High-speed train axle steel with high corrosion fatigue resistance and stable residual stress layer and preparation method thereof

Abstract

The application discloses high-speed train axle steel with high corrosion fatigue resistance and a stable residual stress layer, which is prepared by adopting a high-silicon-medium-chromium-rare earth composite alloying design and combining an innovative heat treatment process of incomplete quenching and two-stage tempering, so that the prepared axle steel has excellent conventional mechanical properties, Rm is greater than or equal to 750 MPa, Rp0.2 is greater than or equal to 600 MPa, and -40 DEG C KV2 is greater than or equal to 60 J, has super-high corrosion fatigue resistance, and has a beneficial surface residual compressive stress, and thus can meet the use requirements in a corrosive environment such as a coastal area and a high-humidity environment.

Description

Technical Field

[0001] This invention belongs to the field of key components and materials for rail transit, specifically relating to a high-speed train axle steel with ultra-high corrosion fatigue resistance and a method for actively introducing beneficial residual compressive stress on the surface through heat treatment, which is used in corrosive environments such as coastal and high-humidity areas. Background Technology

[0002] High-speed trains with speeds of 300 km / h and above not only bear high-cycle rotational bending fatigue loads on their axles, but also face the severe challenge of corrosion fatigue when traversing corrosive environments such as coastal areas and industrial zones, due to the combined effects of corrosive media and alternating stress. The corrosion fatigue limit is usually much lower than the fatigue limit in air, and the crack initiation life is significantly shortened.

[0003] Existing high-speed axle steels (such as EA4T and 30NiCrMoV12) and conventional "quenching + high-temperature tempering" processes primarily optimize the strength and toughness of the matrix, offering limited improvement to the intrinsic corrosion resistance of the material and insufficient ability to actively control the residual stress state of the surface layer. Although post-treatments such as surface rolling and shot peening can introduce compressive stress, this stress layer may loosen during use and increases manufacturing costs and processes. Therefore, there is an urgent need to develop an integrated technical solution that combines the material's high corrosion resistance with the in-situ formation of deep compressive stress during heat treatment. Summary of the Invention

[0004] The purpose of this invention is to provide a novel high-speed train axle and its manufacturing method. Through a "high-silicon-medium-chromium-rare earth" composite alloying design, the inherent corrosion resistance and toughness of the axle steel are significantly improved. Simultaneously, by employing a "partial quenching + two-stage tempering" heat treatment process, a high-value residual compressive stress layer with a depth ≥0.025D ​​(D is the axle diameter) is actively introduced into the axle surface, fundamentally inhibiting the initiation of corrosion fatigue cracks. Ultimately, this achieves the fatigue limit (3×10⁻⁶) of the axle in corrosive environments. 6 The fatigue strength of the simulated corrosion environment is not less than 80% of its fatigue limit in atmospheric environment, while ensuring that it has excellent conventional mechanical properties: tensile strength (Rm) ≥750MPa, yield strength (Rp0.2) ≥600MPa, and impact energy (KV2) at -40℃ ≥60J.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: One of the objectives of this invention is to protect a high-speed train axle steel with high corrosion resistance, fatigue performance, and a stable residual stress layer. The elemental composition and the percentage of each element by mass, calculated as 100% of the total mass, are as follows: C 0.26~0.34%, Si 1.10~1.50%, Mn 0.60~0.90%, P≤0.010%, S≤0.004%, Cr 1.50~1.90%, Mo 0.25~0.40%, Ni 0.70~1.10%, Cu 0.25~0.50%, RE 0.010~0.030%, Ca 0.0010~0.0040%, T[O]≤0.0012%, N≤0.012%, Al 0.020~0.050%, with the remainder being Fe and unavoidable impurities.

[0006] Preferably, the elemental composition and mass percentage of the axle steel, based on a total mass percentage of 100%, are as follows: C 0.28~0.32%, Si 1.20~1.40%, Mn 0.60~0.90%, P≤0.010%, S≤0.004%, Cr 1.60~1.80%, Mo 0.25~0.40%, Ni 0.80~1.00%, Cu 0.25~0.50%, RE 0.015~0.025%, Ca 0.0010~0.0040%, T[O]≤0.0012%, N≤0.012%, Al 0.020~0.050%, with the remainder being Fe and unavoidable impurities.

