US20200024720A1 - Nitrided part and method of production of same - Google Patents

Nitrided part and method of production of same Download PDF

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US20200024720A1
US20200024720A1 US16/337,675 US201716337675A US2020024720A1 US 20200024720 A1 US20200024720 A1 US 20200024720A1 US 201716337675 A US201716337675 A US 201716337675A US 2020024720 A1 US2020024720 A1 US 2020024720A1
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compound layer
less
nitriding
fatigue strength
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Takahide UMEHARA
Masato Yuya
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • 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
    • 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
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • 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
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C—ALLOYS
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • 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
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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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a gas nitrided steel part, in particular a gear, CVT sheave, or other nitrided part excellent in bending straightening ability and bending fatigue strength and a method of production of the same.
  • Steel parts used in automobiles and various industrial machinery etc. are improved in fatigue strength, wear resistance, seizing resistance, and other mechanical properties by carburizing and quenching, high-frequency quenching, nitriding, nitrocarburizing, and other surface hardening heat treatment.
  • Nitriding and nitrocarburizing are performed in the ferrite region of the A 1 point or less. During treatment, there is no phase transformation, so it is possible to reduce the heat treatment strain. For this reason, nitriding and nitrocarburizing are often used for parts requiring high dimensional precision and large sized parts. For example, they are applied to the gears used for transmission parts in automobiles and the crankshafts used for engines.
  • Nitriding is a method of treatment diffusing nitrogen into the surface of a steel material.
  • the medium used for the nitriding there are a gas, salt bath, plasma, etc.
  • gas nitriding is mainly being used since it is excellent in productivity. Due to gas nitriding, the surface of the steel material is formed with a compound layer of a thickness of 10 ⁇ m or more (layer at which Fe 3 N or other nitride is formed). Furthermore, the surface layer of the steel material at the lower side of the compound layer is formed with a nitrogen diffusion layer forming a hardened layer.
  • the compound layer is mainly comprised of Fe 2-3 N (c) and Fe 4 N ( ⁇ ′). The hardness of the compound layer is extremely high compared with the steel of the base material. For this reason, the compound layer improves the wear resistance of a steel part in the initial stage of use.
  • PTL 1 discloses a nitrided part in which the ⁇ ′ phase ratio in the compound layer is made 30 mol % or more to thereby improve the bending fatigue strength.
  • PTL 2 discloses a steel member having a low strain and excellent contact fatigue strength and bending fatigue strength obtained by forming an iron nitride compound layer having a predetermined structure on the steel member.
  • PTL 3 discloses a method of production of a nitrided part optimizing the amounts of elements to thereby raise the fatigue strength after nitriding and suppress deformation after nitriding.
  • the nitrided part of PTL 1 is gas soft nitrided using CO 2 for the atmospheric gas, so the surface layer side of the compound layer easily forms ⁇ phases, therefore the bending fatigue strength is believed to be still not sufficient.
  • the atmosphere is controlled to NH 3 gas: 0.08 to 0.34, H 2 gas: 0.54 to 0.82, and N 2 gas: 0.09 to 0.18 without regard as to the constituents of the steel, so there is a possibility that, depending on the constituents of the steel, the structure or thickness of the compound layer will not become as targeted.
  • the nitriding of PTL 3 does not suitably control the gas conditions at the time of treatment, becomes low in ratio of the ⁇ ′ phases in the compound layer, becomes high in porosity, and leads to easy formation of starting points of pitting and bending fatigue fracture. Further, in the gas nitrocarburizing disclosed in PTL 3, the porosity easily becomes higher.
  • the object of the present invention is to provide a part excellent in bending straightening ability plus rotating bending fatigue strength and a method of production of the same.
  • the inventors took note of the form of the compound layer formed on the surface of the steel material by nitriding and investigated the relationship with the fatigue strength.
  • the inventors discovered that by nitriding steel adjusted in constituents while controlling the nitriding potential considering the amount of C of the material, it is possible to make the vicinity of the surface a phase structure of mainly the ⁇ ′ phases, suppress the formation of a porous layer, and make the compressive residual stress of the surface layer a constant value or more to thereby fabricate a nitrided part having an excellent bending straightening ability and rotating bending fatigue strength.
