US20200407815A1 - Steel - Google Patents

Steel Download PDF

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US20200407815A1
US20200407815A1 US16/976,379 US201916976379A US2020407815A1 US 20200407815 A1 US20200407815 A1 US 20200407815A1 US 201916976379 A US201916976379 A US 201916976379A US 2020407815 A1 US2020407815 A1 US 2020407815A1
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Prior art keywords
steel
content
ferrite
bainite
fraction
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Tatsuya Koyama
Yutaka Neishi
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Nippon Steel Corp
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Nippon Steel Corp
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Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KOYAMA, TATSUYA, NEISHI, YUTAKA
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    • 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
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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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • 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
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    • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
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    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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    • 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
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    • 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
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    • 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
    • C23C8/28—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 more than one element being applied in one step
    • C23C8/30—Carbo-nitriding
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    • 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
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2261/00—Machining or cutting being involved
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    • 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 steel.
  • gears for use in automobiles, construction machinery, industrial machinery, and the like are generally used after receiving carburizing hardening after machining
  • quietness during operation has been more strongly demanded than before, and thus an increase in the dimensional accuracy of gears, especially the dimensional accuracy of teeth, has been demanded.
  • the dimensional accuracy of gear teeth is attributable to deformation associated with heat treatment during carburizing hardening (hereinafter referred to as thermal strain). Since this thermal strain varies and is not uniform for each gear tooth, a symmetrical shape is lost in the same gear, and thus vibrations are created during use, thus losing quietness. Accordingly, demands exist for stabilizing thermal strain on gear teeth so as to provide a symmetrical shape.
  • Patent Document 1 discloses a technology for providing steel having excellent cold forgeability and temper softening resistance. However, Patent Document 1 does not provide a technology for stabilizing thermal strain on gear teeth during carburizing hardening, which is an object of the present invention.
  • Patent Document 2 discloses a technology for providing a hot-rolled steel bar or wire rod composed of a ferrite-pearlite structure, ferrite-pearlite-bainite structure, or ferrite-bainite structure, wherein the standard deviation of ferrite fractions at the time when randomly selected 15 viewing fields of a transverse cross-section are observed and measured with the area per one viewing field being 62500 ⁇ m 2 is 0.10 or less; and when a region from the surface to one-fifth of the radius and a region from the center to one-fifth of the radius in the transverse cross-section are observed, in each of the regions, the amount of Al precipitating as AlN is 0.005% or less, and the density in terms of the number of AlN having a diameter of 100 nm or larger is 5/100 ⁇ m 2 or less.
  • Patent Document 2 uses a pearlite structure to reduce the standard deviation of ferrite fractions. That is to say, according to the technology of Patent Document 2, it is not possible to sufficiently reduce the standard deviation of ferrite fractions while controlling the structures so as not to substantially include pearlite.
  • Patent Document 1 PCT International Publication No. WO 2014/171472
  • Patent Document 2 PCT International Publication No. WO 2011/055651
  • An object of the present invention is to provide steel that stabilizes thermal strain on gear teeth during carburizing hardening.
  • the gist of the present invention is as follows.
  • a steel according to one embodiment of the present invention includes in % by mass:
  • rare earth element 0 to 0.005%
  • structures include ferrite and bainite, an average fraction of the ferrite is in a range of 40 to 70% in terms of area ratio, a total average fraction of the structures other than the ferrite and the bainite is 0% or more and 3% or less on average, and a balance includes bainite; and
  • a standard deviation of a ferrite fraction in the region is 4% or less:
  • R represents a circle equivalent radius of the steel.
  • the steel according to (1) may contain in % by mass one or two or more of:
  • the steel according to (1) or (2) may contain in % by mass one or two or more of:
  • Rare earth element 0.0001 to 0.005%.
  • thermal strain on the teeth of a gear manufactured by carburizing hardening can be stabilized.
  • FIG. 1 is a schematic cross-sectional view of steel for explaining the positions for measuring the average ferrite fraction and the standard deviation of the ferrite fraction.
