Morphology Control for Al 2 O 3 Inclusion Without Ca Treatment in High-Aluminum Steel
- PDF / 1,131,518 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 36 Downloads / 133 Views
INTRODUCTION
ALUMINUM is usually used for melt deoxidation in the secondary refining of steel owing to its high deoxidation efficiency. Nozzle clogging can occur because of the adherence of solid Al2O3 inclusions, formed during deoxidation, to the inner wall of the immersion nozzle. This nozzle clogging can disturb the mold flow, which affects the bloom surface quality, and in extreme cases, the casting sequence can be interrupted.[1–4] Al2O3 inclusions in liquid steel can also be absorbed by mold fluxes during continuous casting. Thus, the practical flux may differ significantly from the predetermined flux in terms of its chemical composition and properties. Performance may become worse, ultimately blocking the continuous casting and even cause production accidents.[5,6] To resolve this, calcium treatment is widely used for Al2O3 inclusions modification in conventional Al-containing steel ([Al] = 0.02 to 0.06 wt pct) to transform solid Al2O3 inclusions into low-melting point Ca-aluminates, which are less harmful to steel properties and increase the castability of steel by minimizing, and ideally eliminating nozzle clogging.[7–10] Numerous theoretical calculations and practical studies have indicated that the required dissolved [Ca] should be proportional to the dissolved [Al] in steel to obtain liquid Ca-aluminates, and the [wt pct Ca]/[wt pct Al]
SHENGPING HE, Associate Professor, GUJUN CHEN and YINTAO GUO, Ph.D Candidates, BOYI SHEN, Master Candidate, and QIAN WANG, Professor, are with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China. Contact e-mail: [email protected] Manuscript submitted March 1, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
ratio should be controlled at ~0.1.[7,11–15] However, for many high-aluminum steels such as 38CrMoAl, transformation induced plasticity steel (TRIP), and twinning induced plasticity steel (TWIP), the dissolved [Al] is above 0.5 wt pct, indicating that the dissolved [Ca] would need to be 0.05 wt pct, which may exceed the solubility of [Ca] in the molten steel,[8,16] and thus, it may be not feasible to completely transform Al2O3 inclusions to liquid Ca-aluminates for high-aluminum steels by calcium treatment. Besides, during the calcium treatment, higher dissolved [Ca] may also react with dissolved [S] to form solid CaS, which may also cause serious nozzle blockage during casting.[7,17–21] Hence, the values of [wt pct S]3 9 [wt pct Al]2 must be controlled below ~0.9 9 109 to 5.6 9 109 in order to get liquid Ca-aluminates rather than solid CaS at casting temperature [1823 K (1550 C)],[19,22–24] which implies a maximum allowable [S] of ~9.7 to 17.8 ppm for a steel containing 1.0 wt pct [Al]. Clearly, it is nearly impossible to obtain liquid Ca-aluminates and avoid the formation of solid CaS by calcium treatment for a steel containing 1.0 wt pct [Al]. In this study, the thermodynamics between aluminum and oxygen in liquid steel as well as slag and steel equilibrium was discussed, and industrial trials were performed to develop an
Data Loading...
