Technical field
[0001]
　The present invention is a steel part that has been subjected to gas nitriding treatment, particularly resistance to pitting resistance and bending gears excellent in fatigue properties, nitrided steel part, such CVT sheaves, and a method of manufacturing the same.
Background technique
[0002]
　The steel parts used in automobiles and various industrial machines, fatigue strength, wear resistance, and to improve the mechanical properties such as galling resistance, carburizing and quenching, induction hardening, nitriding, and soft nitriding such as reduction surface hardening heat treatment is performed.
[0003]
　Nitriding and nitrocarburizing processes, A 1 carried out at points below the ferrite region, since there is no phase transformation during processing, it is possible to reduce the heat treatment distortion. Therefore, nitriding and nitrocarburizing treatment is applied high often used for components and large components with dimensional accuracy, and gears used for example in motor vehicle transmission components, to a crankshaft used in an engine.
[0004]
　Nitriding process is a processing method for entering nitrogen into the steel material surface. The medium used in the nitriding treatment, gas, salt bath, plasma and the like. The automotive transmission parts, mainly, gas nitriding is applied is excellent in productivity. By gas nitriding treatment, the steel material surface is formed thick or more compound layers 10μm is further hardened layer below the steel surface layer of the compound layer is a nitrogen diffusion layer is formed. Compound layer mainly Fe 2 ~ 3 N and Fe 4 consists of N, the hardness of the compound layer is very high compared to steel as a base material. Therefore, compound layer, in the initial stage of use, improving the wear resistance and pitting resistance of the steel part.
[0005]
　However, the compound layer with low toughness, and because deformability is low, sometimes the interface compound layer and the base layer in use is separated, the strength of the parts is reduced. Therefore, it is difficult to use a gas nitriding parts, as part of an impact stress and a large bending stress is loaded.
[0006]
　Therefore, for use as part of an impact stress and a large bending stress is the load, the thickness of the compound layer, and further, it is required to eliminate the compound layer. Incidentally, the thickness of the compound layer, and a process temperature of the nitriding process, NH 3 partial pressure H and 2 nitride potential K is determined by the following equation from the partial pressure N can be controlled by known.
[0007]
　　ＫＮ＝（ＮＨ３分圧）／［（Ｈ２分圧）3/2］
[0008]
　Nitride potential K N if low, thin compound layer, and further it is possible to eliminate the compound layer. However, nitriding potential K N if low, nitrogen is less likely to penetrate into the steel. In this case, the hardness of the hardened layer is lowered, and its depth becomes shallow. As a result, the fatigue strength of the nitride component, abrasion resistance, and seizure resistance is lowered. To address this degradation, by implementing the mechanical polishing or shot blasting or the like to the nitride component after gas nitriding, there is a method for removing the compound layer. However, the production cost is high in this way.
[0009]
　Patent Document 1, with respect to such a problem, the atmosphere of gas nitriding, nitriding different from the nitride potential parameters K N ' = (NH 3 partial pressure) / [(H 2 partial pressure) 1 / 2 controlled by: how to reduce variations in the depth of the hardened layer has been proposed.
[0010]
　Patent Document 2, without the formation of compound layer, cured layer gas nitriding method capable of forming a nitride (layer) has been proposed. The method of Patent Document 2, first to remove the oxide film of the component by fluorination treatment, then it performs a nitriding treatment, non-nitriding material into the processing furnace as a jig for positioning an object to be processed is necessary.
[0011]
　However, even nitride parameters proposed by Patent Document 1 is useful for the control of case depth, does not improve the function of the component.
[0012]
　As proposed in Patent Document 2, providing a non-nitriding of the jig, in the case of the method of performing the fluorine process First, the problem of an increase in the selection and working steps of the jig occurs.
CITATION
Patent Literature
[0013]
Patent Document 1: JP 2006-28588 JP
Patent Document 2: JP 2007-31759 JP
Summary of the Invention
Problems that the Invention is to Solve
[0014]
　An object of the present invention, a compound layer having a low toughness and low deformability thinned, and that increasing the hardened layer depth, and solves both difficult problems, the demand for smaller and lighter or high load capacity of the component can respond is to provide a superior nitrided steel part and nitriding method resistance to pitting resistance and bending fatigue properties.
