Development of Nb bearing high strength steel for buildings by microstructural control through the on-line heating process

This paper presents and discusses the development of high strength steel plates with 780MPa in tensile strength that are suitable for building construction use. Such plates provide an excellent combination of high strength, toughness, deformability and weldability. The key technology to obtain the excellent combination in mechanical properties is the microstructural control of the martensite-austenite (M-A) constituent and the bainitic ferrite dual-phase structure through on-line heat treatment immediately after accelerated cooling during Thermo-Mechanical Controlled Processing (TMCP). The developed steel plates exhibit microstructures of fine M-A dispersed in the bainitic ferrite matrix and basic metallurgical investigations revealed that the transformation behaviour and microstructural morphologies were varied with the cooling step temperatures before the online heating and the on-line heating temperature itself. The paper also briefly discusses the trial production of the developed 780MPa grade steel plate which demonstrated satisfactory combinations of high strength, low yield ratio, toughness and weldability. (AU)