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WO2018179391A1 - Tôle d'acier laminée à chaud, pièce en acier forgé et procédés de production associés - Google Patents

Tôle d'acier laminée à chaud, pièce en acier forgé et procédés de production associés Download PDF

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Publication number
WO2018179391A1
WO2018179391A1 PCT/JP2017/013749 JP2017013749W WO2018179391A1 WO 2018179391 A1 WO2018179391 A1 WO 2018179391A1 JP 2017013749 W JP2017013749 W JP 2017013749W WO 2018179391 A1 WO2018179391 A1 WO 2018179391A1
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WIPO (PCT)
Prior art keywords
steel sheet
less
hot
rolling
rolled steel
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/JP2017/013749
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English (en)
Japanese (ja)
Inventor
龍雄 横井
伸麻 吉川
繁 米村
和也 大塚
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel and Sumitomo Metal Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel and Sumitomo Metal Corp filed Critical Nippon Steel and Sumitomo Metal Corp
Priority to MX2019011711A priority Critical patent/MX2019011711A/es
Priority to EP17904326.0A priority patent/EP3604587A4/fr
Priority to BR112019017622-5A priority patent/BR112019017622A2/pt
Priority to PCT/JP2017/013749 priority patent/WO2018179391A1/fr
Priority to JP2017559726A priority patent/JP6332571B1/ja
Priority to CN201780089327.2A priority patent/CN110475890A/zh
Priority to US16/485,360 priority patent/US20200024679A1/en
Priority to KR1020197028709A priority patent/KR20190126100A/ko
Publication of WO2018179391A1 publication Critical patent/WO2018179391A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
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    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C—CHEMISTRY; METALLURGY
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    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • 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
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • 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
    • C21D8/041—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 fabrication or treatment of ingot or slab
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • 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
    • C21D8/0421—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 working steps
    • C21D8/0426—Hot rolling
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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Definitions

  • the present invention relates to a hot-rolled steel plate, a steel forged part, and a method for producing them.
  • Steel sheets used in automobile body structures are required to have high strength and high press workability from the viewpoint of improving safety and reducing weight.
  • a high-strength steel sheet excellent in hole expansibility (high burring property) better than before has been proposed.
  • a steel sheet excellent in hole expansibility ( ⁇ value) a steel sheet of a ferrite main phase that is precipitation strengthened by fine precipitates such as Ti and Nb and a manufacturing method thereof have been reported.
  • Patent Document 1 discloses a hot-rolled steel sheet having high strength and excellent stretch flangeability.
  • Patent Document 2 discloses a high-formability, high-tensile hot-rolled steel sheet having excellent material uniformity.
  • Patent Document 3 discloses a high-tensile hot-rolled steel sheet having excellent elongation and stretch flangeability.
  • the processing of steel sheets for automobiles is not limited to conventional press processing elements, but new processing is applied to conventional press processing elements such as plate forging. Elements have been combined in a complex way.
  • Conventional press working elements include, for example, deep drawing, hole expansion, stretch forming, bending, and ironing.
  • plate forging is a press work having a composite working element including a working element peculiar to forging, in addition to a working element when pressing a conventional steel plate.
  • the plate thickness of the steel plate remains the original plate thickness, or the steel plate is deformed while being reduced (thinned) by conventional press processing, while the part is being molded,
  • the thickness of the steel sheet is increased (thickening) so that it can be efficiently deformed so that it has the thickness of the steel sheet necessary for its function. The strength of the parts can be ensured.
  • Patent Documents 1 to 3 do not mention any processing including a composite processing element typified by plate forging. Moreover, the winding conditions for manufacturing the hot-rolled steel sheet described in Patent Document 1 are very strict and unrealistic. Furthermore, since the hot-rolled steel sheets described in Patent Documents 2 and 3 contain 0.07% or more of Mo, which is an expensive alloy element, there is a problem that the manufacturing cost is high.
  • High burring steel is known to exhibit good formability in conventional pressing.
  • plate forging which is a forming method that includes elements of forging in the conventional press working, cracks may occur in the steel sheet even when the degree of processing is small.
  • press cracks occur in the areas where sheet thickness constriction (reduction in sheet thickness) occurs, but even in processes that do not involve sheet thickness constriction, such as sheet forging, the material cracks. It has been found that the product may not be obtained due to breakage.
  • the present invention has been made in order to solve the above-mentioned problems, and while maintaining the basic function as a high burring steel, the crack limit of the portion subjected to forging by applying a partial compression force is achieved. It aims at providing the hot rolled steel plate excellent in the plate forgeability which can be improved.
  • the present invention has been made in order to solve the above-mentioned problems, and provides the following hot-rolled steel sheet and forged steel parts and methods for producing them.
  • the chemical composition of the steel sheet is mass%, C: 0.020 to 0.070%, Si: 0.05 to 1.70%, Mn: 0.60 to 2.50%, Al: 0.005 to 0.020%, N: more than 0.0030% and 0.0060% or less, P: 0.050% or less, S: 0.005% or less, Ti: 0.015 to 0.170%, O: 0.0010 to 0.0100%, Nb: 0 to 0.100%, V: 0 to 0.300%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr: 0 to 2.00% Mo: 0 to 1.00%, B: 0 to 0.0100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.1000%, Zr: 0 to 1.000%, Co: 0 to 1.000% Zn: 0 to 1.000%, W: 0 to 1.000% Sn: 0 to 0.050%, and Balance: Fe and impurities, Satisfying the
  • the average equivalent circle diameter of the precipitate containing Ti is 1.00 to 3.00 nm.
  • Tensile strength is 780 MPa or more
  • the product of uniform elongation and tensile strength is 7000 MPa ⁇ % or more
  • the product of the hole expansion ratio and the tensile strength is 50000 MPa ⁇ % or more
  • a method for producing a hot-rolled steel sheet according to any one of (1) to (3) above After performing the melting step, a slab having the chemical composition described in (1) above is cast, and a heating step, a continuous hot rolling step, a first cooling step, a second cooling step, and a winding are performed on the slab.
  • the Si content of the molten steel is set to 0.05 to 0.20% and the dissolved oxygen concentration is set to 0.0020 to 0.0080%, and then deoxidation treatment is performed.
  • the deoxidation treatment after adding Ti, Al is added, and subsequently one or more selected from Ca, Mg and REM are added,
  • the heating step the slab is heated to a temperature of SRTmin ° C.
  • the continuous hot rolling step includes rough rolling and multi-stage finish rolling of three or more stages,
  • the end temperature of the rough rolling is 1100 ° C. or higher,
  • the cumulative strain in the final three stages of rolling in the multistage finish rolling is 0.01 to 0.10,
  • the rolling end temperature of the multistage finish rolling is a temperature of Ar 3 + 30 ° C. or higher obtained by the following formula (ii):
  • In the first cooling step after the multistage finish rolling is completed, cooling is started after 1.00 to 5.00 s, and the temperature is 10 ° C./s or more from the rolling end temperature to a temperature range of 650 to 750 ° C.
