A method for preparing gray cast iron parts adapted to crankcase casting molds

Abstract

This invention provides a method for preparing gray cast iron parts adapted to crankcase casting molds, belonging to the field of casting technology. The method includes melting furnace charge and adjusting the composition of the molten iron before inoculation in the ladle, controlling the contents of C, Si, Mn, S, Cu, Sn, and Cr; performing FeSi75 differential silicon supplementation based on the target final Si content and the amount of Si introduced during inoculation; after high-temperature superheating homogenization, inoculation is performed in the ladle using a barium silicon inoculant to stabilize the final Si within the target range; subsequently, the molten iron is poured into the crankcase casting mold and subjected to in-mold heat preservation and cooling. This invention, through closed-loop control of final Si, Mn / S nucleation control, Cu / Sn pearlite stabilization control, quantitative barium silicon inoculation, and linkage control of pouring temperature and the number of allowable gating boxes, adapts the molten iron to a multi-cavity distribution gating mold with a side-feeding structure. This improves the feeding in areas with thick crankcase sidewalls while reducing the risks of unstable graphite nucleation, insufficient pearlite, hardness fluctuations, and unstable pressure testing.

Description

Technical Field

[0001] This invention belongs to the field of casting technology, and more specifically, relates to a method for preparing gray cast iron parts adapted to crankcase casting molds. Background Technology

[0002] Crankcases are typical gray cast iron box-type components, typically characterized by complex internal structures, long sidewall extensions, significant wall thickness variations, and numerous mounting and reinforcing areas. Thick areas of the sidewalls, wall thickness transition zones, internal corner areas, and localized areas adjacent to mounting points are prone to forming hot spots during casting solidification. For crankcase castings requiring sealing or pressure testing, defects such as shrinkage cavities, porosity, localized looseness, hard spots, or uneven microstructure in these areas can easily manifest as leakage, insufficient dimensional stability, or fluctuations in machining quality during subsequent processing, assembly, or pressure testing.

[0003] Gray cast iron, due to its good casting, vibration damping, and machinability, is commonly used for crankcase-type housing parts. To obtain crankcase gray cast iron parts that meet the requirements for strength, hardness, and machinability, existing processes typically improve the microstructure and properties of the casting by controlling the content of basic elements such as carbon, silicon, manganese, phosphorus, and sulfur, and by employing techniques such as electric furnace melting, inoculation treatment, pouring temperature control, and in-mold cooling. For crankcase castings requiring performance levels up to HT250, the stability of the matrix microstructure is often improved by adjusting the pearlite stabilizing elements and the inoculation method.

[0004] However, crankcase castings have complex structures, and their thick sidewall areas, transitional thickness regions, and localized hot spots are all highly sensitive to solidification feeding and microstructure stability. While ordinary gray cast iron smelting and inoculation processes can achieve certain strength and hardness requirements for the casting as a whole, problems such as insufficient localized graphite nucleation, white iron or hard spots, ferrite ratio fluctuations, insufficient pearlite stability, and significant hardness differences may still exist in relevant areas of the crankcase sidewalls. Especially in multi-part molds or molds with distributed runners, the molten metal travels a long pouring path before entering each cavity, making the later stages of pouring or localized sidewall areas more susceptible to temperature decay and inoculation degradation, resulting in insufficient microstructure and performance stability between castings in different parts or cavities.

[0005] To improve the feeding conditions in thick or transitional areas of the crankcase sidewalls, risers or localized feeding structures can be incorporated into the casting mold. These structures can alleviate shrinkage cavities and porosity in the relevant sidewall areas to some extent, but they also alter the local thermal state of these areas, leading to delayed solidification and cooling. Under these conditions, if ordinary gray cast iron composition control, conventional silicon supplementation methods, and standard inoculation processes are still used, problems such as fluctuating graphite morphology in the sidewall areas, decreased pearlite stability, reduced hardness, or insufficient pressure test stability are likely to occur.

[0006] Therefore, the existing manufacturing process for gray cast iron crankcases still needs further optimization to improve the feeding conditions in thick areas of the crankcase sidewalls while also ensuring stability in graphite nucleation, pearlite matrix, hardness consistency, and pressure testing. This is especially true for gray cast iron crankcases using crankcase casting molds, multi-part casting in a single mold, or those with localized feeding structures. A more sophisticated manufacturing method is needed that integrates composition analysis, silicon supplementation, inoculation, casting, and in-mold cooling. Summary of the Invention

[0007] To address the problem that existing crankcase gray cast iron parts often suffer from uneven shrinkage and microstructure stability during casting, particularly in areas with thick sidewalls, transitional thicknesses, or localized hot spots, leading to shrinkage porosity, hardness fluctuations, localized hard spots, and insufficient stability during pressure testing, this invention provides a method for preparing gray cast iron parts adapted to crankcase casting molds. This method improves the graphite nucleation stability, pearlite stability, and performance consistency of relevant areas on the sidewalls of the crankcase gray cast iron parts through a combination of composition control, final Si closed-loop control, inoculation treatment, casting control, and in-mold cooling control.

[0008] To solve the above problems, the present invention adopts the following technical solution.

[0009] A method for preparing gray cast iron parts adapted to crankcase casting molds includes the following steps: S1. Smelting furnace charge yields gray cast iron molten iron. The molten iron in the furnace is sampled and tested, and its composition is adjusted so that the molten iron in the ladle before inoculation includes, by mass percentage: C 3.25%–3.45%, Si 1.45%–1.65%, Mn 0.90%–1.10%, S 0.075%–0.095%, Cu 0.20%–0.40%, Sn 0.08%–0.12%, P < 0.05%, Cr < 0.15%, with the balance being Fe and unavoidable impurities. S2. Based on the target final Si content and the amount of Si introduced by the subsequent inoculation treatment, adjust the silicon content of the molten iron in the ladle before inoculation so that the Si content of the molten iron in the ladle before inoculation is lower than the target final Si content. S3. After the molten iron is superheated and homogenized, it is tapped from the furnace. During the tapping process, a barium silicon inoculant is used for inoculation treatment inside the ladle to control the final Si content of the molten iron after inoculation to 1.70% to 2.00%. S4. Pour the molten iron in the ladle into the crankcase casting mold for casting. S5. After pouring, keep the gray cast iron crankcase cool inside the mold.

