Preparation method of high-strength and high-toughness casting

Abstract

The invention discloses a preparation method of a high-strength and high-toughness casting, and belongs to the technical field of casting. The method comprises the following steps: a smelting procedure: molten metal is prepared from 2.9%-3.2% of C, 4.0%-4.5% of Si, 0.2%-0.3% of Mn, 0.03%-0.06% of Mg, 0.01%-0.2% of Cu, 0.01%-0.02% of Sn and the balance of Fe; molten metal in the furnace is transferred to a transfer ladle, and a silicon-barium inoculant is adopted for primary inoculation; the molten metal in the transfer ladle is transferred to a pouring ladle, and a silicon-barium inoculant is adopted for secondary inoculation; and the molten metal obtained after secondary inoculation is poured into a cavity through a pouring system, a rare earth inoculant is adopted for instantaneous inoculation in the pouring process, and a casting is formed. The chilling tendency of the casting can be reduced, and the casting with excellent hardness, tensile strength, yield strength and ductility is obtained.

Description

Technical Field

[0001] This invention belongs to the field of casting technology, and more specifically, relates to a method for preparing a high-strength and high-toughness automotive differential housing casting. Background Technology

[0002] As a core load-bearing component of the automotive transmission system, the differential housing has strict material requirements: First, it must possess high strength to transmit torque and resist complex stresses, while also having sufficient toughness to withstand impact loads and prevent brittle fracture. The tensile strength of the casting is typically required to be ≥600MPa, with an elongation ≥10%. Second, it needs appropriate and uniform hardness to ensure wear resistance and good machinability. Uneven or excessive hardness will exacerbate tool wear and increase machining costs. Ideally, the hardness difference between different parts of the same casting should be controlled within 20 HB. Furthermore, the internal structure of the casting must be dense to avoid defects such as shrinkage porosity and shrinkage cavities, which can become stress concentration points, affecting fatigue strength and service life.

[0003] "White iron" refers to the formation of hard and brittle cementite rather than graphite during the solidification of molten iron, which is highly detrimental to the performance of castings. Specifically, white iron structure severely degrades the mechanical properties of castings, especially toughness and elongation, making parts unable to meet usage requirements and potentially leading to premature failure during service, causing safety accidents. The extremely high hardness of white iron structure causes severe tool wear, increasing machining time and tooling costs. Controlling the tendency for white iron formation and obtaining a uniform matrix structure is fundamental to achieving efficient and low-cost machining. For the large-scale manufacturing of automotive parts, production stability and product consistency are crucial. Effectively controlling the tendency for white iron formation means lower scrap rates and higher quality consistency, directly improving production efficiency and market competitiveness. Therefore, reducing the tendency for white iron formation in cast iron differential housings is key to balancing their strength, toughness, and hardness requirements, directly affecting product reliability, machining efficiency, and production costs.

[0004] A search revealed Chinese patent application number 201710003893.9, published on June 9, 2017, which discloses a high-strength, high-toughness as-cast QT700-10 and its production method. The patent describes an original molten iron with the following chemical composition by mass percentage: C 3.8%~4.0%, Si 1.1%~1.7%, P≤0.04%, and CE=C+1 / 3(Si+P)=4.17%~4.47%; Mn 0.2%~0.4%, Cu 0.3%~0.5%, Cr≤0.05%, Sn≤0.013%. This is combined with spheroidizing inoculation and secondary inoculation to obtain as-cast QT700-10. However, the goal of this as-cast ductile iron is to simultaneously achieve high strength (Rm≥700MPa) and high toughness (elongation A≥10%). By optimizing the composition and process (such as multiple inoculations and controlling the AF value), the goal was indeed achieved. However, the yield strength (Rp0.2) is only slightly higher than the standard requirement (420 MPa) and significantly lower than the tensile strength (Rp0.2 / Rm ratio is approximately 0.58~0.62). Low yield strength indicates that the part is prone to permanent deformation (such as bending or elongation) under lower stresses during service, affecting dimensional stability and function. For example, automotive safety components (such as axle housings and steering knuckles) need to maintain their shape under high loads; insufficient yield strength may lead to premature failure.

