Development of alternative as-rolled alloys to replace quenched and tempered steels with tensile strength in the range of 600–800 MPa

Abstract

The fierce competition between steelworks and different alloy design approaches are the motivation behind the development of new microalloyed structural steels which must satisfy increasingly severe mechanical strength and toughness requirements. Other equally desirable aspects for these new materials are the suppression of heat treatments after hot rolling and better weldability, which makes this alloy evolution even more complex. As weldability improvement requires minimization of the carbon content of the steel, these new steels must present hardening mechanisms which does not require the presence of this element. Up to this moment, the most feasible answers to this challenge are steels hardened by copper precipitation, like HSLA-80 or ASTM A710, and the so-called ultra low carbon bainite (ULCB) steels. The aim of this work was to study the effects of some controlled rolling parameters over the mechanical properties of these relatively new steels. The knowledge of these effects certainly will help in the definition of optimised process conditions for these new steel alloy designs. It was verified here that the total strain applied during hot rolling and the finishing temperature were essential to improve the toughness of both alloys; the effect of the former parameter revealed to be more important. The aged HSLA-80 steel showed greater values of yield strength, but lower toughness than the as-rolled ULCB alloy. This fact is certainly due to the aging treatment that is normally applied to the first alloy.