Electric arc furnace steelmaking can be divided into five major stages: the collection of raw materials, the preparation before smelting, the melting period, the oxidation period the reduction period.
Collection of raw materials
Scrap steel is the main material for electric arc furnace steelmaking. The quality of scrap steel directly affects the quality, cost productivity of graphite electrodes. Therefore, the following requirements are imposed on scrap steel:
(1) The surface of the scrap steel should be clean less rust, because the sand other debris in the scrap will reduce the electrical conductivity of the charge, extend the melting time, also affect the dephosphorization effect during the oxidation period corrode the furnace lining. When scrap steel is severely corroded stained with oil, it will reduce the yield of steel alloying elements increase the hydrogen content in the steel.
(2) Lead, tin, arsenic, zinc, copper other non-ferrous metals shall be mixed in the scrap steel. Lead has a high density, a low melting point, is insoluble in molten steel. It is easy to deposit in the cracks at the bottom of the furnace cause a leak. Tin, arsenic copper can easily cause hot brittleness of steel.
(3) Sealed containers, flammable, explosive toxic substances shall be mixed with scrap steel to ensure safe production.
(4) The chemical composition of scrap steel should be clear, the content of sulfur phosphorus should be too high.
(5) The overall dimensions of the scrap steel should be too large (the cross-sectional area should exceed 150mm×150mm, the length should exceed 350mm). In electric arc furnace steelmaking, pig iron is generally used to increase the carbon content of the charge, the amount usually does exceed 30% of the charge.
Preparation before smelting
Ingredients are an indispensable part of the graphite electrode steelmaking process. Whether the ingredients are reasonable is related to whether the steelmaker can perform the smelting operation normally in accordance with the process requirements. Reasonable ingredients can shorten the smelting time. Attention should be paid to the ingredients: First, the ingredients must be calculated correctly the amount of charge must be accurately weighed. The second is that the size of the charge should be matched in proportion to achieve the purpose of good installation quick conversion. The third is that all kinds of charge should be used in combination with steel quality requirements smelting methods. Fourth, the ingredients must meet the process requirements.
In the electric arc furnace steelmaking process, the melting period is called the melting period the beginning of the electrification to the complete melting of the charge. The melting period accounts for about half of the entire smelting time, power consumption accounts for about 2/3 of the total power consumption.
The task of the melting period is to quickly melt the furnace charge with the least power consumption while ensuring the life of the furnace body, make the slag in the burning period, so as to stabilize the arc, prevent gas absorption remove phosphorus in advance.
(1) Starting arc stage. The furnace is filled with furnace material when the arc is energized, the arc is very close to the furnace top. If the input power is too large, the furnace roof is easily burned out if the voltage is too high. Therefore, the intermediate voltage about 2/3 of the rated power of the input transformer are generally selected.
(2) The stage of well penetration. At this stage, the arc is completely surrounded by the charge, almost all of the heat is absorbed by the charge, the furnace lining will be burnt. Generally, the penetration time is about 20 minutes, which accounts for about 1/4 of the total melting time.
(3) The rising stage of the graphite electrode. After the graphite electrode “crosses the well” to the end, a molten pool has been formed at the bottom of the furnace. The lime some elements at the bottom of the furnace are oxidized, causing a layer of slag to form on the surface of the molten steel. The surrounding charge continues to be melted by radiant heat, the molten steel increases. As the liquid level rises, the graphite electrode gradually rises. At this stage, high-power transmission of electric energy is still used, which takes about 1/2 of the total melting time.
(4) Melt the last class. After the charge is melted more than 3/4, the arc can no longer be shielded by the charge, the high temperature area under the three graphite electrodes has been connected into one piece. At this time, if high-power power is used for a long time, the arc will strongly damage the furnace cover furnace wall.
The main task of the melting period is to melt the charge, but in the melting period it is only to make slag, but also an important operation content of the melting period. If only to meet the requirements of covering the molten steel stable electric arc, only 1% to 1.5% of the slag volume is required. It is enough, but considering the requirements of dephosphorization, the molten slag must have a certain degree of oxidation, alkalinity slag amount.