A 400 km / h high-speed rail axle steel and its heat treatment method

Abstract

The invention discloses a 400 km / h high-speed railway axle steel and a heat treatment method thereof, belonging to the technical field of axle steel. The high-speed railway axle steel contains, by weight, the following elements: C 0.20%-0.30%, Mn 0.95%-1.15%, Cr 0.90%-1.10%, Mo 0.55%-0.75%, Ni 1.30%-1.50%, V 0.15%-0.25%, Alt 0.015%-0.040%, and Si 0.15%-0.35%. The high-speed railway axle steel has excellent strength, toughness, and fatigue properties, and has a Young's modulus of ≥211 GPa, a tensile strength of ≥950 MPa, a yield strength of ≥870 MPa, and a KV2 at -60°C of ≥80 J. The high-speed railway axle steel is suitable for manufacturing axles for high-speed railways with a speed of 400 km / h.

Description

Technical Field

[0001] The invention belongs to the technical field of axle steel, and particularly relates to axle steel for a high-speed railway with a speed of 400 kilometers per hour and a heat treatment method thereof. Background Art

[0002] Axles are critical load-bearing components of high-speed rail and must ensure adequate safety, reliability, and a long service life in service environments. The fatigue life of axles on 250 km / h trains reaches 100 million cycles, while that of axles on 350 km / h trains reaches 1 billion cycles. For 400 km / h trains, the fatigue life will reach tens of billions of cycles. This places higher demands on the performance of axle materials, requiring higher strength, toughness, and fatigue resistance. Furthermore, as train speeds increase, energy consumption increases dramatically. To reduce weight, save energy, and promote green travel, axles on 400 km / h trains require a lightweight design with a large-aperture hollow structure. This requires a thinner wall thickness for the hollow axle. Insufficient rigidity can easily cause deformation, impacting vehicle safety. This places even higher technical demands on the rigidity of the axle material.

[0003] Currently, commonly used materials for high-speed rail axles worldwide include EA1N, EA4T, 34CrNiMo6, 30NiCrMoV12, S38C, DZ1, and DZ2. However, EA1N and EA4T have low strength, with yield strengths around 600 MPa, while 34CrNiMo6 and 30NiCrMoV12 have low toughness, with an impact energy of 70 J at room temperature. DZ1 and DZ2 steels offer high strength but low low-temperature toughness, making them unsuitable for manufacturing high-rigidity hollow axles for high-speed trains. The development of rail transit is placing higher demands on high-speed train axle steel, necessitating the development of axle materials with excellent strength, toughness, high stiffness, and long fatigue life.

[0004] Chinese patent CN110284063A discloses a cold-zone high-speed train axle steel and its preparation method. The steel is characterized by its composition, by weight, of 0.23% to 0.28% C, 0.17% to 0.37% Si, 0.60% to 0.77% Mn, 0.95% to 1.20% Cr, 0.20% to 0.30% Mo, 0.10% to 0.25% Ni, 0.02% to 0.04% V, 0.010% to 0.040% Al, and the balance Fe. The steel is normalized at 880°C to 900°C, quenched at 860°C to 880°C, and tempered at 630°C to 650°C. The steel produced by this method contains approximately 15% ferrite, which is detrimental to its strength, resulting in a tensile strength of 587 MPa to 633 MPa. This insufficient strength meets the requirements for high-rigidity high-speed train axles.

[0005] Chinese patent CN102418048A discloses a steel for a hollow axle of a high-speed train and a manufacturing method thereof. The steel is characterized in that, by weight percentage, the chemical composition ratio is C: 0.23-0.29%, Mn: 0.60-0.75%, Si: 0.25-0.40%, Cr: 1.00-1.20%, Mo: 0.20-0.30%, V: 0.020-0.060%, Ni: 0.15-0.30%, P≤0.015%, S≤0.005%, and the remainder is Fe and residual trace impurities. The axle manufactured using the above composition and process has a tensile strength of 705MPa to 785MPa and a room temperature U-type impact energy of 47-68J. The material has insufficient strength and toughness, and the fatigue resistance of the notched specimen is insufficient.

