Middle and low Cr-20 DEG C fracture-resistant high-toughness steel rail and production method thereof

Abstract

The invention relates to a medium-low Cr high-toughness steel rail resistant to-20 DEG C fracture and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.65%-0.85% of C, 0.10%-0.68% of Si, 0.50%-1.05% of Mn, 0.08%-0.35% of Cr, 0.050%-0.200% of Ni, 0.010%-0.150% of Mo, 0.010%-0.035% of Ti, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 1.42%-13.12% of Mn / Cr, preferably 1.43%-9.88% of Mn / Cr and the balance of Fe. And the balance of Fe and inevitable impurities. According to the method, alloy adding is optimized, the solid solution and aging effect of a Cr-Ni-Mo-Ti alloy system is formed, the heat treatment cooling rate is controlled stage by stage, a material matrix is strengthened, the strength and toughness of the steel rail are improved, and the fracture toughness value at the temperature of-20 DEG C reaches 36.1-38.8 MPa.m < 0.5 > under the condition that the alloy cost is not obviously increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel for rail transit, and relates to a rail material, mainly a medium-low Cr high-strength and tough rail resistant to fracture at -20°C for heavy-haul railway operation lines under the condition of -20°C and its production method. Background Art

[0002] The demand for railway construction in tropical regions such as Southeast Asia and South Asia, as well as in temperate regions such as China, Mongolia and Russia, is increasing. This requires that the rail should not only meet the high-temperature environment of 30°C in tropical regions, but also meet the low-temperature environment above -20°C in winter in temperate regions. The thermal changes in the service environment pose new challenges to the stable service of the rail. The application performance of the rail basically does not change under the condition of 30°C high temperature, and the service stability is good. However, the brittleness shows an increasing trend under the condition of -20°C low temperature. Therefore, some foreign users have put forward new technical requirements for rails applied to freight transportation or heavy-haul lines in low-temperature regions, that is, under the service condition of -20°C, the rail needs to meet high strength and high hardness, and also needs to meet higher toughness to resist the problem of increased brittleness of the rail under low-temperature conditions, and prevent brittle fracture failure accidents of the rail under large impact. In order to ensure the safe operation of trains in low-temperature sections as required by users, improve the brittle fracture resistance of the rail under the condition of -20°C, and achieve good matching of the technical indexes of the strength, hardness and -20°C low-temperature fracture toughness of the rail, the research team has carried out the development work of high-strength and tough rails resistant to fracture at -20°C, adopted medium-low chromium and elements to improve toughness and plasticity, and carried out accelerated cooling through an on-line heat treatment unit to further increase the fine-grain strengthening effect, forming a production technology of high-strength and tough rails resistant to fracture at -20°C, and realizing batch and stable production and supply.

