Production method of steel for stable residual austenite frog

Abstract

The present invention discloses a method for producing a stable residual austenite frog steel, comprising the following steps: S1, heating the steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet; S3, heat treating the frog steel obtained after rolling or forging, wherein the initial cooling temperature of the heat treatment is 700-820°C, the cooling rate is 5-15°C / s, and the final cooling temperature is 150-250°C; S4, when the frog steel after heat treatment is cooled to the intermediate cooling temperature, the whole section is heat-insulated, and the heat-insulating treatment time is 240-480h; S5, tempering the cooled frog steel. The method of the present invention can obtain fine, dispersed and stable residual austenite by controlling the morphology and quantity of the residual austenite during the heating, rolling or forging and cooling of the ingot, thereby improving the comprehensive performance of the bainite frog.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a method for producing stable residual ausbainite frog steel. Background Art

[0002] The frog is an important component of the railway track structure. It is a special device for the wheels of locomotives to transfer from one track to another and pass through the intersection of rails. When the locomotive passes through the frog, the frog will be subjected to a huge wheel impact load, and the working conditions are extremely harsh. The frog bears the impact from the wheel load. In addition to the increase in static load, it is also the place on the rail that bears the largest dynamic load. The dynamic load on the rails and turnouts is 2 to 5 times the static load. Therefore, the frog has extremely strict requirements on the performance of the material.

[0003] Existing bainitic rails are all designed with low carbon (carbon content: 0.15-0.25%), supplemented with appropriate amounts of Si, Mn, Cr, Ni, Mo and other elements, to obtain carbide-free bainite or B / M complex phase structure and a small amount of residual austenite under air cooling conditions. Residual austenite refers to a certain amount of austenite that has not been transformed after the transformation of bainite or martensite. Bainitic frogs usually contain 5-30% of residual austenite. The influence of residual austenite on the performance of steel is as follows: ① The hardness of residual austenite is much lower than that of bainite or martensite. Therefore, the bainitic frog decreases with the increase of residual austenite, which has a significant impact on the performance indicators of bainitic frogs; ② Residual austenite has thermal stability and mechanical stability. During tempering or track service wheel-rail contact, residual austenite will transform into martensite structure, which has a profound impact on the tempering process of bainitic frogs and track service; ③ The increase of residual austenite content, after transformation into martensite, usually forms hard particles, and produces core damage inside the product. Large-scale peeling or cracking will occur on the surface of the frog, seriously affecting the fatigue performance and line service performance of the bainite frog; ④ The increase of parasitic friction will improve the toughness of the bainite frog to a certain extent and improve the impact resistance of the bainite frog.

[0004] Therefore, how to control the morphology and quantity of retained austenite has become a technical problem that needs to be solved urgently in the production process of bainitic frog steel. Summary of the invention

[0005] The main purpose of the present invention is to provide a method for producing a stable retained austenite frog steel, by controlling the morphology and amount of retained austenite during heating, rolling or forging and cooling of the ingot, obtaining fine, dispersed and stable retained austenite, and improving the comprehensive performance of the bainite frog.

[0006] In order to solve at least one of the above technical problems, the present invention adopts the following technical solution:

[0007] According to the present invention, a production method of stable retained ausbainite frog steel is provided, comprising the following steps: S1, heating a steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet; S3, heat treating the frog steel obtained after rolling or forging, wherein the initial cooling temperature of the heat treatment is 700-820°C, the cooling rate is 5-15°C / s, and the final cooling temperature is 150-250°C; S4, when the frog steel after the heat treatment is cooled to the final cooling temperature, the entire cross section is subjected to insulation treatment, and the insulation treatment time is 240-480h; S5, tempering the cooled frog steel.

[0008] According to one embodiment of the present invention, the composition of the frog steel is, by mass percentage, 0.24-0.35% C, 1.20-1.8% Si, 1.50-2.50% Mn, 0.002-0.020% P, 0.002-0.020% S, 0.30-1.70% Cr, 0.10-0.60% Mo, 0.002-0.70% Ni, 0.01-0.15% V, 0.001-0.004% Al, and the rest is Fe and unavoidable impurity elements.

[0009] According to an embodiment of the present invention, in step S1, the steel billet is heated at a temperature of 1050-1150° C. and held at a temperature for 200-350 min.

