WO2019098340A1 - 窒化処理部品 - Google Patents
窒化処理部品 Download PDFInfo
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- WO2019098340A1 WO2019098340A1 PCT/JP2018/042548 JP2018042548W WO2019098340A1 WO 2019098340 A1 WO2019098340 A1 WO 2019098340A1 JP 2018042548 W JP2018042548 W JP 2018042548W WO 2019098340 A1 WO2019098340 A1 WO 2019098340A1
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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
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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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- 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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- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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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
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- C22C—ALLOYS
- 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
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- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- 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
Definitions
- the present invention relates to a steel part subjected to gas nitriding treatment.
- Steel parts used in automobiles and various industrial machines are subject to carburizing, induction hardening, nitriding, and nitrocarburizing in order to improve mechanical properties such as fatigue strength, wear resistance, and seizure resistance.
- a surface hardening heat treatment is applied.
- Nitriding and nitrocarburizing process is performed by A 1 point or less of the ferrite region, since there is no phase transformation during processing, it is possible to reduce the heat treatment distortion. Therefore, nitriding treatment and soft nitriding treatment are often used for parts having high dimensional accuracy and large parts, and are applied to, for example, gears used for transmission parts of automobiles and crankshafts used for engines.
- the nitriding treatment is a treatment method in which nitrogen penetrates the surface of the steel material.
- the medium used for the nitriding treatment includes gas, salt bath, plasma and the like.
- the gas nitriding process which is excellent in productivity is mainly applied to transmission parts of automobiles.
- 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 by gas nitriding treatment, and a nitrogen diffusion layer is formed on the surface of the steel under the compound layer.
- a hardened layer is formed.
- the compound layer is mainly composed of Fe 2 to 3 N ( ⁇ ) and Fe 4 N ( ⁇ ′), and the hardness of the compound layer is extremely high compared to the steel core which is a non-nitrided layer. Therefore, the compound layer improves the wear resistance and the surface fatigue strength of the steel component in the early stage of use.
- Patent Document 1 discloses a nitrided component having improved bending fatigue resistance by setting the ratio of the? 'Phase in the compound layer to 30 mol% or more.
- the ratio of the ⁇ ′ phase in the compound layer is 0.5 or more, the thickness of the compound layer is 13 to 30 ⁇ m, and the compound layer thickness / hardened layer depth ⁇ 0.04, A steel member excellent in wear resistance is disclosed.
- the thickness in the compound layer is 3 to 15 ⁇ m
- the area ratio is 50% or more of the ⁇ ′ phase
- the void area ratio from the surface to a depth of 3 ⁇ m is 10
- a nitrided component excellent in rotational bending fatigue strength is disclosed by setting the compressive residual stress on the compound layer surface to 500 MPa or more.
- Patent Document 1 gas nitrocarburizing using CO 2 as an atmosphere gas, so the surface side of the compound layer tends to be ⁇ phase.
- Patent Document 3 The nitrided part of Patent Document 3 is mainly aimed at controlling the ⁇ 'phase ratio of the surface layer portion of the compound layer, and knowledge of the phase ratio and various fatigue strengths in the entire depth direction of the compound layer Because there is not enough, there is room for improvement.
- An object of the present invention is to provide a part that is excellent in surface fatigue strength or wear resistance in addition to rotational bending fatigue strength.
- the present inventors paid attention to the form of the compound layer formed on the surface of the steel material by the nitriding treatment, and investigated the relationship with the fatigue strength.
- the structure of the compound layer produced on the surface layer of the steel after nitriding is mainly made of ⁇ 'phase by nitriding the steel whose component is adjusted under the control of nitriding potential, and the void layer of the surface layer (hereinafter "porous layer") It has been found that a nitrided part having excellent rotational bending fatigue strength and surface fatigue strength or wear resistance can be produced by suppressing the occurrence of (1) and setting the hardness of the compound layer to a certain value or more.
- C 0.05 to 0.35%
- Si 0.05 to 1.50%
- Mn 0.20 to 2.50%
- P 0.025% or less
- S 0.050% or less
- Cr 0.50 to 2.50%
- V 0.05 to 1.30%
- Al 0.050% or less
- N 0.0250% or less
- Mo 0 to 1.
- a nitrided component excellent in surface fatigue strength or wear resistance in addition to rotational bending fatigue strength is suitable for gear parts, and a nitrided part having excellent wear resistance in addition to rotational bending fatigue strength is suitable for CVT and camshaft parts.
- rotational bending fatigue strength it is possible to obtain a nitrided part excellent in wear resistance.
- % representing the content of each component element and the element concentration on the part surface shall mean “mass%”.
- steel core part of the nitriding-treated part concerning this invention is equipped with the same chemical composition as the steel materials used as the raw material.
- C 0.05 to 0.35%
- C is an element necessary to secure the core hardness of the part. Therefore, C needs 0.05% or more.
- the preferable lower limit of the C content is 0.08%.
- the preferable upper limit of C content is 0.30%.
- Si 0.05 to 1.50%
- Si is an element that enhances core hardness by solid solution strengthening. In addition, the resistance to temper softening is increased, and the surface fatigue strength and wear resistance of the surface of the component, which becomes high temperature under wear conditions, are enhanced. In order to exert these effects, Si needs to be 0.05% or more. On the other hand, if the content of Si exceeds 1.50%, the strength after the bar, wire or hot forging becomes too high, and the cutting workability is greatly reduced.
- the preferred lower limit of the Si content is 0.08%.
- the preferred upper limit of the Si content is 1.30%.
- Mn forms a fine nitride (Mn 3 N 2 ) in the compound layer and the diffusion layer by nitriding treatment to increase the hardness, thereby improving the surface fatigue strength, the wear resistance, and the rotational bending fatigue strength. It is an effective element.
- the core hardness is increased by solid solution strengthening. In order to acquire these effects, 0.20% or more of Mn is required.
- Mn when the content of Mn exceeds 2.50%, not only the effect is saturated, but also the hardness after the steel bar, wire rod and hot forging which are materials become too high, so the machinability is greatly reduced.
- the preferable lower limit of the Mn content is 0.40%.
- the preferred upper limit of the Mn content is 2.30%.
- P 0.025% or less
- the content be as small as possible. If the P content exceeds 0.025%, the surface fatigue strength, the wear resistance, and the rotational bending fatigue strength may be reduced.
- the preferable upper limit of P content for preventing the fall of rotational bending fatigue strength is 0.018%. Although the content of P may be 0, it is difficult to make it completely 0, and may be 0.001% or more.
- S 0.050% or less
- S in steel is also an element that combines with Mn to form MnS and improves the machinability. In order to obtain the effect of improving the machinability, it is preferable to contain S 0.003% or more.
- S when the content of S exceeds 0.050%, coarse MnS is easily formed, and the surface fatigue strength, the wear resistance, and the rotational bending fatigue strength are greatly reduced.
- the preferable lower limit of the S content is 0.005%.
- the preferred upper limit of the S content is 0.030%.
- Cr forms a fine nitride (CrN) in the compound layer and the diffusion layer by nitriding treatment to increase the hardness, and is an element effective for improving the surface fatigue strength, the wear resistance, and the rotational bending fatigue strength. It is. In order to obtain these effects, Cr needs to be 0.50% or more. On the other hand, when the content of Cr exceeds 2.50%, not only the effect is saturated, but also the hardness after the steel bar, wire rod and hot forging which are raw materials is too high, so the machinability is significantly reduced. .
- the preferable lower limit of the Cr content is 0.70%.
- the preferred upper limit of the Cr content is 2.00%.
- V forms a fine nitride (VN) in the compound layer and the diffusion layer by nitriding treatment to increase hardness, and is an element effective for improving surface fatigue strength, wear resistance, and rotational bending fatigue strength. It is. In order to obtain these effects, V needs to be 0.05% or more. On the other hand, when the content of V exceeds 1.30%, not only the effect is saturated, but also the hardness after the steel bar, wire rod and hot forging which are raw materials is too high, so the machinability is significantly reduced. .
