Abstract

The ever-increasing demand for better steel quality necessitates the development of cleaner steels. Controlling the as-cast structure, segregation and the number, size, morphology and chemical composition of non-metallic inclusions enhances steel cleanness. This can be achieved by adjusting a wide range of operating practices throughout the steel making process. In this study, the effect of inclusions and segregations on cleanness of the pilot-scale experimental continuously cast and direct-quenched martensitic low-alloyed ultrahigh-strength steel with a tensile strength of 1850 MPa was analyzed. The investigated steel was also aluminum killed and calcium treated. Two different superheats were used to get differences in the as-cast structures and segregation patterns. Samples of these two casting scenarios were hot rolled with two different finish rolling temperatures to distinguish the possible effect of hot rolling temperature from the effect of segregation and as-cast structure. After hot rolling to a thickness of 12 mm, energy dispersive spectrometer analyses on a scanning electron microscope (SEM-EDS) were performed to characterize the inclusion types and sizes through the thickness on longitudinal cross-sections, and the results were compared to optical emission spectrometry (OES) analyses from the as-cast slab in the thickness direction. The main result of this study is that segregation affects the formation of TiN and MnS inclusions by raising the temperature at which they first form from the melt. This promotes their growth especially in the positively macrosegregated columnar-to-equiaxed transition zone and the centerline.