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US20110041959A1 - Steel for machine structure use for surface hardening and steel part for machine structure use - Google Patents

Steel for machine structure use for surface hardening and steel part for machine structure use Download PDF

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Publication number
US20110041959A1
US20110041959A1 US12/734,813 US73481309A US2011041959A1 US 20110041959 A1 US20110041959 A1 US 20110041959A1 US 73481309 A US73481309 A US 73481309A US 2011041959 A1 US2011041959 A1 US 2011041959A1
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steel
layer
machine structure
structure use
inv
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Inventor
Atsushi Mizuno
Masayuki Hashimura
Hajime Saitoh
Shuji Kozawa
Kei Miyanishi
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Nippon Steel Corp
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Individual
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Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HASHIMURA, MASAYUKI, KOZAWA, SHUJI, MIYANISHI, KEI, MIZUNO, ATSUSHI, SAITOH, HAJIME
Publication of US20110041959A1 publication Critical patent/US20110041959A1/en
Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NIPPON STEEL & SUMITOMO METAL CORPORATION MERGER (SEE DOCUMENT FOR DETAILS). Assignors: NIPPON STEEL CORPORATION
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/04—Layered products comprising a layer of metal comprising metal as the main or only constituent of a layer, which is next to another layer of the same or of a different material
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C21D1/09—Surface hardening by direct application of electrical or wave energy; by particle radiation
    • C21D1/10—Surface hardening by direct application of electrical or wave energy; by particle radiation by electric induction
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
    • 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
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C23C8/24—Nitriding
    • C23C8/26—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
    • C23C8/28—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
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/25—Process efficiency

Definitions

  • the present invention relates to steel for machine structure use for surface hardening and in particular a steel part for machine structure use having a high contact fatigue strength applied to a gear, continuously variable transmission, constant velocity joint, hub, or other power transmission part for an automobile etc.
  • Steel parts for machine structure use for example, gears of automatic transmissions, sheaves of continuously variable transmissions, constant velocity joints, hubs, bearings and other power transmission parts require high contact fatigue strength.
  • the above parts have been obtained by working materials such as JIS SCr420, SCM420, or other C: 0.2% or so case hardened steel into parts, then treating them by carburized quenching to form C: 0.8% or so martensite structure hardened layers on the surfaces of the parts and improve the contact fatigue strength.
  • carburized quenching is accompanied with austenite transformation at a 950° C. or so high temperature and is treated for a long period of 5 to 10 hours, in some cases, 10 hours or more, so larger heat treatment deformation due to coarsening of the crystal grains (distortion during quenching) is difficult to avoid.
  • Induction hardening is a method of austenizing and quenching only the required part of the surface layer part of a steel material by heating for a short time and enables a surface hardened part with small distortion during quenching to be obtained with good precision.
  • just induction hardening to obtain a hardness equivalent to that of a carburized quenched material, 0.8% or more of C has to be added.
  • Soft nitriding is a surface hardening method mainly causing nitrogen and carbon to simultaneously diffuse in and permeate the surface of a steel material to form a hardened layer in the 500 to 600° C. temperature range, which is less than the transformation point, and improve the wear resistance, seize resistance, fatigue resistance, etc.
  • the surface of the steel material is formed with nitrides by the diffused nitrogen.
  • a compound layer mainly comprised of Fe 3 N, Fe 4 N, or other Fe nitrides is formed at the surfacemost layer of the steel material.
  • a nitrided layer with N diffused in it is formed inside.
  • Soft nitriding can be performed at a low temperature. Further, compared with carburization, the treatment time is a short one of 2 to 4 hours or so, therefore this is often used for the production of steel parts requiring low distortion. However, with just soft nitriding, the hardened layer depth is shallow, so this cannot be used for transmission gears etc. to which a high contact pressure is applied.
  • PLT 1 discloses the method of combining induction hardening and gas soft nitriding to make up for their respective defects and obtain excellent mechanical properties, in particular, high contact fatigue strength by improvement of the softening resistance.
  • the surface hardness is high, but the concentration of N in the nitrided layer is low, so the high temperature hardness is low, sufficient softening resistance cannot be obtained at the surface of the gear etc. becoming high in temperature during operation, and in the final analysis a high contact fatigue strength cannot be obtained.
