WO2014171472A1 - Case-hardening steel material and case-hardening steel member - Google Patents

Case-hardening steel material and case-hardening steel member Download PDF

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WO2014171472A1
WO2014171472A1 PCT/JP2014/060800 JP2014060800W WO2014171472A1 WO 2014171472 A1 WO2014171472 A1 WO 2014171472A1 JP 2014060800 W JP2014060800 W JP 2014060800W WO 2014171472 A1 WO2014171472 A1 WO 2014171472A1
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steel
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久保田　学
小澤　修司
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新日鐵住金株式会社
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Priority to JP2015512496A priority Critical patent/JPWO2014171472A1/en
Priority to KR1020157029024A priority patent/KR20150126699A/en
Priority to US14/783,292 priority patent/US20160060744A1/en
Priority to CN201480021369.9A priority patent/CN105121687A/en
Publication of WO2014171472A1 publication Critical patent/WO2014171472A1/en

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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D6/00—Heat treatment of ferrous alloys
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    • 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
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • 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
    • C23C8/28—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
    • C23C8/32—Carbo-nitriding of ferrous surfaces
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10—Handling in a vacuum
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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

Definitions

  • the present invention relates to a case-hardening steel material and a case-hardening steel part, and in particular, an automobile, a construction machine, an industry, which has excellent cold forgeability and excellent temper softening resistance after carburizing or carbonitriding quenching / tempering treatment.
  • the present invention relates to a case-hardening steel material and a case-hardening steel part suitable for machine parts.
  • Transmissions used in automobiles, construction machines, etc., and reduction gears used in industrial machines are mainly composed of gears.
  • These parts use medium carbon alloy steel such as JIS SCr420, SCM420, etc., and after carving, tempering, etc. after forming the material into the shape of the part by hot forging, cutting, cold forging, or a combination of these It can be obtained by applying a surface hardening treatment.
  • parts molded by cold forging are subjected to spheroidizing annealing before cold forging for the purpose of improving the die life by softening the material.
  • the subject at the time of performing cold forging is prevention of generation of cracks during cold forging and improvement of die life.
  • Patent Document 1 discloses that the contents of Si, Cr, and Mo are defined, and the temper softening resistance is improved when the total content of these elements exceeds a certain value.
  • Patent Document 2 discloses that when the Si content exceeds 0.15%, deformation resistance during cold forging increases.
  • Patent Documents 4 to 11 describe steels for machine structures in which the size of inclusions is limited. However, neither document describes cold forging.
  • Patent Document 12 describes a steel bar / wire rod that has both cold forgeability and machinability by limiting the size of sulfide-based, oxide-based, nitride-based inclusions, and composite inclusions thereof. ing. However, it does not describe a technique for improving the temper softening resistance.
  • Patent Document 13 describes a steel material for vacuum carburizing or vacuum carbonitriding.
  • the cumulative distribution function predicted by the extreme value statistical method is an oxide represented by [( ⁇ LW / 4) 0.5 ] when 99%, a composite inclusion mainly composed of oxide, nitride, and It is described that the maximum equivalent circular diameter of the composite inclusion mainly composed of nitride is 35 ⁇ m or less.
  • Patent Document 13 assumes vacuum carburization or vacuum carbonitriding. Accordingly, there is still a demand for the development of a steel material that satisfies both the characteristics of improved temper softening resistance and cold forgeability (prevention of cracking and prevention of increase in material hardness).
  • An object of this invention is to provide the case hardening steel materials which were excellent in cold forgeability and temper softening resistance, and the case hardening steel components which consist of the case hardening steel materials in view of said actual condition.
  • excellent resistance to temper softening means that the 300 ° C. tempering hardness of the carburized layer is higher than JIS SCr420 and SCM420.
  • the chemical components are mass%, C: 0.05 to 0.30%, Si: 0.40 to 1.5%, Mn: 0.00. 2 to 1.0%, S: 0.001 to 0.050%, Cr: 1.0 to 2.0%, Mo: 0.02 to 0.8%, Al: 0.001 to 0.20% , N: 0.003 to 0.03%, Nb: 0 to 0.10%, Cu: 0 to 0.2%, Ni: 0 to 1.5%, V: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Sb: 0 to 0.050%, P: 0.030% or less, O: 0.0020% or less, Ti: 0.005% Limiting to the following, the balance is iron and impurities, and satisfies the following formulas ( ⁇ ) and ( ⁇ ); in the inclusion evaluation using the extreme value statistical method, when the predicted area S is 30000 mm 2 , Said Maximum sulfide inclusions diameter
  • Si (%), Mn (%), Cr (%), and Mo (%) in the formulas ( ⁇ ) and ( ⁇ ) are the contents in mass% of the respective elements.
  • the chemical component may contain Nb: 0.015 to 0.10% by mass.
  • the chemical component may contain Si: 0.55 to 1.5% by mass.
  • the chemical component is, by mass, Cu: 0.001 to 0.2%, Ni: 0.001. One or two of ⁇ 1.5% may be contained.
  • the chemical component may contain V: 0.01 to 0.20% in mass%. Good.
  • the chemical component is, by mass, Ca: 0.0001 to 0.0050%, Mg: 0.0001. One or two of ⁇ 0.0050% may be contained.
  • the chemical component may contain Sb: 0.0001 to 0.050% by mass. .
  • the microstructure may have a spheroidized carbide structure.
  • a case-hardened steel part according to another aspect of the present invention is made of the steel for case-hardening as described in any one of (1) to (8) above, and is used for carburizing / quenching / tempering or carbonitriding / quenching / tempering. It has a surface hardened layer formed by processing.
  • case-hardening steel material and a case-hardening steel component in which the carburized layer has a 300 ° C. tempering hardness superior to that of JIS SCr420 or SCM420 and is excellent in cold forgeability. That is, it is possible to provide a case-hardening steel material and a case-hardening steel component that are excellent in temper softening resistance and cold forgeability.
  • the use of these case-hardening steel materials or case-hardening steel parts can reduce the manufacturing cost of gears, and contribute greatly to higher output and improved fuel consumption for automobiles, construction machinery, and industrial machinery. Is possible.
  • SA Spheroidizing annealing
  • the present inventors have clarified the following (a) to (d) as a result of the research.
  • the limit of cold forgeability (the limit of hardness before cold forging) can be determined by an index of the content of each of Si, Mn, Cr, and Mo in consideration of the hardness increasing action.
  • the present inventors perform spheroidizing annealing (SA) on a plurality of steel types in which various alloy elements are contained in 0.2% C steel (steel having a C content of 0.2%), and spheroidize. The influence of each alloy element on the hardness after annealing was quantitatively evaluated.
  • SA spheroidizing annealing
  • carbides in steel constituting pearlite and the like are spheroidized, and the microstructure has a spheroidized carbide structure.
  • the hardness of the steel after spheroidizing annealing can be expressed by the shape of the left side of the following equation (1).
  • the reason why the coefficients of Si and Mn are relatively high is that these alloy elements are dissolved in ferrite and the hardness of the spheroidized annealing material is increased by solid solution strengthening.
  • the coefficients of Cr and Mo are relatively small because these alloy elements are concentrated in cementite during spheroidizing annealing or precipitated in the form of alloy carbides, and the amount of solid solution strengthening is small. This is because the amount of precipitation strengthening is relatively small.
  • the value of the left side of the following formula (1) is 25 or less, the inventors do not excessively increase the hardness of the steel after spheroidizing annealing, but the left side of the following formula (1) It has been clarified that when the value of exceeds 25, the hardness of the steel after spheroidizing annealing becomes excessively high and the cold forgeability is impaired.
  • Si (%), Mn (%), Cr (%), and Mo (%) in the formula (1) are steel contents (mass%) of the respective components.
  • the temper softening resistance (300 ° C. tempering hardness) of steel (particularly carburized layer) can be represented by an index of the content of Si, Mn, and Cr in consideration of the increasing action of each temper softening resistance.
  • Si, Mn, and Cr have a large effect of increasing the temper softening resistance of the carburized layer. This is because when Si, Mn, and Cr are contained, the coarsening of the iron carbide that precipitates during tempering is suppressed.
  • the present inventors simulated a carburized layer and tempered a steel type containing various alloy elements to 0.8% C steel at 300 ° C. The effect of various alloy elements on the hardness after tempering (300 ° C.
  • Si (%), Mn (%), and Cr (%) in the formula (2) are the contents (% by mass) of each component in steel.
  • Si, Mn, Cr, and Mo are contained in a range that satisfies the above formula (1) and the above formula (2) at the same time, thereby increasing the temper softening resistance and lowering the material hardness (cold forgeability). Securing).
  • Specific means for applying the extreme value statistical method to the evaluation of non-metallic inclusions in steel are described in, for example, non-patent literature; influence of metal fatigue micro defects and inclusions, Takayoshi Murakami, etc. It can be carried out according to the method. In this embodiment, it is as follows.
  • the area of one visual field (inspection standard area: S 0 ) is, for example, 10 mm ⁇ 10 mm, and optical microscope observation of 30 visual fields is performed for each specimen so that the area S 0 does not overlap.
  • case hardening steel material according to an embodiment of the present invention (sometimes referred to as a case hardening steel material according to the embodiment) and a case hardening steel component according to an embodiment of the invention (according to the embodiment).
  • a case hardened steel part may be described in detail.
  • the component refers to a core component that is not affected by an increase in the amount of carbon due to carburization of the surface layer portion.
  • % Of content of a component means the mass%.
  • C (C: 0.05-0.30%) C is an essential element for obtaining the strength of the core of the part after carburizing and tempering. Moreover, C content determines the hardness of a core part and also affects the effective hardened layer depth of a carburized layer. Therefore, in this embodiment, the lower limit of the C content is set to 0.05%. However, when there is too much C content, toughness will fall. Therefore, the upper limit of the C content is set to 0.30%. A more desirable C content is 0.10 to 0.25%.
  • Si 0.40 to 1.5%
  • Si is an element effective for improving the temper softening resistance of the carburized layer. Therefore, the lower limit for the Si content is 0.40%. However, when there is too much Si content, the hardness after spheroidizing annealing will rise and cold forgeability will fall. Therefore, the upper limit of Si content is 1.5%.
  • a desirable Si content is 0.45 to 1.0%. In order to suppress the increase in cost and improve the temper softening resistance, it is more desirable that the lower limit of the Si content is 0.55%.
  • Mn 0.2 to 1.0%
  • Mn is an element effective for improving the hardenability of steel. Moreover, Mn improves hot ductility by fixing S in steel as MnS, and prevents generation of scratches in the steel production process (continuous casting, hot rolling). Furthermore, MnS has a function of improving machinability. In order to obtain these effects, the lower limit of the Mn content is set to 0.2%. However, when there is too much Mn content, the hardness after spheroidizing annealing will rise and cold forgeability will fall. Therefore, the upper limit of the Mn content is 1.0%. A desirable Mn content is 0.4 to 0.7%.
  • S has the effect of improving the machinability by forming MnS in steel.
  • the lower limit of the S content is set to 0.001%.
  • the upper limit of the S content is 0.050%.
  • a desirable S content is 0.005 to 0.020%.
  • Cr 1.0-2.0% Cr is an element effective not only for improving hardenability but also for improving temper softening resistance.
  • Cr has a characteristic that even if its content is relatively large, there is little influence on the increase in hardness after spheroidizing annealing. Therefore, the lower limit of the Cr content is 1.0%. However, if the Cr content exceeds 2.0%, the effect of improving the temper softening resistance is saturated, so the upper limit of the Cr content is set to 2.0%.
  • a desirable Cr content is 1.3 to 1.6%.
  • Mo is an element effective for improving hardenability.
  • Si, Mn, and Cr may be selectively oxidized in the steel surface layer during carburizing and heating, thereby reducing the hardenability of the surface layer.
  • an incompletely quenched layer is formed at the time of quenching, which causes a decrease in bending fatigue strength and pitching strength.
  • Mo since Mo has a lower oxidation tendency than the above elements, it is an effective element for reducing the incompletely hardened layer in the surface layer portion. In order to obtain this effect, the lower limit of the Mo content is 0.02%.
  • the upper limit of the Mo content is set to 0.8%.
  • a desirable Mo content is 0.05 to 0.5%.
  • Al (Al: 0.001 to 0.20%)
  • Al has the effect of refining austenite crystal grains by forming fine nitrides in the steel.
  • the lower limit of the Al content is set to 0.001%.
  • the upper limit of the Al content is 0.20%.
  • a desirable Al content is 0.015 to 0.050%.
  • N has the effect of refining austenite crystal grains by forming Al or Nb, V and nitride in steel.
  • the lower limit of the N content is set to 0.003%.
  • the upper limit of N content is 0.03%.
  • a desirable N content is 0.007 to 0.02%.
  • P is an impurity element and is an element that lowers the toughness of steel. Therefore, the P content is limited to 0.030% or less. Desirably, it is limited to 0.020% or less.
  • O is an impurity element and forms an oxide with Al, Si, or the like.
  • the O content increases, the amount of so-called oxide inclusions increases and the size becomes coarse. As will be described later, when coarse oxide inclusions are present, this becomes the starting point of cracking during cold forging. Therefore, the O content is limited to 0.0020% or less. Desirably, the O content is limited to 0.0015% or less, and more desirably 0.0005% or less.
  • Ti is an element which is inevitably mixed in this embodiment and forms a nitride such as TiN. As the amount of Ti increases, the amount of so-called nitride inclusions increases, and the size becomes coarse. If coarse nitride inclusions are present, this becomes the starting point of cracking during cold forging. Therefore, the Ti content is limited to 0.005% or less. Desirably, the Ti content is limited to 0.003% or less.
  • the case-hardening steel according to the present embodiment is based on having the above-described chemical components, but may further contain the following components.
  • the following elements are not necessarily contained. Therefore, there is no need to particularly limit the lower limit of the content, and the lower limit thereof is 0%.
  • Cu is an element effective for improving hardenability like Mo.
  • Cu is an element having a low oxidation tendency, and is an effective element for reducing the incompletely hardened layer in the surface layer portion.
  • the lower limit of the Cu content be 0.001%.
  • the upper limit of Cu content is 0.2%.
  • Ni of about 1 ⁇ 2 of the Cu content is simultaneously contained, the reduction in hot ductility is reduced.
  • a more desirable Cu content is 0.05 to 0.15%.
  • Ni is an element effective for improving hardenability like Mo and Cu.
  • Ni is an element having a low oxidation tendency and is an effective element for reducing the incompletely hardened layer in the surface layer portion.
  • the lower limit of the Ni content be 0.001%.
  • the upper limit of the Ni content is set to 1.5%.
  • a more desirable Ni content is 0.05 to 1.0%.
  • Nb has the effect of forming fine carbides and nitrides in the steel and miniaturizing the austenite crystal grains.
  • the lower limit of the Nb content is preferably set to 0.001%.
  • the lower limit of the Nb content is more preferably 0.015%.
  • the upper limit of Nb content is 0.10%.
  • a desirable upper limit of the Nb content is 0.050%.
  • V (V: 0.20% or less)
  • V has the effect of forming fine carbides and nitrides in the steel and miniaturizing the austenite crystal grains.
  • the lower limit of the V content be 0.01%.
  • the upper limit of V content is 0.20%.
  • a more desirable V content is 0.05 to 0.15%.
  • Ca (Ca: 0.0050% or less) Ca has the effect of preventing the sulfide inclusions from starting as cracks during cold forging by refining so-called sulfide inclusions.
  • the lower limit of the Ca content it is desirable that the lower limit of the Ca content be 0.0001%.
  • the upper limit of Ca content is 0.0050%.
  • a more desirable Ca content is 0.0005 to 0.0015%.
  • Mg refines so-called sulfide inclusions, thereby preventing the sulfide inclusions from starting as cracks during cold forging.
  • the lower limit of the Mg content be 0.0001%.
  • the upper limit of the Mg content is set to 0.0050%.
  • a more desirable Mg content is 0.0005 to 0.0015%.
  • Sb has the effect of suppressing decarburization during hot rolling and spheroidizing annealing.
  • the lower limit of the Sb content is 0.0001%.
  • the upper limit of the Sb content is 0.050%.
  • a more desirable Sb content is 0.001 to 0.010%.
  • the contents of Si, Mn, Cr and Mo are set so as to satisfy the following expression (1), that is, the following expression (1) It is necessary to control so that the value of the left-hand side becomes 25 or less. Because, the limit of the cold forgeability (hardness before cold forging) of the spheroidized annealed material, considering the degree of influence of Si, Mn, Cr, Mo on the hardness of each spheroidized annealed material This is because it must be decided.
  • the desirable range of the left side of the following formula (1) is 24.5 or less, and a more desirable range is 23 or less. 12 ⁇ Si (%) + 25 ⁇ Mn (%) + Cr (%) + 2 ⁇ Mo (%) ⁇ 25 (1)
  • the value on the left side of the following formula (2) is 50 or more.
