Effect of alloy treatment and coiling temperature on microstructure and bending performance of ultra-high strength strip steel

Two different high strength B-containing microalloyed steel strips produced in industrial processing conditions, one treated with Ti and the other treated with Al, processed by controlled rolling, accelerated cooling and coiling in two different temperatures ranges [723 K to 733 K (450 °C to 460 °C)] and [633 K to 653 K (360 °C to 380 °C)] were subjected to bend testing. The Ti treated steel coiled at the higher temperature 733 K (460 °C) showed the best bending performance. The relatively softer (tensile strength of and even {112} in the sub-surface region as well as uniformity of through thickness texture of the rolled sheet improve the bendability. In the presence of crack initiators, like coarse and brittle TiN particles found in the Ti treated steel, a harder microstructure and the presence of Cube and Goss texture in the sub-surface layer, seen for the lower coiling temperature can cause local transgranular cleavage cracking. Finally the post-uniform elongation obtained from tensile testing and bendability follow a good correlation

Development of bimodal grain structures and their effect on toughness in HSLA steel

To understand the formation of bimodal ferrite grain structures (i.e. mixed coarse + fine grain sizes) in TMCR HSLA steel plates, as-continuously cast HSLA steel slabs with varying microalloying additions (Ti, Nb and V) were reheated to temperatures in the range 1000-1300 °C and deformed at 1110 °C and 980 °C temperatures in Gleeble 3500. The interdendritic segregation during continuous casting and the consequent inhomogeneous microalloying precipitate distributions (with interdendritic regions being the precipitate-rich regions) has resulted in severely bimodal austenite grain structures under certain reheat conditions (due to the significantly higher pinning force in solute-rich regions compared to solute-poor regions). The segregation of microalloying elements can also promote bimodality during deformation by affecting the local recrystallisation kinetics. Notch-bend fracture tests were performed at –160 °C to investigate the effect of a bimodal grain size distribution on fracture toughness by comparing local fracture stress values for uniformly fine, uniformly coarse and bimodal ferrite grain structures. Analysis of local fracture stress values suggests that bimodality can raise the scatter in the fracture test results and therefore, it is undesirable. Current methods of measuring bimodality are not useful at consistently quantifying small differences in bimodality between microstructures of steel, and hence, two easy-to-measure parameters (peak height ratio, PHR and peak grain size range, PGSR) have been suggested in this study to quantify bimodality in HSLA steels