RU2416670C2 - Martensite stainless steel - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: steel consists of following elements in wt % : from 0.010 % to 0.018 % of C, from 0.30 % to 0.60 % of Mn, maximum 0.040 % of P, maximum 0.0100 % of S, from 10.00 % to 15.00 % of Cr, from 2.50 % to 5.83 % of Ni, from 1.00 % to 5.00 % of Mo, from 0.050 % to 0.250 % of Ti, from 0.05 % to 1.00 % of Cu, maximum 0.25 % of V, maximum 0.07 % of N, and at least one component from 0.50 % of Si, and maximum 0.10 % of Al, Fe and unavoidable impurities - the rest. Contents of carbon and titanium are stipulated by dependency 6.0 ëñ Ti/C ëñ 10.1. Steel possesses 0.6 % of yield point measured according to ASTM, from 758 MPa to 848 MPa. ^ EFFECT: high strength, resistance to corrosion and to instant fracture under effect of plastic deformation. ^ 3 cl, 2 dwg, 1 tbl, 1 ex

Description

FIELD OF TECHNOLOGY

The present invention relates to martensitic stainless steel, and more particularly to martensitic stainless steel, which can be used in a corrosive medium containing a corrosive substance, such as hydrogen sulfide, gaseous carbon dioxide and chlorine ions.

BACKGROUND

In recent years, there has been deep drilling of an increasing number of oil and gas wells. Steel products used in the form of oil and gas and pipeline pipes in such deep oil and gas wells (hereinafter generally referred to as "oil wells") must have a high yield strength. The steel materials recently used for oil and gas and pipeline pipes should have a high yield stress of the order of 110 ksi (thousand pounds of force per inch) (at which 0.6% of the total yield strength during elongation is from 758 MPa to 862 MPa).

In addition, such oil wells contain hydrogen sulfide, gaseous carbon dioxide and chlorine ions. Therefore, steel materials for oil and gas and pipeline pipes must have high resistance to SSC (sulfide cracking due to stress corrosion) and high resistance to corrosion under the influence of gaseous carbon dioxide.

Typically, oil wells use steel containing many components. In oil wells containing carbon dioxide gas, SUS420 martensitic stainless steel is used, which has high corrosion resistance under the influence of carbon dioxide gas. However, the SUS420 martensitic stainless steel is not suitable for oil wells containing hydrogen sulfide, since its resistance to SSC under the influence of hydrogen sulfide is low.

To eliminate this drawback, martensitic stainless steel products have been developed that possess not only corrosion resistance under the influence of carbon dioxide gas, but also SSC resistance. JP 5-287455 A (hereinafter referred to as "Patent Document 1") describes martensitic stainless steel for oil wells, which is highly resistant to SSC and highly resistant to corrosion by exposure to carbon dioxide gas in oil wells containing substances such as hydrogen sulfide and carbon dioxide gas. To increase resistance to SSC, tensile stress must be reduced. Therefore, as described in Patent Document 1, the tensile stress of martensitic stainless steel is lowered, thereby providing high resistance to SSC. Moreover, the level of fluctuations in tensile stress after tempering is reduced by reducing the magnitude of the tensile stress.

Recently, in the field of stainless steel products for oil and gas and pipeline pipes, there is a need for the ability of steel products not to undergo immediate failure under the influence of plastic deformation caused by externally applied forces, in addition to the above high strength, SSC resistance and corrosion resistance under the influence of gaseous carbon dioxide. More specifically, the value obtained by subtracting the value of the yield stress (0.06% of the total yield strength during elongation) from the value of the tensile stress should be at least 20.7 MPa (= 3 ksi).

The martensitic stainless steel for oil wells described in Patent Document 1 has a low tensile stress. Therefore, in the case when the yield stress of steel is 110 ksi (from 758 MPa to 862 MPa), the value obtained by subtracting the yield strength from the tensile stress is less than 20.7 MPa.

Moreover, as described above, the steel product for oil and gas and pipeline pipes must be resistant to SSC. In the event that the hardness of the same steel product varies greatly, the resistance to SSC is reduced. Therefore, the fluctuation in the hardness of the steel product for oil and gas and pipeline pipes should be prevented.

SUMMARY OF THE INVENTION

The aim of this invention is to obtain a martensitic stainless steel of 110 ksi type (having a yield strength of 758 MPa to 862 MPa), providing a value obtained by subtracting the yield strength from the tensile stress of at least 20.7 MPa and capable of preventing fluctuations hardness.

