Preparation method of extremely cold high-toughness steel rail based on VN microalloying

Abstract

The invention discloses a preparation method of an extremely cold high-toughness steel rail based on VN microalloying. The preparation method comprises the following steps: (1) designing steel rail components in percentage by mass; (2) smelting process; (3) rolling process; (4) a heat treatment process: heating to 930 + / -20 DEG C, preserving heat for 30-45 minutes, discharging and air-cooling to 810 + / -10 DEG C, and carrying out high-pressure air cooling; controlling according to four stages of forced cooling, rapid cooling, slow cooling and natural cooling; tempering treatment: adopting a'low-temperature long-time 'tempering system: preserving heat at 200 DEG C for 6-8 hours; an intermittent heating process of keeping the temperature for 2h, cooling by 50 DEG C and heating again is adopted. Through optimal combination of the components and the process, the impact toughness of the steel rail at-60 DEG C reaches 26.4-28.9 J / cm < 2 >, the tensile strength is 1237-1275 MPa, the brinell hardness is 352-371 HB, and meanwhile, the steel rail has good welding performance and fatigue life.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials technology, and particularly relates to a method for preparing high-toughness steel rails for extreme cold applications based on VN microalloying. Background Technology

[0002] With the development of railway construction and the increase in rail exports, Russia has put forward a demand for Russian standard low-temperature resistant rails. Baogang currently has the production capacity for Russian standard P65 cross-section rails and has already begun mass production. Therefore, developing Russian standard low-temperature resistant (-60℃) rail materials can seize market opportunities and bring positive social benefits to the company. As global railway construction extends to high-altitude and cold regions, the performance of rail materials in extreme low-temperature environments has become a major bottleneck restricting the industry's development. Winter temperatures in Russia can drop as low as -60℃, which places stringent requirements on the low-temperature impact toughness of rail materials. Currently, the service performance of domestic and foreign rail products in such harsh environments is still significantly insufficient. Although China's mainstream U75V / U71Mn rails perform reasonably well in domestic cold-region railways, their impact toughness drops to 5-10 J / cm² at -40℃. 2 It falls far short of meeting the Russian standard GOST 51685-2013, which specifies ≥25 J / cm² at -60℃. 2 Technical requirements. European EN13674-1 standard rails, such as R350HT, improve low-temperature toughness by increasing manganese content, but this leads to deterioration in weldability and a 30%-40% decrease in impact toughness in the heat-affected zone. While Russian-developed E76XC rails achieve certain low-temperature performance through Cr-Si alloying technology, they require a complex secondary tempering heat treatment process, significantly increasing production costs. Regarding existing patented technologies, the Cr-Mo-V alloying scheme proposed in Russian patent RU2682345C1, due to its high final rolling temperature (≥950℃), results in severe grain coarsening, with an impact toughness of only 18-22 J / cm² at -60℃. 2 The Nb microalloying technology used in Chinese patent CN110616370A resulted in uneven distribution of VN precipitates due to improper final rolling temperature control, leading to a sharp 40% decrease in the toughness of the heat-affected zone after welding. European patent EP3287546B1 attempted to improve toughness through high silicon content and aluminum deoxidation, but the excessively high silicon content resulted in unstable welding performance, with an impact toughness pass rate of only 70% at -60℃.

[0003] While Japanese patent JP2019056234 employs an ultra-low carbon design, improving toughness, it results in insufficient strength, with a tensile strength below 1100 MPa, failing to meet the strength requirements of heavy-haul railways. These existing technologies struggle to simultaneously meet the dual demands of high strength and high toughness for rails in extremely cold environments, particularly regarding the crucial indicator of stable impact toughness at -60℃, where significant technical bottlenecks remain. With the development of railway construction and increased rail exports, Russia has expressed a demand for Russian standard low-temperature resistant rails. Currently, Baogang has the capacity to produce Russian standard P65 section rails and is already in mass production.

