WO2023085141A1 - Martensitic stainless steel seamless pipe and method for producing martensitic stainless steel seamless pipe - Google Patents

Abstract

The present invention achieves both high yield strength and excellent pitting corrosion resistance on the inner surface. A martensitic stainless steel seamless pipe according to the present disclosure has a chemical composition described in the specification, a microstructure described in the specification, and a yield strength of 862 MPa or more. The lengths of the pipe axis direction (L direction) and the pipe diameter direction (T direction) are both 1.0 μm. An observation visual field region (50) including an inner surface (10) extending in the L direction comprises an inner surface vicinity region (20), an inner region (30) adjacent below the inner surface vicinity region, and a void region (40). When the observation visual field region (50) is divided into sections divided equally into 256 in the L direction and 256 in the T direction, an inner surface Cu occupancy rate OSCu, which is defined by the number ratio of sections having a Cu concentration greater than 2.0% in the inner surface vicinity region (20), and an inner Cu occupancy rate OICu, which is defined by the number ratio of sections having a Cu concentration greater than 2.0% in the inner region (30), satisfy formula (2).　OSCu/OICu≧1.20　(2)

Description

Seamless martensitic stainless steel pipe and method for producing seamless martensitic stainless steel pipe

The present disclosure relates to a seamless steel pipe and its manufacturing method, and more particularly to a martensitic stainless steel seamless steel pipe and its manufacturing method.

Oil wells and gas wells (hereinafter collectively referred to as “oil wells”) contain corrosive hydrogen sulfide (H ₂ S), carbon dioxide (CO ₂ ), and the like. Here, it is known that chromium (Cr) is effective in improving the carbon dioxide gas corrosion resistance of steel materials. Therefore, in an oil well environment containing a large amount of carbon dioxide, depending on the partial pressure and temperature of carbon dioxide, API L80 13Cr steel (normal 13Cr steel), super 13Cr steel with reduced C content, etc. A martensitic stainless steel material containing about 13% by mass of Cr is used.

In recent years, due to the deepening of oil wells, not only corrosion resistance but also high strength is required for steel materials. For example, steel materials with a yield strength of 110 ksi grade (110 to less than 125 ksi, ie, 758 to 862 MPa) and a yield strength of 125 ksi or more (ie, 862 MPa or more) are beginning to be demanded.

Here, in this specification, an environment containing hydrogen sulfide and carbon dioxide is referred to as a "sour environment". Furthermore, oil well steels used in sour environments are required to have excellent corrosion resistance. In other words, in recent years, there has been a demand for oil well steel materials that have both high strength and excellent corrosion resistance.

International Publication No. 2006/061881 (Patent Document 1), International Publication No. 2008/023702 (Patent Document 2), and International Publication No. 2015/178022 (Patent Document 3) have high strength and excellent corrosion resistance. We propose steel materials for oil wells that are compatible with each other.

The oil-well steel material described in Patent Document 1 is a martensitic stainless steel pipe for oil-well use, and has C: 0.005 to 0.1%, Si: 0.05 to 1%, and Mn: 1.0% by mass. 5-5%, P: 0.05% or less, S: 0.01% or less, Cr: 9-13%, Ni: 0.5% or less, Mo: 2% or less, Cu: 2% or less, Al: It contains 0.001 to 0.1%, N: 0.001 to 0.1%, and the balance consists of Fe and impurities, and has a Cr-depleted region under the surface. As a result, this oil well steel material has a high strength of 655 MPa or more, and has high SCC resistance (stress corrosion cracking resistance) even if it has a Cr-deficient region under the surface. and Patent Document 1 disclose.

The steel material for oil wells described in Patent Document 2 is a martensitic stainless steel having, in mass %, C: 0.010 to 0.030%, Mn: 0.30 to 0.60%, and P: 0.01%. 040% or less, S: 0.0100% or less, Cr: 10.00-15.00%, Ni: 2.50-8.00%, Mo: 1.00-5.00%, Ti: 0.050 ~ 0.250%, V: 0.25% or less, N: 0.07% or less, Si: 0.50% or less, Al: 0.10% or less, and the balance consists of Fe and impurities and satisfies the formula (6.0≤Ti/C≤10.1). Patent Document 2 discloses that this oil well steel has a yield strength of 758 to 862 MPa and is excellent in SSC resistance (sulfide stress cracking resistance) among corrosion resistance.

The steel material for oil well described in Patent Document 3 is a high-strength stainless steel seamless steel pipe for oil well, which contains Cr and Ni, has a chemical composition satisfying the formula (Cr/Ni≦5.3), and has a tempered martensite phase. It has a microstructure with the main phase. In this steel material for oil wells, a phase exhibiting a white color when etched with a Birera corrosive solution has a thickness of 10 to 100 μm in the thickness direction from the outer surface of the steel pipe, and is dispersed in an area ratio of 50% or more on the outer surface of the steel pipe. It has surface tissue. Patent Document 3 discloses that this steel material for oil wells has a yield strength of 654 MPa or more and is excellent in corrosion resistance.

WO2006/061881 WO2008/023702 WO2015/178022

Patent Documents 1 to 3 above propose techniques for achieving both high strength and excellent corrosion resistance. By the way, when a martensitic stainless steel seamless steel pipe is used as an oil well steel pipe, the inner surface of the seamless steel pipe comes into direct contact with the production fluid. Therefore, the inner surface of the seamless steel pipe is particularly required to have corrosion resistance against pitting corrosion and/or crevice corrosion (hereinafter referred to as "pitting corrosion resistance"). However, Patent Documents 1 to 3 do not discuss the pitting corrosion resistance of the inner surface of the seamless steel pipe.

An object of the present disclosure is to provide a seamless martensitic stainless steel pipe that achieves both high strength and excellent pitting corrosion resistance on the inner surface, and a method for producing the seamless martensitic stainless steel pipe. .

The martensitic stainless seamless steel pipe according to the present disclosure is
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%,
O: 0.050% or less,
W: 0 to 2.00%,
Nb: 0 to 0.50%,
Mg: 0-0.0050%,
Rare earth element: 0 to 0.0050%,
B: 0 to 0.0050%, and
Balance: Fe and impurities, and satisfies formula (1),
a microstructure, in volume percent, consisting of 0-15.0% retained austenite, 0-5.0% ferrite, and the balance tempered martensite;
Yield strength is 862 MPa or more,
When the pipe axial direction of the martensitic stainless steel seamless steel pipe is defined as the L direction, and the pipe radial direction of the martensitic stainless steel seamless steel pipe is defined as the T direction,
Including the inner surface of the seamless martensitic stainless steel pipe extending in the L direction, the length of the side extending in the L direction is 1.0 μm, and the length of the side extending in the T direction is 1.0 μm. In the square observation field area,
When the observation visual field region is divided into 256 equal sections in the L direction and 256 equal sections in the T direction,
The observation field of view area is
an inner surface vicinity region having a rectangular shape with the inner surface of the martensitic stainless steel seamless steel pipe as the upper end, 256 sections in the L direction, and 6 sections in the T direction;
the inner surface vicinity region and an inner region adjacent below the inner surface vicinity region;
comprising the inner surface vicinity region and a void region adjacent above the inner surface vicinity region,
Among all the sections in the inner surface vicinity region, the number ratio of sections with a Cu concentration exceeding 2.0% is defined as the inner surface Cu occupancy OS _Cu ,
When the number ratio of sections with a Cu concentration exceeding 2.0% among all the sections in the internal region is defined as the internal Cu occupancy OI _Cu ,
The inner surface Cu occupancy OS _Cu and the internal Cu occupancy OI _Cu satisfy formula (2),
Martensitic stainless seamless steel pipe.
Mo+0.5×W≧2.50 (1)
OS _Cu /OI _Cu ≧1.20 (2)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1) in terms of % by mass.

A method for manufacturing a martensitic stainless seamless steel pipe according to the present disclosure includes:
The method for producing the martensitic stainless seamless steel pipe, comprising:
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%,
O: 0.050% or less,
W: 0 to 2.00%,
Nb: 0 to 0.50%,
Mg: 0-0.0050%,
Rare earth element: 0 to 0.0050%,
B: 0 to 0.0050%, and
Balance: a material preparation step of preparing a material consisting of Fe and impurities and satisfying formula (1);
After heating the prepared material in a heating furnace, hot working in which the cross-sectional reduction rate R defined by formula (A) is 40% or more and the hot working time is 15 minutes or less. A hot working step of manufacturing a mother pipe by carrying out
A quenching step of performing quenching on the raw pipe of ₃ or more points;
and a tempering step of tempering the quenched base pipe under conditions that satisfy formula (B).
Mo+0.5×W≧2.50 (1)
R = {1-(cross-sectional area perpendicular to the axial direction of the tube after hot working/cross-sectional area perpendicular to the axial direction of the raw material before hot working)} x 100 (A)
(T + 273.15) x (20 + log ₁₀ (t/60)) x (1-[Cu]/100) ≤ 17200 (B)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1) in terms of % by mass.
In the formula (B), T is the tempering temperature in degrees Celsius, t is the tempering time in minutes, and [Cu] is the Cu content in the blank in mass %.

The martensitic stainless seamless steel pipe according to the present disclosure can achieve both high strength and excellent pitting corrosion resistance on the inner surface. According to the method for producing a martensitic stainless steel seamless pipe according to the present disclosure, it is possible to produce a martensitic stainless seamless steel pipe that achieves both high strength and excellent pitting corrosion resistance on the inner surface.

FIG. 1 is a schematic diagram showing an example of microstructure observation in a cross section including the inner surface of a martensitic stainless seamless steel pipe and including the pipe axial direction and the pipe radial direction. FIG. 2 is a schematic diagram showing how the observation visual field area is divided into 256 equal parts in the tube radial direction (L direction) and 256 equal parts in the tube axial direction (T direction), and divided into 65536 sections. FIG. 3 is a schematic diagram showing the relationship between the position of each section in the observation field region in the tube radial direction (T direction) and the average Fe concentration in the tube axial direction (L direction).

First, the present inventors studied a martensitic stainless seamless steel pipe that can achieve both high yield strength and excellent pitting corrosion resistance on the inner surface from the viewpoint of chemical composition. As a result, the present inventors found that, in mass %, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050 % or less, Cr: 11.00 to 14.00%, Ni: 5.00 to 7.50%, Mo: 1.50 to 4.50%, Cu: 0.50 to 3.50%, Co: 0 .010-0.500%, Ti: 0.050-0.300%, V: 0.01-1.00%, Ca: 0.0005-0.0050%, Al: 0.001-0.100 %, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 2.00%, Nb: 0 to 0.50%, Mg: 0 to 0.0050%, rare earth elements : 0 to 0.0050%, B: 0 to 0.0050%, and the balance: Fe and impurities. We thought that it might be possible to achieve both excellent pitting corrosion resistance on the surface.

On the other hand, increasing the strength of steel tends to reduce its corrosion resistance. Therefore, even martensitic stainless steel seamless steel pipes having the chemical composition described above may have reduced pitting corrosion resistance as a result of increasing the yield strength to 125 ksi or more. Accordingly, the inventors of the present invention have studied various techniques for improving the pitting corrosion resistance of the seamless martensitic stainless steel pipe having the chemical composition described above while maintaining the yield strength of 125 ksi or more. As a result, the present inventors obtained the following findings.

