Steel materials

Abstract

Provided is a steel material which has exceptional machinability, in which cracking during hot working and melt cracking during induction hardening can be suppressed, and which has exceptional fatigue strength when used as a component for a machine structure. This steel material contains, in terms of mass%, 0.05-0.30% of C, 0.05-0.45% of Si, 0.30-2.00% of Mn, no more than 0.030% of P, 0.010-0.095% of S, 0.01-2.00% of Cr, 0.0051-0.1500% of Bi, and 0.0030-0.0250% of N, and satisfies expression (1). In the steel material, the number density of fine Bi particles is 80-8,000 / mm2, and the number density of coarse Bi particles is 10 / mm2 or less.　[Expression 1]: 0.25≤C+(Si / 10)+(Mn / 5)−(5S / 7)+(5Cr / 22)+1.65V≤1.00

Description

Technical Field

[0001] The present invention relates to steel materials, and more particularly to steel materials used as materials for mechanical structure parts.

Background Art

[0002] Mechanical structure parts are used for components in the undercarriage and axles of automobiles and construction vehicles. High fatigue strength is required for mechanical structure parts.

[0003] In the manufacturing process of mechanical structure parts, cutting may be performed on the steel material used as the material for mechanical structure parts. Therefore, high machinability is required for the steel material used as the material for mechanical structure parts.

[0004] Steel materials used as materials for mechanical structure parts are disclosed in, for example, Japanese Patent Laid-Open No. 57-19366 (Patent Document 1), Japanese Patent Laid-Open No. 2004-18879 (Patent Document 2), and Japanese Patent Laid-Open No. 2008-169411 (Patent Document 3).

[0005] The steel material disclosed in Patent Document 1 contains 0.001 to 0.05% of Ca, 0.02 to 0.15% of Pb and Bi alone or in combination, regulates S to 0.005% or less, and the inclusions are CaS-CaO, Pb, Bi-based inclusions, and suppresses Al2O3 inclusions to less than 0.001%. In this document, a large amount of Ca is continuously added to the molten steel to change the dissolved S to CaS. Also, by the reduction reaction of Ca, Al2O3 is eliminated or made extremely small. Therefore, the inclusions become CaS-CaO-based inclusions. Then, a small amount of one or two of Pb and Bi is added to the molten steel to generate single inclusions of Pb or Bi. It is described in Patent Document 1 that this improves the machinability of the steel material.

[0006] The steel material disclosed in Patent Document 2 contains B: 0.001 to 0.010%, N: 0.002 to 0.010%, and Bi: 0.005 to 0.10% by mass. In this cold forging steel, a total of 15 or more BN with a diameter of 0.7 μm or more and Bi precipitates containing B are present per visual field area of 0.5 mm × 0.5 mm in cross section. In this steel material, by fixing N as BN, the amount of dissolved N is reduced and work hardening is reduced. Further, as described in Patent Document 2, Bi precipitates containing B are generated to improve chip disposability.

[0007] The steel material disclosed in Patent Document 3 contains, by mass, C: 0.15 to 0.55%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.5%, Cu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Cr: 0.01 to 2.5%, Mo: 0.01 to 3.0%, and a total amount of at least one selected from the group consisting of V and W: 0.01 to 1.0%, and the balance consists of Fe and inevitable impurities. This steel material is soaked at 1010°C to 1050°C, then cooled to 500°C to 550°C at a cooling rate of 200°C / min or more, then cooled to 150°C or less at a cooling rate of 100°C / min or more, and then heated in the temperature range of 550°C to 700°C. The LMP that gives the maximum value of the HRC hardness at room temperature of the steel material subjected to these heat treatment and cooling treatments is 17.66 or more. In this steel material, since LMX is 17.66 or more, as described in Patent Document 3, the softening resistance is increased and the thermal fatigue strength is increased.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] Incidentally, an example of the manufacturing process of mechanical structure parts made of steel materials is as follows. The steel material used as the raw material is hot-worked to produce an intermediate product having a rough shape of the mechanical structure parts. The hot working is, for example, hot forging. Machining (cutting) is performed on the manufactured intermediate product to form the intermediate product into a predetermined shape. Quenching and tempering are performed on the intermediate product after cutting. Through the above manufacturing process, mechanical structure parts are manufactured. In the quenching during the above manufacturing process, high-frequency quenching may be performed on the intermediate product (steel material) in order to increase the strength of some parts of the mechanical structure parts. In this case, high-frequency induction heating is performed on the part of the intermediate product (steel material) where the strength is desired to be increased, and then rapid cooling (quenching) is performed.

[0010] However, during high-frequency induction heating, due to the shape of the intermediate product (steel material), the steel material may be locally overheated. And in some cases, a part of the surface layer and the inside of the steel material may melt and cracks may occur. Such cracks are also referred to as "fusion cracks" in this specification. When high-frequency quenching is performed in the manufacturing process of mechanical structure parts, suppression of fusion cracks is required for steel materials.

[0011] Furthermore, during the manufacturing process of steel materials and during the manufacturing process of mechanical structure parts using such steel materials, hot working (for example, hot rolling, hot forging, etc.) is performed. Therefore, for the steel material used as the raw material of mechanical structure parts, not only suppression of fusion cracks but also suppression of cracks during hot working are required. Here, the cracks during hot working are also referred to as "hot working cracks" in this specification. Therefore, for the steel material used as the raw material of mechanical structure parts, excellent machinability is required, and it is required not only that high fatigue strength can be obtained when the steel material is made into mechanical structure parts, but also suppression of hot working cracks and suppression of fusion cracks are required.

[0012] In the above Patent Documents 1 to 3, at least, no consideration has been given to suppression of hot working cracks and suppression of fusion cracks.

[0013] An object of the present invention is to provide a steel material that has excellent machinability, can suppress cracking during hot working, can suppress fusion cracking during high-frequency quenching, and has excellent fatigue strength when used as a component for mechanical structures.

Means for Solving the Problems

[0014] The steel material of the present disclosure is by mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, Bi: 0.0051 to 0.1500%, and N: 0.0030 to 0.0250%, and contains the balance consists of Fe and impurities, satisfies the formula (1), In the steel material, the number density of fine Bi particles having an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm 2 and the number density of coarse Bi particles having an equivalent circle diameter of 10.0 μm or more is 10 particles / mm 2 or less. 0.25 ≦ C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V ≦ 1.00 (1) Here, for each element symbol in the formula, the content of the corresponding element is substituted in mass%. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0015] The steel material of the present disclosure is by mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, Bi: 0.0051 to 0.1500%, and N: 0.0030 to 0.0250%, contains Furthermore, it contains one or more selected from the group consisting of the first group to the fifth group, The balance consists of Fe and impurities, and further satisfies the formula (1). In the steel material, The number density of fine Bi particles with an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm 2 Yes, The number density of coarse Bi particles with an equivalent circle diameter of 10.0 μm or more is 10 particles / mm or less. 2 The following. [First group] Al: 0.060% or less, and Mg: 0.0100% or less, one or more selected from the group consisting of [Second group] Ti: 0.1500% or less, Nb: 0.0800% or less, W: 0.4000% or less, and Zr: 0.2000% or less, one or more selected from the group consisting of [Third group] Ca: 0.0100% or less, Te: 0.0100% or less, B: 0.0050% or less, Sn: 0.0100% or less, and Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Fourth group] Co: 0.0100% or less, Se: 0.0100% or less, Sb: 0.0100% or less, and In: 0.0100% or less, one or more selected from the group consisting of [Fifth group] V: 0.200% or less, Mo: 1.00% or less, Cu: 0.20% or less, and Ni: 0.20% or less, one or more selected from the group consisting of 0.25 ≦ C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V ≦ 1.00 (1) Here, in each element symbol in the formula, the content of the corresponding element is substituted in mass%. When an element is not contained, "0" is substituted in the corresponding element symbol.

Effects of the Invention

[0016] The steel material of the present disclosure has excellent machinability, can suppress cracking during hot working, can suppress fusion cracking during high-frequency quenching, and can obtain excellent fatigue strength when used as a mechanical structure part.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0018] The inventors first examined the chemical composition of a steel material that is excellent in machinability and has excellent fatigue strength when used as a component for mechanical structures. As a result, the inventors found that when the chemical composition is, by mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, N: 0.0030 to 0.0250%, Al: 0 to 0.060%, Mg: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.0800%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Sn: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, Sb: 0 to 0.0100%, In: 0 to 0.0100%, V: 0 to 0.20%, Mo: 0 to 1.00%, Cu: 0 to 0.20%, and Ni: 0 to 0.20%, and the balance consists of Fe and impurities, there is a possibility that excellent machinability and excellent fatigue strength can be obtained when used as a component for mechanical structures.

[0019] However, simply making the steel material have the above chemical composition does not necessarily result in excellent machinability and excellent fatigue strength when used as a component for mechanical structures. Even if the content of each element in the chemical composition is within the above range, if the hardness of the steel material is high, the machinability of the steel material will decrease. On the other hand, even if the content of each element in the chemical composition is within the above range, if the hardness of the steel material is low, the fatigue strength of the mechanical structure component manufactured from the steel material will be low. Therefore, in order to achieve both the fatigue strength of the mechanical structure component and the machinability of the steel material, it is effective to set the hardness of the steel material, which is the material of the mechanical structure component, within an appropriate range.

