RU2647201C1 - Corrosion-resistant pipe from low-carbon pre-peritic steel for oil and gas pipelines and method of its manufacture - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: pipe is made of steel containing, wt %: C less than 0.08, Mn 1.10÷1.60, Si 0.15÷0.50, V 0.030÷0.11, Nb 0.040÷0.080, Al 0.005÷0.060, N 0.005÷0,015, Fe and unavoidable impurities are the rest, when the relation is ([C]+[Mn]/6-[Si]/7+[N]/1,4)≤0.23 and providing a ferrite potential of not less than 1, then the pipe is subjected to hot deformation at 880÷1350°C and heat treatment by heating to A_c3+(30÷45°C), water cooling and subsequent high tempering at temperature A_c1-(50÷150)°C with holding time of at least 4 minutes per 1 mm of thickness of the pipe wall providing for a microstructure consisting of a finely dispersed ferritic carbide mixture.

EFFECT: production of pipes with the required strength and viscoplastic characteristics and high corrosion resistance in a sulfide-containing medium under pressure.

2 cl, 1 dwg, 4 tbl

Description

The invention relates to a method for the production of seamless steel pipes from low-carbon preperitectic steels and can be used for the manufacture of corrosion-resistant pipes from a continuously cast billet with a yield strength of more than 415 MPa (strength group X60, X65 according to API 5L) for oil and gas pipelines.

Known low-alloy steel for the production of high-strength seamless steel pipes, having the following chemical composition, wt. %: 0.15 ÷ 0.18 C; 0.20 ÷ 0.40 Si; 1.40 ÷ 1.60 Mn; not more than 0.05 P; not more than 0.01 S; more than 0.50 to 0.90 Cr; from more than 0.50 to 0.80 Mo; from more than 0.10 to 0.15 V; 0.60 ÷ 1.00 W; 0.0130 ÷ 0.0220 N; iron and due to the smelting of impurities - the rest (RF patent No. 2482211, C22C 38/38, C22C 38/24, publ. 05.20.2013).

Known pipe billet for the production of seamless pipes from low carbon microalloy steel containing, by weight. %: 0.16 ÷ 0.22 C; 1.30 ÷ 1.70 Mn; 0.35 ÷ 0.55 Si; 0.10 ÷ 0.20 V; 0.06 ÷ 0.08 Mo: 0.005 ÷ 0.015 N; 0.0001 ÷ 0.03 As; 0.0001 ÷ 0.02 Sn; 0.0001 ÷ 0.01 Pb; 0.0001 ÷ 0.005 Zn; 0.005 ÷ 0.035 S: iron and inevitable impurities - the rest (RF patent No. 2330895, C21D 8/10, C22C 38/24, C22C 38/60, publ. 10.08.2008).

Known low carbon steel for the production of seamless steel pipes containing, by weight. %: 0.04 ÷ 0.15 C; <= 0.5 Si, 0.5 ÷ 2.00 Mn; 0.1 ÷ 1.0 Mo; <= 0.010 Al; <= 0.012 N; 0.01 ÷ 0.10 V; 0.01 ÷ 0.04 Nb; 0.1 ÷ 0.5 Cr (JPH patent No. 10273723, B21B 17/00, C21D 8/10, C22C 38/00, publ. 13.10.1998).

Known fine ferritic steel for the production of seamless pipes, containing, by weight. %: 0.04 ÷ 0.18 C; 0.20 ÷ 2.00 Mn; 0.01 ÷ 0.9 Si; 0.008 ÷ 0.2 V; 0.008 ÷ 0.2 Nb; <= 0.025 P; <= 0.02 S; 0.005 ÷ 0.1 Al, 0.002 ÷ 0.025 N (JPS patent No. 57134517, C21D 8/00, C21D 9/52, C22C 38/00, publ. 08/19/1982).

The disadvantages of these analogues are not a sufficiently high level of strength and viscoplastic properties, in addition, the peritectic reaction during crystallization does not allow to achieve a high level of corrosion resistance of pipes, as well as obtaining a finely dispersed structure to provide the required characteristics.

A known method of heat treatment of a pipe (RF patent No. 2148660, C21D 9/46, C21D 9/08, publ. 05/10/2000), including normalization from rolling heating at a regulated temperature of the end of rolling, quenching with heating above a critical temperature A ₃ and cooling with water re-quenching with heating to the intercritical temperature range and water cooling, high tempering with air cooling.

The disadvantages of the method include the insufficiently high level of obtained strength and viscoplastic properties, as well as low productivity due to the need for re-heating, which leads to an increase in the cost of pipe products.

