steel

Abstract

To provide a cost-efficient steel that exhibits superior bending fatigue characteristics after being nitrided.SOLUTION: A steel comprises C: 0.04 mass% or more and 0.35 mass% or less, Si: 0.01 mass% or more and 1.20 mass% or less, Mn: 0.30 mass% or more and 1.80 mass% or less, P: 0.1 mass% or less, S: 0.5 mass% or less, Al: 0.010 mass% or more and 0.300 mass% or less and N: 0.0250 mass% or less, with the balance being Fe and impurities, wherein the maximum strength value in the reverse pole figure intensity distribution is 1.5 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to steel. [Background technology]

[0002] In order to address environmental issues such as reducing carbon dioxide emissions, there is a demand for reducing the weight of various industrial product parts. Nitriding is a heat treatment that improves the fatigue properties of steel parts while having the advantage of reducing heat treatment distortion compared to carburizing and quenching. Nitriding is applied to various steel parts, including vehicle gears. Improving fatigue properties enables the miniaturization of steel parts, thereby realizing weight reduction in industrial products such as vehicles. Therefore, improving the fatigue properties of steel parts is desirable.

[0003] Patent Document 1 discloses a structural steel for nitrocarburizing. This structural steel for nitrocarburizing contains, by weight, 0.20-0.50% C, 0.03-0.50% Si, 0.30-3.00% Mn, 0.10-1.00% Cr, 0.03-1.00% Mo, 0.01-0.10% Al, 0.03-0.50% V, 0.015-0.070% S, 0-0.040% Pb, and 15 ppm or less O, with the balance being Fe and unavoidable impurities. In this structural steel for nitrocarburizing, predetermined relationships are defined between the contents of S, Pb, and oxygen and the target material hardness and target core hardness. This structural steel for nitrocarburizing is said to have excellent fatigue strength and machinability, so that it can be used after being machined in an as-rolled, as-forged, or normalized state, and then subjected to nitrocarburizing and shot peening.

[0004] Patent Document 2 describes a steel for soft nitriding that has excellent bending fatigue strength. This soft nitriding steel contains, in mass %, the following alloying elements: C: 0.01% to 0.15%, Si: 0.01% to 1.5%, Mn: 0.15% to 2%, Cu: 0.5% to 2%, N: limited to less than 0.005%, and one or more of Ti: 0.01% to 0.5%, Nb: 0.005% to 0.5%, and V: 0.05% to 0.5%, where C + N ≦ Ti / 4.0 + Nb / 7.7 + V / 4.3, and may further contain, as necessary, one or more of Ni: 0.5% to 2% or less, Cr: 0.1% to 2%, Al: 0.05% to 0.5% or less, S: 0.03% to 0.1%, and Pb: 0.005% to 0.3%, with the balance consisting of Fe and unavoidable impurities. The area ratio of ferrite is 90% or more, with the remainder consisting of carbides and pearlite. The average size of the pearlite is 20 μm or less. In this soft nitriding steel, 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 Application Laid-Open No. 2002-069572 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, if a large amount of Cu is required to be added, as in the case of the nitrocarburizing steel disclosed in Patent Document 2, not only does this increase the manufacturing cost of the steel, but it also leads to high-temperature embrittlement of the steel, which can lead to a decrease in yield due to an increase in cracks after continuous casting and additional costs due to maintenance. If a rare metal such as V is required to be added, as in the case of the nitrocarburizing steel disclosed in Patent Document 1, this can increase the manufacturing cost of the steel and increase the environmental load. Therefore, it is desired to provide nitrided parts that are low-cost and have excellent bending fatigue properties, and a steel that can achieve this.

[0007] The present invention has been made in view of the above circumstances, and its object is to provide a steel that is low in cost and has excellent bending fatigue properties after nitriding. [Means for solving the problem]

[0008] In order to achieve the above object, the steel according to the present invention is as follows.

[0009] [1] The composition is as follows: C: 0.04% by mass or more and 0.35% by mass or less, Si: 0.01% by mass or more and 1.20% by mass or less, Mn: 0.30 mass% or more and 1.80 mass% or less, P: 0.1% by mass or less, S: 0.5% by mass or less, Al: 0.010 mass% or more and 0.300 mass% or less; N: 0.0250 mass% or less, The balance includes Fe and impurities, Steel with a maximum strength value of 1.5 or more in the inverse pole figure strength distribution.

