EP3399063A1 - Acier cémenté, élément cémenté et procédé destiné à la production d'acier cémenté - Google Patents

Acier cémenté, élément cémenté et procédé destiné à la production d'acier cémenté Download PDF

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Publication number
EP3399063A1
EP3399063A1 EP16881816.9A EP16881816A EP3399063A1 EP 3399063 A1 EP3399063 A1 EP 3399063A1 EP 16881816 A EP16881816 A EP 16881816A EP 3399063 A1 EP3399063 A1 EP 3399063A1
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Prior art keywords
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case hardening
carburizing
steel
content
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EP16881816.9A
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German (de)
English (en)
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EP3399063A4 (fr
Inventor
Kei Miyanishi
Akira Shiga
Yutaka Neishi
Hideo Mizukami
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Publication of EP3399063A1 publication Critical patent/EP3399063A1/fr
Publication of EP3399063A4 publication Critical patent/EP3399063A4/fr
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—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
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • C22C38/18—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C23C8/02—Pretreatment of the material to be coated
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C23C8/34—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 more than one step
    • 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
    • C23C8/80—After-treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a case hardening steel, a carburized component, and a manufacturing method of a case hardening steel, and particularly to a case hardening steel which is excellent in coarse grain prevention properties and fatigue properties during carburizing, a manufacturing method thereof, and a carburized component which is obtained from the case hardening steel.
  • Carburized components such as gears, bearing components, rolling components, shafts, and constant-velocity joint components are typically manufactured by a method of forging medium carbon alloy steel for machine structural use specified in, for example, JIS G 4052, JIS G 4104, JIS G 4105, and JIS G 4106, machining the forged steel into a predetermined shape, and performing carburizing and quenching thereon.
  • cold forging including rolling
  • hot forging is performed.
  • the surface skin and dimensional accuracy of a product are good
  • manufacturing costs are lower than in hot forging
  • good yield is achieved.
  • cold forging has been increasingly performed instead of hot forging.
  • the number of carburized components manufactured by performing carburizing and quenching after cold forging has been remarkably increased in recent years.
  • an object of the bearing components and rolling components that receive high contact pressure is to sufficiently ensure fatigue properties such as rolling fatigue properties.
  • internal origin type damage is more frequently seen than surface origin type damage, and thus high depth carburizing is performed in order to suppress damage slightly inward of the surface where shear stress is likely to be maximized.
  • this high depth carburizing typically takes a long period of time of a dozen or so hours to several tens of hours, from the viewpoint of energy saving, a reduction in the carburizing time is required. In order to reduce the carburizing time, an increase in the carburizing temperature is effective.
  • the carburizing time can be reduced by setting the carburizing temperature, which is about 930°C in typical carburizing, to be in a temperature range of 990°C to 1090°C.
  • the carburizing temperature which is about 930°C in typical carburizing
  • the carburizing temperature which is about 930°C in typical carburizing
  • coarse grains are generated, and there may be cases where fatigue properties such as rolling fatigue properties needed for carburized components are not sufficiently obtained. Therefore, there is a demand for case hardening steel which does not cause the generation of coarse grains even when high temperature carburizing is performed and is thus suitable for high temperature carburizing.
  • gears, bearing components, and rolling components which receive high contact pressure are generally large components.
  • Such large components are typically manufactured by hot forging a steel bar, performing a heat treatment such as normalizing as necessary, and performing machining, carburizing and quenching, tempering, and polishing as necessary.
  • a hot forged member after the hot forging needs to be an appropriate material capable of suppressing coarse grains during carburizing. For this, it is necessary to use steel capable of suppressing the generation of coarse grains during carburizing as the material of the steel bar.
  • Patent Document 1 discloses a case hardening steel which contains Ti: 0.05% to 0.2%, S: 0.001% to 0.15%, and N: limited to less than 0.0051%, in which the amount of AlN precipitated after hot rolling is limited to 0.01% or less and coarse grain prevention properties and fatigue properties during carburizing are excellent.
  • Patent Document 2 discloses a case hardening steel which contains Ti: 0.03% to 0.30%, S: 0.010% to 0.10%, N: limited to 0.020% or less, in which the number density of Ti-based sulfides is specified.
  • the present invention has been made taking the foregoing circumstances into consideration, and an object thereof is to provide a case hardening steel having excellent coarse grain prevention properties during carburizing, a carburized component, and a manufacturing method of a case hardening steel.
  • a case of excellent coarse grain prevention properties during carburizing heat treatment strain due to carburizing and quenching can be suppressed even when annealing before the carburizing is omitted, and excellent fatigue properties can be obtained after the carburizing and quenching.
