A 50t axle load freight truck axle designed for use in cold environments with a pressure rating of 1200MPa, along with its heat treatment and manufacturing methods.

Abstract

This invention discloses a 50t axle load freight car axle designed for use in cold environments at 1200MPa, comprising the following chemical components: C, Si, Mn, Cr, Ni, Mo, Nb, V, Cu, Ca, La, [N], and Al. It exhibits good fatigue resistance and excellent low-temperature toughness; the fatigue limit R of a smooth-surfaced specimen is [not specified]. fL Fatigue limit R of specimens with notched surfaces ≥450MPa fE With an impact strength of ≥380MPa and a longitudinal impact energy of KV2≥130J at -40℃, a lateral impact energy of KV2≥110J at -40℃, a longitudinal impact energy of KV2≥110J at -60℃, a lateral impact energy of KV2≥90J at -60℃, a longitudinal impact energy of KV2≥80J at -80℃, and a lateral impact energy of KV2≥60J at -80℃, it can be applied to 50t axle load freight vehicles operating in low-temperature environments above -60℃.

Description

Technical Field

[0001] This invention belongs to the field of freight vehicle axle technology, specifically relating to a 50t axle load freight vehicle axle for use in cold environments with a pressure rating of 1200MPa, and its heat treatment and production methods. Background Technology

[0002] In the domestic and international freight transport process, railway freight car transport remains a very important component. Axle load is the average load weight distributed across each axle of a freight car, and it is the most important indicator for measuring the load-bearing capacity of a freight car. Increasing axle load is the most effective and economical means to improve the efficiency of railway freight transport. However, increasing axle load is also subject to certain constraints. First, there are the conditions of the railway line, such as elevated railways and railway bridges in plateau and mountainous areas, which limit the load-bearing capacity of freight cars. Second, there is the safe load-bearing capacity of the axles.

[0003] With the increase in axle load in freight trains, axles exhibit defects such as low strength and toughness after overall heat treatment. According to relevant standards, the design and strength verification of heavy axle load axles are crucial. The operational safety of the axle is significantly related to the notch sensitivity coefficient q (q = RfL / RfE) and the axle specifications. To ensure safety, the current approach is to increase axle specifications. However, unlimited increases in axle specifications lead to increased train weight, higher axle production costs, and compromised train safety, all of which have limited practical applications.

[0004] From the perspective of operating environment temperature, the lowest temperature in the environment where freight trains operate reaches below -60℃, which poses an extremely severe test to materials. Currently, there is no research on axles with a high axle load at -60℃, nor are there technical specifications for such axle materials, especially low-temperature toughness. If a completely new axle material could be designed, whose low-temperature toughness level could reach the level of pearlitic-ferritic steel axle materials at room temperature, then both the safety of low-temperature service and the safety of 50t axle load freight trains could be ensured.

[0005] Therefore, the following technical problems must be solved for the new generation of 50t axle load freight car axles with good fatigue resistance and 1200MPa grade for use in cold environments: (1) lightweight axle structural design; (2) material and performance design of heavy-duty axles. Therefore, it is urgent to develop a new type of economical, cold-resistant, high-strength, high-toughness, and long-fatigue-life freight train axle made of a new material. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a 50t axle load freight truck axle designed for use in cold environments with a pressure rating of 1200MPa. This axle exhibits good fatigue resistance and excellent low-temperature toughness, with a fatigue limit R0 on a smooth-surfaced specimen. fLFatigue limit R of specimens with notched surfaces ≥450MPa fE With an impact strength of ≥380MPa and a longitudinal impact energy of KV2≥130J at -40℃, a lateral impact energy of KV2≥110J at -40℃, a longitudinal impact energy of KV2≥110J at -60℃, a lateral impact energy of KV2≥90J at -60℃, a longitudinal impact energy of KV2≥80J at -80℃, and a lateral impact energy of KV2≥60J at -80℃, it can be applied to 50t axle load freight vehicles operating in low-temperature environments above -60℃.

[0007] The present invention also provides a heat treatment method and production method for axles of 50t axle load freight vehicles that are used in cold environments with a pressure of 1200MPa.

[0008] The technical solution adopted in this invention is as follows:

[0009] A 50t axle for a freight car designed for use in cold environments with a pressure rating of 1200MPa comprises the following chemical composition by weight percentage: C: 0.46–0.53%, Si: 0.55–0.70%, Mn: 0.45–0.60%, Cr: 1.20–1.40%, Ni: 1.65–1.85%, Mo: 0.45–0.55%, Nb: 0.020–0.050%, V: 0.20–0.30%, Cu: 0.40–0.60%, Ca: 0.002–0.005%, La: 0.010–0.020%, P≤0.010%, S≤0.008%, T[O]≤0.0010%, [N]: 0.015–0.020%, Al: 0.040–0.050%, with the remainder being Fe and other unavoidable impurities.

[0010] The preferred chemical composition includes the following weight percentages: C: 0.48–0.52%, Si: 0.60–0.70%, Mn: 0.50–0.60%, Cr: 1.25–1.35%, Ni: 1.70–1.80%, Mo: 0.48–0.52%, Nb: 0.030–0.040%, V: 0.22–0.28%, Cu: 0.45–0.55%, Ca: 0.003–0.004%, La: 0.012–0.018%, P≤0.008%, S≤0.006%, T[O]≤0.0008%, [N]: 0.016–0.019%, Al: 0.042–0.048%, with the remainder being Fe and other unavoidable impurities.

[0011] The composition of the axle of the 50t axle load freight car serving in the 1200MPa cold environment meets the critical quenching thickness HI≥107.5, HI=25.4×1 / 2×[(0.54×C)×(1.00+0.69×Si)×(1.00+3.41×Mn)×(1.00+1.99×(Cr+V))×(1.00+0.353×(Ni+Cu))×(1.00+2.98×Mo)].

[0012] The axle of the 50t axle load freight vehicle serving in a cold environment with a rated pressure of 1200MPa has a corrosion resistance index I ≥ 6.8, where I = 26.01×Cu + 3.88×Ni + 1.20×Cr + 1.49×Si + 17.28×P - 7.29×Cu×Ni - 9.10×Ni×P - 33.39×Cu 2 .