[0010] [2] The component composition is: Cr: 2.0% by mass or less, Mo: 1.0% by mass or less, Cu: 1.0% by mass or less, Ni: 1.0 mass% or less and B: 0.01% by mass or less, The steel according to [1] above, further comprising one or more selected from the following:

[0011] [3] The component composition is: Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, V: 0.2% by mass or less, Hf: 0.1% by mass or less and Ta: 0.1% by mass or less, The steel according to the above [1] or [2], further comprising one or more selected from the following:

[0012] [4] The component composition is: Sn: 0.1 mass% or less and Sb: 0.1% by mass or less, The steel according to any one of [1] to [3] above, further comprising one or more selected from the following:

[0013] [5] The component composition is: Se: 0.3% by mass or less, Ca: 0.1% by mass or less, Pb: 0.3% by mass or less and Bi: 0.3% by mass or less, The steel according to any one of [1] to [4] above, further comprising one or more selected from the following: [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a steel that is low in cost and has excellent bending fatigue properties after nitriding. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a schematic diagram showing the shape of a test piece. DETAILED DESCRIPTION OF THE INVENTION

[0016] The steel according to this embodiment will be described below.

[0017] First, an overview of the steel according to this embodiment will be described.

[0018] The steel according to this embodiment has a chemical composition (chemical composition) of C (carbon): 0.04% by mass or more and 0.35% by mass or less, Si (silicon): 0.01% by mass or more and 1.20% by mass or less, Mn (manganese): 0.30% by mass or more and 1.80% by mass or less, P (phosphorus): 0.1% by mass or less, S (sulfur): 0.5% by mass or less, Al (aluminum): 0.010% by mass or more and 0.300% by mass or less, and N (nitrogen): 0.0250% by mass or less, with the remainder being Fe (iron) and impurities.

[0019] The steel according to this embodiment has a maximum strength value of 1.5 or more in the inverse pole figure intensity distribution.

[0020] The steel according to this embodiment is low cost and has excellent bending fatigue properties after nitriding.

[0021] The steel according to this embodiment and the soft nitrided parts realized by this steel will be described in detail below.

[0022] An example of a soft-nitrided part realized by the steel according to this embodiment (hereinafter referred to as the soft-nitrided part according to this embodiment) is a part that forms a vehicle such as an automobile. Examples of parts in the automotive field include engine crankshafts, timing gears, etc., transmission gears, ring gears, sun gears, planetary gears, etc., suspension steering pinions, worms, etc., and interior power window worms.

[0023] Nitriding includes both nitriding, in which only nitrogen penetrates steel, and soft nitriding, in which nitrogen and carbon penetrate steel simultaneously, and both refer to treatments that do not cause martensitic transformation of steel. Of these, nitriding in this embodiment refers to soft nitriding.

[0024] As described above, the steel according to this embodiment is nitrided before use. That is, the body of the soft-nitrided component according to this embodiment has a surface layer formed by nitriding the steel according to this embodiment. As an example, this surface layer extends to a depth of about 0.5 mm from the surface of the body. This embodiment also includes cases where the thickness of this surface layer is greater than 0.5 mm.

[0025] The soft-nitrided component according to the present embodiment may be a component obtained by combining with the main body another structure that is made of a metal or a metal alloy and has a non-nitrided surface. The soft-nitrided component according to the present embodiment may be a component obtained by combining with the main body another structure that is not made of metal.

[0026] The steel according to this embodiment forms a portion at a depth of 0.5 mm or more from the surface of the body of the soft-nitrided component according to this embodiment. The surface layer of the body of the soft-nitrided component according to this embodiment may be steel obtained by nitriding the steel according to this embodiment as described above.

[0027] As described above, the steel according to this embodiment contains C, Si, Mn, P, S, Al, and N as its chemical composition, with the balance being Fe and impurities. It is not excluded that this steel may further contain optional elements, which will be described later.

[0028] This steel may contain, as optional components, one or more selected from the group consisting of 2.0 mass% or less of Cr (chromium), 1.0 mass% or less of Mo (molybdenum), 1.0 mass% or less of Cu (copper), 1.0 mass% or less of Ni (nickel), and 0.01 mass% or less of B (boron).