  • Cool at average cooling rate then hold in air for 1-10s
  • cooling is performed at an average cooling rate of 10 ° C./s or more from a temperature range of 600 to 740 ° C.
  • winding is performed at a winding temperature of 450 to 650 ° C. Manufacturing method of hot rolled steel sheet.
  • At least forging processing is performed on the hot-rolled steel sheet according to any one of (1) to (3) above. Manufacturing method of steel forged parts.
  • FIG.1 (a) is a figure which shows the test piece of a simple shear test.
  • FIG.1 (b) is a figure which shows the test piece after a simple shear test.
  • the inventors of the present invention conducted intensive studies to solve the above problems and obtained the following knowledge.
  • (A) Equivalent plastic strain Plate forging includes deformation in a strain range (high strain range) exceeding the fracture strain in the conventional tensile test. Moreover, since plate forging is a complex process, it cannot be evaluated simply by tensile test and shear test data. Therefore, the present inventors introduced “equivalent plastic strain” as an index, and established a new evaluation method.
  • Equivalent plastic strain converts the relationship between the shear stress ⁇ s and the shear plastic strain ⁇ sp in the simple shear test into the relationship between the tensile stress ⁇ and the tensile strain ⁇ in the uniaxial tensile test with different deformation modes. . Then, assuming the relationship between the isotropic hardening rule and the plastic work conjugate, the conversion can be performed as shown in the following equation by using a constant conversion coefficient ( ⁇ ). After calculating the conversion coefficient ( ⁇ ) by the method described later, the equivalent plastic strain is derived.
  • the shear test is performed in multiple stages, and after each stage of the shear test, the starting point of the crack of the test piece generated in the part holding the test piece is machined to crack the test piece.
  • the test results were evaluated by connecting these shear test results in series.
  • conventional tensile testing methods can be applied to tensile stress and tensile strain.
  • a JIS No. 5 test piece based on JIS Z2241 (2011) can be used.
  • TiN a structure mainly composed of ferrite (precipitation strengthened ferrite) that is precipitation strengthened by fine precipitates such as Ti and Nb is used in order to obtain excellent hole expansibility.
  • ferrite precipitation strengthened ferrite
  • fine precipitates such as Ti and Nb
  • TiN coarse TiN precipitates unless a special manufacturing method is used (hereinafter, the deposited TiN is also simply referred to as “TiN”).
  • TiN is a compound in which TiN is thermodynamically very stable and preferential to other compounds at high temperatures such as casting during the steel sheet manufacturing process, hot rolling heating, or rough rolling initial stage. This is because of crystallization or precipitation.
  • TiN is hard enough to be used as a coating for cutting tools, machine parts, plastic molding dies, sports equipment, ornaments, etc., and its hardness is known to be about Hv 2000-2300, and it is a very hard precipitate. It is a thing. Therefore, when subjected to deformation in a high strain region such as plate forging, voids are likely to occur at the interface due to the difference in deformability from the matrix structure.
  • the equivalent plastic strain at break is 0.90 (90%) or more.
  • the equivalent plastic strain at the time of fracture becomes 0.90 (90%) or more, and a certain workability can be obtained even in complex machining such as plate forging. Confirmed that it is possible to secure.
  • the effective cumulative strain is an index that takes into account the temperature during rolling, the recovery of crystal grains due to the rolling reduction of the steel sheet by rolling, recrystallization, and grain growth. Therefore, when obtaining the effective cumulative strain, a constitutive law expressing a static recovery phenomenon over time after rolling was used. Considering that the grains recover statically over time after rolling, the release of energy accumulated as strain in the grains after rolling is due to static recovery due to the disappearance of dislocations in the thermal grains. Because it happens. The disappearance of this thermal dislocation is influenced by the rolling temperature and the elapsed time after rolling. Therefore, taking this static recovery into account, we introduced an index that describes the temperature during rolling, the rolling reduction (logarithmic strain) of the steel sheet due to rolling, and the elapsed time after rolling as parameters, and this is called “effective cumulative strain”. Defined.
  • the desired microstructure can be obtained and the variation in nano hardness is reduced, so by suppressing the generation of voids at the interface between the hard tissue and the soft tissue, Since cracking does not occur even after plate forging, a steel plate excellent in plate forgeability can be obtained.
  • Sequential deoxidation is a method of adding a strong deoxidation element to molten steel in which a weak deoxidation element oxide is present.
  • a weak deoxidation element oxide By reducing the weak deoxidation element oxide by the strong deoxidation element, oxygen is released at a low supply rate and with a low degree of supersaturation. As a result, the oxide generated from the added strong deoxidizing element becomes fine. Deoxidation that maximizes these effects by gradually adding Si, which is a weak deoxidation element, to Ti, Al, then Ca, Mg, or REM, which are strong deoxidation elements, in stages. Is the method.
  • C 0.020 to 0.070%
  • C combines with Nb, Ti, etc. to form precipitates in the steel sheet, and contributes to strength improvement by precipitation strengthening.
  • the C content is less than 0.020%, the effect by the above action cannot be sufficiently obtained.
  • the C content exceeds 0.070%, the iron-based carbide that becomes the starting point of cracking during hole expansion processing increases, and the hole expansion value deteriorates. Therefore, the C content is 0.020 to 0.070%.
  • the C content is preferably 0.025% or more, and more preferably 0.030% or more.
  • it is preferable that C content is 0.060% or less, and it is more preferable that it is 0.050% or less.
  • Si 0.05 to 1.70%
  • Si has a deoxidizing effect and an effect of suppressing precipitation of iron-based carbides such as cementite in the material structure and contributing to improvement of ductility and hole expandability.
  • carbides containing Ti are likely to precipitate at high temperatures. Precipitation of carbides at high temperatures tends to cause variations in the amount of precipitation, resulting in material variations such as strength and hole expandability. Therefore, the Si content is set to 0.05 to 1.70%.
  • the Si content is preferably 0.06% or more, more preferably 0.08% or more, from the viewpoint of suppressing the occurrence of scale defects such as scales and spindle scales. Moreover, it is preferable that Si content is 1.50% or less, and it is more preferable that it is 1.00% or less from a viewpoint of improving chemical conversion property and corrosion resistance after coating.
  • Mn 0.60 to 2.50%
  • Mn is an element that contributes to strengthening ferrite and improving hardenability.
  • the Mn content is set to 0.60 to 2.50%.
  • the Mn content is preferably 1.00% or more, and more preferably 1.50% or more.
  • Mn content is 2.00% or less, and it is more preferable that it is 1.80% or less.
  • Al 0.005 to 0.020%
  • Al like Si, has a deoxidizing effect and an effect of generating ferrite.
  • the Al content is set to 0.005 to 0.020%.
  • the Al content is preferably 0.007% or more, and more preferably 0.010% or more. Further, the Al content is preferably 0.018% or less, and more preferably 0.015% or less.
