IRON & STEEL INDUSTRY
The figure shows a flowchart of the integrated manufacturing process for iron and steel using the blast furnace and basic oxygen furnace (denoted BF and BOF hereinafter, respectively), which is presently the most commonly used method (51% of world steel production). After the BF-BOF process, molten steel is controlled to a target composition and temperature and is then cast by continuous casting machine to produce slabs, blooms, and billets. These castings are rolled to the required dimensions by the rolling mill to produce steel products. The smelting and refining process for iron and steel in the BF-BOF process involves the carbon reduction of iron ore (Fe2O3) in the BF to make molten iron, and decarburization of molten iron in the BOF to make molten steel.
Major reducing agent in the BF is the carbon monoxide gas(CO) generated by the oxidation of the carbon(C) in coke. Consequently, carburization takes place at the same time as reduction, producing hot metal(molten iron) containing about 4% carbon. The hot metal is decarburized to the required carbon content in the BOF. The main reaction in this process is the oxidization of the carbon in the hot metal by both pure oxygen gas (O2) and iron oxide (Fe2O3). The residual oxygen, after contributing to this decarburization reaction, remains in the molten steel. This oxygen is fixed and removed by deoxidation reagents such as silicon and aluminum as SiO2 and Al2O3 or is removed as carbon monoxide gas in the subsequent vacuum degassing process.
In addition to the BF-BOF process, there is another process which utilizes mainly scrap as an iron source, with some direct reduced iron whenever necessary. The direct reduced iron is produced by reducing iron ore with reformed natural gas, whose principal components are hydrogen, carbon monoxide, and methane. The scrap, along with direct reduced iron, is then melted in an electric arc furnace (denoted EAF hereinafter) to produce molten steel which is subsequently processed by the continuous casting machine, as mentioned above.
The molten steel from the BOF and EAF is then deoxidized and alloying elements are added in the prescribed amounts. The molten steel is then held at the target temperature and continuously cast, and the castings obtained are cut to the prescribed length. After heating to the rolling temperature in a reheating furnace, these castings are hot-worked to the required products. Steel shapes, bars, and wire rods are worked on section and bar mills and wire-rod mills equipped with caliber rolls, plates are worked on reversing mills, and hot-rolled steel sheets are worked on hot strip mills. After pickling to remove scale from the surface, the hot-rolled steel sheets are worked to cold-rolled steel sheets on reversing mills or tandem rolling mills, and the cold-rolled steel sheets are tinned or galvanized as required to produce various surface-treated steel sheet products. Steel pipe is produced by forming and welding steel sheets or plates, or by piercing a billet and rolling to the final dimensions without a seam.
Among the elements composing the crust of the earth, iron exists in the largest quantity next to oxygen, silicon, and aluminum. Iron exists as natural ores in the form of oxides, and the estimated amount of ore deposits in the world is approximately 800 billion tons. Typical ores are hematite (Fe2O3) and magnetite (Fe3O4), having theoretical iron contents of 70% and 72%, respectively. The iron content of practical ores is about 65% at maximum, and these ores include 2-6% silica and 1-3% alumina (Al2O3). Representative sources of iron ores are found in mainland China, Brazil, Australia, the former USSR, America, India, Canada, South Africa, and elsewhere. The phosphorus and sulfur contents of ores differ greatly according to their origin.
High-grade iron ore is crushed for sizing, producing both fine ore as well as lump ore. To beneficiate low-grade ore, it is first pulverized into finer particles called pulverized ore. Both fine and pulverized ores are then subjected to pre-treatments before they are charged into the BF; that is, the fine ore is processed into sintered ore by sintering, and pulverized ore is processed into pellets by pelletizing. In Japan, the proportions of iron ores charged into the BF are, at present, 15% lump ore, 10% pellets, and 75% sintered ore. Thus, pretreated iron ores represent a large majority of the ore used.
