JP7684601B2 - steel material - Google Patents

Description

The present invention relates to steel materials, and more specifically, to steel materials that are used as raw materials for mechanical structural components.

Mechanical structural parts used in automobile and construction vehicle suspension components and axles require high fatigue strength.

Furthermore, during the manufacturing process of machine structural parts, cutting may be performed on the steel material that is the raw material for the machine structural parts. Therefore, high machinability is required for the steel material that is the raw material for the machine structural parts.

Steel materials used as raw materials for mechanical structural parts are disclosed, for example, in JP 57-19366 A (Patent Document 1), JP 2004-18879 A (Patent Document 2), and JP 2008-169411 A (Patent Document 3).

The steel material disclosed in Patent Document 1 contains 0.001 to 0.05% Ca, 0.02 to 0.15% Pb and Bi alone or in combination, restricts S to 0.005% or less, makes the inclusions CaS-CaO, Pb, Bi-based inclusions, and suppresses Al ₂ O ₃ inclusions to less than 0.001%. In this document, a large amount of Ca is continuously added to the molten steel to change the dissolved S to CaS. Also, Al ₂ O ₃ is eliminated or extremely reduced by a reduction reaction caused by Ca. Therefore, the inclusions become CaS-CaO-based inclusions. Then, a small amount of one or both of Pb and Bi is added to the molten steel to generate Pb or Bi-based inclusions. Patent Document 1 states that this improves the machinability of the steel material.

The steel material disclosed in Patent Document 2 contains, by mass%, 0.001-0.010% B, 0.002-0.010% N, and 0.005-0.10% Bi. In this cold forging steel, a total of 15 or more BN particles with a diameter of 0.7 μm or more and Bi precipitates containing B are present per field area of 0.5 mm x 0.5 mm cross section. In this steel material, N is fixed as BN, thereby reducing the amount of solute N and reducing work hardening. Furthermore, Patent Document 2 states that Bi precipitates containing B are generated to improve chip disposal.

The steel material disclosed in Patent Document 3 contains, by mass%, 0.15-0.55% C, 0.01-2.0% Si, 0.01-2.5% Mn, 0.01-2.0% Cu, 0.01-2.0% Ni, 0.01-2.5% Cr, 0.01-3.0% Mo, and a total of 0.01-1.0% of at least one selected from the group consisting of V and W, with the balance consisting of Fe and unavoidable impurities. This steel material is soaked at 1010°C to 1050°C, then cooled to 500°C to 550°C at a cooling rate of 200°C/min or more, then cooled to 150°C or less at a cooling rate of 100°C/min or more, and then heated in a temperature range of 550°C to 700°C. The steel material that has been subjected to these heat treatments and cooling treatments has an LMP of 17.66 or more, which gives the maximum HRC hardness at room temperature. Since the LMX of this steel material is 17.66 or more, it is described in Patent Document 3 that the softening resistance and thermal fatigue strength are increased.

Japanese Unexamined Patent Publication No. 57-19366 JP 2004-18879 A JP 2008-169411 A

An example of a manufacturing process for a machine structural component using steel as a raw material is as follows: The raw steel is hot worked to manufacture an intermediate product having a rough shape of the machine structural component. The hot working is, for example, hot forging. The manufactured intermediate product is machined (cut) to give the intermediate product a predetermined shape. The cut intermediate product is quenched and tempered. Through the above manufacturing process, the machine structural component is manufactured.
In the above-mentioned hardening process during the manufacturing process, in order to increase the strength of a portion of a machine structural component, high-frequency hardening may be performed on an intermediate product (steel material). In this case, high-frequency induction heating is performed on the portion of the intermediate product (steel material) where the strength is to be increased, and then the intermediate product is rapidly cooled (hardened).

However, during high-frequency induction heating, due to the shape of the intermediate product (steel), the steel may become locally overheated. This may result in the surface and part of the interior of the steel melting, causing cracks. In this specification, such cracks are also referred to as "fusion cracks." When induction hardening is performed in the manufacturing process for machine structural parts, it is necessary to suppress fusion cracks in the steel.

Furthermore, hot working (e.g., hot rolling, hot forging, etc.) is carried out during the manufacturing process of steel material and during the manufacturing process of machine structural parts using that steel material. Therefore, steel material that is the material for machine structural parts is required to not only suppress fusion cracks, but also suppress cracks during hot working. Here, cracks that occur during hot working are also referred to as "hot working cracks" in this specification. Therefore, steel material that is the material for machine structural parts is required to have excellent machinability, and not only is it required to obtain high fatigue strength when that steel material is used as a machine structural part, but it is also required to suppress hot working cracks and fusion cracks.

The above-mentioned Patent Documents 1 to 3 do not at least consider the suppression of hot working cracks and the suppression of fusion cracks.

The object of the present invention is to provide a steel material which has excellent machinability, is capable of suppressing cracking during hot working, is capable of suppressing melting cracking during induction hardening, and has excellent fatigue strength when used as a mechanical structural part.

The steel material according to the present invention has
The chemical composition, in mass%, is
C: 0.20-0.50%,
Si: 0.01-0.80%,
Mn: 0.50-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-1.30%,
V: more than 0.200 to 0.300%,
Bi: 0.0051 to 0.1500%,
N: 0.0030 to 0.0200%;
The balance is Fe and impurities.
Satisfies formula (1),
In the steel material,
The number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles/ ^mm2 ,
The number density of coarse Bi particles having an equivalent circle diameter of 10.0 μm or more is 10 particles/ ^mm2 or less.
0.80≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.50 (1)
Here, each element symbol in the formula is substituted with the content of the corresponding element in mass %.

The steel material of the present invention has excellent machinability, can suppress cracking during hot working, can suppress melting cracking during induction hardening, and has excellent fatigue strength when used as a mechanical structural part.

FIG. 1 is a schematic diagram of the microstructure of a steel material outside the scope of the present invention after it was heated to 1370° C. at a heating rate of 100° C./sec, held at that temperature for 15 seconds, and then water-cooled. FIG. 2 is a schematic diagram of the microstructure of a steel material within the scope of the present invention after it is heated to 1370° C. at a heating rate of 100° C./sec, held at that temperature for 15 seconds, and then water-cooled. FIG. 3 is a side view of a rotating bending fatigue test piece used in the examples.

The inventors first investigated the chemical composition of a steel material that has excellent machinability and excellent fatigue strength when used as a machine structural part. As a result, the inventors found that the chemical composition is, in mass %, C: 0.20-0.50%, Si: 0.01-0.80%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.010-0.095%, Cr: 0.01-1.30%, V: over 0.200-0.300%, N: 0.0030-0.0200%, Al: 0-0.060%, Mg: 0-0.0100%, Ti: 0-0.0200%, Nb: 0-0.0200%, W: 0-0.4000%, Zr: 0-0.2000%, Ca: 0. The inventors considered that a steel material consisting of Mo: 0-0.0030%, Te: 0-0.0100%, B: 0-0.0050%, Sn: 0-0.0100%, rare earth elements: 0-0.0070%, Co: 0-0.0100%, Se: 0-0.0100%, Sb: 0-0.0100%, In: 0-0.0100%, Mo: 0-0.20%, Cu: 0-0.20%, Ni: 0-0.20%, and the balance being Fe and impurities, would have excellent machinability and could potentially obtain excellent fatigue strength when used as a machine structural part.

However, simply giving a steel material the above-mentioned chemical composition does not necessarily result in excellent machinability or excellent fatigue strength when used as a machine structural part. Even if the content of each element in the chemical composition is within the above-mentioned range, if the hardness of the steel material is high, the machinability of the steel material will decrease. On the other hand, even if the content of each element in the chemical composition is within the above-mentioned range, if the hardness of the steel material is low, the fatigue strength of a machine structural part manufactured using that steel material will decrease. Therefore, in order to achieve both the fatigue strength of a machine structural part and the machinability of the steel material, it is effective to set the hardness of the steel material, which is the material of the machine structural part, within an appropriate range.