[0029] Furthermore, this steel may contain, as an optional component composition, one or more selected from the group consisting of Ti (titanium): 0.1 mass% or less, Nb (niobium): 0.1 mass% or less, V (vanadium): 0.2 mass% or less, Hf (hafnium): 0.1 mass% or less, and Ta (tantalum): 0.1 mass% or less.

[0030] Furthermore, this steel may contain, as an optional component, one or more selected from the group consisting of 0.1 mass % or less of Sn (tin) and 0.1 mass % or less of Sb (antimony).

[0031] Furthermore, this steel may contain, as optional components, one or more selected from the group consisting of 0.3 mass% or less of Se (selenium), 0.1 mass% or less of Ca (calcium), 0.3 mass% or less of Pb (lead), and 0.3 mass% or less of Bi (bismuth).

[0032] The content and effects of each component in the steel will be described in detail below. In the following description, when simply referring to a content, it refers to the content (mass %) in the steel.

[0033] The C content is 0.04% by mass or more and 0.35% by mass or less. In order to increase the central hardness after nitriding, the C content must be 0.04% by mass or more. If the C content exceeds 0.35% by mass, the load during cold working increases, which may shorten the life of the mold. The C content is preferably 0.05% by mass or more and 0.27% by mass or less, more preferably 0.10% by mass or more and 0.25% by mass or less.

[0034] The Si content is 0.01% by mass or more and 1.20% by mass or less. In nitrided steel, Si serves as a deoxidizer. Excessive Si content in steel may reduce the cold workability of the steel material and may also reduce the toughness of the steel, thereby reducing its fatigue strength. The Si content is preferably 0.05% by mass or more and 0.70% by mass or less, more preferably 0.10% by mass or more and 0.50% by mass or less.

[0035] The Mn content is 0.30% by mass or more and 1.80% by mass or less. Mn improves hardenability and strengthens the pre-nitriding structure of steel, thereby strengthening the post-nitriding structure (nitrided steel). To obtain sufficient fatigue strength, the Mn content must be 0.30% by mass or more. If excessive Mn is added to steel, it may result in an increase in deformation resistance. The Mn content is preferably 0.40% by mass or more and 1.70% by mass or less, and more preferably 0.50% by mass or more and 1.30% by mass or less.

[0036] The P content is 0.1% by mass or less. P segregates at the grain boundaries of nitrided steel and reduces toughness, so the lower the P content, the better. The P content is allowed up to 0.1% by mass. The P content is preferably 0.02% by mass or less. There is no problem if there is no particular lower limit for the P content, but since the inclusion of P is usually unavoidable, unnecessary reduction in P content may increase refining time and refining costs. Therefore, it is reasonable and preferable to set the P content to 0.003% by mass or more.

[0037] The S content is 0.5% by mass or less. S exists as sulfide-based inclusions and is an element effective in improving machinability. Excessive S content in steel can lead to a decrease in cold workability. There is no particular lower limit for the S content, but since the inclusion of S is usually unavoidable, excessive reduction in S content can increase refining costs. Therefore, it is reasonable to set the S content to 0.003% by mass or more. The S content is preferably 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.

[0038] The Al content is 0.010% by mass or more and 0.300% by mass or less. Al forms oxides and is an element effective in deoxidizing nitrided steel. Al also has the effect of suppressing the formation of coarse oxide-based 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 may increase the number of inclusions (Al oxides), which may increase the initiation points of fatigue fracture and cause low fatigue strength.

[0039] The N content is 0.0250% by mass or less. N bonds with Al to form nitride (AlN). Finely precipitated AlN increases the hardness of the steel after nitriding (nitrided steel). Excessive N content in steel can lead to surface cracks in the slab after casting. There is no particular lower limit for the N content, but since N content is usually unavoidable, excessively low N content can 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.

[0040] The optional components of the steel will be explained below.