In the sintering process, fine ores 2-3mm in diameter are mixed with coke breeze as a fuel. Burnt limestone powder is used as a flux. These materials are charged in an iron box called a pallet before being ignited. Fine ore particles are partially melted and combined by the combustion heat of the coke to form an agglomerate which is then subjected to crushing and screening processes in order to obtain sintered ore 15-30mm in diameter. The Dwight-Lloyd type of sintering machine is mainly used, sintering being conducted continuously by transferring pallets placed on a caterpillar.
Pelletizing is a process that involves mixing very finely ground particles of ore of less than 200 mesh with fluxing materials such as limestone and dolomite and then shaping them into balls 10-15mm in diameter by a pelletizer, and hardening the balls by firing with heavy oil and/or coal as a fuel. Cold-bond pellets are also produced by pelletizing, and do not require firing. At present, small-scale equipment for producing cold-bond pellets is in operation mainly to treat the dust collected in steel works. This technology offers great promise for the future in terms of energy-saving and reducing environmental pollution.
Compared with sintered ore, pellets have a higher iron- and a lower gang-content, and pelletizing is suitable for treating the very fine ore that will predominate in the future. However, pellets have the disadvantages that more fossil fuel is consumed during pelletizing and it is difficult to control the radial distribution of the thickness of pellets charged in the BF.
In the smelting process for iron and steel, coke serves as the source of carbon, which works as a reducing agent when reducing iron ore in the BF. At the same time, coke acts as the heat source for heating and melting the charged materials. Coke is made by baking coal in a coke oven. Coal is classified into the four grades shown in the figure, anthracite being the highest grade. Typical types are bituminous coal and brown coal. Bituminous coal exists in the largest quantities, having estimated reserves worldwide of approximately 7 trillion (trillion=1012)tons, with confirmed reserves of approximately 2 trillion tons.
The coke used in the BF must have a high carbon content and low ash and sulfur contents, and must have an appropriate porosity as well as good strength to ensure that it gives good reactivity and does not pulverize to choke the gas flow in the BF even at high temperatures. Cokes that meet these requirements are derived from bituminous coals that combine good coking properties with low ash and sulfur contents.
In the coke oven, the raw coal obtained by crushing and blending is charged into the coke chamber, where it is then baked (carbonized) by indirect heating at 1,473-1,573K (1,200-1,300 ) for 14-18 hours to form coke that contains about 90% fixed carbon. The coking process also produces such by-products as gas, coal tar, and pitch which can be refined and treated into useful secondary products such as fuel gas, pure hydrogen gas, chemical products such as benzene, toluene, xylene, naphthalene, dye, and carbon fibers.
The life of a coke oven is about 40 years. In Japan, the lives of the coke ovens now in operation will begin to expire successively about the year 2015, which is expected to result in a shortage of coke. However, to cope with this problem, pulverized coal injection, in which coal with a poor coking property is injected through the tuyeres into the BF, is widely used. In addition, technical developments are being made to provide new technology to (i) produce coke or (ii) establish a cokeless iron making process, both of which will make it possible to select raw coal materials more freely and will cause less environmental pollution.
The blast furnace (BF) has a vertical cylindrical structure externally covered with a shell of thick steel plate and internally lined with refractories. The refractory structure is cooled by water-cooled metal components called staves, which are embedded between the shell and the refractories. The furnace body is composed of (i) the shaft, which tapers outward from the top, (ii) the belly, which is a straight cylinder, (iii) the bosh, which tapers inward toward its bottom and is located immediately under the belly, and (iv) the hearth, at the bottom of the furnace. The shaft, belly, and bosh are usually lined with chamotte brick and silicon-carbide brick, and the hearth is lined with carbon brick. Depending on the size of the furnace, the side wall of the hearth is radially fitted with some 20 to 40 of water-cooled copper tuyeres, which are used to inject the hot blast into the furnace from the hot stoves through the hot-blast main and bustle pipes. Tapholes for discharging hot metal and cinder notches for discharging slag are also installed in the hearth section. The largest BFs at present are about 80m in total height, with a furnace body height of about 35m and a maximum internal diameter of about 16m, and have an internal volume of about 5,200m3. A furnace of this size can produce approximately 10,000 tons of hot metal a day.
