High-strength gray cast iron

Abstract

The application relates to a high-strength gray cast iron, which is composed of the following components 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%, V: 0.03-0.09%, B: 0.005-0.01%, P<=0.18%, S<=0.15%, and the balance of Fe and inevitable impurities, and the percentage by mass of V and B satisfies V / B=4.9-13.1.

Description

Technical Field

[0001] This invention relates to a high-strength gray cast iron, which 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 high-strength gray cast iron, in which the proportion of pearlite in the metallographic structure is more than 98% and the proportion of graphite in the A-type morphology is more than 93%. The gray cast iron of this invention exhibits excellent strength properties, with a cast tensile strength of more than 505 MPa.

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

[0007] The purpose of this invention is to provide a high-strength gray cast iron, the composition of which, 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%, V: 0.03-0.09%, B: 0.005-0.01%, P≤0.18%, S≤0.15%, with the balance being Fe and unavoidable impurities, and the mass percentages of V and B satisfying: V / B = 4.9-13.1.

[0008] The component design principles of this invention will be introduced below.

[0009] 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%.

[0010] 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%.

[0011] 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%.

[0012] 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%.

[0013] 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%.

[0014] 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%.

[0015] Vanadium (V), boron (B), and the V / B ratio: Vanadium helps refine grains, thereby improving the strength of gray cast iron. Trace amounts of boron can improve the hardenability of gray cast iron, effectively inhibiting graphite growth during cooling and controlling graphite size and uniform distribution. The inventors of this invention discovered that by simultaneously adding V and B and synergistically controlling their content relationship, the content and size of type A graphite in gray cast iron can be effectively regulated, thereby maximizing the strength of gray cast iron. By controlling the V / B ratio within a reasonable range, the type A graphite content in the gray cast iron structure can reach over 93%, with a size grade of 5-7, resulting in a near-completely type A graphite morphology with fine dimensions, thus ensuring optimal strength characteristics of the gray cast iron. To achieve the above effects, this invention controls the addition of V to 0.04-0.09% and the addition of B to 0.005-0.001%. Based on extensive experimental analysis, the optimal V / B ratio range was determined to be 4.9-13.1.

[0016] 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%.

[0017] The high-strength gray cast iron of the present invention, having the above-mentioned composition, is tested and analyzed according to GB / T7216-2023. The graphite morphology includes type A graphite, with a proportion of not less than 93%. Furthermore, the type A graphite has a size grade of 5-7, exhibiting a fine characteristic, reducing the fragmentation effect on the microstructure, thereby achieving higher strength performance. Preferably, the high-strength gray cast iron of the present invention has a microstructure consisting entirely of type A graphite.

[0018] The high-strength gray cast iron of this invention, 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.

[0019] High-strength gray cast iron with the above-mentioned graphite morphology and metallographic structure has a tensile strength of over 505 MPa.

[0020] As a further limitation, the mass percentage of Si and C in the high-strength gray cast iron of this invention satisfies the following condition: 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. Taking all the above into consideration, this invention sets the carbon equivalent C + Si / 3 within the range of 3.3-3.8%.

[0021] As a further limitation, the mass percentage of Si and C in the high-strength gray cast iron of the present invention satisfies: Si / C = 0.48-0.62. When within the aforementioned carbon equivalent range, a suitable Si / C ratio is beneficial to improving the tensile strength of gray cast iron.

[0022] By way of non-limiting description, the high-strength gray cast iron of the present invention can be subjected to stress-relief annealing after casting. Through stress-relief annealing, the stress generated during solidification and cooling during the casting process is eliminated, which can effectively improve the service life of the casting.

[0023] As a further description, the high-strength gray cast iron of the present invention is applied to 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 high-strength gray cast iron of the present invention also has outstanding strength properties, making it well-suited to the material performance requirements of high-horsepower diesel engines.

[0024] Furthermore, this invention also provides the application of the aforementioned high-strength gray cast iron in components such as diesel engine cylinder blocks, cylinder heads, camshafts, crankshafts, and bushings. The high-strength 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.

[0025] 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. In particular, the present invention adds V and B elements and synergistically controls the content relationship of V and B, effectively regulating the content and size of type A graphite in gray cast iron, 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 93%, and the size grade of type A graphite is 5-7. The final tensile strength of gray cast iron reaches more than 505 MPa. 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 detailed explanation is provided in conjunction with specific experimental examples.

[0027] The molten iron was smelted according to the design composition and cast into gray cast iron ingots. The smelting temperature was 1485℃ and the pouring temperature was 1365℃. The dimensions of the gray cast iron ingots were 500mm long × 200mm wide × 200mm high. The specific composition is shown in Table 1. The content of P and S was controlled at 0.1±0.01%. The metallographic structure of the obtained gray cast iron ingots was analyzed according to GB / T7216-2023, and the tensile strength was tested according to GB / T228.1-2021. The specific metallographic structure and tensile strength test results are shown in Table 2.

[0028] Table 1: Composition of various gray cast irons, %, balance is Fe.

[0029]

[0030] Table 2: Metallographic structure and strength of various gray cast irons.

