Abstract
The effects of thermomechanical processing in an α+γ region on microstructure and retained austenite characteristics were investigated to enhance the workability limit of a newly developed high strength low alloy TRIP-aided ferrous steel. Ultra fine polygonal ferrite grains of 1 to 2 μm in diameter were formed inside prior martensite lath structure with interlath retained austenite plates or needles when the steels with martensite single phase were subjected to 30-50% reduction at intercritical annealing temperatures between 760 and 800°C, followed by austempering at 375-425°C for 100 s. It was confirmed by EBSP analysis that some of the ultra fine polygonal ferrite grains possessed a large difference in orientation from neighboring grains. Under the same conditions, volume fraction of retained austenite was significantly increased with a small decrease in its carbon concentration, compared to the steel subjected to only heat treatment. The refining of polygonal ferrite grain was principally caused by interlath austenite plates or needles suppressing the grain growth of polygonal ferrite recrystallized just after intercritical reduction.
