JP7759016B2 - steel material - Google Patents

Description

This disclosure relates to steel materials.

As oil and gas wells (hereinafter, collectively referred to as "oil wells") become deeper, there is a demand for higher strength oil well steel materials, such as oil well steel pipes. Specifically, 80 ksi grade (yield strength of 80 to less than 95 ksi, i.e., 552 to less than 655 MPa) and 95 ksi grade (yield strength of 95 to less than 110 ksi, i.e., 655 to less than 758 MPa) oil well steel materials are widely used, and recently there has been an increasing demand for 110 ksi grade (yield strength of 110 to less than 125 ksi, i.e., 758 to less than 862 MPa) oil well steel materials.

Oil wells may also contain corrosive gases such as hydrogen sulfide gas (H ₂ S) and carbon dioxide gas (CO ₂ ). Therefore, steel materials intended for use as oil well steel materials are required to have not only high strength but also excellent corrosion resistance. Furthermore, oil well steel materials are subjected to stress during use. For this reason, sulfide stress cracking resistance (hereinafter referred to as SSC resistance) has been used as an indicator of the excellent corrosion resistance of oil well steel materials.

Technologies for improving the strength and SSC resistance of steel materials have been proposed in JP 2006-28612 A (Patent Document 1), WO 2008/123422 A (Patent Document 2), and JP 2017-166060 A (Patent Document 3).

The steel material disclosed in Patent Document 1 is a steel for steel pipes, and consists, by mass, of C: 0.2-0.7%, Si: 0.01-0.8%, Mn: 0.1-1.5%, S: 0.005% or less, P: 0.03% or less, Al: 0.0005-0.1%, Ti: 0.005-0.05%, Ca: 0.0004-0.005%, N: 0.007% or less, Cr: 0.1-1.5%, Mo: 0.2-1.0%, and the balance being Fe and impurities. This steel material further contains non-metallic inclusions containing Ca, Al, Ti, N, O, and S, with a (Ca%)/(Al%) ratio of 0.55-1.72 and a (Ca%)/(Ti%) ratio of 0.7-19. Patent Document 1 describes that this steel has a high yield strength exceeding 758 MPa and excellent SSC resistance.

The steel disclosed in Patent Document 2 is a low-alloy steel containing, by mass%, 0.10 to 0.20% C, 0.05 to 1.0% Si, 0.05 to 1.5% Mn, 1.0 to 2.0% Cr, 0.05 to 2.0% Mo, 0.10% or less Al, and 0.002 to 0.05% Ti, with Ceq (=C + (Mn/6) + (Cr + Mo + V)/5) being 0.65 or more, the balance being Fe and impurities, including 0.025% or less P, 0.010% or less S, 0.007% or less N, and less than 0.0003% B. This steel further contains 0.1 _or ^less _M23C6 type precipitates having a grain size of 1 μm or more per mm2. Patent Document 2 states that this steel has a yield strength of 654 to 793 MPa and has excellent SSC resistance even in a high-pressure hydrogen sulfide environment.

The steel material disclosed in Patent Document 3 is a material for high-strength oil well steel pipes, and consists, by mass, of C: 0.20-0.45%, Si: 0.05-0.40%, Mn: 0.3-0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005-0.10%, N: 0.001-0.006%, Cr: 0.1-0.8%, Mo: 0.1-1.6%, V: 0.02-0.2%, Nb: 0.001-0.04%, B: 0.0003-0.0030%, O (oxygen): 0.0030% or less, with the balance being Fe and unavoidable impurities. This steel material further has a Rockwell hardness HRC that satisfies the formula (15.6 × [%C] + 29.2 ≦ HRC < 60.5 × [%C] + 31.1). Patent Document 3 states that this steel material makes it possible to obtain steel pipes having a yield strength of 758 to less than 862 MPa and excellent SSC resistance.

Japanese Patent Application Laid-Open No. 2006-28612 WO 2008/123422 JP 2017-166060 A

The techniques disclosed in Patent Documents 1 to 3 above make it possible to obtain oil well steel materials with excellent SSC resistance. However, oil well steel materials with excellent SSC resistance may also be obtained using techniques other than those disclosed in Patent Documents 1 to 3 above.

Furthermore, in recent years, as oil well environments have become more severe, there has been a demand for even higher yield strength in oil well steel materials. Specifically, there is a demand for 150 ksi-class oil well steel materials (yield strength greater than 150 to 170 ksi, i.e., greater than 1034 to 1172 MPa). However, Patent Documents 1 to 3 do not consider achieving both a high 150 ksi-class yield strength and excellent SSC resistance.

The purpose of this disclosure is to provide a steel material that combines high strength of the 150 ksi grade (over 1034 to 1172 MPa) with excellent SSC resistance.

The steel material according to the present disclosure is
In mass%,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.50%,
Mo: 0.40-2.00%,
Ti: 0.002 to 0.020%,
Nb: 0.002-0.100%,
V: 0.05-0.30%,
B: 0.0005-0.0040%,
N: 0.0100% or less,
O: 0.0040% or less,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
W: 0-0.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
the balance being Fe and impurities;
The yield strength is greater than 1034 to 1172 MPa,
In the steel material,
The Al content is 20% or more, the O content is 10% or more, and the number density of Al oxide particles having a major axis of 5.0 μm or more is less than 30 particles/200 ^mm2 , in terms of mass %,
In terms of mass%, the Al content is less than 20%, the Si content is 20% or more, the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 pieces/200 ^mm2 or less.

The steel material disclosed herein combines high strength of 150 ksi grade (over 1034 to 1172 MPa) with excellent SSC resistance.

FIG. 1 is a graph showing the relationship between the number density (pieces/200 mm ² ) of coarse Si oxides (Si oxides with a major axis of 5.0 μm or more) and the number of SSC occurrences (pieces), which is an index of SSC resistance, in this example.

First, the inventors focused on the chemical composition and investigated how to obtain a steel material that combines 150 ksi-class yield strength with excellent SSC resistance. As a result, the inventors discovered a composition, by mass, of C: 0.15-0.45%, Si: 0.05-1.00%, Mn: 0.05-1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005-0.100%, Cr: 0.30-1.50%, Mo: 0.40-2.00%, Ti: 0.002-0.020%, Nb: 0.002-0.100%, V: 0.05-0.30%, B: 0.0005-0. It was thought that if a steel material consisting of the following composition was used, it would be possible to achieve both a 150 ksi-class yield strength and excellent SSC resistance: 0.0040%, N: 0.0100% or less, O: 0.0040% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth elements: 0 to 0.0100%, and the balance being Fe and impurities.

Next, the inventors investigated various methods for improving the SSC resistance of steel materials having the above-mentioned chemical composition and a 150 ksi-class yield strength. Specifically, the inventors considered that if coarse oxide-based inclusions could be reduced, it might be possible to improve SSC resistance while maintaining yield strength. Here, in steel materials having the above-mentioned chemical composition, Al oxides, mainly _{Al2O3 ,} tend to coarsen. Therefore, the inventors first focused on coarse Al oxides.

