steel

Abstract

The present invention provides a steel material which has excellent fusion cracking resistance, excellent resistance to cracking during hot working and excellent machinability, and which enables a component for machine structures to achieve a high fatigue strength in cases where the component is formed of this steel material. A steel material according to the present disclosure has a chemical composition that is set forth in the description, while having an Fn of 0.45-1.05, the Fn being defined by formula (1). With respect to this steel material, the number density of fine Bi particles that have a circle-equivalent diameter of 0.1-1.0 µm is 15.00 per mm2 or more and the number density of coarse Bi particles that have a circle-equivalent diameter of 10.0 µm or more is 0.25 per mm2 or less at a depth of 0.08R (R represents the radius) from the surface of the steel material; and the number density of the fine Bi particles is less than 15.00 per mm2 and the number density of the coarse Bi particles is more than 0.25 per mm2 at a depth of 0.65R from the surface of the steel material.

Description

[Technical Field]

[0001] This disclosure relates to steel materials, and more specifically, to steel materials used as materials for machine structural components. [Background technology]

[0002] Mechanical structural components are used in automotive parts such as crankshafts for automobiles and construction vehicles. High fatigue strength is required for mechanical structural components.

[0003] A technique for improving the fatigue strength of mechanical structural components is disclosed, for example, in Japanese Patent Application Publication No. 2008-169411 (Patent Document 1).

[0004] The steel material for machine structural parts disclosed in Patent Document 1 contains, by mass%, C: 0.15-0.55%, Si: 0.01-2.0%, Mn: 0.01-2.5%, Cu: 0.01-2.0%, Ni: 0.01-2.0%, Cr: 0.01-2.5%, Mo: 0.01-3.0%, and a total amount of at least one selected from the group consisting of V and W: 0.01-1.0%, with the remainder being Fe and unavoidable impurities. This steel is subjected to soaking at 1010°C to 1050°C, then cooled to 500°C to 550°C at a cooling rate of 200°C / min or more, followed by cooling to 150°C or below 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 (Least Maximum Heat Processing Unit) that gives the maximum HRC hardness at room temperature is 17.66 or higher. In this document, the LMP that gives the maximum HRC hardness at room temperature after heat treatment under the above conditions is set to 17.66 or higher, thereby increasing the resistance to softening and improving fatigue properties. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2008-169411 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Incidentally, in order to improve the fatigue strength of mechanical structural components, surface hardening treatments are sometimes applied to them.

[0007] One of the various surface hardening treatments is high-frequency induction hardening. High-frequency induction hardening allows for hardening only the necessary areas. In high-frequency induction hardening, a hardened layer is formed on the surface of the steel material by heating it to a high temperature and then cooling it. Compared to other surface hardening treatments such as soft nitriding, high-frequency induction hardening can achieve a greater hardened layer depth and higher fatigue strength.

[0008] This section describes the high-frequency induction hardening treatment applied to mechanical structural components, using the case of a crankshaft as an example. If the crankshaft, a mechanical structural component, has the shape shown in Figure 1, high-frequency induction hardening is applied to, for example, the fillet R portion 1 of the crankshaft in order to increase its fatigue strength. In this case, a hardened layer is formed on the surface of the fillet R portion 1.

[0009] To increase the hardened layer depth, the output of the high-frequency power can be increased to raise the heating temperature during high-frequency induction hardening. However, when high-frequency induction hardening is performed at high temperatures, the heating temperature tends to become excessively high at the edges of mechanical structural parts. For example, if the mechanical structural part is a crankshaft as shown in Figure 1, the heating temperature becomes excessively high at edge 2. In particular, if the heating rate during high-frequency induction hardening is fast, the heating temperature tends to become excessively high.

[0010] For example, if the heating temperature during high-frequency induction hardening becomes excessively high, exceeding 1350°C, a portion of the steel may melt and crack. Hereinafter, such cracks will be referred to as "molten cracking" in this specification. It is preferable to suppress the occurrence of such molten cracking. In other words, steel materials require excellent resistance to molten cracking.

[0011] Furthermore, steel materials used as components for machine structures undergo hot working during the manufacturing process of the machine structures or the steel materials themselves. Therefore, it is necessary to suppress the occurrence of cracks caused by hot working (hot working cracks) in steel materials. In other words, steel materials require excellent resistance to hot working cracks.

[0012] Furthermore, the steel materials used for machine structural components undergo machining during the manufacturing process. Excellent machinability is sometimes required, particularly within the steel material itself. For example, in the case of a crankshaft, machining such as drilling holes is performed in the center of both end faces. Therefore, excellent machinability is required within the steel material.

[0013] The aforementioned Patent Document 1 does not examine the resistance to melt cracking, the resistance to hot working cracking, and the machinability of steel materials.

[0014] The purpose of this disclosure is to provide a steel material that has excellent resistance to melt cracking, excellent resistance to hot work cracking, and excellent machinability, and that, when used as a material for machine structural parts, allows machine structural parts to obtain high fatigue strength. [Means for solving the problem]

[0015] The steel material according to this disclosure is a steel material having a circular cross-section perpendicular to the axial direction, The chemical composition is expressed in mass percent. C: more than 0.30~0.60%, Si: 0.01~0.90%, Mn: 0.50~1.70%, P:0.030% or less, S: 0.200% or less, Bi: 0.0051~0.2500%, Al: 0.001~0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0~1.30%, V: 0~0.200%, Sn: 0~0.1000%, Sb: 0~0.0500%, As: 0~0.0500%, Pb: 0~0.09%, Mg: 0~0.0100%, Ti: 0~0.0400%, Nb: 0~0.0500%, W: 0~0.4000%, Zr: 0~0.2000%, Ca: 0~0.0100%, Te: 0~0.0100%, B: 0~0.0050%, Rare earth elements: 0~0.0100%, Co: 0~0.0100%, Se: 0~0.0100%, In: 0~0.0100%, Mo: 0~0.30%, Cu: 0~0.50%, Ni: 0~0.50%, and, The remainder consists of Fe and impurities. The Fn defined in equation (1) is between 0.45 and 1.05. When the radius of the steel material is defined as R, at a depth of 0.08R from the surface of the steel material, The number density of fine Bi particles, which are Bi particles with an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 particles / mm³. 2 That's all. The number density of coarse Bi particles, which are Bi particles with an equivalent circular diameter of 10.0 μm or more, is 0.25 particles / mm³. 2 The following: At a depth of 0.65R from the surface of the steel material, The number density of the aforementioned fine Bi particles is 15.00 particles / mm³ 2 It is less than, The number density of the aforementioned coarse Bi particles is 0.25 particles / mm³. 2 It's incredible. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, in equation (1), the mass percentage content of the corresponding element is substituted for each element symbol, and if an element is not present, "0" is substituted for the corresponding element symbol. [Effects of the Invention]

[0016] The steel material disclosed herein has excellent resistance to melt cracking, excellent resistance to hot work cracking, and excellent machinability, and when used as a material for machine structural parts, the machine structural parts can obtain high fatigue strength. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 is a front view of a part of a crankshaft, which is an example of a mechanical structural component. [Figure 2] Figure 2 is a cross-sectional view of the steel material of this embodiment, perpendicular to the axial direction. [Figure 3] Figure 3 is a schematic diagram of the microstructure during the molten cracking evaluation test in the example. [Figure 4] Figure 4 is a schematic diagram of a different microstructure from Figure 3 in the molten cracking evaluation test in the example. [Figure 5] Figure 5 is a side view of the fatigue test specimen used in the fatigue strength evaluation test of the embodiment. [Modes for carrying out the invention]

[0018] The inventors first investigated the chemical composition of steel materials that, when used as materials for mechanical structural components, increase the fatigue strength of those components. As a result, the inventors found that the composition of the materials in mass percent is as follows: C: greater than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.200% or less, Al: 0.001 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0 to 1.30%, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 We considered that if a steel material has a chemical composition of ~0.4000%, Zr:0~0.2000%, Ca:0~0.0100%, Te:0~0.0100%, B:0~0.0050%, rare earth elements:0~0.0100%, Co:0~0.0100%, Se:0~0.0100%, In:0~0.0100%, Mo:0~0.30%, Cu:0~0.50%, Ni:0~0.50%, and the remainder being Fe and impurities, then when machine structural parts are manufactured using this steel material, the machine structural parts may have excellent fatigue strength.

[0019] Next, the inventors investigated means to improve the machinability of steel materials in which the content of each element in the chemical composition is within the above range. As a result, the inventors found that by setting Fn, as defined by formula (1), to 0.45 to 1.05, excellent fatigue strength can be obtained in machine structural parts, and the machinability of the steel material used for machine structural parts is improved. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1)

[0020] The inventors further investigated means to improve the resistance of steel materials to melt cracking during high-frequency induction hardening.

[0021] The carbon (C) content affects the molten cracking that occurs in steel during high-frequency induction hardening. Specifically, carbon segregating at grain boundaries lowers the melting point at those boundaries. As a result, molten cracking becomes more likely. Therefore, the above chemical composition is further enhanced by adding 0.0051 to 0.2500% bicarbonate (Bi). By including Bi within this range, Bi particles (inclusions) are formed in the steel. These fine Bi particles suppress the coarsening of the crystal grains (austenite grains) in the steel during high-frequency induction hardening through a pinning effect. If the crystal grains can be kept fine during high-frequency induction hardening, the reduction in the grain boundary area can be suppressed. If the reduction in the grain boundary area can be suppressed and a certain amount of grain boundary area can be secured, the concentration of carbon segregating at the grain boundaries per unit area decreases. As a result, the occurrence of molten cracking is suppressed.

[0022] However, when steel contains Bi within the aforementioned range, not only fine Bi particles but also coarse Bi particles can be generated. Coarse Bi particles can be the starting point for hot working cracks. Therefore, if there is an excessive amount of coarse Bi particles, the heat working crack resistance of the steel will decrease.

[0023] On the other hand, coarse Bi particles improve the machinability of steel. Considering the above, it seems difficult to simultaneously obtain excellent resistance to melt cracking, excellent resistance to hot work cracking, and excellent machinability in steel having the above-mentioned chemical composition by including Bi within the above-mentioned range.

