A low cost method for purifying and modifying inclusions in steel rails and improving their overall mechanical properties

Abstract

The application discloses a low-cost method for purifying and modifying inclusions of a steel rail and improving comprehensive mechanical properties of the steel rail, and the method comprises adding a rare earth iron alloy with a rare earth content of 30% into molten steel in VD, and performing O determination in the steel before the adding, and the O is required to be less than or equal to 10 ppm, so that the prepared chemical composition of the molten steel for continuous casting is as follows in percentage by mass: C 0.78-0.85%; Si 0.50-0.60%; Mn 1.10-1.30%; P less than or equal to 0.020%; S less than or equal to 0.008%; Cr 0.20-0.30%, RE iron alloy 0.003-0.008% or RE 0.0010-0.0024%; and the rest is Fe and inevitable impurities. The method can obviously reduce A-D type inclusions in the steel, fully plays a purifying and modifying role of the rare earth on the inclusions of the steel, and improves service life of the steel rail.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials, specifically relating to a low-cost method for purifying and modifying inclusions in steel rails and improving their comprehensive mechanical properties. Background Technology

[0002] With the economic development of overseas countries and the continuous increase in transportation routes, the demand for the quality and performance of railway rails is also constantly increasing. Rails used for railway transportation must possess good strength, toughness, cross-sectional hardness, and mechanical properties such as wear and corrosion resistance. Furthermore, they need excellent service performance. Therefore, designing a reasonable chemical composition and fully utilizing the characteristics of the rolling process, based on the chemical composition and production process requirements of material standards, to maximize the strength and toughness of the rails while improving internal metallurgical quality, is of great production and economic significance for extending the service life of the rails. Summary of the Invention

[0003] This invention aims to provide a low-cost method for purifying and modifying inclusions in steel rails and improving their comprehensive mechanical properties. To achieve this goal, this invention utilizes Baogang's own ore and rare earth alloys, rationally designs the composition and adds optimal Si and Mn alloying elements to the rails, and optimizes production process parameters. Based on this, the produced rails meet the standards for strength, elongation, tread hardness, and other properties. Furthermore, the use of rare earth elements during the production process results in rails with excellent cleanliness and toughness, high production efficiency, energy saving and environmental protection, low cost, and good economic benefits. It is suitable for large-scale production and has good promotional value.

[0004] The objective of this invention is achieved through the following measures:

[0005] This invention provides a low-cost method for purifying and modifying inclusions in steel rails and improving their comprehensive mechanical properties. The method includes a steel rail smelting process and a steel rail rolling process. The steel rail smelting process is: molten iron → converter smelting → LF refining → VD → continuous casting. The steel rail rolling process is: billet → sawing → heating → BD1 rolling → BD2 rolling → CCS universal mill continuous rolling → online residual heat quenching → sawing → cooling → head and tail trimming → straightening.

[0006] Rare earth ferroalloys with a rare earth content of 30% are added to the molten steel in the VD process, with an addition amount of 0.003-0.008%. Before addition, the oxygen content in the steel is measured, and the requirement is that O ≤ 10 ppm. The chemical composition of the molten steel prepared for continuous casting is as follows, by mass percentage: C 0.78-0.85%; Si 0.50-0.60%; Mn 1.10-1.30%; P ≤ 0.020%; S ≤ 0.008%; Cr 0.20-0.30%; RE ferroalloy 0.003-0.008% or RE 0.0010-0.0024%; the remainder is Fe and unavoidable impurities.

[0007] In some embodiments, the chemical composition of the molten steel prepared for continuous casting is as follows, by mass percentage: C 0.80–0.82%; Si 0.55–0.57%; Mn 1.17–1.22%; P ≤0.015%; S ≤0.005%; Cr 0.23–0.25%; RE 0.0010–0.0013%; the remainder being Fe and unavoidable impurities.

[0008] In some embodiments, in the smelting and production process of the rails, the converter smelting adopts aluminum-free deoxidation alloying, and the whole process is carried out by normal argon blowing during refining; the vacuum degree is ≤0.10KPa, the deep vacuum time is ≥20min, and the superheat ΔT is ≤28℃.