The present inventors focused on molybdenum (Mo) and tungsten (W) as chemical compositions that improve the pitting corrosion resistance of martensitic stainless seamless steel pipes. Mo forms a solid solution and enhances the pitting corrosion resistance of the seamless steel pipe. Moreover, W forms a solid solution to improve the pitting corrosion resistance of the seamless steel pipe. That is, the present inventors thought that the pitting corrosion resistance of the seamless steel pipe could be improved by increasing the Mo content and the W content.

Here, it is defined as Fn1=Mo+0.5×W. By increasing Fn1, the pitting corrosion resistance of the seamless steel pipe can be improved while maintaining the yield strength of the seamless steel pipe. Therefore, the martensitic stainless seamless steel pipe according to the present embodiment has the chemical composition and microstructure described above, and the chemical composition satisfies the following formula (1). As a result, the martensitic stainless seamless steel pipe according to the present embodiment can achieve both a yield strength of 125 ksi or more and excellent pitting corrosion resistance, provided that other configurations of the present embodiment are satisfied.
Mo+0.5×W≧2.50 (1)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1) in terms of % by mass.

On the other hand, as a result of detailed studies by the present inventors, even martensitic stainless steel seamless steel pipes having the above chemical composition including formula (1) have a yield strength of 125 ksi or more, It has been found that in some cases excellent pitting corrosion resistance cannot be obtained on the surface. Accordingly, the inventors of the present invention conducted a detailed study on a technique for improving the pitting corrosion resistance of the inner surface of a seamless martensitic stainless steel pipe having the above-described chemical composition including formula (1).

First, the present inventors focused on the state of the vicinity of the inner surface of a seamless steel pipe and investigated a method for improving the pitting corrosion resistance of the inner surface. As a result, it was found that copper (Cu) precipitates in the vicinity of the inner surface of the seamless steel pipe may increase the pitting corrosion resistance of the inner surface of the seamless steel pipe. Therefore, the present inventors manufactured various kinds of seamless martensitic stainless steel pipes having the chemical composition described above and in which Cu precipitates were formed in the vicinity of the inner surface, and investigated and studied the pitting corrosion resistance of the inner surface in detail. gone.

As a result of detailed studies by the present inventors, in the martensitic stainless steel seamless steel pipe having the chemical composition described above, instead of simply precipitating Cu precipitates, by unevenly distributing Cu precipitates near the inner surface, It was found that the pitting corrosion resistance of the inner surface can be significantly enhanced. This point will be specifically described with reference to the drawings.

Fig. 1 is a schematic diagram showing an example of microstructure observation in a cross section including the inner surface of the martensitic stainless seamless steel pipe having the chemical composition described above and including the pipe axial direction and the pipe radial direction. The horizontal direction in the observation visual field region 50 in FIG. 1 corresponds to the tube axial direction, and the vertical direction corresponds to the tube radial direction. In this specification, the pipe axial direction of the martensitic stainless seamless steel pipe is also referred to as the "L direction", and the pipe radial direction of the martensitic stainless seamless steel pipe is also referred to as the "T direction". In FIG. 1, the L-direction length of the observation field region 50 shown in the schematic diagram is 1.0 μm, and the T-direction length is 1.0 μm.

In FIG. 1, the inner surface 10 of the seamless steel pipe is near the center in the T direction and can be confirmed as a line segment extending in the L direction. A person skilled in the art can uniquely identify the inner surface 10 of the seamless steel pipe by the method described later. Here, the portion below the inner surface 10 in FIG. 1 is the martensitic stainless seamless steel pipe. Furthermore, the observation visual field region 50 in FIG. 1 is divided into 256 equal sections in the L direction and 256 equal sections in the T direction, resulting in 65536 sections.

Here, the region 20 in Fig. 1 is also referred to as the region near the inner surface of the seamless steel pipe. In this embodiment, the inner surface vicinity region 20 is defined as a rectangle having the inner surface 10 as the upper end, 256 sections in the L direction, and 6 sections in the T direction. Furthermore, the region 30 in FIG. 1 is also referred to as the inner region of the seamless steel pipe. In this embodiment, the interior region 30 is a rectangle that abuts the interior near-surface region 20 below. Region 40 in FIG. 1 is also referred to as void region. The void region 40 corresponds to a through-hole of the martensitic stainless seamless steel pipe. In short, the observation field region 50 of FIG. 1 consists of the inner surface near region 20, the inner region 30 adjacent to the inner surface near region 20 below, and the void region 40 adjacent to the inner surface near region 20 above.

Elemental concentration analysis is performed in each of the 65536 sections of the observation field of view area 50 to identify the concentration of specific metal elements, which will be detailed later, in each section. The ratio of Cu in the obtained specific metal element is obtained in percentage and defined as the Cu concentration in each section. In this specification, the ratio of the number of sections having a Cu concentration exceeding 2.0% among the sections included in the inner surface neighboring region 20 in the observation field region 50 is defined as the inner surface Cu occupancy OS _Cu . Similarly, in this specification, the internal Cu occupancy OI _Cu is defined as the number ratio of sections having a Cu concentration of more than 2.0% among the sections included in the internal region 30 in the observation visual field region 50 .

Define Fn2=OS _Cu /OI _Cu . Fn2 is an index indicating the degree of uneven distribution of Cu precipitates in the region 20 near the inner surface. The larger Fn2 is, the more the Cu precipitates are unevenly distributed in the region 20 near the inner surface, and the pitting corrosion resistance of the inner surface can be effectively improved. As a result of detailed studies, the inventors of the present invention found that if Fn2 is 1.20 or more in a martensitic stainless seamless steel pipe having the chemical composition described above including formula (1), the inner surface of the seamless steel pipe It has been found that the pitting corrosion resistance is remarkably enhanced.

Therefore, the martensitic stainless seamless steel pipe according to the present embodiment has the above-described chemical composition including the formula (1), and furthermore, the inner surface Cu occupancy OS _Cu and the internal Cu occupancy defined as above are OI _Cu satisfies the following formula (2). As a result, the martensitic stainless seamless steel pipe according to the present embodiment can achieve both high yield strength and excellent pitting corrosion resistance on the inner surface.
OS _Cu /OI _Cu ≧1.20 (2)

The details of the reason why the pitting corrosion resistance of the inner surface of the seamless steel pipe increases when the Cu precipitates are unevenly distributed in the region 20 near the inner surface have not been clarified. However, the inventors presume as follows. As described above, a martensitic stainless seamless steel pipe intended for use in an oil well environment has a production fluid passing through it. At this time, H ₂ S gas may come into contact with the inner surface of the seamless steel pipe to form Cu sulfide. Furthermore, Cu precipitates may strengthen the passive film formed on the surface of the martensitic stainless seamless steel pipe. Therefore, the present inventors speculate that if Cu precipitates are unevenly distributed on the inner surface, Cu sulfides are likely to be formed and the pitting corrosion resistance of the inner surface is enhanced.

The gist of the martensitic stainless seamless steel pipe according to the present embodiment completed based on the above knowledge is as follows.

[1]
A martensitic stainless steel seamless steel pipe,
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%,
O: 0.050% or less,
W: 0 to 2.00%,
Nb: 0 to 0.50%,
Mg: 0-0.0050%,
Rare earth element: 0 to 0.0050%,
B: 0 to 0.0050%, and
Balance: Fe and impurities, and satisfies formula (1),
a microstructure, in volume percent, consisting of 0-15.0% retained austenite, 0-5.0% ferrite, and the balance tempered martensite;
Yield strength is 862 MPa or more,
When the pipe axial direction of the martensitic stainless steel seamless steel pipe is defined as the L direction, and the pipe radial direction of the martensitic stainless steel seamless steel pipe is defined as the T direction,
Including the inner surface of the seamless martensitic stainless steel pipe extending in the L direction, the length of the side extending in the L direction is 1.0 μm, and the length of the side extending in the T direction is 1.0 μm. In the square observation field area,
When the observation visual field region is divided into 256 equal sections in the L direction and 256 equal sections in the T direction,
The observation field of view area is
an inner surface vicinity region having a rectangular shape with the inner surface of the martensitic stainless steel seamless steel pipe as the upper end, 256 sections in the L direction, and 6 sections in the T direction;
the inner surface vicinity region and an inner region adjacent below the inner surface vicinity region;
comprising the inner surface vicinity region and a void region adjacent above the inner surface vicinity region,
Among all the sections in the inner surface vicinity region, the number ratio of sections with a Cu concentration exceeding 2.0% is defined as the inner surface Cu occupancy OS _Cu ,
When the number ratio of sections with a Cu concentration exceeding 2.0% among all the sections in the internal region is defined as the internal Cu occupancy OI _Cu ,
The inner surface Cu occupancy OS _Cu and the internal Cu occupancy OI _Cu satisfy formula (2),
Martensitic stainless seamless steel pipe.
Mo+0.5×W≧2.50 (1)
OS _Cu /OI _Cu ≧1.20 (2)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1) in terms of % by mass.

[2]
The martensitic stainless seamless steel pipe according to [1],
W: 0.01 to 2.00%,
Nb: 0.01 to 0.50%,
Mg: 0.0001-0.0050%,
Rare earth element: 0.0001 to 0.0050%, and
B: containing one or more elements selected from the group consisting of 0.0001 to 0.0050%,
Martensitic stainless seamless steel pipe.

[3]
A method for producing a martensitic stainless seamless steel pipe according to [1] or [2],
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%,
O: 0.050% or less,
W: 0 to 2.00%,
Nb: 0 to 0.50%,
Mg: 0-0.0050%,
Rare earth element: 0 to 0.0050%,
B: 0 to 0.0050%, and
Balance: a material preparation step of preparing a material consisting of Fe and impurities and satisfying formula (1);
After heating the prepared material in a heating furnace, hot working in which the cross-sectional reduction rate R defined by formula (A) is 40% or more and the hot working time is 15 minutes or less. A hot working step of manufacturing a mother pipe by carrying out
A quenching step of performing quenching on the raw pipe of ₃ or more points;
a tempering step of performing tempering on the quenched base pipe under conditions that satisfy formula (B);
A method for producing a martensitic stainless seamless steel pipe.
Mo+0.5×W≧2.50 (1)
R = {1-(cross-sectional area perpendicular to the axial direction of the tube after hot working/cross-sectional area perpendicular to the axial direction of the raw material before hot working)} x 100 (A)
(T + 273.15) x (20 + log ₁₀ (t/60)) x (1-[Cu]/100) ≤ 17200 (B)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1) in terms of % by mass.
In the formula (B), T is the tempering temperature in degrees Celsius, t is the tempering time in minutes, and [Cu] is the Cu content in the blank in mass %.

[4]
A method for producing a martensitic stainless seamless steel pipe according to [3],
The material is
W: 0.01 to 2.00%,
Nb: 0.01 to 0.50%,
Mg: 0.0001-0.0050%,
Rare earth element: 0.0001 to 0.0050%, and
B: containing one or more elements selected from the group consisting of 0.0001 to 0.0050%,
A method for producing a martensitic stainless seamless steel pipe.

The martensitic stainless seamless steel pipe according to this embodiment will be described in detail below. In addition, "%" regarding an element means the mass % unless there is particular notice.

[Chemical composition]
The chemical composition of the martensitic stainless seamless steel pipe according to this embodiment contains the following elements.

