The present invention relates to steel and high-frequency hardened steel parts for machine structural, particularly, automobiles, construction machinery, agricultural machinery, wind turbines, power transmission parts used in other industrial machinery, (e.g., gears, bearings, CVT sheaves relates to a machine structural steel which is a high-frequency hardened steel parts and industrial castings used in the shaft, etc.).
　The present application, on October 19, 2015, claiming priority based on Japanese Patent Application No. 2015-205631, filed in Japan, the contents of which are incorporated here.
BACKGROUND
[0002]
　Conventionally, power transmission parts such as gears are often used after the surface hardening treatment. As a method of surface hardening, carburizing, nitriding or induction hardening is employed. In this, "carburizing" is intended aims surface hardening by matrix (base material) is high-carbon the surface layer of material exhibiting high toughness, in order to improve the fatigue strength (surface fatigue strength) It is mainly applied to materials such as CVT and CVJ parts of gears and automotive. However, carburizing treatment is mainly batch processes in a gas atmosphere, and the more energy and cost expended as such heat and a few hours or more holding to example 930 ° C. vicinity. In the actual operation, a problem that involves deterioration of the environment for processing of carburized steel, and inlining had another problem is difficult.
[0003]
　Therefore, for solving these problems, now research is done to obtain the desired strength properties only in induction hardening process. This is because, induction hardening, shortening of the surface hardening treatment time, reduced energy, and because very advantageous cleaner environment.
[0004]
　Conventionally, when manufacturing a component by carburizing, JIS SCr420 and the so-called hardening steel C content such as SCM420 is around 0.2% has been used. The primary reason for using low steel of such C content as a material is a secure machinability. These steels, which have a relatively low C content, after being processed into parts, the C content of the surface layer portion is increased by the carburization, since the surface hardness by being subsequently quenching increases, desired part strength and surface fatigue strength can be obtained.
　On the other hand, the case of obtaining a component strength by high frequency induction quenching without carburizing treatment, to obtain a proper surface hardness, certain C content of the steel must be about 0.4%. However, in this case, since the hardness of the steel material before the cutting is hard, machinability deteriorates. In other words, when producing parts that have been produced by carburization induction hardening far, the biggest challenge is machinability of steel, steel machinability is not degraded even when hard steel material by increasing the C content is required.
[0005]
　It relates induction hardening process, for example, Patent Document 1, 0.50% of Si or less, limiting the Al below 0.10%, control the area fraction of martensite of 70% or more in the induction hardening metal structure before for induction hardening steel is disclosed that. According to the method of Patent Document 1, the strength of the steel is remarkably improved. However, in the technique of Patent Document 1, although in order to obtain a suitable surface hardness for the part to be induction hardening it is necessary to include C of the steel product itself at least 0.35% or more, for improvement of machinability any not been studied. Therefore, in the induction hardening steel material of Patent Document 1, workability, especially low machinability.
[0006]
　Patent Document 2, for induction hardening steel having excellent machinability has been proposed. According to Patent Document 2, ferrite, pearlite, and appropriately controlling the area ratio of bainite, and a distance between particles of the average aspect ratio and the ferrite crystal grains of the ferrite crystal grains is controlled within a specific range, the chemical composition of the steel machinability is described to be improved by the mandatory addition of specific amounts of Al and B as. However, the steel material of Patent Document 2, the area ratio of ferrite is high internal hardness since 1-5% and less. Since internal hardness does not change before and after the induction hardening, that high internal hardness is high hardness before cutting means machinability is low. Such hardness breakage of chipping of the tool during cutting is more likely to occur in a hard steel material, since applicable cutting conditions are limited, there is a case where the productivity is lowered.
[0007]
　Further, Patent Document 3, a technique for improving the machinability of high-frequency hardened steel have been proposed to increase the Al content. Patent Document 3, although the machinability for induction hardening steel has been shown to improve, because compared with machinability machinability and hardening steel of the invention steels has not been made, the skin whether good machinability can be obtained to the same extent as baked steel is unknown. In Patent Document 3, since it is necessary to set the Al content 0.100%, the transformation to austenite is hardly completed during high frequency heating, hardenability is concerned may be reduced.
CITATION
Patent Document
[0008]
Patent Document 1: Japanese Patent 2007-131871 JP
Patent Document 2: Japanese Patent 2012-219334 JP
Patent Document 3: Japanese Patent No. 4659139 Publication
Summary of the Invention
Problems that the Invention is to Solve
[0009]
　The present invention has been made in view of the above circumstances, to provide a high-frequency hardened steel part superior in machinability and excellent mechanical structural steel and surface fatigue strength excellent surface fatigue strength after induction hardening an object of the present invention.
Means for Solving the Problems
[0010]
(1) steel for machine structural use according to one embodiment of the present invention, chemical components, by mass%, C: 0.40 ~ 0.70% , Si: 0.15 ~ 3.00%, Mn: 0. 30 ~ 2.00%, Cr: 0.01 % to less than 0.50%, S: 0.003 ~ 0.070 %, Bi: 0.0001% , more than 0.0050% or less, N: 0. 0030 ~ 0.0075%, Al: 0.003 ~ 0.100%, P: 0.050% or less, B: 0 ~ 0.0050%, Mo: 0 ~ 0.20%, Ni: 0 ~ 1. 00%, Cu: 0 ~ 1.00 %, Ca: 0 ~ 0.0050%, Mg: 0 ~ 0.0050%, Zr: 0 ~ 0.0050%, Rem: 0 ~ 0.0050%, Ti: 0 ~ 0.20%, Nb: 0 ~ 0.20%, V: 0 ~ 0.35%, Sb: 0 ~ 0.015%, Te: 0 ~ 0.20%, and Pb: 0 ~ .50%, wherein the balance being Fe and impurities, satisfies the following formula (1a) and the following formula (2a), the density of lengthwise in a direction parallel to cross, circle equivalent diameter is less than 2.0 .mu.m MnS There 300 / mm 2 at least.
　290 × C + 50 × Si + 430 ≧ 620 · · ·
　(1a) d + 3 [sigma] <20 · · · (2a)
　, however, C in formula (1a), Si is the content by mass%, in the formula (2a), d is the average equivalent circular diameter of the unit μm in circle equivalent diameter 1.0 .mu.m or more MnS, sigma is the standard deviation of circle-equivalent diameter of MnS over the circle equivalent diameter 1.0 .mu.m.
[0011]
(2) The steel for machine structure according to (1), the chemical composition, by mass%, B: 0.0003 ~ 0.0050%, Mo: 0.01 ~ 0.20%, Ni: 0 .05 to 1.00%, and Cu: may contain one or more members selected from the group consisting of 0.05 to 1.00%.
