Engineering microstructures in high-strength low-alloy steels via advanced thermo-mechanical processing is a promising approach to overcome challenges around low work hardenability and toughness in ultrafine-grained mild steels. Recently, multiscale-hierarchical microstructures with ultrafine grains and two populations of precipitates decorating high-angle grain boundaries and dislocation structures were achieved by some of the current authors in a modern Ti-Mo-Nb high-strength low-alloy steel. However, the high-strain rate of 10 s−1 during single-pass plane-strain compression at 600 °C led to the formation of macroscopic shear bands. Here, we propose an optimized advanced multi-hit thermo-mechanical process for achieving homogenous hierarchical microstructures in the same steel without strain localization. This is verified via microscopy and thermo-kinetic modelling, using the software MatCalc. A typical body-centred-cubic rolling texture is achieved in contrast to previous process design. Ultrafine crystallites confined by a mixture of high-angle gain and subgrain boundaries are formed, decorated by two types of precipitates. Large FeMnC-rich cementite particles are found on grain boundaries and smaller TiNbC-rich precipitates on dislocations and subgrain boundaries. It is shown that TiNbC particles transform to a core-shell structure when subjected to direct aging. Thermo-kinetic modelling underpins experimental results concerning the detailed evolution of crystallite size, precipitate morphology and composition, enabling a through-process description of the microstructural evolution.
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This research has received funding by the Australian Research Council DECRA scheme (Project Number DE180100440, DECRA S. Primig) and by the UNSW Sydney Scientia Fellowship scheme. The authors thank Drs Simon Hager and Charlie Kong for technical assistance and use of facilities supported by Microscopy Australia at the Electron Microscope Unit at UNSW Sydney. Dr David Miskovic’s help with carrying out the Gleeble experiments is gratefully acknowledged. The steel used in this study was supplied by voestalpine Stahl Linz GmbH (Austria).
CL designed the study, carried out all experiments and modelling except TEM, and drafted the manuscript. RW carried out TEM investigations and related data analyses. EK guided modelling and helped to revise the manuscript. SP supervised CL, helped to design the study, revised the manuscript and wrote parts of it. All authors approved the final version of the manuscript.
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Manuscript submitted June 21, 2019.
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Ledermueller, C., Kozeschnik, E., Webster, R.F. et al. Advanced Thermo-mechanical Process for Homogenous Hierarchical Microstructures in HSLA Steels. Metall Mater Trans A 50, 5800–5815 (2019). https://doi.org/10.1007/s11661-019-05486-5