  • the present inventors have conducted diligent research on a method for stabilizing thermal strain on the teeth of a gear after carburizing hardening. As a result, it was found that the thermal strain is stabilized by increasing the uniformity of structures in a region of steel that becomes teeth after machining. Accordingly, concerning a method for making uniform the structures of a region corresponding to the gear teeth in steel, the inventors further investigated the effect of changing the chemical components of steel and the manufacturing method. As a result, it was found that by configuring the steel components to be in predetermined ranges and then controlling the casting method and the post-rolling cooling rate, the structures of a region corresponding to the gear teeth in steel can be uniform.
  • the cross-sectional area of casting, the casting rate, and the average cooling rate of the surface from the beginning of casting to the correction point are controlled in a combined manner. This makes it possible to homogenize the cast structures of a region in a bloom that eventually becomes gear teeth. Moreover, concerning the control of the post-rolling cooling rate, the cooling rate of the steel surface is controlled. This makes it possible to homogenize the structures of a region in steel that corresponds to the gear teeth.
  • the C content affects the hardness of the non-carburized portion of a gear. In order to ensure a required hardness, the C content is 0.17% or more. On the other hand, when the C content is excessive, the hardness of the non-carburized portion after carburization is high, resulting in poor impact strength, and thus the C content is 0.21% or less.
  • the preferable lower limit of the C content is 0.175%, 0.18%, 0.185%, or 0.19%.
  • the preferable upper limit of the C content is 0.205%, 0.200%, 0.195%, or 0.19%.
  • Si is an element, the amount of which needs to be strictly limited in steel in order to homogenize the structures of a region corresponding to the teeth of machined gear steel.
  • the Si content needs to be within the range of 0.40 to 0.60%.
  • the preferable lower limit of the Si content is 0.42%, 0.45%, 0.48%, or 0.50%.
  • the preferable upper limit of the Si content is 0.58%, 0.55%, 0.53%, or 0.51%.
  • Mn is an element, the amount of which needs to be strictly limited in steel in order to homogenize the structures of a region corresponding to the teeth of machined gear steel.
  • the Mn content needs to be 0.25% or more.
  • the Mn content is 0.50% or less.
  • the preferable lower limit of the Mn content is 0.27%, 0.30%, 0.32%, or 0.35%.
  • the preferable upper limit of the Mn content is 0.48%, 0.45%, 0.42%, or 0.40%.
  • Cr is an element, the amount of which needs to be strictly limited in steel in order to homogenize the structures of a region corresponding to the teeth of machined gear steel.
  • the Cr content needs to be within the range of 1.35 to 1.55%.
  • the preferable lower limit of the Cr content is 1.37%, 1.40%, 1.42%, or 1.45%.
  • the preferable upper limit of the Cr content is 1.53%, 1.50%, 1.49%, or 1.47%.
  • Mo is an element, the amount of which needs to be strictly limited in steel in order to homogenize the structures of a region corresponding to the teeth of machined gear steel.
  • Mo When Mo is contained in steel together with Nb, which will be described below, Mo suppresses pearlite transformation by increasing the hardenability of steel, and also suppresses coarse austenite crystal grains during the heating of steel. This makes it possible to suitably control hardenability, and obtain the desired bainite structure by suppressing martensite transformation.
  • the Mo content is excessive, the amount of ferrite in steel is insufficient, and the amounts of bainite and the like are increased, resulting in poor workability. In order to obtain the above-described effect, the Mo content needs to be within the range of 0.20 to 0.40%.
  • the preferable lower limit of the Mo content is 0.22%, 0.25%, 0.28%, or 0.30%.
  • the preferable upper limit of the Mo content is 0.38%, 0.35%, 0.32%, or 0.30%.
  • S forms MnS in steel, thereby increasing the machinability of steel.
  • a S content comparable to that of commonly used steel for machine structural use is needed.
  • the S content needs to be within the range of 0.010 to 0.05%.
  • the preferable lower limit of the S content is 0.012%, 0.014%, 0.020%, or 0.022%.
  • the preferable upper limit of the S content is 0.035%, 0.030%, 0.028%, or 0.025%.
  • N has a crystal grain refining effect by forming compounds with Al, Ti, V, Cr, and the like, and thus needs to be contained in an amount of 0.005% or more.
