Low-Cr alloyed high-strength hard steel rail and production method thereof

Abstract

The invention relates to a low-Cr alloying high-strength hard steel rail and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.60%-0.80% of C, 0.05%-0.58% of Si, 0.60%-1.15% of Mn, 0.01%-0.25% of Cr, 0.020%-0.100% of Ni, 0.030%-0.150% of Cu, 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 low-Cr alloyed high-strength hard steel rail is produced through low-Cr alloy component design, pure steelmaking and fine rolling in cooperation with a heat treatment process. The index requirements of passenger and freight mixed transportation lines on the steel rail are met, the service life of the steel rail is prolonged, and the operation and maintenance cost of the railway is saved.

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 low-Cr alloyed high-strength and high-hardness rail for the laying of mixed passenger and freight transportation or heavy-haul railways and its production method. Background Art

[0002] The railway lines in countries such as Southeast Asia, South Asia, West Asia and Africa mainly consist of mixed passenger and freight dedicated lines. In the past, the rails used were mainly hot-rolled rails. In recent years, with the improvement of railway construction levels, in order to improve the service life of rails, reduce the maintenance and replacement costs of rails, and improve the operation efficiency, more and more heat-treated rails with higher strength and hardness, better strength and toughness matching, and better wear resistance are selected as the laying rails, and most of them are obtained by purchasing and importing from countries with rail production capabilities. In order to meet the needs of users and increase the export share of the enterprise's rails, the research team has carried out the development work of low-Cr alloyed high-strength rails, using low chromium and other elements in combination for alloy strengthening, and accelerating cooling through an on-line heat treatment unit to further improve the strength and hardness of the rails, forming a production technology for low-Cr alloyed high-strength rails, and realizing mass production and stable supply.

[0003] At the same time, it is found in the research that the production technology of the present invention makes up for some deficiencies in the related publicly disclosed technologies and shows a certain degree of innovation. For example, CN104195433B discloses "a high-strength and tough pearlitic rail and its production method", in which the tensile strength of the rail does not exceed 1120 MPa, which cannot play a good wear-resistant role for laying on mixed passenger and freight lines, and does not contain the chromium alloying effect. CN104561816B discloses "a rail with excellent high-strength and fatigue-resistant performance and its production method", which shows that the tensile strength of the rail involved is between 1260 MPa and 1420 MPa. The rail shows relatively high strength, but generally, the strength of the rail laid on mixed passenger and freight lines is preferably between 1200 and 1350. Excessive strength will pose certain potential hazards to passenger transportation safety, and at the same time, it is easy to lead to unreasonable resource matching and increase the design cost.

[0004] CN110592496B discloses "a pearlitic rail steel and its preparation method", and CN112501512A discloses "a controlled rolling and controlled cooling high-strength pearlitic rail and its production method". The rails involved show relatively high strength. The silicon content of the former even reaches 1%, which will significantly reduce the toughness and plasticity of the 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 is more suitable for heavy-haul railways. CN107739806A discloses "a hypereutectoid rail with high toughness and plasticity and its manufacturing method", and the rail involved is a hypereutectoid rail for heavy-haul railways. Summary of the Invention

[0005] The present invention develops a low-Cr alloyed high-strength and high-hardness rail and its production method through the combination of alloy strengthening and heat treatment strengthening. Through the design of low-Cr alloy composition, after pure and refined steelmaking and precision rolling, combined with the heat treatment process, a low-Cr alloyed high-strength and high-hardness rail is produced. The strength, toughness and wear resistance of the rail are improved, meeting the requirements of the rail indexes for the passenger and freight mixed transportation line, increasing the service life of the rail and saving the railway operation and maintenance costs.

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

[0007] A low-Cr alloyed high-strength and high-hardness rail, the chemical components in the steel are calculated by weight percentage as follows: C: 0.60% - 0.80%, Si: 0.05% - 0.58%, Mn: 0.60% - 1.15%, Cr: 0.01% - 0.25%, Ni: 0.020% - 0.100%, Cu: 0.030% - 0.150%, P: ≤0.015%, S: ≤0.010%, and the balance is Fe and unavoidable impurities.