- the preferable lower limit of V content is 0.10%.
- the preferable upper limit of the V content is 1.10%.
- Al 0.050% or less
- Al is not an essential element, it is a deoxidizing element and is often contained to some extent in the deoxidized steel. Moreover, it combines with N to form AlN, and the pinning action of austenite grains has the effect of refining the structure of the steel before nitriding treatment and reducing the variation in mechanical characteristics of the nitrided part. In order to obtain the effect of refining the structure of the steel material, it is preferable to contain 0.010% or more.
- Al tends to form hard oxide inclusions, and when the content of Al exceeds 0.050%, the decrease in rotational bending fatigue strength becomes remarkable, and it is desirable even if other requirements are satisfied. The rotational bending fatigue strength can not be obtained.
- the preferred lower limit of the Al content is 0.020%.
- the preferred upper limit of the Al content is 0.040%.
- N 0.0250% or less
- N in the steel combines with Mn, Cr, Al and V to form Mn 3 N 2 , CrN, AlN and VN.
- Al and V which have a high tendency to form nitrides, have the effect of refining the structure of the steel before nitriding treatment and reducing the variation in mechanical characteristics of the nitrided parts by the pinning action of austenite grains.
- the content of N exceeds 0.0250%, coarse AlN is easily formed, so that the above-described effect is difficult to obtain.
- the preferable lower limit of the N content is 0.0050%.
- the preferred upper limit of the N content is 0.0200%.
- the chemical composition of the steel which is the material of the nitriding treatment part concerning the present invention contains the above-mentioned elements, and the balance is Fe and impurities.
- the impurities are components contained in the raw materials or components mixed in in the process of production and are components which are not intentionally contained in steel.
- the impurities are, for example, 0.05% or less of Te, 0.01% or less of W, Co, As, Mg, Zr, and REM.
- the addition of 0.30% or less has no significant effect on the purpose of improving machinability.
- the steel used as the raw material of the nitriding-treated part of the present invention may contain elements shown below instead of a part of Fe.
- Mo forms fine nitrides (Mo 2 N) in the compound layer and diffusion layer formed by nitriding treatment to increase hardness, thereby improving surface fatigue strength, wear resistance, and rotational bending fatigue strength. Is an element effective for In order to acquire these effects, it is preferable to make Mo into 0.01% or more. On the other hand, when the content of Mo exceeds 1.50%, not only the effect is saturated, but also the hardness after the steel bar, wire rod and hot forging which are raw materials becomes too high, so the machinability decreases remarkably. . A more preferable lower limit of the Mo content is 0.10%. The preferred upper limit of the Mo content is 1.10%.
- Cu improves the core hardness of the part as well as the hardness of the nitrogen diffusion layer as a solid solution strengthening element.
- the content is preferably 0.01% or more.
- the content of Cu exceeds 0.50%, the hardness after being used as a material for steel bars, wires and hot forging becomes too high, so the machinability is remarkably reduced and the hot ductility is also reduced. Therefore, during hot rolling, it causes surface damage during hot forging.
- the preferable lower limit of Cu content for maintaining hot ductility is 0.05%.
- the preferred upper limit of the Cu content is 0.40%.
- Ni improves core hardness and surface hardness by solid solution strengthening.
- the content is preferably 0.01% or more.
- the content of Ni exceeds 0.50%, the hardness after a bar, wire or hot forging becomes too high, so that the machinability is remarkably reduced and the alloy cost is increased.
- the preferable lower limit of Ni content for obtaining sufficient machinability is 0.05%.
- the preferred upper limit of the Ni content is 0.40%.
- Nb combines with C and N to form NbC and NbN, and has the effect of refining the structure of the steel before nitriding treatment by the pinning action of austenite grains and reducing the variation in mechanical properties of the nitrided parts .
- Nb is preferably made 0.010% or more.
- the preferable lower limit of the Nb content is 0.015%.
- the preferred upper limit of the Nb content is 0.090%.
- Ti 0 to 0.050%
- Ti combines with N to form TiN and improves core hardness and surface hardness.
- Ti is preferably made 0.005% or more.
- the content of Ti exceeds 0.050%, the effect of improving the core hardness and the surface hardness is saturated and the alloy cost is increased.
- the preferred lower limit of the Ti content is 0.007%.
- the preferred upper limit of the Ti content is 0.040%.
- Solid solution B suppresses grain boundary segregation of P and has an effect of improving toughness.
- BN which combines with N and precipitates improves the machinability.
- B is preferably made 0.0005% (5 ppm) or more.
- the preferable lower limit of the B content is 0.0008%.
- the preferred upper limit of the B content is 0.0080%.
- a machinable element for improving machinability can be contained.
- free-cutting elements include Ca, Pb, Bi, In, and Sn.
- the Ca content is 0.0100% or less
- the Pb content is 0.50% or less
- Bi is The content is 0.50% or less
- the content of In is 0.20% or less
- the content of Sn is 0.100% or less.
- C, Mn, Cr, V and Mo are elements that affect the phase structure and thickness of the compound layer.
- C and Mo have the effect of stabilizing the ⁇ phase and increasing the thickness.
- Mn, Cr and V have the effect of thinning the compound layer. Therefore, by designing these elements in a certain range, the ratio of ⁇ 'phase in the compound layer and the compound layer thickness can be stably controlled, and the surface fatigue strength, the wear resistance and the rotational bending fatigue strength can be reduced. Improve.
- X needs to be 0 or more. If it is less than 0, an effective ratio of ⁇ 'phase can not be obtained for rotational bending fatigue strength. On the other hand, when X exceeds 0.50, the compound layer becomes thin and desired characteristics can not be obtained. The area ratio of the ⁇ ′ phase will be described later.
- the nitriding-treated component according to the present invention is manufactured by processing a steel material into a molded material and then performing nitriding treatment under predetermined conditions.
- the nitrided component according to the present invention comprises a steel core, a nitrogen diffusion layer formed on the steel core, and a compound layer formed on the nitrogen diffusion layer. That is, the nitrided component according to the present invention has a structure in which the compound layer is on the surface, the nitrogen diffusion layer is on the inside of the compound layer, and the steel core is on the inside of the nitrogen diffusion layer.
- the steel core portion is a portion to which the nitrogen which has invaded from the surface did not reach in the nitriding treatment.
- the steel core has the same chemical composition as the steel material that has become the material of the nitrided component.
- the nitrogen diffusion layer is a portion where nitrogen which has invaded from the surface in the nitriding treatment is dissolved in the matrix phase or precipitated as iron nitride and alloy nitride.
- the nitrogen diffusion layer is affected by solid solution strengthening of nitrogen and particle dispersion strengthening of iron nitride and alloy nitride, so the hardness is higher than that of the steel core.
- the compound layer is a layer mainly containing iron nitrides formed by combining nitrogen atoms which have penetrated into steel by nitriding treatment and iron atoms contained in the material.
- the compound layer is mainly composed of iron nitride, but in addition to iron and nitrogen, oxygen mixed from the open air, and each element contained in the steel material of the material (that is, each element contained in the steel core) Or the like) is also included in the compound layer. Generally, 90% or more (mass%) of the elements contained in the compound layer is nitrogen and iron.
- the iron nitride contained in the compound layer is Fe 2-3 N ( ⁇ phase) or Fe 4 N ( ⁇ ′ phase).
- the thickness of the compound layer affects the surface fatigue strength, the wear resistance, and the rotational bending fatigue strength of the nitrided component.
- the compound layer is hard but fragile compared to the inner nitrogen diffusion layer and steel core. If the compound layer is excessively thick, pitting or bending is likely to cause a crack, which is a starting point of fracture, leading to deterioration of surface fatigue strength and rotational bending fatigue strength. On the other hand, when the compound layer is too thin, the contribution of the hard compound layer decreases, so that the surface fatigue strength and the rotational bending fatigue strength also decrease.