C: 0.030% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is over 0%. C enhances the hardenability of the steel material and enhances the strength of the steel material. However, if the C content is too high, C tends to combine with Cr to form Cr carbides. As a result, even if the content of other elements is within the range of the present embodiment, the toughness of the steel material is lowered. Therefore, the C content is 0.030% or less. A preferable lower limit of the C content is 0.001%, more preferably 0.003%, and still more preferably 0.005%. A preferable upper limit of the C content is 0.025%, more preferably 0.020%, and still more preferably 0.015%. The C content is preferably as low as possible.

Si: 1.00% or less Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is over 0%. Si deoxidizes steel. However, if the Si content is too high, the hot workability of the steel deteriorates even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 1.00% or less. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%. A preferable upper limit of the Si content is 0.70%, more preferably 0.50%, still more preferably 0.45%, still more preferably 0.40%.

Mn: 1.00% or less Manganese (Mn) is inevitably contained. That is, the lower limit of the Mn content is over 0%. Mn enhances the hardenability of the steel material and enhances the strength of the steel material. However, if the Mn content is too high, Mn forms coarse inclusions and lowers the toughness of the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the Mn content is 1.00% or less. A preferable lower limit of the Mn content is 0.10%, more preferably 0.20%, and still more preferably 0.25%. A preferable upper limit of the Mn content is 0.80%, more preferably 0.60%, and still more preferably 0.50%.

P: 0.030% or less Phosphorus (P) is an unavoidable impurity. That is, the lower limit of the P content is over 0%. If the P content is too high, even if the content of other elements is within the range of the present embodiment, P will segregate at the grain boundaries and significantly reduce the toughness of the steel material. Therefore, the P content is 0.030% or less. A preferable upper limit of the P content is 0.025%, more preferably 0.020%. The lower the P content is, the better. However, excessive reduction of the P content greatly increases manufacturing costs. Therefore, considering industrial production, the lower limit of the P content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.001%.

S: 0.0050% or less Sulfur (S) is an unavoidable impurity. That is, the lower limit of the S content is over 0%. If the S content is too high, even if the content of other elements is within the range of the present embodiment, S will segregate at the grain boundaries and significantly reduce the toughness of the steel material. Therefore, the S content is 0.0050% or less. A preferable upper limit of the S content is 0.0040%, more preferably 0.0030%, and still more preferably 0.0020%. It is preferable that the S content is as low as possible. However, drastic reduction of the S content greatly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%.

Cr: 11.00-14.00%
Chromium (Cr) increases the pitting resistance of steel. If the Cr content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content is too high, Cr carbides, Cr-containing intermetallics, and Cr oxides are excessively formed. In this case, even if the contents of the other elements are within the range of the present embodiment, the corrosion resistance of the steel material is lowered. Therefore, the Cr content is 11.00-14.00%. A preferable lower limit of the Cr content is 11.05%, more preferably 11.10%, still more preferably 11.50%, still more preferably 11.80%. A preferable upper limit of the Cr content is 13.70%, more preferably 13.50%, still more preferably 13.40%, still more preferably 13.30%.

Ni: 5.00-7.50%
Nickel (Ni) enhances the pitting corrosion resistance of steel materials. Ni is also an austenite-forming element, and converts the microstructure of the steel material to martensite after quenching. If the Ni content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ni content is too high, the above effects will be saturated and the manufacturing cost will increase. Therefore, the Ni content is 5.00-7.50%. A preferable lower limit of the Ni content is 5.10%, more preferably 5.15%, and still more preferably 5.20%. The upper limit of the Ni content is preferably 7.30%, more preferably 7.00%, still more preferably 6.80%, still more preferably 6.60%, still more preferably 6.40 %.

Mo: 1.50-4.50%
Molybdenum (Mo) enhances the pitting corrosion resistance of steel. If the Mo content is too low, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mo content is too high, the above effects will be saturated and the manufacturing cost will increase. Therefore, the Mo content is 1.50-4.50%. A preferable lower limit of the Mo content is 1.60%, more preferably 1.70%, still more preferably 1.80%. A preferred upper limit of the Mo content is 4.30%, more preferably 4.10%, still more preferably 3.90%, still more preferably 3.70%.

Cu: 0.50-3.50%
Copper (Cu) precipitates as Cu precipitates in the steel material. If the Cu precipitates are unevenly distributed in the inner surface vicinity region 20 of the seamless steel pipe, the pitting corrosion resistance of the inner surface of the seamless steel pipe is enhanced. Cu precipitates further increase the strength of the steel. If the Cu content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cu content is too high, the strength of the steel material becomes too high, and the corrosion resistance and/or low temperature toughness of the steel material deteriorates, even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Cu content is 0.50-3.50%. A preferable lower limit of the Cu content is 0.60%, more preferably 0.70%, and still more preferably 0.80%. A preferable upper limit of the Cu content is less than 3.50%, more preferably 3.45%, more preferably 3.40%, still more preferably 3.20%.

Co: 0.010-0.500%
Cobalt (Co) forms a film on the surface of the steel material and enhances the pitting corrosion resistance of the steel material. Co further enhances the hardenability of the steel material and stabilizes the strength of the steel material. If the Co content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Co content is too high, the above effects will saturate and the production cost will increase. Therefore, the Co content is 0.010-0.500%. The lower limit of the Co content is preferably 0.015%, more preferably 0.020%, still more preferably 0.030%. A preferable upper limit of the Co content is 0.450%, more preferably 0.400%.

Ti: 0.050-0.300%
Titanium (Ti) combines with C or N to form carbides or nitrides in the steel material. In this case, the pinning effect suppresses coarsening of crystal grains and increases the strength of the steel material. Furthermore, Ti forms carbides or nitrides to suppress an excessive increase in strength due to excessive formation of V precipitates (carbides, nitrides, carbonitrides). As a result, the pitting corrosion resistance of the steel is enhanced. If the Ti content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ti content is too high, the above effects will saturate and the production cost will increase. If the Ti content is too high, Ti carbides or Ti nitrides are excessively formed, and the toughness of the steel material is lowered. Therefore, the Ti content is 0.050-0.300%. A preferable lower limit of the Ti content is 0.060%, more preferably 0.070%, and still more preferably 0.080%. A preferable upper limit of the Ti content is 0.250%, more preferably 0.200%.

V: 0.01-1.00%
Vanadium (V) forms precipitates (V precipitates) such as carbides, nitrides, and carbonitrides in the steel material to increase the strength of the steel material. If the V content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content is too high, even if the content of the other elements is within the range of the present embodiment, excessive V precipitates are formed and the toughness of the steel decreases. Therefore, the V content is 0.01-1.00%. A preferable lower limit of the V content is 0.02%, more preferably 0.03%. A preferred upper limit of the V content is 0.90%, more preferably 0.80%, still more preferably 0.60%, still more preferably 0.50%.

Ca: 0.0005-0.0050%
Calcium (Ca) renders S in the steel material harmless as sulfides and enhances the hot workability of the steel material. If the Ca content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ca content is too high, even if the contents of other elements are within the ranges of the present embodiment, inclusions in the steel material become coarse and the toughness of the steel material decreases. Therefore, the Ca content is 0.0005-0.0050%. The lower limit of the Ca content is preferably 0.0006%, more preferably 0.0008%, still more preferably 0.0010%. A preferable upper limit of the Ca content is 0.0040%, more preferably 0.0030%.

Al: 0.001-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content is too high, even if the content of other elements is within the range of the present embodiment, coarse Al oxides are formed and the toughness of the steel material is lowered. Therefore, the Al content is 0.001-0.100%. A preferable lower limit of the Al content is 0.002%, more preferably 0.003%, and still more preferably 0.005%. A preferable upper limit of the Al content is 0.095%, more preferably 0.090%, and still more preferably 0.085%. In addition, Al content in this specification is sol. It means the content of Al (acid-soluble Al).

N: 0.0010-0.0500%
Nitrogen (N) enhances the corrosion resistance of steel. If the N content is too low, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the N content is too high, coarse Ti nitrides will form even if the content of other elements is within the range of the present embodiment, and the toughness of the steel material will decrease. Therefore, the N content is 0.0010-0.0500%. A preferable lower limit of the N content is 0.0015%, more preferably 0.0020%, and still more preferably 0.0025%. A preferred upper limit of the N content is 0.0450%, more preferably 0.0400%, still more preferably 0.0350%, still more preferably 0.0300%.

O: 0.050% or less Oxygen (O) is an unavoidable impurity. That is, the lower limit of the O content is over 0%. O forms oxides and lowers the pitting corrosion resistance of steel materials. Therefore, if the O content is too high, the pitting corrosion resistance of the steel is remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the O content is 0.050% or less. A preferable upper limit of the O content is 0.040%, more preferably 0.030%, and still more preferably 0.020%. It is preferable that the O content is as low as possible. However, drastic reduction of O content increases manufacturing cost. Therefore, considering industrial production, the lower limit of the O content is preferably 0.0005%, more preferably 0.001%.

The remainder of the martensitic stainless seamless steel pipe according to this embodiment consists of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when industrially producing steel materials, and are not intentionally included. It means that it is permissible within a range that does not adversely affect the martensitic stainless steel material due to

[Arbitrary element]
The martensitic stainless seamless steel pipe according to the present embodiment may further contain W instead of part of Fe.

W: 0-2.00%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When included, W stabilizes the passive film in a sour environment and inhibits destruction of the passive film by chloride ions and hydrogen sulfide ions. As a result, the pitting corrosion resistance of the steel is enhanced. If even a small amount of W is contained, the above effect can be obtained to some extent. On the other hand, if the W content is too high, W will combine with C to form coarse carbides. In this case, even if the content of other elements is within the range of the present embodiment, the pitting corrosion resistance of the steel material is lowered. Therefore, the W content is 0-2.00%. A preferable lower limit of the W content is 0.01%, more preferably 0.03%, and still more preferably 0.05%. A preferable upper limit of the W content is 1.75%, more preferably 1.50%, and still more preferably 1.20%.

The martensitic stainless seamless steel pipe according to the present embodiment may further contain Nb instead of part of Fe.

Nb: 0-0.50%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When included, Nb combines with C and/or N to form Nb carbides, Nb carbonitrides. In this case, the pinning effect suppresses grain coarsening and increases the yield strength of the steel material. If even a small amount of Nb is contained, the above effect can be obtained to some extent. On the other hand, if the Nb content is too high, Nb carbides and/or Nb carbonitrides are excessively produced even if the other element contents are within the range of the present embodiment. As a result, the pitting corrosion resistance of the steel is lowered. Therefore, the Nb content is 0-0.50%. A preferable lower limit of the Nb content is 0.01%, more preferably 0.02%, and still more preferably 0.03%. A preferable upper limit of the Nb content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.

The martensitic stainless seamless steel pipe according to the present embodiment may further contain one or more elements selected from the group consisting of Mg, rare earth elements (REM), and B, instead of part of Fe. These elements are optional elements, and all of them improve the hot workability of the steel material.

Mg: 0-0.0050%
Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When included, Mg controls the morphology of inclusions and enhances the hot workability of the steel. 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, coarse oxides are formed and the toughness of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Mg content is 0-0.0050%. A preferable lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. A preferable upper limit of the Mg content is 0.0045%, more preferably 0.0040%, and still more preferably 0.0035%.

