Influence of the Temperature of Quenching/Partitioning on the Morphology of 37MnSi5 Steel

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INFLUENCE OF THE TEMPERATURE OF QUENCHING/PARTITIONING ON THE MORPHOLOGY OF 37MnSi5 STEEL H. R. Ghazvinloo1,2 and A. Honarbakhsh-Raouf1 In the last two decades, an extreme trend is observed toward the development of the third-generation of advanced high-strength steels for the automotive industry. As a novel process of heat treatment, quenching and partitioning were originally proposed by Speer, et al. This process is one of the new strategies to produce steels with a combination of high strength and considerable ductility. We study the influence of the temperature of quenching/partitioning on the morphology and microstructure of 37MnSi5 steel in the process of quenching and partitioning. To this end, 210, 238 and 270°C were chosen as quenching temperatures and 300, 400 and 450°C were used as partitioning temperatures. After the heat treatments, the microstructural characteristics were evaluated, compared, and discussed with the use of a scanning electron microscope and a field emission-scanning electron microscope. Keywords: temperature of quenching/partitioning, microstructure, 37MnSi5 steel.

The last few decades have witnessed a significant research effort directed toward the development of advanced high-strength steel (AHSS) grades for automotive applications, as they provide an opportunity for the development of cost-effective and light-weight parts with improved safety and optimized environmental performance [1–3]. The procedure of quenching and partitioning (Q&P) was proposed by Speer, et al. [4, 5]. It is now receiving increased attention as a novel heat treatment to produce AHSS that contain martensite/retained austenite mixtures in the microstructure. The Q&P steel shows a good combination of strength and ductility in which martensite acts as a strengthening phase and retained austenite significantly contributes to ductility. The Q&P process usually consists of a two-step heat treatment. First, steel is held at a temperature in the full austenite (γ ) or intercritical (γ + α) range and then quenched to a temperature between the martensite-start (M s ) and martensite-finish (M f ) tem-

peratures. Second, the quenched steel is held either at the initial quenching temperature or above this temperature in order to enrich the untransformed austenite by carbon diffusion from the supersaturated martensite [6, 7]. The increased amount of carbon in the austenite decreases its M s temperature so that thermally stable austenite is obtained after the final quenching to room temperature. The Q&P process is mainly applied to steels with chemical compositions similar to the chemical compositions of conventional transformation-induced plasticity (TRIP) steels [8, 9]. Significant additions of elements, such as silicon, retard the formation of carbides and give rise to the carbon enrichment of austenite via the partitioning of carbon from supersaturated martensite [8, 10]. Manganese, nickel, and chromium are included in the chemical composition to retard ferrite, pearlite, and bainite formation and to decrease the bainite