Stainless Steel Seamless Pipe and Production Method Therefor

The present disclosure relates to a stainless steel seamless pipe suitable for use in oil wells and gas wells (hereafter simply referred to as oil wells). The present disclosure especially relates to a stainless steel seamless pipe with improved corrosion resistance in high-temperature severe corrosion environments containing carbon dioxide (CO) and chloride ions (Cl), environments containing hydrogen sulfide (HS), and the like.

Stainless steel seamless pipes are widely used for applications such as steel pipes for oil wells. Steel pipes for oil wells are required to have not only excellent yield stress but also excellent low-temperature toughness in view of recent oil field development in cold regions.

Given the possible depletion of energy resources in the near future, oil wells are actively developed in severe corrosion environments such as deep oil fields, environments containing carbon dioxide, and environments containing hydrogen sulfide called sour environments, which have not received much attention in the past. Hence, steel pipes for oil wells are also required to have high corrosion resistance.

Conventionally, 13Cr martensitic stainless steel pipes are commonly used as steel pipes for oil wells for mining in oil and gas fields in environments containing CO, Cl, etc. Recently, however, the development of oil wells of higher temperatures (up to 200° C.) is underway, and 13Cr martensitic stainless steel pipes sometimes lack corrosion resistance. There is thus a demand for a steel pipe for oil wells with higher corrosion resistance, which can be used even in such environments.

In response to this demand, for example, WO 2013/146046 A1 (PTL 1) proposes a stainless steel for oil wells having a composition containing, in mass %, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01% to 1.0%, P: 0.05% or less, S: less than 0.002%, Cr: 16% to 18%, Mo: 1.8% to 3%, Cu: 1.0% to 3.5%, Ni: 3.0% to 5.5%, Co: 0.01% to 1.0%, Al: 0.001% to 0.1%, O: 0.05% or less, and N: 0.05% or less where Cr, Ni, Mo, and Cu satisfy a specific relationship.

WO 2017/168874 A1 (PTL 2) proposes a high-strength stainless steel seamless pipe for oil wells having a composition containing, 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% 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%, and O: 0.010% or less where Cr, Ni, Mo, Cu, C, Si, Mn, and N satisfy a specific relationship.

WO 2018/155041 A1 (PTL 3) proposes a high-strength stainless steel seamless pipe for oil wells having: a composition containing, 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% to 17.5%, Ni: 3.0% to 6.0%, Mo: 2.7% to 5.0%, Cu: 0.3% to 4.0%, W: 0.1% to 2.5%, V: 0.02% to 0.20%, Al: 0.10% or less, N: 0.15% or less, and B: 0.0005% to 0.0100% where C, Si, Mn, Cr, Ni, Mo, Cu, N, and W satisfies a specific relationship; and a microstructure containing, in volume fraction, more than 45% martensite phase as a primary phase, 10% to 45% ferrite phase as a secondary phase, and 30% or less retained austenite phase. According to PTL 3, such a high-strength stainless steel seamless pipe for oil wells has a strength of 862 MPa or more in yield stress YS, and exhibits sufficient corrosion resistance even in high-temperature severe corrosion environments containing CO, Cl, and HS.

WO 2021/065263 A1 (PTL 4) proposes a high-strength stainless steel seamless pipe for oil wells having: a composition containing, in mass %, C: 0.06% or less, Si: 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: more than 15.7% and 18.0% or less, Mo: 1.8% or more and 3.5% or less, Cu: 1.5% or more and 3.5% or less, Ni: 2.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, W: 0.5% or more and 2.0% or less, and Co: 0.01% or more and 1.5% or less where C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a specific relationship; and a microstructure containing, in volume fraction, 25% or more martensite phase, 65% or less ferrite phase, and 40% or less retained austenite phase. According to PTL 4, such a high-strength stainless steel seamless pipe for oil wells has a strength of 758 MPa or more in yield stress YS, and exhibits sufficient corrosion resistance even in high-temperature severe corrosion environments containing CO, Cl, and HS.

