JP7231136B1 - Steel materials used as materials for fastening members, and fastening members - Google Patents

Abstract

A steel material having excellent corrosion resistance and hydrogen embrittlement resistance is provided. The steel material according to the present disclosure 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 to 0.50%, Mo: 0.01 to 0.50%, Ti: 0.001 to 0.100%, one or more selected from the group consisting of Co, Sb, Ge and In: total 0.0013 to less than 0.0065%, N: 0.010% or less, O: 0.015% or less, and the remainder: Fe and impurities, satisfying 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)

Description

TECHNICAL FIELD 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, and washers, and fastening members made of the steel materials.

Fastening members such as bolts, nuts, or washers are used for fastening industrial machines, automobiles, bridges, buildings, and the like. Among these uses, bridges and buildings are sometimes built in coastal areas. The coastal area is a corrosive environment rich in chloride ions. Therefore, even if the fastening member is coated, the coating may peel off and corrosion may progress. Therefore, a fastening member used in a corrosive environment containing chloride ions is required to have excellent corrosion resistance.

Furthermore, hydrogen embrittlement is likely to occur in corrosive environments 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 resistance to hydrogen embrittlement.

Techniques for improving corrosion resistance are proposed in JP-A-2020-180325 (Patent Document 1) and JP-A-2017-226878 (Patent Document 2).

The steel material disclosed in Patent Document 1 has, in mass %, C: 0.15% or more and 0.25% or less, Si: 0.05% or more and 0.30% or less, Mn: 0.50% or more and 1.80%. % 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% 0.0050% or less, N: 0.0020% or more and 0.0100% or less, and O: 0.0025% or less, and the balance is iron and unavoidable impurities. This document describes that the inclusion of Cu, Nb and Sn enhances the corrosion resistance of the steel material in a corrosive environment.

The steel material disclosed in Patent Document 2 has, in mass%, C: 0.15% or more and less than 0.30%, Si: 0.05% or more and 1.00% or less, Mn: 0.20% or more and 2.00%. % 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 more and 1 .000% or less, Nb: 0.005% or more and 0.200% or less, Sn: 0.005% or more and 0.200% or less, N: 0.0010% or more and 0.0100% or less, and the balance being iron and unavoidable impurities. This document describes that the inclusion of Cu, Nb, Sn and Ni increases the corrosion resistance of the steel material in a corrosive environment.

JP 2020-180325 A JP 2017-226878 A

However, the corrosion resistance and hydrogen embrittlement resistance may be enhanced by means different from those of 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 having excellent corrosion resistance and excellent resistance to hydrogen embrittlement.

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

in % by mass,
C: 0.15 to 0.45%,
Si: 0.01 to 1.00%,
Mn: 0.01-1.50%,
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 to 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 to 0.50%,
B: 0 to 0.0050%,
Nb: 0 to 0.300%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0200%, and
Balance: Fe and impurities,
which satisfies the formula (1),
steel.
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 element symbol in formula (1) is substituted with the content in mass% of the corresponding element, and Tx is one or more selected from the group consisting of Co, Sb, Ge and In. The total content in % by weight is substituted. LN in Equation (1) means natural logarithm.

A fastening member according to the present disclosure is constructed from the steel materials described above.

Steels and fasteners according to the present disclosure have excellent corrosion resistance and excellent resistance to hydrogen embrittlement.

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 have investigated and investigated steel materials having excellent corrosion resistance and hydrogen embrittlement resistance. As a result, the following findings were obtained.

In order to improve the corrosion resistance and hydrogen embrittlement resistance of steel, it is effective to reduce the Cr content in the steel as much as possible. Furthermore, Cu, Ni and Sn significantly enhance the corrosion resistance of steel. Based on the above studies from the viewpoint of chemical composition, the present inventors studied the chemical composition of steel having excellent corrosion resistance and hydrogen embrittlement resistance. As a result, the present inventors found that, in terms of % by mass, C: 0.15 to 0.45%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, P: 0.01 to 1.00%. 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 0200% and the balance: Fe and impurities.

However, even steel materials having the chemical composition described above may not provide sufficient corrosion resistance and hydrogen embrittlement resistance in a corrosive environment containing chloride ions. Accordingly, the present inventors investigated and examined the causes of the deterioration of the corrosion resistance and hydrogen embrittlement resistance of the steel material having the chemical composition described above. As a result, the present inventors considered that corrosion proceeds 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 to the surface of the steel materials forming the fastening members, as described above. Furthermore, due to fog, rainfall, and the like, water containing chloride ions that has flown in 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 particularly in the drying process. The dissolved Fe makes the steel surface an acidic environment. That is, as the dissolution of Fe progresses, the pH on the surface of the steel material becomes lower. The lower the pH, the more Fe dissolves. That is, corrosion progresses.

