WO2014030327A1 - 冷間鍛造用丸鋼材 - Google Patents

冷間鍛造用丸鋼材 Download PDF

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WO2014030327A1
WO2014030327A1 PCT/JP2013/004884 JP2013004884W WO2014030327A1 WO 2014030327 A1 WO2014030327 A1 WO 2014030327A1 JP 2013004884 W JP2013004884 W JP 2013004884W WO 2014030327 A1 WO2014030327 A1 WO 2014030327A1
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steel material
less
microstructure
round steel
pearlite
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PCT/JP2013/004884
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English (en)
French (fr)
Japanese (ja)
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江頭　誠
真志 東田
松本　斉
根石　豊
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新日鐵住金株式会社
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Priority to KR1020167025121A priority Critical patent/KR20160111014A/ko
Priority to CN201380042892.5A priority patent/CN104540974B/zh
Priority to KR1020157004118A priority patent/KR101939435B1/ko
Priority to JP2014531495A priority patent/JP5811282B2/ja
Publication of WO2014030327A1 publication Critical patent/WO2014030327A1/ja

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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
    • C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
    • B21J5/00—Methods for forging, hammering, or pressing; Special equipment or accessories therefor
    • 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
    • C21D1/26—Methods of annealing
    • C21D1/32—Soft annealing, e.g. spheroidising
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • 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
    • C21D2211/003—Cementite
    • 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
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • 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
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys

Definitions

  • the present invention relates to a round steel material, and more particularly to a round steel material for cold forging.
  • Structural steel is a material for machine structural parts such as automobile parts, industrial machine parts and construction machine parts. Carbon steel for machine structure and alloy steel for machine structure are used for the structural steel.
  • the spheroidization rate of carbide is usually obtained by subjecting hot-rolled steel to soft annealing (hereinafter referred to as spheroidizing annealing). To increase. Thereby, the hardness of steel materials falls and high cold forgeability is obtained. However, even steel materials that have been subjected to spheroidizing annealing may crack during cold forging.
  • Patent Document 1 Japanese Patent Application Laid-Open No. 2001-240940
  • Patent Document 2 Japanese Patent Application Laid-Open No. 2001-11575
  • Patent Document 3 Japanese Patent Application Laid-Open No. 2011-214130 are steel materials for cold forging having improved cold forgeability after spheroidizing annealing. This is proposed in a gazette (Patent Document 3).
  • the chemical composition of the bar wire for cold forging disclosed in Patent Document 1 is mass%, C: 0.1 to 0.6%, Si: 0.01 to 0.5%, Mn: 0.2 to 1.7%, S: 0.01 to 0.15%, Al: 0.015 to 0.05%, N: 0.003 to 0.025%, and Ni: 3. 5% or less, Cr: 2% or less, Mo: 1% or less, Nb: 0.005 to 0.1%, V: 0.03 to 0.3%, Te: 0.02% or less, Ca: 0.0.
  • the area ratio of ferrite in the region from the surface to the depth of the bar wire radius ⁇ 0.15 is 10% or less, and the balance is substantially one or two of martensite, bainite and pearlite. It consists of the above. Further, the average hardness in the region where the depth is from the rod wire radius x 0.5 to the center is softer than 20 HV compared to the average hardness of the surface layer (region from the surface to the depth of the rod wire radius x 0.15).
  • the chemical composition of the steel bar and the steel wire for machine structure disclosed in Patent Document 2 is mass%, C: 0.1 to 0.5%, Si: 0.01 to 0.15%, Mn: 0.2 -1.7%, Al: 0.0005-0.05%, Ti: 0.005-0.07%, B: 0.0003-0.007%, N: 0.002-0.02% Containing, if necessary, 0.003 to 0.15% S, and / or Cr that is 0.8% or less and the total amount with Mn is 0.3 to 1.3%, P: 0.02% or less, O: 0.003% or less, and the balance consists of Fe and inevitable impurities.
  • the microstructure of the steel bar and steel wire is composed of ferrite and spherical carbide, the ferrite grain size is 8 or more, and the number of spherical carbide per 1 mm 2 of unit area is 1.5 ⁇ 10 5 depending on the amount of C. 6 pieces ⁇ C% or less.
