There are a lot of coarse hypereutectic graphite in pig iron. This coarse graphite has heredity, the melting temperature is low, and the coarse graphite is not easy to be eliminated. The coarse graphite inherits from the liquid to the solid cast iron structure. On the one hand, it reduces the reach of cast iron. Mechanical properties, on the other hand, because of the presence of coarse graphite, the expansion effect of graphitization precipitation that should have been generated during the solidification process is weakened, which increases the shrinkage tendency during the solidification process of molten iron. In addition, it will also cause castings in the subsequent processing process. Coarse graphite is easy to fall off, forming surface pits, which affect the surface finish of castings.
Moreover, there are many impurity elements in pig iron. Such as trace amounts of TI.P.V, B, etc., will have a negative impact on the castings and deteriorate the matrix structure and graphite morphology. Therefore, in the electric furnace smelting, the amount of pig iron charge should be reduced as much as possible, graphite recarburizers should be used to ensure high carbon equivalent, and the amount of scrap steel should be relatively increased.
In this way, cast iron treated with recarburizer under high temperature smelting conditions will generate a large number of dispersed heterogeneous crystalline cores in the molten iron, which reduces the degree of subcooling of the molten iron and promotes the formation of type A graphite The main graphite structure; at the same time, due to the small amount of pig iron, its genetic effect is greatly weakened, so that the A-type graphite flakes are not easy to grow up, which is beneficial to improve the mechanical properties, reduce the shrinkage tendency, and improve the processing performance. Obtain high-quality molten iron by carburizing. Compared with the amount of high scrap steel with the same composition, its mechanical properties are also half a grade lower.
The new smelting process with recarburizer is superior to the traditional large proportion of pig iron in terms of cost and finished product performance.