  • the pitching fatigue strength is improved.
  • the value on the left side is desirably 53 or more, more desirably 55 or more.
  • the sulfide-based inclusion is an inclusion containing S, and includes, for example, MnS, CaS, MgS, (Mn, Ca, Mg) S, TiS, Ti (C, S), FeS, and the like. Point to.
  • the number of sulfide inclusions having a length exceeding 20 ⁇ m and a thickness exceeding 2 ⁇ m needs to be 200 or less per 1 mm 2 . If the length, thickness, or number of sulfide inclusions exceeds the above range, cracks are likely to occur.
  • the major axis is the length and the minor axis is the thickness. MnS having a length of 20 ⁇ m or less does not apply to this limitation in the range where the thickness is small.
  • the thickness is extremely large, for example, when the thickness exceeds 20 ⁇ m, the thickness is long. Because the length becomes the thickness, this restriction is applied.
  • the lower limit of the particle size is 0 ⁇ m.
  • the lower limit of the number density is 0 / mm 2 .
  • the oxide inclusions referred to in the present invention are inclusions containing O, such as Al 2 O 3 , CaO, Cr 2 O 3 , MnO, NbO, SiO 2 , MgO, ZrO 2 , and Ti x O. y , Nb 2 O 5 , FeO x , or a composite thereof.
  • O such as Al 2 O 3 , CaO, Cr 2 O 3 , MnO, NbO, SiO 2 , MgO, ZrO 2 , and Ti x O. y , Nb 2 O 5 , FeO x , or a composite thereof.
  • the predicted value of the maximum oxide inclusion diameter ( ⁇ area) Ox existing in the predicted area S 30000 mm 2. Is preferably 80 ⁇ m or less.
  • Oxide inclusions with an Ox of 80 ⁇ m or less are harmless, but oxide inclusions with an Ox of more than 80 ⁇ m serve as a starting point for cracking. Therefore, it is necessary to define the size of the oxide inclusions as described above.
  • the case-hardened steel component according to the present embodiment is obtained by subjecting the above-described case-hardening steel material to carburizing / quenching / tempering or carbonitriding / quenching / quenching / tempering. That is, the case-hardened steel part is made of steel for case-hardening. Therefore, the case-hardened steel part according to the present embodiment has substantially the same chemical components and inclusions as the chemical components and inclusions of the case-hardening steel material according to the present embodiment described above. Therefore, in order to control the chemical components and inclusions of the case hardening steel part, the case hardening steel material may be controlled to have predetermined chemical components and inclusions. However, the case-hardened steel part has a surface hardened layer because it undergoes carburizing / quenching / tempering or carbonitriding / quenching / tempering, and this is different from the case-hardening steel.
  • the RH vacuum degassing process is performed under the condition that the total processing time is 30 minutes or more, of which the processing time in a reduced pressure atmosphere of 1 Torr or less is 15 minutes or more (refining process).
  • refining process By performing refining under the above-described conditions, the size and number of oxide inclusions can be controlled within a predetermined range. Moreover, in this refining process, it adjusts so that a chemical component may become the preferable range mentioned above.
  • the molten steel which adjusted the chemical component in the refining process is made into a slab by continuous casting (casting process).
  • the casting speed be 0.45 m / min or more.
  • the size and number of sulfide inclusions can be controlled within the above range.
  • the casting speed is less than 0.45 m / min, coarse sulfide inclusions crystallize during solidification of the steel.
  • a desirable casting speed is 0.50 to 1.5 m / min.
  • it is desirable to cool the slab so that the cooling rate from the liquidus temperature to the solidus temperature at 1 ⁇ 4 part in the slab thickness direction is 5 to 200 ° C./min.
  • the slab obtained by the above casting process is subjected to ingot rolling to obtain a steel piece (ingot rolling process).
  • the heating temperature at the time of the block rolling is desirably 1240 ° C. or higher in order to temporarily dissolve coarse sulfides inevitably generated in the matrix.
  • a more desirable heating temperature is 1260 ° C. or higher.
  • a desirable area reduction rate is 45% or more.
  • the cooling rate is required to be 0.7 ° C./s or more.
  • a more desirable cooling rate is 1.5 ° C./s or more. This cooling rate is a cooling rate obtained from the measured value of the surface temperature.
  • steel bar rolling or wire rolling is performed.
  • the heating temperature is set to 1200 ° C. or less in order to prevent MnS growth and coarsening.
  • a more desirable heating temperature is 1000 to 1150 ° C.
  • the total area reduction ratio from the slab to the completion of the steel bar rolling or wire rod rolling is set to 65% or more.
  • the total area reduction is less than 65%, the thickness reduction due to the extension of the sulfide inclusions becomes insufficient, and the number of sulfide inclusions with a large thickness, which is harmful to the occurrence of cold forging cracks. It cannot be reduced.
  • a preferable range of the total area reduction rate is 90% or more.
  • the case-hardened steel part can be obtained by further subjecting the case-hardening steel material to carburizing / quenching / tempering or carbonitriding / quenching / tempering.
  • Carburizing quenching and tempering and carbonitriding quenching and tempering may be performed by known methods.
  • Converter molten steels having the compositions (chemical components) shown in Table 1-1 and Table 1-2 are subjected to RH vacuum degassing treatment under the conditions shown in Table 2, followed by continuous casting under the conditions shown in Table 3, and then A soaking diffusion treatment was performed as necessary, and a 162 mm square rolled material (steel slab) was obtained through a block rolling process.
  • the remainder of Table 1-1 and Table 1-2 is iron and impurities, and the blank indicates that it is not intentionally added.
  • SA spheroidizing annealing processing
  • This cylindrical test piece was subjected to upsetting cold working under conditions of upsetting rate of 50% and strain rate of 1.0.
  • the cold-worked cylindrical specimen was heated and held at 950 ° C. for 5 hours, and immediately cooled with water to freeze the austenite structure after the simulated carburization as a prior austenite grain boundary of the martensite structure.
  • the old austenite grain structure of the cross section in the rolling direction of the test piece subjected to simulated carburizing was observed, and the JIS grain size number was measured.
  • the definition of coarse grains was defined as JIS G 0551 crystal grain size number 5 or less, and any coarse grains that occurred in all fields of view in the cross section were determined to be coarse grains.
  • case-hardened steel and case-hardened steel parts of the present invention may be subjected to SA, but are not essential.
  • SA When cold working is not performed when actually manufacturing a part, or when cold working is possible without performing SA, SA may not be performed. In that case, it is used as high-strength steel. be able to.
  • the Vickers hardness (measuring load 10 kgf) at a 1/4 depth position of the diameter of the steel bar and the forged material was measured according to JIS Z 2244. The number of measurement points was 4 for each material, and the average value was obtained.
  • the hardness is HV155 or more, the deformation resistance at the time of cold forging is increased and the life of the mold is remarkably reduced, so that the cold forgeability is judged to be inferior.
  • the inclusion was measured by observing with an optical microscope at a position in the vicinity of 1/4 of the diameter of the steel bar, and in the case of the forged raw material, at a position in the vicinity of 1/4 of the diameter of the forged raw material.
  • Prediction value of the prediction area S 30,000 mm maximum sulfide inclusions diameter present in 2 ( ⁇ area) S, and a maximum of oxide inclusions diameter ( ⁇ area) Ox is one visual field area (inspection standard Area: S 0 ) is set to 10 mm ⁇ 10 mm, 30 optical microscope observations are performed so that the area S 0 does not overlap, and the diameter ( ⁇ area) of the maximum inclusion existing in each of the 30 visual fields is measured.
  • the number of sulfide inclusions having a length of 20 ⁇ m and a thickness exceeding 2 ⁇ m in each field of view was measured.
  • the total number of 30 fields of view was summed up and divided by the total measurement area (3000 mm 2 ) to measure the number of sulfide inclusions in an area of 1 mm 2 having a length exceeding 20 ⁇ m and a thickness exceeding 2 ⁇ m.
  • the critical compressibility was measured as an index for the occurrence of cracks during cold forging of steel.
  • a test piece for measuring the critical compression ratio ( ⁇ 6mm ⁇ 9mm, notch shape: 30 °, depth 0.8mm, radius of curvature of the tip 0.15mm) from the direction parallel to the longitudinal direction of the steel bar and the forged material Created.
  • To measure the critical compression ratio use a constraining die to perform cold compression at a speed of 10 mm / min, stop the compression when a microcrack with a length of 0.5 mm or more occurs near the notch, and compress at that time The rate was calculated, and this was taken as the compression rate at which cracking occurred.
  • 300 degreeC tempering hardness which is a parameter
  • carburized test pieces ⁇ 20 mm ⁇ 30 mm
  • gas carburization was performed by the shift furnace gas method. Gas carburization has a carbon potential of 0.8%, ambient temperature: 950 ° C, retention time: 5 hours ⁇ ambient temperature: 850 ° C, retention time: 0.5 hour ⁇ 130 ° C oil quenching ⁇ tempering temperature: 150 ° C, retention time : Performed under conditions of 90 minutes.
  • the depth of the incompletely hardened layer is deep, the pitching characteristics are adversely affected, and since the depth of the incompletely hardened layer of JIS-SCr420 is about 25 ⁇ m, the depth of the incompletely hardened layer is more than 25 ⁇ m.
  • the deeper ones were judged to have insufficient improvement in pitching characteristics.
  • tempering temperature: 300 degreeC and holding time: 90 minutes were further performed. Thereafter, the vicinity of the central portion in the longitudinal direction of the test piece was cut in a direction perpendicular to the longitudinal direction, and the Vickers hardness of the cross section was measured. The hardness measurement position was 50 ⁇ m deep from the surface, and the measurement load was 300 gf.
  • JIS-SCr420 has a tempered hardness of 300 ° C. of HV640, it can be regarded as a value that is clearly higher than this value. Those having a value of HV670 or higher are excellent in pitching characteristics, and those having less than HV670 have insufficient pitching characteristics. It was determined that
  • Table 2 summarizes the effects of RH conditions.
  • the RH condition no. In 1-3 both the total processing time of the RH vacuum degassing processing and the processing time in a reduced pressure atmosphere of 1 Torr or less were outside the desirable range. Also, 1-4 was outside the desirable range of processing time in a reduced pressure atmosphere of 1 Torr or less.
  • RH condition No. For 1-B the total processing time of the RH vacuum degassing process was outside the desired range. Manufacturing conditions No. 1 using these conditions. In 20, 23, 42, a, b, c, d, e, and f, the floating removal of the oxide in the molten steel was insufficient, and the oxide inclusions present in the steel bar were large. As a result, the critical compression rate was inferior.
  • the RH condition no. No. 1-1, 1-2, 1-A In 1, 9, and 2, the oxide inclusions were small, and the critical compression ratio of the SA material was good.
  • Table 3 summarizes the influence of casting conditions. Casting condition No. in Table 3 In No. 2-8, the casting speed was out of the desired range. Also, casting conditions No. In No. 2-9, since the cooling rate from the liquidus temperature to the solidus temperature in the 1/4 part of the slab thickness direction was low, the sulfide inclusions present in the bar steel were large. As a result, casting conditions No. 2-8 or No. Production No. 2-9 was adopted. In 64, 65, 66, and 67, the limit compression rate was lowered. On the other hand, the casting condition No. in which the continuous casting condition is appropriate. Manufacturing Nos. 2-1 to 2-7 were adopted. In 1, 2, 53 to 58, the sulfide inclusions were small, and the critical compression ratio was good.
  • Table 4 summarizes the effects of rolling conditions.
  • Rolling condition no. In 3-6 and 3-B the total area reduction rate of the hot rolling was outside the desired range. As a result, the production No. which adopted these conditions was used. In 68 and 69, the reduction in the thickness of MnS due to rolling became insufficient, and there were many sulfide inclusions with a large thickness. Further, as a result, the production No. In 68 and 69, the limit compression rate was lowered.
  • the production conditions No. 3-1 to 3-5, 3-A adopting the rolling condition Nos. 3-1 to 3-5 and 3-A in which the total area reduction ratio of the hot rolling is appropriate. In Nos. 1 and 59 to 63, the thickness was large, the number of elongated sulfide inclusions was small, and the critical compression ratio was good.
  • Tables 5-1, 5-2, 6 and 7 show the measurement results and characteristics of the inclusions in the steel obtained under each production condition.
  • Tables 5-1, 5-2, and 6 show the results of the materials subjected to SA
  • Table 7 shows the results of the materials not subjected to SA.
  • Table 5-1, Table 5-2, and Table 6 all of the production numbers in the scope of the present invention were used. Nos. 1 to 15 and 53 to 63 were all excellent in post-SA hardness, critical compression ratio, 300 ° C. tempered hardness of the carburized layer, and incompletely quenched layer thickness. In addition, production No. including Nb. For 1, 8, 9, and 11, no coarse particles were observed.
  • the production number is at least one of the chemical components or production conditions is out of the desired range.
  • any of the post-SA hardness, the critical compressibility, the 300 ° C. tempered hardness of the carburized layer, and the incompletely quenched layer thickness did not satisfy the target value.
  • production No. In 20, 23, 31, 34, 42, and 45 the O content was high, and the maximum ⁇ area of oxide inclusions was outside the scope of the present invention.
  • production No. 22, 33 and 44 had an S content exceeding the range of the present invention, so the maximum ⁇ area of sulfide inclusions was outside the range of the present invention.
  • case-hardening steel material and case-hardening steel component of the present invention are used, a case-hardening steel material and case-hardening steel component excellent in temper softening resistance and cold forgeability can be provided. Further, by using these, it is possible to reduce the manufacturing cost of the gears, and to greatly contribute to the increase in output and the improvement of fuel consumption for automobiles, construction machines, and industrial machines.

Abstract

A case-hardening steel material has such a property that the predicted value of the largest diameter (√area)S of sulfide-type inclusions that exist in a predicted area (S) is 49 μm or less and the predicted value of the largest diameter (√area)Ox of oxide-type inclusions that exist in the predicted area (S) is 80 μm or less wherein the predicted area (S) is 30000 mm2 in the inclusion rating employing an extreme value statistic method, and also has such a property that the number of sulfide-type inclusions each having a length of more than 20 μm and a thickness of more than 2 μm is limited to 200 per 1 mm2.

Description

肌焼用鋼材と肌焼鋼部品Case-hardening steel and case-hardening steel parts
　本発明は、肌焼用鋼材と肌焼鋼部品とに関し、特に、冷間鍛造性に優れ、かつ浸炭又は浸炭窒化焼入れ・焼戻し処理後に優れた焼戻し軟化抵抗が得られる、自動車、建設機械、産業機械用の部品に好適な肌焼用鋼材及び肌焼鋼部品に関する。
　本願は、２０１３年０４月１８日に、日本に出願された特願２０１３－０８７８５７号に基づき優先権を主張し、その内容をここに援用する。 The present invention relates to a case-hardening steel material and a case-hardening steel part, and in particular, an automobile, a construction machine, an industry, which has excellent cold forgeability and excellent temper softening resistance after carburizing or carbonitriding quenching / tempering treatment. The present invention relates to a case-hardening steel material and a case-hardening steel part suitable for machine parts.
This application claims priority on April 18, 2013 based on Japanese Patent Application No. 2013-087857 for which it applied to Japan, and uses the content here.
　自動車、建設機械等に使用されている変速機、及び産業機械等に使用されている減速機は、主に歯車類によって構成される。これらの部品は、ＪＩＳ　ＳＣｒ４２０、ＳＣＭ４２０等の中炭素合金鋼を素材として用い、熱間鍛造、切削、冷間鍛造、あるいはこれらの組み合わせによって素材を部品の形状に成形した後に、浸炭焼入れ・焼戻し等の表面硬化処理を施すことで得られる。このうち、冷間鍛造で成形される部品は、素材の軟質化による金型寿命の向上を目的として、冷間鍛造の前に球状化焼鈍が行われる。冷間鍛造を行う際の課題は、冷間鍛造時の割れの発生防止及び金型寿命の向上である。従って、割れの起点となる介在物の生成の抑制と、素材の軟質化との両方を達成することができれば、冷間鍛造にかかわるコストを下げることができる。
　一方、自動車等の高出力化及び燃費向上のため、歯車類の高強度化が強く求められている。従来、これらの部品の強度を高めるために、高強度化における大きな課題であった歯車の歯元曲げ疲労強度を向上させる技術の開発が行われてきた。しかしながら、近年、歯元曲げ疲労強度を飛躍的に高めることができるハードショットピーニングの適用拡大に伴い、歯車の高強度化を達成するための課題の重点が、歯元曲げ疲労強度の向上からピッチング強度の向上に移行している。 Transmissions used in automobiles, construction machines, etc., and reduction gears used in industrial machines are mainly composed of gears. These parts use medium carbon alloy steel such as JIS SCr420, SCM420, etc., and after carving, tempering, etc. after forming the material into the shape of the part by hot forging, cutting, cold forging, or a combination of these It can be obtained by applying a surface hardening treatment. Among these, parts molded by cold forging are subjected to spheroidizing annealing before cold forging for the purpose of improving the die life by softening the material. The subject at the time of performing cold forging is prevention of generation of cracks during cold forging and improvement of die life. Therefore, if it is possible to achieve both the suppression of the formation of inclusions as starting points of cracking and the softening of the material, the cost for cold forging can be reduced.