As a result of studies by the present inventors, it has become clear that in a steel material having the above-mentioned chemical composition and a 150 ksi-class yield strength, if the number density of Al oxides having a major axis of 5.0 μm or more is less than 30 particles/200 ^mm2 , SSC resistance may be improved. Herein, particles having an Al content of 20% or more and an O content of 10% or more, in mass%, are also referred to as "Al oxides." Furthermore, in this specification, Al oxides having a major axis of 5.0 μm or more are also referred to as "coarse Al oxides."

Here, Al oxides are hard oxides and tend to reduce the corrosion resistance of steel materials. In particular, when the yield strength is increased to 150 ksi class, the effect of coarse Al oxides tends to become apparent, and SSC resistance tends to decrease significantly. Therefore, in the steel material according to this embodiment having the above-described chemical composition and a 150 ksi-class yield strength, the number density of coarse Al oxides is set to less than 30 particles/200 ^mm2 .

On the other hand, even in steel materials having the above-mentioned chemical composition and a number density of coarse Al oxides of less than 30 particles/200 ^mm2 , when the steel material has a yield strength of 150 ksi, it has sometimes been impossible to obtain stable and excellent SSC resistance. Therefore, the present inventors have investigated ^various methods for stably obtaining excellent SSC resistance for steel materials having the above-mentioned chemical composition, a yield strength of 150 ksi, and a number density of coarse Al oxides of less than 30 particles/200 mm2. As a result of detailed investigations by the present inventors, it has become clear that in steel materials having the above-mentioned chemical composition, a yield strength of 150 ksi, and a number density of coarse Al oxides of less than 30 particles/200 ^mm2 , excellent SSC resistance may be stably obtained if not only the coarse Al oxides but also the coarse Si oxides in the steel material can be reduced.

Herein, in this specification, particles having an Al content of less than 20%, a Si content of 20% or more, and an O content of 10% or more, in mass%, are also referred to as "Si oxides." Furthermore, in this specification, Si oxides having a major axis of 5.0 μm or more are also referred to as "coarse Si oxides." Hereinafter, the relationship between coarse Si oxides and SSC resistance will be specifically explained with reference to the drawings for a steel material having the above-mentioned chemical composition, a 150 ksi-class yield strength, and a number density of coarse Al oxides of less than 30 particles/200 ^mm2 .

Fig. 1 is a diagram showing the relationship between the number density (particles/200 mm) of coarse Si oxides (Si oxides with a major axis of 5.0 µm or more) and the number of SSC occurrences (particles), which is an index of SSC resistance, in this example. Fig. 1 was created using the number density (particles/200 ^{mm )} of coarse Si oxides determined by a method described later and the number of SSC occurrences (particles), also determined by a method described later, for steel materials among the examples described later that satisfy the above-mentioned chemical composition, have a 150 ksi-class yield strength, and have a number density of coarse Al oxides of ^{less than 30} particles/200 mm.

1 , in a steel material having the above-described chemical composition, a 150 ksi-class yield strength, and a number density of coarse Al oxides of less than 30 particles/200 ^mm2 , the number of SSC occurrences was zero when the number density of coarse Si oxides was 5 particles/200 ^mm2 or less, demonstrating excellent SSC resistance. Therefore, in this embodiment, the steel material has the above-described chemical composition and a 150 ksi-class yield strength, the number density of coarse Al oxides is less than 30 particles/200 ^mm2 , and the number density of coarse Si oxides is 5 particles/200 mm2 or ^less . As a result, the steel material according to this embodiment can achieve both a 150 ksi-class yield strength and excellent SSC resistance.

The details of why reducing the number density of coarse Si oxides improves the SSC resistance of steel materials are not clear. However, the present inventors speculate as follows: When manufacturing steel materials having the above-mentioned chemical composition, deoxidation using aluminum (Al) is mainly performed in the steelmaking process. Therefore, for steel materials having the above-mentioned chemical composition, Al oxides, typified by _{Al2O3 ,} have been studied, and the few Si oxides have not been given much attention. However, when the yield strength is increased to the 150 ksi class, there is a possibility that the deterioration of SSC resistance is likely to become apparent not only from coarse Al oxides but also from the few coarse Si oxides. Therefore, the present inventors speculate that by not only reducing the number density of coarse Al oxides to less ^than 30/200 mm2 but also reducing the number density of coarse Si oxides to 5/200 ^mm2 or less, excellent SSC resistance can be stably obtained even with a 150 ksi-class yield strength.

It is possible that the SSC resistance of steel materials is enhanced by a mechanism different from that conjectured by the present inventors. However, it has been proven by the examples described below that in steel materials having the above-mentioned chemical composition, a 150 ksi-class yield strength, and a number density of coarse Al oxides of less than 30 particles/200 ^mm2 , excellent SSC resistance can be obtained by setting the number density of coarse Si oxides to ⁵ particles/200 mm2 or less.

The gist of the steel material of this embodiment, which was completed based on the above findings, is as follows:

[1]
A steel material,
In mass%,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.50%,
Mo: 0.40-2.00%,
Ti: 0.002 to 0.020%,
Nb: 0.002-0.100%,
V: 0.05-0.30%,
B: 0.0005-0.0040%,
N: 0.0100% or less,
O: 0.0040% or less,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
W: 0-0.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
the balance being Fe and impurities;
The yield strength is greater than 1034 to 1172 MPa,
In the steel material,
The Al content is 20% or more, the O content is 10% or more, and the number density of Al oxide particles having a major axis of 5.0 μm or more is less than 30 particles/200 ^mm2 , in terms of mass %,
The Al content is less than 20%, the Si content is 20% or more, and the O content is 10% or more, in mass%, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 pieces/200 ^mm2 or less.
Steel material.

[2]
The steel material according to [1],
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
W: 0.01-0.50%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%, and
Rare earth elements: containing one or more elements selected from the group consisting of 0.0001 to 0.0100%;
Steel material.

[3]
The steel material according to [1] or [2],
The steel material is a seamless steel pipe.
Steel material.

The shape of the steel material according to this embodiment is not particularly limited. The steel material according to this embodiment may be a steel pipe, a round bar (solid material), or a steel plate. Note that round bar refers to a steel bar having a circular cross section perpendicular to the axial direction. The steel pipe may also be a seamless steel pipe or a welded steel pipe.

The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

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

C: 0.15-0.45%
Carbon (C) improves the hardenability of steel and increases its strength. Furthermore, C promotes the spheroidization of carbides during tempering during the manufacturing process, thereby improving the SSC resistance of the steel. If the C content is too low, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is too high, even if the contents of other elements are within the ranges of this embodiment, the amount of carbides will be too large, and the SSC resistance of the steel will decrease. Therefore, the C content is 0.15 to 0.45%. The preferred lower limit of the C content is 0.18%, more preferably 0.20%, even more preferably 0.22%, and even more preferably 0.23%. The preferred upper limit of the C content is 0.40%, more preferably 0.38%, even more preferably 0.35%, and even more preferably 0.30%.

