Structure and properties of structural steels after low-temperature thermomechanical treatment

Abstract

1. Under the conditions of low-temperature thermomechanical treatment used there is no decomposition of austenite or martensite. The phase composition of the steel after thermomechanical treatment without subsequent tempering is the same as that of the quenched steel. 2. The macro- and microstructure of steel subjected to rolling at 550\dgC with 80\2-90% swaging during thermomechanical treatment is anisotropic. The substructure is also anisotropic: martensite blocks of coherent scattering acquire the shape of thin platelets oriented parallel to the plane of the sheet. 3. In the direction perpendicular to the plane of the sheet the martensite blocks of coherent scattering are 2.5\2-3 times smaller after thermomechanical treatment than after ordinary quenching. 4. The anisotropy of the structure is accompanied by the anisotropy of the mechanical properties. 5. The rates of the processes occurring during tempering at temperatures below 500\2-550\dgC are lower after thermomechanical treatment than after ordinary quenching, and the steel is stronger after thermomechanical treatment. At higher tempering temperatures the steel subjected to thermomechanical treatment loses its strength at a higher rate than the steel subjected to ordinary quenching. 6. Martensite formed from deformed austenite acquires many characteristics of cold worked materials: the crystallographic texture, the smaller size of the grains and of the region of coherent scattering, the larger blocks occurring at the relatively high temperatures at which recrystallization and loss of strength occur.