[0007] The second objective of this invention is to protect the method for preparing the high-speed train axle steel, which includes the following steps: (1) Forming: After melting the alloy raw materials, the shaft blank is obtained by casting, rolling and precision forging; (2) Austenitization: The obtained shaft blank is heated in an air furnace to 880~910℃ and held at that temperature to austenitize it; (3) Incomplete quenching: Immerse the axle after step (2) in a strongly stirred quenching liquid and cool it rapidly to a surface temperature of 100~150℃; (4) Two-stage tempering: The axle after the incomplete quenching in step (3) is immediately taken out and slowly cooled to room temperature in the air. Then the axle is reheated to 600~640℃ and cooled to room temperature in the air again.

[0008] Furthermore, the heat preservation time after heating in step (2) is calculated as 1.0~1.3 min / mm.

[0009] Further, the quenching fluid mentioned in step (3) is water or a PAG aqueous solution with a mass concentration of 25%-27%.

[0010] Furthermore, the holding time after reheating in step (4) is calculated as 2.0~2.5 min / mm.

[0011] 1. Ingredient Design: (1) Carbon (C): 0.26~0.34%. Carbon is a fundamental element for ensuring strength. If its content is below 0.26%, it is difficult to achieve the required strength level; if it is above 0.34%, it will significantly impair toughness and weldability. This invention controls the carbon content within the medium range to achieve the best balance between strength and toughness.

[0012] (2) Silicon (Si): 1.10~1.50%. Silicon, as the core element of this invention, plays multiple roles: a. Solid solution strengthening: significantly improves the strength of the ferrite matrix. b. Improves tempering stability: inhibits the precipitation and growth of cementite, enabling the axle steel to maintain high strength after tempering at higher temperatures. c. Enhances corrosion resistance: promotes the formation of a denser and more stable SiO2 protective film on the steel surface, which, in synergy with the Cr oxide film, effectively blocks the intrusion of corrosive media. d. Assists in stress regulation: improves the phase transformation shear strength and enhances the effect of generating surface compressive stress in the subsequent "incomplete quenching" process.

[0013] (3) Manganese (Mn): 0.60~0.90%. Manganese can ensure hardenability, prevent the precipitation of proeutectoid ferrite, and also has a certain solid solution strengthening effect.

[0014] (4) Chromium (Cr): 1.50~1.90%. a. Improves corrosion resistance: It is a key element in the formation of passivation film, significantly improving the corrosion resistance of the matrix. b. Improves hardenability: Ensures uniform performance of large-section axle steel. c. Solid solution strengthening: Contributes to the strength of the matrix.

[0015] (5) Molybdenum (Mo): 0.25~0.40%. a. Improve hardenability and tempering resistance. b. Suppress high-temperature tempering brittleness and ensure that the material still has excellent toughness after tempering.

[0016] (6) Nickel (Ni): 0.70~1.10%. a. Toughening element: significantly improves low-temperature toughness and reduces the ductile-brittle transition temperature. b. When combined with Cu, it prevents hot brittleness. c. It helps to improve corrosion resistance.

[0017] (7) Copper (Cu): 0.25~0.50%. a. Improves atmospheric corrosion resistance: It accumulates in the rust layer, promoting the densification of the rust layer. b. Produces precipitation strengthening: ε-Cu phase is precipitated during aging, which improves strength.

[0018] (8) Rare Earth (RE): 0.010~0.030%. a. Deep purification: Strongly fixes sulfur and oxygen, forming spherical, high-melting-point rare earth oxysulfides. b. Modified inclusions: Spheroidizes and refines harmful inclusions such as elongated MnS, greatly improving the transverse toughness and isotropy of steel, and significantly reducing fatigue crack initiation. c. Microalloying: Trace amounts of solid-solution rare earth can purify grain boundaries and improve grain boundary strength.

[0019] (9) Calcium (Ca): 0.0010~0.0040%. In synergy with RE, it controls the morphology of inclusions, ensures complete globalization of sulfides, and further improves fatigue resistance.

[0020] (10) Phosphorus (P) ≤ 0.010%, sulfur (S) ≤ 0.004%. Strictly limit harmful elements. P easily leads to grain boundary embrittlement, and S forms sulfide inclusions, which is the main origin of fatigue cracks.

