JP4337712B2 - Martensitic stainless steel - Google Patents

Description

  The present invention relates to martensitic stainless steel, and more particularly to martensitic stainless steel used in a corrosive environment containing corrosive substances such as hydrogen sulfide, carbon dioxide, and chlorine ions.

  With the deepening of oil wells and gas wells, martensitic stainless steel used as oil well steel materials such as oil well pipes is required to have high strength and high toughness. Therefore, martensitic stainless steel having a yield stress (0.2% yield strength) of 758 to 860 MPa (hereinafter referred to as 110 ksi class) and martensitic stainless steel having high strength of 110 ksi class or higher have been developed.

  The martensitic stainless steel for oil wells is also required to have high corrosion resistance such as SCC (Stress Corrosion Cracking) resistance and SSC (Sulfide Stress Cracking) resistance. This is because oil wells and gas wells are corrosive environments and contain corrosive substances such as hydrogen sulfide, carbon dioxide, and chlorine ions. In other words, martensitic stainless steel for oil wells is required to have high strength, high toughness, and high corrosion resistance.

  The following patent document 1 (Unexamined-Japanese-Patent No. 2003-3243) is disclosed as martensitic stainless steel having high strength and high corrosion resistance. The martensitic stainless steel disclosed in this document has a higher SSC resistance than conventional martensitic stainless steel by setting the Mo content to 1.5% by mass or more.

  By the way, when Mo content is high, the range of the tempering temperature (henceforth a tempering temperature range) from which 110 ksi grade intensity | strength is obtained becomes very small. FIG. 2 is a diagram showing the relationship between the yield stress and the tempering temperature of martensitic stainless steel having a high Mo content (hereinafter referred to as high Mo martensitic stainless steel). The high Mo martensitic stainless steel of FIG. 2 contains 0.0016% C, 11.8% Cr, 7.2% Ni, 2.9% Mo by mass% with the balance being Fe and impurities. It is. Referring to FIG. 2, the tempering curve C10 has a large slope in the range where the yield stress is in the range of 758 to 860 MPa. Therefore, the tempering temperature must be set in the range of about 580 to about 600 ° C. in order to make the strength of the high Mo martensitic stainless steel 110 ksi class. That is, the tempering temperature range ΔT for making the strength 110 ksi class is very small.

If the tempering temperature range ΔT is small, the productivity decreases. Usually, high Mo martensitic stainless steel is manufactured continuously for several hundred tons. In this case, the high Mo martensitic stainless steel is manufactured from a plurality of heats (molten steel obtained in one steel making process), but the chemical composition of each heat does not completely match and varies slightly. When the tempering temperature range ΔT is small, the tempering temperature must be changed every time the chemical composition changes in order to make the steel have a strength of 110 ksi. In short, to set the strength to 110 ksi class, it is necessary to change the setting of the tempering temperature for each heat. Such a change in setting of the tempering temperature decreases productivity.
JP 2003-3243 A International Publication WO2004 / 57050

  An object of the present invention is to provide a martensitic stainless steel having a large tempering temperature range in which a yield stress of 758 to 860 MPa is obtained.

Means for Solving the Problems and Effects of the Invention

  As a result of various experiments and examinations, the present inventor has obtained the following knowledge.

If the chemical composition as (A) _{A c1} transformation point of martensitic stainless steel is high, the tempering temperature range becomes the yield stress in 758~860MPa widens. This is because, if the _Ac1 transformation point is low, austenite is generated during high-temperature tempering, thereby reducing the strength.

(B) Not only increase the A _c1 transformation point, but also decrease the C content. Thereby, the tempering temperature range in which the yield stress becomes 758 to 860 MPa is further widened. This is because the higher the C content, the greater the gradient of the tempering curve within the yield stress range of 758 to 860 MPa.

