JP7695586B2 - 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 are used for undercarriage parts and axles of automobiles and construction vehicles. Mechanical structural parts are required to have high fatigue strength.

In the manufacturing process of machine structural parts, cutting may be performed on the steel material that will be used to make the machine structural parts. Therefore, high machinability is required for the steel material that will be used to make 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 of the present disclosure is
In mass percent,
C: 0.05-0.30%,
Si: 0.05-0.45%,
Mn: 0.30-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-2.00%,
Bi: 0.0051 to 0.1500%, and
N: 0.0030 to 0.0250%;
the balance being Fe and impurities, satisfying 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.
0.25≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, each element symbol in the formula is substituted with the content of the corresponding element in mass %. When an element is not contained, the corresponding element symbol is substituted with "0".

The steel material of the present disclosure is
In mass percent,
C: 0.05-0.30%,
Si: 0.05-0.45%,
Mn: 0.30-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-2.00%,
Bi: 0.0051 to 0.1500%, and
N: 0.0030 to 0.0250%;
Further, it contains one or more selected from the group consisting of Groups 1 to 5,
The balance is Fe and impurities, and further 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.
[Group 1]
Al: 0.060% or less, and
Mg: 0.0100% or less, one or more selected from the group consisting of [Group 2]
Ti: 0.1500% or less,
Nb: 0.0800% or less,
W: 0.4000% or less, and Zr: 0.2000% or less. [Third Group]
Ca: 0.0100% or less,
Te: 0.0100% or less,
B: 0.0050% or less,
Sn: 0.0100% or less, and
Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Group 4]
Co: 0.0100% or less,
Se: 0.0100% or less,
Sb: 0.0100% or less, and
In: 0.0100% or less, one or more selected from the group consisting of [Group 5]
V: 0.200% or less,
Mo: 1.00% or less,
Cu: 0.20% or less, and
Ni: 0.20% or less, one or more selected from the group consisting of 0.25≦C+(Si/10)+(Mn/5)−(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, the content of the corresponding element in mass% is substituted for each element symbol in the formula. When an element is not contained, "0" is substituted for the corresponding element symbol.

The steel material disclosed herein 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 component.

FIG. 1 is a schematic diagram of the microstructure of a steel material outside the range of the steel material of this embodiment after it is heated to 1390° C. at a heating rate of 100° C./s, held at that temperature for 15 seconds, and then water-cooled. FIG. 2 is a schematic diagram of the microstructure of the steel material of this embodiment after it is heated to 1390° C. at a heating rate of 100° C./s, 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: 0.05-0.30%, Si: 0.05-0.45%, Mn: 0.30-2.00%, P: 0.030% or less, S: 0.010-0.095%, Cr: 0.01-2.00%, N: 0.0030-0.0250%, Al: 0-0.060%, Mg: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.0800%, W: 0-0.4000%, Zr: 0-0.2000%, Ca: 0-0.0100%, Te: 0-0. The present inventors considered that a steel material containing the following elements: 0-0.0100%, B: 0-0.0050%, Sn: 0-0.0100%, rare earth elements: 0-0.0100%, Co: 0-0.0100%, Se: 0-0.0100%, Sb: 0-0.0100%, In: 0-0.0100%, V: 0-0.200%, Mo: 0-1.00%, Cu: 0-0.20%, and Ni: 0-0.20%, with 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.
0.25≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, the content of the corresponding element in mass% is substituted for each element symbol in the formula. When an element is not contained, "0" is substituted for the corresponding element symbol.

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 melting cracks in steel material during induction hardening can be suppressed by further adding 0.0051 to 0.1500% Bi instead of a portion of Fe to the above-mentioned chemical composition. 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 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 during the manufacturing process of steel, or hot forging performed during 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,
In mass percent,
C: 0.05-0.30%,
Si: 0.05-0.45%,
Mn: 0.30-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-2.00%,
Bi: 0.0051 to 0.1500%, and
N: 0.0030 to 0.0250%;
the balance being Fe and impurities, satisfying 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.
0.25≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, the content of the corresponding element in mass% is substituted for each element symbol in the formula. When an element is not contained, "0" is substituted for the corresponding element symbol.