[0003] In the research, it is found that the production technology of the present invention makes up for some deficiencies of the disclosed related technologies and shows a certain degree of innovation. For example, CN104195433B discloses "a high-strength and tough pearlitic steel rail and its production method". In this technology, the tensile strength of the steel rail does not exceed 1120 MPa, which does not meet the strength requirements of the steel rail laid on low-temperature heavy-haul or heavy-haul lines, and the wear resistance is insufficient, seriously reducing the service life. CN104561816B discloses "a steel rail with excellent high strength and fatigue resistance and its production method". This technology shows that the tensile strength of the related steel rail is between 1260 MPa and 1420 MPa, and the steel rail shows relatively high strength, but only emphasizes strength and fatigue resistance, without explaining the effect of low-temperature fracture toughness. At the same time, it is easy to cause unreasonable resource matching and increase the design cost. CN110592496B discloses "a pearlitic steel rail steel and its preparation method", and CN112501512A discloses "a controlled rolling and controlled cooling high-strength pearlitic steel rail and its production method". The related steel rails show relatively high strength. The silicon content of the former even reaches 1%, which will significantly reduce the toughness and plasticity of the steel rail and is prone to potential safety hazards under the cyclic impact of train wheels; the tensile strength of the latter reaches more than 1360 MPa, and the strength meets the requirements but the toughness does not meet the low-temperature service requirements. CN107739806A discloses "a hypereutectoid steel rail with high toughness and plasticity and its manufacturing method". The related steel rail is a hypereutectoid steel rail and is brittle at low temperatures. CN107475616A discloses "a high-strength and tough pearlitic steel rail and its manufacturing method", and CN107675083B discloses "a strong and tough pearlitic steel rail and its manufacturing method". The related steel rails have a certain strength and hardness, but the main strengthening mechanism is the action of microalloying elements, without explaining the combined action of chromium and other elements. CN202211160787 discloses "a smelting and production method of a corrosion-resistant and low-temperature-resistant steel rail for plateau railways", which mainly explains the smelting method and does not explain the low-temperature fracture toughness of the steel rail. CN202310394412 discloses "a manufacturing method and steel rail for improving the low-temperature fracture resistance and contact fatigue damage resistance of steel rails", which involves improving the low-temperature fracture resistance of steel rails, reducing the preparation cost, improving the efficiency, and taking into account the low-temperature fracture resistance and contact fatigue resistance. However, its process is complex and requires wire feeding treatment and nitrogen addition treatment, and the highest content of Cr + Cu + Ni reaches 1.65%, and the content of Cu + Ni reaches 0.85%, resulting in a significant increase in cost. CN202210134161 discloses "a production method of a low-temperature-resistant steel rail in alpine regions", which involves a production method of a low-temperature-resistant steel rail in alpine regions, smelting a steel rail with good strength and toughness ratio and excellent wear resistance, but only using carbon, silicon, and manganese as matrix elements, and the tensile strength cannot meet the high-strength requirements. The fracture toughness has a positive relationship with the strength, so the fracture toughness cannot meet the low-temperature requirements.CN201510750451 discloses "a method for improving the low-temperature fracture toughness of steel rails and the obtained steel rails and their applications", but 1.2% silicon and 2.5% manganese will seriously damage the welding performance, increase the generation of abnormal martensite structure in the matrix, and deteriorate the fracture toughness.

[0004] In order to overcome the deficiencies of the prior art, the present invention provides a high-strength and tough steel rail with medium and low Cr resistance to fracture at -20°C and its production method, and on the basis of not significantly increasing the production cost, produces heat-treated steel rails for use in heavy-haul railway operating lines under -20°C conditions with fracture resistance. Summary of the Invention

[0005] The object of the present invention, a high-strength and tough steel rail with medium and low Cr resistance to fracture at -20°C and its production method, is to optimize alloy addition to form the solution + aging effect of the Cr-Ni-Mo-Ti alloy system, control the heat treatment cooling rate in stages, strengthen the material matrix, improve the strength and toughness of the steel rail, and achieve a -20°C fracture toughness value of 36.1 - 38.8 MPa·m without significantly increasing the alloy cost. 0.5 , so as to meet the requirement that the steel rail maintains stable high strength and toughness under natural high temperature, normal temperature in the natural environment and -20°C low temperature in the natural environment, take into account high hardness and wear resistance, solve the technical requirements put forward by some foreign users for steel rails applied to freight transportation or heavy-haul lines in low-temperature regions, resist the problem of increased brittleness of steel rails under low-temperature conditions, and prevent the steel rail from brittle fracture failure when subjected to a large impact.

[0006] In order to achieve the above object, the present invention is realized by adopting the following technical solutions:

[0007] A high-strength and tough steel rail with medium and low Cr resistance to fracture at -20°C, the chemical composition of the steel is calculated by weight percentage as follows: C: 0.65% - 0.85%, Si: 0.10% - 0.68%, Mn: 0.50% - 1.05%, Cr: 0.08% - 0.35%, Ni: 0.050% - 0.200%, Mo: 0.010% - 0.150%, Ti: 0.010% - 0.035%, P ≤ 0.015%, S ≤ 0.010%, and Mn / Cr: 1.42 - 13.12, preferably in the range of Mn / Cr: 1.43 - 9.88; the balance is Fe and unavoidable impurities.

[0008] The -20°C fracture toughness index of the steel rail is: 36.1 - 38.8 MPa·m 0.5 .

