A metallographic study of isothermal-transformation products formed at various temperatures was made in 2%Si-1%Mn-0.34%C and 2%Si-1%Mn-0.59%C steels with particular emphasis on the formation mechanism of bainitic ferrite. In the 0.34%C steel, the austenite first decomposes into ferrite and then into pearlite in the later stage of transformation at 973 K (just below the A1 temperature). As decreasing the decomposition temperature, the amount of proeutectoid ferrite decreases and the structure becomes fully pearlitic at 873 K. At lower temperatures of 848 K and 823 K, a mixed structure of pearlite and carbide-free bainitic ferrite is observed. At temperatures below 823 K and above the Ms temperature, pearlite does not form and the structure is fully bainitic. This indicates that carbide-free bainitic ferrite has its own formation temperature range different from that for proeutectoid ferrite. It is also demonstrated that at 823 K both pearlite and carbide-free bainitic ferrite exist in the initial stage of transformation and the amount of both phases increases with increasing the isothermal-holding time, unlike the formation process of proeutectoid ferrite and pearlite at 973 K. A similar competitive formation of pearlite and carbide-free bainitic ferrite is observed at 823 K in the 0.59%C steel. These results strongly suggest that the formation mechanism of bainitic ferrite is different from the diffusional mechanism for proeutectoid ferrite and pearlite. The upper critical temperature for the formation of bainite, i.e., the microstructural-Bs, is around 860 K and hardly changes with carbon content.
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