Indian Institute of Technology Kharagpur Comparison of Globularisation Behaviour during Reheating Treatment of Different Pearlitic Microstructures

Pearlitic steels present very interesting mechanical properties when weldability requirements are not too severe. Thin gauge pearlitic steels require at least one intermediate annealing treatment between hot rolling and cold rolling. The globularised cementite in medium carbon steels provides then higher ductility, formability and machinability. On the other hand, in-line induction heating devices are now well known as powerful tools to reheat quickly and efficiently steel flat products. Such a reheating device induces pearlite globularisation after transformation on the run out table, and hence the conventional intermediate annealing treatment presently performed could be avoided. This work deals with the microstructural and mechanical changes that occur due to the reheating of different pearlite ferrite microstructures simulating the conditions of an induction heating device. INTRODUCTION Pearlitic steels present very interesting mechanical properties when weldability requirements are not too severe. Thin gauge pearlitic steels require at least one intermediate annealing treatment between hot rolling and cold rolling. The globularised cementite in medium carbon steels provides then higher ductility, formability and machinability. On the other hand, in-line induction heating devices are now well known as powerful tools to reheat quickly and efficiently steel flat products. Such a reheating device induces pearlite globularisation after transformation on the run out table, and hence the conventional intermediate annealing treatment presently performed could be avoided. This work deals with the microstructural and mechanical changes that occur due to the reheating of different pearlite ferrite microstructures simulating the conditions of an induction heating device. EXPERIMENTAL In order to study the influence of initial microstructure on pearlite globularisation, different thermomechanical and thermal treatments were applied to a 0.44 C, 0.73%Mn 0.18%Si weight per cent steel. The pearlitic steel was manufactured using an usual austenitic hot rolling process with a finishing temperature of 850 oC. Seven passes were performed with a total reduction of 86%. The steel was cooled down to 650 oC at 35 oC/s and then held in a furnace for 2 minutes in order to ensure a thermal homogeneity and the full transformation of the pearlite. After holding, the steel was air cooled. This sample was labeled as PER1. On the other hand, two different microstructures