JP2023160582A - Steel - Google Patents

Abstract

To provide a steel that can prevent hydrogen from infiltrating steel under highly corrosive sour environments.SOLUTION: A steel disclosed herein has a chemical composition comprising, in mass%, C: 0.20-0.45%, Si: 0.60-1.30%, Mn: 0.02-1.00%, P: 0.050% or less, S: 0.0050% or less, Al: 0.010-0.100%, N: 0.0100% or less, Cr: 0.10-3.00%, Mo: 0.35-3.00%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Zr: 0.0010-0.1000%, O: 0.0050% or less, with the balance being Fe and impurities.SELECTED DRAWING: Figure 3

Description

BACKGROUND OF THE INVENTION This disclosure relates to steel, and more particularly to steel used in sour environments.

Some oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") have environments that contain many corrosive substances. The corrosive substance is, for example, a corrosive gas such as hydrogen sulfide. In this specification, an environment containing hydrogen sulfide is referred to as a "sour environment." The temperature of the sour environment ranges from room temperature to about 200°C, depending on the depth of the well. In this specification, normal temperature means 24±3°C.

An example of a steel material used in such a sour environment is steel pipes for oil wells, which are used as oil country tubular goods. When steel materials used in sour environments come into contact with corrosive substances, an electrochemical reaction occurs and hydrogen is generated on the surface of the steel material. Due to this hydrogen, hydrogen embrittlement cracking, typified by sulfide stress corrosion cracking (SSC), is likely to occur in steel materials. In particular, among sour environments, an environment in which the partial pressure of H ₂ S is 1 bar or more is defined herein as a "highly corrosive sour environment."

In a highly corrosive sour environment, hydrogen embrittlement is thought to occur in steel materials through the following process. FIG. 1 is a schematic diagram for explaining a process in which hydrogen penetrates into the interior of a steel material from the surface of the steel material. Referring to FIG. 1, first, when steel material 1 corrodes in a highly corrosive sour environment, Fe in steel material 1 becomes Fe ²⁺ and dissolves in the environment. At this time, electrons e ^- are released to the outside of the steel material 1 (A: occurrence of corrosion). Hydrogen ions H ⁺ present in the environment receive electrons e ⁻ emitted from the steel material 1, are reduced, and are adsorbed on the surface of the steel material 1 as adsorbed hydrogen atoms H _ad (B: adsorption reaction of hydrogen ions). Due to the above mechanism, a plurality of adsorbed hydrogen _atoms Had exist on the surface of the steel material 1. Most of these adsorbed _hydrogen atoms H _ad combine with each other to become hydrogen gas H ₂ and are released from the surface of the steel material 1 into the environment. However, among the plurality of adsorbed hydrogen atoms H _ad existing on the surface of the steel material 1, a small portion of the adsorbed hydrogen atoms H _ad enters into the interior of the steel material 1 from the surface of the steel material 1, and becomes the invading hydrogen atoms H _ab . (C: Invasion of hydrogen). Hydrogen embrittlement occurs in the steel material due to the intruding hydrogen atoms H _ab .

In a highly corrosive sour environment, the above-mentioned penetration of hydrogen into the steel material (invading hydrogen atoms H _ab ) becomes more pronounced, and hydrogen embrittlement cracking is likely to occur. Therefore, for steel materials used in highly corrosive sour environments, it is preferable to be able to suppress hydrogen from penetrating into the steel materials.

Steel materials that can obtain excellent SSC resistance even in highly corrosive sour environments are disclosed in International Publication No. 2017/150251 (Patent Document 1), International Publication No. 2017/150252 (Patent Document 2), and International Publication No. 2018/ This is proposed in No. 043570 (Patent Document 3).

The steel materials disclosed in Patent Document 1 and Patent Document 2 have, in mass%, C: 0.15 to 0.45% or more than 0.45 to 0.65%, Si: 0.10 to 1.0%, Mn: 0.10 to less than 0.90%, P: 0.05% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.010% or less, Cr: 0 .1 to 2.5%, Mo: 0.35 to 3.0%, Co: 0.50 to 3.0% or 0.05 to 5.0%, Cu: 0 to 0.5%, Ni: 0-0.5%, Ti: 0-0.03%, Nb: 0-0.15%, V: 0-0.5%, B: 0-0.003%, Ca: 0-0.004 %, Mg: 0 to 0.004%, Zr: 0 to 0.004%, and rare earth elements: 0 to 0.004%, with the remainder consisting of Fe and impurities, and formula (1) and formula ( It has a chemical composition that satisfies 2), and has a microstructure containing tempered martensite with a volume fraction of 90% or more. Equations (1) and (2) are as follows.
C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+α≧0.50 (1)
(3C+Mo+3Co)/(3Mn+Cr)≧1.0 (2)
Effective B=B-11(N-Ti/3.4)/14 (3)
Here, α in formula (1) is 0.250 when effective B (mass%) defined by formula (3) is 0.0003% or more, and when effective B is less than 0.0003% is 0. The content (mass %) of the corresponding element is substituted for each element symbol in formulas (1) to (3).

Patent Documents 1 and 2 aim to obtain high strength and excellent SSC resistance in a highly corrosive sour environment. These documents state that by containing Co and satisfying formulas (1) and (2) above, it is possible to achieve both high strength and excellent SSC resistance in a highly corrosive sour environment. ing.

The steel material disclosed in Patent Document 3 has, in mass%, C: 0.15 to 0.45%, Si: 0.10 to 1.0%, Mn: 0.10 to 0.8%, and P: 0. .050% or less, S: 0.010% or less, Al: 0.01 to 0.1%, N: 0.010% or less, Cr: 0.1 to 2.5%, Mo: 0.35 to 3 .0%, Co: 0.05-2.0%, Ti: 0.003-0.040%, Nb: 0.003-0.050%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, V: 0-0.5%, B: 0-0.003%, W: 0-1.0%, Ca: 0-0.004%, Mg: 0-0 0.004% and rare earth elements: 0 to 0.004%, with the remainder consisting of Fe and impurities, and has a chemical composition that satisfies formulas (1) and (2). The prior austenite grain size of the microstructure is less than 5 μm. The block size of the microstructure is less than 2 μm. The microstructure contains a total of 90% or more by volume of tempered martensite and tempered bainite. Equations (1) and (2) are as follows.
C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15-Co/6+α≧0.70 (1)
(3C+Mo+3Co)/(3Mn+Cr)≧1.0 (2)
Effective B=B-11(N-Ti/3.4)/14 (3)
Here, α in formula (1) is 0.250 when effective B (mass%) defined by formula (3) is 0.0003% or more, and when effective B is less than 0.0003% is 0. The content (mass %) of the corresponding element is substituted for each element symbol in formulas (1) to (3).

In Patent Document 3, prior austenite grains and blocks are further refined compared to the invention of Patent Document 1. Patent Document 3 states that this provides excellent SSC resistance in a highly corrosive sour environment.

International Publication No. 2017/150251 International Publication No. 2017/150252 International Publication No. 2018/043570

The steel materials disclosed in Patent Documents 1 to 3 mentioned above exhibit excellent SSC resistance in a highly corrosive sour environment. Incidentally, although the above-mentioned Patent Documents 1 to 3 discuss SSC resistance, they do not design steel materials from the viewpoint of hydrogen penetration into the steel materials under a highly corrosive sour environment. As mentioned above, in a highly corrosive sour environment, it is preferable to suppress hydrogen from entering the steel material as much as possible.

By the way, in a steel material under a highly corrosive sour environment, a corrosion product film is formed on the surface of the steel material over time. The corrosion product film inhibits hydrogen from entering the steel material. Here, a state in which a corrosion product film is sufficiently formed on the entire surface of a steel material under a highly corrosive sour environment is referred to as a "steady state." On the other hand, in a highly corrosive sour environment, a state in which a corrosion product film is not formed on the entire surface of a steel material, or a state in which a part of the corrosion product film is peeled off, is referred to as an "unsteady state."

As described above, in a steady state in which a corrosion product film is sufficiently formed on the surface of the steel material, the corrosion product film suppresses hydrogen from penetrating into the steel material. On the other hand, when the steel material is in an unsteady state, a corrosion product film is not sufficiently formed on the entire surface of the steel material. Even when the steel material is in an unsteady state, it is preferable to be able to suppress hydrogen from entering the steel material.

