Medium-strength contact-fatigue-resistant bainite steel rail and preparation method thereof

Abstract

The invention discloses a moderate-strength contact-fatigue-resistant bainite steel rail and a preparation method thereof, and belongs to the technical field of steel rail preparation. The steel rail comprises the following chemical components in percentage by weight: 0.22%-0.26% of C, 1.00%-1.20% of Si, 1.25%-1.45% of Mn, 0.25%-0.6% of Cr, 0.2%-0.4% of Mo, 0.025%-0.040% of Nb, less than or equal to 0.015% of P, less than or equal to 0.010% of S and the balance of Fe and inevitable impurities. The moderate-strength contact fatigue-resistant bainite steel rail for the freight railway curve line is prepared by combining low-cost alloy component design with a controlled cooling process, so that the rolling contact fatigue resistance of the steel rail is improved while the strength and the toughness of the steel rail are ensured; and in the using process, abrasion and rolling contact fatigue damage can be effectively controlled, the service life of the steel rail is remarkably prolonged, and the grinding and maintaining cost of the steel rail is reduced.

Description

Technical Field

[0001] This invention belongs to the field of rail manufacturing technology, specifically relating to a medium-strength, contact fatigue-resistant bainitic rail for use on curved lines of freight railways and its manufacturing method. Background Technology

[0002] With rapid economic development, the axle load and operating density of freight trains are continuously increasing. Traditional pearlitic rails, which improve strength and hardness by refining the spacing between pearlitic lamellars, can reduce wear and extend service life. However, their high strength characteristics easily lead to wheel-rail profile mismatch, causing contact stress concentration and accelerating contact fatigue damage such as spalling on the rail surface, while also exacerbating wheel wear. Against this backdrop, carbide-free bainitic rails, which combine high impact toughness and plasticity, have become the preferred material for heavy-haul freight and curved tracks. Studies have shown that although bainitic rails are not as wear-resistant as pearlitic rails at the same strength, their excellent crack propagation resistance can effectively balance wear and contact fatigue damage, achieving a dual improvement in rail service life and economy.

[0003] Patent publication number CN115261722A discloses a low-carbon multiphase bainitic rail and its preparation method. The microstructure of the rail includes lath bainitic ferrite, proeutectoid ferrite, austenite, and martensite. By volume percentage, the bainitic ferrite is 75%-85%, the proeutectoid ferrite is 10%-15%, the austenite is 3%-8%, and the martensite is less than 3%. The finished rail prepared by this invention has a tensile strength between 980-1180 MPa and an elongation of not less than 15%, exhibiting excellent comprehensive mechanical properties of strength and toughness. It is suitable for non-heavy-haul railways with small curve radii, harsh track conditions, and prominent contact fatigue damage sections. This patent provides a low-carbon multiphase bainitic rail, which contains granular or blocky proeutectoid ferrite and retained austenite in a microstructure of 13%-23%, which has a large difference in mechanical properties from bainitic microstructure. Ferrite at grain boundaries may become crack initiation points. Under wheel-rail contact stress, the rail surface is prone to large plastic deformation layers, leading to fatigue cracks extending into the rail and affecting safety in use.

[0004] In summary, researching and developing a low-cost bainitic rail with good rolling contact fatigue resistance has become an important research topic that urgently needs to be addressed. Summary of the Invention

[0005] Therefore, the purpose of this invention is to provide a medium-strength, contact fatigue-resistant bainitic rail for use on curved freight railway lines and its preparation method. Based on the operating characteristics of freight railways and curved lines, this invention utilizes low-cost alloy composition design and controlled cooling processes to regulate the microstructure of the rail head and the mechanical stability of retained austenite, thus developing a medium-strength, contact fatigue-resistant bainitic rail. Compared with existing hot-rolled bainitic rails of the same strength level, the rail of this invention has a significant advantage in manufacturing cost. While ensuring the strength and toughness of the rail, it improves the rail's resistance to rolling contact fatigue and effectively controls wear and rolling contact fatigue damage during use, significantly extending the service life of the rail and reducing rail grinding and maintenance costs.

