Heat treatment process for removing abnormal structure of net-shaped ferrite of low-carbon steel rail

Abstract

The invention discloses a heat treatment process for removing abnormal structures of low-carbon steel rail network ferrite. The heat treatment process comprises the following steps: (a) entering a heat treatment cooling rail head temperature range of 760-800 DEG C; (b) a first stage: cooling to 550-580 DEG C under the conditions that the P1 air pressure is 12-15KPa and the cooling speed v1 is 2.5-5.0 DEG C / s; the method is characterized in that the cooling speed v1 of the first stage and the thickness H of a rail head meet the condition that v1 = 3.2 / H0.3-5.0 / H0.3; wherein the unit of v1 is DEG C / s, and the unit of H is cm; (c) a second stage: cooling to 460-490 DEG C under the conditions that the air pressure P2 is 15-17KPa and the cooling speed v2 is 1.5-2.5 DEG C / s, wherein P2 / v2 is equal to 6.0 + / -0.5; and (d) obtaining a ferritic structure without continuous network, wherein the pearlite content is greater than or equal to 98.5%. The invention aims to realize effective removal of network ferrite and improve the comprehensive performance of the steel 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 removing abnormal network ferrite structure in low-carbon steel rails. Background Technology

[0002] While existing heat treatment processes for steel rails (such as CN110616338A) improve hardness, they cannot eliminate the continuous network ferrite precipitated along the austenite grain boundaries. According to data from the *Journal of Railway Materials*, rails containing >3% network ferrite, when used in the heavy-haul section of the Daqin Railway, exhibit a crack propagation rate of 0.82 mm / 10,000 cycles, which is 47% ± 5% higher than rails without network structure. Traditional single-stage cooling processes suffer from insufficient cooling rates in the eutectoid transformation zone, leading to the formation of a network of proeutectoid ferrite. A search of domestic and international patents and literature reveals that existing railway rails are mainly produced according to the TB / T2344-2012 standard. Their performance improvement largely depends on the control of trace elements (such as Cr, V, etc.) and conventional heat treatment (such as online residual heat quenching and offline tempering). A typical technology can be found in patent CN103866128A (A high-strength wear-resistant steel rail and its production method). This patent improves hardness by adjusting the C content (0.75-0.85%) and a single-stage quenching process (cooling rate 3-6℃ / s), but it does not design a solution for the network ferrite problem of low-carbon steel rails (C≤0.72%). Patent CN110616338A (two-stage cooling process): Although it mentions controlling the cooling rate (1-3℃ / s), it does not establish the correlation between pressure and cooling rate, resulting in a temperature difference of >100℃ between the core and surface of the rail head in actual production, inducing network ferrite. Another patent CN105296025B (a heat treatment process for heavy-duty rails) discloses a two-stage cooling process, but the initial cooling temperature is 820-860℃, and it does not clearly define the technical means to control network ferrite. As a result, low-carbon rails still have the problem of ferrite precipitating along the grain boundaries under heavy-duty conditions, which manifests as insufficient low-temperature impact toughness (impact energy ≤12J at -40℃) and decreased wear resistance (Burl hardness ≤300HB).

[0003] With the annual transport volume of heavy-haul lines such as the Datong-Qinhuangdao Railway exceeding 400 million tons, the contact stress on the rail tread has increased to over 800 MPa. Rails produced using existing processes frequently suffer from early fatigue cracks caused by network ferrite, resulting in direct economic losses exceeding 1 billion yuan annually due to rail replacements. Therefore, developing a heat treatment process for low-carbon steel rails that can completely remove network ferrite while maintaining both strength and toughness has become an urgent technical challenge. Summary of the Invention

[0004] The purpose of this invention is to provide a heat treatment process for removing abnormal network ferrite structure in low-carbon steel rails. This invention addresses the shortcomings of existing technologies where low-carbon steel rails easily form network ferrite after heat treatment, resulting in poor low-temperature toughness (impact energy less than 15J at -40℃) and insufficient wear resistance (HB≤310), and where existing patents do not offer targeted solutions. This invention provides a two-stage heat treatment process that precisely controls the cooling temperature, pressure, and speed to effectively remove network ferrite and improve the overall performance of the rail.

