Low-yield-ratio steel rail smelting method meeting AREMA standard

Abstract

The invention discloses a smelting method of a low-yield-ratio steel rail meeting the AREMA standard, and belongs to the technical field of metallurgical materials. The method is an American-standard low-yield-ratio steel rail smelting method based on refining process optimization. According to the method, a smelting route of converter-LF furnace (ladle refining furnace)-VD furnace (vacuum degassing) is adopted, and through component design and process control, the yield ratio of the steel rail steel is reduced to 0.74-0.80 on the premise of meeting the requirements of AREMA standard on the chemical components and mechanical properties of the steel rail material, so that the yield ratio of the steel rail steel is reduced to 0.75-0.80; therefore, the internal residual stress peak value of the steel rail is effectively reduced and the fatigue crack resistance is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgical materials, and particularly relates to a method for smelting a low yield ratio rail that meets the AREMA standard. Background Art

[0002] Rail is a key load-bearing component in the railway transportation system and needs to have excellent strength, toughness, and anti-fatigue performance. Currently, high-strength rails widely used in North American heavy-haul railways (such as 136RE heavy rails of the AREMA standard) usually undergo processes such as head hardening to increase hardness, and the ratio of yield strength to tensile strength (yield ratio) is often relatively high (about 0.82 - 0.86). An excessively high yield ratio will cause the residual stress in the rail to be difficult to effectively release during hot rolling straightening, welding, and service operation, easily forming stress concentration areas, which will in turn cause failure modes such as web cracks, head white bands, and rail fractures to occur earlier, threatening the safety of train operation.

[0003] Existing research has shown that reducing the yield ratio of the rail can enhance the plastic buffering ability of the material, which helps the rail to actively release residual stress through small plastic deformations during manufacturing and service, thereby reducing the peak internal stress and improving fatigue performance and service life. Therefore, developing a smelting process for rail steel with a relatively low yield ratio (such as controlled in the range of 0.74 - 0.80) and high purity is of great significance for improving railway operation safety and rail service life.

[0004] Currently, domestic and foreign research on high-strength rails mostly focuses on improving anti-wear and anti-rolling contact fatigue performance, such as adding alloying elements such as Cr, Mo, V through alloying or using a rapid cooling process to prepare a high-hardness structure (such as bainite rail). However, the yield ratio of the rail materials obtained by such methods is often relatively high, and large residual stresses are likely to accumulate during production and service, which is not conducive to the long-term stable operation of the track structure. In addition, the problem of excessive residual stress has been proven to be closely related to phenomena such as web fatigue cracks, weld block shedding, and wheel-rail impact noise radiation.

[0005] From the perspective of mechanical design, the yield ratio is an important parameter for weighing the yield load-bearing capacity and ultimate load-bearing capacity of materials. A lower yield ratio can not only improve the energy absorption and deformation coordination ability of the structure under overload conditions, but also contribute to a more uniform stress distribution, thereby delaying the initiation and propagation of fatigue damage. In the above context, if the yield ratio of the rail can be reduced through metallurgical process means without sacrificing the overall strength and wear resistance of the rail, and the microstructure and composition distribution of the material can be optimized, it will provide a new solution for the control of rail residual stress and the improvement of service life. Summary of the Invention

[0006] The object of the present invention is to provide a method for smelting American Standard low yield ratio steel rails based on refined process optimization. This method adopts a smelting route of converter - LF furnace (ladle refining furnace) - VD furnace (vacuum degassing). Through composition design and process control, on the premise of meeting the requirements of AREMA standard for the chemical composition and mechanical properties of rail materials, the yield ratio of rail steel is reduced to the range of 0.74 - 0.80, thereby effectively reducing the peak value of internal residual stress of the steel rail and improving the anti - fatigue crack performance.