  • N more than 0.0030% and 0.0060% or less If N is contained in a large amount, not only does solute nitrogen remain and ductility decreases, but TiN precipitates and decreases hole expansibility. The smaller the amount, the better. However, excessively reducing the N content increases the cost during refining.
  • the fine oxide obtained by successive deoxidation serves as a nucleus for depositing fine TiN, the N content is allowed to be 0.0060%. Even if the fine oxide obtained by carrying out the sequential deoxidation is utilized to develop the effect of refinement of TiN, if the N content exceeds 0.0060%, TiN becomes coarse. In the case of complex processing such as plate forging, coarse TiN becomes the starting point of voids, which promotes destruction.
  • the N content is more than 0.0030% and 0.0060% or less.
  • the N content is preferably 0.0.0031% or more, and more preferably 0.0035% or more.
  • the N content is preferably 0.0055% or less, more preferably 0.0050% or less.
  • P 0.050% or less
  • P is an impurity contained in the hot metal, and since it segregates at the grain boundaries, it degrades local ductility and weldability. Therefore, the P content is limited to 0.050% or less.
  • the P content is preferably 0.030% or less or 0.020% or less.
  • the lower limit is 0%. However, excessively reducing the content increases the cost during refining, so the lower limit may be made 0.001%.
  • S 0.005% or less
  • S is also an impurity contained in the hot metal, and forms MnS to deteriorate local ductility and weldability. Therefore, the S content is limited to 0.005% or less.
  • the S content may be 0.003% or less or 0.002% or less.
  • the lower limit is 0%. However, excessively reducing the content increases the cost during refining, so the lower limit may be made 0.0005%.
  • Ti 0.015 to 0.170%
  • Ti has the effect that carbonitride or solute Ti delays grain growth during hot rolling, thereby reducing the grain size of the hot-rolled sheet and improving low-temperature toughness. Further, by finely dispersing in the ferrite as TiC, it contributes to increasing the strength of the steel sheet through precipitation strengthening. However, when the content is excessive, in addition to saturation of the effect, TiN that is a hard precipitate is likely to be precipitated. Therefore, the Ti content is set to 0.015 to 0.170%.
  • the Ti content is preferably 0.030% or more, 0.045% or more, or 0.060% or more, 0.070% or more, 0.080% or more, 0.090% or more, or 0.100% or more. It is more preferable that The Ti content is preferably 0.160% or less, 0.150% or less, 0.140% or less, 0.130% or less, or 0.120% or less.
  • O is an element necessary for dispersing a large number of fine oxides during deoxidation of molten steel.
  • O is preferably contained in the final product steel by 0.0010% or more.
  • the O content exceeds 0.0100%, not only the effect is saturated, but also the size of the oxide as an impurity increases and the density also increases. In the complex processing such as plate forging, the oxide becomes the starting point of the void, and the breakage is promoted.
  • the O content is 0.0010 to 0.0100%.
  • the O content is preferably 0.0030% or more, and more preferably 0.0050% or more.
  • the O content is preferably 0.0090% or less, and more preferably 0.0080% or less.
  • Nb 0 to 0.100%
  • Nb has the effect of reducing the grain size of the hot-rolled sheet and improving low-temperature toughness by delaying grain growth during hot rolling by carbonitride or solute Nb.
  • NbC by existing as NbC, it contributes to the strengthening of a steel plate through precipitation strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Nb content is 0.100% or less. If necessary, the Nb content may be 0.080% or less, 0.060% or less, or 0.050% or less.
  • the lower limit is 0%, but the lower limit may be 0.001% or 0.010% in order to sufficiently obtain the above effect.
  • V 0 to 0.300%
  • V is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the V content is set to 0.300% or less. If necessary, the V content may be 0.200% or less, 0.100% or less, or 0.060% or less. The lower limit is 0%, but the lower limit may be 0.001% or 0.010% in order to sufficiently obtain the above effect.
  • Cu 0 to 2.00%
  • Cu is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Cu content is 2.00% or less. In addition, if the Cu content is large, scratches due to scale may occur on the surface of the steel sheet. Therefore, the Cu content may be 1.20% or less, 0.80% or less, 0.50% or less, or 0.25% or less.
  • the lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Cu content may be 0.01%.
  • Ni 0 to 2.00%
  • Ni is an element having an effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Ni content is 2.00% or less. Moreover, when Ni content is contained abundantly, there exists a possibility that ductility may deteriorate. Therefore, the Ni content may be 0.60% or less, 0.35% or less, or 0.20% or less. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Ni content may be 0.01%.
  • Cr 0 to 2.00% Cr is an element having an effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Cr content is 2.00% or less.
  • the upper limit may be set to 1.00%, 0.60%, or 0.30%.
  • the lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Cr content may be 0.01%.
  • Mo 0 to 1.00%
  • Mo is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the Mo content is set to 1.00% or less. In order to further improve economy, the upper limit may be set to 0.60%, 0.30%, or 0.10%. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the Mo content may be 0.005% or 0.01%.
  • B 0 to 0.0100% B segregates at the grain boundaries and improves the low temperature toughness by increasing the grain boundary strength. Therefore, you may make it contain as needed. However, if the content is excessive, the effect is saturated and the economic efficiency is lowered. Therefore, the B content is 0.0100% or less. Further, B is a strong quenching element, and if its content is large, ferrite transformation does not proceed sufficiently during cooling, and sufficient retained austenite may not be obtained. Therefore, the B content may be 0.0050% or less, 0.0020% or less, or 0.0015%. The lower limit is 0%, but in order to sufficiently obtain the above effect, the lower limit of the B content may be 0.0001% or 0.0002%.
  • the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing.
  • Ca, Mg and REM are strong deoxidizing elements. As described above, these elements are added to the molten steel after Ti and Al, and the oxides become fine by sequentially deoxidizing. As a result, TiN deposited as a nucleus also becomes fine, and a high-strength hot-rolled steel sheet excellent in plate forging can be obtained.
  • At least one selected from Ca, Mg and REM is contained so that the total content satisfies the above formula (i).
  • the Ca or Mg content exceeds 0.0100% or the REM content exceeds 0.1000% not only the effect is saturated, but also the size of the oxide containing Al or Ca. As the value increases, the density also increases. As a result, in a complex process such as plate forging, an oxide containing Al or Ca becomes a starting point of a void, and fracture is promoted.
  • the Ca and Mg content is 0.0100% or less, and the REM content is 0.1000% or less.
  • the total content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more.
  • content of Ca and Mg is 0.0090% or less, 0.0080% or less, or 0.0070% or less, respectively.
  • the REM content is preferably 0.0900% or less, 0.0800% or less, or 0.0700% or less.
  • REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means the total content of these elements.
  • the lanthanoid is industrially added in the form of misch metal.
  • Zr 0 to 1.000% Co: 0 to 1.000% Zn: 0 to 1.000% W: 0 to 1.000% It has been confirmed that even if Zr, Co, Zn, and W are each in the range of 1.000% or less, the effects of the present invention are not impaired. These upper limits may be set to 0.300% or 0.10%.
