WO2018066666A1 - 窒化処理部品及びその製造方法 - Google Patents
窒化処理部品及びその製造方法 Download PDFInfo
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- 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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- C21D1/06—Surface hardening
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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Definitions
- the present invention relates to a steel part subjected to gas nitriding, in particular, a nitriding part such as a gear and a CVT sheave having excellent surface fatigue strength 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 and surface fatigue strength of the steel part in the initial stage of use.
- Patent Document 1 discloses a nitriding component having improved bending fatigue strength 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 nitriding low alloy steel having sufficient surface hardness and hardened layer depth by suppressing the formation of a compound layer.
- Patent Document 4 discloses a steel member that has an iron nitride compound layer having a predetermined structure formed on its surface, has high pitting resistance and bending fatigue strength, and has low strain compared to carburizing and carbonitriding. It is disclosed.
- 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.
- the nitriding method of Patent Document 3 is characterized in that the compound layer is thinned by two-step nitriding of high K N value processing and low K N value processing.
- the compound layer is applied by the first nitriding, and the effective hardening layer is deepened by decomposing the compound layer applied by the second nitriding by diffusing N into the hardening layer.
- a complicated process called two-stage nitriding is required.
- the ratio of the ⁇ ′ phase becomes low, and it tends to be a starting point for pitting and bending fatigue failure.
- An object of the present invention is to provide a part excellent in rotational bending fatigue strength in addition to surface fatigue strength 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 a nitrided part having excellent surface fatigue strength and rotational bending fatigue strength can be produced by setting the stress to a certain value or more.
- the present invention has been further studied based on the above findings, and the gist thereof is as follows.
- C 0.05% or more and 0.30% or less
- Si 0.05% or more, 1.5% or less
- Mn 0.2% or more, 2.5% or less
- P 0.00%. 025% or less
- S 0.003% or more
- Cr more than 0.5%
- Al 0.01% or more
- N 0.00.
- Nb 0% or more, 0.1% or less
- B 0% or more, 0.01% or less
- 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, less than 0.50%
- Ti 0% or more, less than 0.05%
- It is a part made of a steel material that is Fe and impurities, and has a compound layer formed on the surface of the steel material and containing iron, nitrogen, and carbon and having a thickness of 3 ⁇ m or more and less than 15 ⁇ m
- the phase structure in the compound layer in the range from the surface to a depth of 5 ⁇ m contains a ⁇ ′ phase in an area ratio of 50% or more, and the void area ratio is less than 10% in the range from the surface to a depth of 3 ⁇ m.
- a nitriding component having a compressive residual stress of 500 MPa or more.
- C 0.05% or more, 0.30% or less
- C is an element necessary for securing the core hardness of the component. If the C content is less than 0.05%, the core strength is too low, and the surface fatigue strength and bending fatigue strength are greatly reduced. On the other hand, when the C content exceeds 0.30%, the thickness of the compound layer increases, and the surface fatigue strength and bending resistance are greatly reduced. A preferred range for the C content is 0.08 to 0.25%.
- Si 0.05% or more, 1.5% or less
- Si increases the core hardness by solid solution strengthening. It also increases the resistance to temper softening and increases the surface fatigue strength of the part surface that becomes high temperature under wear conditions. In order to exhibit these effects, 0.05% or more is contained.
- 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. Further, Mn forms fine nitrides (Mn 3 N 2 ) in the hardened layer during nitriding treatment, and improves wear resistance and surface 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 surface fatigue strength is saturated, but also the steel bar, the wire material, and the hardness after hot forging become too high, so the cutting work 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 surface fatigue strength and the bending fatigue strength are greatly reduced. A preferred range for the S content is 0.005 to 0.03%.
- Cr more than 0.5%, 2.0% or less
- Cr forms fine nitride (CrN) in the hardened layer during nitriding, and improves surface fatigue strength and bending fatigue strength by precipitation strengthening.
- Cr needs to exceed 0.5%.
- the content of Cr exceeds 2.0%, not only the effect of improving the surface fatigue strength is saturated, but also the steel bar, the wire material and the hardness after hot forging become too high, so cutting Workability is significantly reduced.
- a preferable range of the Cr content is 0.7 to 1.8%.
- 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 action of NbC and NbN suppresses the austenite grain coarsening, refines the structure of the steel material before nitriding, and has the effect of reducing variations in the mechanical properties of the nitriding parts. 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 Nb, but in order to obtain the effects 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 nitrides (Mo 2 N) in the hardened layer during nitriding, and improves surface fatigue strength and 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 surface fatigue strength and 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, and its thickness affects the surface fatigue strength and bending strength of the nitriding component. If the compound layer is too thick, it tends to be a starting point of fracture due to pitting or bending. If the compound layer is too thin, the surface residual stress cannot be sufficiently obtained, and the surface fatigue strength and bending strength are reduced. In the nitriding part of the present invention, the thickness of the compound layer is 3 ⁇ m or more and less than 15 ⁇ m from the viewpoint of surface fatigue strength 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.