[0024] However, the inventors believed that by changing the number density of fine Bi particles and coarse Bi particles according to the region of the steel material, excellent resistance to melt cracking, excellent resistance to hot working cracking, and excellent machinability could be obtained simultaneously. Specifically, the surface region of the steel material is prone to melt cracking and hot working cracking. Therefore, in the surface region of the steel material, excellent resistance to melt cracking and excellent resistance to hot working cracking should be obtained. On the other hand, melt cracking and hot working cracking are less likely to occur in the interior region of the steel material. Therefore, in the interior region of the steel material, high machinability should be obtained.

[0025] Based on the above technical concept, we investigated and examined the relationship between the number density of fine and coarse Bi particles in the surface and internal regions of steel materials and the steel's resistance to melt cracking, hot work cracking, and machinability. As a result, for steel materials that satisfy the above chemical composition and have an Fn value of 0.45 to 1.05, the number density of fine Bi particles in the surface region was 15.00 particles / mm³. 2 In summary, the number density of coarse Bi particles is 0.25 particles / mm³. 2 The following is observed, with a number density of fine Bi particles in the internal region of 15.00 particles / mm³. 2 If less than 0.25 coarse Bi particles per millimeter, the number density of coarse Bi particles is 0.25 particles / mm³. 2 The inventors have found that, when using this material, it can simultaneously satisfy excellent resistance to melt cracking, excellent resistance to hot work cracking, and excellent machinability, and furthermore, when used as a material for machine structural parts, it can provide excellent fatigue strength in those machine structural parts.

[0026] Based on the above technical concept, the steel material according to this embodiment has the following configuration.

[0027] [1] A steel material with a circular cross-section perpendicular to the axial direction, The chemical composition is expressed in mass percent. C: more than 0.30~0.60%, Si: 0.01~0.90%, Mn: 0.50~1.70%, P:0.030% or less, S: 0.200% or less, Bi: 0.0051~0.2500%, Al: 0.001~0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0~1.30%, V: 0~0.200%, Sn: 0~0.1000%, Sb: 0~0.0500%, As: 0~0.0500%, Pb: 0~0.09%, Mg: 0~0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, 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%, Rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and the balance consists of Fe and impurities, Fn defined by formula (1) is 0.45 to 1.05, when the radius of the steel material is defined as R, at a depth position of 0.08R from the surface of the steel material, the number density of fine Bi particles, which are Bi particles with an equivalent circle diameter of 0.1 to 1.0 μm, is 15.00 particles / mm 2 or more, the number density of coarse Bi particles, which are Bi particles with an equivalent circle diameter of 10.0 μm or more, is 0.25 particles / mm 2 or less, at a depth position of 0.65R from the surface of the steel material, the number density of the fine Bi particles is less than 15.00 particles / mm 2 and the number density of the coarse Bi particles is 0.25 particles / mm 2 or less, Steel material. Fn = C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V (1) Here, for each element symbol in formula (1), the content in mass% of the corresponding element is substituted. When an element is not contained, "0" is substituted for the corresponding element symbol.

[0028] [2] The steel material according to [1], The aforementioned chemical composition is Cr: 0.01~1.30%, V: 0.001~0.200%, Sn: 0.0001~0.1000%, Sb: 0.0001~0.0500%, As: 0.0001~0.0500%, Pb: 0.01~0.09%, Mg: 0.0001~0.0100%, Ti: 0.0001~0.0400%, Nb: 0.0001~0.0500%, W: 0.0001~0.4000%, Zr: 0.0001~0.2000%, Ca: 0.0001~0.0100%, Te: 0.0001~0.0100%, B: 0.0001~0.0050%, Rare earth elements: 0.0001~0.0100%, Co: 0.0001~0.0100%, Se: 0.0001~0.0100%, In: 0.0001~0.0100%, Mo: 0.01~0.30%, Cu: 0.01~0.50%, Ni: 0.01~0.50%, It contains one or more selected from the group consisting of, Steel material.

[0029] The steel material of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0030] [Features of the steel material of this embodiment] The steel material of this embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition, in mass%, is as follows: C: greater than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.200% or less, Bi: 0.0051 to 0.2500%, Al: 0.001 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0 to 1.30%, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, M The composition is as follows: g: 0-0.0100%, Ti: 0-0.0400%, Nb: 0-0.0500%, W: 0-0.4000%, Zr: 0-0.2000%, Ca: 0-0.0100%, Te: 0-0.0100%, B: 0-0.0050%, rare earth elements: 0-0.0100%, Co: 0-0.0100%, Se: 0-0.0100%, In: 0-0.0100%, Mo: 0-0.30%, Cu: 0-0.50%, Ni: 0-0.50%, and the remainder consists of Fe and impurities. (Feature 2) The value of Fn defined by equation (1) is between 0.45 and 1.05. (Feature 3) When the radius of the steel material is defined as R, the number density of fine Bi particles with an equivalent circular diameter of 0.1 to 1.0 μm at a depth of 0.08 R from the surface of the steel material is 15.00 particles / mm². 2 The above results indicate that the number density of coarse Bi particles, which are Bi particles with an equivalent circular diameter of 10.0 μm or more, is 0.25 particles / mm³. 2 The following applies: (Feature 4) At a depth of 0.65R from the surface of the steel material, the number density of fine Bi particles is 15.00 particles / mm³. 2 The number density of coarse Bi particles is less than 0.25 particles / mm³. 2 It's incredible. Features 1 through 4 are explained below.

[0031] [(Feature 1) Regarding chemical composition] The chemical composition of the steel material in this embodiment contains the following elements:

[0032] C: More than 0.30~0.60% Carbon (C) increases the hardness and fatigue strength of machine structural components manufactured from steel. If the C content is 0.30% or less, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, carbon (C) lowers the melting point of steel. Therefore, if the carbon content exceeds 0.60%, even if the content of other elements is within the range of this embodiment, when high-frequency induction hardening is performed on the steel during the manufacturing process of machine structural parts made from steel, molten cracking is more likely to occur in the steel. Therefore, the C content is between 0.30% and 0.60%. The preferred lower limit for the C content is 0.31%, more preferably 0.35%, even more preferably 0.37%, and even more preferably 0.38%. The preferred upper limit for the C content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0033] Si: 0.01~0.90% Silicon (Si) deoxidizes steel during the steelmaking process. Si further increases the hardness and fatigue strength of machine structural components manufactured from steel. If the Si content is less than 0.01%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, Si has a weak affinity for C. Therefore, if the Si content exceeds 0.90%, even if the content of other elements is within the range of this embodiment, C tends to segregate at the grain boundaries during heating rather than within the grains where Si is dissolved. As a result, when high-frequency induction hardening is performed on steel materials in the manufacturing process of machine structural parts made from steel, molten cracking is more likely to occur in the steel. Therefore, the Si content is 0.01 to 0.90%. The preferred lower limit for the Si content is 0.02%, more preferably 0.05%, more preferably 0.08%, and still more preferably 0.10%. The preferred upper limit for the Si content is 0.70%, more preferably 0.65%, more preferably 0.55%, and still more preferably 0.50%.

[0034] Mn: 0.50~1.70% Manganese (Mn) deoxidizes steel during the steelmaking process. Mn further enhances the hardenability of the steel. As a result, the hardness of machine structural parts manufactured from steel increases, and the fatigue strength of these parts improves. Furthermore, Mn has a strong affinity for carbon (C). Therefore, during heating, C remains within the grains where Mn is dissolved. This suppresses segregation of C at grain boundaries. Consequently, when high-frequency induction hardening is performed on steel during the manufacturing process of machine structural parts made from steel, the occurrence of molten cracking is suppressed. Furthermore, Mn combines with sulfur (S) to form Mn sulfides. Therefore, Mn can suppress the formation of coarse FeS. As a result, the hot workability of steel during hot working improves, and its resistance to hot working cracking increases. If the Mn content is less than 0.50%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, Mn lowers the melting point of steel. Therefore, if the Mn content exceeds 1.70%, even if the content of other elements is within the range of this embodiment, the resistance to melt cracking will decrease when high-frequency induction hardening is performed on the steel during the manufacturing process of machine structural parts made from steel. Furthermore, if the Mn content exceeds 1.70%, even if the content of other elements is within the range of this embodiment, the hardness of the steel will increase excessively. In this case, sufficient machinability of the steel cannot be obtained. Therefore, the Mn content is 0.50-1.70%. The preferred lower limit of the Mn content is 0.70%, more preferably 0.80%, even more preferably 0.85%, and even more preferably 0.90%. The preferred upper limit for the Mn content is 1.65%, more preferably 1.60%, more preferably 1.55%, more preferably 1.50%, more preferably 1.48%, more preferably 1.45%, more preferably 1.43%, and more preferably 1.40%.

[0035] P:0.030% or less Phosphorus (P) is an impurity. P segregates at grain boundaries, lowering the melting point of steel. Therefore, when high-frequency induction hardening is performed on steel during the manufacturing process of machine structural parts made from steel, the steel is more prone to molten cracking. Therefore, the P content is 0.030% or less. A low phosphorus (P) content is preferable. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit for the P content is 0.028%, more preferably 0.026%, more preferably 0.023%, and still more preferably 0.020%.

[0036] S: 0.200% or less Sulfur (S) produces sulfides, which improve the machinability of steel. Even if only a small amount of S is present, the above effect can be obtained to some extent, even if the content of other elements is within the range of this embodiment. However, sulfur (S) lowers the melting point of steel. Therefore, if the S content exceeds 0.200%, even if the content of other elements is within the range of this embodiment, when high-frequency induction hardening is performed on the steel during the manufacturing process of machine structural parts made from steel, molten cracking is more likely to occur in the steel. Therefore, the sulfur content is 0.200% or less. The preferred lower limit of the S content is greater than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The preferred upper limit for the S content is 0.150%, more preferably 0.120%, more preferably 0.095%, more preferably 0.080%, more preferably 0.075%, more preferably 0.055%, and more preferably 0.035%.

[0037] Bi: 0.0051~0.2500% Bismuth (Bi) forms particles in the steel and, through a pinning effect, suppresses the coarsening of crystal grains (austenite grains) in the steel during high-frequency induction hardening. Maintaining fine crystal grains suppresses the reduction of grain boundary area. As a result, the carbon concentration per unit grain boundary area is reduced, and molten cracking during high-frequency induction hardening is suppressed. Bi further improves the machinability of the steel. If the Bi content is less than 0.0051%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Bi content exceeds 0.2500%, even if the content of other elements is within the range of this embodiment, an excessive amount of coarse Bi particles will be generated in the surface region of the steel. These coarse Bi particles in the surface region are prone to becoming the starting point for cracks during hot working in the manufacturing process of steel, or during hot working in the manufacturing process of machine structural parts. As a result, the heat-work crack resistance of the steel decreases. Therefore, the Bi content is between 0.0051% and 0.2500%. The preferred lower limit of the Bi content is 0.0060%, more preferably 0.0070%, even more preferably 0.0100%, even more preferably 0.0150%, and even more preferably 0.0200%. The preferred upper limit for the Bi content is 0.2000%, more preferably 0.1500%, more preferably 0.1250%, more preferably 0.1000%, more preferably 0.0900%, more preferably 0.0750%, and more preferably 0.0600%.