[0020] Therefore, the present inventors examined the content of elements that affect the hardness of a steel material in which the content of each element in the chemical composition is within the above range. Among the elements in the above chemical composition, C, Si, Mn, Cr, and V particularly increase the internal hardness of machine structural parts manufactured using the steel material, and as a result, increase the fatigue strength of the machine structural parts. On the other hand, S decreases the internal hardness. Therefore, the present inventors considered that by setting the content of these elements within an appropriate range, it is possible to achieve both an improvement in the machinability of the steel material and an improvement in the fatigue strength of machine structural parts manufactured using the steel material. As a result of further examination, the present inventors found that in a steel material in which the content of each element in the chemical composition is within the above range, if the formula (1) is satisfied, excellent machinability can be obtained in the steel material, and furthermore, excellent fatigue strength can be obtained when used as a machine structural part. 0.25 ≦ C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V ≦ 1.00 (1) Here, in each element symbol in the formula, the content of the corresponding element is substituted in mass%. When an element is not contained, "0" is substituted in the corresponding element symbol.

[0021] Next, the present inventors examined means for suppressing fusion cracking during high-frequency quenching in a steel material in which the content of each element in the chemical composition is within the above range and satisfies the formula (1). First, the present inventors observed the microstructure of the site where fusion cracking occurred in order to identify the cause of fusion cracking in the steel material during high-frequency quenching. As a result, decarburization did not occur at the site where fusion cracking occurred. On the other hand, fusion cracking did not occur at the decarburized site.

[0022] From this result, the present inventors considered that the C content affects the fusion cracking that occurs in the steel material during high-frequency quenching. Specifically, C segregating at the grain boundaries makes it easier for fusion cracking to occur. Therefore, the present inventors examined means for suppressing the segregation of C at the grain boundaries.

[0023] As a result of investigations, the inventors have found that by further containing 0.0051 to 0.1500% of Bi in place of a part of Fe in the above chemical composition, it is possible to suppress the melting crack of the steel material during high-frequency quenching. The reason is considered as follows. If an appropriate amount of Bi is contained, Bi exists as an inclusion in the steel material. Hereinafter, the inclusion composed of Bi is referred to as a Bi particle. The Bi particle suppresses the coarsening of austenite grains in the steel material during high-frequency quenching by the pinning effect. If the Bi particle is fine, the pinning effect is enhanced. During high-frequency quenching, if the austenite grains are maintained fine, the grain boundary area of the austenite grains increases. If the grain boundary area increases, the concentration of C segregated at the austenite grain boundaries per unit area decreases. As a result, the occurrence of melting cracks is suppressed.

[0024] As described above, by containing an appropriate amount of Bi, the occurrence of melting cracks during high-frequency quenching is suppressed. However, it has been found that cracks may occur when hot working is performed on the steel material. The hot working here is, for example, hot rolling performed during the manufacturing process of the steel material, or hot forging performed during the manufacturing process of machine structure parts. Therefore, an investigation was made on the cause of cracks during hot working. As a result, the inventors obtained the following new findings.

[0025] When Bi is contained in the steel material to suppress melting cracks, there may be a case where large Bi particles having an equivalent circle diameter of 10.0 μm or more are generated together with fine Bi particles (Bi inclusions) having an equivalent circle diameter of 1.0 μm or less in the steel material. The large Bi particles are likely to be the starting points of cracks during hot working. Therefore, if the number density of the large Bi particles is too high, cracks (hot working cracks) are likely to occur during hot working.

[0026] As described above, in the steel material containing Bi, although melting cracks during high-frequency quenching are likely to be suppressed, hot working cracks caused by large Bi particles are likely to occur. If the Bi particles in the steel material are fine, melting cracks during high-frequency quenching are suppressed. On the other hand, if the Bi particles in the steel material are large, hot working cracks are likely to occur.

[0027] Based on the above research results, the inventors considered that by ensuring a certain number density of fine Bi particles in the steel material while suppressing the number density of coarse Bi particles in the steel material as much as possible, it is possible to suppress fusion cracking during high-frequency quenching and also suppress hot working cracking. Therefore, further investigation and examination were carried out on the number density of fine Bi particles and the number density of coarse Bi particles that can fully exhibit these effects. As a result, in the steel material with the above chemical composition, on the premise of satisfying the above formula (1), the number density of fine Bi particles with an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm 2 and the number density of coarse Bi particles with an equivalent circle diameter of 10.0 μm or more is 10 particles / mm 2 or less, the inventors found that it is possible to suppress fusion cracking during high-frequency quenching and also suppress hot working cracking.

[0028] The steel material according to the present embodiment completed based on the above findings has the following configuration.

[0029] [1] A steel material, by mass, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, Bi: 0.0051 to 0.1500%, and, N: 0.0030 to 0.0250%, contains the balance consists of Fe and impurities, satisfies formula (1), in the steel material, the number density of fine Bi particles with an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm 2 and, the number density of coarse Bi particles with an equivalent circle diameter of 10.0 μm or more is 10 particles / mm 2 or less, steel material. 0.25 ≦ C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V ≦ 1.00 (1) Here, for each element symbol in the formula, the content of the corresponding element is substituted in mass%. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0030] [2] A steel material, in mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, Bi: 0.0051 to 0.1500%, and, N: 0.0030 to 0.0250%, contains, further contains one or more selected from the group consisting of the first group to the fifth group, the balance consists of Fe and impurities, satisfies formula (1), in the steel material, the number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm 2 and, the number density of coarse Bi particles having a circle equivalent diameter of 10.0 μm or more is 10 particles / mm 2 or less, Steel material. [First group] Al: 0.060% or less, and, Mg: 0.0100% or less, one or more selected from the group consisting of [Second group] Ti: 0.1500% or less, Nb: 0.0800% or less, W: 0.4000% or less, and, Zr: 0.2000% or less, one or more selected from the group consisting of [Third group] Ca: 0.0100% or less, Te: 0.0100% or less, B: less than or equal to 0.0050%, Sn: less than or equal to 0.0100%, and, rare earth elements: less than or equal to 0.0100%, one or more selected from the group consisting of [Group 4] Co: less than or equal to 0.0100%, Se: less than or equal to 0.0100%, Sb: less than or equal to 0.0100%, and, In: less than or equal to 0.0100%, one or more selected from the group consisting of [Group 5] V: less than or equal to 0.200%, Mo: less than or equal to 1.00%, Cu: less than or equal to 0.20%, and, Ni: less than or equal to 0.20%, one or more selected from the group consisting of 0.25 ≦ C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V ≦ 1.00 (1) Here, for each element symbol in the formula, the content of the corresponding element is substituted in mass%. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0031] [3] The steel material according to [2], containing the said Group 1, steel material.

[0032] [4] The steel material according to [2] or [3], containing the said Group 2, steel material.

[0033] [5] The steel material according to any one of [2] to [4], containing the said Group 3, steel material.

[0034] [6] The steel material according to any one of [2] to [5], containing the said Group 4, steel material.

[0035] [7] The steel material according to any one of items [2] to [6], containing the fifth group, steel material.

[0036] Hereinafter, the steel material of the present embodiment will be described in detail. "%" regarding elements means mass% unless otherwise specified.

[0037] [Features of the steel material of the present embodiment] The steel material of the present embodiment satisfies the following Features 1 to 4. (Feature 1) The chemical composition is, in mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, Bi: 0.0051 to 0.1500%, N: 0.0030 to 0.0250%, Al: 0 to 0.060%, Mg: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.0800%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Sn: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, Sb: 0 to 0.0100%, In: 0 to 0.0100% or less, V: 0 to 0.200%, Mo: 0 to 1.00%, Cu: 0 to 0.20%, and Ni: 0 to 0.20% or less, and the balance consists of Fe and impurities. (Feature 2) On the premise that the content of each element is within the range of Feature 1, formula (1) is satisfied. 0.25 ≦ C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V ≦ 1.00 (1) Here, in each element symbol in the formula, the content of the corresponding element is substituted in mass%. When an element is not contained, "0" is substituted in the corresponding element symbol. (Feature 3) In the steel material, the number density of fine Bi particles having an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm2 It is as follows. (Feature 4) In the steel material, the number density of coarse Bi particles with an equivalent circle diameter of 10.0 μm or more is 10 particles / mm 2 or less. Hereinafter, each of Feature 1 to Feature 4 will be described.

[0038] [(Feature 1) Regarding the chemical composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0039] C: 0.05 to 0.30% Carbon (C) increases the hardness of mechanical structure parts manufactured using the steel material and increases the fatigue strength of mechanical structure parts. If the C content is less than 0.05%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.30%, even if the contents of other elements are within the range of this embodiment, C segregates at the grain boundaries. In this case, the C concentration at the grain boundaries increases. When the C concentration increases, the melting point decreases. Therefore, fusion cracking is likely to occur during high-frequency quenching. Therefore, the C content is 0.05 to 0.30%. The preferable lower limit of the C content is 0.08%, more preferably 0.10%, and even more preferably 0.13%. The preferable upper limit of the C content is 0.28%, more preferably 0.25%, and even more preferably 0.23%.

