CS198380B1 - Process for preparing melt in electric arc furnace for preparing ductible cast iron in cast state - Google Patents

Description

The invention relates to a process for producing a melt in an electric arc furnace as a starting material for the production of ductile cast iron with an elongation of more than 7%, even after casting, without further heat treatment. This ductile iron requires the formation of a grained (spherical) shape of graphite and a corresponding amount of ferrite in the structure. Therefore, the finished castings made of ductile cast iron were annealed additionally by ferritization. However, this led to an increase in annealing costs, to the risk of shape deformation of the castings, to their surface oxidation (scaling), etc.

Until now, the starting melt for producing ductile cast iron with an elongation above 7% has been prepared, for example, in an electric arc furnace, the basic charge of which is selected high-grade pig iron of high chemical grade and high-quality steel waste.

The prior art methods of melt production in electric arc furnaces, which are predominantly synthetic or semi-synthetic, do not allow the production of high quality ductile cast iron with ductility above 7% in the casting state with sufficient operational safety in the prior art oxidation process. The use of high-quality pig iron tends to be accompanied by the risk of melt gasification in the arc furnace and thereby the formation of foundry defects.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a process for preparing a melt for producing ductile iron of 7% ductility in an cast state and to prepare conditions in an electric arc furnace to reduce undesirable and essential elements such as manganese, sulfur, phosphorus, silicon, gases and the like. castings from ductile cast iron had already in the cast state a corresponding amount of ferrite in the structure. The basis of the metal charge is 30 to 70% by weight. pig iron with a maximum silicon content of not more than 1.5% by weight, a maximum of 0.1% by weight of manganese, a maximum of 0.1% by weight of phosphorus, a maximum of 0.1% by weight of phosphorus, and other undesirable elements such as copper, chromium, tin, titanium, arsenic, bismuth in an amount that does not disrupt the structure of ductile cast iron with an elongation above 7% in the cast state and 30 to 70% of quality and unsorted waste, of which unsorted 15 wt. total charge.

SUMMARY OF THE INVENTION The iron ore as an oxidizing additive and lime as a slag-forming additive are added to the metal charge before or during the smelting process in an amount necessary to oxidize the undesirable elements and oxidation is carried out below the carbon boiling point. the slag is stripped off and additional oxidation and granulated additives are added in an amount necessary for the melt oxidation having a carbon content of 2 to 3% by weight, after which oxidation is carried out above a carbon boiling point which is terminated after the carbon content of the melt is reduced 2 wt. and a silicon content below 0.2% by weight, then the slag is stripped off and carbonization, desulphurisation and melt treatment with ferro-alloys are carried out to the desired composition.

The melt produced in this way is a suitable starting material for the production of high-quality ductile cast iron with a ductility in the cast state of more than 7%.

The described technological process enables the preparation of the starting melt with precisely controlled chemical composition before tapping in the electric arc furnace. It is also possible to use pig iron which cannot be used directly for the production of ductile cast iron with an elongation above 7%, for example for a high content of silicon, manganese, phosphorus. The proposed procedure further reduces the time required for carburizing cast iron by up to 50%, reducing the amount of carburizing agents up to half. The annealing and thus the deformation of the castings made of ductile cast iron as well as the single shot blasting of the castings are completely eliminated, thereby shortening their production cycle and reducing the production costs. This process further safely reduces the amount of undesirable elements in the cast iron, allows for control of their amount and thus increases the graphitizing ability of the cast iron, which is ensured by the carbon boiling of the melt with a carbon content of 2 to 3%.

Two examples of the use of the production method according to the invention for the production of lathe chuck and chill castings for the production of steel ingots are described below.

Example 1

The electric arc furnace charge consists of:

pig iron ...... 3500 kg, ie about 54% sorted steel scrap. . . 2500 kg. ie about 39% of steel scrap unsorted. . 500 kg, ie about 7%

Total charge ..... 6500 kg, ie 100%

Specification of feedstock:

Foundry pig iron of composition: C 4,12%, Mn 0,47%; Si 1.05%; P 0.073%; S 0.050%; Cr 0.05%;

sorted steel waste: baled waste of deep-drawing sheets;

unsorted steel waste: commercial grade under the designation cupola scrap.

Melting line:

150 kg of slag-forming additives and 150 kg of iron ore were charged under the charge.

Chemical composition test after melting:

C 2.29%; Mn 0.55%; P 0.040%.

Oxidation at low temperature follows.