  (C) If the C content is lowered, δ ferrite is easily generated, which affects the strength and toughness of the steel. Since 110 ksi grade martensitic stainless steel is used even in an environment where the outside air is below 0 ° C., it requires high strength and high toughness. By making the steel composition martensitic even if the Ac1 transformation point is increased and the C content is decreased, the formation of δ ferrite can be suppressed, and the toughness can be maintained while maintaining the strength of 110 ksi class. Decline can be prevented.

  As a result of examination based on the above knowledge, if the C content is 0.01% or less and the expressions (1) and (2) are satisfied, the tempering temperature range in which the yield stress becomes 758 to 860 MPa is obtained. We found that it can be made larger than before.

  922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-1584.9Al-376.1N ≧ 600 (1)

  30C + 0.5Mn + Ni-1.5Si-Cr-Mo + 7.9 ≧ 0 (2)

  Here, the symbol in a formula | equation is content (mass%) of each element.

The left side of equation (1) (hereinafter, the left side of equation (1) = F1) is an equation for predicting the A _c1 transformation point of the martensitic stainless steel of the present invention. As described above, if the _Ac1 transformation point is increased, it is possible to suppress the precipitation of retained austenite during tempering, and thus it is possible to prevent the yield stress from rapidly decreasing. In other words, the gradient of the tempering curve when the yield stress is in the range of 758 to 860 MPa can be reduced.

  The reason why F1 ≧ 600 is that the tempering process is executed at a tempering temperature of 600 ° C. or lower. If the tempering temperature is set to 600 ° C. or more, fine carbides and intermetallic compounds in the steel become coarse, and the strength is lowered and the toughness is further lowered. Since the tempering temperature is 600 ° C. or lower, it is sufficient that the F1 value is 600 ° C. or higher.

  The left side of Equation (2) is an equation for martensifying the steel after tempering. If the content of C, Mn, and Ni that are austenite forming elements and the content of Si, Cr, and Mo that are ferrite forming elements satisfy the relationship of formula (2), the structure becomes martensite, and δ ferrite Generation can be prevented. Therefore, strength reduction can be prevented and high toughness can be maintained.

  In addition, what is necessary is just to satisfy | fill Formula (3) instead of Formula (1), when a martensitic stainless steel contains Ti. Moreover, what is necessary is just to satisfy | fill Formula (4) instead of Formula (2), when it contains the austenite formation element Cu.

  922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-125.1Ti-1584.9Al-376.1N ≧ 600 (3)

  30C + 0.5Mn + Ni + 0.5Cu-1.5Si-Cr-Mo + 7.9 ≧ 0 (4)

  If these equations are satisfied, a tempering curve such as the curve C1 shown in FIG. 1 can be obtained, and the gradient of the tempering curve in the yield stress range of 758 to 860 MPa can be made smaller than before. Therefore, the tempering temperature range ΔT1 in which the yield stress is 758 to 860 MPa is larger than the tempering temperature range ΔT2 of the conventional tempering curve C2. Therefore, it is possible to suppress a decrease in productivity based on a setting change of the tempering temperature during operation.

  Based on the above findings, the present inventor has completed the following invention.

  The martensitic stainless steel according to the present invention is, in mass%, C: 0.001 to 0.01%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04%. Hereinafter, S: 0.01% or less, Cr: 10-15%, Ni: 4-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, N: 0.00. It contains not more than 07%, and the balance consists of Fe and impurities, satisfies the formulas (1) and (2), and has a yield stress of 758 to 860 MPa.

  922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-1584.9Al-376.1N ≧ 600 (1)

  30C + 0.5Mn + Ni-1.5Si-Cr-Mo + 7.9 ≧ 0 (2)

  Here, the symbol in a formula | equation is content (mass%) of each element. Moreover, 0.2% yield strength is defined as yield stress.

The martensitic stainless steel according to the present invention can reduce the gradient of the tempering curve by setting the C content to 0.01% or less. Furthermore, by satisfying the formula (1), the _Ac1 transformation point can be made higher than before. Therefore, the gradient of the tempering curve becomes small, and the tempering temperature range in which the yield stress becomes 758 to 860 MPa becomes larger than before.

