Transformation behavior of a Ti-Zr deoxidized steel: Microstructure and toughness of simulated coarse grain heat affected zone

Continuous cooling and isothermal transformation behavior of a Ti-Zr deoxidized microalloyed steel was studied and compared with that of C-Mn steel, in conjunction with the evolution of microstructure and measurement of toughness of simulated high heat input welding coarse grain heat-affected zone (CGHAZ). The study indicates that the inclusions in Ti-Zr steel were primarily composite inclusions (ZrO2·MnS) with high density of inclusions of size less than 1 μm. Polygonal ferrite, acicular ferrite, multi-directional bainite, and segmented martensite were the typical microstructures obtained with increase in cooling rate. Each of these phases was analyzed with decreasing isothermal temperature. Intragranular ferrite nucleation was significantly promoted by ZrO2·MnS inclusions. Inclusions of smaller size were more effective with decreasing temperature such that greater fraction of ferrite plates nucleated on one inclusion. Mn-depletion is the operating mechanism and precipitation of TiN was complementary. The simulated CGHAZ was characterized by interlock acicular ferrite microstructure and high impact toughness such that the dimple fracture and fine cleavage facets contributed to high impact energy.