On the other hand, there is a strong demand for higher strength of gears in order to increase the output and fuel consumption of automobiles and the like. Conventionally, in order to increase the strength of these components, a technology for improving the tooth root bending fatigue strength of a gear, which has been a major issue in increasing the strength, has been developed. However, in recent years, with the expansion of the application of hard shot peening that can dramatically increase the root bending fatigue strength, the emphasis on the challenge to achieve higher gear strength is to improve the root bending fatigue strength from pitching. It has shifted to the improvement of strength.
　ピッチング強度を改善する（向上させる）ためには、歯車の浸炭層の焼戻し軟化抵抗を向上させることが有効である。焼戻し軟化抵抗を向上させる手段として、鋼の成分を改良する技術が提案されている。例えば、特許文献１には、Ｓｉ、Ｃｒ、Ｍｏの含有量を規定し、これらの元素の合計含有量が一定値を超えると、焼戻し軟化抵抗が向上することが開示されている。しかしながら、これらの元素の合計含有量が多くなると、冷間鍛造前の素材の硬さが上昇し、変形抵抗が上昇する。また、例えば特許文献２には、Ｓｉの含有量が０．１５％を超えると、冷間鍛造時の変形抵抗が増大することが開示されている。このように、一般に、鋼の各成分（特にＳｉ）の含有量を高めると、焼戻し軟化抵抗向上効果が得られるものの、素材硬さが上昇する。すなわち、焼戻し軟化抵抗の向上と、冷間鍛造性の確保とはトレードオフの関係にある。そのため、焼戻し軟化抵抗と冷間鍛造性とを両立させた鋼材の開発が望まれている。特許文献３には、Ｓｉ、Ｃｒの含有量を増加させて焼戻し軟化抵抗を向上させた上で、Ｓｉ、Ｍｎ、Ｃｒ、Ｍｏの含有量の合計を所定の関係式で規定される値以下に制限することによって、焼戻し軟化抵抗と冷間鍛造性との両立を実現する方法が提案されている。しかしながら、特許文献３の技術は冷間鍛造時の割れの発生防止については考慮していない。そのため、冷間鍛造時に加工率が大きくなる部位に大きな介在物が存在している場合に介在物を起点とする割れが発生するという問題があり、依然として改善の余地が大きい。特許文献４～１１には、介在物の大きさを制限した機械構造用鋼が記載されている。しかしながら、いずれの文献にも、冷間鍛造については記載されていない。特許文献１２には硫化物系、酸化物系、窒化物系介在物やそれらの複合介在物の大きさを制限して冷間鍛造性と被削性とを両立させた棒鋼・線材が記載されている。しかしながら、焼戻し軟化抵抗を向上させる技術については記載されていない。特許文献１３には、真空浸炭または真空浸炭窒化用の鋼材が記載されている。この鋼材では、極値統計法によって予想される累積分布関数が９９％時の〔（πＬＷ／４）０．５〕で表される酸化物、酸化物を主体とする複合介在物、窒化物および窒化物を主体とする複合介在物の最大等価円直径が３５μｍ以下であることが記載されている。しかしながら、特許文献１３では真空浸炭または真空浸炭窒化を前提としている。
　従って、焼戻し軟化抵抗の向上と冷間鍛造性（割れの防止及び素材硬さの上昇防止）との両方の特性を満足する鋼材の開発が依然として要望されている。 In order to improve (improve) the pitching strength, it is effective to improve the temper softening resistance of the carburized layer of the gear. As a means for improving the temper softening resistance, a technique for improving the steel composition has been proposed. For example, Patent Document 1 discloses that the contents of Si, Cr, and Mo are defined, and the temper softening resistance is improved when the total content of these elements exceeds a certain value. However, when the total content of these elements increases, the hardness of the material before cold forging increases, and the deformation resistance increases. For example, Patent Document 2 discloses that when the Si content exceeds 0.15%, deformation resistance during cold forging increases. Thus, generally, when content of each component (especially Si) of steel is raised, although a temper softening resistance improvement effect will be acquired, raw material hardness will rise. That is, there is a trade-off between improving the temper softening resistance and ensuring the cold forgeability. Therefore, it is desired to develop a steel material having both temper softening resistance and cold forgeability. In Patent Document 3, the content of Si and Cr is increased to improve the temper softening resistance, and the total content of Si, Mn, Cr, and Mo is less than the value defined by a predetermined relational expression. By limiting, a method for realizing both temper softening resistance and cold forgeability has been proposed. However, the technique of Patent Document 3 does not consider prevention of cracking during cold forging. For this reason, there is a problem in that cracks originating from inclusions occur when large inclusions are present in a portion where the processing rate becomes large during cold forging, and there is still much room for improvement. Patent Documents 4 to 11 describe steels for machine structures in which the size of inclusions is limited. However, neither document describes cold forging. Patent Document 12 describes a steel bar / wire rod that has both cold forgeability and machinability by limiting the size of sulfide-based, oxide-based, nitride-based inclusions, and composite inclusions thereof. ing. However, it does not describe a technique for improving the temper softening resistance. Patent Document 13 describes a steel material for vacuum carburizing or vacuum carbonitriding. In this steel material, the cumulative distribution function predicted by the extreme value statistical method is an oxide represented by [(πLW / 4) 0.5 ] when 99%, a composite inclusion mainly composed of oxide, nitride, and It is described that the maximum equivalent circular diameter of the composite inclusion mainly composed of nitride is 35 μm or less. However, Patent Document 13 assumes vacuum carburization or vacuum carbonitriding.
Accordingly, there is still a demand for the development of a steel material that satisfies both the characteristics of improved temper softening resistance and cold forgeability (prevention of cracking and prevention of increase in material hardness).
日本国特開２００３－２３１９４３号公報Japanese Laid-Open Patent Publication No. 2003-231943 日本国特開平６－２９９２４１号公報Japanese Unexamined Patent Publication No. 6-299241 日本国特開２００６－１９９９９３号公報Japanese Laid-Open Patent Publication No. 2006-199993 日本国特開２００１－２３４２７５号公報Japanese Unexamined Patent Publication No. 2001-234275 日本国特開２００１－１３１６８５号公報Japanese Patent Laid-Open No. 2001-131585 日本国特開２００１－１３１６８６号公報Japanese Unexamined Patent Publication No. 2001-131686 日本国特開２００３－２６９４６０号公報Japanese Unexamined Patent Publication No. 2003-269460 日本国特開２００６－６３４０２号公報Japanese Unexamined Patent Publication No. 2006-63402 日本国特開２００７－２８９９７９号公報Japanese Unexamined Patent Publication No. 2007-289979 日本国特開２００４－１４３５５０号公報Japanese Unexamined Patent Publication No. 2004-143550 日本国特開２００５－１５４８８６号公報Japanese Unexamined Patent Publication No. 2005-154886 日本国特開２００７－６３５８９号公報Japanese Patent Publication No. 2007-63589 日本国特開２０１０－１５０５６６号公報Japanese Unexamined Patent Publication No. 2010-150566
　本発明は上記の実状を鑑み、冷間鍛造性と焼戻し軟化抵抗とが優れた肌焼用鋼材と、その肌焼用鋼材からなる肌焼鋼部品とを提供することを目的とする。
　なお、本発明において、焼戻し軟化抵抗に優れるとは、浸炭層の３００℃焼戻し硬さがＪＩＳ　ＳＣｒ４２０やＳＣＭ４２０よりも高いことを示す。 An object of this invention is to provide the case hardening steel materials which were excellent in cold forgeability and temper softening resistance, and the case hardening steel components which consist of the case hardening steel materials in view of said actual condition.
In the present invention, excellent resistance to temper softening means that the 300 ° C. tempering hardness of the carburized layer is higher than JIS SCr420 and SCM420.
　本発明者らは、上記課題を解決するため、焼戻し軟化抵抗を向上させるのに適した化学成分の調整と、冷間鍛造時の割れ発生の防止に必要な介在物のサイズ制御とについて鋭意検討した。その結果、（ｉ）Ｓｉ、Ｃｒは浸炭層の焼戻し軟化抵抗を増加する作用が大きいこと、（ｉｉ）球状化焼鈍後の硬さはＳｉ、Ｍｎ、Ｃｒ、Ｍｏの含有量の合計に依存し、それぞれの元素の寄与率が異なること、（ｉｉｉ）鋼中に存在する非金属介在物、特に硫化物系介在物のサイズを適切に制限することによって冷間鍛造時の割れの発生を防止できること等を見出し、本発明の完成に至った。
　本発明の要旨は以下の通りである。 In order to solve the above-mentioned problems, the present inventors diligently studied the adjustment of chemical components suitable for improving the temper softening resistance and the control of the size of inclusions necessary for preventing cracks during cold forging. did. As a result, (i) Si and Cr have a large effect of increasing the temper softening resistance of the carburized layer. (Ii) The hardness after spheroidizing annealing depends on the total content of Si, Mn, Cr and Mo. The contribution ratio of each element is different. (Iii) The occurrence of cracks during cold forging can be prevented by appropriately limiting the size of non-metallic inclusions present in the steel, particularly sulfide inclusions. As a result, the present invention has been completed.
The gist of the present invention is as follows.
　（１）本発明の一態様に係る肌焼用鋼材は、化学成分が、質量％で、Ｃ：０．０５～０．３０％、Ｓｉ：０．４０～１．５％、Ｍｎ：０．２～１．０％、Ｓ：０．００１～０．０５０％、Ｃｒ：１．０～２．０％、Ｍｏ：０．０２～０．８％、Ａｌ：０．００１～０．２０％、Ｎ：０．００３～０．０３％、Ｎｂ：０～０．１０％、Ｃｕ：０～０．２％、Ｎｉ：０～１．５％、Ｖ：０～０．２０％、Ｃａ：０～０．００５０％、Ｍｇ：０～０．００５０％、Ｓｂ：０～０．０５０％を含有し、Ｐ：０．０３０％以下、Ｏ：０．００２０％以下、Ｔｉ：０．００５％以下に制限し、残部が鉄及び不純物であり、下記（α）式、及び（β）式を満足し；極値統計法を用いた介在物評価において、予測面積Ｓを３００００ｍｍ２としたとき、前記予測面積Ｓ中に存在する最大の硫化物系介在物径（√ａｒｅａ）Ｓの予測値が４９μｍ以下であり、前記予測面積Ｓ中に存在する最大の酸化物系介在物径（√ａｒｅａ）Ｏｘの予測値が８０μｍ以下であり；２０μｍを超える長さ及び２μｍを超える厚みを有する硫化物系介在物が１ｍｍ２あたり２００個以下に制限されている。
　１２×Ｓｉ（％）＋２５×Ｍｎ（％）＋Ｃｒ（％）＋２×Ｍｏ（％）≦２５　・・・（α）
　３１×Ｓｉ（％）＋１５×Ｍｎ（％）＋２３×Ｃｒ（％）≧５０　・・・（β）
　ここで、（α）式、及び（β）式中の、Ｓｉ（％）、Ｍｎ（％）、Ｃｒ（％）、Ｍｏ（％）は、それぞれの元素の質量％での含有量である。 (1) In the case-hardening steel according to one aspect of the present invention, the chemical components are mass%, C: 0.05 to 0.30%, Si: 0.40 to 1.5%, Mn: 0.00. 2 to 1.0%, S: 0.001 to 0.050%, Cr: 1.0 to 2.0%, Mo: 0.02 to 0.8%, Al: 0.001 to 0.20% , N: 0.003 to 0.03%, Nb: 0 to 0.10%, Cu: 0 to 0.2%, Ni: 0 to 1.5%, V: 0 to 0.20%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Sb: 0 to 0.050%, P: 0.030% or less, O: 0.0020% or less, Ti: 0.005% Limiting to the following, the balance is iron and impurities, and satisfies the following formulas (α) and (β); in the inclusion evaluation using the extreme value statistical method, when the predicted area S is 30000 mm 2 , Said Maximum sulfide inclusions diameter present in the area S (} area) is the predicted value of S is 49μm or less and a maximum of oxide inclusions diameter (} area) of Ox present in said prediction area S The predicted value is 80 μm or less; the number of sulfide inclusions having a length exceeding 20 μm and a thickness exceeding 2 μm is limited to 200 or less per 1 mm 2 .
12 × Si (%) + 25 × Mn (%) + Cr (%) + 2 × Mo (%) ≦ 25 (α)
31 × Si (%) + 15 × Mn (%) + 23 × Cr (%) ≧ 50 (β)
Here, Si (%), Mn (%), Cr (%), and Mo (%) in the formulas (α) and (β) are the contents in mass% of the respective elements.
　（２）上記（１）に記載の肌焼用鋼材では、前記化学成分が、質量％で、Ｎｂ：０．０１５～０．１０％を含有してもよい。 (2) In the steel for case hardening described in (1) above, the chemical component may contain Nb: 0.015 to 0.10% by mass.
　（３）上記（１）に記載の肌焼用鋼材では、前記化学成分が、質量％で、Ｓｉ：０．５５～１．５％を含有してもよい。 (3) In the steel for case hardening described in (1) above, the chemical component may contain Si: 0.55 to 1.5% by mass.
　（４）上記（１）～（３）のいずれか一項に記載の肌焼用鋼材では、前記化学成分が、質量％で、Ｃｕ：０．００１～０．２％、Ｎｉ：０．００１～１．５％、のうちの１種又は２種を含有してもよい。 (4) In the case-hardening steel according to any one of (1) to (3), the chemical component is, by mass, Cu: 0.001 to 0.2%, Ni: 0.001. One or two of ˜1.5% may be contained.
　（５）上記（１）～（４）のいずれか一項に記載の肌焼用鋼材では、前記化学成分が、質量％で、Ｖ：０．０１～０．２０％、を含有してもよい。 (5) In the case-hardening steel according to any one of (1) to (4), the chemical component may contain V: 0.01 to 0.20% in mass%. Good.
　（６）上記（１）～（５）のいずれか一項に記載の肌焼用鋼材では、前記化学成分が、質量％で、Ｃａ：０．０００１～０．００５０％、Ｍｇ：０．０００１～０．００５０％のうちの１種又は２種を含有してもよい。 (6) In the case-hardening steel according to any one of (1) to (5), the chemical component is, by mass, Ca: 0.0001 to 0.0050%, Mg: 0.0001. One or two of ˜0.0050% may be contained.
　（７）上記（１）～（６）のいずれか一項に記載の肌焼用鋼材では、前記化学成分が、質量％で、Ｓｂ：０．０００１～０．０５０％を含有してもよい。 (7) In the case-hardening steel according to any one of (1) to (6), the chemical component may contain Sb: 0.0001 to 0.050% by mass. .
　（８）上記（１）～（７）のいずれか一項に記載の肌焼用鋼材では、ミクロ組織が球状化炭化物組織を有してもよい。 (8) In the steel for case hardening described in any one of (1) to (7) above, the microstructure may have a spheroidized carbide structure.
　（９）本発明の別の態様に係る肌焼鋼部品は、上記（１）～（８）のいずれか一項に記載の肌焼用鋼材からなり、浸炭焼入れ焼戻し、または浸炭窒化焼入れ焼戻しの処理によって形成された表面硬化層を有する。 (9) A case-hardened steel part according to another aspect of the present invention is made of the steel for case-hardening as described in any one of (1) to (8) above, and is used for carburizing / quenching / tempering or carbonitriding / quenching / tempering. It has a surface hardened layer formed by processing.
　本発明の上記態様によれば、浸炭層の３００℃焼戻し硬さがＪＩＳ　ＳＣｒ４２０やＳＣＭ４２０よりも優れ、かつ冷間鍛造性に優れた肌焼用鋼材及び肌焼鋼部品を提供することができる。すなわち、焼戻し軟化抵抗及び冷間鍛造性に優れた肌焼用鋼材及び肌焼鋼部品を提供することができる。また、これらの肌焼用鋼材または肌焼鋼部品を用いることにより歯車の製造コストを低減することができ、なおかつ自動車、建設機械、産業機械用の高出力化及び燃費向上等に大きく寄与することが可能になる。 According to the above aspect of the present invention, it is possible to provide a case-hardening steel material and a case-hardening steel component in which the carburized layer has a 300 ° C. tempering hardness superior to that of JIS SCr420 or SCM420 and is excellent in cold forgeability. That is, it is possible to provide a case-hardening steel material and a case-hardening steel component that are excellent in temper softening resistance and cold forgeability. In addition, the use of these case-hardening steel materials or case-hardening steel parts can reduce the manufacturing cost of gears, and contribute greatly to higher output and improved fuel consumption for automobiles, construction machinery, and industrial machinery. Is possible.
