RU2447187C1 - Steel of higher corrosion and cold resistance - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel contains the following components in wt %: carbon 0.02 - 0.20, manganese 0.35 - 1.4, silicon 0.01 - 0.40, chromium 0.01 - 0.40, nickel 0.01 - 0.40, copper 0.10 - 0.30, phosphorus 0.005 - 0.035, sulfur not over 0.005, aluminium 0.01 - 0.05, niobium 0.02 - 0.05, calcium 0.0001 - 0.002, zinc, tin, arsenium and lead - not over 0.005 of each, oxygen not over 0.004, iron and unavoidable admixtures making the rest. Copper content depends upon sulfur content in compliance with the following relation: |Cu| ≥ 60|S|, where |Cu| and |S| are absolute content of copper and sulfur. Maximum tolerable density of corrosive nonmetallic calcium aluminte-based inclusions makes 3 incl/mm² of mocroslice area while that of magnesium aluminate based inclusions makes 2 incl/mm² of mocroslice area.

EFFECT: higher corrosion and cold resistance, better weldability and surface quality.

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Description

The invention relates to metallurgy, in particular to the production of low alloy steels for various types of equipment, including for oil tanks, electric pipes of increased corrosion resistance, which can be used for the construction of pipelines transporting corrosive liquids, in particular aqueous media, containing chlorine ions, hydrogen sulfide, carbon dioxide, solids and other components. Such pipes can be used for construction, for pipelines of oil recovery systems, heating networks and for other purposes. Conventional steels in such conditions can be subject to general and local corrosion, stress corrosion cracking, hydrogen embrittlement, corrosion erosion, which leads to through corrosion damage to the pipeline. Therefore, the main requirements for such steels should be their high resistance to various types of corrosion and corrosion-mechanical failure with sufficient strength necessary, in particular, for pipelines operating under pressure, as well as satisfactory weldability, toughness and cold resistance. In meeting these requirements, the considered steels can be used for many operating conditions, providing an increased service life. Considering that many types of metal products are currently used after applying protective coatings on them (zinc, paint, bitumen and others), an additional requirement for such steels is a high surface quality, which should be maintained both in the process of preparing steel for coating and directly when applied. In addition, in the most severe operating conditions, such steels should be used as the main layer of sheets and pipes clad with highly alloyed corrosion-resistant steels. For high-quality joining of the main and cladding layers, a high-quality base surface is also required, as well as high purity of the steel of the main layer by impurities, which during the high-temperature application of the cladding layer can go into the boundary zone and reduce the quality of the connection of the layers.

Known steel for gas and oil pipelines, containing the following components, wt.%:

carbon - 0.02-0.07,

Manganese - 1.45-1.80,

silicon - 0.03-0.1,

vanadium - 0.10-0.12,

aluminum - 0.03-0.05,

sulfur - not more than 0.010,

phosphorus - not more than 0.012,

nitrogen - 0.003-0.005,

calcium + barium + magnesium - 0.07-0.10,

iron and unavoidable impurities are the rest.

Steel provides a high level of mechanical properties and resistance to hydrogen cracking in hydrogen sulfide-containing environments (Patent RU No. 2222630, IPC C22C 38/128, publ. 27.01.2004). However, such steel does not have the required resistance to local corrosion in the form of ulcers and pitting in aqueous media containing chlorine ions, as well as in atmospheric conditions. The result of this can be corrosion damage, which reduces the service life of the equipment, as well as reduce the quality of the surface before applying a protective coating or cladding layer, worsening the quality of the connection.

Known carbon low-alloy steel for electric welded pipes with increased corrosion resistance (Patent RU No. 2203342, IPC C22C 38/42, publ. 04/27/2003), containing the following components, wt.%:

carbon - 0.03-0.25,

Manganese - 0.20-1.70,

silicon - 0.20-0.80,

chrome - 0.01-1.00,

nickel - 0.01-0.60,

copper - 0.01-0.50,

phosphorus - not more than 0.035,

sulfur - not more than 0,025,

aluminum - 0.01-0.06,

iron and unavoidable impurities - the rest,

moreover, the manganese content corresponds to the ratio | Mn | × | S | ≤0.015,

where | Mn | and | S | - the content of manganese and sulfur, respectively, expressed in mass percent,

the density of corrosive non-metallic inclusions in steel is not more than 5 pieces in 1 mm ³ .

