JP7226571B2 - Seamless stainless steel pipe and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a seamless stainless steel pipe suitable for use in oil wells and gas wells (hereinafter simply referred to as oil wells). The present invention improves corrosion resistance and strength at high temperatures, especially in high-temperature severe corrosive environments containing carbon dioxide gas (CO ₂ ) and chloride ions (Cl ⁻ ) and in environments containing hydrogen sulfide (H ₂ S). It relates to a stainless seamless steel pipe.

In recent years, from the perspective of depletion of energy resources expected in the near future, environments that have not been considered in the past, such as deep oil fields, environments containing carbon dioxide gas, and environments containing hydrogen sulfide called sour environments, etc. , the development of oil wells in severely corrosive environments is being actively carried out. Oil well steel pipes used in such environments are required to have high strength and high corrosion resistance.

Conventionally, 13Cr martensitic stainless steel pipes have generally been used as oil well steel pipes for mining in oil and gas fields in environments containing CO ₂ and Cl ⁻ . Recently, however, the development of even higher temperature (up to 200° C.) oil wells has progressed, and in some cases the 13Cr martensitic stainless steel pipe lacks corrosion resistance. There is a demand for oil well steel pipes having high corrosion resistance that can be used in such environments.

In response to such demands, there are techniques described in Patent Documents 1 to 5, for example. In Patent Document 1, in mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.002 %, Cr: 16-18%, Mo: 1.8-3%, Cu: 1.0-3.5%, Ni: 3.0-5.5%, Co: 0.01-1.0 %, Al: 0.001 to 0.1%, O: 0.05% or less, and N: 0.05% or less, and Cr, Ni, Mo, and Cu satisfy a specific relationship. An oil well grade stainless steel with

In addition, in Patent Document 2, in mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0 Less than .005%, Cr: more than 15.0% and 19.0% or less, Mo: more than 2.0% and 3.0% or less, Cu: 0.3 to 3.5%, Ni: 3.0% or more5 less than .0%, W: 0.1 to 3.0%, Nb: 0.07 to 0.5%, V: 0.01 to 0.5%, Al: 0.001 to 0.1%, N : 0.010 to 0.100%, O: 0.01% or less, Nb, Ta, C, N, Cu have a composition that satisfies a specific relationship, and the volume ratio is 45% or more of tempered martensite phase, 20 to 40% ferrite phase, and more than 10% but not more than 25% retained austenite phase. As a result, it is assumed that a high-strength stainless steel seamless steel pipe for oil wells that exhibits a yield strength (YS) of 862 MPa or more and sufficient corrosion resistance even in a high-temperature, severely corrosive environment containing CO ₂ , Cl ⁻ , and H ₂ S can be obtained. there is

In addition, in Patent Document 3, in mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, P: 0.030 % or less, S: 0.005% or less, Cr: 12.0-17.0%, Ni: 4.0-7.0%, Mo: 0.5-3.0%, Al: 0.005- 0.10%, V: 0.005-0.20%, Co: 0.01-1.0%, N: 0.005-0.15%, O: 0.010% or less, Cr , Ni, Mo, Cu, C, Si, Mn, and N satisfy a specific relationship.

In addition, in Patent Document 4, in mass%, 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: 14.5-17.5%, Ni: 3.0-6.0%, Mo: 2.7-5.0%, Cu: 0.3-4.0%, W : 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, N: 0.15% or less, C, Si, Mn, Cr, Ni, Mo, Cu, N, and W have a composition that satisfies a specific relationship, and the volume fraction is more than 45% martensite phase as the main phase, 10 to 45% ferrite phase as the second phase, and a retained austenite phase. A high-strength stainless steel seamless steel pipe for oil wells having a structure containing 30% or less of As a result, it is assumed that a high-strength stainless steel seamless steel pipe for oil wells that exhibits a yield strength (YS) of 862 MPa or more and sufficient corrosion resistance even in a high-temperature, severely corrosive environment containing CO ₂ , Cl ⁻ , and H ₂ S can be obtained. there is

In addition, in Patent Document 5, in mass%, 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: 14.5-17.5%, Ni: 3.0-6.0%, Mo: 2.7-5.0%, Cu: 0.3-4.0%, W : 0.1-2.5%, V: 0.02-0.20%, Al: 0.10% or less, N: 0.15% or less, B: 0.0005-0.0100% , C, Si, Mn, Cr, Ni, Mo, Cu, N, and W have a composition that satisfies a specific relationship, and the volume fraction is more than 45% with a martensite phase as a main phase and a second phase as A high-strength seamless stainless steel pipe for oil wells having a structure containing 10 to 45% ferrite phase and 30% or less retained austenite phase is described. As a result, it is assumed that a high-strength stainless steel seamless steel pipe for oil wells that exhibits a yield strength (YS) of 862 MPa or more and sufficient corrosion resistance even in a high-temperature, severely corrosive environment containing CO ₂ , Cl ⁻ , and H ₂ S can be obtained. there is

WO2013/146046 WO2017/138050 WO2017/168874 WO2018/020886 WO2018/155041

As described above, as the development of oil wells at even higher temperatures continues, oil well steel pipes are expected to have high strength and excellent resistance to carbonation at high temperatures and in severe corrosive environments containing CO ₂ and Cl ⁻ . It is now desired to have both gas corrosion resistance and excellent resistance to sulfide stress cracking (SSC resistance).

