JP6760530B1 - Seamless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a seamless steel pipe having excellent total corrosion resistance in a sour environment containing an H2S partial pressure of more than 0.03 to 0.1 bar. SOLUTION: The seamless steel pipe according to the present disclosure has a chemical composition of mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.01 to 1.00%, P: 0. .030% or less, S: 0.0050% or less, Cr: more than 16.00 to 18.00%, Mo: 1.00 to 3.00%, Cu: 0.50 to 3.50%, Ni: 3 .00 to 5.50%, Co: 0.010 to 0.500%, Al: 0.001 to 0.100%, O: 0.050% or less, N: 0.070% or less, Ca: 0. It is composed of 0001 to 0.0050%, V: 0.01 to 1.00%, and the balance: Fe and impurities, satisfies the formula (1), and has a deactivated pH of 3.00 or less. Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co ≧ 23.0 (1) [Selection diagram] Fig. 2

Description

The present disclosure relates to seamless steel pipes, and more particularly to seamless steel pipes having a microstructure containing ferrite and martensite.

Some oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") have an environment containing a large amount of corrosive substances. The corrosive substance is, for example, a corrosive gas such as hydrogen sulfide and carbon dioxide. In the present specification, an environment containing hydrogen sulfide and carbon dioxide gas is referred to as a “sour environment”. The temperature of the sour environment is about room temperature to 200 ° C., although it depends on the depth of the well. As used herein, the term "normal temperature" means 24 ± 3 ° C.

It is known that chromium (Cr) is effective for improving the carbon dioxide corrosion resistance of steel. Therefore, in an oil well in an environment containing a large amount of carbon dioxide gas, depending on the partial pressure and temperature of carbon dioxide gas, API L80 13Cr steel material (normal 13Cr steel material), super 13Cr steel material having a reduced C content, and the like are represented. A martensitic stainless seamless steel pipe containing about 13% by mass of Cr is used. 13Cr steel or super 13Cr steel is mainly, _{H 2} S partial pressure is used in oil wells following mild sour environment 0.03Bar.

Meanwhile, among the sour environment, a high 0.1bar following environment than _{H 2} S partial pressure 0.03bar that enhanced mild sour environments. Enhanced The mild sour environment, due to the high H _{2 S} partial pressure than mild sour environments, 13Cr steel and the Cr content is higher than Super 13Cr steel duplex stainless seamless steel tube has been applied. However, duplex stainless seamless steel pipes are more expensive than 13Cr steel and super 13Cr steel. Therefore, there is a demand for a seamless steel pipe that can be used in an enhanced mild sour environment even if the Cr content is lower than that of a duplex stainless steel seamless steel pipe.

Therefore, as a steel material that can be used in an enhanced mild sour environment and has a lower Cr content than a duplex stainless seamless steel pipe, a seamless steel pipe containing about 10 to 18% of Cr content (hereinafter, also referred to as 17Cr steel material). Has been proposed.

The 17Cr steel material is disclosed in, for example, Japanese Patent Application Laid-Open No. 2015-110822 (Patent Document 1), Japanese Patent Application Laid-Open No. 2012-149317 (Patent Document 2), and International Publication No. 2017/1688874 (Patent Document 3). ..

The seamless steel pipe disclosed in Patent Document 1 has a mass% of C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less. , S: 0.005% or less, Cr: 15.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 1.5 to 5.0%, Cu: 4.0% or less, W: 0.1 to 2.5%, N: 0.15% or less, C, Si, Mn, Cr, Ni, Mo, Cu, N use the following formula (1), and Cu, Mo, W The following formula (2) is further adjusted and contained in Cu, Mo, W, Cr, and Ni so as to satisfy each of the following formula (3), and has a composition consisting of the balance Fe and unavoidable impurities. Here, the equations (1) to (3) are as follows. Formula (1): -5.9 × (7.82 + 27C-0.91Si + 0.21Mn-0.9Cr + Ni-1.1Mo + 0.2Cu + 11N) ≧ 13.0, formula (2): Cu + Mo + 0.5W ≧ 5.8, formula (3): Cu + Mo + W + Cr + 2Ni ≤ 34.5. Sufficiently generate ferrite phase having excellent pitting resistance by satisfying the equation (1), increase the SSC resistance at high environment of the concentration of H _{2 S} by satisfying the expression (2), satisfies the equation (3) It is described in Patent Document 1 that the formation of retained austenite can be suppressed and the SSC resistance can be further enhanced.

The seamless steel pipe disclosed in Patent Document 2 has a mass% of C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.03% or less. , S: 0.005% or less, Cr: more than 15.5% and 17.5% or less, Ni: 2.5 to 5.5%, Mo: 1.8 to 3.5%, Cu: 0.3 to It contains 3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, has a composition consisting of the balance Fe and unavoidable impurities, and yield strength: 655 to It has a strength of 862 MPa and a yield ratio of 0.90 or more. By setting the C content to 0.01% or less, the Cr content to more than 15.5% to 17.5%, and appropriately containing Ni, Mo, Cu, and V, carbon dioxide corrosion resistance and SSC resistance It is described in Patent Document 2 that

The seamless steel pipe disclosed in Patent Document 3 has a mass% of C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.25 to 1.80%, P. : 0.030% or less, S: 0.005% or less, Cr: 12.0 to 17.0%, Ni: 4.0 to 7.0%, Mo: 0.5 to 3.0%, Al: 0.005 to 0.10%, V: 0.005 to 0.20%, Co: 0.01 to 1.0%, N: 0.005 to 0.15%, O: 0.010% or less It contains and satisfies the following formulas (1) and (2), and has a composition composed of the balance Fe and unavoidable impurities. Here, the equations (1) and (2) are as follows. Formula (1): Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 15.0, Formula (2): Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N ≦ 11. It is described in Patent Document 3 that by satisfying the formula (1), the carbon dioxide gas corrosiveness in a high temperature corrosive environment is enhanced, and by satisfying the formula (2), the hot workability can be enhanced. There is.

Japanese Unexamined Patent Publication No. 2015-110822 Japanese Unexamined Patent Publication No. 2012-149317 International Publication No. 2017/1687874

Meanwhile, 0.03 With enhanced mild sour environment containing _{H 2} S partial pressure of the ultrasonic ～0.1Bar, active dissolution is promoted, corrosion is likely to occur. Further, since the inner surface of the seamless steel pipe comes into direct contact with the production fluid, total corrosion is particularly likely to occur. Thus, 0.03 on the inner surface of the seamless steel pipe used for enhanced mild sour environment containing H _{2 S} partial pressure of the ultrasonic ～0.1Bar, obtained excellent general corrosion resistance. However, Patent Documents 1 to 3, 0.03 Study general corrosion resistance of the inner surface of the seamless steel pipe in enhanced mildness sour environment containing H _{2 S} partial pressure of the ultrasonic ~0.1bar is not performed.

The purpose of the present disclosure, be enhanced mild sour environment containing H _{2 S} partial pressure of 0.03 super ～0.1Bar, it is to provide a seamless steel pipe having excellent general corrosion resistance.

The seamless steel pipe according to this disclosure is
The chemical composition is mass%,
C: 0.050% or less,
Si: 1.00% or less,
Mn: 0.01 to 1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Cr: Over 16.00 to 18.00%,
Mo: 1.00 to 3.00%,
Cu: 0.50-3.50%,
Ni: 3.00 to 5.50%,
Co: 0.010 to 0.500%,
Al: 0.001 to 0.100%,
O: 0.050% or less,
N: 0.070% or less,
Ca: 0.0001 to 0.0050%,
V: 0.01 to 1.00%,
W: 0.01 to 2.00%,
B: 0 to 0.0020%,
Ti: 0-0.010%,
Nb: 0 to 0.300%,
Sn: 0 to 0.300%, and
Remaining: Consisting of Fe and impurities, satisfying formula (1),
In the microstructure of the seamless steel pipe, the volume fraction of ferrite is less than 10 to 50%, the volume fraction of martensite is 50% or more, and the volume fraction of retained austenite is 10% or less.
In the seamless steel pipe, the depassivation pH of the aqueous solution containing 5% by mass of NaCl and 0.41 g / L of CH ₃ COONa and further containing CH ₃ COOH is 3.00 or less.
Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co ≧ 23.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

Seamless steel pipe according to the present disclosure, it is enhanced mild sour environment containing H _{2 S} partial pressure of 0.03 super ～0.1Bar, has excellent general corrosion resistance.

FIG. 1 is a diagram showing the depassivation pH for each test number for steel number A in the examples. FIG. 2 is a diagram showing the relationship between F1 (= Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co) and the depassivated pH. FIG. 3 is a diagram showing an example of the relationship between the natural potential and the pH of the specific test solution. In FIG. 4, in the examples, the content of each element in the chemical composition is within the range of the present embodiment, the formula (1) is satisfied, and the depassivation pH (pHd) is 3.00 or less. It is a figure which shows the relationship between F2 (= (3Ti + B) / N) and absorption energy vE- ₁₀ (J) at −10 ° C. in a steel material. In FIG. 5, the blasting step and the first and second acidic steps are carried out on a steel material in which the content of each element in the chemical composition is within the range of the present embodiment and satisfies the formula (1). The relationship between F1 and depassivation pH (marked with "○" in the figure), and F1 and depassivation when at least one of the blasting step and the first and second acidic steps was not carried out. It is a figure which shows the relationship with the chemical pH (marked with "●" in the figure).

Seam inventors, having 0.03 even enhanced mild sour environment containing H _{2 S} partial pressure of the ultrasonic ～0.1Bar, general corrosion resistance superior to inhibit the activity dissolved in the inner surface A study was conducted on steel-free pipes. First, the chemical composition of seamless steel pipes suitable for oil well applications was examined. As a result, the chemical composition is mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050. % Or less, Cr: more than 16.00 to 18.00%, Mo: 1.00 to 3.00%, Ni: 3.00 to 5.50%, Al: 0.001 to 0.100%, O: 0.050% or less, N: 0.070% or less, Ca: 0.0001 to 0.0050%, V: 0.01 to 1.00%, B: 0 to 0.0020%, Ti: 0 to 0 If it is a seamless steel pipe consisting of 0010%, Nb: 0 to 0.300%, Sn: 0 to 0.300%, and the balance: Fe and impurities, and has a microstructure mainly composed of ferrite and martensite. , I thought it was suitable for oil well applications.

