Study of microstructure and cementite in automobile beam steels produced by compact strip production with flexible technologies

The microstructure and cementite of hot strips of automobile beam steels produced by a compact strip production (CSP) line were studied using scanning electron microscope (SEM), transmission electron microscope (TEM) and electrolytic dissolution technique. The cementite lamella of the as-received hot strip of one kind of automobile beam steel fractured and transformed into discrete particles and the dispersion of fine but non-uniform cementite particles distributed throughout the ferritic matrix of the hot strips, which was the typical phenomenon in the microstructure observed in the experiments. The microstructure with this phenomenon formed by coiling at 540 deg. C with cooling rate 15 K s-1, and led to differences in mechanical properties. The influences of technological parameters on the microstructure were considerably analyzed, and the reason to the formation of this specific characteristic was discussed on the basis of the dislocation-cementite-excess carbon concentration interaction