EFFECT OF CARBON CONTENTS AND Ti ADDITION ON THE MICROSTRUCTURE OF ULTRA-LOW CARBON STEEL

摘要

The microstructures of a series of ultra-low carbon (ULC) steels with various carbon contents (50- 200ppm) and Ti addition (<500ppm) were studied by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The specimens were fabricated under the process of hot rolling, cold rolling and a two-step continuous annealing. The experimental results showed that the increase of carbon content decreased grain size, whereas the addition of Ti resulted in a larger grain size compared to those without Ti addition. Dislocation (cell) structures were introduced by cold rolling, but it was removed by the two-step annealing. Only MnS precipitated from the Ti-free specimens. However, a larger amount of fine TiN, a few coarse TiS and the extremely low number of much coarser Ti_3AlC were observed in the Ti-added specimens. Besides, the reduction of MnS inclusion was obtained by the addition of the Ti.