WO2015060311A1 - Hot-rolled steel sheet having excellent surface hardness after carburizing heat treatment and excellent drawability - Google Patents

Hot-rolled steel sheet having excellent surface hardness after carburizing heat treatment and excellent drawability Download PDF

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Publication number
WO2015060311A1
WO2015060311A1 PCT/JP2014/077983 JP2014077983W WO2015060311A1 WO 2015060311 A1 WO2015060311 A1 WO 2015060311A1 JP 2014077983 W JP2014077983 W JP 2014077983W WO 2015060311 A1 WO2015060311 A1 WO 2015060311A1
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less
crystal grains
steel
hot
steel sheet
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PCT/JP2014/077983
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French (fr)
Japanese (ja)
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梶原　桂
土田　武広
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株式会社神戸製鋼所
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Priority to US15/031,016 priority Critical patent/US20160265078A1/en
Priority to DE112014004834.8T priority patent/DE112014004834T5/en
Priority to MX2016005086A priority patent/MX2016005086A/en
Priority to CN201480057665.4A priority patent/CN105658830B/en
Publication of WO2015060311A1 publication Critical patent/WO2015060311A1/en

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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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • 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
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • 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
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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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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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/08—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
    • 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
    • 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
    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • 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
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces
    • 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
    • C21D2201/00—Treatment for obtaining particular effects
    • C21D2201/05—Grain orientation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a hot-rolled steel sheet that exhibits a good cold workability during processing before heat treatment, and exhibits a predetermined surface hardness and a desired hardness even deep from the surface after carburizing heat treatment.
  • steel materials used as various structural parts especially for parts subjected to surface hardening by carburizing or carbonitriding to improve wear resistance and fatigue resistance, such as automobiles
  • the present invention relates to a hot-rolled steel sheet that is useful as a material for manufacturing a clutch, a damper, a gear (gear), and the like.
  • a case where the present invention is applied to clutches will be taken up as a representative, and the description will proceed.
  • the present invention is not limited to the manufacture of the present invention, and its excellent carburizing hardenability and carbonitriding are also described. It is effectively utilized as a material for manufacturing parts that require high surface hardness and excellent impact properties by making the surface layer hard while maintaining the high toughness of the core by utilizing the permeability.
  • cold working (cold forging) has advantages of higher productivity than hot working and warm working and good dimensional accuracy and yield of steel materials.
  • the problem in manufacturing parts by such cold working is that in order to ensure the strength of the cold-worked parts to be higher than the expected value, the strength, ie deformation, is inevitably required. It is necessary to use a steel material with high resistance. However, the higher the deformation resistance of the steel material to be used, there is a difficulty in reducing the life of the cold working mold.
  • carburizing heat treatment is performed to produce a high-strength part with a predetermined strength and surface hardness.
  • the cold workability before heat treatment is reduced. Therefore, there has been a demand for a solution that ensures both cold workability and improved surface hardness after carburizing heat treatment.
  • Patent Documents 2 to 6 are listed as conventional techniques related to hot-rolled steel sheets.
  • the hot rolled steel sheet disclosed in Patent Document 2 has an average r value of 1.2 or more, an r value (rL) in the rolling direction of 1.3 or more, and an r value (rD) in the 45 ° direction with respect to the rolling direction. 0.9 or more, and the r value (rC) in the direction perpendicular to the rolling direction is 1.2 or more, and each X of ⁇ 111 ⁇ , ⁇ 100 ⁇ , and ⁇ 110 ⁇ of the plate surface in the steel plate 1/2 plate thickness It is said that the workability is improved when the line reflection surface random intensity ratio is 2.0 or more, 1.0 or more, and 0.2 or more, respectively.
  • the hot-rolled steel sheet disclosed in Patent Document 3 has a structure containing a bainite phase of 40 to 95% by volume with the balance being a ferrite phase, and the ferrite has an average crystal grain size of 1.2 ⁇ m or more. By being less than 4 ⁇ m and having at least one of the following characteristics (a) and (b), ductility and burring workability are improved.
  • the hot-rolled steel sheet disclosed in Patent Document 4 has a microstructure in which the average grain size of the ferrite phase is 5 ⁇ m or less, and further the ferrite phase is present in an area ratio of 50% or more, and the 1 ⁇ 4 thickness of the steel sheet
  • the average ODF analysis strength f in the (113) [1-10] to (223) [1-10] orientations of the plate surface at is assumed to be 4 or more, whereby the rigidity is improved.
  • ⁇ r1 defined by the following formula (1) is ⁇ 0.20 or more and 0.20 or less
  • ⁇ r2 defined by the following formula (2) is 0.42 or less. It is said that drawability is improved by assuming that.
  • ⁇ r1 (r0-2r45 + r90) / 2 (1)
  • ⁇ r2 rmax ⁇ rmin (2)
  • r0 r value measured with a test specimen taken in parallel with the rolling direction of the plate surface
  • r45 r value measured with a test piece taken in the direction of 45 ° with respect to the rolling direction of the plate surface
  • r90 r value measured with a test specimen taken in the 90 ° direction with respect to the rolling direction of the plate surface
  • rmax the maximum value of r0, r45 and r90
  • rmin The minimum value of r0, r45, and r90.
  • the hot rolled steel sheet disclosed in Patent Document 6 has an average ferrite grain size of 1 to 10 ⁇ m, a standard deviation of ferrite grain size of 3.0 ⁇ m or less, and an inclusion shape ratio of 2.0 or less. It is said that stretch flangeability is improved.
  • Patent Documents 2 to 6 are excellent in cold workability such as drawing workability, there is no mention of surface hardness after carburizing heat treatment, and the improvement effect is unknown. is there.
  • the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 7 has an average hardness of 170 HV or more up to a depth of 50 ⁇ m in the thickness direction surface layer portion, and has a metal structure of ferrite + pearlite, surface carbon
  • Patent Document 7 Although the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 7 is excellent in surface hardness after carburizing heat treatment, there is no mention of cold workability as well as drawing workability, and its improvement effect Is unknown.
  • Japanese Patent No. 3094856 Japanese Patent No. 3742559 Japanese Patent No. 41161935 Japanese Patent No. 4867257 Japanese Unexamined Patent Publication No. 2013-119635 Japanese Patent No. 4276504 Japanese Unexamined Patent Publication No. 2010-222663
  • an object of the present invention is to provide a hot-rolled steel sheet having both drawability and surface hardness after carburizing heat treatment.
  • the carburizing heat treatment includes not only normal carburizing but also heat treating for carbonitriding.
  • the invention described in claim 1 The plate thickness is 2-10mm, Ingredient composition % By mass (hereinafter the same for chemical components) C: 0.05 to 0.30%, Mn: 0.3 to 3.0%, Al: 0.015 to 0.1%, N: 0.003 to 0.30% included,
  • the balance consists of iron and inevitable impurities,
  • the area ratio of crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more, In the rolling direction, the total area ratio of the crystal grains whose crystal direction is within 10 ° from the ⁇ 001> direction and the crystal grains whose crystal direction is within 10 ° from the ⁇ 110> direction is 25% or less,
  • the average grain size of all the crystal grains is 3 to 50 ⁇ m
  • the hot-rolled steel sheet is excellent in drawing workability
  • the invention described in claim 2 The hot rolled steel sheet according to claim 1, wherein among the inevitable impurities, Si: 0.5% or less, P: 0.030% or less, and S: 0.035% or less.
  • the invention according to claim 3 The hot rolled steel sheet according to claim 1 or 2, wherein the component composition further contains at least one of the following (a) to (f).
  • V selected from the group consisting of 0.5% or less (not including 0%), Ti: 0.1% or less (not including 0%), and Nb: 0.1% or less (not including 0%)
  • At least one At least one selected from the group consisting of Ca: 0.08% or less (excluding 0%) and Zr: 0.08% or less (not including 0%)
  • Mg 0.02%
  • the present invention by controlling the texture of the hot-rolled steel sheet to a predetermined structure form in the structure mainly composed of ferrite and pearlite, by improving the deformability during processing even in a part that requires drawing workability.
  • the steel sheet is less prone to cracking, and the parts obtained after the carburizing heat treatment can provide a hot-rolled steel sheet that can ensure a predetermined surface hardness.
  • the hot-rolled steel sheet according to the present invention (hereinafter also referred to as “the steel sheet of the present invention” or simply “the steel sheet”) will be described in more detail.
  • the steel sheet of the present invention overlaps with the hot forging material (high-strength, high-toughness case hardening steel) described in Patent Document 1 above.
  • the hot forging material high-strength, high-toughness case hardening steel
  • the steel sheet of the present invention has a thickness of 2 to 10 mm. If the plate thickness is less than 2 mm, rigidity as a structure cannot be secured. On the other hand, if the plate thickness exceeds 10 mm, it is difficult to achieve the tissue form defined in the present invention, and the desired effect cannot be obtained.
  • the lower limit of the plate thickness is preferably 3 mm or more, and more preferably 4 mm or more. Further, the upper limit is preferably 9 mm or less, and more preferably 7 mm or less.
  • C is an element indispensable for securing the core strength of the carburized (or carbonitrided) quenching part finally obtained. If it is less than 0.05%, sufficient strength cannot be obtained. However, if it is contained excessively, the toughness is deteriorated, and the machinability and cold forgeability are deteriorated and the workability is impaired, so 0.30% is made the upper limit.
  • a preferable content of C is in the range of 0.08 to 0.25%.
  • Mn is an element effective for deoxidation of molten steel, and in order to exert its effect effectively, it must be contained in an amount of 0.3% or more. However, if it is excessively contained, cold workability and machinability are reduced. In addition to having an adverse effect and increasing the amount of segregation to the crystal grain boundary, it lowers the grain boundary strength and thus adversely affects the impact characteristics, so it must be suppressed to 3.0% or less.
  • a preferable content of Mn is in the range of 0.5 to 2.0%.
  • Al is an element contained in steel as a deoxidizing material for steel, and has an action of binding to N in steel to produce AlN and preventing coarsening of crystal grains. In order to exert such an effect effectively, it must be contained at 0.015% or more, but the effect is saturated at about 0.1%, and beyond that, it combines with oxygen to form a non-metallic inclusion, Since it adversely affects the characteristics, etc., the upper limit was set to 0.1%. Preferably it is 0.08% or less, More preferably, it is 0.06% or less, Most preferably, it is 0.04% or less.
  • N combines with Al, V, Ti, Nb, etc. in steel to produce nitrides, and has the effect of suppressing the coarsening of crystal grains.
  • the effect is by containing 0.003% or more. Effectively demonstrated. Preferably, it is 0.005% or more. However, these effects are saturated at about 0.30%, and if it is contained more than that, nitrides become inclusions and adversely affect the physical properties, so 0.30% was set as the upper limit.
  • it is 0.10% or less, More preferably, it is 0.05% or less, Most preferably, it is 0.03% or less.
  • the steel sheet of the present invention basically contains the above components, and the balance is iron and inevitable impurities, but it is desirable to suppress Si, P and S which are inevitably mixed in as little as possible for the following reasons.
  • Si effectively acts as a strengthening element or a deacidifying element, but promotes grain boundary oxidation to deteriorate bending fatigue properties and adversely affects cold forgeability. Therefore, in order to eliminate such obstacles, the content must be suppressed to 0.5% or less, and particularly when a high level of bending fatigue characteristics is required, it is desirable to suppress the content to 0.1% or less. It is. From such a viewpoint, the more preferable content of Si is in the range of 0.02 to 0.1%.
  • ⁇ P 0.030% or less> P segregates at the grain boundaries and lowers the toughness, so the upper limit was set to 0.030%.
  • the more preferable content of P is 0.020% or less, and further preferably 0.010% or less.
  • S generates MnS and contributes to the improvement of machinability.
  • the S content must be suppressed to 0.035% or less.
  • the more preferable content of S is 0.025% or less, and further preferably 0.020% or less.
  • the steel sheet of the present invention can contain the following permissible components within the range not impairing the action of the present invention.
  • Cr has an excellent effect of improving hardenability
  • Mo is an incompletely hardened structure. This effectively acts to improve the hardenability and the grain boundary strength, and Ni contributes to the improvement of impact resistance by refining the structure after quenching.
  • Such an effect is preferably exhibited by containing at least one of Cr: 0.2% or more, Mo: 0.08% or more, and Ni: 0.2% or more.
  • Cu is an element that effectively acts to improve corrosion resistance, and the effect is preferably exerted by inclusion of 0.3% or more, but the effect is saturated at 2.0%, so it is contained more than that. Is useless. If Cu is contained alone, the hot workability of the steel material tends to deteriorate. Therefore, in order to avoid such adverse effects, Ni having the effect of improving the hot workability should be used in the above content range. Is desirable.
  • Cu and Co are elements that have an effect of strain aging and hardening the steel material, and are effective in improving the strength after processing.
  • these elements are preferably contained in an amount of 0.1% or more, and more preferably 0.3% or more.
  • the Co content is excessive, the effects of strain aging and hardening of the steel material, and the effect of improving the strength after processing are saturated, and there is a possibility of promoting cracking, so the Co content is It is recommended to set it to 5% or less, further 4% or less, especially 3% or less.
  • V 0.5% or less (excluding 0%)
  • Ti 0.1% or less (excluding 0%)
  • Nb at least one selected from the group consisting of 0.1% or less (not including 0%)>
  • V 0.03%
  • Ti 0.005%
  • Nb 0.005%
  • action which improves the impact characteristic of a horizontal eye, since those effects are saturated at 0.08%, respectively, it is 0.08% or less, Furthermore, 0.05% or less, Especially 0.01% or less It is recommended to do.
  • the preferable lower limit for making the said effect of these elements exhibit effectively is Ca: 0.0005% (further 0.001%), Zr: 0.002%.
  • Sb 0.02% or less (excluding 0%)> Sb is an element effective for suppressing the grain boundary oxidation and increasing the bending fatigue strength, but since the effect is saturated at 0.02%, the addition of more is economically useless.
  • a preferable lower limit value for effectively exhibiting the effect of addition of Sb is 0.001%.
  • ⁇ REM 0.05% or less (excluding 0%), Mg: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), Pb: 0.5% or less (excluding 0%), Bi: at least one selected from the group consisting of 0.5% or less (excluding 0%)> REM, like Zr and Ca, is an element that spheroidizes sulfide compound inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability.
  • REM is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected.
  • REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably La and / or Ce.
  • Mg is an element that spheroidizes sulfide compound inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability.
  • Mg is preferably contained in an amount of 0.0002% or more, more preferably 0.0005% or more.
  • 0.02% or less is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.
  • Li like Zr and Ca, can spheroidize sulfide compound inclusions such as MnS to improve the deformability of steel, and lower the melting point of Al-based oxides to make them harmless. It is an element that contributes to improvement.
  • Li is preferably contained in an amount of 0.0002% or more, and more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.
  • Pb is an effective element for improving machinability.
  • Pb is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, if it is contained excessively, production problems such as generation of rolling defects occur, so 0.5% or less, further 0.4% or less, particularly 0.3% or less is recommended.
  • Bi is an element effective for improving the machinability like Pb.
  • Bi is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more.
  • the effect of improving the machinability is saturated even if contained excessively, 0.5% or less, further 0.4% or less, and particularly 0.3% or less are recommended.
  • the steel sheet of the present invention has a structure mainly composed of ferrite and pearlite, and is particularly characterized in that the form of the texture in the steel is more strictly controlled.
  • the structure mainly composed of ferrite and pearlite means that the total amount of ferrite and pearlite is 90% or more in terms of area ratio. If the total amount of ferrite and pearlite is 90% or more in terms of area ratio, a small amount of other structures (bainite, martensite, etc.) may be generated, but it is desirable that the other structures be as small as possible.
  • the deep drawing workability of a thin steel sheet used for a car body outer plate of an automobile is determined by the plastic anisotropy (r value (rankford value): tensile
  • r value crankford value
  • tensile The greater the ratio between the plate width strain and the plate thickness strain in the test), the higher the workability, and the stronger the ⁇ 111 ⁇ plane parallel to the plane orientation in the recrystallized texture, the ⁇ 100 ⁇ plane orientation
  • Patent Documents 2 to 4 various attempts have been made to improve workability by texture control in steel sheets for vehicle body outer plates. No efforts were made.
