Production method of steel for high-toughness frog

Abstract

The present invention discloses a production method of steel for high-toughness frog. The steel for frog contains 0.16 - 0.21% of C and 0.50 - 0.80% of Ni by mass percentage. The method comprises the following steps: S1, smelting, casting, and rolling or forging to obtain the frog; S2, applying a coating layer at the web of the frog, and the coating layer contains kaolin and vanadium-containing tailings with a particle size less than 0.5 mm; S3, heating the frog after applying the coating layer, and quickly putting the heated frog into a salt bath furnace for isothermal treatment. The present invention can improve the toughness of the steel for frog.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgy, and particularly relates to a production method of steel for high-toughness frog. Background Art

[0002] A railway frog is a track plane crossing device that enables a train wheel to switch from one track to another. With the development of railways towards heavy haul and high speed, higher requirements are put forward for frogs. There is an urgent need to develop a new type of frog material with better comprehensive performance than high manganese steel to meet the requirements of high speed, heavy haul and continuous welded rail across sections. Bainitic steel treatment has excellent anti-contact fatigue and wear resistance, especially excellent welding process performance, making it an ideal material for making frogs for heavy-haul high-speed railways.

[0003] Good strength-ductility matching is extremely important for the service safety of bainitic frog materials, especially for frogs used in heavy-haul railways in alpine and high-altitude areas, where strength and ductility are particularly important. Therefore, it is necessary to study a scheme to improve the toughness of steel for frogs. Summary of the Invention

[0004] The main object of the present invention is to provide a production method of steel for high-toughness frog to improve the toughness of the frog.

[0005] According to one aspect of the present invention, a production method of steel for high-toughness frog is provided. The steel for frog contains 0.16 - 0.21% of C and 0.50 - 0.80% of Ni by mass percentage. The method includes the following steps:

[0006] S1, performing smelting, casting and rolling or forging to obtain a frog;

[0007] S2, applying a coating layer at the web of the frog. The coating layer contains kaolin and vanadium-containing tailings with a particle size less than 0.5 mm;

[0008] S3, heating the frog after applying the coating layer, and quickly putting the heated frog into a salt bath furnace for isothermal treatment.

[0009] According to an embodiment of the present invention, by mass percentage, the composition of the steel for frog is: 0.16 - 0.21% of C, 0.70 - 1.0% of Si, 1.00 - 1.50% of Mn, 0.002 - 0.010% of P, 0.002 - 0.010% of S, 0.30 - 0.90% of Cr, 0.30 - 0.60% of Mo, 0.50 - 0.80% of Ni, 0.004 - 0.04% of V, 0.001 - 0.004% of Al, and the rest is Fe and inevitable impurity elements.

[0010] According to an embodiment of the present invention, in step S2, the coating layer contains 60-80% kaolin and 10-20% vanadium-containing tailings by mass percentage, and the thickness of the coating layer is 8-15 mm.

[0011] According to an embodiment of the present invention, in step S3, it is heated to 900-950 °C for heat preservation at a heating rate of 5-15 °C / min, and the heat preservation time is 110-180 min.

[0012] According to an embodiment of the present invention, in step S3, the temperature of the frog leaving the heating furnace is 890-940 °C, and the temperature of the frog entering the salt bath furnace is 850-900 °C.

[0013] According to an embodiment of the present invention, in step S3, the isothermal temperature is 240-280 °C, and the isothermal time is 70-100 min.

[0014] According to an embodiment of the present invention, in step S3, a mixed nitrate is used for salt bath, and the mixed nitrate contains 55% KNO 3 and 45% NaNO 3 .

[0015] According to an embodiment of the present invention, in step S3, the volume of the salt liquid in the salt bath furnace: the volume of the frog ≥ 10:1.

[0016] According to an embodiment of the present invention, in step S3, circulating water is used to cool the furnace wall of the salt bath furnace, and the frog is cooled by swinging.