[0021] (11) Total oxygen (T[O]) ≤ 0.0012%. Strictly control the amount of oxide inclusions, as they are the core origin of fretting and fatigue cracks.

[0022] 2. Heat treatment process: "Incomplete quenching + two-stage tempering" process is adopted. (1) Incomplete quenching: After austenitizing, the axle steel is quenched and cooled, but not to room temperature. Instead, water / oil is applied above the martensitic transformation termination point Mf (usually 100~150℃). At this time, the core has basically completed the martensitic transformation and expanded due to the rapid cooling rate, while the surface layer, due to contact with the medium, has a higher actual temperature than the core layer and remains in the austenitic state or has not fully transformed. This asynchronous phase transformation between the core and the surface is the key to generating residual compressive stress on the surface.

[0023] (2) Two-stage tempering: a. First stage (self-tempering and stress relaxation): This stage does not require external heating. It utilizes the residual heat of the axle steel after it is molten and the subsequent slow cooling to room temperature in the air to transform the untransformed austenite on the surface into martensite, which interacts with the core structure to initially form a favorable stress distribution.

[0024] b. Second stage (stabilization tempering): The microstructure is stabilized by holding the material at the conventional tempering temperature for an extended period of time, allowing carbides to fully precipitate and ultimately “locking in” the beneficial residual compressive stress introduced by the incomplete quenching.

[0025] In summary, this invention systematically solves the industry problem of drastic decline in fatigue performance of high-speed axles in corrosive environments through synergistic innovation in component design and heat treatment processes. Its specific innovations are as follows: 1. Innovation in the "high silicon-medium chromium-rare earth" composite alloying composition system: High-silicon (Si: 1.10~1.50%) design: This design breaks through the limitation that the Si content in traditional axle steel is usually below 0.5%. High silicon not only improves the matrix strength through strong solid solution strengthening, but more importantly, it significantly improves the tempering stability of steel, allowing axle steel to maintain high strength even after tempering at higher temperatures. At the same time, silicon promotes the formation of a denser and more stable SiO2 protective film on the steel surface, which, in synergy with the chromium oxide film, fundamentally enhances the intrinsic corrosion resistance of the material.

[0026] Synergistic corrosion resistance of medium chromium (Cr: 1.50~1.90%) with copper and nickel: By increasing the chromium content to a medium-high range and rationally combining it with Cu and Ni, a synergistic corrosion resistance system was constructed. Cr is the core component for forming the passivation film, Cu promotes the densification of the rust layer, and Ni improves toughness and assists in corrosion resistance. The combination of these three components with high silicon content gives the resulting axle steel an environmental corrosion resistance far exceeding that of conventional EA4T steel.

[0027] Rare Earth (RE: 0.010~0.030%) Deep Purification and Inclusion Modification: Quantitative addition of rare earth elements utilizes their strong deoxidation and desulfurization capabilities to achieve deep purification of molten steel. More importantly, rare earth elements can transform harmful inclusions such as elongated MnS into spherical, dispersed rare earth oxides and sulfides, greatly reducing the initiation sites of fatigue cracks and significantly improving the transverse toughness and fatigue resistance of the material. This is key to improving corrosion fatigue life.

[0028] 2. Innovation in heat treatment process: "Incomplete quenching + two-stage tempering": Abandoning the traditional approach of "complete quenching to room temperature" and reliance on subsequent surface treatment, this process creatively employs an "incomplete quenching" technique that stops quenching above the martensitic transformation termination point (Mf point) (100~150℃). This process utilizes the temperature difference and asynchronous phase transformation between the core and surface of the axle steel during cooling (the core undergoes martensitic transformation and expands before the surface) to actively introduce high residual compressive stress into the surface.

[0029] A unique "two-stage tempering" process was designed: the first stage utilizes the residual heat after liquid removal for "self-tempering," allowing the untransformed austenite on the surface to continue transforming and initially relaxing stress; the second stage involves "stabilization tempering," stabilizing the microstructure and ultimately "locking in" a beneficial residual compressive stress field. This process is simple and easy to implement, yet it can form a highly stable residual compressive stress layer with a depth ≥0.025D ​​on the surface of axle steel, effectively inhibiting the initiation and early propagation of corrosion fatigue cracks from a mechanical perspective.