Accordingly, the present inventors have found that in a seamless steel pipe having a chemical composition described above, in enhanced mild sour environment containing H _{2 S} partial pressure of 0.03 super ～0.1Bar,耐全surface excellent in inner surface A method for obtaining corrosiveness was investigated. As a result, the present inventors further contained 0.50 to 3.50% of Cu, 0.010 to 0.500% of Co, and 0.01 to 2.00 W in the above chemical composition. if content% 0.03 even enhanced mild sour environment containing H _{2 S} partial pressure of the ultrasonic ～0.1Bar, to inhibit the activity dissolved in the inner surface of the seamless steel pipe, it increases the general corrosion resistance I found that there is a possibility. That is, the chemical composition is mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050%. Hereinafter, Cr: more than 16.00 to 18.00%, Mo: 1.00 to 3.00%, Cu: 0.50 to 3.50%, Ni: 3.00 to 5.50%, Co: 0. .010 to 0.500%, Al: 0.001 to 0.100%, O: 0.050% or less, N: 0.070% or less, Ca: 0.0001 to 0.0050%, V: 0. 01 to 1.00%, W: 0.01 to 2.00%, B: 0 to 0.0020%, Ti: 0 to 0.010%, Nb: 0 to 0.300%, Sn: 0 to 0 .300%, and the balance: if seamless steel pipe made of Fe and impurities, even enhanced mild sour environment containing _{H 2} S partial pressure of 0.03 super ～0.1Bar, excellent耐全surface It has been found that corrosiveness may be obtained.

Enhanced mild sour by containing 0.50 to 3.50% of Cu, 0.010 to 0.500% of Co, and 0.01 to 2.00% of W in the steel material having the above chemical composition. The reason why active dissolution is suppressed in the environment and total corrosion resistance is increased has not been clarified in detail. However, the following reasons can be considered. In the above-mentioned enhanced mild sour environment, a passivation film is formed on the surface of the seamless steel pipe having the above-mentioned chemical composition. Under such circumstances, if the seamless steel pipe contains 0.50% or more of Cu, Cu in the steel material reacts with H ₂ S in the enhanced mild sour environment, and Cu sulfurization occurs on the passive film. Things are generated. Similarly, if the seamless steel pipe contains 0.010% or more of Co, Co sulfide is formed on the passive film. If the seamless steel pipe contains 0.01% or more of W, W sulfide is formed on the passive film.

With enhanced mild sour environment, _{H 2} S partial pressure of 0.03 ultra ~0.1bar high. Therefore, the passivation film is easily destroyed by hydrogen sulfide ions (HS ⁻ ) and chloride ions (Cl ⁻ ) in the enhanced mild sour environment. However, in the case of the steel material of the present embodiment, Cu sulfide, Co sulfide, and W sulfide are formed on the passivation film together with Mo sulfide and Ni sulfide. Therefore, it is possible to prevent hydrogen sulfide ions and chloride ions from coming into direct contact with the passive film. Therefore, the destruction of the passivation film by hydrogen sulfide ions and chloride ions is suppressed. As a result, it is considered that it is possible to obtain an excellent general corrosion resistance in enhanced mild sour environment containing H _{2 S} partial pressure of 0.03 super ～0.1Bar.

There is a possibility that the total corrosion resistance is increased by a mechanism different from the above mechanism. However, with the above-mentioned chemical composition, Cu is contained in an amount of 0.50 to 3.50%, Co is contained in an amount of 0.010 to 0.500%, and W is contained in an amount of 0.01 to 2.00%. It has been proved in the examples described later that the total corrosion resistance of the seamless steel pipe of the embodiment is increased.

However, as a result of further investigation, it has been found that even a seamless steel pipe having the above chemical composition may not have excellent total corrosion resistance. Therefore, the present inventors have investigated and examined means for further enhancing the total corrosion resistance of the seamless steel pipe having the above-mentioned chemical composition.

First, the present inventors focused on depassivation pH as an index of total corrosion resistance of seamless steel pipes. In the present specification, the depassivation pH means the lowest pH at which a steel material can maintain a passivation state under a specific environment. The passivation state means a state in which a passivation film is formed on the entire surface of the steel material in an arbitrary region and corrosion can be suppressed over the entire surface. That is, in a pH environment lower than the deactivated pH, the passive film is partially or completely destroyed on the surface of the steel material, active dissolution proceeds, and the steel material is totally corroded. Therefore, the lower the depassivation pH, the more the passivation film is maintained even in a low pH sour environment, and the higher the total corrosion resistance. In addition, in this specification, the depassivation pH is also referred to as "pHd".

Next, the present inventors examined in detail the elements that reduce the depassivation pH of the seamless steel pipe. Among the elements in the above-mentioned chemical composition, Cr, Mo, Ni, Cu, W and Co are examples of elements that stabilize the passive film. Cr forms a passivation film. On the other hand, as described above, Mo, Ni, Cu, W and Co generate sulfides and suppress the destruction of the passive film.

Therefore, the present inventors have described in detail the relationship between the contents of Cr, Mo, Ni, Cu, W and Co and the depassivated pH in a seamless steel pipe having the above-mentioned chemical composition and microstructure. Investigated and examined. As a result, the seamless steel pipe having the above chemical composition and microstructure by satisfying the following equation (1), in enhanced mild sour environment containing H _{2 S} partial pressure of 0.03 ultra ~0.1bar , It was found that the total corrosion resistance can be further stably increased.
Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co ≧ 23.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

It is defined as F1 = Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co. F1 is an index related to the stability of the passivation film. The higher the F1, the more stable the passivation film. If F1 is less than 23.0, the passivation film becomes unstable and the depassivation pH exceeds 3.00. As a result, the total corrosion resistance of the seamless steel pipe is reduced. Therefore, the seamless steel pipe according to the present embodiment has the above-mentioned chemical composition and microstructure, and has an F1 of 23.0 or more.

However, even in a seamless steel pipe having the above-mentioned chemical composition and microstructure and satisfying the formula (1), excellent total corrosion resistance may not be obtained on the inner surface. Therefore, the present inventors have described in detail the relationship between the state near the inner surface of the seamless steel pipe having the above-mentioned chemical composition and microstructure and satisfying the formula (1) and the depassivation pH. Investigated and examined. As a result, the following matters were found.

On the inner surface of the seamless steel pipe, there may be a region where the Cr and Mo concentrations are lower than those of other regions. In particular, in a seamless steel pipe on which a scale is formed, a region in which the concentrations of Cr and Mo are reduced (hereinafter, also referred to as “deelemental layer”) may be formed in a region adjacent to the scale. Cr and Mo stabilize the passivation film of the steel material as described above. Therefore, when a deelement layer is formed on the inner surface of the seamless steel pipe, the passive film of the steel material may become unstable and the total corrosion resistance of the steel material may decrease.

Here, in the manufacturing process of seamless steel pipes for oil well applications in mild sour environment and enhanced mild sour environment, blasting treatment represented by shot blasting is carried out in the final process for the purpose of removing scale on the inner surface of the seamless steel pipe. May be done. Here, the blasting process is a process of mechanically grinding the surface of a steel material. It is possible to remove the scale when the surface layer is blasted on the scale-coated steel material. However, it was newly found that it may not be possible to sufficiently remove even the deelemental layer on the surface layer. That is, in the seamless steel pipe after the blast treatment, the deelement layer may remain on the surface layer of the inner surface.

As described above, Cr and Mo stabilize the passivation film of the steel material. That is, even if the blast treatment is performed, if the deelement layer remains on a part of the surface layer of the inner surface of the seamless steel pipe, the passivation film becomes unstable in the region where the deelement layer remains. As a result, even a seamless steel pipe having a chemical composition satisfying the formula (1) and the above-mentioned microstructure may not have sufficient total corrosion resistance on the inner surface.

Therefore, the present inventors may be able to improve the total corrosion resistance of the inner surface of the seamless steel pipe by removing the scale and the deelement layer from the surface layer by performing a pickling treatment instead of the blast treatment. I wondered if there was one. Specifically, the present inventors considered to carry out the two-step pickling treatment (first pickling step, second pickling step) described in the preferred production method described later. In the first pickling step, the steel material is immersed in an acidic aqueous solution for a long time. As a result, the surface layer of the entire steel material is sufficiently melted. That is, it is considered that the scale and the deelement layer can be removed from the surface layer of the steel material in the first pickling step. Further, in the second pickling step, the surface layer of the steel material is activated. As a result, it is considered that a strong passivation film can be formed on the surface layer of the steel material.

As described above, according to the two-step pickling step (first pickling step, second pickling step), the scale and the deelementation layer are removed from the surface layer of the steel material, and further, the surface layer of the steel material is strong. It can be expected that a passivation film will be formed. In this case, the depassivation pH on the inner surface of the seamless steel pipe should decrease, and the total corrosion resistance on the inner surface of the seamless steel pipe should increase. Based on the above examination results, the relationship between the presence or absence of blasting and pickling treatment and the depassivation pH of a steel material having the above-mentioned chemical composition and microstructure and satisfying the formula (1) is determined. investigated. The results are shown in Table 1.

Table 1 shows an excerpt of the presence or absence of the blasting treatment and the pickling treatment and the depassivation pH of the steel number A among the examples described later. Steel No. A shown in Table 1 had the content of each element in the chemical composition within the above range and satisfied the formula (1). All of the steel materials shown in Table 1 had a microstructure mainly of ferrite and martensite.

“Implementation” in the “blast treatment” column and the “pickling treatment” column in Table 1 means that the treatment was carried out by the method described in the preferred production method described later. The "-" in the "blast treatment" column and the "pickling treatment" column in Table 1 means that the treatment was not performed. In the "Remarks" column of Table 1, the implementation status of the blast treatment and the pickling treatment is summarized. In the "pHd" column of Table 1, the depassivated pH obtained by the method described later is shown.

Further, the results of Table 1 are shown in FIG. FIG. 1 is a diagram showing the depassivation pH of steel number A for each test number.

With reference to FIG. 1, the steel material that was only blasted (test number 23) and the steel material that was only pickled (test number 24) were not blasted or pickled (test number 24). Compared with Test No. 22), the deactivated pH was lowered. However, the steel material subjected to the blasting treatment and the pickling treatment (test number 1) is removed as compared with the steel material subjected to the blasting treatment only (test number 23) and the pickling treatment only (test number 24). The passivation pH was significantly reduced.

As is clear from the above results, if each element content of the chemical composition is within the above range and the seamless steel pipe satisfying the formula (1) is subjected to the blasting treatment and the pickling treatment, it is removed. The present inventors have newly found that the passivation pH is significantly reduced.

