DUCTILITY OF ADVANCED BAINITIC STEELS F.G. Caballero 1 , C. García-Mateo 1 , J. Chao 1 , M.J. Santofimia 2,3 , C. Capdevila 1 and C. García de Andrés 1 1 Materalia Research Group, Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalúrgicas (CENIM), Consejo Superior de Investigaciones Científicas (CSIC), Avda. Gregorio del Amo, 8. E-28040 Madrid, Spain, fgc@cenim.csic.es 2 Netherlands Institute for Metals Research, Mekelweg 2, 2628 CD Delft, Netherlands 3 Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD, Delft, Netherlands Keywords: ductility, TRIP effect, bainite, steels Abstract In this work the effect of the chemical composition, morphology, size, and distribution of the retained austenite on the mechanical stability of this phase has been studied to determine the role that these parameters play on the ductility behaviour of advanced bainitic steels consisting of a bainitic ferrite matrix and a mixture of austenite and martensite. Results suggest that apart of the retained austenite, the morphology of the bainitic matrix is an important factor controlling ductility. Introduction A novel microstructure, carbide free bainite, has achieved the highest strength and toughness combinations to date for bainitic steels in as-rolled conditions. By alloying designing and with the help of phase transformation theory, it was possible to improve simultaneously the strength and toughness because of the ultra-fine grain size of the bainitic ferrite plates (less than 1 µm). Ultimate tensile strengths ranging from 1600 MPa to 1800 MPa were achieved while keeping a total elongation higher than 10 % [1]. Their toughness at room temperature matches tempered martensitic steels, known to be the best-in-class regarding this property. It is believed that tensile elongation in these steels is mainly controlled by the amount of retained austenite [2]. The retained austenite is a ductile phase compared to the bainitic ferrite and would be expected to enhance ductility as far as the austenite is homogeneously distributed between plate boundaries (film austenite). However, isolated pools of austenite (blocky austenite) would influence unfavourably on both elongation and strength presumably, because the strain localization in these areas [3]. Therefore, every effort has to be made to reduce the fraction of blocky austenite. Further improvement can be achieved by TRIP effect i.e. strain induced transformation of retained austenite to martensite. In order to take full advantage of this effect, the mechanical stability of the austenite, i.e. its capability to transform to martensite under strain, must be controlled. In this sense, the role of retained austenite on the mechanical properties of ultra high strength bainitic steels has been analysed in this work. The effect of the chemical composition, morphology, size, and distribution of the retained austenite on the mechanical stability of this phase has been studied. Moreover, the influence of the amount and size of martensite, and the morphology of bainitic ferrite matrix on the ductility behaviour of advanced bainitic sheet steels has been also examined. Materials and Experimental Procedure Chemical composition of these novel bainitic steels was designed to have the same bainitic transformation region in the TTT diagram and the same T o curve [1] than those of Ni2 bainitic steel designed in previous work [4] and taken here as reference (Table 1). The reference steel proved to have impressive mechanical properties when fully transforms to bainite [5]. Designed alloys were manufactured by ARCELOR RESEARCH (France) as 180 x 80 x 12 mm 3 plates. Their actual composition is shown in Table 1. Hot rolling simulations were performed on ARCELOR pilot plant.