Partial austenitisation and TBF steel composed of ferrite, bainitic ferrite, and austenite

Abstract

A TRIP-aided bainitic-ferritic (TBF) steel with a chemical composition of Fe–0.19C–1.7Mn–1.09Si–0.51Al–0.05Nb (wt-%) was partially austenitised from a hot-rolled martensitic initial microstructure. After the hot rolling, the martensitic specimens were reheated to different intercritical temperatures and then austempered at 350°C. Thus, the effect of the initial microstructure of TBF steel on intercritical austenite formation during partial austenitisation was studied. The microstructures were investigated by scanning electron microscopy and electron backscatter diffraction (EBSD), and the tensile properties were tested. Microstructural observations revealed that a final microstructure of fine ferrite, bainitic ferrite, and retained austenite can be obtained. The steel partial austenitised at 770°C showed a good combination of ultimate tensile strength and total elongation.

A Comprehensive Understanding of Bainite Phase Transformation Mechanism in TRIP Bainitic-supported Ferrite Steel

“…An inverse relationship is seen in the YS as the inter critical temperature rises. This tendency is due to the rising levels of RA triggering the TRIP effect when samples deform [33]. Fig.…”

“…4 illustrates the initial idea of cold-rolled TBF steels, which includes complete austenitization followed by rapid quenching to a temperature higher than the austempering stages, resulting in a microstructure with RA grains inside a bainitic matrix. In the partly austenitized TBF steel idea, a multiphase microstructure may be achieved with fine-grained ferrite and RA grains surrounded by bainitic islands [33]. At elevated intercritical temperatures, a decrease in the concentrations of carbon and manganese in the intercritical austenite is expected to cause a shift of the bainitic transformation curve to the left and higher temperatures.…”

“…Consequently, it is necessary to elevate cooling rates and reduce the transformation period by raising the intercritical temperature to promote bainitic transition. An increase in Ms temperature was reported due to reduced quantities of C and Mn in intercritical austenite at higher intercritical temperatures [33].…”

“…Banding may impact the mechanical characteristics and fracture behavior in this scenario. Hot-rolled TBF steels are not as extensively researched as ordinary hot-rolled TRIP steels, despite the potential for enhanced mechanical qualities [33]. Chen et al [160] achieved an ultimate tensile strength of 1219 MPa and a total elongation of 21% using a new steel alloy containing 0.21% carbon, 2.04% manganese, 1.04% silicon, 1.10% aluminum, 0.064% niobium, and 0.2% molybdenum.…”

“…TBF steels, when hot-rolled, need a simpler procedure than traditional TRIP steels to get a bainitic matrix that offers enhanced strength and increased ductility. Mn segregation bands and martensite/austenite islands develop following hot rolling [33]. Producing TRIP steel billets must go through two stages: the smelting and refining stage to create casting billets and the thermomechanical processing stage to create steel plate billets with proper structures [34]- [37].…”