[0017] According to an embodiment of the present invention, the tensile strength of the steel used for the frog is greater than or equal to 1350 MPa, the cross-sectional shrinkage rate is greater than or equal to 45%, the normal temperature impact is greater than or equal to 70 J, the impact at -40 °C is greater than or equal to 40 J, and the low-temperature fracture toughness at -20 °C is greater than or equal to 42 MPa·m 1 / 2 , and the structure is carbide-free bainite structure including bainite ferrite, martensite and retained austenite.

[0018] In the production method of the high-toughness frog steel according to the embodiment of the present invention, by adopting the composition design of 0.16-0.21% C and 0.50-0.80% Ni, it is beneficial to improve the toughness of the frog; by performing isothermal heat treatment on the frog to decompose austenite into the required structure, the toughness of the frog can be further improved; by applying a coating layer containing kaolin and vanadium-containing tailings with a particle size less than 0.5 mm at the web of the frog, it is possible to avoid the generation of hard and brittle martensite structure and internal stress in the structure due to too fast cooling rate at the web, and ensure the high toughness of the whole frog. Brief Description of the Drawings

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0020] Figure 1 The flowchart showing the production method of the steel for high-toughness frog according to the embodiment of the present invention. Detailed implementation manners

[0021] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the following will further elaborate on the embodiments of the present invention in detail with reference to specific embodiments and the accompanying drawings.

[0022] The inventors of the present application have recognized that: toughness represents the ability of a metallic material to resist deformation and fracture, and the level of toughness is determined by the ease or difficulty of crack formation and propagation during the fracture process. Improving the toughness of bainite frog materials can be achieved by reducing stress concentration, improving the uniformity of plastic deformation, regulating the morphology and distribution state of the microstructure, preventing crack propagation along grain boundaries, etc. Specific methods include: refining grains, regulating the microstructure, and adjusting chemical composition.

[0023] (1) Refining grains

[0024] Refining grains can improve the toughness of metallic materials while significantly increasing the strength of the materials. For bainite frog materials, refining grains not only refers to the refinement of the original austenite grain size, but also the refinement of the microstructure such as bainite ferrite, lath bundles and lath blocks of martensite, and martensite-austenite islands is also beneficial to the improvement of toughness.

[0025] Refining grains increases the number of grains per unit volume, and the stress can be shared by more grains. Moreover, it can reduce the strain difference between the inside of the grain and near the grain boundary, increase the uniformity of plastic deformation, and significantly reduce the probability of microcrack formation due to stress concentration. Refining grains can also increase the area of grain boundaries. The disordered atoms on the grain boundaries can generate greater resistance to crack propagation; and affected by the grain orientations on both sides of the grain boundary, the crack propagation path becomes more tortuous, slowing down the crack propagation speed.

[0026] (2) Regulating the microstructure

[0027] The bainite frog material is mainly composed of a bainite and martensite duplex microstructure. The existence of the duplex microstructure increases the phase interface, making crack propagation more difficult. Moreover, the morphology and distribution of martensite-austenite islands and retained austenite in the microstructure can also affect the toughness of the material. Therefore, a deep understanding and regulation of the microstructure are particularly important for improving the toughness of bainite frog materials.

[0028] (3) Adjust the chemical composition

[0029] In order to improve toughness, the C content should not be too high. Additionally, toughness can be improved by adding alloying elements such as Ni.

[0030] Based on the above understanding, the present invention proposes the production method of the steel for high-toughness frog described below. Figure 1 The flowchart showing the production method of the steel for high-toughness frog according to an embodiment of the present invention, the steel for frog contains 0.16 - 0.21% C and 0.50 - 0.80% Ni by mass percentage, as Figure 1 shown, the method includes the following steps:

[0031] S1, perform smelting, casting, and rolling or forging to obtain a frog;

[0032] S2, apply a coating layer at the web of the frog, the coating layer contains kaolin and vanadium-bearing tailings with a particle size less than 0.5 mm;

[0033] S3, heat the frog after applying the coating layer (for example, heat in a gas furnace or a box-type resistance furnace), and quickly put the heated frog into a salt bath furnace for isothermal treatment. Then, take the frog out of the salt bath furnace and cool it naturally to room temperature.