[0006] Chinese patent CN114507822A discloses a heat treatment process for high-strength, high-toughness, and high-ductility axle carbon steel and its forgings. The process features a chemical composition percentage of C: 0.47-0.52 wt%, Si: 0.20-0.35 wt%, Mn: 0.70-0.80 wt%, V: 0.45-0.08 wt%, Al: 0.020-0.040 wt%, S: ≤0.015 wt%, P: ≤0.015 wt%, with the remainder being Fe and residual trace impurities. Normalizing is performed at 880-920°C, quenching is performed at 870-900°C, and tempering is performed at 630-660°C. Using this composition, normalizing and tempering at 560-580°C results in axle tensile strength of 645-710 MPa and impact energy of 10-29 J. However, the material's strength is insufficient and the impact energy is low.

[0007] Although there has been a certain amount of research on high-speed rail axle steel in the existing technology, there are currently problems with low strength, toughness and stiffness, which cannot meet the use requirements of hollow axles for high-speed trains with a speed of 400 kilometers per hour. Summary of the Invention

[0008] In order to solve the above technical problems, the present invention provides a 400 km / h high-speed railway axle steel and a heat treatment method thereof. The high-speed railway axle steel has excellent strength, toughness and fatigue performance and is suitable for manufacturing 400 km / h high-speed railway axles.

[0009] The technical solution adopted by the present invention is as follows:

[0010] A 400 km / h high-speed railway axle steel comprising, by weight, the following: C 0.20%-0.30%, Si 0.15%-0.35%, Mn 0.95%-1.15%, Cr 0.90%-1.10%, Mo 0.55%-0.75%, Ni 1.30%-1.50%, V 0.15%-0.25%, Alt 0.015%-0.040%, P ≤0.015%, S ≤0.015%, N ≤0.008%, O ≤0.002%, with the remainder being Fe and other unavoidable impurities.

[0011] The chemical composition of the 400 km / h high-speed railway axle steel meets the following requirements:

[0012] 2.65%≤A=1.4×%C+1.7×%Mn+(0.65+0.077)×%Cr≤3.00%;

[0013] X=-2.5×%C-1.8×%Si+0.5×%Mn+1.2×%Mo+2.2×%Ni+2.7×%V≥3.75%;

[0014] The metallographic structure of the 400 km / h high-speed railway axle steel is a fine tempered bainite structure.

[0015] The Young's modulus of the 400 km / h high-speed railway axle steel is ≥211 GPa, the tensile strength is ≥950 MPa, the yield strength is ≥870 MPa, and the KV2 at -60°C is ≥80 J, and it has excellent strength and toughness.

[0016] The fatigue limit of a smooth surface sample of the 400 km / h high-speed railway axle steel is ≥430 MPa, and the fatigue limit of a notched surface sample is ≥400 MPa, and the steel has excellent fatigue resistance.

[0017] The present invention also provides a heat treatment method for the 400 km / h high-speed railway axle steel, the heat treatment method comprising the following steps:

[0018] (1) Normalizing: Heat the axle to 940~980℃ and keep it warm, then air cool;

[0019] (2) Tempering and deep cooling: Heat the axle to 840~880℃ and keep it warm, then water cool it and transfer it to liquid nitrogen for deep cooling;

[0020] (3) Tempering.

[0021] The tempering process is as follows: heating the axle to 620-680° C., keeping the temperature, and then air cooling.

[0022] The tempering process is as follows: heating the axle at a speed of 180-240°C / h to 620-680°C, keeping the temperature, and then air cooling.

[0023] The holding time t4 of tempering is determined by the axle diameter D, t4=(2.0~2.4)×D, the unit of t4 is min, and the unit of D is mm.

[0024] In the normalizing step, the axle is heated at a rate of 120-240° C. / h to 940-980° C., kept at that temperature, and then air-cooled.