The inventors of the present invention found that the ratio of the Ti content to the C content in steel and the value (hereinafter also referred to as “TS-YS”) obtained by subtracting the yield stress value (hereinafter also referred to as “YS”) from the tensile stress value (in hereinafter also referred to as “TS”), has a correlation. The following is a description of this discovery.

The authors of this invention have received many types of martensitic stainless steel containing in wt.% From 0.010% to 0.030% C, from 0.30% to 0.60% Mn, a maximum of 0.040% P, a maximum of 0.0100% S, from 10, 00% to 15.00% Cr, from 2.50% to 8.00% Ni, from 1.00% to 5.00% Mo, from 0.050% to 0.250% Ti, maximum 0.25% V, maximum 0 , 07% N, and at least one of a maximum of 0.50% Si and a maximum of 0.10% Al, wherein the balance is Fe and unavoidable impurities, and the Ti / C ratio is from 7.4 to 10.7. During the preparation, tempering-tempering was carried out, and the tempering temperature was controlled in such a way that the yield stress of each type of martensitic stainless steel was 110 ksi (from 758 MPa to 862 MPa). The obtained types of martensitic stainless steel were subjected to tensile tests at room temperatures, and their tensile stresses and yield stresses were established. It should be noted that the yield stress, according to ASTM standard, is taken to be 0.6% of the yield strength with total elongation.

The research results are shown in figure 1. The abscissa in FIG. 1 represents Ti / C, and the ordinate represents TS-YS (ksi). As shown in FIG. 1, Ti / C and TS-YS have a negative correlation. More specifically, as Ti / C decreases, TS-YS rises. Based on this new discovery, the authors of the present invention found that the condition TS-YS≥20.7 MPa (3 ksi) can be fulfilled subject to the following expression (A):

in which character elements represent the content of these elements

(wt.%).

Moreover, the inventors of the present invention first found that if Ti / C is too low, the hardness varies greatly. More specifically, they found that when the Ti / C values are within the corresponding range, the TS-YS value is at least 20.7 MPa, so the level of hardness fluctuation can be reduced.

Based on the above technical facts, the authors created the following invention.

The martensitic stainless steel according to the present invention includes, in wt.%, From 0.010% to 0.030% C, from 0.30% to 0.60% Mn, a maximum of 0.040% P, a maximum of 0.0100% S, from 10.00% up to 15.00% Cr, from 2.50% to 8.00% Ni, from 1.00% to 5.00% Mo, from 0.050% to 0.250% Ti, maximum 0.25% V, maximum 0.07 % N, and at least one species, a maximum of 0.50% Si, and a maximum of 0.10% Al, with the balance being Fe and unavoidable impurities. The martensitic stainless steel according to the present invention also satisfies expression (1) and has a yield stress of 758 MPa to 862 MPa. The yield stress in this invention is 0.6% of the yield strength at total elongation according to ASTM standards.

in which the elements of the symbols represent the content of these elements in

wt.%.

Martensitic stainless steel preferably includes at least one of a maximum of 0.25% Nb and a maximum of 0.25% Zr instead of a portion of Fe.

The martensitic stainless steel preferably further comprises a maximum of 1.00% Cu instead of part Fe.

The martensitic stainless steel preferably further comprises at least one component of a maximum of 0.005% Ca, a maximum of 0.005% Mg, a maximum of 0.005% La and a maximum of 0.005% Ce instead of a portion of Fe.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing the relationship of the value obtained by subtracting the magnitude of the yield stress from the magnitude of the tensile stress, and Ti / C; a

figure 2 is a cross-sectional view in section of a steel pipe, showing the place of measurement of hardness.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

1. The chemical composition

Martensitic stainless steel according to one of the embodiments of the present invention has the following composition. In the further part of the description, "%" referring to elements means "wt.%".

C: from 0.010% to 0.030%

An excess carbon content (C) increases the hardness after tempering too much, thus increasing the sensitivity to sulfide cracking due to stress corrosion. If the C content is too low and the yield strength of the steel is at least 100 ksi (from 758 MPa to 862 MPa), the expression TS-YS≥20.7 MPa cannot be satisfied. Therefore, the content of C should be from 0.010% to 0.030%, preferably from 0.012% to 0.018%.

Mn: 0.30% to 0.60%

Manganese (Mn) improves hot workability. However, with an excess Mn content, further improvement of the action does not occur. Therefore, the Mn content should be between 0.30% and 0.60%.

P: 0.040 or less

Phosphorus (P) is a contaminant that reduces SSC resistance. Therefore, the content of P should be no more than 0.040%.

S: 0.0100% or less

Sulfur (S) is a contaminant that reduces hot workability. Therefore, the content of S is preferably as low as possible. S content should be no more than 0.0100%.

