ensure properties because of insufficient retained austenite.
  • the holding time is less than 1 second, carbon diffusion may be insufficient, similarly. Therefore, it is preferred to hold at a reheating temperature for 1 second or more. If the holding time is more than 150 seconds, carbon may precipitate as cementite, similarly. Therefore, the holding time is preferably 150 seconds or less.
  • the reheating temperature is preferably 320 to 480°C, and more preferably 340 to 460°C.
  • the tempering parameter P is preferably 10,500 to 14,500, and the holding time at this time is preferably 1 to 150 seconds.
  • the tempering parameter P is more preferably 11,000 to 14,000, and the holding time at this time is preferably 1 to 100 seconds, and more preferably 1 to 60 seconds.
  • cooling may be performed to the temperature of 200°C or lower, for example, room temperature.
  • the average cooling rate to 200°C or lower is preferably 10°C/sec.
  • the high-strength sheet according to the embodiment of the present invention can be obtained by the above-mentioned heat treatment.
  • a steel sheet having a thickness of 2.5 mm was produced by multistage rolling after heating to 1,200°C. At this time, the end temperature of hot-rolling was set at 880°C. After cooling to 600°C at 30°C/sec, cooling was stopped and the steel sheet was inserted into a furnace heated to 600°C, held for 30 minutes and then furnace-cooled to obtain a hot-rolled steel sheet.
  • the hot-rolled steel sheet was subjected to pickling to remove the scale on the surface, and then cold-rolled to reduce the thickness to 1.4 mm.
  • This cold rolled sheet was subjected to a heat treatment to obtain samples.
  • the heat treatment conditions are shown in Table 2.
  • the number in parentheses, for example, [2] in Table 2 corresponds to the process of the same number in parentheses in Fig. 1 .
  • samples Nos. 1, 4, 7 and 26 are samples that were not retained at a temperature in a range of 300 to 500°C at a cooling rate of 10°C/sec or less for 10 seconds or more in the step corresponding to [5] of Fig. 1 .
  • Samples 1 and 26 are samples (samples in which the steps corresponding to [5] and [6] in Fig. 1 were skipped) that were immediately cooled to 200°C after starting rapid cooling at 700°C.
  • Sample No. 9 is sample (sample in which the steps corresponding to [6] to [8] in Fig. 1 were skipped) that was cooled to a reheating temperature instead of cooling to a cooling stopping temperature between 100°C or higher and lower than 300°C, followed by holding at the same temperature.
  • Reheating corresponding to [8] was performed by an electric heating method.
  • Cooling rate Parameter °C/sec °C Sec °C/sec °C/sec °C/sec °C/sec °C/sec °C/sec °C/sec °C/sec 1 a 10 850 120 10 700 28 *- *- *- 200 50 100 400 *300 10 12.734 2 a 10 850 120 10 700 28 400 *300 30 200 50 100 440 20 10 12.652 3 a 10 850 120 10 700 28 400 50 *1 200 50 100 400 *300 10 12,734 4 a 10 850 120 10 700 28 400 *3 30 200 50 100 440 20 10 12,652 5 a 10 850 120 10 700 28 *550
  • the ferrite fraction, the total fraction of tempered martensite and tempered bainite (mentioned as "tempered M/B” in Table 3), the amount of retained (amount of retained ⁇ ), the average size of MA, the average size of retained austenite (average size of retained ⁇ ), the ratio of retained austenite having a size of 1.5 ⁇ m or more to the entire austenite (mentioned as "ratio of retained ⁇ having a size of 1.5 ⁇ m or more" in Table 3), and the scattering intensity at the q value of 1 nm -1 in X-ray small angle scattering were determined by the above-mentioned methods.
  • a two-dimensional microfocused X-ray diffraction apparatus RINT-RAPID II manufactured by Rigaku Corporation was used. The results are shown in Table 3.
  • the steel structure (balance structure) other than the steel structure shown in Table 3 is untempered martensite in samples excluding sample No. 9, or untempered bainite in sample No. 9.
  • No. Steel No. Steel structure Ferrite Tempered M/B Amount of retained ⁇ Average size of MA Average size of retained ⁇ Amount of retained ⁇ having size of 1.5 ⁇ m or more Scattering intensity at q value of 1 nm -1 % % % ⁇ m ⁇ m cm -1 1 a 0 70 16.4 0.65 0.78 *0.74 *2.43 2 a 0 69 16.7 *1.30 0.87 3.88 0.71 3 a 0 72 17.8 *14.3 0.82 4.22 *2.42 4 a 0 71 16.3 0.53 0.83 *1.21 0.73 5 a 0 66 18.0 *1.48 0.93 4.11 0.70 6 a 0 76 17.8 0.68 0.76 *0.66 0.73 7 a 0 *55 16.9 *1.50 *1.42 3.30 0.74 8 a
  • Samples Nos. 13, 15, 18, 21 and 28 to 36 are Examples that satisfy all requirements (composition, manufacturing conditions and steel structure) defined in the embodiment of the present invention. All of these samples achieve the tensile strength (TS) of 980 MPa or higher, the yield ratio (YR) of 0.75 or more, TS ⁇ EL of 20,000 MPa% or higher, LDR of 2.05 or more, the hole expansion ratio ( ⁇ ) of 20% or more, the SW cross tension of 6 kN or higher, and the R5 tensile thickness reduction ratio (RA) of 50% or more.
