Production method for improving toughness of hypereutectoid heat treatment steel rail

Abstract

The invention discloses a production method for improving the toughness of a hypereutectoid heat treatment steel rail, and belongs to the technical field of metallurgical materials. The steel rail comprises the following chemical components in percentage by mass: 0.80-1.10% of C; 0.30 to 0.90 percent of Si; mn is 0.55 to 2.25 percent; less than or equal to 0.025% of P; s is less than or equal to 0.025%; the alloy comprises the following components in percentage by weight: 0.20 to 0.90 percent of Cr, 0.04 to 0.30 percent of V, 0.03 to 0.10 percent of Nb and 0.0010 to 0.010 percent of RE; the total mass fraction of the Fe and the impurities is 100%. The production process comprises the following steps: a steelmaking production process: adding a rare earth alloy at a VD station; a steel rail rolling process; an online heat treatment process; and tempering. The invention aims to research parameters of different temperatures, cooling speeds, tempering processes and the like of the steel rail, improve the toughness of the heat-treated hypereutectoid steel rail and ensure the strength of the heat-treated hypereutectoid steel rail.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical materials, and in particular relates to a production method for improving the toughness of a hypereutectoid heat-treated rail. Background Art

[0002] The continuous increase in axle weight, speed, and annual freight volume of heavy-duty trucks poses a serious challenge to the selection of rail materials and their transportation safety. Heavy-duty line rails face problems such as excessive wear and peeling during service, significantly increasing the performance requirements for rails operating under heavy-duty line conditions, including higher requirements for tread hardness and toughness. Currently, the rail materials used in railway transportation are basically medium-to-high-hardness basic rails that meet domestic and international industry standards, and treatment methods are used to improve the strength and toughness of the rails. However, the increase in annual freight volume in recent years has necessitated the development of high-performance rails with superior strength and toughness, exceeding the standards, to meet the needs of the lines.

[0003] Comparative application of rail products worldwide demonstrates that hypereutectoid rails possess high strength and hardness, demonstrating excellent overall serviceability on high-axle-load lines and tight-radius curves, significantly improving upon the current highest-performance H370 rail. However, problems with hypereutectoid rails, such as low toughness and brittle fracture, remain unresolved. Therefore, the development of hypereutectoid heat-treated rails with high strength, wear resistance, and superior toughness is of vital production and economic significance. Summary of the Invention

[0004] The present invention aims to provide a production method for improving the toughness of hypereutectoid heat-treated rails, which can be used to heat treat and temper industrially high-strength and tough hypereutectoid heat-treated rails. The method involves studying parameters such as different temperatures, cooling rates, and tempering processes to improve the toughness of heat-treated hypereutectoid rails while maintaining strength.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The present invention provides a production method for improving the toughness of a hypereutectoid heat-treated rail. The chemical composition of the rail by mass is as follows: C 0.80-1.10%; Si 0.30-0.90%; Mn 0.55-2.25%; P≤0.025%; S≤0.025%; Cr 0.20-0.90%, V 0.04-0.30%, Nb 0.03-0.10%, RE 0.0010-0.010%; the remainder being Fe and impurities, with a total mass fraction of 100%. The production process comprises:

[0007] Steelmaking production process: molten iron → converter smelting → LF refining → VD → LF refining → continuous casting; converter smelting uses aluminum-free deoxidation alloying and double refining, which is different from traditional single refining smelting. The purpose is to better remove inclusions and reduce the risk of hypereutectoid rail rolling fatigue failure; the entire process is argon-blown according to normal refining; vacuum degree ≤ 0.10KPa, deep vacuum time ≥ 15min, superheat ΔT ≤ 25℃, and rare earth alloys are added at the VD station;

[0008] Rail rolling process: billet → sawing → heating → BD1 rolling → BD2 rolling → continuous rolling on a CCS universal rolling mill → online residual heat quenching → sawing → cooling → trimming → straightening → inspection → packaging → weighing → storage; the billet preheating temperature must not exceed 900°C; the heating time must not be less than 3 hours and 15 minutes. The furnace discharge temperature must not be lower than 1150°C, the starting rolling temperature must be ≥1100°C, and the final rolling temperature must be 850-940°C.