Therefore, the present inventors have studied the contents of elements that affect the hardness of steel materials having the above-mentioned ranges of the contents of each element in the chemical composition. Among the elements in the above-mentioned chemical composition, C, Si, Mn, Cr and V in particular increase the internal hardness of machine structural parts manufactured using the steel material as a material, and as a result, increase the fatigue strength of the machine structural parts. On the other hand, S decreases the internal hardness. Therefore, the present inventors thought that by setting the contents of these elements to an appropriate range, it would be possible to improve both the machinability of the steel material and the fatigue strength of the machine structural parts manufactured using the steel material as a material. As a result of further study, the present inventors have found that, in steel materials having the above-mentioned ranges of the contents of each element in the chemical composition, if formula (1) is satisfied, the steel material can have excellent machinability, and further, when used as a machine structural part, excellent fatigue strength can be obtained.
0.80≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.50 (1)
Here, each element symbol in the formula is substituted with the content of the corresponding element in mass %.

Next, the inventors investigated means for suppressing fusion cracks during induction hardening in steel material in which the content of each element in the chemical composition is within the above-mentioned range and which satisfies formula (1). First, the inventors observed the microstructure of the area where fusion cracks occurred in order to identify the cause of fusion cracks occurring in steel material during induction hardening. As a result, no decarburization had occurred in the area where fusion cracks occurred. On the other hand, no fusion cracks occurred in the area where decarburization had occurred.

From these results, the inventors believed that the C content influences the fusion cracks that occur in steel materials during induction hardening. Specifically, C that segregates at grain boundaries makes fusion cracks more likely to occur. Therefore, the inventors investigated means of suppressing the segregation of C at grain boundaries.

As a result of the investigation, the inventors found that by further adding 0.0051 to 0.1500% Bi to the above-mentioned chemical composition, melting cracks in the steel material during induction hardening can be suppressed. The reason for this is believed to be as follows. If an appropriate amount of Bi is contained, Bi exists as inclusions in the steel material. Hereinafter, inclusions made of Bi are referred to as Bi particles. Bi particles suppress the coarsening of austenite grains in the steel material during induction hardening due to the pinning effect. If the Bi particles are fine, the pinning effect is enhanced. If the austenite grains are maintained fine during induction hardening, the grain boundary area of the austenite grains increases. If the grain boundary area increases, the concentration of C segregating at the austenite grain boundaries per unit area decreases. As a result, the occurrence of melting cracks is suppressed.

As described above, by including an appropriate amount of Bi, the occurrence of melting cracks during induction hardening is suppressed. However, it has been found that cracks may occur when hot working is performed on steel. The hot working referred to here is, for example, hot rolling performed in the manufacturing process of steel, or hot forging performed in the manufacturing process of machine structural parts. Therefore, the causes of cracks during hot working were investigated. As a result, the inventors obtained the following new findings.

When steel contains Bi to suppress melting cracks, coarse Bi particles with an equivalent circle diameter of 10.0 μm or more may be generated in the steel along with fine Bi particles (Bi inclusions) with an equivalent circle diameter of 1.0 μm or less. Coarse Bi particles are likely to become the starting point for cracks during hot working. Therefore, if the number density of coarse Bi particles is too high, cracks (hot working cracks) are likely to occur during hot working.

As described above, in steel materials containing Bi, melting cracks during induction hardening are easily suppressed, but hot working cracks caused by coarse Bi particles are easily generated. If the Bi particles in the steel material are fine, melting cracks during induction hardening are suppressed. On the other hand, if the Bi particles in the steel material are coarse, hot working cracks are easily generated.

Based on the above results of the study, the inventors of the present invention thought that by suppressing the number density of coarse Bi particles in the steel material as much as possible while ensuring a certain number density of fine Bi particles in the steel material, it is possible to suppress melting cracks during induction hardening and also suppress hot working cracks. Therefore, further investigation and study was conducted on the number density of fine Bi particles and the number density of coarse Bi particles that fully exert these effects. As a result, the inventors of the present invention found that, in a steel material having the above-mentioned chemical composition, on the premise that the above-mentioned formula (1) is satisfied, if the number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 pieces/mm ² and the number density of coarse Bi particles having a circle equivalent diameter of 10.0 μm or more is 10 pieces/mm ² or less, melting cracks during induction hardening can be suppressed and hot working cracks can also be suppressed.

The steel material of this embodiment, completed based on the above findings, has the following configuration.

[1]
A steel material,
The chemical composition, in mass%, is
C: 0.20-0.50%,
Si: 0.01 to 0.80%,
Mn: 0.50-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-1.30%,
V: more than 0.200 to 0.300%,
Bi: 0.0051 to 0.1500%,
N: 0.0030 to 0.0200%;
The balance is Fe and impurities.
Satisfies formula (1),
In the steel material,
The number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles/ ^mm2 ,
The number density of coarse Bi particles having a circle equivalent diameter of 10.0 μm or more is 10 particles/ ^mm2 or less.
Steel material.
0.80≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.50 (1)
Here, each element symbol in the formula is substituted with the content of the corresponding element in mass %.

[2]
The steel material according to [1],
The chemical composition further contains, instead of a part of Fe,
Al: 0.060% or less,
Mg: 0.0100% or less,
Ti: 0.0200% or less,
Nb: 0.0200% or less,
W: 0.4000% or less,
Zr: 0.2000% or less,
Ca: 0.0030% or less,
Te: 0.0100% or less,
B: 0.0050% or less,
Sn: 0.0100% or less,
Rare earth elements: 0.0070% or less,
Co: 0.0100% or less,
Se: 0.0100% or less,
Sb: 0.0100% or less,
In: 0.0100% or less,
Mo: 0.20% or less,
Cu: 0.20% or less, and
Ni: Contains one or more selected from the group consisting of 0.20% or less;
Steel material.

The steel material of this embodiment will be described in detail below. "%" for elements means mass % unless otherwise specified.

[Features of the steel material according to this embodiment]
The steel material of this embodiment satisfies the following characteristics 1 to 4.
(Feature 1)
The chemical composition is, in mass%, C: 0.20-0.50%, Si: 0.01-0.80%, Mn: 0.50-2.00%, P: 0.030% or less, S: 0.010-0.095%, Cr: 0.01-1.30%, V: over 0.200-0.300%, Bi: 0.0051-0.1500%, N: 0.0030-0.0200%, and the balance being Fe and impurities.
(Feature 2)
Assuming that the content of each element is within the range of characteristic 1, formula (1) is satisfied.
0.80≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.50 (1)
(Feature 3)
In the steel material, the density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8,000 particles/ ^mm2 .
(Feature 4)
In the steel material, the number density of coarse Bi particles having an equivalent circle diameter of 10.0 μm or more is 10 particles/mm ² or less.
Features 1 to 4 will be described below.

[Feature 1: Chemical composition]
The chemical composition of the steel material of this embodiment contains the following elements.

C: 0.20-0.50%
Carbon (C) increases the hardness of machine structural parts manufactured from steel materials and increases the fatigue strength of the machine structural parts. If the C content is less than 0.20%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the C content exceeds 0.50%, C segregates at the grain boundaries even if the contents of other elements are within the ranges of this embodiment. In this case, the C concentration at the grain boundaries increases. If the C concentration increases, the melting point decreases. Therefore, melting cracks are more likely to occur during induction hardening.
Therefore, the C content is 0.20 to 0.50%.
The lower limit of the C content is preferably 0.21%, more preferably 0.22%, and further preferably 0.23%.
The upper limit of the C content is preferably 0.49%, more preferably 0.48%, and further preferably 0.47%.

Si: 0.01~0.80%
Silicon (Si) deoxidizes steel in the steelmaking process. Si also increases the hardness of machine structural parts and increases the fatigue strength of machine structural parts. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, Si has a weak affinity with C. Therefore, if the Si content exceeds 0.80%, C is more likely to segregate at grain boundaries than inside grains where Si is dissolved during heating, even if the contents of other elements are within the ranges of this embodiment. As a result, melting cracks are more likely to occur during induction hardening.
Therefore, the Si content is 0.01 to 0.80%. The lower limit of the Si content is preferably 0.02%, more preferably 0.05%, and further preferably 0.08%.
The upper limit of the Si content is preferably 0.75%, more preferably 0.70%, further preferably 0.65%, and further preferably 0.60%.

