Stainless steel seamless pipe and method for producing same

Abstract

Provided are: a stainless-steel seamless pipe which exhibits high strength, namely a yield strength of 655 MPa or more, excellent CO2 corrosion resistance, and excellent hydrogen embrittlement resistance; and a method for producing same.　This stainless-steel seamless pipe: has a constituent composition that contains, in terms of mass%, 0.05% or less of C, 1.0% or less of Si, 0.01-1.0% of Mn, 0.05% or less of P, 0.005% or less of S, 12.0-14.0% of Cr, more than 3.0% and not more than 7.0% of Ni, more than 0.5% and not more than 3.0% of Mo, 0.10% or less of Al, 0.10% or less of N and 0.010% or less of O, with the remainder comprising Fe and unavoidable impurities; has a structure which contains, in terms of volume ratio, 70% or more of a martensite phase, 20% or less of a retained austenite phase and 10% or less of a ferrite phase and in which the average grain size of prior austenite is 50 μm or less and the maximum grain size of prior austenite is 100 μm or less; and has a yield strength of 655 MPa or more.

Description

Stainless steel seamless pipe and its manufacturing method

[0001] The present invention is ２ When CO is injected underground and isolated, ２ The present invention relates to a stainless steel seamless pipe suitable for use as a pressurized pipe for transporting hydrogen (H ２ ) and CO ２ The present invention relates to a stainless steel seamless pipe having improved resistance to hydrogen embrittlement in environments containing hydrogen.

[0002] As a measure against global warming, CO ２ In order to reduce the amount of CO emitted by industry into the atmosphere, ２ The social implementation of CCS (Carbon Capture and Storage), which captures CO and isolates it underground, is underway. ２ Stainless steel pipes are used to transport CO underground because underground formation water contains chloride ions. ２ When injected underground, CO ２ The acidity of the water dissolves in the water, creating a highly corrosive environment for steel pipes. Furthermore, the high temperatures caused by geothermal heat also exacerbate the corrosive effect. Even in such a high-temperature, acidic environment, corrosion is not a practical problem for stainless steel pipes.

[0003] By the way, CO ２ H that does not emit ２ Hydrogen is known as a clean energy source. Hydrogen produced using energy sources that do not use fossil fuels, such as wind power, does not emit CO ２ Hydrogen is the most effective alternative energy source for combating global warming due to its low emissions, but its high production costs are an issue. On the other hand, hydrogen can also be produced by reforming fossil fuels. This method has low production costs but produces CO as a by-product. ２ This CO ２ By capturing and sequestrating underground, CO ２ Without releasing ２ The H produced in this way can be produced. ２ is called blue hydrogen.

[0004] In the blue hydrogen production process, CO ２ Along with a small amount of H ２ is discharged. ２ The concentration is at most a few vol%. ２ is high pressure, so H ２ Partial pressure can lead to high pressure. ２ CO containing ２ Stainless steel pipes used to transport hydrogen may be at risk of hydrogen embrittlement.

[0005] CCS requires steel pipes to be airtight, and as the required characteristics and form are similar to those required when extracting oil or natural gas from underground, oil well steel pipes are used for CCS injection pipes. ２ The injection pipe must be long enough to reach the stratum where the water can be trapped, so it will be several thousand meters long. When a steel pipe of this length is suspended from the surface, its own weight is placed on the wellhead. In this case, the injection pipe must be strong enough to withstand this weight, so high-strength stainless steel is suitable.

[0006] To meet such demands, for example, UNS S42000 has been proposed. This steel is a martensitic stainless steel containing 13% by mass of Cr. Patent Document 1 discloses a stainless steel seamless pipe having a composition containing, in mass%, 0.015% or less of C, 1.0% or less of Si, 2.0% or less of Mn, 0.020% or less of P, 0.010% or less of S, 0.01 to 0.10% of Al, 10 to 14% of Cr, 3 to 8% or less of Ni, 0.03 to 0.15% of Ti, and 0.015% or less of N, and further containing one or two elements selected from 1 to 4% of Cu, 1 to 4% of Mo, 1 to 4% of W, and 1 to 4% of Co, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, Patent Document 2 states that high strength and excellent hydrogen embrittlement resistance can be obtained by including, in mass %, at least one of C: 0.10% or less, Si: 1.0% or less, Mn: 3.0% or more but less than 7.0%, Cr: 15 to 30%, Ni: 12.0% or more but less than 17.0%, Al: 0.10% or less, N: 0.10 to 0.50%, P: 0.050% or less, S: 0.050% or less, V: 0.01 to 1.0%, and Nb: 0.01 to 0.50%.

[0008] JP 2010-242163 A International Publication No. 2016 / 068009

[0009] The steel represented by the above-mentioned UNS S42000 contains 13 mass % of Cr, and therefore has excellent CO resistance. ２ Although UNS S42000 has corrosion resistance, there is a problem that it does not have sufficient corrosion resistance when the well is deep and the temperature at the bottom of the well is high. Furthermore, the steel described in Patent Document 1 has excellent corrosion resistance, but its hydrogen embrittlement resistance is not necessarily sufficient in hydrogen-containing environments. Furthermore, the steel described in Patent Document 2 has problems such as high alloy costs due to the inclusion of 12.0 mass% or more of Ni, as well as high manufacturing costs due to the need for two cold working steps after pipe making.

[0010] The present invention solves such problems of the prior art and provides high strength with a yield strength of 655 MPa (i.e., 95 ksi) or more and excellent CO resistance. ２ The present invention aims to provide a stainless steel seamless pipe that combines corrosion resistance with excellent hydrogen embrittlement resistance, and a method for manufacturing the same.

[0011] Here, the yield strength is the yield strength obtained in accordance with the API (American Petroleum Institute)-5CT standard. ２ "Corrosive" refers to the corrosion caused by 1 MPa CO2 in an autoclave. ２ -0.04 MPa H ２ Gas (1 MPa CO ２ and 0.04 MPa H ２The test piece is immersed for 720 hours in a 20 mass % NaCl aqueous solution (liquid temperature of the aqueous solution: 150°C) in contact with a gas containing CO and the corrosion product on the test piece is removed after the test, and the corrosion rate determined by the weight loss method is 0.10 mm / y or less. Here, "excellent hydrogen embrittlement resistance" means that the corrosion rate is 0.10 mm / y or less when the test piece is immersed for 720 hours in a 20 mass % NaCl aqueous solution (liquid temperature of the aqueous solution: 150°C) in contact with a gas containing CO and the corrosion product on the test piece is removed after the test, and the corrosion rate is determined by the weight loss method. ２ -0.04 MPa H ２ A test piece conforming to NACE TM0177 (2016) Method D was exposed to the gas for 720 hours, and the fracture toughness value of the test piece obtained was 23 MPa m １／２ This means that the above applies.

[0012] To achieve the above-mentioned objectives, the present inventors have conducted extensive research into the microstructure of stainless steels, particularly into the influence of Mo on hydrogen embrittlement resistance. In particular, they have conducted extensive research into methods for improving the hydrogen embrittlement resistance of 13% Cr steels, which have excellent corrosion resistance. As a result, they have found that refining the microstructure improves hydrogen embrittlement resistance and that the desired hydrogen embrittlement resistance can be achieved without using large amounts of expensive additive elements. In particular, they have found that even if the average grain size, as generally evaluated, is small, the presence of a small number of coarse crystal grains reduces the hydrogen embrittlement resistance of those portions, which significantly affects the hydrogen embrittlement resistance of the entire material. Therefore, they have found that it is important to reduce the maximum grain size.

