A method for producing gray cast iron and gray cast iron produced thereby

Abstract

The application relates to a preparation method of gray cast iron, which comprises the following steps in sequence: smelting qualified component molten iron, teeming and inoculation, and casting; the component of the molten iron is as follows in percentage by mass: C: 2.5-3.5%, Si: 1.0-1.8%, Mn: 0.6-1.5%, Cr<=0.4%, Mo: 0.3-0.7%, Cu: 0.6-1.3%, P<=0.18%, S<=0.15%, and the balance is Fe and inevitable impurities; the teeming and inoculation use a silicon-zirconium-manganese-strontium inoculant, and the component of the silicon-zirconium-manganese-strontium inoculant is as follows in percentage by mass: Si: 60-70%, Zr: 3-5%, Mn: 2.5-4.5%, Sr: 2.0-4.0%, Ca: 0.5-1.5%, Al: 0.5-1.5%, and the balance is Fe.

Description

Technical Field

[0001] This invention relates to a method for preparing gray cast iron. The gray cast iron prepared by this method has excellent strength properties and can be widely used in components such as cylinder blocks, cylinder heads, crankshafts, camshafts, and bushings of high-horsepower diesel engines. Background Technology

[0002] Gray cast iron has a gray cross-section, and the graphite in the matrix exists in the form of flakes. The flake graphite gives gray cast iron good shock absorption and thermal conductivity. Therefore, gray cast iron has become the preferred material for components such as cylinder blocks, cylinder heads, and crankshafts of diesel engines.

[0003] As the market demand for diesel engine horsepower increases, the explosive forces and impacts faced by components such as cylinder blocks and crankshafts during operation are also increasing. Therefore, the requirements for the strength performance of gray cast iron materials for these components are also continuously increasing.

[0004] Based on the above issues, developing high-strength gray cast iron to adapt to the current development trend of diesel engines has become one of the main research directions in the industry. Summary of the Invention

[0005] This invention provides a method for preparing gray cast iron. The gray cast iron prepared by this method has a pearlite content of over 98% and a graphite morphology of over 90% of the metallographic structure. The gray cast iron of this invention exhibits excellent strength properties, with a cast tensile strength of over 500 MPa.

[0006] The technical objective of this invention is achieved through the following means.

[0007] The purpose of this invention is to provide a method for preparing gray cast iron, which includes the following steps: smelting to obtain molten iron with qualified composition, inoculation in a ladle, and casting; the composition of the molten iron, by mass percentage, is: C: 2.5-3.5%, Si: 1.0-1.8%, Mn: 0.6-1.5%, Cr≤0.4%, Mo: 0.3-0.7%, Cu: 0.6-1.3%, P≤0.18%, S≤0.15%, with the balance being Fe and unavoidable impurities; the inoculation in the ladle uses a silicon-zirconium-manganese-strontium inoculant, the composition of which, by mass percentage, is: Si: 60-70%, Zr: 3-5%, Mn: 2.5-4.5%, Sr: 2.0-4.0%, Ca: 0.5-1.5%, Al: 0.5-1.5%, with the balance being Fe.

[0008] The method for preparing gray cast iron according to the present invention will be described below.

[0009] This invention prepares high-strength gray cast iron by combining gray cast iron components and inoculant components.

[0010] First, the component design principle of this invention will be introduced.

[0011] C: Carbon is an element that improves the fluidity of molten cast iron, promotes graphitization, and is also the most basic strengthening element. However, excessive carbon content leads to severe coarsening of graphite in the matrix, which deteriorates the strength of gray cast iron. Conversely, insufficient carbon content results in inadequate strengthening, decreased fluidity of the molten cast iron, increased casting defects, and reduced mechanical properties of the castings. Considering both casting performance and strength requirements, this invention sets the carbon content at 2.5-3.5%.

[0012] Silicon (Si): Similar to carbon, silicon is an element that improves the fluidity of molten cast iron, promoting graphitization and strengthening the matrix. Excessive silicon content leads to an excessively high carbon equivalent, which exacerbates graphite coarsening in the matrix, resulting in a deterioration in the strength of gray cast iron. Conversely, insufficient silicon content results in inadequate strengthening and fluidity of the molten cast iron, increasing casting defects and compromising the mechanical properties of the castings. Considering both casting performance and strength requirements, this invention sets the silicon content at 1.0-1.8%.