In addition, when used at high temperatures, strength at high temperatures (high temperature strength) may also be required. Specifically, the ratio of the yield stress (0.2% yield strength) at 200° C. to the yield stress (0.2% yield strength) at room temperature may be required to be 0.85 or more.

Patent Documents 1 to 5 disclose stainless steels with improved corrosion resistance, but there are cases where the combination of corrosion resistance at high temperatures, high sulfide stress cracking resistance, and high high-temperature strength is not sufficiently achieved. there were.

The present invention solves the problems of the prior art, and provides a seamless stainless steel pipe having a yield strength of 758 MPa (110 ksi) or more, excellent corrosion resistance, and excellent high-temperature strength, and a method for producing the same. intended to provide

Here, "excellent corrosion resistance" means the case of having "excellent carbon dioxide corrosion resistance" and "excellent sulfide stress cracking resistance".

The term “excellent carbon dioxide gas corrosion resistance” as used herein means that a test piece is placed in a test liquid: 20% by mass NaCl aqueous solution (liquid temperature: 200° C., 30 atm _CO2 gas atmosphere) held in an autoclave. The corrosion rate is 0.127 mm/y or less when immersed for 336 hours.

In addition, the term "excellent sulfide stress cracking resistance" as used herein refers to the test liquid held in the autoclave: 0.165% by mass NaCl aqueous solution (liquid temperature: 25 ° C., 0.99 atm CO ₂ gas, 0.01 atm H ₂ S atmosphere), acetic acid + sodium acetate is added to adjust the pH to 3.0, the test piece is immersed in an aqueous solution, and exposed for 720 hours under a load of 90% of the yield stress, It refers to the case where no fracture or crack occurs in the test piece after the test.

In addition, the term "excellent high-temperature strength" as used herein refers to a tensile test in accordance with JIS Z 2241 and a high-temperature tensile test in accordance with JIS G 0567, yield stress at room temperature (0.2% The ratio of the yield stress (0.2% yield strength) at 200°C to the yield strength) is 0.85 or more.
In addition, the method of each test described above is also described in detail in Examples described later.

In order to achieve the above object, the present inventors have extensively studied various factors affecting the high-temperature strength and corrosion resistance of stainless steel. As a result, by containing V in a predetermined amount or more, excellent high-temperature strength was obtained. In addition to containing a predetermined amount or more of Cr, Mo, and Cu, the Mn content is set to a certain amount or less, thereby providing excellent corrosion resistance (excellent carbon dioxide corrosion resistance and excellent sulfide stress cracking resistance). was gotten.

The present invention has been completed based on these findings and further studies. That is, the gist of the present invention is as follows.
[1] in % by mass,
C: 0.06% or less,
Si: 1.0% or less,
Mn: 0.01% or more and 0.90% or less,
P: 0.05% or less,
S: 0.005% or less,
Cr: 15.70% or more and 18.00% or less,
Mo: 1.60% or more and 3.80% or less,
Cu: 1.10% or more and 4.00% or less,
Ni: 3.0% or more and 6.0% or less,
Al: 0.10% or less,
N: 0.10% or less,
O: 0.010% or less,
V: 0.120% or more and 1.000% or less
and C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1),
Having a component composition in which the balance is Fe and unavoidable impurities,
Having a structure containing 30% or more of martensite phase, 60% or less of ferrite phase, and 40% or less of retained austenite phase in volume fraction,
A stainless seamless steel pipe having a yield strength of 758 MPa or more.
Record
13.0 ≤ -5.9 x (7.82 + 27C - 0.91Si + 0.21Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) ≤ 50.0 (1)
Here, C, Si, Mn, Cr, Ni, Mo, Cu, and N: content (% by mass) of each element, and zero if not contained.
[2] The stainless steel seamless pipe according to [1], wherein the martensite phase is 40% or more by volume, the retained austenite phase is 30% or less by volume, and the yield strength is 862 MPa or more.
[3] The seamless stainless steel according to [1] or [2], which further contains, in % by mass, one or more groups selected from Groups A to D below in addition to the component composition. steel pipe.
Group A: W: 3.0% or less Group B: Nb: less than 0.10% Group C: B: 0.010% or less Ta: 0.3% or less Co: 1.5% or less Ti: 0 .3% or less Zr: one or more selected from 0.3% or less Group D: Ca: 0.01% or less REM: 0.3% or less Mg: 0.01% or less , Sn: 1.0% or less, Sb: 1.0% or less [4] The stainless seamless steel pipe according to any one of [1] to [3]. A manufacturing method of
A steel pipe material having the above chemical composition is heated at a temperature in the range of 1100 to 1350° C. and hot-worked to form a seamless steel pipe,
Next, the seamless steel pipe is reheated to a heating temperature in the range of 850 to 1150°C, and quenched by cooling to a cooling stop temperature of 50°C or less at a cooling rate higher than that of air cooling,
Thereafter, tempering temperature: A method for producing a stainless seamless steel pipe, which is tempered by heating to a temperature in the range of 500 to 650°C.

