Microstructural evolution and grain refinement of a high strength low alloy steel produced by compact strip production

A kind of low carbon high strength Nb-Ti microalloyed steel was produced by Compact Strip Production (CSP) at Lianyuan Steel. Optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to analyze the microstructure of the steel. The microstructure mainly consisted of acicular ferrite (AF), granular bainite (GB) and polygonal ferrite (PF). The average grain size of the mixed microstructures was approximately 4 μm. A small amount of Nb and Ti carbide was precipitated in the ferrite matrix, the size being 50-100nm. A large amount of dislocation cells and networks were observed in the matrix. The dislocation cells and networks were formed by means of advanced TMCP and they played an important role in the strengthening of the microalloyed steel. The yield strength of the microalloyed steel was thus more than 600MPa and elongation reached 24%.