[0034] In the production method of the steel for high-toughness frog according to an embodiment of the present invention, by adopting the composition design of 0.16 - 0.21% C and 0.50 - 0.80% Ni, it is beneficial to improve the toughness of the frog; by performing isothermal heat treatment on the frog, the austenite decomposes into the required structure, which can further enhance the toughness of the frog; by applying a coating layer containing kaolin and vanadium-bearing tailings with a particle size less than 0.5 mm at the web of the frog, it is possible to avoid the generation of hard and brittle martensite structure and internal stress in the structure due to the too fast cooling rate at the web, ensuring the high toughness of the whole frog.

[0035] The inventor realized that: The steel for bainite frog is rolled or forged into a profiled rail, and the outer shape presents an "I" shape. Different from the online heat treatment of the rail, which only accelerates the cooling of the rail head and rail bottom, due to the large cross-sectional size of the bainite frog, it can only be accelerated by putting the whole into the quenching liquid. Compared with the rail head and rail bottom, the thickness of the web part is smaller, and the cooling rate of the web will be accelerated due to the reduction of thickness during the accelerated cooling process, resulting in both hard and brittle martensite structure and internal stress in the structure. Therefore, when the bainite frog is put into the quenching liquid as a whole, only high-temperature refractory materials can be coated on the web to balance the cooling of the whole cross-section.

[0036] In an embodiment of the present invention, a coating layer containing kaolin and vanadium-containing tailings with a particle size less than 0.5 mm is adopted. Kaolin is a non-metallic mineral, a kind of clay and clay rock mainly composed of kaolinite group clay minerals. It is white and delicate, also known as dolomite soil. Pure kaolin is white, delicate, and soft, with good plasticity and refractoriness and other physical and chemical properties. Its mineral composition mainly consists of minerals such as kaolinite, halloysite, hydromica, illite, montmorillonite, quartz, and feldspar. During the high-temperature firing process of pure kaolin, pores and deformation will occur inside due to water evaporation. Adding vanadium-containing tailings with a particle size less than 0.5 mm to kaolin can effectively ensure pores and deformation during the sintering process.

[0037] In an embodiment of the present invention, isothermal heat treatment is carried out in step S3. Isothermal heat treatment is a heat treatment process in which the workpiece is heated to the quenching temperature in a heat treatment furnace, and after sufficient heating and holding, it is quickly put into a nitrate furnace for isothermal quenching and held for a certain time to decompose austenite into other tissues. According to different process requirements, the salt bath temperature can be adjusted, and isothermal quenching such as martensite, bainite, and sorbite can be carried out in the nitrate furnace. The heat transfer mode of the workpiece in the furnace is radiative heat transfer, and the heating speed is slower than that in a general electric furnace. It takes a longer time for the workpiece to be thermally penetrated, and the generated thermal stress is smaller, so the deformation of the workpiece is small. During the isothermal quenching process of the workpiece, since the nitrate has been deoxidized and vacuum degassed, and at the same time the salt bath temperature is strictly controlled below the nitrate decomposition temperature, when the workpiece completes the process of decomposing from austenite tissue into other tissues, surface oxidation is avoided. Therefore, the surface of the workpiece after isothermal heat treatment can maintain a metallic luster.

[0038] In some embodiments, by mass percentage, the composition of the frog steel is: 0.16 - 0.21% C, 0.70 - 1.0% Si, 1.00 - 1.50% Mn, 0.002 - 0.010% P, 0.002 - 0.010% S, 0.30 - 0.90% Cr, 0.30 - 0.60% Mo, 0.50 - 0.80% Ni, 0.004 - 0.04% V, 0.001 - 0.004% Al, and the rest is Fe and inevitable impurity elements.

[0039] In some embodiments, in step S2, the coating layer contains 60 - 80% soft kaolin and 10 - 20% vanadium-containing tailings by mass percentage, and the thickness of the coating layer is 8 - 15 mm.

