Mechanical properties and microstructure of lovv-carbon-4% manganese steels

Three vacuum-melted steels based on a composition 0·05–0·08C–4·0Mn, with and without a niobium addition, were controlled rolled to a finish-rolling temperature in the range 700−950°C and air cooled to room temperature. Measurements were made of the ferrite mean free distance, which was correlated with strength and toughness data. Additional qualitative information on precipitation was also gathered. Finished material showed a wide range of strength and toughness. Proof stresses were in the range 610–930 MNm⁻² and ductile-brittle (55 J) transition temperatures varied between −90 and + 30°C. The properties of a given material seem to be governed by the carbon and niobium contents, as well as by the ferrite mean free distance. Up to 0·08%C, very high strength coupled with adequate toughness seemed to be obtained by retaining as much carbon in solution as possible, particularly by avoiding cementite precipitation. A fine NbC precipitation appeared to have a beneficial effect on toughness.