The effects of cooling rate and deformation on microstructures of an X80 pipeline steel were investigated by thermo-mechanically simulated tests heated up to the same temperature and cooled at different rates under 0 and 0.7 true strain deformation.The results reveal that the start temperature for bainite transformation will be increased by 30—80℃higher under 0.7 true strain than under no deformation.But the increase of cooling rate depresses the start temperature for bainite transformation and accelerates the progress of its transformation.It is found that the microstructure of the cooled samples without deformation is all composed of bainite with prior austenite grain boundaries (PAGB),but it is complicated for the cooled samples under 0.7 true strain.In the range of 1—40℃/s, with the increase of cooling rate,microstructures appeare successively such as polygonal ferrite,quasipolygonal ferrite,massive ferrite,granular bainite,acicular ferrite(+granular bainite) and lathy bainite.Full martensite steel can be achieved by quenching.Under current experiment conditions, acicular ferrite can be obtained in the steel at cooling rates of 10—20℃/s.In this cooling rate range, the fraction of high angle grain boundaries(HAGBs) increases and the grain size decreases with the increase of cooling rate.
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Deng et al. (2010) studied this question.
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