  • the microstructure of the rolled steel material is composed of ferrite, lamellar pearlite, and spherical cementite, and the average crystal grain size of ferrite is 10 ⁇ m or less, and the area ratio of the lamellar pearlite in the lamella pearlite with a lamellar spacing of 200 nm or less is 20 to 50%, and the number of spherical cementite is 4 ⁇ 10 5 pieces / mm 2 or more.
  • the surface layer of the steel material after hot rolling is made into a uniform fine structure such as a structure mainly composed of tempered martensite or a structure mainly composed of bainite. More specifically, the steel layer is rapidly cooled to a temperature range greatly below the Ms point so that the steel surface layer region has a structure mainly composed of tempered martensite, or cooling and recuperation are repeated a plurality of times to form the surface layer region.
  • the organization is mainly bainite. In this case, since volume change due to transformation occurs in the steel material, when severe dimensional accuracy and straightness are required, drawing may be necessary before spheroidizing annealing.
  • Patent Document 2 a steel material having a surface temperature of A r3 point to A r3 point + 150 ° C. is rolled.
  • Patent Document 2 it is described that when a steel material having a surface temperature of less than Ar3 is rolled, when rolling in a so-called two-phase region is performed, fine ferrite and pearlite cannot be obtained, which is not preferable.
  • fine ferrite may not be obtained, and the percentage of pearlite in the steel may increase. Therefore, the cold forgeability of the steel material after spheroidizing annealing may be low.
  • the rolled steel material disclosed in Patent Document 3 is suitable for use as a material for parts such as rack bars that require bending strength and impact properties after induction hardening.
  • the ratio of the lamellar pearlite to the entire microstructure of the lamellar pearlite having a lamellar interval of 200 nm or less is as large as 20 to 50%. For this reason, even if the rolled steel material is annealed into a spheroidizing shape, the rolled steel material is not necessarily sufficiently softened, and the excellent cold forgeability required for the steel material for cold forging may not be obtained.
  • An object of the present invention is to provide a cold forging round steel material excellent in cold forgeability after spheroidizing annealing.
  • the round steel for cold forging according to the present embodiment is in mass%, C: 0.15 to 0.60%, Si: 0.01 to 0.5%, Mn: 0.1 to 2.0%, P : 0.035% or less, S: 0.050% or less, Al: 0.050% or less, Cr: 0.02 to 0.5%, N: 0.003 to 0.030%, Cu: 0 to 0 0.5%, Ni: 0 to 0.3%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.0035%, Nb: 0 to 0.050%, and , Ti: 0 to 0.2%, with the balance having a chemical composition consisting of Fe and impurities.
  • the microstructure of the round steel for cold forging is composed of ferrite, pearlite and spherical cementite, the average crystal grain size of ferrite is 10 ⁇ m or less, and the area ratio of pearlite having a lamellar spacing of 200 nm or less in the microstructure is 20%. Is less than. Further, in the microstructure of the cold forging round steel material in the region from the surface to the radius x 0.15 depth, pearlite having an average crystal grain size of ferrite of 5 ⁇ m or less and a lamellar spacing of 200 nm or less is a micro structure in the above region. The area ratio in the tissue is less than 10%, and the number of spherical cementite is 1.0 ⁇ 10 5 pieces / mm 2 or more.
  • the round steel material for cold forging according to this embodiment is excellent in cold forgeability after spheroidizing annealing.
  • FIG. 1 is a schematic diagram of a pearlite colony.
  • FIG. 2A is a plan view of a test piece used in the cold forgeability test of the example.
  • FIG. 2B is a front view of the test piece shown in FIG. 2A.
  • the present inventors conducted various studies in order to solve the above problems. As a result, the present inventors have found the following items (A) to (C).
  • the round steel material for cold forging according to the present embodiment completed based on the findings of (A) to (C) above is C: 0.15 to 0.60%, Si: 0.01 to 0% by mass. 0.5%, Mn: 0.1 to 2.0%, P: 0.035% or less, S: 0.050% or less, Al: 0.050% or less, Cr: 0.02 to 0.5%, N: 0.003 to 0.030%, Cu: 0 to 0.5%, Ni: 0 to 0.3%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 -0.0035%, Nb: 0-0.050%, and Ti: 0-0.2%, with the balance having a chemical composition consisting of Fe and impurities.
  • the microstructure of the round steel for cold forging is composed of ferrite, pearlite and spherical cementite, the average crystal grain size of ferrite is 10 ⁇ m or less, and the area ratio of pearlite having a lamellar spacing of 200 nm or less in the microstructure is 20%. Is less than.