  Furthermore, by satisfy | filling Formula (2), it can prevent that intensity | strength becomes less than 110 ksi and can maintain high toughness. Moreover, since Mo content is high, it has high corrosion resistance.

The martensitic stainless steel according to the present invention is, in mass%, C: 0.001 to 0.01%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04%. Hereinafter, S: 0.01% or less, Cr: 10-15%, Ni: 4-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, N: 0.00. And at least one of Ti: 0.25% or less , V: 0.25% or less , Nb: 0.25% or less , Zr: 0.25% or less , with the balance being Fe And impurities, satisfying the expressions (2) and (3), and having a yield stress of 758 to 860 MPa.

  30C + 0.5Mn + Ni-1.5Si-Cr-Mo + 7.9 ≧ 0 (2)

  922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-125.1Ti-1584.9Al-376.1N ≧ 600 (3)

  Here, the symbol in a formula | equation is content (mass%) of each element. In addition, the numerical value of Ti in Formula (3) may be 0.

  In this case, the tempering temperature range in which the yield stress of 758 to 860 MPa can be obtained can be made larger than before by making the C content 0.01% or less and satisfying the formula (3). Furthermore, by satisfy | filling Formula (2), it can prevent that intensity | strength becomes less than 110 ksi and can maintain high toughness.

The martensitic stainless steel according to the present invention is, in mass%, C: 0.001 to 0.01%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04%. Hereinafter, S: 0.01% or less, Cr: 10-15%, Ni: 4-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, N: 0.00. 07% or less, Ti: 0.25% or less , V: 0.25% or less , Nb: 0.25% or less , Zr: 0.25% or less , and Cu: 1.0% The remainder consists of Fe and impurities, satisfies the formulas (3) and (4), and has a yield stress of 758 to 860 MPa.

  922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-125.1Ti-1584.9Al-376.1N ≧ 600 (3)

  30C + 0.5Mn + Ni + 0.5Cu-1.5Si-Cr-Mo + 7.9 ≧ 0 (4)

  Here, the symbol in a formula | equation is content (mass%) of each element. In addition, the numerical value of Ti in Formula (3) may be 0.

  In this case, the tempering temperature range in which the yield stress of 758 to 860 MPa can be obtained can be made larger than before by making the C content 0.01% or less and satisfying the formula (3). Furthermore, by satisfy | filling Formula (4), intensity | strength can be prevented from becoming less than 110 ksi, and high toughness is maintained.

Preferably, it further contains at least one of Ca: 0.005% or less , Mg: 0.005% or less , La: 0.005% or less , Ce: 0.005% or less .

  In this case, the hot workability of martensitic stainless steel is improved. Even if these elements are contained, the effect of the invention is not affected.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

1. Chemical Composition The martensitic stainless steel according to the embodiment of the present invention has the following composition. Hereinafter, “%” related to elements means “% by mass”.

C: 0.001 to 0.01%
If C is contained excessively, the gradient of the tempering curve becomes steep, and a steel having a yield stress of 758 to 860 MPa cannot be stably obtained. The C content should be limited low. On the other hand, if the C content is less than 0.001%, the production cost increases. Therefore, the C content is 0.001 to 0.01%. A preferable C content is 0.001 to 0.008%.

Si: 0.5% or less Si is effective as a deoxidizer. On the other hand, since Si hardens steel, if the Si content is too high, the toughness and workability of the steel deteriorate. Moreover, since Si is a ferrite forming element, it prevents the martensite formation of steel. Therefore, the Si content is 0.5% or less. A preferable Si content is 0.3% or less.

Mn: 0.1 to 3.0%
Mn contributes to the improvement of hot workability of steel. Further, Mn is an austenite-forming element and contributes to the martensitic transformation of the structure. However, if Mn is contained excessively, the toughness is lowered. Therefore, the Mn content is 0.1 to 3.0%. A preferable Mn content is 0.3 to 1.0%.

