Abstract
The mechanical properties and microstructure of as-cast Nb-Ti microalloyed steel at different temperatures and cooling rates are investigated in this paper. The III brittle 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 850–1 000°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. When 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.
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