[0012]
(3) above (1) or steel for machine structure according to (2), the chemical composition, by mass%, Ca: 0.0003 ~ 0.0050%, Mg: 0.0003 ~ 0.0050% , Zr: 0.0003 to 0.0050%, and Rem: may contain one or more members selected from the group consisting of 0.0003 to 0.0050%.
[0013]
(4) above (1) to (3) any steel for machine structure according to one of, the chemical composition, by mass%, Ti: 0.005 ~ 0.20%, Nb: 0.005 ~ 0.20%, and V: may contain one or more members selected from the group consisting of 0.005 to 0.35%.
[0014]
(5) above (1) to (4) or steel for machine structure according to one of, the chemical composition, in mass%, Sb: 0.0003 ~ 0.015%, Te: 0.0003 ~ 0.20%, Pb: 0.01 ~ 0.50%, and may contain one or more of.
[0015]
(6) the high-frequency hardened steel component according to another aspect of the present invention, chemical components, by mass%, C: 0.40 ~ 0.70% , Si: 0.15 ~ 3.00%, Mn: 0.30 ~ 2.00%, Cr: 0.01 % to less than 0.50%, S: 0.003 ~ 0.070 %, Bi: 0.0001% , more than 0.0050% or less, N: 0.0030 ~ 0.0075%, Al: 0.003 ~ 0.100%, P: 0.050% or less, B: 0 ~ 0.0050%, Mo: 0 ~ 0.20%, Ni: 0 ~ 1.00%, Cu: 0 ~ 1.00 %, Ca: 0 ~ 0.0050%, Mg: 0 ~ 0.0050%, Zr: 0 ~ 0.0050%, Rem: 0 ~ 0.0050%, Ti: 0 ~ 0.20%, Nb : 0 ~ 0.20%, V: 0 ~ 0.35%, Sb: 0 ~ 0.015%, Te: 0 ~ 0.20%, and Pb 0 to 0.50%, wherein the balance being Fe and impurities, satisfies the following formula (1b) and the following formula (2b), circle equivalent diameter in the longitudinal direction parallel to the cross section is less than 2.0μm of MnS the density is 300 pieces / mm 2 at least.
　290 × C + 50 × Si + 430 ≧ 620 · · ·
　(1b) d + 3 [sigma] <20 · · · (2b)
　However, C in the formula (1b), Si is the content by mass%, in the formula (2b), d is the average equivalent circular diameter of the unit μm in circle equivalent diameter 1.0 .mu.m or more MnS, sigma is the standard deviation of circle-equivalent diameter of MnS over the circle equivalent diameter 1.0 .mu.m.
[0016]
(7) the high-frequency hardened steel component according to (6), the chemical composition, by mass%, B: 0.0003 ~ 0.0050%, Mo: 0.01 ~ 0.20%, Ni: 0.05 to 1.00%, and Cu: may contain one or more members selected from the group consisting of 0.05 to 1.00%.
[0017]
(8) high-frequency hardened steel component according to (6) or (7), the chemical composition, by mass%, Ca: 0.0003 ~ 0.0050%, Mg: 0.0003 ~ 0.0050 %, Zr: 0.0003 ~ 0.0050%, and Rem: it may contain one or more members selected from the group consisting of from 0.0003 to 0.0050%.
[0018]
(9) above (6) the high-frequency hardened steel component according to any one of - (8), the chemical composition, by mass%, Ti: 0.005 ~ 0.20%, Nb: 0. 005 to 0.20%, and V: may contain one or more members selected from the group consisting of from 0.005 to 0.35%.
[0019]
(10) (6) above the high-frequency hardened steel component according to any one of - (9), the chemical composition, in mass%, Sb: 0.0003 ~ 0.015%, Te: 0. 0003 to 0.20%, and Pb: 0.01 ~ 0.50% may contain one or more members selected from the group consisting of.
The invention's effect
[0020]
　According to this aspect of the present invention is excellent in machinability and can provide a steel for machine structural use excellent in surface fatigue strength after induction hardening. Further, it is possible to provide a high-frequency hardened steel part superior in surface fatigue strength.
　Mechanical structural steel according to the embodiment of the present invention, C is despite 0.40% or more, so is excellent in machinability when subjected to cutting before induction hardening steel for induction hardening as it preferred. Further, according to the steel for machine structural use according to the aspect of the present invention, an automobile, gears for industrial machinery, shaft, can reduce the rate of cutting costs to total production cost of the steel parts such as pulleys, also part of the quality it is possible to improve.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
[1] Figure 1 after 300 ° C. tempering of the high-frequency hardened steel part is a graph showing the relationship between the Vickers hardness of 50μm depth from the surface and the (290 × C + 50 × Si + 430).
FIG. 2 is a graph showing the line obtained by plotting the relationship of 290 × C + 50 × Si + 430 = 620, the relationship between the evaluation of surface fatigue strength of high-frequency hardened steel parts.
DESCRIPTION OF THE INVENTION
[0022]
　As described above, in the steel parts for machinery structure fabricated through the carburization step, to secure machinability, using a low C content steel as industrial castings. Even at low C content, after processing the steel into the part shape, the surface hardness is increased by carburizing and quenching is performed, sufficient strength of the steel part is obtained. In contrast, for induction hardening steel parts obtain a suitable surface hardness (surface hardness equivalent to steel parts produced by carburizing) is the C content of the steel itself at least 0.4% It must be at least to a degree. In this case, the hardness of the steel material before the cutting is hard, the machinability deteriorates. In other words, in order to obtain a steel for machine structural use excellent in surface fatigue strength after high and induction hardening machinability by high frequency induction quenching is that machinability is not degraded even when hard steel material by increasing the C content Desired.
[0023]
　Conventionally, by adjusting the content of C and Si, excellent steel material surface fatigue strength after carburizing is known to be obtained. However, it has not been possible to achieve both the opposite surface fatigue strength and machinability at a high level with each other. Accordingly, the present inventors have able to achieve both the surface fatigue strength and machinability at a high level, repeatedly surveys and research for the development of mechanical structural steel, as a result, we found the following findings .
[0024]
(A) If the Si content is higher, the surface fatigue strength after induction hardening steel (high-frequency hardened steel component) is increased. As described later, the inclusion of Bi traces, surface fatigue strength is further improved.
[0025]
Vickers hardness of the surface of the (b) after induction hardening steel (high-frequency hardened steel component) is correlated with the C content and the Si content in the steel. Also, the higher the Vickers hardness of the steel part surface, the surface fatigue strength is improved.
[0026]
(C) the machinability is improved by containing the MnS in the steel. It is the improvement factor of machinability MnS Since the precipitation in the diffusion of Mn into the crystallization and precipitation to between dendrite during solidification, machinability by fine dispersion of MnS in the steel (chip disposability sex, it is possible to increase the tool life). Further, to the finely dispersed MnS, it is necessary to shorten the spacing between dendrite trees. Studies on primary arm spacing of dendrite are conventionally performed, it can be represented by the following formula (A).