  • the pearlite in the microstructure in the region from the surface to the radius x 0.15 depth, the pearlite having an average crystal grain size of ferrite of 5 ⁇ m or less and a lamellar spacing of 200 nm or less
  • the area ratio in the microstructure is less than 10%, and the number of spherical cementite is 1.0 ⁇ 10 5 pieces / mm 2 or more.
  • the above round steel material for cold forging is Cu: 0.05-0.5%, Ni: 0.05-0.3%, Mo: 0.05-0.3%, V: 0.05-0. 1% or two or more selected from the group consisting of 3% and B: 0.0005 to 0.0035% may be contained.
  • the cold forging round steel material may contain one or two selected from the group consisting of Nb: 0.005 to 0.050% and Ti: 0.005 to 0.2%. .
  • the chemical composition of the round steel material for cold forging according to the present embodiment contains the following elements.
  • C 0.15-0.60% Carbon (C) increases the strength of the steel material. If the C content is too low, the effect cannot be obtained. On the other hand, if the C content is too high, the area ratio of fine pearlite in the microstructure increases, and the cold forgeability after spheroidizing annealing decreases. Therefore, the C content is 0.15 to 0.60%.
  • the minimum with preferable content of C is 0.20%, More preferably, it is 0.30%, More preferably, it is 0.35%.
  • the upper limit with preferable C content is 0.58%, More preferably, it is 0.55%, More preferably, it is 0.53%.
  • Mn 0.1 to 2.0%
  • Manganese (Mn) increases the strength of the final product (machine structural component) manufactured from the cold forged round steel material. If the Mn content is too low, the strength of the final product will be insufficient. On the other hand, if the Mn content is too high, the hardness of the steel material after spheroidizing annealing will not be sufficiently low. Therefore, the Mn content is 0.1 to 2.0%.
  • the minimum with preferable Mn content is 0.2%, More preferably, it is 0.3%.
  • the upper limit with preferable Mn content is 1.8%, More preferably, it is 1.6%, More preferably, it is 1.4%.
  • Phosphorus (P) is an impurity. P tends to segregate in steel and causes local ductility reduction. Therefore, a lower P content is preferable.
  • the P content is 0.035% or less.
  • P content is preferably 0.030% or less, more preferably 0.025% or less.
  • S 0.050% or less Sulfur (S) is inevitably contained in steel.
  • S sulfur
  • the S content is 0.050% or less.
  • the preferable S content is 0.045% or less.
  • the preferable S content is 0.015% or more.
  • Al 0.050% or less Aluminum (Al) is inevitably contained in steel. Al deoxidizes steel. However, if the Al content is too high, coarse inclusions are generated in the steel, and cracks during cold forging are likely to occur. Therefore, the Al content is 0.050% or less. The preferable Al content is 0.045% or less. When enhancing the deoxidation effect, the preferable Al content is 0.015% or more. In this specification, Al content means content of acid-soluble Al (sol.Al).
  • Chromium (Cr) stabilizes spherical cementite. If the Cr content is too low, the effect cannot be obtained. On the other hand, if the Cr content is too high, the hardness of the steel material after spheroidizing annealing will not be sufficiently low. Therefore, the Cr content is 0.02 to 0.5%.
  • the minimum with preferable Cr content is 0.03%, More preferably, it is 0.05%, More preferably, it is 0.07%.
  • the upper limit with preferable Cr content is 0.45%, More preferably, it is 0.40%, More preferably, it is 0.35%.
  • N 0.003 to 0.030% Nitrogen (N) produces nitrides and refines the crystal grains. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, the above effect is saturated and the manufacturing cost is also increased. Therefore, the N content is 0.003 to 0.030%.
  • the minimum with preferable N content is 0.004%, More preferably, it is 0.005%.
  • the upper limit with preferable N content is 0.022%, More preferably, it is 0.020%, More preferably, it is 0.018%.
  • the round steel material for cold forging of this embodiment contains B which will be described later, if B is combined with N, B cannot exhibit the effect of improving the hardenability of the steel material. In this case, it is necessary to contain a large amount of Ti. Therefore, when it contains B, the one where N content is low is preferable.
  • the upper limit with preferable N content in this case is 0.010%, More preferably, it is 0.008%.
  • the remainder of the chemical composition of the cold forged round steel material of the present embodiment is composed of Fe and impurities.