[0013] Specifically, the average grain size of prior austenite grains, determined by measuring and analyzing the crystal orientation of the structure at the center of the wall thickness on a plane perpendicular to the longitudinal direction of the steel pipe by EBSD, is controlled to 50 μm or less. Furthermore, the maximum grain size of the prior austenite grains is controlled to 100 μm or less. It has been found that these controls result in the steel pipe having excellent hydrogen embrittlement resistance.

[0014] In the present invention, the average grain size of prior austenite grains is measured by EBSD at a single arbitrary position on the microstructure observation surface, over an area of ​​300 μm in the circumferential direction of the steel pipe and 500 μm in the wall thickness direction (actual size) at 1 μm intervals. Next, on an image showing the prior austenite grain boundaries determined by inverse analysis, three 500 μm line segments are drawn in the wall thickness direction of the steel pipe at 50 μm intervals, i.e., three line segments spanning the entire length of the EBSD measurement range in the wall thickness direction. The number of intersections between each 500 μm line segment and the prior austenite grain boundaries is then counted, and 1.5 × (500 ÷ number of intersections) is calculated. The average of the values ​​of 1.5 × (500 ÷ number of intersections) for each of the three line segments is taken as the average grain size of the prior austenite grains. The maximum grain size of the prior austenite grains in the present invention refers to the value obtained by multiplying the actual size of the point with the widest distance between the intersections of three line segments, each having an actual size of 500 μm, used to measure the average grain size described above by 1.5, calculated using a scale bar.

[0015] Next, we will describe a method for manufacturing a steel pipe in which the average grain size of prior austenite grains is 50 μm or less and the maximum grain size of prior austenite grains is 100 μm or less. The Mannesmann pipe-making process is an example of a method for manufacturing seamless steel pipes. In this method, piercing is performed by utilizing the characteristic that a cylindrical material called a round billet is embrittled at its axial center by externally rotating and compressing it. More specifically, the round billet first comes into contact with rolls, which cause deformation by rotary compression. The round billet then comes into contact with a piercing tool and is pierced. The pierced material then undergoes forming rolling and heat treatment to produce a steel pipe product.

[0016] The present inventors investigated the relationship between the manufacturing conditions of seamless steel pipes and the grain size of prior austenite grains. As a result, they found that the microstructure can be refined by ensuring the amount of compression during rotary compression deformation until the round billet contacts the piercing tool. Specifically, they found that by performing piercing under conditions where Db / Dr, the ratio of the initial diameter (Db) of the round billet to the diameter (Dr) of the billet when compressed by rolls and contacting the piercing tool, is 1.01 to 1.10, an average grain size of 50 μm or less and a maximum grain size of 100 μm or less can be stably obtained for the prior austenite grains. The mechanism behind this is presumed to be that the pre-strain introduced by compression during rotary compression further promotes recrystallization in the subsequent piercing process, making the microstructure more likely to be refined.

[0017] The present invention was completed based on these findings and further investigations. The gist of the present invention is as follows. [1] A steel sheet having a component composition containing, by mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.005% or less, Cr: 12.0 to 14.0%, Ni: more than 3.0% but not more than 7.0%, Mo: more than 0.5% but not more than 3.0%, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, with the balance being Fe and unavoidable impurities; and having a structure in which, by volume, a martensite phase: 70% or more, a retained austenite phase: 20% or less, and a ferrite phase: 10% or less, an average grain size of prior austenite is 50 μm or less, and a maximum grain size of prior austenite is 100 μm or less, [2] The stainless steel seamless pipe according to [1], wherein the chemical composition further contains, in mass%, one or more elements selected from the group consisting of Cu: 1.0% or less, V: 0.5% or less, Nb: 0.3% or less, Ti: 0.3% or less, W: 1.0% or less, Co: 1.0% or less, B: 0.010% or less, Ta: 0.3% or less, Zr: 0.3% or less, 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, Zn: 0.1% or less, Pb: 0.1% or less, As: 0.1% or less, and Bi: 0.1% or less. [3] A method for producing a stainless steel seamless pipe having the chemical composition according to [1] or [2] above, comprising: compressing a round billet having the chemical composition and piercing it with a piercing tool under conditions that satisfy the following formula (1) to obtain a seamless steel pipe; then, heating the seamless steel pipe to a quenching temperature of 800 to 1100°C, and subjecting the seamless steel pipe to a quenching treatment in which the seamless steel pipe is cooled from the quenching temperature to 300°C at an average cooling rate of 0.01°C / second or more; and then, subjecting the seamless steel pipe to a tempering temperature of 520 to 700°C.1.01≦Db / Dr≦1.10 ...Equation (1) In equation (1), Db: outer diameter (mm) of the round billet before compression, as viewed in a cross section perpendicular to the billet axial direction, and Dr: outer diameter (mm) of the round billet after compression and before contact with the piercing tool, as viewed in a cross section perpendicular to the billet axial direction.

[0018] According to the present invention, high strength with a yield strength of 655 MPa or more and excellent CO resistance are obtained. ２ It is possible to obtain a stainless steel seamless pipe that combines both corrosion resistance and excellent hydrogen embrittlement resistance. Furthermore, according to the present invention, a stainless steel seamless pipe that combines the above-mentioned properties can be produced by optimizing the piercing conditions in the piercing step.

[0019] The present invention will be described in detail below. The stainless steel seamless pipe of the present invention has a chemical composition containing, by mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.005% or less, Cr: 12.0 to 14.0%, Ni: more than 3.0% but 7.0% or less, Mo: more than 0.5% but 3.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, and the balance consisting of Fe and unavoidable impurities, and has a microstructure containing, by volume fraction, 70% or more of martensite phase, 20% or less of retained austenite phase, and 10% or less of ferrite phase, with an average grain size of prior austenite being 50 μm or less and a maximum grain size of prior austenite being 100 μm or less, and has a yield strength of 655 MPa or more.

[0020] [Component Composition] The stainless steel seamless pipe of the present invention has the above-mentioned component composition. First, the reasons for limiting the component composition will be explained. Hereinafter, unless otherwise specified, "mass %" will be simply written as "%".

[0021] C: 0.05% or less C is not an essential element in the present invention, but is an element that is inevitably contained during steelmaking. If the C content exceeds 0.05%, the desired corrosion resistance, particularly the desired CO resistance, is not obtained. ２Corrosion resistance is not obtained. For this reason, the C content is set to 0.05% or less. The C content is preferably set to 0.04% or less, and more preferably set to 0.03% or less. On the other hand, although there is no lower limit for the C content, excessive reduction of C leads to an increase in manufacturing costs, so the C content is preferably set to 0.003% or more. The C content is more preferably set to 0.005% or more, and further preferably set to 0.008% or more.