[0027]
λ∝（Ｄ×σ×ΔＴ） ０．２５　…（Ａ）
[0028]
　Here, lambda: 1 primary arm spacing of dendrite (μm), D: diffusion coefficient (m 2 / s), sigma: solid-liquid interfacial energy (J / m 2 ), [Delta] T: a freezing range (° C.).
[0029]
　From this equation (A), the primary arm spacing lambda dendrites, depending on the solid-liquid interfacial energy sigma, if it is possible to reduce this sigma lambda is seen to decrease. If it is possible to reduce the lambda, it is possible to reduce the Mn sulfide size crystallizing between dendrite. Also, if sulfides finely dispersed as MnS, surface fatigue strength after induction hardening is increased.
　By containing Bi traces, it is possible to reduce the solid-liquid interfacial energy, as a result, reduces the dendrite spacing, it is possible to miniaturize the MnS crystallizing between dendrite.
[0030]
　Based on the above findings, the present inventors have found that the machinability improves, and, in order to increase the surface fatigue strength after induction hardening, to limit the relation between the C content and the Si content, the Bi it is contained in trace amounts, and found that it is preferable to deposit a large number of fine MnS.
[0031]
　The following describes the machine structural steel and high-frequency hardened steel component according to an embodiment of the present invention (steel for machine structural use and high-frequency hardened steel component according to the present embodiment).
[0032]
　In this embodiment, the machine structural steel is a material which is subjected to induction hardening to obtain a high-frequency hardened steel parts. Further, the high-frequency hardened steel part, refers to those subjected to induction hardening steel for machine structural use (although it may be tempered after induction hardening).
　Frequency hardened steel component according to the present embodiment, the maximum heating temperature in a machine structural steel according to the present embodiment is obtained by performing induction hardening is 850 ~ 1100 ° C.. Frequency hardened steel component according to the present embodiment, for example, use as components of high surface fatigue strength of the gear or the like used in a power transmission for a motor vehicle is required to be assumed.
[0033]
　Mechanical structural steel and high-frequency hardened steel component according to the present embodiment, chemical composition, in mass%, C: 0.40 ~ 0.70% , Si: 0.15 ~ 3.00%, Mn: 0 .30 ~ 2.00%, Cr: 0.01 % to less than 0.50%, S: 0.003 ~ 0.070 %, Bi: 0.0001% , more than 0.0050% or less, N: 0 .0030 ~ 0.0075%, Al: 0.003 ~ 0.100%, P: 0.050% or less, containing, B optionally 0.0050% or less, Mo: 0.20% or less , Ni: 1.00% or less, Cu: 1.00% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.0050% or less, Rem: 0.0050% or less, Ti : 0.20% or less, Nb: 0.20% or less, V: 0.35% or less, Sb: 0.015% or less, Te 0.20% or less, Pb: contains 0.50% or less, the balance being Fe and impurities. Also, and the C content and the Si content satisfies the 290 × C + 50 × Si + 430 ≧ 620.
　Moreover, the machine structural steel and high-frequency hardened steel component according to the present embodiment, the density of MnS of less than the equivalent circle diameter is 2.0μm in the longitudinal direction parallel to cross section 300 / mm 2 occur more than, d the average circle equivalent diameter of a circle equivalent diameter 1.0μm or more MnS, when the standard deviation of circle-equivalent diameter of a circle equivalent diameter 1.0μm or more MnS the sigma, satisfy d + 3σ <20.
[0034]
　First, a description will be given reasons for limiting the content of each element. Hereinafter,% regarding the content is mass%.
[0035]
　
　C is an important element for obtaining the strength of the steel. Further, C is the ferrite fraction (area ratio) was reduced in the structure before induction hardening, to improve the hardenability at the time of induction hardening, an element necessary for increasing the depth of the hardened layer. Ferrite fraction is high in the structure before induction hardening a C content is less than 0.40%, there is a case where curing ability at the time of induction hardening becomes insufficient. Therefore, the C content is 0.40% or more. Preferably 0.45% or more, more preferably 0.50% or more. On the other hand, not only significantly impair the machinability and forgeability C content is too large, a possibility that quenching cracks will occur increases during induction hardening. Therefore, C content is made 0.70%. Preferably, at most 0.65%.
[0036]
　 Si is by improving the temper softening resistance of the quenched layer is an element having an effect of improving the surface fatigue strength after quenching. To attain the effect, the Si content is set to 0.15% or more. Preferably 0.50% or more. On the other hand, decarburization at the time of forging the Si content exceeds 3.00% is significantly. Therefore, Si content is at most 3.00%.
[0037]
　 Mn is a solid solution in steel increases the tensile strength and fatigue strength of the steel, improve the hardenability of steel. Mn is further combined with sulfur in the steel (S) to form a MnS, it enhances the machinability of the steel. To obtain these effects, the Mn content is 0.30% or more. Tensile strength before quenching the steel, when increasing the fatigue strength and hardenability, Mn content is preferably 0.60% or more, more preferably 0.75% or more. On the other hand, if the Mn content is too high, the machinability of the steel is lowered. Therefore, the Mn content is set to 2.00% or less. When increasing the cold forgeability of the steel, Mn content is preferably not more than 1.90%, more preferably not more than 1.70%.
[0038]
　 Cr increases the tensile strength of the steel. Further, Cr increases the hardenability of steel to increase the surface hardness of the steel after induction hardening. To obtain these effects, the Cr content is 0.01% or more. When increasing the hardenability and tensile strength of the steel, the preferred Cr content is 0.03% or more, further preferably 0.10% or more. On the other hand, when the Cr content is too much, the machinability of the steel is lowered. Therefore, the Cr content is less than 0.50%. If further increasing the fatigue strength, Cr content is preferably not more than 0.20%, more preferably not more than 0.10%.
[0039]
　
　S combines with Mn in steel to form a MnS, enhances the machinability of the steel. To obtain this effect, the S content is 0.003% or more. When increasing the machinability of the steel, S content is preferably 0.010% or more, further preferably 0.015% or more. On the other hand, if contained excessively S, fatigue strength of the steel is lowered. Furthermore, when carrying out magnetic particle inspection with respect to the hot forged part after induction hardening, a pseudo pattern is likely to occur on the surface of the hot forged part. Therefore, the S content to less 0.070%. S content is preferably not 0.050% or less, further preferably 0.030% or less.
[0040]
　 Bi is an important element in the present embodiment. By containing Bi traces, steel solidification structure is fine, as a result, MnS is finely dispersed. To obtain a fine effect of MnS, it is necessary to set the Bi content 0.0001 percent. To obtain the MnS finely dispersed effect, it is preferable that the Bi content 0.0010% or more. On the other hand, when the Bi content exceeds 0.0050%, not only the effect of refining the dendrite structure is saturated, the hot workability of the steel is deteriorated, hot rolling becomes difficult. For these reasons, the Bi content is 0.0050% or less.
