Effect of hot rolled grain size on the precipitation kinetics of nitrides in low carbon Al-killed steel

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The precipitation of nitrides plays a general role in the industrial processing of deep drawing quality Al-killed low carbon steels. In this paper, the effect of hot rolled grain size on the precipitation of nitrides has been analysed. To evaluate the effect of grain size on the nitride precipitation kinetics, thermoelectric power based investigations have been performed on hot and cold rolled specimens.In the hot rolled state, the precipitation of nitrides occurs more intensively in the fine grain size microstructure (average grain size=9μm) than in the large grain size microstructure (average grain size=23μm) until the precipitated fraction of nitrides reaches about 70%. In the cold rolled state the effect of grain size is much less significant; probably the precipitation process occurs simultaneously at the grain boundaries and along dislocations. According to the simulation results, significant differences can be found between the precipitated fraction of nitrides in fine and large grain size sheets coiled in the temperature range 550–650°C. In this interval, the precipitated nitride fraction is about two times larger in a fine grain microstructure (9μm) than in sheets with 23μm average grain size. The local position in the coil also affects significantly the precipitated fraction of nitrides. In the outer ring of the coil, less than 20% precipitated fraction is predicted in coiling temperature range 550–700°C. However, in the middle ring of a hot rolled coil, the precipitated fraction changes from 5% to 85% with increasing coiling temperature from 550 to 700°C.