Mn: 0.50-2.00%
Manganese (Mn) deoxidizes steel in the steelmaking process. Mn also has a strong affinity with C. Therefore, during heating, C remains in the grains where Mn is dissolved. This suppresses the segregation of C to grain boundaries, and suppresses the occurrence of melting cracks during induction hardening. If the Mn content is less than 0.50%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Mn content exceeds 2.00%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material will be excessively increased, resulting in a decrease in the machinability of the steel material.
Therefore, the Mn content is 0.50 to 2.00%.
The lower limit of the Mn content is preferably 0.52%, more preferably 0.55%, further preferably 0.57%, and further preferably 0.60%.
The upper limit of the Mn content is preferably 1.98%, more preferably 1.95%, further preferably 1.93%, and further preferably 1.90%.

P: 0.030% or less Phosphorus (P) is an impurity. P segregates at grain boundaries. As a result, P lowers the melting point of steel. As a result, melting cracks are more likely to occur during induction hardening.
Therefore, the P content is 0.030% or less.
The P content is preferably as low as possible. However, excessive reduction in the P content increases the production cost. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%.
The upper limit of the P content is preferably 0.028%, more preferably 0.026%, further preferably 0.023%, and further preferably 0.020%.

S: 0.010-0.095%
Sulfur (S) forms sulfide-based inclusions and improves the machinability of steel. If the S content is less than 0.010%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, S lowers the melting point of the steel material, so if the S content exceeds 0.095%, melting cracks are likely to occur during induction hardening even if the contents of other elements are within the ranges of this embodiment.
Therefore, the S content is 0.010 to 0.095%.
The lower limit of the S content is preferably 0.012%, more preferably 0.015%, more preferably 0.018%, and even more preferably 0.020%. The upper limit of the S content is preferably 0.080%, more preferably 0.075%, and even more preferably 0.070%.

Cr:0.01~1.30%
Chromium (Cr) enhances the hardenability of steel. Therefore, the internal hardness of the machine structural parts is increased. As a result, the fatigue strength of the machine structural parts is increased. Cr also has a strong affinity with C. Therefore, during heating, C remains in the grains in which Cr is dissolved. Therefore, segregation of C to grain boundaries is suppressed, and the occurrence of melting cracks during induction hardening is suppressed. Cr also combines with S to form Cr sulfides. In this case, the formation of coarse FeS is suppressed. As a result, the ductility of the steel during hot working is improved, and hot working cracks are suppressed. If the Cr content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the Cr content exceeds 1.30%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material will be excessively increased, resulting in a decrease in the machinability of the steel material.
Therefore, the Cr content is 0.01 to 1.30%.
The lower limit of the Cr content is preferably 0.02%, more preferably 0.04%, further preferably 0.06%, and further preferably 0.08%.
The upper limit of the Cr content is preferably 1.28%, more preferably 1.26%, and further preferably 1.24%.

V: more than 0.200 to 0.300%
Vanadium (V) precipitates in the ferrite of the steel as V precipitates during the cooling process after hot working in the manufacturing process of the machine structural parts. The V precipitates increase the internal hardness of the machine structural parts. As a result, the fatigue strength of the machine structural parts is increased. Furthermore, V bonds with C to fix C in γ grains. Therefore, V suppresses the occurrence of melting cracks during induction hardening. If the V content is 0.200% or less, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the V content exceeds 0.300%, the hardness of the steel material will be excessively increased even if the contents of other elements are within the ranges of this embodiment. As a result, the machinability of the steel material will decrease. If the V content exceeds 0.300%, the above effects will saturate and the manufacturing cost will increase.
Therefore, the V content is more than 0.200 to 0.300%.
The lower limit of the V content is preferably 0.205%, more preferably 0.210%, still more preferably 0.215%, still more preferably 0.220%, still more preferably 0.225%, and still more preferably 0.230%.
The upper limit of the V content is preferably 0.295%, more preferably 0.290%, and further preferably 0.285%.

Bi:0.0051~0.1500%
Bismuth (Bi) forms inclusions (Bi particles) in steel material. Therefore, melt cracks during induction hardening are suppressed. Bi also improves the machinability of steel material. If the Bi content is less than 0.0051%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Bi content exceeds 0.1500%, coarse Bi particles are generated even if the contents of other elements are within the ranges of this embodiment. The coarse Bi particles are likely to become the starting points of cracks during hot working in the manufacturing process of the steel material, or during hot working in the manufacturing process of machine structural parts manufactured using the steel material as a material. Therefore, hot working cracks are likely to occur.
Therefore, the Bi content is 0.0051 to 0.1500%.
The lower limit of the Bi content is preferably 0.0080%, more preferably 0.0100%, further preferably 0.0120%, further preferably 0.0140%, and further preferably 0.0160%.
The upper limit of the Bi content is preferably 0.1400%, more preferably 0.1350%, and further preferably 0.1300%.

N: 0.0030-0.0200%
Nitrogen (N) forms nitrides and/or carbonitrides during the cooling process after hot working in the manufacturing process of machine structural parts, thereby precipitation strengthening the steel material. As a result, the fatigue strength of the machine structural parts is increased. If the N content is less than 0.0030%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the N content exceeds 0.0200%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the N content is 0.0030 to 0.0200%.
The lower limit of the N content is preferably 0.0032%, more preferably 0.0034%, and further preferably 0.0036%.
The upper limit of the N content is preferably 0.0190%, more preferably 0.0170%, still more preferably 0.0150%, still more preferably 0.0130%, and still more preferably 0.0100%.

The remainder of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when the steel material is industrially manufactured, and are acceptable to the extent that they do not adversely affect the steel material according to this embodiment.

Impurities include any element other than the above-mentioned impurities (P, S). There may be only one type of impurity, or two or more types. Impurities other than the above-mentioned impurities are, for example, as follows: O: 0.0050% or less, Ta and Zn: 0 to 0.01% in total, Pb: 0.09% or less.

[Optional Elements]
The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of the following Groups 1 to 5. All of these elements are optional elements.
[Group 1]
Al: 0.060% or less Mg: 0.0100% or less [Group 2]
Ti: 0.0200% or less Nb: 0.0200% or less W: 0.4000% or less Zr: 0.2000% or less [Group 3]
Ca: 0.0030% or less Te: 0.0100% or less B: 0.0050% or less Sn: 0.0100% or less Rare earth elements: 0.0070% or less [Group 4]
Co: 0.0100% or less Se: 0.0100% or less Sb: 0.0100% or less In: 0.0100% or less [Group 5]
Mo: 0.20% or less Cu: 0.20% or less Ni: 0.20% or less These optional elements will be described below.

[Group 1: Al and Mg]
The steel material of the present embodiment may further contain one or more elements selected from the group consisting of Al and Mg in place of a portion of Fe. These elements are optional elements, and all of them deoxidize the steel.

Al: 0.060% or less Aluminum (Al) is an optional element and may not be contained. In other words, the Al content may be 0%.
When Al is contained, Al deoxidizes the steel, and even if even a small amount of Al is contained, the above effect can be obtained to some extent.
However, if the Al content exceeds 0.060%, even if the contents of other elements are within the ranges of this embodiment, Al forms coarse oxides, which reduce the fatigue strength of machine structural parts.
Therefore, the Al content is 0 to 0.060%, and if Al is contained, the Al content is 0.060% or less.
The lower limit of the Al content is preferably more than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, and even more preferably 0.010%.
The upper limit of the Al content is preferably 0.055%, more preferably 0.050%, and further preferably 0.045%.

Mg: 0.0100% or less Magnesium (Mg) is an optional element and may not be contained. In other words, the Mg content may be 0%.
When Mg is contained, Mg deoxidizes the steel, and even if even a small amount of Mg is contained, the above effect can be obtained to some extent.
However, if the Mg content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, Mg forms coarse oxides, which reduce the fatigue strength of machine structural parts.
Therefore, the Mg content is 0 to 0.0100%, and if contained, the Mg content is 0.0100% or less.
The lower limit of the Mg content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0003%, and further preferably 0.0005%.
The upper limit of the Mg content is preferably 0.0050%, more preferably 0.0045%, and further preferably 0.0040%.

[Group 2: Ti, Nb, W and Zr]
The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of Ti, Nb, W, and Zr in place of a portion of Fe. These elements are optional elements, and all of them form precipitates to increase the toughness of machine structural parts.

