WO2025169959A1 - Steel material and component for mechanical structure - Google Patents

Abstract

Provided is a steel material having excellent cold forgeability. A steel material according to the present disclosure has the chemical composition described in the description. When ND₀ is defined as the number density (molecules/mm²) of Mn sulfide, ND₁ is defined as the number density (molecules/mm²) of Mn sulfide containing Cu-Ni, and ND₂ is defined as the number density (molecules/mm²) of Ti-containing Mn sulfide, ND₁ (molecules/mm²) is 1.00 or greater and satisfies formula (1) and formula (2).　Formula (1): ND₁ + ND₂ ≥ 2.00; Formula (2): (ND₁ + ND₂)/ND₀ < 0.25

Description

Steel materials and machine structural parts

This disclosure relates to steel materials and machine structural components made from such steel materials.

High strength is required for mechanical structural parts used in industrial machinery, automobiles, etc. For this reason, steel is generally used as the material for these parts. Among mechanical structural parts, bolts in particular, cold forging is often used in the manufacturing process when steel is used as the raw material. This is to increase yield and reduce manufacturing costs.

The manufacturing process for a bolt using cold forging is, for example, as follows: The raw steel material is subjected to wire drawing. After wire drawing, the steel is subjected to heat treatment (e.g., annealing) to soften the steel. After heat treatment, the steel is cold forged to produce an intermediate product in the shape of a bolt with a head and shank. The intermediate product is then quenched and tempered to produce the bolt.

In recent years, from the perspective of energy conservation and further reducing manufacturing costs, the omission of heat treatment before the cold forging process has been considered. In order to omit heat treatment before the cold forging process, the steel material used to make the bolts must have excellent cold forgeability. Furthermore, even when the shape of the final product is complex, manufacturing costs can be reduced by using cold forging rather than hot forging. Even in this case, the steel material used to make the bolts must have excellent cold forgeability.

Technology for improving the cold forgeability of steel materials used to make bolts is proposed in Japanese Patent Laid-Open Publication No. 2006-274373 (Patent Document 1) and International Publication No. 2020/090149 (Patent Document 2).

The steel material for bolts disclosed in Patent Document 1 has a composition, by mass, of C: 0.07-0.15%, Si: 0.2% or less, Mn: 0.5-2%, P: 0.015% or less, S: 0.015% or less, Cr: 2% or less, Al: 0.005-0.08%, N: 0.01% or less, a carbon equivalent (Ceq = C + Si/7 + Mn/6 + Cr/9) of 0.50% or less, with the remainder being iron and unavoidable impurities. By limiting the carbon equivalent in the steel, the precipitation of cementite at grain boundaries can be suppressed, reducing embrittlement of the steel material. Furthermore, Patent Document 1 states that by including the above alloying elements within appropriate ranges, excellent cold forgeability can be achieved.

The steel material for bolts disclosed in Patent Document 2 contains, in mass %, C: 0.18-0.24%, Si: 0.10-0.22%, Mn: 0.60-1.00%, Al: 0.010-0.050%, Cr: 0.65-0.95%, Ti: 0.010-0.050%, B: 0.0015-0.0050%, N: 0.0050-0.0100%, P: 0.025% or less (including 0%), S: The steel material for bolts disclosed in Patent Document 2 has a chemical composition containing 0.025% or less (inclusive), 0.20% or less (inclusive), and 0.30% or less (inclusive), with the following ranges satisfying 0.45≦C+Si/24+Mn/6+Ni/40+Cr/5≦0.60 and N≦0.519Al+0.292Ti, with the balance being Fe and unavoidable impurities, and has a microstructure in which bainite accounts for 95% or more of its area fraction. Furthermore, the prior austenite grains in the microstructure have a grain size number of 6 or more, and the strength variation is within 100 MPa. In this steel material, the area fraction of the bainite structure is increased and the prior austenite grains are refined. Patent Document 2 states that this results in a greater Bauschinger effect and reduces deformation resistance during cold forging to form the bolt head.

Japanese Patent Application Laid-Open No. 2006-274373 International Publication No. 2020/090149

However, cold forgeability may also be improved by means other than those of the steel materials disclosed in Patent Documents 1 and 2.

The purpose of this disclosure is to provide steel materials with excellent cold forgeability, and machine structural components that exhibit excellent cold forgeability during the manufacturing process.

The steel material of the present disclosure comprises, in mass%,
C: 0.04 to less than 0.20%
Si: 0.01-0.35%,
Mn: 0.20-1.00%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
Cu: 0.01-0.40%,
Ni: 0.01 to 0.30%,
Cr: 0.01-0.30%,
Mo: 0.001-0.200%,
Sn: 0.001 to 0.100%,
P: 0.040% or less,
S: 0.040% or less,
N: 0.0150% or less,
O: 0.0030% or less,
B: 0 to 0.0010%,
Nb: 0 to 0.050%,
V: 0 to 0.15%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Pb: 0 to 0.090%,
Ca: 0 to 0.0050%, and
Mg: 0 to 0.0050%;
the balance being Fe and impurities;
The number density (pieces/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, and an Mn content of 10% or more, in mass%, is defined as ND _0;
The number density (pieces/mm 2 ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% ^{or more} , in mass%, is defined as ND ₁ ;
When the number density (pieces/mm 2 ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% ^{or more} is defined as ND ₂ ,
ND ₁ (pieces/mm ² ) is 1.00 or more,
Formula (1) and formula (2) are satisfied.
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2)

The machine structural component of the present disclosure comprises, in mass %,
C: 0.04 to less than 0.20%
Si: 0.01-0.35%,
Mn: 0.20-1.00%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
Cu: 0.01-0.40%,
Ni: 0.01 to 0.30%,
Cr: 0.01-0.30%,
Mo: 0.001-0.200%,
Sn: 0.001 to 0.100%,
P: 0.040% or less,
S: 0.040% or less,
N: 0.0150% or less,
O: 0.0030% or less,
B: 0 to 0.0010%,
Nb: 0 to 0.050%,
V: 0 to 0.15%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Pb: 0 to 0.090%,
Ca: 0 to 0.0050%, and
Mg: 0 to 0.0050%;
the balance being Fe and impurities;
The number density (pieces/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, and an Mn content of 10% or more, in mass%, is defined as ND _0;
The number density (pieces/mm 2 ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% ^{or more} , in mass%, is defined as ND ₁ ;
When the number density (pieces/mm 2 ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% ^{or more} is defined as ND ₂ ,
ND ₁ (pieces/mm ² ) is 1.00 or more,
Formula (1) and formula (2) are satisfied.
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2)

The steel material disclosed herein exhibits excellent cold forgeability. The machine structural components disclosed herein exhibit excellent cold forgeability during the manufacturing process.

The inventors first investigated the chemical composition of steel materials that would provide excellent cold forgeability. As a result, the inventors determined that the following components, by mass, were desirable: C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less. It was believed that excellent cold forgeability would be achieved with a chemical composition consisting of S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0-0.0010%, Nb: 0-0.050%, V: 0-0.15%, Sb: 0-0.050%, As: 0-0.050%, Pb: 0-0.090%, Ca: 0-0.0050%, Mg: 0-0.0050%, and the balance being Fe and impurities.

However, it has been discovered that even steel materials that meet the above-mentioned chemical composition may have poor cold forgeability. Therefore, the inventors investigated means of improving cold forgeability from the perspective of the microstructure. As a result, the inventors have come to the following conclusions.