[0006] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a medium-strength, contact fatigue-resistant bainitic steel rail, the chemical composition and weight percentage of which are: C: 0.22%~0.26%, Si: 1.00%~1.20%, Mn: 1.25%~1.45%, Cr: 0.25%~0.6%, Mo: 0.2%~0.4%, Nb: 0.025%~0.040%, P: ≤0.015%, S: ≤0.010%, with the balance being Fe and unavoidable impurities.

[0007] The reasons for the specific composition design of the medium-strength, contact fatigue-resistant bainitic steel rail in this application are as follows: Carbon (C) is the cheapest element used to improve the strength and hardness of the matrix. For the bainitic rail composition design provided by this invention, a C content of less than 0.22% will cause proeutectoid ferrite to be generated in the rail microstructure. The bainitic rail provided by this invention needs to control the generation of proeutectoid ferrite. At the same time, an appropriate C content is beneficial to ensure the strength and wear resistance of the bainitic rail and improve the C content and stability of the retained austenite. However, an excessively high C content will lead to the precipitation of cementite during the phase transformation, reducing the toughness of the bainitic rail. Therefore, this invention selects a C content of 0.22% to 0.26%.

[0008] Si is a solid solution strengthening element that is mainly dissolved in bainitic ferrite. In this invention, the main role of Si is to improve the strength of the microstructure by being a solid solution strengthening element in bainitic ferrite, while inhibiting the precipitation of cementite. Too low a Si content will lead to the precipitation of cementite in the bainitic rail in this invention; therefore, the Si content is selected to be 1.00%~1.20% in this invention.

[0009] Mn can improve the stability of supercooled austenite, reduce the temperature range of bainite transformation, and play a role in solid solution strengthening in bainite, reducing the martensitic transformation temperature and improving the strength and toughness of bainite. However, excessive Mn content can easily lead to local segregation in the rail, affecting the weldability of the rail. Therefore, this invention selects an Mn content of 1.25% to 1.45%.

[0010] Cr can improve hardenability, ensure that the rail obtains bainitic structure at a lower cooling rate, delay bainite transformation, refine the width of bainite laths, and improve the wear resistance of the rail. However, excessive Cr content will increase the proportion of martensite in the structure. Therefore, the Cr content selected in this invention is 0.25%~0.6%.

[0011] Mo can effectively delay the transformation of pearlite, reduce the transformation temperature of bainite, and improve the hardenability of rails, which is beneficial to obtaining uniformity and consistency of bainite microstructure and properties in the rail head section. However, excessive addition of Mo will significantly increase the cost of rails. Therefore, this invention selects a Mo content of 0.2% to 0.4%.

[0012] Nb can effectively refine the austenite grain size, inhibit the formation of proeutectoid ferrite, and refine the bainite structure during rolling. The uniform and fine bainite structure reduces stress concentration and prolongs fatigue life. Therefore, the present invention selects an Nb content of 0.025%~0.040%.

[0013] P is a harmful element in steel rails and is very prone to segregation at grain boundaries. Under the premise of ensuring steelmaking conditions and costs, this invention requires that the P content be ≤0.015%.

[0014] S is a harmful element in the rails of this invention. Excessive S content can easily form large-sized Class A non-metallic inclusions, causing quality defects in the weld heat-affected zone. Therefore, the lower the content, the better, without increasing unnecessary costs. This invention requires S ≤ 0.010%.

[0015] Based on the above technical solution, further, the microstructure of the medium-strength contact fatigue resistant bainitic rail is a bainitic microstructure with a carbon content of 0.05%~0.10% and a lath thickness of 210~280nm, a thin film-like retained austenite with a thickness of 50~80nm, and a certain amount of blocky retained austenite. The total content of retained austenite is 8%~13%, and the content of proeutectoid ferrite and lath martensite is 3%~6%. The rail yield strength (Rp) 0.2 ≥980MPa, tensile strength (Rm) between 1150-1250MPa, elongation (A) ≥14%, rail head tread hardness ≥390HBW, and impact performance at -40℃ Aku ≥85J.