An object of the present disclosure is to provide a steel material that can suppress hydrogen penetration even in an unsteady state under a highly corrosive sour environment.

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

The chemical composition is in mass%,
C: 0.20-0.45%,
Si: 0.60-1.30%,
Mn: 0.02-1.00%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.010-0.100%,
N: 0.0100% or less,
Cr: 0.10-3.00%,
Mo: 0.35-3.00%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Zr: 0.0010-0.1000%,
O: 0.0050% or less,
Sb: 0 to 0.50%,
Co: 0 to 0.50%,
W: 0-0.50%,
Ti: 0 to 0.030%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%,
Ca: 0-0.0040%,
Mg: 0 to 0.0040%, and
Contains rare earth elements: 0 to 0.0040%,
The remainder consists of Fe and impurities,
Steel material.

The steel material according to the present disclosure can suppress the intrusion of hydrogen even in an unsteady state under a highly corrosive sour environment.

FIG. 1 is a schematic diagram for explaining a process in which hydrogen penetrates into the interior of a steel material from the surface of the steel material. FIG. 2 is an image diagram of a progress graph of the hydrogen permeability coefficient obtained by the hydrogen permeation test. FIG. 3 is a diagram showing the relationship between the Zr content of each steel material in Table 1 and the maximum value Per _max of the hydrogen permeability coefficient. FIG. 4 is a diagram showing the relationship between the Zr content of each steel material in Table 1 and the difference ΔPer, which is the value obtained by subtracting the hydrogen permeability coefficient Per ₉₀ from the maximum value Per _max of the hydrogen permeability coefficient. FIG. 5 is a configuration diagram of a cathode-charged hydrogen permeation test device used in the hydrogen permeation test. FIG. 6 is a diagram showing the relationship between Fn1 and the maximum value Per _max of the hydrogen permeability coefficient in a steel material in which the content of each element in the chemical composition is within the range of this embodiment. FIG. 7 is a diagram showing the relationship between Fn1 and the difference ΔPer in hydrogen permeability coefficient in a steel material whose chemical composition contains each element within the range of this embodiment.

The present inventors investigated steel materials that can suppress hydrogen penetration even in a highly corrosive sour environment where the H ₂ S partial pressure is 1 bar or more.

The present inventors first investigated the steel material for use in highly corrosive sour environments from the viewpoint of chemical composition. As a result, the present inventors found that in mass %, C: 0.20-0.45%, Si: 0.60-1.30%, Mn: 0.02-1.00%, P: 0. 050% or less, S: 0.0050% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Cr: 0.10 to 3.00%, Mo: 0.35 to 3. 00%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, O: 0.0050% or less, Sb: 0 to 0.50%, Co: 0 to 0.50% , W: 0-0.50%, Ti: 0-0.030%, Nb: 0-0.150%, V: 0-0.500%, B: 0-0.0030%, Ca: 0- A steel material having a chemical composition of 0.0040%, Mg: 0 to 0.0040%, rare earth elements: 0 to 0.0040%, and the balance consisting of Fe and impurities was considered appropriate. Hereinafter, the above chemical composition will be referred to as a "basic chemical composition."

Therefore, the present inventors investigated elements that can further suppress the penetration of hydrogen into the steel material in an unsteady state, based on the steel material having the basic chemical composition. As a result, the present inventors found that if Zr was included in place of a part of Fe in the basic chemical composition, it was possible to further suppress the intrusion of hydrogen into the steel material in an unsteady state. This finding will be explained below.

[About the effect of containing Zr in steel]
Steel materials having the chemical composition shown in Table 1 were prepared.

Test number 1 in Table 1 is a steel material that satisfies the above-mentioned basic chemical composition. Test numbers 4, 6, and 8 are steel materials that satisfy the above-mentioned basic chemical composition and contain Zr in place of a portion of Fe. Test numbers 1, 4, 6, and 8 were manufactured by the manufacturing method described in Examples below. A hydrogen permeability coefficient measurement test (hereinafter referred to as hydrogen permeation test) was conducted on steel materials with test numbers 1, 4, 6, and 8 using the electrochemical hydrogen permeation method described below, and the hydrogen permeability coefficient from the start of the test was determined. The changes over time were determined. In addition, in the hydrogen permeation test, assuming the use of steel materials in a highly corrosive sour environment, a mixture of 5.0 mass% NaCl and 0.5 mass% CH ₃ COOH saturated with 1 bar H ₂ S gas was used. A test solution that was an aqueous solution and had a pH of 2.7 was used.

The lower the hydrogen permeability coefficient, the more suppressed is the penetration of hydrogen into the steel material. FIG. 2 is an image diagram of a graph of the hydrogen permeability coefficient over time obtained by the hydrogen permeation test. Referring to FIG. 2, the hydrogen permeability coefficient Per usually rises rapidly immediately after the start of the test. After a predetermined period of time has elapsed, the hydrogen permeability coefficient Per decreases over time. That is, in the graph of the hydrogen permeability coefficient Per over time, there is a maximum value (peak value) of the hydrogen permeability coefficient Per. Here, the maximum value of the hydrogen permeability coefficient Per in the graph of the hydrogen permeability coefficient Per over time is defined as "Per _max ".

In the hydrogen permeation test, the hydrogen permeability coefficient after 90 hours from the start of the test is defined as "Per ₉₀ ". The hydrogen permeability coefficient Per ₉₀ means the hydrogen permeability coefficient in the above-mentioned "steady state". In other words, it means the hydrogen permeability coefficient in a state where a corrosion product film is sufficiently formed on the entire surface of the steel material. On the other hand, the maximum value Per _max of the hydrogen permeability coefficient Per means the hydrogen permeability coefficient Per in the above-mentioned "unsteady state". In other words, it means the hydrogen permeability coefficient in a state where a corrosion product film is not sufficiently formed on the entire surface of the steel material. Furthermore, the value obtained by subtracting the hydrogen permeability coefficient Per ₉₀ from the maximum value Per _max of the hydrogen permeability coefficient is defined as the difference ΔPer.

If the maximum value Per _max and the difference ΔPer are small, it means that hydrogen penetration can be sufficiently suppressed in a highly corrosive sour environment even if the steel material is in an unsteady state. For example, when steel pipes are screwed together in the construction of oil country tubular goods, the steel pipes may come into contact with each other, causing flaws on the inner surface and/or inner surface of the steel pipe, and causing part of the corrosion product film to peel off. In this case, the surface of the steel pipe is in an unsteady state. Even if the surface of the steel pipe is in an unsteady state, if the maximum value Per _max and the difference ΔPer are small, it is possible to effectively suppress hydrogen from entering the steel material.

For steel materials used in highly corrosive sour environments, it is possible to not only prevent hydrogen from entering the steel material in a steady state, but also to prevent hydrogen from entering the steel material in an unsteady state due to part of the corrosion product film peeling off for some reason. Even if hydrogen exists, it is preferable to be able to suppress hydrogen from entering the steel material. That is, for steel materials used in a highly corrosive sour environment, it is preferable that the maximum value Per _max and the difference ΔPer of the hydrogen permeability coefficient are small.

FIG. 3 is a diagram showing the relationship between the Zr content and the maximum value Per _max in the steel materials of each test number in Table 1. FIG. 4 is a diagram showing the relationship between the Zr content and the difference ΔPer in the steel materials of each test number in Table 1. Referring to FIGS. 3 and 4, when the content of elements other than Zr in the chemical composition is within the above range, the maximum value Per _max and the difference ΔPer decrease significantly as the Zr content increases.

Based on the above findings, the present inventors considered that the maximum value Per _max and the difference ΔPer can be significantly reduced by further containing Zr in the steel material having the basic chemical composition. Although the reason for this is not clear, the present inventors think as follows. It is thought that Zr contributes to promoting the release of hydrogen adsorbed on the surface of the steel material rather than stabilizing the corrosion product film. Therefore, it is thought that Zr particularly suppresses the intrusion of hydrogen into the steel material in an unsteady state, and as a result, the maximum value Per _max and the difference ΔPer decrease.

