Characterization of Precipitates in Vanadium and Titanium Microalloyed Steels.

Abstract

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 (≅5 nm) which are identified as carbides or carbonitrides of the type M (C, N) (M=V and Cr, where V/Cr≅5). Some relatively coarse particles (≅0.1 μm) 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 (>5μm) were observed together with precipitates of the type MC (M=Ti and Cr) in the form of very fine precipitates (≅5 nm, Ti/Cr≅9) or relatively coarse carbide particles (≅0.1μm, Ti/Cr>30).