[0040] Si: 0.05 to 0.45% Silicon (Si) deoxidizes the steel in the steelmaking process. Si further increases the hardness of mechanical structure parts and increases the fatigue strength of mechanical structure parts. If the Si content is less than 0.05%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the one hand, Si has a weak affinity for C. Therefore, if the Si content exceeds 0.45%, even if the contents of other elements are within the range of this embodiment, during high-frequency induction heating, C tends to segregate at the grain boundaries rather than within the grains in which Si is dissolved. As a result, melting cracks are likely to occur during high-frequency quenching. Therefore, the Si content is 0.05 to 0.45%. The preferable lower limit of the Si content is 0.07%, more preferably 0.10%, and even more preferably 0.13%. The preferable upper limit of the Si content is 0.43%, more preferably 0.40%, and even more preferably 0.38%.

[0041] Mn: 0.30 - 2.00% Manganese (Mn) deoxidizes steel in the steelmaking process. Mn also has a strong affinity for C. Therefore, during heating, C remains within the grains in which Mn is dissolved. As a result, the segregation of C to the grain boundaries is suppressed, and the occurrence of melting cracks during high-frequency quenching is suppressed. If the Mn content is less than 0.30%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Mn content exceeds 2.00%, even if the contents of other elements are within the range of this embodiment, the hardness of the steel material increases excessively. As a result, the machinability of the steel material deteriorates. Therefore, the Mn content is 0.30 - 2.00%. The preferable lower limit of the Mn content is 0.35%, more preferably 0.40%, even more preferably 0.50%, and even more preferably 0.60%. The preferable upper limit of the Mn content is 1.90%, more preferably 1.70%, even more preferably 1.50%, and even more preferably 1.40%.

[0042] P: 0.030% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries. Therefore, P lowers the melting point of the steel material. As a result, melting cracks are likely to occur during high-frequency quenching. Therefore, the P content is 0.030% or less. Preferably, the P content is as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, considering normal industrial production, the preferable lower limit of the P content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferable upper limit of the P content is 0.028%, more preferably 0.026%, even more preferably 0.023%, and even more preferably 0.020%.

[0043] S: 0.010 - 0.095% Sulfur (S) forms sulfide-based inclusions and improves the machinability of steel materials. If the S content is less than 0.010%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, S lowers the melting point of steel materials. Therefore, if the S content exceeds 0.095%, even if the contents of other elements are within the range of this embodiment, melting cracks are likely to occur during high-frequency quenching. Therefore, the S content is 0.010 - 0.095%. The preferable lower limit of the S content is 0.012%, more preferably 0.015%, even more preferably 0.018%, and even more preferably 0.020%. The preferable upper limit of the S content is 0.080%, more preferably 0.070%, and even more preferably 0.060%.

[0044] Cr: 0.01 - 2.00% Chromium (Cr) enhances the hardenability of steel materials. Therefore, the internal hardness of parts for mechanical structures increases. As a result, the fatigue strength of parts for mechanical structures increases. Cr also has a strong affinity with C. Therefore, during heating, C remains within the grains in which Cr is dissolved. Therefore, the segregation of C to the grain boundaries is suppressed, and the occurrence of fusion cracking during high-frequency hardening is suppressed. Cr further combines with S to form Cr sulfides. In this case, the formation of coarse FeS is suppressed. As a result, the ductility of the steel material during hot working is improved, and hot working cracks are suppressed. If the Cr content is less than 0.01%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 2.00%, even if the contents of other elements are within the range of this embodiment, the hardness of the steel material excessively increases. As a result, the machinability of the steel material deteriorates. Therefore, the Cr content is 0.01 - 2.00%. The preferable lower limit of the Cr content is 0.02%, more preferably 0.04%, still more preferably 0.06%, still more preferably 0.08%, and still more preferably 0.10%. The preferable upper limit of the Cr content is 1.90%, more preferably 1.70%, still more preferably 1.50%, and still more preferably 1.20%.

[0045] Bi: 0.0051 - 0.1500% Bismuth (Bi) forms inclusions (Bi particles) in the steel material. Therefore, the occurrence of fusion cracking during high-frequency hardening is suppressed. Bi further enhances the machinability of the steel material. If the Bi content is less than 0.0051%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Bi content exceeds 0.1500%, even if the contents of other elements are within the range of this embodiment, coarse Bi particles are generated. Coarse Bi particles are likely to become the starting points of cracks during hot working in the manufacturing process of the steel material or during hot working in the manufacturing process of parts for mechanical structures manufactured using the steel material as a raw material. Therefore, hot working cracks are likely to occur. Therefore, the Bi content is 0.0051 to 0.1500%. The preferable lower limit of the Bi content is 0.0080%, more preferably 0.0100%, still more preferably 0.0120%, still more preferably 0.0140%, and still more preferably 0.0160%. The preferable upper limit of the Bi content is 0.1400%, more preferably 0.1350%, and still more preferably 0.1300%.

[0046] N: 0.0030 to 0.0250% Nitrogen (N) forms nitrides and / or carbonitrides during the cooling process after hot working in the manufacturing process of mechanical structure parts, thereby precipitation-strengthening the steel material. As a result, the fatigue strength of the mechanical structure parts is increased. If the N content is less than 0.0030%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the N content exceeds 0.0250%, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material deteriorates. Therefore, the N content is 0.0030 to 0.0250%. The preferable lower limit of the N content is 0.0035%, more preferably 0.0040%, still more preferably 0.0050%, and still more preferably 0.0080%. The preferable upper limit of the N content is 0.0240%, more preferably 0.0230%, still more preferably 0.0200%, still more preferably 0.0180%, and still more preferably 0.0150%.

[0047] The balance of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, the impurities are those mixed in from ores, scraps, or manufacturing environments as raw materials during the industrial production of steel materials, and are not intentionally contained, and are allowed within a range that does not adversely affect the steel material according to this embodiment.

[0048] Examples of impurities include all elements other than the above-mentioned impurities (P, S). The impurities may be only one type, or two or more types. Other impurities other than the above-mentioned impurities are, for example, as follows. O: 0.0050% or less, Ta and Zn: 0 to 0.01% in total, Pb: 0 to 0.09%.

[0049] [Regarding Optional Elements] The chemical composition of this embodiment may further contain one or more selected from the group consisting of the first group to the fifth group in place of a part of Fe. [First Group] Al: 0.060% or less, and Mg: 0.0100% or less, one or more selected from the group consisting of [Second Group] Ti: 0.1500% or less, Nb: 0.0800% or less, W: 0.4000% or less, and Zr: 0.2000% or less, one or more selected from the group consisting of [Third Group] Ca: 0.0100% or less, Te: 0.0100% or less, B: 0.0050% or less, Sn: 0.0100% or less, and Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Fourth Group] Co: 0.0100% or less, Se: 0.0100% or less, Sb: 0.0100% or less, and In: 0.0100% or less, one or more selected from the group consisting of [Fifth Group] V: 0.200% or less, Mo: 1.00% or less, Cu: 0.20% or less, and Ni: 0.20% or less, one or more selected from the group consisting of Hereinafter, each optional element will be described.

[0050] [Group 1: Al and Mg] The chemical composition of the steel material of this embodiment may further contain the above-mentioned Group 1 in place of a part of Fe. These elements are optional elements, and all of them deoxidize the steel. Hereinafter, each element of Group 1 will be described.

[0051] Al: 0.060% or less Aluminum (Al) is an optional element and may not be contained. That is, the Al content may be 0%. When contained, that is, when the Al content exceeds 0%, Al deoxidizes the steel. If even a little Al is contained, the above effect can be obtained to some extent. However, if the Al content exceeds 0.060%, even if the contents of other elements are within the range of this embodiment, Al forms coarse oxides. Coarse oxides reduce the fatigue strength of parts for mechanical structures. Therefore, the Al content is 0 to 0.060%. When contained, the Al content is 0.060% or less. The preferable lower limit of the Al content is 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.010%. The preferable upper limit of the Al content is 0.055%, more preferably 0.050%, and more preferably 0.045%.

[0052] Mg: 0.0100% or less Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, that is, when the Mg content exceeds 0%, Mg deoxidizes the steel. If even a little Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0100%, even if the contents of other elements are within the range of this embodiment, Mg forms coarse oxides. Coarse oxides reduce the fatigue strength of parts for mechanical structures. Therefore, the Mg content is 0 to 0.0100%. When contained, the Mg content is 0.0100% or less. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferable upper limit of the Mg content is 0.0090%, more preferably 0.0070%, even more preferably 0.0050%, and even more preferably 0.0040%.

[0053] [Group 2: Ti, Nb, W, and Zr] The chemical composition of the steel material of this embodiment may further contain the above Group 2 in place of a part of Fe. These elements are optional elements, and all of them form precipitates to enhance the toughness of the parts for mechanical structures. Hereinafter, each element of Group 2 will be described.

