The furnace charge is mainly pig iron, scrap steel and return charge, which are melted by medium frequency induction furnace. The target composition of the original molten iron is: 3.7%~4.0% C, 0.7%~0.9% Si, Mn ＜ 0.25%, P ＜ 0.05%, S ＜ 0.04%. Weigh the furnace charge according to the bill of charge; Add pig iron, scrap steel and back-furnace materials into the medium-frequency induction furnace and start smelting. After melting, take the sample before the furnace to detect the content of C, Si, Mn, P and S, and adjust the content of each element in the molten iron in the furnace according to the results; When the temperature in the furnace rises to 1 520~1 550 ℃, the furnace is ready to enter the spheroidization and inoculation process; The pouring temperature is 1 400~1 420 ℃.
1. Preparation of raw materials
(1) Pig iron. Fushun Hanwang pig iron is selected with chemical composition of 4.37% C, 0.66% Si, 0.07% Mn, 0.025% P and 0.019% S. The content of S in pig iron should be as low as possible and should be lower than 0.04% to avoid its reaction with magnesium and rare earth elements to generate magnesium or rare earth sulfide in the process of spheroidization, which can not only reduce the waste of spheroidized alloy, reduce the spheroidization decay rate, but also reduce the formation of slag inclusion, subcutaneous pores and other defects. The P element should be as low as possible and kept below 0.05% to avoid the precipitation of phosphorus eutectic, especially ternary or conforming phosphorus eutectic, and the increase of brittle transition temperature of the material.
(2) Scrap steel. Scrap steel is used to adjust the carbon and silicon content of the molten iron and change the pig iron heredity; Scrap steel shall not contain anti-spheroidizing elements and alloy elements that produce segregation and carbide; The surface of scrap steel shall be free of oil stain, rust, etc.
(3) Returned material. The return material of the same brand shall be selected preferentially, and the return material shall be subject to strong shot blasting before use to prevent the return material from bringing in silicon, sulfur, etc. and forming slag inclusion. It is not allowed to use gray iron as furnace material to prevent excessive sulfur and phosphorus.
2. Spheroidizing process
The spheroidizing process is adopted, and the pit type treatment package is adopted. The spheroidizing agent, inoculant, covering agent and pressing plate are added into the pit at one time. After the molten iron is covered by the nodularizing agent to a certain height, the nodularizing agent starts to react with the molten iron, which increases the uniformity of the reaction between the nodularizing agent and the molten iron, and improves the absorption rate of the nodularizing agent. The rare-earth magnesium spheroidizing agent is used, with the addition ratio of 1.1%~1.3%, the composition (mass fraction) of 40%~50% Si, 5.5%~7.5% Mg, 1.5%~2.5% RE, 2.5%~3.0% Ca, and the rest is Fe, which retains the advantages of rare earth and magnesium spheroidizing elements. The rare earth and magnesium play a complementary role. Due to the purification and anti-interference effects of rare earth, the spherical graphite with better roundness can be obtained with lower residual magnesium content.
3. Inoculation process
The three-time inoculation method is adopted. The multi-element composite inoculant is composed of 70%~75% Si, 1%~1.5% Ca, Al ≤ 1%, and the rest is Fe. Inoculant of 0.4%~0.6% of the mass of molten iron from the furnace shall be put on the nodulizer for one inoculation; After the spheroidizing reaction, immediately transfer the molten iron in the spheroidizing ladle to the pouring ladle, and add 0.4%~0.6% inoculant of the molten iron mass to the molten iron during the ladle pouring process for secondary inoculation; During the pouring process, inoculation is carried out with the flow, and the amount of inoculant added is 0.05%~0.1% of the mass of molten iron. Through three times of inoculation, the graphitization is promoted, the graphite ball with fine and small graphite and high roundness is obtained, and the free cementite is eliminated, and the eutectic cluster is refined.