Low-stress iron alloy casting molding process for high-strength driving unit shell

Abstract

The invention discloses a low-stress iron alloy casting forming process for a high-strength driving unit shell, relates to the technical field of machine manufacturing, and aims to solve the technical problem that the high-strength state and the low-stress state of the current driving unit shell cannot be balanced after casting. Adding foundry returns into the output raw materials, outputting the foundry returns to an electric furnace for smelting, detecting alloy element states in molten iron, adding stream inoculants and instantaneous inoculants according to the alloy element states, preheating a casting mold, and demolding after cooling is completed, so that casting is completed. According to the method, molten iron added with the composite alloy additive is stirred at a constant speed in the third step, it is guaranteed that alloy elements are evenly distributed in the molten iron, local stress concentration caused by coarse grains can be effectively relieved, meanwhile, the spheroidized graphite form after casting can reduce the cutting effect on a matrix, stress concentration generated at the tip end of graphite is avoided, and the service life of the graphite is prolonged. The casting internal stress is further reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of mechanical manufacturing, more particularly, to a low-stress iron alloy casting forming process for a high-strength drive unit shell. BACKGROUND

[0002] The drive unit shell refers to a shell structure for accommodating and protecting internal components of the drive unit. For high-strength use scenarios, it needs to have the characteristics of being able to withstand large stress and having good mechanical properties. The drive unit shell made of iron alloy is often used on heavy vehicles or high-load gearboxes due to its high strength and low cost. For high-strength drive unit shells used on gearboxes, it is necessary to ensure high strength while reducing the load caused by stress. Therefore, when casting the drive unit shell, it is necessary to control the raw material ratio and process to achieve the best state after casting.

[0003] The current problem of the integrated casting forming process for the gearbox is that the air inside cannot be discharged in a timely manner based on physical methods after casting, which requires controlling the casting flow to ensure that the gas can be quickly discharged during single casting. In order to achieve low stress, the integration during casting is required, which makes it difficult to ensure the effective average of the external strength and internal stress during casting, resulting in a large deviation between the strength and stress data of the gearbox shell after casting and the ideal value. In view of this, we propose a low-stress iron alloy casting forming process for a high-strength drive unit shell. SUMMARY

[0004] The present application aims to overcome the shortcomings of the prior art, adapt to the needs of reality, and provide a low-stress iron alloy casting forming process for a high-strength drive unit shell to solve the technical problem of the current drive unit shell after casting that cannot balance high strength and low stress.

[0005] To solve the above technical problems, the present application provides the following technical solution: a low-stress iron alloy casting forming process for a high-strength drive unit shell, which includes the following steps:

[0006] Step 1: Use high-grade iron ore with iron content ≥ 95% and scrap steel with impurity content ≤ 0.5% as the main raw material, and use a composite alloy additive. The non-metallic inclusion content of the output raw material is ≤ 0.15%, and the particle size uniformity is ≥ 90%;

[0007] Step two: the output raw material is added to the electric furnace smelting, and after melting at 1350-1380℃, the composite alloy additive is added twice, mechanical slagging is performed at 1420-1430℃, and after heating to 1530-1550℃, heating is stopped, and high-temperature purification is performed after standing for 5-10 minutes;

[0008] Step three: the purified molten iron is stirred at a stirring rate of 300-500 r / min for 5-8 minutes, and the alloy element state in the molten iron is detected, and according to the alloy element state, the stream inoculant and the instantaneous inoculant are added;

[0009] Step four: the casting mold is preheated, including a casting base, an isolation plate above the casting base, and a cover plate above the isolation plate, so that the casting mold is preheated to 300-400℃, and the water content is ≤0.05%, and the molten iron is cast in a layered casting manner, that is, a bottom casting area is set between the casting base and the isolation plate, and the molten iron is cast into the area and cooled, and a middle casting area is set between the isolation plate and the cover plate, and the molten iron is cast into the area and cooled.

[0010] Step five: after cooling, the mold is removed from top to bottom, the two parts of the casting located in the middle casting area and the bottom casting area are removed, and ultrasonic detection and metallographic analysis are performed to ensure that the internal defect equivalent is ≤Φ2 mm, the pearlite content is ≥90%, the graphite spheroidization rate is ≥95%, the tensile strength of the casting is ≥320 MPa, and the elongation is ≥2%, which indicates that the casting is completed.

