Basic Oxygen Steelmaking is by far the most common process for the conversion of iron into steel. The process dating back to the late 1940's operates under many names, and many variations, but in essence relies upon heat being supplied by oxygen blowing into a bath of molten iron.
Synonyms for the process include BOF, BOS, BOP and LD, the latter being an abbreviation for the first production unit installed at Lintz in Austria during 1952.
Basic oxygen steelmaking has undergone many changes during the late 1980's to improve steel quality, increase the range of steel grades, and improve efficiency. Recent changes have seen the introduction of bottom blowing, combined top blowing and bottom stirring, and increased computerisation of the process.
TOP BLOWN PROCESS
The basic oxygen steelmaking furnace is charged with molten iron and between 15 and 30% scrap, oxygen is blown into the charge at high velocity resulting in temperature increase and turbulence in the metal bath. The high turbulence results in metal droplets being carried away from the central oxygen stream, increasing the metal surface area, and the rate of oxidation of impurities in the molten iron. During this process, the oxidation of silicon, manganese and carbon are all exothermic, resulting in an increase in bath temperature
OTHER BLOWING PROCESSES
Variations to the top blown process incorporate blowing through the furnace bottom. The processes used are many and varied, but all rely on the passage of gas through refractory blocks that are either in the form of tuyeres or permeable bodies.
The Q-BOP and OBM process relies upon the majority of oxygen blowing through the bottom tuyeres usually as two concentric pipes with oxygen passing through the inner and a coolant hydrocarbon gas (natural gas) through the outer.
Top blown processes with bottom bath agitation, provided by either nitrogen or argon gas passing through tuyeres, are newer developments, and enable production of ultra-low carbon steels. All processes using bottom blowing effectively raise bath height, and show different refractory wear profiles to those of only top blown furnaces. Wear of the tuyere blocks and surrounds is often severe in this type of process, and requires the use of erosion resistant high density materials to resist the turbulent flow of molten metal.
REFRACTORY APPLICATIONS
Mechanisms of refractory attack in the BOS process are either the single or combined effects of slag, temperature, oxidising atmosphere and abrasion, and as a result of these agents there are many wear areas. During the first blow of the converter the slag chemistry will become high in FeO, and may be acidic. As the heat progresses, the slag basicity is increased to aid removal of phosphorous and sulphur, and in the final stages, the temperature is increased above 1700oC, to remove carbon.
Since the late 1970's early 1980's, dramatic improvements have been made in converter life for a variety of reasons, which include, process developments, and developments in lining technology. Converters that were traditionally lined with dolomite or pitch impregnated fired magnesite initially saw the introduction of magnesite carbon into the then high wearing trunnion zones. The significant reduction in wear soon led to these materials being incorporated into other areas, until the modern day converter which is primarily built from magnesia carbon materials.
Improvements in life have been from sub 1000 heats to 13000+ heats, with considerable reductions in refractory cost per tonne of steel, and reductions in maintenance costs such as spray costs and downtime.
Slag splashing, a relatively recent innovation, has resulted in dramatic extensions in Converter performance. Slag splashing as the name implies utilises residual slag from the steelmaking process to provide a coating on the refractory lining. Molten slag is forced by means of high pressure gas into the upper reaches of the vessel cone where it becomes more viscous and attaches to the Converter working lining. Slag splashing requires the Converter to be sidelined from steelmaking for several minutes whilst the process takes place. In conjunction with hole chasing using a gunning method, marked improvements in life have and are being obtained.
AREAS OF THE CONVERTER
BOTTOM
The converter bottom is constantly in contact with molten metal during operation and oxidation is not the major cause of wear. During charging with scrap some refractory loss by impact occurs, but the resilience of magnesite carbon is usually sufficient to resist major loss.
Magnesite carbon with neither metal additions nor pitch impregnation offer economical performance in converter bottoms, since, provided the converter is not bath agitated bottom repairs may be readily effected in service by dolomite/slag/chilling.
The QBOP operation causes high stress on the bottom and for this reason fired magnesite products,with modifiers and pitch impregnation are preferred.