[0022] Si: 1.0% or less Si is an element that acts as a deoxidizer. However, if the Si content exceeds 1.0%, hot workability deteriorates. Therefore, the Si content is set to 1.0% or less. The Si content is preferably set to 0.7% or less, more preferably set to 0.5% or less, and even more preferably set to 0.4% or less. On the other hand, the lower limit of the Si content is not particularly limited, but from the viewpoint of enhancing the deoxidizing effect, the Si content is preferably set to 0.03% or more, more preferably set to 0.05% or more, and even more preferably set to 0.10% or more. The Si content is more preferably set to 0.15% or more, and even more preferably set to 0.20% or more.

[0023] Mn: 0.01 to 1.0% Mn is an element that acts as a deoxidizer and desulfurizer and improves hot workability. To obtain the effects as a deoxidizer and desulfurizer, the Mn content is set to 0.01% or more. The Mn content is preferably set to 0.03% or more, more preferably set to 0.05% or more, and even more preferably set to 0.10% or more. The Mn content is more preferably set to 0.20% or more, and even more preferably set to 0.30% or more. On the other hand, even if the Mn content exceeds 1.0%, the effect saturates. For this reason, the Mn content is set to 1.0% or less. The Mn content is preferably set to 0.8% or less, more preferably set to 0.6% or less, and even more preferably set to 0.5% or less.

[0024] P: 0.05% or less P is an impurity and an element that reduces hot workability. In order to suppress defects during manufacturing, the P content is set to 0.05% or less. The P content is preferably set to 0.04% or less, and more preferably set to 0.03% or less. On the other hand, since it is preferable to reduce the P content as much as possible, the lower limit of the P content is not particularly limited and may be 0%. From the viewpoint of manufacturing costs, the P content is preferably set to 0.005% or more. The P content is more preferably set to 0.010% or more, and even more preferably set to 0.014% or more.

[0025] S: 0.005% or less S is an element that significantly reduces hot workability and hinders stable operation of the hot pipe-making process. Furthermore, S exists as sulfide-based inclusions in steel and reduces corrosion resistance. Therefore, the S content is set to 0.005% or less. The S content is preferably set to 0.004% or less, more preferably set to 0.003% or less, and even more preferably set to 0.002% or less. On the other hand, since it is preferable to reduce the S content as much as possible, the lower limit of the S content is not particularly limited and may be 0%. From the viewpoint of production costs, the S content is more preferably set to 0.0005% or more.

[0026] Cr: 12.0 to 14.0% Cr is an element that forms a protective film on the surface of the steel pipe, thereby contributing to improving corrosion resistance. If the Cr content is less than 12.0%, the desired corrosion resistance, particularly the desired CO resistance, is not obtained. ２ Corrosion resistance cannot be ensured. For this reason, the Cr content is set to 12.0% or more. The Cr content is preferably set to 12.2% or more, more preferably set to 12.5% ​​or more, and even more preferably set to 12.6% or more. On the other hand, even if the Cr content exceeds 14.0%, the effect saturates. For this reason, the Cr content is set to 14.0% or less. The Cr content is preferably set to 13.5% or less, more preferably set to 13.2% or less, and even more preferably set to 13.0% or less.

[0027] Ni: More than 3.0% and 7.0% or less Ni improves the hydrogen embrittlement resistance of steel and has the effect of adjusting the phase fraction during steel pipe manufacturing to suppress the occurrence of defects. If the Ni content is 3.0% or less, the desired hydrogen embrittlement resistance and defect suppression effect cannot be obtained. Therefore, the Ni content is set to more than 3.0%. The Ni content is preferably 3.5% or more, more preferably 4.0% or more, and even more preferably 4.2% or more. On the other hand, if the Ni content exceeds 7.0%, the effect saturates and the material cost becomes excessively high. Therefore, the Ni content is set to 7.0% or less. The Ni content is preferably 6.5% or less, more preferably 6.2% or less, and even more preferably 6.0% or less.

[0028] Mo: More than 0.5% and 3.0% or less Mo stabilizes the protective film on the steel pipe surface and prevents Cl － It increases resistance to pitting corrosion due to low pH or low temperature, thereby improving corrosion resistance. If the Mo content is 0.5% or less, the desired corrosion resistance cannot be obtained. Therefore, the Mo content is set to more than 0.5%. The Mo content is preferably 0.8% or more, more preferably 1.5% or more, and even more preferably 1.8% or more. On the other hand, if the Mo content exceeds 3.0%, the effect saturates and the material cost becomes excessively high. Therefore, the Mo content is set to 3.0% or less. The Mo content is preferably 2.5% or less, and more preferably 2.3% or less.

[0029] Al: 0.10% or less Al is an element that acts as a deoxidizer. However, if the Al content exceeds 0.10%, corrosion resistance decreases. Therefore, the Al content is set to 0.10% or less. The Al content is preferably set to 0.07% or less, and more preferably set to 0.05% or less. On the other hand, although the lower limit of the Al content is not particularly limited, from the viewpoint of enhancing the deoxidizing effect, the Al content is preferably set to 0.005% or more, more preferably set to 0.010% or more, and even more preferably set to 0.015% or more.

[0030] N: 0.10% or less N is an element that is inevitably contained in the steelmaking process, but it also increases the strength of steel. However, if the N content exceeds 0.10%, the amount of nitrides formed becomes excessive, which reduces corrosion resistance, particularly CO resistance. ２ The corrosion resistance decreases. Therefore, the N content is set to 0.10% or less. The N content is preferably set to 0.08% or less, more preferably set to 0.06% or less, and even more preferably set to 0.04% or less. On the other hand, although there is no particular limitation on the lower limit of the N content, an extreme reduction in the N content leads to an increase in steelmaking costs. Therefore, the N content is preferably set to 0.002% or more, more preferably set to 0.003% or more, and even more preferably set to 0.005% or more.

[0031] O: 0.010% or less O (oxygen) exists as an oxide in steel and has an adverse effect on various properties. Therefore, in the present invention, it is desirable to reduce the O content as much as possible. In particular, if the O content exceeds 0.010%, hot workability and corrosion resistance decrease. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.007% or less, and more preferably 0.005% or less. Note that the lower limit of the O content is not particularly limited and may be 0%. Since an extreme reduction in the O content leads to an increase in steelmaking costs, the O content is preferably 0.0005% or more. The O content is more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0032] The stainless steel seamless pipe of the present invention contains the above components, with the balance being Fe and inevitable impurities. Here, the stainless steel seamless pipe of the present invention preferably has a composition containing only the above components and the balance being Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be included to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[0033] The components described above are the basic components, and the stainless steel seamless pipe of the present invention can achieve the desired properties with these basic components. In addition to these basic components, the present invention can optionally contain one or more elements selected from the group consisting of Cu, V, Nb, Ti, W, Co, B, Ta, Zr, Ca, REM, Mg, Sn, Sb, Zn, Pb, As, and Bi. Since Cu, V, Nb, Ti, W, Co, B, Ta, Zr, Ca, REM, Mg, Sn, Sb, Zn, Pb, As, and Bi are steel components that can be optionally contained, the content of these elements may be 0%.

[0034] Cu: 1.0% or less Cu has the effect of strengthening the protective coating on the steel pipe surface and increasing corrosion resistance, so it can be added as needed. The Cu content is preferably 0.05% or more, more preferably 0.10% or more, and even more preferably 0.20% or more. The Cu content is more preferably 0.30% or more, and even more preferably 0.40% or more. On the other hand, if the Cu content exceeds 1.0%, the effect saturates and the material cost becomes excessively high. Therefore, when Cu is contained, the Cu content is set to 1.0% or less. The Cu content is preferably 0.90% or less, more preferably 0.80% or less, and even more preferably 0.70% or less.