  • the total content of Zr, Co, Zn and W is preferably 1.000% or less or 0.100%. These contents are not essential, and the lower limit is 0%, but the lower limit may be 0.0001% if necessary.
  • Sn 0 to 0.050% It has been confirmed that the effect of the present invention is not impaired even if Sn is contained in a small amount. However, if it exceeds 0.05%, wrinkles may occur during hot rolling. Therefore, the Sn content is 0.050% or less.
  • the content of Sn is not essential, and the lower limit is 0%, but the lower limit may be 0.001% if necessary.
  • the balance is Fe and impurities.
  • impurities are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially manufacturing steel sheets, and are permitted within a range that does not adversely affect the present invention. Means something.
  • (B) Metal structure The metal structure of the steel plate of this invention is demonstrated.
  • the metallographic structure is 1/4 W or 3/4 W from the end face of the steel sheet when the width and thickness of the steel sheet are W and t, respectively, in a cross section perpendicular to the rolling direction of the steel sheet, and The structure at a position of 1/4 t or 3/4 t from the surface of the steel sheet.
  • “%” means “area%”.
  • Precipitation strengthened ferrite 5 to 70% Supersaturation of Ti carbides when fine precipitates containing Ti (such as finely precipitated Ti carbides, hereinafter also referred to as “fine Ti precipitates”) undergo a ⁇ ⁇ ⁇ transformation during cooling after rolling. With the degree of driving force, Ti carbide is precipitated at the phase interface or homogeneously nucleated in the ferrite, and the pro-eutectoid ferrite in which the Ti carbide is finely dispersed is precipitation strengthened ferrite (hereinafter also referred to as “precipitation strengthened ferrite”). ). Precipitation strengthened ferrite is a structure necessary for achieving both excellent uniform elongation and strength.
  • the area ratio of the precipitation strengthened ferrite is set to 5 to 70%.
  • the area ratio of precipitation strengthened ferrite is preferably 7% or more, and more preferably 10% or more. Further, the area ratio of the precipitation strengthened ferrite is preferably 65% or less, and more preferably 60% or less.
  • the precipitation strengthened ferrite means a ferrite in which the number density of fine Ti precipitates contained in the grains is 1.0 ⁇ 10 16 to 50.0 ⁇ 10 16 pieces / cm 3. . If the number density of fine Ti precipitates contained in the ferrite grains is less than 1.0 ⁇ 10 16 pieces / cm 3 , the effect of precipitation strengthening cannot be sufficiently obtained. On the other hand, when the number density of fine Ti precipitates exceeds 50.0 ⁇ 10 16 pieces / cm 3 , not only the strength is saturated but also the ductility is lowered.
  • the area ratio of precipitation strengthened ferrite is 5 to 70%.
  • the area ratio of ferrite is 5 to 70% and the number density of fine Ti precipitates contained in the ferrite grains is 1.0. It means that it is ⁇ 10 16 to 50.0 ⁇ 10 16 pieces / cm 3 .
  • the average equivalent circle diameter of the fine Ti precipitates contained in the grains of the precipitation strengthened ferrite is preferably 1.00 to 3.00 nm. If the average equivalent circle diameter of the fine Ti precipitate is less than 1.00 nm, it is difficult to obtain the effect of precipitation strengthening. On the other hand, if the average equivalent circle diameter is more than 3.00 nm due to coarse grains, a sufficient amount of fine Ti precipitate is precipitated. This is because things cannot be secured.
  • Bainite 30-95% Bainite is a structure necessary for obtaining a balance between strength and local ductility, and has an effect of suppressing crack propagation. However, if the amount of bainite increases too much, ferrite decreases, and the local elongation is excellent, but the uniform elongation is significantly deteriorated. Therefore, the area ratio of bainite is set to 30 to 95%. The area ratio of bainite is preferably 80% or less, and more preferably 70% or less when the uniform elongation is important.
  • Residual austenite 2% or less
  • the high burring steel is characterized by ensuring high strength and ensuring both strength and workability while ensuring workability due to the presence of precipitation strengthened ferrite and bainite.
  • the presence of thermodynamically stable retained austenite that did not cause martensitic transformation in the steel sheet means that the C concentration of the retained austenite is high, and the retained austenite is formed by work-induced transformation during sheet forging. The hardness of the site becomes too high and promotes the generation of voids. Therefore, the retained austenite is preferably as small as possible, and the area ratio is 2% or less.
  • the area ratio of retained austenite is preferably 1.5% or less, 1% or less, or 0.5% or less. In particular, there is no need to define a lower limit, and the lower limit is 0%, and 0% is most preferable.
  • Martensite 2% or less High burring steel is characterized by ensuring both high strength and workability while ensuring workability due to the presence of precipitation strengthened ferrite and bainite.
  • the area ratio of martensite which is a hard structure, exceeds 2%, voids are likely to be generated at the boundary between martensite and ferrite with an increase in distortion of the steel sheet due to plate forging, and breakage is likely to occur. Therefore, the area ratio of martensite is 2% or less.
  • the area ratio of martensite is preferably 1.5% or less, 1% or less, or 0.5% or less. In particular, it is not necessary to specify a lower limit, and the lower limit is 0%.
  • Pearlite 1% or less Since pearlite is the starting point of fracture during hole expansion molding, its area ratio is 1% or less. The area ratio of pearlite is preferably 0.5% or less. The area ratio of pearlite is preferably reduced as much as possible, and is preferably 0%.
  • Total of precipitation strengthening ferrite and bainite 95% or more High burring steel has precipitation strengthening ferrite that achieves both excellent uniform elongation and strength, and bainite that satisfies both strength and local ductility. This provides excellent strength, uniform elongation and local ductility. If the total area ratio of precipitation strengthened ferrite and bainite is less than 95%, these characteristics deteriorate. Therefore, the total area ratio of precipitation strengthened ferrite and bainite is 95% or more. The total area ratio is preferably 97% or more, and more preferably 98% or more.
  • the area ratio of the metal structure is obtained as follows. As described above, first, a sample is taken from a position of 1/4 W or 3/4 W from the end surface of the steel plate and from a position of 1/4 t or 3/4 t from the surface of the steel plate. And the rolling direction cross section (what is called L direction cross section) of this sample is observed.
  • the sample is subjected to nital etching, and after etching, observation is performed in a 300 ⁇ m ⁇ 300 ⁇ m visual field using an optical microscope. Then, by performing image analysis on the obtained structure photograph, the area ratio A of ferrite, the area ratio B of pearlite, and the total area ratio C of bainite, martensite and retained austenite are obtained.
  • the nital-etched portion is repeller-etched and observed with a 300 ⁇ m ⁇ 300 ⁇ m field of view using an optical microscope.
  • the total area ratio D of a retained austenite and a martensite is computed by performing image analysis with respect to the obtained structure
  • the volume fraction of retained austenite is obtained by X-ray diffraction measurement using a sample that is chamfered from the normal direction of the rolling surface to 1 ⁇ 4 depth of the plate thickness. Since the volume ratio is substantially equal to the area ratio, the volume ratio is defined as the area ratio E of retained austenite.