  • N exceeds 0.020%, compounds are coarse, and the crystal grain refining effect cannot be obtained.
  • the N content needs to be within the range of 0.005 to 0.020%.
  • the preferable lower limit of the N content is 0.0055%, 0.0060%, 0.007%, or 0.010%.
  • the preferable upper limit of the N content is 0.018%, 0.015%, 0.012%, or 0.010%.
  • Al is an element effective for the deoxidation of steel, and is an element that binds to N to form nitride and refine crystal grains.
  • the preferable lower limit of the Al content is 0.004%, 0.007%, 0.010%, or 0.020%.
  • the preferable upper limit of the Al content is 0.080%, 0.050%, 0.040%, or 0.030%.
  • Nb is an element that produces fine compounds with C and N in steel and provides a crystal grain refining effect. Also, Nb when contained in steel together with Mo exerts the above-described synergistic effect (the effect of suppressing pearlite transformation and martensite transformation). When the Nb content is less than 0.001%, this effect is insufficient. When the Nb content exceeds 0.030%, carbonitride is coarse, and this effect cannot be sufficiently obtained. For the above reason, the Nb content needs to be 0.001 to 0.030%. The preferable lower limit of the Nb content is 0.005%, 0.010%, 0.012%, or 0.015%. The preferable upper limit of the Nb content is 0.028%, 0.025%, 0.022%, or 0.020%.
  • the O content is preferably limited to 0.005% or less.
  • the preferable upper limit of the O content is 0.003%, 0.002%, 0.0015%, or 0.001%. Since a smaller O content is more preferable, the lower limit of the O content is 0%. However, removing O more than necessary results in increased manufacturing costs. Accordingly, the lower limit of the O content may be 0.0001%, 0.0002%, 0.0005%, or 0.0008%.
  • the P content is preferably limited to 0.03% or less.
  • the preferable upper limit of the P content is 0.025%, 0.023%, 0.020%, or 0.015%. Since a smaller P content is more preferable, the lower limit of the P content is 0%. However, removing P more than necessary results in increased manufacturing costs. Accordingly, the substantial lower limit of the P content is usually about 0.004% or more.
  • the lower limit of the P content may be 0.005%, 0.007%, 0.010%, or 0.012%.
  • Steel according to the present embodiment may further contain one or two or more selected from the group consisting of Ni, Cu, Co, W, V, Ti, and B in place of a part of Fe in order to increase hardenability or the crystal grain refining effect.
  • the lower limit when these elements are not contained is 0%.
  • Ni is an element effective for imparting necessary hardenability to steel.
  • the Ni content is preferably 0.01% or more.
  • the Ni content exceeds 3.0%, the amount of residual austenite after hardening is large, resulting in poor hardness.
  • the Ni content is 3.0% or less and more preferably 0.01 to 3.0%.
  • the upper limit of the Ni content is more preferably 2.0% and even more preferably 1.8%.
  • a more preferable lower limit of the Ni content is 0.1% and more preferably 0.3%.
  • the Cu is an element effective for increasing the hardenability of steel.
  • the Cu content is preferably 0.01% or more.
  • the Cu content exceeds 1.0%, hot ductility is impaired. Accordingly, the Cu content is 1.0% or less and more preferably 0.01 to 1.0%.
  • a more preferable lower limit of the Cu content is 0.05% and even more preferably 0.1%.
  • Co is an element effective for increasing the hardenability of steel.
  • the Co content is preferably 0.01% or more.
  • the Co content exceeds 3.0%, the effect is saturated. Accordingly, the Co content is 3.0% or less and more preferably 0.01 to 3.0%.
  • a more preferable lower limit of the Co content is 0.05% and even more preferably 0.1%.
  • W is an element effective for increasing the hardenability of steel.
  • the W content is preferably 0.01% or more.
  • the W content exceeds 1.0%, the effect is saturated. Accordingly, the W content is 1.0% or less and more preferably 0.01 to 1.0%.
  • a more preferable lower limit of the W content is 0.05% and even more preferably 0.1%.
  • V is an element that produces fine compounds with C and N in steel and provides a crystal grain refining effect.
  • the V content is preferably 0.01% or more.
