BOF steelmaking accounts for just under 60% of the liquid steel Output in North America. While this figure may decline with the growth of EAF use, the BOF will continue to be a major source of steel for many years. BOFs include conventional top-blown furnaces, Q-BOP (bottom blown) furnaces, and various mixed blowing configurations and inert gas bottom stirring modifications.

Because significantly higher new blast furnace capacity is not expected, steel plants must find ways to meet demand by extending liquid pig iron production. One way to extend production is to optimize both blast furnaces and BOFs, but technological challenges exist. Steelmakers are applying or experimenting with new and emerging technologies that, with more R & D, Gould overcome challenges.

The predominant advantages of the BOF are very high production rates and low-residual-element, low-nitrogen liquid steel tapping. The BOF is fed liquid pig fron, almost always from blast furnaces, in amounts ranging from 65 to 90% of the total metallic charge. The average pig iron is approximately 74% of the charge; the balance is recycled scrap.

Efforts to improve BOF productivity and annual production capacity in recent years have included various automation technologies to optimize the blast furnace and the BOF relationship, better use of secondary refining processes (driven both by productivity and by new steel grades), and improved coordination with downstream facilities.

Advances in slag splashing that extend refractory life and use of post-combustion lances have improved furnace availability. Relines are down to one per year per furnace or less; lining life is in the range of 10,000 to 25,000 heats. Use of the post-combustion lance has reduced the time and effort involved in controlling BOF mouth and lance skulls (a build-up of steel that occurs with use).

Increasing demand for ultra-low-carbon (ULC) steels has made secondary processes more important. Lower interstitial element content in flat-rolled steel is a major worldwide trend. Many shops have focused an coordination among the BOF, tadle treatment station, ladle refming arc furnace, steel desulfurization, and degasser to achieve temperature and chemistry control and timely delivery to the caster. Some shops have found that optimizing secondary processes helps productivity by allowing the BOF to aim for a wider target.

lt may be possible to make ULC steel in the EAF route and muck cheaper if tank degassing can be employed. Tank degassing has been used by several plants to make ULC, but the cycle times are too long. Techniques to improve the kinetics, cycle times, or logistics could reduce cost dramatically.

Another trend is hot metal desulfurization, usually done in the BOF transfer ladle. When 100% desulfurization can be attained, the blast furnace can operate at higher hot-metal sulfur and lower fuel rates, which may reduce hot metal costs.

Meanwhile, steelmakers constantly experiment with BOF oxygen lance configurations, oxygen batching and flux additions practice to achieve better slag making and chemistry, better control of refractory wear, and higher production rates. There is a gradual trend toward softer blowing, or blowing at a lower velocity, with more oxygen nozzles (holes in the lance).

In those areas, technological challenges still exist. Slag splashing has increased furnace life to well beyond the life of the lower hoods. To cope with this incompatibility, shops must consider new maintenance schedules for hoods, environmental control equipment, and new hood materials.

Environmental standards are getting tougher, requiring better air-cleaning technologies for fugitive emission control and in-shop work environments. Current environmental control equipment may not be adequate to meet future standards.

Furnace vessel shell distortion and destruction dunng a long campaign must be overcome. Slag carryover from furnace to ladle, a key to clean steel, should be controlled using electromagnetic sensors and other techniques. This will improve the control and consistency of secondary treatment.

Many shops are beginning to feel the pinch of lower phosphorns specifications. Reducing the recycle of BOF slag to Sinter plants and blast furnace to rcduce steel phosphorus has its benefits and its problems. While lowcring the recycle reduces steel phosphorns, it also increases the amount of slag for landfills as well as hot metal costs by increasing the cost of replacing blast furnace charge materials. Using separate dephosphorization stations, as in some Japanese shops, increases the liquid steel costs and adds another major source of emissions.

Refining of hot metal with a low manganese-silicon ratio reduces recycle of BOF slag to the blast furnace and sinter plants, which produces a low manganese content in the hot metal. This impacts slag formation in the BOF vessel in addition to BOF operating issues.