[0013] The composition of the axle of the 50t axle load freight truck serving in the 1200MPa cold environment satisfies the predicted value of Rm, Y≥1200, Y=9.8×(100C-100(C-0.4) / 3+10Si+25Mo+30Mn+6Ni+20Cr+60V).

[0014] The metallographic structure of the 50t axle load freight car axle used in the 1200MPa cold environment is tempered sorbite and bainite, wherein the volume percentage of tempered sorbite is 90-93%, the volume percentage of bainite is 7-10%, and the content of tempered sorbite in the near surface of the axle from 0 to 40mm is 100%.

[0015] The axle of the 50t axle load freight vehicle serving in cold environments with a load capacity of 1200MPa has a tensile strength ≥1200MPa and a yield strength ≥850MPa; longitudinal impact energy KV2 ≥130J at -40℃, and transverse impact energy KV2 ≥110J at -40℃; longitudinal impact energy KV2 ≥110J at -60℃, and transverse impact energy KV2 ≥90J at -60℃; longitudinal impact energy KV2 ≥80J at -80℃, and transverse impact energy KV2 ≥60J at -80℃; and the fatigue limit R of the smooth-surfaced specimen. fL ≥450MPa; fatigue limit R of specimens with notched surfaces fE ≥380MPa; Notch sensitivity index R fL / R fE ≤1.20.

[0016] This invention also provides a heat treatment method for the axle of a 50t axle load freight car serving in a cold environment with a rated load of 1200MPa, comprising the following steps:

[0017] 1) Preliminary heat treatment, including stress-relieving annealing and normalizing to homogenize the microstructure;

[0018] 2) Performance heat treatment, including two-phase quenching, bi-liquid sub-temperature quenching, and medium-temperature tempering.

[0019] In step 1), the stress-relief annealing specifically involves heating the axle to 580-640°C at a heating rate of 100-120°C / h. The holding time within this temperature range is calculated based on the maximum diameter, at 3 min / mm, followed by furnace cooling. After forging, the axle experiences residual internal stress due to the temperature difference between the surface and core during cooling. This internal stress, combined with the internal attraction generated by subsequent processing factors, causes distortion or cracking of the axle during subsequent performance heat treatment. To eliminate this internal stress, stress-relief annealing is necessary. Stress-relief annealing also reduces hardness and improves dimensional stability.

[0020] In step 1), the homogenization normalizing specifically involves heating the axle at a heating rate of 180–220°C / h to a temperature of 960–1000°C. The holding time within this temperature range is calculated based on the maximum diameter, at a rate of 0.8–1.2 min / mm, followed by air cooling. Normalizing is essentially complete austenitization combined with a pseudo-eutectoid transformation. Because the transformation temperature is lower, normalizing not only refines the grains but also improves the inhomogeneity of the microstructure, preparing the microstructure for subsequent final performance heat treatment. For alloy axles containing carbide-forming elements such as V, Cr, and Nb, a higher heating temperature is necessary to ensure complete dissolution of the carbides during normalizing. Therefore, the normalizing temperature in this invention is set at 960–1000°C.

[0021] In step 2), the two-phase quenching specifically involves heating the axle to 740-780°C at a heating rate of 160-190°C / h. The holding time within this temperature range is calculated at 0.9-1.1 min / mm, based on the maximum diameter. Subsequently, the axle is water-cooled to below 150°C and then air-cooled to room temperature. The purpose of two-phase quenching is primarily to ensure that when heated to A... c1 When the austenite undergoes reverse transformation and begins to nucleate, the temperature is not high, atomic diffusion is inactive, and grain boundary migration is slow. The austenite does not grow rapidly but exists as fine grains. During the subsequent quenching process, it will form fine lath or blocky martensite structures, thereby achieving the effect of refining the grains.

[0022] In step 2), the dual-liquid sub-critical quenching specifically involves heating the axle at a heating rate of 170–200°C / h to a temperature of 840–870°C. The holding time at this temperature is calculated at 0.9–1.1 min / mm, based on the maximum diameter. The axle is then cooled in an alkaline solution for 40–50 seconds before being transferred to an oil bath for further cooling. It is then air-cooled to room temperature until it reaches below 150°C. This process ensures that the axle cools at near-maximum cooling rates in the high-temperature region, rapidly cooling in the most unstable austenite regions such as the pearlite and bainite transformation zones to prevent decomposition, and slowly cooling during the martensitic transformation to reduce the structural stress during the austenite-to-martensite transformation, thus preventing axle distortion and cracking. Simultaneously, it ensures the formation of a fine lath martensitic structure.

[0023] Furthermore, the alkaline solution is a 10% (w / w) NaOH aqueous solution.

[0024] In step 2), the intermediate-temperature tempering specifically involves heating the axle at a heating rate of 130–160°C / h to a temperature of 640–670°C. The holding time within this temperature range is calculated based on the maximum diameter, at a rate of 1.4–1.6 min / mm. The axle is then water-cooled to room temperature to avoid secondary temper brittleness. The preferred tempering temperature is between 630–660°C, when the martensite phase gradually disappears, dislocation density decreases, and carbides fully precipitate and spheroidize. At this temperature, the microstructure has lower strength but significantly increased toughness and ductility. After tempering, a uniform and fine tempered sorbite + bainite microstructure is obtained, resulting in good toughness, ductility, and suitable strength properties.

[0025] The maximum diameter of the heat-treated blank axle is 265mm to 275mm, and the maximum length is 2100 to 2300mm.

[0026] This invention also provides a production method for the 50t axle load freight car axle serving in a cold environment with a rated load of 1200MPa. The production method includes the following steps: smelting in an electric arc furnace or converter → refining in an LF furnace → RH or VD vacuum degassing → continuous casting → billet heating → axle billet rolling → axle blank forging → rough turning of the blank axle → axle end face machining → heat treatment → rough turning of the axle outer diameter → center deep hole machining → finish turning of the axle outer diameter → outer diameter grinding → flaw detection. The heat treatment is performed using the heat treatment method described in this invention.