[0054] Ti: 0.1500% or less Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, that is, when the Ti content exceeds 0%, Ti forms carbides and / or carbonitrides during the cooling process of the hot working process in the manufacturing process of the parts for mechanical structures, thereby refining the crystal grains. As a result, the toughness of the parts for mechanical structures is increased. Even if a small amount of Ti is contained, the above effect can be obtained to a certain extent. However, if the Ti content exceeds 0.1500%, even if the contents of other elements are within the range of this embodiment, the above effect is saturated and the manufacturing cost increases. Therefore, the Ti content is 0 to 0.1500%. When contained, the Ti content is 0.1500% or less. The preferable lower limit of the Ti content is 0.0001%, more preferably 0.0010%, even more preferably 0.0050%, and even more preferably 0.0080%. The preferable upper limit of the Ti content is 0.1400%, more preferably 0.1200%, still more preferably 0.1000%, still more preferably 0.0500%, still more preferably 0.0200%, and still more preferably 0.0150%.

[0055] Nb: 0.0800% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb forms carbides and / or carbonitrides during the cooling process of the hot working process in the manufacturing process of the machine structural parts, thereby refining the crystal grains. As a result, the toughness of the machine structural parts is increased. Even if a little Nb is contained, the above effects can be obtained to a certain extent. However, if the Nb content exceeds 0.0800%, even if the contents of other elements are within the scope of this embodiment, the above effects are saturated and the manufacturing cost becomes high. Therefore, the Nb content is 0 to 0.0800%. When contained, the Nb content is 0.0800% or less. The preferable lower limit of the Nb content is 0.0001%, more preferably 0.0010%, still more preferably 0.0050%, and still more preferably 0.0080%. The preferable upper limit of the Nb content is 0.0700%, more preferably 0.0600%, still more preferably 0.0500%, still more preferably 0.0200%, and still more preferably 0.0150%.

[0056] W: 0.4000% or less Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, that is, when the W content exceeds 0%, W forms carbides and / or carbonitrides during the cooling process of the hot working process in the manufacturing process of the machine structural parts, thereby refining the crystal grains. As a result, the toughness of the machine structural parts is increased. Even if a little W is contained, the above effects can be obtained to a certain extent. However, if the W content exceeds 0.4000%, even if the contents of other elements are within the scope of this embodiment, the above effects will saturate and the manufacturing cost will increase. Therefore, the W content is 0 to 0.4000%. When contained, the W content is 0.4000% or less. The preferable lower limit of the W content is 0.0001%, more preferably 0.0050%, and even more preferably 0.0500%. The preferable upper limit of the W content is 0.3500%, more preferably 0.3000%, and even more preferably 0.2000%.

[0057] Zr: 0.2000% or less Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, that is, when the Zr content exceeds 0%, Zr forms carbides and / or carbonitrides during the cooling process of the hot working process in the manufacturing process of mechanical structure parts, thereby refining the crystal grains. As a result, the toughness of the mechanical structure parts is increased. Even if a small amount of Zr is contained, the above effects can be obtained to a certain extent. However, if the Zr content exceeds 0.2000%, even if the contents of other elements are within the scope of this embodiment, the above effects will saturate and the manufacturing cost will increase. Therefore, the Zr content is 0 to 0.2000%. When contained, the Zr content is 0.2000% or less. The preferable lower limit of the Zr content is 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0050%. The preferable upper limit of the Zr content is 0.1500%, more preferably 0.1000%, even more preferably 0.0500%, and even more preferably 0.0100%.

[0058] [Group 3: Ca, Te, B, Sn and rare earth elements] The chemical composition of the steel material of this embodiment may further contain the above-mentioned Group 3 in place of a part of Fe. These elements are optional elements, and all of them improve the machinability of the steel material. Hereinafter, each element of Group 3 will be described.

[0059] Ca: 0.0100% or less Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca improves the machinability of the steel material. If even a little Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0100%, even if the contents of other elements are within the range of this embodiment, coarse oxides are formed. In this case, the fatigue strength of the parts for mechanical structures decreases. Therefore, the Ca content is 0 to 0.0100%. When contained, the Ca content is 0.0100% or less. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0015%. The preferable upper limit of the Ca content is 0.0090%, more preferably 0.0070%, even more preferably 0.0050%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0060] Te: 0.0100% or less Tellurium (Te) is an optional element and may not be contained. That is, the Te content may be 0%. When contained, that is, when the Te content exceeds 0%, Te improves the machinability of the steel material. If even a little Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.0100%, even if the contents of other elements are within the range of this embodiment, hot working cracks are likely to occur in the steel material. Therefore, the Te content is 0 to 0.0100%. When contained, the Te content is 0.0100% or less. The preferable lower limit of the Te content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0010%. The preferable upper limit of the Te content is 0.0090%, more preferably 0.0085%, and even more preferably 0.0080%.

[0061] B: 0.0050% or less Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B improves the machinability of the steel material. Even if a small amount of B is contained, the above effect can be obtained to a certain extent. However, if the B content exceeds 0.0050%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the range of this embodiment. Therefore, the B content is 0 to 0.0050%. When contained, the B content is 0.0050% or less. The preferable lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0062] Sn: 0.0100% or less Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. When contained, that is, when the Sn content exceeds 0%, Sn improves the machinability of the steel material. Even if a small amount of Sn is contained, the above effect can be obtained to a certain extent. However, if the Sn content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the range of this embodiment. Therefore, the Sn content is 0 to 0.0100%. When contained, the Sn content is 0.0100% or less. The preferable lower limit of the Sn content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the Sn content is 0.0095%, more preferably 0.0090%, even more preferably 0.0085%, and even more preferably 0.0080%.

[0063] Rare earth elements: 0.0100% or less The rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When contained, that is, when the REM content exceeds 0%, REM enhances the machinability of the steel material. Even if a small amount of REM is contained, the above effect can be obtained to a certain extent. However, if the REM content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the range of this embodiment. Therefore, the REM content is 0 to 0.0100%. When contained, the REM content is 0.0100% or less. The preferable lower limit of the REM content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the REM content is 0.0090%, more preferably 0.0070%, and even more preferably 0.0055%.

[0064] In this specification, REM is one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 which are lanthanoids. Also, the REM content in this specification is the total content of these elements.

[0065] [Group 4: Co, Se, Sb and In] The chemical composition of the steel material of this embodiment may further contain the above-mentioned Group 4 in place of a part of Fe. These elements are optional elements, and all of them suppress decarburization of the steel material. Hereinafter, each element of Group 4 will be described.

[0066] Co: 0.0100% or less Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When contained, that is, when the Co content exceeds 0%, Co suppresses decarburization of the steel material during hot working. If even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the range of this embodiment. Therefore, the Co content is 0 to 0.0100%. When contained, the Co content is 0.0100% or less. The preferable lower limit of the Co content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the Co content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0067] Se: 0.0100% or less Selenium (Se) is an optional element and may not be contained. That is, the Se content may be 0%. When contained, that is, when the Se content exceeds 0%, Se suppresses decarburization of the steel material during hot working. If even a small amount of Se is contained, the above effect can be obtained to some extent. However, if the Se content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the range of this embodiment. Therefore, the Se content is 0 to 0.0100%. When contained, the Se content is 0.0100% or less. The preferable lower limit of the Se content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the Se content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0068] Sb: 0.0100% or less Antimony (Sb) is an optional element and may not be contained. That is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb suppresses decarburization of the steel material during hot working. If even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the range of this embodiment. Therefore, the Sb content is 0 to 0.0100%. When contained, the Sb content is 0.0100% or less. The preferable lower limit of the Sb content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the Sb content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0069] In: 0.0100% or less Indium (In) is an optional element and may not be contained. That is, the In content may be 0%. When contained, that is, when the In content exceeds 0%, In suppresses decarburization of the steel material during hot working. If even a small amount of In is contained, the above effect can be obtained to some extent. However, if the In content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the range of this embodiment. Therefore, the In content is 0 to 0.0100%. When contained, the In content is 0.0100% or less. The preferable lower limit of the In content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the In content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0070] [Group 5: V, Mo, Cu, and Ni] The chemical composition of the steel material of this embodiment may further contain the above-mentioned Group 5 in place of a part of Fe. These elements are optional elements, and all of them increase the fatigue strength of mechanical structure parts. Hereinafter, each element of Group 5 will be described.

[0071] V: 0.200% or less Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, that is, when the V content exceeds 0%, V forms precipitates and increases the fatigue strength of mechanical structure parts. V further combines with C to fix C in the austenite grains. Therefore, V suppresses the occurrence of fusion cracking during high-frequency quenching. Even if a little V is contained, the above effects can be obtained to a certain extent. However, if the V content exceeds 0.200%, even if the contents of other elements are within the range of this embodiment, the hardness of the steel material will increase excessively. As a result, the machinability of the steel material decreases. Therefore, the V content is 0 to 0.200%. When contained, the V content is 0.200% or less. The preferable lower limit of the V content is 0.001%, more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.015%. The preferable upper limit of the V content is 0.195%, more preferably 0.190%, even more preferably 0.185%, and even more preferably 0.150%.

[0072] Mo: 1.00% or less Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When it is contained, that is, when the Mo content exceeds 0%, Mo increases the fatigue strength of the machine structural parts. If even a little Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 1.00%, even if the contents of other elements are within the scope of this embodiment, the hardness of the steel material will increase excessively. As a result, the hot workability deteriorates. Therefore, the Mo content is 0 to 1.00%. When it is contained, the Mo content is 1.00% or less. The preferable lower limit of the Mo content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferable upper limit of the Mo content is 0.90%, more preferably 0.80%, even more preferably 0.60%, and even more preferably 0.40%.