P: 0.04% or less P is an impurity. Since P generates SSC, the P content is limited as low as possible. Specifically, the P content is set to 0.04% or less.

S: 0.01% or less S is an impurity. S decreases hot workability. Therefore, the S content is limited as low as possible. Specifically, the S content is set to 0.01% or less.

Cr: 10-15%
Cr contributes to improvement of corrosion resistance in a wet carbon dioxide environment. On the other hand, since Cr is a ferrite-forming element, if it is excessively contained, tempered martensite is difficult to be formed, and strength and toughness are reduced. Therefore, the Cr content is 10 to 15%. A preferable Cr content is 11 to 14%.

Ni: 4-8%
Ni is an austenite-forming element and is necessary to make the structure after tempering martensite. When the Ni content is too low, the structure after tempering contains a large amount of ferrite. On the other hand, when there is too much Ni content, the structure | tissue after tempering contains much austenite. Therefore, the Ni content is 4 to 8%. A preferable Ni content is 4 to 7%.

Mo: 2.8 to 5.0%
Mo is an important element contributing to the improvement of SSC resistance and strength. In the martensitic stainless steel according to the present embodiment, the lower limit of the Mo content is set to 2.8% in order to obtain high SSC resistance. On the other hand, since Mo is a ferrite-forming element, excessive addition hinders the formation of martensite in the structure. Therefore, the upper limit of the Mo content is 5.0%. A preferable Mo content is 2.8 to 4.0%.

Al: 0.001 to 0.10%
Al is effective as a deoxidizer. On the other hand, when there is too much Al content, many inclusions will be produced | generated and corrosion resistance will fall. Therefore, the Al content is 0.001 to 0.10%. A preferable Al content is 0.001 to 0.06%.

N: 0.07% or less N forms nitrides and reduces corrosion resistance. Therefore, the N content is 0.07% or less.

  The balance is composed of Fe and impurities. Impurities are included due to various factors in the manufacturing process.

  The martensitic stainless steel according to the present embodiment further contains at least one of Ti, V, Nb, and Zr as necessary.

Ti: 0.25% or less V: 0.25% or less Nb: 0.25% or less Zr: 0.25% or less Ti, V, Nb, and Zr are selective elements. These elements fix C and reduce variations in strength. On the other hand, when these elements are contained excessively, the structure after tempering is prevented from becoming martensite. Therefore, the content of these elements is set to 0.25% or less . Preferred contents are Ti: 0.005-0.25%, V: 0.005-0.25%, Nb: 0.002-0.25%, Zr: 0.0003-0.25% , more preferably the upper limit of the content is, respectively is 0. 20%.

  The martensitic stainless steel according to the present embodiment further contains Cu as necessary.

Cu: 1.0% or less Cu is a selective element. Cu, like Ni, is an austenite forming element and is effective for martensite formation after tempering. On the other hand, when Cu is contained excessively, hot workability will fall. Therefore, the Cu content is 1.0% or less, preferably 0.02 to 1.0%.

  The martensitic stainless steel according to the present embodiment further contains one or more of Ca, Mg, La, and Ce as necessary.

Ca: 0.005% or less Mg: 0.005% or less La: 0.005% or less Ce: 0.005% or less Ca, Mg, La, and Ce are all selective elements. These elements contribute to the improvement of hot workability. On the other hand, when these elements are contained excessively, coarse oxides are generated, and the corrosion resistance is lowered. Therefore, the content of these elements is 0.005% or less, preferably 0.0002 to 0.005% . Among these elements, Ca and La are elements that particularly contribute to the improvement of hot workability.

2. Manufacturing method Steel having the above chemical composition is melted and refined by a well-known refining process. Subsequently, the molten steel is made into a continuous cast material by a continuous casting method. The continuous cast material is, for example, a slab, bloom or billet. Alternatively, the molten steel is made into an ingot by the ingot-making method.