  • the area ratio of bainite is determined from the difference between the area ratio C and the area ratio D, and the area ratio of martensite is determined from the difference between the area ratio E and the area ratio D.
  • the area ratio of precipitation strengthened ferrite can be determined by the Kernel Average Misoration (KAM) method equipped in EBSP-OIMTM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy).
  • KAM Kernel Average Misoration
  • the analysis conditions for precipitation strengthened ferrite in the present invention are as follows.
  • EBSP-OIMTM the average orientation difference between adjacent pixels is calculated by the third approximation, and the portion where this orientation difference is calculated to be 1 ° or less is precipitation strengthened. Ferrite was used.
  • the formation temperature range of the precipitation-strengthened ferrite of the present invention is that the Ti carbide precipitates at the phase interface or homogeneously nucleates in the ferrite with the driving force as the supersaturation degree of the Ti carbide during ⁇ ⁇ ⁇ transformation during cooling after rolling. It matches the temperature range.
  • Polygonal pro-eutectoid ferrite transformed at high temperature is formed by diffusion transformation, so that the dislocation density is small and the intra-granular strain is small, so the intra-granular difference in crystal orientation is small. Therefore, the crystal orientation difference of the precipitation strengthened ferrite is similarly reduced.
  • the polygonal ferrite area ratio obtained by optical microscope observation and the area where the azimuth difference in the third approximation measured by the KAM method is 1 ° or less can be obtained. This is because the area ratios are almost the same.
  • the measurement of the area fraction of precipitation strengthened ferrite was carried out in detail as follows.
  • a sample collected in the same manner as described in the structure observation was polished with a colloidal silica abrasive for 30 to 60 minutes, and EBSP measurement was performed under measurement conditions of a magnification of 400 times, an area of 160 ⁇ m ⁇ 256 ⁇ m, and a measurement step of 0.5 ⁇ m.
  • the EBSP-OIMTM method irradiates an electron beam onto a highly inclined sample in a scanning electron microscope (SEM), images the Kikuchi pattern formed by backscattering with a high-sensitivity camera, and processes the computer image. It consists of a device and software that measure the crystal orientation of the glass in a short time.
  • the EBSP method allows quantitative analysis of the microstructure and crystal orientation of the bulk sample surface, and the analysis area is an area that can be observed with an SEM. Depending on the resolution of the SEM, the analysis can be performed with a resolution of a minimum of 20 nm. The analysis takes several hours and is performed by mapping tens of thousands of points to be analyzed in a grid at equal intervals. In a polycrystalline material, the crystal orientation distribution and crystal grain size in the sample can be seen.
  • a needle-like sample is prepared from a sample to be measured by cutting and electrolytic polishing using a focused ion beam processing method in combination with an electrolytic polishing method as necessary.
  • the accumulated data can be reconstructed and obtained as an actual distribution image of atoms in real space.
  • the size of the fine Ti precipitate is an equivalent circle diameter calculated by assuming that the fine Ti precipitate is spherical from the number of Ti atoms constituting the fine Ti precipitate and the lattice constant of the fine Ti precipitate.
  • a method for obtaining the equivalent circle diameter (diameter) R of the precipitate using the number of Ti atoms of the fine Ti precipitate obtained by the three-dimensional atom probe measurement method is shown below.
  • the equivalent circle diameter is calculated to be approximately 1 nm.
  • the equivalent circle diameter (diameter) of 30 or more fine Ti precipitates is arbitrarily measured, and the average value is obtained.
  • the number density of fine Ti precipitates is determined using the measurement field of view as the denominator and the number of fine Ti precipitates as the numerator.
  • 5 or more fields of view of 10 nm (plate thickness direction t) ⁇ 40 nm (plate width direction W) ⁇ 60 nm (plate longitudinal direction L) were measured, and the number density (pieces / cm 3 ) The average value was obtained.
  • TiN In the present invention, the existence state of TiN is also defined as follows.
  • the average equivalent circle diameter of TiN 1.0-10.0 ⁇ m
  • the average equivalent circle diameter of TiN is set to 10.0 ⁇ m or less.
  • the average equivalent circle diameter of TiN is preferably 8.0 ⁇ m or less, and more preferably 5.0 ⁇ m or less.
  • TiN is preferably as small as possible, it is not necessary to originally provide a lower limit for the average equivalent circle diameter of TiN. However, in the TiN observation method described later, if the equivalent circle diameter of TiN is less than 1.0 ⁇ m, it is difficult to determine whether it is TiN. Therefore, in the present invention, only those having an equivalent circle diameter of 1.0 ⁇ m or more are set as TiN as measurement targets. Therefore, the average equivalent circle diameter of TiN is 1.0 ⁇ m or more.
  • the average equivalent circle diameter (diameter) of TiN is obtained as follows. As described above, first, a sample is taken from a position of 1/4 W or 3/4 W from the end surface of the steel plate and from a position of 1/4 t or 3/4 t from the surface of the steel plate. Then, the rolling direction cross section (so-called L direction cross section) of the sample is polished and observed without being etched. Specifically, a microstructure photograph is taken at a magnification of 1000 times using an optical microscope, and the microstructure photograph is observed visually or with an image processing apparatus or the like.
  • the equivalent circle diameter (diameter) is obtained for those that can be identified as TiN, and only those having an equivalent circle diameter (diameter) of 1.0 ⁇ m or more are defined as TiN. Then, the visual field of 60 ⁇ m (rolling direction L) ⁇ 40 ⁇ m (sheet thickness direction t) was observed for 20 or more visual fields, and the average of the equivalent circle diameter (diameter) of TiN was averaged. Diameter).
  • Average value of the shortest distance between adjacent TiN 10.0 ⁇ m or more
  • the distance between TiN It is necessary to secure a certain amount. Therefore, the average value of the distance between adjacent TiNs is set to 10.0 ⁇ m or more.
  • the average value is preferably 15.0 ⁇ m or more, and more preferably 20.0 ⁇ m or more.
  • the upper limit is not particularly set, it is inevitable that TiN is deposited to some extent. Therefore, the average value of the shortest distance between adjacent TiNs is preferably set to 1000 ⁇ m or less.
  • the average value of the shortest distance between adjacent TiNs is obtained as follows. Twenty arbitrary TiNs are selected, the distance to the nearest TiN is measured, and the average value is calculated. In addition, the measurement of the shortest distance between TiN is calculated
  • Nano hardness can be measured using, for example, TriscopeScope / TriboIndenter manufactured by Hystron.
  • the nano hardness of 100 points or more can be arbitrarily measured at a load of 1 mN, and the standard deviation of the nano hardness can be calculated from the result.
  • the standard deviation of the nano hardness should be as small as 1.0 GPa or less.
  • the standard deviation of nano hardness is preferably 0.8 GPa or less.
