Superfine pearlite steel rail high and low temperature fracture toughness regulation and control process based on online waste heat zoned controlled cooling

Abstract

The invention discloses a superfine pearlite steel rail high and low temperature fracture toughness regulation and control process based on online waste heat zoned cooling control. The process comprises the following steps: (1) collaborative optimization of component design is carried out to crack the contradiction between strength and toughness of high-carbon steel; (2) innovation of an online waste heat quenching process, wherein the temperature is accurately controlled, specifically, the finish rolling temperature is 910-940 DEG C, and then the steel plate enters a 760-810 DEG C waste heat treatment area immediately; the gradient air cooling technology comprises the following steps: first-stage forced cooling: the air cooling speed is 2.5-5.5 DEG C / s, the air cooling lasts for 40-70 s, and a brittle phase change region is quickly crossed; in the second-stage weak cooling, the cooling speed is smaller than or equal to 1.8 DEG C / s, the outlet rail head temperature is 475-530 DEG C, it is ensured that a'fine grain pearlite + trace ferrite 'tough structure is obtained, and the martensite content is 0.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials technology, and particularly relates to a process for controlling the high and low temperature fracture toughness of ultrafine pearlitic steel rails based on online residual heat zoned cooling. Background Technology

[0002] As my country's high-altitude railway network expands into extreme environments (such as Northeast China below -40℃ and the permafrost region of the Qinghai-Tibet Plateau), low-temperature brittle fracture of rails has become a core bottleneck restricting railway safety. The current situation of the industry highlights three major technical dilemmas: 1. The low-temperature performance of materials is seriously insufficient. The toughness defects of traditional high-carbon rails: the fracture toughness of mainstream U71Mn rails at -40℃ is only 28-32MPa·m1 / 2 (see Journal of Railway Engineering 2022, 44(5):112-118), and the impact energy of the rail head weld area drops sharply to 8-10J, resulting in more than 200 rail failure cases per year in cold regions; the alloy scheme is unbalanced: although the European SS rails have improved toughness by adding 1.2%Mo (fracture toughness at -40℃ ≈38MPa·m1 / 2), the steel rails are still in a state of flux. 0 · 5 However, the cost per ton of steel surged by 35%, and the high Mo content exacerbated segregation in continuous casting (center segregation index ≥ 1.5). 2. Systemic defects exist in the heat treatment process. Offline quenching has low energy efficiency: for example, patent CN112111713A uses secondary heating quenching, which requires an additional 1200-1500 MJ / ton of energy, and the temperature gradient causes a difference in microstructure between the rail head and rail bottom > 18%; improper online cooling control: the water mist quenching process disclosed in patent JP2020-123456 has a cooling rate > 10℃ / s, inducing martensitic phase transformation (volume fraction > 15%), causing a 40% deterioration in impact toughness at -60℃. 3. Lack of a composition-process synergistic mechanism. Existing research (Acta Metallurgica Sinica)

[0003] 2021, 57(8):1029-1038) indicates that simply increasing the Mn content (>1.3%) can expand the austenite region, but it exacerbates the formation of banded structures; V microalloying lacks supporting processes, and the precipitation rate of V(C,N) under traditional controlled cooling processes is less than 60%, and the grain refinement effect does not reach the theoretical value.

[0004] Therefore, the industry urgently needs to develop rail manufacturing technologies that combine ultra-low temperature toughness (fracture toughness at -50℃ ≥33MPa·m1 / 2), low cost (alloy cost ≤300 yuan / ton increment) and production line compatibility to support the future demand for long-life rails for high-altitude and cold-weather railways. Summary of the Invention

[0005] The purpose of this invention is to provide a process for controlling the high and low temperature fracture toughness of ultrafine pearlitic steel rails based on online residual heat zoned cooling. This process controls the high and low temperature fracture toughness through composition optimization and online heat treatment, and is especially suitable for extremely cold environments below -40℃.

[0006] To solve the above technical problems, the "component precision design-online residual heat quenching-gradient air cooling regulation" three-in-one solution is innovatively proposed, and the following technical solutions are adopted:

[0007] The application discloses an online residual heat partition controlled cooling based superfine pearlite rail high and low temperature fracture toughness regulation process, which comprises the following steps:

[0008] 1) Synergistic optimization of component design to break the high carbon steel strength and toughness contradiction

[0009] The chemical composition of the rail and the mass percentage are as follows:

[0010] C 0.60-0.69%: strictly control the upper limit to avoid grain boundary embrittlement, and the lower limit to ensure strength;

[0011] Si 0.30-0.65%: suppress ferrite phase transition, and ≤0.65% to ensure weldability;

[0012] Mn 0.80-1.10%: expand the austenite zone and control the banded structure ≤2 level (better than 3 level of CN113293286A);