  • PLT 2 also discloses the method of combining induction hardening and soft nitriding to produce parts for machine structure use excellent in mechanical strength. With the method of PLT 2, to enable the nitrides to form a solid solution, 900° C. to 1200° C. high temperature induction heating is necessary.
  • PLT 3 discloses a method of production of a part for machine structure use excellent in mechanical strength characterized by treating steel comprised of, by wt %, C: 0.35 to 0.65%, Si: 0.03 to 1.50%, Mn: 0.3 to 1.0%, Cr: 0.1 to 3.0%, and a balance of Fe and impurities by soft nitriding under conditions giving a nitrided layer depth of 150 ⁇ m or more, then by induction hardening under conditions where the nitrided layer austenizes.
  • PLT 4 discloses a method of heat treatment of a machine part characterized by soft nitriding an iron-based material worked into the shape of a part so as to make nitrogen diffuse in and permeate the surface layer and form a compound layer, then induction hardening the part under conditions where the compound layer is consumed, the diffusion layer of the newly formed surface layer is denitrided, and a porous layer is formed at the surfacemost part.
  • PLT 5 discloses a roller support shaft used for a cam follower device made of an iron-based alloy containing Cr, Mo, V, and W in a total of 1.0 to 20.0 wt % and C and N in a total of 0.5 to 1.2 wt % and having a balance of unavoidable impurities and Fe, nitrided at its surface, then induction quenched at the outer peripheral parts other than the two ends.
  • PLT 6 also discloses a method of combining induction hardening and nitriding to obtain excellent mechanical properties.
  • the nitriding in the method of PLT 6 is performed at a high temperature of 600° C. or more, so the compound layer is thin.
  • the N concentration in it is low, so the amount of N diffusing due to decomposition at the time of induction hardening is also small.
  • PLT 7 discloses steel for machine structure use excellent in strength, ductility, toughness, and wear resistance characterized by containing, by mass %, C: over 0.30%, 0.50% or less, Si: 1.0% or less, Mn: 1.5% or less, Mo: 0.3% to 0.5%, Ti: 0.1% or less, and B: 0.0005% to 0.01%, having a balance of Fe and unavoidable impurities, having at its surface a hardened layer of a thickness 50 ⁇ m or less and a Vicker's hardness of 750 or more, and having structures other than said hardened layer with an old austenite grain size of 10 ⁇ m or less, a martensite percentage of 90% or more, and a Vicker's hardness of 450 to less than 750.
  • the steel for machine structure use of PLT 7 does not form the required thickness of nitrided layer and raise the contact fatigue strength, so even if this can be applied to a metal belt of a continuously variable transmission, it is difficult to apply this to gears of automatic transmissions, sheaves of continuously variable transmissions, constant velocity joints, hubs, and other power transmission parts subjected to high contact pressures.
  • PLT 1 Japanese Patent Publication (A) No. 06-172961
  • PLT 2 Japanese Patent Publication (A) No. 07-090363
  • PLT 3 Japanese Patent Publication (A) No. 07-090364
  • PLT 4 Japanese Patent Publication (A) No. 10-259421
  • PLT 5 Japanese Patent Publication (A) No. 2004-183589
  • PLT 7 Japanese Patent Publication (A) No. 2007-177317
  • the present invention in view of this situation, has as its task making up for the defects of the low surface hardness or internal hardness resulting from just induction hardening or soft nitriding by combining induction hardening and soft nitriding and providing a steel part for machine structure use excellent in contact fatigue strength (i) provided with a high surface hardness, internal hardness, and temper softening resistance unable to be obtained by a conventional soft nitrided and induction hardened steel part and, furthermore, (ii) formed with a sufficient lubricating film at its operating surface and a steel for machine structure use for surface hardening use used for said steel part.
  • the compound layer formed at the time of soft nitriding (layer mainly comprised of Fe 3 N, Fe 4 N, or other Fe nitrides) as a source of supply of N and use the later performed induction heating to break down the compound and cause a sufficient amount of N to diffuse in the steel.
  • FIG. 1 shows an example of the cross-sectional distribution of hardness from the surface to the core direction in a soft nitrided material and a soft nitrided and induction hardened material.