TUYERES AND TUYERE SURROUNDS
Bottom blowing and bath agitation through tuyeres contributes to localised wear of the refractory components. The wear is attributable to turbulent flow of molten metal giving rise to erosion of the refractory, and to thermal stress caused by the passage of cold gases.
High density and low porosity pitch bonded, impregnated magnesite carbon based upon fused magnesia are preferred for this application.
CHARGE PAD
The charge pad of the converter is invariably directly opposite the taphole, although in some converters, charging and tapping are carried out on one side of the vessel. The charge pad is subjected to impact by falling scrap, sometimes of heavy gauge for example bloom ends, and erosion by molten iron. In addition sampling probes and temperature dips are often introduced from this side of the tilted vessel leading to slag at high temperature washing the whole charge pad area.
Initial refractory solutions to the charge pad were found with pitch impregnated fired magnesite. As converter lives increased owing to the use of magnesite carbon materials, the charge pad became a major wear area, and resin bonded magnesite carbon with metal additions are giving good performance. These materials offer good resistance to impact, providing resilience coupled with strength, and resistance to slag ingress owing to the presence of Graphite.
TAPPING PAD
The tapping pad is subjected to erosion by steel at high temperature, and to corrosion by molten slag.
Product development in the tapping pad has followed that of the charge pad, traditional materials being replaced with pitch bonded and impregnated magnesite carbon with metal additions.
Since the dominant wear process on the tapping pad involves high temperature slag attack, materials based upon large crystal sized magnesite are rapidly becoming the standard.
Pitch bonded and impregnated materials have been found to give superior performance in the Tapping Pad, owing to the reduction of penetrating slag oxides by the action of carbon in the brick porosity.
SLAG ZONE CROSSOVER
This area of the converter, which occurs at the intersection between the lower tapping pad and upper level of the static bath, is particularly complex since it is subjected to many modes of attack. Slag attack, high temperature and erosion, most attack occurring during the tapping stage. Materials with resistance to high temperature slag attack coupled with oxidation resistance are mandatory.
Pitch bonded, impregnated magnesite carbon based upon large crystal size magnesite, high purity flake graphite and containing metal additions achieve success in this most difficult of regimes. Incorporation of high purity flake graphite is particularly beneficial since it limits 'self oxidation' by the impurity oxides inherently present in flake graphite.
TRUNNIONS
The trunnion areas of the converter are the most difficult to maintain, since they are on the rotational axis, and thus incapable of slag washing. Wear of the refractory material is primarily by loss of the working face by oxidation of the carbon bond. Spraying of this area of the vessel is the only practicable method of maintenance and bricks with high resistance to oxidation are preferred.
MAIN BARREL
The main barrel of the converter may conveniently be split into two areas the Knuckle and the upper sidewall.
KNUCKLE
The knuckle area in the converter is often a major wear area, although design changes are often effective in improving performance. The Knuckle area is invariably subjected to severe slag attack, particularly when the vessel is not being blown, and requires refractory materials with very good resistance to slag corrosion and erosion.
UPPER SIDEWALL
The upper sidewall of the converter is subjected to slag attack, but also to extremes of temperature cycling.
Materials based upon slag resistant materials are of prime importance, but in these areas the refractory lining should also be capable of taking and retaining a slag coating.
SAMPLING SLAGLINES
Areas of the cone distributed either side of the charge pad are subjected to preferential attack by slag when the vessel is tilted to effect temperature measurement and to allow sample dips.
Pitch bonded, impregnated magnesite carbon give good performance in this area of the converter.
CONE
The cone area is subjected to high temperature erosion by high velocity gases carrying with them entrapped particles.
Resin bonded magnesite carbon materials give good performance in the cone area.
UPPER CONE
In the upper cone, the top six to ten rings, the wear mechanism is modified by loss of refractory material during skull removal Damage to refractory bricks in the upper cone occurs during mechanical deskulling either progressively by loss of brick ends or by dislodgement of whole bricks.
Materials with high resilience, combined with strength and oxidation resistance are a prerequisite for this area of the converter. Pitch bonded, impregnated magnesite, sometimes provided with co-moulded metal plates, which expand during oxidation and tighten the brick rings, is the most successful product.