[0073] Cu: 0.20% or less Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When it is contained, that is, when the Cu content exceeds 0%, Cu increases the fatigue strength of the machine structural parts. If even a little Cu is contained, the above effect can be obtained to some extent. However, like Si, Cu promotes the occurrence of fusion cracking during high-frequency quenching. Therefore, if the Cu content exceeds 0.20%, even if the contents of other elements are within the scope of this embodiment, fusion cracking is likely to occur during high-frequency quenching. Therefore, the Cu content is 0 to 0.20%. When it is contained, the Cu content is 0.20% or less. The preferable lower limit of the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the Cu content is 0.15%, more preferably 0.13%, and even more preferably 0.10%.

[0074] Ni: below 0.20% Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni increases the fatigue strength of the parts for mechanical structures. Even if a little Ni is contained, the above effects can be obtained to a certain extent. However, Ni, like Si and Cu, promotes the occurrence of fusion cracking during high-frequency quenching. Therefore, if the Ni content exceeds 0.20%, even if the contents of other elements are within the range of this embodiment, fusion cracking is likely to occur during high-frequency quenching. Therefore, the Ni content is 0 to 0.20%. When contained, the Ni content is 0.20% or less. The preferable lower limit of the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the Ni content is 0.15%, more preferably 0.13%, and even more preferably 0.10%.

[0075] [Method for Measuring Chemical Composition of Steel Material] The chemical composition of the steel material of this embodiment can be measured by a well-known component analysis method conforming to JIS G0321:2017. Specifically, using a drill, chips are collected from inside the steel material with a depth of 1 mm or more from the surface. The collected chips are dissolved in an acid to obtain a solution. For the solution, ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed to conduct elemental analysis of the chemical composition. The C content and S content are determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas fusion - thermal conductivity method. The O content is determined using a well-known inert gas fusion - infrared absorption method.

[0076] In addition, for each element content, based on the significant figures defined in this embodiment, the fractional part of the measured value is rounded off to obtain a value up to the least significant digit of each element content defined in this embodiment. For example, the C content of the steel material in this embodiment is defined by a value up to the second decimal place. Therefore, the C content is a value up to the second decimal place obtained by rounding off the third decimal place of the measured value. Similarly, for the content of elements other than the C content of the steel material in this embodiment, the value obtained by rounding off the fractional part of the measured value to the least significant digit defined in this embodiment is taken as the content of that element. Note that rounding off means discarding if the fractional part is less than 5 and rounding up if the fractional part is 5 or more.

[0077] [(Feature 2) Regarding formula (1)] On the premise that the content of each element of the steel material in this embodiment is within the above range, the steel material in this embodiment satisfies formula (1). 0.25 ≦ C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V ≦ 1.00 (1) Here, for each element symbol in the formula, the content of the corresponding element is substituted in mass%. When an element is not contained, "0" is substituted for the corresponding element symbol. That is, when the optional element V is not contained, formula (1) becomes as follows. 0.25 ≦ C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) ≦ 1.00 (1)

[0078] Define fn1 as follows. fn1 = C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V Note that when the optional element V is not contained, fn1 becomes as follows. fn1 = C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22)

[0079] fn1 is an index of the hardness of the steel material. C, Si, Mn, Cr, and V increase the internal hardness of machine structural parts manufactured from the steel material. On the other hand, S decreases the internal hardness of machine structural parts.

[0080] Even if the content of each element in the steel material is within the range of this embodiment, if fn1 is less than 0.25, the internal hardness of the machine structural parts will excessively decrease. As a result, the fatigue strength of the machine structural parts decreases. On the other hand, even if the content of each element in the steel material is within the range of this embodiment, if fn1 exceeds 1.00, the hardness of the steel material will excessively increase. As a result, the machinability of the steel material decreases. Therefore, fn1 is 0.25 to 1.00. The preferable lower limit of fn1 is 0.28, more preferably 0.30, and even more preferably 0.33. The preferable upper limit of fn1 is 0.98, more preferably 0.95, and even more preferably 0.90.

[0081] [(Feature 3) Regarding fine Bi particles] In the steel material of this embodiment, on the premise of satisfying Feature 1 and Feature 2, the number density of fine Bi particles (hereinafter, also simply referred to as fine Bi particles) having an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm 2 is. If the number density of the fine Bi particles is 80 to 8000 particles / mm 2 is, the occurrence of fusion cracking during high-frequency quenching is suppressed.

[0082] Bi exists in the steel material in the form of particles of Bi alone or particles containing Bi. In this specification, particles of Bi alone or particles containing Bi are collectively defined as Bi particles. In this specification, fine Bi particles mean Bi particles having an equivalent circle diameter of 0.1 to 1.0 μm. Since Bi is a heavy element, Bi particles are observed with higher brightness than the surroundings in the backscattered electron image. The fine Bi particles may exist alone in the steel material without contacting other particles (precipitates or inclusions). Also, the fine Bi particles may exist in the steel material by adhering to or contacting other particles.

[0083] As described above, during high-frequency induction heating, Bi particles pin the austenite grain boundaries. If the equivalent circle diameter of the Bi particles is 0.1 to 1.0 μm, the pinning effect on the austenite grain boundaries is enhanced. During high-frequency quenching, if the austenite grains are maintained fine, the grain boundary area of the austenite grains increases. If the grain boundary area increases, the concentration of C segregating at the grain boundaries decreases. As a result, the occurrence of fusion cracking is suppressed. Even when the steel material satisfies Feature 1 and Feature 2 and further satisfies Feature 4 described later, if the number of fine Bi particles is less than 80 particles / mm 2 , the above effects cannot be obtained sufficiently.

[0084] On the other hand, even when the steel material satisfies Feature 1, Feature 2, and Feature 4, if the number density of the fine Bi particles exceeds 8000 particles / mm 2 , the above effects are saturated, and furthermore, the manufacturing cost increases.

[0085] Therefore, in the steel material of the present embodiment, the number density of the fine Bi particles having an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm 2 .

[0086] The preferable lower limit of the number density of the fine Bi particles is 90 particles / mm 2 , more preferably 95 particles / mm 2 , and even more preferably 100 particles / mm 2 . The preferable upper limit of the number density of the fine Bi particles is 7900 particles / mm 2 , more preferably 6000 particles / mm 2 , and even more preferably 3000 particles / mm 2 , and even more preferably 1000 particles / mm 2 , and even more preferably 900 particles / mm 2 , and even more preferably 800 particles / mm 2 .

[0087] [(Feature 4) Regarding Coarse Bi Particles] In the steel material of the present embodiment, the number density of coarse Bi particles which are Bi particles having an equivalent circle diameter of 10.0 μm or more (hereinafter, also simply referred to as coarse Bi particles) is 10 particles / mm 2 or less. If the number density of the coarse Bi particles is 10 particles / mm 2 or less, cracking (hot working crack) during hot working in the manufacturing process of the steel material or during hot working in the manufacturing process of machine structural parts made of the steel material can be suppressed. The hot working is, for example, hot rolling, hot forging, or the like.

[0088] In this specification, the coarse Bi particles mean Bi particles having an equivalent circle diameter of 10.0 μm or more. In the method for measuring the number density of the coarse Bi particles described later, if the equivalent circle diameter of the particle is 10.0 μm or more and the particle is observed with higher brightness than the surroundings in the backscattered electron image, the particle is determined to be a coarse Bi particle. The coarse Bi particles may exist alone in the steel material without contacting other particles (precipitates or inclusions). Further, the coarse Bi particles may adhere to or contact other particles and exist in the steel material. The upper limit of the equivalent circle diameter of the coarse Bi particles is not particularly limited, but in the case of the chemical composition of the present embodiment, the upper limit of the equivalent circle diameter of the coarse Bi particles is 50.0 μm.

[0089] As described above, the melting crack during high-frequency quenching is suppressed by the fine Bi particles in the steel material. However, Bi in the steel material may form coarse Bi particles instead of fine Bi particles. The coarse Bi particles can be a starting point for hot working cracks in the steel material.

[0090] Even if the steel material satisfies Features 1 to 3, if the number of coarse Bi particles exceeds 10 particles / mm 2 hot working cracks may occur in the steel material. Therefore, in the steel material of the present embodiment, the number density of coarse Bi particles having an equivalent circle diameter of 10.0 μm or more is 10 particles / mm 2 or less.

[0091] The preferable upper limit of the number density of the coarse Bi particles is 8 particles / mm 2 and more preferably 7 particles / mm 2 and still more preferably 6 particles / mm 2and more preferably 5 particles / mm 2 is satisfied.

[0092] The number density of the coarse Bi particles is preferably as low as possible. That is, the number density of the coarse Bi particles is 0 particles / mm 2 is preferable. However, excessive reduction in the number density of the coarse Bi particles increases the manufacturing cost. Therefore, considering normal industrial productivity, the preferable lower limit of the number density of the coarse Bi particles is 1 particle / mm 2 and more preferably 2 particles / mm 2 is satisfied.

[0093] In addition, in the steel material of the present embodiment, not only the above-described fine Bi particles and coarse Bi particles but also intermediate Bi particles having a size exceeding 1.0 μm and less than 10.0 μm (hereinafter also simply referred to as intermediate Bi particles) may be present. However, the intermediate Bi particles hardly affect hot working cracks and melting cracks during high-frequency quenching. Therefore, in suppressing hot working cracks and melting cracks, the intermediate Bi particles do not need to be considered.