  • the surfacemost layer of the nitrided layer (see FIG. 2( a ), FIG. 2( a ) explained later) is formed with a compound layer and, as, shown in FIG. 1 , exhibits an extremely high hardness, but the compound layer is thin.
  • the surface layer structure of a quenched, soft nitrided material is martensite and the core is a ferrite-pearlite structure.
  • the thickness of the compound layer breaking down by induction hardening 10 ⁇ m or more it is possible to increase the thickness of the high N concentration nitrided layer.
  • the compound layer formed by nitridation becomes a brittle compound layer and is degraded in mechanical properties in some cases depending on the nitridation conditions, so usually effort is made to reduce the thickness of the compound layer.
  • the present invention is characterized by deliberately making the thickness of the compound layer greater to deliberately utilize the properties of the compound layer. That is, the present invention increases the thickness of the compound layer to form martensite containing a large amount of N at the time of induction hardening and obtain a structure with a high softening resistance.
  • the softening resistance at the time of a high temperature is strikingly increased.
  • Mn if added in an amount satisfying Mn/S ⁇ 70, suppresses the action of S and exhibits a remarkable effect for formation of a compound layer.
  • Mn/S is preferably 30000 or less.
  • the present invention is characterized by the formation of a compound layer at the surface layer of the steel material by soft nitriding and then using the subsequent induction heating for austenization for hardening and thereby form a nitrided layer.
  • FIG. 2 show an example of a nitride layer observed by an optical microscope and a scan type electron microscope.
  • FIG. 2( a ) shows a nitride layer observed by an optical microscope
  • FIG. 2( b ) shows a nitride layer observed by a scan type electron microscope (enlargement of layer part).
  • the nitride layer is a hard, porous layer with a large number of pores functioning as oil reservoirs due to breakdown of the compound layer. Due to the nitride layer including a large number of pores, the lubricating effect is improved and the wear resistance and durability are greatly improved.
  • the pores in the nitrided layer effectively function as oil reservoirs.
  • the present invention was completed based on the above findings and has as its gist the following:
  • Steel for machine structure use for surface hardening as set forth in any one of (1) to (5), characterized in that said steel for machine structure use for surface hardening is steel which is nitrided, then induction hardened.
  • a steel part for machine structure use obtained by machining steel for machine structure use for surface hardening as set forth in any one of (1) to (7), nitriding it, then induction hardening it, said steel part for machine structure use characterized in that the surface layer from the surface down to a depth of 0.4 mm or more is a nitrided layer and the hardness of the nitrided layer from the surface down to a depth of 0.2 mm is a Vicker's hardness at the time of tempering at 300° C. of 650 or more.
  • the present invention it is possible to provide steel for structural use for surface hardening able to be applied to power transmission parts of automobiles etc. and possible to provide steel parts having high contact fatigue strength, in particular, gears, continuously variable transmission, constant velocity joints, hubs, and other steel parts for machine structures.
  • FIG. 1 is a view showing an example of the distribution of hardness in the cross-section from the surface in the direction of the core in a soft nitrided material and in a soft nitrided and induction hardened material.
  • FIG. 2 are views showing an example of a nitrided layer when observed under an optical microscope and a scan type electron microscope.
  • (a) shows a nitrided layer observed by an optical micrograph
  • (b) shows a nitrided layer (layer part enlarged) observed by a scan type electron microscope.
  • FIG. 3 is a view showing the relationship between the Mn/S and compound layer thickness ( ⁇ m).
  • FIG. 4 is a view showing the relationship between the N concentration (mass %) at a position of 0.2 mm from the surface layer after induction hardening and the 300° C. tempered hardness (Hv).
  • the present invention has as its basic idea to produce a steel part from which a high contact fatigue strength is demanded by nitriding or soft nitriding steel to which suitable amounts of Mn and W are added, then induction hardening it to deeply form a nitrided layer with a high N concentration and improve the hardness and softening resistance.
  • the % means the mass %.
  • C is an important element for obtaining strength of the steel.
  • it is an element required for reducing the ferrite percentage of the structure before induction hardening, improving the hardening ability at the time of induction hardening, and increasing the depth of the hardened layer.