Based on the above study results, the present inventors thought that the dissolution of Fe could be suppressed if the decrease in pH on the surface of the steel material could be suppressed. Therefore, an element capable of suppressing the decrease in pH in the acidic pH range of 0 to 7 was investigated. 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 in a wide range in the acidic region. As a result, the present inventors found that the 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 the steel material. 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 range of 0.0013 to less than 0.0065%, chloride ions The present inventors have found that corrosion resistance and hydrogen embrittlement resistance are further enhanced in a corrosive environment containing In other words, the present inventors have found that the corrosion resistance and hydrogen embrittlement resistance are further enhanced in a corrosive environment containing chloride ions when the steel satisfies feature 1 below.
(Feature 1)
The chemical composition is 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: 0.02-0.30%, Cr: 0.20% or less, Cu: 0.010-0.500%, Ni: 0.01 ~0.50%, Mo: 0.01 to 0.50%, Ti: 0.001 to 0.100%, one or more selected from the group consisting of Co, Sb, Ge and In: 0.5% in total 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 balance: Fe and impurities.

However, the steel material having the chemical composition described above cannot 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 hydrogen embrittlement resistance in the steel material having the chemical composition described above.

As a result of examination, the present inventors have found that, in addition to the above-mentioned feature 1, the steel material that satisfies the following feature 2 can provide excellent corrosion resistance and excellent hydrogen embrittlement resistance.
(Feature 2)
The chemical composition of the steel 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 element symbol in formula (1) is substituted with the content in mass% of the corresponding element, and Tx is one or more selected from the group consisting of Co, Sb, Ge and In. The total content in % by weight is substituted. LN in Equation (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 enhance corrosion resistance in an environment containing chloride ions, and also suppress generation of hydrogen due to reduction reactions caused by corrosion. Suppressing the generation of hydrogen reduces the amount of hydrogen that contacts the surface of the steel material. Therefore, the amount of hydrogen that penetrates into the interior of the steel material from the surface of the steel material is also reduced. As a result, hydrogen embrittlement resistance is enhanced. Therefore, Cu, Sn, Co, Sb, Ge, and In are a group of elements that improve corrosion resistance and suppress hydrogen generation to improve hydrogen embrittlement resistance.

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

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

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

[1]
in % by mass,
C: 0.15 to 0.45%,
Si: 0.01 to 1.00%,
Mn: 0.01-1.50%,
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 to 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 to 0.50%,
B: 0 to 0.0050%,
Nb: 0 to 0.300%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0200%, and
Balance: Fe and impurities,
which satisfies the formula (1),
steel.
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 element symbol in formula (1) is substituted with the content in mass% of the corresponding element, and Tx is one or more selected from the group consisting of Co, Sb, Ge and In. The total content in % by weight is substituted. LN in Equation (1) means natural logarithm.

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

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

The steel material and the fastening member according to this embodiment will be described in detail below. In addition, "%" regarding an element means the mass % unless there is particular notice.

[Characteristics of the steel material of the present embodiment]
The steel material of this embodiment satisfies the following characteristics 1 and 2.
(Feature 1)
The chemical composition is 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: 0.02-0.30%, Cr: 0.20% or less, Cu: 0.010-0.500%, Ni: 0.01 ~0.50%, Mo: 0.01 to 0.50%, Ti: 0.001 to 0.100%, one or more selected from the group consisting of Co, Sb, Ge and In: 0.5% in total 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 balance: Fe and impurities.
(Feature 2)
The chemical composition of the steel 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 element symbol in formula (1) is substituted with the content in mass% of the corresponding element, and Tx is one or more selected from the group consisting of Co, Sb, Ge and In. The total content in % by weight is substituted. LN in Equation (1) means natural logarithm.
Features 1 and 2 will be described below.

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

C: 0.15-0.45%
Carbon (C) enhances the hardenability of the steel material and enhances the strength of the fastening member made of the steel material. If the C content is less than 0.15%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the C content exceeds 0.45%, the cold forgeability of the steel deteriorates even if the content of other elements is within the range of the present embodiment.
Therefore, the C content is 0.15-0.45%.
A preferred lower limit for 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) enhances the strength of a fastening member made of steel through solid-solution strengthening. If the Si content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Si content exceeds 1.00%, the cold forgeability of the steel deteriorates even if the content of other elements is within the range of the present embodiment.
Therefore, the Si content is 0.01-1.00%.
A preferable lower limit of the Si content is 0.02%, more preferably 0.03%, and still more preferably 0.05%.
The preferred upper limit of the Si content is 0.90%, more preferably 0.80%, still more preferably 0.70%, still more preferably 0.60%, still more preferably 0.50 %.