Based on the above findings, the present inventors further investigated the appropriate Zr content when Zr is further included in the above-mentioned basic chemical composition. As a result, by further containing 0.0010 to 0.1000% Zr in the above chemical composition, it is possible to not only suppress hydrogen from entering the steel material in a steady state, but also prevent hydrogen from entering the steel material in an unsteady state. It was found that the intrusion can be sufficiently suppressed.

There is also a possibility that the maximum value Per _max and the difference ΔPer are reduced due to an effect different from the effect of Zr described above. However, by including 0.0010 to 0.1000% Zr in the basic chemical composition mentioned above, it is possible to sufficiently suppress hydrogen from entering the steel material in an unsteady state under a highly corrosive sour environment. This is also proven in the examples described later.

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

[1]
The chemical composition is in mass%,
C: 0.20-0.45%,
Si: 0.60-1.30%,
Mn: 0.02-1.00%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.010-0.100%,
N: 0.0100% or less,
Cr: 0.10-3.00%,
Mo: 0.35-3.00%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Zr: 0.0010-0.1000%,
O: 0.0050% or less,
Sb: 0 to 0.50%,
Co: 0 to 0.50%,
W: 0-0.50%,
Ti: 0 to 0.030%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%,
Ca: 0-0.0040%,
Mg: 0 to 0.0040%, and
Contains rare earth elements: 0 to 0.0040%,
The remainder consists of Fe and impurities,
Steel material.

[2]
The steel material according to [1], further comprising:
Fn1 defined by the following formula (1) is 4.00 or more,
Steel material.
Fn1=(2.0×Si+0.3×Cu+0.9×Ni+Cr+1.2×Mo+8.0×Zr+5.0×Sb+5.0×Co)−(2.0×C+2.0×Mn) (1)
Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element.

[3]
The steel material according to [1] or [2],
The chemical composition is
Sb: 0.01 to 0.50%,
Co: 0.01 to 0.50%,
W: 0.01-0.50%,
Ti: 0.003 to 0.030%,
Nb: 0.003 to 0.150%,
V: 0.005-0.500%,
B: 0.0003 to 0.0030%,
Ca: 0.0003-0.0040%,
Mg: 0.0003 to 0.0040% and
Rare earth elements: 0.0003 to 0.0040%,
Containing one or more selected from the group consisting of
Steel material.
[4]
The steel material according to [1],
The steel material is a steel pipe for oil wells,
Steel material.

In this specification, "oil well steel pipe" means a steel pipe used for oil well applications. Oil well steel pipe is a general term for casings, tubing, and drill pipes used for, for example, drilling oil or gas wells, extracting crude oil or natural gas, and the like. When the steel material of this embodiment is a steel pipe, it is preferably a seamless steel pipe.

Hereinafter, the steel material of this embodiment will be explained in detail. "%" with respect to elements means mass % unless otherwise specified.

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

C: 0.20-0.45%
Carbon (C) improves hardenability and increases the strength of steel materials. C further forms carbides or carbonitrides (carbonitrides, etc.) and increases the strength of the steel material. If the C content is less than 0.20%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the C content exceeds 0.45%, the toughness of the steel material will decrease even if the contents of other elements are within the range of this embodiment. Further, C is an element that promotes hydrogen penetration. Therefore, if the C content exceeds 0.45%, excessive carbonitrides and the like are more likely to be produced. In a corrosive sour environment, if carbonitrides and the like exist in the surface layer of the steel material, the electrical resistance of the region where the carbonitrides and the like are present will be lower than that of the region where the carbonitrides and the like are not present. Therefore, the surface of the steel material becomes electrochemically active, and hydrogen is likely to be generated on the surface of the steel material. As a result, penetration of hydrogen into the steel material is promoted.
Therefore, the C content is 0.20-0.45%.
The preferable lower limit of the C content is 0.21%, more preferably 0.22%, even more preferably 0.23%, still more preferably 0.24%, even more preferably 0.25%. %.
A preferable upper limit of the C content is 0.40%, more preferably 0.38%, still more preferably 0.36%, and still more preferably 0.34%.

Si: 0.60-1.30%
Silicon (Si) deoxidizes steel. Furthermore, Si is an element that suppresses hydrogen penetration and increases the hydrogen embrittlement resistance of steel materials. If the Si content is less than 0.60%, 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 1.30%, the ductility 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 0.60-1.30%.
The lower limit of the Si content is preferably 0.65%, more preferably 0.68%, even more preferably 0.72%, and still more preferably 0.76%.
A preferable upper limit of the Si content is 1.28%, more preferably 1.26%, even more preferably 1.24%, still more preferably 1.20%, and even more preferably 1.18%. %, more preferably 1.16%.

Mn: 0.02-1.00%
Manganese (Mn) deoxidizes steel. Mn further improves the hardenability and increases the strength of the steel material. If the Mn 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, Mn is an element that promotes hydrogen penetration. If the Mn content exceeds 1.00%, Mn sulfide will be excessively produced even if the contents of other elements are within the ranges of this embodiment. Mn sulfide becomes a starting point for pitting corrosion. Therefore, if excessive Mn sulfide is produced, the corrosion rate will increase. As a result, penetration of hydrogen into the steel material is promoted.
Therefore, the Mn content is 0.02-1.00%.
The preferable lower limit of the Mn content is 0.05%, more preferably 0.10%, even more preferably 0.15%, and still more preferably 0.20%.
A preferable upper limit of the Mn content is 0.90%, more preferably 0.80%, preferably 0.70%, and still more preferably 0.60%.

P: 0.050% or less Phosphorus (P) is an impurity that is inevitably contained. In other words, the P content is over 0%.
Like S, P segregates at grain boundaries. If the P content exceeds 0.050%, even if the contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries and 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, if the P content is excessively reduced, manufacturing costs will increase. Therefore, in consideration of 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 0.035%, even more preferably 0.030%, still more preferably 0.025%, and even more preferably 0.020%. %.

S: 0.0050% or less Sulfur (S) is an impurity that is inevitably contained. In other words, the S content is over 0%.
Like P, S segregates at grain boundaries. If the S content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, S segregates at grain boundaries and the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the S content is 0.0050% or less.
It is preferable that the S content is as low as possible. However, if the S content is excessively reduced, manufacturing costs will increase. Therefore, in consideration of industrial production, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002%.
A preferable upper limit of the S content is 0.0040%, more preferably 0.0030%, still more preferably 0.0025%, and still more preferably 0.0020%.

Al: 0.010-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.010%, this effect cannot be obtained even if the content of other elements is within the range of this embodiment.
On the other hand, if the Al content exceeds 0.100%, coarse oxides will be produced 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 Al content is 0.010 to 0.100%.
The preferable lower limit of the Al content is 0.015%, more preferably 0.020%, and still more preferably 0.025%.
A preferable upper limit of the Al content is 0.080%, more preferably 0.070%, still more preferably 0.065%, and still more preferably 0.040%.
The Al content referred to in this specification is sol. It means the content of Al (acid-soluble Al).

N: 0.0100% or less Nitrogen (N) is unavoidably contained. That is, the N content is more than 0%. If the N content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, nitrides will become coarse and the hydrogen embrittlement resistance of the steel material will decrease.
Therefore, the N content is 0.0100% or less.
It is preferable that the N content is as low as possible. However, if the N content is reduced excessively, manufacturing costs will increase. Therefore, considering industrial production, the preferable lower limit of the N content is 0.0001%, more preferably 0.0002%, still more preferably 0.0005%, and still more preferably 0.0020%. It is.
A preferable upper limit of the N content is 0.0080%, more preferably 0.0070%, still more preferably 0.0065%, and still more preferably 0.0060%.

Cr:0.10~3.00%
Chromium (Cr) improves the hardenability of steel materials. Cr is further an element that suppresses hydrogen penetration. Specifically, Cr stabilizes the corrosion product film and suppresses the intrusion of hydrogen. As a result, the hydrogen embrittlement resistance of the steel material is increased. If the Cr content is less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the Cr content exceeds 3.00%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material becomes excessively high, and the hydrogen embrittlement resistance of the steel material decreases. Furthermore, the weldability of the steel material also deteriorates.
Therefore, the Cr content is between 0.10 and 3.00%.
The preferable lower limit of the Cr content is 0.30%, more preferably 0.45%, even more preferably 0.60%, still more preferably 0.80%, even more preferably 0.90%. %.
The preferable upper limit of the Cr content is 2.50%, more preferably 2.00%, even more preferably 1.90%, still more preferably 1.80%, and even more preferably 1.70%. %, more preferably 1.50%.