Ti: 0.0200% or less Titanium (Ti) is an optional element and does not necessarily have to be contained. In other words, the Ti content may be 0%.
When Ti is contained, Ti forms carbides and/or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural part, thereby refining the crystal grains. This increases the toughness of the machine structural part. If even a small amount of Ti is contained, the above effect can be obtained to some extent.
However, if the Ti content exceeds 0.0200%, the above effects become saturated even if the contents of other elements are within the ranges of this embodiment.Furthermore, the manufacturing cost increases.
Therefore, the Ti content is 0 to 0.0200%, and if Ti is contained, the Ti content is 0.0200% or less.
The lower limit of the Ti content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0010%, further preferably 0.0050%, and further preferably 0.0080%.
The upper limit of the Ti content is preferably 0.0180%, more preferably 0.0170%, and further preferably 0.0150%.

Nb: 0.0200% or less Niobium (Nb) is an optional element and may not be contained. In other words, the Nb content may be 0%.
When Nb is contained, Nb forms carbides and/or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural part, thereby refining the crystal grains. This increases the toughness of the machine structural part. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent.
However, if the Nb content exceeds 0.0200%, the above effects become saturated even if the contents of other elements are within the ranges of this embodiment. Furthermore, the manufacturing cost increases.
Therefore, the Nb content is 0 to 0.0200%, and if Nb is contained, the Nb content is 0.0200% or less.
The lower limit of the Nb content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0010%, still more preferably 0.0050%, and still more preferably 0.0080%.
The upper limit of the Nb content is preferably 0.0180%, more preferably 0.0170%, and further preferably 0.0150%.

W: 0.4000% or less Tungsten (W) is an optional element and does not have to be contained. In other words, the W content may be 0%.
When W is contained, W forms carbides and/or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural part, thereby refining the crystal grains. This increases the toughness of the machine structural part. Even if even a small amount of W is contained, the above effect can be obtained to some extent.
However, if the W content exceeds 0.4000%, the above effects become saturated even if the contents of other elements are within the ranges of this embodiment. Furthermore, the manufacturing cost increases.
Therefore, the W content is 0 to 0.4000%, and if W is contained, the W content is 0.4000% or less.
The lower limit of the W content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0050%, and further preferably 0.0500%.
The upper limit of the W content is preferably 0.3500%, more preferably 0.3000%, and further preferably 0.2000%.

Zr: 0.2000% or less Zirconium (Zr) is an optional element and may not be contained. In other words, the Zr content may be 0%.
When Zr is contained, Zr forms carbides and/or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural part, thereby refining the crystal grains. This increases the toughness of the machine structural part. If even a small amount of Zr is contained, the above effect can be obtained to a certain extent.
However, if the Zr content exceeds 0.2000%, the above effects become saturated even if the contents of other elements are within the ranges of this embodiment. Furthermore, the manufacturing cost becomes high.
Therefore, the Zr content is 0 to 0.2000%, and if contained, the Zr content is 0.2000% or less.
The lower limit of the Zr content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0010%, still more preferably 0.0020%, and still more preferably 0.0050%.
The upper limit of the Zr content is preferably 0.1500%, more preferably 0.1000%, further preferably 0.0500%, and further preferably 0.0100%.

[Third group: Ca, Te, B, Sn and rare earth elements]
The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Te, B, Sn, and rare earth elements (REM). These elements are optional elements, and all of them improve the machinability of the steel material.

Ca: 0.0030% or less Calcium (Ca) is an optional element and may not be contained. In other words, the Ca content may be 0%.
When Ca is contained, it improves the machinability of the steel material. Even if even a small amount of Ca is contained, the above effect can be obtained to a certain degree.
However, if the Ca content exceeds 0.0030%, even if the contents of other elements are within the ranges of this embodiment, coarse oxides are formed, and in this case, the fatigue strength of the machine structural part is reduced.
Therefore, the Ca content is 0 to 0.0030%, and if contained, the Ca content is 0.0030% or less.
The lower limit of the Ca content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0010%, and further preferably 0.0015%.
The upper limit of the Ca content is preferably 0.0025%, more preferably 0.0023%, and further preferably 0.0020%.

Tellurium (Te) is an optional element and may not be contained. In other words, the Te content may be 0%.
When Tellurium is contained, Tellurium improves the machinability of the steel material. Even if even a small amount of Tellurium is contained, the above effect can be obtained to a certain degree.
However, if the Te content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Tellurium content is 0 to 0.0100%, and if contained, the Tellurium content is 0.0100% or less.
The lower limit of the Te content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0003%, and further preferably 0.0010%.
The upper limit of the Tellurium content is preferably 0.0090%, more preferably 0.0085%, and further preferably 0.0080%.

B: 0.0050% or less Boron (B) is an optional element and does not have to be contained. In other words, the B content may be 0%.
When B is contained, B improves the machinability of the steel material. If even a small amount of B is contained, the above effect can be obtained to a certain degree.
However, if the B content exceeds 0.0050%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment.
Therefore, the B content is 0 to 0.0050%, and if B is contained, the B content is 0.0050% or less.
The lower limit of the B content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0005%, and further preferably 0.0010%.
The upper limit of the B content is preferably 0.0040%, more preferably 0.0035%, and further preferably 0.0030%.

Sn: 0.0100% or less Tin (Sn) is an optional element and may not be contained. In other words, the Sn content may be 0%.
When Sn is contained, Sn improves the machinability of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to a certain extent.
However, if the Sn content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Sn content is 0 to 0.0100%, and if contained, the Sn content is 0.0100% or less.
The lower limit of the Sn content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0005%, and further preferably 0.0010%.
The upper limit of the Sn content is preferably 0.0095%, more preferably 0.0090%, further preferably 0.0085%, and further preferably 0.0080%.

Rare earth elements: 0.0070% or less Rare earth elements (REM) are optional elements and may not be contained. In other words, the REM content may be 0%.
When REM is contained, the REM enhances the machinability of the steel material. Even if even a small amount of REM is contained, the above effect can be obtained to some extent.
However, if the REM content exceeds 0.0070%, the steel material is prone to hot working cracks even if the contents of other elements are within the ranges of this embodiment.
Therefore, the REM content is 0 to 0.0070%, and if contained, the REM content is 0.0070% or less.
The lower limit of the REM content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0005%, and even more preferably 0.0010%.
The upper limit of the REM content is preferably 0.0065%, more preferably 0.0060%, and further preferably 0.0055%.

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

[Group 4: Co, Se, Sb and In]
The chemical composition of the steel material of the present embodiment may further contain, instead of a part of Fe, one or more elements selected from the group consisting of Co, Se, Sb, and In. These elements are optional elements, and all of them suppress decarburization of the steel material.

Co: 0.0100% or less Cobalt (Co) is an optional element and may not be contained. In other words, the Co content may be 0%.
When Co is contained, Co suppresses decarburization of the steel material during hot working. If even a small amount of Co is contained, the above effect can be obtained to some extent.
However, if the Co content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Co content is 0 to 0.0100%, and if Co is contained, the Co content is 0.0100% or less.
The lower limit of the Co content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0005%, and further preferably 0.0010%.
The upper limit of the Co content is preferably 0.0090%, more preferably 0.0080%, and further preferably 0.0070%.

Se: 0.0100% or less Selenium (Se) is an optional element and may not be contained. In other words, the Se content may be 0%.
When Se is contained, Se suppresses decarburization of the steel material during hot working. Even if even a small amount of Se is contained, the above effect can be obtained to a certain degree.
However, if the Se content exceeds 0.0100%, the steel material is prone to hot working cracks even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Se content is 0 to 0.0100%, and if contained, the Se content is 0.0100% or less.
The lower limit of the Se content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0005%, and further preferably 0.0010%.
The upper limit of the Se content is preferably 0.0090%, more preferably 0.0080%, and further preferably 0.0070%.

Sb: 0.0100% or less Antimony (Sb) is an optional element and may not be contained. In other words, the Sb content may be 0%.
When Sb is contained, that is, when the Sb content exceeds 0%, Sb suppresses decarburization of the steel material during hot working. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent.
However, if the Sb content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Sb content is 0 to 0.0100%, and if contained, the Sb content is 0.0100% or less.
The lower limit of the Sb content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0005%, and further preferably 0.0010%.
The upper limit of the Sb content is preferably 0.0090%, more preferably 0.0080%, and further preferably 0.0070%.