Mn: 0.01-1.50%
Manganese (Mn) enhances the hardenability of steel and enhances the strength of fastening members made of steel. If the Mn content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Mn content exceeds 1.50%, the cold forgeability of the steel deteriorates even if the contents of other elements are within the range of the present embodiment.
Therefore, the Mn content is 0.01-1.50%.
A preferable lower limit of the Mn content is 0.05%, more preferably 0.10%, and still 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. That is, the lower limit of the P content is over 0%.
If the P content exceeds 0.050%, P segregates at grain boundaries even if the content of other elements is within the range of the present embodiment. As a result, the hydrogen embrittlement resistance of the steel is lowered.
Therefore, the P content is 0.050% or less.
The lower the P content is, the better. However, drastic reduction of the P content greatly increases manufacturing costs. Therefore, 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. That is, the lower limit of the S content is over 0%.
If the S content exceeds 0.050%, S segregates at the grain boundaries even if the content of other elements is within the range of the present embodiment. As a result, the hydrogen embrittlement resistance of the steel is lowered.
Therefore, the S content is 0.050% or less.
It is preferable that the S content is as low as possible. However, drastic reduction of the S content greatly increases manufacturing costs. Therefore, considering industrial production, the preferred 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 inevitably contained. That is, the Al content is over 0%.
Al deoxidizes steel. If the Al content is even small, the above effect can be obtained to some extent.
However, if the Al content exceeds 0.100%, coarse Al nitrides are formed even if the content of other elements is within the range of the present embodiment. Coarse Al nitride becomes a starting point of fracture. Therefore, the workability of the steel material is lowered.
Therefore, the Al content is 0.100% or less.
A 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) enhances the corrosion resistance and hydrogen embrittlement resistance of steel under corrosive environments. If the Sn content is less than 0.02%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Sn content exceeds 0.30%, Sn segregates at grain boundaries even if the content of other elements is within the range of the present embodiment. In this case, the hot workability of the steel deteriorates.
Therefore, the Sn content is 0.02-0.30%.
A preferred lower limit for the Sn content is 0.03%, more preferably 0.05%, and still more preferably 0.10%.
The preferred upper limit of the Sn content is 0.25%, more preferably 0.20%, still more preferably 0.15%, still more preferably 0.10%.

Cr: 0.20% or less In the steel material of the present embodiment, the Cr content is over 0%. Cr enhances the hardenability of steel materials. Cr lowers 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 deteriorate even if the contents of other elements are within the ranges of the present embodiment.
Therefore, the Cr content is 0.20% or less.
Cr content is preferably as low as possible. However, drastic reduction of the Cr content significantly increases manufacturing costs. Therefore, considering industrial production, the lower limit of the Cr content is preferably 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%. A preferable upper limit of the Cr content is less than 0.05%, which is more effective for further enhancing the corrosion resistance and hydrogen embrittlement resistance of the steel material.

Cu: 0.010-0.500%
Copper (Cu) enhances the corrosion resistance and hydrogen embrittlement resistance of steel under corrosive environments. If the Cu content is less than 0.010%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Cu content exceeds 0.500%, the steel tends to become red-hot embrittlement. Furthermore, the corrosion resistance and hydrogen embrittlement resistance of the steel are rather lowered.
Therefore, the Cu content is 0.010-0.500%.
The preferred lower limit of the Cu content is 0.050%, more preferably 0.100%, still more preferably 0.150%, still more preferably 0.200%, still 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) enhances the hardenability of steel and enhances the strength of fastening members made of steel. Ni further enhances the corrosion resistance and hydrogen embrittlement resistance of steel. If the Ni content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Ni content exceeds 0.50%, the corrosion resistance and hydrogen embrittlement resistance of the steel are rather lowered.
Therefore, the Ni content is 0.01-0.50%.
The preferred lower limit of the Ni content is 0.05%, more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%, still 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) increases the hardenability of steel and increases the strength of fastening members made of steel. Fastening members for civil engineering and construction applications may exceed 20 mm in diameter. In order to increase the strength of such a thick fastening member, it is necessary to increase the hardenability of the steel material that is the material of the fastening member. Mo tends to increase 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 deteriorate even if the contents of other elements are within the ranges of the present embodiment.
Therefore, the Mo content is 0.01-0.50%.
The lower limit of the Mo content is preferably 0.05%, more preferably 0.10%, still more preferably 0.15%, still 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 nitrides, thereby increasing the strength of the fastening member made of steel. If the Ti content is less than 0.001%, the above effect 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 are formed even if the content of other elements is within the range of the present embodiment. In this case, the corrosion resistance and hydrogen embrittlement resistance of the steel deteriorate.
Therefore, the Ti content is 0.001-0.100%.
The lower limit of the Ti content is preferably 0.005%, more preferably 0.010%, still more preferably 0.015%, still more preferably 0.018%.
A preferred 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) also suppresses the 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 material 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 are enhanced. If the total content of one or more elements selected from the group consisting of Co, Sb, Ge and In is less than 0.0013%, the above effect cannot be sufficiently obtained.
On the other hand, if the total content of one or more elements 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 rather deteriorate.
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 preferred lower limit for 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 still more preferably 0.0020%. Yes, more preferably 0.0022%.
The preferred 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%, still more preferably 0.0059% Yes, more preferably 0.0057%.