[2]
A steel material,
In mass percent,
C: 0.05-0.30%,
Si: 0.05-0.45%,
Mn: 0.30-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-2.00%,
Bi: 0.0051 to 0.1500%, and
N: 0.0030 to 0.0250%;
Further, it contains one or more selected from the group consisting of Groups 1 to 5,
the balance being Fe and impurities, satisfying 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.
[Group 1]
Al: 0.060% or less, and
Mg: 0.0100% or less, one or more selected from the group consisting of [Group 2]
Ti: 0.1500% or less,
Nb: 0.0800% or less,
W: 0.4000% or less, and
Zr: 0.2000% or less, one or more selected from the group consisting of [Group 3]
Ca: 0.0100% or less,
Te: 0.0100% or less,
B: 0.0050% or less,
Sn: 0.0100% or less, and
Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Group 4]
Co: 0.0100% or less,
Se: 0.0100% or less,
Sb: 0.0100% or less, and
In: 0.0100% or less, one or more selected from the group consisting of [Group 5]
V: 0.200% or less,
Mo: 1.00% or less,
Cu: 0.20% or less, and
Ni: 0.20% or less, one or more selected from the group consisting of 0.25≦C+(Si/10)+(Mn/5)−(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, the content of the corresponding element in mass% is substituted for each element symbol in the formula. When an element is not contained, "0" is substituted for the corresponding element symbol.

[3]
The steel material according to [2],
Containing the first group,
Steel material.

[4]
The steel material according to [2] or [3],
Containing the second group,
Steel material.

[5]
The steel material according to any one of [2] to [4],
Containing the third group,
Steel material.

[6]
The steel material according to any one of [2] to [5],
Containing the fourth group
Steel material.

[7]
The steel material according to any one of [2] to [6],
Containing the fifth group
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: 0.05-0.30%, Si: 0.05-0.45%, Mn: 0.30-2.00%, P: 0.030% or less, S: 0.010-0.095%, Cr: 0.01-2.00%, Bi: 0.0051-0.1500%, N: 0.0030-0.0250%, Al: 0-0.060%, Mg: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.0800%, W: 0-0.4000%, Zr: 0-0 .2000%, Ca: 0-0.0100%, Te: 0-0.0100%, B: 0-0.0050%, Sn: 0-0.0100%, rare earth elements: 0-0.0100%, Co: 0-0.0100%, Se: 0-0.0100%, Sb: 0-0.0100%, In: 0-0.0100% or less, V: 0-0.200%, Mo: 0-1.00%, Cu: 0-0.20%, and Ni: 0-0.20% or less, with 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.
0.25≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, the content of the corresponding element in mass% is substituted for each element symbol in the formula. When an element is not contained, "0" is substituted for the corresponding element symbol.
(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: 0.05-0.30%
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 0.05%, 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 0.30%, 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 0.05 to 0.30%.
The lower limit of the C content is preferably 0.08%, more preferably 0.10%, and further preferably 0.13%.
The upper limit of the C content is preferably 0.28%, more preferably 0.25%, and further preferably 0.23%.

Si: 0.05-0.45%
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 0.05%, 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 0.45%, even if the contents of other elements are within the range of this embodiment, C is more likely to segregate at grain boundaries than within grains where Si is dissolved during high-frequency induction heating. As a result, melting cracks are more likely to occur during high-frequency hardening.
Therefore, the Si content is 0.05 to 0.45%.
The lower limit of the Si content is preferably 0.07%, more preferably 0.10%, and further preferably 0.13%.
The upper limit of the Si content is preferably 0.43%, more preferably 0.40%, and further preferably 0.38%.

Mn: 0.30-2.00%
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 0.30%, 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 0.30 to 2.00%.
The lower limit of the Mn content is preferably 0.35%, more preferably 0.40%, further preferably 0.50%, and further preferably 0.60%.
The upper limit of the Mn content is preferably 1.90%, more preferably 1.70%, further preferably 1.50%, and further preferably 1.40%.

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%, further preferably 0.018%, and further preferably 0.020%.
The upper limit of the S content is preferably 0.080%, more preferably 0.070%, and further preferably 0.060%.

Cr:0.01~2.00%
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 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 Cr content is 0.01 to 2.00%.
The lower limit of the Cr content is preferably 0.02%, more preferably 0.04%, further preferably 0.06%, further preferably 0.08%, and further preferably 0.10%.
The upper limit of the Cr content is preferably 1.90%, more preferably 1.70%, further preferably 1.50%, and further preferably 1.20%.

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: 0.0030-0.0250%
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 0.0250%, 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 0.0250%.
The lower limit of the N content is preferably 0.0035%, more preferably 0.0040%, further preferably 0.0050%, and further preferably 0.0080%.
The upper limit of the N content is preferably 0.0240%, more preferably 0.0230%, still more preferably 0.0200%, still more preferably 0.0180%, and still more preferably 0.0150%.

The remainder of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, impurities 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 not intentionally contained, but are permissible within a range that does 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: 0-0.01% in total, Pb: 0-0.09%.

