Materials needed for Steel Production in Basic Oxygen Furnace


Materials needed for Steel Production in Basic Oxygen Furnace

The following types of materials are needed for the production of liquid steel in the basic oxygen furnace (BOF) steelmaking process (Fig 1).

  • Basic raw materials such as hot metal, scrap, and lime etc.
  • Secondary raw materials such as deoxidizers and carburizers.
  • Utility gases such as oxygen, nitrogen, and argon etc.
  • Refractories and Refractory materials such as lining material, gunning material and patching materials etc.
  • Consumable probes such as temperature probes and sampling probes etc.
  • Cooling water for cooling of oxygen blowing lance and exhaust gases.

Materials used for steel production in BOF shop

Fig 1 Materials needed for the production of steel in basic oxygen furnace

Basic raw Materials

The basic raw materials needed for making steel in the BOF converter include (i) hot metal from the blast furnace, (ii) steel scrap and/or any other metallic iron source, (iii) iron ore, and (iv) fluxes.  Scrap, charged from a scrap box, is the first material to be charged into the BOF. The hot metal is then poured into the converter from a hot metal charging ladle, after which the blowing with oxygen gas is started. The fluxes, usually in lump form, are charged into the BOF through a bin system after the start of the oxygen blow. The fluxes can also be injected into the furnace in powder form through bottom tuyeres. The composition and amounts of basic raw materials used in the BOF converter vary from one steel melting shop to another, depending on their availability and the economics of the process.

The hot metal or liquid iron is the primary source of iron units and energy. Hot metal is received from the blast furnaces in either open top or torpedo cars. In case of open top ladles, hot metal is poured in a hot metal mixer to maintain its temperature before its use in the BOF converter. The chemical composition of hot metal can vary substantially, but typically it contains around 3.8 % to 4.5 % carbon, 0. 5 % to 1.5 % silicon, 0.25 % to 1.5 % manganese, 0.05 % to 0.15 % phosphorus and 0.03 % to 0.08 % sulphur.

The sulphur level of hot metal can be reduced to as low as 0.001 % in a hot metal desulphurization plant. The composition of the hot metal depends on the practice and charge in the blast furnace. Generally, there is a decrease in the silicon content and an increase in the sulphur of the hot metal when the blast furnace runs on colder regime. The phosphorus contents of the hot metal increases if the content of phosphorus is high in the blast furnace burden.

Carbon and silicon are the chief contributors of energy. The hot metal silicon affects the amount of scrap that can be charged in the BOF converter heat. For example, if the hot metal silicon is high, there is more amount of heat is generated due to its oxidation, hence more scrap can be charged in the heat. Hot metal silicon also affects the slag volume, and hence the lime consumption and resultant iron yield.

The hot metal is usually saturated with carbon, and its carbon concentration depends on the temperature and the concentration of other solute elements such as silicon and manganese. The carbon content of the hot metal increases with increasing temperature and manganese content, and decreases with increasing silicon content.

It is important to know the temperature and the carbon content of hot metal at the time it is poured into the BOF converter for the purpose of the process control in the BOF converter. The hot metal temperature is normally measured in the hot metal charging ladle before it is charged in the BOF converter. Typically, the temperature of the hot metal is in the range of 1300 deg C to 1350 deg C.

Desulphurization is favoured at high temperatures and low oxygen potentials. Also, the presence of other solute elements in the hot metal such as carbon and silicon increases the activity of sulphur, which in turn enhances desulphurization. Thus low oxygen potential and high carbon and silicon contents make conditions more favourable to remove sulphur from hot metal rather than from steel in the BOF converter. Not all hot metal is desulphurized. Hot metal used for making steel grades with stringent sulphur specifications is desulphurized in the hot metal desulphurization plant where the desulphurizing reagents can reduce hot metal sulphur to as low as 0.001 %, but more typically in the range of to 0.004 % to 0.005 %. It is important that the slag produced after hot metal desulphurization is removed effectively through slag skimming. This slag contains high amounts of sulphur, and any slag carried over into the BOF converter, where conditions are not good for desulphurization, causes sulphur pickup in the liquid steel.

The weighing of the hot metal is done either on a weigh scale before it is poured into the BOF converter. It is very important that the weight of the hot metal is accurately known, as any error can cause problems in turndown chemistry, temperature and heat size in the BOF converter. This weight is also an important input for the static charge model.