Rare earth element (REM): 0-0.0050%
A rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When included, REM, like Mg, controls the morphology of inclusions and enhances the hot workability of the steel. The above effect can be obtained to some extent if REM is contained even in a small amount. However, if the REM content is too high, coarse oxides are formed and the toughness of the steel material is reduced even if the content of other elements is within the range of the present embodiment. Therefore, the REM content is 0-0.0050%. A preferable lower limit of the REM content is 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. A preferred upper limit for the REM content is 0.0045%, more preferably 0.0040%, and still more preferably 0.0035%.

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

B: 0 to 0.0050%
Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When included, B segregates at the austenite grain boundaries to strengthen the grain boundaries and enhance the hot workability of the steel. If even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content is too high, even if the contents of other elements are within the range of the present embodiment, Cr carbide borides are formed, and the toughness of the steel material is lowered. Therefore, the B content is 0-0.0050%. A preferable lower limit of the B content is 0.0001%, more preferably 0.0002%. A preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, still more preferably 0.0035%, still more preferably 0.0030%.

[Formula (1)]
The martensitic stainless seamless steel pipe according to this embodiment has the chemical composition described above and further satisfies the following formula (1).
Mo+0.5×W≧2.50 (1)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1) in terms of % by mass.

By increasing Fn1 (= Mo + 0.5 x W), the pitting corrosion resistance of the seamless steel pipe can be improved while maintaining the yield strength of the seamless steel pipe. Therefore, the seamless martensitic stainless steel pipe according to the present embodiment has the chemical composition and microstructure described above, and has an Fn1 of 2.50 or more. As a result, the martensitic stainless seamless steel pipe according to the present embodiment can achieve both a yield strength of 125 ksi or more and excellent pitting corrosion resistance, provided that other configurations of the present embodiment are satisfied. A preferred lower limit for Fn1 is 2.60, more preferably 2.70. In this embodiment, the upper limit of Fn1 is not particularly limited, but is 4.50, for example.

[Microstructure]
The microstructure of the seamless martensitic stainless steel pipe according to the present embodiment is composed of 0 to 15.0% retained austenite, 0 to 5.0% ferrite, and the balance tempered martensite in terms of volume %. As used herein, "consisting of retained austenite, ferrite and tempered martensite" means that phases other than retained austenite, ferrite and tempered martensite are negligibly small. For example, in the chemical composition of the martensitic stainless seamless steel pipe according to the present embodiment, the volume fraction of precipitates and inclusions is negligible compared to the volume fractions of retained austenite, ferrite, and tempered martensite. small. That is, the microstructure of the seamless martensitic stainless steel pipe according to the present embodiment may contain minute amounts of precipitates, inclusions, and the like in addition to retained austenite, ferrite, and tempered martensite.

As described above, in the microstructure of the seamless martensitic stainless steel pipe according to the present embodiment, the volume fraction of retained austenite is 0 to 15.0%, and the volume fraction of ferrite is 0 to 5.0%. , and the balance consists of tempered martensite. That is, in the microstructure of the seamless martensitic stainless steel pipe according to this embodiment, the volume fraction of tempered martensite is 80 to 100.0%. If the volume fraction of retained austenite and ferrite is too high, it becomes difficult to control the mechanical properties of the steel material. On the other hand, the lower limit of the volume fraction of retained austenite and ferrite may be 0%. That is, the martensitic stainless seamless steel pipe according to the present embodiment may have a microstructure consisting only of tempered martensite.

In the present embodiment, the lower limit of the volume fraction of retained austenite in the microstructure may be 1.0% or 2.0%. Furthermore, in the microstructure, the upper limit of the volume fraction of retained austenite may be 13.0% or 10.0%. In this embodiment, the lower limit of the volume fraction of ferrite in the microstructure may be 0.5%. Furthermore, in the microstructure, the upper limit of the ferrite volume fraction may be 3.0% or 2.0%.

[Method for measuring volume fraction of retained austenite]
The volume fraction (%) of retained austenite in the microstructure of the seamless martensitic stainless steel pipe according to the present embodiment can be obtained by the following method.

　The volume fraction of retained austenite is determined by the X-ray diffraction method. Specifically, a test piece is prepared from the thickness central portion of a martensitic stainless steel seamless steel pipe. Although the size of the test piece is not particularly limited, it is, for example, 15 mm×15 mm×2 mm thick. In this case, the thickness direction of the test piece is parallel to the wall thickness (pipe diameter) direction. Using the prepared test piece, the (200) plane of the α phase (ferrite and martensite), the (211) plane of the α phase, the (200) plane of the γ phase (retained austenite), the (220) plane of the γ phase, The X-ray diffraction intensity of each (311) plane of the γ phase is measured, and the integrated intensity of each plane is calculated.

In the measurement of X-ray diffraction intensity, the target of the X-ray diffractometer is Mo (MoKα ray). After the calculation, the volume fraction Vγ (%) of retained austenite is calculated using the formula (I) for each combination (2×3=6 sets) of each α-phase plane and each γ-phase plane. Then, the average value of the volume fraction Vγ of retained austenite in the six sets is defined as the volume fraction (%) of retained austenite.
Vγ=100/{1+(Iα×Rγ)/(Iγ×Rα)} (I)
where Iα is the integrated intensity of the α phase. Rα is the crystallographically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically calculated value of the γ phase. In this specification, Rα on the (200) plane of the α phase is 15.9, Rα on the (211) plane of the α phase is 29.2, and Rγ on the (200) plane of the γ phase is 35.9. 5. Let Rγ on the (220) plane of the γ phase be 20.8 and Rγ on the (311) plane of the γ phase be 21.8. The volume fraction of retained austenite is the value obtained by rounding the obtained numerical value to the second decimal place.

[Method for measuring volume fraction of ferrite]
The volume fraction (%) of ferrite in the microstructure of the seamless martensitic stainless steel pipe according to the present embodiment can be obtained by the following method.

The volume fraction of ferrite is determined by the point counting method based on ASTM E562 (2019). Specifically, a test piece is prepared from the thickness central portion of a martensitic stainless steel seamless steel pipe. The test piece is not particularly limited as long as it has an observation surface perpendicular to the rolling (tube axis) direction. The specimen is embedded in resin, and the observation surface polished to a mirror surface is immersed in a Villella corrosive solution (mixture of ethanol, hydrochloric acid, and picric acid) for about 60 seconds to expose the tissue by etching. The etched observation surface is observed for 30 fields of view using an optical microscope. Although the field of view area is not particularly limited, it is, for example, 0.03 mm ² per field of view (magnification of 400 times).

In each observation field, ferrite and other phases (retained austenite and tempered martensite) can be distinguished from the contrast by those skilled in the art. Therefore, ferrite in each observation field is specified based on the contrast. The area ratio of the specified ferrite is determined by the point counting method based on ASTM E562 (2019). The arithmetic average value of the ferrite area ratios in the 10 fields of view obtained is defined as the ferrite volume ratio (%). The volume fraction of ferrite is the value obtained by rounding the obtained numerical value to the second decimal place.

[Method for measuring volume fraction of tempered martensite]
The volume fraction (%) of tempered martensite can be obtained by the following method. Specifically, using the volume fraction (%) of retained austenite obtained by the above-described X-ray diffraction method and the volume percentage (%) of ferrite obtained by the above-described point counting method, martensitic stainless steel seamless The volume fraction (%) of tempered martensite in the microstructure of the steel pipe is obtained by the following formula.
Volume fraction of tempered martensite (%) = 100 - {volume fraction of retained austenite (%) + volume fraction of ferrite (%)}

[Yield strength]
The martensitic stainless seamless steel pipe according to this embodiment has a yield strength of 862 MPa or more (125 ksi or more). Yield strength as used herein means a 0.2% offset yield strength obtained in a tensile test. The martensitic stainless steel seamless steel pipe according to the present embodiment has the above chemical composition including formula (1) even if it has a yield strength of 125 ksi or more, and by satisfying formula (2) described later, It exhibits excellent pitting corrosion resistance on the inner surface of seamless steel pipes. In the martensitic stainless seamless steel pipe according to the present embodiment, the upper limit of the yield strength is not particularly limited, but is 1172 MPa, for example.

The yield strength of the martensitic stainless seamless steel pipe according to this embodiment can be obtained by the following method. A tensile test piece is produced from the seamless martensitic stainless steel pipe according to this embodiment in accordance with ASTM E8/E8M (2021). Specifically, a round-bar test piece is produced from the thickness central portion of the seamless steel pipe. The size of the round bar test piece is, for example, a diameter of 8.9 mm at the parallel portion and a gauge length of 35.6 mm. If a round bar test piece cannot be produced from a seamless steel pipe, an arc-shaped test piece is produced. The size of the arc-shaped test piece is, for example, the same thickness as the seamless steel pipe, with a width of 25.4 mm and a gauge length of 50.8 mm. The axial direction of the tensile test piece was parallel to the axial direction of the seamless steel pipe. Using a tensile test piece, a tensile test was performed at room temperature (24±3°C) in accordance with ASTM E8/E8M (2021), and the obtained 0.2% offset yield strength (MPa) was calculated as the yield strength ( MPa). In this specification, the yield strength is a value obtained by rounding off the obtained numerical value to the first decimal place.

[Formula (2)]
The martensitic stainless steel seamless steel pipe according to the present embodiment includes the inner surface of the seamless steel pipe and has a side length of 1.0 μm extending in the L direction and a side length of 1.0 μm extending in the T direction. In a certain square observation field region, when the observation field region is divided into 256 equal sections in the L direction and 256 equal sections in the T direction, the inner surface vicinity region 20 of the martensitic stainless steel seamless steel pipe has , the number ratio of sections with a Cu concentration exceeding 2.0% is defined as the inner surface Cu occupancy OS _Cu , and the number of sections with a Cu concentration exceeding 2.0% in the inner region 30 of the martensitic stainless seamless steel pipe is defined as the internal Cu occupancy OI _Cu , the inner surface Cu occupancy OS _Cu and the internal Cu occupancy OI _Cu satisfy the formula (2).
OS _Cu /OI _Cu ≧1.20 (2)

Fn2 (=OS _Cu /OI _Cu ) is an index indicating the degree of uneven distribution of Cu precipitates in the region 20 near the inner surface. The larger Fn2 is, the more the Cu precipitates are unevenly distributed in the region 20 near the inner surface, and the pitting corrosion resistance of the inner surface can be effectively improved. Therefore, the seamless martensitic stainless steel pipe according to the present embodiment has the above chemical composition including formula (1), a yield strength of 125 ksi or more, and Fn2 of 1.20 or more. As a result, the martensitic stainless seamless steel pipe according to the present embodiment can achieve both high yield strength and excellent pitting corrosion resistance on the inner surface.

Therefore, the martensitic stainless seamless steel pipe according to the present embodiment has a chemical composition that satisfies the formula (1), a yield strength of 862 MPa or more, and Fn2 of 1.20 or more. In the martensitic stainless seamless steel pipe according to the present embodiment, the preferred lower limit of Fn2 is 1.25, more preferably 1.30. Although the upper limit of Fn2 is not particularly limited in this embodiment, it is, for example, 5.00.

