A hypereutectoid steel rail cementite and martensite suppression heat treatment process based on rare earth microalloying and dynamic parameter collaborative control

Abstract

The application discloses a kind of based on rare earth microalloying and dynamic parameter collaborative control's hypereutectoid steel rail cementite and martensite inhibition heat treatment process, belongs to metallurgical material technical field.The method includes: a. into heat treatment cooling rail head temperature range 780-820 ℃;B. first stage: with wind pressure P1 is 14-17KPa, cooling speed v1 is 3.0-5.5 ℃ / s and is cooled to 600-630 ℃;C. second stage: with wind pressure P2 is 16-19KPa, cooling speed v2 is 2.0-3.5 ℃ / s and is cooled to 480-520 ℃, and satisfy P2 / v2=5.5±0.5;D. after the above processing: obtain pearlite content ≥97.5%, no continuous network cementite and martensite content ≤1.0% organization;And limit its mass percentage chemical composition.The purpose of the application is to further realize the effective inhibition of cementite and martensite, improve the comprehensive performance of rail.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials technology, and in particular relates to a heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control. Background Technology

[0002] Existing heat treatment processes for hypereutectoid rails (such as CN110716338A) can improve hardness and strength, but they cannot effectively suppress the proeutectoid cementite precipitating along austenite grain boundaries and the abnormal martensite structure formed during quenching. According to research data from the *Journal of Railway Science and Engineering*, hypereutectoid rails containing >2% proeutectoid cementite experience a reduction of approximately 40% in fatigue crack initiation life when serving on high-speed lines, and the martensitic structure leads to a significant decrease in toughness (impact energy ≤10J at -20℃). Traditional processes often employ single-stage rapid cooling, but this easily induces continuous network precipitation of cementite in hypereutectoid steel, and uneven cooling can lead to the formation of localized martensite, deteriorating rail performance. A search of domestic and international patents reveals that existing hypereutectoid steel rails are mainly produced according to the TB / T2344-2012 standard. Their performance improvement largely depends on high carbon design (C≥0.80%) and conventional quenching-tempering processes. A typical technology can be found in patent CN103866129A (A high carbon wear-resistant steel rail and its manufacturing method). This patent improves hardness by increasing the C content (0.82-0.90%) and single-stage quenching (cooling rate 5-8℃ / s), but it does not propose solutions for the abnormal microstructure of cementite and martensite that precipitates first in hypereutectoid steel. Although patent CN110716338A (three-stage cooling process) controls the cooling rate, it does not introduce rare earth elements to refine the grain boundaries, making it difficult to suppress the precipitation of cementite along the grain boundaries. Another patent CN105296026B (a heat treatment process for hypereutectoid rails) discloses tempering temperature control, but does not involve the synergistic effect of rare earth microalloying and cooling parameters, resulting in the presence of cementite network precipitation and abnormal martensite structure in the rails under high-speed conditions, which manifests as insufficient toughness (impact energy ≤11J at -20℃) and fluctuations in wear resistance (Britell hardness ≥340HB but poor uniformity).

[0003] With the increase in axle load and freight volume on freight railways, the contact stress on the rail tread reaches over 900 MPa. Hypereutectoid rails produced using existing processes suffer from prominent problems of early brittle fracture and fatigue damage due to cementite network precipitation and martensite formation. The economic losses due to early rail replacement and maintenance exceed 1.5 billion yuan annually. Therefore, developing a heat treatment process for hypereutectoid rails that can effectively suppress the abnormal microstructure of pre-precipitated cementite and martensite while maintaining both high hardness and high toughness has become an urgent need for the industry. Summary of the Invention

[0004] To address the shortcomings of existing technologies where hypereutectoid rails tend to develop a network of cementite and abnormal martensite structures after heat treatment, resulting in poor low-temperature toughness (impact energy less than 12J at -20℃) and insufficient hardness uniformity, and the lack of a synergistic control scheme between rare earth microalloying and cooling parameters in existing patents, the present invention aims to provide a synergistic optimization of a two-stage heat treatment process through composition optimization and precise control of cooling temperature, pressure, and speed. This will further effectively suppress cementite and martensite, thereby improving the overall performance of the rails.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention discloses a heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control, comprising:

[0007] a. The temperature range for entering the heat treatment cooling rail head is 780-820℃;

[0008] b. First stage: Cooling to 600-630℃ with air pressure P1 of 14-17KPa and cooling rate v1 of 3.0-5.5℃ / s;

[0009] c. Second stage: Cool to 480-520℃ with wind pressure P2 of 16-19KPa and cooling rate v2 of 2.0-3.5℃ / s, while satisfying P2 / v2=5.5±0.5;

[0010] d. After the above treatment: a microstructure with a pearlite content ≥97.5%, no continuous network cementite, and a martensite content ≤1.0% is obtained;

[0011] The chemical composition of the hypereutectoid rail by mass percentage is as follows: C: 0.82–0.90%, Mn: 0.70–1.00%, Si: 0.20–0.50%, Cr: 0.20–0.40%, V: 0.05–0.10%, RE: 0.01–0.05%; and simultaneously satisfies: RE / C ≥ 0.04, V / Mn = 0.08 ± 0.03; and the Mn and C contents satisfy: Mn / C = 0.95 ± 0.10; the remainder is Fe and unavoidable impurities.

