The Dynamic Continuous Cooling Transformation and Phase Transformation Kinetics of 80 High-Carbon Steel
摘要
The Continuous Cooling Transformation (CCT) diagram of high-carbon steel is an important reference for determining the optimal heat treatment process. By precisely controlling the cooling conditions, refined lamellar pearlite structures can be obtained, thereby optimizing the material's strength and toughness. This study utilized the Gleeble-3500 thermal simulator to investigate the dynamic continuous cooling transformation behavior of 80 high-carbon steel after 40 pct deformation at 950 °C. The cooling rate range was from 0.5 to 80 °C/s, and the JMA (Johnson-Mehl-Avrami) model for pearlite transformation was established. The phase transformation temperatures were measured using the dilatation method, with Ac1 (Austenite start temperature) at 729 °C and Ac3 (Austenite completion temperature) at 886 °C. Dilatation curves and CCT curves were plotted for different cooling rates. By combining Scanning Electron Microscopy (SEM), phase transformation kinetics curves, and Electron Backscatter Diffraction (EBSD) techniques, the microstructural evolution and phase transformation mechanisms at different cooling rates were studied. At cooling rates between 0.5 °C/s and 5 °C/s, only pearlite transformation occurred. As the cooling rate increased, the interlamellar spacing of the pearlite decreased, reaching a minimum of 127 nm at a cooling rate of 5 °C/s. When the cooling rate ranged from 5 to 20 °C/s, both pearlite and bainite transformations occurred simultaneously, with the bainite transformation content gradually increasing as the cooling rate increased. At a cooling rate of 30 °C/s, a three-phase region of pearlite, bainite, and martensite coexisted, and the Kernel Average Misorientation (KAM) value reached its peak, indicating the highest dislocation density. With increasing cooling rates, the grain size of the sample decreased, and both high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs) increased. However, the high cooling rates promoted the transformation to bainite or martensite, leading to a smaller grain orientation deviation, which caused the KAM value to decrease with increasing cooling rates. The pearlite transformation kinetics model fitted by the JMA model showed a high degree of correlation with the experimental data. The results indicate that a cooling rate of 5 °C/s is ideal for producing high-performance 80 high-carbon steel.