  • the present invention was made based on this discovery and after further study. Its gist is as follows:
  • a nitrided part having a steel material as a material comprising, by mass %, C: 0.20% to 0.60% or less, Si: 0.05% to 1.5% or less, Mn: 0.2% to 2.5% or less, P: 0.025% or less, S: 0.003% to 0.05% or less, Cr: 0.05% to 0.50% or less, Al: 0.01% to 0.05% or less, N: 0.003% to 0.025% or less, Nb: 0% to 0.1% or less, B: 0% to 0.01% or less, Mo: 0% to less than 0.50%, V: 0% to less than 0.50%, Cu: 0% to less than 0.50%, Ni: 0% to less than 0.50%, Ti: 0% to less than 0.05% and a balance of Fe and impurities, wherein the nitrided part comprises a compound layer formed on a surface of the steel material, the compound layer containing iron, nitrogen, and carbon, a thickness of the compound layer being 3 ⁇ m
  • FIG. 1 is a view explaining a method of measurement of a depth of a compound layer.
  • FIG. 2 is one example of a structural photograph of a compound layer and diffusion layer.
  • FIG. 3 is a view showing a state of formation of pores in a compound layer.
  • FIG. 4 is one example of a structural photograph where pores are formed in a compound layer.
  • FIG. 5 is a view showing the relationships of nitriding potential with the phase structure of a compound layer and rotating bending fatigue strength.
  • FIG. 6 shows the shape of a four-point bending test piece used for evaluating a bending straightening ability.
  • FIG. 7 shows the shape of a columnar test piece for evaluating rotating bending fatigue strength.
  • C is an element required for securing the core hardness of a part. If the content of C is less than 0.20%, the core strength becomes too low, so the bending straightening ability and bending fatigue strength greatly fall. Further, if the content of C exceeds 0.60%, the compound layer thickness becomes larger and the bending straightening ability and bending resistance greatly fall.
  • the preferable range of the C content is 0.30 to 0.50%.
  • Si raises the core hardness by solution strengthening. To obtain this effect, 0.05% or more is included. On the other hand, if the content of Si exceeds 1.5%, in bars and wire rods, the strength after hot forging becomes too high, so the machinability greatly falls.
  • the preferable range of the Si content is 0.08 to 1.3%.
  • Mn raises the core hardness by solution strengthening. Furthermore, Mn forms fine nitrides (Mn 3 N 2 ) in the hardened layer at the time of nitriding and improves the wear resistance and bending fatigue strength by precipitation strengthening. To obtain these effects, Mn has to be 0.2% or more. On the other hand, if the content of Mn exceeds 2.5%, not only does the effect of raising the bending fatigue strength become saturated, but also the bars and wire rods used as materials become too high in hardness after hot forging, so the machinability greatly falls. The preferable range of the Mn content is 0.4 to 2.3%.
  • P is an impurity and segregates at the grain boundaries to make a part brittle, so the content is preferably small. If the content of P is over 0.025%, sometimes the bending straightening ability and bending fatigue strength fall. The preferable upper limit of the content of P for preventing a drop in the bending fatigue strength is 0.018%. It is difficult to make the content completely zero. The practical lower limit is 0.001%.
  • Cr forms fine nitrides (CrN) in the hardened layer during nitriding and improves the bending fatigue strength by precipitation strengthening.
  • Cr has to be 0.05% or more.
  • the preferable range of the Cr content is 0.10 to 0.30%.
  • Al is a deoxidizing element. For sufficient deoxidation, 0.01% or more is necessary. On the other hand, Al easily forms hard oxide inclusions. If the content of Al exceeds 0.05%, the bending fatigue strength remarkably falls. Even if other requirements are met, the desired bending fatigue strength can no longer be obtained.
  • the preferable range of the Al content is 0.02 to 0.04%.
  • the chemical constituents of the steel used as the material for the nitrided part of the present invention contain the above-mentioned elements and have a balance of Fe and unavoidable impurities.
  • the “unavoidable impurities” mean constituents contained in the raw materials or entering in the process of manufacture which are not intentionally included in the steel.
  • the steel used as the material for the nitrided steel part of the present invention may also contain the elements shown below in place of part of the Fe.
  • B has the effect of suppressing the segregation of P at the grain boundaries and improving the toughness. Further, it bonds with N to form BN and improve the machineability. These effects are obtained by adding B in a trace amount, but to obtain the effect more reliably, B is preferably made 0.0005% or more. If the content of B exceeds 0.01%, not only does the above effect become saturated, but also a large amount of BN segregates and sometimes cracks form in the steel material.
