WO2018066667A1 - Composant nitruré et procédé de production dudit composant nitruré - Google Patents

Composant nitruré et procédé de production dudit composant nitruré Download PDF

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WO2018066667A1
WO2018066667A1 PCT/JP2017/036378 JP2017036378W WO2018066667A1 WO 2018066667 A1 WO2018066667 A1 WO 2018066667A1 JP 2017036378 W JP2017036378 W JP 2017036378W WO 2018066667 A1 WO2018066667 A1 WO 2018066667A1
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nitriding
compound layer
phase
steel
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PCT/JP2017/036378
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Japanese (ja)
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崇秀 梅原
将人 祐谷
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新日鐵住金株式会社
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Priority to BR112019005781A priority Critical patent/BR112019005781A2/pt
Priority to US16/337,675 priority patent/US20200024720A1/en
Priority to EP17858503.0A priority patent/EP3524709A4/fr
Priority to JP2018543972A priority patent/JP6766876B2/ja
Priority to KR1020197002678A priority patent/KR20190022801A/ko
Priority to CN201780058329.5A priority patent/CN109790614A/zh
Publication of WO2018066667A1 publication Critical patent/WO2018066667A1/fr

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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
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    • 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
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    • 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
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    • 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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    • 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 steel part that has been subjected to gas nitriding, in particular, a nitriding part such as a gear and a CVT sheave excellent in bending straightness and bending fatigue strength, and a manufacturing method thereof.
  • Steel parts used in automobiles and various industrial machines such as carburizing and quenching, induction hardening, nitriding, and soft nitriding are used to improve mechanical properties such as fatigue strength, wear resistance, and seizure resistance.
  • a surface hardening heat treatment is applied.
  • Nitriding treatment and soft nitriding treatment are performed in a ferrite region of A 1 point or less, and since there is no phase transformation during the treatment, heat treatment strain can be reduced. Therefore, nitriding treatment and soft nitriding treatment are often used for parts having high dimensional accuracy and large parts, and are applied to gears used for transmission parts of automobiles and crankshafts used for engines, for example.
  • Nitriding is a treatment method in which nitrogen penetrates the steel material surface.
  • the medium used for nitriding include gas, salt bath, and plasma.
  • Gas nitriding treatment with excellent productivity is mainly applied to automobile transmission parts.
  • a compound layer (a layer in which a nitride such as Fe 3 N is deposited) having a thickness of 10 ⁇ m or more is formed on the surface of the steel material, and a nitrogen diffusion layer is formed on the steel material layer below the compound layer.
  • a cured layer is formed.
  • the compound layer is mainly composed of Fe 2-3 N ( ⁇ ) and Fe 4 N ( ⁇ ′), and the hardness of the compound layer is extremely higher than that of steel as a base material. Therefore, the compound layer improves the wear resistance of the steel part in the initial stage of use.
  • Patent Document 1 discloses a nitriding component in which bending fatigue strength is improved by setting the ⁇ ′ phase ratio in the compound layer to 30 mol% or more.
  • Patent Document 2 discloses a steel member having a low strain and excellent surface fatigue strength and bending fatigue strength, in which an iron nitride compound layer having a predetermined structure is formed on the steel member.
  • Patent Document 3 discloses a method for manufacturing a nitrided part by optimizing the element content to increase the fatigue strength after nitriding and suppress deformation after nitriding.
  • the nitriding component of Patent Document 1 is gas soft nitriding using CO 2 as the atmospheric gas, the surface side of the compound layer is likely to be in the ⁇ phase, so that the bending fatigue strength is not yet sufficient.
  • the nitriding component of Patent Document 2 has an NH 3 gas of 0.08 to 0.34, an H 2 gas of 0.54 to 0.82, and an N 2 gas of 0.09 to 0.09 to regardless of the steel components. Since it is controlled to be 0.18, depending on the steel components, the structure and thickness of the compound layer may not be as intended.
  • An object of the present invention is to provide a part excellent in rotational bending fatigue strength in addition to bend straightening and a method for manufacturing the same.
