article · steel research international
This research investigates how adding 0.04 weight per cent niobium influences the hot deformation behaviour, microstructural evolution, and precipitation characteristics of a low-carbon CrNiMnMoB ultrahigh-strength steel. Compression tests conducted between 850 and 1250 degrees Celsius at varying strain rates revealed that niobium addition notably altered thermomechanical performance. Specifically, niobium increased hot flow resistance, delayed dynamic recrystallisation, and elevated deformation activation energy. The resulting processing maps showed that niobium shifts dynamic recrystallisation towards higher temperatures and lower strain rates while suppressing regions prone to flow instability. Furthermore, quantitative kinetics analysis confirmed that niobium carbide precipitation depends strongly on temperature and strain rate, with higher strain rates significantly suppressing precipitation.
Ultrahigh-strength steels require precise processing controls during hot working to prevent defects and achieve target mechanical properties. Understanding how minor microalloying additions like niobium govern recrystallisation and flow behaviour enables metallurgists to pinpoint the exact temperature and strain-rate windows required to process high-performance steels reliably.
This laboratory-stage metallurgical research offers processing maps and precipitation data relevant to primary steelmakers and rolling mills manufacturing ultrahigh-strength components. By defining conditions that avoid flow instability during hot compression, the data can inform the design of industrial hot-rolling regimes. However, because the results stem from simulator compression tests rather than full-scale manufacturing trials, further testing on industrial rolling equipment is needed before commercial use.
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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.
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DOI: 10.1002/srin.70672
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