  • Tensile strength 780 MPa or more
  • the steel sheet according to the present invention preferably has a tensile strength of 780 MPa or more equivalent to that of conventional high burring steel.
  • the upper limit of the tensile strength is not particularly required, but may be 1200 MPa, 1150 MPa, or 1000 MPa. However, the tensile strength indicates the tensile strength of JIS Z 2241 (2011).
  • the uniform elongation is a nominal value at which the value obtained when the nominal stress ⁇ n is differentiated by the nominal strain ⁇ n is zero in the relationship between the nominal stress ⁇ n and the nominal strain ⁇ n in the test specified by JIS Z 2241 (2011).
  • the strain is ⁇ n0, it is expressed by the following formula.
  • Uniform elongation (u-EL) ln ( ⁇ n0 + 1)
  • the hole expansion rate ( ⁇ ) represents ( ⁇ ) of the hole expansion rate according to a test method based on JIS Z 2256 (2010).
  • Equivalent plastic strain 0.9 or more Equivalent plastic strain is the relationship between the shear stress ⁇ s and the shear plastic strain ⁇ sp in the simple shear test, and the tensile stress ⁇ and tensile strain ⁇ in the uniaxial tensile test with different deformation modes. Assuming the relationship between the isotropic hardening rule and the plastic work conjugate, the relationship is converted using a constant conversion coefficient ( ⁇ ).
  • the isotropic hardening law is a work hardening law that assumes that the shape of the yield curve does not change even when strain progresses (that is, expands to a similar shape).
  • the relation of plastic work conjugation is a relation that work hardening is described as a function of only plastic work, and shows the same work hardening amount when given the same plastic work ( ⁇ ⁇ ⁇ ) regardless of the deformation form.
  • the conversion coefficient ⁇ is determined so that the relationship between shear stress and shear plastic strain is similar to the relationship between tensile stress and tensile strain.
  • the conversion coefficient ⁇ can be obtained by the following procedure. First, the relationship between tensile strain ⁇ (actual value) and tensile stress ⁇ (actual value) in a uniaxial tensile test is obtained. Subsequently, the relationship between the shear strain ⁇ s (actual value) and the shear stress ⁇ s (actual value) in the uniaxial shear test is obtained.
  • the tensile strain ⁇ (conversion) obtained from the shear strain ⁇ s (actual value) and the tensile stress ⁇ (conversion) obtained from the shear stress ⁇ s (actual value) are obtained in advance.
  • the tensile stress ⁇ (conversion) is determined when the strain ⁇ (conversion) is between 0.2% and uniform elongation (u-EL).
  • u-EL uniform elongation
  • the equivalent plastic strain ⁇ eq is defined as a value obtained by converting the shear plastic strain ⁇ sp (rupture) at the time of rupture in the simple shear test into the tensile strain ⁇ in the simple tensile test using the obtained ⁇ .
  • the steel plate according to the present invention is characterized by good processing characteristics in a high strain region represented by plate forging, and the equivalent plastic strain ⁇ eq satisfies 0.50 or more. Since the equivalent plastic strain of the conventional TRIP steel is at most about 0.30, it was confirmed that the plate forgeability of the steel sheet according to the present invention is good.
  • the steel plate according to the present invention is mainly used for automobiles and the like, and its thickness range is mainly 1.0 to 4.0 mm. For this reason, the plate thickness range may be 1.0 to 4.0 mm.
  • the lower limit is 1.2 mm, 1.4 mm, or 1.6 mm
  • the upper limit is 3.6 mm, 3.2 mm, or 2. It may be 8 mm.
  • (A) Melting process The manufacturing method preceding hot rolling is not particularly limited except for a deoxidizing method for molten steel.
  • a deoxidation method the composition and size of TiN can be controlled by performing sequential deoxidation described below. Therefore, various secondary smelting processes including sequential deoxidation are performed following the smelting by a blast furnace or an electric furnace to adjust to the above-described component composition. Then, what is necessary is just to manufacture a slab by methods, such as normal continuous casting and thin slab casting. At that time, scrap or the like may be used as a raw material as long as it can be controlled within the component range of the present invention.
  • the dissolved oxygen concentration in the molten steel is adjusted by the amount of Si, which is a weaker deoxidizing element than Ti, before the deoxidation treatment, and the dissolved oxygen concentration balanced with the Si amount is 0.0020 to 0.0080%. And If the dissolved oxygen concentration is less than 0.0020%, a complex oxide sufficient to ultimately reduce the size of the nitride containing Ti cannot be obtained. On the other hand, when the dissolved oxygen concentration exceeds 0.0080%, the effect of reducing the size of the nitride containing Ti is lost due to coarsening of the generated composite oxide.
  • the Si content is less than 0.05%, the dissolved oxygen concentration in equilibrium with Si exceeds 0.0080%.
  • the Si content exceeds 0.2%, the dissolved oxygen concentration that is in equilibrium with Si is less than 0.0020%. Therefore, the Si content before the deoxidation treatment is 0.05 to 0.20%, and the dissolved oxygen concentration is 0.0020 to 0.0080%.
  • Ti is added so that the final content of Ti is 0.015 to 0.170%, and immediately thereafter, the final content of Al is 0.005 to 0.020. Al is added so that it may become%, and a deoxidation process is performed.
  • the Ti oxide formed when Ti is added grows with the passage of time after Ti is added, so that it coarsens and floats, so it is desirable to add Al immediately after adding Ti. .
  • the added amount of Al is such that the final content of Al is less than 0.005%, the Ti oxide grows, agglomerates, and floats.
  • the added amount of Al is such that the final content of Al exceeds 0.020%, the Ti oxide is completely reduced, and finally the above-mentioned composite oxide is sufficiently obtained. I can't get it.
  • Ca which is a stronger deoxidizing element than Ti and Al
  • Ca is preferably added so that the final content of Ca is 0.0005 to 0.0100%.
  • the amount of Ca input is such that the final content of Ca is less than 0.0005%
  • the amount of Ti, Al, Ca or these oxides obtained by sequential deoxidation is small.
  • the amount of TiN precipitated as a nucleus decreases, and a high-strength hot-rolled steel sheet excellent in plate forging cannot be obtained.
  • Mg or REM which is a strong deoxidation element from Ti and Al similarly to Ca instead of Ca. That is, within 5 minutes after adding Al, Mg, which is a stronger deoxidizing element than Ti and Al, is added and deoxidized within a range where the final Mg content is 0.0005 to 0.0100%. May be. Similarly, within 5 minutes after adding Al, REM, which is a stronger deoxidizing element than Ti and Al, is added within a range where the final content of REM is 0.0005 to 0.1000%. May be.
  • two or more selected from Ca, Mg, and REM may be added in combination.
  • the final total content of Ca, Mg and REM becomes 0.0005 or more, the final content of Ca and Mg is 0.0100% or less, and the final content of REM is 0. It may be deoxidized by adding one or more selected from Ca, Mg and REM so as to be 1000% or less.
