Low-cost H340-grade high-strength steel rail rolling and online heat treatment process based on alloy reduction

Abstract

The invention discloses a low-cost H340-grade high-strength steel rail rolling and on-line heat treatment process based on alloy reduction. The rolling key points of the process are as follows: the finish rolling temperature is strictly limited to 910-940 DEG C, and nucleation driving force reduction caused by vanadium reduction is compensated by high-temperature austenitizing; the online heat treatment adopts three-stage cooling, and the core technical parameters of the online heat treatment are as follows: in the first stage, the cooling speed VC1 is equal to [3.5 + 10 * (0.07-V)] + / -0.3 DEG C / s, and the target temperature is 600-630 DEG C; in the second stage, a constant-temperature platform is kept at the temperature of 540-590 DEG C for t = [120-100 * V] seconds; and in the third stage, the cooling speed VC3 is equal to [0.02 * (Mn) 2] + / -0.2 DEG C / s-420-480 DEG C at an outlet. According to the method, through dynamic cooperation of component precise control and three-stage gradient cooling, the H340-grade performance is guaranteed, and meanwhile the vanadium cost is reduced by 40% or above.

Description

Technical Field

[0001] The present invention relates to the technical field of material rolling and heat treatment, and in particular to a low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction. Background Art

[0002] Currently, there are three extreme challenges for rails: high-strength wear resistance: the axle load of heavy-duty trains exceeds 30 tons, and the wheel-rail contact stress reaches 1800-2200MPa, causing the rail head crescent wear rate to accelerate by 3 times; contact fatigue resistance: side wear of rails in curved sections and rolling contact fatigue (RCF) cracks occur simultaneously. 75% of the rails replaced annually on the Daqin Line fail due to fish scale damage; uniformity along the entire length: when the hardness deviation caused by fluctuations in the heat treatment process is greater than 5HRC, the risk of fracture of the rail weld joint increases by 40%.

[0003] Against this backdrop, H340-grade heat-treated rails (represented by U75VH) have become a core material for heavy-duty lines due to their combined tensile strength (≥1180MPa) and hardness (340-390HB). Their performance standards are strictly regulated by TB / T2344-2012. However, the chemical composition of U75VH rails determines the pearlite content; excessively high carbon content increases brittleness; silicon (Si) above 0.75% deteriorates weldability; manganese (Mn) improves hardenability, but excessive amounts induce segregation; and vanadium (V), a core strengthening element, enhances strength through V (C, N) precipitation and solid solution strengthening. The industry generally adopts a "composition upper limit control" strategy: to meet the TB / T2344 performance threshold (such as hardness ≥340HB), the V content is generally controlled at 0.10%-0.12%. This strategy leads to two major problems: cost surge: vanadium iron prices fluctuate sharply (average price of RMB 285,000 / ton in 2023), and vanadium costs per ton of steel account for 12%-15%, significantly higher than Mn (3%-5%) and Si (1%-2%); process redundancy: high V content (>0.08%) inhibits the driving force of phase transformation, requiring extended cooling time to compensate, which is contrary to the development trend of high-efficiency cooling equipment.

[0004] Traditional U75V rails use online residual heat quenching (RQ process), and its cooling path is:

[0005] Finish rolling (950℃) → water mist quenching (>10℃ / s) to 450℃ → air cooling.

[0006] This process has inherent defects: cooling singleness: although strong cooling improves surface hardness, the cooling rate of the core is insufficient (<1℃ / s), resulting in significant structural gradient (pearlite cluster size: 15μm on the surface vs. 35μm in the core); in recent years, high-efficiency cooling equipment has become popular, and the cooling capacity has increased by 30%-40%, which is theoretically achievable: however, there is a serious disconnect between the existing high-V component design and the new equipment: high V inhibits the decomposition of austenite, and a higher cooling rate is required to trigger phase transformation, but accelerated cooling exacerbates stress concentration.

