Studies on hydrogen behaviour in the industrial scale manufacture of alloy steel long products

Abstract

The behaviour of hydrogen during controlled industrial scale secondary steel making process has been examined in a variety of low alloy steels, sensitive to hydrogen flaking. The study examines the role played by the moisture in input raw materials such as the Ferro-alloys, type of carbon additive and fluxes in enhancing the hydrogen content in the ladle furnace. Post alloying, the influence of vacuum degassing parameters such as the vacuum level, vacuum holding time, Argon flow rate, type of porous plug used, slag chemistry and the steel grade were examined. The vacuum degassing process was analysed using a kinetic model, which could justify the trends seen in the vacuum level, holding time and Argon gas flow rate. Finally, the hydrogen pick-up post vacuum degassing through slag cover and the casting tundish was found to be influenced by parameters such as the quality of the tundish spray mass, drying of tundish and casting sequence. It is observed that the hydrogen content decreases significantly during the solid state phase transformation to a cast product. The value is further decreased when the cast product is converted to rolled product. The present study shows that the hydrogen pick-up in an out of circulation ladle shows gt 5ppm H and it decreases in ladles with circulation. When the steel is processed through Ladle furnace, Hydrogen pick up during ferroalloy addition varies between 1.46 to 4.4 ppm H. The addition of graphite flakes against CPC enhances the hydrogen in tundish by 0.5 ppm more. The steel grade influences the hydrogen loss during vacuum degassing. It is observed that the Cr-Mn steel has higher loss [2.2 ppm] as against Cr-Mo steel [1.25 ppm] during melting. newline