Means for Solving the Problems
[0015]
　The present inventors have thin compound layer formed on the surface of the steel material by nitriding treatment, and was examined how to obtain a deep hardened layer. Furthermore, when the nitriding treatment (particularly high K N during treatment with values), in the vicinity of the surface of the steel material, nitrogen was studied also to a method of suppressing the voids gasified is formed. In addition, it was investigated nitriding conditions and resistance to pitting resistance, and bending the relationship between the fatigue properties. As a result, the present inventors have obtained the following findings (a) ~ (d).
[0016]
　(A) K in the gas nitriding process N for values
　Generally, K N values, NH atmosphere in the furnace to carry out the gas nitriding treatment (hereinafter referred to as "nitriding atmosphere", or simply "atmosphere") 3 partial pressure and, H 2 using a partial pressure, defined by the following formula.
[0017]
　　ＫＮ＝（ＮＨ３分圧）／［（Ｈ２分圧）3/2］
[0018]
　K N value can be controlled by the gas flow. However, after setting the gas flow rate, by the nitriding atmosphere reach equilibrium, it is necessary a certain time. Therefore, K N K also until value reaches the equilibrium state N value is changing every moment. Furthermore, K in the middle of the gas nitriding N may change the value, K until it reaches the equilibrium state N value varies.
[0019]
　K as described above N variations in values, compound layer affects the surface hardness and hardening depth. Therefore, K N well target value of the value, K in the gas nitriding N must be controlled within the value range is also predetermined range of variation of.
[0020]
　(B) the compatibility between ensuring of the compound layer suppressing the surface hardness of the product and the case depth
　in the various experiments of the present inventors, the anti-pitting resistance and bending fatigue properties of nitride component, a compound layer the thickness, the gap of the compound layer, the surface hardness and hardened layer depth was involved. Compound layer is thick, and the void of the compound layer is large, easily cracked compound layer as a starting point, pitting strength and bending fatigue strength is lowered.
[0021]
　Furthermore, low surface hardness, the more shallow case depth, crack or cracks are generated starting from the diffusion layer, pitting strength and bending fatigue strength is lowered. That is, the present inventors have found that thin compound layer less voids compound layer is, the surface hardness is high, and was found that excellent resistance to pitting resistance as a deep hardened layer depth.
[0022]
　From the above, to achieve both anti-pitting resistance and bending fatigue properties, not as much as possible produce a compound layer, and it is important to increase the surface hardness and hardening depth.
[0023]
　To suppress the formation of compound layer, in order to secure the hardening depth is once After generating the compound layer, efficient to use the the resulting compound layer as a nitrogen source to decompose the cured layer it is. Specifically, in the first half of the gas nitriding process, high gas nitriding nitride potential (high K N to implement the value processing) to form a compound layer. Then, in the second half of the gas nitriding treatment, high K N value gas nitriding treatment to lower the nitride potential than treatment (low K N implementing value processing). As a result, a high K N compound layer formed by the value processing is decomposed into Fe and N, N promotes the formation of nitrogen diffusion layer (cured layer) by diffusion. Finally, it is possible to thin the compound layer in the nitride component, and increasing the surface hardness, to deepen the hardening depth.
[0024]
　Suppression of the production of (c) void
　high K in the first half of the gas nitriding N when the nitriding treatment in value, there is a case where the layer containing voids compound layer (porous layer) is generated (FIG. 1 (a )). In this case, even after the nitride has been formed nitrogen diffusion layer is decomposed (cured layer), the gap remains as it is a nitrogen diffusion layer. If voids are left in the nitrogen diffusion layer, the fatigue strength of the nitride component is reduced. High K N K in forming the compound layer in the value processing N if an upper limit of values, it is possible to suppress the formation of porous layer and the gap (Figure 1 (b)).
[0025]
　(D) Relationship between the steel component and the compound layer and nitrogen diffusion layer
　on the steel, when C is present, flexural properties of the compound layer is deteriorated. Moreover, the presence of nitride-forming elements such as Mn and Cr, the hardness and the diffusion layer depth of the nitrogen diffusion layer changes. Resistance pitting resistance and bending fatigue properties, higher diffusion layer hardness and the diffusion layer is improved deeper, it is necessary to set the optimum range of the steel component.
[0026]
　The present invention has been completed based on the above findings and has as its gist is as follows.
