QT900-6 nodular cast iron and preparation method and application thereof

Abstract

The invention relates to a QT900-6 nodular cast iron, which is characterized in that the nodular cast iron comprises the following components in percentage by mass: 3.3 to 3.6 percent of C, 2.0 to 2.3 percent of Si, 0.10 to 0.25 percent of Mn, 0.50 to 0.75 percent of Cu, less than 0.03 percent of P, 0.035 to 0.050 percent of S, 0.0025 to 0.0045 percent of O and the balance of Fe and inevitable impurities, and the mass percentage contents [C], [S] and [O] of C, S and O in the nodular cast iron respectively meet the following conditions: [S] / [O] = 9.71 to 16.48 and [C] / (17.6 [S] + 102.5 [O]) = 2.62 to 3.87.

Description

Technical Field

[0001] This invention relates to a QT900-6 ductile iron, which features high strength, high plasticity, and high toughness, achieving an overall and comprehensive improvement in mechanical properties. Compared to steel materials, it has a lower density, making it particularly suitable for use in electric drive axle housings, demonstrating advantages of lightweight and high load-bearing capacity. Background Technology

[0002] The electric drive axle is a core component of the powertrain of new energy vehicles. Its axle housing bears multiple loads, including the weight of the entire vehicle, the transmission of drive torque, and the resistance to road impacts. This places stringent requirements on the strength, plasticity, toughness, and fatigue life of the materials used. Currently, electric drive axle housings are mostly made of cast steel or aluminum alloy. However, cast steel has a high density and weight, which contradicts the trend of vehicle lightweighting. While aluminum alloy has the advantage of being lightweight, it has a low modulus of elasticity and limited load-bearing capacity, making it prone to deformation and failure under heavy load or high torque conditions.

[0003] Ductile iron is an ideal material for replacing steel with iron, as its comprehensive properties are close to those of steel, while its weight reduction exceeds 8%. It is currently widely used in critical components with complex stress and high strength requirements. Furthermore, ductile iron has the natural advantage of being integrally cast, reducing assembly costs. More importantly, it avoids the internal stress, loosening, and fatigue problems associated with welding or bolted connections. Therefore, high-performance ductile iron is a more ideal material for the bridge housing of electric drive bridges.

[0004] Currently, the existing technology proposed by QT900-6 ductile iron can better balance strength and plasticity. However, as new energy vehicles are increasingly developing towards high power, high torque, large size and heavy load, the requirements for the performance of drive axle housings are also getting higher and higher. Ordinary QT900-6 ductile iron cannot meet the more stringent performance requirements.

[0005] Therefore, there is an urgent need to develop a type of ductile iron with superior overall performance. Summary of the Invention

[0006] This invention provides an improved QT900-6 ductile iron. Compared with existing QT900-6 ductile iron, the QT900-6 ductile iron of this invention has higher tensile strength, higher plasticity, and excellent impact toughness, with performance essentially reaching the level of low alloy steel. The high strength-ductility product (i.e., the product of tensile strength and elongation) of this ductile iron confirms its excellent comprehensive performance. When applied to electric drive axle housings, it can "replace steel with iron," achieving a lightweight layout while ensuring performance meets requirements.

[0007] The objective of this invention is achieved as follows.

[0008] A type of QT900-6 ductile iron has a chemical composition by mass percentage containing C: 3.3-3.6%, Si: 2.0-2.3%, Mn: 0.10-0.25%, Cu: 0.50-0.75%, P: less than 0.03%, S: 0.035-0.050%, O: 0.0025-0.0045%, with the balance being Fe and unavoidable impurities. The mass percentages of C, S, and O in the ductile iron [C], [S], and [O] satisfy the following conditions: [S] / [O] = 9.71-16.48 and [C] / (17.6[S]+102.5[O]) = 2.62-3.87, respectively.

[0009] This invention relates to pearlitic ductile iron. Regarding the basic elements, a certain amount of Cu and Mn are added to the ductile iron to stabilize the pearlite and achieve solid solution strengthening, ensuring strength performance. Insufficient Cu or Mn content makes it difficult to guarantee the pearlite matrix content and the strengthening effect; excessive Cu or Mn content makes the material more brittle and difficult to guarantee plasticity.

