Martensitic stainless steel seamless pipe and method for producing same

Abstract

A martensitic stainless steel seamless pipe and a method for producing the martensitic stainless steel seamless pipe are provided. A martensitic stainless steel seamless pipe according to the present invention has a chemical composition containing C, Si, Mn, P, S, Cu, Ni, Cr, Al, Mo, V, Ti, Nb, N, W, and Co and satisfying the formula (1) and the formula (2), the remainder being Fe and incidental impurities, and has a yield strength of 758 MPa or more. 999.65 - 2276.19 x C - 17.36 x Cu - 44.80 x Ni - 34.12 x Cr - 23.22 x Mo + 80.11 x Nb - 932.00 x N - 19.18 x W - 1.77 x Co > 35.00 1.6×Mo+1.35×W+2.38×Co>7.50

Description

Technical Field

[0001] The present invention relates to a martensitic stainless steel seamless pipe and a method for producing the martensitic stainless steel seamless pipe.Background Art

[0002] In conventional oil and gas fields, 13Cr martensitic stainless steel pipes are often used as oil well pipes for use in mining. In recent years, due to depletion of oil fields, oil fields and the like in severer corrosive environments have been developed, and the use of modified 13Cr steel and 17Cr steel with higher carbon dioxide corrosion resistance and SSC resistance has been expanded.

[0003] Furthermore, in recent years, the introduction of carbon capture and storage (CCS) has been promoted worldwide to achieve carbon neutrality. In this technique, a martensitic stainless steel pipe is used to inject recovered carbon dioxide into an aquifer or a depleted oil field. In the CCS, carbon dioxide containing highly corrosive impurities, such as oxygen, sulfur oxide, and nitrogen oxide, is injected at high temperature and pressure. Since oxygen, sulfur oxide, and nitrogen oxide cause localized corrosion, steel pipes are required to be made of a material with higher localized corrosion resistance.

[0004] Also in the oil well pipe application and the CCS application, localized corrosion, such as pitting corrosion, extremely shortens the life of the material, which is a problem. Furthermore, the localized corrosion may develop into stress corrosion cracking (SCC) or sulfide stress cracking (SSC), cause cracking in steel pipes, and should therefore be suppressed.

[0005] In response to such a demand, for example, there are techniques described in Patent Literature 1 and Patent Literature 2. In a martensitic stainless steel material in Patent Literature 1, the SSC resistance can be enhanced by controlling the Cr, Mo, Cu, and Ni contents of modified 13Cr steel in the range of 11.5 ≤ Cr + 2Mo + 2Cu - 1.5Ni ≤ 14.3, and the SSC resistance can be enhanced by controlling the Ti and C contents to be Ti / C ≥ 7.5 to reduce the formation of V carbide. Furthermore, it is stated that the SSC resistance can be further enhanced by controlling the size of each intermetallic compound and each Cr oxide serving as a starting point of SSC to be 5.0 µm 2< or less and controlling the total area fraction of the intermetallic compounds and the Cr oxides to be 3.0% or less.

[0006] In a martensitic stainless steel seamless pipe for an oil well pipe described in Patent Literature 2, the amount of retained γ is reduced by satisfying -35 ≤ -109.37C + 7.307Mn + 6.399Cr + 6.329Cu + 11.343Ni - 13.529Mo + 1.276W + 2.925Nb + 196.775N - 2.621Ti - 120.307 ≤ 45 in the composition of modified 13Cr steel, and this can reduce hardness and enhance the SSC resistance.Citation ListPatent Literature

[0007] PTL 1: International Publication No. WO 2018 / 181404 PTL 2: International Publication No. WO 2019 / 225280 Summary of InventionTechnical Problem

[0008] In Patent Literature 1 and Patent Literature 2, a technique for enhancing the SSC resistance is studied from the perspective of chemical composition, microstructure, and precipitate. In each literature, however, there is no study from the perspective of suppressing the growth of localized corrosion that acts as a starting point of SSC.

[0009] In general, it is thought that a crevice or pitting corrosion where localized corrosion occurs is in an environment with locally a low pH and a high chloride ion concentration, and in such an environment corrosion behavior is different from general corrosion. Thus, an oil well pipe or a CCS injection pipe with low localized corrosion resistance may not be applicable.

[0010] The present invention has been made in view of the above circumstances and aims to provide a martensitic stainless steel seamless pipe with a yield strength of 758 MPa or more and with high localized corrosion resistance and a method for producing the martensitic stainless steel seamless pipe.Solution to Problem

[0011] To achieve these objects, the present inventors have conducted extensive studies on the influence of an alloying element on the corrosion rate in an environment simulating a crevice or the interior of pitting corrosion with a low pH and a high chloride ion concentration using a martensitic stainless steel pipe as a basic composition.