[0005] Therefore, there is an urgent need to develop a method for preparing high-strength and high-toughness castings. Summary of the Invention

[0006] 1. The problem to be solved The purpose of this invention is to provide a method for preparing high-strength and high-toughness castings, which aims to reduce the tendency of white iron castings and achieve castings with a hardness greater than 200 HB, tensile strength ≥600 MPa, yield strength ≥470 MPa and elongation ≥10%.

[0007] 2. Technical Solution To solve the above problems, the technical solution adopted by the present invention is as follows: This invention provides a method for preparing high-strength and high-toughness castings, comprising the following steps: S1. Smelting process: The metal liquid is smelted using a medium-frequency induction furnace. The chemical composition and mass fraction of the metal liquid are as follows: C: 2.9~3.2 wt%, Si: 4.0~4.5 wt%, Mn: 0.2~0.3 wt%, P<0.03 wt%, S<0.015 wt%, Mg: 0.03~0.06 wt%, Cu: 0.01~0.2 wt%, Sn: 0.01~0.02 wt%, with the balance being Fe. S2, First inoculation process: The molten metal in step S1 is transferred to a transfer ladle and inoculated for the first time using barium silicon inoculant; S3. Second inoculation process: The molten metal in the transfer bag in step S2 is transferred to the casting bag and inoculated a second time using barium silicon inoculant. S4. Casting process: The molten metal after the second inoculation in step S3 is poured into the mold cavity through the casting system. Rare earth inoculant is used for instantaneous inoculation during the casting process; that is, the rare earth inoculant is poured into the mold cavity along with the molten metal to form the casting.

[0008] By employing the aforementioned technical solution, a casting process combining high-carbon, high-silicon composition design with three-stage inoculation treatment significantly reduces the tendency of molten iron to form white iron, while ensuring the mechanical properties required for tensile strength ≥600MPa, yield strength ≥470MPa, and elongation ≥10%. Increasing the carbon equivalent enhances the inherent graphitization ability of the molten iron, and the synergistic effect of barium-based long-acting inoculants and rare-earth instantaneous inoculants provides continuous and abundant graphite nucleation sites during the solidification process, effectively inhibiting carbide formation. Furthermore, this solution precisely controls the manganese content (0.2~0.3 wt%) and adds copper-tin composite alloying (Cu: 2.9~3.2 wt%, Sn: 0.01~0.02 wt%), optimizing the matrix structure while ensuring spheroidization, achieving a good balance between high strength and high elongation.

[0009] As one possible implementation, in step S1, the molten metal uses scrap steel and recycled material as the main raw materials, with scrap steel at 40~50 wt% and recycled material at 50~60 wt%.

[0010] Under the above technical solution, to ensure the purity of the molten metal, all raw materials entering the furnace must undergo shot blasting to remove surface rust and adhering sand. Scrap steel thicker than 5 mm must be crushed to a size within 200×200 mm to promote uniform melting and reduce element loss. The proportion of recycled material used is significantly higher than in traditional processes because recycled material itself contains well-developed graphitizing elements and crystal nuclei, which, when properly utilized, can enhance the "genetic graphitization" ability of the molten iron. However, it should be noted that recycled material may contain various trace elements; therefore, the source of recycled material should be strictly controlled to avoid mixing in scrap containing excessive anti-spheroidizing elements (such as Pb, As, Sb, etc.).

[0011] As one possible implementation, in step S1, the smelting process is carried out in a medium-frequency induction furnace, utilizing its electromagnetic stirring effect to promote the homogenization of composition and temperature. The smelting process is divided into three stages: the heating stage (room temperature → 1300℃), the refining stage (1300℃ → 1600℃), and the holding and tapping stage (1600℃ → 1520℃). In the heating stage, a lower power (60-70% of the rated power) is used for slow heating to ensure uniform heating of the furnace charge and avoid local overheating; when the temperature reaches 1300℃, the refining stage begins, at which point the power should be adjusted to 80-90% of the rated power to accelerate the smelting process; in the holding and tapping stage, the power is reduced to 30-40% of the rated power to maintain the molten iron temperature at around 1520℃, in preparation for tapping.