As described above, it is considered that both the scale and the deelemental layer can be removed by performing the pickling treatment. However, the steel material (test number 24) subjected to only the pickling treatment did not have excellent overall corrosion resistance as compared with the steel material (test number 1) subjected to the blasting treatment and the pickling treatment. The details of the reason for this have not been clarified. However, the present inventors think as follows. In the pickling treatment according to the present embodiment, it is considered that the surface layer of the steel material dissolves substantially uniformly as a whole. That is, when the surface layer of the steel material is microscopically roughened by scale formation, the surface layer of the steel material subjected to only the pickling treatment may maintain the microscopically rough state. On the other hand, it is considered that the surface of the blasted steel material is microscopically smoother than that of the steel material subjected to only the pickling treatment.

If the surface of the steel is rough, local corrosion can occur. At the location where corrosion occurs, the pH drops locally and the passivation film is destroyed. In this case, active dissolution proceeds locally, and the passivation state is released (that is, the passivation state is achieved). In short, when the surface of the steel material is microscopically rough, it is depassivated even at a high pH as compared with the case where it is microscopically smooth. Therefore, it is considered that the deactivated pH increases as a result of the surface of the steel material becoming microscopically rough. In this way, although the deelement layer was removed from the steel material that had only been pickled, the surface was rough, so the depassivation pH was high, and there is a possibility that the total corrosion resistance was reduced. , The inventors are thinking.

As described above, the total corrosion resistance of the steel material is affected not only by the stability of the passive film determined by the chemical composition of the steel material, but also by the presence or absence of the deelement layer formed on the surface layer of the steel material and the shape of the surface. Receive. Specifically, as described above, it is considered that the inner surface of the seamless steel pipe is microscopically smoothed by performing the blast treatment. Further, it is considered that if the pickling treatment is carried out, the deelement layer is removed and a strong passivation film is formed on the entire surface of the seamless steel pipe. However, with current technology, it is extremely difficult to identify and measure each of these complex factors. However, with reference to Table 1 and FIG. 1, the steel material that was only blasted (test number 23) and the steel material that was only pickled (test number 24) were blasted and pickled. The numerical value of the deactivated pH is significantly different from that of the steel material (test number 1).

Therefore, in the present embodiment, when a specific test solution (an aqueous solution containing 5% by mass NaCl and 0.41 g / L CH ₃ COONa and further containing CH ₃ COOH) is used in the seamless steel pipe. The seamless steel pipe of this embodiment is defined by defining the deactivated pH. A seamless steel pipe having the above-mentioned chemical composition and microstructure, satisfying the formula (1), and having a depassivation pH of 3.00 or less when a specific test solution is used is excellent. It has been proved from the examples described later that the seamless steel pipe showing the total corrosion resistance and not satisfying these requirements has the low total corrosion resistance.

The seamless steel pipe according to the present embodiment completed based on the above findings has the following configuration.

[1]
It is a seamless steel pipe
The chemical composition is mass%,
C: 0.050% or less,
Si: 1.00% or less,
Mn: 0.01 to 1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Cr: Over 16.00 to 18.00%,
Mo: 1.00 to 3.00%,
Cu: 0.50-3.50%,
Ni: 3.00 to 5.50%,
Co: 0.010 to 0.500%,
Al: 0.001 to 0.100%,
O: 0.050% or less,
N: 0.070% or less,
Ca: 0.0001 to 0.0050%,
V: 0.01 to 1.00%,
W: 0.01 to 2.00%,
B: 0 to 0.0020%,
Ti: 0-0.010%,
Nb: 0 to 0.300%,
Sn: 0 to 0.300%, and
Remaining: Consisting of Fe and impurities, satisfying formula (1),
In the microstructure of the seamless steel pipe, the volume fraction of ferrite is less than 10 to 50%, the volume fraction of martensite is 50% or more, and the volume fraction of retained austenite is 10% or less.
In the seamless steel pipe, the depassivation pH of an aqueous solution containing 5% by mass of NaCl and 0.41 g / L of CH ₃ COONa and further containing CH ₃ COOH is 3.00 or less.
Seamless steel pipe.
Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co ≧ 23.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

[2]
The seamless steel pipe according to [1].
The chemical composition further satisfies the formula (2).
Seamless steel pipe.
(3Ti + B) /N≤0.210 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2). If the corresponding element is not contained, "0" is substituted for the corresponding element symbol.

[3]
The seamless steel pipe according to [1] or [2].
The chemical composition is
B: Contains 0.0001 to 0.0020%,
Seamless steel pipe.

[4]
The seamless steel pipe according to any one of [1] to [3].
The chemical composition is
Ti: 0.001 to 0.010% and
Nb: contains one or more selected from the group consisting of 0.001 to 0.300%.
Seamless steel pipe.

[5]
The seamless steel pipe according to any one of [1] to [4].
The chemical composition is
Sn: contains 0.001 to 0.300%,
Seamless steel pipe.

The seamless steel pipe of the present embodiment may be a seamless steel pipe for oil wells. In the present specification, "seamless steel pipe for oil wells" means a general term for casings, tubing, and drill pipes used for drilling oil wells or gas wells and collecting crude oil or natural gas.

Hereinafter, the seamless steel pipe of the present embodiment will be described in detail. Unless otherwise specified, "%" for an element means mass%.

[Chemical composition]
The chemical composition of the seamless steel pipe of the present embodiment contains the following elements.

C: 0.050% or less Carbon (C) is inevitably contained. That is, the C content is more than 0%. C enhances the hardenability of the steel material and enhances the strength of the steel material. However, if the C content exceeds 0.050%, C is likely to combine with Cr to form Cr carbides. As a result, a Cr-free layer is likely to be formed on the surface layer of the steel material. In this case, even if the content of other elements is within the range of this embodiment, the total corrosion resistance of the steel material is lowered. Therefore, the C content is 0.050% or less. The lower limit of the C content is preferably 0.001%, more preferably 0.004%, still more preferably 0.006%. The preferred upper limit of the C content is 0.045%, more preferably 0.040%, still more preferably 0.035%, still more preferably 0.030%.

Si: 1.00% or less Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si deoxidizes steel. However, if the Si content exceeds 1.00%, the deoxidizing effect is saturated and the hot workability of the steel material is lowered even if the other element content 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%. The preferred upper limit of the Si content is 0.70%, more preferably 0.50%, still more preferably 0.40%.

Mn: 0.01 to 1.00%
Manganese (Mn) enhances the hardenability of the steel material and enhances the strength of the steel material. If the Mn content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 1.00%, even if the content of other elements is within the range of the present embodiment, Mn produces coarse inclusions and the toughness of the steel material is lowered. Therefore, the Mn content is 0.01 to 1.00%. The preferable lower limit of the Mn content is 0.05%, more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%. The preferred upper limit of the Mn content is 0.80%, more preferably 0.70%, still more preferably 0.60%, still more preferably 0.50%.

P: 0.030% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. If the P content exceeds 0.030%, P segregates at the grain boundaries and reduces the toughness of the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the P content is 0.030% or less. The preferred upper limit of the P content is 0.025%, more preferably 0.020%. It is preferable that the P content is as low as possible. However, excessive reduction of P content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and further preferably 0.005%.

S: 0.0050% or less Sulfur (S) is an impurity that is inevitably contained. That is, the S content is more than 0%. Like P, S segregates at the grain boundaries or combines with Mn to form MnS, which is an inclusion. As a result, the toughness and hot workability of the steel material are reduced. When the S content exceeds 0.0050%, the toughness and hot workability of the steel material are significantly lowered even if the content of other elements is within the range of the present embodiment. Therefore, the S content is 0.0050% or less. The preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, still more preferably 0.0020%. It is preferable that the S content is as low as possible. However, excessive reduction of S content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002%, and further preferably 0.0005%.

Cr: Over 16.00 to 18.00%
Chromium (Cr) in the sour environment containing _{H 2} S partial pressure of 0.03 super ～0.1Bar, enhance the general corrosion resistance of the steel to produce a passive film on the surface of the steel material. If the Cr content is 16.00% or less, 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 Cr content exceeds 18.00%, the volume fraction of ferrite in the steel material increases and the strength of the steel material decreases even if the content of other elements is within the range of this embodiment. Therefore, the Cr content is more than 16.00 to 18.00%. The lower limit of the Cr content is preferably 16.05%, more preferably 16.10%, still more preferably 16.15%, still more preferably 16.20%. The preferred upper limit of the Cr content is 17.80%, more preferably 17.60%, still more preferably 17.50%, still more preferably 17.40%.

Mo: 1.00 to 3.00%
Molybdenum (Mo) in the sour environment containing _{H 2} S partial pressure of 0.03 super ～0.1Bar, to produce a sulfide on passive film. Mo sulfide suppresses chloride ion (Cl ⁻ ) and hydrogen sulfide ion (HS ⁻ ) from contacting the inactive film, and suppresses the inactive film from being destroyed by chloride ion and hydrogen sulfide ion. To do. Therefore, Mo suppresses the active melting of the steel material in the above-mentioned sour environment and enhances the total corrosion resistance. If the Mo content is less than 1.00%, 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 exceeds 3.00%, the volume fraction of ferrite in the steel material increases and the strength of the steel material decreases even if the content of other elements is within the range of this embodiment. If the Mo content exceeds 3.00%, a Laves phase (Fe ₂ Mo or the like) is further formed, and the toughness and corrosion resistance of the steel material are lowered. Therefore, the Mo content is 1.00 to 3.00%. The preferred lower limit of the Mo content is 1.10%, more preferably 1.15%, still more preferably 1.20%, still more preferably 1.25%, still more preferably 1.30. %, More preferably 1.40%. The preferred upper limit of the Mo content is 2.90%, more preferably 2.85%, still more preferably 2.80%, still more preferably 2.75%.

Cu: 0.50-3.50%
In copper (Cu) Enhanced mild sour environment containing _{H 2} S partial pressure of 0.03 super ～0.1Bar, to produce a sulfide on passive film. Cu sulfide suppresses chloride ions (Cl ⁻ ) and hydrogen sulfide ions (HS ⁻ ) from coming into contact with the inert film, and suppresses the inert film from being destroyed by chlorine ions and hydrogen sulfide ions. .. Therefore, Cu suppresses the active melting of the steel material in the enhanced mild sour environment and enhances the total corrosion resistance. If the Cu content is less than 0.50%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cu content exceeds 3.50%, the hot workability of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0.50 to 3.50%. The preferable lower limit of the Cu content is 0.55%, more preferably 0.60%, further preferably 0.70%, still more preferably 0.75%, still more preferably 0.80. %. The preferred upper limit of the Cu content is 3.45%, more preferably 3.30%, still more preferably 3.25%, still more preferably 3.20%.

