Steel materials and steel profiles

Abstract

To provide a steel material and steel raw material that are excellent in machinability, having excellent bending fatigue strength and excellent plastic deformability when formed into a nitrided steel component by nitriding used as a raw material.SOLUTION: The steel material according to this embodiment has a chemical composition in mass %: C: 0.33 to 0.45%, Si: 0.15 to 0.35%, Mn: 0.40 to 0.70%, P: 0.035 to 0.070%, S: 0.040 to 0.095%, Cu: 0.25% or less, Ni: 0.25% or less, Cr: 0.04 to less than 0.25%, Al: 0.025% or less, N: 0.010 to 0.020%, and Ti: 0.002 to 0.030%, the remainder being Fe and impurities, and satisfies formula (1). 0.130<0.10 Si+1.7 P+4.7 N<0.230 (1) Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to steel materials and steel semifinished products.

Background Art

[0002] Steel parts typified by crankshafts for automobiles, ships, industrial machines, etc. are required to have high bending fatigue strength. An example of a manufacturing method for steel parts required to have high bending fatigue strength is as follows. First, a steel material used as a raw material is hot forged to produce a steel semifinished product. The produced steel semifinished product is machined (such as cutting) to a shape close to the final product. When obtaining high bending fatigue strength, a surface hardening treatment is performed on the steel semifinished product after machining to increase the strength of the surface layer. The surface hardening treatment is, for example, induction hardening treatment or nitriding treatment. Through the above steps, steel parts are manufactured.

[0003] Induction hardening treatment, which is a type of surface hardening treatment, usually heats to a point A c1 or higher and then rapidly cools. Thereby, the structure is transformed and the surface layer is hardened. On the other hand, nitriding treatment usually heats at a point A c1 or lower. Then, nitrogen is introduced into the surface layer of the steel material, and the surface layer is hardened without transforming the structure.

[0004] As described above, nitriding treatment is a heat treatment that does not involve structure transformation, whereas induction hardening treatment is a heat treatment that involves structure transformation. Therefore, nitriding treatment has less distortion due to heat treatment compared to induction hardening treatment. Therefore, nitriding treatment is performed as the surface hardening treatment for steel parts that require dimensional accuracy. In this specification, steel parts subjected to nitriding treatment are referred to as "nitrided steel parts".

[0005] For the steel material used as the raw material for nitrided steel parts, it is required to obtain sufficient bending fatigue strength when made into nitrided steel parts. Furthermore, as described above, machining typified by cutting is performed during the manufacturing process, so excellent machinability is required.

[0006] Technologies for improving the machinability or bending fatigue strength of steel materials used as the raw material for steel parts are disclosed in Japanese Patent Publication No. 2008-57021 (Patent Document 1) and Japanese Patent Publication No. Hei 7-179986 (Patent Document 2).

[0007] Patent Document 1 describes how the morphology of MnS is changed by incorporating S and Ca into the steel material. This, according to Patent Document 1, improves the machinability of the steel material.

[0008] Patent Document 2 describes increasing the sulfur content in the steel, further reducing the carbon content to 0.30% or less, and setting the carbon equivalent within an appropriate range. This improves the machinability of the steel. Furthermore, to increase the bending fatigue strength, the chromium content is set to 0.40% or more and the vitrification content to 0.03% or more. Patent Document 2 states that by adjusting the chemical composition of the steel as described above, both machinability and bending fatigue strength can be improved. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2008-57021 [Patent Document 2] Japanese Patent Application Publication No. 7-179986 [Overview of the project] [Problems that the invention aims to solve]

[0010] Incidentally, in nitrided steel parts, slight plastic deformation may be applied to slightly modify the shape to match the shape of other steel parts when crimping the mating parts with other steel parts or when combining them. Therefore, the steel material used for nitrided steel parts is required to have excellent plastic deformability when nitrided. Patent documents 1 and 2 do not address the aforementioned plastic deformability.

[0011] The object of the present invention is to provide steel materials and steel profiles that have excellent machinability and, when used as a material and subjected to nitriding treatment to become nitrided steel parts, possess excellent bending fatigue strength and excellent plastic deformation capacity. [Means for solving the problem]

[0012] The steel material according to the present invention is The chemical composition is expressed in mass percent. C: 0.33~0.45%, Si: 0.15~0.35%, Mn: 0.40~0.70%, P: 0.035~0.070% S: 0.040~0.095%, Cu: 0.25% or less, Ni: 0.25% or less, Cr: 0.04-0.25% Al: 0.025% or less, N: 0.010~0.020%, and, It contains Ti: 0.002~0.030%, with the remainder being Fe and impurities. The equation (1) is satisfied. 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0013] The steel material according to the present invention is The chemical composition is expressed in mass percent. C: 0.33~0.45%, Si: 0.15~0.35%, Mn: 0.40~0.70%, P: 0.035~0.070% S: 0.040~0.095%, Cu: 0.25% or less, Ni: 0.25% or less, Cr: 0.04-0.25% Al: 0.025% or less, N: 0.010~0.020%, and, It contains Ti: 0.002 to 0.030%, further contains one or more selected from the group consisting of the first group and the second group, and the balance consists of Fe and impurities, satisfies formula (1). [First group] Ca: 0.0050% or less [Second group] V: 0.02% or less, Mo: 0.02% or less, and, one or more selected from the group consisting of B: 0.0003% or less 0.130 < 0.10Si + 1.7P + 4.7N < 0.230 (1) Here, the content in mass% of the corresponding element is substituted for each element symbol in formula (1).

[0014] The steel shaped material according to the present invention has the above chemical composition, satisfies the said formula (1), In the microstructure, the area ratio of the primary ferrite is 15% or more, and the balance consists of pearlite.

Advantages of the Invention

[0015] The steel material and steel shaped material of the present invention are excellent in machinability, and when used as a material and nitrided to form a nitrided steel part, they have excellent bending fatigue strength and excellent plastic deformation ability.

Brief Description of the Drawings

[0016] [Figure 1] FIG. 1 is a side view of a小野式 rotating bending fatigue test piece used in the bending fatigue test of the embodiment. The unit of the dimensions in the figure is mm. [Figure 2] FIG. 2 is a schematic diagram of a tensile test piece used in the tensile test of the embodiment. The unit of the dimensions in the figure is mm.