[0012] As a result, a chemical composition containing C, Cu, Ni, Cr, Mo, Nb, N, W, and Co adjusted to satisfy an appropriate relational expression and an appropriate range is subjected to quenching treatment and tempering treatment under appropriate conditions. It has been found that this provides a martensitic stainless steel seamless pipe with a desired strength and high localized corrosion resistance in an environment with a low pH and a high chloride ion concentration.

[0013] The present invention has been accomplished on the basis of these findings after further studies, and the gist of the present invention is as follows: [1] A martensitic stainless steel seamless pipe with high localized corrosion resistance, having: a chemical composition containing, on a mass percent basis, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Cu: 3.00% or less, Ni: 3.00% to 10.00%, Cr: 10.00% to 20.00%, Al: 0.100% or less, Mo: 1.00% to 4.00%, V: 1.00% or less, Ti: 0.050% or less, Nb: 0.300% or less, N: 0.100% or less, W: 4.00% or less, and Co: 0.010% to 4.500%, and satisfying the formula (1) and the formula (2), the remainder being Fe and incidental impurities, wherein the martensitic stainless steel seamless pipe has a yield strength of 758 MPa or more, 1.6 × Mo + 1.35 × W + 2.38 × Co > 7.50 wherein each element symbol in the formula (1) and the formula (2) is to be substituted by the corresponding element content (% by mass), and the element content is zero when the corresponding element is not contained. [2] The martensitic stainless steel seamless pipe according to [1], containing, in addition to the chemical composition, on a mass percent basis, one or two selected from Ta: 1.00% or less and REM: 0.200% or less. [3] A method for producing the martensitic stainless steel seamless pipe according to [1] or [2], including: heating a steel pipe material with the chemical composition at a heating temperature in the range of 1200°C to 1300°C and is subjected to pipe making to produce a steel pipe; then heating the steel pipe to a soaking temperature in the range of 920°C to 1200°C and subsequently performing quenching treatment to cool the steel pipe to room temperature; and then performing tempering treatment for tempering at a tempering temperature in the range of 550°C to 640°C. Advantageous Effects of Invention

[0014] The present invention can provide a martensitic stainless steel seamless pipe that has a yield strength of 758 MPa or more and that has high localized corrosion resistance in an environment with a low pH in a crevice or pitting corrosion and with a high chloride ion concentration, and a method for producing the martensitic stainless steel seamless pipe. Furthermore, localized corrosion can be suppressed, and the occurrence of SCC or SSC originating from localized corrosion can also be suppressed.Description of Embodiments

[0015] The present invention is described in detail below. The present invention is not limited to the following embodiments.

[0016] First, the chemical composition of a martensitic stainless steel seamless pipe according to the present invention and the reasons for limitation thereof are described below. Unless otherwise specified, the mass percentage is simply expressed in "%".C: 0.050% or less

[0017] C is an element that increases hardenability and thereby increases the strength of a steel material. However, an excessively high C content results in a decrease in Martensite-start temperature(Ms temperature), no martensite transformation at room temperature, and consequently lower strength. Thus, the C content is 0.050% or less. The C content is preferably 0.040% or less, more preferably 0.035% or less. From the perspective of ensuring the strength, the C content is preferably 0.0050% or more, more preferably 0.010% or more.Si: 1.00% or less

[0018] Si is an element that acts as a deoxidizing agent. A Si content of more than 1.00% results in lower carbon dioxide corrosion resistance and hot workability. Thus, the Si content is 1.00% or less. The Si content is preferably 0.30% or less. From the perspective of ensuring stable strength, the Si content is preferably 0.10% or more.Mn: 1.00% or less

[0019] Mn is an element that improves hot workability and strength. However, an excessive addition of Mn precipitates MnS and reduces sulfide stress corrosion cracking resistance. Thus, the Mn content is 1.00% or less. The Mn content is preferably 0.50% or less, more preferably 0.40% or less. To ensure the strength required in the present invention, the Mn content is preferably 0.10% or more.P: 0.030% or less

[0020] P is an element that lowers the carbon dioxide corrosion resistance, and is desirably decreased as much as possible in the present invention. Thus, the P content is limited to 0.030% or less. The P content is preferably 0.015% or less. The P content has no specific lower limit. However, an extreme decrease results in an increase in production costs, and the P content is therefore preferably 0.005% or more.S: 0.005% or less