[0012] As one possible implementation, in steps S2 and S3, the chemical composition and mass fraction of the barium silicon inoculant are: Si: 68~75 wt%, Ba: 2~3.5 wt%, Ca: 1~2 wt%, Al≤1.5 wt%, and the remainder is Fe.

[0013] As one possible implementation, in step S2, the amount of the barium silicon inoculant accounts for 0.20~0.30% of the total mass of the molten metal in the transshipment package.

[0014] As one possible implementation, in step S2, the temperature of the molten metal is 1480~1520℃ when adding the barium silicon inoculant, the particle size of the barium silicon inoculant is 3~8 mm, and it should be preheated to 200~300℃ before use to prevent molten iron from splashing and to improve the absorption rate.

[0015] As one possible implementation, in step S3, the amount of the barium silicon inoculant accounts for 0.35~0.45% of the total mass of the molten metal in the casting ladle.

[0016] As one possible implementation, in step S3, the temperature of the molten metal is 1430~1450℃ when the barium silicon inoculant is added, and the particle size of the barium silicon inoculant is 1~3 mm. Fine-particle inoculants have a larger specific surface area, allowing for rapid dissolution and effective inoculation, compensating for any potential decline in performance during the first inoculation. The second inoculation, as a supplement and enhancement to the first, focuses on improving the nucleation ability of the molten iron, particularly promoting graphitization during the eutectic solidification stage.

[0017] As one possible implementation, in step S4, the chemical composition and mass fraction of the rare earth inoculant are: Si > 68 wt%, Al < 1.5 wt%, Re 1.5~2.2 wt%, with the remainder being Fe; the particle size of the rare earth inoculant is 0.2~0.7 mm, and it is directly injected into the casting cup using a pneumatic conveying method.

[0018] In the above-mentioned technical solution, instantaneous inoculation is the most critical step, as it provides instantaneous nucleation sites to counteract the effects of inoculation decay. Rare earth elements have strong deoxidation and desulfurization capabilities, which can purify molten iron, and rare earth sulfur oxides can serve as effective nuclei for graphite precipitation. In addition, rare earth elements can neutralize antispheroidizing elements such as antimony, lead, and bismuth, further reducing the tendency for white iron formation.

[0019] As one possible implementation, in step S4, the amount of rare earth inoculant used accounts for 0.15~0.20% of the total mass of the molten metal in the mold cavity.

[0020] 3. Beneficial effects Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The preparation method of the present invention significantly reduces the tendency of ductile iron to turn white through the synergistic effect of multiple mechanisms such as chemical composition optimization, inoculation treatment enhancement and precise control of process parameters.

[0021] (2) In the preparation method of the present invention, the carbon equivalent is controlled in a relatively high range of 2.9~3.2 wt%, which significantly enhances the graphitization ability of molten iron. Increasing the carbon equivalent can increase the eutectic solidification temperature range and reduce the supercooling, thereby inhibiting carbide precipitation. During the eutectic transformation of molten iron with high carbon equivalent, the driving force for graphite precipitation increases and the dendrite spacing of primary austenite decreases, which refines the size of eutectic clusters and reduces the probability of intergranular carbide formation. It is worth noting that although high silicon content (4.0~4.5 wt%) may increase the tendency of ferrite formation during solid-state phase transformation, the brittleness problem that high silicon may cause is effectively avoided by precisely controlling the segregation behavior of silicon and composite inoculation treatment.