  • the lower limit was made 0.3%.
  • the preferable lower limit is 0.36%.
  • the upper limit is 0.53%.
  • Si is an element having the effect of raising the softening resistance of the quenched layer and increasing the contact fatigue strength. To obtain this effect, 0.02% or more has to be added. Preferably, 0.1% or more, more preferably 0.25% or more is added.
  • the preferable upper limit was 1.44%.
  • Mn is an element effective for improving the quenchability and increasing the softening resistance and thereby raising the contact fatigue strength. Furthermore, Mn has the effect of immobilizing the S in the steel as MnS and suppressing the action of S in concentrating at the surface of the steel material to inhibit entry of N and of promoting the formation of a thick compound layer due to nitriding or soft nitriding.
  • Mn has to be added to satisfy Mn/S ⁇ 70.
  • Mn is an element having the effect of lowering the ferrite percentage of the structure before induction hardening and raising the hardening ability at the time of induction hardening.
  • 1.5% or more has to be added.
  • it is 1.55% or more, more preferably 1.6% or more.
  • S is a soft nitriding inhibiting element which has the action of improving the machineability, but concentrating at the surface of the steel material and obstructing entry of N into the steel material at the time of soft nitriding.
  • the action of inhibiting nitriding becomes remarkable and, furthermore, the forgeability also remarkably degrades, so even if added for improving the machineability, it should be kept at 0.025% or less.
  • it is 0.019% or less, more preferably 0.009% or less.
  • the lower limit was made the industrial limit of 0.0001%.
  • Mn/S of the amounts of addition of Mn and S is less than 70, S concentrates at the surface of the steel material and inhibits the formation of the compound layer at the time of nitriding or soft nitriding, so Mn/S was made 70 or more. If Mn/S is 70 or more, the effect of addition is remarkable.
  • the upper limit of Mn and the lower limit of S are set, so there is no particular need to set the upper limit of Mn/S, but when Mn/S ⁇ 30000, the effect of addition of Mn becomes saturated, so the upper limit was made 30000.
  • FIG. 3 is a view showing the relationship between the Mn/S obtained by soft nitriding the steel material under the later explained conditions and the thickness ( ⁇ m) of the compound layer. From FIG. 3 , it is understood that if making Mn/S 70 or more, at the time of soft nitriding, it is possible to obtain a compound layer of a thickness of 10 ⁇ m or more.
  • W is an element with a good affinity with N and having the action of raising the quenchability and promoting the diffusion of N at the time of induction heating to raise the contact fatigue strength. Further, W is an element having the action of lowering the ferrite percentage of the structure before induction hardening and raising the hardening ability at the time of induction hardening.
  • W is an element which does not easily segregate in the steel and has the action of uniformly raising the quenchability of steel and thereby making up for the drop in the quenchability due to the segregation of Mn.
  • 0.0025% or more has to be added.
  • it is 0.01% or more, more preferably 0.05% or more.
  • the preferable upper limit is 0.40%, more preferably 0.25%.
  • Al precipitates and disperses as Al nitrides in the steel and effectively acts for refining the austenite structure at the time of induction hardening.
  • Al is an element effective for improvement of the machineability.
  • 0.001% or more has to be added.
  • it is 0.005% or more, more preferably 0.010% or more.
  • the preferable upper limit is 0.31%, more preferably 0.14%.
  • N is an element forming various types of nitrides and effectively acting to refine the austenite structure at the time of induction hardening.
  • 0.003% or more has to be added.
  • it is 0.005% or more.
  • the preferable upper limit is 0.01%.
  • P is an element segregating at the grain boundaries and acting to reduce the toughness. For this reason, it has to be reduced as much as possible. It is limited to 0.03% or less. Preferably, it is limited to 0.01% or less. The lower limit is made the industrial limit of 0.0001%.
  • O is present in the steel as Al 2 O 3 , SiO 2 , and other oxide-based inclusions, but if O is too great, said oxides end up becoming large in size and form starting points leading to fracture of the power transmission parts, so has to be limited to 0.0050% or less.
  • the lower limit is the industrial limit of 0.0001%.
  • Cr is an element having the effect of raising the nitrided properties of the steel and of raising the softening resistance of the quenched layer to improve the contact pressure fatigue strength.