[0007] To solve the above - mentioned technical problems, the present invention adopts the following technical solutions:

[0008] A method for smelting low yield ratio steel rails conforming to AREMA standard of the present invention includes:

[0009] 1) Hot metal pretreatment

[0010] Perform pretreatment desulfurization on blast furnace hot metal, control the phosphorus and sulfur contents in the hot metal, so that P≤0.120% and S≤0.020% in the pretreated hot metal; select low - phosphorus pig iron and low - sulfur scrap steel as furnace charges, and use deoxidizers with low aluminum content to ensure that the hot metal composition is suitable before entering the converter;

[0011] 2) Converter smelting

[0012] Use a top - blown converter for steelmaking, reduce the carbon content of the hot metal to a lower level to efficiently remove impurities such as phosphorus; when tapping from the converter, the target content of [C] in the molten steel is 0.12%, and the molten steel temperature is 1630±20°C; during the tapping process, argon is introduced to stir the molten steel to keep the slag fluidity and prevent caking; through converter blowing, P in the molten steel is reduced to ≤0.020% to provide low - phosphorus molten steel for subsequent processes;

[0013] 3) LF furnace refining

[0014] Place the converter molten steel in the LF ladle refining furnace for refining, add alloying elements to the molten steel to adjust the composition to the designed range; during the refining process, add an appropriate amount of calcium - silicon alloy for inclusion modification treatment to make the inclusions spherical and finer, thereby improving the cleanliness of the molten steel; heat and stir the molten steel in the LF furnace for 20 - 24 minutes, and control the tapping temperature at 1580±20°C; the composition of the molten steel after refining meets the requirements of the chemical composition of the steel rail in the AREMA standard;

[0015] 4) VD vacuum degassing

[0016] The molten steel refined by LF is subjected to VD furnace vacuum degassing treatment; the vacuum pump is started to keep the molten steel in a deep vacuum state with a vacuum degree of 9 - 11 kPa, and the deep vacuum time is maintained for 16 - 18 minutes to remove hydrogen, oxygen, and nitrogen gas elements in the molten steel to the greatest extent; then argon is introduced for soft stirring, the soft blowing time is 20 - 25 minutes, and the argon flow rate is 90 - 110 NL / min to ensure that the molten steel surface moves slightly without secondary oxidation; after VD degassing, the contents of [H], [O], and [N] in the molten steel are significantly reduced, meeting the strict requirements of rail steel for the contents of gas elements;

[0017] 5) Continuous casting

[0018] The clean molten steel after VD treatment is continuously cast into steel billets; the protective casting process is adopted, and a mold powder with a low aluminum content is used to avoid the introduction of aluminum inclusions and stabilize the heat transfer of the casting stream; during continuous casting, the superheat of the molten steel in the mold ΔT is controlled at 25 °C, the secondary cooling zone uses an extremely weak cooling water ratio and maintains a constant casting speed of 0.59 - 0.61 m / min; the casting machine is equipped with an electromagnetic stirring device, and the electromagnetic stirring is started and the soft reduction technology is implemented during the casting process to refine the internal structure of the casting billet; through the above continuous casting control, steel billets with good surface and internal quality are obtained.

[0019] Further, the vacuum degree is 10 kPa.

[0020] Further, the deep vacuum time is maintained for 17 minutes.

[0021] Further, the soft blowing time is 23 minutes.

[0022] Further, the argon flow rate is 100 NL / min.

[0023] Further, the cross-sectional size of the cast steel billet is 280 mm × 380 mm.

[0024] Further, the casting speed is 0.60 m / min.

[0025] Compared with the prior art, the beneficial technical effects of the present invention:

[0026] The rail steel obtained through the above processes of the present invention has high purity and optimized mechanical properties. The contents of impurities such as phosphorus and sulfur in the steel are low (both P and S are ≤0.020%), the contents of gases such as hydrogen, oxygen, and nitrogen are low ([H] is about 1 - 2 ppm, [O] ≤ 20 ppm, [N] ≤ 50 ppm), and the inclusions are mostly spherically distributed. The improvement of these metallurgical qualities enables the rail material to have a low yield ratio while maintaining sufficiently high strength and toughness. After testing, the yield strength, tensile strength, and elongation rate of the rail steel of the present invention all meet the requirements for heavy rails (such as 136RE grade) in Chapter 4 of the AREMA rail standard. Among them, the yield ratio is controlled within 0.74 - 0.80, effectively reducing the residual stress level of the rail and significantly improving the anti-fatigue performance and service safety of the rail. Detailed implementation mode

[0027] A method for smelting low yield ratio rails conforming to the AREMA standard, comprising:

[0028] 1) Hot metal pretreatment

[0029] Perform pretreatment desulfurization on the blast furnace hot metal, control the phosphorus and sulfur contents in the hot metal, so that P ≤ 0.120% and S ≤ 0.020% in the pretreated hot metal. Select low-phosphorus pig iron and low-sulfur scrap steel as furnace charges, and use deoxidizers with low aluminum content to ensure that the hot metal composition is suitable before entering the converter, as shown in Table 1.

