Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

Abstract

Over many decades, significant efforts have been made to improve the strength-elongation product of advanced high strength steels (AHSSs) by creating tailored multi-phase microstructures. Successive solid-state phase transformations for steels with a well selected chemical composition turned out to be the key instrument in the realisation of such microstructures. In this contribution, we first provide a brief review of the desired microstructures for Transformation-induced plasticity (TRIP), Carbide-free Bainitic (CFB), Quenching & Partitioning (Q&P) and Medium Manganese steels followed by comprehensive discussions on the phase transformations to be used in their creation. The implications for the steel composition to be selected are addressed too. As the presence of the right amount and type of metastable retained austenite (RA) is of crucial importance for the mechanical performance of these AHSSs, special attention is paid to the important role of successive solid-state phase transformations in creating the desired fraction and composition of RA by suitable element partitioning (in particular C and Mn). This critical partitioning not only takes place during final cooling (austenite decomposition) but also during the back transformation (austenite reversion) during reheating.This review aims to be more than just descriptive of the various findings, but to present them from a coherent thermodynamic / thermo-kinetic perspective, such that it provides the academic and industrial community with a rather complete conceptual and theoretical framework to accelerate the further development of this important class of steels. The detailed stepwise treatment makes the review relevant not only for experts but also metallurgists entering the field.

The Effect of Different Annealing Strategies on the Microstructure Development and Mechanical Response of Austempered Steels

“…On the other hand, recent trends in advanced high strength steel production account for the development of lean alloy steels with outstanding mechanical performance reached via novel and efficient heat treatments [11,12]. In order to create retained austenite containing multiphase microstructures, most of the thermo-treatments for the new generation of steels take advantage of the decomposition of austenite and carbon partitioning from bainite and/or martensite [12].…”

“…On the other hand, recent trends in advanced high strength steel production account for the development of lean alloy steels with outstanding mechanical performance reached via novel and efficient heat treatments [11,12]. In order to create retained austenite containing multiphase microstructures, most of the thermo-treatments for the new generation of steels take advantage of the decomposition of austenite and carbon partitioning from bainite and/or martensite [12]. Design and study of bainitic and martensitic based TRIP steels are mainly focused on the evaluation of results obtained by manipulation of low temperature heat treatment parameters (in the range from 200 to 500 • C), after a conventional annealing step (heating rate from 10 to 30 • C/s and soaking time at annealing temperature >60 s).…”

The Effect of Nickel Contents on the Microstructure Evolution and Toughness of 800 MPa Grade Low Carbon Bainite Deposited Metal

“…Moreover, this conclusion can also be verified through the propagation path of the second cracks in impact fracture specimens in the next section (Figure 15). Moreover, it is widely believed that the film-like RA between the bainite ferrite laths will have a positive effect on the toughness of the deposited metals [31]. When the RA film is encounters a crack during the propagation process, the crack tip will be blunted.…”

Effect of Step Quenching Heat Treatments on the Kinetics of Ferrite Formation and Quenching & Partitioning Modeling for a Commercial C-Mn-Si Steel

“…Thus, the automotive industry often seeks the development of advanced designs and materials that are economically viable, manageable, and capable of meeting the required environmental and safety requirements. Therefore, a program to develop a new vehicle must balance safety, performance, fuel economy, environmental sustainability, and a consumer interest design [1][2][3][4] .…”