[0009] In some embodiments, in the rolling production process of the rail, the temperature of the preheating section of the billet is controlled to be no greater than 890°C; the heating time is no less than 2 hours and 45 minutes; the furnace exit temperature is no less than 1160°C; the initial rolling temperature is ≥1120°C; and the final rolling temperature is 820-850°C. After final rolling, the rails are cooled on the cooling bed with a rail spacing of ≥250mm, and then straightened after natural cooling to below 80°C.

[0010] In some embodiments, the rails produced by the method have a Class A inclusion content ≤1.0, a Class B inclusion content ≤1.0, a Class C inclusion content ≤1.0, and a Class D inclusion content ≤1.0.

[0011] In some embodiments, the mechanical properties of the rails produced by the method meet the following requirements: yield strength ≥ 580 MPa, tensile strength ≥ 1080 MPa, elongation ≥ 10%, and Brinell hardness ≥ 315 HB.

[0012] In another aspect, the present invention provides a rail produced by the method described above. Detailed Implementation

[0013] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0014] This invention utilizes Baotou Steel's own ore and rare earth alloys. Through rational design of the composition and the addition of optimal Si and Mn alloying elements to the rails, and optimization of production process parameters, the produced rails meet standards in terms of strength, elongation, and tread hardness. Furthermore, the use of rare earth elements in the production process results in rails with excellent cleanliness and toughness. The production process is highly efficient, energy-saving, environmentally friendly, low-cost, and economically beneficial, making it suitable for large-scale production. The main alloying elements Si, Mn, and Cr added to the rail material provided by this invention aim to improve the rail's strength and ensure a suitable heat treatment line for the phase transformation temperature. The C content in this steel is designed to have a high solid solubility with Fe, providing solid solution strengthening and improving the steel's strength and hardness. However, increasing the C content will reduce the rail's toughness; therefore, the C content in this invention also considers elongation. Mn expands the austenite phase region, increases the stability of the supercooled austenite in the steel, and significantly improves the steel's hardenability. Si (Si) slows the austenite-ferrite transformation rate, improving the steel's strength, elasticity, and tempering resistance. However, considering the weldability of the rails, the Si content should not be too high because Si has poor thermal conductivity, which is detrimental to rail welding. Furthermore, the added RE element is a rare earth alloy. Basic theoretical research on rare earths has proven that the presence of rare earths in steel controls the morphology of sulfides during smelting, forming rare earth sulfur oxides and reducing the formation of other metallic sulfur compounds. These new sulfur compounds do not deform or deform very little during rolling, resulting in grain refinement in the steel. In this invention, the rare earth alloy is added at the VD (Vacuum-Drying) station. Before addition, the O content in the steel is measured, requiring O ≤ 10 ppm. This allows the rare earth to fully exert its grain-refining effect in the steel, further significantly reducing AD-type inclusions, fully utilizing the purification and modification effects of rare earth on steel inclusions, and improving the service life of the rails.

[0015] Example

[0016] The method provided by this invention includes the following processes:

[0017] The smelting process for steel rails is as follows: molten iron → converter smelting → LF refining → VD → continuous casting. Converter smelting employs aluminum-free deoxidation and alloying, with the entire process following normal argon blowing during refining; vacuum degree ≤ 0.10 kPa, deep vacuum time ≥ 20 min, superheat ΔT ≤ 28℃. Rare earth alloys are added during the VD vacuum degassing process, with O content measured in the steel before addition, requiring O ≤ 10 ppm (other process parameters can be performed according to conventional steelmaking methods (e.g., see: Wang Guoxin et al., Research on the Application of Dynamic Light Reduction Technology for Continuous Casting of Steel for Baogang 350 km / h High-Speed ​​Rails, Continuous Casting, 2011.09: 434-440; Zhao Aijun et al., Production Process and Quality Control of Steel Rails for Baogang, Baogang Science and Technology, 2000.09, Vol. 26, No. 3: 6-12, etc.). The chemical composition of the molten steel used for continuous casting in each example is shown in Table 1.