According to the present invention, a seamless stainless steel pipe having a yield strength of 758 MPa (110 ksi) or more, excellent corrosion resistance, and excellent high-temperature strength, and a method for producing the same can be obtained.

The present invention will be described in detail below.

The stainless steel seamless pipe of the present invention has, in % by mass, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 0.90% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.70% or more and 18.00% or less, Mo: 1.60% or more and 3.80% or less, Cu: 1.10% or more and 4.00% or less, Ni: 3 0% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, V: 0.120% or more and 1.000% or less, And C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), the balance is Fe and inevitable impurities, and the volume ratio is 30% or more martensite phase, 60% or less ferrite phase, and 40% or less retained austenite phase, and the yield strength is 758 MPa or more.
13.0 ≤ -5.9 x (7.82 + 27C - 0.91Si + 0.21Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) ≤ 50.0 (1)
Here, C, Si, Mn, Cr, Ni, Mo, Cu, and N: content (% by mass) of each element, and zero if not contained.

First, the reasons for limiting the chemical composition of the seamless stainless steel pipe of the present invention will be explained. Hereinafter, unless otherwise specified, "% by mass" is simply written as "%".

C: 0.06% or less C is an element inevitably contained in the steelmaking process. If the C content exceeds 0.06%, the corrosion resistance is lowered. Therefore, the C content should be 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.04% or less, still more preferably 0.03% or less. Considering decarburization cost, the lower limit of the C content is preferably 0.002%, more preferably 0.003% or more.

Si: 1.0% or less Si is an element that acts as a deoxidizing agent. However, if the Si content exceeds 1.0%, the hot workability and corrosion resistance deteriorate. Therefore, the Si content is set to 1.0% or less. The Si content is preferably 0.7% or less, more preferably 0.5% or less, and still more preferably 0.4% or less. Although there is no particular lower limit as long as the deoxidizing effect can be obtained, the Si content is preferably 0.03% or more, more preferably 0.05% or more, in order to obtain a sufficient deoxidizing effect. be.

Mn: 0.01% to 0.90% Mn is an element that acts as a deoxidizing agent and a desulfurizing agent to improve hot workability. In order to obtain the effects of deoxidation and desulfurization and improve the strength, the Mn content is set to 0.01% or more. On the other hand, if the Mn content exceeds 0.90%, the sulfide stress cracking resistance is lowered. Therefore, the Mn content should be 0.01% or more and 0.90% or less. The Mn content is preferably 0.03% or more, more preferably 0.05% or more. Also, the Mn content is preferably 0.7% or less, more preferably 0.5% or less, and still more preferably 0.4% or less.

P: 0.05% or less P is an element that lowers carbon dioxide corrosion resistance and sulfide stress cracking resistance, and is preferably reduced as much as possible in the present invention, but 0.05% or less is acceptable. . Therefore, the P content should be 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less, still more preferably 0.02% or less.

S: 0.005% or less S is an element that significantly lowers hot workability and hinders stable operation of the hot pipe-making process. In addition, S exists as sulfide-based inclusions in steel and lowers the resistance to sulfide stress cracking. Therefore, it is preferable to reduce S as much as possible, but 0.005% or less is permissible. Therefore, the S content should be 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less, still more preferably 0.002% or less.

Cr: 15.70% or more and 18.00% or less Cr is an element that forms a protective film on the steel pipe surface and contributes to improving corrosion resistance. and sulfide stress cracking resistance cannot be ensured. Therefore, it is necessary to contain 15.70% or more of Cr. On the other hand, if the Cr content exceeds 18.00%, the ferrite fraction becomes too high and the desired strength cannot be secured. Therefore, the Cr content is set to 15.70% or more and 18.00% or less. The Cr content is preferably 16.00% or more, more preferably 16.30% or more. Also, the Cr content is preferably 17.50% or less, more preferably 17.00% or less.

Mo ^: 1.60% to 3.80% Increase stress crack resistance. In order to obtain desired corrosion resistance, it is necessary to contain 1.60% or more of Mo. On the other hand, if Mo is added in excess of 3.80%, the ferrite fraction becomes too high and the desired strength cannot be secured. Therefore, the Mo content should be 1.60% or more and 3.80% or less. The Mo content is preferably 1.80% or more, more preferably 2.00% or more. Also, the Mo content is preferably 3.5% or less, more preferably 3.0% or less, and still more preferably 2.8% or less.

Cu: 1.10% or more and 4.00% or less Cu has the effect of strengthening the protective film on the surface of the steel pipe and increasing the carbon dioxide corrosion resistance and the sulfide stress cracking resistance. In order to obtain the desired strength and corrosion resistance, especially carbon dioxide corrosion resistance, it is necessary to contain 1.10% or more of Cu. On the other hand, if the Cu content is too large, the hot workability of the steel is lowered, so the Cu content is made 4.00% or less. Therefore, the Cu content is set to 1.10% or more and 4.00% or less. The Cu content is preferably 1.80% or more, more preferably 2.00% or more. Also, the Cu content is preferably 3.20% or less, more preferably 3.00% or less, and still more preferably 2.7% or less.

