Optimisation of the mechanical properties of air cooled bainitic steel components through tailoring of the transformation kinetics

The present investigation concerns high performance mechanical components such as crankshafts, injection rails, or many other parts which are manufactured using specially designed steel grades. These steel grades are such that they produce a mostly bainitic microstructure when cooled from a hot-forming process and without the need for heat-treatment. The relationship between cooling conditions (i.e. product dimensions), mechanical properties and bainite transformation kinetics is investigated on laboratory and industrial materials. It is shown that fast transformation kinetics are best to ensure insensitivity to cooling conditions. Material design based on this principles allowed the manufacture of a steel grade that exhibited a constant UTS of approximately 1200 MPa for bars as small as 16 mm up to over 160 mm.