MICROSTRUCTURAL\, EVOLUTION OF Mn-Si-Cr MEDIUM CARBON STEEL DEFORMED UNDER UNDERCOOLED AUSTENITE STATE

An appropriate deformation with low strain rate was applied to a Mn-Si-Cr medium carbon steel at undercooled austenite state. The deformation promotes the formation of pro--eutectoid ferrite, but no lamellar pearlite was observed during deformation. The ferrites nucleate at the grain boundary of austenite and inside the austenite grains, grow in nearly equiaxial shape, and finally divide austenite and connect with one another, leading to the formation of carbon-rich austenite zones. With the increase of strain, ferrites can continue to nucleate inside carbon-rich austenite zones, grow and connect so that the carbon-rich austenite zones are divided continuously and their carbon concentration becomes higher. When the carbon concentration is enough high, the prime spheroidized carbides with 0.5-1 μm in diameter begin to precipitate at the edge of carbon-rich austenite zones. The dynamic recovery and recrystallization of ferrite during deformation process make carbon atoms escape from the Cottrell atmosphere, resulting in precipitation of the secondary spheroidized carbides with size of several decade nanometers within ferrite grains.