[0013] Mn: Manganese contributes to the formation of pearlite, increasing its content, refining the microstructure, and improving the strength of gray cast iron. For this invention, a pearlite-like microstructure (98% pearlite) is desired; therefore, the manganese content is crucial. However, excessive manganese content leads to an increase in carbides, resulting in decreased matrix continuity and deterioration of mechanical strength. To ensure suitable microstructure and strength properties, the manganese content in this invention is set to 0.6-1.5%.

[0014] Cr: Chromium is a pearlite stabilizing element that increases the solubility of carbon in austenite, thereby promoting pearlite formation and refining the pearlite structure, thus improving the strength of gray cast iron. However, chromium easily combines with carbon to form carbides, leading to an increased tendency for gray cast iron to turn white. In this invention, the Cr content is set below 0.4%.

[0015] Mo: Molybdenum can effectively refine the pearlite structure, reduce the interlamellar spacing of pearlite, and prevent graphite coarsening, thereby significantly improving the strength properties of gray cast iron. However, molybdenum's effect on inhibiting ferrite is not significant, so the amount of molybdenum used must be controlled. In this invention, the molybdenum content is set to 0.3-0.7%.

[0016] Cu: Copper is an element that promotes graphitization, reducing the tendency for white cast iron and optimizing the morphology and size of graphite. Furthermore, copper is a pearlite stabilizing element, promoting the acquisition and refinement of pearlite structure. The addition of copper can significantly improve the strength of gray cast iron. However, excessive copper content can cause graphite morphology to transform into B-type, D-type, and E-type, which is detrimental to maintaining the strength of gray cast iron. Therefore, the copper content in this invention is controlled at 0.6-1.3%.

[0017] P and S: Phosphorus and sulfur are impurity elements, unavoidably introduced from the raw materials. Their presence negatively impacts the strength properties of gray cast iron, and phosphorus can also cause brittleness. However, they also have positive effects. For example, phosphorus is a pearlite-forming element, which is beneficial for obtaining pearlite structure. Sulfur can combine with manganese to improve the machinability of gray cast iron and promote graphitization. Considering both cost and performance, the phosphorus content in this invention is limited to below 0.18%, and the sulfur content is limited to below 0.15%.

[0018] When gray cast iron with the above-mentioned components is combined with the inoculant proposed in this invention, gray cast iron with excellent strength can be obtained.

[0019] As a crucial factor in achieving the technical effects of this invention, the inoculant proposed in this invention is specifically a silicon-zirconium-manganese-strontium inoculant, with the following composition: Si: 60-70%, Zr: 3-5%, Mn: 2.5-4.5%, Sr: 2.0-4.0%, Ca: 0.5-1.5%, Al: 0.5-1.5%, and the balance being Fe. By combining the above inoculant components with the gray cast iron composition of this invention, the morphology and size of graphite can be effectively controlled, promoting graphite refinement and uniform distribution, ultimately resulting in gray cast iron with excellent performance and microstructure.

[0020] The inoculant of this invention is added by inverted inoculation, that is, the inoculant is placed at the bottom of the molten iron ladle in advance, and then molten iron that has been smelted and has qualified composition is poured in for inoculation. The inoculation time is usually controlled at 8-15 minutes. The particle size of the inoculant is preferably 3-8 mm. The amount of inoculant added is controlled at 1.2-1.8% of the weight of molten iron. Casting is carried out immediately after inoculation is completed.

[0021] This invention also claims protection for gray cast iron prepared by the above-described gray cast iron preparation method.

[0022] The gray cast iron prepared by the above method, when tested and analyzed according to GB / T7216-2023, exhibits graphite morphology including type A graphite, with the proportion of type A graphite not less than 90%. Furthermore, the size grade of the type A graphite is 6-7, exhibiting a fine characteristic, reducing the fragmentation effect on the microstructure, thereby achieving higher strength performance. Preferably, the gray cast iron of the present invention has a microstructure consisting entirely of type A graphite.

[0023] The gray cast iron prepared by the above method, when tested and analyzed according to GB / T7216-2023, has a pearlite content of not less than 98% in its metallographic structure, which meets the metallographic requirement of 98% pearlite.

[0024] Gray cast iron with the above-mentioned graphite morphology and metallographic structure has a tensile strength of over 500 MPa.

