How Steel Is Made

No Comments » August 4th, 2016 posted by // Categories: Science & Technology



 

THE MAKING OF STEEL


http://www.steel.org/making-steel/how-its-made.aspx

How Steel Is Made

To produce steel, facilities use one of two processes: the basic oxygen furnace (BOF) or the electric arc furnace (EAF). The BOF process uses 25-35 percent old steel (scrap) to make new steel. BOFs make up approximately 40 percent of today’s steelmaking in the U.S. The EAF process uses virtually 100 percent old steel to make new steel. EAFs make up about 60 percent of today’s steelmaking in the U.S.

http://www.steel.org/making-steel/how-its-made/processes/processes-info/electric-arc-furnace-steelmaking.aspx?siteLocation=88e232e1-d52b-4048-9b8a-f687fbd5cdcb

Electric Arc Furnace Steelmaking

By Jeremy A. T. Jones, Nupro Corporation


Courtesy of Mannesmann Demag Corp.

 

FURNACE OPERATIONSThe electric arc furnace operates as a batch melting process producing batches of molten steel known “heats”. The electric arc furnace operating cycle is called the tap-to-tap cycle and is made up of the following operations:

Modern operations aim for a tap-to-tap time of less than 60 minutes. Some twin shell furnace operations are achieving tap-to-tap times of 35 to 40 minutes.

Furnace Charging

The first step in the production of any heat is to select the grade of steel to be made. Usually a schedule is developed prior to each production shift. Thus the melter will know in advance the schedule for his shift. The scrap yard operator will prepare buckets of scrap according to the needs of the melter. Preparation of the charge bucket is an important operation, not only to ensure proper melt-in chemistry but also to ensure good melting conditions. The scrap must be layered in the bucket according to size and density to promote the rapid formation of a liquid pool of steel in the hearth while providing protection for the sidewalls and roof from electric arc radiation. Other considerations include minimization of scrap cave-ins which can break electrodes and ensuring that large heavy pieces of scrap do not lie directly in front of burner ports which would result in blow-back of the flame onto the water cooled panels. The charge can include lime and carbon or these can be injected into the furnace during the heat. Many operations add some lime and carbon in the scrap bucket and supplement this with injection.

The first step in any tap-to-tap cycle is “charging” into the scrap. The roof and electrodes are raised and are swung to the side of the furnace to allow the scrap charging crane to move a full bucket of scrap into place over the furnace. The bucket bottom is usually a clam shell design – i.e. the bucket opens up by retracting two segments on the bottom of the bucket. The scrap falls into the furnace and the scrap crane removes the scrap bucket. The roof and electrodes swing back into place over the furnace. The roof is lowered and then the electrodes are lowered to strike an arc on the scrap. This commences the melting portion of the cycle. The number of charge buckets of scrap required to produce a heat of steel is dependent primarily on the volume of the furnace and the scrap density. Most modern furnaces are designed to operate with a minimum of back-charges. This is advantageous because charging is a dead-time where the furnace does not have power on and therefore is not melting. Minimizing these dead-times helps to maximize the productivity of the furnace. In addition, energy is lost every time the furnace roof is opened. This can amount to 10 – 20 kWh/ton for each occurrence. Most operations aim for 2 to 3 buckets of scrap per heat and will attempt to blend their scrap to meet this requirement. Some operations achieve a single bucket charge. Continuous charging operations such as CONSTEEL and the Fuchs Shaft Furnace eliminate the charging cycle.

Melting

The melting period is the heart of EAF operations. The EAF has evolved into a highly efficient melting apparatus and modern designs are focused on maximizing the melting capacity of the EAF. Melting is accomplished by supplying energy to the furnace interior. This energy can be electrical or chemical. Electrical energy is supplied via the graphite electrodes and is usually the largest contributor in melting operations. Initially, an intermediate voltage tap is selected until the electrodes bore into the scrap. Usually, light scrap is placed on top of the charge to accelerate bore-in. Approximately 15 % of the scrap is melted during the initial bore-in period. After a few minutes, the electrodes will have penetrated the scrap sufficiently so that a long arc (high voltage) tap can be used without fear of radiation damage to the roof. The long arc maximizes the transfer of power to the scrap and a liquid pool of metal will form in the furnace hearth At the start of melting the arc is erratic and unstable. Wide swings in current are observed accompanied by rapid movement of the electrodes. As the furnace atmosphere heats up the arc stabilizes and once the molten pool is formed, the arc becomes quite stable and the average power input increases.

