RU2616756C1 - Method of heat treatment of solid-rolled railroad wheels from alloyed steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method includes heating to the austenitizing temperature, holding at this temperature, intermittent hardening of the rim with water, subsequent cooling of the wheels and drawing-back. Wheels are made of steel containing the following components, wt %: carbon 0.46-0.54, silicon 0.25-0.45, manganese 0.85-1.2, chromium 0.80-1.10, niobium 0.01-0.05, vanadium 0.08-0.20, molybdenum not more than 0.05, titanium not more than 0.01, phosphorus not more than 0.030, sulfur no more than 0.020, nickel and copper not more than 0.25 each, the rest is iron and unavoidable impurities, including hydrogen no more than 0.0002. Hardening of the rim with water is carried out by three-way cooling it from the spraying devices for 290-330 seconds, while the water pressure in the spraying devices is being increased from zero to 0.07-0.09 MPa during 75-85 seconds with further cooling at constant pressure up to formation of martensitic-bainitic-troostite structure in the rim at the depth of the working layer to 30 mm. Subsequent cooling of the wheels is carried out in the air until a ferrite-pearlite structure forming in the disk and hub, and drawing-back is carried out at a temperature of 500-550°C for at least 2 hours.

EFFECT: increased level of the rim strength characteristics, wear resistance, limit of contact fatigue endurance, the required level of plastic and viscous characteristics of the wheel metal is provided.

2 cl, 1 dwg, 6 tbl

Description

The invention relates to a technology for processing highly loaded railway wheels and can be used for hardening heat treatment of solid-rolled railway wheels of various shapes in order to increase physical and mechanical properties and resource. At present, pearlitic carbon steel wheels have exhausted the possibilities of further increasing the level of mechanical properties and durability in operation, in connection with which it is urgent to develop methods for thermally hardening wheels from new economically alloyed steel grades to produce a different type of steel microstructure with new unique product characteristics .

A known method of hardening the working surface of the wheel with a low-temperature plasma arc of direct action. Argon, a mixture of argon with helium, a mixture of argon with carbon-containing gases are used as a plasma-forming gas, while heating is carried out with magnetic oscillation of an arc with a triangular voltage pulse shape, a scanning amplitude of 20-45 mm and a processing distance of 10-30 mm. The hardened surface layer consists of several sublayers: at a depth of 0.2-0.8 mm - the structure of martensite with impregnations of upper bainite, from 0.8 to 1.8 mm - troost-martensitic structure, from 1.8 to 2.5 mm - sorbitol-martensitic structure, from 2.5 to 3 mm - sorbitol, more than 3 m - the structure of the base metal is ferritic-pearlite. The method of surface plasma-arc hardening of railway wheels can significantly increase the productivity of the hardening process, increase wear resistance, reduce the likelihood of the development of hydrogen brittleness of the metal and the tendency to brittle fracture (Patent RU 2430166, IPC C21D 9/34, C21D 1/09, published 09/27/2011) .

This method has several disadvantages: the extremely shallow depth of the hardened layers, as well as the high cost of gas-plasma equipment and its operation.

A known method of processing railway wheels, in which to ensure the required mechanical properties of steel and increase the service life of the wheels, the wheel is first heated to form austenite in all parts of the rim and disk (from 700 to 950 ° C, depending on the composition of the steel). The wheels are held at this temperature for a time sufficient to achieve a fully austenitic structure of all steel. The wheel is then cooled to form a bainite / martensite microstructure on the outer portion of the disk. The wheel is cooled to form a bainite / martensite microstructure on the inner portion of the rim. The wheel is cooled to form a bainite / martensite microstructure on the outer portion of the rim. The cooling steps are performed sequentially to create compressive residual stresses on the outer portion of the rim. In this case, the outer portion of the disk is cooled during quenching for 2-10 minutes, the inner portion of the rim is cooled during quenching for 2-15 minutes, and the outer portion of the rim is cooled at room temperature or alternatively tempered for 1-4 hours. The wheels are made of steel containing (% mass): carbon 0.05-0.30; Manganese 3.00-5.00; silicon 0.45-1.85; iron is the rest. (Patent RU 2495144, IPC C21D 9/34, published October 10, 2013). The mechanical properties of steels of a bainitic-martensitic microstructure are shown: hardness 306-414 HB; dynamic impact strength 70.7-77.2 MPa⋅m ^1/2 .

