WO2023223409A1

WO2023223409A1 - Steel material used as material for fastening member, and fastening member

Info

Publication number: WO2023223409A1
Application number: PCT/JP2022/020477
Authority: WO
Inventors: 清信菅江; 直樹松井; 聡志賀; 勇高畠
Original assignee: 日本製鉄株式会社
Priority date: 2022-05-17
Filing date: 2022-05-17
Publication date: 2023-11-23
Also published as: JPWO2023223409A1; JP7231136B1

Abstract

Provided is a steel material having excellent corrosion resistance and hydrogen embrittlement resistance. A steel material according to the present disclosure comprises: 0.15-0.45 mass% of C; 0.01-1.00 mass% of Si; 0.01-1.50 mass% of Mn: at most 0.050 mass% of P; at most 0.050 mass% of S; at most 0.100% of Al; 0.02-0.30 mass% of Sn; at most 0.20 mass% of Cr; 0.010-0.500 mass% of Cu; 0.01-0.50 mass% of Ni; 0.01-0.50 mass% of Mo; 0.0001-0.100 mass% of Ti; a total of 0.0013-0.0065 mass% (exclusive of 0.0065) of at least one selected from the group consisting of Co, Sb, Ge and In; at most 0.010 mass% of N; and at most 0.015 mass% of O, with the balance consisting of Fe and impurities, and satisfies expression (1).　(1) 23<10×LN(Cu+0.5×Sn+2,000×Tx)+100×(0.5×Ni+Mo)³-100×(0.5×Ni+Mo)²+30×(0.5×Ni+Mo)+10<39

Description

Steel materials used as materials for fastening members and fastening members

The present disclosure relates to steel materials and fastening members, and more particularly to steel materials used as materials for fastening members such as bolts, nuts, washers, etc., and fastening members made of the steel materials.

Fastening members such as bolts, nuts, or washers are used for fastening industrial machinery, automobiles, bridges, buildings, etc. Among these uses, bridges and buildings may be built in coastal areas. Beach areas are corrosive environments with a lot of chloride ions. Therefore, even if the above-mentioned fastening member is painted, the paint may peel off and corrosion may progress. Therefore, fastening members used in corrosive environments containing chloride ions are required to have excellent corrosion resistance.

Furthermore, hydrogen embrittlement is likely to occur in a corrosive environment containing chloride ions. Therefore, fastening members used in corrosive environments containing chloride ions are required to have not only excellent corrosion resistance but also excellent hydrogen embrittlement resistance.

Technologies related to improving corrosion resistance are proposed in JP-A No. 2020-180325 (Patent Document 1) and JP-A No. 2017-226878 (Patent Document 2).

The steel material disclosed in Patent Document 1 has C: 0.15% or more and 0.25% or less, Si: 0.05% or more and 0.30% or less, and Mn: 0.50% or more and 1.80% by mass. % or less, P: 0.002% or more and 0.030% or less, S: 0.0005% or more and 0.0200% or less, Al: 0.010% or more and 0.065% or less, Cu: 0.01% or more and 0. .48% or less, Nb: 0.005% or more and 0.030% or less, Sn: 0.005% or more and 0.200% or less, Ti: 0.005% or more and 0.200% or less, B: 0.0001% N: 0.0020% or more and 0.0100% or less, O: 0.0025% or less, and the remainder has a component composition of iron and inevitable impurities. This document states that containing Cu, Nb, and Sn increases the corrosion resistance of steel in a corrosive environment.

The steel material disclosed in Patent Document 2 has C: 0.15% or more and less than 0.30%, Si: 0.05% or more and 1.00% or less, and Mn: 0.20% or more and 2.00% by mass. % or less, P: 0.001% or more and 0.030% or less, S: 0.0001% or more and 0.0100% or less, Al: 0.010% or more and 0.100% or less, Cu: 0.010% or more1 .000% or less, Nb: 0.005% or more and 0.200% or less, Sn: 0.005% or more and 0.200% or less, and N: 0.0010% or more and 0.0100% or less, with the balance being iron. and has a component composition of unavoidable impurities. This document states that containing Cu, Nb, Sn, and Ni increases the corrosion resistance of the steel material in a corrosive environment.

Japanese Patent Application Publication No. 2020-180325 JP2017-226878A

However, the corrosion resistance and hydrogen embrittlement resistance may be increased by means different from those for the steel materials disclosed in Patent Documents 1 and 2.

An object of the present disclosure is to provide a steel material and a fastening member that have excellent corrosion resistance and excellent hydrogen embrittlement resistance.

The steel material according to the present disclosure has the following configuration.

In mass%,
C: 0.15-0.45%,
Si: 0.01-1.00%,
Mn: 0.01 to 1.50%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.100% or less,
Sn: 0.02-0.30%,
Cr: 0.20% or less,
Cu: 0.010-0.500%,
Ni: 0.01-0.50%,
Mo: 0.01-0.50%,
Ti: 0.001 to 0.100%,
One or more selected from the group consisting of Co, Sb, Ge, and In: 0.0013 to less than 0.0065% in total,
N: 0.010% or less,
O: 0.015% or less,
W: 0-0.50%,
B: 0 to 0.0050%,
Nb: 0 to 0.300%,
Ca: 0-0.0050%,
Mg: 0 to 0.0050%,
Rare earth elements: 0 to 0.0200%, and
The remainder: Fe and impurities,
and satisfies formula (1),
Steel material.
23<10×LN(Cu+0.5×Sn+2000×Tx)+100×(0.5×Ni+Mo) ³ -100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10<39 (1)
Here, the content in mass % of the corresponding element is substituted for the element symbol in formula (1), and Tx is one or more types selected from the group consisting of Co, Sb, Ge, and In. The total content in mass % is substituted. LN in formula (1) means natural logarithm.

The fastening member according to the present disclosure is made of the above-mentioned steel material.

The steel material and fastening member according to the present disclosure have excellent corrosion resistance and excellent hydrogen embrittlement resistance.

FIG. 1 is an example of a photographic image of the microstructure of the steel material of this embodiment. FIG. 2 is a schematic diagram of an example of a graph showing the relationship between the amount of penetrating hydrogen and the breaking load obtained by the SSRT test under hydrogen charging.

The present inventors investigated and studied steel materials that have excellent corrosion resistance and excellent hydrogen embrittlement resistance. As a result, we obtained the following knowledge.

In order to improve the corrosion resistance and hydrogen embrittlement resistance of steel materials, it is effective to reduce the Cr content in the steel materials as much as possible. Furthermore, Cu, Ni, and Sn significantly improve the corrosion resistance of steel materials. Based on the above study from the viewpoint of chemical composition, the present inventors conducted a study on the chemical composition of a steel material having excellent corrosion resistance and excellent hydrogen embrittlement resistance. As a result, the present inventors found that in mass %, C: 0.15 to 0.45%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, P: 0. 050% or less, S: 0.050% or less, Al: 0.100% or less, Sn: 0.02 to 0.30%, Cr: 0.20% or less, Cu: 0.010 to 0.500%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.50%, Ti: 0.001 to 0.100%, N: 0.010% or less, O: 0.015% or less, W : 0-0.50%, B: 0-0.0050%, Nb: 0-0.300%, Ca: 0-0.0050%, Mg: 0-0.0050%, Rare earth elements: 0-0 It was thought that a steel material having a chemical composition consisting of .0200% and the balance: Fe and impurities would have improved corrosion resistance and hydrogen embrittlement resistance even in a corrosive environment containing chloride ions.