N: 0.010% or less Nitrogen (N) is inevitably contained. That is, the N content is over 0%.
N combines with Al or Ti to form nitrides or carbonitrides. These nitrides and carbonitrides suppress coarsening of crystal grains by a pinning effect. As a result, the cold forgeability of the steel material is enhanced.
However, if the N content exceeds 0.010%, coarse nitrides are formed even if the content of other elements is within the range of the present embodiment. Coarse nitrides serve as starting points for fracture and reduce the cold forgeability of the steel material. Furthermore, the hydrogen embrittlement resistance of the bolt is reduced.
Therefore, the N content is 0.010% or less.
A 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%, still more preferably 0.006%.

O: 0.015% or less Oxygen (O) is an unavoidable impurity. That is, the O content is over 0%.
O forms oxides in the steel. If the O content exceeds 0.015%, coarse oxides reduce the hydrogen embrittlement resistance of the bolt even if the content of other elements is within the range of the present embodiment.
Therefore, the O content is 0.015% or less.
The lower limit of the O content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%, 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 according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the steel material is industrially manufactured, and do not adversely affect the steel material according to the present embodiment. Means what is allowed in the range.

[Optional Elements]
The steel material of the present embodiment may further contain one or more elements selected from the following group of elements instead of part of Fe.
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%, and
Rare earth elements: 0-0.0200%.
These elements are described below.

W: 0-0.50%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, W, like Co, Sb, Ge and In, suppresses the 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 effect can be obtained to some extent.
However, if the W content exceeds 0.50%, the corrosion resistance and hydrogen embrittlement resistance of the steel material rather deteriorate even if the contents of other elements are within the ranges of the present embodiment.
Therefore, the W content is 0-0.50%.
A preferable lower limit of the W content is 0.01%, more preferably 0.03%, and still more preferably 0.05%.
A preferable upper limit of the W content is 0.40%, more preferably 0.35%, still more preferably 0.30%, still more preferably 0.25%.

B: 0 to 0.0050%
Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B enhances the hardenability of the steel material and enhances 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 penetration of hydrogen into the steel material under a corrosive environment. B, together with Mo, improves the hardenability of steel materials as a substitute for Cr, and increases the strength of fastening members made of steel materials. If even a small amount of B is contained, the above effect can be obtained to some extent.
However, if the B content exceeds 0.0050%, coarse B nitrides are formed even if the content of other elements is within the range of the present embodiment. Coarse B nitride becomes a starting point of fracture. As a result, the cold forgeability of steel deteriorates.
Therefore, the B content is 0-0.0050%.
A preferable lower limit of the B content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0007%.
The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, still more preferably 0.0030%, still more preferably 0.0025%.

Nb: 0-0.300%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When included, 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 effect can be obtained to some extent.
However, if the Nb content exceeds 0.300%, a large amount of Nb precipitates are formed even if the content of other elements is within the range of the present embodiment. In this case, the amount of hydrogen permeating into the steel increases. As a result, the corrosion resistance and hydrogen embrittlement resistance of the steel are lowered.
Therefore, the Nb content is 0-0.300%.
The preferred lower limit of the Nb content is 0.001%, more preferably 0.003%, still more preferably 0.005%, still more preferably 0.010%, still 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 contained. That is, the Ca content may be 0%. When Ca is contained, that is, when Ca is more than 0%, Ca refines MnS. Therefore, the corrosion resistance and hydrogen embrittlement resistance of the steel are enhanced. If even a little Ca is contained, the above effect can be obtained to some extent.
However, if the Ca content exceeds 0.0050%, coarse Ca oxides are formed even if the content of other elements is within the range of the present embodiment. In this case, the corrosion resistance and hydrogen embrittlement resistance of the steel deteriorate.
Therefore, the Ca content is 0-0.0050%.
The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, 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 contained. That is, the Mg content may be 0%. When Mg is included, that is, when Mg is greater than 0%, Mg refines MnS. Therefore, the corrosion resistance and hydrogen embrittlement resistance of the steel are enhanced. If even a small amount of Mg is contained, the above effect can be obtained to some extent.
However, if the Mg content exceeds 0.0050%, coarse Mg oxides are formed even if the content of other elements is within the range of the present embodiment. In this case, the corrosion resistance and hydrogen embrittlement resistance of the steel deteriorate.
Therefore, the Mg content is 0-0.0050%.
A 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 element (REM): 0-0.0200%
A rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When REM is included, that is, when REM is greater than 0%, REM refines MnS. Therefore, the corrosion resistance and hydrogen embrittlement resistance of the steel are enhanced. The above effect can be obtained to some extent if REM is contained even in a small amount.
However, if the REM content exceeds 0.0200%, coarse oxides are formed even if the content of other elements is within the range of the present embodiment. In this case, the corrosion resistance and hydrogen embrittlement resistance of the steel deteriorate.
Therefore, the REM content is 0-0.0200%.
The preferred lower limit of the REM content is 0.0001%, more preferably 0.0005%, still more preferably 0.0010%, still more preferably 0.0020%, still more preferably 0.0050 %.
A preferred upper limit for the REM content is 0.0150%, more preferably 0.0100%.

