Effect of cooling rate on the slab surface microstructure of microalloyed steel containing Nb and Ti

Cooling rate is a key factor which affects the surface(0 to 5 mm) microstructure of a slab and the distribution of the second phase precipitates after the slab leaves the mould.In order to simulate the thermal cycle when a slab is drawn to the vertical segment,a remelting and solidification cooling experimental device was designed.The influences of cooling rate on the surface microstructure of a slab and the distribution of the second phase precipitates were investigated.It is shown that when the slab is drawninto the vertical segment at a speed of 0.8 m· min^-1 and the slab surface is cooled to less than the A₃ transformation temperature by intensive cooling at a cooling rate of 5℃·s^-1, the surface microstructure of the slab is uniform, the second phase precipitates containing Nb and Ti are well-distributed, and film-like proeutectoid ferrite does not precipitate along the austenite grain boundaries. These help to improve the hot ductility and to alleviate the cracking susceptibility of the slab.