RU2542205C1 - Medium-carbon steel treatment method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: hardening and plastic deformation are performed by rotation forging with relative deformation per pass 5-25% within temperature range 600-500°C. The rotation forging is performed by one or more stages with total actual degree of deformation at least ε~1.2. If the rotation forging is performed by more that one stage, then after each stage the billet is cooled to the room temperature, and re-heating is performed to temperature 600-500°C.

EFFECT: increased impact toughness of steels operating under low temperatures.

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Description

The invention relates to the field of heat-treatment and can be used to increase the toughness of medium-carbon low alloy steels operating at low temperatures. The details of machines and mechanisms operating in the Far North are subject to high requirements for impact strength at low temperatures.

A known method of increasing the impact strength at low temperatures in medium-carbon steels by heat treatment according to the patent of the Russian Federation No. 2178003, IPC C21D 1/28, including normalization and tempering at 655-750 ° C for 120-300 min, cooling in air and re-normalization with exposure of 10-60 minutes Products made by this method over the entire range of heat treatment modes showed an increase in the fracture toughness index by 2-4 times at a temperature of -60 ° C with a slight increase in the level of strength properties compared to traditional heat treatment.

Another Russian patent, which provides a 2-4-fold increase in viscosity properties, - RF No. 2430978, IPC C21D 9/46, is aimed at improving the properties of low-carbon steel. This production method includes the smelting of low-carbon low-alloy steel, preparation of billets, preliminary and final deformation in reverse mode, controlled cooling of rolled products, tempering and final cooling in air to ambient temperature. Controlled cooling of the rolled products was carried out from the temperature of the end of deformation, which was in the range (Ac ₃ +20) - (Ac ₃ +40) ° C, to a temperature of 530-570 ° C at a speed of 30-40 ° / s, and tempering was carried out at a temperature of 665 -695 ° C with a shutter speed of 0.2-4.0 min / mm. As a result, the obtained steel had a 2-4 times higher impact strength at -40 ° C, with some loss of strength and ductility compared to a method involving cooling the steel at a temperature of 760-900 ° C at a speed of 10-60 deg / s to a temperature 300-20 ° C, reheating to a temperature of 590-740 ° C with a shutter speed of 0.2-3.0 min / mm and final cooling in air to ambient temperature.

The closest set of essential features to the claimed invention is the processing method described in the article "Inverse temperature dependence of toughness in ultrafine grain-structure steel" by Y. Kimura, T. Inoue, F. Yin, K. Tsuzaki, including hardening at temperatures above Ac ₃ , tempering at a temperature of 500-600 ° C and grinding the microstructure by plastic deformation to a true degree of 1.7 at a temperature similar to the tempering temperature. This set of operations is called tempforming. A preform made by the described method shows a significant increase in strength, ductility and toughness, including at low temperatures.

The objective of the invention is to expand the arsenal of methods for processing medium-carbon low alloy steels with achieving high impact strength at low temperatures.

The technical result achieved by carrying out the invention consists in grinding the microstructure and forming ferritic grains elongated along the axis of deformation with dispersively distributed carbides, thereby increasing the impact strength of medium-carbon low-alloy steels at low temperatures, and also increasing strength and ductility.

The problem is achieved in that the proposed method, including hardening from a temperature above Ac ₃ and grinding the microstructure of the steel by plastic deformation of the workpiece, has introduced new features: plastic deformation of the workpiece from medium-carbon low alloy steel is carried out by rotational forging with a degree of relative deformation per pass 5-25 % in the temperature range 600-500 ° C with a total true degree of deformation of at least ε ~ 1.2, in 1 or more stages with a total true degree of deformation of at least ε ~ 1.2 in this case, if rotational forging is carried out for more than 1 stage, after each stage the workpiece is cooled to room temperature and re-heated to a temperature in the range of 600-500 ° C.

The choice of the degree of deformation is due to the following reasons. From the point of view of economic feasibility of the technological process, it is unreasonable to use processing with a degree of relative deformation per pass of less than 5%. Relative deformation at 500 ° C per passage of more than 25% leads to the appearance of cracks, which is confirmed during the experiment on the precipitation of hardened steel 40KHGNM. Due to the insufficient technological ductility of medium-carbon steel, deformation at temperatures below 500 ° C is impractical. The choice of the upper limit of the deformation temperature at 600 ° C, in turn, is due to the enlargement of ferrite grains on the one hand due to intensive recrystallization and coagulation of carbides with increasing temperature, and on the other hand, to the heating of the material during rotational forging - the deformation temperature should not exceed Ac ₁ .

