Impacts of Nb on grain refinement in a simulated coarse-grained-heat-affected-zone of ultra-high-strength steels

The impact of Nb contents on grain refinement in a simulated coarse-grained heated-affected zone (CGHAZ) in steels was investigated. The austenite grain grew in multiple directions and then impinged against each other during the α→γ transformation. Subsequently, austenite grains coarsened via the coalescence of small grains and boundary migration. With the increased Nb content, the austenite in CGHAZ became coarser due to the decreased quantity and increased size of Nb precipitates. Furthermore, the austenite decomposed into bainite during the cooling process. Compared with the low-Nb specimen, the crystallographic grain size in CGHAZ of high-Nb specimen is slightly coarser, relating to the coarser prior austenite grain. However, higher Nb content delays the bainite transformation start temperature, leading to multiple variants selection of bainite, increasing the refinement ratio.