The closest solution adopted for the prototype for two objects is the production of seamless pipes, resistant to hydrogen cracking, for pipelines (CN patent No. 104419872, C21D 8/10, C21D 9/08, C22C 38/16, publ. March 18, 2015) made of steel containing, by weight. %: 0.08 ÷ 0.1 C; 0.2 ÷ 0.4 Si; 1.0 ÷ 1.6 Mn; 0.01 ÷ 0.04 Ti; 0.03 ÷ 0.08 V; 0.20 ÷ 0.30 Cu; 0.15 ÷ 0.25 Ni; ≤0.015 P; ≤0.010 S; iron and unavoidable impurities are the rest.

A method of manufacturing seamless pipes includes the following stages - heating the billet in a ring furnace, hot rolling at 1200 ÷ 1300 ° C, subsequent rolling at the MPM mill at a temperature of 1010 ÷ 1030 ° C, quenching in water from a temperature of 900 ÷ 920 ° C (holding at least 20 min), leave at a temperature of 600 ÷ 650 ° C (holding time at least 50 min) and carry out dressing at a temperature of at least 400 ° C. The obtained seamless steel pipes with the indicated chemical composition have a uniform fine-grained structure, a favorable combination of strength and viscoplastic properties — tensile strength 455 ÷ 665 MPa, yield strength 360 ÷ 530 MPa, impact strength at 0 ° C on transverse samples 170 ÷ 190 J and Resistant to hydrogen cracking (HIC) in a neutral environment.

The disadvantage of the prototype is the low level of mechanical characteristics (yield strength less than 415 MPa), as well as the composition of the composition, which does not allow to ensure the future receipt of the required strength and visco-plastic characteristics of the pipes. In addition, this pipe cannot be used in harsher corrosive environments at a pH close to 3 m (acidic medium), since it does not have resistance to sulfide stress corrosion cracking (SSC).

The technical result achieved by the invention is to provide the required strength (yield strength of more than 415 MPa), visco-plastic characteristics and corrosion properties of pipes in a sulfide-containing medium under stress (load of at least 0.80 of the minimum yield strength) by obtaining a finely dispersed structure having morphology low carbon tempered ferritocarbide mixture.

The technical result is achieved due to the fact that the pipe is corrosion-resistant from low-carbon doperectic steel containing carbon, manganese, vanadium, aluminum, nitrogen, iron, according to the invention, is obtained from steel containing components in the following ratio, wt. %:

Carbon less than 0.08

Manganese 1.10-1.60

Silicon 0.15-0.50

Vanadium 0.030-0.11

Niobium 0.040-0.080

Aluminum 0.005-0.060

Nitrogen 0.005-0.015

Iron and unavoidable impurities are the rest,

and having a microstructure consisting of a finely divided low-carbon tempered ferrite-carbide mixture, while the content of carbon, manganese, silicon and nitrogen in steel is in the ratio ([C] + [Mn] / 6- [Si] / 7 + [N] / 1.4) ≤0.23 and provides a ferritic potential of at least 1.

The technical result is also due to the fact that in the method for the production of a corrosion-resistant pipe from low-carbon pre-thermic steel, including hot deformation and heat treatment of the pipe, which consists in heating under austenitization, cooling in water and tempering, according to the invention, the pipe is deformed at a temperature of 880 ÷ 1350 ° C, heating under austenitization is carried out to a temperature of A _С3 + (30 ÷ 45 ° С), and high tempering is carried out when heating to a temperature of A _C1 - (50 ÷ 150) ° C with a holding time of at least 4 min per 1 mm of thickness the walls pipes.

The manufacture of pipes from continuously cast billets (hereinafter referred to as NLZ) with the proposed chemical composition of steel allows crystallization to transfer the steel to the preperitectic class, taking into account the production of ferritic potential F _p = 2.5 (0.5- [C _eq ])> 1 followed by hot rolling pipes and heat treatment according to the proposed regime (C _eq - carbon equivalent).

The ferritic potential F _p = 2.5 (0.5- [C _eq ])> 1 ensures the transfer of steel to the preperitectic class and allows obtaining a finely dispersed structure of a low-carbon tempered ferrite-carbide mixture in the finished pipe, high strength (yield strength more than 415 MPa) and visco-plastic characteristics (impact work at a test temperature of -60 ° C - at least 80 J), an increased level of corrosion resistance of pipes in hydrogen sulfide-containing media (at loads of at least 0.80 of the minimum yield strength).