[0038] Al: 0.001~0.100% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.001%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.100%, even if the content of other elements is within the range of this embodiment, Al will form coarse oxides. These coarse oxides reduce the fatigue strength of machine structural parts manufactured using steel as the material. Therefore, the Al content is between 0.001% and 0.100%. The preferred lower limit of the Al content is 0.002%, more preferably 0.003%, more preferably 0.005%, and still more preferably 0.010%. The preferred upper limit for the Al content is 0.060%, more preferably 0.050%, more preferably 0.040%, more preferably 0.030%, and still more preferably 0.025%.

[0039] N: 0.0250% or less Nitrogen (N) strengthens steel structural components by forming nitrides and / or carbonitrides during the manufacturing process. As a result, the fatigue strength of steel structural components is increased. Even if only a small amount of N is present, the above effect can be obtained to some extent, even if the content of other elements is within the range of this embodiment. On the other hand, if the N content exceeds 0.0250%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.0250% or less. The preferred lower limit of the N content is greater than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0010%, more preferably 0.0020%, more preferably 0.0030%, and more preferably 0.0040%. The preferred upper limit for the N content is 0.0200%, more preferably 0.0190%, more preferably 0.0170%, more preferably 0.0150%, more preferably 0.0130%, and more preferably 0.0100%.

[0040] O: 0.0050% or less Oxygen (O) is an impurity. O forms oxides in steel, reducing the fatigue strength of machine structural components manufactured from steel. Therefore, the O content is 0.0050% or less. A low oxygen content is preferable. However, excessive reduction of the oxygen content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the oxygen content is greater than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferred upper limit for the O content is 0.0030%, more preferably 0.0025%, more preferably 0.0020%, more preferably 0.0015%, and more preferably 0.0012%.

[0041] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities refer to substances that are introduced during the industrial production of steel material from raw materials such as ore, scrap, or the manufacturing environment, and are not intentionally included, but are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[0042] [Optional Elements] The chemical composition of the steel material in this embodiment is further modified by substituting a portion of the Fe with: Cr: 0~1.30%, V: 0~0.200%, Sn: 0~0.1000%, Sb: 0~0.0500%, As: 0~0.0500%, Pb: 0~0.09%, Mg: 0~0.0100%, Ti: 0~0.0400%, Nb: 0~0.0500%, W: 0~0.4000%, Zr: 0~0.2000%, Ca: 0~0.0100%, Te: 0~0.0100%, B: 0~0.0050%, Rare earth elements: 0~0.0100%, Co: 0~0.0100%, Se: 0~0.0100%, In: 0~0.0100%, Mo: 0~0.30%, Cu: 0~0.50%, Ni: 0~0.50%, It may contain one or more selected from the group consisting of the following. The following describes these arbitrary elements.

[0043] [Group 1: Regarding Cr and V] The chemical composition of the steel material in this embodiment may further contain the following first group of elements in place of a portion of Fe. These elements are arbitrary elements and all of them increase the fatigue strength of the mechanical structural components. [Group 1] Cr: 0-1.30%, and, One or more selected from the group consisting of V: ​​0~0.200%.

[0044] Cr: 0~1.30% Chromium (Cr) is an optional element and does not need to be included. In other words, the Cr content may be 0%. When present, i.e., when the chromium content is greater than 0%, chromium enhances the hardenability of steel. Therefore, the hardness of machine structural components manufactured from steel increases, and the fatigue strength of these components improves. Even a small amount of chromium can provide some degree of the above effect. However, if the Cr content exceeds 1.30%, sufficient machinability cannot be obtained in the steel material, even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is between 0% and 1.30%, and if present, the Cr content is 1.30% or less. The preferred lower limit of the Cr content is 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.12%, more preferably 0.14%, more preferably 0.16%, and more preferably 0.18%. The preferred upper limit for the Cr content is 1.25%, more preferably 1.20%, more preferably 1.10%, more preferably 1.00%, and more preferably 0.90%.

[0045] V: 0~0.200% Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. If V is present, that is, if the V content is greater than 0%, V precipitates as V precipitates in the ferrite within the steel during the manufacturing process of machine structural parts made from steel. This increases the hardness of the ferrite in the steel. As a result, the fatigue strength of the machine structural parts increases. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.200%, the above effects saturate, and the manufacturing costs increase further. Therefore, the V content is between 0 and 0.200%, and if present, the V content is 0.200% or less. The preferred lower limit of the V content is 0.001%, more preferably 0.003%, more preferably 0.005%, more preferably 0.007%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The preferred upper limit for the V content is 0.180%, more preferably 0.160%, more preferably 0.140%, more preferably 0.120%, more preferably 0.100%, more preferably 0.080%, more preferably 0.050%, and more preferably 0.049%.

[0046] [Group 2: Regarding Sn, Sb, As, and Pb] The chemical composition of the steel material in this embodiment may further contain the following second group of elements in place of a portion of Fe. All of these elements are arbitrary elements, and all of them improve the machinability of the steel material. [Group 2] Sn: 0~0.1000%, Sb: 0~0.0500%, As: 0~0.0500%, and, One or more selected from the group consisting of Pb: 0-0.09%

[0047] Sn: 0~0.1000% Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. If sn is present, that is, if the sn content is greater than 0%, the sn segregates at the interface between the matrix and the inclusions, embrittles the steel. This increases the machinability of the steel. Even a small amount of sn can provide the above effect to some extent. However, if the Sn content exceeds 0.1000%, excessive segregation of Sn will occur, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the Sn content is between 0 and 0.1000%, and if present, the Sn content is 0.1000% or less. The preferred lower limit for the Sn content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit for the Sn content is 0.0500%, more preferably 0.0100%, more preferably 0.0090%, more preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0048] Sb: 0~0.0500% Antimony (Sb) is an optional element and does not need to be included. In other words, the Sb content may be 0%. If Sb is present, that is, if the Sb content is greater than 0%, Sb segregates at the interface between the matrix phase and the inclusions, embrittles the steel. As a result, the machinability of the steel increases. Even if only a small amount of Sb is present, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.0500%, Sb will segregate excessively, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the Sb content is between 0 and 0.0500%, and if present, the Sb content is 0.0500% or less. The preferred lower limit of the Sb content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit for the Sb content is 0.0400%, more preferably 0.0300%, more preferably 0.0200%, more preferably 0.0100%, more preferably 0.0080%, and more preferably 0.0060%.

[0049] As: 0~0.0500% Arsenic (As) is an optional element and may not be present. In other words, the As content may be 0%. When As is present, that is, when the As content is greater than 0%, As segregates at the interface between the matrix phase and the inclusions, embrittles the steel. This increases the machinability of the steel. Even a small amount of As can provide the above effect to some extent. However, if the As content exceeds 0.0500%, excessive segregation of As will occur, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the As content is between 0 and 0.0500%, and if present, the As content is 0.0500% or less. The preferred lower limit for the As content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit for the As content is 0.0100%, more preferably 0.0070%, more preferably 0.0060%, more preferably 0.0050%, and more preferably 0.0040%.

[0050] Pb: 0~0.09% Lead (Pb) is an optional element and does not need to be included. In other words, the Pb content may be 0%. When Pb is present, that is, when the Pb content is greater than 0%, Pb generates Pb particles in the matrix, making the steel brittle. This increases the machinability of the steel. Even a small amount of Pb can provide some degree of the above effect. However, if the Pb content exceeds 0.09%, excessive Pb particles will be generated, even if the content of other elements is within the range of this embodiment. In this case, the hot workability of the steel material will decrease. Therefore, the Pb content is 0-0.09%, and if present, the Pb content is 0.09% or less. The preferred lower limit of the Pb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Pb content is 0.08%, more preferably 0.07%, even more preferably 0.06%, and even more preferably 0.05%.

[0051] [Group 3: Regarding Mg] The chemical composition of the steel material in this embodiment may further contain the following third group in place of a portion of Fe. [Group 3] Mg: 0~0.0100% Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. If magnesium is present, that is, if the magnesium content is greater than 0%, magnesium will deoxidize the steel. Even if only a small amount of magnesium is present, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0100%, the Mg will form coarse oxides, even if the content of other elements is within the range of this embodiment. These coarse oxides reduce the fatigue strength of machine structural components manufactured from steel. Therefore, the Mg content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit for the Mg content is 0.0050%, more preferably 0.0045%, and even more preferably 0.0040%.

[0052] [Group 4: Ti, Nb, W, and Zr] The chemical composition of the steel material in this embodiment may further contain the following fourth group of elements in place of a portion of Fe. These elements are arbitrary elements, and each forms precipitates, refining the crystal grains in the steel material through a pinning effect, thereby increasing the toughness of the mechanical structural parts manufactured using the steel material. [Group 4] Ti: 0~0.0400%, Nb: 0~0.0500%, W: 0~0.4000%, and, One or more selected from the group consisting of Zr: 0 to 0.2000%.

[0053] Ti: 0~0.0400% Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, the Ti forms precipitates (carbides and / or carbonitrides). These precipitates refine the grain size of the steel material through a pinning effect. This increases the toughness of mechanical structural components. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.0400%, the above effects saturate, and manufacturing costs increase. Therefore, the Ti content is between 0 and 0.0400%, and if present, it is 0.0400% or less. The preferred 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 preferred upper limit for the Ti content is 0.0300%, more preferably 0.0200%, even more preferably 0.0175%, and even more preferably 0.0150%.

[0054] Nb: 0~0.0500% Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. When Nb is present, that is, when the Nb content is greater than 0%, Nb, like Ti, forms precipitates that refine the crystal grains of the steel, thereby increasing the toughness of mechanical structural components. Even a small amount of Nb can provide some degree of the above effect. However, if the Nb content exceeds 0.0500%, the above effect saturates, and manufacturing costs increase. Therefore, the Nb content is between 0 and 0.0500%, and if present, it is 0.0500% or less. The preferred lower limit of the Nb content is 0.0001%, more preferably 0.0010%, even more preferably 0.0050%, and even more preferably 0.0080%. The preferred upper limit for the Nb content is 0.0200%, more preferably 0.0175%, and even more preferably 0.0150%.