  • Mo forms fine nitrides (Mo 2 N) in the hardened layer during nitriding and improves the bending fatigue strength by precipitation strengthening. Further, Mo has the action of age hardening and improves the core hardness at the time of nitriding.
  • the content of Mo for obtaining these effects is preferably 0.01% or more.
  • the content of Mo is 0.50% or more, the bars and wire rods used as materials become too high in hardness after hot forging, so the machinability remarkably falls.
  • the alloy costs increase.
  • the preferable upper limit of the Mo content for securing machinability is less than 0.40%.
  • V 0% to Less than 0.50%
  • V forms fine nitrides (VN) at the time of nitriding and nitrocarburizing, improves the bending fatigue strength by precipitation strengthening, and raises the core hardness of the parts. Further, it has the effect of refining the structure. To obtain these actions, V is preferably made 0.01% or more. On the other hand, if the content of V is 0.50% or more, the bars and wire rods used as materials become too high in hardness after hot forging, so the machinability remarkably falls. In addition, the alloy costs increase. The preferable range of the V content for securing machinability is less than 0.40%.
  • Cu improves the core hardness of the part and the hardness of the nitrogen diffusion layer as a solution strengthening element.
  • inclusion of 0.01% or more is preferable.
  • the content of Cu is 0.50% or more, the bars and wire rods used as materials become too high in hardness after hot forging, so the machinability remarkably falls.
  • the hot ductility falls, so this causes the occurrence of surface defects at the time of hot rolling and at the time of hot forging.
  • the preferable range of the Cu content for maintaining the hot ductility is less than 0.40%.
  • Ni improves the core hardness and the surface layer hardness by solution strengthening. To realize this action of solution strengthening of Ni, inclusion of 0.01% or more is preferable. On the other hand, if the content of Ni is 0.50% or more, the bars and wire rods used as materials become too high in hardness after hot forging, so the machinability remarkably falls. In addition, the alloy costs increase. The preferable range of the Ni content for securing sufficient machinability is less than 0.40%.
  • Ti is preferably made 0.005% or more.
  • the content of Ti is 0.05% or more, the effect of improving the core hardness and surface layer hardness becomes saturated.
  • the alloy costs increase.
  • the preferable range of the Ti content is 0.007 to less than 0.04%.
  • Thickness of Compound Layer 3 ⁇ m to Less than 15 ⁇ m
  • the “compound layer” is the layer of iron nitride formed by the nitriding. Its thickness affects the bending straightening ability and bending strength of the nitrided part. If the compound layer is too thick, it easily becomes the starting point of bending fatigue fracture. If the compound layer is too thin, the residual stress of the surface is not sufficiently obtained and the bending straightening ability and bending fatigue strength fall. In the nitrided part of the present invention, from the viewpoint of the bending straightening ability and bending strength, the thickness of the compound layer is made 3 ⁇ m to less than 15 ⁇ m.
  • the thickness of the compound layer is found by gas nitriding then polishing the vertical cross-section of the test material, etching it, and observing it by an optical microscope. The etching is performed by a 3% Nital solution for 20 to 30 seconds.
  • the compound layer is present at the surface layer of the low alloy steel and observed as a white uncorroded layer. Five fields of a structural photograph captured by the optical microscope by 500 ⁇ (field area: 2.2 ⁇ 10 4 ⁇ m 2 ) are observed. In each field, four points are measured every 30 ⁇ m in the horizontal direction. The average value of the values of the 20 points measured is defined as the “compound thickness ( ⁇ m)”.
  • FIG. 1 shows an outline of the method of measurement
  • FIG. 2 shows one example of a structural photograph of the compound layer and diffusion layer.
  • the ratio of the ⁇ ′ phases is low and the c phase ratio is high at the compound layer from the surface to 5 ⁇ m, the compound layer easily becomes the starting point of fracture at the time of bending straightening and bending fatigue. This is because the fracture toughness value of the ⁇ phases is lower than the ⁇ ′ phases. Further, when the phases near the surface are ⁇ ′ phases, compared to when they are ⁇ phases, the later explained compressive residual stress is easily introduced into the surface and the fatigue strength can be improved.