  • the steel sheet of the present invention is characterized by controlling the orientation and size of the crystal grains within a specific range with respect to the crystal grains of all phases including ferrite and pearlite.
  • all crystal grains including ferrite and pearlite (hereinafter referred to as “all crystal grains”) existing at a position of depth t / 4 (t: plate thickness).
  • the area ratio of crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more>
  • the formation of the texture differs depending on the processing method even if the crystal system is the same.
  • it is expressed by the rolling surface and the rolling direction. That is, as shown below, the rolling surface is represented by ⁇ xxx ⁇ and the rolling direction is represented by ⁇ >.
  • delta) have shown the integer.
  • the expression of each of these directions is described in, for example, edited by Junichi Nagashima: “Aggregation” (published by Maruzen Co., Ltd.).
  • the plate surface orientation is controlled by controlling the area ratio within 10 ° from the (123) plane to 20% or more. Cold workability and drawing workability can be improved.
  • the crystal grains having the (123) plane as the plate surface orientation have an effect of improving the cold workability in the softened state, and in order to effectively exhibit such an effect, the area A rate of 20% or more is required.
  • the area A rate of 20% or more is required.
  • it is 22% or more, more preferably 24% or more, particularly preferably 26% or more.
  • the structure form was defined with a depth position of 1/4 of the sheet thickness as a representative position.
  • crystal grains having a plate plane orientation within 10 ° from the ideal plane orientation ((123) plane) have almost the same action, and thus are defined by the area ratio of crystal grains having a plane plane orientation within that range. It was decided to.
  • the total area ratio of crystal grains whose crystal direction is within 10 ° from the ⁇ 001> direction and crystal grains whose crystal direction is within 10 ° from the ⁇ 110> direction is 25% or less> Since the in-plane anisotropy in drawing workability increases as the total area ratio increases, it is limited to 25% or less, preferably 23% or less, and more preferably 20% or less.
  • the martensitic transformation proceeds while maintaining the crystal orientation relationship of the original crystal grains during quenching after the carburizing heat treatment, so that the transformation strain is reduced.
  • the effect of improving dimensional accuracy can also be obtained.
  • the occurrence of in-plane anisotropy during molding processing indicates that non-uniform strain occurs in the part, which is the cause of carburizing heat treatment, after quenching Since the dimensional accuracy of the component is deteriorated, it is considered that the effect of improving the dimensional accuracy of the component can be obtained by the effect of reducing the in-plane anisotropy by the texture control described above.
  • the average grain size of all the crystal grains is 3 to 50 ⁇ m>
  • the average grain size of all the crystal grains is in the range of 3 to 50 ⁇ m in order to improve the workability (drawing workability, bending workability, press workability) of the steel sheet and satisfy the surface properties after working. is necessary. If the crystal grains become too fine, the deformation resistance becomes too high, so the average grain size is 3 ⁇ m or more, preferably 4 ⁇ m or more, more preferably 5 ⁇ m or more. On the other hand, if the crystal grains become too coarse, the toughness, fatigue characteristics, etc. deteriorate, and even if the crystal orientation is controlled, the press formability such as bending workability and overhang is remarkably reduced, and cracks and rough surfaces during molding occur.
  • the average particle size is 50 ⁇ m or less, preferably 45 ⁇ m or less, and more preferably 40 ⁇ m or less. Similar to the above, there is a crystal grain size distribution in the plate thickness direction, but the average grain size of all crystal grains was defined with a depth position of 1/4 of the plate thickness as a representative position.
  • the crystal plane orientation of the crystal grains is measured and analyzed by SEM-EBSP (Electron Back Scattering Pattern) and EBSD (Electron Back Scattering Diffraction).
  • SEM-EBSP Electron Back Scattering Pattern
  • EBSD Electro Back Scattering Diffraction
  • SEM apparatus for example, SEM (JEOLJSM5410) manufactured by JEOL Ltd. is used, and for example, EBSP: manufactured by TSL (OIM) is used as the EBSP measurement / analysis system.
  • the measurement area of the sample is 300 to 1000 ⁇ m ⁇ 300 to 1000 ⁇ m, and the measurement step interval is 1 to 3 ⁇ m, for example.
  • the crystal direction in the rolling direction of the crystal grains is EBSP measurement with respect to the cross section (side surface) in the rolling direction of the steel sheet, and by analysis, the crystal direction is within the 10 ° direction from the ⁇ 001> direction.
  • the crystal grains whose crystal direction is within 10 ° from the ⁇ 110> direction are identified.
  • the measurement area of the sample is 300 to 1000 ⁇ m ⁇ 300 to 1000 ⁇ m at a quarter part in the plate thickness direction, and the measurement step interval is, for example, 1 to 3 ⁇ m. From the crystal directions of each crystal grain thus identified, those with an orientation within 10 ° from each ideal plane direction are totaled to obtain a total area, and divided by the area of the measurement region, thereby obtaining each ideal crystal direction. The area ratio for each was determined.
  • the average grain size of all the crystal grains is determined by measuring the maximum diameter of each crystal grain observed in a predetermined measurement region using the SEM-EBSP and measurement conditions thereof, and calculating the average value of the average grain diameters. As sought.
  • the steel sheet of the present invention is obtained, for example, by melting and casting raw material steel having the above-mentioned composition to form a slab, and by subjecting the slab as it is or surface chamfered to each step of heating, hot rough rolling, and finish rolling. It can be manufactured as a hot rolled coil rising material. Thereafter, pickling and skin pass may be further performed according to necessary conditions such as surface condition and plate thickness accuracy.
  • a desired oxide can be generated by adding a predetermined alloy element in a predetermined order to molten steel in which the dissolved oxygen amount and the total oxygen amount are adjusted. Particularly in the present invention, it is extremely important to adjust the total oxygen amount after adjusting the dissolved oxygen amount so that coarse oxides are not formed.
  • Dissolved oxygen means oxygen in a free state that does not form oxides and exists in molten steel. Total oxygen means the sum of all oxygen contained in molten steel, that is, free oxygen and oxygen forming oxides.
  • the dissolved oxygen content of the molten steel is adjusted to a range of 0.0010 to 0.0060%.
  • the amount of dissolved oxygen in the molten steel is less than 0.0010%, the amount of dissolved oxygen in the molten steel is insufficient, so that a predetermined amount of Al—O-based oxide cannot be secured, and a desired size distribution cannot be obtained.
  • the amount of dissolved oxygen is set to 0.0010% or more.
  • the dissolved oxygen is preferably 0.0013% or more, more preferably 0.0020% or more.
  • the amount of dissolved oxygen should be suppressed to 0.0060% or less.
  • the amount of dissolved oxygen is preferably 0.0055% or less, more preferably 0.0053% or less.
  • the amount of dissolved oxygen in molten steel primarily refined in a converter or electric furnace usually exceeds 0.010%. Therefore, in the production method of the present invention, it is necessary to adjust the amount of dissolved oxygen in the molten steel to the above range by some method.
  • Examples of the method for adjusting the amount of dissolved oxygen in the molten steel include a method of vacuum C deoxidation using an RH type degassing refining device, a method of adding a deacidifying element such as Si, Mn, and Al.
  • the amount of dissolved oxygen may be adjusted by appropriately combining these methods.
  • a method of adding a deacidifying element such as Si may be adopted to adjust the amount of dissolved oxygen.
  • the deoxidizing element may be added when steel is removed from the converter to the ladle.
  • the molten steel is stirred, and the oxides in the molten steel are floated and separated so that the total oxygen content in the molten steel is 0.0010 to 0.00. Adjust to 0070%.
  • the molten steel in which the amount of dissolved oxygen is appropriately controlled is stirred to remove unnecessary oxides, and then generation of coarse oxides, that is, coarse inclusions can be prevented.
  • the total oxygen amount is set to 0.0010% or more.
  • the total oxygen amount is preferably 0.0015% or more, more preferably 0.0018% or more.
  • the total oxygen amount should be suppressed to 0.0070% or less.
  • the total oxygen amount is preferably 0.0060% or less, more preferably 0.0050% or less.
  • the total amount of oxygen in the molten steel changes generally in correlation with the stirring time of the molten steel, it can be controlled by adjusting the stirring time. Specifically, the total amount of oxygen in the molten steel is appropriately controlled while appropriately measuring the total amount of oxygen in the molten steel after stirring the molten steel and removing the floating oxide.
  • the total oxygen content in the molten steel is adjusted to the above range, and then REM is added before casting.
  • the desired oxide can be obtained by adding the above elements to the molten steel with the total oxygen content adjusted.
  • the form of REM and Ca added to the molten steel is not particularly limited.
  • REM pure La, pure Ce, pure Y, or pure Ca
  • Fe—Si—La alloy, Fe—Si—Ce alloy, An Fe—Si—Ca alloy, Fe—Si—La—Ce alloy, Fe—Ca alloy, Ni—Ca alloy, or the like may be added.
  • Misch metal is a mixture of cerium group rare earth elements, and specifically contains about 40 to 50% Ce and about 20 to 40% La.
  • misch metal often contains Ca as an impurity, when the misch metal contains Ca, it is necessary to satisfy the preferred range defined in the present invention.
  • the stirring time is preferably within 40 minutes.
  • the stirring time is more preferably within 35 minutes, and further preferably within 30 minutes.
  • the lower limit of the stirring time of the molten steel is not particularly limited, but if the stirring time is too short, the concentration of the additive element becomes non-uniform, and the desired effect cannot be obtained as a whole steel material. Accordingly, a desired stirring time corresponding to the container size is required.
  • molten steel with an adjusted composition can be obtained. It casts using the obtained molten steel, and obtains a steel piece.
  • manufacturing is performed by heating, hot rolling including finish rolling, rapid cooling after hot rolling, slow cooling after quenching stop, rapid cooling after slow cooling, and winding.
  • Heating before hot rolling is performed at 1150 to 1300 ° C.
  • An austenite single phase is obtained by this heating.
  • solid solution elements including additive elements such as V and Nb
  • the heating temperature is less than 1150 ° C., it cannot be dissolved in austenite, and coarse carbides are formed, so that the effect of improving fatigue characteristics cannot be obtained.
  • heating temperatures exceeding 1300 ° C. are difficult to operate.
  • Ti is contained as an additive element, the TiC solution solution temperature or higher and 1300 ° C. or lower are necessary also in terms of solid solution of Ti having the highest solution temperature among carbides.
  • a more preferable lower limit of the heating temperature is 1200 ° C.
  • the rough rolling temperature is set to 900 to 1200 ° C. in consideration of securing the subsequent finish rolling temperature, and the proportion of crystal grains having a predetermined crystal orientation is obtained by refining austenite grains in the rough rolling and repeatedly recrystallizing. Can be controlled.
  • the rough rolling temperature is more preferably 900 to 1100 ° C.
  • Hot rolling is performed so that the finish rolling temperature is 800 ° C. or higher. If the finish rolling temperature is too low, ferrite transformation occurs at a high temperature and the precipitated carbides in the ferrite are coarsened, so that a certain finish rolling temperature is required.
  • the finish rolling temperature is more preferably 850 ° C. or higher in order to coarsen austenite grains and increase the grain size of bainite.
  • the rolling reduction in the final pass of hot finish rolling is 10 to 18%, more preferably 11 to 17%, and particularly preferably 12 to 16%.
  • the quenching stop temperature is 680 ° C. or more, the precipitated carbide in the ferrite is coarsened, and the fatigue resistance characteristics cannot be ensured.
  • the quenching stop temperature is preferably 600 to 650 ° C, more preferably 610 to 640 ° C.
  • the slow cooling rate is less than 5 ° C./s
  • the amount of pro-eutectoid ferrite is increased and coarse grains are produced, and coarse grains are produced in the final steel plate, resulting in a non-uniform state of carbides, resulting in cold workability. Deteriorate.
  • the cooling rate is 20 ° C./s or more, a large amount of hard phase (bainite, martensite) is generated, and cold workability is deteriorated.
  • a test steel containing chemical components shown in Table 1 was melted, cast into a 150 kg ingot, and cooled.
  • the components are adjusted for elements other than Al, REM, and Ca, and deoxidized using at least one element selected from C, Si, and Mn.
  • the amount of dissolved oxygen in the molten steel was adjusted.
  • the total amount of oxygen in the molten steel was adjusted by stirring the molten steel in which the amount of dissolved oxygen was adjusted for about 1 to 10 minutes to float and separate oxides in the molten steel.
  • the molten steel which adjusted the component was obtained by adding to the molten steel which adjusted the total oxygen amount.
  • REM was added in the form of a misch metal containing about 25% La and about 50% Ce
  • Ca was added in the form of a Ni—Ca alloy, a Ca—Si alloy, or a Fe—Ca compact. .
  • the obtained ingot was hot-rolled under the conditions shown in Table 2 to produce a hot rolled plate having a predetermined thickness.
  • Table 2 the cooling rate after the rapid cooling stop is not described, but the condition of 10 ° C./s is adopted in each production example.
  • each said hot-rolling board in order to evaluate drawing workability about each said hot-rolling board, it is JIS No. 5 piece so that it may become 0 degree (parallel to a rolling direction), 45 degrees, and 90 degrees (perpendicular to a rolling direction) with respect to a rolling direction.
  • r values r0, r45, r90
  • the average r value and (DELTA) r value were computed with the following formula
  • equation the ⁇ r value is an index for evaluating the in-plane anisotropy of the r value.
  • the carburizing and quenching test was performed under the following conditions.
  • steel no. 1, 2, and 6 to 20 are steels that satisfy the requirements of the compositional composition of the present invention and are manufactured under the recommended hot rolling conditions. Yes, the drawing processability index and the surface hardness after carburizing heat treatment all meet the acceptance criteria, while ensuring good drawing workability, hot rolling showing a predetermined surface hardness (strength) after carburizing heat treatment It was confirmed that a steel plate was obtained.
  • Steel No. Reference numerals 3 to 5 and 21 to 27 are comparative steels that do not satisfy at least one of the component composition and the structure requirements defined in the present invention, and are at least one of the index of drawability and the surface hardness after carburizing heat treatment. Does not meet the acceptance criteria.
  • steel No Although the requirements for component composition 3 are satisfied, the heating temperature before hot rolling, rough rolling temperature, finish rolling temperature, quenching lethal temperature, and winding temperature are all out of the recommended range and are too low in the rolling direction.
  • the crystal directions of ⁇ 001> and ⁇ 110> directions are excessively formed, and the drawing workability, particularly the r value anisotropy, is inferior.
  • steel No. No. 26 steel type v
  • the hot rolling conditions are in the recommended range
  • the Al content is too high and the drawability is also inferior.
  • Steel No. No. 27 (steel type w) has a hot rolling condition in the recommended range, but the N content is too high and the drawability is inferior.
  • the hot-rolled steel sheet according to the present invention exhibits good cold workability during processing, exhibits hardness at a predetermined surface and a deep portion from the surface after carburizing heat treatment, and is excellent in wear resistance and fatigue resistance properties.
  • it is useful for clutches, dampers, gears (gears) and the like used in automobiles.

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Abstract

A hot-rolled steel sheet that has a sheet thickness of 2-10 mm and that contains specific amounts of C, Mn, Al, and N, the remainder comprising iron and unavoidable impurities. With regard to all crystal grains that exist at a depth of t/4 (t being sheet thickness), the average grain diameter of all of the crystal grains is 3-50 μm, the area ratio of crystal grains that have a sheet-plane orientation that is within 10° of a (123) plane is 20% or more, and the total area ratio of crystal grains that, in the rolling direction, have a crystal direction that is within 10° of a <001> direction and crystal grains that, in the rolling direction, have a crystal direction that is within 10° of a <110> direction is 25% or less.