(A) In the manufacturing process of bolts made from steel, a quenching process is carried out to increase the strength of the bolt. Cu and Ni are dispersed in the matrix of the steel, improving the hardenability of the steel. This further increases the strength of bolts manufactured from steel. However, even small amounts of Cu and Ni also increase the strength of the steel after the hot working process. As a result, the cold forgeability of the steel decreases.
On the other hand, when Mn sulfides are present near Cu and Ni, Cu and Ni are concentrated in the Mn sulfides. Therefore, during the hot working process of steel, the Cu and Ni concentrations are locally reduced around the Mn sulfides, resulting in a decrease in hardenability. As a result, the ferrite volume fraction of the steel increases, improving cold forgeability.
Here, Mn sulfides are defined as inclusions having an S content of 10% or more and an Mn content of 10% or more in element concentration analysis in mass% using EDX, which will be described later.

(B) Cu and Ni in steel that did not concentrate into Mn sulfides during the hot working process are re-diffused during heating in the quenching process when manufacturing a bolt. If the number density of Mn sulfides in which Cu and Ni are concentrated (hereinafter referred to as "Cu-Ni-containing Mn sulfides") is already high before the quenching process, concentration of Cu and Ni into Mn sulfides is unlikely to occur during the quenching process. Therefore, localized deterioration of hardenability around the Mn sulfides is suppressed. As a result, sufficient strength can be ensured in bolts made from steel. In other words, in order to improve the cold forgeability of steel while maintaining the strength of bolts made from steel, it is effective to increase the number density of Cu-Ni-containing Mn sulfides in the steel.
Here, the Cu-Ni-containing Mn sulfides are defined as inclusions having an S content of 10% or more, a total Cu and Ni content of 5% or more, and a Mn content of 10% or more, as determined by element concentration analysis in mass% using EDX, which will be described later.

(C) Mn sulfides in steel are generally coarse and elongated in the rolling direction of the steel. Such Mn sulfides become the starting point of cracks during cold forging. Among the Mn sulfides, the above-mentioned Cu—Ni-containing Mn sulfides and Ti-containing Mn sulfides, which are Mn sulfides containing Ti, are finer and less likely to elongate in the rolling direction of the steel compared to other Mn sulfides other than the Cu—Ni-containing Mn sulfides and the Ti-containing Mn sulfides (hereinafter referred to as "normal Mn sulfides"). Therefore, the Cu—Ni-containing Mn sulfides and the Ti-containing Mn sulfides are less likely to become the starting point of cracks during cold forging. In other words, by increasing the number density of the Cu—Ni-containing Mn sulfides and the number density of the Ti-containing Mn sulfides, the cold forgeability of the steel is improved.
Here, the Ti-containing Mn sulfides are defined as inclusions having an S content of 10% or more, a total Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more, as determined by element concentration analysis in mass% using EDX, which will be described later.

(D) In steel with the above-mentioned chemical composition, the Mn sulfides may be primarily composed of Cu-Ni-containing Mn sulfides, Ti-containing Mn sulfides, and ordinary Mn sulfides. In the Mn sulfide generation process, if the number density of Cu-Ni-containing Mn sulfides and the number density of Ti-containing Mn sulfides are high, the amount of S available for the growth of ordinary Mn sulfides will decrease. In this case, the higher the number density of ordinary Mn sulfides, the more the coarsening of individual ordinary Mn sulfides is suppressed. Therefore, the ordinary Mn sulfides also become finer. As a result, ordinary Mn sulfides are less likely to become the starting point for cracks during cold forging, further improving the cold forgeability of the steel.

Based on the above findings, the present inventors investigated and studied the relationship between the number density of Mn sulfides, the number density of Cu-Ni-containing Mn sulfides in the Mn sulfides, the number density of Ti-containing Mn sulfides in the Mn sulfides, and cold forgeability. As a result, they found that when the number density (pieces/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more is defined as ND ₀ , the number density (pieces/mm ² ) of Cu-Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more is defined as ND ₁ , and the number density (pieces/mm ² ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more is defined as ND ₂ , a steel material having excellent cold forgeability can be obtained if ND ₁ (pieces/mm ² ) is 1.00 or more and satisfies formulas (1) and (2).
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2)

The steel material and machine structural components of this embodiment were completed based on the above technical concept and have the following configuration.

The steel material of the first configuration is
In mass%,
C: 0.04 to less than 0.20%
Si: 0.01-0.35%,
Mn: 0.20-1.00%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
Cu: 0.01-0.40%,
Ni: 0.01 to 0.30%,
Cr: 0.01-0.30%,
Mo: 0.001-0.200%,
Sn: 0.001 to 0.100%,
P: 0.040% or less,
S: 0.040% or less,
N: 0.0150% or less,
O: 0.0030% or less,
B: 0 to 0.0010%,
Nb: 0 to 0.050%,
V: 0 to 0.15%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Pb: 0 to 0.090%,
Ca: 0 to 0.0050%, and
Mg: 0 to 0.0050%;
the balance being Fe and impurities;
The number density (pieces/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, and an Mn content of 10% or more, in mass%, is defined as ND _0;
The number density (pieces/mm 2 ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% ^{or more} , in mass%, is defined as ND ₁ ;
When the number density (pieces/mm 2 ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% ^{or more} is defined as ND ₂ ,
ND ₁ (pieces/mm ² ) is 1.00 or more,
Formula (1) and formula (2) are satisfied.
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2)

The steel material of the second configuration is
A steel material of a first configuration,
In mass%,
B: 0.0001 to 0.0010%,
Nb: 0.001 to 0.050%,
V: 0.01-0.15%,
Sb: 0.001 to 0.050%,
As: 0.001 to 0.050%,
Pb: 0.001-0.090%,
Ca: 0.0001 to 0.0050%, and
Mg: 0.0001 to 0.0050%.

The machine structural component of the first configuration comprises:
In mass%,
C: 0.04 to less than 0.20%
Si: 0.01-0.35%,
Mn: 0.20-1.00%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
Cu: 0.01-0.40%,
Ni: 0.01 to 0.30%,
Cr: 0.01-0.30%,
Mo: 0.001-0.200%,
Sn: 0.001 to 0.100%,
P: 0.040% or less,
S: 0.040% or less,
N: 0.0150% or less,
O: 0.0030% or less,
B: 0 to 0.0010%,
Nb: 0 to 0.050%,
V: 0 to 0.15%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Pb: 0 to 0.090%,
Ca: 0 to 0.0050%, and
Mg: 0 to 0.0050%;
the balance being Fe and impurities;
The number density (pieces/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, and an Mn content of 10% or more, in mass%, is defined as ND _0;
The number density (pieces/mm 2 ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% ^{or more} , in mass%, is defined as ND ₁ ;
When the number density (pieces/mm 2 ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% ^{or more} is defined as ND ₂ ,
ND ₁ (pieces/mm ² ) is 1.00 or more,
Formula (1) and formula (2) are satisfied.
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2)

The machine structural component of the second configuration comprises:
A machine structural component having a first configuration,
In mass%,
B: 0.0001 to 0.0010%,
Nb: 0.001 to 0.050%,
V: 0.01-0.15%,
Sb: 0.001 to 0.050%,
As: 0.001 to 0.050%,
Pb: 0.001-0.090%,
Ca: 0.0001 to 0.0050%, and
Mg: 0.0001 to 0.0050%.

The third configuration of the machine structural part is
A machine structural component having a first or second configuration,
The machine structural part is a bolt.

The steel material of this embodiment and the machine structural component of this embodiment will be described below. In the following description, "%" in relation to element content means mass % unless otherwise specified.