  • 0.01% or more is added.
  • it is 0.1% or more, more preferably 0.52% or more.
  • the preferable upper limit is 1.74%, while the more preferable upper limit is 1.30%.
  • Mo is an element which has the effect of raising the softening resistance of the quenched layer to improve the contact fatigue strength and the effect or strengthening and toughening the quenched layer to improve the bending fatigue strength.
  • 0.01% or more has to be added.
  • it is 0.05% or more, more preferably 0.12% or more.
  • the preferable upper limit is 0.80%, the more preferable upper limit is 0.69%.
  • V is an element precipitating and dispersing as nitrides in the steel and effectively acting to refine the austenite structure at the time of induction hardening.
  • 0.01% or more has to be added.
  • it is 0.10% or more, more preferably 0.25% or more.
  • the preferable upper limit is 0.80%, the more preferable upper limit is 0.68%.
  • one or more of Cr: 0.01 to 2.0%, Mo: 0.01 to 1.0%, and V: 0.01 to 1.0% are added.
  • B is an element contributing to the improvement of the quenchability.
  • 0.0005% or more has to be added.
  • it is 0.001% or more.
  • the preferable upper limit is 0.003%.
  • Nb is an element precipitating and dispersing as nitrides in the steel and effectively acting to refine the austenite structure at the time of induction hardening.
  • 0.005% or more has to be added.
  • it is 0.01% or more, more preferably 0.04% or more.
  • the preferable upper limit is 0.2%, while the more preferable upper limit is 0.16%.
  • Ti is an element precipitating and dispersing as nitrides in the steel and effectively acting to refine the austenite structure at the time of induction hardening.
  • 0.005% or more has to be added.
  • it is 0.02% or more, more preferably 0.05% or more.
  • the upper limit was made 0.2%.
  • the preferable upper limit is 0.15%, the more preferable upper limit is 0.11%.
  • Ni is an element further improving the toughness.
  • 0.05% or more has to be added.
  • it is 0.10% or more, more preferably 0.21% or more.
  • the preferable upper limit is 1.5%, the more preferable upper limit is 0.96%.
  • Cu is an element strengthening ferrite and effective for improvement of the quenchability and improvement of the corrosion resistance.
  • 0.01% or more has to be added.
  • it is 0.09% or more, more preferably 0.14% or more.
  • the preferable upper limit is 1.5%, while the more preferable upper limit is 0.95%. Note that, Cu particularly lowers the hot rollability and easily becomes a cause of flaws at the time of rolling, so is preferably added simultaneously with the Ni.
  • Nb 0.005 to 0.3%
  • Ti 0.005 to 0.2%
  • Ni 0.05 to 2.0%
  • Cu 0.01 to 2.0%
  • one or more of Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, Zr: 0.0005 to 0.05%, and Te: 0.0005 to 0.1% is added to the steel material.
  • the above elements are elements suppressing bending fatigue fracture of gears and fatigue fracture of the bottom of the splines of shaft parts caused by flattening of MnS and thereby further improving the bending fatigue strength.
  • Ca: 0.0005% or more, Mg: 0.0005% or more, Zr: 0.0005% or more, or Te: 0.0005% or more has to be added.
  • Ca: 0.0010% or more, Mg: 0.0010% or more, Zr: 0.0010% or more, or Te: 0.0010% or more is added.
  • one or more of Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, Zr: 0.0005 to 0.05%, and Te: 0.0005 to 0.1% may be added.
  • the steel part of the present invention is a steel part obtained by machining steel of the present invention, nitriding or soft nitriding it, then induction hardening it, characterized in that the surface layer from the surface down to a depth of 0.4 mm or more is a nitrided layer and the hardness of the nitrided layer from the surface down to a depth of 0.2 mm is a Vicker's hardness at the time of tempering at 300° C. of 650 or more.
  • the surface layer having a sufficient hardness becomes thinner.
  • internal fracture that is, “spalling”, occurs and the lifetime becomes short, so the surface layer from the surface down to a depth of 0.4mm or more is designated as the “nitrided layer”.
  • Contact fatigue fracture is fracture of a surface starting point formed at a sliding surface raised in temperature (to around 300° C.), so maintaining the high temperature strength, that is, increasing the temper softening resistance, is effective for improving the contact fatigue strength.
