By John Stubbles, Steel Industry Consultant
Accounting for 60% of the world's total output of crude steel, the Basic Oxygen Steelmaking (BOS) process is the dominant steelmaking technology. In the U.S., that figure is 54% and slowly declining due primarily to the advent of the "Greenfield" electric arc furnace (EAF) flat-rolled mills. However, elsewhere its use is growing.
There exist several variations on the BOS process: top blowing, bottom blowing, and a combination of the two. This study will focus only on the top blowing variation.
The Basic Oxygen Steelmaking process differs from the EAF in that it is autogenous, or self-sufficient in energy. The primary raw materials for the BOP are 70-80% liquid hot metal from the blast furnace and the balance is steel scrap. These are charged into the Basic Oxygen Furnace (BOF) vessel. Oxygen (>99.5% pure) is "blown" into the BOF at supersonic velocities. It oxidizes the carbon and silicon contained in the hot metal liberating great quantities of heat which melts the scrap. There are lesser energy contributions from the oxidation of iron, manganese, and phosphorus. The post combustion of carbon monoxide as it exits the vessel also transmits heat back to the bath.
The product of the BOS is molten steel with a specified chemical anlaysis at 2900°F-3000°F. From here it may undergo further refining in a secondary refining process or be sent directly to the continuous caster where it is solidified into semifinished shapes: blooms, billets, or slabs.
Basic refers to the magnesia (MgO) refractory lining which wears through contact with hot, basic slags. These slags are required to remove phosphorus and sulfur from the molten charge.
BOF heat sizes in the U.S. are typically around 250 tons, and tap-to-tap times are about 40 minutes, of which 50% is "blowing time". This rate of production made the process compatible with the continuous casting of slabs, which in turn had an enormous beneficial impact on yields from crude steel to shipped product, and on downstream flat-rolled quality.
II BASIC OPERATION
BOS process replaced open hearth steelmaking. The process predated continuous casting. As a consequence, ladle sizes remained unchanged in the renovated open hearth shops and ingot pouring aisles were built in the new shops. Six-story buildings are needed to house the Basic Oxygen Furnace (BOF) vessels to accommodate the long oxygen lances that are lowered and raised from the BOF vessel and the elevated alloy and flux bins. Since the BOS process increases productivity by almost an order of magnitude, generally only two BOFs were required to replace a dozen open hearth furnaces.
Some dimensions of a typical 250 ton BOF vessel in the U.S. are: height 34 feet, outside diameter 26 feet, barrel lining thickness 3 feet, and working volume 8000 cubic feet. A control pulpit is usually located between the vessels. Unlike the open hearth, the BOF operation is conducted almost "in the dark" using mimics and screens to determine vessel inclination, additions, lance height, oxygen flow etc.
Once the hot metal temperature and chemical analaysis of the blast furnace hot metal are known, a computer charge models determine the optimum proportions of scrap and hot metal, flux additions, lance height and oxygen blowing time.
Figure 2: BOF Vessel in Its Operating Positions. (Ref: Making, Shaping, and Treating of Steel, 11th Edition, Steelmaking And Refining Volume. AISE Steel Foundation, 1998, Pittsburgh PA)
A "heat" begins when the BOF vessel is tilted about 45 degrees towards the charging aisle and scrap charge (about 25 to 30% of the heat weight) is dumped from a charging box into the mouth of the cylindrical BOF. The hot metal is immediately poured directly onto the scrap from a transfer ladle. Fumes and kish (graphite flakes from the carbon saturated hot metal) are emitted from the vessel's mouth and collected by the pollution control system. Charging takes a couple of minutes. Then the vessel is rotated back to the vertical position and lime/dolomite fluxes are dropped onto the charge from overhead bins while the lance is lowered to a few feet above the bottom of the vessel. The lance is water-cooled with a multi-hole copper tip. Through this lance, oxygen of greater than 99.5% purity is blown into the mix. If the oxygen is lower in purity, nitrogen levels at tap become unacceptable.
