Manufacturing method for soft nitrided parts

Abstract

To provide a cost-efficient method for producing a soft nitrided component with superior bending fatigue characteristics.SOLUTION: A method for producing a soft nitrided component includes a cold working step where raw steel material is subjected to cold working, thereby producing a precursor with a maximum strength value in the reverse pole figure intensity distribution of 1.5 or more; and a nitriding step where the precursor is nitrided.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a soft nitrided part.

Background Art

[0002] In response to environmental problems such as reduction of carbon dioxide emissions, weight reduction of parts of various industrial products is required. Nitriding is a heat treatment that improves the fatigue characteristics of steel parts while having the advantage of small heat treatment distortion compared to carburizing quenching and the like. Nitriding is applied to various steel parts including vehicle gears. Improvement of fatigue characteristics enables miniaturization of steel parts, thereby realizing weight reduction of industrial products such as vehicles. Therefore, improvement of fatigue characteristics is desired in steel parts.

[0003] Patent Document 1 discloses a structural steel for soft nitriding. This structural steel for soft nitriding contains, by weight ratio, C: 0.20 to 0.50%, Si: 0.03 to 0.50%, Mn: 0.30 to 3.00%, Cr: 0.10 to 1.00%, Mo: 0.03 to 1.00%, Al: 0.01 to 0.10%, V: 0.03 to 0.50%, S: 0.015 to 0.070%, Pb: 0 to 0.040%, and O: 15 ppm or less, and the balance consists of Fe and inevitable impurities. In this structural steel for soft nitriding, a predetermined relationship is defined between the contents of S, Pb, and oxygen and the target material hardness and the target core hardness. This structural steel for soft nitriding is said to be excellent in fatigue strength and machinability for use after being machined in a state such as as-rolled, as-forged, or normalized, and then subjected to soft nitriding treatment and shot peening.

[0004] Patent document 2 describes a steel for soft nitriding that exhibits excellent bending fatigue strength. This steel for soft nitriding contains alloying elements in mass percent as follows: C: 0.01%~0.15%, Si: 0.01%~1.5%, Mn: 0.15%~2%, Cu: 0.5%~2%, N: limited to less than 0.005%, and containing one or more elements from Ti: 0.01%~0.5%, Nb: 0.005%~0.5%, and V: 0.05%~0.5%, with C+N ≤ Ti / 4.0 + Nb / 7.7 + V / 4.3. Furthermore, if necessary, it may contain one or more elements from Ni: 0.5%~2% or less, Cr: 0.1%~2%, Al: 0.05%~0.5%, S: 0.03%~0.1%, and Pb: 0.005%~0.3%, with the remainder being Fe and unavoidable impurities. Furthermore, the ferrite area ratio is 90% or more, with the remainder consisting of carbides and pearlite structures. The average size of the pearlite is 20 μm or less. In this steel for soft nitriding, Cu is an element that contributes to the age hardening of the core hardness during soft nitriding and is considered essential for obtaining high fatigue strength. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 09-227992 [Patent Document 2] Japanese Patent Publication No. 2002-069572 [Overview of the project] [Problems that the invention aims to solve]

[0006] For example, if a large amount of Cu is required, as in the soft nitriding steel disclosed in Patent Document 2, it not only increases the manufacturing cost of the steel but can also lead to high-temperature embrittlement of the steel, resulting in a decrease in yield due to increased cracking after continuous casting and an increase in additional costs due to maintenance. If the addition of rare metals such as V is required, as in the soft nitriding steel disclosed in Patent Document 1, it can lead to an increase in the manufacturing cost of the steel and an increase in the environmental burden. Therefore, there is a need for a method of manufacturing soft nitrided parts at low cost with excellent bending fatigue properties.

[0007] This invention has been made in view of the above circumstances, and its purpose is to provide a method for manufacturing soft nitrided parts that are low cost and have excellent bending fatigue characteristics. [Means for solving the problem]

[0008] The method for manufacturing soft nitride components according to the present invention, in order to achieve the above objective, is as follows.

[0009] [1] A cold working process to obtain a precursor in which the maximum intensity of the inverse pole figure intensity distribution is 1.5 or more by cold working the raw steel material, A method for manufacturing a soft nitride component, comprising a nitriding step of nitriding the aforementioned precursor.

[0010] [2] The method for manufacturing a soft nitrided part according to [1] above, wherein the cold working includes at least one element from cold tensile working, drawing, and forging.

[0011] [3] The method for manufacturing a soft nitrided part according to [1] or [2] above, wherein in the cold working step, a portion of the precursor having a maximum value of 1.5 or more is formed.