[0041] The Cr content may be 2.0% by mass or less. Cr contributes to improving hardenability and temper softening resistance, and is also a useful element for promoting spheroidization of carbides. If the Cr content exceeds 2.0% by mass, the Cr nitride layer formed on the surface after soft nitriding becomes thick, which may hinder the penetration of N into the interior and result in 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% by mass or less. Mo improves hardenability and strengthens the pre-nitriding structure, thereby strengthening the post-nitriding structure. However, if the Mo content is 1.0% by mass or more, the hardenability becomes excessive, the hardness after rolling increases, and workability and machinability may decrease. In order to realize the effect of Mo in improving the strength of the steel material, it is preferable to contain Mo in an amount of 0.01% by mass or more in the steel. The Mo content is more preferably 0.03% by mass or more and 0.50% by mass or less, and even more preferably 0.05% by mass or more and 0.25% by mass or less.

[0043] The Cu content may be 1.0 mass% or less. Cu improves hardenability and strengthens the pre-nitriding structure, thereby strengthening the post-nitriding structure. To achieve this effect, it is preferable to contain 0.01 mass% or more of Cu in the steel. If the Cu content exceeds 1.0 mass%, the surface of the rolled material may become rough, which may leave residual flaws. The Cu content is more preferably 0.015 mass% or more and 0.5 mass% or less, and even more preferably 0.03 mass% or more and 0.3 mass% or less.

[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 contain 0.01 mass% or more of Ni in the steel. Even if the Ni content in the steel exceeds 1.0 mass%, the above effects will saturate. The Ni content is more preferably 0.015 mass% or more and 0.5 mass% or less, and even more preferably 0.03 mass% or more and 0.3 mass% or less.

[0045] The B content may be 0.01% by mass or less. B segregates at grain boundaries and suppresses diffusional transformation, thereby effectively improving hardenability. In addition, B strengthens grain boundaries, suppresses the initiation and propagation of fatigue cracks, and improves fatigue strength. To obtain this effect of B, it is preferable to include 0.0003% by mass or more of B in the steel. If the B content exceeds 0.01% by mass, the toughness of the steel decreases, so the B content is preferably 0.01% by mass or less. The B content is more preferably 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 to form fine precipitates during soft nitriding, thereby improving the strength of the steel. However, even if the Ti content in the steel exceeds 0.1% by mass, the effect saturates. 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 mass% or less. Nb bonds with carbon and nitrogen to form fine precipitates during soft nitriding, thereby improving the strength of the steel. However, even if the Nb content in the steel exceeds 0.1 mass%, the effect is saturated. The Nb content is preferably 0.005 mass% or more and 0.08 mass% or less, and more preferably 0.01 mass% or more and 0.06 mass% or less.

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

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

[0050] The Ta content may be 0.1% by mass or less. Ta combines with carbon and nitrogen to form fine precipitates during soft nitriding, thereby improving the strength of the steel. To obtain this effect of Ta, it is preferable to include at least 0.003% by mass of Ta in the steel. On the other hand, if the Ta content in the steel exceeds 0.1% by mass, cracks are more likely to occur during casting and 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 Sn content may be 0.1% by mass or less. Sn is an element effective for improving the corrosion resistance of the steel surface. From the viewpoint of improving corrosion resistance, it is preferable that the Sn content in the steel is 0.003% by mass or more. Excessive Sn content in the steel reduces 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.

[0052] The Sb content may be 0.1% by mass or less. Sb is an element that is effective in suppressing decarburization of the steel surface and preventing a decrease in surface hardness. To achieve this effect, it is preferable to contain 0.0003% by mass or more of Sb in the steel. If an excessive amount of Sb is contained in the steel, 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.

[0053] The Se content may be 0.3% by mass or less. Se improves machinability by combining with Mn and Cu and dispersing as precipitates in the steel. To achieve this effect, it is preferable that the steel contains at least 0.001% by mass of Se. Even if the Se content exceeds 0.3% by mass, the effect saturates. The Se content is more preferably 0.005% by mass to 0.1% by mass, and even more preferably 0.008% by mass to 0.09% by mass.

[0054] The Ca content may be 0.1% by mass or less. Ca improves machinability by combining with S and dispersing as sulfides in the steel. To achieve this effect, it is preferable to include at least 0.0005% by mass of Ca in the steel. Even if the Ca content in the steel exceeds 0.1% by mass, the effect saturates. 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 improving chip disposability. To achieve this effect, it is preferable to include 0.01% by mass or more of Pb in the steel. Even if an excessive amount of Pb is included in the steel, the effect of improving chip disposability 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 improving chip disposability. To achieve this effect, it is preferable to contain Bi at 0.01% by mass or more. Even if excessive Bi is contained in steel, the effect of improving chip disposability 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 balance other than the elements described above is Fe and impurities. The impurities are substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment during industrial production of steel, and are allowed to a degree that does not adversely affect the properties of this embodiment.

