JP7678315B2 - Steel for high-frequency hardening, high-frequency hardening steel parts and manufacturing method thereof - Google Patents

Description

The present invention relates to steel for induction hardening, induction hardening steel parts, and their manufacturing method.

High fatigue strength is required for machine structural parts used in crankshafts for automobiles and construction vehicles. Therefore, to improve fatigue strength, surface hardening treatment is sometimes applied to machine structural parts.

Among the various surface hardening treatments, induction hardening can harden only the necessary areas. 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 has been put into practical use to perform induction hardening on the fillet R portion 1 shown in Figure 1 in order to improve the fatigue strength of crankshafts, which are one type of machine structural part.

In recent years, the industrial world has been calling for further improvements in the fatigue strength of machine structural parts. In order to increase the depth of the hardened layer using induction hardening, the output of the high-frequency power during induction hardening can be increased to raise 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 or inside of the steel material may melt and cause cracks. Hereinafter, such cracks are referred to as "melting cracks" in this specification. Parts with melting cracks are not suitable for practical use. Therefore, it is necessary to suppress melting cracks.

The following method can be given as an example of a method for manufacturing machine structural parts that undergo induction hardening and tempering, 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.

Normally, induction hardened steel parts are required to have excellent fatigue strength. Technology for improving fatigue strength is disclosed, for example, in Patent Document 1.

Patent Document 1 claims that the wear resistance and bending fatigue strength can be improved by performing cutting finishing under specific conditions on the base material obtained by induction hardening and tempering.

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, extending the tool life and reducing tool costs are urgent issues for the base material obtained by induction hardening and tempering, and the base material is required to have excellent machinability.

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 the base material, to have machinability when cutting the base material obtained by induction hardening, and to have high fatigue strength when made into induction hardened steel parts.

Technology relating to steel materials for machine structures is disclosed, for example, in Patent Document 2 and Patent Document 3.

The steel for machine structural use disclosed in Patent Document 2 contains 0.001-0.05% Ca, 0.02-0.15% Pb and Bi, either alone or in combination, restricts B to 0.005% or less, makes inclusions CaS-CaO, Pb, Bi-based inclusions, and suppresses Al ₂ O ₃ 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, Al ₂ O ₃ is eliminated or extremely reduced by a reduction reaction caused by Ca. As a result, 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 form independent inclusions of Pb or Bi or to cause them to precipitate finely around the CaS--CaO, thereby improving machinability.

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 mechanical structural use has an area of sulfide-based inclusions containing 1.0% or more by weight that are in contact with oxide-based inclusions with a CaO content of 0.2 ^to 62% by weight, the area of which is 2.0 x ^10-4 mm2 or more per 3.5 ^mm2 field area, and the MnS inclusions are finely dispersed, improving machinability and chip crushability.

JP 2017-082299 A Japanese Unexamined Patent Publication No. 57-19366 JP 2003-226934 A

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

The present invention aims to provide a steel for induction hardening, induction hardening steel parts, and a manufacturing method thereof, which have the objective of suppressing melting cracks during induction hardening and maintain the machinability after induction hardening and the fatigue strength when made into induction hardened steel parts.

Focusing on tool wear, which is the most important index from the viewpoint of ensuring machinability after induction hardening, the tool was observed after the cutting test, and it was 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 the melting point, the oxides soften to a certain extent and adhere to the tool when the temperature rises during cutting. The adhered oxides become a protective film, suppressing tool wear. Furthermore, when this oxide exists in the steel in a form combined with sulfides containing a certain amount of CaS, the protective film action when it adheres to the tool is enhanced, and the effect of suppressing tool wear is increased.

However, in steel materials that contain inclusions that contain oxides with low melting points, melting cracks can occur. When the causes of melting cracks were investigated, it was found that when the steel material is heated to high temperatures by high-frequency heating, melting cracks occur starting from oxides with low melting points. In other words, in order to suppress melting cracks, it is better to limit the density of low-melting-point oxides and instead increase the melting point of the oxides.

