Points for MELTING SCRAP FOR DUCTILE IRON

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

1.Charge Selection

(1) Steel Scrap

The scrap used in the production of ductile iron should be carbon steel. They should not contain elements that inhibit graphitization, such as chromium. Should not contain anti-globular alloy elements. Scrap should be about 30% of the block size is thick sheet-like, can be placed in the furnace.

(2) Carburizing agent

A.) High-Quality Coke. Sulfur content should be low, block degree 60 ~ 80 mm, with high strength, burning red cannot be crushed immediately after.

B.)Waste electrode block. It is better to have a certain length for cutting into the liquid iron.

(3)Pig Iron

Mainly used to adjust the carbon content of liquid iron. Wish to use WC > 4% high carbon low sulfur pig iron. Pig Iron in the silicon content is appropriate higher some better, the production of ductile iron can add less ferrosilicon.

(4) Nodulizer and inoculant

According to the use of traditional technology, subject to the actual situation of the plant.

2.Smelting

SMELTING PROCESS: Coke Block + Scrap Steel → electrode block → heating → stripping Coke Block and electrode block → sampling analysis of carbon → pig iron → recharging → sampling analysis of Si, MN → heating → FERROMANGANESE + ferrosilicon → tapping → inoculation + spheroidization → casting.

The following considerations apply in the smelting process:

(1) coke blocks are installed at the bottom of the furnace in order to create a longer soaking time for coke blocks in molten iron. The consumption of coke is about 5% of scrap steel. Scrap steel melting about 50 %, coke above the application of thick sheet scrap cover. It is also possible to use scrap electrode blocks of 60 ~ 80 mm instead of coke as the carburizing agent in the furnace bottom, which has lower sulfur content and is more advantageous to obtain high-quality ductile iron.

(2) the proportion of scrap used is mainly determined by the efficiency of carbon increase. The manufacturers who have started using this process to produce ductile iron should start at around 30% . After increasing the efficiency of carbon increase, the proportion of scrap used is gradually increased. Excessive use of scrap, if the carbon efficiency is not high, the impact of the end-product carbon silicon equivalent.

(3) after the scrap is melted, a cutting rod is inserted into the electrode, the temperature in the furnace is properly increased, and the carbon-increasing efficiency is increased. However, the furnace temperature is too high, increase power consumption, is not conducive to the furnace lining.

(4) when the carbon content is high enough, the coke block and electrode bar are removed, and the carbon content is sampled and analyzed.

(5) according to the results of the analysis, the quantity of pig iron added and the quantity of spheroidal graphite cast iron used are calculated.

(6) to decide whether the total amount of molten iron can be replenished with the recycled material according to the estimation of silicon content.

(7) the content of SI, MN, and other alloy elements is analyzed by sampling.

(8) properly increase the temperature of molten iron, according to the results of the analysis to complement alloy elements, after reaching the temperature out of the furnace before adding ferrosilicon.

(9) ferrosilicon melts to a molten state, immediately out of the furnace.

(10) spheroidizing, inoculation, and pouring shall be carried out as usual.

3. Adjust your calculations

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. The chemical composition is the internal cause, is fundamental, is variable, almost every furnace is not the same. Therefore, to the chemical composition must strengthen the examination, the earnest adjustment, not suitable uses “the fixed proportion production”. In the chemical composition, carbon equivalent is particularly important. It is especially important to adjust carbon equivalent because it is difficult to control increasing carbon efficiency and carbon equivalent fluctuates greatly.

(1) calculating the quantity of pig iron added, calculating the quantity of pig iron added according to the carbon content. In effect, the carbon equivalent of ductile iron is adjusted so that the molten carbon equivalent formed by pig iron and scrap is equal to the carbon equivalent required for ductile iron.

Pig Iron addition = scrap addition (C content required for ductile iron-analysis of c content) / (c content in pig IRON-C content required for ductile iron)

(2) the carbon equivalent of the SPHEROIDAL graphite cast iron is normal, and the adding amount is not big. Therefore, after the addition of recycled iron, the carbon content of the entire molten iron is qualified.

The feeding amount of SPHEROIDAL graphite cast iron is equal to the total charge-scrap-adding amount of pig iron.

(3) the estimate of silicon content is as follows:

Si Content ≈(scrap + pig iron + SPHEROIDAL graphite cast iron addition) SI content ≤100% the allowable SI content of the original Liquid Iron

When the silicon content estimated is too high, the amount of nodular cast iron is little added, and the total amount of burden is supplemented with pig iron and scrap whose proportion is 9∶1.

(4) the amount of ferrosilicon is calculated as follows:

Ferro-silicon (FE) content = (total charge) / ferro-silicon (SI content required for ductile iron)

(5) the quantities of manganese and other alloying elements are calculated as follows:

Alloying iron addition = total charge (Ductile Iron Alloy content-analysis content) / alloying element content in Alloy Iron

The above calculation does not take into account the burning. This is because the induction furnace melting ductile iron, the elements burn little loss, iron is also a certain loss. If encounter special circumstances should pay attention to burn damage.

EXAMPLES OF COMPOSITION ADJUSTMENT APPLICATIONS:

1-ton induction furnace was used to produce pearlite ductile iron qt800-2 with the cast the ng weight of 1000kg.

The controlled chemical compositions were 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% .

The composition of the charge used is shown in the attached table.

  • ductile iron ball

The carbon content of “scrap steel liquid” is 2.5% after carburizing.

Pig Iron addition = 300(3.5%-2.5%) / (4%-3.5%) = 600(kg)

The usage amount of SPHEROIDAL graphite cast iron is 1000-300-600 = 100(kg)

Estimated silicon content = (3000.3% + 6001.8% + 1002.5%)100% = 1.42%

If the silicon content of nodulizer is 0.6% and the silicon content of inoculant is 0.4 %, then the silicon content in the primary iron should be 1.5 %, which is higher than the estimated silicon content. The carbon equivalent can be adjusted by using Z18 cast pig iron, and the total charge can be replenished by using recycled ductile iron.

Silicon content was 1.38% and manganese content was 0.15 %.

Addition of ferrosilicon = (1000(2.5%-0.6%-0.4%-1.38%)) / 75% = 1.6 kg

Addition of FERROMANGANESE = (1000(0.5%-0.15%)) / 65% = 5.4 kg

4.Conclusion

In induction furnace production, we use scrap to produce QT800-2 with the strength of 800 ~ 1000MPA and elongation of > 4 %.

At present, the market scrap many, pig iron expensive, high-quality pig iron is extremely scarce. Therefore, we suggest that the production of ductile iron manufacturers can refer to the above assumption, “steel instead of iron” production of ductile iron.