[0094] [Method for measuring the number density of fine Bi particles and coarse Bi particles] The number density of the fine Bi particles and the coarse Bi particles can be measured by the following method. Among the cross-sections perpendicular to the axial direction (rolling direction) of the steel material (bar steel), a test piece including the R / 2 part is sampled. Here, the R / 2 part means the central part of the radius R in the cross-section perpendicular to the axial direction of the steel material. Of the surface of the sampled test piece, the surface corresponding to the cross-section perpendicular to the axial direction of the steel material is defined as the observation surface. The observation surface is mirror-polished. Using a scanning electron microscope (SEM), observe 20 fields of the R / 2 part of the mirror-polished observation surface at a magnification of 1000 times. The area of each field is 100 μm × 120 μm.

[0095] Based on the backscattered electron images of each field of view obtained by SEM observation, the number density of coarse Bi particles and fine Bi particles is examined using a well-known particle analysis method of image analysis. Specifically, the particles in the steel material are identified based on the interface between the matrix phase and the particles of the steel material. The particles referred to here are inclusions or precipitates. Image analysis is performed to obtain the equivalent circle diameter of the identified particles. Specifically, the area of each identified particle is obtained. The diameter of a circle with the same area as the obtained area is defined as the equivalent circle diameter (μm) of the particle.

[0096] Since Bi is a heavy element, it is observed with high brightness in the backscattered electron image. Therefore, among the particles observed in the backscattered electron image obtained by the above SEM observation, particles with an equivalent circle diameter of 0.1 to 1.0 μm and observed with higher brightness than the surroundings are identified as fine Bi particles. Also, among the particles observed in the backscattered electron image obtained by SEM observation, particles with an equivalent circle diameter of 10.0 μm or more and observed with higher brightness than the surroundings are identified as coarse Bi particles.

[0097] In the examples described later, as a result of analyzing the chemical compositions of the fine Bi particles and coarse Bi particles identified by the above method using an energy dispersive X-ray analyzer (EDX: Energy Dispersive X-ray spectroscopy) equipped with the SEM, it has been confirmed that all are Bi particles. The beam diameter of the EDX at the time of confirmation was 0.1 to 1.0 μm.

[0098] The fine Bi particles and coarse Bi particles are identified by the above method. Based on the total number of fine Bi particles identified in each field of view and the total area of 20 fields of view (0.24 mm 2 ), the number of fine Bi particles per unit area (particles / mm 2 ) is obtained. Also, based on the total number of coarse Bi particles identified in each field of view and the total area of 20 fields of view (0.24 mm 2 ), the number of coarse Bi particles per unit area (particles / mm 2 ) is obtained.

[0099] [Effect of the steel material of the present embodiment] As described above, the steel material of the present embodiment satisfies Features 1 to 4. Therefore, in the steel material of the present embodiment, the machinability is excellent, cracking during hot working and fusion cracking during high-frequency quenching can be suppressed, and it has excellent fatigue strength when used as a mechanical structure part.

[0100] [Preferred uses of the steel material of the present embodiment] The steel material of the present embodiment can be widely applied, for example, as a material for mechanical structure parts. The steel material of the present embodiment is particularly suitable when high-frequency quenching is carried out in the manufacturing process of mechanical structure parts. However, even when high-frequency quenching is not carried out, the steel material of the present embodiment can be applied as a material for mechanical structure parts.

[0101] [Manufacturing method] An example of the manufacturing method of the steel material of the present embodiment will be described. The manufacturing method of the steel material described hereinafter is an example for manufacturing the steel material according to the present embodiment. Therefore, the steel material having the above-described configuration may be manufactured by other manufacturing methods other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferred example of the manufacturing method of the steel material according to the present embodiment.

[0102] An example of the manufacturing method of the steel material according to the present embodiment includes the following steps. (Step 1) Refining step (Step 2) Casting step (Step 3) Hot working step Note that the hot working step is an optional step. Hereinafter, each step will be described.

[0103] [(Step 1) Refining step] In the refining step, molten steel having a chemical composition that satisfies the above-described Features 1 and 2 is produced. The refining step includes a primary refining step and a secondary refining step. In the primary refining process, the molten pig iron produced by a well-known method is refined in a converter. In the secondary refining process, alloying elements are added to the molten steel so that the chemical composition of the molten steel satisfies Feature 1 and Feature 2. Specifically, in the secondary refining process, while stirring the molten steel by a well-known refining method, the component adjustment of the molten steel other than Bi is carried out. Then, while stirring the molten steel, Bi is added to the molten steel by wire, and the component adjustment of Bi is performed.

[0104] In the secondary refining process, the following conditions are satisfied. (Condition) After adding Bi to the molten steel, the time T from the addition of Bi to the end of stirring in the secondary refining process is set to be more than 15 minutes and less than 60 minutes.

[0105] In the secondary refining process, after adding Bi, the time from the addition of Bi to the end of stirring in the secondary refining process is more than 15 minutes and less than 60 minutes. When the time from the addition of Bi to the end of stirring in the secondary refining process is 15 minutes or less, Bi does not diffuse sufficiently in the molten steel. In this case, excessive amounts of coarse Bi particles are generated in the steel material. When the time from the addition of Bi to the end of stirring in the secondary refining process is 60 minutes or more, the fine Bi particles are likely to aggregate. Therefore, the number density of the fine Bi particles decreases.

[0106] In the secondary refining process, if the time from the addition of Bi to the end of stirring in the secondary refining process is more than 15 minutes, Bi diffuses sufficiently in the molten steel. Therefore, sufficient fine Bi particles are generated in the steel material. Further, in the secondary refining process, if the time from the addition of Bi to the end of stirring in the secondary refining process is less than 60 minutes, the aggregation of the fine Bi particles can be sufficiently suppressed. Therefore, the number density of the fine Bi particles is 80 particles / mm 2 or more, and the number density of the coarse Bi particles is 10 particles / mm 2 or less.

[0107] After adding Bi to the molten steel, the preferable upper limit of the time from the addition of Bi to the end of stirring in the secondary refining process is 50 minutes, and more preferably 40 minutes. After adding Bi, the preferable lower limit of the time from the addition of Bi to the end of stirring in the secondary refining process is 20 minutes, and more preferably 30 minutes.

[0108] After adding Bi, the temperature of the molten steel until the end of stirring in the secondary refining process is 1510 - 1560°C.

[0109] [(Process 2) Casting Process] In the casting process, using the molten steel, a slab or bloom (casting piece) or an ingot (steel ingot) is manufactured by a well-known casting method. The casting method is, for example, the continuous casting method or the ingot-making method.

[0110] [(Process 3) Hot Working Process] The hot working process is an optional process. That is, the hot working process may or may not be carried out. When the hot working process is carried out, in the hot working process, hot working is carried out on the slab or ingot manufactured in the above casting process to manufacture the steel material of the present embodiment. The steel material of the present embodiment is, for example, a bar. The hot working process may be, for example, hot rolling or hot forging.

[0111] When hot rolling is carried out in the hot working process, for example, only the rough rolling process may be carried out, or the rough rolling process and the finish rolling process may be carried out. The rough rolling process is, for example, block rolling. The finish rolling process is, for example, finish rolling using a continuous rolling mill. In a continuous rolling mill, for example, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. The heating temperature in the rough rolling process and the finish rolling process is, for example, 1000 - 1300°C.

[0112] Through the above manufacturing process, the steel material of the present embodiment is manufactured. As described above, the hot working process may be omitted in this manufacturing method. That is, the steel material of the present embodiment may be a casting (slab or ingot). Also, the steel material of the present embodiment may be manufactured by carrying out the hot working process.

[0113] [Manufacturing Method of Machine Structure Parts] As described above, the steel material of the present embodiment serves as a material for mechanical structure parts. The mechanical structure parts are, for example, parts for automotive applications. The mechanical structure parts are, for example, parts of the undercarriage, axles, crankshafts, etc.

[0114] The mechanical structure parts using the steel material of the present embodiment are manufactured, for example, by the following well-known manufacturing methods.

[0115] First, the steel material of the present embodiment is hot-worked to produce an intermediate product with a rough shape of the mechanical structure part. The hot working is, for example, hot forging. The produced intermediate product is cut into a predetermined shape by machining. High-frequency quenching and tempering are performed on the machined intermediate product. Through the above steps, the mechanical structure parts are manufactured.

[0116] In the steel material of the present embodiment, the content of each element in the chemical composition is within the range of the present embodiment and satisfies formula (1). Furthermore, the number density of fine Bi particles with an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8000 particles / mm 2 and the number density of coarse Bi particles with an equivalent circle diameter of 10.0 μm or more is 10 particles / mm 2 or less. That is, the steel material of the present embodiment satisfies features 1 to 4. Therefore, excellent machinability can be obtained in the steel material of the present embodiment. Furthermore, excellent fatigue strength can be obtained in the mechanical structure parts manufactured using the steel material of the present embodiment. Furthermore, hot working cracks during the manufacturing process of the steel material or during the manufacturing process of the mechanical structure parts are suppressed. Furthermore, when manufacturing mechanical structure parts using the steel material of the present embodiment, even if high-frequency quenching is performed, melting cracks are suppressed.