[0010] Further, in S2, the expected final Si content is calculated based on the target final Si content, the amount of barium silicon inoculant added, the Si content of the barium silicon inoculant, and the Si recovery rate; when the expected final Si content is lower than the target final Si content, FeSi75 ferrosilicon silicon supplement agent is added to the furnace before the superheating homogenization treatment.

[0011] Furthermore, the amount m of the FeSi75 ferrosilicon additive is calculated according to the following formula: m=M×ΔSi / 100÷(w×η); Where m is the amount of FeSi75 ferrosilicon silicon supplement agent added, M is the mass of molten iron to be treated, ΔSi is the difference in the mass percentage of Si to be supplemented, w is the mass fraction of Si in the FeSi75 ferrosilicon silicon supplement agent, and η is the Si recovery rate.

[0012] Furthermore, the molten iron in the ladle before inoculation, by mass percentage, satisfies the following: C 3.30%–3.40%, Si 1.50%–1.60%, Mn 0.95%–1.05%, S 0.08%–0.09%, Cu 0.25%–0.35%, Sn 0.095%–0.11%, P < 0.05%, Cr < 0.15%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, the mass ratio of Mn to S in the molten iron before inoculation in the ladle is controlled to be 10.6 to 13.1.

[0014] Furthermore, the total content of Cu and Sn in the molten iron before inoculation in the ladle is controlled to be 0.345% to 0.46%, and the mass ratio of Cu to Sn is controlled to be 2.3 to 3.7.

[0015] Furthermore, in step S3, the molten iron is heated to 1530-1550°C and held at that temperature for more than 5 minutes for superheating and homogenization treatment, and the furnace exit temperature is controlled to be greater than 1510°C; the particle size of the barium silicon inoculant is 3-8 mm, and the amount added is 0.35%-0.45% of the mass of the molten iron.

[0016] Furthermore, the crankcase casting mold is a crankcase casting mold with a side-feeding structure, the side-feeding structure including a side-connecting gating channel disposed on the side wall of the cavity module and an independent riser connected to the side-connecting gating channel.

[0017] Furthermore, the crankcase casting mold is a multi-cavity distribution runner mold, which includes a gate, a runner, a liquid supply runner, and multiple cavity modules. The gate is connected to the liquid supply runner through the runner, and the liquid supply runner is connected to the cavity distribution runners of the multiple cavity modules. Each cavity module is provided with the side shrinkage compensation structure.

[0018] Furthermore, during casting, the mass of molten iron poured into each sand box is controlled to be 335-345 kg, and the casting time for each sand box is less than 35 seconds. The number of sand boxes allowed to be cast in the same ladle is controlled according to the casting temperature: when the casting temperature is 1400-1420℃, 6 boxes are allowed; when the casting temperature is 1390-1399℃, 5 boxes are allowed; when the casting temperature is 1380-1389℃, 4 boxes are allowed; when the casting temperature is 1370-1379℃, 3 boxes are allowed; when the casting temperature is 1365-1369℃, 1 box is allowed; casting is stopped when the casting temperature is below 1360℃; the gray cast iron crankshaft box is kept in the mold for cooling for more than 4 hours.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention enables gray cast iron to be compatible with crankcase casting molds featuring side-feeding structures by employing pre-inoculation composition control within the ladle, closed-loop silicon supplementation for final Si, Mn / S nucleation control, Cu / Sn pearlite stabilization control, quantitative inoculation with barium silicon, linkage control between pouring temperature and the number of allowable pouring boxes, and in-mold heat preservation and cooling. This method improves the feeding conditions in thick areas of the crankcase sidewalls by utilizing the side-feeding structure, while reducing the risks of graphite nucleation instability, increased ferrite, insufficient pearlite, hardness fluctuations, and unstable pressure testing caused by localized post-solidification and slow cooling. This results in improved microstructure stability, hardness consistency, feeding effect, and sealing reliability of the gray cast iron crankcase. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the preparation method of the present invention.

[0021] Figure 2 This is a schematic diagram of a crankcase casting mold used to implement the method of the present invention.

[0022] Figure 3 This is a comparison diagram of the metallographic structure of the sidewall region in the example and the comparative example.

[0023] Figure 4 This is a comparison chart of the hardness distribution in the sidewall region.

[0024] Figure 5 This is a comparison chart of the stress test pass rates for the example and the comparative example.

[0025] Figure 6 This is a comparison chart of shrinkage porosity levels in areas with thick sidewalls. Detailed Implementation

[0026] The present invention will be further described below. The following embodiments are used to illustrate the technical solution and working mechanism of the present invention, and are not intended to limit the scope of protection of the present invention. Without departing from the concept of the present invention, those skilled in the art can make adaptive adjustments to the furnace charge composition, pouring weight, inoculation method and pouring temperature control method according to the specific structure of the crankcase casting, wall thickness distribution, number of mold cavities, gating path and production cycle.

[0027] This embodiment provides a method for preparing gray cast iron parts adapted to crankcase casting molds. This method is particularly suitable for gray cast iron crankcase castings where stable feeding is required in areas with thick sidewalls, transitional wall thickness areas, or hot spot areas on the side of the crankcase.

[0028] like Figure 2 As shown, in a typical embodiment, the crankcase casting mold is a multi-cavity distribution runner mold with a side-feeding structure. The mold includes a template, a gate, a runner, a supply runner, multiple cavity modules, and cavity distribution runners corresponding to each cavity module. After entering through the gate, the molten metal is distributed to the supply runner via the runner, then enters the cavity distribution runner of each cavity module, and finally enters the corresponding cavity, thereby achieving the casting and molding of multiple crankcase castings in one mold.