Ni: 3.00 to 5.50%
In nickel (Ni) Enhanced mild sour environment containing _{H 2} S partial pressure of 0.03 super ～0.1Bar, to produce a sulfide on passive film. Ni sulfide suppresses chloride ions (Cl ⁻ ) and hydrogen sulfide ions (HS ⁻ ) from coming into contact with the inert film, and suppresses the inert film from being destroyed by chloride ions and hydrogen sulfide ions. To do. Therefore, Ni suppresses the active melting of the steel material in the enhanced mild sour environment and enhances the total corrosion resistance. If the Ni content is less than 3.00%, 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 Ni content exceeds 5.50% other elements content be in the range of this embodiment, A _c1 transformation point becomes too low. In this case, retained austenite is likely to be excessively produced. As a result, the steel material may not have the desired mechanical properties. Therefore, the Ni content is 3.00 to 5.50%. The lower limit of the Ni content is preferably 3.05%, more preferably 3.10%, still more preferably 3.15%, still more preferably 3.20%, still more preferably 3.25. %. The preferred upper limit of the Ni content is 5.00%, more preferably 4.90%, still more preferably 4.85%, still more preferably 4.80%.

Co: 0.010 to 0.500%
In cobalt (Co) Enhanced mild sour environment containing _{H 2} S partial pressure of 0.03 super ～0.1Bar, to produce a sulfide on passive film. Co sulfide suppresses chloride ion (Cl ⁻ ) and hydrogen sulfide ion (HS ⁻ ) from contacting the inactive film, and suppresses the inactive film from being destroyed by chloride ion and hydrogen sulfide ion. To do. Therefore, Co suppresses the active melting of the steel material in the enhanced mild sour environment and enhances the total corrosion resistance. If the Co content is less than 0.010%, these effects 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 exceeds 0.500%, the toughness of the steel material decreases even if the content of other elements is within the range of the present embodiment. Therefore, the Co content is 0.010 to 0.500%. The preferable lower limit of the Co content is 0.030%, more preferably 0.050%, further preferably 0.070%, further preferably 0.080%, still more preferably 0.100. %, More preferably 0.120%. The preferred upper limit of the Co content is 0.450%, more preferably 0.400%, still more preferably 0.350%, still more preferably 0.300%, still more preferably 0.250. %.

Al: 0.001 to 0.100%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.001%, the above effect cannot be sufficiently obtained even if the other element content is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.100%, coarse oxides are generated 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 Al content is 0.001 to 0.100%. The lower limit of the Al content is preferably 0.002%, more preferably 0.003%, still more preferably 0.010%, still more preferably 0.015%. The preferred upper limit of the Al content is 0.070%, more preferably 0.055%, still more preferably 0.050%, still more preferably 0.045%. In addition, the Al content in this specification is referred to as sol. It means the content of Al (acid-soluble Al).

O: 0.050% or less Oxygen (O) is an impurity that is inevitably contained. That is, the O content is more than 0%. O forms coarse oxide-based inclusions and reduces the toughness of the steel material. Therefore, the O content is 0.050% or less. The upper limit of the O content is preferably 0.030%, more preferably 0.020%, still more preferably 0.010%. The O content is preferably as low as possible. However, excessive reduction of O content greatly increases the manufacturing cost. Therefore, when industrial production is taken into consideration, the preferable lower limit of the O content is 0.001%.

N: 0.070% or less Nitrogen (N) is inevitably contained. That is, the N content is more than 0%. If the N content exceeds 0.070%, N forms a coarse nitride and reduces the toughness of the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the N content is 0.070% or less. The preferable lower limit of the N content is 0.001%, more preferably 0.002%, still more preferably 0.005%. The preferred upper limit of the N content is 0.060%, more preferably 0.050%, still more preferably 0.040%, still more preferably 0.030%.

Ca: 0.0001 to 0.0050%
Calcium (Ca) controls the morphology of inclusions and enhances the hot workability of steel. Controlling the morphology of inclusions is, for example, spheroidizing inclusions. If the Ca content is less than 0.0001%, 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 exceeds 0.0050%, a coarse Ca oxide is produced. In this case, the toughness of the steel material decreases even if the content of other elements is within the range of this embodiment. Therefore, the Ca content is 0.0001 to 0.0050%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0010%, still more preferably 0.0015%. The preferred upper limit of the Ca content is 0.0045%, more preferably 0.0040%, still more preferably 0.0035%.

V: 0.01 to 1.00%
Vanadium (V) enhances the hardenability of the steel material and enhances the strength of the steel material. If the V content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content exceeds 1.00%, the toughness of the steel material decreases even if the content of other elements is within the range of the present embodiment. Therefore, the V content is 0.01 to 1.00%. The lower limit of the V content is preferably 0.02%, more preferably 0.03%. The preferred upper limit of the V content is 0.50%, more preferably 0.30%, still more preferably 0.10%.

W: 0.01 to 2.00%
Tungsten (W) in the enhanced mild sour environment containing _{H 2} S partial pressure of 0.03 super ～0.1Bar, to produce a sulfide on passive film. W sulfide suppresses chloride ion (Cl ⁻ ) and hydrogen sulfide ion (HS ⁻ ) from contacting the inactive film, and suppresses the inactive film from being destroyed by chloride ion and hydrogen sulfide ion. To do. Therefore, W suppresses the active melting of the steel material in the enhanced mild sour environment and enhances the total corrosion resistance. If the W content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the W content exceeds 2.00%, coarse W carbides are generated 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 W content is 0.01 to 2.00%. The lower limit of the W content is preferably 0.02%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.15%. The preferred upper limit of the W content is 1.80%, more preferably 1.60%, still more preferably 1.50%, still more preferably 1.40%.

The rest of the chemical composition of the seamless steel pipe according to this embodiment consists of Fe and impurities. Here, the impurities are those that are mixed in from ore, scrap, or the manufacturing environment as raw materials when the seamless steel pipe is industrially manufactured, and are not intentionally contained, and are not intentionally contained. It means what is allowed as long as it does not adversely affect the effect of the seamless steel pipe of the form.

[About Optional Elements]
The chemical composition of the seamless steel pipe according to the present embodiment may further contain B instead of a part of Fe.

B: 0 to 0.0020%
Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When B is contained, B dissolves in the steel material to improve the hot workability of the steel material. If B is contained even in a small amount, the above effect can be obtained to some extent. However, if the B content exceeds 0.0020%, coarse B nitride is produced even if the content of other elements is within the range of the present embodiment, and the low temperature toughness of the steel material is lowered. Therefore, the B content is 0 to 0.0020%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, still more preferably 0.0005%, still more preferably 0.0008%. The preferred upper limit of the B content is 0.0019%, more preferably 0.0015%.

The chemical composition of the seamless steel pipe according to the present embodiment may further contain one or more selected from the group consisting of Ti and Nb instead of a part of Fe. All of these elements are optional elements and increase the strength of the steel material.

Ti: 0 to 0.010%
Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When Ti is contained, Ti combines with C and / or N to form carbides, nitrides or carbonitrides (hereinafter referred to as carbonitrides and the like). In this case, the coarsening of crystal grains is suppressed by the pinning effect, and the strength of the steel material is increased. When B is contained, Ti further binds to N to suppress B from binding to N and secure the amount of solid solution B. If even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.010%, even if the content of other elements is within the range of this embodiment, coarse Ti carbonitride or the like is generated, and the low temperature toughness of the steel material is lowered. .. Therefore, the Ti content is 0 to 0.010%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.002%. The preferred upper limit of the Ti content is 0.008%, more preferably 0.006%.

Nb: 0 to 0.300%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with C to form Nb carbides. In this case, the coarsening of crystal grains is suppressed by the pinning effect, and the strength of the steel material is increased. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.300%, the above effect is saturated. Therefore, the Nb content is 0 to 0.300%. The preferable lower limit of the Nb content is 0.001%, more preferably 0.002%, still more preferably 0.010%. The preferred upper limit of the Nb content is 0.250%, more preferably 0.200%, still more preferably 0.150%.

The seamless steel pipe according to the present embodiment may further contain Sn instead of a part of Fe.

Sn: 0 to 0.300%
Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. If contained, Sn is suppressed activity dissolution of steel in enhanced mild sour environment containing _{H 2} S partial pressure of 0.03 super ～0.1Bar, enhance the general corrosion resistance. If even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.300%, the toughness of the steel material decreases even if the content of other elements is within the range of the present embodiment. Therefore, the Sn content is 0 to 0.300%. The preferred lower limit of the Sn content is 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.015%. The preferred upper limit of the Sn content is 0.250%, more preferably 0.200%, still more preferably 0.150%.

[About equation (1)]
The chemical composition of the seamless steel pipe of the present embodiment further satisfies the formula (1).
Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co ≧ 23.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

It is defined as F1 = Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co. F1 is an index showing the stability of the passivation film in the seamless steel pipe on the premise that the content of each element in the chemical composition is within the range of the present embodiment. The relationship between F1 and depassivated pH (pHd) in a seamless steel pipe in which the content of each element in the chemical composition is within the range of the present embodiment will be described with reference to figures. FIG. 2 is a diagram showing the relationship between F1 and depassivated pH. In FIG. 2, in the examples described later, the content of each element in the chemical composition is within the range of the present embodiment, and F1 and the depassivated pH are obtained for the steel material produced by the preferable production method described later. Created using.

With reference to FIG. 2, when the content of each element in the chemical composition is within the range of the present embodiment and the steel material produced by the preferable production method described later has an F1 of less than 23.0, it is in a passivation state. The deactivated pH does not fluctuate so much even if the F1 value fluctuates because the deactivated pH exceeds 3.00. On the other hand, when F1 becomes 23.0 or more, the depassivation pH becomes 3.00 or less, and further, as F1 increases, the depassivation pH rapidly decreases. That is, in the graph of FIG. 2, there is an inflection point near F1 = 23.0. Therefore, if the content of each element in the chemical composition is within the range of this embodiment and the steel material produced by the preferable production method described later has F1 of 23.0 or more, the depassivation pH is 3. It becomes 00 or less and shows excellent total corrosion resistance. Therefore, in the seamless steel pipe according to the present embodiment, F1 is set to 23.0 or more.