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 increase the melting point of 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 the heat generated during cutting is greater than that of soft steel because the steel material after induction hardening is hard. As a result, it was found that the oxides in the inclusions can be regarded as ternary oxides of CaO-Al ₂ O ₃ -SiO ₂ , and that the melting point of the oxide is mainly determined by the content of Al ₂ O ₃ and CaO, so that fusion cracks can be prevented by controlling the content of Al ₂ O ₃ and CaO, and machinability can be improved by controlling the content of CaS.

Furthermore, they found that inclusions with a particle size of 1 μm or more have a large effect on machinability and melting cracks, and considered the oxides in the inclusions with a particle size of 1 μm or more to be ternary oxides of CaO-Al ₂ O ₃ -SiO _2. They found an equation that shows the melting point of the oxide, and discovered that it is sufficient to satisfy the following equation (4) so that the melting point is within the appropriate temperature range.
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 while suppressing 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.")

On the other hand, from the viewpoint of avoiding melting cracks, it has been found that it is sufficient to limit inclusions containing oxides having a composition that satisfies formula (5), which indicates that the melting point of the oxide is in the low melting point range (hereinafter, may be referred to as "low melting point oxides").
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 the inclusions to be preferable.

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.

Based on the above findings, the present invention was completed, and its gist is as follows:

[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, 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...Formula (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 the element symbol.
[2]
The steel for induction hardening according to the above [1], characterized in that, among the oxide-containing inclusions present in the steel and having an equivalent circle diameter of 1.0 μm or more, the number density of the oxide-containing inclusions satisfying formula (4) is 0.15 pieces/ ^mm2 or more and the number density of the oxide-containing inclusions satisfying formula (5) is 0.15 pieces/ ^mm2 or less, the proportion of the number of composite inclusions containing CaS: 10 mass% or more among the oxide-containing inclusions satisfying 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: The content (mass%) of Al 2 _{O 3} in the oxide when the oxide is regarded as a ternary oxide of CaO-Al ₂ O ₃ -SiO ₂
B: The content (mass%) of CaO in the oxide when the oxide is regarded as a ternary oxide of CaO-Al ₂ O ₃ -SiO ₂
d+3σ≦20...Formula (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 items [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 items [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, instead of a part of the Fe,
Ti: 0.039% or less,
Nb: 0.050% or less; and
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, instead of a part of the Fe,
Mo: 0.095% or less,
Cu: 0.50% or less, and
A steel for induction hardening containing one or more selected from the group consisting of Ni: 0.50% or less.
[8]
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 having a maximum compressive residual stress of 300 MPa or more in a region from the surface to a depth of 200 μm.
[9]
A method for manufacturing an induction-hardened steel part according to the above [8], comprising the steps of: manufacturing a member by processing a steel for induction hardening having a chemical composition according to any one of the above [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.

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 has excellent fatigue strength when made into induction hardened steel parts.

FIG. 1 is a front view showing a portion of a crankshaft which is an induction hardened steel part. FIG. 2 is a schematic diagram of the microstructure of a test piece taken from a simulated rough member obtained using a conventional steel for induction hardening, which was heated to 1,350° C. or higher at a heating rate of 100° C./sec, held at that temperature for 10 seconds, and then water-cooled. FIG. 3 is a schematic diagram of the microstructure of a test piece taken from a simulated rough member obtained by using a steel for induction hardening, which is an example of the present invention, after heating to 1350° C. or higher at a heating rate of 100° C./sec, holding the temperature for 10 seconds, and then water-cooling. FIG. 4 is a schematic diagram of a rotating bending fatigue test piece obtained by taking from each simulated rough member, followed by induction hardening and tempering, and then cutting.

The induction hardening steel, induction hardening steel parts, and manufacturing method thereof according to this embodiment will be described in detail below. 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 if the corresponding element is not contained.