[0025] In the normalizing step, the holding time t1 is determined by the axle diameter D, t1=(1.3-1.7)×D, where the unit of t1 is min and the unit of D is mm.

[0026] In the tempering and deep freezing step, the axle is water-cooled to below 100° C. and then transferred to liquid nitrogen for deep freezing. The deep freezing holding time t3 is determined by the axle diameter D, t3=(1.6~2.0)×D, where t3 is in min and D is in mm.

[0027] In the tempering and deep cooling step, the axle is heated at a rate of 240-360° C. / h to 840-880° C. and kept at this temperature.

[0028] In the tempering and deep cooling step, the holding time t2 is determined by the axle diameter D, t2=(0.8~1.2)×D, the unit of t2 is min, and the unit of D is mm.

[0029] The present invention also provides a production method for high-speed rail axles with a speed of 400 kilometers per hour, which includes the following steps: smelting → continuous casting of round billets → heating of round billets → rolling of axle billets → heating of axle billets → forging of axle billets → rough turning of axles → heat treatment of axles → fine turning of axles → inner hole processing of axles → grinding → flaw detection, wherein the heat treatment of the axles is carried out using the heat treatment method described in the present invention.

[0030] In the high-speed railway axle steel provided by the present invention, the functions and controls of the various components are as follows:

[0031] Carbon (C) is the cheapest strengthening element in steel. Every 0.1% increase in solid-solution C increases strength by approximately 450 MPa. C forms precipitation phases with alloying elements in steel, contributing to precipitation strengthening. C significantly improves hardenability. Extensive testing has determined that every 0.01% increase in carbon increases hardenability at J9 by 0.9 HRC, enabling the formation of a martensitic structure in the core of large-size axles. However, increasing C content decreases plasticity and toughness, so the C content is controlled between 0.20% and 0.30%.

[0032] Si: Si is an effective solid-solution strengthening element in steel, increasing its rigidity. It also acts as a deoxidizer during steelmaking and is a commonly used deoxidizer. However, Si tends to segregate at austenite grain boundaries, reducing intergranular bonding and causing brittleness. Furthermore, Si can cause elemental segregation in steel. Therefore, the Si content is controlled between 0.15% and 0.35%.

[0033] Mn: Mn has a solid solution strengthening effect. As an austenite stabilizing element, Mn can significantly improve the hardenability of steel. Mn can significantly increase the rigidity of steel. Mn combined with sulfur can prevent sulfur-induced hot brittleness. Therefore, the Mn content is controlled between 0.95% and 1.15%.

[0034] Cr: Cr is a carbide-forming element that forms various complex carbides in steel, which contributes to dispersion strengthening and significantly improves the steel's rigidity. Cr also forms an oxide film on the material's surface, enhancing the steel's corrosion resistance. This property improves the axle's surface integrity during operation, reducing microcracks and corrosion damage, thereby enhancing fatigue performance. The Cr content should be controlled between 0.90% and 1.10%.

[0035] Mo: Mo primarily improves the hardenability and tempering toughness of steel. Mo dissolved in the matrix enables the steel structure to maintain high stiffness during tempering and reduces the segregation of impurity elements at grain boundaries, thereby increasing the toughness of the steel and reducing temper brittleness. Therefore, the Mo content is controlled at 0.55% to 0.75%.

[0036] Ni: Ni forms an infinitely miscible solid solution with Fe. It is an austenite-stabilizing element, expanding the phase region, increasing the stability of supercooled austenite, shifting the C curve to the right, and improving the hardenability of the steel. Ni can also refine the width of martensite laths, increasing strength. Ni can significantly lower the ductile-brittle transition temperature of steel and improve low-temperature toughness. The Ni content should be controlled between 1.30% and 1.50%.

[0037] V: V can dissolve in steel and form compounds with C and N, strengthening and toughening it. It can also form interfaces with high mismatch angles, increasing the crack propagation work and thus improving fatigue life. Therefore, the V content is controlled at 0.15% to 0.25%.