  • the Vicker's hardness when tempering at 300° C. is less than 650, the nitrided layer cannot withstand a high contact pressure, so at the nitrided layer from the surface down to a depth of 0.2 mm, the Vicker's hardness when tempering at 300° C. is made 650 or more.
  • induction hardening can be judged by (a) the distribution of structures observed under an optical microscope after obtaining a micro sample from the steel part and corroding it by a Nital corrosive solution, (b) the distribution of hardness measured from the surface to the core, and, furthermore, (c) the N concentration from the surface to the core measured by EPMA.
  • nitrided layer with a high N concentration to obtain a high contact fatigue strength
  • high frequency heat treatment layer mainly comprised of Fe 3 N, Fe 4 N, or other Fe nitrides
  • the nitriding or soft nitriding is necessary and important treatment.
  • the soft nitriding temperature is made less than 600° C. If the soft nitriding temperature is a low temperature, it is possible to prevent heat treatment deformation, grain boundary oxidation, etc. of the steel material, so from this point as well, the soft nitriding temperature is made less than 600° C.
  • the soft nitriding temperature is preferably 500° C. or more.
  • the depth of the nitrided layer reaches the saturated state even if soft nitriding for a long time, so the soft nitriding time is preferably 1 to 3 hours.
  • the cooling after the soft nitriding may be performed by any method of air cooling, N 2 gas cooling, oil cooling, etc.
  • gas soft nitriding or salt bath soft nitriding may be used as the soft nitriding.
  • nitriding As the method for supplying nitrogen to the surface of the steel material and forming a 10 ⁇ m or more compound layer at the surface layer of the steel material, not only soft nitriding, but also nitriding may be used.
  • the “nitriding” referred to here is not a method, like soft nitriding, of treatment in a mixed atmosphere of NH 3 and CO 2 (and sometimes also N 2 ), but a surface hardening method of treatment by NH 3 for a long time and is an industrially different method.
  • the heating conditions at the time of the induction hardening have to be set considering the breakdown of the compound layer.
  • the heating temperature has to be made the austenization temperature to less than 900° C. and the holding time 0.05 to 5 seconds.
  • FIG. 4 shows the relationship of the N concentration at a position 0.2 mm from the surface after induction hardening and the Vicker's hardness at the time of tempering at 300° C. From FIG. 4 , it will be understood that to make the Vicker's hardness 650 or more, the N concentration has to be made 0.35% or more.
  • the heating temperature is 900° C. or more, N unnecessarily diffuses to the inside and the N concentration at the nitrided layer from the surface down to 0.2 mm required for improvement of the contact fatigue strength becomes less than 0.35%.
  • the Vicker's hardness when tempering at 300° C. becomes less than 650 and, furthermore, the increase in the oxide layer causes deterioration of the mechanical properties. If the heating temperature is less than the austenization temperature, martensite transformation does not occur and a high surface hardness cannot be obtained.
  • the holding time is less than 0.05 second, the breakdown of the compound layer and diffusion of the N produced become insufficient.
  • N is unnecessarily diffused inward and the N concentration at the nitrided layer from the surface down to 0.2 mm required for improvement of the contact fatigue strength becomes less than 0.35%.
  • the Vicker's hardness when tempering at 300° C. becomes less than 650.
  • the frequency of the induction is around 400 kHz if a small sized steel part and around 5 kHz if a large sized steel part.
  • the coolant used for the quenching is preferably water, a polymer quenching agent, or other water-based one with a large cooling ability.
  • After induction hardening usually it is preferable to temper the steel part at a low temperature of around 150° C. based on a carburized quenched part so as to secure the toughness of the part.
  • the steel part of the present invention is a steel part obtained by soft nitriding, then induction hardening, characterized in that a surface layer from the surface down to a depth of 5 ⁇ m or more includes pores of a circle equivalent diameter of 0.1 to 1 ⁇ m in an amount of 10000/mm 2 or more.
  • lubrication of the operating surfaces is important. If the lubrication is insufficient, the steel materials will contact each other, seizing and sticking will occur, and the contact fatigue strength falls.