[Features of the steel material according to this embodiment]
The steel material of this embodiment satisfies the following characteristics.
(Feature 1)
The chemical composition is, in mass%, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities.
(Feature 2)
The number density _ND1 of the Cu-Ni-containing Mn sulfides is 1.00 pieces/ ^mm2 or more.
(Feature 3)
The number density ND ₀ (pieces/mm ² ) of Mn sulfides, the number density ND ₁ (pieces/mm ² ) of Cu—Ni-containing Mn sulfides, and the number density ND ₂ (pieces/mm ² ) of Ti-containing Mn sulfides satisfy the formulas (1) and (2).
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2)
Features 1 to 3 will be explained below.

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

C: 0.04 to less than 0.20% Carbon (C) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the C content is less than 0.04%, 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 C content is 0.20% or more, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be excessively increased, and therefore the cold forgeability of the steel material will be reduced.
Therefore, the C content is 0.04 to less than 0.20%.
The lower limit of the C content is preferably 0.06%, and more preferably 0.08%.
The upper limit of the C content is preferably 0.19%, more preferably 0.17%, and even more preferably 0.15%.

Si: 0.01~0.35%
Silicon (Si) solid-solution strengthens steel. Si also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Si content exceeds 0.35%, the strength of the steel material will be excessively increased even if the contents of other elements are within the ranges of this embodiment, and therefore the cold forgeability of the steel material will be reduced.
Therefore, the Si content is 0.01 to 0.35%.
The lower limit of the Si content is preferably 0.02%, and more preferably 0.03%.
The upper limit of the Si content is preferably 0.32%, more preferably 0.30%, and even more preferably 0.25%.

Mn: 0.20-1.00%
Manganese (Mn) solid-solution strengthens steel. Mn also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Mn content is less than 0.20%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Mn content exceeds 1.00%, coarse Mn sulfides are formed in excess, and therefore the cold forgeability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Mn content is 0.20 to 1.00%.
The lower limit of the Mn content is preferably 0.22%, more preferably 0.25%, and even more preferably 0.30%.
The upper limit of the Mn content is preferably 0.95%, and more preferably 0.90%.

Al: 0.001-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.001%, 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 Al content exceeds 0.100%, coarse Al-based inclusions are generated even if the contents of other elements are within the ranges of this embodiment. The coarse Al-based inclusions become the starting points for cracks during cold forging. As a result, the cold forgeability of the steel material is reduced.
Therefore, the Al content is 0.001 to 0.100%.
The lower limit of the Al content is preferably 0.005%, and more preferably 0.010%.
The upper limit of the Al content is preferably 0.080%, and more preferably 0.070%.
In this embodiment, the Al content means the total Al (Total-Al) content.

Ti: 0.001-0.100%
Titanium (Ti) forms Ti-containing Mn sulfides. As described above, the formation of Ti-containing Mn sulfides suppresses coarsening of Mn sulfides. Furthermore, Ti-containing Mn sulfides are less likely to elongate during processing than normal Mn sulfides. Therefore, in steel materials during cold forging, the occurrence of cracks originating from elongated Mn sulfides is suppressed. As a result, the cold forgeability of the steel material is improved. If the Ti content is less than 0.001%, 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 Ti content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, coarse Ti carbides, Ti nitrides, and Ti carbonitrides are formed. These Ti inclusions or Ti precipitates become the starting points for cracks during cold forging. As a result, the cold forgeability of the steel material is reduced.
Therefore, the Ti content is 0.001 to 0.100%.
The lower limit of the Ti content is preferably 0.003%, more preferably 0.010%, and even more preferably 0.015%.
The upper limit of the Ti content is preferably 0.080%, and more preferably 0.070%.

Cu: 0.01~0.40%
Copper (Cu) solid-solution strengthens steel. Cu also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Cu content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Cu content exceeds 0.40%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes excessively high, and therefore the cold forgeability of the steel material deteriorates.
Therefore, the Cu content is 0.01 to 0.40%.
The lower limit of the Cu content is preferably 0.02%, and more preferably 0.04%.
The upper limit of the Cu content is preferably 0.38%, more preferably 0.34%, and even more preferably 0.30%.

Ni: 0.01~0.30%
Nickel (Ni) solid-solution strengthens steel. Ni also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. If the Ni content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Ni content exceeds 0.30%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes excessively high, and therefore the cold forgeability of the steel material deteriorates.
Therefore, the Ni content is 0.01 to 0.30%.
The lower limit of the Ni content is preferably 0.02%, and more preferably 0.04%.
The upper limit of the Ni content is preferably 0.29%, more preferably 0.27%, even more preferably 0.24%, and still more preferably 0.20%.

Cr:0.01~0.30%
Chromium (Cr) solid-solution strengthens steel. Cr also improves the hardenability of steel. As a result, the strength of bolts manufactured using the steel is increased. 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 ranges of this embodiment.
On the other hand, if the Cr content exceeds 0.30%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes excessively high, and therefore the cold forgeability of the steel material deteriorates.
Therefore, the Cr content is 0.01 to 0.30%.
The lower limit of the Cr content is preferably 0.02%, and more preferably 0.04%.
The upper limit of the Cr content is preferably 0.27%, more preferably 0.25%, even more preferably 0.20%, and still more preferably 0.15%.

Mo: 0.001-0.200%
Molybdenum (Mo) improves the hardenability of steel and increases the strength of bolts manufactured using the steel. If the Mo content is less than 0.001%, 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 Mo content exceeds 0.200%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes excessively high, and therefore the cold forgeability of the steel material deteriorates.
Therefore, the Mo content is 0.001 to 0.200%.
The lower limit of the Mo content is preferably 0.005%, more preferably 0.008%, and even more preferably 0.010%.
The upper limit of the Mo content is preferably 0.180%, more preferably 0.150%, and even more preferably 0.100%.

Sn: 0.001-0.100%
Tin (Sn) segregates at the interface between the matrix and Mn sulfides, embrittling the steel material. This improves the machinability of the steel material. If the Sn content is less than 0.001%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
However, if the Sn content exceeds 0.100%, Sn will segregate excessively, and therefore the cold forgeability of the steel material will be reduced even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Sn content is 0.001 to 0.100%.
The lower limit of the Sn content is preferably 0.002%, and more preferably 0.004%.
The upper limit of the Sn content is preferably 0.092%, more preferably 0.090%, even more preferably 0.080%, and still more preferably 0.070%.

P: 0.040% or less Phosphorus (P) is an impurity. If the P content exceeds 0.040%, P segregates excessively at grain boundaries, reducing grain boundary strength. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material will be reduced.
Therefore, the P content is 0.040% 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%, even more preferably 0.002%, and still more preferably 0.003%.
The upper limit of the P content is preferably 0.035%, more preferably 0.030%, and even more preferably 0.025%.

S: 0.040% or less Sulfur (S) is an impurity. S combines with Mn to form Mn sulfides. If the S content exceeds 0.040%, excessive coarse Mn sulfides are formed. The coarse Mn sulfides become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced.
Therefore, the S content is 0.040% or less.
The S content is preferably as low as possible. However, if the S content is reduced too much, the production cost increases. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%.
The upper limit of the S content is preferably 0.035%, more preferably 0.030%, and even more preferably 0.025%.

N: 0.0150% or less Nitrogen (N) is an impurity. N combines with Al, Ti, B, etc. to form nitrides. If the N content exceeds 0.0150%, excessive coarse nitrides are generated. These coarse nitrides become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced.
Therefore, the N content is 0.0150% or less.
The N content is preferably as low as possible. However, if the N content is reduced too much, the production cost increases. Therefore, in consideration of normal industrial production, the lower limit of the N content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0010%, and still more preferably 0.0020%.
The upper limit of the N content is preferably 0.0130%, more preferably 0.0100%, and even more preferably 0.0080%.

