Steel for high-frequency hardening, high-frequency hardening steel parts and manufacturing method thereof

Abstract

To suppress melting cracks in induction hardening.SOLUTION: Steel for induction hardening has a predetermined chemical composition and has melting cracks and machinability improved by satisfying expressions (1)-(3) below. Further, an inclusion of an equivalent circle diameter≥1.0 μm present in the steel preferably contains a proper melting point oxide≥0.15 / mm2 satisfying expression (4), a low melting point oxide≤0.15 / mm2 satisfying expression (5), and a CaS composite inclusion≥50% among the proper melting point oxide. The average value d and the standard deviation of the equivalent circle diameter preferably satisfy expression (6). 0.01≤Ca / Al≤0.12:(1), Ca-0.0008×Ln(S)≤0.00493:(2), Al+1091×S3-61.5×S2+1.59×S≤0.0567:(3), 805≤10.4×A+1.4×B≤1004:(4), 10.4×A+1.4×B≤404:(5), A: content (mass%) of Al2O3, B: content (mass%) of CaO, d+3σ≤20:(6)SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a steel for induction hardening, an induction hardening steel part and a method for producing the same. [Background technology]

[0002] High fatigue strength is required for machine structural parts used in crankshafts of automobiles and construction vehicles, etc. Therefore, in order to improve the fatigue strength, the machine structural parts are sometimes subjected to a surface hardening treatment.

[0003] Among various surface hardening treatments, induction hardening can harden only the necessary parts. Furthermore, because induction hardening involves heating at high temperatures and then cooling, it is possible to obtain a deeper hardened layer and higher fatigue strength than other surface hardening treatments such as soft nitriding. For this reason, induction hardening is often performed on machine structural parts. For example, a technology for induction hardening the fillet R part 1 shown in Figure 1 has been put to practical use in order to improve the fatigue strength of a crankshaft, which is a type of machine structural part.

[0004] In recent years, the industrial world has been demanding further improvement in fatigue strength of machine structural parts. In order to increase the depth of the hardened layer by using induction hardening, the output of high frequency power may be increased in induction hardening to increase the heating temperature. However, when induction hardening is performed at high temperatures, the heating temperature is likely to become excessively high at the edge of the machine structural part (the edge corresponds to the part indicated by the symbol 2 in the case of a crankshaft as shown in FIG. 1, for example). In particular, when the heating rate during induction hardening is fast, the heating temperature is likely to become excessively high. For example, when the heating temperature during induction hardening becomes excessively high and reaches 1350°C or higher, a part of the surface layer or inside of the steel material may melt and cause cracks. Hereinafter, such cracks are referred to as "fusion cracks" in this specification. Parts with fusion cracks are not suitable for practical use. Therefore, it is required to suppress fusion cracks.

[0005] The following method can be given as an example of a method for manufacturing machine structural parts to be induction hardened and tempered, i.e., induction hardened steel parts. That is, first, a rough part that is the rough shape of the final product is manufactured, and then an intermediate part with a shape closer to the final product is manufactured by processing. The intermediate part thus manufactured is induction hardened and tempered to obtain a base material. The base material is then finished (cut or grinded) to obtain an induction hardened steel part.

[0006] Generally, induction hardened steel parts are required to have excellent fatigue strength. A technique for improving fatigue strength is disclosed in, for example, Patent Document 1.

[0007] Patent Document 1 describes how abrasion resistance and bending fatigue strength can be improved by subjecting a raw material obtained by induction hardening and tempering to a cutting finish under specific conditions.

[0008] However, when cutting the base material obtained by induction hardening and tempering, expensive CBN tools are generally used, resulting in huge tool costs.In other words, it is an urgent task to extend the tool life and reduce the tool costs of the base material obtained by induction hardening and tempering, and the base material is required to have excellent machinability.

[0009] Therefore, the steel for induction hardening, which is the raw material for induction hardened steel parts, is required to suppress melting cracks when induction hardening is performed on the intermediate part to produce a base material, to have machinability when cutting the base material obtained by induction hardening, and to have high fatigue strength when made into an induction hardened steel part.

[0010] Techniques relating to steel materials for machine structures are disclosed in, for example, Patent Documents 2 and 3.

[0011] The steel for machine structural use disclosed in Patent Document 2 contains 0.001-0.05% Ca, 0.02-0.15% Pb and Bi alone or in combination, restricts B to 0.005% or less, makes inclusions CaS-CaO, Pb, Bi-based inclusions, and suppresses Al2O3 inclusions to less than 0.001%, thereby improving machinability. In this document, a large amount of Ca is continuously added to molten steel to change dissolved S to CaS. Also, Al2O3 is eliminated or extremely reduced by a reduction reaction with Ca. Therefore, the inclusions become CaS-CaO-based and are finely and uniformly dispersed. Then, a small amount of one or both of Pb and Bi is added to generate single inclusions of Pb or Bi or to finely precipitate around CaS-CaO, thereby improving machinability.

[0012] The steel for machine structural use disclosed in Patent Document 3 contains, by mass%, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Al: 0.001-0.02%, Ca: 0.0005-0.02%, O: 0.0005-0.01%, and N: 0.001-0.04%, as well as one or two of Ti: 0.002-0.020% and Zr: 0.002-0.040%, with the balance being Fe and unavoidable impurities. This steel for machine structural use has a field area of ​​3.5 mm2 where the area occupied by sulfide-based inclusions containing 1.0% or more Ca and which are in contact with oxide-based inclusions with a CaO content of 0.2-62% by weight is 3.5 mm2 or less. 2 2.0x10 -4 mm 2 As described above, the fine dispersion of MnS inclusions improves machinability and chip breakability. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] JP 2017-082299 A [Patent Document 2] Japanese Patent Application Publication No. 57-19366 [Patent Document 3] JP 2003-226934 A Summary of the Invention [Problem to be solved by the invention]

[0014] However, the above-mentioned Patent Documents 2 and 3 do not at least consider the prevention of melting cracks during induction hardening and the machinability when cutting the base material obtained by induction hardening.

[0015] The present invention has an object to provide a steel for induction hardening, which has an object to suppress melting cracks during induction hardening, and which maintains machinability after induction hardening and fatigue strength when made into an induction hardened steel part, and a manufacturing method thereof. [Means for solving the problem]

[0016] Focusing on tool wear, which is the most important index from the viewpoint of ensuring machinability after induction hardening, we conducted tool observations after cutting tests and found that it is important to control the state of inclusions in the steel. That is, the temperature rises due to heat generation at the cutting interface between the tool and the steel during cutting, so the inclusions in the steel are affected by the temperature rise. In particular, by controlling the composition of the oxides in the inclusions, specifically the content of CaO and Al2O3, and optimizing the melting point of the oxides by lowering it, the oxides soften to a certain extent when the temperature rises during cutting and adhere to the tool. The adhered oxides act as a protective film and suppress tool wear. Furthermore, when these oxides exist in the steel in a form combined with sulfides containing a certain amount of CaS, the protective film effect when they adhere to the tool is enhanced, and the effect of suppressing tool wear is greater.

[0017] However, in steel materials in which inclusions containing oxides with a low melting point are present, melting cracks may occur. When the cause of melting cracks was investigated, it was found that when the steel material is heated to a high temperature by high-frequency heating, melting cracks occur starting from oxides with a low melting point. In other words, in order to suppress melting cracks, it is better to limit the density of low-melting-point oxides and instead make the oxides have a high melting point.

[0018] As described above, it is desirable to lower the melting point of the oxides in the inclusions to improve machinability, but conversely, it is desirable to have a higher melting point for the oxides to suppress fusion cracks. In order to solve the problem of achieving both improved machinability and suppression of fusion cracks, the inventors conducted various studies, focusing on the fact that steel after induction hardening is hard and therefore generates more heat during cutting than in the case of soft steel. As a result, it was found that the oxides in the inclusions can be regarded as ternary oxides of CaO-Al2O3-SiO2, and that the melting point of the oxides is mainly determined by the contents of Al2O3 and CaO, so that fusion cracks can be prevented by controlling the contents of Al2O3 and CaO, and machinability can be improved by controlling the CaS content.

[0019] Furthermore, it was found that inclusions with a diameter of 1 μm or more have a large effect on machinability and fusion cracking. The oxides in inclusions with a diameter of 1 μm or more were regarded as ternary oxides of CaO-Al2O3-SiO2, and an equation was found to show the melting point of the oxide. It was also found that the melting point should be within the appropriate temperature range by satisfying the following equation (4). 805≦10.4×A+1.4×B≦1004: Formula (4) A: Al2O3 content in the oxide (mass%) B: CaO content in the oxide (mass%) In other words, to have a melting point that improves machinability and suppresses melting cracks, it is necessary to increase the amount of inclusions that contain oxides that satisfy formula (4). (Hereinafter, oxides that have a composition that satisfies formula (4) may be referred to as "oxides with appropriate melting points.")

