Change in Yield Point Elongation and Yield Strength of Nb-Added Extra-Low- Carbon Steels by Temper-Rolling

This study investigates the effect of light temper rolling on the yield behavior of Nb added extra-low carbon steel sheets containing 11 ppm and 28 ppm of solute carbon. Tensile deformation analysis revealed that the yield strength increased monotonically in the 11 ppm steel, whereas the 28 ppm steel exhibited an initial decrease followed by an increase with increasing temper rolling reduction. A higher temper rolling reduction produced a larger number of plastic deformation bands with reduced strain per band, resulting in more uniform deformation in the 11 ppm steel and localized deformation in the 28 ppm steel. Yield point elongation in the 28 ppm steel disappeared once the strain in the localized deformation regions decreased to levels comparable to those of the uniformly deforming 11 ppm steel. These findings indicate that temper rolling induced plastic regions facilitate the propagation of deformation fronts during tensile testing, which is similar to that observed in dual phase steels comprising mixed hard and soft regions.