Medium-low-Cr high-strength steel rail capable of resisting fracture at low temperature of minus 40 DEG C and production method thereof

Abstract

The invention relates to a medium-low-Cr high-strength steel rail resistant to fracture at the low temperature of minus 40 DEG C and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.78%-0.85% of C, 0.50%-0.60% of Si, 0.80%-0.90% of Mn, 0.16%-0.28% of Cr, 0.010%-0.025% of Ti, 0.005%-0.015% of Sb, 40-50ppm of N, 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. On the premise that the strength of the steel rail is not reduced, the low-temperature toughness, especially the fracture toughness at the low temperature of minus 40 DEG C, is improved. The steel rail can keep stable obdurability under the low-temperature condition of-40 DEG C in the natural environment, and the technical problem that the brittleness of the steel rail is reduced under the extremely cold condition is solved. And meanwhile, the design cost of the high-toughness steel rail serving in the low-temperature severe environment is greatly reduced.

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 and low Cr high-strength rail resistant to low-temperature fracture for heavy-haul railway operation lines under -40°C frigid conditions and its production method. Background Art

[0002] With the continuous advancement of China's railway towards high-quality development, railway construction has reached its peak of development. The construction of the "eight horizontal and eight vertical" high-speed railway network has been completed, and the construction of heavy-haul railways has entered a new stage. As an important part of the railway, rails have withstood the tests of stable service in complex environments such as barren mountainous areas, coastal environments, and humid tunnels. In recent years, to meet the needs of economic construction and development, the railway construction in cold regions has been continuously growing. With the increase in the volume of operations and the emergence of non-cold-resistant quality problems in the service of rails, railway users have put forward new requirements for the service performance of rails, which need to meet high strength, toughness, and wear resistance under low-temperature environmental conditions. As is well known, theoretically, with the increase in carbon content in steel materials, the ductility and plasticity decrease; as the environmental temperature decreases, the ductility and plasticity become worse, and brittle fracture is likely to occur. If the alloy composition design of steel materials is optimized and intervened through production process control, its ductility and plasticity will be significantly improved, and the low-temperature fracture resistance performance will also be greatly enhanced. Therefore, to meet the application performance requirements of rails in cold regions put forward by railway users, the present invention proposes a medium and low Cr high-strength rail resistant to -40°C low-temperature fracture and its production method to achieve stable service of rails under -40°C frigid conditions.