  • the present inventors focused on 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 steel with adjusted components is nitrided under nitriding potential control that takes into account the C content of the material, so that the vicinity of the surface becomes a phase structure mainly composed of ⁇ 'phase, suppressing the generation of porous material and compressing residual surface layers. It has been found that by setting the stress to a certain value or more, a nitrided part having excellent bend straightening properties and rotational bending fatigue strength can be produced.
  • the present invention has been further studied based on the above findings, and the gist thereof is as follows.
  • C 0.20 to 0.60%, Si: 0.05 to 1.5%, Mn: 0.2 to 2.5%, P: 0.025% or less, S: 0.003 -0.05%, Cr: 0.05-0.50%, Al: 0.01-0.05%, N: 0.003-0.025%, Nb: 0-0.1%, B: 0 to 0.01%, Mo: 0% or more, less than 0.50%, V: 0% or more, less than 0.50%, Cu: 0% or more, less than 0.50%, Ni: 0% or more, 0 Less than 50% and Ti: 0% or more and less than 0.05%, with the balance being Fe and impurities, and the thickness containing iron, nitrogen and carbon formed on the steel surface
  • the compound layer has a compound layer of 3 ⁇ m or more and less than 15 ⁇ m, and the phase structure in the compound layer having a depth of 5 ⁇ m to the surface contains ⁇ ′ phase in an area ratio of 50% or more, and at a depth of 3 ⁇ m to the surface A n
  • C 0.20% or more, 0.60% or less
  • C is an element necessary for securing the core hardness of the component. If the C content is less than 0.20%, the core strength is too low, so that the bending straightness and bending fatigue strength are greatly reduced. On the other hand, when the C content exceeds 0.60%, the thickness of the compound layer increases, and the bending straightness and bending resistance are greatly reduced. A preferable range of the C content is 0.30 to 0.50%.
  • Si 0.05% or more, 1.5% or less
  • Si increases the core hardness by solid solution strengthening. In order to exhibit this effect, 0.05% or more is contained.
  • the Si content exceeds 1.5%, the strength after steel bar, wire, and hot forging becomes too high, so that the machinability is greatly reduced.
  • a preferred range for the Si content is 0.08 to 1.3%.
  • Mn 0.2% or more and 2.5% or less
  • Mn increases the core hardness by solid solution strengthening. Furthermore, Mn forms fine nitrides (Mn 3 N 2 ) in the hardened layer during nitriding, and improves wear resistance and bending fatigue strength by precipitation strengthening. In order to obtain these effects, Mn needs to be 0.2% or more.
  • Mn if the content of Mn exceeds 2.5%, not only the effect of increasing the bending fatigue strength is saturated, but also the hardness after the steel bar, wire rod and hot forging as the material becomes too high. The performance is greatly reduced.
  • a preferable range of the Mn content is 0.4 to 2.3%.
  • P 0.025% or less
  • P is an impurity and segregates at the grain boundaries to embrittle the part. Therefore, the content is preferably small. If the P content exceeds 0.025%, the bending straightness and bending fatigue strength may be reduced. The upper limit with preferable P content for preventing the fall of bending fatigue strength is 0.018%. It is difficult to make the content completely zero, and the practical lower limit is 0.001%.
  • S 0.003% to 0.05%
  • S combines with Mn to form MnS and improves the machinability. In order to obtain this effect, S needs to be 0.003% or more. However, when the S content exceeds 0.05%, coarse MnS is easily generated, and the bending straightness and bending fatigue strength are greatly reduced. A preferred range for the S content is 0.005 to 0.03%.
  • Cr 0.05% or more and 0.50% or less
  • Cr forms fine nitride (CrN) in the hardened layer during nitriding, and improves bending fatigue strength by precipitation strengthening.
  • Cr needs to be 0.05% or more.
  • the Cr content exceeds 0.5%, the hardness after the steel bars, wire rods, and hot forging as raw materials becomes too high, so that the bending straightness decreases.
  • a preferable range of the Cr content is 0.10 to 0.30%.
  • Al 0.01% or more, 0.05% or less
  • Al is a deoxidizing element, and 0.01% or more is necessary for sufficient deoxidation.