The preferred lower limit of F1 is 23.5, more preferably 24.0, still more preferably 24.5, even more preferably 25.0, even more preferably 26.0, even more preferably. It is 26.9. The upper limit of F1 is not particularly limited, but in the case of the chemical composition of the present embodiment, it is 36.0. F1 is obtained by rounding off the second decimal place.

[Deactivated pH (pHd)]
As described above, in the present specification, the lowest pH at which a steel material can maintain a passivation state under a specific environment is referred to as a passivation pH under that environment. As described above, in the present specification, the depassivated pH is also referred to as "pHd". When the pH of the environment in which the steel material is exposed is lower than the depassivation pH, the passivation film of the steel material is destroyed, active dissolution proceeds, and total corrosion progresses. Therefore, the lower the deactivated pH, the more the passivated film is maintained even in a low pH sour environment, and the higher the total corrosion resistance of the seamless steel pipe.

The seamless steel pipe according to the present embodiment contains 5% by mass of NaCl and 0.41 g / L of CH ₃ COONa, and further, the depassivation pH in the aqueous solution containing CH ₃ COOH is 3.00 or less. is there. In the present specification, "an aqueous solution containing 5% by mass of NaCl and 0.41 g / L of CH ₃ COONa and further containing CH ₃ COOH" is also referred to as a "specific test solution".

The seamless steel pipe of the present embodiment has the above-mentioned chemical composition satisfying the formula (1), reduces the deelementation layer of the inner surface surface layer, and microscopically smoothes the inner surface surface layer. The depassivation pH of the specific test solution can be set to 3.00 or less. In addition, as described in the preferable manufacturing method described later, in this embodiment, by carrying out both the blast treatment and the pickling treatment, the deelement layer of the inner surface surface layer is reduced, and the inner surface surface layer is microscopically viewed. Can be smoothed. In this way, the seamless steel pipe according to the present embodiment can have a depassivation pH of 3.00 or less on the inner surface.

As described above, the total corrosion resistance of the steel material is affected not only by the stability of the passive film determined by the chemical composition of the steel material, but also by the presence or absence of the deelement layer formed on the surface layer of the steel material and the surface shape. However, with current technology, it is extremely difficult to identify and measure each of these complex factors. Therefore, in the present embodiment, as described above, when a specific test solution (an aqueous solution containing 5% by mass of NaCl and 0.41 g / L of CH ₃ COONa and further containing CH ₃ COOH) is used. The passivation pH is defined as an index of the stability of the passivation film, the presence or absence of the deelementation layer on the surface layer, and the surface shape.

As described above, there is no seam having the above-mentioned chemical composition and microstructure, satisfying the formula (1), and having a depassivation pH of 3.00 or less when a specific test solution is used. It is proved from the examples described later that the steel pipe exhibits excellent total corrosion resistance, and the seamless steel pipe that does not meet these requirements has low total corrosion resistance.

[Measurement method of depassivated pH]
The depassivated pH (pHd) can be measured by the following method. A test piece is prepared in which only the region corresponding to the inner surface of the seamless steel pipe is exposed. Specifically, a test piece including the inner surface of the seamless steel pipe is collected. The size of the test piece is not particularly limited. For example, a disk-shaped test piece having a thickness of 1 mm and a diameter of 15 mm may be used, or a plate-shaped test piece may be used. A coating is formed on a region other than the region corresponding to the inner surface of the seamless steel pipe in the test piece. The coating is not particularly limited as long as it is inert to corrosion in an enhanced mild sour environment. The coating is, for example, a resin coating. In this way, a test piece in which only the region corresponding to the inner surface of the seamless steel pipe is exposed is produced. The test piece is energized in the subsequent electrochemical measurement. Therefore, when forming a non-conductor coating such as a resin coating, a conducting wire for energization is connected to an arbitrary portion of the test piece in a region other than the region corresponding to the inner surface.

Prepare a plurality of specific test solutions containing 5% by mass of NaCl (sodium chloride) and 0.41 g / L of CH ₃ COONa (sodium acetate), and further containing different concentrations of CH ₃ COOH (acetic acid). .. The pH of each of the prepared specific test solutions is measured with a pH meter to determine the pH of each specific test solution. Prepare a plurality of specific test solutions having a pH of approximately 0.2 pitch.

The natural potential of the test piece in each specific test solution is measured by the following method. First, prepare an electrolytic cell. The electrolytic cell is a glass cell (800 mL). Put the specific test solution in the electrolytic cell and degas with high-purity Ar for 1 hour or more. After degassing, the test gas _{(H 2} S partial pressure is the 0.1 bar, balance CO ₂₎ was bubbled for 30 minutes or more, is saturated. The specific test solution at the time of the test is kept at room temperature (24 ± 3 ° C.), and the electrolytic cell is sealed. The counter electrode is platinum, and the reference electrode is an SCE saturated calomel electrode. Immerse the test piece in the test solution and measure the natural potential using a potentiostat. During the test, the test gas is aerated in the test solution at a flow rate of about 10 mL / min to maintain saturation.

After 4 hours have passed when the natural potential is sufficiently stabilized, the natural potential is measured. A test piece corresponding to each specific test solution having a pH of approximately 0.2 pitch is prepared, and the natural potential of each specific test solution is determined. The relationship between the obtained natural potential and the pH of the specific test solution is plotted.

FIG. 3 is a diagram showing an example of the relationship between the obtained natural potential and the pH of the specific test solution. With reference to FIG. 3, the pH of the plot immediately before the natural potential rises sharply is defined as "depassivation pH". Specifically, with reference to FIG. 3, the specific test solution having the lowest pH is defined as the first specific test solution, and the second specific test solution, the third specific test solution, and the nth specific test are in descending order of pH. Defined as a solution. The natural potential of the first specific test solution is defined as V1, the natural potential of the second specific test solution is defined as V2, the natural potential of the nth specific test solution is defined as Vn, and the natural potential of the n + 1 specific test solution is defined. Is defined as Vn + 1. In FIG. 3, the natural potential is remarkably increased from the natural potential Vn to the natural potential Vn + 1, and the potential difference between the natural potential Vn and the natural potential (Vn + 1, Vn + 2, Vn + 3, ...) After the natural potential Vn + 1 is , The potential difference between the natural potential Vn and the natural potential before the natural potential Vn-1 (Vn-1, Vn-2, Vn-3 ...) Is significantly larger. In this case, the pH of the nth specific test solution is defined as "depassivation pH".

[Microstructure]
The microstructure of the seamless steel pipe according to this embodiment is mainly composed of ferrite and martensite, and the balance is composed of retained austenite. More specifically, in the microstructure of the seamless steel pipe according to the present embodiment, the volume fraction of ferrite is less than 10 to 50%, the volume fraction of martensite is 50% or more, and the volume fraction of retained austenite is 10. % Or less. In addition, in this specification, martensite includes not only fresh martensite but also tempered martensite.

If the volume fraction of ferrite is too high, the yield strength of the steel material will decrease. Further, if the volume fraction of retained austenite is too high, Cr carbides are likely to be formed at the grain boundaries of retained austenite. In this case, the total corrosion resistance is reduced. In the microstructure of the steel material in which the content of each element in the chemical composition is within the range of the present embodiment, the volume fraction of ferrite is less than 10 to 50%, the volume fraction of martensite is 50% or more, and retained austenite. When the volume fraction of is 10% or less, the yield strength is 756 MPa (110 ksi) or more, and on the premise that the formula (1) is satisfied, excellent total corrosion resistance can be obtained in an enhanced mild sour environment.

The preferred lower limit of the volume fraction of ferrite is 15%, more preferably 20%, and even more preferably 25%. The preferred upper limit of the volume fraction of ferrite is 49%, more preferably 45%. The preferred lower limit of the volume fraction of martensite is 51%, more preferably 53%. The preferred upper limit of the volume fraction of martensite is 85%, more preferably 80%, still more preferably 75%, still more preferably 70%. The preferred upper limit of the volume fraction of retained austenite is 9%, more preferably 8%, still more preferably 7%. The volume fraction of retained austenite may be 0%.

[Measurement method of volume fraction of ferrite, martensite, retained austenite]
The volume fraction of ferrite (vol.%), The volume fraction of martensite (vol.%) And the volume fraction of retained austenite (vol.%) In the microstructure of the seamless steel pipe of the present embodiment are measured by the following methods.

[Volume fraction of retained austenite]
The volume fraction of retained austenite is determined by X-ray diffraction. Specifically, a test piece is collected from the center position of the wall thickness of the seamless steel pipe. The size of the test piece is not particularly limited, but is, for example, 15 mm × 15 mm × thickness 2 mm. In this case, the thickness direction of the test piece is the pipe diameter direction. Using the obtained test piece, each of the α-phase (200) plane, the α-phase (211) plane, the γ-phase (200) plane, the γ-phase (220) plane, and the γ-phase (311) plane. The X-ray diffraction intensity of is measured, and the integrated intensity of each surface is calculated. In the measurement of the X-ray diffraction intensity, the target of the X-ray diffractometer is Mo (MoKα ray), and the output is 50 kV-40 mA. After the calculation, the volume fraction Vγ (%) of retained austenite is calculated by using the formula (I) for each combination (2 × 3 = 6 sets) of each surface of the α phase and each surface of the γ phase. Then, the average value of the volume fraction Vγ of the six sets of retained austenite is defined as the volume fraction (%) of the retained austenite.
Vγ = 100 / {1+ (Iα × Rγ) / (Iγ × Rα)} (I)
Here, Iα is the integral intensity of the α phase. Rα is a crystallographic theoretically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is a crystallographic theoretically calculated value of the γ phase. In the present 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. 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 rounded off to the first decimal place of the obtained numerical value.

[Volume fraction of ferrite]
The volume fraction of ferrite is determined by the following method. Collect the test piece from the center position of the wall thickness of the seamless steel pipe. The size of the test piece is not particularly limited. Of the surfaces of the test pieces, the surface parallel to the longitudinal direction (tube axial direction) of the seamless steel pipe and corresponding to the cross section perpendicular to the wall thickness direction (tube radial direction) is defined as the observation surface. Mirror polish the observation surface. The observed surface after polishing is etched with a mixed solution of aqua regia and glycerin. Arbitrary three fields of view of the observation surface after etching are observed using a 100x optical microscope. The observation field of view is, for example, 1000 μm × 1000 μm. On the etched observation surface, ferrite and other phases (martensite, retained austenite) can be distinguished based on contrast. Identify ferrite based on contrast. For the specified ferrite, the area ratio of ferrite is determined by a point calculation method based on JIS G 0555 (2003). The arithmetic mean value of the area fractions of the three ferrites obtained is regarded as the volume fraction of ferrites (vol.%). The volume fraction of ferrite is rounded off to the first decimal place of the obtained numerical value.

