Steel material and crankshaft using the same

Abstract

Provided is a steel material having excellent machinability, bending fatigue strength, wear resistance, and bending straightening property. The steel material contains, in mass%, C: 0.25% to 0.35%, Si: 0.05% to 0.35%, Mn: 0.85% to 1.20%, P: 0.080% or less, S: 0.030% to 0.100%, Cr: 0.10% or less, Ti: 0.050% or less, Al: 0.050% or less, N: 0.005% to 0.024%, O: 0.0100% or less, and the remainder composed of Fe and impurities, Fn1 defined in the specification is 1.00 to 2.05, and Fn2 is 0.42 to 0.60. The number density of MnS single inclusions and MnS complex inclusions having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 The total number of the MnS single inclusions and the MnS complex inclusions is 70% or more of the total number of inclusions, and the number of the MnS complex oxides is 30% or more of the total number of oxides.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel material and a crankshaft, and more particularly to a steel material that becomes a raw material of a crankshaft and a crankshaft manufactured by subjecting the steel material to nitriding treatment. BACKGROUND

[0002] In conveyances typified by automobiles, trucks, and construction machines, a crankshaft is used. The crankshaft is required to have excellent bending fatigue strength. Also, in recent years, in order to reduce environmental burdens, an idling stop technique that repeatedly performs starting and stopping of an engine is becoming widespread. If the frequency of repeatedly performing starting and stopping of the engine is increased, the frequency of operation of the crankshaft is increased before an oil film (an oil film formed by engine oil) is sufficiently formed at a sliding portion such as a pin portion and a journal portion of the crankshaft. Also, in recent years, in order to improve fuel economy, the viscosity of engine oil is being reduced. Therefore, there is a tendency that the thickness of the oil film that protects the sliding portion of the crankshaft is reduced. Thus, the crankshaft is required not only to have excellent bending fatigue strength but also to have excellent wear resistance.

[0003] Also, in conjunction with the above-described requirement for improvement in fuel economy, parts of conveyances are being reduced in weight. As a result, a crankshaft that has a complex shape that has not been used in the past and that is difficult to machine has appeared. Thus, a steel material that becomes a raw material of the crankshaft is required to have excellent machinability.

[0004] As a technique for improving the bending fatigue strength and the wear resistance of a crankshaft among the above-described bending fatigue strength, wear resistance, and machinability, nitriding treatment is known. Among them, the nitriding treatment in the present specification also includes soft nitriding treatment. The nitriding treatment is a heat treatment technique in which nitrogen (or nitrogen and carbon) is diffused and permeated into a surface layer of a steel material at a temperature of equal to or lower than the A1 transformation point. A nitriding layer composed of a compound layer and a diffusion layer is formed at the surface layer of the crankshaft on which the nitriding treatment is performed. The compound layer is formed at the outermost surface of the crankshaft, is mainly composed of nitrides represented by Fe3N, and has a depth of several tens of μm to about 30 μm. The diffusion layer is formed at a position inside the compound layer, is a region in which nitrogen diffused into the inside of the steel material is solidified, and has a depth of about several hundred μm. The nitriding treatment has the following characteristics: a strain generated after heat treatment is smaller compared to other surface solidification heat treatments such as high-frequency quenching treatment and carburizing quenching treatment.

[0005] However, even with the nitriding treatment, the strain after heat treatment cannot be completely eliminated. Also, the crankshaft is particularly required to have high straightness. Therefore, the crankshaft after the nitriding treatment is generally subjected to a bending correction process to improve the straightness of the crankshaft. If a crack is generated in the crankshaft at the time of bending correction, the bending fatigue strength is significantly reduced. Thus, in the steel material for the nitriding treatment, excellent bending correction properties, that is, a property of suppressing generation of a crack at the bending correction process, are required.

[0006] Techniques for improving the bending fatigue strength and the wear resistance of a nitrided component represented by a crankshaft are disclosed in International Publication No. 2016 / 182013 (Patent Document 1) and Japanese Patent Application Publication No. 2013-7077 (Patent Document 2).

[0007] For the nitrided component disclosed in Patent Document 1, the compound layer is set to be a γ' phase (Fe4N) main body by controlling the nitriding potential in a nitriding furnace, and the compound layer of the γ' phase main body is thickened. It is described in Patent Document 1 that by setting the compound layer to be the γ' phase main body, the fatigue strength of the nitrided component can be maintained, and the wear resistance can be improved.

[0008] In Patent Document 2, after a pretreatment consisting of fluorination treatment is performed, a nitriding treatment is performed. Thereby, on the surface layer of the steel material, a wear resistant layer (1st diffusion layer) in which nitrogen is also enriched in a state in which carbon is enriched, and a diffusion layer (2nd diffusion layer) in which carbon is the main body and in which the nitrogen concentration is low, which is located at a position inside the steel material than the 1st diffusion layer, are formed. It is described in Patent Document 2 that by forming a nitrided layer having such a structure, the fatigue strength and the wear resistance are excellent.

[0009] Prior art documents

[0010] Patent documents

[0011] Patent Document 1: International Publication No. 2016 / 182013

[0012] Patent Document 2: Japanese Patent Application Publication No. 2013-7077 SUMMARY

[0013] Problems to be solved by the invention

[0014] The fatigue strength and the wear resistance of a crankshaft can also be improved by techniques other than those disclosed in Patent Document 1 and Patent Document 2. However, in Patent Document 1 and Patent Document 2, research on the machinability of a steel material that becomes a raw material of a crankshaft, and the bending straightening property of a crankshaft is not performed.

[0015] An object of the present application is to provide a steel material that becomes a raw material of a crankshaft, and a crankshaft that uses the steel material as a raw material, which has excellent machinability, and has excellent bending fatigue strength, excellent wear resistance, and excellent bending straightening property when a nitriding treatment is performed to form the crankshaft.

[0016] Solution to the problem

[0017] The steel material of the present application contains, in mass%:

[0018] C: 0.25% to 0.35%,

[0019] Si: 0.05% to 0.35%,

[0020] Mn: 0.85% to 1.20%,

[0021] P: 0.080% or less,

[0022] S: 0.030% to 0.100%,

[0023] Cr: 0.10% or less,

[0024] Ti: 0.050% or less,

[0025] Al: 0.050% or less,

[0026] N: 0.005% to 0.024%, and

[0027] O: 0.0100% or less,

[0028] the remainder consisting of Fe and impurities,

[0029] Fn1 defined by mathematical expression (1) is 1.00 to 2.05,

[0030] Fn2 defined by mathematical expression (2) is 0.42 to 0.60,

[0031] in the steel material,

[0032] an inclusion in which the total of the Mn content and the S content is 80.0% or more by mass is defined as a MnS single inclusion,

[0033] an inclusion in which the total of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as a MnS complex inclusion,

[0034] an inclusion in which the total of the Al content, the Ca content, and the O content is 80.0% or more by mass and the total of the Mn content and the S content is less than 15.0% by mass is defined as a single oxide,

[0035] an inclusion in which the total of the Al content, the Ca content, and the O content is 15.0% to less than 80.0% by mass and the total of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as a MnS complex oxide, in this case,

[0036] in the steel material,

[0037] The number density of the total of the MnS single inclusions having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusions having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 The above,

[0038] The proportion of the total number of the MnS single inclusions having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusions having a circular equivalent diameter of 1.0 μm or more with respect to the total number of inclusions having a circular equivalent diameter of 1.0 μm or more is 70% or more,

[0039] The proportion of the number of the MnS complex oxides having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the single oxides having a circular equivalent diameter of 1.0 μm or more and the MnS complex oxides having a circular equivalent diameter of 1.0 μm or more is 30% or more,

[0040] Fn1 = Mn + 7.24Cr + 6.53Al ··· (1)

[0041] Fn2 = C + 0.10Si + 0.19Mn + 0.23Cr - 0.34S ··· (2)

[0042] In the mathematical expression (1) and the mathematical expression (2), the content of each element is substituted with the corresponding element in mass %.

[0043] The crankshaft of the present application comprises:

[0044] a pin portion;

[0045] a journal portion; and

[0046] an arm portion disposed between the pin portion and the journal portion,

[0047] at least the pin portion and the journal portion comprise:

[0048] a nitrided layer formed on a surface layer; and

[0049] a core portion located at a position inside the nitrided layer,

[0050] the core portion contains, in mass %:

[0051] C: 0.25% to 0.35%,

[0052] Si: 0.05% to 0.35%,

[0053] Mn: 0.85% to 1.20%,

[0054] P: 0.080% or less,

[0055] S: 0.030% to 0.100%,

[0056] Cr: 0.10% or less,

[0057] Ti: 0.050% or less,

[0058] Al: 0.050% or less,

[0059] N: 0.005% to 0.024%, and

[0060] O: 0.0100% or less,

[0061] the remainder consisting of Fe and impurities,

[0062] Fn1 defined by mathematical expression (1) is 1.00 to 2.05,

[0063] Fn2 defined by mathematical expression (2) is 0.42 to 0.60,

[0064] in the inclusions in the core portion,

[0065] an inclusion in which the total of the Mn content and the S content is 80.0% or more by mass is defined as an MnS single inclusion,

[0066] an inclusion in which the total of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as an MnS complex inclusion,

[0067] an inclusion in which the total of the Al content, the Ca content, and the O content is 80.0% or more by mass and the total of the Mn content and the S content is less than 15.0% by mass is defined as a single oxide,

[0068] an inclusion in which the total of the Al content, the Ca content, and the O content is 15.0% to less than 80.0% by mass and the total of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as an MnS complex oxide, in this case,

[0069] in the core portion,

[0070] the number density of the total of the MnS single inclusion having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 or more,

[0071] the proportion of the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 1.0 μm or more with respect to the total number of inclusions having a circular equivalent diameter of 1.0 μm or more is 70% or more,

[0072] the number of the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more is 30% or more relative to the total number of the single oxides having a circle equivalent diameter of 1.0 μm or more and the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more,

[0073] Fn1 = Mn + 7.24Cr + 6.53Al...(1)

[0074] Fn2 = C + 0.10Si + 0.19Mn + 0.23Cr - 0.34S...(2)

[0075] wherein the content of the corresponding element is substituted for each element symbol in mathematical formula (1) and mathematical formula (2) in mass %.

[0076] Effects of the invention

[0077] The steel material of the present application is excellent in machinability, and has excellent bending fatigue strength, excellent wear resistance, and excellent bending straightening property in the case where a crankshaft is formed by performing nitriding treatment. The crankshaft of the present application has excellent bending fatigue strength, excellent wear resistance, and excellent bending straightening property. BRIEF DESCRIPTION OF DRAWINGS

[0078] Figure 1 is a schematic view for explaining the position of a sample for determination of inclusions collected from a steel material which becomes a raw material of a crankshaft.

[0079] Figure 2 is a view showing one example of a main part of a crankshaft of the present embodiment.

[0080] Figure 3 is a cross-sectional view of the surface layer vicinity of a pin portion or a journal portion of a crankshaft in Figure 2

[0081] Figure 4 is a side view of a bending fatigue test piece for a rotary bending fatigue test of Example No. 1.

[0082] Figure 5 are a front view, a side view, and a plan view of a bending test piece for a 4-point bending test of Example.

[0083] Figure 6 is a perspective view showing a ring block method wear tester in Example. DETAILED DESCRIPTION

[0084] The inventors have researched a steel material which becomes a raw material of a crankshaft, which obtains excellent machinability in a manufacturing process of a crankshaft, and exhibits excellent bending fatigue strength, excellent wear resistance, and excellent bending straightening property in the case where a crankshaft is formed by performing nitriding treatment.​

[0085] First, the inventors studied the chemical composition of a steel material that can improve the machinability described above, and can improve the bending fatigue strength, wear resistance, and bending straightening property in the case of forming a crankshaft. As a result, it was considered that as long as a steel material having a chemical composition containing, in mass %, C: 0.25 to 0.35%, Si: 0.05 to 0.35%, Mn: 0.85 to 1.20%, P: 0.080% or less, S: 0.030 to 0.100%, Cr: 0.10% or less, Ti: 0.050% or less, Al: 0.050% or less, N: 0.005 to 0.024%, O: 0.0100% or less, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Mo: 0 to 0.10%, Nb: 0 to 0.050%, Ca: 0 to 0.0100%, Bi: 0 to 0.30%, Te: 0 to 0.0100%, Zr: 0 to 0.0100%, Pb: 0 to 0.09%, and the remainder consisting of Fe and impurities, the machinability can be improved, and in the case of forming a crankshaft by nitriding treatment, there is a possibility that the bending fatigue strength, wear resistance, and bending straightening property can be improved. Therefore, the machinability, bending fatigue strength, wear resistance, and bending straightening property were studied based on the chemical composition described above.

[0086] The bending fatigue strength after nitriding treatment has a positive correlation with the hardness of the nitrided layer formed on the surface of the steel material after nitriding treatment and the hardness of the core portion inside the nitrided layer. On the other hand, the bending straightening property after nitriding treatment has a negative correlation with the hardness of the nitrided layer of the steel material after nitriding treatment. Furthermore, the machinability has a negative correlation with the hardness of the steel material before nitriding treatment (that is, the core portion of the steel material after nitriding treatment that is not affected by the nitriding treatment). Thus, in order to improve the bending fatigue strength, wear resistance, and bending straightening property after nitriding treatment, and to improve the machinability of the steel material in the manufacturing process of the crankshaft, it is necessary to control the hardness of the nitrided layer of the steel material after nitriding treatment and the hardness of the core portion of the steel material after nitriding treatment within a certain range.

[0087] The hardness of the nitrided layer of the steel material after nitriding treatment is determined by the hardness of the steel material before nitriding treatment and the increase in the hardness of the surface layer of the steel material due to nitriding treatment. Among them, the "increase in the hardness of the surface layer of the steel material due to nitriding treatment" refers to the difference between the hardness of the nitrided layer formed due to nitriding treatment and the hardness of the steel material before nitriding treatment. That is, the higher the hardness of the steel material before nitriding treatment (that is, the core portion of the steel material after nitriding treatment) and the greater the increase in the hardness of the surface layer of the steel material due to nitriding treatment, the higher the hardness of the nitrided layer of the steel material after nitriding treatment.

[0088] Here, the inventors have found that, in a steel material having the above chemical composition, the hardness of the steel material before nitriding treatment (i.e., the core portion after nitriding treatment) depends on the contents of elements that increase the hardness of the steel material by solid solution strengthening, i.e., C, Si, Mn, and Cr, and the content of an element that embrittles the steel material, i.e., S. Also, the inventors have found that the amount of increase in the hardness of the surface layer of the steel material due to nitriding treatment depends on the contents of elements having a high affinity with nitrogen, i.e., Mn, Cr, and Al.