- the thickness of the compound layer is set to 5 to 15 ⁇ m from the above viewpoint.
- the thickness of the compound layer is measured by polishing, etching, and observing with a scanning electron microscope (SEM) the vertical cross section of the test material after gas nitriding treatment.
- the etching is performed with a 3% nital solution for 20 to 30 seconds.
- the compound layer is present on the surface of the low alloy steel and is observed as an uncorroded layer.
- the compound layer is observed from 10 fields of view (photograph area: 6.6 ⁇ 10 2 ⁇ m 2 ) taken at 4000 ⁇ magnification, and the thickness of the compound layer is measured at three points every 10 ⁇ m in the horizontal direction. Then, the average value of the 30 measured points is defined as the compound layer thickness ( ⁇ m).
- FIG. 1 shows an outline of the measurement method
- FIG. 2 shows an example of a photograph of the structure of the compound layer and the nitrogen diffusion layer. As shown in FIG. 2, the compound layer which is not etched by etching and the nitrogen diffusion layer which has been corroded clearly have different contrasts and can be discrimin
- the compound layer and the nitrogen diffusion layer between the nitrogen diffusion layer into which nitrogen has penetrated by the nitriding treatment and the steel core part to which the penetration does not reach. It is difficult to identify the boundary with the steel core.
- the hardness profile in the depth direction is measured, the area where the hardness decreases continuously with the depth is the nitrogen diffusion layer, and the area where the hardness becomes constant regardless of the depth is the steel core is there. If the difference between the Vickers hardness value at a certain point A and the Vickers hardness value at a point B 50 ⁇ m deeper than the surface at a certain point A is within 1% in the nitrided parts, the point A and the point B and It may be determined that both are in the steel core. Alternatively, since nitrogen does not infiltrate 5.0 mm or more from the surface under normal nitriding conditions, the point 5.0 mm deep from the surface may be the steel core.
- the ⁇ 'phase has an fcc structure, and is richer in toughness than the ⁇ phase which is an hcp structure.
- the ⁇ phase has a wider solid solution range of N and C and higher hardness than the ⁇ ′ phase. Therefore, the present inventors have conducted research and research focusing on clarifying the structure of a compound layer effective for surface fatigue strength and rotational bending fatigue strength. As a result, as shown in FIG. 3, it was found that the rotational bending fatigue strength increased as the ratio of the ⁇ ′ phase in the compound layer increased. In particular, it has been found that the ratio of the ⁇ ′ phase effective for rotational bending fatigue strength is 50% or more in the area ratio in the surface vertical cross section.
- the ratio of the ⁇ ′ phase forms a peak around 70% in the above area ratio, and at least the surface fatigue strength decreases at most by the ⁇ ′ phase.
- the area ratio of the ⁇ 'phase is determined by image processing of the tissue photograph. Specifically, ⁇ in the compound layer is obtained for 10 texture photographs of a cross section perpendicular to the surface of the surface of the nitrided part taken at 4000 times by backscattered electron diffraction (EBSD). The 'phase and ⁇ phase are determined, and the area ratio of the ⁇ ' phase in the compound layer is determined by binarization by image processing. Then, an average value of the measured area ratio of the ⁇ 'phase of 10 views is defined as the area ratio (%) of the ⁇ ' phase.
- EBSD backscattered electron diffraction
- the void is formed by separating N 2 gas from the surface of the steel material along grain boundaries from an energy stable place such as a grain boundary on the surface of the steel material with small binding force by the base material.
- the generation of N 2 is more likely to occur as the nitriding potential K N described later is higher. This is in accordance with K N increases, bcc ⁇ ⁇ ' ⁇ occur phase transformation epsilon, gamma' for toward 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
- the outline of formation of voids in the compound layer in FIG. 5 (Dieter Rietke et al .: “Nitriding and soft nitriding of iron”, Agne Technical Center, Tokyo, (2011), P. 21); Shows a photo of the organized tissue.
- the void area ratio can be measured by a scanning electron microscope (SEM).
- SEM scanning electron microscope
- the ratio of the total area of the voids (void area ratio, unit:%) in the area 90 ⁇ m 2 in the range of 3 ⁇ m deep from the outermost surface is determined by analysis using an image processing application. And the average value of 10 measured visual fields is defined as a void area ratio (%). Even in the case where the compound layer is less than 3 ⁇ m, the measurement target is similarly to a depth of 3 ⁇ m from the surface.
- the void area ratio is preferably 5% or less, more preferably 2% or less, still more preferably 1% or less, and most preferably 0.
- a steel material having the above-described components is subjected to gas nitriding treatment.
- the treatment temperature of the gas nitriding treatment is 550 to 620 ° C., and the treatment time of the entire gas nitriding treatment is 1.5 to 10 hours.
- the temperature of the gas nitriding process (nitriding temperature) mainly correlates with the diffusion rate of nitrogen and affects the surface hardness and the depth of the hardened layer. If the nitriding temperature is too low, the diffusion rate of nitrogen is low, the surface hardness is low, and the depth of the hardened layer is shallow. On the other hand, if the nitriding temperature exceeds the A C1 point, an austenitic phase ( ⁇ phase) in which the diffusion rate of nitrogen is smaller than that of the ferrite phase ( ⁇ phase) is generated in the steel, the surface hardness is lowered, and the hardened layer depth It gets shallow. Therefore, in the present embodiment, the nitriding temperature is 550 to 620 ° C. around the ferrite temperature range. In this case, reduction in surface hardness can be suppressed, and reduction in depth of the hardened layer can be suppressed.
- Treatment time of whole gas nitriding treatment 1.5 to 10 hours
- the gas nitriding treatment is performed in an atmosphere containing NH 3 , H 2 and N 2 .
- the time of the entire nitriding treatment correlates with the formation and decomposition of the compound layer and the diffusion and penetration of nitrogen, and influences the surface hardness and the depth of the hardened layer. Exert. If the treatment time is too short, the surface hardness will be low and the hardened layer depth will be shallow. On the other hand, if the treatment time is too long, the void area ratio on the surface of the compound layer increases, and the surface fatigue strength and the rotational bending fatigue strength decrease. If the treatment time is too long, the manufacturing cost will further increase. Therefore, the treatment time of the entire nitriding treatment is 1.5 to 10 hours.
- the atmosphere gas nitriding process of the present embodiment includes other NH 3, H 2 and N 2, inevitably oxygen, an impurity such as carbon dioxide.
- a preferred atmosphere is 99.5% (volume%) or more in total of NH 3 , H 2 and N 2 .
- the nitriding potential is controlled.
- the area ratio of the ⁇ ′ phase in the compound layer can be set within a predetermined range, and the void area ratio in the range of 3 ⁇ m from the surface can be 10% or less.
- the nitriding potential K N of the gas nitriding treatment 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 atmosphere of gas nitriding can be controlled by adjusting the flow rate of gas.
- the nitriding potential of the gas nitriding treatment affects the thickness, phase structure and void area ratio of the compound layer, and the optimum nitriding potential has a lower limit of 0.15 and an upper limit of 0.40. , It was found that the average was 0.18 or more and less than 0.30.
- the ⁇ 'phase ratio in the compound layer can be stably increased without complicating the nitriding conditions, and the surface is 3 ⁇ m deep from the surface.
- the void area ratio in the range of height can be 10% or less. Therefore, it is possible to obtain a nitrided component having an excellent rotational bending fatigue strength, preferably a surface fatigue strength of 2400 MPa or more and a rotational bending fatigue strength of 600 MPa or more.
- the rotational bending fatigue strength can be enhanced by increasing the proportion of the ⁇ 'phase in the compound layer.
- the surface fatigue (contact fatigue with tangential force due to slip) strength is such that the proportion of ⁇ 'phase forms a peak around 70% in area ratio, and at least the surface fatigue strength decreases at most by ⁇ ' phase It turned out to be. This seems to be because it is desirable that the hardness of the compound layer is high in order to secure the surface fatigue strength.