Si: 0.05-1.00%
Silicon (Si) deoxidizes steel. If the Si content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, a large number of coarse Si oxides are formed, and the SSC resistance of the steel may decrease, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.05 to 1.00%. A preferred lower limit of the Si content is 0.10%, more preferably 0.15%, and even more preferably 0.20%. A preferred upper limit of the Si content is 0.85%, more preferably 0.75%, even more preferably 0.60%, even more preferably 0.50%, and even more preferably 0.40%.

Mn: 0.05-1.00%
Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, coarse sulfide-based inclusions are formed, reducing the SSC resistance of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.05 to 1.00%. A preferred lower limit of the Mn content is 0.06%, more preferably 0.08%, and even more preferably 0.10%. A preferred upper limit of the Mn content is 0.90%, more preferably 0.80%, even more preferably 0.70%, even more preferably 0.60%, and even more preferably 0.50%.

P: 0.030% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries, reducing the SSC resistance of the steel material. Therefore, the P content is 0.030% or less. A preferred upper limit of the P content is 0.025%, more preferably 0.020%, even more preferably 0.015%, and even more preferably 0.010%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

S: 0.0050% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is greater than 0%. If the S content is too high, even if the contents of other elements are within the ranges of this embodiment, S segregates at grain boundaries, reducing the SSC resistance of the steel material. Therefore, the S content is 0.0050% or less. A preferred upper limit of the S content is 0.0040%, more preferably 0.0032%, even more preferably 0.0030%, even more preferably 0.0020%, and even more preferably 0.0015%. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

Al: 0.005-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect is not sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel material is reduced. On the other hand, if the Al content is too high, a large number of coarse Al oxides are formed, and the SSC resistance of the steel material is reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.005 to 0.100%. A preferred lower limit of the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. A preferred upper limit of the Al content is 0.080%, more preferably 0.060%, even more preferably 0.040%, and even more preferably 0.035%. As used herein, the "Al" content refers to the content of "acid-soluble Al," that is, "sol. Al."

Cr: 0.30~1.50%
Chromium (Cr) improves the hardenability of steel. Cr also increases the temper softening resistance of steel, enabling high-temperature tempering. As a result, the SSC resistance of the steel is improved. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the SSC resistance of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.30 to 1.50%. A preferred lower limit of the Cr content is 0.35%, more preferably 0.40%, and even more preferably 0.50%. A preferred upper limit of the Cr content is 1.40%, more preferably 1.30%, even more preferably 1.20%, even more preferably 1.10%, and even more preferably 1.05%.

Mo: 0.40~2.00%
Molybdenum (Mo) improves the hardenability of steel. Mo also increases the temper softening resistance of steel, enabling high-temperature tempering. As a result, the SSC resistance of steel is improved. If the Mo content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, the above effects saturate. Therefore, the Mo content is 0.40 to 2.00%. A preferred lower limit of the Mo content is 0.45%, more preferably 0.50%, even more preferably 0.51%, even more preferably 0.55%, and even more preferably 0.60%. A preferred upper limit of the Mo content is 1.80%, even more preferably 1.60%, even more preferably 1.40%, and even more preferably 1.30%.

Ti: 0.002-0.020%
Titanium (Ti) bonds with N to form nitrides, which refine the grains of the steel material through a pinning effect. As a result, the strength of the steel material is increased. If the Ti content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, the Ti nitrides become coarse, and the SSC resistance of the steel material decreases. Therefore, the Ti content is 0.002 to 0.020%. The preferred lower limit of the Ti content is 0.003%, and more preferably 0.004%. The preferred upper limit of the Ti content is 0.018%, more preferably 0.015%, even more preferably 0.010%, and even more preferably 0.008%.

Nb: 0.002-0.100%
Niobium (Nb) combines with C and/or N to form carbides, nitrides, or carbonitrides (hereinafter referred to as "carbonitrides, etc."). Carbonitrides, etc., refine the grains of steel through a pinning effect, thereby improving the SSC resistance of the steel. Nb also forms fine carbides during tempering, thereby improving the temper softening resistance of the steel and increasing its strength. If the Nb content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content is too high, excessive carbonitrides, etc. are formed, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel decreases. Therefore, the Nb content is 0.002 to 0.100%. A preferred lower limit of the Nb content is 0.005%, more preferably 0.010%, even more preferably 0.015%, and still more preferably 0.020%. The upper limit of the Nb content is preferably 0.080%, more preferably 0.060%, and even more preferably 0.040%.

V: 0.05-0.30%
Vanadium (V) forms carbonitrides and the like. Carbonitrides and the like have a pinning effect, refining the grain size of the steel material and improving the SSC resistance of the steel material. V also forms fine carbides during tempering, increasing the temper softening resistance of the steel material and improving the strength of the steel material. If the V content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive carbonitrides and the like are formed, reducing the SSC resistance of the steel material. Therefore, the V content is 0.05 to 0.30%. A preferred lower limit of the V content is 0.06%, more preferably 0.07%, and even more preferably 0.08%. A preferred upper limit of the V content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

B: 0.0005-0.0040%
Boron (B) dissolves in steel to improve the hardenability and strength of the steel. If the B content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, coarse nitrides are formed, reducing the SSC resistance of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0.0005 to 0.0040%. The preferred lower limit of the B content is 0.0006%, and more preferably 0.0008%. The preferred upper limit of the B content is 0.0035%, more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%.

N: 0.0100% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is greater than 0%. N combines with Ti to form nitrides, which refine the grains of the steel material through a pinning effect. As a result, the strength of the steel material is increased. However, if the N content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed, and the SSC resistance of the steel material is reduced. Therefore, the N content is 0.0100% or less. A preferred upper limit of the N content is 0.0080%, more preferably 0.0060%, even more preferably 0.0050%, and even more preferably 0.0040%. To more effectively obtain the above effects, a preferred lower limit of the N content is 0.0005%, more preferably 0.0010%, even more preferably 0.0015%, and even more preferably 0.0020%.

O: 0.0040% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is greater than 0%. If the O content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse oxides are formed, and the SSC resistance of the steel material decreases. Therefore, the O content is 0.0040% or less. A preferred upper limit of the O content is 0.0035%, more preferably 0.0033%, even more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, "impurities" refers to substances that are mixed in from raw materials such as ore or scrap, or the manufacturing environment, during the industrial production of steel material, and are acceptable to the extent that they do not adversely affect the steel material according to this embodiment.

[Optional element]
The chemical composition of the steel material may further contain one or more elements selected from the group consisting of Cu and Ni in place of a portion of Fe. These elements are optional elements and improve the hardenability of the steel material.

Cu: 0-0.50%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu improves the hardenability of the steel material and increases the strength of the steel material. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content is too high, the SSC resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.50%. The preferred lower limit of the Cu content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Cu content is 0.35%, more preferably 0.25%, even more preferably 0.15%, even more preferably 0.10%, and even more preferably 0.05%.