[0011] Preferably, the composite alloy additive in step one is composed of at least two or more of manganese iron, silicon iron, copper-chromium alloy, and copper-tin alloy, the particle size after crushing in step one is 20-50 mm, and before the output raw material, multi-layer screening and strong magnetic separation are required, wherein the screening apertures of the multi-layer screening are 60 mm, 30 mm, and 20 mm respectively, and the magnetic field strength of the strong magnetic separation is 12000-15000Gs.

[0012] Preferably, in step two, the electric furnace adopts a medium-frequency induction electric furnace with a frequency of 200-500 Hz, and the temperature is raised to 1350-1380℃ at a rate of 5-8℃ / min, the interval time for the two additions of the composite alloy additive is 10-15 minutes, and after adding the second composite alloy additive, a carburizing agent and silicon iron are added, and in step two, the slagging rate is guaranteed to be ≥98%, and the carbon, silicon, and manganese contents in the molten iron and the carbon equivalent are detected by a direct-reading spectrometer, and the carbon equivalent is 3.6-3.8%.

[0013] Preferably, in step two, argon gas is introduced into the molten iron for gas washing during high-temperature purification, and the flow rate of the argon gas is 5-10 L / min, and the carburizing agent in step two is a graphitized carburizing agent with a fixed carbon content of ≥98% and a sulfur content of ≤0.05%.

[0014] Preferably, the alloy element detection in the third step is based on metal element detector operation, and the detection content and data requirement value are respectively copper 0.6%-0.8%, tin 0.04%-0.06%, and chromium 0.25%-0.30%; when the alloy element content is not in the range, elements or reaction agents can be added according to the data difference size.

[0015] Preferably, the stream inoculant content in the third step is 0.3%-0.6% of the molten iron content and is continuously added in the pouring gate, the transient inoculant content in the third step is 0.1%-0.15% of the molten iron content and is batch added in the pouring cup, the stream inoculant in the third step is silicon particles with a purity of ≥99.5%, and the transient inoculant in the third step is silicon particles with a particle size of 0.5-1mm.

[0016] Preferably, the casting mode of the bottom casting area and the middle casting area in the fourth step is based on gradient temperature control mode casting, wherein the initial temperature is 1380-1400℃, the middle period is 1350-1380℃, and the final period is 1300-1350℃, the pouring speed is adjusted according to "initial 5-8L / min, middle period 10-15L / min, and final period 5-8L / min", and the cooling is performed in a segmented slow cooling mode, that is, the cooling rate is ≤5℃ / h above 200℃, and the natural cooling is adopted below 200℃.

[0017] Preferably, after the casting in the fourth step is completed, the cooling is performed to 650-700℃ for 4-6h, and the casting stress is detected based on a stress detector; when the casting stress is ≤30MPa, the cooling is performed to below 200℃ at ≤5℃ / h, and if the casting stress is ≥30MPa, the temperature is continuously maintained at 650-700℃ and the detection is continuously performed.

[0018] Preferably, low-temperature aging treatment is performed before the cooling and demolding in the fifth step, specifically 180-220℃ for 8-12h, the ultrasonic detection in the fifth step is specifically based on an ultrasonic detector to detect the internal defects at a frequency of 2.5-5MHz, and the metallographic analysis in the fifth step is specifically to analyze the graphite state by using an image analyzer to ensure that the graphite length is ≤200μm and there is no dendritic graphite.

[0019] Preferably, positioning holes are arranged between the bottom casting area and the isolation plate and between the isolation plate and the cover plate in the fourth step, and a pouring pipe is arranged on the cover plate.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] 1、The present application ensures that the alloying elements are evenly distributed in the molten iron by uniformly stirring the molten iron to which the composite alloy additive is added in step three, avoids the "weak strength area" caused by local component deficiency, provides a component basis for the overall strength of the casting to meet the standard, at the same time, the uniform molten iron state can avoid loose structure caused by local component difference during subsequent solidification, further ensures the strength to guarantee the strength of the later pouring, at the same time prevents delamination, reduces "component uneven internal stress" from the source, avoids the risk of cracking caused by stress concentration, at the same time, the stream inoculant continuously refines the grains during the flow of the molten iron, the fine and uniform grain structure can reduce the "structure stress" during the solidification process, the finer the grain, the larger the grain boundary area, and the more dispersed the stress distribution, which can effectively alleviate the local stress concentration caused by coarse grains, at the same time, the spheroidized graphite morphology after pouring can reduce the "cutting effect" on the matrix, avoid stress concentration caused by graphite sharp tip, further reduce the internal stress of the casting, and solve the technical problem that the current drive unit shell cannot balance high strength and low stress state after pouring.

