Production method of steel for Beima complex phase frog

Abstract

The present invention discloses a production method of a Bainite-martensite composite frog steel, wherein the frog steel contains 0.26-0.35% C, 1.0-1.5% Cr and 0.30-0.50% Mo by mass percentage, and the method comprises the following steps: S1, heating a steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet to obtain a frog; S3, cooling the frog obtained after rolling or forging at a cooling rate of 5-15°C / s, wherein the initial cooling temperature is 780-820°C and the final cooling temperature is 100-250°C; S4, tempering the cooled frog. The present invention can produce frog steel with a bainite / martensite composite structure and improve the comprehensive performance of the frog steel.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a production method of steel for a Bema complex phase frog. Background Art

[0002] During the service, the frog is subjected to complex alternating loads from the wheels, and the working conditions of the frog are very harsh. The frog is the most severely stressed and damaged component in the railway line.

[0003] The traditional high manganese steel frog cast structure is a mixed structure of austenite, pearlite and carbide. In order to obtain a single-phase austenite structure, high manganese steel is usually treated with water toughening. The high manganese steel frog is heated to the austenitizing temperature, kept warm for a certain period of time, and then quickly cooled to room temperature. After water toughening, the strength and plastic toughness of the high manganese steel frog are significantly improved. The high manganese steel frog has good processability. Under the interaction of impact load and contact stress, the dislocation density increases, the dislocations are crossed and piled up, and the solute atoms interact with the dislocations, so that the high manganese steel frog is strengthened. After strengthening, the hardness of the high manganese steel frog increases sharply, and high-density dislocations and twins are generated on the surface, which improves the wear resistance of the surface and maintains good toughness in the core. The high manganese steel frog must be fully hardened under huge impact loads or pressure. The method of explosive hardening is usually used to increase the hardness of the surface of the high manganese steel frog. After explosion hardening, the service life of the high manganese steel frog is significantly improved. However, there are certain defects such as shrinkage cavities and looseness after casting, and cracks or collapses will appear on the surface, resulting in reduced wear resistance of high manganese steel frogs.

[0004] Therefore, high manganese steel frogs are difficult to meet the increasingly improved requirements for frog performance, and it is necessary to study a frog steel with better comprehensive performance. Summary of the invention

[0005] The main purpose of the present invention is to provide a production method of Bema composite frog steel to achieve good comprehensive mechanical properties of frog steel.

[0006] According to one aspect of the present invention, a method for producing a steel for a Bema complex phase frog is provided, wherein the steel for the frog contains 0.26-0.35% C, 1.0-1.5% Cr and 0.30-0.50% Mo by mass percentage, and the method comprises the following steps:

[0007] S1, heating the steel billet obtained after smelting and casting;

[0008] S2, rolling or forging the heated steel billet to obtain a frog;

[0009] S3, cooling the frog obtained after rolling or forging at a cooling rate of 5 to 15°C / s, wherein the initial cooling temperature is 780 to 820°C, and the final cooling temperature is 100 to 250°C;

[0010] S4, tempering the cooled frog.

[0011] According to one embodiment of the present invention, the composition of the frog steel is, by mass percentage, 0.26-0.35% C, 0.70-2.1% Si, 1.60-2.50% Mn, 0.002-0.020% P, 0.002-0.020% S, 1.0-1.50% Cr, 0.30-0.50% Mo, 0.002-0.70% Ni, 0.02-0.15% V, 0.001-0.004% Al, 0.002-0.006% B, and the rest is Fe and unavoidable impurity elements.

[0012] According to an embodiment of the present invention, in step S1, heating is performed at 1250-1300° C. for a heating time of 200-500 min.

[0013] According to one embodiment of the present invention, in step S4, the tempering temperature is 200-350° C., and the tempering time is 48-96 hours.

[0014] According to an embodiment of the present invention, the tensile strength of the frog steel is greater than or equal to 1450 MPa, and the elongation is greater than or equal to 12%.

[0015] According to an embodiment of the present invention, the metallographic structure of the frog steel contains 60-80% bainite and 5-20% martensite.

[0016] According to one embodiment of the present invention, protective casting is performed throughout the entire casting process.