[0012] [4] The method for manufacturing a soft nitrided part according to any one of [1] to [3], wherein the cold working step is performed two or more times.

[0013] [5] The cold working step is repeated until the maximum value becomes 1.5 or more. A method for manufacturing a soft-nitrided part according to any one of [1] to [4]. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a method for manufacturing soft nitrided parts that have excellent bending fatigue characteristics at low cost. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram showing the shape of the test specimen.

Embodiment for Carrying out the Invention

[0016] Hereinafter, a method for manufacturing a soft nitrided component according to the present embodiment will be described.

[0017] First, an outline of the method for manufacturing a soft nitrided component according to the present embodiment will be described.

[0018] The method for manufacturing a soft nitrided component according to the present embodiment includes a cold working step of cold working a raw material steel to obtain a precursor having a maximum value of the intensity of the inverse pole figure intensity distribution of 1.5 or more, and a nitriding step of nitriding the precursor.

[0019] According to the method for manufacturing a soft nitrided component according to the present embodiment, a soft nitrided component with excellent bending fatigue characteristics at low cost can be provided.

[0020] Hereinafter, the method for manufacturing a soft nitrided component according to the present embodiment and the soft nitrided component realized by this manufacturing method will be described in detail.

[0021] An example of a soft nitrided component (hereinafter referred to as the soft nitrided component according to the present embodiment) realized by the method for manufacturing a soft nitrided component according to the present embodiment is a component that forms a vehicle such as an automobile. For example, when exemplifying components in the automotive field, components such as the crankshaft of an engine, timing gears, etc., the transmission gears of a transmission, ring gears, sun gears, planetary gears, etc., the steering pinion and worm of the undercarriage, and the worm for power windows in the interior can be mentioned.

[0022] Nitriding includes both nitriding treatment in which only nitrogen is infiltrated into steel and soft nitriding treatment in which nitrogen and carbon are simultaneously infiltrated into steel, and both are treatments that do not cause martensitic transformation of steel. Among these, nitriding in the present embodiment refers to soft nitriding treatment.

[0023] In the method for manufacturing a soft nitrided component according to the present embodiment, as described above, a raw material steel is nitrided to manufacture a soft nitrided component. That is, the main body of the soft nitrided component according to the present embodiment has a surface layer in which steel is nitrided. This surface layer is, for example, about 0.5 mm from the surface of the main body. The case where the thickness of this surface layer is greater than 0.5 mm is also included in the present embodiment. Hereinafter, the raw material steel may be simply referred to as steel.

[0024] The soft nitrided component according to the present embodiment may be a combination of this main body and a structure different from the main body, which is formed of a metal or a metal alloy and whose surface is not nitrided. The soft nitrided component according to the present embodiment may be a combination of this main body and another structure that is not made of metal.

[0025] The steel used in the method for manufacturing a soft nitrided component according to the present embodiment may contain Fe (iron) as the main component and, as other component compositions, C (carbon), Si (silicon), Mn (manganese), P (phosphorus), S (sulfur) and impurities. It is not excluded that this steel further contains any of the components described later.

[0026] This steel may contain, as optional components, one or more selected from Al (aluminum), N (nitrogen), Cr (chromium), Ti (titanium), Nb (niobium), V (vanadium), Hf (hafnium), Ta (tantalum), Sn (tin), Sb (antimony), Se (selenium), Ca (calcium), Pb (lead), Bi (bismuth).

[0027] An example of the steel that can be used in the method for manufacturing a soft nitrided component according to the present embodiment is carbon steel for machine structural use (S45C) defined in JIS G4051.

[0028] Hereinafter, among the steels that can be used in the method for manufacturing a soft nitrided component according to the present embodiment, the content and effects of each component in a particularly suitable steel will be described in detail. In the following description, when simply described as content, it is the content (mass%) in the steel. Note that the steel that can be used in the method for manufacturing a soft nitrided component according to the present embodiment is not limited to the steel described below.

[0029] The steel that can be used in the manufacturing method of soft nitrided parts according to this embodiment preferably contains, in terms of its component composition (chemical composition), C: 0.04% to 0.35% by mass, Si: 0.01% to 1.20% by mass, Mn: 0.30% to 1.80% by mass, P: 0.1% or less by mass, S: 0.5% or less by mass, Al: 0.010% to 0.300% by mass, and N: 0.0250% or less by mass, with the remainder being Fe (iron) and impurities.