[0022] 2、The present application also preheats the mold in step four and uses the layered pouring method, the preheating temperature of 300-400 DEG C can reduce the temperature difference between the mold and the molten iron, the molten iron slowly solidifies from the surface to the inside, greatly reduces the temperature difference stress, at the same time, the layered pouring can make the casting slowly solidify in different areas, each area has enough space to release the shrinkage stress when cooling, avoids the stress superposition when the whole casting, at the same time, the layered cooling can make the shrinkage amount of each part of the casting more uniform, reduce the uneven stress caused by the thickness difference, at the same time, during the layered pouring, the molten iron parameters can be adjusted according to the performance requirements of different areas of the bottom and the middle part, to ensure that each area can form a high pearlite structure and a high graphite spheroidization rate, and ensure the consistency of the overall strength. BRIEF DESCRIPTION OF DRAWINGS

[0023] Fig. 1 The flowchart of the present application;

[0024] Fig. 2 The overall structure diagram of the pouring mold of the present application;

[0025] Fig. 3 The unfolded state diagram of the pouring mold of the present application.

[0026] Explanation of reference numerals in the figure: 100, pouring base; 110, positioning hole; 120, bottom pouring area; 200, isolation plate; 210, middle pouring area; 300, cover plate; 310, pouring pipe. DETAILED DESCRIPTION

[0027] As Figs. 1 to 3 shown, the present application relates to a low-stress iron alloy pouring forming process for a high-strength drive unit shell, which comprises the following steps:

[0028] Step 1: Select high-grade iron ore with an iron content of ≥95% and scrap steel with an impurity content of ≤0.5% as the main raw materials, and prepare composite alloy additives. Perform jaw crushing pretreatment on the raw materials. The non-metallic inclusion content in the output raw materials is ≤0.15%, and the particle size uniformity is ≥90%.

[0029] Step 2: Add the output raw materials to the recycled materials and output them to the electric furnace for smelting. After melting at 1350-1380℃, add the composite alloy additives in two batches. When the temperature reaches 1420-1430℃, mechanically remove the slag. After heating to 1530-1550℃, stop heating and let it stand for 5-10 minutes for high-temperature purification.

[0030] Step 3: Stir the purified molten iron at a speed of 300-500 r / min for 5-8 min. Detect the state of alloying elements in the molten iron and add in-flow inoculant and instantaneous inoculant according to the state of alloying elements.

[0031] Step 4: Preheat the casting mold, including the casting base 100, the isolation plate 200 above it, and the cover plate 300 above the isolation plate 200, to preheat the casting mold to 300-400℃, keeping the moisture content ≤0.05% and casting the molten iron in a layered casting method. That is, a bottom casting zone 120 is set between the casting base 100 and the isolation plate 200, where the molten iron is poured and then cooled. A middle casting zone 210 is set between the isolation plate 200 and the cover plate 300, where the molten iron is poured and then cooled.

[0032] Step 5: After cooling, demold the casting by removing the two parts of the casting located in the middle casting area 210 and the bottom casting area 120 from top to bottom. Perform ultrasonic testing and metallographic analysis to ensure that the internal defect equivalent is ≤ Φ2mm, the pearlite content is ≥90%, the graphite spheroidization rate is ≥95%, and the tensile strength of the casting is ≥320MPa and the elongation is ≥2%. This indicates that the casting is complete.

[0033] This invention, through step three, involves uniformly stirring the molten iron with added composite alloy additives to ensure the uniform distribution of alloying elements within the molten iron. This avoids "weak areas" caused by localized component deficiencies, providing a compositional basis for achieving the overall strength of the casting. Simultaneously, the uniform state of the molten iron prevents structural loosening due to localized compositional differences during subsequent solidification, further guaranteeing strength for later casting. It also prevents delamination, reducing "compositionally inhomogeneous internal stress" from the source and avoiding the risk of cracking due to stress concentration. Furthermore, the inoculant continuously refines the grains during the molten iron's flow. The fine, uniform grain structure reduces "structural stress" during solidification. The finer the grains, the larger the grain boundary area, and the more dispersed the stress distribution, effectively alleviating localized stress concentration caused by coarse grains. Additionally, the spheroidized graphite morphology after casting reduces the "splitting effect" on the matrix, preventing stress concentration at graphite tips and further reducing internal stress in the casting. This solves the current technical problem of the inability to balance high strength and low stress states in the current drive unit housing after casting.