[0016] Secondly, the present invention provides a method for preparing the above-mentioned medium-strength contact fatigue resistant bainitic rail, including continuous casting billet pretreatment, continuous casting billet heating, rail rolling, controlled cooling, straightening, and tempering; After final rolling, the rails are cooled to 700-800℃. First, forced cooling is applied to the rail head tread and both sides at a cooling rate of 4-7℃ / s. When the surface temperature of the rail head drops to 500-530℃, forced cooling is applied to the rail head tread and both sides at a cooling rate of 2-4℃ / s. When the surface temperature of the rail head drops to 380-400℃, forced cooling is stopped. After air cooling to room temperature, the rails are straightened. The straightened rails are then tempered and held at 345-365℃ for 15-30 hours before being removed from the furnace and cooled to room temperature.

[0017] Based on the above technical solution, the rail is further cooled to 750~790℃ after rolling, and the cooling method is air cooling.

[0018] Based on the above technical solution, the forced cooling method is further described as forced cooling by air jets.

[0019] Based on the above technical solution, the continuous casting billet is further defined as a cross-sectional continuous casting billet with a diameter of 280~320mm × 380~410mm.

[0020] Based on the above technical solution, the pretreatment of the continuous casting billet specifically involves holding the continuous casting billet at 500~750℃ for 20~30h and then cooling it to room temperature with the furnace.

[0021] Based on the above technical solution, further, the homogenization temperature of the continuously cast billet during the heating process is 1210~1240℃, and the holding time is 2~4 hours.

[0022] Based on the above technical solution, the continuously cast billet is further descaled by high-pressure water after heating.

[0023] Based on the above technical solution, the rail rolling process further adopts the universal method or the die rolling method to roll rails with a cross section of 50~75kg / m, the rail rolling compression ratio is greater than 11, the BD1 initial rolling temperature is 1100~1140℃, and the final rolling temperature is 850~890℃.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention prepares a medium-strength, contact fatigue-resistant bainitic rail for freight railway curved lines by designing a low-cost alloy composition and controlling the cooling process. The pretreatment of the continuous casting billet reduces the hydrogen content in the rail, thereby reducing the risk of hydrogen embrittlement during rail straightening and use. The use of a lower final rolling temperature combined with Nb microalloying effectively refines the original austenite grain size. Through a two-stage controlled cooling process, the first stage uses a larger cooling rate to suppress the formation of proeutectoid ferrite, and the second stage uses a moderate cooling rate to control the supercooled austenite to undergo bainitic phase transformation at a lower temperature, thereby refining the bainitic structure and increasing the C content in the bainitic structure. At the same time, it reduces the content of blocky martensite in the steel, ensuring the microstructure composition of the rail of this invention.

[0025] (2) The rail of the present invention adopts a two-stage cooling. In the first stage, the overall temperature of the rail is reduced to above the phase transformation start temperature of the cooling rate with a large cooling rate, which inhibits the formation of proeutectoid ferrite, shortens the rail cooling time, and improves the rail production rhythm. In the second stage, a moderate cooling rate is adopted to avoid the material surface cooling rate being too large to form a large amount of martensite structure. At the same time, a certain degree of supercooling is maintained so that the supercooled austenite begins bainitic phase transformation in a lower temperature range, which refines the bainitic structure and improves the hardness and toughness of the rail. After being air-cooled to room temperature and straightened, it is sent to a 345~365℃ holding furnace for 15~30h. This holding process mainly plays the role of stabilizing the residual austenite, and at the same time, it has a tempering effect on the martensite generated during the straightening process, reduces the residual stress of the rail, and ensures that the residual austenite in the rail has high stability during use. The phase transformation of the residual austenite will only occur under large strain, which inhibits the propagation of cracks into the material.

