Subsurface Porosity in Continuously Cast Slabs
The importance of porosity in continuously cast slabs is often underestimated and should not be overlooked. Porosity can act as a stress accumulation zone, render the outer coating of the slab prone to failure, and cause defects such as cracks. Subsurface porosity can be categorized into two general types: shrinkage and gas porosity. Shrinkage occurs due to volumetric changes during solidification and cooling, while gas porosity results from entrapped gas during casting.
Shrinkage Porosity
Shrinkage porosity is the most common type of subsurface porosity found in continuously cast slabs. This type of porosity occurs due to differences between the densities of liquid steel and the solidified steel. During solidification, liquid steel contracts and solidifies, causing a decrease in volume. This difference in volumes generates macro-scale voids in the shape of flakes or dendrites. These voids are mostly confined within the near-surface regions, forming a skin-like layer at the top and bottom of the slab (Fig. 1). Additionally, heterogeneity in casting parameters and the flow of the liquid steel will produce a variety of smaller micro-voids or micro-pockets of shrinkage porosity.
Fig. 1. Schematic representation of the micro-structure of shrinkage porosity in a slab
The size, shape and principal orientation of shrinkage porosity depend mainly on the casting speed, solidification rate and cooling rate, among other factors. Fast casting speed and slow solidifications tend to produce large and curved macro-voids due to the high pressure of liquid steel in the mold. As the cooling rate is reduced, the size of the macro-voids increases, while their number and orientation remain the same.
Gas Porosity
Gas porosity is secondary type of porosity in continuously cast slabs, which is caused by entrapped gas during the casting process. Entrapped gas can be generated by a number of sources, including gas bubbles generated by dissolution reactions between liquid steel and the refractory material, gas released from mold lubricants and contamination from the atmosphere, among other sources. In addition, gas bubbles can also be created due to coalescence of small molten droplets and from desorption of hydrogen from the slag during solidification.
Gas porosity is often found as oblong-shaped cavities located near the outer surface of the slabs in the two-phase regions of solid and liquid (Fig. 2). Due to the differences in densities between liquid steel and the solidified steel, the entrapped gas rises towards the surface of the slabs, forming a network of interconnected pores on the surface. The presence of gas porosity in slabs can lead to the formation of macro-scale cracks or defects, particularly when subjected to thermal stresses.
Fig. 2. Schematic representation of gas porosity in a slab
In order to minimize the effect of porosity on the quality of the continuously cast slabs, it is important to use the appropriate casting parameters and optimize the refractory lining of the caster. Additionally, higher casting speeds should be avoided and the cooling rate should be carefully adjusted depending on the particular slab specifications.