[0034] This invention also utilizes the preheating of the mold and layered casting in step four. The preheating temperature of 300-400℃ reduces the temperature difference between the mold and the molten iron, allowing the molten iron to solidify slowly from the surface to the interior, significantly reducing thermal stress. At the same time, layered casting allows the casting to solidify slowly in different areas, with each area having sufficient space to release shrinkage stress during cooling, avoiding stress superposition during overall casting. Furthermore, layered cooling makes the shrinkage of different parts of the casting more uniform, reducing uneven shrinkage stress caused by thickness differences. Additionally, during layered casting, the molten iron parameters can be adjusted according to the performance requirements of different areas at the bottom and middle, ensuring that each area can form a high pearlite and high graphite spheroidization structure, guaranteeing consistent overall strength.

[0035] Specifically, the composite alloy additive in step one is composed of at least two or more of ferromanganese, ferrosilicon, copper-chromium alloy, and copper-tin alloy. The particle size after crushing in step one is 20-50mm, and the raw material needs to be screened and subjected to strong magnetic separation before output. The sieve apertures of the multi-layer screening are 60mm, 30mm, and 20mm, respectively, and the magnetic field strength of the strong magnetic separation is 12000-15000Gs.

[0036] By combining multiple alloying agents, the mechanical properties of cast iron can be synergistically controlled. For example, ferromanganese can improve strength and hardness, ferrosilicon can improve casting performance, copper-chromium alloy can enhance wear resistance and heat resistance, and copper-tin alloy can optimize toughness. Compared with a single alloying agent, it can more accurately match the comprehensive performance requirements of castings, avoid the limitations of single-element performance control, and broaden the application scenarios of castings.

[0037] The crushing particle size is controlled at 20-50mm, which can ensure that the raw materials melt quickly and evenly in the subsequent smelting, avoid the component segregation caused by insufficient melting of large pieces of raw materials, and also prevent the raw materials from being too fine, which would cause dust loss or agglomeration during smelting.

[0038] Multi-layer screening can accurately screen out raw materials with qualified particle size, remove ultra-coarse impurities and excessively fine powder, ensure the uniformity of raw material particle size, and provide a foundation for stable smelting;

[0039] High-intensity magnetic separation can efficiently remove ferromagnetic impurities from raw materials, such as scrap iron and magnetic slag, preventing impurities from entering the molten iron and affecting the purity of castings, and reducing defects such as cracks and porosity in subsequent castings caused by impurities.

[0040] Furthermore, in step two, the electric furnace is a 200-500Hz medium-frequency induction furnace, and the temperature is increased to 1350-1380℃ at a rate of 5-8℃ / min. The interval between the two additions of the composite alloy additive is 10-15min. After the second addition of the composite alloy additive, the carbon raiser and ferrosilicon are added. In step two, the slag removal rate is guaranteed to be ≥98%, and the carbon, silicon, and manganese content and carbon equivalent in the molten iron are determined to be 3.6-3.8% based on the detection by a direct-reading spectrometer.

[0041] The 200-500Hz medium-frequency induction furnace has high heating efficiency and precise temperature control, which can avoid the problem of large temperature fluctuations in the industrial frequency furnace, ensure stable molten iron temperature, and reduce component segregation caused by uneven temperature.

[0042] A slow heating rate of 5-8℃ / min can avoid thermal stress caused by excessively rapid heating of the raw materials, prevent local overheating of molten iron leading to oxidation and burn-off, and at the same time give sufficient melting time to the low melting point components in the raw materials, thereby improving the uniformity of molten iron.

[0043] A smelting temperature of 1350-1380℃ can ensure that the molten iron is fully melted and has good fluidity, while avoiding excessive oxidation or element loss caused by excessive temperature.

[0044] The alloy additive is added in two stages with an interval of 10-15 minutes. This allows the alloy added in the first stage to fully dissolve and diffuse, avoiding excessive local composition caused by adding a large amount at once. Subsequent addition of carbon raisers and ferrosilicon can precisely control the carbon and silicon content, further optimizing the composition of the molten iron.