  • the case hardening steel according to the aspect of the present invention has a predetermined chemical composition, and the maximum diameter of the Ti-based precipitates is controlled to be in a predetermined range, thereby achieving excellent coarse grain prevention properties during carburizing. Therefore, with the case hardening steel according to the aspect of the present invention, heat treatment strain due to carburizing and quenching can be suppressed, and excellent fatigue properties are obtained after the carburizing and quenching.
  • the carburized component according to the aspect of the present invention has less heat treatment strain and has excellent fatigue properties.
  • the case hardening steel which has excellent coarse grain prevention properties during carburizing can be manufactured.
  • the case hardening steel obtained by the manufacturing method can suppress heat treatment strain due to carburizing and quenching and achieve excellent fatigue properties after the carburizing and quenching.
  • Ti-based precipitates are first solid-solubilized in a matrix by setting the heating temperature during the hot rolling to a high temperature and the precipitation temperature range of the Ti-based precipitates after the hot rolling is set such that slow cooling is achieved. Through the heating, rolling, and cooling, the formation of bainite can be suppressed, and a large amount of Ti-based precipitates can be formed and finely dispersed.
  • Nb carbonitride is first solid-solubilized in a matrix by setting the heating temperature during the hot rolling to a high temperature and the precipitation temperature range of the Nb carbonitride is set such that slow cooling is achieved. Through the heating, rolling, and cooling, a large amount of Nb carbonitride can be finely dispersed.
  • bainite is formed in the hot rolled structure, it is difficult for the Nb carbonitride to precipitate at phase interfaces. Therefore, it is preferable to suppress the formation of bainite as much as possible.
  • the present invention has been made based on the above-described novel findings.
  • case hardening steel according to an embodiment of the present invention case hardening steel according to this embodiment
  • a carburized component according to the embodiment of the present invention a carburized component according to this embodiment
  • a manufacturing method thereof will be described in detail.
  • C is an element effective in improving the strength of steel.
  • a preferable tensile strength for example, about 900 MPa
  • the C content exceeds 0.30%, the steel becomes hard, the cold workability deteriorates, and the toughness of a core portion after the carburizing and quenching deteriorates. Therefore, it is necessary to set the C content to be in a range of 0.10% to 0.30%.
  • Si is an element effective in deoxidizing steel.
  • Si is an element effective in imparting necessary strength and hardenability to steel and improving the temper softening resistance of the steel.
  • the Si content is less than 0.02%, the effect is not sufficiently obtained.
  • the Si content exceeds 1.50%, the hardness of the steel increases and the cold forgeability deteriorates. For the above reasons, it is necessary to set the Si content to be in a range of 0.02% to 1.50%.
  • a suitable range of the Si content is 0.02% to 0.30%.
  • it is more desirable that the Si content is set to be in a range of 0.02% to 0.15%.
  • Si is an element effective in increasing the grain boundary strength
  • Si is an element effective in increasing the service life by suppressing the microstructural change and deterioration of the material in a rolling fatigue process of the carburized components.
  • a suitable range of the Si content is 0.20% to 1.50%.
  • the Si content is set to be in a range of 0.40% to 1.50%.
  • the effect of the inclusion of Si on the suppression of the microstructural change and deterioration of the material in the rolling fatigue process of the bearing component or the rolling component is particularly significant when the amount of retained austenite (so-called retained ⁇ amount) in the structure after being subjected to carburizing and quenching is 30% to 40%.
  • retained ⁇ amount the amount of retained austenite in the structure after being subjected to carburizing and quenching is 30% to 40%.
  • the nitriding treatment after the carburizing is appropriately performed under the condition that the nitrogen concentration on the surface is in a range of 0.2% to 0.6%. It is desirable that the carbon potential during carburizing in this case is set to be in a range of 0.9% to 1.3%.
  • Mn is an element effective in deoxidizing steel.
  • Mn is an element effective in imparting necessary strength and hardenability to steel.
  • the Mn content is set to 0.30% or more, and is desirably 0.50% or more.
  • the Mn content exceeds 1.80%, not only is the effect saturated, but also the cold forgeability deteriorates due to an increase in the hardness of the steel. Therefore, it is necessary to set the Mn content to 1.80% or less, and desirably 1.20% or less. In a case where the cold forgeability of the steel is considered as being important, it is desirable to set the Mn content to be in a range of 0.50% to 0.75%.
  • P is an element which causes deterioration of cold forgeability by increasing deformation resistance during cold forging and thus causing deterioration of toughness.
  • P is an element which causes deterioration of fatigue strength by embrittling grain boundaries of a component after quenching and tempering. Therefore, it is desirable to reduce the P content as much as possible.