[0021] S is an element that significantly lowers hot workability and is desirably decreased as much as possible. Decreasing the S content to 0.005% or less makes it possible to produce a pipe by hot working, and the S content in the present invention is therefore limited to 0.005% or less. The S content is preferably 0.002% or less. The S content has no specific lower limit. However, an excessive decrease results in an increase in production costs, and the S content is therefore preferably 0.0005% or more.Cu: 3.00% or less

[0022] Cu is an element that strengthens a protective film and improves the carbon dioxide corrosion resistance. However, a Cu content of more than 3.00% results in lower hot workability due to precipitation of metal Cu. Thus, the Cu content is 3.00% or less. The Cu content is preferably 2.60% or less. From the perspective of effectively obtaining the above effects, the Cu content is preferably 1.00% or more.Ni: 3.00% to 10.00%

[0023] Ni is an element that strengthens a protective film, improves the carbon dioxide corrosion resistance, and increases the strength of steel by solid solution. Such effects require a Ni content of 3.00% or more. However, a Ni content of more than 10.00% results in an energetically stable austenite phase, a martensite phase with lower stability, and lower strength. Thus, the Ni content is limited to the range of 3.00% to 10.00%. The Ni content is preferably 5.00% or more and preferably 8.00% or less. The Ni content is more preferably 7.00% or less.Cr: 10.00% to 20.00%

[0024] Cr is an element that forms a protective film and improves the carbon dioxide corrosion resistance. A Cr content of 10.00% or more can result in carbon dioxide corrosion resistance required for an oil well pipe or a CCS injection pipe. However, an excessively high Cr content results in a lower Ms temperature, an unstable martensite phase, and consequently lower strength. Thus, the Cr content is limited to the range of 10.00% to 20.00%. The Cr content is preferably 11.00% or more, more preferably 12.00% or more. The Cr content is preferably 17.00% or less, more preferably 15.00% or less.Al: 0.100% or less

[0025] Al is an element that acts as a deoxidizing agent. To obtain such an effect, it is effective to have an Al content of 0.01% or more. However, an Al content of more than 0.100% adversely affects toughness due to excessive precipitation of an oxide. Thus, the Al content in the present invention is limited to 0.100% or less. The Al content is preferably 0.040% or less. The Al content is preferably 0.010% or more. The term "Al content", as used herein, refers to the total Al content.Mo: 1.00% to 4.00%

[0026] Mo is an element that forms a dense corrosion product film and thereby improves the localized corrosion resistance. To achieve localized corrosion resistance required for a severe corrosive environment, the Mo content needs to be 1.00% or more. However, an excessively high Mo content of more than 4.00% results in precipitation of an intermetallic compound and lower toughness or localized corrosion resistance. Thus, the Mo content is limited to the range of 1.00% to 4.00%. The Mo content is preferably 1.40% or more, more preferably 3.00% or more. The Mo content is preferably 3.60% or less.V: 1.00% or less

[0027] V is an element that improves the strength of steel by precipitation strengthening. However, a V content of more than 1.00% results in lower toughness due to excessive precipitation of a carbide. Thus, the V content in the present invention is limited to 1.00% or less. The V content is preferably 0.07% or less. From the perspective of effectively obtaining the above effects, the V content is preferably 0.01% or more.Ti: 0.050% or less

[0028] Ti is an element that forms TiN, covers an oxide or sulfide inclusion, and thereby reduces the occurrence of pitting corrosion. However, the addition of more than 0.050% of Ti results in an excessive amount of inclusion and lower SSC resistance. Thus, the Ti content is 0.050% or less. The Ti content is preferably 0.020% or less. From the perspective of effectively obtaining the above effects, the Ti content is preferably 0.001% or more.Nb: 0.300% or less

[0029] Nb is an element that can form a carbide, reduce the amount of solid solution carbon, and improve the Ms temperature. However, an excessively high Nb content may result in lower toughness due to the precipitation of a carbide or nitride. Thus, the Nb content is 0.300% or less. The Nb content is preferably 0.100% or less. The Nb content may be 0%. From the perspective of effectively obtaining the above effects, the Nb content is preferably 0.005% or more, more preferably 0.050% or more.N: 0.100% or less

[0030] N is an element that dissolves in steel and increases the strength. However, a N content of more than 0.100% results in large amounts of various nitride inclusions and lower carbon dioxide corrosion resistance. Thus, the N content in the present invention is limited to 0.100% or less. The N content is preferably 0.050% or less. From the perspective of effectively obtaining the above effects, the N content is preferably 0.005% or more, more preferably 0.010% or more.W: 4.00% or less

[0031] W is an element that forms a dense corrosion product film and thereby improves localized corrosion resistance. However, an excessively high W content may result in lower toughness or localized corrosion resistance due to the precipitation of an intermetallic compound. Thus, the W content is 4.00% or less. The W content is preferably 3.00% or less, more preferably 2.00% or less. The W content may be 0%. From the perspective of effectively obtaining the above effects, the W content is preferably 1.0% or more.Co: 0.010% to 4.500%

[0032] Co is an element that forms a dense corrosion product film and thereby improves the localized corrosion resistance. To obtain such an effect, the Co content needs to be 0.010% or more. However, an excessive addition of more than 4.500% of Co increases production costs. Thus, the Co content ranges from 0.010% to 4.500%. The Co content is preferably 0.20% or more and preferably 4.00% or less. The Co content is more preferably 0.40% or more and more preferably 3.00% or less.

