Steel for soft nitriding and soft-nitrided component

Abstract

To provide a steel for soft nitriding that makes it possible to manufacture a soft-nitrided component having excellent fatigue strength without impairing machinability before soft nitriding treatment.SOLUTION: A steel material for soft nitriding contains, in mass%, C: 0.08% to 0.20%, Si: 0.05% to 0.40%, Mn: less than 1.55%, Cr: 0.80% to 1.60%, Mo: 0.20% to 0.70%, V: 0.15% to 0.40%, and Al: 0.005% to 0.200%, with the remainder being Fe and unavoidable impurities, satisfies the following formulas (1), (2), and (3), and has a metal structure in which a bainite area ratio exceeds 90%. 40[C]+7[Mn]+8[Cr]+9[Mo]+12[V]<30.0 Formula (1), 1200[C]+112[Cr]+134[Mo]+560[V]-700[Al]>280 Formula (2), and [Cr]+2[Mo]+20[V]+4[Al]>4.0 Formula (3) (in the above formulas, [ ] represents a content in mass% of an element in the brackets).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel material for nitrocarburizing and a nitrocarburized part using the same. [Background technology]

[0002] Power transmission parts such as gears are required to have high strength to resist surface fatigue caused by contact between tooth surfaces and bending fatigue at the tooth base. For this reason, power transmission parts are made of materials with high internal toughness and are subjected to surface treatments such as carburizing and nitriding to increase the hardness of the surface. Among these surface treatments, soft nitriding, which is performed at a low temperature of around 600°C, causes little heat treatment distortion associated with the surface treatment and can reduce noise caused by meshing of gears, etc. However, the strength of parts that have been soft-nitrided (soft-nitrided parts) is not necessarily sufficient, and soft-nitrided parts can only be applied to parts that require a relatively low level of fatigue strength.

[0003] The following points can be cited as problems in further increasing the fatigue strength of soft nitrided parts. One known method for increasing the fatigue strength of components is to apply additional shot peening to the surface layer to increase compressive residual stress. The compressive residual stress imparted by shot peening has the effect of strengthening stress concentration areas that are the starting points for initial cracks and suppressing the occurrence of initial cracks. However, conventional soft-nitrided components lack hardness near the surface, which can lead to unevenness and cracks during shot peening, which can actually reduce fatigue strength. It is also necessary to increase the core hardness of the soft-nitrided parts, but adding an excessive amount of alloying components to increase the core hardness is likely to impair machinability in cutting processes and the like that are carried out prior to the soft-nitriding treatment.

[0004] Incidentally, the following Patent Document 1 discloses a technology related to the present invention. Patent Document 1 discloses a nitriding steel that has good machinability before nitriding and can improve the surface hardness of nitrided parts after nitriding, but Patent Document 1 does not disclose any working examples that satisfy the alloy composition of the present invention. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-077333 Summary of the Invention [Problem to be solved by the invention]

[0006] With the above circumstances as a background, the present invention aims to provide a steel material for nitrocarburizing that can be used to manufacture nitrocarburized parts with excellent fatigue strength without impairing the machinability before nitrocarburizing treatment, and to provide nitrocarburized parts using the same. [Means for solving the problem]

[0007] The inventors have taken into consideration the effect of each alloying element on the hardness before nitriding, and on the surface hardness and core hardness after nitriding, and have discovered that by properly balancing the amount of each alloying element added, the overall effect is to increase the fatigue strength of the nitrided parts while ensuring the machinability before nitriding.

[0008] The gist of the present invention is as follows.

[0009] [1] A steel material for soft nitriding containing, by mass%, C: 0.08 to 0.20%, Si: 0.05 to 0.40%, Mn: less than 1.55%, Cr: 0.80 to 1.60%, Mo: 0.20 to 0.70%, V: 0.15 to 0.40%, Al: 0.005 to 0.200%, and the balance being Fe and unavoidable impurities, and satisfying the following formulas (1), (2), and (3), and having a metal structure with a bainite area ratio of more than 90%. 40[C]+7[Mn]+8[Cr]+9[Mo]+12[V]<30.0...Formula (1) 1200[C]+112[Cr]+134[Mo]+560[V]-700[Al]>280...Formula (2) [Cr]+2[Mo]+20[V]+4[Al]>4.0 …Formula (3) (In the above formula, [ ] represents the mass % content of the element in [ ])

[0010] [2] The steel material for nitrocarburizing according to [1], containing, by mass%, 0.030 to 0.060% of Al.

