A medium-strength steel rail with high yield strength and a production method thereof

Abstract

The present invention belongs to the field of rail production, and specifically relates to a medium-strength rail with high yield strength and a production method thereof. The medium-strength rail with high yield strength described in the present invention has: yield strength ≥ 820MPa, tensile strength of 1200-1300MPa, and elongation ≥ 10%. The medium-strength rail involved in the present invention obtains high yield strength through online heat treatment process design without adding multiple alloy elements, which can effectively reduce the probability of contact fatigue damage in the use of passenger and freight mixed lines, and can effectively improve the service performance and service life of the rail, and improve the safety of train operation. At the same time, the production method of the medium-strength rail with high yield strength provided by this patent is simple and easy to operate, which is conducive to its large-scale promotion and application in passenger and freight mixed lines.

Description

Technical Field

[0001] The invention relates to the field of rail production, in particular to a medium-strength rail with high yield strength and a production method thereof. Background Art

[0002] my country's railways are in a stage of rapid development, with a large increase in passenger-only lines and heavy-load freight lines. However, in order to achieve all-round development in the railway field, existing passenger-freight mixed lines are also undergoing upgrades and renovations, with higher requirements for freight transport volume, passenger safety, and overall operational stability. This trend has led to an increase in surface contact fatigue damage such as rail peeling, which has seriously affected the service performance and service life of the rails, making it difficult to ensure railway transportation efficiency and safety.

[0003] At present, in order to improve the service performance and service life of rails, high-performance heat-treated pearlite rails are mainly used in mixed passenger and freight lines and dedicated freight lines at home and abroad. The tensile strength and hardness of the rails are improved by online or offline heat treatment to improve the wear resistance and contact fatigue resistance. However, the existing medium-strength pearlite heat-treated rails rarely take into account the improvement of the rail's resistance to plastic deformation brought by high yield strength and its effect of fundamentally reducing the occurrence of surface contact fatigue damage. On the other hand, high-strength heat-treated pearlite rails with high yield strength are difficult to take into account the reduced safety caused by the increased wheel wear caused by their excessively high tensile strength and hardness when used in mixed passenger and freight lines.

[0004] Patent CN 106086622A "A heat treatment production method for passenger and freight mixed railway rails and the resulting rails" discloses a passenger and freight mixed railway rail and its production method, the chemical composition of the rail includes by weight percentage: C: 0.71-0.82%, Si: 0.13-0.60%, Mn: 0.65-1.25%, Cr: 0.05-0.25%, P: ≤0.020%, S: ≤0.015%, Al: ≤0.1%, and the balance is Fe and unavoidable impurities. This patent adopts an online heat treatment method to perform accelerated cooling, slow cooling and air cooling on the center of the rail head tread, both sides of the rail head and the center of the rail bottom of the rail in sequence. The starting temperature of the accelerated cooling is 650-950°C, the cooling rate is 2.5-7°C / s, and the final cooling temperature is 400-630°C. The cooling rate of the slow cooling is 0.1-1.5°C / s, and the final cooling temperature is 180-300°C. The aluminum content in the rails described in this patent is relatively high, which can easily cause core damage and fracture of the rails; and the production method of the rails requires accelerated cooling of the rail bottom, which has high requirements on equipment and a complex production method.

[0005] Patent CN 104988405A "A mixed passenger and freight rail and its production method and application" discloses a mixed passenger and freight rail and its preparation method, the chemical composition of the rail includes by weight percentage: C: 0.71-0.78%, Si: 0.30-0.80%, Mn: 0.80-1.1%, Cr: 0.1-0.3%, V: 0.04-0.2%, P: ≤0.020%, S: ≤0.015%, the balance is Fe and Avoidable impurities, the rail adopts an online heat treatment method: the rail after final rolling is quickly cooled, and then the gauge angle is quickly heated and kept warm; the conditions for rapid cooling include: the initial cooling temperature is 800-880℃, the cooling rate is 2-6℃ / s, and the final cooling temperature is 430-470℃; the conditions for rapid heating and insulation include: raising the gauge angle temperature to 700-800℃ at a heating rate of 3.0-4.0℃ / s and keeping warm for 2-4min. The rails described in this patent need to add high-priced alloy elements such as V, which is costly. At the same time, the rail head is heated multiple times during the rail production process, the production method is complicated, and the secondary heating has a significant negative impact on the performance stability of the rail head, which seriously affects the service safety performance of the rail line.

