RU2352680C1 - Ferrite corrosion-resistant steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to ferrite corrosion-resistant steel, used in mechanical engineering for products, for which there are raise a demand of providing of high solidity and corrosion resistance while sufficient plasticity. Steel contains carbon, chrome, nickel, titanium, molybdenum, aluminium, lanthanum, yttrium and iron, at following ratio of components, wt %: carbon till 0.03, chrome 12-25, nickel 5-18, molybdenum 0.8-6, titanium 0.25-0.5, aluminium 3-9.2, lanthanum + yttrium till 0.05, iron - the rest.

EFFECT: there are increased strength properties of steel in tempered and wear out conditions while keeping resistance against brittle behavior, aroused by heating.

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Description

The invention relates to the field of metallurgy, that is, to the search for alloys used in mechanical engineering for products that are required to provide high hardness and corrosion resistance with sufficient ductility.

Chromium alloyed ferritic steels are used for the manufacture of products operating in oxidizing environments, for household appliances, in the food and light industry, and for heat transfer equipment in power engineering. These steels have high corrosion resistance in nitric acid, aqueous solutions of ammonia, in ammonium nitrate, a mixture of nitric, phosphoric and hydrofluoric acids, as well as in other aggressive environments [1].

Known analogues of the invention [2-9], allowing to obtain ferritic corrosion-resistant steels with increased ductility, yield strength and productivity of pipe welding [2], resistance to thermal cyclic stress and oxidation at elevated temperatures [3], etc. However, all these steels have insufficient strength.

Currently, among the domestic ferritic corrosion-resistant steels, the most famous are 12X17, 08X18T and 015X18M2B. During high-temperature heating, austenite may form in 12X17 steel, which is undesirable for this type of steel, since martensitic transformation occurs during cooling, which increases hardness, reduces ductility, and causes a tendency to intergranular corrosion. To prevent this phenomenon, the carbon content is reduced or titanium, niobium, and molybdenum are introduced, which contribute to obtaining a single-phase structure, and the formation of titanium and niobium carbides reduces the tendency to grain growth and improves the corrosion resistance, in particular, welds (08X18T and 015X18M2B) [1] .

In foreign practice, steels with a low total carbon and nitrogen content (0.025-0.035%) containing 18-28% Cr and 2-4% Mo, stabilized with Ti or Nb have been developed. These steels are called superferrites; they have high resistance in many aggressive environments, resistant to stress corrosion, pitting and crevice corrosion [1].

Chromium ferritic steels have a major drawback: they can be embrittled in the process of process heating and long shutter speeds at elevated temperatures during operation. They can be brittle at holdings in the temperature range 400-500 ° C, brittle at 600-800 ° C (due to the formation of the σ-phase) and brittleness due to the formation of excessively large grains, for example, during welding. The brittleness of chromium ferritic steels is difficult, and often it is impossible to eliminate subsequent processing, which narrows the possibilities of their practical use and imposes restrictions on technological operations [1].

The prototype of the invention is ferritic stainless steel [10], containing in wt.%: Carbon 0.02-0.09, chromium 5.0-13.0, silicon 1.0-2.5, aluminum 0.9-1, 65, titanium 0.2-0.8, molybdenum 0.07-0.35, vanadium 0.07-0.15, iron - the rest, which has increased ductility, weldability, heat resistance in the environment of combustion products and corrosion resistance in salt and acidic environments, but insufficient strength.

The problem to which the invention is directed is to create ferritic corrosion-resistant steel having a higher complex of physico-mechanical properties (strength, corrosion resistance) in a hardened and aged state, and at the same time that would not be susceptible to brittleness when heated.

The problem is achieved in that the corrosion-resistant ferritic steel containing carbon, chromium, molybdenum, titanium, aluminum and iron, additionally contains nickel, lanthanum and yttrium in the following ratio of components

in wt.%: carbon up to 0.03, chromium 12-25, nickel 5-18, molybdenum 0.8-6, titanium 0.25-0.5, aluminum 3-9.2, lanthanum + yttrium ≤0, 05, iron - the rest. ^* ( ^* Exclusion from the chemical composition of cobalt and an increase in the amount of aluminum leads to a change in the structural class and a sharp increase in the strength properties in the state: quenching with subsequent aging [14].)

Carbon is not specifically introduced into steel, it is a harmful impurity, and the carbon content in steel should not exceed 0.03% to ensure high ductility.

When the chromium content is less than 8%, the corrosion properties of stainless steel are not ensured. With a high chromium content (more than 25%), steel becomes more expensive and there is a danger of the formation of the σ phase, which leads to a decrease in ductility.

