Effect of morphological change of carbide on elongation of boron-bearing Al-killed steel sheets

It has been reported that boron has a beneficial effect on the mechanical properties of continuously annealed Al-killed steel sheets. However, the effect of boron on elongation of boron-bearing steels had not been fully investigated. In this study, the effect of boron on elongation was investigated for boron-bearing Al-killed steel sheet by analyzing the substantial effects of fine carbides precipitating in matrix and at the grain boundaries. Using scanning electron microscope and image analyzer, an investigation was made for the precipitation site and the size distribution of fine carbides caused by the variation of excess boron and over-aging temperature. The effects of fine carbides on the tensile elongation of the steels were examined. The carbide morphology and the precipitation site changed with excess boron since the excess boron accelerated carbide precipitation in matrix by segregating at the grain boundaries. In the case where uniform elongation and post-uniform elongation were evaluated by n-value and m-value respectively, n-value decreased with increasing amount of fine carbides in matrix. Furthermore, excess boron deteriorated n-value irrespective of carbide morphology. On contrast, m-value rose with a decrease in the diameter of carbide at the grain boundary irrespective of small amount of excess boron. It was indicated that the deterioration of elongation by the small amount of excess boron was caused by a decrease in uniform elongation, which might be attributed to fine carbides in matrix and boron in solution.