[0006] Patent CN 112301205A "A high yield ratio pearlite rail and its preparation method" discloses a high yield ratio pearlite rail and its manufacturing method. The components of the rail are as follows by weight percentage: C: 0.70-0.85%, Si: 0.2-0.8%, Mn: 0.8-1.1%, Cr: 0.5-0.7%, Cu: 0.01-0.1%, Nb: 0.01-0.05%, P: ≤0.020%, S: ≤0.015%, Al≤0.005%, and the balance is Fe and unavoidable impurities. The rail adopts an online heat treatment method, and the heat treatment is a multi-stage cooling process, and the rail is cooled from 850-950°C to room temperature at different cooling rates. The steel rails described in this patent have a high yield strength, but their tensile strength exceeds 1300MPa, which can easily lead to rapid wear of wheels when used on low axle-load mixed passenger and freight lines, thereby increasing line maintenance costs. At the same time, the steel rails contain a large amount of alloy elements such as Cr, Cu, Nb, etc., and the production cost is high, making it difficult to promote production.

[0007] Among the current patents related to heat-treated pearlite rails and their production methods, although the rails published in most patents have good strength and hardness, the research on the yield strength of the rails is relatively rough. The improvement in yield strength is mainly driven by the improvement in overall tensile properties. It does not take into account the situation that excessive tensile strength in the application of rails in low axle weight lines will lead to reduced service safety. The heat-treated pearlite rails obtained by them still cannot fully meet the performance requirements after the upgrade and reconstruction of passenger and freight mixed transportation lines, and the chemical composition system and production process are relatively complex, making it difficult to achieve large-scale application. Summary of the invention

[0008] In view of the defects and shortcomings of the prior art, the object of the present invention is to provide a medium-strength rail with high yield strength and a production method thereof to meet the standard sound insulation and shock absorption requirements, and has the advantages of faster construction speed, stronger adhesion, lower cost, and more stable sound insulation effect.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] A medium-strength steel rail with high yield strength has the following characteristics: yield strength ≥ 820 MPa, tensile strength of 1200-1300 MPa, and elongation ≥ 10%.

[0011] In one or more embodiments, the rail head microstructure of the medium strength rail is fully pearlite.

[0012] In one or more embodiments, the surface hardness of the rail top surface of the medium strength rail is 350-390 HB, and the hardness of the 10 mm deep section of the rail top surface and the gauge angle is 35.5-41.0 HRC.

[0013] In one or more embodiments, the chemical composition of the rail, by weight percentage, should be: C: 0.65-0.85%, Si: 0.15-0.60%, Mn: 0.50-1.30%, Cr: 0.05-0.20%, P: ≤0.020%, S: ≤0.015%, and the balance is Fe.

[0014] The present invention also provides a production method for medium-strength rails with high yield strength, which comprises in sequence: converter smelting, LF furnace refining, RH vacuum treatment, continuous casting to obtain steel billets, rolling the steel billets, online heat treatment and processing.

[0015] In one or more embodiments, the in-line thermal processing includes:

[0016] a. First stage cooling: The rails to be final rolled are cooled, and the cooling treatment includes accelerated cooling of the rail top surface, the two upper fillets of the rail head, the two side surfaces of the rail head, and the two lower fillets of the rail head until the rail top surface temperature is 630-750℃;

[0017] b. Second stage cooling: the rail cooled in step a is subjected to accelerated cooling treatment at a cooling rate of 3.0-6.0°C / s on the rail top surface, the two upper fillets of the rail head, the two side surfaces of the rail head, and the two lower fillets of the rail head until the rail top surface temperature reaches 400-500°C;

[0018] c. The third stage of cooling: Place the rail after cooling in step c on a cooling bed and air cool it to room temperature.

[0019] In one or more embodiments, the cooling medium used in the online heat treatment is at least one of water mist, compressed air, and a mixture of compressed air and water mist.

