Transverse corner cracks have been frequently observed and are extremely difficult to prevent in some microalloyed steels produced on some vertical bending type continuous slab casters. The cracks are usually found on the fixed (outer) side corner of the slabs through inspection of the acid pickled surface and macroscopic examination. In the present study, the slab surface microstructure was investigated, and the results show that the causes of the crack formation are chain-like precipitations and film-like proeutectoid ferrite in the austenite grain boundaries when the slabs are bent. Initially, when the temperature of the slabs dropped after solidification, the Nb, V or Ti carbides and/or nitrides precipitated in chain-like way, then pinned onto the austenite grain boundaries. This process hindered grain boundary slippage and lowered the force on the grain boundaries. Second, due to the stress mismatch between the matrix and the fine precipitates during bending operations, the chain-like precipitated carbides and/or nitrides in the grain boundaries increased the cracking susceptibility of the slabs. Meanwhile, the film-like proeutectoid ferrite precipitated along the austenite grain boundaries during the austenite-ferrite transformation disrupted the continuity of the austenite matrix. In view of the fact that the strength of proeutectoid ferrite film is lower than that of the austenite grains, when the slabs were subjected to bending stress, the cracks were generated along the film-like proeutectoid ferrite.

In the process of continuous casting, the cooling rate is the key factor affecting and deciding the slab surface microstructure. In this study, confocal laser scanning microscopy was used for in situ observation of, and research on, slab surface microstructure evolution under different cooling rates. It was found that the slab microstructure was uniform, and there was no obvious filmlike proeutectoid ferrite or chainlike microalloyed element precipitation in the prior austenite grain boundary under the cooling rate of 3-6uC s 21 . The results of hot tensile experiments showed that this microstructure had a higher hot ductility, which contributed to the virtual disappearance of the brittle trough and reduction of the steel's cracking susceptibility.