WO 2021/187331 A1 (PTL 5) proposes a high-strength stainless steel seamless pipe for oil wells having: a composition containing, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.2% or more and 18.5% or less, Mo: 1.5% or more and 4.3% or less, Cu: 1.1% or more and 3.5% or less, Ni: 3.0% or more and 6.5% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, and Sb: 0.001% or more and 1.000% or less where C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a specific relationship; and a microstructure containing, in volume fraction, 30% or more martensite phase, 65% or less ferrite phase, and 40% or less retained austenite phase. According to PTL 5, such a high-strength stainless steel seamless pipe for oil wells has a strength of 758 MPa or more in yield stress YS, and exhibits sufficient corrosion resistance even in high-temperature severe corrosion environments containing CO, Cl, and HS.

The conventional techniques proposed in PTL 1 to PTL 5 can improve the corrosion resistance of stainless steel, but the performance is still insufficient.

As mentioned above, steel pipes for oil wells are required to have not only excellent yield stress and low-temperature toughness but also high corrosion resistance to withstand use in severe corrosion environments.

For example, steel pipes for oil wells are required to have excellent corrosion resistance in carbon dioxide environments (COcorrosion resistance), and particularly required to have excellent COcorrosion resistance even in high-temperature environments.

Steel pipes for oil wells are also required to have resistance (SSC resistance) to sulfide stress cracking (SSC) in hydrogen sulfide environments. Particularly in offshore oil fields, cold seawater has a high specific gravity and stays near the seabed, so that the pipes for oil wells are exposed to temperatures lower than the atmospheric temperature of the region. Accordingly, steel pipes for oil wells are required to have excellent SSC resistance even in low-temperature environments.

There are cases where, when extracting petroleum, the properties (mainly permeability) of the layer in which petroleum is stored (petroleum reservoir) are poor and sufficient production volume cannot be obtained, or the expected production volume cannot be obtained due to, for example, clogging in the reservoir. One way of improving productivity is acidizing, i.e. a treatment of injecting an acid such as hydrochloric acid into the reservoir. Steel pipes for oil wells are therefore also required to have excellent corrosion resistance in acid environments.

With the conventional techniques, however, it is impossible to obtain a steel pipe having sufficient levels of yield stress, low-temperature toughness, high-temperature COcorrosion resistance, low-temperature SSC resistance, and corrosion resistance in acid environments.

In particular, the technique described in PTL 5 is supposed to provide high strength, high-temperature corrosion resistance, and corrosion resistance in acid environments, but the SSC resistance is not necessarily sufficient. The reason for this is considered as follows: If the phase fraction during steel pipe production is not appropriate, the hot workability is insufficient, and cracking occurs on the inner and outer surfaces of the steel pipe. In the case where such a steel pipe is used in an oil well, corrosive ions stay inside the cracks and further concentrate with the progress of corrosion. This causes insufficient SSC resistance.

It could therefore be helpful to provide a stainless steel seamless pipe having high strength of 758 MPa (110 ksi) or more in yield stress and excellent low-temperature toughness and corrosion resistance.

In the present disclosure, “excellent corrosion resistance” means excellent in all of high-temperature COcorrosion resistance, low-temperature SSC resistance, and corrosion resistance in acid environments.

Herein, “excellent high-temperature COcorrosion resistance” means that the corrosion rate when a test piece is immersed in a test liquid: 20 mass % NaCl aqueous solution (liquid temperature: 200° C., 30 atm COgas atmosphere) held in an autoclave for an immersion time of 336 hours is 0.127 mm/y or less.

Moreover, “excellent low-temperature SSC resistance” means that, when a C-shaped test piece conforming to NACE TM0177 Method C is immersed in an aqueous solution obtained by adding acetic acid+sodium acetate to a 0.165 mass % NaCl aqueous solution (liquid temperature: 7° C., 0.995 atm COgas, 0.005 atm HS atmosphere) to adjust the pH to 3.0 for an immersion time of 720 hours under load of 100% of yield stress as load stress, there is no cracking in the test piece after the test.

