RU2387727C2 - Modifying agent for carbon and low-alloyed steel for rolled products and tubes from steel with increased corrosion resistance - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention can be used during fabrication of carbon and low-alloyed steels for rolled products and tubes with increased mechanical properties and resistance to various types of common and local corrosion. Such metal products are used in construction engineering, for pipelines of oilfield systems, heating systems and for other applications. Modifying agent contains the following component ratio, wt %: calcium - 0.5-15, rare-earth metals- 17-40, silica - 5-50, iron- the rest; at that, calcium content meets the following condition: Ca≤0.7 rare-earth metals, where Ca and rare-earth metals-content of calcium and rare-earth metals respectively. In addition, modifying agent can contain magnesium in quantity of 0.5-5.0 wt %.

EFFECT: invention allows increasing corrosion resistance of steel by forming modified non-metallic inclusions which are not corrosion active non-metallic inclusions, at maintaining its manufacturability and physical and mechanical properties, including strength and cold resistance.

2 cl, 2 tbl

Description

The invention relates to ferrous metallurgy and can be used in the production of carbon and low alloy steels for rolled products and pipes with improved mechanical properties and resistance to various types of general and local corrosion. Such metal products are used in construction, for pipelines of oil recovery systems, heating networks and for other purposes.

Conventional steels under such conditions can be subject to general and local corrosion, stress corrosion cracking, hydrogen embrittlement, and 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 required for pipelines operating under pressure, as well as viscosity and cold resistance.

Given that the pipelines under consideration have a considerable length, which is associated with the need to use significant volumes of pipes, the cost of such metal products should be relatively low, which excludes the possibility of using steels containing significant amounts of expensive alloying elements. At present, manganese is one of the main alloying elements providing increased strength of steel. In turn, the increased content of manganese can lead to the appearance in the steel of a significant amount of particles of manganese sulfide, which reduce the viscosity and cold resistance of the steel. Therefore, technological methods aimed at modifying sulfide inclusions are widely used for the considered types of metal products. To this end, the introduction of modifiers containing calcium that effectively modify sulfide inclusions is widely used at the ladle treatment stage. This leads to an increase in the viscosity and cold resistance of steel, as well as its resistance to sulfide cracking and hydrogen embrittlement.

At the same time, as shown by numerous studies of recent years (Rodionova I.G., Baklanova O.N., Zaitsev A.I. On the role of nonmetallic inclusions in accelerating local corrosion processes in oilfield pipelines made of carbon and low alloy steels. Metals, 2004, No. 5, p.13-18), the presence in the steel of a part of non-metallic inclusions of a certain chemical composition, formed during processing with calcium-containing modifiers, can lead to an abnormal decrease in the resistance of steel to local corrosion in a number of environments (in particular Typical of oilfield pipelines, heating systems and others.). These inclusions are called corrosion-active non-metallic inclusions (CANV). Therefore, if it is necessary to ensure a high complex of steel properties, including its strength, toughness, cold resistance, as well as resistance to various types of corrosion damage, in particular against local corrosion, modifiers of a certain chemical composition should be used to ensure the formation of modified inclusions that are not CANV. Moreover, the treatment of steel with a modifier should not reduce the technological characteristics of steel - its spillability, hot ductility, etc.

Known ligature used for deoxidation, refining and modification of steel and cast iron, containing the following components, wt.%:

Calcium 5-15 Rare earth metals 20-34 Aluminum 10-25 Nickel 2-9 Manganese 2-14 Iron rest

(RF patent No. 2239669, IPC: C22C 35/00).

The use of this ligature allows you to increase the strength and plastic properties of cast metal with a high degree of assimilation of the ligature components of the processed metal. At the same time, the ratio between the calcium and aluminum content in the modifier can lead to the fact that the modification products will be the first type of CANV (inclusions based on calcium aluminates). In this case, the steel will have a low resistance to local corrosion. In addition, the presence of nickel in the modifier increases its value, which will inevitably affect the cost of the metal being processed.