After this first stage of the oxidation period, the slag was withdrawn and a new batch of 150 kg of oxidation and 150 kg of slag-forming additive was charged. After the temperature was raised, the second phase, carbon boiling, took place. After the oxidation was finished, the chemical composition test was taken - duration of the oxidation period 40 min. C 2.05%; Mn 0.12%; Si 0.12%; P 0.025%.

After the slag was pulled off, 130 kg of pitch coke was emitted. After finishing in 25 min. followed by treatment of the chemical composition with ferro-alloys, desulphurisation with desulphurization carbide slag (lime + fluorspar and pitch coke) and processing of the prepared melt by magnesium master alloy into ductile cast iron.

Final chemical composition of ductile iron produced:

C 3.43%; Mn 0.26%; Si 2.59%; P 0.032%; S 0.009%; Cr 0.02%; Ni 0.01%; Cu 0.05%; Sn 0.005%; Mg 0.062%.

Metallographic structure from block Y 2 (according to ČSN) in cast state:

fully grained graphite, ferrite content 45 to 50%.

Achieved mechanical values of produced ductile iron in block Y 2 in cast state:

yield strength - 360 MPa strength - 583 MPa elongation - 13,3%

The ductile iron produced was used for casting molds with a gross weight of about 2.51 and an increase in durability over conventional materials was achieved.

Example 2

The electric arc furnace charge consists of:

pig iron. ..... 3500 kg, ie about 54% sorted steel scrap. . . 2000 kg, ie about 31% non-sorted steel scrap. . 1000 kg, ie about 15%

Total charge ..... 6500 kg, ie about 100%

Specification of feedstock:

Foundry pig iron of the following composition:

C 4.38%; Mn 0.87%; Si 1.18%; P 0.045%; S 0.049%; Cu 0,04% sorted steel scrap: baled deep-drawn sheet metal waste unsorted steel scrap: commercial grade under the designation cupola scrap.

Melting line:

150 kg of slag-forming additives and 100 kg of iron ore were charged under the charge.

Chemical composition test after melting:

C 2.98%; Mn 0.62%; P 0.034%.

After melting, oxidation was carried out at low temperature. After this, the slag was withdrawn and a new batch of 100 kg of oxidation and 170 kg of slag-forming additive was introduced. After the temperature increase, the second phase of the oxidation period, ie carbon boiling, took place.

After completion of oxidation chemical test was taken - duration of oxidation period 45 min .:

C 2.30%; Mn 0.21%; P 0.021%.

After the slag was withdrawn, 120 kg of pitched coke was melted. After digging in 40 min. followed by treatment of the chemical composition with ferro-alloys, desulphurisation with desulphurization carbide slag (lime + fluorspar + pitch coke) and processing of the prepared melt by magnesium master alloy into ductile cast iron.

Final chemical composition of ductile iron produced:

C 3.52%; Mn 0.66%; Si 2.64%; P 0.029%; S 0.009%; Cr 0.03%; Ni 0.02%; Cu 0.03%; Sn 0.006%; Mg 0.059%.

Metallographic structure from block Y 2 (according to ČSN) in cast state:

fully grained graphite, ferrite content 45 to 50%.

Achieved mechanical values of produced ductile iron in block Y 2 in cast state:

yield strength - 379 MPa strength - 606 MPa elongation - 15,8%

The ductile iron produced was used for castings of lathe chucks, gross weight of 17 to 84 kg.

Claims (1)

SUBJECT OF THE INVENTION Process for producing a melt in an electric arc furnace as a starting material for the production of ductile cast iron with ductility above 7% in a cast state, in which the base of the metal charge comprises on the one hand 30 to 70% by weight of pig iron with a silicon content 1.5% by weight, manganese maximum 1.2% by weight, phosphorus maximum 0.1% by weight, sulfur maximum 0.1% by weight and other undesirable elements such as copper, chromium, tin, titanium, arsenic, bismuth in an amount which it does not disturb the structure of ductile iron with an elongation above 7% in the cast state, and 30 to 70% of quality and unsorted waste, of which unsorted waste is at most 15% by weight of the total charge, the iron ore as an oxidizing additive and lime as a slag-forming additive in the amount required to oxidize the undesirable elements and oxidation is carried out below the carbon boiling point. melt having a carbon content of 2 to 3% by weight, followed by an oxide Above a carbon boiling point which is terminated after the carbon content of the melt has been reduced by at least 0.2% by weight and the silicon content is below 0.2% by weight, the slag is stripped off and the carbonation, desulphurisation and ferroalloys are treated to the desired composition.