Description

絞り加工性と浸炭熱処理後の表面硬さに優れる熱延鋼板Hot-rolled steel sheet with excellent drawability and surface hardness after carburizing heat treatment
　本発明は、熱処理前の加工中は良好な冷間加工性を示しつつ、浸炭熱処理後は、所定の表面硬さ、および表面から深いところでも所望の硬さを示す熱延鋼板に関し、詳しくは、各種の構造用部品として用いられる鋼材のうち、特に耐摩耗性や耐疲労特性を改善するため、浸炭焼入れまたは浸炭窒化焼入れ処理による表面硬質化処理の行なわれる部品、例えば自動車などの各部に用いられるクラッチ、ダンパー、歯車（ギア）等を製造するための素材として有用な熱延鋼板に関するものである。なお、以下の説明では、クラッチ類に適用する場合を代表的に取り上げて説明を進めるが、本発明はもとより、その製造に限定されるものではなく、その優れた浸炭焼入性や浸炭窒化焼入性を活用し、芯部の高靭性を維持しつつ表層部を硬質化して、高い表面硬さと優れた衝撃特性の求められる部品を製造するための素材として有効に活用される。 The present invention relates to a hot-rolled steel sheet that exhibits a good cold workability during processing before heat treatment, and exhibits a predetermined surface hardness and a desired hardness even deep from the surface after carburizing heat treatment. Of steel materials used as various structural parts, especially for parts subjected to surface hardening by carburizing or carbonitriding to improve wear resistance and fatigue resistance, such as automobiles The present invention relates to a hot-rolled steel sheet that is useful as a material for manufacturing a clutch, a damper, a gear (gear), and the like. In the following description, a case where the present invention is applied to clutches will be taken up as a representative, and the description will proceed. However, the present invention is not limited to the manufacture of the present invention, and its excellent carburizing hardenability and carbonitriding are also described. It is effectively utilized as a material for manufacturing parts that require high surface hardness and excellent impact properties by making the surface layer hard while maintaining the high toughness of the core by utilizing the permeability.
　近年、環境保護の観点から、自動車の燃費向上を目的として、自動車用の各種部品、例えばギアなどのトランスミッション部品やケース等に用いられる鋼材の軽量化、すなわち高強度化に対する要求が益々高まっている。このような軽量化・高強度化の要請に応えるために、一般に用いられる鋼材としては、棒鋼を熱間鍛造した鋼材（熱間鍛造材）が用いられてきた（例えば、特許文献１参照）。また、部品製造工程におけるＣＯ２の排出量削減のため、これまで熱間鍛造によって加工されていたギヤなどの部品の冷間鍛造化に関する要求も高まっている。 In recent years, from the viewpoint of environmental protection, for the purpose of improving the fuel efficiency of automobiles, there are increasing demands for weight reduction of steel materials used for various parts for automobiles, for example, transmission parts such as gears and cases, that is, higher strength. . In order to meet such demands for weight reduction and high strength, steel materials obtained by hot forging steel bars (hot forging materials) have been used as steel materials that are generally used (see, for example, Patent Document 1). In addition, in order to reduce CO 2 emissions in the component manufacturing process, there is an increasing demand for cold forging of components such as gears that have been processed by hot forging.
　ところで、冷間加工（冷間鍛造）は、熱間加工や温間加工に比較して生産性が高く、しかも寸法精度および鋼材の歩留まりがともに良好な利点がある。しかし、このような冷間加工によって部品を製造する場合に問題となるのは、冷間加工された部品の強度を期待される所定値以上に確保するためには、必然的に強度、すなわち変形抵抗の高い鋼材を用いる必要があることである。ところが、使用する鋼材の変形抵抗が高いものほど冷間加工用金型の寿命短縮を招く難点がある。 By the way, cold working (cold forging) has advantages of higher productivity than hot working and warm working and good dimensional accuracy and yield of steel materials. However, the problem in manufacturing parts by such cold working is that in order to ensure the strength of the cold-worked parts to be higher than the expected value, the strength, ie deformation, is inevitably required. It is necessary to use a steel material with high resistance. However, the higher the deformation resistance of the steel material to be used, there is a difficulty in reducing the life of the cold working mold.
　上記背景より、トランスミッション部品の分野では、従来からの棒鋼の鍛造品（熱間鍛造、冷間鍛造等）から、部品の軽量化や低コスト化を狙いとして鋼板を用いた部品製造に切り替える検討も進んでいる。中でも、ギアやダンパー、クラッチ等で表面に面圧がかかる部品では、耐摩耗性や耐疲労特性を付与するために、鋼板を部品加工した後に浸炭熱処理することにより、表面硬さを高くすることが行われている。これらの部品製造用の鋼板としては、従来は一般的な軟鋼（ＳＰＨＣなど）が用いられていたが、さらなる高強度化、高硬度化が求められている。 From the above background, in the field of transmission parts, it is also considered to switch from conventional steel bar forgings (hot forging, cold forging, etc.) to parts manufacturing using steel plates with the aim of reducing the weight and cost of parts. Progressing. Above all, in parts where surface pressure is applied by gears, dampers, clutches, etc., the surface hardness should be increased by carburizing heat treatment after the steel plate is processed to provide wear resistance and fatigue resistance. Has been done. Conventionally, a general mild steel (SPHC or the like) has been used as a steel plate for manufacturing these parts, but higher strength and higher hardness are required.
　鋼板を所定形状に冷間加工（プレス成形など）した後、浸炭熱処理を行うことで、所定の強度、表面硬さが確保された高強度部品が製造される。浸炭表面の硬度を上げるためには、Ｃ量を中心とした主要成分や添加元素の量を増加させることが考えられるが、そうすると熱処理前の冷間加工性が低下する。したがって、冷間加工性の確保と浸炭熱処理後の表面硬さの向上を両立させる解決策が望まれていた。 After cold working (press forming, etc.) the steel sheet into a predetermined shape, carburizing heat treatment is performed to produce a high-strength part with a predetermined strength and surface hardness. In order to increase the hardness of the carburized surface, it is conceivable to increase the amount of main components and additive elements centering on the amount of C. However, if this is done, the cold workability before heat treatment is reduced. Therefore, there has been a demand for a solution that ensures both cold workability and improved surface hardness after carburizing heat treatment.
　上述したように、本発明は熱延鋼板を対象とするが、熱延鋼板に関する従来技術として例えば下記特許文献２～６が挙げられる。 As described above, the present invention is intended for hot-rolled steel sheets, but the following Patent Documents 2 to 6 are listed as conventional techniques related to hot-rolled steel sheets.
　特許文献２に開示された熱延鋼板は、平均ｒ値が１．２以上、圧延方向のｒ値（ｒＬ） が１．３以上、圧延方向に対して４５゜方向のｒ値（ｒＤ）が、０．９以上、圧延方向と直角方向のｒ値（ｒＣ）が１．２以上であり、鋼板１／２板厚における板面の｛１１１｝、｛１００｝、および｛１１０｝の各Ｘ線反射面ランダム強度比が、それぞれ２．０以上、１．０以上、および０．２以上であるものとすることで、加工性が向上するとされている。 The hot rolled steel sheet disclosed in Patent Document 2 has an average r value of 1.2 or more, an r value (rL) in the rolling direction of 1.3 or more, and an r value (rD) in the 45 ° direction with respect to the rolling direction. 0.9 or more, and the r value (rC) in the direction perpendicular to the rolling direction is 1.2 or more, and each X of {111}, {100}, and {110} of the plate surface in the steel plate 1/2 plate thickness It is said that the workability is improved when the line reflection surface random intensity ratio is 2.0 or more, 1.0 or more, and 0.2 or more, respectively.
　特許文献３に開示された熱延鋼板は、体積率で４０～９５％のベイナイト相を含有して残部がフェライト相からなる組織を有し、上記フェライトの平均結晶粒径が１．２μｍ以上で４μｍ未満であって、かつ下記の（ａ）と（ｂ）の少なくとも一方の特徴を有するものとすることで、延性およびバーリング加工性が向上するとされている。
（ａ） 鋼板の表面から板厚方向に１／８の厚さの位置におけるフェライトの平均粒径（ｄｓ）と板厚中心におけるフェライトの平均粒径（ｄｃ）との比率（ｄｓ／ｄｃ）が０．３～０．７であること。
（ｂ） 鋼板の表面から板厚方向に１／８の厚さの位置における｛１１０｝＜１１１＞、｛１１０｝＜００１＞および｛２１１｝＜１１１＞の極密度の和が集合組織をもたないものの５倍以上であり、かつ、それぞれが１．５倍以上であること。 The hot-rolled steel sheet disclosed in Patent Document 3 has a structure containing a bainite phase of 40 to 95% by volume with the balance being a ferrite phase, and the ferrite has an average crystal grain size of 1.2 μm or more. By being less than 4 μm and having at least one of the following characteristics (a) and (b), ductility and burring workability are improved.
(A) The ratio (ds / dc) of the average particle diameter (ds) of ferrite at the position of 1/8 thickness from the surface of the steel sheet to the average thickness (dc) of ferrite at the center of the sheet thickness is 0.3 to 0.7.
(B) The sum of the polar densities of {110} <111>, {110} <001>, and {211} <111> at a position of 1/8 thickness from the surface of the steel plate also has a texture. It should be 5 times or more of that which is not good, and each must be 1.5 times or more.
　特許文献４に開示された熱延鋼板は、フェライト相の平均粒径が５μｍ以下であり、さらにフェライト相が面積率で５０％以上存在するミクロ組織を有し、かつ鋼板の１／４板厚における板面の（１１３）［１－１０］～（２２３）［１－１０］方位における平均のＯＤＦ解析強度ｆが４以上であるものとすることで、剛性が向上するとされている。 The hot-rolled steel sheet disclosed in Patent Document 4 has a microstructure in which the average grain size of the ferrite phase is 5 μm or less, and further the ferrite phase is present in an area ratio of 50% or more, and the ¼ thickness of the steel sheet The average ODF analysis strength f in the (113) [1-10] to (223) [1-10] orientations of the plate surface at is assumed to be 4 or more, whereby the rigidity is improved.
　特許文献５に開示された熱延鋼板は、下記式（１）で規定されるΔｒ１が－０．２０以上０．２０以下であり、下記式（２）で規定されるΔｒ２が０．４２以下であるものとすることで、絞り成形性が向上するとされている。
　Δｒ１＝（ｒ０－２ｒ４５＋ｒ９０）／２ ・・・ （１） 
　Δｒ２＝ｒｍａｘ－ｒｍｉｎ ・・・ （２）
　ここで、式中の各記号は以下の値を表す： 
　ｒ０：板面の圧延方向に対して平行に採取した試験片で測定したｒ値、 
　ｒ４５：板面の圧延方向に対して４５°方向に採取した試験片で測定したｒ値、 
　ｒ９０：板面の圧延方向に対して９０°方向に採取した試験片で測定したｒ値、 
　ｒｍａｘ：ｒ０、ｒ４５およびｒ９０のうち最大の値、 
　ｒｍｉｎ：ｒ０、ｒ４５およびｒ９０のうち最小の値。  In the hot-rolled steel sheet disclosed in Patent Document 5, Δr1 defined by the following formula (1) is −0.20 or more and 0.20 or less, and Δr2 defined by the following formula (2) is 0.42 or less. It is said that drawability is improved by assuming that.
Δr1 = (r0-2r45 + r90) / 2 (1)
Δr2 = rmax−rmin (2)
Where each symbol in the formula represents the following value:
r0: r value measured with a test specimen taken in parallel with the rolling direction of the plate surface,
r45: r value measured with a test piece taken in the direction of 45 ° with respect to the rolling direction of the plate surface,
r90: r value measured with a test specimen taken in the 90 ° direction with respect to the rolling direction of the plate surface,
rmax: the maximum value of r0, r45 and r90,
rmin: The minimum value of r0, r45, and r90.
　特許文献６に開示された熱延鋼板は、フェライト平均粒径が１～１０μｍ、フェライト粒径の標準偏差が３．０μｍ以下、介在物の形状比が２．０以下であるものとすることで、伸びフランジ性が向上するとされている。 The hot rolled steel sheet disclosed in Patent Document 6 has an average ferrite grain size of 1 to 10 μm, a standard deviation of ferrite grain size of 3.0 μm or less, and an inclusion shape ratio of 2.0 or less. It is said that stretch flangeability is improved.
　しかしながら、上記特許文献２～６に開示された熱延鋼板は、絞り加工性などの冷間加工性に優れるものの、浸炭熱処理後の表面硬さについては何ら言及がなく、その改善効果は不明である。 However, although the hot-rolled steel sheets disclosed in Patent Documents 2 to 6 are excellent in cold workability such as drawing workability, there is no mention of surface hardness after carburizing heat treatment, and the improvement effect is unknown. is there.
　一方、特許文献７に開示された熱延鋼板（浸炭鋼帯）は、板厚方向表層部の５０μｍ深さまでの平均硬さが１７０ＨＶ以上で、且つ、金属組織がフェライト＋パーライトであり、表面炭素濃度ＣＳ（質量％）と鋼中平均炭素濃度ＣＭ（質量％）の差ΔＣ＝ＣＳ－ＣＭが０．１質量％以上であるものとすることで、打ち抜き時の「だれ」を軽減するとともに、打ち抜き後の浸炭処理を省略できるとされている。 On the other hand, the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 7 has an average hardness of 170 HV or more up to a depth of 50 μm in the thickness direction surface layer portion, and has a metal structure of ferrite + pearlite, surface carbon The difference between the concentration CS (mass%) and the average carbon concentration CM (mass%) in steel ΔC = CS-CM is 0.1 mass% or more to reduce the “dripping” at the time of punching, It is said that carburizing after punching can be omitted.
　しかしながら、上記特許文献７に開示された熱延鋼板（浸炭鋼帯）は、浸炭熱処理後の表面硬さに優れるものの、絞り加工性はもとより冷間加工性については何ら言及がなく、その改善効果は不明である。 However, although the hot-rolled steel sheet (carburized steel strip) disclosed in Patent Document 7 is excellent in surface hardness after carburizing heat treatment, there is no mention of cold workability as well as drawing workability, and its improvement effect Is unknown.
　上記のように、絞り加工性と浸炭熱処理後の表面硬さを兼備する熱延鋼板については、これまでほとんど検討がなされていなかった。 As described above, there has been little investigation so far regarding hot-rolled steel sheets having both drawability and surface hardness after carburizing heat treatment.
日本国特許第３０９４８５６号公報Japanese Patent No. 3094856 日本国特許第３７４２５５９号公報Japanese Patent No. 3742559 日本国特許第４１６１９３５号公報Japanese Patent No. 41161935 日本国特許第４８６７２５７号公報Japanese Patent No. 4867257 日本国特開２０１３－１１９６３５号公報Japanese Unexamined Patent Publication No. 2013-119635 日本国特許第４２７６５０４号公報Japanese Patent No. 4276504 日本国特開２０１０－２２２６６３号公報Japanese Unexamined Patent Publication No. 2010-222663
　そこで、本発明は、絞り加工性と浸炭熱処理後の表面硬さを兼備する熱延鋼板を提供することを目的とする。なお、本発明において浸炭熱処理は、通常の浸炭のためのほか、浸炭窒化のための熱処理の場合も含むものである。 Therefore, an object of the present invention is to provide a hot-rolled steel sheet having both drawability and surface hardness after carburizing heat treatment. In the present invention, the carburizing heat treatment includes not only normal carburizing but also heat treating for carbonitriding.
　請求項１に記載の発明は、 
　板厚が２～１０ｍｍであり、 
　成分組成が、 
　質量％で（以下、化学成分について同じ。）、
　Ｃ　：０．０５～０．３０％、 
　Ｍｎ：０．３～３．０％、 
　Ａｌ：０．０１５～０．１％、 
　Ｎ　：０．００３～０．３０％を含み、 
　残部は鉄および不可避的不純物からなり、 
　フェライト及びパーライトを主体とする組織であって、
　深さｔ／４（ｔ：板厚）の位置に存在する、フェライトおよびパーライトを含む全ての結晶粒（以下、「全結晶粒」という。）に関し、
　板面方位が（１２３）面から１０°以内の結晶粒の面積率が２０％以上であり、 
　圧延方向で、結晶方向が＜００１＞方向から１０°以内の結晶粒と、結晶方向が＜１１０＞方向から１０°以内の結晶粒との合計面積率が２５％以下であるとともに、
　前記全結晶粒の平均粒径が３～５０μｍである 
ことを特徴とする絞り加工性と浸炭熱処理後の表面硬さに優れる熱延鋼板である。  The invention described in claim 1
The plate thickness is 2-10mm,
Ingredient composition
% By mass (hereinafter the same for chemical components)
C: 0.05 to 0.30%,
Mn: 0.3 to 3.0%,
Al: 0.015 to 0.1%,
N: 0.003 to 0.30% included,
The balance consists of iron and inevitable impurities,
An organization mainly composed of ferrite and pearlite,
Regarding all the crystal grains including ferrite and pearlite (hereinafter referred to as “all crystal grains”) existing at a depth t / 4 (t: plate thickness),
The area ratio of crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more,
In the rolling direction, the total area ratio of the crystal grains whose crystal direction is within 10 ° from the <001> direction and the crystal grains whose crystal direction is within 10 ° from the <110> direction is 25% or less,
The average grain size of all the crystal grains is 3 to 50 μm
The hot-rolled steel sheet is excellent in drawing workability and surface hardness after carburizing heat treatment.