  • the V content exceeds 0.3%, compounds are coarse, and the crystal grain refining effect cannot be obtained. Accordingly, the V content is 0.3% or less and more preferably 0.01 to 0.3%.
  • a more preferable lower limit of the V content is 0.1% and even more preferably 0.15%.
  • Ti is an element that produces fine compounds with C and N in steel and provides a crystal grain refining effect.
  • the Ti content is preferably 0.001% or more.
  • the Ti content exceeds 0.3%, the effect is saturated.
  • the Ti content is 0.3% or less and more preferably 0.001 to 0.3%.
  • a more preferable upper limit of the Ti content is 0.25% and even more preferably 0.2%.
  • B functions to suppress the grain boundary segregation of P.
  • B also has the effect of increasing grain boundary strength and intragranular strength and the effect of increasing hardenability, and these effects increase the fatigue strength of steel.
  • the B content is preferably 0.0001% or more.
  • the B content exceeds 0.005%, the effect is saturated.
  • the B content is 0.005% or less and preferably 0.0001 to 0.005%.
  • a more preferable upper limit of the B content is 0.0045% and even more preferably 0.004%.
  • the chemical composition of steel according to the present embodiment may further contain one or two or more selected from the group consisting of Pb, Bi, Ca, Mg, Zr, Te, and rare earth elements (REMs) in place of a part of Fe.
  • Pb Pb
  • Bi Bi
  • Ca Ca
  • Mg Mg
  • Zr Te
  • REMs rare earth elements
  • Pb is an element that increases machinability by being molten and embrittled during cutting.
  • the Pb content is preferably 0.01% or more.
  • a more preferable lower limit of the Pb content is 0.05% and even more preferably 0.1%.
  • the preferable upper limit of Pb is 0.4% and even more preferably 0.3%.
  • Bi is an element that increases machinability due to finely dispersed sulfide.
  • the Bi content is preferably 0.0001% or more.
  • the Bi content is 0.5% and more preferably 0.0001 to 0.5%.
  • a more preferable lower limit is 0.0001% and even more preferably 0.001%.
  • the preferable upper limit of Bi is 0.4% and even more preferably 0.3%.
  • the Ca is an element effective for the deoxidation of steel and reduces the Al 2 O 3 content in oxide.
  • the Ca content is preferably 0.0001% or more.
  • the Ca content exceeds 0.01%, a large amount of Ca-containing coarse oxide appears and causes a shortened rolling fatigue life.
  • the Ca content needs to be within the range of 0.0001 to 0.01%.
  • the preferable lower limit of the Ca content is 0.0003% and more preferably 0.0005%.
  • the preferable upper limit of the Ca content is 0.008% and more preferably 0.006%.
  • Mg is a deoxidizing element and produces oxide in steel. Moreover, Mg-based oxide formed by Mg likely becomes a nucleus for crystallization and/or precipitation of MnS. Also, the sulfide of Mg makes MnS spherical by becoming a complex sulfide of Mn and Mg. Thus, Mg is an element effective for controlling the dispersion of MnS and improving machinability. In order to obtain this effect, the Mg content is preferably 0.0001% or more. However, when the Mg content exceeds 0.01%, a large amount of MgS is produced, and the machinability of steel decreases.
  • the Mg content needs to be 0.01% or less.
  • the preferable upper limit of the Mg content is 0.008% and more preferably 0.006%.
  • the preferable lower limit of the Mg content is 0.0005% and more preferably 0.001%.
  • Zr is a deoxidizing element and forms oxide. Moreover, Zr-based oxide formed by Zr likely becomes a nucleus for crystallization and/or precipitation of MnS. Thus, Zr is an element effective for controlling the dispersion of MnS and improving machinability.
  • the Zr content is preferably 0.0001% or more. However, when the amount of Zr exceeds 0.05%, the effect is saturated. Thus, in order to obtain the above-described effect by containing Zr, the Zr content is 0.05% or less and more preferably 0.0001 to 0.05%.
  • the preferable upper limit of the Zr content is 0.04% and more preferably 0.03%.
  • the preferable lower limit of the Zr content is 0.0005% and more preferably 0.001%.