[0011] [3] The steel material for nitrocarburizing according to [1], further containing, by mass%, one or more of Ti: 0.002 to 0.040%, Cu: 0.01 to 1.0%, and Ni: 0.01 to 1.0%.

[0012] [4] A soft-nitrided part made of the steel material according to any one of [1] to [3] and having a soft-nitrided layer on a surface layer, A soft nitrided part with a surface hardness of over 600 HV at a depth of 0.2 mm from the surface of the part and a core hardness of over 280 HV. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing a test piece for evaluating bending fatigue strength. [Figure 2] FIG. 1 is a diagram showing a test piece for evaluating contact fatigue strength together with a roller pitting test method. DETAILED DESCRIPTION OF THE INVENTION

[0014] The steel material for soft nitriding according to this embodiment contains predetermined amounts of C, Si, Mn, Cr, Mo, V, and Al, with the balance being Fe and unavoidable impurities. In addition, the steel material may further contain Ti, Cu, and Ni. The reasons for limiting each chemical component in the steel material for nitrocarburizing according to this embodiment will be described in detail below. In the following description, "%" means "mass %" unless otherwise specified.

[0015] C: 0.08 to 0.20% C is an element effective in ensuring internal (core) hardness. To obtain the required hardness, 0.08% or more is contained. However, if added in excess, the hardness becomes too high, resulting in a decrease in forgeability and machinability, so the upper limit is set at 0.20%. The preferred range of C is 0.08 to 0.14%.

[0016] Si: 0.05 to 0.40% Silicon is an effective deoxidizing element for steel. To achieve this effect, the content must be 0.05% or more. However, if the Si content is too high, the hardness increases and forgeability decreases, so the upper limit is set at 0.40%.

[0017] Mn: Less than 1.55% Mn has the effect of ensuring the bending fatigue strength and core hardness of soft nitrided parts, as well as a deoxidizing effect. However, if the Mn content is too high, the hardness increases, resulting in a decrease in forgeability and machinability, so an upper limit has been set, making the Mn content less than 1.55%.

[0018] Cr: 0.80~1.60% Cr combines with N during soft nitriding to form fine nitrides, which increases the surface hardness. Cr also suppresses the formation of ferrite during cooling after forging, thereby improving the area ratio of bainite. To achieve these effects, Cr is contained in an amount of 0.80% or more, preferably 0.95% or more. However, if the Cr content is too high, the hardness increases, resulting in a decrease in forgeability and machinability, so the Cr content must be 1.60% or less, preferably 1.50% or less, and more preferably 1.25% or less.

[0019] Mo: 0.20 to 0.70% Mo combines with C in steel at the soft nitriding temperature to form carbides, which increase the core hardness through age hardening. It also suppresses the formation of ferrite during cooling after forging, thereby improving the area ratio of bainite. To achieve these effects, Mo is added in an amount of 0.20% or more. However, if the Mo content is too high, the hardness increases, resulting in a decrease in forgeability and machinability. For this reason, the Mo content must be 0.70% or less. It is preferably 0.60% or less, and more preferably 0.45% or less.

[0020] V: 0.15 to 0.40% V combines with C and / or N during soft nitriding to form carbides, nitrides, and carbonitrides, thereby increasing the hardness of the surface layer. It also combines with C in the steel to form carbides through age hardening at the soft nitriding temperature, increasing the hardness of the core. To obtain these effects, the V content must be 0.15% or more, preferably 0.20% or more. However, if the V content is too high, the hardness increases, resulting in a decrease in forgeability and machinability. Therefore, the V content must be 0.40% or less, and preferably 0.30% or less.

[0021] Al: 0.005 to 0.200% Al is sometimes used as a deoxidizer during melting. It also combines with N to form AlN during soft nitriding, which has the effect of increasing surface hardness. To achieve these effects, an Al content of 0.005% or more is required. However, if the Al content is too high, hard and coarse Al2O3 is formed, which not only reduces forgeability but also reduces the effective hardened layer during soft nitriding, resulting in a decrease in bending fatigue strength and pitting strength. Therefore, an upper limit has been set, limiting the Al content to 0.005 to 0.200%. The preferred Al range is 0.030 to 0.060%.

[0022] Ti: 0.002 to 0.040% Ti contributes to preventing coarsening of austenite grains during forging, so it can be added in an amount of 0.002% or more as needed. However, since excessive addition increases costs, the upper limit is set at 0.040%.