However, even with steel materials having the above chemical composition, sufficient corrosion resistance and sufficient hydrogen embrittlement resistance may not be obtained in corrosive environments containing chloride ions. Therefore, the present inventors investigated and studied the cause of the decrease in corrosion resistance and hydrogen embrittlement resistance of steel materials having the above-mentioned chemical composition. As a result, the present inventors believed that corrosion progresses by the following mechanism in a corrosive environment containing chloride ions.

When fastening members such as bolts, nuts, or washers are used in buildings such as bridges in coastal areas, chloride ions fly onto the surface of the steel materials that make up the fastening members, as described above. Furthermore, water containing chloride ions that comes flying in due to fog, rain, etc. adheres to the surface of the steel material. As a result, the surface of the steel material becomes wet (wetting process), and then water containing chloride ions dries from the surface of the steel material (drying process).

Under such an environment, the dissolution of Fe on the surface of the steel material is promoted, especially in the drying process. The dissolved Fe makes the surface of the steel material an acidic environment. In other words, as the dissolution of Fe progresses, the pH at the surface of the steel material decreases. The lower the pH, the more Fe dissolves. In other words, corrosion progresses.

Based on the above study results, the present inventors believed that dissolution of Fe could be suppressed if the decrease in pH on the surface of the steel material could be suppressed. Therefore, we investigated elements that can suppress the decrease in pH in the acidic range of pH 0 to 7. As a result, if any one or more of Co, Sb, Ge, and In is contained in addition to the above-mentioned Cu, Ni, and Sn, it is possible to suppress the decrease in pH over a wide range in the acidic region. As a result, the present inventors discovered that dissolution of Fe can be suppressed in the drying process.

Based on the above study results, the present inventors further studied the chemical composition of steel materials. As a result, if the above chemical composition further contains one or more selected from the group consisting of Co, Sb, Ge, and In in a total amount of less than 0.0013 to 0.0065%, chloride ions The present inventors have discovered that corrosion resistance and hydrogen embrittlement resistance are further enhanced in a corrosive environment containing. In other words, the present inventors have found that when the steel material satisfies the following characteristic 1, corrosion resistance and hydrogen embrittlement resistance are further enhanced in a corrosive environment containing chloride ions.
(Feature 1)
The chemical composition is in mass%, C: 0.15 to 0.45%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, P: 0.050% or less, S : 0.050% or less, Al: 0.100% or less, Sn: 0.02 to 0.30%, Cr: 0.20% or less, Cu: 0.010 to 0.500%, Ni: 0.01 ~0.50%, Mo: 0.01~0.50%, Ti: 0.001~0.100%, one or more selected from the group consisting of Co, Sb, Ge, and In: total of 0. 0013 to less than 0.0065%, N: 0.010% or less, O: 0.015% or less, W: 0 to 0.50%, B: 0 to 0.0050%, Nb: 0 to 0.300% , Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.0200%, and the remainder: Fe and impurities.

However, with the steel material having the above-mentioned chemical composition, it was not possible to sufficiently achieve both excellent corrosion resistance and excellent hydrogen embrittlement resistance in a corrosive environment containing chloride ions. Therefore, the present inventors further investigated means for obtaining excellent corrosion resistance and excellent hydrogen embrittlement resistance in steel materials having the above-mentioned chemical composition.

As a result of studies, the present inventors have found that in a steel material that satisfies the above feature 1, by further satisfying the following feature 2, excellent corrosion resistance and excellent hydrogen embrittlement resistance can be obtained.
(Feature 2)
The chemical composition of the steel material further satisfies formula (1).
23<10×LN(Cu+0.5×Sn+2000×Tx)+100×(0.5×Ni+Mo) ³ -100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10<39 (1)
Here, the content in mass % of the corresponding element is substituted for the element symbol in formula (1), and Tx is one or more types selected from the group consisting of Co, Sb, Ge, and In. The total content in mass % is substituted. LN in formula (1) means natural logarithm.

Define F1=10×LN(Cu+0.5×Sn+2000×Tx)+100×(0.5×Ni+Mo) ³ −100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10. Among the elements in the chemical composition that satisfy Feature 1, Cu, Sn, Co, Sb, Ge, and In increase corrosion resistance in an environment containing chloride ions, and also suppress the generation of hydrogen due to reduction reactions caused by corrosion. If hydrogen generation is suppressed, the amount of hydrogen that comes into contact with the steel surface will be reduced. Therefore, the amount of hydrogen penetrating into the steel material from the surface of the steel material is also reduced. As a result, hydrogen embrittlement resistance increases. Therefore, Cu, Sn, Co, Sb, Ge, and In are an element group that not only increases corrosion resistance but also suppresses hydrogen generation and increases hydrogen embrittlement resistance.

On the other hand, among the elements in the chemical composition that satisfies feature 1, Ni and Mo have a synergistic effect with Sn in the chemical composition that satisfies feature 1, and depending on their content, increases or decreases corrosion resistance and hydrogen embrittlement resistance. or Specifically, in a chemical composition that satisfies Feature 1, corrosion resistance and hydrogen embrittlement resistance increase when the contents of Ni and Mo are in a certain range, but conversely, corrosion resistance and hydrogen embrittlement resistance decrease when the Ni and Mo contents are in a different range. The level of corrosion resistance and hydrogen embrittlement resistance due to Ni and Mo is considered to be influenced by the synergistic effect of Ni and Mo with Sn. “100×(0.5×Ni+Mo) ³ −100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10” in F1 is a cubic equation. This cubic equation shows the relationship between the Ni and Mo contents and the corrosion resistance and hydrogen embrittlement resistance in the chemical composition of Feature 1 containing Sn.

When F1 is higher than 23 and lower than 39, the relationship between the contents of Cu, Sn, Co, Sb, Ge, and In, and the contents of Ni and Mo is appropriate in the chemical composition that satisfies characteristic 1. . Therefore, both excellent corrosion resistance and excellent hydrogen embrittlement resistance can be achieved in a corrosive environment containing chloride ions.

The steel material according to this embodiment, which was completed based on the above findings, has the following configuration.

[1]
In mass%,
C: 0.15-0.45%,
Si: 0.01-1.00%,
Mn: 0.01 to 1.50%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.100% or less,
Sn: 0.02-0.30%,
Cr: 0.20% or less,
Cu: 0.010-0.500%,
Ni: 0.01-0.50%,
Mo: 0.01-0.50%,
Ti: 0.001 to 0.100%,
One or more selected from the group consisting of Co, Sb, Ge, and In: 0.0013 to less than 0.0065% in total,
N: 0.010% or less,
O: 0.015% or less,
W: 0-0.50%,
B: 0 to 0.0050%,
Nb: 0 to 0.300%,
Ca: 0-0.0050%,
Mg: 0 to 0.0050%,
Rare earth elements: 0 to 0.0200%, and
The remainder: Fe and impurities,
and satisfies formula (1),
Steel material.
23<10×LN(Cu+0.5×Sn+2000×Tx)+100×(0.5×Ni+Mo) ³ -100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10<39 (1)
Here, the content in mass % of the corresponding element is substituted for the element symbol in formula (1), and Tx is one or more types selected from the group consisting of Co, Sb, Ge, and In. The total content in mass % is substituted. LN in formula (1) means natural logarithm.