[0035] V: 0.5% or less V is an element that increases strength without impairing toughness by forming carbonitrides. Furthermore, because V easily forms carbonitrides, it prevents corrosion-resistant elements such as Cr from forming carbonitrides, thereby reducing the effective amount of carbonitrides that contribute to corrosion resistance. This allows for excellent corrosion resistance, so V can be included as needed. However, even if V is included in an amount exceeding 0.5%, the effect saturates. Therefore, when V is included, the V content is set to 0.5% or less. The V content is preferably set to 0.4% or less, and more preferably set to 0.3% or less. While the lower limit of the V content is not particularly limited, the V content is preferably set to 0.01% or more, and more preferably set to 0.03% or more. The V content is more preferably set to 0.10% or more, and even more preferably set to 0.20% or more.

[0036] Nb: 0.3% or less Nb is an element that forms carbonitrides and improves strength and corrosion resistance, and can be contained as needed. However, since Nb carbonitrides tend to reduce low-temperature toughness, when Nb is contained, the Nb content is set to 0.3% or less. The Nb content is preferably set to 0.2% or less, and more preferably set to 0.1% or less. The Nb content is preferably set to 0.01% or more. The Nb content is more preferably set to 0.03% or more, and even more preferably set to 0.05% or more.

[0037] Ti: 0.3% or less Ti is an element that increases strength and corrosion resistance and can be contained as needed. However, if Ti is contained in an amount exceeding 0.3%, low-temperature toughness decreases. Therefore, when Ti is contained, the Ti content is set to 0.3% or less. The Ti content is preferably set to 0.2% or less, and more preferably set to 0.15% or less. The Ti content is preferably set to 0.001% or more, and more preferably set to 0.01% or more. The Ti content is more preferably set to 0.03% or more, and even more preferably set to 0.05% or more.

[0038] W: 1.0% or less W is an element that contributes to improving the strength of steel and stabilizes the protective coating on the steel pipe surface to enhance corrosion resistance, and can be contained as needed. However, if the W content exceeds 1.0%, the effect saturates and the material cost becomes excessively high. Therefore, when W is contained, the W content is set to 1.0% or less. The W content is preferably set to 0.8% or less, more preferably 0.7% or less. The W content is preferably set to 0.1% or more, and even more preferably 0.2% or more. The W content is more preferably set to 0.3% or more, and even more preferably 0.5% or more.

[0039] Co: 1.0% or less Co is an element that improves corrosion resistance and can be contained as needed. However, even if Co is contained in an amount exceeding 1.0%, the effect saturates. Therefore, when Co is contained, the Co content is set to 1.0% or less. The Co content is preferably set to 0.5% or less, more preferably set to 0.3% or less, and even more preferably set to 0.1% or less. The Co content is preferably set to 0.01% or more. The Co content is more preferably set to 0.03% or more. The Co content is more preferably set to 0.05% or more, and even more preferably set to 0.07% or more.

[0040] B: 0.010% or less B is an element that contributes to improving hot workability and has the effect of suppressing the occurrence of cracks and breakages during the pipe-making process, and can be contained as needed. However, if the B content exceeds 0.010%, low-temperature toughness decreases. Therefore, when B is contained, the B content is set to 0.010% or less. The B content is preferably set to 0.007% or less. The B content is preferably set to 0.0005% or more, more preferably 0.0010% or more. The B content is further preferably set to 0.0030% or more.

[0041] Ta: 0.3% or less Ta is an element that has the effect of increasing strength and improving corrosion resistance, and can be contained as needed. However, even if Ta is contained in an amount exceeding 0.3%, the effect saturates. Therefore, when Ta is contained, the Ta content is set to 0.3% or less. The Ta content is preferably set to 0.20% or less, and more preferably set to 0.15% or less. The Ta content is preferably set to 0.001% or more. The Ta content is more preferably set to 0.01% or more, and even more preferably set to 0.05% or more.

[0042] Zr: 0.3% or less Zr is an element that increases strength and can be contained as needed. Zr also has the effect of improving SSC resistance. However, even if Zr is contained in an amount exceeding 0.3%, this effect saturates. Therefore, when Zr is contained, the Zr content is set to 0.3% or less. The Zr content is preferably set to 0.0005% or more. The Zr content is more preferably set to 0.01% or more, and even more preferably set to 0.05% or more.

[0043] Ca: 0.01% or less Ca is an element that improves hot workability by controlling the morphology of sulfides and also has the effect of suppressing the occurrence of cracks and fractures during the pipe-making process, and can be contained as needed. To achieve these effects, the Ca content is preferably 0.001% or more. The Ca content is more preferably 0.002% or more, even more preferably 0.003% or more, and even more preferably 0.005% or more. On the other hand, even if Ca is contained in an amount exceeding 0.01%, these effects saturate, and no effect commensurate with the content can be expected. Therefore, when Ca is contained, the Ca content is 0.01% or less. The Ca content is preferably 0.008% or less, more preferably 0.007% or less.

[0044] REM: 0.3% or less. REM (rare earth metal) is an element that contributes to improving stress corrosion cracking resistance by controlling the morphology of sulfides and can be contained as needed. However, if the REM content exceeds 0.3%, the effect saturates and no effect commensurate with the content can be expected. Therefore, when REM is contained, the REM content is set to 0.3% or less. The REM content is preferably 0.0005% or more. The REM content is more preferably 0.01% or more, and even more preferably 0.05% or more. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The chemical composition of the stainless steel seamless pipe of the present invention can optionally contain at least one of the above REMs. Therefore, the REM content in the present invention refers to the total content of the above elements.

[0045] Mg: 0.01% or less Mg is an element that improves corrosion resistance and can be contained as needed. However, even if the Mg content exceeds 0.01%, the effect saturates and no effect commensurate with the content can be expected. Therefore, when Mg is contained, the Mg content is set to 0.01% or less. The Mg content is preferably set to 0.0005% or more. The Mg content is more preferably set to 0.0010% or more, and even more preferably set to 0.0030% or more.

[0046] Sn: 1.0% or less Sn is an element that improves corrosion resistance and can be contained as needed. However, even if the Sn content exceeds 1.0%, the effect saturates and no effect commensurate with the content can be expected. Therefore, when Sn is contained, the Sn content is set to 1.0% or less. The Sn content is preferably set to 0.001% or more. The Sn content is more preferably set to 0.01% or more, and even more preferably set to 0.1% or more. The Sn content is more preferably set to 0.3% or more, and even more preferably set to 0.5% or more.

[0047] Sb: 1.0% or less Sb is an element that improves corrosion resistance and can be contained as needed. However, even if the Sb content exceeds 1.0%, the effect saturates and no effect commensurate with the content can be expected. Therefore, when Sb is contained, the Sb content is set to 1.0% or less. The Sb content is preferably set to 0.001% or more. The Sb content is more preferably set to 0.01% or more, and even more preferably set to 0.05% or more. The Sb content is more preferably set to 0.3% or more, and even more preferably set to 0.5% or more.

