22MnB5 high‐strength steel is widely used in automotive safety components, yet the continuous cooling pathway from solidification to final microstructure and hardness under controlled cooling rates remains unclear. Here, phase transformations and microstructure evolution of 22MnB5 were tracked from liquid to room temperature by combining in situ high‐temperature confocal laser scanning microscopy (HT‐CLSM), CALPHAD thermodynamic simulations, differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD) and hardness testing under cooling at 1°C/s, 7°C/s, 15°C/s and 30°C/s. In situ observations reveal a transformation sequence L → δ → γ → ferrite/upper bainite/lower bainite/martensite, with all start and finish temperatures decreasing systematically as cooling rate increases. A high‐temperature MB 2 phase and NaCl‐type Ti(C, N) carbonitrides are dynamically captured and correlated with DSC exotherms and calculated stability windows, providing direct evidence of precipitation–transformation coupling. EBSD shows grain refinement, increased high‐angle boundary fraction, and higher local lattice distortion with increasing cooling rate, indicating a transition from diffusion‐controlled ferrite formation to bainitic and finally shear‐dominated martensitic transformation. Vickers hardness rises from 250 HV at 1°C/s to 570 HV at 30°C/s, linking cooling rate, transformation, microstructure, and mechanical response and enabling the design of cooling schedules to control bainitic or martensitic microstructures in 22MnB5 components.
Liu et al. (Thu,) studied this question.
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