In: 0.0100% or less Indium (In) is an optional element and may not be contained. That is, the In content may be 0%.
When In is contained, In suppresses decarburization of the steel material during hot working. If even a small amount of In is contained, the above effect can be obtained to a certain degree.
However, if the In content exceeds 0.0100%, hot working cracks are likely to occur in the steel material even if the contents of other elements are within the ranges of this embodiment.
Therefore, the In content is 0 to 0.0100%, and if In is contained, the In content is 0.0100% or less.
The lower limit of the In content is preferably more than 0%, more preferably 0.0001%, further preferably 0.0005%, and further preferably 0.0010%.
The upper limit of the In content is preferably 0.0090%, more preferably 0.0080%, and further preferably 0.0070%.

[Group 5: Mo, Cu and Ni]
The chemical composition of the steel material of the present embodiment may further contain one or more elements selected from the group consisting of Mo, Cu, and Ni, instead of a portion of Fe. These elements are optional elements, and all of them increase the fatigue strength of machine structural parts.

Mo: 0.20% or less Molybdenum (Mo) is an optional element and may not be contained. In other words, the Mo content may be 0%.
When Mo is contained, Mo increases the fatigue strength of machine structural parts. Even if even a small amount of Mo is contained, the above effect can be obtained to a certain degree.
However, if the Mo content exceeds 0.20%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material increases excessively, resulting in a decrease in hot workability.
Therefore, the Mo content is 0 to 0.20%, and if Mo is contained, the Mo content is 0.20% or less.
The lower limit of the Mo content is preferably more than 0%, more preferably 0.01%, further preferably 0.05%, and further preferably 0.10%.
The upper limit of the Mo content is preferably 0.19%, more preferably 0.17%, and further preferably 0.15%.

Cu: 0.20% or less Copper (Cu) is an optional element and may not be contained. In other words, the Cu content may be 0%.
When Cu is contained, Cu increases the fatigue strength of the machine structural part. Even if only a small amount of Cu is contained, the above effect can be obtained to a certain degree.
However, Cu, like Si, promotes the occurrence of melting cracks during induction hardening, so if the Cu content exceeds 0.20%, melting cracks are likely to occur during induction hardening even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Cu content is 0 to 0.20%, and if contained, the Cu content is 0.20% or less.
The lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, further preferably 0.02%, and further preferably 0.03%.
The upper limit of the Cu content is preferably 0.15%, more preferably 0.13%, and further preferably 0.10%.

Ni: 0.20% or less Nickel (Ni) is an optional element and may not be contained. In other words, the Ni content may be 0%.
When Ni is contained, Ni enhances the fatigue strength of the machine structural parts. Even if even a small amount of Ni is contained, the above effect can be obtained to a certain degree.
However, Ni, like Si and Cu, promotes the occurrence of melting cracks during induction hardening, so if the Ni content exceeds 0.20%, melting cracks are likely to occur during induction hardening even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Ni content is 0 to 0.20%, and if Ni is contained, the Ni content is 0.20% or less.
The lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, further preferably 0.02%, and further preferably 0.03%.
The upper limit of the Ni content is preferably 0.15%, more preferably 0.13%, and further preferably 0.10%.

[(Feature 2) Regarding Formula (1)]
The steel material of the present embodiment further satisfies formula (1) on the premise that the content of each element is within the above-mentioned range, that is, on the premise that characteristic 1 is satisfied.
0.80≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.50 (1)
Here, each element symbol in the formula is substituted with the content of the corresponding element in mass %.

fn1 is defined as C + (Si/10) + (Mn/5) - (5S/7) + (5Cr/22) + 1.65V. fn1 is an index of the hardness of steel. C, Si, Mn, Cr and V increase the internal hardness of machine structural parts manufactured using steel as the material. On the other hand, S decreases the internal hardness of machine structural parts.

Even if the content of each element in the steel material is within the range of this embodiment, if fn1 is less than 0.80, the internal hardness of the machine structural component is excessively reduced. As a result, the fatigue strength of the machine structural component is reduced. On the other hand, even if the content of each element in the steel material is within the range of this embodiment, if fn1 exceeds 1.50, the hardness of the steel material is excessively increased. As a result, the machinability of the steel material is reduced.
Therefore, fn1 is between 0.80 and 1.50.
The lower limit of fn1 is preferably 0.81, more preferably 0.82, and even more preferably 0.85.
The upper limit of fn1 is preferably 1.48, more preferably 1.45, and even more preferably 1.43.

[Feature 3: Fine Bi particles]
In the steel material of this embodiment, on the premise that the content of each element is within the above range and formula (1) is satisfied, the number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm (hereinafter also simply referred to as fine Bi particles) is 80 to 8000 particles/mm ^2. If the number density of the fine Bi particles is 80 to 8000 particles/mm ² , the occurrence of melting cracks during induction hardening is suppressed.

In the steel material, Bi exists in the form of particles of simple Bi or particles containing Bi at a high concentration. In this specification, particles of simple Bi or particles containing Bi at a high concentration are collectively defined as Bi particles. In this specification, fine Bi particles refer to Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm. Since Bi is a heavy element, Bi particles are observed with a higher brightness than the surroundings in a backscattered electron image. Fine Bi particles may exist alone in the steel material without contacting other particles (precipitates or inclusions). Fine Bi particles may also exist in the steel material attached to or in contact with other particles.

As described above, during high-frequency induction heating, Bi particles pin the austenite grain boundaries. If the circle-equivalent diameter of the Bi particles is 0.1 to 1.0 μm, the pinning effect of the austenite grain boundaries is enhanced. If the austenite grains are kept fine during high-frequency hardening, the grain boundary area of the austenite grains increases. If the grain boundary area increases, the concentration of C segregated at the grain boundaries decreases. As a result, the occurrence of melting cracks is suppressed. Even if the content of each element in the chemical composition of the steel is within the range of this embodiment, formula (1) is satisfied, and the number of coarse Bi particles having a circle-equivalent diameter of 10.0 μm or more is 10 pieces/ ^mm2 or less, the above effect cannot be sufficiently obtained if the number of fine Bi particles is less than 80 pieces/ ^mm2 .

On the other hand, even if the content of each element in the chemical composition of the steel material is within the range of this embodiment, formula (1) is satisfied, and the number of coarse Bi particles having an equivalent circle diameter of 10.0 μm or more is 10 particles/ ^mm2 or less, if the number density of fine Bi particles exceeds 8000 particles/ ^mm2 , the above effect becomes saturated and the manufacturing cost becomes higher.

Therefore, in the steel material of this embodiment, the number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles/mm ² .

The lower limit of the number density of the fine Bi particles is preferably 90 particles/ ^mm2 , more preferably 95 particles/ ^mm2 , and even more preferably 100 particles/ ^mm2 .
The upper limit of the number density of the fine Bi particles is preferably 7900 particles/ ^mm2 , more preferably 6000 particles/ ^mm2 , more preferably 3000 particles/ ^mm2 , more preferably 1000 particles/ ^mm2 , more preferably 900 particles/ ^mm2 , and more preferably 800 particles/ ^mm2 .

[Feature 4: Large Bi particles]
In the steel material of this embodiment, the number density of coarse Bi particles (hereinafter simply referred to as coarse Bi particles), which are Bi particles with an equivalent circle diameter of 10.0 μm or more, is 10 particles/ ^mm2 or less. If the number density of the coarse Bi particles is 10 particles/ ^mm2 or less, cracks (hot working cracks) can be suppressed during hot working in the manufacturing process of the steel material, or during hot working in the manufacturing process of machine structural parts made from the steel material. Hot working is, for example, hot rolling, hot forging, etc.

In this specification, coarse Bi particles refer to Bi particles having a circle-equivalent diameter of 10.0 μm or more. In the method for measuring the number density of coarse Bi particles described later, if the circle-equivalent diameter of a particle is 10.0 μm or more and is observed with a higher brightness than the surroundings, the particle is judged to be a coarse Bi particle. The coarse Bi particles may exist alone in the steel material without contacting other particles (precipitates or inclusions). The coarse Bi particles may also exist in the steel material by adhering to or contacting other particles. There is no particular limit to the upper limit of the circle-equivalent diameter of the coarse Bi particles, but in the case of the chemical composition of this embodiment, the upper limit of the circle-equivalent diameter of the coarse Bi particles is 50.0 μm.