[0012] Furthermore, at the end of the second stage, the pearlite lamellar spacing is 90-130 nm.

[0013] Furthermore, the Si content and the rail head thickness H (in mm) satisfy the following condition: Si ≤ 0.35 + 0.015H.

[0014] Furthermore, in the first stage: the cooling rate v1 and the rail head thickness H satisfy the following:

[0015] V1 = (4.0~6.0) / H0.25 The rail head thickness H is measured in cm.

[0016] Furthermore, the chemical composition of the hypereutectoid rail by mass percentage is as follows: C: 0.85%, Mn: 0.88%, Si: 0.42%, Cr: 0.32%, V: 0.075%, RE: 0.035%; RE / C: 0.041, V / Mn = 0.085; and the Mn and C contents satisfy: Mn / C = 0.966; the remainder is Fe and unavoidable impurities.

[0017] Furthermore, the chemical composition of the hypereutectoid rail by mass percentage is as follows: C: 0.87%, Mn: 0.90%, Si: 0.40%, Cr: 0.35%, V: 0.080%, RE: 0.035%; RE / C: 0.040, V / Mn = 0.089; and the Mn and C contents satisfy: Mn / C = 1.035; the remainder is Fe and unavoidable impurities.

[0018] Furthermore, the chemical composition of the hypereutectoid rail by mass percentage is as follows: C: 0.84%, Mn: 0.86%, Si: 0.44%, Cr: 0.30%, V: 0.070%, RE: 0.040%; RE / C: 0.048, V / Mn = 0.081; and the Mn and C contents satisfy: Mn / C = 1.024; the remainder is Fe and unavoidable impurities.

[0019] Furthermore, the performance indicators meet the following requirements: impact energy at -20℃ reaches 13.0~15.5J, Brinell hardness is 405~425HB, yield strength and tensile strength are better matched, σ0.2 / σb≈0.65, avoiding the risk of brittle fracture.

[0020] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0021] Compared with the prior art (including the aforementioned comparative patents), the present invention has the following outstanding advantages:

[0022] Targeted solutions to cementite and martensite problems: Through rare earth microalloying + initial temperature of 780-820℃ + two-stage differentiated cooling rates, the amount of cementite precipitation is reduced by ≥85% and the amount of martensite formation is eliminated compared to the single-stage cooling of CN103866129A, achieving a "no network cementite, no martensite" microstructure.

[0023] Performance indicators surpass existing technologies: -20℃ impact energy reaches 13.0~15.5J (more than 25% higher than CN105296026B), Brinell hardness is 405~425HB (meeting the requirement of ≥370HB for ultra-high capacity lines), hardness uniformity is improved (standard deviation ≤3HB), and the matching of yield strength and tensile strength is better (σ0.2 / σb≈0.65, avoiding the risk of brittle fracture);

[0024] High process compatibility: It adopts high-pressure air cooling medium, which does not require modification of existing production lines and is suitable for industrial mass production; the addition of rare earth elements is low-cost and effective, and improves the service life of rails. Attached Figure Description

[0025] The present invention will be further described below with reference to the accompanying drawings.

[0026] Figure 1 The metallographic structure (network ferrite + pearlite) is shown in the comparative example.

[0027] Figure 2 The metallographic structure of the railhead in Example 2 is (small amount of ferrite + pearlite).

[0028] Figure 3 The spacing between the metallographic plates at the track head in Example 2 is 110 nm. Detailed Implementation

[0029] The chemical composition of the heat treatment cooling process for the test rails is shown in Table 1.

[0030] Table 1. Components (mass percentage / %) of each example and comparative example

[0031] Example C Si Mn Cr V RE RE / C V / Mn Mn / C Comparative Example 0.88 0.52 0.85 0.15 0.02 0 0 0.024 0.966 Example 1 0.85 0.42 0.88 0.32 0.075 0.035 0.041 0.085 1.035 Example 2 0.87 0.4 0.9 0.35 0.08 0.035 0.040 0.089 1.034 Example 3 0.84 0.44 0.86 0.3 0.07 0.04 0.048 0.081 1.024

[0032] In this embodiment, the rail head width is 76.0 mm according to the 75 kg / m specification in the TB / T2344.1-2020 standard. The heat treatment and cooling process is shown in Table 2.

