WO2013061543A1 - High-tension hot-rolled steel plate and method for manufacturing same - Google Patents

High-tension hot-rolled steel plate and method for manufacturing same Download PDF

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Publication number
WO2013061543A1
WO2013061543A1 PCT/JP2012/006645 JP2012006645W WO2013061543A1 WO 2013061543 A1 WO2013061543 A1 WO 2013061543A1 JP 2012006645 W JP2012006645 W JP 2012006645W WO 2013061543 A1 WO2013061543 A1 WO 2013061543A1
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steel sheet
rolled steel
hot
mass
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PCT/JP2012/006645
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French (fr)
Japanese (ja)
Inventor
珠子 有賀
船川　義正
永明 森安
貴幸 村田
浩 大和田
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Ｊｆｅスチール株式会社
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Priority to JP2013540638A priority Critical patent/JP5610089B2/en
Publication of WO2013061543A1 publication Critical patent/WO2013061543A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—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
    • C21D8/0463—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 following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—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
    • C21D8/0478—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 involving a particular surface treatment
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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

Definitions

  • the present invention relates to a high-strength hot-rolled steel sheet suitable for transport materials such as automobile parts and structural materials, and a production method thereof (high-strength-hot-rolled-steel-sheet-and-method-for-producing-the-same).
  • undercarriage parts have a complicated shape
  • steel sheets as materials for undercarriage parts are regarded as important in workability as well as strength
  • high-tensile steel sheets with excellent workability such as elongation and stretch flangeability are used. It has been demanded.
  • Patent Document 1 discloses that by weight, C: 0.03-0.25%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.007% or less, Al: 0.07% or less, and Cr : A composition containing 1.0% or less, a composite structure composed of ferrite and the second phase, and by specifying the hardness, volume ratio, and particle size of the second phase, the tensile strength (TS) is 490 N / mm A technique for improving fatigue properties and stretch flangeability of high-strength hot-rolled steel sheets exceeding 2 (490 MPa) has been proposed.
  • the second phase is one or more of pearlite, bainite, martensite, and retained austenite.
  • Patent Document 2 wt%, C: 0.01 to 0.10%, Si: 1.5% or less, Mn: more than 1.0% to 2.5%, P: 0.15% or less, S: 0.008% or less, Al: 0.01 to
  • the chemical composition contains 0.08%, one or two of Ti and Nb: 0.10 to 0.60%, the ferrite content is 95% or more in area ratio, and the average crystal grain size of ferrite is 2.0 to 10.0 ⁇ m
  • TS tensile strength
  • Patent Document 3 in mass%, C: 0.01 to 0.1%, S ⁇ 0.03%, N ⁇ 0.005%, Ti: 0.05 to 0.5%, Si: 0.01 to 2%, Mn: 0.05 to 2% , P ⁇ 0.1%, Al: 0.005 to 1.0%, and a composition containing Ti in a range that satisfies Ti-48 / 12C-48 / 14N-48 / 32S ⁇ 0%, and the particle size in steel is 5nm or more by the minimum interval average size at 10 1 ⁇ 10 3 nm of precipitates containing the Ti or less 10 1 nm ultra 10 4 nm, tensile strength (TS) of the high-strength hot-rolled steel sheet is not less than 640MPa Techniques for improving Burring formability and fatigue properties have been proposed.
  • TS tensile strength
  • Patent Document 1 when a steel sheet is pressed to form a desired part shape, the interface between the soft ferrite and the hard second phase is the starting point for cracking during processing. It has the problem that it is easy and processability is not stable. Further, the technique proposed in Patent Document 1 also has a problem that when the tensile strength (TS) of the steel sheet is increased to 590 MPa class, the workability, particularly the stretch flangeability, is insufficient for the current requirements. (See the example of Patent Document 1).
  • TS tensile strength
  • the present invention advantageously solves the above-mentioned problems of the prior art, and is suitable as a material for automobile parts.
  • An object of the present invention is to provide a high-tensile hot-rolled steel sheet having a hole expansion ratio ⁇ : 100% or more and a method for producing the same.
  • the Ti content of the carbide containing Ti exceeds the C content by atomic ratio, the carbide is liable to be coarsened and adversely affect hot-rolled steel sheet characteristics.
  • the Ti, N, and S contents with respect to the C content in the steel used as the material are within a predetermined range. It is effective to control to (C / 12> Ti / 48-N / 14-S / 32).
  • the present invention has been completed based on the above findings, and the gist thereof is as follows.
  • ⁇ By mass% C: 0.010% or more and 0.050% or less, Si: 0.2% or less, Mn: 0.1% to 0.8%, P: 0.025% or less, S: 0.01% or less, N: 0.01% or less, Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, containing S, N, and Ti so that the following formula (1) is satisfied, with the balance consisting of Fe and inevitable impurities, and the ferrite phase High tensile hot rolling with a matrix having an area ratio of 95% or more with respect to the entire structure and a structure in which fine carbides containing Ti and having an average particle diameter of less than 10 nm are dispersed and having a tensile strength of 590 MPa or more. steel sheet.
  • Ti ⁇ 0.04+ (N / 14 ⁇ 48 + S / 32 ⁇ 48) (1) (S, N, Ti: content of each element
  • a high-tensile-strength hot-rolled steel sheet that contains B: 0.0035% or less in mass% so as to satisfy the following formula (2) in [1].
  • B ⁇ 0.0003-0.00025Mn (2) (Mn, B: content of each element (mass%))
  • Mn, B content of each element (mass%)
  • the steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling is cooled, wound, and hot-rolled steel sheet.
  • the steel material in mass%, C: 0.010% or more and 0.050% or less, Si: 0.2% or less, Mn: 0.1% to 0.8%, P: 0.025% or less, S: 0.01% or less, N: 0.01% or less, Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, S, N, and Ti are contained so as to satisfy the following formula (1), and the balance is composed of Fe and inevitable impurities,
  • the final rolling temperature of the finish rolling is 880 ° C.
  • a method for producing a hot-rolled steel sheet Ti ⁇ 0.04+ (N / 14 ⁇ 48 + S / 32 ⁇ 48) (1) (S, N, Ti: content of each element (mass%))
  • [12] A method for producing a high-tensile hot-rolled steel sheet, wherein the composition in [9] or [10] satisfies the following formula (3): C / 12> Ti / 48-N / 14-S / 32 (3) (C, S, N, Ti: content of each element (mass%))
  • the hot-rolled steel sheet of the present invention has a matrix in which the ferrite phase is 95% or more in area ratio with respect to the entire structure, and a structure in which fine carbides containing Ti and having an average particle diameter of less than 10 nm are dispersed and precipitated in the matrix.
  • Ferrite phase 95% or more in area ratio with respect to the entire structure
  • formation of a ferrite phase is essential to ensure the workability (stretch flangeability) of a hot-rolled steel sheet.
  • it is effective to make the structure of the hot-rolled steel sheet a ferrite phase having a low dislocation density and excellent ductility.
  • the structure of the hot-rolled steel sheet is preferably a ferrite single-phase structure, but even if it is not a complete ferrite single-phase structure, it is substantially a ferrite single-phase structure, that is, If the area ratio with respect to the entire structure is 95% or more of the ferrite phase, the above effect is sufficiently exhibited. Therefore, the area ratio of the ferrite phase to the entire structure is 95% or more. Preferably, it is 97% or more.
  • examples of the structure other than the ferrite phase include cementite, pearlite, bainite phase, martensite phase, retained austenite phase, and the like. However, it is acceptable if it is preferably about 3% or less.
  • Fine carbides containing Ti tend to be fine carbides with an extremely small average particle size. For this reason, in the present invention for increasing the strength of a hot-rolled steel sheet by dispersing and precipitating fine carbides in the hot-rolled steel sheet, fine carbides containing Ti are used as the fine carbides to be dispersed and precipitated.
  • Average particle diameter of fine carbide Less than 10 nm
  • the average particle diameter of fine carbide is extremely important for imparting desired strength (tensile strength: 590 MPa or more) to a hot-rolled steel sheet.
  • fine carbide containing Ti is used.
  • the average particle size of is less than 10 nm.
  • the fine carbide acts as a resistance to dislocation movement that occurs when deformation is applied to the steel sheet, strengthening the hot-rolled steel sheet, but the average particle diameter of the fine carbide is 10 nm. If it is less than the above, the above action becomes even more remarkable. Therefore, the average particle diameter of the fine carbide containing Ti is set to less than 10 nm. More preferably, it is 5 nm or less.
  • the volume ratio with respect to the entire structure is 0.0005 or more. However, if the volume ratio exceeds 0.003, the strength becomes too high and the stretch flangeability may be deteriorated. Therefore, the volume ratio is preferably 0.0005 or more and 0.003 or less.
  • precipitation form of fine carbide containing Ti in addition to the row precipitation that is the main precipitation form, even if there is a mixture of randomly precipitated fine carbide, without affecting the characteristics at all, Regardless of the form of precipitation, various precipitation forms are collectively referred to as dispersion precipitation.
  • C 0.010% or more and 0.050% or less
  • C is an essential element for forming fine carbides and strengthening the hot-rolled steel sheet. If the C content is less than 0.010%, a tensile strength of 590 MPa or more cannot be obtained. On the other hand, when the C content exceeds 0.050%, the strength increases and pearlite is easily formed in the steel sheet, and it is difficult to obtain excellent stretch flangeability. Therefore, the C content is 0.010% or more and 0.050% or less. Preferably they are 0.020% or more and 0.035% or less. More preferably, it is 0.020% or more and 0.030% or less.
  • Si 0.2% or less Si is a solid solution strengthening element and is an element effective for increasing the strength of steel.
  • the Si content is 0.2% or less.
  • it is 0.05% or less.
  • it is preferably 0.005% or more for strengthening the solid solution.
  • Mn 0.1% or more and 0.8% or less Since Mn is a solid solution strengthening element and is an element effective for increasing the strength of steel, it is desirable to increase the Mn content from the viewpoint of strengthening the hot-rolled steel sheet. If the Mn content is less than 0.1%, solid solution strengthening cannot be obtained. On the other hand, if the Mn content is less than 0.1%, the Ar 3 transformation point becomes too high, and the fine carbide containing Ti tends to become coarse as will be described later. On the other hand, when the Mn content exceeds 0.8%, segregation is likely to occur, and a phase other than the ferrite phase, that is, a hard phase is formed, and stretch flangeability is deteriorated. Therefore, the Mn content is 0.1% or more and 0.8% or less. Preferably they are 0.1% or more and 0.5% or less. More preferably, it is 0.1% or more and 0.45% or less.
  • P 0.025% or less
  • P is a solid solution strengthening element and is an element effective for increasing the strength of steel.
  • the P content is 0.025% or less.
  • it is 0.02% or less. Further, it is preferably 0.005% or more for strengthening the solid solution.
  • S 0.01% or less S is an element that lowers hot workability (hot rollability), increases the hot cracking susceptibility of slabs, and is present as MnS in steel and stretch flangeability of hot rolled steel sheets. Deteriorate. Therefore, in the present invention, it is preferable to reduce S as much as possible, and set it to 0.01% or less. Preferably it is 0.005% or less.
  • N 0.01% or less N is a harmful element in the present invention and is preferably reduced as much as possible.
  • the N content is 0.01% or less.
  • it is 0.006% or less.
  • Al 0.06% or less
  • Al is an element that acts as a deoxidizer. In order to acquire such an effect, it is desirable to contain 0.001% or more, but inclusion exceeding 0.06% reduces elongation and stretch flangeability. For this reason, Al content shall be Al: 0.06% or less.
  • Ti 0.05% or more and 0.10% or less Ti is the most important element in the present invention. Ti is an element that contributes to increasing the strength of the steel sheet while maintaining excellent stretch flangeability by forming carbides. If the Ti content is less than 0.05%, the desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more) cannot be ensured. On the other hand, when the Ti content exceeds 0.10%, the stretch flangeability tends to decrease. Therefore, the Ti content is 0.05% or more and 0.10% or less. Preferably they are 0.065% or more and 0.095% or less.
  • the hot-rolled steel sheet of the present invention contains S, N, and Ti in the above-described range and satisfying the expression (1).