　請求項２に記載の発明は、
　前記不可避的不純物のうち、Ｓｉ：０．５％以下、Ｐ：０．０３０％以下、Ｓ：０．０３５％以下である請求項１に記載の熱延鋼板である。 The invention described in claim 2
The hot rolled steel sheet according to claim 1, wherein among the inevitable impurities, Si: 0.5% or less, P: 0.030% or less, and S: 0.035% or less.
　請求項３に記載の発明は、
　成分組成が、さらに、下記（ａ）～（ｆ）の少なくとも１種を含むものである請求項１または２に記載の熱延鋼板である。
（ａ）Ｃｒ：３．０％以下（０％を含まない）、Ｍｏ：１．０％以下（０％を含まない）及びＮｉ：３．０％以下（０％を含まない）よりなる群から選択される少なくとも１種
（ｂ）Ｃｕ：２．０％以下（０％を含まない）及びＣｏ：５％以下（０%を含まない）よりなる群から選択される少なくとも１種
（ｃ）Ｖ：０．５％以下（０％を含まない）、Ｔｉ：０．１％以下（０％を含まない）及びＮｂ：０．１％以下（０%を含まない）よりなる群から選ばれる少なくとも１種 
（ｄ）Ｃａ：０．０８％以下（０％を含まない）及びＺｒ：０．０８％以下（０％を含まない） よりなる群から選択される少なくとも１種
（ｅ）Ｓｂ：０．０２％以下（０％を含まない）
（ｆ）ＲＥＭ：０．０５％以下（０％を含まない）、Ｍｇ：０．０２％以下（０％を含まない）、Ｌｉ：０．０２％以下（０％を含まない）、Ｐｂ：０．５％以下（０％を含まない）及びＢｉ：０．５％以下（０％を含まない）よりなる群から選ばれる少なくとも１種 The invention according to claim 3
The hot rolled steel sheet according to claim 1 or 2, wherein the component composition further contains at least one of the following (a) to (f).
(A) A group consisting of Cr: 3.0% or less (not including 0%), Mo: 1.0% or less (not including 0%), and Ni: 3.0% or less (not including 0%) (B) at least one selected from the group consisting of Cu: 2.0% or less (not including 0%) and Co: 5% or less (not including 0%) (c) V: selected from the group consisting of 0.5% or less (not including 0%), Ti: 0.1% or less (not including 0%), and Nb: 0.1% or less (not including 0%) At least one
(D) At least one selected from the group consisting of Ca: 0.08% or less (excluding 0%) and Zr: 0.08% or less (not including 0%) (e) Sb: 0.02 % Or less (excluding 0%)
(F) REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0.02% or less (not including 0%), Pb: 0.5% or less (not including 0%) and Bi: at least one selected from the group consisting of 0.5% or less (not including 0%)
　本発明によれば、フェライト＋パーライト主体の組織において、熱延鋼板の集合組織を所定の組織形態に制御することで、絞り加工性が求められる部品においても、加工中の変形能を高めることにより、金型の寿命が延長されるとともに、鋼板に割れが発生しにくく、浸炭熱処理後に得られる部品は所定の表面硬さを確保できる熱延鋼板を提供できるようになった。 According to the present invention, by controlling the texture of the hot-rolled steel sheet to a predetermined structure form in the structure mainly composed of ferrite and pearlite, by improving the deformability during processing even in a part that requires drawing workability. In addition to extending the life of the mold, the steel sheet is less prone to cracking, and the parts obtained after the carburizing heat treatment can provide a hot-rolled steel sheet that can ensure a predetermined surface hardness.
　以下、本発明に係る熱延鋼板（以下、「本発明鋼板」、あるいは、単に「鋼板」ともいう。）について、さらに詳細に説明する。本発明鋼板は、上記特許文献１に記載された熱間鍛造材（高強度高靭性肌焼き用鋼）と成分組成が重複するが、組織をフェライト＋パーライト主体組織としたうえで、熱延鋼板の集合組織を所定の組織形態に制御するとともに、結晶粒を微細化する点で異なっている。 Hereinafter, the hot-rolled steel sheet according to the present invention (hereinafter also referred to as “the steel sheet of the present invention” or simply “the steel sheet”) will be described in more detail. The steel sheet of the present invention overlaps with the hot forging material (high-strength, high-toughness case hardening steel) described in Patent Document 1 above. Are different from each other in that the texture is controlled to a predetermined structure and crystal grains are refined.
〔本発明鋼板の板厚：２～１０ｍｍ〕
　まず、本発明鋼板は、板厚が２～１０ｍｍのものを対象とする。板厚が２ｍｍ未満では、構造体としての剛性が確保できなくなる。一方、板厚が１０ｍｍを超えると、本発明で規定する組織形態を達成することが難しく、所望の効果が得られなくなる。板厚の下限は３ｍｍ以上が好ましく、４ｍｍ以上が更に好ましい。また、上限は、９ｍｍ以下が好ましく、７ｍｍ以下がより好ましい。 [Thickness of the steel sheet of the present invention: 2 to 10 mm]
First, the steel sheet of the present invention has a thickness of 2 to 10 mm. If the plate thickness is less than 2 mm, rigidity as a structure cannot be secured. On the other hand, if the plate thickness exceeds 10 mm, it is difficult to achieve the tissue form defined in the present invention, and the desired effect cannot be obtained. The lower limit of the plate thickness is preferably 3 mm or more, and more preferably 4 mm or more. Further, the upper limit is preferably 9 mm or less, and more preferably 7 mm or less.
　次に、本発明鋼板を構成する成分組成について説明する。以下、化学成分の単位はすべて質量％である。 Next, the component composition constituting the steel sheet of the present invention will be described. Hereinafter, all the units of chemical components are mass%.
〔本発明鋼板の成分組成〕
＜Ｃ：０．０５～０．３０％＞ 
　Ｃは、最終的に得られる浸炭（もしくは浸炭窒化）焼入れ部品としての芯部強度を確保するうえで欠くことのできない元素であり、０．０５％未満では十分な強度が得られなくなる。しかし、過剰に含有させると靭性が劣化するほか、被削性や冷間鍛造性が低下して加工性を損なうので０．３０％を上限とする。Ｃの好ましい含有量は０．０８～０．２５％の範囲である。 [Component composition of the steel sheet of the present invention]
<C: 0.05 to 0.30%>
C is an element indispensable for securing the core strength of the carburized (or carbonitrided) quenching part finally obtained. If it is less than 0.05%, sufficient strength cannot be obtained. However, if it is contained excessively, the toughness is deteriorated, and the machinability and cold forgeability are deteriorated and the workability is impaired, so 0.30% is made the upper limit. A preferable content of C is in the range of 0.08 to 0.25%.
＜Ｍｎ：０．３～３．０％＞
　Ｍｎは、溶鋼の脱酸に有効な元素であり、その効果を有効に発揮させるには０．３％以上含有させなければならないが、過度に含有させると、冷間加工性や被削性に悪影響を与えるとともに、結晶粒界への偏析量の増大によって粒界強度を低下させ、ひいては衝撃特性に悪影響を及ぼすようになるので、３．０％以下に抑えなければならない。Ｍｎの好ましい含有量は０．５～２．０％の範囲である。 <Mn: 0.3 to 3.0%>
Mn is an element effective for deoxidation of molten steel, and in order to exert its effect effectively, it must be contained in an amount of 0.3% or more. However, if it is excessively contained, cold workability and machinability are reduced. In addition to having an adverse effect and increasing the amount of segregation to the crystal grain boundary, it lowers the grain boundary strength and thus adversely affects the impact characteristics, so it must be suppressed to 3.0% or less. A preferable content of Mn is in the range of 0.5 to 2.0%.
＜Ａｌ：０．０１５～０．１％＞
　Ａｌは鋼材の脱酸材として鋼中に含まれてくる元素であり、鋼中のＮと結合してＡｌＮを生成し、結晶粒の粗大化を防止する作用を有している。こうした効果を有効に発揮させるには０．０１５％以上含有させなければならないが、その効果は０．１％程度で飽和し、それを超えると酸素と結合して非金属系介在物となり、衝撃特性等に悪影響を及ぼすようになるので、０．１％を上限と定めた。好ましくは０．０８％以下であり、さらに好ましくは０．０６％以下、特に好ましくは０．０４％以下である。 <Al: 0.015 to 0.1%>
Al is an element contained in steel as a deoxidizing material for steel, and has an action of binding to N in steel to produce AlN and preventing coarsening of crystal grains. In order to exert such an effect effectively, it must be contained at 0.015% or more, but the effect is saturated at about 0.1%, and beyond that, it combines with oxygen to form a non-metallic inclusion, Since it adversely affects the characteristics, etc., the upper limit was set to 0.1%. Preferably it is 0.08% or less, More preferably, it is 0.06% or less, Most preferably, it is 0.04% or less.
＜Ｎ：０．００３～０．３０％＞ 
　Ｎは鋼中でＡｌ，Ｖ，Ｔｉ，Ｎｂ等と結合して窒化物を生成し、結晶粒の粗大化を抑制する作用を有しており、その効果は０．００３％以上含有させることによって有効に発揮される。好ましくは、０．００５％以上である。しかし、それらの効果は０．３０％程度で飽和し、それ以上に含有させると窒化物が介在物となって物性に悪影響を及ぼすようになるので、０．３０％を上限と定めた。好ましくは０．１０％以下であり、さらに好ましくは０．０５％以下、特に好ましくは０．０３％以下である。 <N: 0.003-0.30%>
N combines with Al, V, Ti, Nb, etc. in steel to produce nitrides, and has the effect of suppressing the coarsening of crystal grains. The effect is by containing 0.003% or more. Effectively demonstrated. Preferably, it is 0.005% or more. However, these effects are saturated at about 0.30%, and if it is contained more than that, nitrides become inclusions and adversely affect the physical properties, so 0.30% was set as the upper limit. Preferably it is 0.10% or less, More preferably, it is 0.05% or less, Most preferably, it is 0.03% or less.
　本発明鋼板は上記成分を基本的に含有し、残部が鉄および不可避的不純物であるが、不可避的に混入してくるＳｉ，ＰおよびＳは、下記の理由からそれぞれできるだけ少なく抑えることが望ましい。 The steel sheet of the present invention basically contains the above components, and the balance is iron and inevitable impurities, but it is desirable to suppress Si, P and S which are inevitably mixed in as little as possible for the following reasons.
＜Ｓｉ：０．５％以下＞
　Ｓｉは、強化元素あるいは脱酸性元素として有効に作用する反面、粒界酸化を助長して曲げ疲労特性を劣化させるとともに冷間鍛造性にも悪影響を及ぼす。したがってこうした障害をなくすにはその含有量を０．５％以下に抑えなければならず、特に高レベルの曲げ疲労特性が求められるときは、その含有量を０．１％以下に抑えることが望まれる。こうした観点から、Ｓｉのより好ましい含有量は０．０２～０．１％の範囲である。 <Si: 0.5% or less>
Si effectively acts as a strengthening element or a deacidifying element, but promotes grain boundary oxidation to deteriorate bending fatigue properties and adversely affects cold forgeability. Therefore, in order to eliminate such obstacles, the content must be suppressed to 0.5% or less, and particularly when a high level of bending fatigue characteristics is required, it is desirable to suppress the content to 0.1% or less. It is. From such a viewpoint, the more preferable content of Si is in the range of 0.02 to 0.1%.
＜Ｐ：０．０３０％以下＞ 
　Ｐは結晶粒界に偏析して靭性を低下させるので、その上限は０．０３０％と定めた。Ｐのより好ましい含有量は０．０２０％以下、さらに好ましくは０．０１０％以下である。 <P: 0.030% or less>
P segregates at the grain boundaries and lowers the toughness, so the upper limit was set to 0.030%. The more preferable content of P is 0.020% or less, and further preferably 0.010% or less.
＜Ｓ：０．０３５％以下＞ 
　ＳはＭｎＳを生成し、被削性の向上に寄与するが、本発明を歯車等に適用する場合は、縦目の衝撃特性だけでなく横目の衝撃特性も重要であり、横目の衝撃特性向上には異方性の低減が必要となり、そのためにはＳ含有量を０．０３５％以下に抑えなければならない。Ｓのより好ましい含有量は０．０２５％以下、さらに好ましくは０．０２０％以下である。 <S: 0.035% or less>
S generates MnS and contributes to the improvement of machinability. However, when the present invention is applied to gears and the like, not only the impact characteristics of the vertical eyes but also the impact characteristics of the horizontal eyes are important, and the impact characteristics of the horizontal eyes are improved. In order to reduce the anisotropy, the S content must be suppressed to 0.035% or less. The more preferable content of S is 0.025% or less, and further preferably 0.020% or less.
　また本発明鋼板には、上記の基本成分に加えて、本発明の作用を損なわない範囲で、以下の許容成分を含有させることができる。 In addition to the above basic components, the steel sheet of the present invention can contain the following permissible components within the range not impairing the action of the present invention.
＜Ｃｒ：３．０％以下（０％を含まない）、
Ｍｏ：１．０％以下（０％を含まない）、
Ｎｉ：３．０％以下（０％を含まない）よりなる群から選択される少なくとも１種＞
　これらの元素は、焼入性を高めあるいは焼入れ組織を微細化する作用を有する点で有用元素であり、特にＣｒは優れた焼入性向上効果を有しており、またＭｏは不完全焼入れ組織の低減と焼入性の向上、さらには粒界強度の向上に有効に作用し、さらにＮｉは焼入れ後の組織を微細化して耐衝撃性の向上に寄与する。こうした効果は、好ましくはＣｒ：０．２％以上、Ｍｏ：０．０８％以上、Ｎｉ：０．２％以上のうち１少なくとも１種を含有させることによって有効に発揮されるが、Ｃｒ量が３．０％を超えるとＣｒが炭化物を生成して粒界偏析を起こし、粒界強度を低下させて靭性に悪影響を及ぼし、Ｍｏの上記効果は約１．０％で飽和し、またＮｉの上記効果も３．０％で飽和するので、それ以上の添加は経済的に全く無駄である。 <Cr: 3.0% or less (excluding 0%),
Mo: 1.0% or less (excluding 0%),
Ni: at least one selected from the group consisting of 3.0% or less (not including 0%)>
These elements are useful elements in that they have a function of enhancing hardenability or refining the hardened structure, particularly Cr has an excellent effect of improving hardenability, and Mo is an incompletely hardened structure. This effectively acts to improve the hardenability and the grain boundary strength, and Ni contributes to the improvement of impact resistance by refining the structure after quenching. Such an effect is preferably exhibited by containing at least one of Cr: 0.2% or more, Mo: 0.08% or more, and Ni: 0.2% or more. If it exceeds 3.0%, Cr generates carbides to cause grain boundary segregation, lowers the grain boundary strength, adversely affects toughness, and the above effect of Mo is saturated at about 1.0%. Since the above effect is saturated at 3.0%, addition beyond that is economically useless.
＜Ｃｕ：２．０％以下（０％を含まない）、および／または、
Ｃｏ：５％以下（０%を含まない）＞
　Ｃｕは耐食性の向上に有効に作用する元素であり、その効果は好ましくは０．３％以上含有させることによって有効に発揮されるが、その効果は２．０％で飽和するのでそれ以上の含有は無駄である。なおＣｕを単独で含有させると、鋼材の熱間加工性が悪くなる傾向があるので、こうした弊害を回避するには、熱間加工性向上効果を有するＮｉを前記含有量の範囲で併用することが望ましい。 <Cu: 2.0% or less (not including 0%), and / or
Co: 5% or less (excluding 0%)>
Cu is an element that effectively acts to improve corrosion resistance, and the effect is preferably exerted by inclusion of 0.3% or more, but the effect is saturated at 2.0%, so it is contained more than that. Is useless. If Cu is contained alone, the hot workability of the steel material tends to deteriorate. Therefore, in order to avoid such adverse effects, Ni having the effect of improving the hot workability should be used in the above content range. Is desirable.