Mo: 0.35-3.00%
Molybdenum (Mo) stabilizes the corrosion product film formed on the surface of steel materials in a highly corrosive sour environment and suppresses hydrogen from penetrating into the steel materials. Mo further forms fine precipitates to increase the temper softening resistance of the steel and increase the strength of the steel. If the Mo content is less than 0.35%, 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 Mo content exceeds 3.00%, the above effects will be saturated.
Therefore, the Mo content is 0.35-3.00%.
The lower limit of the Mo content is preferably 0.37%, more preferably 0.40%, and still more preferably 0.42%.
A preferable upper limit of the Mo content is 2.50%, more preferably 2.00%, and still more preferably 1.50%.

Cu: 0.01~0.50%
In a highly corrosive sour environment, copper (Cu) concentrates at the interface between the corrosion product film and the base metal, suppresses the surface activity of the base metal, and suppresses hydrogen from penetrating into the steel material. Furthermore, Cu is dissolved in the steel material to improve the hardenability of the steel material, thereby increasing the strength of the steel material. If the Cu content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment.
On the other hand, if the Cu content exceeds 0.50%, Cu will precipitate in the steel material even if the contents of other elements are within the range of this embodiment. Precipitated Cu tends to trap hydrogen. Therefore, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the Cu content is 0.01-0.50%.
The lower limit of the Cu content is preferably 0.02%, more preferably 0.03%, and still more preferably 0.05%.
A preferable upper limit of the Cu content is 0.49%, more preferably 0.45%, still more preferably 0.40%, and still more preferably 0.35%.

Ni: 0.01~0.50%
In a highly corrosive sour environment, nickel (Ni) concentrates at the interface between the corrosion product film and the base metal, suppresses the surface activity of the base metal, and suppresses hydrogen from penetrating into the steel material. Ni further improves the hardenability of the steel material by forming a solid solution in the steel material, thereby increasing the strength 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%, even if the contents of other elements are within the ranges of this embodiment, local corrosion tends to progress and the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the Ni content is 0.01 to 0.50%.
The preferable lower limit of the Ni content is 0.02%, more preferably 0.03%, and still more preferably 0.05%.
A preferable upper limit of the Ni content is 0.49%, more preferably 0.48%, and still more preferably 0.45%.

Zr: 0.0010-0.1000%
Zirconium (Zr) suppresses hydrogen from entering the steel material and increases the hydrogen embrittlement resistance of the steel material. Zr particularly suppresses hydrogen from entering the steel material in an unsteady state. If the Zr content is less than 0.0010%, 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 Zr content exceeds 0.1000%, the hydrogen embrittlement resistance of the steel material will be saturated even if the other element contents are within the ranges of this embodiment. Furthermore, manufacturing costs are increased.
Therefore, the Zr content is 0.0010-0.1000%.
The lower limit of the Zr content is preferably 0.0030%, more preferably 0.0050%, even more preferably 0.0060%, even more preferably 0.0080%, and even more preferably 0.0100%. %.
A preferable upper limit of the Zr content is 0.0800%, more preferably 0.0700%, still more preferably 0.0600%, and still more preferably 0.0550%.

O: 0.0050% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is over 0%. If the O content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, coarse oxides will be formed and the hydrogen embrittlement resistance of the steel material will deteriorate.
Therefore, the O content is 0.0050% or less.
It is preferable that the O content is as low as possible. However, extreme reduction in O content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.
A preferable upper limit of the O content is 0.0040%, more preferably 0.0030%, still more preferably 0.0025%, and still more preferably 0.0020%.

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 chemical composition of the steel material of this embodiment further includes:
In place of a part of Fe,
Sb: 0 to 0.50%,
Co: 0 to 0.50%,
W: 0-0.50%,
Ti: 0 to 0.030%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%,
Ca: 0-0.0040%,
Mg: 0 to 0.0040%, and
Rare earth elements: 0 to 0.0040%,
It may contain one or more elements selected from the group consisting of.
These arbitrary elements will be explained below.

[About the first group (Sb, Co and W)]
The chemical composition of the steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Sb, Co, and W in place of a part of Fe. These elements are optional elements. All of these elements suppress hydrogen from entering the steel material. Hereinafter, Sb, Co and W will be explained.

Sb: 0-0.50%
Antimony (Sb) is an optional element and may not be included. That is, the Sb content may be 0%. When contained, Sb functions as a hydrogen penetration suppressing element. Specifically, Sb suppresses hydrogen from penetrating into the steel material in a highly corrosive sour environment and increases hydrogen embrittlement resistance. If even a small amount of Sb is contained, the above effects can be obtained to some extent.
However, if the Sb content exceeds 0.50%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the Sb content is 0 to 0.50%.
The preferable lower limit of the Sb content is 0.01%, more preferably 0.03%, and still more preferably 0.05%.
A preferable upper limit of the Sb content is 0.48%, more preferably 0.45%, still more preferably 0.43%, and still more preferably 0.40%.

Co: 0-0.50%
Cobalt (Co) is an optional element and may not be included. That is, the Co content may be 0%. When contained, Co functions as a hydrogen penetration inhibiting element. Specifically, in a highly corrosive sour environment, Co concentrates at the interface between the corrosion product film and the base metal, suppresses the surface activity of the base metal, and suppresses hydrogen from penetrating into the steel material. Furthermore, Co dissolves in the steel material, improves the hardenability of the steel material, and increases the strength of the steel material. If even a small amount of Co is contained, the above effects can be obtained to some extent.
However, if the Co content exceeds 0.50%, the effect is saturated.
Therefore, the Co content is 0-0.50%.
The preferable lower limit of the Co content is 0.01%, more preferably 0.05%, and even more preferably 0.08%.
A preferable upper limit of the Co content is 0.48%, more preferably 0.45%, and still more preferably 0.43%.

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 functions as a hydrogen penetration suppressing element. Specifically, W forms a corrosion film on the surface of the steel material. This suppresses hydrogen from entering the steel material. Therefore, the hydrogen embrittlement resistance of the steel material increases. 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%, coarse carbides will be produced even if the contents of other elements are within the range of this embodiment. Therefore, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the W content is 0 to 0.50%.
The lower limit of the W content is preferably 0.01%, more preferably 0.05%, and still more preferably 0.08%.
The upper limit of the W content is preferably 0.45%, more preferably 0.35%, even more preferably 0.25%, and still more preferably 0.20%.

[About the second group (Ti, Nb, V)]
The chemical composition of the steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Ti, Nb, and V in place of a part of Fe. These elements are optional elements. All of these elements form precipitates that increase the strength of the steel. Ti, Nb and V will be explained below.

Ti: 0-0.030%
Titanium (Ti) is an optional element and may not be included. That is, the Ti content may be 0%. When contained, Ti combines with C and/or N to form carbonitrides and the like. These carbonitrides etc. refine the crystal grains due to the pinning effect and increase the strength of the steel material. If even a small amount of Ti is contained, the above effects can be obtained to some extent.
However, if the Ti content exceeds 0.030%, the Ti nitride etc. will become coarse even if the content of other elements is within the range of this embodiment. Therefore, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the Ti content is 0 to 0.030%.
The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%.
A preferable upper limit of the Ti content is 0.025%, more preferably 0.020%, still more preferably 0.015%, and still more preferably 0.010%.

Nb: 0-0.150%
Niobium (Nb) is an optional element and may not be included. That is, the Nb content may be 0%. When contained, Nb combines with C and/or N to form carbonitrides and the like. These carbonitrides etc. refine the crystal grains due to the pinning effect and increase the strength of the steel material. If even a small amount of Nb is contained, the above effects can be obtained to some extent.
However, if the Nb content exceeds 0.150%, Nb carbonitrides etc. will become coarse even if the content of other elements is within the range of this embodiment. Therefore, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the Nb content is between 0 and 0.150%.
The preferable lower limit of the Nb content is 0.001%, more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.007%.
A preferable upper limit of the Nb content is 0.120%, more preferably 0.090%, even more preferably 0.060%, still more preferably 0.050%, and even more preferably 0.030%. %, more preferably 0.020%.