In the martensitic stainless seamless steel pipe according to the present embodiment, the inner surface Cu occupancy OS _Cu and the internal Cu occupancy OI _Cu can be obtained by the following method. A thin film test piece for observing the inner surface is produced from the seamless martensitic stainless steel pipe according to the present embodiment. A thin film test piece is produced by focused ion beam (Focused Ion Beam, hereinafter also referred to as “FIB”) processing. Moreover, the shape of the thin film test piece is not particularly limited as long as an observation surface described later can be obtained. The size of the observation surface of the thin film test piece is, for example, 10 μm×10 μm, and the thickness of the thin film test piece is, for example, 150 nm. In FIB processing, a protective film (so-called deposition film) for protecting the inner surface is further formed on the inner surface.

On the observation surface of the obtained thin film test piece, an observation visual field region of L direction: 1.0 μm×T direction: 1.0 μm=1.0 μm ² is specified. The observation field of view is adjusted so as to include the inner surface of the seamless steel pipe. Preferably, as shown in FIG. 1, the observation field region 50 is specified so that the inner surface 10 of the seamless steel pipe is positioned near the center of the observation field region 50 in the T direction and extends in the L direction. Here, "the inner surface 10 of the seamless steel pipe is located near the center of the observation field region 50 in the T direction" means that when the observation field region 50 is specified, the inner surface 10 of the seamless steel pipe that can be confirmed by observation It means that it is positioned approximately in the center of the observation visual field area 50 in the T direction. For example, when the observation visual field region 50 is divided into 5 equal parts in the T direction, if the inner surface 10 of the seamless steel pipe falls in the third region from the top in the T direction over the entire length in the L direction, the inner surface of the seamless steel pipe 10 can be said to be located near the center of the observation visual field area 50 in the T direction. In addition, "the inner surface 10 of the seamless steel pipe extends in the L direction of the observation field region 50" means that when the observation field region 50 is specified, the inner surface 10 of the seamless steel pipe that can be confirmed by observation extends to the observation field region 50. It means that it is approximately parallel to the L direction of the . In this embodiment, arbitrary four observation visual fields are specified from the observation surface of the thin film test piece.

　Perform tissue observation with a transmission electron microscope (hereinafter also referred to as "TEM") for the specified four observation field areas. Although the conditions for tissue observation are not particularly limited, for example, the acceleration voltage is set to 200 kV.

The observation visual field region in which the tissue was observed by the TEM was divided into 65536 sections, which were equally divided into 256 sections in the L direction and 256 sections in the T direction. One section is a square of L direction: 4 nm x T direction: 4 nm = 16 nm ² . In this specification, each section is represented by (n, m) with the upper left corner of the observation field region as the origin. Here, n (integer) means the L-direction position in the observation visual field area, with 1 being the left end of the observation visual field area and 256 being the right end. Similarly, m (integer) means the T-direction position in the observation visual field area, with 1 being the upper end of the observation visual field area and 256 being the lower end. Hereinafter, it demonstrates concretely using drawing.

FIG. 2 is a schematic diagram showing how the observation visual field area 50 is divided into 256 equal parts in the L direction and 256 equal parts in the T direction, and divided into 65536 sections. Referring to FIG. 2, the n-th section from the left end of observation visual field area 50 and the m-th section from the upper end of observation visual field area 50 is expressed as (n, m). Further, with reference to FIG. 2 , the upper left section of the observation visual field area 50 is (1, 1), the upper right section of the observation visual field area 50 is (256, 1), and the lower left section of the observation visual field area 50 is The edge segment is (1,256) and the lower rightmost segment of viewing field 50 is (256,256).

In addition, as described above, in the seamless steel pipe according to the present embodiment, the inner surface 10 extends in the L direction of the observation visual field area 50 . In short, in the seamless steel pipe according to the present embodiment, the observation field region 50 is specified so that the inner surface 10 of the seamless steel pipe that can be confirmed by observation is substantially parallel to the L direction of the observation field region 50 . Therefore, the observation visual field region 50 is specified so that the section in the T direction including the inner surface 10 of the seamless steel pipe that can be confirmed by observation is as small as possible over the entire length in the L direction. For example, the inner surface 10 of the seamless steel pipe according to the present embodiment may be included in 5 sections in the T direction, more preferably in 3 sections in the T direction, over the entire length in the L direction. Most preferably, the inner surface 10 of the seamless steel pipe according to this embodiment is included in one section in the T direction over the entire length in the L direction.

Next, elemental concentration analysis is performed on each section of the observation field region by EDS (Energy Dispersive X-ray Spectroscopy) attached to the TEM. The target elements are quantified as Fe, Cr, Ni, Mo, and Cu. From the results of the EDS analysis, the Fe concentration C _Fe (n,m) and the Cu concentration C _Cu (n,m) are specified in terms of relative intensities for the section (n,m). Specifically, the Fe concentration C _Fe (n, m) and the Cu concentration C _Cu (n, m) are defined by the following equations (3) and (4).
C _Fe (n, m) = 100 × [Fe] _{(n, m)} / ([Fe] _{(n, m)} + [Cr] _{(n, m)} + [Ni] _{(n, m)} + [Mo] _{(n, m)} + [Cu] _{(n, m)} ) (3)
C _Cu (n, m) = 100 × [Cu] _{(n, m)} / ([Fe] _{(n, m)} + [Cr] _{(n, m)} + [Ni] _{(n, m)} + [Mo] _{(n, m)} + [Cu] _{(n, m)} ) (4)
where [Fe] _{(n, m)} , [Cr] _{(n, m)} , [Ni] _{(n, m)} , [Mo] _{(n, m)} and , [Cu] _{(n, m)} are substituted with the detected intensities of Fe, Cr, Ni, Mo, and Cu in the section (n, m) determined by the EDS analysis.

Using the determined Fe concentration C _Fe (n, m) of each section, the inner surface of the seamless steel pipe in the observation field region can be specified. Concretely, for example, it is also possible to obtain the L-direction average value of the Fe concentration C _Fe (n, m) and obtain it from the plot. More specifically, the arithmetic average of the Fe concentrations C _Fe (1, m) to C _Fe (256, m) at the T-direction position m is obtained, and defined as the L-direction average value A _Fe (m) of the Fe concentration. . The L-direction average A _Fe (m) of the obtained 256 Fe concentrations is plotted against the T-direction position m.

FIG. 3 is a schematic diagram showing the relationship between the tube radial direction (T direction) position m of each section in the observation field region and the tube axis direction (L direction) average value A _Fe (m) of the Fe concentration. Referring to FIG. 3, in region 100, the L-direction average Fe concentration A _Fe (m) changes abruptly. Further, referring to FIG. 3, on the right side of the region 100 (that is, in the positive direction of the T-direction position m), the L-direction average Fe concentration A _Fe (m) is relatively stable. This is because the Fe concentration differs greatly between the gap region 40 and the inner surface vicinity region 20 in the observation field region, and the Fe content in the inner region 30 is relatively stable.

Thus, the inner surface of the seamless steel pipe can be specified from the shape of the plot of the L-direction average value A _Fe (m) of the Fe concentration with respect to the T-direction position m. Specifically, as shown in FIG. 3, a region 100 is specified in which the L-direction average value A _Fe (m) of the Fe concentration changes sharply. It is naturally possible for a person skilled in the art to specify the region 100 where the L-direction average value A _Fe (m) of the Fe concentration changes sharply. Next, the maximum and minimum values of the L-direction average value A _Fe (m) of the Fe concentration in the region 100 are obtained, and the arithmetic average value A _Fe-ave thereof is obtained. Furthermore, in the region 100, when the value of the L-direction average value A _Fe (m) of the Fe concentration first exceeds A _Fe-ave , that m is specified as the T-direction position of the inner surface. More specifically, when the T-direction position of the inner surface is k (integer), the L-direction average value of Fe concentration A _Fe (m) and A _Fe-ave are given by the following equations (5) and (6): It holds.
A _Fe (k−1)≦A _Fe-ave (5)
A _Fe (k) > A _Fe-ave (6)

In this embodiment, the region 20 near the inner surface is defined as a rectangle with the inner surface of the martensitic stainless seamless steel pipe as the upper end, 256 sections in the L direction, and 6 sections in the T direction. In other words, in this embodiment, the upper left end is the section (1, k), the upper right end is the section (256, k), and the lower left end is the section (1, k+5 ) and the lower right corner is the section (256, k+5) is defined as the inner surface vicinity region 20 .

Further, the observation visual field region 50 is composed of the inner surface vicinity region 20, the inner region 30, and the gap region 40. Here, the internal region 30 is adjacent to the inner surface vicinity region 20 below the inner surface vicinity region 20 . That is, in this embodiment, the upper left end is section (1, k+6), the upper right end is section (256, k+6), and the lower left end is section (1 , 256) and the lower right corner is the section (256, 256).

Similarly, the void region 40 is adjacent to the inner surface vicinity region 20 above the inner surface vicinity region 20 . That is, in this embodiment, the upper left end is section (1, 1), the upper right end is section (256, 1), and the lower left end is section (1 , k−1) and the lower right corner is the section (256, k−1) is defined as the void region 40 . As described above, the void regions 40 correspond to the through holes of the martensitic stainless seamless steel pipe.

Among all the sections of the inner surface vicinity region 20 defined as above, the number ratio of the sections having the Cu concentration exceeding 2.0% is defined as the inner surface Cu occupancy OS _Cu . Here, the total number of sections in the inner surface vicinity region 20 is 256 sections in the L direction×6 sections in the T direction=1536 sections. Of all rectangular sections with section (1, k) as the upper left corner and section (256, k + 5) as the lower right corner, the sections with a Cu concentration C _Cu (n, m) exceeding 2.0% are counted, The number ratio to the total number of partitions 1536 is calculated. As described above, in this embodiment, arbitrary four observation visual field regions are specified from the observation surface of the thin film test piece. Therefore, the arithmetic average value of the number ratios obtained in the four observation field regions is defined as the inner surface Cu occupancy OS _Cu . In this specification, the inner surface Cu occupancy OS _Cu is a value obtained by rounding the obtained numerical value to the second decimal place.

Similarly, the ratio of the number of sections with a Cu concentration exceeding 2.0% to all the sections of the inner region 30 defined as described above is defined as the internal Cu occupancy OI _Cu . Here, as described above, the internal region 30 is defined as the Cu concentration C _Cu (n, m) of all the rectangular sections with the section (1, k+6) as the upper left corner and the section (256, 256) as the lower right corner. Count the compartments in which is more than 2.0%, and determine the number ratio to the total number of compartments. As described above, in this embodiment, arbitrary four observation visual field regions are specified from the observation surface of the thin film test piece. Therefore, the arithmetic average value of the number ratios obtained in the four observation field regions is defined as the internal Cu occupancy _OICu . In this specification, the internal Cu occupancy OI _Cu is a value obtained by rounding the obtained numerical value to the third decimal place.

Here, as described above, in FIB processing, a protective film is formed on the inner surface. On the other hand, in the present embodiment, the Fe concentration C 2 _Fe is used to specify the inner surface 10 of the seamless steel pipe as described above. Therefore, elements other than Fe (for example, carbon: C, tungsten: W, or platinum: Pt) are preferable for the elements constituting the protective film formed in FIB processing. These elements are used as ordinary vapor deposition elements for thin film specimens on which structural observation is performed by TEM, and those skilled in the art can naturally select and use them.

