Sebastián F. Medina

Bio: Sebastián F. Medina is an academic researcher from Spanish National Research Council. The author has contributed to research in topics: Microalloyed steel & Austenite. The author has an hindex of 26, co-authored 99 publications receiving 2185 citations.

Improved Model for Static Recrystallization Kinetics of Hot Deformed Austenite in Low Alloy and Nb/V Microalloyed Steels

Abstract: Using torsion tests a improved model has been constructed to predict the static recrystallization kinetics of deformed austenite in low alloy and microalloyed steels. The model quantifies the influence of the most common elements (C, Si, Mn, Mo) in low alloy steels and the typical elements (V, Nb) in microalloyed steels, when they are in solution. Activation energy (Q) is the parameter sensitive to the content and nature of each alloying element, and an expression for Q is shown as a function of the percentage of each one. Nb is the element that contributes most to increasing the value of Q, and thus that which most delays recrystallization kinetics. C is seen to be the only alloying element that contributes to lowering the value of Q, and thus to accelerating recrystallization kinetics. Extrapolation of the expression of Q to pure iron in the austenitic phase gives a value of 148 637 J mol-1, which is similar to other values found in the literature for the grain boundary self-diffusion energy of pure Feγ . Static recrystallization kinetics follow Avrami's law and expressions are given for the parameter t0.5 and the exponent n.

Basal plane texture and flow pattern in friction stir weld of a magnesium alloy

Abstract: The microtextures in a friction-stir-welded magnesium (Mg) alloy, AZ61, with a nugget-shaped stir zone were analyzed by orientation imaging microscopy (OIM). The base material had a (0002) 〈uvi0〉 texture ((0002) parallel to the sheet plane, 〈uvi0〉 parallel to the welding direction). Friction stir welding produced texture components different from those of the base material in the stir zone. Except for the upper surface of the plate, most of the stir zone had a texture with a strong tendency for the (0002) basal plane to align with the surface of the hard pin of the welding tool. Formation of this texture component was attributable to shear deformation arising from the rotation of the hard pin. The (0002) planes tended to align with an ellipsoidal surface in the nugget-shaped stir zone. The present study suggests that the nugget shape, which is a characteristic feature of the stir zone, is strongly related to formation of the ellipsoidal surface described by the (0002) basal plane.

Phase Transformation and Mechanical Properties of Si-Free CMnAl Transformation-Induced Plasticity-Aided Steel

Abstract: Conventional CMnSi transformation-induced plasticity (TRIP)-aided steels are a promising solution for producing lighter, crash-resistant car bodies, due to their high-strength and large uniform elongation. The CMnSi TRIP-aided steels, with more than 1 mass pct Si, have the drawback of poor galvanizability due to the presence of complex Si-Mn oxides on the surface. The full substitution of the Si by Al in cold-rolled and intercritically annealed TRIP-aided steels, therefore, was evaluated in detail. The phase-transformation kinetics during the intercritical annealing and the isothermal bainitic transformation were investigated by means of dilatometry. The allotropic phase-boundary was determined both by thermodynamic calculations and the experimental determination of the C content in the retained austenite. The results imply that short isothermal bainitic transformation times are sufficient to obtain the TRIP microstructure and that the processing of CMnAl TRIP-aided steels in a continuous annealing line not equipped for overaging is possible. The mechanical properties were evaluated for CMnAl TRIP-aided steels obtained using an industrial thermal cycle: the properties matched those of the conventional CMnSi TRIP-aided steels, where it was found that the high-Al CMnAl TRIP-aided steel had a high strain-hardening coefficient of 0.25, which was stable up to a true strain of 0.25.