[0058] When steel is cold-worked, crystal orientation rotation occurs according to the processing method. As the amount of processing increases, crystal orientation rotation becomes more pronounced, and the concentration of crystal orientations progresses. The inverse pole figure intensity distribution obtained by EBSD (electron backscatter diffraction) is a representative index that shows the degree of concentration of crystal orientations. When the maximum strength value in the inverse pole figure intensity distribution is 1.5 or higher, the fatigue strength of the steel is improved. To sufficiently improve the fatigue strength of steel, the maximum strength value 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 steel according to this embodiment, the maximum strength value in the inverse pole figure intensity distribution measured at a depth of 1 / 4 of the diameter from the surface of the body of a steel bar, or at a depth of 1 / 4 of the width and 1 / 4 of the thickness from the surface of the body of a steel plate, in a direction parallel to the surface of the part, is 1.5 or more. [Example]

[0060] Hereinafter, the steel according to this embodiment will be described based on examples, but the steel according to this embodiment is not limited to these examples.

[0061] First, steels (Steel Nos. 1 to 31) having the chemical compositions shown in Table 1 were melted and formed into round bars having a diameter of 20 mm by hot rolling. Among the chemical composition values ​​shown in Table 1, the underlined values ​​are outside the ranges specified in this embodiment.

[0062] This round bar was then subjected to cold tension processing (an example of cold processing) to produce a round bar with a diameter of 16 mm, after which an Ono-type rotating bending fatigue test specimen (hereinafter simply referred to as the test specimen) with a gripping diameter of 12 mm and a parallel diameter of 10 mm was cut from the center.

[0063] FIG. 1 is a schematic diagram showing the shape of the test piece of this example. The test piece is linear along the axis G, and its cross section perpendicular to the axis G is a circular rod shape whose center overlaps with the axis G. Both ends of the test piece in the direction along the axis G are gripping portions 1, 1, which will be described later. A parallel portion 2 is disposed between the gripping portions 1, 1 in the direction along the axis G. In the test piece shown in FIG. 1, the diameter of the parallel portion 2 is 10 mm. The diameter of the gripping portions 1, 1 is 12 mm. The length of the gripping portion 1 in the direction along the axis G is 25 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 curved to have a radius of 15 from the end of the gripping portion 1 to the center in the axial direction.

[0064] [Table 1]

[0065] The specimen was then measured using the EBSD method to determine the inverse pole figure intensity distribution and the maximum intensity value. The inverse pole figure intensity distribution was determined for a cross section that overlaps with the central axis (axis G shown in Figure 1) of a cross section perpendicular to the longitudinal direction at the gripping part of the specimen.

[0066] The cross section was measured by cutting the grip portion along the longitudinal direction to expose the cross section.

[0067] The calculation of the inverse pole figure intensity distribution based on the measurement results by the EBSD method was carried out using the crystal orientation analysis software OIM manufactured by TSL, Inc. Note that a scanning electron microscope (manufactured by JEOL Ltd., model: JSM-7001F) was used for the measurements by the EBSD method.

[0068] The measurement field of view for the EBSD method was a square area with sides of 0.5 mm, and the inverse pole figure intensity distribution was analyzed in the direction along the central axis (axis center G) (the same as the tensile axis direction in the fatigue test described below) using the spherical harmonics method under the conditions of Series Rank: 13, Gaussian Smoothing: 5.0°, and Sample Symmetry: Triclinic.

[0069] Next, this test piece was subjected to gas soft-nitriding heat treatment (nitriding) at 570°C for 3 hours to obtain a post-treatment test piece (an example of a soft-nitrided part). The nitriding treatment was carried out in a mixed gas containing ammonia and carbon monoxide.

[0070] Next, the treated test specimens of each component composition were separated into those with a maximum strength value of 1.5 or more in the inverse pole figure strength distribution and those with a maximum strength value of not less than 1.5. Then, the treated test specimens with a maximum strength value of not less than 1.5 in the pole figure strength distribution were subjected to a rotating bending fatigue test, and the fatigue limits (1 × 10 7 The stress (equivalent to the number of times of stress) was evaluated.