[0055] W: 0~0.4000% Tungsten (W) is an optional element and does not need to be included. In other words, the W content may be 0%. When present, i.e., when the W content is greater than 0%, W, like Ti, forms precipitates that refine the crystal grains of the steel, thereby increasing the toughness of mechanical structural components. Even a small amount of W can provide some degree of the above effect. However, if the W content exceeds 0.4000%, the above effect saturates, and manufacturing costs increase. Therefore, the W content is between 0 and 0.4000%, and if present, it is 0.4000% or less. The preferred lower limit of the W content is 0.0001%, more preferably 0.0050%, and even more preferably 0.0500%. The preferred upper limit for the W content is 0.3500%, more preferably 0.3000%, and even more preferably 0.2000%.

[0056] Zr: 0~0.2000% Zirconium (Zr) is an optional element and does not need to be included. In other words, the Zr content may be 0%. When Zr is present, that is, when the Zr content is greater than 0%, Zr, like Ti, forms precipitates that refine the crystal grains of the steel, thereby increasing the toughness of mechanical structural components. Even a small amount of Zr will provide some degree of the above effect. However, if the Zr content exceeds 0.2000%, the above effects saturate, and manufacturing costs increase. Therefore, the Zr content is between 0 and 0.2000%, and if present, it is less than 0.2000%. The preferred 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 preferred upper limit for the Zr content is 0.1500%, more preferably 0.1000%, even more preferably 0.0500%, and even more preferably 0.0100%.

[0057] [Group 5: Ca, Te, B, and rare earth elements (REM)] The chemical composition of the steel material in this embodiment may further contain the following fifth group of elements in place of a portion of Fe. These elements are arbitrary elements and all of them improve the machinability of the steel material. [Group 5] Ca: 0~0.0100%, Te: 0~0.0100%, B: 0~0.0050%, and, Rare earth elements: One or more selected from the group consisting of 0 to 0.0100%.

[0058] Ca: 0~0.0100% Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. When calcium is present, i.e., when the calcium content is greater than 0%, calcium improves the machinability of steel. Even a small amount of calcium will provide some degree of this effect. However, if the Ca content exceeds 0.0100%, coarse oxides will form even if the content of other elements is within the limits of this practice. Coarse oxides reduce the fatigue strength of machine structural components manufactured from steel. Therefore, the Ca content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit for the Ca content is 0.0085%, more preferably 0.0070%, even more preferably 0.0050%, and even more preferably 0.0030%.

[0059] Te: 0~0.0100% Tellurium (Te) is an optional element and does not need to be included. In other words, the Te content may be 0%. When present, i.e., when the Te content is greater than 0%, Te improves the machinability of steel. Even a small amount of Te will provide some degree of the above effect. However, if the Te content exceeds 0.0100%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Te content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit for the Te content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0010%. The preferred upper limit for the Te content is 0.0090%, more preferably 0.0085%, even more preferably 0.0080%, and even more preferably 0.0040%.

[0060] B: 0~0.0050% Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When present, i.e., when the B content is greater than 0%, B combines with N to form BN, improving the machinability of the steel. Furthermore, B segregates at grain boundaries, contributing to grain boundary strengthening and increasing the fatigue strength of machine structural parts manufactured from steel. Even a small amount of B can provide some of the above effects. However, if the B content exceeds 0.0050%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the B content is 0-0.0050%, and if present, it is 0.0050% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0061] Rare earth elements (REM): 0~0.0100% Rare earth elements (REMs) are optional elements and do not need to be included. In other words, the REM content may be 0%. When REM is present, that is, when the REM content is greater than 0%, REM improves the machinability of the steel. Even if only a small amount of REM is present, the above effect can be obtained to some extent. However, if the REM content exceeds 0.0100%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the REM content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit for the REM content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the REM content is 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, and even more preferably 0.0040%.

[0062] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this specification, REM content refers to the total content of these elements.

[0063] [Group 6: Co, Se, and In] The chemical composition of the steel material in this embodiment may further contain the following sixth group of elements in place of a portion of Fe. These elements are arbitrary elements and all suppress decarburization of the steel material. [Group 6] Co: 0~0.0100%, Se: 0~0.0100%, and, One or more selected from the group consisting of In: 0 to 0.0100%.

[0064] Co: 0~0.0100% Cobalt (Co) is an optional element and may not be present. In other words, the Co content may be 0%. If Co is present, that is, if the Co content is greater than 0%, Co suppresses decarburization of the steel during the manufacturing process. Even if only a small amount of Co is present, the above effect can be obtained to some extent. However, if the Co content exceeds 0.0100%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Co content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the Co content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0030%. The preferred upper limit for the Co content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0065] Se: 0~0.0100% Selenium (Se) is an optional element and does not need to be present. In other words, the Se content may be 0%. If Se is present, that is, if Se is greater than 0%, Se suppresses decarburization of steel during the manufacturing process. Even if only a small amount of Se is present, the above effect can be obtained to some extent. However, if the Se content exceeds 0.0100%, hot working cracks will occur even if the content of other elements is within the range of this embodiment. Therefore, the Se content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the Se content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit for the Se content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0066] In: 0~0.0100% Indium (In) is an optional element and may not be present. In other words, the In content may be 0%. If present, i.e., if the amount of In is greater than 0%, In suppresses decarburization of steel during the manufacturing process. Even if only a small amount of In is present, the above effect can be obtained to some extent. However, if the In content exceeds 0.0100%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the In content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the In content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the In content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0067] [Group 7: Mo, Cu, and Ni] The chemical composition of the steel material in this embodiment may further contain the following seventh group of elements in place of a portion of Fe. These elements are arbitrary elements and all of them increase the fatigue strength of machine structural parts manufactured using steel as the material. [Group 7] Mo: 0~0.30%, Cu: 0~0.50%, and, One or more species selected from the group consisting of Ni: 0-0.50%.

[0068] Mo: 0~0.30% Molybdenum (Mo) is an optional element and does not need to be included. In other words, the Mo content may be 0%. When present, i.e., when the Mo content is greater than 0%, Mo increases the fatigue strength of machine structural components manufactured from steel. Even a small amount of Mo will provide some degree of the above effect. However, if the Mo content exceeds 0.30%, even if the content of other elements is within the range of this embodiment, the hardness of the steel becomes excessively high, and the hot workability of the steel decreases. Therefore, the Mo content is 0-0.30%, and if present, it is 0.30% or less. The preferred lower limit for the Mo content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the Mo content is 0.20%, more preferably 0.17%, and even more preferably 0.15%.

[0069] Cu: 0~0.50% Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When present, i.e., when the Cu content is greater than 0%, Cu increases the fatigue strength of machine structural components manufactured from steel. Even a small amount of Cu can provide some degree of this effect. However, if the Cu content exceeds 0.50%, even if the content of other elements is within the range of this embodiment, molten cracking is more likely to occur during high-frequency induction hardening. Therefore, the Cu content is 0-0.50%, and if present, it is 0.50% or less. The preferred lower limit for the Cu content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Cu content is 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0070] Ni: 0~0.50% Nickel (Ni) is an optional element and does not need to be included. In other words, the Ni content may be 0%. When present, i.e., when the Ni content is greater than 0%, Ni increases the fatigue strength of machine structural components manufactured from steel. Even a small amount of Ni will provide some degree of this effect. However, if the Ni content exceeds 0.50%, even if the content of other elements is within the range of this embodiment, molten cracking is more likely to occur during high-frequency induction hardening. Therefore, the Ni content is 0-0.50%, and if present, it is 0.50% or less. The preferred lower limit for the Ni content is 0.01%, and more preferably 0.02%. The preferred upper limit for the Ni content is 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0071] [(Feature 2) About Fn] In the steel material of this embodiment, Fn, as defined by formula (1), is further 0.45 to 1.05. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element. If an element is not present, "0" is substituted for the corresponding element symbol.

[0072] Fn is an index of the fatigue strength of machine structural components manufactured from steel and the machinability of the steel. If Fn is less than 0.45, even if the steel satisfies characteristics 1, 3, and 4, sufficient fatigue strength cannot be obtained in machine structural components manufactured from steel. On the other hand, if Fn exceeds 1.05, even if the steel satisfies characteristics 1, 3, and 4, the machinability of the steel decreases. Therefore, Fn is between 0.45 and 1.05. A preferred lower limit for Fn is 0.50, more preferably 0.55, and even more preferably 0.60. A preferred upper limit for Fn is 1.00, more preferably 0.95, and even more preferably 0.90.

[0073] [(Feature 3) 0.08R depth position D 0.08R [Number density of Bi particles in this location] Figure 2 is a cross-sectional view of the steel material in this embodiment, perpendicular to the axial direction. As shown in Figure 2, the cross-section 10 perpendicular to the axial direction of the steel material is circular. The radius of the cross-section 10 is defined as R. Surface D of the steel material 0.00R The position at a depth of 0.08R, starting from this point, is called "0.08R depth position D". 0.08R It is defined as " In the steel material of this embodiment, the depth position D is 0.08R. 0.08R In this case, the number density of fine Bi particles, which are Bi particles with an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 particles / mm³. 2 The above results indicate that the number density of coarse Bi particles, which are Bi particles with an equivalent circular diameter of 10.0 μm or more, is 0.25 particles / mm³. 2 The following applies:

[0074] As shown in Figure 2, the surface D of the steel material 0.00R From 0.08R depth position D0.08R The region up to this point is defined as the surface region SA. The surface region SA of steel is the region near the surface of the steel that comes into contact with processing tools such as molds or rolls during the hot working process in the manufacturing process of steel, or during the hot working process in the manufacturing process of machine structural parts made from steel. Therefore, sufficient resistance to hot working cracks is required in the surface region SA. Furthermore, the surface region SA is the region where the temperature becomes high due to high-frequency induction heating when high-frequency induction hardening is performed in the manufacturing process of machine structural parts made from steel. Therefore, sufficient resistance to melt cracks is required in the surface region SA. Thus, sufficient resistance to hot working cracks and sufficient resistance to melt cracks are required in the surface region SA of steel.