  • the present invention is characterized by soft nitriding the surface layer of the steel material to form a compound layer mainly comprised of Fe 3 N, Fe 4 N, and Fe nitrides, then induction heating it to austenize and quench it to form a nitrided layer.
  • the nitrided layer is formed by the breakdown of the compound layer, but at this time, the N in the compound layer diffuses inside to form a nitrided layer and the old compound layer becomes a hard porous layer in which a large number of pores are dispersed. These pores function as oil reservoirs resulting in an improvement in the lubricating effect and a greater improvement in the wear resistance and durability.
  • the soft nitriding conditions and induction heating conditions have to be suitably controlled.
  • the compound layer formed by the soft nitriding also has some pores, so an oil reservoir effect is expressed, but said compound layer is extremely fragile and cannot withstand a large load, so a high contact fatigue strength cannot be obtained.
  • the size of the pores was limited to a circle equivalent diameter of 1 ⁇ m or less.
  • the pores are too small, the pores do not sufficiently function as oil reservoirs, so the size of the pores has to be a circle equivalent diameter of 0.1 ⁇ m or more.
  • the pores do not effectively function as oil reservoirs, so there have to be 10000/mm 2 or more at the nitrided layer from the surface down to 5 [ 2 m or more in depth.
  • the gear faces of gears and other sliding members in normal operation, are worn down by 5 ⁇ m or more before the end of their lifetime, so it is necessary that there be pores in the amount of 10000/mm 2 or more at the surface down to a depth of 5 ⁇ m or more. If the pores are large in size, the density depends on the soft nitriding conditions and induction heating conditions.
  • soft nitriding is performed at 580° C. to less than 600° C.
  • induction heating is performed at 880° C. to less than 900° C. for 1 second to 4 seconds.
  • the structure of the steel member has to be made one with a surface layer of martensite and a core remaining as a ferrite-pearlite structure. If quenching only the surface layer to cause martensite transformation and giving the surface layer compressive residual stress, the contact fatigue strength is improved. If the core as well is made to transform to martensite, the surface layer falls in compressive residual stress and the contact fatigue strength falls.
  • test pieces of a diameter of 26 mm and a length of 100 mm were fabricated for use for hardness tests for investigating the softening resistance by tempering.
  • the small roller test pieces and large roller test pieces were soft nitrided (nitrided in N 2 (0.45 Nm 3 /h)+NH 3 (0.5 Nm 3 /h)+CO 2 (0.05 Nm 3 /h) atmosphere at a predetermined temperature for 2 hours and cooled in N 2 gas), then induction hardened (frequency 100 kHz).
  • the test pieces of the large rollers and small rollers were used for a standard contact fatigue test, that is, roller pitching fatigue test.
  • the roller pitching fatigue test was conducted by applying various Hertz stresses (contact pressures) to the small rollers and pushing the large rollers, making the rotating directions of the two test pieces the same at the contact parts, and making the slip rate ⁇ 40% (at the contact parts, the peripheral speeds of the large rollers being 40% larger than the peripheral speeds of the small roller test pieces).
  • the oil temperature of the gear oil fed to the above contact parts was made 90° C.
  • the test was usually ended at 10,000,000 ⁇ (10 7 ⁇ ) showing the fatigue limit of steel.
  • the maximum Hertz stress where pitching did not occur at the small roller and a speed of 10,000,000 was reached was made the fatigue limit of the small roller.
  • the occurrence of pitching was detected by a vibration meter attached to the tester. After detecting vibration, the two rollers were stopped from rotating and the occurrence of pitching and the speed were confirmed.
  • test pieces for measurement of hardness were soft nitrided and induction hardened under the same conditions as the case of the small roller test pieces and large roller test pieces. After this, the pieces were quenched at 300° C. for 60 minutes and cut. The cut cross-sections were measured for the distribution of hardness from the surfaces to the cores by a Vicker's hardness meter. Note that the quenched surface layer structure was martensite and the non-quenched core remained a ferrite-pearlite structure.
  • N concentration at a position 0.2 m from the surface was measured by EPMA.