[0027] The composition of the axle for a 50t axle load freight vehicle operating in cold environments at 1200MPa, provided by this invention, includes the following functions and control measures for each component:

[0028] Carbon (C): Carbon is essential for steel to achieve high strength and hardness. Axles with a load capacity of 50 tons or less typically have a higher C content. While a high C content is beneficial for the strength and hardness of steel, it is extremely detrimental to its plasticity and toughness, reducing the yield strength ratio, increasing decarburization sensitivity, and worsening fatigue resistance and machinability. Therefore, appropriately reducing the C content in steel improves its plasticity and toughness. However, excessively low C content can affect the steel's strength. Considering the requirements for the HI value and the addition of other elements, the C content is controlled between 0.46% and 0.53%, with a further preferred range of 0.48% to 0.52%.

[0029] Si: Si is the main deoxidizing element in steel and has a strong solid solution strengthening effect. However, excessive Si content will reduce the plasticity and toughness of steel, increase the activity of carbon, promote decarburization and graphitization during forging and heat treatment, make smelting difficult and easily form inclusions, and deteriorate the fatigue resistance of steel. Taking all factors into consideration, the Si content should be controlled at 0.55-0.70%, and more preferably 0.60-0.70%.

[0030] Mn is a major alloying element in steel and an effective element for deoxidation and desulfurization. Mn improves the stability of austenite in steel, as well as its hardenability and strength. However, during the tempering of quenched steel, Mn and P have a strong tendency to co-segregate at grain boundaries, promoting temper brittleness and deteriorating the toughness of the steel. Considering all factors, the Mn content is controlled at 0.45–0.60%, and more preferably 0.50–0.60%.

[0031] Cr: Cr can effectively improve the hardenability and tempering resistance of steel to obtain the required high strength; at the same time, Cr can reduce the activity of C, which can reduce the tendency of steel surface decarburization during heating, rolling and heat treatment, and is conducive to obtaining high fatigue resistance. However, excessive content will deteriorate the toughness of steel. Taking all factors into consideration, the Cr content is controlled at 1.20-1.40%, and more preferably 1.25-1.35%.

[0032] Ni: A major alloying element in steel. Ni can improve the strength and toughness of steel, strengthen grain boundaries at low temperatures, and is an essential alloying element for obtaining high toughness and low-temperature toughness. It also lowers the impact toughness transition temperature. Ni can improve the hardenability and corrosion resistance of steel and ensure its toughness at low temperatures. Taking all factors into consideration, the Ni content is controlled at 1.65–1.85%, with a further preferred value of 1.70–1.80%.

[0033] Mo: Mo is a substitutional solid solution alloying element. When dissolved in austenite, it improves the hardenability of steel, enhances tempering resistance, and prevents temper brittleness. If the Mo content is too low, these effects are limited; if the Mo content is too high, these effects are saturated, and the cost of the steel increases. Considering all factors, the Mo content is controlled at 0.45–0.55%, with a further preferred value of 0.48–0.52%.

[0034] Nitrogen b (Nb): Nitrogen b is a highly effective microalloying element for refining grain size. In steel, Nb primarily increases the recrystallization temperature of austenite, thereby refining the austenite grains and improving the steel's strength and ductility. This invention also utilizes the relatively stable carbides of Nb to fix carbon, promoting the dissolution of alloying elements such as chromium and molybdenum into the solid solution and facilitating solid solution strengthening at high temperatures. However, excessive Nb diminishes its strengthening effect and increases the steel's crack susceptibility. The Nb content is controlled at 0.020–0.050%, more preferably 0.030–0.040%.

[0035] V content: The strengthening and toughening effect of V on steel is mainly manifested in precipitation strengthening. Firstly, V(C,N) precipitates during forging and rolling to refine austenite grains. Secondly, during reheating after heat treatment, a large amount of V(CN) nano-second phase precipitates, refining the grain size of reheated austenite. Excessive V content leads to excessively high V(CN) precipitation temperature, excessive precipitation amount, and coarse particle size, which is detrimental to austenite grain refinement and negatively impacts the steel's strength and toughness. Too low a V content results in negligible overall effects. Considering all factors, the V content is controlled between 0.20% and 0.30%, with a further preferred range of 0.22% to 0.28%.

[0036] Cu Content: Copper is also a non-carbide-forming element in steel, promoting austenite formation. The solubility of copper in steel varies greatly, exhibiting both solid solution strengthening and precipitation dispersion strengthening effects, thus improving yield strength and tensile strength. Simultaneously, the cathodic contact between steel and secondary precipitated Cu on the surface promotes anodizing and forms a well-protected rust layer, improving corrosion resistance. This is especially true when Cu is combined with Ni, Cr, Mo, V, etc., significantly enhancing the corrosion resistance of the steel. Cu and Ni can form an infinite solid solution, increasing the melting point of the solid solution and preventing surface cracking. Below 0.40%, Cu's effect is minimal, resulting in poor corrosion resistance; above 0.60%, Cu content easily causes surface cracking. Considering all factors, the Cu content should be controlled between 0.40% and 0.60%, with 0.45% to 0.55% being a further preferred range.

[0037] Ca (Ca) plays a role in deoxidation, desulfurization, and modification of non-metallic inclusions, thereby improving the toughness and fatigue resistance of steel. A Ca content less than 0.002% does not achieve these effects, but a content exceeding 0.005% becomes quite difficult to add and increases the amount of inclusions. Therefore, the Ca content is controlled at 0.0020–0.0050%, and more preferably 0.0030–0.0040%.

[0038] La (La): Adding an appropriate amount of La to steel can transform inclusions such as MnS and Al2O3 into rare earth inclusions, resulting in good deoxidation and desulfurization effects. The tiny solid particles of La provide heterogeneous crystal nuclei or segregate at the crystallization interface, hindering cell growth and improving the room-temperature mechanical properties of the steel. Excessive La has no significant effect. The La content is controlled at 0.010–0.020%, with a further preferred La content of 0.012–0.018%.

[0039] P: P can form micro-segregates during the solidification of molten steel, and then agglomerate at the grain boundaries when heated to the austenitizing temperature, which significantly increases the brittleness of the steel. Therefore, the content of P is controlled to be below 0.010%, and more preferably below 0.008%.