[0040] In some embodiments, in step S3, it is heated to 900 - 950°C for holding at a heating rate of 5 - 15°C / min, and the holding time is 110 - 180 min. The heating temperature cannot be too high to prevent the austenite grain size from being too large.

[0041] In some embodiments, in step S3, the temperature of the frog leaving the heating furnace is 890 - 940 °C, and the temperature of the frog entering the salt bath furnace is 850 - 900 °C.

[0042] In some embodiments, in step S3, the isothermal temperature is 240 - 280 °C, and the isothermal time is 70 - 100 min. In this embodiment, in step S3, a mixed nitrate salt is used for salt bath, and the mixed nitrate salt contains 55% KNO 3 and 45% NaNO 3 by mass percentage. The melting point of the mixed nitrate salt with this composition and proportion is 218 °C, and the use temperature is 230 - 550 °C, which can be used in the isothermal temperature range of 240 - 280 °C. The isothermal temperature is related to the melting point of the mixed nitrate salt, and it is necessary to ensure that the isothermal temperature is higher than the melting point of the mixed nitrate salt, while the melting point of the mixed nitrate salt is related to its formula. Therefore, it is necessary to design the isothermal temperature and the formula of the mixed nitrate salt comprehensively.

[0043] In some embodiments, in step S3, the volume ratio of the salt liquid in the salt bath furnace to the volume of the frog is ≥ 10:1, ensuring that the salt liquid can cool the frog sufficiently.

[0044] In some embodiments, in step S3, circulating water is used to cool the furnace wall of the salt bath furnace, and the frog is cooled by swinging.

[0045] In some embodiments, the smelting process adopts low sulfur control of 0.002 - 0.010%, and protective casting is carried out throughout the casting process. In some embodiments, the hydrogen content of the finished frog is ≤ 1.2 ppm; the nitrogen content of the finished frog is ≤ 60 ppm.

[0046] According to the above description, the present invention is improved in terms of composition design, isothermal heat treatment process, and applying a coating layer at the web of the frog, etc., so as to obtain a carbide-free bainite structure composed of bainite ferrite, martensite, retained austenite, etc., refine the structure, and improve the strength and toughness of the frog. In the embodiments of the present invention, the tensile strength of the frog steel produced according to the above method is greater than or equal to 1350 MPa, the reduction of area is greater than or equal to 45%, the impact at room temperature is greater than or equal to 70 J, the impact at - 40 °C is greater than or equal to 40 J, and the low temperature fracture toughness at - 20 °C is greater than or equal to 42 MPa·m 1 / 2 , and the structure is a carbide-free bainite structure including bainite ferrite, martensite, and retained austenite, which is particularly suitable as the material for frogs used in heavy-haul railways in alpine and high-altitude areas.

[0047] The following is described according to specific embodiments.

[0048] The frogs in the embodiments and comparative examples of the present invention are subjected to full-process protective casting, and low sulfur control of 0.002 - 0.010% is preferably adopted, and their chemical compositions are shown in Table 1.

[0049] Table 1 Chemical composition of frog for examples and comparative examples / %

[0050]

[0051] The same rolling or forging process was adopted for the examples and comparative examples, and different web coating processes were adopted, as shown in Table 2.

[0052] Table 2 Web coating process of frog for examples and comparative examples

[0053]

[0054] The finished frogs of the examples and comparative examples were sent into a gas furnace or a box-type resistance furnace and heated to 900 - 950°C at a rate of 5 - 15°C / min, and the holding time was 110 - 180 min. Then, different heat treatment processes were adopted, as shown in Table 3.

[0055] Table 3 Heat treatment process of steel for bainite frog of examples and comparative examples

[0056]

[0057]

[0058] The examples and comparative examples processed tensile specimens according to the sampling positions of tensile specimens required by TB / T 2344 "Ordering Technical Conditions for Rails of 43 kg / m - 75 kg / m" and inspected them. At the same time, metallographic structure inspection was carried out according to the inspection positions of metallographic specimens. The statistics of tensile and metallographic data are shown in Table 4.