[0033] Furthermore, in the present invention, C, Cu, Ni, Cr, Mo, Nb, N, W, and Co are in the above ranges and are contained so as to satisfy the following formula (1).

[0034] Each element symbol in the formula (1) is to be substituted by the corresponding element content (% by mass), and the element content is zero when the corresponding element is not contained.

[0035] The formula (1) is correlated with the Ms temperature and is derived as a result of intensive studies by the present inventors. By satisfying this formula, the free energy difference between an austenite phase and a martensite phase exceeds the energy required for deformation accompanying transformation in a region of 35°C or more and 300°C or less. This can cause martensite transformation even during cooling to room temperature in quenching treatment described later, and can ensure sufficient strength. Thus, the value on the left side of the formula (1) (the value of "999.65 - 2276.19 x C - 17.36 x Cu - 44.80 x Ni - 34.12 x Cr - 23.22 x Mo + 80.11 x Nb - 932.00 x N - 19.18 x W - 1.77 x Co") is more than 35.00. The value on the left side of the formula (1) is preferably 45.00 or more, more preferably 55.00 or more. The value on the left side of the formula (1) has no specific upper limit. From the perspective of reducing the cost of reducing C and N, the value on the left side of the formula (1) is preferably 300.00 or less, more preferably 200.00 or less.

[0036] In the present invention, Mo, W, and Co are contained so as to satisfy the following formula (2). 1.6 × Mo + 1.35 × W + 2.38 × Co > 7.50

[0037] Each element symbol in the formula (2) is to be substituted by the corresponding element content (% by mass), and the element content is zero when the corresponding element is not contained.

[0038] The formula (2) is correlated with the localized corrosion resistance and is derived as a result of intensive studies by the present inventors. Satisfying this formula can result in high localized corrosion resistance due to the formation of a dense corrosion product film. Thus, the value on the left side of the formula (2) (the value of "1.6 x Mo + 1.35 x W + 2.38 x Co") is more than 7.50. The value on the left side of the formula (2) is preferably 7.80 or more, more preferably 8.00 or more. The value on the left side of the formula (2) has no specific upper limit. An excessive addition of an alloy promotes the precipitation of an intermetallic compound, and the value on the left side of the formula (2) is therefore preferably 20.00 or less.

[0039] As described above, a crevice or the interior of pitting corrosion is an environment with a low pH and a high chloride ion concentration. Thus, to reproduce this environment, a 5.4% by mass CrCl 3 solution (pH: 2.1, temperature of liquid: 25°C, H 2 S: 0.1 bar, CO 2 : 0.9 bar) is used as a test liquid in the present invention. Thus, the phrase "high localized corrosion resistance", as used herein, refers to a corrosion rate of 2.5 mm / y or less when a test piece is immersed in this environment (that is, in this test liquid) for 24 hours. A method of measuring the corrosion rate is described in detail later in Examples.

[0040] In the present invention, the remainder other than these components is composed of Fe and incidental impurities.

[0041] The components described above are base components, and a martensitic stainless steel seamless pipe according to the present invention with these base components can have desired characteristics. In the present invention, in addition to these base components, the following optional elements can be contained as required. The following components Ta and REM can be contained as required and may be 0%.

[0042] One or two selected from Ta: 1.00% or less and REM: 0.200% or lessTa: 1.00% or less

[0043] Ta is an element that precipitates a nitride together with Ti, covers an oxide or sulfide inclusion, and thereby reduces the occurrence of pitting corrosion, and can be selectively contained as required. Even when Ta is excessively added, the effects are saturated. Thus, when Ta is contained, the Ta content is 1.00% or less. The Ta content is preferably 0.70% or less. The Ta content is preferably 0.10% or more.REM: 0.200% or less

[0044] A rare-earth metal (REM) is an element that modifies an oxide or sulfide inclusion and reduces the occurrence of pitting corrosion, and can be selectively contained as required. However, an excessive addition of REM lowers toughness. Thus, when REM is contained, the REM content is 0.200% or less. The REM content is preferably 0.100% or less, more preferably 0.060% or less. The REM content is preferably 0.001% or more.