V: 0-0.500%
Vanadium (V) is an optional element and may not be included. That is, the V content may be 0%. When contained, V combines with C and/or N to form carbonitrides and the like. These carbonitrides refine the crystal grains due to the pinning effect and increase the strength of the steel material. If even a small amount of V is contained, the above effects can be obtained to some extent.
However, if the V content exceeds 0.500%, the toughness of the steel material will decrease even if the contents of other elements are within the range of this embodiment.
Therefore, the V content is 0-0.500%.
The lower limit of the V content is preferably 0.001%, more preferably 0.005%, even more preferably 0.010%, even more preferably 0.015%, and still more preferably 0.020%. %.
A preferable upper limit of the V content is 0.400%, more preferably 0.300%, even more preferably 0.200%, still more preferably 0.150%, and even more preferably 0.120%. %.

[About the third group (B)]
The chemical composition of the steel material according to this embodiment may further contain B in place of a part of Fe.
B: 0-0.0030%
Boron (B) is an optional element and may not be included. That is, the B content may be 0%. When contained, B forms a solid solution in the steel material, improves the hardenability of the steel material, and increases the strength of the steel material. If even a small amount of B is contained, the above effects can be obtained to some extent.
However, if the B content exceeds 0.0030%, coarse B nitrides will be produced even if the contents of other elements are within the range of this embodiment. Therefore, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the B content is 0 to 0.0030%.
The lower limit of the B content is preferably 0.0001%, more preferably 0.0003%, even more preferably 0.0005%, and still more preferably 0.0008%.
A preferable upper limit of the B content is 0.0028%, more preferably 0.0025%, and still more preferably 0.0023%.

[Group 4 (Ca, Mg, rare earth elements (REM))]
The chemical composition of the steel material according to this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, and rare earth elements (REM) in place of a part of Fe. These elements are optional elements. All of these elements refine the Mn sulfide and improve the hydrogen embrittlement resistance of the steel material.

Ca: 0-0.0040%
Calcium (Ca) is an optional element and may not be included. That is, the Ca content may be 0%. When contained, Ca combines with S in the steel to refine Mn sulfides. This increases the hydrogen embrittlement resistance of the steel material. If even a small amount of Ca is contained, the above effects can be obtained to some extent.
However, if the Ca content exceeds 0.0040%, the oxides in the steel material will become coarse even if the contents of other elements are within the range of this embodiment. Therefore, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the Ca content is 0 to 0.0040%.
The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.
A preferable upper limit of the Ca content is 0.0030%, more preferably 0.0025%, still more preferably 0.0020%, and still more preferably 0.0015%.

Mg: 0-0.0040%
Magnesium (Mg) is an optional element and may not be included. That is, the Mg content may be 0%. When contained, Mg refines Mn sulfide in the steel material and increases the hydrogen embrittlement resistance of the steel material. If even a small amount of Mg is contained, the above effects can be obtained to some extent.
However, if the Mg content exceeds 0.0040%, the oxides in the steel material will become coarse even if the contents of other elements are within the range of this embodiment. Therefore, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the Mg content is 0 to 0.0040%.
The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.
The upper limit of the Mg content is preferably 0.0030%, more preferably 0.0025%, even more preferably 0.0020%, and still more preferably 0.0015%.

Rare earth elements: 0-0.0040%
Rare earth elements (REM) are optional elements and may not be included. That is, the REM content may be 0%. When contained, REM refines Mn sulfide in the steel material and increases the hydrogen embrittlement resistance of the steel material. If even a small amount of REM is contained, the above effects can be obtained to some extent.
However, if the REM content exceeds 0.0040%, the oxides in the steel material will become coarse even if the contents of other elements are within the range of this embodiment. Therefore, the hydrogen embrittlement resistance of the steel material decreases.
Therefore, the REM content is between 0 and 0.0040%.
The lower limit of the REM content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.
A preferable upper limit of the REM content is 0.0032%, more preferably 0.0028%, still more preferably 0.0026%, and still more preferably 0.0024%.

REM in this specification refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids (lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71). It contains at least one element, and the REM content means the total content of these elements.

[Effects of steel material of this embodiment]
In the steel material of this embodiment, even in an unsteady state under a highly corrosive sour environment, it is possible to suppress hydrogen from penetrating into the steel material. Here, "it is possible to suppress the intrusion of hydrogen into the steel material even in an unsteady state in a highly corrosive sour environment" means that 5.0% by mass of NaCl saturated with 1 bar of H ₂ S gas and 0.0% by mass of NaCl saturated with 1 bar of H 2 S gas are used. The maximum value Per _max of the hydrogen permeability coefficient obtained in a hydrogen permeation test using a test liquid that is a mixed aqueous solution with 5% by mass of CH ₃ COOH and whose pH is 2.7 is 8.25 μA/cm or less. Yes, which also means that the difference ΔPer is 2.70 μA/cm or less.

[Method of measuring hydrogen permeability coefficient]
The hydrogen permeability coefficient Per ₉₀ and the difference ΔPer, which are indicators of the amount of hydrogen penetrating into the steel material under a highly corrosive sour environment, can be measured by the following method.
To measure the hydrogen permeability coefficient Per ₉₀ , a double-cell type cathode-charged hydrogen permeation test device 10 shown in FIG. 5 is used.

When the steel material is a steel pipe, a test piece 11 is taken from the center of the wall thickness. When the steel material is a steel plate, a test piece 11 is taken from the center of the plate thickness. When the steel material is a round steel (steel bar), a test piece 11 is taken from the center of the cross section perpendicular to the rolling direction. The size of the test piece 11 is not particularly limited. For example, it may be a disk-shaped test piece 11 with a thickness L11 of 0.1 cm (1 mm) and a diameter of 25 mm, or it may be a plate-like test piece.

The cathode charge hydrogen permeation test device 10 in FIG. 5 includes a hydrogen entry cell 20 and a hydrogen detection cell 30. The test piece 11 is sandwiched between the hydrogen entry side cell 20 and the hydrogen detection side cell 30 and fixed. Test liquid 21, which is a mixed aqueous solution (NACE A solution) of 5.0 mass% NaCl and 0.5 mass% CH ₃ COOH and whose pH is adjusted to 2.7, is prepared. A test liquid 21 is injected into the hydrogen entry cell 20. At this time, the liquid level of the test liquid 21 should be at a position lower than the lower end of the gas exhaust piping 222 of the gas supply and exhaust device 22 and higher than the lower end of the gas supply piping 221 of the gas supply and exhaust device 22. Make it so that After the test liquid 21 is injected into the hydrogen entry cell 20, the inside of the hydrogen entry cell 20 is degassed with high-purity Ar for one hour or more. After degassing, test gas (1 bar H ₂ S gas) is passed through the test liquid 21 for 30 minutes or more from the gas supply piping 221 of the gas supply/discharge device 22 to saturate it. The test liquid 21 saturated with 1 bar of _H2S is referred to as the "highly corrosive test liquid" 21. The highly corrosive test liquid 21 during the test is maintained at room temperature (24±3° C.), and the hydrogen entry side cell 20 is sealed. The reference electrode RE of the hydrogen entry cell 20 is a saturated KCl silver silver chloride reference electrode. Further, a 0.1 N (0.1 normal) NaOH aqueous solution 31 is injected into the hydrogen detection side cell 30 . The counter electrode CE of the hydrogen detection side cell 30 is made of platinum, and the reference electrode RE is made of a saturated KCl silver silver chloride reference electrode. In addition, in the test piece 11, the area A11 in contact with the highly corrosive test liquid 21 is defined as the test area A11. The test area A11 is, for example, 2.3 cm ² . In the test piece 11, the area in contact with the 0.1 N (0.1 normal) NaOH aqueous solution 31 is equal to or larger than the test area A11. Further, the specific liquid volume of both the hydrogen entry side cell 20 and the hydrogen detection side cell 30 is set to 30 mL/cm ² or more.