Scrap is the second largest source of iron units in the BOF converter after hot metal. Scrap is basically recycled iron or steel, that is either generated within the steel plant (e.g. crops at CCM, mill scrap, recovered scrap from the steel melting shop waste, or maintenance scrap), or purchased from an outside source.

It is important that the various types of scraps are loaded in correct amounts to meet the requirements of scrap mix in the scrap box. The scrap box is weighed to know the exact amount of scrap in the scrap box. The scrap mix and the scrap weight are important parameters; otherwise the turndown performance of the heat in the BOF converter gets adversely affected.

Generally, the lighter scrap is loaded in the front, and the heavier scrap in the rear end of the scrap box. This causes the lighter scrap to land first in the BOF converter as the scrap box is tilted. It is preferable that the lighter scrap fall on the refractory lining first, before the heavier scrap, to minimize its impact and hence damage to the refractory lining. Also, since heavy scrap is more difficult to melt than light scrap, it is preferable that it sits on top so that it is closest to the area of oxygen jet impingement and hence can melt faster. Scrap pieces that are too large to be charged into the furnace are cut into smaller pieces by means of shears, flame cutting, or by oxygen lancing. Thin, small pieces of scrap such as sheet shearings and punchings are compressed into bales using special hydraulic presses. Normally, larger, heavier pieces of scrap are more difficult to melt than lighter, smaller ones. Scrap which is not melted can cause significant problems in the process control. It can result into high temperatures or missed chemistries at turndown.

Combined blowing practice in BOF converter can significantly enhance the mixing characteristics and hence improves the melting of larger pieces of scrap. Certain elements present in scrap, such as copper, molybdenum, tin and nickel get introduced in the BOF converter through scrap charge. These elements cannot be oxidized and hence cannot be removed during the blowing of the BOF heat. These elements dissolve evenly in the liquid bath during the oxygen blow. Certain other elements such as aluminum, silicon and zirconium present in the scrap can be fully oxidized during the blowing process and become incorporated in the slag. Elements which fall in the middle category in terms of their tendency to react, such as phosphorus, manganese and chromium distribute themselves between the metal and slag. Zinc and lead are mostly removed during the blowing of the BOF heat as vapour. Steel melting shops typically use about 10 % to 35 % of their total metallic charge as scrap, with the exact amount depending on the local conditions and economics.  Technically the scrap hot metal ratio in the BOF metallic charge depends on factors like the silicon, carbon and temperature of the hot metal, use of a post combustion lance.

Direct reduced iron (DRI) is used in some steel melting shops as a coolant as well as a source of iron units. DRI typically contains about 89 % to 94 % of total iron (around 88 % to 96 % of metallization), 0.1 % to 4 % carbon, 2.8 % to 6 % alumina and silica combined, 3 % to 8% FeO and small amounts of CaO and MgO. DRI may contain phosphorus in the range of 0.005 % to 0.09 %, sulphur in the range 0.001 % to 0.03 % and low concentrations of nitrogen (usually less than 20 ppm).

DRI is normally fed into the BOF either in lump form or in briquetted form of size at around 25 mm to 30 mm. The DRI briquettes are passivated to eliminate any tendency to spontaneous burning so that they can be handled conveniently in the steel melting shop. DRI is usually fed into the BOF converter through the bin system.

In some steel melting shop, pig iron is also used as source of iron units. Pig iron needs heat for its melting and once it gets melted it behaves in the BOF converter as hot metal. Pig iron is charged in the converter through scrap box along with other scrap mix.

Iron ore is usually charged in the form of lump into the BOF converter as a coolant and it is often used as a scrap substitute. Iron ores are useful scrap substitutes as they contain lower amounts of residual elements such as copper, zinc, nickel, and molybdenum. The cooling effect of iron ore is about three times higher than scrap. The reduction of the iron oxide in the ore is endothermic and higher amounts of hot metal and lower amounts of scrap are required when iron ore is used for cooling. Iron ores are to be charged early in the blow when the carbon content in the liquid bath is high so as to effectively reduce the iron oxide in the iron ore. The reduction of the iron oxides in the ore produces significant amounts of gas, and consequently there is increased tendency of slag foaming and slopping. Late addition of the iron ore results into a detrimental effect on the iron yield and end point slag chemistry. If iron ore is used only as a coolant just before tapping of the heat from the converter then  the slag becomes highly oxidized and fluid, enhancing slag carryover into the ladle. The delay in the cooling reaction from the unreduced iron ore causes a sudden decrease in temperature or a violent ladle reaction resulting in over-oxidation of the liquid steel.

