Speaker
Description
Niobium being a strong carbide former has the propensity of significantly contributing to strength increase by dispersing copious ultra-fine particles in the iron matrix. According to the well-known Ashby-Orowan theory, a homogeneous particle dispersion with closest inter-particle spacing results in the highest strength contribution. From that point of view, it is important to understand how NbC precipitates nucleate and which are the ideal thermo-mechanical processing conditions to achieve the best strengthening potential. In this contribution, a variety of thermo-mechanical treatments will be presented for analyzing the strengthening contribution of niobium precipitation in different low carbon steels. A method to derive precipitation strengthening from the true stress-strain curve originating from tensile testing will be detailed. This one allows identifying direct influences of precipitates on the early yielding behavior and the subsequent Lüders deformation. Applying this methodology to a large variety of niobium microalloyed low carbon steels resulted in a robust data set indicating the expectable precipitation strengthening capability under various processing conditions. The data reveal that the strengthening contribution most typically is in the range of 150-180 MPa. Under particular processing conditions a strength increase of 200-220 MPa could be achieved. These findings based on tensile tests are complemented by transmission microscopic analysis. Evidently, the particle distribution is related to the thermo-mechanical processing history. While adding more niobium to the alloy does not necessarily further augment the strength the combination with other alloying elements can have a positive effect. The gathered insights allow working out recommendations for reliable alloying and processing concepts optimizing precipitation strengthening.