  Hot-work slabs, blooms, and ingots into billets. At this time, the billet may be formed by hot rolling or may be formed by hot forging.

  A billet obtained by continuous casting or hot working is hot worked into an oil well pipe. For example, the Mannesmann method is performed as hot working. As the hot working, a hot extrusion such as a Eugene-Segenel method may be performed, or a forged pipe manufacturing method such as an Erhardt method may be performed. Quench and temper the oil well pipe after hot working. The quenching process is performed by a known method. For example, the quenching temperature is 900 to 950 ° C. However, other temperature ranges may be used.

  In the tempering treatment, a preferable lower limit of the tempering temperature is 500 ° C. On the other hand, if the tempering temperature is too high, retained austenite precipitates and the yield stress cannot be 758 to 860 MPa. Therefore, the upper limit of the preferable tempering temperature is 600 ° C.

  Furthermore, the martensitic stainless steel according to the embodiment of the present invention satisfies the following formulas (1) and (2).

  922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-1584.9Al-376.1N ≧ 600 (1)

30C + 0.5Mn + Ni-1.5Si-Cr-Mo + 7.9 ≧ 0 (2)
It satisfies the equation _(1), since the higher the transformation point _{A c1,} the yield stress can be reduced the gradient of the tempering curves in the range of 758～860MPa. Moreover, if the formula (2) is satisfied, the formation of martensite in the structure can be accelerated. Therefore, if the formulas (1) and (2) are satisfied, the tempering temperature range in which a yield stress of 758 to 860 MPa can be obtained can be made larger than before. Therefore, it is possible to suppress a decrease in productivity based on a setting change of the tempering temperature during operation.

  Furthermore, high toughness required as oil well steel can be obtained by satisfying the formula (2).

  In addition, when the martensitic stainless steel by this Embodiment contains Ti, Formula (3) is satisfy | filled instead of Formula (1). Moreover, when a martensitic stainless steel contains Cu, Formula (4) is satisfy | filled instead of Formula (2).

  922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-125.1Ti-1584.9Al-376.1N ≧ 600 (3)

  30C + 0.5Mn + Ni + 0.5Cu-1.5Si-Cr-Mo + 7.9 ≧ 0 (4)

  In the above description, martensitic stainless steel is used as the steel pipe, but martensitic stainless steel may be used as the steel plate.

Test materials having the chemical composition shown in Table 1 were produced, and the tempering temperature range in which the yield stress was 758 to 860 MPa was investigated in each test material. Furthermore, the toughness and corrosion resistance of each specimen were investigated.

  Steel having the chemical composition shown in Table 1 was melted. As shown in Table 1, the chemical compositions of the test materials 1 to 11 were within the range of the chemical composition of the present invention.

  Here, F1 and F2 of each test material were obtained with the left side of Formula (1) as F1 and the left side of Formula (2) as F2. At this time, the left side of formula (3) = F3 was obtained instead of F1 for the test material containing Ti. Further, for the specimen containing Cu, the left side of formula (4) = F4 was determined instead of F2.

  As for the test material 1-the test material 11, all F1-F4 was in the range of this invention. Specifically, F1 and F3 were 600 or more, and F2 and F4 were 0 or more.

  On the other hand, in the test materials 12 and 13, although the chemical composition was within the range of the present invention, the F3 value was less than 600. In the test materials 14 to 16, the C content exceeded the upper limit of the present invention. Further, the sample material 14 had an F3 value of less than 600, and the sample material 15 had an F4 value of less than 0.

  The molten steels of test materials 1 to 16 were cast into continuous cast materials. The manufactured continuous cast material was hot forged and hot rolled into a plurality of steel plates having a thickness of 15 mm, a width of 120 mm, and a length of 1000 mm. The steel sheet after hot forging and hot rolling was air-cooled to room temperature. The following test was implemented using the obtained steel plate.