[0045] A slag removal rate of ≥98% can remove oxide slag and molten slag from the surface of molten iron to the maximum extent, avoid slag from being rolled into the casting and forming slag inclusion defects, and at the same time reduce the adsorption of molten iron components by slag (such as the adsorption of silicon and manganese elements by slag), ensuring the stability of molten iron composition.

[0046] Furthermore, in step two, argon gas is introduced into the molten iron for gas washing during high-temperature purification, with an argon gas flow rate of 5-10 L / min. The carburizing agent in step two is specifically a graphitized carburizing agent with a fixed carbon content ≥98% and a sulfur content ≤0.05%.

[0047] Argon gas washing at a rate of 5-10 L / min can utilize the inertness and fluidity of argon to remove gaseous impurities such as hydrogen and nitrogen from molten iron, reducing porosity defects in castings. At the same time, argon gas stirring can promote uniformity of molten iron composition and avoid local component segregation.

[0048] Graphitized recarburizing agents with a fixed carbon content of ≥98% have high carbon content and high absorption efficiency, which can quickly and accurately increase the carbon content of molten iron and reduce the amount of recarburizing agent used. Sulfur content ≤0.05% can avoid the introduction of too much sulfur element by the recarburizing agent (sulfur can easily cause hot brittleness of cast iron), and ensure the toughness and strength of castings.

[0049] Furthermore, the alloy element detection in step three is based on the operation of a metal element detector. The required values ​​for detection are 0.6%-0.8% for copper, 0.04%-0.06% for tin, and 0.25%-0.30% for chromium. When the alloy element content is not within this range, elements or reactants can be added according to the difference in data.

[0050] By accurately detecting and controlling the content of copper, tin, and chromium within a specified range using a metal element analyzer, it is possible to ensure that the castings achieve the target performance. Timely adjustments based on content differences can prevent performance deviations caused by insufficient or excessive elements, thereby improving the casting qualification rate.

[0051] It is worth noting that the in-flow inoculant in step three contains 0.3%-0.6% of the molten iron content and is continuously added in the gating system; the instantaneous inoculant in step three contains 0.1%-0.15% of the molten iron content and is added in batches in the pouring cup; the in-flow inoculant in step three is silicon particles with a purity ≥99.5%; and the instantaneous inoculant in step three is silicon particles with a particle size of 0.5-1mm.

[0052] The inoculant is continuously added in the gating system and can be evenly dispersed with the flow of molten iron, continuously refining the graphite structure and avoiding insufficient local inoculation in the casting; high-purity silicon particles can reduce the introduction of impurities and ensure stable inoculation effect.

[0053] The instant inoculant is added in batches to the pouring cup, which can quickly replenish the inoculum before the molten iron enters the mold, further refine the graphite, prevent the appearance of white iron on the surface of the casting, and the 0.5-1mm diameter silicon particles have a moderate dissolution rate, which can avoid undissolved particles that are too large or washed away particles that are too small, thus ensuring inoculum efficiency.

[0054] It is worth mentioning that the casting method of the bottom casting area 120 and the middle casting area 210 in step four is based on gradient temperature control. The initial temperature is 1380-1400℃, the middle temperature is 1350-1380℃, and the final temperature is 1300-1350℃. The casting speed is adjusted according to "initial 5-8L / min, middle 10-15L / min, and final 5-8L / min". The cooling adopts a segmented slow cooling method, that is, the cooling rate above 200℃ is ≤5℃ / h, and the temperature below 200℃ is naturally cooled.

[0055] Gradient temperature control casting can reduce thermal stress caused by excessive temperature gradient in castings and prevent cracks. Segmented slow cooling allows internal stress in castings to be released slowly, avoiding stress concentration caused by excessive cooling. Natural cooling below 200℃ balances efficiency and stress control, further reducing the risk of casting cracking.

[0056] It is worth noting that after casting is completed in step four, the casting is kept at 650-700℃ for 4-6 hours. The stress of the casting is detected by a stress detector. If the stress of the casting is ≤30MPa, it can be cooled to below 200℃ at ≤5℃ / h with the furnace. If the stress of the casting is ≥30MPa, it is kept at 650-700℃ and the stress is continuously detected.