- the ratio of ⁇ 'phase in the compound layer is 50% or more and 80% or less in area ratio in a cross section perpendicular to the surface Specify.
- the present inventors deposit nitrides such as CrN and VN in the compound layer, or make a solid solution element of substitution type element in the compound layer, so that the hardness is also obtained in the compound layer of 50-80% of the ⁇ 'phase. It has been found that it is possible to enhance Specifically, the hardness of the compound layer is increased and the surface fatigue strength is enhanced by satisfying 0 ⁇ X ⁇ 0.25 for the value X of the content ratio of C, Mn, Cr, V and Mo. it can.
- the area ratio of the ⁇ ′ phase of the iron nitride in the compound layer is 50% or more and 80% or less, and 0 ⁇ X ⁇ 0.25.
- the surface fatigue strength and the rotational bending fatigue strength can be compatible at a high level as compared with the prior art.
- the hardness of the compound layer can be 730 HV or more, but the hardness of the compound layer is preferably harder, and specifically, it is preferably 750 Hv or more.
- Nitrided parts having excellent rotational bending fatigue strength can be enhanced by increasing the proportion of the ⁇ 'phase in the compound layer. Therefore, for products in which surface fatigue strength is not required so much (products in which the tangential force and the contact surface pressure are below a certain level), the proportion of ⁇ 'phase in the compound layer in the nitrided part according to the present invention is perpendicular to the surface. It is desirable that the area ratio in the cross section be 80% or more. However, in a product in which the tangential force and the contact surface pressure are equal to or less than a predetermined value, when the ⁇ ′ phase is 80% or more, the wear resistance becomes a problem instead of the surface fatigue strength. As described above, in addition to the hardness of the ⁇ 'phase being lower than that of the ⁇ phase, when the ⁇ ' phase is 80% or more, the thickness of the compound layer becomes insufficient, resulting in insufficient wear resistance. Was there.
- the present inventors appropriately control not only the hardness of the compound layer but also the necessary compounds by appropriately controlling the value of X, specifically by setting 0.25 ⁇ X ⁇ 0.50. It has been found that the thickness of the layer can be secured. That is, also in the nitrided part according to the present invention, the area ratio of the ⁇ ′ phase of the iron nitride in the compound layer is particularly 80% or more, in particular, 0.25 ⁇ X ⁇ 0.50. In this way, both the rotational bending fatigue strength and the wear resistance can be achieved at a high level.
- the hardness of the compound layer can be 710 HV or more, but the hardness of the compound layer is preferably harder, and specifically, it is preferably 730 Hv or more.
- Example 1 In the first embodiment, in particular, a nitrided component excellent in rotational bending fatigue strength and surface fatigue strength will be described.
- the nitrided parts according to the present invention in particular, 0 ⁇ X ⁇ 0.25, and the area ratio of the ⁇ ′ phase of iron nitride in the compound layer is 50% or more and 80% or less. .
- Ingots of steels a to ag having chemical components shown in Tables 1-1 to 1-2 were manufactured using a 50 kg vacuum melting furnace.
- a to y in Table 1-1 are steels having the chemical components specified in this example.
- steels z to ag shown in Table 1-2 are steels of comparative examples which deviate from the chemical components specified in the present embodiment by at least one element or more.
- the ingot was hot forged to form a round bar with a diameter of 40 mm. Hot forging was performed at a temperature between 1000 ° C. and 1100 ° C., and after forging, it was allowed to cool in the air. Subsequently, each round bar was annealed and then subjected to cutting work to produce a small roller for a roller pitting test for evaluating the surface fatigue strength shown in FIG.
- a plurality of small rollers are made from one ingot for the roller pitting test, in which cross-sectional observation (measurement of compound layer thickness and void area ratio, measurement of ⁇ 'phase ratio, and compound layer hardness Of the number of small rollers required for the roller pitting test. Furthermore, the cylindrical test piece for evaluating the rotational bending fatigue strength shown in FIG. 9 was produced by using the same round bar as a raw material. A plurality of cylindrical specimens were also made from one ingot for rotational bending fatigue testing.
- the small roller which is a roller pitting test piece, includes a test surface with a diameter of 26 mm and a width of 28 mm and a grip with a diameter of 22 provided on both sides of the test surface.
- the test surface was brought into contact with the large roller, and was rotated after applying a predetermined surface pressure.
- Gas nitriding was performed on the collected test pieces under the following conditions.
- the test piece was charged into a gas nitriding furnace, and each gas of NH 3 , H 2 and N 2 was introduced into the furnace to carry out nitriding treatment under the conditions shown in Tables 2-1 and 2-2.
- Test No. 42 was gas soft nitriding treatment in which 3% of CO 2 gas was added in volume ratio in the atmosphere. Oil-cooling was performed using the oil of 80 degreeC with respect to the test piece after gas nitriding treatment.
- the H 2 partial pressure in the atmosphere was measured using a heat conduction H 2 sensor directly attached to the gas nitriding furnace. The difference in thermal conductivity between the standard gas and the measurement gas was converted to the gas concentration and measured. The H 2 partial pressure was measured continuously during the gas nitriding process.
- NH 3 partial pressure was measured using an infrared absorption NH 3 analyzer attached to the outside of the furnace. The NH 3 partial pressure was measured continuously during the gas nitriding process. As for Test No. 42, which is in the atmosphere of a CO 2 gas mixture, (NH 4 ) 2 CO 3 may be precipitated in the infrared absorption type NH 3 analyzer and there is a risk that the device may break down. The NH 3 partial pressure was measured every 10 minutes using an analyzer.
- the NH 3 flow rate and the N 2 flow rate were adjusted such that the nitriding potential K N calculated in the apparatus converges to the target value.
- the nitriding potential K N was recorded every 10 minutes, and the lower limit value, the upper limit value, and the average value were derived.
- the test surface (the position of 26 in FIG. 7) was cut at a plane perpendicular to the longitudinal direction, and the obtained cross section was mirror-polished and etched.
- the etched cross section was observed using a scanning electron microscope (SEM, manufactured by JEOL Ltd .; JSM-7100F) to measure the compound layer thickness and confirm the presence or absence of voids in the surface layer. Etching was performed for 20-30 seconds with a 3% nital solution.
- the compound layer can be identified as an uncorroded layer present in the surface layer.
- the compound layer was observed at a magnification of 4000 ⁇ from 10 fields of view (area of view: 6.6 ⁇ 10 2 ⁇ m 2 ) taken with a scanning electron microscope, and the thickness of three compound layers was measured every 10 ⁇ m. . Then, the average value of the 30 measured points was defined as the compound layer thickness ( ⁇ m).
- the ratio of the total area of voids (void area ratio, unit%) in the area 90 ⁇ m 2 in the range of depth 3 ⁇ m from the outermost surface It calculated
- tissue photograph was measured by remove
- the ⁇ 'phase ratio was determined by image processing of the tissue photograph. Specifically, the cross-sectional view perpendicular to the surface of the nitrided component obtained at 4000 ⁇ was analyzed by backscattered electron diffraction (Electron Back Scatter Diffraction: EBSD, manufactured by EDAX), and a phase map was drawn. The ⁇ ′ phase and the ⁇ phase in the compound layer were determined with respect to 10 sheets of the phase map, and the area ratio of the ⁇ ′ phase in the compound layer was determined by binarization by image processing. Then, the average value of the measured area ratio of the ⁇ ′ phase of 10 fields of view was defined as the ⁇ ′ phase ratio (%).
- the hardness of the compound layer was measured by the following method using a nanoindentation apparatus (Hysitron; TI950). At a position near the center in the thickness direction of the compound layer, 50 points were randomly indented at an indentation load of 10 mN. Indenter is triangular pyramid (Berkovich) shape, hardness derivation complies with ISO14577-1, the conversion from nanoindentation hardness H IT to Vickers hardness HV, was performed by the following equation.
- the average value of the measured 50 points was defined as the hardness (HV) of the compound layer.