[0033] Table 2 Heat treatment processes for each embodiment

[0034] Example Inlet temperature ℃ First stage wind pressure (kPa) First stage cooling rate ℃ / s Cool to temperature ℃ Second stage wind pressure (kPa) Second stage cooling rate ℃ / s Cool to temperature ℃ Comparative Example 795 15 3.8 620 17 2.8 500 Example 1 810 16 3.6 615 18 3.0 495 Example 2 805 15 3.5 625 17 3.2 510 Example 3 812 17 3.7 610 19 2.9 485

[0035] Properties of heat-treated steel specimens: Tensile specimens had a diameter of d0 = 10 mm and a gauge length of L0 = 5d0. Tread hardness was measured on the rail, with specimens 250 mm long and 0.5 mm ground off the top surface of the rail head. Five test points were used for Brinell hardness testing, and the average value was calculated. The test environment temperature was 20℃ ± 5℃. Impact sampling was performed at the center of the tread surface, longitudinally, with dimensions of 10 mm × 10 mm × 50 mm, using an A~KU2~ type notch. The experimental results are shown in Table 3.

[0036] Table 3 Mechanical properties of each embodiment

[0037] Example Yield strength σ ~ 0.2 ~ (MPa) Tensile strength σb (MPa) Elongation δ (%) Brinell hardness (HB) Impact energy at -20℃, J Microstructure Comparative Example 850 1420 8.5 398 8.5 5.2% network cementite + pearlite Example 1 890 1450 10.0 425 14.3 1.0% dotted cementite + pearlite Example 2 910 1480 10.5 428 15.5 0.8% dotted cementite + pearlite Example 3 895 1445 10.5 420 14.8 0.9% dotted cementite + pearlite

[0038] As shown in Table 3, each embodiment exhibits good strength, low-temperature impact toughness, and mechanical properties, with a microstructure free of network cementite. The rails produced using these rails meet the technological development requirements of ultra-high-capacity freight railways.

[0039] 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 heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control, characterized in that, include: a. The temperature range for entering the heat treatment cooling rail head is 780-820℃; b. First stage: Cooling to 600-630℃ with air pressure P1 of 14-17KPa and cooling rate v1 of 3.0-5.5℃ / s; c. Second stage: Cool to 480-520℃ with wind pressure P2 of 16-19KPa and cooling rate v2 of 2.0-3.5℃ / s, while satisfying P2 / v2=5.5±0.5; d. After the above treatment: a microstructure with a pearlite content ≥97.5%, no continuous network cementite, and a martensite content ≤1.0% is obtained; The chemical composition of the hypereutectoid rail by mass percentage is as follows: C: 0.82–0.90%, Mn: 0.70–1.00%, Si: 0.20–0.50%, Cr: 0.20–0.40%, V: 0.05–0.10%, RE: 0.01–0.05%; and simultaneously satisfies: RE / C ≥ 0.04, V / Mn = 0.08 ± 0.03; and Mn and C contents satisfy: Mn / C = 0.95 ± 0.10; the remainder is Fe and unavoidable impurities.

2. The heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control as described in claim 1, characterized in that, At the end of the second stage, the pearlite lamellar spacing is 90-130 nm.

3. The heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control as described in claim 1, characterized in that, The Si content and the rail head thickness H (in mm) must satisfy the following condition: Si ≤ 0.35 + 0.015H.

4. The heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control as described in claim 1, characterized in that, First stage: Cooling rate v1 and rail head thickness H satisfy: V1 = (4.0~6.0) / H 0.25 The rail head thickness H is measured in cm.

5. The heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control as described in claim 1, characterized in that, The chemical composition of the hypereutectoid rail by mass percentage is as follows: C: 0.85%, Mn: 0.88%, Si: 0.42%, Cr: 0.32%, V: 0.075%, RE: 0.035%; RE / C: 0.041, V / Mn = 0.085; and the Mn and C contents satisfy: Mn / C = 0.966; the remainder is Fe and unavoidable impurities.

6. The heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control as described in claim 1, characterized in that, The chemical composition of the hypereutectoid rail by mass percentage is as follows: C: 0.87%, Mn: 0.90%, Si: 0.40%, Cr: 0.35%, V: 0.080%, RE: 0.035%; RE / C: 0.040, V / Mn = 0.089; and the Mn and C contents satisfy: Mn / C = 1.035; the remainder is Fe and unavoidable impurities.

7. The heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control as described in claim 1, characterized in that, The chemical composition of the hypereutectoid rail by mass percentage is as follows: C: 0.84%, Mn: 0.86%, Si: 0.44%, Cr: 0.30%, V: 0.070%, RE: 0.040%; RE / C: 0.048, V / Mn = 0.081; and the Mn and C contents satisfy: Mn / C = 1.024; the remainder is Fe and unavoidable impurities.

8. The heat treatment process for suppressing cementite and martensite in hypereutectoid rails based on rare earth microalloying and dynamic parameter synergistic control as described in claim 1, characterized in that, Performance indicators meet the following requirements: impact energy of 13.0~15.5J at -20℃, Brinell hardness of 405~425HB, better matching between yield strength and tensile strength, σ0.2 / σb≈0.65, avoiding the risk of brittle fracture.

Images