[0017] In the production method of the Bema composite frog steel according to the embodiment of the present invention, the microstructure is controlled by controlling the composition and cooling rate of the frog steel, which is beneficial to producing the Bema composite frog steel with good comprehensive mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1A flow chart showing a method for producing steel for a Bema composite frog according to an embodiment of the present invention;

[0020] Figure 2 Show the use Figure 1 Metallographic diagram of steel for Bema composite frog produced by the method. DETAILED DESCRIPTION

[0021] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0022] Bainitic frog steel refers to a steel type that is heated to austenitization and then air-cooled or heat-treated to form a bainitic structure. Bainitic frog steel has high strength and high toughness. Bainitic frog steel is optimized mainly in two ways: one is alloying, which is based on air-cooled bainitic steel and supplemented by adding alloying elements such as Si, Cr, and Ni to obtain bainitic structure under air-cooling conditions; the other is heat treatment, which is to accelerate the cooling of bainite after austenitization, and achieve the purpose of bainitic frog steel structure and performance by isothermal treatment, air cooling, controlled cooling, etc.

[0023] The inventor of the present application realizes that, under the effect of microalloying, the steel can be heat treated to postpone the high temperature transformation, expand the medium temperature transformation, and significantly reduce the starting transformation temperature of the bainite structure; after subsequent tempering treatment, a bainite / martensite complex steel with good comprehensive mechanical properties can be obtained; the steel treated by heat treatment process with high alloy composition design contains multiphase structures such as bainite, martensite and retained austenite. On this basis, the present application proposes a production method for bainite-martensite complex frog steel as described below.

[0024] Figure 1 A flow chart showing a method for producing a Bema composite frog steel according to an embodiment of the present invention, wherein the frog steel comprises 0.26-0.35% C, 1.0-1.5% Cr and 0.30-0.50% Mo by mass percentage, such as Figure 1 As shown, the method comprises the following steps:

[0025] S1, heating the steel billet obtained after smelting and casting;

[0026] S2, rolling or forging the heated steel billet to obtain a frog; wherein the steel billet may be dephosphorized before rolling or forging;

[0027] S3, cooling the frog obtained after rolling or forging at a cooling rate of 5-15°C / s, wherein the start cooling temperature is 780-820°C and the final cooling temperature is 100-250°C; the heat treatment in step S3 can be performed using the residual heat of rolling or forging, that is, using the residual heat to start cooling from the start cooling temperature without first cooling and then heating to the start cooling temperature;

[0028] S4, tempering the cooled frog; wherein the cooled frog may be straightened first and then tempered.

[0029] The present invention is mainly aimed at medium carbon bainitic alloy steel (carbon content ≥ 0.25%), and by increasing the C content and Cr content, the hardenability is significantly improved, the martensite content in the structure is increased, and the strength is further improved. In the production method of the Bema complex phase frog steel according to the embodiment of the present invention, a high carbon and high hardenability alloy component design such as 0.26-0.35% C, 1.0-1.5% Cr and 0.30-0.50% Mo is adopted, and a high cooling rate design is adopted during the heat treatment process of step S3, so as to facilitate the acquisition of the Bema complex phase structure, so that the frog steel has both the high toughness of bainite and the high hardness of martensite, and is suitable as a frog material for heavy-duty railways. Among them, the final cooling temperature is set at 100-250°C, which is conducive to ensuring the precipitation of martensite.

[0030] Heating the steel billet in step S1 is beneficial to improving the strength of the steel. In some embodiments, the steel billet may be heated at 1250-1300° C. for 200-500 min.

[0031] In step S3, a cooling rate of 5 to 15°C / s is used throughout the cooling process, that is, a faster cooling rate is used before and after the phase change, so that the organizational ratio between bainite (toughness organization) and martensite (strength index) can be well regulated to obtain better product performance. The cooling process can be cooled by a high-speed water flow at a temperature of 15 to 25°C, and the water pressure is 1 to 2MPa. The start cooling temperature in step S3 is 780 to 820°C. The start cooling temperature is relatively high, so that the hardenability of the product is relatively improved, and the martensite content is relatively high, so that the strength of the frog is relatively high and the toughness is relatively low.

[0032] Tempering in step S4 is beneficial to ensure the toughness of the steel. In some embodiments, the tempering temperature is 200-350° C. and the tempering time is 48-96 hours.

[0033] In some embodiments, the entire casting process is performed by protective casting, and low sulfur control is adopted to control the sulfur content in the steel to 0.002-0.010 wt%.