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

C: 0.31% to 0.60%
Carbon (C) is preferably contained in an amount of 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 a large amount of C is contained, 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%.

Si: 0.51% to 1.00%
Silicon (Si) deoxidizes steel in the steelmaking process. Since Si further increases the strength of ferrite by solid solution strengthening and increases the fatigue strength of induction hardened steel parts, 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 it easier for melt cracks 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%.

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 a large amount, melting cracks are likely to occur during induction hardening, so the Mn content should be 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 preferable 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%.

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 since excessive reduction of the P content increases the manufacturing cost, the preferred lower limit of the P content may be more than 0%, 0.001%, or 0.002%, taking into account the economic efficiency of refining.

S: 0.006% to 0.040%
Sulfur (S) generates 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 is preferable to be 0.040% or less. Therefore, the S content is 0.006% or more and 0.040% or less. The preferable lower limit of the S content is 0.008%, 0.010%, 0.013%, 0.015%, or 0.020%. The preferable upper limit of the S content may be 0.035%, 0.030%, 0.025%, or 0.023%.

Cr: 0% to 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 in which 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, making melting cracks more 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 upper limit of the Cr content is preferably 0.18%, 0.17%, 0.16%, or 0.15%.

Ca: 0.0006% to 0.0023%
Calcium (Ca) is a deoxidizing element and generates soft Ca oxides. It also generates 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 high, the composite inclusions become coarse and the fatigue properties deteriorate, 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%.

Al: 0.021% to 0.050%
Aluminum (Al) is a deoxidizing element and forms _Al2O3 in steel. If too much _Al is added, oxygen (O) is consumed, which may make it difficult to generate 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 generated, 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%.

V: 0% to 0.099%
Vanadium (V) precipitates in the ferrite in the steel as V precipitates during the cooling process after hot forging, increasing the strength of the ferrite, and therefore the fatigue strength of the induction-hardened steel parts is increased, but it is not necessary to include it. On the other hand, if the V content is high, the amount of V nitrides 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. In order to obtain the above effect, if V is contained, it should be 0.001% or more, and the preferred 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%.

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 contain 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 preferable lower limit of the N content is 0.0001%, 0.0005%, 0.0010%, 0.0020%, 0.0030%, 0.0040%, or 0.0050%. The upper limit of the N content is preferably 0.0230%, 0.0200%, 0.0180%, 0.0150%, 0.0120%, 0.0100%, or 0.0080%.

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 the manufacturing cost, 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 generate Ca oxide, it is preferable to satisfy 0.25≦Ca/O≦5.0. Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula.

In addition to the above elements, the remainder is composed of Fe and impurities. Here, impurities refer to components that are mixed in from the raw materials ore, scrap, or the manufacturing environment when the steel material is industrially manufactured, and also include elements that are not intentionally included. They are permissible as long as they do not impair the properties of the steel material and induction hardened steel parts according to this embodiment.

The steel material of this embodiment may further contain the following elements in place of a portion of Fe. However, the parts of 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 their content is 0%.

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 is saturated and the manufacturing cost increases, so the Ti content should be 0.039% or less. A more preferable upper limit of the Ti content is 0.038%, 0.035%, or 0.030%.

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 the 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 is saturated and the manufacturing cost increases, so the Nb content should be 0.050% or less. A more preferable upper limit of the Nb content is 0.040% or 0.030%.

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 the 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 should be 0.0019% or less. The more preferable upper limit of the Zr content is 0.0017% or 0.0015%.

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 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%.

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 preferred 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 preferred upper limit of the Cu content is 0.40%, 0.30%, 0.20%, 0.17%, 0.13%, 0.10%, 0.07%, or 0.05%.

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 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%.

Next, to control inclusions, the relationship between the Ca, Al, and S contents is set to satisfy formulas (1) to (3).

