Design of a high strength, high ductility 12 wt% Mn medium manganese steel with hierarchical deformation behaviour

Abstract

A novel medium Mn steel of composition Fe-12Mn-4.8Al-2Si-0.32C-0.3V was manufactured with 1.09 GPa yield strength, 1.26 GPa tensile strength and 54% elongation. The thermomechanical process route was designed to be industrially translatable and consists of hot and then warm rolling before a 30 min intercritical anneal. The resulting microstructure comprised of coarse elongated austenite grains in the rolling direction surrounded by necklace layers of fine austenite and ferrite grains. The tensile behaviour was investigated by in-situ neutron diffraction and the evolution of microstructure studied with Electron Backscattered Diffraction (EBSD). It was found that the coarse austenite grains contributed to the first stage of strain hardening by transforming into martensite and the fine austenite necklace grains contributed to the second stage of strain hardening by a mixture of twinning and transformation induced plasticity (TWIP and TRIP) mechanisms. This hierarchical deformation behaviour contributed to the exceptional ductility of this steel.

Microstructure Evolution and Tensile Behaviour of a Cold Rolled 8 Wt Pct Mn Medium Manganese Steel

“…The presence of YPE strongly indicates that TRIP rather than TWIP was the dominant deformation mechanism at yielding. [17,[35][36][37] The yield strength, tensile strength and strain hardening rate increased significantly with increasing cold rolling reduction. The strain hardening rate in the as-annealed sample was not constant throughout the tensile curve, most likely due to different deformation mechanisms being active at different stages but averaged out to approximately 2 GPa.…”

“…[2,3,14,18] A summary of the tensile properties of the investigated steels is shown in Table II including the energy absorption parameter U30 which is determined as the area under the true tensile curve up to a true strain of 0.3. [35] C. Carbide Precipitation Microstructure…”

“…This also explains why the YPE in CR20 was shorter than CR10 as the CR20 sample contains more SIM nucleation sites than CR10 which will result in a higher local strain hardening rate within the Lu¨ders band and therefore a higher band velocity. [36] D. Industrial Relevance In our previous work, [35] we introduced a medium Mn steel of composition Fe-12Mn-4.8Al-2Si-0.32C-0.3V and found that the high price of V greatly increased the alloying cost of the steel. Many researchers have also added V to medium Mn steels and achieved large amounts of precipitation strengthening.…”

“…13-U30 diagram comparing alloying cost and energy absorption between several medium Mn steels in the literature and commerical steels. [2,3,5,7,12,13,35,39,40,[63][64][65][66][67][68][69] *ThyssenKrupp dual phase steels.…”

A Scale-up Study on Chemical Segregation and the Effects on Tensile Properties in Two Medium Mn Steel Castings

“…Tight composition control in medium Mn steels is important as slight variations in austenite composition may lead to different active deformation mechanisms, i.e., transformation induced plasticity (TRIP), TWIP, or TWIP þ TRIP. [9] The TWIP + TRIP mechanism is usually sought after in medium Mn steels as it provides the optimal balance between strength and ductility. However, the composition window where the TWIPþTRIP mechanism is active is usually very narrow.…”