Abstract
Effects of different strengthening factors such as ferrite grain refinement, solid solution element, pearlite volume fraction, and transformation micro-constituent on ductility were investigated in order to find the most desirable strengthening method for high strength steel with good ductility.
Carbon, silicon, manganese, and chromium contents were independently changed in C-Si-Mn steel. Ferrite grain sizes were varied in the range of d-1/2=410 by annealing the hot rolled or cold rolled sheets at temperatures between 600°C and 1 100°C. Strengthening by solid solution and pearlite volume fraction (carbon content) was investigated at the same grain size. Transformation micro-constituent was continuously changed from ferrite·pearlite to bainite by holding the austenitized sheets in salt bath kept between 300°C and 650°C for long enough to finish transformation. Strength and total elongation of these materials were investigated.
Ferrite grain refining was the most desirable strengthening method because it enhanced strength and elongation. Strengthening by the others were accompanied by decrease in elongation though its degree was different with each other. Solid solution strengthening by Si showed least decrease in elongation, followed by strengthenings by pearlite volume fraction and by solid solution of Mn. Strengtheningby the transformation micro-constituent was not desirable for ductility. Solid solution of Cr was not effective in strengthening.
Improvement in ductility by ferrite grain refining is due to the increase in local elongation. Decrease in ductility by the other strengthening methods is largely influenced by the decrease in uniform elongation.
