Enhancement of quality of steel sheet through process optimisation

Abstract

In spite of threat from aluminium, composites and plastics, aluminium killed steel (AKS) sheets are mainstay in automotive sector because they possess excellent formability and unique combination of strength and toughness besides being available at economic cost. By judicious design of chemistry and adopting suitable thermomechanical control processing (TMCP) higher r values (r being a measure of formability) can be achieved in these steels. Improvements in secondary steelmaking processes have made possible achievement of the most suitable chemistry and cleanliness in these steels. TMCP includes hot deformation, cold deformation and annealing. The amount of deformation during high temperature rolling in the single phase g region followed by high temperature finish rolling and finally low temperature coiling develop both uniform ferrite grains of particular size and hot rolled crystallographic texture and hot band structure, are indicative of the quality of the hot strip products. Higher ¯r value in the cold rolled and annealed sheets depends on the quality of the hot rolled product. The amount of cold deformation of the hot rolled strip and the two steps short time protective atmosphere annealing allow the AIN precipitation to occur prior to recrystallisation. This enhances the intensity of (III) texture and the formation of typical microstructure (pancake type). These result in through thickness texture strengthening and higher ¯r value in the thermomechanically processed AKS sheets, to meet the demand of automotive industry with respect to deep and extra deep drawing properties.