[0009] The room-temperature tensile strength (Rm) of the steel rail is 1235 - 1320 MPa, and the yield strength (Rp 0.2)820 - 920 MPa, elongation after fracture is 13.0% - 16.0%, hardness of the tread is 366 - 388 HBW, hardness of the cross-section is 357 - 388 HBW, hardness transition is uniform, without abnormal steep high points.

[0010] The microstructure of the rail head of the said rail consists of sorbite, troostite and pearlite. The interlamellar spacing on the surface of the rail head is between 90 - 100 nm, and the interlamellar spacing in the intermediate transition region is between 100 - 110 nm. The structure is uniformly transitional and does not contain other structures.

[0011] Through the design of medium and low Cr alloy composition and the addition of molybdenum, nickel and titanium elements in the present invention, a Cr-Ni-Mo-Ti alloy system is formed. During the mutual cooperation process of the rolling and heat treatment processes of this alloy system, a solid solution + aging effect is generated, significantly strengthening the strength and toughness of the rail, especially improving the toughness under the condition of -20 °C, and realizing the safe service of the rail on the line with large temperature fluctuations. The specific functions of the components are as follows:

[0012] C is an element to improve the hardness of the matrix. Its function in the present invention is to strengthen the matrix and ensure that the basic hardness of the rail meets the requirements of heat treatment. In the present invention, when the C content is lower than 0.65%, even with the intervention of heat treatment, it is impossible to ensure that the hardness of the rail meets the strength requirements of the heavy-haul freight lines with large axle loads in the exporting countries, and the wear resistance will also be significantly reduced; when the C content is higher than 0.85%, it will significantly increase the low-temperature brittleness of the rail, and abnormal structures are likely to appear during heat treatment. Therefore, the present invention selects the C content to be 0.65% - 0.85%.

[0013] Si is a solid-solution strengthening element. Its function in the present invention is to dissolve in ferrite during casting and cooling and dissolve in pearlite during the rolling process, which can improve the hardness and strength of the rail and replace high-cost alloy elements at the same time. When the Si content is lower than 0.10%, the solid-solution strengthening effect is not obvious; when the Si content is higher than 0.68%, the toughness and plasticity of the rail significantly decrease. Therefore, the present invention selects the Si content to be 0.10% - 0.68%.

[0014] Mn is a carbide-forming element that can increase the hardness of cementite. Cr is a matrix-strengthening element that can increase the hardenability and hardening capacity of the rail. The elements Mn and Cr are components of the Cr-Ni-Mo-Ti alloy system in the present invention and play an important role in achieving the effects of the present invention. At the same time, by combining the elements Mn and Cr and controlling the manganese-chromium ratio (Mn / Cr) to be between 1.42 and 13.12, the eutectoid transformation temperature can be increased, the free energy during the pearlite transformation process can be increased, and the nucleation rate can be increased to refine the pearlite structure. In the present invention, when the manganese content is lower than 0.50%, the matrix strength and hardness will be significantly reduced; when the manganese content is higher than 1.05%, the positive segregation degree of manganese elements will increase, resulting in abnormal martensite structure. When the chromium content is lower than 0.08%, the function of chromium cannot be effectively exerted; when the chromium content is higher than 0.35%, the effect of the manganese-chromium ratio will be weakened. Therefore, in the present invention, the Cr content is selected to be 0.08% - 0.35%, and the Mn content is selected to be 0.50% - 1.05%.

[0015] P is a harmful element in rail steel, which is likely to cause segregation and "cold brittleness". The lower it is, the better under the premise of ensuring steelmaking conditions, steelmaking costs, etc. The present invention requires that the P content ≤ 0.015%.

[0016] S is a harmful element in rail steel and is the main forming element of type A inclusions. At the same time, it is prone to "hot brittleness" during rolling. Therefore, the lower it is, the better under the premise of ensuring no increase in unnecessary costs. The present invention requires that S ≤ 0.010%.