Plastic deformation at 500 ° C is preferable to obtain a finer-grained structure than in the case of deformation at 600 ° C. However, for rods of large diameter, for example, if the initial diameter is 35 mm, as shown in Example 4 below, when plastic deformation is performed at 500 ° C, there is a significant heterogeneity of plastic deformation over the cross section. In this case, the initial deformation at a temperature of 600 ° C ensures the formation of an ultrafine-grained structure uniform in cross section due to the active passage of dynamic recrystallization. For more intensive grinding of the microstructure and, accordingly, to ensure an increase in the strength characteristics, it is possible to divide the treatment into several stages and conduct subsequent stages of deformation with repeated heating to a temperature below 600 ° C.

It should be noted that the impact strength of the workpieces obtained by rotational forging according to the proposed method is significantly higher than that of workpieces after quenching and high tempering: the impact work of samples at a temperature of -40 and -65 ° C after rotational forging is 9-11 times higher than the impact work of the samples after quenching and tempering at a temperature similar to the deformation temperature.

In the process of heating and warming up the workpiece to a deformation temperature, the steel, which is initially in a hardened state, is released. The liberated carbide particles serve to intensify the processes of grain refinement by providing barriers to the migration of boundaries. It is worth noting that the use of the tempering operation before rotational forging would lead to a more complete precipitation of carbides and some increase in their size, however, the inclusion of this operation in the bar production cycle is impractical in view of the increase in time costs.

Graphic materials.

FIG. 1. Photos of samples of steel 40KHGNM after quenching and precipitation at a temperature of T = 500 ° C to a degree of 25% (a) and 50% (b), in FIG. (b) cracks are visible, which confirms the choice of the degree of deformation for the passage.

FIG. 2. The microstructure of steel 40HGNM after processing according to the mode of example 4 in cross section: a - center, b - edge. Images were obtained using a FEI Quanta-600 scanning electron microscope.

FIG. 3. Photographs of the microstructure of steel 40KHGNM after processing according to the regime of Example 5 in transverse (a) and longitudinal (b) sections. Images were obtained using a JEOL JEM 2100 transmission electron microscope.

The essence of the proposed technical solution is illustrated by examples of specific performance.

Example 1

The initial billet is a bar of medium-carbon steel 40KHGNM, containing wt.%: C - 0.37-0.43; Cr 0.6-0.9; Mn - 0.5-0.8; Ni - 0.7-1.1; Mo - 0.15-0.25; Si 0.17-0.37; S - up to 0.035; P - up to 0.035 - measuring 65 × 500 mm. The initial microstructure is grains of ferrite and perlite colony. The critical temperatures for this steel are: A _C3 = 761 ° C, A _C1 = 776 ° C.

The billet is heated in an oven to a temperature of 840 ° C, i.e. higher than A _C3 , and maintained at this temperature until a uniform austenite solid solution forms. Then the workpiece is hardened to prevent pearlite transformation. As a result of quenching, martensite is formed.

Next, the workpiece is heated in a furnace to a temperature of 600 ° C and subjected to deformation on a rotary forging machine with a step of 4-6 mm in diameter: ⌀65 → ⌀59 → ⌀54 → ⌀49 → ⌀44 → ⌀39 → ⌀35, true degree strain ε ~ 1.2. During heating and deformation, martensite decomposes with the formation of a ferrite-cementite mixture, a fragmented structure forms during deformation, and dynamic recrystallization develops in ferrite. At the end of the deformation, the preform is cooled in air to room temperature. After deformation, the structure is small ferrite grains with a size of ~ 710 nm, the grain-subgrain structure has a size of ~ 420 nm, and dispersively distributed carbide particles ~ 45 nm.

The mechanical properties of steel are shown in table 1.

Example 2

For this example, the workpiece was the starting material, the thermal and deformation-heat treatment of which is described in detail in Example 1.

Next, the billet is heated in a furnace to a temperature of 600 ° C and subjected to deformation on a rotary forging machine with a step of 4 mm in diameter: ⌀35 → ⌀31 → ⌀27 → ⌀23, respectively, the true degree of deformation ε ~ 2.2. At the end of the deformation, the preform is cooled in air to room temperature. After deformation, the preform structure consists of ferrite grains with an average grain size of ~ 600 nm, the average grain-subgrain size is ~ 380 nm, and carbides ~ 55 nm. The mechanical properties of steel are shown in table 1, and the values of the impact at different test temperatures are shown in table 2.

Example 3

For this example, the workpiece was the starting material, the heat and strain-heat treatment of which is described in detail in Example 2.