  • Ti ⁇ 0.04+ (N / 14 ⁇ 48 + S / 32 ⁇ 48) (1)
  • the above formula (1) is a requirement to be satisfied in order to bring the fine carbide containing Ti into the above-described desired precipitation state, and is an extremely important index in the present invention.
  • the amount of Ti that becomes the precipitation nuclei of fine carbides decreases with the precipitation of the nitrides and sulfides, and the fine content containing Ti is reduced. It becomes difficult to sufficiently precipitate the carbide.
  • the Ti, N and S contents are controlled so as to satisfy the formula (1) Ti ⁇ 0.04+ (N / 14 ⁇ 48 + S / 32 ⁇ 48).
  • Ti ⁇ 0.04+ (N / 14 ⁇ 48 + S / 32 ⁇ 48) As a result, a sufficient amount of Ti serving as a nucleus for precipitation of fine carbide can be secured, and the fine carbide can be stably precipitated with an average particle size of less than 10 nm.
  • the steel material is heated to the austenite region before hot rolling to dissolve carbides in the steel material, and carbides containing Ti are precipitated simultaneously with the subsequent austenite ⁇ ferrite transformation.
  • the austenite ⁇ ferrite transformation temperature is high, the precipitated carbide containing Ti becomes coarse. Therefore, in the present invention, it is preferable to precipitate the carbide containing Ti at the time of winding by adjusting the temperature of the austenite ⁇ ferrite transformation (Ar 3 transformation point) to the winding temperature range. Thereby, the coarsening can be suppressed, and a carbide having an average particle diameter of less than 10 nm can be obtained.
  • B 0.0035% or less is contained so as to satisfy the following formula (2) It is preferable to do.
  • B 0.0035% or less
  • B is an element that lowers the Ar 3 transformation point of steel.
  • the carbide containing Ti is refined. be able to.
  • the B content is preferably 0.0003% or more.
  • the above effect is saturated even if the content exceeds 0.0035%. Therefore, the B content is preferably 0.0035% or less. More preferably, it is 0.0003% or more and 0.0020% or less.
  • B when B is contained, it is also important to control the ratio of the B content and the Mn content in the steel within an appropriate range.
  • the present inventors studied a means for finely dispersing (including an average particle diameter of less than 10 nm) a carbide containing Ti in a matrix having an area ratio of 95% or more with respect to the entire structure of the ferrite phase. As a result, adjusting the temperature of the austenite ⁇ ferrite transformation (Ar 3 transformation point) in the hot rolling process to the coiling temperature range described later refines the carbide containing Ti to an average particle size of less than 10 nm. It was newly discovered that it is a very effective means.
  • the content of Mn which is a solid solution strengthening element
  • a desired steel plate strength (tensile strength: 590 MPa or more) can be ensured without using the effect of B described above. it can.
  • the Mn content is 0.35% or less, it may be difficult to secure a desired steel sheet strength without using the above-described effect of B. Therefore, when the Mn content is 0.35% or less, it is preferable to contain B for the purpose of making the carbide containing Ti finer.
  • C, S, N, and Ti it is preferable to adjust the contents of C, S, N, and Ti so as to satisfy the expression (3) within the above range.
  • the carbide containing Ti has a strong tendency to become a fine carbide having an extremely small average particle diameter.
  • the Ti bonded to C becomes C or more in atomic ratio, the carbide is easily coarsened. And it becomes difficult to ensure desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more) with the coarsening of carbides.
  • the present invention it is preferable to define the contents of C, Ti, N, and S as shown in formula (3). That is, in the present invention, for C and Ti contained in the steel material, the atomic% of C (C / 12) is more than the atomic% of Ti that can contribute to carbide formation (Ti / 48-N / 14-S / 32). It is preferable to increase the amount. Thereby, the coarsening of the fine carbide containing Ti can be suppressed.
  • one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, and V are combined in a total of 0.1% or less, preferably 0.03. % Or less.
  • Components other than the above are Fe and inevitable impurities.
  • the steel sheet of the present invention may have a plating film on the surface.
  • a plating film By forming a plating film on the surface of the steel sheet, the corrosion resistance of the hot-rolled steel sheet is improved, and a hot-rolled steel sheet suitable for a part exposed to a severe corrosive environment, for example, a material for an undercarriage part of an automobile is obtained.
  • the plating film include a hot dip galvanizing film and an alloyed hot dip galvanizing film.
  • hot rolling consisting of rough rolling and finish rolling is performed on a steel material, and after finishing rolling, the steel material is cooled and wound to obtain a hot rolled steel sheet.
  • the finish rolling temperature of the finish rolling is 880 ° C. or higher
  • the average cooling rate of the cooling is 10 ° C./s or higher
  • the winding temperature of the winding is 550 ° C. or higher and lower than 800 ° C. To do.
  • the melting method of the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Moreover, after melting, it is preferable to use a slab (steel material) by a continuous casting method because of problems such as segregation, but a slab can also be formed by a known casting method such as ingot-bundling rolling or thin slab continuous casting. good. In addition, when hot-rolling the slab after casting, the slab may be rolled after being reheated in a heating furnace, and when the temperature is maintained at a predetermined temperature or higher, direct rolling without heating the slab You may do it.
  • the steel material obtained as described above is subjected to rough rolling and finish rolling.
  • the heating temperature of the steel material is preferably set to 1150 ° C. or higher. However, if the heating temperature of the steel material is excessively high, the surface is excessively oxidized and TiO 2 is generated and Ti is consumed. Is preferably 1300 ° C. or lower.
  • the step of heating the steel material before rough rolling is It can be omitted.
  • the rough rolling conditions are not particularly limited.
  • Finishing rolling temperature 880 ° C. or more Optimization of the finishing rolling temperature is important for securing elongation and stretch flangeability of the hot-rolled steel sheet and reducing the rolling load of finishing rolling.
  • the finish rolling temperature is 880 ° C. or higher.
  • it is 900 degreeC or more. If the finish rolling temperature becomes excessively high, the crystal grains become coarse and adversely affect the securing of the desired steel sheet strength (tensile strength: 590 MPa or more). Therefore, the finish rolling temperature is preferably 1000 ° C. or less.
  • the average cooling rate 10 ° C / s or higher If the average cooling rate from 880 ° C or higher to the coiling temperature is less than 10 ° C / s after finishing rolling, the Ar 3 transformation point increases and carbides containing Ti Is not sufficiently refined. Therefore, the average cooling rate is 10 ° C./s or more. Preferably it is 30 ° C./s or more. Further, it is preferably less than 200 ° C./s in order to obtain a ferrite structure.
  • Winding temperature 550 ° C or higher and less than 800 ° C This is extremely important in forming a structure in which fine matrixes containing Ti and an average particle diameter of less than 10 nm are dispersed and precipitated.
  • the coiling temperature is set to 550 ° C. or higher and lower than 800 ° C.
  • they are 550 degreeC or more and less than 700 degreeC, More preferably, they are 580 degreeC or more and less than 700 degreeC.
  • high-tensile hot-rolled steel sheet with excellent workability that can be applied as a material for undercarriage parts with a tensile strength (TS) of 590 MPa or more and a complicated cross-sectional shape.
  • TS tensile strength
  • Ti Ti ⁇ 0.04+ (N / 14 ⁇ 48 + S / 32 ⁇ 48)) more than a predetermined amount with respect to the N and S content in the steel that is the material of the hot-rolled steel sheet.
  • fine carbides with an average particle size of less than 10 nm can be obtained by adding the B and Mn contents in the steel used as the material of the hot-rolled steel sheet so as to satisfy a predetermined relationship (B ⁇ 0.0003-0.00025Mn). Is controlled so as to sufficiently disperse and precipitate.
  • the present invention it is possible to disperse and precipitate fine carbides having an average particle diameter of less than 10 nm over the entire width direction without strictly specifying the production conditions of the hot-rolled steel sheet. Uniform and good characteristics (tensile strength, stretch flangeability) are imparted over the entire direction.
  • the hot-rolled steel sheet manufactured as described above may be plated to form a plating film on the surface of the steel sheet.
  • a hot dip galvanizing process may be performed as a plating process to form a hot dip galvanized film, or an alloyed hot dip galvanized film may be formed on the surface of the steel sheet by further alloying after the hot dip galvanizing process. .
  • Molten steel having the composition shown in Table 1 was melted and continuously cast by a generally known technique to obtain a slab (steel material) having a thickness of 250 mm. These slabs are heated to 1250 ° C, roughly rolled, and subjected to finish rolling at the finish rolling temperature shown in Table 2. After finishing rolling, the temperature range from 880 ° C to the coiling temperature is shown in Table 2. The steel sheet was cooled at the average cooling rate shown and wound at the winding temperature shown in Table 2 to obtain a hot rolled steel sheet having a plate thickness of 2.3 mm.
  • hot-rolled steel sheets (hot-rolling numbers a2, b2, and c2) are immersed in a 480 ° C zinc plating bath (0.1% Al-Zn) and molten zinc with an adhesion amount of 45 g / m 2 per side. After forming the plating film, alloying treatment was performed at 520 ° C. to obtain an alloyed hot-dip galvanized steel sheet.
  • Samples are collected from the hot-rolled steel sheet obtained as described above, microstructure observation, tensile test, hole expansion test, ferrite phase area ratio, average particle diameter and volume ratio of fine carbide containing Ti, tensile strength, The hole expansion rate (stretch flangeability) was determined.
  • the test method was as follows.
  • the particle size of fine carbide containing Ti is calculated based on the observation results of 30 fields of view at 260,000 times, and the individual particle size is obtained by image processing using circular approximation, and the obtained particle size is arithmetically averaged to obtain the average particle size.
  • the diameter The volume ratio of fine carbide containing Ti is the weight of Ti carbide based on the extraction residue analysis of the residue collected by electrolysis of the ground iron using 10% acetylacetone-1% tetramethylammonium chloride-methanol solution (AA solution). The volume was determined by dividing this by the density of Ti carbide (TiC), and this volume was divided by the volume of the dissolved iron.
  • Each of the examples of the present invention is a hot-rolled steel sheet having both a high strength of tensile strength TS: 590 MPa or more and an excellent stretch flangeability of a hole expansion ratio ⁇ : 100% or more.
  • a predetermined high strength cannot be ensured or the hole expansion rate ⁇ cannot be ensured.

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Abstract

A high-tension hot-rolled steel plate having: a composition comprising, in mass%, 0.010% to 0.050% of C, 0.2% or less of S, 0.1% to 0.8% of Mn, 0.025% or less of P, 0.01% or less of S, 0.01% or less of N, 0.06% or less of Al, and 0.05% to 0.10% of Ti, so that S, N, and Ti satisfy Ti ≥ 0.04 + (N/14 × 48 + S/32 × 48), with the remainder being Fe and unavoidable impurities; a matrix in which the area ratio of the ferrite phase is equal to or greater than 95% relative to the entire structure; and a structure in which a fine carbide which contains Ti and in which the average particle diameter is less than 10 nm is dispersed and precipitated.

Description

高張力熱延鋼板およびその製造方法High-tensile hot-rolled steel sheet and manufacturing method thereof
　本発明は、自動車用部品等の輸送機材、構造材の素材に好適な、高張力熱延鋼板およびその製造方法(high strength hot rolled steel sheet and method for producing the same)に関する。 The present invention relates to a high-strength hot-rolled steel sheet suitable for transport materials such as automobile parts and structural materials, and a production method thereof (high-strength-hot-rolled-steel-sheet-and-method-for-producing-the-same).
地球環境保全の観点からCO2排出量を削減すべく、自動車車体の強度を維持しつつその軽量化を図り、自動車の燃費を改善することが、自動車業界においては常に重要な課題とされている。自動車車体の強度を維持しつつ車体の軽量化を図るうえでは、自動車部品用素材となる鋼板の高強度化により、鋼板を薄肉化することが有効である。例えば、自動車の足回り部品用鋼板の高強度薄肉化は、自動車車体の大幅な軽量化につながるため、自動車燃費向上に極めて有効な手段である。そのため、これらの部品用素材に対する高強度化の要望は非常に強い。 In order to reduce CO 2 emissions from the viewpoint of global environmental conservation, maintaining the strength of the car body while reducing its weight and improving the fuel efficiency of the car has always been an important issue in the automobile industry. . In order to reduce the weight of the vehicle body while maintaining the strength of the automobile body, it is effective to reduce the thickness of the steel sheet by increasing the strength of the steel sheet used as a material for automobile parts. For example, the reduction in the strength and thickness of steel plates for undercar parts of automobiles is a very effective means for improving automobile fuel consumption because it leads to a significant reduction in weight of the automobile body. Therefore, there is a strong demand for increasing the strength of these component materials.