[0041]
　 Al is by precipitating dispersed in the steel as a nitride is an element effective for grain refinement of the austenite structure at the time of induction hardening. Further, Al is an element to increase the depth of the hardened layer to increase the hardenability. The Al is an element effective in improving the machinability. To obtain these effects, the Al content is 0.003% or more. Preferably 0.010% or more. Furthermore Al is an N and compounds formed during nitriding, it is effective to increase the N concentration of the surface layer portion, which is an effective element in surface fatigue strength improvement. In this respect, the Al content is 0.003% or more. On the other hand, when the Al content exceeds 0.100% transformation to austenite is less likely complete at the time of high frequency heating, but rather hardenability is reduced. Therefore, Al content is made 0.100% or less.
[0042]
　
　P is contained as an impurity. P Since segregated in the grain boundary to lower the toughness of the steel, it is necessary to minimized, the less preferred. Since there is significant decrease in toughness exceeds 0.050% P content, it limits the P content to 0.050% or less. Although the P content is desirably small, so it is difficult to 0%, the lower limit of the P content may be 0.0001% of industrial limitations.
[0043]
<290 × C + 50 × Si + 430 ≧ 620>
　The inventors have after 300 ° C. tempering of the high-frequency hardened steel components, and 50μm depth of Vickers hardness (Hv) from the surface, the relationship between the C content and the Si content They were examined. As a result, as shown in FIG. 1, it was found to be organized in 290 × C + 50 × Si + 430. Also, Figure 2 shows a line obtained by plotting the relationship of 290 × C + 50 × Si + 430 = 620, the relationship between the evaluation of surface fatigue strength. As shown in FIG. 2, across the relationship line, it was able to be classified into a defective non-defective surface fatigue strength. That is, the present inventors have studied intensively, the value of the left side of the formula (1) were found to be those substantially corresponding to the Vickers hardness of the tempered high-frequency hardened steel parts of the surface at 300 ° C.. In addition, as a result of investigating the relationship between the surface fatigue strength and the 300 ° C. Vickers hardness after tempering at roller pitting fatigue test, if the Vickers hardness is not less than 620Hv, the surface fatigue strength and the surface fatigue strength of a conventional gas carburizing gear it was found to be equal to or higher than that. That is, if the value of 290 × C + 50 × Si + 430 620 or more, to have a sufficient surface fatigue strength after induction hardening. If it is less than the value of 290 × C + 50 × Si + 430 620, the surface fatigue strength is lowered, and pitching occurs. Therefore, in the steel for machine structural use according to the present embodiment, in addition to the content of each element, it is necessary to control the C content, Si content so as to satisfy the following formula (1).
[0044]
２９０×Ｃ＋５０×Ｓｉ＋４３０≧６２０　　（１）
[0045]
　Mechanical structural steel according to the present embodiment contains the above chemical components, and the balance basically in that it consists of Fe and impurities. However, the mechanical structural steel according to this embodiment, optionally, B: 0 ~ 0.0050%, Mo: 0 ~ 0.20%, Ni: 0 ~ 1.00%, Cu: 0 ~ 1 .00%, Ca: 0 ~ 0.0050%, Mg: 0 ~ 0.0050%, Zr: 0 ~ 0.0050%, Rem: 0 ~ 0.0050%, Ti: 0 ~ 0.20%, Nb : 0 ~ 0.20%, V: 0 ~ 0.35%, Sb: 0 ~ 0.015%, Te: 0 ~ 0.20%, Pb: is selected from 0 to the group consisting of 0.50% it may contain one or two or more. However, since these elements do not necessarily have to be contained, the lower limit is 0%.
[0046]
　Here, the impurities, in producing the steel industrially, from raw materials such as ores or scraps, or a component that mixes the various environments of the manufacturing process, in a range that does not adversely affect the steel means what is acceptable.
[0047]
　
　B contributes to improving the machinability by combined with N in steel to precipitate as BN. Also, B may be degraded from BN when heated induction hardening, B, and the greater enhance hardenability. When hardenability at the time of induction hardening is increased, the surface fatigue strength of the steel after induction hardening is increased. To obtain these effects, B content that is preferably 0.0003% or more. On the other hand, when the B content exceeds 0.0050%, the effect is not only saturated, also it causes cracking during rolling or forging rather. Therefore, even if the inclusion of B, B content to be 0.0050% or less.
[0048]
　
　Mo, by improving the temper softening resistance of the quenched layer has the effect of improving the surface fatigue strength of the steel after induction hardening. Further, Mo has the effect of improving the bending fatigue strength by toughening the quenched layer. To obtain these effects, it is preferable that the Mo content is 0.01% or more, and more preferably 0.05% or more. On the other hand, when the Mo content exceeds 0.20%, on the effect is saturated, economy is impaired. Therefore, even if the inclusion of Mo, the Mo content is set to less 0,20%.
[0049]
　 Ni
　is enriched surface of the steel material at the time of oxidation, by suppressing the subsequent oxidation reaction is an element having an effect of improving the corrosion prevention ability. To reliably exhibit this effect, it is preferable that the Ni content is 0.05% or more. On the other hand, when the Ni content exceeds 1.00%, the machinability is deteriorated. Therefore, even if the inclusion of Ni, the Ni content is set to 1.00% or less.
[0050]
　 Cu
　is enriched surface of the steel material at the time of oxidation, by suppressing the subsequent oxidation reaction, has the effect of improving the corrosion prevention ability. To reliably exhibit this effect, it is preferable that the Cu content is 0.05% or more. On the other hand, when the Cu content exceeds 1.00%, since the hot ductility is lowered, likely flaws formed during rolling. Therefore, even if the inclusion of Cu, the Cu content is set to 1.00% or less.
[0051]
　 Ca, Mg, Te is to prevent the MnS is stretched during rolling, induction hardening bending fatigue strength is more element that improves later. In order to obtain this effect reliably, alone or combined, Ca content 0.0003% or more, Mg content 0.0003% or more, a Te content be 0.0003% or more preferable. However, if the content of each element is too large, on the effect is saturated, economy is impaired. Therefore, Ca, even if the inclusion of Mg and / or Te, a Ca content of 0.0050% or less, a Mg content of 0.0050% or less, the Te content to 0.20% or less.
[0052]
　 Zr, by precipitating dispersed in the steel as nitrides, an element having the effect of fine austenite structure at the time of induction hardening. To obtain this effect, the Zr content is preferably made 0.0003% or more. On the other hand, Zr content is steel embrittlement precipitates are coarsened to exceed 0.0050%. Therefore, even if the inclusion of Zr, the Zr content is 0.0050% or less.
