Abstract: Nanostructured bainitic steels consisting of extremely fine bainite plates (<50 nm) together with fine interlath films of retained austenite have become well established in high carbon steels due to their remarkable mechanical properties. However, the necessity of prolonged isothermal holding and poor weldability restrict the industrial applications of such steels. Recently, nanostructured bainite concept has been extended to low/medium carbon steels by several researchers through specific composition and process designs with the primary aim to accelerate the bainitic transformation kinetics. The fineness of bainitic ferrite plates and retained austenite films originates from low-temperature isothermal transformation and provides an excellent combination of ultrahigh tensile strength (1700–2400 MPa), adequate ductility (up to 30 %) and good fracture toughness (30–50 MPa.m^1/2).

Among different methodologies for acceleration of bainitic phase transformation kinetics, one approach to greatly refine the bainitic ferrite plates is to apply isothermal transformation of austenite to bainite below Ms in steels, which has been explored with various carbon contents. Earlier studies suggest that the transformation rate will increase when some martensite is introduced in the microstructure prior to bainite formation. Besides, the combined effects of ausforming and transformation below Ms in low and medium carbon steels have been reported recently. However, strong misorientation gradients developed by ausforming below Ms actually delayed the onset of austenite to bainite transformation and even hindered the growth of bainite, since the misorientation gradients faded the orientation relationship between austenite and new bainite.

In the present study, a series of physical simulation experiments were carried out in a Gleeble 3800 ® thermomechanical simulator in order to systematically investigate the characteristics of nanostructured bainite formed in a 0.4 wt.% C steel (Ms= ≈280°C) via isothermal holding above and below Ms following ausforming. A series of experiments combining ausforming (~ 0.5 true strain) and isothermal holding at the same temperature were conducted at five different temperatures in the range 235 - 325°C. Dilatation measurements were carefully recorded on all the specimens during the course of isothermal holding as well as cooling stages, both prior to and after the holding, to discern the phase transformation behaviour of the steels. Vickers hardness was measured on all the specimens following Gleeble simulations and corroborated with the microstructures examined and analyzed by scanning and transmission electron microscopy, electron backscatter diffraction and X-ray diffraction.

As expected, the results suggest that ausforming above Ms reduced the time for onset of bainite transformation, whereas the effect was just the opposite below Ms with the onset times delayed by introducing prior ausforming. However, significant refinement of bainitic ferrite plates and retained austenite films was observed in the ausformed samples below Ms, besides noticeable increase in hardness. Whereas the formation of nanostructured bainite below Ms introduced nanotwins in the adjacent austenite, the samples ausformed above Ms exhibited nanotwins only in secondary martensite formed during final cooling. Primary martensite in samples ausformed below Ms also contained a small fraction of transition (Fe2C/η) carbides.

Keywords: Nanostructured Bainite; Retained austenite; Martensite; Transition carbide; Ausforming; Phase transformation.