3. Melting process, quality control and improvement
   3.1 Control of C increase rate and use of carburizer
   When using medium frequency furnace to melt gray cast iron, it is generally believed that as long as the chemical composition and temperature of the molten iron are controlled in front of the furnace, high-quality molten iron can be melted, but this is not the case. The most important thing for medium frequency furnace to melt gray cast iron is to control the core role of carburizer. The higher the C increase rate, the better the performance of the molten iron. The C increase rate mentioned here refers to the C added to the molten iron in the form of carburizer, not the C brought into the charge. Production practice shows that the higher the proportion of pig iron in the charge ratio, the greater the tendency of white cast iron; the higher the proportion of carburizer, the smaller the tendency of white cast iron. This requires more cheap scrap steel and recycled materials to be used in the batch, and less or no pig iron. There are a large number of fine and dispersed heterogeneous crystal nuclei in the molten iron using scrap steel C increase process, which reduces the supercooling of the molten iron and promotes the formation of graphite structure dominated by A-type graphite. At the same time, the reduction in the amount of pig iron also reduces the adverse genetic effects of coarse graphite in pig iron, and the performance of gray cast iron also improves with the increase in the amount of scrap steel.
   C exists in the original iron liquid mainly in the form of fine graphite and C atoms. From the perspective of graphite refinement, it is not desirable to have too many C atoms in the original iron liquid, which will inevitably reduce the number of graphite cores, and C atoms are more likely to form cementite during the cooling process. Fine graphite can directly serve as heterogeneous nucleation cores. Refining graphite and increasing cores are the key to achieving high performance of cast iron. Increasing the amount of recarburizer can increase the number of nucleation cores, thereby laying a solid foundation for refining graphite. Therefore, in actual production, the use of recarburizer and the effect of increasing C should be emphasized. To this end, the following points should be noted:
   (1) The absorption rate of the recarburizer is directly related to its w (C) amount. The higher the w (C) amount, the higher the absorption rate.
   (2) The particle size of the recarburizer is the main factor affecting its dissolution into the molten iron. Practice has shown that the particle size of the recarburizer is generally controlled at 1 to 4 mm. The recarburizer with fine powder or coarse particles has a poor carbon increasing effect.
   (3) Si has a great influence on the carbon increasing effect. High Si molten iron has poor carbon increasing performance and slow carbon increasing speed. Therefore, FeSi should be added after the carbon increasing is in place. The principle of increasing carbon first and then adding Si should be followed.
   (4) S can hinder the absorption of C. The carbon increasing speed of high S molten iron is much slower than that of low S molten iron.
   (5) Graphite recarburizer can improve the nucleation ability of molten iron. The absorption rate is also more than 10% higher than that of non-graphite recarburizer. Therefore, low N graphite recarburizer should be selected.
   (6) The method of using the recarburizer is recommended to use the furnace addition method, that is, first add a certain amount of small pieces of recycled materials and scrap steel to the bottom of the furnace, then add all the recarburizer according to the batching amount, and then press a layer of small pieces of scrap steel and pig iron on top, and then add the furnace charge while melting. This method is simple and easy to operate, with high production efficiency and an absorption rate of up to 90%. If the amount of carburizer added is large, it can be added in two batches, first adding 60% to 70% to the scrap steel cushion at the bottom of the furnace, and the remaining carburizer is added during the process of adding scrap steel. Carburizer can also be added when the molten iron temperature is 1400-1430℃, the purpose is to increase the amount of w (C) in the molten iron to the upper limit required by the grade.
   (7) The carburizer should not be added too late. Adding carburizer in the later stage of smelting will cause the carburizer to burn easily and the C absorption rate is low. The carburizer added later requires additional melting and absorption time, which slows down the adjustment of chemical composition and heating time, reduces production efficiency, increases power consumption, and may also cause harm due to excessive heating.
   (8) Stirring the molten iron can promote C increase, especially the graphite agglomerates attached to the furnace wall. If the molten iron is not overheated and the molten iron is kept warm for a certain period of time, it is not easy to dissolve in the molten iron. The strong electromagnetic stirring of the medium frequency furnace is beneficial to C increase.