Ni: 0-0.50%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni improves the hardenability and strength of the steel material. Ni also dissolves in the steel to improve the SSC resistance of the steel material. Even if even a small amount of Ni is contained, these effects can be obtained to some extent. However, if the Ni content is too high, localized corrosion is promoted and the SSC resistance of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Ni content is 0.30%, more preferably 0.20%, even more preferably 0.10%, and even more preferably 0.05%.

The chemical composition of the above-mentioned steel may further contain W instead of part of the Fe.

W: 0 to 0.50%
Tungsten (W) is an optional element and may not be included. That is, the W content may be 0%. When W is included, it forms a protective corrosion film in sour environments and suppresses hydrogen penetration into the steel material. This improves the SSC resistance of the steel material. Even if even a small amount of W is included, the above effect can be achieved to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse carbides will form in the steel material, reducing the SSC resistance of the steel material. Therefore, the W content is 0 to 0.50%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the W content is less than 0.50%, even more preferably 0.48%.

The chemical composition of the above-mentioned steel may further contain, in place of a portion of the Fe, one or more elements selected from the group consisting of Ca, Mg, Zr, and rare earth elements. All of these elements are optional and render the S in the steel harmless as sulfides. As a result, these elements enhance the SSC resistance of the steel.

Ca: 0~0.0100%
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca renders S in the steel harmless as sulfides, thereby improving the SSC resistance of the steel. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel will coarsen, reducing the SSC resistance of the steel. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The preferred upper limit of the Ca content is 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

Mg: 0-0.0100%
Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg neutralizes S in the steel material as sulfides, improving the SSC resistance of the steel material. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the SSC resistance of the steel material. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The preferred upper limit of the Mg content is 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

Zr: 0~0.0100%
Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When contained, Zr neutralizes S in the steel material by converting it into sulfides, thereby improving the SSC resistance of the steel material. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the SSC resistance of the steel material. Therefore, the Zr content is 0 to 0.0100%. The preferred lower limit of the Zr content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The preferred upper limit of the Zr content is 0.0040%, even more preferably 0.0025%, and even more preferably 0.0020%.

Rare earth elements (REM): 0 to 0.0100%
Rare earth elements (REM) are optional elements and may not be included. That is, the REM content may be 0%. When included, REM neutralizes S in the steel material as sulfides, thereby improving the SSC resistance of the steel material. REM also combines with P in the steel material to suppress P segregation at grain boundaries. Therefore, a decrease in the SSC resistance of the steel material due to P segregation is suppressed. Even if even a small amount of REM is included, the above effects can be achieved to some extent. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the SSC resistance of the steel material. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0006%. The upper limit of the REM content is preferably 0.0040%, more preferably 0.0025%, and even more preferably 0.0020%.

In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and the lanthanides lanthanum (La), atomic number 57, to lutetium (Lu), atomic number 71. In addition, in this specification, REM content refers to the total content of these elements.

[Yield strength]
The steel material according to this embodiment has a yield strength of more than 1034 to 1172 MPa (more than 150 to 170 ksi). Yield strength as used herein means the stress at 0.65% elongation (0.65% proof stress) obtained in a tensile test at room temperature (25°C) in accordance with ASTM E8/E8M (2021). The steel material according to this embodiment has the above-mentioned chemical composition and satisfies the number density of coarse Al oxides and the number density of coarse Si oxides described below, and thereby has excellent SSC resistance even when the yield strength is more than 1034 to 1172 MPa.

The yield strength of the steel material according to this embodiment is determined by the following method. First, a round bar test specimen is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the round bar test specimen is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test specimen is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, the round bar test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the steel pipe. If the steel material is a round bar, the round bar test specimen is prepared from the R/2 position. In this specification, the R/2 position refers to the center position of the radius R in a cross section perpendicular to the axial direction of the round bar. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the round bar. The size of the round bar test specimen is, for example, 8.9 mm in parallel diameter and 35.6 mm in gauge length. Using the prepared round bar test specimen, a tensile test is performed at room temperature (25°C) in the air according to a method in accordance with ASTM E8/E8M (2021), and the obtained stress at 0.65% elongation (0.65% proof stress) is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is calculated by rounding the obtained value to one decimal place.

[Number density of coarse Al oxide particles]
The steel material according to this embodiment has the above-described chemical composition and a yield strength of more than 1034 to 1172 MPa, and further has a number density of coarse Al oxides of less than 30 particles/200 ^mm2 . As described above, in this specification, particles having an Al content of 20% or more and an O content of 10% or more, by mass%, are also referred to as "Al oxides." As described above, in this specification, Al oxides having a major axis of 5.0 μm or more are also referred to as "coarse Al oxides." In other words, coarse Al oxides refer to particles having an Al content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more, by mass%.

As described above, when manufacturing a steel material having the above-described chemical composition, deoxidation using mainly aluminum (Al) is carried out in the steelmaking process. Therefore, a large amount of Al oxides is likely to form in a steel material having the above-described chemical composition. Furthermore, Al oxides are hard oxides and are likely to reduce the corrosion resistance of the steel material. In particular, when the steel material has a high yield strength of 150 ksi class, the effect of coarse Al oxides is likely to become apparent, and SSC resistance is likely to be significantly reduced. Therefore, in the steel material according to this embodiment, which has the above-described chemical composition and a yield strength of more than 1034 to 1172 MPa, the number density of coarse Al oxides is set to less than 30 particles/200 ^mm² .

In this embodiment, the upper limit of the number density of coarse Al oxides is preferably 28 particles/200 ^mm² , more preferably 25 particles/200 ^mm² , and even more preferably 22 particles/200 ^mm² . In this embodiment, the lower limit of the number density of coarse Al oxides is not particularly limited and may be 0 particles/200 ^mm² . The lower limit of the number density of coarse Al oxides may be, for example, 5 particles/200 ^mm² , 7 particles/200 ^mm² , or 9 particles/200 ^mm² . A method for determining the number density of coarse Al oxides will be described later.

[Number density of coarse Si oxide particles]
The steel material according to this embodiment has the above-described chemical composition and a yield strength of more than 1034 to 1172 MPa, the number density of coarse Al oxides is less than 30 particles/200 ^mm2 , and the number density of coarse Si oxides is 5 particles/200 mm2 or ^less . As described above, in this specification, particles having an Al content of less than 20%, a Si content of 20% or more, and an O content of 10% or more, by mass%, are also referred to as "Si oxides." As described above, in this specification, Si oxides having a major axis of 5.0 μm or more are also referred to as "coarse Si oxides." In other words, coarse Si oxides refer to particles having an Al content of less than 20%, a Si content of 20% or more, an O content of 10% or more, and a major axis of 5.0 μm or more, by mass%.

As described above, silicon oxides have not received much attention until now due to their small number. However, when a steel has a high yield strength of 150 ksi class, not only coarse Al oxides but also a small number of coarse Si oxides may be prone to a significant decrease in SSC resistance. Therefore, by not only reducing the number density of coarse Al oxides to less than 30 particles/200 ^mm2 but also reducing the number density of coarse Si oxides to 5 particles/200 mm2 or ^less , it is possible to stably obtain excellent SSC resistance even when the yield strength is increased to 150 ksi class. Therefore, the steel material according to this embodiment has the above-described chemical composition and a yield strength of more than 1034 to 1172 MPa, and the number density of coarse Al oxides in the steel material is less than 30 particles/200 ^mm2 and further the number density of coarse Si oxides is 5 particles/200 ^mm2 or less.