[0040] S: An unavoidable impurity in steel, forming MnS inclusions and segregating at grain boundaries, which deteriorates the toughness and fatigue resistance of the steel. Therefore, its content is controlled to be below 0.008%, and more preferably below 0.006%.

[0041] Oxygen (T[O]): Oxygen forms various oxide inclusions in steel. Under stress, stress concentration easily occurs at these oxide inclusions, leading to the initiation of microcracks and thus deteriorating the mechanical properties of the steel, especially its toughness and fatigue resistance. Therefore, measures must be taken in metallurgical production to reduce its content as much as possible. Considering economic efficiency, its content is controlled below 0.0010%. More preferably, it is below 0.0008%.

[0042] [N]: N forms carbonitrides with V and Al in steel, which can effectively inhibit austenite grain growth. However, excessive N content will lead to a deterioration in the toughness and fatigue resistance of steel. Taking all factors into consideration, the N content is controlled within the range of 0.015 to 0.020%. More preferably, it is 0.016 to 0.019%.

[0043] Al: Besides reducing dissolved oxygen in molten steel, aluminum can also refine grain size. However, excessive Al content can reduce harmful elements such as Ti in the steel, and during continuous casting, it can easily cause secondary oxidation and contamination of the molten steel. Considering all factors, the Al content should be controlled between 0.040% and 0.050%, with a further preferred value of 0.042% to 0.048%.

[0044] Compared to the AAR M-101F axle used in 50t freight car axles operating in cold environments at 1200MPa, the axle provided by this invention has a reduced C content to improve the steel's ductility and toughness. Trace amounts of Nb, V, and N are added to enhance NbV(CN) precipitation strengthening, refine grains, and improve the steel's toughness and yield strength, thereby improving its fatigue resistance and anti-peeling properties. Cr and Mo are added to improve the steel's oxidation resistance and corrosion resistance, while also improving its hardenability and tempering resistance, increasing the axle's surface fatigue limit. Appropriate amounts of Ni and Cu are added; Ni improves the steel's strength and toughness, strengthens grain boundaries at low temperatures, achieving high low-temperature toughness and lowering the impact toughness transition temperature. The cathodic contact between the steel and the secondary Cu precipitation on the surface promotes anodizing and forms a protective rust layer, improving corrosion resistance. Simultaneously, Cu and Ni form an infinite solid solution, increasing the melting point of the solid solution and preventing surface cracking. Adding RE and La elements to steel reduces the segregation of harmful impurities at grain boundaries, improves and strengthens grain boundaries, promotes the spheroidization of inclusions, further enhances the toughness of the steel, and reduces the notch sensitivity index of the material. Strict control of the content of impurity elements such as T[O], P, and S in the steel further improves its fatigue resistance.

[0045] Mn, Si, Ni, Cr, Mo, Cu, and V are the main elements affecting the hardenability of steel. Each element has a different influence on hardenability. The purpose of this invention is to achieve a tempered sorbite + bainite matrix composed of uniform, fine-grained cementite and polygonal ferrite in the cross-section of axles after heat treatment, exhibiting excellent fatigue resistance. This is achieved by considering the maximum diameter of the axle cross-section (￠275mm) and the diameter of the central deep hole (￠60mm~￠80mm). The maximum effective thickness H of the axle from the surface to the inner surface of the center hole is ≤107.5mm, and the critical quenching thickness HI=25.4×1 / 2×[(0.54C)×(1.00+0.69×Si)×(1.00+3.41×Mn)×(1.00+1.99×(Cr+V))×(1.00+0.353×(Ni+Cu))×(1.00+2.98×Mo)] should be ≥107.5.

[0046] Meanwhile, to ensure good corrosion resistance of the axle, the corrosion resistance index (I) of the steel needs to be guaranteed. Based on the influence factors of various elements on the corrosion resistance of the axle, Cr can form a dense oxide film on the steel surface, improving the steel's passivation ability. Cu can increase the corrosion potential of the steel, significantly improving corrosion resistance. Through the reasonable matching of effective elements, a corrosion resistance formula is formed, setting the corrosion resistance index I of this steel to ≥6.8, I=26.01×Cu+3.88×Ni+1.20×Cr+1.49×Si+17.28×P-7.29×Cu×Ni-9.10×Ni×P-33.39×Cu2 .

[0047] With the increase in train axle load, the fatigue life requirements for axles increase exponentially. For axle loads below 45t, the fatigue performance requirement is 10 cycles. 7 Compared to non-fracture axle loads, the fatigue life requirement for axles on a 50t axle-loaded vehicle is increased by one order of magnitude, meaning the fatigue performance requirement is 10 cycles. 8 No fracture, for high-frequency 10 7 Fatigue, its fatigue strength is greater than R fL / Rm≈0.50, while for ultra-high week 10 8 Fatigue, its fatigue strength is greater than R fL / Rm≈0.40. Therefore, to meet the requirements of a 50t axle load vehicle axle, the tensile strength Rm should be ≥1200MPa. Based on the role of alloying elements in steel, the predicted value of Rm, Y = 9.8 × (100C - 100(C - 0.4) / 3 + 10Si + 25Mo + 30Mn + 6Ni + 20Cr + 60V), should be ≥1200. Y is an index used to evaluate the influence of each element on the tensile strength of quenched and tempered steel by weighting and summing the results.

[0048] In the above formula, the value of each element is the content of the corresponding element in the above component × 100;

[0049] The heat treatment method for 50t axle load freight car axles used in cold environments at 1200MPa provided by this invention, compared with the normalizing + tempering method used for AAR M-101F axles, adopts an overall quenching and tempering heat treatment technology of "preliminary heat treatment (stress-relieving annealing + microstructure homogenization normalizing) + performance heat treatment (two-phase zone quenching + double liquid sub-temperature quenching + medium temperature tempering)". This makes the entire cross section of the axle obtain tempered sorbite + bainite composed of uniform fine-grained cementite and polygonal ferrite matrix. Compared with "pearlite + ferrite", it obtains high hardness and high strength, while also having strong toughness and a high yield strength ratio, further improving the fatigue resistance of the axle.

