EFFECT OF AUSTENITE DEGRADATION CONDITIONS ON THE PROPERTIES OF STEEL 20

Purpose. To investigate the nature of property changes in thermally strengthened coiled steel 20 depending on the mechanism of austenite degradation. Research methods. The material for study was a wire with a diameter of 3 mm, made of low-carbon steel, with 0.22 % С, 0.46 % Mn, 0.088 % Si, 0.1 % Cr, 0.03 % S, 0.012 % P. The different structure state of the steel was obtained after certain processing. The samples, 0.3 m long, were subjected to austenitization at a temperature of 920 °C for 8 min, after that for 10 min followed isotherm exposure, at temperatures of 650–200 °C and tempering at temperature of isothermal transformation, duration by 60 min. The microstructure was studied using light and electron microscopy. Dimensions of structural element were determined by quantitative metallography methods. The mechanical properties were determined from analyzing tensile curves at room temperature and strain rate of 10-3 s-1. Results. The change at structure and properties is largely determined by the mechanism of austenite transformation under isothermal conditions. During diffusion transformation of austenite, the dispersion of the phase components is accompanied by an increase at super saturation of the solid solution by carbon. For the intermediate transformation region, a change at phase composition of the steel is added. The subsequent tempering determines the kinetics and degree of completion processes of structure formation of the thermally hardened rolled steel. At low tempering temperatures of steel with a bainite structure, hardening is due to the development of aging processes. With an increase at tempering temperature, the development of softening processes is accelerated. Scientific novelty. The increase plasticity of the steel with a bainite structure is due to a decrease of dispersion of cementite particles, dislocation density, and the development of polygonization at initial stages of recrystallization. The softening of steel with a ferrite-pearlite structure is determined by a decrease in phase hardening during austenite transformation. Practical value. Mechanism analysis of structural transformations has determined that manufacture of thermally strengthened rolled steel, the expected softening after coiling is recommended to compensate by a corresponding decrease of temperature of the end of accelerated cooling.