[Regarding optional elements]
The chemical composition of this embodiment may further contain one or more elements selected from the group consisting of first to fifth groups, in place of a portion of Fe.
[Group 1]
Al: 0.060% or less, and
Mg: 0.0100% or less, one or more selected from the group consisting of [Group 2]
Ti: 0.1500% or less,
Nb: 0.0800% or less,
W: 0.4000% or less, and
Zr: 0.2000% or less, one or more selected from the group consisting of [Group 3]
Ca: 0.0100% or less,
Te: 0.0100% or less,
B: 0.0050% or less,
Sn: 0.0100% or less, and
Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Group 4]
Co: 0.0100% or less,
Se: 0.0100% or less,
Sb: 0.0100% or less, and
In: 0.0100% or less, one or more selected from the group consisting of [Group 5]
V: 0.200% or less,
Mo: 1.00% or less,
Cu: 0.20% or less, and
Ni: 0.20% or less, one or more selected from the group consisting of: Each optional element will be described below.

[Group 1: Al and Mg]
The chemical composition of the steel material of the present embodiment may further contain the above-mentioned first group elements in place of a portion of Fe. These elements are optional elements, and all of them deoxidize the steel. Each element of the first group will be described below.

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, that is, when the Al content exceeds 0%, Al deoxidizes the steel. 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%. If Al is contained, the Al content is 0.060% or less.
The lower limit of the Al content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%, and still 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 contained, that is, when the Mg content is more than 0%, Mg deoxidizes the steel. 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%. If Mg is contained, the Mg content is 0.0100% or less.
The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%, and further preferably 0.0005%.
The upper limit of the Mg content is preferably 0.0090%, more preferably 0.0070%, further preferably 0.0050%, and further preferably 0.0040%.

[Group 2: Ti, Nb, W and Zr]
The chemical composition of the steel material of this embodiment may further contain the above-mentioned second group elements in place of a portion of Fe. These elements are optional elements, and all of them form precipitates to increase the toughness of the machine structural parts. Each element of the second group will be described below.

Ti: 0.1500% 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, that is, when the Ti content is more than 0%, 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. Even if Ti is contained even a small amount, the above effect can be obtained to some extent.
However, if the Ti content exceeds 0.1500%, the above effects become saturated and the manufacturing cost becomes high even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Ti content is 0 to 0.1500%. If Ti is contained, the Ti content is 0.1500% or less.
The lower limit of the Ti content is preferably 0.0001%, more preferably 0.0010%, further preferably 0.0050%, and further preferably 0.0080%.
The upper limit of the Ti content is preferably 0.1400%, more preferably 0.1200%, more preferably 0.1000%, more preferably 0.0500%, more preferably 0.0200%, and more preferably 0.0150%.

Nb: 0.0800% 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, that is, when the Nb content is more than 0%, 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 0.0800%, the above effects become saturated and the manufacturing cost becomes high even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Nb content is 0 to 0.0800%. If Nb is contained, the Nb content is 0.0800% or less.
The lower limit of the Nb content is preferably 0.0001%, more preferably 0.0010%, further preferably 0.0050%, and further preferably 0.0080%.
The upper limit of the Nb content is preferably 0.0700%, more preferably 0.0600%, further preferably 0.0500%, further preferably 0.0200%, 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, that is, when the W content is more than 0%, 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 a certain extent.
However, if the W content exceeds 0.4000%, the above effects become saturated and the manufacturing cost becomes high even if the contents of other elements are within the ranges of this embodiment.
Therefore, the W content is 0 to 0.4000%. If W is contained, the W content is 0.4000% or less.
The lower limit of the W content is preferably 0.0001%, more 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, that is, when the Zr content is more than 0%, 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. Even 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 and the production cost becomes high even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Zr content is 0 to 0.2000%. If Zr is contained, the Zr content is 0.2000% or less.
The lower limit of the Zr content is preferably 0.0001%, more preferably 0.0010%, further preferably 0.0020%, and further 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 the above-mentioned third group elements in place of a portion of Fe. These elements are optional elements, and all of them improve the machinability of the steel material. Each element of the third group will be described below.