[0031]

[0032] The above embodiments and comparative examples are further analyzed and explained below with reference to Tables 1-2.

[0033] The gray cast irons tested in series 1-10 all met the requirements of this invention. Metallographic analysis revealed that the gray cast irons all met the requirements of having a type A graphite content of over 93%, a type A graphite size grade of 5-7, and a pearlite content of over 98%. Mechanical property testing showed that the tensile strength of the gray cast irons was over 505 MPa, demonstrating outstanding mechanical properties, good metallographic morphology, and excellent mechanical performance characteristics.

[0034] Tests 11-13 and 17 adjusted the V and B contents of tests 2, 3, 4, and 9, respectively. The adjusted V and B contents were still within the requirements of this invention, but the V / B ratio no longer met the requirements of this invention. The final test results showed that the amount of type A graphite in gray cast iron could not meet the requirements of the invention, the tensile strength of gray cast iron could not reach 505 MPa, and the size of type A graphite also showed varying degrees of coarsening. The size of type A graphite in tests 11, 12, 13, and 17 exceeded the scope of the requirements of this invention.

[0035] Test No. 14 adjusted the V content of Test No. 5. The adjusted V content is not within the scope of this invention, and the V / B ratio no longer meets the requirements of this invention. The final test results show that the amount of type A graphite in gray cast iron cannot meet the requirements of the invention, the tensile strength of gray cast iron cannot reach 505 MPa, and the size of type A graphite has become coarser.

[0036] Tests 15-16 adjusted the B content of tests 7-8. The adjusted V / B ratio met the requirements of the present invention, but the B content was not within the scope of the present invention. The final test results showed that the amount of type A graphite in gray cast iron could not meet the requirements of the invention, the tensile strength of gray cast iron could not reach 505 MPa, and the size of type A graphite also showed varying degrees of coarsening. The size of type A graphite in test 16 exceeded the scope of the requirements of the present invention.

[0037] By comparing Examples 1-10 and Comparative Examples 11-17, it is easy to see that controlling V, B, and V / B within appropriate ranges is crucial for obtaining type A graphite with specific quantity and size, as well as gray cast iron with excellent tensile strength.

[0038] Furthermore, the gray cast irons tested in this invention (numbers 1, 5, and 10) simultaneously meet the requirements of C+Si / 3 being in the range of 3.3-3.8% and Si / C being in the range of 0.48-0.62, exhibiting superior mechanical properties. The gray cast irons meeting the above requirements have a tensile strength of over 535 MPa, indicating that controlling appropriate C+Si / 3 and Si / C is more conducive to improving the tensile strength performance of gray cast iron.

[0039] In summary, this invention effectively regulates the content and size of type A graphite in gray cast iron by adding V and B elements and synergistically controlling their content relationship, thereby maximizing the strength of gray cast iron. Through compositional control, this invention ensures that the proportion of type A graphite in gray cast iron is not less than 93%, and the size grade of type A graphite is 5-7. The resulting gray cast iron exhibits a tensile strength exceeding 505 MPa. It possesses excellent strength characteristics and metallographic structure, making it particularly suitable for applications in diesel engine cylinder blocks, cylinder heads, camshafts, crankshafts, and bushings.

[0040] 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.

[0041] 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 high-strength gray cast iron, characterized in that, The composition of the high-strength gray cast iron, by mass percentage, is 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%, V: 0.03-0.09%, B: 0.005-0.01%, P≤0.18%, S≤0.15%, with the balance being Fe and unavoidable impurities, and the mass percentages of V and B satisfy: V / B=4.9-13.

1.

2. The high-strength gray cast iron according to claim 1, characterized in that, The graphite morphology of the high-strength gray cast iron includes type A graphite, and the proportion of type A graphite is not less than 93%.

3. The high-strength gray cast iron according to claim 2, characterized in that, The size grade of the type A graphite is 5-7.

4. The high-strength gray cast iron according to claim 1, characterized in that, The proportion of pearlite in the microstructure of the high-strength gray cast iron is not less than 98%.

5. The high-strength gray cast iron according to claim 1, characterized in that, The high-strength gray cast iron has a tensile strength of 505 MPa or higher.

6. The high-strength gray cast iron according to claim 1, characterized in that, The mass percentage of Si and C in the high-strength gray cast iron satisfies: Si / C = 0.48-0.

62.

7. The high-strength gray cast iron according to claim 1, characterized in that, The mass percentage of Si and C in the high-strength gray cast iron satisfies: C + Si / 3 = 3.3-3.8%.

8. The high-strength gray cast iron according to claim 1, characterized in that, The high-strength gray cast iron can undergo stress-relief annealing after casting.

9. The high-strength gray cast iron according to claim 1, characterized in that, The high-strength gray cast iron is used in diesel engine cylinder blocks, cylinder heads, camshafts, crankshafts, and bushing components.

10. The application of the high-strength gray cast iron as described in any one of claims 1-9 in diesel engine cylinder blocks, cylinder heads, camshafts, crankshafts, and bushing components.

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