As blowing begins, an ear-piercing shriek is heard. This is soon muffled as silicon from the hot metal is oxidized forming silica, SiO2, which reacts with the basic fluxes to form a gassy molten slag that envelops the lance. The gas is primarily carbon monoxide (CO) from the carbon in the hot metal. The rate of gas evolution is many times the volume of the vessel and it is common to see slag slopping over the lip of the vessel, especially if the slag is too viscous. Blowing continues for a predetermined time based on the metallic charge chemistry and the melt specification. This is typically 15 to 20 minutes, and the lance is generally preprogrammed to move to different heights during the blowing period. The lance is then raised so that the vessel can be turned down towards the charging aisle for sampling and temperature tests. Static charge models however do not ensure consistent turndown at the specified carbon and temperature because the hot metal analysis and metallic charge weights are not known precisely. Furthermore, below 0.2% C, the highly exothermic oxidation of iron takes place to a variable degree along with decarburization. The "drop" in the flame at the mouth of the vessel signals low carbon, but temperature at turndown can be off by +/- 100°F.
In the past, this meant delays for reblowing or adding coolants. Today, with more operating experience, better computer models, more attention to metallic input quality, and the availability of ladle furnaces that adjust for temperature, turndown control is more consistent. In some shops, sublances provide a temperature-carbon check about two minutes before the scheduled end of the blow. This information permits an "in course" correction during the final two minutes and better turn-down performance. However, operation of sublances is costly, and the required information is not always obtained due to malfunctioning of the sensors.
Once the heat is ready for tapping and the preheated ladle is positioned in the ladle car under the furnace, the vessel is tilted towards the tapping aisle, and steel emerges from the taphole in the upper "cone" section of the vessel. The taphole is generally plugged with material that prevents slag entering the ladle as the vessel turns down. Steel burns through the plug immediately. To minimize slag carryover into the ladle at the end of tapping, various "slag stoppers" have been designed. These work in conjunction with melter's eyeballs, which remain the dominant control device. Slag in the ladle results in phosphorus reversion, retarded desulfurization, and possibly "dirty steel". Ladle additives are available to reduce the iron oxide level in the slag but nothing can be done to alter the phosphorus.
After tapping steel into the ladle, and turning the vessel upside down and tapping the remaining slag into the "slag pot", the vessel is returned to the upright position. In many shops residual slag is blown with nitrogen to coat the barrel and trunion areas of the vessel. This process is known as "slag splashing". Near the end of a campaign, gunning with refractory materials in high wear areas may also be necessary. Once vessel maintenance is complete the vessel is ready to receive the next charge.
III BASIC CHEMISTRY AND HEAT BALANCE
A heat size of 250 tons is used as the basis for the following calculations. This is close to the average heat size for the 50 BOFs which were operable in the U.S. in 1999. The following charge chemistry is assumed:
Table 1 illustrates the heat balance PER TON OF HOT METAL. It assumes a 75% hot metal in a total charge of 275 tons which yields 250 tons of liquid steel (without alloys). If the oxygen were supplied as air, the heat required to take N2 from room temperature to 2900°F would be about 500,000 Btu per NTHM, which illustrates that the BOS is a Bessemer process with cold scrap substituted for cold nitrogen. (NTHM one short ton or 2000 pounds of hot metal).
TABLE I. HEAT BALANCE PER NET TON OF HOT METAL75% HOT METAL IN CHARGE
The actual percentage of hot metal in the charge is very sensitive to the silicon content and temperature of the hot metal and obviously increases as these decrease.
The oxygen required per heat is shown in Table II, as #/NTHM and as a percentage for the various reactions. 181#/NTHM corresponds to about 18.6 tons/per heat or 1800 scf/tapped ton. Oxygen consumption increases if end-point control is poor and reblows are necessary.
TABLE II. OXYGEN REQUIREMENTS PER NTHM
The final calculation for yield losses is shown in TABLE III. The metalloids and Mn are oxidized out of the hot metal, the scrap is often coated with Zn which volatilizes, and iron units are lost to the slag, fume, and slopping. To tap 250 tons of liquid steel, 250/0.91 or 275 charge tons are required, of which 206 will be hot metal, and the balance scrap.
TABLE III YIELD LOSSES IN A BOF HEAT
IV RAW MATERIALS
i) HOT METAL
Hot metal is liquid iron from the blast furnace saturated with up to 4.3% carbon and containing 1% or less silicon, Si. It is transported to the BOF shop either in torpedo cars or ladles. The hot metal chemistry depends on how the blast furnace is operated and what burden (iron-bearing) materials are charged to it. The trend today is to run at high productivity with low slag volumes and fuel rates, leading to lower silicon and higher sulfur levels in the hot metal. If BOF slag is recycled, P and Mn levels rise sharply since they report almost 100% to the hot metal. U.S. iron ores are low in both elements.