REM in this specification refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide lanthanide (La) with atomic number 57 to lutetium with atomic number 71 ( Lu) is 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, using a drill, chips are collected from the inside at a depth of 1 mm or more from the surface of the steel material. 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 obtained by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content and O content are determined using the well-known inert gas fusion-thermal conductivity method.

It should be noted that each element content, based on the significant digits defined in this embodiment, rounded off the measured numerical value, the numerical value up to the minimum digit of each element content defined in this embodiment do. For example, the C content of the steel material of this embodiment is defined by a numerical value up to the second decimal place. Therefore, the C content is a numerical value to the second decimal place obtained by rounding the measured numerical value to the third decimal place.

Similarly, the content of elements other than the C content of the steel material of the present embodiment is a value obtained by rounding off the numerical value to the minimum digit specified in the present embodiment with respect to the measured value. is the content of the element.

Rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[(Feature 2) Regarding Formula (1)]
The chemical composition of the steel material of the present embodiment further satisfies the formula (1) on the premise that the content of each element is within the range of the present 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 element symbol in formula (1) is substituted with the content in mass% of the corresponding element, and Tx is one or more selected from the group consisting of Co, Sb, Ge and In. The total content in % by weight is substituted. LN in Equation (1) means natural logarithm. A natural logarithm is a logarithm with Napier's number e as a base.

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.

Among the elements in the chemical composition that satisfy feature 1, Cu, Sn, Co, Sb, Ge, and In enhance corrosion resistance in an environment containing chloride ions, and also suppress generation of hydrogen due to reduction reactions caused by corrosion. Suppressing the generation of hydrogen reduces the amount of hydrogen that contacts the surface of the steel material. Therefore, the amount of hydrogen that penetrates into the interior of the steel material from the surface of the steel material is also reduced. As a result, hydrogen embrittlement resistance is enhanced. Therefore, Cu, Sn, Co, Sb, Ge, and In improve corrosion resistance, suppress generation of hydrogen, and improve resistance to hydrogen embrittlement.

Furthermore, among the elements in the chemical composition of feature 1, Ni and Mo increase or decrease corrosion resistance and hydrogen embrittlement resistance depending on their content through a synergistic action with Sn in the chemical composition that satisfies feature 1. or Specifically, in the chemical composition that satisfies feature 1, corrosion resistance and hydrogen embrittlement resistance are increased when the Ni and Mo contents are within certain ranges, but corrosion resistance and hydrogen embrittlement resistance are decreased when the contents are different. The level of corrosion resistance and hydrogen embrittlement resistance due to Ni and Mo is considered to be affected by the synergistic action of Ni, Mo, and Sn.
“100×(0.5×Ni+Mo) ³ −100×(0.5×Ni+Mo) ² +30×(0.5×Ni+Mo)+10” in F1 is a cubic expression. This cubic expression shows the relationship between the Ni and Mo contents, corrosion resistance, and hydrogen embrittlement resistance in the chemical composition of feature 1 containing Sn.

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

Therefore, F1 is higher than 23 and less than 39.
A preferred lower limit for F1 is 24, more preferably 25, and even more preferably 26.
The preferred upper limit of F1 is 38, more preferably 36, and even 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.

[Regarding the microstructure of the steel material of the present 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 the annealed steel is enhanced. Therefore, it is possible to manufacture a fastening member by performing cold forging using the steel material of the present 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, the area ratio of pearlite is 10% or less in the microstructure at a depth of D/4 in the radial direction from the surface of the steel material in the cross section perpendicular to the longitudinal direction of the steel material. , the part other than the pearlite consists of one or more selected from the group consisting of polygonal ferrite, bainitic ferrite, acicular ferrite, and martensite. In this specification, perlite includes pseudo perlite. FIG. 1 shows an example of a photographic image of the microstructure at the D/4 depth position of the steel material of this embodiment. The low-lightness portion in FIG. 1 is pearlite, and the portion other than pearlite is composed 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 microstructure described above.

