Ultra-High-Strength Interstitial-Free Steel Processed by Equal-Channel Angular Pressing at Large Equivalent Strain

The billets of interstitial-free (IF) steel are deformed by equal-channel angular pressing (ECAP) at 298 K (25 °C) adopting the route B_C up to an equivalent strain (ε _vm) of 24. The evolution of microstructures and their effects on the mechanical properties are examined. The microstructural refinement involves the elongation of grains, the subdivision of grains to the bands with high dislocation density, and the splitting of bands into the cell blocks and then cell blocks into the cells. The widths of the bands and the size of cells decrease with strain. The degree of reduction in the grain size is highest at the low strain level. However, most of the boundaries at this stage are of low-angle boundaries (at ε _vm = 3). Thereafter, the misorientation angle increases by progressive lattice rotation with strain. The coarse bands transform step by step from the lamellar structure to the ribbon-shaped grains and finally to the near-equiaxed grain structures with the subgrains of a saturated low-angle grain boundary fraction of 0.34 at very large strain >15. The as-received coarse-grained microstructure (grain size of 57.6 ± 21 µm) has been refined to 257 ± 48 nm at an equivalent strain of 24. The strength increases considerably up to ε _vm = 3 due to grain refinement and high dislocation density. However, the strengthening at later stages is mainly due to the increase in misorientation angle and refinement. Initial yield strength of 227 MPa is increased to a record value of 895 MPa on straining up to ε _vm = 24 at 298 K (25 °C). Uniform elongation decreases drastically at low equivalent strain but it regains marginally later. The ECAPed sample fails by a ductile fracture at ε _vm = 0.6 to 6 but by a mixed mode of ductile–brittle fracture at larger strain of 9 to 24.