Regulation on Annealing Multiscale Laminar Structure and Mechanical Characteristics of Plain Medium Carbon Steel by Warm Pre-deformation Extent

A heterogeneous structure of multi-scale ferrite and granular or short rod cementite was obtained by warm-rolled pseudo-eutectoid microstructure, and the recrystallization of deformed ferrite and the dissolution and precipitation of cementite were controlled by subsequent short-time annealing treatment. The results show that the ferrite grains of annealed samples with different deformation extent distributed in the multiscale laminar structure, and the grain size trans from nanoscale to micron-scale. With the increase of deformation, the recrystallization ratio increases. The annealing sample with deformation of 84 pct obtains the optimal microstructure of multi-scale lamellar ferrite cementite that effectively improves the strain-strengthening ability, and realizes the optimal combination of strength and plasticity. The accumulation of more GNDs in a gradient distribution at the heterogeneous interface provides good strain reinforcement with the highest boundary density, which helps to provide additional strength and plasticity. The multi-stage regulation of ferrite recrystallization and cementite distribution after annealing provides good elongation to achieve a combination of HDI mechanism, fine grain strengthening, and Orowan strengthening mechanism. Various models were used to fit the true stress–true strain, and the Voce-Kocks model accurately described the stress–strain of plain medium carbon steel with a heterogeneous laminar structure. Graphical Abstract