Fe-0.002%C, Fe-0.01%C, Fe-1.5%Mn-0.001%C and Fe-1.5%Mn-0.01%C steels were vacuum-melted and examined. Fe-0.05%C and Fe-1.5%Mn-0.05%C steels were also melted and used for comparison. The effects of cooling rate and chemical composition on microstructures were examined by optical and electron microscopy. Autoradiography of boron was also performed in order to know relation between gamma and alpha grain boundaries and effects of microstructures on discontinuous yielding were also examined. The fraction of quasi-polygonal ferrite (alpha(q)) increases with the cooling rate and the contents of C and Mn. Dislocation density in alpha(q) is higher than that in polygonal ferrite (alpha(p)) and dislocations tend to form cell structures in alpha(q) matrix, whereas dislocations which do not form cell structures were also observed. Normal cooling rate dependence of ferrite grain size was not observed in Fe-0.002%C and Fe-1.5%Mn-0.001%C steels. By autoradiography of boron it was known that not a small fraction of grain boundaries of ferrite might be coherent. Even in the ultra-low carbon steels cooled at 360 degrees C/s, cementite was observed on ferrite grain boundaries. Discontinuous yielding was suppressed by bainite transformed during continuous cooling, while Mn enhanced discontinuous yielding. Bainite and martensite could be observed in the specimens of Fe-0.002%C and Fe-1.5Mn-0.001C steels cooled at the rate higher than 360 degrees C/s. In such specimens, the continuous change from alpha(q) to bainite was observed. Martensite can be distinguished from bainite by its higher microhardness and thinner lath width.
