Role of silicon in influencing strength and impact behaviour of ferrite and its likely influence at ultrafine grain size

Abstract

Plates of fine grained Nb containing steels having Si contents of 0.03, 0.1, and 0.51% were austenitised at 920°C and cooled at 40 and 7.5 K min^-1 through the γ to αtransformation temperature range. Increasing the Si level from 0.03 to 0.1%resulted in a decrease in the yield strength and an improvement in impact performance for both cooling rates. Raising the Si level further to 0.51%Si caused the strength to increase and the impact behaviour to deteriorate. The results are explained in terms of Si segregating to the ferrite boundaries, increasing the activity of C and N, and thereby displacing these atoms from the boundary regions. In this way, although Si increases σ₀in the Hall–Petch relationship the K _y value is reduced. Consequently, at fine grain size, Si additions can actually reduce the strength of steels and, at ultrafine grain sizes (1 µm), this would be expected to result in very substantial softening.