[0017] The element Ni and the element Fe can be infinitely solid-solved, which can expand the austenite region of iron, improve the low-temperature toughness of the rail at -20°C through solid-solution strengthening, and at the same time is a component of the Cr-Ni-Mo-Ti alloy system, which can reduce the critical transformation temperature, reduce the diffusion rate of other elements, improve the hardenability, increase the pearlite content while strengthening the ferrite, and improve the strength and plasticity of the rail at the same time. In order to achieve the above-mentioned invention effects for the rail of the present invention, the Ni content is controlled at 0.050% - 0.200%.

[0018] The element Mo is a component of the Cr-Ni-Mo-Ti alloy system in the present invention. Its function is to increase the stability of supercooled austenite in the high-temperature transformation zone, cooperate effectively with the heat treatment process, and form fine sorbite structure in the high-temperature zone through an accelerated cooling process during the heat treatment process, effectively refining the pearlite lamellae, and then accelerating the formation of finer troostite structure in the medium and low-temperature transformation zones, strengthening the normal-temperature toughness and low-temperature toughness of the rail. When the molybdenum content is lower than 0.010%, the function cannot be effectively exerted; when the molybdenum content is higher than 0.150%, the function reaches the peak value and the cost will increase. Therefore, in the present invention, the Mo content is selected to be 0.010% - 0.150%.

[0019] In the present invention, Ti is an element of the Cr-Ni-Mo-Ti alloy system. Since titanium has a relatively high affinity for sulfur, titanium can form hard titanium sulfides, which can prevent the adverse effects caused by elongated manganese sulfides, thereby reducing the degree of anisotropy of the toughness of the rail and increasing the fracture toughness value at -20°C. When the titanium content is less than 0.010%, the effect cannot be effectively exerted; when the titanium content is higher than 0.035%, the low-temperature fracture toughness will be significantly reduced and the cost will be increased. Therefore, in the present invention, the Ti content is selected to be 0.010% - 0.035%.

[0020] On the basis of the above composition design, in order to achieve the effects and purposes of the present invention and produce a high-strength and tough rail with medium and low Cr resistance to fracture at -20°C, it is necessary to cooperate with smelting processes, rolling processes, and online heat treatment processes that match the composition of the present invention. The entire process design is closely combined with the composition design, effectively realizing the hardness and toughness of the rail under the conditions of the present invention. The designed rail is an online heat-treated rail. The specific processes and technological innovation features are as follows:

[0021] A production method of a high-strength and tough rail with medium and low Cr resistance to fracture at -20°C includes the following method steps:

[0022] 1) Rail smelting, refining, vacuum degassing, and continuous casting processes: For the smelted hot metal, desulfurization pretreatment is adopted, and it is smelted using a converter or an electric furnace, deoxidized with a silicon-barium multi-element alloy. During the tapping process, 60 - 70 kg / ton of steel of quicklime is added, aiming to promote the floating of the slag, reduce the sulfide content in the steel, and at the same time ensure that the furnace temperature is not lost, reducing the heating cost during the refining process. LF refining is adopted to further purify the molten steel and precisely control each alloy component to ensure that the alloy components meet the design requirements. During the tapping process, 4.00 - 4.80 kg / ton of steel of quicklime is added, and the refining time is 39.0 - 41.0 minutes. VD or RH vacuum degassing is adopted to ensure that the hydrogen content is below 1.5 ppm and the oxygen content is below 18 ppm, preventing hydrogen-induced cracks in the rail. The number of coarse and fine series of non-metallic inclusions is controlled below 1.5. The continuous casting billet is used for the cast billet, the cross-sectional size is not less than 280 mm × 380 mm, and the casting speed is controlled at 0.5 - 0.8 m / min to ensure the surface quality of the cast billet without cracks. Only through the above treatment methods can the beneficial effects of the present invention be realized, and the high-strength and tough rail with medium and low Cr resistance to fracture at -20°C of the present invention can be obtained in cooperation with the subsequent processes.