Steel additionally contains calcium in an amount of 0.0001-0.008%.

Steel contains structurally free ferrite with a grain number of at least 8. Steel has satisfactory resistance against local corrosion, in particular in aqueous media with a high content of chlorine ions, strength, toughness and weldability. However, its resistance to general corrosion, as well as resistance to atmospheric corrosion, may be insufficient. The impact strength of such steel at low temperatures (especially at -40 ° C and below) can be low, which indicates insufficient cold resistance.

The closest analogue of the claimed invention is "Steel with high corrosion resistance" (Patent RU No. 2344194, IPC C22C 38/58, publ. 10.04.2008 - prototype). Steel contains the following components, wt.%:

carbon - 0.03-0.25,

Manganese - 0.15-1.60,

silicon - 0.01-0.80,

chrome - 0.01-0.50,

nickel - 0.01-0.60,

copper - 0.01-0.30,

phosphorus - not more than 0.035,

sulfur - not more than 0.010,

aluminum - 0.01-0.06,

oxygen - not more than 0.005,

iron and unavoidable impurities - the rest,

moreover, the manganese content is determined depending on the sulfur content in accordance with the condition:

| Mn | ≤2.0-145 | S |,

where | Mn | and | S | - the absolute values of the content of manganese and sulfur, respectively, while the maximum allowable density of corrosive non-metallic inclusions in steel N _KANV , inclusions / mm ² , is determined depending on the oxygen content in the steel in accordance with the condition:

| N _CANV | ≤7-1000 | O ₂ |,

where | N _CANV | - the absolute value of the density of corrosive non-metallic inclusions,

| About ₂ | - the absolute value of the oxygen content.

Steel additionally contains niobium in an amount of 0.01-0.07% or vanadium in an amount of 0.01-0.10%, as well as calcium in an amount of 0.0001-0.008%.

Steel has satisfactory resistance to general and local corrosion, in particular in aqueous media with a high content of chlorine ions, strength, toughness and weldability. However, such steel may have low resistance to atmospheric corrosion, which is the reason for the appearance of defects on the surface, reducing the quality of the application of protective coatings or cladding layer. The impact strength of such steel and its welded joints at low temperatures (especially at -60 ° C and below) can be low, which indicates insufficient cold resistance and weldability.

The problem solved by this invention is to provide corrosion resistance, strength, toughness, cold resistance and weldability of steel with high surface quality.

The technical result of this invention is to increase the corrosion resistance, cold resistance, weldability and surface quality of low alloy steel while maintaining strength and toughness.

The technical result is achieved by the fact that the known steel containing carbon, manganese, silicon, chromium, nickel, copper, phosphorus, sulfur, aluminum, niobium, calcium, iron and inevitable impurities, including oxygen, tin, zinc, arsenic and lead, while limiting the maximum permissible density of corrosive non-metallic inclusions in steel according to the invention contains components in the following ratio, wt.%:

carbon - 0.02-0.20,

Manganese - 0.35-1.4,

silicon - 0.01-0.40,

chromium - 0.01-0.40,

nickel - 0.01-0.40,

copper - 0.10-0.30,

phosphorus - 0.005-0.035,

sulfur - not more than 0.005,

aluminum - 0.01-0.05,

niobium - 0.02-0.05,

calcium - 0.0001-0.002,

zinc, tin, arsenic and lead - not more than 0.005 each,

oxygen - not more than 0.004,

iron and unavoidable impurities - the rest,

moreover, the copper content is determined depending on the sulfur content in accordance with the condition:

where | Cu | and | S | - absolute values of the copper and sulfur content, respectively, while the maximum permissible density of CANV based on calcium aluminates is 3 incl./mm ^{2 of} the microsection area, and the maximum permissible density of CANV based on magnesium aluminates is 2 incl./mm ^{2 of} the microsection .

The content of carbon and manganese in the proposed range allows you to obtain the required level of strength of steel without reducing its corrosion resistance. At a lower content of these elements, the required strength of steel may not be provided. At their higher content, corrosion resistance and weldability are reduced.

The content of silicon, aluminum in the proposed range determines the necessary degree of deoxidation of steel with a limited amount of oxides that adversely affect the corrosion resistance of steel. With their lower content, it is difficult to obtain the required oxygen content in the steel. In addition, the presence of silicon in the solid solution makes a certain contribution to the increase in strength characteristics. This is also the result of a certain aluminum content in steel, which, by binding nitrogen to aluminum nitride particles, inhibits grain growth and, thus, also leads to an increase in strength and toughness. A higher value of these elements than provided by the claims, reduces the corrosion resistance of steel.