[0023] Cu: 0.01 to 1.00% Cu has the effect of improving hardenability, so it can be added in an amount of 0.01% or more if necessary. However, since excessive addition of Cu leads to a decrease in hot forgeability, the upper limit is set to 1.00%.

[0024] Ni: 0.01 to 1.00% Ni can be added in an amount of 0.01% or more as needed to improve hardenability and ductility / toughness, but since excessive addition increases the cost of steel, the upper limit is set at 1.00%.

[0025] 40[C]+7[Mn]+8[Cr]+9[Mo]+12[V]<30.0...Formula (1) The value on the left side of equation (1) is an index representing the hardness after hot working such as hot forging (hereinafter simply referred to as after hot forging), more specifically before soft nitriding treatment. The larger the value, the higher the hardness after hot forging, and the smaller the value, the lower the hardness after hot forging. By adjusting the alloying elements so that the value of the left side of formula (1) is less than 30.0, the hardness of the steel material after hot forging can be made less than 280 HV, and this ensures machinability in cutting processes and the like that are carried out prior to the soft nitriding treatment.

[0026] 1200[C]+112[Cr]+134[Mo]+560[V]-700[Al]>280...Formula (2) The value on the left side of equation (2) is an index representing the core hardness after soft nitriding, and by making this value greater than 280, the core hardness after soft nitriding can be increased to greater than 280 HV. This increases the strength of soft nitrided parts against bending fatigue and surface fatigue. Below 280 HV, the bending fatigue strength decreases, and the practically required 10 7 The bending fatigue strength, which is the lifespan, decreases.

[0027] [Cr]+2[Mo]+20[V]+4[Al]>4.0 …Formula (3) The value on the left side of equation (3) is an index representing the surface hardness after soft nitriding (more specifically, the hardness at a depth of 0.2 mm from the surface), and by making this value greater than 4.0, the hardness at a depth of 0.2 mm from the surface after soft nitriding can be made to exceed 600 HV. By making the surface hardness over 600HV, it becomes possible to perform shot peening on the surface of the part while suppressing the occurrence of surface irregularities and cracks on the soft nitrided part. By applying compressive residual stress to the part surface by shot peening, it is possible to suppress the occurrence and propagation of cracks when repeated stress is applied to the part, thereby improving fatigue strength. If the surface hardness is below 600HV, cracks will occur on the surface due to shot peening, which will become the starting point of fracture in the subsequent fatigue test, reducing fatigue strength and preventing the practically required 10 7 The bending fatigue strength, which is the lifespan of the steel, is reduced. 7 The number of times will be less than 100.

[0028] The metal structure has a bainite area ratio of over 90% In the steel material for soft nitriding of this embodiment, the structure after hot forging is substantially a bainite single-phase structure. Specifically, when hot forging is performed and then air-cooling is performed to room temperature, the area ratio of the bainite structure is set to more than 90%. By making the metal structure a bainite structure, higher toughness can be obtained than with a ferrite-pearlite structure of the same hardness. However, if ferrite structure is mixed in the structure, the yield strength ratio and durability ratio will also decrease, which may lead to a decrease in fatigue strength, so the area ratio of the bainite structure is set to more than 90%.

[0029] The soft-nitrided part according to this embodiment is manufactured by using a steel material having the above alloy composition through the steps of, for example, hot forging, machining, and gas soft-nitriding. Gas soft-nitriding is a process in which nitrogen and carbon penetrate the surface layer of a part by heating it to a temperature below the A1 transformation point in an atmosphere containing NH3, hardening the surface layer by dissolving nitrogen or precipitating fine carbonitrides. The soft-nitrided layer formed on the surface of the part consists of a compound layer mainly made up of Fe2-3N (ε phase) and Fe4N (γ' phase), and a nitrogen diffusion layer formed directly below that where nitrogen is diffused into the matrix.

[0030] In the soft-nitrided component of this embodiment, the surface layer is hardened by the soft-nitriding treatment, and the occurrence of cracks in, for example, a gear usage environment can be suppressed. Here, the surface hardness of the soft-nitrided component (more specifically, the hardness at a position 0.20 mm from the surface of the component) is set to exceed 600 HV. In this way, if additional shot peening is performed on the surface layer, it is possible to avoid the problem of unevenness and cracks occurring, which would actually reduce fatigue strength. In other words, with the soft-nitrided component of this embodiment, the level of fatigue strength can be effectively increased by performing shot peening.