[0010] Unlike existing technologies that control the S and O content in ductile iron to keep it as low as possible, this invention intentionally controls the O and S content in ductile iron within a certain range. This is because, through repeated experiments, the inventors discovered that fine oxides and sulfides can act as graphite nuclei, increasing the number of graphite spheres and improving the graphite size grade, thereby significantly improving the strength, plasticity, and toughness of ductile iron and ensuring that the material has a high strength-ductility product.

[0011] Furthermore, the inventors discovered that in order to fully realize the aforementioned effects of S and O, it is not enough to simply control the content of S and O. It is also necessary to coordinately control the content relationship between the two and the content relationship between S, O and C in order to maximize the corresponding effects and thus ensure the achievement of the technical effects of the present invention.

[0012] This invention obtains pearlitic ductile iron with excellent spheroidization grade, graphite spheroid size, and graphite spheroid quantity by controlling the element content and the corresponding element content relationship of ductile iron. This ductile iron has excellent tensile strength and elongation, excellent impact heat resistance at 20℃, and a high product of tensile strength and elongation, exhibiting excellent comprehensive performance. The ductile iron of this invention has one or more of the following properties or microstructure characteristics.

[0013] 1) The tensile strength of ductile iron is above 980MPa and the elongation is above 7.5%.

[0014] 2) The product of tensile strength and elongation of ductile iron, i.e., the strength-ductility product, is above 8000 MPa·%.

[0015] 3) Impact toughness ak of ductile iron at 20℃ VAt 15J / cm 2 above.

[0016] 4) The pearlite content of ductile iron reaches grade 95.

[0017] 5) The spheroidal graphite cast iron has a spheroidal graphite grade of 1 (corresponding to a spheroidal graphite rate ≥ 95%) or 2 (corresponding to a spheroidal graphite rate of 90%-94%).

[0018] 6) The graphite size of ductile iron is grade 7 (actual graphite size > 0.015 mm and ≤ 0.03 mm) or grade 8 (actual graphite size ≤ 0.015 mm).

[0019] 7) The number of graphite spheres in ductile iron reaches 270-350 per mm. 2 .

[0020] Furthermore, the present invention also provides a method for preparing the aforementioned ductile iron, comprising the following steps: 1) Smelting: Pig iron, scrap steel and carbon raiser are put into the smelting furnace for smelting. After the temperature is stabilized above 1550℃, the composition is tested. The composition is adjusted according to the test results and the amount of spheroidizing agent and inoculant added is determined. The furnace is removed after the composition meets the standard. 2) Spheroidization and inoculation: Place rare earth magnesium silicon iron spheroidizing agent and silicon iron inoculant into the spheroidizing ladle in sequence, and pour molten iron into the spheroidizing ladle. The weight of rare earth magnesium silicon iron spheroidizing agent added is 1.2-1.5% of the weight of molten iron, and the weight of silicon iron inoculant added is 1.5-2.5% of the weight of molten iron. 3) Oxygen and sulfur determination: The composition of the molten iron after spheroidization and inoculation in step 2) is analyzed, and the oxygen and sulfur are controlled within the corresponding range by feeding calcium wire or adding FeS powder and FeO powder; 4) Casting: Pour the molten iron from step 3) into the mold, and remove it after cooling.

[0021] The above-mentioned ductile iron preparation process is basically the same as the existing ductile iron preparation process. The difference is that, due to the process requirements of precise control of O and S, the present invention specifically sets up a step of determining oxygen and sulfur after spheroidization and inoculation and before casting, so as to ensure that O and S are within the range required by the present invention and meet the content relationship of O, S and C, O and S required by the present invention. Casting is carried out after determining oxygen and sulfur.

[0022] In the oxygen and sulfur determination process, if one or both of O and S are too high, the principle of removal followed by replenishment is followed. First, calcium wire is fed in and the O and S are checked to ensure that they are within or below the required range of the invention composition. This ensures that neither O nor S is above the required range. If O or S is below the required range, FeS powder or FeO powder is added accordingly to adjust O or S to the required range before casting. If both O and S are within the required range, no adjustment is needed before casting. If both O and S are below the required range, or O is below the required range but S is within the required range, or S is below the required range but O is within the required range, FeS powder or FeO powder is added accordingly to adjust O or S to the required range before casting.

