1. The introduction of columbium into medium manganese steel strongly decreases its propensity towards temper brittleness. The optimum columbium content for this steel is 0. 25%. 2. The impact toughness of manganese steel at subzero temperatures is increased by a content of 0.20–0.98% Cb. 3. Manganese steels with 0.4–0.5% C are temper brittle and upon double etching, reveal the boundaries of the former austenitic grain; the carbides are located around these boundaries. In steels with less carbon, grain boundary carbides are practically absent but there is a higher saturation of the solid solution. Manganese steels containing columbium are free from temper brittleness and under the same etching conditions, do not show the boundaries of the original austenite grain. 4. The introduction of columbium into medium manganese steel broadens the area of its application in machine building.