A heat treatment process for a rare earth microalloyed high-strength rail

Abstract

The application discloses a heat treatment process of a rare earth micro-alloyed high-strength rail, which comprises the following steps: first, heating the rail to 920-940 DEG C; then, air cooling to 730-780 DEG C; through two-stage cooling module gradient cooling, the cooling speed is controlled to avoid the generation of martensite due to too fast cooling or the formation of reticular cementite due to too slow cooling; finally, the outlet temperature is 494-510 DEG C, and the structure is stable as pearlite plus a small amount of ferrite; and the chemical composition of the rail is as follows: C: 0.90-0.95%, Si: 0.5-0.6%, Mn: 0.85-0.95%, Cr: 0.1-0.3%, Nb: 0.01-0.02%, Ni: 0.1-0.2%, and Ce: 0.0015-0.0025%. Through accurate control of austenitizing temperature, air cooling temperature and gradient cooling parameters, the application solves the problems of the generation of reticular cementite and excessive residual stress of high-carbon steel rails.

Description

Technical Field

[0001] This invention belongs to the field of rail manufacturing technology, and particularly relates to a heat treatment process for rare earth microalloyed high-strength rails. Background Technology

[0002] Traditional rail materials, such as carbon steel and alloy steel, are prone to wear, fatigue, and plastic deformation under high-intensity loads. For example, prolonged heavy-load operation can cause surface wear on the rails, reducing their service life, and may also lead to fatigue cracks inside the rails, potentially causing serious safety accidents. Heavy-haul railways typically use more robust rails, and the design of sleepers and ballast beds must meet higher load-bearing requirements. Heavy-haul railway tracks should possess stronger compressive, bending, and shear resistance to withstand the high-stress environment under heavy loads. Therefore, developing new high-strength, wear-resistant heavy-haul rail materials is particularly important.

[0003] Rare earth elements possess unique chemical and physical properties due to their special electronic structure. They are commonly used as additives in alloys to improve the properties of metallic materials. They can significantly alter the microstructure of materials, improving strength, toughness, and wear resistance. Studies have shown that rare earth elements can significantly improve the microstructure of steel, enhancing its mechanical properties. For example, rare earth elements can enhance the strength and toughness of steel by refining grains and improving its phase composition. Research indicates that adding small amounts of rare earth elements (such as Ce and La) can effectively inhibit the aggregation of impurities in steel, promote the formation of a uniform alloy phase, and thus improve the overall performance of the steel. Furthermore, rare earth elements play a crucial role in the iron and steel metallurgical process. They can effectively remove impurity elements such as sulfur and oxygen from steel, thereby improving the smelting quality. Studies have shown that rare earth elements have significant deoxidation and desulfurization effects during smelting, significantly improving the toughness and corrosion resistance of steel. In addition, rare earth elements also have a significant impact on the phase transformation behavior of steel, contributing to improved heat treatment performance and further enhancing its mechanical properties.

[0004] Rare-earth microalloyed high-strength steel rails have broad application prospects in heavy-haul railways due to their high strength and good wear resistance. However, high-carbon steel rails are prone to the formation of network cementite during heat treatment, leading to a decrease in toughness. Excessive residual stress can also affect the service life of the rails. Existing heat treatment processes struggle to precisely control the cooling rate and temperature range, resulting in unstable rail performance. Therefore, it is necessary to optimize the heat treatment process to ensure that the rail microstructure and performance meet standards. Summary of the Invention

[0005] The purpose of this invention is to provide a heat treatment process for rare earth microalloyed high-strength steel rails, which solves the problems of network cementite formation and excessive residual stress in high-carbon steel rails by precisely controlling the austenitizing temperature, air cooling temperature and gradient cooling parameters.

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

[0007] This invention discloses a heat treatment process for rare-earth microalloyed high-strength steel rails. First, the rail is heated to 920-940℃ for austenitization, ensuring full solid solution of the alloying elements. It is then air-cooled to 730-780℃ to prepare for subsequent cooling. Gradient cooling is achieved through a two-stage cooling module to control the cooling rate, preventing excessively rapid cooling from producing martensite or excessively slow cooling from forming a network of cementite. The final outlet temperature is 494-510℃, ensuring a stable microstructure of pearlite with trace amounts of ferrite.

[0008] The gradient cooling is as follows: the first module is set to a temperature of 600-620℃ and rapidly cools down at a cooling rate of 3.8-4.2℃ / s; after entering the second module, the temperature is adjusted to 530-545℃ and the cooling rate is slowed down to 2.3-2.7℃ / s.