[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

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 Al ₂ O ₃ , it is preferable to contain sufficient Ca relative to the Al content in order to generate a large amount of oxides with appropriate melting points. On the other hand, if the Ca content is too high relative to the Al content, a large amount of low-melting-point oxides will be generated, causing melting cracks. Therefore, in order to properly control the composition of oxides, F1 should be in the range of 0.01 to 0.12. The preferable lower limit of F1 should be 0.02 or 0.03. The preferable lower limit of F1 should be 0.11, 0.10, or 0.09.

[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, the explanation will be given assuming that Ca-0.0008×Ln(S) is F2. Note that Ln means the natural logarithm.
F2=Ca-0.0008×Ln(S)

In order to form a protective film by depositing oxide-containing inclusions on the tool during cutting after induction hardening and reduce tool wear, it is advisable to generate CaS and generate composite inclusions of appropriate melting point oxide and CaS in a specified ratio. If the Ca content is too high compared to the S content, the CaS will aggregate and become coarse, resulting in fewer composite inclusions. Furthermore, the size of the composite inclusions will become too large, deteriorating 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 should preferably be set to 0.00483.

[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. Hereinafter, Al+1091×S ³ -61.5×S ² +1.59×S will be explained as F3.
F3=Al+1091×S ³ -61.5×S ² +1.59×S

As described above, Al is a deoxidizing element and forms _Al2O3 , so when _the Al content is high, Ca oxides are less likely to be generated. In addition, S combines with Ca to generate CaS, so when the S content is high, Ca oxides are less likely to be generated. When the value of F3 is large, not only is it not possible to obtain a sufficient amount of oxides with appropriate melting points, but CaS aggregates and the size of the composite inclusions becomes too large, which deteriorates the fatigue properties. Therefore, in order to generate a sufficient amount of oxides with appropriate melting points, it is preferable to contain Al and S so that F3 is 0.0567 or less. The preferable upper limit of F3 is 0.0517 or 0.0467.

[Number density of inclusions containing oxides with appropriate melting point: 0.15 pieces/ ^mm2 or more]
The oxides contained in steel can be regarded as mainly ternary oxides of CaO-Al ₂ O ₃ -SiO ₂ , and the melting point of the oxide is mainly determined by the content of Al ₂ O ₃ and CaO. In particular, the influence of inclusions with a particle size of 1 μm or more is large. In order to form a protective film by depositing inclusions containing oxides on the tool during cutting after induction hardening to reduce tool wear, it is preferable that the inclusions with a particle size of 1 μm or more are softened to a certain extent. Therefore, it is preferable that the number density of inclusions with a particle size of 1 μm or more that contain oxides with a suitable melting point is 0.15 pieces/mm ² or more.
805≦10.4×A+1.4×B≦1004: Formula (4)
In the formula (4), A and B are as follows:
A: The content (mass%) of Al 2 _{O 3} in the oxide when the oxide is regarded as a ternary oxide of CaO-Al ₂ O ₃ -SiO ₂
B: The content (mass%) of CaO in the oxide when the oxide is regarded as a ternary oxide of CaO-Al ₂ O ₃ -SiO ₂
Here, the following description will be given assuming that 10.4×A+1.4×B is F4.
F4=10.4×A+1.4×B

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 is softened too much by heat generated during cutting after induction hardening, making it difficult to adhere to the tool. The inventors have confirmed that it is good to have a sufficient number (0.15 pieces/ ^mm2 or more) of oxides with appropriate melting points having a composition with an F4 value in the range of 805 to 1004.

The more oxides there are, the more sufficiently the inclusions accumulate on the tool, and the more effective the tool wear can be. The preferable lower limit of the number density of the inclusions containing the oxides with appropriate melting points is 0.20 pieces/ ^mm2 , 0.30 pieces/ ^mm2 , 0.40 pieces/ ^mm2 , 0.50 pieces/ ^mm2 , 0.60 ^pieces / ^mm2 , 0.70 pieces/mm2, or 0.75 pieces/ ^mm2 .