[0071] In this rotating bending fatigue test, if the fatigue limit is 780 MPa or more, it can be evaluated as having excellent fatigue properties. As shown in Table 1, the treated test specimen (nitrocarburized part) formed from the steel according to this embodiment has excellent fatigue properties. Among the fatigue limit values ​​shown in Table 1, the underlined values ​​are those less than 780 MPa.

[0072] It is also clear that the treated test pieces (nitrocarburized parts) made of steel according to this embodiment have significantly better bending fatigue properties than those made of steel not according to this embodiment.

[0073] Thus, the steel forming the post-treatment test piece (main body of the soft nitrided part) according to this embodiment does not require the inclusion of a large amount of a relatively expensive metal such as Cu, and is low-cost, yet has excellent fatigue properties.

[0074] Furthermore, the steel forming the post-treatment test piece (main body of the soft nitrided part) according to this embodiment does not necessarily require the addition of a rare metal such as V, is low cost, and yet has excellent fatigue properties.

[0075] The inverse pole figure intensity distribution of the treated specimens was measured using the EBSD method, and the maximum intensity value was determined. The maximum intensity value of the inverse pole figure intensity distribution of each treated specimen was the same as the maximum intensity value of the inverse pole figure intensity distribution of the corresponding pre-nitrided specimen (the state of each treated specimen before nitriding).

[0076] The inverse pole figure intensity distribution was determined for a portion of the gripped portion of the treated test piece at a depth of 0.5 mm from the surface. Measurements for the portion of the gripped portion of the treated test piece at a depth of 0.5 mm from the surface were performed after mirror polishing a cross section perpendicular to the axis G at the center of the length of the gripped portion.

[0077] That is, the maximum intensity value of the inverse pole figure intensity distribution inside the test piece (in this embodiment, the portion 0.5 mm deep from the surface) was the same before and after nitriding the test piece.

[0078] In this manner, it is possible to provide a steel that is low in cost and has excellent bending fatigue properties after nitriding.

[0079] It should be noted that the embodiments disclosed in this specification are merely examples, and the present invention is not limited to these embodiments, and can be modified as appropriate within the scope of the purpose of the present invention. [Industrial Applicability]

[0080] The present invention is applicable to steel. [Explanation of symbols]

[0081] 1: Grip part 2: Parallel section G: Axial center

Claims

1. The component composition is as follows: C: 0.04% by mass or more and 0.35% by mass or less, Si: 0.10% by mass or more and 1.20% by mass or less, Mn: 0.30% by mass or more and 0.90% by mass or less, P: 0.1% by mass or less, S: 0.5% by mass or less, Al: 0.010 mass% or more and 0.300 mass% or less; N: 0.0250 mass% or less, The balance consists of Fe and impurities, Steel having a maximum strength value of 1.5 or more in an inverse pole figure intensity distribution measured in a direction parallel to the surface at a depth of 0.5 mm from the surface.

2. The component composition is Cr: 2.0% by mass or less, Mo: 1.0% by mass or less, Cu: 1.0% by mass or less, Ni: 1.0 mass% or less; and B: 0.01% by mass or less, 2. The steel of claim 1, further comprising one or more of the following:

3. The component composition is Nb: 0.1% by mass or less, V: 0.09% by mass or less, Hf: 0.1% by mass or less and Ta: 0.1% by mass or less, 2. The steel of claim 1, further comprising one or more of the following:

4. The component composition is Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, V: 0.2% by mass or less, Hf: 0.1% by mass or less and Ta: 0.1% by mass or less, 3. The steel of claim 2, further comprising one or more selected from the group consisting of:

5. The component composition is Sn: 0.1% by mass or less; and Sb: 0.1% by mass or less, The steel according to any one of claims 1 to 4, further comprising one or more selected from the following:

6. The component composition is Se: 0.3% by mass or less, Ca: 0.1% by mass or less, Pb: 0.3% by mass or less and Bi: 0.3% by mass or less, The steel according to any one of claims 1 to 4, further comprising one or more selected from the following:

7. The component composition is Se: 0.3% by mass or less, Ca: 0.1% by mass or less, Pb: 0.3% by mass or less and Bi: 0.3% by mass or less, 6. The steel of claim 5, further comprising one or more selected from the group consisting of:

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