All BFs have auxiliary equipment such as (i) belt conveyors for transporting raw materials (ore and coke) to the furnace top, (ii) hoppers for temporarily storing these raw materials, (iii) a bell-type or bell-less-type device for charging the raw materials into the furnace with appropriate distribution in the radial direction, (iv) hot stoves for heating the blast, (v) blowers for feeding the blast, and (vi) equipment for dust removal, and recovering and storing the gas from the furnace top. Blast furnaces in which pulverized coal is injected from the tuyeres (PCI = pulverized-coal injection) are provided with equipment for pulverizing the coal and feeding it under pressure. With bell-type charging equipment, the raw materials enter the furnace through the gap created by moving down a small inverted bell. This bell closes and a larger bell (big-end-down) opens to allow material to fall into the shaft below. With bell-less charging equipment, the raw materials are dropped into the furnace through a rotating chute. The hot stove is a cylindrical furnace about 12m in diameter and some 55m in height, and has a chamber filled with checkered silica bricks. The hot stove is a type of heat exchanger in which the heat produced by combustion of the BF gas is stored in the checker-work chamber, after which cold air is blown through the hot checker-work to produce the preheated hot air blast to the furnace. Two or more stoves are operated on alternate cycles, providing a continuous source of hot blast to the furnace.
A BF is usually operated with a furnace-top pressure of about 250 kilopascals. To recover the energy from the large volume of high-pressure exhaust gas, the BF is equipped, after dust removal, with a top-pressure recovery turbine (TRT), for generating electric power by utilizing the pressure difference between the furnace-top and gas storing holder.
A total of about 1,600 kg/ton-hot metal of such iron-bearing materials as sintered ore, lump ore and pellets, and about 380 kg/ton-hot metal of coke as the reductant are charged in alternate layers from the top of the BF. It has recently become common practice to inject usually 90-120 kg/ton-hot metal of pulverized coal as part of the reductant from the tuyeres in the lower part of the furnace. At present, heavy-oil injection from the tuyeres is rarely used for economic reasons. Approximately 1,000 Nm3/ton-hot metal of hot blast is also blown through the tuyeres after preheating to 1,423-1,523K (1,150-1,250 ) at the hot stoves. The humidity and oxygen concentration of the hot blast are also controlled.
The hot blast reacts with the coke and pulverized coal in the belly and bosh of the BF to form a mixture of carbon monoxide and nitrogen. This mixture ascends in the furnace while exchanging heat and reacting with the raw materials descending from the furnace top. The gas is eventually discharged from the furnace top and recovered for use as fuel in the works. During this process, the layer-thickness ratio of iron-bearing materials to coke charged from the furnace top and their radial distribution are controlled so that the hot blast can pass with appropriate radial distribution. During the descent of the burden in the furnace, the iron-bearing materials are indirectly reduced by carbon monoxide gas in the low-temperature zone of the upper furnace. In the lower part of the furnace, carbon dioxide, produced by the reduction of the remaining iron ore by carbon monoxide is instantaneously reduced by coke (C) into carbon monoxide which again reduces the iron oxide. The overall sequence can be regarded as direct reduction of iron ore by solid carbon in the high-temperature zone of the lower furnace. The reduced iron simultaneously melts, drips, and collects as hot metal at the hearth. The hot metal and molten slag are then discharged at fixed intervals (usually 2-5 hours) by opening the tapholes and cinder notches in the furnace wall.
The materials discharged from the BF are hot metal at 1,803K (1,530 ), about 300 kg/ton-hot metal of molten slag, and dust-bearing exhaust gas discharged from the furnace top. Hot metal is poured into a torpedo car, where it is subjected to hot metal pretreatment, and then transferred to the steel making plant. Molten slag is crushed after cooling and is recycled as a material for roadbed and cement. After dust removal, the exhaust gas is used as a fuel for the reheating furnaces.
