Microstructural and micro hardness variation during low temperature recovery annealing in C-Mn high strength Steels

Recovery was studied in four different grades of steel, ULC, HSLA and DP600 and DP800 with varying C-Mn content. All the steels samples were 80% cold rolled and annealed at 300 and 500°C for 30, 60,120, 300 minutes, mainly to study the hardness and microstructural changes. Specimens were further tested using micro Vickers hardness tester. Microstructural changes were observed using scanning electron microscope. The cold rolled microstructure consisted of ferrite in ULC and ferrite-pearlite in HSLA, DP600 and DP800 steels. All the grains were pancaked and aligned in the rolling direction. The Hardness values increases for annealed specimens at 300 and 500°C in HSLA, DP600 and DP800 compared to cold rolled condition, which can be attributed to hardening of the ferrite matrix due to pinning of dislocations by carbon atoms. The hardness values for annealed specimens in ULC and DP800 decreases at 300 and 500°C which is attributed to softening.