RU2464324C1 - Cryogenic treatment method of austenitic steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: austenitic steel containing the following component ratio, wt %, is made: carbon 0.01-0.20, chrome 12.0-16.0, nickel 11.0-15.0, aluminium 0.2-2.7, molybdenum 1.5-2.5, manganese up to 0.3, titanium up to 0.3, silicon up to 0.3, copper up to 0.2, sulphur up to 0.03, phosphorus up to 0.03, iron and inevitable impurities are the rest when the following conditions are met: sum of nickel and molybdenum is equal to 13.0-16.5, ratio of aluminium and nickel is equal to 0.018-0.18; steel hardening is performed; plastic deformation is performed at cryogenic temperatures at several stages; low-temperature tempering is performed after each stage; high-temperature tempering and ultrasound treatment is performed in the range of operating frequencies f=26-27 kHz.

EFFECT: increasing yield strength and magnetic properties.

2 tbl, 2 ex

Description

The invention relates to the field of ferrous metallurgy, and more particularly to methods of deformation and ultrasound treatment of high-strength austenitic steels with martensitic and diffusion transformations at cryogenic temperatures, and can be used, for example, in the manufacture of highly loaded parts in mechanical engineering and power engineering, precision parts in instrumentation.

A known method of deformation processing of steels, including hardening, plastic deformation with liquid nitrogen and tempering (RF Patent No. 2049126, C21 / D 9/22 02/05/1993). The main disadvantages of this method are the low strength characteristics of the treated steels and high internal microstresses.

The prototype of the claimed invention in technical essence is a method of cryogenic-deformation processing of steel, including hardening, plastic deformation in several stages at a temperature of liquid nitrogen, low-temperature tempering after each stage and high-temperature tempering at the final stage of processing steel billets containing components in the following ratio, wt %: carbon 0.01-0.20; chrome 13.0-18.0; nickel 5.0-10.0; vanadium 3.0-6.0; molybdenum 3.0-6.0; manganese 3.0-6.0; titanium 0.1-2.0; aluminum up to 0.3; silicon up to 0.3; copper up to 0.2; sulfur up to 0.03; phosphorus up to 0.03; iron and inevitable impurities rest; when the condition is met: the sum of vanadium and molybdenum is 7.0-11.0, the sum of nickel and manganese is 9.0-15.0 (RF Patent No. 2394922, C21D 8/00 07/20/2010). The application of this method leads to increased strength and operational reliability of corrosion-resistant steel. The disadvantages of this invention are the relatively low yield strength of steels and low saturation magnetization.

The technical result of the invention is the production of austenitic steels with high values of yield strength and saturation magnetization while maintaining a satisfactory resistance to embrittlement.

The specified technical result is achieved by the fact that in the method of deformation processing of steel, including hardening, plastic deformation at cryogenic temperatures in several stages, low-temperature tempering after each stage and high-temperature tempering, characterized in that they process steel containing components in the following ratio, wt.% :

Carbon 0.01-0.20 Chromium 12.0-16.0 Nickel 11.0-15.0 Aluminum 0.2-2.7 Molybdenum 1.5-2.5 Manganese up to 0.3 Titanium up to 0.3 Silicon up to 0.3 Copper up to 0.2 Sulfur up to 0.03 Phosphorus up to 0.03 Iron and the Inevitable impurities rest

when the condition is met: the sum of nickel and molybdenum is 13.0-16.5; the ratio of aluminum and nickel is 0.018-0.18,

after high-temperature tempering, ultrasonic treatment of steel is carried out in the operating frequency range f = 26-27 kHz.

Distinctive features of the proposed method from the prototype are:

- smelting of chromium-nickel austenitic steel of a given chemical composition: nickel content of 11.0-15.0%; the molybdenum content is 1.5-2.5%; the sum of nickel and molybdenum is 13.0-16.5;

- the ratio of aluminum and nickel is 0.018-0.18;

- carrying out after high-temperature tempering ultrasonic processing in the range of operating frequencies f = 26-27 kHz.

These distinctive features, together with the optimal chemical composition of austenitic steel, make it possible to obtain nanocrystalline steels with high values of yield strength and saturation magnetization.