[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. For example, CN104561816B discloses "A rail with excellent high strength and fatigue resistance and its production method". The technology shows that the tensile strength of the involved rail is between 1260 MPa and 1420 MPa. The rail exhibits 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. CN114635072A discloses "A production method of low-temperature resistant rail for alpine regions". Its chemical composition is in mass percentage: C: 0.67 - 0.69%, Si: 0.36 - 0.39%, Mn: 0.97 - 1.13%, P ≤ 0.015%, S ≤ 0.010%, and the rest is Fe and inevitable impurities. This patent improves the tensile strength, elongation, tread hardness and other properties of the rail through conventional production processes by reducing the content of C element, reasonably designing the composition and adding the optimal Si and Mn alloying elements to the rail. However, under the existing composition system and process, the improvement range of its performance is limited. CN107739806A discloses "Hypereutectoid rail with high toughness and plasticity and its manufacturing method". The involved rail is a hypereutectoid rail, which is relatively brittle under low-temperature conditions. CN107475616A discloses "High-strength and tough pearlitic rail and its manufacturing method", and CN107675083B discloses "Strong and tough pearlitic rail and its manufacturing method". The involved rails have certain strength and hardness, but the main strengthening mechanism is the effect of microalloying elements, without explaining the cooperative effect of chromium and other elements. CN112239831A discloses "A high-toughness and alpine railway rail and its production method". The weight percentage content of its alloying elements is: C: 0.40 - 0.65%, Si: 0.10 - 0.60%, Mn: 0.5 - 1.10%, Cr: 0.1 - 0.3%, P ≤ 0.020%, S ≤ 0.020%, V: 0.004 - 0.006%, and the balance is iron and inevitable impurities. This patent improves the low-temperature toughness index of the rail by reducing the C content, controlling the content of harmful substances such as P, S, H, O, N, improving the steel purity, reducing inclusions, and using the rolling waste heat for accelerated cooling to refine the structure. However, in order to improve the low-temperature toughness, this patent reduces the C content to a relatively low level. Although appropriate amounts of Mn, Cr, and V strengthening elements are added, the strength improvement is limited, and it is impossible to balance high strength and high toughness. CN202211160787 discloses "A smelting and production method of corrosion-resistant and low-temperature resistant rail for plateau railway", which mainly explains the smelting method and does not explain the low-temperature fracture toughness of the rail.CN202310394412 discloses "Manufacturing Method and Rail for Improving Low-Temperature Fracture Resistance and Resistance to Contact Fatigue Damage of Rails", which involves enhancing the low-temperature fracture resistance of rails, reducing preparation costs, improving efficiency, and taking into account both low-temperature fracture resistance and contact fatigue resistance. However, its process is complex, requiring wire feeding treatment and nitrogen addition treatment, and the maximum of Cr + Cu + Ni reaches 1.65%, and Cu + Ni reaches 0.85%, resulting in a significant increase in cost. CN109402520A discloses "A Rare Earth-Containing Heat-Treated Rail with Low-Temperature Wear Resistance and Its Preparation Method", and the weight percentage content of alloying elements in its basic alloy system is: C: 0.63 - 0.69%; Si: 0.35 - 0.60%; Mn: 0.95 - 1.20%; P ≤ 0.025%; S ≤ 0.025%; RE alloy: 0.0002 - 0.001%; the rest are Fe and inevitable impurities. This patent appropriately reduces the existing carbon content level of the rail, optimizes the content of Si and Mn alloying elements, and adds rare earth elements to further refine the grain size, thereby simultaneously improving the toughness and strength and hardness indexes of the rail. However, although the addition of rare earth elements can modify inclusions, the distribution of its inclusions still has the characteristics of unevenness and large size, and its grain size is uneven, resulting in an unstable improvement effect on the toughness of rail steel. CN202210134161 discloses "A Production Method of Low-Temperature Resistant Rails in Alpine Regions", which involves a production method of low-temperature resistant rails in alpine regions, smelting rails with good strength and toughness ratio and excellent wear resistance, but only using carbon, silicon, and manganese elements as matrix elements, the tensile strength cannot meet the high-strength requirements, and 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 Rails and the Obtained Rails and Their Applications", but 1.2% silicon and 2.5% manganese will not only seriously damage the welding performance, increase the generation of abnormal martensite structure in the matrix, and deteriorate the fracture toughness. Summary of the Invention

[0004] The present invention provides a medium-low Cr high-strength rail with -40°C low-temperature fracture resistance and its production method, aiming to improve low-temperature toughness, especially -40°C low-temperature fracture toughness, without reducing the strength of the rail. It meets the requirement that the rail maintains stable strength and toughness under the low-temperature condition of -40°C in the natural environment, and solves the technical problem of reducing the brittleness of the rail under extremely cold conditions. At the same time, it greatly reduces the design cost of high-strength and tough rails serving in such a low-temperature and harsh environment.

[0005] To achieve the above objectives, the present invention adopts the following technical solutions:

[0006] A medium and low Cr high-strength steel rail resistant to low-temperature fracture at -40°C. The chemical components in the steel are as follows by weight percentage: C: 0.78% - 0.85%, Si: 0.50% - 0.60%, Mn: 0.80% - 0.90%, Cr: 0.16% - 0.28%, Ti: 0.010% - 0.025%, Sb: 0.005% - 0.015%, N: 40 - 50 ppm, P ≤ 0.015%, S ≤ 0.010%, and the balance is Fe and unavoidable impurities.

[0007] The fracture toughness index of the steel rail is: the fracture toughness index at -40°C is 36.8 - 38.2 MPa·m 0.5 .

[0008] The tensile strength (R m ) of the steel rail at room temperature is 1200 - 1240 MPa, the yield strength (Rp 0.2 ) is 800 - 820 MPa, and the elongation after fracture is 12.0% - 14.0%.

[0009] The hardness of the tread surface of the steel rail is 369 - 380 HB, and the hardness of the cross-section is 36.1 - 39.5 HRC.

[0010] The metallographic structure of the rail head consists of fine lamellar pearlite, and the lamellar spacing of the rail head is between 110 - 135 nm.