New technologies have been under development. Work is being conducted to improve chemistry, temperature, and process control in the BOF. The use of in-blow sensors with possible feedback control is being developed to improve carbon and temperature control to measure lance height and detect the advent of slopping. Improving techniques of adding alloys, usually with the aid of secondary processing, will increase control over chemistry levels to meet new grade demands and allow consolidation of grades. Upgrading computer and expert systems will also help operators achieve consistent process control.

Using inert gas bottom stiri-ing achieves better iron yields and alloy recovery through reduction of furnace slag iron oxide, but maintaining effective stirring continues to be a major inconvenience in many shops that have tried the technique.

Many shops need techniques that enable the aggressive use of post-combustion lances or supplemental fuels to extend the use of hot metal. These techniques will increase their capacity without requiring investment in new hot metal capacity . These techniques would also minimize production loss during periods of blast furnace relines.

Other technologies that support oxygen steelmaking include scrap preparation and handling, fluxes and methods of additions, recycling of waste oxides, and process sensors with feedback capability (for example, light meter, lasers, infrared temperature detectors).

Scrap handling is unique in each plant. Use of home scrap usually requires preparation before recharging into the BOF. Use of outside or purchased scrap is subject to the saure demands and problems experienced by EAF operators. In addition, the trend in scrap prices (especially for premium scrap) in recent years has been upward due to competitive buying pressures from EAF Shops.

Seme integrated plants are experimenting with lower-grade, higher-residual (and thus cheaper) scrap because it can be diluted by low-residual hot metal.

Flux quality, size, and method of introduction are becoming more important because of increased demands an slagmaking, both for refractory maintenance and for control of sulfur and phosphorns. Investigations of flux batching and related oxygen lance schedules are contributing to ongoing improvements in charge recipe calculations and the consistency of slagmaking.

Increasingly, recycling in-plant waste oxides in the BOF is addressing environmental pressures and presenting opportunities for low-cost sources of iron and/or coolants in the furnaces.

The use of industrial gases is also increasing. Many shops have nitrogen circuits tied into the main lance circuit for slag splashmg as well as for bottom stirring. Nitrogen gas can be used for nitrogen chemical control when required an certain grades to replace expensive nitrided ferro-manganese.

Several technological challenges also exist. BOF steelmaking improvements must overcome many technological challenges. One area that requires technological development is scrap systems. Scrap delivery and analysis systems are complicated, unreliable, and ineflicient. Without better systems, the amount of scrap used and effect an quality is limited.

Also challenging BOF steelmaking is the difficulty of maintaining reliable sensors and automated systems. The lack of fully developed, reliable automatic flux batching systems, particularly bin level detectors for dusty environments, limits slag making consistency. Also, the hostility of the BOF environment for lasers and other sensors makes it difficult to find suitable protection and locations for these sensors. Without speedy, comprehensive optical sensors, scanning and reviewing historical data an the condition of furnaces and ladles is difficult.

Another technological challenge that must be overcome is slag analysis. A speedy, reliable slag oxide analysis teehnique is not available, particularly for fron oxides or for controlling lance height, making slag, and calculating alloy efficiencies. In addition, slag analysis is expensive and slow and involves sampling separation problems related to the use of iron versus iron oxides.

How to utilize the high levels (up to 85%) of uncombusted carbon monoxidc leaving the BOF vessel is another dificult technological challenge.

Recent and developing BOF process improvements primarily affect scrap and process sensors. Additionally, development is ongoing in bumers and nozzles for the BOF process. This work could improve post-combustion performance.

Preheating techniques and quality improvements lead the emerging scrap technologies. A number of investigations are looking for economical ways to remove residual elements from scrap to replace expensive but successful techniques, such as detinned bundles. Also, effective scrap preheating techniques are being developed. Smelt-refining processes and EAFs have developed submerged dust injection and scrap prpheating technologies, but their application to BOF steelmaking remains unexplored.

Light meters, lasers and infrared cameras and sensors are being studied to control carbon, temperature, slopping, waste gas composition, and lance height above the bath. Improved sensors, instrumentation, computer power, and process models are used to provide data that enable an operator to consistently optimize production processes. Industry-sponsored work (though AISI and DOE) an vahous sensors in BOFs that apply laser measurement and in-lance cameras are presently undergoing commercialization tests.