[0030] This steel may contain one or more of the following components as an optional component: Cr (chromium): 2.0% by mass or less, Mo (molybdenum): 1.0% by mass or less, Cu (copper): 1.0% by mass or less, Ni (nickel): 1.0% by mass or less, and B (boron): 0.01% by mass or less.

[0031] Furthermore, this steel may contain one or more of the following as optional components: Ti (titanium): 0.1% by mass or less, Nb (niobium): 0.1% by mass or less, V (vanadium): 0.2% by mass or less, Hf (hafnium): 0.1% by mass or less, and Ta (tantalum): 0.1% by mass or less.

[0032] Furthermore, this steel may contain one or more of the following components as an optional component: Sn (tin): 0.1% by mass or less and Sb (antimony): 0.1% by mass or less.

[0033] Furthermore, this steel may contain one or more of the following components as an optional component: Se (selenium): 0.3% by mass or less, Ca (calcium): 0.1% by mass or less, Pb (lead): 0.3% by mass or less, and Bi (bismuth): 0.3% by mass or less.

[0034] The carbon content may be between 0.04% by mass and 0.35% by mass. To increase the hardness of the core after nitriding, the carbon content should preferably be 0.04% by mass or more. If the carbon content exceeds 0.35% by mass, the load during cold working will increase, which may lead to a reduction in the lifespan of the mold. The carbon content is preferably between 0.05% by mass and 0.27% by mass, and more preferably between 0.10% and 0.25% by mass.

[0035] The Si content may be between 0.01% by mass and 1.20% by mass. In nitrided steel, Si contributes as a deoxidizing agent. Excessive Si content in steel can reduce the cold workability of the steel and may also reduce fatigue strength through a decrease in toughness. The Si content is preferably between 0.05% by mass and 0.70% by mass, and more preferably between 0.10% by mass and 0.50% by mass.

[0036] The Mn content may be between 0.30% by mass and 1.80% by mass. Mn improves hardenability and strengthens the pre-nitrided structure of steel, thereby increasing the strength of the post-nitrided structure (nitrided steel). To obtain sufficient fatigue strength, the Mn content should be 0.30% by mass or more. Excessive Mn content in steel may lead to an increase in deformation resistance. The Mn content is preferably between 0.40% by mass and 1.70% by mass, and more preferably between 0.50% by mass and 1.30% by mass.

[0037] The P content may be 0.1% by mass or less. Since P segregates at the grain boundaries of nitrided steel and reduces its toughness, a lower P content is desirable. A P content of up to 0.1% by mass is acceptable. Preferably, the P content is 0.02% by mass or less. Although there is no particular problem in not setting a lower limit for the P content, since P content is usually unavoidable, unnecessarily reducing P may increase the refining time and refining costs. For this reason, it is reasonable and preferable to have a P content of 0.003% by mass or more.

[0038] The sulfur (S) content may be 0.5% by mass or less. S exists as a sulfide inclusion and is an element effective in improving machinability. Excessive S content in steel may lead to a decrease in cold workability. There is no particular lower limit to the S content, but since S content is usually unavoidable, excessively low S content may increase refining costs. For this reason, it is reasonable to set the S content at 0.003% by mass or more. Preferably, the S content is 0.004% by mass or more and 0.3% by mass or less, and more preferably 0.005% by mass or more and 0.09% by mass or less.

[0039] The Al content may be between 0.010% by mass and 0.300% by mass. Al forms oxides and is an effective element for deoxidizing nitrided steel. In addition, Al has the effect of suppressing the formation of coarse oxide inclusions in nitrided steel. If the Al content is less than 0.010% by mass, these effects may not be obtained. Excessive Al content in steel can lead to an increase in inclusions (Al oxides), which can increase the starting points for fatigue fracture and cause low fatigue strength.

[0040] The N content may be 0.0250% by mass or less. N combines with Al to form nitride (AlN). Finely precipitated AlN has the effect of increasing the hardness of steel after nitriding (nitrided steel). Excessive N content in steel may lead to surface cracking of steel billets after casting. There is no particular lower limit to the N content, but since N content is usually unavoidable, excessively low N content may increase refining costs. The N content is 0.0010% by mass or more, preferably 0.0015% by mass or more. The N content is preferably 0.0180% by mass or less, more preferably 0.0020% by mass or more and 0.0150% by mass or less.

[0041] The Cr content may be 2.0% by mass or less. Cr is an element that contributes to improving hardenability and resistance to tempering softening, and is also useful in promoting the spheroidization of carbides. If the Cr content exceeds 2.0% by mass, the Cr nitride layer formed on the surface after soft nitriding becomes thicker, which can inhibit the penetration of N into the interior and lead to insufficient hardness. The Cr content is preferably 0.40% by mass or more and 1.90% by mass or less, and more preferably 0.70% by mass or more and 1.75% by mass or less.