When the carbon concentration is from 0.10 to 0.16%, under cooling, the isothermal peritectic reaction L + δ → γ proceeds from the liquid state of the peritectic class steel with the formation of austenite, the carbon concentration in which corresponds to 0.16% (see figure, which represents the top of the iron-carbon diagram). The excess δ-ferrite phase transforms into the γ-iron phase in the temperature range below 1499 ° C to temperatures limited by the line of complete transition completely to the austenitic state. The formation of two solid solutions of carbon in δ- and γ-iron with different crystal lattices (body-centered and face-centered, respectively) contributes to a greater likelihood of a defect structure due to the appearance of imperfections in the crystal lattice - vacancies, interstitial displaced atoms, dislocations (free nodes of the lattice), packaging defects, and others, which leads to a decrease in corrosion resistance.

In addition, as a result of intermediate peritectic δ → γ transformations, the metal volume changes during solidification, which can also cause surface defects of the continuously cast billet. A change in the volume of the metal (shrinkage) adversely affects the passage of the metal through the primary cooling zone - a copper crystallizer. In this case, the formation of an air gap between the surface of the mold and the outer cortical layer of the workpiece is possible. This affects the heat removal, negatively affects the thickness of the cortical layer and the macrostructure of the workpiece as a whole, worsening the density of the central zone.

When cooling from the liquid state region of steels of the preperitectic class containing less than 0.10% carbon, primary crystallization occurs by converting the liquid into δ-ferrite and ends at solidus line temperatures. In the process of subsequent cooling, δ-ferrite undergoes transformation into the γ-iron phase (austenite) in the temperature range δ → γ of the transformation. With a decrease in the carbon content, the temperature range of the existence of δ ferrite and, correspondingly, the length of metal stay in this region increase. Considering the fact that the diffusion mobility of carbon atoms and other dissolved impurities in δ ferrite is several orders of magnitude (≈ 10 times) higher than the diffusion rate in austenite, an increase in the length of metal stay in the region of δ ferrite leads to greater homogenization, redistribution of impurity atoms from interdendritic areas throughout the volume. In this case, the likelihood of internal imperfections in the metal is minimal. The homogeneity of the chemical composition incorporated in the process of crystallization of the workpiece by the doperitectic mechanism is one of the main factors that can increase the corrosion resistance of pipes in hydrogen sulfide-containing media.

To assess the guaranteed formation during crystallization of δ-ferrite, the ferrite potential is used (Control of surface Quality of 0.08% <C <0.12% Steel Slabs in Continuous Casting / Vicent Guyot, JF Martin, A. Ruelle ea // ISIJ International, Vol. 36. - 1996. - Supplemtnt, P. S227-S230.), Calculated by the formula:

where C _eq is the carbon equivalent.

C _eq = (% C) +0.04 (% Mn) +0.1 (% Ni) +0.7 (% N) -0.14 (% Si) -0.04 (% Cr) -0.1 (% Mo) -0.24 (% Ti) - 0.7 (% S).

Based on the above formula, the higher the carbon equivalent C _eq , the lower the resulting ferrite potential. Thus, to reduce the carbon equivalent, it is necessary to establish a certain content of components that make the main contribution to the ferrite potential — carbon, manganese, silicon, and nitrogen.

To ensure the required carbon equivalent value, the ratio of these components should not exceed 0.23:

When these boundary conditions are met, the ferrite potential will be more than 1, which guarantees the transition of the steel class to the subperithetical region during crystallization.

Carbon in this composition is one of the main elements that make it possible to ensure the transition of the steel class to the preperitectic region. With a carbon content of more than 0.08% and a minimum content of manganese, silicon and nitrogen, the ratio [2] will be 0.25, the carbon potential will increase and will help reduce the ferritic potential of less than 1.

The content of manganese and silicon within the specified limits provides alloying of the solid solution, its hardening, increasing the level of strength and viscoplastic characteristics of the pipes, as well as obtaining the required combination of strength and ductility and transferring the steel class to the preperitic region.

The content of vanadium and niobium within the specified limits provides a decrease in the degree of anisotropy, interdendritic heterogeneity of steel and related banding (line layout of its individual elements), obtaining a hereditarily fine-grained structure with an austenite grain size of not more than 9 points and a final finely dispersed structure due to a change in the kinetics of martensite decay during the tempering process, and influencing the complex of physical and mechanical characteristics and corrosion properties. When the content of vanadium is less than 0.030% and niobium is less than 0.040%, a sufficient formation of carbonitride phases is not ensured, contributing to the refinement of the structure and obtaining the required characteristics. The vanadium content of more than 0.11% and niobium of more than 0.080% leads to the fact that these elements do not bind to carbides and pass into a solid solution, weakening the interatomic bond forces. In addition, the excess content of vanadium and niobium leads to an unreasonable increase in the cost of finished products.