Modes for Carrying Out the Invention

[0017] It should be noted that "小野式" in the original text seems to be a specific name or term that may not be a common English expression. If there is a more accurate English equivalent, it can be further adjusted. Also, the reference numbers like etc. are left unchanged as required.The inventors of this invention investigated a steel material that exhibits excellent machinability and, when used as a raw material and subjected to nitriding treatment to become a nitrided steel part, possesses excellent bending fatigue strength and excellent plastic deformation capacity.

[0018] The inventors initially investigated the relationship between bending fatigue strength and plastic deformability in nitrided steel parts made from steel. Since these are conflicting mechanical properties, it seemed difficult to increase both bending fatigue strength and plastic deformability simultaneously.

[0019] Here, the inventors examined the roles of the nitrided layer and the core of a nitrided steel component, separating them into their respective components. Bending fatigue strength is primarily influenced by the hardness of the core. On the other hand, the plastic deformability required for a nitrided steel component is, as mentioned above, required to slightly modify its shape to match the shape of other steel components when combined with them. In other words, the required plastic deformability is a minute plastic deformation, and this plastic deformation is primarily required in the nitrided layer.

[0020] Based on the above considerations, the inventors considered that it would be effective to increase the hardness of the core to improve bending fatigue strength while suppressing the hardness of the nitrided layer to improve plastic deformability. Therefore, they investigated steel materials from the perspective of chemical composition to ensure that the core and nitrided layer have the above-described configuration when used as nitrided steel parts. As a result, the inventors obtained the following findings.

[0021] Cr, Mn, Si, P, and N are elements that increase the core hardness of nitrided steel parts when nitrided. All of these elements increase the core hardness through solid solution strengthening. However, among these elements, Cr and Mn form nitrides in the nitrided layer during the nitriding process, increasing the hardness of the nitrided layer as well. Therefore, increasing the content of Cr, Mn, Si, P, and N increases not only the hardness of the core but also the hardness of the nitrided layer. As a result, while sufficient bending fatigue strength can be obtained in nitrided steel parts, sufficient plastic deformation capacity cannot be obtained.

[0022] On the other hand, among Cr, Mn, Si, P, and N, P does not form nitrides, so its influence on the hardness of the nitrided layer is small. Also, if the amount of Si is small, it does not form many nitrides in the nitriding temperature range. Therefore, its influence on the hardness of the nitrided layer is small. Furthermore, since the N concentration of the nitrided layer is determined by the amount of N that penetrates from the surface, the N content contained in the steel material has almost no effect on the hardness of the nitrided layer. Consequently, although Si, P, and N increase the hardness of the core of nitrided steel parts, their contribution to improving the hardness of the nitrided layer is significantly smaller compared to Cr and Mn.

[0023] Based on the above findings, the inventors considered that by suppressing the Cr and Mn content while setting the Si, P, and N content within appropriate ranges in the chemical composition of the steel material used for nitrided steel parts, it would be possible to achieve both sufficient bending fatigue strength and sufficient plastic deformability when the steel is made into nitrided steel parts. Therefore, based on the above findings, the chemical composition of the steel material was investigated. As a result, we considered that if the chemical composition contains, by mass%, C: 0.33~0.45%, Si: 0.15~0.35%, Mn: 0.40~0.70%, P: 0.035~0.070%, S: 0.040~0.095%, Cu: 0.25% or less, Ni: 0.25% or less, Cr: less than 0.04~0.25%, Al: 0.025% or less, N: 0.010~0.020%, and Ti: 0.002~0.030%, and furthermore, if any arbitrary elements are included, one or more selected from the groups consisting of the first and second groups mentioned above are included in place of a portion of Fe, with the remainder being Fe and impurities, then it is possible to achieve both excellent bending fatigue strength and excellent plastic deformability when used as nitrided steel parts.

[0024] However, even with steel materials that satisfy the above-mentioned chemical composition, when nitrided steel parts were produced, sufficient plastic deformability could be obtained, but sufficient bending fatigue strength could not. Furthermore, in the manufacturing process of nitrided steel parts using steel materials, sufficient machinability could not be obtained in the steel profiles produced.

[0025] Therefore, the inventors further investigated means to obtain sufficient properties in terms of plastic deformability, bending fatigue strength, and machinability. As a result, they obtained the following findings.

[0026] Depending on the shape of the nitrided steel part, the stress gradient from the surface to the interior when bending is applied becomes gentler. Therefore, to increase the bending fatigue strength of nitrided steel parts, it is effective to increase the hardness of the core as well as the nitrided layer. As mentioned above, if Cr and Mn are included to increase the hardness of the core, the hardness of the nitrided layer also increases, and the plastic deformability decreases. On the other hand, as mentioned above, Si, P, and N increase the hardness of the core similarly to Cr and Mn, but their effect on the hardness of the nitrided layer is smaller compared to Cr and Mn. Therefore, by increasing the content of Si, P, and N, it is possible to increase bending fatigue strength while maintaining plastic deformability. However, if the content of Si, P, and N is too high, the hardness of the core becomes excessively high. In this case, sufficient bending fatigue strength can be obtained, but machinability decreases.

[0027] Therefore, the inventors conducted a detailed study on the relationship between Si content, P content, and N content and machinability and bending fatigue strength in steel materials satisfying the above-mentioned chemical composition. As a result, they found that when the Si content, P content, and N content satisfy formula (1), sufficient machinability can be obtained in steel materials (steel profiles), and sufficient bending fatigue strength and sufficient plastic deformation capacity can be obtained in nitrided steel parts. 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0028] Based on the above findings, the steel material and steel profile of this embodiment have the following configuration.

[0029] [1] The chemical composition is expressed in mass percent. C: 0.33~0.45%, Si: 0.15~0.35%, Mn: 0.40~0.70%, P: 0.035~0.070% S: 0.040~0.095%, Cu: 0.25% or less, Ni: 0.25% or less, Cr: 0.04-0.25% Al: 0.025% or less, N: 0.010~0.020%, and, It contains Ti: 0.002~0.030%, with the remainder being Fe and impurities. The equation (1) satisfies, Steel material. 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0030] [2] The chemical composition is expressed in mass percent. C: 0.33~0.45%, Si: 0.15~0.35%, Mn: 0.40~0.70%, P: 0.035~0.070% S: 0.040~0.095%, Cu: 0.25% or less, Ni: 0.25% or less, Cr: 0.04-0.25% Al: 0.025% or less, N: 0.010~0.020%, and, It contains Ti: 0.002~0.030%, Furthermore, it contains one or more selected from the groups consisting of Group 1 and Group 2, with the remainder being Fe and impurities. The equation (1) satisfies, Steel material. [Group 1] Ca: 0.0050% or less [Group 2] V: 0.02% or less, Mo: 0.02% or less, and, B: One or more selected from the group consisting of 0.0003% or less. 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0031] [3] [2] The steel material described above, The chemical composition includes the first group, Steel material.