Known modifier for steel, containing the following components, wt.%:

Calcium 1-10 Rare earth metals 6-15 Aluminum 1-10 Manganese 11-25 Vanadium 10-30 Silicon 8-20 Nitrogen 0.5-2.0 Boron 0.2-2.0 Iron rest

(USSR author's certificate No. 5222258, IPC: C22C 35/00).

The modifier is used to increase the physicomechanical properties of steel, primarily strength. At the same time, part of the modifier components is involved in the modification of non-metallic inclusions (calcium, rare-earth metals, aluminum), and the other part is in alloying to increase strength (manganese, vanadium, silicon, nitrogen, and boron). At the same time, the combination of steel alloying processes and the modification of non-metallic inclusions does not always lead to the desired result due to a significant difference in the required content of the considered components in steel. This complicates and lengthens the technology compared to options where the final stage of the technology is the processing of the modifier only with the aim of modifying non-metallic inclusions. The presence in the modifier of a significant number of components intended for alloying significantly reduces the range of steel grades for which this modifier can be used. Finally, a large number of modifier components can make it difficult to control the formation of non-metallic inclusions, lead to the appearance of CANV and, therefore, to reduce the corrosion resistance of steel.

A known method of producing carbon or low alloy steel with high corrosion resistance, including the smelting of steel containing carbon, manganese, silicon, chromium, nickel, aluminum, copper, phosphorus, sulfur, iron and inevitable impurities, with a regulated content of manganese and sulfur, its after-furnace treatment, continuous casting into slabs, hot rolling into strips or sheets, ending in the range of 800-950 ° C, and cooling (RF Patent No. 2184155, IPC: C21D 8/10). This technical solution is the closest analogue of the claimed invention.

In the process of out-of-furnace treatment (as a possible embodiment of the invention), the steel is blown with a powder containing calcium, or a wire containing calcium is introduced with regulated technological parameters.

In addition, in the process of out-of-furnace treatment, liquid steel is purged with an inert gas, and the purge time is prescribed depending on the amount of calcium introduced into the steel.

In the absence of input of a calcium-containing component, the steel is characterized by high purity according to CANV, which guarantees its satisfactory resistance against local corrosion. However, the presence in the steel of unmodified particles of manganese sulfide reduces the cold resistance of steel, as well as resistance to corrosion and mechanical damage (hydrogen cracking, sulfide corrosion cracking under stress SKRN). When a calcium-containing component is introduced, the cold resistance and resistance of steel against corrosion and mechanical damage increase, however, to prevent significant contamination of KANV steel, strict regulation of technological parameters of ladle processing and a high production culture are required. Otherwise, the appearance of CANV in steel will inevitably occur and the resistance to local corrosion will decrease.

The problem to which the invention is directed, is to provide a high complex of steel properties, including its strength, toughness, cold resistance, as well as resistance to various types of corrosion damage, in particular against local corrosion, by increasing the purity of steel according to CANV. This implies the use of modifiers of a certain chemical composition, providing the formation of modified inclusions that are not CANV. At the same time, metal processing with a modifier should not reduce the technological characteristics of steel - its spillability, hot ductility, etc.

The technical result of the invention is to increase the corrosion resistance of steel by forming modified non-metallic inclusions that are not CANV, while maintaining its manufacturability and physico-mechanical properties, including strength and cold resistance.

The technical result is achieved by the fact that the modifier for carbon and low carbon steel for rolled products and pipes of increased corrosion resistance, containing calcium, according to the invention additionally contains rare earth metals, silicon and iron in the following ratio, wt.%:

calcium 0.5-15 rare earth metals 17-40 silicon 5-50 iron rest

and the calcium content meets the condition:

[Ca] ≤0.7 [REM], where [Ca] and [REM] are the contents of calcium and rare earth metals, respectively

also the fact that it additionally contains magnesium in an amount of 0.5-5.0 wt.%.

Calcium within the proposed range has a deoxidizing effect, increases the viscosity and cold resistance of the metal and does not participate in the formation of CANV, that is, does not lead to a decrease in resistance to local corrosion.