The capacity of a BF is expressed by the weight of hot metal that can be produced per day. Representative technical indicators stand for (i) the tapping ratio, which shows the tapped quantity of hot metal per day per m3 of inner volume of the furnace, and(ii) the fuel ratio, which shows the consumption of coke and auxiliary fuel required per ton of hot metal. In Japanese BFs,tapping ratios of 1.7-2.1 ton/m3/day and fuel ratios of 470-500 kg/ton are typical.
The greatest tasks to be achieved in blast-furnace operation are to decrease the unit energy consumption, ensure stable operation in terms of the tapped quantity, composition, and temperature, and to extend the life of the furnace. In recent large-scale BFs, unit energy consumption has been decreased to 13 gigajoules/ton-hot metal, out of which the energy required for ore reduction accounts for about 60%.
Careful preparation of raw materials for physical and chemical consistency is an effective way of stabilizing operation over long periods. It is also necessary to understand the physical and chemical behavior accurately in each part of the furnace during operation. For this purpose, monitoring the conditions inside the furnace and applying artificial intelligence for data processing and judgment have been put into practical use with great success. For the future, it will be necessary to understand the complex conditions inside the furnace and use such data to further stabilize furnace operations.
Because blast-furnace relining is extremely expensive, total production costs can be reduced substantially by extending furnace life. Technological advances in operation and maintenance to date have extended the life of BFs to as long as 16 years (current record), but further technical development is desired to extend furnace life to twenty years or more.
Increasingly stringent quality requirements have heightened the demand for steels with very low levels of impurities such as phosphorus, sulfur, hydrogen, nitrogen, and oxygen, and of nonmetallic inclusions such as MnS, SiO2, and Al2O3. Such high purity cannot be attained by BOF blowing for decarburization since its refining capability is limited. Hot metal produced in the BF is conventionally transferred either to a ladle or to a vessel called a torpedo car and is then charged into the BOF. The oxygen blowing process in which hot metal is decarburized and converted to steel is carried out mostly in the BOF. However, a method for dividing the refining capability and allocating the divided function to processes before and after the BOF has been put into practical use.
The processes in which impurities are removed from the hot metal are called hot metal pretreatment, whereas the processes in which the molten steel tapped from the BOF is subjected to further refining and degassing are called secondary refining. At present, an integrated process of smelting in the BF, hot metal pretreatment, decarburizing in the BOF, and secondary refining has become the standard manufacturing process for high grade steels.
Hot metal pretreatment includes the desiliconization, dephosphorization, and desulfurization of hot metal. The silicon in the hot metal is oxidized in the BOF, where it reacts with added lime (CaO) and iron oxide (FeO) to form a CaO-FeO-SiO2 slag. If the silicon content of the hot metal is low, this reaction is shortened in the BOF, the production efficiency is improved, and the volume of slag generated is small; therefore, decarburizing with a high iron yield is possible. Desiliconization is therefore conducted as a pretreatment process by adding iron oxides such as mill scale and sintered ore fines to hot metal in the runners in the casthouse of the BF or in the transfer vessel.
Dephosphorization is usually carried out by injecting a dephosphorizing agent containing lime, iron oxide, fluorspar, etc. into the hot metal in the transfer ladle or torpedo car together with a gas. This promotes the transfer of the phosphorus in the hot metal to the slag phase, which is then discharged. Dephosphorization is usually carried out after desiliconization, because the dephosphorization reaction proceeds more quickly at lower silicon contents. Although hot metal is desulfurized to some extent by the dephosphorization treatment, extra low sulfur steels require further desulfurization, which is performed by separate injection of desulfurizing agents such as CaO, Na2CO3, CaC2, and Mg into the hot metal.
