δ-TRIP Steel

Abstract

A new concept in steels which are considered by transformation plasticity was recently developed entirely by calculation. The steel relies on the presence of δ-ferrite in solidification, bainitic ferrite and retained austenite in the microstructure. δ-TRIP steel contains a small amount of silicon to avoid surface quality problems during hot rolling and galvanizing. The original concept steel was found to possess excellent properties for automotive applications. The limited experimental validation of this concept has been extended in the present work, almost to the point of advanced commercialisation.An attempt to reproduce the work highlighted a number of δ-ferrite, the solidification process has been investigated in order to understand the stability of δ-ferrite as a function of solute content and non equilibrium cooling conditions. The solid-state transformation of δ-ferrite into austenite occurs without the required partitioning of solutes and this is responsible for the development of non-equilibrium microstructures, a conclusion supported by microanalytical data and through calculations of limiting phase diagrams based on para equilibrium 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.Further, the stability of this δ-ferrite has been investigated during reheating into a temperature range typical of hot rolling conditions since the alloys are always used in the rolled condition. It is found that contrary to expectations from calculated phase diagrams, the steel becomes fully austenitic under these conditions and that a better balance of ferrite promoting solutes is necessary in order to stabilise the dendritic structure. New alloys aredesigned for this purpose and are found suitable for hot rolling in the two-phase field over the temperature range 900-1200 oC.Furthermore, the heat treatment profiles have been studied to achieve good properties for automotive application, which is consistent with practical production processes. The alloys exhibit impressive combinations of tensile strength and elongation. The alloys rely on significant concentrations of ferrite-stabilising solutes so that δ-ferrite which forms during solidification is retained in the microstructure. Except the role of retained austenite playing as TRIP effect, its plastic deformation is found to contribute to the overall mechanical behavior as well. The mechanical stabilisation phenomenon of retained austenite has been firstly observed experimentally in TRIP-assisted steels here. A physical model has been proposed for the estimation of mechanical stabilisation. The result of modeling is consistent with the experimental observations. This explains the reason why retained austenite cannot transform fully into martensite in TRIP-assisted steels. The δ-TRIP steel is expected to be weldable even though its carbon equivalent is much higher than the conventionally permitted value 0.4 wt% because a dual phase (ferrite + martensite) is supposed to be achieved in both heat affected zone and fusion zone by introducing δ-ferrite into the microstructure. The δ-ferrite can persist in the entire temperature range during spot welding by addition of strong ferrite stabilising element, aluminium. The spot weldability of δ-TRIP alloys has therefore been investigated in this research with changing the aluminium contents.Hot-press forming steels are formed in a fully austenitic state followed by die-quenching in order to generate martensite to achieve high strength. The ductility therefore, tends to be limited. A novel steel design, based on chemical compositions of a δ-TRIP alloy but different heat treatment, in which the forming operation is in the two-phase austenite and ferrite field, so that the quenching results in a dual-phase ferrite and martensite microstructure at ambient temperature. It is demonstrated that slightly better properties are achieved compared with current hot-press forming steels. The interpretation of the mechanisms of deformation during tensile testing indicates that the ductility can be further enhanced without compromising strength. The new steel also can be heated to temperature which is lower than that used for conventional hot-press forming steels, before transfer into the forming press.