[0053]
　 Rem (rare earth element) suppresses MnS is to stretch during rolling, is an element to bend further improve fatigue strength. In order to obtain this effect reliably, it is preferable that the Rem content 0.0003% or more. However, if the content of each element exceeds the above, on the effect is saturated, inclusion size generated is promoted composite oxide of sulfides and oxides become coarse, therefore, even if to be contained, the Rem content is 0.0050% or less. The Rem, La, refers to an element of lanthanoid such as Ce, Rem content refers to the total content of these elements. When the addition of these elements, even by using the mischmetal these elements are mixed, not in any way what the effect is to change.
[0054]
　
　Ti, by precipitation in the steel as nitrides, an element having the effect of fine austenite structure at the time of induction hardening. To obtain this effect, the Ti content is preferably made 0.005% or more. On the other hand, the steel becomes brittle and precipitates Ti content exceeds 0.20% is coarse. Therefore, even if the inclusion of Ti, the Ti content is set to 0.20% or less.
[0055]
　
　Nb, by precipitation in the steel as nitrides, an element having the effect of fine austenite structure at the time of induction hardening. To obtain this effect, the Nb content is preferably made 0.005% or more. On the other hand, on the Nb content exceeds 0.20%, the effect is to be saturated, economy is impaired. Therefore, even if the inclusion of Nb, the Nb content is set to 0.20% or less.
[0056]
　
　V, by precipitating dispersed in the steel in as a nitride, an element having the effect of fine austenite structure at the time of induction hardening. To obtain this effect, the V content is preferably made 0.005% or more. On the other hand, on the V content exceeds 0.35%, the effect is to be saturated, economy is impaired. Therefore, even if the inclusion of V, the V content is set to 0.35% or less.
[0057]
　 Sb
　is a strong element of the surface segregation tendency, is an element effective for preventing oxidation by oxygen adsorption from the outside. To reliably exhibit the antioxidant effect, it is preferable that the Sb content 0.0003% or more. On the other hand, the effect is saturated when the Sb content exceeds 0.015%. Therefore, in consideration of efficiency, even when the inclusion of Sb, the Sb content is set to 0.015% or less.
[0058]
　 Pb is an element to improve the machinability of steel. If it contains Pb at all, but the effect can be obtained, since the effect is large of 0.01% or more, when to obtain the effect, the Pb content is preferably set to 0.01% or more. On the other hand, if Pb is excessively contained, it lowers the toughness and the hot ductility of the steel. Therefore, even if to be contained, the content of Pb and 0.50% or less. Preferred Pb content is 0.25% or less.
[0059]
　Even if the induction hardening steel for machine structural use according to the present embodiment, the chemical composition does not change. Therefore, the chemical composition of the high-frequency hardened steel component according to the present embodiment is the same as the chemical composition of the steel for machine structural use according to the present embodiment.
[0060]
　Next, a description will be given MnS mechanical structural steel and high-frequency hardened steel part of the metal structure according to the present embodiment comprises.
[0061]
Circle the density (number density) of MnS of equivalent diameter of less than 2.0μm is 300 / mm 2 or
　more] MnS are the useful for improving machinability, it is necessary to ensure the number density is there. However, the machinability and to increase the S content is improved, coarse MnS is increased. Coarse MnS, along with decreasing the machinability, decreases the surface fatigue strength of the steel after induction hardening. Therefore, in order to improve the machinability, not only the number density of the MnS, it is necessary to control the size. Specifically, MnS of less than 2.0μm in equivalent circle diameter of 300 / mm 2 to be present in the steel in the above the density (number density), wear of the tool is inhibited. Since improves machinability equivalent circle diameter the more MnS of less than 2.0 .mu.m, it is not necessary to define the upper limit of the density.
　Circle equivalent diameter of MnS is the diameter of a circle having an area equal to the area of MnS, it can be determined by image analysis. Similarly, the number density of the MnS is determined by image analysis. Specifically, in the longitudinal direction parallel to the cross section of the hot-forging steel, and photographed at 100 times by an optical microscope, 0.9 mm 2Inspection reference area images (areas) providing 10 fields of view, and selected 10 pieces in the descending order from the MnS in the observation field of view (image), to calculate the equivalent circle diameter of each were selected MnS, these the size (diameter), it is possible to obtain the equivalent circle diameter by converting the equivalent circle diameter indicates the diameter of a circle having the same area as the area of ​​the deposit. The number density is calculated by dividing the observation field area, the number of MnS. It inclusions is MnS may be confirmed by energy dispersive X-ray spectroscopy device that comes with the scanning electron microscope (EDS). In realistically general purpose device, to deal with the size and composition of the particles statistically, it is preferably not less than 1.0μm circle equivalent diameter of MnS which an observation target.
[0062]
　[Equation (2)]
　as described above, to reduce dendrite primary arm spacing, by a fine sulfides crystallizing from between the dendrite, the surface fatigue strength of the steel after induction hardening is increased. More specifically, Eliminating the 20μm or more MnS at the maximum equivalent circle diameter, thereby improving machinability.
　The inventors have observed field 9 mm 2 variations of equivalent circle diameter of the sulfide to be detected per the standard deviation sigma, the value obtained by adding the average circle equivalent diameter d to three times the standard deviation (3 [sigma]), as in equation (2) was defined as F1.
[0063]
　Ｆ１＝ｄ＋３σ　　（２）
[0064]
　Here, d in the formula (2) is the average circle equivalent diameter of MnS over 1.0μm in equivalent circle diameter ([mu] m), sigma is the standard deviation of circle-equivalent diameter of a circle equivalent diameter 1.0μm or more MnS it is. Further, F1 value, the observation field 9 mm 2 is observed within the predicted standard deviation of circle-equivalent diameter and equivalent circle diameter of the sulfide, mechanical structural steel or high-frequency hardened steel component according to the present embodiment It indicates the maximum equivalent circle diameter of sulfide 99.7% of the number of the number of observable sulfide with an optical microscope that exists. That is, if the F1 value is less than 20 ([mu] m), 20 [mu] m or more sulfide at the maximum equivalent circle diameter indicates that there is little. Such steel has high machinability, excellent surface fatigue strength after induction hardening. Circle equivalent diameter of MnS is the diameter of a circle having an area equal to the area of MnS, it can be determined by image analysis as described above. The circle equivalent diameter of an observation target the MnS was above 1.0μm, the above 1.0μm is at realistically general purpose equipment, it is in a range capable of handling the size and components of the particles statistically it, and, because the influence is small to give it into smaller sulfide surface fatigue strength by controlling the and chip control.
[0065]
　[Dendrite structure]
　Continuous casting slab used in the manufacture of machine structural steel of the present embodiment, the solidified structure exhibits dendrite form. MnS of machine structural use in steel (molten steel) procoagulant, or be crystallized during solidification often greatly influenced by the dendrite primary arm spacing. That is, the smaller the dendrite primary arm spacing, MnS crystallizing between trees is reduced. Mechanical structural steel according to the present embodiment, it is desirable dendrite primary arm spacing in the stage of the slab is less than 600 .mu.m. By refining the dendrite structure, MnS crystallizing from dendrite primary arm is miniaturized, the maximum equivalent circle diameter of MnS is less than 20 [mu] m.