[0008] The selection of the addition amount (weight percentage) of the above elements and their action explanations are as follows:

[0009] C is the basic element to improve the matrix hardness in the steel. Its function in the present invention is to strengthen the matrix and ensure that the basic hardness of the rail meets the heat treatment requirements. In the present invention, when the C content is lower than 0.60%, even with the intervention of heat treatment, it is impossible to ensure that the hardness of the rail meets the use requirements of the passenger and freight mixed transportation line in the export country, and the wear resistance and service life are significantly reduced; when the C content is higher than 0.80%, it will seriously weaken the strengthening effect of the Cr element, increase the risk of brittle fracture of the rail, and form abnormal retained austenite structure during the accelerated cooling process. Therefore, the present invention selects the C content to be 0.60% - 0.80%.

[0010] 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 rolling, 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 0.05%, the solid solution strengthening effect is not obvious; when the Si content is higher than 0.58%, the toughness and plasticity of the rail decrease significantly, and the welding performance is seriously damaged. Therefore, the present invention selects the Si content to be 0.05% - 0.58%.

[0011] Mn is a carbide-forming element. Its function in the present invention is to increase the hardness of cementite and promote the precipitation strengthening of copper. In the present invention, when the manganese content is lower than 0.60%, the above functions cannot be exerted; when the manganese content is higher than 1.15%, it will increase the positive segregation degree of manganese and carbon, and it is easy to generate abnormal martensite structure. Therefore, the present invention selects the Mn content to be 0.60% - 1.15%.

[0012] Cr is an element that improves the hardness of the matrix. Its function in the present invention is to increase the hardenability and hardening ability of the rail during the heat treatment process, and improve the wear resistance of the rail. At the same time, combined with the carbon, silicon and manganese in the present invention, it can improve the strength of the rail steel, delay the pearlite transformation time, making the rail easy to be heat-treated, saving energy and reducing costs. When the chromium content is lower than 0.01%, the above properties are significantly inhibited; when the chromium content is higher than 0.25%, the elongation after fracture of the rail will be significantly reduced. Therefore, the present invention selects the Cr content to be 0.01% - 0.25%.

[0013] 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%.

[0014] 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%.

[0015] The functions of Cu and Ni elements in the present invention are to alleviate the problems of reduced toughness and plasticity brought by the Cr element. By adding a small amount of copper and nickel elements in cooperation, the concentration of eutectoid carbon can be maintained at a relatively low level, while playing a good solid solution strengthening role, improving the toughness and plasticity of the rail. Combined with the on-line heat treatment process, it can increase the elongation after fracture by about 2% on the basis of the carbon, silicon, manganese and chromium components, and at the same time increase the yield strength (Rp0.2) by about 50 MPa without reducing the tensile strength. In order to obtain the above-mentioned invention effects for the rail of the present invention, the Ni content is controlled at 0.020% - 0.100%, and the Cu content is controlled at 0.030% - 0.150%.

[0016] The fracture toughness index of the said rail at -20 °C is: up to 36.0 MPa·m 0.5 , and the average value reaches 34.98 MPa·m 0.5 .

[0017] The room temperature tensile strength of the said rail is 1230 - 1296 MPa, the elongation after fracture is 11.0% - 15.0%, the hardness of the tread is 356 - 378 HBW, the hardness of the cross-section is 351 - 366 HBW, the hardness transition is uniform, and there is no abnormal steep high point.

[0018] The metallographic structure of the rail head of the said rail is composed of pearlite and a small amount of ferrite. The proportion of pearlite exceeds 95%. The pearlite lamellar spacing on the surface of the rail head is between 100 - 110 nm, and the pearlite lamellar spacing in the intermediate transition region is between 110 - 120 nm. The structure is uniformly transitional and does not contain other structures.

[0019] Friction and wear and contact fatigue tests of the rail carried out on the GPM-30 rolling contact fatigue test rig: After 250,000 total counter-grinding tests, the rail wear was 0.8436 - 0.8898 g, and the number of spalled chunks was 0.

[0020] Based on the above composition design, in order to achieve the effects and purposes of the present invention and produce a low-Cr alloyed high-strength and high-hardness rail, 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 hardness of the rail under the conditions of the present invention. The designed rail is an on-line heat-treated rail. The specific process and technological innovation features are as follows:

[0021] A production method of a low-Cr alloyed high-strength and high-hardness rail, comprising the following method steps:

[0022] 1) Rail smelting, refining, vacuum degassing, and continuous casting processes: The molten iron for smelting is subjected to desulfurization pretreatment, smelted using a converter or an electric furnace, deoxidized with ferrosilicon aluminum, and 0.20 - 0.30 kg / ton of steel of calcium carbonate is added during the tapping process. The purpose is to promote the floating of CaS and reduce the sulfide content in the steel, while ensuring that the furnace temperature is not lost and reducing the heating cost during the refining process. LF refining is to further purify the molten steel, precisely control each alloy component, ensure that the alloy components meet the design requirements, deoxidize with ferrosilicon aluminum, add 5.00 - 6.00 kg / ton of steel of calcium carbonate during the tapping process, refine for 40 - 45 minutes, ensure the full floating of the slag, achieve the purity of the molten steel, and effectively control non-metallic inclusions. VD or RH vacuum degassing is used to ensure that gases such as hydrogen and oxygen are below 2 ppm and 20 ppm respectively, prevent hydrogen-induced cracks in the rail, and control the B-type inclusion grade below 1.0. The continuous casting billet is used, with a cross-sectional size not less than 280*410 mm, and the casting speed is controlled at 0.4 - 0.7 m / min to ensure the surface quality of the casting billet without cracks. Only through the above treatment methods can the beneficial effects of the present invention be achieved, and the high-strength and high-hardness rail of the present invention can be obtained in cooperation with subsequent processes.

[0023] 2) Rail rolling: The billet is descaled by high-pressure water and rolled into rails using a three-stand, five-stand or seven-stand rolling mill. For the first heavy reduction rolling of the rails: The rolling temperature is 1080 - 1130 °C, a large number of dislocations are formed, and the austenite grains are initially fragmented; for the second heavy reduction rolling: The rolling temperature is 1030 - 1070 °C, the austenite grains are refined, and the dislocation density is increased; for the third heavy reduction rolling: The rolling temperature is 985 - 1025 °C, the newly grown austenite is fragmented, and at the same time, copper and nickel elements promote the nucleation and growth of the newly fragmented austenite. Blocked by the accumulation of dislocations, the grains do not grow significantly, effectively refining the austenite grains. In this process, Cr plays a role, forming alloy cementite (Fe·Cr)3C with Fe, further enhancing the matrix hardness, wear resistance, and stability, laying a foundation for further strengthening by subsequent on-line heat treatment;

[0024] 3) On-line heat treatment of rails: After the rail rolling is completed, it enters the on-line heat treatment unit using the rolling residual temperature, and the cooling medium is air. The on-line heat treatment unit compresses the air to complete the under-speed on-line. The unit is divided into 6 sections, numbered from section 1 to section 6 in sequence. In order to ensure that the temperature of the rails entering the unit meets the under-speed quenching conditions, the present invention adds carbon, manganese, and nickel elements and makes them play a synergistic role, expanding the austenite region and reducing the eutectoid temperature, thereby reducing the starting temperature of on-line heat treatment, that is, reducing the tapping temperature of the previous process. The temperature of the rails entering the unit is controlled at 650 - 820 °C, effectively saving energy and reducing production costs. In the above heat treatment process, the process is specified for each section of the unit: The cooling rate of the rail head tread in section 1 of the unit is 2.0 - 3.0 °C / s; the cooling rate of the rail head tread in section 2 of the unit is 1.5 - 3.0 °C / s; the cooling rate of the rail head tread in section 3 of the unit is 1.5 - 2.5 °C / s; the cooling rate of the rail head tread in section 4 of the unit is 1.5 - 2.0 °C / s; the cooling rate of the rail head tread in section 5 of the unit is 1.0 - 2.0 °C / s; the cooling rate of the rail head tread in section 6 of the unit is 1.0 - 1.5 °C / s. The temperature of the rails leaving the unit is controlled at 450 - 550 °C. During this cooling process, nickel and copper elements cooperate with each other to play a precipitation strengthening role, and at the same time, promote the right shift of the C curve position, reduce the critical cooling rate, improve the hardenability of the steel, and ensure the high strength and toughness of the final rails; chromium elements play a role in strong hardenability and hardenability. Finally, the rails are cooled to room temperature.

[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 positive role of conventional elements such as carbon, manganese, chromium, nickel, and copper in the rolling and heat treatment processes, and cooperates with the production process, complementing each other. Beneficial effects are obtained while saving alloy costs, 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 rails to meet the design expectations and realizing mass production applications. The technical innovation advantages of the present invention are concentrated in the following aspects:

[0026] First, it meets the requirements of high-strength, tough and wear-resistant steel rails for passenger and freight mixed transportation lines in Southeast Asia, South Asia, West Asia, Africa and other countries. By means of technical design, the unusual functions of conventional elements are brought into play. Especially without adding a large amount of micro-alloying elements and precious metal elements, through the mutual cooperation of carbon, manganese, chromium, nickel and copper elements, their respective mechanism functions in the rolling stage and heat treatment stage are exerted, ensuring the microstructure and properties of the steel rails, laying a compositional foundation for obtaining higher strength, toughness, plasticity and hardness, and reducing the alloy cost.