  • Te promotes the spheroidization of MnS and thus improves the machinability of steel.
  • the Te content is preferably 0.0001% or more.
  • the Te content is 0.1% or less and more preferably 0.0001 to 0.1%.
  • the preferable upper limit of the Te content is 0.08% and more preferably 0.06%.
  • the preferable lower limit of the Te content is 0.0005% and more preferably 0.001%.
  • Rare earth elements are elements that promote the production of MnS by producing sulfide in steel and this sulfide becoming a precipitation nucleus for MnS, and improve the machinability of steel.
  • the total amount of rare earth elements is preferably 0.0001% or more.
  • the total amount of rare earth elements exceeds 0.005%, sulfide is coarse, reducing the fatigue strength of steel. Accordingly, the total amount of rare earth elements is 0.005% or less and more preferably 0.0001 to 0.005%.
  • the preferable upper limit of the total amount of rare earth elements is 0.004% and more preferably 0.003%.
  • the preferable lower limit of the total amount of rare earth elements is 0.0005% and more preferably 0.001%.
  • the rare earth element as used herein is a collective term referring to 17 elements including 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 in addition to yttrium (Y) and scandium (Sc) in the periodic table.
  • the amount of rare earth elements means the total amount of one or two or more of these elements.
  • Steel according to the present embodiment contains the above-described alloying components, and the balance includes Fe and impurities. Elements other than the above-described alloying components are allowable in steel as impurities from raw materials and manufacturing equipment as long as the amounts thereof do not affect the properties of steel.
  • the region of steel corresponding to the gear teeth is a region including a part from the tooth tip to the tooth root of a gear after forging or cutting, and is a region satisfying 0.7R ⁇ r ⁇ 0.9R in rolled steel, wherein r is the distance from the center of the cross-section of steel that is perpendicular to the length direction, and R is a circle equivalent radius in the cross-section of steel that is perpendicular to the length direction of steel.
  • the uniform structures suitable for improvement of thermal strain are structures including ferrite and bainite, and that the structure fractions are in suitable ranges.
  • the thermal strain was stabilized when, in the 0.7R ⁇ r ⁇ 0.9R region, the average value of the ferrite fraction (average fraction) in terms of area ratio is in the range of 40 to 70%, the total of the average fractions of structures other than ferrite and bainite is 0% or more and 3% or less on average, the balance includes bainite, and the standard deviation of the average ferrite fraction in the 0.7R ⁇ r ⁇ 0.9R range is 4% or less, as determined by the measurement method described below.
  • a “fraction” with respect to a metal structure means the average value of a structure fraction (unit: area %) in the cross-section of steel determined by the means described below.
  • the “fraction” does not mean an average value in the entirety of a cross-section but means the fraction in each measured visual field, as will be described below.
  • the preferable lower limit of the ferrite fraction is 42% and more preferably 45%.
  • the preferable upper limit of the ferrite fraction is 68% and more preferably 65%.
  • a lower standard deviation of the ferrite fraction in the 0.7R ⁇ r ⁇ 0.9R range is more preferable, and thus the lower limit is 0%.
  • the preferable upper limit of the standard deviation of the ferrite fraction in the 0.7R ⁇ r ⁇ 0.9R range is 3.5% and more preferably 3%.
  • “bainite” means, among the structures obtained by heating steel to form an austenite single phase structure and then cooling it to room temperature by continuous cooling, a structure excluding a ferrite structure, a pearlite structure, and a martensite structure, and means a collective term referring to an upper bainite structure, a lower bainite structure, or a mixed structure of an upper bainite structure and a lower bainite structure.
  • pearlite is contained in the structures of steel according to the present embodiment because it deteriorates carburizing hardenability.
  • steel composed of mixed structures of ferrite, pearlite, and bainite is carburizing-hardened, the austenite crystal grain structure in a region corresponding to the teeth becomes non-uniform during heating. Accordingly, deformation after carburizing hardening, i.e., thermal strain, is increased.
  • the area ratio of pearlite needs to be limited as much as possible.
  • the total of structures other than ferrite and bainite is specified to be 0% or more and 3% or less.
  • steel according to the present embodiment is steel having a ferrite-bainite structure.