In this embodiment, the upper limit of the number density of the coarse Si oxides is preferably 4 particles/200 ^mm² , and more preferably 3 particles/200 ^mm² . In this embodiment, the lower limit of the number density of the coarse Si oxides is not particularly limited and may be 0 particles/200 ^mm² . The lower limit of the number density of the coarse Si oxides may be, for example, 1 particle/200 ^mm² .

In this embodiment, the number density of coarse Al oxides and the number density of coarse Si oxides in a steel material can be determined by the following method. First, a test piece is prepared from the steel material according to this embodiment, with the observation surface being a surface including the rolling direction and the reduction direction. Specifically, if the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the observation surface being a surface including the rolling direction and the plate thickness direction. If the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the observation surface being a surface including the pipe axial direction and the pipe radial direction. If the steel material is a round bar, a test piece is prepared with the R/2 position in the center and the observation surface being a surface including the axial and radial directions.

The observation surface of the prepared test piece is polished to a mirror finish, and then measurements are performed. The area of the observation surface is not limited, but may be, for example, 300 mm ² (20 mm × 15 mm). The number of Si oxides with a major axis of 5.0 μm or more on the observation surface is determined. Specifically, particles on the observation surface are first identified based on contrast. Element concentration analysis (EDS analysis) is performed on each identified particle. In EDS analysis, an acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb. Based on the EDS analysis results of each particle, if the Al content is 20% or more and the O content is 10% or more by mass, the particle is identified as an "Al oxide." Based on the EDS analysis results of each particle, if the Al content is less than 20%, the Si content is 20% or more, and the O content is 10% or more, in mass%, the particle is identified as a "Si oxide."

Of the Al oxides identified on the observation surface, Al oxides with a major axis of 5.0 μm or more (coarse Al oxides) are identified, and the total number of coarse Al oxides is calculated. Furthermore, of the Si oxides identified on the observation surface, Si oxides with a major axis of 5.0 μm or more (coarse Si oxides) are identified, and the total number of coarse Si oxides is calculated. The major axes of Al oxides and Si oxides can be calculated using well-known methods. In this specification, the major axis of Al oxides and Si oxides refers to the longest line segment (μm) connecting any two points on the periphery of Al oxides and Si oxides on the observation surface.

The number density of the coarse Al oxides (particles/200 mm ² ) is determined based on the total number of coarse Al oxides and the total area of the observation surface. Furthermore, the number density of the coarse Si oxides (particles/200 mm ^{2 ) is determined based on the total number of coarse Si oxides and the total area of the observation surface. In this embodiment, the number density of the coarse Al oxides (particles/200 mm 2 )} and the number density of the coarse Si oxides (particles/200 mm ² ) are both determined by rounding the obtained values to the nearest tenth. The number densities of the coarse Al oxides and the coarse Si oxides can be measured using a scanning electron microscope equipped with a composition analysis function (SEM-EDS device). For example, an automatic analyzer manufactured by FEI (ASPEX) under the trade name Metals Quality Analyzer can be used as the SEM-EDS device.

[SSC resistance]
The SSC resistance of the steel material according to this embodiment can be evaluated by an SSC resistance test performed in accordance with NACE TM0177-2016 Method A. Specifically, it can be evaluated by the following method.

The test solution is a mixed aqueous solution (NACE solution D) of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate, adjusted to pH 5.0 with hydrochloric acid. Round bar test specimens are prepared from the steel material according to this embodiment. When the steel material is a steel plate, the round bar test specimen is prepared from the center of the plate thickness. In this case, the axial direction of the round bar test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the round bar test specimen is prepared from the center of the wall thickness. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the round bar test specimen is prepared from the R/2 position. In this case, the axial direction of the round bar test specimen is parallel to the axial direction of the round bar. The size of the round bar test specimen is, for example, 6.35 mm in diameter and 25.4 mm in length of the parallel portion.

A stress equivalent to 90% of the actual yield stress is applied to the prepared round bar test specimen. A test solution at 24°C is poured into a test vessel so that the stressed round bar test specimen is immersed, forming a test bath. After degassing the test bath, a mixed gas of 0.01 atm _H2S gas and 0.99 atm _CO2 gas is blown into the test bath to saturate it. The test bath saturated with the mixed gas is maintained at 24°C for 720 hours. In the SSC resistance test conducted under the above conditions, the steel material according to this embodiment shows no cracks after 720 hours. In this specification, "no cracks are observed" means that no cracks are observed when the test specimen is observed with the naked eye after the test.

[Microstructure]
The microstructure of the steel material according to this embodiment has a total volume fraction of tempered martensite and tempered bainite of 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. If the microstructure of a steel material having the above-described chemical composition contains a total volume fraction of tempered martensite and tempered bainite of 90% or more, it can achieve both a 150 ksi-class yield strength and excellent SSC resistance, provided that the other configurations of this embodiment are satisfied. In other words, in this embodiment, if a steel material achieves both a 150 ksi-class yield strength and excellent SSC resistance, it is determined that the microstructure has a total volume fraction of tempered martensite and tempered bainite of 90% or more.

When determining the volume fraction of tempered martensite and tempered bainite by observation, they can be determined by the following method. First, a test piece having an observation surface is prepared from the steel material according to this embodiment. If the steel material is a steel plate, a test piece is prepared from the center of the plate thickness, with the observation surface being a plane including the rolling direction and the plate thickness direction. If the steel material is a steel pipe, a test piece is prepared from the center of the wall thickness, with the observation surface being a plane including the pipe axial direction and the pipe radial direction. If the steel material is a round bar, a test piece is prepared with the R/2 position in the center and the observation surface being a plane including the axial and radial directions.

The observation surface of the test specimen is polished to a mirror finish, and then immersed in a nital etching solution for approximately 10 seconds to reveal the structure by etching. The etched observation surface is observed using a scanning electron microscope (SEM) to obtain secondary electron images from 10 fields of view. The field area is, for example, 0.01 mm ² (magnification: 1000x). In each field of view, tempered martensite and tempered bainite are identified based on contrast. The area fractions of the identified tempered martensite and tempered bainite are calculated. The method for calculating the area fractions is not particularly limited, and any known method may be used. For example, the area fractions of tempered martensite and tempered bainite can be calculated by image analysis. In this embodiment, the arithmetic mean value of the area fractions of tempered martensite and tempered bainite calculated in all fields of view is defined as the volume fraction of tempered martensite and tempered bainite.

[Manufacturing method]
A method for manufacturing a steel material according to this embodiment will be described. Below, a method for manufacturing a seamless steel pipe will be described as an example of a steel material according to this embodiment. The method for manufacturing a seamless steel pipe includes a step of preparing a material (steelmaking step), a step of hot-working the material to manufacture a mother pipe (hot-working step), and a step of quenching and tempering the mother pipe to produce a seamless steel pipe (quenching step and tempering step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be described in detail below.