Ni: 3.0% or more and 6.0% or less Ni increases the strength of steel through solid-solution strengthening and improves the toughness of steel. In order to secure the toughness required for oil country tubular goods, it is necessary to contain 3.0% or more of Ni. On the other hand, if the Ni content exceeds 6.0%, the stability of the martensite phase is lowered, and the strength is lowered. Therefore, the Ni content is set to 3.0% or more and 6.0% or less. The Ni content is preferably 3.5% or more, more preferably 4.0% or more, still more preferably 4.5% or more. Also, the Ni content is preferably 5.5% or less, more preferably 5.2% or less.

Al: 0.10% or less Al is an element that acts as a deoxidizing agent. However, when the Al content exceeds 0.10%, the corrosion resistance is lowered. Therefore, the Al content is set to 0.10% or less. The Al content is preferably 0.07% or less, more preferably 0.05% or less, still more preferably 0.04% or less. Although there is no particular lower limit as long as the deoxidizing effect is obtained, the Al content is preferably 0.005% or more, more preferably 0.01% or more, in order to obtain a sufficient deoxidizing effect. be.

N: 0.10% or less N is an element that is unavoidably contained in the steelmaking process, but it is also an element that increases the strength of steel. However, if the N content exceeds 0.10%, nitrides are formed and the corrosion resistance is lowered. Therefore, the N content is set to 0.10% or less. The N content is preferably 0.08% or less, more preferably 0.05% or less, still more preferably 0.03% or less. There is no particular lower limit for the N content, but an extreme reduction in the N content invites an increase in steelmaking costs. Therefore, the N content is preferably 0.002% or more, more preferably 0.003% or more.

O: 0.010% or less O (oxygen) exists as an oxide in steel, and thus has an adverse effect on various properties. Therefore, in the present invention, it is desirable to reduce it as much as possible. In particular, when the O content exceeds 0.010%, the hot workability and corrosion resistance deteriorate. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.005% or less.

V: 0.120% or more and 1.000% or less V is an important element in the present invention that improves high-temperature strength. V forms carbonitrides, and due to precipitation strengthening, high strength can be obtained not only at room temperature but also at high temperatures. In order to obtain the desired high temperature strength, the V content is 0.120% or more. On the other hand, even if the V content exceeds 1.000%, the effect is saturated. Therefore, in the present invention, the V content is set to 0.120% or more and 1.000% or less. Also, the V content is preferably 0.180% or more, more preferably 0.250% or more, and still more preferably 0.300% or more. Also, the V content is preferably 0.500% or less, more preferably 0.400% or less, and still more preferably 0.300% or less.

In the present invention, in addition to satisfying the above component composition, C, Si, Mn, Cr, Ni, Mo, Cu, and N are contained so as to satisfy the following formula (1).
13.0 ≤ -5.9 x (7.82 + 27C - 0.91Si + 0.21Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) ≤ 50.0 (1)
Here, C, Si, Mn, Cr, Ni, Mo, Cu, and N: content (% by mass) of each element, and zero if not contained.
"-5.9 x (7.82 + 27C - 0.91Si + 0.21Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N)" of formula (1) (hereinafter simply referred to as "middle polynomial of formula (1)" or "median value") is obtained as an index showing the tendency of ferrite phase formation. If the alloying elements shown in the formula (1) are adjusted so as to satisfy the formula (1), a composite structure consisting of a martensite phase and a ferrite phase, or further a retained austenite phase can be stably realized. can be done. When the alloying element described in the formula (1) is not contained, the value of the polynomial in the middle of the formula (1) shall be treated as the content of the element concerned as 0%.

If the value of the middle polynomial in the above formula (1) is less than 13.0, the ferrite phase will decrease in the hot working temperature range, resulting in a decrease in production yield. On the other hand, if the value of the middle polynomial in the above formula (1) exceeds 50.0, the volume fraction of the ferrite phase exceeds 60%, making it impossible to ensure the desired strength. Therefore, in the formula (1) defined in the present invention, the lower limit left-side value is set to 13.0, and the upper limit right-side value is set to 50.0. The lower limit of the left-side value of formula (1) defined in the present invention is preferably 15.0, more preferably 20.0. Also, the right-side value is preferably 45.0, more preferably 40.0. That is, the value of the central polynomial in formula (1) should be 13.0 or more and 50.0 or less. Preferably, it is 15.0 or more and 45.0 or less. More preferably, it is 20.0 or more and 40.0 or less.

In the present invention, the balance other than the component composition described above consists of Fe and unavoidable impurities.

With the above essential elements, the stainless steel seamless pipe of the present invention can obtain the intended properties. In the present invention, for the purpose of further improving the characteristics, in addition to the above-described basic composition, the following selective elements (W, Nb, B, Ta, Co, Ti, Zr, Ca , REM, Mg, Sn, Sb) may be contained.

Specifically, in the present invention, W: 3.0% or less can be contained in addition to the above component composition.
Further, in the present invention, in addition to the component composition described above, Nb: less than 0.10% can be contained.
Furthermore, in the present invention, in addition to the above component composition, B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less. It can contain one or more selected from 3% or less.
Furthermore, in the present invention, in addition to the component composition described above, Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less. One or more selected from 0% or less can be contained.

