Effect of cool deformation on mechanical properties and microstructure of Nb microalloyed steels

The improvements in mechanical properties of microalloyed steels result mainly from the refinement of ferrite grain size together with a controlled amount of precipitation strengthening. Applying small amounts of deformation, at very low hot working temperatures (e.g. coiling temperature), in the ferrite region (i.e. cool deformation) is found to improve strength and toughness by controlling the final microstructure of the steel. The main reason is thought to be the much finer and more uniformly dispersed precipitates in the steel matrix.;It has been found that these alloying elements respond very well to cool deformation, with the strength being highest in steels containing both Nb and Cu. However, there was also a cool deformation effect in the non Nb and Cu bearing steel. It was confirmed that precipitation plays a key role in the effect of cool deformation, with much of the precipitation taking place dynamically. However, static processes also seem to have a measurable effect on room temperature properties. Even low amounts of copper (e.g. ∼0.4 wt%) can contribute to strengthening of the steel. The Cu addition is found to affect the mechanical properties by affecting the precipitation and growth of Nb compounds. However, it was not possible to analyze the composition of the very small precipitates (below 10nm), and it cannot be discounted that some Cu and Ti bearing precipitation was also occurring during cool deformation.;In the present study, the effects of Nb and Cu on mechanical properties and corresponding microstructures in steels with different levels of cool deformation are investigated. The mechanical properties of the samples were determined with shear punch testing and the microstructure was examined by scanning and transmission electron microscopy. Thermodynamics simulations with FactSage were done to further analyze the possibility of precipitation of different elements.