O: 0.0030% or less Oxygen (O) is an impurity. O combines with other elements in the steel to form oxides. If the O content exceeds 0.0030%, excessive coarse oxides are formed. These coarse oxides become the starting points for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel is reduced.
Therefore, the O content is 0.0030% or less.
The O content is preferably as low as possible. However, if the O content is reduced too much, the production cost increases. Therefore, in consideration of normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0001%, and even more preferably 0.0002%.
The upper limit of the O content is preferably 0.0027%, more preferably 0.0024%, and even more preferably 0.0020%.

The balance of the chemical composition of the steel material of this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, when steel material is industrially manufactured, and are acceptable within the range that does not adversely affect the steel material of this embodiment.

[Optional Elements]
The chemical composition of the steel material of this embodiment further contains, instead of a part of Fe,
B: 0 to 0.0010%,
Nb: 0 to 0.050%,
V: 0 to 0.15%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Pb: 0 to 0.090%,
Ca: 0 to 0.0050%, and
Mg: 0 to 0.0050%.
These elements are all optional elements, and will be explained below.

[Group 1: B, Nb and V]
The chemical composition of the steel material of the present embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of B, Nb, and V. Any of these elements increases the strength of a bolt manufactured using the steel material as a raw material.

B: 0-0.0010%
Boron (B) is an optional element and may not be contained, that is, the B content may be 0%.
When boron (B) is contained, that is, when the B content is more than 0%, it improves the hardenability of the steel material. As a result, the strength of the bolt manufactured from the steel material is increased. Even if even a small amount of B is contained, the above effect can be obtained to some extent.
However, if the B content exceeds 0.0010%, coarse B nitrides are formed. These coarse B nitrides become the starting points for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced.
Therefore, the B content is 0 to 0.0010%, and if B is contained, the B content is 0.0010% or less.
The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.
The upper limit of the B content is preferably 0.0009%, more preferably 0.0008%, and even more preferably 0.0006%.

Nb: 0-0.050%
Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%.
When Nb is contained, that is, when the Nb content is more than 0%, Nb forms Nb precipitates such as carbides, nitrides, and carbonitrides. The Nb precipitates increase the strength of bolts manufactured from the steel material through precipitation strengthening. 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 0.050%, coarse Nb precipitates are formed. The coarse Nb precipitates become the starting points for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced.
Therefore, the Nb content is 0 to 0.050%, and when Nb is contained, the Nb content is 0.050% or less.
The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.005%.
The upper limit of the Nb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

V: 0 to 0.15%
Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%.
When V is contained, that is, when the V content exceeds 0%, V forms V precipitates such as carbides and carbonitrides. The V precipitates increase the strength of bolts manufactured from the steel material through precipitation strengthening. Even if even a small amount of V is contained, the above effect can be obtained to some extent.
However, if the V content exceeds 0.15%, coarse V precipitates are formed. These coarse V precipitates become the starting points for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced.
Therefore, the V content is 0 to 0.15%, and if V is contained, the V content is 0.15% or less.
The lower limit of the V content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.04%.
The upper limit of the V content is preferably 0.13%, more preferably 0.10%, and even more preferably 0.08%.

[Group 2: Sb, As and Pb]
The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Sb, As and Pb in place of a portion of Fe. All of these elements improve the machinability of the steel material.

Sb: 0 to 0.050%
Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%.
When Sb is contained, that is, when the Sb content exceeds 0%, Sb segregates at the interface between the matrix and Mn sulfides, embrittling the steel material. This improves the machinability of the steel material. 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 0.050%, Sb segregates excessively, which reduces the cold forgeability of the steel material even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Sb content is 0 to 0.050%, and when Sb is contained, the Sb content is 0.050% or less.
The lower limit of the Sb content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.004%.
The upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

As: 0~0.050%
Arsenic (As) is an optional element and may not be contained, that is, the As content may be 0%.
When As is contained, that is, when the As content exceeds 0%, As segregates at the interface between the matrix and Mn sulfides, embrittling the steel material. As a result, the machinability of the steel material is improved. Even if even a small amount of As is contained, the above effect can be obtained to some extent.
However, if the As content exceeds 0.050%, As segregates excessively, and therefore the cold forgeability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the As content is 0 to 0.050%, and when As is contained, the As content is 0.050% or less.
The lower limit of the As content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.004%.
The upper limit of the As content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

Pb: 0-0.090%
Lead (Pb) is an optional element and may not be contained, that is, the Pb content may be 0%.
When Pb is contained, that is, when the Pb content exceeds 0%, Pb segregates at the interface between the matrix and Mn sulfides, embrittling the steel material. This improves the machinability of the steel material. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent.
However, if the Pb content exceeds 0.090%, excessive Pb segregation occurs, and therefore the cold forgeability of the steel material deteriorates even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Pb content is 0 to 0.090%, and if Pb is contained, the Pb content is 0.090% or less.
The lower limit of the Pb content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.004%.
The upper limit of the Pb content is preferably 0.080%, more preferably 0.070%, and even more preferably 0.060%.

[Third group: Ca and Mg]
The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. Any of these elements refines Mn sulfides in the steel material and improves the cold forgeability of the steel material.

Ca: 0-0.0050%
Calcium (Ca) is an optional element and may not be contained. In other words, the Ca content may be 0%. When Ca is contained, that is, when the Ca content is more than 0%, Ca refines Mn sulfides. Therefore, the cold forgeability of the steel material is improved. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent.
However, if the Ca content exceeds 0.0050%, coarse Ca oxides are generated, which become the starting points for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced.
Therefore, the Ca content is 0 to 0.0050%, and when Ca is contained, the Ca content is 0.0050% or less.
The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%.
The upper limit of the Ca content is preferably 0.0040%, and more preferably 0.0030%.

Mg: 0-0.0050%
Magnesium (Mg) is an optional element and may not be contained. In other words, the Mg content may be 0%. When Mg is contained, that is, when Mg exceeds 0%, Mg refines Mn sulfides. Therefore, the cold forgeability of the steel material is improved. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent.
However, if the Mg content exceeds 0.0050%, coarse Mg oxides are generated, which become the starting points for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material is reduced.
Therefore, the Mg content is 0 to 0.0050%, and when Mg is contained, the Mg content is 0.0050% or less.
The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%.
The upper limit of the Mg content is preferably 0.0040%, and more preferably 0.0030%.

[(Feature 2) Number density ND ₁ of Cu—Ni-containing Mn sulfides]
As described above, inclusions (particles) having an S content of 10% or more, a Cu and Ni total content of 5% or more, and a Mn content of 10% or more in element concentration analysis in mass% using EDX, which will be described later, are defined as Cu-Ni-containing Mn sulfides.
Furthermore, in the steel material of this embodiment, the number density ND ₁ (pieces/mm ² ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more is 1.00 or more.

When Mn sulfides are present in the vicinity of Cu and Ni, Cu and Ni are usually concentrated in the Mn sulfides. As a result, Cu-Ni-containing Mn sulfides are formed. As described above, in order to improve the cold forgeability of a steel material while maintaining the strength of a bolt made from the steel material, it is preferable that the number density ND ₁ (pieces/mm ² ) of the Cu-Ni-containing Mn sulfides is high. If the number density ND ₁ (pieces/mm ² ) of the Cu-Ni-containing Mn sulfides is 1.00 or more, excellent cold forgeability can be obtained, provided that the steel material satisfies Features 1 and 3.