[0038] Al: Al is a strong deoxidizing element and also improves the steel's oxidation resistance. However, as the Al content increases, the amount of coarse carbonitride inclusions increases. The Al content should be controlled between 0.015% and 0.040%.

[0039] O and N: TO forms oxide inclusions in steel, so control TO≤0.0020%; N precipitates Fe4N in steel, which diffuses slowly, causing aging of the steel. At the same time, N also reduces the cold working performance of the steel, so control N≤0.0080%.

[0040] In order to ensure sufficient stiffness of the axle, it is necessary to reasonably match the elements that increase stiffness and the elements that are harmful to stiffness. C can significantly increase stiffness, with a stiffness coefficient of 1.4. Mn is also a major element that increases stiffness, and the stronger substitution effect of Mn increases stiffness more significantly, with a stiffness coefficient of 1.7. Since Cr increases the stiffness of steel through solid solution and carbides, the solid solution strength improvement coefficient is 0.65, and the carbide improvement coefficient is 0.077. However, too high stiffness will significantly reduce the toughness of steel, so the stiffness parameter (A) should be controlled within 2.65%≤A≤3.00%,

[0041] A=1.4×%C+1.7×%Mn+(0.65+0.077)×%Cr.

[0042] In order to ensure that the steel has high toughness and fatigue performance, the ratio of C, Si, Mn, Mo, Ni and V needs to be limited. Since C has a high stiffness and has a certain deteriorating effect on toughness and fatigue performance, the influence coefficient of C on toughness and fatigue performance is -2.5. Si is segregated in steel, which is not conducive to toughness and fatigue, so the coefficient is -1.8. Mn can significantly refine the substructure of steel and improve toughness, but the improvement in stiffness is obviously not conducive to toughness, so the coefficient is 0.5. Mo is beneficial to toughness and fatigue performance mainly by improving tempering stiffness and refining carbides in steel, so the coefficient of Mo is 1.2. Ni can increase stacking fault energy and significantly improve low-temperature toughness, so the coefficient of Ni is 2.2. V can improve toughness and fatigue properties by refining grains and improving precipitation phases, so the coefficient is 2.7; that is, X=-2.5×%C-1.8×%Si+0.5×%Mn+1.2×%Mo+2.2×%Ni+2.7×%V≥3.75%.

[0043] The heat treatment method for axle steel for high-speed railways with a speed of 400 kilometers per hour provided by the present invention adopts a process of normalizing + quenching and tempering deep cooling + tempering for heat treatment. Normalizing can stabilize the structure of the axle after forging and prepare the structure for performance heat treatment. The high-speed railway axle steel provided by the present invention has a high alloy content. Conventional water cooling alone cannot complete the transformation of the structure formed under cooling into a fine martensitic structure, and some residual austenite will be left. Deep freezing treatment with liquid nitrogen can completely transform the austenite into a fine martensitic structure.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The axle steel for 400 km / h high-speed rail provided by this invention has a Young's modulus ≥ 211 GPa, a tensile strength ≥ 950 MPa, a yield strength ≥ 870 MPa, and a KV2 at -60°C ≥ 80 J, demonstrating excellent toughness and high rigidity. The fatigue limit of smooth-surface specimens is ≥ 430 MPa, and that of notched-surface specimens is ≥ 400 MPa, demonstrating excellent fatigue resistance.

[0046] The high-speed rail axle steel provided by the present invention has excellent toughness, fatigue performance and high rigidity, and is suitable for manufacturing axles of high-speed rails with a speed of 400 kilometers per hour. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is the microstructure of the high-speed railway axle steel after tempering in Example 2. DETAILED DESCRIPTION

[0048] The present invention provides a 400 km / h high-speed railway axle steel, which contains, by weight percentage, the following: C 0.20% to 0.30%, Si 0.15% to 0.35%, Mn 0.95% to 1.15%, Cr 0.90% to 1.10%, Mo 0.55% to 0.75%, Ni 1.30% to 1.50%, V 0.15% to 0.25%, Alt 0.015% to 0.040%, P ≤0.015%, S ≤0.015%, N ≤0.008%, O ≤0.002%, and the remainder is Fe and other inevitable impurities;

[0049] The chemical composition of the 400 km / h high-speed railway axle steel meets the following requirements:

[0050] 2.65%≤A=1.4×%C+1.7×%Mn+(0.65+0.077)×%Cr≤3.00%;

[0051] X=-2.5×%C-1.8×%Si+0.5×%Mn+1.2×%Mo+2.2×%Ni+2.7×%V≥3.75%.

