Microanalysis of TiNb carbonitrides in HSLA steels

The microchemistry and morphology of Ti- and Nb-rich carbonitrides formed during continuous casting in some HSLA steels, differing only in their Nb content, have been investigated in order to characterize the effect of Nb on the nucleation and growth process of these particles. Precipitates were extracted using the carbon extraction replica technique and analyzed in a VG-HB 5 STEM by EDS. They exhibit large composition gradients and their morphology is roughly correlated to their size. A model based on the formalism of ternary solubility products in liquid iron and in austenite is developed to explain satisfactorily these composition gradients, and gives the composition of newly formed nuclei in austenite and that of the last deposited layer of precipitates. Nuclei appear as spherical particles, grow by interface instability to polyhedral particles, then to dendrites, with secondary protrusions developed from the primary arms also as a result of an interface instability process.