Examples

[0117] The effects of the steel material of the present embodiment will be further specifically described by examples. The conditions in the following examples are one example of the conditions adopted to confirm the feasibility and effects of the steel material of the present embodiment. Therefore, the steel material of the present embodiment is not limited to this one example of conditions.

[0118] Steel materials having the chemical compositions shown in Tables 1 to 4 were produced. Table 2 is a continuation of Table 1, and Table 4 is a continuation of Table 3. In any of the test numbers, the O (oxygen) content was 0.0050% or less by mass. Also, the total content of Ta and Zn was 0 to 0.01% by mass. Further, the Pb content was 0 to 0.09% by mass.

[0119]

Table 1

[0120]

Table 2

[0121]

Table 3

[0122]

Table 4

[0123] Specifically, using a 70-ton converter, a refining process (primary refining process and secondary refining process) was carried out. In the primary refining process, refining in the converter was carried out under the same conditions for the hot metal produced by a well-known method. In the secondary refining process, elements were added to produce molten steel having the chemical compositions shown in Tables 1 to 4. Specifically, secondary refining was carried out by a well-known method, and while stirring the molten steel, the components of elements other than Bi were adjusted. Thereafter, while further stirring the molten steel, Bi was added by wire to adjust the component of Bi in the molten steel.

[0124] After adding Bi to the molten steel, the time T (minutes) until the end of stirring in the secondary refining was as shown in Tables 5 and 6. In addition, after adding Bi to the molten steel, the temperature of the molten steel until the end of stirring in the secondary refining was 1510 to 1560 °C. By the above process, molten steel having the chemical compositions shown in Tables 1 to 4 was produced.

[0125]

Table 5

[0126]

Table 6

[0127] Using molten steel, a slab (bloom) with a cross-section of 300 mm × 400 mm was produced by continuous casting. After heating this slab, the slab was block-rolled to produce billets with a cross-section of 180 mm × 180 mm. The heating temperature of the slab during block rolling was 1250°C.

[0128] Furthermore, hot forging equivalent to finish rolling was performed on the billets to produce steel bars with a diameter of 80 mm. The heating temperature of the billets during hot forging was 1250°C. Through the above manufacturing process, steel materials were produced.

[0129] [Evaluation Test] The following evaluation tests were performed on the steel materials of each test number. (Test 1) Chemical Composition Analysis Test (Test 2) Hot Work Cracking Evaluation Test (Test 3) Measurement Test of Number Densities of Fine Bi Particles and Coarse Bi Particles (Test 4) Melting Cracking Evaluation Test (Test 5) Machinability Evaluation Test (Drill Life Test) (Test 6) Fatigue Strength Evaluation Test (Rotating Bending Fatigue Test) The following describes each evaluation test.

[0130] [(Test 1) Chemical Composition Analysis Test] For the steel materials of each test number, the chemical composition was measured by a well-known component analysis method described in the above [Measurement Method of Chemical Composition of Steel Materials]. As a result, the chemical composition of the steel materials of each test number was as described in Tables 1 to 4.

[0131] [(Test 2) Hot Work Cracking Evaluation Test] The surface of the steel materials for each test number produced was visually observed. As a result of the visual observation, when three or more distinct cracks per meter in the longitudinal direction of the steel material were observed on the surface of the steel material, it was determined that hot working cracks had occurred. As a result of the visual observation, when three or more distinct cracks per meter in the longitudinal direction of the steel material were not observed on the surface of the steel material, it was determined that hot working cracks were suppressed.

[0132] The evaluation results of hot working cracks are shown in the "Hot Working Cracks" column of Tables 5 and 6. When hot working cracks are suppressed, it is denoted as "E" (Excellent). When hot working cracks occur, it is denoted as "NA" (Not Accepted).

[0133] [(Tests 3 to 6) Number density measurement test of fine Bi particles and coarse Bi particles, fusion crack evaluation test, machinability evaluation test, and fatigue strength evaluation test] [Manufacture of simulated intermediate products for machine structural parts] Heat treatment simulating hot forging during the manufacturing process of machine structural parts made of steel materials with each test number was carried out. Specifically, the steel material was heated and held at 1100 °C for 30 minutes. Then, the steel material was air-cooled. The steel material subjected to the above heat treatment is hereinafter referred to as "simulated intermediate product for machine structural parts (or simply simulated intermediate product)". The simulated intermediate product for machine structural parts was a round steel bar with a diameter of 80 mm.

[0134] [(Test 3) Number density measurement test of fine Bi particles and coarse Bi particles] [Number density of fine Bi particles and coarse Bi particles in steel materials] Using the steel materials with each test number, based on the method described in the above [Method for measuring the number density of fine Bi particles and coarse Bi particles], the number density of fine Bi particles (particles / mm 2 ), and the number density of coarse Bi particles (particles / mm 2 ) of the steel materials with each test number were determined. Among the cross-sections (transverse sections) perpendicular to the axial direction (rolling direction) of the steel material, test pieces including the R / 2 part were sampled. Using this test piece, the number density of fine Bi particles (particles / mm 2 ), and the number density of coarse Bi particles (particles / mm2 ) was determined. The results of the number density of the obtained fine Bi particles are shown in the "Fine Bi Particle Number Density (particles / mm 2 )" column in the "Steel Material" columns of Tables 5 and 6. The results of the number density of the obtained coarse Bi particles are shown in the "Coarse Bi Particle Number Density (particles / mm 2 )" column in the "Steel Material" columns of Tables 5 and 6.

[0135] [Number Density of Fine and Coarse Bi Particles in Simulated Intermediate Products] Using the simulated intermediate products of the mechanical structure parts for each test number, based on the method described in the above [Measurement Method of Number Density of Fine and Coarse Bi Particles], the number density of fine Bi particles (particles / mm 2 ) and the number density of coarse Bi particles (particles / mm 2 ) were determined. Among the cross-sections (transverse sections) perpendicular to the axial direction (rolling direction) of the simulated intermediate products (bar steel), test pieces including the R / 2 part were collected. Using this test piece, the number density of fine Bi particles (particles / mm 2 ) and the number density of coarse Bi particles (particles / mm 2 ) for each test number were determined. The results of the number density of the obtained fine Bi particles are shown in the "Fine Bi Particle Number Density (particles / mm 2 )" column in the "Simulated Intermediate Product" columns of Tables 5 and 6. The results of the number density of the obtained coarse Bi particles are shown in the "Coarse Bi Particle Number Density (particles / mm 2 )" column in the "Simulated Intermediate Product" columns of Tables 5 and 6.

[0136] Note that the simulated intermediate products of the mechanical structure parts were manufactured by performing a heat treatment that simulates hot forging on the steel material as the raw material. Performing only the heat treatment at 1100 °C that simulates hot forging does not affect the number density of fine Bi particles and the number density of coarse Bi particles. Therefore, the number density of fine Bi particles and the number density of coarse Bi particles of the simulated intermediate products of the mechanical structure parts are substantially the same as the number density of fine Bi particles and the number density of coarse Bi particles of the steel material.

[0137] [(Test 4) Molten Crack Evaluation Test] A test piece with a width of 10 mm, a thickness of 3 mm, and a length of 10 mm was taken from the R / 2 part of a cross-section perpendicular to the axial direction (rolling direction) of the simulated intermediate product of the mechanical structure part. The longitudinal direction of the test piece was parallel to the axial direction (rolling direction) of the simulated intermediate product of the mechanical structure part. Also, the central axis parallel to the longitudinal direction of the test piece coincided with the R / 2 part.

[0138] Using a thermal cycle test device manufactured by Fuji Denpa Koki Co., Ltd., a simulation test of high-frequency quenching was carried out on the test piece. Specifically, the test piece was heated to 1390 °C at a heating rate of 100 °C / second using a high-frequency coil. Then, the test piece was held at 1390 °C for 15 seconds. After that, the test piece was water-cooled.

[0139] The water-cooled test piece was cut in a direction perpendicular to the longitudinal direction at the central position in the longitudinal direction of the test piece. Then, the cut surface was used as the observation surface. The observation surface was mechanically polished. The mechanically polished observation surface was corroded with Picral reagent. The corroded observation surface was observed with an optical microscope at 400 times magnification, and the presence or absence of fusion cracking was visually confirmed. The number of observation fields was two. Each observation field was 250 μm × 400 μm.

[0140] If a clearly corroded area (corroded area) with a width of 5 μm or more at the grain boundary was observed in at least one of the two observation fields of the observation surface, it was judged that fusion cracking had occurred. The clearly corroded area with a width of 5 μm or more at the grain boundary means an area such as the corroded area 10 at the grain boundary in the field of view, where the maximum width is 5 μm or more, as shown in Fig. 1. On the other hand, as shown in Fig. 2, if no corroded area was observed at the grain boundary in either of the two observation fields, it was judged that fusion cracking was suppressed.

[0141] The evaluation results of fusion cracking are shown in the "Fusion Cracking" column of Tables 5 and 6. When fusion cracking is suppressed, it is denoted by "E". When fusion cracking occurs, it is denoted by "NA". The "-" in the "Fusion Cracking" column of Table 6 means that the fusion cracking evaluation test has not been carried out.

