Inter-critical Annealing of a Lean Composition Steel under Controlled Cooling to Produce Multiphase Microstructure

Abstract

In the past years, there has been growth in the search and use of new advanced materials in the transport industry. So far, conventional steel is the main material for car bodies, but there is a growing demand for other materials to decrease the weight of cars and thereby saving costs, energy and environment. Also, the increasing safety requirements in the automotive industry has forced search for new materials. The different parts need to be as light as possible, but with sufficient strength and ductility. Hereby, multiphase steels like dual phase and TRIP (Transformed Induced Plasticity) steels are important because of their high strength in combination with good formability. Various methods have been used for the production of these steels to get best outcomes. However, extremely limited work has been reported for the production of dual/ multiphase steels with controlled cooling and using a lean chemistry. The present experimental work reports on the methods of production of dual/ multiphase steels (tensile strength range 500–800 MPa; ductility in the range 12–33%) with ferrite/ martensite, ferrite/ bainite/ martensite and ferrite/ martensite/ austenite/ bainite multiphase structures. These structures were obtained in a normalized steel of very lean chemistry (0.11C, 1.8Mn, and 0.325 Si) subjected to inter-critical annealing and soaking followed by controlled cooling in an annealing simulator. Dual phase structures were produced by inter-critical annealing followed by direct cooling to room temperature to achieve high strengths. Multiphase microstructures were produced by holding in the bainitic and martensitic range to get various combinations of strength and ductility (multi-functional). The present work also compares the results for annealing process obtained through experimental investigations and through software predictions. Finally, micro-mechanical modeling has been done for steel with dual phase structure The true stress- strain curves obtained through actual tensile experiments closely matched with the true stress- strain curves predicted by micro-mechanical modelling.