Microstructure and Mechanical Properties of Semi-continuous Equal-channel Angular Extruded Interstitial-free Steel

Abstract

An innovative method called semi-continuous equal-channel angular extrusion (SC-ECAE) has been developed to produce ultrafine grained steel by inducing severe plastic deformation. In contrast to the external forces that are exerted on specimens in traditional ECAE, the driving forces are applied on the dies in the novel SC-EACE process. Commercial interstitial-free steel sheets with width of 160 mm and thickness of 2 mm were processed repeatedly to various passes at room temperature using this method. The microstructural evolution was characterized using high-resolution electron backscatter diffraction (EBSD), and the mechanical properties were investigated by tensile testing. The EBSD images indicated that the fraction of high-angle boundaries (HABs) began to increase gradually after four passes; after six passes, elongated HAB structures with nearly submicron-scale average spacings were formed. The tensile testing results showed that strengthening was accompanied by a decrease in tensile ductility, but no significant anisotropy was observed. After 10 passes, a final HAB fraction of about 90% and an overall grain size of 0.55 μm, yield strength of 638.7 MPa, an ultimate tensile strength (UTS) of 710.3 MPa, and a total elongation of 12.0 % were obtained.