[0030] In summary, this invention, through the organic combination of components and processes, forms a complete, efficient, and easily industrialized technical solution.

[0031] Testing revealed that the axle steel produced by this invention possesses the following properties: 1) Conventional mechanical properties: tensile strength (Rm) ≥780MPa, yield strength (Rp0.2) ≥630MPa, elongation after fracture (A) ≥18%, reduction of area (Z) ≥5%, impact energy at -40℃ (KV2) ≥65J.

[0032] 2) Corrosion fatigue performance: In a 3.5% NaCl aqueous solution environment, the rotational bending fatigue limit (3×10⁻⁶) 6 (Cycles) ≥310MPa, reaching more than 81% of its atmospheric fatigue limit (~380MPa).

[0033] 3) Stable and deep residual compressive stress layer: The maximum residual compressive stress value on the surface is ≥-500MPa, the depth of the residual compressive stress layer is ≥0.025D ​​(for φ200mm axle, the depth is ≥5mm), and the stress distribution is stable.

[0034] 4) High corrosion resistance and structural purity: Through cyclic immersion corrosion testing, its corrosion rate is reduced by more than 40% compared to conventional EA4T steel. Metallographic observation shows that inclusions are fine spherical, and the original austenite grain size is ≥8 (see...). Figure 1 ). Attached Figure Description

[0035] Figure 1 Metallographic image of the axle steel prepared according to the present invention. Detailed Implementation

[0036] A high-speed train axle steel with high corrosion resistance, fatigue performance, and a stable residual stress layer, has the following elemental composition and mass percentage of each element, calculated as 100% by mass: C 0.26~0.34%, Si 1.10~1.50%, Mn 0.60~0.90%, P≤0.010%, S≤0.004%, Cr 1.50~1.90%, Mo 0.25~0.40%, Ni 0.70~1.10%, Cu 0.25~0.50%, RE 0.010~0.030%, Ca 0.0010~0.0040%, T[O]≤0.0012%, N≤0.012%, Al 0.020~0.050%, with the remainder being Fe and unavoidable impurities.

[0037] The method for preparing the axle steel for high-speed trains includes the following steps: (1) Forming: After melting the alloy raw materials, the shaft blank is obtained by casting, rolling and precision forging; (2) Austenitization: The obtained shaft blank is heated in an air furnace to 880~910℃ and held at 1.0~1.3 min / mm (based on the maximum diameter of the shaft); (3) Incomplete quenching: Immerse the axle after step (2) in a strongly stirred quenching liquid (water or PAG aqueous solution with a mass concentration of 25%-27%) and cool it rapidly to a surface temperature of 100~150℃; (4) Two-stage tempering: Take out the axle after the incomplete quenching in step (3) immediately, pile it in the air to cool slowly to room temperature, then put the axle back into the tempering furnace, heat it to 600~640℃, keep it at 2.0~2.5 min / mm, and then air cool it to room temperature.

[0038] To make the content of this invention easier to understand, the technical solution of this invention will be further described below with reference to specific embodiments, but this invention is not limited thereto.

[0039] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0040] Prepare steel samples according to the composition content given in Table 1 (the balance not shown in Table 1 is Fe and unavoidable impurities).

[0041] Table 1. Elemental composition and mass percentage (%) of steel samples prepared in the examples and comparative examples.

[0042] The specific production process flow of the embodiment is as follows: Smelting in an electric arc furnace or converter → refining in an LF furnace → vacuum degassing in an RH or VD furnace → feeding rare earth wire → continuous casting → billet heating → rolling of axle billets → forging of axle blanks → rough turning of axle blanks → machining of axle end faces → incomplete quenching + two-stage tempering → rough turning of axle outer diameter → finish turning of axle outer diameter → grinding of outer diameter → flaw detection.

[0043] Table 2 shows the heat treatment process parameters for preparing the steel samples in the examples.

[0044] Table 2. Heat treatment process parameters for preparing steel samples in the examples.