[0004] Testing revealed that the embodiments of this invention achieve an impact energy of 26-28 J at -60℃ and a tensile strength of 1200-1240 MPa, fully meeting the GOST 51685-2013 standard (impact energy ≥25 J at -60℃, tensile strength ≥1180 MPa) and exhibiting excellent comprehensive performance. Therefore, developing a low-temperature (-60℃) high-impact toughness rail material that meets Russian standards can seize market opportunities and bring positive social benefits to enterprises. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing high-toughness steel rails for extreme cold conditions based on VN microalloying. Through innovative composition design and process control, it effectively solves the technical problems of insufficient impact toughness and difficulty in balancing strength and toughness of existing steel rail materials under ultra-low temperature conditions.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention discloses a method for preparing high-toughness steel rails for extremely cold weather applications based on VN microalloying, comprising:

[0008] (1) Design of rail composition by mass percentage: C: 0.72-0.78%; Si: 0.50-0.60%; Mn: 1.05-1.25%; Cr: 0.15-0.20%; V: 0.08-0.15%; N: 0.012-0.015%, and the V / N ratio is strictly controlled at 6.5-8.0; P≤0.025%; S≤0.025%; the remainder is Fe and unavoidable trace elements;

[0009] VN composite addition: V and N form VN precipitates with a size of less than 50nm, which refine the grain size to above ASTM 10 through grain boundary pinning, achieving fine grain strengthening; Cr-Si-Mn synergy: Cr improves hardenability, Si enhances solid solution strengthening, and Mn stabilizes austenite, and the three work together to ensure strength; Si / Mn ratio optimized to 0.45~0.55 to achieve the best match between weldability and strength; Low carbon design: The carbon content is controlled below 0.78%, which significantly reduces the low-temperature brittle transition temperature.

[0010] (2) Smelting process:

[0011] Based on the composition design, the smelting process employs a three-step method: "pre-deoxidation → alloying → refining".

[0012] 1) Pre-deoxidation stage: Add Al-Mg composite deoxidizer and control [O] ≤ 20ppm;

[0013] 2) Alloying stage: VN alloy is added in two stages. The first addition is 70% of the total mass after the steel ladle is put into the refining process. The second addition is the remaining 30% after 2 / 3 of the refining process time has elapsed.

[0014] 3) Refining stage: Hold at 1600℃±5℃ for 30 minutes, with electromagnetic stirring intensity ≥200rpm;

[0015] Component uniformity control:

[0016] 1) Online monitoring was conducted using laser-induced breakdown spectroscopy (LIBS), with compositional fluctuations ≤ ±0.02%;

[0017] 2) The superheat of the molten steel should be controlled within the range of 30–50℃;

[0018] 3) Argon gas protection during the casting process prevents secondary oxidation and ensures uniform composition;

[0019] (3) Rolling process:

[0020] Ingot heating regime: 1250±50℃ for 3-4 hours to ensure uniform austenitization; Temperature control: furnace exit temperature ≥1150℃, initial rolling temperature ≥1050℃, final rolling temperature 910~940℃; Deformation distribution: 60~70% deformation in roughing stage, 30~40% deformation in finishing stage.

[0021] (4) Heat treatment process:

[0022] Heating to 930±20℃, holding for 30~45min, then air-cooling to 810±10℃ using high-pressure air cooling; controlled in four stages: "forced cooling → rapid cooling → slow cooling → natural cooling".

[0023] 1) Strong cooling stage 810~730℃: 5~8℃ / s, suppressing proeutectoid ferrite;

[0024] 2) Rapid cooling stage 730~570℃: 2~4.5℃ / s, promotes the transformation of fine pearlite;

[0025] 3) Slow cooling stage 430~570℃: 1~2.5℃ / s, to control the coarsening of pearlite lamellars and further complete the pearlite transformation;

[0026] 4) Natural cooling ≤430℃: 0.35~1.2℃ / s, natural cooling;

[0027] Tempering treatment: adopt a "low temperature and long time" tempering regime: hold at 200℃ for 6 to 8 hours; adopt an intermittent heating process of holding for 2 hours → cooling down by 50℃ → reheating to eliminate residual stress; obtain an ideal microstructure of fine lath bainite + a small amount of retained austenite.

[0028] Furthermore, the rolled plate has a thickness of 20mm.

[0029] Further, the rail composition design by mass percentage is as follows: C: 0.75%, Si: 0.55%, Mn: 1.15%, Cr: 0.18%, V: 0.10%, N: 0.013%, P: 0.018%, S: 0.008%, with the remainder being Fe and unavoidable impurities; the V / N ratio is 7.69.