Ca: 0.0100% or less Calcium (Ca) is an optional element and may not be contained. In other words, the Ca content may be 0%.
When contained, that is, when the Ca content is more than 0%, Ca improves the machinability of the steel material. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent.
However, if the Ca content exceeds 0.0100%, 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 0.0100%. If Ca is contained, the Ca content is 0.0100% or less.
The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0010%, and further preferably 0.0015%.
The upper limit of the Ca content is preferably 0.0090%, more preferably 0.0070%, further preferably 0.0050%, further preferably 0.0030%, 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 contained, that is, when the Te content is more than 0%, Te enhances the machinability of the steel material. Even if even a small amount of Te is contained, the above effect can be obtained to some extent.
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%. If Tellurium is contained, the Tellurium content is 0.0100% or less.
The lower limit of the Tellurium content is preferably 0.0001%, more 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 contained, that is, when the B content exceeds 0%, B enhances the machinability of the steel material. Even 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%. When B is contained, the B content is 0.0050% or less.
The lower limit of the B content is preferably 0.0001%, more 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 contained, that is, when the Sn content is more than 0%, Sn enhances the machinability of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some 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%. If Sn is contained, the Sn content is 0.0100% or less.
The lower limit of the Sn content is preferably 0.0001%, more 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: 0.0100% or less Rare earth elements (REM) are optional elements and may not be contained. In other words, the REM content may be 0%.
When contained, that is, when the REM content is more than 0%, 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 0.0100%, 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 REM content is 0 to 0.0100%. If contained, the REM content is 0.0100% or less.
The lower limit of the REM content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.
The upper limit of the REM content is preferably 0.0090%, more preferably 0.0070%, 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 this embodiment may further contain the above-mentioned fourth group elements in place of a portion of Fe. These elements are optional elements, and all of them suppress decarburization of the steel material. Each element of the fourth group will be described below.

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, that is, when the Co content exceeds 0%, Co suppresses decarburization of the steel material during hot working. Even 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%. If Co is contained, the Co content is 0.0100% or less.
The lower limit of the Co content is preferably 0.0001%, more 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 contained, that is, when the Se content exceeds 0%, 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 some extent.
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%. If Se is contained, the Se content is 0.0100% or less.
The lower limit of the Se content is preferably 0.0001%, more 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 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%. If Sb is contained, the Sb content is 0.0100% or less.
The lower limit of the Sb content is preferably 0.0001%, more 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 contained, that is, when the In content exceeds 0%, In suppresses decarburization of the steel material during hot working. Even 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%. If In is contained, the In content is 0.0100% or less.
The lower limit of the In content is preferably 0.0001%, more 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: V, Mo, Cu and Ni]
The chemical composition of the steel material of this embodiment may further contain the above-mentioned Group 5 elements in place of a portion of Fe. These elements are optional elements, and all of them increase the fatigue strength of machine structural parts. Each element of Group 5 will be described below.

V: 0.200% or less Vanadium (V) is an optional element and does not necessarily have to be contained. In other words, the V content may be 0%.
When V is contained, that is, when the V content is more than 0%, V forms precipitates and increases the fatigue strength of machine structural parts. V also bonds with C to fix C in austenite grains. Therefore, V suppresses the occurrence of melting cracks during induction hardening. Even if even a small amount of V is contained, the above effect can be obtained to a certain extent.
However, if the V content exceeds 0.200%, the hardness of the steel material will be excessively increased even if the contents of other elements are within the ranges of this embodiment, resulting in a decrease in the machinability of the steel material.
Therefore, the V content is 0 to 0.200%. If V is contained, the V content is 0.200% or less.
The lower limit of the V content is preferably 0.001%, more preferably 0.005%, further preferably 0.010%, and further preferably 0.015%.
The upper limit of the V content is preferably 0.195%, more preferably 0.190%, further preferably 0.185%, and further preferably 0.150%.

Mo: 1.00% or less Molybdenum (Mo) is an optional element and may not be contained. In other words, the Mo content may be 0%.
When contained, that is, when the Mo content exceeds 0%, 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 1.00%, 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 1.00%. If Mo is contained, the Mo content is 1.00% or less.
The lower limit of the Mo content is preferably 0.01%, more preferably 0.05%, and further preferably 0.10%.
The upper limit of the Mo content is preferably 0.90%, more preferably 0.80%, further preferably 0.60%, and further preferably 0.40%.

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 contained, that is, when the Cu content is more than 0%, Cu enhances the fatigue strength of the machine structural part. Even if even 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%. If contained, the Cu content is 0.20% or less.
The lower limit of the Cu content is preferably 0.01%, more 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 contained, that is, when the Ni content is more than 0%, Ni enhances the fatigue strength of the machine structural part. 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%. If Ni is contained, the Ni content is 0.20% or less.
The lower limit of the Ni content is preferably 0.01%, more 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%.

[Method of measuring the chemical composition of steel]
The chemical composition of the steel material of this embodiment can be measured by a known component analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the inside of the steel material at a depth of 1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by a known inert gas fusion-thermal conductivity method. The O content is determined by a known inert gas fusion-infrared absorption method.

