Effect of Annealing Time on Microstructure and Mechanical Properties of Cold-Rolled Niobium and Titanium Bearing Micro-alloyed Steel Strips

Effects of annealing time on microstructure of cold-rolled niobium-titanium bearing micro-alloyed steel strips were investigated by optical microscopy,scanning electron microscopy,electron back-scatter diffraction(EBSD)and transmission electron microscopy.The complete recrystallization annealing temperature of 670℃ and complete annealing time of 9 min were determined using Vickers-hardness testing and EBSD analysis.The ferrite microstructure with spheric cementite particles and nano-scale precipitates of Nb(C,N)in matrix was obtained.The kinetics of the ferrite grain growth is lowered due to ferrite grain boundaries pinned by the cementite particles,so the ferrite grain size of 5.5μm remains unchanged among the annealing time ranging from 9 to 30 min.In addition,the strength of tested steel also keeps unchanged with the increase of annealing time.The higher yield strength of approximately 420MPa can be obtained by grain refinement and precipitation hardening and the higher elongation of approximately 40%and work-hardening exponent of approximately 0.2 can be gained due to grain refinement and presence of cementite particles,indicating that the balance of strength,ductility and forming property is realized.