[0075] As described above, during heating in high-frequency induction hardening, fine Bi particles with an equivalent circle diameter of 0.1 to 1.0 μm suppress grain coarsening in the surface region SA due to the pinning effect. By increasing the number density of fine Bi particles in the surface region SA, the grains can be maintained in a fine state due to the pinning effect. Therefore, the reduction of grain boundary area is suppressed, and the occurrence of molten cracking is suppressed. 0.08R depth position D in the surface region SA 0.08R In this case, the number density of fine Bi particles is 15.00 particles / mm³. 2 Under these conditions, the crystal grains can be maintained finely during heating in high-frequency induction hardening. As a result, sufficient resistance to melt cracking can be obtained in the steel material.

[0076] Furthermore, the surface region SA is subjected to significant external forces from the hot working tool during hot working. In the surface region SA of the steel material, where external forces are greatest during hot working, coarse Bi particles, which have an equivalent circular diameter of 10.0 μm or more, become the starting point for hot working cracks. Therefore, it is preferable to have as low a number density of coarse Bi particles as possible in the surface region SA of the steel material. (0.08R depth position D within the surface region SA) 0.08R In this case, the number density of coarse Bi particles is 0.25 particles / mm³. 2 If the following conditions are met, the number density of coarse Bi particles in the surface region SA is sufficiently low. Therefore, sufficient heat-work crack resistance can be obtained in the steel material.

[0077] Therefore, 0.08R depth position D 0.08R The number density of fine Bi particles is 15.00 particles / mm³. 2 The above results indicate that the number density of coarse Bi particles is 0.25 particles / mm³. 2 The following applies:

[0078] 0.08R Depth Position D 0.08R The preferred lower limit for the number density of fine Bi particles is 20.00 particles / mm³. 2 And more preferably 25.00 pieces / mm 2 And more preferably 30.00 pieces / mm 2 That is the case. 0.08R Depth Position D 0.08R The upper limit of the number density of fine Bi particles is not particularly limited. If the steel material satisfies features 1 and 2, then 0.08R depth position D 0.08R The upper limit of the number density of fine Bi particles is, for example, 1000.00 particles / mm³. 2 That is the case.

[0079] 0.08R Depth Position D 0.08R The preferred upper limit for the number density of coarse Bi particles is 0.20 particles / mm³. 2 And more preferably 0.16 pieces / mm 2 And more preferably 0.12 pieces / mm 2 And more preferably 0.08 pieces / mm 2 That is the case. 0.08R Depth Position D 0.08R A lower number density of coarse Bi particles is preferable. 0.08R depth position D 0.08R The preferred lower limit for the number density of coarse Bi particles is 0.03 particles / mm³. 2 And more preferably 0.00 pieces / mm 2 That is the case.

[0080] Furthermore, the surface region SA contains not only the fine and coarse Bi particles mentioned above, but also Bi particles with an equivalent circle diameter greater than 1.0 to less than 10.0 μm. However, in the surface region SA, the number density of fine and coarse Bi particles correlates more strongly with hot working cracks and molten cracks during high-frequency induction hardening than the number density of Bi particles with an equivalent circle diameter greater than 1.0 to less than 10.0 μm. Therefore, in this embodiment, the position D at a depth of 0.08R within the surface region SA is considered. 0.08R The number density of fine and coarse Bi particles in this sample is used as an indicator of hot working cracks and molten cracks during high-frequency induction hardening of steel materials.

[0081] [(Feature 4) 0.65R depth position D 0.65R [Number density of Bi particles in this location] Refer to Figure 2, the surface D of the steel material. 0.00R The position at a depth of 0.65R, starting from this point, is called "0.65R depth position D". 0.65R It is defined as "the surface D of the steel material". 0.00R The position at a depth of 1.00R, starting from this point, is called "1.00R depth position D". 1.00R It is defined as "

[0082] In the steel material of this embodiment, the depth position D is 0.65R. 0.65R In this case, the number density of fine Bi particles, which are Bi particles with an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 particles / mm³. 2 The number density of coarse Bi particles, which are less than 10.0 μm and have an equivalent circular diameter of 10.0 μm or more, is 0.25 particles / mm³. 2 It's incredible.

[0083] As shown in Figure 2, the 0.65R depth position D 0.65R From 1.00R depth position D 1.00R The region up to this point is defined as the internal region CA. Compared to the surface region SA, the internal region CA of steel is less susceptible to external forces during hot working and heat during high-frequency induction hardening. Therefore, in the internal region CA, resistance to hot working cracks and resistance to melt cracks are not required compared to the surface region SA. On the other hand, in the manufacturing process of machine structural parts made from steel, the internal region CA may be subjected to machining. In this case, sufficient machinability is required in the internal region CA.

[0084] During machining, coarse Bi particles act as the starting point for separating chips from the steel material. Therefore, coarse Bi particles improve the machinability of the steel. Consequently, a higher number density of coarse Bi particles in the internal region CA is preferable. (0.65R depth position D within the internal region CA) 0.65R In this case, the number density of coarse Bi particles is 0.25 particles / mm³. 2 If it is ultra-high quality, sufficient machinability can be obtained in steel materials.

[0085] On the other hand, the influence of fine Bi particles on the machinability of steel is low. Furthermore, if the number density of fine Bi particles is high, the number density of coarse Bi particles will be lower. Therefore, in the internal region CA, a low number density of fine Bi particles is preferable. (0.65R depth position D within the internal region CA) 0.65R In this case, the number density of fine Bi particles is 15.00 particles / mm³. 2 If the value is less than the specified threshold, coarse Bi particles are likely to be generated in the internal CA region. As a result, sufficient machinability can be obtained in the steel material.

[0086] Therefore, 0.65R depth position D 0.65R The number density of fine Bi particles is 15.00 particles / mm³. 2 The number density of coarse Bi particles is less than 0.25 particles / mm³. 2 It's incredible.

[0087] 0.65R Depth Position D 0.65R The preferred upper limit for the number density of fine Bi particles is 14.00 particles / mm³. 2 And more preferably 10.00 pieces / mm 2 And more preferably 7.50 pieces / mm 2 That is the case. 0.65R Depth Position D 0.65R A lower number density of fine Bi particles is preferable. 0.65R depth position D 0.65R The preferred lower limit for the number density of fine Bi particles is 5.00 particles / mm³. 2 And more preferably 3.00 pieces / mm 2 And more preferably 1.50 pieces / mm2 And more preferably 0.00 pieces / mm 2 That is the case.

[0088] 0.65R Depth Position D 0.65R The preferred lower limit for the number density of coarse Bi particles is 0.26 particles / mm³. 2 And more preferably 0.30 pieces / mm 2 And more preferably 0.40 pieces / mm 2 And more preferably 0.50 pieces / mm 2 That is the case. 0.65R Depth Position D 0.65R There is no particular upper limit to the number density of coarse Bi particles in . However, if the steel material satisfies features 1 and 2, then 0.65R depth position D 0.65R The upper limit of the number density of coarse Bi particles is, for example, 10.00 particles / mm³. 2 And more preferably 7.00 pieces / mm 2 That is the case.

[0089] Furthermore, within the internal region CA, not only the fine and coarse Bi particles mentioned above are present, but also Bi particles with an equivalent circle diameter greater than 1.0 to less than 10.0 μm. However, in the internal region CA, the number density of coarse Bi particles correlates more strongly with machinability than the number density of Bi particles with an equivalent circle diameter greater than 1.0 to less than 10.0 μm. Therefore, in this embodiment, the machinability is determined at a depth position D of 0.65R within the internal region CA. 0.65R In this method, the number density of coarse Bi particles is used as an indicator of the machinability of the steel material.

[0090] [0.08R depth position D 0.08R [Method for measuring the number density of fine and coarse Bi particles] 0.08R Depth Position D 0.08R The number density of fine Bi particles and coarse Bi particles can be measured by the following method.

[0091] In a cross-section parallel to the axial direction of the steel material and including the central axis of the steel material, the depth position D is 0.08R. 0.08RCollect a test piece containing it. Among the collected test pieces, a cross-section that is parallel to the axial direction of the steel material and includes the central axis of the steel material is used as the observation surface.

[0092] Mirror-polish the observation surface. Using a Scanning Electron Microscope (SEM), at a magnification of 1000 times, among the observation surfaces after mirror polishing, at a depth position D of 0.08R 0.08R Select a rectangular observation region including it. The area of the observation region is 25.6 mm 2 Let it be. Select the observation region so that the depth position D of 0.08R is arranged at the center position of the observation region. 0.08R Furthermore, divide the observation region into 624 parts (26×24 division) in a rectangular field of view of 202.5 μm×202.5 μm.

[0093] Based on the backscattered electron images of each field obtained by SEM observation, using a well-known particle analysis method of image analysis, examine the number density of coarse Bi particles and fine Bi particles. Specifically, based on the interface between the matrix phase and particles of the steel material, identify the particles in the steel material. The particles mentioned here are inclusions or precipitates. Perform image analysis and obtain the equivalent circle diameter of the identified particles. Specifically, obtain the area of each identified particle. The diameter of a circle with the same area as the obtained area is defined as the equivalent circle diameter (μm) of the particle.

[0094] Among the particles observed in the backscattered electron images obtained by the above SEM observation, particles with an equivalent circle diameter of 0.1 to 1.0 μm and, as a result of point analysis of the composition of the particles using an Energy Dispersive X-ray spectroscopy (EDX) equipped with the SEM, particles with a Bi content of 50% or more by mass are identified as fine Bi particles. Also, among the particles observed in the backscattered electron images obtained by SEM observation, particles with an equivalent circle diameter of 10.0 μm or more and, as a result of point analysis of the composition of the particles using EDX, particles with a Bi content of 50% or more by mass are identified as coarse Bi particles. The acceleration voltage for EDX analysis is 20 kV. Since Bi is a heavy element, it is observed with high brightness in the backscattered electron image. Therefore, Bi particles may be identified based on the brightness.

[0095] Based on the total number of fine Bi particles identified in each field of view and the total area (25.6 mm 2 ) of the plurality of fields of view constituting the observation region, the number density (number / mm 0.08R ) of fine Bi particles at the 0.08R depth position D 2 ) is determined. Similarly, based on the total number of coarse Bi particles identified in each field of view and the total area (25.6 mm 2 ) of the plurality of fields of view, the number density (number / mm 0.08R ) of coarse Bi particles at the 0.08R depth position D 2 ) is determined.