  • the ⁇ ′ phase ratio in the compound layer is found by electron back scatter diffraction (EBSD). Specifically, the area of 150 ⁇ m 2 from the outermost surface of compound layer down to a depth of 5 ⁇ m is measured by EBSD and an analysis diagram for discriminating the ⁇ ′ phases and ⁇ phases is prepared. Further, the obtained EBSD analysis image is used to find the area ratio of the ⁇ ′ phases using an image processing application. This is defined as the “ ⁇ ′ phase ratio (%)”. In EBSD measurement, it is suitable to measure about 10 fields by a power of about 4000 ⁇ .
  • the above ⁇ ′ phase ratio means the ratio of the ⁇ ′ phases of the “compound layer” from the surface to a depth of 5 ⁇ m. That is, if the thickness of the compound layer is less than 5 ⁇ m from the surface, the ⁇ ′ phase ratio at the region of the compound layer thickness is calculated. As one example, if the thickness of the compound is 3 ⁇ m from the surface, the ratio of ⁇ ′ phases of the compound layer from the surface to a depth of 3 ⁇ m becomes the ⁇ ′ phase ratio.
  • the ⁇ ′ phase ratio is preferably 60% or more, more preferably 65% or more, still more preferably 70% or more.
  • the ⁇ ′ phase ratio may be found by the method of using X-ray diffraction.
  • measurement by X-ray diffraction becomes vague in measurement region and cannot find the accurate ⁇ ′ phase ratio. Therefore, in the present invention, the ⁇ ′ phase ratio of the compound layer is made one found by EBSD.
  • the pore area ratio has to be made less than 10%.
  • Pores are formed due to N 2 gas desorbing from the surface of the steel material along the grain boundaries from the grain boundaries and other energy stable locations at the surface of the steel material where the binding force by the matrix is small.
  • N 2 is more easily generated the higher the later explained nitriding potential K N . This is because as the K N becomes higher, a bcc ⁇ ′ ⁇ phase transformation occurs and the ⁇ phases become larger in amount of solid solution of N 2 compared with the ⁇ ′ phases and thus more easily generate N 2 gas.
  • FIG. 3 shows an outline of formation of pores in the compound layer
  • FIG. 4 shows a structural photograph of the formation of pores.
  • the pore area ratio can be measured by observation by an optical microscope. Specifically, the span from the surface to 3 ⁇ m at the cross-section of a test material is measured at five fields by a power of 1000 ⁇ (field area: 5.6 ⁇ 10 3 ⁇ m 2 ). At each field, the ratio of the pores in the range from the outermost surface to a depth of 3 ⁇ m is made the “pore area ratio”.
  • the pore area ratio is preferably less than 5%, more preferably less than 2%, still more preferably less than 1%. 0 is most preferable.
  • Compressive Residual Stress of Surface of Compound Layer 500 MPa or More
  • the nitrided part of the present invention is nitrided to harden the surface of the steel and given compressive residual stress at the surface layer part of the steel to improve the fatigue strength and wear resistance of the part.
  • the nitrided part of the present invention becomes one having an excellent bending fatigue strength by improving the compound layer in the above way and further introducing compressive residual stress of 500 MPa or more into the surface. The method of production for introducing such compressive residual stress into the surface of the part will be explained later.
  • a steel material having the above-mentioned constituents is gas nitrided.
  • the treatment temperature of the gas nitriding is 550 to 620° C., while the treatment time of the gas nitriding as a whole is 1.5 to 10 hours.
  • the temperature of the gas nitriding is mainly correlated with the diffusion rate of the nitrogen and affects the surface hardness and hardened layer depth. If the nitriding temperature is too low, the diffusion rate of the nitrogen becomes slower, the surface hardness becomes lower, and the hardened surface depth becomes shallower. On the other hand, if the nitriding temperature exceeds the A d point, austenite phases ( ⁇ phases) with diffusion rates of nitrogen smaller than the ferrite phases (a phases) are formed in the steel, the surface hardness becomes lower, and the hardened layer depth becomes shallower. Therefore, in the present embodiment, the nitriding temperature is the 550 to 620° C. around the ferrite temperature region. In this case, the surface hardness can be kept from becoming lower and the hardened layer depth can be kept from becoming shallower.
  • the gas nitriding is performed in an atmosphere containing NH 3 , H 2 , and N 2 .