[Number density of inclusions containing low melting point oxides: 0.15 pieces/ ^mm2 or less]
On the other hand, when the 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 melting cracks. Therefore, it is sufficient to limit the number of low-melting-point oxides so that F4, which is an index of the melting point of oxides, is 404 or less. In other words, in order to suppress melting cracks, it is sufficient to limit the number of inclusions that contain low-melting-point oxides with a particle size of 1 μm or more and satisfy formula (5) to 0.15 pieces/ ^mm2 or less.
10.4×A+1.4×B≦404: Equation (5) where A and B in equation (5) are as follows and are the same as equation (4).
A: The content (mass%) of Al 2 _{O 3} in the oxide when the oxide is regarded as a ternary oxide of CaO-Al ₂ O ₃ -SiO ₂
B: The content (mass%) of CaO in the oxide when the oxide is regarded as a ternary oxide of CaO-Al ₂ O ₃ -SiO ₂
That is, F4≦404.

The preferred upper limit of the number density of inclusions containing low melting point oxides is 0.12 or 0.10 pieces/ ^mm2 , and more preferably 0.08 pieces/ ^mm2 , 0.06 pieces/ ^mm2 , 0.05 pieces/ ^mm2 , 0.04 ^pieces /mm2, 0.03 pieces/ ^mm2 , 0.02 pieces/ ^mm2 , or 0.01 pieces/ ^mm2 .

[The proportion of composite inclusions containing CaS: 10% by mass or more among oxides with appropriate melting point 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 a composite inclusion in which this oxide exists in combination with a sulfide 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.

[The average value and standard deviation of the circle equivalent diameter of CaS composite inclusions satisfy formula (6)]
As described above, CaS composite inclusions are important for improving machinability after induction hardening. On the other hand, if these CaS composite 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 composite inclusions so as to satisfy formula (6).
d+3σ≦20: Formula (6)
Here, d and σ in the formula (6) are as follows:
d: average value of equivalent circle diameters of CaS composite inclusions having a circle equivalent diameter (μm) of 1.0 μm or more; σ: standard deviation of equivalent circle diameters of CaS composite inclusions (μm);

The value of d+3σ is derived from the equivalent circle diameter and the standard deviation of the equivalent circle diameter of the CaS composite inclusions observed within the observation field of view of the inclusion measurement method described later, and indicates that most of the observable CaS composite inclusions have a smaller equivalent circle diameter 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 an equivalent circle diameter exceeding 20 μm. By satisfying formula (6), the fatigue strength of induction hardened steel parts can be increased. The preferred upper limit of the value of d+3σ is 18 μm, 16 μm, 15 μm, 14 μm, or 13 μm.

In this specification, oxides are mainly composed of Al ₂ O ₃ , CaO, and SiO ₂ , and sulfides are mainly composed of MnS and CaS. Therefore, in the measurement method described below, 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 S is converted to CaS. Next, the amount of Ca consumed by CaS is subtracted from the analytical value of Ca, and the remaining Ca is converted to CaO. Next, the analytical values of Al and Si are converted to Al ₂ O ₃ and SiO ₂ , respectively. From such a determination, the mass % contents of Al ₂ O ₃ , CaO, SiO ₂ , MnS and CaS of each inclusion can be obtained.

To determine A and B in formulas (4) and (5), the mass percentages of _{Al2O3 ,} CaO, and _SiO2 are converted so that the sum of these is 100%, and the converted _Al2O3 content ( _% ) is A and the converted CaO content (%) is B.

Among inclusions containing oxides with appropriate melting points, composite inclusions containing 10% or more by mass of CaS (CaS composite inclusions) refer to inclusions, among inclusions containing oxides with appropriate melting points, in which CaS accounts for 10% _or more when the sum of the mass percentages of _Al2O3 , CaO, _SiO2 , MnS and CaS is taken as 100%.

