ABSTRACT

A Nb-Ti-B microalloyed steel with a lean chemical composition was designed in order to achieve high strength thin hot rolled strips (Yield Strength > 700 MPa), exploiting the peculiar thermo-mechanical process by Arvedi I.S.P. technology. Results of metallurgical modelling, laboratory tests, pilot trials and industrial trials were used to identify the optimum conditions to obtain microstructures with the desired mechanical properties directly from the hot strip mill. The lean chemical composition was designed to decrease the flow stress of austenite and, at the same time, to ensure enough hardenability to produce a fully bainitic microstructure. Suitable set-up of laminar cooling sections along the run out table was established by a mathematical model describing simultaneously the strip temperature evolution and austenite phase transformation in order to develop the proper bainitic microstructure. Industrial trials were performed in order to validate the prediction capability of the model and to assess the microstructure and the relevant mechanical properties with given steel and process conditions.Optimum hot rolling and cooling conditions to produce hot-rolled high strength steel (Yield Strength > 700 MPa) were identified for strips with thickness in the range 2.0 mm-4.0 mm.