[Microstructure Observation Method]
The microstructure of steel can be observed by the following method.
In the cross section perpendicular to the longitudinal direction of the steel material, the surface including the D/4 depth position in the radial direction from the surface of the steel material is defined as the observation plane. Here, as described above, D means the diameter of the steel material. Take a sample containing the viewing surface. The observation surface of the sample is mirror-polished. Etching is performed on the mirror-polished observation surface using 3% nitric acid alcohol (nital etchant). An arbitrary observation field (0.5 mm×0.5 mm) of the etched observation surface is observed with a 500-fold optical microscope. Identify each tissue in the observation field and determine the area ratio (%).

[Uses of the steel material of the present embodiment]
The steel material of this embodiment can be applied as a material for fastening members for industrial machinery, automobiles, bridges, buildings, and the like. 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, a wire rod, or a steel plate.

[Manufacturing method of steel]
An example of the method for manufacturing the steel material of the present embodiment will be described. The steel material manufacturing method described below is an example for manufacturing the steel material of the present embodiment. Therefore, the steel material having the above 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 the steel material manufacturing method of the present embodiment.

An example of the steel manufacturing method of the present embodiment includes the following steps.
(Step 1) Step of preparing materials (material preparation step)
(Step 2) Step of hot working the raw material to produce steel (hot working step)
Each step will be described below.

[(Step 1) Material preparation step]
In the material preparation step, the material for the steel material of the present embodiment is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition is within the range of the present embodiment. A refining method is not particularly limited, and a well-known method may be used. For example, molten iron produced by a 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 contents of alloying elements in molten steel are adjusted to produce molten steel having a chemical composition in which the contents of each element are within the range of the present embodiment.

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

[(Step 2) Hot working step]
In the hot working process, the material (ingot, bloom or billet) prepared in the material preparing process is hot worked to manufacture the steel material of the present 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, as an example, a case where the steel material is a steel bar or a wire rod will be described. However, even if the steel material is a steel plate, it can be manufactured by a similar hot working process.

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

[(Step 21) blooming rolling step]
In the blooming process, the raw material is hot rolled to produce a billet.
Specifically, in the blooming step, the material is hot rolled (blooming) by a blooming mill to produce a billet. When a continuous rolling mill is arranged downstream of the blooming mill, the billet after blooming is further hot-rolled using the continuous rolling mill to produce a smaller billet. may In a continuous rolling mill, horizontal stands with a pair of horizontal rolls and vertical stands with a pair of vertical rolls are alternately arranged in a row. As described above, in the blooming process, the raw material is manufactured into billets by using a blooming mill or by using a blooming mill and a continuous rolling mill.

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

[(Step 22) Finish rolling step]
In the finish rolling step, first, the billet cooled to room temperature is heated using a heating furnace. The billet after heating is subjected to hot rolling 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 well-known range. The heating temperature is, for example, 900-1050.degree. In the finish rolling process, hot rolling (finish rolling) is performed by a continuous rolling mill having a plurality of rolling stands arranged in a row. In hot rolling using a continuous rolling mill, the steel material temperature at the delivery side of the stand that finally reduces the steel material is defined as finishing temperature (° C.). Finishing temperatures may be within known ranges. The finishing temperature is for example less than 800-900°C.

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

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

[Regarding 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 the characteristics 1 and 2. As described above, fastening members are, for example, bolts, nuts, washers, and the like.

[Manufacturing method of fastening member made of steel according to the present embodiment]
The method of manufacturing the fastening member made of steel according to the present embodiment is a well-known manufacturing method. A method of manufacturing a fastening member includes, for example, the following steps.
(Step 31) Wire drawing step (Step 32) Cold forging step (Step 33) Quenching and tempering step Each step will be described below.

[(Step 31) wire drawing step]
In the wire drawing process, a steel wire is manufactured by performing a well-known wire drawing process on the above-described steel material. The wire drawing process may be only the primary wire drawing process, or a plurality of wire drawing processes such as the secondary wire drawing process may be performed.

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

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

[Quenching]
Quenching is performed by well-known methods. The quenching temperature and the holding time at the quenching temperature may be within known ranges. The quenching temperature is, for example, 840-970°C. The holding time at the quenching temperature is, for example, 15 minutes to 360 minutes (6 hours). After the holding time has passed, the intermediate product is quenched. Specifically, the intermediate product is water-cooled or oil-cooled.

[Tempering]
The intermediate product after quenching is subjected to tempering. The tempering temperature and the holding time at the tempering temperature may be within a well-known range. The tempering temperature is, for example, 400-550°C. The holding time at the tempering temperature is 0.5-6.0 hours.

