Development of 780MPa Grade Steel Plates with Excellent Deformability by Microstructural Control through the On-Line Heating Process

摘要

High strength steel plates with 780MPa in tensile strength, which were suitable for building construction use taking seismic design into account, have been developed. The steel plates provide excellent combination of high strength, toughness, deformability, resistance to fracture in earthquake and weldability. The key technology to obtain the excellent combination in mechanical properties of the steel is the microstructural control of M-A (martensite-austenite constituent) and the bainitic ferrite dual-phase structure, through the on-line heat treatment immediately after the accelerated cooling in TMCP (Thermo-mechanical controlled processing). The developed steel plates have microstructure of fine M-A particles dispersed in the bainitic ferrite matrix. Basic metallurgical research revealed that microstructural morphologies were varied with the cooling stop temperature before the on-line heating, and the on-line heating temperature itself. Trial production of the developed 780MPa grade steel plates was carried out with the actual plate mill. The obtained plates showed the satisfactory combination of high strength, low yield ratio, toughness and weldability.