[0009] Online heat treatment process: The cooling medium for online residual heat quenching is air or a mixture of mist and air. After the final rolling of the rail, the residual heat treatment temperature starts at 720℃-820℃. After 140-200s of online heat treatment, the rail leaves the heat treatment line. In the first stage of online heat treatment, the top surface, both sides, gauge angle jaw, and rail bottom of the rail are cooled. The actual cooling rate in the cooling section is 3℃ / s-5℃ / s, and the cooling time is 50s-80s to ensure that the final microstructure of the rail is pearlite. In the second stage, weak cooling is carried out to cool the top surface, both sides of the rail head, gauge angle jaw, and rail bottom of the rail. The cooling rate is ≤2℃ / s. The outlet rail head temperature after cooling is 420℃-520℃, and then it is naturally cooled to room temperature.

[0010] Tempering process: rail loading → heating tempering furnace → insulation → unloading → cooling and transportation; after heating the tempering furnace to 240-400℃, insulation is carried out for 6-24 hours, then unloading and air cooling are carried out for shipment.

[0011] Furthermore, the added rare earth alloy is a high-purity FeCe alloy or a lanthanum-cerium mixed rare earth.

[0012] Furthermore, the chemical composition of the rail by mass percentage is: C 0.90%; Si 0.54%; Mn 1.00%; P 0.013%; S 0.007%; Cr 0.29%, V 0.042%, Nb 0.021%, RE 0.0024%; the rest is Fe and impurities, with a total mass fraction of 100%.

[0013] Furthermore, the chemical composition of the rail by mass percentage is: C 0.91%; Si 0.48%; Mn 0.98%; P 0.011%; S 0.003%; Cr 0.29%, V 0.045%, Nb 0.029%, RE 0.0028%; the rest is Fe and impurities, with a total mass fraction of 100%.

[0014] Furthermore, the chemical composition of the rail by mass percentage is: C 0.93%; Si 0.57%; Mn 1.01%; P 0.019%; S 0.003%; Cr 0.27%, V 0.048%, Nb 0.032%, RE 0.0035%; the rest is Fe and impurities, with a total mass fraction of 100%.

[0015] Furthermore, the chemical composition of the rail by mass percentage is: C 0.94%; Si 0.60%; Mn 0.99%; P 0.017%; S 0.005%; Cr 0.30%, V 0.051%, Nb 0.027%, RE 0.0021%; the rest is Fe and impurities, with a total mass fraction of 100%.

[0016] Furthermore, the chemical composition of the rail by mass percentage is: C 0.93%; Si 0.59%; Mn 1.02%; P 0.009%; S 0.004%; Cr 0.34%, V 0.047%, Nb 0.041%, RE 0.0033%; the rest is Fe and impurities, with a total mass fraction of 100%.

[0017] This material is based on C-Si-Mn and contains the following alloying elements: Cr, V, Nb, and RE. The carbon content in this steel is designed to improve the wear resistance and rolling contact fatigue of heavy-duty rails. The carbon content is designed to be >0.77%, a hypereutectoid carbon content, which increases the steel's strength and hardness. Mn expands the austenite phase, increasing the stability of the steel's undercooled austenite, allowing for temperature control and stable production during online heat treatment. Si reduces the austenite-to-ferrite transformation rate and increases undercooling, while also controlling the cross-sectional hardness of the rail. V and Nb alloying elements refine the material's microstructure, precipitating C and N compounds to enhance strength and toughness, while also improving toughness during offline tempering. Nb, in particular, plays a crucial role in initial grain refinement and can replace some of the more expensive V alloys. Furthermore, the RE element, a rare earth alloy, controls the morphology of sulfides during the smelting process and improves the steel's oxidation resistance during subsequent low-temperature tempering.