[0027] Second, the chromium, copper and nickel elements are effectively combined with the rolling process, that is, according to the element mechanism function, the rolling process of the present invention is designed. Through three large-deformation rollings, the initial fragmentation of austenite is realized, the dislocation density is increased, and the newly grown austenite is fragmented again, effectively refining the austenite grains and laying a foundation for fine-grain strengthening. At the same time, the copper and nickel elements promote the nucleation and growth of the newly fragmented austenite. Blocked by the accumulation of dislocations, the grains will not grow significantly, effectively refining the austenite grains; Cr plays the role of forming alloy cementite (Fe·Cr)3C with Fe, further enhancing the matrix hardness, wear resistance and stability.

[0028] Third, the carbon, manganese, nickel, chromium and copper elements are effectively combined with the heat treatment process. That is, according to the element mechanism function, the heat treatment process of the present invention is designed. By adding carbon, manganese and nickel elements and exerting their synergistic effects, the austenite region is expanded and the eutectoid temperature is reduced, thereby reducing the starting temperature of on-line heat treatment, that is, reducing the tapping temperature of the previous process, and controlling the rail entering temperature at 650-820°C; during the cooling process, the nickel and copper elements cooperate with each other to exert the precipitation strengthening effect, and at the same time promote the right shift of the C curve position and the reduction of the critical cooling rate, improving the hardenability of the steel and ensuring the high strength and toughness of the final rail; the chromium element plays a role in strong hardenability and hardenability, and controls the rail exit unit temperature at 450-550°C.

[0029] Fourth, the composition design is effectively combined with the on-line heat treatment. The combined strengthening effect of alloy elements and heat treatment process is effectively exerted, reducing the alloy cost and thermal energy consumption.

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] 1) The fracture toughness index of the steel rail of the present invention is: up to 36.00 MPa·m 0.5 , with an average value of 34.98 MPa·m 0.5 .

[0032] 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 , Ambient temperature: 21 - 23 °C. During the test, the span S = 4W, displacement control, loading rate V = 1.0 mm / min.

[0033] 2) Tensile properties and hardness of the steel rail of the present invention: Tensile strength at room temperature is 1230 - 1296 MPa, elongation after fracture is 11.0 - 15.0%, hardness of the tread surface is 356 - 378 HBW, hardness of the cross-section is 351 - 366 HBW, and the hardness transition is uniform without abnormal steep high points.

[0034] 3) Microstructure composition of the steel rail of the present invention: The microstructure of the rail head consists of pearlite and a small amount of ferrite, and the proportion of pearlite exceeds 95%. The pearlite lamellar spacing on the rail head surface is between 100 - 110 nm, and the pearlite lamellar spacing in the intermediate transition region is between 110 - 120 nm. The microstructure is uniformly transitional and does not contain other microstructures.

[0035] 4) Friction and wear and rolling contact fatigue tests of the steel rail of the present invention carried out on the GPM - 30 rolling contact fatigue test bench: After 250,000 total counter - grinding tests, the wear of the steel rail of the present invention is 0.8436 - 0.8898 g, and the number of spalled and chipped pieces is 0, showing excellent hardness and toughness.

[0036] The experimental conditions are as follows: The specimen is an annular specimen with a thickness of 10 mm, an outer diameter of 60 mm, and an inner diameter of 30 mm, as shown in Figure 2 , where the friction and wear specimen has a smooth surface, and the rolling contact fatigue specimen has a 5 - mm - high groove in the center of the surface; test load: 1000 KN; slip: 5%; material of the counter - grinding specimen: wheel steel with a hardness of 300 - 310 HBW; rotation rate: 220 revolutions per minute; total number of wear times: 250,000 times. The fatigue crack growth rate test is carried out in accordance with TB / T2344 - 2012. Description of the Drawings

[0037] Figure 1 It is the metallographic structure diagram of Example 1.

[0038] Figure 2 It is the structure diagram of the friction and wear and rolling contact fatigue specimen of the present invention. Detailed Description of the Invention

[0039] In order to make the purpose, technical solutions and advantages of the present invention clearer, the following further explains the specific implementation manners of the present invention in combination with 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.