[0020] On the other hand, from the viewpoint of avoiding melting cracks, it has been found that it is sufficient to limit inclusions containing oxides (hereinafter sometimes referred to as "low melting point oxides") having a composition that satisfies formula (5), which indicates that the melting point of the oxide is in the low melting point range. 10.4×A+1.4×B≦404: Formula (5) Furthermore, it was revealed that by optimizing the relationship between the Ca, Al, and S contents, it is possible to control them to produce desirable inclusions.

[0021] In addition, we also investigated ways to improve fatigue strength. To improve fatigue strength, we found a relationship between the contents of C, Si, Mn, and V in the steel in order to control the microstructure of the induction hardened layer and the hardness after tempering, as well as the hardness of the non-hardened layer.

[0022] Based on the above findings, the present invention has been completed, the gist of which is as follows.

[0023] [1] A steel for induction hardening, having a chemical composition, in mass%, of: C: 0.31-0.60%, Si: 0.51-1.00%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.006~0.040%, Cr: 0-0.19%, Ca: 0.0006~0.0023%, Al: 0.021 to 0.050%, V: 0~0.099%, N: 0.0250% or less, and O: 0.0050% or less and the balance being Fe and impurities, A steel for induction hardening characterized by satisfying the following formulas (1) to (3): 0.01≦Ca / Al≦0.12 ··· Formula (1) Ca-0.0008×Ln(S)≦0.00493...Equation (2) Al+1091×S 3 -61.5×S 2 +1.59×S≦0.0567...Equation (3) Here, the content (mass%) of each corresponding element is substituted for each element symbol in formulas (1) to (3), and when the corresponding element is not contained, "0" is substituted for that element symbol. [2] The steel for induction hardening according to the above [1], wherein the number density of oxide-containing inclusions that contain oxides and have an equivalent circle diameter of 1.0 μm or more and satisfy formula (4) is 0.15 pieces / mm2 In the above, the density of inclusions containing oxides that satisfy formula (5) is 0.15 pieces / mm 2 the ratio of the number of composite inclusions containing CaS: 10% by mass or more among the inclusions containing oxides that satisfy formula (4) is 50% or more, and the average value d and standard deviation σ of the equivalent circle diameters of the composite inclusions satisfy formula (6). 805≦10.4×A+1.4×B≦1004...Equation (4) 10.4×A+1.4×B≦404 Formula (5) Here, A and B in the formulas (4) and (5) are as follows: A: The content of Al2O3 in the oxide (mass%) when the oxide is considered to be a ternary oxide of CaO-Al2O3-SiO2. B: The CaO content (mass%) in the oxide when the oxide is considered to be a ternary oxide of CaO-Al2O3-SiO2. d+3σ≦20...Equation (6) [3] The steel for induction hardening according to the above [1] or [2], characterized in that it satisfies the following formula (7): 60×C+5.5×Si+29Mn-29V≧58...Formula (7) Here, the content (mass%) of the corresponding element is substituted for each element symbol in formula (7), and when the corresponding element is not contained, "0" is substituted for the element symbol. [4] The steel for induction hardening according to any one of the above [1] to [3], characterized in that it satisfies the following formula (8): 244≦462×C+102×Si+7×Mn≦316...Formula (8) Here, the content (mass%) of the corresponding element is substituted for each element symbol in formula (8), and when the corresponding element is not contained, "0" is substituted for the element symbol. [5] The steel for induction hardening according to any one of the above [1] to [4], characterized in that it satisfies the following formula (9): 149×C+36×Si+70×Mn+76×V≧155...Formula (9) Here, the content (mass%) of the corresponding element is substituted for each element symbol in formula (9), and when the corresponding element is not contained, "0" is substituted for the element symbol. [6] The steel for induction hardening according to any one of the items [1] to [5], The chemical composition further contains, in place of a portion of the Fe, Ti: 0.039% or less, Nb: 0.050% or less, and Steel for induction hardening containing one or more selected from the group consisting of Zr: 0.0019% or less. [7] The steel for induction hardening according to any one of the items [1] to [6], The chemical composition further contains, in place of a portion of the Fe, Mo: 0.095% or less Cu: 0.50% or less, and Steel for induction hardening containing one or more selected from the group consisting of Ni: 0.50% or less. [8] An induction-hardened steel part, comprising a steel having a chemical composition according to any one of the above [1] to [7], and comprising a portion in which the maximum compressive residual stress in a region from the surface to a depth of 200 μm is 300 MPa or more. [9] A method for manufacturing an induction-hardened steel part according to the above [8], comprising the steps of: processing a steel for induction hardening having a chemical composition according to any one of the above [1] to [7] to manufacture a member; induction hardening the member; tempering the induction-hardened member; and cutting the tempered member to a depth of 0.05 to 0.40 mm from the surface in the thickness direction. Effect of the Invention

[0024] The steel for induction hardening of the present invention can suppress the occurrence of melting cracks during induction hardening, has excellent machinability after induction hardening, and further has excellent fatigue strength when made into induction hardened steel parts. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a front view showing a portion of a crankshaft which is an induction hardened steel part. [Diagram 2] FIG. 2 is a schematic diagram of the microstructure of a test piece taken from a simulated rough component obtained using conventional induction hardening steel, which was heated to 1,350°C or higher at a heating rate of 100°C / s, held at that temperature for 10 seconds, and then water-cooled. [Diagram 3] FIG. 3 is a schematic diagram of the microstructure of a test piece taken from a simulated rough component obtained using a steel for induction hardening, which is an example of the present invention, after heating to 1,350° C. or higher at a heating rate of 100° C. / s, holding the temperature for 10 seconds, and then water cooling. [Figure 4] FIG. 4 is a schematic diagram of a rotating bending fatigue test piece that was taken from each simulated rough member, then induction hardened and tempered, and then machined. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Hereinafter, the induction hardening steel, induction hardening steel part, and the manufacturing method thereof according to the present embodiment will be described in detail. Unless otherwise specified, "%" in the content of the chemical composition means "mass %". Furthermore, unless otherwise specified, each element symbol in a formula using element symbols is substituted with the content (mass %) of the corresponding element, and 0 (zero) is substituted when the corresponding element is not contained.

[0027] [Chemical composition] The chemical composition of the steel for induction hardening of this embodiment contains the following elements.

[0028] C: 0.31% to 0.60% Carbon (C) is preferably contained at 0.31% or more in order to increase the fatigue strength of induction hardened steel parts. On the other hand, C lowers the melting point of steel material, so if it is contained in a large amount, melting cracks are likely to occur during induction hardening, so the C content is preferably 0.60% or less. Therefore, the C content is 0.31 to 0.60%. The preferred lower limit of the C content is 0.32%, 0.33%, 0.34%, 0.35%, 0.36%, 0.37%, or 0.38%. The preferred upper limit of the C content is 0.59%, 0.58%, 0.57%, 0.56%, 0.55%, 0.54%, 0.53%, 0.52%, 0.51%, 0.50%, 0.49%, or 0.48%.

[0029] Silicon: 0.51% to 1.00% Silicon (Si) deoxidizes steel in the steelmaking process. Si further increases the strength of ferrite through solid solution strengthening, and therefore increases the fatigue strength of induction hardened steel parts, so it is preferable to contain 0.51% or more. On the other hand, Si has a weak affinity with C, and when heated, C segregates at the grain boundaries rather than inside the grains where Si is solid-solved, lowering the melting point near the grain boundaries and making melting cracks more likely to occur during induction hardening, so the Si content should be 1.00% or less. Therefore, the Si content is 0.51 to 1.00%. The preferred lower limit of the Si content is 0.52%, 0.54%, 0.56%, 0.58%, or 0.60%. The preferred upper limit of the Si content is 0.95%, 0.90%, 0.85%, 0.80%, 0.75%, or 0.70%.