　To fine dendrite structure may contain a Bi traces, it is effective to reduce the solid-liquid interfacial energy in the molten steel.
[0066]
　Frequency hardened steel component according to the present embodiment is obtained by performing the induction hardening steel for machine structural use according to the present embodiment.
　The high frequency hardened steel component according to the present embodiment, the residual γ and nitrides, and does not have a heterogeneous surface abnormal layer containing grain boundary oxidation, generation of surface anomaly layer is minimized the thing was. The high frequency hardened steel component according to the present embodiment, at a position from the surface of 50μm depth even after the 300 ° C. tempering has a more hardness 720Hv in Vickers hardness.
[0067]
　[Production Method]
　Next, a preferred manufacturing method of a steel for machine structural use and high-frequency hardened steel component according to the present embodiment.
　Method for producing a machine structural steel according to the present embodiment has the above chemical composition, and a step of dendrite primary arm spacing casting slab is less than 600μm in the range from the surface layer of 15 mm, the cast pieces and a step of hot working. Here, the hot working may include hot rolling.
[0068]
[Casting Process]
　produced by the chemical composition and the continuous casting method slabs of steel which satisfies the equation (1). It may be ingot (steel ingot) by the ingot-making method. Casting conditions, for example, by using a mold of 220 × 220 mm square, the superheat of the molten steel in the tundish and 10 ~ 50 ° C., the casting speed can be exemplified conditions that 1.0 ~ 1.5 m / min.
　Furthermore, in order to make the dendrite primary arm spacing described above to below 600 .mu.m, when casting molten steel having the above chemical composition, the temperature of the slab surface to the solidus temperature of the liquidus temperature at a depth of 15mm region it is desirable to average cooling rate to 100 ° C. / min or higher 500 ° C. / min or less. Is less than the average cooling rate is 100 ° C. / min, it becomes difficult to dendrite primary arm spacing of less than 600μm at a depth position of 15mm from the slab surface, it may be impossible to finely disperse MnS. On the other hand, in the 500 ° C. / min greater, too in MnS to crystallize from between dendrite fine, there is a possibility that the machinability is lowered.
　Further, since the decrease center segregation, it may be added the reduction in the course of the continuous casting solidification stage.
[0069]
　The temperature range from liquidus temperature to the solidus temperature is the temperature range from the solidification start to the solidification ends. Therefore, the average cooling rate in this temperature range means the average solidification rate of the slab. The average cooling rate mentioned above, for example, mold section size, it casting speed, etc. are controlled to a proper value, or cast immediately after, can be achieved by means such as increasing the amount of cooling water used for water cooling. This is applicable to a continuous casting method and ingot casting method both.
[0070]
　Cooling rate at the position of 15mm depth from above the slab surface, etching the cross section of the cast slab in picric acid, 5 mm pitch in the direction the cast for each position of the depth of 15mm from the slab surface in dendrite secondary arm spacing lambda 2 to ([mu]
[0071]
　　lambda 2 = 710 × A-0.39 (3)
[0072]
　Therefore, the optimal casting conditions are, for example, to produce a plurality of cast pieces obtained by changing the casting conditions, the cooling rate in each slab calculated by the equation (3) can be determined from the obtained cooling rates.
[0073]
[Hot working step]
　Then, by performing a hot working slabbing or the like cast slab or ingot obtained in the casting process, to produce a billet (slab). Further, by hot rolling the billet, and steel bars and wire rods is a machine structural steel of the present embodiment. There is no particular limitation on the reduction ratio in the hot working.
[0074]
　Hot rolling, for example, after heating 1.5 hours at a heating temperature of the billet 1250 ~ 1300 ° C., performs finishing temperature of 900 ~ 1100 ° C.. After finishing rolling, in air, cooled in a condition where the cooling rate is allowed to cool or less. To increase the productivity, when it reached 600 ° C., cooling, mist cooling and water cooling may be cooled by appropriate means. Each of the above heating temperature and the heating time is meant the average temperature and standing furnace time in the furnace. Moreover, finishing temperature of hot rolling, means a surface temperature of the rod wire of the final stand outlet of the rolling mill comprising a plurality of stands. The cooling rate after the finish rolling refers to the cooling rate at the surface of the bar wire (steel bar or wire rod).
[0075]
　Thus, the machine structural steel of the present embodiment can be obtained.
[0076]
　Moreover, steel bars and wire rods produced the (mechanical structural steel) and hot forged to produce intermediate product crude form. The refining the intermediate product may be carried out. Furthermore, the intermediate product is machined, the intermediate product into a predetermined shape. Machining for example, cutting or drilling.
　Next, the induction hardening was performed on the intermediate product to cure the surface of the intermediate product. Thus, the surface hardened layer on the surface of the intermediate product is formed. Then, carrying out the finishing against high-frequency hardened intermediate product. Finishing is a grinding or polishing.
[0077]
　In the step of performing induction hardening, quenching temperature (maximum heating temperature) and 850 ~ 1100 ° C., performs the normal temperature near the temperature range, for example, cooled to 25 ° C. or less. When the quenching temperature is lower than 850 ° C., it can not be subjected to sufficient quenching formed and fabricated material by induction hardening, the pro-eutectoid ferrite appears. When the pro-eutectoid ferrite is present, the hardness of the surface hardened layer becomes nonuniform, it does not improve the surface fatigue strength. Moreover, when the quenching temperature is lower than 850 ° C., the surface layer portion is not sufficiently austenitizing, it is impossible to obtain a desired hardened layer depth. On the other hand, if the quenching temperature exceeds 1100 ° C., the oxidation of the surface layer portion becomes remarkable, it is not sufficiently ensured smoothness of surface texture. Again, the surface fatigue strength decreases. Further, in order to austenitize the surface layer sufficiently, the time to be 850 ° C. or more, preferably within 1 minute 0.5 seconds.
[0078]
　Frequency hardened steel parts are produced according to the present embodiment by the above steps. Frequency hardened steel component according to the present embodiment has the same chemical composition as the steel for machine structural use, the density of MnS of less than the equivalent circle diameter of 2.0μm is 300 / mm 2 not less than, d + 3 [sigma] <20 [mu] m to satisfy. Also it has a surface hardened layer.
[0079]
　As described above, in the high-frequency hardened steel component materials become mechanical structural steel (steel bar in the above example), it is necessary to a maximum equivalent circle diameter of MnS is less than 20 [mu] m. If the forging material (the bars), MnS in the steel is refined according to forging molding ratio. However, high-frequency hardened steel parts are often those with complex shapes, forging molding ratio is not uniform for the entire material. Accordingly, in the forged in materials, parts are hardly wrought, i.e., forging molding ratio very small portion occurs. In such a portion, in order to improve the machinability, it is necessary largest circle equivalent diameter of MnS of machine structural use in steel as a material is less than 20 [mu] m. Mechanical structural steel according to the present embodiment, irrespective of the processing amount of hot working, it is possible to machinability improvement and surface improving fatigue strength.