The sulfur level from the blast furnace can be 0.05% but an efficient hot metal desulfurizing facility ahead of the BOF will reduce this to below .01%. The most common desulfurizing reagents, lime, calcium carbide and magnesium - used alone or in combination - are injected into the hot metal through a lance. The sulfur containing compounds report to the slag; however, unless the sulfur-rich slag is skimmed before the hot metal is poured into the BOF, the sulfur actually charged will be well above the level expected from the metal analysis.
In autogenous BOS operation, scrap is by far the largest heat sink. At 20 - 25% of the charge it is one of the most important and costly components of the charge.
Steel scrap is available in many forms. The major categories are "home scrap", generated within the plant. With the advent of continuous casting, the quantity of home scrap has diminished and it is now necessary for integrated mills to buy scrap on the market. Flat rolled scrap is generally of good quality and it's impact on the chemistry of BOF operations can almost be ignored. There is a yield loss of about 2% due to the zinc coating on galvanized scrap. "Prompt scrap" is generated during the manufacturing of steel products. It finds its way into the recycling stream very quickly. Many steel mills have agreements with manufacturers to buy their prompt scrap. "Obsolete" or "post consumer" scrap returns to the market after a product has ended its useful life. Cans return to the market very quickly but autos have an average life of 12 years.
Scrap also comes in many sizes, varying chemical analyses and a variety of prices. All of which makes the purchase and melting of scrap a very complex issue. Very large pieces of scrap can be difficult to melt and may damage the vessel when charged. Some scrap may contain oil or surface oxidation. Obsolete scrap may contain a variety of other objects which could be hazardous or explosive. Obviously the chemical analysis of obsolete scrap is imprecise.
Scrap selection is further complicated by the wide variety of steel products. Deep drawing steels limit the maximum residual (%Cu +%Sn + %Ni +%Cr +%Mo) content to less than 0.13%. While other products allow this to range as high as 0.80%. Since these elements cannot be oxidized from the steel, their content in the final product can only be reduced by dilution with very high purity scrap or hot metal. The use of low residual hot metal in the BOS, with its inherent dilution effect, is one of the features that distinguish BOF from EAF steelmaking.
Fluxes serve two important purposes. First they combine with SiO2 which is oxidized from the hot metal to form a "basic" slag that is fluid at steelmaking temperatures. This slag absorbs and retains sulfur and phosphorus from the hot metal.
Lime (95+% CaO) and dolomite (58%CaO, 39% MgO) are the two primary fluxes. They are obtained by calcining the carbonate minerals, generally offsite in rotary kilns. Calcining CaCO3 and MgCO3 liberates CO2 leaving CaO or MgO. Two types, "soft" and "hard" burned lime, are available. A lump of soft burned lime dissolves quickly in a cup of water liberating heat. Hard burned material just sits there. Soft burned fluxes form slag more quickly than hard-burned, and in the short blowing cycle, this is critical for effective sulfur and phosphorus removal. The amount of lime charged depends on the Si content of the hot metal.
In BOS steelmaking a high CaO/SiO2 ratio in the slag is desirable, e.g. 3. A rule of thumb is 6 X the weight of Si charged. The MgO addition is designed to be about 8 to 10% of the final slag weight. This saturates the slag with MgO, thus reducing chemical erosion of the MgO vessel lining.
Limestone, scrap, and sponge iron are all potential coolants that can be added to a heat that has been overblown and is excessively hot. The economics and handling facilities dictate the selection at each shop.
Bulk alloys are charged from overhead bins into the ladle. The common alloys are ferromanganese (80%Mn, 6%C, balance Fe), silicomanganese (66%Mn, 16%Si, 2%C, balance Fe), and ferrosilicon (75% Si, balance Fe). Aluminum can be added as shapes and/or injected as rod. Sulfur, carbon, calcium, and special elements like boron and titanium are fed at the ladle furnace as powders sheathed in a mild steel casing about 1/2 inch in diameter.
The basis for most refractory bricks for oxygen steelmaking vessels in the U.S. today is magnesia, MgO, which can be obtained from minerals or seawater. Only one dolomite (MgO + CaO) deposit is worked in the U.S (near Reading, PA). For magnesia, the lower the boron oxide content, and the lower the impurity levels (but with a CaO/SiO2 ratio above 2 to avoid low melting point intergranular phases), the greater the hot strength of the brick. Carbon is added as pitch (tar) or graphite.