[0089] Therefore, the inventors have studied the relationship between the contents of elements that increase the hardness of the surface layer of the steel material after nitriding treatment (Mn, Cr, and Al), the contents of elements that affect the hardness of the core portion after nitriding treatment (C, Si, Mn, Cr, and S), and machinability, bending fatigue strength, wear resistance, and bending straightening property, for a steel material in which the contents of each element in the chemical composition are within the above ranges. As a result, the inventors have obtained the following findings.

[0090] Fn1 is defined by mathematical expression (1), and Fn2 is defined by mathematical expression (2).

[0091] Fn1 = Mn + 7.24 Cr + 6.53 Al (1)

[0092] Fn2 = C + 0.10 Si + 0.19 Mn + 0.23 Cr - 0.34 S (2)

[0093] Here, the contents of the corresponding elements are substituted for each element symbol in mathematical expression (1) and mathematical expression (2) in mass %.

[0094] For Fn1, in a steel material in which the contents of each element in the chemical composition are within the above ranges, Fn1 is an index of the amount of increase in the hardness of the surface layer of the steel material due to nitriding treatment. That is, for Fn1, it is assumed that the contents of each element in the chemical composition of the steel material are within the above ranges, and Fn1 is related to the bending fatigue strength and bending straightening property of the steel material after nitriding treatment. If Fn1 is less than 1.00, then even if the contents of each element in the chemical composition are within the ranges of the present embodiment and Fn2 is within the range of the present embodiment, sufficient bending fatigue strength cannot be obtained in the steel material after nitriding treatment, i.e., the crankshaft. On the other hand, if Fn1 exceeds 2.05, then even if the contents of each element in the chemical composition are within the ranges of the present embodiment and Fn2 is within the range of the present embodiment, the bending straightening property of the steel material after nitriding treatment is reduced. If Fn1 is 1.00 to 2.05, then it is assumed that the contents of each element in the chemical composition are within the ranges of the present embodiment and Fn2 is within the range of the present embodiment, and sufficient bending fatigue strength and sufficient bending straightening property can be obtained in the crankshaft.

[0095] Fn2 is an index of the hardness of the steel material before nitriding treatment (that is, the core of the steel material after nitriding treatment) for Fn2 in the steel material in which the content of each element in the chemical composition is within the above range. Fn2 is related to the machinability of the steel material and the bending fatigue strength of the steel material after nitriding treatment for Fn2 on the premise that the chemical composition of the steel material is within the above range. If Fn2 is less than 0.42, even if the content of each element in the chemical composition is within the range of the present embodiment and Fn1 is within the range of the present embodiment, sufficient bending fatigue strength cannot be obtained in the steel material after nitriding treatment, that is, the crankshaft. On the other hand, if Fn2 exceeds 0.60, even if the content of each element in the chemical composition is within the range of the present embodiment and Fn1 is within the range of the present embodiment, sufficient machinability cannot be obtained in the steel material. If Fn2 is 0.42 to 0.60, on the premise that the content of each element in the chemical composition is within the range of the present embodiment and Fn1 is within the range of the present embodiment, sufficient machinability can be obtained in the steel material, and sufficient bending fatigue strength can be obtained in the crankshaft.

[0096] As described above, by setting the chemical composition to an appropriate range, it is possible to some extent to improve the machinability of the steel material, the bending fatigue strength of the steel material after nitriding treatment, and the bending straightening property. Therefore, the inventors also conducted a study of improving the machinability of the steel material and the wear resistance of the steel material after nitriding treatment using elements other than the chemical composition. Among them, the inventors conducted a study focusing on inclusions not only for machinability but also for wear resistance. As a result, the following insights were obtained for inclusions that affect machinability and wear resistance. In the following description, inclusions are defined as follows.

[0097] (a) Inclusions in which the total content of Mn and S is 80.0% or more in mass% on the premise that the mass% of the inclusion is 100% are defined as "MnS single inclusions".

[0098] (b) Inclusions in which the total content of Mn and S is 15.0% to less than 80.0% in mass% on the premise that the mass% of the inclusion is 100% are defined as "MnS complex inclusions".

[0099] (c) Inclusions in which the total content of Al, Ca, and O is 80.0% or more in mass% and the total content of Mn and S is less than 15.0% in mass% on the premise that the mass% of the inclusion is 100% are defined as "single oxides".

[0100] (d) When the mass percentage of inclusions is set to 100%, inclusions with a total mass percentage of Mn and S of 15.0% to less than 80.0% and a total mass percentage of Al, Ca and O of 15.0% to less than 80.0% are defined as “MnS composite oxides”.

[0101] Hereinafter, both individual MnS inclusions and MnS composite inclusions will be collectively referred to as "MnS-based inclusions". Furthermore, as defined above, MnS composite oxides are included within MnS composite inclusions.

[0102] Machinability is affected not only by the hardness of the steel before nitriding (the core of the nitrided steel) but also by inclusions. Specifically, the number density (numbers / mm²) of MnS-based inclusions (individual MnS inclusions and MnS composite inclusions) present in the steel is a significant factor. 2 The higher the number density of MnS inclusions, the better the machinability. However, if the size of the MnS inclusions is too small, their impact on machinability is minimal. Specifically, if the equivalent diameter of the MnS inclusions is less than 5.0 μm, their impact on the machinability of the steel is negligible. Therefore, increasing the number density of MnS inclusions with an equivalent diameter of 5.0 μm or greater is effective in improving the machinability of steel. Furthermore, the equivalent diameter refers to the diameter of a circle with the same area when the area of ​​each inclusion is converted into the diameter of a circle with the same area.

[0103] Furthermore, inclusions also affect the wear resistance of nitrided steel. A compound layer forms on the outermost layer of the nitrided layer on the surface of the nitrided steel. In crankshafts manufactured using nitriding, cracks form and progress in this compound layer, leading to its peeling and consequently, wear development. The compound layer is formed when the steel, originally a component of the steel, is altered by the nitriding process, resulting in a high nitrogen content. If inclusions are present on the surface of the steel before nitriding, and this surface is altered into a compound layer through nitriding, the inclusions will be contained within the compound layer.

[0104] The inventors have found that the cracks of the compound layer are not caused by the inclusions in the compound layer. Therefore, the inventors have focused on the types of the inclusions and studied the relationship between the types of the inclusions and the generation of the cracks of the compound layer. As a result, the inventors have found that the cracks of the compound layer, which are the cause of the wear, are mostly initiated from the hard oxides. In addition, the inventors have found that the soft MnS-based inclusions are not easy to be the initiation points of the cracks of the compound layer, and the MnS complex oxides, which are the complex inclusions of the MnS-based inclusions and the oxides, are not easy to be the initiation points of the cracks of the compound layer. Therefore, the inventors have found that, in the crankshaft manufactured by the nitriding treatment, in order to improve the wear resistance, it is effective to reduce the oxides as much as possible or to make the oxides complex inclusions (MnS complex oxides) with the MnS-based inclusions.

[0105] However, the oxides in the molten steel become the nuclei of the MnS-based inclusions, and thus, the oxides in the molten steel are required to some extent for the generation of the MnS-based inclusions. Therefore, the oxides are also generated to some extent in the steel material. Therefore, the inventors have further studied the relationship between the inclusions in the steel material and the machinability and the wear resistance of the steel material after the nitriding treatment, focusing on the above-described MnS-based inclusions (MnS single inclusions and MnS complex inclusions), the oxides, and the MnS complex oxides. As a result, the inventors have found that, if the inclusions in the steel material satisfy the following (I) to (III), which are premised on the fact that the element contents of the chemical compositions are within the ranges of the present embodiment and Fn1 and Fn2 are within the ranges of the present embodiment, the machinability of the steel material and the wear resistance of the crankshaft manufactured by the nitriding treatment of the steel material can be further improved.

[0106] (I) In the steel material, the number density of the total of the MnS single inclusions having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusions having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 or more.

[0107] (II) In the steel material, the proportion of the total number of the MnS single inclusions having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusions having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the inclusions having a circular equivalent diameter of 1.0 μm or more is 70% or more.

[0108] (III) In the steel material, the proportion of the number of the MnS complex oxides having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the oxides having a circular equivalent diameter of 1.0 μm or more and the MnS complex oxides having a circular equivalent diameter of 1.0 μm or more is 30% or more.

[0109] As described above, the steel used as the raw material for the crankshaft in this embodiment and the crankshaft were studied with regard to chemical composition and inclusions that may become the starting point for cracks in the nitrided layer (especially the compound layer), and were finally completed with the following configuration.

[0110] [1] A type of steel, which contains, by mass percent:

[0111] C: 0.25%–0.35%

[0112] Si: 0.05%~0.35%

[0113] Mn: 0.85%–1.20%

[0114] P: below 0.080%

[0115] S: 0.030%~0.100%

[0116] Cr: less than 0.10%

[0117] Ti: below 0.050%

[0118] Al: below 0.050%

[0119] N: 0.005%–0.024%, and

[0120] O: Below 0.0100%,

[0121] The remaining portion consists of Fe and impurities.

[0122] Fn1, defined by mathematical formula (1), is 1.00 to 2.05.

[0123] Fn², as defined by mathematical formula (2), is 0.42–0.60.

[0124] In the inclusions in the steel,

[0125] Inclusions whose combined Mn and S content, expressed as a percentage by mass, is 80.0% or higher are defined as MnS inclusions.

[0126] Inclusions with a combined Mn and S content of 15.0% to less than 80.0% by mass are defined as MnS composite inclusions.

[0127] Inclusions with a combined Al, Ca, and O content of 80.0% or more by mass and a combined Mn and S content of less than 15.0% by mass are defined as individual oxides.

[0128] The inclusion in which the total of the Al content, the Ca content, and the O content is 15.0% to less than 80.0% by mass and the total of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as the MnS complex oxide when

[0129] In the steel material,

[0130] The number density of the total of the MnS single inclusion having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 The above,

[0131] The proportion of the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 1.0 μm or more with respect to the total number of inclusions having a circular equivalent diameter of 1.0 μm or more is 70% or more,

[0132] The proportion of the number of the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the single oxide having a circular equivalent diameter of 1.0 μm or more and the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more is 30% or more,

[0133] Fn1 = Mn + 7.24Cr + 6.53Al ··· (1)

[0134] Fn2 = C + 0.10Si + 0.19Mn + 0.23Cr - 0.34S ··· (2)

[0135] In the mathematical expression (1) and the mathematical expression (2), the content of the corresponding element is substituted by mass % for each element symbol.

[0136] [2] The steel material according to [1], wherein

[0137] The steel material contains one element or two or more elements selected from the group consisting of the following elements to replace a part of the Fe:

[0138] Cu: 0.20% or less,

[0139] Ni: 0.20% or less,

[0140] Mo: 0.10% or less,

[0141] Nb: 0.050% or less,

[0142] Ca: 0.0100% or less,

[0143] Bi: 0.30% or less,

[0144] Te: 0.0100% or less,

[0145] Zr: 0.0100% or less, and

[0146] Pb: 0.09% or less.

[0147] [3] A crankshaft comprising:

[0148] a pin portion;

[0149] a journal portion; and

[0150] an arm portion disposed between the pin portion and the journal portion,

[0151] at least the pin portion and the journal portion comprise:

[0152] a nitrided layer formed on a surface layer; and

[0153] a core portion located inward of the nitrided layer,

[0154] the core portion contains, in mass%:

[0155] C: 0.25% to 0.35%,

[0156] Si: 0.05% to 0.35%,

[0157] Mn: 0.85% to 1.20%,

[0158] P: 0.080% or less,

[0159] S: 0.030% to 0.100%,

[0160] Cr: 0.10% or less,

[0161] Ti: 0.050% or less,

[0162] Al: 0.050% or less,

[0163] N: 0.005% to 0.024%, and

[0164] O: 0.0100% or less,

[0165] the remainder consisting of Fe and impurities,

[0166] Fn1 defined by mathematical expression (1) is 1.00 to 2.05,

[0167] Fn2 defined by mathematical expression (2) is 0.42 to 0.60,

[0168] in the inclusions in the core portion,

[0169] an inclusion in which the sum of the Mn content and the S content is 80.0% or more by mass is defined as an MnS single inclusion,

[0170] an inclusion in which the sum of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as an MnS complex inclusion,

[0171] an inclusion in which the sum of the Al content, the Ca content, and the O content is 80.0% or more by mass and the sum of the Mn content and the S content is less than 15.0% by mass is defined as a single oxide,

[0172] an inclusion in which the sum of the Al content, the Ca content, and the O content is 15.0% to less than 80.0% by mass and the sum of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as an MnS complex oxide, in this case,

[0173] in the core portion,

[0174] the number density of the sum of the MnS single inclusion having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 above,

[0175] the proportion of the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 1.0 μm or more with respect to the total number of inclusions having a circular equivalent diameter of 1.0 μm or more is 70% or more,

[0176] the proportion of the number of the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the single oxide having a circular equivalent diameter of 1.0 μm or more and the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more is 30% or more,

[0177] Fn1 = Mn + 7.24Cr + 6.53Al ··· (1)

[0178] Fn2 = C + 0.10Si + 0.19Mn + 0.23Cr - 0.34S ··· (2)

[0179] wherein the content of the corresponding element is substituted by mass % for each element symbol in the mathematical expression (1) and the mathematical expression (2).

[0180] [4] The crankshaft according to [3], wherein

[0181] the core portion further contains one element or two or more elements selected from the group consisting of the following elements to replace a part of the Fe:

[0182] Cu: less than 0.20%

[0183] Ni: below 0.20%

[0184] Mo: 0.10% or less,

[0185] Nb: below 0.050%

[0186] Ca: below 0.0100%

[0187] Bi: below 0.30%

[0188] Te: less than 0.0100%

[0189] Zr: below 0.0100%, and

[0190] Pb: below 0.09%.

[0191] The following describes the steel used as the raw material for the crankshaft in this embodiment and the crankshaft itself. Furthermore, unless otherwise stated, "%" related to elements means mass percentage. Additionally, in this specification, "nitriding treatment" also includes soft nitriding treatment.

[0192] [Chemical Composition]

[0193] The steel used in this embodiment is used as the raw material for the crankshaft. The chemical composition of the steel used in this embodiment contains the following elements.

[0194] C: 0.25%~0.35%

[0195] Carbon (C) improves the bending fatigue strength of nitrided steel (crankshaft). If the C content is less than 0.25%, the above-mentioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the C content exceeds 0.35%, the hardness of the crankshaft core becomes too high, and the hardness of the nitrided layer also becomes too high, even if the contents of other elements are within the range of this embodiment. In this case, the bending correctivity of the crankshaft decreases. Therefore, the C content is 0.25% to 0.35%. The preferred lower limit of the C content is 0.26%, and more preferably 0.27%.