[0023] Rare earth magnesium ferrosilicon spheroidizing agents can be conventional commercially available products, with typical compositions such as Si: 45-49%, Mg: 5.1-5.4%, RE: 2.0-2.4%, Ca: 2.0-2.2%, and Fe balance. Ferrosilicon inoculants can also be conventional commercially available products, with typical compositions of Si: 74-80% and Fe balance.

[0024] This invention also provides the application of the aforementioned QT900-6 ductile iron in the manufacture of electric drive axle housings. As mentioned earlier, the QT900-6 ductile iron of this invention has excellent comprehensive strength and plasticity properties, and outstanding impact toughness, which can meet the performance requirements of electric drive axle housings under high power, high torque, and heavy load conditions. Under the premise that the performance meets the operating conditions, the ductile iron of this invention also has the outstanding advantages of lightweight and low cost compared with alloy steel.

[0025] The beneficial effects of this invention are mainly reflected in the following aspects: By intentionally controlling the O and S content in ductile iron within a certain range, and by synergistically controlling the content relationship between O and S, as well as the content relationship between S, O, and C, fine oxides and sulfides can act as graphite nuclei, increasing the number of graphite spheres and improving the graphite size grade. This significantly improves the strength, plasticity, and toughness of ductile iron, ensuring that the material has a high strength-ductility product, giving ductile iron excellent comprehensive performance. When using iron instead of steel as the material for electric drive bridge housings, it has outstanding advantages in lightweighting and cost. Detailed Implementation

[0026] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, the following description is provided in conjunction with specific experiments.

[0027] First, prepare several ductile iron castings according to the following steps: 1) Smelting: Pig iron, scrap steel and carbon raiser are put into the smelting furnace for smelting. After the temperature is stabilized at 1560℃, the composition is tested. The composition is adjusted according to the test results and the amount of spheroidizing agent and inoculant to be added is determined. The furnace is removed after the composition meets the standard. 2) Spheroidization and inoculation: Rare earth magnesium silicon iron spheroidizing agent and silicon iron inoculant are placed in the spheroidizing ladle in sequence. Molten iron is then poured into the spheroidizing ladle. The weight of rare earth magnesium silicon iron spheroidizing agent added is 1.4% of the weight of molten iron, and the weight of silicon iron inoculant added is 2.1% of the weight of molten iron. The rare earth magnesium ferrosilicon spheroidizing agent has the following composition: Si: 48%, Mg: 5.2%, RE: 2.2%, Ca: 2.0%, and Fe balance. The ferrosilicon inoculant has the following composition: Si: 75%, and Fe balance. 3) Oxygen and sulfur determination: The composition of the molten iron after spheroidization and inoculation in step 2) is analyzed. Based on the results of the O and S tests, the oxygen and sulfur are controlled within the corresponding range by feeding calcium wire or adding FeS powder and FeO powder. 4) Casting: Pour the molten iron from step 3) into the mold. After cooling, remove the mold and perform composition analysis. Simultaneously, determine the room temperature tensile strength and elongation, as well as the 20℃ impact toughness (V-notch), of the ductile iron according to GB / T 228.1-2021 and GB / T229-2020, and calculate the strength-ductility product. Determine the metallographic type, nodularity, graphite grade, and number of graphite nodules of the ductile iron according to GB / T9441-2021.

[0028] The composition of each ductile iron is shown in Table 1. [S] / [O] is calculated as formula A, and [C] / (17.6[S]+102.5[O]) is calculated as formula B. The microstructure of each ductile iron is shown in Table 2. The mechanical properties of each ductile iron are shown in Table 3.

[0029] Table 1. Composition of various ductile irons (mass percentage, %, balance Fe).

[0030]

[0031] Table 2. Microstructure characteristics of various ductile irons.

[0032]

[0033] Table 3 Mechanical properties of various ductile irons.

