Effect of equalisation time and temperatures on microstructures of simulated directly and conventionally charged V - microalloyed steels
Two equalisation or reheating temperatures (1050°C and 1150°C) and three equalisation or reheating times (53 min, 318 min, and 1333 min) were applied to three V- and V-Ti- microalloyed steels, which contain a low carbon content (0.067 ~ 0.073 wt%) and high nitrogen content (0.017 ~ 0.021 wt%), by the simulated direct charging or conventional cold charging processes. The experimental results show that in directly charged V-microalloyed steels, MnS provides the main pinning effect but does not have a sufficient ability to prevent the austenite grains from growing during the equalisation. Because of the slow precipitation, AIN only precipitates at the longest equalisation time and is the main compound which has an obvious pinning effect on the austenite grain growth in V-microalloyed steels. Austenite grain growth appears more likely to be abnormal as a result of the direct charging than of the cold charging due to the precipitation of fine AIN particles during the phase transformations. The experimental results also show that the longer equalisation or reheating time in the furnace does not result in a significant change in the microstructures and in the austenite grain size, because of the precipitation of AIN during equalisation or reheating. However, in V-Ti-microalloyed steels, the existence of titanium can promote the precipitation of complex (Ti×V₁₋×)N particles during equalisation or reheating. In the cold charged V-Ti-N microalloyed steel, the fine austenite grains produced (<10μm) can be observed after the reheating and water-quenching due to the significant pinning effect of (Ti×V₁₋×)N. When the reheating time of V-Ti-N microalloyed steels at 1150°C is longer than 318 min, the coarsening of (Ti×V₁₋×)N precipitates occurs and leads to abnormal austenite grain growth. The TEM results also show that cruciform (Ti×V₁₋×)N particles can only be found indirectly charged steels but not in cold charged steels.