- void area ratio of compound layer of surface to 3 ⁇ m: less than 10% If voids are present in the compound layer of the surface to 3 ⁇ m, stress concentration occurs, which becomes the starting point for pitting and bending fatigue failure. Therefore, the void area ratio needs to be less than 10%.
- 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. 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.
- 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 surface fatigue strength and rotational bending fatigue strength by improving the compound layer as described above and 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 10%, 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 is satisfied, the phase structure of the compound layer becomes a ⁇ ′ phase ratio of 50% or more, and the nitriding component has high rotational bending fatigue strength and surface 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 j (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 40 mm. Subsequently, after each round bar was annealed, cutting was performed, and a small roller for a roller pitting test for evaluating the surface fatigue strength shown in FIG. 6 and a large roller shown in FIG. 7 were produced. Furthermore, the cylindrical test piece for evaluating the bending fatigue strength shown in FIG. 8 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 34 was a gas soft nitriding treatment in which CO 2 gas was added by 3% by volume in the atmosphere.
- Test number 35 was a two-stage nitriding treatment in which the nitriding conditions were changed between the first half and the second half.
- the test number 36 corresponds to Example 16 in Patent Document 3. 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
- roller pitting test was carried out under the conditions shown in Table 4 with a combination of the above small roller pitting test roller and the large roller pitting test roller having the shape shown in FIG.
- the large roller for the roller pitting test is made of a steel that satisfies the standard of JIS SCM420, and is a general manufacturing process, that is, “normalizing ⁇ test piece processing ⁇ eutectoid carburizing by gas carburizing furnace ⁇ low temperature tempering ⁇ polishing
- the Vickers hardness Hv at a position of 0.05 mm from the surface, that is, at a depth of 0.05 mm, is 740 to 760, and the depth of the Vickers hardness Hv is 550 or more.
- Table 4 shows the test conditions for evaluating the surface fatigue strength.
- the number of test censoring is 2 ⁇ 10 7 times indicating the general fatigue source of steel, and the maximum surface pressure that reaches 2 ⁇ 10 7 times without occurrence of pitting in the small roller test piece is the same as that of the small roller test piece.
- the fatigue limit is 2 ⁇ 10 7 times indicating the general fatigue source of steel, and the maximum surface pressure that reaches 2 ⁇ 10 7 times without occurrence of pitting in the small roller test piece is the same as that of the small roller test piece. The fatigue limit.
- Detecting the occurrence of pitting was performed using a vibrometer provided in the testing machine. After the occurrence of vibration, the rotation of both the small roller test piece and the large roller test piece was stopped, and the occurrence of pitting and the number of rotations were confirmed.
- the maximum surface pressure at the fatigue limit was set to 2000 MPa or more.
- the maximum stress at the fatigue limit was set to 600 MPa or more.
- Test results The results are shown in Table 2.
- Test Nos. 1 to 25 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 area 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 surface fatigue strength of 2000 MPa or more and a rotational bending fatigue strength of 600 MPa or more.
- Test No. 30 had a lower lower limit of the nitriding potential, a sufficient compound layer thickness could not be obtained, and the residual stress on the surface was lowered, so that the surface fatigue strength and rotational bending fatigue strength were lowered.
- Test No. 32 had a high upper limit of the nitriding potential, the void area ratio increased, and the rotary bending fatigue strength decreased.
- Test No. 33 the upper and lower limits of the nitriding potential were not appropriate, the compound layer thickness was increased, and the void area ratio was increased, so that the surface fatigue strength and rotational bending fatigue strength were reduced.
- Test No. 34 was soft nitriding treatment, and almost no ⁇ 'phase was generated on the surface and the residual stress was low, so that the surface fatigue strength and the rotational bending fatigue strength were low.
- test number 35 the average K N was appropriate and the ⁇ ′ phase ratio was high, but the upper limit of K N during nitriding was high, and the void area ratio increased.
- Test No. 40 had an excessively high amount of P and S in the steel, and was destroyed early due to the segregation of P grain boundaries and the generation of coarse MnS.
- test number 42 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. 43 had low C content, Mn content, and Cr content of steel, and the upper limit of nitriding potential was high and the void area ratio was increased, so that the surface fatigue strength and rotational bending fatigue strength were low.