The magnesia lime type refractories used in lining oxygen steelmaking vessels are selected mainly for their compatibility with the highly basic finishing slags required to remove and retain phosphorus in solution. During refining, the refractories are exposed to a variety of slag conditions ranging from 1 to 4 basicity as silicon is oxidizes from the bath and combines with lime. The iron oxide, FeO, content of the bath increases with blowing time especially as the carbon in the steel falls below 0.2 % and Fe is oxidized. Although all refractory materials are dissolved by FeO, MgO forms a solid solution with FeO, meaning they coexist as solids within a certain temperature range. The high concentrations of FeO formed late in the blow, however, will oxidize the carbon in the brick.
The original bricks were tar bonded, where the MgO grains were coated with tar and pressed warm represented a great step forward for the BOS process. Tempering removed volatiles. In service, the tar was coked and the residual intergranular carbon resisted slag wetting and attack by FeO. In addition, as the tar softened during vessel heat-up, the lining was relieved of expansive stresses. Hot strength was increased by sintering bricks made from pure MgO grains at a high temperature and then impregnating them with tar under a vacuum. However, for environmental reasons these types of bricks are no longer used in oxygen steelmaking.
Today's working lining refractories are primarily resin-bonded magnesia-carbon bricks made with high quality sintered magnesite and high purity flake graphite. Resin-bonded brick are unfired and contain 5% to 25% high purity flake graphite and one or more powdered metals. These brick require a simple curing step at 350 to 400°F to "thermoset" the resin that makes them very strong and therefore easily handled during installation. Further refinements include using prefused grains in the mix. Small additions of metal additives (Si, Al, and Mg) protect the graphite from oxidation because they are preferentially oxidized. Metallic carbides, nitrides, and magnesium-aluminate spinel form in service at the hot face of the brick filling voids, and adding strength and resistance to slag attack.
The rate of solution of a refractory by the slag is dependent on its properties. These properties are directly related to the purity and crystal sizes of the starting ingredients as well as the manufacturing process. Additions of up to 15% high purity graphite to MgO-carbon refractories provide increased corrosion resistance. Beyond 15% this trend is reversed due to the lower density of the brick. Ultimately, the cost per ton of steel for brick and gunning repair materials, coupled with the need for vessel availability, dictate the choice of lining.
The penetration of slag and metal between the refractory grains, mechanical erosion by liquid movement, and chemical attack by slags all contribute to loss of lining material. Over the years, there have been numerous operating developments designed to counteract this lining wear:
i) Critical wear zones (impact and tap pads, turndown slag lines, and trunion areas) in furnaces have been zoned with bricks of the highest quality.
ii) "Slag splashing" whereby residual liquid slag remaining after the tap is splashed onto the lining with high pressure nitrogen blown through the oxygen lance. This seemingly simple practice has increased lining life beyond all expectations, from a few thousand to over 20,000 heats per campaign.
iii) Instruments are now available to measure lining contours in a short time period, to maximize gunning effectiveness using MgO slurries.
iv) Dolomite (40%MgO) is added to the flux addition to create slags with about 8% MgO, which is close to the MgO saturation level of the slag.
v) Improved end-point control resulting in lower FeO levels and shorter oxygen-off to charge intervals have reduced refractory deterioration.
None of the above would be significant however, without the improvements in quality and type of basic brick available to the industry.
Today, the refractory industry is undergoing major structural changes. Companies are being continually acquired and the total number of North American suppliers is greatly reduced. A very high percentage of refractory materials are being produced off shore, with China being the most significant newcomer.
Environmental challenges at BOS shops include: (1) the capture and removal of contaminants in the hot and dirty primary off-gas from the converter; (2) secondary emissions associated with charging and tapping the furnaces; (3) control of emissions from ancillary operations such as hot metal transfer, desulfurization, or ladle metallurgy operations; (4) the recycling and/or disposal of collected oxide dusts or sludges; and (5) the disposition of slag.