[0007] Ansteel, a steel company in the industry, previously experimented with V = 0.05% (the lower limit of the standard), but the hardness only reached 325-335HB (less than 340HB). This was attributed to insufficient hardenability due to the failure to coordinate the Mn / Si ratio; the cooling process continued to use high V parameters, failing to realize the potential for low-temperature phase transformation. Furthermore, some attempts were made to partially replace V with Nb (0.02%-0.04%), but the high precipitation temperature of Nb (C, N) (>950°C) deteriorated rolling plasticity and increased the scrap rate to 2.3% (compared to the industry average of 0.8%). Baosteel's rare earth iron ore has low impurities: P ≤ 0.008% and S ≤ 0.005% (only one-third of the standard limit), which significantly reduces the risk of grain boundary embrittlement. It also contains trace amounts of 0.001%-0.003% rare earth (Ce / La), which refines the original austenite grains (ASTM grade ≥ 10).

[0008] In summary, the current H340 rail production faces the triple dilemma of "high cost, low coordination, and idle resources". Therefore, an innovative process that deeply integrates resource characteristics, precise composition control, and gradient cooling is urgently needed to achieve cost reduction and efficiency improvement while ensuring H340-level performance. Summary of the Invention

[0009] The purpose of the present invention is to provide a low-cost H340-grade high-strength rail rolling and online heat treatment process based on alloy reduction. The present invention breaks through the traditional path of "high vanadium to maintain performance" and pioneers a "low vanadium critical cooling rate control" technical route. Through the dynamic synergy of precise composition control (V 0.04-0.07%) and three-stage gradient cooling, the vanadium cost is reduced by more than 40% while ensuring H340-grade performance (σb ≥ 1230MPa, HB ≥ 355).

[0010] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0011] The present invention provides a low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction, the process mainly comprising: billet → heating → BD1 rolling → BD2 rolling → CCS universal rolling mill continuous rolling → online heat treatment → cooling → straightening; the characteristics are:

[0012] Key points of rolling: The final rolling temperature is strictly limited to 910-940℃, and high-temperature austenitization is used to compensate for the decrease in nucleation driving force caused by vanadium reduction;

[0013] Online heat treatment uses three-stage cooling, and its core technical parameters are as follows:

[0014] Stage 1: Cooling rate VC1 = [3.5 + 10 × (0.07 - V)] ± 0.3 °C / s → target 600-630 °C, where V is the mass percentage content;

[0015] The second stage: constant temperature platform 540-590℃, maintained for t = [120-100×V] seconds;

[0016] The third stage: cooling rate VC3 = [0.02 × (Mn) 2 ]±0.2℃ / s→export 420-480℃, where Mn is the value of its mass percentage content.

[0017] Furthermore, the chemical composition of the low-cost H340 grade high-strength rail is as follows by weight: C 0.6-0.9%, Si 0.58-0.65%, V 0.04-0.07%, Mn 0.85-0.95%, P≤0.015%, and the remainder is Fe and unavoidable impurities.

[0018] Furthermore, the chemical composition of the low-cost H340 grade high-strength rail is as follows by mass percentage: C: 0.78%, Si: 0.65%, V = 0.042%, Mn = 0.92%, P = 0.008%, and the remainder is Fe and unavoidable impurities.

[0019] Furthermore, the process performs:

[0020] Finish rolling temperature 935℃: 50℃ higher than the conventional lower limit, expanding the austenite recrystallization window;

[0021] Three-stage cooling dynamic calculation:

[0022] In the first stage, VC1 = 3.5 + 10 × (0.07-0.042) = 3.78 °C / s → the measured rail head is 620 °C;

[0023] The second stage is a constant temperature of 550°C, and the constant temperature time is t = 120-100×0.042 = 115.8 seconds → 115 seconds in actual time;

[0024] The third stage VC3 = 0.02 × (0.92) 2 =0.017℃ / s→actual 0.02℃ / s;

[0025] The performance results meet the following requirements: tensile strength 1265MPa; tread hardness is usually 357.2-359.3HB, hardness fluctuation along the entire length is ≤2.1HB, and elongation is 11.5%-13.5%.

