Advanced Thermo-mechanical Process for Homogenous Hierarchical Microstructures in HSLA Steels
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ODUCTION
IN 2017, 1689 million tones crude steel were produced, with ~50 pct used for building and infrastructure, followed by 15 pct for mechanical equipment and 12 pct for automotive applications.[1,2] It is expected that the world’s population will grow by another 2.7 billion people until 2050, resulting in expanding urban communities. Therefore, novel higher performance steels are needed for enabling mechanical design with thinner structural elements, leading to weight reduction and, thus, reduced fuel consumption and CO2 emissions in
CARINA LEDERMUELLER and SOPHIE PRIMIG are with the School of Materials Science & Engineering, UNSW Sydney, Sydney, 2052 NSW, Australia. Contact e-mail: [email protected] ERNST KOZESCHNIK is with the Institute of Materials Science and Technology, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria. RICHARD F. WEBSTER is with the Electron Microscopy Unit, Mark Wainwright Analytical Centre, UNSW Sydney, Sydney, 2052 NSW, Australia Manuscript submitted June 21, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
transport applications. Due to their exceptional cost-performance ratio, modern high-strength low-alloy (HSLA) steels are excellent candidates to cover the corresponding growing demands for advanced steels for construction and transportation.[1,2] HSLA steels are conventionally manufactured by a sophisticated temperature and deformation schedule known as thermo-mechanical processing (TMP), which allows to precisely control the microstructural evolution. Here, engineering of a designed target microstructure can lead to the desired mechanical properties, i.e., high strength and elongation. Therefore, during TMP, final rolling passes are carried out in the austenite non-recrystallization region to produce pancaked grains with high-defect densities, which can act as nucleation sites for fine ferrite grains. The phase transformation from austenite to ferrite results in product grain sizes of 2 to 5 lm, and yield strengths of > 500 MPa are frequently achieved.[3,4] Following the Hall–Petch-relationship,[5] it is obvious that grain sizes below 2 lm will achieve superior yield strength, with exceptional mechanical properties in the range of ultrafine grains (< 1 lm). However, this is not possible via conventional TMP.
In contrast to conventional TMP, advanced TMP (aTMP) follows a different approach: Lowering the rolling temperatures to the ferrite region or annealing of cold-rolled martensite has been shown to have a significant effect on grain refinement in mild steels, leading to grain sizes in the submicron regime, resulting in exceptional yield strength.[6–8] It has been proposed that the mechanism behind the grain fragmentation is continuous dynamic recrystallization, a dynamic restoration process governed by extended recovery.[6,9,10] However, challenges such as low work-hardening capability and delamination due to rather large cementite particles still need to be overcome in such steels.[6,11] It is further known that steels with hierarchical microstructures, which are defined as micros
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