[0018] Table 1: Components of each example (mass percentage / %)

[0019]

[0020] The rolling process of steel rails is as follows: billet → sawing → heating → BD1 rolling → BD2 rolling → CCS universal mill continuous rolling → online residual heat quenching → sawing → cooling → head and tail trimming → straightening → inspection → packaging → weighing → warehousing. The billet preheating temperature is no higher than 890℃; the heating time is no less than 2 hours and 45 minutes; the furnace exit temperature is no lower than 1160℃, the initial rolling temperature is ≥1120℃, and the final rolling temperature is 820~850℃; after final rolling, the rails are cooled on the cooling bed with a rail spacing of ≥250mm, and straightened after natural cooling to below 80℃ (other operating processes can be carried out according to conventional rolling methods (for example, see: Wang Guoxin et al., Research on the Application of Dynamic Light Pressure Reduction Technology for Continuous Casting of 350km / h High-Speed ​​Rail Steel of Baogang, Continuous Casting, 2011.09: 434-440; Zhao Aijun et al., Production Process and Quality Control of Rails of Baogang, Baogang Science and Technology, 2000.09, Vol.26, No.3: 6-12, etc.)); compared with the existing rolling process, the final rolling temperature is reduced, and rare earth elements are used to refine the steel grains; the cooling interval after rolling is increased, which enhances the temperature drop rate of the rails after rolling and refines the internal microstructure.

[0021] Performance of rolled rail specimens: Tensile specimens were manufactured according to the American standard AREMA, with a diameter d0 = 12.5 mm and a gauge length Lo = 5 do. Tread hardness was measured by randomly sampling the rail, with a specimen length of 250 mm and a 0.5 mm reduction on the top surface of the rail head. Five test points were used for Brinell hardness testing, and the average value was calculated. The test temperature was 20℃ ± 5℃. The experimental results are shown in Table 2. Inclusions were tested according to method A, and the results are shown in Table 3. In Tables 2 and 3, the average value without rare earth elements refers to the average value obtained in Examples 1-3 without the addition of rare earth elements.

[0022] Table 2: Mechanical properties of each example

[0023]

[0024] Table 3: Inclusions in each example

[0025]

[0026] As can be seen from Tables 2 and 3, each example has good strength, hardness and mechanical properties. The rails produced from them meet the technical requirements and the yield strength is improved, which plays an important role in reducing and minimizing the formation of early peeling cracks. In addition, the AD type inclusions in the steel are significantly reduced, which fully demonstrates the purification and modification effect of rare earth on steel inclusions and improves the service life of the rails.

[0027] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-cost method for purifying and modifying inclusions in rails and improving their overall mechanical properties, the method comprising: The smelting and rolling processes for steel rails, wherein the smelting process is: molten iron → converter smelting → LF refining → VD → continuous casting; and the rolling process is: billet → sawing → heating → BD1 rolling → BD2 rolling → CCS universal mill continuous rolling → online residual heat quenching → sawing → cooling → head and tail trimming → straightening; characterized in that: Rare earth ferroalloys with a rare earth content of 30% are added to the molten steel in the VD process, with an addition amount of 0.003-0.008%. Before addition, the oxygen content in the steel is measured, and the requirement is that O ≤ 10 ppm. The chemical composition of the molten steel prepared for continuous casting is as follows, by mass percentage: C 0.80-0.82%; Si 0.55-0.57%; Mn 1.17-1.22%; P ≤ 0.015%; S ≤ 0.005%; Cr 0.23-0.25%; RE 0.0010-0.0013%; the remainder being Fe and unavoidable impurities. Furthermore, in the rolling production process of the rails, the temperature of the preheating section of the billet is controlled to be no greater than 890℃; the heating time is no less than 2 hours and 45 minutes; the furnace exit temperature is no less than 1160℃; the initial rolling temperature is ≥1120℃; and the final rolling temperature is 820~850℃. After final rolling, the rails are cooled on the cooling bed with a rail spacing of ≥250 mm, and are straightened after natural cooling to below 80℃. The obtained rails have the following inclusion sizes: A-type inclusions ≤ 1.0, B-type inclusions ≤ 1.0, C-type inclusions ≤ 1.0, and D-type inclusions ≤ 1.

0. Furthermore, the mechanical properties must meet the following requirements: yield strength ≥ 580 MPa, tensile strength ≥ 1080 MPa, elongation ≥ 10%, and Brinell hardness ≥ 315HB. In the smelting and production process of the rails, the converter smelting adopts aluminum-free deoxidation alloying, and the whole process is carried out by normal argon blowing during refining; the vacuum degree is ≤0.10 KPa, the deep vacuum time is ≥20 min, and the superheat ΔT is ≤28℃.

2. A type of steel rail, characterized in that, The rail is produced by the method described in claim 1.

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