[0018] Compared with the prior art, the present invention has the following beneficial technical effects:

[0019] This invention rationally designs the steel composition and incorporates optimized Si, Mn, V, Nb, and RE alloying elements into the rails. It also utilizes an online waste heat heat treatment process to enhance rail strength and hardness. This rolling process improves rail tensile strength, elongation, tread hardness, and wear resistance. A rational tempering process eliminates internal stresses from heat treatment and refines the microstructure, improving the material's fracture toughness without substantially compromising strength or hardness. The resulting hypereutectoid rails exhibit excellent strength, hardness, and toughness, making them suitable for heavy-duty railway applications and promising for widespread adoption. DETAILED DESCRIPTION

[0020] The steel production process is as follows: molten iron → converter smelting → LF refining → VD → LF refining → continuous casting. Converter smelting utilizes aluminum-free deoxidation and alloying, with argon purge throughout the entire process, as is typical for refining. Vacuum level ≤ 0.10 kPa, deep vacuum time ≥ 15 minutes, and superheat ΔT ≤ 25°C. The chemical compositions of each example are shown in Table 1.

[0021] Table 1 Composition of each embodiment (mass percentage / %)

[0022]

[0023]

[0024] The rail rolling process is as follows: billet → sawing → heating → BD1 rolling → BD2 rolling → continuous rolling on a CCS universal rolling mill → online residual heat quenching → sawing → cooling → trimming → straightening → inspection → packaging → weighing → storage. The billet preheating temperature must not exceed 900°C, and the heating time must be no less than 3 hours and 15 minutes. The furnace discharge temperature must be no less than 1150°C, the start rolling temperature must be ≥1100°C, and the final rolling temperature must be 850-940°C.

[0025] The cooling medium for online residual heat quenching is wind or a mixture of mist and wind. After the final rolling of the rail, the residual heat treatment temperature starts at 720℃-820℃. After 140-200s of online heat treatment, the rail leaves the heat treatment production line. In the first stage of online heat treatment, the top surface, both sides, the lower jaw of the gauge angle, and the bottom of the rail are cooled. The actual cooling section has a cooling rate of 3℃ / s-5℃ / s and a cooling time of 50s-80s. This ensures that the final microstructure of the rail is pearlite. In the second stage, weak cooling is used to cool the top surface, both sides of the rail head, the lower jaw of the gauge angle, and the bottom of the rail. The cooling rate is ≤2℃ / s. The temperature of the rail head at the exit after cooling is 420℃-520℃, and then it is naturally cooled to room temperature.

[0026] Table 2 Rail material rolling process examples

[0027]

[0028]

[0029] The properties of the rolled steel samples, including tensile strength, tread hardness, and fracture toughness, were tested according to the industry standard TB / T2344.1-2020, "Steel Rails, Part 1, 43kgm-75kgm Rails." The experimental results are shown in Table 3.

[0030] Table 3 Mechanical properties of each embodiment

[0031]

[0032] It can be seen from Table 3 that each embodiment has good strength, hardness and normal structure, but the tensile elongation and fracture toughness are relatively low.

[0033] The rail material tempering production process is as follows: rail loading → heating the tempering furnace → holding the rails → removing the rails from the furnace → cooling and transporting. The tempering furnace, powered by natural gas and other energy sources, is heated to 240-400°C, then held at that temperature for 6-24 hours. The rails are then removed from the furnace, air-cooled, and shipped. The experimental results are shown in Table 4.

[0034] Table 4 Mechanical properties of each embodiment after tempering

[0035]

[0036]

[0037] It can be seen from Table 4 that after tempering, the strength and hardness of each embodiment are basically unchanged, but the tensile elongation and fracture toughness are relatively increased.