- the roller pitting test was conducted under the conditions shown in Table 3 by combining the small roller for the roller pitting test and the large roller for the roller pitting test having the shape shown in FIG. In addition, a large roller is created on the conditions different from this invention, and is not this invention item.
- the unit of dimensions in FIGS. 7 and 8 is “mm”.
- the large roller for the roller pitting test is a general manufacturing process using steel meeting the SCM420 standard of JIS G 4053 (2016), that is, “normalize ⁇ test piece processing ⁇ eutectoid carburization by gas carburizing furnace ⁇
- the Vickers hardness HV is 740 to 760 at a position of 0.05 mm from the surface, that is, a position of 0.05 mm in depth, and the Vickers hardness Hv is 550.
- the above depth was in the range of 0.8 to 1.0 mm.
- Table 3 shows the test conditions under which the surface fatigue strength was evaluated.
- the number of times of test discontinuation was 2 ⁇ 10 7 showing the fatigue limit of general steel, and the maximum contact pressure reached 2 ⁇ 10 7 without pitting in the small roller test piece was used for the small roller test piece It was a fatigue limit.
- the surface pressure was tested in increments of 50 MPa, particularly near the fatigue limit. That is, in the pitting strength values shown in Tables 2-1 and 2-2, in the target test numbers, no pitting occurred in the small roller test pieces tested under the same surface pressure, but the same surface pressure It is shown that pitting occurred in the small roller test pieces tested at a surface pressure higher than 50 MPa.
- the pitting occurrence was detected by a vibrometer provided in the tester, and after the occurrence of vibration, the rotation of both the small roller test piece and the large roller test piece was stopped to confirm the pitting occurrence and the number of rotations.
- the surface pressure at the fatigue limit in the roller pitting test shown in Table 3 is 2400 MPa or more.
- the stress at the fatigue limit in the Ono type rotational bending fatigue test was aimed to be 600 MPa or more.
- Test results The results are shown in Tables 2-1 and 2-2.
- the test numbers 1 to 31 are the components of steel and the conditions of gas nitriding treatment within the range assumed in the present embodiment, the compound layer thickness is 5 to 15 ⁇ m, and the ⁇ ′ phase ratio of the compound layer is 50% to 80%.
- the void area ratio of the compound layer was 10% or less.
- the hardness of the compound layer was 730 Hv or more (measured load: 10 mN), and good results were obtained with a surface fatigue strength of 2400 MPa or more and a rotational bending fatigue strength of 600 MPa or more.
- the composition of the steel and part of the conditions for gas nitriding treatment are out of the range assumed in the present embodiment, and any of the thickness of the compound layer, the ⁇ ′ phase, and the void area ratio Or several characteristics did not reach the target value.
- the surface fatigue strength or the rotational bending fatigue strength did not meet the target.
- the atmosphere in the gas nitriding treatment is carbon dioxide-containing and soft nitriding treatment is performed, the formed compound layer is thick and the ratio of ⁇ 'phase is low ( ⁇ phase is formed And the void area ratio was high, and sufficient characteristics were not obtained in terms of pitting strength and rotational bending fatigue strength.
- Test No. 46 is a comparative example in which the surface fatigue strength does not reach the target value, but is a component suitable as a nitrided component excellent in rotational bending fatigue strength and wear resistance of Example 2 described later.
- the steel ac used for the test No. 46 is also the steel b of the invention example of the second embodiment.
- Example 2 In the second embodiment, a nitrided part which is particularly excellent in rotational bending fatigue strength and wear resistance will be described.
- the nitrided parts according to the present invention in particular, 0.25 ⁇ X ⁇ 0.50, and the area ratio of the ⁇ ′ phase of the iron nitride in the compound layer is 80% or more. .
- Ingots of steels a to ag having chemical components shown in Tables 4-1 to 4-2 were produced in a 50 kg vacuum melting furnace.
- a to y in Table 4-1 are steels having the chemical components specified in this example.
- steels z to ag shown in Table 4-2 are steels of comparative examples in which at least one element or more deviates from the chemical components specified in this example.
- the ingot was hot forged to form a round bar with a diameter of 40 mm.
- hot forging was performed at a temperature between 1000 ° C. and 1100 ° C., and after forging, it was allowed to cool in the air.
- each round bar was annealed and then subjected to a cutting process to produce a small roller for a roller pitting test for evaluating the abrasion resistance shown in FIG.
- the quantity used for cross-sectional observation was also produced under the same conditions.
- the cylindrical test piece for evaluating the rotational bending fatigue strength shown in FIG. 9 was produced by using the same round bar as a raw material.
- Gas nitriding was performed on the collected test pieces under the following conditions.
- the test piece was charged into a gas nitriding furnace, and NH 3 , H 2 and N 2 gases were introduced into the furnace, and the nitriding treatment was carried out under the conditions shown in Tables 5-1 to 5-2.
- Test No. 42 was gas soft nitriding treatment in which 3% of CO 2 gas was added in volume ratio in the atmosphere. Oil-cooling was performed using the oil of 80 degreeC with respect to the test piece after gas nitriding treatment.
- the thickness of the compound layer, the ratio (area ratio) of the ⁇ 'phase in the compound layer, the void area ratio, and the hardness of the compound layer were measured by the same method as in Example 1 using the small roller after gas nitriding treatment .
- the abrasion resistance evaluation test was evaluated by a roller pitting tester (manufactured by Komatsu Equipment Co., Ltd .; RP 102) according to the following method.
- the small roller for the roller pitting test was subjected to finish processing of the grip portion for the purpose of removing heat treatment distortion, and then subjected to the roller pitting test piece.
- the shape after finishing is the same as that of Example 1 shown in FIG.
- the roller pitting test was conducted under the conditions shown in Table 6 using a combination of the above-mentioned small roller for the pitting test and the large roller for the roller pitting test of the shape shown in FIG.
- a large roller is created on the conditions different from this invention, and is not this invention item.
- the unit of dimensions in FIGS. 7 and 8 is “mm”.
- the large roller for the roller pitting test is a general manufacturing process using steel meeting the SCM420 standard of JIS G 4053 (2016), that is, “normalize ⁇ test piece processing ⁇ eutectoid carburization by gas carburizing furnace ⁇
- the Vickers hardness HV is 740 to 760 at a position of 0.05 mm from the surface, that is, a position of 0.05 mm in depth, and the Vickers hardness Hv is 550.
- the above depth was in the range of 0.8 to 1.0 mm.
- Table 6 shows the test conditions under which the abrasion resistance was evaluated.
- the number of repetitions is 2 ⁇ 10 6
- the wear portion of the small roller is scanned along the main axis direction using a roughness meter, and the maximum wear depth is measured.
- the average value of In this example assuming application to a CVT or a camshaft part, it was aimed that the wear depth by the roller pitting test shown in Table 6 is 10 ⁇ m or less.
- the wear depth is 10 ⁇ m or less and the maximum stress at fatigue limit is 640 MPa or more did.
- Test results The results are shown in Tables 5-1 to 5-2.
- the test numbers 1 to 31 are the composition of steel and the conditions of gas nitriding treatment in the range assumed in this embodiment, the compound layer thickness is 5 to 15 ⁇ m, the ⁇ ′ phase ratio of the compound layer is 80% or more, the compound The layer void area ratio was 10% or less.
- the hardness of the compound layer was 710 Hv (measured load: 10 mN), and a favorable result was obtained with an abrasion depth of 10 ⁇ m or less and a rotational bending fatigue strength of 640 MPa or more.
- the composition of the steel and part of the conditions for gas nitriding treatment are out of the range assumed in the present embodiment, and any of the thickness of the compound layer, the ⁇ ′ phase, and the void area ratio Or several characteristics did not reach the target value.
- the abrasion resistance or rotational bending fatigue strength did not meet the target.
- the atmosphere in the gas nitriding treatment contains carbon dioxide and is soft nitriding treatment, the proportion of the ⁇ 'phase in the formed compound layer becomes low ( ⁇ phase is formed), Sufficient characteristics were not obtained in terms of rotational bending fatigue strength.