  • the time of the nitriding as a whole that is, the time from the start to the end of the nitriding (treatment time) is correlated with formation and breakdown of the compound layer and the diffusion and cementation of nitrogen and affects the surface hardness and hardened layer depth. If the treatment time is too short, the surface hardness becomes lower and the hardened layer depth becomes shallower. On the other hand, if the treatment time is too long, denitrification and decarburization occur and the surface hardness of the steel falls. If the treatment time is too long, further, the production cost rises. Therefore, the treatment time of the nitriding as a whole is 1.5 to 10 hours.
  • the atmosphere of the gas nitriding of the present embodiment includes NH 3 , H 2 , and N 2 and also unavoidably oxygen, carbon dioxide, and other impurities.
  • the preferable atmosphere contains NH 3 , H 2 , and N 2 in a total of 99.5% (vol %) or more.
  • the ⁇ phases with high solid solubility limits of C are preferentially formed.
  • the ⁇ ′ phases cannot take in almost any C in solid solution, so if performing the nitrocarburizing, the compound layer becomes the single ⁇ phases. Further, since the growth rate of the ⁇ phases is faster than the ⁇ ′ phases, with gas nitrocarburizing where ⁇ phases are stably formed, the compound layer is formed thicker than required. Therefore, in the present invention, rather than gas nitrocarburizing, as explained later, it is necessary to perform gas nitriding controlling the nitriding potential K N .
  • the nitriding is performed at a nitriding potential controlled considering the amount of C of the material. Due to this, it is possible to make the phase structure at the compound layer from the surface to a depth of 5 ⁇ m a ⁇ ′ phase ratio of 50% or more, make the pore area ratio from the surface to a depth of 3 ⁇ m less than 1%, and make the compressive residual stress of the surface of the compound layer 500 MPa or more.
  • the nitriding potential K N of the gas nitriding is defined by the following formula:
  • K N (atm ⁇ 1/2 ) ((NH 3 partial pressure (atm))/[(H 2 partial pressure (atm)) 3/2 ]
  • the partial pressures of the NH 3 and H 2 of the atmosphere of the gas nitriding can be controlled by adjusting the flow rates of the gases.
  • the K N at the time of gas nitriding be a certain value or more, but as explained above, if the K N becomes too high, the ratio of the ⁇ phases easily generating N 2 gas becomes greater and the pores become greater. Therefore, it is important to provide the condition of K N and suppress the formation of pores.
  • the nitriding potential of the gas nitriding has an effect on the phase structure of the compound layer and the rotating bending fatigue strength of the nitrided part and that the optimal nitriding potential is determined by the C content of the steel.
  • the phase structure of the compound layer becomes one with a ⁇ ′ phase ratio of 50% or more and further the nitrided part has a high bending straightening ability and rotating bending fatigue strength.
  • the ⁇ ′ phase ratio at the compound layer will not become 50% or more.
  • FIG. 5 shows the results of investigation of the relationships of the nitriding potential with the ⁇ ′ ratio of the compound layer and rotating bending fatigue strength.
  • FIG. 5 relates to the steel “a” (Table 1) of the later described examples.
  • gas nitriding is performed at a nitriding potential K N corresponding to the amount of C of the steel used as a material. Due to this, it becomes possible to stably impart ⁇ ′ phases to the surface of the steel and obtain a nitrided part having excellent bending straightening ability and rotating bending fatigue strength, preferably a bending straightening ability of 1.2% or more and a rotating bending fatigue strength of 520 MPa or more.
  • the ingots were hot forged to obtain round bars of a diameter of 25 mm.
  • each round bar was annealed, then machined to fabricate rectangular test pieces shown in FIG. 2 for evaluation of the bending straightening ability.
  • columnar test pieces were fabricated for evaluation of the bending fatigue resistance shown in FIG. 3 .
  • test piece was gas nitrided under the next conditions.
  • the test piece was loaded into a gas nitriding furnace, NH 3 , H 2 , and N 2 gases were introduced into the furnace, and nitriding was carried out under the conditions shown in Table 2.
  • CO 2 gas was added to the atmosphere by a volume ratio of 3% for performing gas nitrocarburizing.
  • the gas nitrided test piece was oil cooled using 80° C. oil.
  • the H 2 partial pressure in the atmosphere was measured using a heat conduction type H 2 sensor directly attached to the gas nitriding furnace body. The difference in heat conductivities of the standard gas and measured gas was measured converted to the gas concentration. The H 2 partial pressure was measured continuously during the gas nitriding.
  • the NH 3 partial pressure was measured by attaching a manual glass tube-type NH 3 analyzer to the outside of the furnace.