[0052] The heat treatment method for 400 km / h high-speed railway axle steel comprises the following steps:

[0053] (1) Normalizing: Heat the axle at a rate of 120-240℃ / h to 940-980℃, keep warm, and then air cool. The holding time t1 is determined by the axle diameter D, t1=(1.3-1.7)×D, t1 is in min, D is in mm;

[0054] (2) Tempering and deep cooling: heat the axle at a rate of 240-360℃ / h to 840-880℃, keep warm, and the holding time t2 is determined by the axle diameter, t2=(0.8-1.2)×D. Then water cool the axle until it reaches below 100℃, transfer it to liquid nitrogen for deep cooling, and the deep cooling holding time t3 is determined by the axle diameter D, t3=(1.6-2.0)×D. The units of t2 and t3 are both min.

[0055] (3) Tempering: Heat the axle to 620~680℃ at a rate of 180~240℃ / h, and then air cool. The holding time t4 is determined by the axle diameter D, t4=(2.0~2.4)×D, and the unit of t4 is min.

[0056] The present invention is described in detail below with reference to the embodiments.

[0057] The composition and weight percentage of the high-speed railway axle steel in each embodiment and comparative example are shown in Table 1, and the remainder is Fe and other inevitable impurities.

[0058]

[0059] The production process of the high-speed railway axle steel in each embodiment and comparative example is as follows:

[0060] Electric furnace smelting: oxygen is determined before tapping, and steel retention is adopted during tapping to avoid slag;

[0061] LF furnace: elements such as C, Si, Mn, Cr, Ni, Mo, and V are adjusted to target values;

[0062] Vacuum degassing: Pure degassing time ≥ 15 minutes, ensuring that the [H] content after vacuum treatment is ≤ 1.5ppm, to avoid the appearance of white spots in the steel and cause hydrogen embrittlement;

[0063] Continuous casting: The target temperature of the tundish molten steel is controlled at 10~40℃ above the liquidus temperature, and continuously cast into Φ380mm~Φ700mm round billets.

[0064] Rolling route: round billet heating → high-pressure water descaling → billet opening → 200mm×200mm~280mm×280mm square billet → slow cooling.

[0065] Forging route: billet heating → high-pressure water descaling → forging → slow cooling.

[0066] Axle processing route: axle rough turning → axle heat treatment → axle fine turning → axle inner hole processing → grinding → flaw detection → packaging and warehousing.

[0067] The heat treatment is shown in Table 2.

[0068]

[0069] The performance testing method is as follows:

[0070] Organization: Samples were taken from the cross section of the axle end at 1 / 2 of the wall thickness for metallographic and grain size analysis.

[0071] Performance: Take tensile, impact and fatigue specimens from the cross section of the axle end at 1 / 2 of the wall thickness, and conduct mechanical property tests in accordance with GB / T228, GB / T229 and GB / T4337.

[0072] The mechanical properties of the high-speed railway axle steels in the embodiments and comparative examples are shown in Tables 3 and 4.

[0073]

[0074]

[0075] From the above, it can be seen that the chemical composition and production methods of the steels of Examples 1 to 3 are properly controlled, and the steels have good rigidity, plasticity, toughness and fatigue properties.

[0076] Although the chemical composition of the axle steels in Comparative Examples 1 to 3 meets the composition range required by this application, they cannot fully meet the requirements of Formulas A and X, resulting in insufficient low-temperature toughness.