[0080]
　As described above, the machine structural steel of the present embodiment is excellent in machinability and excellent surface fatigue strength when a high-frequency hardened steel parts.
Example
[0081]
　Examples illustrate the present invention below. Conditions in examples are 1 example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited only to this one example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention may employ various conditions.
[0082]
　Steel having the chemical compositions shown in Table 1 and Table 2 No. The a ~ ii smelted in 270ton converter, and carrying out continuous casting using a continuous casting machine to produce a slab of 220 × 220 mm square. It was added pressure in the course of the continuous casting solidification stage. In continuous casting of the slab, the average cooling rate of temperature range of from the surface of the slab to the solidus temperature of the liquidus temperature at the depth position of 15 mm, was changed by changing the amount of cooling water of the mold.
[0083]
　Then, charged with slab produced in the heating furnace, after heating at a heating temperature of 1250 ~ 1300 ° C. over 10 hours to obtain a slabbing and billet.
[0084]
　Then, after heating 1.5 hours at a heating temperature of the billet 1250 ~ 1300 ° C., and hot-rolled finishing temperature of 900 ~ 1100 ° C., and a round bar having a diameter of 40 mm. In this way, test No. Steel was prepared for the mechanical structure of 1 to 31.
[0085]
[Solidification structure observed]
　were observed slab solidification structure used in the manufacture of machine structural steel. Specifically, etching the slab cross-section at picric acid, at a position of 15mm in the depth direction from the slab surface, the dendrite primary arm spacing at 5mm pitch in casting direction was measured at 100 points, the average value I was determined.
[0086]
[Microstructure test]
　each test No. Microstructure of the round bar (steel for machine structural use) was observed. After cut perpendicular to the rod axis direction (longitudinal direction), D / 4 position: cut parallel to the (D diameter) with respect to the axial direction, and a test piece for microstructure observation. Test surface is a longitudinal cross section parallel to the rolling.
　Specifically, a polishing test piece vertical 10mm × horizontal 10mm were prepared 10 by polishing the cut surface of the test piece, and photographed a predetermined position of the polishing test piece at 100 times by an optical microscope, 0.9 mm 2 test reference area of the image (area) was prepared 10 fields of view. The observation field of view selected 10 pieces in the descending order from the MnS in (image), to calculate the equivalent circle diameter of each were selected MnS. These dimensions (diameter), was converted to the equivalent circle diameter indicates the diameter of a circle having the same area as the area of the deposit. Further, the particle size distribution of the detected MnS, and calculate the average circle equivalent diameter and the standard deviation of the sulfide.
　Determination of MnS observes the steel metal structure by an optical microscope to determine the contrast in the tissue. However, for confirmation, some were identified MnS with a scanning electron microscope and energy dispersive X-ray spectrometer (EDS).
[0087]
[Machinability and surface fatigue strength evaluation test]
　Next, each test No. Using a round bar (mechanical structural steel), it was investigated the drill life as machinability.
　In addition, it was carried out roller pitching fatigue test for the surface fatigue strength evaluation.
[0088]

　For the evaluation of the drill life, and cutting from the center of a round bar of φ40mm diameter 38mm, machinability evaluation test piece height 21 mm, were subjected to drill tests. Tool uses drill Fujikoshi manufactured model number SD3.0 Ltd., 1 0.25 mm feed per rotation, one well perforation depth 9 mm, lubricating performs drilling test using a water-soluble cutting oil , it was evaluated the machinability of each steel. A metric until a cumulative hole depth 1000mm adopted maximum cutting speed VL1000 possible cutting, up to cutting speed VL1000 good than 40 m / min, it was assessed less than 40 m / min as a defective.
[0089]

　The roller pitching fatigue test was cutting a round bar φ40mm after the above heat treatment diameter 26mm from the center, the small roller test piece having a cylindrical portion of the width of 28mm. After the small roller test pieces taken in the manner described above were subjected to induction hardening under the conditions shown in Table 3, performs tempered for 1 hour at 0.99 ° C., was evaluated surface fatigue strength at a roller pitting test.
[0090]
　Specifically, was roller pitting fatigue test is a standard surface fatigue strength test using a large roller test piece separately prepared small roller test piece prepared (carburized after surface grinding of SCM722) above. Roller pitting fatigue test, against a large roller test piece surface pressure of various Hertzian stress in the small roller test piece, the peripheral speed direction of the both roller test piece at the contact portion with the same direction, the slip rate -40% (peripheral speed of the contact portion toward the large roller test piece than the small roller test piece 40% greater) were rotated in the test as. Oil temperature supplied as lubricating oil ATF (AT lubricating oil) to the contact portion and 80 ° C., the contact stress between the large roller test piece and the small roller test piece was 3000 MPa. Test truncation number 10 million times (10 7 and times), if the pitching is in the small roller test piece reached 10 million times speed without causing the surface fatigue strength is sufficiently high, the durability of the small roller test piece it is determined that the (roller pitching fatigue durability) has been sufficiently secured. Detection of pitting is performed by the vibration meter that is installed in the test machine to check the speed and the occurrence of pitting damage by stopping the rotation of both rollers after vibration detection.
[0091]
　As shown in Table 1 to Table 3, the examples of the invention, are excellent both in surface fatigue strength after machinability and induction hardening. On the other hand, in the comparative examples, do not satisfy the chemical components of the present invention, and / or to the state of existence of MnS does not satisfy the present invention range, the results do not satisfy at least one of the surface fatigue strength after machinability or induction hardening became.
[0092]
　Test No. 18 and 19, it did not contain the Bi. Also, less number density of MnS of less than 2.0 .mu.m, also d + 3 [sigma] was 20 or more. As a result, the machinability was not sufficient.
　Test No. 20 had less C content. Further, 290 × C + 50 × Si + 430 was less than 620. As a result, the surface fatigue strength is not sufficient.
　Test No. 21, the content of Cr exceeds the upper limit. Therefore, baked unevenness occurs in the tissue after high frequency hardening, sufficient surface fatigue strength can not be obtained.
　Test No. 22-25, 290 × C + 50 × Si + 430 was less than 620. As a result, the surface fatigue strength is not sufficient.
　Test No. 26 had less S content. Therefore, it is impossible to ensure sufficient MnS, sufficient machinability can not be obtained.
　Test No. 27, since the S content is too large, surface fatigue fracture starting from the MnS occurs, the surface fatigue strength is not sufficient.