[0034]

[0035] It is evident from the above embodiments that the ductile iron conforming to the composition of this invention has a high spheroidization rate, fine graphite, and a large graphite quantity. In terms of performance, its tensile strength is above 980 MPa, its elongation is greater than 7.5%, and its impact toughness at 20°C is above 15 J / cm. 2The product of tensile strength and elongation, i.e., the strength-ductility product, reaches over 8000 MPa·%, confirming that the ductile iron of this invention has excellent strength, ductility, and toughness, with outstanding comprehensive performance, and can meet the application conditions of high horsepower, high load, and high power of electric drive axle housings.

[0036] In contrast, the comparative examples of this invention show that at least one of the following components of ductile iron—O, S, [S] / [O], or [C] / (17.6[S]+102.5[O])—fail to meet the requirements of this invention. From a microstructural perspective, these ductile irons exhibit low spheroidization, coarse graphite, and insufficient graphite quantity. In terms of mechanical properties, their strength, elongation, and toughness all show varying degrees of decline, and their strength-ductility product cannot meet the requirements of this invention. This demonstrates that the intentional control of O and S and the synergistic control of O, S, and C content in this invention play a crucial role in ensuring the excellent overall comprehensive performance of ductile iron.

[0037] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0038] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A type of QT900-6 ductile iron, characterized in that, The composition of the ductile iron, by mass percentage, includes C: 3.3-3.6%, Si: 2.0-2.3%, Mn: 0.10-0.25%, Cu: 0.50-0.75%, P: less than 0.03%, S: 0.035-0.050%, O: 0.0025-0.0045%, with the balance being Fe and unavoidable impurities. Furthermore, the mass percentages of C, S, and O in the ductile iron [C], [S], and [O] satisfy the following conditions: [S] / [O] = 9.71-16.48 and [C] / (17.6[S]+102.5[O]) = 2.62-3.87, respectively.

2. The QT900-6 ductile iron according to claim 1, characterized in that, The ductile iron has a tensile strength of over 980 MPa and an elongation of over 7.5%.

3. A QT900-6 ductile iron according to any one of claims 1-2, characterized in that, The product of the tensile strength and elongation of the ductile iron, i.e., the strength-ductility product, is above 8000 MPa·%.

4. A QT900-6 ductile iron according to any one of claims 1-3, characterized in that, The impact toughness ak of the ductile iron at 20°C V At 15J / cm 2 above.

5. A QT900-6 ductile iron according to any one of claims 1-4, characterized in that, The pearlite content of the ductile iron reaches grade 95.

6. A QT900-6 ductile iron according to any one of claims 1-5, characterized in that, The ductile iron has a spheroidization grade of 1 (spheroidization rate ≥ 95%) or 2 (spheroidization rate 90%-94%).

7. A QT900-6 ductile iron according to any one of claims 1-6, characterized in that, The graphite size of the ductile iron is grade 7 (actual graphite size > 0.015 mm and ≤ 0.03 mm) or grade 8 (actual graphite size ≤ 0.015 mm).

8. A QT900-6 ductile iron according to any one of claims 1-7, characterized in that, The number of graphite spheres in the ductile iron reaches 270-350 per mm. 2 .

9. A method for preparing QT900-6 ductile iron according to any one of claims 1-8, comprising the following steps: 1) Smelting: Pig iron, scrap steel and carbon raiser are put into the smelting furnace for smelting. After the temperature is stabilized above 1550℃, the composition is tested. The composition is adjusted according to the test results and the amount of spheroidizing agent and inoculant added is determined. The furnace is removed after the composition meets the standard. 2) Spheroidization and inoculation: Place rare earth magnesium silicon iron spheroidizing agent and silicon iron inoculant into the spheroidizing ladle in sequence, and pour molten iron into the spheroidizing ladle. The weight of rare earth magnesium silicon iron spheroidizing agent added is 1.2-1.5% of the weight of molten iron, and the weight of silicon iron inoculant added is 1.5-2.5% of the weight of molten iron. 3) Oxygen and sulfur determination: The composition of the molten iron after spheroidization and inoculation in step 2) is analyzed, and the oxygen and sulfur are controlled within the corresponding range by feeding calcium wire or adding FeS powder and FeO powder; 4) Casting: Pour the molten iron from step 3) into the mold, and remove it after cooling.

10. The application of the QT900-6 ductile iron according to any one of claims 1-9 in the preparation of electric drive bridge housings.

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