[0029] Each cavity module has a side feeding structure in its side region. This side feeding structure includes a side connecting gating extending along the side wall of the cavity module and an independent riser communicating with the side connecting gating. The independent riser is arranged in a vertical column shape, with its lower end communicating with the side connecting gating, which in turn communicates with the corresponding cavity. After pouring, the molten metal in the independent riser can replenish the area adjacent to the side wall of the cavity module in the corresponding cavity through the side connecting gating, thereby improving the feeding conditions in thick areas, transitional areas, or hot spots on the side wall of the crankcase, and reducing the risk of shrinkage cavities, porosity, and post-processing leakage in these areas.

[0030] Further investigation during research and development revealed that the aforementioned side-feeding structure, while improving feeding in the sidewall area, also alters the local thermal state of that region. Independent risers and side-connected gating systems possess a certain storage capacity and heat capacity for molten metal, which, after pouring, easily leads to relatively delayed solidification and cooling conditions in the relevant areas of the crankcase sidewall. If the conventional gray cast iron smelting, silicon supplementation, and inoculation control methods are still used, this area is prone to problems such as unstable graphite nucleation, graphite coarsening, increased ferrite proportion, decreased pearlite stability, or hardness fluctuations, thereby affecting the machining stability and pressure testing stability of the crankcase gray cast iron parts.

[0031] Based on the aforementioned thermal state changes, this invention establishes a preparation method that combines composition control, final Si closed-loop control, inoculation treatment, casting control, and in-mold cooling. This method addresses the localized post-solidification and relatively slow cooling conditions created by crankcase casting molds with side-feeding structures. This allows the relevant areas of the crankcase sidewalls to achieve improved feeding while maintaining relatively stable graphite nucleation, pearlite matrix, hardness range, and pressure testing performance.

[0032] like Figure 1 As shown, the preparation method of this embodiment includes the following steps.

[0033] First, the furnace charge is added to an induction furnace for smelting to obtain gray cast iron molten iron. The furnace charge is composed of pig iron, scrap steel, recycled materials, carbon raiser, ferrosilicon, ferromanganese, sulfur raiser, electrolytic copper, and tin. In actual production, appropriate materials are selected from the above furnace charge according to the target molten iron composition to ensure that the molten iron in the ladle before inoculation reaches the composition range defined by this invention.

[0034] When the molten iron is heated to 1470±10℃, samples of the molten iron in the furnace are taken for testing, and the chemical composition is adjusted according to the test results. The molten iron in the ladle before inoculation is as follows by mass percentage: C 3.25%~3.45%, Si 1.45%~1.65%, Mn 0.90%~1.10%, S 0.075%~0.095%, Cu 0.20%~0.40%, Sn 0.08%~0.12%, P<0.05%, Cr<0.15%, with the balance being Fe and unavoidable impurities.

[0035] Preferably, the molten iron in the ladle before inoculation has the following mass percentages: C 3.30%–3.40%, Si 1.50%–1.60%, Mn 0.95%–1.05%, S 0.08%–0.09%, Cu 0.25%–0.35%, Sn 0.095%–0.11%, P < 0.05%, Cr < 0.15%, with the balance being Fe and unavoidable impurities.

[0036] C and Si are used to control the carbon equivalent, graphitization ability, and fluidity of gray cast iron. For crankcase-type castings, too low a C content will reduce the fluidity of the molten iron and increase the tendency for white iron to form, while too high a C content will easily lead to coarse graphite, reduced strength, or microstructure fluctuations. Therefore, in this embodiment, C is controlled at 3.25% to 3.45%, preferably 3.30% to 3.40%.

[0037] The Si content before inoculation in the ladle is controlled at 1.45%–1.65%, preferably 1.50%–1.60%. This range is lower than the final Si range before final pouring, aiming to reserve space for subsequent silicon replenishment and Si introduction during inoculation, and to avoid premature formation of a high Si state in the furnace. When the initial Si in the furnace is too high, the molten iron is prone to excessive graphitization tendency, coarsening of graphite morphology, or insufficient space for inoculation adjustment during the overheating and holding stages; when the initial Si is too low, it is difficult for the subsequent final Si to stably reach the target range. Therefore, this invention adopts a closed-loop control method with lower initial Si and final Si to ensure that the molten iron has suitable graphitization capacity before pouring, while avoiding Si adjustment through excessive inoculant.

[0038] After completing the in-furnace sampling and testing, based on the target final Si content and the amount of Si introduced by the subsequent inoculation treatment, silicon supplementation is performed on the molten iron in the ladle before inoculation to make the Si content of the molten iron in the ladle before inoculation lower than the target final Si content.

[0039] Specifically, the expected final Si content is calculated based on the target final Si content, the amount of barium silicon inoculant added, the Si content in the barium silicon inoculant, and the Si recovery rate. When the expected final Si content is lower than the target final Si content, FeSi75 ferrosilicon additive is added to the furnace before the superheating homogenization treatment. The amount m of FeSi75 ferrosilicon additive added is calculated according to the following formula: m=M×ΔSi / 100÷(w×η).

[0040] Where m is the amount of FeSi75 ferrosilicon silicon supplement agent added, in kg; M is the mass of molten iron to be treated, in kg; ΔSi is the difference in the mass percentage of Si required to be supplemented; w is the mass fraction of Si in the FeSi75 ferrosilicon silicon supplement agent; and η is the Si recovery rate.

[0041] The Si recovery rate η refers to the proportion of the mass of Si actually entering the molten iron and being counted in the Si content of the molten iron from the silicon supplement, relative to the theoretical Si mass in the silicon supplement. The Si recovery rate η is determined based on the furnace type, addition temperature, addition method, molten iron weight, melting time, and historical on-site detection data. Under the conditions of silicon supplementation in a medium-frequency electric furnace, η is taken as 0.80 to 0.95, preferably 0.85 to 0.92.

[0042] For example, if the mass of molten iron to be treated, M, is 1000 kg, the required Si difference ΔSi to reach the target final Si level is 0.10%, the Si mass fraction w in FeSi75 is 0.75%, and the Si recovery rate η is 0.90, then the amount of FeSi75 added, m, is: m=1000×0.10 / 100÷(0.75×0.90)=1.48kg.