Such treatments can be made more effective by identifying and enhancing the elementary rate steps that control the dephosphorization and desulfurization processes. A good understanding of thermodynamics and transport phenomena is indispensable in achieving these objectives.
The basic oxygen furnace (BOF), whose profile is shown in the figure, is a tiltable vessel lined with refractories such as magnesia carbon brick. Auxiliary equipment includes a chute for scrap charging, hoppers for alloys and fluxes, a lance for injecting pure oxygen gas, a sublance for measuring the temperature and carbon concentration of the molten steel, lifting devices for the lance and sublance, equipment for tilting the vessel, and equipment for recovering and cleaning the exhaust gas. The BOF capacity is expressed as the weight of crude steel that can be decarburized per heat. Most BOFs in Japan have a capacity of 150-300 tons.
The main function of the BOF is to decarburize the hot metal using pure oxygen gas. In the top-blown BOF, pure oxygen is injected as a high-velocity jet against the surface of the hot metal, allowing penetration of the impinging jet to some depth into the metal bath. Under these conditions, the oxygen reacts directly with carbon in the hot metal to produce carbon monoxide. The pure oxygen top-blown BOF can decarburize 200 tons of hot metal from 4.3% C to 0.04% C in about 20 minutes. As a result of this high productivity, the BOF replaced the open hearth furnace, which was a much slower process.
The injected pure oxygen gas first oxidizes silicon and then carbon in hot metal. When the carbon concentration of the hot metal is decreased to about 1%, the oxidation of iron begins in parallel with that of carbon. The oxidation of iron becomes marked at carbon concentrations of less than 0.1%, decreasing both the oxygen efficiency for decarburization and the decarburization rate, while increasing iron loss into the slag. The problem with the top-blown BOF is thus the oxidation of iron when a low carbon concentration is reached, and the resulting decrease in the decarburization rate. When the iron oxide content of the slag increases excessively, it can react too quickly with carbon in the molten steel and cause sudden gas evolution, forming a mix of slag and molten steel that sometimes erupts from the vessel in a phenomenon called "slopping" or "spitting".
The use of an oxygen lance with multiple holes at the tip has proven very effective in delocalizing the oxygen supply and increasing the decarburization rate while restraining excessive oxidation of the molten steel and preventing slopping and spitting. However, the effectiveness of this lance was still inadequate, and the bottom-blown oxygen process was developed, in which pure oxygen gas is injected into the molten steel from the bottom of the BOF. The bottom blowing enhances the stirring of the hot metal and thereby shortens the average mixing time in the molten steel bath, and promotes transport of solute carbon in the bath, preventing the over-oxidation of slag, which is the cause of slopping and spitting. Consequently, the bottom blowing enhances decarburization efficiency, especially at low carbon concentrations. The bottom-blowing is performed with bottom tuyeres of concentric double-wall pipe. The inner pipe is used to blow pure oxygen gas along with pulverized limestone as a slag-forming agent, while propane gas is blown through the outer pipe as a coolant to prevent tuyere burn back, since propane undergoes an endothermic reaction during decomposition, which results in cooling and reduced burning of the tuyeres. These improvements have made the production of low-carbon steels much easier.
The top-and-bottom blown BOF, which combines the advantages of both types of BOF, has recently become prominent in oxygen steel making The combined blowing BOFs mostly use bottom-blown inert gases in place of oxygen gas for stirring. Various methods of bottom-blowing have been adopted. As one example, a ceramic plug with embedded multiple small pipes or multiple slits is used in the bottom tuyeres. Irrespective of the type of the BOF, the exhaust gas, which is high in CO content, is either combusted in the throat of the BOF and passes through a waste-heat boiler installed in the upper part of the throat to recover the sensible heat and the heat of combustion, or is recovered un combusted through exhaust-gas recovery equipment and stored in a gas tank for later use as fuel.