The aluminum content within the specified limits ensures the production of a hereditarily fine-grained structure due to the formation of finely dispersed complex calcium aluminates, which serve as centers of nucleation of austenitic grains, as well as increased corrosion properties due to the presence of finely dispersed inclusions of no more than 0.5 points and their uniform distribution over the billet cross section. When the aluminum content is less than 0.005%, oxygen binding to aluminum oxides is not fully ensured to provide a sufficient number of crystallization centers and to obtain the hereditarily fine-grained structure necessary to achieve the required pipe characteristics; when the aluminum content is more than 0.060%, the number of nonmetallic inclusions increases and their coarsening occurs. which adversely affects the characteristics of the pipes, in particular on corrosion resistance.

It is known that the main factor affecting the corrosion resistance of linear pipes is the purity of the metal according to non-metallic inclusions (Konakova MA, Teplinsky Yu.A. Corrosion cracking under stress of pipe steels. - S-Pb: 2004. - 358 p.) . Non-metallic inclusions, being “traps” of atomic hydrogen, along with other imperfections of the crystal lattice, lead to its accumulation, a local decrease in the strength of the material, i.e. the presence of hydrogen contributes to brittle fracture due to cleavage, intergranular separation, plastic deformations with a high degree of localization, and the formation of embrittlement phases. In this regard, the transfer of steel to the preperitectic class and the accompanying increased level of purity for nonmetallic inclusions, homogenization of the pipe metal in chemical composition and, as a result, the uniformity of the cross-sectional structure, allows achieving high strength characteristics and corrosion resistance of pipes in hydrogen sulfide-containing media under voltage.

The nitrogen content within the specified limits ensures the formation of nitrides in steel, contributing to the refinement of the structure. The lower limit of 0.005% is limited by the capabilities of production technology, the upper limit of 0.015% is due to the need to obtain the specified strength and ductility characteristics of the pipes, as well as the metallurgical quality of the pipe products.

A change in the class of steel does not affect the level of strength and viscoplastic properties, making it possible to obtain a high level of resistance of pipes to stress corrosion cracking in sulfide-containing media.

The resulting tubular billet, taking into account the preparation of a ferrite potential of crystallization F _p = 2.5 (0.5- [C _eq ])> 1, is subjected to hot deformation at a temperature of 880–1350 ° C, followed by cooling in still air. Heat treatment tubes is heated by austenitizing at a temperature A _C3 + (30 ÷ 45 ° C), cooled in water and then tempered at a temperature A _C1 - (50 ÷ 150) ° C with an exposure of not less than 4 minutes per 1 mm thickness pipe wall. The resulting pipe has the following mechanical properties - temporary tensile strength 517 ÷ 760 MPa, yield strength 415 ÷ 600 MPa, elongation of more than 25%, impact on transverse samples according to Charpy at a test temperature of 0 ° C of at least 150 J, at a test temperature of 60 ° C - at least 80 J. The pipe is resistant to cracking in a sulfide-containing medium under stress according to NACE TM 0177 / ASTM G39 at a load of 0.80 of the minimum yield strength (solution "A", method G39, test duration 720 hours with a positive gas pressure H ₂ S, H = 3).

Hot deformation at a temperature of 880 ÷ 1350 ° C allows for the selected steel composition to produce a shape change guaranteed in the austenitic region. At temperatures below 880 ° C, the plastic properties of steel decrease, which leads to an increase in the load on the mill drives for rolling pipes, an increase in the number of defects in pipe-rolling origin and an unfavorable microstructure. An increase in temperature above 1350 ° C leads to overheating of the metal, an increase in the size of austenitic grain, degradation of the structure, and a decrease in the level of required strength characteristics of pipes.

Hardening from the austenitic region at a temperature of A _C3 + (30 ÷ 45 ° C) allows for the proposed low-carbon corrosion-resistant steel to provide complete austenitic transformation with the formation of a homogeneous finely dispersed structure along the pipe wall thickness necessary to provide the required strength and viscoplastic characteristics and resistance against hydrogen sulfide cracking at loads not lower than 0.80 of the minimum yield strength. At the same time, the size of austenitic grain is ensured no more than 9 points.

Subsequent high tempering when heated to a temperature of A _C1 - (50 ÷ 150) ° C provides the microstructure of a finely divided low-carbon tempered ferrite-carbide mixture due to the ongoing processes of coagulation, spheroidization of the carbide component with the release of finely dispersed complex vanadium, niobium carbides and allows you to obtain the required pipe characteristics . Exposure during tempering, which is at least 4 min per 1 mm of the pipe wall thickness, ensures complete homogenization of the chemical composition, decomposition of the supersaturated solid solution with the occurrence of structural stress removal processes, and the release of special niobium and vanadium carbides.