[0050] Traditional 45t axle load vehicles use a solid axle design. This invention adopts a lightweight design with a central deep hole of Φ80mm in diameter. This reduces the impact of harmful structures in the axle center on the fatigue performance of the axle, while also reducing the axle's own weight, reducing the unsprung weight of the bogie, reducing wear between the wheels and the rails, and allowing for direct online periodic ultrasonic testing without disassembling the bogie to check for internal defects in the axle, ensuring the safety of the wheelset.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] The cold-resistant freight axle produced using the chemical composition, process flow, and heat treatment parameters of this invention has the advantages of cold resistance, high strength, and excellent fatigue resistance compared with the prior art.

[0053] (1) It can achieve a high strength of over 1200MPa, and its plasticity and toughness are significantly better than those of commercial steel. Its fatigue limit is significantly higher than that of commercial steel, exhibiting a good balance of strength and toughness and excellent fatigue resistance. Among them: tensile strength (Rm) ≥ 1200MPa, yield strength ≥ 850MPa, longitudinal impact energy KV2 (notch depth 2mm) at -40℃ ≥ 130J, transverse impact energy KV2 (notch depth 2mm) at -40℃ ≥ 110J, longitudinal impact energy KV2 (notch depth 2mm) at -60℃ ≥ 110J, transverse impact energy KV2 (notch depth 2mm) at -60℃ ≥ 90J, longitudinal impact energy KV2 (notch depth 2mm) at -80℃ ≥ 80J, transverse impact energy KV2 (notch depth 2mm) at -80℃ ≥ 60J, and fatigue limit R of the smooth surface specimen. fL Fatigue limit R of specimens with notched surfaces ≥450MPa fE ≥380MPa, notch sensitivity index R fL / R fE With a strength of ≤1.20, it is used on axles of 50t axle load vehicles. Compared with axles of 45t axle load vehicles, it can meet the safety performance requirements without increasing the specifications, and at the same time has a ductile-brittle transition temperature of -80℃.

[0054] (2) The austenite grain size is greater than or equal to 10.0. The microstructure of the steel after heat treatment is tempered sorbite and bainite, of which the volume percentage of tempered sorbite is 90-93% and the volume percentage of bainite is 7-10%. The tempered sorbite content near the surface of the axle from 0 to 40 mm is 100%.

[0055] (3) Compared with axles with a load of less than 45 axles, it can meet the safety performance requirements without increasing the specifications, and has a ductile-brittle transition temperature of -60℃. Attached Figure Description

[0056] Figure 1 The metallographic structure 40 mm below the surface of the axle in Example 1 is 100% tempered sorbite;

[0057] Figure 2 The microstructure 40 mm below the surface of the axle in Comparative Example 1 is pearlite + ferrite. Detailed Implementation

[0058] This invention provides a 50t axle for freight vehicles operating in cold environments with a load capacity of 1200MPa, comprising the following chemical composition by weight percentage: C: 0.46–0.53%, Si: 0.55–0.70%, Mn: 0.45–0.60%, Cr: 1.20–1.40%, Ni: 1.65–1.85%, Mo: 0.45–0.55%, Nb: 0.020%–0.050%, V: 0.20–0.30%, Cu: 0.40–0.60%, Ca: 0.002–0.005%, La: 0.010–0.020%, P≤0.010%, S≤0.008%, T[O]≤0.0010%, [N]: 0.015–0.020%, Al: 0.040–0.050%, with the remainder being Fe and other unavoidable impurities.

[0059] The preferred chemical composition includes the following weight percentages: C: 0.48–0.52%, Si: 0.60–0.70%, Mn: 0.50–0.60%, Cr: 1.25–1.35%, Ni: 1.70–1.80%, Mo: 0.48–0.52%, Nb: 0.030%–0.040%, V: 0.22–0.28%, Cu: 0.45–0.55%, Ca: 0.003–0.004%, La: 0.012–0.018%, P≤0.008%, S≤0.006%, T[O]≤0.0008%, [N]: 0.016–0.019%, Al: 0.042–0.048%, with the remainder being Fe and other unavoidable impurities.

[0060] The composition of the axle of the 50t axle load freight car serving in the 1200MPa cold environment meets the critical quenching thickness HI≥107.5, HI=25.4×1 / 2×[(0.54×C)×(1.00+0.69×Si)×(1.00+3.41×Mn)×(1.00+1.99×(Cr+V))×(1.00+0.353×(Ni+Cu))×(1.00+2.98×Mo)].

[0061] The axle of the 50t axle load freight vehicle serving in a cold environment with a rated pressure of 1200MPa has a corrosion resistance index I ≥ 6.8, where I = 26.01×Cu + 3.88×Ni + 1.20×Cr + 1.49×Si + 17.28×P - 7.29×Cu×Ni - 9.10×Ni×P - 33.39×Cu 2 .

[0062] The composition of the axle of the 50t axle load freight truck serving in the 1200MPa cold environment satisfies the predicted value of Rm, Y≥1200, Y=9.8×(100C-100(C-0.4) / 3+10Si+25Mo+30Mn+6Ni+20Cr+60V).

[0063] The heat treatment method for the axle of the 50t axle load freight car used in cold environments with a rated pressure of 1200MPa includes the following steps:

[0064] 1) Preliminary heat treatment, including stress-relieving annealing and normalizing to homogenize the microstructure;

[0065] 2) Performance heat treatment, including two-phase quenching, bi-liquid sub-temperature quenching, and medium-temperature tempering.

[0066] In step 1), the stress-relief annealing specifically involves heating the axle to a temperature of 580-640°C at a heating rate of 100-120°C / h, with the holding time within this temperature range calculated based on the maximum diameter at 3 min / mm, followed by furnace cooling.

[0067] In step 1), the homogenization normalizing of the tissue specifically involves heating the axle to a temperature of 960-1000℃ at a heating rate of 180-220℃ / h, and then air-cooling the axle. The heating and holding time in this temperature range is based on the maximum diameter and is calculated at 0.8-1.2 min / mm.

[0068] In step 2), the two-phase quenching specifically involves heating the axle to a temperature of 740-780°C at a heating rate of 160-190°C / h, and then holding it at this temperature for 0.9-1.1 min / mm based on the maximum diameter. After that, the axle is water-cooled to below 150°C and then air-cooled to room temperature.