The content of each element is determined by rounding off the measured value based on the significant figures defined in this embodiment to the lowest digit of the content of each element defined in this embodiment. For example, the C content of the steel material in this embodiment is defined to the second decimal place. Therefore, the C content is determined to the second decimal place by rounding off the measured value to the third decimal place.
Similarly, the contents of other elements other than the C content of the steel material of this embodiment are determined by rounding off the measured value to the minimum digit specified in this embodiment, and this is the content of that element.
Here, rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[(Feature 2) Regarding Formula (1)]
The steel material of this embodiment further satisfies formula (1), provided that the content of each element is within the above-mentioned range.
0.25≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, the content of the corresponding element is substituted for each element symbol in the formula in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol. In other words, when the optional element V is not contained, formula (1) becomes as follows.
0.25≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)≦1.00 (1)

fn1 is defined as follows:
fn1=C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V
When the optional element V is not contained, fn1 is as follows:
fn1=C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)

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 a 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 0.25, 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 1.00, the hardness of the steel material is excessively increased. As a result, the machinability of the steel material is reduced.
Therefore, fn1 is between 0.25 and 1.00.
The lower limit of fn1 is preferably 0.28, more preferably 0.30, and even more preferably 0.33.
The upper limit of fn1 is preferably 0.98, more preferably 0.95, and even more preferably 0.90.

[Feature 3: Fine Bi particles]
In the steel material of this embodiment, assuming that Features 1 and 2 are 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. In this specification, particles of simple Bi or particles containing Bi 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). Also, fine Bi particles may 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 equivalent circle 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 maintained 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 fusion cracks is suppressed. Even if the steel material satisfies Features 1 and 2 and further satisfies Feature 4 described below, if the fine Bi particles are less than 80 particles/ ^mm2 , the above effect cannot be sufficiently obtained.

On the other hand, even if a steel material satisfies Features 1, 2, and 4, if the density of fine Bi particles exceeds 8,000 particles/ ^mm2 , the above effects become 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 preferred lower limit of the number density of the fine Bi particles is 90 particles/ ^mm2 , more preferably 95 particles/ ^mm2 , and even more preferably 100 particles/ ^mm2 . The preferred upper limit of the number density of the fine Bi particles is 7900 particles/ ^mm2 , more preferably 6000 particles/ ^mm2 , more preferably 3000 particles/ ^mm2 , more preferably 1000 particles/ ^mm2 , more preferably 900 particles/mm2, and even ^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 in a backscattered electron image, 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. The upper limit of the circle-equivalent diameter of the coarse Bi particles is not particularly limited, 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 be the starting point for hot working cracks in steel.

Even if a steel material satisfies Features 1 to 3, if the coarse Bi particles exceed 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/ ^mm2 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 part is taken from a cross section perpendicular to the axial direction (rolling direction) of the steel material (steel bar). Here, the R/2 part means the center part 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 part of the observation surface after mirror polishing 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.

Because Bi is a heavy element, it is 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 a circle-equivalent 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 a circle-equivalent 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.

[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 manufacturing a steel material according to this embodiment will be described. The method for manufacturing a steel material described below is one example for manufacturing a steel material according to this embodiment. Therefore, a steel material having the above-mentioned configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel material according to this embodiment.

An example of a method for manufacturing a steel material according to this embodiment includes the following steps.
(Step 1) Refining step (Step 2) Casting step (Step 3) Hot working step The hot working step is an optional 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 process, the time from when Bi is added until stirring is completed is more than 15 minutes and less than 60 minutes. If the time from when Bi is added until stirring is completed in the secondary refining process is 15 minutes or less, Bi does not diffuse sufficiently in the molten steel. In this case, an excessive number of coarse Bi particles are generated in the steel. If the time from when Bi is added until stirring is completed in the secondary refining process is 60 minutes or more, fine Bi particles tend to aggregate together. As a result, the number density of fine Bi particles decreases.

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 an 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 process, for example, only a rough rolling process may be performed, or a rough rolling process and a finish rolling process may be performed. The rough rolling process is, for example, blooming. The finish rolling process 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 arranged alternately in a row. The heating temperature in the rough rolling process and the finish rolling process 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 were manufactured having the chemical compositions shown in Tables 1 to 4. Table 2 is a continuation of Table 1, and Table 4 is a continuation of Table 3. Note that for all test numbers, the O (oxygen) content was 0.0050% or less by mass. The total Ta and Zn content was 0-0.01% by mass. The Pb content was 0-0.09% by mass.

Specifically, the refining process (primary refining process and secondary refining process) was carried out using a 70-ton converter. In the primary refining process, molten pig iron produced by a known method was refined in a converter under the same conditions. In the secondary refining process, elements were added to produce molten steel having the chemical composition shown in Tables 1 to 4. Specifically, secondary refining was carried out by a known method, and the components of elements other than Bi were adjusted while the molten steel was stirred. Thereafter, Bi was further added with a wire while the molten steel was stirred, and the component of Bi in the molten steel was adjusted.

