Temperature and deformation conditions of gas bubbles welding at hot rolling

Abstract

At production of hot-rolled product from middle-carbon and low-carbon steel, a surface defect “rolled-out bubble” is formed sometimes. To decrease rejections by surface defects like this one, it is necessary to know the mechanism of their origination. The comprehensive study by methods of micro-X-ray-spectral and X-ray-phase analysis on internal surface of the rolled-out bubbles and areas, located in the direct vicinity of the defects mentioned, allowed to determine the content of isolations, localized at the internal surface of the defects. The content was as follows (%, mas.): quartz - 24, tridymite - 28, magnetite - 48. By the method of differential scanning calorimetry the melting temperature of isolations on the internal surface of the rolled-out bubbles determined, amounted to 1178 dC, which corresponds to melting temperature of iron-silicon eutectic Fe 3 O 4 -SiO 2 . A mechanism of the defect “rolled-out bubble” formation proposed. According to the mechanism proposed, the inclusions of silicates, localized at the internal surface of the bubbles and complex inclusions based on silicon and iron oxides, form the low-melting eutectic of Fe 3 O 4 -SiO 2 type when interacting with oxygen of air during the heating before rolling. As a result of expanding forces during the rolling, in the surface metal layer, which has a liquid eutectic at the rolling temperatures, discontinuity flaws are formed. To avoid the defects it was proposed to keep the reheating and rolling temperature lower the melting temperature of the eutectic of Fe 3 O 4 -SiO 2 type. The corrected rolling regime allowed to exclude practically completely the rejections by defect “rolled-out bubble” and to decrease the metal consumption coefficient at production of rolled products from S355J2G3 and C45R steels.