[0011] The selection of the addition amount (by weight percentage) of each of the above elements and its function description are as follows:

[0012] C is an element that improves the strength and hardness of the matrix. Its function in the present invention is to strengthen the matrix and ensure that the basic hardness of the steel rail meets the heat treatment requirements. In the present invention, when the C content is lower than 0.78%, the hardness of the steel rail cannot be guaranteed, and the wear resistance will be significantly reduced; when the C content is higher than 0.85%, the low-temperature toughness of the steel rail will be damaged, and the matching degree with the heat treatment process is relatively low. Therefore, the present invention selects the C content to be 0.78% - 0.85%.

[0013] Si is a solid-solution strengthening element. Its function in the present invention is to dissolve in ferrite during pouring and cooling and dissolve in pearlite during the rolling process, which can improve the hardness and strength of the steel rail. At the same time, it cooperates with the subsequent segmented cooling process to improve the hardenability of the steel rail and replace high-cost alloy elements, reducing the alloy cost. When the Si content is lower than 0.50%, the solid-solution strengthening effect is not obvious; when the Si content is higher than 0.60%, the toughness and plasticity of the steel rail will significantly decrease. Therefore, the present invention selects the Si content to be 0.50% - 0.60%.

[0014] Mn is a carbide-forming element that can increase the hardness of cementite. Mn is a matrix-strengthening element that can increase the hardenability and hardening capacity of the rail. In the present invention, when the manganese content is less than 0.80%, the matrix strength and elongation after fracture will be significantly reduced; when the manganese content is higher than 0.90%, the segregation of manganese elements will increase, which will affect the corresponding degree of uniformity. Therefore, the present invention selects the Mn content to be 0.80% - 90%.

[0015] Cr is a strengthening element. When the chromium content is less than 0.16%, the function of chromium cannot be effectively exerted; when the chromium content is higher than 0.28%, the strengthening effect of chromium reaches the upper limit, and further increase will significantly increase the cost. Therefore, the present invention selects the Cr content to be 0.16% - 0.28%.

[0016] Ti is a fine grain strengthening element that can inhibit the growth of austenite grains. In the present invention, it is combined with carbon and nitrogen to form titanium carbide and titanium nitride, which pin the austenite grain boundaries under high-temperature conditions, inhibit the growth of austenite, and ensure the performance of heat treatment. When the titanium content is less than 0.010%, the effects of fine grain strengthening and inhibiting the growth of austenite grains are not obvious; when the titanium content is higher than 0.025%, large carbide is easily formed in high-carbon steel, weakening the toughness of the material. Therefore, the present invention selects the titanium content to be 0.010% - 0.025%.

[0017] Sb is a corrosion-resistant element and also a sulfide modification element. In the present invention, the addition of antimony element is to change the morphology of manganese sulfide, refine the size of manganese sulfide, improve the strength and toughness, and at the same time improve the corrosion resistance. When the antimony element content is less than 0.005%, this effect is not obvious; when the antimony element content is higher than 0.015%, the toughness of the rail will be significantly reduced. Therefore, the present invention selects the Sb content to be 0.005% - 0.015%.

[0018] P is a harmful element in rail steel, which is likely to cause segregation and "cold brittleness". The lower the better under the premise of ensuring steelmaking conditions, steelmaking cost, etc. The present invention requires that the P content ≤ 0.015%.

[0019] S is a harmful element in rail steel and is the main forming element of type A inclusions. At the same time, it is easy to produce "hot brittleness" during rolling. Therefore, the lower the better under the premise of not increasing unnecessary costs. The present invention requires that S ≤ 0.010%.

[0020] N is a strengthening element, especially when combined with the microalloying element Ti to form titanium nitride, which improves the strength and toughness of the rail. When the nitrogen content is less than 40 ppm, the effect is not obvious; when the nitrogen content is higher than 50 ppm, the effect of titanium nitride will not be significantly improved, and further increase is likely to lead to an increase in microcracks in the steel billet. Therefore, the present invention selects the N content to be controlled at 40 - 50 ppm.

[0021] Based on the above composition design, to achieve the effects and purposes of the present invention and produce high-strength and tough steel rails with medium and low Cr resistance to fracture under -40°C severe cold conditions, it is necessary to cooperate with smelting processes, rolling processes, and on-line 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 strength and toughness of the steel rails under the conditions of the present invention. The designed steel rails are on-line heat-treated steel rails.