[0096] [Method for measuring the number density of fine and coarse Bi particles at the 0.65R depth position D 0.65R The number density of fine Bi particles and the number density of coarse Bi particles at the 0.65R depth position D 0.65R can be measured by the following method.

[0097] In a cross-section parallel to the axial direction of the steel material and including the central axis of the steel material, a test piece including the 0.65R depth position D 0.65R is sampled. Among the sampled test pieces, a cross-section parallel to the axial direction of the steel material and including the central axis of the steel material is used as the observation surface.

[0098] The observation surface is mirror-polished. Using SEM, at a magnification of 1000 times, a rectangular observation region including the 0.65R depth position D 0.65R is selected from the mirror-polished observation surface. The area of the observation region is set to 25.6 mm 2 . The observation region is selected so that the 0.65R depth position D 0.65R is arranged at the center position of the observation region. Further, the observation region is divided into​​​​Based on backscattered electron images of each field of view obtained by SEM observation, the number density of coarse and fine Bi particles is investigated using well-known image analysis particle analysis methods. Specifically, particles in the steel material are identified based on the interface between the matrix phase and the particles. These particles are inclusions or precipitates. Image analysis is performed to determine the equivalent circular diameter of the identified particles. Specifically, the area of ​​each identified particle is determined. The diameter of a circle with the same area as the determined area is defined as the equivalent circular diameter (μm) of that particle.

[0100] Of 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 whose Bi content is 50% or more by mass, as determined by point analysis of the particle composition using EDX, are identified as fine Bi particles. Furthermore, of the particles observed in the backscattered electron image obtained by the SEM observation, particles with an equivalent circle diameter of 10.0 μm or more and whose Bi content is 50% or more by mass, as determined by point analysis of the particle composition using EDX, are identified as coarse Bi particles. The acceleration voltage for EDX analysis is set to 20 kV. Since Bi is a heavy element, it is observed as high brightness in the backscattered electron image. Therefore, Bi particles may also be identified based on their brightness.

[0101] The total number of fine Bi particles identified in each field of view, and the total area (25.6 mm²) of the multiple fields of view constituting the above observation region. 2 Based on ), 0.65R depth position D 0.65R Number density of fine Bi particles (particles / mm³) 2 Similarly, determine the total number of coarse Bi particles identified in each field of view and the total area of ​​the multiple fields of view (25.6 mm²). 2 Based on ), 0.65R depth position D 0.65R Number density of coarse Bi particles (particles / mm³) 2 )

[0102] [Effects of the steel material in this embodiment] As described above, the steel material of this embodiment satisfies features 1 to 4. Therefore, the steel material simultaneously provides excellent resistance to melt cracking, excellent resistance to hot working cracking, and excellent machinability. Furthermore, high fatigue strength can be obtained in machine structural parts manufactured using this steel material.

[0103] [Preferred uses and shapes of the steel material of this embodiment] The steel material of this embodiment can be widely applied, for example, as a material for machine structural parts. The steel material of this embodiment is particularly suitable when hot working such as hot forging, high-frequency induction hardening, and cutting are performed in the manufacturing process of machine structural parts. However, even if one or more of the hot working, high-frequency induction hardening, and cutting processes are not performed, the steel material of this embodiment can still be applied as a material for machine structural parts.

[0104] The cross-section of the steel material perpendicular to its axial direction is circular. The diameter of the cross-section perpendicular to the axial direction of the steel material is not particularly limited, but for example, it is 10 to 200 mm.

[0105] [Manufacturing method] An example of a steel manufacturing method according to this embodiment will be described. Steel materials satisfying features 1 to 4 may be manufactured by other manufacturing methods other than those described below. However, the manufacturing methods described below are preferred examples of steel manufacturing methods according to this embodiment.

[0106] An example of the steel manufacturing method of this embodiment includes the following steps. Note that step 3 is an optional step and may not be performed. (Process 1) Refining process (Process 2) Casting Process (Process 3) Hot working process The following describes each step.

[0107] [(Process 1) Refining Process] The refining process produces molten steel having a chemical composition that satisfies the above-described characteristics 1 and 2. The refining process includes a primary refining process and a secondary refining process. In the primary refining process, molten steel is produced by refining molten iron, manufactured by a well-known method, in a converter. In the secondary refining process, alloying elements are added to the molten steel to adjust its chemical composition to satisfy characteristics 1 and 2. Specifically, in the secondary refining process, the components of the molten steel other than Bi are adjusted while stirring the molten steel using a well-known refining method. After that, Bi is added to the molten steel using a wire, and then the molten steel is stirred to adjust the Bi component. The following conditions are met in the secondary refining process. (Condition 1) After adding Bi to the molten steel, the time t0 until the end of stirring in the secondary refining process is set to be more than 15 minutes but less than 60 minutes. (Condition 2) The stirring power density ε of the molten steel after adding Bi is assumed to be 10 to 100 W / t. Here, the stirring power density ε (W / t) is defined by the following equation (A). ε=0.0285×Q×T / W×log(1+513.5×Z / V1) (A) Here, in equation (A), Q is substituted with the gas flow rate (NL / min) blown into the ladle containing the molten steel. T is substituted with the molten steel temperature (K). W is substituted with the molten steel mass (t). Z is substituted with the molten steel depth (m) in the ladle. V1 is substituted with the degree of vacuum (torr) in the atmosphere containing the molten steel during stirring. Conditions 1 and 2 are explained below.

[0108] [Condition 1: Regarding time t0] In the secondary refining process, the time t0 from the addition of Bi until the end of stirring in the secondary refining process is between 15 minutes and 60 minutes. If the time t0 from the addition of Bi until the end of stirring in the secondary refining process is 15 minutes or less, Bi will not diffuse sufficiently in the molten steel. In this case, an excess of coarse Bi particles will be generated in the manufactured steel. Therefore, at a depth position D of 0.08R, 0.08R The number density of coarse Bi particles becomes excessively high.

[0109] On the one hand, when the time t0 from the addition of Bi to the end of stirring in the secondary refining process is 60 minutes or more, Bi in the molten steel tends to aggregate. In this case, in the produced steel material, the number density of fine Bi particles decreases. Therefore, the number density of fine Bi particles at the depth position D of 0.08R 0.08R becomes excessively low.

[0110] In the secondary refining process, if the time t0 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, Bi diffuses sufficiently in the molten steel. Therefore, on the premise of satisfying Condition 2 and Conditions 3 and 4 described later, at the depth position D of 0.08R in the steel material 0.08R and at the depth position D of 0.65R 0.65R the number densities of fine Bi particles and coarse Bi particles are within an appropriate range.

[0111] Note that after adding Bi, the temperature of the molten steel until the end of stirring in the secondary refining process is 1510 - 1630°C.

[0112] [Condition 2: Regarding the stirring power density ε] The stirring power density ε of the molten steel after adding Bi to the molten steel is 10 - 100 W / t. If the stirring power density ε of the molten steel after adding Bi to the molten steel is less than 10 W / t, Bi does not diffuse sufficiently in the molten steel. In this case, in the produced steel material, coarse Bi particles are excessively generated. Therefore, the number density of coarse Bi particles at the depth position D of 0.08R 0.08R becomes excessively high.

[0113] On the other hand, if the stirring power density ε of the molten steel after adding Bi to the molten steel exceeds 100 W / t, Bi in the molten steel tends to aggregate. In this case, in the produced steel material, the number density of fine Bi particles decreases. Therefore, the number density of fine Bi particles at the depth position D of 0.08R 0.08R becomes excessively low.

[0114] If the stirring power density ε of the molten steel after adding Bi is 10 to 100 W / t, then Bi will diffuse sufficiently in the molten steel. Therefore, assuming that condition 2 and conditions 3 and 4 described later are met, the position D at a depth of 0.08R in the steel material is determined. 0.08R and 0.65R depth position D 0.65R The number densities of fine Bi particles and coarse Bi particles are within an appropriate range.

[0115] [(Process 2) Casting Process] In the casting process, molten steel is used to produce cast slabs using a well-known continuous casting method. The casting process is carried out under the following conditions: (Condition 3) When the surface solidification and cooling rate is defined as the solidification and cooling rate at a depth of 15 mm from the surface in a cross section perpendicular to the longitudinal direction of the cast slab, the surface solidification and cooling rate shall be 550°C / min or higher. (Condition 4) In a cross-section perpendicular to the longitudinal direction of the cast slab, if the cross-sectional shape is rectangular, the internal solidification and cooling rate is defined as the solidification and cooling rate at the midpoint between the slab surface and the center of the cast slab, on the line midway along the width of the long side of the rectangular cross-section. If the cross-sectional shape is circular, the internal solidification and cooling rate is defined as the solidification and cooling rate at the midpoint of the radius R (i.e., at a depth of R / 2). In this case, the internal solidification and cooling rate shall be 100°C / min or less. Conditions 3 and 4 will be explained below.

[0116] [Condition 3: Regarding the surface solidification and cooling rate] Bi particles are generated during solidification. In other words, Bi particles are generated by crystallization. Therefore, in order for the steel material to satisfy characteristic 3, it is preferable to increase the solidification and cooling rate of the surface portion of the cast slab so that the steel solidifies before the Bi particles coarseen. Accordingly, in the casting process, the cooling rate of the cast slab in the mold of the continuous casting apparatus is adjusted so that the solidification and cooling rate at a depth of 15 mm from the surface of the manufactured cast slab is 550°C / min or more.

[0117] Here, the cooling rate from the liquidus temperature to the solidus temperature within the temperature range of steel in the casting process is defined as the solidification cooling rate (°C / min). As described above, the solidification cooling rate at a depth of 15 mm from the surface of the cast slab is defined as the "surface solidification cooling rate." The surface solidification cooling rate is determined by the following method.