[2]
The steel material according to [1],
W: 0.01-0.50%,
B: 0.0001 to 0.0050%,
Nb: 0.001-0.300%,
Ca: 0.0001-0.0050%,
Mg: 0.0001 to 0.0050%, and
Rare earth elements: 0.0001-0.0200%,
Containing one or more selected from the group consisting of
Steel material.

[3]
A fastening member made of the steel material according to [1] or [2].

Hereinafter, the steel material and fastening member according to this embodiment will be described in detail. Note that "%" regarding elements means mass % unless otherwise specified.

[Characteristics of the steel material of this embodiment]
The steel material of this embodiment satisfies the following characteristics 1 and 2.
(Feature 1)
The chemical composition is in mass%, C: 0.15 to 0.45%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, P: 0.050% or less, S : 0.050% or less, Al: 0.100% or less, Sn: 0.02 to 0.30%, Cr: 0.20% or less, Cu: 0.010 to 0.500%, Ni: 0.01 ~0.50%, Mo: 0.01~0.50%, Ti: 0.001~0.100%, one or more selected from the group consisting of Co, Sb, Ge, and In: total of 0. 0013 to less than 0.0065%, N: 0.010% or less, O: 0.015% or less, W: 0 to 0.50%, B: 0 to 0.0050%, Nb: 0 to 0.300% , Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.0200%, and the remainder: Fe and impurities.
(Feature 2)
The chemical composition of the steel material further satisfies formula (1).
23<10×LN(Cu+0.5×Sn+2000×Tx)+100×(0.5×Ni+Mo) ³ -100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10<39 (1)
Here, the content in mass % of the corresponding element is substituted for the element symbol in formula (1), and Tx is one or more types selected from the group consisting of Co, Sb, Ge, and In. The total content in mass % is substituted. LN in formula (1) means natural logarithm.
Feature 1 and Feature 2 will be explained below.

[(Feature 1) Regarding the content of each element in the chemical composition]
The chemical composition of the steel material according to this embodiment contains the following elements.

C: 0.15-0.45%
Carbon (C) improves the hardenability of steel and increases the strength of fastening members made of steel. If the C content is less than 0.15%, 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 C content exceeds 0.45%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the C content is 0.15-0.45%.
The preferable lower limit of the C content is 0.18%, more preferably 0.20%, and still more preferably 0.22%.
A preferable upper limit of the C content is 0.43%, more preferably 0.41%, and still more preferably 0.39%.

Si: 0.01~1.00%
Silicon (Si) increases the strength of fastening members made of steel through solid solution strengthening. 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 range of this embodiment.
On the other hand, if the Si content exceeds 1.00%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the Si content is 0.01 to 1.00%.
The preferable lower limit of the Si content is 0.02%, more preferably 0.03%, and even more preferably 0.05%.
The preferable upper limit of the Si content is 0.90%, more preferably 0.80%, even more preferably 0.70%, still more preferably 0.60%, and still more preferably 0.50%. %.

Mn: 0.01-1.50%
Manganese (Mn) improves the hardenability of steel materials and increases the strength of fastening members made of steel materials. If the Mn 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 Mn content exceeds 1.50%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the Mn content is 0.01-1.50%.
The lower limit of the Mn content is preferably 0.05%, more preferably 0.10%, and even more preferably 0.15%.
A preferable upper limit of the Mn content is 1.40%, more preferably 1.30%, and still more preferably 1.20%.

P: 0.050% or less Phosphorus (P) is an impurity. In other words, the lower limit of the P content is over 0%.
If the P content exceeds 0.050%, P will segregate at grain boundaries even if the contents of other elements are within the ranges of this embodiment. As a result, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the P content is 0.050% or less.
It is preferable that the P content is as low as possible. However, extreme reduction in P content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and still more preferably 0.003%.
A preferable upper limit of the P content is 0.040%, more preferably 0.030%, and still more preferably 0.020%.

S: 0.050% or less Sulfur (S) is an impurity. In other words, the lower limit of the S content is over 0%.
If the S content exceeds 0.050%, S will segregate at grain boundaries even if the contents of other elements are within the range of this embodiment. As a result, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the S content is 0.050% or less.
It is preferable that the S content is as low as possible. However, extreme reduction in S content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.002%, and still more preferably 0.003%.
A preferable upper limit of the S content is 0.040%, more preferably 0.030%, and still more preferably 0.020%.

Al: 0.100% or less Aluminum (Al) is unavoidably contained. That is, the Al content is over 0%.
Al deoxidizes steel. If the Al content is even small, the above effects can be obtained to some extent.
However, if the Al content exceeds 0.100%, coarse Al nitrides will be produced even if the contents of other elements are within the range of this embodiment. Coarse Al nitrides become a starting point for destruction. Therefore, the workability of the steel material decreases.
Therefore, the Al content is 0.100% or less.
The preferable lower limit of the Al content is 0.001%, more preferably 0.010%, and still more preferably 0.015%.
A preferable upper limit of the Al content is 0.090%, more preferably 0.080%, and still more preferably 0.070%.
In the chemical composition of the steel material of this embodiment, the Al content means the total Al (Total-Al) content.

Sn: 0.02-0.30%
Tin (Sn) increases the corrosion resistance and hydrogen embrittlement resistance of steel materials in corrosive environments. If the Sn content is less than 0.02%, 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 Sn content exceeds 0.30%, Sn will segregate at grain boundaries even if the contents of other elements are within the range of this embodiment. In this case, the hot workability of the steel material decreases.
Therefore, the Sn content is 0.02-0.30%.
The preferable lower limit of the Sn content is 0.03%, more preferably 0.05%, and still more preferably 0.10%.
A preferable upper limit of the Sn content is 0.25%, more preferably 0.20%, still more preferably 0.15%, and still more preferably 0.10%.

Cr: 0.20% or less In the steel material of this embodiment, the Cr content is more than 0%. Cr improves the hardenability of steel. Cr reduces the corrosion resistance and hydrogen embrittlement resistance of steel materials in corrosive environments. If the Cr content exceeds 0.20%, the corrosion resistance and hydrogen embrittlement resistance of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the Cr content is 0.20% or less.
It is preferable that the Cr content is as low as possible. However, extreme reduction in Cr content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the Cr content is 0.01%, more preferably 0.02%, and still more preferably 0.03%.
A preferable upper limit of the Cr content is 0.15%, more preferably 0.10%, and still more preferably 0.07%. The preferred upper limit of the Cr content, which is more effective for further increasing the corrosion resistance and hydrogen embrittlement resistance of steel materials, is less than 0.05%.

Cu: 0.010-0.500%
Copper (Cu) increases the corrosion resistance and hydrogen embrittlement resistance of steel materials in corrosive environments. If the Cu content is less than 0.010%, 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 Cu content exceeds 0.500%, the steel material will easily become red-hot embrittled. Furthermore, the corrosion resistance and hydrogen embrittlement resistance of the steel material are rather reduced.
Therefore, the Cu content is 0.010 to 0.500%.
The preferable lower limit of the Cu content is 0.050%, more preferably 0.100%, even more preferably 0.150%, even more preferably 0.200%, even more preferably 0.250%. %.
A preferable upper limit of the Cu content is 0.450%, more preferably 0.400%.

