CHARACTERIZATION OF PRECIPITATES IN VANADIUM AND TITANIUM MICROALLOYED STEELS

Transmission electron microscopic observations were carried out on carbon extraction replicas prepared from two commercial V-microalloyed steels and one commercial Ti-microalloyed steel hot rolled to 10 mm diameter bars at different rolling conditions. The presence of about 0.13% Cu in V- or Ti-microalloyed steels led to the formation of copper sulphides preferentially rather than manganese sulphides although the steels containing more than 1% Mn. In Ti-microalloyed steel, globular titanium carbosulphides of the type Ti4C2S2 are observed in addition to copper sulphide. In V-steels, transmission electron microscopy revealed very fine precipitates in the pro-eutectoid ferrite (congruent-to 5 nm) which are identified as carbides or carbonitrides of the type M (C, N) (M = V and Cr, where V/Cr congruent-to 5). Some relatively coarse particles (congruent-to 0.1-mu-m) were also observed (with V/Cr > 15) which are suggested to be V-nitrides or carbonitrides formed at relatively high temperatures in austenite. In Ti-steel, coarse titanium nitride particles (> 5-mu-m) were observed together with precipitates of the type MC (M = Ti and Cr) in the form of very fine precipitates (congruent-to 5 nm, Ti/Cr congruent-to 9) or relatively coarse carbide particles (congruent-to 0.1-mu-m, Ti/Cr > 30).