[0032] [4] The steel materials described in [2] or [3], The aforementioned chemical composition contains the second group, Steel material.

[0033] [5] Having the chemical composition described in any one of items [1] to [4], The above equation (1) is satisfied, In the microstructure, the area ratio of protereminate ferrite is 15% or more, and the remainder consists of pearlite. Steel profile.

[0034] The steel materials and steel profiles of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0035] [Regarding the steel material used in this invention] The steel material of this embodiment satisfies the following characteristics. (Feature 1) The chemical composition is within the range of this embodiment. (Feature 2) The chemical composition further satisfies equation (1). 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element. The following describes each of its features.

[0036] [(Feature 1) Regarding chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements:

[0037] C: 0.33~0.45% Carbon (C) enhances the hardenability of steel and increases the bending fatigue strength of nitrided steel parts manufactured using steel as a material. If the C content is less than 0.33%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the carbon content exceeds 0.45%, the machinability of the steel profile manufactured using steel as a material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the C content is 0.33-0.45%. The preferred lower limit of the C content is 0.35%, more preferably 0.36%, even more preferably 0.37%, and even more preferably 0.38%. The preferred upper limit for the C content is 0.43%, more preferably 0.42%, and even more preferably 0.41%.

[0038] Si: 0.15~0.35% Silicon (Si) dissolves in protereminate ferrite and ferrite in pearlite, increasing the bending fatigue strength of nitrided steel parts made from steel. If the Si content is less than 0.15%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 0.35%, the machinability of the steel material made from steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.15-0.35%. The preferred lower limit for the Si content is 0.17%, more preferably 0.18%, and even more preferably 0.20%. The preferred upper limit for the Si content is 0.34%, and more preferably 0.33%.

[0039] Mn: 0.40~0.70% Manganese (Mn) forms nitrides during the nitriding process in the manufacturing of nitrided steel parts made from steel, thereby increasing the hardness of the nitrided layer of the nitrided steel parts. Furthermore, Mn improves hardenability, increasing the strength of the core of the nitrided steel parts and increasing the bending fatigue strength of the nitrided steel parts. Mn also combines with sulfur to form MnS, improving the machinability of steel profiles made from steel. If the Mn content is less than 0.40%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 0.70%, the hardness of the nitrided layer becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, sufficient plastic deformation capacity cannot be obtained in the nitrided steel part. Therefore, the Mn content is 0.40-0.70%. The preferred lower limit for the Mn content is 0.45%, more preferably 0.48%, and even more preferably 0.50%. The preferred upper limit for the Mn content is 0.65%, more preferably 0.63%, and even more preferably 0.60%.

[0040] P: 0.035~0.070% Phosphorus (P) does not contribute to improving the hardness of the nitrided layer of nitrided steel parts made from steel, but rather increases the hardness of the core. Therefore, P increases the bending fatigue strength of nitrided steel parts while maintaining the plastic deformability of the nitrided steel parts. If the P content is less than 0.035%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the P content exceeds 0.070%, the core hardness of the steel profile made from steel becomes excessively high, and the machinability of the steel profile decreases. Therefore, the P content is 0.035-0.070%. The preferred lower limit for the P content is 0.040%, more preferably 0.43%, and even more preferably 0.045%. The preferred upper limit for the P content is 0.067%, and more preferably 0.065%.

[0041] S: 0.040~0.095% Sulfur (S) combines with Mn to form MnS, which improves the machinability of steel profiles made from steel. If the S content is less than 0.040%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the sulfur content exceeds 0.095%, coarse MnS is formed even if the content of other elements is within the range of this embodiment. In this case, the bending fatigue strength of nitrided steel parts made from steel material decreases. Therefore, the sulfur content is 0.040-0.095%. The preferred lower limit for the S content is 0.045%, more preferably 0.048%, and even more preferably 0.050%. The preferred upper limit for the S content is 0.090%, more preferably 0.085%, even more preferably 0.080%, and even more preferably 0.070%.

[0042] Cu: 0.25% or less Copper (Cu) dissolves in ferrite, increasing the bending fatigue strength of nitrided steel parts made from steel. Even a small amount of Cu is sufficient to achieve this effect to some extent. However, if the Cu content exceeds 0.25%, even if the content of other elements is within the range of this embodiment, segregation at the grain boundaries of the steel material may occur during the manufacturing process of the steel material, or during the hot working process in the manufacturing process of steel profiles using steel material, causing hot cracking. Therefore, the copper content is 0.25% or less. The preferred lower limit of the Cu content is greater than 0%, more preferably 0.01%, and even more preferably 0.05%. The preferred upper limit for the Cu content is 0.20%, and more preferably 0.10%.

[0043] Ni: 0.25% or less Nickel (Ni) dissolves in ferrite, increasing the bending fatigue strength of nitrided steel parts made from steel. Furthermore, Ni suppresses the occurrence of hot cracks caused by copper when the steel contains copper. Even a small amount of Ni is sufficient to achieve the above effects to some extent. However, if the Ni content exceeds 0.25%, the above effects saturate, and manufacturing costs increase. Therefore, the Ni content is 0.25% or less. The preferred lower limit for the Ni content is greater than 0%, more preferably 0.01%, and even more preferably 0.05%. The preferred lower limit for the Ni content is 0.20%, and more preferably 0.15%.

[0044] Cr: 0.04-0.25% Chromium (Cr) combines with nitrogen to form nitrides, increasing the hardness of the nitrided layer. Therefore, it increases the bending fatigue strength of nitrided steel parts made from steel. If the Cr content is less than 0.04%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content is 0.25% or more, the hardness of the nitrided layer of the nitrided steel part becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the plastic deformability of the nitrided steel part decreases. Therefore, the Cr content is less than 0.04-0.25%. The preferred lower limit for the Cr content is 0.05%, more preferably 0.07%, and even more preferably 0.09%. The preferred upper limit for the Cr content is 0.20%, more preferably 0.18%, and even more preferably 0.16%.

