The effects of heat treatment condition and Si distribution on order-disorder transition in high Si steels

Abstract

The decomposition and formation of ordered phases in steels with 5 to 6.5 %Si were investigated by a combined analysis of TEM and electrical resistivity measurements in order to determine the optimal heat treatment conditions for removal of ordered phases. TEM diffraction pattern study revealed that B2 ordered phase in 6.5 %Si steel was sufficiently dissolved by heat treatment at 850 degreesC for 1 h, and then rapidly re-formed during cooling. The critical cooling rate, above which the suppression of B2 phase formation was possible, increased rapidly with Si content higher than 5.6 %. D0(3) phase appeared only in the case of as-cast samples containing Si content as high as 6.5 %. The measurement of electrical resistivity change during the heat treatment showed that the atomic movement was substantially spurred above 800 degreesC, resulting in an active order-disorder transition. The removal of solidification segregation is necessary to reduce the amount of B2 ordered phase in the as-cast or hot-rolled state, to lower the heat treatment temperature for dissolution of B2 ordered phase, and to shorten the heat treatment time.