[0057] The stress relief temperature range of 650-700℃ is the range for cast iron, which can initially eliminate casting stress. By accurately judging the stress value with a stress detector, slow cooling can further stabilize the stress when the stress is ≤30MPa, avoiding deformation during subsequent use. When the stress is ≥30MPa, continued heat preservation can further release the stress, preventing the casting from cracking during processing or use due to excessive stress, and ensuring the dimensional stability and service life of the casting.

[0058] It is worth noting that the low-temperature aging treatment before cooling and demolding in step five is specifically a heat treatment at 180-220℃ for 8-12 hours. The ultrasonic testing in step five is specifically based on an ultrasonic testing instrument at a frequency of 2.5-5MHz to detect internal defects. The metallographic analysis in step five is specifically based on an image analyzer to analyze the graphite state, ensuring that the graphite length is ≤200μm and there is no dendritic graphite.

[0059] Low-temperature aging treatment at 180-220℃ for 8-12 hours can further eliminate residual stress in castings, stabilize the microstructure and dimensions of castings, and prevent deformation after demolding or in subsequent processing.

[0060] 2.5-5MHz ultrasonic testing can efficiently detect defects such as porosity, cracks, and shrinkage cavities inside castings, preventing unqualified castings from flowing into subsequent processes and reducing cost losses.

[0061] It is worth noting that in step four, positioning holes 110 are provided between the bottom casting area 120 and the isolation plate 200, and between the isolation plate 200 and the cover plate 300, and a through casting pipe 310 is provided on the cover plate 300.

[0062] The positioning hole 110 can accurately position the relative positions of the bottom casting area 120, the isolation plate 200 and the cover plate 300, avoid misalignment of the mold due to assembly deviation, ensure the dimensional accuracy of the casting, and reduce the defects of uneven wall thickness of the casting caused by assembly deviation. The through casting pipe 310 on the cover plate 300 can guide the molten iron to flow smoothly into the mold, ensure that the molten iron fills the mold, and reduce the defects of incomplete pouring.

[0063] Example 1

[0064] Step 1: Select iron ore with an iron content of 98% and scrap steel with an impurity content of 0.5%. Use ferromanganese and copper-chromium alloy as composite alloy additives. Pre-treat the raw materials by jaw crushing. The particle size after crushing is 20mm. The non-metallic inclusion content in the output raw material is 0.15%, and the particle size uniformity is 94%.

[0065] Step 2: The output raw materials are added to the remelting material and sent to a 300Hz medium-frequency induction furnace for melting. The temperature is increased to 1350℃ at a rate of 8℃ / min. Ferromanganese and copper-chromium alloy are added at 15-minute intervals, followed by a carbon raiser with a fixed carbon content of 98% and a sulfur content of 0.05% and ferrosilicon. Mechanical slag removal is performed at 1430℃, with a slag removal rate of 98%. The carbon, silicon, and manganese content and carbon equivalent in the molten iron are detected by a direct-reading spectrometer and are 3.8%. Heating is stopped after the temperature is increased to 1530℃ and the mixture is allowed to stand for 5 minutes for high-temperature purification. During high-temperature purification, argon gas is introduced into the molten iron for gas washing at a flow rate of 8L / min.

[0066] Step 3: Stir the purified molten iron at a stirring speed of 400 r / min for 6 min. Analyze the alloying elements in the molten iron using a metal element analyzer. The results are: copper 0.6%, tin 0.04%, chromium 0.30%. Add silicon particles with a purity of 99.5% (0.3% of the molten iron content) continuously in the gating system and silicon particles with a content of 1 mm (0.15% of the molten iron content) in batches in the pouring cup as in-flow inoculant and transient inoculant, respectively.

[0067] Step 4: Preheat the casting mold, including the casting base 100, the isolation plate 200 above it, and the cover plate 300 above the isolation plate 200. Preheat the casting mold to 400℃, maintaining a moisture content of 0.05%. Use a layered casting method to pour molten iron. A bottom casting zone 120 is set between the casting base 100 and the isolation plate 200, where the molten iron is poured and then cooled. A middle casting zone 210 is set between the isolation plate 200 and the cover plate 300, where the molten iron is poured and then cooled. Gradual temperature control is used during casting to maintain... The initial temperature of the molten iron during casting is 1400℃, the middle temperature is 1380℃, and the final temperature is 1300℃. The pouring speed is adjusted according to "8L / min initially, 12L / min in the middle stage, and 8L / min in the final stage". During cooling, a segmented slow cooling method is adopted, that is, the cooling rate is 5℃ / h above 200℃, and natural cooling is performed below 200℃. The temperature is held at 650℃ for 6 hours. Based on the stress detection instrument, the stress of the casting is measured to be 28MPa. The temperature is then cooled to below 200℃ at 5℃ / h in the furnace, and then held at 180℃ for 8 hours for further cooling.