The content of rare earth metals (REM) within the proposed limits determines the necessary degree of modification of non-metallic inclusions to ensure cold resistance and resistance to corrosion and mechanical damage. A higher content of rare-earth metals reduces the technological characteristics of steel, its spillability.

Silicon in the modifier within the proposed range has a deoxidizing effect, allows you to vary the strength of steel depending on the requirements of the consumer. A higher silicon content can lead to a decrease in the corrosion resistance of steel.

An additional restriction of calcium content depending on the content of rare-earth metals allows avoiding the presence of CANW in steel: a complex oxide or oxysulfide inclusion containing calcium and rare-earth metals becomes corrosive and leads to a decrease in resistance to local corrosion if the calcium content in it becomes more than 0.7 [REM].

Magnesium in the modifier within the proposed range due to intensive evaporation contributes to the effective mixing of liquid steel when introducing the modifier, its better absorption, ensuring uniform chemical composition of the steel throughout the melting volume.

Examples of specific modifier compositions and their effect on steel properties

The tested modifiers, the composition of which is shown in table 1, in the form of a cored wire was introduced into steel type 20, microalloyed with niobium. One metal melting was processed by a modifier, the composition of which corresponded to the prototype, another metal melting was performed without its processing by modifiers. In order to compare the effectiveness of the action and the nature of the effect on the properties of steel of the studied modifiers and the most commonly used for the modification of non-metallic inclusions SiCa30, the metal of one heat was processed with this ingredient.

The smelting of the metal was carried out in a vacuum induction furnace company BALZERS in a corundum crucible. The weight of one ingot was about 1 kg. Ingot size: ⌀ = 35 mm, h = 110 mm. As the charge used billets of steel of the following chemical composition.

The chemical composition of the steel grade 20-KSX used as a charge in the experimental melting, wt.%

FROM Si Mn P S Cr Ni Cu Al Nb Fe 0.21 0.2 0.54 0.007 0.004 0.18 0,035 0.08 0,032 0,044 rest

The charge was melted under a vacuum of ~ 10 ^-1 -10 ^-2 mm Hg. After entering the required weighed modifier (3 g / 1 kg) and holding the melt for 3 minutes, the steel was poured into ingots.

The ingots were rolled at the DUO 300 laboratory mill in three passes to a thickness of 5 mm.

Samples for tensile and impact bending tests (specimens with notch “V” type 12 according to GOST 9454) were made from the obtained strips at a temperature of +20 and -60 ° C, for the study of CANV and corrosion tests. In tensile tests, the correspondence of the obtained variants was evaluated to K52 strength class (yield strength of at least 350 N / mm ² , tensile strength of at least 510 N / mm ² , elongation δ _{5 of} at least 20%). When testing for impact bending, values above 100 J / cm ² at + 20 ° C and above 70 J / cm ² at -60 ° C were considered satisfactory, although these values are significantly higher than the requirements of NTD (such a high level of impact strength, as a rule , may indicate the resistance of steel to corrosion and mechanical destruction - SKRN and hydrogen cracking).

To identify the CANV, as well as to assess the resistance of steel to local corrosion, we used the electrochemical method, which consists in potentiodynamic tests (when the potential values change in a certain range with a given speed) of samples with a polished surface along a section parallel to the rolling axis. After the tests, the number of surface corrosion lesions whose formation is caused by the presence of CANV was analyzed. The density of such lesions was taken as the number of such lesions per 1 mm ² microsection area. If the density of CANV was less than 2 incl./mm ² , then according to this indicator, steel was considered satisfactorily resistant to local corrosion. Another indicator of the resistance of steel to local corrosion is the maximum current density during electrochemical measurements, which, as shown in previous studies, correlates well with the real rate of local corrosion as applied to oil fields in Western Siberia (with the actual life of them before the formation of through corrosion damage). When the current density value is not more than 3 mA / cm ^2, the local corrosion rate does not exceed 0.5 mm / year, and the metal can be considered satisfactorily resistant to local corrosion.