By the manufacturing method described above, the fastening member made of the steel material of the present embodiment can be manufactured. The manufactured fastening member provides sufficient corrosion resistance and sufficient resistance to hydrogen embrittlement in a corrosive environment containing chloride ions.

The effects of the steel material of the present embodiment will be described more specifically by way of examples. The conditions in the following examples are examples of conditions adopted for confirming the feasibility and effect of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one condition example.

[Material preparation process]
Molten steel having chemical compositions shown in Tables 1-1 and 1-2 was produced. A raw material (slab) was produced by continuous casting using molten steel.

[Hot working process]
A blooming rolling process was performed on the produced material to produce a billet. In the blooming step, after heating the material to 1100 to 1300° C., hot rolling was performed using a blooming mill and a continuous rolling mill. A billet produced by the blooming process was allowed to cool to room temperature.

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

The steel material of each test number was manufactured by the above manufacturing process.

[About the evaluation test]
The following steel material evaluation tests (tests 1 to 4) were performed on the manufactured steel materials of 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 described 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 [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 material (steel bar) of each test number. When cracks occurred during hot working, it was determined that the hot workability was low (described as "B" (Bad) in the "Hot workability" column in Table 2). On the other hand, when no cracks occurred in the steel material during the hot working process, it was judged to be excellent in 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 material of each test number was evaluated by the following tests.
In the corrosion resistance evaluation test, in consideration of easiness of evaluation of corrosion resistance, steel plates of each test number were manufactured by the following method, instead of using steel bars as raw materials, instead of steel bars. Specifically, the raw materials having the chemical compositions shown in Tables 1-1 and 1-2 were subjected to a finish rolling process and a cooling process to produce steel sheets of each test number with a thickness of 6 mm. The steel sheet 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.

The steel plate was subjected to quenching and tempering simulating the bolt manufacturing process. Quenching 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 passed, the steel plate was immersed in oil at 60°C and quenched. The atmosphere in the heat treatment furnace was filled with Ar gas to suppress decarburization of the steel sheet. Tempering was performed after the quenching treatment. 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 of 100 mm x 60 mm x 3 mm in thickness was taken from a steel plate that had been quenched and tempered. Shot blasting was performed on the surface of the sampled plate-shaped test piece, and the surface of the plate-shaped test piece was adjusted so that the ten-point average roughness Rzjis based on JIS B0601:2001 was 75 μm.

The surface of the plate-shaped test piece after shot blasting is coated with a 120 μm thick undercoat (manufactured by Shinto Paint Co., Ltd., trade name: Neogosei #2300PS), and a 30 μm thick intermediate coat (Shinto Paint Co., Ltd. (trade name: Syntoflon #100) and a 25 μm-thick topcoat (trade name: Syntoflon #100 manufactured by Shinto Toryo Co., Ltd.).

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

Using a plate-shaped test piece having coating film defects, a corrosion test was carried out in accordance with US standard SAE J2334 using a dry-wet repeated tester capable of being immersed in salt water. 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 kept in an environment of 50° C. and 100% RH for 6 hours.
(Step 2: salt water immersion step)
The plate specimens after step 1 are immersed in an aqueous pH 8 solution containing 0.5% NaCl, 0.1% CaCl ₂ and 0.075% NaHCO ₃ for 15 minutes.
(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. The plate-shaped test piece after holding is dried.

The steps 1 to 3 were defined as one cycle, and 80 cycles were performed.

Of the coating film on the plate-shaped test piece after 80 cycles, the coating film portion peeled off from the test piece surface starting from the coating film defect (hereinafter referred to as the coating film peeling portion) was removed with a cutter. After removing the peeled portion of the coating film, an image of the coating film of the plate-shaped test piece in plan view was generated. Image processing was used to distinguish between the area where the coating film remained and the area where the steel plate was exposed (coating film peeled portion) on the surface of the plate-shaped test piece. Then, the total area (peeled area) of the peeled coating film was determined. Based on the total area of the peeled paint film and the area of the surface of the plate-shaped test piece on which the paint film was formed, the peeled area ratio (%) of the paint film was determined. The determined peeled area ratio is shown in the column of "Peeled area ratio (%)" in Table 2.

[(Test 4) Hydrogen embrittlement resistance evaluation test]
The hydrogen embrittlement resistance of the steel material of each test number was evaluated by the following tests.
First, a steel material (steel bar) was subjected to quenching and tempering simulating a bolt manufacturing process. Quenching 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 passed, the steel material was immersed in oil at 60°C and oil quenched. The atmosphere in 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 penetrated hydrogen in the steel material of each test number subjected to the above quenching treatment and tempering treatment was measured by the following method (infiltration hydrogen amount investigation test).
First, a test piece was taken from the central position in the cross section perpendicular to the axial direction of the steel material (steel bar). A round bar test piece having a diameter of 7 mm and a length of 100 mm was used as the test piece. 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 carried out according to US standard SAE J2334, and the amount of hydrogen that penetrated into each test piece after the corrosion test was measured.