本発明の実施例で適用した、球状化焼鈍（ＳＡ：Ｓｐｈｅｒｏｉｄｉｚｉｎｇ　ａｎｎｅａｌｉｎｇ）のパターンを示す図である。It is a figure which shows the pattern of spheroidizing annealing (SA: Spheroidizing annealing) applied in the Example of this invention.
　本発明者らは研究の結果、以下の（ａ）～（ｄ）について明らかにした。 The present inventors have clarified the following (a) to (d) as a result of the research.
　（ａ）冷間鍛造性の限界（冷間鍛造前の硬さの限界）は、Ｓｉ、Ｍｎ、Ｃｒ、Ｍｏの、それぞれの硬さの上昇作用を考慮した含有量の指標で決定できる。
　本発明者らは、０．２％Ｃ鋼（Ｃ含有量が０．２％の鋼）に種々の合金元素を含有させた複数の鋼種に対して球状化焼鈍（ＳＡ）を行い、球状化焼鈍後の硬さに及ぼす各合金元素の影響を定量的に評価した。球状化焼鈍を行うと、パーライトなどを構成する鋼中の炭化物が球状化し、ミクロ組織が球状化炭化物組織を有することになる。炭化物が球状化すると、転位運動の障害となる炭化物間の間隔が大きくなり、それによって硬さが低下するため、望ましい。
　調査の結果、本発明者らは、球状化焼鈍後の鋼の硬さは下記（１）式の左辺の形で表現できることを明らかにした。Ｓｉ、Ｍｎの係数が比較的高い理由は、これらの合金元素がフェライトに固溶し、固溶強化によって球状化焼鈍材の硬さを上げるためである。一方で、Ｃｒ、Ｍｏの係数が比較的小さいのは、これらの合金元素が球状化焼鈍時にセメンタイト中に濃化したり、合金炭化物の形で析出するので固溶強化量が小さいこと、及び、これらの炭化物は大きいため、析出強化量としては相対的に小さいことによる。
　本発明者らは、下記（１）式の左辺の値が２５以下である場合には、球状化焼鈍後の鋼の硬さが過度に高くなることはないが、下記（１）式の左辺の値が２５を超えると球状化焼鈍後の鋼の硬さが過度に高くなり、冷間鍛造性を損なうことを明らかにした。 (A) The limit of cold forgeability (the limit of hardness before cold forging) can be determined by an index of the content of each of Si, Mn, Cr, and Mo in consideration of the hardness increasing action.
The present inventors perform spheroidizing annealing (SA) on a plurality of steel types in which various alloy elements are contained in 0.2% C steel (steel having a C content of 0.2%), and spheroidize. The influence of each alloy element on the hardness after annealing was quantitatively evaluated. When spheroidizing annealing is performed, carbides in steel constituting pearlite and the like are spheroidized, and the microstructure has a spheroidized carbide structure. When the carbides are spheroidized, it is desirable because the distance between the carbides that hinders the dislocation movement is increased, thereby reducing the hardness.
As a result of the investigation, the present inventors have clarified that the hardness of the steel after spheroidizing annealing can be expressed by the shape of the left side of the following equation (1). The reason why the coefficients of Si and Mn are relatively high is that these alloy elements are dissolved in ferrite and the hardness of the spheroidized annealing material is increased by solid solution strengthening. On the other hand, the coefficients of Cr and Mo are relatively small because these alloy elements are concentrated in cementite during spheroidizing annealing or precipitated in the form of alloy carbides, and the amount of solid solution strengthening is small. This is because the amount of precipitation strengthening is relatively small.
When the value of the left side of the following formula (1) is 25 or less, the inventors do not excessively increase the hardness of the steel after spheroidizing annealing, but the left side of the following formula (1) It has been clarified that when the value of exceeds 25, the hardness of the steel after spheroidizing annealing becomes excessively high and the cold forgeability is impaired.
　１２×Ｓｉ（％）＋２５×Ｍｎ（％）＋Ｃｒ（％）＋２×Ｍｏ（％）≦２５　・・・（１）
　ここで、（１）式中の、Ｓｉ（％）、Ｍｎ（％）、Ｃｒ（％）、Ｍｏ（％）は、それぞれの成分の鋼中含有量（質量％）である。 12 × Si (%) + 25 × Mn (%) + Cr (%) + 2 × Mo (%) ≦ 25 (1)
Here, Si (%), Mn (%), Cr (%), and Mo (%) in the formula (1) are steel contents (mass%) of the respective components.
　（ｂ）鋼（特に浸炭層）の焼戻し軟化抵抗（３００℃焼戻し硬さ）は、Ｓｉ、Ｍｎ、Ｃｒの、それぞれの焼戻し軟化抵抗の増加作用を考慮した含有量の指標で表すことができる。
　Ｓｉ、Ｍｎ、Ｃｒは浸炭層の焼戻し軟化抵抗を増加させる作用が大きい。これは、Ｓｉ、Ｍｎ、Ｃｒが含有されると、焼戻し時に析出する鉄炭化物の粗大化が抑制されるためである。本発明者らは、各合金元素の影響を定量的に評価するため、浸炭層を模擬して、０．８％Ｃ鋼に対して種々の合金元素を含有させた鋼種に対して３００℃焼戻しを行い、焼戻し後の硬さ（３００℃焼戻し硬さ）に及ぼす各種の合金元素の影響を調査した。その結果、本発明者らは、各合金元素による浸炭層の３００℃焼戻し硬さの増加作用は、下記（２）式の左辺の形で表現できることを明らかにした。また、その左辺の値が５０以上である場合には３００℃焼戻し硬さが一般の浸炭部品よりも明瞭に向上し、優れたピッチング強度が得られるのに対して、５０未満である場合にはピッチング強度の改善が不十分であることを明らかにした。 (B) The temper softening resistance (300 ° C. tempering hardness) of steel (particularly carburized layer) can be represented by an index of the content of Si, Mn, and Cr in consideration of the increasing action of each temper softening resistance.
Si, Mn, and Cr have a large effect of increasing the temper softening resistance of the carburized layer. This is because when Si, Mn, and Cr are contained, the coarsening of the iron carbide that precipitates during tempering is suppressed. In order to quantitatively evaluate the influence of each alloy element, the present inventors simulated a carburized layer and tempered a steel type containing various alloy elements to 0.8% C steel at 300 ° C. The effect of various alloy elements on the hardness after tempering (300 ° C. tempered hardness) was investigated. As a result, the present inventors have clarified that the effect of increasing the 300 ° C. tempering hardness of the carburized layer by each alloy element can be expressed in the form of the left side of the following equation (2). Moreover, when the value of the left side is 50 or more, the 300 ° C. tempering hardness is clearly improved as compared with general carburized parts, and excellent pitching strength is obtained, whereas when it is less than 50, It was clarified that the improvement of pitching strength was insufficient.
　３１×Ｓｉ（％）＋１５×Ｍｎ（％）＋２３×Ｃｒ（％）≧５０　・・・（２）
ここで、（２）式中の、Ｓｉ（％）、Ｍｎ（％）、Ｃｒ（％）は、それぞれの成分の鋼中の含有量（質量％）である。 31 × Si (%) + 15 × Mn (%) + 23 × Cr (%) ≧ 50 (2)
Here, Si (%), Mn (%), and Cr (%) in the formula (2) are the contents (% by mass) of each component in steel.
　上記より、Ｓｉ、Ｍｎ、Ｃｒ、Ｍｏを上記（１）式と、上記（２）式とを同時に満たす範囲で含有させることによって、焼戻し軟化抵抗の増加と素材硬さの低下（冷間鍛造性の確保）とが両立できる。 From the above, Si, Mn, Cr, and Mo are contained in a range that satisfies the above formula (1) and the above formula (2) at the same time, thereby increasing the temper softening resistance and lowering the material hardness (cold forgeability). Securing).
　（ｃ）鋼中に存在する非金属介在物（硫化物系、酸化物系、窒化物系）、特に硫化物系介在物のサイズを制限することによって冷間鍛造時の割れの発生を防止できる。
　鋼中に存在する大きな介在物は、割れの起点となる。そのため、工業的な規模で安定した量産を行うためには、部品の素材について、広い領域で介在物の分布状況を評価する必要がある。割れの起点となる大きな介在物の存在は「極値統計法」で推定することができる。極値統計法とは、ある母集団から複数個の試験片を採取し、個々の試験片に存在する最大の介在物の大きさを顕微鏡法にて測定し、その面積の平方根を極値確率紙にプロットすることにより、母集団あるいは任意の面積（または体積）中に存在する最大の介在物の粒径（√ａｒｅａ）を予測する方法である。鋼中の非金属介在物の評価に極値統計法を適用する具体的な手段としては、例えば、非特許文献；金属疲労　微小欠陥と介在物の影響、村上敬宜著、等に記載された方法に準じて行うことができる。本実施形態では、以下の通りとした。（ｉ）１視野の面積（検査基準面積：Ｓ０）を例えば１０ｍｍ×１０ｍｍとし、面積Ｓ０が重複しないように各供試材につきそれぞれ３０視野の光学顕微鏡観察を行う。（ｉｉ）３０視野のそれぞれに存在している最大介在物の粒径の測定を行ってその面積の平方根（√ａｒｅａ）を極値確率紙にプロットを行う。（ｉｉｉ）予測面積Ｓを３００００ｍｍ２として最大介在物の粒径（√ａｒｅａ）を予測する。
　なお、介在物の測定は、酸化物、硫化物のそれぞれの介在物について行う必要がある。これは、酸化物の粒径分布・硫化物の粒径分布はそれぞれ異なるものであり、別々に評価するべきであるからである。極値統計法は、比較的簡便であり、かつ信頼性が高い。 (C) It is possible to prevent cracks during cold forging by limiting the size of non-metallic inclusions (sulfide-based, oxide-based, nitride-based) existing in steel, particularly sulfide-based inclusions. .
Large inclusions present in the steel serve as starting points for cracking. For this reason, in order to perform stable mass production on an industrial scale, it is necessary to evaluate the distribution of inclusions in a wide area with respect to the component materials. Existence of large inclusions as the starting point of cracking can be estimated by the “extreme value statistical method”. The extreme value statistical method is to collect multiple specimens from a population, measure the size of the largest inclusion in each specimen by microscopy, and determine the square root of the area as the extreme probability. This is a method for predicting the particle size (√area) of the largest inclusion existing in a population or an arbitrary area (or volume) by plotting on paper. Specific means for applying the extreme value statistical method to the evaluation of non-metallic inclusions in steel are described in, for example, non-patent literature; influence of metal fatigue micro defects and inclusions, Takayoshi Murakami, etc. It can be carried out according to the method. In this embodiment, it is as follows. (I) The area of one visual field (inspection standard area: S 0 ) is, for example, 10 mm × 10 mm, and optical microscope observation of 30 visual fields is performed for each specimen so that the area S 0 does not overlap. (Ii) The particle size of the maximum inclusion existing in each of the 30 visual fields is measured, and the square root (√area) of the area is plotted on the extreme probability paper. (Iii) Estimating the particle size (√area) of the maximum inclusion by setting the predicted area S to 30000 mm 2 .
In addition, it is necessary to measure the inclusions for each inclusion of oxide and sulfide. This is because the particle size distribution of oxides and the particle size distribution of sulfides are different and should be evaluated separately. The extreme value statistical method is relatively simple and highly reliable.
（ｄ）硫化物系介在物については、その存在頻度が大きい。そのため、冷間鍛造時の割れの発生を防止するためには、極値統計法で推定される最大サイズに加えて、ある大きさ以上の硫化物系介在物の単位面積あたりの数（個数密度）についても制限する必要がある。 (D) The presence of sulfide inclusions is large. Therefore, in order to prevent the occurrence of cracks during cold forging, in addition to the maximum size estimated by the extreme value statistical method, the number of sulfide inclusions per unit area of a certain size (number density) ) Must also be restricted.
　以下、本発明の一実施形態に係る肌焼用鋼材（本実施形態に係る肌焼用鋼材と言う場合がある。）及び本発明の一実施形態に係る肌焼鋼部品（本実施形態に係る肌焼鋼部品と言う場合がある。）について詳細に説明する。まず、本実施形態に係る肌焼用鋼材の成分の限定理由について説明する。成分は、表層部の浸炭による炭素量の増加の影響を受けない芯部の成分を指す。成分の含有量の％は質量％を意味する。 Hereinafter, a case hardening steel material according to an embodiment of the present invention (sometimes referred to as a case hardening steel material according to the embodiment) and a case hardening steel component according to an embodiment of the invention (according to the embodiment). A case hardened steel part may be described in detail. First, the reasons for limiting the components of the case-hardening steel according to this embodiment will be described. The component refers to a core component that is not affected by an increase in the amount of carbon due to carburization of the surface layer portion. % Of content of a component means the mass%.
（Ｃ：０．０５～０．３０％）
　Ｃは浸炭焼入れ・焼戻し後の、部品の芯部の強度を得るために必須の元素である。また、Ｃ含有量は、芯部の硬さを決定し、浸炭層の有効硬化層深さにも影響する。そこで、本実施形態ではＣ含有量の下限を０．０５％とする。しかし、Ｃ含有量が多すぎると靭性が低下する。そのため、Ｃ含有量の上限を０．３０％とする。より望ましいＣ含有量は、０．１０～０．２５％である。 (C: 0.05-0.30%)
C is an essential element for obtaining the strength of the core of the part after carburizing and tempering. Moreover, C content determines the hardness of a core part and also affects the effective hardened layer depth of a carburized layer. Therefore, in this embodiment, the lower limit of the C content is set to 0.05%. However, when there is too much C content, toughness will fall. Therefore, the upper limit of the C content is set to 0.30%. A more desirable C content is 0.10 to 0.25%.
（Ｓｉ：０．４０～１．５％）
　Ｓｉは浸炭層の焼戻し軟化抵抗を向上させるのに有効な元素である。そのため、Ｓｉ含有量の下限を０．４０％とする。しかし、Ｓｉ含有量が多すぎると球状化焼鈍後の硬さが上昇し、冷間鍛造性が低下する。そのため、Ｓｉ含有量の上限を１．５％とする。望ましいＳｉ含有量は、０．４５～１．０％である。コストの増加を抑えて焼戻し軟化抵抗を向上させる場合、Ｓｉ含有量の下限を０．５５％とすることがより望ましい。 (Si: 0.40 to 1.5%)
Si is an element effective for improving the temper softening resistance of the carburized layer. Therefore, the lower limit for the Si content is 0.40%. However, when there is too much Si content, the hardness after spheroidizing annealing will rise and cold forgeability will fall. Therefore, the upper limit of Si content is 1.5%. A desirable Si content is 0.45 to 1.0%. In order to suppress the increase in cost and improve the temper softening resistance, it is more desirable that the lower limit of the Si content is 0.55%.
（Ｍｎ：０．２～１．０％）
　Ｍｎは鋼の焼入性を向上させるのに有効な元素である。また、Ｍｎは、鋼中のＳをＭｎＳとして固定することによって熱間延性を改善し、鋼の製造工程（連続鋳造、熱間圧延）におけるキズの発生を防止する。更に、ＭｎＳは切削性を向上させる働きを有する。これらの効果を得るため、Ｍｎ含有量の下限を０．２％とする。しかし、Ｍｎ含有量が多すぎると球状化焼鈍後の硬さが上昇し、冷間鍛造性が低下する。そのため、Ｍｎ含有量の上限を１．０％とする。望ましいＭｎ含有量は、０．４～０．７％である。 (Mn: 0.2 to 1.0%)
Mn is an element effective for improving the hardenability of steel. Moreover, Mn improves hot ductility by fixing S in steel as MnS, and prevents generation of scratches in the steel production process (continuous casting, hot rolling). Furthermore, MnS has a function of improving machinability. In order to obtain these effects, the lower limit of the Mn content is set to 0.2%. However, when there is too much Mn content, the hardness after spheroidizing annealing will rise and cold forgeability will fall. Therefore, the upper limit of the Mn content is 1.0%. A desirable Mn content is 0.4 to 0.7%.
（Ｓ：０．００１～０．０５０％）
　Ｓは鋼中でＭｎＳを形成して切削性を向上させる効果がある。この効果を得るため、Ｓ含有量の下限を０．００１％とする。しかし、Ｓ含有量が多すぎるとＭｎＳ等の、いわゆる硫化物系介在物の量が多くなり、またそのサイズも粗大化する。後述のように、粗大な硫化物系介在物が数多く存在する場合には、冷間鍛造時にその粗大な硫化物系介在物が割れの起点となる。そのため、Ｓ含有量の上限を０．０５０％とする。望ましいＳ含有量は、０．００５～０．０２０％である。 (S: 0.001 to 0.050%)
S has the effect of improving the machinability by forming MnS in steel. In order to obtain this effect, the lower limit of the S content is set to 0.001%. However, when the S content is too large, the amount of so-called sulfide inclusions such as MnS increases, and the size thereof becomes coarse. As will be described later, when there are a large number of coarse sulfide inclusions, the coarse sulfide inclusions become the starting point of cracking during cold forging. Therefore, the upper limit of the S content is 0.050%. A desirable S content is 0.005 to 0.020%.