Moreover, “excellent corrosion resistance in acid environments” means that the corrosion rate when a test piece is immersed in a 15 mass % hydrochloric acid solution heated to 80° C. for an immersion time of 40 minutes is 600 mm/y or less.

Moreover, “excellent low-temperature toughness” means that the Charpy absorbed energy vEat −10° C. is 40 J or more. The Charpy absorbed energy vEis measured by the following procedure. First, three V-notch test pieces (10 mm thick) whose longitudinal direction is perpendicular to the pipe axis and whose notch is on a plane perpendicular to the pipe axis are collected per one stainless steel seamless pipe in accordance with ASTM E23. These test pieces are then subjected to a Charpy impact test at a test temperature of −10° C., and the lowest value of the absorbed energies of the three test pieces is taken to be the Charpy absorbed energy vEat −10° C.

Upon careful examination on various factors influencing the corrosion resistance, especially the SSC resistance and the corrosion resistance in acid environments, of stainless steel, we discovered that excellent corrosion resistance can be achieved by containing at least predetermined amounts of Cr, Mo, Sb, Co, and Ca and limiting the amount of Ni, which affects the phase fraction of steel, to a predetermined range.

The present disclosure is based on these discoveries and further studies. We thus provide the following.

1. A stainless steel seamless pipe comprising: a chemical composition containing (consisting of), in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.2% or more and 18.0% or less, Mo: 1.5% or more and 4.3% or less, Cu: 1.2% or more and 3.5% or less, Ni: 3.5% or more and 5.2% or less, V: 0.5% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, Sb: 0.001% or more and 1.000% or less, Co: 0.01% or more and 1.00% or less, and Ca: 0.001% or more and 0.030% or less, with a balance consisting of Fe and inevitable impurities; 30% or more martensite phase, 50% or less ferrite phase, and 40% or less retained austenite phase in volume fraction; a yield stress of 758 MPa or more; and a Charpy absorbed energy vEat −10° C. of 40 J or more.

2. The stainless steel seamless pipe according to 1., wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of Nb: 0.07% or less, Ti: 0.2% or less, W: 0.9% or less, B: 0.01% or less, Ta: 0.3% or less, Zr: 0.3% or less, REM: 0.3% or less, Mg: 0.01% or less, and Sn: 1.0% or less.

3. The stainless steel seamless pipe according to 1, or 2., comprising: 50% or more martensite phase, 50% or less ferrite phase, and 25% or less retained austenite phase in volume fraction; and a yield stress of 862 MPa or more.

4. A production method for a stainless steel seamless pipe, the production method comprising: making a seamless steel pipe from a steel material having the chemical composition according to 1, or 2.; heating the seamless steel pipe to a quenching temperature of 850° C. to 1150° C.; cooling the seamless steel pipe after the heating to a cooling stop temperature of 50° C. or less at a cooling rate of 0.01° C./s or more; and heating the seamless steel pipe after the cooling to a tempering temperature of 500° C. to 650° C., to produce a stainless steel seamless pipe having: 30% or more martensite phase, 50% or less ferrite phase, and 40% or less retained austenite phase in volume fraction; a yield stress of 758 MPa or more; and a Charpy absorbed energy vE.at −10° C. of 40 J or more.

5. The production method for a stainless steel seamless pipe according to 4., wherein the stainless steel seamless pipe has: 50% or more martensite phase, 50% or less ferrite phase, and 25% or less retained austenite phase in volume fraction; and a yield stress of 862 MPa or more.

It is thus possible to provide a stainless steel seamless pipe having high strength of 758 MPa (110 ksi) or more in yield stress and excellent low-temperature toughness and corrosion resistance.

The presently disclosed techniques will be described in detail below.

A stainless steel seamless pipe according to the present disclosure has the foregoing chemical composition. First, the reasons for limiting the chemical composition will be described below. Hereafter, “mass %” is simply written as “%” unless otherwise noted.

