Heterogeneous Precipitation on Nb Microalloyed Steels: Thermodynamic Analysis and TEM Observations

A. MANNUCCI (EXPOSITOR); A. OTERO; A.H.COPPOLA OWEN

Shanghai, China

Conferencia; Second International Conference on Advanced Structural Steels (ICASS 2004); 2004

High strength low alloy steels (HSLA) are steels containing small amounts, singly or in combination, of such elements as vanadium, titanium, niobium (called microalloying elements, MA), which present chemical affinity to carbon and nitrogen. These MA elements form carbides and nitrides, which effectively make the austenite grain size to decrease, refining the products formed during the decomposition of austenite. Grain refinement is an essential component of HSLA steels because it is the only strengthening mechanism making simultaneously impact transition temperature (ITT) to decrease and strength to increase. Three steels with different Nb content and almost similar remaining composition were cast ad hoc in the continuous casting machines in Dalmine Steel works and the influence that Nb addition has on both carbonitride and aluminium nitride precipitation evolution was predicted and discussed by utilizing thermodynamic calculations and ThermoCalc software. As a result, a very accentuated transition from almost nitride to almost carbide equilibrium composition of carbonitrides was found to take place in a short range of temperatures, suggesting that precipitation could take place in a heterogeneous way during cooling. Experimental observations with TEM microscopy and SAEDP analysis has also been carried out in order to corroborate such heterogeneous precipitation, and two clearly different types of carbonitrides were found for Nb steels: a richTi-richN type, that forms at higher temperatures, and a richV-richC type, that forms at lower temperatures in the austenite range.