RU2716922C1 - Austenitic corrosion-resistant steel with nitrogen - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to production of corrosion-resistant austenitic steels used for production of articles operated in strong oxidative and alkaline media. Steel contains carbon, silicon, manganese, chromium, nickel, nitrogen, magnesium or boron, iron and unavoidable impurities, including sulfur, phosphorus, copper, tin, antimony, arsenic, lead and bismuth, in following ratio of components, wt. %: carbon ≤ 0.030, silicon ≤ 0.50, manganese 2.0-4.0, chromium 17.0–21.0, nickel 8.0–10.0, nitrogen 0.25–0.35, magnesium 0.05 or boron 0.005, sulfur ≤ 0.015, phosphorus ≤ 0.015, copper ≤ 0.20, tin ≤ 0.005, antimony ≤ 0.005, arsenic ≤ 0.005, lead ≤ 0.005, bismuth ≤ 0.005, iron – the rest.

EFFECT: providing a stable austenite structure, up to cryogenic temperatures, high strength, as well as high viscosity and resistance to general and intergranular corrosion, including corrosion under stress.

2 cl, 4 dwg, 4 tbl

Description

The invention relates to metallurgy, in particular to the production of corrosion-resistant austenitic steels, possessing, along with a stable austenitic structure (up to cryogenic temperatures), increased strength, as well as high viscosity and resistance to general and intergranular corrosion in highly oxidizing and alkaline environments and products made of them.

Corrosion-resistant austenitic steels not alloyed with nitrogen used in the domestic and foreign industry are known from the prior art: 03X18H11 (analogue 1), 08X18H10T, 12X18H10T, 03X17H14M3, AISI 304, AISI 304L, AISI 316, AISI 316L, AISI 321 and others. The main disadvantages of these steels are low short-term and long-term mechanical properties. As well as their relatively low resistance to pitting corrosion, intergranular corrosion (MCC) and stress corrosion cracking.

Nitrogen-containing steels are also known in the prior art, for example: 03X20H16AG-6 (analogue 2) and 07X21G7AN5 (EP222), the main disadvantage of which is the high manganese content. Due to the high toxicity and explosiveness of manganese vapor when it is added to molten steel, it is necessary to provide furnace equipment with sophisticated dust collection and air purification systems to ensure proper personnel safety, which is a significant drawback and complicates the production technology of such steel. It should also be noted that many additional requirements for ensuring the labor safety of the personnel of metallurgical plants were established by document No. 586-91 of June 31, 1991, “Sanitary Rules for the Production of Lead, Selenium and Manganese Steel,” which dictate special requirements when working with high manganese steels.

In addition, the known steel RU 2409697 C1 (publ. 20.01.2011 - analog 3)

Steel has the following chemical composition (wt.%):

carbon 0.01-0.10 silicon 0.05-2.0 manganese 0.1-3.0 chromium 17.0-26.0 nickel 11.0-24.5 molybdenum 1.0-5.0 nitrogen 0.05-0.40 vanadium 0.01-0.25 cerium 0.01-0.05 calcium 0.001-0.150 iron and inevitable impurities rest.

Steel is intended for the production of high-strength long products, forged blanks and other products. The disadvantage of this steel is the high content of expensive elements of Nickel and molybdenum.

Known steel RU 2413031 C1 (publ. 02.27.2011 - analog 4)

Steel has the following chemical composition (wt.%):

carbon ≤0.02 silicon ≤0.80 manganese 1.0-2.0 chromium 16.0-18.0 nickel 8.0-9.5 molybdenum 2.5-4.0 nitrogen 0.10-0.20 copper 0.30-0.90 boron 0.001-0.005 hafnium 0.001-0.010 iron and inevitable impurities rest.

Steel is intended for the production of sheet metal parts and welded structures from them. The disadvantage of this steel, on the one hand, is the insufficient content of chromium and nitrogen to ensure corrosion resistance and mechanical properties, and, on the other hand, alloying it with molybdenum and hafnium significantly increases the cost of metal.

The closest chemical composition to the proposed steel is steel according to WO 2011155296 A1, (publ. 15.12.2011 - prototype), containing, by weight. %:

carbon ≤0.20 silicon ≤ 2.00 manganese 0.1-3.0 chromium 14.0-28.0 nickel 6.0-30.0 iron and inevitable impurities rest.