[0022] A production method of high-strength steel rails with medium and low Cr resistance to fracture at -40°C low temperature, including smelting, rolling, and on-line heat treatment, the specific method is as follows:

[0023] 1) Steel rail smelting, refining, vacuum degassing, and continuous casting processes: The molten iron for smelting adopts desulfurization pretreatment, uses a converter or an electric furnace for smelting, deoxidizes with ferrosilicon alloy, and adds 0.85 - 0.90 kg / t of calcium carbonate during the tapping process. The purpose is to promote the floating of slag, reduce the sulfur content in the steel, reduce the phosphorus content, and at the same time ensure that the furnace temperature is not lost and reduce the heating cost during the refining process. LF refining is adopted to further purify the molten steel and accurately control each alloy component to ensure that the alloy components meet the design requirements. Add 3.80 - 3.90 kg / t of dolomite during the tapping process and refine for 24 - 26 minutes. VD or RH vacuum degassing is used to ensure that hydrogen and oxygen are below 1.5 ppm and 18 ppm respectively, prevent hydrogen-induced cracks in the steel rails, and control the coarse and fine series grades of non-metallic inclusions below 1.5. The continuous casting billet is used for the billet, with a cross-sectional size of 280×380 mm, and the drawing speed is controlled at 0.50 - 0.60 m / min to ensure the surface quality of the 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 steel rails with medium and low Cr resistance to fracture at -40°C of the present invention can be obtained in cooperation with the subsequent processes.

[0024] 2) The rolling described above includes: The billet is descaled by high-pressure water, and the steel rail is rolled with three large deformations. The rolling deformation rate of rough rolling is 40% - 45% to refine austenite grains and increase the dislocation density; the rolling deformation rate of semi-finishing rolling of the steel rail is 60% - 65% of the rough-rolled billet to further refine austenite grains and form dislocation pile-ups; the rolling deformation rate of finish rolling of the steel rail is 50% - 55% of the semi-finished rolled billet. In this process, the alloy elements are effectively combined with the rolling process, giving play to the strengthening and toughening effects of the "C-Si-Mn-Cr" series of alloy elements and the strengthening effects of the C-Ti-N combination to precipitate titanium nitride and titanium carbide at high temperatures. Through the cooperation of alloy elements in this process, the grain boundaries are pinned by carbonitrides of titanium, making the supercooled austenite more stable and not easy to grow in the high-temperature transformation zone, laying a composition and rolling foundation for on-line segmented heat treatment.

[0025] 3) The online heat treatment described above includes: after the rail rolling is completed, taking advantage of the remaining rolling temperature to enter the online heat treatment unit. The cooling medium is air. The unit is divided into 6 sections, numbered from section 1 to section 6 in sequence. The temperature of the rail entering is controlled at 780 - 800 °C. Process limitations are set for each section of the unit: for the section 1 unit, the cooling rate of the rail head tread is 1.5 - 3.0 °C / s and it is maintained for 9 - 14 s, preferably 10 s; for the section 2 unit, the cooling rate of the rail head tread is 0.5 - 1.2 °C / s and it is maintained for 15 - 20 s, preferably 20 s; for the section 3 unit, the cooling rate of the rail head tread is 0.5 - 2.0 °C / s and it is maintained for 8 - 14 s, preferably 8 s; for the section 4 unit, the cooling rate of the rail head tread is 1.0 - 1.5 °C / s and it is maintained for 12 - 15 s, preferably 13 s; for the section 5 unit, the cooling rate of the rail head tread is 0.8 - 1.2 °C / s and it is maintained for 12 - 15 s, preferably 12 s; for the section 6 unit, the cooling rate of the rail head tread is 0.5 - 0.8 °C / s until the temperature of the rail leaving the unit is controlled at 530 - 550 °C, and finally the rail is air-cooled to room temperature. Through the combined action of segmented cooling and alloy, the rail matrix is strengthened, and the strength and toughness of the rail are improved. Without significantly increasing the alloy cost, the -40 °C fracture toughness value reaches 36.8 - 38.2 MPa·m 0.5 , meeting the requirement of maintaining high and stable strength and toughness of the rail under extremely cold (-40 °C) conditions.