The lower limit of the number density _ND1 is preferably 1.10, more preferably 1.30, and even more preferably 1.50.
There is no particular limitation on the upper limit of the number density ND _1. When the steel material satisfies the characteristics 1 and 3, the upper limit of the number density ND ₁ is, for example, 5.00, for example, 4.00, or for example, 3.00.

[(Feature 3) Equation (1) and Equation (2)]
As described above, in the element concentration analysis in mass% using EDX described later, inclusions (particles) having an S content of 10% or more and an Mn content of 10% or more are defined as Mn sulfides.
In element concentration analysis in mass% using EDX, which will be described later, inclusions (particles) having an S content of 10% or more, a total Cu and Ni content of less than 5%, a Ti content of 10% or more, and a Mn content of 10% or more are defined as Ti-containing Mn sulfides.
Furthermore, in the steel material of this embodiment, the number density ND ₀ (pieces/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more, the number density ND ₁ (pieces/mm 2 ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, and the number density ND ₂ (pieces/mm ² ) of Ti-containing Mn sulfides having an equivalent circle diameter of ^1.0 μm or more satisfy formulas (1) and (2).
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2)
The formulas (1) and (2) will be explained below.

[Formula (1)]
It is defined as F1 = ND ₁ + ND _2. As described above, Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides are finer and less likely to elongate in the axial direction of the steel material than normal Mn sulfides. Therefore, Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides are less likely to become the starting point of cracks during cold forging. Furthermore, if the number density of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides is high, the amount of S used for the growth of normal Mn sulfides is reduced. Therefore, on the premise that the above-mentioned formula (2) is satisfied, coarsening of normal Mn sulfides is suppressed. As a result, the cold forgeability of the steel material is improved.
If the total value F1 of the number density _ND1 of the Cu-Ni-containing Mn sulfides and the number density _ND2 of the Ti-containing Mn sulfides is 2.00 or more, the number densities of the Cu-Ni-containing Mn sulfides and the Ti-containing Mn sulfides are sufficiently high, and therefore, excellent cold forgeability can be obtained in the steel material.

The lower limit of F1 is preferably 2.10, more preferably 2.20, even more preferably 2.40, and still more preferably 2.60.
The upper limit of F1 is not particularly limited. When the steel material satisfies the characteristics 1 and 2, the upper limit of F1 is, for example, 10.00, for example, 8.00, for example, 7.00, or for example, 6.50.
Note that F1 is the value obtained by rounding off the obtained numerical value to two decimal places (i.e., the value to two decimal places).

[Formula (2)]
It is defined as F2 = (ND ₁ + ND ₂ ) / ND _0. F2 means the ratio of the number density of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides to the number density of Mn sulfides. When the total value F1 of the number densities of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides satisfies formula (1), if the number density ND ₀ of Mn sulfides is high, the number density of Mn sulfides is usually high. As described above, when the total value F1 of the number densities of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides satisfies formula (1), the amount of S available for the growth of Mn sulfides is usually reduced.
If F2 is less than 0.25, the number density of the normal Mn sulfides is sufficiently high while the amount of S available for the growth of the normal Mn sulfides is limited. Therefore, the normal Mn sulfides become sufficiently fine. Therefore, excellent cold forgeability can be obtained in the steel material, provided that formula (1) is satisfied.

The lower limit of F2 is not particularly limited, and is preferably 0.01, more preferably 0.03, even more preferably 0.04, and still more preferably 0.05.
The upper limit of F2 is preferably 0.24, more preferably 0.23, even more preferably 0.22, and still more preferably 0.20.
Note that F2 is the value obtained by rounding off the obtained numerical value to two decimal places (i.e., the value to two decimal places).

[Method for measuring number densities _ND0 , _ND1 and _ND2 ]
The number density ND ₀ of Mn sulfides, the number density ND ₁ of Cu—Ni-containing Mn sulfides, and the number density ND ₂ of Ti-containing Mn sulfides are determined by the following method.
Five test pieces are taken, each having a cross section including the axial and radial directions of the steel material, and a surface including an R/2 depth portion from the surface of the steel material. The R/2 depth portion refers to the center of a line segment (i.e., radius R) connecting the surface of the steel material to the center position of the steel material. The size of each test piece is not particularly limited. Of the surfaces of each test piece, the surface that includes the axial and radial directions of the steel material and that includes the above-mentioned R/2 depth portion is defined as the target surface. The R/2 depth portion corresponds to the center position of the target surface.

The collected test specimen is embedded in resin. The target surface of the resin-embedded test specimen is polished. The observation area within the polished target surface is observed using a scanning electron microscope (SEM) equipped with composition analysis capabilities. The observation area is a rectangle measuring 1200 μm x 960 μm, centered at the R/2 depth. The long side of the observation area corresponds to the axial direction of the steel material. During observation, the observation area is divided into 36 non-overlapping fields of 200 μm x 160 μm, and each field is observed at 500x magnification.

In the observation area, particles (precipitates or inclusions) with an equivalent circle diameter of 1.0 μm or more are identified based on the Z contrast of the backscattered electron image. In the backscattered electron image, particles appear with a darker contrast than the matrix. The equivalent circle diameter refers to the diameter of a circle when the area of a particle is converted into a circle with the same area. Each identified particle is subjected to elemental concentration analysis using energy dispersive X-ray spectroscopy (EDX) to identify Mn sulfides. The EDX analysis (elemental concentration analysis) uses the EDX-standardless method. The accelerating voltage is set to 20 kV, and the quantified elements are C, Si, Mn, P, S, Cr, Ti, Cu, Ni, Ca, N, O, and Al.

In the EDX analysis results of each particle, when the total content in mass% of the above quantified elements is taken as 100%, if the S content is 10% or more and the Mn content is 10% or more in mass%, the particle is identified as a Mn sulfide.
When the total mass % content of the above quantified elements is taken as 100%, if the S content is 10% or more, the total Cu and Ni content is 5% or more, and the Mn content is 10% or more, in mass %, the particle is identified as a Cu-Ni-containing Mn sulfide.
When the total content of the above quantified elements in mass % is taken as 100%, if the S content is 10% or more, the total content of Cu and Ni is less than 5%, the Ti content is 10% or more, and the Mn content is 10% or more in mass %, the particle is identified as a Ti-containing Mn sulfide.

The total number of Mn sulfides having an equivalent circle diameter of 1.0 μm or more identified in each observation region of the five test pieces is determined. Based on the total number of Mn sulfides obtained and the total area of each observation region of the five test pieces, the number density ND ₀ (numbers/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more is determined.
Similarly, the total number of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more identified in each observation region of the five test pieces is determined. Based on the total number of Cu—Ni-containing Mn sulfides obtained and the total area of each observation region of the five test pieces, the number density ND ₁ (numbers/mm ² ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more is determined.
Similarly, the total number of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more identified in each observation region of the five test pieces is calculated. Based on the total number of Ti-containing Mn sulfides thus calculated and the total area of each observation region of the five test pieces, the number density ND ₂ (numbers/mm ² ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more is calculated.
The number densities ND ₀ , ND ₁ , and ND ₂ are values obtained by rounding the obtained numerical values to two decimal places (that is, values to one decimal place).

[Effects of the steel material according to this embodiment]
As described above, the steel material of this embodiment satisfies Features 1 to 3. Therefore, the steel material of this embodiment has excellent cold forgeability.