[0048] Zn: 0.1% or less Zn is an impurity and reduces hot workability, so if Zn is contained, the Zn content is set to 0.1% or less. Since the lower the Zn content, the better, no lower limit is set, but the Zn content is preferably 0.0005% or more, more preferably 0.001% or more. The Zn content is more preferably 0.003% or more, and even more preferably 0.005% or more.

[0049] Pb: 0.1% or less Pb is an impurity that reduces hot workability, so if Pb is contained, the Pb content is set to 0.1% or less. Since the lower the Pb content, the better, no lower limit is set, but the Pb content is preferably 0.0005% or more, and more preferably 0.001% or more. The Pb content is more preferably 0.01% or more, and even more preferably 0.03% or more.

[0050] As: 0.1% or less Since As is an impurity and reduces hot workability, when As is contained, the As content is set to 0.1% or less. Since the lower the As content, the better, no lower limit is set, but the As content is preferably 0.0005% or more, and more preferably 0.001% or more. The As content is more preferably 0.01% or more, and even more preferably 0.03% or more.

[0051] Bi: 0.1% or less Because Bi is an impurity and reduces hot workability, when Bi is contained, the Bi content is set to 0.1% or less. Since the lower the Bi content, the better, no lower limit is set, but the Bi content is preferably 0.0005% or more, and more preferably 0.001% or more. The Bi content is more preferably 0.01% or more, and even more preferably 0.03% or more.

[0052] [Structure] Next, the reasons for limiting the structure of the stainless steel seamless pipe of the present invention will be explained.

[0053] The structure of the stainless steel seamless pipe in one embodiment of the present invention is, in volume fraction, 70% or more of a martensite phase, 20% or less of a retained austenite phase, and 10% or less of a ferrite phase, and the average grain size of the prior austenite determined using EBSD described later is 50 μm or less and the maximum grain size of the prior austenite is 100 μm or less.The structure of the stainless steel seamless pipe may be, in volume fraction, 70% or more of a martensite phase, 20% or less of a retained austenite phase, and 10% or less of a ferrite phase, and the average grain size of the prior austenite determined using EBSD described later is 50 μm or less and the maximum grain size of the prior austenite is 100 μm or less.

[0054] Martensite phase: 70% or more by volume If the volume fraction of the martensite phase is less than 70%, the desired strength cannot be ensured. Therefore, the volume fraction of the martensite phase is set to 70% or more. The volume fraction of the martensite phase is preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more. On the other hand, the upper limit of the volume fraction of the martensite phase is not particularly limited, but the volume fraction of the martensite phase is preferably 99.8% or less, and more preferably 99.5% or less. The volume fraction of the martensite phase is preferably 98% or less, more preferably 97.8% or less, even more preferably 97.5% or less, and even more preferably 97% or less. The volume fraction of the martensite phase is preferably 96.8% or less, and more preferably 96.5% or less.

[0055] Retained austenite phase: 20% or less by volume The presence of retained austenite phase is not essential, but some austenite that has not been completely transformed into martensite may remain in the steel as retained austenite. If a large amount of retained austenite phase, exceeding 20% ​​by volume, is present, the desired hydrogen embrittlement resistance may not be ensured. Furthermore, if the volume fraction of the retained austenite phase exceeds 20%, the desired strength cannot be obtained. Therefore, the volume fraction of the retained austenite phase is set to 20% or less. The volume fraction of the retained austenite phase is preferably 15% or less, and more preferably 10% or less. On the other hand, the lower limit of the volume fraction of the retained austenite phase is not particularly limited, but the volume fraction of the retained austenite phase is preferably 2% or more, and more preferably 3% or more.

[0056] Ferrite phase: 10% or less by volume. The presence of a ferrite phase is not essential, but some ferrite that cannot be completely transformed into austenite or martensite during heat treatment may remain in the steel as a ferrite phase. Because the ferrite phase contains more Cr, the Cr concentration in the martensite or austenite phase surrounding the ferrite phase decreases, resulting in reduced corrosion resistance. Therefore, the volume fraction of the ferrite phase is set to 10% or less. The volume fraction of the ferrite phase is preferably set to 7% or less, more preferably set to 5% or less, and even more preferably set to 3% or less. On the other hand, although there is no particular lower limit for the volume fraction of the ferrite phase, the volume fraction of the ferrite phase is preferably set to 0.2% or more, more preferably set to 0.5% or more.

[0057] Here, the volume fraction of each of the phases can be measured by the following method. First, a steel piece is cut from a surface of a stainless steel seamless pipe perpendicular to the longitudinal direction (pipe axis direction) of the steel pipe, and the steel piece is embedded in resin and mirror-polished to prepare a sample for microstructure observation. This observation surface is immersed in a KOH solution (i.e., a mixture of 35 g of KOH and 100 g of pure water) at a voltage of 3 A / cm. ２The specimen was then subjected to electrolytic corrosion at a current density of 0.015 g for 35 seconds, followed by corrosion with Villela's reagent (i.e., a mixture of 2 g picric acid, 5 ml hydrochloric acid, and 50 ml ethanol, respectively) for 30 seconds. The structure of the specimen for microstructural observation was then imaged at 400x magnification using an optical microscope. An image was cut from any point on the obtained optical microscope photograph, measuring 300 μm in the circumferential direction of the steel pipe and 200 μm in the wall thickness direction of the steel pipe, and analyzed using image analysis software (ImageJ 1.52p, National Institute of Health), to calculate the ferrite phase structure fraction (area fraction (%)). In the above analysis, the ferrite phase can be extracted by using the Weka Trainable Segmentation function on the optical microscope photograph, using three bright ferrite regions and three dark martensite regions as training data, and automatically classifying the other regions using the Segmentation function. The area fraction of the ferrite phase extracted in this way is defined as the volume fraction (%) of the ferrite phase.

[0058] Next, an X-ray diffraction test piece taken from the stainless steel seamless pipe is ground and polished so that a cross section perpendicular to the pipe axis (i.e., C-section) serves as the measurement surface, and the structural fraction of the retained austenite (γ) phase is measured using X-ray diffraction. Specifically, the volume fraction of the retained austenite phase is calculated from the integrated intensities of the austenite (220) plane and the ferrite (211) plane using the following formula: Vγ (%) = 100 / (1 + (IαRγ / IγRα)) where Vγ is the volume fraction of the retained austenite phase, Iα is the integrated intensity of the ferrite (211) plane, Iγ is the integrated intensity of the austenite (220) plane, Rα is the theoretically calculated value of α (34.15), and Rγ is the theoretically calculated value of γ (22.33).

[0059] The remainder other than the ferrite phase and the retained austenite phase determined by the above measurement method is defined as the martensite phase fraction. The above observation method for each structure is also applied to the examples described later.

[0060] Average grain size of prior austenite: 50 μm or less Maximum grain size of prior austenite: 100 μm or less The stainless steel seamless pipe of the present invention has a structure in which the average grain size of prior austenite is 50 μm or less and the maximum grain size is 100 μm or less.