  • the P content exceeds 0.050%, deterioration of the cold forgeability and fatigue strength becomes significant, so that the P content is limited to 0.050% or less.
  • a suitable range of the P content is 0.015% or less.
  • the P content may also be 0%.
  • S is an element that forms MnS in steel. Since MnS can be the origin of bending fatigue fracture of a carburized component, it is necessary to prevent the formation of MnS. Therefore, the S content is set to 0.020% or less, and the relationship between the S content and the Ti content is set to be in a range satisfying Expression (1).
  • S in steel is present as Ti-based carbosulfide, and thus excellent fatigue properties are obtained after carburizing and quenching.
  • the S content is more preferably 0.015% or less.
  • the Ti-based carbosulfide has the pinning effect that contributes to the prevention of the generation of coarse grain.
  • Ti in Expression (1) is the Ti content (mass%)
  • S in the Expression (1) is the S content (mass%)
  • Cr is an element effective in improving the strength and hardenability of steel. Furthermore, in a case where case hardening steel is used as the material of a carburized component such as a bearing component and a rolling component, Cr increases the retained ⁇ amount after carburizing and quenching and suppresses the microstructural change and deterioration of the material in a rolling fatigue process. Therefore, Cr is an element that contributes to an increase in the fatigue life of the carburized component. When the Cr content is less than 0.40%, the effect is insufficient. Therefore, it is necessary to set the Cr content to 0.40% or more. The Cr content is preferably 0.70% or more. On the other hand, when the Cr content exceeds 2.00%, the cold forgeability deteriorates due to an increase in the hardness of the steel. Therefore, it is necessary to set the Cr content to 2.00% or less. The Cr content is preferably 1.60% or less.
  • the effect of the inclusion of Cr on the suppression of the microstructural change and deterioration of the material in the rolling fatigue process of the bearing component or the rolling component is particularly significant when the retained ⁇ amount in the structure after being subjected to carburizing and quenching is 30% to 40%.
  • it is effective to perform a nitriding treatment after the carburizing under the condition that the nitrogen concentration on the surface is in a range of 0.2% to 0.6%.
  • Al is an element effective as a deoxidizing agent.
  • the Al content is set to 0.005% or more.
  • the Al content is preferably 0.025% or more.
  • the Al content exceeds 0.050%, a portion of AlN remains unsolubilized during heating before hot rolling performed at the time of manufacturing of case hardening steel and becomes a precipitation site of precipitates of Ti (Ti and Nb in a case where Nb is contained). In this case, fine dispersion of the Ti-based precipitates (Ti-based precipitates and Nb carbonitride in the case where Nb is contained) is inhibited such that grains coarsen during carburizing. Therefore, it is necessary to set the Al content to 0.050% or less.
  • the Al content is preferably 0.040% or less.
  • Ti is an element that forms fine Ti-based carbide and Ti-based carbosulfide such as TiC, TiCS, and Ti 4 C 2 S 2 in steel and is an element effective for achieving ⁇ grain refinement during carburizing.
  • Ti content is less than 0.06%, the effect is insufficient, and thus the Ti content is set to 0.06% or more.
  • the Ti content exceeds 0.20%, precipitation hardening by TiC significant occurs and the cold workability deteriorates significantly.
  • the formation of precipitates mainly containing TiN significantly occurs, and the rolling fatigue properties after carburizing and quenching deteriorate. Therefore, it is necessary to set the Ti content to 0.20% or less.
  • the Ti content is preferably less than 0.15%.
  • Ti(C, N) When the case hardening steel according to this embodiment or a forged member obtained by forging the case hardening steel is subjected to carburizing and quenching, a solid-soluted Ti reacts with carbon and nitrogen infiltrating during the carburizing such that a large amount of fine TiC and TiN (hereinafter, sometimes referred to as "Ti(C, N)") precipitates on the carburized layer.
  • Ti(C, N) contributes to the improvement of rolling fatigue life in a carburized component such as a bearing component or a rolling component obtained by performing carburizing and quenching on the case hardening steel.
  • the carbonitriding treatment is a treatment in which the above-described carburizing and nitriding in a diffusion treatment after the carburizing are performed, and in the nitriding treatment, the condition that the nitrogen concentration on the surface is in a range of 0.2% to 0.6% is appropriate.
  • Bi is an important element in the case hardening steel according to this embodiment.
  • sulfide is finely dispersed as the solidification structure of the steel (mainly dendrite structure) becomes refined.
  • the Bi content is preferably 0.0010% or more.
  • the Bi content exceeds 0.0050%, the effect of refining the solidified structure is saturated and the hot workability of the steel deteriorates. This makes it difficult to perform the hot rolling at the time of manufacturing of the case hardening steel.
