Formation Dynamics Of M-A Constituent In HSLA Steel And It's Effect On Mechanical Property

The research and exploitation of high strength low alloy steel (HSLA) is very important in the development of material science. HSLA steel is developed for large engineering components (such as ships, bridges, pressure vessels, etc.) to reduce the structural weight, increase the reliability and save steel materials. Now in the shortage of resources, lack of energy background, HSLA steel acts as a kind of high-performance steel, and its excellent performance demonstrates the great potential for development. It is already widely applied in the ship, bridge engineer area. But most of the high strength low alloy steel products in the state of hot rolling are poor in low temperature toughness. The main reason is: there is a great deal of granular bainite in the microstructure.In this theis two kinds of high-strength low-alloy steel are chosen. The continuous cooling transformation process of experimental steel was simulated in different cooling rates with Gleeble-3500. Phase transition temperature range of M-A constituent in experimental steel was tested. The microstructure of M-A constituent was observed with general corrosion or colour corrosion by optical microscope, SEM and TEM. The area percentage and average size of the M-A islands were count in this paper. The nanoindentation hardness of M-A islands and the matrix, which is near the islands were tested. Vickers hardness and impack toughness of experimental steel were tested in the normalization state and tempering state.The results show that there is M-A constituent in the cooling rate range between 0.167℃/s and 15℃/s when experimental steel is in the normalizing continuous cooling process, and the M-A transformation temperature range is 420℃-120℃. As the cooling rate increased, the starting and finishing point show lifting tendency. But the tendency is affected by pearlite transformation or martensite transformation. As the cooling rate increased, M-A islands change from massive shape gradually into lath shape, the amount reduced, the size refined, the difference in hardness between islands and matrix decreased, theCharpy-V-Notch impact energy increased. When the normalized experimental steel was tempered, the M-A islands completely decompose, the hardness decreases slightly, the impact energy greatly. Accelerating cooling rates and tempering could improve the impact energy of the hot rolling steel or normalized steel and it is an effective technological method to improve the toughness of HSLA steel.