As optional components, the inventors note:

molybdenum ≤5.0 tungsten ≤10.0 copper ≤5.0 nitrogen ≤0.30 vanadium ≤1.0 niobium ≤1.5 titanium ≤ 0.50 calcium ≤0.02 magnesium ≤0.02 aluminum ≤0.30 zirconium ≤ 0.50 boron ≤0.02 REM ≤0.10

The disadvantage of the prototype is the insufficiently high level of strength properties of steel, as well as the lack of standards for harmful impurities such as sulfur, phosphorus, tin, antimony, arsenic, lead, bismuth, which negatively affect the service properties of parts made of this steel. Also, the inventors of WO 2011155296 A1 set a very high boron content (not more than 0.02 wt.%) As a valid additive. Studies of various chromium-nickel and chromium-nickel-manganese steels, widely covered in the technical literature, have shown a significant effect of low boron concentrations on the resistance of metal to intergranular corrosion, as well as its strong influence on the technological ductility of steel. Our own studies show that exceeding the threshold of 0.0025 wt. % boron (which differs by an order of magnitude from the data of WO 2011155296 A1) during microalloying of austenitic steel has a significant negative effect on its technological and operational properties. So, for example, experimentally confirmed that the introduction of the proposed steel 0.01 wt. % boron increases the rate of intergranular corrosion under the conditions of the DU test (GOST 6032-2017) from 0.252 mm / year (steel without boron) to 1.328 mm / year (steel with 0.01 wt.% boron), i.e. 5.3 times. Technological plasticity is reduced by more than 2 times. The same conclusion can be drawn about the excessively allowed maximum concentration of rare-earth metals and zirconium in steel.

The technical problem to which the present invention is directed is the development of corrosion-resistant austenitic steel, which, along with a stable austenitic structure (up to cryogenic temperatures), has increased strength, as well as high viscosity and resistance to general and intergranular corrosion in highly oxidizing and alkaline environments and products made from the proposed steel.

To achieve this goal, corrosion resistant austenitic steel 03Kh20N9G3A0.30 is proposed, containing carbon, silicon, manganese, chromium, nickel, nitrogen, the rest of iron and unavoidable impurities, including sulfur, phosphorus, copper, tin, antimony, arsenic, lead and bismuth , in the following ratio of components, wt.%:

carbon ≤ 0,030 silicon ≤ 0.50 manganese 2.0-4.0 sulfur ≤ 0.015 phosphorus ≤ 0.015 chromium 17.0-21.0 nickel 8.0-10.0 nitrogen 0.25-0.35 copper ≤0.20 tin ≤0.005 antimony ≤0.005 arsenic ≤0.005 lead ≤0.005 bismuth ≤0.005 iron rest.

In order to improve technological ductility, steel is additionally micro-alloyed with magnesium by 0.05 wt. % (calculated) or cerium to 0.04 wt. % (calculated) or yttrium to 0.04 wt. % (calculated) or boron to 0.005 wt. % (by calculation).

The essence of the invention lies in the selection of optimal ratios of the main alloying elements to ensure a complex of physical and mechanical properties, as well as in the normalization of harmful impurities dissolved in a metal, to ensure its purity for non-metallic inclusions and to ensure high quality products.

The content of alloying elements is selected from the following principles.

Chrome in corrosion-resistant steels is the main element that gives them the necessary properties. Depending on the chromium content, the critical temperatures A ₃ and A ₁ change, and at its concentration of about ~ 13 wt.% (And higher), the region of existence of the γ phase is closed, i.e. chromium is a strong ferrite-forming element.

Chromium refers to metals that are easily passivated in oxidizing environments with a change in negative potential to positive. An increase in the chromium content in steel from 13 to 17% greatly increases its corrosion resistance in nitric acid. An increase in chromium concentration over 21% leads to the formation of δ ferrite in the structure of austenitic steel, which has a negative effect on the hot ductility of the metal and its properties. Thus, the optimum chromium content in steel, providing its high corrosion resistance and mechanical properties, is from 17 to 21 wt. %

Nickel is a corrosion resistant metal. It is well resistant to the action of water, solutions of salts and alkalis. Its addition to iron increases the corrosion resistance of alloys in solutions of sulfuric, hydrochloric and in a number of organic acids. Nickel is one of the elements that, when introduced into iron, leads to an abrupt improvement in the corrosion resistance of the alloy in sulfuric acid. Nickel, like chromium, has the ability to passivate and changes the electrode potential of a metal in solutions of nitric acid, sodium chloride with hydrogen peroxide, and when stripping under a solution, it changes the positive potential to negative. The passivating ability of nickel is less than that of chromium or molybdenum in chromium-nickel steels of the X18H10 type.

Nickel, in contrast to chromium, is a strong austenite-forming element that stabilizes the austenitic structure of steel. To achieve an austenitic structure at room temperature in the Soviet Union and other world powers, the first corrosion-resistant chromium-nickel steels with a chromium content of 18 wt. % and nickel 10-12 wt.%. To ensure a stable austenitic structure in the proposed steel, the nickel content was reduced to 8-10 wt.%, Which is sufficient to ensure a stable austenitic structure and corrosion resistance of the metal in solutions of sulfuric, hydrochloric and in a number of organic acids. A smaller amount of nickel does not allow for a stable austenitic structure in steel (inclusions of δ-ferrite are formed), and a larger amount of nickel significantly increases its cost.

