Achieving ultra-fine grained microalloyed steel by severe plastic process

Latterly, it is of interest to enhance combination of strength and toughness of steel for advanced products, therefor the objective of this research is to improve the mechanical properties of high strength low - or ultra - low alloy (HSLA) steels through grain refinement. The ultra-fine grained S420NL steel were produced by high speed high pressure torsion (HSHPT) at room temperature. This severe plastic deformation method was chosen because it is one of the most feasible and rapid ways for nanostructuring various metallic materials down to ultrafine grain range. The procedure key parameters of deformation are described in detail in the paper. The influence of severe plastic deformation applied in cast state steel on microstructure changes and deformation behaviour was investigated. Optical microscopy, SEM and energy dispersive x-ray spectroscopy (EDX) analysis carried out on the samples revealed that severe plastic deformation led to the subsequent formation of an ultrafine-grained microstructure. Sample hardness increased rapidly with progress of torsion deformation degree. (paper)