[0020] In one or more embodiments, in step a, when the temperature of the top surface of the rail after final rolling is between 800-950° C., cooling care is performed.

[0021] In one or more embodiments, during the cooling process, the temperature is lowered at a cooling rate of 1.0-3.0° C. / s.

[0022] In one or more embodiments, the cooling medium used in the in-line heat treatment is compressed air.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention adopts a method of controlling the chemical composition of the rail and an online heat treatment process, and can obtain a medium-strength rail with high yield strength without adding multiple alloy elements, and its yield strength is ≥820MPa, the tensile strength is 1200-1300MPa, and the elongation is ≥10%. Therefore, the medium-strength rail of the present invention can effectively reduce the probability of contact fatigue damage in the use of passenger and freight mixed transportation lines, can effectively improve the service performance and service life of the rail, and improve the safety of train operation. At the same time, the production method of the medium-strength rail with high yield strength provided by this patent is simple and easy to operate, which is conducive to its large-scale promotion and application in passenger and freight mixed transportation lines. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is further described in detail below in conjunction with specific implementation methods.

[0026] The invention provides a medium-strength steel rail with high yield strength. The medium-strength steel rail has high yield strength, wherein the yield strength is ≥820MPa, the tensile strength is 1200-1300MPa, and the elongation is ≥10%.

[0027] Furthermore, the rail head microstructure of the medium strength rail is full pearlite.

[0028] Furthermore, the surface hardness of the rail top surface of the medium-strength rail is 350-390HB, and the hardness of the 10mm deep section of the rail top surface and the gauge angle is 35.5-41.0HRC.

[0029] Furthermore, for a medium-strength rail with high yield strength, the chemical composition of the rail, measured in weight percentage, should be: C: 0.65-0.85%, Si: 0.15-0.60%, Mn: 0.50-1.30%, Cr: 0.05-0.20%, P: ≤0.020%, S: ≤0.015%, and the balance is Fe and unavoidable impurities.

[0030] As a preferred solution, the chemical composition of the steel rail is, by weight percentage, C: 0.70-0.85%, Si: 0.30-0.60%, Mn: 0.95-1.25%, Cr: 0.05-0.20%, P: ≤0.020%, S: ≤0.015%, and the balance is Fe and unavoidable impurities. The reasons for limiting the contents of the main chemical elements of the steel rail in the present invention are described in detail below.

[0031] C is the most important and cheapest element in pearlite rails that enables the rails to obtain good comprehensive mechanical properties and promote pearlite transformation. When the C content is less than 0.65%, under the production process of the present invention, it is impossible to ensure that the rails have appropriate hardness and the anti-contact fatigue performance of the rails cannot be guaranteed; when the C content is greater than 0.85%, under the production process of the present invention, the carbide ratio of the rails is too high, the tensile strength is excessive, and the strength index is excessive, which reduces the anti-contact fatigue performance of the rails during service and has an adverse effect on the safe use of the rails; therefore, the carbon content in the present invention is limited to 0.65-0.85%.

[0032] The main role of Si in steel is to inhibit the formation of cementite and act as a solid solution strengthening element to increase the hardness of the ferrite matrix and improve the strength and hardness of steel. When the Si content is less than 0.15%, its solid solution content is low, resulting in an insignificant strengthening effect, and abnormal structures such as martensite are likely to appear in the rails; when the Si content is greater than 0.60%, local segregation is likely to occur, which will reduce the toughness and weldability of the steel and have a negative impact on the safe use of the rails. Therefore, the Si content in the present invention is limited to 0.15-0.60%.

[0033] Mn is essential for improving the strength of ferrite and austenite in steel. When the Mn content is less than 0.50%, it is difficult to achieve the effect of increasing the hardness of carbides and thus increasing the strength and hardness of steel; when the Mn content is greater than 1.30%, it will coarsen the grain size, affect the structural changes of rail steel during heat treatment, and significantly reduce the toughness and plasticity of steel; at the same time, Mn has a significant effect on the diffusion of C in steel, and abnormal structures such as bainite or martensite may be produced in the Mn segregation area, which will affect the welding performance of the rail. Therefore, the Mn content in the present invention is limited to 0.50-1.30%.