[0043] This invention arranges FeSi75 silicon supplementation before the superheated homogenization process, allowing the silicon supplement to fully melt and evenly distribute in the high-temperature molten iron. This method avoids problems such as insufficient melting, compositional fluctuations, or localized high-Si areas that occur during temporary silicon supplementation within the ladle. Furthermore, the amount of barium silicon inoculant added is primarily determined based on the inoculation effect, rather than being used as a material for significantly adjusting silicon content. Increasing the amount of barium silicon inoculant to supplement final Si can easily lead to excessive inoculant addition, increasing the risks of inclusions, slag porosity, over-inoculation, and instability during pressure testing.

[0044] In this embodiment, Mn is controlled at 0.90% to 1.10%, and S is controlled at 0.075% to 0.095%. More preferably, Mn is controlled at 0.95% to 1.05%, S is controlled at 0.08% to 0.09%, and the mass ratio of Mn to S is controlled at 10.6 to 13.1.

[0045] In gray cast iron, sulfur (S) needs to be controlled at an appropriate level. Excessive S increases the risk of harmful inclusions and brittleness; insufficient S leads to insufficient sulfide nucleation sites in the molten iron, reducing graphite nucleation points. This is especially problematic in long-flow casting, multi-cavity casting, or areas with localized temperature decay, where insufficient graphite nucleation, increased white cast iron tendency, or hardness fluctuations are likely to occur. This implementation method does not simply pursue extremely low S levels, but rather controls the Mn / S mass ratio while controlling the S content, ensuring that S primarily exists in the form of relatively stable and dispersed MnS-type sulfides or complex sulfides.

[0046] These sulfides serve as the basic nucleation base for graphite formation, further enhancing the graphite nucleation capability under the subsequent inoculation effect of silicon and barium. For crankcase casting molds with side-feeding structures, the side-connected gating system and independent risers improve the feeding in the sidewall region, but also relatively delay the solidification and cooling process in this area. If the graphite nucleation base is insufficient, graphite morphology fluctuations, uneven microstructure, or hardness differences are prone to occur in this region. This invention, through Mn / S ratio control, ensures that the relevant areas of the crankcase sidewall still have a stable graphite nucleation base under localized post-solidification conditions, thereby reducing the risks of white iron, hard spots, and microstructure fluctuations.

[0047] In this embodiment, Cu is controlled at 0.20%–0.40%, and Sn is controlled at 0.08%–0.12%. More preferably, Cu is controlled at 0.25%–0.35%, and Sn is controlled at 0.095%–0.11%. Preferably, the total content of Cu and Sn is controlled at 0.345%–0.46%, and the mass ratio of Cu to Sn is controlled at 2.3–3.7.

[0048] Both Cu and Sn promote and stabilize the pearlite matrix. Cu's pearlite-promoting effect is relatively mild, which is beneficial for improving the matrix's strength and hardness stability; Sn has a stronger stabilizing effect on pearlite, but excessive Sn content can easily increase brittleness or cause over-strengthening of the microstructure. Therefore, this invention achieves a synergistic effect in stabilizing pearlite through the combined control of Cu and Sn.

[0049] For ordinary gray cast iron crankcases, Cu / Sn control can improve pearlite content and mechanical properties. However, in this invention, Cu / Sn control is also used to adapt crankcase casting molds with side-feeding structures. Because the side-connected gating system and independent risers increase the local heat capacity of relevant areas of the crankcase sidewall, creating relatively slow cooling conditions in these areas, slow cooling may lead to decreased pearlite stability, increased ferrite proportion, or decreased hardness. This invention, through Cu / Sn composite control, ensures that the thick areas of the crankcase sidewall maintain a pearlite-dominated matrix structure under improved feeding and local slow cooling conditions, reducing hardness decrease and hardness fluctuations in different areas.

[0050] Meanwhile, this embodiment controls the Cr content to less than 0.15%. Although Cr can improve hardness, it also tends to increase carbides and white cast iron content, especially in areas with significant temperature decay, thin-walled regions, or locally supercooled areas, where hard spots may form, affecting processability and performance stability. This invention stabilizes pearlite with Cu / Sn without relying on increasing the Cr content to achieve hardness, thus balancing pearlite stability, processability, and local microstructure uniformity.

[0051] After completing silicon supplementation and composition adjustment, the molten iron is heated to 1530-1550℃ and held for more than 5 minutes for superheating and homogenization treatment. Then, the furnace exit temperature is controlled to be greater than 1510℃.

[0052] The superheated homogenization treatment serves two purposes. First, it ensures the complete dissolution of additives such as FeSi75 ferrosilicon, ferromanganese, sulfur enhancers, electrolytic copper, and tin, improving the uniformity of element distribution in the molten iron. Second, it reduces the impact of localized component segregation on subsequent graphite nucleation, pearlite formation, and hardness stability. For multi-cavity distribution runner molds, the molten metal undergoes a specific flow path before entering each cavity. If the composition within the furnace is not uniform, microstructural differences are more likely to occur in different cavities or different parts. Therefore, this invention provides a stable molten iron foundation for subsequent inoculation and casting through superheated homogenization treatment.

[0053] During the tapping process, a barium silicon inoculant is added to the molten iron for in-ladle inoculation treatment. The particle size of the barium silicon inoculant is 3-8 mm, and the amount added is 0.35%-0.45% of the molten iron mass. The final Si content of the molten iron after in-ladle inoculation is controlled at 1.70%-2.00%, preferably 1.75%-1.90%.

[0054] Barium silicon inoculant is used to improve graphite nucleation ability, reduce white cast iron tendency, and mitigate the impact of inoculation fading on the microstructure stability of complex crankcase castings. While the Si in the barium silicon inoculant contributes to the final Si content, this invention does not achieve final Si replenishment by significantly increasing the amount of barium silicon inoculant. Instead, it achieves closed-loop control of final Si through a combination of FeSi75 silicon replenishment and Si introduction by the barium silicon inoculant. This ensures that the barium silicon inoculant is maintained within an appropriate inoculation dosage range, reducing the risk of over-inoculation and inclusions.