[Shape of steel material according to this embodiment]
The steel material in this embodiment is a steel bar or wire rod. The steel bar or wire rod is a steel material that has a circular cross section perpendicular to the axial direction and extends in a rod shape. The steel material may be wound in a coil shape or cut to a predetermined length. The diameter of the cross section of the steel material is, for example, 4 to 20 mm.

[Use of the steel material according to this embodiment]
The steel material of this embodiment can be used as a material for machine structural parts such as bolts. The steel material of this embodiment is particularly suitable as a material for bolts, which are fastening means for industrial machinery, automobiles, bridges, buildings, etc. The steel material of this embodiment may also be used for applications other than those described above.

[Steel 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 the steel material according to this embodiment. Therefore, a steel material having the above-described 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 the method for manufacturing the steel material according to this embodiment includes the following steps.
(Process 1) Refining process (Process 2) Casting process (Process 3) Hot working process

Each step will be described below.
[(Process 1) Refining process]
In the refining process, molten steel having a chemical composition that satisfies the above-mentioned feature 1 is produced. The refining method is not particularly limited, and any known method may be used. For example, molten pig iron produced by a known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to known secondary refining. Through the above-mentioned processes, molten steel having a chemical composition that satisfies feature 1 is produced.

[(Process 2) Casting process]
In the casting process, the molten steel produced in the refining process is used to produce blooms (blooms) by continuous casting.

[(Step 3) Hot working step]
In the hot working process, the bloom is hot worked to produce a steel product. The hot working process includes the following steps:
(Step 31) Blooming step (Step 32) Finishing rolling step Each step will be explained below.

[(Step 31) Blooming rolling step]
In the blooming process, the bloom is hot-rolled (blooming) using a blooming mill to produce a billet. If a continuous rolling mill having a plurality of rolling stands arranged in a line is disposed downstream of the blooming mill, the billet may be hot-rolled using the continuous rolling mill to further reduce the size of the billet. The heating temperature in the blooming process may be within a known temperature range. For example, the heating temperature is 1100 to 1300°C. The billet produced in the blooming process is allowed to cool (air-cool) to room temperature before the finish rolling process.

[(Step 32) Finish rolling step]
In the finish rolling process, the billet is hot rolled (finish rolling) to produce the steel material of this embodiment. Specifically, the billet after the blooming process is heated using a heating furnace. The heating temperature is, for example, 1000 to 1250°C. The heated billet is hot rolled (finish rolling) using a finish rolling mill to produce a steel material having a circular cross section with a desired diameter. The finish rolling mill is, for example, a continuous rolling mill in which multiple rolling stands are arranged in a row so that the rolling direction of adjacent rolling stands is perpendicular.

[Manufacturing conditions for the manufacturing method of this embodiment]
The above-described manufacturing method satisfies the following conditions.
(Condition 1)
In the hot working step and subsequent steps, the total residence time t1 during which the material is heated to a temperature of 1200°C or higher is set to 120 minutes or less.
(Condition 2)
Of the total area reduction ratio TRR (Total Reduction Ratio) in the finish rolling process, the ratio X (%) of the total area reduction ratio RR ₉₈₀ in the finish rolling process at a steel material temperature of 980°C or less is defined by the following formula.
X= _RR980 /TRR×100
In this case, the following formula (A) is satisfied.
X-√((Ti+Cu+Ni+1)/(2×S))≧0 (A)
Here, each element symbol in formula (A) is substituted with the content of the corresponding element in mass %.
(Condition 3)
In the finish rolling process, the maximum area reduction rate in one pass when the steel material temperature exceeds 1000°C is defined as Y (%). At this time, the following formula (B) is satisfied.
Y-15/√(Cu+0.5×Ni+0.2)>0 (B)
Here, the content of each element in formula (B) in terms of mass % is substituted for each element symbol.
Conditions 1 to 3 will be explained below.

[Condition 1: total residence time t1 when the heating temperature of the material is 1200 ° C or higher]
If the total residence time t1 of the material at a heating temperature of 1200°C or higher exceeds 120 minutes in the steps subsequent to the hot working step (i.e., from the start of heating of the steel material in the blooming step to the completion of finish rolling in the finish rolling step and the cooling of the steel material after finish rolling to room temperature), the Cu precipitates, Ni precipitates, and Ti precipitates will coarsen. The coarsening of these precipitates occurs due to Ostwald ripening of the precipitates. During the Ostwald ripening process, fine Cu precipitates, Ni precipitates, and Ti precipitates are incorporated into the coarsened precipitates and disappear. Therefore, the amounts of Cu, Ni, and Ti available for forming Cu—Ni-containing Mn sulfides and Ti-containing Mn sulfides will decrease. As a result, the number densities of the Cu—Ni-containing Mn sulfides and Ti-containing Mn sulfides will decrease. In this case, the formula (1) will not be satisfied in the produced steel. Therefore, the total residence time t1 at 1200° C. or higher is set to 120 minutes or less.

The upper limit of the total residence time t1 at 1200° C. or higher is preferably 110 minutes, more preferably 100 minutes, and even more preferably 90 minutes.
The lower limit of the total residence time t1 at 1200°C or higher is not particularly limited. A preferred lower limit is 0 minutes, but considering industrial production, it is more preferably 10 minutes, even more preferably 20 minutes, and even more preferably 30 minutes. Note that if the heating temperature of the material does not reach 1200°C or higher in the processes after the hot working process, the total residence time t1 at 1200°C or higher is set to 0 minutes.

[Condition 2: Formula (A)]
FA is defined as X - √((Ti + Cu + Ni + 1) / (2 × S)). FA is an index that indicates the ease with which Mn sulfides are fragmented in the finish rolling process. In the finish rolling process, Mn sulfides tend to elongate in the rolling direction of the steel material. In the finish rolling process, Mn sulfides are likely to elongate, particularly when the steel material temperature is 980°C or lower. Furthermore, compared with Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides, normal Mn sulfides are significantly more likely to elongate.

If FA satisfies formula (A), a sufficient amount of rolling reduction has been applied to the steel when the steel temperature is below 980°C. In this case, Mn sulfides, in particular, typically elongate in the rolling direction and then break into multiple pieces, becoming finer, reducing the number of coarse Mn sulfides that can become crack initiation points during cold forging. As a result, the number density of Mn sulfides increases compared to the number densities of Cu-Ni-containing Mn sulfides and Ti-containing Mn sulfides, and the manufactured steel satisfies formula (2).

The lower limit of FA is preferably 0.5, more preferably 0.8, and even more preferably 1.0.
The upper limit of FA is not particularly limited, but is preferably 10.0, more preferably 8.0, even more preferably 6.0, and even more preferably 5.0.
In consideration of normal industrial production, the lower limit of X is, for example, 3.0%, and the upper limit of X is, for example, 15.0%.

[Condition 3: Formula (B)]
FB is defined as Y-15/√(Cu+0.5×Ni+0.2). FB is an index that indicates the ease with which Cu and Ni concentrate in Mn sulfides in the finish rolling process. In hot rolling using a finish rolling mill equipped with multiple rolling stands, the area reduction rate for each rolling stand of a billet passing through each rolling stand is defined as the "area reduction rate in one pass."

If the steel temperature exceeds 1000°C during the finish rolling process, Cu and Ni in the steel are more likely to diffuse. Therefore, rolling at a steel temperature above 1000°C and with a large area reduction further promotes the diffusion of Cu and Ni, accelerating the concentration of Cu and Ni in Mn sulfides. If FB is 0 or less, the concentration of Cu and Ni in Mn sulfides is insufficient. In this case, the steel does not satisfy Feature 2. Therefore, FB must be greater than 0.