[0026] (3) The rail microstructure prepared by this invention consists of bainitic microstructure with a carbon content of 0.05%~0.10% and an average lath thickness of 210~280 nm, thin-film retained austenite with a thickness of 50~80 nm, and a certain amount of blocky retained austenite. The total content of retained austenite is 8%~13%, and the content of proeutectoid ferrite and lath martensite is 3%~6%. The rail yield strength (Rp) 0.2 ≥980MPa, tensile strength (Rm) is 1150~1250MPa, elongation (A) is ≥14%, rail head tread hardness is ≥390HBW, and impact performance at -40℃ is Aku≥85J. Attached Figure Description

[0027] To more clearly illustrate the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below.

[0028] Figure 1 This is a microstructure diagram of the rail prepared in Example 2 of the present invention. Detailed Implementation

[0029] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0030] This invention provides a medium-strength, contact fatigue-resistant bainitic steel rail with the following chemical composition and weight percentages: C: 0.22%~0.26%, Si: 1.00%~1.20%, Mn: 1.25%~1.45%, Cr: 0.25%~0.6%, Mo: 0.2%~0.4%, Nb: 0.025%~0.040%, P: ≤0.015%, S: ≤0.010%, with the balance being Fe and unavoidable impurities.

[0031] Based on the above technical solution, further, the microstructure of the medium-strength contact fatigue resistant bainitic rail is a bainitic microstructure with a carbon content of 0.05%~0.10% and a lath thickness of 210~280nm, a thin film-like retained austenite with a thickness of 50~80nm, and a certain amount of blocky retained austenite. The total content of retained austenite is 8%~13%, and the content of proeutectoid ferrite and lath martensite is 3%~6%. The rail yield strength (Rp) 0.2 ≥980MPa, tensile strength (Rm) between 1150-1250MPa, elongation (A) ≥14%, rail head tread hardness ≥390HBW, and impact performance at -40℃ AKu ≥85J.

[0032] The present invention also provides a method for preparing the above-mentioned medium-strength contact fatigue resistant bainitic rail, including continuous casting billet pretreatment, continuous casting billet heating, rail rolling, controlled cooling, straightening, and tempering; After final rolling, the rails are cooled to 700-800℃. First, forced cooling is applied to the rail head tread and both sides at a cooling rate of 4-7℃ / s. When the surface temperature of the rail head drops to 500-530℃, forced cooling is applied to the rail head tread and both sides at a cooling rate of 2-4℃ / s. When the surface temperature of the rail head drops to 380-400℃, forced cooling is stopped. After air cooling to room temperature, the rails are straightened. The straightened rails are then tempered and held at 345-365℃ for 15-30 hours before being removed from the furnace and cooled to room temperature.

[0033] Based on the above technical solution, the rail is further cooled to 750~790℃ after rolling, and the cooling method is air cooling.

[0034] Based on the above technical solution, the forced cooling method is further described as forced cooling by air jets.

[0035] Based on the above technical solution, the continuous casting billet is further defined as a cross-sectional continuous casting billet with a diameter of 280~320mm × 380~410mm.

[0036] Based on the above technical solution, the pretreatment of the continuous casting billet specifically involves holding the continuous casting billet at 500~750℃ for 20~30h and then cooling it to room temperature with the furnace.

[0037] Based on the above technical solution, further, the homogenization temperature of the continuously cast billet during the heating process is 1210~1240℃, and the holding time is 2~4 hours.

[0038] Based on the above technical solution, the continuously cast billet is further descaled by high-pressure water after heating.

[0039] Based on the above technical solution, the rail rolling process further adopts the universal method or the die rolling method to roll rails with a cross section of 50~75kg / m, the rail rolling compression ratio is greater than 11, the BD1 initial rolling temperature is 1100~1140℃, and the final rolling temperature is 850~890℃.

