A. Basic tasks of steelmaking
The basic tasks of steelmaking can be summarized as: “four removals” (decarburization, oxygen, phosphorus, and sulfur), “two removals” (degassing and inclusion removal), and “two adjustments” (adjustment of composition and temperature). That is, decarburization, dephosphorization, desulfurization, deoxidation, removal of harmful gases and non-metallic inclusions, an increase of temperature, and adjustment of composition.
1 – Decarburization and adjust its content to a certain range. Carbon content is the most important element to control the performance of steel. The increase of carbon content in steel will increase the hardness, strength, and brittleness, while the ductility will decrease; on the contrary, the decrease of carbon content will decrease the hardness and strength and increase the ductility. Therefore, the steelmaking process must oxidize carbon to a certain range according to the steel specification.
2 – Dephosphorization and desulfurization. For most steel types, P and S are harmful impurities. P can cause “cold brittleness” of steel, and S can cause “hot brittleness” of steel. Therefore, it is required to remove them as much as possible in the steelmaking process.
3 – Deoxidation. In the process of oxidation steelmaking, a large amount of oxygen is input into the molten pool to oxidize impurities, so that a certain amount of oxygen is dissolved in molten steel, which will greatly affect the quality of steel. Therefore, it is necessary to reduce the oxygen content in the steel.
4 – Remove the gas and non-metallic inclusions. Gas in steel mainly refers to hydrogen and nitrogen dissolved in steel, which will cause hydrogen embrittlement and aging of steel respectively. Non-metallic inclusions include oxides, sulfides, phosphides, nitrides, and their complex compounds, which will destroy the continuity of steel and reduce its mechanical properties of steel. Therefore, the task of steelmaking is to remove gas and inclusions.
5 – Adjust the composition of molten steel. In order to ensure the various physical and chemical properties of the steel, in addition to controlling the carbon content of molten steel and reducing the impurity content, an appropriate amount of alloying elements should be added to make the content reach the steel specification range to improve the performance of the steel.
6 – Adjust the temperature of molten steel. In order to complete the above tasks and ensure the smooth pouring of the molten steel, the molten steel must be heated and kept within a certain high-temperature range, and at the same time, the temperature of the molten steel must be adjusted to a suitable tapping temperature range according to the requirements of the smelting process.
B. Classification of converter steelmaking Converters can be divided into basic converters and acid converters according to the properties of lining refractory materials, air and oxygen converters according to the type of oxidizing gas supplied, and top-blown, bottom-blown, side-blown, and compound-blown converters according to the gas supply parts. The heat sources are divided into self-heating and external fuel-heating converters. At present, the main converter steelmaking methods in the world are the oxygen top-blown converter steelmaking method, oxygen bottom-blown converter steelmaking method, and top-bottom composite blowing converter steelmaking method. In my country, the LD method and composite blowing method are mainly used.