  • the Bi content is set to 0.0050% or less.
  • the Bi content is preferably 0.0040% or less.
  • N When N is bonded to Ti in steel, coarse TiN which hardly contributes to the prevention of coarsening of grains is formed.
  • TiN becomes a precipitation site of Ti-based precipitates mainly containing TiC and TiCS, and NbC and NbN mainly containing NbC (hereinafter, sometimes referred to as "Nb(C, N)") and inhibits fine precipitation of Ti-based precipitates and Nb(C, N).
  • Nb(C, N) mainly containing NbC
  • This adverse effect is particularly significant in a case where the N content exceeds 0.0060%. For the above reasons, it is necessary to set the N content to 0.0060% or less.
  • the N content is preferably less than 0.0051 %.
  • the N content may also be 0%.
  • the oxide inclusions become the origin of rolling fatigue fracture. Therefore, the lower the O content of the case hardening steel, the longer the rolling life of the carburized component. Therefore, in a case where the case hardening steel is used as the material of the carburized component such as the bearing component or the rolling component, it is desirable to limit the O content to 0.0012% or less.
  • the O content may also be 0%.
  • the case hardening steel according to this embodiment basically includes the above-described elements and the remainder consisting of Fe and impurities.
  • one or more elements selected from the group consisting of Mo, Ni, V, B, and Nb may be contained in the following ranges in addition to the above-mentioned elements.
  • these elements are not necessarily contained. Therefore, the lower limits thereof are 0%.
  • the elements do not impair the properties of the case hardening steel and are thus allowed.
  • the impurities are components incorporated from raw materials such as ore or scrap or from various environments in a manufacturing process when steel is industrially manufactured and are allowed in a range in which the steel is not adversely affected.
  • the chemical composition of the case hardening steel according to this embodiment may further include one or more of Mo, Ni, V, B, and Nb as necessary in the following ranges.
  • Mo is an element effective in improving the strength and hardenability of steel. Furthermore, Mo is an element effective in an increase in the fatigue life by increasing the retained ⁇ amount in a bearing component or a rolling component obtained after carburizing and suppressing the microstructural change and deterioration of the material in a rolling fatigue process. In a case of obtaining these effects, it is preferable to set the Mo content to 0.02% or more. The Mo content is more preferably 0.05% or more. However, when the Mo content exceeds 1.50%, machinability and cold forgeability deteriorate due to an increase in hardness. For the above reasons, even in the case where Mo is contained, the Mo content is set to be in a range of 1.50% or less. The Mo content is preferably 0.50% or less.
  • the effect of the inclusion of Mo on the suppression of the microstructural change and deterioration of the material in the rolling fatigue process of the bearing component or the rolling component is particularly significant when the retained ⁇ amount in the structure after being subjected to carburizing and quenching is 30% to 40%, like the above-described effect of Cr.
  • Ni is an element effective in improving the strength and hardenability of steel. In a case of obtaining the effect, it is preferable to set the Ni content to 0.10% or more.
  • the Ni content is more preferably 0.20% or more.
  • the Ni content exceeds 3.50%, machinability and cold forgeability deteriorate due to an increase in hardness. Therefore, even in a case where Ni is contained, the Ni content is set to be in a range of 3.50% or less.
  • the Ni content is preferably 2.00% or less.
  • V is an element effective in improving the strength and hardenability of steel. In a case of obtaining the effect, it is preferable to set the V content to 0.02% or more. However, when the V content exceeds 0.50%, machinability and cold forgeability deteriorate due to an increase in hardness. Therefore, even in a case where V is contained, the V content is set to be in a range of 0.50% or less. The V content is preferably 0.20% or less.
  • B is an element effective in improving the strength and hardenability of steel.
  • B forms boron iron carbide in a steel bar or a wire rod in a cooling process after rolling and thus increases the growth rate of ferrite, thereby providing an effect of softening the rolled steel.
  • B improves the grain boundary strength of a carburized material and also has an effect of improving fatigue strength and impact strength as a carburized component.
  • the B content is more preferably 0.0005% or more.
  • the B content is set to be in a range of 0.0050% or less.
  • the B content is preferably 0.0030% or less.
  • Nb is an element that is bonded to C and N in steel during carburizing to form Nb(C, N) and is thus effective in suppressing coarsening of grains.
  • Nb the effect of preventing coarse grains due to Ti-based precipitates is further increased. This is because Nb is solid-solubilized in the Ti-based precipitates and thus suppresses coarsening of the Ti-based precipitates.
  • the effect of the inclusion of Nb increases as the Nb content is increased.
  • Nb causes deterioration of machinability and cold forgeability, and deterioration of carburizing properties. In particular, when the Nb content is 0.040% or more, the hardness of the material increases and the machinability and the cold forgeability deteriorate.