  • an impurity means the thing mixed from the ore as a raw material, a scrap, or a manufacturing environment, etc. when manufacturing steel materials industrially.
  • the round steel material for cold forging of this embodiment may further contain one or more selected from the group consisting of Cu, Ni, Mo, V, and B instead of a part of Fe. All of these elements increase the strength of machine structural parts manufactured from cold forged round steel.
  • Cu 0 to 0.5% Copper (Cu) is an optional element and may not be contained. Cu increases the strength of machine structural parts by solid solution strengthening. However, if the Cu content is too high, the hot workability decreases. Therefore, the Cu content is 0 to 0.5%.
  • the minimum with preferable Cu content for acquiring the said effect more effectively is 0.05%, More preferably, it is 0.10%.
  • the upper limit with preferable Cu content is 0.4%, More preferably, it is 0.3%.
  • Nickel (Ni) is an optional element and may not be contained. Ni increases the strength of machine structural parts by solid solution strengthening. However, if the Ni content is too high, the economy is impaired. Therefore, the Ni content is 0 to 0.3%.
  • the minimum with preferable Ni content for acquiring the said effect more effectively is 0.05%, More preferably, it is 0.10%.
  • the upper limit with preferable Ni content is 0.25%, More preferably, it is 0.2%.
  • Mo 0 to 0.3%
  • Molybdenum (Mo) is an optional element and may not be contained. Mo increases the strength of machine structural parts by solid solution strengthening. However, if the Mo content is too high, the effect is saturated and the economy is impaired. Therefore, the Mo content is 0 to 0.3%.
  • the minimum with preferable Mo content for acquiring the said effect more effectively is 0.05%, More preferably, it is 0.1%.
  • the upper limit with preferable Mo content is 0.25%, More preferably, it is 0.20%.
  • V 0 to 0.3%
  • Vanadium (V) is an optional element and may not be contained. V increases the strength of mechanical structural parts by precipitation strengthening. However, if the V content is too high, the hardness of the steel material becomes too high and the cold forgeability decreases. Therefore, the V content is 0 to 0.3%.
  • the minimum with preferable V content for acquiring the said effect more effectively is 0.05%, More preferably, it is 0.1%.
  • the upper limit with preferable V content is 0.25%, More preferably, it is 0.20%.
  • B 0 to 0.0035%
  • Boron (B) is an optional element and may not be contained.
  • B increases the hardenability of the steel material and increases the strength of the final product (machine structural component) manufactured from the steel material.
  • the B content is 0 to 0.0035%.
  • the minimum with preferable B content for improving the said effect is 0.0005%, More preferably, it is 0.0010%.
  • the upper limit with preferable B content is 0.0030%.
  • the round steel material for cold forging of this embodiment may further contain one or two selected from the group consisting of Nb and Ti instead of a part of Fe. All of these elements form carbonitrides to refine crystal grains.
  • Niobium (Nb) is an optional element and may not be contained. Nb forms carbonitride and refines crystal grains. Due to the refinement of crystal grains, the cold forgeability of steel is increased. However, if the Nb content is too high, the carbonitride becomes coarse. Coarse carbonitride becomes a starting point of cracking during cold forging. Therefore, the Nb content is 0 to 0.050.
  • the minimum with preferable Nb content for improving the said effect more is 0.005%, More preferably, it is 0.010%.
  • the upper limit with preferable Nb content is 0.035%, More preferably, it is 0.030%.
  • Titanium (Ti) is an optional element and may not be contained. Ti forms carbonitrides and refines crystal grains. When the round steel material for cold forging of this embodiment contains B, Ti couple
  • Ti suppresses the binding of B to N. Therefore, when B is contained, Ti is preferably also contained.
  • the microstructure of the round steel material for cold forging of the present embodiment having the above-described chemical composition is composed of ferrite, pearlite, and spherical cementite.
  • the average crystal grain size of ferrite is 10 ⁇ m or less
  • the area ratio of pearlite (fine pearlite) having a lamellar spacing of 200 nm or less in the pearlite is less than 20%.
  • the average crystal grain size of ferrite is 5 ⁇ m or less, and fine pearlite occupies the microstructure in the surface region.
  • the area ratio is less than 10%.
  • the number of spherical cementite in the microstructure of the surface layer region is 1.0 ⁇ 10 5 pieces / mm 2 or more.
