Converter Steelmaking
Converter steelmaking is a steelmaking process that uses hot metal, scrap steel, and ferroalloys as the main raw materials. It does not rely on external energy sources but instead utilizes the physical heat of the molten iron itself and the heat generated by chemical reactions between the components of the iron liquid to complete the steelmaking process in the converter. Converters are classified by refractory lining as acid or basic; by the location of gas blowing into the furnace as top-blown, bottom-blown, or side-blown; and by the type of gas used as air converters or oxygen converters. Basic oxygen top-blown and combined top-bottom blown converters are the most widely used steelmaking equipment due to their fast production speed, large output, high single-furnace yield, low cost, and low investment. Converters are primarily used for producing carbon steel, alloy steel, and for smelting copper and nickel.
Converter Steelmaking Raw Materials
The raw materials for converter steelmaking are divided into metallic materials, non-metallic materials, and gases. Metallic materials include hot metal, scrap steel, and ferroalloys. Non-metallic materials include slag-forming agents, fluxes, and coolants. Gases include oxygen, nitrogen, argon, carbon dioxide, etc. Non-metallic materials are added during the converter steelmaking process to remove impurities such as phosphorus and sulfur and to control the process temperature. They mainly include slag-forming agents (lime, dolomite), fluxes (fluorspar, mill scale), coolants (iron ore, limestone, scrap steel), carburizers, and fuels (coke, graphite, coal, heavy oil).
Converter Steelmaking Product Quality
Varieties and Quality of Basic Oxygen Top-Blown Converter Steel
Gases and inclusions in steel are the main indicators for evaluating the metallurgical quality of steel. In basic oxygen top-blown converter steelmaking, the reaction rate is fast and the boiling is intense, so the H, N, and O contents in the steel are relatively low: [H] is (3–5) × 10⁻⁴%, [N] is (20–40) × 10⁻⁴%, and [O] in low-carbon steel is 0.06%–0.10%. Inclusions are related to deoxidation and solidification operations. An important factor affecting the nitrogen content of top-blown converter steel is the purity of oxygen, as can be seen from the data in Table 4. Therefore, the oxygen used for converter steelmaking should be more than 99% pure oxygen.
Low-carbon steel is the main product of converter steelmaking. Because the converter decarburizes quickly and the gas content in the steel is low, the steel has good plasticity and low-temperature plasticity, as well as excellent deep drawing and welding properties. Converter steel used to produce hot-rolled sheets, cold-rolled sheets, galvanized sheets, automotive sheets, cold-formed sections, low-carbon soft wire rods, etc., all exhibit good performance.
Although there are some difficulties in smelting medium- and high-carbon steel in converters, the quality of the steel can still be ensured. Various structural steels, bearing steels, hard wire rods, etc., made from converter steel have been widely used. The difficulties in smelting high-carbon steel are carbon control and dephosphorization. When C > 0.2%, it is difficult to control carbon accurately by experience. If a sublance is available, it can be used for control; without a sublance, rapid analysis at the furnace front is required, which causes delays. The end-point (FeO) in high-carbon steel is low, and the dephosphorization time is short. Therefore, a double-slag operation is required, i.e., tapping the initial slag at the beginning of the decarburization period to remove the phosphorus that has entered the slag in the early stage. However, the double-slag operation loses a large amount of heat and iron in the slag and should not be used unless absolutely necessary. The carburizing method is another operation method for smelting medium- and high-carbon steel. In this case, the blowing operation is the same as for low-carbon steel, but carburizer is added in the ladle to increase the carbon content to the required level. The carburizers are coke, petroleum coke, etc. The small amount of carburization for medium-carbon steel is easy to accomplish. For high-carbon steel, carburization must be well controlled, but the quality of rail steel, hard wire rods, etc., can meet the requirements when smelted by the carburizing method.
There are no special difficulties in smelting low-alloy steel in converters. When smelting alloy steel, a large amount of ferroalloys needs to be added to the ladle for alloying, which lowers the steel temperature. Increasing the tapping temperature excessively adversely affects dephosphorization. Therefore, smelting alloy steel should be combined with secondary refining, using a ladle furnace to complete alloying. In addition, as the requirements for controlling the composition of steel become increasingly stringent, in order to reduce fluctuations in steel properties, the narrower the composition range, the better. This also requires fine-tuning of alloy composition during ladle refining.
There are still difficulties in smelting ultra-low-carbon steel (C < 0.03%) in top-blown converters. First, because below the critical carbon content, the decarburization rate decreases and the bath stirring weakens. Intensifying oxygen supply only promotes iron oxidation rather than carbon removal. Second, [%C] × [%O] = 0.0025. When [%C] = 0.01%, [%O] = 0.25%, which is already the saturated concentration of [O]. That is to say, 0.01% C is the theoretical limit of decarburization. To further decarburize, the partial pressure of CO in the gas phase must be reduced, which requires the use of secondary refining methods.