[0013] Cr 0.10-0.35%: widen the phase transition window, and increase the online heat treatment temperature tolerance by ±15℃;

[0014] V 0.01-0.08%: key innovation point, pin Austenite grain boundary through V(C, N) precipitation, and realize pearlite lamella spacing ≤150nm (traditional process >200nm);

[0015] P≤0.025%, S≤0.025%, and the rest is Fe and impurities;

[0016] 3) Online residual heat quenching process innovation (to solve the defects of high energy consumption and poor uniformity of offline treatment)

[0017] Rolling process: billet → sawing → heating → BD1 rolling → BD2 rolling → CCS universal rolling mill continuous rolling → online residual heat quenching → subsequent process; wherein:

[0018] Temperature accurate control: finish rolling temperature 910-940℃ → immediately enter the 760-810℃ residual heat treatment zone; (avoid secondary heating, energy saving ≥30%).

[0019] Gradient air cooling technology:

[0020] First stage strong cooling: air cooling speed 2.5-5.5℃ / s, lasting for 40-70s, quickly passing through the brittle phase transition zone;

[0021] Second stage weak cooling: cooling rate ≤1.8℃ / s, outlet rail head temperature 475-530℃, to ensure obtaining "fine grain pearlite + trace ferrite" tough structure, martensite content is 0.

[0022] Further, the chemical composition of the steel rail and the mass percentage are as follows: C 0.63%, Si 0.53%, Mn 1.0%, Cr 0.10%, V 0.012%, and the rest is Fe and impurities.

[0023] Further, the chemical composition of the steel rail and the mass percentage are as follows: C 0.64%, Si 0.47%, Mn 0.96%, Cr 0.12%, V 0.020%, and the rest is Fe and impurities.

[0024] Further, the chemical composition of the steel rail and the mass percentage are as follows: C 0.62%, Si 0.42%, Mn 1.10%, Cr 0.13%, V 0.025%, and the rest is Fe and impurities.

[0025] Further, the chemical composition of the steel rail and the mass percentage are as follows: C 0.60%, Si 0.65%, Mn 0.80%, Cr 0.35%, V 0.08%, and the rest is Fe and impurities.

[0026] Further, the chemical composition of the steel rail and the mass percentage are as follows: C 0.69%, Si 0.30%, Mn 1.10%, Cr 0.10%, V 0.01%, and the rest is Fe and impurities.

[0027] Further, the low-temperature toughness of the steel rail is: the fracture toughness at-40℃ reaches 33.7-36.1MPa·m 0.5 .

[0028] Further, the steel rail still maintains the elongation of 15-17% under the tensile strength of 1083-1115MPa.

[0029] Compared with the prior art, the beneficial technical effects of the present application are:

[0030] Through the above synergistic effect:

[0031] Low-temperature toughness: the fracture toughness at-40℃ reaches 33.7-36.1MPa·m 0 · 5 (more than 40% higher than U71Mn, the same as the high-end steel rail containing Mo but the cost is reduced by 35%);

[0032] Strength and toughness balance: the steel rail still maintains the elongation of 15-17% under the tensile strength of 1083-1115MPa (resolving the contradiction of "strength ↑ toughness ↓" of traditional high-carbon steel).

[0033] The core advantage of the present application is different from the prior art:

[0034] Technical features The invention Typical patent CN112111713A Heat treatment mode Straight-through online quenching after rolling Offline secondary heating Cooling medium High-pressure air (zero water resource consumption) Water-based quenching liquid Uniformity of structure Full-face hardness difference ≤ 5 HB Head / bottom hardness difference ≥ 15 HB Low-temperature phase transformation control Complete inhibition of martensite Martensite content > 8% DETAILED DESCRIPTION

[0035] An ultra-fine pearlite rail high and low temperature fracture toughness regulation process based on online residual heat partition controlled cooling, comprising:

[0036] Steel smelting process: molten iron → converter smelting → LF refining → VD → continuous casting. The converter adopts aluminum-free deoxidization alloying and full-process argon blowing; the vacuum degree is ≤0.10 KPa, the deep vacuum time is ≥18 min, and the superheat degree ΔT is ≤30℃.

[0037] Rolling and heat treatment process:

[0038] Square billet → sawing → heating → BD1 rolling → BD2 rolling → CCS universal rolling mill continuous rolling → online residual heat quenching → sawing → cooling → head and tail cutting → straightening → inspection → packaging → weighing → warehouse.

