Abstract: Low carbon ferritic steel samples were heat treated at 880 ℃ for 0.5 h and then water quenched,tempered at 660 ℃ for 10 h,then isothermal aged at 400 ℃ for 11000 h.The characteristics and relations of different crystal structure of nano-rich Cu phase precipitated in low carbon ferrite steel were analyzed by using high resolution transmission electron microscope (HRTEM) and energy dispersive spectroscopy (EDS).The short rod of monoclinic structure of 9R nano Cu-rich phase with mutualling twin structure were observed, which the long axis is about 9.5 nm, the short axis is about 6.6 nm.It is indicate that the early precipitation of Cu is preferentially transformed into the monoclinic structure of 9R with twin relationship, At the same time, it is observed that the fcc twin structure of Cu, as well as the fcc non-twin structure of Cu with Moire fringe, It is showed that the 9R structure not only retreat into the fcc non-twin structure, also can be directly transformed into fcc structure with twin relationship.

Key words: Low carbon ferritic steel samples were heat treated at 880 ℃ for 0.5 h and then water quenched,tempered at 660 ℃ for 10 h,then isothermal aged at 400 ℃ for 11000 h.The characteristics and relations of different crystal structure of nano-rich Cu phase precipitated in low carbon ferrite steel were analyzed by using high resolution transmission electron microscope (HRTEM) and energy dispersive spectroscopy (EDS).The short rod of monoclinic structure of 9R nano Cu-rich phase with mutualling twin structure were observed, which the long axis is about 9.5 nm, the short axis is about 6.6 nm.It is indicate that the early precipitation of Cu is preferentially transformed into the monoclinic structure of 9R with twin relationship, At the same time, it is observed that the fcc twin structure of Cu, as well as the fcc non-twin structure of Cu with Moire fringe, It is showed that the 9R structure not only retreat into the fcc non-twin structure, also can be directly transformed into fcc structure with twin relationship.