Development of sintered steels with twip effect

TWIP Steels (TWinning Induced Plasticity) are Advanced High Strength Steels (AHSS) steels that combine extremely high strength with high ductility and formability. They have high manganese content (17-24%) that causes the steel to be completely austenitic at room temperature. This microstructure is responsible for twins to form inside the grains during deformation, which causes a high value of the strain hardening coefficient as the microstructure becomes finer. The mechanical resistance can reach values even greater than 1000 MPa. Often, resistance is achieved at the expense of formability and ductility and thus, currently a global investigation is directed towards TWIP steels. The fact that TWIP steels contain a high degree of alloying elements complicates their production process. Currently, the only route of manufacture is by continuous casting, followed by hot and cold rolling, and annealing heat treatment. In each of the stages many obstacles are to be overcome. Some of the problems encountered are dendritic segregation during solidification, delayed fracture and selective oxidation. From the point of view of mechanical properties, TWIP steels have an anisotropic behavior in tension-compression that manifests as spring-back during cold forming and a low elastic limit that restricts its use in elements that are at risk of collision. This thesis aims at using mechanical milling as an alternative route to develop TWIP steels to avoid the problems caused by the conventional method. The use of powders will avoid problems of segregation and all those associated with melting and casting. In this thesis, different kind of metallic powders with a global composition Fe-22Mn-1.5Si-1.5Al-0.8C and Fe-22Mn-1.5Si-0.8C were blended and milled in a planetary mill at different conditions to get an ultrafine powder. The powder was cold compacted and subsequently sintered at 1200°C during 1 hour. The microstructure was characterized by EBSD and XRD to confirm a fully austenitic microstructure and the increase in strength was determined by hardness tests. Increasing milling time was found to favor the transformation of Ferrite (Fe-α) to Austenite (Fe-γ). Increasing milling time also increased the relative density. Increasing milling time increased the density of twins and helps create steels with finer grains with increased resistance. Aluminum was found to impede the formation of austenite and its presence made FeSi stable at milling times strictly inferior to 25h. The samples were subjected to selective oxidation even when the experiments were carried out in inert (Ar gas) atmosphere