  • the points where the circumferences having 0.7R+0.25 mm, 0.8R, and 0.9R ⁇ 0.25 mm intersect straight lines radially dividing, from the center of the cross-section of steel, the cross-section into eight equal parts (central angle 45°) were regarded as measurement points, and rectangular regions having 0.5 mm ⁇ 1 mm 0.5 mm 2 were regarded as measurement regions such that the respective measurement points were at the centers of the rectangles. There are 24 measurement regions.
  • the ferrite fraction and the standard deviation of the ferrite fraction in the 0.7R ⁇ r ⁇ 0.9R range were determined by observation using an optical microscope with respect to a sample obtained by mirror-polishing the cross-section of steel and corroding it with nital.
  • each measurement region of the nital-corroded sample was visually observed, and in each measurement region, the 0.5 mm 2 area in an image captured at 100 observation magnification (captured at 400 observation magnification when the boundary of structures is unclear) was binarized using image processing software Winroof 2015 so as to have ferrite and bainite as bright regions to derive the area ratios of the bright regions, and thereby the ferrite fraction and the bainite fraction of each measurement region were obtained.
  • the area obtained by excluding the area of non-metallic structures such as MnS from the test area was regarded as an evaluated area, and the respective proportions of the areas of the ferrite structure and the bainite structure relative to the evaluated area were regarded as the area ratios of the ferrite structure and the bainite structure.
  • the average value of the ferrite fraction of the 24 measurement regions was regarded as the ferrite fraction, and the average value of the bainite fraction of the 24 measurement regions was regarded as the bainite fraction.
  • the area ratio of structures other than ferrite and bainite were determined by 100 ⁇ (Ferrite fraction+Bainite fraction).
  • the standard deviation of the ferrite fraction in the 24 measurement points was regarded as the standard deviation of the ferrite fraction of the 0.7R ⁇ r ⁇ 0.9R range.
  • V is the casting rate, and the unit is m/min
  • A is the casting size (the cross-sectional area of the bloom), and the unit is mm 2
  • C is the average cooling rate of the bloom from immediately after casting to the bending correction point.
  • the average cooling rate of the bloom is a value obtained by dividing the temperature difference between the casting temperature of molten steel and the surface temperature of the bloom at the bending correction point by the time required to reach the correction point from immediately below the mold.
  • the unit is ° C./min.
  • the bending correction point is a position where the shape of the bloom is corrected from a curved shape to a straight shape in curved continuous casting.
  • the range of V ⁇ A 0.5 /C needs to be controlled to 6.0 to 20.0.
  • the preferable lower limit is 6.2 or more and more preferably 6.5 or more.
  • the preferable upper limit is 19.0 or less and more preferably 18.0 or less. It is impossible to actually measure the internal temperature during casting, but the use of this formula enables the internal temperature to be estimated in consideration of the items that can be actually measured and the casting size, thereby enabling cast control of a region corresponding to the gear teeth during casting.
  • post-rolling cooling it is important to control the average cooling rate when the surface temperature of steel during cooling is between 800° C. and 300° C.
  • a uniform structure can be obtained by controlling the average cooling rate to 0.1 to 1.0° C./sec when the surface temperature of steel is between 800° C. and 300° C., and, moreover, the ferrite fraction can be within a predetermined range. When the average cooling rate exceeds this range, a uniform structure cannot be obtained, and thermal strain is increased.
  • the preferable lower limit of the post-rolling cooling rate is 0.15° C./sec or faster and more preferably 0.2° C./sec or faster.
  • the preferable upper limit of the post-rolling cooling rate is 0.9° C./sec or slower and more preferably 0.8° C./sec or slower.
  • Molten steel is cast using a curved continuous casting machine (a casting step).
  • the mold size, the casting rate, and the cooling rate during casting are controlled as described above, and are desirably in the following ranges from the viewpoint of productivity.
  • the mold size is 30000 mm 2 or more and 400000 mm 2 or less, the casting rate is 0.2 m/min or faster and 3.0 m/min or slower, and the cooling rate from casting to the correction point is 4.0° C./min or faster and 100° C./min or slower.