[0030] Further, the rail composition design by mass percentage is as follows: C: 0.73%, Si: 0.58%, Mn: 1.20%, Cr: 0.16%, V: 0.12%, N: 0.014%, P: 0.015%, S: 0.006%, with the remainder being Fe and unavoidable impurities; the V / N ratio is 8.57.

[0031] Further, the rail composition design by mass percentage is as follows: C: 0.77%, Si: 0.52%, Mn: 1.10%, Cr: 0.20%, V: 0.08%, N: 0.012%, P: 0.020%, S: 0.010%, with the remainder being Fe and unavoidable impurities; the V / N ratio is 6.67.

[0032] Furthermore, the smelting process employs a 25kg vacuum induction furnace.

[0033] Further, the heat treatment process: reheat to 930℃, hold for 30-45 minutes, and then air-cool to 810℃ after being taken out of the furnace and subjected to high-pressure air cooling.

[0034] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0035] By precisely controlling the V / N ratio and Cr content, nanoscale VN precipitates are formed in the pearlite matrix, achieving a synergistic effect of grain refinement and precipitation reinforcement.

[0036] A "high-temperature rolling + controlled cooling" process was developed, which, while ensuring the pearlite microstructure, refined the pearlite lamellar spacing to 0.15–0.20 μm.

[0037] By optimizing the isothermal transformation process, short rod-shaped cementite was obtained, which significantly improved the low-temperature toughness of the material.

[0038] The established composition-process-structure-performance relationship model can achieve a stable impact energy of ≥25J at -60℃ while maintaining the strength characteristics of traditional pearlitic steel rails.

[0039] The advantages of this invention are:

[0040] This invention, through the optimized combination of the above-mentioned components and processes, enables the rail to achieve an impact toughness of 26.4–28.9 J / cm at -60℃. 2 With a tensile strength of 1237–1275 MPa and a Brinell hardness of 352–371 HB, it fully meets the requirements of GOSTPR51685-2013 standard, and also exhibits good weldability and fatigue life. Compared with existing technologies, while maintaining the same strength level, its low-temperature impact toughness is improved by more than 40%, demonstrating significant technological advancement and industrial application value. Detailed Implementation

[0041] Example 1

[0042] I. Rail Composition (by weight):

[0043] C: 0.75%, Si: 0.55%, Mn: 1.15%, Cr: 0.18%, V: 0.10%, N: 0.013%, P: 0.018%, S: 0.008%, with the remainder being Fe and unavoidable impurities. The V / N ratio is 7.69.

[0044] II. Smelting process:

[0045] Smelting was carried out using a 25kg vacuum induction furnace;

[0046] Pre-deoxidation: Add Al-Mg composite deoxidizer to control the [O] content to 18ppm;

[0047] Alloying: VN alloy is added in two stages (the first stage is 70% of the total mass after the steel ladle is put into the refining process, and the remaining 30% is added after 2 / 3 of the refining time).

[0048] Refining: Hold at 1600℃ for 30 minutes, with electromagnetic stirring intensity at 220 rpm;

[0049] Casting: Molten steel superheated to 40℃, cast under argon protection.

[0050] III. Rolling process:

[0051] Steel ingot heating: Hold at 1250℃ for 3.5 hours;

[0052] Temperature control: furnace exit temperature 1160℃, initial rolling temperature 1070℃, final rolling temperature 925℃;

[0053] Deformation distribution: 65% deformation in rough rolling, 35% deformation in finish rolling;

[0054] Rolled thickness: 20mm sheet.

[0055] IV. Heat treatment process:

[0056] Reheat: Hold at 930℃ for 35 minutes;

[0057] Cooling control:

[0058] Forced cooling phase (810~730℃): Cooling rate 6℃ / s;

[0059] Rapid cooling phase (730~570℃): Cooling rate 3.5℃ / s;

[0060] Slow cooling phase (570~430℃): Cooling rate 2℃ / s;

[0061] Natural cooling (≤430℃): Cooling rate 0.8℃ / s;

[0062] Tempering treatment: Hold at 200℃ for 7 hours, using an intermittent heating process of holding for 2 hours → cooling down by 50℃ → reheating.