一方、鋼板を素材とする自動車部品の多くは、プレス加工やバーリング加工等によって成形されるため、自動車部品用鋼板には優れた伸びおよび伸びフランジ性(stretch-flange formability)を有することが要求される。例えば、足回り部品は複雑な形状を有することから、足回り部品用素材としての鋼板では、強度とともに加工性が重要視され、伸び、および伸びフランジ性等の加工性に優れた高張力鋼板が求められている。 On the other hand, since many automotive parts made of steel plates are formed by pressing, burring, etc., steel plates for automobile parts are required to have excellent elongation and stretch-flange formability. The For example, undercarriage parts have a complicated shape, steel sheets as materials for undercarriage parts are regarded as important in workability as well as strength, and high-tensile steel sheets with excellent workability such as elongation and stretch flangeability are used. It has been demanded.
しかしながら、一般的に鉄鋼材料は高強度化に伴い加工性が低下する。そのため、高張力熱延鋼板を足回り部品等に適用するうえでは、強度と加工性を兼備した高張力熱延鋼板の開発が必須となり、現在までに多くの研究が為され、様々な技術が提案されている。 However, in general, the workability of steel materials decreases with increasing strength. Therefore, in order to apply high-tensile hot-rolled steel sheets to undercarriage parts, etc., it is essential to develop high-tensile hot-rolled steel sheets that have both strength and workability. Proposed.
　例えば、特許文献１には、重量％で、C：0.03～0.25％、Si：2.0％以下、Mn：2.0％以下、P：0.1％以下、S：0.007％以下、Al：0.07％以下及びCr：1.0％以下を含有する組成とし、フェライトと第２相からなる複合組織とし、第２相の硬さ、体積率、および粒径を規定することにより、引張強さ(TS)が490N/mm2(490MPa)を超える高強度熱延鋼板の疲労特性(Fatigue property)と伸びフランジ性を向上させる技術が提案されている。前記第２相は、パーライト、ベイナイト、マルテンサイト、残留オーステナイト(retained austenite)の１種以上である。 For example, Patent Document 1 discloses that by weight, C: 0.03-0.25%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.007% or less, Al: 0.07% or less, and Cr : A composition containing 1.0% or less, a composite structure composed of ferrite and the second phase, and by specifying the hardness, volume ratio, and particle size of the second phase, the tensile strength (TS) is 490 N / mm A technique for improving fatigue properties and stretch flangeability of high-strength hot-rolled steel sheets exceeding 2 (490 MPa) has been proposed. The second phase is one or more of pearlite, bainite, martensite, and retained austenite.
　また、特許文献２には、wt％で、C：0.01～0.10％、Si：1.5％以下、Mn：1.0％超～2.5％、P：0.15％以下、S：0.008％以下、Al：0.01～0.08％、Ti，Nbの１種又は２種の合計：0.10～0.60％を含む化学成分とし、フェライト量が面積率で95％以上であり、かつフェライトの平均結晶粒径が2.0～10.0μm であり、マルテンサイトおよび残留オーステナイトを含まない組織とすることにより、引張強さ(TS)が490MPa以上である高強度熱延鋼板の疲労強度、特に伸びフランジ性を向上させる技術が提案されている。そして、特許文献２で提案された技術では、Mn含有量を1.0％超～2.5％とすることにより、鋼板強度が向上するとともに微細フェライト粒が得られるとされている。 In Patent Document 2, wt%, C: 0.01 to 0.10%, Si: 1.5% or less, Mn: more than 1.0% to 2.5%, P: 0.15% or less, S: 0.008% or less, Al: 0.01 to The chemical composition contains 0.08%, one or two of Ti and Nb: 0.10 to 0.60%, the ferrite content is 95% or more in area ratio, and the average crystal grain size of ferrite is 2.0 to 10.0μm There has been proposed a technique for improving the fatigue strength, particularly the stretch flangeability of a high-strength hot-rolled steel sheet having a tensile strength (TS) of 490 MPa or more by making the structure free from martensite and retained austenite. In the technique proposed in Patent Document 2, the strength of the steel sheet is improved and fine ferrite grains are obtained by setting the Mn content to more than 1.0% to 2.5%.
また、特許文献３には、質量％にて、C：0.01～0.1％、S≦0.03％、N≦0.005％、Ti：0.05～0.5％、Si：0.01～2％、Mn：0.05～2％、P≦0.1％、Al：0.005～1.0％を含み、さらにTi－48/12C－48/14N－48/32S≧0％を満たす範囲でTiを含有する組成とし、鋼中の粒子で5nm以上のTiを含む析出物の平均サイズを101～103nmで最小間隔を101nm超104 nm以下とすることにより、引張強さ(TS)が640MPa以上である高強度熱延鋼板のバーリング加工性(Burring formability)と疲労特性を向上させる技術が提案されている。 In Patent Document 3, in mass%, C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, Ti: 0.05 to 0.5%, Si: 0.01 to 2%, Mn: 0.05 to 2% , P ≤ 0.1%, Al: 0.005 to 1.0%, and a composition containing Ti in a range that satisfies Ti-48 / 12C-48 / 14N-48 / 32S ≥ 0%, and the particle size in steel is 5nm or more by the minimum interval average size at 10 1 ~ 10 3 nm of precipitates containing the Ti or less 10 1 nm ultra 10 4 nm, tensile strength (TS) of the high-strength hot-rolled steel sheet is not less than 640MPa Techniques for improving Burring formability and fatigue properties have been proposed.
特開平４－３２９８４８号公報JP-A-4-329848 特開２０００－３２８１８６号公報JP 2000-328186 A 特開２００２－１６１３４０号公報JP 2002-161340 A
　しかしながら、特許文献１で提案された技術では、鋼板にプレス加工等を施して所望の部品形状に成形する際、軟質のフェライトと硬質の第２相との界面が、加工時の割れ発生起点となり易く、加工性が安定しないという問題を有する。また、特許文献１で提案された技術では、鋼板の引張強さ(TS)を590MPa級に高めた場合、加工性、特に伸びフランジ性が現状の要求に対して不十分であるという問題も見られる（特許文献１の実施例参照）。 However, in the technique proposed in Patent Document 1, when a steel sheet is pressed to form a desired part shape, the interface between the soft ferrite and the hard second phase is the starting point for cracking during processing. It has the problem that it is easy and processability is not stable. Further, the technique proposed in Patent Document 1 also has a problem that when the tensile strength (TS) of the steel sheet is increased to 590 MPa class, the workability, particularly the stretch flangeability, is insufficient for the current requirements. (See the example of Patent Document 1).
また、特許文献２で提案された技術では、鋼板のMn含有量が高いため、鋼板の板厚中央部にMnが偏析し、鋼板のプレス成形の際、加工時に割れを誘発するので、優れた伸びフランジ性を安定的に確保することが困難であり、必ずしも十分な伸びフランジ性を得ることができない。また、特許文献２で提案された技術では、Tiを所定含有量としてTi炭化物を形成することにより、伸びフランジ性に悪影響を及ぼす固溶Cの低減化を図っているが、Cに対して過剰のTiを含有させると、Ti炭化物が粗大化し易くなり、所望の強度を安定的に得られないという問題が見られる。 Further, in the technique proposed in Patent Document 2, since the Mn content of the steel sheet is high, Mn segregates in the central part of the thickness of the steel sheet, and when the steel sheet is press-formed, cracks are induced during processing, which is excellent. It is difficult to stably secure stretch flangeability, and sufficient stretch flangeability cannot always be obtained. In the technique proposed in Patent Document 2, Ti carbide is formed with Ti as a predetermined content to reduce solid solution C that adversely affects stretch flangeability. When Ti is contained, Ti carbide tends to be coarsened, and there is a problem that a desired strength cannot be stably obtained.
また、特許文献３で提案された技術では、鋼板に含まれる析出物のサイズの分布が大きく、所望の強度を安定的に確保することができないという問題が見られる。また、特許文献３で提案された技術では、鋼板の伸びフランジ性が不十分である（特許文献３の実施例参照）。 Moreover, in the technique proposed by patent document 3, the size distribution of the precipitate contained in a steel plate is large, and the problem that desired intensity | strength cannot be ensured stably is seen. Moreover, with the technique proposed by patent document 3, the stretch flangeability of a steel plate is inadequate (refer the Example of patent document 3).
大量生産される自動車部品に対しては、その素材を安定的に供給すべく熱延鋼板を工業的に大量生産する必要があるが、上記した従来技術では、引張強さ(TS)が590MPa以上であり且つ優れた加工性（伸びフランジ性）を有する高張力熱延鋼板を、安定的に供給することが困難である。本発明は、上記した従来技術が抱える問題を有利に解決し、自動車部品用の素材として好適な、引張強さ(TS)：590MPa以上であり且つ優れた加工性（伸びフランジ性）、具体的には穴拡げ率(Hole expansion ratio)λ：100％以上を有する高張力熱延鋼板およびその製造方法を提供することを目的とする。 For automobile parts that are mass-produced, it is necessary to industrially mass-produce hot-rolled steel sheets in order to stably supply the materials, but the above-mentioned conventional technology has a tensile strength (TS) of 590 MPa or more. In addition, it is difficult to stably supply a high-tensile hot-rolled steel sheet having excellent workability (stretch flangeability). The present invention advantageously solves the above-mentioned problems of the prior art, and is suitable as a material for automobile parts. Tensile strength (TS): 590 MPa or more and excellent workability (stretch flangeability), specifically An object of the present invention is to provide a high-tensile hot-rolled steel sheet having a hole expansion ratio λ: 100% or more and a method for producing the same.
　上記課題を解決すべく、本発明者らは、熱延鋼板の高強度化と加工性（伸びフランジ性）に及ぼす各種要因について鋭意検討した。その結果、以下のような知見を得た。
１）鋼板組織を転位密度が低い加工性に優れたフェライト単相組織とし、更に、微細炭化物を分散析出させて析出強化すると、熱延鋼板の伸びフランジ性を維持したまま、強度が向上すること。
２）加工性に優れ且つ引張強さ(TS)：590MPa以上の高強度を有する熱延鋼板を得るためには、析出強化に有効な平均粒子径が10nm未満である微細炭化物を十分に分散析出させる必要があること。
３）析出強化に寄与する微細炭化物としては、強度確保等の観点からは、Tiを含む炭化物が有効であること。 In order to solve the above-mentioned problems, the present inventors diligently studied various factors affecting the high strength and workability (stretch flangeability) of a hot-rolled steel sheet. As a result, the following findings were obtained.
1) When the steel sheet structure is a ferrite single-phase structure with low dislocation density and excellent workability, and fine carbides are dispersed and precipitated to strengthen the precipitation, the strength is improved while maintaining the stretch flangeability of the hot-rolled steel sheet. .
2) To obtain hot-rolled steel sheets with excellent workability and high tensile strength (TS): 590 MPa or more, fine carbides with an average particle diameter of less than 10 nm effective for precipitation strengthening are sufficiently dispersed and precipitated. That you need to do.
3) As fine carbides contributing to precipitation strengthening, carbides containing Ti are effective from the viewpoint of ensuring strength and the like.
４）Tiを含む炭化物を平均粒子径：10nm未満とし、且つ590MPa以上の引張強さが得られるよう十分に分散析出させるためには、析出核となるTi炭化物を形成するTi量を確保する必要があり、素材となる鋼中のN,S含有量に対して所定量以上のTi（Ti ≧ 0.04＋(N/14×48＋S/32×48)）を含有させる必要があること。
５）フェライト相中に、Tiを含む炭化物を微細（平均粒子径：10nm未満）に析出させるうえでは、素材となる鋼中のB含有量とMn含有量とを所望の比率（B ≧ 0.0003－0.00025Mn）に制御することが有効であること。
６）Tiを含む炭化物のTi含有量が、原子比でC含有量超となると、炭化物が粗大化し易くなり、熱延鋼板特性に悪影響を及ぼすこと。
７）Tiを含む炭化物のTi含有量を原子比でC含有量未満として炭化物の粗大化を抑制するうえでは、素材となる鋼中のC含有量に対するTi,N,S含有量を所定の範囲（C/12 ＞ Ti/48－N/14－S/32）に制御することが有効であること。 4) In order to disperse and precipitate carbide containing Ti with an average particle size of less than 10 nm and sufficient tensile strength of 590 MPa or more, it is necessary to secure the amount of Ti that forms Ti carbide as a precipitation nucleus. It is necessary to contain Ti (Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48)) more than a predetermined amount with respect to the N and S content in the steel material.