[Volume fraction of martensite]
The volume fraction of martensite is calculated by the following method. Based on the volume fraction of retained austenite and the volume fraction of ferrite obtained by the above method, the volume fraction of martensite (vol.%) Is determined using the following formula.
Volume fraction of martensite = 100- (volume fraction of retained austenite + volume fraction of ferrite)

[Preferable form of seamless steel pipe of this embodiment]
In the seamless steel pipe of the present embodiment, the content of each element in the chemical composition is preferably within the range of the present embodiment and satisfies the formula (1) and further satisfies the formula (2).
(3Ti + B) /N≤0.210 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2). If the corresponding element is not contained, "0" is substituted for the corresponding element symbol.

It is defined as F2 = (3Ti + B) / N. F2 is an index of low temperature toughness in a seamless steel pipe on the premise that the content of each element in the chemical composition is within the range of the present embodiment. As described above, B enhances the hot workability of the steel material, and Ti enhances the strength of the steel material. However, both Ti and B are elements that easily bond with N and easily form a nitride. When Ti and B combine with N to generate an excessive amount of coarse nitride, the low temperature toughness of the steel material decreases. In FIG. 4, among the examples described later, the content of each element in the chemical composition is within the range of this embodiment, the formula (1) is satisfied, and the depassivation pH (pHd) is 3.00 or less. It is a figure which shows the relationship between F2 and the absorbed energy vE- ₁₀ (J) at _-10 degreeC in the steel material. In FIG. 4, among the examples described later, the content of each element in the chemical composition is within the range of this embodiment, the formula (1) is satisfied, and the depassivation pH (pHd) is 3.00 or less. It was prepared by using F2 and absorbed energy vE- ₁₀ (J).

With reference to FIG. 4, in a steel material in which the content of each element in the chemical composition is within the range of the present embodiment, the formula (1) is satisfied, and the depassivation pH (pHd) is 3.00 or less. When F2 is 0.210 or less, the absorbed energy vE- ₁₀ (J) is as high as 150J or more, and even if F2 fluctuates, the absorbed energy vE- ₁₀ (J) does not fluctuate so much. On the other hand, if F2 exceeds 0.210, the absorbed energy vE- ₁₀ (J) drops sharply to less than 100J. That is, in the graph of FIG. 2, there is an inflection point near F2 = 0.210. Therefore, in a steel material in which the content of each element in the chemical composition is within the range of the present embodiment, the formula (1) is satisfied, and the depassivation pH (pHd) is 3.00 or less, F2 is further 0. If it is .210 or less, not only excellent total corrosion resistance can be obtained, but also excellent low temperature toughness can be obtained.

The preferred upper limit of F2 is 0.205, more preferably 0.200, even more preferably 0.190, even more preferably 0.180, even more preferably 0.170, even more preferably. It is 0.160. In addition, F2 rounds off the fourth decimal place of the obtained numerical value.

[Evaluation method of low temperature toughness]
The low temperature toughness of the seamless steel pipe according to this embodiment is evaluated by a Charpy impact test according to ASTM E23 (2018). Specifically, a V-notch test piece is produced from the center position of the wall thickness of the seamless steel pipe according to the present embodiment. The V-notch test piece is manufactured in accordance with API 5CRA (2010). A Charpy impact test based on ASTM E23 (2018) is carried out on a V-notch test piece prepared according to API 5CRA (2010) to obtain an absorbed energy vE- ₁₀ (J) at −10 ° C. In the present embodiment, if the absorbed energy vE- ₁₀ (J) is 150 J or more, it is determined that the product has excellent low temperature toughness.

In the seamless steel pipe of the present embodiment, the content of each element in the chemical composition is within the range of the present embodiment, the formula (1) is satisfied, and the depassivation pH (pHd) is 3.00 or less. If so, it has excellent overall corrosion resistance even if the formula (2) is not satisfied. Therefore, in the seamless steel pipe of the present embodiment, the content of each element in the chemical composition is within the range of the present embodiment, the formula (1) is satisfied, and the depassivation pH (pHd) is 3.00 or less. If so, it is not necessary to satisfy the equation (2). As described above, the seamless steel pipe of the present embodiment preferably has the content of each element in the chemical composition within the range of the present embodiment, satisfies the formula (1), and has a depassivated pH (pHd). Is 3.00 or less, and the equation (2) is satisfied. In this case, not only excellent total corrosion resistance but also excellent low temperature toughness can be obtained.

[Yield strength]
The yield strength of the seamless steel pipe of the present embodiment is not particularly limited. The preferred yield strength is 758 MPa or more (110 ksi or more). The upper limit of the yield strength is not particularly limited, but the upper limit of the yield strength of the seamless steel pipe according to the present embodiment is, for example, less than 1000 MPa (less than 145 ksi). The lower limit of the yield strength is preferably 793 MPa (115 ksi), more preferably 827 MPa (120 ksi), still more preferably 862 MPa (125 ksi). The preferred upper limit of the yield strength is 965 MPa (140 ksi), more preferably 931 MPa (135 ksi).

As used herein, yield strength means 0.2% offset proof stress (MPa) obtained by a tensile test at room temperature (24 ± 3 ° C.) according to ASTM E8 / E8M (2013). Specifically, the yield strength is obtained by the following method. Take a tensile test piece from the center position of the wall thickness of the seamless steel pipe. The tensile test piece is, for example, a round bar tensile test piece having a parallel portion diameter of 6.0 mm and a parallel portion length of 40.0 mm. The longitudinal direction of the parallel portion of the round bar tensile test piece shall be parallel to the pipe axis direction of the seamless steel pipe. A tensile test is performed at room temperature (24 ± 3 ° C.) in accordance with ASTM E8 / E8M (2013) using a round bar tensile test piece to determine a 0.2% offset proof stress (MPa). The obtained 0.2% offset proof stress is defined as the yield strength (MPa).

[Use of seamless steel pipe]
The use of the seamless steel pipe according to this embodiment is not particularly limited. The seamless steel pipe according to this embodiment is suitable for a seamless steel pipe for oil wells. Seamless steel pipes for oil wells are, for example, casings, tubing, drill pipes and the like used for drilling oil wells or gas wells, collecting crude oil or natural gas, and the like.

[Production method]
An example of a method for manufacturing a seamless steel pipe according to this embodiment will be described. The manufacturing method described below is an example, and the manufacturing method of the seamless steel pipe of the present embodiment is not limited to this. That is, as long as the seamless steel pipe of the present embodiment having the above configuration can be manufactured, the manufacturing method is not limited to that described below. However, the manufacturing method described below is a suitable manufacturing method for manufacturing the seamless steel pipe of the present embodiment.

An example of the method for manufacturing a seamless steel pipe of the present embodiment includes a step of preparing a raw pipe (a raw pipe preparation step) and a step of performing post-treatment on the raw pipe (post-treatment step). Hereinafter, each step will be described in detail.

[Base pipe preparation process]
In the raw tube preparation step, a raw tube in which the content of each element in the chemical composition is within the range of the present embodiment and satisfies the formula (1) is prepared. Preferably, a raw tube is prepared in which the content of each element in the chemical composition is within the range of the present embodiment and satisfies the formulas (1) and (2). The method for manufacturing the raw tube is not particularly limited. As the raw pipe, a pipe supplied from a third party may be used, or a pipe manufactured by the raw pipe manufacturing process described below may be used.

[Material pipe manufacturing process]
When manufacturing a raw pipe, the raw pipe manufacturing step includes a material preparation step, a hot working step, and a heat treatment step. Hereinafter, each step will be described.

[Material preparation process]
In the material preparation step, first, a molten steel in which the content of each element in the chemical composition is within the range of the present embodiment and satisfies the formula (1) or the formulas (1) and (2) is well known. Manufactured by refining method. A 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 ingot may be manufactured by the ingot method using the molten steel. If desired, slabs, blooms or ingots may be hot rolled to produce billets. The material (slab, bloom, or billet) is manufactured by the above manufacturing process.

[Hot working process]
In the hot working process, the prepared material is hot-worked. First, the material is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300 ° C. A raw pipe (seamless steel pipe) is manufactured by performing hot working on the material extracted from the heating furnace. For example, the Mannesmann method is carried out as hot working to manufacture a bare tube. In this case, the billet is drilled and rolled by a drilling machine. In the case of drilling and rolling, the drilling ratio is not particularly limited, but is, for example, 1.0 to 4.0. The billet after perforation rolling is stretch-rolled using a mandrel mill. Further, if necessary, the billet after stretch rolling is subjected to constant diameter rolling using a reducer or a sizing mill. A bare tube is manufactured by the above steps. The cumulative surface reduction rate in the hot working process is not particularly limited, but is, for example, 20 to 70%.

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

[Heat treatment process]
The heat treatment step includes a quenching step and a tempering step.

[Quenching process]
In the heat treatment step, first, the steel material produced in the hot working step is hardened (quenching step). Quenching is carried out by a well-known method. Specifically, the hot-worked steel material is charged into a heat treatment furnace and maintained at a quenching temperature. Quenching temperature is _{A C3} transformation point or higher, for example, 900 to 1000 ° C.. After holding the steel material at the quenching temperature, it is rapidly cooled (quenched). The holding time at the quenching temperature is not particularly limited, but is, for example, 10 to 60 minutes. The quenching method is, for example, water cooling. The quenching method is not particularly limited. For example, the raw pipe may be rapidly cooled by immersing it in a water tank or an oil tank, or cooling water may be poured or sprayed onto the outer surface and / or inner surface of the steel pipe by shower cooling or mist cooling. May be rapidly cooled.

After hot working, quenching (direct quenching) may be performed immediately after hot working without cooling the raw pipe to room temperature, or it may be supplemented before the temperature of the raw pipe after hot working drops. Quenching may be carried out after charging into a heating furnace and maintaining the quenching temperature.

The quenching temperature described above means the furnace temperature when a heat treatment furnace or a reheating furnace is used, and means the temperature of the outer surface of the raw pipe in the case of direct quenching. The holding time means the time in the furnace (the time from the charging to the heat treatment furnace or the reheating furnace to the extraction).

