High strength gray cast iron in China

As the largest automobile production and consumption country in the world, China has huge development space and broad prospects for the cast iron auto parts industry. With the continuous development and progress of science and technology, people continue to put forward higher requirements for the performance and quality of gray iron castings. The international industrial cycle has resulted in an oversupply of medium and low-grade gray cast iron castings in the domestic and international markets, while the demand for high-strength and high-quality gray cast iron castings is increasing day by day. This is not only a challenge but also a major development opportunity for the casting auto parts industry. According to the national industrial development direction of high efficiency and energy saving, the gray cast iron cylinder block products should be developed in the direction of light weight, thin wall and artistry.

In 2009, China surpassed the United States for the first time, becoming the world’s largest auto producer and consumer, and has maintained a strong momentum. In this context, as the core engine of automobile, people put forward higher requirements and expectations. The key components of automobile engine will be developed in the future, such as bearing complex loads, sufficient stiffness and strength, easy cutting, thin wall, good heat resistance and thermal fatigue performance. However, with the lightweight development of the whole vehicle and the improvement of the specific power of the engine, the limitations of its application are becoming larger and larger. The main reason is that the current technical level is difficult to ensure that the hardness difference of the body section with a wall thickness of 3-5mm is less than 40hbs and the structure is fine and uniform. At the same time, the existence of coarse flake graphite makes it extremely difficult to further improve its strength, initial property and wear resistance, and it is difficult to realize the manufacturing of high-strength and lightweight cylinder block. Therefore, it is necessary to use advanced casting equipment and optimize the casting process, especially in terms of material, so that the mechanical, physical and technological properties of gray iron castings can meet the needs of the development of the automotive industry.

With the development of the times, people have higher and higher requirements for the performance of automotive engines, so it is necessary to further improve the strength and wear resistance of the cylinder block. Most domestic and foreign scholars have made various alloying on cast iron, that is, on the basis of the original composition, the proportion of Cr, Ni, Sn, Cu and other elements is increased, and the content of S and P is strictly controlled, which not only improves the wear resistance and tensile strength of the cylinder body, but also improves the casting performance. Wu Hong, Liu Zhenyi and Yu Ling have studied that adding CR, Mo, Cu and other elements to gray cast iron can significantly improve the tensile strength and section uniformity of gray cast iron. Inoculation treatment is an important way to improve the mechanical properties and machinability of cylinder block and cylinder head gray cast iron castings. Many scholars have done a lot of research on the effect of inoculation on the nucleation and growth of austenite, graphite and pearlite during solidification and crystallization of cast iron. Jiangjinwen et al. Studied and filled the effect of the addition of composite inoculants (75sife, Si Ba, Si Sr, re Ca Ba) on the thin-walled high-strength gray cast iron, and found that the composite addition of multiple inoculants was more beneficial to the improvement of the properties of gray cast iron. It is found that inoculants have a great influence on the formation of primary austenite, and the dendrite morphology and distribution have a great influence on the microstructure and properties of cast iron. When used with dilute inoculant, the primary and Eutectic Transformation of austenite can be changed to improve the properties of cast iron. Single inoculant and single inoculant adding method are difficult to meet the needs of the production of high-strength cylinder block gray cast iron castings. Two or more inoculants, especially the new inoculants containing rare earth, are used to improve the pearlite content in the cast iron matrix by combining multiple inoculation methods, such as ladle inoculation, flow inoculation and in mold inoculation, Refine the matrix grain and flake graphite size, improve the casting properties, reduce the wall thickness sensitivity, and gradually improve the mechanical properties and machinability of high-strength cylinder block castings.

In foreign countries, due to the differences in production technology, most large automobile enterprises have applied high-strength gray cast iron cylinder blocks in the new generation of automobiles. However, in the process of research, there are still white structures in the places with large thin wall of the cylinder block, which are difficult to meet the requirements. The key parts are burned, slag inclusion Defects such as porosity, high rejection rate of gray cast iron castings, and difficult to control process stability.