[0118] A test specimen is taken from a cross section perpendicular to the longitudinal direction of a slab manufactured by continuous casting, including a point 15 mm deep from the surface of the slab. For example, if the cross section perpendicular to the longitudinal direction of the slab is rectangular, a test specimen is taken from the center of the width of the slab's surface (or the center of the width of the surface corresponding to the longer side if the cross section is rectangular) including a point 15 mm deep. Of the test specimen, the surface corresponding to the cross section perpendicular to the longitudinal direction of the slab is designated as the observation surface. Within the observation surface, a 5 mm × 5 mm area centered at a depth of 15 mm from the surface of the slab is designated as the observation region. In the observation region, the spacing between 10 dendrite secondary arms is measured, and its arithmetic mean is taken as λ² (μm). Using the measured dendrite secondary arm spacing λ² (μm), the surface solidification and cooling rate V (°C / min) is determined. V = (λ² / 770) -1 / 0.41

[0119] The region within 15 mm of the surface of the cast slab solidifies as it passes through the mold of the continuous casting apparatus. Therefore, the surface solidification and cooling rate is controlled by the cooling mechanism in the mold. There is no particular upper limit to the surface solidification and cooling rate. A preferred lower limit for the surface solidification and cooling rate is 600°C / min or higher, and a more preferred lower limit is 700°C / min or higher. As an example of a preferred casting speed to obtain a surface solidification and cooling rate of 550°C / min or higher, a rate of 0.6 m / min or less can be cited.

[0120] [Condition 4: Regarding the internal solidification and cooling rate] As mentioned above, Bi particles are generated during solidification. Therefore, in order for the steel material to satisfy characteristic 4, it is preferable to slow down the solidification and cooling rate inside the cast slab to coarseen the Bi particles before solidifying the steel. Accordingly, in the casting process, the cooling rate of the cast slab is adjusted so that the internal solidification and cooling rate is 100°C / min or less.

[0121] Here, the internal solidification cooling rate based on the definition above is determined by the following method. For slabs manufactured by continuous casting, if the cross-sectional shape is rectangular, a test specimen is taken that includes the midpoint between the slab surface and the center of the slab, on the line midway along the width of the long side of the rectangular cross-section. If the cross-sectional shape is circular, a test specimen is taken that includes the midpoint of the radius R (R / 2 depth position). Of the test specimens, the surface corresponding to the cross-section perpendicular to the longitudinal direction of the slab is designated as the observation surface. Within the observation surface, a 5 mm × 5 mm area centered on the aforementioned midpoint is designated as the observation region. In the observation region, the spacing between 10 dendrite secondary arms is measured, and the arithmetic mean is taken as λ² (μm). Using the measured dendrite secondary arm spacing λ² (μm), the internal solidification and cooling rate V (°C / min) is determined. V = (λ² / 770) -1 / 0.41

[0122] The internal solidification and cooling rate can be adjusted, for example, by changing the size of the mold. Specifically, the internal solidification and cooling rate can be adjusted by adjusting the area of ​​the cross-sectional surface of the mold. Furthermore, in the group of rolls arranged downstream of the mold in a continuous casting machine, multiple fluid nozzles for cooling the cast slab are arranged between the rolls. Therefore, the internal solidification and cooling rate can be adjusted by adjusting the flow rate of the fluid (coolant such as water, air, or a mixture of coolant and air) sprayed from these multiple fluid nozzles. The lower limit of the internal solidification and cooling rate is not particularly limited, but a preferred lower limit is 10°C / min or higher. A preferred upper limit is 50°C / min or lower, and a more preferred upper limit is 25°C / min or lower.

[0123] [(Process 3) Hot working process] The hot working process is optional. In other words, the hot working process may or may not be performed. If the hot working process is performed, the hot working process is carried out on the cast slab produced in the casting process to produce steel material.

[0124] The hot working process may, for example, only be a well-known rough rolling process, or may include a well-known rough rolling process and a well-known finish rolling process carried out after the rough rolling process. In the rough rolling process, for example, billets are produced from heated slabs or steel ingots by block rolling, or by hot rolling with a continuous rolling mill after block rolling. In the finish rolling process, for example, finish rolling is carried out on the heated billets using a well-known continuous rolling mill to produce steel materials (bar steel). The heating temperature in the rough rolling process is, for example, 1000 - 1300°C. The heating temperature in the finish rolling process is, for example, 1000 - 1300°C.

[0125] In the above hot working process, steel materials are produced by hot rolling. However, steel materials may also be produced by other hot working processes other than hot rolling. For example, instead of hot rolling, steel materials may be produced by hot forging. Also, steel materials may be produced by carrying out hot rolling and hot forging. Even when hot forging is carried out in the hot working process, the heating temperature is, for example, 1000 - 1300°C.

[0126] Through the above manufacturing process, the steel materials of this embodiment are produced. As described above, the hot working process may be omitted. That is, the steel materials of this embodiment may be cast products (slabs).

[0127] [Method for manufacturing parts for mechanical structures] As described above, the steel materials of this embodiment are used as materials for parts for mechanical structures. The method for manufacturing parts for mechanical structures is well-known and, for example, is as follows.

[0128] The steel materials of this embodiment are hot worked to produce intermediate products with a rough shape of parts for mechanical structures (for example, crankshafts). The hot working is, for example, hot forging. The produced intermediate products are air-cooled.

[0129] After cooling, the intermediate product is subjected to machining to cut it into a predetermined shape. The machined intermediate product is then subjected to well-known high-frequency induction hardening (tempering omitted), or well-known high-frequency induction hardening and well-known tempering. Through these processes, a machine structural component is manufactured. [Examples]

[0130] The effects of the steel material of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

[0131] Steel materials having the chemical compositions shown in Tables 1-1 to 1-3 were manufactured.

[0132] [Table 1-1]

[0133] [Table 1-2]

[0134] [Table 1-3]

[0135] Specifically, a 70-ton converter was used to carry out the refining process (primary refining process and secondary refining process). In the primary refining process, molten iron produced by a well-known method was refined in the converter under the same conditions.

[0136] After the primary refining process, a secondary refining process was carried out. Specifically, refining was performed using an LF (Ladle Furnace), followed by RH vacuum degassing. Through these processes, the composition of elements other than Bi was adjusted. Subsequently, Bi was added to the molten steel using a wire, and the molten steel was stirred to further adjust the Bi composition.

[0137] The time t0 (minutes) from the addition of Bi to the molten steel until the end of stirring in the secondary refining process is shown in Table 2. Furthermore, the stirring power density ε (W / t) during stirring is also shown in Table 2. The molten steel temperature from the addition of Bi to the end of stirring in the secondary refining process was 1510-1630°C. Molten steel was produced by the above process.

[0138] [Table 2]

[0139] A slab (bloom) was produced using molten steel by continuous casting. The solidification and cooling rate (°C / min) was adjusted during casting. The surface solidification and cooling rates and the internal solidification and cooling rates are shown in Table 2. The surface solidification and cooling rates and the internal solidification and cooling rates were determined using the methods described in [Condition 3: Surface Solidification and Cooling Rate] and [Condition 4: Internal Solidification and Cooling Rate] above.

[0140] Hot working was performed on the manufactured bloom. Specifically, rough rolling was performed on the bloom to produce a billet with a cross-section of 180 mm x 180 mm. The heating temperature of the bloom during rough rolling was 1250°C.

[0141] Furthermore, hot forging equivalent to finish rolling was performed on the billet to produce a steel bar with a diameter of 97 mm. The billet was heated to 1250°C during hot forging. Forging was carried out using an air hammer (model 600HP) manufactured by Otani Machinery Works Co., Ltd. After heating the billet at 1250°C for 1 hour, it was forged to a cross-section of 120 mm x 120 mm. Then, after heating it again at 1250°C for 1 hour, it was forged to a cross-section of an octagon circumscribed around a circle with a diameter of 97 mm. Then, after heating it again at 1250°C for 1 hour, it was forged to a steel bar (round steel) with a diameter of 97 mm and a circular cross-section. The steel material was manufactured through the above manufacturing process.

[0142] [Evaluation Test] The following evaluation tests were conducted on the steel materials for each test number. (Test 1) 0.08R depth position D 0.08R Number density measurement test of fine and coarse Bi particles (Test 2) 0.65R depth position D 0.65R Number density measurement test of fine and coarse Bi particles (Test 3) Hot working crack evaluation test (Test 4) Molten Cracking Evaluation Test (Test 5) Machinability evaluation test (drill life test) (Test 6) Fatigue strength evaluation test (rotational bending fatigue test) The following describes each evaluation test.

[0143] [(Test 1) 0.08R depth position D 0.08R [Number density measurement test of fine and coarse Bi particles] For the steel material of each test number, the above-mentioned [0.08R depth position D 0.08R The method for measuring the number density of fine Bi particles and coarse Bi particles at depth position D is as described in []. 0.08R Number density of fine Bi particles (particles / mm³) 2 ) and 0.08R depth position D 0.08R Number density of coarse Bi particles (particles / mm³) 2 The result was calculated. The results are shown in Table 2.

[0144] [(Test 2) 0.65R depth position D 0.65R [Number density measurement test of fine and coarse Bi particles] For the steel material of each test number, the above-mentioned [0.65R depth position D 0.65R The method for measuring the number density of fine and coarse Bi particles at depth position D is as described in []. 0.65R Number density of fine Bi particles (particles / mm³) 2 ) and 0.65R depth position D 0.65R Number density of coarse Bi particles (particles / mm³) 2 The result was calculated. The results are shown in Table 2.

[0145] [(Test 3) Hot Working Cracking Evaluation Test] The surface of each test sample was observed. If two or more clear cracks were observed per meter along the longitudinal direction of the steel material, it was determined that hot working cracks had occurred. Conversely, if no two or more clear cracks were observed per meter along the longitudinal direction of the steel material, it was determined that hot working cracks had been suppressed. Here, a clear crack refers to a crack of 3 mm or longer observed with the naked eye or using a simple magnifying glass.

[0146] The evaluation results for hot working cracking are shown in the "Hot Working Cracking" column of Table 2. "NG" indicates that hot working cracking occurred, and "OK" indicates that hot working cracking was suppressed.

[0147] [(Tests 4-6) Regarding the molten cracking evaluation test, machinability evaluation test, and fatigue strength evaluation test] [Manufacturing of intermediate parts for simulated mechanical structures] A heat treatment simulating hot forging during the manufacturing process of machine structural parts was performed using the steel material of each test number. Specifically, the steel material was heated and held at 1100°C for 30 minutes. After that, the steel material was allowed to cool in the atmosphere. The steel material subjected to the above heat treatment will hereafter be referred to as a "simulated intermediate product of a machine structural part (or simply a simulated intermediate product)." The simulated intermediate product of a machine structural part was a steel bar (round steel) with a diameter of 97 mm.

[0148] [(Test 4) Molten Cracking Evaluation Test] A test specimen measuring 10 mm in width, 3 mm in thickness, and 100 mm in length was fabricated by machining from the surface region SA of a simulated intermediate product of a mechanical structural component. The longitudinal direction of the test specimen was parallel to the longitudinal direction of the simulated intermediate product. Furthermore, the central axis parallel to the longitudinal direction of the test specimen coincided with the 0.08R depth position.