[0045] The specific production process flow for Comparative Examples 1-2 is as follows: Smelting in an electric arc furnace or converter → refining in an LF furnace → RH or VD vacuum degassing → continuous casting → billet heating → rolling of axle billets → forging of axle blanks → rough turning of axle blanks → machining of axle end faces → complete quenching + high-temperature tempering → finish turning of axle outer diameter → grinding of outer diameter → flaw detection.

[0046] Table 3 shows the heat treatment process parameters for preparing steel samples in Comparative Examples 1-2.

[0047] Table 3 Heat treatment process parameters for steel samples prepared in the comparative example

[0048] The mechanical properties, fatigue properties, and residual compressive stress depth of the obtained steel samples were tested in accordance with GB / T 13299, GB / T 6394, GB / T 228, GB / T 229, GB / T231, and GB / T 21143. The results are shown in Table 4.

[0049] Table 4 Mechanical properties, fatigue properties, and residual compressive stress depth of the steel samples obtained in the examples and comparative examples.

[0050] As can be seen from Table 1, the steel samples prepared in the examples have better performance in all aspects than the comparative examples, with Example 2 showing better corrosion fatigue resistance.

[0051] As can be seen from the above, this invention, through a unique "high-silicon-medium-chromium-rare-earth" composition design and an innovative and easy-to-implement "incomplete quenching + two-stage tempering" process, successfully introduces a deep and stable beneficial residual compressive stress layer on the surface of the axle, significantly improving the intrinsic corrosion resistance of the material. This compositional system is unique, exhibits superior performance, and demonstrates strong process robustness, making it highly suitable for large-scale industrial production and providing a reliable guarantee for the safe operation of high-speed trains in harsh environments.

[0052] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A high-speed train axle steel with high corrosion fatigue resistance and a stable residual stress layer, characterized in that, Based on a total mass percentage of 100%, the elemental composition and mass percentage of the axle steel are as follows: C 0.26~0.34%, Si 1.10~1.50%, Mn 0.60~0.90%, P≤0.010%, S≤0.004%, Cr 1.50~1.90%, Mo 0.25~0.40%, Ni 0.70~1.10%, Cu 0.25~0.50%, RE 0.010~0.030%, Ca 0.0010~0.0040%, T[O]≤0.0012%, N≤0.012%, Al 0.020~0.050%, with the remainder being Fe and unavoidable impurities.

2. The high-speed train axle steel according to claim 1, characterized in that, Based on a total mass percentage of 100%, the elemental composition and mass percentage of the axle steel are as follows: C 0.28~0.32%, Si 1.20~1.40%, Mn 0.60~0.90%, P≤0.010%, S≤0.004%, Cr 1.60~1.80%, Mo 0.25~0.40%, Ni 0.80~1.00%, Cu 0.25~0.50%, RE 0.015~0.025%, Ca 0.0010~0.0040%, T[O]≤0.0012%, N≤0.012%, Al 0.020~0.050%, with the remainder being Fe and unavoidable impurities.

3. The high-speed train axle steel according to claim 1, characterized in that, The axle steel has a tensile strength ≥750MPa, a yield strength ≥600MPa, and an impact energy ≥60J at -40℃.

4. A method for preparing high-speed train axle steel as described in claim 1, characterized in that, After the alloy raw materials are melted, the axle blanks are obtained by casting, rolling and precision forging. The blanks are then heated to 880~910℃ and held at that temperature to austenitize them. After incomplete quenching and two-stage tempering, the high-speed train axle steel is obtained.

5. The method for preparing high-speed train axle steel according to claim 4, characterized in that, The austenitizing holding time is calculated as 1.0~1.3 min / mm.

6. The method for preparing high-speed train axle steel according to claim 4, characterized in that, The incomplete quenching involves immersing the austenitized axle steel in a vigorously stirred quenching liquid and rapidly cooling it to a surface temperature of 100-150°C.

7. The method for preparing high-speed train axle steel according to claim 6, characterized in that, The quenching fluid is water or a PAG aqueous solution with a mass concentration of 25%-27%.

8. The method for preparing high-speed train axle steel according to claim 4, characterized in that, The two-stage tempering process involves air-cooling the quenched axle steel to room temperature, then heating it to 600-640°C, followed by a second air-cooling.

9. The method for preparing high-speed train axle steel according to claim 8, characterized in that, The holding time after reheating is calculated as 2.0~2.5 min / mm.

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