[0069] In step 2), the dual-liquid sub-temperature quenching specifically involves heating the axle to a temperature of 840-870°C at a heating rate of 170-200°C / h. The heating and holding time in this temperature range is calculated based on the maximum diameter at 0.9-1.1 min / mm. Subsequently, the axle is cooled in an alkaline solution for 40-50 seconds and then transferred to an oil bath for further cooling until it reaches below 150°C and is then air-cooled to room temperature.

[0070] Furthermore, the alkaline solution is a 10% (w / w) NaOH aqueous solution.

[0071] In step 2), the intermediate-temperature tempering specifically involves heating the axle at a heating rate of 130–160°C / h to a temperature of 640–670°C. The holding time in this temperature range is based on the maximum diameter, and the holding time is calculated at 1.4–1.6 min / mm. Then, the axle is water-cooled to room temperature.

[0072] The production method of the 50t axle load freight car axle for service in cold environments of 1200MPa includes the following steps: electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting → billet heating → axle billet rolling → axle blank forging → rough turning of the blank axle → axle end face machining → heat treatment → rough turning of the axle outer circle → center deep hole machining → finish turning of the axle outer circle → outer circle grinding → flaw detection, wherein the heat treatment is carried out using the heat treatment method described in this invention.

[0073] The present invention will now be described in detail with reference to the embodiments.

[0074] Examples 1-4

[0075] A 50t axle for a freight truck with good fatigue resistance and designed for use in cold environments at 1200MPa is shown in Table 1. The chemical composition and weight percentage of the axle are as follows: The balance not shown in Table 1 is Fe and unavoidable impurities.

[0076] Comparative Examples 1-3

[0077] The chemical composition and weight percentage of an axle for a heavy-duty freight vehicle with an axle load of less than 45t are shown in Table 1. The balance not shown in Table 1 is Fe and unavoidable impurities.

[0078] Table 1. Chemical composition (wt%), critical quenching thickness (in), and corrosion resistance index of the smelting processes in the examples and comparative examples.

[0079]

[0080]

[0081] Examples 1-4

[0082] In the embodiment, the maximum diameter of the axle is 265mm to 275mm and the maximum length is 2100mm to 2300mm.

[0083] Production follows the process flow: electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting → billet heating → axle billet rolling → axle blank forging → rough turning of axle blank → axle end face machining → stress relief annealing + microstructure homogenization normalizing + two-phase zone quenching + double liquid sub-temperature quenching + medium temperature tempering → rough turning of axle outer diameter → center deep hole machining → finish turning of axle outer diameter → outer diameter grinding → flaw detection.

[0084] The specific heat treatment process for each embodiment is as follows:

[0085] Example 1:

[0086] Stress-relief annealing: Heat at 120℃ / h to 640℃, hold for 795min, and furnace cool to below 100℃.

[0087] Homogenization normalizing: Heat at 180℃ / h to 1000℃, hold for 290min, and air cool to below 200℃.

[0088] Two-phase quenching: Heat to 780℃ at 160℃ / h, hold for 250min, water cool to below 150℃, and air cool to room temperature.

[0089] Dual-liquid sub-temperature quenching: Heat to 870℃ at 200℃ / h, hold for 275min, cool with 10% NaOH aqueous solution for 40s, then transfer to quenching oil to cool to below 150℃, and finally air cool to room temperature.

[0090] Medium-temperature tempering: Heat at 130℃ / h to 660℃, hold for 410 minutes, and cool to room temperature with water to avoid secondary tempering brittleness.

[0091] Example 2:

[0092] Stress-relief annealing: Heat to 580℃ at 100℃ / h, hold for 825 minutes, and then furnace cool to below 100℃.

[0093] Homogenization normalizing: Heat at 220℃ / h to 960℃, hold for 330min, and air cool to below 200℃.

[0094] Two-phase quenching: Heat to 740℃ at 190℃ / h, hold for 275min, water cool to below 150℃, and air cool to room temperature.

[0095] Dual-liquid sub-temperature quenching: Heat to 840℃ at 170℃ / h, hold for 250min, cool with 10% NaOH aqueous solution for 45s, then transfer to quenching oil to cool to below 150℃, and finally air cool to room temperature.

[0096] Medium-temperature tempering: Heat to 640℃ at 130℃ / h, hold for 420 minutes, and cool to room temperature with water to avoid secondary tempering brittleness.

[0097] Example 3:

[0098] Stress-relief annealing: Heat at 110℃ / h to 590℃, hold for 800min, and furnace cool to below 100℃.

[0099] Homogenization normalizing: Heat at 210℃ / h to 970℃, hold for 300min, and air cool to below 200℃.

[0100] Two-phase quenching: Heat to 750℃ at 180℃ / h, hold for 290min, water cool to below 150℃, and air cool to room temperature.

[0101] Dual-liquid sub-temperature quenching: Heat to 850℃ at 180℃ / h, hold for 260min, cool with 10% NaOH aqueous solution for 44s, then transfer to quenching oil to cool to below 150℃ and air cool to room temperature.

[0102] Medium-temperature tempering: Heat at 140℃ / h to 640℃, hold for 430 minutes, and cool to room temperature with water to avoid secondary tempering brittleness.

[0103] Example 4:

[0104] Stress-relief annealing: Heat to 620℃ at 110℃ / h, hold for 810min, and furnace cool to below 100℃.

[0105] Homogenization normalizing: Heat at 190℃ / h to 990℃, hold for 290min, and air cool to below 200℃.

[0106] Two-phase quenching: Heat to 770℃ at 170℃ / h, hold for 300min, water cool to below 150℃, and air cool to room temperature.

[0107] Dual-liquid sub-temperature quenching: Heat to 860℃ at 190℃ / h, hold for 280min, cool with 10% NaOH aqueous solution for 48s, then transfer to quenching oil to cool to below 150℃ and air cool to room temperature.

[0108] Medium-temperature tempering: Heat at 150℃ / h to 650℃, hold for 430 minutes, and cool to room temperature with water to avoid secondary tempering brittleness.