It is possible to use mill scale as a coolant in the BOF converter in suitable amounts. Mill scale is found to be very effective in increasing the hot metal to scrap ratio. However, it causes heavy slopping during the process. Mill scale and other iron oxide additions are reduced during the main blow releasing iron and oxygen. This additional oxygen becomes available for carbon removal thus speeding up the overall reaction. Slopping is likely caused by the increased slag volume associated with using more hot metal (more quantities of silicon and carbon generate more SiO2 and CO, respectively) and by the increased reaction rate.

During steelmaking in BOF converter the consumption of calcined lime depends on the hot metal silicon, the hot metal to scrap ratio in the converter charge, the initial (hot metal) and final (steel aim) sulphur and phosphorus contents. Calcined lime is produced by calcining of the limestone. The quality of calcined lime needed in BOF converter steel making is described in separate article available at link  http://ispatguru.com/quality-of-lime-for-steelmaking-in-converter/.

Since a large quantity of calcined lime is charged into the BOF converter within a short period of time, careful selection of the lime quality is important to improve its dissolution in the slag. In general, small lumps with high porosity have higher reactivity and promote rapid slag formation. The most common quality problems with calcined lime are uncalcined inner cores, hydration, excess fines and too low a reactivity.

Calcined dolomite is charged with the calcined lime to saturate the slag with MgO, and reduce the dissolution of MgO from the furnace refractories into the slag. Typically calcined dolomite contains around 36 % to 40 % of MgO and 54 % to 58 % of CaO. The addition of calcined dolomite in the BOF converter bath is to be such that it keeps the MgO level of slag above the saturation limit. MgO level of the slag above saturation limit makes the slag less corrosive and reduces/eliminates the chemical attack of the slag on the furnace refractories.

In some steel melting shops, raw dolomite is added directly into the BOF converter. This acts as a coolant and as a source of MgO to saturate the slag but there is a delayed effect since calcining reaction takes place in the BOF converter. When raw dolomite is heated, the endothermic calcining reaction occurs, causing a temperature drop in the BOF converter.

Calcined dolomite is also added for the conditioning of the slag prior to slag splashing. It is important that the chemistry and size of the calcined dolomite is controlled.

In some BOF converter shops limestone or raw dolomite is frequently used as a coolant rather than as a flux. Limestone is commonly used to cool the bath if the turndown temperature is higher than the specified aim. When limestone is heated, the endothermic calcining reaction occurs producing CaO and CO2, causing a temperature drop in the BOF converter. The extent of the temperature drop just before tap depends on the heat size and the condition of the slag. For example, in a 150 ton heat size, 1 ton addition of limestone causes drop of the temperature of the bath by around 12 deg C.

Calcium fluoride or fluorspar (CaF2) is a slag fluidizer that reduces the viscosity of the slag. When added to the BOF it promotes rapid lime dissolution in the slag by dissolving the dicalcium silicate (2CaO.SiO2) layer formed around the lime particles which retards the dissolution of the lime in the slag. These days, fluorspar is used very sparingly because of its very corrosive attack of all types of refractories, including both BOF converter and the steel teeming ladle. Also, the fluorides form strong acids in the waste gas collection system which corrode structural parts and which are also undesirable emissions.

Secondary raw materials

Secondary raw materials are deoxidizers and carburizers. These are normally added in the steel teeming ladle during the tapping of the heat from the converter.

Deoxidation is the last stage in steelmaking. During making of steel, the steel bath at the time of tapping contains 400 to 800 ppm activity of oxygen. Deoxidation is carried out during tapping by adding into the teeming ladle appropriate amounts of ferro alloys or other special deoxidizers.

Deoxidizers are usually the bulk ferro alloys such as ferro-silicon, silico-manganese and ferro-manganese. They are used in steel making for deoxidation as well as for the introduction of alloying elements. They are the most economical way for introducing alloying element into the steel. Ferroalloys impart distinctive qualities to steels.

Ferroalloys are also added for grain size control as well as for improvement in the mechanical properties of steel. Depending upon the process of steelmaking and the type of steel being made, the requirement of different ferroalloys varies widely. The addition of ferroalloys to steel increases its resistance to corrosion and oxidation, improves its hardenability, tensile strength at high temperature, wear and abrasion resistance with added carbon and increases other desired properties in the steel such as creep strength etc. Ferroalloys are vital inputs for producing all types of steel. They are used as raw material in the production of alloys steel and stainless steel.

