The effect of Si on the microstructural evolution of discontinuously cooled strip steels

The effects of silicon (0.16 and 1.4 wt%) and thermomechanical processing schedules on the transformation history and final microstructure of a low carbon, Nb microalloyed steels were investigated. Two initial austenite conditions were produced in the steels: recrystallised and nonrecrystallised. Complex microstructures consisting of ∼50% ferrite, granular bainite and/or acicular ferrite were obtained using discontinuous cooling schedules. The bainite transformation temperatures were ∼50-70°C higher in the steel with low Si content, regardless of the thermomechanical processing schedule. Both the Vickers hardness and the microhardness of second phase were higher in the steel with the higher Si content due to a solution strengthening and a finer microstructure. Increased silicon content promotes formation of carbide free phases: polygonal ferrite and acicular ferrite.