  • Al tends to form hard oxide inclusions, and if the Al content exceeds 0.05%, the bending fatigue strength is significantly reduced, and the desired bending can be achieved even if other requirements are satisfied. Fatigue strength cannot be obtained.
  • a preferable range of the Al content is 0.02 to 0.04%.
  • N combines with Al and V to form AlN and VN.
  • AlN and VN have the effect of refining the structure of the steel material before nitriding by the pinning action of austenite grains and reducing the variation in mechanical properties of the nitriding parts. This effect is difficult to obtain when the N content is less than 0.003%.
  • the content of N exceeds 0.025%, coarse AlN is likely to be formed, and thus the above effect is difficult to obtain.
  • a preferable range of the N content is 0.005 to 0.020%.
  • the chemical composition of steel used as the material for the nitriding component of the present invention contains the above elements, and the balance is Fe and inevitable impurities. Inevitable impurities are components contained in raw materials or mixed in during the manufacturing process, and are components not intentionally contained in steel.
  • the steel used as the material of the nitriding component of the present invention may contain the following elements instead of part of Fe.
  • Nb 0% or more, 0.1% or less
  • Nb combines with C and N to form NbC and NbN.
  • the pinning effect of NbC and NbN suppresses the austenite grain coarsening, refines the structure of the steel material before nitriding, and reduces the variation in mechanical properties of the nitriding component.
  • This effect can be obtained by adding a small amount of Nb, but in order to obtain the effect more reliably, Nb is preferably 0.01% or more. If the Nb content exceeds 0.1%, coarse NbC and NbN are likely to be formed, making it difficult to obtain the above effect.
  • B has the effect of suppressing grain boundary segregation of P and improving toughness. Moreover, it combines with N to form BN and improve machinability. These effects can be obtained by adding a small amount of B, but in order to obtain the effect more reliably, B is preferably 0.0005% or more. When the content of B exceeds 0.01%, not only the above effect is saturated, but also a large amount of BN segregates, which may cause cracks in the steel material.
  • Mo forms fine nitride (Mo 2 N) in the hardened layer during nitriding, and improves bending fatigue strength by precipitation strengthening.
  • Mo exhibits an age hardening action during nitriding to improve the core hardness.
  • the Mo content for obtaining these effects is preferably 0.01% or more.
  • the Mo content is 0.50% or more, the hardness after the steel bar, wire rod, and hot forging as raw materials becomes too high, so that the machinability is remarkably lowered and the alloy cost is increased.
  • the upper limit with preferable Mo content for ensuring machinability is less than 0.40%.
  • V forms fine nitride (VN) during nitriding and soft nitriding, improves bending fatigue strength by precipitation strengthening, and increases the core hardness of the component. It also has the effect of refining the structure. In order to obtain these actions, V is preferably 0.01% or more. On the other hand, if the V content is 0.50% or more, the hardness of the raw steel bar, wire, and hot forging becomes too high, so that the machinability is remarkably lowered and the alloy cost is increased. A preferable range of the V content for ensuring the machinability is less than 0.40%.
  • Cu 0% or more and less than 0.50%
  • the content is preferably 0.01% or more.
  • the Cu content is 0.50% or more, the hardness after the steel bar, wire rod, and hot forging becomes too high, so that the machinability is remarkably lowered and the hot ductility is also lowered. It causes surface scratches during hot rolling and hot forging.
  • a preferable range of the Cu content for maintaining hot ductility is less than 0.40%.
  • Ni improves the core hardness and surface hardness by solid solution strengthening.
  • the content is preferably 0.01% or more.
  • the Ni content is 0.50% or more, the hardness after steel bar, wire, and hot forging becomes too high, so that the machinability is remarkably lowered and the alloy cost is increased.
  • a preferable range of the Ni content for obtaining sufficient machinability is less than 0.40%.
  • Ti 0% or more and less than 0.05%
  • Ti combines with N to form TiN and improves core hardness and surface hardness.
  • Ti is preferably 0.005% or more.
  • the Ti content is 0.05% or more, the effect of improving the core hardness and the surface layer hardness is saturated, and the alloy cost increases.