[0030] Mn: 0.50% to 2.00% Manganese (Mn) has a strong affinity with C, so that when heated, C remains in the grains where Mn is dissolved. Therefore, segregation of C to grain boundaries is suppressed, and the occurrence of melting cracks during induction hardening can be suppressed, so it is preferable to contain 0.50% or more. On the other hand, Mn lowers the melting point of steel, and if contained in large amounts, melting cracks are likely to occur during induction hardening, so the Mn content is preferably 2.00% or less. Therefore, the Mn content is 0.50% or more and 2.00% or less. The preferable lower limit of the Mn content is 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%, 0.95%, 0.98%, 1.00%, 1.01%, 1.02%, 1.03%, 1.04%, or 1.05%. A preferred upper limit of the Mn content is 1.95%, 1.80%, 1.85%, 1.75%, 1.70%, 1.65%, 1.60%, 1.55%, 1.50%, 1.45%, or 1.40%.

[0031] P:0.050% or less Phosphorus (P) is an impurity that not only lowers the melting point of steel but also segregates at grain boundaries, making it easier for melting cracks to occur during induction hardening, so the P content should be 0.050% or less. The P content is preferably as low as possible. The preferred upper limit of the P content is 0.040%, 0.035%, 0.030%, 0.025%, 0.020%, or 0.015%. The P content may be desirably 0%, but an excessive reduction in the P content increases manufacturing costs, so considering the economics of refining, the preferred lower limit of the P content may be more than 0%, 0.001%, or 0.002%.

[0032] S: 0.006%~0.040% Sulfur (S) forms sulfides and improves machinability, so it is preferable to contain 0.006% or more. On the other hand, if a large amount of S is contained, it lowers the melting point of the steel material and makes it easier for melting cracks to occur during induction hardening, so the S content should be 0.040% or less. Therefore, the S content is 0.006% or more and 0.040% or less. The preferred lower limit of the S content is 0.008%, 0.010%, 0.013%, 0.015%, or 0.020%. The preferred upper limit of the S content may be 0.035%, 0.030%, 0.025%, or 0.023%.

[0033] Cr: 0%~0.19% Chromium (Cr) may not be contained. However, like Mn, Cr has a strong affinity with C, so that C remains in the grains where Cr is dissolved during heating. Therefore, segregation of C to grain boundaries is suppressed, and the occurrence of melting cracks during induction hardening can be suppressed. On the other hand, Cr lowers the melting point of the steel material, and melting cracks are likely to occur during induction hardening, so the Cr content should be 0.19% or less. Therefore, the Cr content is 0% or more and 0.19% or less. In order to obtain the above effect, if Cr is contained, the Cr content should be 0.01% or more, and the more preferable lower limit should be 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, or 0.10%. The preferable upper limit of the Cr content should be 0.18%, 0.17%, 0.16%, or 0.15%.

[0034] Ca: 0.0006% to 0.0023% Calcium (Ca) is a deoxidizing element and produces soft Ca oxides. It also produces CaS, which forms composite inclusions with the Ca oxides. In cutting after induction hardening, these composite inclusions accumulate on the tool to form a protective film and reduce tool wear, so the Ca content should be 0.0006% or more. On the other hand, if the Ca content is too high, the composite inclusions become coarse and deteriorate the fatigue properties, so the Ca content should be 0.0023% or less. Therefore, the Ca content is 0.0006% or more and 0.0023% or less. The preferable lower limit of the Ca content is 0.0007%, 0.0008%, 0.0009%, or 0.0010%. The upper limit of the Ca content is preferably 0.0022%, 0.0021%, 0.0020%, 0.0019%, 0.0018%, 0.0017%, 0.0016%, or 0.0015%.

[0035] Al: 0.021% to 0.050% Aluminum (Al) is a deoxidizing element that forms Al2O3 in steel. Adding too much Al consumes oxygen (O), which may make it difficult to form Ca oxides, which are necessary to improve machinability, especially after induction hardening, so the Al content should be 0.050% or less. On the other hand, if the Al content is too low, a lot of oxides with low melting points are formed, making it easier for melting cracks to occur, so the Al content should be 0.021% or more. Therefore, the Al content is 0.021% or more and 0.050% or less. The preferred lower limit of the Al content is 0.022%, 0.023%, 0.024%, or 0.025%. The upper limit of the Al content is preferably 0.048%, 0.046%, 0.044%, 0.042%, 0.040%, 0.038%, 0.037%, 0.036%, 0.035%, 0.031%, or 0.028%.

[0036] V: 0%~0.099% Vanadium (V) precipitates in the ferrite in the steel as V precipitates during the cooling process after hot forging, and increases the strength of the ferrite, so that the fatigue strength of the induction hardened steel parts is increased, but it is not necessary to contain it. On the other hand, if the V content is high, the amount of V nitride increases, which increases the amount of ferrite in the grains, and the ferrite may remain even after induction hardening, reducing the fatigue strength of the induction hardened steel parts, so the V content should be 0.099% or less. Therefore, the V content is 0% or more and 0.099% or less. To obtain the above effect, if V is contained, it should be 0.001% or more, and the preferable lower limit should be 0.002%, 0.003%, 0.004%, 0.005%, 0.006%, 0.007%, 0.008%, 0.009%, or 0.010%. The upper limit of the V content is preferably 0.098%, 0.095%, 0.090%, 0.080%, 0.070%, 0.060%, 0.050%, 0.040%, 0.030%, or 0.020%.

[0037] N: 0.0250% or less Nitrogen (N) forms nitrides and / or carbonitrides during cooling after hot forging to strengthen the steel material by precipitation, and increases the fatigue strength after induction hardening, but it is not necessary to include it. On the other hand, if the N content is high, the hot workability of the steel material decreases, so the N content may be 0.0250% or less. Since excessive reduction in the N content increases the manufacturing cost, taking into account the economic efficiency of refining, the preferred lower limit of the N content is 0.0001%, 0.0005%, 0.0010%, 0.0020%, 0.0030%, 0.0040%, or 0.0050%. The preferred upper limit of the N content is 0.0230%, 0.0200%, 0.0180%, 0.0150%, 0.0120%, 0.0100%, or 0.0080%.

[0038] O: 0.0050% or less Oxygen (O) is an impurity, and the less the better. If the O content is high, it forms oxides in the steel and reduces the fatigue strength of induction hardened steel parts, so the O content should be 0.0050% or less. The preferred upper limit of the O content is 0.0030% or 0.0020%. Since excessive reduction of the O content increases manufacturing costs, taking economics into consideration, the preferred lower limit of the O content is 0.0001%, 0.003%, 0.0005%, or 0.0008%. In order to produce Ca oxide, it is preferable to satisfy 0.25≦Ca / O≦5.0, where each element symbol in the formula is substituted with the content (mass%) of the corresponding element.

[0039] In addition to the above elements, the balance is Fe and impurities. Here, the impurities are components that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of steel material, and also include elements that are not intentionally contained, and are permissible as long as they do not impair the properties of the steel material and induction hardened steel parts according to this embodiment.

[0040] The steel material of this embodiment may further contain the following elements in place of a portion of Fe. However, the part according to this embodiment can solve the problem without containing the elements exemplified below. Therefore, the elements exemplified below do not need to be contained, and the lower limit of the content thereof is 0%.

[0041] Ti: 0.039% or less Titanium (Ti) forms carbides and / or carbonitrides during the cooling process of the hot forging process to refine the crystal grains. This can increase the toughness of the induction hardened steel parts. To obtain this effect, the Ti content may be preferably 0.001% or more, 0.002% or more, 0.003% or more, 0.005% or more, 0.007% or more, 0.009% or more, 0.010% or more, 0.011% or more, 0.015% or more, 0.020% or more, or 0.021% or more. On the other hand, even if the Ti content is increased, the above effect saturates and the manufacturing cost increases, so the Ti content is preferably 0.039% or less. More preferably, the upper limit of the Ti content is 0.038%, 0.035%, or 0.030%.

[0042] Nb: 0.050% or less Niobium (Nb) forms carbides and / or carbonitrides during the cooling process of the hot forging process to refine the crystal grains. This increases the toughness of induction hardened steel parts. If even a small amount of Nb is contained, the above effect can be obtained to a certain extent. The preferable lower limit of the Nb content to obtain the above effect is 0.001%, 0.002%, 0.003%, 0.005%, 0.007%, 0.010%, 0.013%, 0.016%, or 0.020%. On the other hand, even if the Nb content is increased, the above effect saturates and the manufacturing cost increases, so the Nb content is preferably 0.050% or less. The more preferable upper limit of the Nb content is 0.040% or 0.030%.