[0034] Cr, as a carbide-forming element, can form a variety of carbides with carbon in steel; at the same time, Cr can evenly distribute carbides in steel, reduce carbide size, and improve the wear resistance of rails. When the Cr content is less than 0.05%, the hardness and proportion of the formed carbides are low; when the Cr content is greater than 0.20%, the hardenability of the rail is too high, which can easily cause the rail to produce harmful bainite and martensite structures, while reducing the mechanical properties of the rail, and it is impossible to ensure that the rail is a full pearlite structure. Therefore, the Cr content in the present invention is limited to 0.05-0.20%.

[0035] P and S are both impurity elements that cannot be completely removed from the rail. P will be concentrated at the grain boundary of the rail structure, seriously reducing the toughness of the rail; S is easy to form MnS inclusions in steel, which is harmful to the wear resistance and contact fatigue resistance of the rail. Therefore, the P content in the present invention needs to be controlled below 0.020%; the S content needs to be controlled below 0.015%.

[0036] The present invention also provides a production method for the above-mentioned medium-strength rail with high yield strength, which comprises in sequence: converter smelting, LF furnace refining, RH vacuum treatment, continuous casting to obtain steel billets, rolling the steel billets, online heat treatment and processing.

[0037] Furthermore, in the above-mentioned method for producing the medium-strength rail with high yield strength, the online heat treatment comprises:

[0038] a. First stage cooling: When the top surface temperature of the rail is between 800-950℃ after final rolling, the top surface of the rail, the two upper fillets of the rail head, the two side surfaces of the rail head, and the two lower fillets of the rail head are cooled at a cooling rate of 1.0-3.0℃ / s until the top surface temperature is 630-750℃;

[0039] b. Second stage cooling: the rail cooled in step a is subjected to accelerated cooling treatment at a cooling rate of 3.0-6.0°C / s on the rail top surface, the two upper fillets of the rail head, the two side surfaces of the rail head, and the two lower fillets of the rail head until the rail top surface temperature reaches 400-500°C;

[0040] c. The third stage of cooling: Place the rail after cooling in step c on a cooling bed and air cool the rail to room temperature.

[0041] Furthermore, the cooling medium used in the online heat treatment of this patent is at least one of water mist, compressed air, and a mixture of compressed air and water mist.

[0042] The inventors of the present invention have found through extensive research that:

[0043] ① Cooling in the first stage of the online heat treatment process: When the rail head tread temperature is 800-950℃, the rail has not yet started the pearlite phase transformation. In order to obtain a rail with high yield strength and not excessively increase the tensile strength of the rail, a lower cooling rate is required to uniformly reduce the temperature of the rail. To ensure that the rail head temperature drops uniformly, the cooling rate needs to be controlled between 1.0-3.0℃ / s;

[0044] ② For the second stage cooling of the online heat treatment process: When the rail head tread temperature is cooled to 630-750℃ in the first stage, the rail has begun to undergo pearlite phase transformation. Considering the heat transfer from the unaccelerated cooling part of the rail to the rail head, a higher cooling rate is required to obtain a rail with high yield strength. The cooling rate in the second stage needs to be controlled between 3.0-6.0℃ / s;

[0045] ③ For the third stage cooling: After the first two cooling stages, the internal temperature of the rail head is in the range of 400-500℃. At this time, the rail has completed the pearlite phase transformation process, and there is no obvious meaning in continuing to accelerate the cooling. The rail can be air-cooled to room temperature for subsequent processing.

[0046] The complete production process of the production method of the medium-strength rail with high yield strength of the present invention can be: using low-sulfur vanadium-containing molten steel, smelting in a converter or an electric furnace, LF refining, RH or VD vacuum treatment, large square billet protection continuous casting, billet heating in a billet heating furnace, high-pressure water descaling before billet rolling, universal rolling mill rolling, online heat treatment of the rail, room temperature air cooling on a walking cooling bed, horizontal and vertical composite straightening, rail specification inspection, processing line processing, surface inspection and warehousing.