[0059] Table 4 Tensile properties and metallographic structure of frog for examples and comparative examples

[0060]

[0061]

[0062] The examples and comparative examples processed tensile specimens according to the sampling positions of impact specimens and inspected them. At the same time, inspection was carried out according to the inspection positions of fracture toughness specimens. The statistics of impact and fracture toughness data are shown in Table 5.

[0063] Table 5 Impact and -20°C low-temperature fracture toughness data of examples and comparative examples

[0064]

[0065] Combined with Tables 1 to 5, the relevant parameters of the component design, web coating process, and isothermal heat treatment process of the comparative examples are not within the preset range, resulting in the tensile properties of the produced frog being worse than those of the examples to varying degrees. Generally speaking, adopting the production method of the present invention improves the strength of the bainite frog, and at the same time improves the toughness, and the running safety of the frog is improved.

[0066] Those of ordinary skill in the art should understand that: the discussion of any above embodiment is only exemplary, and is not intended to imply that the scope (including the claims) disclosed by the embodiments of the present invention is limited to these examples; under the idea 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 variations in different aspects of the embodiments of the present invention as described above, and they are not provided in detail for the sake of brevity. Therefore, any omission, modification, equivalent replacement, improvement, etc. made within the spirit and principle of the embodiments of the present invention shall be included in the protection scope of the embodiments of the present invention.

Claims

1. A production method of steel for high-toughness frog, characterized in that, the steel for frog contains 0.16 - 0.21% of C and 0.50 - 0.80% of Ni by mass percentage, and the method comprises the following steps: S1, performing smelting, casting and rolling or forging to obtain a frog; S2, applying a coating layer at the web of the frog, and the coating layer contains kaolin and vanadium-bearing tailings with a particle size less than 0.5 mm; S3, heating the frog after applying the coating layer, and quickly putting the heated frog into a salt bath furnace for isothermal treatment; In step S2, the coating layer contains 60 - 80% of kaolin and 10 - 20% of vanadium-bearing tailings by mass percentage, and the thickness of the coating layer is 8 - 15 mm.

2. The method according to claim 1, characterized in that, by mass percentage, the composition of the steel for frog is: 0.16 - 0.21% of C, 0.70 - 1.0% of Si, 1.00 - 1.50% of Mn, 0.002 - 0.010% of P, 0.002 - 0.010% of S, 0.30 - 0.90% of Cr, 0.30 - 0.60% of Mo, 0.50 - 0.80% of Ni, 0.004 - 0.04% of V, 0.001 - 0.004% of Al, and the rest is Fe and inevitable impurity elements.

3. The method according to claim 1, characterized in that, in step S3, heating at a heating rate of 5 - 15 °C / min to 900 - 950 °C for heat preservation, and the heat preservation time is 110 - 180 min.

4. The method according to claim 1, characterized in that, in step S3, the temperature of the frog leaving the heating furnace is 890 - 940 °C, and the temperature of the frog entering the salt bath furnace is 850 - 900 °C.

5. The method according to claim 1, characterized in that, in step S3, the isothermal temperature is 240 - 280 °C, and the isothermal time is 70 - 100 min.

6. The method according to claim 5, characterized in that, In step S3, a salt bath is carried out using a mixed nitrate salt, and the mixed nitrate salt contains 55% of KNO by mass percentage 3 and 45% of NaNO 3 .

7. The method according to claim 1, characterized in that, in step S3, the volume of the salt solution in the salt bath furnace: the volume of the frog ≥ 10:

1.

8. The method according to claim 1, characterized in that, in step S3, using circulating water to cool the furnace wall of the salt bath furnace, and the frog is cooled by swinging.

9. The method according to claim 2, characterized in that, The steel for frog has a tensile strength of not less than 1350 MPa, an area reduction of not less than 45%, a Charpy impact energy at room temperature of not less than 70 J, a Charpy impact energy at -40°C of not less than 40 J, and a low-temperature fracture toughness at -20°C of not less than 42 MPa·m 1 / 2 , and the microstructure is carbide-free bainite structure including bainitic ferrite, martensite and retained austenite.

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