The time T (minutes) from the addition of Bi to the molten steel until the end of stirring in the secondary refining was as shown in Tables 5 and 6. The temperature of the molten steel from the addition of Bi to the molten steel until the end of stirring in the secondary refining was 1510-1560°C. Through the above process, molten steel with the chemical composition shown in Tables 1 to 4 was produced.

A bloom with a cross section of 300 mm x 400 mm was produced by continuous casting using molten steel. After heating, the bloom was rolled into a billet with a cross section of 180 mm x 180 mm. The heating temperature of the bloom during the blooming process was 1250°C.

Furthermore, hot forging equivalent to finish rolling was performed on the billet to produce a steel material (steel bar) with a diameter of 80 mm. The heating temperature of the billet during hot forging was 1250°C. The steel material was produced by the above manufacturing process.

[Evaluation test]
The following evaluation tests were carried out on the steel materials with each test number.
(Test 1) Chemical composition analysis test (Test 2) Hot working crack evaluation test (Test 3) Number density measurement test of fine Bi particles and coarse Bi particles (Test 4) Melting crack evaluation test (Test 5) Machinability evaluation test (drill life test)
(Test 6) Fatigue strength evaluation test (rotating bending fatigue test)
Each evaluation test will be described below.

[(Test 1) Chemical composition analysis test]
The chemical composition of the steel material having each test number was measured by the well-known composition analysis method described in the above [Method of measuring chemical composition of steel material]. As a result, the chemical composition of the steel material having each test number was as shown in Tables 1 to 4.

[(Test 2) Hot working crack evaluation test]
The surface of the steel material of each test number produced was visually observed. If the visual observation revealed that three or more clear cracks were observed on the surface of the steel material per meter in the longitudinal direction of the steel material, it was determined that hot working cracks had occurred. If the visual observation revealed that three or more clear cracks were not observed on the surface of the steel material per meter in the longitudinal direction of the steel material, it was determined that hot working cracks had been suppressed.

The evaluation results for hot working cracks are shown in the "Hot working cracks" column of Tables 5 and 6. If hot working cracks were suppressed, they are indicated as "E" (Excellent). If hot working cracks occurred, they are indicated as "NA" (Not Accepted).

[(Tests 3 to 6) Measurement of the number density of fine and coarse Bi particles, evaluation of melting cracks, evaluation of machinability, and evaluation of fatigue strength]
[Manufacturing of simulated intermediate products of machine structural parts]
A heat treatment was carried out to simulate hot forging during the manufacturing process of a machine structural part using the steel material of each test number as a material. Specifically, the steel material was heated and held at 1100°C for 30 minutes. The steel material was then allowed to cool in the air. The steel material subjected to the above heat treatment is hereinafter referred to as a "simulated intermediate product of a machine structural part (or simply a simulated intermediate product)." The simulated intermediate product of a machine structural part was a steel bar with a diameter of 80 mm.

[(Test 3) Measurement of the number density of fine Bi particles and coarse Bi particles]
[Number density of fine Bi particles and coarse Bi particles in steel material]
Using the steel material of each test number, the number density (pieces/mm 2 ) of the fine Bi particles and the number density (pieces/mm ² ) of the coarse Bi particles of the steel material of each test number were obtained based on the method described in ^the above-mentioned [Method for measuring the number density of fine Bi particles and coarse Bi particles]. A test piece including the R/2 part was taken from a cross section (transverse section) perpendicular to the axial direction (rolling direction) of the steel material. Using this test piece, the number density (pieces/mm ² ) of the fine Bi particles and the number density (pieces/mm ² ) of the coarse Bi particles of each test number were obtained. The obtained results of the number density of the fine Bi particles are shown in the "Fine Bi particle number density (pieces/mm ² )" column of the "Steel material" column in Tables 5 and 6. The obtained results of the number density of the coarse Bi particles are shown in the "Coarse Bi particle number density (pieces/mm ² )" column of the "Steel material" column in Tables 5 and 6.

[Number density of fine Bi particles and coarse Bi particles in simulated intermediate product]
Using the simulated intermediate products of the machine structural parts of each test number, the number density (pieces/mm2) of the fine Bi particles and the number density (pieces/ ^mm2 ) of the coarse Bi particles of each test number were determined based on the method described in the above-mentioned [Method for measuring the number density of fine Bi particles and coarse Bi particles]. A test piece including the R/ ² portion was taken from a cross section (transverse section) perpendicular to the axial direction (rolling direction) of the simulated intermediate product (steel bar). Using this test piece, the number density (pieces/ ^mm2 ) of the fine Bi particles and the number density (pieces/ ^mm2 ) of the coarse Bi particles of each test number were determined. The obtained results of the number density of the fine Bi particles are shown in the "Fine Bi particle number density (pieces/ ^mm2 )" column in the "Simulated intermediate product" column of Tables 5 and 6. The obtained results of the number density of the coarse Bi particles are shown in Tables 5 and 6 in the column "Number density of coarse Bi particles (pieces/mm ² )" in the column "Simulated intermediate product".