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 material and the outer surface in the longitudinal section of the steel material) of the steel material (steel bar). Of the surfaces of the taken sample, the surface corresponding to the longitudinal section of the steel material is used as the observation surface. After mirror-polishing the observation surface, the observation surface after mirror-polishing is observed at a magnification of 500 times using a scanning electron microscope (SEM). The observation area is preferably 32 ^mm2 or more.

Based on the backscattered electron image obtained by SEM observation, the number density is examined using a well-known image analysis type particle analysis method. 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.

The composition of the inclusions and/or precipitates is further analyzed using an EDX equipped in the SEM. In this embodiment, the composition analysis is performed using EDX at an acceleration voltage of 20 kV.

The inclusions are identified by the above method, and the number per unit area (inclusions/mm ² ) is calculated based on the number of inclusions identified and the measured area.

If 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 formulas (7) to (9).

[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. When 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.

Because induction hardening is a rapid, 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 hardness unevenness and hardness insufficiency. The 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 hardenability by observing the microstructure of steel materials with 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, making it easier to prevent hardness unevenness and hardness insufficiency of the induction hardened steel parts. Therefore, it is preferable to set F7 to 58 or more.

[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.

After induction hardening, tempering is performed. In most 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 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 parts that have been induction hardened and tempered 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 parts becomes about 580HV or more, which is too hard and tends to reduce the machinability when cutting after induction hardening. Therefore, it is preferable to set F8 to 244 to 316.

[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.

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

[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.

[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, a refining process using LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is first performed. Furthermore, a Ruhrstahl-Hausen (RH) vacuum degassing process is carried out, after which the alloy components are finally adjusted.

[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.

[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 cast piece or steel ingot produced in the casting step to produce a steel for induction hardening (e.g., a 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.

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

The above manufacturing process produces steel for induction hardening. As mentioned above, the hot working process may be omitted in this manufacturing method. In other words, the steel for induction hardening in this embodiment may be a cast product (a piece or an ingot).

The hardness of induction hardening steel (steel material) corresponds to the internal hardness of induction hardening steel parts made from induction hardening steel. Internal hardness here refers to the hardness of the area not hardened by induction hardening. Low internal hardness can reduce fatigue strength. For this reason, the hardness of induction hardening steel is preferably 210 HV or more. The area not hardened by induction hardening refers to, for example, an area equal to or greater than the full hardened layer depth as described in JIS 0559, Method for measuring hardened layer depth after flame hardening and induction hardening of steel.

[Method of manufacturing 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.

[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.

Frequency during high frequency heating: 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.

Heating time during high frequency heating: 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 polymeric compounds such as polyalkylene glycol, polyethylene glycol, polyvinyl alcohol, etc. The liquid temperature is preferably in the range of 20 to 40°C.

[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.

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

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 the cutting tool is greater than -5°, the tool may be easily chipped during cutting. On the other hand, if the rake angle is less than -30°, 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°.

Tool nose radius: 0.4 to 1.2 mm
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.

Feed: 0.1 to 0.4 mm/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.

Cutting speed: 50 to 500 m/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.

Depth of cut: 0.05 to 0.40 mm
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 the upper limit is more preferably 0.20 mm.

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 toward 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.

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 the 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 Electric Automate (using a Cr tube) with a collimator φ1 mm using the 2θ sin2ψ method. Further, the compressive residual stress in the depth direction is measured by performing electrolytic polishing. 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 made from the surface to a depth of 200 μm at 10 μm intervals to determine the maximum compressive residual stress.

The effects of the induction hardening steel, induction hardening steel parts, and manufacturing method thereof of this embodiment will be explained in more detail using 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 of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

Steel having the chemical composition in Table 1 was melted and hot forged to produce steel material (steel bar) with a diameter of 55 mm. A "-" in the "Chemical composition" column in Table 1 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 is rounded to the significant figures (numbers to the least significant digits) specified in the above embodiment.