[0043] Zr: 0.0019% or less Zirconium (Zr) forms carbides and / or carbonitrides during the cooling process of the hot forging process to refine the crystal grains. This can increase the toughness of induction hardened steel parts. If even a small amount of Zr is contained, the above effect can be obtained to a certain extent. The preferable lower limit of the Zr content to obtain the above effect is 0.0001%, and more preferably 0.0003%, 0.0005%, 0.0007%, or 0.0008%. On the other hand, even if the Zr content is increased, the above effect saturates and the manufacturing cost increases, so the Zr content is preferably 0.0019% or less. The more preferable upper limit of the Zr content is 0.0017% or 0.0015%.

[0044] Mo: 0.095% or less Molybdenum (Mo) increases the fatigue strength of induction hardened steel parts. Even if even a small amount of Mo is contained, the above effect can be obtained to a certain extent. The preferred lower limit of the Mo content to obtain the above effect is 0.001%, 0.005%, 0.010%, 0.015%, or 0.020%. On the other hand, if the Mo content is too high, the hot workability may decrease, so the Mo content should be 0.095% or less. The more preferred upper limit of the Mo content is 0.090%, 0.080%, 0.070%, 0.060%, or 0.050%.

[0045] Cu: 0.50% or less Copper (Cu) increases the fatigue strength of induction hardened steel parts. Even if even a small amount of Cu is contained, the above effect can be obtained to a certain extent. The preferable lower limit of the Cu content to obtain the above effect is 0.01% or 0.02%. On the other hand, if the Cu content is high, melting cracks may easily occur during induction hardening, so the Cu content should be 0.50% or less. The more preferable upper limit of the Cu content is 0.40%, 0.30%, 0.20%, 0.17%, 0.13%, 0.10%, 0.07%, or 0.05%.

[0046] Ni: 0.50% or less Nickel (Ni) increases the fatigue strength of induction hardened steel parts. Even if even a small amount of Ni is contained, the above effect can be obtained to a certain extent. The preferable lower limit of the Ni content to obtain the above effect is 0.01% or 0.02%. On the other hand, if the Ni content is too high, melting cracks may easily occur during induction hardening, so the Ni content should be 0.50% or less. The more preferable upper limit of the Ni content is 0.40%, 0.30%, 0.20%, 0.10%, or 0.05%.

[0047] Next, in order to control inclusions, the relationship between the contents of Ca, Al and S is set to satisfy the formulas (1) to (3).

[0048] [0.01≦Ca / Al≦0.12] The above chemical composition further satisfies formula (1). 0.01≦Ca / Al≦0.12: Formula (1) Here, the content (mass%) of the corresponding element is substituted for each element symbol in formula (1). When the corresponding element is not contained, "0" is substituted for the element symbol. Hereinafter, Ca / Al will be described as F1. F1=Ca / Al

[0049] In order to form a protective film by depositing oxide-containing inclusions on the tool during cutting after induction hardening to reduce tool wear, it is preferable to generate Ca oxides in the steel, which have a low melting point and become soft during cutting. On the other hand, since the presence of Al forms Al2O3, it is preferable to contain sufficient Ca relative to the Al content in order to generate a large amount of oxides with a suitable melting point. On the other hand, if the Ca content is too high relative to the Al content, a large amount of low-melting-point oxides is generated, causing melting cracks. Therefore, in order to properly control the composition of the oxides, F1 should be in the range of 0.01 to 0.12. The preferable lower limit of F1 is 0.02 or 0.03. The preferable lower limit of F1 is 0.11, 0.10, or 0.09.

[0050] [Ca-0.0008×Ln(S)≦0.00493] The above chemical composition further satisfies formula (2). Ca-0.0008×Ln(S)≦0.00493: Formula (2) Here, the content (mass%) of the corresponding element is substituted for each element symbol in formula (2). When the corresponding element is not contained, "0" is substituted for the element symbol. Hereinafter, Ca-0.0008×Ln(S) will be explained as F2. Note that Ln means the natural logarithm. F2 = Ca-0.0008 x Ln(S)

[0051] In order to form a protective film by depositing oxide-containing inclusions on the tool to reduce tool wear during cutting after induction hardening, it is advisable to generate CaS and generate a composite inclusion of the oxide with a suitable melting point and CaS in a predetermined ratio. If the Ca content is too high compared to the S content, the CaS aggregates and becomes coarse, and the composite inclusions become fewer. Furthermore, the size of the composite inclusions becomes too large, which deteriorates the fatigue properties. For this reason, it is advisable to include Ca and S so that F2 is 0.00493 or less. The upper limit of F2 is preferably set to 0.00483.

[0052] [Al+1091×S 3 -61.5×S 2 +1.59×S≦0.0567] The above chemical composition further satisfies formula (3). Al+1091×S 3 -61.5×S 2 +1.59×S≦0.0567: Formula (3) Here, the content (mass%) of the corresponding element is substituted for each element symbol in formula (3). When the corresponding element is not contained, "0" is substituted for the element symbol. Here, Al + 1091 × S 3 -61.5×S 2 +1.59×S is explained as F3. F3=Al+1091×S 3 -61.5×S 2 +1.59×S

[0053] As mentioned above, Al is a deoxidizing element and forms Al2O3, so if the Al content is high, Ca oxides are less likely to form. Also, S combines with Ca to form CaS, so if the S content is high, Ca oxides are less likely to form. If the F3 value is large, not only is it not possible to obtain a sufficient amount of oxides with the appropriate melting point, but CaS aggregates and the size of the composite inclusions becomes too large, deteriorating the fatigue properties. Therefore, in order to generate a sufficient amount of oxides with the appropriate melting point, it is recommended that Al and S be contained so that F3 is 0.0567 or less. The preferred upper limit of F3 is 0.0517 or 0.0467.

[0054] [Number density of inclusions containing oxides with appropriate melting point: 0.15 pieces / mm 2 End] The oxides contained in steel can be regarded as mainly ternary oxides of CaO-Al2O3-SiO2, and the melting point of the oxides is mainly determined by the content of Al2O3 and CaO. In particular, inclusions with a particle size of 1 μm or more have a large effect. In cutting after induction hardening, in order to form a protective film by depositing inclusions containing oxides on the tool to reduce tool wear, it is desirable for inclusions with a particle size of 1 μm or more to soften to a certain extent. Therefore, it is considered that inclusions with a particle size of 1 μm or more that contain oxides with an appropriate melting point have a number density of 0.15 pieces / mm 2 It would be good if that were the case. 805≦10.4×A+1.4×B≦1004: Formula (4) Here, A and B in formula (4) are as follows: A: The content of Al2O3 in the oxide (mass%) when the oxide is considered to be a ternary oxide of CaO-Al2O3-SiO2. B: The CaO content (mass%) in the oxide when the oxide is considered to be a ternary oxide of CaO-Al2O3-SiO2. Here, the following explanation will be given assuming that 10.4×A+1.4×B is F4. F4=10.4×A+1.4×B

[0055] When the F4 value is large, the oxide has a high melting point and does not soften even when heated during cutting after induction hardening, and does not form a protective film on the tool. On the other hand, when the F4 value is small, the oxide has a low melting point and softens too much due to heat generated during cutting after induction hardening, making it difficult for the oxide to adhere to the tool. The inventors have determined that a sufficient number (0.15 pieces / mm) of oxides with appropriate melting points having a composition with an F4 value in the range of 805 to 1004 are present. 2 It was confirmed that it is good to have it present in

[0056] The more oxides there are, the more sufficient the inclusions will be deposited on the tool, and the more effective it will be in suppressing tool wear. The preferred lower limit of the density of inclusions containing oxides with appropriate melting points is 0.20 pieces / mm 2 , 0.30 pieces / mm 2 , 0.40 pieces / mm 2 , 0.50 pieces / mm 2 , 0.60 pieces / mm 2 , 0.70 pieces / mm 2 , or 0.75 pieces / mm 2 It is recommended to do so.

[0057] [Number density of inclusions containing low melting point oxides: 0.15 pieces / mm 2 below] On the other hand, when steel material is heated to a high temperature by high-frequency heating, low-melting-point oxides melt during heating and become the starting point of fusion cracks. Therefore, it is sufficient to limit the number of low-melting-point oxides so that F4, an index of the oxide melting point, is 404 or less. In other words, in order to suppress fusion cracks, it is necessary to limit the number of inclusions that have a grain size of 1 μm or more and contain low-melting-point oxides that satisfy formula (5) to 0.15 pieces / mm 2 It would be better to do the following: 10.4×A+1.4×B≦404: Formula (5) where A and B in formula (5) are as follows and are the same as formula (4). A: The content of Al2O3 in the oxide (mass%) when the oxide is considered to be a ternary oxide of CaO-Al2O3-SiO2. B: The CaO content (mass%) in the oxide when the oxide is considered to be a ternary oxide of CaO-Al2O3-SiO2. That is, F4≦404.