As mentioned above, fine Bi particles in steel suppress melting cracks during induction hardening. However, Bi in steel may form coarse Bi particles instead of fine Bi particles. Coarse Bi particles can become the starting point for hot working cracks in steel.

Even if the content of each element in the chemical composition of the steel material is within the range of this embodiment, formula (1) is satisfied, and the number of fine Bi particles is 80 to 8000 particles/ ^mm2 , if the number of coarse Bi particles exceeds 10 particles/ ^mm2 , hot working cracks may occur in the steel material.
Therefore, in the steel material of this embodiment, the number density of coarse Bi particles having an equivalent circle diameter of 10.0 μm or more is 10 particles/mm ² or less.

The upper limit of the number density of the coarse Bi particles is preferably 8 particles/ ^mm2 , more preferably 7 particles/ ^mm2 , even more preferably 6 particles/ ^mm2 , and even more preferably 5 particles/ ^mm2 .

It is preferable that the number density of the coarse Bi particles is as low as possible. That is, the number density of the coarse Bi particles is preferably 0 particles/ ^mm2 . However, excessive reduction in the number density of the coarse Bi particles increases the manufacturing cost. Therefore, taking into consideration normal industrial productivity, the preferred lower limit of the number density of the coarse Bi particles is 1 particle/ ^mm2 , and more preferably 2 particles/ ^mm2 .

In addition to the fine Bi particles and coarse Bi particles described above, the steel material of this embodiment may also contain intermediate Bi particles (hereinafter simply referred to as intermediate Bi particles) that are greater than 1.0 μm and less than 10.0 μm. However, intermediate Bi particles are less likely to affect hot working cracks and fusion cracks during induction hardening. Therefore, intermediate Bi particles do not need to be taken into consideration when suppressing hot working cracks and fusion cracks.

[Method for measuring the number density of fine Bi particles and coarse Bi particles]
The number density of fine Bi particles and coarse Bi particles can be measured by the following method.
A test piece including the R/2 portion is taken from a cross section perpendicular to the axial direction (rolling direction) of the steel material (steel bar). Here, the R/2 portion means the center portion of the radius R in the cross section perpendicular to the axial direction of the steel material. Of the surfaces of the taken test piece, the surface corresponding to the cross section perpendicular to the axial direction of the steel material is used as the observation surface.
The observation surface is mirror-polished. Using a scanning electron microscope (SEM), the R/2 portion of the mirror-polished observation surface is observed in 20 fields of view at a magnification of 1000 times. The area of each field of view is 100 μm×120 μm.

Based on the backscattered electron images of each field obtained by SEM observation, the number density of coarse Bi particles and fine Bi particles is examined using a well-known particle analysis method of image analysis. Specifically, particles in the steel are identified based on the interface between the steel matrix and the particles. The particles referred to here are inclusions or precipitates. Image analysis is performed to determine the circular equivalent diameter of the identified particles. Specifically, the area of each identified particle is determined. The diameter of a circle with the same area as the determined area is defined as the circular equivalent diameter (μm) of the particle.

As mentioned above, Bi is a heavy element and is therefore observed with high brightness in the backscattered electron image. Therefore, among the particles observed in the backscattered electron image obtained by the above SEM observation, particles with an equivalent circle diameter of 0.1 to 1.0 μm and observed with a higher brightness than the surrounding particles are identified as fine Bi particles. Furthermore, among the particles observed in the backscattered electron image obtained by SEM observation, particles with an equivalent circle diameter of 10.0 μm or more and observed with a higher brightness than the surrounding particles are identified as coarse Bi particles.

In the examples described below, the chemical compositions of the fine Bi particles and the coarse Bi particles identified by the above method were analyzed using an energy dispersive X-ray analyzer (EDX: Energy Dispersive X-ray spectroscopy) equipped in the SEM, and it was confirmed that both were Bi particles. The EDX beam diameter at the time of confirmation was 0.1 to 1.0 μm.

Fine Bi particles and coarse Bi particles are identified using the above method. The number of fine Bi particles per unit area (particles/mm2) is calculated based on the total number of fine Bi particles identified in each visual field and the total area of the 20 visual fields (0.24 ^mm2 ). The number of coarse Bi particles per unit area (particles/ ^mm2 ) is also calculated based on the total number of coarse Bi particles identified in each visual field and the total area of the 20 visual fields ( ^{0.24 mm2} ).

[Effects of the steel material according to this embodiment]
As described above, the steel material of this embodiment satisfies Features 1 to 4. Therefore, the steel material of this embodiment has excellent machinability, can suppress cracks during hot working and melting cracks during induction hardening, and has excellent fatigue strength when used as a machine structural part. These effects will be described below.

[Preferable uses of the steel material according to this embodiment]
The steel material of this embodiment is widely applicable, for example, as a material for machine structural parts. The steel material of this embodiment is particularly suitable for the case where high-frequency hardening is performed in the manufacturing process of the machine structural parts. However, even if high-frequency hardening is not performed, the steel material of this embodiment is applicable as a material for machine structural parts.

[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 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.

An example of the method for manufacturing a steel material according to the present embodiment includes the following steps: Note that step 3 is an optional step and may not be performed.
(Step 1) Refining step (Step 2) Casting step (Step 3) Hot working step Each step will be described below.

[(Step 1) Refining step]
In the refining process, molten steel is produced having a chemical composition that satisfies the above-mentioned features 1 and 2. The refining process includes a primary refining process and a secondary refining process.
In the primary refining process, molten iron produced by a known method is refined in a converter. In the secondary refining process, alloy elements are added to the molten steel so that the chemical composition of the molten steel satisfies Features 1 and 2. Specifically, in the secondary refining process, the components of the molten steel other than Bi are adjusted while the molten steel is stirred by a known refining method. Thereafter, Bi is added to the molten steel by a wire while stirring the molten steel, and the Bi component is adjusted.

In the secondary refining process, the following conditions are met:
(conditions)
The time T from the addition of Bi to the molten steel until the end of stirring in the secondary refining step is set to more than 15 minutes and less than 60 minutes.

In the secondary refining step, the time from the addition of Bi to the end of stirring in the secondary refining step is more than 15 minutes and less than 60 minutes.
If the time from the addition of Bi to the end of stirring in the secondary refining process is 15 minutes or less, Bi does not diffuse sufficiently in the molten steel, and in this case, an excessive number of coarse Bi particles are generated in the steel material.
If the time from the addition of Bi to the end of stirring in the secondary refining step is 60 minutes or longer, the fine Bi particles tend to aggregate with each other, resulting in a decrease in the number density of the fine Bi particles.
If the time from the addition of Bi in the secondary refining process until the end of stirring in the secondary refining process is more than 15 minutes, Bi is sufficiently diffused in the molten steel. Therefore, fine Bi particles are sufficiently generated in the steel. Furthermore, if the time from the addition of Bi in the secondary refining process until the end of stirring in the secondary refining process is less than 60 minutes, aggregation of the fine Bi particles can be sufficiently suppressed. Therefore, the number density of the fine Bi particles is 80 particles/ ^mm2 or more, and the number density of the coarse Bi particles is 10 particles/ ^mm2 or less.

After adding Bi to the molten steel, the preferred upper limit of the time until the end of stirring in the secondary refining process is 50 minutes, more preferably 40 minutes. After adding Bi, the preferred lower limit of the time until the end of stirring in the secondary refining process is 20 minutes, more preferably 30 minutes.

After Bi is added, the temperature of the molten steel until the end of stirring in the secondary refining process is 1510 to 1560°C.

[(Step 2) Casting step]
In the casting process, the molten steel is used to produce a slab (or bloom) or a steel ingot (ingot) by a well-known casting method, such as a continuous casting method or an ingot casting method.

[(Step 3) Hot working step]
The hot working step is an optional step, i.e., the hot working step may or may not be performed.
When the hot working step is carried out, the slab or steel ingot produced in the casting step is subjected to hot working in the hot working step to produce the steel material of the present embodiment. The steel material of the present embodiment is, for example, a steel bar. The hot working step may be, for example, hot rolling or hot forging.
When hot rolling is performed in the hot working step, for example, only a rough rolling step may be performed, or a rough rolling step and a finish rolling step may be performed. 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.