[0042] The Mo content may be 1.0 mass% or less. Mo improves hardenability and strengthens the post-nitrided structure by strengthening the pre-nitrided structure. However, if the Mo content is 1.0 mass% or more, the hardenability becomes excessive, the hardness after rolling increases, and the workability and machinability may decrease. In order to exhibit the effect of Mo on improving the strength of the steel, it is preferable to include Mo in the steel at a concentration of 0.01 mass% or more. The Mo content is more preferably 0.03 mass% to 0.50 mass%, and even more preferably 0.05 mass% to 0.25 mass%.

[0043] The Cu content may be 1.0 mass% or less. Cu improves hardenability and strengthens the structure after nitriding by strengthening the pre-nitriding structure. To obtain this effect, it is preferable to include Cu in the steel at a concentration of 0.01 mass% or more. If the Cu content exceeds 1.0 mass%, the surface of the rolled material may become rough, raising concerns that defects may remain. The Cu content is more preferably 0.015 mass% to 0.5 mass%, and even more preferably 0.03 mass% to 0.3 mass%.

[0044] The Ni content may be 1.0 mass% or less. Ni is an element useful for improving toughness. To obtain these effects, it is preferable to include Ni in the steel at a concentration of 0.01 mass% or more. The above effects saturate even if the Ni content exceeds 1.0 mass%. The Ni content is more preferably 0.015 mass% to 0.5 mass%, and even more preferably 0.03 mass% to 0.3 mass%.

[0045] The B content may be 0.01% by mass or less. B is effective in improving hardenability by segregating at grain boundaries and suppressing diffusion-type transformation. In addition, it also strengthens grain boundaries, suppresses the occurrence and propagation of fatigue cracks, and improves fatigue strength. To obtain this effect from B, it is preferable to include B in the steel at a concentration of 0.0003% by mass or more. If the B content exceeds 0.01% by mass, the toughness of the steel decreases, so it is preferable to keep the B content at 0.01% by mass or less. More preferably, the B content is 0.0005% by mass or more and 0.005% by mass or less, and even more preferably 0.0007% by mass or more and 0.002% by mass or less.

[0046] The Ti content may be 0.1% by mass or less. Ti combines with carbon and nitrogen, and improves the strength of the steel by forming fine precipitates during soft nitriding. However, the effect saturates even if the amount of Ti in the steel exceeds 0.1% by mass. The Ti content is preferably 0.005% by mass or more and 0.08% by mass or less, and more preferably 0.01% by mass or more and 0.06% by mass or less.

[0047] The Nb content may be 0.1% by mass or less. Nb combines with carbon and nitrogen, and improves the strength of the steel by forming fine precipitates during soft nitriding. However, the effect saturates even if the Nb content exceeds 0.1% by mass. The Nb content is preferably 0.005% by mass or more and 0.08% by mass or less, and more preferably 0.01% by mass or more and 0.06% by mass or less.

[0048] The V content may be 0.2 mass% or less. V combines with carbon and nitrogen, and improves the strength of the steel by forming fine precipitates during soft nitriding. To obtain this effect from V, it is preferable to include V in the steel at least 0.003 mass% or more. Including more than 0.2 mass% of V in the steel only increases the alloy cost, and the effect saturates. The V content is more preferably 0.005 mass% to 0.15 mass%, and even more preferably 0.01 mass% to 0.10 mass%.

[0049] The Hf content may be 0.1% by mass or less. Hf combines with carbon and nitrogen, and improves the strength of the steel by forming fine precipitates during soft nitriding. To obtain this effect of Hf, it is preferable to include at least 0.003% by mass or more of Hf in the steel. If the Hf content exceeds 0.1% by mass, coarse precipitates may be generated during casting solidification, leading to a deterioration of fatigue strength. The Hf content is more preferably 0.005% by mass or more and 0.06% by mass or less, and even more preferably 0.01% by mass or more and 0.05% by mass or less.

[0050] The Ta content may be 0.1% by mass or less. Ta combines with carbon and nitrogen, and improves the strength of the steel by forming fine precipitates during soft nitriding. To obtain this effect from Ta, it is preferable to include at least 0.003% by mass or more of Ta in the steel. On the other hand, if the Ta content exceeds 0.1% by mass in the steel, cracks are more likely to occur during casting solidification, and defects may remain even after rolling and forging. The Ta content is more preferably 0.005% by mass or more and 0.06% by mass or less, and even more preferably 0.01% by mass or more and 0.05% by mass or less.