[0009] The chemical composition of the rare earth microalloyed steel rail by mass percentage is as follows: C: 0.90-0.95%, Si: 0.5-0.6%, Mn: 0.85-0.95%, Cr: 0.1-0.3%, Nb: 0.01-0.02%, Ni: 0.1-0.2%, Ce: 0.0015-0.0025%, with the remainder being Fe and impurities.

[0010] Furthermore, the rail to be treated is placed smoothly into a continuous heating furnace and heated at a uniform rate of 8℃ / min. When the temperature inside the furnace reaches 930℃, it is kept at this temperature for 10 minutes to allow the pearlite and ferrite structure inside the rail to fully transform into uniform austenite.

[0011] Furthermore, the air cooling stage ends when the surface temperature of the rail drops to 750°C.

[0012] Furthermore, the steel rail, with a temperature of 750°C, is rapidly fed into a dedicated temperature-controlled cooling device, passing through two temperature control modules in sequence.

[0013] Furthermore, the first module is set to a temperature of 610℃ and is rapidly cooled at a cooling rate of 4℃ / s.

[0014] Furthermore, upon entering the second module, the temperature is adjusted to 538℃, and the cooling rate is slowed down to 2.5℃ / s.

[0015] Furthermore, the temperature of the rail stabilizes at 500°C when it leaves the device, and then the rail is moved to a normal temperature environment to air cool to room temperature.

[0016] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0017] This invention solves the problems of network cementite formation and excessive residual stress in high-carbon steel rails by precisely controlling the austenitizing temperature, air cooling temperature, and gradient cooling parameters.

[0018] The rails treated by this process have a tensile strength of 1371~1410MPa, a tread hardness of 392~411HB, an elongation of 9~10%, and a residual stress of ≤206MPa. All properties meet the requirements of heavy-haul railways, and the process is stable and controllable, making it suitable for industrial production. Attached Figure Description

[0019] The present invention will be further described below with reference to the accompanying drawings.

[0020] Figure 1 This refers to the rail hardness value.

[0021] Figure 2 Sampling locations for rail structure and head, waist, and bottom structures;

[0022] Figure 3 This is a photo showing the spacing between the images;

[0023] Figure 4 Location for full-section hardness sampling;

[0024] Figure 5 These are the hardness values ​​of each line on the rail head.

[0025] Figure 6 A surface scan image of rare earth elements. Detailed Implementation

[0026] Example

[0027] A heat treatment process for rare earth microalloyed high-strength steel rails:

[0028] The rolled rare earth microalloyed steel rail has the following chemical composition: C: 0.92%, Si: 0.55%, Mn: 0.90%, Cr: 0.2%, Nb: 0.015%, Ni: 0.15%, Ce: 0.0020%, with the remainder being iron and impurities.

[0029] Heat treatment process:

[0030] (1) Austenitization: The rail to be treated is placed smoothly into a continuous heating furnace and heated at a uniform rate of 8℃ / min. When the temperature inside the furnace reaches 930℃, the temperature is maintained for 10 minutes to allow the pearlite, ferrite and other structures inside the rail to be fully transformed into uniform austenite, thus preparing the structure for subsequent cooling phase transformation.

[0031] (2) Air cooling: The austenitized rails are taken out of the heating furnace and placed in a well-ventilated room temperature environment for natural air cooling. The air cooling stage ends when the surface temperature of the rails drops to 750°C.

[0032] (3) Gradient cooling: The rail at 750℃ is quickly sent into a special temperature control cooling device and passes through two temperature control modules in sequence: the first module is set to 610℃ and is cooled rapidly at a cooling rate of 4℃ / s; after entering the second module, the temperature is adjusted to 538℃ and the cooling rate is slowed down to 2.5℃ / s. The precise control of the rail structure is achieved through gradient cooling.

[0033] (4) Final cooling: Strictly control the outlet temperature of the cooling device to ensure that the temperature of the rail is stable at 500℃ when it leaves the device. Then move the rail to a normal temperature environment and let it air cool to room temperature to complete the entire heat treatment process.

[0034] The tensile properties and hardness test results of the test rails are shown in Table 1.

[0035] Table 1. Laboratory tensile properties of the test rails

[0036]

[0037] The tensile strength and elongation of the test rail were above 1330MPa and 9% respectively, reaching the target values, and the tread hardness reached the target of 390HB.

[0038] like Figure 1 As shown, the hardness of the test steel was compared with that of U78CrV heat-treated steel rail, and the average hardness was 16 HB higher.