Manganese slightly impairs the corrosion resistance of chromium and chromium-nickel steels in oxidizing and reducing environments when it is introduced in sufficiently large quantities (8-16%). When introduced into steel up to 6% Mn, its effect does not significantly affect the corrosion resistance. There are many environments in which chromium nickel manganese steels exhibit high corrosion resistance. At the same time, at many enterprises of domestic metallurgy high manganese steels are not produced due to the high toxicity of manganese vapors and the explosion hazard of dispersed manganese-containing dust. The steel producer is required to provide furnace equipment with sophisticated dust collection and air purification systems. A similar trend has developed in Europe and America, where manganese steels have not found any serious consumer and their production has practically been curtailed. The main reason for this situation is that in Western countries the use of manganese steels in food engineering and industry is prohibited by the sanitary services because of lower corrosion resistance, especially in chloride-containing environments, and also because the production of stainless steels with high (≥8 wt. %) manganese content requires significant environmental costs. The optimal addition of manganese to steel, which, on the one hand, only has a positive effect on the properties of steel, and on the other hand does not cause significant harm to personnel and the environment, is 2.0-4.0 wt.%. It should be noted that manganese is also introduced into the composition of the steel to provide increased nitrogen uptake, therefore, when the manganese content is less than 2.0 wt.%, The ingots of the proposed steel are affected by gas bubbles due to nitrogen released from the melt during cooling and crystallization of the steel after casting.

Nitrogen, according to various researchers, has an ambiguous effect on the corrosion resistance of austenitic steel, which is explained by various research methods. Our own studies have shown that the addition of nitrogen to the proposed steel significantly reduces its tendency to intergranular corrosion in a highly oxidizing environment. This is due to its effect on a decrease in carbon activity, as well as a change in the temperature-time parameters of the precipitation of excess phases and, above all, chromium carbide Cr ₂₃ C ₆ (an increase in the critical time to the formation of carbides along grain boundaries). It was found that when the nitrogen content is from 0.25 to 0.35 wt.%, The strength properties of steel significantly increase (with a slight decrease in the ductility of the metal), as well as its corrosion resistance. With a lower nitrogen content, the necessary properties are not provided and the corrosion resistance advantages of the proposed steel are not realized, and with a higher nitrogen content, it is released in the form of bubbles in the crystallizing metal.

Carbon, silicon, sulfur and phosphorus significantly reduce the ductility, toughness and corrosion resistance of austenitic steel, therefore, they are harmful impurities in the proposed steel. With their increased content, sulfur and phosphorus can be part of harmful non-metallic inclusions and form fusible eutectics (therefore, the inventive steel has a sulfur and phosphorus content limited to 0.015 wt.% Each), and carbon and silicon can be part of Cr ₂₃ C ₆ carbides released during slow cooling of products in air in a dangerous temperature range (450-750 ° C), as well as:

1. during heat treatment (including after welding and in the heat-affected zone);

2. during prolonged operation of products from the proposed steel in the specified temperature range.

The copper atoms of tin, lead, antimony, arsenic and bismuth predominantly segregate at grain boundaries. Moreover, they have a low melting point and also create fusible eutectics. Grain bonds are weakened, as a result of which they are destroyed during prolonged high-temperature loading.

Limiting the content of copper, tin, antimony, arsenic, lead, bismuth, sulfur and phosphorus allows you to get cleaner grain boundaries and higher high-temperature long-term ductility and strength.

Examples of carrying out the invention

Experienced steel, within the claimed composition, was smelted in an open induction furnace with a capacity of 40 kg The chemical composition of the experimental melts is shown in Table 1. The smelting technology included the melting of charge materials (iron, chromium, nickel), an additive of manganese and ferrosilicon, followed by an addition of nitrided ferrochrome and microalloying elements. Casting was carried out in molds, which were cooled in calm air. The deformation of the ingots was carried out by the method of free forging and rolling to workpieces of the required cross section according to the regimes used for the deformation of stainless austenitic steel grades with metal cooling in air.