（Ｃｒ：１．０～２．０％）
　Ｃｒは焼入性を向上させるだけでなく、焼戻し軟化抵抗を向上させるのに有効な元素である。加えて、Ｃｒは比較的含有量が多くても球状化焼鈍後の硬さの上昇への影響が少ないという特徴がある。そのため、Ｃｒ含有量の下限を１．０％とする。しかし、Ｃｒ含有量が２．０％を超えると焼戻し軟化抵抗の向上効果は飽和するため、Ｃｒ含有量の上限を２．０％とする。望ましいＣｒ含有量は、１．３～１．６％である。 (Cr: 1.0-2.0%)
Cr is an element effective not only for improving hardenability but also for improving temper softening resistance. In addition, Cr has a characteristic that even if its content is relatively large, there is little influence on the increase in hardness after spheroidizing annealing. Therefore, the lower limit of the Cr content is 1.0%. However, if the Cr content exceeds 2.0%, the effect of improving the temper softening resistance is saturated, so the upper limit of the Cr content is set to 2.0%. A desirable Cr content is 1.3 to 1.6%.
（Ｍｏ：０．０２～０．８％）
　Ｍｏは焼入性を向上させるのに有効な元素である。Ｓｉ、Ｍｎ、Ｃｒは浸炭加熱時に鋼表層部において選択酸化されることによって表層部の焼入性を低下させる場合がある。そのような場合、焼入れ時に不完全焼入れ層が形成され、曲げ疲労強度、ピッチング強度低下の要因となる。一方、Ｍｏは上記元素よりも酸化傾向が低いため、表層部の不完全焼入れ層の低減に有効な元素である。この効果を得るため、Ｍｏ含有量の下限を０．０２％とする。しかし、Ｍｏ含有量が多すぎると球状化焼鈍後の硬さが上昇し、冷間鍛造性が低下するため、Ｍｏ含有量の上限を０．８％とする。望ましいＭｏ含有量は、０．０５～０．５％である。 (Mo: 0.02-0.8%)
Mo is an element effective for improving hardenability. Si, Mn, and Cr may be selectively oxidized in the steel surface layer during carburizing and heating, thereby reducing the hardenability of the surface layer. In such a case, an incompletely quenched layer is formed at the time of quenching, which causes a decrease in bending fatigue strength and pitching strength. On the other hand, since Mo has a lower oxidation tendency than the above elements, it is an effective element for reducing the incompletely hardened layer in the surface layer portion. In order to obtain this effect, the lower limit of the Mo content is 0.02%. However, if the Mo content is too large, the hardness after spheroidizing annealing increases and the cold forgeability decreases, so the upper limit of the Mo content is set to 0.8%. A desirable Mo content is 0.05 to 0.5%.
（Ａｌ：０．００１～０．２０％）
　Ａｌは鋼中で微細な窒化物を形成することによってオーステナイト結晶粒を微細化する効果がある。この効果を得るため、Ａｌ含有量の下限を０．００１％とする。しかし、Ａｌ含有量が０．２０％を超えるとその効果が飽和する。そのため、Ａｌ含有量の上限を０．２０％とする。望ましいＡｌ含有量は、０．０１５～０．０５０％である。 (Al: 0.001 to 0.20%)
Al has the effect of refining austenite crystal grains by forming fine nitrides in the steel. In order to obtain this effect, the lower limit of the Al content is set to 0.001%. However, when the Al content exceeds 0.20%, the effect is saturated. Therefore, the upper limit of the Al content is 0.20%. A desirable Al content is 0.015 to 0.050%.
（Ｎ：０．００３～０．０３％）
　Ｎは鋼中でＡｌあるいはＮｂ、Ｖと窒化物を形成することによってオーステナイト結晶粒を微細化する効果を有する。この効果を得るため、Ｎ含有量の下限を０．００３％とする。しかし、Ｎ含有量が過剰になると鋼の熱間延性が低下し、鋼の製造工程（連続鋳造、熱間圧延）におけるキズの発生が顕著になる。そのため、Ｎ含有量の上限を０．０３％とする。望ましいＮ含有量は、０．００７～０．０２％である。 (N: 0.003-0.03%)
N has the effect of refining austenite crystal grains by forming Al or Nb, V and nitride in steel. In order to obtain this effect, the lower limit of the N content is set to 0.003%. However, when the N content is excessive, the hot ductility of the steel is lowered, and the occurrence of scratches in the steel production process (continuous casting, hot rolling) becomes significant. Therefore, the upper limit of N content is 0.03%. A desirable N content is 0.007 to 0.02%.
（Ｐ：０．０３０％以下）
　Ｐは不純物元素であり、鋼の靭性を低下させる元素である。そのため、Ｐ含有量は０．０３０％以下に制限する。望ましくは、０．０２０％以下に制限する。 (P: 0.030% or less)
P is an impurity element and is an element that lowers the toughness of steel. Therefore, the P content is limited to 0.030% or less. Desirably, it is limited to 0.020% or less.
（Ｏ：０．００２０％以下）
　Ｏは不純物元素であり、Ａｌ、Ｓｉ等と酸化物を形成する。Ｏ含有量が増加すると、いわゆる酸化物系介在物の量が多くなり、またそのサイズも粗大になる。後述するように、粗大な酸化物系介在物が存在する場合にはそれが冷間鍛造時の割れの起点となる。そのため、Ｏ含有量を０．００２０％以下に制限する。望ましくは、Ｏ含有量を０．００１５％以下、より望ましくは、０．０００５％以下に制限する。 (O: 0.0020% or less)
O is an impurity element and forms an oxide with Al, Si, or the like. As the O content increases, the amount of so-called oxide inclusions increases and the size becomes coarse. As will be described later, when coarse oxide inclusions are present, this becomes the starting point of cracking during cold forging. Therefore, the O content is limited to 0.0020% or less. Desirably, the O content is limited to 0.0015% or less, and more desirably 0.0005% or less.
（Ｔｉ：０．００５％以下）
　Ｔｉは、本実施形態においては不可避的に混入し、ＴｉＮのような窒化物を形成する元素である。Ｔｉの量が増加すると、いわゆる窒化物系介在物の量が多くなり、またそのサイズも粗大になる。粗大な窒化物系介在物が存在する場合にはそれが冷間鍛造時の割れの起点となる。そのため、Ｔｉ含有量を０．００５％以下に制限する。望ましくは、Ｔｉ含有量を０．００３％以下に制限する。 (Ti: 0.005% or less)
Ti is an element which is inevitably mixed in this embodiment and forms a nitride such as TiN. As the amount of Ti increases, the amount of so-called nitride inclusions increases, and the size becomes coarse. If coarse nitride inclusions are present, this becomes the starting point of cracking during cold forging. Therefore, the Ti content is limited to 0.005% or less. Desirably, the Ti content is limited to 0.003% or less.
　本実施形態に係る肌焼用鋼材は、上述の化学成分を有することを基本とするが、さらに以下の成分を含有していてもよい。以下の元素は必ずしも含有させる必要はない。そのため、含有量の下限を特に制限する必要はなく、それらの下限は０％である。 The case-hardening steel according to the present embodiment is based on having the above-described chemical components, but may further contain the following components. The following elements are not necessarily contained. Therefore, there is no need to particularly limit the lower limit of the content, and the lower limit thereof is 0%.
（Ｃｕ：０．２％以下）
　ＣｕはＭｏと同様に焼入性を向上させるのに有効な元素である。また、Ｃｕは酸化傾向が低い元素であり、表層部の不完全焼入れ層を低減するために有効な元素である。これらの効果を得る場合、Ｃｕ含有量の下限を０．００１％とすることが望ましい。しかし、Ｃｕ含有量が多すぎると鋼の熱間延性が低下し、鋼の製造工程（連続鋳造、熱間圧延）におけるキズの発生が顕著になる。そのため、Ｃｕ含有量の上限を０．２％とする。なお、Ｃｕを含有させる場合にはＣｕ含有量の１／２程度のＮｉを同時に含有させると、熱間延性の低下が軽減される。より望ましいＣｕ含有量は、０．０５～０．１５％である。 (Cu: 0.2% or less)
Cu is an element effective for improving hardenability like Mo. Cu is an element having a low oxidation tendency, and is an effective element for reducing the incompletely hardened layer in the surface layer portion. When obtaining these effects, it is desirable that the lower limit of the Cu content be 0.001%. However, when there is too much Cu content, the hot ductility of steel will fall and the generation | occurrence | production of the damage | wound in the manufacturing process (continuous casting, hot rolling) of steel will become remarkable. Therefore, the upper limit of Cu content is 0.2%. In addition, in the case of containing Cu, when Ni of about ½ of the Cu content is simultaneously contained, the reduction in hot ductility is reduced. A more desirable Cu content is 0.05 to 0.15%.
（Ｎｉ：１．５％以下）
　ＮｉはＭｏ、Ｃｕと同様に焼入性を向上するのに有効な元素である。また、Ｎｉは酸化傾向が低い元素であり、表層部の不完全焼入れ層を低減するために有効な元素である。これらの効果を得る場合、Ｎｉ含有量の下限を０．００１％とすることが望ましい。しかし、Ｎｉは、コストへの影響が大きい元素であるため、Ｎｉ含有量の上限を１．５％とする。より望ましいＮｉ含有量は、０．０５～１．０％である。 (Ni: 1.5% or less)
Ni is an element effective for improving hardenability like Mo and Cu. Ni is an element having a low oxidation tendency and is an effective element for reducing the incompletely hardened layer in the surface layer portion. When obtaining these effects, it is desirable that the lower limit of the Ni content be 0.001%. However, since Ni is an element having a large influence on the cost, the upper limit of the Ni content is set to 1.5%. A more desirable Ni content is 0.05 to 1.0%.
（Ｎｂ：０．１０％以下）
　Ｎｂは鋼中で微細な炭化物、窒化物を形成し、オーステナイト結晶粒を微細化する効果を有する。この効果を得る場合、Ｎｂ含有量の下限を０．００１％とすることが望ましい。特に、冷間鍛造後に焼準や焼鈍を行わない場合、あるいは浸炭温度が９３０℃よりも高温の場合などにはオーステナイト結晶粒の粗大化が起こりやすいため、粗大化の防止のためにＮｂ炭窒化物の量を増加させることが有効である。そのため、Ｎｂ含有量の下限を０．０１５％とすることがより望ましい。しかし、Ｎｂ含有量が０．１０％を超えるとその効果が飽和する。そのため、Ｎｂ含有量の上限を０．１０％とする。望ましいＮｂ含有量の上限は、０．０５０％である。 (Nb: 0.10% or less)
Nb has the effect of forming fine carbides and nitrides in the steel and miniaturizing the austenite crystal grains. When obtaining this effect, the lower limit of the Nb content is preferably set to 0.001%. In particular, when normalization or annealing is not performed after cold forging, or when the carburizing temperature is higher than 930 ° C., austenite crystal grains are likely to be coarsened. Therefore, Nb carbonitride is used to prevent coarsening. It is effective to increase the amount of objects. Therefore, the lower limit of the Nb content is more preferably 0.015%. However, when the Nb content exceeds 0.10%, the effect is saturated. Therefore, the upper limit of Nb content is 0.10%. A desirable upper limit of the Nb content is 0.050%.
（Ｖ：０．２０％以下）
　Ｖは鋼中で微細な炭化物、窒化物を形成し、オーステナイト結晶粒を微細化する効果を有する。この効果を得る場合、Ｖ含有量の下限を０．０１％とすることが望ましい。しかし、Ｖ含有量が０．２０％を超えるとその効果が飽和する。そのため、Ｖ含有量の上限を０．２０％とする。より望ましいＶ含有量は、０．０５～０．１５％である。 (V: 0.20% or less)
V has the effect of forming fine carbides and nitrides in the steel and miniaturizing the austenite crystal grains. When obtaining this effect, it is desirable that the lower limit of the V content be 0.01%. However, when the V content exceeds 0.20%, the effect is saturated. Therefore, the upper limit of V content is 0.20%. A more desirable V content is 0.05 to 0.15%.
（Ｃａ：０．００５０％以下）
　Ｃａはいわゆる硫化物系介在物を微細化することによって、硫化物系介在物が冷間鍛造時に割れの起点となることを防止する効果を有する。この効果を得る場合、Ｃａ含有量の下限を０．０００１％とすることが望ましい。しかし、Ｃａ含有量が０．００５０％を超えるとその効果が飽和する。そのため、Ｃａ含有量の上限を０．００５０％とする。より望ましいＣａ含有量は０．０００５～０．００１５％である。 (Ca: 0.0050% or less)
Ca has the effect of preventing the sulfide inclusions from starting as cracks during cold forging by refining so-called sulfide inclusions. When obtaining this effect, it is desirable that the lower limit of the Ca content be 0.0001%. However, when the Ca content exceeds 0.0050%, the effect is saturated. Therefore, the upper limit of Ca content is 0.0050%. A more desirable Ca content is 0.0005 to 0.0015%.
（Ｍｇ：０．００５０％以下）
　Ｍｇはいわゆる硫化物系介在物を微細化することによって、硫化物系介在物が冷間鍛造時に割れの起点となることを防止する効果を有する。この効果を得る場合、Ｍｇ含有量の下限を０．０００１％とすることが望ましい。しかし、Ｍｇ含有量が０．００５０％を超えるとその効果が飽和する。そのため、Ｍｇ含有量の上限を０．００５０％とする。より望ましいＭｇ含有量は０．０００５～０．００１５％である。 (Mg: 0.0050% or less)
Mg refines so-called sulfide inclusions, thereby preventing the sulfide inclusions from starting as cracks during cold forging. When obtaining this effect, it is desirable that the lower limit of the Mg content be 0.0001%. However, when the Mg content exceeds 0.0050%, the effect is saturated. Therefore, the upper limit of the Mg content is set to 0.0050%. A more desirable Mg content is 0.0005 to 0.0015%.
（Ｓｂ：０．０５０％以下）
　Ｓｂは熱間圧延、球状化焼鈍時の脱炭を抑制する効果を有する。この効果を得る場合、Ｓｂ含有量の下限を０．０００１％とすることが望ましい。しかしＳｂ含有量が０．０５０％を超えるとその効果が飽和する。そのため、Ｓｂ含有量の上限を０．０５０％とする。より望ましいＳｂ含有量は、０．００１～０．０１０％である。 (Sb: 0.050% or less)
Sb has the effect of suppressing decarburization during hot rolling and spheroidizing annealing. When obtaining this effect, it is desirable that the lower limit of the Sb content is 0.0001%. However, when the Sb content exceeds 0.050%, the effect is saturated. Therefore, the upper limit of the Sb content is 0.050%. A more desirable Sb content is 0.001 to 0.010%.
　次に、本実施形態に係る肌焼用鋼材における、Ｓｉ、Ｍｎ、Ｃｒ及びＭｏの含有量について、冷間鍛造性及び焼戻し軟化抵抗の観点から説明する。 Next, the contents of Si, Mn, Cr and Mo in the case-hardening steel according to this embodiment will be described from the viewpoint of cold forgeability and temper softening resistance.
　本実施形態に係る肌焼用鋼材において、冷間鍛造性の観点からは、Ｓｉ、Ｍｎ、Ｃｒ及びＭｏの含有量を、下記（１）式を満足するように、すなわち、下記（１）式の左辺の値が２５以下になるように、制御する必要がある。なぜなら、球状化焼鈍材の冷間鍛造性（冷間鍛造前の硬さ）の限界は、Ｓｉ、Ｍｎ、Ｃｒ、Ｍｏの、それぞれの球状化焼鈍材の硬さへの影響度を考慮して決定しなければならないからである。下記（１）の左辺においてＳｉ、Ｍｎ、Ｃｒ及びＭｏの各元素の係数が異なるのは、元素によって冷間鍛造性（冷間鍛造前の硬さ）へ寄与する程度が異なるからである。
　なお、下記（１）式の左辺の望ましい範囲は２４．５以下、より望ましい範囲は２３以下である。
　１２×Ｓｉ（％）＋２５×Ｍｎ（％）＋Ｃｒ（％）＋２×Ｍｏ（％）≦２５　・・・（１） In the case-hardening steel according to the present embodiment, from the viewpoint of cold forgeability, the contents of Si, Mn, Cr and Mo are set so as to satisfy the following expression (1), that is, the following expression (1) It is necessary to control so that the value of the left-hand side becomes 25 or less. Because, the limit of the cold forgeability (hardness before cold forging) of the spheroidized annealed material, considering the degree of influence of Si, Mn, Cr, Mo on the hardness of each spheroidized annealed material This is because it must be decided. The reason why the coefficient of each element of Si, Mn, Cr and Mo is different in the left side of the following (1) is that the degree of contribution to cold forgeability (hardness before cold forging) varies depending on the element.