Chemical energy is be supplied via several sources including oxy-fuel burners and oxygen lances. Oxy-fuel burners burn natural gas using oxygen or a blend of oxygen and air. Heat is transferred to the scrap by flame radiation and convection by the hot products of combustion. Heat is transferred within the scrap by conduction. Large pieces of scrap take longer to melt into the bath than smaller pieces. In some operations, oxygen is injected via a consumable pipe lance to “cut” the scrap. The oxygen reacts with the hot scrap and burns iron to produce intense heat for cutting the scrap. Once a molten pool of steel is generated in the furnace, oxygen can be lanced directly into the bath. This oxygen will react with several components in the bath including, aluminum, silicon, manganese, phosphorus, carbon and iron. All of these reactions are exothermic (i.e. they generate heat) and supply additional energy to aid in the melting of the scrap. The metallic oxides that are formed will end up in the slag. The reaction of oxygen with carbon in the bath produces carbon monoxide, which either burns in the furnace if there is sufficient oxygen, and/or is exhausted through the direct evacuation system where it is burned and conveyed to the pollution control system. Auxiliary fuel operations are discussed in more detail in the section on EAF operations.

Once enough scrap has been melted to accommodate the second charge, the charging process is repeated. Once the final scrap charge is melted, the furnace sidewalls are exposed to intense radiation from the arc. As a result, the voltage must be reduced. Alternatively, creation of a foamy slag will allow the arc to be buried and will protect the furnace shell. In addition, a greater amount of energy will be retained in the slag and is transferred to the bath resulting in greater energy efficiency.

Once the final scrap charge is fully melted, flat bath conditions are reached. At this point, a bath temperature and sample will be taken. The analysis of the bath chemistry will allow the melter to determine the amount of oxygen to be blown during refining. At this point, the melter can also start to arrange for the bulk tap alloy additions to be made. These quantities are finalized after the refining period.

Refining

Refining operations in the electric arc furnace have traditionally involved the removal of phosphorus, sulfur, aluminum, silicon, manganese and carbon from the steel. In recent times, dissolved gases, especially hydrogen and nitrogen, been recognized as a concern. Traditionally, refining operations were carried out following meltdown i.e. once a flat bath was achieved. These refining reactions are all dependent on the availability of oxygen. Oxygen was lanced at the end of meltdown to lower the bath carbon content to the desired level for tapping. Most of the compounds which are to be removed during refining have a higher affinity for oxygen that the carbon. Thus the oxygen will preferentially react with these elements to form oxides which float out of the steel and into the slag.
In modern EAF operations, especially those operating with a “hot heel” of molten steel and slag retained from the prior heat, oxygen may be blown into the bath throughout most of the heat. As a result, some of the melting and refining operations occur simultaneously.

Phosphorus and sulfur occur normally in the furnace charge in higher concentrations than are generally permitted in steel and must be removed. Unfortunately the conditions favorable for removing phosphorus are the opposite of those promoting the removal of sulfur. Therefore once these materials are pushed into the slag phase they may revert back into the steel. Phosphorus retention in the slag is a function of the bath temperature, the slag basicity and FeO levels in the slag. At higher temperature or low FeO levels, the phosphorus will revert from the slag back into the bath. Phosphorus removal is usually carried out as early as possible in the heat. Hot heel practice is very beneficial for phosphorus removal because oxygen can be lanced into the bath while its temperature is quite low. Early in the heat the slag will contain high FeO levels carried over from the previous heat thus aiding in phosphorus removal. High slag basicity (i.e. high lime content) is also beneficial for phosphorus removal but care must be taken not to saturate the slag with lime. This will lead to an increase in slag viscosity, which will make the slag less effective. Sometimes fluorspar is added to help fluidize the slag. Stirring the bath with inert gas is also beneficial because it renews the slag/metal interface thus improving the reaction kinetics.

