Morphology Control for AlSubscript2/SubscriptO

Authors: Shengping He Gujun Chen Yintao Guo Boyi Shen Qian Wang

Publish Date: 2015/01/06

Volume: 46, Issue: 2, Pages: 585-594

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Abstract

Nozzle blockage is a major problem during continuous casting of Alcontaining steel Herein we analyzed the thermodynamic equilibrium behavior between aluminum and oxygen in steel at 1873 K 1600 °C and demonstrated that the dissolved O initially decreases with increasing the dissolved Al until approximately 01 wt pct Al and after that the dissolved O increases with dissolved Al Thus for highaluminum steel with 10 wt pct dissolved Al the precipitation of Al2O3 inclusion can be avoided during cooling from deoxidation temperature to the liquidus temperature if the actual dissolved O can be kept from increasing when the dissolved Al further increases from 01 to 10 wt pct Hence a method of inclusion control for highaluminum steel without traditional Ca treatment technology was proposed based on the thermodynamic analysis Industrial tests confirmed that lowmelting point Caaluminate inclusions were observed typically through a slag washing with SiO2minimized highbasicity slag during tapping accompanied by twostep Aladding process for production of highaluminum steel Moreover there was no nozzle clogging occurred for five heats of continuous castingAluminum 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 interrupted1 2 3 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 accidents56 To resolve this calcium treatment is widely used for Al2O3 inclusions modification in conventional Alcontaining steel Al = 002 to 006 wt pct to transform solid Al2O3 inclusions into lowmelting point Caaluminates which are less harmful to steel properties and increase the castability of steel by minimizing and ideally eliminating nozzle clogging7 8 9 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 Caaluminates and the wt pct Ca/wt pct Al ratio should be controlled at ~01711 12 13 14 15 However for many highaluminum steels such as 38CrMoAl transformation induced plasticity steel TRIP and twinning induced plasticity steel TWIP the dissolved Al is above 05 wt pct indicating that the dissolved Ca would need to be 005 wt pct which may exceed the solubility of Ca in the molten steel816 and thus it may be not feasible to completely transform Al2O3 inclusions to liquid Caaluminates for highaluminum 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 717 18 19 20 21 Hence the values of wt pct S3 × wt pct Al2 must be controlled below ~09 × 10−9 to 56 × 10−9 in order to get liquid Caaluminates rather than solid CaS at casting temperature 1823 K 1550 °C1922 23 24 which implies a maximum allowable S of ~97 to 178 ppm for a steel containing 10 wt pct Al Clearly it is nearly impossible to obtain liquid Caaluminates and avoid the formation of solid CaS by calcium treatment for a steel containing 10 wt pct AlIn 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 alternate process for improving the castability of highaluminum steel 38CrMoAl without calcium treatment The proposed process incorporates slag washing with SiO2minimized highbasicity slag during tapping accompanied by twostep Aladding process and appropriate topslag refining process to control the shape of Al2O3 inclusions and thereby create favorable conditions to satisfy the requirement of castability for production of highaluminum steel in the absence of calcium treatment

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