[pitting corrosion resistance]
The martensitic stainless seamless steel pipe according to this embodiment has excellent pitting corrosion resistance on the inner surface. In the present embodiment, excellent pitting corrosion resistance on the inner surface is defined as follows.

In this embodiment, the pitting potential of the inner surface of the seamless steel pipe is measured to evaluate the pitting corrosion resistance. Specifically, a test piece for pitting potential measurement is produced from the seamless steel pipe according to the present embodiment. The test piece includes an area of 1.0 cm ² of the inner surface of the seamless steel pipe as a test surface. The shape of the test piece is not particularly limited as long as it includes the test surface described above. For example, the test piece may have an inner surface area of 1.0 cm ² or more and a thickness equal to the wall thickness of the seamless steel pipe. The area of the test piece other than the test surface shall be covered with an insulator. Here, the insulator is not particularly limited, and a well-known insulator that can be used in the test environment described later may be used. For example, resin may be used as the insulator.

An electrochemical test is performed using a test piece for pitting potential measurement to measure the anodic polarization curve. Specifically, the test solution is a 25% by mass sodium chloride aqueous solution whose pH is adjusted to 4.5 with 0.08 g/L sodium hydrogen carbonate. The test solution should be degassed before use. Enclose the specimen in an autoclave. In an autoclave, a test piece is immersed in a test solution having a specific liquid volume of 500 mL/cm ² or more to prepare a test bath. After degassing the test bath, a mixed gas of 0.03 atm H ₂ S gas and 10 atm CO ₂ gas is pressurized into the autoclave, and the test bath is stirred to create a corrosive environment.

The test bath is heated to 175°C. For electrochemical studies, a saturated KCl-Silver Chloride Electrode is used as the Reference Electrode. Furthermore, a platinum electrode is used as a counter electrode. After the immersion potential stabilizes, the anodic polarization curve is measured by a potentiostat at a potential sweep rate of 20 mV/min in the anodic direction from the immersion potential. The anode polarization curve is measured until the anode current density reaches 1000 μA/cm ² .

From the obtained anodic polarization curve, the potential when the anodic current density reaches 1000 μA/cm ² is determined. Similar measurements are performed three times, and the arithmetic average value of the obtained potentials is defined as the pitting corrosion potential V'c1000 (mV). In this embodiment, if the pitting potential V'c1000 defined above is -230 mV or more, the seamless steel pipe is judged to have excellent pitting corrosion resistance on the inner surface.

[Applications of seamless steel pipes]
Applications of the martensitic stainless seamless steel pipe according to the present embodiment are not particularly limited. The martensitic stainless seamless steel pipe according to the present embodiment is suitable for oil well seamless steel pipes. Seamless steel pipes for oil wells are, for example, casings, tubings, drill pipes, etc. used for drilling oil wells or gas wells, extracting crude oil or natural gas, and the like.

[Production method]
An example of a manufacturing method for the martensitic stainless seamless steel pipe according to the present embodiment will be described. The manufacturing method described below is merely an example, and the method for manufacturing the martensitic stainless seamless steel pipe of the present embodiment is not limited to this. That is, as long as the martensitic stainless steel seamless steel pipe of the present embodiment having the above-described structure can be manufactured, it is not limited to the manufacturing method described below, and may be manufactured by other manufacturing methods. Preferably, the method for manufacturing the martensitic stainless steel seamless pipe according to the present embodiment includes a material preparation step, a hot working step, and a heat treatment step (quenching step and tempering step). Hereinafter, a case where the manufacturing method includes a material preparation process, a hot working process, and a heat treatment process will be described in detail.

[Material preparation process]
In the material preparation step, a material having the chemical composition described above is prepared. Here, the chemical composition of the material is the same as the chemical composition of the martensitic stainless seamless steel pipe according to this embodiment. Specifically, the material according to the present embodiment is, in mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0 .0050% or less, Cr: 11.00 to 14.00%, Ni: 5.00 to 7.50%, Mo: 1.50 to 4.50%, Cu: 0.50 to 3.50%, Co : 0.010-0.500%, Ti: 0.050-0.300%, V: 0.01-1.00%, Ca: 0.0005-0.0050%, Al: 0.001-0 .100%, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 2.00%, Nb: 0 to 0.50%, Mg: 0 to 0.0050%, Rare earth element: 0 to 0.0050%, B: 0 to 0.0050%, and balance: Fe and impurities. The manufacturing method is not particularly limited as long as the material has the chemical composition described above. Materials may be prepared by manufacturing or purchased from a third party. That is, the method of preparing the material is not limited.

When manufacturing and preparing materials, for example, manufacture by the following method. Molten steel having the above chemical composition is produced by a well-known refining method. A cast slab is manufactured by a continuous casting method using the manufactured molten steel. Here, the slab is a slab, bloom, or billet. Instead of the slab, the molten steel may be used to produce an ingot by an ingot casting method. If desired, the slab, bloom or ingot may be hot rolled to produce a billet. A raw material (slab, bloom, or billet) is manufactured by the manufacturing process described above.

[Hot working process]
In the hot working process, the prepared material is hot worked. First, the material is heated in a heating furnace. Although the heating temperature is not particularly limited, it is, for example, 1100 to 1300.degree. A blank pipe (seamless steel pipe) is manufactured by subjecting the raw material extracted from the heating furnace to hot working. Specifically, in the present embodiment, piercing-rolling is performed as hot working to manufacture the mother tube. A well-known method can be used for piercing-rolling, and is not particularly limited. For example, the piercing ratio in piercing-rolling is not particularly limited.

In the hot working process according to the present embodiment, the piercing-rolled mother tube is further hot-rolled as necessary. Specifically, the blank tube after piercing-rolling may be stretched-rolled and then shaped-rolled. In this case, a mandrel mill or a plug mill may be used in the elongation rolling. Moreover, you may implement elongation rolling using an elongator mill as needed. Furthermore, you may carry out elongation-rolling which combined these two or more. For example, the mother tube after piercing-rolling may be stretch-rolled using an elongator mill and then stretch-rolled using a plug mill. A stretch reducer may be used, a sizing mill may be used, or a plurality of these may be used in combination in the constant rolling performed on the mother pipe after stretch rolling. A blank tube is manufactured by the above steps.

Preferably, in the hot working step according to the present embodiment, the cumulative area reduction rate R due to hot working is 40% or more. The area reduction rate R is defined by the following formula (A).
R = {1-(cross-sectional area perpendicular to the axial direction of the tube after hot working/cross-sectional area perpendicular to the axial direction of the material before hot working)} x 100 (A)

The "mother pipe after hot working" in formula (A) means the mother pipe after the final hot working is completed. The "material before hot working" in formula (A) means the material before hot working. That is, in the hot working process according to the present embodiment, the cross-sectional area reduction rate R is defined by the cross-sectional area perpendicular to the axial direction of the material changed by the hot working.

If the cross-sectional reduction rate R in the hot working process is large, a strong shearing force is applied to the inner surface of the mother tube during working, and many precipitation sites for Cu precipitates are formed on the inner surface of the mother tube. As a result, Cu precipitates tend to be unevenly distributed on the inner surface of the manufactured martensitic stainless steel seamless pipe. Specifically, if other preferable manufacturing conditions are satisfied and the cross-section reduction rate R is set to 40% or more, Fn2 can be set to 1.20 or more in the manufactured martensitic stainless seamless steel pipe. .

Therefore, in the hot working process according to the present embodiment, it is preferable to set the cross-sectional reduction rate R to 40% or more. In the hot working process according to the present embodiment, the upper limit of the cross-sectional reduction rate R is not particularly limited, but is, for example, 80%.

Preferably, in the hot working process according to this embodiment, the working time is 15 minutes or less. Processing time (minutes) means the time from when the material is extracted from the heating furnace to when the final hot working is completed. If the working time is too long, the number of precipitation sites for Cu precipitates on the inner surface of the mother pipe decreases during hot working. As a result, Cu precipitates are less likely to be unevenly distributed on the inner surface of the manufactured martensitic stainless steel seamless pipe. On the other hand, if other preferable manufacturing conditions are satisfied and the processing time is set to 15 minutes or less, Fn2 can be 1.20 or more in the manufactured martensitic stainless seamless steel pipe.

Therefore, in the hot working process according to this embodiment, it is preferable to set the working time to 15 minutes or less. A more preferable upper limit of the processing time is 13 minutes, more preferably 10 minutes. In the hot working process according to this embodiment, the lower limit of the working time is not particularly limited, but is, for example, 5 minutes.

As described above, in the hot working process according to the present embodiment, the material is pierced and rolled, and then hot rolled as necessary. Specifically, the raw material may be pierced-rolled, stretch-rolled, and then shaped-rolled. Moreover, since the hot working process according to the present embodiment is carried out by combining a plurality of hot rollings, it also includes a conveying process between the plurality of hot rollings. Furthermore, if necessary, the blank tube may be heated using a reheating furnace or a heating furnace. In other words, the working time in the hot working process according to the present embodiment means not only the time for a plurality of hot rollings, but also the total time including the time required for transportation, heating, etc. during the time. In short, in the hot working process according to the present embodiment, the total time required for piercing-rolling, stretching-rolling, shaping-rolling, and other transportation, heating, etc. is 15 minutes or less.

[Heat treatment process]
The heat treatment process includes a quenching process and a tempering process. In the heat treatment process, first, the blank tube produced in the hot working process is quenched (quenching process). After quenching, the tube is tempered (tempering step). The quenching process and the tempering process will be described below.

[Quenching process]
In the quenching process, quenching is performed by a well-known method. In the present specification, "quenching" means quenching a blank tube having a point of _A3 or higher. Quenching may be performed immediately after hot working without cooling the mother pipe to room temperature after hot working (direct quenching), or before the temperature of the mother pipe after hot working decreases. Quenching may be performed after the tube is brought to the quenching temperature by charging into a heat treatment furnace or a reheating furnace.

The quenching temperature is above the A _C3 transformation point, eg, 900-1000°C. Here, the quenching temperature means the furnace temperature in the case of using a heat treatment furnace or a reheating furnace, and means the temperature of the outer surface of the mother tube in the case of direct quenching. In the case of using a heat treatment furnace or a reheating furnace, the time for holding the blank tube at the quenching temperature is not particularly limited, but is, for example, 10 to 120 minutes.

The quenching method is not particularly limited, but for example, water cooling. As a method of quenching by water cooling, specifically, the blank pipe may be immersed in a water tank or an oil tank to be rapidly cooled. Alternatively, the blank pipe may be rapidly cooled by shower cooling or mist cooling by pouring or jetting cooling water against the outer surface and/or the inner surface of the blank pipe.

[Tempering process]
In the tempering process, the quenched mother pipe is tempered to adjust the yield strength. As used herein, the term "tempering" means reheating and holding the quenched mother tube at A _c1 point or lower. In the tempering process according to the present embodiment, the tempering temperature is 500° C. to the A _c1 transformation point. In the tempering process according to this embodiment, the tempering time is 10 to 180 minutes. In this specification, the tempering temperature means the furnace temperature (° C.) in the heat treatment furnace. In this specification, the tempering time means the time during which the mother tube is held at the tempering temperature.

