SURFACE PHENOMENA PECULIARITIES IN THE BOF STEELMAKING BATH

Abstract

BOF steelmaking technology is largely determined with processes taking place in the LD-converter reaction zone, which consists of “primary” and “secondary” sub-zones. The “primary” zone is a crater formed as result of supersonic gas stream impact on metal melt sur- face, fulfilled with metal droplets of 0.1 – 2 mm in diameter. Surrounding in the “secondary” zone consists of the large amount of gas bubbles of 0.2 – 4 mm in diameter. The total surface area of droplets and bubbles is by four orders of magnitude larger as compared with the stable metal surface of magnitude larger as compared with the stable metal surface place. This suggests important role of interface phenomena at steel refining processes. The reaction zone structure and its temperature distribution were studied with “hot” modeling method, where molten cast iron was blown with oxygen in transparent quartz crucible. Each blow was accomplished with photo- and cinema filming through crucible wall. Besides temperature distribution obtained material also allowed study of metal bath hydrodinamics directly in blowing zone. The most unexpected result here was the motion trajectory of bubbles in the “secondary” zone. They moved normally to the crater surface, i.e. almost in a horizontal direction instead of vertical float as it was noticed at “cold” modeling with water. This very important phenomenon is caused by surface tension in homogeneity, due to which the bubbles are moved at higher temperatures direction. Surface tension forces in front of and behind gas bubble in liquid with temperature gradient are different. Because contact forces behind bubble are larger as compared that in front, it is pushed out in direction of surface tension decrease. Surface tension inhomogeneity is generated with temperature (up to 1200 °C) and oxygen concentration gradients in the “secondary” reaction zone. Iron-carbon surface tension changes with temperature rise inconsistently. Surface tension increases with temperature rise up to 1550 °C. At reaching 1550 – 1600 °C there is a bend, which after surface tension begin to decrease. This bend point is as higher as lower carbon concentration in alloy gas bubbles and heterogeneous phase’s motion in surface decrease direction starts from 1550 °C isotherm. So it is outward border of “secondary” reaction zone, which separates it from main metal bath. Inside it resulting surface tension forces push gas bubbles and slag particles into accelerating motion with mass of metal melt in horizontal direction to the crater. This phenomenon determines whole steelmaking bath hydrodynamics with oxygen redistribution between molten metal components and hence the steel refining process in general.