[0045] Al: 0.025% or less Aluminum (Al) deoxidizes steel during the steelmaking process in the manufacturing of steel materials. Even a small amount of Al is sufficient to achieve this effect to some extent. However, if the Al content exceeds 0.025%, even if the content of other elements is within the range of this embodiment, an excessive amount of Al oxide will be generated in the steel material. In this case, the machinability of the steel profile made from the steel material will decrease. Therefore, the Al content is 0.025% or less. The preferred lower limit of the Al content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred lower limit for the Al content is 0.020%, more preferably 0.015%, and even more preferably 0.010%.

[0046] N: 0.010~0.020% Nitrogen (N) dissolves in the steel material, increasing the hardness of the core of nitrided steel parts made from steel. As a result, the bending fatigue strength of nitrided steel parts is increased. If the N content is less than 0.010%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the N content exceeds 0.020%, even if the content of other elements is within the range of this embodiment, bubbles due to nitrogen gas may form in the nitrided steel parts, which may reduce the bending fatigue strength. Therefore, the N content is 0.010-0.020%. The preferred lower limit for the N content is 0.012%, more preferably 0.013%, and even more preferably 0.015%. The preferred upper limit for the N content is 0.019%, more preferably 0.018%, and even more preferably 0.017%.

[0047] Ti: 0.002~0.030% Titanium (Ti) combines with nitrogen to form TiN, which suppresses the coarsening of austenite grains during hot forging. Suppression of coarse austenite grain formation reduces hardenability, allowing for sufficient protereminate ferrite, and consequently improving machinability. If the Ti content is less than 0.002%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ti content exceeds 0.030%, the TiN becomes coarser, degrading the fatigue properties. Therefore, the Ti content is between 0.002% and 0.030%. The preferred lower limit for the Ti content is 0.003%, more preferably 0.004%, and even more preferably 0.005%. The preferred upper limit for the Ti content is 0.025%, more preferably 0.020%, and even more preferably 0.010%.

[0048] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are introduced during the industrial production of steel material from raw materials such as ore, scrap, or the manufacturing environment, and are not intentionally included, but are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[0049] [Optional Elements] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the groups consisting of Group 1 and Group 2, in place of a portion of Fe. [Group 1] Ca: 0.0050% or less [Group 2] V: 0.02% or less, Mo: 0.02% or less, and, B: One or more selected from the group consisting of 0.0003% or less. The following is a description of arbitrary elements.

[0050] [Group 1: Ca] The chemical composition of the steel material in this embodiment may further contain Ca in place of some of the Fe. Ca: 0.0050% or less Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. When calcium is present, that is, when the calcium content is greater than 0%, calcium softens oxides by transforming them into complex oxides. Therefore, it is possible to extend the life of tools used when machining steel profiles made from steel, and the machinability of the steel profiles is improved. Even if only a small amount of calcium is present, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse Ca oxides will be formed, even if the content of other elements is within the range of this embodiment. In this case, the machinability of the steel profile will actually decrease. Therefore, the Ca content is between 0 and 0.0050%, and if present, the Ca content is 0.0050% or less. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit for the Ca content is 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, and even more preferably 0.0030%.

[0051] [Group 2: V, Mo, and B] The chemical composition of the steel material in this embodiment may further include one or more elements selected from the group consisting of V, Mo, and B, in place of some of the Fe. These elements are arbitrary and all increase the bending fatigue strength of nitrided steel parts made from steel. Each element will be described below.

[0052] V:0.02% or less Vanadium (V) is an optional element and does not need to be present. In other words, the V content may be 0%. When present, i.e., when the V content is greater than 0%, V forms nitrides in the nitrided layer of nitrided steel parts made from steel, thereby increasing the bending fatigue strength of the nitrided steel parts. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.02%, the hardness of the nitrided layer of the nitrided steel part becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the plastic deformability of the nitrided steel part decreases. Therefore, the V content is 0-0.02%, and if present, the V content is 0.02% or less. The preferred lower limit for the V content is greater than 0%, and more preferably 0.01%.

[0053] Mo: 0.02% or less Molybdenum (Mo) is an optional element and does not need to be included. In other words, the Mo content may be 0%. When present, i.e., when the Mo content is greater than 0%, Mo forms nitrides in the nitrided layer of nitrided steel parts made from steel, thereby increasing the bending fatigue strength of the nitrided steel parts. Even a small amount of Mo can provide the above effect to some extent. However, if the Mo content exceeds 0.02%, the hardness of the nitrided layer of the nitrided steel part becomes excessively high, even if the content of other elements is within the range of this embodiment. In this case, the plastic deformability of the nitrided steel part decreases. Therefore, the Mo content is 0-0.02%, and if present, the Mo content is 0.02% or less. The preferred lower limit for the Mo content is greater than 0%, and more preferably 0.01%.

[0054] B: 0.0003% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When present, i.e., when the B content is greater than 0%, B increases the strength of the steel. Therefore, the bending fatigue strength of nitrided steel parts made from steel increases. Even if only a small amount of B is present, the above effect can be obtained to some extent. However, if the B content exceeds 0.0003%, bainite may form in steel profiles made from steel, even if the content of other elements is within the range of this embodiment. In this case, the machinability of the steel profile decreases. Therefore, the B content is 0-0.0003%, and if present, the B content is 0.0003% or less. The preferred lower limit for the B content is greater than 0%, and more preferably 0.0001%. The preferred upper limit for B content is 0.0002%.

[0055] [Method for measuring the chemical composition of steel materials] The chemical composition of the steel material in this embodiment can be measured by a well-known component analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the inside of the steel material to a depth of 1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. Elemental analysis of the chemical composition is performed on the solution using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The C and S content is determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas melting-thermal conductivity method. The O content is determined using a well-known inert gas melting-infrared absorption method.

[0056] Furthermore, the content of each element shall be rounded to the minimum number of digits of the element content specified in this embodiment, based on the significant figures defined in this embodiment. For example, the carbon content of the steel material in this embodiment is defined to two decimal places. Therefore, the carbon content shall be the number obtained by rounding the third decimal place of the measured value to two decimal places.