Ni: 0.01~0.50%
Nickel (Ni) improves the hardenability of steel materials and increases the strength of fastening members made of steel materials. Ni further increases the corrosion resistance and hydrogen embrittlement resistance of the steel material. 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 range of this embodiment.
On the other hand, if the Ni content exceeds 0.50%, the corrosion resistance and hydrogen embrittlement resistance of the steel material will deteriorate.
Therefore, the Ni content is 0.01 to 0.50%.
The preferable lower limit of the Ni content is 0.05%, more preferably 0.10%, even more preferably 0.15%, even more preferably 0.20%, even more preferably 0.25%. %.
A preferable upper limit of the Ni content is 0.45%, more preferably 0.40%.

Mo: 0.01~0.50%
Molybdenum (Mo) improves the hardenability of steel and increases the strength of fastening members made of steel. Fastening members for civil engineering and architectural applications may have a diameter exceeding 20 mm. In order to increase the strength of such thick fastening members, it is necessary to improve the hardenability of the steel material from which they are made. Mo tends to improve the hardenability of steel materials.
On the other hand, if the Mo content exceeds 0.50%, the corrosion resistance and hydrogen embrittlement resistance of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the Mo content is 0.01 to 0.50%.
The lower limit of the Mo content is preferably 0.05%, more preferably 0.10%, even more preferably 0.15%, and even more preferably 0.20%.
A preferable upper limit of the Mo content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.

Ti: 0.001-0.100%
Titanium (Ti) combines with N to form Ti nitride, increasing the strength of fastening members made of steel. If the Ti content is less than 0.001%, the above effects cannot be sufficiently obtained.
On the other hand, if the Ti content exceeds 0.100%, an excessive amount of Ti precipitates such as carbides and carbonitrides will be generated even if the contents of other elements are within the ranges of this embodiment. In this case, the corrosion resistance and hydrogen embrittlement resistance of the steel material decrease.
Therefore, the Ti content is 0.001 to 0.100%.
The lower limit of the Ti content is preferably 0.005%, more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.018%.
A preferable upper limit of the Ti content is 0.080%, more preferably 0.060%, still more preferably 0.040%, and still more preferably 0.030%.

One or more selected from the group consisting of Co, Sb, Ge, and In: 0.0013 to less than 0.0065% in total Cobalt (Co), antimony (Sb), germanium (Ge), and indium (In) It also suppresses dissolution of Fe in the steel material in an acidic environment. More specifically, these elements dissolve preferentially over Fe in an acidic environment. These dissolved elements adhere to the steel surface as oxides or metals. Therefore, dissolution of Fe in the steel material is suppressed. As a result, the corrosion resistance and hydrogen embrittlement resistance of the steel material increases. If the total content of one or more selected from the group consisting of Co, Sb, Ge, and In is less than 0.0013%, the above effects cannot be sufficiently obtained.
On the other hand, if the total content of one or more selected from the group consisting of Co, Sb, Ge, and In is 0.0065% or more, the corrosion resistance and hydrogen embrittlement resistance of the steel material will deteriorate on the contrary.
Therefore, the total content of one or more selected from the group consisting of Co, Sb, Ge, and In is 0.0013 to less than 0.0065%.
A preferable lower limit of the total content of one or more selected from the group consisting of Co, Sb, Ge, and In is 0.0015%, more preferably 0.0017%, and even more preferably 0.0020%. 0.0022%, more preferably 0.0022%.
A preferable upper limit of the total content of one or more selected from the group consisting of Co, Sb, Ge, and In is 0.0063%, more preferably 0.0061%, and still more preferably 0.0059%. 0.0057%, more preferably 0.0057%.

N: 0.010% or less Nitrogen (N) is unavoidably contained. That is, the N content is more than 0%.
N combines with Al or Ti to form nitride or carbonitride. These nitrides and carbonitrides suppress coarsening of crystal grains due to their pinning effect. As a result, the cold forgeability of the steel material is improved.
However, if the N content exceeds 0.010%, coarse nitrides will be produced even if the contents of other elements are within the range of this embodiment. Coarse nitrides become a starting point for fracture and reduce the cold forgeability of steel materials. Furthermore, the bolt's resistance to hydrogen embrittlement is reduced.
Therefore, the N content is 0.010% or less.
The preferable lower limit of the N content is 0.001%, more preferably 0.002%, and still more preferably 0.003%.
A preferable upper limit of the N content is 0.009%, more preferably 0.008%, still more preferably 0.007%, and still more preferably 0.006%.

O: 0.015% or less Oxygen (O) is an impurity that is inevitably contained. In other words, the O content is more than 0%.
O forms oxides in steel materials. If the O content exceeds 0.015%, even if the contents of other elements are within the ranges of this embodiment, coarse oxides will reduce the hydrogen embrittlement resistance of the bolt.
Therefore, the O content is 0.015% or less.
The preferable lower limit of the O content is 0.001%, more preferably 0.002%, still more preferably 0.003%, and still more preferably 0.004%.
A preferable upper limit of the O content is 0.013%, more preferably 0.011%, and still more preferably 0.009%.

The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition are those that are mixed in from ores used as raw materials, scrap, or the manufacturing environment when steel products are manufactured industrially, and do not have a negative effect on the steel products according to this embodiment. means permissible within range.

[Optional Elements]
The steel material of this embodiment may further contain one or more elements selected from the following element groups in place of a part of Fe.
W: 0-0.50%,
B: 0 to 0.0050%,
Nb: 0 to 0.300%,
Ca: 0-0.0050%,
Mg: 0 to 0.0050%, and
Rare earth elements: 0 to 0.0200%.
These elements will be explained below.

W: 0-0.50%
Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. When contained, W, like Co, Sb, Ge, and In, suppresses dissolution of Fe in the steel material in a corrosive environment containing chloride ions. This increases the corrosion resistance and hydrogen embrittlement resistance of the steel material. If even a small amount of W is contained, the above effects can be obtained to some extent.
However, if the W content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the corrosion resistance and hydrogen embrittlement resistance of the steel material will deteriorate.
Therefore, the W content is 0 to 0.50%.
The preferable lower limit of the W content is 0.01%, more preferably 0.03%, and still more preferably 0.05%.
The upper limit of the W content is preferably 0.40%, more preferably 0.35%, even more preferably 0.30%, and still more preferably 0.25%.

B: 0-0.0050%
Boron (B) is an optional element and may not be included. That is, the B content may be 0%. When contained, B increases the hardenability of the steel material and increases the strength of the fastening member made of the steel material. In the steel material of this embodiment, the Cr content is suppressed in order to suppress hydrogen from entering the steel material in a corrosive environment. Together with Mo, B improves the hardenability of steel materials as a substitute for Cr, thereby increasing the strength of fastening members made of steel materials. If even a small amount of B is contained, the above effects can be obtained to some extent.
However, if the B content exceeds 0.0050%, coarse B nitrides will be produced even if the contents of other elements are within the range of this embodiment. Coarse B nitrides become a starting point for destruction. As a result, the cold forgeability of the steel material decreases.
Therefore, the B content is 0 to 0.0050%.
The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0007%.
A preferable upper limit of the B content is 0.0045%, more preferably 0.0040%, still more preferably 0.0030%, and still more preferably 0.0025%.