  • a preferred range for the Ti content is 0.007 to less than 0.04%.
  • the compound layer is an iron nitride layer formed by nitriding treatment, and the thickness of the compound layer affects the bending straightness and bending strength of the nitriding component. If the compound layer is too thick, it tends to be a starting point for bending fatigue fracture. If the compound layer is too thin, sufficient residual stress on the surface cannot be obtained, and the bending straightness and bending fatigue strength will be reduced.
  • the thickness of the compound layer is set to 3 ⁇ m or more and less than 15 ⁇ m from the viewpoint of bending straightness and bending strength.
  • the thickness of the compound layer is measured by gas nitriding treatment, polishing a vertical section of the test material, etching and observing with an optical microscope. Etching is performed with a 3% nital solution for 20-30 seconds.
  • the compound layer exists in the surface layer of the low alloy steel and is observed as a white uncorroded layer. Observe 5 visual fields (field area: 2.2 ⁇ 10 4 ⁇ m 2 ) of the tissue photograph taken at 500 times with an optical microscope. In each field of view, four points are measured every 30 ⁇ m in the horizontal direction. The average value of the 20 measured values is defined as the compound thickness ( ⁇ m).
  • FIG. 1 shows an outline of the measurement method
  • FIG. 2 shows an example of a structure photograph of the compound layer and the diffusion layer.
  • the ⁇ ′ phase ratio in the compound layer is obtained by backscattered electron diffraction (Electron Backscatter Diffraction: EBSD). Specifically, EBSD measurement is performed on an area of 150 ⁇ m 2 from the outermost surface of the compound layer to a depth of 5 ⁇ m, and an analysis diagram for discriminating the ⁇ ′ phase and the ⁇ phase is created. Then, for the obtained EBSD analysis image, the area ratio of the ⁇ ′ phase is obtained using an image processing application, and this is defined as the ⁇ ′ phase ratio (%). In EBSD measurement, it is appropriate to measure about 10 fields of view at a magnification of about 4000 times.
  • the above ⁇ ′ phase ratio means the ratio of the “compound layer” ⁇ ′ phase having a depth of 5 ⁇ m from the surface. That is, when the thickness of the compound layer is less than 5 ⁇ m from the surface, the ⁇ ′ phase ratio in the region corresponding to the thickness of the compound layer is calculated. As an example, if the thickness of the compound is 3 ⁇ m from the surface, the ratio of the ⁇ ′ phase of the compound layer having a depth of 3 ⁇ m to the surface is the ⁇ ′ phase ratio.
  • the ⁇ ′ phase ratio is preferably 60% or more, more preferably 65% or more, and even more preferably 70% or more.
  • a method of obtaining the ⁇ ′ phase ratio using X-ray diffraction is also conceivable.
  • the measurement region becomes ambiguous, and an accurate ⁇ ′ phase ratio cannot be obtained. Therefore, the ⁇ 'phase ratio in the compound layer in the present invention is determined by EBSD.
  • the void is formed by desorbing N 2 gas from the surface of the steel material along the grain boundary from a location that is stable in terms of energy, such as a grain boundary, on the surface of the steel material having a small restraining force by the base material.
  • the generation of N 2 becomes easier as the nitriding potential K N described later increases.
  • K N 'occurs phase transformation ⁇ ⁇ , ⁇ ' bcc ⁇ ⁇ accordance becomes high due towards the epsilon phase than phase is larger amount of dissolved N 2, towards the epsilon phase N 2 gas It is because it is easy to generate.
  • FIG. 3 shows an outline in which voids are formed in the compound layer
  • FIG. 4 shows a structure photograph in which voids are formed.
  • the void area ratio can be measured by observation with an optical microscope. Specifically, 5 fields of view (field area: 5.6 ⁇ 10 3 ⁇ m 2 ) were measured at a magnification of 1000 times from the surface to 3 ⁇ m in the cross section of the test material, and 3 ⁇ m from the outermost surface for each field of view. The ratio of the voids in the depth range is defined as the void area ratio.
