Procesado y caracterización de propiedades mecánicas de acero twip mediante técnicas pulvimetalúrgicas

Resumen

Twinning induced plasticity (TWIP) steels possess both, very high tensile strength and large elongation together with high formability and an outstanding high strain hardening. The unique combination of its distinctive features makes possible to reduce cross section of components providing as well enhanced impact energy absorption ability, which in turns make them attractive for its commercialization. Nevertheless, their complicated fabrication process has limited the widespread of their industrial production. In this thesis it has been explore whether TWIP steel production is feasible by means of four powder metallurgy processes, as alternative to conventional fabrication route because of the multiple issues presented along the fabrication process of these kind of steels. It has been obtained samples from the mixture of elemental and ferroalloy powders processed by conventional powder metallurgy and by mechanical alloying techniques. It has been obtained samples as well from prealloyed Fe-22Mn-0,4C-1,5Al-1,5Si atomized powder processed through mechanical milling technique and metal injection molding technology. Manufactured bulk samples have been evaluated to determine its consolidation degree. In order to validate obtaining a TWIP steel, chemical composition and X-ray diffraction analyses have been performed, as well as microstructural and mechanical characterization. Microstructural characterization has been carried out by optical microscopy, scanning electron microscopy and EBSD analyses, and the mechanical properties have been evaluated by microhardness and microtensile tests. In general, in the conventional powder metallurgy and mechanical alloying techniques it has been formed complex microstructures due to diffusion and oxidation issues. On the other hand, in the mechanical milling and metal injection molding techniques it has been developed the typical crystallographic phase and microstructure of TWIP steels. However, it has been observed that both, the mechanical milling technique as well as the metal injection molding technology require quite high sintering temperatures in order to achieve the usual relative density of those techniques. In the case of metal injection molding, it has been used a bimodal powder distribution that improved the feedstock packing density in such a way that make it possible to increase the metallic loading for the same flow characteristics. In the final pieces the microtensile test have demonstrated that strain hardening rate vales are sustained along the plastic regime, typical in TWIP steels, due to the activation of mechanical twinning. The tensile strength, yield strength and microhardness values are like those of TWIP steels obtained by conventional fabrication process. Regarding the samples manufactured by mechanical milling, they have had higher strain hardening rate, yield strength and microhardness values, but less ductility. By means of metal injection molding technology it has been possible to manufacture final pieces with microstructural and mechanical properties similar to those obtained by the conventional fabrication route, on the other hand through mechanical milling technique it has been possible to manufacture pieces with higher yield strength that enables them for industrial applications.