The steel material of this embodiment is manufactured by the above manufacturing process. As mentioned above, the hot working process may be omitted in this manufacturing method. In other words, the steel material of this embodiment may be a cast product (a piece of cast steel or an ingot). The steel material of this embodiment may also be manufactured by carrying out a hot working process.

[Method of manufacturing machine structural parts]
As described above, the steel material of the present embodiment is used as a material for machine structural parts. The machine structural parts are, for example, parts for automobiles. The machine structural parts are, for example, suspension parts, axles, crankshafts, etc.

Mechanical structural components using the steel material of this embodiment as the material are manufactured, for example, by the following well-known manufacturing method.

First, the steel material of this embodiment is hot worked to produce an intermediate product having a rough shape for a machine structural component. The hot working is, for example, hot forging. The produced intermediate product is cut into a predetermined shape by machining. After cutting, the intermediate product is subjected to induction hardening and tempering. Through the above steps, a machine structural component is produced.

In the steel material of this embodiment, the content of each element in the chemical composition is within the range of this embodiment and satisfies formula (1). Furthermore, the number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles/ ^mm2 , and the number density of coarse Bi particles having a circle equivalent diameter of 10.0 μm or more is 10 particles/ ^mm2 or less. In other words, the steel material of this embodiment satisfies features 1 to 4. Therefore, the steel material of this embodiment has excellent machinability. Furthermore, machine structural parts manufactured using the steel material of this embodiment as a material have excellent fatigue strength. Furthermore, hot working cracks during the manufacturing process of the steel material or the manufacturing process of the machine structural parts are suppressed. Furthermore, when manufacturing machine structural parts using the steel material of this embodiment as a material, melting cracks are suppressed even if induction hardening is performed.

The effects of the steel material 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 steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

Steel materials having the chemical compositions in Tables 1 to 4 were manufactured using the following manufacturing process. Note that for all test numbers, the O (oxygen) content was 0.0050% or less by mass. The total Ta and Zn content was 0 to 0.01% by mass. The Pb content was 0.09% or less by mass.

In Tables 1 to 4, the "-" portion means that the content of the corresponding element is 0% in significant figures (numerical values to the least significant digit) as specified in the embodiment. In other words, the content of the corresponding element is 0% when the fraction in significant figures (numerical values to the least significant digit) as specified in the above embodiment is rounded off.
For example, the Mo content specified in this embodiment is specified as a numerical value up to two decimal places, which means that the measured Mo content in test number 1 in Table 2 was 0% when rounded off to two decimal places.
In addition, the Mg content specified in this embodiment is specified as a numerical value up to the fourth decimal place, so that in the case of test number 1 in Table 1, the measured Mg content was 0% when rounded off to the fifth decimal place.
Here, rounding off means that if the digit (fraction) to the right of the specified minimum digit is less than 5, it is rounded down, and if it is 5 or more, it is rounded up.

The refining process (primary refining process and secondary refining process) was carried out using molten steel and a 70-ton converter. In the primary refining process, refining was carried out in a converter on molten pig iron produced by a known method. In the secondary refining process, alloy elements were added to produce molten steel having the chemical composition of the steel material of this embodiment. Specifically, secondary refining was carried out by a known method, and the components of the molten steel other than Bi were adjusted while stirring the molten steel. Then, Bi was added with a wire while stirring the molten steel, and the Bi components were adjusted. The time T (min) from the addition of Bi in the secondary refining process until the end of stirring in the secondary refining process was as shown in Tables 5 and 6. Note that the molten steel temperature until the end of stirring after adding Bi was 1510 to 1560°C. A cast piece (bloom) having a cross section of 300 mm x 400 mm was produced by a continuous casting method.

The produced cast pieces were heated and then bloomed to produce billets with a cross section of 180 mm x 180 mm. The billets were heated to 1250°C and then hot forged to produce steel materials (steel bars) with a diameter of 80 mm and the chemical composition shown in Table 1.

[Manufacturing intermediate parts for simulated mechanical structures]
A heat treatment was carried out to simulate hot forging in the manufacturing process of machine structural parts made of steel. 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 intermediate products of the simulated machine structural parts. The intermediate products of the simulated machine structural parts for each test number were steel materials (steel bars) with a diameter of 80 mm.

[Evaluation test]
The following evaluation tests were carried out on the steel materials of each test number and the intermediate products of the simulated machine structural parts of each test number.
(Test 1) Hot working crack evaluation test (Test 2) Measurement of the number density of fine Bi particles and coarse Bi particles (Test 3) Melting crack evaluation test (Test 4) Machinability test (drill life test)
(Test 5) Fatigue strength evaluation test (rotating bending fatigue test)
Tests 1 to 5 will be described below.

[(Test 1) Hot working crack evaluation test]
The surface of the produced steel material was visually observed. When three or more clear cracks were not observed per meter in the axial direction (rolling direction) of the steel material on the surface of the steel material as a result of the visual observation, it was determined that hot working cracks were sufficiently suppressed (indicated as "E" (Excellent) in the "Hot working cracks" column in Tables 5 and 6).
On the other hand, when visual observation revealed that three or more clear cracks per meter were observed on the surface of the steel in the axial direction (rolling direction) of the steel, it was determined that hot working cracks could not be sufficiently suppressed (indicated as "NA" (Not Accepted) in the "Hot working cracks" column in Tables 5 and 6).
In addition, when hot working cracking could not be sufficiently suppressed in the hot working cracking evaluation test, Tests 3 to 5 were not performed (indicated by "-" in the "melting cracking", "machinability", and "fatigue strength" columns of Tables 5 and 6).

[(Test 2) Measurement of the number density of fine Bi particles and coarse Bi particles]
Based on the method described in the above [Method for measuring the number density of fine Bi particles and coarse Bi particles], the number density (pieces/ ^mm2 ) of fine Bi particles and the number density (pieces/ ^mm2 ) of coarse Bi particles of the steel material of each test number were determined. Note that test pieces were taken from the steel material (steel bar) before the heat treatment simulating hot forging was performed. The obtained number density results of fine Bi particles are shown in the "Fine Bi particle number density (pieces/ ^mm2 )" column of Tables 5 and 6. The obtained number density results of coarse Bi particles are shown in the "Coarse Bi particle number density (pieces/ ^mm2 )" column of Tables 5 and 6.

In addition, the intermediate product of the simulated mechanical structure part is a steel material with a diameter of 80 mm, which is subjected to a heat treatment simulating hot forging. Simply performing a heat treatment at 1100°C simulating hot forging does not affect the number density of fine Bi particles and coarse Bi particles in the steel material. Therefore, the number density of fine Bi particles and coarse Bi particles of the intermediate product of the simulated mechanical structure part is substantially the same as the number density of fine Bi particles and coarse Bi particles of the steel material with a diameter of 80 mm. In fact, based on the method described in the above [Method for measuring the number density of coarse Bi particles and fine Bi particles], the number density (pieces/ ^mm2 ) of fine Bi particles and the number density (pieces/ ^mm2 ) of coarse Bi particles of the intermediate product of the simulated mechanical structure part of each test number were obtained. As a result, the number density of fine Bi particles (pieces/ ^mm2 ) and the number density of coarse Bi particles (pieces/ ^mm2 ) of the intermediate products of the simulated mechanical structural parts with each test number were approximately the same as the number density of fine Bi particles (pieces/ ^mm2 ) and the number density of coarse Bi particles (pieces/ ^mm2 ) of the steel with the corresponding test number.

[(Test 3) 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 taken, including the R/2 portion of the cross section perpendicular to the axial direction (rolling direction) of the intermediate product of the simulated mechanical structural part of each test number. The longitudinal direction of the test piece was parallel to the axial direction (rolling direction) of the intermediate product of the simulated mechanical structural part. In addition, the central axis parallel to the longitudinal direction of the test piece coincided with the R/2 portion.

A simulated high-frequency hardening test was conducted on the test piece using a thermal 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. 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. Of the corroded observation surface, a field of view corresponding to the R/2 portion was observed under an optical microscope at 400x magnification. The presence or absence of melt cracks was visually confirmed in the observed field of view. The area of the field of view was 250 μm × 400 μm.