[0047] The present invention will be described in detail below by way of examples, but the scope of the present invention is not limited thereto.

[0048] Examples 1-3 and Comparative Examples 1-3 correspond to steel rails with chemical compositions numbered 1-3 below, and the manufacturing process is the same as above. The specific chemical compositions are shown in Table 1.

[0049] Table 1

[0050] serial number C Si Mn Cr P S 1 0.80 0.50 1.09 0.08 0.011 0.006 2 0.74 0.55 1.02 0.15 0.017 0.012 3 0.78 0.48 0.99 0.11 0.009 0.003

[0051] The balance is Fe and inevitable impurities.

[0052] The heat treatment process parameters of Examples 1-3 and Comparative Examples 1-3 are shown in Table 2. The differences between the smelting process and the rolling process of the Examples and Comparative Examples are negligible.

[0053] Table 2

[0054]

[0055] In the present invention, the yield strength, tensile strength and elongation of the rail are tested according to GB / T 228.1 "Metallic material tensile test Part 1: Room temperature test method", the 10mm section hardness of the rail is tested according to GB / T 230.1 "Metallic material Rockwell hardness test Part 1: Test method", and the rail top surface hardness of the rail is tested according to GB / T 231.1 "Metallic material Brinell hardness test Part 1: Test method". The tensile properties, rail top surface hardness and 10mm section hardness of Examples 1-3 and Comparative Examples 1-3 are shown in Table 3.

[0056] Table 3

[0057]

[0058]

[0059] By comparing the embodiments and comparative examples, it can be seen that in the embodiments of the present invention, under the same chemical composition and smelting process, different online heat treatment methods of the rails after rolling will have a significant impact on the final performance of the rails. The rails obtained by the method described in the present invention have high yield strength, the yield strength is ≥820MPa, the tensile strength is 1200-1300MPa, and the elongation is ≥10%; while the rails in the comparative examples have yield strength, tensile strength or microstructure that do not meet the requirements.

[0060] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention.

Claims

1. A method for producing a medium-strength rail with high yield strength, characterized in that: The chemical composition of the rail should be: C: 0.65-0.85%, Si: 0.15-0.60%, Mn: 0.50-1.30%, Cr: 0.05-0.20%, P: ≤0.020%, S: ≤0.015%, and the balance is Fe. The production method comprises in sequence: converter smelting, LF furnace refining, RH vacuum treatment, continuous casting to obtain steel billets, rolling the steel billets, online heat treatment and processing, wherein the online heat treatment comprises: a. First stage cooling: cooling the rail after final rolling, wherein the cooling process includes accelerating the cooling process of the rail top surface, the two upper fillets of the rail head, the two side surfaces of the rail head, and the two lower fillets of the rail head at a cooling rate of 1.0-3.0°C / s until the rail top surface temperature reaches 630-750°C; b. Second stage cooling: the rail cooled in step a is subjected to accelerated cooling treatment at a cooling rate of 3.0-6.0°C / s on the rail top surface, the two upper fillets of the rail head, the two side surfaces of the rail head, and the two lower fillets of the rail head until the rail top surface temperature reaches 400-500°C; c. Cooling in the third stage: placing the steel rail cooled in step c on a cooling bed, air cooling the steel rail to room temperature to obtain a medium-strength steel rail, wherein the medium-strength steel rail has a yield strength of ≥820 MPa, a tensile strength of 1200-1300 MPa, and an elongation of ≥10%, and a surface hardness of 350-390 HB on the rail top surface, and a hardness of 35.5-41.0 HRC on the rail top surface and at the gauge angle at a depth of 10 mm.

2. The production method according to claim 1, characterized in that The cooling medium used in the online heat treatment is at least one of water mist, compressed air, and a mixture of compressed air and water mist.

3. The production method according to claim 1, characterized in that In step a, when the temperature of the top surface of the rail after final rolling is between 800-950° C., cooling care is performed.

4. The production method according to claim 1, characterized in that The cooling medium used in the online heat treatment is compressed air.

5. The production method according to claim 1, characterized in that: The rail head microstructure of the medium-strength rail is full pearlite.

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