The ultra-fast cooling technology of large section bars and the microstructure for different cooling patterns were studied by optical microscope,transmission electron microscope and energy spectrometer.The results indicated that the large section bars were passed through the zone of secondary carbide precipitation quickly by ultra-fast cooling technology(UFC)at instantaneous cooling rate of about 200 ℃/s and the finishing cooling temperature was higher than Ms.The lamellar spacing of pearlite decreased and the microhardness increased with decreasing the re-reddening temperature.The precipitation of network carbide was restrained when re-reddening temperature was 690 ℃.And fine laminated pearlite was obtained through transformation of pseudopearlition that induced the reduction of the diameter of pearlite grain and refinement of the lamellar spacing of pearlite,so ideal microstructures of promoting spheroidizing annealing were obtained.
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