The presence of chromium, nickel and copper in steel has a positive effect on the resistance of steel against general corrosion and against pitting corrosion in some environments. A related limitation of the lower limit of their content in steel. Higher values of the content of these elements than those provided by the claims lead to a decrease in the weldability of steel.

The microalloying of steel with niobium leads to an increase in the strength, toughness and cold resistance of steel due to the release of fine particles of carbonitrides or carbides, including at the final stage of hot rolling and due to the corresponding refinement of the grain structure.

The limitation of the upper limits of the content of phosphorus and sulfur is associated with the need to provide a certain level of viscosity and cold resistance of steel. The limitation of the lower limit of the phosphorus content is due to the fact that phosphorus, increasing the activity of carbon, contributes to a more intensive release of niobium carbonitride particles during hot rolling, which increases the strength of steel.

Limiting the oxygen content in the steel leads to an increase in resistance to general corrosion.

When the content of zinc, tin, arsenic and lead is more than 0.005% of each, the toughness of the welded joint decreases, especially at low temperatures.

An additional limitation of the copper content depending on the sulfur content (see equation 1) allows the formation of a favorable morphology of sulfide precipitates, which, in addition to manganese and sulfur, contain copper. This significantly increases the resistance to pitting corrosion, the viscosity and cold resistance of steel.

Limiting the density of corrosive nonmetallic inclusions (CANF) based on calcium aluminates detected by special methods (Patent RU No. 2149400, IPC G01N 33/20, publ. 05.20.2000), which accelerate local corrosion to the greatest extent in aqueous media containing chlorine ions , as well as corrosion erosion processes, can significantly reduce the speed of these processes and increase the durability of pipelines. In this case, the permissible value of the CANV density should not exceed 3 on / mm ² .

CANV based on magnesium aluminates, the formation of which was possible by reducing magnesium from the lining or from slag during the processing of steel in the ladle at a low level of oxidation of metal and slag, negatively affects the resistance of steel to general and local corrosion in aqueous chlorine-containing environments, in atmospheric conditions on the quality of the surface, and therefore on the quality of the application of protective coatings and cladding layer. The permissible density of the CANV based on magnesium aluminates should not exceed 2 incl./mm ² .

When steel is treated with calcium in an amount that ensures its content in the limits specified in the formula, the inclusions of manganese sulfide and also inclusions based on magnesium aluminates are modified, which increases the viscosity and cold resistance characteristics of rolled steel and pipes. With a lower calcium content, sufficient modification of non-metallic inclusions does not occur. With its larger content, the contamination of CANV steel based on calcium aluminates increases.

Examples of specific performance of the method.

Eight variants of low alloy steels were smelted in a laboratory furnace and poured into sheet pimps, which were then rolled on a mill into strips 5 mm thick.

The chemical composition of the eight studied steel melts, the dependence of the copper content on the sulfur content and the density of the CANW are given in table 1.

Samples of welded joints were made from rolled steel of the indicated options by welding with high-frequency currents. Complex samples of mechanical and corrosion tests were carried out on samples taken from rolled products and from welded joints — tensile tests in accordance with GOST 1497, impact strength at minus 60 ° C and minus 10 ° C in accordance with GOST 9455, as well as special corrosion tests according to the procedure, consisting in determining the mass of the samples as a result of in-situ corrosion tests in an aqueous medium containing 0.17 mol / L NaCl, 0.13 mol / L KCl, 8 mol / L NaHCO ₃ and 0.8 mol / L Na ₂ SO ₄ , in within 90 days (method 1). In addition, the local corrosion rate was determined according to method 2 (Lipovskikh V.M., Kashinsky V.I., Reformatskaya I.I., Florianovich G.M., Podobaev A.I. and Ascheulova I.I. Dependence of corrosion resistance of heat pipes from carbon steel from the water regime of the heating system. Protection of metals. 1999, v. 35, No. 6, p. 653-655): the rate of development of pits in hot water (steam) was determined at a temperature of 135 ± 15 ° C, containing 50 mg / l chlorine ion, 50 mg / l of sulfate ion and 20 mg / l of oxygen, pH 8.5-9.5, field tests lasted 3 months.

