Quantitative analysis of mixed niobium-titanium carbonitride solubility in HSLA steels based on atom probe tomography and electrical resistivity measurements

Solubility of mixed niobium-titanium carbonitrides in commercially relevant High-Strength Low-Alloy (HSLA) steel was investigated by combined use of electrical resistivity measurements and APT after interrupted quenching from soaking temperatures between 950 and 1250 °C. Increasing electrical resistivity of the bulk material towards higher soaking temperatures was proportional to the nominal niobium addition which was varied between 0.002 and 0.022–0.043–0.085 wt.-%. Correlative APT analysis of the solutes in the steel matrix showed good agreement with electrical resistivity. Investigating numerous precipitate particles, APT also derived a precise composition for mixed niobium-titanium-carbonitrides which constitute the steel microstructure after casting/before soaking. The scavenging of microalloy elements by insoluble titanium nitrides was corrected by means of combined APT analysis of such precipitate and a quantitative image analysis for the estimation of the total volume fraction of titanium nitrides. For the first time, solute and precipitate composition together were used for solubility calculations of such mixed carbonitrides to derive an experimental solubility product. This was compared to solubility products of well-established simple carbides and nitrides and theoretical calculations of the solubility of multicomponent carbonitrides. The large discrepancy between experimentally derived and modelled solubility emphasizes the necessity of a robust methodology for the quantification of microalloy precipitation in HSLA steels.