[0196] Si: 0.05%~0.35%

[0197] Silicon (Si) improves the bending fatigue strength of the crankshaft. Si also deoxidizes the steel. If the Si content is less than 0.05%, the above-mentioned effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the Si content exceeds 0.35%, the hardness of the nitrided layer of the crankshaft becomes too high and the bending straightening property of the crankshaft decreases even if the contents of the other elements are within the ranges of the present embodiment. Thus, the Si content is 0.05% to 0.35%. The preferable lower limit of the Si content is 0.07%, further preferably 0.09%, further preferably 0.10%. The preferable upper limit of the Si content is 0.33%, further preferably 0.31%, further preferably 0.30%.

[0198] Mn: 0.85% to 1.20%

[0199] Manganese (Mn) improves the bending fatigue strength of the crankshaft. Mn also deoxidizes the steel. If the Mn content is less than 0.85%, the above-mentioned effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the Mn content exceeds 1.20%, the hardness of the nitrided layer of the crankshaft becomes too high and the bending straightening property of the crankshaft decreases even if the contents of the other elements are within the ranges of the present embodiment. Thus, the Mn content is 0.85% to 1.20%. The preferable lower limit of the Mn content is 0.87%, further preferably 0.89%, further preferably 0.90%. The preferable upper limit of the Mn content is 1.18%, further preferably 1.16%, further preferably 1.14%.

[0200] P: 0.080% or less

[0201] Phosphorus (P) is an impurity that is inevitably contained. That is, the P content exceeds 0%. If the P content exceeds 0.080%, the bending fatigue strength of the crankshaft decreases even if the contents of the other elements are within the ranges of the present embodiment. Thus, the P content is 0.080% or less. The preferable upper limit of the P content is 0.050%, further preferably 0.030%. The P content is preferably as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Thus, the preferable lower limit of the P content is 0.001%, further preferably 0.002%.

[0202] S: 0.030% to 0.100%

[0203] Sulfur (S) improves machinability of the steel. If the S content is less than 0.030%, the above-mentioned effect cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the S content exceeds 0.100%, the castability of the steel is reduced even if the contents of the other elements are within the ranges of the present embodiment. Thus, the S content is 0.030% to 0.100%. The preferable lower limit of the S content is 0.035%, further preferably 0.037%, further preferably 0.040%. The preferable upper limit of the S content is 0.095%, further preferably 0.090%, further preferably 0.085%, further preferably 0.080%.

[0204] Cr: 0.10% or less

[0205] Chromium (Cr) is an impurity that is inevitably contained. That is, the Cr content exceeds 0%. If the Cr content exceeds 0.10%, the bend straightening property of the crankshaft is reduced even if the contents of the other elements are within the ranges of the present embodiment. Thus, the Cr content is 0.10% or less. The Cr content is preferably as low as possible. However, excessive reduction of the Cr content increases the manufacturing cost. Thus, the preferable lower limit of the Cr content is 0.01%, further preferably 0.02%.

[0206] Ti: 0.050% or less

[0207] Titanium (Ti) is inevitably contained. That is, the Ti content exceeds 0%. Ti forms TiN by combining with N, suppresses the coarsening of the crystal grains due to the pinning effect, and improves the bend fatigue strength of the crankshaft. If Ti is contained even in a small amount, the above-mentioned effect can be obtained to some extent. However, if the Ti content exceeds 0.050%, coarse TiN is formed even if the contents of the other elements are within the ranges of the present embodiment, which results in a reduction in the bend fatigue strength of the crankshaft. Thus, the Ti content is 0.050% or less. The preferable lower limit of the Ti content is 0.001%, further preferably 0.003%, further preferably 0.005%. The preferable upper limit of the Ti content is 0.045%, further preferably 0.040%, further preferably 0.030%.

[0208] Al: 0.050% or less

[0209] Aluminum (Al) is inevitably contained. That is, the Al content exceeds 0%. Al combines with nitrogen at the time of nitriding treatment to form AlN, increasing the hardness of the nitrided layer of the crankshaft and increasing the bending fatigue strength of the crankshaft. If Al is contained even in a small amount, the above effects can be obtained to some extent. However, if the Al content exceeds 0.050%, the hardness of the nitrided layer of the crankshaft becomes excessively high even if the contents of the other elements are within the ranges of the present embodiment, and the bending straightening property of the crankshaft decreases. Thus, the Al content is 0.050% or less. The preferable upper limit of the Al content is 0.045%, further preferably 0.040%, further preferably 0.035%, further preferably 0.030%. The preferable lower limit of the Al content is 0.001%, further preferably 0.002%, further preferably 0.005%. The Al content referred to here means the content of Al including oxides in the steel (total Al).

[0210] N: 0.005% to 0.024%

[0211] Nitrogen (N) combines with Ti to form TiN, inhibiting the coarsening of grains due to pinning effects, and increasing the bending fatigue strength of the crankshaft. If the N content is less than 0.005%, the above effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the N content exceeds 0.024%, the hot workability of the steel material decreases even if the contents of the other elements are within the ranges of the present embodiment. Thus, the N content is 0.005% to 0.024%. The preferable lower limit of the N content is 0.006%, further preferably 0.008%, further preferably 0.010%. The preferable upper limit of the N content is 0.022%, further preferably 0.021%, further preferably 0.020%.

[0212] O: 0.0100% or less

[0213] Oxygen (O) is an impurity that is inevitably contained. That is, the O content exceeds 0%. O forms oxides in the steel material. If the O content exceeds 0.0100%, coarse oxides are formed even if the contents of the other elements are within the ranges of the present embodiment, resulting in a decrease in the bending fatigue strength of the crankshaft and a decrease in the wear resistance. Thus, the O content is 0.0100% or less. The preferable upper limit of the O content is 0.0080%, further preferably 0.0060%, further preferably 0.0050%. The O content is preferably as low as possible. However, an excessive decrease in the O content increases the manufacturing cost. Thus, the preferable lower limit of the O content is 0.0001%, further preferably 0.0005%.

[0214] The remaining portion of the chemical composition of the steel material of the present embodiment is composed of Fe and impurities. Among them, the impurities refer to components mixed from ores, waste materials, or manufacturing environments, etc. as raw materials at the time of industrial manufacturing of the steel material and are not components intentionally contained in the steel material. As such impurities, for example, the following impurities are present. Co: 0.02% or less, Sn: 0.02% or less, Zn: 0.02% or less.

[0215] [For arbitrary elements]

[0216] [Group 1 arbitrary elements]

[0217] Also, the chemical composition of the steel material of the present embodiment can further contain one element or two or more elements selected from the group consisting of Cu, Ni, Mo, and Nb in place of a portion of Fe. These elements are arbitrary elements, and each of them can improve the bending fatigue strength of the crankshaft.

[0218] Cu: 0.20% or less

[0219] Copper (Cu) is an arbitrary element, and it can also not be contained. That is, the Cu content can also be 0%. In the case where it is contained, that is, in the case where the Cu content exceeds 0%, Cu is solid-solved in the steel material to improve the bending fatigue strength of the crankshaft. For the Cu content, if it is contained even in a small amount, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.20%, the bending straightening property of the crankshaft is reduced even if the contents of the other elements are within the range of the present embodiment. Thus, the Cu content is 0.20% or less. That is, the Cu content is 0 to 0.20%. The preferable lower limit of the Cu content exceeds 0%, and it is further preferably 0.01%, further preferably 0.02%, further preferably 0.05%, and further preferably 0.07%. The preferable upper limit of the Cu content is 0.19%, further preferably 0.18%, and further preferably 0.17%.

[0220] Ni: 0.20% or less

[0221] Nickel (Ni) is an optional element and can not be contained. That is, the Ni content can also be 0%. In the case of being contained, that is, in the case of the Ni content exceeding 0%, Ni is solid-solved in the steel material to improve the bending fatigue strength of the crankshaft. For the Ni content, if it is contained even in a small amount, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.20%, the bending straightening property of the crankshaft is reduced even if the contents of the other elements are within the range of the present embodiment. Thus, the Ni content is 0.20% or less. That is, the Ni content is 0 to 0.20%. The preferable lower limit of the Ni content exceeds 0%, further preferably 0.01%, further preferably 0.02%, further preferably 0.05%, further preferably 0.07%. The preferable upper limit of the Ni content is 0.19%, further preferably 0.18%, further preferably 0.17%.

[0222] Mo: 0.10% or less

[0223] Molybdenum (Mo) is an optional element and can not be contained. That is, the Mo content can also be 0%. In the case of being contained, that is, in the case of the Mo content exceeding 0%, Mo is solid-solved in the steel material to improve the bending fatigue strength of the crankshaft. For the Mo content, if it is contained even in a small amount, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.10%, the bending straightening property of the crankshaft is reduced even if the contents of the other elements are within the range of the present embodiment. Thus, the Mo content is 0.10% or less. That is, the Mo content is 0 to 0.10%. The preferable lower limit of the Mo content exceeds 0%, further preferably 0.01%, further preferably 0.02%, further preferably 0.03%. The preferable upper limit of the Mo content is 0.09%, further preferably 0.08%.

[0224] Nb: 0.050% or less

[0225] Niobium (Nb) is an optional element and can not be contained. That is, the Nb content can also be 0%. In the case of being contained, that is, in the case of the Nb content exceeding 0%, Nb forms a carbide, nitride, or carbonitride to refine the crystal grains due to a pinning effect, and the bending fatigue strength of the crankshaft can be improved. If Nb is contained even in a small amount, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.050%, the bending straightening property of the crankshaft is reduced even if the contents of the other elements are within the range of the present embodiment. Thus, the Nb content is 0.050% or less. That is, the Nb content is 0 to 0.050%. The preferable lower limit of the Nb content exceeds 0%, further preferably 0.001%, further preferably 0.003%, further preferably 0.005%. The preferable upper limit of the Nb content is 0.040%, further preferably 0.030%.

[0226] [Group 2 optional elements]

[0227] Also, the steel material of the present embodiment can further contain one or two or more elements selected from the group consisting of Ca, Bi, Te, Zr, and Pb in place of a part of Fe. These elements are optional elements, and all of them can improve the machinability of the steel material.

[0228] Ca: 0.0100% or less

[0229] Calcium (Ca) is an optional element, and can not be contained. That is, the Ca content can also be 0%. In the case of being contained, that is, in the case where the Ca content exceeds 0%, Ca improves the machinability of the steel material. If Ca is contained even in a small amount, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0100%, even if the contents of the other elements are within the ranges of the present embodiment, coarse oxides are formed, which results in a decrease in the bending fatigue strength of the crankshaft. Thus, the Ca content is 0.0100% or less. That is, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content exceeds 0%, and is further preferably 0.0001%, further preferably 0.0002%, and further preferably 0.0003%. The preferable upper limit of the Ca content is 0.0090%, and is further preferably 0.0080%.

[0230] Bi: 0.30% or less

[0231] Bismuth (Bi) is an optional element, and can not be contained. That is, the Bi content can also be 0%. In the case of being contained, that is, in the case where the Bi content exceeds 0%, Bi improves the machinability of the steel material. If Bi is contained even in a small amount, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.30%, even if the contents of the other elements are within the ranges of the present embodiment, the bending fatigue strength of the crankshaft decreases. Thus, the Bi content is 0.30% or less. That is, the Bi content is 0 to 0.30%. The preferable lower limit of the Bi content exceeds 0%, and is further preferably 0.01%, further preferably 0.02%, and further preferably 0.05%. The preferable upper limit of the Bi content is 0.27%, and is further preferably 0.25%.

[0232] Te: 0.0100% or less

[0233] Tungsten (W) is an optional element and can not be contained. That is, the W content can also be 0%. In the case of being contained, that is, in the case of the W content exceeding 0%, W improves the machinability of the steel material. If W is contained even in a small amount, the above effect can be obtained to some extent. However, if the W content exceeds 0.0300%, the bending fatigue strength of the crankshaft decreases even if the contents of the other elements are within the ranges of the present embodiment. Thus, the W content is 0.0300% or less. That is, the W content is 0 to 0.0300%. The preferable lower limit of the W content exceeds 0%, further preferably is 0.0001%, further preferably is 0.0002%, further preferably is 0.0003%. The preferable upper limit of the W content is 0.0290%, further preferably is 0.0280%.

[0234] Zr: 0.0100% or less

[0235] Zirconium (Zr) is an optional element and can not be contained. That is, the Zr content can also be 0%. In the case of being contained, that is, in the case of the Zr content exceeding 0%, Zr improves the machinability of the steel material. If Zr is contained even in a small amount, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.0100%, the bending fatigue strength of the crankshaft decreases even if the contents of the other elements are within the ranges of the present embodiment. Thus, the Zr content is 0.0100% or less. That is, the Zr content is 0 to 0.0100%. The preferable lower limit of the Zr content exceeds 0%, further preferably is 0.0001%, further preferably is 0.0002%, further preferably is 0.0003%. The preferable upper limit of the Zr content is 0.0090%, further preferably is 0.0080%.

[0236] Pb: 0.09% or less

[0237] Lead (Pb) is an optional element and can not be contained. That is, the Pb content can also be 0%. In the case of being contained, that is, in the case of the Pb content exceeding 0%, Pb improves the machinability of the steel material. If Pb is contained even in a small amount, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, the bending fatigue strength of the crankshaft decreases even if the contents of the other elements are within the ranges of the present embodiment. Thus, the Pb content is 0.09% or less. That is, the Pb content is 0 to 0.09%. The preferable lower limit of the Pb content exceeds 0%, further preferably is 0.01%, further preferably is 0.02%, further preferably is 0.05%. The preferable upper limit of the Pb content is 0.08%, further preferably is 0.07%.

[0238] [For Fn1 and Fn2]

[0239] The chemical composition of the steel material of the present embodiment is further premised that the content of each element in the chemical composition is within the range of the present embodiment, and Fn1 defined by mathematical expression (1) is 1.00 to 2.05, and Fn2 defined by mathematical expression (2) is 0.42% to 0.60%.

[0240] Fn1 = Mn + 7.24Cr + 6.53Al...(1)

[0241] Fn2 = C + 0.10Si + 0.19Mn + 0.23Cr - 0.34S...(2)

[0242] In mathematical expression (1) and mathematical expression (2), the content of each element corresponding to the symbol of each element is substituted with mass %.

