RU2405840C1 - Hardening method of austenitic non-magnetic steel - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: steel is heated to 1150-1250C, cooled to 950-1100C, and 30% plastic deformation at the above temperatures with further exposure in the air during 605 seconds and cooling in water is performed. ^ EFFECT: increasing yield point of austenitic steel at maintaining high level of ductility and impact strength characteristics. ^ 1 ex, 1 tbl

Description

The invention relates to metallurgy, in particular to the heat treatment of metals and alloys, and can be used in engineering and other industries that are consumers of austenitic steels of increased strength and ductility.

In the metallurgy of structural materials, various hardening methods are known and widely used, including plastic deformation.

A known method of hardening steel, including hardening and cold (at room temperature) deformation [Dzugutov M.Ya. Plastic deformation of high alloy steels and alloys. M .: Metallurgy, 1977, 479 p.]. This method of hardening allows increasing the strength properties of steel (tensile strength σ _B (tensile strength) and yield strength σ _0.2 ), but at the same time the ductility characteristics (elongation δ and relative narrowing ψ) are reduced, and impact strength KCV is also reduced. The indicated changes in the mechanical properties occur as a result of an increase in the dislocation density in the matrix.

Since cold plastic deformation leads to an uneven distribution of the dislocation, strong stresses can arise in the areas of their accumulation, which cause the formation of micro and macrocracks. The appearance of cracks leads to premature failure of steels with a decrease in the above parameters of ductility and toughness.

There is also known a method of hardening, including hardening and warm (at 400-600 ° C) deformation [Orlov A.R., Tyurin L.N., Gribovsky V.K., Chernicha L.E., Lysov D.S. Warm deformation of metals. Minsk: Science and Technology, 1978, 415 pp.]. With this hardening method, due to elevated deformation temperatures, the dislocation density in steel decreases, therefore, the decrease in the characteristics of δ, ψ and KCV is weaker than during cold deformation. The heterogeneity in the distribution of the dislocation also decreases, which reduces the likelihood of cracking.

However, it is still not possible to obtain simultaneously high values of strength properties (σ _0.2 , σ _B ), ductility characteristics and impact strength (δ, ψ, KCV).

Closest to the claimed is a method of hardening austenitic non-magnetic steel, including heating the material to temperatures corresponding to the region of stable austenite (1150-1250 ° C), cooling to temperatures 950-1100 ° C, at which high temperature deformation is carried out, in particular, 30% , followed by cooling in water [Bernstein M.L. Thermomechanical processing of steel. Volume 2. S.695-1024. M.: Metallurgy, 1968]. In this method of high-temperature thermomechanical processing (HTMO), due to high deformation temperatures (950-1100 ° С), the dislocation density decreases even more than in the hardening method, which includes warm (at 400-600 ° С) deformation. This is due to the fact that at high deformation temperatures, significant dislocation mobility is realized, which easily interact with each other and annihilate. As a result of these processes, the plastic and viscous properties (δ, ψ, KCV) decrease slightly and there are no cracks in the material.

However, this known method has the following disadvantages. So steel, hardened by this method, does not have a sufficiently high level of yield strength, the value of which affects the permissible load on products made of this steel. This is due to the fact that during HTMT, austenite softens due to the facilitated proceeding of the process of annihilation of dislocations, which are responsible for the strength properties.

The basis of the invention is to increase the yield strength of austenitic steels while maintaining a high level of ductility and toughness.

The problem is solved in that in the method of hardening austenitic non-magnetic steel, including its heating to temperatures of 1150-1250 ° C, corresponding to the region of stable austenite, cooling to temperatures of 950-1100 ° C, plastic deformation of 30% at these temperatures and cooling in water , according to the invention, after plastic deformation at 950-1000 ° C, the steel is held in air for 60 ± 5 seconds.

The technical result in the proposed method of hardening is achieved by the fact that after heating the material to 1150-1250 ° C, temperatures corresponding to the region of stable austenite, and subsequent high-temperature plastic deformation at 950-1100 ° C, a 30-minute pause is made, during which the material cools down in air for 60 ± 5 seconds in the range of decomposition temperatures of a supersaturated solid solution. Then the material is cooled in water.

Heating to temperatures of 1150-1250 ° C, corresponding to the region of stable austenite, is carried out in order to dissolve in the austenite chromium and vanadium nitrides that are released during the hot redistribution of the metal.

Subsequent high-temperature plastic deformation at 950-1100 ° C is carried out to shape and harden austenite due to the appearance of dislocations in it.

The authors found that holding the steel in air for 60 ± 5 seconds after high-temperature plastic deformation ensures its cooling to temperatures at which the decomposition of the supersaturated solid solution occurs, which additionally strengthens the steel due to the release of intermetallic compounds - vanadium and chromium nitrides in austenite .

In addition, it should be noted that the proposed new method of hardening does not require the use of any additional equipment and can be carried out on the same rolling mill, as in the case of using the known method of hardening.