  • the round steel material for cold forging of this embodiment has the above microstructure. Therefore, in the cold forging performed after spheroidizing annealing, generation
  • (1) the microstructure in the entire steel material and (2) the microstructure in the surface layer region of the steel material will be described in detail.
  • the microstructure of the steel material is a mixed structure made of ferrite, pearlite, and spherical cementites. Therefore, the hardness of the microstructure is low compared to martensite and bainite.
  • the average crystal grain size of ferrite in the microstructure is 10 ⁇ m or less. Therefore, the diffusion distance of C is short, and cementite tends to be spheroidized during spheroidizing annealing.
  • the lamella spacing is determined at any three locations.
  • line segment L ⁇ b> 1 is drawn in a direction perpendicular to the extending direction of cementite 2 at measurement point P ⁇ b> 1.
  • both end points P L1 and P L1 of the line segment L1 are arranged at the center of the width of each of the pair of cementite 2 closest to the boundary 10 of the pearlite colony 1 at the measurement location P1.
  • the length of the line segment L1 and the number N of cementite crossing the line segment L1 are obtained, and the lamella interval (nm) at the measurement point P1 is obtained by the following equation.
  • Lamella interval at measurement point P1 L1 / (N-1)
  • the lamella spacing means the distance between adjacent cementites. At the measurement location P1, the number N of cementite that intersects the line segment L1 is “4”.
  • a line segment L2 is drawn at the measurement point P2.
  • both end points of the line segment L2 are respectively arranged at the center of the width of each of the pair of cementite 2 closest to the boundary 10 of the pearlite colony 1 at the measurement location P2.
  • the cementite number N at this time is “5”. Based on the above formula, the lamella interval at the measurement point P2 is obtained. Similarly, the lamella interval of the measurement location P3 is also obtained.
  • the average of the lamella intervals determined at the measurement points P1 to P3 is defined as the “lamellar interval” (nm) of pearlite colony 1.
  • a pearlite colony having a lamella spacing of less than 200 ⁇ m is defined as “fine pearlite”.
  • the average crystal grain size of ferrite, the area ratio of fine pearlite, and the number of spherical cementite in the microstructure of the surface layer region are defined as follows: To do.
  • the average crystal grain size of ferrite in the microstructure of the surface layer region exceeds 5 ⁇ m, the cold forgeability in the surface layer region is lowered, and cracks may occur during cold forging. Therefore, the average crystal grain size of ferrite in the microstructure of the surface layer region is 5 ⁇ m or less.
  • the number of spherical cementite in the microstructure of the surface region is 1.0 ⁇ 10 5 pieces / mm 2 or more.
  • spherical cementite in the surface layer region becomes a nucleus, and spherical cementite is likely to be generated and grow. Therefore, the spheroidization rate of the surface layer region after spheroidizing annealing is further increased.
  • the identification of the phase of the microstructure, the average crystal grain size of ferrite, the area ratio of fine pearlite, and the number of spherical cementite can be obtained by the following methods.
  • the cross section of the round steel material (cross section perpendicular to the axial direction of the round steel material) is mirror-polished to obtain an observation surface.
  • the mirror-polished observation surface is corroded with 3% nitric alcohol (nitral liquid) to reveal a microstructure.
  • the revealed microstructure is observed with a scanning electron microscope (SEM).
  • the observation surface of the round steel material is mirror-polished. After polishing, the observation surface is corroded with picric alcohol (picral liquid). Using the SEM of 5,000 times, a microscopic photographed image is generated for 15 fields of view in the same manner as the above-described phase identification.
  • the major axis L and the minor axis W of each cementite in each visual field are measured by image processing using the captured image of each visual field.
  • cementite having an L / W of 2.0 or less is defined as spherical cementite.
  • the average of the crystal grain size of ferrite in a total of six fields of view at the positions Q1 and Q2 is determined and defined as the average crystal grain size ( ⁇ m) of ferrite in the surface layer region.
  • the area ratio of fine pearlite is measured by the following method.
  • a pearlite colony is identified (sectioned) in each of the 15 fields of view (25 ⁇ m ⁇ 20 ⁇ m).
  • the pearlite colony is identified by image processing.
  • the lamella spacing (nm) is determined by the method described above.
  • the pearlite colony whose lamella interval is 200 nm or less is specified as "fine pearlite”.
  • the area Af ( ⁇ m 2 ) of the specified fine pearlite is obtained, and the fine pearlite area ratio in each field of view is obtained based on the formula (1).
