High aluminum transformation-induced plasticity (TRIP) steels offer a unique combination of high tensile strength and ductility, high impact energy absorption and good formability. The surface of the slab is prone to depressions and longitudinal cracks during continuous casting due to the high Al content in steels. Surface depressions of the 1.35 wt.% Al-TRIP steel slab in a steel works were investigated by scanning electronic microscopy (SEM) and mold fluxes with different Al2O3/SiO2 ratios were researched by thermodynamic calculations and high-temperature static balance experiments. The results show that some micro-cracks were distributed along the grain boundary in the surface depression of the slab. Inclusions containing K and Na, which were probably from mold flux, were found in the depression samples. Meanwhile, the components of reactive mold flux showed significant variation in their chemical composition during the continuous casting process of the Al-TRIP steel. A large number of depressions and irregular oscillation marks on the Al-TRIP steel slab surface were generated due to serious deterioration in the physical properties of the mold flux. Since the TRIP steel is a typical hypo-peritectic steel, the overly large thermal contraction and volume contraction during initial solidification is the intrinsic cause of surface depression. The change of mold flux properties during casting aggravates the formation of depressions.
A water model of slab mold was established to analyze the flow field behaviors. Particle image velocimetry measurement, fluctuation sensors, and industrial experiments were adopted to analyze the slag entrainment and inclusion adsorption. The risk of surface slag entrainment was discussed from the perspectives of flow patterns, surface fluctuation, and velocity. The ability of mold slag to adsorb inclusions was discussed by electrolysis on slab samples. The results reflected that the flow field determined the fluctuated amplitude, surface pattern and flow velocity of the mold surface, playing an important role in the slag entrainment and inclusion adsorption. Uneven surface fluctuations and downward vertical velocity lead to high slag entrapping risk. High surface horizontal velocity in mold leads to a higher inclusion concentration gradient at the slagsteel interface, which leads to a stronger ability of mold slag to adsorb inclusions.
Particle image velocimetry (PIV) system was adopted to investigate the relationship between the mixing phenomenon and the flow field of a 210 t RH degasser by a 1:4 scale water model. The results of mixing simulation experiments indicated that the mixing time decreased with the increase of gas blowing rate. However, with the increase of Snorkel immersion depth (SID), the mixing time presented a decreasing rend firstly and then increased. The measurement of flow fields of RH ladle by PIV system can explain the phenomenon above. According to the characteristics of the flow field in RH ladle, the flow field can be divided into the mixing layer, the transition layer, and the inactive layer. On the one hand, the stirring power in RH ladle and vacuum chamber both increases with more gas blowing rate, leading to the decrease of mixing time. On the other hand, with SID increases from 400 mm to 480 mm, the gas blowing depth increase results in the mixing power increases, and the mixing time decreases at the beginning. Because of too much-molten steel in the vacuum chamber and the expanding of the inactive layer in RH ladle, however, the utilization rate of the gas driving force begins to decrease. Therefore, the mixing time started to increases with the increase of SID.
The reaction between [Al] in molten steel and (SiO2) in the liquid slag layer was one of the restrictive factors in the quality control for high Al-TRIP steel continuous casting. In this work, the composition and property variations of two slags during a slag-steel reaction were analyzed. Accordingly, the crystalline morphologies of slag were discussed and the solid layer lubrication performance was evaluated by Jackson α factors. In addition, a simple kinetics equilibrium model was established to analyze the factors which affected SiO2 consumption. The results reflected that slag-steel reacted rapidly in the first 20 minutes, resulting in the variation of viscosity and the melting temperature of slags. The slag-steel reaction also affected the crystal morphology significantly. Slag was precipitated as crystals with a higher melting temperature, a higher Jackson α factor, and a rougher boundary with the consumption of SiO2 and the generation of Al2O3. In other words, although generated Al2O3 acted as a network modifier to decrease the viscosity of the liquid slag layer adjacent slab shell, the consumption of SiO2 led to the deterioration of the lubrication performance in the solid slag layer adjacent copper, which was detrimental to the quality control for high Al-TRIP steel. Finally, a kinetics equilibrium model indicated that it is possible to reduce a slag-steel reaction by adjusting factors, such as the diffusion coefficient k, cSiO2, ρf and Lf, during the actual continuous casting process.
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