Preferably, in the tempering step according to the present embodiment, tempering temperature T (°C) and tempering time t (minutes) are adjusted so as to satisfy the following formula (B).
(T + 273.15) x (20 + log ₁₀ (t/60)) x (1-[Cu]/100) ≤ 17200 (B)
Here, the tempering temperature (°C) is substituted for T in the formula (B), the tempering time (minutes) is substituted for t, and the Cu content (% by mass) of the mother pipe is substituted for [Cu]. be done.

Define FnB=(T+273.15)×(20+log ₁₀ (t/60))×(1−[Cu]/100). FnB is an index showing the yield strength of the manufactured martensitic stainless seamless steel pipe. If FnB is too large, the desired yield strength may not be obtained. On the other hand, if FnB is set to 17200 or less while satisfying other preferable manufacturing conditions, the produced martensitic stainless seamless steel pipe can stably have a yield strength of 862 MPa or more.

Therefore, in the tempering process according to this embodiment, it is preferable to set FnB to 17200 or less. The upper limit of FnB is more preferably 17,100, more preferably 17,000. A preferable upper limit of FnB is 16,700 when stably obtaining a yield strength of 965 MPa or more. Although the lower limit of FnB is not particularly limited, it is 14350, for example.

The martensitic stainless steel seamless steel pipe according to the present embodiment can be manufactured by the above steps. As described above, the martensitic stainless steel seamless steel pipe may be produced by a method other than the production method described above. Further, the martensitic stainless seamless steel pipe thus produced may be post-treated as necessary. The post-treatment is, for example, descaling to remove oxide scale formed on the surface of the steel material. EXAMPLES The present invention will be described in more detail below with reference to examples.

A molten steel having the chemical composition shown in Table 1 was produced. "-" in Table 1 means that the content of the corresponding element was 0% when the numerical values listed in Table 1 were rounded off. Specifically, it means that the W content of Steel No. 3 was 0% by rounding off to the third decimal place. The Nb content of Steel No. 1 was rounded to the third decimal place, meaning that it was 0%. The Mg content, REM content, and B content of Steel No. 1 were rounded to the fifth decimal place, meaning that they were 0%. In addition, Table 1 shows the chemical composition described in Table 1 and the Fn1 obtained from the above definition.

Using the manufactured molten steel of each steel number, steel ingots were manufactured by the ingot casting method. An ingot of each steel number was heated at 1250° C. for 3 hours and hot forged to produce a round billet with a diameter of 200 mm.

After heating the round billet of each test number at 1230°C, hot rolling was performed as hot working to produce a blank pipe (seamless steel pipe). Tables 2 to 4 show the area reduction rate R of hot working (hot rolling) for each test number. In the section reduction rate R column of Tables 2 to 4, "A (Acceptable)" means that the reduction rate R was 40% or more. In Tables 2 to 4, the values listed in the section reduction rate R column mean the section reduction rate R (%). Furthermore, Tables 2 to 4 show the time (working time) from the extraction of the round billet from the heating furnace to the end of the final hot working (hot rolling). In the column of hot working time in Tables 2 to 4, "A (Acceptable)" means that the hot working time was 15 minutes or less. In the hot working time columns of Tables 2 to 4, "NA (Not Acceptable)" means that the hot working time exceeded 15 minutes.

Quenching was performed on the blank pipe of each test number. Quenching was performed by reheating the blank tube in a heat treatment furnace and immersing it in a water bath. For each test number, the quenching temperature (furnace temperature of the heat treatment furnace) was 900° C., and the time for holding the quenching temperature was 15 minutes. After quenching, tempering was performed on the blank tube of each test number. Tempering was performed by reheating the quenched tube in a tempering furnace and holding the tube. For each test number, tempering temperature T (° C.), tempering time t (minutes), tempering temperature T (° C.), tempering time t (minutes), Cu content (% by mass) and the above definitions Tables 2 to 4 show the FnB. Through the above manufacturing process, martensitic stainless seamless steel pipes of each test number were manufactured.

[Evaluation test]
A tensile test, a microstructure observation test, an inner surface observation test, and a pitting corrosion resistance test were carried out on the manufactured seamless steel pipes of each test number.

[Tensile test]
A tensile test was performed on the seamless steel pipes of each test number in accordance with ASTM E8/E8M (2021). Specifically, a round-bar tensile test piece was produced from the thickness central portion of the seamless steel pipe of each test number. The diameter of the parallel portion of the round bar tensile test piece was 8.9 mm, and the gauge length was 35.6 mm. The longitudinal direction of the round bar tensile test piece was parallel to the rolling direction (tube axial direction) of the seamless steel pipe. A tensile test was performed at room temperature (25° C.) in the atmosphere using a round bar tensile test piece of each test number to obtain a 0.2% offset yield strength (MPa). The obtained 0.2% offset yield strength was defined as the yield strength (MPa). The obtained yield strength of each test number is shown in the "YS (MPa)" column of Tables 2 to 4.

[Microstructure Observation Test]
A microstructure volume ratio measurement test was performed on the seamless steel pipes of each test number to obtain the volume ratios of retained austenite and ferrite. For the seamless steel pipes of each test number, the volume fraction (%) of retained austenite was determined by the X-ray diffraction method described above. The volume fraction (%) of retained austenite in the obtained seamless steel pipes of each test number is shown in the "retained γ (%)" column of Tables 2 to 4. Furthermore, for the seamless steel pipes of each test number, the volume fraction (%) of ferrite was obtained by the point counting method based on the above-mentioned ASTM E562 (2019). The obtained volume fraction (%) of ferrite for each test number is shown in the "Ferrite (%)" column of Tables 2 to 4.

[Inner surface observation test]
An inner surface observation test was performed on the seamless steel pipes of each test number, except for test numbers 62 to 64, to obtain the inner surface Cu occupancy OS _Cu , the internal Cu occupancy OI _Cu , and Fn2. . For the seamless steel pipes of each test number, a thin film test piece was prepared by the method described above, and TEM observation was performed. A thin film test piece used for TEM observation was produced by FIB processing. Gallium (Ga) ions were used for FIB processing. In order to protect the inner surface during FIB processing, a carbon protective film was formed on the inner surface. The observation surface size of the thin film test piece was 10 μm×10 μm, and the thickness of the thin film test piece was 150 nm. Furthermore, the observation field of view area of L direction: 1.0 μm, T direction: 1.0 μm was divided into 256 equal parts in the L direction and 256 equal parts in the T direction into 65536 sections, and EDS analysis was performed on each section. . The T-direction position of the inner surface in the observation field region was specified by the above-described method, and the inner surface vicinity region and the inner region were specified. Among the partitions including the identified inner surface vicinity region and the inner region, the number ratio of partitions with a Cu concentration exceeding 2.0% is obtained, and the inner surface Cu occupancy OS _Cu and the internal Cu occupancy OI _Cu are obtained. Defined. Fn2 was determined from the determined inner surface Cu occupancy OS _Cu , internal Cu occupancy OI _Cu , and the above equation (2). Tables 2 to 4 show the obtained inner surface Cu occupancy OS _Cu , internal Cu occupancy OI _Cu , and Fn2. The seamless steel pipes of test numbers 62 to 64 did not contain Cu, so the inner surface observation test was not performed.

[Pitting corrosion resistance test]
A pitting corrosion resistance test was performed on the seamless steel pipes of each test number to determine the pitting potential V'c1000 (mV). Specifically, from the seamless steel pipe of each test number, a test piece for pitting potential measurement, which includes an inner surface area of 1.0 cm ² or more as a test surface and has the same thickness as the wall thickness of the seamless steel pipe, is prepared. made. The test solution was a pre-degassed 25 mass % sodium chloride aqueous solution whose pH was adjusted to 4.5 with 0.08 g/L sodium bicarbonate. In an autoclave, a test piece was immersed in a test solution having a specific liquid volume of 500 mL/cm ² or more to prepare a test bath. After degassing the test bath, a mixed gas of 0.03 atm H ₂ S gas and 10 atm CO ₂ gas was pressurized into the autoclave, and the test bath was stirred.

The test bath was heated to 175°C. In the electrochemical tests, a saturated KCl silver-silver chloride electrode was used as the reference electrode and a platinum electrode was used as the counter electrode. After the immersion potential was stabilized, the anodic polarization curve was measured with a potentiostat at a potential sweep rate of 20 mV/min in the anode direction from the immersion potential. From the obtained anodic polarization curve, the potential when the anodic current density reached 1000 μA/cm ² was determined. Similar measurements were performed three times, and the arithmetic average value of the obtained potentials was taken as the pitting potential V'c1000 (mV). Tables 2 to 4 show the pitting potential V'c1000 (mV) determined for each test number.

[Test results]
With reference to Tables 1 to 4, the seamless steel pipes of test numbers 1 to 11, 13, 14, 16 to 22, 24 to 32, 34 to 39, 42 to 46, 48 to 51, and 54 to 60 are , the chemical composition was appropriate, the Fn1 was 2.50 or more, and the manufacturing method was also the preferred manufacturing method described in the specification. As a result, these seamless steel pipes have a yield strength of 862 MPa or more, and in the microstructure, the volume fraction of retained austenite is 0 to 15.0% and the volume fraction of ferrite is 0 to 5.0%. rice field. These seamless steel pipes also had Fn2 of 1.20 or more. As a result, these seamless steel pipes had a pitting potential V'c1000 of -230 mV or higher in the pitting corrosion resistance test. That is, these seamless steel pipes had high strength of 862 MPa or more and excellent pitting corrosion resistance on the inner surface.

On the other hand, the seamless steel pipes of test numbers 12, 15, 23, 33, 40, 41 and 52 exceeded 17200 in FnB in the manufacturing method. As a result, the yield strength of these seamless steel pipes was less than 862 MPa, and the desired strength was not obtained.

The FnB of the seamless steel pipes of test numbers 47 and 53 exceeded 17200 in the manufacturing method. As a result, these seamless steel pipes had a ferrite volume fraction of more than 5.0% and a yield strength of less than 862 MPa. That is, these seamless steel pipes did not have the desired strength.

The Co content of the seamless steel pipe of test number 61 was too low. As a result, the seamless steel pipe had a yield strength of less than 862 MPa, failing to obtain the desired strength.

The seamless steel pipes of test numbers 62-64 had too low a Cu content. As a result, these seamless steel pipes had a pitting potential V'c1000 of less than -230 mV in the pitting corrosion resistance test. That is, these seamless steel pipes did not have excellent pitting corrosion resistance on the inner surface.

The Fn1 of the seamless steel pipes of test numbers 65 and 66 was too low. As a result, in the pitting corrosion resistance test, the pitting potential V'c1000 was less than -230 mV. That is, this seamless steel pipe did not have excellent pitting corrosion resistance on the inner surface.

The seamless steel pipes of test numbers 67 to 72 had a cross-sectional reduction rate R of less than 40% in the manufacturing method. As a result, these seamless steel pipes had Fn2 of less than 1.20. As a result, in the pitting corrosion resistance test, the pitting potential V'c1000 was less than -230 mV. That is, these seamless steel pipes did not have excellent pitting corrosion resistance on the inner surface.

For the seamless steel pipes of test numbers 73 to 76, the hot working time exceeded 15 minutes in the manufacturing method. As a result, these seamless steel pipes had Fn2 of less than 1.20. As a result, in the pitting corrosion resistance test, the pitting potential V'c1000 was less than -230 mV. That is, these seamless steel pipes did not have excellent pitting corrosion resistance on the inner surface.