[Steelmaking process]
In the steelmaking process, first, molten pig iron produced by a known method is refined in a converter (primary refining). The molten steel produced by the primary refining is then subjected to secondary refining. In the secondary refining, alloy elements are added to adjust the composition, thereby producing molten steel that satisfies the above-mentioned chemical composition.

Secondary refining involves, for example, RH (Ruhrstahl-Hausen) vacuum degassing treatment. Final adjustment of the alloy composition is then carried out. Combined refining may also be carried out during secondary refining. In this case, prior to the RH vacuum degassing treatment, refining is carried out using, for example, an LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) treatment.

Materials are manufactured using molten steel that has undergone secondary refinement. Specifically, cast pieces (slabs, blooms, or billets) are produced using continuous casting using molten steel that has undergone secondary refinement. In continuous casting, molten steel is first poured from a ladle into a tundish. At this time, packing sand is usually enclosed in the ladle nozzle to seal it. As a result, packing sand may be mixed in with the molten steel from the ladle to the tundish. Furthermore, when manufacturing materials with the above-mentioned chemical composition, silicon oxides may be used as packing sand. In this case, there is a concern that silicon oxides may be introduced into the manufactured material.

Therefore, in this embodiment, the molten steel and the Si oxide are separated to prevent the Si oxide sealed in the ladle nozzle from being introduced into the tundish. The method for separating the Si oxide is not particularly limited, but the following method can be used, for example. A sloped metal plate is placed below the ladle nozzle and above the opening of the tundish. When the ladle nozzle is opened, the Si oxide is discharged first from the nozzle, followed by the molten steel. Here, Si oxide is lighter than the molten steel. Therefore, the Si oxide discharged from the nozzle is guided along the slope of the metal plate and out of the opening of the tundish. The slope of the metal plate can be achieved, for example, by placing a metal plate machined into a bottomless cone shape with its apex directly below the ladle nozzle, or by other methods. Furthermore, a single metal plate can be used, or multiple metal plates can be stacked. Furthermore, the thickness of the metal plate is not particularly limited, but is, for example, approximately 1 to 10 mm.

After the silicon oxides are discharged from the nozzle, the molten steel is discharged. At this time, the molten steel discharged from the nozzle is introduced into the tundish through the opening together with the metal plate. That is, in this embodiment, part or all of the metal plate may be introduced into the tundish and mixed into the molten steel. Therefore, in this embodiment, it is preferable that the metal plate be a metal plate made of alloy elements contained in the molten steel. As a metal plate made of alloy elements contained in the molten steel, for example, an aluminum plate can be used. Note that in this specification, aluminum plate means a metal plate made of aluminum and the remainder made of impurities.

Preferably, after the Si oxides are discharged from the nozzle and before the molten steel is discharged, the metal plate is removed from below the nozzle. In this case, Si oxides adhering to the metal plate can be prevented from being mixed into the molten steel. The method for removing the metal plate from below the nozzle is not particularly limited. For example, a hole may be formed in part of the metal plate and the metal plate may be removed using a rod with a hook at its tip. In this case, the hook at the tip of the rod can be hooked into the hole in the metal plate and the rod can be pulled to remove the metal plate. In this way, the Si oxides can be separated from the molten steel and the molten steel can be introduced into the tundish. The method for separating Si oxides from molten steel is not limited to the above method.

Next, the prepared molten steel is cast to produce the raw material. The casting method is not particularly limited, but an example is continuous casting. When producing the raw material using continuous casting, it is preferable to carry out the following method.

The casting speed in a continuous casting machine is preferably 1.0 to 3.0 m/min. If the casting speed is too slow, an accumulation zone of Al oxides may form in the material. In this case, the produced steel will contain a large number of coarse Al oxides, reducing the SSC resistance of the steel. On the other hand, if the casting speed is too fast, the Al oxides may not rise to the surface of the molten steel, leaving a large number of Al oxides in the material. In this case, the produced steel will contain a large number of coarse Al oxides, reducing the SSC resistance of the steel. Therefore, the casting speed in a continuous casting machine is preferably 1.0 to 3.0 m/min.

When producing a material using the continuous casting method, it is preferable to further electromagnetically stir the molten steel in the mold. Specifically, by performing electromagnetic stirring in the mold at a current value of 330 to 450 A, it becomes less likely that an aluminum oxide accumulation zone will form in the material. If the current value for electromagnetic stirring in the mold is too low, the molten steel will not be stirred sufficiently, and an aluminum oxide accumulation zone may form in the material. In this case, the produced steel will contain a large amount of coarse aluminum oxide, reducing the SSC resistance of the steel. On the other hand, if the current value for electromagnetic stirring in the mold is too high, it may place too much strain on the manufacturing equipment. Therefore, in this embodiment, it is preferable to electromagnetically stir in the mold at a current value of 330 to 450 A. Molten steel is cast using the above method to produce a material.

[Hot working process]
In the hot working process, the prepared material is hot worked to produce an intermediate steel material. When the steel material is a seamless steel pipe, the intermediate steel material corresponds to a mother pipe. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The billet extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). The hot working method is not particularly limited, and a well-known method may be used.

For example, a mother pipe may be manufactured by carrying out the Mannesmann process as hot working. In this case, a round billet is pierced and rolled using a piercing mill. When piercing and rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced and rolled round billet is further hot rolled using a mandrel mill, reducer, sizing mill, etc. to produce a mother pipe. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%.

Other hot working methods may also be used to produce a blank pipe from the billet. For example, in the case of short, thick-walled steel material such as a coupling, the blank pipe may be produced by forging using the Erhardt method or other methods. The blank pipe is produced through the above process. The wall thickness of the blank pipe is not particularly limited, but is, for example, 9 to 60 mm.

When the steel material is round steel, the material is first heated in a heating furnace. There are no particular restrictions on the heating temperature, but it is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot processed to produce intermediate steel material with a circular cross section perpendicular to the axial direction. Hot processing is, for example, blooming using a blooming mill, or hot rolling using a continuous rolling mill. A continuous rolling mill has an alternating arrangement of horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction.

When the steel material is a steel plate, the raw material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The raw material extracted from the heating furnace is hot rolled using a blooming mill and a continuous rolling mill to produce intermediate steel in the shape of a steel plate.

The mother pipe produced by hot working may be air-cooled (as-rolled). The mother pipe produced by hot working may be quenched directly after hot working without being cooled to room temperature, or may be quenched after being reheated after hot working.

When quenching is performed directly after hot working, or after reheating and then quenching, cooling may be stopped or slow cooling may be performed during quenching. In this case, the occurrence of quench cracks in the mother pipe can be suppressed. When quenching is performed directly after hot working, or after reheating and then quenching, stress relief annealing (SR) may also be performed after quenching and before the next heat treatment step. In this case, residual stress in the mother pipe is removed.