[0022] (2) In the preparation method of the present invention, the three inoculation treatments constitute a complete anti-white iron system. The barium element in the barium silicon inoculant has a long-lasting inoculation effect. Barium has a large atomic radius and a slow diffusion rate in molten iron, which can continuously release nucleation cores for a long time, effectively delaying inoculation decline. Calcium has a strong deoxidation and desulfurization ability, which can purify molten iron. At the same time, the sulfur oxides of calcium have a high degree of matching with the graphite lattice, making them good heterogeneous nucleation cores. The rare earth elements (cerium, yttrium, etc.) added in the third inoculation can effectively neutralize interfering elements. Anti-spheroidizing elements such as lead, antimony, and bismuth tend to agglomerate at the boundaries of eutectic clusters, hindering graphitization. Rare earth elements can form stable compounds with these elements, eliminating their anti-graphitization effect. In addition, rare earth elements can also improve the graphite morphology, increase the spheroidization rate, and further reduce the tendency of white iron.

[0023] (3) The phenomenon of reverse white iron formation often occurs in the center or thick parts of the casting. This is because the cooling rate in these areas is slower and compositional segregation is more likely to occur. By adding elements such as copper (0.01~0.2 wt%) and tin (0.01~0.02 wt%), the pearlite tendency of the matrix can be improved to a certain extent. However, by precisely controlling the amount added and strengthening the inoculation, the formation of carbides caused by excessive alloying elements can be avoided. Copper can slightly promote graphitization and improve the matrix strength. Although tin has a strong ability to promote pearlite formation, its reverse graphitization effect can be avoided by controlling it below 0.02% and using effective inoculation treatment. Attached Figure Description

[0024] Figure 1 A three-dimensional structural diagram and a cross-sectional view of the differential housing; Figure 2 This is a schematic diagram of the combination of the core, casting, and chill of the present invention; Figure 3 This is a schematic diagram of the combination of the core and casting of the present invention; Figure 4 This is a schematic diagram of the moving mold and the fixed mold of the core box mold of the present invention; Figure 5 This is a schematic diagram of the structure of a pair of movable blocks in the core box mold of the present invention; Figure 6 This is a schematic diagram of the casting system of the present invention; Figure 7 The image shows the spheroidization of the casting obtained in Example 1 under a metallographic microscope. Figure 8 The image shows the basic microstructure of the casting obtained in Example 1 under a metallographic microscope. Figure 9 The image shows the spheroidization of the casting obtained in Example 2 under a metallographic microscope. Figure 10 The image shows the basic microstructure of the casting obtained in Example 2 under a metallographic microscope. In the picture: 1. Pour plate; 2. Cross runner; 3. Filter slag sheet; 4. Riser; 5. Core; 51. Sand injection port; 52. Loose block; 6. Chill; 7. Casting. Detailed Implementation

[0025] The present invention will be further described below with reference to specific embodiments.

[0026] In a specific implementation, taking the casting of a high-strength, high-toughness differential housing as an example, such as... Figure 1The diagram shows the structure of the differential housing. The right side shows a longitudinal cross-sectional view of the left differential housing. It can be seen that the differential housing structure is complex and the wall thickness is uneven, with thicker sections and thinner sections. Cast clay cores cannot be directly formed; they need to be made into movable parts.

[0027] like Figure 2 The diagram shows the assembly of the core and the casting. The central hole in the casting is formed using a chill (6). Since the molten iron is poured into the mold cavity, the chill (6) will not melt. Therefore, the chill (6) can be removed by hammering after forming. The contact surfaces between the casting (7) and the chill (6) are then machined. Figure 3 The diagram shows the assembly of the core and casting after removing chill 7.

[0028] Core 5 is made of silica-coated sand, which is mainly composed of high-quality selected natural quartz sand, thermoplastic phenolic resin, hexamethylenetetramine, and reinforcing agents. The required average fineness is 53-60; gas evolution is ≤15 ml / g.

[0029] Figure 4 This is the core box mold for core 5, with the moving mold on the left and the fixed mold on the right. It includes a sand injection port 51, a movable block 52, and a core box chamber. Silica-coated sand enters the core box chamber through the sand injection port 51. The movable block 52 is located within the core box chamber and is used to shape the outer surface of core 5, assisting in the shaping of core 5. Figure 5 This is a schematic diagram of a pair of live blocks 52.

