Formation mechanism of surface upwarping and black line defects on low carbon hot rolled plate

Abstract

To improve the surface quality of low-carbon hot-rolled steel plates, the upwarping and black line defects were investigated by means of XRF (X-ray diffractometer), OM (optical microscopy), SEM (scanning electron microscopy) and EBSD (electron back scatter diffraction). The results showed that the micro area compositions under surface upwarping and black line defects were mainly made of secondary oxide particles, steelmaking slag and mould flux respectively. According to the metallographic structure, inclusion compositions and grains texture analysis, it could be concluded that surface defects on hot rolled plates were attributed to the entrapment of ladle slag or mould flux, gas bubbles, micro-cracks of casting slab in the steelmaking process, rolled-in scale and unreasonable sizing press rolling parameters during the rolling process, and the position and formation process of typical surface defects on hot rolled plates were discussed in detail. Finally, it concluded that the slag entrapment and solidification hook should be controlled during continuous casting.