[0057] Similarly, for the carbon content of the steel material in this embodiment, the content of other elements is determined by rounding the measured value to the minimum digit specified in this embodiment.

[0058] Rounding means that if the fractional part is less than 5, it is rounded down, and if the fractional part is 5 or greater, it is rounded up.

[0059] [Regarding (Feature 2) Equation (1)] The chemical composition of the steel material in this embodiment further satisfies formula (1). 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0060] F1 is defined as F1 = 0.10Si + 1.7P + 4.7N. F1 is an index for obtaining sufficient bending fatigue strength in nitrided steel parts while improving the machinability of the steel profiles produced during the manufacturing process of nitrided steel parts using steel materials that satisfy characteristic 1.

[0061] As described above, in nitrided steel parts having a nitrided layer, increasing the hardness of the core increases the bending fatigue strength. However, if the hardness of the nitrided layer increases along with the hardness of the core, the plastic deformability of the nitrided steel part decreases. Therefore, in the steel material of this embodiment, the content of Cr and Mn, which are alloying elements that increase the hardness of nitrided steel parts by forming nitrides during nitriding treatment, is kept within the above range, while the content of Si, P and N, which are alloying elements that increase the hardness of nitrided steel parts by solid solution strengthening without substantially forming nitrides, is appropriately increased. Among the three alloying elements described above, the ratio of solid solution strengthening ability in the core to precipitation strengthening ability in the nitrided layer increases in the order of N, P, and Si. In F1, the coefficients of N, P, and Si are adjusted considering the ratio of solid solution strengthening ability in the core to precipitation strengthening ability in the nitrided layer.

[0062] If F1 is 0.130 or less, even if the steel material satisfies characteristic 1, the hardness of the core of the nitrided steel part is insufficient. Therefore, sufficient bending fatigue strength cannot be obtained in the nitrided steel part. On the other hand, if F1 is 0.230 or more, even if the steel material satisfies characteristic 1, the hardness of the steel profile becomes excessively high. In this case, sufficient machinability cannot be obtained in the steel profile. Therefore, F1 should be higher than 0.130 and less than 0.230.

[0063] A preferred lower limit for F1 is 0.132, more preferably 0.135, even more preferably 0.140, and even more preferably 0.150. A preferred upper limit for F1 is 0.220, more preferably 0.210, and even more preferably 0.200.

[0064] [Effects of the steel material in this embodiment] As described above, the steel material of this embodiment satisfies features 1 and 2. Therefore, while improving the machinability of the steel profiles produced during the manufacturing process of nitrided steel parts using the steel material, sufficient plastic deformability and sufficient bending fatigue strength can be obtained in the nitrided steel parts.

[0065] [Applications of the steel material of this embodiment] The steel material of this embodiment can be widely applied as a material for nitrided steel parts, such as those used in machine structural components. In particular, it can be applied as a material for nitrided steel parts manufactured by nitriding treatment.

[0066] [Shape of the steel material in this embodiment] The shape of the steel material in this embodiment is not particularly limited. The steel material in this embodiment may be a steel plate, a steel bar (steel bar or wire), or a steel pipe. Preferably, the steel material in this embodiment is a steel bar (steel bar or wire).

[0067] [Regarding the microstructure of the steel material of this embodiment] The microstructure of the steel material in this embodiment is not particularly limited. When the steel material in this embodiment is used as the material for the nitrided steel parts described above, the steel material is heated to the austenite temperature range, and then hot working, such as hot forging, is performed to produce intermediate steel profiles. In this hot working process, the microstructure of the steel material is reset. Therefore, the microstructure of the steel material in this embodiment is not particularly limited.

[0068] [Regarding the steel profiles of this invention] The steel profile of this embodiment is manufactured during the manufacturing process of nitrided steel parts using the steel material of this embodiment described above. The steel profile of this embodiment satisfies the following features 1 to 3. (Feature 1) The chemical composition is within the range of this embodiment. (Feature 2) The chemical composition further satisfies equation (1). 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element. (Feature 3) In the microstructure, the area ratio of protereminate ferrite is 15% or more, with the remainder consisting of pearlite.

[0069] Here, features 1 and 2 are the same as features 1 and 2 of the steel material of this embodiment described above. This is because the steel profile of this embodiment is manufactured using the steel material of this embodiment as the raw material. Feature 3 of the steel profile of this embodiment will now be described.

[0070] [(Feature 3) Microstructure of steel profiles] In the steel profile of this embodiment, the area ratio of protereminate ferrite in the microstructure is 15% or more, and the remainder consists of pearlite. In other words, the area ratio of pearlite is 85% or less.

[0071] In the microstructure of the steel profile of this embodiment, bainite and / or martensite are not contained. If bainite and / or martensite are present in the microstructure of the steel profile, the hardness of the steel profile will be excessively high. As a result, sufficient machinability cannot be obtained in the steel profile. Furthermore, if the area ratio of ferrite in the microstructure of the steel profile is less than 15%, the area ratio of pearlite will exceed 85%. In this case as well, the hardness of the steel profile will be excessively high. As a result, the machinability of the steel profile will decrease. If the area ratio of protereminate ferrite in the microstructure of the steel profile is 15% or more, and the remainder is pearlite, the hardness of the steel profile will be sufficiently suppressed. As a result, sufficient machinability can be obtained in the steel profile.

[0072] In the microstructure of the steel profile, the preferred lower limit of the area percentage of protereminate ferrite is 16%, more preferably 18%, more preferably 20%, more preferably 22%, and more preferably 25%. There is no particular upper limit to the area ratio of protereminate ferrite. A preferred upper limit for protereminate ferrite is 45%, more preferably 42%, even more preferably 38%, and even more preferably 35%.

[0073] In the microstructure of the steel profile, the preferred lower limit of the area ratio of pearlite is 55%, more preferably 58%, more preferably 62%, and still more preferably 65%. The preferred upper limit for the area ratio of perlite is 84%, more preferably 82%, more preferably 80%, more preferably 78%, and more preferably 75%.

[0074] [Method for observing the microstructure of steel profiles and measuring the area ratio of ferrite] The microstructure of the steel profile in this embodiment and the area ratio of ferrite are measured by the following method.