[0034] In some embodiments, the composition of the frog steel is, by mass percentage, 0.26-0.35% C, 0.70-2.1% Si, 1.60-2.50% Mn, 0.002-0.020% P, 0.002-0.020% S, 1.0-1.50% Cr, 0.30-0.50% Mo, 0.002-0.70% Ni, 0.02-0.15% V, 0.001-0.004% Al, 0.002-0.006% B, and the rest is Fe and unavoidable impurity elements.

[0035] In an embodiment of the present invention, the tensile strength of the frog steel produced according to the above method is greater than or equal to 1450 MPa, and the elongation is greater than or equal to 12%. The metallographic structure of the frog steel comprises bainite, martensite and residual austenite, wherein the content of bainite is 60-80%, and the content of martensite is 5-20%.

[0036] The following is a description based on specific embodiments.

[0037] The frogs of the embodiments of the present invention and the comparative examples are cast with full protection, preferably with 0.002-0.010% low sulfur control, and their chemical compositions are shown in Table 1.

[0038] Table 1 Chemical composition of frogs in embodiments and comparative examples / %

[0039]

[0040]

[0041] The embodiment and the comparative example adopt the same heating process, the heating temperature is 1250-1300°C, and the heating time is controlled at 200-500min. Different heat treatment processes are adopted, as shown in Table 2.

[0042] Table 2 Heat treatment process of examples and comparative examples

[0043]

[0044]

[0045] The frogs of the embodiment and the comparative example adopt the same tempering process. After being cooled and straightened, the frogs are tempered at a temperature of 200 to 350° C. for a tempering time of 48 to 96 hours.

[0046] The embodiments and comparative examples are carried out according to the requirements of TB / T 2344 "Technical Conditions for Ordering 43kg / m to 75kg / m Rails". The tensile specimens are processed and inspected according to the tensile specimen sampling positions, and the metallographic structure is inspected according to the metallographic specimen inspection positions. The metallographic photos are shown in FIG. Figure 2The tensile test data and metallographic data statistics are shown in Table 3.

[0047] Table 3 Tensile properties of rails in embodiments and comparative examples

[0048]

[0049]

[0050] Combined with Tables 1 to 3, the elongation of the comparative examples is smaller than that of the embodiments; the C content, Cr content, and Mo content of Comparative Examples 1, 3, and 5 are relatively low, resulting in a relatively low strength of the obtained steel and a martensite content of 0 in the microstructure; the too small or too large cooling rate in Comparative Examples 9 and 10 also affects the strength or elongation of the steel and the content of bainite and martensite to varying degrees.

[0051] In general, the production method of the present invention improves the toughness of the bainite frog and increases the depth of the hardened layer, thereby improving the overall performance of the rail and the safety of rail operation.

[0052] A person skilled in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples; under the concept of the embodiments of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and there are many other changes in different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of simplicity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included in the protection scope of the embodiments of the present invention.

Claims

1. A method for producing steel for Bema composite frog, characterized in that: The following steps are involved: S1, heating the steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet to obtain a frog; S3, cooling the frog obtained after rolling or forging at a cooling rate of 5 to 15°C / s, wherein the initial cooling temperature is 780 to 820°C, and the final cooling temperature is 100 to 250°C; S4, tempering the cooled frog; In step S3, cooling is performed from the start cooling temperature using the residual heat from rolling or forging; The composition of the frog steel is as follows, by mass percentage: 0.32-0.35% C, 1.50-2.1% Si, 1.60-2.50% Mn, 0.002-0.020% P, 0.002-0.020% S, 1.0-1.50% Cr, 0.35-0.50% Mo, 0.002-0.70% Ni, 0.07-0.15% V, 0.001-0.004% Al, 0.005-0.006% B, and the rest is Fe and unavoidable impurity elements; In step S4, the tempering temperature is 200-350° C., and the tempering time is 48-96 hours.

2. The method according to claim 1, characterized in that In step S1, heating is performed at 1250-1300°C for 200-500 minutes.

3. The method according to claim 1, characterized in that The tensile strength of the frog steel is greater than or equal to 1450 MPa, and the elongation is greater than or equal to 12%.

4. The method according to claim 1, characterized in that: The metallographic structure of the frog steel has a bainite content of 60-80% and a martensite content of 5-20%.

5. The method according to claim 1, characterized in that Protective casting is carried out throughout the casting process.

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