Medium Mn steels
Creators: name: Kwok Wie Jie, Thomas
Publisher: name: Imperial College London
Year: 2022

description: Medium Mn steels containing 4-12 wt\% Mn are a relatively new class of steels which has gained significant research attention over the past decade. These steels are typically duplex (austenite$+$ferrite) and can be processed in various ways to produce different microstructure variants. Additional Twinning Induced Plasticity (TWIP) and/or Transformation Induced Plasticity (TRIP) effects can also be enabled in the austenite phase. Medium Mn steels therefore have great flexibility in design and can be processed to exhibit a large range of tensile properties. This thesis describes the alloy development journey to develop novel TWIP-assisted medium Mn steels with high strength and high ductility. The aim was to produce industrially scalable medium Mn steels compared to high Mn TWIP steels (16-30 wt\% Mn) which faced many problems during industrial trials due to the high Mn content. Various tools such as Thermo-Calc were used to guide an iterative alloy design process that involved vacuum arc melting, hot rolling, tensile testing and electron microscopy. Findings from each alloy would then be used to guide the next iteration. As a result, a series of novel medium Mn steels with decreasing Mn contents and different strain hardening mechanisms have been developed. It is hoped that the relationships between composition, processing and tensile properties established in this work will continue to guide future medium Mn steel development. In Chapter 3, a novel medium Mn steel with 8 wt\% Mn was produced that displayed both TWIP and TRIP effects. Cold rolling was investigated as a means to improve strength. It was observed that cold rolling introduced a larger density of twins and therefore twin intersections into the microstructure compared to uniaxial tension. It was postulated that the higher density of twin intersections which can act as $\alpha'$-martensite nucleation sites led to an enhanced TRIP response in the cold rolled samples, improving strain hardenability while retaining a signifcant amount of ductility ($\sim30\%$). Chapter 4 describes a scaling up study on the same steel. It was found that after a standard slab reheating cycle of 1250 \degree C for 2 h, some $\delta$-ferrite remained untransformed but the matrix was mostly compositionally homogeneous, even in Mn. When thermomechanically processed, the $\delta$-ferrite was observed to form stringers, resulting in a slight loss in yield and tensile strength but the strain hardening behaviour was preserved. Finally, the relative effects of TWIP and TRIP were investigated in chapter 5. A new medium Mn steel with 5 wt\% Mn was developed and processed differently to produce two microstructure types (lamellar and mixed equiaxed$+$lamellar). Electron microscopy on interrupted tensile specimens showed different twinning kinetics but a similar $\alpha'$-martensite nucleation and growth mechanism between the two microstructure variants. However, constitutive modelling of their strain hardening rate curves showed that the contribution to strength from twinning was insignificant compared to the TRIP effect, questioning the relevance of the TWIP effect in medium Mn steels.