  • Fine pearlite area ratio (%) Af / field of view ⁇ 100 (1)
  • the area Af can be obtained by using well-known image processing by marking the boundary 10 and the inside of the pearlite colony 1 in FIG.
  • the average of the fine pearlite area ratio of each visual field determined based on the formula (1) is defined as the area ratio (%) of the fine pearlite in the microstructure.
  • the average of the fine pearlite area ratios (total 6 fields of view) at the positions Q1 and Q2 obtained based on the formula (1) is defined as the area ratio (%) in the microstructure in the surface area of the fine pearlite.
  • Number of spherical cementite The number of spherical cementite (L / W is 2.0 or less cementite) at positions Q1 and Q2 (total 6 fields of view) is counted. Based on the total number of spherical cementites in 6 fields of view, the number of spherical cementites per 1 mm 2 area (pieces / mm 2 ) is calculated. The number obtained is defined as the number of spherical cementites (pieces / mm 2 ) in the microstructure in the surface layer region.
  • the preferred area ratio of fine pearlite in the microstructure of the entire round steel material is less than 15%.
  • a preferable area ratio of fine pearlite in the microstructure of the surface layer region to the microstructure of the surface layer region is 8% or less. In order to improve cold forgeability, these area ratios are preferably as small as possible, and may be 0%.
  • a preferable number of spherical cementite in the microstructure of the surface layer region is 2.0 ⁇ 10 5 pieces / mm 2 or more.
  • the upper limit is substantially 1.0 ⁇ 10 7 pieces / mm 2 .
  • a material (for example, billet) having the above chemical composition is heated in a heating furnace.
  • the heated raw material is extracted from the heating furnace, and hot rolled using a continuous rolling mill to produce a cold rolled forging steel.
  • the continuous rolling mill includes a plurality of arranged rolling mills (stands).
  • the cold forging round steel material is manufactured based on the all-continuous rolling method.
  • the all-continuous rolling method means that the material extracted from the heating furnace is continuously rolled without stopping halfway until it leaves the final stand of the continuous rolling mill and becomes a round steel material for cold forging. Means the method.
  • manufacturing conditions in the all continuous rolling method will be described.
  • the material is heated so that the heating temperature of the material before hot rolling (that is, the surface temperature of the material) is 810 ° C. or lower. In this case, rolling in a two-phase region is performed. By carrying out rolling in the two-phase region, the ferrite grains in the round steel material after rolling can be made fine. On the other hand, if the heating temperature is too low, the load on the continuous rolling mill becomes excessive. Therefore, the minimum of the heating temperature of the preferable raw material before hot rolling is 670 degreeC.
  • Total area reduction in all continuous rolling method The total area reduction in the all-continuous rolling method is made higher than 30%.
  • the total area reduction rate (%) is defined by equation (2).
  • Total area reduction ratio (cross-sectional area of material-cross-sectional area of round steel) / cross-sectional area of material x 100 (2)
  • the cross-sectional area (mm 2 ) of the material means the area of a cross section perpendicular to the central axis of the material.
  • the cross-sectional area (mm 2 ) of the round steel material means an area of a cross section perpendicular to the central axis of the round steel material manufactured by the all continuous rolling method.
  • the temperature of the round steel material immediately after rolling in the two-phase region that is, the surface temperature of the round steel material on the final rolling mill exit side is set to Ac 3 points or more.
  • the processed structure is once reverse transformed.
  • the surface temperature of the material rises due to processing heat generation.
  • the surface temperature of the round steel material on the outlet side of the final rolling mill is set to Ac3 point or higher.
  • the structure of the round steel material once becomes an austenite single phase. Ferrite refined by dynamic recrystallization becomes fine austenite by reverse transformation.
  • the surface temperature of the round steel material is set to Ar 3 to 600 ° C. within 5 seconds by a water cooling device disposed on the exit side of the final rolling mill.
  • a water cooling device disposed on the exit side of the final rolling mill.
  • austenite produced by reverse transformation becomes coarse.
  • austenite becomes coarse fine ferrite cannot be obtained even if the surface temperature of the round steel material is made Ar3 or lower after that.
  • the cooling time is not particularly limited as long as it is within 5 seconds.
  • the surface temperature of the round steel material may be set to Ar 3 to 600 ° C. in 3 seconds. After the surface temperature of the round steel material is set at Ar 3 to 600 ° C., the cooling by the water cooling device is stopped.