[0040] Examples 1-5 This embodiment provides a method for preparing a medium-strength, contact fatigue-resistant bainitic steel rail. The chemical composition and weight percentage of the steel rail are shown in Table 1. Table 1. Chemical composition and weight percentage (wt%) of the rails in Examples 1-5

[0041] This includes continuous casting billet pretreatment, continuous casting billet heating, rail rolling, controlled cooling, straightening, and tempering; The continuous casting billet adopts a large cross-section of 280~320mm×380~410mm. The pretreatment of the continuous casting billet is as follows: the continuous casting billet is held at 500-750℃ for 20-30h and then cooled to room temperature in the furnace; the soaking temperature of the continuous casting billet is 1210~1240℃ and the holding time is 2.5~3.5 hours; after high pressure water descaling, it is rolled into the required cross-section rails of 60~75kg / m by the universal rolling method, ensuring that the rail rolling compression ratio is greater than 11. The BD1 initial rolling temperature is 1100~1140℃ and the final rolling temperature of the universal rolling mill is 850~890℃. After final rolling, the rails are air-cooled to 750-790℃. Forced cooling with air jets at a rate of 4-7℃ / s is then applied to the rail head tread and both sides. When the surface temperature of the rail head drops to 500-530℃, forced cooling with air jets at a rate of 2-4℃ / s is applied to the rail head tread and both sides. Forced cooling is stopped when the surface temperature of the rail head drops to 380-400℃. The rails are then air-cooled to room temperature for straightening. After straightening, the rails are heat-treated in a furnace at 345-365℃ for 15-30 hours. After being removed from the furnace and allowed to return to room temperature, the rails undergo flaw detection, processing, and other procedures to obtain the finished rails.

[0042] Table 2. Main process parameters for pretreatment, heating, and rolling of continuously cast billets in Examples 1-5

[0043] Table 3. Controlled cooling and tempering process parameters for Examples 1-5

[0044] Table 4. Microstructure of the rails prepared in Examples 1-5

[0045] Table 5. Mechanical properties and wear resistance results of the rails prepared in Examples 1-5.

[0046] Note: The rails were tested for tensile strength according to the TB / T2344-2020 standard, and the tread hardness samples were taken. The wear test of the rails of this invention used hollow circular specimens with a thickness of 10 mm and a diameter of 36 mm. The test load was 170 kg, and the test was conducted in a dry friction environment with a slip of 5% and a cycle count of 200,000 revolutions. The contact stress of the rolling contact fatigue tester was 1400 MPa.

[0047] The rail yield strength (Rp) prepared within the composition range given in this invention and according to the controlled cooling process given in this invention. 0.2 ≥980MPa, tensile strength (Rm) is 1150~1250MPa, elongation (A) is ≥14%, rail head tread hardness is ≥390HBW, and impact performance at -40℃ is Aku≥85J.

[0048] Comparative Examples 1-3 This comparative example provides a conventional method for preparing bainitic rails. The chemical composition and weight percentage of the rails in Comparative Examples 1-3 are shown in Table 6. Table 6 Chemical composition and weight percentage (wt%) of rails used in Comparative Examples 1-3

[0049] This includes continuous casting billet pretreatment, continuous casting billet heating, rail rolling, and cooling; The continuously cast billet adopts a cross-section of 280×380mm. The pretreatment of the continuously cast billet is as follows: the billet is held at 500-750℃ for 20-30h and then cooled to room temperature in the furnace. The soaking temperature of the continuously cast billet is 1250~1270℃, and the holding time is 2.5~3.5h. After descaling by high pressure water, it is rolled into the required cross-section rails of 60~75kg / m by the universal method, ensuring that the rail rolling compression ratio is greater than 11. The BD1 development temperature is 1135~1165℃, and the final rolling temperature of the universal rolling mill is 970~1020℃. After air cooling to room temperature, it is straightened and then tempered at 310~360℃ for 30~50h.

[0050] Table 7. Main process parameters for pretreatment, heating, and rolling of continuously cast billets in Comparative Examples 1-3

[0051] Table 8 Tempering process parameters for Comparative Examples 1-3

[0052] Table 9. Microstructure of rails prepared in Comparative Examples 1-3

[0053] Table 10 Mechanical properties and wear resistance of the rails prepared in Comparative Examples 1-3

[0054] Note: The rails were tested for tensile strength according to the TB / T2344-2020 standard, and the tread hardness samples were taken. The wear test of the rails of this invention used hollow circular specimens with a thickness of 10 mm and a diameter of 36 mm. The test load was 170 kg, and the test was conducted in a dry friction environment with a slip of 5% and a cycle count of 200,000 revolutions. The contact stress of the rolling contact fatigue tester was 1400 MPa.