[0031] On the other hand, in the positions other than the soft-nitrided layer, the hardness is increased by the age hardening effect at the soft-nitriding temperature, i.e., by the formation of carbides, and the core hardness is set to exceed 280 HV. By increasing the core hardness to exceed 280 HV, the bending fatigue strength can be improved. Here, the core hardness is the hardness at a position that is not affected by nitrogen and carbon that penetrate during soft nitriding. [Example]

[0032] Next, examples of the present invention are described below. Test pieces were prepared for 16 examples and comparative examples with different alloy compositions through the processes of melting and casting → hot forging → machining → gas soft nitriding treatment → shot peening, and the structure was observed, and surface hardness, internal hardness, bending fatigue strength, and surface fatigue strength were evaluated.

[0033] 1. Preparation of Test Specimens A 150 kg steel ingot with the chemical composition shown in Table 1 below was melted in a vacuum induction melting furnace, then hot forged to a diameter of 70 mm after heating at 1250°C for 4 hours, then further heated at 1250°C for 2 hours, then forged to a diameter of 22-30 mm, and then air cooled at a controlled air speed of 1°C / min to produce a steel bar exhibiting a bainite structure. This steel bar was then machined to produce a test piece 10 for use in a bending fatigue strength test, with the shape shown in Figure 1. Similarly, machining was performed to prepare a test piece 11 for use in a contact fatigue strength test, which has the shape shown in Fig. 2(A). In the figure, the diameter of the contact portion 11a is d1 = φ26 mm, the diameter of the small diameter portions 11b on both sides is d2 = φ23 mm, the width of the contact portion 11a is w1 = 28 mm, and the width of the small diameter portions 11b is w2 = 51 mm.

[0034] [Table 1]

[0035] 2. Gas soft nitriding treatment The test pieces 10 and 11 prepared as described above were subjected to gas soft nitriding treatment. The gas soft-nitriding treatment was carried out using a multipurpose surface modification device manufactured by Oriental Engineering Co., Ltd., using a mixed gas with a gas ratio of NH3:N2:H2:CO = 50:20:15:15, and holding the temperature at 540°C for 4 hours to form soft-nitrided layers on the surfaces of test pieces 10 and 11. Thereafter, the test pieces were cooled in oil at 120°C.

[0036] 3.Shot peening treatment Next, an air-type shot peening device equipped with a jet nozzle was used to perform a shot peening treatment on the shaft portion of the test piece 10 and the contact portion of the test piece 11. The shot material was steel (HV750) with a diameter of 0.9 mm, the spray pressure was 0.5 MPa, and the shot duration was set to achieve a coverage (the percentage of the area that was actually hit by the shot) of 300%.

[0037] 4. Evaluation of Test Specimens Test pieces 10 and 11 were used to evaluate the structure, surface hardness, internal hardness, bending fatigue strength, and surface fatigue strength by the following methods.

[0038] <Structural observation> Using the test piece 10 after gas soft nitriding treatment (before shot peening treatment), the internal structure of the test piece 10 was etched with nital and then observed with an optical microscope (magnification 400x) to measure the bainite area ratio. The evaluation results are shown in Table 2 below.

[0039] <Surface hardness and internal hardness> The hardness was measured using a Vickers hardness tester according to the test method specified in JIS Z 2244. The test load was 300 g. As with the above-mentioned microstructural observation, test piece 10 after gas nitrocarburizing (before shot peening) was used, and the hardness was measured at a position 0.2 mm below the surface on a cut surface cut perpendicular to the axial direction, and the average of these five points was taken as the surface hardness. The hardness was also measured at a position 3 mm below the surface on the same cut surface, and the average of these five points was taken as the internal hardness (core hardness) after nitriding. The hardness was also measured at a similar position (3 mm below the surface) before gas nitrocarburizing, and the average of these five points was taken as the internal hardness before nitriding. These evaluation results are shown in Table 2 below.

[0040] <Bending fatigue strength> Using the test piece 10 after the shot peening treatment, an Ono type rotating bending fatigue test was carried out in accordance with a method conforming to JIS Z 2274. The test conditions were a rotation speed of 3600 rpm and a test temperature of room temperature. Rotating bending fatigue tests were conducted under various load conditions until the test piece broke, and the SN curve was taken. 7 The maximum stress at which the bearing did not break after several cycles was taken as the life strength (unit: MPa). The target life strength was 550 MPa or more, which would prevent breakage from the tooth base when used as a gear. The evaluation results are shown in Table 2 below.