  • the surface temperature of the steel material is cooled to a temperature not exceeding Ar 3 point and not lower than 600 ° C. within 5 seconds after the end of rolling by the all-continuous rolling method, and then water cooling by the water cooling device is stopped.
  • a method that does not have such a high cooling rate that martensite and bainite are generated, for example, cooling may be performed.
  • the round steel material for cold forging having the above-mentioned microstructure is manufactured.
  • the manufactured round steel for cold forging is subjected to spheroidizing annealing and then cold forged into a final product (structural machine part or the like). Since the round steel material for cold forging of this embodiment is provided with the above-mentioned chemical composition and microstructure, it is excellent in cold forgeability after spheroidizing annealing.
  • a square billet (a cross section of 140 mm ⁇ 140 mm and a length of 10 m) made of steels A to H having the chemical composition shown in Table 1 was prepared.
  • the chemical compositions of the steels A to E, G, and H were within the range of the chemical composition of the cold forging round steel material of the present embodiment.
  • the chemical composition of the steel F the C content deviated from the range of the C content defined in the present embodiment.
  • Table 1 shows the Ar 3 point and Ac 3 point of each steel.
  • the surface temperature (° C.) of the square billet (material) extracted from the heating furnace (before continuous rolling) is described.
  • post-rolling temperature the surface temperature (° C.) of the round steel material on the outlet side of the final rolling mill (stand) among the continuous rolling mills is described.
  • the “post-rolling temperature” was obtained by measuring with a radiation thermometer arranged on the exit side of the final rolling mill.
  • temperature after cooling the surface temperature (° C.) of the round steel material 5 seconds after leaving the final rolling mill is described.
  • the “temperature after cooling” was obtained by measuring the surface temperature of the round steel material with a radiation thermometer when 5 seconds passed.
  • the “total area reduction rate” from the square billet (material) calculated by the equation (2) was 96%.
  • the water cooling conditions between the rolling mills (stands) in the continuous rolling mill are adjusted so that the surface temperature of the round steel material on the exit side of the final rolling mill becomes Ac 3 or higher. Adjusted. Furthermore, after the rolling by the final rolling mill is completed, the cooling rate is controlled by the amount of water using a water cooling device, and cooling is performed so that the surface temperature of the steel material is 3 points or less of Ar and less than 600 ° C. within 5 seconds. Then, cooling by the water cooling device was stopped. After the cooling by the water cooling device was stopped, the round steel material was allowed to cool in the atmosphere.
  • test numbers 11 and 12 water cooling conditions between the stands were adjusted, and water cooling was performed after rolling. However, the post-rolling temperature of test number 10 was less than Ac3 point. Test No. 11 had a temperature after water cooling of less than 600 ° C.
  • test piece having a length of 20 mm was cut out from each round steel material after spheroidizing annealing.
  • the surface corresponding to the longitudinal section of the round steel material was embedded in a resin so as to be an observation surface, and mirror-polished.
  • the sample was corroded with picric alcohol (picral solution), and a microscopic photographed image was generated for 15 fields of view using the SEM of 5000 times, similar to the above phase identification.
  • picric alcohol picral solution
  • the major axis L and the minor axis W of each cementite were individually measured using the captured images. Then, the ratio of the number of cementite (ie, spherical cementite) having L / W of 2.0 or less to the number of cementite in the photographed image (each field of view described later) was determined and used as the spheroidization rate (%).
  • the observed position is a position Q1 having a depth of 1 mm (radius ⁇ 0.067 depth) from the surface, a position Q2 having a depth of 2.25 mm (radius ⁇ 0.15 depth) from the surface, and from the surface.
  • a total of 15 visual fields were observed with 3 visual fields per site. The area of each visual field was 25 ⁇ m ⁇ 20 ⁇ m.
  • the average value of the spheroidizing ratios in the six visual fields at positions Q1 and Q2 among the spheroidizing ratios obtained in each field of view was defined as the surface layer spheroidizing ratio (%) after spheroidizing annealing.
  • the average value of the spheroidizing ratio in nine visual fields at positions Q3 to Q5 was defined as the internal spheroidizing ratio (%) after spheroidizing annealing.
  • FIGS. 2A and 2B A test piece shown in FIGS. 2A and 2B was produced from each round steel material after the spheroidizing annealing treatment.
  • FIG. 2A is a plan view of the test piece
  • FIG. 2B is a front view of the test piece.