[0010] According to one embodiment of the present invention, in step S2, the final cooling temperature of rolling or forging is 950-1000°C, and the rolling reduction ratio or forging ratio is ≥5.

[0011] According to an embodiment of the present invention, in step S4, during the heat preservation treatment, a guide rail is used to constrain the rail waist.

[0012] According to an embodiment of the present invention, in step S4, the frog steel after the heat preservation treatment is naturally cooled.

[0013] According to an embodiment of the present invention, in step S5, the cooled frog steel is tempered at a temperature of 200-400° C. for 5-120 hours.

[0014] According to an embodiment of the present invention, in step S5, the tempered frog steel is naturally cooled.

[0015] According to an embodiment of the present invention, during the natural cooling process, the spacing between adjacent frog steels is 100 mm.

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

[0017] In the production method of stable residual austenite steel for bainite frogs according to an embodiment of the present invention, by controlling the morphology and amount of residual austenite during heating, rolling or forging and cooling of the ingot, fine, dispersed and stable residual austenite is obtained, thereby improving the comprehensive performance of the bainite frog. The bainite frog produced by this method has a tensile strength of ≥1350MPa, an elongation of ≥12%, and a residual austenite content of ≤12%. After 5 million physical frog fatigue tests, the residual austenite conversion rate is as low as less than 10%. Compared with conventional bainite frogs or bainite rails, the early damage rate in service is reduced by 70-90%, which greatly improves the mechanical and organizational stability of bainite steel and enhances the service stability of bainite frogs. 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 1 A flow chart showing a method for producing a stable retained ausbainite frog steel according to an embodiment of the present invention. DETAILED DESCRIPTION

[0020] 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.

[0021] The effect of retained austenite on strength, the strengthening methods in steel include: solid solution strengthening, substructure dislocation strengthening, grain boundary strengthening, precipitation strengthening, fine grain strengthening and multiphase microstructure strengthening. The strength of bainitic frog steel containing retained austenite mainly comes from the hard phase matrix structure in the steel, mainly including ferrite, bainite and martensite. During the stretching process, the retained austenite in the steel will undergo phase transformation with the increase of deformation to form a hard phase martensite structure, resulting in an increase in tensile strength.

[0022] The influence of retained austenite on plasticity: when the bainite frog is under the action of plastic deformation, the volume of retained austenite increases, strain hardening occurs, martensitic phase transformation occurs, local hardness increases, and further deformation becomes difficult. At the same time, with the continuous development of phase transformation, the material acquires high plasticity. When the retained austenite phase transforms, the dislocation density increases, dislocation strengthening occurs, and the strength also increases.

[0023] The effect of retained austenite on toughness: With the increase of deformation, martensitic phase transformation occurs, which increases the work hardening rate of the necking zone, widens the necking zone and makes the necking zone smooth, thereby improving the toughness of the steel. Retained austenite is a soft phase, which can relax the local concentrated stress through plastic deformation, delay the initiation of microcracks, and hinder the expansion and growth of microcracks. During crack propagation, the retained austenite will turn, absorb more energy, and improve the fracture toughness of the bainite frog.

[0024] Figure 1 A flow chart showing a method for producing a stable retained ausbainite frog steel according to an embodiment of the present invention is shown. The method generally comprises the following steps:

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

[0026] S2, rolling or forging the heated steel billet;

[0027] S3, heat treating the frog steel obtained after rolling or forging, wherein the heat treatment has a cooling start temperature of 700 to 820° C., a cooling rate of 5 to 15° C. / s, and a final cooling temperature of 150 to 250° C.;

[0028] S4, when the heat-treated frog steel is cooled to the final cooling temperature, the entire cross section is subjected to insulation treatment, and the insulation treatment time is 240 to 480 hours;

[0029] S5, tempering the cooled frog steel.

[0030] In the production method of the stable retained ausbainite frog steel according to the embodiment of the present invention, the composition of the frog steel is preferably: 0.24-0.35% C, 1.20-1.8% Si, 1.50-2.50% Mn, 0.002-0.020% P, 0.002-0.020% S, 0.30-1.70% Cr, 0.10-0.60% Mo, 0.002-0.70% Ni, 0.01-0.15% V, 0.001-0.004% Al, and the rest is Fe and unavoidable impurity elements.