[0026] To achieve the technical effects of the present invention, the present invention integrates production technologies such as alloy composition design, steelmaking, rolling, and online heat treatment. In particular, it gives full play to the roles of conventional elements such as carbon, silicon, manganese, and chromium in the rolling and heat treatment processes, and cooperates with the production process to give full play to the advantages of each process, reflecting the systematicness and innovation from design to production. According to the element characteristics, the rolling process and the segmented cooling process are designed, enabling the technical indicators of the rail to meet the design expectations and realizing mass production applications. The technical innovation advantages of the present invention are concentrated in the following aspects:

[0027] First, an alloy system of "C - Si - Mn - Cr" is designed, which meets the requirements of high strength and toughness of the rail in the extremely cold environment of -40 °C in heavy - haul railways. Without adding high - cost metal elements, it gives full play to the mechanism functions of conventional elements in the rolling stage and the heat treatment stage, ensuring the microstructure and properties of the rail, and laying a technical foundation for obtaining -40 °C anti - low - temperature fracture rails.

[0028] Second, the alloy system of "C - Si - Mn - Cr" is effectively coordinated with the rolling process, that is, according to the element mechanism functions, the rolling process of the present invention is designed. Through three large - deformation rollings, the dislocation density is increased and fine - grain strengthening is achieved; through the cooperation of silicon and chromium elements, solid - solution strengthening is completed, and the -40 °C fracture toughness index is improved.

[0029] Thirdly, the "C-Si-Mn-Cr" alloy system is effectively combined with the heat treatment process. That is, according to the elemental mechanism, the heat treatment process of the present invention is designed. By designing the "C-Si-Mn-Cr" alloy system and giving full play to its synergistic effect, the pearlite lamellae can be effectively refined, and the low-temperature toughness of the rail can be improved.

[0030] Fourthly, the combination of "C-Ti-N" obtains the strengthening effect of precipitating titanium nitride and titanium carbide at high temperature, realizes the pinning of austenite grain boundaries and the refinement of austenite, which not only improves the strength of the rail but also enhances its toughness.

[0031] Fifthly, the modification effect of Sb element on manganese sulfide. By adding an appropriate amount of Sb element, the morphology and size of sulfides are changed, the formation of long-sized manganese sulfide is prevented, and the strength and toughness of the rail are improved.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] 1) Fracture toughness index at -40°C: 36.8 - 38.2 MPa·m 0.5 。

[0034] Conditions for fatigue pre-crack: 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: -40°C. During testing, span S = 4W, displacement control, loading rate V = 1.0 mm / min.

[0035] 2) Tensile properties and hardness: Tensile strength (R m ) at room temperature is 1200 - 1240 MPa, yield strength (Rp 0.2 ) is 800 - 820 MPa, elongation after fracture is 12.0% - 14.0%, hardness of the tread surface is 369 - 380 HB, hardness of the cross-section is 36.1 - 39.5 HRC, and the hardness transition is uniform without abnormal steep high points.

[0036] 3) Microstructure composition: The metallographic structure of the rail head consists of fine lamellar pearlite, the lamellar spacing of the rail head is between 110 - 135 nm, and the structure is uniformly transitioned without containing other structures. Description of the Drawings

[0037] Figure 1 It is the microscopic SEM morphology diagram of the rail in Example 1. Detailed Embodiments

[0038] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further illustrates the specific implementation manners of the present invention in conjunction with embodiments. The following embodiments are used to specifically illustrate the content of the present invention. These embodiments are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0039] In the present invention, the Cr component is designed in the process of increasing carbon and reducing manganese, and is combined with "titanium carbonitride". Antimony modification treatment is carried out, and the cooling rate of stage-controlled heat treatment is combined to strengthen the material matrix, ensure the strength of the rail while improving the toughness, and achieve a fracture toughness value of 36.8 - 38.2 MPa·m at 40°C. 0.5 Thus, the rail can serve safely and stably under extremely cold conditions.

[0040] The production processes, tensile properties, hardness, -40°C fracture toughness, etc. corresponding to the chemical components of the embodiments of the present invention are specifically shown in Tables 1 to 4. The microscopic SEM morphology of the rail in Example 1 is shown in Figure 1 .

[0041] Table 1 Chemical Components of Embodiments

[0042]

[0043] Table 2 Key Parameters of Steelmaking Process Control in Embodiments

[0044]

[0045]

[0046] Table 3 Key Parameters of Rolling and Online Heat Treatment Process Control in Embodiments

[0047]

[0048] The preferred time is adopted for the segmented cooling time in the embodiment.