  • the Nb content is set to be less than 0.040%.
  • workability such as machinability and cold forgeability is considered as being important
  • an appropriate range of the Nb content is less than 0.030%.
  • carburizing properties are considered as being important in addition to workability
  • an appropriate range of the Nb content is less than 0.020%.
  • an appropriate range of the Nb content is less than 0.010%.
  • the Nb content is a small amount such as less than 0.030%, less than 0.020%, or less than 0.010%, Nb significantly improves the coarse grain prevention properties compared to a case where Nb is not contained. Therefore, in a case where it is desirable to obtain the above-described effect, the Nb content may be more than 0%.
  • the Nb content In order to achieve both the coarse grain prevention properties and the workability, it is preferable to adjust the Nb content according to the Ti content. Specifically, it is preferable to set the total content (Ti + Nb) of the Nb content and the Ti content to 0.07% to 0.20%. In particular, in a case where the case hardening steel is subjected to carburizing at a high temperature or cold forging, a desirable range of the total content of the Nb content and the Ti content is more than 0.091% and less than 0.17%.
  • the maximum diameter of Ti-based precipitates predicted by extreme value statistics under the condition that an inspection standard area is 100 mm 2 , a number of inspections is 16 visual fields, and an area where prediction is performed is 30,000 mm 2 is set to 40 ⁇ m or less.
  • One of the required properties of the carburized component obtained from the case hardening steel which is an object of this embodiment is the improvement of contact fatigue strength such as rolling fatigue properties and surface fatigue strength.
  • contact fatigue strength such as rolling fatigue properties and surface fatigue strength.
  • the maximum diameter of the Ti-based precipitates predicted by extreme value statistics under the above condition is set to be 40 ⁇ m or less, and is preferably 30 ⁇ m or less.
  • a method of measuring and predicting the maximum diameter of precipitates using extreme value statistics is based on the method described in pp. 233 to 239 of "Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions" published by YOKENDO Ltd., on March 8, 1993 .
  • the two-dimensional inspection method in which maximum precipitates observed in a predetermined area (an area where prediction is performed: 30,000 mm 2 ) by two-dimensional inspection are estimated is used. Detailed measurement procedures will be described in Examples.
  • the area where prediction is performed is set in consideration of a risk volume of a general component.
  • the structure fraction (area ratio) of bainite is 30% or less.
  • the bainitic structure in the case hardening steel is small from the viewpoint of improving cold workability.
  • An adverse effect of the bainitic structure in the case hardening steel is particularly significant when the structure fraction of the bainite exceeds 30%. For the above reasons, it is preferable to limit the structure fraction of the bainite to 30% or less.
  • a suitable range of the structure fraction of the bainite is 20% or less.
  • a suitable range of the structure fraction of the bainite is 10% or less.
  • the bainitic structure may also be 0%.
  • a structure other than the bainite is preferably a structure mainly containing ferrite and pearlite.
  • the grain size number of ferrite contained in the metallographic structure is preferably No. 8 to No. 11 specified in JIS G 0552.
  • austenite grains are excessively refined during carburizing.
  • the driving force for grain growth increases and coarse grains tend to be formed.
  • the ferrite grain size exceeds No. 11 specified in JIS G 0552, this tendency becomes significant.
  • the ferrite grain size is less than No. 8 specified in JIS G 0552, the ferrite is coarse, the ductility deteriorates, and thus cold forgeability also deteriorates.
  • it is preferable to set the ferrite grain size number to be in a range of No. 8 to No. 11 specified in JIS G 0552.
  • case hardening steel according to this embodiment Since the case hardening steel according to this embodiment has excellent coarse grain prevention properties during carburizing, heat treatment strain due to carburizing and quenching can be suppressed. In addition, when the carburizing and quenching is performed, a carburized component having excellent fatigue properties is obtained. Even when the case hardening steel according to this embodiment is subjected to high temperature carburizing, the generation of coarse grains during the carburizing can be suppressed. Therefore, by performing the high temperature carburizing after forging, the carburizing time can be reduced. Furthermore, even for a carburized component in the related art in which switching from hot forging to cold forging is not performed due to deterioration of dimensional accuracy caused by heat treatment strain, switching to cold forging can also be performed. Moreover, annealing performed in the related art to suppress heat treatment strain after cold forging can be omitted.
  • a carburized component according to this embodiment includes the case hardening steel according to this embodiment.
  • the carburized component according to this embodiment is manufactured, for example, by a method of forging the case hardening steel according to this embodiment, machining the forged steel into a predetermined shape, and performing carburizing, quenching and tempering thereon.
