Development of Bulk Ultrafine-Grained Low Carbon Steel through Equal-Channel Angular Pressing and Post Processing

Abstract

The objectives of the present investigation are to optimize processing conditions to develop ultrafine-grained (UFG) low carbon steel through equal-channel angular pressing(ECAP) and post processing by cold rolling and cryorolling to further grain refinement to obtain ultra high strength in the material. Another objective is to regain the ductility in UFG low carbon steel with maintaining high strength through developing bimodal grain size distribution of ultrafine grains and micronsized grains by suitable thermal treatments of UFG low carbon steel. Finally, correlations among microstructure, texture and mechanical properties will be established. Low carbon (0.8wt%C) steel work pieces of 15 mm diameter is deformed through an ECAP die upto an equivalent strain, εvm =16.8 adopting route Bc at room temperature. The ECAPed material is further deformed by coldrolling upto 80% reduction in area and cryorolling at -50°C upto 75% reduction in area followed by flash annealing at various temperatures (475-675°C). Microstructures are examined by optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD) from the bulk specimens. EBSD Data is analysed for misorientation, grain boundary fraction and microtexture. Bulk texture, dislocation density and elastic stored energy are measured from X-ray diffraction (XRD) study. Mechanical properties are evaluated by microhardness measurement,tensile testing, and fractography. Ultrafine-grained structure of average grain size of 0.2μm with high angle of misorientation, ~40°, has been achieved in the coarse-grained low carbon steel by by ECAP at εvm=16.8 at room temperature. The rate of refinement is high at initial stages of deformation but it decreases with the amount of imposed strain. At initial stages of deformation most of the grain boundaries are of a low angle of misorientation. Lowest level of low angle grain boundaryfraction is < 20% at εvm=16.8. Defect density increases with imposed strain and reaches a maximum value at intermediate strain but decreases to lower level at higher strain.