[0077] Figure 1 This is the microstructure of the high-speed railway axle steel after tempering in Example 2; as can be seen from the figure, its structure is finely dispersed tempered bainite, and the dispersed tiny precipitates enhance the stiffness and fatigue performance of the material.

[0078] The above-mentioned detailed description of the 400 km / h high-speed railway axle steel and its heat treatment method with reference to the embodiments is illustrative rather than restrictive. Several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A 400 km / h high-speed railway axle steel, characterized in that: Contains by weight: C 0.20%-0.30%, Si 0.15%-0.35%, Mn 0.95%-1.15%, Cr 0.90%-1.10%, Mo 0.55%-0.75%, Ni 1.30%-1.50%, V 0.15%-0.25%, Alt 0.015%-0.040%, P ≤0.015%, S ≤0.015%, N ≤0.008%, O ≤0.002%, the rest is Fe and other unavoidable impurities; The chemical composition of the 400 km / h high-speed railway axle steel meets the following requirements: 2.65%≤A=1.4×%C+1.7×%Mn+(0.65+0.077)×%Cr≤3.00%; X=-2.5×%C-1.8×%Si+0.5×%Mn+1.2×%Mo+2.2×%Ni+2.7×%V≥3.75%; The metallographic structure of the 400 km / h high-speed railway axle steel is a tempered bainite structure.

2. The 400 km / h high-speed railway axle steel according to claim 1, characterized in that: The Young's modulus of the 400 km / h high-speed railway axle steel is ≥211 GPa, the tensile strength is ≥950 MPa, the yield strength is ≥870 MPa, and the KV2 at -60°C is ≥80 J.

3. The 400 km / h high-speed railway axle steel according to claim 1, characterized in that: The fatigue limit of smooth surface specimens of high-speed rail axle steel with a speed of 400 kilometers per hour is ≥430MPa, and the fatigue limit of surface notched specimens is ≥400MPa.

4. A heat treatment method for axle steel of a 400 km / h high-speed railway according to any one of claims 1 to 3, characterized in that: The heat treatment method comprises the following steps: (1) Normalizing: Heat the axle to 940~980℃ and keep it warm, then air cool; (2) Tempering and deep cooling: Heat the axle to 840~880℃ and keep it warm, then water cool it and transfer it to liquid nitrogen for deep cooling; (3) Tempering.

5. The heat treatment method according to claim 4, characterized in that The tempering process is as follows: heating the axle to 620-680° C., keeping the temperature, and then air cooling.

6. The heat treatment method according to claim 5, characterized in that The tempering process is as follows: heating the axle at a speed of 180-240°C / h to 620-680°C, keeping the temperature, and then air cooling.

7. The heat treatment method according to claim 5 or 6, characterized in that The holding time t4 of tempering is determined by the axle diameter D, t4=(2.0~2.4)×D, the unit of t4 is min, and the unit of D is mm.

8. The heat treatment method according to any one of claims 4 to 6, characterized in that: In the normalizing step, the axle is heated at a rate of 120-240° C. / h to 940-980° C., kept at that temperature, and then air-cooled.

9. The heat treatment method according to any one of claims 4 to 6, characterized in that: In the normalizing step, the holding time t1 is determined by the axle diameter D, t1=(1.3-1.7)×D, where the unit of t1 is min and the unit of D is mm.

10. The heat treatment method according to any one of claims 4 to 6, characterized in that: In the tempering and deep freezing step, the steel is water-cooled to below 100° C. and then transferred to liquid nitrogen for deep freezing. The deep freezing holding time t3 is determined by the axle diameter D, t3=(1.6~2.0)×D, where t3 is in min and D is in mm.

11. The heat treatment method according to any one of claims 4 to 6, characterized in that: In the tempering and deep cooling step, the axle is heated at a rate of 240-360° C. / h to 840-880° C. and kept at this temperature.

12. The heat treatment method according to any one of claims 4 to 6, characterized in that: In the tempering and deep cooling step, the holding time t2 is determined by the axle diameter D, t2=(0.8~1.2)×D, the unit of t2 is min, and the unit of D is mm.

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