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

REFERENCE SIGNS LIST 10 inner surface 20 inner surface vicinity area 30 inner area 40 void area 50 observation visual field area

The gist of the martensitic stainless seamless steel pipe according to the present embodiment can also be described as follows.

[1]
A martensitic stainless steel seamless steel pipe,
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%,
O: 0.050% or less, and
Balance: Fe and impurities, and satisfies formula (1A),
a microstructure, in volume percent, consisting of 0-15.0% retained austenite, 0-5.0% ferrite, and the balance tempered martensite;
Yield strength is 862 MPa or more,
When the pipe axial direction of the martensitic stainless steel seamless steel pipe is defined as the L direction, and the pipe radial direction of the martensitic stainless steel seamless steel pipe is defined as the T direction,
Including the inner surface of the seamless martensitic stainless steel pipe extending in the L direction, the length of the side extending in the L direction is 1.0 μm, and the length of the side extending in the T direction is 1.0 μm. In the square observation field area,
When the observation visual field region is divided into 256 equal sections in the L direction and 256 equal sections in the T direction,
The observation field of view area is
an inner surface vicinity region having a rectangular shape with the inner surface of the martensitic stainless steel seamless steel pipe as the upper end, 256 sections in the L direction, and 6 sections in the T direction;
the inner surface vicinity region and an inner region adjacent below the inner surface vicinity region;
comprising the inner surface vicinity region and a void region adjacent above the inner surface vicinity region,
Among all the sections in the inner surface vicinity region, the number ratio of sections with a Cu concentration exceeding 2.0% is defined as the inner surface Cu occupancy OS _Cu ,
When the number ratio of sections with a Cu concentration exceeding 2.0% among all the sections in the internal region is defined as the internal Cu occupancy OI _Cu ,
The inner surface Cu occupancy OS _Cu and the internal Cu occupancy OI _Cu satisfy formula (2),
Martensitic stainless seamless steel pipe.
Mo≧2.50 (1A)
OS _Cu /OI _Cu ≧1.20 (2)
Here, the content of the corresponding element is substituted for the symbol of the element in the formula (1A) in terms of % by mass.

[2]
A martensitic stainless steel seamless steel pipe,
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%, and
O: 0.050% or less, and further,
W: 2.00% or less,
Nb: 0.50% or less,
Mg: 0.0050% or less,
Rare earth element: 0.0050% or less, and
B: contains one or more elements selected from the group consisting of 0.0050% or less,
balance: Fe and impurities, and satisfies formula (1B),
a microstructure, in volume percent, consisting of 0-15.0% retained austenite, 0-5.0% ferrite, and the balance tempered martensite;
Yield strength is 862 MPa or more,
When the pipe axial direction of the martensitic stainless steel seamless steel pipe is defined as the L direction, and the pipe radial direction of the martensitic stainless steel seamless steel pipe is defined as the T direction,
Including the inner surface of the seamless martensitic stainless steel pipe extending in the L direction, the length of the side extending in the L direction is 1.0 μm, and the length of the side extending in the T direction is 1.0 μm. In the square observation field area,
When the observation visual field region is divided into 256 equal sections in the L direction and 256 equal sections in the T direction,
The observation field of view area is
an inner surface vicinity region having a rectangular shape with the inner surface of the martensitic stainless steel seamless steel pipe as the upper end, 256 sections in the L direction, and 6 sections in the T direction;
the inner surface vicinity region and an inner region adjacent below the inner surface vicinity region;
comprising the inner surface vicinity region and a void region adjacent above the inner surface vicinity region,
Among all the sections in the inner surface vicinity region, the number ratio of sections with a Cu concentration exceeding 2.0% is defined as the inner surface Cu occupancy OS _Cu ,
When the number ratio of sections with a Cu concentration exceeding 2.0% among all the sections in the internal region is defined as the internal Cu occupancy OI _Cu ,
The inner surface Cu occupancy OS _Cu and the internal Cu occupancy OI _Cu satisfy formula (2),
Martensitic stainless seamless steel pipe.
Mo+0.5×W≧2.50 (1B)
OS _Cu /OI _Cu ≧1.20 (2)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1B) in mass %.

[3]
A method for producing a martensitic stainless seamless steel pipe according to [1],
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%,
O: 0.050% or less, and
Balance: a material preparation step of preparing a material consisting of Fe and impurities and satisfying formula (1A);
After heating the prepared material in a heating furnace, hot working in which the cross-sectional reduction rate R defined by formula (A) is 40% or more and the hot working time is 15 minutes or less. A hot working step of manufacturing a mother pipe by carrying out
A quenching step of performing quenching on the raw pipe of ₃ or more points;
a tempering step of performing tempering on the quenched base pipe under conditions that satisfy formula (B);
A method for producing a martensitic stainless seamless steel pipe.
Mo≧2.50 (1A)
R = {1-(cross-sectional area perpendicular to the axial direction of the tube after hot working/cross-sectional area perpendicular to the axial direction of the raw material before hot working)} x 100 (A)
(T + 273.15) x (20 + log ₁₀ (t/60)) x (1-[Cu]/100) ≤ 17200 (B)
Here, the content of the corresponding element is substituted for the symbol of the element in the formula (1A) in terms of % by mass.
In the formula (B), T is the tempering temperature in degrees Celsius, t is the tempering time in minutes, and [Cu] is the Cu content in the blank in mass %.

[4]
A method for producing a martensitic stainless seamless steel pipe according to [2],
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%, and
O: 0.050% or less, and further,
W: 2.00% or less,
Nb: 0.50% or less,
Mg: 0.0050% or less,
Rare earth element: 0.0050% or less, and
B: contains one or more elements selected from the group consisting of 0.0050% or less,
Balance: a material preparation step of preparing a material consisting of Fe and impurities and satisfying formula (1B);
After heating the prepared material in a heating furnace, hot working in which the cross-sectional reduction rate R defined by formula (A) is 40% or more and the hot working time is 15 minutes or less. A hot working step of manufacturing a mother pipe by carrying out
A quenching step of performing quenching on the raw pipe of ₃ or more points;
a tempering step of performing tempering on the quenched base pipe under conditions that satisfy formula (B);
A method for producing a martensitic stainless seamless steel pipe.
Mo+0.5×W≧2.50 (1B)
R = {1-(cross-sectional area perpendicular to the axial direction of the tube after hot working/cross-sectional area perpendicular to the axial direction of the raw material before hot working)} x 100 (A)
(T + 273.15) x (20 + log ₁₀ (t/60)) x (1-[Cu]/100) ≤ 17200 (B)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1B) in mass %.
In the formula (B), T is the tempering temperature in degrees Celsius, t is the tempering time in minutes, and [Cu] is the Cu content in the blank in mass %.

Claims (4)

A martensitic stainless steel seamless steel pipe,
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%,
O: 0.050% or less,
W: 0 to 2.00%,
Nb: 0 to 0.50%,
Mg: 0-0.0050%,
Rare earth element: 0 to 0.0050%,
B: 0 to 0.0050%, and
Balance: Fe and impurities, and satisfies formula (1),
a microstructure, in volume percent, consisting of 0-15.0% retained austenite, 0-5.0% ferrite, and the balance tempered martensite;
Yield strength is 862 MPa or more,
When the pipe axial direction of the martensitic stainless steel seamless steel pipe is defined as the L direction, and the pipe radial direction of the martensitic stainless steel seamless steel pipe is defined as the T direction,
Including the inner surface of the seamless martensitic stainless steel pipe extending in the L direction, the length of the side extending in the L direction is 1.0 μm, and the length of the side extending in the T direction is 1.0 μm. In the square observation field area,
When the observation visual field region is divided into 256 equal sections in the L direction and 256 equal sections in the T direction,
The observation field of view area is
an inner surface vicinity region having a rectangular shape with the inner surface of the martensitic stainless steel seamless steel pipe as the upper end, 256 sections in the L direction, and 6 sections in the T direction;
the inner surface vicinity region and an inner region adjacent below the inner surface vicinity region;
comprising the inner surface vicinity region and a void region adjacent above the inner surface vicinity region,
Among all the sections in the inner surface vicinity region, the number ratio of sections with a Cu concentration exceeding 2.0% is defined as the inner surface Cu occupancy OS _Cu ,
When the number ratio of sections with a Cu concentration exceeding 2.0% among all the sections in the internal region is defined as the internal Cu occupancy OI _Cu ,
The inner surface Cu occupancy OS _Cu and the internal Cu occupancy OI _Cu satisfy formula (2),
Martensitic stainless seamless steel pipe.
Mo+0.5×W≧2.50 (1)
OS _Cu /OI _Cu ≧1.20 (2)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1) in terms of % by mass. The martensitic stainless seamless steel pipe according to claim 1,
W: 0.01 to 2.00%,
Nb: 0.01 to 0.50%,
Mg: 0.0001-0.0050%,
Rare earth element: 0.0001 to 0.0050%, and
B: containing one or more elements selected from the group consisting of 0.0001 to 0.0050%,
Martensitic stainless seamless steel pipe. A method for producing a martensitic stainless steel seamless steel pipe according to claim 1 or 2,
in % by mass,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 11.00 to 14.00%,
Ni: 5.00 to 7.50%,
Mo: 1.50-4.50%,
Cu: 0.50-3.50%,
Co: 0.010 to 0.500%,
Ti: 0.050 to 0.300%,
V: 0.01 to 1.00%,
Ca: 0.0005 to 0.0050%,
Al: 0.001 to 0.100%,
N: 0.0010 to 0.0500%,
O: 0.050% or less,
W: 0 to 2.00%,
Nb: 0 to 0.50%,
Mg: 0-0.0050%,
Rare earth element: 0 to 0.0050%,
B: 0 to 0.0050%, and
Balance: a material preparation step of preparing a material consisting of Fe and impurities and satisfying formula (1);
After heating the prepared material in a heating furnace, hot working in which the cross-sectional reduction rate R defined by formula (A) is 40% or more and the hot working time is 15 minutes or less. A hot working step of manufacturing a mother pipe by carrying out
A quenching step of performing quenching on the raw pipe of ₃ or more points;
a tempering step of performing tempering on the quenched base pipe under conditions that satisfy formula (B);
A method for producing a martensitic stainless seamless steel pipe.
Mo+0.5×W≧2.50 (1)
R = {1-(cross-sectional area perpendicular to the axial direction of the tube after hot working/cross-sectional area perpendicular to the axial direction of the raw material before hot working)} x 100 (A)
(T + 273.15) x (20 + log ₁₀ (t/60)) x (1-[Cu]/100) ≤ 17200 (B)
Here, the content of the corresponding element is substituted for the symbol of the element in formula (1) in terms of % by mass.
In the formula (B), T is the tempering temperature in degrees Celsius, t is the tempering time in minutes, and [Cu] is the Cu content in the blank in mass %. A method for producing a martensitic stainless steel seamless steel pipe according to claim 3,
The material is
W: 0.01 to 2.00%,
Nb: 0.01 to 0.50%,
Mg: 0.0001-0.0050%,
Rare earth element: 0.0001 to 0.0050%, and
B: containing one or more elements selected from the group consisting of 0.0001 to 0.0050%,
A method for producing a martensitic stainless seamless steel pipe.

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