[0023] 2) Rail rolling: The continuous casting billet is descaled by high-pressure water and then rolled into rails using a three-stand, five-stand or seven-stand rolling mill. For the first heavy reduction rolling of the rails: the temperature is 1150 - 1220 °C, which preliminarily crushes the austenite grains and prepares for the age hardening of the Cr-Ni-Mo-Ti alloy system; for the second heavy reduction rolling: the temperature is 1080 - 1145 °C, which refines the austenite grains and increases the dislocation density, and the elements of the Cr-Ni-Mo-Ti alloy are completely dissolved; for the third heavy reduction rolling: the rolling temperature is 990 - 1060 °C, the newly grown austenite is crushed, and then air-cooled for 30 - 60 s, preferably 60 s. At this time, the Cr-Ni-Mo-Ti alloy system simultaneously exerts the effects of solution and age hardening. Through the cooperation of alloying elements, this process makes the cold austenite more stable in the high-temperature transformation zone. Titanium combines with sulfur to form relatively hard titanium sulfides, and the sulfide morphology is spindle-shaped and not significantly elongated, thereby reducing the anisotropy of the rail toughness and laying a compositional foundation for subsequent on-line heat treatment strengthening.

[0024] 3) On-line heat treatment of rails: After the rail rolling is completed, the rails enter the on-line heat treatment unit using the rolling afterheat, and the cooling medium is air. The on-line heat treatment unit compresses the air to complete the on-line under-speed quenching; the unit is divided into 8 sections, numbered from section 1 to section 8 in sequence. In order to ensure that the temperature of the rails entering the unit meets the conditions for under-speed quenching, in this invention, by forming the Cr-Ni-Mo-Ti alloy system and exerting its solution and age hardening effects, the temperature of the rails entering the unit is controlled at 750 °C - 850 °C; process limitations are set for each section of the unit: for section 1 of the unit, the cooling rate of the rail head tread is 3.5 - 4.0 °C / s and maintained for 7 - 15 s, preferably 10 s; for section 2 of the unit, the cooling rate of the rail head tread is 1.5 - 3.0 °C / s and maintained for 7 - 15 s, preferably 10 s; for section 3 of the unit, the cooling rate of the rail head tread is 1.5 - 2.0 °C / s and maintained for 7 - 15 s, preferably 10 s; for section 4 of the unit, the cooling rate of the rail head tread is 0.5 - 1.2 °C / s and maintained for 15 - 25 s, preferably 20 s; for section 5 of the unit, the cooling rate of the rail head tread is 0.5 - 2.0 °C / s and maintained for 5 - 15 s, preferably 8 s; for section 6 of the unit, the cooling rate of the rail head tread is 1.0 - 1.5 °C / s and maintained for 7 - 15 s, preferably 10 s; for section 7 of the unit, the cooling rate of the rail head tread is 1.0 - 1.5 °C / s and maintained for 7 - 15 s, preferably 10 s; for section 8 of the unit, the cooling rate of the rail head tread is 0.5 - 0.8 °C / s and maintained for 15 - 25 s, preferably 20 s. The temperature of the rails leaving the unit is controlled at 450 - 500 °C, and finally the rails are cooled to room temperature. Through the combined action of segmented cooling and alloying, the rail matrix is strengthened, and the strength and toughness of the rails are improved. Without significantly increasing the alloy cost, the fracture toughness value at -20 °C reaches 36.1 - 38.8 MPa·m 0.5 , meeting the requirements for the rails to maintain stable high strength and toughness under natural high temperature, natural ambient temperature and natural ambient temperature at -20 °C.

[0025] To achieve the technical effects of the present invention, the present invention integrates production technologies such as alloy composition design, steelmaking, rolling, and on-line heat treatment. In particular, it gives full play to the role of Cr-Ni-Mo-Ti in the rolling and heat treatment processes, cooperates with the production process, gives play to the advantages of each process, reflects the systematicness and innovation from design to production. According to the element characteristics, the rolling process and the segmented cooling process are designed, so that the technical indicators of the rail reach the design expectations and realize mass production applications.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1) The present invention designs a Cr-Ni-Mo-Ti alloy system, which meets the requirements of high strength and toughness of export rails under the temperature difference of 50 °C

[0028] (-20 °C to 30 °C) in heavy-haul railways. Without adding a large amount of precious metal elements, it gives full play to the mechanism of each element of the present invention in the rolling stage and the heat treatment stage, ensures the microstructure and properties of the rail, and lays a technical foundation for obtaining rails with anti-low-temperature fracture at -20 °C.