  • the bloom obtained by the casting step is subjected to bloom rolling to obtain a billet (a bloom-rolling step).
  • the heating temperature during bloom rolling is desirably 1100° C. or higher.
  • a more preferable heating temperature is 1200° C. or higher.
  • an excessively high heating temperature results in coarse crystal grains, and thus the upper limit of the heating temperature is desirably 1280° C.
  • the bloom-rolling reduction of area is desirably 30% or more and more preferably 40% or more.
  • bar rolling or wire rod rolling is performed.
  • the heating temperature of bar rolling or wire rod rolling is desirably 1100° C. or higher.
  • a more preferable heating temperature is 1150° C. or higher.
  • an excessively high heating temperature results in coarse crystal grains, and thus the upper limit of the heating temperature is desirably 1250° C.
  • the post-rolling cooling rate is controlled such that the average cooling rate when the surface temperature of steel is between 800° C. and 300° C. is 0.1 to 1.0° C./sec.
  • a carburized gear is obtained by performing machining on the above steel to form a gear shape and then performing carburizing hardening and tempering.
  • a method for forming a gear shape hot forging, cold forging, cutting, or grindstone processing may be performed. Also, in order to increase workability, normalizing and annealing may be performed. Moreover, these may be combined.
  • carburizing hardening any carburizing method such as gas carburizing and vacuum carburizing can be used. Moreover, carbonitriding may be performed. Any type of gear may be created, such as spur gears, helical gears, bevel gears, external teeth, and internal teeth.
  • Test Nos.1 to 19 of the inventive examples had good thermal strain.
  • Test Nos. 20 to 23, 33, and 34 of the comparative examples good thermal strain was not obtained because the chemical component ranges were outside the scope of the present invention.
  • any one or more of the ferrite fraction, the fractions of structures other than ferrite and bainite, and the variation in ferrite fraction were outside the scope of the invention, and thus it was not possible to suppress the variation in helix deviation.
  • molten steels having the chemical components shown in Steel Nos. 1, 3, and 24 of Table 1 were cast under the conditions shown in Production Conditions 1 to 12 of Table 2 to obtain blooms. Thereafter, the blooms were heated to 1250° C. and bloom-rolled to obtain billets having 162 mm per side. These billets were heated to 1200° C., bar-rolled to have a shape (a post-rolling diameter) shown in Production Conditions 1 to 12 of Table 2, and cooled under the cooling conditions shown in the same table to obtain steels 1, 24 to 32, 35, and 36.
  • Test No. 32 is a test example corresponding to Production No. 1 of PCT International Publication No. WO 2014/171472.
  • Test Nos. 1 and 24 to 28 of the inventive examples had good thermal strain.
  • the production conditions were not desirable in Test Nos. 29 to 32, 35, and 36 of the comparative examples, good thermal strain was not obtained.
  • Test No. 35 the variation in ferrite fraction was excessive. This is presumably because the post-rolling cooling rate was too fast, and thus it was not possible to achieve structural uniformity. Accordingly, in Test No. 35, it was not possible to suppress the variation in helix deviation.
  • Test No. 36 the fraction of a structure other than ferrite and bainite was excessive.
  • the structure other than ferrite and bainite was pearlite. This is presumably because V ⁇ A 0.5 /C was too small, thus it was not possible to eliminate segregation, and moreover the post-rolling cooling rate was too small. Accordingly, in Test No. 36, it was not possible to suppress the variation in helix deviation.
  • Test No. 36 the variation in ferrite fraction was suppressed despite V ⁇ A 0.5 /C being too small This is considered to be because the structure included pearlite.
  • pearlite also causes an increased variation in helix deviation, and thus it cannot be said that the steel of Test No. 36 is steel that stabilizes thermal strain.

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CN111868281B (zh) 2022-05-10
EP3770291B1 (fr) 2024-01-17
WO2019182054A1 (fr) 2019-09-26
EP3770291A4 (fr) 2021-12-22
CN111868281A (zh) 2020-10-30
JPWO2019182054A1 (ja) 2020-12-17
KR102463278B1 (ko) 2022-11-07
KR20200118854A (ko) 2020-10-16
JP6919762B2 (ja) 2021-08-18

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