Corner crack control for thin slab continuous casting of microalloyed steel

Abstract

Thin slab continuous casting and rolling process is an important way to produce hot-rolled strips. Recently, the process has been widely used to produce Nb/V/Ti/B bearing microalloyed steel. However, during the continuous casting of the thin slabs of the microalloyed steel, there are frequent cracks on the corners of the slabs, which would cause quality defects, such as scars and cracks at the edges of the hot-rolled coils. These defects are a common technical issue in the steel industry. In this paper, the characteristics of the microstructure and carbonitride precipitation of the thin slab corner of QStE380TM low carbon niobium–titanium bearing microalloyed steel, as well as the reduction of area of the steel under different cooling and tensile rates, were detected. Moreover, the evolutions of the temperature of the solidified shell in different structure molds and secondary cooling processes, as well as the stress of the thin slab surface during liquid core reduction, were numerically simulated. The results show that there is a significant third brittle temperature zone during continuous casting of microalloyed steel thin slabs, and the greater the deformation rate of the thin slab, the more significant the third brittle temperature zone is. Under the conventional thin slab continuous casting process, the cooling rate of the thin slab corners in the upper part of the mold and the secondary cooling zone from the mold exit to the liquid core reduction segment is lower than 5 °C·s−1, which is the key factor to lead a chain of niobium–titanium carbonitrides precipitate at the grain boundaries of the corners. As a result, the plasticity of the thin slab corners is greatly reduced. During the process of liquid core reduction, the low plasticity corners of the thin slab crack because of large deformation and stress. Applying the Gaussian concave curved surface mold, which the narrow face copper plates could efficiently compensate the shell shrinkage, the narrow face-foot roll zone hard cooling process can increase the cooling rates of the thin slab corners over 10 and 20 °C·s−1 in the mold and the narrow face-foot roller cooling zone, respectively. As a result, the carbonitrides precipitate in the thin slab corners disperses, and the stress of the thin slab corners reduces since the new mold promotes the metal flow of slab narrow surface broadsiding during the liquid core reduction. Finally, the cracking rate of the thin slab corners during the microalloyed steel thin slab casting has been reduced significantly.

Keywords

• microalloyed steel
• thin slab continuous casting
• corner cracks
• mold
• liquid core reduction
• hard cooling