Due to the corrosion reaction that spontaneously progresses while the test piece 11 is naturally immersed, hydrogen is generated on the surface of the test piece 11 on the hydrogen entry side cell 20 side. A part of the generated hydrogen passes through the test piece 11 as interstitial hydrogen atoms H _ab and is oxidized into hydrogen ions in the hydrogen detection cell 30 . The oxidation current value at this time is measured using the potentio stud 40. In the hydrogen detection side cell 30, the test piece 11 is kept at a constant potential of +0.148 V with respect to the reference electrode (saturated KCl silver silver chloride reference electrode) RE, and the permeated hydrogen element (intruded hydrogen atom H _ab ) is converted into hydrogen ion. oxidizes to The hydrogen permeation current J (μA/cm ² ) is determined by dividing the measured oxidation current value by the test area A11 (2.3 cm ² ). Then, the hydrogen permeation coefficient Per (μA/cm) is determined by multiplying the hydrogen permeation current J by the thickness L11 (0.1 cm) of the test piece. During the test, a test gas (H ₂ S gas at 1 bar) is passed through the highly corrosive test liquid 21 from the gas supply piping 221 of the gas supply/discharge device 22 at a flow rate of about 10 mL/min to maintain saturation.

After the test piece 11 is brought into contact with the highly corrosive test liquid 21 and the NaOH aqueous solution 31, the hydrogen permeability coefficient Per is measured over time from the start of the test, and a graph of the hydrogen permeability coefficient Per over time shown in FIG. 2 is created. The test is terminated after 90 hours have passed from the start of the test.

In the created graph of the hydrogen permeability coefficient over time, the hydrogen permeability coefficient Per ₉₀ (μA/cm) at the time point of 90 hours from the start of the test is determined. Furthermore, the maximum value Per _max (μA/cm) of the hydrogen permeability coefficient Per is determined. The hydrogen permeability coefficient Per ₉₀ is subtracted from the obtained maximum value Per _max to obtain the difference ΔPer (μA/cm).

As mentioned above, the steel material in this embodiment is a mixed aqueous solution of 5.0 mass% NaCl and 0.5 mass% CH ₃ COOH saturated with 1 bar of H ₂ S gas, and has a pH of 2.7. The maximum value Permax of the hydrogen permeability coefficient obtained in a hydrogen permeation test using the test liquid obtained is 8.25 μA/cm or less, and the difference ΔPer is 2.70 μA/cm or less.

[Preferred form of this embodiment]
Preferably, in the chemical composition of the steel material of this embodiment, Fn1 defined by the following formula (1) is 4.00 or more.
Fn1=(2.0×Si+0.3×Cu+0.9×Ni+Cr+1.2×Mo+8.0×Zr+5.0×Sb+5.0×Co)−(2.0×C+2.0×Mn) (1)

FIG. 6 is a diagram showing the relationship between Fn1 and the maximum value Per _max of the hydrogen permeability coefficient in a steel material in which the content of each element in the chemical composition is within the range of this embodiment. FIG. 7 is a diagram showing the relationship between Fn1 and the difference ΔPer in hydrogen permeability coefficient in a steel material whose chemical composition contains each element within the range of this embodiment. FIGS. 6 and 7 were created based on the results obtained in the examples described later.

Referring to FIGS. 6 and 7, when the content of each element in the chemical composition is within the range of this embodiment, the maximum value Per _max and the difference ΔPer decrease significantly as Fn1 increases. If Fn1 is 4.00 or more, the maximum value Per _max will be 6.50 μA/cm or less, and the difference ΔPer will be 1.00 μA/cm or less. Therefore, if Fn1 is 4.00 or more, even in an unsteady state in a highly corrosive sour environment, it is possible to further suppress hydrogen from penetrating into the steel material.

A more preferable lower limit of Fn1 is 4.50. Referring to FIGS. 6 and 7, when Fn1 becomes 4.50 or more, even if Fn1 increases, the maximum value Per _max and the difference ΔPer do not decrease much. That is, in the graphs of FIGS. 6 and 7, there is an inflection point near Fn1=4.50. Therefore, more preferably, in the chemical composition of the steel material of this embodiment, Fn1 is 4.50 or more. In this case, the maximum value Per _max of the hydrogen permeability coefficient becomes 6.10 μA/cm or less, and the difference ΔPer becomes 0.75 μA/cm or less. Therefore, even in an unsteady state in a highly corrosive sour environment, it is possible to further suppress hydrogen from entering the steel material.

A more preferable lower limit of Fn1 is 4.60. In this case, the maximum value Per _max of the hydrogen permeability coefficient becomes 6.00 μA/cm or less, and the difference ΔPer becomes 0.60 μA/cm or less. Therefore, even in an unsteady state under a highly corrosive sour environment, hydrogen penetration into the steel material can be significantly suppressed. A more preferable lower limit of Fn1 is 4.70, still more preferably 4.80, and still more preferably 4.90.

[About microstructure]
The microstructure of the steel material of this embodiment is not particularly limited. If the content of each element in the chemical composition of the steel material of this embodiment is within the above-mentioned range, it is possible to suppress hydrogen from penetrating into the steel material based on electrochemical action.

Note that the microstructure of the steel material having the chemical composition of the present embodiment is, for example, mainly composed of one or more of ferrite, pearlite, martensite, and bainite, and the remainder is retained austenite and/or MA (Martensite-Austenite Constituent). It is. Here, martensite also includes tempered martensite. Bainite also includes tempered bainite. "Mainly consisting of one or more of ferrite, pearlite, martensite, and bainite" means that the total area ratio of ferrite, pearlite, martensite, and bainite is 80% or more.

However, the suppression of hydrogen penetration into the steel material, which is a feature of the steel material of this embodiment, largely depends on the chemical composition as described above, and has little effect on the microstructure. Therefore, the microstructure of the steel material of this embodiment is not particularly limited.

In addition, when the steel material of this embodiment is required to have high strength, preferably the total area ratio of martensite and bainite in the microstructure of the steel material is 80%, more preferably 90%, and still more preferably 95%. %.

[About the method of measuring the total area ratio of martensite and bainite]
The total area ratio of martensite and bainite in the microstructure can be determined by well-known microstructure observation.
A test piece is taken from the center of the thickness of the steel material. The center of thickness is the center of wall thickness when the steel material is a steel pipe, the center of thickness when the steel material is a steel plate, and the center of thickness perpendicular to the rolling direction when the steel material is a round steel (bar). This is the center of the cross section. The observation surface of the test piece is, for example, 10 mm x 10 mm. The observation plane is a cross section perpendicular to the rolling direction of the steel material.

Polish the observation surface of the test piece to a mirror surface. The surface to be observed after polishing is immersed in a nital corrosive solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed in 10 fields of view using a secondary electron image using a scanning electron microscope (SEM). The field of view area is, for example, 10000 μm ² (1000x magnification).

In each field of view, martensite and bainite can be distinguished from other structures (ferrite, pearlite, etc.) based on their morphology. Specifically, a structure having a lamellar structure can be identified as pearlite, a structure including laths or lenses can be identified as martensite and bainite, and a structure without a substructure within the grains can be identified as ferrite. Identify martensite and bainite in each field. The total area ratio (%) of martensite and bainite is determined based on the total area of the 10 visual fields and the specified total area of martensite and bainite.

[Yield strength]
The yield strength of the steel material of this embodiment is not particularly limited.
A preferred yield strength is 655 MPa or more (95 ksi or more). A preferred lower limit of yield strength is 758 MPa (110 ksi). The upper limit of the yield strength is not particularly limited, but in the case of the above chemical composition, the upper limit of the yield strength is, for example, less than 862 MPa.

In this specification, yield strength means 0.2% offset yield strength (MPa) obtained by a tensile test at room temperature (24±3° C.) in accordance with ASTM E8/E8M (2021).

[Method of measuring yield strength]
Yield strength is measured by the following method. Specifically, a tensile test is performed in accordance with ASTM E8/E8M (2021). Specifically, a tensile test piece is taken from the steel material. The size of the tensile test piece is not particularly limited. The tensile test piece is, for example, a round bar tensile test piece with a parallel part diameter of 6.0 mm and a gage length of 30.0 mm.
If the steel material is a steel pipe, take a tensile test piece from the center of the wall thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the steel pipe. If the steel material is a steel plate, take a tensile test piece from the center of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel material is round steel, a tensile test piece is taken from section R/2. In this specification, round steel means a steel bar whose cross section perpendicular to the axial direction is circular. The R/2 section means the center of the radius R in the cross section perpendicular to the rolling direction of the round steel. The longitudinal direction of the tensile test piece is parallel to the axial direction of the round steel.
Using the collected tensile test piece, a tensile test is conducted in the atmosphere at room temperature (24±3°C). The obtained 0.2% offset proof stress (MPa) is defined as yield strength (MPa).

