Abstract
The paper is devoted to the study of physical metallurgy conditions of the M/A component formation, created in connection with austenite matrix decomposition in granular bainite of the Mn-Mo-Nb structural steel type during continuous cooling. It was demonstrated, that the important part of the M/A component is a high-carbon martensi-te containing internal transformation twins. Further, the paper describes the technique of volume fraction evaluation of this component after application of a special etching method. Simultaneously, a qualitative relationship between volume fraction of the M/A component and the microfractography of fracture surfaces of notch impact specimens is determined. The conditions leading to the modification of the M/A component formation in granular bainite are defined. The analysis includes the influence of chemical composition, the state of austenite matrix and the applied cooling rate respectively.

Key words
M/A component formation, Mn-Mo-Nb structural steels, physical metallurgy

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