[0058] The preferred upper limit of the density of inclusions containing low melting point oxides is 0.12, 0.10 pieces / mm 2 and more preferably 0.08 pieces / mm 2 , 0.06 pieces / mm 2 , 0.05 pieces / mm 2 , 0.04 pieces / mm 2 , 0.03 pieces / mm 2 , 0.02 pieces / mm 2 , or 0.01 pieces / mm 2 It is.

[0059] [Among the oxides with appropriate melting points, the proportion of composite inclusions containing CaS: 10% by mass or more is 50% or more] The oxide with the appropriate melting point softens to a certain extent when the temperature rises during cutting, and adheres to the tool to form a protective film, suppressing tool wear. Furthermore, in the case of composite inclusions in which this oxide exists in combination with sulfides containing CaS, the protective film action when it adheres to the tool is enhanced, and the effect of suppressing tool wear is increased. In particular, the proportion of the number of composite inclusions containing 10 mass% or more of CaS (hereinafter, sometimes referred to as CaS composite inclusions) is preferably 50% or more of the inclusions containing the oxide with the appropriate melting point. The proportion of the number of CaS composite inclusions is preferably 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, or 80% or more.

[0060] [The average value and standard deviation of the circle equivalent diameter of CaS composite inclusions satisfy formula (6)] As mentioned above, CaS complex inclusions are important for improving machinability after induction hardening. On the other hand, if these CaS complex inclusions become coarse, they have a negative effect on the fatigue properties of induction hardened steel parts. In order to improve the fatigue properties, it is effective to limit the size of the CaS complex inclusions so that they satisfy formula (6). d+3σ≦20: Formula (6) Here, d and σ in the formula (6) are as follows: d: The average value of the equivalent circle diameter (μm) of CaS composite inclusions that are 1.0 μm or more σ: Standard deviation of the circle equivalent diameter of CaS composite inclusions (μm)

[0061] The value of d+3σ is derived from the circle equivalent diameter and the standard deviation of the circle equivalent diameter of the CaS composite inclusions observed within the observation field range of the inclusion measurement method described later, and indicates that most of the observable CaS composite inclusions have a circle equivalent diameter smaller than this value. In other words, if the value of d+3σ is 20 (μm) or less, it indicates that there are very few coarse CaS composite inclusions in the steel with a circle equivalent diameter exceeding 20 μm. By satisfying formula (6), the fatigue strength of the induction hardened steel parts can be improved. The preferable upper limit of the value of d+3σ is 18 μm, 16 μm, 15 μm, 14 μm, or 13 μm.

[0062] In this specification, oxides are mainly composed of Al2O3, CaO, and SiO2, and sulfides are mainly composed of MnS and CaS. Therefore, in the measurement method described later, oxides and sulfides are determined as follows using an energy dispersive X-ray spectrometry (EDX). First, the analytical value of Mn detected by EDX is converted to MnS. Next, the amount of S consumed by MnS is subtracted from the analytical value of S, and the remaining amount of S is converted to CaS. Next, the amount of Ca consumed by CaS is subtracted from the analytical value of Ca, and the remaining amount of Ca is converted to CaO. Next, the analytical values ​​of Al and Si are converted to Al2O3 and SiO2, respectively. From such determination, the contents of Al2O3, CaO, SiO2, MnS, and CaS in mass% of each inclusion can be obtained.

[0063] To calculate A and B in formulas (4) and (5), convert the mass percentages of Al2O3, CaO, and SiO2 so that the total is 100%, and the converted Al2O3 content (%) is A and the converted CaO content (%) is B.

[0064] Among inclusions containing oxides with suitable melting points, composite inclusions containing 10% or more by mass of CaS (CaS composite inclusions) refer to inclusions containing oxides with suitable melting points which contain 10% or more of CaS when the sum of the mass percentages of Al2O3, CaO, SiO2, MnS and CaS is taken as 100%.

[0065] The number density of inclusions can be measured by the following method. A sample is taken from the R / 2 position (the center position of the straight line (radius R) connecting the central axis of the steel and the outer surface in the longitudinal section of the steel) of the steel (steel bar). Of the surfaces of the taken sample, the surface corresponding to the longitudinal section of the steel is used as the observation surface. After mirror polishing the observation surface, the mirror-polished observation surface is observed at a magnification of 500 times using a scanning electron microscope (SEM). The observation area is 32 mm 2 It would be good if that were the case.

[0066] Based on the backscattered electron image obtained by SEM observation, the number density is examined using a well-known particle analysis method based on image analysis. Specifically, image analysis is performed based on the interface between the parent phase of the steel material and the inclusions and / or precipitates, and the circle-equivalent diameter of the inclusions and / or precipitates is calculated. Here, the circle-equivalent diameter means the diameter of a circle when the area of ​​each inclusion and / or precipitate is converted into a circle having the same area. The circle-equivalent diameter of the inclusions observed is set to 1.0 μm or more because controlling inclusions smaller than this has little effect on machinability, fusion cracking, and fatigue strength.

[0067] Furthermore, the components of the inclusions and / or precipitates are analyzed using an EDX equipped in the SEM. In this embodiment, the component analysis is performed by EDX at an acceleration voltage of 20 kV.

[0068] The inclusions were identified using the above method, and the number per unit area (inclusions / mm 2 ) is required.

[0069] When it is necessary to obtain even better fatigue strength when the steel is made into an induction hardened steel part, the relationship between the contents of C, Si, Mn and V should satisfy the formulas (7) to (9).

[0070] [60×C+5.5×Si+29Mn-29V≧58] 60×C+5.5×Si+29Mn-29V≧58: Formula (7) Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula. If the corresponding element is not contained, "0" is substituted for the element symbol. Here, 60×C+5.5×Si+29Mn-29V is explained below as F7.

[0071] Since induction hardening is a rapid and short-time heating and hardening process, the hardness after hardening is likely to be uneven or insufficient, and if this occurs, the fatigue strength of the induction hardened steel parts may decrease. Since induction heating is short, if the pro-eutectoid ferrite region is large, C atoms cannot diffuse to all parts of the ferrite during induction heating. As a result, a martensite structure with low hardness is generated, causing uneven hardness and insufficient hardness. The present inventors investigated the relationship between the contents of C, Si, Mn, and V that affect the structure and the pro-eutectoid ferrite area ratio that affects the hardenability by observing the microstructure of steel materials having various chemical components. As a result, it was found that by setting F7 to 58 or more, the pro-eutectoid ferrite area ratio of the steel for induction hardening and the intermediate member before induction hardening can be about 25% or less, and hardness unevenness and hardness insufficiency of the induction hardened steel parts can be easily prevented. Therefore, it is preferable to set F7 to 58 or more.

[0072] [244≦462×C+102×Si+7×Mn≦316] 244≦462×C+102×Si+7×Mn≦316: Formula (8) Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula. If the corresponding element is not contained, "0" is substituted for the element symbol. Here, 462×C+102×Si+7×Mn is explained as F8 in the following.

[0073] Tempering is performed after induction hardening, and in many cases the tempering temperature is about 150 to 200°C, but for example, parts such as crankshafts may be tempered at a relatively high temperature of about 300°C. Therefore, tempered hardness is important to obtain high fatigue strength. The present inventors investigated the relationship between the 300°C tempered hardness after induction hardening and the contents of C, Si, Mn and V, and quantitatively correlated the two. As a result, it was found that by setting F8 to 244 or more, the hardness of the surface layer of the induction hardened steel part that has been subjected to induction hardening and tempering becomes about 510HV or more, and high fatigue strength can be obtained. On the other hand, if F8 exceeds 316, the hardness of the surface layer of the induction hardened steel part becomes about 580HV or more, which is too hard, and there is a tendency for the machinability during cutting after induction hardening to decrease. Therefore, it is preferable that F8 is 244 to 316.

[0074] [149×C+36×Si+70×Mn+76×V≧155] 149×C+36×Si+70×Mn+76×V≧155: Formula (9) Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula. If the corresponding element is not contained, "0" is substituted for the element symbol. Here, 149×C+36×Si+70×Mn+76×V is explained below as F9.