The disadvantage of this method is the low productivity of the process, the difficulty in organizing differentiated cooling of various elements of the wheel and the guaranteed provision of the necessary level of residual compressive stresses in the rim.

A known method of heat treatment of solid-rolled railway wheels, including heating them to austenitic temperature, vertical intermittent hardening and tempering ("Equipment of thermal shops, heat-treatment and combined processing of metal products". Bolshakov VI, Dolzhenkov I.E., Zaitsev A.V. ., Dnepropetrovsk, RIA Dnepr-VAL, 2010, pp. 395-400). When hardening, three-sided cooling of the rim, rotating in the vertical plane of the wheel, is carried out with water from spraying devices. Moreover, in order to ensure a given level of mechanical properties of the wheels, the cooling time during quenching, tempering parameters are set depending on the total content of carbon wheels in the steel plus 1/4 manganese. Any steel grade according to this indicator, taking into account the range of C and Mn contents, is divided into several groups according to the assigned intermittent hardening and tempering modes. Duration of hardening of wheels with a minimum and maximum content in steel (C + 1/4 Mn) of the mass. % may vary up to two or more times. All this leads to a violation of the rhythm of the work of production shops, based on the need for float processing and acceptance of wheels, to complicate the organization of the float flow in them, the marriage of the wheels when they lag behind the stream. A significant drawback is also that, despite this division of the wheels into groups within each steel grade, it is not possible to achieve the same level of mechanical properties on the wheels of various swimming trunks.

A known method of manufacturing solid-rolled railway wheels from continuously cast ingots (patent RU No. 2119841 IPC B21H 1/04, C21D 9/34, C21C 7/00, publ. 10.10.1998). The method includes smelting a number of converter steel melts, out-of-furnace treatment of wheel steel with bringing the amount (С + 1/4 Mn) to a certain value, continuous casting into ingots, rolling of wheels, heat treatment by heating the wheels to the temperature of hardening, cooling for a certain time and vacation. During the out-of-furnace treatment, the sum (C + 1/4 Mn) in the steel is brought to a constant value for all melts equal to 0.815-0.864. The heat treatment of the wheels of different melts is carried out according to a single regime of heat hardening with a constant cooling and tempering time. At all stages of the manufacture of solid-rolled railway wheels, ingots obtained from different heats are simultaneously used. The technical result is the provision of reducing the marriage of wheels, increasing and stabilizing the level of mechanical properties of the wheels, increasing their service life.

The disadvantage of the method of manufacturing wheels from continuously cast billets according to patent RU No. 2119841 is that when the values of the sum value (С + 1/4 Mn) are scattered at 0.05% or more to level the effect of the spread of carbon and manganese contents and provide the necessary level and stabilization In practice, the mechanical properties of steel are additionally alloyed with vanadium, chromium, and molybdenum. It should also be noted that the method according to this patent relates to the manufacture of wheels of steel grade 2 according to GOST 10791-2011 and cannot be used in the manufacture of wheels with higher strength characteristics of the rim.

Closest to the claimed method is a method of manufacturing wheels according to patent JP 2005-350769 A, IPC C22C 38/00, B60B 17/00, publ. 12/22/2005. This patent discloses a wheel in which its rolling surface has a structure composed of a bainitic structure, a tempering martensite structure, or a mixed structure of bainite and tempering martensite. For the manufacture of wheels used steel of the following chemical composition in wt. %: carbon 0.35-0.55; manganese 0.40-0.80; silicon 0.10-0.60; vanadium 0.005-0.30; phosphorus ≤0.030; sulfur ≤0.030; molybdenum 0.20-0.60; nickel 0.05-1.0; chrome 0.50-1.5; copper 0.05-1.0; aluminum 0.005-0.10; titanium 0.005-0.30; niobium 0.005-0.30; iron is the rest.