　またＣｕとＣｏは、いずれも鋼材をひずみ時効させ、硬化させる作用があり、加工後強度を向上させるのに有効な元素である。このような作用を有効に発揮させるためには、これらの元素は、それぞれ０．１％以上、さらには０．３％以上含有させることが好ましい。しかし、Ｃｏの含有量が過剰であると、鋼材をひずみ時効および硬化させる効果、さらに、加工後強度を向上させる効果が飽和し、また、割れを促進させるおそれがあるため、Ｃｏの含有量は５％以下、さらには４％以下、特に３％以下とすることが推奨される。 Also, Cu and Co are elements that have an effect of strain aging and hardening the steel material, and are effective in improving the strength after processing. In order to effectively exhibit such an action, these elements are preferably contained in an amount of 0.1% or more, and more preferably 0.3% or more. However, if the Co content is excessive, the effects of strain aging and hardening of the steel material, and the effect of improving the strength after processing are saturated, and there is a possibility of promoting cracking, so the Co content is It is recommended to set it to 5% or less, further 4% or less, especially 3% or less.
＜Ｖ：０．５％以下（０％を含まない）、 
Ｔｉ：０．１％以下（０％を含まない）、 
Ｎｂ：０．１％以下（０%を含まない）よりなる群から選ばれる少なくとも１種＞
　これらの元素はＣやＮと結合して炭化物や窒化物を生成し、結晶粒を微細化して靭性（耐衝撃性）の向上に寄与するが、それぞれ上限値付近でその効果は飽和し、かえって被削性や冷間加工性に悪影響を及ぼすおそれがでてくるので、それぞれ上限値以下に抑えなければならない。これら元素の添加効果を有効に発揮させるための好ましい下限値はＶ：０．０３％、Ｔｉ：０．００５％およびＮｂ：０．００５％である。 <V: 0.5% or less (excluding 0%),
Ti: 0.1% or less (excluding 0%),
Nb: at least one selected from the group consisting of 0.1% or less (not including 0%)>
These elements combine with C and N to form carbides and nitrides, refine the crystal grains and contribute to the improvement of toughness (impact resistance), but the effect is saturated near the upper limit, respectively. Since there is a risk of adversely affecting the machinability and cold workability, each must be suppressed to the upper limit value or less. Preferred lower limit values for effectively exhibiting the effect of addition of these elements are V: 0.03%, Ti: 0.005%, and Nb: 0.005%.
＜Ｃａ：０．０８％以下（０％を含まない）、および／または、 
Ｚｒ：０．０８％以下（０％を含まない）＞ 
　Ｃａは、硬質の介在物を柔軟な介在物で包み込み、またＺｒはＭｎＳを球状化させ、いずれも被削性の向上に寄与するほか、両元素ともＭｎＳの球状化による異方性の低減によって横目の衝撃特性を高める作用を有しているが、それらの効果はそれぞれ０．０８％で飽和するので、それぞれ０．０８％以下、さらには０．０５％以下、特に０．０１％以下とすることが推奨される。なおこれらの元素の上記効果を有効に発揮させるための好ましい下限値は、Ｃａ：０．０００５％（さらには０．００１％）、Ｚｒ：０．００２％である。 <Ca: 0.08% or less (excluding 0%), and / or
Zr: 0.08% or less (excluding 0%)>
Ca wraps hard inclusions with flexible inclusions, and Zr spheroidizes MnS, both of which contribute to improving machinability, and both elements are reduced by anisotropy due to spheroidization of MnS. Although it has the effect | action which improves the impact characteristic of a horizontal eye, since those effects are saturated at 0.08%, respectively, it is 0.08% or less, Furthermore, 0.05% or less, Especially 0.01% or less It is recommended to do. In addition, the preferable lower limit for making the said effect of these elements exhibit effectively is Ca: 0.0005% (further 0.001%), Zr: 0.002%.
＜Ｓｂ：０．０２％以下（０％を含まない）＞
　Ｓｂは、粒界酸化を抑制して曲げ疲労強度を高めるうえで有効な元素であるが、その効果は０．０２％で飽和するので、それ以上の添加は経済的に無駄である。該Ｓｂの添加効果を有効に発揮させるための好ましい下限値は０．００１％である。 <Sb: 0.02% or less (excluding 0%)>
Sb is an element effective for suppressing the grain boundary oxidation and increasing the bending fatigue strength, but since the effect is saturated at 0.02%, the addition of more is economically useless. A preferable lower limit value for effectively exhibiting the effect of addition of Sb is 0.001%.
＜ＲＥＭ：０．０５％以下（０％を含まない）、
Ｍｇ：０．０２％以下（０％を含まない）、
Ｌｉ：０．０２％以下（０％を含まない）、
Ｐｂ：０．５％以下（０％を含まない）、
Ｂｉ：０．５％以下（０％を含まない）よりなる群から選ばれる少なくとも１種＞
　ＲＥＭは、Ｚｒ及びＣａと同様にＭｎＳなどの硫化化合物系介在物を球状化させ、鋼の変形能を高めるとともに、被削性の向上に寄与する元素である。このような作用を有効に発揮させるためには、ＲＥＭは、０．０００５％以上、さらには０．００１％以上含有させることが好ましい。しかし、過剰に含有しても、その効果が飽和し、含有量に見合う効果が期待できないため、０．０５％以下、さらには０．０３％以下、特に０．０１％以下が推奨される。
　なお、本発明において、ＲＥＭとは、ランタノイド元素（ＬａからＬｎまでの１５元素）およびＳｃ（スカンジウム）とＹ（イットリウム）を含む意味である。これらの元素のなかでも、Ｌａ、ＣｅおよびＹよりなる群から選ばれる少なくとも１種の元素を含有することが好ましく、より好ましくはＬａおよび／またはＣｅを含有するのがよい。 <REM: 0.05% or less (excluding 0%),
Mg: 0.02% or less (excluding 0%),
Li: 0.02% or less (excluding 0%),
Pb: 0.5% or less (excluding 0%),
Bi: at least one selected from the group consisting of 0.5% or less (excluding 0%)>
REM, like Zr and Ca, is an element that spheroidizes sulfide compound inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, REM is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected. Therefore, 0.05% or less, further 0.03% or less, particularly 0.01% or less is recommended.
In the present invention, REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium). Among these elements, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y, more preferably La and / or Ce.
　Ｍｇは、Ｚｒ及びＣａと同様にＭｎＳなどの硫化化合物系介在物を球状化させ、鋼の変形能を高めるとともに、被削性の向上に寄与する元素である。このような作用を有効に発揮させるためには、Ｍｇは、０．０００２％以上、さらには０．０００５％以上含有させることが好ましい。しかし、過剰に含有しても、その効果が飽和し、含有量に見合う効果が期待できないため、０．０２％以下、さらには０．０１５％以下、特に０．０１％以下が推奨される。 Mg, like Zr and Ca, is an element that spheroidizes sulfide compound inclusions such as MnS to increase the deformability of steel and contribute to the improvement of machinability. In order to effectively exhibit such an action, Mg is preferably contained in an amount of 0.0002% or more, more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.
　Ｌｉは、Ｚｒ及びＣａと同様にＭｎＳなどの硫化化合物系介在物を球状化させ、鋼の変形能を高めることができ、また、Ａｌ系酸化物を低融点化して無害化して被削性の向上に寄与する元素である。このような作用を有効に発揮させるためには、Ｌｉは、０．０００２％以上、さらには０．０００５％以上含有させることが好ましい。しかし、過剰に含有しても、その効果が飽和し、含有量に見合う効果が期待できないため、０．０２％以下、さらには０．０１５％以下、特に０．０１％以下が推奨される。 Li, like Zr and Ca, can spheroidize sulfide compound inclusions such as MnS to improve the deformability of steel, and lower the melting point of Al-based oxides to make them harmless. It is an element that contributes to improvement. In order to effectively exhibit such an action, Li is preferably contained in an amount of 0.0002% or more, and more preferably 0.0005% or more. However, even if contained excessively, the effect is saturated and an effect commensurate with the content cannot be expected, so 0.02% or less, further 0.015% or less, and particularly 0.01% or less is recommended.
　Ｐｂは、被削性を向上させるために有効な元素である。このような作用を有効に発揮させるためには、Ｐｂは０．００５％以上、さらには０．０１％以上含有させることが好ましい。しかし、過剰に含有させると、圧延疵の発生等の製造上の問題を生じるため、０．５％以下、さらには０．４％以下、特に０．３％以下が推奨される。 Pb is an effective element for improving machinability. In order to effectively exhibit such an action, Pb is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, if it is contained excessively, production problems such as generation of rolling defects occur, so 0.5% or less, further 0.4% or less, particularly 0.3% or less is recommended.
　Ｂｉは、Ｐｂと同様に、被削性を向上させるために有効な元素である。このような作用を有効に発揮させるためには、Ｂｉは０．００５％以上、さらには０．０１％以上含有させることが好ましい。しかし、過剰に含有させても被削性向上の効果が飽和するため、０．５％以下、さらには０．４％以下、特に０．３％以下が推奨される。 Bi is an element effective for improving the machinability like Pb. In order to effectively exhibit such an action, Bi is preferably contained in an amount of 0.005% or more, and more preferably 0.01% or more. However, since the effect of improving the machinability is saturated even if contained excessively, 0.5% or less, further 0.4% or less, and particularly 0.3% or less are recommended.
　次に、本発明鋼板を特徴づける組織について説明する。 Next, the structure characterizing the steel sheet of the present invention will be described.
〔本発明鋼板の組織〕
　上述したとおり、本発明鋼板は、フェライト及びパーライトを主体とする組織とするものであるが、特に、鋼中の集合組織の形態をより厳密に制御することを特徴とする。なお、本発明において、フェライト及びパーライトを主体とする組織とはフェライトとパーライトの合計量が面積率で９０％以上であることを意味する。フェライトとパーライトの合計量が面積率で９０％以上であれば、他の組織（ベイナイト、マルテンサイトなど）が少量生成していても構わないが、他の組織は出来るだけ少ない方が望ましい。 [Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention has a structure mainly composed of ferrite and pearlite, and is particularly characterized in that the form of the texture in the steel is more strictly controlled. In the present invention, the structure mainly composed of ferrite and pearlite means that the total amount of ferrite and pearlite is 90% or more in terms of area ratio. If the total amount of ferrite and pearlite is 90% or more in terms of area ratio, a small amount of other structures (bainite, martensite, etc.) may be generated, but it is desirable that the other structures be as small as possible.
　一般に、従前より鋼板の成形性向上のための集合組織制御に関しては、自動車の車体外板に用いられる薄鋼板の深絞り加工性は、材料の塑性異方性（ｒ値（Ｌａｎｋｆｏｒｄ値）：引張り試験における板幅ひずみと板厚ひずみの比）が大きいほど、その加工性が高くなること、さらに再結晶集合組織において板面方位に平行な｛１１１｝面を強く発達させ、｛１００｝面方位を弱めることが深絞り性の向上に不可欠であることが実験的にも理論的にも明らかにされている(日本鉄鋼協会：「再結晶・集合組織とその組織制御への応用再結晶研究の最前線」，１９９９年３月，ｐ．２０８参照)。 In general, with regard to texture control for improving the formability of a steel sheet, the deep drawing workability of a thin steel sheet used for a car body outer plate of an automobile is determined by the plastic anisotropy (r value (rankford value): tensile The greater the ratio between the plate width strain and the plate thickness strain in the test), the higher the workability, and the stronger the {111} plane parallel to the plane orientation in the recrystallized texture, the {100} plane orientation It has been experimentally and theoretically revealed that weakening the strength of the steel is essential for the improvement of deep drawability (Japan Iron and Steel Institute: “Recrystallization / texture and its application to microstructure control Frontline ”, March 1999, p. 208).
　このため、車体外板用の鋼板では、上記特許文献２～４に開示されるように、集合組織制御による加工性向上の取り組みは種々行われているものの、浸炭熱処理用鋼板では、このような取り組みは行われていなかった。 For this reason, as disclosed in the above-mentioned Patent Documents 2 to 4, various attempts have been made to improve workability by texture control in steel sheets for vehicle body outer plates. No efforts were made.
〔本発明鋼板の集合組織〕
　本発明鋼板は、フェライトおよびパーライトを含む全ての相の結晶粒に関し、結晶粒の方位およびサイズをそれぞれ特定範囲に制御することを特徴とする。 [A texture of the steel sheet of the present invention]
The steel sheet of the present invention is characterized by controlling the orientation and size of the crystal grains within a specific range with respect to the crystal grains of all phases including ferrite and pearlite.
＜深さｔ／４（ｔ：板厚）の位置に存在する、フェライトおよびパーライトを含む全ての結晶粒（以下、「全結晶粒」という。）に関し、
板面方位が（１２３）面から１０°以内の結晶粒の面積率が２０％以上＞
　集合組織のでき方は結晶系が同じでも加工法によって異なり、圧延材の場合は圧延面と圧延方向で表現される。つまり、下記に示すように圧延面は｛○○○｝で、圧延方向は＜△△△＞で、それぞれ表現される。なお、○や△は整数を示している。これら各方位の表現については、長島晋一編著：「集合組織」（丸善株式会社刊）などに記載されている。 <Regarding all crystal grains including ferrite and pearlite (hereinafter referred to as “all crystal grains”) existing at a position of depth t / 4 (t: plate thickness).
The area ratio of crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more>
The formation of the texture differs depending on the processing method even if the crystal system is the same. In the case of a rolled material, it is expressed by the rolling surface and the rolling direction. That is, as shown below, the rolling surface is represented by {xxx} and the rolling direction is represented by <ΔΔΔ>. In addition, (circle) and (triangle | delta) have shown the integer. The expression of each of these directions is described in, for example, edited by Junichi Nagashima: “Aggregation” (published by Maruzen Co., Ltd.).
　本発明では、深さｔ／４の位置に存在する全結晶粒に関し、板面方位が、（１２３）面から１０°以内の面積率を２０％以上に制御することで、浸炭熱処理用鋼板の冷間加工性、絞り加工性を向上させることができる。 In the present invention, with respect to all the crystal grains existing at the position of depth t / 4, the plate surface orientation is controlled by controlling the area ratio within 10 ° from the (123) plane to 20% or more. Cold workability and drawing workability can be improved.
　従来から、結晶粒の板面方位に関し、板面に平行な（１１１）面方位を強く発達させる一方、（００１）面方位を弱めることが深絞り性の向上に有効であることが知られている。冷間圧延工程と焼鈍工程を施す工程ではこのような板面方位の制御が可能であったが、本発明鋼板では、このような板面方位の制御が難しかった。 Conventionally, with regard to the crystal plane orientation of the crystal grains, it has been known that the (111) plane orientation parallel to the plane is strongly developed while the (001) plane orientation is effective in improving the deep drawability. Yes. Such plate surface orientation control was possible in the cold rolling step and the annealing step, but it was difficult to control such plate surface orientation in the steel sheet of the present invention.
　そこで本発明では、新たに（１２３）面の板面方位を有する結晶粒を導入することによって、本発明鋼板での集合組織制御を可能にし、軟質化状態での冷間加工性の向上を実現できるようになった。 Therefore, in the present invention, by newly introducing crystal grains having a (123) plane orientation, it is possible to control the texture of the steel sheet of the present invention and to improve cold workability in a softened state. I can do it now.
　上述したように、板面方位として（１２３）面を有する結晶粒は、軟質化状態での冷間加工性を向上させる作用を有し、このような作用を有効に発揮させるためには、面積率で２０％以上が必要である。好ましくは２２％以上、さらに好ましくは２４％以上、特に好ましくは２６％以上である。 As described above, the crystal grains having the (123) plane as the plate surface orientation have an effect of improving the cold workability in the softened state, and in order to effectively exhibit such an effect, the area A rate of 20% or more is required. Preferably it is 22% or more, more preferably 24% or more, particularly preferably 26% or more.
　なお、板材では、板厚方向に組織分布を有するため、板厚の１／４の深さ位置を代表位置として組織形態を規定した。また、板面方位が上記理想面方位（（１２３）面）から１０°以内の結晶粒はほぼ同等の作用を有すると考えられるので、その範囲の板面方位を有する結晶粒の面積率で規定することとした。 In addition, since the sheet material has a structure distribution in the sheet thickness direction, the structure form was defined with a depth position of 1/4 of the sheet thickness as a representative position. In addition, it is considered that crystal grains having a plate plane orientation within 10 ° from the ideal plane orientation ((123) plane) have almost the same action, and thus are defined by the area ratio of crystal grains having a plane plane orientation within that range. It was decided to.
