RU2594769C1 - Corrosion-resistant steel for seamless hot-rolled tubing and casing pipes high operational reliability and pipe made therefrom - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, specifically to corrosion-resistant steels for production of seamless hot-rolled tubing and casing pipes operating under conditions of high concentration of carbon dioxide and hydrogen sulphide in composition of pumped hydrocarbon medium at deposits, located in Arctic areas. Steel contains following component ratio, wt%: carbon 0.14-0.23, silicon 0.17-0.4, manganese 0.4-0.7, chromium more than 1.0-5.1, molybdenum 0.15-0.5, vanadium 0.04-0.06, nickel 0.1-0.7, copper 0.15-0.5, aluminium 0.02-0.05, sulphur not more than 0.007, phosphorus not more than 0.015, nitrogen not more than 0.014, iron - balance. Coefficient of operational reliability of steel, defined by expression R=0.8×[Cr]+3.5×[Mo]+2.5×[Cu], is 2.0÷5.5, and content of sulphur must not be over S_max=0.01-0.01×[Cu], wt%.

EFFECT: improved operational reliability of pipes due to increased resistance to carbon-dioxide corrosion while maintaining resistance to sulphide corrosion, high cold resistance and prevention of hot brittleness during hot rolling of steel pipes.

3 cl, 4 tbl

Description

The invention relates to metallurgy, in particular to corrosion-resistant steels, and can be used for the production of hot-rolled seamless tubing and casing pipes of increased operational reliability operating in conditions of high concentration of carbon dioxide and hydrogen sulfide in the composition of the pumped hydrocarbon medium in fields located in arctic areas.

In accordance with the standard of the American Petroleum Institute ANSI / APISpec 5CT / ISO 11960 for hydrocarbon media containing carbon dioxide (CO ₂ ), casing and tubing of strength group L80 13Cr are used from steel containing, by weight. %: carbon 0.15-0.22; manganese 0.25-1.00; chrome 12.0-14.0; nickel not more than 0.50; copper no more than 0.25; sulfur not more than 0.010; phosphorus no more than 0,020; silicon not more than 1.00. The disadvantages of this steel composition are low resistance to sulfide stress cracking (hereinafter referred to as RNC) in environments containing hydrogen sulfide, and the cold resistance of pipes made from it (especially when increasing the strength class).

In the patent of the Russian Federation No. 2243284, C22C 38/42, publ. 12/27/2004 the composition of economically alloyed steel containing, by weight. %: carbon 0.07-0.30; manganese 0.35-1.50; silicon 0.15-0.70; chrome 0.05-1.00; nickel 0.05-0.50; copper 0.05-0.50; aluminum 0.01-0.05; sulfur not more than 0.010; phosphorus no more than 0,020; calcium 0.0008-0.0020; iron and unavoidable impurities, including oxygen - the rest, for the manufacture of seamless corrosion-resistant pipes. Steel with the specified chemical composition has high resistance to NRC, but does not provide the required corrosion resistance to carbon dioxide corrosion and cold resistance.

Known corrosion-resistant steel for tubing and casing (RF patent No. 2371508, C22C 38/26, F16L 9/02, publ. 10/27/2009) with the following components, wt. %: carbon no more than 0.16; silicon 0.30-0.50; manganese 0.50-0.70; chrome 3.00-6.00; molybdenum 0.40-1.00; vanadium 0.04-0.10; niobium 0.04-0.10; aluminum 0.020-0.050; REM 0.005-0.015; iron and unavoidable impurities are the rest. Tubing made of this steel, after heat treatment, has mechanical properties of strength group E according to GOST 633-80 and TU 14-161-195-2001 (yield strength should be from 552 to 758 MPa), have high resistance to SRN and cold resistance (KCV ^{-60 ° С} not less than 100 J / cm ² ). In this case, the pipe metal has insufficient corrosion resistance in a medium containing carbon dioxide - the rate of uniform corrosion is 0.34 mm / year (according to GOST 9.502, steel with a given corrosion rate has a reduced corrosion resistance).

Closest to the proposed steel is corrosion-resistant steel for seamless hot-rolled pipes of oil grade, including tubing and casing (US No. 200880286504 A1, C22C 38/18, publ. 20.11.2008), selected as a prototype, containing the following components, wt. %: carbon 0.03-0.3; silicon 0.01-0.8; manganese 0.3-2.5; chrome 1.0 or less; molybdenum 0.5 or less; vanadium 0.3 or less; aluminum 0.001-0.1; nickel 1.0 or less; copper 1.0 or less; sulfur 0.01 or less; phosphorus 0.03 or less; nitrogen 0.01 or less; iron is the rest.

Oil and gas assortment pipes of the specified steel have insufficient corrosion resistance in a medium containing carbon dioxide, and also have insufficient strength - yield strength less than 552 MPa.

