Influence of Boron on the Precipitation Kinetics in Advanced Ultra-High Strength Steels
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Influence of Boron on the Precipitation Kinetics in Advanced Ultra-High Strength Steels G. Altamirano1, I. Mejía1, A. Hernández-Expósito2,3, J.M. Cabrera2,3 Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo. Edificio “U-5”, 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. 1
ABSTRACT In the present work, the stress relaxation method was employed to determine the influence of B addition on the kinetics of strain-induced precipitation and its interaction with the static austenite recrystallization. For this purpose, the behavior of two low carbon advanced ultra-high strength steels was analyzed during stress relaxation tests at different temperatures and constant pre-strain rate. The precipitation start (Ps) and finish (Pf) times were determined from the relaxation curves and then the corresponding precipitation-time-temperature diagrams were constructed for each steel. Transmission Electron Microscopy was used to determine the chemical nature and evolution of precipitation. In general, the results show that the addition of B retards the austenite recrystallization, tends to accelerate the precipitation kinetics of carbonitrides and leads to a finer and denser distribution of precipitates. These results are discussed in terms of the driving force for the nucleation of precipitation, which in turn is controlled by the degree of supersaturation of microalloying element and as a function of B segregation and B-vacancy complexes to dislocations and grain boundaries. INTRODUCTION The advanced ultra-high strength steels grades are usually microalloyed steels mainly characterized by multiple microstructures, derived from very specific chemical compositions in combination with an appropriate thermomechanical treatment [1]. The high strength is achieved by various mechanisms such as grain refinement, multiple phase coexistence and precipitation hardening of carbides, nitrides and/or complex carbonitrides [2].The strain-induced precipitation of the microalloying elements plays an important role in controlling the final microstructure and hence, in the mechanical properties of steel [3]. Besides affecting the austenite decomposition, the strain-induced precipitation retards or inhibits the austenite recrystallization and is responsible for the hardening and obtaining of fine microstructures in thermomechanically processed microalloyed steels [4, 5]. For all these reasons, the precipitation kinetics of the most commonly used microalloying elements such as Nb, V and Ti have been widely studied and the precipitation-time-temperature diagrams determined for various steel compositions and experimental conditions [6-18]. Amongst all the microalloying elements, most studies agree the Nb carbonitrides exhibit the strongest effect in retarding the austenite recrystal
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