[0075] A test specimen is taken from a steel profile, starting from a depth of 1 mm or more from the surface. An arbitrary surface of the taken test specimen is mirror-polished. The mirror-polished surface is used as the observation surface. The observation surface is etched with Nital to reveal the microstructure. The etched observation surface is observed at 200x magnification using an optical microscope. Specifically, photographic images of five arbitrary fields of view of the observation surface are generated. The size of each field of view is, for example, 0.60 mm × 0.48 mm.

[0076] Based on the contrast of the photographic images in each field of view, each tissue (ferrite, pearlite, bainite, and martensite) is identified. Furthermore, a square grid pattern with a mesh spacing of 50 μm is superimposed on the photographic images in each field of view. The tissue is identified at the locations where it overlaps with the grid points. The tissues at all grid points in the five fields of view are counted (aggregated) for each tissue. The ratio of the number of ferrite counts to the total number of grid points is defined as the ferrite area percentage (%). Similarly, the ratio of the number of pearlite counts to the total number of grid points is defined as the pearlite area percentage (%).

[0077] Note that the size of each field of view is not limited to the sizes mentioned above, but the size of each field of view should be set so that the total number of grid points in all five fields of view is 500 or more.

[0078] [Effects of the steel profile according to this know-how] As described above, the steel profile of this embodiment satisfies features 1 to 3. Therefore, the steel profile of this embodiment provides excellent machinability. Furthermore, in nitrided steel parts manufactured by performing nitriding treatment on the steel profile as the material, sufficient plastic deformability and sufficient bending fatigue strength can be obtained.

[0079] [Method for manufacturing steel materials and steel profiles of this invention] An example of a method for manufacturing steel materials and steel profiles of this embodiment will be described. The method for manufacturing steel materials and steel profiles described below is an example for manufacturing steel materials and steel profiles of this embodiment. Therefore, steel materials and steel profiles having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing steel materials and steel profiles of this embodiment. In this embodiment, a method for manufacturing steel bars will be described as an example of steel materials.

[0080] [Methods for manufacturing steel materials] An example of a method for manufacturing steel materials according to this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Hot working process The following describes each step.

[0081] [(Process 1) Material preparation process] In the material preparation process, the material for the steel of this embodiment is prepared. Specifically, molten steel whose chemical composition satisfies features 1 and 2 is produced. The refining method is not particularly limited, and any well-known method may be used. For example, molten iron produced by a well-known method is subjected to refining in a converter (primary refining). A well-known secondary refining is performed on the molten steel tapped from the converter. In secondary refining, alloying elements are added to the molten steel to adjust the composition and produce molten steel having a chemical composition that satisfies features 1 and 2.

[0082] The raw material is manufactured using the molten steel produced by the refining method described above, by a well-known casting method. For example, an ingot is manufactured using the ingot-making method with the molten steel. Alternatively, a bloom or billet may be manufactured using the continuous casting method with the molten steel. The raw material (ingot, bloom, or billet) is manufactured by the above method.

[0083] [(Process 2) Hot working process] Steel is manufactured by hot working the manufactured material. The hot working process usually involves one or more hot working steps. If multiple hot working steps are performed, the first hot working step may be, for example, rolling using bract rolling or hot forging, and subsequent hot working steps may be rolling using a continuous rolling mill. A continuous rolling mill is equipped with multiple rolling stands arranged in a row. The steel material is cooled to room temperature after hot working. Billets may be produced by rough rolling and rolling using a continuous rolling mill, and then the billets may be reheated and further finished rolling using a continuous rolling mill to produce steel material of the desired size. Alternatively, steel material may be manufactured from raw material solely by hot forging. The heating temperature of the material during hot working is not particularly limited, but is, for example, 1100 to 1300°C.

[0084] The steel material of this embodiment is manufactured through the above process.

[0085] [Method for manufacturing steel profiles] An example of the steel profile material of this embodiment includes the following steps. (Process 3) Steel profile forming process

[0086] [(Process 3) Steel Profile Forming Process] In the steel profile forming process, the steel material of this embodiment is used to form the steel profile. The forming method is, for example, hot forging. Specifically, first, the steel material is heated. The heating temperature is, for example, 1000 to 1300°C. The heated steel material is hot forged to form a steel material of a predetermined shape. The formed steel material is air-cooled. The cooling rate by air cooling is adjusted as appropriate using a fan. By adjusting the cooling rate by air cooling as appropriate, a steel profile that satisfies feature 3 can be manufactured. Note that if accelerated cooling such as water cooling is performed on the steel material after hot working, the ferrite area ratio will be less than 15%, and the pearlite area ratio will be more than 85%.

[0087] [Manufacturing method for nitrided steel parts using steel as the material] An example of a method for manufacturing nitrided steel parts using the above-mentioned steel material (or steel profiles manufactured using the above-mentioned steel material) includes the following steps: (Process 4) Machining process (Step 5) Nitriding process The following describes each step.

[0088] [(Process 4) Machining process] In the machining process, the steel profile is machined to shape it into a form close to the final product shape. Specifically, the shape of the steel profile is adjusted by performing well-known cutting and / or grinding processes.

[0089] [(Step 5) Nitriding process] In the nitriding process, a well-known nitriding treatment is performed on the steel profile after the machining process. Well-known nitriding treatments include, for example, gas nitriding, gas soft nitriding, salt bath soft nitriding, and plasma nitriding. The gas used for nitriding may be NH3 only, or it may be a well-known mixed gas containing NH3, N2, H2, CO2, and various hydrocarbons. The cooling method for the steel profile after nitriding may be water cooling, oil cooling, or furnace cooling. Nitrided steel parts are manufactured through the above manufacturing process.

[0090] [About nitrided steel parts] The nitrided steel component comprises a nitrided layer formed on the surface and a core portion located inside the nitrided layer. The chemical composition of the core portion is the same as that of the steel material in the above-described embodiment.

[0091] Nitrided steel parts manufactured using the steel material or steel profile of this embodiment as a material exhibit excellent bending fatigue strength. Furthermore, when plastic deformability is required when the nitrided steel parts are combined with other steel parts by fitting or the like, they exhibit excellent plastic deformability. [Examples]

[0092] The effects of the steel materials and steel profiles of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the steel materials and steel profiles of this embodiment. Therefore, the steel materials and steel profiles of this embodiment are not limited to this one example of conditions.

[0093] [Steel manufacturing] Steel materials having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured.