The simulated intermediate product of the machine structural part was manufactured by carrying out a heat treatment simulating hot forging on the raw material steel. Simply carrying out a heat treatment at 1100°C simulating hot forging does not affect the number density of the fine Bi particles and the number density of the coarse Bi particles. Therefore, the number density of the fine Bi particles and the number density of the coarse Bi particles of the simulated intermediate product of the machine structural part are substantially the same as the number density of the fine Bi particles and the number density of the coarse Bi particles of the steel.

[(Test 4) 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 from the R/2 part of the cross section perpendicular to the axial direction (rolling direction) of the simulated intermediate product of the machine structural part. The longitudinal direction of the test piece was parallel to the axial direction (rolling direction) of the simulated intermediate product of the machine structural part. In addition, the central axis parallel to the longitudinal direction of the test piece coincided with the R/2 part.

A simulation test of high-frequency hardening was carried out on the test piece using a thermal cycle test device manufactured by Fuji Electric Industrial Co., Ltd. Specifically, the test piece was heated to 1390°C at a heating rate of 100°C/sec using a high-frequency coil. The test piece was then held at 1390°C for 15 seconds. The test piece was then water-cooled.

After water cooling, the test piece was cut in a direction perpendicular to the longitudinal direction at the center of the longitudinal direction of the test piece. The cut surface was used as the observation surface. The observation surface was mechanically polished. After mechanical polishing, the 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. Two observation fields were used. Each observation field was 250 μm x 400 μm.

When a clearly corroded area (corroded area) having a width of 5 μm or more was observed at the grain boundary in at least one of the two observation fields of the observation surface, it was determined that a melting crack had occurred. A clearly corroded area having a width of 5 μm or more at the grain boundary means, for example, an area with a maximum width of 5 μm or more, such as the corroded area 10 at the grain boundary in the field of view as shown in Figure 1. On the other hand, when no corroded area was observed at the grain boundary in either of the two observation fields of view as shown in Figure 2, it was determined that a melting crack had been suppressed.

The evaluation results of melting cracks are shown in the "Melting Cracks" column of Tables 5 and 6. If melting cracks were suppressed, it is indicated with "E". If melting cracks occurred, it is indicated with "NA". A "-" in the "Melting Cracks" column of Table 6 means that a melting crack evaluation test was not conducted.

[(Test 5) Machinability evaluation test (drill life test)]
A test piece for machinability evaluation test was cut out from a simulated intermediate product of a machine structural part. Specifically, a drill was used to drill a hole at a depth of 21 mm in the radial direction from the outer surface in a cross section perpendicular to the axial direction (rolling direction) of a simulated intermediate product with a diameter of 80 mm. A drill of model number SD3.0 manufactured by Fujikoshi Corporation was used as a tool. The feed rate per revolution was set to 0.25 mm/rev as a drilling condition. 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.

Drilling was performed under the above drilling conditions to evaluate the machinability of the steel material. The maximum cutting speed VL1000 (m/min) was used as the evaluation index. The maximum cutting speed VL1000 means the fastest cutting speed of a drill capable of drilling a hole 1000 mm long.

When the maximum cutting speed VL1000 was 35 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 35 m/min, it was determined that sufficient machinability was not obtained (indicated by "NA" in the "machinability" column in Tables 5 and 6). Note that the "-" in the "machinability" column in Table 6 means that a machinability evaluation test was not conducted.

[(Test 6) Fatigue strength evaluation test (rotating bending fatigue test)]
The fatigue strength was evaluated by the following test method using fatigue test pieces assumed to be machine structural parts manufactured from steel materials.

Rotating bending fatigue test specimens were taken from the simulated intermediate products of the machine structural parts. Figure 3 shows a side view of the rotating bending fatigue test specimens taken from the intermediate products of each simulated machine structural part. The "φ" value in Figure 3 indicates the diameter (mm) at that location.

The fatigue test specimen was a round bar test specimen, with a diameter of the parallel part being 8 mm and a diameter of the gripping part being 12 mm. The longitudinal direction of the fatigue test specimen was parallel to the axial direction of the simulated intermediate product. Specifically, the parallel part was produced by cutting to a depth of 3.5 mm from the surface of the simulated intermediate product 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 fatigue test specimen was assumed to be a machine structural part after cutting processing was performed on an intermediate product after hot working in the manufacturing process of a machine structural part using steel material. The parallel part of the 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 in accordance with JIS B 0601 (2001).