Nb: 0-0.300%
Niobium (Nb) is an optional element and may not be included. That is, the Nb content may be 0%. When contained, Nb forms Nb precipitates such as carbides and carbonitrides. Nb precipitates increase the strength of fastening members made of steel. If the Nb content is even small, the above effects can be obtained to some extent.
However, if the Nb content exceeds 0.300%, a large amount of Nb precipitates will be generated even if the content of other elements is within the range of this embodiment. In this case, the amount of hydrogen penetrating into the steel material increases. As a result, the corrosion resistance and hydrogen embrittlement resistance of the steel material decrease.
Therefore, the Nb content is 0-0.300%.
The lower limit of the Nb content is preferably 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.020%. %.
A preferable upper limit of the Nb content is 0.250%, more preferably 0.200%, and still more preferably 0.150%.

Ca: 0-0.0050%
Calcium (Ca) is an optional element and may not be included. That is, the Ca content may be 0%. When Ca is contained, that is, when Ca is more than 0%, Ca makes MnS fine. Therefore, the corrosion resistance and hydrogen embrittlement resistance of the steel material increases. 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 will be produced even if the contents of other elements are within the range of this embodiment. In this case, the corrosion resistance and hydrogen embrittlement resistance of the steel material decrease.
Therefore, the Ca content is 0 to 0.0050%.
The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, and still more preferably 0.0005%.
A preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%.

Mg: 0-0.0050%
Magnesium (Mg) is an optional element and may not be included. That is, the Mg content may be 0%. When Mg is contained, that is, when Mg is more than 0%, Mg refines MnS. Therefore, the corrosion resistance and hydrogen embrittlement resistance of the steel material increases. 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 will be produced even if the contents of other elements are within the range of this embodiment. In this case, the corrosion resistance and hydrogen embrittlement resistance of the steel material decrease.
Therefore, the Mg content is between 0 and 0.0050%.
The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0005%.
A preferable upper limit of the Mg content is 0.0040%, more preferably 0.0030%.

Rare earth elements (REM): 0 to 0.0200%
Rare earth elements (REM) are optional elements and may not be included. That is, the REM content may be 0%. When REM is contained, that is, when REM is more than 0%, REM refines MnS. Therefore, the corrosion resistance and hydrogen embrittlement resistance of the steel material increases. If even a small amount of REM is contained, the above effects can be obtained to some extent.
However, if the REM content exceeds 0.0200%, coarse oxides will be produced even if the contents of other elements are within the range of this embodiment. In this case, the corrosion resistance and hydrogen embrittlement resistance of the steel material decrease.
Therefore, the REM content is between 0 and 0.0200%.
The lower limit of the REM content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0050%. %.
A preferable upper limit of the REM content is 0.0150%, more preferably 0.0100%.

In this specification, REM refers to scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanoids such as lanthanum (La) with an atomic number of 57 to lutetium (with an atomic number of 71). One or more elements selected from the group consisting of Lu). The REM content in this specification is the total content of these elements.

[Method for measuring chemical composition of steel]
The chemical composition of the steel material of this embodiment can be measured by a well-known component analysis method based on 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 the well-known high frequency combustion method (combustion-infrared absorption method). The N content and O content are determined using the well-known inert gas melting-thermal conductivity method.

In addition, the content of each element is determined by rounding off the fraction of the measured value based on the significant figures specified in this embodiment, and calculates the value to the smallest digit of the content of each element specified in this embodiment. do. For example, the C content of the steel material of this embodiment is defined as a value to the second decimal place. Therefore, the C content is a value obtained by rounding off the measured value to the second decimal place.

Similarly, the content of other elements other than C content in the steel material of this embodiment is a value obtained by rounding off the measured value to the smallest digit specified in this embodiment. Let be the content of the element.

Note that rounding 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 chemical composition of the steel material of this embodiment further satisfies formula (1) on the premise that the content of each element is within the range of this embodiment.
23<10×LN(Cu+0.5×Sn+2000×Tx)+100×(0.5×Ni+Mo) ³ -100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10<39 (1)
Here, the content in mass % of the corresponding element is substituted for the element symbol in formula (1), and Tx is one or more types selected from the group consisting of Co, Sb, Ge, and In. The total content in mass % is substituted. LN in formula (1) means natural logarithm. The natural logarithm is a logarithm whose base is Napier's number e.

Define F1=10×LN(Cu+0.5×Sn+2000×Tx)+100×(0.5×Ni+Mo) ³ −100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10. F1 is an index of corrosion resistance and hydrogen embrittlement resistance of steel materials.

Among the elements in the chemical composition that satisfy Feature 1, Cu, Sn, Co, Sb, Ge, and In increase corrosion resistance in an environment containing chloride ions, and also suppress the generation of hydrogen due to reduction reactions caused by corrosion. If hydrogen generation is suppressed, the amount of hydrogen that comes into contact with the steel surface will be reduced. Therefore, the amount of hydrogen penetrating into the steel material from the surface of the steel material is also reduced. As a result, hydrogen embrittlement resistance increases. Therefore, Cu, Sn, Co, Sb, Ge, and In increase corrosion resistance and suppress hydrogen generation to increase hydrogen embrittlement resistance.

Furthermore, among the elements in the chemical composition of feature 1, Ni and Mo have a synergistic effect with Sn in the chemical composition that satisfies feature 1, and can increase or decrease corrosion resistance and hydrogen embrittlement resistance depending on their content. or Specifically, in a chemical composition that satisfies characteristic 1, corrosion resistance and hydrogen embrittlement resistance increase when the content of Ni and Mo is in a certain range, but conversely, corrosion resistance and hydrogen embrittlement resistance decrease in a different range. The level of corrosion resistance and hydrogen embrittlement resistance due to Ni and Mo is considered to be influenced by the synergistic effect of Ni and Mo with Sn.
“100×(0.5×Ni+Mo) ³ −100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10” in F1 is a cubic equation. This cubic equation shows the relationship between the Ni and Mo contents and the corrosion resistance and hydrogen embrittlement resistance in the chemical composition of Feature 1 containing Sn.

When F1 is higher than 23 and lower than 39, the relationship between the contents of Cu, Sn, Co, Sb, Ge, and In, and the contents of Ni and Mo is appropriate in the chemical composition that satisfies characteristic 1. . Therefore, it is possible to achieve both excellent corrosion resistance and excellent hydrogen embrittlement resistance in a corrosive environment containing chloride ions.

Therefore, F1 is higher than 23 and less than 39.
The lower limit of F1 is preferably 24, more preferably 25, and still more preferably 26.
The upper limit of F1 is preferably 38, more preferably 36, and still more preferably 34.

The F1 value shall be an integer. That is, the F1 value is a value obtained by rounding off the obtained value to the first decimal place.

[About the microstructure of the steel material of this embodiment]
The microstructure of the steel material of this embodiment is not particularly limited. When the steel material of this embodiment is used as a fastening member, if the hardness of the steel material is too high, annealing treatment is performed. The cold forgeability of annealed steel increases. Therefore, it is possible to manufacture a fastening member by performing cold forging using the steel material of this embodiment as a raw material. Therefore, the microstructure of the steel material of this embodiment is not particularly limited.