[0030] Table 1 Hot metal situation for converter smelting of high-carbon steel rails

[0031]

[0032] 2) Converter smelting

[0033] Use a top-blown converter for steelmaking, and reduce the carbon content of the hot metal to a lower level to efficiently remove impurities such as phosphorus. When tapping from the converter, the molten steel [C] is required to be about 0.12%, and the molten steel temperature is about 1630°C. Argon is introduced to stir the molten steel during tapping to keep the slag fluidity and prevent caking. Through converter blowing, P in the molten steel is reduced to ≤0.020% to provide low-phosphorus molten steel for subsequent processes. The typical tapping composition after converter blowing and the converter smelting process table are shown in Table 2.

[0034] Table 2 Converter smelting process table for high-carbon steel rails

[0035]

[0036] 3) LF furnace refining

[0037] The converter molten steel is refined in an LF ladle furnace. Alloying elements are added to the molten steel to adjust the composition to the designed range (for example, [C] is increased to about 0.75 - 0.80%, [Mn] is about 1.1 - 1.3%, [Si] is about 0.4 - 0.6%, and a small amount of alloying elements such as Cr are added as needed). During the refining process, an appropriate amount of calcium-silicon alloy (CaSi) is added for inclusion modification treatment to spheroidize and refine the inclusions, thereby improving the cleanliness of the molten steel. The LF furnace heats and stirs the molten steel for about 22 minutes, and the tapping temperature is controlled at about 1580°C. After refining, the composition of the molten steel meets the chemical composition requirements of the rail steel according to the AREMA standard (such as the contents of C and Mn are within the specified range, and P and S are lower than 0.020%, etc.).

[0038] 4) VD vacuum degassing

[0039] The molten steel after LF refining is subjected to VD furnace vacuum degassing treatment. The vacuum pump is started to keep the molten steel in a deep vacuum state (vacuum degree about 10 kPa), and the deep vacuum time is maintained for about 17 minutes to remove gas elements such as hydrogen, oxygen, and nitrogen in the molten steel to the greatest extent. Subsequently, argon gas is introduced for soft stirring (soft blowing), the soft blowing time is about 23 minutes, and the argon gas flow rate is about 100 NL / min to ensure that the molten steel surface moves slightly without secondary oxidation. After VD degassing, the contents of [H], [O], and [N] in the molten steel are significantly reduced, meeting the strict requirements of the rail steel for the contents of gas elements.

[0040]

[0041] 5) Continuous casting

[0042] The clean molten steel after VD treatment is continuously cast into steel billets. The protective casting process is adopted, and a mold powder with a low aluminum content is used to avoid the introduction of aluminum inclusions and stabilize the heat transfer of the casting stream. The cross-sectional size of the cast steel billet is about 280 mm × 380 mm (typical heavy rail steel billet size). During continuous casting, the superheat ΔT of the molten steel in the mold is controlled at about 25°C, a very weak cooling water ratio is used in the secondary cooling zone, and a constant casting speed of about 0.60 m / min is maintained. The casting machine is equipped with an electromagnetic stirring device, and electromagnetic stirring is started during the casting process and the soft reduction technology is implemented to refine the internal structure of the cast billet. Through the above continuous casting control, steel billets with good surface and internal quality are obtained.

[0043] Table 3 Composition of high carbon rail steel billets %

[0044]

[0045] The rail steel obtained by the above process has high purity and optimized mechanical properties. The contents of impurities such as phosphorus and sulfur in the steel are low (both P and S ≤ 0.020%), and the contents of gases such as hydrogen, oxygen and nitrogen are low ([H] is about 1 - 2 ppm, [O] ≤ 20 ppm, [N] ≤ 50 ppm), and the inclusions are mostly distributed in spherical shape. The improvement of these metallurgical qualities enables the rail material to have a lower yield ratio while maintaining sufficiently high strength and toughness. After testing, the yield strength, tensile strength and elongation of the rail steel of the present invention all meet the requirements for heavy rails (such as 136RE grade) in the AREMA Chapter 4 Rail Standard, and the yield ratio is controlled at 0.74 - 0.80, effectively reducing the residual stress level of the rail and significantly improving the anti-fatigue performance and service safety of the rail.