C is an element inevitably contained in the steelmaking process. If the C content is more than 0.06%, the corrosion resistance decreases. The C content is therefore 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.04% or less, and further preferably 0.03% or less. Since the C content is desirably as low as possible from the viewpoint of the corrosion resistance, no lower limit is placed on the C content. From the viewpoint of decarburization cost, however, the C content is preferably 0.002% or more, more preferably 0.003% or more, and further preferably 0.005% or more.

Si is an element that acts as a deoxidizer. If the Si content is more than 1.0%, the hot workability and the corrosion resistance decrease. The Si content is therefore 1.0% or less, preferably 0.7% or less, more preferably 0.5% or less, and further preferably 0.4% or less. Although no lower limit is placed on the Si content, the Si content is preferably 0.03% or more, more preferably 0.05% or more, and further preferably 0.1% or more from the viewpoint of enhancing the deoxidizing effect.

Mn is an element that acts as a deoxidizing material and a desulfurizing material and improves the hot workability. To achieve its effect as a deoxidizing and desulfurizing material and improve the strength, the Mn content is 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more, and further preferably 0.1% or more. If the Mn content is more than 1.0%, the effect is saturated. The Mn content is therefore 1.0% or less, preferably 0.8% or less, more preferably 0.6% or less, and further preferably 0.4% or less.

P is an element that decreases the COcorrosion resistance and the SSC resistance. To achieve the desired corrosion resistance, the P content is 0.05% or less, preferably 0.04% or less, and more preferably 0.03% or less. Since it is desirable to reduce the P content as much as possible, no lower limit is placed on the P content, and the P content may be 0%. Since excessive reduction causes an increase in cost, the P content is preferably 0.005% or more and more preferably 0.010% or more from the viewpoint of cost.

S is an element that significantly decreases the hot workability and hinders stable operation in the hot pipe making process. Moreover, S exists as sulfide-based inclusions in the steel, and decreases the corrosion resistance. The S content is therefore 0.005% or less, preferably 0.004% or less, more preferably 0.003% or less, and further preferably 0.002% or less. Since it is desirable to reduce the S content as much as possible, no lower limit is placed on the S content, and the S content may be 0%. Since excessive reduction causes an increase in cost, the S content is preferably 0.0003% or more and more preferably 0.0005% or more from the viewpoint of cost.

Cr is an element that forms a protective coating on the steel pipe surface and contributes to improved corrosion resistance. If the Cr content is less than 15.2%, the desired COcorrosion resistance and SSC resistance cannot be ensured. The Cr content is therefore 15.2% or more, preferably 15.5% or more, more preferably 16.0% or more, and further preferably 16.30% or more. If the Cr content is more than 18.0%, the ferrite fraction is excessively high, and the desired strength cannot be ensured. The Cr content is therefore 18.0% or less, preferably 17.5% or less, more preferably 17.2% or less, and further preferably 17.0% or less.

Mo stabilizes the protective coating on the steel pipe surface and increases the resistance to pitting corrosion caused by Cland low pH, thus enhancing the corrosion resistance. To achieve the desired corrosion resistance, the Mo content is 1.5% or more, preferably 1.8% or more, more preferably 2.0% or more, and further preferably 2.3% or more. If the Mo content is more than 4.3%, the ferrite fraction is excessively high, and the desired strength cannot be ensured. The Mo content is therefore 4.3% or less, preferably 4.0% or less, more preferably 3.5% or less, and further preferably 3.0% or less.

Cu has the effect of enhancing the COcorrosion resistance and the SSC resistance by strengthening the protective coating on the steel pipe surface. To achieve the desired strength and corrosion resistance, in particular COcorrosion resistance, the Cu content is 1.2% or more, preferably 1.8% or more, more preferably 2.0% or more, and further preferably 2.3% or more. If the Cu content is excessively high, the hot workability of the steel decreases and outer surface flaws occur during pipe making, making it impossible to achieve the desired SSC resistance. The Cu content is therefore 3.5% or less, preferably 3.2% or less, more preferably 3.0% or less, and further preferably 2.7% or less.