5) In order to precipitate carbide containing Ti finely in the ferrite phase (average particle size: less than 10 nm), the desired ratio of B content and Mn content in the raw steel (B ≧ 0.0003- It is effective to control to 0.00025Mn).
6) When the Ti content of the carbide containing Ti exceeds the C content by atomic ratio, the carbide is liable to be coarsened and adversely affect hot-rolled steel sheet characteristics.
7) In order to suppress the coarsening of carbides by setting the Ti content of carbides containing Ti to less than the C content by atomic ratio, the Ti, N, and S contents with respect to the C content in the steel used as the material are within a predetermined range. It is effective to control to (C / 12> Ti / 48-N / 14-S / 32).
　本発明は上記の知見に基づき完成されたものであり、その要旨は次のとおりである。
　　　　・　質量％で、
C ：0.010％以上0.050％以下、    Si：0.2％以下、
Mn：0.1％以上0.8％以下、        P ：0.025％以下、
S ：0.01％以下、　　　　　　　   N ：0.01％以下、
Al：0.06％以下、               Ti：0.05％以上0.10％以下
を、S、N、およびTiが下記(1)式を満足するように含有し、残部がFeおよび不可避的不純物からなる組成と、フェライト相の組織全体に対する面積率が95％以上であるマトリックスと、Tiを含み平均粒子径が10nm未満である微細炭化物が分散析出した組織とを有し、引張強さが590MPa以上である高張力熱延鋼板。
　　　　　　　Ti ≧ 0.04＋(N/14×48＋S/32×48)　 ・・・ (1)
　　　　　　　　（S、N、Ti：各元素の含有量（質量％）） The present invention has been completed based on the above findings, and the gist thereof is as follows.
・ By mass%
C: 0.010% or more and 0.050% or less, Si: 0.2% or less,
Mn: 0.1% to 0.8%, P: 0.025% or less,
S: 0.01% or less, N: 0.01% or less,
Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, containing S, N, and Ti so that the following formula (1) is satisfied, with the balance consisting of Fe and inevitable impurities, and the ferrite phase High tensile hot rolling with a matrix having an area ratio of 95% or more with respect to the entire structure and a structure in which fine carbides containing Ti and having an average particle diameter of less than 10 nm are dispersed and having a tensile strength of 590 MPa or more. steel sheet.
Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
(S, N, Ti: content of each element (mass%))
［２］　前記［１］において、質量％でB ：0.0035％以下を、下記(2)式を満足するように含有する、高張力熱延鋼板。
　　　　　　　　　　B ≧ 0.0003－0.00025Mn ・・・ (2)
（Mn、B：各元素の含有量（質量％））
［３］前記Bが、0.0003％以上、0.0020％以下である、［２］に記載の高張力熱延鋼板。 [2] A high-tensile-strength hot-rolled steel sheet that contains B: 0.0035% or less in mass% so as to satisfy the following formula (2) in [1].
B ≧ 0.0003-0.00025Mn (2)
(Mn, B: content of each element (mass%))
[3] The high-tensile hot-rolled steel sheet according to [2], wherein B is 0.0003% or more and 0.0020% or less.
［４］　前記［１］または［２］において、前記組成が、下記(3)式を満足する、高張力熱延鋼板。
C/12 ＞ Ti/48－N/14－S/32 ・・・ (3) 
　　　　　　　（C、S、N、Ti：各元素の含有量（質量％）） [4] A high-tensile hot-rolled steel sheet according to [1] or [2], wherein the composition satisfies the following formula (3).
C / 12> Ti / 48-N / 14-S / 32 (3)
(C, S, N, Ti: content of each element (mass%))
［５］前記［１］または［２］において、前記微細炭化物の組織全体に対する体積比が0.0005以上である、高張力熱延鋼板。
［６］前記体積比が、0.0005以上、0.003以下である、［５］に記載の高張力熱延鋼板。 [5] The high-tensile hot-rolled steel sheet according to [1] or [2], wherein a volume ratio of the fine carbide to the entire structure is 0.0005 or more.
[6] The high-tensile hot-rolled steel sheet according to [5], wherein the volume ratio is 0.0005 or more and 0.003 or less.
［７］　前記［１］または［２］において、前記組成に加えてさらに、質量％で、Cu、Sn、Ni、Ca、Mg、Co、As、Cr、W、Nb、Pb、Ta、Mo、V、のいずれか１種以上を合計で0.1％以下含有する、高張力熱延鋼板。 [7] In the above [1] or [2], in addition to the composition, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, A high-strength hot-rolled steel sheet containing at least 0.1% of any one of V.
［８］　前記［１］または［２］において、鋼板表面にめっき皮膜を有する高張力熱延鋼板。 [8] The high-tensile hot-rolled steel sheet having a plating film on the steel sheet surface in [1] or [2].
［９］　鋼素材に、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻き取り、熱延鋼板とする高張力熱延鋼板の製造方法において、
前記鋼素材を、質量％で、
C ：0.010％以上0.050％以下、     Si：0.2％以下、
Mn：0.1％以上0.8％以下、        P ：0.025％以下、
S ：0.01％以下、　　　　　　　   N ：0.01％以下、
Al：0.06％以下、               Ti：0.05％以上0.10％以下
を、S、N、およびTiが下記(1)式を満足するように含有し、残部がFeおよび不可避的不純物からなる組成とし、
前記仕上げ圧延の仕上げ圧延温度を880℃以上とし、前記冷却の平均冷却速度を10℃/s以上とし、前記巻取り温度を550℃以上800℃未満とする、引張強さが590MPa以上の高張力熱延鋼板の製造方法。
　　　　　　　Ti ≧ 0.04＋(N/14×48＋S/32×48)　 ・・・ (1)
　　　　　　　　（S、N、Ti：各元素の含有量（質量％）） [9] In the method for producing a high-strength hot-rolled steel sheet, the steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling is cooled, wound, and hot-rolled steel sheet.
The steel material in mass%,
C: 0.010% or more and 0.050% or less, Si: 0.2% or less,
Mn: 0.1% to 0.8%, P: 0.025% or less,
S: 0.01% or less, N: 0.01% or less,
Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, S, N, and Ti are contained so as to satisfy the following formula (1), and the balance is composed of Fe and inevitable impurities,
The final rolling temperature of the finish rolling is 880 ° C. or higher, the average cooling rate of the cooling is 10 ° C./s or higher, the winding temperature is 550 ° C. or higher and lower than 800 ° C., and the tensile strength is 590 MPa or higher. A method for producing a hot-rolled steel sheet.
Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
(S, N, Ti: content of each element (mass%))
［１０］　前記［９］において、前記組成に加えてさらに、質量％でB ：0.0035％以下を、下記(2)式を満足するように含有する高張力熱延鋼板の製造方法。
　　　　　　　　　　B ≧ 0.0003－0.00025Mn ・・・ (2)
　　　　　　　　　（Mn、B：各元素の含有量（質量％））
［１１］前記Bが、0.0003％以上、0.0020％以下である、［１０］に記載の高張力熱延鋼板の製造方法。 [10] The method for producing a high-tensile hot-rolled steel sheet according to [9], further including B: 0.0035% or less by mass% in addition to the composition so as to satisfy the following expression (2).
B ≧ 0.0003-0.00025Mn (2)
(Mn, B: content of each element (mass%))
[11] The method for producing a high-tensile hot-rolled steel sheet according to [10], wherein B is 0.0003% or more and 0.0020% or less.
［１２］　前記［９］または［１０］において、前記組成が、下記(3)式を満足する高張力熱延鋼板の製造方法。
C/12 ＞ Ti/48－N/14－S/32 ・・・ (3) 
　　　　　　　（C、S、N、Ti：各元素の含有量（質量％）） [12] A method for producing a high-tensile hot-rolled steel sheet, wherein the composition in [9] or [10] satisfies the following formula (3):
C / 12> Ti / 48-N / 14-S / 32 (3)
(C, S, N, Ti: content of each element (mass%))
［１３］　前記［９］または［１０］において、前記組成に加えてさらに、質量％で、Cu、Sn、Ni、Ca、Mg、Co、As、Cr、W、Nb、Pb、Ta、Mo、Vのいずれか１種以上を合計で0.1％以下含有する高張力熱延鋼板の製造方法。 [13] In the above [9] or [10], in addition to the composition, in addition to mass, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, A method for producing a high-tensile hot-rolled steel sheet containing at least 0.1% of any one of V in total.
　本発明によれば、自動車用鋼板等に好適な、引張強さ(TS)：590MPa以上で、かつ、プレス時の断面形状が複雑な足回り部品等の素材として十分に適用可能な優れた加工性（伸びフランジ性）を有する高張力熱延鋼板を提供することが可能となり、産業上格段の効果を奏する。 According to the present invention, excellent processing that can be applied satisfactorily as a material for undercarriage parts, etc. suitable for automobile steel sheets and the like, having a tensile strength (TS) of 590 MPa or more and a complicated sectional shape at the time of pressing. It is possible to provide a high-tensile hot-rolled steel sheet having the properties (stretch flangeability), and there is a remarkable industrial effect.
　以下、本発明について詳細に説明する。
　まず、本発明鋼板の組織および炭化物の限定理由について説明する。
　本発明の熱延鋼板は、フェライト相が組織全体に対する面積率で95％以上であるマトリックスと、該マトリックスにTiを含み平均粒子径が10nm未満である微細炭化物が分散析出した組織を有する。 Hereinafter, the present invention will be described in detail.
First, the structure of the steel sheet of the present invention and the reasons for limiting the carbide will be described.
The hot-rolled steel sheet of the present invention has a matrix in which the ferrite phase is 95% or more in area ratio with respect to the entire structure, and a structure in which fine carbides containing Ti and having an average particle diameter of less than 10 nm are dispersed and precipitated in the matrix.
　フェライト相：組織全体に対する面積率で95％以上
　本発明においては、熱延鋼板の加工性（伸びフランジ性）を確保する上でフェライト相の形成が必須となる。熱延鋼板の伸びおよび伸びフランジ性の向上には、熱延鋼板の組織を、転位密度の低い延性に優れたフェライト相とすることが有効である。特に、伸びフランジ性の向上には、熱延鋼板の組織をフェライト単相組織とすることが好ましいが、完全なフェライト単相組織でない場合であっても、実質的にフェライト単相組織、すなわち、組織全体に対する面積率で95％以上がフェライト相であれば、上記の効果を十分に発揮する。したがって、フェライト相の組織全体に対する面積率は95％以上とする。好ましくは、97％以上である。 Ferrite phase: 95% or more in area ratio with respect to the entire structure In the present invention, formation of a ferrite phase is essential to ensure the workability (stretch flangeability) of a hot-rolled steel sheet. In order to improve the elongation and stretch flangeability of the hot-rolled steel sheet, it is effective to make the structure of the hot-rolled steel sheet a ferrite phase having a low dislocation density and excellent ductility. In particular, to improve stretch flangeability, the structure of the hot-rolled steel sheet is preferably a ferrite single-phase structure, but even if it is not a complete ferrite single-phase structure, it is substantially a ferrite single-phase structure, that is, If the area ratio with respect to the entire structure is 95% or more of the ferrite phase, the above effect is sufficiently exhibited. Therefore, the area ratio of the ferrite phase to the entire structure is 95% or more. Preferably, it is 97% or more.
なお、本発明の熱延鋼板において、フェライト相以外の組織としては、セメンタイト、パーライト、ベイナイト相、マルテンサイト相、残留オーステナイト相等が挙げられ、これらの合計は組織全体に対する面積率で5％程度以下、好ましくは3％程度以下であれば許容される。 In the hot-rolled steel sheet of the present invention, examples of the structure other than the ferrite phase include cementite, pearlite, bainite phase, martensite phase, retained austenite phase, and the like. However, it is acceptable if it is preferably about 3% or less.
