The process methods of basic EAF steelmaking can be divided into oxidation method, non-oxidation method (also known as charging method), and return oxygen blowing method.
1. Characteristics of oxidation smelting
The oxidation smelting operation consists of six stages: furnace filling, charging, melting period, oxidation period, reduction period, and steel tapping. Its characteristics are that there is an oxidation period, and ore or oxygen blowing is used for dephosphorization and decarburization. Boiling the molten pool can reduce the gas in the steel and discharge inclusions. After deoxidation reduction and adjustment of the chemical composition and temperature of the molten steel, the steel is tapped. This method of smelting can obtain steel with low phosphorus content and low gas and inclusion content, and cheap scrap steel raw materials can also be used. Therefore, most general steel grades are smelted by oxidation. The disadvantage is that if there is alloy return material in the charge, some of the alloy elements will be oxidized and lost in the slag. The process of smelting using oxidation method is as follows.
① Furnace filling
After steel tapping, the damaged parts of the furnace body need to be repaired quickly to ensure the normal smelting of the next furnace of steel.
②Charging
Before charging, add lime about 2% of the weight of the charge into the furnace. Then raise the electrode, lift the furnace cover, open the furnace body or lift the furnace cover and unscrew it, and lift the charge (charge tank) with a crane to load the charge into the furnace at one time.
③Melting period
The stage from power on to the complete melting of the charge is the melting period. Its main task is to quickly melt all the charge, and it is required to remove part of the phosphorus to ensure full melting of carbon and temperature. In order to accelerate the melting of the charge and save electricity, oxygen blowing is generally used to assist melting during the melting period. In addition, during the melting period, it is necessary to replace the water ring, connect and release the electrode, block the steel outlet, change the slag bucket, etc., prepare the necessary tools and make various preparations.
④Oxidation period
After the charge is completely melted, take samples for analysis, and enter the oxidation period after temperature measurement. The main tasks of this stage are to minimize the phosphorus content in the molten steel; remove gases (nitrogen, hydrogen) and inclusions in the steel; accurately control the carbon at the end of the oxidation period, and heat the molten steel to a temperature greater than or equal to the steel tapping temperature. In order to complete the above tasks, lime and ore must be added to the furnace, and oxygen blowing and slag flow operations must be performed. When the oxidation period is over, the slag must be removed.
⑤ Reduction period
After the power is turned off and the oxidized slag is removed, new slag is made with lime and fluorite, and the reduction period begins. The main tasks of the reduction period are deoxidation, removal of sulfur in steel, and adjustment of the chemical composition and temperature of the molten steel. At the beginning of the reduction period, high-power power is used to melt the slag, and ferromanganese and ferroalloys required for smelting steel are added to the molten pool. Carbon powder and ferrosilicon powder are evenly added to the slag surface in batches to make the slag white, and the white slag is maintained. The material is taken from the white slag for analysis, the composition and temperature are adjusted, and the steel is deoxidized by inserting aluminum.
⑥ Steel tapping
The refined molten steel is poured into the steel barrel from the steel tapping port. When tapping, the slag and steel are required to be mixed and violently mixed to further purify the molten steel for pouring. When operating electric furnace steelmaking, in addition to controlling the chemical composition of steel, special attention should be paid to the adjustment of smelting temperature and slag. Because temperature runs through the entire smelting process, it determines the chemical reaction rate at each stage of steelmaking and is a key factor in ensuring the quality of steel, shortening smelting time, and reducing production costs. The temperature can be controlled by adjusting the power supply voltage and current.
The slag composition is adjusted according to the needs of each stage of smelting. Adding more lime can increase the alkalinity and viscosity of the slag, and adding fluorite can improve the fluidity of the slag. According to the process needs, part of the slag can be removed or scraped off to re-slag. Adjusting the slag composition is related to the chemical reaction rate at the slag-steel interface and directly affects the quality of steel. Generally speaking, steelmaking is actually slag smelting.
2. Characteristics of non-oxidation smelting method
The non-oxidation method has no oxidation period during the smelting process. It can fully recover the alloy elements in the raw materials, so a large amount of alloy steel cutting heads, cutting tails, waste ingots, excess steel and pouring channels can be added to the charge to reduce the amount of ferroalloys and reduce the production cost of steel. After the charge is melted, the steel liquid composition and temperature are adjusted by reduction, and then the steel can be tapped. Because there is no oxidation period and the smelting time is short, low alloy steel, stainless steel and high-speed tool steel can all be smelted by this method. Its disadvantage is that the dephosphorization, impurity removal and degassing are not as effective as the oxidation method. The requirements for the charge are high, and clean, rust-free and low-phosphorus steel materials must be added. All measures must be taken to prevent air inhalation during the smelting process. Since the chemical composition of the molten steel basically depends on the ingredients, the chemical composition and weighing of the ingredients must be accurate. For the above reasons, alloy steel is not smelted by this method, and it is generally used for the smelting of low alloy steel. The output of smelting by the non-oxidation method accounts for a small proportion of the total output of electric furnace steelmaking.
3. Characteristics of return oxygen blowing smelting method
The return oxygen blowing method is to add a large amount of alloy steel return material to the charge. According to the theory that the affinity of carbon and oxygen is greater than the affinity of certain alloy elements and oxygen under certain temperature conditions, when the molten steel rises to a certain temperature, oxygen is blown into the molten steel to strengthen the smelting method, so as to achieve the purpose of decarburization, degassing and removing inclusions while recovering a large amount of alloy elements. This can reduce costs and improve quality. The return oxygen blowing method is often used in the smelting of high alloys such as stainless steel and high-speed tool steel. Because if these high alloy steels are smelted by oxidation, the amount of ferroalloys must be increased during the reduction period, especially stainless steel must add low-carbon ferroalloys, which not only increases the cost but also makes the reduction period operation extremely difficult. The return oxygen blowing method ideally solves the smelting problem of high alloy steel.