As described above, in the hot working process, the prepared material is hot worked to produce intermediate steel. The quenching process is described in detail below.

[Quenching process]
In the quenching process, the prepared intermediate steel material (blank pipe) is quenched. In this specification, "quenching" means rapidly cooling the intermediate steel material at or above the _A3 point. The preferred quenching temperature is 800 to 1000°C. If the quenching temperature is too high, the prior γ grains may become coarse, which may reduce the SSC resistance of the steel material. Therefore, the quenching temperature is preferably 800 to 1000°C.

In this specification, the quenching temperature corresponds to the surface temperature of the intermediate steel material measured with a thermometer installed at the outlet of the equipment that performs the final hot working, when quenching is performed directly after hot working. Furthermore, when quenching is performed after supplementary heating or reheating after hot working, the quenching temperature corresponds to the temperature of the furnace where supplementary heating or reheating is performed.

The quenching method involves, for example, continuously cooling the intermediate steel material (raw pipe) from the quenching start temperature, thereby continuously lowering the surface temperature of the raw pipe. The method of continuous cooling is not particularly limited and may be any well-known method. Examples of continuous cooling methods include immersing the raw pipe in a water tank for cooling, or accelerating the cooling of the raw pipe by shower water cooling or mist cooling.

If the cooling rate during quenching is too slow, the microstructure will not be primarily martensite and bainite, and the mechanical properties specified in this embodiment (yield strength of over 1034 to 1172 MPa) will not be obtained. In this case, excellent SSC resistance will also not be obtained.

Therefore, as described above, in the steel manufacturing method according to this embodiment, the intermediate steel is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate in the range of 800 to 500°C in the surface temperature of the intermediate steel (blank pipe) during quenching is defined as the cooling rate during quenching CR _800-500 . More specifically, the cooling rate during quenching CR _800-500 is determined from the temperature measured at the location within the cross section of the intermediate steel to be quenched that cools the slowest (for example, the center of the thickness of the intermediate steel when both surfaces are forcibly cooled).

The preferred cooling rate during quenching, CR _800-500 , is 300°C/min or more. The lower limit of the cooling rate during quenching, CR _800-500 , is more preferably 450°C/min, and even more preferably 600°C/min. The upper limit of the cooling rate during quenching, CR _800-500, is not particularly specified, but is, for example, 60,000°C/min.

Preferably, the mother pipe is heated multiple times in the austenite region and then quenched. In this case, the austenite grains before quenching are refined, thereby improving the SSC resistance of the steel material. By quenching multiple times, heating in the austenite region may be repeated multiple times, or by performing normalizing and quenching, heating in the austenite region may be repeated multiple times. Quenching and tempering, as described below, may also be combined and performed multiple times. In other words, quenching and tempering may be performed multiple times. In this case, the SSC resistance of the steel material is further improved. The tempering process is described in detail below.

[Tempering process]
In the tempering process, the quenched mother pipe is tempered. In this specification, "tempering" means reheating the quenched intermediate steel material to a temperature below the A _c1 point and holding the temperature. The tempering temperature corresponds to the furnace temperature when the quenched intermediate steel material is heated and held. The tempering time means the time for which the intermediate steel material is held at the tempering temperature.

The tempering temperature is adjusted appropriately depending on the chemical composition of the seamless steel pipe and the desired yield strength. In other words, for a base pipe having the chemical composition of this embodiment, the tempering temperature is adjusted to adjust the yield strength of the seamless steel pipe to greater than 1034 to 1172 MPa. It is naturally possible for a person skilled in the art to adjust the yield strength of a seamless steel pipe to greater than 1034 to 1172 MPa by adjusting the tempering temperature. Specifically, in the tempering process according to this embodiment, the preferred tempering temperature is 640 to 660°C.

If the tempering time is too short, a microstructure mainly composed of tempered martensite and tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the above effects will saturate. Therefore, in the tempering process of this embodiment, the tempering time is preferably 10 to 90 minutes. A more preferred lower limit for the tempering time is 15 minutes. A more preferred upper limit for the tempering time is 80 minutes.

The steel material according to this embodiment can be manufactured using the above manufacturing method. Note that the above manufacturing method has been described as an example of a method for manufacturing a seamless steel pipe. However, the steel material according to this embodiment may be a steel plate or other shape. Similar to the above manufacturing method, manufacturing methods for steel plate or other shapes also include, for example, a preparation process, a quenching process, and a tempering process. Furthermore, the above manufacturing method is an example, and the steel material may be manufactured using other manufacturing methods.

The present invention will now be explained in more detail with reference to the following examples.

Molten steel was produced having the chemical composition shown in Tables 1-1 and 1-2. Note that "-" in Table 1-2 indicates that the content of each element was at the impurity level. Specifically, the Cu content, Ni content, and W content of Steel A were rounded to two decimal places to mean 0%. Furthermore, the Ca content, Mg content, Zr content, and rare earth element (REM) content of Steel A were rounded to five decimal places to mean 0%.

The molten steel was used to produce round billets by continuous casting. During continuous casting, when molten steel was introduced from the ladle into the tundish, a metal plate processed into a baseless cone shape was placed above the opening of the tundish, with its apex positioned directly below the nozzle of the ladle. Table 2 indicates whether or not a metal plate of the above shape was placed above the opening of the tundish. Specifically, when a metal plate of the above shape was placed above the opening of the tundish, an "A" is indicated in the "Metal Plate" column of Table 2. When a metal plate of the above shape was not placed above the opening of the tundish, a "B" is indicated in the "Metal Plate" column of Table 2. The metal plate of the above shape placed above the opening of the tundish was an aluminum plate. Specifically, three 2 mm thick aluminum plates were used, stacked one on top of the other. When a metal plate was placed, it was removed from below the nozzle using a rod with a hook at its tip after silicon oxide was discharged from the nozzle and before the molten steel was discharged. Furthermore, the molten steel was cast into a round billet at the casting speed shown in Table 2. At this time, electromagnetic stirring was carried out in the mold at the current value shown in Table 2.

The produced round billet for each test number was held at 1,250°C for 1 hour and then hot-rolled using the Mannesmann-mandrel method to produce a mother pipe (seamless steel pipe) for each test number. The resulting mother pipe for each test number was then quenched. Specifically, the mother pipe for each test number was held at the temperature (°C) for the time (minutes) listed in the "Quenching Process" column of Table 2, followed by quenching by shower water cooling. For test number 3, after the above-mentioned quenching, the mother pipe was held at 900°C for 10 minutes, followed by further quenching by shower water cooling. The cooling rate during quenching, CR _800-500 , for each test number was within the range of 480 to 30,000°C/min. The temperature (°C) in the quenching process was the temperature (°C) of the heat treatment furnace in which the mother pipe was heated. The time (minutes) in the quenching process was the time (minutes) the mother pipe was held at the quenching temperature.

