Study on the Hot Ductility of High Nb and Ti Micro-alloyed Steel

The hot ductility of high Nb and Ti micro-alloyed steel had been investigated through the high temperature tensile test and optical microscopy,scanning electron microscopy,transmission electron microscopy and micro-hardness tester.The results show that the relationship between the rate of reduction in area and the temperature range from 600~1 200℃ was V-shape for the high Nb and Ti micro-alloyed steel.The rate of reduction in area was the lowest at 800℃ while that was the highest at 1 200℃.The third brittle zone for the high Nb and Ti micro-alloyed steel was 680~910℃.The main reason for the embrittlement was the large-size carbonitrides and pro-eutectoid ferrite precipitated at the grain boundaries which weakened the grain boundary bonding force and provided conditions for crack generation and expansion.The micron-sized TiN particles in the sample were the main reason for the low rate of reduction in area of the high Nb and Ti micro-alloyed steel.The hot ductility at the third brittle zone can be improved by increasing the strain rate,shortening the time for the precipitation of pro-eutectoid ferrite and the growth of TiN particles and weakening the stress concentration during the tensile test.