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

Abstract 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 KQ (conditional fracture toughness), J-integral (crack initiation energy) and Kee (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: Kee = 68.9MPa√m, J = 81.4 kJ/m2, YS = 923MPa, %El. = 13.6; IF steel: Kee = 72MPa√m, J = 87.7 kJ/m2, YS = 623Mpa and %El. = 19). This is ascribed to the development of homogeneously distributed submicron size (0.69μm) ferritic + martensitic structure in the microalloyed steel and submicron size (0.83μm) 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

Hot deformation characteristic and strain dependent constitutive flow stress modelling of Ti + Nb stabilized interstitial free steel

Abstract An effort has been made to establish a relation between Zener–Hollomon parameter, flow stress and dynamic recrystallization (DRX). In this context, the plastic flow behavior of Ti + Nb stabilized interstitial free (IF) steel was investigated in a temperature range of 650–1100 °C and at constant true strain rates in the range 10−3–10 s−1, to a total true strain of 0.7. The flow stress curves can be categorized into two distinct types, i.e. with/without the presence of steady-state flow following peak stress behavior. A novel constitutive model comprising the strain effect on the activation energy of DRX and other material constants has been established to predict the constitutive flow behavior of the IF steel in both α and γ phase regions, separately. Predicted flow stress seems to correlate well with the experimental data both in γ and α phase regions with a high correlation coefficient (0.982 and 0.936, respectively) and low average absolute relative error (7 and 11%, respectively) showing excellent fitting. A detailed analysis of the flow stress, activation energy of DRX and stress exponent in accord with the modelled equations suggests that dislocation glide controlled by dislocation climb is the dominant mechanism for the DRX, as confirmed by the transmission electron microscopy analysis

High cycle fatigue performance, crack growth and failure mechanisms of an ultrafine-grained Nb+Ti stabilized, low-C microalloyed steel processed by multiphase controlled rolling and forging

Abstract An effort has been made to examine the high cycle fatigue (HCF) properties including crack propagation characteristics and related fracture mechanisms of submicron-grained (SG) Nb + Ti stabilized low C steel processed through advanced multiphase-controlled rolling (MCR) and multiaxial forging (MAF). The HCF and other mechanical properties have been correlated with microstructural features characterized by light optical (LOM), transmission electron (TEM) and scanning electron microscopy (SEM), aided with electron backscatter diffraction (EBSD). TEM analysis near the fracture zones of the fatigue tested samples and corresponding fractographic analysis corroborated well in explaining the improved fatigue life of the SG steel. The fatigue strength was found to have a linear relationship with the tensile strength in both types of processed samples. The fatigue strength of the forged specimens was estimated to be nearly twice than that of the untreated annealed steel, demonstrating significantly different fracture characteristics. Intergranular fracture is found to be dominant in the rolled/forged specimens, in comparison to the transgranular fracture observed in the as-received steel. Such variances in fatigue strength and fracture characteristics have been endorsed to their microstructural constituents. Superior combinations of yield strength (YS), tensile strength (UTS), elongation (% El.) and high cycle fatigue strength (σf) (YS = 1027 MPa, %El. = 8.3%, σf = 355 MPa) were obtained in multiphase-controlled 15-cycle multiaxially forged (MAFed) specimens (processed in intercritical α+γ phase regime). An enhancement of the fatigue strength can be ascribed to the formation of evenly dispersed nano-sized fragmented cementite (Fe₃C) particles (~35 nm size) present in the SG ferritic matrix (average ~280 nm size). The fine dislocation substructures/cells together with the nano-sized Fe₃C particles could efficiently block the initiation and propagation of cracks thereby enhancing the fatigue endurance limit of the steel. Superior mechanical properties together with high fatigue resistance in the SG material render the present steels highly beneficial for high-strength structural applications