  • the density of pores of a circle equivalent diameter of 0.1 to 1 ⁇ m was found by taking test pieces soft nitrided and induction hardened under the same conditions as the small roller test pieces and large roller test pieces, cutting them at surfaces perpendicular to the rolling direction, embedding these in a resin, mirror polishing them, and quantifying the resultant surface layer structures by image processing.
  • the measurement was performed by a power of 3000 ⁇ for a field of 50 ⁇ m 2 for 40 fields.
  • the measurement value was converted to the number of pores per mm 2 to calculate the density.
  • the invention examples of Examples 1 to 31 all have a lifetime of the roller pitching fatigue test of 10,000,000 (10 7 ) or more and an excellent contact fatigue strength (high fatigue test life) and give good results compared with the comparative examples of Examples 32 to 53.
  • the Mn/S is low and the surface concentration of S cannot be prevented.
  • the compound layer formed by the soft nitriding is thin, the thickness of the nitrided layer of the steel part is a small one of less than 0.4 mm after induction hardening, the N concentration from the surface down to a depth of 0.2 mm is low, and the Vicker's hardness with 300° C. tempering is a value less than 650.
  • the induction heating temperature is too high, N unnecessarily diffuses inside, and the thickness of the nitrided layer is 0.65 mm or in the scope of the present invention, but the N concentration of the steel part from the surface down to a depth of 0.2 mm is a low 0.20%, the 300° C. tempered Vicker's hardness is also 575, that is, does not reach 650, and furthermore an oxide layer is formed at the surface layer and the fatigue life is short.
  • the induction heating time is too long and N unnecessarily diffuses inside and the thickness of the nitrided layer is 0.70 mm or in the scope of the present invention, but the N concentration of the steel part from the surface down to a depth of 0.2 mm is a low 0.27%, the 300° C. tempered Vicker's hardness is a low 471, and the fatigue life is short.
  • the chemical composition is in the range defined by the present invention, the N concentration from the surface down to a depth of 0.2 mm is a high 0.36%, and the Vicker's hardness when tempering at 300° C. is also a high value of 651, but the soft nitriding temperature is too low, so the compound layer is thin, the nitrided layer of the steel part is also a thin 0.25 mm, and the fatigue life is short.
  • Example 45 where the chemical composition and the soft nitriding conditions exceed the scope of the present invention, the Mn/S is small and the soft nitriding temperature is too high, so the compound layer is thin. Furthermore, W is not added, so the nitrided layer of the steel part is also a thin 0.15 mm, the N concentration from the surface down to a depth of 0.2 mm is a low 0.06%, the Vicker's hardness when tempering at 300° C. is also 388, that is, does not reach 650, and the fatigue life is short.
  • the compound layer is thin, so the nitrided layer of the steel part is thin, the 300° C. tempered hardness from the surface down to a depth of 0.2 mm is low, and the fatigue life does not reach 10,000,000.
  • the Mn/S is small, so the compound layer is thin. Further, W is not added and the induction heating temperature is high, so the thickness of the nitrided layer of the steel part is a small 0.12 mm, the N concentration from the surface down to a depth of 0.2 mm is 0.18%, the 300° C. tempered hardness is a low 580, and the fatigue life is short.
  • the compound layer is thin, so the nitrided layer of the steel part is thin, the 300° C. tempered hardness from the surface down to a depth of 0.2 mm is low, and the fatigue life does not reach 10,000,000.
  • the Mn/S is 70 or more, but the soft nitriding temperature is high, so the compound layer is thin. Further, W is not added and the induction heating temperature is high, so the nitrided layer of the steel part is a thin 0.30 mm, the N concentration from the surface down to 0.2 mm is 0.08%, the 300° C. tempered hardness is a low 535, and the fatigue life is extremely short.
  • the present invention it is possible to provide steel for structural use for surface hardening able to be applied to power transmission parts of automobiles etc. and provide steel parts having a high contact fatigue strength, in particular gears, continuously variable transmissions, constant velocity joints, hubs, bearings and other steel parts for machine structure use.
  • the present invention greatly contributes to the higher output and lower cost of automobiles, so the industrial applicability is large.

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WO2010070958A1 (ja) 2010-06-24
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AU2009311051A1 (en) 2010-07-08
CN101842507B (zh) 2013-03-27
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US9156231B2 (en) 2015-10-13
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