[Measurement of the number density of appropriate melting point oxides and low melting point oxides, the number ratio of appropriate melting point oxides existing 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 central position of a 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 mirror polishing the observation surface, inclusions and oxides were measured using an SEM by the above-mentioned method. The results obtained are shown in Table 2.

[Measurement of ferrite area ratio and hardness of steel for induction hardening]
The mirror-polished observation surface was etched with a nital solution, and the microstructure was observed with an optical microscope. 20 fields of view were photographed at 400 times magnification (field of view of about 0.32 mm x 0.24 mm) with an 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 hardness was measured by measuring the Vickers hardness using a sample obtained by polishing the observation surface. The results are shown in Table 2.

[Manufacturing of simulated rough parts]
The manufactured steel for induction hardening was subjected to a heat treatment simulating hot forging in a process for 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. Thereafter, the steel was allowed to cool in the air to manufacture a simulated rough part. The simulated rough part was a steel bar having a diameter of 55 mm.

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

A simulation test of high-frequency hardening was carried out on the above test piece using a test device manufactured by Fuji Electric Industrial Co., Ltd. (product name: "Thermal Cycle Test Device"). 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. The test piece was then water-cooled.

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 under an optical microscope at 400x magnification, and the presence or absence of melt cracks was visually confirmed. The observation surface was 250 μm x 400 μm.

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 a fusion crack had not occurred. The evaluation results of fusion cracks are shown in the "fusion cracks" column in Table 2. Cases where a fusion crack occurred were marked with "X", and cases where a fusion crack did not occur were marked with "O".

[Manufacturing of simulated raw materials]
A cylindrical round bar test piece with a diameter of 35 mm and a length of 300 mm was manufactured by machining the simulated rough member 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).

[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 raw material after induction hardening and tempering was turned using a general-purpose lathe. The cutting tool used was 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 wear tip was measured, and this was taken as the amount of flank wear. In this measurement, a flank wear amount of 100 μm or less is considered to be acceptable. The results of the machinability evaluation are shown in the "Tool wear" column of Table 2. The flank wear amount is preferably 90 μm or less.

[Fatigue strength evaluation test (rotating bending fatigue test)]
A rotating bending fatigue test piece was taken from the manufactured simulated rough member. The rotating bending fatigue test piece was prepared so that the direction of the central axis of the rotating bending fatigue test piece coincided with the direction of the central axis of the simulated rough member. The rotating bending fatigue test piece was subjected to induction hardening 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 using a 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 bottom of the notch was 6.72 mm, but the test pieces were prepared with a diameter at the bottom of the notch of 6.92 mm before induction hardening, and then machined to a cut depth of 0.1 mm after induction hardening to obtain the shape shown in FIG.

The Ono-type rotating bending fatigue test was performed at room temperature (23°C) in an air atmosphere at a rotation speed of 3600 rpm in both directions. Fatigue tests were performed on multiple test pieces by applying different stresses, and the highest stress at which the test pieces did not break after ¹⁰⁷ cycles was taken as the fatigue strength (MPa).

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 have passed. A fatigue strength of 530 MPa or more is preferable, 560 MPa or more is more preferable, and 590 MPa or more is even more preferable.

[Measurement of hardness of induction hardened steel parts]
Using the above Ono-type rotating bending fatigue test piece that had not been subjected to the fatigue test, 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, and then the Vickers hardness was measured at a position 50 μm from the surface layer. The results are shown in Table 2.

[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 Ono-type rotating bending fatigue test pieces that had not been subjected to the fatigue test. In this example, the maximum compressive residual stress was 300 MPa or more in all cases.

[Test Results]
Table 2 shows the test results. Referring to Table 2, the steel materials of test numbers 1 to 16 had appropriate chemical compositions and satisfied formulas (1) to (3). Therefore, in the steel materials of each test number, the content of CaO and Al ₂ O ₃ was such that the number density of the appropriate melting point oxide was 0.15 pieces/mm ² or more, the number density of the low melting point oxide was 0.15 pieces/mm ² or less, the proportion of the number of the appropriate melting point oxides present as composite inclusions containing CaS: 10% or more in the content in mass% 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 the machinability was high. Furthermore, the fatigue strength in the rotating bending fatigue test was 500 MPa or more, and the fatigue strength was high.