In the U.S., most BOF primary gas handling systems are designed to generate plant steam from the water-cooled hood serving the primary system. About half of the systems are open combustion designs where excess air is induced at the mouth of the hood to completely burn the carbon monoxide. The gases are then cooled and cleaned either in a wet scrubber or a dry electrostatic precipitator. The remainder of U.S. systems are suppressed combustion systems where gases are handled in an uncombusted state and cleaned in a wet scrubber before being ignited prior to discharge. In both cases, the cleaned gases must meet EPA-mandated levels for particulate matter.
Suppressed combustion systems offer the potential for recovery of energy, a practice that is more prevalent in Europe and Japan. However, in the U.S., other than steam generation, no attempt is made to capture the chemical or sensible heat in the off-gas leaving the vessel. While this represents the loss of a considerable amount of energy (about 0.7 million Btu/ton), the pay-back on capital required, either for the conversion of open combustion to suppressed combustion systems or the addition of necessary gas collection facilities for suppressed combustion systems, is over 10 years. In addition, the necessity of taking shops out of service to make these changes is not practical. Most BOF shops in the U.S. pre-date the energy crises of the 1970s, and even today, energy in the U.S. is relatively less expensive than it is abroad.
Secondary fugitive emissions associated with charging and tapping the BOF vessel, or emissions escaping the main hood during oxygen blowing, may be captured by exhaust systems serving local hoods or high canopy hoods located in the trusses of the shop or both. Typically a fabric collector, or baghouse, is use for the collection of these fugitive emissions. Similarly, ancillary operations such as hot metal transfer stations, desulfurization, or ladle metallurgy operations are usually served by local hood systems exhausted to fabric filters.
The particulate matter captured in the primary system, whether in the form of sludge from wet scrubbers or dry dust from precipitators, must be processed before recycling. Sludge from wet scrubbers requires an extra drying step. Unlike EAF dust, BOF dust or sludge is not a listed hazardous waste. If the zinc content is low enough, it can be recycled to the blast furnace or BOF vessel after briquetting or pelletizing. Numerous processes for recycling the particulate are in use or under development.
BOF slag typically contains about 5% MnO and 1% P2O5 and are often can be recycled through the blast furnace. Because lime in steel slag absorbs moisture and expands on weathering, its use as an aggregate material is limited, but other commercial uses are being developed to minimize the amount that must be disposed.
The BOS has been a pivotal process in the transformation of the U.S. steel industry since World War II. Although it was not recognized at the time, the process made it possible to couple melting with continuous casting. The result has been that melt shop process and finishing mill quality and yields improved several percent, such that the quantity of raw steel required per ton of product decreased significantly.
The future of the BOS depends on the availability of hot metal, which in turn depends on the cost and availability of coke. Although it is possible to operate BOFs with reduced hot metal charges, i.e. < 70%, there are productivity penalties and costs associated with the supply of auxiliary fuels. Processes to replace the blast furnace are being constantly being unveiled, and the concept of a hybrid BOF-EAF is already a reality at the Saldahna Works in South Africa. However, it appears that the blast furnace and the BOS will be with us for many decades into the future.
The American Iron and Steel Institute acknowledges, with thanks, the contributions of Teresa M. Speiran, Senior Research Engineer, Refractories and Bruce A. Steiner, Senior Environmental Advisor, Collier Shannon Scott PLLC.
HISTORY OF THE BASIC OXYGEN STEELMAKING PROCESS
Basic Oxygen Steelmaking is unquestionably the "son of Bessemer", the original pneumatic process patented by Sir Henry Bessemer in 1856. Because oxygen was not available commercially in those days, air was the oxidant. It was blown through tuyeres in the bottom of the pear shaped vessel. Since air is 80% inert nitrogen, which entered the vessel cold but exited hot, removed so much heat from the process that the charge had to be almost 100% hot metal for it to be autogenous. The inability of the Bessemer process to melt significant quantities of scrap became an economic handicap as steel scrap accumulated. Bessemer production peaked in the U.S. in 1906 and lingered until the 1960s.
There are two interesting historical footnotes to the original Bessemer story:
William Kelly was awarded the original U.S. patent for pneumatic steelmaking over Bessemer in 1857. However, it is clear that Kelly's "air boiling" process was conducted at such low blowing rates that the heat generation barely offset the heat losses. He never developed a commercial process for making steel consistently.
