Development of high strength low alloy TRIP-aided steels with ultra fine grain

The effects of thermomechanical processing and prior structure on microstructure and retained austenite characteristics of high-strength low alloy 0.20C-1.5Si-1.5Mn TRIP-aided steels were investigated to enhance the workability limit. Ultra fine polygonal ferrite grain of I to 2mu m in diameter were completed inside prior martensite lath structure adjacent to interlath retained austenites in the steels with a prior structure of martensite single phase which were subjected to 30-50% reduction at intercritical annealing temperatures between 760 and 820degreesC, followed by austempering at 375-450degreesC for 100 seconds. Under the same processing conditions, volume fraction of retained austenite was significantly increased with a small decrease in its carbon concentration. It was found in the steels that relatively thick austenite plates recrystallized during intercritical annealing suppressed the migration of prior martensite lath boundary and the resultant grain growth of recrystallized polygonal ferrite. Also, It was confirmed by EBSP analysis that a difference in orientation between the ultra fine polygonal ferrite grains was relatively large.