Features of Nonmetallic Inclusion (Precipitates) Evolution During Heating of Structural Steel for Rolling

Evolution of nonmetallic inclusions on heating a billet for rolling is studied using contemporary methods of electron microscopy and local x-ray spectral analysis for steels 20-KSKh and 13KhFA from an industrial melt. It is established that it is controlled by the occurrence of three main processes. The first consists in the deposition of manganese sulfide on the surface of aluminum-magnesium spinel and other oxide composites MgO-Al2O3-CaO both from solid solution and from soluble MnS precipitates, as a result of which inclusions become corrosion-active and their amount increases. An increase in metal heating duration leads to the coalescence of sulfide components from smaller to larger sizes. As a result of this, the number of inclusions exhibiting corrosion activity decreases. The majority of titanium nitride inclusions are retained with the metal composition outside a dependence on billet heating temperature and duration, and there may be formation of complex TiN inclusions with niobium nitride, with calcium and manganese sulfides, and also of even more complex composition, including aluminum-magnesium spinel, and these inclusions may have an unfavorable effect on the steel's properties. In contrast, a favorable process is deposition during subsequent cooling on the surface of sulfide and other forms of inclusions of cementite and other carbides, which may lead to a marked increase in strength properties without a reduction in steel ductility. On the basis of results obtained, a number of effective production methods are suggested for improving steel resistance to local corrosion and other service properties.