[0055] For molds with side-feeding structures, the side-connected runners and independent risers improve the feeding conditions in the crankcase sidewall region. However, the local cooling process in this region is relatively delayed, requiring higher stability of graphite nucleation. The combination of barium silicon inoculant and sulfide nucleation cores formed under Mn / S control can increase the number and stability of graphite nuclei, enabling stable graphite structures to be obtained even in thick areas of the crankcase sidewall under localized post-solidification conditions.

[0056] In some embodiments, an in-flow supplementary inoculation step is also included. The in-flow inoculator is a ferrosilicon inoculator with a particle size of 0.2–0.7 mm, and the amount added is 0.05%–0.10% of the molten iron mass. In-flow supplementary inoculation is suitable for production conditions with long pouring paths, a large number of cavities, or significant temperature decay in the later stages of pouring. By using in-flow supplementary inoculation, the inoculation effect can be further shifted to before and after the molten iron enters the pouring system, improving the microstructure stability of distant cavities or complex side regions.

[0057] The molten iron, after inoculation in the ladle, is poured into the crankcase casting mold for casting. The crankcase casting mold is a crankcase casting mold with a side-feeding structure. This side-feeding structure includes a side-connecting gating system located on the side wall of the cavity module and an independent riser connected to the side-connecting gating system. The side-connecting gating system and the independent riser are used to improve the feeding conditions in the thick areas, transitional areas, or hot spots of the crankcase side wall.

[0058] In one embodiment, the crankcase casting mold is a multi-cavity distribution runner mold. This multi-cavity distribution runner mold includes a gate, a runner, a liquid supply runner, and multiple cavity modules. The gate is connected to the liquid supply runner via the runner, and the liquid supply runner is connected to the cavity distribution runners of the multiple cavity modules. Each cavity module is provided with a side-feeding structure.

[0059] The method of this invention has good adaptability to this type of mold. The side-feeding structure improves the feeding in the crankcase sidewall area, but it also increases the local heat capacity in this area; the multi-cavity distribution gating structure can improve production efficiency, but there are differences in flow path, pouring sequence and temperature decay when molten iron is distributed among multiple cavities. Therefore, this invention, through the combination of composition, final Si closed-loop, inoculation, pouring and cooling control, ensures that the molten iron has sufficient fluidity and graphitization ability when entering this type of mold, while maintaining the stability of the pearlite matrix in the local post-solidification region.

[0060] During casting, the mass of molten iron poured into each sand box should be controlled at 335–345 kg, and the casting time for each sand box should be less than 35 seconds. The initial casting temperature should be controlled at 1400–1420℃. When casting multiple sand boxes consecutively in the same ladle, the allowable number of sand boxes cast in the same ladle should be controlled according to the casting temperature. When the pouring temperature is 1400~1420℃, it is permissible to pour 6 boxes; When the pouring temperature is 1390~1399℃, 5 boxes are allowed to be poured; When the pouring temperature is 1380~1389℃, it is permissible to pour 4 boxes; When the pouring temperature is 1370~1379℃, it is permissible to pour 3 boxes; When the pouring temperature is 1365~1369℃, one box is allowed to be poured; Stop pouring when the pouring temperature is below 1360℃.

[0061] The linkage control between temperature and the allowable number of casting boxes is not simply a field management rule, but rather a match to the casting characteristics of the crankcase multi-cavity distribution runner mold. If multiple casting boxes continue to be poured after the ladle temperature decreases, the molten iron enters the crankcase cavity after passing through the distribution runner, the supply runner, and the cavity distribution runner. This can easily lead to insufficient filling, cold shuts, inoculation fading, white cast iron tendency, or hardness fluctuations in the far-end cavities, areas with thick sidewalls, or local hot spots. This invention limits the allowable number of casting boxes by a temperature range, ensuring that the molten iron entering the crankcase casting mold maintains sufficient superheat and graphitization capacity, thereby reducing the risk of defects in the post-casting box castings.

[0062] After casting, the gray cast iron crankcase is kept in the mold for heat preservation and cooling for more than 4 hours.

[0063] The crankcase has a complex internal cavity, transitional areas on the side walls, and locally thick structures. If the sand is removed too early after casting, the temperature difference between the inside and outside of the casting and the thermal stress between areas of different wall thicknesses will be large, which can easily lead to deformation, microcracks, or unstable pressure testing. In-mold insulation and cooling can reduce abrupt changes in the cooling rate, mitigate the thermal stress in the complex structure of the crankcase, and improve dimensional and sealing stability.

[0064] However, in-mold insulation and cooling can also lead to insufficient pearlite stability in locally slow-cooling areas. This is especially true near the side-connected gating system and independent risers, where the local heat capacity is large, further reducing the cooling rate. To avoid ferrite increase, decreased hardness, or coarsening of the microstructure in this area, this invention utilizes the combined effects of Cu / Sn composite pearlite stabilization, Mn / S nucleation control, low Cr control, and final Si closed-loop inoculation control to ensure that the crankcase casting maintains a stable pearlite matrix and hardness range after in-mold slow cooling.

[0065] Therefore, this invention focuses on the synergistic control of feeding and microstructure stability in the crankcase sidewall region. While the side-feeding structure improves feeding conditions in the crankcase sidewall region, it also leads to localized post-solidification and relatively slow cooling in this area. To address this thermal state change, this invention employs a closed-loop control of lower initial and final Si levels to maintain stable graphitization capacity in the molten iron before casting, avoiding the introduction of inclusions and over-inoculation risks due to excessive inoculant addition; it controls the Mn / S ratio to form a stable sulfide nucleation base, and enhances graphite nucleation capacity through silicon-barium inoculation; it stabilizes the pearlite matrix through Cu / Sn composite control, and reduces the risk of carbide hard spots through low Cr control; it reduces the impact of post-casting temperature drop on mold filling and microstructure stability through linkage control of casting temperature and the number of allowable casting boxes; and it reduces thermal stress in the complex crankcase structure through in-mold insulation and cooling. These measures work together to ensure that the gray cast iron crankcase maintains relatively stable microstructure, hardness, and pressure testing performance while improving feeding in the sidewall region.

