The Smelting Process Of Scrap Tempered Ductile Iron

Publish Time: 07/01 2021 Author: Site Editor Visit: 14910

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

The choice of charge

(1) Scrap Steel

Scrap used in the production of ductile iron should be carbon scrap. It should not contain elements that hinder graphitization, such as chromium. It should not contain anti-spheroidizing alloying elements. The lumps of scrap steel should be about 30% in the form of thick flakes, which can be laid flat in the furnace.

(2) Recarburizer

① High-quality coke. The sulphur content should be low, the lumpiness is 60-80mm, and it has high strength. It cannot be crushed after being burned.
②Spent electrode block. It is best to have a certain length to facilitate cutting into the molten iron.

(3) Cast Iron

It is mainly used to adjust the carbon content of molten iron. Hope to use high-carbon low-sulfur pig iron with wC>4%. The silicon content in the pig iron is appropriately higher, and the production of nodular cast iron can be less ferrosilicon.

(4) Spheroidizing

Spheroidizing agent and inoculant are used according to traditional techniques, and are determined in accordance with the actual conditions of each factory.

Smelt

Smelting process: Coke block + scrap steel → electrode block → temperature increase → pull out coke block and electrode block → sample and analyze carbon content → pig iron → return material → sample and analyze Si, Mn → temperature increase → ferromanganese + ferrosilicon → discharge →Inoculation + spheroidization → pouring.　

The precautions during the smelting process are as follows:

The coke block is installed at the bottom of the furnace, the purpose is to create a longer soaking time of the coke block in the molten iron. The amount of coke block is about 5% of the scrap. About 50% of the scrap is melted, and the coke should be covered with thick flake scrap. It is also possible to use waste electrode blocks of 60-80 mm instead of coke to be installed at the bottom of the furnace as a recarburizer, with lower sulfur content, which is more beneficial to obtain high-quality ductile iron.
The proportion of scrap steel used is mainly determined by the efficiency of carbon increase. For manufacturers who start to use this method to produce ductile iron, about 30% is better. After improving the carbon increase efficiency, gradually increase the proportion of scrap steel. Excessive use of scrap steel, if the carbon increase efficiency is not high, will affect the carbon silicon equivalent of the final product.
After the scrap steel is melted, insert the electrode rod to appropriately increase the temperature in the furnace and increase the carbon efficiency. However, the furnace temperature is too high, which increases power consumption and is also detrimental to the furnace lining.
After the carbon content is estimated to be high enough, take out the coke block and electrode rod, and sample and analyze the carbon content.
According to the analysis results, calculate the amount of pig iron added and the amount of recycled ductile iron used.
Determine whether the total amount of molten iron can be made up with the refurbished material based on the estimation of the silicon content.
After the reheating material is melted, take a sample and analyze the content of Si, Mn and other alloying elements.
Properly increase the temperature of the molten iron, supplement the alloying elements according to the analysis results, and add ferrosilicon before the furnace reaches the furnace temperature.
The ferrosilicon is melted to a molten state and is immediately released from the furnace.
Spheroidization, inoculation, and pouring are carried out as usual.

Composition adjustment calculation

As we all know, the mechanical properties of ductile iron mainly depend on the chemical composition, cooling rate (as-cast ductile iron) and heat treatment process. The chemical composition is the internal cause, the root, and the variable, almost every furnace is different. Therefore, it is necessary to strengthen the detection of chemical components and make careful adjustments, and it is not appropriate to adopt "fixed ratio production". In the chemical composition, carbon equivalent is particularly important. In particular, nodular cast iron produced by using scrap steel as the main charge, because the efficiency of carbon increase is difficult to control, and the carbon equivalent fluctuates greatly, so special attention should be paid to adjusting the carbon equivalent.

Calculate the amount of pig iron added. Calculate the amount of pig iron added based on the analysis of the carbon content. In fact, it is to adjust the carbon equivalent of nodular cast iron so that the carbon equivalent of the melt formed by both pig iron and scrap steel reaches the carbon equivalent required for nodular cast iron.The amount of pig iron added = the amount of scrap added × (the required carbon content of nodular cast iron-analysis of carbon content) / (the carbon content of pig iron-the required carbon content of nodular cast iron)
Calculation of the added amount of recycled ductile iron The carbon equivalent of recycled ductile iron is basically normal, and the amount added is not large. Therefore, after the reheated iron is added, the carbon content of the molten iron in the entire furnace is qualified.The amount of nodular cast iron added to the furnace = the total amount of charge-the amount of scrap steel-the amount of pig iron added
Estimation of silicon content The calculation formula is as follows:Silicon content ≈ ((scrap steel + pig iron + nodular cast iron addition amount) × silicon content / total charge] × 100% ≤ allowable silicon content of the original molten ironWhen the estimated silicon content is too high, add a small amount of recycled ductile iron, and use the same amount of pig iron and scrap with a ratio of 9:1 to make up the total charge.
Calculation of Ferrosilicon Addition The calculation formula is as follows:The amount of ferrosilicon added = [total amount of charge × (required silicon content of ductile iron-increased silicon content by inoculation-increased silicon content by spheroidization-analysis of silicon content)] / silicon content of ferrosilicon
The amount of manganese and other alloying elements added is calculated as follows:The amount of alloy iron added = the total amount of charge × (the required alloy content of ductile iron-analysis content)/alloy element content in alloy ironThe burning loss is not included in the above calculations.

This is because nodular cast iron is melted in the induction furnace, and the elements are not burnt much, and the iron is also burned to a certain extent. If you encounter special circumstances, you should pay attention to burning.

Composition adjustment application example:

Using a 1t induction electric furnace to produce pearlitic ductile iron QT800-2, the casting weight is 1000kg.
"The control chemical composition is: wC=3.5%, wSi=2.5%, wMn=0.5%, wP<0.07%, wS<0.02%, wMg=0.03%～0.06%, wRE=0.02%～0.04%.
After the recarburization, the carbon content of the "scrap molten steel" in the furnace is 2.5%.
The amount of pig iron added = 300 × (3.5%-2.5%) / (4%-3.5%) = 600 (kg)
The amount of nodular cast iron used in the furnace = 1000-300-600=100 (kg)
Estimated silicon content = (300×0.3%＋600×1.8%＋100×2.5%)×100%=1.42%
If the silicon content of the nodulizer is 0.6% and the silicon content of the inoculant is 0.4%, the silicon content of the original molten iron should be 1.5%, which is higher than the estimated silicon content. For this, Z18 cast iron can be used to adjust the carbon equivalent, and the re-fired ductile iron can be used to make up the total amount of charge.
The silicon content is 1.38% and the manganese content is 0.15% before the furnace is released.
Ferrosilicon added amount = [1000 (2.5%-0.6%-0.4%-1.38%)] / 75% = 1.6kg
Ferromanganese added amount = [1000 (0.5%-0.15%)] / 65% = 5.4kg

4. Concluding remarks

In the production of induction electric furnace, we use scrap steel to produce QT800-2, the strength is 800～1000MPa, and the elongation is more than 4%.

At present, there is a lot of scrap steel in the market, pig iron is expensive, and high-quality pig iron is even more scarce. Therefore, we suggest that manufacturers with nodular cast iron production tasks can refer to the above-mentioned assumption and "substituting steel for iron" to produce nodular cast iron.

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