[0039] Metallographic samples were taken from the test rails according to TB / T2344 standard for non-metallic inclusion and microstructure testing. The test results of the decarburized layer of the test rails all met the requirement of TB / T 2344 for a decarburized layer ≤0.5mm, with an average of 0.32mm.

[0040] Depend on Figure 2 It can be seen that the microstructure of the rail head is pearlite with trace amounts of ferrite, but the web has cementite microstructure, some of which are in the form of a network. The steel was in the furnace for 4 hours.

[0041] The inter-rail spacing was measured by taking samples from the test rail head, and the inter-rail spacing was 0.27 μm.

[0042] Table 2 Residual stress of the test rails

[0043]

[0044] Note: + indicates tensile stress.

[0045] The test results show that the residual tensile stress along the centerline of the rail base meets the requirements of TB / T 2344.

[0046] The crack propagation rate of the rails was sampled and tested according to the requirements of TB / T 2344-2012 standard, and the results are shown in Table 5-19. The test results satisfy the condition that when ΔK = 10 MPa·m 1 / 2 and ΔK = 13.5 MPa·m 1 / 2 At that time, da / dN(m / Gc) should be less than or equal to 17 and 55 respectively.

[0047] Table 3. Crack propagation rate of rails da / dN (m / Gc)

[0048]

[0049] Take rail samples for full-section hardness testing. Test locations are as follows: Figure 4 As shown.

[0050] The hardness values ​​at points A, B, C, D, and E of the test rail head were determined by... Figure 5 As shown, the hardness values ​​of each line show a decreasing trend without significant fluctuations. The range of line A is 41.7~35.6 HRC, line B is 41.8~35.8 HRC, line C is 41.2~36.3 HRC, line D is 41.6~35.4 HRC, and line E is 41.7~35.1 HRC.

[0051] To analyze the rare earth content in steel rails, a sample (rare earth detection level of 10 ppm) was subjected to surface scanning. The results are shown below. Figure 6 .

[0052] 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 rare earth microalloyed high-strength steel rails, characterized in that: First, the rail is heated to 920-940℃ for austenitization to ensure full solid solution of alloying elements; then it is air-cooled to 730-780℃ to prepare for subsequent cooling; gradient cooling is performed through a two-stage cooling module to control the cooling rate and avoid excessively rapid cooling that produces martensite or excessively slow cooling that forms network cementite; the final outlet temperature is 494-510℃ to ensure a stable microstructure of pearlite with trace amounts of ferrite. The gradient cooling is as follows: the first module is set to a temperature of 600-620℃ and rapidly cools down at a cooling rate of 3.8-4.2℃ / s; after entering the second module, the temperature is adjusted to 530-545℃ and the cooling rate is slowed down to 2.3-2.7℃ / s. The chemical composition of the rare earth microalloyed steel rail by mass percentage is as follows: C: 0.90-0.95%, Si: 0.5-0.6%, Mn: 0.85-0.95%, Cr: 0.1-0.3%, Nb: 0.01-0.02%, Ni: 0.1-0.2%, Ce: 0.0015-0.0025%, with the remainder being Fe and impurities.

2. The heat treatment process for rare earth microalloyed high-strength steel rails according to claim 1, characterized in that: The rail to be treated is placed smoothly into a continuous heating furnace and heated at a uniform rate of 8℃ / min. When the temperature inside the furnace reaches 930℃, it is kept at this temperature for 10 minutes to allow the pearlite and ferrite structure inside the rail to fully transform into uniform austenite.

3. The heat treatment process for rare earth microalloyed high-strength steel rails according to claim 1, characterized in that: The air cooling stage ends when the surface temperature of the rail drops to 750℃.

4. The heat treatment process for rare earth microalloyed high-strength steel rails according to claim 1, characterized in that: The steel rail, with a temperature of 750℃, is quickly fed into a special temperature-controlled cooling device and passes through two temperature control modules in sequence.

5. The heat treatment process for rare earth microalloyed high-strength steel rails according to claim 1, characterized in that: The first module is set to a temperature of 610℃ and is rapidly cooled at a rate of 4℃ / s.

6. The heat treatment process for rare earth microalloyed high-strength steel rails according to claim 1, characterized in that: After entering the second module, the temperature is adjusted to 538℃, and the cooling rate is slowed down to 2.5℃ / s.

7. The heat treatment process for rare earth microalloyed high-strength steel rails according to claim 1, characterized in that: When the rail leaves the device, the temperature is stable at 500℃. The rail is then moved to a normal temperature environment and allowed to air cool to room temperature.

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