In addition, we evaluated the presence of defects (corrosion damage) on the surface of the rolled product after aging for 1 month in atmospheric conditions.

The results of determining the yield strength, tensile strength, impact strength at minus 60 ° C and minus 10 ° C, local and general corrosion rates (methods 1 and 2, respectively), as well as the presence of defects on the surface of the rolling stock for the studied melts are presented in table 2.

It can be seen that options 1 and 2, corresponding to the claims, provide high mechanical characteristics and corrosion resistance of rolled steel and pipes.

Reduced, compared with the provided claims, the copper content in the heat 3, as well as the failure of the condition | Cu | ≥60 | S | in melts 3 and 4, it leads to a decrease in the resistance of steel to general corrosion (melt 3), against pitting corrosion in some environments, and to a decrease in cold resistance. In addition, the low copper content in smelting 3 caused defects on the surface.

The increased, compared with the calculated, the content of zinc and tin in smelting 5 leads to a decrease in the toughness of steel.

The phosphorus content below the minimum allowable in melt 6 leads to a decrease in the strength characteristics of steel.

The increased, in comparison with the calculated, calcium content in smelting 7 leads to increased contamination of the KANV steel based on calcium aluminates (7 instead of 3 incl. / Mm ² ). This, in turn, dramatically increases the rate of both general and local corrosion.

If there is a significant amount of CANV based on magnesium aluminates - 4 inclusions instead of 1 (melting 8), the local corrosion rate increases by more than 2 times, the general corrosion rate - by more than 1.5 times, surface defects appear.

The reduced corrosion resistance of steel in the presence of corrosive non-metallic inclusions is the main reason for premature failure of pipelines of oil recovery systems and heating mains, especially with an increased content of oxygen and sulfur.

Thus, the use of the present invention significantly increases the corrosion resistance and cold resistance of low alloy steels while maintaining their strength, toughness and weldability. Ultimately, this will lead to a significant increase in the period of trouble-free operation of pipelines.

table 2 No. of swimming trunks Yield strength σ _T , N / mm ² Temporary resistance σ _B , N / mm ² Impact strength KCU _-60C , J / cm ² Impact strength KCV- _10C , J / cm ² The rate of general corrosion, g · m ² / hour (method 1) The rate of local corrosion, mm / year (method 2) Defects on repeat hire welded joint hire welded joint hire welded joint hire welded joint one 480 460 580 570 190 160 200 190 0.04 0.2-0.4 no 2 430 420 540 520 140 130 160 150 0.05 0.3-0.5 no 3 440 410 530 520 70 fifty 80 60 0.15 1.0-1.2 there is four 460 450 560 540 90 80 one hundred 90 0.08 1.1-1.3 no 5 470 450 570 550 80 70 one hundred 80 0,07 0.2-0.5 no 6 370 350 490 460 170 150 180 170 0.06 0,3-05 no 7 450 430 530 510 160 140 150 130 0.18 1.6-1.8 no 8 480 460 560 540 140 120 120 80 0.23 1.3-1.5 there is

Claims (1)

Steel of increased corrosion and cold resistance, containing carbon, manganese, silicon, chromium, nickel, copper, phosphorus, sulfur, aluminum, niobium, calcium, iron and unavoidable impurities, including oxygen, tin, zinc, arsenic and lead, with the limitation the maximum permissible density of corrosive non-metallic inclusions in steel, characterized in that it contains components in the following ratio, wt.%:
carbon 0.02-0.20 manganese 0.35-1.4 silicon 0.01-0.40 chromium 0.01-0.40 nickel 0.01-0.40 copper 0.10-0.30 phosphorus 0.005-0.035 sulfur no more than 0,005 aluminum 0.01-0.05 niobium 0.02-0.05 calcium 0.0001-0.002 zinc, tin, arsenic and lead not more than 0.005 each oxygen no more than 0,004 iron and inevitable impurities rest,
moreover, the copper content is determined depending on the sulfur content in accordance with the condition:
| Cu | ≥60 | S |,
where | Cu | and | S | - absolute values of copper and sulfur, respectively,
in this case, the maximum allowable density value of corrosive non-metallic inclusions (CANF) based on calcium aluminates is 3 incl./mm ^{2 of} the microsection area, and the maximum allowable density value of CANW based on magnesium aluminates is 2 incl./mm ^{2 of} the microsection.