W: 3.0% or less W is an element that contributes to the improvement of the strength of steel and stabilizes the protective film on the surface of the steel pipe, thereby enhancing carbon dioxide corrosion resistance and sulfide stress cracking resistance. . When W is contained in combination with Mo, the corrosion resistance is remarkably improved. W can be contained as necessary in order to obtain the above effects. On the other hand, even if W content exceeds 3.0%, the effect is saturated. Therefore, when W is contained, the W content is preferably 3.0% or less. The W content is more preferably less than 1.5%, still more preferably 1.0% or less. Moreover, when W is contained, the W content is more preferably 0.05% or more, and still more preferably 0.10% or more.

Nb: less than 0.10% Nb is an element that increases strength and improves corrosion resistance, and can be contained as necessary. On the other hand, if the Nb content is 0.10% or more, desired high-temperature strength may not be obtained. Therefore, when Nb is contained, the Nb content is preferably less than 0.10%. The Nb content is more preferably 0.05% or less, still more preferably 0.03% or less. Also, the Nb content is more preferably 0.005% or more, and still more preferably 0.010% or more.

B: 0.010% or less B is an element that increases strength and can be contained as necessary. B also contributes to the improvement of hot workability, and has the effect of suppressing the occurrence of cracks and fissures in the tube-making process. On the other hand, even if the content of B exceeds 0.010%, not only does the effect of improving hot workability hardly appear, but also the low temperature toughness decreases. Therefore, when B is contained, the B content is preferably 0.010% or less. The B content is more preferably 0.008% or less, still more preferably 0.007% or less. Also, the B content is more preferably 0.0005% or more, and still more preferably 0.0010% or more.

Ta: 0.3% or less Ta is an element that increases strength and improves corrosion resistance, and can be contained as necessary. In order to obtain such effects, it is preferable to contain 0.001% or more of Ta. On the other hand, even if the content of Ta exceeds 0.3%, the effect is saturated. Therefore, when Ta is contained, it is preferable to limit the Ta content to 0.3% or less. The Ta content is more preferably 0.25% or less, more preferably 0.06% or less, more preferably 0.050% or less, still more preferably 0.025% or less. More preferably, it is 0.005% or more.

Co: 1.5% or less Co is an element that increases strength and can be contained as necessary. In addition to the effects described above, Co also has the effect of improving corrosion resistance. In order to obtain such effects, the Co content is preferably 0.0005% or more. The Co content is more preferably 0.005% or more, still more preferably 0.010% or more. On the other hand, even if the content of Co exceeds 1.5%, the effect is saturated. Therefore, when Co is contained, it is preferable to limit the Co content to 1.5% or less. The Co content is more preferably less than 0.150%.

Ti: 0.3% or less Ti is an element that increases strength and can be contained as necessary. In order to obtain such effects, it is preferable to contain 0.0005% or more of Ti. On the other hand, if the Ti content exceeds 0.3%, the toughness is lowered. Therefore, when Ti is contained, it is preferable to limit the Ti content to 0.3% or less.

Zr: 0.3% or less Zr is an element that increases strength and can be contained as necessary. In addition to the effects described above, Zr also has the effect of improving sulfide stress cracking resistance. In order to obtain such effects, it is preferable to contain 0.0005% or more of Zr. On the other hand, even if the Zr content exceeds 0.3%, the effect is saturated. Therefore, when Zr is contained, it is preferable to limit the Zr content to 0.3% or less.

Ca: 0.01% or less Ca is an element that contributes to the improvement of sulfide stress cracking resistance through morphology control of sulfides, and can be contained as necessary. In order to obtain such effects, it is preferable to contain 0.0005% or more of Ca. On the other hand, even if the content of Ca exceeds 0.01%, the effect saturates, and the effect commensurate with the content cannot be expected. Therefore, when Ca is contained, it is preferable to limit the Ca content to 0.01% or less.

REM: 0.3% or less REM (rare earth metal) is an element that contributes to improvement of sulfide stress cracking resistance through morphology control of sulfides, and can be contained as necessary. In order to obtain such effects, it is preferable to contain 0.0005% or more of REM. On the other hand, even if the content of REM exceeds 0.3%, the effect is saturated, and the effect corresponding to the content cannot be expected. Therefore, when REM is contained, it is preferable to limit the REM content to 0.3% or less.
In the present invention, REM includes scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. Lanthanoids. The REM concentration in the present invention is the total content of one or more elements selected from the above REMs.

Mg: 0.01% or less Mg is an element that improves corrosion resistance and can be contained as necessary. In order to obtain such effects, it is preferable to contain 0.0005% or more of Mg. On the other hand, even if the content of Mg exceeds 0.01%, the effect is saturated, and the effect corresponding to the content cannot be expected. Therefore, when Mg is contained, it is preferable to limit the Mg content to 0.01% or less.