[0045] Next, the steel pipe microstructure of a martensitic stainless steel seamless pipe according to the present invention and the reasons for limitation thereof are described below.

[0046] A steel pipe microstructure in the present invention has a martensite phase as a main phase. The term "main phase", as used herein, refers to a volume fraction of 55% or more based on the entire steel pipe microstructure. The upper limit of the martensite phase is preferably, but not limited to, a volume fraction of 95% or less because the stress corrosion cracking resistance may decrease.

[0047] The steel pipe microstructure may contain, in addition to the martensite phase, a ferrite phase with a volume fraction of 40% or less. Due to its softness, a ferrite phase with a volume fraction of more than 40% reduces the strength of steel. The volume fraction of the ferrite phase is preferably 30% or less, more preferably 10% or less. The lower limit of the ferrite phase is preferably, but not limited to, a volume fraction of 0.1% or more, more preferably 1.0% or more, for stable production. A retained austenite phase with a volume fraction of 40% or less may also be contained. Due to the softness of the austenite phase, a retained austenite phase with a volume fraction of more than 40% reduces the strength of steel. The volume fraction of the retained austenite phase is preferably 30% or less, more preferably 20% or less. The lower limit of the retained austenite phase is preferably, but not limited to, a volume fraction of 1% or more, more preferably 5% or more, for stable production.

[0048] Next, an embodiment of a method for producing a martensitic stainless steel seamless pipe according to the present invention is described below. In the following description of the production method, unless otherwise specified, the temperature (°C) is the surface temperature of a steel pipe material or a steel pipe (that is, a seamless steel pipe after pipe production). The surface temperature can be measured with a radiation thermometer or the like.

[0049] In the present invention, a steel pipe material with the chemical composition described above is used. The steel pipe material may be produced by any method. For example, preferably, a molten steel with the above chemical composition is obtained by steelmaking using a melting method, such as a converter, and is then formed into a steel pipe material, such as a billet, by a continuous casting method, an ingot casting and blooming method, or the like.

[0050] Subsequently, the steel pipe material is heated in a heating step, and the heated steel pipe material is formed into a hollow mother pipe by a Mannesmann-plug mill process or a Mannesmann-mandrel mill process using a piercer and is then subjected to hot working and pipe production in a pipe production step. Through these steps, a seamless steel pipe with desired dimensions (that is, a predetermined shape) and with the chemical composition is produced. A seamless steel pipe may also be produced by hot extrusion by a pressing method.

[0051] In the heating step of the steel pipe material, the heating temperature preferably ranges from 1200°C to 1300°C. At a heating temperature of less than 1200°C, a large stress is required for deformation, and it is therefore difficult to produce a steel pipe. On the other hand, a high heating temperature of more than 1300°C results in an excessively high amount of ferrite and steel with lower workability, which makes the steel pipe production difficult. From the perspective of temperature homogenization and heating cost, the heating time at the heating temperature preferably ranges from 10 to 200 minutes.

[0052] When the temperature decreases in the pipe production step, the heating step may be performed once or more in the middle of the pipe production step.

[0053] A seamless steel pipe after pipe production is preferably cooled to room temperature at a cooling rate equal to or higher than natural cooling. This can induce martensite transformation and achieve a desired strength.

[0054] Subsequently, the seamless steel pipe (hereinafter referred to as a "steel pipe") is subjected to quenching treatment and tempering treatment.[Quenching Treatment]

[0055] More specifically, the steel pipe is heated to a soaking temperature of 920°C or more and 1200°C or less, is held for a predetermined time, and is then subjected to quenching treatment to decrease the steel pipe surface temperature to room temperature.

[0056] When the heating temperature of the quenching treatment (that is, the soaking temperature) is less than 920°C, the melting temperature of a precipitated intermetallic compound may not be reached, and the desired localized corrosion resistance cannot be achieved. Thus, the soaking temperature is limited to 920°C or more. The soaking temperature is preferably 950°C or more. A soaking temperature of more than 1200°C results in a coarsened microstructure and a steel with lower strength. Thus, the soaking temperature is limited to 1200°C or less. The soaking temperature is preferably 1170°C or less.

[0057] From the perspective of ensuring temperature uniformity, the soaking temperature is preferably held for 5 minutes or more. The holding time is preferably 300 minutes or less.