On the other hand, in test number 17, the C content was too high. As a result, melting cracks occurred.

In test number 18, the C content was too low, resulting in low fatigue strength.

In test number 19, the Si content was too high. As a result, melting cracks occurred.

In test number 20, the Si content was too low, resulting in low fatigue strength.

In test number 21, the Mn content was too high. As a result, melting cracks occurred.

In test number 22, the Mn content was too low. As a result, melting cracks occurred.

In test number 23, the P content was too high. As a result, melting cracks occurred.

In test number 24, the S content was too high. Therefore, 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). Therefore, the machinability and fatigue strength were low.

In test number 25, the S content was too low. As a result, machinability was poor.

In test number 26, the Cr content was too high. As a result, melting cracks occurred.

In test number 27, the Ca content was too high. As a result, the fatigue strength was low. Furthermore, because formula (2) was not satisfied, the proportion of composite inclusions containing 10% or more CaS in terms of 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). As a result, the machinability and fatigue strength were low.

In test number 28, the Ca content was too low. As a result, the machinability was poor.

In test number 29, the Ca content was too low. Therefore, the machinability was poor. Furthermore, the 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.

In test number 30, the Al content was too high. Therefore, the machinability was poor. Furthermore, because formula (3) was not satisfied, the content of CaO and _Al2O3 was low in the number density of _oxides with appropriate melting points, 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.

In test number 31, the Al content was too low. As a result, melting cracks occurred.

In the test number 32, the Al content was too low. Furthermore, the formula (1) was not satisfied, specifically, F1 was too high, so that the content of CaO and _Al2O3 was high and the number density _of low melting point oxides was high. Therefore, melting cracks occurred.

In test number 33, the V content was too high, which resulted in reduced fatigue strength.

In test number 34, the O content was too high. As a result, the fatigue strength was low.

Test No. 35 did not satisfy formula (2), so the proportion of composite inclusions containing 10% or more CaS in mass percent 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). As a result, the machinability and fatigue strength were low.

In test number 36, the content of CaO and _Al2O3 was low, and the number density _of oxides with appropriate melting points did not satisfy formula (3), 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.

The above describes the embodiments of the present invention. However, 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.

The present invention can be used in the steel industry, which produces steel for induction hardening, and in machinery manufacturing industries such as the automobile industry, which produces and uses induction hardening steel parts.

1 Fillet R portion 2 Edge portion of crankshaft 10 Melting crack

Claims (9)

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, 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...Formula (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. The steel for induction hardening according to claim 1, further characterized in that, among the oxide-containing inclusions present in the steel and having an equivalent circle diameter of 1.0 μm or more, the number density of the oxide-containing inclusions satisfying formula ( ⁴ ) is 0.15 pieces/ ^mm2 or more and the number density of the oxide-containing inclusions satisfying formula (5) is 0.15 pieces/mm2 or less, the proportion of the number of composite inclusions containing CaS: 10 mass% or more among the oxide-containing inclusions satisfying 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: The content (mass%) of Al 2 _{O 3} in the oxide when the oxide is regarded as a ternary oxide of CaO-Al ₂ O ₃ -SiO ₂
B: The content (mass%) of CaO in the oxide when the oxide is regarded as a ternary oxide of CaO-Al ₂ O ₃ -SiO ₂
d+3σ≦20...Formula (6) 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. 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. 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. 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. 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. A high-frequency hardened steel part having the chemical composition described in any one of claims 1 to 7, characterized in that the maximum compressive residual stress in the region from the surface to a depth of 200 μm is 300 MPa or more. A method for manufacturing induction-hardened steel parts according to claim 8, comprising the steps of manufacturing a member by processing a steel for induction hardening having the 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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