  • the diameter D1 of the test piece was 29 mm
  • the length L4 was 44 mm.
  • a cutout portion extending in the axial direction was formed on the outer peripheral surface of the test piece.
  • the notch angle A1 of the notch was 30 °
  • the corner radius R1 of the groove bottom portion of the notch was 0.15 mm.
  • the depth D2 of the notch was 0.8 mm.
  • the compression test was performed in the cold (room temperature) using a test piece and a press.
  • the specimen was first compressed to 15% in the axial direction. Thereafter, the test piece was unloaded each time 1.5 to 2.5% of axial compression was applied to the test piece, and the test piece was observed for cracking. The compression, unloading and observation were repeated until cracking occurred.
  • a fine crack length 0.5-1.0 mm
  • Five test pieces were prepared for each test number, and the above compression test was performed on the five test pieces. The average value of the compression ratios of the five test pieces when cracking occurred was defined as “limit compression ratio”. When the critical compression ratio exceeded 50%, it was evaluated that the cold forgeability was excellent.
  • Table 2 shows the test results.
  • “F” in the “Phase” column of the “Total microstructure” column indicates ferrite
  • “LP” indicates lamellar pearlite
  • “SC” indicates spherical cementite.
  • crystal grain size the ferrite average crystal grain size ( ⁇ m) in the microstructure of the entire round steel material in each test number is described.
  • fine LP ratio the area ratio (%) of the fine pearlite in the entire microstructure is described.
  • the “crystal grain size” column in the “microstructure in the surface region” column lists the ferrite average crystal grain size ( ⁇ m) in the surface layer region for each test number.
  • the “fine LP ratio” column an area ratio (%) of the surface area of the fine pearlite in the microstructure is described.
  • SC number the number of spherical cementite (pieces / mm 2 ) in the microstructure of the surface layer region is described.
  • the chemical compositions of the steel materials of Test Nos. 1 to 7 were appropriate, and the production conditions (total area reduction rate, heating temperature, temperature after rolling, temperature after cooling) were also appropriate. Therefore, the microstructures of the round steel materials of test numbers 1 to 7 are composed of ferrite, pearlite and spherical cementite, the average grain size of ferrite in the microstructure of the whole round steel material is 10 ⁇ m or less, and the fine LP rate is also less than 20%. Met.
  • the average crystal grain size of ferrite in the microstructure of the surface region of test numbers 1 to 7 is 5 ⁇ m or less, the fine LP rate is less than 10%, and the number of spherical cementite is 1.0 ⁇ 10 5 / mm 2 or more. Therefore, the surface spheroidization rate after spheroidizing annealing was as high as 80% or more, and the internal spheroidization rate was as high as 70% or more. As a result, the limit compression ratio of the round steel materials of test numbers 1 to 7 exceeded 50%, and excellent cold forgeability was exhibited.
  • test number 8 the C content of the steel material was too high. Therefore, the fine LP rate in the microstructure of the surface layer region was 10% or more. As a result, the critical compression ratio was 50% or less.
  • test number 10 Although the chemical composition of the steel material was appropriate, the temperature after cooling was too high. Therefore, spherical cementite did not exist in the microstructure of the round steel material, and the ferrite was also coarse. Therefore, the critical compression rate was 50% or less.
  • test number 11 Although the chemical composition of the steel material was appropriate, the temperature after rolling was too low. Therefore, the fine LP rate in the whole round steel material and the microstructure of the surface layer region was too high. Therefore, the surface spheroidization rate and the internal spheroidization rate after spheroidizing annealing were low, and the critical compression rate was 50% or less.
  • the round steel material for cold forging of this embodiment has a high spheroidization rate and is excellent in cold forgeability after spheroidizing annealing. Therefore, it can be widely applied to applications requiring excellent cold forgeability.
  • the round steel material for cold forging of the present embodiment is used as a material for machine structural parts such as automobile parts, industrial machine parts, construction machine parts, etc., which have been manufactured in the hot forging process and the cutting process so far. be able to. Particularly when used in such applications, the cold forged round steel material of the present embodiment can contribute to the near net shaping of parts.

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EP3279355A4 (en) * 2015-03-31 2018-09-05 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Steel wire for mechanical structural parts
KR101934176B1 (ko) 2014-06-13 2018-12-31 신닛테츠스미킨 카부시키카이샤 냉간 단조용 강재
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