Specifically, Steps 1 to 3 of Test 3 were regarded as one cycle, and the amount of penetrated hydrogen was measured for the test piece after 56 cycles of the test. After the corrosion test, the test piece was immersed in liquid nitrogen until immediately before measuring the amount of intruding hydrogen so that the hydrogen that had penetrated into the test piece after the corrosion test would not be released. Before measuring the amount of penetrating hydrogen, sandblasting was used to completely remove corrosion products adhering to the surface of the test piece. A thermal desorption spectrometer was used to measure the amount of penetrating hydrogen on the test piece from which the corrosion products had been removed. Specifically, the amount of diffusible hydrogen detected by the desorption reaction from room temperature to 200° C. was measured by a temperature programmed desorption spectrometer, and was taken as the amount of penetrating hydrogen. The arithmetic average value of the amount of penetrated hydrogen in the obtained two test pieces was defined as the amount of penetrated hydrogen He (ppm) of the steel material with the test number.

Subsequently, a hydrogen embrittlement resistance evaluation test was carried out by the following method.

A test piece was taken from the central position of the cross section perpendicular to the axial direction of the steel material (steel bar) subjected to the above quenching and tempering. 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 in the central position of the test piece in the longitudinal direction. The notch depth 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 performed using the prepared circular notched round bar test piece. Specifically, a hydrogen charge solution was prepared by adding 3 g/L of NH ₄ SCN to a 3% NaCl solution. The amount of hydrogen penetration into the test piece was adjusted by adjusting the current density applied to the test piece in a state in which the round bar test piece with annular notches was immersed in the hydrogen charging solution.

A round-bar test piece with an annular notch charged with hydrogen at each current density was plated so as not to desorb hydrogen. The plated circular notched round bar test piece was left at room temperature for 8 hours or more. After that, a tensile test was performed at a speed of 0.005 mm/min to break the circular notched round bar test piece. After breaking, the penetrating hydrogen content (ppm) of the circular notched round bar test piece was measured using a thermal desorption spectrometer.

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

Furthermore, a tensile test was performed at a rate of 0.005 mm / min on the circular notched round bar test piece that was not charged with hydrogen of each test number, and the circular notched round bar test piece was broken. A load (σ ₀ ) was obtained.

The breaking load ratio (σ _2He /σ ₀ ) was obtained by dividing the breaking load σ _2He when charged with hydrogen by the breaking load (σ ₀ ) when not charged with hydrogen.

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

[Evaluation results]
Table 2 shows the evaluation results.
In test numbers 1-26, the chemical composition was appropriate and F1 satisfied formula (1). Therefore, the peeled area ratio was 40% or less, and sufficient corrosion resistance was obtained. Furthermore, sufficient hydrogen embrittlement resistance was obtained. The microstructures of these test numbers all have a pearlite area ratio of 10% or less, and the portion other than the pearlite is from the group consisting of polygonal ferrite, bainitic ferrite, acicular ferrite, and martensite. It was a tissue consisting of one or more selected species. Therefore, all of these test numbers were excellent in hot workability.

On the other hand, in Test No. 27, the Sn content was low. Therefore, the peeled area ratio exceeded 40%, and the corrosion resistance was low. Furthermore, sufficient hydrogen embrittlement resistance was not obtained.

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

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

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

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

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

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

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

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

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

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

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

Claims (3)

in % by mass,
C: 0.15 to 0.45%,
Si: 0.01 to 1.00%,
Mn: 0.01-1.50%,
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 to 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 to 0.50%,
B: 0 to 0.0050%,
Nb: 0 to 0.300%,
Ca: 0 to 0.0050%,
Mg: 0-0.0050%,
Rare earth elements: 0 to 0.0200%, and
Balance: Fe and impurities,
which satisfies the formula (1),
steel.
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 element symbol in formula (1) is substituted with the content in mass% of the corresponding element, and Tx is one or more selected from the group consisting of Co, Sb, Ge and In. The total content in % by weight is substituted. LN in Equation (1) means natural logarithm. The steel material according to claim 1,
W: 0.01 to 0.50%,
B: 0.0001 to 0.0050%,
Nb: 0.001 to 0.300%,
Ca: 0.0001 to 0.0050%,
Mg: 0.0001 to 0.0050%, and
Rare earth element: 0.0001 to 0.0200%,
containing one or more selected from the group consisting of
steel. A fastening member made of the steel material according to claim 1 or 2.

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