Sn: 1.0% or less Sn is an element that improves corrosion resistance and can be contained as necessary. In order to obtain such effects, it is preferable to contain 0.001% or more of Sn. On the other hand, even if the Sn content exceeds 1.0%, the effect is saturated, and the effect corresponding to the content cannot be expected. Therefore, when Sn is contained, it is preferable to limit the Sn content to 1.0% or less.

Sb: 1.0% or less Sb is an element that improves corrosion resistance and can be contained as necessary. In order to obtain such an effect, it is preferable to contain 0.001% or more of Sb. On the other hand, even if the content of Sb exceeds 1.0%, the effect is saturated, and the effect corresponding to the content cannot be expected. Therefore, when Sb is contained, it is preferable to limit the Sb content to 1.0% or less.

Next, the reasons for limiting the structure of the seamless stainless steel pipe of the present invention will be described.

The stainless seamless steel pipe of the present invention has the above composition and has a structure containing 30% or more of the martensite phase, 60% or less of the ferrite phase, and 40% or less of the retained austenite phase in terms of volume fraction. have.

In the seamless stainless steel pipe of the present invention, the martensite phase is 30% or more by volume in order to ensure the desired strength. The martensite phase is preferably 40% or more. The martensite phase is preferably 70% or less, more preferably 65% or less.

Moreover, in the present invention, a ferrite phase of 60% or less by volume is included. When ferrite phase is contained, the progress of sulfide stress cracking can be suppressed, and excellent corrosion resistance can be obtained. On the other hand, if a large amount of ferrite phase precipitates exceeding 60% in volume ratio, it may become impossible to ensure the desired strength. Preferably, the ferrite phase is 5% or more by volume. More preferably, it is 10% or more. Moreover, preferably, the ferrite phase is 50% or less in volume ratio. More preferably, it is 45% or less.

Furthermore, in the present invention, in addition to the martensite phase and the ferrite phase, the austenite phase (retained austenite phase) with a volume fraction of 40% or less is included. The presence of retained austenite phase improves ductility and toughness. On the other hand, if a large amount of austenite phase precipitates exceeding 40% by volume, the desired strength cannot be ensured. Therefore, the volume fraction of the retained austenite phase is set to 40% or less. Preferably, the retained austenite phase is at least 5% by volume. Also preferably, the volume fraction of the retained austenite phase is 35% or less. More preferably, the volume fraction of the retained austenite phase is 30% or less.

Here, the measurement of the structure of the seamless stainless steel pipe of the present invention can be performed by the following method. First, a test piece for tissue observation was corroded with Vilera's reagent (picric acid, hydrochloric acid, and ethanol mixed at a ratio of 2 g, 10 ml, and 100 ml, respectively), and the tissue was imaged with a scanning electron microscope (magnification: 1000 times). , an image analyzer is used to calculate the structure fraction (area ratio (%)) of the ferrite phase. This area ratio is defined as the volume ratio (%) of the ferrite phase.

Then, the X-ray diffraction test piece was ground and polished so that the cross section perpendicular to the tube axis direction (C cross section) was the measurement surface, and the structure of the retained austenite (γ) phase was analyzed using the X-ray diffraction method. measure the rate. The structure fraction of the retained austenite phase is obtained by measuring the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α (ferrite) and converting it using the following formula.
γ (volume ratio) = 100/(1 + (IαRγ/IγRα))
Here, Iα: integrated intensity of α, Rα: theoretical crystallographically calculated value of α, Iγ: integrated intensity of γ, and Rγ: theoretically calculated crystallographic value of γ.

The remainder other than the ferrite phase and the residual γ phase determined by the above measuring method is defined as the fraction of the martensite phase.

A preferred method for manufacturing the seamless stainless steel pipe of the present invention will be described below.

It is preferable to melt the molten steel having the chemical composition described above by a conventional melting method such as a converter, and to make a steel pipe material such as a billet by a normal method such as continuous casting method, ingot casting-slabbing rolling method, or the like. The heating temperature of the steel pipe material before hot working is preferably 1100 to 1350°C. This makes it possible to achieve both hot workability during pipe making and low-temperature toughness of the final product.
Then, the obtained steel pipe material is hot-worked using a pipe-making process of the Mannesmann-plug mill method or the Mannesmann-mandrel mill method, which is a generally known pipe-making method, to make a pipe of a desired size. A seamless steel pipe having the composition described above. A cooling treatment may be performed after the hot working. This cooling treatment (cooling process) does not have to be particularly limited. Within the range of the component composition of the present invention described above, it is preferable to cool to room temperature at a cooling rate similar to that of air cooling after hot working.

In the present invention, the obtained seamless steel pipe is further subjected to heat treatment including quenching treatment and tempering treatment.

The quenching process is a process of reheating to a temperature in the range of 850 to 1150° C. and then cooling at a cooling rate faster than air cooling. The cooling stop temperature at this time is 50° C. or less as the surface temperature of the seamless steel pipe.