Tiを含む微細炭化物
　Tiを含む炭化物は、その平均粒子径が極めて小さい微細炭化物となる傾向が強い。そのため、熱延鋼板中に微細炭化物を分散析出させることにより熱延鋼板の高強度化を図る本発明においては、分散析出をさせる微細炭化物として、Tiを含む微細炭化物とする。 Fine carbides containing Ti Carbides containing Ti tend to be fine carbides with an extremely small average particle size. For this reason, in the present invention for increasing the strength of a hot-rolled steel sheet by dispersing and precipitating fine carbides in the hot-rolled steel sheet, fine carbides containing Ti are used as the fine carbides to be dispersed and precipitated.
微細炭化物の平均粒子径：10nm未満
熱延鋼板に所望の強度（引張強さ：590MPa以上）を付与するうえでは微細炭化物の平均粒子径が極めて重要であり、本発明においてはTi を含む微細炭化物の平均粒子径を10nm未満とする。マトリックス中に微細炭化物が析出すると、その微細炭化物が、鋼板に変形が加わった際に生じる転位の移動に対する抵抗として作用することにより熱延鋼板が強化されるが、微細炭化物の平均粒子径を10nm未満とすると、上記の作用がより一層顕著となる。したがって、Ti を含む微細炭化物の平均粒子径は10nm未満とする。より好ましくは5nm以下である。 Average particle diameter of fine carbide: Less than 10 nm The average particle diameter of fine carbide is extremely important for imparting desired strength (tensile strength: 590 MPa or more) to a hot-rolled steel sheet. In the present invention, fine carbide containing Ti is used. The average particle size of is less than 10 nm. When fine carbide precipitates in the matrix, the fine carbide acts as a resistance to dislocation movement that occurs when deformation is applied to the steel sheet, strengthening the hot-rolled steel sheet, but the average particle diameter of the fine carbide is 10 nm. If it is less than the above, the above action becomes even more remarkable. Therefore, the average particle diameter of the fine carbide containing Ti is set to less than 10 nm. More preferably, it is 5 nm or less.
熱延鋼板強度を安定して得るためには、Tiを含む微細炭化物の分散析出状態を制御することが有効であり、本発明においては、Tiを含み平均粒子径が10nm未満である微細炭化物の、組織全体に対する体積比が0.0005以上となるように分散析出させることが好ましい。但し、上記体積比が0.003を超えると、強度が高くなり過ぎ、伸びフランジ性が低下するおそれがあるため、上記体積比は0.0005以上0.003以下とすることが好ましい。 In order to stably obtain hot-rolled steel sheet strength, it is effective to control the dispersion and precipitation state of fine carbides containing Ti.In the present invention, fine carbides containing Ti and having an average particle size of less than 10 nm are effective. It is preferable to disperse and precipitate so that the volume ratio with respect to the entire structure is 0.0005 or more. However, if the volume ratio exceeds 0.003, the strength becomes too high and the stretch flangeability may be deteriorated. Therefore, the volume ratio is preferably 0.0005 or more and 0.003 or less.
なお、本発明において、Tiを含む微細炭化物の析出形態として、主たる析出形態である列状析出のほか、ランダムに析出している微細炭化物が混在していても、なんら特性に影響を与えず、析出の形態は問わず、種々析出形態を合わせて分散析出と称することとする。 In the present invention, as the precipitation form of fine carbide containing Ti, in addition to the row precipitation that is the main precipitation form, even if there is a mixture of randomly precipitated fine carbide, without affecting the characteristics at all, Regardless of the form of precipitation, various precipitation forms are collectively referred to as dispersion precipitation.
　次に、本発明熱延鋼板の成分組成の限定理由について説明する。なお、以下の成分組成を表す％は、特に断らない限り質量％を意味するものとする。
　C ：0.010％以上0.050％以下
　Cは、微細炭化物を形成し、熱延鋼板を強化するうえで必須の元素である。C含有量が0.010％未満であると、590MPa以上の引張強さが得られなくなる。一方、C含有量が0.050％を超えると、強度が上昇するとともに、鋼板中にパーライトが形成され易くなり、優れた伸びフランジ性を得ることが困難となり易い。したがって、C含有量は0.010％以上0.050％以下とする。好ましくは0.020％以上0.035％以下である。より好ましくは0.020％以上0.030％以下である。 Next, the reason for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C: 0.010% or more and 0.050% or less C is an essential element for forming fine carbides and strengthening the hot-rolled steel sheet. If the C content is less than 0.010%, a tensile strength of 590 MPa or more cannot be obtained. On the other hand, when the C content exceeds 0.050%, the strength increases and pearlite is easily formed in the steel sheet, and it is difficult to obtain excellent stretch flangeability. Therefore, the C content is 0.010% or more and 0.050% or less. Preferably they are 0.020% or more and 0.035% or less. More preferably, it is 0.020% or more and 0.030% or less.
Si：0.2％以下
Siは、固溶強化元素であり、鋼の高強度化に有効な元素である。しかしながら、Si含有量が0.2％を超えると、フェライト相からのC析出が促進され、粒界に粗大なFe炭化物が析出し易くなり、伸びフランジ性が低下する。また、過剰なSiは、めっき性に悪影響を及ぼす。したがって、Si含有量は0.2％以下とする。好ましくは0.05％以下である。また、固溶強化にために0.005％以上であるのが好ましい。 Si: 0.2% or less
Si is a solid solution strengthening element and is an element effective for increasing the strength of steel. However, when the Si content exceeds 0.2%, C precipitation from the ferrite phase is promoted, coarse Fe carbides are likely to precipitate at the grain boundaries, and stretch flangeability is deteriorated. In addition, excessive Si adversely affects the plating properties. Therefore, the Si content is 0.2% or less. Preferably it is 0.05% or less. Further, it is preferably 0.005% or more for strengthening the solid solution.
Mn：0.1％以上0.8％以下
Mnは、固溶強化元素であり、鋼の高強度化に有効な元素であるため、熱延鋼板を強化する観点からはMn含有量を高めることが望ましい。Mn含有量が0.1％未満では、固溶強化を得ることができない。また、Mn含有量が0.1％未満であると、Ar3変態点が高くなり過ぎて、後述するようにTiを含む微細炭化物が粗大化し易くなる。一方、Mn含有量が0.8％を超えると偏析が生じ易くなり、且つ、フェライト相以外の相、すなわち硬質相が形成され、伸びフランジ性が低下する。したがって、Mn含有量は0.1％以上0.8％以下とする。好ましくは0.1％以上0.5％以下である。より好ましくは0.1％以上0.45％以下である。 Mn: 0.1% or more and 0.8% or less
Since Mn is a solid solution strengthening element and is an element effective for increasing the strength of steel, it is desirable to increase the Mn content from the viewpoint of strengthening the hot-rolled steel sheet. If the Mn content is less than 0.1%, solid solution strengthening cannot be obtained. On the other hand, if the Mn content is less than 0.1%, the Ar 3 transformation point becomes too high, and the fine carbide containing Ti tends to become coarse as will be described later. On the other hand, when the Mn content exceeds 0.8%, segregation is likely to occur, and a phase other than the ferrite phase, that is, a hard phase is formed, and stretch flangeability is deteriorated. Therefore, the Mn content is 0.1% or more and 0.8% or less. Preferably they are 0.1% or more and 0.5% or less. More preferably, it is 0.1% or more and 0.45% or less.
P ：0.025％以下
Pは、固溶強化元素であり、鋼の高強度化に有効な元素であるが、P含有量が0.025％を超えると偏析が顕著になり、伸びフランジ性が低下する。したがって、P含有量は0.025％以下とする。好ましくは0.02％以下である。また、固溶強化にために0.005％以上であるのが好ましい。 P: 0.025% or less
P is a solid solution strengthening element and is an element effective for increasing the strength of steel. However, when the P content exceeds 0.025%, segregation becomes significant and stretch flangeability deteriorates. Therefore, the P content is 0.025% or less. Preferably it is 0.02% or less. Further, it is preferably 0.005% or more for strengthening the solid solution.
S ：0.01％以下
　Sは、熱間加工性（熱間圧延性）を低下させる元素であり、スラブの熱間割れ感受性を高めるほか、鋼中にMnSとして存在して熱延鋼板の伸びフランジ性を劣化させる。そのため、本発明ではSを極力低減することが好ましく、0.01％以下とする。好ましくは0.005％以下である。 S: 0.01% or less S is an element that lowers hot workability (hot rollability), increases the hot cracking susceptibility of slabs, and is present as MnS in steel and stretch flangeability of hot rolled steel sheets. Deteriorate. Therefore, in the present invention, it is preferable to reduce S as much as possible, and set it to 0.01% or less. Preferably it is 0.005% or less.
N ：0.01％以下
　Nは、本発明においては有害な元素であり、極力低減することが好ましい。特にN含有量が0.01％を超えると、鋼中に粗大な窒化物が生成することに起因して、伸びフランジ性が低下する。したがって、N含有量は0.01％以下とする。好ましくは0.006％以下である。 N: 0.01% or less N is a harmful element in the present invention and is preferably reduced as much as possible. In particular, when the N content exceeds 0.01%, the stretch flangeability deteriorates due to the formation of coarse nitrides in the steel. Therefore, the N content is 0.01% or less. Preferably it is 0.006% or less.
Al：0.06％以下
　Alは、脱酸剤として作用する元素である。このような効果を得るためには0.001％以上含有することが望ましいが、0.06％を超える含有は、伸びおよび伸びフランジ性を低下させる。このため、Al含有量はAl：0.06％以下とする。 Al: 0.06% or less Al is an element that acts as a deoxidizer. In order to acquire such an effect, it is desirable to contain 0.001% or more, but inclusion exceeding 0.06% reduces elongation and stretch flangeability. For this reason, Al content shall be Al: 0.06% or less.
Ti：0.05％以上0.10％以下
Tiは、本発明において最も重要な元素である。Tiは、炭化物を形成することにより、優れた伸びフランジ性を維持しつつ、鋼板の高強度化に寄与する元素である。Ti含有量が0.05％未満では、所望の熱延鋼板強度（引張強さ：590MPa以上）を確保することができない。一方、Ti含有量が0.10％を超えると、伸びフランジ性が低下する傾向にある。したがって、Ti含有量は0.05％以上0.10％以下とする。好ましくは0.065％以上0.095％以下である。 Ti: 0.05% or more and 0.10% or less
Ti is the most important element in the present invention. Ti is an element that contributes to increasing the strength of the steel sheet while maintaining excellent stretch flangeability by forming carbides. If the Ti content is less than 0.05%, the desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more) cannot be ensured. On the other hand, when the Ti content exceeds 0.10%, the stretch flangeability tends to decrease. Therefore, the Ti content is 0.05% or more and 0.10% or less. Preferably they are 0.065% or more and 0.095% or less.
本発明の熱延鋼板は、S、N、Tiを、上記した範囲で且つ(1)式を満足するように含有する。
Ti ≧ 0.04＋(N/14×48＋S/32×48)　 ・・・ (1)
（S、N、Ti：各元素の含有量（質量％））
上記(1)式は、Tiを含む微細炭化物を、上記した所望の析出状態とするために満足すべき要件であり、本発明において極めて重要な指標である。 The hot-rolled steel sheet of the present invention contains S, N, and Ti in the above-described range and satisfying the expression (1).
Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
(S, N, Ti: content of each element (mass%))
The above formula (1) is a requirement to be satisfied in order to bring the fine carbide containing Ti into the above-described desired precipitation state, and is an extremely important index in the present invention.
