Abstract The fine structure of the adiabatic shear band (ASB) in the serrated chips of low alloy steel 30CrNi 3 MoV under orthogonal cutting conditions was observed by means of SEM and TEM. Microhardness measurement was performed on the ASBs. The results show that the structure between the center of the ASB and matrix is different. The average microhardness values in the shear band exceeded quenched martensite hardness. In the central region of the ASB, the recrystallization microstructure which consists mainly of martensite, χ-Fe 5 C 2 , F 3 C and austenite was observed. In the transitional region between the center of the ASB and the matrix, the large deformed microstructure was observed and a large amount of dislocations was found on the boundaries of martensite laths. Investigative results indicate that dynamic recrystallization and martensite phase transformation took place in the ASB. The ASBs formed in orthogonal cutting may be classified as transformation bands.