Abstract: In order to explore the forming mechanism of crack on high strength slab containing titanium and niobium, the morphology of crack was revealed by macro-etching, the microstructure of slab and crack fractograph were examined by metallographic analysis and the characteristic of continuous cooling transformation was tested based on the thermal expansion method via GLEEBLE-2000. Consequently forming reason of macro-etching crack on high strength steel slab containing titanium and niobium was elucidated from aspects of temperature variation and phase transformation. The results showed that cooling rate of the slab could reach as high as 5℃/s within the range of 30mm from torch cutting plane after torch cutting; high cooling rate made Bainitic and Martensitic phase transformation taking place on the slab; and the combination of stress caused by the high cooling rate and the phase transformation led to the severe macro-etching cracks on the high strength slab bearing titanium and niobium.

Key words: high strength steel containing titanium and niobium, slab, phase transformation, crack