[0055] The results above show that the composition design of this invention can reduce the amount of alloy used, thereby reducing costs. The controlled cooling manufacturing process avoids the adverse effects of reduced alloy content. The impact toughness and elongation of the tested rails are improved, while the tread tensile strength is slightly reduced. The tread hardness remains at the same level, and the rail wear is slightly increased. This indicates that the rails produced by the method provided by this invention have higher resistance to rolling contact fatigue while maintaining good toughness. In summary, this invention can effectively improve the contact fatigue damage resistance of rails, significantly reduce rail production costs, and is suitable for freight railways and curved tracks.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A medium-strength, contact fatigue-resistant bainitic steel rail, characterized in that, Its chemical composition and weight percentage are as follows: C: 0.22%~0.26%, Si: 1.00%~1.20%, Mn: 1.25%~1.45%, Cr: 0.25%~0.6%, Mo: 0.2%~0.4%, Nb: 0.025%~0.040%, P: ≤0.015%, S: ≤0.010%, with the balance being Fe and unavoidable impurities; The microstructure of the medium-strength, contact fatigue-resistant bainitic rail consists of bainitic microstructure with a carbon content of 0.05%~0.10% and a lath thickness of 210~280 nm, thin-film retained austenite with a thickness of 50~80 nm, and a certain amount of blocky retained austenite. The total content of retained austenite is 8%~13%, and the content of proeutectoid ferrite and lath martensite is 3%~6%. The rail yield strength (Rp) 0.2 ≥980MPa, tensile strength (Rm) is 1150-1250MPa, elongation (A) is ≥14%, rail head tread hardness is ≥390HBW, and impact performance at -40℃ is Aku≥85J; The preparation method of the medium-strength contact fatigue resistant bainitic steel rail includes continuous casting billet pretreatment, continuous casting billet heating, rail rolling, controlled cooling, straightening, and tempering; After the rail is finally rolled, it is cooled to 700~800℃. First, forced cooling is applied to the rail head tread and both sides of the rail at a cooling rate of 4~7℃ / s. When the surface temperature of the rail head drops to 500~530℃, forced cooling is applied to the rail head tread and both sides of the rail at a cooling rate of 2~4℃ / s. When the surface temperature of the rail head drops to 380~400℃, forced cooling is stopped. After air cooling to room temperature, the rail is straightened. The straightened rail is then tempered and held at 345~365℃ for 15~30 hours before being taken out of the furnace and cooled to room temperature. The rail rolling process produces rails with a cross-sectional area of ​​50~75kg / m, a rail rolling compression ratio greater than 11, an initial rolling temperature of 1100~1140℃ for BD1, and a final rolling temperature of 850~890℃.

2. The medium-strength, contact fatigue-resistant bainitic steel rail according to claim 1, characterized in that, After the rails are rolled, they are cooled to 750~790℃ by air cooling.

3. The medium-strength, contact fatigue-resistant bainitic steel rail according to claim 1, characterized in that, The forced cooling method is forced air cooling.

4. The medium-strength, contact fatigue-resistant bainitic steel rail according to claim 1, characterized in that, The continuous casting billet is a cross-section continuous casting billet of 280~320mm×380~410mm; the specific pretreatment of the continuous casting billet is to keep the continuous casting billet at 500~750℃ for 20~30h and then cool it to room temperature with the furnace.

5. The medium-strength, contact fatigue-resistant bainitic steel rail according to claim 1, characterized in that, During the heating process of the continuously cast billet, the homogenization temperature of the billet is 1210~1240℃, and the holding time is 2~4 hours.

6. The medium-strength, contact fatigue-resistant bainitic steel rail according to claim 1, characterized in that, After heating, the continuously cast billet is descaled by high-pressure water.

Images