Improvement of fracture toughness of Ti+Nb stabilized microalloyed and interstitial free steels processed through single phase regime control multiaxial forging

Abstract

Objective of the current study is to enhance the mechanical properties, with a special emphasis on fracture toughness, of Ti + Nb stabilized interstitial free and microalloyed steels through microstructural modification by single-phase controlled multiaxial forging at large cumulative strains. Analysis of fracture toughness was executed through calculating KQ (conditional fracture toughness), Kee (equivalent energy fracture toughness) and J-integral (crack initiation energy) values from single-edge bend test data of the forged specimens. The effect of strain hardening rate and strain hardening exponent on deformation behavior were examined to correlate the yield strength (YS) and uniform elongation. Also, theoretically calculated YS (obtained from analysis of strengthening mechanisms) was correlated well with the experimentally obtained results. The quantitative measurement of grain size, low- and high-angle grain boundaries and their distribution in the deformed state were investigated through EBSD/TEM analysis. Superior combinations of the YS, ductility (%El.) and fracture toughness were obtained through intercritical (α+γ) phase regime (~Ar1) control 15 cycles multiaxially forged (MAFed) microalloyed steel (YS = 1027 MPa, %El. = 8.3% and Kee = 90 MPa√m) and pure α-ferritic region (<Ar1) control 18 cycles MAFed IF steel (YS = 881 MPa, %El. = 11.2% and Kee = 97 MPa√m) specimens. Enhancement of the fracture toughness is ascertained to the formation of uniformly distributed nanosize fragmented cementite (Fe3C) particles (~35 nm size) within the submicron size (~280 nm size) ferritic microstructure in the microalloyed steel; whereas in case of the IF steel, this is attributed to the formation of ultrafine ferrite grain (~320 nm) along with dense dislocation substructures. These dislocation cells and fine substructures as well as nanosize Fe3C could effectively block the crack initiation and propagation and thereby enhance the fracture toughness.