[0243] [For Fn1]

[0244] For Fn1 defined by mathematical expression (1), in the chemical composition, premised that the content of each element is within the range of the present embodiment and Fn2 is within the range of the present embodiment, the Fn1 becomes an index of the hardness of the nitrided layer formed in the surface layer of the steel material (crankshaft) after the nitriding treatment. Thus, in the steel material in which the content of each element in the chemical composition is within the range of the present embodiment, Fn1 is related to the bending fatigue strength of the crankshaft and the bending straightening property of the crankshaft. Specifically, if Fn1 is less than 1.00, even if the content of each element of the chemical composition is within the range of the present embodiment and Fn2 is within the range of the present embodiment, a sufficient bending fatigue strength cannot be obtained in the crankshaft. On the other hand, if Fn1 exceeds 2.05, even if the content of each element of the chemical composition is within the range of the present embodiment and Fn2 is within the range of the present embodiment, the bending straightening property of the crankshaft is reduced. If Fn1 is 1.00 to 2.05, premised that the content of each element of the chemical composition is within the range of the present embodiment and Fn2 is within the range of the present embodiment, a sufficient bending fatigue strength can be obtained in the crankshaft, and the bending straightening property of the crankshaft is also sufficiently improved. The preferable lower limit of Fn1 is 1.02, and further preferably 1.03. The preferable upper limit of Fn1 is 2.03, and further preferably 2.01, and further preferably 2.00.

[0245] [For Fn2]

[0246] For Fn2 defined by mathematical expression (2), in the chemical composition, with each element content being within the range of the present embodiment and Fn1 being within the range of the present embodiment, Fn2 becomes an index of the hardness of the steel material before the nitriding treatment (that is, the core portion of the crankshaft). Thus, in the steel material in which each element content of the chemical composition is within the range of the present embodiment and Fn1 is within the range of the present embodiment, Fn2 is related to the bending fatigue strength of the crankshaft and the machinability of the steel material. Specifically, if Fn2 is less than 0.42, even if each element content of the chemical composition is within the range of the present embodiment and Fn1 is within the range of the present embodiment, a sufficient bending fatigue strength cannot be obtained in the crankshaft. On the other hand, if Fn2 exceeds 0.60, even if each element content of the chemical composition is within the range of the present embodiment and Fn1 is within the range of the present embodiment, a sufficient machinability cannot be obtained in the steel material. If Fn2 is 0.42 to 0.60, with each element content of the chemical composition being within the range of the present embodiment and Fn1 being within the range of the present embodiment, a sufficient bending fatigue strength can be obtained in the crankshaft, and the machinability of the steel material is also sufficiently improved. The preferable lower limit of Fn2 is 0.43, further preferably 0.44, further preferably 0.45. The preferable upper limit of Fn2 is 0.58, further preferably 0.57, further preferably 0.56.

[0247] [For inclusions in the steel material]

[0248] In the steel material of the present embodiment, it is defined as follows.

[0249] (a) Inclusions in which the total content of Mn and S is 80.0% or more in mass% when the mass% of the inclusion is taken as 100% are defined as "MnS single inclusions".

[0250] (b) Inclusions in which the total content of Mn and S is 15.0% to less than 80.0% in mass% when the mass% of the inclusion is taken as 100% are defined as "MnS complex inclusions".

[0251] (c) Inclusions in which the total content of Al, Ca, and O is 80.0% or more in mass%, and the total content of Mn and S is less than 15.0% in mass% when the mass% of the inclusion is taken as 100% are defined as "single oxides".

[0252] (d) Inclusions in which the total content of Mn and S is 15.0% to less than 80.0% in mass%, and the total content of Al, Ca, and O is 15.0% to less than 80.0% in mass% when the mass% of the inclusion is taken as 100% are defined as "MnS complex oxides".

[0253] As defined above, the MnS complex oxide is included in the MnS complex inclusion.

[0254] In the steel material of the present embodiment, the inclusions satisfy the following regulation.

[0255] (I) In the steel material, the number density of the total of the MnS single inclusion having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 or more.

[0256] (II) In the steel material, the proportion of the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the inclusions having a circular equivalent diameter of 1.0 μm or more is 70% or more.

[0257] (III) In the steel material, the proportion of the number of the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the single oxide having a circular equivalent diameter of 1.0 μm or more and the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more is 30% or more.

[0258] Hereinafter, (I) to (III) are described.

[0259] [For (I)]

[0260] The MnS single inclusion and the MnS complex inclusion are defined as "MnS-based inclusions". The MnS-based inclusions improve the machinability of the steel material. Therefore, if the number density (pieces / mm 2 ) of the MnS-based inclusions is increased, the machinability of the steel material is improved. However, if the size of the MnS-based inclusions is too small, it does not contribute to the improvement of the machinability of the steel material. In the case of the steel material having the chemical composition in which the contents of the above-described respective elements are within the ranges of the present embodiment and Fn1 and Fn2 are within the ranges of the present embodiment, the MnS-based inclusions having a circular equivalent diameter of less than 5.0 μm are difficult to contribute to the improvement of the machinability of the steel material. On the other hand, the MnS-based inclusions having a circular equivalent diameter of 5.0 μm or more significantly improve the machinability of the steel material.

[0261] The number density of the MnS-based inclusions (MnS single inclusion and MnS complex inclusion) having a circular equivalent diameter of 5.0 μm or more is defined as the number density SN (pieces / mm 2 ). If the number density SN is 20 pieces / mm 2The above enables the machinability of the steel material having the chemical composition in which the content of each element described above is within the range of the present embodiment and Fn1 and Fn2 are within the range of the present embodiment to be sufficiently improved. The preferable lower limit of the number density of the MnS-based inclusions having a circular equivalent diameter of 5.0 μm or more is 22 pieces / mm 2 , and further preferably 25 pieces / mm 2 . In addition, the upper limit of the number density of the MnS-based inclusions having a circular equivalent diameter of 5.0 μm or more is not particularly limited, and in the case of the steel material having the chemical composition in which the content of each element described above is within the range of the present embodiment and Fn1 and Fn2 are within the range of the present embodiment, the upper limit of the number density of the MnS-based inclusions having a circular equivalent diameter of 5.0 μm or more is, for example, 250 pieces / mm 2 , and preferably 200 pieces / mm 2 . In addition, in the present embodiment, the upper limit of the circular equivalent diameter of the inclusions is not particularly limited. However, in the case of the steel material having the chemical composition in which the content of each element described above is within the range of the present embodiment and Fn1 and Fn2 are within the range of the present embodiment, the upper limit of the circular equivalent diameter of the MnS-based inclusions is, for example, 75 μm.

[0262] [For (II)]

[0263] The crankshaft of the present embodiment has a nitrided layer in the surface layer. The nitrided layer is formed to a predetermined depth from the surface of the steel material due to nitriding treatment. The nitrided layer has a compound layer and a diffusion layer. The compound layer is formed in a range of a predetermined depth from the surface of the nitrided layer. The diffusion layer is formed at a position inside the steel material further than the compound layer. A portion inside the crankshaft further than the nitrided layer is referred to as a core portion. Among them, in the steel material before the nitriding treatment, inclusions exist in a region for forming the compound layer. Therefore, inclusions certainly remain in the compound layer after the nitriding treatment. Oxides among the inclusions included in the compound layer easily become a starting point of cracks of the compound layer of the pin portion and the journal portion of the crankshaft during use of the crankshaft. Therefore, the oxides reduce the wear resistance of the crankshaft. Thus, if the proportion of the total number of the MnS-based inclusions with respect to the total number of the inclusions in the steel material is increased, the proportion of the number of the oxides can be reduced, and the wear resistance of the pin portion and the journal portion of the crankshaft can be improved.

[0264] wherein the proportion of the total number of the MnS single inclusions and the MnS complex inclusions with respect to the total number of the inclusions having a circular equivalent diameter of 1.0 μm or more is defined as "MnS-based inclusion number proportion RA MnS". Inclusions having a circular equivalent diameter of less than 1.0 μm do not greatly affect the wear resistance of the crankshaft having the nitrided layer (compound layer). On the other hand, inclusions having a circular equivalent diameter of 1.0 μm or more can affect the wear resistance of the crankshaft having the nitrided layer (compound layer). Therefore, the circular equivalent diameter of the inclusions that are the object of the proportion RA MnS of the number of MnS-based inclusions is set to 1.0 μm or more. Further, in the present embodiment, the upper limit of the circular equivalent diameter of the inclusions is not particularly limited. However, in the case of a steel material having a chemical composition in which the contents of the respective elements described above are within the ranges of the present embodiment and Fn1 and Fn2 are within the ranges of the present embodiment, the upper limit of the circular equivalent diameter of the inclusions is, for example, 75 μm.

[0265] In the steel material having a chemical composition in which the contents of the respective elements described above are within the ranges of the present embodiment and Fn1 and Fn2 are within the ranges of the present embodiment, if the proportion of the total number of MnS single inclusions and MnS complex inclusions with respect to the total number of inclusions having a circular equivalent diameter of 1.0 μm or more (that is, the proportion RA MnS of the number of MnS-based inclusions) is 70% or more, the wear resistance of the crankshaft can be sufficiently improved. The lower limit of the proportion RA MnS of the number of MnS-based inclusions is preferably more than 70%, further preferably 72%, and further preferably 73%. The upper limit of the proportion RA MnS of the number of MnS-based inclusions is not particularly limited and can be 100%.

[0266] [For (III)]

[0267] In the present specification, the collective term of the single oxides and the MnS complex oxides is defined as "oxides". In the crankshaft described above, even if the proportion of the number of MnS-based inclusions in the total number of inclusions is high, if the proportion of the number of MnS complex oxides in the oxides is low, the proportion of the number of single oxides in the oxides is high. In this case, the proportion of the hard single oxides present in the compound layer is high. The single inclusions easily become the starting point of cracks of the compound layer. Therefore, if the proportion of the single oxides in the oxides present in the compound layer increases, the wear resistance of the crankshaft having the nitrided layer decreases. Thus, the approach of not only increasing the proportion RA MnS of the number of MnS-based inclusions, but also increasing the proportion of the number of MnS complex oxides with respect to the total number of oxides (single oxides and MnS complex oxides) can improve the wear resistance of the crankshaft having the nitrided layer.

[0268] The number ratio of the number of MnS complex oxides having a circular equivalent diameter of 1.0 μm or more to the total number of oxides (single oxides and MnS complex oxides) having a circular equivalent diameter of 1.0 μm or more in the steel material is defined as the MnS complex oxide number ratio RA OX In the steel material having the chemical composition in which the content of each element described above is within the range of the present embodiment and Fn1 and Fn2 are within the range of the present embodiment, if the above (I) and (II) are satisfied and further the ratio of the number of MnS complex oxides having a circular equivalent diameter of 1.0 μm or more to the total number of oxides (single oxides and MnS complex oxides) having a circular equivalent diameter of 1.0 μm or more in the steel material (MnS complex oxide number ratio RA OX ) is 30% or more, sufficient wear resistance can be obtained in the crankshaft. The lower limit of the MnS complex oxide number ratio RA OX is preferably 32.0%, further preferably 34.0%, further preferably 35.0%. The upper limit of the MnS complex oxide number ratio RA OX is not particularly limited and can be 100.0%. Further, in the present embodiment, the upper limit of the circular equivalent diameter of the oxides is not particularly limited. However, in the case of the steel material having the chemical composition in which the content of each element described above is within the range of the present embodiment and Fn1 and Fn2 are within the range of the present embodiment, the upper limit of the circular equivalent diameter of the oxides is, for example, 75 μm.

[0269] [Measurement method of inclusions]

[0270] The number density SN, the MnS-based inclusion number ratio RA MnS , and the MnS complex oxide number ratio RA OX can be found by the following method.

[0271] The number of MnS-based inclusions (MnS single inclusions and MnS complex inclusions) and the number of oxides (single oxides and MnS complex oxides) in the steel can be measured by the following method. A sample is collected from the steel material. Specifically, as shown in Figure 1 , the sample is collected from a position at a distance of R / 2 from the center axis C1 of the steel material 1 in the radial direction (R is the radius of the steel material 1). The size of the observation surface of the sample is not particularly limited. The observation surface of the sample is, for example, L1 x L2, L1 is set to 10 mm, and L2 is set to 5 mm. The thickness L3 of the sample in the direction perpendicular to the observation surface is, for example, set to 5 mm. The normal line N of the observation surface is set to be perpendicular to the center axis C1 (that is, the observation surface is parallel to the axial direction of the steel material), and the R / 2 position is set to be the substantially central position of the observation surface.

[0272] The observed surface of the collected sample was mirror-polished, and 50 fields of view (field of view area 125 μm x 75 μm per field of view) were randomly observed at a magnification of 2000 times using a scanning electron microscope (SEM).

[0273] Each inclusion in the fields of view was determined. The inclusions were determined by contrast. Energy dispersive X-ray spectroscopy (EDX) was used to determine MnS single inclusions, MnS complex inclusions, single oxides, MnS complex oxides for each inclusion determined. Specifically, each inclusion in the field of view was irradiated with a beam, characteristic X-rays were detected, and elemental analysis in the inclusion was performed. The inclusions were determined based on the results of elemental analysis of each inclusion as follows.

[0274] (a) In the case where the mass % of the inclusion is 100%, the total of the Mn content and the S content in the inclusion is 80.0% or more in mass %, the inclusion is defined as "MnS single inclusion".

[0275] (b) In the case where the mass % of the inclusion is 100%, the total of the Mn content and the S content in the inclusion is 15.0% to less than 80.0% in mass %, the inclusion is defined as "MnS complex inclusion".

[0276] (c) In the case where the mass % of the inclusion is 100%, the total of the Al content, the Ca content, and the O content in the inclusion is 80.0% or more in mass %, and the total of the Mn content and the S content is less than 15.0% in mass %, the inclusion is defined as "single oxide".

[0277] (d) In the case where the mass % of the inclusion is 100%, the total of the Al content, the Ca content, and the O content in the inclusion is 15.0% to less than 80.0% in mass %, and the total of the Mn content and the S content is 15.0% to less than 80.0% in mass %, the inclusion is defined as "MnS complex oxide".

[0278] The inclusions to be determined as described above were inclusions having a circular equivalent diameter of 1.0 μm or more. The circular equivalent diameter refers to the diameter of a circle having the same area as each inclusion. The circular equivalent diameter (μm) of each inclusion determined was obtained using a well-known image analysis.

[0279] In this embodiment, the EDX beam diameter used for inclusion determination is set to approximately 50 nm. As a result, for inclusions with a circular equivalent diameter less than 1.0 μm, the composition of the iron matrix can sometimes be detected using EDX, failing to achieve sufficient accuracy in elemental analysis. Furthermore, inclusions with a circular equivalent diameter less than 1.0 μm have little impact on machinability and wear resistance. Therefore, in this embodiment, as described above, inclusions with a circular equivalent diameter of 1.0 μm or more are designated as the target for determination.

[0280] Calculate the total number of individual MnS inclusions and composite MnS inclusions (i.e., MnS-based inclusions with a circular equivalent diameter of 5.0 μm or more) identified in 50 fields of view. Based on the total number of MnS-based inclusions with a circular equivalent diameter of 5.0 μm or more and the total area of ​​the 50 fields of view, calculate the number density SN (inclusions / mm) of MnS-based inclusions with a circular equivalent diameter of 5.0 μm or more. 2 In addition, the number density SN is set to the value obtained by rounding the first decimal place.