[0038] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A production method for improving the toughness of hypereutectoid heat-treated rails, characterized in that: The chemical composition of the rail by mass percentage is: C 0.80-1.10%; Si 0.30-0.90%; Mn 0.55-2.25%; P≤0.025%; S≤0.025%; Cr 0.20-0.90%, V 0.04-0.30%, Nb 0.03-0.10%, RE 0.0010-0.010%; The rest is Fe and impurities, with a total mass fraction of 100%. The production process includes: Steelmaking production process: molten iron → converter smelting → LF refining → VD → LF refining → continuous casting; converter smelting adopts aluminum-free deoxidation alloying and double refining, which is different from traditional single refining smelting. The purpose is to better remove inclusions and reduce the risk of hypereutectoid rail rolling fatigue damage failure; the whole process is argon blowing as normal refining; Vacuum degree ≤ 0.10KPa, deep vacuum time ≥ 15min, superheat ΔT ≤ 25℃, rare earth alloy is added at the VD station; Rail rolling process: billet → sawing → heating → BD1 rolling → BD2 rolling → continuous rolling on a CCS universal rolling mill → online residual heat quenching → sawing → cooling → trimming → straightening → inspection → packaging → weighing → storage; the billet preheating temperature shall not exceed 900°C; the heating time shall not be less than 3 hours and 15 minutes; the furnace discharge temperature shall not be lower than 1150°C, the starting rolling temperature shall be ≥1100°C, and the finishing rolling temperature shall be 850-940°C; Online heat treatment process: The cooling medium for online residual heat quenching is air or a mixture of mist and air. After the final rolling of the rail, the residual heat treatment temperature starts at 720℃-820℃. After 140-200s of online heat treatment, the rail leaves the heat treatment line. In the first stage of online heat treatment, the top surface, both sides, gauge angle jaw, and rail bottom of the rail are cooled. The actual cooling rate in the cooling section is 3℃ / s-5℃ / s, and the cooling time is 50s-80s to ensure that the final microstructure of the rail is pearlite. In the second stage, weak cooling is carried out to cool the top surface, both sides of the rail head, gauge angle jaw, and rail bottom of the rail. The cooling rate is ≤2℃ / s. The outlet rail head temperature after cooling is 420℃-520℃, and then it is naturally cooled to room temperature. Tempering process: rail loading → heating tempering furnace → insulation → unloading → cooling and transportation; after heating the tempering furnace to 240-400℃, insulation is carried out for 6-24 hours, then unloading and air cooling are carried out for shipment.

2. The production method for improving the toughness of hypereutectoid heat-treated rail according to claim 1, characterized in that: The added rare earth alloy is a high-purity FeCe alloy or a lanthanum-cerium mixed rare earth.

3. The production method for improving the toughness of hypereutectoid heat-treated rail according to claim 1, characterized in that: The chemical composition of the rail by mass percentage is: C 0.90%; Si 0.54%; Mn 1.00%; P 0.013%; S 0.007%; Cr 0.29%, V 0.042%, Nb 0.021%, RE 0.0024%; the rest is Fe and impurities, with a total mass fraction of 100%.

4. The production method for improving the toughness of hypereutectoid heat-treated rail according to claim 1, characterized in that: The chemical composition of the rail by mass percentage is: C 0.91%; Si 0.48%; Mn 0.98%; P 0.011%; S 0.003%; Cr 0.29%, V 0.045%, Nb 0.029%, RE 0.0028%; the rest is Fe and impurities, with a total mass fraction of 100%.

5. The production method for improving the toughness of hypereutectoid heat-treated rail according to claim 1, characterized in that: The chemical composition of the rail by mass percentage is: C 0.93%; Si 0.57%; Mn 1.01%; P 0.019%; S 0.003%; Cr 0.27%, V 0.048%, Nb 0.032%, RE 0.0035%; the rest is Fe and impurities, with a total mass fraction of 100%.

6. The production method for improving the toughness of hypereutectoid heat-treated rail according to claim 1, characterized in that: The chemical composition of the rail by mass percentage is: C 0.94%; Si 0.60%; Mn 0.99%; P 0.017%; S 0.005%; Cr 0.30%, V 0.051%, Nb 0.027%, RE 0.0021%; the rest is Fe and impurities, with a total mass fraction of 100%.

7. The production method for improving the toughness of hypereutectoid heat-treated rail according to claim 1, characterized in that: The chemical composition of the rail by mass percentage is: C 0.93%; Si 0.59%; Mn 1.02%; P 0.009%; S 0.004%; Cr 0.34%, V 0.047%, Nb 0.041%, RE 0.0033%; the rest is Fe and impurities, with a total mass fraction of 100%.

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