[0039] Heating system: preheating section ≤800℃, total time ≥3h, discharge temperature ≥1100℃, rolling start ≥1050℃, and final rolling 910-940℃;

[0040] Online quenching: immediately entering the 760-810℃ air cooling zone after final rolling, controlled in two stages:

[0041] Strong cooling stage: rail head top surface / sides / rail gauge angle lower jaw / rail bottom, cooling speed 2.5-5.5℃ / s, time length 40-70s;

[0042] Weak cooling stage: full-section cooling speed ≤1.8℃ / s, outlet rail head temperature 475-530℃, air cooling to room temperature.

[0043] Examples 1-5 and comparative examples

[0044]

[0045] The fracture toughness of examples 1-5 at -40℃ is all ≥33.7 MPa·m 0 · 5 , which is increased by more than 40% compared with the comparative examples; the toughness of the comparative examples is seriously deteriorated due to the absence of V grain refinement + water mist quenching martensite.

[0046] The full-section hardness difference of examples 1-5 is ≤5HB (such as rail head 323HB / rail bottom 318HB); the hardness difference of the rail head / rail bottom of the comparative examples reaches 28HB (uneven distribution of martensite).

[0047] The above described embodiments are only to illustrate the preferred modes of the present application, and are not intended to limit the scope of the present application. Any modification and improvement made by those skilled in the art to the technical solutions of the present application without departing from the design spirit of the present application shall fall within the protection scope of the present application.

Claims

1. A high and low temperature fracture toughness regulation process of superfine pearlitic rail based on online residual heat partition controlled cooling, characterized in that, Comprise: 1) Synergistic optimization of ingredient design to break the contradiction between strength and toughness of high carbon steel The chemical composition and mass percentage of the steel rail are as follows: C 0.60-0.69%, Si 0.30-0.65%, Mn 0.80-1.10%, Cr 0.10-0.35%, V 0.01-0.08%, P≤0.025%, S≤0.025%, and the rest is Fe and impurities; 2) Online residual heat quenching process innovation Rolling process: billet → sawing → heating → BD1 rolling → BD2 rolling → CCS universal mill continuous rolling → online residual heat quenching → subsequent process; wherein: Temperature precise control: finish rolling temperature 910-940℃ → immediately enter 760-810℃ residual heat treatment zone; Gradient air cooling technology: First stage strong cooling: air cooling speed 2.5-5.5℃ / s, lasting for 40-70s, quickly pass through the brittle phase transition zone; Second stage weak cooling: cooling speed ≤1.8℃ / s, outlet rail head temperature 475-530℃, to ensure obtaining "fine pearlite + trace ferrite" toughness structure, and the content of martensite is 0. The chemical composition and mass percentage of the steel rail are as follows: C 0.63%, Si 0.53%, Mn 1.0%, Cr 0.10%, V 0.012%, and the rest is Fe and impurities.

2. The ultra-fine pearlitic rail high and low temperature fracture toughness regulating process based on online residual heat partitioned controlled cooling according to claim 1, characterized in that, The chemical composition and mass percentage of the steel rail are as follows: C 0.64%, Si 0.47%, Mn 0.96%, Cr 0.12%, V 0.020%, and the rest is Fe and impurities.

3. The ultra-fine pearlitic rail high and low temperature fracture toughness regulating process based on online residual heat partitioned controlled cooling according to claim 1, characterized in that, The chemical composition and mass percentage of the steel rail are as follows: C 0.62%, Si 0.42%, Mn 1.10%, Cr 0.13%, V 0.025%, and the rest is Fe and impurities.

4. The ultra-fine pearlitic rail high and low temperature fracture toughness regulating process based on online residual heat partitioned controlled cooling according to claim 1, characterized in that, The chemical composition and mass percentage of the steel rail are as follows: C 0.60%, Si 0.65%, Mn 0.80%, Cr 0.35%, V 0.08%, and the rest is Fe and impurities.

5. The ultra-fine pearlitic rail high and low temperature fracture toughness regulating process based on online residual heat partitioned controlled cooling according to claim 1, characterized in that, The chemical composition and mass percentage of the steel rail are as follows: C 0.69%, Si 0.30%, Mn 1.10%, Cr 0.10%, V 0.01%, and the rest is Fe and impurities.

6. The ultra-fine pearlitic rail high and low temperature fracture toughness regulating process based on online residual heat partitioned controlled cooling according to claim 1, characterized in that, The steel rail still maintains elongation of 15-17% under tensile strength of 1083-1115MPa.

7. The ultra-fine pearlitic rail high and low temperature fracture toughness regulating process based on online residual heat partitioned controlled cooling according to claim 1, characterized in that, Rail low temperature toughness: -40°C fracture toughness of 33.7-36.1 MPa-m 0.5 .

8. The ultra-fine pearlitic rail high and low temperature fracture toughness regulating process based on online residual heat partitioned controlled cooling according to claim 1, characterized in that, ​

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