- test number 46 is a comparative example in which the rotational bending fatigue strength does not reach the target value, the target value of the rotational bending fatigue strength in Example 1 described above (an example in which the gear parts are assumed) is cleared. It is a component suitable as a nitrided component excellent in rotational bending fatigue strength and surface fatigue strength.
- the steel ac used for the test No. 46 is also the steel k of the invention example of the example 1.
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Abstract
Description
Cは、部品の芯部硬さを確保するために必要な元素である。そのため、Cは0.05%以上が必要である。一方、Cの含有量が0.35%を超えると、熱間鍛造後の強度が高くなりすぎるため、切削加工性が大きく低下する。C含有量の好ましい下限は0.08%である。また、C含有量の好ましい上限は0.30%である。
Siは、固溶強化によって、芯部硬さを高める元素である。また、焼戻し軟化抵抗を高め、摩耗条件下で高温となる部品表面の面疲労強度、および耐摩耗性を高める。これらの効果を発揮させるため、Siは0.05%以上が必要である。一方、Siの含有量が1.50%を超えると、棒鋼、線材や熱間鍛造後の強度が高くなりすぎるため、切削加工性が大きく低下する。Si含有量の好ましい下限は0.08%である。Si含有量の好ましい上限は1.30%である。
Mnは、窒化処理によって、化合物層や拡散層中に微細な窒化物(Mn3N2)を形成し、硬さを高めるため、面疲労強度や耐摩耗性、および回転曲げ疲労強度の向上に有効な元素である。また、固溶強化によって、芯部硬さを高める。これらの効果を得るため、Mnは0.20%以上が必要である。一方、Mnの含有量が2.50%を超えると、効果が飽和するだけでなく、素材となる棒鋼、線材や熱間鍛造後の硬さが高くなりすぎるため、切削加工性が大きく低下する。Mn含有量の好ましい下限は0.40%である。Mn含有量の好ましい上限は2.30%である。
Pは不純物であって、粒界偏析して部品を脆化させるので、含有量は少ない方が好ましい。Pの含有量が0.025%を超えると、面疲労強度や耐摩耗性、および回転曲げ疲労強度が低下する場合がある。回転曲げ疲労強度の低下を防止するためのP含有量の好ましい上限は0.018%である。Pの含有量は0でもよいが、完全に0とするのは難しく、0.001%以上含有してもよい。
Sは必須の元素ではないが、意図的に添加しなくても通常不純物として含有される。鋼中のSはMnと結合してMnSを形成し、切削加工性を向上させる元素でもある。切削加工性を向上させる効果を得るために、Sは0.003%以上含有させるのが好ましい。しかしながら、Sの含有量が0.050%を超えると、粗大なMnSを生成しやすくなり、面疲労強度や耐摩耗性、および回転曲げ疲労強度が大きく低下する。S含有量の好ましい下限は0.005%である。S含有量の好ましい上限は0.030%である。
Crは、窒化処理によって、化合物層や拡散層中に微細な窒化物(CrN)を形成し、硬さを高めるため、面疲労強度や耐摩耗性、および回転曲げ疲労強度の向上に有効な元素である。これらの効果を得るため、Crは0.50%以上が必要である。一方、Crの含有量が2.50%を超えると、効果が飽和するだけでなく、素材となる棒鋼、線材や熱間鍛造後の硬さが高くなりすぎるため、切削加工性が著しく低下する。Cr含有量の好ましい下限は0.70%である。Cr含有量の好ましい上限は2.00%である。
Vは、窒化処理によって、化合物層や拡散層中に微細な窒化物(VN)を形成し、硬さを高めるため、面疲労強度や耐摩耗性、および回転曲げ疲労強度の向上に有効な元素である。これらの効果を得るため、Vは0.05%以上が必要である。一方、Vの含有量が1.30%を超えると、効果が飽和するだけでなく、素材となる棒鋼、線材や熱間鍛造後の硬さが高くなりすぎるため、切削加工性が著しく低下する。V含有量の好ましい下限は0.10%である。V含有量の好ましい上限は1.10%である。
Alは必須の元素ではないが、脱酸元素であり、脱酸後の鋼中にも、多くの場合はある程度含有される。また、Nと結合してAlNを形成し、オーステナイト粒のピンニング作用により、窒化処理前の鋼材の組織を微細化し、窒化処理部品の機械的特性のばらつきを低減する効果を持つ。鋼材の組織を微細化する効果を得るためには、0.010%以上含有させるのが好ましい。一方で、Alは硬質な酸化物系介在物を形成しやすく、Alの含有量が0.050%を超えると、回転曲げ疲労強度の低下が著しくなり、他の要件を満たしていても所望の回転曲げ疲労強度が得られなくなる。Al含有量の好ましい下限は0.020%である。Al含有量の好ましい上限は0.040%である。
Nは、必須の元素ではないが、意図的に添加しなくても通常不純物として含有される。鋼中のNは、Mn、Cr、Al、Vと結合してMn3N2、CrN、AlN、VNを形成する。中でも窒化物形成傾向の高いAl、Vはオーステナイト粒のピンニング作用により、窒化処理前の鋼材の組織を微細化し、窒化処理部品の機械的特性のばらつきを低減する効果を持つ。鋼材の組織を微細化する効果を得るためには、0.0030%以上含有させるのが好ましい。一方で、Nの含有量が0.0250%を超えると、粗大なAlNが形成されやすくなるため、上記の効果は得難くなる。N含有量の好ましい下限は0.0050%である。N含有量の好ましい上限は0.0200%である。
Moは、窒化処理によって形成される化合物層や拡散層中に微細な窒化物(Mo2N)を形成し、硬さを高めるため、面疲労強度や耐摩耗性、および回転曲げ疲労強度の向上に有効な元素である。これらの効果を得るため、Moは0.01%以上とするのが好ましい。一方、Moの含有量が1.50%を超えると、効果が飽和するだけでなく、素材となる棒鋼、線材や熱間鍛造後の硬さが高くなりすぎるため、切削加工性が著しく低下する。Mo含有量のより好ましい下限は0.10%である。Mo含有量の好ましい上限は1.10%である。
Cuは、固溶強化元素として部品の芯部硬さならびに窒素拡散層の硬さを向上させる。Cuの固溶強化の作用を発揮させるためには0.01%以上の含有が好ましい。一方、Cuの含有量が0.50%を超えると、素材となる棒鋼、線材や熱間鍛造後の硬さが高くなりすぎるため、切削加工性が著しく低下する他、熱間延性が低下するため、熱間圧延時、熱間鍛造時に表面傷発生の原因となる。熱間延性維持のためのCu含有量の好ましい下限は0.05%である。Cu含有量の好ましい上限は0.40%である。
Niは、固溶強化により芯部硬さ及び表面硬さを向上させる。Niの固溶強化の作用を発揮させるためには0.01%以上の含有が好ましい。一方、Niの含有量が0.50%を超えると、棒鋼、線材や熱間鍛造後の硬さが高くなりすぎるため、切削加工性が著しく低下する他、合金コストが増大する。十分な切削加工性を得るためのNi含有量の好ましい下限は0.05%である。Ni含有量の好ましい上限は0.40%である。
Nbは、CやNと結合してNbCやNbNを形成し、オーステナイト粒のピンニング作用により、窒化処理前の鋼材の組織を微細化し、窒化処理部品の機械的特性のばらつきを低減する効果を持つ。この作用を得るため、Nbは0.010%以上とするのが好ましい。一方、Nbの含有量が0.100%を超えると、粗大なNbC、NbNが形成されるため、上記の効果は得難くなる。Nb含有量の好ましい下限は0.015%である。Nb含有量の好ましい上限は0.090%である。
Tiは、Nと結合してTiNを形成し、芯部硬さ及び表面硬さを向上させる。この作用を得るため、Tiは0.005%以上とするのが好ましい。一方、Tiの含有量が0.050%を超えると、芯部硬さ及び表面硬さを向上させる効果が飽和する他、合金コストが増大する。Ti含有量の好ましい下限は0.007%である。Ti含有量の好ましい上限は0.040%である。