1. Tempering temperature range First, the obtained steel sheets were quenched. At this time, the quenching temperature was 910 ° C. Subsequently, tempering was performed on the quenched steel sheet. At this time, the tempering temperature was changed within a temperature range of 450 to 650 ° C. Tensile tests were performed using steel plates tempered at each tempering temperature. Specifically, a round bar test piece having a parallel part diameter of 6.35 mm and a parallel part length of 25.4 mm was prepared from the steel plate. A tensile test was performed at room temperature based on JIS Z2241 using the prepared round bar test piece, and yield stress was obtained. After the tensile test, a tempering temperature range ΔT in which the yield stress was in the range of 758 to 860 MPa was determined for each specimen. The 0.2% yield strength was defined as the yield stress.

Table 2 shows the tempering temperature range in which the yield stress in each specimen is 758 to 860 MPa.

  ΔT in Table 2 is a difference value between the highest temperature and the lowest temperature among the tempering temperatures at which the yield stress of each specimen becomes 758 to 860 MPa. The unit is ° C.

  As shown in Table 2, all of the test materials 1 to 11 had ΔT of 40 ° C. or higher. On the other hand, since the test materials 12 and 13 had an F3 value of less than 600, ΔT was less than 40 ° C. Since the specimen 14 had a high C content and an F3 value of less than 600, ΔT was less than 40 ° C. Since the test materials 15 and 16 had a high C content, ΔT was less than 40 ° C.

  FIG. 1 shows the relationship between the tempering temperature and the yield stress in the specimen 1 and specimen 14. As shown in FIG. 1, the tempering curve C1 of the specimen 1 having an F3 value of 600 or more had a small gradient in the yield stress range of 758 to 860 MPa, and the tempering temperature range ΔT1 was 110 ° C. On the other hand, the slope of the tempering curve C2 in the yield stress range of 758 to 860 MPa of the specimen 14 having an F3 value of less than 600 was large, and the tempering temperature range ΔT2 was as small as 20 ° C.

2. Toughness Table 3 shows the results of determining the toughness value of each specimen.

  The toughness test was conducted as follows. The obtained steel plate was quenched at 910 ° C. and subsequently tempered so that the yield stress was a value shown in Table 3. A 10 mm wide V-notch test piece based on JISZ2202 was produced from the tempered steel plate.

  A Charpy impact test was performed at −40 ° C. based on JIS Z2242 using the prepared V-notch test piece, and the absorbed energy was obtained.

  The unit of absorbed energy in Table 3 is J. Since all of the test materials 1 to 11 had an F2 or F4 value of 0 or more, the absorbed energy exceeded 100 J and exhibited high toughness. On the other hand, since the sample material 15 had an F4 value of less than 0, the absorbed energy was low.

3. Corrosion resistance Corrosion resistance in a wet carbon dioxide environment was evaluated by carrying out the following carbon dioxide corrosion test. A test piece having a width of 20 mm, a thickness of 3 mm, and a length of 50 mm was cut out from a steel plate that had been quenched and tempered under the same conditions as in toughness evaluation. The surface of the cut test piece was polished with # 600 emery paper, and then degreased and dried.

The prepared test piece was immersed in a 25% NaCl aqueous solution saturated with 9.73 atm CO ₂ gas and 0.014 atm H ₂ S for 720 hours. During the test, the temperature of the aqueous solution was maintained at 165 ° C.

  After the test, the corrosion weight loss of the test piece was determined. Specifically, the value obtained by subtracting the weight of the test piece after the test from the weight of the test piece before the test was taken as the corrosion weight loss. Furthermore, the presence or absence of local corrosion on the surface of the test piece was confirmed visually. If the corrosion weight loss was less than 7.7 g and no local corrosion occurred, it was judged that the corrosion resistance in a wet carbon dioxide environment was high.