[0281] Furthermore, the total number of inclusions with a circular equivalent diameter of 1.0 μm or more among the inclusions identified in 50 fields of view was determined. Also, the total number of individual MnS inclusions and composite MnS inclusions with a circular equivalent diameter of 1.0 μm or more among the inclusions identified in 50 fields of view was determined. Based on the total number of inclusions with a circular equivalent diameter of 1.0 μm or more, the total number of individual MnS inclusions with a circular equivalent diameter of 1.0 μm or more, and the total number of composite MnS inclusions with a circular equivalent diameter of 1.0 μm or more, the proportion RA of MnS inclusions was calculated using the following mathematical formula. MnS (%).

[0282] RA MnS = (Total number of individual MnS inclusions and composite MnS inclusions with a circular equivalent diameter of 1.0 μm or more) / (Total number of inclusions with a circular equivalent diameter of 1.0 μm or more) × 100

[0283] In addition, the proportion of MnS-based inclusions RA MnS Let this be the value obtained by rounding the first decimal place.

[0284] Moreover, the total number of the individual oxides having a circle equivalent diameter of 1.0 μm or more and the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more among the inclusions determined in the 50 fields of view was found. Moreover, the total number of the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more among the inclusions determined in the 50 fields of view was found. Based on the total number of the individual oxides having a circle equivalent diameter of 1.0 μm or more and the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more (that is, the total number of the oxides having a circle equivalent diameter of 1.0 μm or more), and the total number of the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more, and using the following mathematical expression, the MnS complex oxide number ratio RA OX (%) was found.

[0285] RA OX = (the total number of the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more) / (the total number of the oxides having a circle equivalent diameter of 1.0 μm or more) x 100

[0286] Moreover, the MnS complex oxide number ratio RA OX was set to a value obtained by rounding off the first decimal place.

[0287] As described above, for the steel material of the present embodiment, each element is within the range of the present embodiment, and Fn1 defined by mathematical expression (1) is 1.00 to 2.05, Fn2 defined by mathematical expression (2) is 0.42 to 0.60, and the following (I) to (III) are satisfied.

[0288] (I) In the steel material, the number density of the total of the MnS individual inclusions having a circle equivalent diameter of 5.0 μm or more and the MnS complex inclusions having a circle equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 or more.

[0289] (II) In the steel material, the ratio of the total number of the MnS individual inclusions having a circle equivalent diameter of 1.0 μm or more and the MnS complex inclusions having a circle equivalent diameter of 1.0 μm or more to the total number of the inclusions having a circle equivalent diameter of 1.0 μm or more is 70% or more.

[0290] (III) In the steel material, the ratio of the number of the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more to the total number of the individual oxides having a circle equivalent diameter of 1.0 μm or more and the MnS complex oxides having a circle equivalent diameter of 1.0 μm or more is 30% or more.

[0291] By having the above-described configuration, in the steel material of the present embodiment, excellent machinability can be obtained, and in the case where the steel material is subjected to nitriding treatment to form a crankshaft, excellent wear resistance, excellent bending fatigue strength, and excellent bending straightening property can be obtained.

[0292] [For a crankshaft]

[0293] The crankshaft of the present embodiment is manufactured by subjecting the steel material of the present embodiment described above to hot forging and then to nitriding treatment. Figure 2 is a view that shows one example of a main portion of the crankshaft of the present embodiment. Referring to Figure 2 , the crankshaft 10 of the present embodiment is provided with a pin portion 11, a journal portion 12, and an arm portion 13. The journal portion 12 is disposed coaxially with the rotation axis of the crankshaft 10. The pin portion 11 is disposed offset from the rotation axis of the crankshaft 10. The arm portion 13 is disposed between the pin portion 11 and the journal portion 12, and is connected to the pin portion 11 and the journal portion 12. The crankshaft 10 can be provided with a round corner portion, not shown, at the adjacent portion of the pin portion 11 adjacent to the arm portion 13, or can be provided with a round corner portion, not shown, at the adjacent portion of the journal portion 12 adjacent to the arm portion 13.

[0294] The journal portion 12 is rotatably supported by a bearing, not shown, and is connected to a driving source such as an engine. The pin portion 11 is inserted into the large end portion of a connecting rod, not shown. The crankshaft 10 is rotated about the axis by the driving force from the driving source, thereby causing the connecting rod to perform up-and-down movement. At this time, the pin portion 11 and the journal portion 12 slide while receiving external force.

[0295] Figure 3 is a cross-sectional view of the vicinity of the surface layer of the pin portion 11 or the journal portion 12 of the crankshaft 10 in Figure 2 . At least the pin portion 11 and the journal portion 12 of the crankshaft 10 are provided with a nitrided layer 20 formed at the surface layer, and a core portion 23 located at a position inside the nitrided layer 20. The nitrided layer 20 is formed by nitriding treatment, and includes a compound layer 21 and a diffusion layer 22. The compound layer 21 is formed at the outermost surface layer of the crankshaft 10, and contains ε phase as Fe nitride. The diffusion layer 22 is formed at a position inside the compound layer, and is strengthened by solid-solution N and / or Al nitride, Cr nitride, Mo nitride, and the like. The core portion 23 is a base material portion inside the nitrided layer 20, and is a portion that has not been affected by the nitriding treatment.

[0296] The depth of the nitrided layer 20 can be appropriately adjusted depending on the conditions of the nitriding treatment.

[0297] [For chemical composition of core portion]

[0298] The chemical composition of the core portion of the pin portion and the journal portion of the crankshaft is the same as the chemical composition of the steel material of the present embodiment. That is, the chemical composition of the core portion of the crankshaft contains, in mass %, C: 0.25 to 0.35 %, Si: 0.05 to 0.35 %, Mn: 0.85 to 1.20 %, P: 0.080 % or less, S: 0.030 to 0.100 %, Cr: 0.10 % or less, Ti: 0.050 % or less, Al: 0.050 % or less, N: 0.005 to 0.024 %, O: 0.0100 % or less, Cu: 0 to 0.20 %, Ni: 0 to 0.20 %, Mo: 0 to 0.10 %, Nb: 0 to 0.050 %, Ca: 0 to 0.0100 %, Bi: 0 to 0.30 %, Te: 0 to 0.0100 %, Zr: 0 to 0.0100 %, Pb: 0 to 0.09 %, and the remainder consists of Fe and impurities, Fn1 defined by mathematical expression (1) is 1.00 to 2.05, and Fn2 defined by mathematical expression (2) is 0.42 to 0.60.

[0299] The following (I) to (III) are also satisfied in the core portion.

[0300] (I) In the core portion, the number density SN of the MnS single inclusion having a circular equivalent diameter of 5.0 μm or more and the MnS compound inclusion having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 or more.

[0301] (II) In the core portion, the proportion (that is, the MnS-based inclusion number proportion RA MnS ) of the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS compound inclusion having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the inclusions having a circular equivalent diameter of 1.0 μm or more is 70 % or more.

[0302] (III) In the core portion, the proportion (that is, the MnS compound oxide number proportion RA OX ) of the number of the MnS compound oxide having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the oxides (the single oxide and the MnS compound oxide) having a circular equivalent diameter of 1.0 μm or more is 30 % or more.

[0303] The conditions of (I) to (III) of the core portion of the pin portion and the journal portion of the crankshaft are the same as (I) to (III) of the steel material. Thus, the preferable lower limit value of the number density SN, the preferable lower limit value of the MnS-based inclusion number proportion RA MnS , and the preferable lower limit value of the MnS compound oxide number proportion RA OX in the core portion are the same as the preferable lower limit value of the number density SN, the preferable lower limit value of the MnS-based inclusion number proportion RA MnSPreferred lower limit value of the number ratio RA of MnS complex oxides OX The preferred lower limit value is the same as that of the number ratio RA of MnS complex oxides.

[0304] [Manufacturing method]

[0305] Hereinafter, one example of a manufacturing method of the steel material and one example of a manufacturing method of the crankshaft according to the present embodiment will be described. Furthermore, the steel material and the crankshaft according to the present embodiment can have the above-described configuration, and the manufacturing method is not limited to the manufacturing method described below. However, the manufacturing method described below is one example that is preferable for manufacturing the steel material and the crankshaft according to the present embodiment.

[0306] First, one example of a manufacturing method of the steel material according to the present embodiment will be described. One example of the manufacturing method of the steel material includes a steel manufacturing step and a hot working step. Hereinafter, each step will be described.

[0307] [Steel manufacturing step]

[0308] The steel manufacturing step includes a refining step and a continuous casting step.

[0309] [Refining step]

[0310] In the refining step, primary refining using a converter is performed, and thereafter, secondary refining using an LF (Ladle Furnace) and an RH (Ruhrstahl-Hausen) is performed.

[0311] [Primary refining]

[0312] In the refining step, first, molten iron manufactured by a well-known method is subjected to well-known molten iron pretreatment to perform desulfurization treatment, desiliconization treatment, and dephosphorization treatment. The molten iron that has been subjected to the desulfurization treatment, the desiliconization treatment, and the dephosphorization treatment is subjected to refining using a converter (primary refining) and molten steel is manufactured. An alloying element can also be added to the molten steel at the time of the primary refining or after the primary refining to adjust the composition of the molten steel.

[0313] [Secondary refining]

[0314] The molten steel after the primary refining is subjected to secondary refining. In the secondary refining, refining using an LF is performed, and thereafter, RH vacuum degassing treatment is performed so that the morphology of the inclusions of the steel material satisfies (I) to (III).

[0315] [Refining using an LF]

[0316] In the secondary refining, first, desulfurization treatment by the LF is performed to further remove inclusions in the molten steel. For the refining using the LF, the operation is performed in such a manner as to satisfy the following conditions.

[0317] (i) The oxygen content of the molten steel in the refining with LF is set to 40 ppm or less.

[0318] (ii) The temperature of the molten steel in the refining with LF is set to 1550°C or more.

[0319] [For condition (i)]

[0320] The oxygen content and the temperature of the molten steel in the refining with LF have an influence on the morphology of the MnS-based inclusions. If the oxygen content of the molten steel in the refining with LF exceeds 40 ppm, even if the temperature of the molten steel is 1550°C or more, coarse blocky MnS-based inclusions are crystallized. In this case, the blocky MnS-based inclusions are floated and absorbed by the slag, and the number of the MnS-based inclusions (MnS single inclusions and MnS complex inclusions) in the steel product decreases. Alternatively, the MnS-based inclusions remain in the steel in a coarse form, and thus the number of the MnS-based inclusions in the steel product decreases. As a result, the number density SN of the MnS-based inclusions having a circular equivalent diameter of 5.0 μm or more in the steel product is less than 20 pieces / mm 2 .

[0321] [For condition (ii)]

[0322] Similarly, if the temperature of the molten steel in the refining with LF is less than 1550°C, even if the oxygen content of the molten steel is 40 ppm or less, coarse blocky MnS-based inclusions are crystallized. In this case, the blocky MnS-based inclusions are floated and absorbed by the slag, or the MnS-based inclusions remain in the steel in a coarse form, and thus the number of the MnS-based inclusions in the steel product decreases. As a result, the number density SN of the MnS-based inclusions having a circular equivalent diameter of 5.0 μm or more in the steel product is less than 20 pieces / mm 2 .

[0323] The oxygen content of the molten steel in the refining with LF is adjusted to 40 ppm or less, and the temperature of the molten steel in the refining with LF is adjusted to 1550°C or more, so that the crystallization of the MnS-based inclusions during the refining with LF is suppressed. Further, it is also possible that, during the refining with LF, an alloying element is added to the molten steel to perform composition adjustment.

[0324] [RH vacuum degassing treatment]

[0325] After the refining with the LF, the RH (Ruhrstahl-Hausen) vacuum degassing treatment is performed to perform the degassing (removal of N, H in the molten steel) and the separation and removal of the inclusions. In the RH vacuum degassing treatment, the alloying elements are charged into the molten steel as necessary to perform the composition adjustment. In the RH vacuum degassing treatment, the operation is performed in such a manner as to satisfy the following conditions (iii) to (v).

[0326] (iii) The temperature of the molten steel in the RH vacuum degassing treatment is set to 1550°C or higher.

[0327] (iv) The dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment is in the range of 40 ppm to 120 ppm.

[0328] (v) The deoxidation treatment is performed by charging Al into the molten steel before the end of the RH vacuum degassing treatment, and the deoxidation treatment time achieved by the Al charging is set to 5 minutes or less.

[0329] [For condition (iii)]

[0330] If the temperature of the molten steel in the RH vacuum degassing treatment is less than 1550°C, even if the oxygen content of the molten steel is 40 ppm to 120 ppm, coarse blocky MnS-based inclusions are crystallized. In this case, the blocky MnS-based inclusions are floated and absorbed by the slag, or the MnS-based inclusions remain in the steel in a coarse form, and thus the number of MnS-based inclusions in the steel product is reduced. As a result, the number density SN of the MnS-based inclusions having a circular equivalent diameter of 5.0 μm or more in the steel product is less than 20 pieces / mm 2 .

[0331] [For condition (iv)]

[0332] If the dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment is less than 40 ppm, many MnSs without oxide as a nucleus are produced, and the amount of the MnS complex oxide is reduced. Thus, in the steel product, the proportion of the number of the MnS complex oxides having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the oxides (the individual oxides and the MnS complex oxides) having a circular equivalent diameter of 1.0 μm or more (that is, the MnS complex oxide number proportion RA OX ) is less than 30%.

[0333] On the other hand, if the dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment exceeds 120 ppm, coarse MnS-based inclusions are generated. In this case, since coarse MnS-based inclusions are generated in the steel material, the number of MnS-based inclusions itself becomes small. As a result, the number density SN of MnS-based inclusions having a circular equivalent diameter of 5.0 μm or more in the steel material is less than 20 pieces / mm 2 . In addition, in the steel material as a product, the proportion of the total number of MnS single inclusions having a circular equivalent diameter of 1.0 μm or more and MnS complex inclusions having a circular equivalent diameter of 1.0 μm or more with respect to the total number of inclusions having a circular equivalent diameter of 1.0 μm or more (that is, the MnS-based inclusion number proportion RA MnS ) is less than 70.0%.

[0334] [For condition (v)]

[0335] In a case where the deoxidation treatment time realized by the Al addition before the end of the RH vacuum degassing treatment exceeds 5 minutes, many coarse single oxides are generated in the molten steel. In this case, in the casting process, the coarse single oxides do not function as a nucleus of MnS-based inclusions. As a result, MnS single inclusions not combined with the single oxides are generated, and the generation of MnS complex oxides is suppressed. As a result, in the steel material as a product, the proportion of the number of MnS complex oxides having a circular equivalent diameter of 1.0 μm or more with respect to the total number of oxides having a circular equivalent diameter of 1.0 μm or more (that is, the MnS complex oxide number proportion RA OX ) is less than 30%.