In addition, the desirable range of the left side of the following formula (1) is 24.5 or less, and a more desirable range is 23 or less.
12 × Si (%) + 25 × Mn (%) + Cr (%) + 2 × Mo (%) ≦ 25 (1)
　また、本実施形態に係る肌焼用鋼材において、焼戻し軟化抵抗の観点からは、Ｓｉ、Ｍｎ、Ｃｒの含有量を、下記（２）式の左辺の値が５０以上になるように制御する必要がある。ギヤやＣＶＴのようなパワートレイン部品は、使用中に他部品と接触する位置が接触によって局部的に発熱し、焼戻しを受けて軟化する。この軟化がピッチング疲労特性の劣化の支配因子である。従って、ピッチング疲労強度を向上させるためには浸炭層の焼戻し軟化抵抗である３００℃焼戻し硬さを向上させることが有効である。（２）式の左辺の値が５０以上であれば、ピッチング疲労強度が向上する。左辺の値は、望ましくは５３以上、より望ましくは５５以上である。
　３１×Ｓｉ（％）＋１５×Ｍｎ（％）＋２３×Ｃｒ（％）≧５０　・・・（２） Moreover, in the steel for case hardening according to the present embodiment, from the viewpoint of temper softening resistance, it is necessary to control the content of Si, Mn, and Cr so that the value on the left side of the following formula (2) is 50 or more. There is. In powertrain components such as gears and CVTs, the position of contact with other components during use generates heat locally due to contact, and is softened by tempering. This softening is a dominant factor in the deterioration of the pitching fatigue characteristics. Therefore, in order to improve the pitching fatigue strength, it is effective to improve the 300 ° C. tempering hardness which is the temper softening resistance of the carburized layer. If the value of the left side of the formula (2) is 50 or more, the pitching fatigue strength is improved. The value on the left side is desirably 53 or more, more desirably 55 or more.
31 × Si (%) + 15 × Mn (%) + 23 × Cr (%) ≧ 50 (2)
　次に、本実施形態に係る肌焼用鋼材における、硫化物系介在物の大きさ、及び数について説明する。 Next, the size and number of sulfide inclusions in the case-hardening steel according to this embodiment will be described.
　本実施形態において硫化物系介在物とは、Ｓを含有する介在物であって、例えばＭｎＳ、ＣａＳ、ＭｇＳ、（Ｍｎ，Ｃａ，Ｍｇ）Ｓ、ＴｉＳ、Ｔｉ（Ｃ，Ｓ）、ＦｅＳ等を指す。
　本実施形態に係る肌焼用鋼材においては、極値統計法を用いた介在物評価を行った場合に、予測面積Ｓ＝３００００ｍｍ２中に存在する最大の硫化物系介在物径である（√ａｒｅａ）Ｓの予測値が４９μｍ以下であり、かつ２０μｍを超える長さであると共に２μｍを超える厚みを有する硫化物系介在物が１ｍｍ２あたり２００個以下である必要がある。 In the present embodiment, the sulfide-based inclusion is an inclusion containing S, and includes, for example, MnS, CaS, MgS, (Mn, Ca, Mg) S, TiS, Ti (C, S), FeS, and the like. Point to.
In the case-hardening steel according to this embodiment, when inclusion evaluation using the extreme value statistical method is performed, it is the maximum sulfide inclusion diameter existing in the predicted area S = 30000 mm 2 (√ area) It is necessary that the predicted value of S is 49 μm or less, and the number of sulfide inclusions having a length exceeding 20 μm and a thickness exceeding 2 μm is 200 or less per 1 mm 2 .
　冷間鍛造によって大きな塑性加工を受けた場合、大きな硫化物系介在物が鋼中に存在していると、介在物とマトリックスとの界面が割れの起点となり、最終的に大きな冷鍛割れに成長する場合がある。しかしながら、硫化物系介在物の径が４９μｍ以下であれば、割れの起点とならず無害である。一方でが、４９μｍを超える硫化物系介在物は割れの起点となる。そのため、（√ａｒｅａ）Ｓの上限を４９μｍとした。 When subjected to large plastic working by cold forging, if large sulfide inclusions are present in the steel, the interface between the inclusions and the matrix becomes the starting point of cracking and eventually grows into a large cold forging crack. There is a case. However, if the diameter of the sulfide inclusion is 49 μm or less, it does not become a starting point of cracking and is harmless. On the other hand, sulfide inclusions exceeding 49 μm serve as starting points for cracking. Therefore, the upper limit of (√area) S is set to 49 μm.
　硫化物系介在物は、後述する酸化物系介在物や窒化物系介在物と比べて量が多いため存在頻度が高い。また、硫化物系介在物は熱間加工によって細長く伸長するため、冷鍛割れへの影響が大きい。例えば、長さ２０μｍ、厚み２μｍの硫化物系介在物の（√ａｒｅａ）Ｓは６．３μｍであり、上記で制限した最大硫化物系介在物径（４９μｍ）より小さいが、これ以上の長さ・厚みを持つ硫化物系介在物が１ｍｍ２あたり２００個を超えて存在している場合は、実質大きな介在物が存在していることと変わらず、冷間加工時に割れの発生が頻発する。従って、硫化物系介在物については、介在物径のみならず、ある大きさ以上の介在物の数についても規定する必要がある。すなわち、２０μｍを超える長さ及び２μｍを超える厚み、を有する硫化物系介在物を１ｍｍ２あたり２００個以下にする必要がある。硫化物系介在物の長さ及び厚み、または、個数が上記範囲を超えると、割れが発生しやすい。硫化物系介在物のサイズを計測する際には、長径を長さ、短径を厚みとする。
　長さが２０μｍ以下のＭｎＳは、その厚みが小さい範囲ではこの制限には当てはまらないが、仮にその厚みが極めて大きい場合、例えば２０μｍを超えるものが存在している場合を考えると、厚みが長さ、長さが厚みになるので、この制限に当てはまることになる。 
　なお、硫化物系介在物、酸化物系介在物については、小さい方が望ましいので、その粒径の下限は０μｍであある。また、２０μｍを超える長さ及び２μｍを超える厚みを有する硫化物系介在物は、少ない方が望ましいので、その個数密度の下限は０個／ｍｍ２である。 Since sulfide inclusions are larger in amount than oxide inclusions and nitride inclusions, which will be described later, the existence frequency is high. Further, since sulfide-based inclusions are elongated by hot working, the influence on cold forging cracks is large. For example, (√area) S of a sulfide type inclusion having a length of 20 μm and a thickness of 2 μm is 6.3 μm, which is smaller than the maximum sulfide type inclusion diameter (49 μm) limited as described above, but longer than this. -When there are more than 200 sulfide inclusions with a thickness per 1 mm 2 , cracks frequently occur during cold working, as is the case with substantially large inclusions. Therefore, for sulfide inclusions, it is necessary to define not only the diameter of inclusions but also the number of inclusions having a certain size or more. That is, the number of sulfide inclusions having a length exceeding 20 μm and a thickness exceeding 2 μm needs to be 200 or less per 1 mm 2 . If the length, thickness, or number of sulfide inclusions exceeds the above range, cracks are likely to occur. When measuring the size of sulfide inclusions, the major axis is the length and the minor axis is the thickness.
MnS having a length of 20 μm or less does not apply to this limitation in the range where the thickness is small. However, if the thickness is extremely large, for example, when the thickness exceeds 20 μm, the thickness is long. Because the length becomes the thickness, this restriction is applied.
In addition, about the sulfide type inclusion and oxide type inclusion, since the smaller one is desirable, the lower limit of the particle size is 0 μm. Moreover, since it is desirable that the number of sulfide inclusions having a length exceeding 20 μm and a thickness exceeding 2 μm is desirable, the lower limit of the number density is 0 / mm 2 .
　次に、本実施形態に係る肌焼用鋼材における、酸化物系介在物の大きさについて説明する。 Next, the size of oxide inclusions in the case-hardening steel according to this embodiment will be described.
　本発明で言う酸化物系介在物とは、Ｏを含有する介在物であって、例えばＡｌ２Ｏ３、ＣａＯ、Ｃｒ２Ｏ３、ＭｎＯ、ＮｂＯ、ＳｉＯ２、ＭｇＯ、ＺｒＯ２、ＴｉｘＯｙ、Ｎｂ２Ｏ５、ＦｅＯｘ、あるいはこれらの複合したもの等を指す。 The oxide inclusions referred to in the present invention are inclusions containing O, such as Al 2 O 3 , CaO, Cr 2 O 3 , MnO, NbO, SiO 2 , MgO, ZrO 2 , and Ti x O. y , Nb 2 O 5 , FeO x , or a composite thereof.
　本実施形態に係る肌焼用鋼材においては、極値統計法を用いた介在物評価において、予測面積Ｓ＝３００００ｍｍ２中に存在する最大の酸化物系介在物径（√ａｒｅａ）Ｏｘの予測値が８０μｍ以下であることが好ましい。 In the steel for case hardening according to the present embodiment, in the inclusion evaluation using the extreme value statistical method, the predicted value of the maximum oxide inclusion diameter (√area) Ox existing in the predicted area S = 30000 mm 2. Is preferably 80 μm or less.
　これは、冷間鍛造によって大きな塑性加工を受けた場合、大きな酸化物系介在物が鋼中に存在すると、介在物とマトリックスとの界面が割れの起点となり、最終的には大きな冷鍛割れに成長するためである。（√ａｒｅａ）Ｏｘが８０μｍ以下の酸化物系介在物は無害であるが、８０μｍを超える酸化物系介在物は割れの起点となる。従って、酸化物系介在物のサイズを上記のように規定する必要がある。 This is because if a large oxide inclusion is present in the steel when subjected to a large plastic working by cold forging, the interface between the inclusion and the matrix becomes the starting point of cracking, and eventually a large cold forging crack occurs. It is to grow up. (√area) Oxide inclusions with an Ox of 80 μm or less are harmless, but oxide inclusions with an Ox of more than 80 μm serve as a starting point for cracking. Therefore, it is necessary to define the size of the oxide inclusions as described above.
　本実施形態に係る肌焼鋼部品は、上記の肌焼用鋼材に対して、浸炭焼入れ焼戻し、または浸炭窒化焼入れ焼戻しの処理を施すことによって得られる。すなわち、肌焼鋼部品は、肌焼用鋼材からなる。そのため、本実施形態に係る肌焼鋼部品は、上述した本実施形態に係る肌焼用鋼材の化学成分、介在物と、実質的に同一の化学成分、介在物を有している。従って、肌焼鋼部品の化学成分、介在物を制御するためには、肌焼用鋼材において、所定の化学成分、介在物を有するように制御すればよい。
　ただし、肌焼鋼部品は、浸炭焼入れ焼戻し、または浸炭窒化焼入れ焼戻し処理を経るので、表面硬化層を有しており、この点が、肌焼用鋼材とは異なる。 The case-hardened steel component according to the present embodiment is obtained by subjecting the above-described case-hardening steel material to carburizing / quenching / tempering or carbonitriding / quenching / quenching / tempering. That is, the case-hardened steel part is made of steel for case-hardening. Therefore, the case-hardened steel part according to the present embodiment has substantially the same chemical components and inclusions as the chemical components and inclusions of the case-hardening steel material according to the present embodiment described above. Therefore, in order to control the chemical components and inclusions of the case hardening steel part, the case hardening steel material may be controlled to have predetermined chemical components and inclusions.
However, the case-hardened steel part has a surface hardened layer because it undergoes carburizing / quenching / tempering or carbonitriding / quenching / tempering, and this is different from the case-hardening steel.
　本実施形態に係る肌焼用鋼材及び、肌焼鋼部品の好ましい製造条件について説明する。 Favorable manufacturing conditions for the case hardening steel material and the case hardening steel component according to the present embodiment will be described.
　本実施形態では、二次精錬において、ＲＨ真空脱ガス処理を、総処理時間が３０分以上、そのうち、１Ｔｏｒｒ以下の減圧雰囲気での処理時間が１５分以上となる条件で行う（精錬工程）。上述の条件で精錬を行うことで、酸化物系介在物の大きさ、及び数を所定の範囲に制御することができる。また、この精錬工程では、化学成分が上述した好ましい範囲となるように調整する。 In the present embodiment, in the secondary refining, the RH vacuum degassing process is performed under the condition that the total processing time is 30 minutes or more, of which the processing time in a reduced pressure atmosphere of 1 Torr or less is 15 minutes or more (refining process). By performing refining under the above-described conditions, the size and number of oxide inclusions can be controlled within a predetermined range. Moreover, in this refining process, it adjusts so that a chemical component may become the preferable range mentioned above.
　続いて、精錬工程において化学成分を調整した溶鋼を連続鋳造により鋳片にする（鋳造工程）。連続鋳造によって鋳片を製造するにあたって、鋳造速度を０．４５ｍ／ｍｉｎ以上にすることが望ましい。鋳造速度を０．４５ｍ／ｍｉｎ以上とすることで、硫化物系介在物の大きさ及び数を、上述の範囲に制御することができる。鋳造速度が０．４５ｍ／ｍｉｎ未満の場合には鋼の凝固時に粗大な硫化物系介在物が晶析出する。望ましい鋳造速度は０．５０～１．５ｍ／ｍｉｎである。
　さらに、鋳造に際し、鋳片厚み方向１／４部における液相線温度から固相線温度までの冷却速度が５～２００℃／ｍｉｎとなるように鋳片を冷却することが望ましい。冷却速度が５℃／ｍｉｎ未満では、硫化物系介在物が粗大に析出するだけでなく、連続鋳造の生産性も悪化するため望ましくない。また、冷却速度が２００℃／ｍｉｎ超であると、連続鋳造時に鋳片に割れが生じる可能性が高まるため望ましくない。
　冷却条件と２次デンドライトアーム間隔とには、相関がある。そのため、２次デンドライトアーム間隔を測定することで、上記冷却速度を算出することができる。具体的には、冷却速度は凝固後の鋳片厚み方向凝固組織の２次デンドライトアームの間隔を用いて、下記（３）式により計算で求めることができる。
　Ｒｃ＝（λ２／７７０）（－１／０．４１）・・・（３）
　Ｒｃ：冷却速度（℃／ｍｉｎ）、λ２：２次デンドライトアームの間隔（μｍ） Then, the molten steel which adjusted the chemical component in the refining process is made into a slab by continuous casting (casting process). In producing a slab by continuous casting, it is desirable that the casting speed be 0.45 m / min or more. By setting the casting speed to 0.45 m / min or more, the size and number of sulfide inclusions can be controlled within the above range. When the casting speed is less than 0.45 m / min, coarse sulfide inclusions crystallize during solidification of the steel. A desirable casting speed is 0.50 to 1.5 m / min.
Further, during casting, it is desirable to cool the slab so that the cooling rate from the liquidus temperature to the solidus temperature at ¼ part in the slab thickness direction is 5 to 200 ° C./min. When the cooling rate is less than 5 ° C./min, not only the sulfide inclusions are coarsely precipitated but also the productivity of continuous casting deteriorates, which is not desirable. Further, if the cooling rate is more than 200 ° C./min, the possibility of cracking in the slab increases during continuous casting, which is not desirable.
There is a correlation between the cooling conditions and the secondary dendrite arm spacing. Therefore, the cooling rate can be calculated by measuring the secondary dendrite arm interval. Specifically, the cooling rate can be calculated by the following equation (3) using the interval between the secondary dendrite arms of the solidified structure in the slab thickness direction after solidification.
Rc = (λ2 / 770) (−1 / 0.41) (3)
Rc: Cooling rate (° C./min), λ2: Secondary dendrite arm spacing (μm)
　上記鋳造工程により得られた鋳片に対し、分塊圧延を行い、鋼片を得る（分塊圧延工程）。分塊圧延の際の加熱温度は、不可避的に生じた粗大な硫化物を一旦マトリックスに固溶させるため、１２４０℃以上にすることが望ましい。さらに望ましい加熱温度は、１２６０℃以上である。分塊圧延の減面率は、硫化物系介在物の厚みを減少させるため、４０％以上にする必要がある。望ましい減面率は４５％以上である。また、分塊圧延中、又は分塊圧延後の冷却速度が遅い場合には、固溶したＭｎＳが再び粗大な硫化物として析出することから、分塊圧延及びその後の冷却過程における１２４０～１０００℃までの冷却速度を０．７℃／ｓ以上にする必要がある。さらに望ましい冷却速度は、１．５℃／ｓ以上である。この冷却速度は、表面温度の実測値から求められる冷却速度である。 分 The slab obtained by the above casting process is subjected to ingot rolling to obtain a steel piece (ingot rolling process). The heating temperature at the time of the block rolling is desirably 1240 ° C. or higher in order to temporarily dissolve coarse sulfides inevitably generated in the matrix. A more desirable heating temperature is 1260 ° C. or higher. In order to reduce the thickness of the sulfide inclusions, it is necessary to make the area reduction rate of the block rolling 40% or more. A desirable area reduction rate is 45% or more. In addition, when the cooling rate is low during the partial rolling or after the partial rolling, the dissolved MnS precipitates again as coarse sulfides. Therefore, 1240 to 1000 ° C. in the partial rolling and the subsequent cooling process. The cooling rate is required to be 0.7 ° C./s or more. A more desirable cooling rate is 1.5 ° C./s or more. This cooling rate is a cooling rate obtained from the measured value of the surface temperature.
