EVOLUTION OF ACICULAR FERRITIC MICROSTRUCTURE IN A TITANIUM BEARING HSLA STEEL

An attempt has been made to design and develop acicular ferritic steels by adopting three-stage controlled rolling, with titanium alone as microadditive. Process parameters such as finish rolling temperature (FRT), amount of deformation during the last pass and also the cooling rate after finishing have been altered to obtain different microstructural effects. Optical as well as transmission electron microscopy studies indicated that acicular ferritic microstructure can be engendered by applying a minimum deformation of 25 % at the finishing pass followed by water quenching. Decreasing the FRT from 850-degrees-C, for water quenched steels deformed 33 %, resulted in the refinement of polygonal ferrite, whereas the reverse is the case with the aspect ratio of acicular ferrite. The volume fraction of acicular ferrite progressively increases as the FRT is lowered. Evaluation of tensile properties suggested that yield strength as high as approximately 600 MPa with an elongation of approximately 25 %, could be obtained by finishing at a lower temperature of 750-degrees-C. An impact transition temperature (ITT of -53-degrees-C indicated that the steel possesses adequate toughness properties. As weldability is one of the criteria for structural applications, this aspect too has been investigated.