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Medium-Alloy Manganese-Rich Transformation-Induced Plasticity Steels
DONG WOO SUH, JOO HYUN RYU, MIN SUNG JOO, HONG SEOK YANG, KYOOYOUNG LEE, and H.K.D.H. BHADESHIA
The manganese concentration of steels which rely on transformation-induced plasticity is generally less than 2 wt pct. Recent work has highlighted the potential for strong and ductile alloys containing some 6 wt pct of manganese, but with aluminum additions in order to permit heat treatments which are amenable to rapid production. However, large concentrations of aluminum also cause diculties during continuous casting. Alloy design calculations have been carried out in an eort to balance these conicting requirements, while maintaining the amount of retained austenite and transformation kinetics. The results indicate that it is possible by adjusting the carbon and manganese concentrations to reduce the aluminum concentration, without compromising the mechanical properties or transformation kinetics. The deformation-induced transformation of retained austenite is explained quantitatively, for a range of alloys, in terms of a driving force which takes into account the very ne state of the retained austenite.
DOI: 10.1007/s11661-012-1402-3 The Minerals, Metals & Materials Society and ASM International 2012
I. INTRODUCTION
WORK by Miller[1] established the possibility of alloys containing a medium manganese concentration, Fe-0.11C-5.7 Mn wt pct, in order to generate a mixed microstructure of ferrite and austenite by intercritical annealing. The plasticity obtained during the deformation-induced martensitic transformation of austenite (TRIP) led to impressive values of total elongation (34 to 19 pct) for tensile strengths in the range 875 to 1200 MPa. The time periods for intercritical annealing were in the range 1 to 16 hour.
In an eort to make the process more amenable to industrial production, Suh and co-workers[2] added up to 3 wt pct of aluminum to the steel, on the basis that this solute increases the free energy change associated with the austenite to ferrite transformation.[3] This has the eect of increasing the temperature at which the steel can be intercritically annealed, thus permitting the time period of annealing to be reduced to just two minutes.[2]
However, it is considered that 3 wt pct is too large a concentration from the point of view of steel manufacture by continuous casting because of the possibility of nozzle clogging.[4] The purpose of the present work was to see if a...