[0068] Step 5: After cooling, demold the casting by removing the two parts of the casting located in the middle casting area 210 and the bottom casting area 120 from top to bottom. Perform ultrasonic testing and metallographic analysis to ensure that the internal defect equivalent is ≤ Φ2mm, the pearlite content is ≥90%, the graphite spheroidization rate is ≥95%, and the tensile strength of the casting is ≥320MPa and the elongation is ≥2%. This indicates that the casting is complete.

[0069] The strength of the gearbox housing cast using the above steps was compared with that of the gearbox housing produced by gravity casting, as shown in the table below:

[0070]

[0071] Based on the table above, the following conclusions can be drawn:

[0072] The maximum load-bearing strength of the shell under rated load is 18.2% higher than that of the casting produced by gravity casting. It exhibits higher strength reserve under lower stress, effectively avoids the risk of plastic deformation and fracture under static load, and extends the service life of the shell.

[0073] At 1500rpm, the dynamic load-bearing capacity of the housing is increased by 25% compared to the casting produced by gravity casting. It has better resistance to dynamic impact under lower dynamic stress, which greatly reduces the damage to the housing caused by vibration and impact during high-speed operation and improves the stability and safety of the drive unit in long-term operation.

[0074] 10 7The fatigue strength of the casting produced by gravity casting is increased by 28.6% under the fatigue strength limit of the second cycle. The shell has significantly enhanced resistance to fatigue cracking under long-term cyclic loads. It can effectively solve the problem of early damage caused by fatigue failure of the shell in the casting produced by gravity casting, and is suitable for the long service life of the drive unit.

[0075] Compared to castings produced by gravity casting, the stress corrosion cracking strength threshold under salt spray environment is 33.3% higher, and the resistance to corrosion failure is significantly enhanced, making it suitable for harsh outdoor or industrial environments with humid conditions and high salt spray levels.

[0076] After temperature cycling from -40℃ to 120℃, the thermal load-bearing strength is 55.6% higher than that of castings produced by gravity casting. It has higher thermal load-bearing strength under lower thermal stress and significantly optimized thermal impact resistance, and can stably adapt to extreme working conditions of alternating high and low temperatures.

[0077] Compared to castings produced by gravity casting, the local crack resistance at the abrupt change in shell wall thickness is increased by 62.5%. Under lower local stress, its local crack resistance is stronger, solving common failure problems such as local cracking and extrusion deformation in existing technologies and ensuring the overall structural integrity of the shell.

[0078] The embodiments disclosed in this invention are preferred embodiments, but are not limited thereto. Those skilled in the art can easily understand the spirit of this invention based on the above embodiments and make different extensions and variations, but as long as they do not depart from the spirit of this invention, they are all within the protection scope of this invention.

Claims

1. A low-stress iron alloy casting process for a high-strength drive unit housing, characterized in that, The casting process includes the following steps: Step 1: Select high-grade iron ore with an iron content of ≥95% and scrap steel with an impurity content of ≤0.5% as the main raw materials, and prepare composite alloy additives. Perform jaw crushing pretreatment on the raw materials. The non-metallic inclusion content in the output raw materials is ≤0.15%, and the particle size uniformity is ≥90%. Step 2: Add the output raw materials to the recycled materials and output them to the electric furnace for smelting. After melting at 1350-1380℃, add the composite alloy additives in two batches. When the temperature reaches 1420-1430℃, mechanically remove the slag. After heating to 1530-1550℃, stop heating and let it stand for 5-10 minutes for high-temperature purification. Step 3: Stir the purified molten iron at a speed of 300-500 r / min for 5-8 min. Detect the state of alloying elements in the molten iron and add in-flow inoculant and instantaneous inoculant according to the state of alloying elements. Step 4: Preheat the casting mold, including the casting base, the isolation plate above it, and the cover plate above the isolation plate, to 300-400℃, keeping the moisture content ≤0.05%. Cast the molten iron using a layered casting method, that is, set up a bottom casting zone between the casting base and the isolation plate, where the molten iron is poured and then cooled, and set up a middle casting zone between the isolation plate and the cover plate, where the molten iron is poured and then cooled. Step 5: After cooling, demold the casting by removing the two parts of the casting located in the middle casting area and the bottom casting area from top to bottom. Perform ultrasonic testing and metallographic analysis to ensure that the internal defect equivalent is ≤ Φ2mm, the pearlite content is ≥90%, the graphite spheroidization rate is ≥95%, and the tensile strength of the casting is ≥320MPa and the elongation is ≥2%. This indicates that the casting is complete.

2. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 1, characterized in that, The composite alloy additive in step one is composed of at least two or more of ferromanganese, ferrosilicon, copper-chromium alloy, and copper-tin alloy. The particle size after crushing in step one is 20-50mm, and the raw material needs to be screened in multiple layers and subjected to strong magnetic separation before output. The sieve apertures of the multiple layers are 60mm, 30mm, and 20mm, respectively, and the magnetic field strength of the strong magnetic separation is 12000-15000Gs.

3. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 1, characterized in that, In step two, a 200-500Hz medium-frequency induction furnace is used, and the temperature is increased to 1350-1380℃ at a rate of 5-8℃ / min. The interval between the two additions of the composite alloy additive is 10-15min. After the second addition of the composite alloy additive, a carburizing agent and ferrosilicon are added. In step two, the slag removal rate is guaranteed to be ≥98%, and the carbon, silicon, and manganese content and carbon equivalent in the molten iron are determined to be 3.6-3.8% based on the detection of a direct-reading spectrometer.

4. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 3, characterized in that, In step two, argon gas is introduced into the molten iron for gas washing during high-temperature purification. The flow rate of argon gas is 5-10 L / min. The carburizing agent in step two is a graphitized carburizing agent with a fixed carbon content ≥98% and a sulfur content ≤0.05%.

5. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 1, characterized in that, The alloy element detection in step three is based on the operation of a metal element detector. The required values ​​for detection are 0.6%-0.8% for copper, 0.04%-0.06% for tin, and 0.25%-0.30% for chromium. If the alloy element content is not within this range, elements or reactants can be added according to the difference in data.

6. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 5, characterized in that, The in-flow inoculant in step three contains 0.3%-0.6% of the molten iron content and is continuously added in the gating system. The instantaneous inoculant in step three contains 0.1%-0.15% of the molten iron content and is added in batches in the pouring cup. The in-flow inoculant in step three is silicon particles with a purity ≥99.5%, and the instantaneous inoculant in step three is silicon particles with a particle size of 0.5-1mm.

7. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 6, characterized in that, The casting method for the bottom and middle casting areas in step four is based on gradient temperature control. The initial temperature is 1380-1400℃, the middle temperature is 1350-1380℃, and the final temperature is 1300-1350℃. The casting speed is adjusted according to "initial 5-8L / min, middle 10-15L / min, and final 5-8L / min". The cooling is carried out in stages with slow cooling, that is, the cooling rate above 200℃ is ≤5℃ / h, and the temperature below 200℃ is naturally cooled.

8. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 7, characterized in that, After casting is completed in step four, the casting is cooled to 650-700℃ and held for 4-6 hours. The stress of the casting is detected by a stress detector. If the stress of the casting is ≤30MPa, it can be cooled to below 200℃ at ≤5℃ / h with the furnace. If the stress of the casting is ≥30MPa, it is held at 650-700℃ and the stress is continuously detected.

9. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 1, characterized in that, In step five, a low-temperature aging treatment is performed before cooling and demolding, specifically by holding at 180-220℃ for 8-12 hours. The ultrasonic testing in step five specifically involves using an ultrasonic testing instrument at a frequency of 2.5-5MHz to detect internal defects. The metallographic analysis in step five specifically involves using an image analyzer to analyze the graphite state, ensuring that the graphite length is ≤200μm and that there is no dendritic graphite.

10. The low-stress iron alloy casting process for a high-strength drive unit housing according to claim 1, characterized in that, In step four, positioning holes are provided between the bottom casting area and the isolation plate, and between the isolation plate and the cover plate. A through casting pipe is provided on the cover plate.

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