If the heating temperature is less than 850° C., the reverse transformation from martensite to austenite does not occur, and the transformation from austenite to martensite does not occur during cooling, making it impossible to ensure the desired strength. On the other hand, if the heating temperature exceeds 1150° C., the crystal grains become coarse. Therefore, the heating temperature for the quenching treatment is set to a temperature in the range of 850 to 1150.degree. Preferably, the heating temperature for the quenching treatment is 900° C. or higher. Preferably, the heating temperature for the hardening treatment is 1100° C. or lower. Further, if the cooling stop temperature exceeds 50° C., transformation from austenite to martensite does not occur sufficiently, and the retained austenite fraction becomes excessive. Therefore, in the present invention, the cooling stop temperature in cooling in the quenching treatment is set to 50° C. or lower. Here, the “cooling rate equal to or higher than air cooling” is 0.01° C./s or higher.

Further, in the quenching treatment, the soaking time is preferably 5 to 30 minutes in order to uniformize the temperature in the thickness direction and prevent the material from fluctuating.

The tempering treatment is to heat the quenched seamless steel pipe to a tempering temperature of 500 to 650°C. Moreover, after this heating, it can be left to cool.

If the tempering temperature is less than 500°C, the temperature is too low and the desired tempering effect cannot be expected. On the other hand, when the tempering temperature exceeds 650° C., intermetallic compounds precipitate and excellent low-temperature toughness cannot be obtained. Therefore, the tempering temperature should be in the range of 500 to 650°C. Preferably, the tempering temperature is 520°C or higher. Preferably, the tempering temperature is 630°C or less.

Further, in the tempering treatment, the holding time (soaking holding time) is preferably 5 to 90 minutes in order to uniform the temperature in the thickness direction and prevent the material from fluctuating.

By performing the heat treatment (quenching treatment and tempering treatment) described above, the structure of the seamless steel pipe becomes a structure containing a martensite phase, a ferrite phase, and a retained austenite phase specified by a predetermined volume fraction. As a result, a seamless stainless steel pipe having desired strength and excellent corrosion resistance can be obtained.

As described above, the stainless seamless steel pipe obtained by the present invention is a high-strength steel pipe having a yield strength of 758 MPa or more, and has excellent corrosion resistance and high-temperature strength. Preferably, the yield strength is 862 MPa or more. Preferably, the yield strength is 1034 MPa or less. The seamless stainless steel pipe of the present invention can be used as a seamless stainless steel pipe for oil wells (high-strength seamless stainless steel pipe for oil wells).

The present invention will be further described below based on examples. In addition, the present invention is not limited to the following examples.

A steel pipe material was cast using molten steel having the composition shown in Tables 1-1 and 1-2. Thereafter, the steel pipe material was heated and hot-rolled using a model seamless rolling mill to form a seamless steel pipe having an outer diameter of 83.8 mm and a wall thickness of 12.7 mm, followed by air cooling. At this time, the heating temperature of the steel pipe material before hot working was set to 1250°C.

A test piece material was cut out from the obtained seamless steel pipe, reheated to a heating temperature of 960°C, soaked for 20 minutes, and quenched by cooling (water cooling) to a cooling stop temperature of 30°C. . Then, the heating temperature (tempering temperature) is 575°C for a soaking time of 20 minutes, or the heating temperature (tempering temperature) is 525°C for a soaking time of 20 minutes, or the heating temperature (tempering temperature) is 620°C. The steel sheet was subjected to a soaking holding time of 40 minutes and then air-cooled for tempering. The cooling rate for water cooling during quenching treatment was 11° C./s, and the cooling rate for air cooling (air cooling) during tempering treatment was 0.04° C./s. The blanks in Tables 1-1 and 1-2 indicate that they are not intentionally added, and include not only the case of no content (0%) but also the case of unavoidable inclusion.

Each test piece was taken from the obtained heat-treated test piece material (seamless steel pipe), and a structure observation, a tensile test, a high-temperature tensile test, and a corrosion resistance test were performed. The test method was as follows.

(1) Observation of Structure From the obtained heat-treated test material, a test piece for observing structure was taken so that the cross section orthogonal to the tube axis direction was the observation surface. The specimen for tissue observation obtained was corroded with Vilera's reagent (picric acid, hydrochloric acid and ethanol mixed at a ratio of 2 g, 10 ml and 100 ml, respectively), and the tissue was imaged with a scanning electron microscope (magnification: 1000 times). Then, using an image analyzer, the ferrite phase structure fraction (area ratio (%)) was calculated. This area ratio was taken as the volume ratio (%) of the ferrite phase.

In addition, an X-ray diffraction test piece is taken from the obtained heat-treated test material, ground and polished so that the cross section (C section) perpendicular to the tube axis direction becomes the measurement surface, and the X-ray diffraction method is performed. was used to measure the structural fraction of the retained austenite (γ) phase. The structure fraction of the retained austenite phase was obtained by measuring the diffraction X-ray integrated intensity of the (220) plane of γ and the (211) plane of α (ferrite) and converting it using the following formula.
γ (volume ratio) = 100/(1 + (IαRγ/IγRα))
Here, Iα: integrated intensity of α, Rα: theoretical crystallographically calculated value of α, Iγ: integrated intensity of γ, and Rγ: theoretically calculated crystallographic value of γ.
The fraction of the martensite phase is the remainder other than the ferrite phase and the residual γ phase.