The preferred method for the production of steel is vacuum degassing and continuous casting. Heat treatment of the wheels is carried out by heating to austenitization temperature with holding at this temperature. Hardening of the rim is carried out with continuous cooling with water, or isothermal transformation of austenite when the wheel is immersed in a salt bath until a martensitic or bainitic or mixed martensitic-bainitic structure is formed in the rim, followed by tempering.

The disadvantage of this method of heat treatment of steel of the specified composition is that the method of continuous cooling of the rim is declarative in nature, based on the description it cannot be implemented in practice, and when heat treated with immersion in a salt bath, the wheel production process is much more complicated, productivity decreases and cost increases. Another significant drawback is that when carbon and chromium are contained in steel closer to the upper limit, the formation of bainite / martensite structures in the wheel disks is possible according to both heat treatment options, which can reduce their fatigue strength, reliability and operational safety in comparison with pearlitic disks structure. When the content of carbon, chromium, manganese, vanadium is closer to the lower limits, the probability of obtaining the microstructure declared in the patent is low. The high contents of chromium, molybdenum, vanadium, niobium, nickel, copper due to the increase in the cost of the wheels make them uncompetitive. It is also impossible to obtain compressive stresses in the rim, since the preparation of a martensitic or mixed martensitic-bainitic structure, in contrast to a pearlitic one, occurs with an increase in volume due to a lower specific gravity.

The disadvantages of the method include a wide range of content of steel elements, which does not allow its continuous casting in batches of heat for melting - with serial casting at the beginning of casting, the steel of each heat is mixed with the remainder of the steel in the intermediate ladle from the previous heat. With a certain difference in the content of elements in swimming trunks during continuous continuous casting along the length of the obtained continuously cast billets, it is possible to obtain such a variation in the content of elements that, upon heat treatment of the wheels of each melting in a single mode, will result in rejection by their mismatch with the required level of properties.

The objective of the present invention is to develop a technology for heat hardening wheels made of economically alloyed steel of a certain chemical composition, which achieves a significant increase in the level of mechanical and operational properties of the wheels (wear resistance, contact fatigue endurance of the rim metal, endurance of the disk, the possibility of using wheels under freight cars when high axle loads). The objective of the invention is also to significantly simplify the technology of production of wheels, increase productivity and reduce the cost of their production.

When using the claimed invention, the following technical result is achieved:

- increase in the strength characteristics of the rim (section hardness, temporary tensile strength) by - at least 10%, wear resistance by - at least 50% compared to standard wheels for freight cars;

- increase the limit of contact fatigue endurance of the rim metal by not less than 25%, the endurance limit of the disk up to 50% in comparison with standard wheels;

- ensuring the level of plastic and viscous characteristics of the metal wheels at the level of values or higher than standard;

- the possibility of continuous casting of wheel steel by series of melting and melting and heat hardening of wheels made from them according to the same parameters of heating, hardening and tempering, while increasing productivity, reducing costs and stabilizing the level of properties of wheels;

- the possibility of increasing the axle loads of freight cars from 22.5 to 27 tp

To solve this problem and achieve a technical result, a method for heat treatment of solid-rolled railway wheels made of alloy steel, including heating to austenitic temperature, holding at this temperature, intermittent quenching of the rim with water, subsequent cooling of the wheels and tempering, the wheels are made of steel, containing the following components, mass. %: carbon 0.46-0.54, silicon 0.25-0.45, manganese 0.85-1.2, chromium 0.80-1.10, niobium 0.01-0.05, vanadium 0, 08-0.20, molybdenum not more than 0.05, titanium not more than 0.01, phosphorus not more than 0.030, sulfur not more than 0.020, nickel and copper not more than 0.25 each, the rest is iron and inevitable impurities, including hydrogen is not more than 0.0002, quenching of the rim with water is carried out by means of its tripartite cooling from spraying devices for 290-330 seconds, while the water pressure in the spraying devices is increased from zero to 0.07-0.09 MPa for 75-85 seconds with further cooling

at constant pressure until a martensitic-bainitic-troostite structure is formed in the rim at a depth of the working layer up to 30 mm, the wheels are subsequently cooled in air until a ferrite-pearlite structure is formed in the disk and the hub, and tempering is carried out at a temperature of 500-550 ° С for at least 2 hours.