The chemical composition of the steel was selected after detailed experiments on cryogenic deformation of austenitic steels with different alloying elements. The carbon content in the range of 0.01-0.20% provides carbide hardening during heat treatment of steel. At a lower carbon content, carbide formation does not occur; at a higher carbon content, a decrease in the ductility of steel is observed. The nickel content is in the range of 11.0-15.0%, provided that the sum of nickel and molybdenum is 13.0-16.5%, provides the formation of particles of reinforcing intermetallic nanophases in martensite during cryogenic deformation and subsequent tempering, which increases hardness . When the content of Nickel and molybdenum outside the specified intervals, this effect is not observed. When the nickel content is less than 11.0%, insufficient austenite is formed and subsequently converted to martensite during tempering necessary to ensure the plastic properties of the steel. With a nickel content of more than 15.0%, the steel structure consists of stabilized austenite, which does not turn into martensite during cryogenic deformation, which does not allow to obtain high strength properties. Aluminum is introduced into steel in an amount of 0.2-2.7% to ensure the release of hardening intermetallic phases during high temperature tempering. When the aluminum content is less than 0.2%, the volume fraction of intermetallic compounds at high tempering is small and hardening is ineffective. When the aluminum content is more than 2.7%, a decrease in ductility and embrittlement of steel is observed. Molybdenum is a surface-active element and is introduced into steel in an amount of 1.5-2.5% to increase the ductility of steel. When alloying steel with molybdenum in an amount of less than 1.5%, the effect is not observed. When its content is more than 2.5%, the formation of sparingly soluble compounds of the μ-phase type occurs, which requires quenching after heating to very high temperatures of 1200-1250 ° C. The ratio of aluminum and nickel in the composition of the steel is 0.018-0.18, which leads to the formation of a sufficient amount of intermetallic compounds during tempering and to achieve the required level of strength. When the ratio of aluminum and nickel is less than 0.018%, the volume fraction of the hardening intermetallic phases based on Ni and Al (type NiAl) is insufficient to obtain a high saturation magnetization of steel. Alloying steel with aluminum and nickel in a ratio of more than 0.18% leads to a decrease in resistance to delayed brittle fracture and a decrease in the service life of parts and products made of processed austenitic steel.

The chemical composition of the steel according to the invention provides the most intensive formation of a nanocrystalline structure in the austenitic matrix during deformation at cryogenic temperatures. As a result, just such a composition leads to the most noticeable hardening of steel, which after deformation has a two-phase austenitic-martensitic structure. The presence of the initial austenitic phase provides sufficient ductility of the alloy, although the size of the austenite fragments is also in the nanometric range. The proposed chemical composition, as shown by the studies, also ensures the release of nanocrystalline intermetallic phases during high-temperature tempering, which leads to additional nanophase hardening. The precipitation of intermetallic compounds occurs in steel without deformation processing, however, after cryogenic deformation processing, the dispersion and volume fraction of phase particles increase, which leads to an increase in the yield strength and saturation magnetization.

Plastic deformation of steel at cryogenic temperatures provides the formation of a nanocrystalline structure, which leads to a significant increase in strength and magnetic characteristics (namely, yield strength and saturation magnetization). Deformation is carried out in at least two stages in combination with low-temperature tempering, as this avoids a decrease in ductility and an increase in internal microstresses.

Carrying out low-temperature tempering after each stage of deformation is a mandatory operation, as it provides a significant removal of internal microstresses caused by deformation processing, and helps to increase the service life of parts made of austenitic steel.

During high-temperature tempering, dispersed intermetallic phases are released, which leads to additional nanophase hardening and an increase in the yield strength after steel processing according to the proposed method.

After ultrasonic treatment (RCD) after high-temperature tempering, in the operating frequency range f = 26–27 kHz, the physicomechanical properties of austenitic steel are improved. In the process of ultrasonic treatment in the steel, ultrasonic stresses occur. The action of ultrasound leads to the formation in the volume of the material of an additional amount of α-martensite (10-12%). The additional γ → α transformation associated with ultrasonic deformation is observed mainly in microregions with an increased level of stresses (stacking faults, twin intersection). Moreover, “ultrasonic” martensite in these areas is characterized by a higher degree of dispersion than martensite “cryogenic”, which contributes to an increase in the magnetic properties of steel. In this case, an increase in the saturation magnetization of the material is observed, which is an important characteristic for magnetically semi-solid and magnetically solid materials. The use of RCDs with a frequency of less than 26 kHz does not increase the properties of steel. After sonication with a frequency of more than 27 kHz, a decrease in ductility occurs and embrittlement of steel is observed.

The effectiveness of the application of this invention is shown in the following variants of the processing method.

The processing method No. 1. Melting of austenitic steel with a chemical composition, wt.%: 0.05 C, 12.5 Cr, 12.2 Ni, 0.7 Al, 1.7 Mo, 0.2 Mn, 0.2 Ti, 0.15 Si, 0.1 Cu, 0.003 S, 0.01 P, ∑Cr + Ni = 24.7; Al / Ni ratio = 0.057 (the rest is iron and inevitable impurities), smelted by vacuum induction, homogenization and stripping of the ingots was carried out, the ingots were forged at a temperature of 1050-950 ° C per square bar. The bar (12 mm square) was quenched from 1000 ° С in air, deformation was carried out by rolling at a liquid nitrogen temperature of 196 ° С in two passes (in two stages) with a total degree of deformation ε = 50% (from 12 to 6, 0 mm), after each pass, low-temperature tempering was carried out at a temperature of 200 ° С for 1 h, high-temperature tempering was carried out at a temperature of 520 ° С for 2 h, the samples were subjected to ultrasonic treatment (RCD) with an operating frequency of f = 26.3 kHz. Table 1 shows the data of x-ray examination of steel after processing by the proposed method. The results of determining the mechanical and magnetic properties of samples of steel processed by the proposed method are shown in table 2, p. 1.