[0030] The production parameters for the core box mold heat-forming clay core 5 are as follows: core box moving mold temperature: 250±20℃; core box fixed mold temperature: 250±20℃; sand injection pressure: 0.3-0.6MPa; sand injection time: 2±1s; curing time: 180±20s.

[0031] Figure 6 This is a schematic diagram of the gating system for vertical DISA production. It includes a gating bowl 1, a transverse runway 2, a filter slag sheet 3, and risers 4 connected in sequence. Molten iron is poured into the gating system from the gating bowl 1, and then diverted to the three risers 4 through the transverse runway 2. Filter slag sheets 3 are installed between the transverse runway 2 and each riser 4 to filter impurities in the molten iron. Two side risers 4 are provided on the side of each cavity to feed the hot spots of the casting. The casting can be well fed, the casting is dense, and the drilling is less prone to shrinkage defects.

[0032] The preparation method of the above-mentioned high-strength and high-toughness differential housing casting includes the following steps: S1. Smelting process: Scrap steel and recycled materials are used as the main raw materials, with 40~50 wt% scrap steel and 50~60 wt% recycled materials. They are smelted into molten metal in a medium-frequency induction furnace, in the following stages: heating stage (room temperature → 1300℃), refining stage (1300℃ → 1600℃), and holding and tapping stage (1600℃ → 1520℃).

[0033] The chemical composition and mass fraction of the molten metal are as follows: C: 2.9~3.2 wt%, Si: 4.0~4.5 wt%, Mn: 0.2~0.3 wt%, P<0.03 wt%, S<0.015 wt%, Mg: 0.03~0.06 wt%, Cu: 0.01~0.2 wt%, Sn: 0.01~0.02 wt%, with the balance being Fe.

[0034] S2, First inoculation process: When the molten metal in step S1 is transferred to the transfer ladle and the temperature of the molten metal is 1480~1520℃, 0.20~0.30% of the total mass of the molten metal in the transfer ladle is added for the first inoculation: wherein the particle size of the silicon barium inoculator is 3~8 mm. The chemical composition and mass fraction of the barium silicon inoculant are as follows: Si: 68~75 wt%, Ba: 2~3.5 wt%, Ca: 1~2 wt%, Al≤1.5 wt%, and the remainder is Fe.

[0035] S3. Second Inoculation Process: The molten metal from the transfer container in step S2 is transferred to the casting ladle. When the temperature of the molten metal is 1430~1450℃, 0.35~0.45% of a silicon-barium inoculant (based on the total mass of the molten metal in the casting ladle) is added for a second inoculation, wherein: The particle size of the barium silicon inoculant is 1~3 mm; The chemical composition and mass fraction of the barium silicon inoculant are as follows: Si: 68~75 wt%, Ba: 2~3.5 wt%, Ca: 1~2 wt%, Al≤1.5 wt%, and the remainder is Fe.

[0036] S4. Casting process: The molten metal after the second inoculation in step S3 is injected into the mold cavity through the above-mentioned casting system. At the same time, it is directly injected into the casting system by pneumatic conveying. Rare earth inoculant is used for instantaneous inoculation during the casting process; that is, the rare earth inoculant is poured into the mold cavity along with the molten metal to form a casting.

[0037] The chemical composition and mass fraction of the rare earth inoculant are: Si > 68 wt%, Al < 1.5 wt%, Re 1.5~2.2 wt%, with the remainder being Fe. In specific embodiments, Re is cerium or yttrium. The particle size of the rare earth inoculant is 0.2~0.7 mm.

[0038] The preparation method described above is further described below through examples and comparative examples: The metal liquid compositions of Examples 1-3 and Comparative Example 1 are shown in Table 1: Table 1. Metal liquid composition of Examples 1-3 and Comparative Example 1

[0039] Table 2 Chemical composition of the barium silicon inoculant used in the first and second incubation processes of Examples 1-3 and Comparative Example 1

[0040] Table 3 Chemical composition of rare earth inoculants in Examples 1-3 and Comparative Example 1

[0041] Table 4. Casting performance testing of Examples 1-3 and Comparative Example 1

[0042] The above description is merely a preferred embodiment of this application and an explanation of the technical principles used. Those skilled in the art should understand that the scope involved in this application is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the inventive concept. For example, technical solutions formed by replacing the above-mentioned features with technical features with similar functions disclosed in this application (but not limited to) each other.

