SEM-based automatic image analysis of sulfide inclusions in hot-rolled carbon steels

Abstract

In the present study automatic image analysis (AIA) with a scanning electron microscope (SEM) proved to be a useful technique for quantitative characterization of the chemical composition, volume fraction, size, and shape of large numbers of nonmetallic inclusions in a series of 0.1 and 0.2 wt.% carbon, 1.0 wt.% manganese steels containing various sulfur levels and, in two steels, rare-earth additions. The inclusions in the rare-earth-treated steels were predominantly rare-earth sulfides or oxysulfides. The volume fractions of inclusions in the different steels determined by SEM-based AIA were in good agreement with those expected on the basis of sulfur and oxygen contents. The inclusions in the steels without the rare-earth additions were triaxial ellipsoids and had length/width (aspect) ratios significantly greater than those of the inclusions in the rare-earth-treated steels, which were oblate spheroids of revolution. The size and area-fraction distributions of the inclusions appeared to obey a log-normal distribution. Compared with untreated steels, steels with rare-earth additions contained a narrow range of inclusion sizes, with fewer large and small inclusions. The steels with low sulfur contents had an inclusion size distribution in between those obtained for the higher sulfur steels with and without rare-earth additions.