[0061] The grain size of prior austenite is determined using the following method. First, a grain size measurement specimen taken from a stainless steel seamless pipe is ground and polished so that a cross section perpendicular to the pipe axis (i.e., C-section) serves as the measurement surface. The crystal orientation of the martensite phase is measured using electron backscatter diffraction (EBSD). At a single point of interest on the microstructure observation surface, EBSD measurements are performed at 1-μm intervals over an area measuring 300 μm in the circumferential direction of the steel pipe and 500 μm in the wall thickness direction (actual size). Because the martensite phase has a crystal structure similar to a body-centered cubic (BCC) lattice, it cannot be distinguished from the ferrite phase, which also has a body-centered cubic structure. However, if the volume fraction of the ferrite phase described above is 10% or less, its influence on the measurement results is negligible. Therefore, the BCC measurement results can be used directly for subsequent analysis.

[0062] EBSD provides information about the crystal orientation of the martensite phase. Here, martensite is austenite transformed during heat treatment, and there is a specific relationship between the crystal orientations of martensite and austenite. Therefore, by performing an inverse analysis of the orientation relationship from the crystal orientation of the martensite phase, information about the crystal orientation of the austenite (prior austenite) before transformation at each position can be constructed. The average grain size is calculated using an intercept method from an image showing the grain boundaries after inverse analysis. Specifically, on the image showing the grain boundaries of the prior austenite grains obtained by inverse analysis, three 500 μm line segments are drawn in actual size in the thickness direction of the steel pipe, i.e., three line segments spanning the entire length of the EBSD measurement range in the thickness direction, at 50 μm intervals. The number of intersections between each 500 μm line segment and the grain boundaries of the prior austenite is counted, and the result is 1.5 × (500 ÷ number of intersections). The average value of 1.5 × (500 ÷ number of intersections) for each of the three line segments is defined as the average grain size of the prior austenite. The maximum grain size of the prior austenite grains is determined by multiplying the actual size of the point with the widest interval between the intersections of the three line segments, each 500 μm in actual size, used to measure the average grain size described above by 1.5, using a scale bar.

[0063] Although the detailed mechanism is unclear, the desired hydrogen embrittlement resistance can be obtained by setting the average grain size to 50 μm or less and the maximum grain size to 100 μm or less. This is presumably because the refined structure suppresses the occurrence of intergranular cracking, which occurs at the grain boundaries, making the material less susceptible to fracture even in a hydrogen environment. In this regard, even if the average grain size, which is generally evaluated as being small, is small, if a small amount of coarse crystal grains are present, the hydrogen embrittlement resistance of that portion is low, and this characteristic significantly affects the hydrogen embrittlement resistance of the entire material. Therefore, it is considered particularly important to reduce the maximum grain size.

[0064] For the reasons described above, it is essential for the microstructure of the present invention that the average grain size of this prior austenite be 50 μm or less and the maximum grain size be 100 μm or less. The smaller the grain size of the prior austenite, the more improved hydrogen embrittlement resistance can be expected, and the average grain size of the prior austenite is preferably 45 μm or less, and more preferably 40 μm or less. The maximum grain size of the prior austenite is preferably 90 μm or less, more preferably 80 μm or less, and even more preferably 70 μm or less.

[0065] As described above, since it is desirable that the average grain size and maximum grain size of the prior austenite are as small as possible, there are no particular restrictions on the lower limits of the average grain size and maximum grain size of the prior austenite. The average grain size of the prior austenite is preferably 1 μm or more. The average grain size of the prior austenite is more preferably 3 μm or more. The average grain size of the prior austenite is further preferably 10 μm or more, and even more preferably 20 μm or more. The maximum grain size of the prior austenite is preferably 10 μm or more. The maximum grain size of the prior austenite is more preferably 15 μm or more. The maximum grain size of the prior austenite is further preferably 20 μm or more, and even more preferably 40 μm or more.

[0066] [Yield strength] Yield strength: 655 MPa or more The stainless steel seamless pipe of the present invention has a yield strength of 655 MPa or more. The yield strength is preferably more than 655 MPa. The upper limit of the yield strength is not particularly limited, but it is preferably 896 MPa or less. The yield strength can be measured by the method described in the examples.

[0067] The stainless steel seamless pipe of the present invention can be used for any purpose without any particular limitation. ２ CO for CCS underground sequestration ２ In addition to press-fitting steel pipes, ２ The steel pipe can be extremely suitably used as a steel pipe for transporting hydrogen underground for temporary storage of the hydrogen contained therein.

[0068] [Manufacturing Method] Next, one embodiment of the manufacturing method for the stainless steel seamless pipe of the present invention will be described.

[0069] The stainless steel seamless pipe of the present invention can be produced by forming a seamless steel pipe from a steel material (round billet) and then subjecting the seamless steel pipe to quenching and tempering treatments under specific conditions.

[0070] A round billet is used as the steel material, and the steel material has the above-mentioned composition.

[0071] The method for producing the above-mentioned steel material (round billet) is not particularly limited, and any method can be used. For example, molten steel having the above-mentioned composition is produced by a melting method using a converter or the like, and then formed into a round billet by a method such as continuous casting or ingot making-blooming rolling. For example, steel scrap heated and melted in an electric furnace may be used to produce the molten steel. Alternatively, for example, after the above-mentioned melting, the round billet may be directly produced by casting into a cylindrical shape.

[0072] [Pipe Making] The obtained steel material (round billet) is made into a seamless steel pipe. In the present invention, the pipe making is performed by hot working. Specifically, in the hot working, the steel material is heated, and the heated steel material is formed into a mother pipe (i.e., a hollow mother pipe) using a piercing machine. The mother pipe is then rolled, such as by forming, to produce a seamless steel pipe of the desired dimensions. The method for hot working the steel material to produce a seamless steel pipe is not particularly limited, and any method can be used. For example, a seamless steel pipe can be obtained using either the Mannesmann plug mill method or the Mannesmann mandrel mill method. The heating temperature in this hot working is not particularly limited, but from the viewpoint of achieving both high levels of hot workability during pipe making and low-temperature toughness of the final product, it is preferably 1100 to 1350°C. After the heating, piercing is performed to drill holes in the steel material.

[0073] In the following description of the manufacturing method, temperatures (°C) refer to the surface temperatures of the steel pipes (seamless steel pipes after pipe making) unless otherwise specified. These surface temperatures can be measured using a radiation thermometer or the like.

[0074] (Piercing) In the present invention, a seamless steel pipe is obtained by piercing a round billet in the piercing step. Specifically, in the present invention, a seamless steel pipe is obtained by compressing the round billet and piercing it with a piercing tool under the condition that satisfies the following formula (1): 1.01≦Db / Dr≦1.10 (1) In formula (1), Db: outer diameter (mm) of the round billet before compression, as viewed in a cross section perpendicular to the billet axial direction (hereinafter also referred to as initial diameter), Dr: outer diameter (mm) of the round billet after compression and before contact with the piercing tool, as viewed in a cross section perpendicular to the billet axial direction (hereinafter also referred to as billet diameter).

[0075] In the present invention, when producing this seamless steel pipe, it is important to optimize the amount of rolling of the round billet before the start of piercing in the piercing process. Specifically, the conditions of the piercing process are controlled so that Db / Dr, which is the ratio of the initial diameter (Db) of the round billet to the diameter (Dr) of the billet before it is compressed and contacted with the piercing tool, is 1.01 to 1.10. By appropriately controlling the piercing condition Db / Dr in the piercing process to satisfy 1.01 to 1.10, the average grain size of the prior austenite can be reduced to 50 μm or less, and the maximum grain size of the prior austenite can be reduced to 100 μm or less. The compression can be performed by rolls, which come into contact with the round billet and rotate and move it forward. The shape of the piercing tool is not particularly limited, as long as it can pierce the round billet and control the inner diameter of the seamless steel pipe after piercing.