[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 removing abnormal network ferrite structure from low-carbon steel rails, comprising:

[0007] (a) The temperature range of the heat treatment cooling rail head is 760-800℃;

[0008] (b) First stage: Cooling to 550-580℃ with a wind pressure of 12-15KPa and a cooling rate of 2.5-5.0℃ / s using a P1 air pressure; characterized in that the cooling rate v1 in stage 1 satisfies the following condition:

[0009] v1 = 3.2 / H 0.3 -5.0 / H 0.3

[0010] Where v1 is in °C / s and H is in cm;

[0011] (c) Second stage: Cool to 460-490℃ with wind pressure P2 of 15-17KPa and cooling rate v2 of 1.5-2.5℃ / s, and satisfy P2 / v2=6.0±0.5;

[0012] (d) Obtain a pearlite content ≥98.5% and no continuous network ferrite structure.

[0013] Furthermore, the chemical composition of the low-carbon steel rail by mass percentage is as follows: C: 0.67–0.72%, Mn: 0.85–1.05%, Cr: 0.15–0.30%, V: 0.03–0.07%, N: 0.0040–0.0060%; and must simultaneously satisfy: Cr / C ≥ 0.35, V / 10N = 1.0 ± 0.5; the Si content and rail head thickness Hmm satisfy: Si ≤ (0.45 + 0.01H)%; Mn / C = 1.25 ± 0.10; the remainder is Fe and unavoidable impurities.

[0014] Furthermore, the chemical composition of the low-carbon steel rail by mass percentage is as follows: C: 0.68%, Si: 0.42%, Mn: 0.98%, Cr: 0.25%, V: 0.052%, N: 0.0042%; the remainder is Fe and unavoidable impurities.

[0015] Furthermore, the chemical composition of the low-carbon steel rail by mass percentage is as follows: C: 0.70%, Si: 0.38%, Mn: 0.92%, Cr: 0.28%, V: 0.048%, N: 0.0048%; the remainder is Fe and unavoidable impurities.

[0016] Furthermore, the chemical composition of the low-carbon steel rail by mass percentage is as follows: C: 0.69%, Si: 0.43%, Mn: 0.92%, Cr: 0.29%, V: 0.040%, N: 0.0039%; the remainder is Fe and unavoidable impurities.

[0017] Furthermore, the impact energy of this low-carbon steel rail at -40℃ reaches 13.7~16.1J.

[0018] Furthermore, the low-carbon steel rail has a Brinell hardness of 315–323 HB.

[0019] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0020] Targeted solution to the network ferrite problem: By using an initial temperature of 760-800℃ and two stages of differentiated cooling rates, the amount of ferrite precipitation is reduced by ≥90% compared to the single-stage cooling of CN103866128A, achieving a "network-free ferrite" microstructure.

[0021] Performance indicators surpass existing technologies: -40℃ impact energy reaches 13.7~16.1J (more than 20% higher than CN105296025B), Brinell hardness is 315~323HB (meeting the requirement of ≥310HB for heavy-load lines), and the yield strength and tensile strength are better matched (σ0.2 / σb≈0.6, avoiding the risk of brittle fracture);

[0022] High process compatibility: It uses high-pressure air cooling medium, so there is no need to modify the existing production line, making it suitable for industrial mass production. Attached Figure Description

[0023] The present invention will be further described below with reference to the accompanying drawings.

[0024] Figure 1 For comparison, the metallographic structure of the example is (network ferrite + pearlite);

[0025] Figure 2 The metallographic structure of the railhead in Example 2 (small amount of ferrite + pearlite);

[0026] Figure 3 Example 2: The metallographic interlayer spacing of the railhead is 110 nm. Detailed Implementation

[0027] The chemical composition of the heat treatment cooling process for the test rails is shown in Table 1.