　Test No. 28, were less Mn content. Therefore, it is impossible to ensure sufficient MnS, sufficient machinability can not be obtained. In addition, the test No. At 28, the surface fatigue strength is not sufficient. This, S is generated by FeS to reduce the hot ductility because it was not sufficiently secured, premature fatigue failure is estimated to be due caused starting from the crack found to have been generated in the test piece that.
　Test No. 29, since the Mn content was often, strength before induction hardening becomes excessively high, sufficient machinability can not be obtained.
　Test No. 30, due to the cooling rate during casting was slow, less number density of MnS of less than 2.0 .mu.m, also d + 3 [sigma] becomes 20 or more. Therefore, sufficient machinability can not be obtained.
　Test No. 31 had less C content. As a result, the surface fatigue strength is not sufficient.

WE claims

Chemical composition, in
mass%,
C: 0.40
~ 0.70%, Si: 0.15 ~ 3.00%, Mn: 0.30
~ 2.00%, Cr: 0.01% or more, 0 less than%
.50, S: 0.003
~ 0.070%, Bi: 0.0001%, more than 0.0050% or
less,
N: 0.0030 ~ 0.0075%, Al: 0.003 ~ 0.100
%, P: 0.050% or
less,
B:
0 ~ 0.0050%, Mo: 0 ~
0.20%, Ni: 0 ~ 1.00%, Cu: 0 ~
1.00%, Ca: 0 ~
%
0.0050,
Mg: 0 ~ 0.0050%, Zr: 0 ~
0.0050%, Rem: 0 ~ 0.0050%,
Ti: 0 ~ 0.20%, Nb: 0 ~ 0.20%,
V:
0 ~ 0.35%, Sb:
0 ~ 0.015%, Te: 0 ~ 0.20%, and
Pb: 0 ~ 0.50%,
wherein the remainder being F It consists of e and impurities,
　Formula (1a) and the following formula satisfies the (2a),
　in the longitudinal direction parallel to cross, circle equivalent diameter the density of MnS of less than 2.0μm is 300 / mm 2 at least
, characterized in that the machine structural steel.
　290 × C + 50 × Si + 430 ≧ 620 · · ·
　(1a) d + 3 [sigma] <20 · · · (2a)
　, however, C in formula (1a), Si is the content by mass%, in the formula (2a), d is the average equivalent circular diameter of the unit μm in circle equivalent diameter 1.0 .mu.m or more MnS, sigma is the standard deviation of circle-equivalent diameter of MnS over the circle equivalent diameter 1.0 .mu.m.
[Requested item 2]
　The chemical composition, by
mass%,
B: 0.0003 ~ 0.0050%,
Mo: 0.01 ~ 0.20%, Ni: 0.05 ~ 1.00%, and
Cu: 0.05 ~ 1 .00%
of one or machine structural steel according to contain two or more to claim 1, wherein is selected from the group consisting of.
[Requested item 3]
　The chemical composition, by
mass%,
Ca: 0.0003 ~
0.0050%, Mg: 0.0003 ~ 0.0050%, Zr: 0.0003 ~ 0.0050%, and
Rem: 0.0003 ~ 0 .0050%
one or containing two or more, characterized in claim 1 or 2 steel for machine structural use according to selected from the group consisting of.
[Requested item 4]
　The chemical composition, by
mass%,
Ti: 0.005 ~ 0.20%, Nb: 0.005 ~ 0.20%, and
V: 0.005 ~ 0.35%
1 selected from the group consisting of steel for machine structural use according to any one of claims 1 to 3, characterized in that it contains more species or in combination.
[Requested item 5]
　The chemical composition, in
mass%,
Sb: 0.0003 ~ 0.015%,
Te: 0.0003 ~ 0.20%, Pb: 0.01 ~ 0.50%, and
one of or two steel for machine structural use according to any one of claims 1 to 4, characterized in that it contains more species.
[Requested item 6]
　Chemical composition, in
mass%,
C: 0.40
~ 0.70%, Si: 0.15 ~ 3.00%, Mn: 0.30
~ 2.00%, Cr: 0.01% or more, 0 less than%
.50, S: 0.003
~ 0.070%, Bi: 0.0001%, more than 0.0050% or
less,
N: 0.0030 ~ 0.0075%, Al: 0.003 ~ 0.100
%, P: 0.050% or
less,
B:
0 ~ 0.0050%, Mo: 0 ~
0.20%, Ni: 0 ~ 1.00%, Cu: 0 ~
1.00%, Ca: 0 ~
%
0.0050,
Mg: 0 ~ 0.0050%, Zr: 0 ~
0.0050%, Rem: 0 ~ 0.0050%,
Ti: 0 ~ 0.20%, Nb: 0 ~ 0.20%,
V:
0 ~ 0.35%, Sb:
0 ~ 0.015%, Te: 0 ~ 0.20%, and
Pb: 0 ~ 0.50%,
wherein the remainder being F It consists of e and impurities,
　Formula (1b) and the following formula satisfies the (2b),
　the density of MnS of a circle equivalent diameter of less than 2.0μm in the longitudinal direction parallel to cross section 300 / mm 2 at least
, characterized in that induction hardening Nyuhagane parts.
　290 × C + 50 × Si + 430 ≧ 620 · · ·
　(1b) d + 3 [sigma] <20 · · · (2b)
　However, C in the formula (1b), Si is the content by mass%, in the formula (2b), d is the average equivalent circular diameter of the unit μm in circle equivalent diameter 1.0 .mu.m or more MnS, sigma is the standard deviation of circle-equivalent diameter of MnS over the circle equivalent diameter 1.0 .mu.m.
[Requested item 7]
　The chemical composition, by
mass%,
B: 0.0003 ~ 0.0050%,
Mo: 0.01 ~ 0.20%, Ni: 0.05 ~ 1.00%, and
Cu: 0.05 ~ 1 .00%
frequency hardened steel part according to claim 6, characterized in that it contains one or more members selected from the group consisting of.
[Requested item 8]
　The chemical composition, by
mass%,
Ca: 0.0003 ~
0.0050%, Mg: 0.0003 ~ 0.0050%, Zr: 0.0003 ~ 0.0050%, and
Rem: 0.0003 ~ 0 .0050%
frequency hardened steel component according to claim 6 or 7, characterized in that it contains one or more members selected from the group consisting of.
[Requested item 9]
　The chemical composition, by
mass%,
Ti: 0.005 ~ 0.20%, Nb: 0.005 ~ 0.20%, and
V: 0.005 ~ 0.35%
1 selected from the group consisting of frequency hardened steel component according to any one of claims 6-8, characterized in that it contains more species or in combination.
[Requested item 10]
　The chemical composition, in
mass%,
Sb: 0.0003 ~ 0.015%, Te: 0.0003 ~ 0.20%, and
Pb: 0.01 ~ 0.50%,
is selected from the group consisting of one or high-frequency hardened steel component according to any one of claims 6-9, characterized by containing two or more.