[0076] The value of Db / Dr is set to 1.01 or more, preferably 1.02 or more, more preferably 1.03 or more, and even more preferably 1.04 or more. The larger the value of Db / Dr, the smaller the average grain size of prior austenite can be. However, if the value of Db / Dr becomes extremely large, that is, if the billet reduction before piercing becomes large, the amount of cracking occurring in the center of the billet increases due to the so-called Mannesmann effect, and the quality of the inner surface of the steel pipe after pipemaking is likely to deteriorate. Therefore, the value of Db / Dr is set to 1.10 or less. The value of Db / Dr is preferably set to 1.09 or less, more preferably 1.08 or less, and even more preferably 1.07 or less.

[0077] When pipe making is performed by hot working, a cooling treatment may be performed after pipe making. The cooling treatment can be performed under any conditions without any particular limitations. For example, as the cooling treatment, after hot working, it is preferable to cool the steel pipe at an average cooling rate equal to or faster than air cooling until the surface temperature of the steel pipe reaches room temperature. An average cooling rate equal to or faster than air cooling means 0.01°C / second or more.

[0078] [Quenching and Tempering] The seamless steel pipe is then subjected to heat treatment including quenching and tempering under specific conditions. The conditions for the quenching and tempering treatments are described below.

[0079] (Quenching Treatment) First, the seamless steel pipe is heated to a quenching temperature of 800 to 1100°C, and the heated seamless steel pipe is cooled from the quenching temperature to a steel plate surface temperature of 300°C at an average cooling rate of 0.01°C / second or more.

[0080] Quenching temperature: 800 to 1100°C If the heating temperature in the quenching treatment (i.e., the quenching temperature) is less than 800°C, reverse transformation from martensite to austenite does not occur, and transformation from austenite to martensite does not occur during cooling, resulting in the desired strength not being ensured. Therefore, the quenching temperature is set to 800°C or higher. The quenching temperature is preferably set to 900°C or higher. On the other hand, if the quenching temperature is higher than 1100°C, the ferrite fraction becomes too high, resulting in a decrease in corrosion resistance. Therefore, the quenching temperature is set to 1100°C or lower. The quenching temperature is preferably set to 1050°C or lower.

[0081] In the above-mentioned quenching treatment, the seamless steel pipe may be heated to the above-mentioned quenching temperature, and then subjected to a soaking treatment in which the pipe is maintained at the quenching temperature. By performing the soaking treatment, the temperature of the seamless steel pipe in the wall thickness direction can be made uniform, thereby reducing variations in the material properties. The time for maintaining the pipe at the quenching temperature (i.e., the soaking time) is not particularly limited, but is preferably 3 minutes or more, and more preferably 5 minutes or more. Furthermore, a longer soaking time may increase the prior austenite grain size and thereby reduce the fracture toughness value, so the time is preferably 30 minutes or less, more preferably 20 minutes or less, and even more preferably 15 minutes or less.

[0082] Average cooling rate: 0.01°C / second or more In the quenching treatment, if the average cooling rate from the quenching temperature to 300°C is less than 0.01°C / second, the desired structure cannot be obtained. Therefore, the average cooling rate from the quenching temperature to 300°C is set to 0.01°C / second or more. The average cooling rate is preferably 1.0°C / second or more, more preferably 5.0°C / second or more, and even more preferably 10.0°C / second or more. There is no particular upper limit, but the average cooling rate is preferably 80.0°C / second or less, and more preferably 40.0°C / second or less.

[0083] The cooling method is not particularly limited and can be performed by any method. For example, cooling is preferably performed by at least one of air cooling and water cooling, with water cooling being more preferred. The average cooling rate (°C / sec) is calculated by subtracting 300 (°C) from the quenching temperature (cooling start temperature) (°C) and dividing the result by the cooling time (seconds) from the cooling start temperature to 300°C. That is, the average cooling rate (°C / sec) is calculated by (quenching temperature - 300) (°C) / cooling time (seconds) from the quenching temperature to 300°C. In practice, cooling is performed to a temperature below 300°C, but the desired strength can be obtained by water cooling or air cooling with an average cooling rate of 0.01°C / second or more from the quenching temperature to 300°C. Therefore, the average cooling rate in the present invention is defined as above.

[0084] Cooling stop temperature: 300°C or less In the present invention, as described above, the average cooling rate from the quenching temperature to 300°C is set to 0.01°C / second or more, so the cooling stop temperature in the quenching treatment is essentially 300°C or less. If the cooling stop temperature is higher than 300°C, the desired structure cannot be obtained. If the cooling stop temperature is high, the transformation from austenite to martensite does not occur sufficiently, and the residual austenite fraction becomes excessive. Therefore, the cooling stop temperature in the above quenching treatment is set to 300°C or less. The cooling stop temperature is preferably 200°C or less, more preferably 100°C or less, and even more preferably 50°C or less. In addition, there is no particular limitation on the lower limit of the cooling stop temperature, but the cooling stop temperature is preferably 0°C or more, more preferably 3°C or more, and even more preferably 5°C or more. The cooling stop temperature is the surface temperature of the seamless steel pipe.

[0085] (Tempering Treatment) Next, the seamless steel pipe after the quenching treatment is subjected to tempering treatment by heating to a tempering temperature of 520 to 700°C.

[0086] Tempering temperature: 520 to 700°C If the tempering temperature is less than 520°C, a sufficient tempering effect cannot be obtained, resulting in a deterioration of low-temperature toughness. Therefore, the tempering temperature is set to 520°C or higher. The tempering temperature is preferably set to 550°C or higher. On the other hand, if the tempering temperature is higher than 700°C, the tempering effect per unit time becomes large, increasing the risk of insufficient strength due to variations in heating time in actual operation, which is an industrial disadvantage. Therefore, the tempering temperature is set to 700°C or lower. The tempering temperature is preferably set to 650°C or lower.

[0087] In the tempering treatment, the seamless steel pipe can be heated to the tempering temperature and then held at the tempering temperature. The time for holding at the tempering temperature (i.e., holding time) is not particularly limited. The holding time may be controlled from the viewpoint of uniforming the temperature in the wall thickness direction and preventing material variations. Specifically, the holding time is preferably 5 minutes or more. The holding time is more preferably 10 minutes or more, and even more preferably 15 minutes or more. Furthermore, the holding time is preferably 90 minutes or less. The holding time is more preferably 60 minutes or less, and even more preferably 40 minutes or less.

[0088] After the tempering treatment, the steel sheet can be air-cooled (naturally cooled).

[0089] By carrying out the above-mentioned quenching and tempering treatments, the steel sheet has the above-mentioned strength and excellent CO resistance. ２ A stainless steel seamless pipe having both corrosion resistance and hydrogen embrittlement resistance can be obtained. Furthermore, the stainless steel seamless pipe of the present invention can also have excellent low-temperature toughness.

[0090] The stainless steel seamless pipe of the present invention obtained as described above may have an outer diameter (diameter) of 30 mm or more, but is not particularly limited thereto. The outer diameter (diameter) may be 600 mm or less. The stainless steel seamless pipe of the present invention may have a wall thickness (wall thickness) of 3 mm or more, but is not particularly limited thereto. The wall thickness may be 30 mm or less.