  • the carburized component according to this embodiment has the same chemical composition and Ti-based precipitates as those of the case hardening steel according to this embodiment.
  • the carburized component according to this embodiment is obtained through carburizing and quenching, the carburized component according to this embodiment is different from the case hardening steel in that a carburizing and quenching layer is formed on the surface.
  • the manufacturing method described below is merely an example, and as long as a case hardening steel satisfying the scope of this embodiment can be obtained, the manufacturing method of the case hardening steel according to this embodiment is not limited to the following manufacturing conditions.
  • Steel having the above-mentioned chemical composition is melted (melting process) according to a typical method such as a converter or an electric furnace, and was cast into a bloom having the above-mentioned chemical composition (casting process). Thereafter, blooming is performed as necessary (blooming process), thereby obtaining a rolled material to be hot rolled into a wire rod or a steel bar.
  • the size of the bloom, the cooling rate during solidification, and the blooming conditions are not particularly limited.
  • the rolled material having the above-described chemical composition is heated under the following conditions, hot rolled into a wire rod or a steel bar, and cooled, thereby obtaining case hardening steel.
  • the rolled material having the above-mentioned chemical composition is heated at a temperature of 1 150°C or higher for a holding time of 10 minutes or longer (heating process), and the heated rolled material is hot rolled into a wire rod or a steel bar (hot rolling process).
  • the hot rolling when the heating temperature is 1150°C or higher and the holding time is 10 minutes or longer, the Ti-based precipitates can be sufficiently solid-solubilized in a matrix.
  • the heating temperature before the hot rolling is lower than 1150°C and/or the holding time is shorter than 10 minutes, the Ti-based precipitates and AlN (in a case where Nb is contained, Ti-based precipitates, Nb precipitates, and AlN) cannot be sufficiently solid-solubilized in the matrix.
  • coarse Ti-based precipitates once formed in the casting process remain unsolubilized in the steel after being hot rolled and cooled, and the Ti-based precipitates (Ti-based precipitates and Nb-based precipitates in the case where Nb is contained) cannot be finely precipitated.
  • coarsening of the Ti-based precipitates remaining unsolubilized in the heating process before the hot rolling proceeds due to Ostwald growth.
  • the finish temperature (finish rolling temperature) of the hot rolling is preferably set to 840°C to 1000°C.
  • finish temperature of the hot rolling is preferably set to 840°C to 1000°C.
  • the finish temperature is lower than 840°C, the ferrite grain size becomes too fine, and coarse grains are likely to be generated during carburizing.
  • the finish temperature exceeds 1000°C, the ferrite grains becomes coarse, and the hardness of the steel after being hot rolled and cooled increases, resulting in deterioration of cold forgeability.
  • the finish temperature of the hot rolling is 840°C to 1000°C.
  • the finish temperature is preferably 920°C to 1000°C.
  • the finish temperature is preferably 840°C to 920°C.
  • the steel After the hot rolling, the steel is cooled (cooling process).
  • a cooling rate average cooling rate
  • the time for which the Ti-based precipitates pass a precipitation temperature range is sufficiently secured, and dispersion of fine Ti-based precipitates is promoted.
  • the structure fraction of bainite can be suppressed. As a result, steel in which the structure fraction of bainite is 30% or less and excellent coarse grain prevention properties are achieved during carburizing is obtained.
  • the cooling rate exceeds 1.00 °C/s in the above temperature range, there is concern that the structure fraction of the bainite may increase to more than 30%.
  • the cooling rate in the above temperature range is 0.70 °C/s or less.
  • the cooling rate in a range of 800°C to 500°C may exceed 1.00 °C/s. Therefore, it is preferable to control the cooling rate to decrease.
  • a method of reducing the cooling rate for example, a method in which a heat insulation cover or a heat insulation cover with a heat source is installed in a rear stage of a hot rolling line and slow cooling of the steel is performed after the hot rolling by the heat insulation cover may be employed.
  • Spheroidizing annealing may be performed on the steel ((wire rod or steel bar): case hardening steel) after the cooling process as necessary.
  • the steel By performing the spheroidizing annealing, the steel is softened, and thus the load during cold forging can be reduced.
  • the case hardening steel according to this embodiment is obtained.
  • This case hardening steel is suitable as the material of a carburized component.
  • the carburized component according to this embodiment can be manufactured by the method in which the case hardening steel according to this embodiment is forged, is machined into a predetermined shape, and is subjected to carburizing and quenching.
  • the carburizing and quenching may be performed after hot forging, or the carburizing and quenching may be performed after cold forging.
  • the carburized component can be manufactured by hot forging the case hardening steel (wire rod or steel bar), performing a heat treatment such as normalizing as necessary, and performing machining, carburizing and quenching, tempering, and polishing as necessary.