Most European iron ores and therefore hot metal was high in sulfur and phosphorus and no processes to remove these from steel had been developed in the 1860s. As a result, Bessemer's steel suffered from both "hot shortness" (due to sulfur) and "cold shortness" (due to phosphorus) that rendered it unrollable. For his first commercial plant in Sheffield, 1866, Bessemer remelted cold pig iron imported from Sweden as the raw material for his hot metal. This charcoal derived pig iron was low in phosphorus and sulfur, and (fortuitously) high in manganese which acted as a deoxidant. In contrast the U.S. pig iron was produced using low sulfur charcoal and low phosphorus domestic ore. Therefore, thanks to the engineering genius of Alexander Holley, two Bessemer plants were in operation by 1866. However, the daily output of remotely located charcoal blast furnaces was very low. Therefore, hot metal was produced by remelting pig iron in cupolas and gravity feeding it to the 5 ton Bessemer vessels.
The real breakthrough for Bessemer occurred in 1879 when Sidney Thomas, a young clerk from a London police court, shocked the metallurgical establishment by presenting data on a process to remove phosphorus (and also sulfur) from Bessemer's steel. He developed basic linings produced from tar-bonded dolomite bricks. These were eroded to form a basic slag that absorbed phosphorus and sulfur, although the amounts remained high by modern standards. The Europeans quickly took to the "Thomas Process" because of their very high-phosphorus hot metal, and as a bonus, granulated the phosphorus-rich molten slag in water to create a fertilizer. In the U.S., Andrew Carnegie, who was present when Thomas presented his paper in London, befriended the young man and cleverly acquired the U.S. license, which squelched any steelmaking developments in the South where high phosphorus ores are located.
Although Bessemer's father had jokingly suggested using pure oxygen instead of air (U.K.patent 2207, Oct 5,1858), this possibility was to remain a dream until "tonnage oxygen" became available at a reasonable cost. A 250 ton BOF today needs about 20 tons of pure oxygen every 40 minutes. Despite its high cost, oxygen was used in Europe to a limited extent in the 1930's to enrich the air blast for blast furnaces and Thomas converters. It was also used in the U.S for scarfing, and welding.
The production of low cost tonnage oxygen was stimulated in World War II by the German V2 rocket program. After the war, the Germans were denied the right to manufacture tonnage oxygen, but oxygen plants were shipped to other countries. The bottom tuyeres used in the Bessemer and Thomas processes could not withstand even oxygen-enriched air, let alone pure oxygen. In the late 1940s, Professor Durrer in Switzerland pursued his prewar idea of injecting pure oxygen through the top of the vessel. Development now moved to neighboring Austria where developers wanted to produce low nitrogen, flat-rolled sheet, but a shortage of scrap precluded open hearth operations. Following pilot plant trials at Linz and Donawitz, a top blown pneumatic process for a 35 ton vessel using pure oxygen was commercialized by Voest at Linz in 1952. The nearby Dolomite Mountains also provided an ideal source of material for basic refractories.
The new process was officially dubbed the "LD Process" and because of its high productivity was seen globally as a viable, low capital process by which the war torn countries of Europe could rebuild their steel industries. Japan switched from a rebuilding plan based on open hearths to evaluate the LD, and installed their first unit at Yawata in 1957.
Two small North American installations started at Dofasco and McLouth in 1954. However, with the know-how and capital invested in 130 million tons of open hearth capacity, plans for additional open hearth capacity well along, cheap energy, and heat sizes greater by an order of magnitude (300 versus 30 tons), the incentive to install this untested, small-scale process in North America was lacking. The process was acknowledged as a breakthrough technically but the timing, scale, and economics were wrong for the time. The U.S., which manufactured about 50% of the world's total steel output, needed steel for a booming post-war economy.
There were also acrimonious legal actions over patent rights to the process and the supersonic lance design, which was now multihole rather than single hole. Kaiser Industries held the U.S. patent rights but in the end, the U.S. Supreme Court supported lower court decisions that considered the patent to be invalid.
Nevertheless, the appeal of lower energy, labor, and refractory costs for the LD process could not be denied and although oxygen usage in the open hearth delayed the transition to the new process in the U.S., oxygen steelmaking tonnage grew steadily in the 1960's. By 1969, it exceeded that of the open hearth for the first time and has never relinquished its position as the dominant steelmaking process in the U.S. but the name LD never caught on in the U.S.
Technical developments over the years include improved computer models and instrumentation for improved turn-down control, external hot metal desulfurization, bottom blowing and stirring with a variety of gases and tuyeres, slag splashing, and improved refractories.