The mechanical properties and microstructure of as-cast Nb-Ti microalloyed steel at different temperatures and cooling rates are investigated in this paper.TheⅢbrittle zone(700-900℃) is revealed.The reduction of the hot ductility is due to the film-like proeutectoid ferrite or the Nb(C,N) precipitates along the austenitic grain boundaries.In the temperature range of 850-1000℃,with the increase of the cooling rate,the hot ductility decreases.However,in the range of 650-850℃,the appearance of large volume fractions of ferrite on austenite grain boundaries minimizes the effect of cooling rate on hot ductility.When the cooling rate is 10℃/s, austenite transforms more quickly to ferrite and at a lower temperature a larger amount of ferrite nucleates and precipitates in the grain,which leads to a sharper improvement in the hot ductility at 650℃. The mechanical properties and microstructure of as-cast Nb-Ti microalloyed steel at different temperatures and cooling rates are investigated in this paper. The IIIbrittle zone (700-900 ° C) is revealed. The reduction of the hot ductility is due to the film-like proeutectoid ferrite or the Nb (C, N) precipitates along the austenitic grain boundaries. In the temperature range of 850-1000 ° C, with the increase of the cooling rate, the hot ductility decreases.However, in the range of 650-850 ° C , the appearance of large volume fractions of ferrite on austenite grain boundaries minimizes the effect of cooling rate on hot ductility. At the cooling rate is 10 ° C./s, austenite transforms more quickly to ferrite and at a lower temperature a larger amount of ferrite nucleates and precipitates in the grain, which leads to a sharper improvement in the hot ductility at 650 ° C.