The wheels are made of steel containing 0.49-0.52 mass. % carbon, while the carbon and manganese content are related by the ratio, mass. %: carbon +1/4 manganese = 0.75-0.78.

The choice of intermittent hardening of the rim is due to the fact that it is the most optimal from the point of view of simplicity of technology, productivity and cost of the process, stability in obtaining the necessary structure and level of mechanical properties. The chemical composition of steel for wheels, heat-strengthened by the present method, is selected taking into account the fact that when cooling in air in a wheel disk, the cooling speeds will not be lower than critical in the entire range of element contents, which guarantees the ferrite-pearlite structure of the steel, providing high fatigue strength of the wheels.

The selected parameters of hardening and tempering make it possible to obtain a martensitic-bainitic-troostite structure in the wheel rim of steel of a given chemical composition to the entire depth of the working layer, which significantly increases the mechanical and operational properties of the wheels compared to standard wheels (increased strength, hardness combined with high ductility and viscosity, high wear resistance and endurance to contact fatigue damage). This guarantees the creation in the rim of the wheels of compression stresses that prevent the nucleation and propagation of fatigue cracks from the rolling surface into its interior.

The lower content of molybdenum, niobium, copper, chromium, nickel, vanadium compared with the steel presented in the prototype, significantly reduces the cost of steel and manufactured wheels, and at the same time is sufficient to achieve a technical result with the claimed method of heat hardening. The limitation of hydrogen content is not more than 0.0002 wt. % due to the fact that with such a content of steel acquires immunity against the formation of flocs.

The use of steel in a narrower range of carbon content in the range of 0.49-0.52 (wt.%) And values of carbon +1/4 manganese (wt.%) Equal to 0.75-0.78, can significantly simplify the production technology wheels due to the possibility of continuous casting became a series of smelting for melting and heat treatment of wheels made from them according to the uniform modes of heating, intermittent quenching and tempering.

The use of sprayers and pressure regulation during hardening positively affects the maintenance of a constant temperature of the cooling medium and the uniformity of cooling of the surface of the wheel, and the increase in pressure of cooling water in the spraying devices during the initial 75-85 seconds due to the uniform cooling rate of the metal over the cross section of the rim prevents the appearance of the surface riding a rough martensitic structure. The quenching time, depending on the temperature of the cooling water, is 290-330 seconds, the tempering temperature is 500-550 degrees for at least 2 hours selected experimentally, it is proved that the technical result is achieved only within the above ranges.

In FIG. 1 shows the microstructure of the metal of the rim and the disk of the railway wheel made of experimental steel grade B, where:

1 - at a depth of 5 mm from the rolling surface, the microstructure is represented by fine-needle martensite and troostite;

2 - at a depth of 15 mm from the skating surface, the microstructure is represented by fine-needle martensite, bainite and troostite;

3 - at a depth of 30 mm from the skating surface, the microstructure is represented by bainite and troostite;

4 - at a depth of 50 mm from the rolling surface, the microstructure is represented by perlite and ferrite along the boundaries of former austenitic grains;

5 - microstructures of the disk and hub are represented by perlite and ferrite along the boundaries of former austenitic grains.

An example of a specific implementation.

An experimental steel melting was obtained with the claimed chemical composition shown in table 1, conditionally named grade B.

Wheel hardening

The wheels were quenched to a temperature of 850 ° C for 2 hours until an austenitic structure formed. The rim was thermally strengthened by three-way spray cooling of the rims of the wheels with water for 300 seconds, while the water pressure in the spray devices was proportionally increased to 0.07-0.09 MPa for 80 seconds with further cooling at constant pressure until a martensitic-bainitic formation in the rim a troostite structure at a depth of the working layer up to 30 mm, the subsequent cooling of the wheels is carried out in air until a ferrite-pearlite structure is formed in the disk and the hub, and tempering is carried out at a temperature of 500-550 ° C for of not less than 2 hours.