[0094] [Table 1-1]

[0095] [Table 1-2]

[0096] In Table 1-2, "-" indicates that the corresponding element content is 0% when rounded to the least significant digit as defined in the embodiment. In other words, it means that the corresponding element content is 0% when rounded to the least significant digit as defined in the embodiment described above. For example, the Ca content specified in this embodiment is given as a value to two decimal places. Therefore, in test number 7 in Table 1, it means that when the measured Ca content was rounded to the fifth decimal place, it was 0%. Rounding means that if the digit below the specified minimum digit (the fractional part) is less than 5, it is truncated; if it is 5 or greater, it is rounded up.

[0097] The steel materials for each test number were manufactured using the following method: A 50 kg ingot was produced using a vacuum melting furnace. After heating the ingot to 1250°C, hot working was performed to produce the steel material. Specifically, hot forging was performed to create a steel bar with a cross-section perpendicular to the axial direction of 75 mm x 75 mm. The steel bar after hot forging was allowed to cool to room temperature. The steel bar after cooling was heated to 1250°C. The heated steel bar was hot forged to produce a circular steel bar with a cross-section perpendicular to the axial direction of 60 mm in diameter. The steel bar after hot forging was allowed to cool to room temperature. The steel material (steel bar) was manufactured through the above manufacturing process.

[0098] [Manufacturing of steel profiles] Using the steel materials of each test number that were manufactured, the steel profiles of each test number were produced in the following manner.

[0099] To simulate the steel profile forming process described in step 3 above, the steel material was held at 1150°C for 60 minutes. After holding, the steel material was cooled to room temperature by blowing air on it with a fan (indicated as "Normal Cooling" in the "Cooling Method" column of Table 2). For steel materials of test numbers 22 and 23, after being held at 1150°C for 60 minutes, they were water-cooled to room temperature (accelerated cooling) (indicated as "Accelerated Cooling" in the "Cooling Method" column of Table 2).

[0100] [Table 2]

[0101] Through the above process, steel materials and steel profiles for each test number were manufactured.

[0102] [About the evaluation test] The following evaluation tests (Tests 1 to 5) were performed on the steel materials and steel profiles of each test number that were manufactured. [Steel material evaluation test] (Test 1) Chemical composition measurement test of steel material (Test 2) Microstructural observation test of steel profiles (Test 3) Machinability evaluation test (Test 4) Bending fatigue strength evaluation test (Test 5) Plastic Deformation Ability Evaluation Test The following describes each test.

[0103] [(Test 1) Chemical composition measurement test of steel material] The chemical composition of the steel material for each test number was analyzed based on the [Method for Measuring the Chemical Composition of Steel Material] described above. As a result, the chemical composition of the steel material for each test number was as shown in Tables 1-1 and 1-2. Since the steel profiles for each test number were manufactured using the steel material for the corresponding test number as the raw material, the chemical composition of the steel profiles for each test number was also the same as the chemical composition of the corresponding test number in Tables 1-1 and 1-2.

[0104] [(Test 2) Microstructural observation test of steel profiles] For each steel profile with a given test number, the microstructure of the steel profile was observed based on the [Method for Observing the Microstructure of Steel Materials] described above, and the area percentage of proecution ferrite and pearlite was determined. The test specimens were taken from the radial center position (R / 2 position) in the cross section perpendicular to the axial direction of the steel profile. The obtained results are shown in the "Ferrite Area Percentage (%)" and "Pearlite Area Percentage (%)" columns of the "Steel Profile" in Table 2.

[0105] [(Test 3) Machinability Evaluation Test] The following Vickers hardness tests were performed on the steel profiles of each test number. Each steel profile with a test number was cut perpendicular to the axial direction. A Vickers hardness test, in accordance with JIS Z 2244:2009, was performed at five arbitrary locations within the center of the cut surface (within a circular area of ​​0.10R (R is the radius) centered on the center position). The test force was 9.8 N. The arithmetic mean of the five hardnesses obtained was defined as the Vickers hardness (Hv) for the test number. The obtained Vickers hardness is shown in the "Vickers Hardness (HV)" column of Table 2. If the Vickers hardness was 205 HV or less, it was determined that sufficient machinability was obtained. On the other hand, if the Vickers hardness exceeded 205 HV, it was determined that sufficient machinability was not obtained.

[0106] [(Test 4) Bending fatigue strength evaluation test] For each test number, an Ono-type rotary bending fatigue test specimen was prepared from the midpoint of the radius (R / 2 position) of the cross-section perpendicular to the axial direction of the steel profile (60 mm diameter steel bar). The numbers in Figure 1 indicate dimensions (in mm). In Figure 1, "φ" means diameter. "R1" means that the radius of curvature of the notch bottom is 1 mm. The length of the Ono-type rotary bending fatigue test specimen was 22 mm, the diameter of the parallel section was 8 mm, and the diameter of the gripping section was 12 mm.

[0107] Specifically, intermediate Ono-type rotary bending fatigue test specimens were fabricated by machining (cutting) the steel profiles of each test number. Nitriding treatment was then performed on these intermediate specimens to produce the Ono-type rotary bending fatigue test specimens shown in Figure 1. The conditions for the nitriding treatment were as follows: During the nitriding treatment, the intermediate specimens were held at 590°C for 2 hours in an atmosphere of RX gas and ammonia gas in a 1:1 ratio. After holding, the intermediate specimens were oil-cooled. Through these steps, Ono-type rotary bending fatigue test specimens simulating nitrided steel parts were fabricated.

[0108] Ono-type rotary bending fatigue tests were performed using Ono-type rotary bending fatigue test specimens for each test number. Multiple Ono-type rotary bending fatigue test specimens were prepared for each test number. Fatigue tests were conducted by varying the stress applied to each specimen, and the results were obtained for 10 million cycles (10 7After several cycles, the highest stress at which fracture did not occur was defined as the bending fatigue strength (MPa). In the Ono rotary bending fatigue test, the rotation speed was set to 3000 rpm, and the stress ratio was set to bidirectional. The obtained bending fatigue strengths are shown in the "Bending Fatigue Strength (MPa)" column under the "Nitrided Steel Parts" column in Table 2.