固溶Bは、Pの粒界偏析を抑制し、靭性を向上させる効果を持つ。また、Nと結合して析出するBNは、切削性を向上させる。これらの作用を得るため、Bは0.0005%(5ppm)以上とすることが好ましい。一方、Bの含有量が0.0100%を超えると、上記効果が飽和するだけでなく、多量なBNが偏析することで鋼材に割れが生じることがある。B含有量の好ましい下限は0.0008%である。B含有量の好ましい上限は0.0080%である。
その他、必要に応じて被削性を向上させるための快削性元素を含有させることができる。快削性元素としては、Ca、Pb、Bi、In、及びSnが挙げられる。被削性向上のためには、Ca、Pb、Bi、In、及びSnの1種類以上の元素を、それぞれ0.005%以上含有させることが好ましい。快削性元素は多量に添加しても効果は飽和し、また、熱間延性が低下するので、Caの含有量は0.0100%以下、Pbの含有量は0.50%以下、Biの含有量は0.50%以下、Inの含有量は0.20%以下、Snの含有量は0.100%以下とする。
化合物層の厚さは、窒化処理部品の面疲労強度や耐摩耗性、回転曲げ疲労強度に影響する。化合物層は、内側の窒素拡散層および鋼芯部に比べると硬質だが割れやすい性質を持つ。化合物層が過度に厚いと、ピッティングや曲げによって亀裂が生じやすく、破壊起点となりやすく、面疲労強度、回転曲げ疲労強度の劣化につながる。一方、化合物層が薄すぎると、硬い化合物層の寄与が小さくなるために、やはり面疲労強度や回転曲げ疲労強度が低下する。本発明にかかる窒化処理部品においては、上記の観点から、化合物層の厚さは5~15μmとする。
γ’相はfcc構造であり、hcp構造であるε相に比べ靭性に富む。一方で、ε相はγ’相に比べ、N及びCの固溶範囲が広く、高硬度である。そこで、本発明者らは、面疲労強度及び回転曲げ疲労強度に有効な化合物層の構造を明らかにすることを主眼とした調査、研究を重ねた。その結果、図3に示すように、化合物層におけるγ’相の割合が高まるほど回転曲げ疲労強度が高まることを知見した。特に、回転曲げ疲労強度に有効なγ’相の割合は、表面な垂直な断面における面積率で50%以上であることを知見した。
表面から3μmの深さまでの範囲の化合物層に存在する空隙には応力集中が生じ、ピッティングや曲げ疲労破壊の起点となりやすい。そのため、空隙面積率は10%以下とする必要がある。
ガス窒化処理の温度(窒化処理温度)は、主に、窒素の拡散速度と相関があり、表面硬さ及び硬化層深さに影響を及ぼす。窒化処理温度が低すぎれば、窒素の拡散速度が遅く、表面硬さが低くなり、硬化層深さが浅くなる。一方、窒化処理温度がAC1点を超えれば、フェライト相(α相)よりも窒素の拡散速度が小さいオーステナイト相(γ相)が鋼中に生成され、表面硬さが低くなり、硬化層深さが浅くなる。したがって、本実施形態では、窒化処理温度はフェライト温度域周囲の550~620℃である。この場合、表面硬さが低くなるのを抑制でき、かつ、硬化層深さが浅くなるのを抑制できる。
ガス窒化処理は、NH3、H2、N2を含む雰囲気で実施する。窒化処理全体の時間、つまり、窒化処理の開始から終了までの時間(処理時間)は、化合物層の形成及び分解と窒素の拡散浸透と相関があり、表面硬さ及び硬化層深さに影響を及ぼす。処理時間が短すぎると表面硬さが低くなり、硬化層深さが浅くなる。一方、処理時間が長すぎれば、化合物層表面の空隙面積率が増加し、面疲労強度や回転曲げ疲労強度が低下する。処理時間が長すぎればさらに、製造コストが高くなる。したがって、窒化処理全体の処理時間は1.5~10時間である。
本発明にかかる窒化処理部品の窒化処理方法では、窒化ポテンシャルを制御する。これにより、化合物層中のγ’相の面積率を所定の範囲内とし、表面から3μmの深さの範囲における空隙面積率を10%以下とすることができる。
上述したように、化合物層におけるγ’相の割合を高めることで回転曲げ疲労強度を高めることができる。反面、面疲労(すべりによる接線力を伴う接触疲労)強度は、γ’相の割合が面積率で70%付近にピークを形成し、それよりもγ’相が多くとも少なくとも面疲労強度が低下することが判明した。これは、面疲労強度を確保するうえでは化合物層の硬さが高いほうが望ましいことに由来すると思われる。すなわち、γ’相が70%を超えて過度に多くなると、γ’相に比べて硬いε相の割合が減少し、特に80%を超えると化合物層の硬さが不十分となり、その結果、面疲労強度が低下するものと思われる。反面、上述したように、靱性に富むγ’相を少なくして50%未満とすると、回転曲げ疲労強度が不十分となる。本発明にかかる窒化処理部品において、特に面疲労強度が要求される窒化処理部品については、化合物層におけるγ’相の割合を、表面に垂直な断面における面積率で50%以上、80%以下と規定する。
上述したように、化合物層におけるγ’相の割合を高めることで回転曲げ疲労強度を高めることができる。そのため、面疲労強度がそれほど要求されない製品(接線力や接触面圧が一定以下である製品)には、本発明にかかる窒化部品において、さらに化合物層におけるγ’相の割合を、表面に垂直な断面における面積率で80%以上とすることが望ましい。しかしながら、接線力や接触面圧が一定以下の製品において、γ’相を80%以上とした場合には、面疲労強度に代えて、耐摩耗性が問題となる。上述したように、γ’相はε相に比べて硬度が低いことに加え、γ’相が80%以上の場合には化合物層の厚さが不十分となり、結果として耐摩耗性が不十分であることがあった。
実施例1では、特に回転曲げ疲労強度及び面疲労強度に優れた窒化処理部品について説明する。本発明にかかる窒化処理部品の中でも、特に、0≦X≦0.25、かつ、化合物層における鉄窒化物のγ’相の面積率が50%以上、80%以下であることを特徴とする。
ガス窒化処理後の小ローラーにおいて、試験面部(図7のφ26の位置)を長手方向に垂直な面にて切断し、得られた断面を鏡面研磨し、エッチングした。走査型電子顕微鏡(Scanning Electron Microscope:SEM、日本電子社製;JSM-7100F)を用いてエッチングされた断面を観察し、化合物層厚さの測定及び表層部の空隙の有無の確認を行った。エッチングは、3%ナイタール溶液で20~30秒間行った。
γ’相比率は、組織写真を画像処理することにより求めた。具体的には、後方散乱電子回折法(Electron Back Scatter Diffraction:EBSD、EDAX社製)により、4000倍で取得した窒化処理部品の表面に垂直な断面視野を解析し、相マップを作図した。この相マップ10枚に対して、化合物層中のγ’相、ε相を判別し、化合物層中に占めるγ’相の面積比率を、画像処理により2値化して求めた。そして、測定された10視野のγ’相の面積比率の平均値を、γ’相比率(%)と定義した。
化合物層の硬さは、ナノインデンテーション装置(Hysitron社製;TI950)により、次の方法で測定した。化合物層の厚さ方向中央近傍位置において、押込み荷重10mNにてランダムに50点インデントした。圧子は三角錐(バーコビッチ)形状であり、硬さ導出はISO14577-1に準拠し、ナノインデンテーション硬さHITからビッカース硬さHVへの換算を、次式により行った。
面疲労強度は、ローラーピッティング試験機(小松設備社製;RP102)により、次の方法で評価した。ローラーピッティング試験用小ローラーを、熱処理ひずみを除く目的で掴み部の仕上げ加工を行った後、それぞれローラーピッティング試験に供した。仕上げ加工後の形状を図7に示す。
ガス窒化処理に供した円柱試験片に対し、JIS Z 2274(1978)に準拠した小野式回転曲げ疲労試験を実施した。回転数は3000rpm、試験打ち切り回数は、一般的な鋼の疲労限を示す1×107回とし、回転曲げ疲労試験片において、破断が生じずに1×107回に達した最大応力を回転曲げ疲労試験片の疲労限とした。回転曲げ疲労試験においては、特に疲労限付近では応力を10MPa刻みで試験を行った。すなわち、表2-1~2-2に示す回転曲げ疲労強度の値は、対象試験番号において、同応力下にて試験を行った円柱試験片には破断が生じなかったが、同応力よりも10MPa高い応力下で試験を行った円柱試験片には破断が生じたことを示している。
結果を表2-1~2-2に示す。試験番号1~31は鋼の成分、及びガス窒化処理の条件が本実施例で想定する範囲内であり、化合物層厚さが5~15μm、化合物層のγ’相比率が50%以上80%以下、化合物層空隙面積率が10%以下であった。その結果、化合物層の硬さが730Hv以上(測定荷重10mN)となり、面疲労強度が2400MPa以上、回転曲げ疲労強度が600MPa以上と良好な結果が得られた。
実施例2では、特に回転曲げ疲労強度及び耐摩耗性に優れた窒化処理部品について説明する。本発明にかかる窒化処理部品の中でも、特に、0.25≦X≦0.50、かつ、前記化合物層における前記鉄窒化物のγ’相の面積率が80%以上であることを特徴とする。