For example, when the diameter of the steel material is D, in the cross section perpendicular to the longitudinal direction of the steel material, the area ratio of pearlite in the microstructure at a depth of D/4 in the radial direction from the surface of the steel material is 10% or less. The portion other than pearlite is made of one or more selected from the group consisting of polygonal ferrite, bainitic ferrite, acicular ferrite, and martensite. Note that in this specification, pearlite includes pseudo pearlite. FIG. 1 shows an example of a photographic image of the microstructure of the steel material of this embodiment at a depth of D/4. The portion with low brightness in FIG. 1 is pearlite, and the portion other than pearlite is made of one or more selected from the group consisting of polygonal ferrite, bainitic ferrite, acicular ferrite, and martensite. However, the microstructure of the steel material of this embodiment is not limited to the above-mentioned microstructure.

[Microstructure observation method]
The microstructure of steel can be observed using the following method.
In a cross section perpendicular to the longitudinal direction of the steel material, a surface including a position at a depth of D/4 in the radial direction from the surface of the steel material is defined as an observation surface. Here, as mentioned above, D means the diameter of the steel material. Collect a sample including the observation surface. Mirror-polish the observation surface of the sample. Etching is performed on the mirror-polished observation surface using 3% nitric alcohol (nital etching solution). An arbitrary observation field (0.5 mm x 0.5 mm) of the etched observation surface is observed with a 500x optical microscope. Identify each tissue in the observation field and calculate the area ratio (%).

[Applications of steel material of this embodiment]
The steel material of this embodiment is applicable as a material for fastening members of industrial machines, automobiles, bridges, buildings, etc. Here, the fastening members include bolts, nuts, and washers. The shape of the steel material of this embodiment is not particularly limited. The shape of the steel material may be a steel bar or wire rod, or may be a steel plate.

[Manufacturing method of steel materials]
An example of the method for manufacturing steel materials of this embodiment will be described. The method for manufacturing steel materials described below is an example for manufacturing the steel materials of this embodiment. Therefore, the 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 preferable example of the method for manufacturing the steel material of this embodiment.

An example of the method for manufacturing steel materials of this embodiment includes the following steps.
(Process 1) Process of preparing materials (material preparation process)
(Process 2) Process of manufacturing steel products by hot processing materials (hot processing process)
Each step will be explained below.

[(Process 1) Material preparation process]
In the material preparation step, a material for the steel material of this embodiment is prepared. Specifically, molten steel whose chemical composition contains each element within the range of this embodiment is manufactured. The refining method is not particularly limited, and any known method may be used. For example, molten metal produced by a well-known method is subjected to refining (primary refining) in a converter. Well-known secondary refining is performed on the molten steel tapped from the converter. In the secondary refining, the content of alloying elements in the molten steel is adjusted to produce molten steel having a chemical composition in which the content of each element is within the range of this embodiment.

Using the molten steel produced by the above-mentioned refining method, a material is produced by a well-known casting method. For example, an ingot may be manufactured by an ingot-forming method using molten steel. Alternatively, blooms or billets may be manufactured by continuous casting using molten steel. A material (ingot, bloom, or billet) is manufactured by the above method.

[(Step 2) Hot processing step]
In the hot working step, hot working is performed on the material (ingot, bloom, or billet) prepared in the material preparation step to manufacture the steel material of this embodiment. Although the shape of the steel material is not particularly limited, it is, for example, a steel bar or a wire rod. In the following description, a case where the steel material is a steel bar or a wire rod will be described as an example. However, even if the steel material is a steel plate, it can be manufactured using the same hot working process.

The hot working process includes the following steps. The main conditions in each step are as follows.
(Step 21) Blooming rolling step (Step 22) Finish rolling step (Step 23) Cooling step Each step will be explained below.

[(Step 21) Blooming rolling step]
In the blooming process, a billet is manufactured by hot rolling a material.
Specifically, in the blooming process, a billet is manufactured by hot rolling (blending) the material using a blooming mill. When a continuous rolling mill is installed downstream of the blooming mill, the billet after blooming is further hot-rolled using the continuous mill to produce a billet with a smaller size. It's okay. In a continuous rolling mill, horizontal stands having a pair of horizontal rolls and vertical stands having a pair of vertical rolls are alternately arranged in a line. As described above, in the blooming rolling process, a material is manufactured into a billet using a blooming mill or a blooming mill and a continuous rolling mill.

The heating temperature in the blooming process may be within a known temperature range. The heating temperature is, for example, 1100 to 1300°C. The billet produced by the blooming process is allowed to cool (air cool) to room temperature before the finish rolling process.

[(Step 22) Finish rolling step]
In the finish rolling process, first, the billet, which has been cooled to room temperature, is heated using a heating furnace. The heated billet is hot rolled using a continuous rolling mill to produce a steel bar or wire rod.

The heating temperature in the heating furnace in the finish rolling process may be within a known range. The heating temperature is, for example, 900 to 1050°C. In the finish rolling process, hot rolling (finish rolling) is performed using a continuous rolling mill equipped with a plurality of rolling stands arranged in a row. In hot rolling using a continuous rolling mill, the temperature of the steel material at the exit side of the stand where the steel material is finally rolled is defined as the finishing temperature (°C). The finishing temperature may be within a known range. The finishing temperature is, for example, less than 800-900°C.

[(Step 23) Cooling step]
In the cooling process, the steel material after the finish rolling process is cooled. Here, the arithmetic mean value (°C/sec) of the cooling rate at the finishing temperature FT to 300°C is defined as the average cooling rate. The average cooling rate may be within a known range. The average cooling rate CR1 is, for example, 0.6 to 1.8°C/sec.

In addition, in the hot working step of the above-mentioned manufacturing process, a finish rolling step may be performed without implementing the blooming rolling step. In other words, the blooming process is an arbitrary process. For example, when a billet is prepared in the material preparation process, the blooming process may be omitted and the finish rolling process may be performed.

[About the fastening member of this embodiment]
The fastening member of this embodiment is made of the steel material of this embodiment described above. In other words, the fastening member of this embodiment satisfies Feature 1 and Feature 2. As mentioned above, the fastening members are, for example, bolts, nuts, washers, and the like.

[Method for manufacturing fastening member made of steel material of this embodiment]
The method of manufacturing the fastening member made of steel according to this embodiment is a well-known manufacturing method. For example, the method for manufacturing a fastening member includes the following steps.
(Step 31) Wire drawing process (Step 32) Cold forging process (Step 33) Quenching and tempering process Each process will be explained below.

[(Step 31) Wire drawing process]
In the wire drawing process, a well-known wire drawing process is performed on the above-mentioned steel material to produce a steel wire. The wire drawing process may be only a primary wire drawing process, or may be performed multiple times, such as a secondary wire drawing process.

[(Step 32) Cold forging process]
In the cold forging process, the steel wire after the wire drawing process is subjected to well-known cold forging to produce an intermediate product having the shape of a fastening member.

[(Step 33) Quenching and tempering step]
In the quenching and tempering process, the intermediate product is quenched and tempered.

[Quenching]
Hardening is performed in a known manner. The quenching temperature and the holding time at the quenching temperature may be within known ranges. The quenching temperature is, for example, 840 to 970°C. The holding time at the quenching temperature is, for example, 15 minutes 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.

[Tempered]
Tempering is performed on the intermediate product after quenching. The tempering temperature and the holding time at the tempering temperature may be within known ranges. The tempering temperature is, for example, 400 to 550°C. The holding time at the tempering temperature is 0.5 to 6.0 hours.

With the above manufacturing method, a fastening member made of the steel material of this embodiment can be manufactured. The manufactured fastening member has sufficient corrosion resistance and sufficient hydrogen embrittlement resistance in a corrosive environment containing chloride ions.

