An Investigation Of Grain Refinement Mechanism Of Low-carbon Steel By Equal Channel Angular Processing

Equal Channel Angular Pressing (ECAP) is a processing procedure in which a material is subjected to intense plastic straining but without the introduction of any change in the cross-sectional dimensions of the sample. It is a deformation route of serve pure shear. ECAP is successfully applied in the production of Submicrometer Grain Materials(SGM).ECAP of low carbon steel(0.15wt%C) was successfully carried out in this study. An equivalent true strain of ~9.2 is obtained after 8 passes of ECAP in this study. The microstructure evolution in ECAP was investigated by means of transmission electron microscope. The results show that:1. The grain sizes of low carbon steel(0.15wt%C) was reduced to the submicrometer range by using ECAP at room temperature.2. Processing route is a very important fact of grain refinement in ECAP besides temperature. A near-equiaxed cell microstructure achieved after two passes in plane T. Equiaxed grain with clarity boundaries and average grain size of 0.25 u m formed after four passes. Same results appears in plane L.By means of route C, structure demonstrated the characters of distinct band after eleven passes. The result illuminates that Route Be leads most expeditiously to a microstructure of reasonably equiaxed ultrafine grain.3. Microstructure evolution using route Be is investigated by four planes of T, L, S and R. Experimental observation of each plane is different and plane S , R isfirst conducted. Dislocation cell most appears in plane S but deformation cross band appeared in plane R after two passes. The same phenomena is achieved in latter passes. The results suggest that cross slip of different pass is not a mechanism of grain refinement, cross deformation band on plane R and double slip or many slip is other mechanism of grain refinement.4. At the initial stage of pressing, submicrometerorder ferrite grains enclosed by serrated and lowangled boundaries were formed by operation of the multi-slip systems belonging to the {110}<111>and {112}<111> slip system family. As the pass number of pressing increases, the portion of high-angle grain boundaries increased. It indicates that a portion of high angle boundary increased with increasing the pass number by either the conversion of existing low angle boundary to high angle boundary or new formation of high angle boundary. Subgrain rotation is more favorable than intragranular strain for accommodating a large amount of strain since serrated boundaries restrict dislocation movement causing intragranular strain.5. Microstructure achieved in higher number passes of ECAP, the corresponding selected area diffract pattern showed that intensity of {110},{112}and {123} slip planes are more than other plane. This show that these slip plane exert key function one side, the other side grain misorientation of microstructure material formed by ECAP is not random absolutely, it has tightness relations with slip plane of deformation.