  • the void area ratio is preferably less than 5%, more preferably less than 2%, even more preferably less than 1%, and most preferably 0.
  • the steel surface is hardened by nitriding treatment, and compressive residual stress is introduced into the surface layer portion of the steel, so that the fatigue strength and wear resistance of the component are improved.
  • the nitriding component of the present invention has excellent bending fatigue strength by improving the compound layer as described above and further introducing a compressive residual stress of 500 MPa or more on the surface. A manufacturing method for introducing such compressive residual stress into the surface of the component will be described later.
  • a gas nitriding treatment is performed on a steel material having the above-described components.
  • the gas nitriding treatment temperature is 550 to 620 ° C., and the entire gas nitriding treatment time is 1.5 to 10 hours.
  • the gas nitriding temperature (nitriding temperature) is mainly correlated with the diffusion rate of nitrogen and affects the surface hardness and the hardened layer depth. If the nitriding temperature is too low, the diffusion rate of nitrogen is slow, the surface hardness is low, and the hardened layer depth is shallow. On the other hand, if it exceeds nitriding temperature the C1 point A, ferrite phase (alpha phase) the nitrogen diffusion rate is small austenite phase than (gamma phase) is generated in the steel, the surface hardness becomes low, hardening depth Becomes shallower. Therefore, in this embodiment, the nitriding temperature is 550 to 620 ° C. around the ferrite temperature range. In this case, it can suppress that surface hardness becomes low, and can suppress that hardened layer depth becomes shallow.
  • Total gas nitriding treatment time 1.5 to 10 hours
  • the gas nitriding treatment is performed in an atmosphere containing NH 3 , H 2 , and N 2 .
  • the entire time of nitriding treatment that is, the time from the start to the end of nitriding treatment (treatment time) correlates with the formation and decomposition of the compound layer and the diffusion and penetration of nitrogen, and affects the surface hardness and the depth of the hardened layer. Effect.
  • processing time is too short, surface hardness will become low and the hardening layer depth will become shallow.
  • the treatment time is too long, denitrification and decarburization occur and the surface hardness of the steel decreases. If the processing time is too long, the manufacturing cost is further increased. Accordingly, the processing time of the entire nitriding process is 1.5 to 10 hours.
  • the atmosphere of the gas nitriding treatment of the present embodiment inevitably contains impurities such as oxygen and carbon dioxide in addition to NH 3 , H 2 and N 2 .
  • a preferable atmosphere is 99.5% (volume%) or more in total of NH 3 , H 2 and N 2 .
  • nitriding is performed under a nitriding potential controlled in consideration of the C content of the material.
  • the phase structure in the compound layer having a depth of 5 ⁇ m to the surface is set to a ⁇ ′ phase ratio of 50% or more, the void area ratio in the depth of 3 ⁇ m to the surface is less than 1%, and the compressive residual stress on the surface of the compound layer is set to 500 MPa. This can be done.
  • the nitriding potential K N of the gas nitriding process is defined by the following equation.
  • K N (atm ⁇ 1/2 ) ((NH 3 partial pressure (atm)) / [(H 2 partial pressure (atm)) 3/2 ]
  • the partial pressure of NH 3 and H 2 in the gas nitriding atmosphere can be controlled by adjusting the gas flow rate.
  • K N at the time of gas nitriding treatment needs to be a certain value or more. As described above, if K N becomes too high, N 2 gas is likely to be generated. The proportion of phases increases and voids increase. Thus, provided the conditions of K N, it is important to suppress the generation of voids.
  • the nitriding potential of the gas nitriding treatment affects the phase structure of the compound layer and the rotational bending fatigue strength of the nitriding component, and the optimum nitriding potential is determined by the C content of the steel. I found it.
  • the nitriding potential during the gas nitriding treatment is always 0.15 ⁇ K N ⁇ ⁇ 0.17 during the gas nitriding treatment. It was found that if ⁇ ln (mass% C) +0.20 was satisfied, the phase structure of the compound layer would be a ⁇ ′ phase ratio of 50% or more, and that the nitriding component had high bending straightening and rotational bending fatigue strength. .