3.2 Control of S and N
3.2.1 Control of w(S)
There is no S source for smelting cast iron in medium frequency furnace, and the w(S) content of molten iron is low. Therefore, it has great advantages for producing ductile iron, but for gray cast iron, low S and high Mn will increase casting stress, greatly increase the probability of cracks, and the appropriate amount of S in molten iron can improve the inoculation effect. The medium frequency furnace produces gray cast iron, not only does not increase S, but also makes the w(S) content lower (about 0.04%) due to the large amount of scrap steel used. When w(S) <0.06% in gray cast iron, it is easy to cause poor graphite morphology, difficult to inoculate, and a large tendency to shrinkage and white cast iron. In order to obtain normal graphite morphology by smelting molten iron in medium frequency furnace, it is necessary to have a suitable w(S) content. If the content of S and sulfide is low, the number of crystal nuclei will decrease, the graphite nucleation ability will decrease, the white cast iron will increase, the A-type graphite will decrease, the D and E-type undercooled graphite and ferrite will increase, the grains will be coarse, and the strength will decrease. Moreover, as the holding time of high-temperature molten iron increases, the degree of supercooling continues to increase. The higher the grade of gray cast iron, the more significant the influence of holding temperature and time on supercooling. The w (S) content of molten iron is low and the number of eutectic groups is small. As the w (S) content increases, the number of eutectic groups increases sharply. The more eutectic groups there are and the smaller their size, the better the mechanical properties of cast iron [9-11]. Therefore, when smelting gray cast iron in a medium-frequency furnace, the w (S) content should generally be increased to close to 0.1% to give full play to the beneficial role of S, improve the inoculation effect, increase the number of nuclei in molten iron, and increase the metallographic structure of the casting to be mainly A-type graphite. The volume fraction of pearlite in the matrix structure increases, thereby improving the strength and cutting performance of cast iron. Generally, after adjusting the chemical composition, FeS is added to increase S, or coke is used as a carburizer. While increasing C, the w (S) content is also increased to more than 0.06%. However, the w (S) content should not be too high, as it will increase white cast iron. When the amount of w(S) is high, as the amount of w(Mn) increases, the generated MnS fully plays a role in heterogeneous nucleation, creating conditions for inoculation treatment. However, when the amount of w(Mn) is greater than 1%, too much MnS is generated and concentrated at the grain boundary, weakening the grain boundary, and even generating slag inclusions, reducing the strength of cast iron. From the perspective of reducing MnS slag inclusions, the amount of w(S) should be controlled to be less than 0.1%, so that the amount of w(Mn) allowed to exist is higher, which is beneficial to improving the performance of gray cast iron.
3.2.2 Control of w(N) amount
Due to the large amount of scrap steel used in the medium frequency furnace to melt gray cast iron, and as the proportion of scrap steel increases, the amount of carburizer also increases, and the w(N) amount of carburizer is high, so the w(N) amount of the medium frequency furnace molten iron is high. When the amount of w(N) in the molten iron is greater than 100×10-6, the casting is prone to cracking, shrinkage and crack-like subcutaneous pore defects. An effective way to control the amount of w (N) in molten iron is to keep the molten iron at high temperature. As the time of keeping the temperature increases, the amount of w (N) will gradually decrease. However, long-term keeping of high-temperature molten iron will increase the degree of supercooling and the tendency of white cast iron, so generally, a graphite recarburizer with a low amount of w (N) is selected in production. If necessary, 10% Fe2O3 powder can be added to the coating to eliminate the influence of high N. However, N in gray cast iron is a limiting element like S. A trace amount of N in molten iron can refine the grains and eutectic groups of gray cast iron, increase the volume fraction of pearlite in the matrix, and improve the mechanical properties. It plays a positive role in improving the graphite morphology of gray cast iron and promoting the pearlitization of the matrix structure. Nitrogen compounds can also serve as crystal nuclei to create growth conditions for graphite nucleation. Generally, the amount of w (N) should be controlled below 0.008%.