  • (B) Heating process The manufactured slab is hot-rolled to obtain a hot-rolled steel sheet.
  • the slab is heated.
  • the slab is heated to a temperature of SRTmin ° C. or higher and 1260 ° C. or lower represented by the following formula (i).
  • SRTmin means the solution temperature of TiC.
  • SRTmin 7000 / ⁇ 2.75-log (Ti ⁇ C) ⁇ -273 (i)
  • the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing.
  • the end temperature of rough rolling is 1100 ° C. or higher so that precipitates containing Ti do not precipitate.
  • the multistage finish rolling is performed by continuous rolling of three or more stages (for example, six stages or seven stages). Then, multi-stage finish rolling is performed so that the cumulative strain (effective cumulative strain) in the final three-stage rolling is 0.01 to 0.10.
  • the effective cumulative strain is the change in crystal grain size due to rolling temperature, rolling reduction of the steel sheet due to rolling, and change in crystal grain size where the crystal grains recover statically over time after rolling. It is an index that takes into account.
  • the effective cumulative strain ( ⁇ eff) can be obtained by the following equation.
  • Effective cumulative strain ( ⁇ eff) ⁇ i (ti, Ti) (iii)
  • ⁇ i is expressed by the following equation.
  • ⁇ i (ti, Ti) ei / exp ((ti / ⁇ R) 2/3 )
  • ti Time from the last i-th rolling to the start of primary cooling after the last rolling (s)
  • Q Q:
  • the desired microstructure can be obtained and the variation in nano hardness can be reduced.
  • it suppresses the growth of voids generated at the interface between the hard and soft tissues, makes it difficult to bond even when voids grow, and does not generate cracks even after plate forging. Steel plate can be obtained.
  • the end temperature of multi-stage finish rolling may be Ar 3 (° C.) + 30 ° C. or higher using Ar 3 obtained by the following equation (ii). This is because the intended precipitation-strengthened ferrite and bainite are obtained in the present invention.
  • Ar 3 970-325 ⁇ C + 33 ⁇ Si + 287 ⁇ P + 40 ⁇ Al ⁇ 92 ⁇ (Mn + Mo + Cu) ⁇ 46 ⁇ (Cr + Ni) (ii)
  • the element symbol in the said formula represents content (mass%) in the hot-rolled steel plate of each element, and shall substitute 0 when not containing.
  • the average cooling rate in the first cooling step is less than 10 ° C./s, pearlite is easily generated.
  • the upper limit of the cooling rate is not particularly limited, but may be 300 ° C./s or less in order to avoid overcooling.
  • the holding temperature in the atmosphere is lower than 650 ° C., bainite is easily generated, and the bainite area ratio is increased.
  • the holding temperature in the atmosphere exceeds 750 ° C., pearlite is easily generated.
  • holding in the atmosphere includes that the hot-rolled steel sheet is limited to air cooling or cooling in the cooling facility to a minimum, and the lower limit of the cooling rate at this time is ideally 0 ° C./s.
  • the upper limit is 8 ° C./s.
  • (E) Second (accelerated) cooling step After holding in the air, cooling is performed at an average cooling rate of 10 ° C./s or more from a temperature range of 600 to 740 ° C.
  • the cooling start temperature is less than 600 ° C.
  • the ferrite transformation does not proceed sufficiently and the precipitation of fine Ti precipitates becomes insufficient.
  • the cooling start temperature exceeds 740 ° C.
  • the ferrite transformation proceeds excessively, and pearlite is generated, which may deteriorate the hole expandability.
  • the average cooling rate is less than 10 ° C./s, pearlite is generated and the hole expandability may be deteriorated.
  • the upper limit of the average cooling rate is not particularly limited, but it may be 1000 ° C./s or less because there is a concern that the steel sheet warps due to thermal strain due to thermal deviation.
  • (F) Winding step Thereafter, the cooled hot-rolled steel sheet is wound at a winding temperature of 450 to 650 ° C.
  • the conditions after the winding process are not particularly limited.
  • Tables 2 and 3 steel having the chemical composition shown in Table 1 is melted to produce a slab, and the slab is hot-rolled under the conditions shown in Tables 2 and 3 and then cooled. Then, it was wound up to produce a hot rolled steel sheet.
  • Tables 4 and 5 show the thickness of the obtained hot-rolled steel sheet.
  • Metal structure The metal structure of the obtained hot rolled steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, in the cross section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, 1/4 W from the end face of the steel sheet and 1 from the surface of the steel sheet A specimen for observing the metal structure was cut out from the position of / 4t.
  • the rolling direction cross section (so-called L direction cross section) of the above test piece was subjected to nital etching, and after etching, observation was performed in a 300 ⁇ m ⁇ 300 ⁇ m visual field using an optical microscope. Then, by performing image analysis on the obtained structure photograph, the area ratio A of ferrite, the area ratio B of pearlite, and the total area ratio C of bainite, martensite and retained austenite were obtained.
  • the nital-etched portion was repeller-etched and observed with a 300 ⁇ m ⁇ 300 ⁇ m field of view using an optical microscope.
  • the total area rate D of a retained austenite and a martensite was computed by performing image analysis with respect to the obtained structure
  • the volume ratio of the retained austenite was calculated
  • the area ratio of bainite was determined from the difference between the area ratio C and the area ratio D, and the area ratio of martensite was determined from the difference between the area ratio E and the area ratio D. By this method, the area ratios of ferrite, bainite, martensite, retained austenite, and pearlite were determined.
  • the area ratio of precipitation-strengthened ferrite was measured by EBSP after polishing the test piece with a colloidal silica abrasive and measuring a field of view of 160 ⁇ 256 ⁇ m at a magnification of 400 ⁇ under a measurement step of 0.5 ⁇ m. I asked for.
  • the fine Ti precipitate was also subjected to electrolytic polishing of the test piece and measured by a three-dimensional atom probe measurement method, and the equivalent circle diameter and number density were obtained.
  • the test specimen was observed at 20 magnifications at a magnification of 1000 times and a 60 ⁇ 40 ⁇ m visual field, and the average equivalent circular diameter of TiN was determined by image processing. Further, the shortest distance between TiNs was obtained by observing the same part as that in the structure investigation with a 500 times metal microscope.
  • the tensile strength properties (tensile strength (TS), uniform elongation (u-EL), hole expansion rate ( ⁇ )) are 1 / (2) from one end of the plate to the plate width direction when the plate width is W.
  • TS tensile strength
  • u-EL uniform elongation
  • ⁇ hole expansion rate
  • the test piece of the simple shear test is a direction (width direction) orthogonal to the rolling direction at a position of 1/4 W or 3/4 W from one end of the plate to the plate width direction when the plate width of the steel plate is W. Is taken as the longitudinal direction.
  • An example of a test piece is shown to Fig.1 (a).
  • the test piece of the simple shear test shown in FIG. 1 has a rectangular thickness of 23 mm in the width direction of the steel plate and 38 mm in the rolling direction of the steel plate so that both sides are evenly ground so that the thickness is 2.0 mm. It processed so that it might become a test piece.