  • the hot forging can be performed at a heating temperature of 1150°C or higher.
  • conditions during the carburizing and quenching are not particularly limited, for example, high temperature carburizing may be performed such that the carburizing temperature is in a temperature range of 950°C to 1090°C.
  • the carbon potential during the carburizing may be set to be high in a range of 0.9% to 1.3%.
  • a carbonitriding treatment in which nitriding is performed in a diffusion treatment after the carburizing may be performed. In the nitriding treatment after the carburizing, the condition that the nitrogen concentration on the surface is in a range of 0.2% to 0.6% is appropriate for improving the rolling fatigue life.
  • the billet was heated at a heating temperature shown in Table 2 for a holding time of 10 minutes or longer, was hot rolled at a finish temperature for hot rolling shown in Table 2, and was cooled at a cooling rate shown in Table 2 in a temperature of 800°C to 500°C after the hot rolling, thereby manufacturing a steel bar with a diameter of 24 to 30 mm.
  • each steel bar (case hardening steel) after being hot rolled and cooled was observed, the structure was identified by the following method, and the structure fraction of bainite was measured.
  • the ferrite grain size was measured according to the specification of JIS G 0552 and the grain size number was examined.
  • the Vickers hardness was measured as an index of cold workability by the following method.
  • the ⁇ grain size number, the rolling fatigue life, and the rotational bending fatigue strength were evaluated as the quality of the material after the carburizing by the following method.
  • a sample was taken by cutting (traversing) each steel bar (case hardening steel) in a direction perpendicular to the axial direction. After burying the obtained sample in a resin, the cut surface (observed section) was polished. The observed section after being polished was corroded with Nital to expose and observe the microstructure, and a bainitic structure in the microstructure was identified. Furthermore, the area ratio of the bainitic structure in the observed section was obtained and was used as the structure fraction (%) of bainite.
  • a structure other than the bainite was ferrite, or ferrite and pearlite.
  • Prediction of the maximum diameter of the Ti-based precipitates using extreme value statistics was performed by the following method. Whether or not the precipitates were based on Ti was determined by the difference in contrast in an optical microscope. The validity of the identification method based on the difference in contrast was confirmed in advance by a scanning electron microscope with an energy dispersive X-ray spectrometer.
  • a test piece was taken from each steel bar (case hardening steel), and a region of a 100 mm 2 inspection standard area (region of 10 mm x 10 mm) was prepared in advance for 16 visual fields in a longitudinal section of the steel bar.
  • the maximum precipitate of Ti-based precipitates in each 100 mm 2 inspection standard area was detected and photographed with an optical microscope at a magnification of 1,000-fold. This was repeated 16 times (that is, the number of inspections was 16 visual fields) for the visual field of each 100 mm 2 inspection standard area. From the obtained photograph, the diameter of the maximum precipitate in each inspection standard area was measured.
  • the geometric mean of the major axis and the minor axis is obtained and taken as the diameter of the precipitate.
  • 16 pieces of data of the 16 diameters of the maximum precipitates obtained were plotted on extreme value probability paper by the method described in pp. 233 to 239 of "Fatigue of Metals: Effects of Fine Defects and Inclusions" published by YOKENDO LTD. PUBLISHERS , a maximum precipitate distribution line (a linear function of maximum precipitate diameter and extreme value statistics standardized variable) was obtained, and the maximum precipitate distribution line was subjected to extrapolation such that the diameter of the maximum precipitates in an area of 30,000 mm 2 where prediction was performed was predicted.
  • a sample was taken by cutting (traversing) each steel bar (case hardening steel) after rolling in the direction perpendicular to the axial direction. After burying the obtained sample in a resin, the cut surface (observed section) was polished. Regarding a portion at a diameter 1/4 depth from the surface of the observed section after being polished, the Vickers hardness was measured five times in total under a load of 10 kg based on "Vickers hardness test - Test method" in JIS Z 2244 (2009), and the average value was taken as the Vickers hardness. When the Vickers hardness is 230 HV or less, excellent cold forgeability was determined.
  • Each steel bar (case hardening steel) was subjected to spheroidizing annealing, and thereafter an upsetting test piece was prepared. After performing upsetting at a rolling reduction of 50%, a carburizing simulation was conducted under the following conditions.
  • the heating temperature was set to three temperatures, 1000°C, 1050°C, and 1100°C, and in a case of any of the heating temperatures, heating was performed for five hours, followed by water cooling.
  • the cut surface of each test piece after the carburizing simulation was polished and then corroded, and prior austenite grain sizes were observed to obtain a grain coarsening temperature (coarse grains generation temperature).