＜圧延方向で、結晶方向が＜００１＞方向から１０°以内の結晶粒と、結晶方向が＜１１０＞方向から１０°以内の結晶粒との合計面積率が２５％以下＞
　この合計面積率が大きくなるほど絞り加工性における面内異方性が大きくなるので、２５％以下、好ましくは２３％以下、さらに好ましくは２０％以下に制限する。 <In the rolling direction, the total area ratio of crystal grains whose crystal direction is within 10 ° from the <001> direction and crystal grains whose crystal direction is within 10 ° from the <110> direction is 25% or less>
Since the in-plane anisotropy in drawing workability increases as the total area ratio increases, it is limited to 25% or less, preferably 23% or less, and more preferably 20% or less.
　また、上記面内異方性の低減効果により、浸炭熱処理後の焼き入れ時において、マルテンサイト変態は元の結晶粒の結晶方位関係を維持しつつ進行するので、変態ひずみを低減し、部品の寸法精度を向上する効果も得られる。また、別の解釈として、成形加工時に面内異方性が生じることは、部品内でひずみの不均一が生じていることを示しており、それが原因となって、浸炭熱処理、焼入れ後の部品の寸法精度を悪化させていることから、上記の集合組織制御による面内異方性の低減効果により、部品の寸法精度を向上する効果が得られるとも考えられる。 In addition, due to the effect of reducing the in-plane anisotropy, the martensitic transformation proceeds while maintaining the crystal orientation relationship of the original crystal grains during quenching after the carburizing heat treatment, so that the transformation strain is reduced. The effect of improving dimensional accuracy can also be obtained. Further, as another interpretation, the occurrence of in-plane anisotropy during molding processing indicates that non-uniform strain occurs in the part, which is the cause of carburizing heat treatment, after quenching Since the dimensional accuracy of the component is deteriorated, it is considered that the effect of improving the dimensional accuracy of the component can be obtained by the effect of reducing the in-plane anisotropy by the texture control described above.
＜前記全結晶粒の平均粒径が３～５０μｍ＞
　全結晶粒の平均粒径は、鋼板の加工性（絞り加工性、曲げ加工性、プレス加工性）を向上させるとともに、加工後の表面性状を満足させるため、３～５０μｍの範囲であることが必要である。結晶粒が細かくなりすぎると、変形抵抗が高くなりすぎるため、その平均粒径は３μｍ以上、好ましくは４μｍ以上、さらに好ましくは５μｍ以上とする。一方、結晶粒が粗大化しすぎると、靱性、疲労特性などが劣化するとともに、結晶方位を制御しても、曲げ加工性や張出などのプレス成形性が著しく低下し、成形時の割れや肌荒れなどの不良が生じ易いため、その平均粒径は５０μｍ以下、好ましくは４５μｍ以下、さらに好ましくは４０μｍ以下とする。なお、上記と同様、板厚方向で結晶粒のサイズ分布が存在するが、板厚の１／４の深さ位置を代表位置として全結晶粒の平均粒径を規定した。 <The average grain size of all the crystal grains is 3 to 50 μm>
The average grain size of all the crystal grains is in the range of 3 to 50 μm in order to improve the workability (drawing workability, bending workability, press workability) of the steel sheet and satisfy the surface properties after working. is necessary. If the crystal grains become too fine, the deformation resistance becomes too high, so the average grain size is 3 μm or more, preferably 4 μm or more, more preferably 5 μm or more. On the other hand, if the crystal grains become too coarse, the toughness, fatigue characteristics, etc. deteriorate, and even if the crystal orientation is controlled, the press formability such as bending workability and overhang is remarkably reduced, and cracks and rough surfaces during molding occur. The average particle size is 50 μm or less, preferably 45 μm or less, and more preferably 40 μm or less. Similar to the above, there is a crystal grain size distribution in the plate thickness direction, but the average grain size of all crystal grains was defined with a depth position of 1/4 of the plate thickness as a representative position.
〔結晶粒の板面方位の測定方法〕
　結晶粒の板面方位は、ＳＥＭ－ＥＢＳＰ（Ｅｌｅｃｔｒｏｎ　Ｂａｃｋ　Ｓｃａｔｔｅｒｉｎｇ　Ｐａｔｔｅｒｎ）と、ＥＢＳＤ（Ｅｌｅｃｔｒｏｎ　Ｂａｃｋ　Ｓｃａｔｔｅｒｉｎｇ　Ｄｉｆｆｒａｃｔｉｏｎ）によって、測定・解析される。ＳＥＭ装置としては、例えば日本電子社製ＳＥＭ（ＪＥＯＬＪＳＭ５４１０）、ＥＢＳＰ測定・解析システムとして、例えばＥＢＳＰ：ＴＳＬ社製（ＯＩＭ）を各々用いる。また、結晶粒の大きさにもよるが試料の測定領域は３００～１０００μｍ×３００～１０００μｍとし、測定ステップ間隔は例えば１～３μｍとする。このようにして同定した各結晶粒の結晶方位より、上記各理想面方位から１０°以内の方位のものを集計して合計面積を求め、測定領域の面積で除すことにより、各理想面方位ごとの面積率を求めた。 [Measurement method of crystal plane orientation]
The crystal plane orientation of the crystal grains is measured and analyzed by SEM-EBSP (Electron Back Scattering Pattern) and EBSD (Electron Back Scattering Diffraction). As the SEM apparatus, for example, SEM (JEOLJSM5410) manufactured by JEOL Ltd. is used, and for example, EBSP: manufactured by TSL (OIM) is used as the EBSP measurement / analysis system. Although it depends on the size of the crystal grains, the measurement area of the sample is 300 to 1000 μm × 300 to 1000 μm, and the measurement step interval is 1 to 3 μm, for example. From the crystal orientations of the respective crystal grains identified in this way, those with orientations within 10 ° from each of the ideal plane orientations are totaled to obtain a total area, and divided by the area of the measurement region, thereby obtaining each ideal plane orientation. The area ratio for each was determined.
〔結晶粒の圧延方向での結晶方向の測定方法〕
　また、結晶粒の圧延方向での結晶方向は、鋼板の圧延方向の断面（側面）についてＥＢＳＰ測定を行い、解析によって、圧延方向で、結晶方向が＜００１＞方向から１０°以内の結晶粒と、結晶方向が＜１１０＞方向から１０°以内の結晶粒を同定する。測定方法としては、結晶粒の大きさにもよるが、試料の測定領域は、板厚方向の１／４部で３００～１０００μｍ×３００～１０００μｍとし、測定ステップ間隔は例えば１～３μｍとする。このようにして同定した各結晶粒の結晶方向より、上記各理想面方向から１０°以内の方位のものを集計して合計面積を求め、測定領域の面積で除すことにより、各理想結晶方向ごとの面積率を求めた。 [Method of measuring crystal orientation in the rolling direction of crystal grains]
Further, the crystal direction in the rolling direction of the crystal grains is EBSP measurement with respect to the cross section (side surface) in the rolling direction of the steel sheet, and by analysis, the crystal direction is within the 10 ° direction from the <001> direction. The crystal grains whose crystal direction is within 10 ° from the <110> direction are identified. As a measuring method, although it depends on the size of the crystal grains, the measurement area of the sample is 300 to 1000 μm × 300 to 1000 μm at a quarter part in the plate thickness direction, and the measurement step interval is, for example, 1 to 3 μm. From the crystal directions of each crystal grain thus identified, those with an orientation within 10 ° from each ideal plane direction are totaled to obtain a total area, and divided by the area of the measurement region, thereby obtaining each ideal crystal direction. The area ratio for each was determined.
〔前記全結晶粒の平均粒径の測定方法〕
　前記全結晶粒の平均粒径は、上記ＳＥＭ－ＥＢＳＰと、その測定条件を用い、所定の測定領域内に観察される各結晶粒の最大直径を各々測定し、それらの平均値を平均粒径として求めた。 [Measuring method of average grain size of all crystal grains]
The average grain size of all the crystal grains is determined by measuring the maximum diameter of each crystal grain observed in a predetermined measurement region using the SEM-EBSP and measurement conditions thereof, and calculating the average value of the average grain diameters. As sought.
　次に、上記本発明鋼板を得るための好ましい製造方法を以下に説明する。 Next, a preferred manufacturing method for obtaining the steel sheet of the present invention will be described below.
〔本発明鋼板の好ましい製造方法〕
　本発明鋼板は、例えば、上記成分組成を有する原料鋼を溶解、鋳造してスラブとし、スラブまま、または、表面面削したスラブを、加熱、熱間粗圧延、仕上げ圧延の各工程を経て得られた熱延コイル上がり材として製造することができる。その後、表面状態や板厚精度等の必要条件に応じて、さらに、酸洗、スキンパスを施してもよい。 [Preferred production method of the steel sheet of the present invention]
The steel sheet of the present invention is obtained, for example, by melting and casting raw material steel having the above-mentioned composition to form a slab, and by subjecting the slab as it is or surface chamfered to each step of heating, hot rough rolling, and finish rolling. It can be manufactured as a hot rolled coil rising material. Thereafter, pickling and skin pass may be further performed according to necessary conditions such as surface condition and plate thickness accuracy.
［溶鋼の調製］
　まず、溶存酸素量と全酸素量を調整した溶鋼に、所定の順番で所定の合金元素を添加することによって、所望の酸化物を生成させることができる。特に本発明では、粗大な酸化物が生成しないように、溶存酸素量を調整した後、全酸素量を調整することが極めて重要である。 [Preparation of molten steel]
First, a desired oxide can be generated by adding a predetermined alloy element in a predetermined order to molten steel in which the dissolved oxygen amount and the total oxygen amount are adjusted. Particularly in the present invention, it is extremely important to adjust the total oxygen amount after adjusting the dissolved oxygen amount so that coarse oxides are not formed.
　溶存酸素とは、酸化物を形成しておらず、溶鋼中に存在するフリーな状態の酸素を意味する。全酸素とは、溶鋼に含まれる全ての酸素、すなわち、フリー酸素と酸化物を形成している酸素の総和を意味する。 “Dissolved oxygen” means oxygen in a free state that does not form oxides and exists in molten steel. Total oxygen means the sum of all oxygen contained in molten steel, that is, free oxygen and oxygen forming oxides.
　まず、溶鋼の溶存酸素量を０．００１０～０．００６０％の範囲に調整する。溶鋼の溶存酸素量が０．００１０％未満では、溶鋼中の溶存酸素量が不足するため、Ａｌ－Ｏ系酸化物を所定量確保することができず、所望のサイズ分布が得られない。また、溶存酸素量が不足すると、ＲＥＭを添加する場合は、ＲＥＭが硫化物を形成するため、介在物が粗大となり特性を劣化させる原因となる。したがって、上記溶存酸素量は０．００１０％以上とする。上記溶存酸素は、好ましくは０．００１３％以上、より好ましくは０．００２０％以上である。 First, the dissolved oxygen content of the molten steel is adjusted to a range of 0.0010 to 0.0060%. When the amount of dissolved oxygen in the molten steel is less than 0.0010%, the amount of dissolved oxygen in the molten steel is insufficient, so that a predetermined amount of Al—O-based oxide cannot be secured, and a desired size distribution cannot be obtained. In addition, when the amount of dissolved oxygen is insufficient, when REM is added, REM forms sulfides, so that inclusions become coarse and deteriorate characteristics. Therefore, the amount of dissolved oxygen is set to 0.0010% or more. The dissolved oxygen is preferably 0.0013% or more, more preferably 0.0020% or more.
　一方、上記溶存酸素量が０．００６０％を超えると、溶鋼中の酸素量が多くなりすぎるため、溶鋼中の酸素と上記元素の反応が激しくなって溶製作業上好ましくないばかりか、粗大な酸化物を生成して却って特性を劣化させる。したがって、上記溶存酸素量は０．００６０％以下に抑えるべきである。上記溶存酸素量は、好ましくは０．００５５％以下、より好ましくは０．００５３％以下とする。 On the other hand, if the amount of dissolved oxygen exceeds 0.0060%, the amount of oxygen in the molten steel becomes too large, and the reaction between the oxygen in the molten steel and the above elements becomes violent, which is not preferable for melting work, and is coarse. Oxide is produced and the characteristics are deteriorated. Therefore, the amount of dissolved oxygen should be suppressed to 0.0060% or less. The amount of dissolved oxygen is preferably 0.0055% or less, more preferably 0.0053% or less.
　ところで、転炉や電気炉で一次精錬された溶鋼中の溶存酸素量は、通常０．０１０％を超えている。そこで本発明の製法では、溶鋼中の溶存酸素量を何らかの方法で上記範囲に調整する必要がある。 By the way, the amount of dissolved oxygen in molten steel primarily refined in a converter or electric furnace usually exceeds 0.010%. Therefore, in the production method of the present invention, it is necessary to adjust the amount of dissolved oxygen in the molten steel to the above range by some method.
　溶鋼中の溶存酸素量を調整する方法としては、例えばＲＨ式脱ガス精錬装置を用いて真空Ｃ脱酸する方法や、ＳｉやＭｎ、Ａｌなどの脱酸性元素を添加する方法などが挙げられ、これらの方法を適宜組み合わせて溶存酸素量を調整してもよい。また、ＲＨ式脱ガス精錬装置の代わりに、取鍋加熱式精錬装置や簡易式溶鋼処理設備などを用いて溶存酸素量を調整してもよい。この場合、真空Ｃ脱酸による溶存酸素量の調整はできないため、溶存酸素量の調整にはＳｉ等の脱酸性元素を添加する方法を採用すればよい。Ｓｉ等の脱酸性元素を添加する方法を採用するときは、転炉から取鍋へ出鋼する際に脱酸性元素を添加しても構わない。 Examples of the method for adjusting the amount of dissolved oxygen in the molten steel include a method of vacuum C deoxidation using an RH type degassing refining device, a method of adding a deacidifying element such as Si, Mn, and Al. The amount of dissolved oxygen may be adjusted by appropriately combining these methods. Moreover, you may adjust the amount of dissolved oxygen using a ladle heating type refining apparatus, a simple molten steel processing facility, etc. instead of the RH type degassing refining apparatus. In this case, since the amount of dissolved oxygen cannot be adjusted by vacuum C deoxidation, a method of adding a deacidifying element such as Si may be adopted to adjust the amount of dissolved oxygen. When employing a method of adding a deoxidizing element such as Si, the deoxidizing element may be added when steel is removed from the converter to the ladle.
　溶鋼の溶存酸素量を０．００１０～０．００６０％の範囲に調整した後は溶鋼を攪拌し、溶鋼中の酸化物を浮上分離することによって溶鋼中の全酸素量を０．００１０～０．００７０％に調整する。このように本発明では、溶存酸素量が適切に制御された溶鋼を撹拌し、不要な酸化物を除去してから、粗大な酸化物、すなわち、粗大な介在物の生成を防止できる。 After the dissolved oxygen content of the molten steel is adjusted to the range of 0.0010 to 0.0060%, the molten steel is stirred, and the oxides in the molten steel are floated and separated so that the total oxygen content in the molten steel is 0.0010 to 0.00. Adjust to 0070%. As described above, in the present invention, the molten steel in which the amount of dissolved oxygen is appropriately controlled is stirred to remove unnecessary oxides, and then generation of coarse oxides, that is, coarse inclusions can be prevented.
　上記全酸素量が０．００１０％未満では、所望の酸化物量不足になるため、介在物の微細なサイズ分布に寄与する酸化物量を確保することができない。したがって、上記全酸素量は０．００１０％以上とする。上記全酸素量は、好ましくは０．００１５％以上、より好ましくは０．００１８％以上である。 If the total oxygen amount is less than 0.0010%, the desired amount of oxide is insufficient, and the amount of oxide that contributes to the fine size distribution of inclusions cannot be ensured. Therefore, the total oxygen amount is set to 0.0010% or more. The total oxygen amount is preferably 0.0015% or more, more preferably 0.0018% or more.