[0336] If the temperature of the molten steel in the RH vacuum degassing treatment is adjusted to 1550°C or more, and the dissolved oxygen amount in the molten steel in the RH vacuum degassing treatment is adjusted so that the dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment becomes 40 ppm to 120 ppm, and the treatment time of the deoxidation treatment realized by the Al addition implemented before the end of the RH vacuum degassing treatment is set to 5 minutes or less, it is possible to suppress the generation of coarse MnS-based inclusions in the molten steel before the next process, that is, the casting process, and it is possible to generate many fine oxides that function as a nucleus of MnS generation in the next process, that is, the casting process.

[0337] [Continuous casting process]

[0338] In the continuous casting process, the molten steel after the refining process described above is used, and a steel ingot is manufactured by a continuous casting method. In the continuous casting process, casting is implemented under the following conditions.

[0339] (vi) The casting speed from the start of the continuous casting to the end of the continuous casting is set to 0.6 m / min to 1.0 m / min.

[0340] [for condition (vi)]

[0341] If the casting speed in the continuous casting step is less than 0.6 m / min, the casting speed is too slow. In this case, although MnS-based inclusions are generated in the solidification stage, they are coarsened, and as a result, the number of MnS-based inclusions themselves becomes small. As a result, in the steel material as a product, the proportion of the total number of MnS single inclusions having a circle equivalent diameter of 1.0 μm or more and MnS complex inclusions having a circle equivalent diameter of 1.0 μm or more with respect to the total number of inclusions having a circle equivalent diameter of 1.0 μm or more (that is, the proportion of the number of MnS-based inclusions RA MnS ) is less than 70%.

[0342] On the other hand, if the casting speed in the continuous casting step exceeds 1.0 m / min, the casting speed is too fast, and thus, MnS-based inclusions are generated in the enriched molten steel. At this time, MnS is not combined with single oxides, but is generated as MnS single inclusions. As a result, in the steel material as a product, the proportion of the number of MnS complex oxides having a circle equivalent diameter of 1.0 μm or more with respect to the total number of oxides having a circle equivalent diameter of 1.0 μm or more (that is, the proportion of the number of MnS complex oxides RA OX ) is less than 30%.

[0343] A steel ingot containing inclusions satisfying the above (I) to (III) is manufactured through the above refining step and the casting step.

[0344] [hot working step]

[0345] In the hot working step, a steel ingot manufactured through the continuous casting step is subjected to hot working to manufacture a steel material. The shape of the steel material is a bar steel.

[0346] The hot working step includes a rough rolling step and a finish rolling step. In the rough rolling step, a raw material is subjected to hot working to manufacture a billet. The rough rolling step uses, for example, an initial rolling mill. The initial rolling mill is used to manufacture a billet by subjecting a steel ingot to initial rolling. In a case where a continuous rolling mill is provided downstream of the initial rolling mill, the billet after the initial rolling is further subjected to hot rolling using the continuous rolling mill, and a billet having a smaller size is further manufactured. In the continuous rolling mill, horizontal stands each having a pair of horizontal rollers are alternately arranged in a line with vertical stands each having a pair of vertical rollers. Through the above steps, a billet is manufactured from a steel ingot in the rough rolling step. The heating temperature of the heating furnace in the rough rolling step is not particularly limited, and is, for example, 1100°C to 1300°C.

[0347] In the finish rolling process, the billet is first heated using a heating furnace. The heated billet is subjected to hot rolling using a continuous rolling mill to produce a bar steel as a steel product. The heating temperature of the heating furnace in the finish rolling process is not particularly limited, and is, for example, 1000°C to 1250°C. In addition, in the finish rolling, the temperature of the steel product on the exit side of the rolling stand at which the final reduction is performed is defined as the final temperature. At this time, the final temperature is, for example, 900°C to 1150°C. The final temperature is measured by a temperature gauge provided on the exit side of the rolling stand at which the final reduction is performed. The steel product after the finish rolling is cooled at a cooling rate of natural cooling or the like to produce the steel product of the present embodiment.

[0348] Further, in the production method described above, the rough rolling process and the finish rolling process are performed in the hot working process to produce the steel product. However, the finish rolling process in the hot working process can be omitted. In addition, the hot working process in the production method described above can be omitted. Even in these production methods, the steel product of the present embodiment having the chemical composition in which the content of each element is within the range of the present embodiment and Fn1 and Fn2 are within the range of the present embodiment and satisfying (I) to (III) described above can be produced.

[0349] [Production method of crankshaft]

[0350] Next, one example of the production method of the crankshaft of the present embodiment using the steel product of the present embodiment will be described.

[0351] One example of the production method of the crankshaft of the present embodiment includes a hot forging process, a cutting process, and a nitriding treatment process.

[0352] [Hot forging process]

[0353] The steel product of the present embodiment described above is subjected to hot forging to produce an intermediate product having a shape of a crankshaft. The heating temperature of the steel product before the hot forging is, for example, 1100°C to 1350°C. The heating temperature referred to herein is the furnace temperature (°C) of a heating furnace. The holding time of the heating temperature is not particularly limited, and is held until the temperature of the steel product becomes equal to the furnace temperature. The final temperature of the hot forging is, for example, 1000°C to 1300°C.

[0354] The intermediate product after the hot forging is cooled in a well-known manner. The cooling method is, for example, natural cooling. As needed, the intermediate product after the cooling is subjected to a shot blasting treatment such as shot blasting to remove the oxide skin generated at the time of the hot forging.

[0355] [Cutting process]

[0356] The intermediate product after the hot forging process is subjected to cutting. The intermediate product is shaped to further approach the product shape by the cutting.

[0357] [Nitriding treatment process]

[0358] The intermediate product after the cutting process is subjected to nitriding treatment. In the present embodiment, a well-known nitriding treatment is employed. The nitriding treatment is, for example, gas nitriding, salt bath nitriding, ion nitriding, or the like. The furnace atmosphere gas in the nitriding can be either NH3 alone or a mixed gas containing NH3, N2, and / or H2. In addition, the gases can be made to contain carburizing gas to perform soft nitriding treatment. That is, the so-called nitriding treatment in the present specification includes soft nitriding treatment.

[0359] In the case where the gas soft nitriding treatment is performed, for example, an atmosphere gas in which an endothermic conversion gas (RX gas) and ammonia gas are mixed at 1 : 1 is used, the nitriding treatment temperature is set to 500°C to 650°C, and the holding time of the nitriding treatment temperature is set to 0.5 hours to 8.0 hours. The intermediate product after the nitriding treatment is subjected to quenching. The quenching method is water quenching or oil quenching. The nitriding treatment conditions are not limited to the above conditions, and can be appropriately adjusted in such a manner that the nitriding layer becomes a desired depth.

[0360] The crankshaft in which the nitriding layer is formed on the surface layer is manufactured through the above nitriding treatment process.

[0361] Example

[0362] Hereinafter, the effects of the steel material and the crankshaft of the present embodiment are further specifically described using examples (Example 1 and Example 2). The conditions in the following examples are one example of conditions employed in order to confirm the implementability and effects of the steel material and the crankshaft of the present embodiment. Thus, the steel material and the crankshaft of the present embodiment are not limited to this one example of conditions.

[0363] [Example 1]

[0364] [Manufacture of Test Material]

[0365] A molten steel having the chemical compositions of Table 1 and Table 2 was smelted using a 70-ton converter.

[0366] [Table 1]

[0367] Table 1

[0368]

[0369] [Table 2]

[0370] Table 2

[0371]

[0372] The content of any element is indicated in the column of "Others" in Table 1. For example, in the case of being described as "0.20 Cu", it means that the content of Cu is 0.20%. In the case of being described as "-", it means that the content of any element is less than the detection limit, or any element is not contained. After the molten steel is subjected to primary refining, secondary refining is performed. In the secondary refining, first, refining using LF is performed. The temperature of the molten steel in the refining using LF is indicated in the column of "Temperature of molten steel (°C)" in the column of "LF" in Table 3. The content of oxygen of the molten steel in the refining using LF is indicated in the column of "Content of dissolved oxygen (ppm)" in the column of "LF" in Table 3.

[0373] [Table 3]

[0374] Table 3

[0375]

[0376] After the refining using LF, RH vacuum degassing treatment is performed. The temperature of the molten steel in the RH vacuum degassing treatment is indicated in the column of "Temperature of molten steel (°C)" in the column of "RH" in Table 3. The content of dissolved oxygen of the molten steel 5 minutes before the end of the RH vacuum degassing treatment is indicated in the column of "Content of dissolved oxygen (ppm)" in the column of "RH" in Table 3. The deoxidation treatment time realized by the Al input before the end of the RH vacuum degassing treatment is indicated in the column of "Al deoxidation time (min)" in the column of "RH" in Table 3. In the column of "Temperature of molten steel (°C)" in the column of "LF", "X1-X2" means that the temperature of the molten steel in the refining using LF varies in the range of X1°C to X2°C. In the column of "Content of dissolved oxygen (ppm)" in the column of "LF", "X3-X4" means that the content of oxygen of the molten steel in the refining using LF varies in the range of X3 ppm to X4 ppm. In the column of "Temperature of molten steel (°C)" in the column of "RH", "X5-X6" means that the temperature of the molten steel in the RH vacuum degassing treatment varies in the range of X5°C to X6°C. In the column of "Content of dissolved oxygen (ppm)" in the column of "RH", "X7-X8" means that the content of dissolved oxygen of the molten steel 5 minutes before the end of the RH vacuum degassing treatment varies in the range of X7 ppm to X8 ppm. In the column of "Al deoxidation time (min)" in the column of "RH", "X9" means that the deoxidation treatment time realized by the Al input before the end of the RH vacuum degassing treatment is X9 minutes.

[0377] The molten steel after the secondary refining is used and a steel ingot is manufactured by continuous casting. The casting speed from the start to the end of the continuous casting is indicated in the column of "Casting speed (m / min)" in the column of "Continuous casting" in Table 3. In the column of "Casting speed (m / min)" in the column of "Continuous casting", "X10-X11" means that the casting speed from the start to the end of the continuous casting varies in the range of X10 m / min to X11 m / min.

[0378] The steel ingot thus produced was subjected to a rough rolling process, thereby producing a billet having a rectangular shape with a cross section perpendicular to the longitudinal direction of 180 mm x 180 mm. The heating temperature in the rough rolling process was in the range of 1200°C to 1260°C. The billet thus produced was subjected to a finish rolling process, and was naturally cooled in the atmosphere to produce a steel material as a bar steel having a diameter of 80 mm. The heating temperature in the finish rolling process was 1050°C to 1200°C, and the final temperature was 900°C to 1150°C. The steel material to be a raw material of the crankshaft was produced by the above production process.

[0379] The steel materials of each test number were subjected to the following evaluation tests.

[0380] [evaluation tests]

[0381] [Inclusion measurement test]

[0382] The number density SN, the proportion of the number of MnS complex oxides RA MnS , and the proportion of the number of MnS complex oxides RA OX were found for the steel materials of each test number using the following methods.

[0383] A sample was collected from the steel material of each test number. Specifically, as shown in Figure 1 , the sample was collected from a position at a distance of R / 2 from the center axis C1 of the steel material in the radial direction (R is the radius of the steel material). The observation surface of the sample was L1 x L2, L1 was set to 10 mm, L2 was set to 5 mm, and the sample thickness L3 in the direction perpendicular to the observation surface was set to 5 mm. The normal line N of the observation surface was perpendicular to the center axis C1 (that is, the observation surface was parallel to the axial direction of the steel material), and the R / 2 position was set to the approximate center position of the observation surface.

[0384] The observation surface of the collected sample was mirror polished, and 50 fields of view (field of view area of each field of view: 125 μm x 75 μm) were randomly observed at a magnification of 2000 times using a scanning electron microscope (SEM).

[0385] In each field of view, inclusions were determined based on contrast. Next, MnS single inclusions, MnS complex inclusions, and MnS complex oxides were determined from the determined inclusions using energy dispersive X-ray spectroscopy (EDX). Specifically, each inclusion in the field of view was irradiated with a light beam to detect characteristic X-rays, and elemental analysis in the inclusion was performed. The inclusions were determined based on the elemental analysis results of each inclusion as follows.

[0386] (a) In the case where the total of the Mn content and the S content in the inclusion is 80.0% or more in mass%, the inclusion is defined as "MnS single inclusion".

[0387] (b) in the case where the total of the Mn content and the S content in the inclusion is 15.0% to less than 80.0% by mass, the inclusion is defined as "MnS complex inclusion".

[0388] (c) in the case where the total of the Al content, the Ca content, and the O content in the inclusion is 80.0% or more by mass, and the total of the Mn content and the S content is less than 15.0% by mass, the inclusion is defined as "single oxide".

[0389] (d) in the case where the total of the Al content, the Ca content, and the O content in the inclusion is 15.0% to less than 80.0% by mass, and the total of the Mn content and the S content is 15.0% to less than 80.0% by mass, the inclusion is defined as "MnS complex oxide".

[0390] The inclusion to be the above-mentioned determination object is set to an inclusion having a circular equivalent diameter of 1.0 μm or more. The beam diameter of the EDX used for the determination of the inclusion is set to about 50 nm.

[0391] [Determination of number density SN]

[0392] The total number of the MnS single inclusion having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 5.0 μm or more among the inclusions determined in the 50 fields of view is found. The number density SN (pieces / mm 2 ) is found based on the total number of the MnS single inclusion having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 5.0 μm or more and the total area of the 50 fields of view.

[0393] [Determination of MnS-based inclusion number ratio RA MnS (%)]

[0394] The total number of the inclusion having a circular equivalent diameter of 1.0 μm or more among the inclusions determined in the 50 fields of view is found. Also, the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 1.0 μm or more among the inclusions determined in the 50 fields of view is found. The MnS-based inclusion number ratio RA MnS (%) is found based on the total number of the inclusion having a circular equivalent diameter of 1.0 μm or more and the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 1.0 μm or more and using the following mathematical expression.

[0395] RA MnS= (total number of MnS single inclusions having a circular equivalent diameter of 1.0 μm or more and total number of MnS complex inclusions having a circular equivalent diameter of 1.0 μm or more) / (total number of inclusions having a circular equivalent diameter of 1.0 μm or more) x 100

[0396] [MnS complex oxide number ratio RA OX determination]

[0397] The total number of oxides (single oxides and MnS complex oxides) having a circular equivalent diameter of 1.0 μm or more among the inclusions determined in the 50 fields of view was found. Also, the total number of MnS complex oxides having a circular equivalent diameter of 1.0 μm or more among the inclusions determined in the 50 fields of view was found. The MnS complex oxide number ratio RA was found on the basis of the total number of oxides having a circular equivalent diameter of 1.0 μm or more and the total number of MnS complex oxides having a circular equivalent diameter of 1.0 μm or more and by using the following mathematical formula OX (%).