  • the chucking part 2 on both sides is chucked by 10 mm toward the long piece side (rolling direction) of the test piece in the short piece direction (width direction), and a shear width of 3 mm (shear deformation generating part 1) is formed at the center of the test piece. It was made to provide. In addition, when the plate thickness was less than 2.0 mm, the plate thickness was tested as it was without grinding. Moreover, the center of the test piece was marked with a straight line with a pen or the like in the short piece direction (width direction).
  • FIG. 1B shows an example of a test piece subjected to shear deformation.
  • shear strain ⁇ s tan ( ⁇ )
  • the simple shear test In the simple shear test, a simple shear tester (maximum displacement 8 mm) was used. Therefore, there is a limit on the stroke (displacement) of the testing machine. In addition, due to the occurrence of cracks at the end of the test piece or at the chuck part, in one shear test, the test may not be performed until the test piece breaks. Therefore, as described above, the “multi-stage shear test method” is adopted, which repeats a series of operations such as loading of the shear test load, unloading of the load, cutting off the end of the chuck part of the test piece in a straight line, and reloading of the load. did.
  • Shear modulus is taken into account from the shear strain ( ⁇ s) obtained in each stage of the shear test.
  • the shear plastic strain ( ⁇ sp) obtained by subtracting the shear elastic strain ( ⁇ se) was determined as follows, and the shear plastic strain ( ⁇ s) at each stage was combined and joined together.
  • Shear plastic strain ⁇ sp Shear strain ⁇ s-Shear elastic strain ⁇ se
  • Shear elastic strain ⁇ se ⁇ s / G ⁇ s: Shear stress
  • G Shear elastic modulus
  • G Shear elastic modulus
  • G E / 2 (1 + ⁇ ) ⁇ 78000 (MPa).
  • the test is performed until the specimen breaks. In this way, the relationship between the shear stress ⁇ s and the shear plastic strain ⁇ sp can be traced.
  • the shear plastic strain when the test piece breaks is ⁇ spf.
  • the standard deviation of nano hardness was measured.
  • the specimen for observing the metallographic structure was ground again, and at a load of 1 mN (loading 10 s, unloading 10 s), a 1/4 depth position (1 / 4t part), a measurement area of 25 ⁇ m ⁇ 25 ⁇ m was measured at intervals of 5 ⁇ m. From the results, the average value of nano hardness and the standard deviation of nano hardness were calculated.
  • the measurement of nano hardness was carried out using a Triscope or TriboIndenter manufactured by Hystron.
  • the hot-rolled steel sheet according to the present invention has a tensile strength (TS) of 780 MPa or more, a product of uniform elongation u-EL and tensile strength TS (TS ⁇ u -EL) is 7000 MPa ⁇ % or more, the product (TS ⁇ ⁇ ) of the hole expansion ratio ⁇ and the tensile strength TS is 50000 MPa ⁇ % or more, and a hot-rolled steel sheet having well-balanced characteristics is obtained. Moreover, it was confirmed that the hot-rolled steel sheet according to the present invention is a steel sheet that has an equivalent plastic strain exceeding 0.90 (90%) and can withstand high strain region processing such as plate forging.
  • the hot-rolled steel sheet according to the present invention can be widely used for machine parts and the like.
  • the remarkable effect can be obtained.

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Abstract

L'invention concerne une tôle d'acier laminée à chaud, la composition chimique de ladite tôle d'acier étant, en % en masse, C : de 0,020 à 0,070 %, Si : de 0,05 à 1,70 %, Mn : de 0,60 à 2,50 %, Al : de 0,005 à 0,020 %, N : de plus de 0,0030 à 0,0060 %, P ≤ 0,050 %, S ≤ 0,005 %, Ti : de 0,015 à 0,170 %, Nb : de 0 à 0,100 %, V : de 0 à 0,300 %, Cu : de 0 à 2,00 %, Ni : de 0 à 2,00 %, Cr : de 0 à 2,00 %, Mo : de 0 à 1,00 %, B : de 0 à 0,0100 %, Ca : de 0 à 0,0100 %, Mg : de 0 à 0,0100 %, terres rares : de 0 à 0,1000 %, Zr : de 0 à 1,000 %, Co : de 0 à 1,000 %, Zn : de 0 à 1,000 %, W : de 0 à 1,000 %, Sn : de 0 à 0,050 %, le reste étant du Fe et des impuretés, et Ca + Mg + REM ≥ 0,0005. La structure métallique de la tôle d'acier comprend, en % en surface, de 5 à 70 % de ferrite, de 30 à 95 % de bainite, 2 % ou moins de phase gamma résiduelle, 2 % ou moins de martensite, 1 % ou moins de perlite, avec ferrite + bainite ≥ 95 %. La densité en nombre de micro-dépôts de Ti dans les grains de ferrite est de 1,0 × 1016 à 50,0 × 1016 /cm3 ; le diamètre circulaire équivalent moyen des dépôts de TiN dans la tôle d'acier est de 1,0 à 10,0 µm ; la valeur moyenne de la plus courte distance entre dépôts de TiN adjacents est d'au moins 10,0 µm ; et l'écart-type de la nano-dureté est de 1,00 GPa ou moins.
PCT/JP2017/013749 2017-03-31 2017-03-31 Tôle d'acier laminée à chaud, pièce en acier forgé et procédés de production associés Ceased WO2018179391A1 (fr)

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MX2019011711A MX2019011711A (es) 2017-03-31 2017-03-31 Lamina de acero laminada en caliente, pieza forjada de acero y metodo de produccion.
EP17904326.0A EP3604587A4 (fr) 2017-03-31 2017-03-31 Tôle d'acier laminée à chaud, pièce en acier forgé et procédés de production associés
BR112019017622-5A BR112019017622A2 (pt) 2017-03-31 2017-03-31 Chapa de aço laminada a quente, peça de aço forjado, e métodos de produção da mesma
PCT/JP2017/013749 WO2018179391A1 (fr) 2017-03-31 2017-03-31 Tôle d'acier laminée à chaud, pièce en acier forgé et procédés de production associés
JP2017559726A JP6332571B1 (ja) 2017-03-31 2017-03-31 熱間圧延鋼板および鋼製鍛造部品ならびにそれらの製造方法
CN201780089327.2A CN110475890A (zh) 2017-03-31 2017-03-31 热轧钢板和钢制锻造部件及其制造方法
US16/485,360 US20200024679A1 (en) 2017-03-31 2017-03-31 Hot rolled steel sheet, steel forged part and production method therefor
KR1020197028709A KR20190126100A (ko) 2017-03-31 2017-03-31 열간 압연 강판 및 강제 단조 부품 및 그들의 제조 방법

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MX2019011711A (es) 2019-11-21
US20200024679A1 (en) 2020-01-23
CN110475890A (zh) 2019-11-19
EP3604587A4 (fr) 2020-09-09

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