[0058] The cooling method and cooling conditions in the quenching treatment are not particularly limited. For example, the cooling may be natural cooling or water cooling. The term "natural cooling", as used herein, refers to a cooling rate of 0.05°C / s or more and 20°C / s or less, and the term "water cooling", as used herein, refers to a cooling rate of 5°C / s or more and 100°C / s or less.[Tempering Treatment]

[0059] Subsequently, a steel pipe subjected to the quenching treatment is subjected to tempering treatment. More specifically, the tempering treatment includes heating to a temperature of 550°C or more and 640°C or less (that is, a tempering temperature), holding for a predetermined time, and then natural cooling.

[0060] A tempering temperature of more than 640°C results in austenite reverse transformation, thus failing to achieve a desired strength. Thus, the tempering temperature is limited to 640°C or less. The tempering temperature is preferably 620°C or less. A tempering temperature of less than 550°C results in precipitation of carbide and lower pitting corrosion resistance. Thus, the tempering temperature is limited to 550°C or more. The tempering temperature is preferably 580°C or more.

[0061] From the perspective of ensuring temperature uniformity, the tempering temperature is preferably held for 10 minutes or more. The holding time is preferably 200 minutes or less.EXAMPLES

[0062] The present invention is described below on the basis of Examples. The following contents are merely examples for carrying out the present invention, and the present invention is not limited to these embodiments. Thus, the present invention can be implemented by appropriately modifying the following embodiments without departing from the gist of the present invention.

[0063] 50 kg of a molten steel with the chemical composition shown in Table 1 was obtained by steelmaking in a vacuum furnace and was formed into an ingot by ingot casting. The ingot was heated at 1250°C for 60 minutes and was rolled to a wall thickness of 105 mm. It was then heated again at 1250°C for 60 minutes to produce a sheet material with a length of 1500 mm, a width of 160 mm, and a sheet thickness of 15 mm. The steps for producing the sheet material may be considered to be the same as the heating step and the pipe production step performed on the steel pipe material described above. The sheet material was then subjected to quenching treatment and tempering treatment under the conditions shown in Table 2 to produce a test material. The quenching treatment and the tempering treatment performed on the sheet material may be considered to be the same as the quenching treatment and the tempering treatment performed on a seamless steel pipe.

[0064] The test material was subjected to a tensile test and a localized corrosion resistance test by the methods described below.[Tensile Test]

[0065] A Japanese Industrial Standards (JIS) No. 14A test piece for tensile test (a round bar test piece for tensile test with a parallel portion diameter of 6 mm and a parallel portion length of 25 mm) was taken from the test material. The longitudinal direction of the parallel portion of the test piece was the rolling direction of the sheet material. The test piece was subjected to a tensile test at normal temperature (25°C) to determine the yield strength (YS) (unit: MPa). The yield strength was 0.2% proof stress. The tensile test was performed in accordance with the provisions of JIS Z 2241: 2022. One with a yield strength of 758 MPa or more was judged to be acceptable, and one with a yield strength of less than 758 MPa was judged to be unacceptable.[Localized Corrosion Resistance Test]

[0066] A test piece with a width of 20 mm, a length of 50 mm, and a thickness of 3 mm was taken from the test material.

[0067] The test piece was immersed in a test liquid, a 5.4% by mass CrCl 3 aqueous solution (pH: 2.1, temperature of liquid: 25°C, H 2 S: 0.1 bar, CO 2 : 0.9 bar), and was subjected to a corrosion test for a test period (immersion period) of 24 hours. This is a corrosion test simulating the environment of a crevice or the interior of pitting corrosion with a low pH and a high chloride ion concentration. The weight of the test piece after the test was measured to determine the corrosion rate from the weight loss due to the corrosion test. One with a corrosion rate of 2.50 mm / y or less was judged to be acceptable, and one with a corrosion rate of more than 2.50 mm / y was judged to be unacceptable.[Measurement of Microstructure]

[0068] A test piece for microstructure observation was prepared from the test material, and the microstructure was measured. An observation surface of the microstructure was a cross section perpendicular to the rolling direction (C cross section). First, the test piece for microstructure observation was corroded with a Vilella's reagent (more specifically, a mixed reagent of 2 g of picric acid, 10 ml of hydrochloric acid, and 100 ml of ethanol), and an image of the microstructure was taken with an optical microscope. The magnification was 400 times. The microstructure fraction (% by area) of a ferrite phase was calculated using an image analyzer. The area fraction of the ferrite phase was regarded as the volume fraction (%) of the ferrite phase.

