Effect of composition and morphology of non-metallic inclusions on fracture toughness in as-cast AHSS

Abstract

A set of water-cooling copper-chill mold equipment is designed to study the precipitation behavior of different inclusion types in test steel under different cooling conditions, as well as its effects on the steel mechanical properties. As the results reveal, slow cooling treatment near the solidus temperature of test steel is conducive to forming more MnS + Al2O3 composite inclusions. The impact energy (−16 °C) of slow-cooled cast ingots at the core position is 28% higher than that of the air-cooled ingots. Scanning electron microscopy analysis of the specimens shows that small cracks are formed around single Al2O3 inclusions, but no cracks were found around composite inclusions. A calculation model of the tessellated stress of composite inclusions in steel is introduced to verify this phenomenon. The calculation demonstrates that, with a cover of ductile MnS, stress concentration around the composite inclusion is significantly decreased as compared with the single Al2O3 inclusion in the test steel.