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。Cの含有量が0.05%未満では、芯部強度が低くなりすぎるため、面疲労強度や曲げ疲労強度が大きく低下する。また、Cの含有量が0.30%を超えると、化合物層厚さが大きくなり、面疲労強度や耐曲げ性が大きく低下する。C含有量の好ましい範囲は0.08~0.25%である。
Siは、固溶強化によって、芯部硬さを高める。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を高める。これらの効果を発揮させるため、0.05%以上を含有させる。一方、Siの含有量が1.5%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含有量の好ましい範囲は0.08~1.3%である。
Mnは、固溶強化によって、芯部硬さを高める。さらに、Mnは、窒化処理時には、硬化層中に微細な窒化物(Mn3N2)を形成し、析出強化によって耐摩耗性及び面疲労強度を向上させる。これらの効果を得るため、Mnは0.2%以上が必要である。一方、Mnの含有量が2.5%を超えると、面疲労強度を高める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含有量の好ましい範囲は0.4~2.3%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、曲げ矯正性や曲げ疲労強度が低下する場合がある。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。含有量を完全に0とするのは難しく、現実的な下限は0.001%である。
Sは、Mnと結合してMnSを形成し、切削加工性を向上させる。この効果を得るために、Sは0.003%以上が必要である。しかしながら、Sの含有量が0.05%を超えると、粗大なMnSを生成しやすくなり、面疲労強度や曲げ疲労強度が大きく低下する。S含有量の好ましい範囲は0.005~0.03%である。
Crは、窒化処理時に、微細な窒化物(CrN)を硬化層中に形成し、析出強化によって面疲労強度及び曲げ疲労強度を向上させる。これらの効果を得るため、Crは0.5%超が必要である。一方、Crの含有量が2.0%を超えると、面疲労強度を向上させる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含有量の好ましい範囲は0.7~1.8%である。
Alは、脱酸元素であり、十分な脱酸のために0.01%以上が必要である。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0.05%を超えると、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曲げ疲労強度が得られなくなる。Al含有量の好ましい範囲は0.02~0.04%である。
Nは、Al、Vと結合してAlN、VNを形成する。AlN、VN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。Nの含有量が0.003%未満ではこの効果は得難い。一方で、Nの含有量が0.025%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含有量の好ましい範囲は0.005~0.020%である。
Nbは、CやNと結合してNbCやNbNを形成する。NbC、NbNのピンニング作用により、オーステナイト粒の粗大化が抑制され、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。この効果はNbを微量添加すれば得られるが、より確実に効果を得るためには、Nbは0.01%以上とするのが好ましい。Nbの含有量が0.1%を超えると、粗大なNbC、NbNが形成されやすくなるため、上記の効果は得にくくなる。
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BNを形成し切削性を向上させる。これらの効果はNbを微量添加すれば得られるが、より確実に効果を得るためには、Bは0.0005%以上とすることが好ましい。Bの含有量が0.01%を超えると、上記効果が飽和するだけでなく、多量のBNが偏析することで鋼材に割れが生じることがある。
Moは、窒化時に微細な窒化物(Mo2N)を硬化層中に形成し、析出強化によって面疲労強度及び曲げ疲労強度を向上させる。また、Moは、窒化時に時効硬化作用を発揮して芯部硬さを向上させる。これらの効果を得るためのMo含有量は0.01%以上とするのが好ましい。一方、Moの含有量が0.50%以上では、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、合金コストが増大する。切削加工性確保のためのMo含有量の好ましい上限は0.40%未満である。
Vは、窒化及び軟窒化時に微細な窒化物(VN)を形成し、析出強化によって面疲労強度及び曲げ疲労強度を向上させ、部品の芯部硬さを高くする。また、組織微細化の効果も有する。これらの作用を得るため、Vは0.01%以上とするのが好ましい。一方、Vの含有量が0.50%以上では、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、合金コストが増大する。切削加工性確保のためのV含有量の好ましい範囲は0.40%未満である。
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%以上では、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含有量の好ましい範囲は0.40%未満である。
Niは、固溶強化により芯部硬さ及び表層硬さを向上させる。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%以上では、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含有量の好ましい範囲は0.40%未満である。
Tiは、Nと結合してTiNを形成し、芯部硬さ及び表層硬さを向上させる。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0.05%以上では、芯部硬さ及び表層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含有量の好ましい範囲は0.007~0.04%未満である。