　上記鋼片を肌焼用鋼材（棒鋼または線材）とするため、棒鋼圧延または線材圧延を行う。棒鋼圧延または線材圧延の際の加熱時には、ＭｎＳの成長、粗大化を防止するため、加熱温度を１２００℃以下にすることが望ましい。より望ましい加熱温度は、１０００～１１５０℃である。また、鋳片から棒鋼圧延または線材圧延完了までの総減面率（分塊圧延と、棒鋼圧延または線材圧延とのトータルの減面率）は６５％以上にする。総減面率が６５％未満の場合には硫化物系介在物の伸長に伴う厚みの減少が不十分となり、冷鍛割れの発生に対して有害な、大きな厚みを持つ硫化物系介在物の数を減らすことができない。総減面率の好適範囲は９０％以上である。 棒 In order to use the steel slab as a case-hardening steel (steel or wire), steel bar rolling or wire rolling is performed. At the time of heating at the time of steel bar rolling or wire rod rolling, it is desirable to set the heating temperature to 1200 ° C. or less in order to prevent MnS growth and coarsening. A more desirable heating temperature is 1000 to 1150 ° C. Further, the total area reduction ratio from the slab to the completion of the steel bar rolling or wire rod rolling (total area reduction ratio between the block rolling and the bar steel rolling or wire rod rolling) is set to 65% or more. If the total area reduction is less than 65%, the thickness reduction due to the extension of the sulfide inclusions becomes insufficient, and the number of sulfide inclusions with a large thickness, which is harmful to the occurrence of cold forging cracks. It cannot be reduced. A preferable range of the total area reduction rate is 90% or more.
　上記肌焼用鋼材に対し、さらに、浸炭焼入れ焼戻し、または、浸炭窒化焼入れ焼戻しの処理を施すことで、肌焼鋼部品が得られる。浸炭焼入れ焼戻し、浸炭窒化焼入れ焼戻しは、公知の方法で行えばよい。 The case-hardened steel part can be obtained by further subjecting the case-hardening steel material to carburizing / quenching / tempering or carbonitriding / quenching / tempering. Carburizing quenching and tempering and carbonitriding quenching and tempering may be performed by known methods.
　以下に、実施例により本発明を更に説明する。
　表１－１、表１－２に示す組成（化学成分）を有する転炉溶製鋼について、表２の条件でＲＨ真空脱ガス処理を行い、引き続いて表３の条件で連続鋳造を行い、その後必要に応じて均熱拡散処理を行い、分塊圧延工程を経て１６２ｍｍ角の圧延素材（鋼片）を得た。なお、表１－１、表１－２の残部は鉄及び不純物であり、空欄は、意図的に添加していないことを示す。 In the following, the present invention will be further explained by examples.
Converter molten steels having the compositions (chemical components) shown in Table 1-1 and Table 1-2 are subjected to RH vacuum degassing treatment under the conditions shown in Table 2, followed by continuous casting under the conditions shown in Table 3, and then A soaking diffusion treatment was performed as necessary, and a 162 mm square rolled material (steel slab) was obtained through a block rolling process. The remainder of Table 1-1 and Table 1-2 is iron and impurities, and the blank indicates that it is not intentionally added.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　次に表４に示す条件の熱間圧延によって加工を行って棒鋼形状とし、その後、一部の棒鋼については、図１の条件で球状化焼鈍処理（ＳＡ）を行った。また、一部の棒鋼についてはＳＡを行わない棒鋼に対して熱間鍛造（加熱温度：１２５０℃、据え込み率５０％）を行い、円盤状の鍛造素形材に成形し、その後鍛造素形材に対してＳＡを行った。その他、一部の棒鋼及び鍛造素形材に対してはＳＡを行わなかった。このようにして製造した棒鋼及び鍛造素形材を素材とし、種々の特性を評価した。
　また、その後、素材から直径１６ｍｍ、長さ２４ｍｍの円柱試験片を切削加工によって採取した。この円柱試験片に据え込み率５０％、歪速度１．０の条件で据込み冷間加工を行った。次いで浸炭を模擬するため、冷間加工を行った円柱試験片を９５０℃で５時間加熱保定した後、直ちに水冷して模擬浸炭後のオーステナイト組織をマルテンサイト組織の旧オーステナイト粒界として凍結した。次に模擬浸炭を行った試験片の圧延方向断面の旧オーステナイト粒組織を観察し、ＪＩＳ結晶粒度番号を測定した。粗大粒の定義をＪＩＳ　Ｇ　０５５１の結晶粒度番号で５番以下とし、断面内の全ての視野において一つでも粗大粒が発生しているものを粗大粒有りと判定した。 Next, it processed by the hot rolling of the conditions shown in Table 4, and it was set as the bar steel shape, and spheroidizing annealing processing (SA) was performed on the conditions of FIG. In addition, for some steel bars, hot forging (heating temperature: 1250 ° C., upsetting rate 50%) is performed on steel bars not subjected to SA, and formed into a disk-shaped forging material, and then forging SA was performed on the material. In addition, SA was not performed on some steel bars and forged blanks. Various properties were evaluated using the steel bar and the forged material thus produced as raw materials.
Thereafter, a cylindrical test piece having a diameter of 16 mm and a length of 24 mm was collected from the material by cutting. This cylindrical test piece was subjected to upsetting cold working under conditions of upsetting rate of 50% and strain rate of 1.0. Next, in order to simulate carburization, the cold-worked cylindrical specimen was heated and held at 950 ° C. for 5 hours, and immediately cooled with water to freeze the austenite structure after the simulated carburization as a prior austenite grain boundary of the martensite structure. Next, the old austenite grain structure of the cross section in the rolling direction of the test piece subjected to simulated carburizing was observed, and the JIS grain size number was measured. The definition of coarse grains was defined as JIS G 0551 crystal grain size number 5 or less, and any coarse grains that occurred in all fields of view in the cross section were determined to be coarse grains.
　本発明の肌焼用鋼材及び肌焼鋼部品はＳＡを行ってよいが必須ではない。実際に部品を製造する際に冷間加工を行わない場合、又はＳＡを行わなくても冷間加工が可能である場合にはＳＡを行わなくても良く、その場合は高強度鋼として使用することができる。 The case-hardened steel and case-hardened steel parts of the present invention may be subjected to SA, but are not essential. When cold working is not performed when actually manufacturing a part, or when cold working is possible without performing SA, SA may not be performed. In that case, it is used as high-strength steel. be able to.
　まず、棒鋼及び鍛造素形材の直径の１／４深さ位置のビッカース硬さ（測定荷重１０ｋｇｆ）を、ＪＩＳ　Ｚ　２２４４に準じて測定した。測定点数は一つの材料につき４点とし、平均値を求めた。硬さがＨＶ１５５以上のものは、冷間鍛造時の変形抵抗が大きくなり金型の寿命が顕著に低下するので、冷間鍛造性が劣ると判定した。 First, the Vickers hardness (measuring load 10 kgf) at a 1/4 depth position of the diameter of the steel bar and the forged material was measured according to JIS Z 2244. The number of measurement points was 4 for each material, and the average value was obtained. When the hardness is HV155 or more, the deformation resistance at the time of cold forging is increased and the life of the mold is remarkably reduced, so that the cold forgeability is judged to be inferior.
　棒鋼の直径の１／４近傍の位置、鍛造素形材の場合は鍛造素形材の直径の１／４近傍の位置において、光学顕微鏡で観察を行い、介在物測定を行った。予測面積Ｓ＝３００００ｍｍ２中に存在する最大の硫化物系介在物径（√ａｒｅａ）Ｓ、及び最大の酸化物系介在物径（√ａｒｅａ）Ｏｘの予測値は、１視野の面積（検査基準面積：Ｓ０）を１０ｍｍ×１０ｍｍとし、面積Ｓ０が重複しないように３０視野の光学顕微鏡観察を行い、３０視野のそれぞれに存在している最大介在物の径（√ａｒｅａ）の測定を行って極値確率紙にプロットを行い、予測面積Ｓを３００００ｍｍ２として最大介在物の径（√ａｒｅａ）を予測することによって決定した。介在物の測定は、酸化物（酸化物系介在物）、硫化物（硫化物系介在物）のそれぞれについて独立して評価を行った。 The inclusion was measured by observing with an optical microscope at a position in the vicinity of 1/4 of the diameter of the steel bar, and in the case of the forged raw material, at a position in the vicinity of 1/4 of the diameter of the forged raw material. Prediction value of the prediction area S = 30,000 mm maximum sulfide inclusions diameter present in 2 (} area) S, and a maximum of oxide inclusions diameter (} area) Ox is one visual field area (inspection standard Area: S 0 ) is set to 10 mm × 10 mm, 30 optical microscope observations are performed so that the area S 0 does not overlap, and the diameter (√area) of the maximum inclusion existing in each of the 30 visual fields is measured. Then, plotting was performed on extreme value probability paper, and the predicted area S was set to 30000 mm 2 to predict the maximum inclusion diameter (√area). Inclusions were measured independently for oxides (oxide inclusions) and sulfides (sulfide inclusions).
　上記の介在物の測定と同時に、各視野中の２０μｍの長さ及び２μｍを超える厚みを有する硫化物系介在物の個数を測定した。この個数を３０視野全て合計し、総測定面積（３０００ｍｍ２）で割ることにより、２０μｍを超える長さ及び２μｍを超える厚みを有する硫化物系介在物の面積１ｍｍ２中の存在個数を測定した。 Simultaneously with the measurement of the inclusions, the number of sulfide inclusions having a length of 20 μm and a thickness exceeding 2 μm in each field of view was measured. The total number of 30 fields of view was summed up and divided by the total measurement area (3000 mm 2 ) to measure the number of sulfide inclusions in an area of 1 mm 2 having a length exceeding 20 μm and a thickness exceeding 2 μm.
　次に、鋼材の冷間鍛造時の割れ発生に対する指標として、限界圧縮率を測定した。棒鋼及び鍛造素形材の長手方向に平行な向きから限界圧縮率測定用の試験片（φ６ｍｍ×９ｍｍ、切り欠き形状：３０°、深さ０．８ｍｍ、先端部の曲率半径０．１５ｍｍ）を作成した。限界圧縮率の測定は拘束ダイスを使用して１０ｍｍ／ｍｉｎのスピードで冷間圧縮を行い、切り欠き近傍に長さ０．５ｍｍ以上の微小割れが生じたときに圧縮を停止し、その時の圧縮率を算出し、これを割れ発生の圧縮率とした。この試験を１つの水準についてｎ＝１０行い、累積破損確率が５０％の圧縮率を求めることによって限界圧縮率とし、限界加工率の指標とした。ＪＩＳ－ＳＣｒ４２０のＳＡ材の限界圧縮率がおよそ６５％であるので、この値よりも明らかに高い値と見なせる、６８％以上の高い値を示すものは限界加工率が優れると判断し、逆に６８％未満のものは劣ると判定した。 Next, the critical compressibility was measured as an index for the occurrence of cracks during cold forging of steel. A test piece for measuring the critical compression ratio (φ6mm × 9mm, notch shape: 30 °, depth 0.8mm, radius of curvature of the tip 0.15mm) from the direction parallel to the longitudinal direction of the steel bar and the forged material Created. To measure the critical compression ratio, use a constraining die to perform cold compression at a speed of 10 mm / min, stop the compression when a microcrack with a length of 0.5 mm or more occurs near the notch, and compress at that time The rate was calculated, and this was taken as the compression rate at which cracking occurred. This test was performed for one level, n = 10, and a compression rate with a cumulative failure probability of 50% was obtained to obtain a critical compression rate, which was used as an index of the critical processing rate. Since the critical compression ratio of SA material of JIS-SCr420 is about 65%, it can be considered that the value is clearly higher than this value. Those less than 68% were judged to be inferior.
　次に、浸炭後の部品の耐ピッチング特性の指標である３００℃焼戻し硬さを測定した。３００℃焼戻し硬さを測定するため、まず素材の棒鋼（ＳＡ材、及びＳＡなし材）から、浸炭用の試験片（φ２０ｍｍ×３０ｍｍ）を採取した。その後、変成炉ガス方式によるガス浸炭を行った。ガス浸炭はカーボンポテンシャル０．８％で、雰囲気温度：９５０℃、保持時間：５時間　→　雰囲気温度：８５０℃、保持時間：０．５時間　→　１３０℃油焼入れ　→　焼戻し温度：１５０℃、保持時間：９０分となる条件で行った。その後、表層部の組織を調査するため、試験片の長手方向の中央部近傍を長手方向と垂直方向に切断し、断面の顕微鏡試料を作成した。組織観察のためこの試料に２％ナイタールで腐食を行い、浸炭層の表層部を顕微鏡によって観察した。浸炭層の表層部に生成している不完全焼入れ層（主にパーライト及び／またはベイナイトからなる非マルテンサイト組織が存在している層）の深さを測定した。不完全焼入れ層の深さが深い場合にはピッチング特性に悪影響を与えること、及びＪＩＳ－ＳＣｒ４２０の不完全焼入れ層の深さが２５μｍ程度であることから、不完全焼入れ層の深さが２５μｍよりも深いものはピッチング特性の向上が不十分であると判定した。
　また、３００℃焼戻し硬さを求めるため、更に、焼戻し温度：３００℃、保持時間：９０分の焼戻しを行った。その後、試験片の長手方向の中央部近傍を長手方向と直角方向に切断し、断面のビッカース硬さを測定した。硬さの測定位置は表面から５０μｍ深さの位置とし、測定荷重は３００ｇｆとした。また、１つの試験片について５個所を測定し、平均値を求めた。ＪＩＳ－ＳＣｒ４２０の３００℃焼戻し硬さがＨＶ６４０であるので、この値よりも明らかに高い値と見なせる、ＨＶ６７０以上の値を示すものはピッチング特性に優れ、ＨＶ６７０に満たないものはピッチング特性が不十分であると判定した。 Next, 300 degreeC tempering hardness which is a parameter | index of the anti-pitching characteristic of the parts after carburizing was measured. In order to measure the tempering hardness at 300 ° C., first, carburized test pieces (φ20 mm × 30 mm) were collected from the raw steel bars (SA material and SA-free material). After that, gas carburization was performed by the shift furnace gas method. Gas carburization has a carbon potential of 0.8%, ambient temperature: 950 ° C, retention time: 5 hours → ambient temperature: 850 ° C, retention time: 0.5 hour → 130 ° C oil quenching → tempering temperature: 150 ° C, retention time : Performed under conditions of 90 minutes. Then, in order to investigate the structure | tissue of a surface layer part, the center part vicinity of the longitudinal direction of a test piece was cut | disconnected in the vertical direction with the longitudinal direction, and the microscope sample of the cross section was created. For structural observation, this sample was corroded with 2% nital, and the surface layer of the carburized layer was observed with a microscope. The depth of an incompletely quenched layer (a layer in which a non-martensite structure mainly composed of pearlite and / or bainite exists) generated in the surface layer portion of the carburized layer was measured. When the depth of the incompletely hardened layer is deep, the pitching characteristics are adversely affected, and since the depth of the incompletely hardened layer of JIS-SCr420 is about 25 μm, the depth of the incompletely hardened layer is more than 25 μm. The deeper ones were judged to have insufficient improvement in pitching characteristics.
