The quality of furnace charge mainly depends on the content of trace elements in raw materials. The trace elements in cast iron mainly come from pig iron and scrap steel. Pig iron itself contains coarse graphite flakes whose melting point is above 2000 ° C. during remelting, the graphite cannot be completely dissolved. The unmelted graphite becomes the core of graphite crystallization in the crystallization process, making the graphite coarse again. The coarse graphite flakes have the function of splitting the matrix, which greatly reduces the strength of cast iron. Therefore, the tensile strength of gray cast iron will decrease significantly with the increase of pig iron. The experiments of Liu Youping and others in Nanchang diesel engine plant show that when pig iron is added, the tensile strength is relatively low.
In foreign countries, more scrap steel is added before the melting of gray cast iron, so that there are more low-carbon micro areas in the molten iron. The pre eutectic austenite pre storage core formed by this makes the pre eutectic austenite easier to form and develop, so as to refine the pearlite at the boundary of eutectic group, and then improve the tensile strength of cast iron.
In order to obtain high-quality molten iron, firstly, the tapping temperature should be high, which is conducive to the refinement and precipitation of graphite and improve the flow performance of molten iron, so as to obtain perfect castings, improve the mechanical properties of castings, make the matrix structure more and more fine, improve the tensile strength and reduce the Poisson’s ratio, The ratio of liquid iron quality coefficient, maturity and hardening degree is increased.
At present, the most widely used smelting method in China is cupola melting, and the key to cupola melting is to improve the temperature of molten iron. Foreign cupola basically adopts hot air supply, and some also add oxygen. The quality of coke is good, and the temperature of molten iron is high, which can reach above; Compared with foreign countries, the melting technology of domestic cupola is relatively backward, mostly using cold air supply, paying no attention to the quality of coke, and the melting temperature of molten iron is low, generally about 1450 ° C.