Effect of boron on the continuous cooling transformation kinetics in a low carbon advanced ultra-high strength steel (A-
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Effect of boron on the continuous cooling transformation kinetics in a low carbon advanced ultra-high strength steel (A-UHSS) G. Altamirano1, I. Mejía1, A. Hernández-Expósito2,3, J. M. Cabrera2,3 1 Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo. Edificio “U”, Ciudad Universitaria, Morelia, Michoacán, México. 2 Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, ETSEIB – Universitat Politècnica de Catalunya. Av. Diagonal 647, Barcelona, Spain. 3 Fundació CTM Centre Tecnològic, Av. de las Bases de Manresa, 1, Manresa, Spain. ABSTRACT The aim of the present research work is to investigate the influence of B addition on the phase transformation kinetics under continuous cooling conditions. In order to perform this study, the behavior of two low carbon advanced ultra-high strength steels (A-UHSS) is analyzed during dilatometry tests over the cooling rate range of 0.1-200°C/s. The start and finish points of the austenite transformation are identified from the dilatation curves and then the continuous cooling transformation (CCT) diagrams are constructed. These diagrams are verified by microstructural characterization and Vickers micro-hardness. In general, results revealed that for slower cooling rates (0.1-0.5 °C/s) the present phases are mainly ferritic-pearlitic (F+P) structures. By contrast, a mixture of bainitic-martensitic structures predominates at higher cooling rates (50-200°C/s). On the other hand, CCT diagrams show that B addition delays the decomposition kinetics of austenite to ferrite, thereby promoting the formation of bainiticmartensitic structures. In the case of B microalloyed steel, the CCT curve is displaced to the right, increasing the hardenability. These results are associated with the ability of B atoms to segregate towards austenitic grain boundaries, which reduce the preferential sites for nucleation and development of F+P structures. INTRODUCTION The advanced ultra-high strength steels (A-UHSS) such as dual phase (DP), complex phase (CP), boron steels (BS) and martensitic steels (MART) are taking a strong interest in various industrial sectors, particularly the automotive industry. These latest generation steels with multiphase microstructures are characterized by an excellent combination of high strength, good toughness and ductility [1]. However, in order to ensure the industrial application is necessary to know important aspects such as its phase transformation kinetics under nonequilibrium conditions. The overall CCT kinetics can be readily described by the CCT phase diagrams [2]. This kind of diagrams provides precise information on the nature of microconstituents formed from the anisothermal decomposition of austenite. The formation of each transformation product is mainly described by the transformation-start temperature and by the formation cooling rate range. In the practical situation these diagrams are mainly used to predict the microstructure and hence the mechanical properties of steel after thermal and/or thermo-mechanical
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