According to the results of tensile tests, all investigated options meet the requirements. Moreover, lower ductility (at the highest strength) was obtained for the option corresponding to the prototype (option 1). Of the investigated variants, slightly lower impact toughness values were obtained both at + 20 ° C and at -60 ° C for melting without modification (option 2), as well as for melts with a minimum content of calcium and rare-earth metals in the modifier (options 1 and 5 correspond to REM in the modifier is less than according to the claims). All other options showed a very high level of viscosity and cold resistance, which may indicate a high resistance to stress corrosion cracking. CANV in an amount more permissible was found in the metal of three heats (options 1, 3, and 6), where the modifier contained an increased amount of calcium compared with that recommended by the claims (general - option 3, or with respect to rare-earth metals - options 1 and 6 correspond to modifiers containing calcium in an amount of more than 0.7 of the content of rare-earth metals). The results of determining the maximum current density correlate well with the density of the CANV. The permissible value of the current density as well as the CANV density is exceeded for options 1.3 and 6. Thus, only steel obtained using a modifier that fully complies with the claims (option 4) meets all the requirements for mechanical properties, cold resistance and corrosion resistance.

It should be noted that in order to average the chemical composition and uniform distribution of non-metallic inclusions after modification, for all the considered options, mixing of the steel with argon flushing was required. On the samples taken before purging, accumulations of inclusions and uneven chemical composition were observed. Additionally, 1.5% magnesium was added to the modifier of embodiment 4 corresponding to claim 1. When such a modifier (corresponding to claim 2) is introduced into steel due to the active evaporation of magnesium and the associated stirring of the metal, additional purging was not necessary to average the chemical composition and uniform distribution of non-metallic inclusions.

Thus, only modifiers corresponding to the claims provide the required set of steel properties, including strength characteristics, cold resistance and corrosion resistance. Moreover, the modifier according to claim 2 of the claims is more technologically advanced. When using it, purging the metal with an inert gas after modification to average the chemical composition and uniform distribution of non-metallic inclusions may be unnecessary.

Thus, the use of this proposal significantly increases the corrosion resistance of steel by forming modified non-metallic inclusions that are not CANV, while maintaining its manufacturability and physico-mechanical properties, including strength and cold resistance.

Table 1 The chemical composition of the investigated modifier,% wt. No. composition. modif. Ca P3m Si Al Mn V N AT Fe Ca≤0.7PЗM one 8 8 fifteen 5 fifteen eighteen one one rest 8> 5.6 (prototype) 2 - - - - - - - - - - (without mod.) 3 thirty - 70 - - - - - - - (SiCa) four 2 thirty thirty - - - - - rest 2 <21 5 8 fourteen 25 - - - - - rest 8 <9.8 6 fifteen twenty 25 - - - - - rest 15> 14

table 2 Mechanical properties and corrosion resistance of steel of the investigated modification options Option No. σ _t , N / mm ² σ _in , N / mm ² δ ₅ ,% KCV ^-20 KCV ^-60 Density CANV, incl./mm ² I _cor. 5 mA / cm ² one 470 600 21 75 45 four 7.5 2 450 560 25 80 thirty <1 2.5 3 460 580 26 120 95 8 eleven four 470 590 26 130 110 <1 1,6 5 440 560 23 90 fifty 1,5 2,8 6 450 570 28 115 105 3 5,0 Req. ur ≥350 ≥510 ≥20 ≥100 ≥70 ≤2 ≤3

Claims (2)

1. Modifier for carbon and low alloy steel for rolled products and pipes of increased corrosion resistance, containing calcium, characterized in that it additionally contains rare earth metals, silicon and iron in the following ratio, wt.%:
calcium 0.5-15 rare earth metals 17-40 silicon 5-50 iron Rest
and the calcium content meets the condition:
Ca <0.7 REM, where Ca and REM are the contents of calcium and rare earth metals, respectively. 2. The modifier according to claim 1, characterized in that it further comprises magnesium in an amount of 0.5-5.0 wt.%.