The effects of the steel material of this embodiment will be explained in more detail with examples. The conditions in the following examples are examples 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 condition.

[Material preparation process]
Molten steel having the chemical composition shown in Tables 1-1 and 1-2 was produced. Materials (slabs) were manufactured using molten steel using a continuous casting method.

[Hot processing process]
The produced material was subjected to a blooming rolling process to produce a billet. In the blooming rolling process, the material was heated to 1100 to 1300°C and then hot rolled using a blooming mill and a continuous rolling mill. The billet produced by the blooming process was allowed to cool to room temperature.

A finish rolling process was performed on the manufactured billet. In the finish rolling process, the billet was heated at 900 to 1050°C. The heated billet was hot rolled using a continuous rolling mill to produce a steel material (steel bar) with a diameter of 22 mm. The finishing temperature during hot rolling was 800 to less than 900°C. Cooling was performed on the steel material (steel bar) after hot rolling. At this time, the average cooling rate from the finishing temperature to 300°C was 0.6 to 1.8°C/sec.

Through the above manufacturing process, steel materials of each test number were manufactured.

[About the evaluation test]
The following steel evaluation tests (Test 1 to Test 4) were conducted on the manufactured steel materials with each test number.
(Test 1) Steel chemical composition measurement test (Test 2) Hot workability evaluation test (Test 3) Corrosion resistance evaluation test (Test 4) Hydrogen embrittlement resistance evaluation test Each test will be explained below.

[(Test 1) Steel chemical composition measurement test]
The chemical composition of the steel material (steel bar) of each test number was analyzed based on the above-mentioned [method for measuring chemical composition of steel material]. As a result, the chemical compositions of all test numbers were as shown in Tables 1-1 and 1-2.

[(Test 2) Hot workability evaluation test]
It was confirmed whether or not cracks occurred during the hot working process for the steel materials (steel bars) of each test number. When cracking occurred during hot working, hot workability was determined to be low (described as "B" (Bad) in the "hot workability" column in Table 2). On the other hand, if no cracks occurred in the steel material during the hot working process, it was determined that the steel material had excellent hot workability (described as "E" (Excellent) in the "hot workability" column in Table 2).

[(Test 3) Corrosion resistance evaluation test]
The corrosion resistance of the steel materials of each test number was evaluated by the following test.
In the corrosion resistance evaluation test, considering the ease of evaluating corrosion resistance, steel plates of each test number were manufactured using the following method instead of using steel bars as the material. Specifically, a finish rolling process and a cooling process were performed on the materials having the chemical compositions shown in Tables 1-1 and 1-2 to produce steel plates having a thickness of 6 mm and each test number. The steel plate temperature at the start of finish rolling was 900 to 1050°C. The finishing temperature was below 800-900°C. The average cooling rate from finishing temperature to 300°C was 0.6-1.8°C/sec.

Hardening and tempering were performed on the steel plate, simulating the manufacturing process of bolts. Hardening was performed using a heat treatment furnace. The quenching temperature was 880°C, and the holding time at the quenching temperature was 60 minutes. After the holding time had elapsed, the steel plate was immersed in oil at 60°C to perform quenching. The inside of the heat treatment furnace was filled with Ar gas to suppress decarburization of the steel sheet. Tempering was performed after the hardening process. Tempering was performed using a heat treatment furnace. The tempering temperature was 450°C, and the holding time at the tempering temperature was 90 minutes.

A plate-shaped test piece measuring 100 mm x 60 mm x 3 mm in thickness was taken from the steel plate that had been quenched and tempered. Shot blasting was performed on the surface of the sampled plate-shaped test piece, and the ten-point average roughness Rzjis based on JIS B0601:2001 was adjusted to 75 μm on the surface of the plate-shaped test piece.

The surface of the plate-shaped test piece after shot blasting was coated with a 120 μm thick undercoat (trade name: Neo Gosei #2300PS, manufactured by Shinto Paint Co., Ltd.) and a 30 μm thick intermediate coat (trade name: Shinto Paint Co., Ltd.). A coating film was formed consisting of a 25 μm thick topcoat (trade name: Syntoflon #100, manufactured by Shinto Paint Co., Ltd.).

A cutter was used to form coating film defects that reached the base (steel plate). The total length of the coating film defect was 500 mm.

A corrosion test based on the American standard SAE J2334 was conducted using a dry-wet cycle tester capable of immersion in salt water using a plate-shaped test piece having a coating film defect. Specifically, a test was conducted in which one cycle consisted of the following three steps (24 hours in total).
(Step 1: Wetting process)
The plate-shaped test piece is held in an environment of 50° C. and 100% RH for 6 hours.
(Step 2: Salt water immersion process)
The plate specimen after step 1 is immersed for 15 minutes in an aqueous solution containing 0.5% NaCl, 0.1% CaCl ₂ and 0.075% NaHCO ₃ at pH 8.
(Step 3: Drying process)
The plate-shaped test piece after step 2 is held in an environment of 60° C. and relative humidity of 50% RH for 17.75 hours. Dry the plate-shaped specimen after holding.

Eighty cycles were performed, with the steps 1 to 3 described above being one cycle.

Of the coating film on the plate-shaped test piece after 80 cycles, the part of the coating film that had peeled off from the surface of the test piece (hereinafter referred to as the peeled part of the coating film) starting from the coating film defect was removed using a cutter. After removing the peeled part of the paint film, an image of the paint film of the plate-shaped test piece in plan view was generated. By image processing, on the surface of the plate-shaped test piece, a region where the paint film remained and a region where the steel plate was exposed (paint film peeled part) were distinguished. Then, the total area of the peeled part of the paint film (peeled area) was determined. The peeled area ratio (%) of the paint film was determined based on the total area of the peeled part of the paint film and the area of the surface of the plate-shaped test piece on which the paint film was formed. The obtained peeled area ratio is shown in the "peeled area ratio (%)" column of Table 2.

[(Test 4) Hydrogen embrittlement resistance evaluation test]
The hydrogen embrittlement resistance of the steel materials of each test number was evaluated by the following test.
First, the steel material (steel bar) was quenched and tempered to simulate the manufacturing process of bolts. Hardening was performed using a heat treatment furnace. The quenching temperature was 880°C, and the holding time at the quenching temperature was 60 minutes. After the holding time had elapsed, the steel material was immersed in oil at 60°C to perform oil quenching. Note that the inside of the heat treatment furnace was filled with Ar gas to suppress decarburization of the steel material. Tempering was performed after quenching. Tempering was performed using a heat treatment furnace. The tempering temperature was 450°C, and the holding time at the tempering temperature was 90 minutes.

The amount of penetrating hydrogen in the steel materials of each test number subjected to the above quenching and tempering treatments was measured by the following method (invading hydrogen amount investigation test).
First, a test piece was taken from the center of a cross section perpendicular to the axial direction of a steel material (steel bar). The test piece was a round bar test piece with a diameter of 7 mm and a length of 100 mm. The central axis of the test piece was coaxial with the steel material. Two test pieces were prepared for each test number.

A corrosion test was conducted in accordance with the American standard SAE J2334, and the amount of hydrogen penetrating each specimen after the corrosion test was measured.

