Steelmaking Practice - Steel Making - Lecture Notes, Study notes of Material Engineering

Download Steelmaking Practice - Steel Making - Lecture Notes and more Study notes Material Engineering in PDF only on Docsity! Lecture 12 Converter Steelmaking Practice & combined blowing Contents: Refining of hot metal Composition and temperature during the blow Physico-chemical interactions Developments in Top blown steelmaking practice Concept of bottom stirring in top blowing Top blowing attributes Characteristic feature of converter steelmaking Environmental issues in oxygen steelmaking Causes of high turnover rates of BOF Key words: Top blown steelmaking, combined blowing, bottom stirring, hot metal refining Refining of hot metal After the previous heat is tapped and slag is drained, lining is inspected. Scrap and hot metal are charged. Converter is tilted into the vertical position and the lance is lowered in the vessel to start the blowing. Selection of the starting lance distance is such that the concentration of the force at the bath level should not cause ejection of tiny iron particles (sparking) and at the same time maximum bath surface area is covered by the oxygen jet. The starting lance distance(Xi) for specific oxygen blowing rate 3 Nm 3 ton ×min can be calculated by Xi = 0.541(db)1.04 db is bath diameter in meter. For 150 Tons converter, db = 4.87 m and Xi = 2.8 m, when oxygen flow rate in approximately 450 Nm3/min. Initially oxygen is blown soft by keeping lance distance higher to promote slag formation and to avoid ejection of small particles, because hot metal is not covered by slag. Lime may be added either at the beginning of the blow or in portion during the blow. Oxygen is blown for nearly 15-20 minutes by progressively decreasing the lance distance such that slag foaming remains under control and oxidation reactions occur uninterruptedly. Slag and metal samples are analyzed. Composition and temperature during the blow Docsity.com Typical variation of composition of metal with the blowing time is shown in figure 12.1 Figure 12.1: Variation of composition of metal during the blow The following observations can be made: • Impurities like C, Si, Mn begin to oxidize simultaneously. Si and Mn oxidize faster relative to C. Also Fe oxidizes to FeO. Rate of carbon removal is low in the beginning. • Formation of slag begins with the oxidation of Si − SiO2, Mn → MnO and Fe → FeO • Dissolution of lime increases during the blow. In the initial periods FeO helps lime dissolution. • Formation of basic and limy slag promotes removal of P. It may be noted that once slag is formed both C and P removal occur simultaneously. • In the initial stages, carbon removal rate is kept lower than P removal since P removal is favoured at lower temperatures. If carbon is removed at a faster rate in the beginning bath temperature would increase which impedes dephosphorization. Once phosphorus removal is complete, carbon removal rate can be increased. • Note that Mn content of metal decreases initially but at later periods of blow Mn content of bath increases. This is due to the onset of the following reaction: (MnO) + [Fe] = [Mn] + (FeO) In the later stages of the blow bath temperature increases due to decrease in carbon content and at the sametime FeO content of slag decreases. Both conditions are responsible for increase in Mn content of the bath. To overcome, sometimes iron ore additions are made to increase the FeO content of slag to adjust the Mn content of steel. • Temperature of the bath increases continuously. Physico-chemical interactions Physico- chemical interactions of molten bath with oxygen jet depends on the lance profile i.e. change of lance height during the blow. The lance profile is specific to each converter and depends on converter profile, hot metal composition, oxygen flow rate, hot metal chemistry and steel of desired composition. Nevertheless, in all converters initial lance distance is such as to promote iron oxidation so that dissolution of CaO commences. The idea is to create a basic and limy slag at the early part of the blow to onset dephosphorization. Shallow jet penetration covers the larger bath surface and is favorable more for iron and silicon oxidation. Small amount of carbon may be removed. Once slag is formed, lance is lowered. Oxygen jet penetrates into the bath and carbon reaction favours because oxygen is available now deep into the bath. At the same time, force of the oxygen jet creates metal droplets and as a Docsity.com