Investigation on the crystallography of reverted structure and its effect on the properties of low carbon steel

A new type of reverted structure was found to form in upper bainite steel with 0.06% C, 1.5wt% Cu and 2.0wt% Ni through annealing at the temperature between A1 and A3 (two-phase region heat treatment) with slow cooling. Tensile and charpy tests showed that the sample with reverted structure exhibited high improvement in −20°C impact toughness and slight decrease in yield strength. Microstructure observed in detail by TEM suggests that Cu precipitates are helpful to keep the yield strength level. By using SEM and EBSD techniques, two conclusions were gained from the analysis focusing on morphology and crystallography. (I) Together with toughness improvement, reverted structure is formed and dispersed along the prior bainite lath boundaries, apparently increases the density of high angle boundary, especially the boundary above 45° which can effectively stop the propagation of micro-cracks and hence inhibit the initiation of brittle fracture. Thus increasing the density of high angle boundary by generating this new type of reverted structure could be considered as another available option for improving low temperature impact toughness in addition to generating high amount of retained austenite film. (II) Reverted structure and its nearby parallel un-reverted matrix compose “reverted packet”, within which orientations from all the reverted structure and the matrix closely follow the orientations’ distribution of the crystallographic packet described in coherent transformation. Due to this particular orientations’ distribution, high angle boundaries forming in this “reverted packet” are effective in refining the effective grain size for trans-granular cleavage fracture but relatively ineffective in increasing yield strength, thereby theoretically beneficial to decrease the DBTT.