The figure shows an example of the material balance of a top-and-bottom blown BOF. The low scrap ratio operation normally practiced in Japan consists of the following sequence. A small amount of scrap is charged in advance of the pretreated low-silicon hot metal as the main raw material, and the melt is refined by blowing pure oxygen gas. To produce 1 ton of molten steel, 1,033 kg of hot metal, 28 kg of scrap, 11kg of ferro alloys, 23 kg of burnt lime, and 50 Nm3 of pure oxygen gas are required. In the area where higher scrap operation is more economical, the scrap ratio can be increased up to about 15wt %. After blowing for 20 minutes, the carbon concentration is decreased from about 4% to 0.05%, and the temperature rises from 1,473K (1,200 ) to 1,903K (1,630 ). The purpose of blowing in the BOF is to decarburize and attain a sufficiently high tapping temperature. Hence, blowing is finished when the carbon concentration and temperature of the molten steel have reached these target values. On tapping, alloys and deoxidizers such as silicomanganese and/or aluminum are added to the molten steel in ladle. In the subsequent secondary refining process, the molten steel is degassed and alloys are added to make the final adjustments needed to reach target compositions.
The operation of the BOF starts with tilting of the vessel. Scrap and then hot metal are charged into the vessel, the vessel is returned to the upright position, and the multi-hole lance for top-blowing pure oxygen is inserted from the throat and lowered to near the surface of the hot metal. Blowing starts with a supersonic jet of pure oxygen gas impinging on the metal bath and, at the same time, an inert gas is blown from the furnace bottom to stir the bath. In the initial stage of blowing, the silicon in the hot metal is oxidized to form silica which reacts with the burnt lime and iron oxide additions and forms a CaO-SiO2-FeO slag. At the same time, the temperature in the furnace rises and the scrap starts to melt. The carbon concentration of the hot metal is high in the initial stage of blowing, so the pure oxygen gas reacts efficiently with the carbon to form carbon monoxide and decarburization proceeds. At this stage, decarburization is controlled by the pure oxygen feed rate, and the bath temperature rises progressively as decarburization proceeds. With the progress of decarburization and the consequent decrease in carbon concentration, the decarburization reaction is controlled by the rate of carbon transfer in the molten steel to the oxygen gas/molten steel interface. If the transfer of carbon by the stirring of the molten steel is insufficient, the pure oxygen gas is consumed to oxidize iron rather than reacting with carbon. This results in an increase of iron oxide in the slag and a decrease in the yield of iron. To prevent this, gas blowing from the furnace bottom is increased.
Oxygen blowing from the main lance was controlled in the past by using a static blowing model, which incorporates the composition and temperature of the charged materials, thermodynamic quantities of reactions involved, the degree of wear of the furnace refractories, the combustion ratio of the exhaust gas, and other factors. The model was based on the material balance, heat balance, and calculated thermodynamic factors and reaction rates. In the static blowing model, the quantity of pure oxygen gas to be blown in is determined by computing these balances and fine tuned by inputting heat data to the model each time blowing is carried out. It is more common now to use, on top of the static control, dynamic control in which the carbon concentration and temperature are measured by the sublance near the end of the blowing. The amount of pure oxygen gas to be injected is then adjusted using measured values, and blowing comes to an end when the target value has been attained. The essential points of BOF operation include:
(i) How to raise the rate of hitting the target values of carbon concentration and temperature at the end of blowing with only one blowing operation.
(ii) How to raise the oxygen efficiency for decarburization, the yield ratio, and production performance.
(iii) How to reduce wear of the furnace refractories, the consumption of auxiliary raw materials and pure oxygen gas, and heat loss.
After the end of blowing, the vessel is tilted and the molten steel is poured from the taphole into a ladle. At this time, ferro alloys, and deoxidizing and desulfurizing agents are added to the molten steel in the ladle. In the final stage of tapping, various kinds of slag stoppers are used to prevent the BOF slag from flowing into the ladle, since the slag has a strong oxidizing power and re oxidizes the molten steel.