Cr: from 10.00% to 15.00%

Chromium (Cr) improves the resistance to corrosion caused by gaseous carbon dioxide. However, an excess Cr content prevents the formation of a martensitic phase in the structure after tempering. Therefore, the Cr content should be between 10.00% and 15.00%.

Ni: 2.50% to 8.00%

Nickel (Ni) effectively promotes the formation of a mainly martensitic phase in the structure after tempering. If the Ni content is too low, a large amount of the ferrite phase precipitates in the tempered structure. On the other hand, excess Ni content mainly causes the formation of an austenitic phase in the tempered structure. Therefore, the Ni content should be from 2.50% to 8.00%, preferably from 4.00% to 7.00%.

Mo: 1.00% to 5.00%

Molybdenum (Mo) improves the SSC resistance of high-strength steel in a medium containing hydrogen sulfide. However, with an excess content of Mo, no further improvement in action occurs. Therefore, the Mo content should be between 1.00% and 5.00%.

Ti: 0.050% to 0.250%

Titanium (Ti) improves toughness, preventing the formation of large grains in the structure. However, an excess Ti content prevents the formation of a mainly martensitic phase in the structure after tempering; therefore, toughness and corrosion resistance (SSC resistance and corrosion resistance caused by carbon dioxide gas) are reduced. Therefore, the Ti content should be from 0.050% to 0.250%, preferably from 0.050% to 0.150%.

N: 0.07% or less

Nitrogen (N) is an impurity. Excessive N content causes a large amount of nitrogen-based inclusions to precipitate in steel, which reduces corrosion resistance. Therefore, the N content should be no more than 0.07%, preferably no more than 0.03%, more preferably no more than 0.02%, even more preferably no more than 0.01%.

V: 0.25% or less

Vanadium (V) binds C in steel to form carbide, and thus increases the tempering temperature and resistance to SSC. However, an excess V content prevents the formation of a martensitic phase. Thus, the content of V should be no more than 0.25%. The lower limit of the V content is preferably 0.01%.

The martensitic stainless steel according to this embodiment contains at least one of elements such as Si and Al.

Si: 0.50% or less

Al: 0.10% or less

Both silicon (Si) and aluminum (Al) effectively act as a deoxidizing agent. However, an excess Si content reduces the toughness and hot workability. Excess Al content causes the formation of a large number of inclusions in the steel, which reduces corrosion resistance. Therefore, the Si content should be no more than 0.50%, and the Al content should be no more than 0.10%. The lower limit of the Si content is preferably 0.10%, and the lower limit of the Al content is preferably 0.001%. It should be noted that if the content of Si and / or Al is less than the specified lower limits, the above effect is achieved only to some extent.

The balance of martensitic stainless steel according to the present invention includes Fe. It should be noted that for various reasons, contaminants other than the inevitable impurities described above may be contained in the steel.

Moreover, the Ti content and the C content in the above chemical composition satisfy the following expression (I):

in which the elements of the symbols represent the content of these elements (in

wt.%).

As shown in FIG. 1, as Ti / C decreases, TS-YS rises. In the event that the Ti / C exceeds 10.1, the expression TS-YS≥20.7 MPa cannot be satisfied.

On the other hand, if Ti / C is too low, the oscillation of hardness increases. More specifically, the hardness fluctuation (HRC) determined by the following expression (2) is at least 2.5.

Hardness swing

According to the present invention, Hmax and Hmin are measured in the following manner. As shown in FIG. 2, the hardness of the cross section corresponding to the center of the steel pipe is measured using the Rockwell C hardness scale (hereinafter referred to simply as “Rockwell hardness” and expressed in HRC units), in the central sections of the thickness P1-P4 with a gap of 90 ° in the direction along the circle. Of the four Rockwell hardness values obtained, the maximum value is Hmax and the minimum value is Hmin.

With a hardness fluctuation of at least 2.5, resistance to SSC tends to decrease. If Ti / C is at least 6.0, the fluctuation in hardness is less than 2.5 and can be prevented. Despite the lack of a clear explanation, this may occur for the following reason. If Ti / C is too low, the Ti content in the steel is low. Therefore, a large amount of VC precipitates during the holidays. Precipitated VCs are unevenly sized, depending on where they precipitate in the steel pipe. As a result, hardness varies greatly. On the other hand, if Ti / C is high, the Ti content in the steel is high. Therefore, Ti / C precipitates during tempering, and VC precipitation is suppressed. Therefore, the level of fluctuation in hardness is reduced.

The martensitic stainless steel according to the present invention satisfies the expression (1), therefore, TS-YS is not less than 20.7 MPa, and the variation in hardness is less than 2.5.

