Gray cast iron preparation method and gray cast iron prepared through gray cast iron preparation method

Abstract

The invention relates to a preparation method of gray cast iron. The preparation method sequentially comprises the following steps that molten iron with qualified components is obtained through smelting, ladle-to-ladle inoculation is conducted, and casting is conducted. The molten iron comprises the following components in percentage by mass: 2.5 to 3.5 percent of C, 1.0 to 1.8 percent of Si, 0.6 to 1.5 percent of Mn, less than or equal to 0.4 percent of Cr, 0.3 to 0.7 percent of Mo, 0.6 to 1.3 percent of Cu, less than or equal to 0.18 percent of P, less than or equal to 0.15 percent of S and the balance of Fe and inevitable impurities; a silicon-zirconium-manganese-strontium inoculant is used for ladle-to-ladle inoculation, and the silicon-zirconium-manganese-strontium inoculant comprises the following components in percentage by mass: 60 to 70 percent of Si, 3 to 5 percent of Zr, 2.5 to 4.5 percent of Mn, 2.0 to 4.0 percent of Sr, 0.5 to 1.5 percent of Ca, 0.5 to 1.5 percent of Al and the balance of Fe.

Description

Technical Field

[0001] The invention relates to a method for preparing grey cast iron. The grey cast iron prepared by the method has excellent strength characteristics and can be widely used in components such as cylinder blocks, cylinder heads, crankshafts, camshafts, shaft sleeves, etc. of high-horsepower diesel engines. Background Art

[0002] The cross section of gray cast iron is gray, and the graphite in the matrix structure exists in the form of flakes. The flake graphite gives gray cast iron good shock absorption and thermal conductivity. Therefore, gray cast iron becomes the preferred material for diesel engine cylinder blocks, cylinder heads, crankshafts and other components.

[0003] As the market demand for diesel engine horsepower becomes higher and higher, the explosive force and impact force faced by its cylinder block, crankshaft and other components during operation are also increasing. Therefore, the requirements for the strength performance of gray cast iron materials of these components are also continuously improved.

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

[0005] The present invention provides a method for preparing gray cast iron. The gray cast iron prepared by the method has a pearlite content of more than 98% in its metallographic structure, and a graphite content of more than 90% in type A form. The gray cast iron of the present invention exhibits excellent strength performance, and the as-cast tensile strength reaches more than 500 MPa.

[0006] The technical objectives of the present invention are achieved by the following means.

[0007] The object of the present invention is to provide a method for preparing gray cast iron, which comprises the following steps in sequence: smelting to obtain molten iron with qualified composition, pouring ladle inoculation, and casting; the composition of the molten iron is calculated by mass percentage as follows: 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 unavoidable impurities; the pouring ladle inoculation uses a silicon-zirconium-manganese-strontium inoculant, and the composition of the silicon-zirconium-manganese-strontium inoculant is calculated by mass percentage as follows: 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.

[0008] Next, the method for preparing gray cast iron of the present invention is introduced.

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

[0010] First, the composition design principle of the present invention is introduced.

[0011] C: Carbon is an element that improves the fluidity of molten iron, promotes graphitization, and is also the most basic strengthening element. However, too high a carbon content will cause serious coarsening of graphite in the matrix structure, which will deteriorate the strength of gray cast iron. Too low a carbon content will result in insufficient strengthening effect, reduced fluidity of molten iron, increased casting defects, and reduced mechanical properties of castings. Considering the casting performance and strength requirements, the present invention sets the carbon content to 2.5-3.5%.

[0012] Si: Silicon has a similar effect to carbon. It is also an element that improves the fluidity of molten iron and has the function of promoting graphitization and strengthening the matrix. Too much silicon will lead to too high carbon equivalent, thereby aggravating the coarsening of graphite in the matrix and deteriorating the strength of gray cast iron. Too low silicon content will lead to insufficient strengthening effect and fluidity of molten iron, increase casting defects, and the mechanical properties of castings cannot be guaranteed. Comprehensive casting performance to obtain strength requirements, the present invention sets the silicon content to 1.0-1.8%.

[0013] Mn: Manganese helps to obtain pearlite structure, can increase the content of pearlite, refine the structure, and improve the strength of gray cast iron. For the present invention, it is expected to obtain the metallographic structure of pearl 98, so the content of manganese is crucial to the present invention. However, too high a manganese content will lead to an increase in the number of carbides, resulting in a decrease in the continuity of the matrix and a deterioration in mechanical strength. In order to ensure a suitable metallographic structure and strength performance, the manganese content of the present invention is set to 0.6-1.5%.

[0014] Cr: Chromium is a pearlite stabilizing element that can increase the solubility of carbon in austenite, thereby promoting the formation of pearlite and refining the pearlite structure, thereby increasing the strength of gray cast iron. However, chromium easily combines with carbon to form carbides, resulting in an increased tendency of gray cast iron to white cast iron. The present invention sets the Cr content to less than 0.4%.

[0015] Mo: Molybdenum can effectively refine the pearlite structure, reduce the interlamellar spacing of pearlite, and prevent graphite from coarsening, thereby significantly improving the strength properties of gray cast iron. However, the effect of molybdenum on inhibiting ferrite is not obvious, so the amount of molybdenum should be controlled. The content of molybdenum in the present invention is set to 0.3-0.7%.