  • sample No. 1 exhibited insufficient amount of retained austenite having a size of 1.5 ⁇ m or more, thus failing to obtain sufficient deep drawability since retention was not performed at a temperature in a range of 300°C to 500°C after austenitizing. Since the holding time [7] was as long as 300 seconds, carbide (cementite) was precipitated. Because of large scattering intensity of X-ray small angle scattering, it can be said that cementite having a size of about 1 nm has a large volume fraction. As a result, collision resistance properties (thickness reduction ratio) were degraded.
  • Sample No. 2 exhibited excessive average size of MA, thus failing to obtain sufficient hole expansion ratio since the holding temperature [5] was as long as 300 seconds.
  • Sample No. 3 exhibited excessive average size of MA, thus failing to obtain sufficient hole expansion ratio since the cooling rate [6] was as low as 1°C/sec. Since the holding time [7] was as long as 300 seconds, carbide (cementite) was precipitated. Because of large scattering intensity of X-ray small angle scattering, it can be said that cementite having a size of about 1 nm has a large volume fraction. As a result, collision resistance properties (thickness reduction ratio) were degraded.
  • Sample No. 4 exhibited insufficient amount of retained austenite having a size of 1.5 ⁇ m or more, thus failing to obtain sufficient deep drawability since the holding time [5] was as short as 3 seconds.
  • Sample No. 5 exhibited excessive average size of MA, thus failing to obtain sufficient hole expansion ratio and sufficient deep drawability since the holding temperature [5] was as high as 550°C.
  • Sample No. 6 exhibited insufficient amount of retained austenite having a size of 1.5 ⁇ m or more, thus failing to obtain sufficient deep drawability since the holding temperature [5] was as low as 250°C.
  • Sample No. 7 exhibited insufficient total amount of tempered martensite and tempered bainite, and excessive average size of MA and excessive average size of retained austenite since the cooling stopping temperature [6] was as high as 350°C. As a result, sufficient hole expansion ratio and deep drawability could not be obtained.
  • Sample No. 8 exhibited excessive amount of ferrite and insufficient total amount of tempered martensite and tempered bainite, thus failing to obtain sufficient tensile strength and yield ratio since the heating temperature [1] was as low as 780°C.
  • Sample No. 9 did not form martensite and bainite, and exhibited excessive average size of MA and excessive average size of retained austenite since the cooling stopping temperature [6] was as high as 400°C. As a result, sufficient tensile strength and yield ratio could not be obtained. Furthermore, a small amount of carbide was formed because of holding at the same temperature for 300 seconds (holding time [9]). As a result, ⁇ decreased.
  • Sample No. 10 exhibited reduced amount of retained ⁇ and insufficient amount of retained austenite having a size of 1.5 ⁇ m or more since the cooling stopping temperature [5] was as low as 20°C. As a result, sufficient value of TS ⁇ EL and sufficient deep drawability could not be obtained.
  • Sample No. 12 exhibited excessive amount of ferrite and insufficient total amount of tempered martensite and tempered bainite, thus failing to obtain sufficient tensile strength and yield ratio since the rapid cooling starting temperature [4] was as low as 580°C.
  • Sample No. 14 exhibited excessive amount of ferrite, insufficient total amount of tempered martensite and tempered bainite, and excessive average size of MA since the cooling rate [4] was as low as 8°C/sec. As a result, sufficient tensile strength and yield ratio could not be obtained.
  • Sample No. 19 exhibited the parameter increased to 14,604 since the reheating temperature [7] was as high as 550°C. Therefore, the amount of retained ⁇ decreased and the amount of retained austenite having a size of 1.5 ⁇ m or more was insufficient. As a result, TS ⁇ EL and deep drawability were degraded. Because of large scattering intensity of X-ray small angle scattering, it can be said that cementite having a size of about 1 nm has a large volume fraction. As a result, collision resistance properties (thickness reduction ratio) were degraded.
  • Sample No. 20 exhibited the parameter decreased to 9,280 since the reheating temperature [8] was as low as 250°C. Therefore, the sample exhibited insufficient carbon diffusion, decreased amount of retained ⁇ , and insufficient amount of retained austenite having a size of 1.5 ⁇ m or more. As a result, TS ⁇ EL and deep drawability were degraded.
  • Sample No. 22 exhibited small C amount, insufficient amount of retained austenite and insufficient amount of retained austenite having a size of 1.5 ⁇ m or more, thus failing to obtain sufficient TS ⁇ EL and deep drawability.
  • Sample No. 23 exhibited large Mn amount and insufficient amount of retained austenite, thus failing to obtain sufficient TS ⁇ EL.
  • Sample No. 24 exhibited small Mn amount, excessive amount of ferrite, and insufficient total amount of tempered martensite and tempered bainite. As a result, sufficient tensile strength and yield ratio could not be obtained.
  • Sample No. 25 exhibited small amount of Si + Al, insufficient total amount of tempered martensite and tempered bainite, small amount of retained austenite, excessive average size of MA, and excessive average size of retained austenite. As a result, sufficient TS ⁇ EL, hole expansion ratio and deep drawability could not be obtained.
  • Sample No. 26 exhibited excessive C amount, thus failing to obtain sufficient SW cross tensile strength since retention was not performed at a temperature in a range of 300°C to 500°C after austenitizing.
  • Sample No. 27 exhibited excessive amount of Si + Al, thus failing to obtain sufficient TS ⁇ EL.
  • the manufacturing method according to the embodiment of the present invention enables the production of the steel sheet that satisfies the composition and the steel structure defined in the embodiment of the present invention.

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