  • the measurement of the prior austenite grain sizes was performed according to JIS G 0551, about 10 visual fields were observed at a magnification of 400-fold, and the generation of coarse grains was determined when even a single coarse grain having a grain size number of No. 5 or less was present.
  • a grain coarsening temperature of higher than 1100°C was determined as good coarse grain prevention properties, and a grain coarsening temperature of 1100°C or lower was determined as inferior coarse grain prevention properties.
  • the grain coarsening temperatures are shown in Table 2.
  • each steel bar (case hardening steel) was subjected to cold forging at a rolling reduction of 50%, a columnar rolling fatigue test piece having a diameter of 12.2 mm and an Ono type rotating bending test piece (with an R1.14 notch) having a parallel portion with a diameter of 9 mm were prepared, and carburizing was performed under the condition of five hours and a carbon potential of 0.8% at 1050°C.
  • the temperature of a quenching oil was 130°C, and tempering was performed at 180°C for two hours.
  • the ⁇ (austenite) grain size of the carburized layer was investigated by the following method.
  • a samples was taken by cutting (traversing) the parallel portion subjected to the Ono type rotating bending after the carburizing, quenching and tempering, in the direction perpendicular to the axial direction. After burying the obtained sample in a resin, the cut surface (observed section) was polished. Corrosion was caused to expose austenite grains from the observed section after being polished, and the austenite grain size was measured in a visual field centered on a position at a depth of 200 ⁇ m from the surface according to the specification of JIS G 0551.
  • the rolling fatigue properties were evaluated using a point contact type rolling fatigue testing machine (Hertz maximum contact stress 5884 MPa).
  • L10 life defined as "number of stress repetitions at which fatigue fracture occurs at a cumulative failure probability of 10% obtained by plotting test results on Weibull probability paper" was used.
  • the rolling fatigue life represents a relative value of the L10 life of each material when the L10 life of Comparative Steel No. 17 as was set to 1.
  • the bending fatigue strength was evaluated using an Ono type rotating bending fatigue testing machine.
  • the material that withstood a stress of 550 MPa 10,000,000 times was evaluated as "OK", and the material that was fractured was evaluated as "NG”.
  • the grain coarsening temperatures of steels of the present invention (Nos. 1 to 12, 22, and 23) were higher than 1100°C
  • the ⁇ grain size of the carburized material heated at 1050°C was also No. 7 or higher in terms of grain size number, which means fine grains, and the rolling fatigue life and the result of the rotating bending fatigue test were good.
  • Comparative Steel No. 13 did not contain Bi, the grain coarsening temperature was lower than those of the steels of the present invention.
  • Comparative Steel No. 15 had a large S content and did not satisfy Expression (1). Therefore, fatigue fracture originated from MnS had occurred, and the rolling fatigue life of the result of the rotating bending fatigue test were inferior to those of the steels of the present invention.
  • precipitates of Ti-based carbonitride effective in preventing coarsening caused by the formation of a large amount of Ti-based sulfide could not be sufficiently obtained, and the grain coarsening temperature was lower than those of the steels of the present invention.
  • Comparative Steel No. 18 the N content was high, and coarse TiN was formed. Therefore, the rolling fatigue properties and the rotating bending fatigue properties were inferior to those of the steels of the present invention. Furthermore, in No. 18, the amount of precipitates of Ti-based carbonitride effective in preventing coarse grains was reduced due to the formation of coarse TiN. Therefore, the grain coarsening temperature was lower than those of the steels of the present invention.
  • Comparative Steel No. 20 had a large N content and thus had coarse TiN formed, so that the rolling fatigue properties were inferior to those of the steels of the present invention.
  • Comparative Steel No. 21 had a large Nb content, the carburizing properties were degraded, and a sufficient carbon concentration could not be obtained. As a result, the strength was insufficient, and the rolling fatigue life and the result of the rotating bending fatigue test were inferior to those of the steels of the present invention.
  • the case hardening steel according to the present invention has a predetermined chemical composition, and the maximum diameter of Ti-based precipitates is controlled to be in a predetermined range, thereby achieving excellent coarse grain prevention properties during carburizing. Therefore, with the case hardening steel according to the present invention, heat treatment strain due to carburizing and quenching can be suppressed, and excellent fatigue properties are obtained after the carburizing and quenching.
  • the carburized component manufactured by performing carburizing and quenching on the case hardening steel of the present invention has less heat treatment strain and has excellent fatigue properties.
  • the case hardening steel which has excellent coarse grain prevention properties during carburizing can be manufactured.
  • This case hardening steel can suppress heat treatment strain due to carburizing and quenching and achieves excellent fatigue properties after the carburizing and quenching.
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