[0026] Furthermore, the chemical composition of the low-cost H340 grade high-strength rail is as follows by mass percentage: C: 0.78%, V = 0.048%, S = 0.004%, Si = 0.62%, Mn = 0.92%, and the remainder is Fe and unavoidable impurities.

[0027] Furthermore, the cooling rate in the first stage was fine-tuned based on the Si content: VC1′ = VC1×[1-0.3×(Si-0.60)] → actual 3.42°C / s;

[0028] The second stage is a constant temperature of 570°C, and the constant temperature time is t = 120-100×0.042 = 115.8 seconds → 115 seconds in actual time.

[0029] The third stage VC3 = 0.02 × (0.92) 2 =0.017℃ / s→actual 0.02℃ / s;

[0030] The performance results meet the following requirements: tensile strength 1248MPa; tread hardness 356.8-358.1HB, full-length hardness fluctuation ≤2.3HB, elongation 11.5%-12.5%.

[0031] Furthermore, the chemical composition of the low-cost H340 grade high-strength rail is as follows by mass percentage: C: 0.79%, Si: 0.58%, Mn: 0.85%, V: 0.041%, P: 0.006%, S: 0.004%, V = 0.041%, and the rest is Fe and unavoidable impurities.

[0032] Furthermore, the final rolling temperature is increased to 935℃.

[0033] Three-stage cooling process execution:

[0034] First stage cooling rate VC1 = 3.5 + 10 × (0.07-0.041) = 3.89 ° C / s → measured rail head temperature 625 ° C;

[0035] The second stage is kept at 580°C, 120-100×0.041=115.9 seconds → shortened by 7.9 seconds by increasing the Mn content;

[0036] The third stage cooling rate VC3 = 0.02 × (0.85) 2 =0.014℃ / s→slowly cool to outlet temperature 465℃;

[0037] The performance results meet the following requirements: tensile strength 1256MPa; tread hardness 359.8-361.1HB, full-length hardness fluctuation ≤1.3HB, elongation 11.5%-13.5%.

[0038] Compared with the prior art, the present invention has the following beneficial technical effects:

[0039] This invention breaks through the traditional path of "high vanadium to maintain performance" and pioneers the "low vanadium critical cooling rate control" technical route. Through the dynamic coordination of precise composition control (V 0.04-0.07%) and three-stage gradient cooling, it achieves a vanadium cost reduction of more than 40% while ensuring H340-grade performance (σb ≥ 1230MPa, HB ≥ 355). DETAILED DESCRIPTION

[0040] The rolling process of the rail material of the present invention mainly comprises: billet→heating→BD1 rolling→BD2 rolling→CCS universal rolling mill continuous rolling→online heat treatment→cooling→straightening.

[0041] This invention breaks through the traditional path of "high vanadium to maintain performance" and pioneers the "low vanadium critical cooling rate control" technology route. Through the dynamic synergy of precise composition control (V 0.04-0.07%) and three-stage gradient cooling, it achieves a vanadium cost reduction of more than 40% while ensuring H340-grade performance (σb ≥ 1230MPa, HB ≥ 355). The core innovation lies in:

[0042] The critical phase transformation window of low-vanadium steel was found: when V≤0.07%, the bainite transformation termination temperature (Bf) rises to 480-510℃ (conventional high-vanadium steel Bf=430-460℃), and the cooling path needs to be reconstructed to avoid brittle phase precipitation;

[0043] The first "rapid cooling-constant temperature-slow cooling" three-stage model: based on the CCT curve characteristics of low-vanadium steel (austenite instability zone expanded by 20%), it accurately matches the cooling rate and phase transformation dynamics requirements.

[0044] Technical Solution

[0045] Exclusive ingredient design (mass fraction%, differentiated from standard U75VH):

[0046]

[0047] Key points of rolling:

[0048] The finishing rolling temperature is strictly limited to 910-940℃, and high-temperature austenitization is used to compensate for the decrease in nucleation driving force caused by vanadium reduction;

[0049] Core technical parameters of online heat treatment three-stage cooling

[0050] Stage 1: Cooling rate VC1 = [3.5 + 10 × (0.07 - V)] ± 0.3 °C / s → target 600-630 °C;

[0051] The second stage: constant temperature platform 540-590℃, maintained for t = [120-100×V] seconds;

[0052] The third stage: cooling rate VC3 = [0.02 × (Mn%) 2 ]±0.2℃ / s→Outlet 420-480℃.