The processing method No. 2. Melting austenitic steel with a chemical composition, wt.%: 0.13 C, 16.0 Cr, 13.5 Ni, 1.9 Al, 1.8 Mo, 0.2 Mn, 0.2 Ti, 0.15 Si , 0.1 Cu, 0.01 S, 0.01 P, ∑Cr + Ni = 29.5; Al / Ni ratio = 0.140 (the rest is iron and inevitable impurities), smelted by the vacuum induction method, the ingots were homogenized and cleaned, the ingots were forged at a temperature of 1050–950 ° C onto a square bar. The bar (12 mm square) was quenched from 1000 ° С in air, deformation was carried out by rolling at a liquid nitrogen temperature of -196 ° С in three passes (in three stages) with a total degree of deformation ε = 60% (from 12 mm to 5 , 2 mm), after each pass, low tempering was carried out at a temperature of 250 ° С for 1 h, high-temperature tempering was carried out at a temperature of 500 ° С for 2 h, the samples were subjected to ultrasonic treatment (RCD) with an operating frequency of f = 26.8 kHz . The test results of samples of steel processed by the proposed method using rolling and ultrasonic treatment are shown in table 2, p. 2.

The connection between changes in the structure, mechanical and magnetic properties of the proposed steel after cryogenic deformation and ultrasonic treatment is studied.

An electron microscopic study of the proposed steel showed that after cryogenic deformation, martensite and austenite nanocrystals up to 100 nm in size are observed in the structure, and after high-temperature tempering at 500-520 ° С, 2 h, additionally, NiAl intermetallic nanophase particles up to 20 nm in size are observed. They stand out mainly at interphase boundaries and inside fragments of austenitic and martensitic crystals. In total, the appearance of three nanocrystalline phases of different nature in the structure leads to a substantial increase in the yield strength while maintaining a satisfactory ductility of steel. The use of RCDs increases the volume fraction of martensite and its degree of dispersion, which provides additional hardening and an increase in magnetic properties.

Table 1 shows the data of x-ray analysis of the proposed steel after deformation of 50%. It is seen that a significant amount of martensite appeared in the structure (more than 30%). Thus, after plastic deformation at a liquid nitrogen temperature of -196 ° С (cryogenic temperature), a two-phase nanocrystalline austenitic-martensitic structure is formed in which hardening intermetallic phases are released after high-temperature tempering. After RCD, the amount of martensite increases, which leads to an increase in strength and magnetic properties.

The comparison of the mechanical and magnetic properties of the proposed steel and steel machined according to the prototype and having the appropriate chemical composition, which includes hardening from 1050 ° C, deformation by rolling at a temperature of liquid nitrogen in 2 stages with a reduction ratio of up to ε = 50%, tempering at 200 ° C, 1 h and at 530 ° C, 2 h. The test results of samples of austenitic steel treated according to the prototype are shown in table 2, p. 3.

The results obtained (table 2) indicate that the treatment of austenitic steel according to the proposed method in comparison with the prototype allows to increase the yield strength by 100-130 MPa and saturation magnetization by 40-50%, while maintaining satisfactory ductility. Processing by the proposed method increases the operational reliability and service life of parts and products from austenitic steel after cryogenic processing.

Table 1 The amount of austenite and martensite in the structure of austenitic steel manufactured and processed by the proposed method, and austenitic steel processed according to the prototype. No. Processing method % vol. γ phase % vol. α phase β (222) γ-phases β (220) α phase one According to the proposed method 66.0 ± 0.2 34.0 ± 0.2 28.0 49.0 2 63.5 ± 0.2 36.5 ± 0.2 29.0 48.0 3 According to the prototype 67.5 ± 0.2 32.5 ± 0.2 33.0 52.0 Note: γ-phase - austenite, α-phase - martensite.

table 2 The mechanical and magnetic properties of austenitic steel manufactured and processed by the proposed method, and austenitic steel processed by the prototype. No. Processing method Mechanical properties Magnetic properties σ _in , MPa σ _0.2 , MPa δ,% HRC H _C , kA / m B _max , T one According to the proposed method 2200 1900 9 69 8.9 0.95 2 2210 1930 9 69 9.3 0.97 3 According to the prototype 2200 1800 9 69 5.2 0.55 Note: σ _B is the tensile strength, σ _0.2 is the yield strength, δ is the elongation, HRC is Rockwell hardness, H _C is the coercive force, B _max is the saturation magnetization.

Claims (1)

The method of cryogenic treatment of austenitic steel containing components in the following ratio, wt.%:
carbon 0.01-0.20 chromium 12.0-16.0 nickel 11.0-15.0 aluminum 0.2-2.7 molybdenum 1.5-2.5 manganese up to 0.3 titanium up to 0.3 silicon up to 0.3 copper up to 0.2 sulfur up to 0.03 phosphorus up to 0.03 iron and inevitable impurities rest
when the conditions are met: the sum of nickel and molybdenum is 13.0-16.5, and the ratio of aluminum and nickel is 0.018-0.18,
including hardening, plastic deformation at cryogenic temperatures in several stages, low-temperature tempering of steel after each stage and high-temperature tempering, while after high-temperature tempering, ultrasonic treatment is performed in the operating frequency range f = 26-27 kHz.