  • the partial pressure of the residual NH 3 was measured every 10 minutes. Simultaneously, the nitriding potential K N was calculated. The NH 3 flow and N 2 flow were adjusted so that these converged to the target values. Every 10 minutes when measuring the NH 3 partial pressure, the nitriding potential K N was calculated and the NH 3 flow and N 2 flow were adjusted so that these converged to the target values.
  • the cross-section of a gas nitrided small roller in the direction vertical to the length direction was polished to a mirror surface and etched.
  • the etched cross-section was examined using a scanning electron microscope (SEM), measured for compound layer thickness, and checked for any pores in the surface layer part.
  • SEM scanning electron microscope
  • the compound layer can be confirmed as a white uncorroded layer present at the surface layer.
  • the compound layer was observed from 10 fields of a structural photograph taken at 4000 ⁇ (field area: 6.6 ⁇ 10 2 ⁇ m 2 ).
  • the thicknesses of the compound layer at three points were respectively measured every 10 ⁇ m.
  • the average value of the 30 points measured was defined as the compound thickness ( ⁇ m).
  • pore area ratio the ratio of the total area of the pores in an area of 90 ⁇ m 2 in a range of 3 ⁇ m depth from the outermost surface
  • the ⁇ ′ phase ratio in the compound layer was found by electron back scatter diffraction (EBSD).
  • EBSD electron back scatter diffraction
  • the area of 150 ⁇ m 2 from the outermost surface of the compound layer to a depth of 5 ⁇ m was measured by EBSD to prepare an analysis diagram for discriminating between the ⁇ ′ phases and ⁇ phases.
  • the obtained EBSD analysis image was measured for the ⁇ ′ phase ratio (%) using an image processing application. In EBSD measurement, 10 fields were measured at 4000 ⁇ power.
  • the average value of the ⁇ ′ phase ratios of the 10 fields measured was defined as the “ ⁇ ′ phase ratio (%)”. If the compound layer is less than 5 ⁇ m, the ⁇ ′ phase ratio at the region of the compound layer thickness part was calculated.
  • the nitrided small roller contact part was measured for the residual stresses ⁇ ⁇ ′ , ⁇ ⁇ , and ⁇ m of the ⁇ ′ phases, ⁇ phases, and matrix under the conditions of Table 3 using a micro-area X-ray residual stress measurement system. Furthermore, the residual stress ⁇ c found by the following formula using the area ratios V ⁇ ′ , V ⁇ , and V m of the ⁇ ′ phases, ⁇ phases, and matrix in the area 90 ⁇ m 2 in the range from the outermost surface to a depth of 3 ⁇ m found by EBSD was defined as the “residual stress of the surface”.
  • the square test piece used for gas nitriding was subjected to a static bending test.
  • the static bending test was performed by four-point bending by a distance between inside support points of 30 mm and a distance between outside support points of 80 mm.
  • the strain rate was made 2 mm/min.
  • a strain gauge was attached to the R part in the longitudinal direction of the square test piece. The maximum strain (%) when the R part cracked and measurement by the strain gauge becomes no longer possible was found as the “bending straightening ability”.

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US12031577B2 (en) 2020-02-25 2024-07-09 Nippon Steel Corporation Crankshaft and method of manufacturing the same

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JPWO2020090999A1 (ja) * 2018-11-02 2021-12-02 パーカー熱処理工業株式会社 窒化鋼部材並びに窒化鋼部材の製造方法及び製造装置
JP7415154B2 (ja) * 2019-12-24 2024-01-17 日本製鉄株式会社 窒化処理鋼部品の製造方法
WO2021172177A1 (ja) * 2020-02-25 2021-09-02 日本製鉄株式会社 クランクシャフト及びその製造方法

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WO2014136307A1 (ja) * 2013-03-08 2014-09-12 新日鐵住金株式会社 高周波焼入れ部品の素形材及びその製造方法
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EP3118346B1 (en) * 2014-03-13 2019-12-18 Nippon Steel Corporation Nitriding method and nitrided part production method
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US12031577B2 (en) 2020-02-25 2024-07-09 Nippon Steel Corporation Crankshaft and method of manufacturing the same
EP4054059A1 (de) * 2021-03-05 2022-09-07 Siemens Aktiengesellschaft Magnetblech für ein blechpaket, blechpaket, elektrische maschine und verfahren zur herstellung eines magnetblechs

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