Effect of vanadium and other microalloying elements on the microstructure and properties of bainitic HSLA steels, The

Abstract

Modern high-strength low-alloy (HSLA) steels, used extensively in commercial products such as high-strength linepipe and structural construction components, are coiled at temperatures ranging from 400 – 600 °C. The microstructures produced are bainitic, often do not contain iron carbides, and contain microalloying additions such as vanadium (V). This study attempted to characterize the evolution of vanadium-containing precipitates in low-carbon, HSLA bainitic steels. Furthermore, the effect of varying nitrogen (N) concentration on precipitation was considered. The resulting microstructural characterization led to a more fundamental understanding of the role of microalloying elements during processing, especially with respect to vanadium, in temperature ranges corresponding with hot-rolled strip coiling. A series of HSLA steels containing 0.06 wt pct carbon was considered: a base composition with no microalloying additions and multiple steels with variable microalloy content. Two steels contained only a nominal V microalloying addition of 0.06 wt pct and 75 or 150 ppm N and another steel contained multiple microalloying elements with nominal 0.06 wt pct V, 0.04 wt pct niobium (Nb), and 0.01 wt pct titanium (Ti). Initial characterization and processing of the investigated steels included prior austenite grain analysis, flow stress comparisons during simulated thermomechanical controlled processing (TMCP), and thermodynamic simulations. Hardness increases of approximately 20 – 30 HV in an extensive time-temperature study were then correlated with precipitation after reaustenitizing, rapid cooling to produce bainite, and holding at 550 °C in the vanadium-containing steels. Quantitative dark-field transmission electron microscopy (TEM) was employed to characterize disk-shaped precipitates approximately 2 – 4 nm in diameter and 1.0 – 1.5 nm in thickness with a rock-salt crystal structure. The volume fraction of precipitates reached approximately 10-3 after holding for 5 h at 550 °C. An Avrami analysis was also applied to the measured volume fraction changes. This analysis correlated with TEM observations indicating growth of the precipitate phase through increases in diameter with minimal change in thickness. Compositional analysis of the precipitates via 3D atom probe (3DAP) analysis indicated the presence of both vanadium and chromium (Cr) in the precipitate phase. Furthermore, only nitrogen-containing compositions were found and no carbonitride compositions. A V-Cr solubility model was considered to show that a mixed nitride phase would be predicted. Additionally, a cross-correlative TEM and 3DAP analysis confirmed that a secondary precipitate phase and not solute clusters were present at the times and temperatures investigated. The importance of soluble nitrogen on the precipitation characteristics was highlighted by the 3DAP compositional analyses and multiple comparative experiments. Reductions in volume fraction of precipitates in bainitic ferrite at 550 °C were observed in both the vanadium containing steel containing lower nitrogen content (75 ppm) as well as in a V-Nb-Ti microalloyed steel in which nitrogen in solution was reduced from precipitation of an additional Nb-rich phase during high-temperature processing. The differences in measured volume fractions were confirmed by calculations utilizing differences in nitrogen content. Additionally, a reduction of available vanadium for precipitation in ferrite was confirmed by compositional analysis via energy dispersive spectroscopy in the V-Nb-Ti steel due to formation of the additional phase at high temperatures. Precipitation during simulated TMCP, which included high-temperature deformation and an accelerated cooling step to the coiling temperature, was also investigated in one vanadium-containing steel. No differences were found in precipitate morphologies, volume fractions, and number densities after a simulated coiling hold at 550 °C when compared with a processing route involving only heat treatments. Furthermore, quantitative analysis with scanning transmission electron microscopy revealed no measurable differences in dislocation densities in the bainitic ferrite between the two processing routes investigated (i.e. with and without high-temperature deformation). A recovery model was considered that accounted for interactions between precipitates and dislocations and indicated a delay in recovery for specific time ranges at 550 °C. At coiling temperatures below approximately 500 °C, experimental evidence did not indicate the presence of vanadium-containing precipitates, thus indicating that vanadium would not delay recovery in the form of a precipitate phase at lower coiling temperatures. Overall, the results highlighted the efficacy of vanadium as a nitride former which can prevent recovery during the first few hours of coiling at temperatures around 550 °C in industrial strip production.