Behavior of Non-metallic Inclusions in Steel during Hot Deformation and the Effects of Deformed Inclusions on Local Ductility

Abstract

In order to clarify both the behavior of non-metallic inclusions during hot deformation and the effects of non-metallic inclusions on the local ductility of steel with aluminum deoxidized and containing lower sulfur content at about 0.002–0.01% were investigated. Both the commercial-quality 440 MPa-class plain carbon steel and super ultra-low carbon steel were studied. To investigate the distribution, morphology, and chemical composition, along with the change in such characteristics, during the hot deformation of non-metallic inclusions, thermo-mechanical treatment with a compression test was carried out. Moreover, the reduction of area with the tensile test species, which refers to the local ductility of steel, was examined, and the effect of the distribution, morphology, and chemical compositions of both the Al2O3 inclusions and the elongated MnS inclusions were studied.Consequently, metal sulfur content of higher than 60 ppm and elongated MnS inclusions of over 10 regarding the aspect ratio were observed. In addition, the elongated MnS inclusions had a stronger influence on local ductility than the smaller Al2O3 inclusions, and drastic effects on the nucleation of voids. Thus, a fracture is most likely to be initiated by void formation at the interface of the elongated MnS inclusions and metal matrix notched by MnS, and thus would experience coalescence in accordance with a brittle fracture in the soft MnS inclusions. The local ductility in steel including elongated MnS inclusions is small because the fracturing and deformation of metal are most likely to be related to the elongated MnS inclusions.