Direct-reduced iron (denoted DRI hereinafter) is obtained when fine ore and lump ore are reduced in a solid state at the relatively low temperature of about 1,273K (1,000 ) using reformed natural gas. The methods now used include the FIOR, FINMET and CIRCORED processes and IRON CARBIDE process, all of which reduce fine ore in a fluidized bed; the HYL-I process, and HYL-II process, which use a retort bed, and the Midrex process and the HYL-III process, which use a countercurrent shaft furnace to reduce pellets and lump ore, and others. Of these, the Midrex, HYL-I and HYL-III processes have been successfully industrialized in large scale production. The Midrex and HYL-III processes are now most commonly used for direct reduction, the former having the largest manufacturing share. Production of DRI totaled 31 million tons in 1995.
The Midrex process is shown in the figure. Reforming natural gas has a H2/CO ratio of 1.6, the temperature is 1,173K (900 ), the in-furnace pressure of the countercurrent shaft furnace is 100 kilopascals, and the energy necessary for reduction is 10.5 gigaJoules/ton-DRI. Part of the exhaust gas is mixed with natural gas and reformed, and the remainder is used as the fuel for the reformer furnace. In the HYL-III process, the H2/CO of the reformed gas is 3, the temperature is 1,203K (930 ), the in-furnace pressure of the countercurrent shaft furnace is 450 kilopascals, and the energy necessary for reduction is basically the same as in the Midrex process. In both processes, higher furnace temperatures result in higher productivity, because the metal is reduced by an endothermic reaction. However, an excessive furnace temperature will cause the pellets and lump ore to melt during reduction and agglomeration. The maximum reduction rate is about 95%, and the carbon content is limited to about 2.5%.
Plant locations have been confined to places where natural gas is available, although the demand for steel in such places was not necessarily great. Furthermore, the large specific area of the active surface of spongy DRI makes it sensitive to re oxidation and ignition when it comes into contact with air and water, especially sea water. Handling and transportation were therefore difficult and potentially hazardous, making large-volume export unprofitable. As a result, the production of DRI has failed to reach expectations. To overcome this difficulty, a hot-briquetting facility to minimize the specific area by compaction was developed and industrialized, and has been installed in the lower part of the countercurrent shaft furnaces since 1984. This had two repercussions. Hot briquetted iron (HBI) has minimized the risk of ignition and substantially reduced re oxidation, making handling and transportation of DRI much easier, and enabled DRI to be used as a substitute for scrap in steel making by the electric furnace. Subsequently, as minimills began to produce steel sheet, DRI was no longer a mere substitute for scrap, and began to be used as a material for high-grade steel with deep drawing quality, because of its low contents of residuals such as Cu, Sn, As, Sb, Bi, Zn, and Pb which all deteriorate the quality of steel products.
According to the statistics of the International Iron and Steel Institute (IISI), the world's DRI production more than tripled from 9.1millon tons in 1984 to 31millon tons in 1995. During this period, the world's hot metal production leveled off at approximately 500 million tons. Consequently, the ratio of DRI production to world hot metal production has increased from 2% to 6%.
Additional attempts are underway to remove geographic restriction of the reductant by replacing natural gas by coal. The SL/RN process, which utilizes rotary kiln to reduce lump ore, pellets and sand iron with coal, is in commercial production. This process suffers, however, from relatively big heat loss and facility size, and hence finds limited acceptance of 2 million tons/year. A new attempt, called FASTMET process, mixes fine coal powder in green pellets, reduces the pellets by firing in rotating hearth furnace in a very short period of time, aiming at commercialization in the near future.
In 1997, the world's DRI production is estimated to reach a high 4.4 million tons.