The obtained blank pipes of each test number were tempered. Specifically, the blank pipes of each test number were tempered by holding them at the temperature (°C) for the time (minutes) listed in the "Tempering Process" column of Table 2. Here, the tempering temperature (°C) listed in Table 2 is the temperature (°C) of the tempering furnace in which the blank pipes were heated. Furthermore, the tempering time (minutes) listed in Table 2 is the time (minutes) the blank pipes were held at the tempering temperature. Through the above manufacturing process, seamless steel pipes of each test number were obtained.

[Evaluation test]
The seamless steel pipes having the respective test numbers after tempering were subjected to a tensile test, a test for measuring the number density of coarse Al oxides and coarse Si oxides, and an SSC resistance test, all of which will be described below.

[Tensile test]
A tensile test was conducted on each seamless steel pipe to determine its yield strength. The tensile test was conducted in accordance with ASTM E8/E8M (2021). A round bar test specimen with a parallel section diameter of 8.9 mm and a gauge length of 35.6 mm was prepared from the center of the wall thickness of each seamless steel pipe. The axial direction of the round bar test specimen was parallel to the axial direction of the seamless steel pipe. A tensile test was conducted at room temperature (25°C) in air using the prepared round bar test specimen to obtain the yield strength (MPa) of each seamless steel pipe. In this example, the stress at 0.65% elongation (0.65% proof stress) obtained in the tensile test was defined as the yield strength. The obtained yield strength (MPa) is shown in Table 3 as "YS (MPa)."

[Number density measurement test of coarse Al oxides and coarse Si oxides]
A number density measurement test for coarse Al oxides and coarse Si oxides was conducted on the seamless steel pipes of each test number to determine the number densities of Al oxides (coarse Al oxides) with a major diameter of 5.0 μm or more and Si oxides (coarse Si oxides) with a major diameter of 5.0 μm or more. The number densities of coarse Al oxides and coarse Si oxides were determined by the above-described method using test specimens prepared from the central portion of the wall thickness of the seamless steel pipes of each test number. The obtained number densities (particles/200 ^mm2 ) of coarse Al oxides are shown in the "Coarse Al oxides (particles/200 mm2 ⁾ " column of Table 3. The obtained number densities (particles/200 ^mm2 ) of coarse Si oxides are shown in the "Coarse Si oxides (particles/200 ^mm2 )" column of Table 3.

[SSC resistance test]
For each seamless steel pipe with each test number, an SSC resistance test was performed in accordance with NACE TM0177-2016 Method A to evaluate the SSC resistance. Specifically, a round bar test piece with a diameter of 6.35 mm and a parallel portion length of 25.4 mm was prepared from the center of the wall thickness of each seamless steel pipe with each test number. The SSC resistance test was performed on three of the prepared test pieces. The axial direction of the test piece was parallel to the pipe axis direction.

Tensile stress was applied to the round bar specimens of each test number in the axial direction. The applied stress was adjusted to 90% of the actual yield stress of each steel plate. The test solution used was a mixed aqueous solution of 5.0 mass% sodium chloride and 0.4 mass% sodium acetate (NACE solution D), adjusted to pH 5.0 with hydrochloric acid. The test solution at 24°C was poured into three test vessels to form test baths. The three stressed round bar specimens were immersed in the test baths of different test vessels. After degassing each test bath, a mixed gas of 0.01 atm _H2S gas and 0.99 atm _CO2 gas was blown into the test bath to saturate it. The test baths saturated with the mixed gas were maintained at 24°C for 720 hours.

After 720 hours of holding, the round bar test specimens of each test number were observed for the occurrence of sulfide stress cracking (SSC). Specifically, after 720 hours of holding, the round bar test specimens were visually inspected. For each test number, the number of round bar test specimens in which SSC occurred out of the three test specimens is shown in the "Number of SSC-Induced Specimens" column in Table 3.

[Evaluation results]
With reference to Tables 1-1, 1-2, 2, and 3, the seamless steel pipes of test numbers 1 to 12 had appropriate chemical compositions, and their manufacturing methods also satisfied the above-mentioned preferred conditions. As a result, these seamless steel pipes had yield strengths of more than 1034 to 1172 MPa, coarse Al oxide number densities of less than 30 particles/200 ^mm² , and coarse Si oxide number densities of 5 particles/200 ^mm² or less. As a result, the number of SSC occurrences in these seamless steel pipes was zero in the SSC resistance test. That is, the seamless steel pipes of test numbers 1 to 12 had yield strengths of more than 1034 to 1172 MPa and excellent SSC resistance. It was determined that the sum of the volume fractions of tempered martensite and tempered bainite in the microstructure of these seamless steel pipes was 90% or more.

On the other hand, the casting speed in the steelmaking process for the seamless steel pipes of test numbers 13 and 14 was too fast. As a result, the number density of coarse Al oxides in these seamless steel pipes was 30 particles/200 mm2 or ^more . As a result, the number of SSC occurrences in these seamless steel pipes was one or more in the SSC resistance test, and these seamless steel pipes did not have excellent SSC resistance.

For seamless steel pipes of test numbers 15 to 17, no metal plate was used in the steelmaking process. As a result, the number density of coarse Si oxides in these seamless steel pipes exceeded 5 particles/200 ^mm2 . As a result, these seamless steel pipes showed one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

The seamless steel pipe of test number 18 had an excessively high O content. As a result, this seamless steel pipe experienced one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

The seamless steel pipe of test number 19 had an excessively low Mo content. As a result, this seamless steel pipe experienced one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

The seamless steel pipe of test number 20 had an excessively high S content. As a result, this seamless steel pipe experienced one or more SSC occurrences in the SSC resistance test, and did not have excellent SSC resistance.

The above describes embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims (3)

A steel material,
In mass%,
C: 0.15-0.45%,
Si: 0.05-1.00%,
Mn: 0.05-1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.100%,
Cr: 0.30-1.50%,
Mo: 0.40-2.00%,
Ti: 0.002 to 0.020%,
Nb: 0.002-0.100%,
V: 0.05-0.30%,
B: 0.0005-0.0040%,
N: 0.0100% or less,
O: 0.0040% or less,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%,
W: 0-0.50%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
Rare earth elements: 0 to 0.0100%, and
the balance being Fe and impurities;
The yield strength is greater than 1034 to 1172 MPa,
In the microstructure of the steel material, the total volume fraction of tempered martensite and tempered bainite is 90% or more,
In the steel material,
The Al content is 20% or more, the O content is 10% or more, and the number density of Al oxide particles having a major axis of 5.0 μm or more is less than 30 particles/200 ^mm2 , in terms of mass %,
In terms of mass%, the Al content is less than 20%, the Si content is 20% or more, and the O content is 10% or more, and the number density of Si oxides having a major axis of 5.0 μm or more is 5 pieces/200 ^mm2 or less.
Steel material. The steel material according to claim 1,
Cu: 0.01 to 0.50%,
Ni: 0.01-0.50%,
W: 0.01-0.50%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%, and
Rare earth elements: containing one or more elements selected from the group consisting of 0.0001 to 0.0100%;
Steel material. The steel material according to claim 1 or claim 2,
The steel material is a seamless steel pipe.
Steel material.