[Tempering process]
A tempering step is further carried out on the raw pipe after quenching. In the tempering process, the yield strength of the raw pipe is adjusted. In the seamless steel pipe of the present embodiment, the tempering temperature is set to 500 ° C. to the _Ac1 transformation point. The preferred lower limit of the tempering temperature is 510 ° C, more preferably 520 ° C. The preferred upper limit of the tempering temperature is 630 ° C, more preferably 620 ° C. The holding time at the tempering temperature is not particularly limited, but is, for example, 10 to 180 minutes. By appropriately adjusting the tempering temperature according to the chemical composition, the yield strength of the seamless steel pipe having the above-mentioned chemical composition satisfying the formula (1) can be adjusted. Preferably, the tempering conditions are adjusted so that the yield strength of the seamless steel pipe is 758 MPa (110 ksi) or more.

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

By the above steps, a bare tube can be manufactured.

[Post-treatment process]
In the post-treatment step, a blasting step and a pickling step are carried out on the raw pipe prepared by the raw pipe preparation step. The pickling step includes a first pickling step and a second pickling step. Hereinafter, the blasting step and the pickling step (first pickling step and second pickling step) will be described in detail.

[Blasting process]
In the blasting step, the inner surface of the prepared raw pipe is blasted. In the blasting step, the scale formed in the above-mentioned heat treatment step is mechanically ground and removed by the blasting treatment. In the blasting process, the inner surface of the raw tube is further microscopically smoothed. As a result, the inner surface of the seamless steel pipe after the pickling treatment described in the subsequent stage is microscopically smoothed, and the overall corrosion resistance is enhanced. Preferably, the outer surface of the raw tube is also blasted. In this case, the seamless steel pipe after the pickling step described later has excellent total corrosion resistance not only on the inner surface but also on the outer surface.

In the blasting process, the type of blasting treatment is not particularly limited as long as the surface of the raw pipe can be mechanically ground. The blasting process performed in the blasting step may be, for example, shot blasting, sandblasting, or shot peening. A preferred blasting process performed in the blasting process is shot blasting.

In the blasting process, a projection material is sprayed on the inner surface of the raw tube. The projecting material is, for example, steel, cast steel, stainless steel, glass, silica sand, alumina, amorphous, zirconia and the like. The shape of the projecting material may be spherical, cut wire, round cut wire, or grid. As a method of spraying the projection material, for example, the projection material may be sprayed on the surface of the steel material by using compressed air, centrifugal force by an impeller (impeller type), high-pressure water, ultrasonic waves, or the like. It is possible for those skilled in the art to appropriately adjust the conditions of the blasting treatment to appropriately remove the scale and to microscopically smooth the inner surface of the raw tube.

As described above, in the blasting step, the scale can be removed from the inner surface of the raw tube and the inner surface of the raw tube can be microscopically smoothed. On the other hand, in the blasting step, it is difficult to completely remove the deelement layer from the surface layer on the inner surface of the raw tube. Therefore, in the preferred method for producing a seamless steel pipe according to the present embodiment, the remaining deelement layer is removed by the first pickling step described later. Hereinafter, the first pickling step will be described in detail.

[First pickling process]
In the first pickling step, the raw pipe that has been blasted is pickled with a sulfuric acid solution. Specifically, a sulfuric acid tank in which a sulfuric acid solution is stored is prepared. The sulfuric acid solution is, for example, an aqueous solution containing sulfuric acid having a concentration of 5.0 to 30.0% by mass. The temperature of the sulfuric acid solution in the sulfuric acid tank is adjusted to 50.0 to 80.0 ° C., and the raw tube is immersed in the sulfuric acid tank. The immersion time in the sulfuric acid tank is, for example, 20 to 40 minutes. By immersing the raw tube in the sulfuric acid tank, the deelement layer on the surface of the raw tube is removed. After the above immersion time has elapsed, the raw pipe is pulled up from the sulfuric acid tank. Then, the raw pipe is immersed in a washing tank in which water is stored, and the raw pipe is washed with water.

[Second pickling process]
In the second pickling step, the raw pipe that has been pickled with a sulfuric acid solution in the first pickling step is pickled with a mixed acid of nitric acid and fluoroacid. Specifically, a treatment tank for storing the treatment solution is prepared. The treatment solution contains nitric acid and phosphoric acid. The nitric acid concentration of the treatment solution is, for example, 5.0 to 15.0% by mass. The fluoride concentration of the treatment solution is, for example, 2.0 to 7.0% by mass. The treatment solution is an aqueous solution containing nitric acid and phosphoric acid.

The temperature of the treatment solution in the treatment tank is adjusted to room temperature (24 ± 3 ° C.) to 50 ° C., and the raw tube is immersed in the treatment tank. The immersion time in the treatment tank is, for example, 10 to 45 minutes. By immersing the raw tube in the treatment tank, the surface of the raw tube is activated. As a result, a strong passivation film is formed on the surface of the steel material after washing with water, which will be described later. As a result, it becomes easier to uniformly form a passivation film on the surface of the steel material.

After the above immersion time has elapsed, the raw pipe is pulled up from the treatment tank. Then, the raw pipe is immersed in a washing tank in which water is stored, and the raw pipe is washed with water. The method of washing with water is not particularly limited, and a well-known method may be used. For example, as in the first pickling step, the raw pipe may be immersed in a water washing tank in which water is stored. For example, further, shower washing with high-pressure water may be performed.

By carrying out the above blasting step, the first pickling step and the second pickling step, the scale of the surface layer of the steel material and the deelementation layer are sufficiently removed, and a strong passivation film is uniformly formed. It becomes easy to be done. In this case, the surface of the steel material is further microscopically smooth. As a result, the inner surface of the seamless steel pipe according to the present embodiment has a depassivation pH of 3.00 or less in the specific test solution. As a result, a seamless steel pipe of the present embodiment, even enhanced mild sour environment containing H _{2 S} partial pressure of 0.03 super ～0.1Bar, it shows excellent general corrosion resistance in inner surface.

On the other hand, when only the blasting treatment is performed on the raw pipe, the scale of the inner surface of the seamless steel pipe is removed, but the deelement layer on the inner surface of the seamless steel pipe cannot be sufficiently removed. , A stable passivation film is not formed. On the other hand, when only the pickling treatment is performed on the raw pipe, the deelement layer on the inner surface of the seamless steel pipe is removed and a stable passivation film is uniformly formed, but the inside of the seamless steel pipe. The surface becomes microscopically rough. Therefore, even if only the blasting treatment or the pickling treatment is performed on the raw pipe, the obtained seamless steel pipe has a depassivation pH of more than 3.00 in the specific test solution on the inner surface. Excellent total corrosion resistance cannot be obtained.

FIG. 5 shows the above-mentioned blasting step and the first and second acidity in a steel material in which the content of each element in the chemical composition is within the range of the present embodiment and satisfies the formula (1) in the examples described later. The relationship between F1 and depassivation pH when the steps were carried out (marked with "○" in the figure), the above-mentioned blast step, and when at least one of the first and second acidic steps was not carried out. It is a figure which shows the relationship between F1 and depassivation pH (marked with "●" in the figure).

With reference to FIG. 5, even if the content of each element in the chemical composition is within the range of the present embodiment and the formula (1) is satisfied, the above-mentioned blasting step and the first and second pickling steps When at least one of the above is not carried out (marked with “●” in the figure), the deactivated pH (pHd) exceeds 3.00. On the other hand, when the content of each element in the chemical composition is within the range of this embodiment and the above-mentioned blasting step and the first and second acidic steps are carried out on the steel material satisfying the formula (1) (in the figure). “○” mark), depassivation pH (pHd) is 3.00 or less, and excellent total corrosion resistance can be obtained.

The manufacturing method described above is an example, and the manufacturing method of the seamless steel pipe of the present embodiment is not limited to this. As long as the seamless steel pipe of the present embodiment having the above configuration can be manufactured, the manufacturing method is not particularly limited.

In the seamless steel pipe of the present embodiment, the content of each element in the chemical composition is within the range of the present embodiment, the formula (1) is satisfied, and the depassivation pH (pHd) is 3.00 or less. Is. Therefore, the seamless steel pipe of the present embodiment has excellent total corrosion resistance. Preferably, the seamless steel pipe of the present embodiment has the content of each element in the chemical composition within the range of the present embodiment, satisfies the formula (1), and further satisfies the formula (2). In this case, the seamless steel pipe of the present embodiment not only has excellent total corrosion resistance, but also has excellent low temperature toughness.

A molten steel having the chemical composition shown in Table 2 was produced.

“-” In Table 2 means that the corresponding element content was below the detection limit. For example, the Co content of steel number R in Table 2 means that it was "0"% when the fourth decimal place was rounded off. The Ti content of steel number A in Table 2 means that it was "0"% when the fourth decimal place was rounded off. The B content of steel number A in Table 2 means that it was "0"% when the fifth decimal place was rounded off. The Nb content of steel number A in Table 2 means that it was "0"% when the fourth decimal place was rounded off. The Sn content of steel number A in Table 2 means that it was "0"% when the fourth decimal place was rounded off.

The molten steel was melted in a 50 kg vacuum furnace to produce an ingot by the ingot method. The ingot was heated at 1250 ° C. for 3 hours. A block was manufactured by performing hot forging on the heated ingot. The block after hot forging was held at 1230 ° C. for 15 minutes, and hot rolling was carried out. In this way, a steel material (plate material) having a thickness of 13 mm simulating a seamless steel pipe was manufactured. One of the surfaces perpendicular to the thickness direction of the steel material was designated as a surface simulating the inner surface of the seamless steel pipe (hereinafter, also referred to as “simulated surface”).

Quenching was performed on the steel materials of each test number. The quenching temperature was 900 ° C. in all test numbers, and the holding time at the quenching temperature was 15 minutes in all test numbers. Tempering was performed on the steel materials of each test number after quenching. The tempering temperature was in the range of 540 to 600 ° C., and the holding time at the tempering temperature was 30 minutes in all the test numbers. The yield strength was adjusted by changing the tempering temperature of the steel material of each test number.

The steel materials of each test number after tempering were blasted and pickled. In the blasting process, shot blasting was performed on the simulated surface. As the projecting material, alumina having a particle size of # 14 was used. Table 3 shows whether or not the blasting treatment is performed for the steel materials of each test number. “Implementation” in the “Blasting” column in Table 3 means that the blasting has been performed. A "-" in the "blast processing" column in Table 3 means that the blast processing was not performed.

