Effect of temperature of reheating deformation on microstructure of microalloyed steel
Among the various strengthening mechanisms of iron and steel materials, grain refinement is the only method that can improve the strength and toughness at the same time. The main methods for obtaining ultrafine grains include dynamic recrystallization and transformation of austenite, strain-induced ferrite transformation, two-phase rolling and warm rolling in ferrite zone. However, most of the existing studies are limited to small specification products that are easy to achieve large deformation at low temperature and rapid cooling. In the production of thick plates, due to the uneven temperature and shape variables in the thickness direction, it is difficult to achieve refinement on the full thickness, but it is feasible to obtain ultra-fine crystals on the surface of the steel plate, and its overall performance can also be greatly improved.
The researchers studied the microstructure evolution of microalloyed steel when it was heated to the two-phase zone and analyzed the influence of heating temperature on it through a single pass compression deformation thermal simulation test. OM, SEM and EBSD were used to analyze the microstructure and orientation distribution of the test steel.
The results show that the austenite transformation occurs at the grain boundary after heating. The volume fraction of austenite is about 20% at 740 ~ 800℃, and increases to 50% at 830℃. When the deformation is heated to the two-phase zone, dynamic recovery or dynamic recrystallization occurs in the deformed ferrite. With the increase of deformation temperature, the deformation ferrite changes from dynamic recovery to dynamic recrystallization, and the subgrain boundary decreases. The proportion of large Angle grain boundary reaches 91.2% at 830℃, and uniform fine crystal structure is obtained after cooling with an average effective grain diameter of 3.9μm.