Furthermore, the SSC resistance in a wet hydrogen sulfide environment was evaluated by carrying out the following SSC test. A tensile test piece having a parallel part diameter of 6.3 mm and a parallel part length of 25.4 mm was prepared from a steel sheet which had been quenched and tempered under the same conditions as those during toughness evaluation. A proof ring test was performed based on NACE TM0177-96 Method A using the prepared tensile test pieces. At this time, the test piece was immersed in a 20% NaCl aqueous solution saturated with 0.03 atm of H ₂ S (CO ₂ bal.) For 720 hours. The pH of the aqueous NaCl solution was 4.5, and the temperature of the aqueous solution was maintained at 25 ° C. during the test. After the test, the presence or absence of cracks was confirmed visually.

Table 4 shows the results of the corrosion resistance test.

  In the table, “◯” in the carbon dioxide gas corrosion test indicates that the corrosion weight loss is 7.7 g or less and that local corrosion has not occurred. In addition, “◯” in the SSC corrosion test indicates that no crack has occurred. All of the test materials 1 to 11 had high corrosion resistance.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

  The martensitic stainless steel according to the present invention can be used as a steel material used in a corrosive environment containing corrosive substances such as hydrogen sulfide, carbon dioxide gas, and chlorine ions. In particular, the present invention is applicable to steel materials for production equipment, steel materials for geothermal power generation equipment, steel materials for carbon dioxide removal equipment, and steel pipes used as oil well pipes in wet hydrogen sulfide environments and wet carbon dioxide environments such as oil wells and gas wells.

It is a figure which shows the relationship between the yield stress and tempering temperature of the specimen 1 and the specimen 14 in the Example of this invention. It is a figure which shows the relationship between the yield stress and tempering temperature of the conventional high Mo martensitic stainless steel.

Claims (4)

In mass%, C: 0.001 to 0.01%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 4-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, N: 0.07% or less, the balance being Fe And satisfying the formula (1) and the formula (2),
Martensitic stainless steel having a yield stress of 758 to 860 MPa.
922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-1584.9Al-376.1N ≧ 600 (1)
30C + 0.5Mn + Ni-1.5Si-Cr-Mo + 7.9 ≧ 0 (2)
Here, the symbol in a formula | equation is content (mass%) of each element. In mass%, C: 0.001 to 0.01%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 4-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, N: 0.07% or less,
Ti: 0.25% or less , V: 0.25% or less , Nb: 0.25% or less , Zr: one or more of 0.25% or less , and the balance consists of Fe and impurities, Satisfying formula (2) and formula (3),
Martensitic stainless steel having a yield stress of 758 to 860 MPa.
30C + 0.5Mn + Ni-1.5Si-Cr-Mo + 7.9 ≧ 0 (2)
922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-125.1Ti-1584.9Al-376.1N ≧ 600 (3)
Here, the symbol in a formula | equation is content (mass%) of each element. In mass%, C: 0.001 to 0.01%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 4-8%, Mo: 2.8-5.0%, Al: 0.001-0.10%, N: 0.07% or less,
One or more of Ti: 0.25% or less , V: 0.25% or less , Nb: 0.25% or less , Zr: 0.25% or less ,
Cu: 1.0% or less , with the balance being Fe and impurities, satisfying formula (3) and formula (4),
Martensitic stainless steel having a yield stress of 758 to 860 MPa.
922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr-81.1Ni + 95.8Mo-125.1Ti-1584.9Al-376.1N ≧ 600 (3)
30C + 0.5Mn + Ni + 0.5Cu-1.5Si-Cr-Mo + 7.9 ≧ 0 (4)
Here, the symbol in a formula | equation is content (mass%) of each element. The martensitic stainless steel according to any one of claims 1 to 3 , further comprising Ca: 0.005% or less , Mg: 0.005% or less , La: 0.005% or less , Ce : Martensitic stainless steel characterized by containing one or more of 0.005% or less .

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