Investigation of the mechanisms of gas bubbles/inclusions entrapment in the solidified steel shell

A major problem related to the production of low and ultra-low carbon steel is the formation of blister and sliver defects appearing in the final product after the cold rolling and annealing operations. These defects originate during the solidification in and below the mould where both non-metallic inclusions and gas bubbles can be entrapped in the solidifying shell. The major aims of this project were to get a better understanding of this entrapment mechanism according to different casting parameters and to more deeply analyse the role of process parameters on the occurrence of blisters and sliver defects on the final products. The cold trapping mode' (IRSID) and the hot entrapment simulator (CRM) were developed in order to simulate the trapping of argon bubbles in the solidifying shell. The major role of the tilting angle of the copper plate on the argon bubble trapping was shown. Extensive work was also performed (Corus, CRM, IRSID) in order to vizualise the primary solidification structure and solidification hooks of low and ultra-low carbon steel and to find a possible relationship with the argon bubble entrapment. The dendritic structure was found for LC, ELC and higher alloyed ULC steels. It appeared that the detected small argon bubbles were entrapped in the interdendritic area and were surrounded by a primary solidification area. Mathematical modelling was also performed to calculate the steel flow velocity near the solidification front and to predict the entrapment of argon bubbles and inclusions in the solidifying shell (IRSID, Corus). Finally, extensive work was carried out by TKS in order to study the role of process parameters on the occurrence of blisters and sliver defects on the final products. A large number of investigations were performed on slabs, hot rolled and cold rolled products, and after annealing.