[0063] Example 2

[0064] I. Rail Composition (by weight):

[0065] C: 0.73%, Si: 0.58%, Mn: 1.20%, Cr: 0.16%, V: 0.12%, N: 0.014%, P: 0.015%, S: 0.006%, with the remainder being Fe and unavoidable impurities. The V / N ratio is 8.57.

[0066] II. Smelting process:

[0067] Smelting was carried out using a 25kg vacuum induction furnace;

[0068] Pre-deoxidation: Add Al-Mg composite deoxidizer to control the [O] content to 15ppm;

[0069] Alloying: VN alloy is added in two stages (the first stage is 70% of the total mass after the steel ladle is put into the refining process, and the remaining 30% is added after 2 / 3 of the refining time).

[0070] Refining: Hold at 1605℃ for 30 minutes, with electromagnetic stirring intensity at 210 rpm;

[0071] Casting: Molten steel superheated to 35℃, cast under argon protection.

[0072] III. Rolling process:

[0073] Steel ingot heating: Hold at 1250℃ for 3.5 hours;

[0074] Temperature control: furnace exit temperature 1170℃, initial rolling temperature 1080℃, final rolling temperature 935℃;

[0075] Deformation distribution: 68% deformation in rough rolling, 32% deformation in finish rolling;

[0076] Rolled thickness: 20mm sheet.

[0077] IV. Heat treatment process:

[0078] Reheat: Hold at 930℃ for 40 minutes;

[0079] Cooling control:

[0080] Forced cooling phase (810~730℃): Cooling rate 7℃ / s;

[0081] Rapid cooling phase (730~570℃): Cooling rate 4℃ / s;

[0082] Slow cooling phase (570~430℃): Cooling rate 2.2℃ / s;

[0083] Natural cooling (≤430℃): Cooling rate 1.0℃ / s;

[0084] Tempering treatment: Hold at 200℃ for 6.5 hours, using intermittent heating with 2 hours of holding → 50℃ cooling → reheating.

[0085] Example 3

[0086] I. Rail Composition (by weight):

[0087] C: 0.77%, Si: 0.52%, Mn: 1.10%, Cr: 0.20%, V: 0.08%, N: 0.012%, P: 0.020%, S: 0.010%, with the remainder being Fe and unavoidable impurities. The V / N ratio is 6.67.

[0088] II. Smelting process:

[0089] Smelting was carried out using a 25kg vacuum induction furnace;

[0090] Pre-deoxidation: Add Al-Mg composite deoxidizer to control the [O] content to 20ppm;

[0091] Alloying: VN alloy is added in two stages (the first stage is 70% of the total mass after the steel ladle is put into the refining process, and the remaining 30% is added after 2 / 3 of the refining time).

[0092] Refining: Hold at 1595℃ for 30 minutes, with electromagnetic stirring intensity at 200 rpm;

[0093] Casting: Molten steel superheated to 45℃, cast under argon protection.

[0094] III. Rolling process:

[0095] Steel ingot heating: Hold at 1250℃ for 3.5 hours;

[0096] Temperature control: furnace exit temperature 1150℃, initial rolling temperature 1050℃, final rolling temperature 915℃;

[0097] Deformation distribution: 62% deformation in rough rolling, 38% deformation in finish rolling;

[0098] Rolled thickness: 20mm sheet.

[0099] IV. Heat treatment process:

[0100] Reheat: Hold at 930℃ for 30 minutes;

[0101] Cooling control:

[0102] Forced cooling phase (810~730℃): Cooling rate 5.5℃ / s;

[0103] Rapid cooling phase (730~570℃): Cooling rate 3℃ / s;

[0104] Slow cooling phase (570~430℃): Cooling rate 1.8℃ / s;

[0105] Natural cooling (≤430℃): Cooling rate 0.5℃ / s;

[0106] Tempering treatment: Hold at 200℃ for 8 hours, using an intermittent heating process of holding for 2 hours → cooling down by 50℃ → reheating.

[0107] Table 1: Performance test results for each embodiment

[0108]

[0109] Note: All tests were performed in accordance with GOST 51685-2013 standard.

