NbC and TiN precipitation in continuously cast microalloyed steels

Abstract

With high yield strength, toughness and good weldability, microalloyed steels are widely used in the automotive, pipeline and transportation industries. Microalloying elements such as niobium (Nb), titanium (Ti) and vanadium (V) in concentrations of less than 0.1 wt. pct. are typical. For optimal benefits in the final product, it is usually desired for Ti to form fine precipitates during and after solidification and for Nb to be in solution prior to hot-rolling. Vanadium precipitates at lower temperatures and is less involved in the solidification/casting process. In one aspect of the investigation, the effects of cooling rate on the titanium nitride (TiN) precipitation size distribution were investigated in a Ti-added low-carbon steel. Prior research reported an inverse relationship between the average TiN precipitation size and the post-solidification cooling rate and the present work was undertaken to examine this behavior over a wider range of cooling rates. Using the Gleeble® 3500's casting simulation capabilities along with controlled cooling rates, the TiN precipitation behavior in thick-slab, thin-slab and thin-strip material was simulated using a commercially produced 0.04C, 1.23Mn steel with near-stoichiometric Ti and N levels. Transmission electron microscopy (TEM) investigation of carbon extraction replicas was carried out to characterize the influence of cooling rates on precipitate size distributions. Decreasing particle sizes with increasing cooling rates were found. Average particle sizes as low as 6.7 nm were present in thin-strip simulations and might be of interest, as fine particles could contribute to strengthening of rapidly cooled steels. In a second aspect of the investigation, niobium carbide (NbC) precipitation during the compact strip production (CSP) process was investigated in two Nb-added low-carbon steels. Instead of industrial sampling, the Gleeble® was used for casting simulations using two CMn(Nb) steels with high and low-Nb contents (0.046 wt. pct. and 0.014 wt. pct.). TEM investigation of carbon extraction replicas and inductively coupled plasma mass spectrometry (ICP-MS) measurements of electrochemically-dissolved precipitates were performed to analyze the NbC precipitation behavior at the slab centerline, columnar region and surface in the as-cast and prior-to-rolling condition (following tunnel furnace equalization). In the centerline and columnar slab regions of the as-cast condition of both steels no precipitation was found. In the slab surface, temperature fluctuations (related to intermittent water spray cooling) enhance the formation of finer particles and increase the percentages of precipitation. An increase of Nb concentration generally led to larger average particle sizes and larger percentages of precipitation. After an initial coarsening, particles dissolved in the tunnel furnace, and the extent of dissolution increased at lower Nb contents.