In the traditional production process of ductile iron, about 10% carbon scrap is generally used for batching.This paper only deals with some problems that should be paid special attention to in the production of ductile iron in induction furnace after increasing the proportion of carbon scrap used.According to the production practice, the author puts forward his own view, for peer reference.

1.Selection of furnace charge

(1) Scrap steel used in the production of ductile iron shall be carbon scrap.It should not contain elements that prevent graphitization, such as chromium.And should not contain anti-spheroidizing alloy elements.The size of scrap steel should be about 30% thick flake, can be placed flat in the furnace.(2) Carburizer high quality coke.The sulfur content should be low, the lumpiness is 60 ~ 80mm, has the higher strength, after burning red can not be smashed.Waste electrode block.It is better to have a certain length, which can be inserted into the iron solution.(3) Pig iron is mainly used to adjust the carbon content of molten iron.Hope to use wC > 4% high carbon low sulfur pig iron.The silicon content in pig iron is suitably high some better, produces the ductile iron may add the silicon iron less.(4) Spheroidizing agent and inoculant should be used according to the traditional process, according to the actual situation of each plant.

2.Smelting and smelting process: coke block + scrap steel → electrode block → extracting coke block and electrode block → sampling and analysis of carbon content → pig iron → return charge → sampling and analysis of Si, Mn→ tempering → ferro manganese + ferrosilicon → coming out of furnace → inoculation + spindling → pouring.The precautions during smelting are as follows :(1) the coke block is installed at the bottom of the furnace to create a longer immersion time for the coke block in the molten iron.Coke block consumption is about 5% of scrap steel.Scrap melting about 50%, coke top with thick sheet scrap cover.The waste electrode block of 60 ~ 80 mm can be used instead of coke to be installed at the bottom of the furnace as carbonizing agent, which has lower sulfur content and is more favorable for obtaining high quality ductile iron.(2) The proportion of scrap steel used is mainly determined by the carbon increment efficiency.The manufacturer that begins to use this method to produce ductile iron, with 30% left and right sides had better.After increasing the carbon increasing efficiency, the proportion of scrap steel used is gradually increased.Excessive use of scrap steel, if the carbon increase efficiency is not high, will affect the final product carbon silicon equivalent.(3) After the waste steel is melted, stick it into the electrode rod to properly increase the temperature in the furnace and increase the carbon increasing efficiency.But, the furnace temperature is too high, increases the power consumption, also disadvantageous to the furnace lining.(4) After the estimated carbon content is high enough, remove the coke block and electrode bar and sample the carbon content.(5) According to the analysis results, calculate the amount of pig iron added and the amount of return ductile iron used.(6) According to the silicon content estimation, decide whether to use the return charge to make up the total amount of molten iron.(7) After the return charge is melted down, the content of Si, Mn and other alloy elements is analyzed by sampling.(8) Appropriately raise the temperature of molten iron, make up the alloy elements according to the analysis results, and add ferrosilicon before reaching the furnace temperature.(9) Melt silicon iron to molten state, immediately out of the oven.(10) Spheroidization, inoculation and pouring shall be carried out according to the routine.

3.It is well known that the mechanical properties of ductile iron mainly depend on the chemical composition, cooling rate (as cast ductile iron) and heat treatment process.Chemical composition is internal cause, fundamental, and variable, almost every furnace is different.Therefore, it is necessary to strengthen the detection and careful adjustment of chemical composition, and it is not suitable to adopt "fixed proportion production".Carbon equivalent is particularly important in chemical composition.Especially for the ductile cast iron produced with scrap steel as the main charge, it is difficult to control the carbon increment efficiency and the carbon equivalent fluctuates greatly, so we should pay special attention to adjust the carbon equivalent.

(1) Calculate the amount of pig iron added according to the analysis of carbon content.In fact, it is to adjust the carbon equivalent of ductile iron so that the liquid carbon equivalent formed by pig iron and scrap steel reaches the carbon equivalent required by ductile iron.Cast iron content = scrap steel content × (carbon content required for ductile iron - carbon content analyzed)/(carbon content required for cast iron - carbon content required for ductile iron)

(2) Calculation of the amount of return furnace ductile iron, the carbon equivalent of return furnace ductile iron is basically normal, and the amount of addition is small.Therefore, the carbon content of molten iron in the whole furnace is qualified after the addition of reboiler iron.The amount of ductile iron in the furnace = the total amount of charge - the amount of scrap steel - the amount of cast iron

(3) Calculation formula of silicon content estimation is as follows: silicon content is ≈ ((scrap steel + cast iron + amount of return furnace ductile iron added) × silicon content/total amount of charge) × 100% ≤ The amount of silicon allowed in the original liquid iron is too high. If an appropriate amount of return furnace ductile iron is added less and equal proportion of cast iron and waste steel is used to make up the total amount of charge.

(4) The calculation formula for the amount of ferrosilicon added is as follows: Ferrosilicon added = [total amount of charge × (required silicon content of ductile iron - amount of silicon increased by inoculation - amount of silicon increased by spheroidization - amount of silicon analyzed)] / Ferrosilicon contained by silicon

(5) The calculation formula of the amount of manganese and other alloying elements added is as follows: the amount of alloying iron added = total charge × (ductile iron required alloy content - analytical content)/alloying element content in alloyed iron is excluded from the above calculation.This is because the induction furnace melting nodular cast iron, each element burn loss is not big, at the same time the iron also has a certain burning loss.In case of special circumstances, attention should be paid to burning.

Application example of composition adjustment: using 1T induction furnace to produce pearlite nodular cast iron QT800 -- 2, casting weight 1000kg.Controlled chemical composition: wC = 3.5%, wSi = 2.5%, wMn = 0.5%, wP < 0.07%, wS < 0.02%, wMg = 0.03% ~ 0.06%, and wRE = 0.02% ~ 0.04%.The composition of the charge to be used is shown in the table.

After carburizing, the carbon content of "waste steel liquid" in the furnace is 2.5%.The amount of pig iron added = 300× (3.5%-2.5%)/(4%-3.5%) = 600 (kg) usage of backfurnace ductile iron = 1000 -- 300 -- 600 = 100 (kg) Estimated silicon content = (300× 0.3% +600× 1.8% +100× 2.5%) × 100% = 1.42% If the amount of silicon increased by spheroidizing agent is 0.6% and the amount of silicon increased by inbreeding agent is 0.4%, then the amount of silicon contained in the original molten iron should be 1.5%, which is higher than the estimated amount of silicon.For this purpose, Z18 cast pig iron can be used to adjust the carbon equivalent and return ductile iron can be used to make up the total charge amount.The silicon content and manganese content were 1.38% and 0.15% respectively.Content of ferrosilicon = 1000 (2.5% - 0.6% - 0.4% - 1.38%) / 75% = 1.6kg of ferromanganese = 1000 (0.5% - 0.15%) / 65% = 5.4kg

4.Conclusion in induction furnace production, we use scrap production QT800-2, strength in 800 ~ 1000MPa, elongation > 4%.At present, there are a lot of scrap steel in the market, pig iron is expensive, high quality pig iron is scarce.Therefore, we suggest that manufacturers with nodular cast iron production tasks, can refer to the above assumption, "steel instead of iron" production of nodular cast iron.