[0149] A heat treatment test simulating high-frequency induction hardening was performed on a test specimen using a thermal cycle testing apparatus manufactured by Fuji Denpa Koki Co., Ltd. Specifically, the test specimen was heated to 1400°C at a heating rate of 100°C / second. The test specimen was then held at 1400°C for 15 seconds. After that, the test specimen was water-cooled.

[0150] After water cooling, the specimen was cut perpendicular to its longitudinal direction at the center of the specimen's longitudinal direction. The cut surface was then used as the observation surface. The observation surface was mechanically polished. The mechanically polished observation surface was then etched with picral reagent. The center of the etched observation surface was observed with a 400x optical microscope, and the presence or absence of fusion cracks was visually confirmed. The observation field of view was set to 250 μm × 400 μm.

[0151] In the observation surface, if a clearly corroded region (corrosion area) with a length of 5 μm or more was observed at the grain boundary, it was determined that molten cracking had occurred. A clearly corroded region with a width of 5 μm or more at the grain boundary refers, for example, to the region indicated by reference numeral 15 in Figure 3. On the other hand, if no corrosion area was observed at the grain boundary, as shown in Figure 4, it was determined that molten cracking had been suppressed. The evaluation results for molten cracking are shown in the "Molten Cracking" column of Table 2. If molten cracking occurs, "NG" is indicated. If molten cracking is suppressed, "OK" is indicated.

[0152] [(Test 5) Machinability Evaluation Test (Drill Life Test)] Test specimens for machinability evaluation were cut from simulated intermediate parts of machine structural components. Specifically, a drill was used to drill holes at arbitrary positions in the internal region CA of the cross-section perpendicular to the longitudinal direction of the 97 mm diameter simulated intermediate part. A drill of model SD3.0 manufactured by Fujikoshi Corporation was used as the tool. The drilling conditions were a feed rate of 0.25 mm / rev per revolution and a drilling depth of 9 mm per hole. A water-soluble cutting oil was used as the lubricant.

[0153] The machinability of the steel material was evaluated by performing drilling under the drilling conditions described above. The maximum cutting speed VL1000 (m / min) was used as the evaluation index. The maximum cutting speed VL1000 is the maximum cutting speed of a drill that can drill a hole with a cumulative depth of 1000 mm in length.

[0154] Based on the maximum cutting speed VL1000, the machinability was evaluated as follows. VL1000 at 20 m / min or more: Excellent machinability ("OK") VL1000 less than 20 m / min: Low machinability ("NG") The "Machinability" column in Table 2 shows the evaluation results as "OK" or "NG".

[0155] [(Test 6) Fatigue Strength Evaluation Test (Rotational Bending Fatigue Test)] The fatigue strength was evaluated using fatigue test specimens simulating mechanical structural parts manufactured from steel, according to the following test method.

[0156] A fatigue test specimen, as shown in Figure 5, was prepared from a simulated intermediate product of a mechanical structural component. The fatigue test specimen was a round bar specimen with a parallel section diameter D1 of 8 mm and a gripping section diameter of 12 mm. The fatigue test specimen was prepared by machining from the R / 2 position (i.e., the center of the radius) of the cross-section perpendicular to the longitudinal direction of the simulated intermediate product of the mechanical structural component. The longitudinal direction of the fatigue test specimen was parallel to the longitudinal direction of the simulated intermediate product. It is common technical knowledge to those skilled in the art that if the rotational bending fatigue strength of the test specimen before high-frequency induction hardening is sufficiently high, the rotational bending fatigue strength of the test specimen after high-frequency induction hardening will also be sufficiently high.

[0157] The parallel sections of the fatigue test specimens were polished to adjust the surface roughness. Specifically, the mean surface roughness (Ra) was set to within 3.0 μm and the maximum height (Rmax) to within 9.0 μm, in accordance with JIS B 0601:2001.

[0158] Using fatigue test specimens, an Ono-type rotary bending fatigue test was performed at room temperature (23°C), in an atmospheric environment, under the condition of alternating rotation at a rotation speed of 3600 rpm. Fatigue tests were conducted on multiple specimens with varying stresses applied, and 10 7 The fatigue strength (MPa) was defined as the highest stress at which fracture did not occur after the cycle.

[0159] If the obtained fatigue strength was 300 MPa or higher, 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 Table 2. A fatigue strength of 300 MPa or higher was marked as "OK," and a fatigue strength of less than 300 MPa was marked as "NG."

[0160] [Test Results] The test results are shown in Tables 1-1 to 1-3 and Table 2.

[0161] Referring to Tables 1-1 to 1-3 and Table 2, the steel materials for test numbers 1 to 38 had an appropriate chemical composition and satisfied formula (1). Furthermore, the manufacturing conditions were also appropriate. Therefore, the steel materials for each test number satisfied characteristics 1 to 4. As a result, no hot working cracks or melt cracks were observed, and excellent hot working crack resistance and excellent melt crack resistance were obtained. Furthermore, the maximum cutting speed VL1000 of the steel materials was 20 m / min or more, and excellent machinability was obtained. In addition, the fatigue strength of the steel materials was 300 MPa or more, and the fatigue strength of the machine structural parts manufactured using the steel materials was high.

[0162] On the other hand, in test numbers 39 and 40, Fn was less than 0.45. Therefore, the fatigue strength of the machine structural components manufactured using steel as the material was low.

[0163] In tests 41 and 42, the Fn value was greater than 1.05. Therefore, the machinability of the steel material was poor.

[0164] In tests 43 and 44, the time t0 from the addition of Bi to the end of stirring was 15 minutes or less. Therefore, the depth position D of 0.08R 0.08R The number density of coarse Bi particles is 0.25 particles / mm³ 2 It was too extreme. As a result, hot working cracks occurred.

[0165] In tests 45 and 46, the time t0 from the addition of Bi to the end of stirring was 60 minutes or more. Therefore, the depth position D of 0.08R 0.08R The number density of the fine Bi particles is 15.00 particles / mm³ 2 It fell below a certain level. As a result, molten cracking occurred.

[0166] In tests 47 and 48, the stirring power density ε of the molten steel after Bi addition was less than 10 W / t. Therefore, the depth position D of 0.08R was 0.08R The number density of coarse Bi particles is 0.25 particles / mm³ 2It was too extreme. As a result, hot working cracks occurred.

[0167] In tests 49 and 50, the stirring power density ε of the molten steel after Bi addition exceeded 100 W / t. Therefore, the depth position D of 0.08R was 0.08R The number density of the fine Bi particles is 15.00 particles / mm³ 2 It fell below a certain level. As a result, molten cracking occurred.

[0168] In tests 51 and 52, the surface solidification and cooling rate during casting was less than 550°C / min. Therefore, the depth position D of 0.08R was 0.08R The number density of the fine Bi particles is 15.00 particles / mm³ 2 It is less than 0.08R depth position D 0.08R The number density of coarse Bi particles is 0.25 particles / mm³ 2 It became excessive. As a result, hot working cracks and molten cracks occurred.

[0169] In tests 53 and 54, the internal solidification and cooling rate during casting exceeded 100°C / min. Therefore, the 0.65R depth position D 0.65R The number density of the fine Bi particles is 15.00 particles / mm³ 2 That is all, 0.65R depth position D 0.65R The number density of coarse Bi particles is 0.25 particles / mm³ 2 The results were as follows. As a result, the machinability of the steel material was low.

[0170] In test number 55, the Bi content was too low. Therefore, the depth position D was 0.08R. 0.08R The number density of the fine Bi particles is 15.00 particles / mm³ 2 It is less than 0.65R depth position D 0.65R The number density of coarse Bi particles is 0.25 particles / mm³ 2 The following occurred. As a result, molten cracking occurred, and the machinability of the steel material was poor.

[0171] In test number 56, the Bi content was too high. Therefore, the depth position D was set to 0.08R. 0.08R The number density of coarse Bi particles is 0.25 particles / mm³ 2It became excessive. As a result, hot working cracks occurred.

[0172] Embodiments of the present invention have been described above. However, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and the embodiments described above can be appropriately modified and implemented without departing from the spirit of the invention. [Explanation of symbols]

[0173] 1. Fillet radius 2. Crankshaft edge 15. Molten cracking

Claims

1. A steel material with a circular cross-section perpendicular to the axial direction, The chemical composition is expressed in mass percent. C: more than 0.30 to 0.60%, Si: 0.01-0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.200% or less, Bi: 0.0051-0.2500%, Al: 0.001-0.100%, N: 0.0250% or less, O: 0.0050% or less, Cr: 0 to 1.30%, V: 0-0.200%, Sn: 0-0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0-0.4000%, Zr: 0 to 0.2000%, Ca: 0-0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, Rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0-0.50%, and, The remainder consists of Fe and impurities. The Fn defined in equation (1) is between 0.45 and 1.

05. When the radius of the steel material is defined as R, at a depth of 0.08R from the surface of the steel material, The number density of fine Bi particles, which are Bi particles with an equivalent circular diameter of 0.1 to 1.0 μm, is 15.00 particles / mm³. ２ That's all. The number density of coarse Bi particles, which are Bi particles with an equivalent circular diameter of 10.0 μm or more, is 0.25 particles / mm². ２ The following: At a depth of 0.65R from the surface of the steel material, The number density of the aforementioned fine Bi particles is 15.00 particles / mm². ２ It is less than, The number density of the aforementioned coarse Bi particles is 0.25 particles / mm³. ２ It is super, Steel material. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element, and if an element is not present, "0" is substituted for the corresponding element symbol.

2. The steel material according to claim 1, The aforementioned chemical composition is Cr: 0.01-1.30%, V: 0.001-0.200%, Sn: 0.0001 to 0.1000%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Pb: 0.01-0.09%, Mg: 0.0001-0.0100%, Ti: 0.0001 to 0.0400%, Nb: 0.0001 to 0.0500%, W: 0.0001-0.4000%, Zr: 0.0001 to 0.2000%, Ca: 0.0001-0.0100%, Te: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, Rare earth elements: 0.0001 to 0.0100%, Co: 0.0001 to 0.0100%, Se: 0.0001 to 0.0100%, In: 0.0001 to 0.0100%, Mo: 0.01-0.30%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, It contains one or more selected from the group consisting of, Steel material.

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