[0027]
　[1] by mass%, C: 0.05 ~ 0.25%, Si: 0.05 ~ 1.5%, Mn: 0.2 ~ 2.5%, P: 0.025% or less, S: 0.003 ~ 0.05%, Cr: 0.5 super ~ 2.0%, Al: 0.01 ~ 0.05%, and N: containing 0.003 to 0.025%, the balance being Fe and steel as an impurity and material, which is formed on the steel surface, comprising iron, nitrogen and thickness 3μm following compound layer containing carbon, and a cured layer formed under the compound layer, effective hardening nitrided steel part a layer depth characterized in that it is a 160 ~ 410 .mu.m.
[0028]
　[2] The steel product instead of a part of Fe, Mo: less than 0.01 to 0.50%, V: characterized by containing one or two of lower than 0.01 to 0.50% nitrided steel part of the [1] to.
[0029]
　[3] The steel material instead of a part of Fe, Cu: less than 0.01 to 0.50%, Ni: characterized by containing one or two of lower than 0.01 to 0.50% nitrided steel part according to [1] or [2] to.
[0030]
　[4] The steel product instead of a part of Fe, Ti: wherein characterized in that it contains less than 0.005 to 0.05 percent [1] either the nitriding parts to [3].
[0031]
　[5] by mass%, C: 0.05 ~ 0.25% , Si: 0.05 ~ 1.5%, Mn: 0.2 ~ 2.5%, P: 0.025% or less, S: from 0.003 to 0.05% or less, Cr: 0.5 super ~ 2.0%, Al: 0.01 ~ 0.05%, and N: containing 0.003 to 0.025%, the balance being the steel is Fe and impurities as a material, NH 3 , H 2 and N 2 gas the steel in a gas atmosphere containing heated to 550 ~ 620 ° C., the total processing time a of 1.5 to 10 hours nitride comprising a step of performing processing, the gas nitriding treatment, the treatment time high K and X times N and value processing, high K N subsequent processing time value processing low K and Y time N consists value processing, the high K N value processing, nitriding potential K is determined by equation (1) NX is from 0.15 to 1.50 calculated by equation (2) The nitriding potential K is because NX average value K NXave a is 0.30 to 0.80, the low K N value processing, Equation (3) Potential nitride obtained by the K NY is 0.02-0.25 , and the above nitriding potential K is determined by equation (4) NY average value K NYave is 0.03 to 0.20, and the average value K nitride potential obtained by equation (5) Nave 0.07 to method for producing a nitrided steel part, which is a 0.30.
　　　K NX = (NH 3 partial pressure) X / [(H 2 partial pressure) 3/2 ] X　　　· · · (1)
[Equation 1]

[Equation 2]

　Here, in equation (2) and (4) the subscript i is a number representing the measured times at predetermined time intervals, X 0 nitride potential K NX measurement interval (time), Y 0 nitride potential K NY measurement interval (time), K NXi high KN i th nitride potential in the measurement within the value processing, K NYI low K N is a nitride potential at i th measurement in value processing.
[0032]
　[6] The gas atmosphere, NH 3 , H 2 and N 2 method for producing a nitrided steel part of the [5], characterized in that it comprises in total 99.5% by volume or more.
[0033]
　[7] The steel product instead of a part of Fe, Mo: less than 0.01 to 0.50%, V: characterized by containing one or two of lower than 0.01 to 0.50% the [5] or [6] the method for producing a nitrided steel part according to.
[0034]
　[8] The steel product instead of a part of Fe, Cu: less than 0.01 to 0.50%, Ni: characterized by containing one or two of lower than 0.01 to 0.50% the [5] to any of the manufacturing method of the nitrided steel part of [7] to.
[0035]
　[9] The steel product instead of a part of Fe, Ti: 0.005 above, wherein the containing less than ~ 0.05% [5] to the manufacture of any of the nitrided components [8] Method.
Effect of the invention
[0036]
　According to the present invention, thin compound layer is produced suppression of voids (porous layer) further includes a high surface hardness and deep hardened layer, resistance to pitting resistance, and bending excellent fatigue characteristics nitriding it is possible to obtain the steel parts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037]
[Figure 1] is a view showing a compound layer after the nitriding treatment, (a) shows the example in which the porous layer is generated containing voids compound layer, (b) the example production of porous layer and voids is suppressed it is.
[2] High K N average K nitride potential value processing NXave illustrates a, the relationship between the surface hardness and the compound layer thickness.
[3] Low K N average value K values nitriding potential processing NYave illustrates a, the relationship between the surface hardness and the compound layer thickness.
[4] The average value K nitride potential Nave shows a, the relationship between the surface hardness and the compound layer thickness.
