Microstructure evolution and tensile behaviour of a cold rolled 8 wt\% Mn medium manganese steel

Abstract

A novel medium manganese steel named Novalloy with composition Fe-8.3Mn-3.8Al-1.8Si-0.5C-0.06V-0.05Sn was developed and thermomechanically processed through hot rolling and intercritical annealing. The steel possessed a yield strength of 1 GPa, tensile strength of 1.13 GPa and ductility of 41%. In order to study the effect of cold rolling after intercritical annealing on subsequent tensile properties, the steel was further cold rolled up to 20% reduction. After cold rolling, it was observed that the strain hardening rate increased continuously with increasing cold rolling reduction but without a significant drop in ductility during subsequent tensile tests. The microstructural evolution with cold rolling reduction was analysed to understand the mechanisms behind this enhanced TRIP effect. It was found that cold rolling activated additional twinning systems which provided a large number of potent nucleation sites for strain induced martensite to form during subsequent tensile tests.

A scale up study on chemical segregation and the effects on tensile properties in two medium Mn steel castings

“…Two ingots weighing 400 g (60 × 23 × 23 mm) and 5 kg (250 × 70 × 30 mm) with nominal composition Fe-8Mn-4Al-2Si-0.5C-0.07V-0.05Sn were cast using vacuum arc melting and Vacuum Induction Melting (VIM) respectively. The choice of elements and alloying concept was described in a previous paper [21]. The 400 g ingot was prepared using pure elements and solidified in a copper mould, while the 5 kg ingot was prepared using ferroalloys and solidified in a mild steel mould.…”

“…Thermo-Calc is often used during the alloy design stage to give an indication of phase fractions and approximate compositions of each phase, especially within the intercritical temperature regime. In most medium Mn steels in the literature, the phases and compositions obtained during intercritical annealing are retained to room temperature [21,[24][25][26]. From Figure 2, it can be seen that the slight differences in composition between the 400 g and 5 kg ingot did not result in a significant change in the expected phase fractions at the chosen IA temperature of 750 • C. Thermo-Calc also predicted that δ-ferrite should not be expected below 1370 • C but it was clearly observed in the as-cast micrographs of both 400 g and 5 kg ingots in Figure 3.…”

“…Tensile samples were obtained from rolled and IA strips after homogenising a bar from the 400 g and 5 kg ingot for 2 h at 1250 • C, hereby known as 400 g (2 h) and 5 kg (2 h). The tensile curves are shown in Figure 9 together with a tensile curve obtained from previous work on a fully homogenised sample [21]. The fully homogenised sample, hereby known as 400 g (24 h), was produced using the same thermomechanical processing steps but from a 400 g ingot which was homogenised for 24 h. A summary of the tensile properties is shown in Table 2.…”

“…However, the austenite phase fraction was not constant across all three samples (Table 3), suggesting that the composition of the austenite could not have been identical under normal circumstances. Of the alloying additions in medium Mn steel, C is able to significantly alter the SFE and Md 30 of austenite, even in small concentrations [15,21,30]. Therefore, in order for the austenite phase in the partially homogenised samples to maintain the same C content as the fully homogenised sample while having a lower austenite fraction, it was likely that the excess C in the partially homogenised samples precipitated in the form of carbides along the δ-ferrite interphase boundaries (Figure 10d).…”

“…Building upon previous work on a 8 wt% Mn medium Mn steel [21], this study aims to elucidate some of the problems which may be encountered during scale up of medium Mn steels from small scale to larger scale melts with cooling rates more representative of industrial practices. In this study, two laboratory castings of different sizes and therefore different Secondary Dendrite Arm Spacings (SDAS) were produced in order to study the effect of chemical microsegregation, homogenisation duration and δ-ferrite transformation on tensile properties.…”