Nature of large (Ti, Nb)(C, N) particles precipitated during the solidification of Ti, Nb HSLA steel

To investigate the microsegregation phenomena and complex (Ti, Nb)(C, N) precipitation behavior during continuous casting, a unidirectional solidification unit was employed to simulate the solidification process. The samples of Ti, Nb-addition steels after unidirectional solidification were examined using field emission scanning electron microscope (FE-SEM) and electron probe X-ray microanalyzer (EPMA). In such specimens, dendrite structure and mushy zone can be detected along the solidification direction. It shows that the addition of titanium, niobium to high-strength low-alloyed (HSLA) steel results in undesirable (Ti, Nb)(C, N) precipitation because of microsegregation. The effect of cooling rate on (Ti, Nb)(C, N) precipitation was investigated. The composition of large precipitates was determined using FE-SEM with EDS. Large (Ti, Nb)(C, N) precipitates could be divided into three kinds according to the composition and morphology. With the cooling rate increasing, Ti-rich (Ti, Nb)(C, N) precipitates are transformed to Nb-rich (Ti, Nb)(C, N) precipitates.