[0066] The effects of the present invention are further illustrated below through examples. To verify the improvement effect of the preparation method of the present invention on the feeding stability, microstructure stability, and pressure test stability of the side wall region of the gray cast iron crankcase, the same crankcase product was used for testing. Unless otherwise specified, all embodiments and comparative examples used a multi-cavity distribution gating mold with a side feeding structure for casting. Each cavity module in the mold is provided with a side connecting gating and an independent riser. The side connecting gating is located on the side wall of the cavity module and communicates with the cavity. The lower end of the independent riser communicates with the side connecting gating.

[0067] Each group of experiments used the same furnace of molten iron to continuously pour six sand molds, with each mold containing six pieces, resulting in a total of 36 crankcase castings. In performance testing, tensile strength was the average value of randomly selected samples from each group; sidewall hardness was determined by the range of test results from the thickest areas of the sidewalls of selected castings in the near-gate cavity, intermediate cavity, and distal cavity, with the sidewall hardness difference being the difference between the maximum and minimum hardness values ​​at the test points in that group; pearlite and ferrite proportions were the average values ​​from metallographic analysis of the sidewall areas; shrinkage porosity was the evaluation result after X-ray non-destructive testing of the thickest areas of the sidewalls of six randomly selected crankcase castings from each group, with lower values ​​indicating less shrinkage porosity; the pressure test pass rate was the result of full inspection of all 36 crankcase castings in each group.

[0068]

[0069]

[0070] In Examples 1 to 5, the overheating and heat preservation time was 6 minutes, the single-box casting mass was 340 kg, the single-box casting time was 30 to 34 seconds, and the particle size of the barium silicon inoculant was 3 to 8 mm.

[0071] All comparative examples below are based on Example 1. Except for the differences explicitly stated in each comparative example, all other unaltered aspects, such as composition, furnace charge smelting, sampling and testing, superheating and homogenization, silicon-barium inoculation, casting quality, casting time, in-mold heat preservation and cooling, and testing methods, are performed in accordance with Example 1.

[0072]

[0073]

[0074] As shown in Tables 1 to 5, Examples 1 to 5 all fall within the composition and process range defined in claim 1, with Examples 3 and 4 further falling within the preferred control range for the total Cu+Sn content and the Cu / Sn mass ratio. All examples employ closed-loop final Si control, quantitative inoculation with barium silicon, superheating homogenization, casting control, and in-mold cooling processes, and are all adapted to multi-cavity distribution runner molds with side-feeding structures. In all examples, the tensile strength of the gray cast iron crankcase is not less than 250 MPa, the hardness of the sidewall region is maintained within the range of 193–225 HBW, the pearlite proportion is maintained above 92.0%, and the pressure test pass rate is not less than 97.2%. This demonstrates that the present invention can obtain stable microstructure and performance within a relatively wide range of composition and process ranges.

[0075] Among them, Example 1 is the preferred embodiment. As shown in Table 4, the average tensile strength of Example 1 is 306 MPa, the hardness of the sidewall region is 205-211 HBW, the hardness difference of the sidewall is only 6 HBW, the pearlite ratio reaches 98.0%, and the ferrite ratio is only 0.5%. As shown in Table 5, the shrinkage porosity grade of Example 1 is Grade 1, and all 36 crankcase castings passed the pressure test. These results indicate that, even with the improvement of sidewall feeding through a side-connected gating system and independent risers, Example 1 can still maintain a refined microstructure, stable pearlite, and minimal hardness fluctuation in the sidewall region, achieving a balance between improved feeding and stable microstructure properties.

[0076] In Example 2, the levels of C, Si, Cu, and Sn were relatively low, resulting in a decrease in the hardness and pearlite ratio in the sidewall region compared to Example 1. However, the pressure test pass rate still reached 97.2%, indicating that significantly better overall performance than the comparative example could still be obtained even at lower compositional boundaries. In Example 5, the contents of C, Si, Cu, and Sn were relatively high, leading to relatively high tensile strength and hardness, with a slight tendency for carbides appearing locally. However, the pressure test pass rate was still 97.2%, demonstrating the feasibility of the invention across a wider range. Examples 3 and 4 were within the preferred compositional range, both achieving a 100.0% pass rate in the pressure test and exhibiting no obvious white cast iron. The hardness difference in the sidewall region was 9 HBW in both examples, indicating that the preferred compositional range could further improve the consistency of the microstructure and properties in the sidewall region.

[0077] Comparative Example 1 reduced the S content from 0.085% to 0.050% and increased the Mn / S mass ratio from 11.76 to 20.00. As shown in Table 4, the hardness of the sidewall region in Comparative Example 1 increased to 190–223 HBW, with a sidewall hardness difference of 33 HBW, significantly higher than the 6 HBW in Example 1. As shown in Table 5, Comparative Example 1 exhibited localized white iron and hard spots, and the pressure test pass rate decreased to 91.7%. These results indicate that in crankcase molds with side-compacting structures, simply reducing the S content does not improve microstructure stability. On the contrary, excessively low S content weakens the sulfide nucleation base, making the sidewall region more susceptible to insufficient graphite nucleation, localized white iron, and hardness fluctuations under localized post-solidification conditions.

[0078] Comparative Example 2 did not undergo FeSi75 differential silicon supplementation or final Si closed-loop control, and the measured final Si content after inoculation in the ladle was 1.69%. As shown in Table 4, the hardness of the sidewall region of Comparative Example 2 ranged from 186 to 218 HBW, with a hardness difference of 32 HBW. The pearlite proportion decreased to 88.0%, while the ferrite proportion increased to 7.0%. As shown in Table 5, Comparative Example 2 exhibited localized hard spots, and the pressure test pass rate decreased to 88.9%. These results indicate that for crankcase molds with side-feeding structures, the sidewall region experiences a relatively slow cooling state due to the heat capacity of the independent riser and the side-connected gating system, making it more sensitive to final Si and graphite nucleation stability. Simply adding barium silicon inoculant cannot guarantee stable graphitization ability of the molten iron before casting.