Next, the workpiece is heated in a furnace to a temperature of 600 ° C and subjected to deformation on a rotary forging machine with a step of 3-5 mm by a diameter: ⌀23 → ⌀18 → ⌀15, respectively, the true degree of deformation ε ~ 2.9. At the end of the deformation, the preform is cooled in air to room temperature. The blank after deformation has the following characteristics: the average grain size of the ferrite is ~ 580 nm, the average size of the grain-subgrain structure is ~ 460 nm, and the carbides are ~ 75 nm.

The mechanical properties of steel are shown in table 1, and the values of the impact at different test temperatures are shown in table 2.

Example 4

For this example, the workpiece was the starting material, the thermal and deformation-heat treatment of which is described in detail in Example 1.

Next, the workpiece is heated in an oven to a temperature of 500 ° C and subjected to deformation on a rotary forging machine with a step of 3-5 mm in diameter: ⌀35 → ⌀33 → ⌀31 → ⌀27 → ⌀21, the true degree of deformation ε ~ 2.2 . At the end of the deformation, the preform is cooled in air to room temperature. After deformation, a significant cross-sectional heterogeneity is observed in the workpiece structure. In the central region, the structure consists of ferrite grains with a size of ~ 705 nm, the grain-subgrain structure has a size of ~ 380 nm, and dispersively distributed carbide particles ~ 50 nm (Fig. 2).

The mechanical properties of steel are shown in table 1, and the values of the impact at different test temperatures are shown in table 2.

Example 5

For this example, the starting material was a workpiece, the heat and strain-heat treatment of which is described in detail in Example 4.

Next, the billet is heated in a furnace to a temperature of 600 ° C and subjected to deformation on a rotary forging machine with a step of 3 mm in diameter: ⌀21 → ⌀18 → ⌀15, the true degree of deformation ε ~ 2.9. At the end of the deformation, the preform is cooled in air to room temperature. After deformation, the structure of the first preform is ferrite grains with a size of ~ 500 nm, the grain-subgrain structure has a size of ~ 400 nm, and the dispersed particles of carbides are ~ 55 nm (Fig. 3).

The mechanical properties of steel are shown in table 1, and the values of the impact at different test temperatures are shown in table 2.

Table 1 The mechanical properties of steel 40HGNM after various modes of rotational forging in comparison with the properties of steel processed by a known method and prototype Example No. Steel Processing Conditions KV, J σ _0.2 , MPa σ _B , MPa δ,% - Hardening 840 ° C, tempering 600 ° C 1 h 57 840 980 fifteen prototype Quenching, tempering 500 ° C 1 h fourteen 1470 1770 10 prototype Tempforming at 500 ° C, ε ~ 1.7 226 1840 1850 fifteen one T = 600 ° C, ε ~ 1.2 143 980 1020 17.7 2 T = 600 ° C, ε ~ 2.2 212 1030 1060 16.1 3 T = 600 ° C, ε ~ 2.9 223 830 900 22 four T = 500 ° C, ε ~ 2.2 158 1090 1100 16.7 5 T = 500 ° C, ε ~ 2.9 235 960 1000 19 Where KV is the stroke work, J; σ _0.2 - yield strength, MPa; σ _B - ultimate strength, MPa; δ is the relative strain,%.

table 2 The values of the impact in the low temperature region in steel treated with various modes according to the proposed method, in comparison with the properties of steel processed by a known method and prototype Example No. Steel Processing Conditions Test temperature ° C +20 -twenty -40 -65 -one hundred Work Impact, J - Hardening 840 ° C, tempering 600 ° C 1 h 57 37 thirty 26 eighteen prototype Quenching, tempering 500 ° C 1 h fourteen fourteen - 12 7 prototype Tempforming at 500 ° C, ε ~ 1.7 226 291 - 285 88 2 T = 600 ° C, ε ~ 2.2 212 219 198 267 183 3 T = 600 ° C, ε ~ 2.9 223 258 270 217 261 four T = 500 ° C, ε ~ 2.2 158 146 169 206 122 5 T = 500 ° C, ε ~ 2.9 235 278 202 280 291

Thus, the task to expand the arsenal of methods for processing medium-carbon low-alloy steels with the achievement of high impact toughness at low temperatures has been solved.

Claims (1)

A method of processing medium-carbon low alloy steels, including hardening from a temperature above Ac ₃ , grinding the microstructure by plastic deformation, characterized in that plastic deformation is carried out by rotational forging with a degree of relative deformation per pass of 5-25% at a temperature of 600-500 ° C, while rotational forging is carried out in one or more stages with a total true degree of deformation ε of at least 1.2, moreover, when carrying out rotational forging in more than one stage, after each stage, cool down Preparations to room temperature and reheating is carried out to a temperature in the range 600-500 ° C.

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