When even one corroded area (corroded area) with a width of 5 μm or more was observed in the grain boundary of the structure in the observed field of view, it was determined that fusion cracking was not sufficiently suppressed (indicated by "E" in the "fusion cracking" column in Tables 5 and 6). A corroded area with a width of 5 μm or more in the grain boundary means an area with a maximum width of 5 μm or more, such as the corroded area 10 in the grain boundary GB in the field of view, as shown in Figure 1. On the other hand, when no corroded area was observed in the grain boundary GB, as shown in Figure 2, it was determined that fusion cracking was sufficiently suppressed (indicated by "NA" in the "fusion cracking" column in Tables 5 and 6).

[(Test 4) Machinability evaluation test (drill life test)]
A test piece for machinability evaluation was taken from the intermediate product of the simulated machine structural part of each test number. Specifically, a drill was drilled at a depth of 21 mm from the outer surface of the steel material in a cross section perpendicular to the longitudinal direction of the intermediate product of the simulated machine structural part with a diameter of 80 mm. A drill of model SD3.0 manufactured by Fujikoshi Corporation was used, and the feed rate per revolution was set to 0.25 mm/rev. The drilling depth of one hole was set to 9 mm. During drilling, water-soluble cutting oil was continuously supplied to the drilled portion as a lubricant.
The drill was drilled under the above conditions to evaluate the machinability of the steel material. The maximum cutting speed VL1000 (m/min) was used as an evaluation index. The maximum cutting speed VL1000 means the fastest cutting speed of a drill capable of drilling a hole of 1000 mm in length.
When the maximum cutting speed VL1000 was 15 m/min or more, it was determined that excellent machinability was obtained (indicated by "E" in the "machinability" column in Tables 5 and 6). On the other hand, when the maximum cutting speed VL1000 was less than 15 m/min, it was determined that excellent machinability was not obtained (indicated by "NA" in the "machinability" column in Tables 5 and 6).

[(Test 5) Fatigue strength evaluation test (rotating bending fatigue test)]
Rotating bending fatigue test pieces were taken from the intermediate products of the manufactured simulated machine structural parts. Figure 3 is a side view of the rotating bending fatigue test pieces taken from the intermediate products of each simulated machine structural part. The numerical value of "φ" in Figure 3 means the diameter (mm) at that part.

The rotating bending fatigue test specimen had a parallel part with a diameter of 8 mm and a gripping part with a diameter of 12 mm. Specifically, the parallel part was created by cutting the surface of the intermediate product of the simulated mechanical structural part to a depth of 3.5 mm by lathe processing. Therefore, the surface of the parallel part was at least within a range of a depth of 5 mm from the surface of the steel bar. The longitudinal direction of the parallel part of the rotating bending fatigue test specimen was parallel to the longitudinal direction of the simulated mechanical structural part. The above-mentioned rotating bending fatigue test specimen was assumed to be an intermediate product after cutting processing was performed on the intermediate product after hot working in the manufacturing process of the mechanical structural part using steel material. The parallel part of the collected rotating bending fatigue test specimen was subjected to finish polishing to adjust the surface roughness. Specifically, the center line average roughness (Ra) of the surface was set to within 3.0 μm, and the maximum height (Rmax) was set to within 9.0 μm.

It is common knowledge among those skilled in the art that if the fatigue strength is sufficiently high in a test using a rotating bending fatigue test piece taken from an intermediate product of a simulated machine structural part before induction hardening, then the machine structural part after induction hardening will also have excellent fatigue strength. Therefore, using the above rotating bending fatigue test piece, an Ono-type rotating bending fatigue test was performed under alternating conditions of room temperature (23°C), air atmosphere, and a rotation speed of 3600 rpm. Fatigue tests were performed on multiple test pieces by applying different stresses, and the highest stress at which the test piece did not break after ¹⁰⁷ cycles was determined as the fatigue strength (MPa).

If the fatigue strength obtained was 550 MPa or more, it was determined that excellent fatigue strength was obtained (indicated by "E" in the "Fatigue strength" column in Tables 5 and 6). If the fatigue strength was less than 550 MPa, it was determined that excellent fatigue strength was not obtained (indicated by "NA" in the "Fatigue strength" column in Tables 5 and 6).

[Evaluation Results]
With reference to Tables 1 to 6, the steel materials of test numbers 1 to 43 satisfied features 1 to 4. Therefore, hot working cracks were sufficiently suppressed, and fusion cracks were sufficiently suppressed. Furthermore, in the machinability evaluation test, the maximum cutting speed VL1000 was 15 m/min or more, and excellent machinability was obtained. Furthermore, in the fatigue strength evaluation test, the fatigue strength was 550 MPa or more, and excellent fatigue strength was obtained.

On the other hand, in test number 44, the C content was too high, which caused melting cracks.

In test number 45, the C content was too low, resulting in low fatigue strength.

In test number 46, the Si content was too high, which caused melting cracks.

In test number 47, the Mn content was too high. As a result, the machinability of the steel was poor.

In test number 48, the Mn content was too low, which caused melting cracks.

In test number 49, the P content was too high, which caused melting cracks.

In test number 50, the S content was too high, which caused melting cracks.

In test number 51, the S content was too low. As a result, the machinability of the steel was poor.

In test number 52, the Cr content was too high. As a result, the machinability of the steel was poor.

In test number 53, the V content was too high. As a result, the machinability of the steel was poor.

In test number 54, the Bi content was too high. Therefore, the number density of coarse Bi particles exceeded 10 particles/ ^mm2 . As a result, hot working cracks occurred.

In test number 55, the Bi content was too low. Therefore, the machinability of the steel material was low. Furthermore, the number density of fine Bi particles was less than 80 particles/ ^mm2 . As a result, melting cracks occurred.

In test number 56, the N content was too high, which resulted in hot working cracks.

In test number 57, the N content was too low, resulting in low fatigue strength.

In test numbers 58 and 59, the value of fn1 was too high. In other words, fn1 did not satisfy formula (1). As a result, the machinability of the steel was low.

In test numbers 60 and 61, the value of fn1 was too low. In other words, fn1 did not satisfy formula (1). As a result, the fatigue strength was low.

In test numbers 62 to 64, the time T (min) from the addition of Bi to the end of stirring in the refining process was too short. As a result, the number density of coarse Bi particles exceeded 10 particles/ ^mm2 . As a result, hot working cracks occurred.

In test numbers 65 and 66, the time T (min) from the addition of Bi to the end of stirring in the refining step was too long, so the number density of fine Bi particles was less than 80 particles/ ^mm2 , and melting cracks occurred.

The above describes the embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims (2)

A steel material that is used as a material for machine structural parts ,
The chemical composition, in mass%, is
C: 0.20-0.50%,
Si: 0.01-0.80%,
Mn: 0.50-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-1.30%,
V: more than 0.200 to 0.300%,
Bi: 0.0051 to 0.1500%,
N: 0.0030 to 0.0200%;
The balance is Fe and impurities.
Satisfies formula (1),
In the steel material,
The number density of fine Bi particles having a circle equivalent diameter of 0.1 to 1.0 μm is 80 to 8000 particles/ ^mm2 ,
The number density of coarse Bi particles having a circle equivalent diameter of 10.0 μm or more is 10 particles/ ^mm2 or less.
Steel material.
0.80≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.50 (1)
Here, each element symbol in the formula is substituted with the content of the corresponding element in mass %. The steel material according to claim 1,
The chemical composition further contains, instead of a part of Fe,
Al: 0.060% or less,
Mg: 0.0100% or less,
Ti: 0.0200% or less,
Nb: 0.0200% or less,
W: 0.4000% or less,
Zr: 0.2000% or less,
Ca: 0.0030% or less,
Te: 0.0100% or less,
B: 0.0050% or less,
Sn: 0.0100% or less,
Rare earth elements: 0.0070% or less,
Co: 0.0100% or less,
Se: 0.0100% or less,
Sb: 0.0100% or less,
In: 0.0100% or less,
Mo: 0.20% or less,
Cu: 0.20% or less, and
Ni: Contains one or more selected from the group consisting of 0.20% or less;
Steel material.

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