[0029] 2) The present invention effectively combines the Cr-Ni-Mo-Ti alloy system with the rolling process, that is, according to the mechanism of the elements, the rolling process of the present invention is designed. Through three large-deformation rollings, the primary fragmentation of austenite is realized, the dislocation density is increased, and grain refinement strengthening is achieved; through the cooperation of these four elements, precipitation aging strengthening is completed, the -20 °C fracture toughness index is improved, and the stability of the strength and toughness of the rail in the hot and cold environments is further enhanced. At the same time, titanium forms hard titanium sulfide with sulfur, preventing manganese sulfide from being elongated during rolling, reducing the anisotropy degree of the rail toughness, and increasing the -20 °C fracture toughness value.

[0030] 3) The present invention effectively combines the Cr-Ni-Mo-Ti alloy system with the heat treatment process. That is, according to the mechanism of the elements, the heat treatment process of the present invention is designed. By designing the Cr-Ni-Mo-Ti alloy system and giving full play to its synergistic effect, a fine sorbite structure is formed, effectively refining the pearlite lamellae, and then accelerating the formation of finer troostite structure in the medium and low temperature transformation regions, strengthening the room temperature toughness and low temperature toughness of the rail.

[0031] 4) The -20 °C fracture toughness index of the rail of the present invention is: 36.1 - 38.8 MPa·m 0.5 .

[0032] Conditions for fatigue prefabricated cracks: Span S = 4W, sine wave, f = 31 Hz, r = 0.1, ΔK control: Pre-crack Initial Kmax = 23 MPa·m 0.5, Pre-crack Final Kmax = 16.5 MPa·m 0.5 , Test temperature: -20°C. During testing, the span S = 4W, displacement control, loading rate V = 1.0 mm / min.

[0033] 5) Tensile properties and hardness of the rail of the present invention: Tensile strength at room temperature (Rm) is 1235 - 1320 MPa, yield strength (Rp0.2) is 820 - 920 MPa, elongation after fracture is 13.0 - 16.0%, hardness of the tread surface is 366 - 388 HBW, hardness of the cross-section is 357 - 388 HBW, and the hardness transition is uniform without abnormal steep high points.

[0034] 6) Microstructure composition of the rail of the present invention: The microstructure of the rail head consists of sorbite, troostite, and pearlite. The lamellar spacing on the surface of the rail head is between 90 - 100 nm, and the lamellar spacing in the intermediate transition region is between 100 - 110 nm. The microstructure is uniformly transitional and does not contain other microstructures. Description of the Drawings

[0035] Figure 1 It is the microstructure diagram of Example 1. Detailed Embodiments

[0036] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the following further illustrates the detailed embodiments of the present invention in conjunction with the examples. The following examples are used to specifically illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0037] Table 1 lists the chemical compositions of the steels in the examples; Table 2 lists the key parameters for controlling the steelmaking process of the steels in the examples; Table 3 lists the key parameters for controlling the rolling and on-line heat treatment processes of the steels in the examples; Table 4 lists the mechanical properties, hardness, and fracture toughness at -20°C of the examples. The microstructure of Example 1 is shown in Figure 1 .

[0038] Table 1 Chemical Compositions of the Steels in the Examples

[0039]

[0040] Table 2 Key Parameters for Controlling the Steelmaking Process of the Examples

[0041]

[0042] Table 3 Key Parameters for Controlling the Rolling and On-line Heat Treatment Processes of the Examples

[0043]

[0044]

[0045] Table 4 Mechanical properties, hardness, and fracture toughness at -20°C of the steel in the examples

[0046]

[0047]

Claims

1. A high-strength and tough rail with medium and low Cr resistance to fracture at -20°C, characterized in that, The chemical components in the steel are by weight percentage: C: 0.65% - 0.85%, Si: 0.10% - 0.68%, Mn: 0.50% - 1.05%, Cr: 0.08% - 0.35%, Ni: 0.050% - 0.200%, Mo: 0.010% - 0.150%, Ti: 0.010% - 0.035%, P ≤ 0.015%, S ≤ 0.010%, and Mn / Cr: 1.42 - 13.12, with the balance being Fe and unavoidable impurities.

