Precautions for smelting gray cast iron (ductile iron) in medium frequency induction heating furnace

Use scrap steel, scrap iron (gray iron, ductile iron), steel chips, recycled iron, and new iron as raw materials to smelt gray cast iron (ductile iron) in an induction furnace. The molten iron has a higher superheat temperature than the molten iron in the cupola furnace. , the melting and holding time is long, which can easily cause decarburization. Castings with the same chemical composition and the same mold have higher strength and hardness than those smelted in cupola furnaces; have a greater tendency to whiten when chilled; the length of graphite is shorter, and it is easy to produce D and E type graphite; the fluidity of molten iron is poor , increased shrinkage can easily cause various casting defects (pores, cold insulation, shrinkage, honeycomb pores, etc.). When the amount of scrap steel used is greater than 50%, the melting and holding time of the molten iron increases, and the above phenomenon becomes more serious. There are several explanations for the reason: N theory (O theory attached), the N content in the molten iron in induction furnaces and cupola furnaces is different; core-eutectic group theory, the crystallization core disappears during long-term overheating and heat preservation, thus making the eutectic The number of crystal clusters decreases, the depth of quenching increases, and the shrinkage intensifies.

Using cheap scrap steel as raw material, in order to meet the carbon content of the gray cast iron (ductile iron) grade, a carbonaceous carburizing agent (or waste electrode rod) must be added during the smelting process to increase the carbon content so that the carbon content meets the grade requirements ( Carbon content upper limit or slightly exceeds). There are a large number of dispersed heterogeneous crystalline cores in the gray cast iron molten iron after carburization, which reduces the supercooling degree of the molten iron and promotes the formation of a graphite structure dominated by A-type graphite; at the same time, due to the reduction in pig iron dosage , its undesirable genetic effects are reduced, so the A-type graphite flake branches are less likely to grow up, making the graphite short and uniform. The amount of scrap steel increases (different scrap steel has different N content), which increases the N content. N prevents the formation of ferrite by changing the graphite structure in cast iron and promotes the formation of pearlite, while N compounds crystallize as graphite. The core creates nucleation and growth conditions for graphite. Gray cast iron contains higher N, which not only stabilizes pearlite, but also makes the ends of graphite relatively bald.

When using a medium frequency induction heating furnace to smelt gray cast iron (ductile iron), on the premise of ensuring the chemical composition requirements of the gray cast iron (ductile iron) grade, the quantity, size, distribution, shape and matrix structure characteristics of the graphite must also be firmly grasped. Therefore, Pay attention to the following points.

① When using a medium frequency induction heating furnace to smelt ductile iron and gray cast iron, because a large amount of cheap scrap steel (scrap steel chips) can be used, the melting time should be shortened as much as possible, the melting speed should be accelerated, and the high-temperature holding time should be short. Do not use the process of melting cast steel. When melting cast iron, a covering layer should be added to keep it warm at around 1300°C.

② Scrap steel, steel cutting, recycled materials, etc. should use rollers to remove rust and impurities as much as possible; pay attention to the composition and special elements of scrap steel, recycled materials, recycling parts; if the amount of scrap steel added is more than 50%, it can be mixed with Ti, Al, B and other recycled materials that have a greater chemical affinity with N are used to control the increase in the amount of N. It is better to add some new iron (or ductile iron returned scrap castings) when batching. It can be added together when feeding; it can also be added after melting and before slag removal.

③ When the amount of scrap steel increases, a carburizing agent (carbonaceous carburizing agent or scrap electrode rod block, but with a low N content) must be added. According to the grade requirements of the smelting casting, the carbon equivalent must be reached. The carburizing agent can be added during charging. Add; it can also be added when the molten iron is 1400~1430°C. Its carbon equivalent is 0.2% to 0.4% higher than that of cupola furnace smelting.

④The impact of recycling scrap steel, scrap iron, returned iron, and steel cuttings to melt the molten iron on C, Si, Mn, P, and S. Generally, induction furnace smelting does not have a sulfur-increasing process, and its S content is relatively low, close to that of molten steel. When the sulfur content is high, it will cause difficulty in inoculation. The use of 75FeSi has little effect on the inoculation of molten iron. Use an inoculant containing a small amount of Mg and RE or add high-sulfur pig iron to the molten iron. After melting, the S content can be increased. , thereby improving its incubation performance.

⑤ Due to the different scrap steel, scrap steel chips, returned castings, and various raw materials in each factory, the intermediate frequency induction heating furnace has different acid linings (Si02) and alkaline linings (Mgo or Al203); although the chemical composition requirements of the brand are standard When smelting gray cast iron (ductile iron), based on the actual situation of the factory, different smelting processes are used to combine the effects of C, Si, Mn, P, and S. Then, FeSi or FeSiCa inoculation treatment and rare earth magnesium are used before pouring. Spheroidizing agent spheroidizing treatment.