The upper limit of Ti / C is preferably 9.6, more preferably 9.0.

The martensitic stainless steel according to this embodiment optionally further comprises at least one of elements such as Nb and Zr instead of a part of Fe.

Nb: 0.25% or less

Zr: 0.25% or less

Both niobium (Nb) and zirconium (Zr) are optional elements. These elements form carbide, binding C in steel, and reduce fluctuations in hardness after tempering. However, the excessive content of these elements prevents the formation of a mainly martensitic phase in the tempered structure. Therefore, the total content of Nb and Zr should be no more than 0.25% for each. Preferred lower limits for Nb and Zr are 0.005% each. It should be noted that if the content of each of Nb and Zr is less than 0.005%, the above effect can be achieved only to some extent.

The martensitic stainless steel according to this embodiment optionally further contains Cu instead of part Fe.

Cu: 1.00% or less

Copper (Cu) is an optional element. Like Ni, Cu effectively promotes the formation of a martensitic phase in the structure after tempering. However, excess Cu reduces hot workability. Therefore, the content of Cu should be no more than 1.00%. The lower limit of the content of Cu is preferably 0.05%. It should be noted that if the content of Cu is less than 0.05%, the above effect is achieved only to some extent.

The martensitic stainless steel according to this embodiment optionally further contains at least one component of Ca, Mg, La and Ce instead of a part of Fe.

Ca: 0.005% or less

Mg: 0.005% or less

La: 0.005% or less

Ce: 0.005% or less

Calcium (Ca), magnesium (Mg), lanthanum (La) and cerium (Ce) are optional elements. These elements improve hot workability. However, with excessive content of these elements, large oxides are formed, and corrosion resistance is reduced. Therefore, the content of each of these elements should be no more than 0.005%. The lower limit of the content of each of these elements is preferably 0.0002%. It should be noted that if the content of Ca, Mg, La and Ce is less than 0.0002%, the above effect can be achieved only to some extent. Of these elements, Ca and / or La is preferred.

2. The method of obtaining

The following is a description of a method for producing martensitic stainless steel according to this embodiment. From a molten steel having the chemical composition specified in paragraph 1 above, in a manner such as continuous casting, a slab or billet is obtained. Alternatively, molten steel is obtained as an ingot by casting into ingots. The slab or ingot is subjected to hot processing in such a way as blooming, receiving a bill (blank).

The resulting billet is heated in a heating furnace, and the billet extracted from the heating furnace is stitched axially on a piercing mill. Then, the billet or billet is prepared in the form of a seamless steel pipe having the desired size, using a mill for rolling seamless pipes on a mandrel, a crimping device or the like. Then carry out heat treatment (hardening and tempering). At this stage, the quenching and tempering temperatures are controlled so that 0.6% of the total yield strength for elongation of tempered martensitic stainless steel is in the range of 758 MPa to 862 MPa (110 ksi).

It should be noted that the above description relates to a method for producing a seamless martensitic stainless steel pipe, while a welded steel pipe can be obtained by any other well-known methods.

Example

Seamless steel pipes having various chemical compositions were obtained, after which TS-YS and fluctuations in the hardness of the obtained seamless steel pipes were investigated.

Research Method

By melting, billets were obtained from various types of steel having the chemical compositions indicated in the table under test numbers. Each of the billets obtained was hot forged and hot rolled to obtain seamless steel pipes.

Then, quenching and tempering are carried out in such a way that 0.6% of the total yield strength when elongating each of the stainless steel pipes is in the range of 758 MPa to 862 MPa. More specifically, the quenching temperature is 910 ° C, and the tempering temperature is controlled in the range from 560 ° C to 630 ° C. After quenching and tempering, 0.6% of the total elongation yield strength (YS) and tensile stress (TS) of each of the stainless steel pipes are measured. In the axial direction, a sample is taken from each seamless steel pipe in the form of a round rod (according to ASTM A370), while the parallel part of the sample has a length of 25.4 mm and a cross-sectional diameter of 6.35 mm in the axial direction of the seamless steel pipe. The resulting samples in the form of a round rod are subjected to tensile tests at room temperature and measure 0.6% of the total yield strength at elongation YS (MPa) and tensile stress TS (MPa) according to ASTM standard. After measurement, TS-YS is obtained for each sample under test numbers.

