Fracture toughness characteristics of ultrafine grained Nb-Ti stabilized microalloyed and interstitial free steels processed by advanced multiphase control rolling

Aim of the current study is to analyze the fracture toughness values along with other mechanical properties and correlating the microstructures of ultrafine grained (UFG) microalloyed and interstitial free (IF) steels produced through advanced 3-steps control multiphase rolling. The analysis of fracture toughness was carried out through computing K-Q (conditional fracture toughness), J-integral (crack initiation energy) and K-ee (equivalent energy fracture toughness) values from 3-point bend test data of rolled specimens. Microstructural analysis was performed through transmission electron microscopy (TEM) along with selected area electron diffraction (SAED) and Electron backscatter diffraction (EBSD). The quantitative measurement of low and high angle grain boundaries and their distribution in the deformed state were determined through EBSD analysis. The good combinations of fracture toughness, yield strength (YS) and percent elongation (%El.) (i.e. ductility) were achieved through innovative 3-phase control rolling (microalloyed steel: K-ee = 68.9MPaVm, J = 81.4 kJ/m(2), YS = 923MPa, %El. = 13.6; IF steel: K-ee = 72MPa root Vm, J = 87.7 kJ/m(2), YS = 623Mpa and %El. = 19). This is ascribed to the development of homogeneously distributed submicron size (0.69 mu m) ferritic + martensitic structure in the microalloyed steel and submicron size (0.83 mu m) ferritic grains along with high density dislocation substructure in the IF steel. These dislocation cells and substructures could effectively block the crack initiation and propagation. The development of UFG microstructure has been analyzed in the light of deformation induced ferrite transformation (DIFT) and dynamic recrystallization (DRX) mechanisms. Superior fracture toughness of the UFG steels along with better combination of mechanical properties is very demanding for high strength structural applications.