EFFECT OF TEMPERATURE-STRAIN PARAMETERS ON THE STRUCTURE AND MECHANICAL PROPERTIES OF Nb, V, Ni - ALLOYED STEELS IN TERMS OF HIGH TEMPERATURE THERMOMECHANICAL PROCESSING

摘要

The effect of rolling temperature (800-1000 °C) - strain accumulation (number of rolling passes: 3÷5) - cooling rate (8.6 - 25°C/s) parameters at the High Temperature Thermomechanical Processing (HTMP) on the microstructure, mechanical properties at the room and low temperatures of C-Ni-Nb and C-Ni-V steels intended for the usage as a high- strength material for production of the shelf constructions, strips and other special welded products has been studied. Experiment planning method (EPM) has been used to study the effect of the mentioned above parameters on the mechanical properties. By results of experiments the regression equations described the qualitative and quantitative effect of HTMP parameters on the mechanical properties of steel are constructed. The analysis of microstructure and SEM analysis of the fracture at +20°C and -40°C temperatures has been carried out. It was shown that the highest strength (YS = 987 MPa) in combination with a high impact strength (W = 154 J) has a Ni-Nb-bearing steel with lower bainite structure formed result in maximum passes and highest deformation temperature and cooling rate realization. The best cold resistance has steel with low bainite structure transformed at the accelerate cooling from the fine fragmented hot deformed austenite.