耐摩耗性は、ローラーピッティング試験機(小松設備社製;RP102)により、次の方法で評価した。ローラーピッティング試験用小ローラーを、熱処理ひずみを除く目的で掴み部の仕上げ加工を行った後、それぞれローラーピッティング試験片に供した。仕上げ加工後の形状は、図7に示した実施例1のものと同じである。
ガス窒化処理に供した円柱試験片に対し、JIS Z 2274(1978)に準拠した小野式回転曲げ疲労試験を実施した。回転数は3000rpm、試験打ち切り回数は、一般的な鋼の疲労限を示す1×107回とし、回転曲げ疲労試験片において、破断が生じずに1×107回に達した最大応力を回転曲げ疲労試験片の疲労限とした。
結果を表5-1~5-2に示す。試験番号1~31は鋼の成分、及びガス窒化処理の条件が本実施例で想定する範囲内であり、化合物層厚さが5~15μm、化合物層のγ’相比率が80%以上、化合物層空隙面積率10%以下となった。その結果、化合物層の硬さが710Hv(測定荷重10mN)となり、摩耗深さが10μm以下、回転曲げ疲労強度が640MPa以上と良好な結果が得られた。
Claims (3)
- 質量%で、
C :0.05~0.35%、
Si:0.05~1.50%、
Mn:0.20~2.50%、
P :0.025%以下、
S :0.050%以下、
Cr:0.50~2.50%、
V :0.05~1.30%、
Al:0.050%以下、
N :0.0250%以下、
Mo:0~1.50%、
Cu:0~0.50%、
Ni:0~0.50%、
Nb:0~0.100%、
Ti:0~0.050%、
B :0~0.0100%、
Ca:0~0.0100%、
Pb:0~0.50%、
Bi:0~0.50%、
In:0~0.20%、及び
Sn:0~0.100%
を含有し、残部がFe及び不純物である鋼芯部と、
前記鋼芯部の上に形成された窒素拡散層と、
前記窒素拡散層の上に形成された、鉄窒化物を主として含有する厚さ5~15μmの化合物層を有し、
前記化合物層の表面から垂直な断面において、表面から3μmまでの深さの範囲における空隙面積率が10%以下であり、
前記鋼芯部におけるC、Mn、Cr、V、Moの含有量に基づいて定められるXを、
X=-2.1×C+0.04×Mn+0.5×Cr+1.8×V-1.5×Mo
と定義すると、
(i)0≦X≦0.25、かつ、前記化合物層における前記鉄窒化物のγ’相の面積率が50%以上、80%以下である、または、
(ii)0.25≦X≦0.50、かつ、前記化合物層における鉄窒化物のγ’相の面積率が80%以上である
ことを特徴とする窒化処理部品。 - 0≦X≦0.25、かつ、前記化合物層における前記鉄窒化物のγ’相の面積率が50%以上、80%以下であることを特徴とする請求項1に記載の窒化処理部品。
- 0.25≦X≦0.50、かつ、前記化合物層における鉄窒化物のγ’相の面積率が80%以上であることを特徴とする請求項1に記載の窒化処理部品。
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US (1) | US11371132B2 (ja) |
EP (1) | EP3712287B1 (ja) |
JP (1) | JP6922998B2 (ja) |
KR (1) | KR20200062317A (ja) |
CN (1) | CN111406123B (ja) |
WO (1) | WO2019098340A1 (ja) |
Cited By (7)
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JP2021101040A (ja) * | 2019-12-24 | 2021-07-08 | 日本製鉄株式会社 | 窒化処理鋼部品およびその製造方法 |
WO2021154360A1 (en) * | 2020-01-30 | 2021-08-05 | Cummins Inc. | Two-stage gas nitriding process for improved wear and erosion resistance |
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 |
JP2021147688A (ja) * | 2020-03-23 | 2021-09-27 | 日本製鉄株式会社 | 機械構造用鋼、機械構造部品およびその製造方法 |
DE112020006870T5 (de) | 2020-03-11 | 2022-12-29 | Nippon Steel Corporation | Gasweichnitrierbehandeltes bauteil und herstellungsverfahren davon |
CN115605629A (zh) * | 2020-05-15 | 2023-01-13 | 杰富意钢铁株式会社(Jp) | 钢和钢部件 |
WO2023203838A1 (ja) * | 2022-04-18 | 2023-10-26 | ジヤトコ株式会社 | 歯車 |
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KR102458518B1 (ko) * | 2022-04-05 | 2022-10-25 | 신승호 | 내마모성 유압 브레이커용 프론트 커버 |
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JP2021101040A (ja) * | 2019-12-24 | 2021-07-08 | 日本製鉄株式会社 | 窒化処理鋼部品およびその製造方法 |
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 |
WO2021154360A1 (en) * | 2020-01-30 | 2021-08-05 | Cummins Inc. | Two-stage gas nitriding process for improved wear and erosion resistance |
CN115210401A (zh) * | 2020-01-30 | 2022-10-18 | 康明斯公司 | 用于改善耐磨性和耐腐蚀性的两阶段气体氮化方法 |
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CN115151737B (zh) * | 2020-02-25 | 2024-07-26 | 日本制铁株式会社 | 曲轴及其制造方法 |
CN115151737A (zh) * | 2020-02-25 | 2022-10-04 | 日本制铁株式会社 | 曲轴及其制造方法 |
US12031577B2 (en) | 2020-02-25 | 2024-07-09 | Nippon Steel Corporation | Crankshaft and method of manufacturing the same |
JP7488491B2 (ja) | 2020-02-25 | 2024-05-22 | 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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WO2023203838A1 (ja) * | 2022-04-18 | 2023-10-26 | ジヤトコ株式会社 | 歯車 |
Also Published As
Publication number | Publication date |
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KR20200062317A (ko) | 2020-06-03 |
JPWO2019098340A1 (ja) | 2020-12-03 |
EP3712287A1 (en) | 2020-09-23 |
EP3712287B1 (en) | 2023-07-19 |
US11371132B2 (en) | 2022-06-28 |
CN111406123A (zh) | 2020-07-10 |
EP3712287A4 (en) | 2021-03-24 |
US20200362447A1 (en) | 2020-11-19 |
JP6922998B2 (ja) | 2021-08-18 |
CN111406123B (zh) | 2021-11-26 |
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