[0075] The fatigue strength of induction-hardened steel parts is affected not only by the hardness of the part hardened by induction hardening, but also by the hardness of the inside that is not hardened by induction hardening. Therefore, in order to improve the fatigue strength, it is necessary to increase the hardness of the steel material. Therefore, the inventors investigated the relationship between the hardness of the steel material and the contents of C, Si, Mn and V, and quantitatively correlated the two. As a result, it was found that by making F9 155 or more, the hardness of the steel material, that is, the internal hardness of the induction-hardened steel part, becomes about 210HV or more, and high fatigue strength tends to be obtained. Therefore, it is preferable to make F9 155 or more.

[0076] [Manufacturing method] An example of a method for producing a steel material according to the present embodiment is as follows. The method for producing a steel material for induction hardening according to the present embodiment includes a refining step, a casting step, and a hot working step. The hot working step is an optional step and may not be performed. Each step will be described below.

[0077] [Refining process] In the refining process, molten steel having the above-mentioned chemical composition is produced. Existing methods can be applied to the refining process. For example, the refining process is as follows. The refining process includes a primary refining process and a secondary refining process. In the primary refining process, refining is performed in a converter on molten iron produced by a known method. Specifically, oxygen is blown onto the molten iron to remove carbon. In the secondary refining process, alloy elements for component adjustment are added to produce molten steel having a chemical composition of the steel material of this embodiment. Specifically, after the primary refining process, deoxidation is performed on the molten steel tapped from the converter. After the deoxidation, slag removal is performed. After the slag removal, secondary refining is performed. For example, composite refining is performed as the secondary refining. For example, first, refining is performed using a Ladle Furnace (LF) or a Vacuum Arc Degassing (VAD). Furthermore, a Ruhrstahl-Hausen (RH) vacuum degassing process is performed. Then, final adjustment of the alloy components is performed.

[0078] [Casting process] In the casting process, molten steel is used to produce a cast piece (slab or bloom) or a steel block (ingot) by a well-known casting method. The casting method may be any existing method, such as a continuous casting method or an ingot casting method.

[0079] [Hot processing process] The hot working step is an optional step. That is, the hot working step may or may not be performed. The hot working step can be performed by applying an existing hot working method. For example, it is as follows. When the hot working step is performed, in the hot working step, hot working is performed on the slab or steel ingot produced in the casting step to produce steel for induction hardening (for example, steel bar). The hot working step may be, for example, only a rough rolling step, or may include a rough rolling step and a finish rolling step. The rough rolling step is, for example, blooming. The finish rolling step is, for example, finish rolling using a continuous rolling mill. In the continuous rolling mill, for example, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. The heating temperature in the rough rolling step and the finish rolling step is, for example, 1000 to 1300°C.

[0080] In the above-mentioned hot working step, the steel material is manufactured by hot rolling. However, instead of hot rolling, the steel material may be manufactured by hot forging. Also, the steel material may be manufactured by performing hot forging after hot rolling. Even when hot forging is performed in the hot working step, the heating temperature is 1000 to 1300°C.

[0081] The steel for induction hardening is manufactured by the above manufacturing steps. As described above, the hot working step may be omitted in this manufacturing method. In other words, the steel for induction hardening of this embodiment may be a cast product (a cast piece or an ingot).

[0082] The hardness of the steel for induction hardening (steel material) corresponds to the internal hardness of induction hardened steel parts made of the steel for induction hardening. The internal hardness here refers to the hardness of the area not hardened by induction hardening. Low internal hardness may reduce fatigue strength. For this reason, the hardness of the steel for induction hardening is preferably 210HV or more. The area not hardened by induction hardening refers to, for example, an area equal to or greater than the total hardened layer depth described in the JIS0559 method for measuring the hardened layer depth of flame hardened and induction hardened steel.

[0083] [Manufacturing method for induction hardened steel parts] An example of a method for manufacturing an induction hardened steel part using the induction hardening steel of this embodiment is as follows. The above-mentioned induction hardening steel (cast piece, ingot, or steel bar) is hot forged and cooled in air to manufacture a crude part of an induction hardened steel part (e.g., a crankshaft). The crude part is machined to obtain an intermediate part. The intermediate part is induction hardened to obtain a base material. Furthermore, the base material is subjected to finishing processing (cutting, grinding). Through the above steps, an induction hardened steel part is manufactured.

[0084] [High-frequency hardening treatment] In the induction hardening treatment, induction heating is first performed, and then hardening is performed. The induction heating and hardening are preferably performed under the following conditions.

[0085] High frequency heating frequency: 10 to 300 kHz If the frequency is too low, the heating range will be wide, and distortion during hardening may become large. On the other hand, if the frequency is too high, the heating range will be concentrated only on the surface layer. In this case, the hardened layer will become thin, and fatigue strength may decrease. Therefore, the frequency during high-frequency heating is preferably 10 to 300 kHz.

[0086] High frequency heating time: 0.5 to 60 seconds The heating time is the time from when heating of the intermediate member starts to when water cooling starts. If the heating time during high-frequency heating is too long, the austenite grains may become coarse and the fatigue strength may decrease. On the other hand, if the heating time is too short, the cementite may not be sufficiently dissolved, and ferrite may remain. Therefore, the heating time during high-frequency heating is preferably 0.5 to 60 seconds. After high-frequency heating, quenching is performed by water cooling or by using a water-soluble quenching coolant of polymer compounds such as polyalkylene glycol, polyethylene glycol, polyvinyl alcohol, etc. The liquid temperature is preferably in the range of 20 to 40°C.

[0087] [Tempering] Tempering after induction hardening is preferably performed, for example, under conditions of 150 to 350° C. for 0.5 to 3 hours. This induction hardening and tempering makes it possible to obtain a preform having a hardness of about 450 to 800 HV at a depth of 0.05 to 0.40 mm from the surface.

[0088] [Cutting] After induction hardening and tempering, the surface layer of the base material is cut to obtain an induction hardened steel part with a surface hardness adjusted to about 450 to 800 HV. It is not necessary to cut the entire surface layer of the part, but it can be selectively applied to parts that require dimensional accuracy and strength. Depending on the part, grinding may be performed as necessary.

[0089] The cutting process is preferably carried out under the following conditions. Cutting tool rake angle α: -30°<α≦-5° If the rake angle α of a cutting tool is larger than -5°, the tool may be easily chipped during cutting. On the other hand, if the rake angle is -30° or less, the cutting resistance may become too large, resulting in increased tool wear. Therefore, it is preferable that the rake angle α is -30°<α≦-5°.

[0090] Tool nose R: 0.4~1.2mm If the nose R of the tool is too small, the surface roughness becomes too large, and the fatigue strength of the part may decrease. On the other hand, if the nose R of the tool is too large, the cutting resistance becomes large, and the tool wear may increase. Therefore, the nose R of the tool is preferably 0.4 to 1.2 mm.

[0091] Feed: 0.1~0.4mm / rev (revolution) If the feed is too small, the cutting efficiency may decrease, resulting in a decrease in manufacturing efficiency. On the other hand, if the feed is too large, the cutting resistance may increase, resulting in increased tool wear. Therefore, the feed is preferably 0.1 to 0.4 mm / rev.

[0092] Cutting speed: 50~500m / min If the cutting speed is too high, the cutting temperature will rise and tool wear may occur. On the other hand, if the cutting speed is too low, the cutting efficiency will decrease and manufacturing efficiency will decrease, and the cutting temperature will be low and a protective film on the tool due to adhesion of inclusions in the steel may not be formed. Therefore, the cutting speed is preferably 50 to 500 m / min.

[0093] Depth of cut: 0.05~0.40mm If the depth of cut is too small, the cutting efficiency may decrease, resulting in a decrease in manufacturing efficiency. On the other hand, if the depth of cut is too large, the cutting resistance may increase, resulting in increased tool wear. Therefore, the depth of cut is preferably 0.05 to 0.40 mm, and more preferably the upper limit is 0.20 mm.

[0094] In the cross section of the induction hardened steel part obtained in this embodiment, the hardness at a depth of 50 μm from the surface in the thickness direction (from the surface to the center in the case of a cylindrical part) is defined as the surface hardness. In order to obtain excellent fatigue strength, it is preferable that the surface hardness is 510 HV or more.