[0079] Comparative Example 3 did not use FeSi75 differential silicon supplementation; instead, the amount of barium silicon inoculant added was increased to 0.75%, resulting in a final Si content of 1.86%. Table 4 shows that the pearlite ratio and hardness stability of Comparative Example 3 were improved compared to Comparative Example 2. However, Table 5 shows that its pressure test pass rate was only 86.1%, with the main defects being an increase in slag porosity and inclusions, and an increase in the defect level in some casting sidewall areas. These results indicate that even if the final Si content reaches the target range, using excessive barium silicon inoculant to supplement Si will still increase slag content, inclusions, and the risk of over-inoculation, thus affecting the stability of the pressure test.

[0080] Comparative Example 4 reduced the Sn content from 0.103% to 0.040%, increasing the Cu / Sn mass ratio from 2.91 to 7.50. Table 4 shows that in Comparative Example 4, the pearlite content decreased to 78.0%, the ferrite content increased to 18.0%, the sidewall hardness decreased to 181–196 HBW, and the average tensile strength decreased to 258 MPa. Table 5 shows that the pressure test pass rate of Comparative Example 4 decreased to 83.3%, with the main defects being low sidewall hardness and increased leakage. These results indicate that after improving feeding with side-connected gating systems and independent risers, a relatively slow cooling condition is created in the crankcase sidewall region. Under these conditions, without Cu / Sn synergistic pearlite stabilization control, the sidewall region is prone to increased ferrite content, decreased hardness, and performance fluctuations.

[0081] Combination Figure 3 It can be further observed that in Example 1, the graphite distribution in the sidewall region is relatively uniform, and the pearlite matrix is ​​relatively stable. In Comparative Example 1, due to the low S content and the Mn / S mass ratio deviating from the range of this invention, the graphite nucleation in the sidewall region is insufficient, and local white spots or hard spots tend to appear. In Comparative Example 4, due to the low Sn content and the imbalance of the Cu / Sn synergistic relationship, the proportion of ferrite in the sidewall region is significantly increased, and the pearlite content is insufficient. This indicates that the Mn / S nucleation control, silicon-barium inoculation, and Cu / Sn pearlite stabilization control of this invention have a synergistic effect on the stability of the sidewall region structure.

[0082] Comparative Example 5 eliminated the linkage control between pouring temperature and the number of allowable pouring boxes, continuing pouring up to 6 boxes even when the temperature of the same ladle dropped to 1368℃. Table 4 shows that the hardness range of the sidewall region in Comparative Example 5 expanded to 188–224 HBW, with a hardness difference reaching 36 HBW. Table 5 shows that Comparative Example 5 exhibited localized white iron, cold shuts, and hard spots in the post-pouring box, with shrinkage porosity reaching level 2–3, and the pressure test pass rate decreased to 80.6%. These results indicate that in multi-cavity distribution runner molds, molten iron needs to pass through the distribution runner, the supply runner, and the cavity distribution runner before entering the crankcase cavity. Temperature decay in the later stages of pouring significantly affects filling, inoculation, and microstructure stability.

[0083] Figure 4 The sidewall hardness distribution of Examples 1, 2, 4, and 5 at the near-gate cavity, intermediate cavity, and distal cavity is shown. Figure 4 It can be seen that the sidewall hardness of Example 1 varies little at different cavity locations, indicating that the process of the present invention can maintain good hardness consistency in multi-cavity distribution runner molds. Comparative Examples 2, 4 and 5 all show more obvious hardness fluctuations due to insufficient final Si closed-loop control, insufficient Cu / Sn synergistic control and lack of linkage control between pouring temperature and the number of allowable pouring boxes, respectively.

[0084] Comparative Example 6 used a standard riser mold without a side-connecting gating system or an independent riser connected to the side-connecting gating system. As shown in Table 4, Comparative Example 6, due to the use of the same materials and process control as Example 1, maintained good levels in pearlite ratio, hardness range, and tensile strength. However, as shown in Table 5, Comparative Example 6 had a shrinkage porosity grade of 3, and the pressure test pass rate was only 77.8%. The main defects were significant shrinkage porosity and leakage in the thick sidewall areas. These results indicate that simply relying on material composition, silica supplementation, inoculation, and casting control is insufficient to fully address the problem of insufficient shrinkage compensation in the thick sidewall areas or transitional wall areas of the crankcase.

[0085] Figure 5 The stress test pass rates for each embodiment and comparative example are shown. Figure 5 It can be seen that the pressure test pass rates of Examples 1 to 5 remained at a high level, while the pressure test pass rates of each comparative example decreased to varying degrees. This indicates that deviating from Mn / S nucleation control, final Si closed-loop control, Cu / Sn pearlite stability control, pouring temperature linkage control, or side feeding structure will weaken the overall stability of the crankcase gray cast iron parts.

[0086] Figure 6 The diagram shows a comparison of shrinkage porosity levels in areas with thick sidewalls. Figure 6It can be seen that the shrinkage porosity grade of Example 1 is the lowest. Comparative Example 6, due to the use of a common riser mold and the lack of a side-connecting gating system and an independent riser, has a significantly higher shrinkage porosity grade in the thick sidewall area. Although Comparative Examples 2, 3 and 5 did not change the side-feeding structure, the shrinkage porosity grade and pressure test stability were still affected by the final Si stability, the method of adding inoculant or improper control of pouring temperature.