[0109] [(Test 5) Plastic Deformability Evaluation Test] Tensile test specimens, as shown in Figure 2, were prepared from the radial center position (R / 2 position) of the cross-section perpendicular to the axial direction of each steel profile for each test number. The width of the parallel section of the tensile test specimen was 4 mm, and the thickness of the tensile test specimen was 2 mm.

[0110] Specifically, intermediate tensile test specimens were prepared by machining (cutting) the steel profiles of each test number. Nitriding treatment was then performed on these intermediate specimens to produce the tensile test specimens shown in Figure 2. The conditions for the nitriding treatment were as follows: During the nitriding treatment, the intermediate specimens were held at 590°C for 2 hours in an atmosphere of RX gas and ammonia gas in a 1:1 ratio. After holding, the intermediate specimens were oil-cooled. Through these steps, tensile test specimens simulating nitrided steel parts were produced. A strain gauge with a gauge length of 2 mm for fracture detection was attached to the parallel section of the fabricated tensile test specimen, with the longitudinal direction of the gauge parallel to the axial direction of the tensile test specimen. A tensile test was performed on the tensile test specimen with the strain gauge attached, in accordance with JIS Z 2241:2011. The tensile test was performed at room temperature (approximately 25°C), in air, and at a tensile speed of 1.5 mm / min, and the nominal strain until the strain gauge broke was determined. The obtained nominal strain is shown in the "Nominal Strain" column of the "Nitrided Steel Parts" column in Table 2.

[0111] [Evaluation Results] The evaluation results are shown in Table 2. Referring to Tables 1-1, 1-2, and 2, the steel material met Feature 1 and Feature 2 in test numbers 1 to 11. Furthermore, the steel profile met Feature 1 to Feature 3. As a result, the Vickers hardness was 205 HV or less, and sufficient machinability was obtained for the steel profile made from steel material. Furthermore, the bending fatigue strength was 270 MPa or more, and sufficient bending fatigue strength was obtained for the nitrided steel parts. Furthermore, the nominal strain was 0.030 or more, and sufficient plastic deformation capacity was obtained for the nitrided steel parts.

[0112] On the other hand, in test number 12, the Mn content was too high. As a result, the Vickers hardness exceeded 205 HV, and sufficient machinability could not be obtained. Furthermore, the nominal strain was less than 0.030, and sufficient plastic deformation capacity could not be obtained.

[0113] In test number 13, the phosphorus content was too low. As a result, the bending fatigue strength was less than 270 MPa, and sufficient bending fatigue strength was not obtained.

[0114] In test number 14, the Cr content was too high. As a result, the nominal strain was less than 0.030, and sufficient plastic deformation capacity could not be obtained.

[0115] In test number 15, the nitrogen content was too low. Furthermore, the boron content was too high. As a result, the ferrite area ratio in the steel profile was too low. Consequently, sufficient machinability could not be obtained.

[0116] In test number 16, the V content was too high. As a result, the nominal strain was less than 0.030, and sufficient plastic deformation capacity could not be obtained.

[0117] In test number 17, the nitrogen content was too low. As a result, the bending fatigue strength was less than 270 MPa, and sufficient bending fatigue strength was not obtained.

[0118] In test number 18, the Si content was too high. As a result, the Vickers hardness exceeded 205 HV, and sufficient machinability could not be obtained.

[0119] In test number 19, the P content was too high. As a result, the Vickers hardness exceeded 205 HV, and sufficient machinability could not be obtained.

[0120] In test number 20, the Cr content was too low. As a result, the bending fatigue strength was less than 270 MPa, and sufficient bending fatigue strength was not obtained.

[0121] In test number 21, the Ti content was too low. As a result, the ferrite area ratio in the steel profile was too low. Consequently, sufficient machinability could not be obtained.

[0122] In tests 22 and 23, accelerated cooling (water cooling) was performed during the manufacturing process of the steel profiles. As a result, the ferrite area ratio in the steel profiles was too low. Consequently, sufficient machinability could not be obtained.

[0123] Test sample number 24 had low C, P, S, N, and Ti content. As a result, its bending fatigue strength was less than 270 MPa, and sufficient bending fatigue strength was not achieved.

[0124] In tests 25 and 26, F1 was below the lower limit. Therefore, the bending fatigue strength was less than 270 MPa, and sufficient bending fatigue strength was not obtained.

[0125] In tests 27 and 28, F1 exceeded the upper limit. As a result, the Vickers hardness exceeded 205 HV, and sufficient machinability could not be obtained.

[0126] Embodiments of the present invention have been described above. However, the embodiments described above are merely illustrative examples for carrying out the present disclosure. Therefore, the present disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of the present invention.

Claims

1. The chemical composition is expressed in mass percent. C: 0.33-0.45%, Si: 0.15-0.35%, Mn: 0.40-0.70%, P: 0.035-0.070%, S: 0.040-0.095%, Cu: more than 0% and less than 0.25%, Ni: more than 0% and less than 0.25%, Cr: 0.04-0.25% Al: more than 0% and less than 0.025%, N: 0.010–0.020%, and, It contains Ti: 0.002 to 0.030%, with the remainder consisting of Fe and impurities. The equation (1) satisfies, Steel material. 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

2. The chemical composition is expressed in mass percent. C: 0.33-0.45%, Si: 0.15-0.35%, Mn: 0.40-0.70%, P: 0.035-0.070%, S: 0.040-0.095%, Cu: more than 0% and less than 0.25%, Ni: more than 0% and less than 0.25%, Cr: 0.04-0.25% Al: more than 0% and less than 0.025%, N: 0.010–0.020%, and, It contains Ti: 0.002 to 0.030%, Furthermore, it contains one or more selected from the group consisting of Group 1 and Group 2, with the remainder being Fe and It consists of impurities, The equation (1) satisfies, Steel material. [Group 1] Ca: 0.0050% or less [Group 2] V: 0.02% or less, Mo: 0.02% or less, and, B: One or more selected from the group consisting of 0.0003% or less. 0.130<0.10Si+1.7P+4.7N<0.230 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

3. The steel material according to claim 2, The aforementioned chemical composition contains the first group, Steel material.

4. The steel material according to claim 2, The aforementioned chemical composition contains the second group, Steel material.

5. Having the chemical composition described in any one of claims 1 to 4, The above equation (1) is satisfied, In the microstructure, the area ratio of protereminate ferrite is 15% or more, with the remainder being pearlite. Consists of, Steel profile.

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