　一方、上記全酸素量が０．００７０％を超えると、溶鋼中の酸化物量が過剰となり、粗大な酸化物、すなわち、粗大な介在物が生成して特性が劣化する。したがって、上記全酸素量は０．００７０％以下に抑えるべきである。上記全酸素量は、好ましくは０．００６０％以下、より好ましくは０．００５０％以下とする。 On the other hand, if the total oxygen amount exceeds 0.0070%, the amount of oxide in the molten steel becomes excessive, and coarse oxides, that is, coarse inclusions are generated, and the characteristics deteriorate. Therefore, the total oxygen amount should be suppressed to 0.0070% or less. The total oxygen amount is preferably 0.0060% or less, more preferably 0.0050% or less.
　溶鋼中の全酸素量は、概ね溶鋼の攪拌時間に相関して変化することから、撹拌時間を調整するなどして制御することができる。具体的には、溶鋼を撹拌し、浮上してきた酸化物を除去した後の溶鋼中の全酸素量を適宜測定しながら、溶鋼中の全酸素量を適切に制御する。 Since the total amount of oxygen in the molten steel changes generally in correlation with the stirring time of the molten steel, it can be controlled by adjusting the stirring time. Specifically, the total amount of oxygen in the molten steel is appropriately controlled while appropriately measuring the total amount of oxygen in the molten steel after stirring the molten steel and removing the floating oxide.
　鋼材にＲＥＭ及びＣａを添加する場合は、溶鋼中の全酸素量を上記範囲に調整した後に、ＲＥＭを添加してから鋳造する。全酸素量を調整した溶鋼へ上記の元素を添加することによって所望とする酸化物が得られる。 When REM and Ca are added to the steel material, the total oxygen content in the molten steel is adjusted to the above range, and then REM is added before casting. The desired oxide can be obtained by adding the above elements to the molten steel with the total oxygen content adjusted.
　溶鋼へ添加するＲＥＭ及びＣａの形態は特に限定されず、例えば、ＲＥＭとして、純Ｌａや純Ｃｅ、純Ｙなど、あるいは純Ｃａ、さらにはＦｅ－Ｓｉ－Ｌａ合金、Ｆｅ－Ｓｉ－Ｃｅ合金、Ｆｅ－Ｓｉ－Ｃａ合金、Ｆｅ－Ｓｉ－Ｌａ－Ｃｅ合金、Ｆｅ－Ｃａ合金、Ｎｉ－Ｃａ合金などを添加すればよい。また、溶鋼へミッシュメタルを添加してもよい。ミッシュメタルとは、セリウム族希土類元素の混合物であり、具体的には、Ｃｅを４０～５０％程度、Ｌａを２０～４０％程度含有している。ただし、ミッシュメタルには不純物としてＣａを含むことが多いので、ミッシュメタルがＣａを含む場合は、本発明で規定する好適範囲を満足する必要がある。 The form of REM and Ca added to the molten steel is not particularly limited. For example, as REM, pure La, pure Ce, pure Y, or pure Ca, and further Fe—Si—La alloy, Fe—Si—Ce alloy, An Fe—Si—Ca alloy, Fe—Si—La—Ce alloy, Fe—Ca alloy, Ni—Ca alloy, or the like may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of cerium group rare earth elements, and specifically contains about 40 to 50% Ce and about 20 to 40% La. However, since misch metal often contains Ca as an impurity, when the misch metal contains Ca, it is necessary to satisfy the preferred range defined in the present invention.
　本発明でＲＥＭを添加した場合は、粗大な酸化物の除去を促進する目的で、ＲＥＭを添加した後は、４０分を超えない範囲で溶鋼を攪拌することが好ましい。攪拌時間が４０分を超えると、微細な酸化物が溶鋼中で凝集・合体するため酸化物が粗大化し、特性が劣化する。したがって、攪拌時間は４０分以内とすることが好ましい。攪拌時間は、より好ましくは３５分以内であり、さらに好ましくは３０分以内である。溶鋼の攪拌時間の下限値は特に限定されないが、攪拌時間が短過ぎると添加元素の濃度が不均一となり、鋼材全体として所望の効果が得られない。したがって、容器サイズに応じた所望の攪拌時間が必要となる。 When REM is added in the present invention, it is preferable to stir the molten steel within a range not exceeding 40 minutes after the addition of REM for the purpose of promoting the removal of coarse oxides. When the stirring time exceeds 40 minutes, fine oxides aggregate and coalesce in the molten steel, so that the oxides become coarse and the characteristics deteriorate. Therefore, the stirring time is preferably within 40 minutes. The stirring time is more preferably within 35 minutes, and further preferably within 30 minutes. The lower limit of the stirring time of the molten steel is not particularly limited, but if the stirring time is too short, the concentration of the additive element becomes non-uniform, and the desired effect cannot be obtained as a whole steel material. Accordingly, a desired stirring time corresponding to the container size is required.
　以上のようにして、成分組成が調整された溶鋼が得られる。得られた溶鋼を用いて鋳造し、鋼片を得る。 As described above, molten steel with an adjusted composition can be obtained. It casts using the obtained molten steel, and obtains a steel piece.
　次に、加熱、仕上げ圧延を含む熱間圧延、熱延後の急冷、急冷停止後の緩冷、緩冷後の急冷、巻取りを行って製造する。 Next, manufacturing is performed by heating, hot rolling including finish rolling, rapid cooling after hot rolling, slow cooling after quenching stop, rapid cooling after slow cooling, and winding.
［加熱］
　熱間圧延前の加熱は１１５０～１３００℃で行う。この加熱によりオーステナイト単相とする。これにより固溶元素（Ｖ、Ｎｂなどの添加元素を含む）は、オーステナイトに固溶させる。加熱温度が１１５０℃未満ではオーステナイトに固溶できず、粗大な炭化物が形成されるため疲労特性改善効果が得られない。一方、１３００℃を超える加熱温度は操業上困難である。また、添加元素としてＴｉが含まれる場合、炭化物のうち最も溶体化温度の高いＴｉを固溶させる点でも、ＴｉＣの溶体化温度以上１３００℃以下が必要である。加熱温度のより好ましい下限は１２００℃である。 [heating]
Heating before hot rolling is performed at 1150 to 1300 ° C. An austenite single phase is obtained by this heating. Thereby, solid solution elements (including additive elements such as V and Nb) are dissolved in austenite. If the heating temperature is less than 1150 ° C., it cannot be dissolved in austenite, and coarse carbides are formed, so that the effect of improving fatigue characteristics cannot be obtained. On the other hand, heating temperatures exceeding 1300 ° C. are difficult to operate. Moreover, when Ti is contained as an additive element, the TiC solution solution temperature or higher and 1300 ° C. or lower are necessary also in terms of solid solution of Ti having the highest solution temperature among carbides. A more preferable lower limit of the heating temperature is 1200 ° C.
［熱間粗圧延］
　粗圧延では、本発明で規定する所定の結晶方位を有する結晶粒の割合を確保するため、再結晶オーステナイトの組織制御を行う。粗圧延温度は、以後の仕上げ圧延温度の確保も考慮して９００～１２００℃とし、粗圧延でのオーステナイト粒の微細化、繰り返し再結晶化させることで、所定の結晶方位を有する結晶粒の割合を制御することができる。粗圧延温度は、より好ましくは９００～１１００℃である。 [Hot rough rolling]
In rough rolling, the structure control of recrystallized austenite is performed in order to ensure the proportion of crystal grains having a predetermined crystal orientation defined in the present invention. The rough rolling temperature is set to 900 to 1200 ° C. in consideration of securing the subsequent finish rolling temperature, and the proportion of crystal grains having a predetermined crystal orientation is obtained by refining austenite grains in the rough rolling and repeatedly recrystallizing. Can be controlled. The rough rolling temperature is more preferably 900 to 1100 ° C.
［熱間仕上げ圧延］
　熱間圧延は、仕上げ圧延温度が８００℃以上になるように行う。仕上げ圧延温度を低温化しすぎるとフェライト変態が高温で起るようになり、フェライト中の析出炭化物が粗大化するため、一定以上の仕上げ圧延温度が必要である。仕上げ圧延温度は、オーステナイト粒を粗大化してベイナイトの粒径を大きくするため、８５０℃以上とするのがより好ましい。 [Hot finish rolling]
Hot rolling is performed so that the finish rolling temperature is 800 ° C. or higher. If the finish rolling temperature is too low, ferrite transformation occurs at a high temperature and the precipitated carbides in the ferrite are coarsened, so that a certain finish rolling temperature is required. The finish rolling temperature is more preferably 850 ° C. or higher in order to coarsen austenite grains and increase the grain size of bainite.
［熱間仕上げ圧延の最終パスの圧下率］
　熱間仕上げ圧延の最終パスの圧下率が高すぎると、本発明の集合組織の形態が得られず、異方性が大きくなる。逆に圧下率が低すぎると、集合組織が発達しない。このため、熱間仕上げ圧延の最終パスの圧下率は１０～１８％とし、より好ましくは１１～１７％、特に好ましくは１２～１６％とする。 [Rolling ratio of the final pass of hot finish rolling]
If the rolling reduction in the final pass of hot finish rolling is too high, the texture structure of the present invention cannot be obtained and the anisotropy increases. Conversely, if the rolling reduction is too low, the texture does not develop. For this reason, the rolling reduction in the final pass of hot finish rolling is 10 to 18%, more preferably 11 to 17%, and particularly preferably 12 to 16%.
［熱延後の急冷］
　上記仕上げ圧延終了後、５ｓ以内に２０℃／ｓ以上の冷却速度（急冷速度）で急冷し、５８０℃以上６８０℃未満の温度（急冷停止温度）で急冷を停止する。フェライト変態の開始温度を低温化することによりフェライト中に形成される析出炭化物を微細化するためである。冷却速度（急冷速度）が２０℃／ｓ未満ではパーライト変態が促進され、または、急冷停止温度が５８０℃未満ではパーライト変態またはベイナイト変態が促進され、冷間加工性が低下する。一方、急冷停止温度が６８０℃以上になるとフェライト中の析出炭化物が粗大化してしまい、耐疲労特性が確保できない。急冷停止温度は、好ましくは６００～６５０℃、さらに好ましくは６１０～６４０℃である。 [Rapid cooling after hot rolling]
After the finish rolling, quenching is performed at a cooling rate (quenching rate) of 20 ° C./s or more within 5 s, and the quenching is stopped at a temperature of 580 ° C. or more and less than 680 ° C. (quenching stop temperature). This is because the precipitation carbide formed in the ferrite is refined by lowering the starting temperature of the ferrite transformation. When the cooling rate (quenching rate) is less than 20 ° C./s, pearlite transformation is promoted, or when the quenching stop temperature is less than 580 ° C., pearlite transformation or bainite transformation is promoted, and cold workability is lowered. On the other hand, when the quenching stop temperature is 680 ° C. or more, the precipitated carbide in the ferrite is coarsened, and the fatigue resistance characteristics cannot be ensured. The quenching stop temperature is preferably 600 to 650 ° C, more preferably 610 to 640 ° C.
［急冷停止後の緩冷］
　上記急冷停止後は、５℃／ｓ以上２０℃／ｓ未満の冷却速度（緩冷速度）で緩冷する。緩冷速度を５℃／ｓ以上とすることで、熱延中における初析フェライトの形成を抑制し、フェライト中の析出炭化物を適度に微細化させること、熱延板での結晶粒組織を制御することにより、最終鋼板における集合組織形態を制御するためである。緩冷速度が５℃／ｓ未満では、初析フェライトの形成量が多くなり、粗大粒が生成するとともに、最終鋼板で粗大粒が生成し、炭化物の不均一状態を生じ、冷間加工性を劣化させる。また、２０℃／ｓ以上の冷却速度とすると、硬質相（ベイナイト、マルテンサイト）が多く生成されるようになり、冷間加工性が低下する。 [Slow cooling after rapid cooling stop]
After the rapid cooling stop, it is slowly cooled at a cooling rate (slow cooling rate) of 5 ° C./s or more and less than 20 ° C./s. By setting the slow cooling rate to 5 ° C./s or more, the formation of pro-eutectoid ferrite during hot rolling is suppressed, the precipitated carbides in ferrite are appropriately refined, and the grain structure of the hot rolled sheet is controlled. This is to control the texture form in the final steel sheet. When the slow cooling rate is less than 5 ° C./s, the amount of pro-eutectoid ferrite is increased and coarse grains are produced, and coarse grains are produced in the final steel plate, resulting in a non-uniform state of carbides, resulting in cold workability. Deteriorate. On the other hand, when the cooling rate is 20 ° C./s or more, a large amount of hard phase (bainite, martensite) is generated, and cold workability is deteriorated.
［緩冷後の急冷、巻取り］
　上記緩冷後、５５０℃超６５０℃以下で巻き取る。巻取り温度が６５０℃超では、表面酸化スケールが多く形成され、表面性状が劣化し、一方５５０℃未満では、マルテンサイトが多く形成され、冷間加工性が低下する。 [Rapid cooling after slow cooling, winding]
After the above-described slow cooling, the film is wound at over 550 ° C and below 650 ° C. When the coiling temperature is higher than 650 ° C., many surface oxide scales are formed and the surface properties are deteriorated. On the other hand, when it is less than 550 ° C., many martensites are formed and cold workability is lowered.
　以下、本発明を実施例によってさらに詳細に説明するが、下記実施例は本発明を限定する性質のものではなく、前・後記の趣旨に適合し得る範囲で適当に変更して実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。 Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.
　下記表１に示す成分組成の鋼を真空溶解法により溶製し、厚さ１２０ｍｍのインゴットに鋳造し、これを下記表２に示す条件にて熱間圧延を施し熱延鋼板を作製した。なお、いずれの試験においても、急冷停止後の冷却は１０℃／ｓ以下の冷却速度で５～２０ｓ緩冷する条件であった。 Steel having the component composition shown in the following Table 1 was melted by a vacuum melting method, cast into an ingot having a thickness of 120 mm, and hot rolled under the conditions shown in Table 2 to produce a hot rolled steel sheet. In all the tests, the cooling after the rapid cooling stop was a condition of slow cooling for 5 to 20 s at a cooling rate of 10 ° C./s or less.
　真空溶解炉（容量１５０ｋｇ）を用い、表１に示した化学成分を含有する供試鋼を溶製し、１５０ｋｇのインゴットに鋳造して冷却した。真空溶解炉で供試鋼を溶製するに当っては、Ａｌ、ＲＥＭ、Ｃａ以外の元素について成分調整するとともに、Ｃ，ＳｉおよびＭｎから選ばれる少なくとも１種の元素を用いて脱酸して溶鋼の溶存酸素量を調整した。溶存酸素量を調整した溶鋼を１～１０分程度攪拌して溶鋼中の酸化物を浮上分離させることによって溶鋼の全酸素量を調整した。 Using a vacuum melting furnace (capacity 150 kg), a test steel containing chemical components shown in Table 1 was melted, cast into a 150 kg ingot, and cooled. In melting the test steel in a vacuum melting furnace, the components are adjusted for elements other than Al, REM, and Ca, and deoxidized using at least one element selected from C, Si, and Mn. The amount of dissolved oxygen in the molten steel was adjusted. The total amount of oxygen in the molten steel was adjusted by stirring the molten steel in which the amount of dissolved oxygen was adjusted for about 1 to 10 minutes to float and separate oxides in the molten steel.
　なお、ＲＥＭおよびＣａを添加する場合は、全酸素量を調整した溶鋼に添加することによって成分調整した溶鋼を得た。なお、ＲＥＭはＬａを約２５％とＣｅを約５０％含有するミッシュメタルの形態で、ＣａはＮｉ－Ｃａ合金、またはＣａ－Ｓｉ合金、またはＦｅ－Ｃａ圧粉体の形態で、それぞれ添加した。 In addition, when adding REM and Ca, the molten steel which adjusted the component was obtained by adding to the molten steel which adjusted the total oxygen amount. REM was added in the form of a misch metal containing about 25% La and about 50% Ce, and Ca was added in the form of a Ni—Ca alloy, a Ca—Si alloy, or a Fe—Ca compact. .
　そして、得られたインゴットを表２に示す各条件で熱間圧延して所定板厚の熱延上がり板を製造した。なお、 表２には急冷停止後の冷却速度を記載していないが、各製造例ともに１０℃／ｓの条件を採用している。 The obtained ingot was hot-rolled under the conditions shown in Table 2 to produce a hot rolled plate having a predetermined thickness. In Table 2, the cooling rate after the rapid cooling stop is not described, but the condition of 10 ° C./s is adopted in each production example.