[0087] The results in Tables 4 and 5 show that the technical effect of this invention does not stem from a single factor. Comparative Example 1 indicates that a lack of adequate S and Mn / S nucleation windows leads to hardness fluctuations and localized white iron formation; Comparative Example 2 indicates that a lack of final Si closed-loop control leads to decreased microstructure and hardness stability; Comparative Example 3 indicates that excessive Si supplementation with barium silicon inoculant increases the risk of inclusions and pressure testing; Comparative Example 4 indicates that a lack of Cu / Sn synergistic control leads to insufficient pearlite and decreased hardness in the slow-cooling area of ​​the sidewalls; Comparative Example 5 indicates that a lack of temperature and number-of-casting-unit linkage control leads to fluctuations in the quality of the post-casting box; Comparative Example 6 indicates that even with good material and process control, the sidewall area feeding problem is still difficult to fully resolve when a side feeding structure is lacking.

[0088] Therefore, this invention addresses the unique thermal state of crankcase casting molds with side-feeding structures by implementing a synergistic control scheme for composition, silicon supplementation, inoculation, pouring, and cooling. While the side-feeding structure improves feeding in the crankcase sidewall region, it introduces localized post-solidification and relatively slow cooling conditions. This invention, through closed-loop control of final Si, Mn / S nucleation control, Cu / Sn pearlite stabilization control, quantitative inoculation of silicon and barium, linkage control of pouring temperature and allowable number of pouring boxes, and in-mold heat preservation and cooling, enables the gray cast iron crankcase to maintain stable graphite nucleation, pearlite matrix, hardness range, and pressure testing performance while improving feeding in the sidewall region.

Claims

1. A method for preparing gray cast iron parts adapted to crankcase casting molds, characterized in that, Includes the following steps: S1. Smelting furnace charge yields gray cast iron molten iron. The molten iron in the furnace is sampled and tested, and its composition is adjusted so that the molten iron in the ladle before inoculation includes, by mass percentage: C 3.25%–3.45%, Si 1.45%–1.65%, Mn 0.90%–1.10%, S 0.075%–0.095%, Cu 0.20%–0.40%, Sn 0.08%–0.12%, P < 0.05%, Cr < 0.15%, with the balance being Fe and unavoidable impurities. S2. Based on the target final Si content and the amount of Si introduced by the subsequent inoculation treatment, adjust the silicon content of the molten iron in the ladle before inoculation so that the Si content of the molten iron in the ladle before inoculation is lower than the target final Si content. S3. After the molten iron is superheated and homogenized, it is tapped from the furnace. During the tapping process, a barium silicon inoculant is used for inoculation treatment inside the ladle to control the final Si content of the molten iron after inoculation to 1.70% to 2.00%. S4. Pour the molten iron in the ladle into the crankcase casting mold for casting. S5. After pouring, keep the gray cast iron crankcase cool inside the mold.

2. The preparation method according to claim 1, characterized in that, In S2, the expected final Si content is calculated based on the target final Si content, the amount of barium silicon inoculant added, the Si content of the barium silicon inoculant, and the Si recovery rate; when the expected final Si content is lower than the target final Si content, FeSi75 ferrosilicon silicon supplement agent is added to the furnace before the superheating homogenization treatment.

3. The preparation method according to claim 2, characterized in that, The amount m of the FeSi75 ferrosilicon additive is calculated according to the following formula: m=M×ΔSi / 100÷(w×η); Where m is the amount of FeSi75 ferrosilicon silicon supplement agent added, M is the mass of molten iron to be treated, ΔSi is the difference in the mass percentage of Si to be supplemented, w is the mass fraction of Si in the FeSi75 ferrosilicon silicon supplement agent, and η is the Si recovery rate.

4. The preparation method according to claim 1, characterized in that, The molten iron in the ladle before inoculation meets the following mass percentage requirements: C 3.30%–3.40%, Si 1.50%–1.60%, Mn 0.95%–1.05%, S 0.08%–0.09%, Cu 0.25%–0.35%, Sn 0.095%–0.11%, P < 0.05%, Cr < 0.15%, with the balance being Fe and unavoidable impurities.

5. The preparation method according to claim 1 or 4, characterized in that, The mass ratio of Mn to S in the molten iron before inoculation in the ladle is controlled to be 10.6 to 13.

1.

6. The preparation method according to claim 1 or 4, characterized in that, The total content of Cu and Sn in the molten iron before inoculation in the ladle is controlled at 0.345% to 0.46%, and the mass ratio of Cu to Sn is controlled at 2.3 to 3.

7.

7. The preparation method according to claim 1, characterized in that, In step S3, the molten iron is heated to 1530-1550℃ and held for more than 5 minutes for superheating and homogenization treatment, and the furnace exit temperature is controlled to be greater than 1510℃; the particle size of the barium silicon inoculant is 3-8mm, and the amount added is 0.35%-0.45% of the mass of the molten iron.

8. The preparation method according to claim 1, characterized in that, The crankcase casting mold is a crankcase casting mold with a side feeding structure. The side feeding structure includes a side connecting gating channel provided on the side wall of the cavity module and an independent riser connected to the side connecting gating channel.

9. The preparation method according to claim 8, characterized in that, The crankcase casting mold is a multi-cavity distribution runner mold, which includes a gate, a runner, a liquid supply runner, and multiple cavity modules. The gate is connected to the liquid supply runner through the runner, and the liquid supply runner is connected to the cavity distribution runners of the multiple cavity modules. Each cavity module is provided with the side shrinkage compensation structure.

10. The preparation method according to claim 9, characterized in that, During casting, the mass of molten iron poured into each sand box is controlled to be 335-345 kg, and the casting time for each sand box is less than 35 seconds. The number of sand boxes allowed to be cast in the same ladle is controlled according to the casting temperature: when the casting temperature is 1400-1420℃, 6 boxes are allowed; when the casting temperature is 1390-1399℃, 5 boxes are allowed; when the casting temperature is 1380-1389℃, 4 boxes are allowed; when the casting temperature is 1370-1379℃, 3 boxes are allowed; when the casting temperature is 1365-1369℃, 1 box is allowed; casting is stopped when the casting temperature is below 1360℃; the gray cast iron crankshaft box is kept in the mold for cooling for more than 4 hours.

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