The interest on TWIP steels has steadily increased since they were first developed about a decade ago. The main characteristics of these steels is their elevated ductility, above 50%, and a high strain hardening coefficient, which leads to ultimate tensile strengths above 1400MPa. According to these mechanical properties, TWIP steels are promising for applications in the automotive industry, allowing weight reduction of the components, and thus lowering consumption and CO2 emissions, while their energy absorption capability will increase the safety of the passengers in the event of a collision. To reach their exceptional properties, TWIP steels are alloyed with 18-25%Mn, in addition to other elements such as C and/or Al-Si. The objective of these compositions is the generation of austenitic microstructures at room temperature and the control of the stacking fault energy of the alloy to assure that twinning is a likely deformation mechanism. In fact, twinning is the origin of the excellent combination of high strength and ductility of TWIP steels. However, the fabrication of TWIP steels implies some difficulties due to the high alloying degree, as well as the nature of the alloying elements. TWIP steels are now produced as thin sheets following the continuous casting plus hot and cold rolling route. In terms of processing, the main problems which exhibit TWIP steels are Mn losses during casting, big segregation during solidification, hot shortness, elevated reheating temperatures and times prior to hot rolling and tendency to generate heterogeneous microstructures. From the point of view of the mechanical properties, TWIP steels have some limitations, such as their low yield strength, delayed fracture after cold forming operations and low corrosions resistance. This project is a comprehensive evaluation of the different factors which could mitigate some of the problems previously described, exploring the benefits of applying alternative processing routes. For this purpose, different compositions will be designed based on thermodynamic calculation, a reference Fe-Mn-Al-Si-C TWIP steel and three other improved TWIP steels with improved based on their composition (through microalloying and Cr additions). Theses steels will produced following the conventional route as well as other alternative routes based on powder metallurgy, because segregation can be minimized and more homogeneous microstructures can be obtained. In particular, MIM technology has been chosen and powders will be produced following different procedures: atomization, mixture of ferroalloys and mechanical milling of previously atomized powders. Moreover, the severe plastic deformation route by ECAP will also be explored to improve the properties trough the generation of ultra-fine grained microstructures. The subsequent evaluation of the mechanical behavior of the steels subjected to the different processing routes will be used to establish new limits to the properties which can be achieved for TWIP steels.
Plan Estatal de Investigación Científica y Técnica y de Innovación 2013-2016
Programa Estatal de I+D+i Orientada a los Retos de la Sociedad
Retos de Investigación: Proyectos de I+D+i
Gobierno De España. Ministerio De Economía Y Competitividad, Mineco