[0091] The present invention will be further described below based on examples, but the present invention is not limited to the following examples.

[0092] First, seamless steel pipes were produced from steel materials having the chemical compositions shown in Table 1 according to the following procedure.

[0093]

[0094] Specifically, a steel material was cast using molten steel having the chemical composition shown in Table 1. The steel material was then heated and hot-worked using a model seamless rolling mill to form a seamless steel pipe having an outer diameter of 177.8 mm and a wall thickness of 16.0 mm, which was then air-cooled. The heating temperature of the steel material before hot-working was 1250°C. The ratio Db / Dr of the initial diameter (Db) of the round billet to the billet diameter (Dr) was the value shown in Table 2.

[0095] The resulting seamless steel pipe was then subjected to quenching and tempering under the following conditions to obtain a stainless steel seamless pipe.

[0096] [Quenching Treatment] The obtained seamless steel pipe was subjected to quenching treatment under the conditions shown in Table 2. Specifically, the seamless steel pipe was heated to the quenching temperature shown in Table 2 and held at the quenching temperature for the soaking time shown in Table 2. Next, the pipe was cooled by water cooling. During the cooling, the cooling rate was measured using a thermocouple attached to the end of the pipe. Table 2 shows the average cooling rate from the time of removal from the heating furnace until the temperature reached 300°C. That is, the average cooling rate shown in Table 2 was calculated by (quenching temperature - 300) (°C) / cooling time (seconds) from the quenching temperature to 300°C.

[0097] [Tempering Treatment] The cooled seamless steel pipe was then heated to the tempering temperature shown in Table 2 and held at the tempering temperature for the holding time shown in Table 2. The seamless steel pipe was then air-cooled (i.e., allowed to cool naturally). The average cooling rate during air-cooling was 0.04°C / sec.

[0098] Test pieces were taken from the obtained stainless steel seamless pipe and subjected to the following methods: (1) microstructural observation, (2) tensile test, (3) CO ２ (3) A corrosion test and (4) a hydrogen embrittlement resistance evaluation test were carried out.

[0099] (1) Microstructural observation Using the obtained stainless steel seamless pipe, the volume fraction of each phase was measured by the method described above. The amount of martensite phase was calculated as "100% - volume fraction (%) of ferrite phase - volume fraction (%) of retained austenite phase."

[0100] Furthermore, the average grain size and maximum grain size of the prior austenite were calculated using the method described above.

[0101] (2) Tensile test Arc-shaped tensile test specimens were taken from the obtained stainless steel seamless pipe in accordance with the API (American Petroleum Institute)-5CT standard so that the pipe axis direction was the tensile direction, and a tensile test was carried out to determine the yield strength (YS). Here, specimens with a yield strength YS of 655 MPa or more were considered to be high strength and passed, and specimens with a yield strength YS of less than 655 MPa were considered to have failed.

[0102] (3) CO ２ Corrosion test CO resistance ２ To evaluate the corrosiveness, the following tests were carried out.

[0103] A test piece having a thickness of 3 mm, a width of 25 mm and a length of 50 mm was prepared from the obtained stainless steel seamless pipe by machining. ２ A corrosion test was carried out. ２ The corrosion test was carried out in an autoclave using a 20 mass % NaCl aqueous solution (liquid temperature: 150°C, CO ２ -0.04 MPa H ２ Gas atmosphere (1 MPa CO ２ and 0.04 MPa H ２ The test specimens were immersed in a gas atmosphere containing 0.10 mm / y or less for 30 days (i.e., 720 hours). After the test, the corrosion products on the test specimens were removed and the corrosion rate was determined by the weight loss method. A corrosion rate of 0.10 mm / y or less was judged as passing, and a rate exceeding this value was judged as failing.

[0104] (4) Hydrogen embrittlement resistance evaluation test Test pieces were prepared from the obtained stainless steel seamless pipes in accordance with NACE TM0177 (2016) Method D, and a hydrogen embrittlement resistance evaluation test was carried out. The hydrogen embrittlement resistance evaluation test was carried out by subjecting the pipes to a CO2 atmosphere of 1 MPa in an autoclave. ２ -0.04 MPa H ２ A test piece conforming to NACE TM0177 (2016) Method D was exposed to the gas for 720 hours, and the fracture toughness value obtained was 23 MPa m １／２It should be noted that NACE TM0177 (2016) Method D is a standard in which the test is valid only if a crack propagates in the test environment, but in this test, the fracture toughness value was calculated using the initial pre-crack length even when the crack did not propagate.

[0105] The results obtained are shown in Table 2.

[0106]

[0107] As can be seen from the results shown in Table 2, all of the stainless steel seamless pipes that satisfy the conditions of the present invention have high strength with a yield strength of 655 MPa or more and excellent CO resistance. ２ Therefore, the stainless steel seamless pipe of the present invention has excellent corrosion resistance and hydrogen embrittlement resistance. ２ CO for CCS underground sequestration ２ In addition to press-fitting steel pipes, ２ The steel pipe can be extremely suitably used as a steel pipe for transporting hydrogen underground for temporary storage of the hydrogen contained therein.

Claims

1. A steel sheet having a composition containing, by mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.005% or less, Cr: 12.0 to 14.0%, Ni: more than 3.0% but not more than 7.0%, Mo: more than 0.5% but not more than 3.0%, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, with the balance being Fe and unavoidable impurities; and having a structure in which, by volume, the martensite phase is 70% or more, the retained austenite phase is 20% or less, and the ferrite phase is 10% or less, the average grain size of the prior austenite is 50 μm or less, and the maximum grain size of the prior austenite is 100 μm or less. A stainless steel seamless pipe having a yield strength of 655 MPa or more.

2. A stainless steel seamless pipe as described in claim 1, wherein the chemical composition further contains one or more elements selected from the group consisting of, in mass%, Cu: 1.0% or less, V: 0.5% or less, Nb: 0.3% or less, Ti: 0.3% or less, W: 1.0% or less, Co: 1.0% or less, B: 0.010% or less, Ta: 0.3% or less, Zr: 0.3% or less, 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, Zn: 0.1% or less, Pb: 0.1% or less, As: 0.1% or less, Bi: 0.1% or less.

3. A method for producing a stainless steel seamless pipe having the chemical composition defined in claim 1 or 2, comprising the steps of: compressing a round billet having said chemical composition and piercing it with a piercing tool to obtain a seamless steel pipe under conditions that satisfy the following formula (1); then heating the seamless steel pipe to a quenching temperature of 800 to 1100°C and subjecting it to a quenching treatment in which the seamless steel pipe is cooled from the quenching temperature to 300°C at an average cooling rate of 0.01°C / second or more; and then subjecting it to a tempering treatment in which the seamless steel pipe is heated to a tempering temperature of 520 to 700°C. 1.01≦Db / Dr≦1.10 ... formula (1) In formula (1), Db: outer diameter (mm) of the round billet before compression, as viewed in cross section perpendicular to the axial direction of the billet, Dr: outer diameter (mm) of the round billet after compression and before contact with the piercing tool, as viewed in cross section perpendicular to the axial direction of the billet.

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