WO2014171472A1 - Matière d'acier de cémentation et élément d'acier de cémentation - Google Patents

Matière d'acier de cémentation et élément d'acier de cémentation 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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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 KR1020157029024A priority Critical patent/KR20150126699A/ko
Priority to CN201480021369.9A priority patent/CN105121687A/zh
Priority to US14/783,292 priority patent/US20160060744A1/en
Priority to JP2015512496A priority patent/JPWO2014171472A1/ja
Publication of WO2014171472A1 publication Critical patent/WO2014171472A1/fr

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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
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    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces
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    • 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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    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/18—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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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—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.

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Abstract

L'invention concerne une matière d'acier de cémentation qui a une telle propriété que la valeur prévue du plus grand diamètre (√surface)S d'inclusions de type sulfure qui existent dans une surface prévue (S) est 49 µm ou moins et la valeur prévue du plus grand diamètre (√surface)Ox d'inclusions de type oxyde qui existent dans la surface prévue (S) est 80 µm ou moins dans laquelle la surface prévue (S) est 30 000 mm2 dans le classement d'inclusion employant une méthode statistique de valeurs extrêmes, et a également une telle propriété que le nombre d'inclusions de type sulfure ayant chacune une longueur de plus de 20 µm et une épaisseur de plus de 2 µm est limité à 200 par 1 mm2.
PCT/JP2014/060800 2013-04-18 2014-04-16 Matière d'acier de cémentation et élément d'acier de cémentation WO2014171472A1 (fr)

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