[0142] [(Test 5) Machinability Evaluation Test (Drill Life Test)] Test pieces for evaluating machinability were cut out from the simulated intermediate products of parts for mechanical structures. Specifically, a hole was drilled to a depth of 21 mm in the radial direction from the outer surface in a cross-section perpendicular to the axial direction (rolling direction) of a simulated intermediate product with a diameter of 80 mm using a drill. As the tool, a drill with model number SD3.0 manufactured by Nachi-Fujikoshi Corporation was used. As the drilling condition, the feed per revolution was set to 0.25 mm / rev. Also, the drilling depth of one hole was set to 9 mm. During drilling, a water-soluble cutting oil was continuously supplied as a lubricant to the drilling location.

[0143] Drill drilling was carried out under the above-mentioned drilling conditions to evaluate the machinability of the steel material. As the evaluation index, the maximum cutting speed VL1000 (m / min) was used. The maximum cutting speed VL1000 means the fastest cutting speed of a drill capable of drilling a 1000-mm-long hole.

[0144] When the maximum cutting speed VL1000 was 35 m / min or more, it was judged that excellent machinability was obtained (denoted by "E" in the "Machinability" column in Tables 5 and 6). On the other hand, when the maximum cutting speed VL1000 was less than 35 m / min, it was judged that sufficient machinability was not obtained (denoted by "NA" in the "Machinability" column in Tables 5 and 6). Note that "-" in the "Machinability" column in Table 6 means that the machinability evaluation test was not carried out.

[0145] [(Test 6) Fatigue Strength Evaluation Test (Rotating Bending Fatigue Test)] The fatigue strength was evaluated using fatigue test pieces assuming parts for mechanical structures manufactured from steel material by the following test method.

[0146] Rotating bending fatigue test pieces were taken from the simulated intermediate products of parts for mechanical structures. Figure 3 is a side view of the rotating bending fatigue test pieces taken from the intermediate products of each simulated mechanical structure part. The numerical value of "φ" in Figure 3 means the diameter (mm) at that part.

[0147] The fatigue test piece was a round bar test piece, with a diameter of the parallel part being 8 mm and a diameter of the gripping part being 12 mm. The longitudinal direction of the fatigue test piece was parallel to the axial direction of the simulated intermediate product. Specifically, the parallel part was prepared by cutting to a depth of 3.5 mm from the surface of the simulated intermediate product by lathe processing. Therefore, the surface of the parallel part was at least within a range of a depth of 5 mm from the surface of the steel bar. The fatigue test piece was assumed to be a machine structural part after cutting processing was performed on an intermediate product after hot working in the manufacturing process of a machine structural part using steel material. The parallel part of the fatigue test piece was subjected to finish polishing to adjust the surface roughness. Specifically, the center line average roughness (Ra) of the surface was set to within 3.0 μm and the maximum height (Rmax) was set to within 9.0 μm in accordance with JIS B 0601 (2001).

[0148] It is common technical knowledge among those skilled in the art that if the fatigue strength is sufficiently high in a test using a rotating bending fatigue test piece taken from an intermediate product of a simulated machine structural part before induction hardening, then excellent fatigue strength can also be obtained in the machine structural part after induction hardening. Therefore, using the fatigue test piece, an Ono-type rotating bending fatigue test was performed under alternating conditions of room temperature (23°C), air atmosphere, and a rotation speed of 3600 rpm. Fatigue tests were performed on multiple test pieces by applying different stresses, and 10 7 The highest stress at which the specimen did not break after cycling was taken as the fatigue strength (MPa).

[0149] If the fatigue strength obtained was 230 MPa or more, it was determined that sufficient fatigue strength had been obtained. The results of the fatigue strength evaluation are shown in the "Fatigue strength" column of Tables 5 and 6. If the fatigue strength was 230 MPa or more, it was determined that excellent fatigue strength had been obtained (indicated by "E"). On the other hand, if the fatigue strength was less than 230 MPa, it was determined that sufficient fatigue strength had not been obtained (indicated by "NA"). Note that "-" in the "Fatigue strength" column of Table 6 means that a fatigue strength evaluation test was not conducted.

[0150] [Test Results] Referring to Tables 1 to 6, the steel materials with test numbers 1 to 46 satisfied Features 1 to 4. Therefore, hot working cracks and melting cracks were sufficiently suppressed. Furthermore, the maximum cutting speed VL1000 was 35 m / min or more, and excellent machinability was obtained. Furthermore, the fatigue strength was 230 MPa or more, and excellent fatigue strength was obtained.

[0151] On the other hand, in Test No. 47, the C content was too high. Therefore, melting cracks occurred.

[0152] In Test No. 48, the C content was too low. Therefore, the fatigue strength was low.

[0153] In Test No. 49, the Si content was too high. Therefore, melting cracks occurred.

[0154] In Test No. 50, the Mn content was too high. Therefore, the machinability was low.

[0155] In Test No. 51, the Mn content was too low. Therefore, melting cracks occurred.

[0156] In Test No. 52, the P content was too high. Therefore, melting cracks occurred.

[0157] In Test No. 53, the S content was too high. Therefore, melting cracks occurred.

[0158] In Test No. 54, the S content was too low. Therefore, the machinability was low.

[0159] In Test No. 55, the Cr content was too high. Therefore, the machinability was low.

[0160] In Test No. 56, the Bi content was too high. Therefore, the number density of coarse Bi particles exceeded 10 particles / mm 2 and hot working cracks occurred.

[0161] In Test No. 57, the Bi content was too low. Therefore, the machinability was low. Furthermore, the number density of fine Bi particles was less than 80 particles / mm 2 and as a result, fusion cracking occurred.

[0162] In Test No. 58, the N content was too high. Therefore, hot working cracks occurred.

[0163] In Test Nos. 59 and 60, the value of fn1 was too high. That is, fn1 did not satisfy Equation (1). Therefore, sufficient machinability could not be obtained.

[0164] In Test Nos. 61 and 62, the value of fn1 was too low. That is, fn1 did not satisfy Equation (1). Therefore, sufficient fatigue strength could not be obtained.

[0165] In Test Nos. 63 to 65, in the refining process, the time T (minutes) from the addition of Bi to the end of stirring was too short. Therefore, the number density of coarse Bi particles exceeded 10 particles / mm 2 and as a result, hot working cracks occurred.

[0166] In Test Nos. 66 to 68, in the refining process, the time T (minutes) from the addition of Bi to the end of stirring was too long. Therefore, the number density of fine Bi particles was less than 80 particles / mm 2 and fusion cracking occurred.

[0167] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. A steel material, by mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, Bi: 0.0051 to 0.1500%, and, N: 0.0030 to 0.0250%, contains, the balance consists of Fe and impurities, satisfies formula (1), in the steel material, the number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 pieces / mm ２ and, the number density of coarse Bi particles having a circle equivalent diameter of 10.0 μm or more is 10 pieces / mm ２ or less, Steel material. 0.25 ≦ C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V ≦ 1.00 (1) Here, for each element symbol in the formula, the content of the corresponding element is substituted in mass%. When an element is not contained, "0" is substituted for the corresponding element symbol.

2. A steel material, by mass%, C: 0.05 to 0.30%, Si: 0.05 to 0.45%, Mn: 0.30 to 2.00%, P: 0.030% or less, S: 0.010 to 0.095%, Cr: 0.01 to 2.00%, Bi: 0.0051 to 0.1500%, and, N: 0.0030 to 0.0250%, contains, further contains one or more selected from the group consisting of the first group to the fifth group, The remainder consists of Fe and impurities and satisfies formula (1). In the steel material The number density of fine Bi particles having an equivalent circle diameter of 0.1 to 1.0 μm is 80 to 8,000 particles / mm ２ and The number density of coarse Bi particles having an equivalent circle diameter of 10.0 μm or more is 10 particles / mm ２ or less. Steel material. [Group 1] Al: 0.060% or less, and Mg: 0.0100% or less, one or more selected from the group consisting of [Group 2] Ti: 0.1500% or less, Nb: 0.0800% or less, W: 0.4000% or less, and Zr: 0.2000% or less, one or more selected from the group consisting of [Group 3] Ca: 0.0100% or less, Te: 0.0100% or less, B: 0.0050% or less, Sn: 0.0100% or less, and Rare earth element: 0.0100% or less, one or more selected from the group consisting of [Group 4] Co: 0.0100% or less, Se: 0.0100% or less, Sb: 0.0100% or less, and In: 0.0100% or less, one or more selected from the group consisting of [Group 5] V: 0.200% or less, Mo: 1.00% or less, Cu: 0.20% or less, and Ni: 0.20% or less, one or more selected from the group consisting of 0.25 ≦ C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V ≦ 1.00 (1) Here, in each element symbol in the formula, the content of the corresponding element is substituted in mass%. When an element is not contained, "0" is substituted in the corresponding element symbol.

3. The steel material according to claim 2, containing the first group, steel material.

4. The steel material according to claim 2, containing the second group, steel material.

5. The steel material according to claim 2, containing the third group, steel material.

6. The steel material according to claim 2, containing the fourth group, steel material.

7. The steel material according to claim 2, containing the fifth group, steel material.

Images