Specifically, Steps 1 to 3 of Test 3 were taken as one cycle, and the amount of hydrogen penetrating was measured for the test piece after the 56-cycle test. In order to prevent the hydrogen that had entered the test piece after the corrosion test from leaving, the test piece after the corrosion test was immersed in liquid nitrogen until just before the amount of hydrogen that had entered was measured. Before measuring the amount of penetrating hydrogen, the corrosion products adhering to the surface of the test piece were completely removed using sandblasting. The amount of penetrating hydrogen was measured using a temperature programmed desorption analyzer for the test piece from which corrosion products had been removed. Specifically, the amount of diffusible hydrogen detected by a desorption reaction from room temperature to 200° C. was measured using a temperature-programmed desorption analyzer, and was defined as the amount of intruded hydrogen. The arithmetic mean value of the amount of penetrating hydrogen of the two obtained test pieces was defined as the amount of penetrating hydrogen He (ppm) of the steel material of that test number.

Subsequently, a hydrogen embrittlement resistance evaluation test was conducted using the following method.

A test piece was taken from the center position of the cross section perpendicular to the axial direction of the steel material (steel bar) that had been quenched and tempered as described above. The test piece was a round bar test piece with an annular notch having a diameter of 7 mm and a length of 70 mm. An annular notch was formed at the longitudinal center of the test piece. The depth of the notch was 1.4 mm, the notch angle was 60°, and the radius of curvature of the notch bottom was 0.175 mm.

An SSRT (Slow Strain Rate Technique) test was conducted using the prepared round bar test piece with an annular notch. Specifically, a hydrogen charge solution was prepared by adding 3 g/L of NH ₄ SCN to a 3% NaCl solution. With the round bar test piece with an annular notch immersed in the hydrogen charging solution, the current density applied to the test piece was adjusted to adjust the amount of hydrogen penetrating into the test piece.

Round bar specimens with annular notches charged with hydrogen at various current densities were plated to prevent hydrogen from desorbing. After the plating treatment, the round bar test piece with an annular notch was left at room temperature for 8 hours or more. Thereafter, a tensile test was performed at a speed of 0.005 mm/min to break the round bar test piece with an annular notch. After the fracture, the amount of hydrogen (ppm) intruding into the round bar test piece with an annular notch was measured using a temperature programmed desorption analyzer.

Through the above tests, a graph of the amount of penetrating hydrogen (ppm) and breaking load (kN) as illustrated in FIG. 2 was created. Based on the created graph, the breaking load (σ _2He ) at a hydrogen amount (2He) that is twice the amount of invaded hydrogen (He) obtained in the intruded hydrogen amount investigation test was determined.

Furthermore, a tensile test was performed on the round bar specimen with an annular notch that had not been charged with hydrogen at a speed of 0.005 mm/min to break the round bar specimen with an annular notch of each test number. The load (σ ₀ ) was determined.

The breaking load σ _2He when charged with hydrogen was divided by the breaking load (σ ₀ ) when not charged with hydrogen to obtain the breaking load ratio (σ _2He /σ ₀ ).

Hydrogen embrittlement resistance was evaluated based on the breaking load ratio (σ _2He /σ ₀ ). The evaluation results are shown in the "Hydrogen embrittlement resistance" column in Table 2. If the breaking load ratio was 0.8 or more, it was judged that the hydrogen embrittlement resistance was excellent (indicated by "E" in Table 2). If the breaking load ratio was less than 0.8, it was determined that the hydrogen embrittlement resistance was low (indicated by "B" in Table 2).

[Evaluation results]
The evaluation results are shown in Table 2.
In test numbers 1 to 26, the chemical composition was appropriate and F1 satisfied formula (1). Therefore, the peeling area ratio was 40% or less, and sufficient corrosion resistance was obtained. Furthermore, sufficient hydrogen embrittlement resistance was obtained. In addition, the microstructures of these test numbers all have a pearlite area ratio of 10% or less, and the parts other than pearlite are made from the group consisting of polygonal ferrite, bainitic ferrite, acicular ferrite, and martensite. The tissue consisted of one or more selected types. Therefore, all of these test numbers had excellent hot workability.

On the other hand, in test number 27, the Sn content was low. Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, sufficient hydrogen embrittlement resistance could not be obtained.

In test number 28, the Sn content was high. Therefore, hot workability was low.

In test number 29, the Cr content was too high. Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, sufficient hydrogen embrittlement resistance could not be obtained.

In test number 30, the Cu content was too low. Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, sufficient hydrogen embrittlement resistance could not be obtained.

In test number 31, the Cu content was too high. Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, sufficient hydrogen embrittlement resistance could not be obtained.

In test number 32, the total content Tx of Co, Sb, Ge, and In was too high. The peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, sufficient hydrogen embrittlement resistance could not be obtained.

In test numbers 33 and 34, the total content Tx of Co, Sb, Ge, and In was too low. Therefore, the peeled area ratio exceeded 40% and the corrosion resistance was low. Furthermore, sufficient hydrogen embrittlement resistance could not be obtained.

In test number 35, the Ni content was too high. Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, sufficient hydrogen embrittlement resistance could not be obtained.

In test number 36, the Mo content was too high. Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, the hydrogen embrittlement resistance also decreased.

In test number 37, the W content was too high. Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, the hydrogen embrittlement resistance also decreased.

In test numbers 38 and 39, F1 exceeded the upper limit of formula (1). Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, the hydrogen embrittlement resistance also decreased.

In test numbers 40 and 41, F1 was less than the lower limit of formula (1). Therefore, the peeling area ratio exceeded 40%, and the corrosion resistance was low.

The embodiments of the present disclosure have been described above. However, the embodiments described above are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the embodiments described above, and the embodiments described above can be modified and implemented as appropriate without departing from the spirit thereof.

Claims

In mass%,
C: 0.15-0.45%,
Si: 0.01-1.00%,
Mn: 0.01 to 1.50%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.100% or less,
Sn: 0.02-0.30%,
Cr: 0.20% or less,
Cu: 0.010-0.500%,
Ni: 0.01-0.50%,
Mo: 0.01-0.50%,
Ti: 0.001 to 0.100%,
One or more selected from the group consisting of Co, Sb, Ge, and In: 0.0013 to less than 0.0065% in total,
N: 0.010% or less,
O: 0.015% or less,
W: 0-0.50%,
B: 0 to 0.0050%,
Nb: 0 to 0.300%,
Ca: 0-0.0050%,
Mg: 0 to 0.0050%,
Rare earth elements: 0 to 0.0200%, and
The remainder: Fe and impurities,
and satisfies formula (1),
Steel material.
23<10×LN(Cu+0.5×Sn+2000×Tx)+100×(0.5×Ni+Mo) 3 -100×(0.5×Ni+Mo) 2 +30×(0.5×Ni+Mo)+10<39 (1)
Here, the content in mass % of the corresponding element is substituted for the element symbol in formula (1), and Tx is one or more types selected from the group consisting of Co, Sb, Ge, and In. The total content in mass % is substituted. LN in formula (1) means natural logarithm.
The steel material according to claim 1,
W: 0.01-0.50%,
B: 0.0001 to 0.0050%,
Nb: 0.001-0.300%,
Ca: 0.0001-0.0050%,
Mg: 0.0001 to 0.0050%, and
Rare earth elements: 0.0001-0.0200%,
Containing one or more selected from the group consisting of
Steel material.
A fastening member made of the steel material according to claim 1 or claim 2.