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 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 taken as the fatigue strength (MPa).

If the fatigue strength obtained was 230 MPa or more, it was determined that sufficient fatigue strength had been obtained. The results of the fatigue strength evaluation are shown in the "Fatigue strength" column of Tables 5 and 6. If the fatigue strength was 230 MPa or more, it was determined that excellent fatigue strength had been obtained (indicated by "E"). On the other hand, if the fatigue strength was less than 230 MPa, it was determined that sufficient fatigue strength had not been obtained (indicated by "NA"). Note that a "-" in the "Fatigue strength" column of Table 6 means that a fatigue strength evaluation test was not conducted.

[Test Results]
Referring to Tables 1 to 6, the steel materials of Test Nos. 1 to 46 satisfied Features 1 to 4. Therefore, hot working cracks and fusion cracks were sufficiently suppressed. Furthermore, the maximum cutting speed VL1000 was 35 m/min or more, and excellent machinability was obtained. Furthermore, the fatigue strength was 230 MPa or more, and excellent fatigue strength was obtained.

On the other hand, in test number 47, the C content was too high, which caused melting cracks.

In test number 48, the C content was too low, resulting in low fatigue strength.

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

In test number 50, the Mn content was too high, resulting in poor machinability.

In test number 51, the Mn content was too low, which caused melting cracks.

In test number 52, the P content was too high, which caused melting cracks.

In test number 53, the S content was too high, which caused melting cracks.

In test number 54, the S content was too low, resulting in poor machinability.

In test number 55, the Cr content was too high, resulting in poor machinability.

In test number 56, 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 57, the Bi content was too low. Therefore, the machinability was poor. Furthermore, the number density of fine Bi particles was less than 80 particles/ ^mm2 . As a result, melting cracks occurred.

In test number 58, the N content was too high, which resulted in hot working cracks.

In test numbers 59 and 60, the value of fn1 was too high. In other words, fn1 did not satisfy formula (1). As a result, sufficient machinability was not obtained.

In test numbers 61 and 62, the value of fn1 was too low. In other words, fn1 did not satisfy formula (1). As a result, sufficient fatigue strength was not obtained.

In test numbers 63 to 65, 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 66 to 68, the time T (min) from the addition of Bi to the end of stirring in the refining process 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 (7)

A steel material,
In mass percent,
C: 0.05-0.30%,
Si: 0.05-0.45%,
Mn: 0.30-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-2.00%,
Bi: 0.0051 to 0.1500%, and
N: 0.0030 to 0.0250%;
the balance being Fe and impurities, satisfying 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.
0.25≦C+(Si/10)+(Mn/5)-(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, the content of the corresponding element in mass% is substituted for each element symbol in the formula. When an element is not contained, "0" is substituted for the corresponding element symbol. A steel material,
In mass percent,
C: 0.05-0.30%,
Si: 0.05-0.45%,
Mn: 0.30-2.00%,
P: 0.030% or less,
S: 0.010-0.095%,
Cr: 0.01-2.00%,
Bi: 0.0051 to 0.1500%, and
N: 0.0030 to 0.0250%;
Further, it contains one or more selected from the group consisting of Groups 1 to 5,
the balance being Fe and impurities, satisfying 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.
[Group 1]
Al: 0.060% or less, and
Mg: 0.0100% or less, one or more selected from the group consisting of [Group 2]
Ti: 0.1500% or less,
Nb: 0.0800% or less,
W: 0.4000% or less, and
Zr: 0.2000% or less, one or more selected from the group consisting of [Group 3]
Ca: 0.0100% or less,
Te: 0.0100% or less,
B: 0.0050% or less,
Sn: 0.0100% or less, and
Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Group 4]
Co: 0.0100% or less,
Se: 0.0100% or less,
Sb: 0.0100% or less, and
In: 0.0100% or less, one or more selected from the group consisting of [Group 5]
V: 0.200% or less,
Mo: 1.00% or less,
Cu: 0.20% or less, and
Ni: 0.20% or less, one or more selected from the group consisting of 0.25≦C+(Si/10)+(Mn/5)−(5S/7)+(5Cr/22)+1.65V≦1.00 (1)
Here, the content of the corresponding element in mass% is substituted for each element symbol in the formula. When an element is not contained, "0" is substituted for the corresponding element symbol. The steel material according to claim 2,
Containing the first group,
Steel material. The steel material according to claim 2,
Containing the second group,
Steel material. The steel material according to claim 2,
Containing the third group,
Steel material. The steel material according to claim 2,
Containing the fourth group,
Steel material. The steel material according to claim 2,
Containing the fifth group
Steel material.

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