Formation Mechanisms of Ultrafine Grained Structure in Severe Plastic Deformation of Low Carbon Steel

摘要

It has been well-known that severe plastic deformation (SPD) over equivalent strain of 4 can realize ultrafine grained (UFG) bulk metallic materials with mean grain sizes smaller than 1 μm. However, the formation mechanism of the UFGs during SPD has not yet been fully understood but rather confused, since it seems significantly different from conventional recrystallization. The purpose of the present paper is to clarify the formation mechanism of the UFGs in SPD of low-carbon steel with ferrite (BCC) matrix. An ultra-low carbon interstitial free (IF) steel sheet was intensely deformed up to equivalent strain of 5.6 by the accumulative roll bonding (ARB) process, which is a SPD process using rolling deformation. The microstructure and crystallographic features of the specimens deformed to various strains were analyzed by means of FE-SEM/EBSD. The evolution of the microstructure together with the misorientation evolution in ferrite during SPD was clarified. It could be concluded from the experimental results that the formation mechanism of the UFGs is 'ultrafine grain subdivision' essentially. Consequently, the UFGs fabricated by SPD have characters of both the 'grains' having large misorientations to each other and the deformation microstructure. The microstructural observations on the ARB processed and annealed specimens indicated that the annealing process of SPD materials is greatly different from discontinuous recrystallization characterized by nucleation and growth but rather continuous.