[5] in the form of a small roller for roller pitting test used to evaluate resistance to pitting resistance.
6 is a shape of a large roller for roller pitting test used to evaluate resistance to pitting resistance.
A cylindrical test piece for evaluating FIG. 7 flexural fatigue properties.
DESCRIPTION OF THE INVENTION
[0038]
　It will be described in detail below each requirement of the present invention. First, a description will be given chemical composition of the steel as a material. Hereinafter, "%" representing the element concentration in the content and the component surface of the component element means "mass%".
[0039]
　[C: 0.05 ~ 0.25%]
　C is an element necessary for ensuring the core hardness of the component. When the content of C is less than 0.05%, the core strength is too low, pitting strength and bending fatigue strength is significantly decreased. When the content of C exceeds 0.25%, the high K N values during processing tends to increase the compound layer thickness, and the low K N compound layer is less likely to degrade during value processing. Therefore, it becomes difficult to reduce the compound layer thickness after nitriding, pitting strength and bending fatigue strength may be reduced. Moreover, since the strength after hot forging is too high, cutting resistance is greatly reduced. The preferred range of C content is from 0.08 to 0.20 percent.
[0040]
　[Si: 0.05 ~
　1.5%] Si is by solid solution strengthening, enhancing the core hardness. In addition, there is also a deoxidizing element. To exhibit these effects, it is contained more than 0.05%. On the other hand, when the content of Si exceeds 1.5%, steel bars, since the strength after wire or hot forging is too high, cutting resistance is greatly reduced. A preferred range of Si content is 0.08 to 1.3%.
[0041]
　[Mn: 0.2 ~
　2.5%] Mn is by solid solution strengthening, enhancing the core hardness. Furthermore, Mn, at the time of nitriding treatment, fine nitrides in the cured layer (Mn 3 N 2 to form a), To obtain these effects, Mn is required more than 0.2%. On the other hand, when the content of Mn exceeds 2.5%, precipitation strengthening ability is saturated. Furthermore, since the effective hardened layer depth becomes shallower, the pitting strength and bending fatigue strength decreases. Further, since the steel bar as a material, the wire material and the hardness after hot forging too high, cuttability is greatly reduced. A preferred range of Mn content is 0.4 to 2.3%.
[0042]
　[P: 0.025% or less]
　P is an impurity, since the brittle parts by grain boundary segregation, the content is preferably small. When the content of P exceeds 0.025%, the bending straightening property and bending fatigue strength is lowered. The preferable upper limit of the P content to prevent the deterioration of the bending fatigue strength is 0.018%. The complete content 0 and it is difficult to realistic lower limit is 0.001%.
[0043]
　[S: 0.003 ~ 0.05%]
　S forms MnS, in combination with Mn, to improve the machinability. To obtain this effect, S is a required 0.003% or more. However, when the content of S exceeds 0.05%, it becomes easy to generate a coarse MnS, pitting strength and bending fatigue strength is significantly decreased. A preferred range of S content is 0.005 to 0.03%.
[0044]
　[Cr: 0.5 super ~
　2.0%] Cr is during nitriding, fine nitrides (Cr 2 a N) is formed in the hardened layer, to improve the pitting strength and bending fatigue strength by precipitation strengthening . To obtain these effects, Cr is required than 0.5%. On the other hand, when the content of Cr exceeds 2.0%, precipitation strengthening ability is saturated. Furthermore, since the effective hardened layer depth becomes shallower, the pitting strength and bending fatigue strength decreases. Further, since the steel bar as a material, the wire material and the hardness after hot forging too high, cutting workability is remarkably lowered. The preferred range of Cr content is 0.6 to 1.8%.
[0045]
　[Al: 0.01 ~
　0.05%] Al is a deoxidizing element, is required 0.01% or more for sufficient deoxidation. On the other hand, Al is liable to form a hard oxide inclusions, the content of Al exceeds 0.05%, bending decrease in fatigue strength is significantly desired bending even meet the other requirements fatigue strength can not be obtained. A preferred range of the Al content is 0.02 to 0.04%.
[0046]
　[N: 0.003 ~ 0.025%]
　N is, Al, V, combines with Ti to form AlN, VN, a TiN. AlN, VN, TiN by pinning action of the austenite grains, the steel of tissue prior to nitriding treatment finer and have the effect of reducing the variation in mechanical properties of the nitrided steel part. This effect is in a content less than 0.003% of N is inaccessible. On the other hand, when the content of N exceeds 0.025%, the order likely to be coarse AlN is formed, the above effect becomes difficult to obtain. A preferred range of N content is 0.005 to 0.020%.

