Nanostructured bainitic (NB) steels are promising materials in respect of meeting the ever-increasing demand for tough, ultrahigh strength steels for various applications, such as automotive components, rails and crossings for railways. These steels, consisting of extremely fine bainite plates at nanometre level together with a small fraction of fine film-like retained austenite (RA) have already become well-established in high carbon (C) steels during the past two decades. However, prolonged isothermal bainitizing treatments, often over several days, restrict the industrial processing of such steels on a commercial scale. Also, the possibility of retaining coarse blocky austenite with relatively low stability can result in the deterioration of ductility and toughness. 

Recently, the nanostructured bainite concept proposed in medium carbon steels has been attempted by several researchers through specific composition and process designs with the primary aim of accelerating the bainite transformation kinetics, where the fineness of bainitic ferrite plates and RA films originates from low-temperature isothermal transformation in relatively short durations. Among different methodologies for acceleration of bainitic phase transformation kinetics, an approach to refine the bainitic ferrite plates through isothermal transformation of austenite into bainite below Ms temperature has been explored in our recent study using medium C (~0.4 wt.%) steels [1]. Acceleration of transformation kinetics was realized due to the presence of a small fraction of martensite prior to bainite formation [1]. Besides, the combined effect of prior ausforming and isothermal bainitizing, both above and below Ms temperature, has also been explored. Overall, the results suggest that ausforming above Ms reduced the time for onset of bainite formation, while the effect was just the opposite below Ms, as the onset times were increased by prior ausforming. However, significant refinement of bainitic ferrite plates as well as retained austenite films was observed (Figure 1a,b) in the samples ausformed below Ms with a concomitant increase in Vickers hardness.

Based on Gleeble results, laboratory rolling simulations were investigated comprising of thermomechanical rolling, followed by low temperature ausforming and isothermal holding close to Ms temperature in order to investigate the combined effect on the bainite transformation characteristics and resultant properties. The morphology of bainite and RA in rolled samples were found to be extremely fine. Optimal processing provided an excellent combination of ultrahigh tensile strength (1600–1800 MPa), adequate ductility (~15-20 %) and good impact toughness (~20-30 J×cm1/2) in these steels (Figure 1c). The paper presents an account of the current results and highlights the future work to build a link between the ausforming conditions, microstructures and properties.

References

[1] P. Kaikkonen, S. Ghosh, M. Somani, J. Kömi, J. Mater. Res. & Tech, 2023, 23, 466-490.