THE STEELMAKING AND CASTING OF Ti STABILIZED STAINLESS STEELS

摘要

1. Careful control of the process chemistry at each stage of the steelmaking process has demonstrated improvements in the castability and final product quality of Ti stabilized grades, as evidenced in improved sequence ratios, and the reduction in grinding losses and incidence of metallurgical defects respectively. 2. Understanding the interaction of Ti with O and N in the steel, and Ti-oxides or nitrides with slags and fluxes is an important prerequisite to decreasing inclusion formation and minimizing surface defects. 3. Properties such as interfacial tension between steel and slags/fluxes and steel and inclusions, play an important role in determining the slag-steel reactions, inclusion removal and surface defects, and need to be considered when choosing slag and flux compositions. The effects of slag chemistry, viscosity and interfacial tension on the absorption of inclusions have been explained in detail. 4. Solubility of TiOx is limited by the solubility of CaO.TiO_2 in ladle slags and this has been determined to be upto 15% as TiO_2 from plant studies, experimental work and by modeling. 5. Effective control of steel temperature, tapping procedure, gas stirring, raw material quality and the time and quantity of Ti additions, is necessary to minimize and control primary Ti loss and the generation of oxides and nitrides. 6. The TiO_2 absorption capacity of the tundish slags has been determined and found to be adequately modeled using the ladle slag solubility prediction. Changes in the CaO/SiO_2 and Al_2O_3/SiO_2 ratios, together with the initial slag chemistry have been shown to be important in ensuring the maximum TiO_2 absorption in the tundish. 7. The solubility of TiOx in mould fluxes has been described and modeled. Good predictions are possible for mould fluxes having a CaO/SiO_2 ratio <1. 8. Inclusion modeling has proved a useful tool for determining what solubility products precipitate and at what temperature. The use of models to predict and control the formation of solubility product inclusions has been and will remain a key tool in further improving steel quality. Using such tools it is possible to manipulate the precipitation of inclusions at different stages of the steelmaking, by control of steel and slag/flux chemistry. 9. Despite the current level of understanding of Ti interactions, more work on Ti speciation and solution thermodynamics remains to be done.