[0051] The Sb content may be 0.1% by mass or less. Sb is an effective element for suppressing decarburization on the surface of steel materials and preventing a decrease in surface hardness. To achieve this effect, it is preferable to include 0.0003% by mass or more of Sb in the steel. If the steel contains too much Sb, the workability of the steel will decrease. The Sb content is more preferably 0.001% by mass or more and 0.05% by mass or less, and even more preferably 0.0015% by mass or more and 0.035% by mass or less.

[0052] The Sn content may be 0.1% by mass or less. Sn is an effective element for improving the corrosion resistance of the steel surface. From the viewpoint of improving corrosion resistance, it is preferable to include 0.003% by mass or more of Sn in the steel. Including too much Sn in the steel will reduce workability. The Sn content is more preferably 0.0010% by mass or more and 0.050% by mass or less, and even more preferably 0.0015% by mass or more and 0.035% by mass or less.

[0053] The Se content may be 0.3 mass% or less. Se combines with Mn and Cu and disperses as precipitates in the steel, thereby improving machinability. To obtain this effect, it is preferable to include at least 0.001 mass% or more of Se in the steel. The effect saturates even if the Se content exceeds 0.3 mass%. The Se content is more preferably 0.005 mass% to 0.1 mass%, and even more preferably 0.008 mass% to 0.09 mass%.

[0054] The Ca content may be 0.1% by mass or less. Ca combines with S and disperses as sulfides in the steel, improving machinability. To obtain this effect, it is preferable to include at least 0.0005% by mass or more of Ca in the steel. The effect saturates even if the Ca content exceeds 0.1% by mass. The Ca content is more preferably 0.0010% by mass or more and 0.0500% by mass or less, and even more preferably 0.0015% by mass or more and 0.0300% by mass or less.

[0055] The Pb content may be 0.3% by mass or less. Pb has the effect of refining chips during cutting. Adding Pb is effective when you want to improve chip handling performance. To obtain this effect, it is preferable to include 0.01% by mass or more of Pb in the steel. If you include too much Pb in the steel, the effect of improving chip handling performance will saturate. The Pb content is preferably 0.01% by mass or more and 0.2% by mass or less, and more preferably 0.01% by mass or more and 0.1% by mass or less.

[0056] The Bi content may be 0.3% by mass or less. Bi has the effect of refining chips during cutting. Adding Bi is effective when you want to improve chip handling performance. To obtain this effect, it is preferable to include Bi at a concentration of 0.01% by mass or more. If you include too much Bi in the steel, the effect of improving chip handling performance will saturate. The Bi content is preferably 0.01% by mass or more and 0.2% by mass or less, and more preferably 0.01% by mass or more and 0.1% by mass or less.

[0057] The remaining elements, other than those described above, are Fe and impurities. Impurities are substances that are introduced during the industrial production of steel materials from raw materials such as ore, scrap, or the manufacturing environment, and are acceptable as long as they do not adversely affect the properties of this embodiment.

[0058] Cold working of steel causes crystal orientation rotation according to the working method. As the amount of work increases, the crystal orientation rotation becomes more pronounced, and the accumulation of crystal orientations progresses. The inverse pole figure intensity distribution obtained by EBSD (electron beam backscatter diffraction) is a typical indicator of the degree of crystal orientation accumulation. When the maximum intensity in the inverse pole figure intensity distribution is 1.5 or higher, the fatigue strength of the steel improves. To sufficiently improve the fatigue strength of the steel, the maximum intensity in the inverse pole figure intensity distribution is preferably 1.7 or higher, and more preferably 2.0 or higher.

[0059] In the soft nitrided component of this embodiment, the maximum value of the intensity in the inverse pole figure intensity distribution measured in a direction parallel to the component surface at a depth of 0.5 mm from the surface of the main body is 1.5 or greater.

[0060] Cold working in this embodiment includes processes that include at least one of the following: cold tensile working (cold tensile working), cold drawing, and cold forging (cold forging). Specifically, for example, gear forming, spline forming, drawing (diameter reduction) forming, forward extrusion forming, and backward extrusion forming by cold forging, as well as gear forming and spline forming by cold rolling, are also included in the cold working in this embodiment.

[0061] There is no limit to the number of cold working steps performed on steel. For example, a single cold working step on steel may be performed to achieve a maximum intensity of 1.5 or higher in the inverse pole figure intensity distribution of the precursor.

[0062] Alternatively, for example, the steel may be cold-worked multiple times (two or more times) to achieve a maximum intensity of 1.5 or more in the inverse pole figure intensity distribution of the precursor.

