Effect of adding scrap steel on Microstructure and properties of gray cast iron

Gray cast iron is one of the most commonly used metal materials for automotive parts such as engine, because of its unique structure and properties. As people have higher and higher requirements for the performance of engines as the heart of automobiles, the optimization of the structure and mechanical properties of gray cast iron for engines has attracted wide attention. In recent years, many scholars at home and abroad have carried out a lot of research on the optimization of smelting process, inoculation process, pouring process and casting process parameters, and the development of new technology innovation, and have achieved some important research results. The results show that the pearlite matrix in gray cast iron can be refined by adding appropriate amount of other alloy elements while increasing the carbon equivalent (CE) of gray iron, so as to improve its mechanical properties; The inoculation effect of gray cast iron can be greatly improved by using high-efficiency Si SR or re inoculant and secondary inoculation process; Adopting the numerical simulation of casting process, adjusting the casting process parameters, improving the casting process and mechanical properties of thin-walled cylinder block, and so on. Although the improvement of these casting processes and the application of new technologies are beneficial to improve the mechanical properties of gray cast iron, they often encounter problems such as high cost and difficult process control in actual production.

In recent years, the proportioning method of using scrap steel as the main raw material and properly adding reheating material and carburizing agent has attracted much attention at home and abroad. Scrap steel has low price and small composition change. The addition of carburizing agent can promote the melting of carbon in molten iron and the nucleation and growth of graphite, which can effectively solve the problems of high cost and difficult process control in the production of high-performance castings. In foreign countries, the batching method of scrap + carburizer has been widely used in the production of iron castings, but in China, the industrialization of this method is still very few, and there are still many technical and theoretical problems to be solved.

By adjusting the ratio of scrap steel and carburizing agent in the smelting raw materials of gray cast iron, controlling the addition amount of returned materials, reducing the heredity of coarse graphite and hard inclusions in the returned materials, and improving the casting and mechanical properties of gray cast iron castings. The structure, mechanical properties and fracture process of gray iron castings with different scrap content were studied. The effect of raw material selection on the structure and properties of gray iron castings was explored, and the related mechanism was studied.

  1. grey cast iron is produced by using the proportioning method of scrap steel + carburizing agent. When changing the addition ratio, the hardness and tensile strength of grey cast iron will be improved with the increase of scrap steel content, and the mechanical properties of grey cast iron with 100% scrap steel will increase most significantly.
  2. the composition, structure and energy conditions of graphite nucleation and growth during solidification of gray cast iron molten iron added with 100% scrap are superior. Most of the graphite sheets in the cast iron are fine, curved short-film A-type graphite W and a small amount of D-type graphite. The pearlite based structure is refined to form sorbite texture, MNS The amount and size of tin and other non-metallic inclusions are small, and the microstructure of gray cast iron has been significantly improved.
  3. according to the evaluation of smelting quality, mechanical properties, casting properties and processability of grey cast iron, the grey cast iron with 100% scrap steel has the best comprehensive properties.
  4. in the gray cast iron with 100% scrap, the refinement and bending of graphite lead to the change of crack propagation path at the graphite tip, which increases the applied stress of crack propagation and improves the tensile strength.
  5. the increase of pearlite content and the refinement of lamellar spacing in the gray cast iron with 100% scrap lead to the diversification of crack propagation modes in the cast iron matrix and the obvious increase of crack propagation resistance, which is also one of the main reasons for the significant improvement of the mechanical properties of the castings.