[0398] RA OX = (total number of MnS complex oxides having a circular equivalent diameter of 1.0 μm or more) / (total number of oxides having a circular equivalent diameter of 1.0 μm or more) x 100

[0399] [bending fatigue test]

[0400] A hot-forged bar of each test number (a bar steel of 80 mm in diameter) was subjected to a hot-forging process that assumes a manufacturing process of a crankshaft. Specifically, the steel was heated at 1200°C. The heated steel was subjected to a hot-forging process, and a forged bar of 50 mm in diameter was manufactured by natural cooling to normal temperature in the atmosphere. The final temperature of the hot-forged bar was 1000°C to 1050°C.

[0401] A small Nishiyama rotary bending fatigue test piece (hereinafter referred to as a fatigue test piece) shown in Fig. 1 was collected from the R / 2 position of the forged bar. Figure 4 The length direction of the fatigue test piece was parallel to the length direction of the forged bar. The central axis of the fatigue test piece was substantially identical to the R / 2 position. Figure 4 In Fig. 1, the numerical value with mm indicates a dimension (unit: mm). Figure 4 In Fig. 1, φ indicates a diameter, and R indicates a radius of curvature.

[0402] A soft nitriding treatment was performed on the fatigue test piece produced, which was a nitriding treatment that simulates the manufacturing process of a crankshaft. The treatment temperature of the soft nitriding treatment was set to 580°C to 600°C, and the holding time of the treatment temperature was set to 1.5 hours to 2.0 hours. The atmosphere gas of the soft nitriding treatment was set to a well-known atmosphere gas (NH3+RX gas). The fatigue test piece after the holding time was subjected to water cooling to produce a fatigue test piece that simulates a crankshaft.

[0403] A rotary bending fatigue test of the Ono type was performed using the produced fatigue test piece. Specifically, in the atmosphere at normal temperature, the rotation speed was set to 3000 rpm (50 Hz), and the number of test stop times was set to 1 x 10 7 times. The stress amplitude was performed at 600 MPa, 630 MPa, and 660 MPa, and was set so that the number of test times N = 2 at each stress amplitude. The bending fatigue strength was evaluated based on the obtained results as follows.

[0404] Evaluation A: At a stress amplitude of 660 MPa, both times did not break (endurance)

[0405] Evaluation B: At a stress amplitude of 630 MPa, both times did not break (endurance), and at a stress amplitude of 660 MPa, broke 1 time or more

[0406] Evaluation C: At a stress amplitude of 600 MPa, both times did not break (endurance), and at a stress amplitude of 630 MPa, broke 1 time or more

[0407] Evaluation D: At a stress amplitude of 600 MPa, broke 1 time or more

[0408] In the case of Evaluation A to Evaluation C, it was judged that the rotary bending fatigue strength was excellent, and in the case of Evaluation D, it was judged that the rotary bending fatigue strength was low.

[0409] [Bending straightening evaluation test]

[0410] A hot-forged piece was produced by performing a hot-forging process that simulates the manufacturing process of a crankshaft on each test number of steel (a bar steel of 80 mm in diameter). Specifically, the steel was heated at 1200°C. The heated steel was subjected to a hot-forged piece, and was naturally cooled to normal temperature in the atmosphere to produce a forged piece of 50 mm in diameter. The final temperature of the hot-forged piece was 1000°C to 1050°C.

[0411] From the R / 2 position of the forged piece, a 4-point bending test piece shown in FIG. 1 was collected. Figure 5 Figure 5 ​A front view 210, a side view 220, and a plan view 230 of the 4-point bending test piece are shown. Values in the drawing with "mm" indicate dimensions. Values in the drawing with "R" mean a radius of curvature. A semicircular notch portion (a radius of curvature of a notch bottom 3 mm, a depth 2 mm) extending in a direction perpendicular to the length direction is provided at a central position in the length direction of the 4-point bending test piece.

[0412] A soft nitriding treatment was performed on the 4-point bending test piece produced, which was subjected to a manufacturing process of a crankshaft. The treatment temperature of the soft nitriding treatment was set to 580°C to 600°C, and the holding time of the treatment temperature was set to 1.5 hours to 2.0 hours. The atmosphere gas of the soft nitriding treatment was set to a well-known atmosphere gas (NH3+RX gas). The fatigue test piece after the holding time was subjected to water cooling to produce a 4-point bending test piece simulating a crankshaft.

[0413] A bending correction test was performed on the 4-point bending test piece produced. First, a strain gauge with a gauge length of 2 mm was attached (bonded) to the notch bottom of the notch portion of the 4-point bending test piece. Thereafter, a 4-point bending test in which a tensile strain was applied to the notch bottom was performed in a 4-point bending manner until the strain gauge was broken. In the 4-point bending test, a 4-point bending in which the distance between the inner supports was set to 30 mm and the distance between the outer supports was set to 80 mm was performed. The strain rate at the time of the 4-point bending was set to 2 mm / min. The maximum strain amount (με) at the time of the breaking of the strain gauge was found. For the 4-point bending test, 10 times were performed for each test number, and the average of the maximum strain amounts obtained in the 10 tests was set to the bending correction strain amount. The bending correction property was evaluated based on the obtained bending correction strain amount as follows.

[0414] Evaluation A: The bending correction strain amount was 40,000 με or more.

[0415] Evaluation B: The bending correction strain amount was 30,000 με to less than 40,000 με.

[0416] Evaluation C: The bending correction strain amount was 20,000 με to less than 30,000 με.

[0417] Evaluation D: The bending correction strain amount was less than 20,000 με.

[0418] In the case of Evaluation A to Evaluation C, it was judged that the bending correction property was excellent, and in the case of Evaluation D, it was judged that the bending correction property was poor.

[0419] [Cutting property evaluation test]

[0420] Hot forging of each test number steel material (bar steel of 80 mm in diameter) was performed to obtain a hot forged bloom which assumes a manufacturing process of a crankshaft. Specifically, the steel material was heated at 1200°C. The hot forging of the heated steel material was performed, and the hot forged bloom of 50 mm in diameter was manufactured by natural cooling to normal temperature in the atmosphere. The final temperature of the hot forging was 1000°C to 1050°C. A sample of 50 mm in diameter and 200 mm in length was collected by cutting the hot forged bloom in a direction perpendicular to the length direction.

[0421] Drilling using a deep hole drill was performed at the R / 2 position of the surface (cross section) of the sample perpendicular to the length direction, and the machinability was evaluated. Specifically, a standard deep hole drill (TUNGALOY CORPORATION, without breaker) of 9.5 mm in diameter was used, and drilling was performed at the R / 2 position in parallel with the axial direction. The cutting speed at the time of drilling was set to 107 mm / min (the drill bit rotation speed was 3600 rpm), the feed speed was set to 0.023 mm / rev, and the perforation distance was set to 90 mm / hole. After 200 holes were drilled under the above conditions, the wear amount of the relief surface of the deep hole drill was measured. The machinability was evaluated based on the obtained wear amount as follows.

[0422] Evaluation A: Wear amount is less than 30 μm

[0423] Evaluation B: Wear amount is 30 μm to less than 40 μm

[0424] Evaluation C: Wear amount is 40 μm to less than 50 μm

[0425] Evaluation D: Wear amount is 50 μm or more

[0426] In the case of Evaluation A to Evaluation C, it was judged that the machinability was excellent, and in the case of Evaluation D, it was judged that the machinability was poor.

[0427] [Resistance to wear evaluation test]

[0428] A block material of 10 mm x 15 mm x 6.35 mm was collected from the R / 2 position of the hot forged bloom of 50 mm in diameter manufactured in the machinability evaluation test. The test surface of 15 mm x 6.35 mm was set to be parallel to the central axis of the hot forged bloom.

[0429] Soft nitriding treatment which assumes a manufacturing process of a crankshaft was performed on the block material. The treatment temperature of the soft nitriding treatment was set to 580°C to 600°C, and the holding time of the treatment temperature was set to 1.5 hours to 2.0 hours. The atmosphere gas of the soft nitriding treatment was a well-known atmosphere gas (NH3+ RX gas). The block test piece which assumes a crankshaft was manufactured by water cooling the block material after the holding time.

[0430] A grinding process was performed on a test surface (10 mm x 6.35 mm) of the block-shaped test piece so that the arithmetic average roughness Ra of the test surface became 0.2. Here, the arithmetic average roughness Ra was measured in accordance with JIS B 0601 (2013), and the reference length was set to 5 mm.

[0431] A ring block method wear test was performed using the sample material and Figure 6 as shown in the drawing. Referring to Figure 6 , the ring block method wear tester 100 is provided with a ring test piece 103 and a bath 101 in which a lubricating oil 102 is stored. The lubricating oil 102 uses a commercially available engine oil having a viscosity of 0W-20. The raw material of the ring test piece 103 is a general bearing metal material, namely, an Al alloy. The Al alloy contains 12% of Sn and 3% of Si in terms of mass%, and the remainder is Al. The outer diameter D of the ring test piece 103 is 35 mm, and the width W of the ring test piece 103 is 8.7 mm.

[0432] As shown in the drawing, Figure 6 the lower portion of the ring test piece 103 was dipped into the lubricating oil 102 in the bath 101. Then, the block-shaped test piece 50 was arranged above the ring test piece 103. At this time, the block-shaped test piece 50 was arranged in such a manner that the test surface 51 of the block-shaped test piece 50 faced the ring test piece 103. In a state in which the block-shaped test piece 50 was pressed to the outer peripheral surface of the ring test piece 103 with a load P of 100 N from above toward below of the block-shaped test piece 50, the ring test piece 103 was rotated to perform a wear test. At this time, the rotation speed of the ring test piece 103 was set to 700 rpm, and the sliding speed was set to 1.28 m / sec. The following behavior was repeatedly performed, and the test was continued until the total of the sliding time (test time) became 100 hours: the test was interrupted every 60 minutes from the start of the test, and the lubricating oil in the contact portion 52 of the test surface 51 of the block-shaped test piece 50, which contacted the outer peripheral surface of the ring test piece 103, was wiped off, and then the test was started again. When 100 hours had passed from the sliding time (test time), the test was ended.

[0433] With respect to the contact portion 52 of the test surface 51 of the block-shaped test piece 50 after the test was ended, 5 fields of view (each field of view was 250 pm x 150 pm) were observed at a magnification of 1000 times using an SEM, and the presence or absence of peeling of the compound layer and the presence or absence of fine cracks on the compound layer were investigated. Based on the investigation results, the wear resistance was evaluated as follows.

[0434] Evaluation A: no peeling, no fine cracks

[0435] Evaluation B: no peeling, fine cracks

[0436] Evaluation D: peeling

[0437] In the case of Evaluation A and Evaluation B, it was judged that the wear resistance was excellent, and in the case of Evaluation D, it was judged that the wear resistance was poor.

[0438] [Results of Test]

[0439] The results of the test are shown in Table 4 and Table 5.

[0440] [Table 4]

[0441] Table 4

[0442]

[0443] [Table 5]

[0444] Table 5

[0445]

[0446] Referring to Table 4 and Table 5, the content of each element in the chemical composition of Test No. 1 to Test No. 63 was appropriate, Fn1 was 1.00 to 2.05, and Fn2 was 0.42 to 0.60. Also, the manufacturing conditions were appropriate. Therefore, the number density SN was 20 pieces / mm 2 The above, the number ratio RA of the MnS inclusions MnS was 70% or more, the number ratio RA of the MnS complex oxides OX was 30% or more. Therefore, excellent rotational bending fatigue strength was obtained, excellent bending straightening property was obtained, excellent machinability was obtained, and excellent wear resistance was obtained.

[0447] On the other hand, the content of C of Test No. 64 was too high. Therefore, the bending straightening strain amount was less than 20,000 με, and the bending straightening property was low.

[0448] The content of C of Test No. 65 was too low. Therefore, in the Ono-type rotational bending fatigue test, it was broken before 1 x 10 7 times was reached at a stress amplitude of 600 MPa, and the bending fatigue strength was low.

[0449] The content of Si of Test No. 66 was too high. Therefore, the bending straightening strain amount was less than 20,000 με, and the bending straightening property was low.

[0450] The content of Si of Test No. 67 was too low. Therefore, in the Ono-type rotational bending fatigue test, it was broken before 1 x 10 7 times was reached at a stress amplitude of 600 MPa, and the bending fatigue strength was low.

[0451] The content of Mn of Test No. 68 was too high. Therefore, the bending straightening strain amount was less than 20,000 με, and the bending straightening property was low.

[0452] The Mn content in test number 69 was too low. Therefore, in the Ono-style rotating bending fatigue test, at a stress amplitude of 600 MPa, the Mn content was insufficient to reach 1×10⁻⁶. 7 The fracture occurred before the second fracture, and the bending fatigue strength was relatively low.

[0453] Test number 70 had an excessively high P content. Therefore, in the Ono-style rotating bending fatigue test, at a stress amplitude of 600 MPa, the P content reached 1×10⁻⁶. 7 The fracture occurred before the second fracture, and the bending fatigue strength was relatively low.

[0454] The sulfur content in test number 71 was too low. Therefore, in the machinability evaluation test, the wear on the flank face of the deep hole drill exceeded 50 μm, indicating low machinability.

[0455] The Cr content in test number 72 was too high. Therefore, the bending correction strain was less than 20,000 με, indicating low bending correction performance.

[0456] Test number 73 had an excessively high Ti content. Therefore, in the Ono-style rotating bending fatigue test, at a stress amplitude of 600 MPa, the Ti content reached 1×10⁻⁶. 7 The fracture occurred before the second fracture, and the bending fatigue strength was relatively low.

[0457] The Al content in test number 74 was too high. Therefore, the bending correction strain was less than 20,000 με, indicating low bending correction performance.

[0458] The nitrogen content in test number 75 was too low. Therefore, in the Ono-style rotating bending fatigue test, at a stress amplitude of 600 MPa, the nitrogen content was insufficient to reach 1 × 10⁻⁶ MPa. 7 The fracture occurred before the second fracture, and the bending fatigue strength was relatively low.

[0459] The O content in test number 76 was too high. Therefore, in the Ono-style rotating bending fatigue test, at a stress amplitude of 600 MPa, the O content reached 1×10⁻⁶. 7 The fracture occurred before the test, and the bending fatigue strength was low. Furthermore, delamination of the compound layer was observed on the test surface of the block test piece after the ring-block wear test, indicating low wear resistance.