In general, if low phosphorus levels are a requirement for a particular steel grade, the scrap is selected to give a low level at melt-in. The partition of phosphorus in the slag to phosphorus in the bath ranges from 5 to 15. Usually the phosphorus is reduced by 20 to 50 % in the EAF.

Sulfur is removed mainly as a sulfide dissolved in the slag. The sulfur partition between the slag and metal is dependent on slag chemistry and is favored at low steel oxidation levels. Removal of sulfur in the EAF is difficult especially given modern practices where the oxidation level of the bath is quite high. Generally the partition ratio is between 3 and 5 for EAF operations. Most operations find it more effective to carry out desulfurization during the reducing phase of steelmaking. This means that desulfurization is performed during tapping (where a calcium aluminate slag is built) and during ladle furnace operations. For reducing conditions where the bath has a much lower oxygen activity, distribution ratios for sulfur of between 20 and 100 can be achieved.

Control of the metallic constituents in the bath is important as it determines the properties of the final product. Usually, the melter will aim at lower levels in the bath than are specified for the final product. Oxygen reacts with aluminum, silicon and manganese to form metallic oxides, which are slag components. These metallics tend to react with oxygen before the carbon. They will also react with FeO resulting in a recovery of iron units to the bath. For example:

Mn + FeO = MnO + Fe
Manganese will typically be lowered to about 0.06 % in the bath.

The reaction of carbon with oxygen in the bath to produce CO is important as it supplies a less expensive form of energy to the bath, and performs several important refining reactions. In modern EAF operations, the combination of oxygen with carbon can supply between 30 and 40 % of the net heat input to the furnace. Evolution of carbon monoxide is very important for slag foaming. Coupled with a basic slag, CO bubbles are tapped in the slag causing it to “foam” and helping to bury the arc. This gives greatly improved thermal efficiency and allows the furnace to operate at high arc voltages even after a flat bath has been achieved. Burying the arc also helps to prevent nitrogen from being exposed to the arc where it can dissociate and enter into the steel.

If the CO is evolved within the steel bath, it helps to strip nitrogen and hydrogen from the steel. Nitrogen levels in steel as low as 50 ppm can be achieved in the furnace prior to tap. Bottom tapping is beneficial for maintaining low nitrogen levels because tapping is fast and a tight tap stream is maintained. A high oxygen potential in the steel is beneficial for low nitrogen levels and the heat should be tapped open as opposed to blocking the heat.
At 1600 C, the maximum solubility of nitrogen in pure iron is 450 ppm. Typically, the nitrogen levels in the steel following tapping are 80 – 100 ppm.

Decarburization is also beneficial for the removal of hydrogen. It has been demonstarted that decarburizing at a rate of 1 % per hour can lower hydrogen levels in the steel from 8 ppm down to 2 ppm in 10 minutes.
At the end of refining, a bath temperature measurement and a bath sample are taken. If the temperature is too low, power may be applied to the bath. This is not a big concern in modern meltshops where temperature adjustment is carried out in the ladle furnace.
De-Slagging

De-slagging operations are carried out to remove impurities from the furnace. During melting and refining operations, some of the undesirable materials within the bath are oxidized and enter the slag phase.
It is advantageous to remove as much phosphorus into the slag as early in the heat as possible (i.e. while the bath temperature is still low). The furnace is tilted backwards and slag is poured out of the furnace through the slag door. Removal of the slag eliminates the possibility of phosphorus reversion.
During slag foaming operations, carbon may be injected into the slag where it will reduce FeO to metallic iron and in the process produce carbon monoxide which helps foam the slag. If the high phosphorus slag has not been removed prior to this operation, phosphorus reversion will occur. During slag foaming, slag may overflow the sill level in the EAF and flow out of the slag door.