[0069] A test piece for X-ray diffraction was ground and electropolished such that the cross section perpendicular to the rolling direction (C cross section) became a measurement surface, and the amount of retained austenite (γ) was measured by X-ray diffractometry. The integrated X-ray diffraction intensities of the (200) plane, the (220) plane, and the (311) plane of γ, and the (200) plane and the (211) plane of α (ferrite) were measured to calculate the amount of retained austenite using the following formula. γ volume fraction = 100 / 1 + IαRγ / IγRα

[0070] Iα denotes the integral intensity of α, Rα denotes the crystallographic theoretically calculated value of α, Iγ denotes the integral intensity of γ, and Rγ denotes the crystallographic theoretically calculated value of γ.

[0071] The remainder other than the ferrite phase and the retained austenite phase obtained by the above measurement method was defined as the fraction (unit: volume fraction) of a martensite phase.

[0072] Table 2 shows the results. [Table 1]Steel No.Chemical composition (% by mass)CSiMnPSCuNiCrtotal AlMoVTiNbNWCoTaREMFormula (1) *1Formula (2) *2A0.0310.150.480.0150.00092.536.0512.600.0343.610.0600.0050.2300.05151.620.400039.048.92B0.0370.160.460.0160.00082.595.8213.070.0343.630.0600.0050.2290.05030.790.400035.107.83C0.0110.150.480.0140.00132.556.3711.990.0403.670.0590.0040.0490.03011.260.4000101.658.53D0.0110.250.320.0150.00072.466.1512.500.0311.990.0580.0040.0820.04892.980.420086.778.21E0.0190.260.360.0140.00081.566.1912.280.0313.920.0600.0140.0680.03481.371.890085.3812.62F0.0130.120.480.0130.00091.536.9011.640.0311.130.0600.0030.0890.01262.313.7300155.4613.80G0.0310.270.320.0150.00071.065.1812.010.0301.220.0580.0100.0730.03163.911.7900138.7511.49H0.00880.140.210.0140.00092.536.5212.450.0363.560.0550.00300.010300.8000125.137.60I0.0110.290.260.0150.00082.086.5014.580.0401.630.0600.0010.0580.03380.794.350062.2814.03J0.0120.120.280.0150.00091.985.5114.230.0323.810.0600.0140.0980.03351.250.0130069.757.81K0.0100.210.310.0150.00100.124.8019.500.0341.140.0710.0030.1300.01271.022.120043.228.25L0.0420.160.280.0140.00090.565.4110.680.0403.060.0580.0060.0560.03691.110.7300203.387.69M0.0260.310.360.0130.00091.033.3314.900.0312.860.0550.0070.0730.02370.960.7700162.597.70N0.0100.260.400.0150.00080.089.6212.960.0311.250.0490.0040.1210.01440.582.630053.809.04O0.0230.280.310.0140.00072.536.7813.270.0393.120.0580.0070.0760.01641.460.970.063035.509.27P0.0340.200.320.0150.00072.495.4011.740.0322.940.0580.0040.0650.04322.332.5000.005084.1113.80Q0.00930.240.320.0140.00102.536.2012.570.0351.960.0550.0090.0780.04703.070.410.0570.004885.248.26R0.02580.160.470.0140.00082.586.0012.580.0323.550.0560.0120.2300.04881.640.400.0620.005256.468.85S0.0330.150.470.0150.00102.595.8113.280.0323.560.0560.0110.2300.04940.850.400.0620.004838.887.80T0.00920.150.480.0130.00092.596.3512.010.0373.550.0550.0100.0460.03301.250.400.0620.0048105.308.32U0.0310.240.290.0150.00091.046.0512.010.0304.220.0600.0100.0840.03892.110.430061.4710.62V0.0170.130.270.0140.00082.517.3212.630.0370.880.0600.0150.0860.01212.272.460085.8010.33W0.0130.280.240.0150.00111.126.8513.160.0311.340.0600.0160.1270.02114.580.790064.8710.21X0.0230.210.240.0130.00092.045.5014.420.0371.670.0600.0040.0820.01391.774.810085.8516.51Y0.0100.170.350.0140.00091.496.8913.650.0353.560.0600.0070.0780.01152.2700045.948.76Z0.0240.250.310.0150.00071.037.5512.880.0423.480.0560.0130.1610.03041.440.200025.227.99AA0.0110.250.320.0150.00082.496.3612.690.0411.450.0580.0040.0810.01253.000.4100116.387.35AB0.0370.160.460.0160.00082.595.8213.070.0343.630.0600.0050.2290.05030.790.400035.107.83AC0.0110.150.480.0140.00132.556.3711.990.0403.670.0590.0040.0490.03011.260.4000101.658.53*1. 999.65 − 2276.19 × C − 17.36 × Cu − 44.80 × Ni − 34.12 × Cr − 23.22 × Mo + 80.11 × Nb − 932.00 × N − 19.18 × W − 1.77 × Co > 35.00 *2. 1.6 × Mo + 1.35 × W + 2.38 × Co > 7.50 [Table 2] Test material No.Steel No.Heating temperature (°C)Quenching treatmentTempering treatmentFerrite fraction (% by volume)Volume fraction of retained austenite (% by volume)Value of formula (1)Value of formula (2)YS (MPa)Corrosion rate (mm / y)NoteSoaking temperature (°C)Soaking time (min)Tempering temperature (°C)Tempering time (min)1A1250120020600305.87.239.048.929462.15Example2B1250117020600306.31135.107.839262.41Example3C1250105020600302.98101.658.5311022.12Example4D1250112020600301.214.386.778.2111121.91Example5E1250114020590307.727.485.3812.629591.98Example6F1250120020580303.38.2155.4613.809352.06Example7G1250110020610301.95.1138.7511.4910211.95Example8H125096020600300.97.2125.137.6010002.05Example9I1250117020630302.515.662.2814.038292.46Example10J12501190206003021.313.169.757.819602.19Example11K12501010206103032.410.943.228.257981.98Example12L1250103020570301.52.5203.387.697632.31Example13M12501060205903027.63.9162.597.708372.37Example14N125097020610300.713.253.809.048132.26Example15O1250108020590304.51735.509.2710632.28Example16P1250116020610301.916.584.1113.8010302.01Example17Q1250112020600301.714.285.248.2611322.06Example18R1250120020600306.621.856.468.8510052.40Example19S1250117020600307.75.338.887.8010232.47Example20T1250105020600302.21.6105.308.3211092.30Example21C1210105020600302.710.1101.658.5311602.15Example22C1290105020600303.69.8101.658.5310582.35Example23B1250117010600305.110.335.107.839332.47Example24H1250960300600301.27125.137.609462.28Example25I1250117020630201.411.362.2814.038662.41Example26F12501200205502003.313.2155.4613.809101.96Example27U1250120020610301013.461.4710.629863.12Comparative example28v125096020610300.324.485.8010.338653.65Comparative example29W1250100020590305.324.764.8710.219683.42Comparative example30X12501140206003010.22.385.8516.519552.56Comparative example31Y12501170206303014.27.945.948.768934.83Comparative example32Z1250103520600301.662.225.227.993901.54Comparative example33AA1250117020600300.217.4116.387.3510912.59Comparative example34AB125091020600306.328.435.107.839262.68Comparative example35AC1250105020650302.945.9101.658.537152.12Comparative example36B12501220206003020.110.435.107.837332.42Comparative example37I1250117020540301.44.962.2814.039513.08Comparative example