claims
　By
　　mass%, C:
　　0.05 ~
　　0.25%, Si: 0.05 ~ 1.5%, Mn: 0.2
　　~ 2.5%, P: 0.025% or
　　less, S: 0.003 ~
　　0.05%, Cr: 0.5 ultra
　　2.0% ~, Al: 0.01 ~ 0.05%, and
　　N: 0.003 ~ 0.025%
containing the balance of Fe and impurities certain steel as raw material,
　which is formed on the steel surface, comprising iron, the thickness 3μm following compound layer containing nitrogen and carbon, and a cured layer formed under the compound layer,
　effective case depth There is a 160 ~ 410 .mu.m
nitrided steel part, characterized in that.
[Claim 2]
　The steel material instead of a part of Fe, Mo: less than 0.01 to 0.50%, V: claims, characterized in that it contains one or two of lower than 0.01 to 0.50% nitrided steel part according to claim 1.
[Claim 3]
　The steel material instead of a part of Fe, Cu: less than 0.01 to 0.50%, Ni: claims, characterized in that it contains one or two of lower than 0.01 to 0.50% nitrided steel part according to claim 1 or 2.
[Claim 4]
　The steel material instead of a part of Fe, Ti: 0.005 nitrided component according to any one of claims 1 to 3, characterized in that it contains less than to 0.05%.
[Claim 5]
　Area 25μm ranging from the outermost surface of 5μm depth of the steel 2 nitrided component according to any one of claims 1 to 4, the proportion of the voids occupying the is equal to or less than 10%.
[Claim 6]
　By
　　mass%, C:
　　0.05 ~
　　0.25%, Si: 0.05 ~ 1.5%, Mn: 0.2
　　~ 2.5%, P: 0.025% or
　　less, S: 0.003 ~
　　0.05%, Cr: 0.5 ultra
　　2.0% ~, Al: 0.01 ~ 0.05%, and
　　N: 0.003 ~ 0.025%
contained, the balance of Fe and impurities there steel was used as a raw
　material, NH 3 , H 2 and N 2 step heating the steel in a gas atmosphere containing the 550 ~ 620 ° C., subjected to gas nitriding treatment of the entire processing time a of 1.5 to 10 hours the provided,
　the gas nitriding treatment, high K is the processing time and the time X N and value processing, high K N subsequent processing time value processing low K and Y time N consists value processing,
　the high K N value treatment, nitriding potential K is determined by equation (1) NX is 0.15 to 1.50 determined by equation (2) Is the nitriding potential K NX average value K NXave is 0.30 to 0.80
　the low K N value processing, nitriding potential K is determined by equation (3) NY in the 0.02-0.25 There, the nitriding potential K is determined by equation (4) NY average value K NYave is 0.03 to 0.20, and the average value K nitride potential obtained by equation (5) Nave is 0.07 to 0 a .30
method of manufacturing nitrided steel part, characterized in that.
　　　K NX = (NH 3 partial pressure) X / [(H 2 partial pressure) 3/2 ] X　　　· · · (1)
[Equation 1]

[Equation 2]

　Here, in equation (2) and (4) the subscript i is a number representing the measured times at predetermined time intervals, X 0 nitride potential K NX measurement interval (time), Y 0 nitride potential K NY measurement interval (time), K NXi high KN i th nitride potential in the measurement within the value processing, K NYI low K N is a nitride potential at i th measurement in value processing.
[Claim 7]
　The gas atmosphere, NH 3 , H 2 and N 2 method for producing a nitrided steel part according to claim 6, characterized in that it comprises in total 99.5% by volume or more.
[8.]
　The steel material instead of a part of Fe, Mo: less than 0.01 to 0.50%, V: claims, characterized in that it contains one or two of lower than 0.01 to 0.50% method for producing a nitrided steel part according to claim 6 or 7.
[Claim 9]
　The steel material instead of a part of Fe, Cu: less than 0.01 to 0.50%, Ni: claims, characterized in that it contains one or two of lower than 0.01 to 0.50% method for producing a nitrided steel part according to any one of claim 6-8.
[Claim 10]
　The steel material instead of a part of Fe, Ti: 0.005 ~ method of manufacturing nitrided component according to any one of claims 6-9, characterized in that it contains less than 0.05%.