The proposed method allows to obtain the required level of strength and viscoplastic characteristics of the pipes, as well as to provide high resistance to hydrogen sulfide cracking under stress at a load of at least 0.80 of the minimum yield strength due to the obtaining of finely dispersed pipe structure due to an increase in the level of homogeneity and purity of non-metallic inclusions of the initial LLZ used for the production of pipes.

The proposed method for the production of corrosion-resistant pipes from low carbon doperectic steel was tested in the production of pipes with dimensions of 168.3 ÷ 426.0 × 7.9 ÷ 31.8 mm at TPA 159-426 of Volzhsky Pipe Plant JSC.

A pipe is made of steel containing components in the following ratio, wt. %: carbon - 0.05; silicon - 0.33; Manganese - 1.31; vanadium - 0.05; niobium - 0.04; aluminum - 0.027; nitrogen - 0.006; iron and unavoidable impurities are the rest. Hot deformation was carried out at a temperature of 1030 ÷ 1190 ° C and was cooled in calm air. Then the pipes were heat treated along the route: quenching from a temperature of at least 850 ÷ 900 ° C from the austenitic region to water and subsequent high tempering when heated to a temperature of at least 580 ÷ 600 ° C with holding at a given temperature for more than 80 ÷ 140 minutes.

The ferritic potential [1], obtained due to the optimal selection of components of the chemical composition, amounted to 1.08; the ratio ([C] + [Mn] / 6- [Si] / 7 + [N] /1.4) was 0.22; the microstructure after heat treatment is a uniform, uniformly distributed finely divided ferrite-carbide mixture, the size of the austenitic grain was 9 points according to GOST 5639. The contamination of the pipe metal with non-metallic inclusions did not exceed 0.5 points for point oxides according to GOST 1778.

The mechanical properties of pipes after heat treatment were: temporary resistance - 517 ÷ 760 MPa, yield strength - 415 ÷ 600 MPa, elongation - more than 26%, impact work on transverse Charpy samples at a test temperature of 0 ° C - not less than 200 J, at a test temperature of -60 ° C - at least 100 J. The pipe is resistant to cracking in a sulfide-containing medium under stress at loads of 0.72 from the minimum yield strength (standard requirement) and 0.80 from the minimum yield strength (more stringent requirement) NACE TM 0177 / ASTM G39 The tests were carried out in solution "A", method G39, the duration of the tests was 720 hours at a positive gas pressure of H ₂ S, the test results are shown in tables 1-4.

The use of a corrosion-resistant pipe made of low-carbon doperectic steel for oil and gas pipelines manufactured by the proposed method provides a high level of strength and viscoplastic characteristics of the pipes, as well as a high level of resistance to stress cracking in a sulfide-containing medium.

Claims (6)

1. The pipe is corrosion-resistant from low carbon doperectic steel containing carbon, manganese, silicon, vanadium, aluminum, nitrogen, iron, characterized in that the pipe is obtained from steel containing components in the following ratio, wt. %: carbon less than 0.08 manganese 1.10-1.60 silicon 0.15-0.50 vanadium 0,030-0,11 niobium 0,040-0,080 aluminum 0.005-0.060 nitrogen 0.005-0.015 iron and inevitable impurities rest, and has a microstructure consisting of a finely divided low-carbon tempered ferrite-carbide mixture, while the content of carbon, manganese, silicon and nitrogen in steel is in the ratio ([C] + [Mn] / 6- [Si] / 7 + [N] / 1.4) ≤0.23 and provides a ferritic potential of at least 1. 2. A method of manufacturing a corrosion-resistant pipe from low-carbon doperitectic steel, including hot deformation and heat treatment of the pipe by heating under austenitization, cooling in water and tempering, characterized in that the pipe is obtained from steel containing components in the following ratio, wt. %: carbon less than 0.08 manganese 1.10-1.60 silicon 0.15-0.50 vanadium 0,030-0,11 niobium 0,040-0,080 aluminum 0.005-0.060 nitrogen 0.005-0.015 iron and inevitable impurities rest, when the ratio ([C] + [Mn] / 6- [Si] / 7 + [N] / 1.4) ≤0.23 and the ferritic potential is not less than 1, while the pipe is deformed at a temperature of 880 ÷ 1350 ° C, heating under austenitization is carried out to a temperature of A _C3 + (30 ÷ 45 ° C) and high tempering is carried out when heated to a temperature of A _C1 - (50 ÷ 150) ° C with a holding time of at least 4 minutes per 1 mm of the pipe wall thickness.

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