The nickel content in the amount of 5-18% increases the ductility, viscosity; Nickel is also part of the hardening phase. Nickel, together with chromium, increases the corrosion resistance in weakly oxidizing or non-oxidizing solutions of chemicals. The use of nickel as a base makes it possible to obtain alloys with high corrosion resistance in strong aggressive acids [11].

Molybdenum increases strength, relaxation resistance, contributes to increased corrosion resistance and heat resistance, suppresses reversible temper brittleness [12-13].

Additional hardening is obtained as a result of dispersion hardening. For this, aluminum and titanium are introduced into the steel, and titanium, as the most powerful element in terms of affinity for carbon, forms an insignificant amount of TiC carbides. Independent Ti intermetallic does not form, although it is possible that it can be a part of nickel aluminide, forming a more complex intermetallic hardening phase. Since the carbon content is limited (≤0.03%), the amount of Ti can also be reduced to 0.25%. In the steel under study, the hardening phase — the NiAl intermetallic compound (or of a more complex composition) is released from the bcc phase, as in martensitic-aging steels.

Lanthanum and yttrium are introduced into steel to grind ferrite grains.

In accordance with the structural diagram for stainless cast chromium-nickel steels by A. Scheffler (Fig. 1), when the aluminum content changes, the studied steel falls into the 100% ferrite region (shaded). The relative contribution of each element to the establishment of the structure is determined by the nickel and chrome equivalent according to the following formulas [15]:

% Ni Equivalent =% Ni +% Co + 30 (% C) +25 (% N) +0.5 (% Mn) +0.3 (% Cu)

% Cr equivalent =% Cr + 2 (% Si) +1.5 (% Mo) +5 (% V) +5.5 (% Al) +1.5 (% Nb)

+1.75 (% Ti) +0.75 (% W)

Example. Samples from the studied steel 03X13H8M2YU3T were smelted in induction furnaces of the Tamman type weighing 1-1.5 kg. Then they were subjected to quenching heating in the temperature range 900-1200 ° C for 15 min, followed by cooling in water. Vickers hardness of the samples after quenching varied from 400 to 475 HV _{5 / 12.5} . An X-ray diffraction study showed that the structure of the steel under study consists practically of 100% ferrite and a small amount of the hardening ordered intermetallic phase NiAl.

Samples quenched from 1000 ° С to water underwent aging in the temperature range 300-600 ° С. The study showed that the greatest hardening is achieved after aging at 500 ° C for 1 h. The hardness of quenched samples after aging increased from 475 to 530 HV _{5 / 12.5} and the microhardness from 650 to 800 HV. As shown by the results of X-ray diffraction analysis, embrittlement inherent in ferritic steels was not observed in the temperature range 400–500 ° C, since aluminum leads to suppression of σ phase evolution [16]. The hardening obtained during aging occurs due to the additional isolation of the same intermetallic phase NiAl from the bcc phase (ferrite).

To assess the behavior during deformation, the samples of the studied hardened steel 03X13H8M2UZT were subjected to deformation by compression to ε = 0.6 without destroying the integrity of the sample (figure 2). The yield strength increased from 565 in the quenched state to 1125 MPa.

The studies showed that, despite the high strength properties, the studied steel has satisfactory ductility and can withstand low total deformations.

Subsequent aging of deformed steel at 500 ° C led to an increase in microhardness to 920 HV.

Comparative tests for corrosion resistance showed that the steel under study is superior in corrosion resistance to industrial stainless steels 12X18H10T, 30X13.

Thus, it is advisable to use the proposed steel in the hardening + aging state, after which it was possible to obtain a high complex of strength properties while maintaining a sufficient plasticity margin, which makes it possible to use the studied steel as a material for high-strength, corrosion-resistant, and heat-resistant parts for instrument making and precision engineering in hardened and aged condition.

Bibliography

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11. Babakov A.A., Pridantsev M.V. Corrosion resistant steels and alloys. M .: Metallurgy, 1971, 200 p.

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13. Rakhstadt A.G. Spring steel and alloys. M.: Metallurgy, 1982, 400 p.

14. Patent No. 2252977. Russia. Publication No. 15 of 05/27/2005, class C22C 38/52. High strength corrosion resistant austenitic steel.

15. MITOM No. 10, 1997. B. Voronenko Modern corrosion-resistant austenitic-ferritic steels.

16. Falcon I.Ya. Two-phase steels. M .: Metallurgy, 1964, 215 p.

Claims (1)

Ferritic corrosion-resistant steel containing carbon, chromium, molybdenum, titanium, aluminum and iron, characterized in that it additionally contains nickel, lanthanum and yttrium in the following ratio, wt.%:
carbon up to 0.03 chromium 12-25 nickel 5-18 molybdenum 0.8-6 titanium 0.25-0.5 aluminum 3-9,2 lanthanum + yttrium up to 0.05 iron rest

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