Table 1

Mechanical properties were tested according to GOST 1497-84. The test results are shown in table 2. From table 2 it is seen that the yield strength of the proposed steel did not fall below 380 MPa, reaching 410 MPa, which is 55-70% higher compared to steel 03X18H11 not alloyed with nitrogen, and also to 30 % is higher than that of other nitrogen-containing steels, as are the analogues considered above, of other nitrogen-containing steels known in the prior art according to GOST 5632-2014, for example: 03X17AN9 (EK177), 10X14AG15 (DI13), 03X17H9AM3 (AISI 316LN), 12X17G9AN4 (EI878 )

Technological plasticity and deformation resistance were evaluated by two methods: hot twisting method (Fig. 1) according to GOST 3565-80 and hot stretching method (Fig. 2) on a plastometer (samples according to GOST 9651-84).

table 2

As can be seen from FIG. 1 and 2b, the technological plasticity of nitrogen-containing steels is much less than that of steel 03X18H11 not alloyed with nitrogen. This leads to the formation of surface defects during deformation of nitrogen steel ingots. It should be noted that the achieved level of technological ductility of the proposed steel (curves 2 and 3 in Figs. 1, 2b) is superior to similar nitrogen-containing steels (for example, 03Х20Н16АГ6 - curve 4 in Figs. 1, 2b) with higher strength of the proposed steel ( Fig. 2a). The proposed microalloying of steel 03Kh20N9G3A0.30 with boron or rare-earth (cerium or yttrium) or alkaline-earth metals (magnesium) in the above ranges allowed us to improve the technological ductility of steel (by the relative narrowing of the sample and the number of revolutions during twisting) by 30-50% over almost the entire temperature range tests, equating the proposed steel for technological ductility with steel analog 03X18H11 not alloyed with nitrogen (region 5 in Fig. 1, 2b), with consistently high strength.

Thus, the resulting complex of mechanical properties exceeds all considered analogues and prototype.

Tests of the corrosion resistance of the metal were carried out in accordance with the DU method (according to GOST 6032-2017). The test results in table 3. It is seen that the resistance to intergranular corrosion of the proposed steel is 30% higher than that of steel 03X18H11.

Table 3

Tests for stress corrosion cracking were carried out (table 4, Fig. 3) in a boiling 42% solution of MgCl ₂ at a solution temperature of 150 ° C. The base of the test took an exposure value of 1000 hours. If after 1000 hours no cracks were found on the sample, the tests were stopped.

It can be seen from the above data that, at voltages of 280 MPa, the resistance of nitrous steel 03Kh20N9G3A0.30 turned out to be more than 8 times higher than that of steel 03Kh18N11, and at a voltage of 100 MPa almost 10 times.

Table 4

The cracking during the test is clearly transcrystalline (Fig. 4a), inherent in the destruction of austenitic steels in chlorides. An electron-microfractographic study of fractures of samples after testing for corrosion cracking revealed signs of brittle fracture with a characteristic "brook" pattern (Fig.4b). It should be noted that the nature of the destruction of the proposed steel and 03X18H11 is identical, but the time required for the initiation and development of cracks differs by an order of magnitude, as can be seen from table 4 and FIG. 3.

Thus, the proposed steel has a significant advantage over steel known from the prior art, both non-alloyed with nitrogen and alloyed with it.

The technical and economic efficiency of the invention in comparison with analog steels consists in increasing operational characteristics by increasing short-term and long-term strength and increasing resistance against MKK and pitting formation. At the same time, the proposed steel can be smelted in open induction furnaces and does not require special dust-collecting equipment or equipment to create an excess nitrogen pressure above the melt. In this case, the deformation of the ingots is allowed to be carried out according to the regimes adopted for austenitic steels.

Brief Description of the Drawings

The following drawings are used in the present invention:

1. FIG. 1 - The results of tests of technological plasticity of austenitic steels by hot twisting.

2. FIG. 2 - The results of tests of deformation resistance (a) and technological plasticity (b) of austenitic steels.

3. FIG. 3 - The dependence of the time before the appearance of the first cracks on stress when tested for corrosion stress cracking of steel.

4. FIG. 4 - The characteristic microstructure (a) and microfractographic image of the fracture (b) of the steel sample 03Kh20N9G3A0,30 after corrosion cracking tests.

Claims (3)

1. Corrosion-resistant austenitic steel containing carbon, silicon, manganese, chromium, nickel, nitrogen, iron and inevitable impurities, including sulfur, phosphorus, copper, tin, antimony, arsenic, lead and bismuth, characterized in that it additionally contains magnesium or boron in the following ratio of components, wt.%: carbon ≤ 0,030 silicon ≤ 0.50 manganese 2.0-4.0 chromium 17.0-21.0 nickel 8.0-10.0 nitrogen 0.25-0.35 magnesium 0.05 or boron 0.005 sulfur ≤ 0.015 phosphorus ≤ 0.015 copper ≤ 0.20 tin ≤ 0.005 antimony ≤ 0.005 arsenic ≤ 0.005 lead ≤ 0.005 bismuth ≤ 0.005 iron rest 2. The product is made of corrosion-resistant austenitic steel, characterized in that it is made of steel according to claim 1.

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