[0031] Among the above components, 0.24-0.35% C can significantly reduce the martensitic transformation Ms point and improve the stability of austenite. 1.50-2.50% Mn can not only expand the austenitic phase area, but also reduce the martensitic transformation starting temperature Ms. 1.20-1.8% Si and 0.002-0.020% P can also reduce the martensitic transformation starting temperature Ms, while 0.001-0.004% Al can increase the Ms temperature.

[0032] In some embodiments, the following parameters may be used in steps S1 to S2: After smelting and casting, the steel billet is heated to 1050-1150°C, and the holding time is 200-350 minutes. The steel billet is rolled or forged after dephosphorization, wherein the final cooling temperature of rolling or forging is 950-1000°C, and the rolling compression ratio or forging ratio is ≥5. Small-sized residual austenite is obtained by low-temperature heating of the ingot, rolling with a large compression ratio, and accelerated cooling by heat treatment in step S3. The smaller the austenite grains, the lower the Ms temperature. In addition, during the deformation process, large-sized residual austenite has poor stability, and martensitic phase transformation will occur under low strain. Small-sized residual austenite has strong stability and does not undergo phase transformation under high strain.

[0033] When the temperature of the frog steel is cooled to the final cooling temperature after heat treatment, the entire section is subjected to a long-term heat preservation treatment in step S4. The entire section means that the steel is subjected to heat preservation treatment as a whole. The frog steel is in the shape of an "I" character and is divided into three parts: the rail head, the rail waist and the rail bottom. Traditional processes either heat treat the rail head or the rail head and the rail bottom. The material of the present invention is a composite structure such as bainite + martensite + paratenite, and the full-section heat preservation treatment is used to improve the overall performance of the product, reduce internal stress, etc. In some embodiments, during the heat preservation treatment, a guide rail is used to constrain the rail waist. The frog steel after the heat preservation treatment can be cooled to room temperature by natural cooling, that is, the spacing between adjacent frog steels is 100 mm, and there is no rapid cooling device to accelerate the cooling.

[0034] When the frog steel of the composition of the present invention is used, after rapid cooling, a certain amount of residual austenite will appear in the austenite structure due to the influence of the composition and the cooling rate. The residual austenite structure retained at room temperature is in an unstable state, and will be transformed into a hard martensite structure again during the subsequent heating or deformation process. After rapid cooling, long-term heat preservation treatment is conducive to the transformation of residual austenite from an unstable state to a stable state, or from a residual austenite structure to a bainite structure with better comprehensive performance.

[0035] In step S4, after the cooled frog steel is straightened, it is tempered at 200-400°C for 5-120 hours to eliminate residual stress. The frog steel after tempering can also be cooled to room temperature by natural cooling. Among them, the tempering time is mainly related to the heating medium, furnace temperature, chemical composition of steel, structure, sample size and shape, furnace loading method and furnace loading amount, and the sample size and shape and structure have the most obvious influence on the tempering time. The cross-sectional thickness of frog steel can usually reach 116mm. According to the commonly used empirical formula for tempering (1):

[0036] T=a×K×D (1)

[0037] In the formula, T is the heating time (min); a is the heating coefficient (min / mm), which is related to the workpiece size, heating medium, and chemical composition of the steel; K is the furnace correction coefficient (take 1.5-2.0); D is the effective thickness of the part (mm)

[0038] The minimum calculated tempering time is 300 minutes, i.e. 5 hours.

[0039] At the same time, in the embodiment of the present invention, the structure of the frog steel is a composite structure of bainite + martensite + paratenite, the content of martensite and paratenite in the structure is small, and the transformation during the tempering process is affected by the extrusion of the bainite structure, the transformation rate is extremely low, and the required tempering time is long. Experimental research shows that the longest tempering time is 120h, and the longest time for international bainite material is more than 30 days.

[0040] In some embodiments, protective casting is performed throughout the casting process to control the nitrogen, hydrogen, and oxygen contents of the frog steel.

[0041] The following is a description based on specific embodiments.

