In situ observation of reheated austenite grain growth and bainite transformation in DH36 ship plate steel

The evolution of microstructure during heating, holding, and cooling stages directly determines the mechanical properties of steels. To achieve effective microstructural control during slab hot rolling, the high-temperature confocal laser scanning microscope is used for in situ observation of austenite grain growth behavior in DH36 ship plate steel during heating/holding stages and phase transformation evolution under different cooling rates. The results demonstrate that austenite grain size increases significantly with the elevated heating temperatures and prolonged holding time. When the heating temperature is in the range of 1 150-1 200 ℃, the average austenite grain size is 30.6-42.4 μm. When the heating temperature is raised to 1 250 ℃, the austenite grains grow significantly, reaching an average size of 253.4 μm. The grain growth exhibited a two-stage pattern: rapid growth during the initial holding period(0-5 min), followed by a significant decrease in the growth rate until it stabilized(5-30 min). A kinetic model for austenite grain growth was established based on these findings. Furthermore, room-temperature microstructures show marked variations under different cooling rates(360 and 720 ℃/min). At 360 ℃/min, granular bainite dominates the microstructure, while increasing to 720 ℃/min yields predominantly lath bainite. Bainite primarily nucleates along austenite grain boundaries, with additional nucleation modes observed including intragranular nucleation at inclusions and nucleation at broad sides of pre-existing bainite plates.