[0063] Alternatively, the cold working process may be repeated until the maximum intensity in the inverse pole figure intensity distribution of the precursor reaches 1.5 or higher.

[0064] When cold working steel, the amount of deformation per cold working step is arbitrary.

[0065] Crystal orientation rotation due to cold working occurs regardless of the composition and microstructure of the steel, and the orientation accumulation behavior depends on the processing method. Therefore, the method for manufacturing soft-nitrided parts according to this embodiment is applicable to all types of steel materials used as raw materials, and is not limited to the examples of this embodiment. [Examples]

[0066] The steel according to this embodiment will be described below based on the examples. However, the steel according to this embodiment is not limited to this embodiment.

[0067] (Example 1) Carbon steel for machine structures (S45C) as specified in JIS G4051 was formed into a 20mm diameter round bar (an example of raw material) by hot rolling.

[0068] Next, this round bar was cold-drawn (an example of cold working) to produce round bar precursors. As shown in Table 1, precursors were produced by changing the number of cold-drawn passes from one to three, and by changing the diameter of the cold-drawn die (precursors No. 2 to 6). For comparison with the precursors produced by cold-drawn, an uncold-drawn round bar was also prepared (precursor No. 1).

[0069] Fifteen samples each of precursors No. 1 through 6 were prepared.

[0070] Table 1 shows the diameters of precursors No. 2 to 6 and the cumulative cross-sectional area reduction rate (%) during repeated cold drawing. The cumulative cross-sectional area reduction rate is the reduction in the area of ​​the cross-sections intersecting in the axial direction when a 20 mm diameter round bar is processed as a precursor.

[0071] Next, a portion of these round bars or precursors (precursors No. 1 to 6) was cut to obtain Ono-type rotary bending fatigue test specimens (hereinafter simply referred to as "test specimens") with a parallel section diameter of 10 mm.

[0072] Figure 1 shows a schematic diagram illustrating the shape of the test specimen. The test specimen is a straight line along the axis G, and its cross-section perpendicular to the axis G is a circular rod shape where the axis G and its center overlap. Both ends of the test specimen in the direction along the axis G are gripping portions 1,1, which will be described later. A parallel portion 2 is positioned between the gripping portions 1,1 in the direction along the axis G. In this embodiment, the diameter of the parallel portion 2 shown in Figure 1 is 10 mm. The diameter of the gripping portions 1,1 is 12 mm. The length between the gripping portions 1,1 including the parallel portion 2 is 30 mm. Both ends of the parallel portion 2 in the direction along the axis G are tapered into a curved surface with a radius of R15 from the end of the gripping portion 1 to the central part in the axial direction.

[0073] [Table 1]

[0074] Next, five randomly selected specimens from the 15 specimens corresponding to each precursor were measured using the EBSD method to determine the inverse pole figure intensity distribution, and the maximum intensity of each specimen was determined. Then, the average of the maximum intensity values ​​of the five specimens corresponding to each precursor (hereinafter sometimes referred to as the average intensity) was calculated. The inverse pole figure intensity distribution was determined for the cross-sections that coincide with the central axis of the cross-section perpendicular to the longitudinal direction (axis G shown in Figure 1) at the gripping portion of the specimen.

[0075] The measurement of this cross-section was performed by cutting the gripping portion along its longitudinal direction to expose the cross-section.

[0076] The inverse pole figure intensity distribution, based on the EBSD measurement results, was calculated using TSL's crystal orientation analysis software, OIM. A scanning electron microscope (JEOL Ltd., model: JSM-7001F) was used for the EBSD measurements.

[0077] The measurement field of view in the EBSD method was defined as a 0.5 mm square area, and the inverse pole figure intensity distribution was analyzed using the spherical harmonic function method in the direction along the central axis (axis G) (the same as the tensile axis direction in the fatigue test described later) under the conditions of Series Rank: 13, Gaussian Smoothing: 5.0°, and Sample Symmetry: Triclinic.

[0078] Next, of the 15 test specimens corresponding to each precursor, the remaining 10 specimens were subjected to gas soft nitriding heat treatment at 570°C for 3 hours (nitriding) to obtain post-treatment test specimens (an example of a soft nitrided component) (an example of the nitriding process). The nitriding treatment was carried out in a mixed gas containing ammonia and carbon monoxide.