[Shape of steel material]
The shape of the steel material of this embodiment is not particularly limited. The steel material of this embodiment may be a steel pipe, a steel plate, or a round steel.

Preferably, the steel material of this embodiment is a steel pipe for oil wells. Steel pipes for oil wells are, for example, casings, tubing, drill pipes, etc. used for drilling oil or gas wells, extracting crude oil or natural gas, and the like. More preferably, the oil well steel pipe of this embodiment is a seamless steel pipe.

[Production method]
An example of the method for manufacturing steel materials of this embodiment will be described. Note that the manufacturing method described below is an example, and the method for manufacturing the steel material of this embodiment is not limited thereto. That is, as long as the steel material of this embodiment having the above-mentioned configuration can be manufactured, the manufacturing method is not limited to the manufacturing method described below. However, the manufacturing method described below is a suitable manufacturing method for manufacturing the steel material of this embodiment.

An example of the method for manufacturing a steel material according to the present embodiment includes a material preparation step, a hot working step, and a heat treatment step. Each step will be explained in detail below.

[Material preparation process]
In the material preparation step, first, molten steel having the above chemical composition is produced by a well-known refining method. The produced molten steel is used to produce slabs by continuous casting. Here, the slab is a slab, bloom, or billet. Instead of the slab, an ingot may be manufactured using the above-mentioned molten steel by an ingot forming method. If necessary, the slab, bloom or ingot may be hot rolled to produce a billet. Through the above manufacturing process, a material (slab, bloom, or billet) is manufactured.

[Hot processing process]
In the hot working step, the prepared material is hot worked. When the final product is a steel pipe, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. Hot processing is performed on the material extracted from the heating furnace to produce raw pipes (seamless steel pipes). For example, the Mannesmann method is performed as hot working to produce a blank pipe. In this case, the billet is pierced and rolled using a piercer. In the case of piercing rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The billet after piercing and rolling is subjected to elongation rolling using a mandrel mill. Furthermore, if necessary, the billet after elongation rolling is subjected to sizing rolling using a reducer or a sizing mill. Through the above steps, a raw pipe is manufactured. The cumulative area reduction rate in the hot working process is not particularly limited, but is, for example, 20 to 70%.

A raw pipe may be manufactured from a billet by a hot working method other than the Mannesmann method. For example, in the case of a short thick-walled steel material such as a coupling, the raw tube may be manufactured by forging such as the Erhard method, or the raw tube may be manufactured by hot extrusion.

When the final product is a steel plate, the steel plate is manufactured by hot rolling the material (slab) using, for example, one or more rolling mills including a pair of roll groups. The heating temperature before hot rolling is not particularly limited, but is, for example, 1100 to 1300°C.

When the final product is a round steel, for example, the material (bloom) is subjected to blooming using a blooming mill and/or hot rolling using a continuous rolling mill. In other words, the raw material may be subjected to blooming rolling to form a round steel. The raw material may not be subjected to blooming rolling, but may be hot rolled using a continuous rolling mill to form a round steel. The material may be subjected to blooming using a blooming mill and hot rolling using a continuous rolling mill to form a round steel. A continuous rolling mill has a plurality of rolling stands arranged in a row, and each stand includes a pair of rolling rolls. When performing blooming rolling, the heating temperature before blooming rolling is not particularly limited, but is, for example, 1100 to 1300°C. When performing hot rolling using a continuous rolling mill, the heating temperature before hot rolling is not particularly limited, but is, for example, 1100 to 1300°C.

[Heat treatment process]
A heat treatment step may or may not be performed. That is, the steel material of this embodiment may be an as-rolled material. When implementing a heat treatment process, the heat treatment process includes a quenching process and a tempering process.

[Quenching process]
In the heat treatment process, first, a quenching process is performed on the steel material manufactured in the hot working process. Hardening is carried out by a well-known method. Specifically, the steel material after hot working is charged into a heat treatment furnace and maintained at the quenching temperature. The quenching temperature is above the A _C3 transformation point, for example, 900 to 1000°C. After holding the steel material at the quenching temperature, it is rapidly cooled (quenched). The holding time at the quenching temperature is not particularly limited, but is, for example, 10 to 60 minutes. The quenching method is, for example, water cooling. The quenching method is not particularly limited. If the steel material is a steel pipe, the raw pipe may be rapidly cooled by immersing it in a water tank or oil tank, or by pouring or spraying cooling water onto the outside and/or inside of the steel pipe using shower cooling or mist cooling. Then, the steel pipe may be rapidly cooled.

Note that after hot working, quenching (direct quenching) may be performed immediately after hot working without cooling the steel material to room temperature, or the steel material may be heated in a reheating furnace before the temperature of the steel material after hot working falls. The quenching may be carried out after charging the material to a quenching temperature and maintaining the quenching temperature.

The quenching temperature described above means the furnace temperature when a heat treatment furnace or reheating furnace is used, and the temperature of the steel material when direct quenching is used. Holding time means in-furnace time (time from charging to a heat treatment furnace or reheating furnace until extraction).

[Tempering process]
The steel material after quenching is further subjected to a tempering step. In the tempering process, the yield strength of the steel material is adjusted. In the steel material of this embodiment, the tempering temperature is, for example, 500° C. to the A _C1 transformation point. The holding time at the tempering temperature is not particularly limited, but is, for example, 10 to 180 minutes. By appropriately adjusting the tempering temperature according to the chemical composition, the yield strength of the steel material having the above chemical composition that satisfies formula (1) is adjusted. Preferably, the yield strength of the steel material is adjusted to 655 MPa or more.

The tempering temperature explained above means the furnace temperature (° C.) in the heat treatment furnace, and the holding time at the tempering temperature means the time in the furnace (the time from charging to the heat treatment furnace until extraction).

Note that quenching and tempering may be performed once or multiple times. For example, after hardening and tempering, hardening and tempering may be performed again. Hardening and tempering may be performed only once, or as described above, hardening and tempering may not be performed.

By carrying out the above manufacturing process, the steel material of this embodiment can be manufactured.

The effects of the steel material of this embodiment will be explained in more detail with reference to 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.

Steel materials (seamless steel pipes) having the chemical compositions shown in Tables 2-1 and 2-2 were manufactured.

The "-" part in Tables 2-1 and 2-2 indicates that the content of the corresponding element is 0% when rounding off the numbers listed in Tables 2-1 and 2-2. It means that it was. For example, the Zr content of Test No. 1 was rounded to the fifth decimal place, meaning it was 0%. The Sb content of Test No. 1 was rounded to the second decimal place, meaning that it was 0%. The Ti content of Test No. 1 was rounded to the fourth decimal place, meaning that it was 0%. The B content of Test No. 1 was rounded to the fifth decimal place, meaning it was 0%.

The above-mentioned molten steel was melted in a 50 kg vacuum furnace, and an ingot was produced by an ingot forming method. The ingot was heated at 1250°C for 3 hours. Hot forging was performed on the heated ingot to produce a block. The block after hot forging was soaked at 1230° C. for 15 minutes and hot rolled to produce a steel material (steel plate) having a thickness of 15 mm.

Hardening was performed on the steel material. The quenching temperature was 950° C. in all test numbers, and the holding time at the quenching temperature was 15 minutes in all test numbers. The steel material after quenching was tempered by holding at a tempering temperature (° C.) of 560° C. for 30 minutes to produce a steel material (steel plate). A steel material was manufactured by the above manufacturing process.

The total area ratio of martensite and bainite for each test number was determined by the method described in [Method for measuring the total area ratio of martensite and bainite]. Note that a test piece was taken from the center of the thickness of the steel material (steel plate). The observation surface of the test piece was 10 mm x 10 mm. Moreover, the visual field area was 10000 μm ² . As a result, the total area ratio of martensite and bainite was 90% or more in all test numbers.

Furthermore, the yield strength of each test number was measured by the method described in [Method of measuring yield strength]. The size of the round bar test piece was 6.0 mm in parallel part diameter and 30.0 mm in gage distance. The longitudinal direction of the round bar test piece was parallel to the pipe axis direction of the steel material (seamless steel pipe). As a result, the yield strength was 655 to less than 862 MPa in all test numbers.