Moreover, in order to obtain | require 300 degreeC tempering hardness, tempering temperature: 300 degreeC and holding time: 90 minutes were further performed. Thereafter, the vicinity of the central portion in the longitudinal direction of the test piece was cut in a direction perpendicular to the longitudinal direction, and the Vickers hardness of the cross section was measured. The hardness measurement position was 50 μm deep from the surface, and the measurement load was 300 gf. Moreover, five places were measured about one test piece, and the average value was calculated | required. Since JIS-SCr420 has a tempered hardness of 300 ° C. of HV640, it can be regarded as a value that is clearly higher than this value. Those having a value of HV670 or higher are excellent in pitching characteristics, and those having less than HV670 have insufficient pitching characteristics. It was determined that
　表２に、ＲＨ条件の影響をまとめた。表２のＲＨ条件Ｎｏ．１－３では、ＲＨ真空脱ガス処理の総処理時間と１Ｔｏｒｒ以下の減圧雰囲気での処理時間とがいずれも望ましい範囲を外れていた。また、１－４は１Ｔｏｒｒ以下の減圧雰囲気での処理時間が望ましい範囲を外れていた。また、ＲＨ条件Ｎｏ．１－Ｂは、ＲＨ真空脱ガス処理の総処理時間が望ましい範囲を外れていた。これらの条件を採用した製造条件Ｎｏ．２０、２３、４２、ａ、ｂ、ｃ、ｄ、ｅ、ｆでは、溶鋼中の酸化物の浮上除去が不十分であり、棒鋼中に存在している酸化物系介在物が大きかった。また、その結果として限界圧縮率が劣っていた。これに対して、ＲＨ条件が適正であるＲＨ条件Ｎｏ．１－１、１－２、１－Ａを採用した製造Ｎｏ．１、９、２では酸化物系介在物が小さく、ＳＡ材の限界圧縮率も良好であった。 Table 2 summarizes the effects of RH conditions. In Table 2, the RH condition no. In 1-3, both the total processing time of the RH vacuum degassing processing and the processing time in a reduced pressure atmosphere of 1 Torr or less were outside the desirable range. Also, 1-4 was outside the desirable range of processing time in a reduced pressure atmosphere of 1 Torr or less. In addition, RH condition No. For 1-B, the total processing time of the RH vacuum degassing process was outside the desired range. Manufacturing conditions No. 1 using these conditions. In 20, 23, 42, a, b, c, d, e, and f, the floating removal of the oxide in the molten steel was insufficient, and the oxide inclusions present in the steel bar were large. As a result, the critical compression rate was inferior. On the other hand, the RH condition no. No. 1-1, 1-2, 1-A In 1, 9, and 2, the oxide inclusions were small, and the critical compression ratio of the SA material was good.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　表３に鋳造条件の影響をまとめた。表３の鋳造条件Ｎｏ．２－８は、鋳造速度が望ましい範囲から外れていた。また、鋳造条件Ｎｏ．２－９は鋳片厚み方向１／４部における液相線温度から固相線温度までの冷却速度が小さいため、棒鋼中に存在している硫化物系介在物が大きかった。その結果、鋳造条件Ｎｏ．２－８またはＮｏ．２－９を採用した製造Ｎｏ．６４、６５、６６、６７は、いずれも限界圧縮率が低下していた。これに対して、連続鋳造条件が適正である鋳造条件Ｎｏ．２－１～２－７を採用した製造Ｎｏ．１、２、５３～５８では、硫化物系介在物が小さく、限界圧縮率も良好であった。 Table 3 summarizes the influence of casting conditions. Casting condition No. in Table 3 In No. 2-8, the casting speed was out of the desired range. Also, casting conditions No. In No. 2-9, since the cooling rate from the liquidus temperature to the solidus temperature in the 1/4 part of the slab thickness direction was low, the sulfide inclusions present in the bar steel were large. As a result, casting conditions No. 2-8 or No. Production No. 2-9 was adopted. In 64, 65, 66, and 67, the limit compression rate was lowered. On the other hand, the casting condition No. in which the continuous casting condition is appropriate. Manufacturing Nos. 2-1 to 2-7 were adopted. In 1, 2, 53 to 58, the sulfide inclusions were small, and the critical compression ratio was good.
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
　表４に圧延条件の影響をまとめた。表４の圧延条件Ｎｏ．３－６、３－Ｂは熱間圧延の総減面率が望ましい範囲を外れていた。その結果、これらの条件を採用した製造Ｎｏ．６８、６９では、圧延によるＭｎＳの厚みの減少が不十分となり、厚みの大きな硫化物系介在物が数多く存在していた。また、これにより、製造Ｎｏ．６８、６９は、限界圧縮率が低下していた。これに対して、熱間圧延の総減面率が適正である圧延条件Ｎｏ、３－１～３－５、３－Ａを採用した製造Ｎｏ．１、５９～６３では、厚みが大きく、かつ伸長している硫化物系介在物の数が少なく、限界圧縮率も良好であった。 Table 4 summarizes the effects of rolling conditions. Rolling condition no. In 3-6 and 3-B, the total area reduction rate of the hot rolling was outside the desired range. As a result, the production No. which adopted these conditions was used. In 68 and 69, the reduction in the thickness of MnS due to rolling became insufficient, and there were many sulfide inclusions with a large thickness. Further, as a result, the production No. In 68 and 69, the limit compression rate was lowered. On the other hand, the production conditions No. 3-1 to 3-5, 3-A adopting the rolling condition Nos. 3-1 to 3-5 and 3-A in which the total area reduction ratio of the hot rolling is appropriate. In Nos. 1 and 59 to 63, the thickness was large, the number of elongated sulfide inclusions was small, and the critical compression ratio was good.
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
　表５－１、表５－２、表６、表７に各製造条件で得られた鋼の介在物測定結果及び特性を示す。表５－１、表５－２、表６はＳＡを行った素材、表７はＳＡを行わなかった素材の結果を示している。
　表５－１、表５－２、表６から分かるように、全てが本願発明の範囲である製造Ｎｏ．１～１５、５３～６３はＳＡ後硬さ、限界圧縮率、浸炭層の３００℃焼戻し硬さ、不完全焼入れ層厚さの全てが優れていた。また、Ｎｂを含む製造Ｎｏ．１、８、９、１１については、さらに粗大粒も観察されなかった。
　これに対し、化学成分または製造条件の少なくとも１つが望ましい範囲を外れた製造Ｎｏ．１６～５２、６４～６９、ａ～ｆについては、ＳＡ後硬さ、限界圧縮率、浸炭層の３００℃焼戻し硬さ、不完全焼入れ層厚さのいずれかが目標値を満たしていなかった。さらに、製造Ｎｏ．２０、２３、３１、３４、４２、４５ではＯ含有量が高く、酸化物系介在物の最大√ａｒｅａが本願発明の範囲を外れていた。また、製造Ｎｏ．２２、３３、４４はＳ含有量が本願発明の範囲を超えていたため、硫化物系介在物の最大√ａｒｅａが本願発明の範囲を外れていた。
　また、表７から分かるように、ＳＡを行わなかった素材についても、全てが本願発明の範囲である製造Ｎｏ．１０１～１１５は浸炭層の３００℃焼戻し硬さ、不完全焼入れ層厚さに優れていた。一方で、化学成分または製造条件の少なくとも１つが望ましい範囲を外れた製造Ｎｏ．１１６～１１８、１２４、１２６、１２８、１２９、１３５、１３６、１３８、１３９、１４６、１４８、１５０、１５１では、３００℃焼戻し硬さまたは不完全焼入れ層硬さが劣っていた。この傾向は、ＳＡ材と同様であった。 Tables 5-1, 5-2, 6 and 7 show the measurement results and characteristics of the inclusions in the steel obtained under each production condition. Tables 5-1, 5-2, and 6 show the results of the materials subjected to SA, and Table 7 shows the results of the materials not subjected to SA.
As can be seen from Table 5-1, Table 5-2, and Table 6, all of the production numbers in the scope of the present invention were used. Nos. 1 to 15 and 53 to 63 were all excellent in post-SA hardness, critical compression ratio, 300 ° C. tempered hardness of the carburized layer, and incompletely quenched layer thickness. In addition, production No. including Nb. For 1, 8, 9, and 11, no coarse particles were observed.
On the other hand, if the production number is at least one of the chemical components or production conditions is out of the desired range. For 16 to 52, 64 to 69, and a to f, any of the post-SA hardness, the critical compressibility, the 300 ° C. tempered hardness of the carburized layer, and the incompletely quenched layer thickness did not satisfy the target value. Furthermore, production No. In 20, 23, 31, 34, 42, and 45, the O content was high, and the maximum √area of oxide inclusions was outside the scope of the present invention. In addition, production No. 22, 33 and 44 had an S content exceeding the range of the present invention, so the maximum √area of sulfide inclusions was outside the range of the present invention.
Further, as can be seen from Table 7, all of the materials that were not subjected to SA were manufactured No. in the scope of the present invention. Nos. 101 to 115 were excellent in 300 ° C. tempering hardness and incomplete quenching layer thickness of the carburized layer. On the other hand, a production No. in which at least one of chemical components or production conditions is out of the desired range. 116 to 118, 124, 126, 128, 129, 135, 136, 138, 139, 146, 148, 150, 151 had inferior 300 ° C. tempered hardness or incompletely hardened layer hardness. This tendency was the same as that of the SA material.
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000008
Figure JPOXMLDOC01-appb-T000008
Figure JPOXMLDOC01-appb-T000009
Figure JPOXMLDOC01-appb-T000009
　本発明の肌焼用鋼材及び肌焼鋼部品を用いれば、焼戻し軟化抵抗と冷間鍛造性とに優れた肌焼用鋼材及び肌焼鋼部品を提供することができる。また、これらを用いることにより歯車の製造コストを低減することができ、なおかつ自動車、建設機械、産業機械用の高出力化及び燃費向上等に大きく寄与することが可能になる。 If the case-hardening steel material and case-hardening steel component of the present invention are used, a case-hardening steel material and case-hardening steel component excellent in temper softening resistance and cold forgeability can be provided. Further, by using these, it is possible to reduce the manufacturing cost of the gears, and to greatly contribute to the increase in output and the improvement of fuel consumption for automobiles, construction machines, and industrial machines.

Claims (9)

  1. 　化学成分が、質量％で、
    Ｃ：０．０５～０．３０％、
    Ｓｉ：０．４０～１．５％、
    Ｍｎ：０．２～１．０％、
    Ｓ：０．００１～０．０５０％、
    Ｃｒ：１．０～２．０％、
    Ｍｏ：０．０２～０．８％、
    Ａｌ：０．００１～０．２０％、
    Ｎ：０．００３～０．０３％、
    Ｎｂ：０～０．１０％、
    Ｃｕ：０～０．２％、
    Ｎｉ：０～１．５％、
    Ｖ：０～０．２０％、
    Ｃａ：０～０．００５０％、
    Ｍｇ：０～０．００５０％、
    Ｓｂ：０～０．０５０％
    を含有し、
    Ｐ：０．０３０％以下、
    Ｏ：０．００２０％以下、
    Ｔｉ：０．００５％以下
    に制限し、残部が鉄及び不純物であり、下記（１）式、及び（２）式を満足し；
    　極値統計法を用いた介在物評価において、予測面積Ｓを３００００ｍｍ２としたとき、前記予測面積Ｓ中に存在する最大の硫化物系介在物径（√ａｒｅａ）Ｓの予測値が４９μｍ以下であり、前記予測面積Ｓ中に存在する最大の酸化物系介在物径（√ａｒｅａ）Ｏｘの予測値が８０μｍ以下であり；
    　２０μｍを超える長さ及び２μｍを超える厚みを有する硫化物系介在物が１ｍｍ２あたり２００個以下に制限されている；
    ことを特徴とする肌焼用鋼材。
    　１２×Ｓｉ（％）＋２５×Ｍｎ（％）＋Ｃｒ（％）＋２×Ｍｏ（％）≦２５　・・・（１）
    　３１×Ｓｉ（％）＋１５×Ｍｎ（％）＋２３×Ｃｒ（％）≧５０　・・・（２）
    　ここで、（１）式及び（２）式中の、Ｓｉ（％）、Ｍｎ（％）、Ｃｒ（％）、Ｍｏ（％）は、それぞれの元素の質量％での含有量である。     Chemical composition is mass%,
    C: 0.05 to 0.30%
    Si: 0.40 to 1.5%,
    Mn: 0.2 to 1.0%,
    S: 0.001 to 0.050%,
    Cr: 1.0 to 2.0%,
    Mo: 0.02 to 0.8%,
    Al: 0.001 to 0.20%,
    N: 0.003-0.03%,
    Nb: 0 to 0.10%,
    Cu: 0 to 0.2%,
    Ni: 0 to 1.5%,
    V: 0 to 0.20%,
    Ca: 0 to 0.0050%,
    Mg: 0 to 0.0050%,
    Sb: 0 to 0.050%
    Containing
    P: 0.030% or less,
    O: 0.0020% or less,
    Ti: limited to 0.005% or less, the balance being iron and impurities, satisfying the following formulas (1) and (2);
    In the inclusion evaluation using the extreme value statistical method, when the predicted area S is 30000 mm 2 , the maximum sulfide inclusion diameter (√area) S existing in the predicted area S is 49 μm or less. Yes, the predicted value of the maximum oxide inclusion diameter (√area) Ox existing in the predicted area S is 80 μm or less;
    The number of sulfide inclusions having a length exceeding 20 μm and a thickness exceeding 2 μm is limited to 200 or less per 1 mm 2 ;
    This is a steel for skin hardening.
    12 × Si (%) + 25 × Mn (%) + Cr (%) + 2 × Mo (%) ≦ 25 (1)
    31 × Si (%) + 15 × Mn (%) + 23 × Cr (%) ≧ 50 (2)
    Here, Si (%), Mn (%), Cr (%), and Mo (%) in the formulas (1) and (2) are contents in mass% of the respective elements.
  2. 　前記化学成分が、質量％で、
    Ｎｂ：０．０１５～０．１０％
    を含有することを特徴とする請求項１に記載の肌焼用鋼材。     The chemical component is mass%,
    Nb: 0.015 to 0.10%
    The steel material for case hardening according to claim 1, comprising:
  3. 　前記化学成分が、質量％で、
    Ｓｉ：０．５５～１．５％
    を含有することを特徴とする請求項１に記載の肌焼用鋼材。     The chemical component is mass%,
    Si: 0.55 to 1.5%
    The steel material for case hardening according to claim 1, comprising:
  4. 　前記化学成分が、質量％で、
    Ｃｕ：０．００１～０．２％、
    Ｎｉ：０．００１～１．５％、
    のうちの１種又は２種を含有することを特徴とする、請求項１～３のいずれか一項に記載の肌焼用鋼材。     The chemical component is mass%,
    Cu: 0.001 to 0.2%,
    Ni: 0.001 to 1.5%,
    The steel material for case hardening according to any one of claims 1 to 3, characterized in that it contains one or two of them.
  5. 　前記化学成分が、質量％で、
    Ｖ：０．０１～０．２０％、
    を含有することを特徴とする、請求項１～４のいずれか一項に記載の肌焼用鋼材。     The chemical component is mass%,
    V: 0.01-0.20%,
    The steel material for case hardening according to any one of claims 1 to 4, characterized by comprising:
  6. 　前記化学成分が、質量％で、
    Ｃａ：０．０００１～０．００５０％、
    Ｍｇ：０．０００１～０．００５０％
    のうちの１種又は２種を含有することを特徴とする、請求項１～５のいずれか一項に記載の肌焼用鋼材。     The chemical component is mass%,
    Ca: 0.0001 to 0.0050%,
    Mg: 0.0001 to 0.0050%
    The steel for case hardening according to any one of claims 1 to 5, characterized by containing one or two of them.
  7. 　前記化学成分が、質量％で、
    Ｓｂ：０．０００１～０．０５０％
    を含有することを特徴とする、請求項１～６のいずれか一項に記載の肌焼用鋼材。     The chemical component is mass%,
    Sb: 0.0001 to 0.050%
    The steel material for case hardening according to any one of claims 1 to 6, characterized by comprising:
  8. 　ミクロ組織が球状化炭化物組織を有することを特徴とする、請求項１～７のいずれか一項に記載の肌焼用鋼材。 The steel for case hardening according to any one of claims 1 to 7, wherein the microstructure has a spheroidized carbide structure.
  9. 　請求項１～８のいずれか一項に記載の肌焼用鋼材からなり、浸炭焼入れ焼戻し、または浸炭窒化焼入れ焼戻しの処理によって形成された表面硬化層を有することを特徴とする、肌焼鋼部品。 A case-hardened steel part comprising the surface hardened layer made of the steel for case hardening according to any one of claims 1 to 8 and formed by carburizing / quenching / tempering or carbonitriding / quenching / quenching / tempering. .
PCT/JP2014/060800 2013-04-18 2014-04-16 Case-hardening steel material and case-hardening steel member WO2014171472A1 (en)

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JP2016186120A (en) * 2015-03-27 2016-10-27 株式会社神戸製鋼所 Steel material for carbonitriding, and carbonitrided component
JP2016204752A (en) * 2015-04-22 2016-12-08 Ｊｆｅスチール株式会社 Case-hardened steel and production method thereof
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US11174543B2 (en) 2016-05-31 2021-11-16 Jfe Steel Corporation Case hardening steel, method of producing case hardening steel, and method of producing gear part
JP2018176241A (en) * 2017-04-17 2018-11-15 新日鐵住金株式会社 Method for production of steel material for machine structure
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