Relation between manganese oxide inclusions and microstructure of low-carbon rimmed steel sheet

Abstract

The influence of manganese oxide inclusions on the microstructure of cold-rolled low-carbon rimmed steel sheet containing 0.30 pct Mn and a high level of oxygen was studied in the laboratory by annealing full-hard samples in the 1200‡ to 1400‡F range. Microprobe results showed that the oxide inclusions deplete the surrounding matrix of its manganese and thereby permit sulfur to remain in solid solution after prior hot rolling. It was found that the dissolved sulfur precipitates during subsequent annealing after cold rolling and produces bands of fine grains in the inclusion area. It was determined that the manganese oxide inclusions, which are generally concentrated toward the center of the sheet cross section, establish a gradient of manganese in solid solution between the surface and center of the steel samples studied. As demonstrated by the electron microprobe and photomicrographic evidence, this manganese gradient caused carbon to diffuse to the sheet surfaces during annealing. The resulting size, shape, and distribution of carbides at the sheet surfaces depended largely on the annealing treatment.