The lower limit of FB is preferably 0.1, more preferably 0.2, even more preferably 0.3, and still more preferably 0.4.
The upper limit of FB is not particularly limited, but is preferably 15.0, more preferably 12.0, and even more preferably 8.0.
In consideration of normal industrial production, the lower limit of Y is, for example, 15.0%, and the upper limit of Y is, for example, 35.0%.

The steel material of this embodiment is manufactured through the above manufacturing process.

[Regarding the machine structural component of this embodiment]
The machine structural component of this embodiment uses the steel material of this embodiment as a material. The machine structural component of this embodiment is, for example, a bolt. The machine structural component of this embodiment may also be, for example, a nut. The machine structural component of this embodiment may also be, for example, a hollow component, or a cup-shaped component with one end open.

[Features of the machine structural component of this embodiment]
The machine structural component of this embodiment satisfies the following characteristics.
(Feature 4)
The chemical composition is, in mass%, C: 0.04 to less than 0.20%, Si: 0.01 to 0.35%, Mn: 0.20 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Cr: 0.01 to 0.30%, Mo: 0.001 to 0.200%, Sn: 0.001 to 0.100%, P: 0.040% or less, S: 0.040% or less, N: 0.0150% or less, O: 0.0030% or less, B: 0 to 0.0010%, Nb: 0 to 0.050%, V: 0 to 0.15%, Sb: 0 to 0.050%, As: 0 to 0.050%, Pb: 0 to 0.090%, Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities.
(Feature 5)
The number density _ND1 of the Cu-Ni-containing Mn sulfides is 1.00 pieces/ ^mm2 or more.
(Feature 6)
The number density ND ₀ (pieces/mm ² ) of Mn sulfides, the number density ND ₁ (pieces/mm ² ) of Cu—Ni-containing Mn sulfides, and the number density ND ₂ (pieces/mm ² ) of Ti-containing Mn sulfides satisfy the formulas (1) and (2).
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2)

The number density _ND0 of Mn sulfides, the number density _ND1 of Cu-Ni-containing Mn sulfides, and the number density _ND2 of Ti-containing Mn sulfides in a machine structural component are determined based on the method described in [Method for measuring number densities _ND0 , _ND1 , and _ND2 ]. However, the test piece used for the measurement is taken so that the target surface is a cross section including the depth direction from the surface of the machine structural component. The center position of the target surface corresponds to a position 1 mm deep from the surface of the machine structural component. The observation area on the target surface is a rectangle of 1200 μm × 960 μm centered at a position 1 mm deep from the surface of the machine structural component. The long side of the observation area is perpendicular to the depth direction from the surface of the machine structural component.

The effect of each element in Feature 4 is the same as the effect of the corresponding element in Feature 1 of the steel material of this embodiment. Feature 5 has the same technical significance as Feature 2 of the steel material of this embodiment. Feature 6 has the same technical significance as Feature 3 of the steel material of this embodiment. Therefore, the machine structural component of this embodiment, which satisfies Features 4 to 6, achieves excellent cold forgeability during the manufacturing process.

[Method of manufacturing machine structural parts]
The machine structural component of this embodiment is manufactured by a known method using the steel material of this embodiment as a raw material. The chemical composition of the machine structural component manufactured by a known method is the same as the chemical composition of the steel material used as the raw material. Furthermore, the number density ND ₀ of Mn sulfides, the number density ND ₁ of Cu-Ni-containing Mn sulfides, and the number density ND ₂ of Ti-containing Mn sulfides in the machine structural component manufactured by a known method are almost unchanged from those of the steel material used as the raw material. Therefore, the machine structural component of this embodiment can satisfy Features 4 to 6 if manufactured by a known method using the steel material of this embodiment that satisfies Features 1 to 3 as a raw material.

A manufacturing process for a bolt will be described as an example of a machine structural component of this embodiment. The manufacturing method for a bolt, which is a machine structural component of this embodiment, includes, for example, a wire drawing process, a cold forging process, and a quenching and tempering process.
In the wiredrawing process, a well-known wiredrawing process is performed on the steel material of this embodiment to produce a steel wire. The wiredrawing process may be a primary wiredrawing process only, or multiple wiredrawing processes such as a secondary wiredrawing process may be performed. In the cold forging process, a well-known cold forging (heading) is performed on the steel wire after the wiredrawing process to produce a bolt-shaped intermediate product.
In the quenching and tempering process, the intermediate product is quenched and tempered. Quenching is performed by a well-known method. The quenching temperature is, for example, 840 to 970°C. The holding time at the quenching temperature is, for example, 15 to 360 minutes (6 hours). After the holding time has elapsed, the intermediate product is rapidly cooled. Specifically, the intermediate product is water-cooled or oil-cooled. The quenched intermediate product is then tempered. The tempering temperature is, for example, 400 to 600°C. The holding time at the tempering temperature is, for example, 0.5 to 6.0 hours.

The above manufacturing method can be used to manufacture the bolt, which is the machine structural component of this embodiment. The machine structural component of this embodiment achieves excellent cold forgeability during the manufacturing process. Therefore, in the manufacturing process for the machine structural component of this embodiment, cracking of the material and deterioration of the mold during cold forging are suppressed, even without performing heat treatment for softening before cold forging.

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.

A well-known refining process was carried out to produce steel having the chemical composition shown in Table 1 (Table 1A and Table 1B).

Specifically, blooms were produced using the produced molten steel by continuous casting. The produced blooms were then subjected to blooming to produce billets. In the blooming process, the blooms were heated to 1100-1300°C and then hot rolled using a blooming mill. The produced billets were allowed to cool to room temperature. The produced billets were then subjected to a finish rolling process. In the finish rolling process, the billets were heated to 1000-1250°C. The heated billets were then hot rolled using a finish rolling mill to produce wire rod (steel material) with a diameter of 10.0 mm. Steel materials with each test number were produced using the above production process.

The total residence time t1 (minutes) during which the temperature of the steel (bloom, billet, and steel material) was 1200°C or higher from the start of heating of the bloom in the blooming process until the steel material after the finish rolling process was cooled to room temperature was as shown in the "t1 (minutes)" column in Table 2. In the finish rolling process, the proportions X (%) of the total area reduction rate RR ₉₈₀ and FA in the finish rolling process when the steel material temperature was 980°C or lower were as shown in the "X (%)" and "FA" columns in Table 2. In the finish rolling process, the maximum area reduction rates Y (%) and FB in one pass when the billet temperature exceeded 1000°C were as shown in the "Y (%)" and "FB" columns in Table 2.

[Evaluation test]
The following evaluation tests were carried out on the manufactured steel materials with each test number.
(Test 1) Number density ND ₀ , ND ₁ and ND ₂ measurement test (Test 2) Limit compression test Each test will be explained below.

[(Test 1) Measurement Test of Number Densities ND ₀ , ND ₁ and ND ₂ ]
Based on the method described above in [Method for measuring number densities _ND0 , _ND1 , and _ND2 ], the number density _ND0 (pieces/ ^mm2 ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more, the number density _ND1 (pieces/ ^mm2 ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, and the number density _ND2 (pieces/ ^mm2 ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more were determined. The results obtained are shown in the " _ND0 (pieces/ ^mm2 )" column, the " _ND1 (pieces/ ^mm2 )" column, and the " _ND2 (pieces/ ^mm2 )" column in Table 2, respectively.