　Ti ≧ 0.04＋(N/14×48＋S/32×48)　 ・・・ (1)
　先述のとおり、本発明においては熱延鋼板中にTiを含む微細炭化物を分散析出させるが、この微細炭化物は、熱延前の加熱で鋼素材中の炭化物を溶解し、主に熱間圧延後の巻取り時に析出させる。ここで、上記微細炭化物を、そのサイズを平均粒子径10nm未満として安定的に分散析出させるためには、微細炭化物の析出核となるTi量が十分に確保されている必要がある。しかしながら、高温域では、Tiは炭化物よりも窒化物や硫化物を形成し易い。そのため、鋼素材のN、S含有量に対してTi含有量が不十分であると、上記窒化物や硫化物の析出に伴い微細炭化物の析出核となるTi量が減少し、Tiを含む微細炭化物を十分に析出させることが困難となる。 Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
As described above, in the present invention, fine carbide containing Ti is dispersed and precipitated in the hot-rolled steel sheet, but this fine carbide dissolves the carbide in the steel material by heating before hot rolling, mainly after hot rolling. Precipitated during winding. Here, in order to stably disperse and precipitate the fine carbide with an average particle diameter of less than 10 nm, it is necessary to secure a sufficient amount of Ti as a precipitation nucleus of the fine carbide. However, at a high temperature range, Ti is easier to form nitrides and sulfides than carbides. Therefore, if the Ti content is insufficient with respect to the N and S contents of the steel material, the amount of Ti that becomes the precipitation nuclei of fine carbides decreases with the precipitation of the nitrides and sulfides, and the fine content containing Ti is reduced. It becomes difficult to sufficiently precipitate the carbide.
　そこで、本発明においては、Ti、N、S含有量を(1)式Ti ≧ 0.0４＋(N/14×48＋S/32×48)を満足するように制御する。これにより、微細炭化物の析出の核となるTi量が十分に確保され、上記微細炭化物を、そのサイズを平均粒子径10nm未満として安定的に析出させることができる。 Therefore, in the present invention, the Ti, N and S contents are controlled so as to satisfy the formula (1) Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48). As a result, a sufficient amount of Ti serving as a nucleus for precipitation of fine carbide can be secured, and the fine carbide can be stably precipitated with an average particle size of less than 10 nm.
また、本発明においては、熱延前に鋼素材をオーステナイト域まで加熱して鋼素材中の炭化物を溶解し、その後のオーステナイト→フェライト変態と同時にTiを含む炭化物を析出させる。しかしながら、オーステナイト→フェライト変態温度が高いと、析出したTiを含む炭化物が粗大なものとなる。そのため、本発明においては、オーステナイト→フェライト変態の温度（Ar3変態点）を巻取り温度範囲に調整することにより、Tiを含む炭化物を巻き取り時に析出させることが好ましい。これにより、その粗大化を抑制することが可能となり、平均粒子径10nm未満の炭化物を得ることができる。 In the present invention, the steel material is heated to the austenite region before hot rolling to dissolve carbides in the steel material, and carbides containing Ti are precipitated simultaneously with the subsequent austenite → ferrite transformation. However, when the austenite → ferrite transformation temperature is high, the precipitated carbide containing Ti becomes coarse. Therefore, in the present invention, it is preferable to precipitate the carbide containing Ti at the time of winding by adjusting the temperature of the austenite → ferrite transformation (Ar 3 transformation point) to the winding temperature range. Thereby, the coarsening can be suppressed, and a carbide having an average particle diameter of less than 10 nm can be obtained.
　オーステナイト→フェライト変態の温度（Ar3変態点）を巻取り温度範囲に調整するうえでは、上記した組成に加えてさらに、B ：0.0035％以下を、次の(2)式を満足するように含有することが好ましい。
B ≧ 0.0003－0.00025Mn ・・・ (2) In order to adjust the temperature of the austenite → ferrite transformation (Ar 3 transformation point) to the coiling temperature range, in addition to the above-mentioned composition, B: 0.0035% or less is contained so as to satisfy the following formula (2) It is preferable to do.
B ≧ 0.0003-0.00025Mn (2)
　B ：0.0035％以下
　Bは、鋼のAr3変態点を低下させる元素であり、本発明では、Bを添加して鋼のAr3変態点を下げることによって、Tiを含む炭化物の微細化を図ることができる。このような効果を得るためには、B含有量を0.0003％以上とすることが好ましい。一方、0.0035％を超えて含有しても上記の効果が飽和する。したがって、B含有量は0.0035％以下とすることが好ましい。より好ましくは0.0003％以上0.0020％以下である。 B: 0.0035% or less B is an element that lowers the Ar 3 transformation point of steel. In the present invention, by adding B to lower the Ar 3 transformation point of steel, the carbide containing Ti is refined. be able to. In order to obtain such an effect, the B content is preferably 0.0003% or more. On the other hand, the above effect is saturated even if the content exceeds 0.0035%. Therefore, the B content is preferably 0.0035% or less. More preferably, it is 0.0003% or more and 0.0020% or less.
　B ≧ 0.0003－0.00025Mn ・・・ (2)
　本発明において、Bを含有する場合には、鋼中のB含有量とMn含有量との比率を適正範囲に制御することも重要である。本発明者らは、フェライト相の組織全体に対する面積率が95％以上であるマトリックス中に、Tiを含む炭化物を微細（平均粒子径が10nm未満）に分散析出させる手段について検討した。その結果、熱間圧延過程におけるオーステナイト→フェライト変態の温度（Ar3変態点）を、後述の巻取り温度範囲に調整することが、Tiを含む炭化物を平均粒子径：10nm未満にまで微細化する極めて有効な手段であることを新たに知見した。 B ≧ 0.0003-0.00025Mn (2)
In the present invention, when B is contained, it is also important to control the ratio of the B content and the Mn content in the steel within an appropriate range. The present inventors studied a means for finely dispersing (including an average particle diameter of less than 10 nm) a carbide containing Ti in a matrix having an area ratio of 95% or more with respect to the entire structure of the ferrite phase. As a result, adjusting the temperature of the austenite → ferrite transformation (Ar 3 transformation point) in the hot rolling process to the coiling temperature range described later refines the carbide containing Ti to an average particle size of less than 10 nm. It was newly discovered that it is a very effective means.
また、本発明者らは更に検討を進めた結果、本発明の鋼組成においては、鋼素材のB含有量とMn含有量とが所望の関係を満たすように制御することにより、鋼のAr3変態点を目的とする範囲に調整できることを明らかにした。ここで、上式において、右辺（0.0003－0.00025Mn）の値がゼロ以下となる場合には、右辺の値をゼロと見なすものとする。 Further, the present inventors have found that further studying, in the steel composition of the present invention, by which the B content and Mn content of the steel material is controlled to satisfy the desired relationship, steel Ar 3 It was clarified that the transformation point can be adjusted to the target range. Here, in the above equation, when the value on the right side (0.0003-0.00025Mn) is less than or equal to zero, the value on the right side is assumed to be zero.
なお、本発明において、固溶強化元素であるMnの含有量が0.35％超であれば、上記したBの効果を用いることなく所望の鋼板強度（引張強さ：590MPa以上）を確保することができる。しかしながら、Mnの含有量が0.35％以下では、上記したBの効果を利用せずに所望の鋼板強度を確保することが困難な場合がある。そこで、Mnの含有量が0.35％以下の場合には、Tiを含む炭化物をより微細化させる目的でBを含有することが好ましい。 In the present invention, if the content of Mn, which is a solid solution strengthening element, exceeds 0.35%, a desired steel plate strength (tensile strength: 590 MPa or more) can be ensured without using the effect of B described above. it can. However, if the Mn content is 0.35% or less, it may be difficult to secure a desired steel sheet strength without using the above-described effect of B. Therefore, when the Mn content is 0.35% or less, it is preferable to contain B for the purpose of making the carbide containing Ti finer.
また、本発明においては、C、S、N、Tiの含有量を、上記した範囲で且つ(3)式を満足するように調整することが好ましい。
C/12 ＞ Ti/48－N/14－S/32 ・・・ (3) 
（C、S、N、Ti：各元素の含有量（質量％））
先述のとおり、Tiを含む炭化物は、その平均粒子径が極めて小さい微細炭化物となる傾向が強い。しかしながら、Cと結合するTiが原子比でC以上になると、炭化物が粗大化し易くなる。そして、炭化物の粗大化に伴って、所望の熱延鋼板強度（引張強さ：590MPa以上）を確保することが困難となる。 In the present invention, it is preferable to adjust the contents of C, S, N, and Ti so as to satisfy the expression (3) within the above range.
C / 12> Ti / 48-N / 14-S / 32 (3)
(C, S, N, Ti: content of each element (mass%))
As described above, the carbide containing Ti has a strong tendency to become a fine carbide having an extremely small average particle diameter. However, when the Ti bonded to C becomes C or more in atomic ratio, the carbide is easily coarsened. And it becomes difficult to ensure desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more) with the coarsening of carbides.
そのため、本発明では、C、Ti、N、S含有量を(3)式のように規定することが好ましい。すなわち、本発明では、鋼素材に含まれるCおよびTiについて、Cの原子％（C/12）を、炭化物生成に寄与できるTiの原子％（Ti/48－N/14－S/32）よりも多くすることが好ましい。これにより、Tiを含む微細炭化物の粗大化を抑制することができる。 Therefore, in the present invention, it is preferable to define the contents of C, Ti, N, and S as shown in formula (3). That is, in the present invention, for C and Ti contained in the steel material, the atomic% of C (C / 12) is more than the atomic% of Ti that can contribute to carbide formation (Ti / 48-N / 14-S / 32). It is preferable to increase the amount. Thereby, the coarsening of the fine carbide containing Ti can be suppressed.
　本発明の鋼板においては、Cu、Sn、Ni、Ca、Mg、Co、As、Cr、W、Nb、Pb、Ta、Mo、Vのいずれか１種以上を合計で0.1％以下、好ましくは0.03％以下含有してもよい。また、上記以外の成分は、Feおよび不可避的不純物である。 In the steel sheet of the present invention, one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, and V are combined in a total of 0.1% or less, preferably 0.03. % Or less. Components other than the above are Fe and inevitable impurities.
　また、本発明の鋼板は、表面にめっき皮膜を有するものとしてもよい。鋼板表面にめっき皮膜を形成することにより、熱延鋼板の耐食性が向上し、厳しい腐食環境に晒される部品、例えば自動車の足回り部品の素材に好適な熱延鋼板が得られる。なお、めっき皮膜としては、例えば溶融亜鉛めっき皮膜や合金化溶融亜鉛めっき皮膜等が挙げられる。 Further, the steel sheet of the present invention may have a plating film on the surface. By forming a plating film on the surface of the steel sheet, the corrosion resistance of the hot-rolled steel sheet is improved, and a hot-rolled steel sheet suitable for a part exposed to a severe corrosive environment, for example, a material for an undercarriage part of an automobile is obtained. Examples of the plating film include a hot dip galvanizing film and an alloyed hot dip galvanizing film.
　次に、本発明の熱延鋼板の製造方法について説明する。
本発明は、鋼素材に、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻き取り、熱延鋼板とする。この際、前記仕上げ圧延の仕上げ圧延温度を880℃以上とし、前記冷却の平均冷却速度を10℃/s以上とし、前記巻き取りの巻取り温度を550℃以上800℃未満とすることを特徴とする。 Next, the manufacturing method of the hot rolled steel sheet of the present invention will be described.
In the present invention, hot rolling consisting of rough rolling and finish rolling is performed on a steel material, and after finishing rolling, the steel material is cooled and wound to obtain a hot rolled steel sheet. At this time, the finish rolling temperature of the finish rolling is 880 ° C. or higher, the average cooling rate of the cooling is 10 ° C./s or higher, and the winding temperature of the winding is 550 ° C. or higher and lower than 800 ° C. To do.
　本発明において、鋼素材の溶製方法は特に限定されず、転炉、電気炉等、公知の溶製方法を採用することができる。また、溶製後、偏析等の問題から連続鋳造法によりスラブ（鋼素材）とするのが好ましいが、造塊－分塊圧延法、薄スラブ連鋳法等、公知の鋳造方法でスラブとしても良い。なお、鋳造後にスラブを熱間圧延するにあたり、加熱炉でスラブを再加熱した後に圧延しても良いし、所定温度以上の温度を保持している場合には、スラブを加熱することなく直送圧延しても良い。 In the present invention, the melting method of the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Moreover, after melting, it is preferable to use a slab (steel material) by a continuous casting method because of problems such as segregation, but a slab can also be formed by a known casting method such as ingot-bundling rolling or thin slab continuous casting. good. In addition, when hot-rolling the slab after casting, the slab may be rolled after being reheated in a heating furnace, and when the temperature is maintained at a predetermined temperature or higher, direct rolling without heating the slab You may do it.
