PSI - Issue 69

Zeynab Aalipour et al. / Procedia Structural Integrity 69 (2025) 105–112

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1. Introduction Advanced high-strength steels (AHSS) have attracted considerable attention in automotive and structural engineering applications due to their potential to deliver an exceptional combination of strength and ductility. Among various AHSS grades, quenching and partitioning (Q&P) steels have emerged as a particularly promising class, thanks to their ability to retain austenite in the microstructure [1]. Retained austenite (RA) imparts enhanced ductility through the Transformation-Induced Plasticity (TRIP) effect, which is crucial for energy absorption and overall formability during service [2]. However, realizing a stable fraction of RA depends on both the alloy composition, especially silicon content and the specific processing parameters, notably the partitioning time. In the Q&P process, a fully or partially austenitized steel is quenched to a temperature below the martensite start (Ms), producing a controlled amount of martensite, followed by a “partitioning” step at an intermediate temperature. Deformation Quenching and Partitioning (DQ&P) is a recently developed thermomechanical processing technique that integrates deformation with quenching and partitioning. This innovative approach is designed to enhance the work-hardening capacity and uniform elongation of high-strength hot-rolled structural steel. The key advantage of DQ&P processing lies in its ability to refine the martensite packet and block size while shortening and randomizing the lath structure. These microstructural modifications result in significantly improved mechanical properties in the steel [3,4]. During partitioning, carbon diffuses from the supersaturated martensite into the untransformed austenite, thereby stabilizing it against subsequent transformation upon cooling [5,6]. The extent and rate of carbon diffusion are strongly influenced by alloying elements such as silicon, manganese, and chromium, which can either promote or suppress the formation of competing phases like bainite or carbides [7,8]. Silicon is known to reduce the kinetics of cementite precipitation, allowing more carbon to remain available for stabilizing the austenite. Despite growing interest in Q&P steels, relatively few studies have systematically isolated the effect of partitioning time across different silicon levels in medium-carbon alloys [9]. Moreover, the interplay between retained austenite fraction, secondary martensite formation, and potential bainitic reactions remains incompletely understood. A deeper knowledge of these relationships is vital for optimizing strength– ductility balances in medium-carbon steels, where trade-offs in phase fractions can significantly influence mechanical properties. In this work, we explore the role of silicon content and partitioning time in controlling the stability of RA in a deformation-assisted Q&P process. Two medium-carbon steels with different silicon levels, i.e., high-silicon (H-Si) and low-silicon (L-Si) undergo DQ&P for various durations. X-ray diffraction (XRD) is employed to quantify the evolution of phase fractions, secondary martensite, and bainite, while lattice-parameter measurements enable calculation of the carbon content in RA. To correlate these macroscopic observations with underlying microstructural features, electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) are used to map phase distributions and morphology. 2. Experimental The material used in this study was medium-carbon steels obtained from OCAS NV (Zelzate, Belgium) as 70 kg vacuum-cast ingots. The definitive chemical compositions of the experimental steels were determined by spark optical-emission spectroscopy (SOES) and carbon combustion analysis; the results are listed in Table 1. Samples were machined to dimensions of 6 mm × 9 mm for the Gleeble thermomechanical simulations. The Direct Quenching and Partitioning (DQ&P) process was carried out using a Gleeble 3800® thermomechanical simulator (Dynamic Systems Inc., Poestenkill, NY, USA). Figure 1 outlines the thermo-mechanical schedule. Specimens were first austenitised at 1150 °C for 2 min, then cooled at 5 °C s⁻¹ to 850 °C and held for 10 s. While at 850 °C they were hot-compressed to a total true strain of 0.6 in three passes (≈0.2 per pass) at a strain rate of 1 s⁻¹, with 25 s inter-pass delays. The samples were immediately quenched to 150 °C, after which a partitioning treatment was applied: 300 °C for the high-silicon (H-Si) steel and 250 °C for the low-silicon (L-Si) steel, as determined in preliminary optimisation trials. Partitioning times varied and were set to 10, 100, 1000, and 10000 s to allow for carbon redistribution within the microstructure. Microstructural analyses were conducted using EBSD and SEM to examine the morphology and features of the processed samples. Phase identification and quantification were performed using X-ray diffraction (XRD), and the retained austenite fraction