[0043] Apart from the technical features described in the specification, the other technical features are known to those skilled in the art. To highlight the innovative features of this invention, the other technical features will not be described in detail here.

Claims

1. A method for preparing a high-strength, high-toughness casting, characterized in that: The steps are as follows: S1. Smelting process: The metal liquid is smelted using a medium-frequency induction furnace. The chemical composition and mass fraction of the metal liquid are as follows: C: 2.9~3.2 wt%, Si: 4.0~4.5 wt%, Mn: 0.2~0.3 wt%, P<0.03 wt%, S<0.015 wt%, Mg: 0.03~0.06 wt%, Cu: 0.01~0.2 wt%, Sn: 0.01~0.02 wt%, with the balance being Fe. S2, First inoculation process: The molten metal in step S1 is transferred to a transfer ladle and inoculated for the first time using barium silicon inoculant; S3. Second inoculation process: The molten metal in the transfer bag in step S2 is transferred to the casting bag and inoculated a second time using barium silicon inoculant. S4. Casting process: The molten metal after the second inoculation in step S3 is poured into the mold cavity through the casting system. Rare earth inoculant is used for instantaneous inoculation during the casting process to form the casting.

2. The method for preparing a high-strength, high-toughness casting according to claim 1, characterized in that: In steps S2 and S3, the chemical composition and mass fraction of the barium silicon inoculant are as follows: Si: 68~75 wt%, Ba: 2~3.5 wt%, Ca: 1~2 wt%, Al≤1.5 wt%, and the remainder is Fe.

3. The method for preparing a high-strength, high-toughness casting according to claim 1, characterized in that: In step S2, the amount of the barium silicon inoculant accounts for 0.20~0.30% of the total mass of the molten metal in the transshipment package.

4. The method for preparing a high-strength, high-toughness casting according to claim 1, characterized in that: In step S2, the temperature of the molten metal is 1480~1520℃ and the particle size of the barium silicon inoculant is 3~8 mm when the barium silicon inoculant is added.

5. The method for preparing a high-strength, high-toughness casting according to claim 1, characterized in that: In step S3, the amount of the barium silicon inoculant accounts for 0.35 to 0.45% of the total mass of the molten metal in the casting ladle.

6. The method for preparing a high-strength, high-toughness casting according to claim 1, characterized in that: In step S3, the temperature of the molten metal is 1430~1450℃ and the particle size of the barium silicon inoculant is 1~3 mm when the barium silicon inoculant is added.

7. A method for preparing a high-strength, high-toughness casting according to claim 1, characterized in that: In step S4, the chemical composition and mass fraction of the rare earth inoculant are: Si > 68 wt%, Al < 1.5 wt%, Re 1.5~2.2 wt%, and the remainder is Fe; the particle size of the rare earth inoculant is 0.2~0.7 mm, and it is directly injected into the casting cup by pneumatic conveying.

8. The method for preparing a high-strength, high-toughness casting according to claim 1, characterized in that: In step S4, the amount of rare earth inoculant used accounts for 0.15 to 0.20% of the total mass of the molten metal in the mold cavity.

9. A method for preparing a high-strength, high-toughness casting according to any one of claims 1 to 8, characterized in that: In step S1, the molten metal uses scrap steel and recycled materials as the main raw materials, with scrap steel at 40-50 wt% and recycled materials at 50-60 wt%.

10. The method for preparing a high-strength, high-toughness casting according to claim 9, characterized in that: In step S1, the smelting process is divided into three stages: heating stage: the heating temperature is raised from room temperature to 1300℃; refining stage: the heating temperature is raised from 1300℃ to 1600℃; holding and unloading stage: the heating temperature is lowered from 1600℃ to 1520℃.

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