[0049] Table 4 Mechanical Properties, Hardness, -40°C Fracture Toughness of Embodiments

[0050]

Claims

1. A medium and low Cr high-strength steel rail resistant to low-temperature fracture at -40°C, characterized in that: The chemical components in the steel are by weight percentage: C: 0.78% - 0.85%, Si: 0.50% - 0.60%, Mn: 0.80% - 0.90%, Cr: 0.16% - 0.28%, Ti: 0.010% - 0.025%, Sb: 0.005% - 0.015%, N: 40 - 50 ppm, P ≤ 0.015%, S ≤ 0.010%, and the balance is Fe and inevitable impurities.

2. The medium and low Cr high-strength steel rail resistant to low-temperature fracture at -40°C according to claim 1, wherein: The rail fracture toughness index is as follows: the fracture toughness index at -40°C is 36.8 - 38.2 MPa·m 0.5 .

3. A medium and low Cr high-strength steel rail resistant to low-temperature fracture at -40°C according to claim 1, characterized in that: The tensile strength of the rail at room temperature is 1200 - 1240 MPa, the yield strength is 800 - 820 MPa, and the elongation after fracture is 12.0% - 14.0%.

4. A medium and low Cr high-strength steel rail resistant to low-temperature fracture at -40°C according to claim 1, characterized in that: The hardness of the rail tread is 369 - 380 HB, and the hardness of the cross-section is 36.1 - 39.5 HRC.

5. A medium and low Cr high-strength steel rail resistant to -40°C low-temperature fracture according to claim 1, characterized in that: The metallographic structure of the rail head consists of fine lamellar pearlite, and the pearlite lamellar spacing in the rail head is between 110 - 135 nm.

6. A production method of a medium and low Cr high-strength rail resistant to -40°C low-temperature fracture as described in any one of claims 1 - 5, including smelting, rolling, and online heat treatment, characterized in that: The rolling includes: the rolling reduction rate of rough rolling is 40% - 45%, the rolling reduction rate of semi-finish rolling of the rail is 60% - 65% of the rough rolling billet, and the rolling reduction rate of finish rolling of the rail is 50% - 55% of the semi-finish rolling billet; The online heat treatment includes: after the rail rolling is completed, it enters the online heat treatment unit, the cooling medium is air, the temperature when the rail enters is controlled at 780 - 800°C; the temperature when the rail exits the unit is controlled at 530 - 550°C.

7. The production method of a medium and low Cr high-strength steel rail resistant to -40°C low-temperature fracture according to claim 6, characterized in that: In the smelting process, desulfurization pretreatment is adopted for the molten iron, and 0.85 - 0.90 kg / t of calcium carbonate is added during the tapping process.

8. The production method of a medium and low Cr high-strength steel rail resistant to -40 °C low-temperature fracture according to claim 6, characterized in that: In the smelting process, LF refining is adopted, 3.80 - 3.90 kg / t of dolomite is added during the tapping process, refined for 24 - 26 min, and VD or RH vacuum degassing is carried out.

9. The production method of a medium and low Cr high-strength steel rail resistant to -40 °C low-temperature fracture according to claim 6, characterized in that: In the smelting process, continuous casting billets are used for the cast billets, and the drawing speed is controlled at 0.50 - 0.60 m / min.

10. The production method of a medium and low Cr high-strength steel rail resistant to low-temperature fracture at -40°C according to claim 6, characterized in that: In the online heat treatment process, the unit is divided into 6 sections, numbered from section 1 to section 6 in sequence. The cooling rate of the rail head tread of section 1 unit is 1.5 - 3.0°C / s and is maintained for 9 - 14 s; the cooling rate of the rail head tread of section 2 unit is 0.5 - 1.2°C / s and is maintained for 15 - 20 s; the cooling rate of the rail head tread of section 3 unit is 0.5 - 2.0°C / s and is maintained for 8 - 14 s; the cooling rate of the rail head tread of section 4 unit is 1.0 - 1.5°C / s and is maintained for 12 - 15 s, the cooling rate of the rail head tread of section 5 unit is 0.8 - 1.2°C / s and is maintained for 12 - 15 s, and the cooling rate of the rail head tread of section 6 unit is 0.5 - 0.8°C / s.

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