The following table shows the typical constituents of an EAF slag:

 

 

Component Source Composition Range
CaO Charged 40 – 60 %
SiO2 Oxidation product 5 – 15 %
FeO Oxidation product 10 – 30 %
MgO Charged as dolomite 3 – 8 %
CaF2 Charged – slag fluidizer
MnO Oxidation product 2 – 5%
S Absorbed from steel
P Oxidation product

 

Tapping

 

 

Once the desired steel composition and temperature are achieved in the furnace, the tap-hole is opened, the furnace is tilted, and the steel pours into a ladle for transfer to the next batch operation (usually a ladle furnace or ladle station). During the tapping process bulk alloy additions are made based on the bath analysis and the desired steel grade. De-oxidizers may be added to the steel to lower the oxygen content prior to further processing. This is commonly referred to as “blocking the heat” or “killing the steel”. Common de-oxidizers are aluminum or silicon in the form of ferrosilicon or silicomanganese. Most carbon steel operations aim for minimal slag carry-over. A new slag cover is “built” during tapping. For ladle furnace operations, a calcium aluminate slag is a good choice for sulfur control. Slag forming compounds are added in the ladle at tap so that a slag cover is formed prior to transfer to the ladle furnace. Additional slag materials may be added at the ladle furnace if the slag cover is insufficient.
Furnace Turn-around

Furnace turn-around is the period following completion of tapping until the furnace is recharged for the next heat. During this period, the electrodes and roof are raised and the furnace lining is inspected for refractory damage. If necessary, repairs are made to the hearth, slag-line, tap-hole and spout. In the case of a bottom-tapping furnace, the taphole is filled with sand. Repairs to the furnace are made using gunned refractories or mud slingers. In most modern furnaces, the increased use of water-cooled panels has reduced the amount of patching or “fettling” required between heats. Many operations now switch out the furnace bottom on a regular basis (2 to 6 weeks) and perform the hearth maintenance off-line. This reduces the power-off time for the EAF and maximizes furnace productivity. Furnace turn-around time is generally the largest dead time (i.e. power off) period in the tap-to-tap cycle. With advances in furnace practices this has been reduced from 20 minutes to less than 5 minutes in some newer operations.

 


 

http://www.steel.org/making-steel/how-its-made/processes/processes-info/the-basic-oxygen-steelmaking-process.aspx?siteLocation=88e232e1-d52b-4048-9b8a-f687fbd5cdcb

 

The Basic Oxygen Steelmaking (BOS) Process

By John Stubbles, Steel Industry Consultant

I INTRODUCTION

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.

Figure 1: Charging aisle of a Basic Oxygen Steelmaking Plant showing scrap being charged into the BOF vessel. A ladle full of hot metal is seen to the right.

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.

Figure 3: Section through the BOF vessel during oxygen blowing. (Ref: Making, Shaping, and Treating of Steel, 11th Edition, Steelmaking And Refining Volume. AISE Steel Foundation, 1998, Pittsburgh PA)

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.

Figure 4: A ladle of molten steel leaving for the ladle metallurgical facility or the caster.

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:

%C
%Si
%Mn
%S
%P
%Al
Residuals
Hot metal
4.5
.75
1.0
.01
.05
0
0
Scrap
.05
.05
.4
.015
.01
.03
0.1

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 METAL
75% HOT METAL IN CHARGE

HEAT AVAILABLE
Btu (000’s)
HEAT REQUIRED
Btu (000’s)
C —> CO
366
H.M 2400—>2900 F _
220
Si —> SiO2
204
FLUXES —>2900 F
110
Mn —> MnO
60
O2 —>2900 F
120
P —>P2O5
10
HEAT LOSSES
50
Fe—>FeO
110
SCRAP —>2900 F
415
CO—>CO2
130
SLAG FORMATION
35
TOTAL
915
915

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

REACTION
#/NTHM
% OF TOTAL
C —>CO
120
66
Si—>SiO2
17
9
Fe—>FeO (SLAG)
16
9
CO—>CO2
12
7
Fe—>FeO (FUME)
8
4
Mn,P—>MnO,P2O5
7
4
DISSOLVED OXYGEN
1
1
181
100

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

LOSS
% CHARGE
METALLOIDS IN HM (6.3%)
4.7
DEBRIS,COATINGS ON SCRAP ( 2.5 %)
0.6
IRON TO SLAG
2.2
IRON TO FUME
1
IRON TO SLOPPING
0.5
TOTAL
9

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.

ii) SCRAP

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.

iii) FLUXES

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.

iv) COOLANTS

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.

v) ALLOYS

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.
V REFRACTORIES

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.
VI ENVIRONMENTAL ISSUESEnvironmental 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.
VII CONCLUSION

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.
ADDENDUM

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.


Leave a Reply

Home | About | Contact | Login