[0042] The frog steels of the embodiments and comparative examples of the present invention are cast through full-process protection, wherein the main chemical components of embodiments 1 to 5 are shown in Table 1 in terms of mass percentage, and the components of comparative examples 1 to 2 are the same as those of embodiments 1 to 2.

[0043] Table 1 Chemical composition of frog steel in the embodiments and comparative examples (%)

[0044]

[0045] The embodiment and comparative example adopt the same smelting, billet heating, rolling or forging process. The billet soaking temperature is 1050-1150℃, and the holding time is 200-350min; the final cooling temperature of rolling or forging is 950~1000℃, and the forging ratio or compression ratio is ≥5. After using waste heat or reheating, the cooling temperature range is 700-820℃, the cooling rate is 5-15℃ / s, and the final cooling temperature is 150-250℃. The heat treatment process of the embodiment and comparative example is shown in Table 2.

[0046] Table 2 Heat treatment process of examples and comparative examples

[0047]

[0048] After the bainite frogs of the embodiments and comparative examples are heat-insulated, cooled, and straightened, they are tempered at 200-400°C for 5-120 hours. According to the requirements of TB / T 2344 "Technical Conditions for Ordering 43kg / m~75kg / m Rails", the tensile specimens are taken and tested. At the same time, a fatigue test machine for the rails is used to perform 5 million fatigue tests on the bainite frogs. Before and after the bainite frog test, metallographic samples are taken from the fillet of the rail head, and XRD is used to compare and analyze the changes in the residual austenite before and after the test. The content of residual austenite is ≤12%. After 5 million physical frog fatigue tests, the residual austenite conversion rate is as low as less than 10%. The tensile and metallographic data statistics are shown in Table 3.

[0049] Table 3 Tensile properties of rails in embodiments and comparative examples

[0050]

[0051] Combined with Tables 1 to 3, the relevant parameters of the composition design, heating process, and rolling process of the comparative example are not within the predetermined range. As shown in Table 4, the tensile strength of the bainitic frog produced by this method is ≥1350MPa, the elongation is ≥12%, and the residual austenite content is ≤12%. After 5 million physical frog fatigue tests, the residual austenite conversion rate is as low as less than 10%. Compared with conventional bainitic frogs or bainitic rails, the early service damage rate is reduced by 70-90%, which greatly improves the mechanical and organizational stability of bainitic steel and enhances the service stability of bainitic frogs.

[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 a stable residual ausbainite frog steel, characterized in that: The frog steel has the following components by mass percentage: 0.24-0.35% C, 1.20-1.8% Si, 1.50-2.50% Mn, 0.002-0.020% P, 0.002-0.020% S, 0.30-1.70% Cr, 0.10-0.60% Mo, 0.002-0.70% Ni, 0.01-0.15% V, 0.001-0.004% Al, and the rest is Fe and unavoidable impurity elements. The method comprises the following steps: S1, heating the steel billet obtained after smelting and casting; S2, rolling or forging the heated steel billet; S3, heat treating the frog steel obtained after rolling or forging, wherein the heat treatment has a cooling start temperature of 700-820°C, a cooling rate of 5-15°C / s, and a final cooling temperature of 150-250°C; S4, when the heat-treated frog steel is cooled to the final cooling temperature, the entire cross section is subjected to insulation treatment, and the insulation treatment time is 240 to 480 hours; S5, tempering the cooled frog steel.

2. The method according to claim 1, characterized in that In step S1, the steel billet has an even heating temperature of 1050-1150°C and a holding time of 200-350 minutes.

3. The method according to claim 1, characterized in that In step S2, the final cooling temperature of rolling or forging is 950-1000°C, and the rolling reduction ratio or forging ratio is ≥5.

4. The method according to claim 1, characterized in that: In step S4, during the heat preservation process, the rail waist is constrained by using a guide rail.

5. The method according to claim 4, characterized in that In step S4, the frog steel after the heat preservation treatment is naturally cooled.

6. The method according to claim 1, characterized in that In step S5, the cooled frog steel is tempered at a temperature of 200-400° C. for 5-120 hours.

7. The method according to claim 6, characterized in that In step S5, the tempered frog steel is naturally cooled.

8. The method according to claim 5 or 7, characterized in that: During the natural cooling process, the spacing between adjacent frog steels is 100 mm.

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

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