[0053] Example 1 (Low-vanadium-high-manganese synergistic process)

[0054] Composition: C: 0.78%, Si: 0.65%, V = 0.042% (approaching the lower limit), Mn = 0.92% (mid-upper limit), P = 0.008% (Baotou Steel's advantage)

[0055] Process Execution:

[0056] Finish rolling temperature 935℃: 50℃ higher than the conventional lower limit, expanding the austenite recrystallization window;

[0057] Three-stage cooling dynamic calculation:

[0058] In the first stage, VC1 = 3.5 + 10 × (0.07-0.042) = 3.78 °C / s → the actual rail head temperature is 620 °C (meets the standard);

[0059] The second stage is a constant temperature of 550°C, and the constant temperature time is t = 120-100×0.042 = 115.8 seconds → 115 seconds in actual time;

[0060] The third stage VC3 = 0.02 × (0.92) 2 =0.017°C / s→actual 0.02°C / s (no martensite detected).

[0061] Performance results: tensile strength 1265MPa; tread hardness is usually 357.2-359.3HB, hardness fluctuation along the entire length is ≤2.1HB, and elongation is 11.5%-13.5%.

[0062] Example 2 (maximum utilization of resources)

[0063] Composition: C: 0.78%, V = 0.048%, S = 0.004% (Baotou steel ore S ≤ 0.005% characteristics), Si = 0.62% (middle limit), Mn = 0.92% (middle upper limit); final rolling temperature 932 ° C.

[0064] Fine-tune the cooling rate in the first stage based on the Si content: VC1' = VC1 × [1 - 0.3 × (Si - 0.60)] → actual 3.42 °C / s;

[0065] The second stage is a constant temperature of 570°C, and the constant temperature time is t = 120-100×0.042 = 115.8 seconds → 115 seconds in actual time.

[0066] The third stage VC3 = 0.02 × (0.92) 2 =0.017°C / s→actual 0.02°C / s (no martensite detected).

[0067] Performance results: tensile strength 1248MPa; tread hardness is usually 356.8-358.1HB, hardness fluctuation along the entire length is ≤2.3HB, and elongation is 11.5%-12.5%.

[0068] Example 3:

[0069] Chemical composition design (wt%):

[0070] C: 0.79%, Si: 0.58%, Mn: 0.85%, V: 0.041%, P: 0.006%, S: 0.004%, where V = 0.041% (approaching the lower critical value);

[0071] The final rolling temperature is increased to 935℃;

[0072] Three-stage cooling process execution:

[0073] First stage cooling rate VC1 = 3.5 + 10 × (0.07-0.041) = 3.89°C / s → measured rail head temperature 625°C (target 600-630°C);

[0074] The second stage is maintained at a constant temperature of 580°C, with a holding time of t = 120-100 × 0.041 = 115.9 seconds (shortened by 7.9 seconds by increasing the Mn content);

[0075] The third stage cooling rate VC3 = 0.02 × (0.85) 2 =0.014℃ / s→slowly cool to outlet temperature 465℃.

[0076] Performance results: tensile strength 1256MPa; tread hardness is usually 359.8-361.1HB, hardness fluctuation along the entire length is ≤1.3HB, elongation is 11.5%-13.5%.

