The composition of molten steel should first meet the requirements of steel grade specifications. However, molten steel that meets the specification composition may not be completely suitable for continuous casting. For example, the sulfur content of ordinary carbon steel mold casting molten steel is less than or equal to 0.050% to meet the specification requirements, while the sulfur content of ordinary carbon steel continuous casting molten steel is required to be less than or equal to 0.030% or lower.
Therefore, the composition of continuous casting molten steel must be strictly controlled according to the requirements of the continuous casting process and the quality of the billet. Its main control principles:
- Composition stability: The composition of molten steel must be controlled within a narrow range to ensure the relative stability of the composition of molten steel in each furnace during multi-furnace continuous casting, and ensure uniform and consistent performance of the billet.
- Crack sensitivity: The continuous casting billet is not only subject to the static pressure of molten steel and mechanical stress during straightening, but once stress concentration occurs in the weak parts, it will cause surface cracks or internal crack defects. Therefore, it is necessary to strictly control the elements in the steel that affect the tendency of hot cracks, that is, avoid the crack sensitive areas of each component; reduce the content of elements harmful to cracks, and add a third element to eliminate the influence of harmful elements.
- Castability of molten steel: For example, when casting silicon-killed steel in small square billets, in order to prevent water plugging and subcutaneous bubbles, the acid-soluble aluminum in the steel should be controlled to [AlS] ≤ 0.006%.
Control of carbon, silicon and manganese content
Carbon is the basic element in steel and has the greatest impact on the structure and performance. Especially for steel that needs to be used in a heat-treated state, its influence is more prominent. Therefore, the carbon content of molten steel must be accurately controlled. When multiple furnaces are continuously cast, the difference in carbon content of molten steel between each furnace and batch is required to be less than 0.02%.
The silicon and manganese content of continuous casting molten steel is controlled within a narrow range to ensure the relative stability of silicon and manganese content in continuous casting furnaces. For example, a factory stipulates that the difference in manganese content of two furnaces of continuous casting is not more than 0.15%. If the difference in manganese content is greater than 0.15%, the connecting billet is judged to be unqualified.
Strictly control the content of C, Si, Mn, etc., and the composition fine-tuning measures of refining outside the furnace must be taken to meet the requirements. In electric arc furnace steelmaking, since it has a relatively strong ability to reduce carbon, sulfur and phosphorus, it only needs to use Ladle Refining Furnace (LRF) to assist in adjusting the composition; in medium frequency furnace steelmaking, since its own refining capacity is almost non-existent, it is necessary to add our company’s independently developed LOD furnace, and use IF+LOD+LRF steelmaking process to play a role in reducing carbon.
Control of phosphorus and sulfur content
The sulfur content of continuous casting molten steel is one of the most important components affecting the continuous casting process and the quality of the billet. Production experience shows that when the sulfur content in steel exceeds 0.020%, the hot cracks on the surface of the slab increase significantly. The S and P contents should be controlled according to the lower limit according to the product quality requirements, such as strict billet control (P+S) ≤0.040% for quality requirements. For this reason, the molten steel supplied to continuous casting must undergo molten iron pre-desulfurization and refining outside the furnace to obtain pure molten steel.
Control of residual element content
The elements that are not intentionally added to the steel composition but brought in by the raw materials and cannot be removed during the steelmaking and refining process are called residual elements. Such as Cu, Sn, As and Sb.
These residual elements are brought in by scrap steel in electric furnace steelmaking. Since the chemical stability of Cu, Sn, Sb and As is stronger than that of iron, they will not be removed during refining. Surface cracks will be caused during continuous casting and hot rolling.
Measures to control residual elements:
- Select scrap steel, and dilute the charge with scrap steel of higher purity for scrap steel with too high residual element content. A particularly effective measure is to add sponge iron;
- On the premise that the product can meet the standard, add an appropriate amount of Ni to the molten steel to offset the harmful effects of copper;
- The most effective method is to control the solidification structure of the surface of the billetand the appropriate surface temperature of the billet, avoid thermal stress and reduce the oxidation of the surface of the billetto reduce the enrichment of copper. The retention time of the billet at high temperature in the heating furnace should be as short as possible, and it is best to use continuous casting-continuous rolling.