[0025] As a further limitation, the mass percentages of Si and C in the gray cast iron prepared by the above method satisfy the following: C + Si / 3 = 3.3-3.8%. If the carbon equivalent is too high, although the molten iron has good fluidity and fewer casting defects, the graphite coarsening is severe, leading to a decrease in the strength properties of the gray cast iron. If the carbon equivalent is too low, the molten iron has poor fluidity and more casting defects, also resulting in a decrease in the strength properties of the gray cast iron. Considering the above, this invention sets the carbon equivalent C + Si / 3 within the range of 3.3-3.8%.

[0026] As a further limitation, the mass percentage of Si and C in the gray cast iron prepared by the above preparation method satisfies: Si / C = 0.48-0.62. When it is within the aforementioned carbon equivalent range, a suitable Si / C ratio is beneficial to improving the tensile strength of gray cast iron.

[0027] By way of non-limiting description, the gray cast iron prepared by the above preparation method can be subjected to stress-relief annealing after casting. By stress-relief annealing, the stress generated during solidification and cooling during the casting process can be eliminated, which can effectively improve the service life of the casting.

[0028] As a further description, the gray cast iron prepared by the above method is used in components such as diesel engine cylinder blocks, cylinder heads, camshafts, crankshafts, and bushings. These components are typically made of gray cast iron materials with excellent wear resistance and thermal conductivity. In addition to the excellent wear resistance and thermal conductivity of conventional gray cast iron, the gray cast iron of this invention also has outstanding strength properties, making it well-suited to the material performance requirements of high-horsepower diesel engines.

[0029] Furthermore, this invention also provides the application of the aforementioned gray cast iron in components such as diesel engine cylinder blocks, cylinder heads, camshafts, crankshafts, and bushings. The gray cast iron of this invention exhibits excellent strength and outstanding impact resistance, and can be widely used in related parts of high-horsepower diesel engines.

[0030] The present invention has the following beneficial effects: The present invention uses Cu and Mo to alloy gray cast iron, which effectively improves the tensile strength of gray cast iron. The present invention also proposes the composition of the inoculant. By designing the composition of gray cast iron and combining it with the inoculant proposed in the present invention, the content and size of type A graphite in gray cast iron can be effectively controlled, thereby maximizing the strength of gray cast iron. Through composition control, the present invention ensures that the proportion of type A graphite in gray cast iron is not less than 90%, and the size grade of type A graphite is 6-7. The final tensile strength of gray cast iron reaches more than 500 MPa.

Claims

1. A method for preparing gray cast iron, characterized in that, The preparation method of the gray cast iron includes the following steps in sequence: smelting to obtain molten iron with qualified composition, inoculation in a ladle, and casting; The composition of the molten iron, by mass percentage, is: C: 2.5-3.5%, Si: 1.0-1.8%, Mn: 0.6-1.5%, Cr≤0.4%, Mo: 0.3-0.7%, Cu: 0.6-1.3%, P≤0.18%, S≤0.15%, with the balance being Fe and unavoidable impurities; The inverted packaging inoculation uses a silicon-zirconium-manganese-strontium inoculant, the composition of which, by mass percentage, is: Si: 60-70%, Zr: 3-5%, Mn: 2.5-4.5%, Sr: 2.0-4.0%, Ca: 0.5-1.5%, Al: 0.5-1.5%, with the balance being Fe.

2. The method for preparing gray cast iron according to claim 1, characterized in that, The amount of inoculant added is 1.2-1.8% of the mass of the molten iron.

3. A gray cast iron prepared by the method for preparing gray cast iron according to claim 1 or 2.

4. The gray cast iron according to claim 3, characterized in that, The graphite morphology of the gray cast iron includes type A graphite, and the proportion of type A graphite is not less than 90%.

5. The gray cast iron according to claim 4, characterized in that, The size grade of the type A graphite is 6-7.

6. The gray cast iron according to claim 3, characterized in that, The proportion of pearlite in the microstructure of the gray cast iron is not less than 98%.

7. The gray cast iron according to claim 3, characterized in that, The gray cast iron has a tensile strength of 500 MPa or higher.

8. The gray cast iron according to claim 3, characterized in that, The mass percentage of Si and C in the gray cast iron satisfies: Si / C = 0.48-0.

62.

9. The gray cast iron according to claim 3, characterized in that, The mass percentage of Si and C in the gray cast iron satisfies: C + Si / 3 = 3.3-3.8%.

10. The gray cast iron according to claim 3, characterized in that, The gray cast iron is used in diesel engine cylinder blocks, cylinder heads, camshafts, crankshafts, and bushing components.