　上記の如く得られた鋼素材に、粗圧延および仕上げ圧延を施すが、本発明においては、粗圧延前に鋼素材中の炭化物を溶解する必要がある。炭化物形成元素であるTiを含有する本発明においては、鋼素材の加熱温度を1150℃以上とすることが好ましい。但し、鋼素材の加熱温度が過剰に高くなると、表面が過剰に酸化されTiO2が生じてTiが消費され、鋼板にした場合に表面近傍の硬さの低下が生じ易くなるため、上記加熱温度は1300℃以下とすることが好ましい。また、先述のとおり、粗圧延前の鋼素材が、所定温度以上の温度を保持しており、鋼素材中の炭化物が溶解している場合には、粗圧延前の鋼素材を加熱する工程は省略可能である。なお、粗圧延条件については特に限定する必要はない。 The steel material obtained as described above is subjected to rough rolling and finish rolling. In the present invention, it is necessary to dissolve carbides in the steel material before rough rolling. In the present invention containing Ti which is a carbide forming element, the heating temperature of the steel material is preferably set to 1150 ° C. or higher. However, if the heating temperature of the steel material is excessively high, the surface is excessively oxidized and TiO 2 is generated and Ti is consumed. Is preferably 1300 ° C. or lower. In addition, as described above, when the steel material before rough rolling maintains a temperature equal to or higher than a predetermined temperature, and the carbide in the steel material is dissolved, the step of heating the steel material before rough rolling is It can be omitted. The rough rolling conditions are not particularly limited.
仕上げ圧延温度：880℃以上
　仕上げ圧延温度の適正化は、熱延鋼板の伸びおよび伸びフランジ性の確保、並びに、仕上げ圧延の圧延荷重の低減化を図るうえで重要となる。仕上げ圧延温度が880℃未満であると、熱延鋼板表層の結晶粒が粗大粒となり、伸びフランジ性が損なわれる。また、未再結晶温度域で圧延が行われるため、旧オーステナイト粒界に粗大なTiの炭化物が析出し、伸びフランジ性が低下する。したがって、仕上げ圧延温度は880℃以上とする。好ましくは900℃以上である。なお、仕上げ圧延温度が過剰に高くなると、結晶粒が粗大化して所望の鋼板強度（引張強さ：590MPa以上）の確保に悪影響を及ぼすため、仕上げ圧延温度は1000℃以下とすることが望ましい。 Finishing rolling temperature: 880 ° C. or more Optimization of the finishing rolling temperature is important for securing elongation and stretch flangeability of the hot-rolled steel sheet and reducing the rolling load of finishing rolling. When the finish rolling temperature is less than 880 ° C., the crystal grains of the hot rolled steel sheet surface layer are coarse and the stretch flangeability is impaired. In addition, since rolling is performed in the non-recrystallization temperature range, coarse Ti carbide precipitates at the prior austenite grain boundaries, and stretch flangeability deteriorates. Accordingly, the finish rolling temperature is 880 ° C. or higher. Preferably it is 900 degreeC or more. If the finish rolling temperature becomes excessively high, the crystal grains become coarse and adversely affect the securing of the desired steel sheet strength (tensile strength: 590 MPa or more). Therefore, the finish rolling temperature is preferably 1000 ° C. or less.
　平均冷却速度：10℃/s以上
　仕上げ圧延終了後、880℃以上の温度から巻取り温度までの平均冷却速度が10℃/s未満であると、Ar3変態点が高くなり、Tiを含む炭化物が十分に微細化されない。したがって、上記平均冷却速度は10℃/s以上とする。好ましくは30℃/s以上である。また、フェライト組織を得るために200℃/s未満であるのが好ましい。 Average cooling rate: 10 ° C / s or higher If the average cooling rate from 880 ° C or higher to the coiling temperature is less than 10 ° C / s after finishing rolling, the Ar 3 transformation point increases and carbides containing Ti Is not sufficiently refined. Therefore, the average cooling rate is 10 ° C./s or more. Preferably it is 30 ° C./s or more. Further, it is preferably less than 200 ° C./s in order to obtain a ferrite structure.
巻取り温度：550℃以上800℃未満
　巻取り温度の適正化は、熱延鋼板の組織を、熱延鋼板の幅方向全域にわたり所望の組織、すなわち、フェライト相が組織全体に対する面積率で95％以上であるマトリックスと、Tiを含み平均粒子径が10nm未満である微細炭化物が分散析出した組織とする上で、極めて重要である。 Winding temperature: 550 ° C or higher and less than 800 ° C This is extremely important in forming a structure in which fine matrixes containing Ti and an average particle diameter of less than 10 nm are dispersed and precipitated.
　巻取り温度が550℃未満であると、過冷却状態となり易い圧延材幅方向端部において、微細炭化物の析出が不十分となり、所望の鋼板強度（引張強さ：590MPa以上）を付与することが困難となる。また、ランナウトテーブル上の走行安定性を確保し難くなるという問題を生じる。一方、巻取り温度が800℃以上となると、パーライトが生じ、フェライト相が組織全体に対する面積率で95％以上であるマトリックスとすることが困難となる。したがって、巻取り温度は550℃以上800℃未満とする。好ましくは550℃以上700℃未満、より好ましくは580℃以上700℃未満である。 When the coiling temperature is less than 550 ° C, precipitation of fine carbides becomes insufficient at the end in the width direction of the rolled material, which tends to be supercooled, and the desired steel sheet strength (tensile strength: 590 MPa or more) can be imparted. It becomes difficult. Moreover, the problem that it becomes difficult to ensure the running stability on a run-out table arises. On the other hand, when the coiling temperature is 800 ° C. or higher, pearlite is generated, and it becomes difficult to obtain a matrix in which the ferrite phase is 95% or more in terms of the area ratio with respect to the entire structure. Therefore, the coiling temperature is set to 550 ° C. or higher and lower than 800 ° C. Preferably they are 550 degreeC or more and less than 700 degreeC, More preferably, they are 580 degreeC or more and less than 700 degreeC.
　以上のように、引張強さ(TS)：590MPa以上で、かつ、断面形状が複雑な足回り部品等の素材としても適用可能な優れた加工性（伸びフランジ性）を有する高張力熱延鋼板を製造するうえでは、平均粒子径が10nm未満である微細炭化物を鋼板幅方向全域にわたり分散析出させる必要がある。 As described above, high-tensile hot-rolled steel sheet with excellent workability (stretch flangeability) that can be applied as a material for undercarriage parts with a tensile strength (TS) of 590 MPa or more and a complicated cross-sectional shape. In order to manufacture, it is necessary to disperse and precipitate fine carbides having an average particle diameter of less than 10 nm over the entire width direction of the steel sheet.
　しかしながら、本発明においては、熱延鋼板の素材となる鋼中のN,S含有量に対して所定量以上のTi（Ti ≧ 0.04＋(N/14×48＋S/32×48)）を含有させ、或いは更に熱延鋼板の素材となる鋼中のB,Mn含有量が所定の関係（B ≧ 0.0003－0.00025Mn）を満足するように含有させることにより、平均粒子径が10nm未満である微細炭化物が十分に分散析出するような組成に制御されている。そのため、本発明によると、熱延鋼板の製造条件をさほど厳密に規定しなくても、幅方向全域にわたって平均粒子径が10nm未満である微細炭化物を分散析出させることが可能となり、熱延鋼板幅方向全域にわたり均一かつ良好な特性（引張強さ、伸びフランジ性）が付与される。 However, in the present invention, Ti (Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48)) more than a predetermined amount with respect to the N and S content in the steel that is the material of the hot-rolled steel sheet. In addition, fine carbides with an average particle size of less than 10 nm can be obtained by adding the B and Mn contents in the steel used as the material of the hot-rolled steel sheet so as to satisfy a predetermined relationship (B ≧ 0.0003-0.00025Mn). Is controlled so as to sufficiently disperse and precipitate. Therefore, according to the present invention, it is possible to disperse and precipitate fine carbides having an average particle diameter of less than 10 nm over the entire width direction without strictly specifying the production conditions of the hot-rolled steel sheet. Uniform and good characteristics (tensile strength, stretch flangeability) are imparted over the entire direction.
なお、本発明においては、以上のようにして製造された熱延鋼板に対し、めっき処理を施すことにより、鋼板表面にめっき皮膜を形成してもよい。例えば、めっき処理として溶融亜鉛めっき処理を施し溶融亜鉛めっき皮膜を形成し、或いは溶融亜鉛めっき処理後、更に合金化処理を施すことにより、鋼板表面に合金化溶融亜鉛めっき皮膜を形成してもよい。 In the present invention, the hot-rolled steel sheet manufactured as described above may be plated to form a plating film on the surface of the steel sheet. For example, a hot dip galvanizing process may be performed as a plating process to form a hot dip galvanized film, or an alloyed hot dip galvanized film may be formed on the surface of the steel sheet by further alloying after the hot dip galvanizing process. .
　表１に示す組成の溶鋼を通常公知の手法により溶製、連続鋳造して肉厚250mmのスラブ（鋼素材）とした。これらのスラブを、1250℃に加熱後、粗圧延し、表２に示す仕上げ圧延温度とする仕上げ圧延を施し、仕上げ圧延終了後、880℃の温度から巻取り温度までの温度域を表２に示す平均冷却速度で冷却し、表２に示す巻取り温度で巻取り、板厚：2.3mmの熱延鋼板とした。なお、一部の熱延鋼板（熱延番号a2,b2,c2）については、480℃の亜鉛めっき浴（0.1％Al-Zn）中に浸漬し、片面当たり付着量45g/m2の溶融亜鉛めっき皮膜を形成した後、520℃で合金化処理を行い、合金化溶融亜鉛めっき鋼板とした。 Molten steel having the composition shown in Table 1 was melted and continuously cast by a generally known technique to obtain a slab (steel material) having a thickness of 250 mm. These slabs are heated to 1250 ° C, roughly rolled, and subjected to finish rolling at the finish rolling temperature shown in Table 2. After finishing rolling, the temperature range from 880 ° C to the coiling temperature is shown in Table 2. The steel sheet was cooled at the average cooling rate shown and wound at the winding temperature shown in Table 2 to obtain a hot rolled steel sheet having a plate thickness of 2.3 mm. Some hot-rolled steel sheets (hot-rolling numbers a2, b2, and c2) are immersed in a 480 ° C zinc plating bath (0.1% Al-Zn) and molten zinc with an adhesion amount of 45 g / m 2 per side. After forming the plating film, alloying treatment was performed at 520 ° C. to obtain an alloyed hot-dip galvanized steel sheet.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　上記により得られた熱延鋼板から試験片を採取し、組織観察、引張試験、穴拡げ試験を行い、フェライト相の面積率、Tiを含む微細炭化物の平均粒子径および体積比、引張強さ、穴拡げ率（伸びフランジ性）を求めた。試験方法は次のとおりとした。 Samples are collected from the hot-rolled steel sheet obtained as described above, microstructure observation, tensile test, hole expansion test, ferrite phase area ratio, average particle diameter and volume ratio of fine carbide containing Ti, tensile strength, The hole expansion rate (stretch flangeability) was determined. The test method was as follows.
（i）組織観察
　得られた熱延鋼板から試験片を採取し、試験片の圧延方向と平行な断面を機械的に研磨し、ナイタールで腐食した後、走査型電子顕微鏡（SEM）で倍率：3000倍にて撮影した組織写真（SEM写真）を用い、画像解析装置によりフェライト相、フェライト相以外の組織の種類、および、それらの面積率を求めた。
　また、熱延鋼板から作製した薄膜を透過型電子顕微鏡（TEM）によって倍率： 260000倍で観察し、Tiを含む微細炭化物の粒子径を求めた。
　Tiを含む微細炭化物の粒子径は、260000倍での30視野の観察結果をもとに、円近似を用いた画像処理で個々の粒子径を求め、求めた粒子径を算術平均し、平均粒子径とした。
　Tiを含む微細炭化物の体積比は、10％アセチルアセトン－1％塩化テトラメチルアンモニウム－メタノール溶液（AA溶液）を用いて地鉄を電解し、ろ過捕集した残渣の抽出残渣分析によりTi炭化物の重量を求め、これをTi炭化物（TiC）の密度で割ることによって体積を求め、この体積を溶解した地鉄の体積で除することによって求めた。 (I) Microstructure observation A test piece was collected from the obtained hot-rolled steel sheet, a cross section parallel to the rolling direction of the test piece was mechanically polished, corroded with nital, and then magnified with a scanning electron microscope (SEM): Using a structure photograph (SEM photograph) taken at a magnification of 3000 times, the type of the structure other than the ferrite phase and the ferrite phase and the area ratio thereof were determined by an image analyzer.