2. The high-strength and tough steel rail with medium and low Cr resistant to fracture at -20°C according to claim 1, characterized in that, Mn / Cr: 1.43 - 9.

88.

3. A high-strength and tough steel rail resistant to fracture at -20°C with medium and low Cr according to claim 1, characterized in that, The fracture toughness index of the rail at -20°C is: 36.1 - 38.8 MPa·m 0.5 .

4. A high-strength and tough steel rail with medium and low Cr resistance to fracture at -20°C according to claim 1, characterized in that The tensile strength of the rail at room temperature is 1235 - 1320 MPa, the yield strength is 820 - 920 MPa, the elongation after fracture is 13.0% - 16.0%, the hardness of the tread surface is 366 - 388 HBW, and the hardness of the cross-section is 357 - 388 HBW.

5. A high-strength and tough steel rail resistant to fracture at -20°C with medium and low Cr according to claim 1, characterized in that The metallographic structure of the rail head consists of sorbite, troostite, and pearlite, and the interlamellar spacing on the surface of the rail head is between 90 - 100 nm.

6. A production method of a high-strength and tough steel rail with medium and low Cr resistance to fracture at -20°C as described in any one of claims 1-5, characterized in that, It includes the following method steps: 1) Rail rolling: The first large reduction rolling of the rail: the temperature is 1150 - 1220 °C; the second large reduction rolling: the temperature is 1080 - 1145 °C; the third large reduction rolling: the temperature is 990 - 1060 °C, and then air cooling. 2) On-line heat treatment of the rail: After the rail rolling is completed, it enters the on-line heat treatment unit by using the remaining rolling temperature, the cooling medium is air, the on-line heat treatment unit compresses the air to complete the on-line under-speed quenching; the temperature of the rail entering the unit is controlled at 750 °C - 850 °C; the temperature of the rail leaving the unit is controlled at 450 - 500 °C.

7. The production method of a high-strength and tough steel rail with medium and low Cr resistance to fracture at -20°C according to claim 6, characterized in that, When smelting hot metal, 0.6 - 0.7 kg of quicklime is added per ton of steel during the tapping process.

8. The production method of a high-strength and tough steel rail with medium and low Cr resistant to fracture at -20°C according to claim 6, characterized in that, LF refining is adopted, 4.00 - 4.80 kg of quicklime is added per ton of steel during the tapping process, and the refining time is 39.0 - 41.0 minutes; VD or RH vacuum degassing is adopted.

9. The production method of a high-strength and tough steel rail with medium and low Cr resistance to fracture at -20°C according to claim 6, characterized in that, The continuous casting billet is used for the casting billet, and the drawing speed is controlled at 0.5 - 0.8 m / min.

10. The production method of a high-strength and tough steel rail with medium and low Cr resistant to fracture at -20°C according to claim 6, characterized in that, In the on-line heat treatment process of the rail described above, the unit is divided into 8 sections, numbered from section 1 to section 8 in sequence. The cooling rate of the tread surface of the rail head by the section 1 unit is 3.5 - 4.0 °C / s and is maintained for 7 - 15 s; the cooling rate of the tread surface of the rail head by the section 2 unit is 1.5 - 3.0 °C / s and is maintained for 7 - 15 s; the cooling rate of the tread surface of the rail head by the section 3 unit is 1.5 - 2.0 °C / s and is maintained for 7 - 15 s; the cooling rate of the tread surface of the rail head by the section 4 unit is 0.5 - 1.2 °C / s and is maintained for 15 - 25 s; the cooling rate of the tread surface of the rail head by the section 5 unit is 0.5 - 2.0 °C / s and is maintained for 5 - 15 s; the cooling rate of the tread surface of the rail head by the section 6 unit is 1.0 - 1.5 °C / s and is maintained for 7 - 15 s; the cooling rate of the tread surface of the rail head by the section 7 unit is 1.0 - 1.5 °C / s and is maintained for 7 - 15 s; the cooling rate of the tread surface of the rail head by the section 8 unit is 0.5 - 0.8 °C / s and is maintained for 15 - 25 s.

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