[0460] In test number 77, although the content of each element was within the range of this embodiment, Fn1, as defined by mathematical formula (1), exceeded the upper limit. Therefore, the bending correction strain was less than 20000με, and the bending correction performance was low.

[0461] In test number 78, although the content of each element was within the range of this embodiment, Fn1, as defined by mathematical formula (1), was less than the lower limit. Therefore, in the Ono-style rotating bending fatigue test, at a stress amplitude of 600 MPa, it reached 1×10 7 The fracture occurred before the second fracture, and the bending fatigue strength was relatively low.

[0462] In Test No. 79, although the content of each element was within the range of the present embodiment, Fn2 defined by mathematical expression (2) exceeded the upper limit. Therefore, the wear amount of the relief surface of the deep hole drill became 50 μm or more in the machinability evaluation test, and the machinability was low.

[0463] In Test No. 80, although the content of each element was within the range of the present embodiment, Fn2 defined by mathematical expression (2) was less than the lower limit. Therefore, the steel broke at a stress amplitude of 600 MPa before reaching 1 x 107 cycles in the small- diameter rotary bending fatigue test, and the bending fatigue strength was low. 7

[0464] In Test No. 81, although the content of each element of the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the dissolved oxygen amount in the refining using LF exceeded 40 ppm. Therefore, the number density SN was less than 20 pieces / mm2. As a result, the wear amount of the relief surface of the deep hole drill became 50 μm or more in the machinability evaluation test, and the machinability was low. 2

[0465] In Test No. 82, although the content of each element of the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the casting speed in the continuous casting process was less than 0.6 m / min. Therefore, the proportion RA of the number of MnS inclusions was less than 70%. As a result, peeling of the compound layer was observed on the test surface of the block test piece after the ring block wear test, and the wear resistance was low. MnS

[0466] In Test No. 83, although the content of each element of the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment was less than 40 ppm. Therefore, the proportion RA of the number of MnS complex oxides was less than 30%. As a result, peeling of the compound layer was observed on the test surface of the block test piece after the ring block wear test, and the wear resistance was low. OX

[0467] [Second Embodiment]

[0468] [Manufacture of Test Material]

[0469] A molten steel having the chemical composition of Table 6 was melted using a 70-ton converter.

[0470] [Table 6]

[0471] Table 6

[0472] ​​​​

[0473] The molten steel was subjected to secondary refining. In the secondary refining, first, refining with LF was performed. The oxygen content of the molten steel in the refining with LF is indicated in the "dissolved oxygen amount (ppm)" column of the "LF" column in Table 7, and the temperature of the molten steel in the refining with LF is indicated in the "molten steel temperature (°C)" column of the "LF" column in Table 7.

[0474] [Table 7]

[0475] Table 7

[0476]

[0477] After the refining with LF, RH vacuum degassing treatment was performed. The temperature of the molten steel in the RH vacuum degassing treatment is indicated in the "molten steel temperature (°C)" column of the "RH" column in Table 7. The dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment is indicated in the "dissolved oxygen amount (ppm)" column of the "RH" column in Table 7. The deoxidation treatment time achieved by the Al input before the end of the RH vacuum degassing treatment is indicated in the "Al deoxidation time (min)" column of the "RH" column in Table 7. In the "molten steel temperature (°C)" column of the "LF" column, "X1-X2" means that the temperature of the molten steel in the refining with LF varied in the range of X1°C to X2°C. In the "dissolved oxygen amount (ppm)" column of the "LF" column, "X3-X4" means that the oxygen content of the molten steel in the refining with LF varied in the range of X3 ppm to X4 ppm. In the "molten steel temperature (°C)" column of the "RH" column, "X5-X6" means that the temperature of the molten steel in the RH vacuum degassing treatment varied in the range of X5°C to X6°C. In the "dissolved oxygen amount (ppm)" column of the "RH" column, "X7-X8" means that the dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment varied in the range of X7 ppm to X8 ppm. In the "Al deoxidation time (min)" column of the "RH" column, "X9" means that the deoxidation treatment time achieved by the Al input before the end of the RH vacuum degassing treatment was X9 minutes.

[0478] The molten steel after the secondary refining was used and a steel ingot was manufactured by continuous casting. The casting speed from the start to the end of the continuous casting is indicated in the "casting speed (m / min)" column of the "continuous casting" column in Table 7. In the "casting speed (m / min)" column of the "continuous casting" column, "X10-X11" means that the casting speed from the start to the end of the continuous casting varied in the range of X10 m / min to X11 m / min.

[0479] A rough rolling process was performed on the manufactured steel ingot, thereby manufacturing a billet having a rectangular shape with a cross section perpendicular to the length direction of 180 mm x 180 mm. The heating temperature in the rough rolling process was in the range of 1200°C to 1260°C.

[0480] Using the manufactured billets, finish rolling was performed, and the steel materials were manufactured as bar steels with a diameter of 80 mm by natural cooling in the atmosphere. The following evaluation tests were performed on the steel materials of each test number.

[0481] [evaluation tests]

[0482] [inclusion measurement tests]

[0483] For the steel materials of each test number, the number density SN, the MnS-based inclusion number ratio RA MnS , and the MnS complex oxide number ratio RA OX were found using the same method as that of the first embodiment.

[0484] [cutting property evaluation tests]

[0485] In each test number, cutting property evaluation tests were performed in the same manner as that of the first embodiment, and the cutting property was evaluated on the same basis as that of the first embodiment.

[0486] [wear resistance evaluation tests]

[0487] In each test number, wear resistance evaluation tests were performed in the same manner as that of the first embodiment, and the wear resistance was evaluated on the same basis as that of the wear resistance evaluation tests of the first embodiment.

[0488] [results of tests]

[0489] The results of tests are shown in Table 7. Referring to Table 7, the content of each element in the chemical composition of test numbers 84 to 90 was appropriate, Fn1 was 1.00 to 2.05, and Fn2 was 0.42 to 0.60. Also, the manufacturing conditions were appropriate. Therefore, the number density SN was 20 or more per mm 2 , the MnS-based inclusion number ratio RA MnS was 70.0% or more, and the MnS complex oxide number ratio RA OX was 30.0% or more. Thus, excellent rotational bending fatigue strength was obtained, excellent bending straightening property was obtained, excellent cutting property was obtained, and excellent wear resistance was obtained.

[0490] On the other hand, in test number 91, although the content of each element in the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the temperature of the molten steel in the refining using an LF was less than 1550°C. Therefore, the number density SN was less than 20 per mm 2 . As a result, the amount of wear of the relief surface of the deep hole drill became 50 μm or more in the cutting property evaluation test, and the cutting property was low.

[0491] In Test No. 92, although the content of each element of the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the dissolved oxygen amount in the refining using LF exceeded 40 ppm. Therefore, the number density SN was less than 20 pieces / mm 2 . As a result, the wear amount of the flank face of the deep hole drill became 50 μm or more in the cutting property evaluation test, and the cutting property was low.

[0492] On the other hand, in Test No. 93, although the content of each element of the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the molten steel temperature in the RH vacuum degassing treatment was less than 1550°C. Therefore, the number density SN was less than 20 pieces / mm 2 . As a result, the wear amount of the flank face of the deep hole drill became 50 μm or more in the cutting property evaluation test, and the cutting property was low.

[0493] In Test No. 94, although the content of each element of the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment exceeded 120 ppm. Therefore, the number density SN was less than 20 pieces / mm 2 . Moreover, the number ratio RA of MnS inclusions MnS was less than 70%. As a result, peeling of the compound layer was observed on the test surface of the block test piece after the ring block wear test, and the wear resistance was low. Moreover, the wear amount of the flank face of the deep hole drill became 50 μm or more in the cutting property evaluation test, and the cutting property was low.

[0494] In Test No. 95, although the content of each element of the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the dissolved oxygen amount of the molten steel 5 minutes before the end of the RH vacuum degassing treatment was less than 40 ppm. Therefore, the number ratio RA of MnS complex oxides OX was less than 30%. As a result, peeling of the compound layer was observed on the test surface of the block test piece after the ring block wear test, and the wear resistance was low.

[0495] In Test No. 96, although the content of each element of the chemical composition was within the range of the present embodiment and Fn1 and Fn2 were also within the range of the present embodiment, the deoxidation treatment time realized by the Al addition before the end of the RH vacuum degassing treatment exceeded 5 minutes. Therefore, the number ratio RA of MnS complex oxides OX was less than 30%. As a result, peeling of the compound layer was observed on the test surface of the block test piece after the ring block wear test, and the wear resistance was low.

[0496] In Test No. 97, although the content of each element of the chemical composition was within the range of the embodiment and Fn1 and Fn2 were also within the range of the embodiment, the casting speed in the continuous casting process exceeded 1.0 m / min. Therefore, the number ratio RA of MnS complex oxides OX was less than 30%. As a result, peeling of the compound layer was observed on the test surface of the block test piece after the ring block abrasion test, and the wear resistance was low.

[0497] In Test No. 98, although the content of each element of the chemical composition was within the range of the embodiment and Fn1 and Fn2 were also within the range of the embodiment, the casting speed in the continuous casting process was less than 0.6 m / min. Therefore, the number ratio RA of MnS complex oxides MnS was less than 70%. As a result, peeling of the compound layer was observed on the test surface of the block test piece after the ring block abrasion test, and the wear resistance was low.

[0498] The above describes the embodiment of the present application. However, the above-described embodiment is only an example for implementing the present application. Thus, the present application is not limited to the above-described embodiment, and the above-described embodiment can be appropriately changed and implemented within the scope of the gist thereof.

[0499] Explanation of reference signs

[0500] 1, Steel material; 10, Crankshaft; 11, Pin portion; 12, Journal portion; 13, Arm portion; 20, Nitriding layer; 23, Core portion.

Claims

1. A steel material, wherein, The steel material contains, in mass %: C：0.25％～0.35％、 Si: 0.05 to 0.35%, Mn: 0.85 to 1.20%, P: 0.080% or less, S：0.030％～0.100％、 Cr: 0.10% or less, Ti: 0.050% or less, Al: 0.050% or less, N: 0.005 to 0.024%, and O: 0.0100% or less, the remainder consisting of Fe and impurities, Fn1 defined by mathematical expression (1) is 1.00 to 2.05, Fn2 defined by mathematical expression (2) is 0.42 to 0.60, among the inclusions in the steel material, an inclusion in which the total of the Mn content and the S content is 80.0% or more in mass % is defined as an MnS single inclusion, an inclusion in which the total of the Mn content and the S content is 15.0% to less than 80.0% in mass % is defined as an MnS complex inclusion, an inclusion in which the total of the Al content, the Ca content, and the O content is 80.0% or more in mass % and the total of the Mn content and the S content is less than 15.0% in mass % is defined as a single oxide, an inclusion in which the total of the Al content, the Ca content, and the O content is 15.0% to less than 80.0% in mass % and the total of the Mn content and the S content is 15.0% to less than 80.0% in mass % is defined as an MnS complex oxide, and in this case, in the steel material, The number density of the total of the MnS single inclusions having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusions having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 The above, the proportion of the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 1.0 μm or more with respect to the total number of inclusions having a circular equivalent diameter of 1.0 μm or more is 70% or more, the proportion of the number of the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the single oxide having a circular equivalent diameter of 1.0 μm or more and the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more is 30% or more, Fn1 = Mn + 7.24Cr + 6.53Al...(1) Fn2 = C + 0.10Si + 0.19Mn + 0.23Cr - 0.34S...(2) wherein the content of the corresponding element in mass % is substituted for each element symbol in mathematical expression (1) and mathematical expression (2).

2. The steel material according to claim 1, wherein the steel material contains one element or two or more elements selected from the group consisting of the following elements in place of a part of the Fe: Cu: 0.20% or less, Ni: 0.20% or less, Mo: 0.10% or less, Nb: 0.050% or less, Ca: 0.0100% or less, Bi: 0.30% or less, Te: 0.0100% or less, Zr: 0.0100% or less, and Pb: 0.09% or less.

3. A crankshaft, wherein, The crankshaft is provided with: a pin portion; a journal portion; and an arm portion disposed between the pin portion and the journal portion, at least the pin portion and the journal portion are provided with: a nitrided layer formed in a surface layer; and a core portion located at a position deeper than the nitrided layer, the core portion contains, in mass %: C：0.25％～0.35％、 Si: 0.05 to 0.35%, Mn: 0.85 to 1.20%, P: 0.080% or less, S：0.030％～0.100％、 Cr: 0.10% or less, Ti: 0.050% or less, Al: 0.050% or less, N: 0.005 to 0.024%, and O: 0.0100% or less, the remainder consisting of Fe and impurities, Fn1 defined by mathematical expression (1) is 1.00 to 2.05, Fn2 defined by mathematical expression (2) is 0.42 to 0.60, in the core portion, an inclusion in which the total of the Mn content and the S content is 80.0% or more by mass is defined as an MnS single inclusion, an inclusion in which the total of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as an MnS complex inclusion, an inclusion in which the total of the Al content, the Ca content, and the O content is 80.0% or more by mass and the total of the Mn content and the S content is less than 15.0% by mass is defined as a single oxide, an inclusion in which the total of the Al content, the Ca content, and the O content is 15.0% to less than 80.0% by mass and the total of the Mn content and the S content is 15.0% to less than 80.0% by mass is defined as an MnS complex oxide, in this case, in the core portion, The number density of the total of the MnS single inclusions having a circular equivalent diameter of 5.0 μm or more and the MnS complex inclusions having a circular equivalent diameter of 5.0 μm or more is 20 pieces / mm 2 The above, the proportion of the total number of the MnS single inclusion having a circular equivalent diameter of 1.0 μm or more and the MnS complex inclusion having a circular equivalent diameter of 1.0 μm or more with respect to the total number of inclusions having a circular equivalent diameter of 1.0 μm or more is 70% or more, the proportion of the number of the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more with respect to the total number of the single oxide having a circular equivalent diameter of 1.0 μm or more and the MnS complex oxide having a circular equivalent diameter of 1.0 μm or more is 30% or more, Fn1 = Mn + 7.24Cr + 6.53Al...(1) Fn2 = C + 0.10Si + 0.19Mn + 0.23Cr - 0.34S...(2) wherein the content of the corresponding element is substituted by the element symbol for each element in mathematical expression (1) and mathematical expression (2) in mass %.

4. The crankshaft according to claim 3, wherein the core portion further contains one element or two or more elements selected from the group consisting of the following elements in place of a part of the Fe: Cu: 0.20% or less, Ni: 0.20% or less, Mo: 0.10% or less, Nb: 0.050% or less, Ca: 0.0100% or less, Bi: 0.30% or less, Te: 0.0100% or less, Zr: 0.0100% or less, and Pb: 0.09% or less.

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