Ni improves the low-temperature toughness of the steel. Ni also contributes to an increased austenite fraction, and thus influences the hot workability during hot rolling. To achieve the desired toughness, the Ni content is 3.5% or more, preferably 3.8% or more, more preferably 4.0% or more, and further preferably 4.3% or more. If the Ni content is more than 5.2%, the austenite fraction is excessively high, and the hot workability of the steel decreases. Consequently, flaws tend to occur during hot rolling, and the desired SSC resistance may not be achieved. The Ni content is therefore 5.2% or less, and preferably 5.0% or less.

V is an element that forms carbonitrides to thus increase the strength without impairing the toughness. V also has the effect of improving the corrosion resistance. This is because, as a result of V preferentially forming carbonitrides, corrosion-resistant elements such as Cr are prevented from forming carbonitrides and consequently a decrease in the amount effective for the corrosion resistance is suppressed. If the V content is more than 0.5%, the effect is saturated. The V content is therefore 0.5% or less, preferably 0.2% or less, and further preferably 0.1% or less. Although no lower limit is placed on the V content, the V content is preferably 0.01% or more, and more preferably 0.03% or more.

Al is an element that acts as a deoxidizer. If the Al content is more than 0.10%, the corrosion resistance decreases. The Al content is therefore 0.10% or less, preferably 0.07% or less, and more preferably 0.05% or less. Although no lower limit is placed on the Al content, the Al content is preferably 0.005% or more, more preferably 0.01% or more, and further preferably 0.015% or more from the viewpoint of enhancing the deoxidizing effect.

N is an element that is inevitably contained in the steelmaking process but also enhances the strength of the steel. If the N content is more than 0.10%, the amount of nitride formed is excessive and the corrosion resistance decreases. The N content is therefore 0.10% or less, preferably 0.07% or less, more preferably 0.05% or less, and further preferably 0.03% or less. Although no lower limit is placed on the N content, excessive reduction of the N content causes an increase in steelmaking cost. The N content is therefore preferably 0.002% or more, more preferably 0.003% or more, and further preferably 0.005% or more.

O (oxygen) exists as oxides in the steel, and accordingly adversely affects various properties. In the present disclosure, it is desirable to reduce the O content as much as possible. In particular, if the O content is more than 0.010%, the hot workability and the corrosion resistance decrease. The O content is therefore 0.010% or less. Since excessive reduction causes an increase in cost, the O content is preferably 0.00005% or more and more preferably 0.001% or more from the viewpoint of cost.

Sb is an element necessary to improve the corrosion resistance in acid environments. To achieve the desired corrosion resistance, the Sb content is 0.001% or more, preferably 0.003% or more, more preferably 0.005% or more, and further preferably 0.010% or more. If the Sb content is more than 1.000%, the effect is saturated. The Sb content is therefore 1.000% or less, preferably 0.500% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

Co is an element that improves the corrosion resistance. To achieve the desired corrosion resistance, the Co content is 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more. If the Co content is more than 1.00%, the effect is saturated. The Co content is therefore 1.00% or less, preferably 0.50% or less, more preferably 0.30% or less, and further preferably 0.10% or less.

Ca is an element that improves the hot workability through sulfide morphological control, and suppresses flaws during pipe making to thus contributes to improved SSC resistance of the steel pipe. To achieve the effect, the Ca content is 0.001% or more, preferably 0.003% or more, more preferably 0.005% or more, even more preferably more than 0.010%, further preferably 0.012% or more, and most preferably 0.014% or more. If the Ca content is more than 0.030%, the effect is saturated, and the effect commensurate with the amount cannot be expected. The Ca content is therefore 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less.

In one embodiment of the present disclosure, a stainless steel seamless pipe has a chemical composition containing the foregoing components with the balance consisting of Fe and inevitable impurities.