Moreover, the thin film produced from the hot-rolled steel sheet was observed with a transmission electron microscope (TEM) at a magnification of 260,000 times, and the particle diameter of fine carbide containing Ti was determined.
The particle size of fine carbide containing Ti is calculated based on the observation results of 30 fields of view at 260,000 times, and the individual particle size is obtained by image processing using circular approximation, and the obtained particle size is arithmetically averaged to obtain the average particle size. The diameter.
The volume ratio of fine carbide containing Ti is the weight of Ti carbide based on the extraction residue analysis of the residue collected by electrolysis of the ground iron using 10% acetylacetone-1% tetramethylammonium chloride-methanol solution (AA solution). The volume was determined by dividing this by the density of Ti carbide (TiC), and this volume was divided by the volume of the dissolved iron.
（ii）引張試験
　得られた熱延鋼板から、圧延方向に対して直角方向を引張方向とするJIS　5号引張試験片（JIS Z 2201）を採取し、JIS Z 2241の規定に準拠した引張試験を行い、引張強さ(TS)を測定した。 (Ii) Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece (JIS Z 2201) with the direction perpendicular to the rolling direction as the tensile direction was sampled and a tensile test in accordance with the provisions of JIS Z 2241. The tensile strength (TS) was measured.
（iii）穴拡げ試験
　得られた熱延鋼板から、試験片（大きさ：130mm×130mm）を採取し、該試験片に初期直径d0：10mmφの穴を打ち抜き加工で形成した。これら試験片を用いて、穴拡げ試験を実施した。すなわち、該穴に頂角：60°の円錐ポンチを挿入し、該穴を押し広げ、亀裂が熱延鋼板（試験片）を貫通したときの穴の径dを測定し、次式で穴拡げ率λ(％)を算出した。
　　　　　　　　　穴拡げ率λ(％)＝{(d－d0)/d0}×100
　得られた結果を表３に示す。 (Iii) Hole expansion test From the obtained hot-rolled steel sheet, a test piece (size: 130 mm x 130 mm) was collected, and a hole having an initial diameter d 0 : 10 mmφ was formed in the test piece by punching. Using these test pieces, a hole expansion test was performed. That is, a conical punch with an apex angle of 60 ° is inserted into the hole, the hole is expanded, and the diameter d of the hole when the crack penetrates the hot-rolled steel sheet (test piece) is measured. The rate λ (%) was calculated.
Hole expansion rate λ (%) = {(d−d 0 ) / d 0 } × 100
The obtained results are shown in Table 3.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　本発明例は何れも、引張強さTS：590MPa以上の高強度と、穴拡げ率λ：100％以上の優れた伸びフランジ性を兼備した熱延鋼板となっている。一方、本発明の範囲を外れる比較例は、所定の高強度が確保できていないか、穴拡げ率λが確保できていない。
 
 
 
  Each of the examples of the present invention is a hot-rolled steel sheet having both a high strength of tensile strength TS: 590 MPa or more and an excellent stretch flangeability of a hole expansion ratio λ: 100% or more. On the other hand, in a comparative example outside the scope of the present invention, a predetermined high strength cannot be ensured or the hole expansion rate λ cannot be ensured.



Claims (13)

  1. 質量％で、
    C ：0.010％以上0.050％以下、     Si：0.2％以下、
    Mn：0.1％以上0.8％以下、         P ：0.025％以下、
    S ：0.01％以下、　　　　　　　    N ：0.01％以下、
    Al：0.06％以下、                Ti：0.05％以上0.10％以下
    を、S、N、およびTiが下記(1)式を満足するように含有し、残部がFeおよび不可避的不純物からなる組成と、フェライト相の組織全体に対する面積率が95％以上であるマトリックスと、Tiを含み平均粒子径が10nm未満である微細炭化物が分散析出した組織とを有し、引張強さが590MPa以上である、高張力熱延鋼板。
    　　　　　　　Ti ≧ 0.04＋(N/14×48＋S/32×48)　 ・・・ (1)
    　　　　　　　　（S、N、Ti：各元素の含有量（質量％））     % By mass
    C: 0.010% or more and 0.050% or less, Si: 0.2% or less,
    Mn: 0.1% to 0.8%, P: 0.025% or less,
    S: 0.01% or less, N: 0.01% or less,
    Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, containing S, N, and Ti so that the following formula (1) is satisfied, with the balance consisting of Fe and inevitable impurities, and the ferrite phase High tensile heat, having a matrix with an area ratio of 95% or more with respect to the entire structure of the above and a structure in which fine carbides containing Ti and an average particle size of less than 10 nm are dispersed and precipitated, and a tensile strength of 590 MPa or more Rolled steel sheet.
    Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
    (S, N, Ti: content of each element (mass%))
  2. 前記組成に加えてさらに、質量％でB ：0.0035％以下を、下記(2)式を満足するように含有する、請求項１に記載の高張力熱延鋼板。
    　　　　　　　　　　B ≧ 0.0003－0.00025Mn ・・・ (2)
    　　　　　　　　　（Mn、B：各元素の含有量（質量％））     The high-tensile hot-rolled steel sheet according to claim 1, further containing, in addition to the composition, B: 0.0035% or less by mass% so as to satisfy the following expression (2).
    B ≧ 0.0003-0.00025Mn (2)
    (Mn, B: content of each element (mass%))
  3. 前記Bが、0.0003％以上、0.0020％以下である、請求項２に記載の高張力熱延鋼板。 The high-tensile hot-rolled steel sheet according to claim 2, wherein B is 0.0003% or more and 0.0020% or less.
  4. 前記組成が、下記(3)式を満足する請求項１または２に記載の高張力熱延鋼板。
    C/12 ＞ Ti/48－N/14－S/32 ・・・ (3) 
    　　　　　　　（C、S、N、Ti：各元素の含有量（質量％））     The high-tensile hot-rolled steel sheet according to claim 1 or 2, wherein the composition satisfies the following formula (3).
    C / 12> Ti / 48-N / 14-S / 32 (3)
    (C, S, N, Ti: content of each element (mass%))
  5. 前記微細炭化物の組織全体に対する体積比が0.0005以上である、請求項１または２に記載の高張力熱延鋼板。 The high-tensile hot-rolled steel sheet according to claim 1 or 2, wherein a volume ratio of the fine carbide to the entire structure is 0.0005 or more.
  6. 前記体積比が、0.0005以上、0.003以下である、請求項５に記載の高張力熱延鋼板。 The high-tensile hot-rolled steel sheet according to claim 5, wherein the volume ratio is 0.0005 or more and 0.003 or less.
  7. 前記組成に加えてさらに、質量％で、Cu、Sn、Ni、Ca、Mg、Co、As、Cr、W、Nb、Pb、Ta、Mo、Vのいずれか１種以上を合計で0.1％以下含有する、請求項１または２に記載の高張力熱延鋼板。 In addition to the above composition, in addition to mass, any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, V is 0.1% or less in total. The high-tensile hot-rolled steel sheet according to claim 1 or 2, which is contained.
  8. 　鋼板表面にめっき皮膜を有する、請求項１または２に記載の高張力熱延鋼板。 The high-tensile hot-rolled steel sheet according to claim 1 or 2, wherein the steel sheet surface has a plating film.
  9. 鋼素材に、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻き取り、熱延鋼板とする高張力熱延鋼板の製造方法において、
    前記鋼素材を、質量％で、
    C ：0.010％以上0.050％以下、     Si：0.2％以下、
    Mn：0.1％以上0.8％以下、         P ：0.025％以下、
    S ：0.01％以下、　　　　　　　    N ：0.01％以下、
    Al：0.06％以下、                Ti：0.05％以上0.10％以下
    を、S、N、およびTiが下記(1)式を満足するように含有し、残部がFeおよび不可避的不純物からなる組成とし、
    前記仕上げ圧延の仕上げ圧延温度を880℃以上とし、前記冷却の平均冷却速度を10℃/s以上とし、前記巻取り温度を550℃以上800℃未満とする、引張強さが590MPa以上の高張力熱延鋼板の製造方法。
    　　　　　　　Ti ≧ 0.04＋(N/14×48＋S/32×48)　 ・・・ (1)
    　　　　　　　　（S、N、Ti：各元素の含有量（質量％））     In the method for producing a high-tensile hot-rolled steel sheet, the steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling is cooled, wound, and hot-rolled steel sheet.
    The steel material in mass%,
    C: 0.010% or more and 0.050% or less, Si: 0.2% or less,
    Mn: 0.1% to 0.8%, P: 0.025% or less,
    S: 0.01% or less, N: 0.01% or less,
    Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, S, N, and Ti are contained so as to satisfy the following formula (1), and the balance is composed of Fe and inevitable impurities,
    The final rolling temperature of the finish rolling is 880 ° C or higher, the average cooling rate of the cooling is 10 ° C / s or higher, the winding temperature is 550 ° C or higher and lower than 800 ° C, and the tensile strength is 590 MPa or higher. A method for producing a hot-rolled steel sheet.
    Ti ≧ 0.04+ (N / 14 × 48 + S / 32 × 48) (1)
    (S, N, Ti: content of each element (mass%))
  10. 前記組成に加えてさらに、質量％でB ：0.0035％以下を、下記(2)式を満足するように含有する、請求項９に記載の高張力熱延鋼板の製造方法。
    　　　　　　　　　　B ≧ 0.0003－0.00025Mn ・・・ (2)
    　　　　　　　　　（Mn、B：各元素の含有量（質量％））     The method for producing a high-tensile hot-rolled steel sheet according to claim 9, further comprising, in addition to the composition, B: 0.0035% or less by mass% so as to satisfy the following formula (2).
    B ≧ 0.0003-0.00025Mn (2)
    (Mn, B: content of each element (mass%))
  11. 前記Bが、0.0003％以上、0.0020％以下である、請求項１０に記載の高張力熱延鋼板の製造方法。 The method for producing a high-tensile hot-rolled steel sheet according to claim 10, wherein B is 0.0003% or more and 0.0020% or less.
  12. 前記組成が、下記(3)式を満足する、請求項９または１０に記載の高張力熱延鋼板の製造方法。
    C/12 ＞ Ti/48－N/14－S/32 ・・・ (3) 
    　　　　　　　（C、S、N、Ti：各元素の含有量（質量％））     The method for producing a high-tensile hot-rolled steel sheet according to claim 9 or 10, wherein the composition satisfies the following formula (3).
    C / 12> Ti / 48-N / 14-S / 32 (3)
    (C, S, N, Ti: content of each element (mass%))
  13. 前記組成に加えてさらに、質量％で、Cu、Sn、Ni、Ca、Mg、Co、As、Cr、W、Nb、Pb、Ta、Mo、Vのいずれか１種以上を合計で0.1％以下含有する、請求項９または１０に記載の高張力熱延鋼板の製造方法。
          In addition to the above composition, in addition to mass, any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Ta, Mo, V is 0.1% or less in total. The manufacturing method of the high-tensile-strength hot-rolled steel sheet according to claim 9 or 10.
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WO2015118863A1 (en) * 2014-02-05 2015-08-13 Ｊｆｅスチール株式会社 High-strength hot-rolled steel sheet and production method therefor
JPWO2015118863A1 (en) * 2014-02-05 2017-03-23 Ｊｆｅスチール株式会社 High strength hot rolled steel sheet and method for producing the same
JP2017106121A (en) * 2014-02-05 2017-06-15 Ｊｆｅスチール株式会社 High-strength hot-rolled steel sheet and production method therefor
CN104480391A (en) * 2014-11-24 2015-04-01 江苏省沙钢钢铁研究院有限公司 Hot-rolled pickled plate and production method thereof
CN104480397A (en) * 2014-12-19 2015-04-01 山东钢铁股份有限公司 700MPa-grade hot-rolled automobile structure steel and preparation method thereof

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