[Evaluation test]
The following evaluation tests were conducted on the manufactured steel materials.

[Hydrogen permeation test]
A test piece was taken from the center of the plate thickness of the steel material of each test number. The test piece had a disk shape with a thickness of 1 mm (0.1 cm) and a diameter of 25 mm.

A cathode charge hydrogen permeation test apparatus 10 shown in FIG. 5 was prepared. The test piece 11 was sandwiched between the hydrogen entry side cell 20 and the hydrogen detection side cell 30 and fixed. A test solution was prepared which was a mixed aqueous solution of 5.0% by mass of NaCl and 0.5% by mass of CH ₃ COOH and whose pH was adjusted to 2.7. A test liquid 21 was injected into the cell 20 on the hydrogen entry side. At this time, the liquid level of the test liquid 21 should be at a position lower than the lower end of the gas exhaust piping 222 of the gas supply and exhaust device 22 and higher than the lower end of the gas supply piping 221 of the gas supply and exhaust device 22. I made it so that After the test liquid 21 was injected into the hydrogen entry cell 20, the inside of the hydrogen entry cell 20 was degassed with high-purity Ar for one hour or more. After degassing, test gas (1 bar H ₂ S gas) was passed through the test liquid 21 for 30 minutes or more from the gas supply piping 221 of the gas supply/discharge device 22 to saturate it. The test liquid (highly corrosive test liquid) 21 saturated with 1 bar of H ₂ S during the test was kept at room temperature (24±3° C.), and the hydrogen entry side cell 20 was kept in a sealed state. The reference electrode RE of the hydrogen entry cell 20 was a saturated KCl silver silver chloride reference electrode. Further, a 0.1N (0.1 normal) NaOH aqueous solution 31 was injected into the hydrogen detection side cell 30. The counter electrode CE of the hydrogen detection side cell 30 was made of platinum, and the reference electrode RE was made of a saturated KCl silver silver chloride reference electrode. In addition, in the test piece 11, the area A11 in contact with the highly corrosive test liquid 21 was defined as the test area A11. The test area A11 was 2.3 ^cm2 . In the test piece 11, the area in contact with the 0.1 N (0.1 normal) NaOH aqueous solution 31 was set to be equal to or larger than the test area A11. Further, the specific liquid volume of both the hydrogen entry side cell 20 and the hydrogen detection side cell 30 was set to 110 mL/cm ² .

Due to the corrosion reaction that spontaneously progressed while the test piece 11 was naturally immersed, hydrogen was generated on the surface of the test piece 11 on the hydrogen entry side cell 20 side. A part of the generated hydrogen passed through the test piece 11 as hydrogen atoms and was oxidized to hydrogen ions in the hydrogen detection cell 30, so the oxidation current value at this time was measured using the potentio stud 40. In the hydrogen detection side cell 30, the test piece 11 was kept at a constant potential of +0.148 V with respect to the reference electrode (saturated KCl silver silver chloride reference electrode) RE, and the permeated hydrogen element was oxidized to hydrogen ions. The measured oxidation current value was divided by the test area A11 (2.3 cm ² ) to determine the hydrogen permeation current J (μA/cm ² ). Then, the hydrogen permeation coefficient Per (μA/cm) was determined by multiplying the hydrogen permeation current J by the thickness L11 (0.1 cm) of the test piece. During the test, a test gas (H ₂ S gas at 1 bar) was passed through the gas supply piping 221 of the gas supply/discharge device 22 into the highly corrosive test liquid 21 at a flow rate of about 10 mL/min to maintain saturation.

The hydrogen permeability coefficient Per was measured over time from the start of the test, and a graph of the hydrogen permeability coefficient over time shown in FIG. 2 was created. The test was terminated 90 hours after the start of the test. In the created graph of the hydrogen permeability coefficient over time, the hydrogen permeability coefficient Per ₉₀ at the time point of 90 hours from the start of the test was determined. Furthermore, the maximum value Per _max (μA/cm) of the hydrogen permeability coefficient Per was determined. The hydrogen permeability coefficient Per ₉₀ was subtracted from the obtained maximum value Per _max to obtain the difference ΔPer (μA/cm).

[Test results]
The test results are shown in Table 3. In addition, in Table 3, Fn1 of each test number is shown in the "Fn1" column.

Referring to Table 2-1, Table 2-2, and Table 3, in test numbers 4 to 24, the chemical composition was appropriate. Therefore, a hydrogen permeation test was conducted using a test solution that was a mixed aqueous solution of 5.0% by mass NaCl and 0.5% by mass CH ₃ COOH saturated with 1 bar of H ₂ S gas and whose pH was 2.7. The maximum value Per _max of the hydrogen permeability coefficient obtained was 8.25 μA/cm or less, and the difference ΔPer was 2.70 μA/cm or less. Therefore, it was possible to sufficiently suppress hydrogen from entering the steel material in an unsteady state.

Furthermore, among test numbers 4 to 24, Fn1 was 4.00 or higher in test numbers 8, 11 to 16, and 19 to 24. Therefore, in these test numbers, the maximum value Per _max of the hydrogen permeability coefficient is 6.50 μA/cm or less, and the difference ΔPer is 1.00 μA/cm or less, which prevents hydrogen from penetrating into the steel material in an unsteady state. I was able to suppress it further.

Furthermore, among test numbers 4 to 24, Fn1 was 4.50 or higher in test numbers 11 to 15, 19, and 22 to 24. Therefore, in these test numbers, the maximum value Per _max of the hydrogen permeability coefficient is 6.10 μA/cm or less, and the difference ΔPer is 0.75 μA/cm or less, which prevents hydrogen from penetrating into the steel material in an unsteady state. I was able to suppress it further.

Further, among test numbers 4 to 24, Fn1 was 4.60 or higher in test numbers 11 to 14, 19, and 22 to 24. Therefore, in these test numbers, the maximum value Per _max of the hydrogen permeability coefficient is 6.00 μA/cm or less, and the difference ΔPer is 0.60 μA/cm or less, which prevents hydrogen from penetrating into the steel material in an unsteady state. It was significantly suppressed.

On the other hand, in the steel materials of test numbers 1 to 3, the Zr content was too low. Therefore, a hydrogen permeation test was conducted using a test solution that was a mixed aqueous solution of 5.0% by mass NaCl and 0.5% by mass CH ₃ COOH saturated with 1 bar of H ₂ S gas and whose pH was 2.7. The maximum value Per _max of the hydrogen permeability coefficient greatly exceeded 8.25 μA/cm, and the difference ΔPer greatly exceeded 2.70 μA/cm.

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 (4)

The chemical composition is in mass%,
C: 0.20-0.45%,
Si: 0.60-1.30%,
Mn: 0.02-1.00%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.010-0.100%,
N: 0.0100% or less,
Cr: 0.10-3.00%,
Mo: 0.35-3.00%,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
Zr: 0.0010-0.1000%,
O: 0.0050% or less,
Sb: 0 to 0.50%,
Co: 0 to 0.50%,
W: 0-0.50%,
Ti: 0 to 0.030%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%,
Ca: 0-0.0040%,
Mg: 0 to 0.0040%, and
Contains rare earth elements: 0 to 0.0040%,
The remainder consists of Fe and impurities,
Steel material. The steel material according to claim 1, further comprising:
Fn1 defined by the following formula (1) is 4.00 or more,
Steel material.
Fn1=(2.0×Si+0.3×Cu+0.9×Ni+Cr+1.2×Mo+8.0×Zr+5.0×Sb+5.0×Co)−(2.0×C+2.0×Mn) (1)
Here, each element symbol in formula (1) is substituted with the content in mass % of the corresponding element. The steel material according to claim 1 or claim 2,
The chemical composition is
Sb: 0.01 to 0.50%,
Co: 0.01 to 0.50%,
W: 0.01-0.50%,
Ti: 0.003 to 0.030%,
Nb: 0.003 to 0.150%,
V: 0.005-0.500%,
B: 0.0003 to 0.0030%,
Ca: 0.0003-0.0040%,
Mg: 0.0003 to 0.0040% and
Rare earth elements: 0.0003 to 0.0040%,
Containing one or more selected from the group consisting of
Steel material. The steel material according to claim 1,
The steel material is a steel pipe for oil wells,
Steel material.

Images