The evolution of precipitates in Nb-Ti microalloyed steels during solidification and post-solidification cooling

Abstract

A series of Nb-Ti steels containing titanium in the range 0.005 to 0.038 wt% and nitrogen in the range 0.005 to 0.011 wt% were prepared using a base composition of 0.06 wt% carbon and 0.027 wt% niobium. Small ingots were cooled slowly in order to simulate larger castings. Cooling was interrupted at various temperatures in the austenite phase field, either by quenching or by holding isothermally before quenching. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to investigate the morphology, distribution, composition, and particle size of carbonitrides in processed ingots. Samples of the quenched ingots were tempered for 1 hr at 600°C. Tempered hardness was used to estimate the amount of microalloy elements in solution in austenite at the moment of quenching. Ingots containing 0.013-0.017 wt% Ti were found to be undersaturated with respect to (NbxTi1-x)(CyN1-y), and Nb-rich carbonitride precipitates were present after solidification. These were stable during cooling, and supersaturation did not occur until the ingot temperature fell below approximately 1 200°C. It was found that the Nb-rich precipitates became Ti rich in the ingots quenched from 1 400°C, when either the nitrogen content was increased from 0.005 to 0.011 wt% or the titanium content was 0.038 wt%. Carbonitride precipitates were not found in ingots containing less than 0.011 wt% Ti for any quench temperature down to 1 000°C, although Nb-rich precipitates were present at 800°C and in mould-cooled ingots. Holding the ingots isothermally at temperatures down to 950°C for 3 hr brought the composition of precipitates and the concentration of solutes closer to equilibrium. It has concluded that, for the base composition and Ti content less than a critical value of 0.012 wt%, as-cast austenite was more supersaturated with microalloy precipitants than expected from equilibrium considerations. Increasing the Ti content above the critical value led to interdendritic precipitation of carbonitride that precipitated during cooling and reduced the degree of supersaturation.