化合物層とは窒化処理により形成された鉄窒化物の層であり、その厚さは、窒化処理部品の面疲労強度や曲げ強度に影響する。化合物層が厚すぎると、ピッティングや曲げによる破壊の起点となりやすい。化合物層が薄すぎると、表面の残留応力が十分に得られず、面疲労強度や曲げ強度が低下する。本発明の窒化処理部品においては、面疲労強度や曲げ強度の観点から、化合物層の厚さは3μm以上15μm未満とする。
表面~5μmの化合物層においてγ’相の比率が低く、ε相比率が高いと、ピッティングや曲げ疲労破壊の起点となりやすくなる。これは、ε相の破壊靭性値がγ’相と比べ低いためである。また、表面付近の相がγ’相であるとε相である場合に比べ、後述する圧縮残留応力を表面に導入しやすくなり、疲労強度を向上させることが可能となる。
表面~3μmの化合物層に空隙が存在すると応力集中が生じ、ピッティングや曲げ疲労破壊の起点となる。そのため、空隙面積率は10%未満とする必要がある。
本発明の窒化処理部品は、窒化処理により鋼の表面が硬化するとともに、鋼の表層部に圧縮残留応力が導入され、部品の疲労強度、耐摩耗性が向上する。本発明の窒化処理部品は、化合物層を上述した向上とし、さらに表面に圧縮残留応力を500MPa以上導入することにより、優れた面疲労強度、回転曲げ疲労強度を有するものとなる。部品の表面にこのような圧縮残留応力を導入するための製造方法は後述する。
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、脱窒や脱炭が発生して鋼の表面硬さが低下する。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。
本発明の窒化処理方法では、生地のC量を考慮して制御された窒化ポテンシャルの下で窒化処理が施される。これにより、表面~5μmの深さの化合物層における相構造をγ’相比率50%以上とし、表面~3μmの深さにおける空隙面積率を10%未満とし、化合物層表面の圧縮残留応力を500MPa以上とすることができる。
ガス窒化処理後の小ローラーの、長さ方向に垂直な方向の断面を鏡面研磨し、エッチングした。走査型電子顕微鏡(Scanning Electron Microscope:SEM)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。
化合物層中のγ’相比率を、後方散乱電子回折法(Electron BackScatter Diffraction:EBSD)で求めた。化合物層の最表面から5μm深さまでの、面積150μm2についてEBSD測定を行い、γ’相、ε相を判別する解析図を作成し、得られたEBSD解析像について、画像処理アプリケーションを用いてγ’相比率(%)を決定した。EBSD測定では、4000倍の倍率で10視野測定した。
窒化後の小ローラー接触部に対し、微小部X線残留応力測定装置を用いて、表3の条件でγ’相、ε相及び母層(matrix)の残留応力σγ’、σε、σmを測定した。さらに、EBSDにて求めた、最表面から3μm深さ範囲の面積90μm2中に占めるγ’相、ε相及び母層の面積比Vγ’、Vε、Vmを用いて、以下の式で求まる残留応力σcを表面の残留応力とした。
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を図2に示す。
ガス窒化処理に供した円柱試験片に対し、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2に示す。試験番号1~25は鋼の成分、及びガス窒化処理の条件が本発明の範囲内であり、化合物厚さが3~15μm、化合物層のγ’層比率が50%以上、化合物層空隙面積率10%未満、化合物層の圧縮残留応力が500MPa以上となった。その結果、面疲労強度が2000MPa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。
Claims (1)
- 質量%で、
C :0.05%以上、0.30%以下、
Si:0.05%以上、1.5%以下、
Mn:0.2%以上、2.5%以下、
P :0.025%以下、
S :0.003%以上、0.05%以下、
Cr:0.5%超、2.0%以下、
Al:0.01%以上、0.05%以下、
N :0.003%以上、0.025%以下、
Nb:0%以上、0.1%以下、
B :0%以上、0.01%以下、
Mo:0%以上、0.50%未満、
V :0%以上、0.50%未満、
Cu:0%以上、0.50%未満、
Ni:0%以上、0.50%未満、及び
Ti:0%以上、0.05%未満
を含有し、残部がFe及び不純物である鋼材を素材とした部品であって、
鋼材の表面に形成された、鉄、窒素及び炭素を含有する厚さ3μm以上15μm未満の化合物層を有し、
表面から5μmの深さまでの範囲の化合物層における相構造がγ’相を面積率で50%以上含有し、
表面から3μmの深さまでの範囲において空隙面積率が10%未満であり、
化合物層表面の圧縮残留応力が500MPa以上である
ことを特徴とする窒化処理部品。
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US16/338,383 US20200040439A1 (en) | 2016-10-05 | 2017-10-05 | Nitrided part and method of production of same |
BR112019006046A BR112019006046A2 (pt) | 2016-10-05 | 2017-10-05 | peça nitretada e método para produzir a mesma |
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JP2020033638A (ja) * | 2018-08-27 | 2020-03-05 | Jfeスチール株式会社 | 部品およびその製造方法 |
JP2020084206A (ja) * | 2018-11-15 | 2020-06-04 | 日本製鉄株式会社 | 鋼部品及びその製造方法 |
JP2020117756A (ja) * | 2019-01-22 | 2020-08-06 | 日本製鉄株式会社 | 軟窒化処理部品及びその製造方法 |
WO2021181570A1 (ja) * | 2020-03-11 | 2021-09-16 | 日本製鉄株式会社 | ガス軟窒化処理部品及びその製造方法 |
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US20230193414A1 (en) * | 2020-05-15 | 2023-06-22 | Jfe Steel Corporation | Steel and steel component |
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