Thus, the proposed method of hardening allows to increase the yield strength of austenitic steels while maintaining a high level of ductility and toughness.

And, in addition, it is quite technological and simple in the conditions of real production.

Example. As a material hardened by known and proposed methods, we use austenitic steel of the following composition: (wt%) 0.05 C; 19.0 Cr; 10.0 Mn; 5.88 Ni; 1.62 Mo; 0.5 N; 0.32 V; 0.18 Nb; 0.005 S; 0.012 P; rest Fe.

An ingot weighing 25 kg of laboratory smelting, made at the pilot production of the Prometey Central Research Institute, was forged into a billet, which was then cut into 14 × 60 × 80 mm plates. The plates were placed in a furnace heated to 1200 ° C, kept in it for 20 min, and then transferred to another furnace located near the rolling mill and heated to a certain deformation temperature Td (950, 1000, 1050, 1100 ° C). After isothermal holding for 15 minutes at each TD, the preforms were deformed by 30% in one pass and cooled in water (a known method of hardening). Another batch of blanks after deformation at TD was kept in air for 60 ± 5 seconds (pause), after which the plate was cooled in water (the claimed method of hardening).

Samples for tensile tests on the FP-100 installation and impact bending on the PSV-30 installation were made from hardened billets. Tensile strength σ _B , yield strength σ _0.2 , elongation δ, elongation ψ were determined on five-fold samples with a working part diameter of 4 mm. The impact strength KCV was determined at room temperature on samples with a cross section of 10 × 10 mm and a length of 55 mm. A V-shaped incision was made in the middle of the impact specimen with a depth of 2 mm. In addition, Δσ _0.2 / σ _0.2 (in%) was calculated, where Δσ _0.2 = σ * _0.2 -σ _0.2 ; σ * _0.2 and σ _0.2 are the yield strengths after the proposed and known hardening methods, respectively. The test results are presented in the table.

After hardening in a known manner, a structure appears in steel that is typical of materials after high-temperature thermomechanical processing [Bernstein M.L. Thermomechanical processing of steel. Volume 2. S.695-1024. M .: Metallurgy, 1968]: elongated grains, the boundaries of which in some cases have a toothed structure. A rather high dislocation density is observed inside the grains. After quenching from 1200 ° C in water, the mechanical properties of the steel have the following values: σ _B = 921 MPa; σ _0.2 = 551 MPa; δ = 52.1%; ψ = 63.7%; KCV = 166.9 J / cm ² . Due to the appearance of dislocations in austenite after hardening according to the known method, σ _B and σ _0.2 increase, while δ, ψ and KCV decrease.

In the proposed method, after conducting high-temperature deformation, the sample was kept for 60 ± 5 seconds in air (pause) and then quenched in water. During a pause, the sample is in the temperature range at which the supersaturated solid solution decomposes with the release of vanadium and chromium nitrides, which leads to an additional increase in the yield strength. At all deformation temperatures T _D (950, 1000, 1050, 1100 ° C) a one-minute pause gives a gain in yield strength. The greatest increase in yield strength is achieved at T _D = 1100 ° C. A decrease or increase in Td leads to a smaller increase in yield strength. In the first case, due to a decrease in the degree of decomposition of the solid solution, and in the second, due to the strong development of the processes of recovery and recrystallization.

Table treatment TD, ° C σ _0.2 MPa σ _in MPa δ% ψ% KCV J / cm ² Δσ _0.2 MPa Δσ _0.2 / σ _0.2 % No. one Quenching (from 1200 ° C in water), 950 870 1075 31.3 51.0 81 cooling to 1000 863 1019 31.7 51.0 104.6 temperature 1050 828 990 31.7 53.3 111.3 deformations 1100 729 941 34.3 51.0 120.3 T _D rolling on 30% at T _D , cooling in water (famous method hardening) 2 Quenching (from 1200 ° C in water), 950 913 1061 29.7 49.0 82.5 43 5 cooling to 1000 923 1072 26.3 41.2 86.7 60 7 temperature 1050 864 1015 31.3 49.0 111.7 36 four deformations 1100 822 1015 34.7 60,2 109.3 93 13 TD, rolling on 30% at T _D , minute pause, cooling in water (claimed method hardening)

Compare the yield strength of steel obtained after the known and proposed methods of hardening (see table) at T _D = 1100 ° C. In the second case, the yield strength increases by 13% (from 729 to 822 MPa), while the plastic properties (δ, ψ), KCV) remain at a high level (table).

The advantage of the proposed method also lies in the fact that for the hardening of steels in industrial conditions, a low power rolling mill is required, since T _{D is} high.

Claims (1)

The method of hardening austenitic non-magnetic steel, including its heating to 1150-1250 ° C, cooling to temperatures of 950-1100 ° C, plastic deformation by 30% at these temperatures, subsequent exposure to air for 60 ± 5 s and cooling in water.