Design of advanced steels

Significance 

Medium Mn steels are a promising material for developing advanced high strength steels with increased formability and reduced weight. Generally, the mechanical properties of medium Mn steels depend on the austenite morphology and stability, and thus, modification of the particle sizes and the austenite stability can yield better mechanical properties. To date, several methods for austenite stabilization have been proposed. They mainly focus on improving the material’s properties by reducing the density, increasing the annealing temperatures, and inhibiting carbide precipitation during cooling. Consequently, the relationship between processing, microstructure, and properties of medium Mn steels have recently attracted significant research attention. Nevertheless, despite the extensive research, little work has been conducted on systematic optimization of chemical composition and heat treatment processes to fabricate Mn steels with desired microstructural characteristics. This can be attributed to overdependence on exhaustive experimental methods and provisional computation techniques.

Motivated by the promising results from previous research work, Professor Gregory Haidemenopoulos and John Aristeidakis who is a PhD candidate, both from the University of Thessaly in Greece developed a new ICME design approach for the composition and processing of medium Mn steels. The main objective was to develop ferrite containing medium Mn steel with an optimized microstructure as per the design and austenite stability requirements. Their research work is currently published in the research journal, Acta Materialia.

In their approach, a novel medium Mn TRIP/TWIP steel was developed via a combination of multi-objective genetic optimization and CALPHAD-based thermodynamic and kinetic modeling techniques. The austenite stacking fault energy was predicted using a newly developed sub-regular solution model. This model takes into consideration the effects of carbon, manganese, nickel, aluminum, silicon, and chromium metals. The implementation of the stack fault energy was done in the MATLAB environment and the source code has been made available to provide more insights and further understanding of the results. Finally, the proposed modeling approach was validated using both data derived from the literature and in-house experiments.

The authors managed to identify several annealing temperatures and Pareto optimal solutions. The optimal intercritical annealing conditions of the single medium Mn steel was identified by solving additional optimization problems in the domain of time. Hot-rolling and solidification simulations revealed the presence of considerable δ-ferrite fractions just before annealing. This resulted in the segregation of elements in the high-temperature austenite, which further transformed to primary martensite upon quenching. Additionally, the formation of austenite bands was observed upon intercritical annealing due to the growth of austenite from martensite that came in contact with the δ-ferrite. Furthermore, austenite at the boundary exhibited a coarser structure and enhanced stability attributed to the carbon and manganese partitioning from the δ-ferrite. In contrast, the interior austenite exhibited faster kinetics that grew in fine laminar structures. The experimental results were consistent with the results derived from the literature.

In summary, the study reported a strategy for the composition and processing design of medium-Mn steels based on a combination of CALPHAD, SFE modeling, and genetic optimization techniques. Results showed that the proposed modeling allowed the design of new medium Mn steels as per the requirements. In a statement to Advances in Engineering, Professor Gregory Haidemenopoulos said their study will widen the application of modern computational techniques coupled with generic optimization in addressing the design and processing challenges encountered in the development of medium Mn steels and other materials.

Design of advanced steels - Advances in Engineering

About the author

Gregory N. Haidemenopoulos has been Professor of Physical Metallurgy and Director of the Laboratory of Materials (LoM) at University of Thessaly, Greece since 1992. His research is concerned with processing-structure-properties in metallic materials, dealing with transformation kinetics in TRIP steels, corrosion-induced hydrogen trapping in aluminum alloys. More recent research focuses on computational alloy and process simulation with applications in the design of homogenization of extrudable aluminum alloys, the design of medium-Mn steels and the thermomechanical process design of HSLA steels. In addition to teaching and research, Prof. Haidemenopoulos has provided consulting services to industry in the fields of process design, failure analysis, materials selection and corrosion control.

Prof. Haidemenopoulos has supervised several PhD students and has earned the University of Thessaly’s Mechanical Engineering Departmental Best Teaching Award for the years 2007, 2008, 2009, 2011, 2014, and 2015. He has published the textbook Physical Metallurgy – Principles and Design, CRC Press, Taylor & Francis, USA, 2018, ISBN 978-1-138-62768-0. He is Member of Editorial Board of European Journal of Materials (Taylor and Francis), Materials (MDPI), The Open Corrosion Journal and International Journal of Metallurgical and Materials Engineering, published 10 book chapters, 90 papers in refereed journals, and presented many keynote and invited lectures worldwide. Prof. Haidemenopoulos received his PhD in Metallurgy, MSc in Metallurgy, and MSc in Naval Architecture and Marine Engineering from MIT.

About the author

John S. Aristeidakis was born in Thessaloniki, Greece in the year 1994. He graduated with a diploma degree in Mechanical Engineering from the Polytechnic School of the University of Thessaly, Greece, in 2017. Currently he is a PhD student and occupies a research assistant position at the Laboratory of Materials at the University of Thessaly. The subject of his dissertation is the development of general methodologies based on computational modeling, simulation, and optimization, for the design of highly formable, high strength medium Mn TRIP/TWIP steels for lightweight automotive applications, via tailoring of the retained austenite stability at room temperature. More specifically, John is involved in CALPHAD based thermodynamic end kinetic modeling of the microstructural evolution, constitutive modeling of the mechanical behavior resulting from the activation of TRIP & TWIP in medium Mn steels as well as uncertainty propagation and optimization.

He has worked on several industrially and publicly funded projects, including the EU RFCS “Light Chassis” project that aims to develop affordable, lightweight automotive components from novel medium Mn steels, using a bottom up approach. In the past, John has also been involved in the characterization of the microstructure and simulation of the process chain of extrudable 6xxx-Al alloys, aiming to improve extrudability. Lately, he started working on the solidification of austenitic stainless steels during additive manufacturing as well as on the purification of Al scrap via impurity precipitation and electromagnetic separation in the liquid phase, during recycling of Al alloys. John Aristeidakis is interested in the implementation of Integrated Computational Materials Engineering (ICME) approaches for accelerating the development of new materials with optimized properties.

Reference

Aristeidakis, J., & Haidemenopoulos, G. (2020). Composition and processing design of medium-Mn steels based on CALPHAD, SFE modeling, and genetic optimizationActa Materialia, 193, 291-310.

Go To Acta Materialia

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