The fluctuations in hardness of each of the seamless steel pipes are determined. More specifically, each seamless steel pipe is cut in the center in the transverse direction. As shown in FIG. 2, the cross sectional hardness of the cut seamless steel pipe is measured using the Rockwell C hardness scale (HRC) in the central sections of the thickness P1-P4 with a gap of 90 ° in the circumferential direction. Of the four Rockwell hardness values obtained, the maximum value is Hmax and the minimum value is Hmin. Using the thus obtained Hmax and Hmin, using the expression (2) get the value of the fluctuation of hardness (HRC).

Research results

The research results are presented in the table. In this table, “Ti / C” is the ratio of the Ti content (wt.%) To the C content (wt.%) For each of the samples under test numbers. In this table, “TS” represents the tensile stress (MPa) of each sample under test numbers, and “YS” represents 0.6% of the total elongation yield strength (MPa) obtained by subtracting 0.6% of the total elongation yield strength from elongation from tensile stress values. In this table, "hardness fluctuation" is the hardness fluctuation obtained using expression (2). It should be noted that the underlined numerical values are outside the range defined by this invention.

In the 0.6% table, the total elongation yield strength (YS) is within the range of 758 MPa to 862 MPa.

Seamless steel pipes under No. 1-49 have chemical compositions within the range defined by this invention, and their Ti / C values satisfy expression (1). Therefore, TS-YS of any of the seamless steel pipes is at least 20.7 MPa, and the hardness fluctuation is less than 2.5.

On the other hand, seamless steel pipes nos. 50 and 51 have chemical compositions within the range defined by this invention, however, their Ti / C values do not satisfy expression (1), or the Ti / C of each pipe exceeds 10.1. Therefore, TS-YS is less than 20.7 MPa.

The content of C in all seamless steel pipes under No. 52-69 is less than the lower limit of the content of C defined by this invention. Therefore, TS-YS of any of the pipes is less than 20.7 MPa.

Seamless steel pipes No. 70-73 have chemical compositions within the range defined by this invention, however, all their Ti / C values are less than 6.0. Therefore, the hardness fluctuation is at least 2.5.

Seamless steel pipes nos. 1-49 and 70-73 in the table are subjected to SSC tests and their resistance to SSC is evaluated. More specifically, a tensile test specimen with a parallel portion having a diameter of 6.3 mm and a length of 25.4 mm is taken from each of the seamless steel pipes. Using the obtained samples for tensile tests, verification ring tests are carried out according to NACE TM0177-96, method A. For this, the samples are immersed for 720 hours in a 20% aqueous solution of NaCl saturated with H ₂ S (ball. CO ₂ ) under a pressure of 0.03 atm. The pH of the aqueous NaCl solution is 4.5, and the temperature of the aqueous solution during the tests is maintained at 25 ° C. After testing, the samples are examined visually for cracks.

According to the test results, in none of the samples for tensile testing under No. 1-49 cracks appeared. And vice versa, cracks were found in the samples for tensile tests under No. 70-73.

Despite the above description of an embodiment of the present invention, it serves only as an illustration and example and should not be construed as limiting. The present invention may be subjected to various modifications, provided that they do not violate the essence and scope of the present invention.

INDUSTRIAL APPLICABILITY

The martensitic stainless steel according to this invention is widely used as steel products used in a corrosive environment containing a corrosive substance, such as hydrogen sulfide, gaseous carbon dioxide and chlorine ions. More specifically, such steel is suitable for the manufacture of steel products used in production equipment for oil or natural gas, as well as installations for generating geothermal electricity. Such steel is particularly suitable for use in the form of oil and gas production and pipeline pipes in oil and gas wells.

Claims (3)

1. Martensitic stainless steel, including, wt.%: From 0.010 to 0.018% C, from 0.30 to 0.60% Mn, a maximum of 0.040% P, a maximum of 0.0100% S, from 10.00 to 15.00 % Cr, from 2.50 to 5.83% Ni, from 1.00 to 5.00% Mo, from 0.050 to 0.250% Ti, from 0.05 to 1.00% Cu, maximum 0.25% V, a maximum of 0.07% N and at least one component of a maximum of 0.50% Si and a maximum of 0.10% Al, with the balance being Fe and unavoidable impurities, said martensitic stainless steel satisfying the expression 6.0 ≤ Ti / C ≤10.1, in which the symbol elements represent the element content in wt.%, And has a 0.6% yield strength, measured according to ASTM, from 758 to 848 MPa. 2. The martensitic stainless steel according to claim 1, further comprising at least one of a maximum of 0.25% Nb and a maximum of 0.25% Zr. 3. The martensitic stainless steel according to any one of claims 1 and 2, comprising at least one component of a maximum of 0.005% Ca, a maximum of 0.005% Mg, a maximum of 0.005% La and a maximum of 0.005% Ce.

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