Quantitative SAXS Analysis of Precipitate Characteristics Limiting Hot Ductility in HSLA Steels Containing V, Nb & NbTi

Hot ductility behavior between 700 and 1 050°C is governed by precipitation and phase transformation, which can cause cracking and limit steel processing in other ways. Characterization of this precipitation with conventional microscopy techniques is difficult due to the limited particle size, usually occurring on the nanometer scale. Casting simulation and hot tension testing were performed using a Gleeble 3500 to generate a property profile for high-strength low alloy (HSLA) steels containing Nb, Ti, V, and N. Small-angle X-ray scattering (SAXS) was then used to characterize precipitates in Gleeble-tested samples in order to evaluate relationships between hot ductility measurements and precipitate size, spacing, and volume fraction. SAXS results showed that interparticle spacing and volume fraction were the most significant factors influencing ductility in all tested grades. Specimens with reduction-in-area measurements ranging from 3–90% exhibited a range of particle spacing data from 5–80 nm and precipitate volume fraction from 0.001–0.03%. It was observed that particle spacing of 10–20 nm and a volume fraction of 0.01% were the most detrimental to hot ductility. Significant outcomes of the current study are that precipitate density may be the most significant factor limiting ductility and that laboratory-scale SAXS measurement represents a viable method for bulk precipitate characterization.