Subsequently, the steel materials of each test number were pickled. In the pickling treatment, a two-step pickling treatment was carried out as described in the above-mentioned preferable production method. Specifically, the following steps were carried out. First, the steel material was immersed in a sulfuric acid solution at 60 ° C. containing 20.0% by mass of sulfuric acid for 30 minutes. After the immersion time had elapsed, the steel material was pulled out of the sulfuric acid solution and washed with water. The steel material after washing with water was immersed in a treatment solution at room temperature (24 ° C.) containing 10.0% by mass of nitric acid and 5.0% by mass of phosphoric acid for 30 minutes. The steel material after the lapse of the immersion time was washed with water. Table 3 shows whether or not the steel material of each test number is pickled. “Implementation” in the “pickling treatment” column in Table 3 means that the above-mentioned pickling treatment has been carried out. A "-" in the "pickling treatment" column in Table 3 means that the pickling treatment was not performed.

Through the above manufacturing process, a steel material simulating a seamless steel pipe was manufactured.

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

[Measurement test of the volume fraction of ferrite in the microstructure, the volume fraction of martensite, and the volume fraction of retained austenite]
A test piece having a thickness of 15 mm × 15 mm × thickness 2 mm was collected from the center position of the steel plate thickness of each test number. The thickness direction of the test piece corresponded to the plate thickness direction of the steel material (that is, the direction perpendicular to the simulated surface). Using the obtained test piece, each of the α-phase (200) plane, the α-phase (211) plane, the γ-phase (200) plane, the γ-phase (220) plane, and the γ-phase (311) plane. The X-ray diffraction intensity of was measured, and the integrated intensity of each surface was calculated. In the measurement of the X-ray diffraction intensity, the target of the X-ray diffractometer was Mo (MoKα ray), and the output was 50 kV-40 mA. After the calculation, the volume fraction Vγ (%) of retained austenite was calculated using the formula (I) for each combination (2 × 3 = 6 pairs) of each surface of the α phase and each surface of the γ phase. Then, the average value of the volume fraction Vγ of the 6 sets of retained austenite was defined as the volume fraction (%) of the retained austenite.
Vγ = 100 / {1+ (Iα × Rγ) / (Iγ × Rα)} (I)
In formula (I), 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 35.5. The Rγ on the (220) plane of the γ phase was set to 20.8, and the Rγ on the (311) plane of the γ phase was set to 21.8. The volume fraction of retained austenite was rounded off to the first decimal place of the obtained numerical value.

The volume fraction of ferrite of each test number was measured by the following method. A test piece was collected from the center position of the steel plate thickness of each test number. Of the surface of the test piece, a cross section parallel to the longitudinal direction (rolling direction) of the steel material and perpendicular to the plate thickness direction was defined as the observation surface. The observation surface was mirror-polished. The observed surface after polishing was etched with a mixed solution of aqua regia and glycerin. Arbitrary three fields of view of the observation surface after etching were observed using a 100x optical microscope. The observation field of view was 1000 μm × 1000 μm. In each observation field, ferrite was identified based on contrast. For the specified ferrite, the area ratio of ferrite was determined by a point calculation method based on JIS G 0555 (2003). The arithmetic mean value of the area fractions of the three ferrites obtained was regarded as the volume fraction (%) of the ferrites. The volume fraction of ferrite was rounded off to the first decimal place of the obtained numerical value.

Furthermore, based on the volume fraction of retained austenite and the volume fraction of ferrite obtained by the above method, the volume fraction (%) of martensite was determined using the following formula.
Volume fraction of martensite = 100- (volume fraction of retained austenite + volume fraction of ferrite)

The volume fraction (%) of ferrite is shown in the "ferrite" column of the "microstructure (volume%)" column in Table 3. The volume fraction (%) of martensite is shown in the "martensite" column. The volume fraction (%) of retained austenite is shown in the "Retained austenite" column.

[Tensile test]
Tensile test pieces were collected from the center position of the steel plate thickness of each test number. The tensile test piece was a round bar tensile test piece having a parallel portion diameter of 6 mm and a parallel portion length of 40 mm. The longitudinal direction of the parallel portion of the round bar tensile test piece was parallel to the longitudinal direction (rolling direction) of the steel material. A tensile test was performed at room temperature (24 ± 3 ° C.) in accordance with ASTM E8 / E8M (2013) using a round bar tensile test piece to determine the yield strength YS (MPa). The yield strength YS was 0.2% offset proof stress. The yield strength YS (MPa and ksi) obtained is shown in Table 3.

[Deactivated pH measurement test]
A test piece including a simulated surface was collected from the steel material of each test number. The size of the test piece was a disk-shaped test piece having a thickness of 1 mm and a diameter of 15 mm. A resin film was formed on a region other than the simulated surface of the disk-shaped test piece. A lead wire was connected in advance to a region other than the simulated surface of the disk-shaped test piece. A plurality of specific test solutions containing 5% by mass of NaCl (sodium chloride) and 0.41 g / L of CH ₃ COONa (sodium acetate) and further containing different concentrations of CH ₃ COOH (acetic acid) were prepared. .. The pH of the prepared specific test solutions was measured with a pH meter to specify the pH of each specific test solution. A plurality of specific test solutions having a pH of approximately 0.2 pitch were prepared.

The natural potential of the test piece in each specific test solution was measured by the following method. An electrolytic cell was prepared. The electrolytic cell was a glass cell (800 mL). A specific test solution was placed in an electrolytic cell and degassed with high-purity Ar for 1 hour or more. Thereafter, the test gas _{(H 2} S partial pressure 0.1 bar, balance CO ₂₎ was bubbled over 30 minutes and allowed to saturate. The specific test solution at the time of the test was kept at room temperature (24 ± 3 ° C.), and the electrolytic cell was sealed. The counter electrode was platinum and the reference electrode was an SCE saturated calomel electrode. The test piece was immediately immersed in a specific test solution, and the natural potential was measured using a potentiostat. During the test, the test gas was aerated in the solution at a flow rate of about 10 mL / min to maintain saturation.

The natural potential was measured 4 hours after the natural potential was sufficiently stabilized. A test piece corresponding to each specific test solution having a pH of approximately 0.2 pitch was prepared, and the natural potential of each specific test solution was determined. The relationship between the obtained natural potential and the pH of the specific test solution was plotted. Based on the plotted graph, as described above, the pH of the specific test solution in the plot immediately before the natural potential rises sharply was defined as "depassivation pH". The determined depassivation pH is shown in the "pHd" column of Table 3.

[Test results]
With reference to Tables 2 and 3, the content of each element in the chemical compositions of Test Nos. 1-17 was appropriate, and F1 satisfied formula (1). In addition, the manufacturing conditions were appropriate. Therefore, the depassivation pH of the steel materials of these test numbers was 3.00 or less, and excellent total corrosion resistance was obtained. In the microstructures of the steel materials of test numbers 1 to 17, the ferrite volume fraction was less than 10 to 50%, the martensite volume fraction was 50% or more, and the retained austenite volume fraction was 10% or less. It was.

Of the test numbers 1 to 17, the steel materials of test numbers 1 to 14 further had a chemical composition satisfying the formula (2). Therefore, the absorbed energy vE- ₁₀ (J) at −10 ° C. of these test numbers was 150 J or more, and not only excellent total corrosion resistance but also excellent low temperature toughness was obtained.

On the other hand, in Test Nos. 18 to 20, although the content of each element in the chemical composition was appropriate and the production conditions were appropriate, the chemical composition did not satisfy the formula (1). As a result, the depassivation pH exceeded 3.00, and the total corrosion resistance was low.

In test number 21, the Co content was too low. As a result, the depassivation pH exceeded 3.00, and the total corrosion resistance was low.

In Test Nos. 22, 25 and 28 to 32, the content of each element in the chemical composition was appropriate, and although F1 satisfied the formula (1), neither the blasting treatment nor the pickling treatment was carried out. Therefore, the depassivation pH exceeded 3.00, and the total corrosion resistance was low.

In test numbers 23 and 26, the content of each element in the chemical composition was appropriate, and although F1 satisfied the formula (1), the pickling treatment was not carried out. Therefore, the depassivation pH exceeded 3.00, and the total corrosion resistance was low.

In test numbers 24 and 27, the content of each element in the chemical composition was appropriate, and although F1 satisfied the formula (1), the blast treatment was not carried out. Therefore, the depassivation pH exceeded 3.00, and the total corrosion resistance was low.

The embodiments of the present invention have been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

Claims (5)

It is a seamless steel pipe
The chemical composition is mass%,
C: 0.050% or less,
Si: 1.00% or less,
Mn: 0.01 to 1.00%,
P: 0.030% or less,
S: 0.0050% or less,
Cr: Over 16.00 to 18.00%,
Mo: 1.00 to 3.00%,
Cu: 0.50-3.50%,
Ni: 3.00 to 5.50%,
Co: 0.010 to 0.500%,
Al: 0.001 to 0.100%,
O: 0.050% or less,
N: 0.070% or less,
Ca: 0.0001 to 0.0050%,
V: 0.01 to 1.00%,
W: 0.01 to 2.00%,
B: 0 to 0.0020%,
Ti: 0-0.010%,
Nb: 0 to 0.300%,
Sn: 0 to 0.300%, and
Remaining: Consisting of Fe and impurities, satisfying formula (1),
In the microstructure of the seamless steel pipe, the volume fraction of ferrite is less than 10 to 50%, the volume fraction of martensite is 50% or more, and the volume fraction of retained austenite is 10% or less.
In the seamless steel pipe, the depassivation pH of an aqueous solution containing 5% by mass of NaCl and 0.41 g / L of CH ₃ COONa and further containing CH ₃ COOH is 3.00 or less.
Seamless steel pipe.
Cr + 2.0Mo + 0.5Ni + 2.0Cu + W + 0.5Co ≧ 23.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). The seamless steel pipe according to claim 1.
The chemical composition further satisfies the formula (2).
Seamless steel pipe.
(3Ti + B) /N≤0.210 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2). If the corresponding element is not contained, "0" is substituted for the corresponding element symbol. The seamless steel pipe according to claim 1 or 2.
The chemical composition is
B: Contains 0.0001 to 0.0020%,
Seamless steel pipe. The seamless steel pipe according to any one of claims 1 to 3.
The chemical composition is
Ti: 0.001 to 0.010% and
Nb: contains one or more selected from the group consisting of 0.001 to 0.300%.
Seamless steel pipe. The seamless steel pipe according to any one of claims 1 to 4.
The chemical composition is
Sn: contains 0.001 to 0.300%,
Seamless steel pipe.

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