[0079] Next, treated specimens corresponding to each precursor were subjected to a rotational bending fatigue test at a stress amplitude of 500 MPa. The fatigue strength was measured as the fatigue failure life (cycles), and the average fatigue failure life (average fatigue strength, cycles) of 10 treated specimens corresponding to each precursor was calculated. A higher average fatigue failure life indicates better bending fatigue characteristics and is therefore preferable. The average fatigue failure lives corresponding to each precursor are also shown in Table 1.

[0080] Furthermore, the fatigue fracture life improvement rate due to cold drawing was calculated for each post-treatment test specimen corresponding to precursors No. 2 to 6. The fatigue fracture life improvement rate due to cold drawing is calculated by dividing the average fatigue fracture life of precursors No. 2 to 6 by the average fatigue fracture life of the round bar (precursor No. 1). A higher fatigue fracture life improvement rate is preferable as it indicates superior bending fatigue characteristics. The fatigue fracture life improvement rates corresponding to each precursor are also shown in Table 1.

[0081] If the fatigue failure life of the treated test specimen (soft-nitrided part) is extended compared to the fatigue failure life of the treated test specimen (precursor No. 1) that was nitrided without cold drawing (i.e., if the improvement rate of fatigue failure life exceeds 1), then the bending fatigue characteristics of the treated test specimen can be said to be superior.

[0082] As shown in Table 1, the improvement in fatigue fracture life increases as the average strength of the test specimen (precursor) increases. Therefore, it can be seen that when the average strength increases, the bending fatigue characteristics of the treated test specimen (soft-nitrided part) become superior.

[0083] Of these, if the fatigue failure life of the treated test piece (soft-nitrided part) is more than twice that of the treated test piece (precursor No. 1) that was nitrided without cold drawing (i.e., the fatigue failure life improvement rate is 2 or more), then the bending fatigue characteristics of the treated test piece can be said to be particularly superior.

[0084] As shown in Table 1, when the average strength of the test specimen (precursor) is 1.5 or higher, the fatigue fracture life improvement rate is 2 or higher. Therefore, it can be seen that when the average strength is 1.5 or higher, the bending fatigue characteristics of the treated test specimen (soft nitrided part) are excellent. Furthermore, the fatigue fracture life improvement rate increases as the average strength of the test specimen (precursor) increases, indicating that the bending fatigue characteristics of the treated test specimen (soft nitrided part) are excellent as the average strength of the test specimen (precursor) increases.

[0085] (Example 2) First, steels with the component compositions shown in Table 2 (steels No. 1 to 31) were melted and then hot-rolled to form round bars with a diameter of 20 mm (an example of raw material steel).

[0086] Then, this round bar was cold-drawn (an example of cold working) to obtain a round bar precursor with a diameter of 16 mm (an example of a cold working process). After that, a portion of this precursor was cut to obtain an Ono-type rotary bending fatigue test specimen (hereinafter simply referred to as the test specimen) with a parallel section diameter of 10 mm.

[0087] In this embodiment, the test specimen has the same shape as the test specimen in Example 1 (see Figure 1).

[0088] [Table 2]

[0089] Then, measurements were performed on this specimen using the EBSD method to determine the inverse pole figure intensity distribution, and the maximum intensity was determined. The inverse pole figure intensity distribution was determined for the cross section of the specimen's gripping portion that coincides with the central axis of the cross section perpendicular to the longitudinal direction (axis G shown in Figure 1).

[0090] The measurement of this cross-section was performed by cutting the gripping portion along its longitudinal direction to expose the cross-section.

[0091] The inverse pole figure intensity distribution based on the measurement results using the EBSD method was calculated in the same manner as in Example 1.

[0092] The measurement field of view range and the conditions for analyzing the inverse pole figure intensity distribution using the EBSD method were the same as in Example 1.

[0093] Next, this test specimen was subjected to gas soft nitriding heat treatment at 570°C for 3 hours (nitriding) to obtain a post-treatment test specimen (an example of a soft nitrided part) (an example of the nitriding process). The nitriding treatment was carried out in a mixed gas containing ammonia and carbon monoxide.

[0094] Next, the treated specimens for each component composition were separated into those with a maximum intensity of 1.5 or higher in the inverse pole figure intensity distribution and those without. Then, the treated specimens with a maximum intensity of 1.5 or higher in the pole figure intensity distribution and those without were subjected to rotational bending fatigue tests, and their respective fatigue limits (1 × 10⁻¹⁰) were determined. 7 We evaluated the stress (equivalent to 50°C).

[0095] As shown in Table 2, it can be seen that post-treatment test specimens (soft-nitrided parts, with a maximum intensity of 1.5 or higher in the pole figure intensity distribution) manufactured by the manufacturing method of soft-nitrided parts according to this embodiment have significantly superior bending fatigue properties compared to those manufactured by a manufacturing method not according to this embodiment.