The technical problem solved by the invention is to increase the operational reliability of pipes by reducing the rate of metal corrosion in environments containing carbon dioxide, while maintaining resistance to sulfide corrosion and improving the technological properties of seamless hot-rolled tubing and casing pipes.

The problem is solved due to the fact that corrosion-resistant steel for seamless hot-rolled tubing and casing, containing carbon, silicon, manganese, chromium, molybdenum, vanadium, aluminum, nickel, copper, sulfur, phosphorus and iron, according to the invention contains components in the following ratio, wt. %:

carbon 0.14-0.23 silicon 0.17-0.40 manganese 0.40-0.70 chromium from more than 1.0 to 5.1 molybdenum 0.15-0.50 vanadium 0.04-0.06 nickel 0.10-0.70 copper 0.15-0.50 aluminum 0.02-0.05 sulfur no more than 0,007 phosphorus no more than 0.015 nitrogen no more than 0,014 iron rest

the coefficient of operational reliability of steel R, determined by the expression R = 0.8 × [Cr] + 3.5 × [Mo] + 2.5 × [Cu], is 2.0 ÷ 5.5, where [Cr] , [Mo] and [Cu] - the content in the steel of chromium, molybdenum and copper, respectively, wt. %

In addition, for the content of sulfur and copper, the condition [S] = 0.01-0.01 × [Cu] is fulfilled, where [S] and [Cu] are the content of sulfur and copper in steel, respectively, wt. %

The problem is also solved due to the fact that a seamless hot-rolled pipe, made in the form of a tubing or casing, is made of the specified corrosion-resistant steel.

The proposed composition and content of elements in steel are determined by the following factors.

The carbon content in the proposed range provides the required level of strength properties of the pipes after heat treatment, including hardening or normalization with subsequent tempering. The carbon content is less than the declared concentration of 0.14 wt. % leads to a decrease in strength properties below an acceptable level, and a content above 0.23 wt. % - to reduce corrosion resistance and resistance to NRC, caused by an increase in strength properties and volume fraction of the carbide phase in the microstructure of steel.

Silicon and aluminum within the specified limits provide the required degree of deoxidation of steel. With their lower content, complete deoxidation of the steel is not ensured, the oxygen concentration in the steel increases, which leads to an increase in the number of non-metallic inclusions of the oxide type and, as indicated in the scientific and technical literature, to a decrease in resistance to NRC. When the content of silicon and aluminum in an amount is greater than the upper limit of each element, non-metallic inclusions of the silicate type are formed, as well as large aluminum nitrides and carbonitrides, which negatively affect the corrosion resistance of steel.

The molybdenum content within the specified limits provides high cold resistance and a low tendency to reversible temper brittleness, and improves the resistance to RNS. In addition, molybdenum gives the steel a tendency to passivation, both in reducing and in oxidizing environments. This allows the use of molybdenum-alloyed steels for the production of pipes operated in environments that simultaneously contain hydrogen sulfide and carbon dioxide. The molybdenum content is higher than the declared concentration of 0.50 wt. % complicates the softening of steel during tempering, which may cause a decrease in cold resistance of pipes and resistance to RNS. In addition, it is known that increasing the molybdenum content significantly increases the cost of pipes, which reduces their competitiveness. When the molybdenum content in the steel is below 0.15 wt. % its effect becomes ineffective, which leads to a decrease in corrosion resistance and resistance to NRC.

The vanadium content within the indicated limits allows to simultaneously increase the toughness and yield strength of steel by reducing grain size during the formation of dispersed carbonitride phases. The increase in the content of vanadium above the declared concentration of 0.06 wt. % can lead to noticeable dispersion hardening and grain boundary precipitates, causing a decrease in toughness and an increase in the tendency of pipe metal to brittle fracture and RNS. With an insufficient content of vanadium in steel - below 0.04 wt. % its effect becomes ineffective, which leads to a decrease in strength properties and impact strength.

Manganese is contained in steel in an amount of 0.40-0.70 wt. %., usually used for structural steels.

The Nickel content in the specified range provides high impact strength at low temperatures due to the beneficial effect on the characteristics of the crystal lattice of steel and, as a result, increases resistance to NRC, also nickel reduces the negative effect of copper on the red crack. Nickel content less than 0.10 wt. % does not have a positive effect on the toughness of steel, and when the nickel content is above 0.70 wt. % there is no further increase in mechanical and corrosion properties.

The chromium content within the specified limits provides high corrosion resistance of steel in environments containing carbon dioxide, since it promotes self-passivation of the surface due to the formation of an oxide protective film enriched with chromium. The positive effect of the proposed chromium content is manifested when the carbon content is limited, since due to this it is possible to ensure the presence of most of the chromium in the solid solution, and not in the carbide phases. The chromium content is below 1.0 wt. % leads to a decrease in resistance to carbon dioxide corrosion, and above 5.10 wt. % - to reduce resistance to NRC and cold resistance.