[0040] 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, fracture toughness, and wear of the examples. The metallographic structure of Example 1 is shown in Figure 1 .

[0041] Table 1 Chemical Compositions of the Steels in the Examples

[0042]

[0043]

[0044] Table 2 Key Control Parameters for the Steelmaking Process of the Examples

[0045]

[0046]

[0047] Table 3 Key Control Parameters for the Rolling and On-line Heat Treatment Processes of the Examples

[0048]

[0049]

[0050] Table 4 Mechanical Properties, Hardness, Fracture Toughness, and Wear of the Steels in the Examples

[0051]

Claims

1. A high-strength and high-hardness rail with low Cr alloying, characterized in that, The chemical components in the steel are by weight percentage: C: 0.60% - 0.80%, Si: 0.05% - 0.58%, Mn: 0.60% - 1.15%, Cr: 0.01% - 0.25%, Ni: 0.020% - 0.100%, Cu: 0.030% - 0.150%, P: ≤0.015%, S: ≤0.010%, and the balance is Fe and inevitable impurities.

2. The low-Cr alloyed high-strength and high-hardness steel rail according to claim 1, wherein The fracture toughness index of the rail at -20°C is: up to 36.00 MPa·m 0.5 , and the average value reaches 34.98 MPa·m 0.5 .

3. A low-Cr alloyed high-strength and high-hardness steel rail according to claim 1, characterized in that, The room temperature tensile strength of the rail is 1230 - 1296 MPa, the elongation after fracture is 11.0% - 15.0%, the hardness of the tread is 356 - 378 HBW, and the hardness of the cross-section is 351 - 366 HBW.

4. A low-Cr alloyed high-strength and high-hardness steel rail according to claim 1, characterized in that, The metallographic structure of the rail head consists of pearlite and a small amount of ferrite, and the pearlite lamellar spacing on the surface of the rail head is between 100 - 110 nm.

5. A low-Cr alloyed high-strength and high-hardness steel rail according to claim 1, characterized in that, The friction and wear and contact fatigue test of the rail was carried out on the GPM-30 rolling contact fatigue test bench: After 250,000 total counter-grinding tests, the wear of the rail was 0.8436 - 0.8898 g, and the number of peeled-off blocks was 0.

6. A production method of a low-Cr alloyed high-strength and high-hardness steel rail as described in any one of claims 1 to 5, characterized in that, It includes the following method steps: 1) Rail rolling: The first rolling temperature of the rail is 1080 - 1130 °C; the second rolling temperature is 1030 - 1070 °C; the third rolling temperature is 985 - 1025 °C; 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 rolling residual temperature, the cooling medium is air, the on-line heat treatment unit compresses the air to complete the on-line under-speed; the temperature of the rail entering the unit is controlled at 650 - 820 °C, and the temperature of the rail leaving the unit is controlled at 450 - 550 °C.

7. The production method of a low-Cr alloyed high-strength and high-hardness steel rail according to claim 6, characterized in that, When smelting hot metal, 0.20 - 0.30 kg of calcium carbonate is added per ton of steel during the tapping process.

8. The production method of a low-Cr alloyed high-strength and high-hardness steel rail according to claim 6, characterized in that, LF refining is adopted, 5.00 - 6.00 kg of calcium carbonate is added per ton of steel during the tapping process, refined for 40 - 45 minutes, and VD or RH vacuum degassing is adopted.

9. The production method of a low-Cr alloyed high-strength and high-hardness steel rail according to claim 6, characterized in that, The continuous casting billet is used for the casting billet, and the drawing speed is controlled at 0.4 - 0.7 m / min.

10. The production method of a low-Cr alloyed high-strength and high-hardness steel rail according to claim 6, characterized in that, In the on-line heat treatment process of the rail, the unit is divided into 6 sections, numbered 1 section to 6 sections in sequence. The cooling rate of the 1st section unit for the tread of the rail head is 2.0 - 3.0 °C / s; the cooling rate of the 2nd section unit for the tread of the rail head is 1.5 - 3.0 °C / s; the cooling rate of the 3rd section unit for the tread of the rail head is 1.5 - 2.5 °C / s; the cooling rate of the 4th section unit for the tread of the rail head is 1.5 - 2.0 °C / s; the cooling rate of the 5th section unit for the tread of the rail head is 1.0 - 2.0 °C / s; the cooling rate of the 6th section unit for the tread of the rail head is 1.0 - 1.5 °C / s.

Images