The disk was cooled in air to a temperature of 300-450 ° C. The wheels were released at a temperature of 500-520 ° C for 2.5 hours. The microstructure along the cross section of the rim and wheel disc of grade B steel is shown in FIG. 1. The mechanical properties of the wheels obtained during testing and for comparison of wheels according to GOST 10791-2011 are presented in Table 2. The characteristics of the wheels that determine their operational properties are presented in Tables 3, 4, 5, 6.

The hydrogen content in the steel was 1.7 ppm or 0.00017%. The value of the ratio C + 1 / 4Mn was 0.77%.

The formation of a martensitic-bainitic-troostite structure in the rim at a depth of the working layer of up to 30 mm in the process of hardening of alloy steel wheels of grade B allows to obtain increased strength, hardness in combination with high ductility and viscosity compared to standard wheels, and a ferrite-pearlite disk structure provides its high fatigue strength and reliability of the wheels in operation.

The hardness of the rim of the test wheels made by the present method of grade B steel is at a high level over the entire working thickness of the rim and reaches: in the crest - up to 400 HB, at a depth of 10 mm from the rolling surface - up to 396 HB, at a control depth of 30 mm - up to 346 HB, at a depth of 50 mm - up to 321 HB, surpasses wheels made of steel of grade T by 12%, of steel of grade 2 - by 26%.

In terms of wear resistance, experimental wheels are superior to wheels made of steel of grade 2 by more than 2 times, of steel of grade T by more than 1.5 times.

The endurance limit of the disk of the experimental wheels in comparison with the wheels of steel grade 2 is more than 1.65 times.

In terms of contact fatigue endurance, wheels made of steel of grade B are superior to wheels of steel of grade 2 by 1.5-2 times, of steels of grades T and L, by 25-50%.

Impact strength of the rim and disk of the test wheels significantly exceeds the minimum acceptable level according to GOST 10791-2011 for all steel grades specified in the standard.

The crack resistance (fracture toughness) of the experimental wheels is comparable with the same indicator of wheels made of serial steel grades “T” and “2” (57 and 66 MPa × m ^1/2, respectively), meets the requirements of GOST 10791-2011.

According to the results of fatigue tests, the endurance limit of wheel disks made of grade “B” steel was 1200 kN, which is 1.65 times higher than that for standard wheels made of grade “2” steel according to GOST 10791-2004 with an identical loading pattern (726 kN), which meets the requirements of GOST 10791-2011 for solid-rolled wheels of freight cars, designed for operation with a maximum axial load of 27 tf.

Metallographic studies confirmed the ferrite-pearlite structure of the metal in the disk and the mixed martensitic-bainitic-troostite structure in the rim for the entire working depth 30 mm from the rolling surface.

The obtained test results show that the inventive method of heat treatment of wheels of the proposed economically alloy steel ensure the achievement of the technical result of the invention.

Claims (15)

1. The method of heat treatment of solid-rolled railway wheels made of alloy steel, including heating to austenitic temperature, holding at this temperature, intermittent quenching of the rim with water, subsequent cooling of the wheels and tempering, characterized in that the wheels are made of steel containing the following components, wt. .%: carbon 0.46-0.54 silicon 0.25-0.45 Manganese 0.85-1.2 chrome 0.80-1.10 niobium 0.01-0.05 vanadium 0.08-0.20 molybdenum not more than 0.05 titanium no more than 0.01 phosphorus no more than 0,030 sulfur no more than 0,020 nickel no more than 0.25 copper no more than 0.25 the rest is iron and inevitable impurities, including hydrogen no more than 0.0002, the rim is quenched with water by cooling it from its spraying devices three-way for 290-330 seconds, while the water pressure in the spraying devices is increased from zero to 0.07- 0.09 MPa for 75-85 seconds with further cooling at constant pressure until a martensitic-bainitic-troostite structure is formed in the rim at a depth of the working layer of up to 30 mm, the subsequent cooling of the wheels is carried out in air until ferrite-perlite is formed in the disk and hub second structure, and rental carried out at 500-550 ° C for at least 2 hours. 2. The method according to p. 1, characterized in that the wheels are made of steel containing 0.49-0.52 wt.% Carbon, while the carbon and manganese contents are related by the ratio, wt.%: Carbon +1/4 manganese = 0.75-0.78.

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