[0095] If cutting is performed after induction hardening, a large compressive residual stress can be imparted to the machined surface. Therefore, a large compressive residual stress is obtained on the surface of the induction hardened steel part obtained in this embodiment. Specifically, the maximum compressive residual stress in a depth region of 200 μm or less from the surface is 300 MPa or more. This increases the fatigue strength of the induction hardened steel part. For example, the compressive residual stress can be measured as follows. A 2 mm x 2 mm range on the surface of the part is masked so that it can be measured, and the compressive residual stress is measured for the 2 mm x 2 mm range using a Rigaku Automate (using a Cr tube) with a collimator of φ1 mm by the 2θ·sin2ψ method. Further, electrolytic polishing is performed to measure the compressive residual stress in the depth direction. An electrolyte containing 11.6% ammonium chloride, 35.1% glycerin, and 53.3% water is prepared. Using this electrolyte, electrolytic polishing is performed on the surface at a voltage of +20 V. The amount of polishing is adjusted by changing the time of electrolytic polishing. Next, the residual stress of the electrolytically polished bottom surface is measured. Measurements are taken from the surface to a depth of 200 μm at 10 μm intervals to determine the maximum compressive residual stress. EXAMPLES

[0096] The effects of the induction hardening steel, induction hardening steel parts, and manufacturing method thereof according to the present embodiment will be described in more detail with reference to examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the induction hardening steel, induction hardening steel parts, and manufacturing method thereof according to the present embodiment. Therefore, the steel material of the present embodiment is not limited to this one example of conditions.

[0097] Steel having the chemical composition shown in Table 1 was melted and hot forged to produce a steel material (steel bar) having a diameter of 55 mm. In Table 1, "-" in the "Chemical composition" column means that the corresponding element content is 0% in the significant figures (numbers to the least significant digits) specified in the embodiment. In other words, the corresponding element content is 0% when the fraction in the significant figures (numbers to the least significant digits) specified in the above embodiment is rounded off.

[0098] [Measurement of the number density of oxides with appropriate melting points and oxides with low melting points, the number ratio of oxides with appropriate melting points that exist as composite inclusions containing 10% or more of CaS, and the value of d+σ] A sample was taken from the R / 2 position of the longitudinal section of the manufactured steel for induction hardening (the center position of the straight line (radius R) connecting the central axis of the steel for induction hardening and the outer surface in the longitudinal section of the steel for induction hardening). Of the surfaces of the taken sample, the surface corresponding to the longitudinal section of the steel for induction hardening was used as the observation surface. After the observation surface was mirror-polished, inclusions and oxides were measured using an SEM by the above-mentioned method. The results obtained are shown in Table 2.

[0099] [Measurement of ferrite area ratio and hardness of steel for induction hardening] The mirror-polished observation surface was etched with nital solution, and the microstructure was observed with an optical microscope. 20 fields of view were photographed at 400x magnification (approximately 0.32mm x 0.24mm field of view) with the optical microscope, the area of ​​the ferrite region was measured by image analysis, and the ratio of the area of ​​the ferrite region to the total photographed area was calculated. The Vickers hardness was measured using a sample of the above observation surface that had been polished. The results are shown in Table 2.

[0100] [Manufacturing of simulated rough parts] The manufactured steel for induction hardening was subjected to a heat treatment simulating the hot forging process used in manufacturing induction hardened steel parts from the steel for induction hardening. Specifically, the steel was heated to 1100°C and held at that temperature for 30 minutes. The steel was then allowed to cool in the air to produce a simulated rough part. The simulated rough part was a steel bar with a diameter of 55 mm.

[0101] [Melting crack evaluation test] A test piece with a width of 10 mm, a thickness of 3 mm, and a length of 10 mm was machined from the R / 2 position of the cross section perpendicular to the longitudinal direction of the simulated rough part. The longitudinal direction of the test piece was parallel to the longitudinal direction of the simulated rough part. In addition, the central axis of the test piece parallel to the longitudinal direction coincided with the R / 2 position. This test piece corresponds to the simulated intermediate part.

[0102] A simulation test of high-frequency hardening was performed on the above test piece using a test device (product name "Heat Cycle Test Device") manufactured by Fuji Electric Industrial Co., Ltd. Specifically, the test piece was heated to 1370°C at a heating rate of 100°C / sec using a high-frequency coil. The test piece was then held at 1370°C for 15 seconds. Thereafter, the test piece was water-cooled.

[0103] After water cooling, the cross section (observation surface) perpendicular to the longitudinal direction of the test piece was mechanically polished. The mechanically polished observation surface was corroded with picral reagent. The corroded observation surface was observed with an optical microscope at 400x magnification, and the presence or absence of melt cracks was visually confirmed. The observation surface was 250 μm × 400 μm.

[0104] When a clearly corroded area (corroded area) with a width of 5 μm or more was observed at the grain boundary of the structure of the observed surface, it was determined that a fusion crack had occurred. A clearly corroded area with a width of 5 μm or more at the grain boundary means, for example, an area such as fusion crack 10 in Figure 2. On the other hand, when no corroded area was observed at the grain boundary as in Figure 3, it was determined that no fusion crack had occurred. The evaluation results of fusion cracks are shown in the "fusion cracks" column in Table 2. Cases where fusion cracks occurred were marked with "X", and cases where fusion cracks did not occur were marked with "O".

[0105] [Manufacturing of imitation materials] A cylindrical round bar test piece with a diameter of 35 mm and a length of 300 mm was manufactured by machining a simulated rough part with a diameter of 55 mm. The round bar test piece was subjected to high-frequency heating under conditions of a frequency of 100 kHz and a heating time of 2.0 seconds, and then the test piece was quenched after the high-frequency heating by spraying a water-soluble quenching coolant with a diluted concentration of 5 to 15% onto the test piece, and then tempered at 300°C for 2 hours to produce a machinable test piece (simulated base material).

[0106] [Machinability evaluation test (wear evaluation of turning tools after induction hardening)] The machinability test was evaluated by the amount of flank wear (μm) of the cutting tool. The simulated blanks after induction hardening and tempering were turned using a general-purpose lathe. The cutting tool used was a 4NC-DNGA150412-BNC200 (manufactured by Sumitomo Electric Hardmetal Co., Ltd.). The cutting conditions were a cutting depth of 0.15 mm, a cutting speed of 400 m / min, and a feed of 0.2 mm / rev, and the cutting was performed in a dry state. One pass of cutting was performed for each test piece, and cutting was repeated for multiple test pieces until the total cutting time reached 9 minutes, after which the amount of flank wear of the cutting tool was measured. A microscope was used to measure the amount of flank wear. The tool was set so that the tool flank was parallel to the measurement table, and the worn part was observed at a magnification of 200 times. At this time, the distance from the cutting edge of the part where the wear was maximum near the center of the worn part to the tip of the worn part was measured, and this was taken as the amount of flank wear. In this measurement, a tool passing the test is one in which the amount of flank wear is 100 μm or less. The results of the machinability evaluation are shown in the "Tool wear" column in Table 2. It is preferable that the amount of flank wear is 90 μm or less.

[0107] [Fatigue strength evaluation test (rotating bending fatigue test)] Rotating bending fatigue test pieces were taken from the manufactured simulated rough material. The rotating bending fatigue test pieces were prepared so that the direction of the central axis of the rotating bending fatigue test pieces was aligned with the direction of the central axis of the simulated rough material. The rotating bending fatigue test pieces were high-frequency hardened so that the effective hardened layer depth was 1 mm ± 0.2 mm, and then tempered at 300 ° C for 90 minutes. Then, using a tool (4NC-VNGA160404-BNC200: manufactured by Sumitomo Electric Hardmetal Co., Ltd.), the notched portion of the rotating bending fatigue test piece was cut at a cutting speed of 50 m / min, a cutting depth of 0.1 mm, and water-soluble cutting oil. Figure 4 is a schematic diagram of the notched Ono-type rotating bending fatigue test piece used in the fatigue test. The diameter at the notch bottom is 6.72 mm, but the test piece was prepared with a diameter at the notch bottom of 6.92 mm before induction hardening, and then machined with a cutting depth of 0.1 mm after induction hardening to obtain the shape shown in Figure 4.

[0108] The Ono-type rotating bending fatigue test was conducted at room temperature (23°C) in the air at a rotation speed of 3600 rpm. Fatigue tests were conducted on multiple test pieces by applying different stresses. 7 The highest stress that did not cause fracture after cycling was taken as the fatigue strength (MPa).

[0109] The results of the fatigue strength evaluation are shown in the "Fatigue strength" column in Table 2. A fatigue strength of 500 MPa or more was deemed to be acceptable. The fatigue strength is preferably 530 MPa or more, more preferably 560 MPa or more, and even more preferably 590 MPa or more.

[0110] [Measurement of hardness of induction hardened steel parts] Using the above Ono-type rotating bending fatigue test piece that had not been subjected to fatigue testing, the notch bottom was cut in a cross section perpendicular to the longitudinal direction of the test piece, embedded in resin, and the surface layer was polished, after which the Vickers hardness was measured at a position 50 μm from the surface layer. The results are shown in Table 2.