[(Test 2) Limit Compression Test]
To evaluate cold forgeability, a limit compression test was conducted to determine the limit compression ratio of the steel material. Specifically, multiple limit compression ratio measurement specimens were taken from the steel material (wire rod) of each test number. The limit compression ratio measurement specimens were cylindrical, with a diameter of 8 mm and a length of 12 mm. The longitudinal direction of the limit compression ratio measurement specimen was parallel to the axial direction of the steel material of each test number. The central axis of the limit compression ratio measurement specimen corresponded to the central axis of the steel material of each test number. A single longitudinal notch was formed on the side (circumferential surface) of the test piece. The notch angle was 30 degrees, the notch depth was 0.46 mm, and the radius of curvature of the notch tip was 0.15 mm. The notch length was 12 mm, the same as the length of the test piece. Furthermore, a recess was formed on each of the pair of end faces of the test piece to secure the test piece to an end face restraint die. The depressions were formed at the center of each end face and had a conical shape. The diameter of the opening of the depression at the end face (corresponding to the base of the cone) was 2 mm, and the angle of the apex in a cross section including the central axis of the conical depression was 120°.

A 500-ton hydraulic press was used for the limit compression test. The limit compression test was conducted on the prepared limit compression ratio measurement specimens using the following method. Each specimen was cold compressed at a speed of 15 mm/s using an end-face restraint die with a convex portion corresponding to the depression in the specimen. Compression was stopped when microcracks of 0.5 mm or more appeared at the notch base, and the compression ratio (%) at that time was calculated. This measurement was performed a total of five times to determine the compression ratio (%) at which the cumulative failure probability was 50%. A limit compression ratio of 55% or greater was rated "E (Excellent)," indicating that excellent cold forgeability had been achieved. On the other hand, a limit compression ratio of less than 55% was rated "NA (Not Accepted)," indicating that excellent cold forgeability had not been achieved. The evaluation results are shown in the "Limit Compression Ratio" column in Table 2.

[Test Results]
With reference to Tables 1A, 1B, and 2, the steel materials of Test Nos. 1 to 15 satisfied Features 1 to 3. Therefore, the steel materials of these Test Nos. obtained excellent cold forgeability.

On the other hand, test number 16 had too high a C content. As a result, excellent cold forgeability was not achieved.

In test number 17, the Si content was too high. As a result, excellent cold forgeability was not achieved.

In test number 18, the Mn content was too high. As a result, excellent cold forgeability was not achieved.

In test number 19, the Al content was too high. As a result, excellent cold forgeability was not achieved.

In test number 20, the Ti content was too high. As a result, excellent cold forgeability was not achieved.

In test number 21, the Cu content was too high. Furthermore, FA was too low. As a result, F2 did not satisfy formula (2). As a result, excellent cold forgeability was not achieved.

In test number 22, the Ni content was too high. Furthermore, FA was too low. As a result, F2 did not satisfy formula (2). As a result, excellent cold forgeability was not achieved.

In test number 23, the Cr content was too high. As a result, excellent cold forgeability was not achieved.

In test number 24, the Mo content was too high. As a result, excellent cold forgeability was not achieved.

In test number 25, the Sn content was too high. As a result, excellent cold forgeability was not achieved.

In test number 26, the P content was too high. As a result, excellent cold forgeability was not achieved.

In test number 27, the S content was too high. As a result, excellent cold forgeability was not achieved.

In test number 28, the N content was too high. As a result, excellent cold forgeability was not achieved.

In test number 29, the O content was too high. As a result, excellent cold forgeability was not achieved.

In test numbers 30 and 31, the total residence time t1 at 1200°C or above was too long. As a result, F1 did not satisfy formula (1). As a result, excellent cold forgeability was not achieved.

In test numbers 32 and 33, FA was too low. As a result, F2 did not satisfy formula (2). As a result, excellent cold forgeability was not achieved.

In test numbers 34 and 35, FB was too low, and therefore _ND1 was too low, resulting in failure to obtain excellent cold forgeability.

The above describes an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by modifying the above-described embodiment as appropriate within the scope of the spirit of the present invention.

Claims (5)

In mass%,
C: 0.04 to less than 0.20%
Si: 0.01-0.35%,
Mn: 0.20-1.00%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
Cu: 0.01-0.40%,
Ni: 0.01 to 0.30%,
Cr: 0.01-0.30%,
Mo: 0.001-0.200%,
Sn: 0.001 to 0.100%,
P: 0.040% or less,
S: 0.040% or less,
N: 0.0150% or less,
O: 0.0030% or less,
B: 0 to 0.0010%,
Nb: 0 to 0.050%,
V: 0 to 0.15%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Pb: 0 to 0.090%,
Ca: 0 to 0.0050%, and
Mg: 0 to 0.0050%;
the balance being Fe and impurities;
The number density (pieces/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, and an Mn content of 10% or more, in mass%, is defined as ND _0;
The number density (pieces/mm 2 ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% ^{or more} , in mass%, is defined as ND ₁ ;
When the number density (pieces/mm 2 ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% ^{or more} is defined as ND ₂ ,
ND ₁ (pieces/mm ² ) is 1.00 or more,
Satisfying formula (1) and formula (2),
Steel material.
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2) The steel material according to claim 1,
In mass%,
B: 0.0001 to 0.0010%,
Nb: 0.001 to 0.050%,
V: 0.01-0.15%,
Sb: 0.001 to 0.050%,
As: 0.001 to 0.050%,
Pb: 0.001-0.090%,
Ca: 0.0001 to 0.0050%, and
Mg: 0.0001 to 0.0050%;
Steel material. In mass%,
C: 0.04 to less than 0.20%
Si: 0.01-0.35%,
Mn: 0.20-1.00%,
Al: 0.001-0.100%,
Ti: 0.001 to 0.100%,
Cu: 0.01-0.40%,
Ni: 0.01 to 0.30%,
Cr: 0.01-0.30%,
Mo: 0.001-0.200%,
Sn: 0.001 to 0.100%,
P: 0.040% or less,
S: 0.040% or less,
N: 0.0150% or less,
O: 0.0030% or less,
B: 0 to 0.0010%,
Nb: 0 to 0.050%,
V: 0 to 0.15%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Pb: 0 to 0.090%,
Ca: 0 to 0.0050%, and
Mg: 0 to 0.0050%;
the balance being Fe and impurities;
The number density (pieces/mm ² ) of Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, and an Mn content of 10% or more, in mass%, is defined as ND _0;
The number density (pieces/mm 2 ) of Cu—Ni-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of 5% or more, and an Mn content of 10% ^{or more} , in mass%, is defined as ND ₁ ;
When the number density (pieces/mm 2 ) of Ti-containing Mn sulfides having an equivalent circle diameter of 1.0 μm or more, an S content of 10% or more, a total content of Cu and Ni of less than 5%, a Ti content of 10% or more, and a Mn content of 10% ^{or more} is defined as ND ₂ ,
ND ₁ (pieces/mm ² ) is 1.00 or more,
Satisfying formula (1) and formula (2),
Mechanical structural parts.
ND ₁ + ND ₂ ≧2.00 (1)
(ND ₁ + ND ₂ )/ND ₀ <0.25 (2) The machine structural component according to claim 3,
In mass%,
B: 0.0001 to 0.0010%,
Nb: 0.001 to 0.050%,
V: 0.01-0.15%,
Sb: 0.001 to 0.050%,
As: 0.001 to 0.050%,
Pb: 0.001-0.090%,
Ca: 0.0001 to 0.0050%, and
Mg: 0.0001 to 0.0050%;
Mechanical structural parts. The machine structural component according to claim 3 or 4,
The machine structural component is a bolt.
Mechanical structural parts.