Effect of Microalloying Elements (B, Nb, V and Ti) on the Strain Hardening Behavior of High-Manganese TWIP Steels
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Effect of Microalloying Elements (B, Nb, V and Ti) on the Strain Hardening Behavior of High-Manganese TWIP Steels. F. Reyes-Calderón1, I. Mejía1 and J.M. Cabrera2,3 1
Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, Edificio “U-5”, Ciudad Universitaria. 58060-Morelia, Michoacán. México. [email protected], [email protected]. 2 Departament de Ciència del Materials i Enginyeria Metal·lúrgica, ETSEIB, Universitat Politècnica de Catalunya, Av. Diagonal 647. 08028-Barcelona, Spain. [email protected]. 3 Fundació CTM Centre Tecnològic, Av. de les Bases de Manresa 1. 08242-Manresa (Barcelona), Spain. ABSTRACT The present research work analyses the influence of microalloying elements (B, Nb, V and Ti) on the tensile strength and the strain hardening behavior of a high-manganese TWIP steel. The analysis was carried out by means of true stress-true strain curves derived from uniaxial tension tests. The work hardening exponent was determined by using the Hollomon and differential Crussard-Jaoul models. Metallographic characterization was carried out to determine the metallurgical changes associated with n values. The results indicate that the Hollomon analysis results in strain hardening exponent values up to 0.46. On the other hand, the differential Crussard-Jaoul analysis establishes a clear distinction of n value for two stages of plastic deformation which are determined by a sharp slope change in the plot of ln(dV/dH)-lnH. INTRODUCTION TWIP steels have been widely considered for many industrial applications, particularly in the automotive sector. These steels can provide both high strength and large ductility because twins can produce plastic deformation and, at the same time, twin boundaries act as an obstacle for dislocation movement [1]. The occurrence of twinning depends strongly on the stacking fault energy (SFE), which in turn depends on chemical composition and temperature [2]. The twinning effect produces a high value of the strain hardening exponent n. Xiong et al. [3], investigated the dynamic tensile properties of TWIP steels and determined that n increases with strain rate due to formation of twins during deformation, which then become an obstacle for dislocation slip. The strain hardening evolves with deformation such that: (i) at the beginning of plastic yielding deformation by dislocation slip is the dominant deformation mode and n decreases; (ii) once the stress required to initiate twinning is reached, primary twins with similar orientations proliferate thereby increasing the n value at a constant rate; (iii) at even higher stresses, secondary twins are formed and intersect primary twins and further increases the n exponent. When secondary twinning occurs while primary twinning is still active, the two mechanisms overlap and this causes a gradual decrease in n value [4, 5]; (iv) further straining of previously formed twins hinder the production of new twins thereby decreasing n. It must be emphasized that the relative extent of each of these strain harden
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