1. Production equipment and burden ratio
A 3T medium frequency induction furnace is used in the smelting process, and the surface of the charge is required to be free of paint and rust. Because the paint containing charge will increase the mass fraction of lead in the molten iron, form widmanstatten graphite, deteriorate the graphite morphology and affect the material properties. In addition, the rusty charge will increase the mass fraction of oxygen in molten iron and generate a large amount of slag.
Use petroleum coke carburizing agent (nitrogen mass fraction 0.5% ~ 0.8%) to ensure the nitrogen mass fraction in molten iron. Some data show that the tensile strength of gray cast iron increases with the increase of nitrogen content, which is mainly because it is easy to obtain pearlite and passivate graphite after the increase of nitrogen mass fraction, so as to avoid increasing the length of graphite and increasing the cleavage of graphite to the matrix, which will affect the material properties.
Silicon carbide is used to supplement the mass fraction of silicon. The use cost of silicon carbide is low. Adding silicon carbide to molten iron can coarsen and shorten the graphite length of gray cast iron and reduce the undercooling of molten iron after inoculation. Silicon carbide has deoxidation effect and a series of metallurgical reactions in molten iron, which can reduce the harmful effects of oxides in charge to a certain extent. The burden ratio used in this test is 70% ~ 80% scrap steel, 20% ~ 30% machine iron, 5% ~ 10% Z18 pig iron and 1.0% ~ 1.5% silicon carbide (silicon carbide mass fraction > 97%, particle size 1 mm ~ 5 mm)
2. Smelting process control
The feeding sequence is scrap steel, mechanical iron and pig iron, and the melting temperature is less than 1380 ℃; After melting, sample and test, and the sampling temperature shall be controlled between 1420 ℃ and 1440 ℃; The superheat temperature is guaranteed to be between 1500 ℃ and 1520 ℃.
Silicon strontium inoculant is selected for iron tapping inoculation. The mass fraction of inoculant is 0.30% and the particle size of inoculant is 3 mm ~ 10 mm 75 ferrosilicon is used for flow inoculation treatment, the mass fraction is 0.155% ~ 0.20%, and the particle size of 75 ferrosilicon is 10 mm ~ 20 mm
3. Inoculation treatment
Inoculation treatment is mostly used in the production of high-strength gray cast iron. The carbon equivalent of this kind of gray cast iron is relatively low. The solidified structure of untreated molten iron contains a large amount of undercooled graphite and cementite. Only after inoculation can fine A-type graphite be obtained. The furnace charge in this test contains more scrap and uses high nitrogen carburizing agent. The nitrogen content of molten iron is high, and nitrogen hinders the diffusion of carbon atoms. In addition, the superheated temperature of molten iron melted in electric furnace is high, and most graphite particles have been dissolved. Therefore, a strong inoculant is required to obtain the expected effect. Strontium can enhance the graphitization ability of silicon inoculants and does not significantly increase the number of eutectic groups. Because the eutectic group interface is the main part of micro porosity, reducing the number of eutectic groups is conducive to reducing intergranular porosity defects. It is suitable for the production of gray cast iron with air tightness requirements. Strontium ferrosilicon has a significant effect on the ability of inhibiting white cast iron, which can reduce the section sensitivity and ensure the uniform microstructure and hardness of gray cast iron. It is very effective to use it for inoculation treatment in low-carbon high-strength gray cast iron. Therefore, silicon strontium inoculant is used in this test, with a mass fraction of 0.30%. The inoculation method is inoculation in iron tapping tank, which can make the inoculant evenly distributed in molten iron. The parameters of silicon strontium inoculant are shown in the table.
| Category | Si | Al | Ca | Sr | Fe | Particle size / mm |
| Silicon strontium inoculant | 75 | 0.45 | 0.08 | 0.7 | Other | 3-8 |
4. Flow inoculation
Because the ladle inoculation takes a long time from inoculation to solidification, and the role of inoculant can not be brought into full play, the flow inoculation method is gradually applied. This method shortens the inoculation to pouring time, so the inoculation effect is more stable and the amount of inoculant is less. Through experimental comparison, flow inoculation can better reduce the white tendency of molten iron and improve the nucleation rate of molten iron, effectively improve the morphology of graphite, promote the uniform precipitation of graphite, reduce or eliminate the free distribution of cementite and phosphorus eutectic in the matrix, and make the lamellar spacing of pearlite structure uniform.