(2) Tensile test From the obtained heat-treated test material, a rod-shaped test piece was taken so that the tube axis direction was the tensile direction, and a tensile test was performed in accordance with the provisions of JIS Z 2241 (2011). Yield stress (YS) was determined as 0.2% yield strength. Here, those with a yield strength YS of 758 MPa or more were regarded as having high strength and were regarded as acceptable, and those with a yield strength of less than 758 MPa were regarded as unacceptable.

(3) High-temperature tensile test A rod-shaped test piece was taken from the obtained heat-treated test material so that the tube axis direction was the tensile direction, and a tensile test was performed at 200 ° C. in accordance with the provisions of JIS G 0567 (2012). was carried out and the yield stress (YS) was determined as 0.2% proof stress. Here, the same steel is subjected to the same heat treatment, and the ratio of the 0.2% proof stress at 200 ° C. to the yield stress (0.2% proof stress) obtained in the tensile test of (2) is 0.85 or more. One case was considered a pass, and one less than 0.85 was considered a fail.

(4) Corrosion resistance test (carbon dioxide corrosion resistance test and sulfide stress cracking resistance test)
A corrosion test piece having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm was machined from the obtained heat-treated test material. A corrosion test was performed using the corrosion test piece to evaluate the carbon dioxide corrosion resistance.

A corrosion test for evaluating carbon dioxide corrosion resistance is performed by immersing the above corrosion test piece in a test liquid: 20 mass% NaCl aqueous solution (liquid temperature: 200 ° C., 30 atm CO ₂ gas atmosphere) held in an autoclave. and the immersion period was 14 days (336 hours). After the test, the weight of the test piece was measured, and the corrosion rate calculated from the weight loss before and after the corrosion test was obtained. A corrosion rate of 0.127 mm/y or less was accepted, and a corrosion rate exceeding 0.127 mm/y was rejected.

Furthermore, from the obtained test piece material, a round bar-shaped test piece (diameter: 3.81 mm) was produced by machining and subjected to a sulfide stress cracking test (SSC (Sulfide Stress Cracking) test).

_The SSC resistance test was carried out by adding _acetic acid The test piece is immersed in an aqueous solution adjusted to pH: 3.0 by adding sodium acetate, exposed for 720 hours under a load of 90% of the yield stress, and the presence or absence of breakage or cracking of the test piece after the test is observed. bottom. Those with no breakage or cracks were accepted (indicated by the symbol "○" in Tables 2-1 and 2-2), and those with breakage or cracks were rejected (in Tables 2-1 and 2-2, the symbol " ×”).

The results obtained are shown in Tables 2-1 and 2-2.

As shown in Tables 2-1 and 2-2, all of the examples of the present invention have a high yield strength YS of 758 MPa or more and a high temperature corrosive environment of 200 ° C. containing CO ₂ and Cl ^- The stainless seamless steel pipe had excellent corrosion resistance (carbon dioxide gas corrosion resistance), excellent sulfide stress cracking resistance, and excellent high-temperature strength.

Claims (4)

in % by mass,
C: 0.06% or less,
Si: 1.0% or less,
Mn: 0.01% or more and 0.90% or less,
P: 0.05% or less,
S: 0.005% or less,
Cr: 15.70% or more and 18.00% or less,
Mo: 1.60% or more and 3.80% or less,
Cu: 1.10% or more and 4.00% or less,
Ni: 3.0% or more and 6.0% or less,
Al: 0.10% or less,
N: 0.10% or less,
O: 0.010% or less,
V: 0.180% or more and 1.000% or less
and C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1),
Having a component composition in which the balance is Fe and unavoidable impurities,
Having a structure containing, in volume fraction, 30% or more martensite phase, 5% or more and 60% or less ferrite phase, and 40% or less retained austenite phase,
A stainless seamless steel pipe having a yield strength of 758 MPa or more.
13.0 ≤ -5.9 x (7.82 + 27C - 0.91Si + 0.21Mn - 0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N) ≤ 50.0 (1)
Here, C, Si, Mn, Cr, Ni, Mo, Cu, and N: content (% by mass) of each element, and zero if not contained. 2. The stainless steel seamless pipe according to claim 1, wherein the martensite phase is 40% or more by volume, the retained austenite phase is 30% or less by volume, and the yield strength is 862 MPa or more. 3. The stainless steel seamless pipe according to claim 1, further comprising, in mass %, one or more groups selected from Groups A to D below in addition to said component composition.
Group A: W: 3.0% or less Group B: Nb: less than 0.10% Group C: B: 0.010% or less Ta: 0.3% or less Ti : 0.3% or less Zr: One or more selected from 0.3% or less Group D: Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% Below, one or more selected from Sb: 1.0% or less A method for manufacturing a stainless steel seamless pipe according to any one of claims 1 to 3,
A steel pipe material having the above chemical composition is heated at a temperature in the range of 1100 to 1350° C. and hot-worked to form a seamless steel pipe,
Next, the seamless steel pipe is reheated to a heating temperature in the range of 850 to 1150°C, and quenched by cooling to a cooling stop temperature of 50°C or less at a cooling rate higher than that of air cooling,
Thereafter, tempering temperature: A method for producing a stainless seamless steel pipe, which is tempered by heating to a temperature in the range of 500 to 650°C.