[0096] Thus, the steel used to form the post-processed test specimen (the body of the soft-nitrided part) according to this embodiment does not require the inclusion of large amounts of relatively expensive metals such as Cu, and is therefore low-cost, yet exhibits excellent fatigue properties.

[0097] Furthermore, the steel used to form the post-processed test specimen (the body of the soft-nitrided component) according to this embodiment does not require the addition of rare metals such as V, and is low-cost, yet exhibits excellent fatigue properties.

[0098] Furthermore, when the inverse pole figure intensity distribution was determined by measuring the treated specimens using the EBSD method and finding the maximum intensity, the maximum intensity of the inverse pole figure intensity distribution for each treated specimen was the same as the maximum intensity of the inverse pole figure intensity distribution for the corresponding pre-nitriding specimen (the pre-nitriding state of each treated specimen).

[0099] The inverse pole figure intensity distribution was determined for the portion of the gripping area of ​​the treated specimen at a depth of 0.5 mm from the surface. To measure the portion at a depth of 0.5 mm from the surface of the gripping area of ​​the treated specimen, a portion of the surface of the gripping area (for example, position 3 shown in Figure 1) was cut 0.5 mm below the surface to expose the relevant portion.

[0100] In other words, the maximum intensity of the inverse pole figure intensity distribution inside the specimen (in this embodiment, the portion at a depth of 0.5 mm from the surface) was the same before and after nitriding the specimen.

[0101] As described above, a method for manufacturing soft nitrided parts with excellent bending fatigue characteristics at low cost can be provided.

[0102] The embodiments disclosed herein are illustrative examples, and the embodiments of the present invention are not limited thereto. They can be modified as appropriate without departing from the purpose of the present invention. [Industrial applicability]

[0103] This invention can be applied to a method for manufacturing soft nitride components. [Explanation of Symbols]

[0104] 1: Grip part 2: Parallel section G: Axial center

Claims

1. The raw material steel, comprising a composition including C: 0.04% to 0.35% by mass, Si: 0.10% to 1.20% by mass, Mn: 0.30% to 1.80% by mass, P: 0.1% or less by mass, S: 0.5% or less by mass, Al: 0.010% to 0.300% by mass, and N: 0.0250% or less by mass, with the remainder being Fe and impurities, is cold-worked to obtain a precursor having a maximum intensity of 1.5 or more in the inverse pole figure intensity distribution. A method for manufacturing a soft nitride component, comprising a nitriding step of nitriding the aforementioned precursor.

2. The component composition is as follows: Cr: 2.0% by mass or less, Mo: 1.0% by mass or less, Cu: 1.0% by mass or less, Ni: 1.0% by mass or less, B: 0.01% by mass or less, Nb: 0.1% by mass or less, V: 0.09% by mass or less, Hf: 0.1 mass% or less, Ta: 0.1 mass% or less, Sn: 0.1 mass% or less, Sb: 0.1 mass% or less, Se: 0.3 mass% or less, Ca: 0.1 mass% or less, Pb: 0.3 mass% or less, and Bi: 0.3 mass% or less. A method for manufacturing a soft nitride component according to claim 1, further comprising one or more selected from among the following.

3. The method for manufacturing a soft nitrided part according to claim 1, wherein the cold working includes at least one element from cold tensile working, drawing, and forging.

4. The method for manufacturing a soft nitrided part according to claim 2, wherein the cold working includes at least one element from cold tensile working, drawing, and forging.

5. The method for manufacturing a soft nitrided part according to claim 1, wherein in the cold working step, a portion of the precursor has a maximum value of 1.5 or more.

6. The method for manufacturing a soft nitrided part according to claim 2, wherein in the cold working step, a portion of the precursor has a maximum value of 1.5 or more.

7. The method for manufacturing a soft nitrided part according to claim 3, wherein in the cold working step, a portion of the precursor has a maximum value of 1.5 or more.

8. The method for manufacturing a soft nitride component according to claim 4, wherein in the cold working step, a portion of the precursor has a maximum value of 1.5 or more.

9. The method for manufacturing a soft nitrided part according to any one of claims 1 to 8, wherein the cold working step is performed two or more times.

10. The method for manufacturing a soft nitrided part according to any one of claims 1 to 8, wherein the cold working step is repeated until the maximum value becomes 1.5 or more.

11. The method for manufacturing a soft nitrided part according to claim 9, wherein the cold working step is repeated until the maximum value becomes 1.5 or more.

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