Strain‐Induced NbC Precipitation as a Governing Mechanism for Hot Deformation Resistance in CrNiMnMoB Ultrahigh‐Strength Steels

Two low‐C CrNiMnMoB ultrahigh‐strength steels, one Nb‐free (0Nb) and one microalloyed with 0.04 wt.% Nb (4Nb), were developed to investigate the role of Nb in hot deformation behaviour, strain‐induced precipitation, and microstructural evolution. Isothermal single‐hit compression tests were conducted over 850–1250 °C and strain rates of 0.01–50 s −1 using a Gleeble 3800 simulator, complemented by Thermo‐Calc predictions and EBSD/TEM analyses. Nb addition significantly modified the thermomechanical response by increasing flow resistance, delaying dynamic recrystallization (DRX), and elevating the deformation activation energy. Processing maps revealed that Nb shifted DRX domains toward higher temperatures and lower strain rates while suppressing regions of flow instability. Quantitative precipitation‐kinetics analysis demonstrated that the solubility of Nb in austenite is negligible at 850 °C (≈0.00049 wt.%) and increases progressively with temperature, reaching full dissolution (≈100% of total Nb, 0.04 wt.%) at 1250 °C. Increasing strain rate significantly suppressed strain‐induced NbC precipitation, reducing the normalized precipitation fraction ( X p ) from nearly 50% at 0.01 s −1 to below ~10% at 1 s −1 and 3%–5% at 50 s −1 . These results confirm that NbC precipitation is strongly temperature‐ and strain‐rate‐dependent and governs hot deformation resistance in Nb‐microalloyed CrNiMnMoB steels.