　このようにして得られた各熱延上がり板について、上記［発明を実施するための形態］の項で説明した測定方法により、結晶粒の板面方位、結晶粒の圧延方向での結晶方向、および、全結晶粒の平均粒径を調査した。なお、表３に示している鋼Ｎｏ．１～２７はいずれもフェライトとパーライトの合計量が面積率で９０％以上であった（フェライト及びパーライトを主体とする組織であった）ことを確認している。 For each hot rolled plate thus obtained, by the measurement method described in the above section [Mode for carrying out the invention], the crystal plane direction of the crystal grains, the crystal direction in the rolling direction of the crystal grains, And the average particle diameter of all the crystal grains was investigated. In addition, steel No. shown in Table 3 Nos. 1-27 confirmed that the total amount of ferrite and pearlite was 90% or more in area ratio (it was a structure mainly composed of ferrite and pearlite).
　また、上記各熱延上がり板について、絞り加工性を評価するため、圧延方向に対して０°（圧延方向に平行）、４５°、９０°（圧延方向に垂直）になるようにＪＩＳ５号片を採取し、引張試験を行って、それぞれの角度でのｒ値（ｒ０、ｒ４５、ｒ９０）を求めた。そして、以下の式で平均ｒ値およびΔｒ値を算出した。ここで、Δｒ値は、ｒ値の面内異方性を評価するための指標である。
　　平均ｒ値＝（ｒ０＋２×ｒ４５＋ｒ９０）／４
　　Δｒ値＝（ｒ０＋ｒ９０）／２－ｒ４５
　そして、ｒ０、ｒ４５、ｒ９０、および、平均ｒ値が、全て０．８５以上で、かつΔｒ値が±０．１以内のものを合格とした。 Moreover, in order to evaluate drawing workability about each said hot-rolling board, it is JIS No. 5 piece so that it may become 0 degree (parallel to a rolling direction), 45 degrees, and 90 degrees (perpendicular to a rolling direction) with respect to a rolling direction. Were collected and subjected to a tensile test to obtain r values (r0, r45, r90) at respective angles. And the average r value and (DELTA) r value were computed with the following formula | equation. Here, the Δr value is an index for evaluating the in-plane anisotropy of the r value.
Average r value = (r0 + 2 × r45 + r90) / 4
Δr value = (r0 + r90) / 2−r45
Then, r0, r45, r90, and the average r value were all 0.85 or more and the Δr value was within ± 0.1.
　さらに、上記各熱延上がり板について、浸炭熱処理後の表面硬さを評価するため、以下の条件にて浸炭焼き入れ試験を行った。 Furthermore, in order to evaluate the surface hardness after the carburizing heat treatment, the carburizing and quenching test was performed under the following conditions.
〔浸炭焼き入れ条件〕
　カーボンポテンシャル（ＣＰ値）＝０．８％のガス雰囲気中で、９００℃×２．５ｈ保持後さらに８５０℃×０．５ｈ保持して浸炭処理を施した後、１００℃で油焼き入れをし、その後１６０℃×２ｈ保持して焼き戻し処理を施した後、空冷した。 [Carburizing and quenching conditions]
In a gas atmosphere of carbon potential (CP value) = 0.8%, after holding at 900 ° C. × 2.5 h and further carburizing by holding at 850 ° C. × 0.5 h, oil quenching was performed at 100 ° C. Then, after holding at 160 ° C. for 2 hours and performing a tempering treatment, it was air-cooled.
＜浸炭熱処理後の表面硬さ＞
　そして、ビッカース硬さ試験機を用いて、荷重：１０００ｇ、測定位置：鋼板表面から０．８ｍｍ深さの位置を測定回数：５回の条件で、ビッカース硬さ（Ｈｖ）を測定し、３５０Ｈｖ以上のものを合格とした。ここで、測定位置を表面から０．８ｍｍ深さの位置としたのは、浸炭熱処理後において、表面から深いところでも所望の硬さ（強度）を示すことを必要条件としたことによるものである。 <Surface hardness after carburizing heat treatment>
Then, using a Vickers hardness tester, load: 1000 g, measurement position: position at a depth of 0.8 mm from the surface of the steel sheet, measurement number: 5 times, Vickers hardness (Hv) is measured, 350 Hv or more Was accepted. Here, the measurement position was set at a depth of 0.8 mm from the surface because, after carburizing heat treatment, it was a necessary condition to show a desired hardness (strength) even deep from the surface. .
　これらの測定結果を下記表３に示す。  These measurement results are shown in Table 3 below.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　表３に示すように、鋼Ｎｏ．１、２、６～２０はいずれも、本発明の成分組成規定の要件を満足する鋼種を用い、推奨の熱間圧延条件で製造した結果、本発明の組織規定の要件を充足する発明鋼であり、絞り加工性の指標および浸炭熱処理後の表面硬さはすべて合格基準を満たしており、良好な絞り加工性を確保しつつ、浸炭熱処理後は所定の表面硬さ（強度）を示す熱延鋼板が得られることが確認できた。 As shown in Table 3, steel no. 1, 2, and 6 to 20 are steels that satisfy the requirements of the compositional composition of the present invention and are manufactured under the recommended hot rolling conditions. Yes, the drawing processability index and the surface hardness after carburizing heat treatment all meet the acceptance criteria, while ensuring good drawing workability, hot rolling showing a predetermined surface hardness (strength) after carburizing heat treatment It was confirmed that a steel plate was obtained.
　これに対し、鋼Ｎｏ．３～５、２１～２７は本発明で規定する成分組成および組織の要件のうち少なくともいずれかを満足しない比較鋼であり、絞り加工性の指標および浸炭熱処理後の表面硬さのうち少なくともいずれかが合格基準を満たしていない。 In contrast, Steel No. Reference numerals 3 to 5 and 21 to 27 are comparative steels that do not satisfy at least one of the component composition and the structure requirements defined in the present invention, and are at least one of the index of drawability and the surface hardness after carburizing heat treatment. Does not meet the acceptance criteria.
　例えば、鋼Ｎｏ．３は、成分組成の要件は満たしているものの、熱延前の加熱温度、粗圧延温度、仕上げ圧延温度、急冷致死温度および巻取り温度のいずれもが推奨範囲を外れて低すぎ、圧延方向での結晶方向が＜００１＞方向と＜１１０＞方向の結晶粒が過剰に形成され、絞り加工性、中でもｒ値の異方性が劣っている。 For example, steel No. Although the requirements for component composition 3 are satisfied, the heating temperature before hot rolling, rough rolling temperature, finish rolling temperature, quenching lethal temperature, and winding temperature are all out of the recommended range and are too low in the rolling direction. The crystal directions of <001> and <110> directions are excessively formed, and the drawing workability, particularly the r value anisotropy, is inferior.
　また、鋼Ｎｏ．４は、成分組成の要件は満たしているものの、熱延後の板厚が規定範囲を外れて大きすぎ、（１２３）面の板面方位を有する結晶粒が不足するとともに、結晶粒が粗大化し、絞り加工性が劣っている。 Steel No. No. 4, although the requirements of the component composition are satisfied, the plate thickness after hot rolling is too large outside the specified range, the crystal grains having the (123) plane plane orientation are insufficient, and the crystal grains are coarsened. The drawing processability is inferior.
　また、鋼Ｎｏ．５は、成分組成の要件は満たしているものの、仕上げ圧延の最終パスにおける圧下率が推奨範囲を外れて小さすぎ、（１２３）面の板面方位を有する結晶粒が不足し、圧延方向での結晶方向が＜００１＞方向と＜１１０＞方向の結晶粒が過剰に形成され、絞り加工性、中でもｒ値の異方性が劣っている。 Steel No. 5, although the requirements of the component composition are satisfied, the rolling reduction in the final pass of finish rolling is too small outside the recommended range, the crystal grains having the (123) plane plate orientation are insufficient, and in the rolling direction Crystal grains with <001> direction and <110> direction are formed excessively, and drawing workability, especially r value anisotropy is inferior.
　また、鋼Ｎｏ．２１（鋼種ｑ）は、熱延条件は推奨範囲にあるものの、Ｃ含有量が低すぎ、浸炭熱処理後の表面硬さが劣っている。 Steel No. Although 21 (steel type q) has a hot rolling condition in the recommended range, the C content is too low and the surface hardness after carburizing heat treatment is inferior.
　一方、鋼Ｎｏ．２２（鋼種ｒ）は、熱延条件は推奨範囲にあるものの、Ｃ含有量が高すぎ、絞り加工性が劣っている。  On the other hand, steel No. Although 22 (steel type r) has a hot rolling condition in the recommended range, the C content is too high and the drawability is inferior.
　また、鋼Ｎｏ．２３（鋼種ｓ）は、熱延条件は推奨範囲にあるものの、Ｍｎ含有量が低すぎ、浸炭熱処理後の表面硬さが劣っている。 Steel No. Although 23 (steel type s) has a hot rolling condition in the recommended range, the Mn content is too low and the surface hardness after carburizing heat treatment is inferior.
　一方、鋼Ｎｏ．２４（鋼種ｔ）は、熱延条件は推奨範囲にあるものの、Ｍｎ含有量が高すぎ、絞り加工性が劣っている。  On the other hand, steel No. Although 24 (steel type t) has a hot rolling condition in the recommended range, the Mn content is too high and the drawability is inferior.
　また、鋼Ｎｏ．２５（鋼種ｕ）は、熱延条件は推奨範囲にあるものの、Ａｌ含有量が低すぎ、絞り加工性が劣っている。  Steel No. Although 25 (steel type u) has a hot rolling condition in the recommended range, the Al content is too low and the drawability is inferior.
　一方、鋼Ｎｏ．２６（鋼種ｖ）は、熱延条件は推奨範囲にあるものの、Ａｌ含有量が高すぎ、やはり絞り加工性が劣っている。  On the other hand, steel No. No. 26 (steel type v), although the hot rolling conditions are in the recommended range, the Al content is too high and the drawability is also inferior.
　また、鋼Ｎｏ．２７（鋼種ｗ）は、熱延条件は推奨範囲にあるものの、Ｎ含有量が高すぎ、絞り加工性が劣っている。  Steel No. No. 27 (steel type w) has a hot rolling condition in the recommended range, but the N content is too high and the drawability is inferior.
　以上より、本発明の適用性が確認できた。 From the above, the applicability of the present invention was confirmed.
　本発明を詳細にまた特定の実施態様を参照して説明したが、本発明の精神と範囲を逸脱することなく様々な変更や修正を加えることができることは当業者にとって明らかである。
　本出願は、２０１３年１０月２２日出願の日本特許出願（特願２０１３－２１９４６８）に基づくものであり、その内容はここに参照として取り込まれる。 Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on October 22, 2013 (Japanese Patent Application No. 2013-219468), the contents of which are incorporated herein by reference.
　本発明の熱延鋼板は、加工中に良好な冷間加工性を示しつつ、浸炭熱処理後に所定の表面および表面から深い部分での硬さを示し、耐摩耗性や耐疲労特性にも優れるため、例えば自動車に用いられるクラッチ、ダンパー、歯車（ギア）等に有用である。 The hot-rolled steel sheet according to the present invention exhibits good cold workability during processing, exhibits hardness at a predetermined surface and a deep portion from the surface after carburizing heat treatment, and is excellent in wear resistance and fatigue resistance properties. For example, it is useful for clutches, dampers, gears (gears) and the like used in automobiles.

Claims (3)

  1. 　板厚が２～１０ｍｍであり、 
    　成分組成が、 
    　質量％で（以下、化学成分について同じ。）、
    　Ｃ　：０．０５～０．３０％、 
    　Ｍｎ：０．３～３．０％、 
    　Ａｌ：０．０１５～０．１％、 
    　Ｎ　：０．００３～０．３０％を含み、 
    　残部は鉄および不可避的不純物からなり、 
    　フェライト及びパーライトを主体とする組織であって、
    　深さｔ／４（ｔ：板厚）の位置に存在する、フェライトおよびパーライトを含む全ての結晶粒（以下、「全結晶粒」という。）に関し、
    　板面方位が（１２３）面から１０°以内の結晶粒の面積率が２０％以上であり、 
    　圧延方向で、結晶方向が＜００１＞方向から１０°以内の結晶粒と、結晶方向が＜１１０＞方向から１０°以内の結晶粒との合計面積率が２５％以下であるとともに、
    　前記全結晶粒の平均粒径が３～５０μｍである 
    ことを特徴とする絞り加工性と浸炭熱処理後の表面硬さに優れる熱延鋼板。     The plate thickness is 2-10mm,
    Ingredient composition
    % By mass (hereinafter the same for chemical components)
    C: 0.05 to 0.30%,
    Mn: 0.3 to 3.0%,
    Al: 0.015 to 0.1%,
    N: 0.003 to 0.30% included,
    The balance consists of iron and inevitable impurities,
    An organization mainly composed of ferrite and pearlite,
    Regarding all the crystal grains including ferrite and pearlite (hereinafter referred to as “all crystal grains”) existing at a depth t / 4 (t: plate thickness),
    The area ratio of crystal grains whose plate plane orientation is within 10 ° from the (123) plane is 20% or more,
    In the rolling direction, the total area ratio of the crystal grains whose crystal direction is within 10 ° from the <001> direction and the crystal grains whose crystal direction is within 10 ° from the <110> direction is 25% or less,
    The average grain size of all the crystal grains is 3 to 50 μm
    A hot-rolled steel sheet excellent in drawing workability and surface hardness after carburizing heat treatment.
  2. 　前記不可避的不純物のうち、Ｓｉ：０．５％以下、Ｐ：０．０３０％以下、Ｓ：０．０３５％以下である請求項１に記載の熱延鋼板。 The hot rolled steel sheet according to claim 1, wherein among the inevitable impurities, Si: 0.5% or less, P: 0.030% or less, and S: 0.035% or less.
  3. 　成分組成が、さらに、下記（ａ）～（ｆ）の少なくとも１種を含むものである請求項１または２に記載の熱延鋼板。
    （ａ）Ｃｒ：３．０％以下（０％を含まない）、Ｍｏ：１．０％以下（０％を含まない）及びＮｉ：３．０％以下（０％を含まない）よりなる群から選択される少なくとも１種
    （ｂ）Ｃｕ：２．０％以下（０％を含まない）及びＣｏ：５％以下（０%を含まない）よりなる群から選択される少なくとも１種
    （ｃ）Ｖ：０．５％以下（０％を含まない）、Ｔｉ：０．１％以下（０％を含まない）及びＮｂ：０．１％以下（０%を含まない）よりなる群から選ばれる少なくとも１種 
    （ｄ）Ｃａ：０．０８％以下（０％を含まない）及びＺｒ：０．０８％以下（０％を含まない） よりなる群から選択される少なくとも１種
    （ｅ）Ｓｂ：０．０２％以下（０％を含まない）
    （ｆ）ＲＥＭ：０．０５％以下（０％を含まない）、Ｍｇ：０．０２％以下（０％を含まない）、Ｌｉ：０．０２％以下（０％を含まない）、Ｐｂ：０．５％以下（０％を含まない）及びＢｉ：０．５％以下（０％を含まない）よりなる群から選ばれる少なくとも１種     The hot rolled steel sheet according to claim 1 or 2, wherein the component composition further comprises at least one of the following (a) to (f).
    (A) A group consisting of Cr: 3.0% or less (not including 0%), Mo: 1.0% or less (not including 0%), and Ni: 3.0% or less (not including 0%) (B) at least one selected from the group consisting of Cu: 2.0% or less (not including 0%) and Co: 5% or less (not including 0%) (c) V: selected from the group consisting of 0.5% or less (not including 0%), Ti: 0.1% or less (not including 0%), and Nb: 0.1% or less (not including 0%) At least one
    (D) At least one selected from the group consisting of Ca: 0.08% or less (excluding 0%) and Zr: 0.08% or less (not including 0%) (e) Sb: 0.02 % Or less (excluding 0%)
    (F) REM: 0.05% or less (not including 0%), Mg: 0.02% or less (not including 0%), Li: 0.02% or less (not including 0%), Pb: 0.5% or less (not including 0%) and Bi: at least one selected from the group consisting of 0.5% or less (not including 0%)
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