[0028] Table 1. Components of each example (mass percentage / %)

[0029]

[0030] The present invention produces rail heads with a width of 70.8 mm according to the 60 kg / m specification in the TB / T2344.1-2020 standard. The heat treatment and cooling process is shown in Table 2.

[0031] Table 2 Heat treatment processes for each example

[0032]

[0033] Properties of heat-treated steel samples: Tensile test specimens had a diameter d0 = 10 mm and a gauge length L0 = 5d0. Tread surface hardness was measured on the rail, with a specimen length of 250 mm. 0.5 mm of the rail head was ground off. 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, designated as A. KU2 Type notch. The experimental results are shown in Table 2.

[0034] Table 3 Mechanical properties of each example

[0035]

[0036] As shown in Table 3, each example exhibits good strength, low-temperature impact toughness, and mechanical properties, with a microstructure free of network ferrite. Rails produced using these materials meet Russian standard technical requirements.

[0037] 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 removing abnormal network ferrite structure from low-carbon steel rails, characterized in that, include: (a) The temperature range of the heat treatment cooling rail head is 760-800℃; (b) First stage: Cool to 550-580℃ with a P1 air pressure of 12-15KPa and a cooling rate v1 of 2.5-5.0℃ / s; The characteristic is that the cooling rate v1 in stage 1 and the rail head thickness H satisfy the following: v1=3.2 / H 0.3 -5.0 / H 0.3 Where v1 is in °C / s and H is in cm; (c) Second stage: Cool to 460-490℃ with wind pressure P2 of 15-17KPa and cooling rate v2 of 1.5-2.5℃ / s, and satisfy P2 / v2=6.0±0.5; (d) Obtain a pearlite content ≥98.5% and no continuous network ferrite structure.

2. The heat treatment process for removing abnormal network ferrite structure from low-carbon steel rails according to claim 1, characterized in that, The chemical composition of the low-carbon steel rail by mass percentage is as follows: C: 0.67-0.72%, Mn: 0.85-1.05%, Cr: 0.15-0.30%, V: 0.03-0.07%, N: 0.0040-0.0060%; and must simultaneously satisfy: Cr / C ≥ 0.35, V / 10N = 1.0 ± 0.5; Si content and rail head thickness Hmm satisfy: Si ≤ (0.45 + 0.01H)%; Mn / C = 1.25 ± 0.10; the remainder is Fe and unavoidable impurities.

3. The heat treatment process for removing abnormal network ferrite structure from low-carbon steel rails according to claim 1, characterized in that, The chemical composition of the low-carbon steel rail by mass percentage is as follows: C: 0.68%, Si: 0.42%, Mn: 0.98%, Cr: 0.25%, V: 0.052%, N: 0.0042%; the remainder is Fe and unavoidable impurities.

4. The heat treatment process for removing abnormal network ferrite structure from low-carbon steel rails according to claim 1, characterized in that, The chemical composition of the low-carbon steel rail by mass percentage is as follows: C: 0.70%, Si: 0.38%, Mn: 0.92%, Cr: 0.28%, V: 0.048%, N: 0.0048%; the remainder is Fe and unavoidable impurities.

5. The heat treatment process for removing abnormal network ferrite structure from low-carbon steel rails according to claim 1, characterized in that, The chemical composition of the low-carbon steel rail by mass percentage is as follows: C: 0.69%, Si: 0.43%, Mn: 0.92%, Cr: 0.29%, V: 0.040%, N: 0.0039%; the remainder is Fe and unavoidable impurities.

6. The heat treatment process for removing abnormal network ferrite structure from low-carbon steel rails according to claim 1, characterized in that, The low-carbon steel rail has an impact energy of 13.7 to 16.1 J at -40℃.

7. The heat treatment process for removing abnormal network ferrite structure from low-carbon steel rails according to claim 1, characterized in that, The low-carbon steel rail has a Brinell hardness of 315–323 HB.

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