Contribution of grain boundary misorientation to intragranular globular austenite reversion and resultant in grain refinement in a high-strength low-alloy steel

This paper elucidates the determining role of grain boundary types/misorientations on formation of intragranular globular austenite, which has random orientation or large deviation from the parent austenite, in a high-strength low-alloy bainitic steel during reaustenitization. Results revealed that within the parent austenite with lath bainite, reverted austenite with random orientation or large deviation from the parent austenite can nucleate at the packet and block boundaries with high misorientations, and rapidly evolve to become globular austenite with increase of temperature, making a significant contribution in final austenite grain refinement. While in granular bainite, the low misoriented boundaries belonging to the same Bain group are the preferred nucleation sites for the reverted austenite, which is detrimental to the formation of intragranular globular austenite. In addition, the precipitation mode of cementite is different in the reverse process, that is, cementite is precipitated from the matrix of lath bainite, but it is precipitated from retained austenite or martensite/austenite (M/A) particles in granular bainite, which will also lead to the different or consistent variant orientation of reverted austenite and matrix. Therefore, the final austenite grain refinement in initial granular bainite was mainly due to the formation of intergranular globular reverted austenite, and the refining effect is much lower than that of lath bainite.