Mechanical properties and microstructure of ULCB steels affected by thermomechanical rolling, quenching and tempering

A new family of high-strength low- and medium-alloyed bainitic steels with ultra-low carbon content and copper or micro-alloy additions has been developed. An increased amount of deformation below the austenite recrystalization temperature during finish rolling of ULCB–Mn and ULCB–Ni steels has contributed to the increase in the tensile properties of YS>690 MPa and to the impact toughness being greater than 50 J at 190 K. This was attributed to the formation of dislocated lath martensite and acicular bainitic type ferrite from a homogenous austenite upon cooling in the air or water quenching. For steels having more carbon taken into solution, the tempering parameters after quenching are very important in the value of the fracture toughness stress intensity factor KC and the impact strength resistance to fracture KV. The local microsegregations of Ni, Mn and Cu at and above the AS temperature were superimposed on carbon segregation into the new austenite formed in the inter-critical range (α+γ). This newly formed austenite contributes to the retention of retained austenite islands and untransformed inter-laths when the steel is quenched or cooled down after rolling or ageing to room temperature. A marked improvement in toughness is associated with the formation of the new austenite, which gives the TRIP steel effect during deformation. For steel aged at temperatures of greater than 665°C, a second strengthening peak has been observed that is associated strongly with hardenability and transformation strengthening as well as particle strengthening caused by ε-Cu precipitation. Comparison of the mechanical properties and toughness of US and Polish grades of bainitic steels that have been produced recently has been made. Future applications of these steels might be in shipbuilding, in the constructions of offshore platforms, pressure vessels, and other high performance structural members, having as they do values of YS>650 MPa and Charpy V energy 200 J at 213 K.