[0073] All the examples had a yield strength (YS) of 758 MPa or more and had high localized corrosion resistance in an environment with a low pH and a high chloride ion concentration. By contrast, in Comparative Examples outside the scope of the present invention, at least one of the yield strength (YS) and the localized corrosion resistance did not have a desired value.Industrial Applicability

[0074] A steel material according to the present invention is useful as an oil well pipe material and a CCS injection pipe material for which high strength and localized corrosion resistance are required.

Claims

1. A martensitic stainless steel seamless pipe with high localized corrosion resistance, comprising: a chemical composition containing, on a mass percent basis, C: 0.050% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.005% or less, Cu: 3.00% or less, Ni: 3.00% to 10.00%, Cr: 10.00% to 20.00%, Al: 0.100% or less, Mo: 1.00% to 4.00%, V: 1.00% or less, Ti: 0.050% or less, Nb: 0.300% or less, N: 0.100% or less, W: 4.00% or less, and Co: 0.010% to 4.500%, and satisfying the formula (1) and the formula (2), the remainder being Fe and incidental impurities, wherein the martensitic stainless steel seamless pipe has a yield strength of 758 MPa or more, 1.6 × Mo + 1.35 × W + 2.38 × Co > 7.50 wherein each element symbol in the formula (1) and the formula (2) is to be substituted by the corresponding element content (% by mass), and the element content is zero when the corresponding element is not contained.

2. The martensitic stainless steel seamless pipe according to Claim 1, comprising, in addition to the chemical composition, on a mass percent basis, one or two selected from Ta: 1.00% or less and REM: 0.200% or less.

3. A method for producing the martensitic stainless steel seamless pipe according to Claim 1 or 2, comprising: heating a steel pipe material with the chemical composition at a heating temperature in the range of 1200°C to 1300°C and is subjected to pipe making to produce a steel pipe; then heating the steel pipe to a soaking temperature in the range of 920°C to 1200°C and subsequently performing quenching treatment to cool the steel pipe to room temperature; and then performing tempering treatment for tempering at a tempering temperature in the range of 550°C to 640°C.

Images