The biggest feature of continuous casting process is forced cooling of molten steel, solidifying molten steel into billet in a short time. Therefore, the continuous casting process itself has more stringent requirements on the quality of molten steel. It must not only ensure stable and appropriate molten steel temperature and deoxidation degree to meet castability, but also minimize the sulfur and phosphorus impurities and gas and inclusion content in steel to ensure smooth casting and billet quality. In other words, ensuring timely supply of qualified molten steel is the basis and prerequisite for continuous casting production.
Continuous casting molten steel temperature control
The casting temperature of molten steel is an important process parameter for casting, and the requirements of continuous casting on molten steel temperature are much stricter than those of mold casting. This is because:
- Appropriate casting temperature is the prerequisite for smooth continuous casting. If the casting temperature of molten steel is too low during continuous casting, it will cause the water inlet of the tundish to freeze, forcing the casting terminal; if it is too high, it will not only easily cause the water inlet of the ladle to lose control, but also cause the thinning of the billetshell and uneven thickness, resulting in steel leakage.
- Appropriate casting temperature is the basis for obtaining good casting quality. Both theory and practice show that high pouring temperature will aggravate the secondary oxidation of molten steel and the erosion of molten steel on the lining refractory material, thereby increasing the number of non-metallic inclusions in the casting, and promoting the occurrence of various defects such as rhombus deformation, bulging, internal cracks, center looseness and center segregation. Low pouring temperature will easily form a cold shell at the surface of the steel liquid in the crystallizer, deteriorate the surface quality of the casting, and have an adverse effect on the purity of the steel.
Control of the composition of molten steel in continuous casting and requirements for deoxidation of molten steel
The operating characteristics of the continuous casting process are: (1) continuous operation, continuous pouring of multiple furnaces; (2) forced cooling throughout the process; (3) the molding and gradual solidification of the casting are completed in a specific movement (including certain deformation).
The control of the composition of molten steel and the requirements for deoxidation of molten steel in the continuous casting process are mainly:
- Stability of composition. Continuous casting requires that the composition of molten steel in the ladle must be uniform, so as to ensure the consistency of the composition of the billet; for molten steel cast in multiple furnaces, the national standard for continuous casting billets requires that the continuous casting of the tundish must be the same type of steel.
- Good molten steel castability. The diameter of the water inlet of the continuous casting tundish is small, especially for the small square billet continuous casting machine, which puts higher requirements on the fluidity of molten steel. The fluidity of molten steel is the actual reflection of the castability of molten steel. Under the condition of proper temperature control, the fluidity of molten steel reflects the purity of the molten steel itself. Molten steel with few inclusions and low N, H, and O gas content has better fluidity.
- Anti-crack sensitivity. During the solidification process of the continuous casting billetshell growth, it is not only affected by the thermal stress and organizational stress generated by forced cooling, but also by the combined effect of multiple stresses such as the static pressure of molten steel, the mechanical tensile stress during billet drawing, and the bending stress during straightening. Once stress concentration is caused in the weak part, it will cause defects such as surface cracks and internal cracks. Therefore, the elements in the steel that affect the tendency of hot cracks must be strictly controlled.
Applying refining technology outside the furnace to provide purified molten steel for continuous casting
With the continuous improvement of industrial technology and living standards, industries such as transportation, energy, aviation, construction, automobiles, machinery manufacturing and electronic technology have higher and higher quality requirements for steel products, and the use of steel products is becoming more and more extensive. In order to meet the needs of the market, modern steel mills need to be able to produce steel grades with ultra-low carbon, ultra-low sulfur, extremely low gas content and a very narrow composition control range.
The development of refining technology outside the furnace optimizes the metallurgical process flow, which not only provides an effective means for the production of the above-mentioned steel grades, but also reduces energy efficiency and improves labor productivity. The current mainstream optimization process with converter and electric arc furnace as the main steelmaking method is:
- Blast furnace molten iron-> molten iron pretreatment-> converter steelmaking-> molten steel secondary refining-> continuous casting-> continuous rolling;
- Scrap steel, pig iron, metallized pellets-> high power or ultra-high power electric arc furnace smelting-> eccentric furnace bottom steel tapping-> molten steel secondary refining-> continuous casting-> continuous rolling
For induction furnace steelmaking, our company Shanghai Metallurgy Equipment Group, SME Group independently developed the LOD furnace, combined with LRF, to form a combination of furnace secondary refining, forming the IF+LOD+LRF process: scrap steel, (~20%) pig iron-> induction furnace melting-> steel tapping-> LOD-> LRF-> continuous casting-> continuous rolling
This process greatly reduces the requirements for scrap steel quality in induction furnace steelmaking, enabling it to produce qualified steel billets with low carbon, low sulfur and low phosphorus, greatly improving the market competitiveness of induction furnace steel mills.