[0109] Other processes are carried out using conventional techniques.

[0110] Comparative Example 1 - Comparative Example 2

[0111] Production follows the process flow: electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting → billet heating → axle billet rolling → axle blank forging → rough turning of blank axle → axle end face machining → normalizing + normalizing + tempering → axle outer diameter finish turning → outer diameter grinding → flaw detection.

[0112] The heat treatment processes for the cold-resistant freight axles produced in Comparative Example 1 and Comparative Example 2 both included two normalizing and tempering processes. The specific heat treatment process parameters are shown below:

[0113] Comparative Example 1:

[0114] Normalizing: Heat at 120℃ / h to 870℃, hold for 280min, and air cool to below 200℃;

[0115] Normalizing: Heat at 120℃ / h to 820℃, hold for 280min, and air cool to below 200℃;

[0116] Tempering: Heat at 100℃ / h to 560℃, hold for 420 minutes, and then air cool to room temperature.

[0117] Comparative Example 2:

[0118] Normalizing: Heat at 110℃ / h to 860℃, hold for 280min, and air cool to below 200℃;

[0119] Normalizing: Heat at 120℃ / h to 800℃, hold for 280min, and air cool to below 200℃;

[0120] Tempering: Heat at 100℃ / h to 550℃, hold for 420 minutes, and then air cool to room temperature.

[0121] Comparative Example 3

[0122] Production follows the process flow: electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting → billet heating → axle billet rolling → axle blank forging → rough turning of blank axle → axle end face machining → normalizing + quenching + tempering → axle outer diameter finish turning → outer diameter grinding → flaw detection.

[0123] The heat treatment process for the heavy-duty axles produced in Comparative Example 3 all included normalizing, quenching, and tempering. The specific heat treatment process parameters are shown below:

[0124] Normalizing: Heat at 70℃ / h to 870℃, hold for 360min, and then air cool at 400℃ / h to below 200℃;

[0125] Quenching: Heat at 70℃ / h to 850℃, hold at that temperature for 390min, and then cool to room temperature with water;

[0126] Tempering: Heat at 70℃ / h to 670℃, hold for 560min, air cool at 400℃ / h to below 150℃, and then air cool to room temperature.

[0127] Comparative Examples 4-5:

[0128] The maximum diameter of the axle in Comparative Examples 4 and 5 is 265mm to 275mm, and the maximum length is 2100mm to 2300mm.

[0129] Production follows the process flow: electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting → billet heating → axle billet rolling → axle blank forging → rough turning of axle blank → axle end face machining → stress relief annealing + microstructure homogenization normalizing + two-phase zone quenching + double liquid sub-temperature quenching + medium temperature tempering → rough turning of axle outer diameter → center deep hole machining → finish turning of axle outer diameter → outer diameter grinding → flaw detection.

[0130] The specific heat treatment process for each embodiment is as follows:

[0131] Stress-relief annealing: Heat to 580℃ at 100℃ / h, hold for 825 minutes, and then furnace cool to below 100℃.

[0132] Homogenization normalizing: Heat at 220℃ / h to 960℃, hold for 330min, and air cool to below 200℃.

[0133] Two-phase quenching: Heat to 740℃ at 190℃ / h, hold for 275min, water cool to below 150℃, and air cool to room temperature.

[0134] Dual-liquid sub-temperature quenching: Heat to 840℃ at 170℃ / h, hold for 250min, cool with 10% NaOH aqueous solution for 45s, then transfer to quenching oil to cool to below 150℃, and finally air cool to room temperature.

[0135] Medium-temperature tempering: Heat to 670℃ at 130℃ / h, hold for 420 minutes, and cool to room temperature with water to avoid secondary tempering brittleness.

[0136] Comparative Example 5:

[0137] Stress-relief annealing: Heat to 620℃ at 110℃ / h, hold for 810min, and furnace cool to below 100℃.

[0138] Homogenization normalizing: Heat at 190℃ / h to 990℃, hold for 290min, and air cool to below 200℃.

[0139] Two-phase quenching: Heat to 770℃ at 170℃ / h, hold for 300min, water cool to below 150℃, and air cool to room temperature.

[0140] Dual-liquid sub-temperature quenching: Heat to 860℃ at 190℃ / h, hold for 280min, cool with 10% NaOH aqueous solution for 48s, then transfer to quenching oil to cool to below 150℃ and air cool to room temperature.

[0141] Medium-temperature tempering: Heat at 150℃ / h to 650℃, hold for 430 minutes, and cool to room temperature with water to avoid secondary tempering brittleness.

[0142] The performance indicators of the axles in the embodiments and comparative examples are shown in Tables 2, 3 and 4.

[0143] Table 2 Mechanical properties and corrosion resistance of the examples and comparative examples

[0144]

[0145]

[0146] Table 3. Metallographic structures and hardness values ​​and deviations of axle cross sections in the examples and comparative examples.

[0147]

[0148]

[0149] The above-mentioned organizational and performance testing methods are as follows:

[0150] Performance tests were conducted in accordance with GB / T 13299, GB / T 6394, GB / T 228, GB / T 229, GB / T231, GB / T 21143, GB / T19746, and YB / T5345.

[0151] As can be seen from the above, because the chemical composition and production methods of the steels in Examples 1-4 were appropriately controlled, their chemical composition ensured HI≥107.5 and I≥6.8, resulting in good strength, plasticity, toughness, and contact fatigue resistance. Comparative Examples 1 and 2 had unsuitable chemical compositions and heat treatment processes, while Comparative Example 3 had an unsuitable heat treatment process. The improper control of chemical composition and heat treatment processes in Comparative Examples 1 and 2 resulted in excessively low strength, cross-sectional hardness, and fatigue resistance. Comparative Example 3 had the same chemical composition as Example 2, but its unreasonable heat treatment process led to lower strength. Comparative Example 4 had a chemical composition and heat treatment process that met the requirements, but the Y value did not meet the strength prediction formula requirements, so the final mechanical properties did not meet the requirements. Comparative Example 5 had a chemical composition and heat treatment process that met the requirements, but the I value did not meet the corrosion resistance index formula requirements.