If at the end of the blow the carbon content of the steel is below specifications, the liquid steel is also recarburized. This is done by the controlled addition of carburizers in the teeming ladle. Common carburizers are coke breeze and petroleum coke.

However, large additions in the teeming ladle have the adverse effect on the temperature of the liquid steel.

Utility gases

In the BOF converter steelmaking process a water-cooled lance is used to inject oxygen at very high velocities onto the liquid bath to produce steel. With the increasing demands to produce higher quality steels with lower impurity levels, oxygen of very high purity is to be supplied. Therefore, the oxygen for steelmaking is to be at least 99.5 % pure, and ideally 99.7 % to 99.8 % pure. The remaining parts are 0.005 % to 0.01 % nitrogen and the rest is argon.

In the BOF converter, the oxygen is jetted at supersonic velocities (Mach>1) with convergent/divergent (Laval) nozzles at the tip of the water-cooled lance. A forceful gas jet penetrates the slag and impinges onto the liquid metal surface to refine the steel. Today, most BOFs operate with lance tips containing four to five nozzles and with oxygen flow rates that range from 640 N cum/min to 900 N cum/min.

Nitrogen is normally used in BOF converter for combined blowing and slag splashing. Nitrogen gas needed for improving the mixing of the metal bath is blown through the bottom mounted tuyeres or permeable elements. The stirring of the bath is performed with nitrogen gas in the high carbon range of the melt in the bath. The bottom flow rates are normally lower than 0.2 N Cum/t minute. In typical practice nitrogen gas is introduced through the bottom in the first 60 % to 80 % of the oxygen blow. The rapid evolution of the CO gas in the first part of the oxygen flow prevents nitrogen pick up in the steel.

Nitrogen is also used for splashing of the conditioned liquid slag onto the walls of the BOF converter after the tapping of the heat from the BOF converter for creating a protective layer of slag coating over the refractories.

Argon is normally used in BOF converter for combined blowing. Argon gas needed for improving the mixing of the metal bath is blown through the bottom mounted tuyeres or permeable elements. The stirring of the bath is performed with argon gas in the low carbon range of the melt in the bath. The bottom flow rates are normally lower than 0.2 N Cum/t minute. In typical practice argon gas is introduced through the bottom in the last 20 % to 40 % of the oxygen blow.

Refractories and refractory materials

There are three types of refractory materials needed for BOF converter steelmaking. These are basic bricks usually magnesia carbon refractories, magnesia based gunning compound for the gunniting of the damaged portion of the refractories, and patching material (usually broken used bricks) for patching of the eroded bottom of the converter. These are described in article given under link http://ispatguru.com/refractory-lining-of-a-basic-oxygen-furnace/.

Consumable probes

Consumables needed for the steel production in the BOF converter are disposable type of probes for sampling the steel for analysis after the blow is over and for measuring temperatures of the hot metal in the hot metal charging ladle and of liquid steel in the BOF converter after the blow is over.

Cooling water

Water is needed for the steel production in the BOF converter for the cooling of the oxygen blowing lance and nozzle as well as for the cooling of the exhaust gases.

Cooling water is necessary in the lance to prevent the burning up of the oxygen lance in the BOF converter. Both the copper lance nozzle and the steel lance are cooled by recirculating water at a pressure of around 6 kg/sq cm. The important components of a lance are the water cooling channels where the cooling water flows through the center of the nozzle and exits through the outer pipe of the lance. It is designed to get maximum velocity of cooling water in the nozzle area, which is exposed to the highest temperatures.  Cooling water is critical for maintaining high lance life. The flow rates need to be maintained at the design rate. The cooling water outlet temperature is not to exceed 60 deg C to 65 deg C.

The CO rich gas coming out of the converter is first cooled in the converter hood indirectly either by cooling water or by evaporative cooling system (ECS) to bring down its nominal temperature from 1600 -1700 deg C to around 900 deg C. In case of evaporative cooling system demineralized water is needed for the cooling of the exhaust gases since in this system heat is recovered in the form of steam.

In some steel melting shops, the top cone of the BOF converter is water cooled. The two components in the top cone of the BOF converter which can be benefitted from water cooling as a means to maintain their low operating temperature are the conical shell itself and the lip ring at the top corner of the cone.

The quality of water is an important parameter. If the water is contaminated with oxides or dirt, deposits normally form inside the pipes, resulting in a negative effect on heat transfer.