Abstract
Interstitial-free steel workpieces are deformed by equal-channel angular pressing (ECAP) for equivalent strain εvm = 3 and εvm = 21 followed by flash annealing. Microstructures are analyzed by optical microscopy, scanning electron microscopy and transmission electron microscopy. Mechanical properties are evaluated by hardness testing. Yield strength of materials is calculated from hardness values. Flash annealing (at 675 °C) of ECAPed samples for εvm = 3 and εvm = 21 results in abnormal subgrain growth and abnormal grain growth, respectively. Flash annealing at 700 °C of ECAPed (at εvm = 3) IF steel converts abnormally grown subgrains to grains which serve as nuclei for recrystallization and that result in bimodal grain size distribution. Bimodal grain size distribution is also produced when ECAPed IF steel for εvm = 21 is flash annealed at 675 °C due to abnormal grain growth or secondary recrystallization. Flash annealing of IF steel samples ECAPed for low εvm, in the temperature range 600-675 °C, decreases the hardness continuously with increase in the annealing temperature but it increases at high εvm. The former is due to annihilation of defects but the later is caused by ordering of nonequilibrium boundaries. The hardening and strengthening behaviors are similar.
Highlights
Interstitial-free (IF) steel is one of the recently developed steels used widely in the automotive industry because of its excellent deep drawability and high planar isotropy when the microstructure contains micrometer size grains (Ref 1) but it possesses low strength (Ref [2, 3])
When ECAP3 sample is flash annealed at 650 °C, the morphology of grains is maintained (Fig. 2c) and the bands grow to 459 ± 87 nm by grain boundary migration
On flash annealing of ECAP21 sample at 600 °C, fine grains increase to average size of 377 ± 94 nm (Fig. 4a, 5a) but the elongated nature is maintained
Summary
Interstitial-free (IF) steel is one of the recently developed steels used widely in the automotive industry because of its excellent deep drawability and high planar isotropy when the microstructure contains micrometer size grains (Ref 1) but it possesses low strength (Ref [2, 3]). Previous reports have shown that the ductility of UFG materials can be enhanced without compromising the strength by appropriate annealing treatments after SPD (Ref [22,23,24,25]).This improvement was attributed to the bimodal grain size distribution introduced into the deformed microstructure with the combination of fine and coarse grains (Ref [25,26,27,28,29]). In case of commercial purity UFG Al, considerable efforts to introduce a bimodal grain structure failed (Ref 31), since grain coarsening occurred in a more or less homogeneous manner The reason for this is reported as strong and rapid initial recovery and no left driving
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