[0111] [Measurement of residual stress in induction hardened steel parts] The maximum compressive residual stress was measured at the notch root in a region 200 μm deep from the surface using the above-mentioned Ono-type rotating bending fatigue test pieces that had not been subjected to fatigue testing. In this example, the maximum compressive residual stress was 300 MPa or more in all cases.

[0112] [Test Results] The test results are shown in Table 2. Referring to Table 2, the steel materials with test numbers 1 to 16 had appropriate chemical compositions and satisfied formulas (1) to (3). Therefore, the steel materials with each test number had CaO and Al2O3 contents that were within the appropriate melting point, and the oxide number density was 0.15 particles / mm 2 The density of low-melting-point oxide particles is 0.15 particles / mm 2The ratio of the number of oxides with appropriate melting points that exist as composite inclusions containing 10% or more of CaS in terms of mass% content in the average composition was 50% or more, and the average value and standard deviation of the circle equivalent diameter of the composite inclusions satisfied formula (6). Therefore, no melting cracks occurred. Furthermore, the tool wear amount was 100 μm or less, and machinability was high. Furthermore, the fatigue strength in the rotating bending fatigue test was 500 MPa or more, and fatigue strength was high.

[0113] On the other hand, in test number 17, the C content was too high, which caused melting cracks.

[0114] In test number 18, the C content was too low, which resulted in low fatigue strength.

[0115] In test number 19, the Si content was too high, which caused melting cracks.

[0116] In test number 20, the Si content was too low, which resulted in low fatigue strength.

[0117] In test number 21, the Mn content was too high, which caused melting cracks.

[0118] In test number 22, the Mn content was too low, which caused melting cracks.

[0119] In test number 23, the P content was too high, which caused melting cracks.

[0120] In test number 24, the S content was too high. As a result, melting cracks occurred. Furthermore, because formula (3) was not satisfied, the number density of oxides with appropriate melting points for the CaO and Al2O3 contents was low, and the average value and standard deviation of the circle equivalent diameter of the composite inclusions did not satisfy formula (6). As a result, the machinability and fatigue strength were low.

[0121] In test number 25, the S content was too low, which resulted in poor machinability.

[0122] In test number 26, the Cr content was too high, which caused melting cracks.

[0123] In test number 27, the Ca content was too high. Therefore, the fatigue strength was low. Furthermore, because formula (2) was not satisfied, the proportion of the number of composite inclusions containing CaS: 10% or more in mass% content in the average composition among the oxides with appropriate melting points was low, and the average value and standard deviation of the circle equivalent diameter of the composite inclusions did not satisfy formula (6). Therefore, the machinability and fatigue strength were low.

[0124] In test number 28, the Ca content was too low, which resulted in poor machinability.

[0125] In test number 29, the Ca content was too low. Therefore, the machinability was poor. Furthermore, formula (1) was not satisfied, specifically, F1 was too low, so the content of CaO and Al2O3 was low and the number density of oxides with proper melting points was low. Therefore, the machinability was poor.

[0126] In test number 30, the Al content was too high. Therefore, the machinability was poor. Furthermore, because formula (3) was not satisfied, the number density of oxides with appropriate melting points for the CaO and Al2O3 contents was low, and the average value and standard deviation of the circle equivalent diameter of the composite inclusions did not satisfy formula (6). Therefore, the machinability and fatigue strength were poor.

[0127] In test number 31, the Al content was too low, which caused melting cracks.

[0128] In test number 32, the Al content was too low. Furthermore, formula (1) was not satisfied, specifically, F1 was too high, so the content of CaO and Al2O3 was high and the number density of low melting point oxides was high. Therefore, melting cracks occurred.

[0129] In test number 33, the V content was too high, which resulted in a decrease in fatigue strength.

[0130] In test number 34, the O content was too high, which resulted in low fatigue strength.

[0131] In test number 35, formula (2) was not satisfied, so the proportion of the number of composite inclusions containing 10% or more CaS in mass% content in the average composition among the oxides with appropriate melting points was small, and the average value and standard deviation of the circle equivalent diameter of the composite inclusions did not satisfy formula (6). Therefore, the machinability and fatigue strength were low.

[0132] In test number 36, the content of CaO and Al2O3 did not satisfy formula (3), so the number density of oxides with appropriate melting points was low, and the average value and standard deviation of the circle equivalent diameter of the composite inclusions did not satisfy formula (6). Therefore, the machinability and fatigue strength were low.

[0133] The embodiment of the present invention has been described above. However, the above-mentioned embodiment does not necessarily represent the present invention. The above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

[0134] [Table 1]

[0135] [Table 2] [Industrial Applicability]

[0136] The present invention can be utilized in the steel industry which produces steel for induction hardening, and in the machinery manufacturing industry such as the automobile industry which produces and uses induction hardening steel parts. [Explanation of symbols]

[0137] 1 Fillet R 2. Crankshaft edge 10 Melting crack

Claims

1. A steel for induction hardening, having a chemical composition, in mass%, of: C: 0.31-0.60%, Si: 0.51-1.00%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.006-0.040%, Cr: 0-0.19%, Ca: 0.0006-0.0023%, Al: 0.021-0.050%, V: 0 to 0.099%, N: 0.0250% or less, and O: 0.0050% or less and the balance being Fe and impurities, A steel for induction hardening characterized by satisfying the following formulas (1) to (3): 0.01≦Ca / Al≦0.12...Formula (1) Ca-0.0008×Ln(S)≦0.00493...Formula (2) Al + 1091×S ３ -61.5×S ２ +1.59×S ≤ 0.0567... Equation (3) Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula, and when the corresponding element is not contained, "0" is substituted for that element symbol.

2. Furthermore, the number density of oxide-containing inclusions that are present in the steel and have an equivalent circle diameter of 1.0 μm or more and satisfy the formula (4) is 0.15 pieces / mm ２ The number density of the inclusions containing oxides satisfying the formula (5) is 0.15 pieces / mm ２ the ratio of the number of composite inclusions containing CaS: 10% by mass or more among the inclusions containing oxides that satisfy formula (4) is 50% or more, and the average value d and standard deviation σ of the equivalent circle diameters of the composite inclusions satisfy formula (6). 805≦10.4×A+1.4×B≦1004...Formula (4) 10.4×A+1.4×B≦404...Formula (5) Here, A and B in the formulas (4) and (5) are as follows: A: CaO-Al oxide 2 O 3 -SiO ２ When considered as a ternary oxide of the above, the Al in the oxide 2 O 3 Content (mass%) B: CaO-Al oxide 2 O 3 -SiO ２ When considered as a ternary oxide, the content of CaO in the oxide (mass%) d+3σ≦20...Formula (6)

3. 3. The steel for induction hardening according to claim 1 or 2, further satisfying the following formula (7): 60×C+5.5×Si+29Mn-29V≧58...Formula (7) Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula, and when the corresponding element is not contained, "0" is substituted for that element symbol.

4. The steel for induction hardening according to any one of claims 1 to 3, further satisfying the following formula (8): 244≦462×C+102×Si+7×Mn≦316...Formula (8) Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula, and when the corresponding element is not contained, "0" is substituted for that element symbol.

5. The steel for induction hardening according to any one of claims 1 to 4, further satisfying the following formula (9): 149×C+36×Si+70×Mn+76×V≧155...Formula (9) Here, the content (mass %) of the corresponding element is substituted for each element symbol in the formula, and when the corresponding element is not contained, "0" is substituted for that element symbol.

6. The chemical composition further contains, instead of a part of the Fe, Ti: 0.039% or less, Nb: 0.050% or less; and The steel for induction hardening according to any one of claims 1 to 5, further comprising one or more selected from the group consisting of Zr: 0.0019% or less.

7. The chemical composition further contains, instead of a part of the Fe, Mo: 0.095% or less, Cu: 0.50% or less, and The steel for induction hardening according to any one of claims 1 to 6, further comprising one or more selected from the group consisting of Ni: 0.50% or less.

8. 8. An induction hardened steel part having a chemical composition according to any one of claims 1 to 7, characterized in that the maximum compressive residual stress in a region from the surface to a depth of 200 μm is 300 MPa or more.

9. A method for manufacturing an induction hardened steel part according to claim 8, comprising the steps of: manufacturing a member by processing a steel for induction hardening having a chemical composition according to any one of claims 1 to 7; induction hardening the member; tempering the induction hardened member; and cutting the tempered member to a depth of 0.05 to 0.40 mm from the surface in the thickness direction.

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