PSI - Issue 37

E. Entezari et al. / Procedia Structural Integrity 37 (2022) 145–152 E.Entezari et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction Advanced High Strength Steels (AHSS) are widely used in the automotive parts industry, specifically to manufacture lightweight vehicles that have to meet high safety standards and provide better fuel efficiency. Advanced high strength steels are obtained by heat treatments such as Quenching and Partitioning (Q-P), Quenching-Partitioning Tempering (Q-P-T), Twinning Induced Plasticity (TWIP), Transformation Induced Plasticity (TRIP), and Dual Phase (DP). Recent studies have focused on the Q-P steels and Q-P-T heat treatments due to their promising results (Schmitt et al. (2018)). Q-P treatment produces high strength multiphase steels with a reasonable amount of residual austenite at room temperature. According to Edmonds et al. (2006), Q-P treatment starts with austenization, followed by quenching to a temperature between martensite start (M S ) and martensite finish (M f ) temperature, causing carbon diffusion from supersaturated martensite to the metastable austenite, resulting in an increased amount of stabilized austenite phase at room temperature. And then, partitioning treatment is done at a temperature above the M S and, finally, water quenching to room temperature. This procedure allows obtaining steels with an excellent combination of strength and ductility as compared to other conventional steel production processes. Tsuchiyama et al. (2012) observed that Q-P treatment could improve the strength-ductility balance of low carbon steel. Further studies demonstrated that the bainitic and martensitic transformations that occurred during the Q-P treatment promote carbon diffusion from carbon-enriched martensite to stable austenite (Gouné et al. (2013)). Generally, the volume fraction of austenite and its morphology are the main factors in enhancing the ductility of Q-P treated steels. Shen et al. (2015) showed that the mechanical stability of retained austenite and a higher volume fraction of film austenite that occurred during the Q-P treatment resulted in strength-ductility balance. Besides the volume fraction of film austenite, the thickness of the bainite sheaves plays a vital role in increasing ultimate tensile strength. Avishan et al. (2012) observed that thinner bainite sheaves enhanced mechanical properties in nanostructured bainitic steel. So far, the researchers have focused on the strength-ductility balance of AHSS steels during the development of the Q-P heat treatment, finding that the optimum combination of mechanical properties can be achieved by obtaining a multiphase microstructure containing bainite, tempered martensite, and residual austenite with enriched carbon, as concluded by Mandal et al. (2016). Therefore, by controlling the Q-P temperatures, partitioning time, and cooling rate, it is possible to produce AHSS steels having an excellent combination of strength and toughness. The present work investigates the effect of quenching temperature on microstructural characteristics and mechanical properties of low carbon steel after quenching partitioning treatment and identifies the operating strengthening mechanisms that occur during the Q-P treatment. 2. Materials and methods The design of novel chemical composition was done using the MUCG83™ thermodynamic model to produce low carbon steel with low enough B S and M S temperatures and a proper difference between the two temperatures, according to our previous research work (Mousalou et al. (2018)). Low carbon steel was fabricated in the laboratory by melting scrap and ferro-alloys in an induction furnace with an inert gas atmosphere and further cast in metallic cube molds. The cast steel was electroslag remelted at 23-25 V and 140 to 180 A, with a synthetic slag composed of %70 Al 2 O 3 + %30 CaF 2 to obtain clean steel. A high solidification rate after the electroslag remelting process was used to decrease the segregation of alloying elements, as described by Entezari et al. (2018). The steel coupons were homogenized at 1200 °C for 4 hours and hot-rolled at 900-1000 °C in several passes to obtain plates of approximately 17 mm of thickness. The final chemical composition of steel was 0.26 C, 1.70 Mn, 1.42 Si, 1.10 Cr, 1.10 Ni, 0.94 Cu, 0.24 Mo, 0.1 V, Bal. Fe (in Wt.%). Dilatometric samples were machined from a hot-rolled plate perpendicular to the rolling direction to prepare samples with a 4 mm diameter and 10 mm length. Then, the prepared samples were tested by a Bahr Dil805A™ instrument in which samples were heated up to 930 °C with a rate of 5 °C s -1 and were isothermally held for 20 min and finally, were cooled to the ambient temperature with a rate of 20 °C s -1 . The results obtained from dilatometric