Non-equilibrium solidification and ferrite in δ-TRIP steel

Abstract

Microscopy and microanalysis experiments on two cast alloys, designed on the basis of equilibrium to contain substantial amounts of δ-ferrite, reveal zero or much reduced fractions of this phase in the solidified condition. It appears that the solid state transformation of δ-ferrite into austenite occurs without the required partitioning of solutes and that this is responsible for the development of non-equilibrium microstructures. This conclusion is supported by microanalytical data and through calculations of limiting phase diagrams based on paraequilibrium rather than equilibrium. Kinetic simulations confirm that this interpretation is consistent with the majority of austenite growing in the solid state without the partitioning of the substitutional solutes.

Retention of δ-ferrite in aluminium-alloyed TRIP-assisted steels

“…A few issues have been resolved that should assist in the future design of the so-called d-TRIP steels and indeed in the generic problem of d-ferrite retention: -It has been demonstrated, both by conducting elevated temperature equilibrium heat-treatment experiments, and by rationalizing previous data (Yi et al 2010), that the recent thermodynamic databases are able to correctly represent phase stabilities, specifically in the context of the d-TRIP steels. -It has been shown by simulation of new solidification experiments that the retention of excess d-ferrite cannot be attributed to the growth process during peritectic reaction for the range of alloys studied.…”

“…Recent attempts, at designing low-alloy steels in which the microstructure retains a large fraction of d-ferrite, have come up against Yi et al (2010) difficulties in which the quantity of d-ferrite is sometimes grossly overestimated using standard phase diagram calculations (Yi et al 2010), thus necessitating the empirical study of a series of seven alloys with varying aluminium contents (Yi et al 2011b;Jung et al 2012). An attempt was made to explain the discrepancies using kinetic simulations and microanalytical data, resulting in a conclusion that the problem arises owing to the inadequate partitioning of substitutional solute as austenite grows from d-ferrite.…”

Mechanical properties and austenite stability in hot-rolled 0.2C–1.6/3.2Al–6Mn–Fe TRIP steel

“…Furthermore, Al in TRIP steels facilitates the presence of ferrite [11] and δ-ferrite during solidification and contributes to excellent tensile properties [12,13]. For instance, Suh et al reported that Fe-6Mn-0.1C-3Al (wt%) steel achieved an excellent combination of high tensile strength (1000 MPa) and ductility (30%) [14].…”

Extraordinary ductility in Al-bearing δ-TRIP steel

“…They basically help ensure the presence of d-ferrite under equilibrium conditions, at all temperatures in the solid state. Aluminium plays a key role in this and experience based on experiments (Yi et al 2010a) has indicated that such calculations overestimate the amount of d-ferrite expected when the real alloys solidify during casting under conditions which deviate from equilibrium. Therefore, the two alloys studied here (table 1) have larger aluminium concentrations than in the original work (Chatterjee et al 2007).…”

“…Subsequent experiments highlighted the fact that the alloy system is sensitive to non-equilibrium solidification which meant that d-ferrite could not be reliably retained in the microstructure (Yi et al 2010a). Further intense research (Yi et al 2010b) was necessary in order to characterize the transformation behaviour during and after hot-rolling in the two-phase field; the rolling deformation is necessary to apply the steel in sheet form for use in the manufacture of automobiles.…”