Ultra-High-Strength Interstitial-Free Steel Processed by Equal-Channel Angular Pressing at Large Equivalent Strain
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INTERSTITIAL-FREE (IF) steels find applications in the automotive industries for the fabrication of complicated body parts due to their high formability and high planar anisotropy. However, the lower strength of the material demands higher section thickness leading to an increased mass and consequent decreased fuel efficiency.[1,2] It has been suggested that the strength can be improved significantly by the grain refinement.[3] The grain size can be reduced to an ultrafine level by severe plastic deformation (SPD) techniques,[4] among which equal-channel angular pressing (ECAP) has become a popular technique.[2–20] In general, the microstructural refinement is dependent on the amount of strain, type of routes as well as the geometry of the die.[4,21,22] The degree of refinement in IF steel increases with increasing the amount of total equivalent strain[7,10,16] and equivalent strain per pass and increasing the severity or decreasing the intersection DEEPA VERMA, Research Scholar, N.K. MUKHOPADHYAY and G.V.S. SASTRY, Professors, and R. MANNA, Associate Professor, are with the Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi, Uttar Pradesh 221005, India. Contact e-mail: rmanna.met@iitbhu. ac.in Manuscript submitted January 9, 2015. Article published online February 1, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
angles.[8] Among these, the amount of strain has a strong effect on the microstructural modification. Various IF steels have been deformed up to a maximum equivalent strain of 9.2 to attain a microstructural refinement up to 200 to 300 nm.[2,5–7,12,14,16–19,23] The rate of microstructural refinement is found to be more in case of the route BC than the other routes.[6,7,20,24–26] Yoshinori et al.[6] have reported that the microstructural refinement and the evolution of the high-angle grain boundaries (HAGBs) are more in route BC due to the duality in shearing directions. In route BC, the orthogonal change in the shear direction activates new slip systems and yield strength increases due to the resistance from the previous dense dislocation walls (DDWs).[20,24–26] However, upon interaction of old DDWS by the newly formed dislocations, flow softening occurs, leading to plastic instability and the formation of shear bands. The intersection of different shear bands formed between two orthogonal orientations of the billet fragments the grains into smaller grains. In route C, the direct reversal of the strain path lowers the rate of the grain refinement due to the injection of dislocations of opposite sign. Therefore, the grain refinement is faster in B than in A or C. In route BA, the billets are rotated by 90 deg in an alternative direction between the two consecutive passes and the dislocations of opposite sign interact and nullify. Therefore, the route BC is more effective than the route BA for the effective grain refinement. VOLUME 47A, APRIL 2016—1803
Usually, the deformation of billet is uniform at the center only. The front end or nose and back end or tail are
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