[0016] Cu: Copper is an element that promotes graphitization, can reduce the tendency of white cast iron and optimize the shape and size of graphite. Copper is a pearlite stabilizing element, which can promote the acquisition of pearlite structure and refine pearlite. The addition of copper can greatly improve the strength of gray cast iron. However, if the copper content is too high, the graphite morphology tends to transform to type B, type D, and type E, which is not conducive to maintaining the strength of gray cast iron. In summary, the copper content of the invention is controlled at 0.6-1.3%.

[0017] P, S: Phosphorus and sulfur are impurity elements, which are inevitably introduced by the raw materials. The presence of the two has a negative impact on the strength properties of gray cast iron, and phosphorus can also cause cold brittleness. However, they also have positive effects. For example, phosphorus is a pearlite-forming element, which is conducive to the acquisition of pearlite structure. Sulfur can be combined with manganese to improve the machinability of gray cast iron and promote graphitization. Considering the cost and performance comprehensively, the phosphorus content of the present invention is limited to less than 0.18%, and the sulfur content is limited to less than 0.15%.

[0018] The gray cast iron with the above-mentioned composition is matched with the inoculant proposed in the present invention to obtain gray cast iron with excellent strength.

[0019] As an important factor for obtaining the technical effect of the present invention, the inoculant proposed in the present invention is specifically a silicon-zirconium-manganese-strontium inoculant, and its composition 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%, and the balance is Fe. By combining the above-mentioned inoculant components with the gray cast iron components of the present invention, the morphology and size of graphite can be effectively regulated, and the graphite can be miniaturized and evenly distributed, and finally gray cast iron with excellent performance and structure can be obtained.

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

[0021] The present invention also claims protection for the grey cast iron prepared by the grey cast iron preparation method.

[0022] The gray cast iron prepared by the above preparation method, according to the detection and analysis of GB / T7216-2023, the graphite morphology includes type A graphite, the proportion of type A graphite is not less than 90%, and the size grade of the type A graphite is 6-7, showing a small feature, reducing the splitting effect on the organization, thereby obtaining higher strength performance. Preferably, the gray cast iron of the present invention has a full type A graphite organization morphology.

[0023] The gray cast iron prepared by the above preparation method is tested and analyzed according to GB / T7216-2023, and the proportion of pearlite in the metallographic structure is not less than 98%, that is, it reaches the metallographic requirement level of pearlite 98.

[0024] The gray cast iron having the above-mentioned graphite morphology and metallographic structure has a tensile strength of more than 500 MPa.

[0025] As a further limitation, the mass percentage of Si and C in the gray cast iron prepared by the above preparation method satisfies: C+Si / 3=3.3-3.8%. If the carbon equivalent is too high, although the molten iron has good fluidity and small casting defects, the graphite is seriously coarsened, resulting in reduced strength performance of the gray cast iron. If the carbon equivalent is too low, the molten iron has poor fluidity and many casting defects, which also leads to reduced strength performance of the gray cast iron. Taking the above into consideration, the present invention sets the carbon equivalent C+Si / 3 in the range of 3.3-3.8%.

[0026] As a further limitation, the mass percentages of Si and C in the gray cast iron prepared by the above preparation method satisfy: Si / C=0.48-0.62. Within the aforementioned carbon equivalent range, the appropriate Si / C is beneficial to improving the tensile strength of the gray cast iron.

[0027] As a non-limiting description, the gray cast iron prepared by the above preparation method can be subjected to stress relief annealing after casting. Through the stress relief annealing, the stress generated by 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 preparation method is applied to diesel engine cylinder block, cylinder head, camshaft, crankshaft, sleeve and other parts. These parts are usually 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 the present invention also has excellent strength performance, so that the gray cast iron of the present invention can be well adapted to the performance requirements of high-horsepower diesel engines for materials.

[0029] In addition, the present invention also provides the application of the gray cast iron in diesel engine cylinder blocks, cylinder heads, camshafts, crankshafts, sleeves and other components. The gray cast iron of the present invention has 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 adopts Cu and Mo to alloy the gray cast iron, and effectively improves the tensile strength of the gray cast iron. The present invention also proposes the component composition of the inoculant. Through the design of the gray cast iron composition and the inoculant proposed by the present invention, the content and size of type A graphite in the gray cast iron are effectively regulated, thereby maximizing the strength of the gray cast iron. The present invention controls the composition so that the proportion of type A graphite in the gray cast iron is not less than 90%, and the size grade of the type A graphite is 6-7, and the tensile strength of the gray cast iron finally obtained reaches more than 500MPa.

Claims

1. A method for preparing grey cast iron, characterized in that: The preparation method of the grey cast iron comprises the following steps in sequence: smelting to obtain molten iron with qualified components, pouring and inoculating, and casting; The composition of the molten iron is as follows by mass percentage: 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 unavoidable impurities; The inverted package inoculation uses a silicon-zirconium-manganese-strontium inoculant, and the components of the silicon-zirconium-manganese-strontium inoculant are as follows by mass percentage: 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.

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

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

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

5. The grey cast iron according to claim 3, characterized in that: The size grade of the A-type graphite is 6-7.

6. The grey cast iron according to claim 3, characterized in that: The proportion of pearlite in the metallographic structure of the grey cast iron is not less than 98%.

7. The grey cast iron according to claim 3, characterized in that: The tensile strength of the grey cast iron is above 500 MPa.

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

62.

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

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