The copper content in the specified amount leads to a decrease in the absorption of hydrogen on the surface and prevents its penetration into steel, thereby increasing its resistance to RNS. In addition, in conjunction with the addition of chromium and molybdenum, copper increases the resistance to carbon dioxide corrosion. The content in copper steel is less than 0.15 wt. % does not have a positive effect on corrosion resistance and resistance to NRC, and with a copper content of more than 0.50 wt. % its positive impact on these characteristics does not increase.

The proposed content of chromium, molybdenum and copper in steel is regulated by the operational reliability coefficient R. It has been experimentally established that the value of the coefficient R, which is determined by the formula R = 0.8 × [Cr] + 3.5 × [Mo] + 2.5 × [ Cu], should be from 2.0 to 5.5 to ensure increased operational reliability of the pipes. When the coefficient R is less than 2.0, the required corrosion resistance of steel in media containing both carbon dioxide and hydrogen sulfide is not provided, and when the value of R is more than 5.5, there is no increase in corrosion resistance, but the technological properties of steel in the manufacture of pipes are reduced.

It is known that an increased copper content (over 0.30 wt.%) Leads to a deterioration in the technological properties of steel, namely, the manifestation of red brittleness. During hot deformation of steel containing copper, copper atoms can accumulate under a layer of oxides during the formation of scale. Under the influence of tensile stresses arising from hot deformation, copper atoms penetrate into the steel along the grain boundaries and thereby cause the formation of surface cracks. To eliminate the red breaking of steels containing copper, the sulfur content is limited, since sulfur is one of the main elements enhancing the negative effect of copper. As studies have shown, for the content of sulfur and copper, the condition [S] = 0.01-0.01 × [Cu], wt. %

Table 1 shows the options for the chemical composition of the proposed steel and the values of the operational reliability coefficient R. The choice of the compositions of the proposed steel was made so that the reliability coefficient R covered the entire declared interval. For steel with a chemical composition corresponding to melts 6 and 7, the condition for limiting the sulfur content [S] = 0.01-0.01 × [Cu], wt. %

In the factory, tubing pipes of 73.02 × 5.51 mm in size and pipe blanks for couplings of 88.9 × 13.0 mm in size were manufactured. Pipe rental was carried out on a continuous pipe rolling unit with regulation of the temperature at which deformation ended. Heat treatment was carried out according to the following regime: quenching or normalization at a temperature of from 880 to 920 ° C, followed by tempering at a temperature of from 630 to 690 ° C (depending on the required yield strength).

To confirm the high operational reliability of the pipes of the proposed steel grade, the mechanical properties of the proposed steel grade and prototype steel were tested (Table 2), the corrosion resistance of steel of various melts of the proposed steel grade and steel of the prototype (table 3), and the redness of the proposed steel grade was tested depending on the content of sulfur and copper (table 4).

Thus, the use of the proposed steel allows to provide:

- reducing the rate of uniform corrosion of the pipe metal in an environment containing carbon dioxide to 0.20 mm / year or less;

- resistance of pipe metal to sulfide stress cracking at a threshold voltage of 90% of the minimum guaranteed yield strength;

- high cold resistance (impact strength of at least 100 J / cm ² at a test temperature of minus 60 ° C);

- prevention of the phenomenon of red brittleness during hot rolling of pipes, provided that the maximum sulfur content is limited depending on the copper content.

Claims (3)

1. Corrosion-resistant steel for seamless hot-rolled tubing and casing, containing carbon, silicon, manganese, chromium, molybdenum, vanadium, aluminum, nickel, copper, sulfur, phosphorus and iron, characterized in that it contains components in the following ratio, wt.%:
carbon 0.14-0.23 silicon 0.17-0.40 manganese 0.40-0.70 chromium from more than 1.0 to 5.1 molybdenum 0.15-0.50 vanadium 0.04-0.06 nickel 0.10-0.70 copper 0.15-0.50 aluminum 0.02-0.05 sulfur no more than 0,007 phosphorus no more than 0.015 nitrogen no more than 0,014 iron rest
the coefficient of operational reliability of steel R, determined by the expression R = 0.8 × [Cr] + 3.5 × [Mo] + 2.5 × [Cu], is 2.0 ÷ 5.5, where [Cr] , [Mo] and [Cu] - the content in the steel of chromium, molybdenum and copper, respectively, wt.%. 2. Steel according to claim 1, characterized in that for the sulfur and copper content the condition [S] = 0.01-0.01 × [Cu] is fulfilled, where [S] and [Cu] are the content of sulfur and copper in the steel accordingly, wt.%. 3. Seamless hot-rolled pipe made in the form of a tubing or casing made of corrosion-resistant steel, characterized in that it is made of steel according to claim 1 or 2.