  • the ⁇ ′ phase ratio in the compound layer does not become 50% or more when taking a nitriding potential value that does not satisfy the above formula even once.
  • FIG. 5 shows the results of investigating the relationship between the nitriding potential, the ⁇ ′ ratio of the compound layer, and the rotational bending fatigue strength.
  • FIG. 5 is about the steel a (Table 1) of the Example mentioned later.
  • the gas nitriding treatment is performed under the nitriding potential K N corresponding to the C amount of the steel as the material.
  • K N nitriding potential
  • a to aa having chemical components shown in Table 1 were melted in a 50 kg vacuum melting furnace to produce molten steel, and the molten steel was cast to produce an ingot.
  • a to s are steels having chemical components defined in the present invention.
  • the steels t to aa are comparative steels that are at least one element or more out of the chemical components defined in the present invention.
  • This ingot was hot forged into a round bar with a diameter of 25 mm. Subsequently, after each round bar was annealed, cutting was performed to prepare a square test piece for evaluating the bending straightness shown in FIG. Furthermore, the cylindrical test piece for evaluating the bending fatigue strength shown in FIG. 3 was produced.
  • a gas nitriding treatment was performed on the collected specimen under the following conditions.
  • the test piece was placed in a gas nitriding furnace, NH 3 , H 2 , and N 2 gases were introduced into the furnace, and nitriding was performed under the conditions shown in Table 2.
  • the test number 32 was a gas soft nitriding treatment in which CO 2 gas was added at 3% by volume in the atmosphere. Oil cooling was performed using 80 ° C. oil on the test piece after the gas nitriding treatment.
  • 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 thermal conductivity between the standard gas and the measurement gas was measured in terms of gas concentration.
  • the H 2 partial pressure was continuously measured during the gas nitriding process.
  • the NH 3 partial pressure was measured by attaching a manual glass tube NH 3 analyzer outside the furnace.
  • the partial pressure of residual NH 3 was measured every 10 minutes, and at the same time, the nitriding potential K N was calculated, and the NH 3 flow rate and the N 2 flow rate were adjusted so as to converge to the target value.
  • the nitriding potential K N was calculated every 10 minutes when the NH 3 partial pressure was measured, and the NH 3 flow rate and the N 2 flow rate were adjusted so as to converge to the target value.
  • the compound layer can be confirmed as a white uncorroded layer present in the surface layer.
  • the compound layer was observed from 10 visual fields (field area: 6.6 ⁇ 10 2 ⁇ m 2 ) photographed at a magnification of 4000 times, and the thickness of three compound layers was measured every 10 ⁇ m. And the average value of 30 points measured was defined as the compound thickness ( ⁇ m).
  • the ratio of the total area of voids in the area of 90 ⁇ m 2 in the range of 3 ⁇ m depth from the outermost surface was obtained by binarizing with an image processing application. And the measured average value of 10 visual fields was defined as the void area ratio (%). Even in the case where the compound layer was less than 3 ⁇ m, the measurement object was similarly measured from the surface to a depth of 3 ⁇ m.
  • the ⁇ ′ phase ratio in the compound layer was determined by backscattered electron diffraction (EBSD).
  • EBSD measurement is performed for an area of 150 ⁇ m 2 from the outermost surface of the compound layer to a depth of 5 ⁇ m, an analysis diagram for discriminating the ⁇ ′ phase and the ⁇ phase is created, and the obtained EBSD analysis image is subjected to ⁇ ′ using an image processing application.
  • the phase ratio (%) was determined. In EBSD measurement, 10 fields of view were measured at a magnification of 4000 times.
  • the average value of the measured ⁇ ′ phase ratios of the 10 visual fields was defined as the ⁇ ′ phase ratio (%).
  • the ⁇ ′ phase ratio in the region corresponding to the thickness of the compound layer was calculated.
  • ⁇ c V ⁇ ′ ⁇ ⁇ ′ + V ⁇ ⁇ ⁇ + V m ⁇ m
  • a static bending test was carried out on the square specimen subjected to the gas nitriding treatment.