[0110] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing high-toughness steel rails for extremely cold weather applications based on VN microalloying, characterized in that, include: (1) Design of rail composition by mass percentage: C: 0.72-0.78%; Si: 0.50-0.60%; Mn: 1.05-1.25%; Cr: 0.15-0.20%; V: 0.08-0.15%; N: 0.012-0.015%, and the V / N ratio is strictly controlled at 6.5-8.0; P≤0.025%; S≤0.025%; the remainder is Fe and unavoidable trace elements; (2) Smelting process: Based on the composition design, the smelting process employs a three-step method: "pre-deoxidation → alloying → refining". 1) Pre-deoxidation stage: Add Al-Mg composite deoxidizer and control [O] ≤ 20ppm; 2) Alloying stage: VN alloy is added in two stages. The first addition is after the steel ladle is put into the refining process, and the second addition is after 2 / 3 of the refining process time. The first addition is 70% and the second addition is the remaining 30%. 3) Refining stage: Hold at 1600℃±5℃ for 30 minutes, with electromagnetic stirring intensity ≥200rpm; Component uniformity control: 1) Online monitoring was conducted using laser-induced breakdown spectroscopy (LIBS), and the compositional fluctuation was ≤ ±0.02%. 2) The superheat of the molten steel should be controlled within the range of 30–50℃; 3) Argon gas protection during the casting process prevents secondary oxidation and ensures uniform composition; (3) Rolling process: Ingot heating regime: 1250±50℃ for 3-4 hours to ensure uniform austenitization; Temperature control: furnace exit temperature ≥1150℃, initial rolling temperature ≥1050℃, final rolling temperature 910~940℃; Deformation distribution: 60~70% deformation in roughing stage, 30~40% deformation in finishing stage. (4) Heat treatment process: Heat to 930±20℃, hold for 30~45min, then air cool to 810±10℃ using high-pressure air cooling; control the process in four stages: "forced cooling → rapid cooling → slow cooling → natural cooling". 1) Strong cooling stage 810~730℃: 5~8℃ / s, suppressing proeutectoid ferrite; 2) Rapid cooling stage 730~570℃: 2~4.5℃ / s, promotes the transformation of fine pearlite; 3) Slow cooling stage 430~570℃: 1~2.5℃ / s, to control the coarsening of pearlite lamellars and further complete the pearlite transformation; 4) Natural cooling ≤430℃: 0.35~1.2℃ / s, natural cooling; Tempering treatment: adopt a "low temperature and long time" tempering regime: hold at 200℃ for 6 to 8 hours; adopt an intermittent heating process of holding for 2 hours → cooling down by 50℃ → reheating to eliminate residual stress; obtain an ideal microstructure of fine lath bainite + a small amount of retained austenite.

2. The method for preparing high-toughness steel rails for extreme cold weather based on VN microalloying according to claim 1, characterized in that, Rolled sheet with a thickness of 20mm.

3. The method for preparing high-toughness steel rails for extremely cold weather based on VN microalloying according to claim 1, characterized in that, Rail composition design by weight percentage: C: 0.75%, Si: 0.55%, Mn: 1.15%, Cr: 0.18%, V: 0.10%, N: 0.013%, P: 0.018%, S: 0.008%, with the remainder being Fe and unavoidable impurities; The V / N ratio is 7.

69.

4. The method for preparing high-toughness steel rails for extremely cold weather based on VN microalloying according to claim 1, characterized in that, Rail composition design by weight percentage: C: 0.73%, Si: 0.58%, Mn: 1.20%, Cr: 0.16%, V: 0.12%, N: 0.014%, P: 0.015%, S: 0.006%, with the remainder being Fe and unavoidable impurities; The V / N ratio is 8.

57.

5. The method for preparing high-toughness steel rails for extremely cold weather applications based on VN microalloying according to claim 1, characterized in that, Rail composition design by weight percentage: C: 0.77%, Si: 0.52%, Mn: 1.10%, Cr: 0.20%, V: 0.08%, N: 0.012%, P: 0.020%, S: 0.010%, with the remainder being Fe and unavoidable impurities; The V / N ratio is 6.

67.

6. The method for preparing high-toughness steel rails for extremely cold weather based on VN microalloying according to claim 1, characterized in that, The smelting process uses a 25kg vacuum induction furnace.

7. The method for preparing high-toughness steel rails for extremely cold weather applications based on VN microalloying according to claim 1, characterized in that, Heat treatment process: reheat to 930℃, hold for 30-45 minutes, remove from the furnace and air cool to 810℃, then use high-pressure air cooling.

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