[0077] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction, the process mainly comprises: billet → heating → BD1 rolling → BD2 rolling → CCS universal rolling mill → online heat treatment → cooling → straightening; characterized by: in: Key points of rolling: The final rolling temperature is strictly limited to 910-940℃, and high-temperature austenitization is used to compensate for the decrease in nucleation driving force caused by vanadium reduction; Online heat treatment uses three-stage cooling, and its core technical parameters are as follows: Stage 1: Cooling rate VC1 = [3.5 + 10 × (0.07 - V)] ± 0.3 °C / s → target 600-630 °C, where V is the mass percentage content; The second stage: constant temperature platform 540-590℃, maintained for t = [120-100×V] seconds; The third stage: cooling rate VC3 = [0.02 × (Mn) 2 ]±0.2℃ / s→export 420-480℃, where Mn is the value of its mass percentage content.

2. The low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction according to claim 1 is characterized in that: The chemical composition of the low-cost H340 grade high-strength rail is as follows by weight: C 0.6-0.9%, Si 0.58-0.65%, V 0.04-0.07%, Mn 0.85-0.95%, P≤0.015%, and the remainder is Fe and unavoidable impurities.

3. The low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction according to claim 2 is characterized in that: The chemical composition of the low-cost H340 grade high-strength rail is as follows by mass percentage: C: 0.78%, Si: 0.65%, V = 0.042%, Mn = 0.92%, P = 0.008%, and the remainder is Fe and unavoidable impurities.

4. The low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction according to claim 3 is characterized by: Process Execution: Finish rolling temperature 935℃: 50℃ higher than the conventional lower limit, expanding the austenite recrystallization window; Three-stage cooling dynamic calculation: In the first stage, VC1 = 3.5 + 10 × (0.07-0.042) = 3.78 °C / s → the measured rail head is 620 °C; The second stage is a constant temperature of 550°C, and the constant temperature time is t = 120-100×0.042 = 115.8 seconds → 115 seconds in actual time; The third stage VC3 = 0.02 × (0.92) 2 =0.017℃ / s→actual 0.02℃ / s; The performance results meet the following requirements: tensile strength 1265MPa; tread hardness is usually 357.2-359.3HB, hardness fluctuation along the entire length is ≤2.1HB, and elongation is 11.5%-13.5%.

5. The low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction according to claim 2 is characterized in that: The chemical composition of the low-cost H340 grade high-strength rail is as follows by mass percentage: C: 0.78%, V = 0.048%, S = 0.004%, Si = 0.62%, Mn = 0.92%, and the rest is Fe and unavoidable impurities.

6. The low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction according to claim 5 is characterized in that: Fine-tune the cooling rate in the first stage based on the Si content: VC1' = VC1 × [1 - 0.3 × (Si - 0.60)] → actual 3.42 °C / s; The second stage is a constant temperature of 570°C, and the constant temperature time is t = 120-100×0.042 = 115.8 seconds → 115 seconds in actual time. The third stage VC3 = 0.02 × (0.92) 2 =0.017℃ / s→actual 0.02℃ / s; The performance results meet the following requirements: tensile strength 1248MPa; tread hardness 356.8-358.1HB, full-length hardness fluctuation ≤2.3HB, elongation 11.5%-12.5%.

7. The low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction according to claim 2 is characterized in that: The chemical composition of the low-cost H340 grade high-strength rail is as follows by mass percentage: C: 0.79%, Si: 0.58%, Mn: 0.85%, V: 0.041%, P: 0.006%, S: 0.004%, V = 0.041%, and the rest is Fe and unavoidable impurities.

8. The low-cost H340 grade high-strength rail rolling and online heat treatment process based on alloy reduction according to claim 7 is characterized in that: The final rolling temperature is increased to 935℃. Three-stage cooling process execution: First stage cooling rate VC1 = 3.5 + 10 × (0.07-0.041) = 3.89 ° C / s → measured rail head temperature 625 ° C; The second stage is kept at 580°C, and the holding time is t = 120-100 × 0.041 = 115.9 seconds → shortened by 7.9 seconds by increasing the Mn content; The third stage cooling rate VC3 = 0.02 × (0.85) 2 =0.014℃ / s→slowly cool to outlet temperature 465℃; The performance results meet the following requirements: tensile strength 1256MPa; tread hardness 359.8-361.1HB, full-length hardness fluctuation ≤1.3HB, elongation 11.5%-13.5%.

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