Precipitation of Acicular Ferrite Pipeline Steel X70 in Thermomechanically Controlled Processing

The precipitation behaviors of micro alloy Nb,Ti elements in acicular ferrite pipeline steel X70 in thermomechanically controlled processing were investigated by using transmission electron microscopy.The precipitates observed by TEM were in two forms.The large precipitates were complex compounds comprising TiN precipitate of square shape as core with Nb containing precipitate nucleated on pre-existing TiN precipitate as caps on one or more faces at high temperatures.The fine and spherical Nb carbides and/or carbonitrides precipitated heterogeneously on dislocations and sub-boundaries at low temperatures.The thermomechanically controlled processing promoted the precipitation of the secondary phase particles.