Recovery, recrystallisation and formability of ferritic stainless steels.

The effects of additions of C, N, Mo, Ti and Nb on recovery, recrystallisation and formability have been investigated in 17% and 25% Cr steels after 35%, 50% and 75% reduction.In general additions of molybdenum retarded recovery at both chromium and interstitial levels, while additions of titanium increased the rate of recovery. Additions of niobium increased the rate of recovery at the lower interstitial level and decreased the rate of recovery at the higher interstitial level. In the majority of steels a distinct hardness plateau was observed between recovery and recrystallisation, the extent of which depends on composition degree of cold reduction and temperature, and was associated with the pinning of subgrain boundaries by precipitating particles during recovery.The end of the plateau coincided with the incubation period for recrystallisation and was interpreted in terns of the unpinning of subgrain boundaries leading to the formation of recrystallised grains.Additions of Mo, Nb and increasing the interstitial content in general retarded recrystallisation while additions of Ti and increasing the chromium had relatively little effect.A comprehensive determination of the cold worked and recrystallisation textures has been made. Alloying elements had little effect on the development of deformation textures but considerably affected the development of recrystallisation textures. Additions of niobium, titanium and molybdenum in general favoured the development of {111} recrystallisation textures because the nucleation of {111} oriented grains was favoured at particle/matrix interfaces. Swift cupping and Erichsen tests have been used as a measure of formability in these steels. The recrystallisation textures have been related to r values and LDR. Good correlations have been obtained between R values, LDR and the ratio of the I(111):I(100) texture and between r values and LDR. Both deep drawability and r value increased as the ratio of I(100):I(100) increased. The LDR was increased by increasing the degree of cold reduction, by additions of titanium and niobium at the lower interstitial level and molybdenum because of the increased the I(100):I(100)texture ratio. The Erichsen value was increased with increasing cold reduction but decreasing the interstitial content did not improve the Erichsen value in the base 17% Cr and 17% Cr molybdenum steels. A quantitative relationship has been obtained between the yield stress composition and recrystallised grain size. Additions of molybdenum and chromium increased the yield stress because of their effects on lattice dilation, while titanium additions decreased the yield stress because it removed interstitials from solid solution.