  • the static bending test was performed by four-point bending with an inner fulcrum distance of 30 mm and an outer fulcrum distance of 80 mm, and the strain rate was 2 mm / min.
  • a strain gauge was attached to the R portion in the longitudinal direction of the square test piece, and the maximum strain amount (%) when a crack occurred in the R portion and the strain gauge could not be measured was determined as the bending straightness.
  • the aim was to have a bending straightness of 1.2% or more.
  • the maximum stress at the fatigue limit was set to 520 MPa or more.
  • Test results The results are shown in Table 2.
  • Test Nos. 1 to 23 are steel components and gas nitriding conditions within the scope of the present invention, the compound thickness is 3 to 15 ⁇ m, the ⁇ ′ layer ratio of the compound layer is 50% or more, and the void ratio of the compound layer Less than 10%, the compressive residual stress of the compound layer was 500 MPa or more.
  • good results were obtained with a bending straightness of 1.2% or more and a rotational bending fatigue strength of 520 MPa or more.
  • Test No. 30 had a lower lower limit of the nitriding potential, a sufficient compound layer thickness was not obtained, and the residual stress on the surface was lowered, so that the rotational bending fatigue strength was lowered.
  • Test No. 32 had a high upper limit of the nitriding potential, an increase in the void area ratio, and a decrease in bending straightness and rotational bending fatigue strength.
  • the upper limit of the nitriding potential was too high, the thickness of the compound layer was increased, the ⁇ ′ phase ratio was low, and the void area ratio was increased, so that the bending straightness and the rotational bending fatigue strength were low.
  • Test No. 34 was soft nitriding, and almost no ⁇ 'phase was formed on the surface, and the residual stress was low, so that the bending straightness and the rotational bending fatigue strength were low.
  • Test No. 39 had an excessive amount of P and S in the steel, and was destroyed early due to segregation of P grain boundaries and generation of coarse MnS.
  • test number 41 the amount of Al in the steel was too high, oxide inclusions were generated, and the base layer was used as the starting point, and it was destroyed early.
  • Test No. 42 had a low C content and Mn content of steel, and a high Cr content. Therefore, the hardness of the base material was increased, and the bending straightness and rotational bending fatigue strength were decreased.

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Abstract

L'invention concerne un composant nitruré présentant une excellente résistance à la fatigue par flexion en rotation en plus de propriétés de flexion/redressage, et est caractérisé en ce que : un matériau en acier possédant une composition chimique prescrite est utilisé comme matériau associé ; une couche de composé qui contient du fer, de l'azote et du carbone et possède une épaisseur supérieure ou égale à 3 µm et inférieure à 15 µm est formée sur la surface d'acier ; la structure de phase de la couche de composé dans la plage allant de la surface à une profondeur de 5 µm contient une phase γ' dans une quantité égale ou supérieure à 50 % par rapport de surface ; le rapport de surface de vide dans la plage allant de la surface à une profondeur de 3 µm est inférieur à 1 % ; et la contrainte résiduelle de compression au niveau de la surface de couche de composé est d'au moins 500 MPa. L'invention concerne en outre un procédé de production dudit composant nitruré.
PCT/JP2017/036378 2016-10-05 2017-10-05 Composant nitruré et procédé de production dudit composant nitruré WO2018066667A1 (fr)

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BR112019005781A BR112019005781A2 (pt) 2016-10-05 2017-10-05 parte nitretada e método de produção da mesma
US16/337,675 US20200024720A1 (en) 2016-10-05 2017-10-05 Nitrided part and method of production of same
EP17858503.0A EP3524709A4 (fr) 2016-10-05 2017-10-05 Composant nitruré et procédé de production dudit composant nitruré
JP2018543972A JP6766876B2 (ja) 2016-10-05 2017-10-05 窒化処理部品及びその製造方法
KR1020197002678A KR20190022801A (ko) 2016-10-05 2017-10-05 질화 처리 부품 및 그의 제조 방법
CN201780058329.5A CN109790614A (zh) 2016-10-05 2017-10-05 氮化处理部件及其制造方法

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EP3524709A4 (fr) 2020-02-19
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