Ordering-disordering phenomena and micro-hardness characteristics of B2 phase in Fe-(5-6.5%)Si alloys

The disappearing and forming behavior of B2 ordered phase in high Si steels with Si-levels between 5 and 6.5% (by weight, unless specified otherwise) was investigated by observing TEM and measuring electrical resistivity and micro- and nano-hardness. The critical cooling rate for suppressing the formation of B2 ordered phase increased exponentially with Si-content. The ordered phase coarsening was observed to occur actively above 800 °C, where atoms have enough mobility. The removal of solidification segregation is necessary in order to reduce the amount of B2 ordered phase in the as-cast or hot-rolled state, to lower the annealing temperature for the dissolution of B2 ordered phase, and to shorten the annealing time. Micro-Vickers hardness depended only on Si-content even under various heat treatment conditions, and the nano-hardness had a small difference between B2 and A2 phases. In the high Si steels, the short-range order in A2 disordered phase was likely to cause a comparable hardness as that of B2 ordered phase.