Materials 2023, 16, 3852. https://doi.org/10.3390/ma16103852 www.mdpi.com/journal/materials Article Mechanical Properties of High Carbon Low-Density Steels Jiří Hájek 1, *, Zbyšek Nový 1 , Ludmila Kučerová 2 , Hana Jirková 3 , Črtomir Donik 4 and Zdeněk Jansa 5 1 COMTES FHT a. s., 334 41 Dobrany, Czech Republic; zbysek.novy@comtes2t.cz 2 Regional Technological Institute, University of West Bohemia, 301 00 Plzen, Czech Republic; skal@rti.zcu.cz 3 Department of Materials and Engineering Metallurgy, University of West Bohemia, 301 00 Plzen, Czech Republic; hstankov@fst.zcu.cz 4 Institute of Metals and Technology (IMT), 1000 Ljubljana, Slovenia 5 New Technologies Research Centre, University of West Bohemia, 301 00 Plzen, Czech Republic; zjansa@ntc.zcu.cz * Correspondence: jiri.hajek@comtes2t.cz; Tel.: +420-377-197-333 Abstract: The paper presents the possibilities of heat treatment of low-density structural steels usa- ble for springs. Heats have been prepared with chemical compositions 0.7 wt% C and 1 wt% C, as well as 7 wt% Al and 5 wt% Al. Samples were prepared from ingots weighing approximately 50 kg. These ingots were homogenised, then forged, and hot rolled. Primary transformation temperatures and specific gravity values were determined for these alloys. For low-density steels, there usually needs to be a solution to achieve the required ductility values. At cooling rates of 50 °C/s and 100 °C/s, the kappa phase is not present. A SEM analysed the fracture surfaces for the presence of transit carbides during tempering. The martensite start temperatures ranged from 55–131 °C, depending on the chemical composition. The densities of the measured alloys were 7.08 g/cm 3 and 7.18 g/cm 3 , respectively. Therefore, heat treatment variation was carried out to achieve a tensile strength of over 2500 MPa, with ductility of almost 4%. Hardnesses above 60 HRC were achieved for 1 wt% C heats using the appropriate heat treatment. Keywords: low density steels; heat treatment; tensile test 1. Introduction Low-density steels are an emerging class of structural materials for applications mainly in the automotive, chemical, and aerospace industries [1,2]. With the growing de- mand for optimising fuel consumption and the ever-tightening regulations dealing with CO2 emissions, the automotive industry is increasingly emphasising the need to reduce product weight while maintaining existing qualities, especially concerning occupant safety. According to a study by Ivan Gutierrez-Urrutia [3], reducing the weight of a vehi- cle by 100 kg reduces CO2 emissions by approximately 8.5 g per km. Various strategies are being adopted to reduce the weight of cars, such as structural modifications to remove redundant materials or replacing materials with beKer properties. High-strength steel, which provides the same mechanical properties while using thinner walls, is often used as a substitute for conventional steel. Another option is to use a lower-density material, which reduces the product’s weight while maintaining the material’s volume [4]. Such materials, today, include lightweight alloys, such as aluminium–magnesium alloys and various composites. However, these materials only sometimes meet the high mechanical and thermal stress requirements. Therefore, research nowadays focuses on developing lower-weight steels with high strength and low density [3,5]. The principle concept of alloying in low-density steels is simple, but the metallurgical feed technology or subsequent heat treatment is complicated. The addition of Al to the Fe–C system has a large effect on the phase fields and the phase constituent [6]. The ranges of composition and temperature for the high-temperature peritectic transformation are Citation: Hájek, J.; Nový, Z.; Kučerová, L.; Jirková, H.; Donik, Č.; Jansa, Z. Mechanical Properties of High Carbon Low-Density Steels. Materials 2023, 16, 3852. https://doi.org/ 10.3390/ma16103852 Academic Editor: Alexander Yu Churyumov Received: 19 April 2023 Revised: 12 May 2023 Accepted: 18 May 2023 Published: 19 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, SwiWerland. This article is an open access article distributed under the terms and conditions of the Creative Commons AKribution (CC BY) license (hKps://creativecommons.org/license s/by/4.0/).