[0041] <Surface fatigue strength> The test piece 11 (see Figure 2(A)) after shot peening was used as a small roller, and as shown in Figure 2(B), the contact part 11a was brought into contact with the opposing large roller 12 (material: SUJ2) with a diameter D0 = φ130 mm and a width W0 = 18 mm at a high surface pressure (here, 2.5 GPa, which corresponds to the actual usage environment), and they were rotated at a slip ratio of 40% using a roller pitting tester, and the pitting life until pitting occurred was evaluated. The target pitting life was 1 x 10 7 The evaluation results are shown in Table 2 below. The other conditions were as follows: Small roller rotation speed: 1500 rpm, lubricant type: ATF, oil temperature: 100°C

[0042] [Table 2]

[0043] The evaluation results in Tables 1 and 2 reveal the following: Comparative Example 1 is an example in which the Cr content exceeds the upper limit of the range specified in this embodiment and does not satisfy the requirement of formula (1). In this Comparative Example 1, the hardness after hot forging (internal hardness before nitriding) is higher than 280 HV, and machinability is not ensured.

[0044] Comparative Example 2 is an example in which the C content exceeds the upper limit of the range specified in this embodiment and does not satisfy the requirement of formula (1). In this Comparative Example 2, the hardness after hot forging (internal hardness before nitriding) is also higher than 280 HV, and machinability is not ensured.

[0045] Comparative Example 3 is an example in which the Mo content and V content exceed the upper limits of the ranges specified in this embodiment and do not satisfy the requirement of formula (1). In this Comparative Example 3, the hardness after hot forging (internal hardness before nitriding) is also higher than 280 HV, and machinability is not ensured.

[0046] In Comparative Example 4, the Cr content and V content were below the lower limits of the ranges specified in this embodiment, and the requirements of formulas (2) and (3) were not satisfied. In this Comparative Example 4, the bainite area fraction was lower than 90%, and both the surface hardness and internal hardness (core hardness) after nitriding were low. Despite being subjected to shot peening, both the bending fatigue strength and surface fatigue strength did not achieve the targets.

[0047] Comparative Example 5 is an example in which the Mo content is below the lower limit of the range specified in this embodiment and does not satisfy the requirement of formula (2). In this Comparative Example 5, the bainite area fraction was lower than 90%, the internal hardness (core hardness) after nitriding was lower than 280 HV, and the target bending fatigue strength was not achieved even after shot peening treatment. As described above, Comparative Examples 1 to 5 have problems with either the machinability when manufacturing soft nitrided parts or the fatigue strength of the test pieces.

[0048] In contrast, in Examples 1 to 11, in which the chemical composition of the steel falls within the range specified in this embodiment, the internal hardness before nitriding was lower than 280 HV, ensuring machinability, and the test pieces that were subjected to gas soft-nitriding and then shot peening achieved the targets for bending fatigue strength and surface fatigue strength. In other words, it is clear that by using the steels of the Examples, it is possible to manufacture soft-nitrided parts with excellent fatigue strength without compromising the machinability before soft-nitriding.

[0049] Although the present invention has been described in detail above, the present invention is not limited to the above-described embodiments and examples, and various modifications are possible within the scope of the invention.

Claims

1. By mass% C: 0.08-0.20% Si: 0.05-0.40% Mn: less than 1.55% Cr:0.80~1.60% Mo: 0.20-0.70% V: 0.15-0.40% Al: 0.005-0.200% and the balance being Fe and unavoidable impurities, and the steel material for soft nitriding satisfies the following formulas (1), (2), and (3), and has a metal structure with a bainite area ratio of more than 90%. 40[C]+7[Mn]+8[Cr]+9[Mo]+12[V]<30.0...Formula (1) 1200[C]+112[Cr]+134[Mo]+560[V]-700[Al]>280...Formula (2) [Cr]+2[Mo]+20[V]+4[Al]>4.0...Formula (3) (In the above formula, [ ] represents the mass % content of the element in [ ])

2. In claim 1, in mass % The steel material for soft nitriding contains 0.030 to 0.060% of Al.

3. In claim 1, in mass % Ti: 0.002-0.040% Cu: 0.01~1.0% Ni: 0.01~1.0% The steel material for soft nitriding further contains one or more of the following.

4. A soft-nitrided part made of the steel material according to any one of claims 1 to 3 and having a soft-nitrided layer on a surface layer, A soft nitrided part having a surface hardness of more than 600 HV at a depth of 0.2 mm from the surface of the part and a core hardness of more than 280 HV.

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