[0152] The above detailed description of a 50t axle load freight truck axle serving in a cold environment of 1200MPa and its heat treatment and production methods is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A 50t axle load freight truck axle designed for use in cold environments with a pressure rating of 1200MPa, characterized in that: The chemical composition includes the following weight percentages: C: 0.46~0.53%, Si: 0.55~0.70%, Mn: 0.45~0.60%, Cr: 1.20~1.40%, Ni: 1.65~1.85%, Mo: 0.45~0.55%, Nb: 0.020~0.050%, V: 0.20~0.30%, Cu: 0.40~0.60%, Ca: 0.002~0.005%, La: 0.010~0.020%, P≤0.010%, S≤0.008%, T[O]≤0.0010%, [N]: 0.015~0.020%, Al: 0.040~0.050%, with the remainder being Fe and other unavoidable impurities; The composition of the axle of the 50t axle load freight truck serving in cold environments with a rated capacity of 1200MPa meets the following requirements: The critical quenching thickness HI≥107.5, HI=25.4×1 / 2×[(0.54×C)×(1.00+0.69×Si)×(1.00+3.41×Mn)×(1.00+1.99×(Cr+V))×(1.00+0.353×(Ni+Cu))×(1.00+2.98×Mo)]; Corrosion resistance index I ≥ 6.8, I = 26.01 × Cu + 3.88 × Ni + 1.20 × Cr + 1.49 × Si + 17.28 × P - 7.29 × Cu × Ni - 9.10 × Ni × P - 33.39 × Cu 2 ; The predicted value of Rm is Y≥1200, Y=9.8×(100C-100(C-0.4) / 3+10Si+25Mo+30Mn+6Ni+20Cr+60V). The axle of the 50t axle load freight vehicle serving in cold environments with a rated strength of 1200MPa has a tensile strength ≥1200MPa and a yield strength ≥850MPa.

2. The axle of a 50t freight truck with a load capacity serving in cold environments at 1200MPa, as described in claim 1, is characterized in that... The chemical composition includes the following weight percentages: C: 0.48~0.52%, Si: 0.60~0.70%, Mn: 0.50~0.60%, Cr: 1.25~1.35%, Ni: 1.70~1.80%, Mo: 0.48~0.52%, Nb: 0.030~0.040%, V: 0.22~0.28%, Cu: 0.45~0.55%, Ca: 0.003~0.004%, La: 0.012~0.018%, P≤0.008%, S≤0.006%, T[O]≤0.0008%, [N]: 0.016~0.019%, Al: 0.042~0.048%, with the remainder being Fe and other unavoidable impurities.

3. The axle of a 50t freight truck with a 1200MPa-class cold environment as described in claim 1 or 2, characterized in that, The metallographic structure of the axle of the 50t axle load freight car serving in a cold environment of 1200MPa is tempered sorbite and bainite.

4. The axle of a 50t freight truck with a 1200MPa-class cold environment as described in claim 1 or 2, characterized in that, The longitudinal impact energy (KV2) of the axle of the 50t freight truck operating in a 1200MPa-level cold environment is ≥130J at -40℃ and ≥110J at -40℃; the longitudinal impact energy (KV2) is ≥110J at -60℃ and ≥90J at -60℃; the longitudinal impact energy (KV2) is ≥80J at -80℃ and ≥60J at -80℃; the fatigue limit (R) of the smooth-surfaced specimen is... fL ≥450MPa; fatigue limit R of specimens with notched surfaces fE ≥380MPa; Notch sensitivity index R fL / R fE ≤1.

20.

5. The heat treatment method for the axle of a 50t axle load freight car operating in a cold environment of 1200MPa as described in any one of claims 1-4, characterized in that, The heat treatment method includes the following steps: 1) Preliminary heat treatment, including stress-relieving annealing and normalizing to homogenize the microstructure; 2) Performance heat treatment, including two-phase quenching, bi-liquid sub-temperature quenching, and medium-temperature tempering; In step 1), the stress-relief annealing specifically involves heating the axle to a temperature of 580-640°C at a heating rate of 100-120°C / h, with the holding time within this temperature range calculated based on the maximum diameter at 3 min / mm, followed by furnace cooling. In step 1), the homogenization normalizing of the tissue specifically involves heating the axle to a temperature of 960-1000℃ at a heating rate of 180-220℃ / h, and the holding time in this temperature range is calculated based on the maximum diameter, with a holding time of 0.8-1.2 min / mm, followed by air cooling. In step 2), the two-phase quenching specifically involves heating the axle to a temperature of 740-780°C at a heating rate of 160-190°C / h, and then holding it at this temperature for 0.9-1.1 min / mm based on the maximum diameter. After that, the axle is water-cooled to below 150°C and then air-cooled to room temperature. In step 2), the dual-liquid sub-temperature quenching specifically involves heating the axle at a heating rate of 170~200℃ / h to a temperature of 840~870℃. The heating and holding time in this temperature range is calculated based on the maximum diameter at 0.9~1.1min / mm. Then, the axle is cooled in an alkaline solution for 40~50s and transferred to an oil bath for further cooling until it reaches below 150℃ and is then air-cooled to room temperature. In step 2), the intermediate-temperature tempering specifically involves heating the axle at a heating rate of 130~160℃ / h to a temperature of 640~670℃. The holding time in this temperature range is based on the maximum diameter, and the holding time is calculated at 1.4~1.6min / mm. Then, the axle is water-cooled to room temperature.

6. The heat treatment method according to claim 5, characterized in that, The maximum diameter of the heat-treated blank axle is 265mm~275mm and the maximum length is 2100~2300mm.

7. The method for producing axles for 50t freight trucks operating in cold environments with a load capacity of 1200MPa as described in any one of claims 1-4, characterized in that, The production method includes the following steps: 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 → heat treatment → rough turning of axle outer diameter → machining of center deep holes → finish turning of axle outer diameter → grinding of outer diameter → flaw detection, wherein the heat treatment is performed using the heat treatment method described in claim 5 or 6.

8. The application of the 50t axle load freight truck axle, which is designed for use in cold environments with a capacity of 1200MPa, as described in any one of claims 1-4, in freight vehicles.

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