[0046] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solution of the present invention should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for smelting low yield ratio steel rails that comply with AREMA standards, characterized in that: Including: 1) Hot metal pretreatment The hot metal from the blast furnace is pretreated for desulfurization to control the phosphorus and sulfur contents in the hot metal, so that after pretreatment, P≤0.120% and S≤0.020% in the hot metal; low-phosphorus pig iron and low-sulfur scrap steel are selected as furnace charges, and a deoxidizer with a low aluminum content is used to ensure that the hot metal composition is suitable before entering the converter; 2) Converter smelting A top-blown converter is used for steelmaking to reduce the carbon content of the hot metal to a lower level for efficient removal of phosphorus impurities; when tapping from the converter, the target content of [C] in the molten steel is 0.12%, and the molten steel temperature is 1630±20°C; argon is introduced during tapping to stir the molten steel and keep the slag fluidity to prevent caking; through converter blowing, P in the molten steel is reduced to ≤0.020% to provide low-phosphorus molten steel for subsequent processes; 3) LF furnace refining The molten steel from the converter is placed in an LF ladle refining furnace for refining, and alloying elements are added to the molten steel to adjust the composition to the designed range; during refining, an appropriate amount of calcium-silicon alloy is added for inclusion modification treatment to spheroidize and refine the inclusions, thereby improving the cleanliness of the molten steel; the LF furnace heats and stirs the molten steel for 20 - 24 minutes, and the tapping temperature is controlled at 1580±20°C; the composition of the molten steel after refining meets the chemical composition requirements of the rail steel in the AREMA standard; 4) VD vacuum degassing The molten steel refined by LF is subjected to VD furnace vacuum degassing treatment; the vacuum pump is started to keep the molten steel in a deep vacuum state, with a vacuum degree of 9 - 11 kPa, and the deep vacuum time is maintained for 16 - 18 minutes to remove the hydrogen, oxygen, and nitrogen gas elements in the molten steel to the greatest extent; then argon is introduced for soft stirring, with a soft blowing time of 20 - 25 minutes and an argon flow rate of 90 - 110 NL / min to ensure that the molten steel surface moves slightly without secondary oxidation; after VD degassing, the contents of [H], [O], and [N] in the molten steel are significantly reduced, meeting the strict requirements of the rail steel for the content of gas elements; 5) Continuous casting The clean molten steel after VD treatment is continuously cast into steel billets; a protective casting process is adopted, and a mold powder with a low aluminum content is used to avoid the introduction of aluminum inclusions and stabilize the heat transfer of the casting stream; during continuous casting, the superheat ΔT of the molten steel in the mold is controlled at 25°C, and in the secondary cooling zone, an extremely weak cooling water ratio is used and the constant casting speed is maintained at 0.59 - 0.61 m / min; the casting machine is equipped with an electromagnetic stirring device, and the electromagnetic stirring is started and the soft reduction technology is implemented during the casting process to refine the internal structure of the casting billet; through the above continuous casting control, steel billets with good surface and internal quality are obtained.

2. The method for smelting low yield ratio steel rails meeting the AREMA standard according to claim 1, characterized in that: The vacuum degree is 10 kPa.

3. The method for smelting a low yield ratio rail meeting the AREMA standard according to claim 1, characterized in that: The deep vacuum time is maintained for 17 minutes.

4. The method for smelting low yield ratio steel rails meeting the AREMA standard according to claim 1, characterized in that: The soft blowing time is 23 minutes.

5. The method for smelting a low yield ratio steel rail meeting the AREMA standard according to claim 1, characterized in that: The argon flow rate is 100 NL / min.

6. The method for smelting a low yield ratio rail meeting the AREMA standard according to claim 1, characterized in that: The cross-sectional size of the cast steel billet is 280 mm×380 mm.

7. The method for smelting a low yield ratio rail meeting the AREMA standard according to claim 1, characterized in that: The casting speed is 0.60 m / min.

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