Heating in an electric furnace is made by electric energy. Raw ferrous materials consist mostly of scrap, some cold pig iron and DRI. For this reason, the electric furnace plays an important role in the recovery and recycling of waste iron resources. In areas where an abundant supply of scrap and electric power are available, the proportion of steel making via the electric furnace route is relatively high, because both energy consumption and equipment investment are substantially smaller than via the integrated route using a BF and BOF to produce steel from ore. Electric furnaces are classified as arc furnaces or induction furnaces, according to the heating method. The arc furnace is used far more extensively for steel making because its capacity is large and production efficiency is high.
In addition to melting, both oxidation refining and reduction refining are possible in the arc furnace; the former is used for decarburization, dephosphorization, and dehydrogenation, and the latter for desulfurization and deoxidation. The arc furnace is also capable of melting a higher fraction of alloy scraps. For this reason, it is often used to refine high-alloy steels, such as stainless steel. However, with the introduction of secondary refining processes such as the argon oxygen decarburization (denoted AOD hereinafter) and vacuum oxygen decarburization (VOD) processes, which are exclusively used for refining stainless steel, the role of the arc furnace has been limited to high-efficiency melting in the upstream process. Even with commercial grades of carbon steel, it is common to conduct high-efficiency melting and decarburization in the arc furnace and to finish the process with a separate secondary refining furnace.
The efficiency of heating, melting, and decarburization in the arc furnace has been substantially increased by adopting an ultra high-power transformer and oxy-fuel burner, as well as by injecting coal powder and pure oxygen gas. Cooling and protecting the furnace walls and ceiling with water-cooled panels has also been enhanced, enabling an increase in production efficiency from 80 to 120 ton/h. Recent trends have seen a shift from the alternating-current arc furnace to the direct-current arc furnace, the introduction of preheating and continuous charging equipment for scrap, and the adoption of the eccentric furnace-bottom tapping. The DC arc furnace offers lower unit consumption of power, electrodes, and refractories, and both noise and flicker are also lower. The preheating and continuous charging equipment for scrap decreases the energy consumption because preheating is carried out by the high-temperature exhaust gas, and heat loss by opening the furnace lid during conventional scrap charging can be prevented. The eccentric bottom-tapping allows efficient tapping without tilting the vessel, and is desirable for maintaining the cleanliness of the molten steel, because the carry over of oxidizing slag into the ladle during tapping can be prevented.
In the production of high-grade steel, refining under vacuum was initially introduced to remove such gas components as hydrogen before casting the molten steel tapped from the converter. This is called vacuum degassing because the gas components in the molten steel are removed by reducing the balanced partial pressures during and after pouring the molten steel into a reduced-pressure vessel. The functions of temperature control, final refining, and composition control were subsequently added to the secondary refining equipment because the function of the converter is increasingly concentrated on decarburization, and further reductions in impurity elements and nonmetallic inclusions should therefore be performed by other means. The allowable ranges of target temperature and composition have also become tighter requiring fine tuning. Thus, secondary refining has recently become the standard process for producing high-grade steels. The most important functions of secondary refining are final desulfurization, degassing of oxygen, nitrogen, hydrogen, etc., removal of inclusions, and final decarburization for ultra-low carbon steel.
Desulfurization is conducted by adding CaO, Na2CO3, CaF2, etc. in a similar manner to that used in the hot metal pretreatment process. Denitrification and dehydrogenation are achieved by treating the molten steel under reduced pressure in a vacuum vessel. Deoxidation is conducted by adding silicon and aluminum to the molten steel to form nonmetallic inclusions of silica (SiO2) and alumina (Al2O3), which are coagulated by stirring the molten steel for enhanced flotation. These are then absorbed into the top slag and removed. Additional decarburization, if required, is carried out by blowing pure oxygen gas onto or into the molten steel in the vacuum vessel to remove the carbon as carbon monoxide.
Secondary refining equipment typically used in the mass production of high-purity steel at integrated steel mills includes the RH (Ruhrstahl-Hausen) vacuum degasser and LF (ladle furnace). The RH equipment injects argon gas into one (suction tube) of the two tubes (snorkels) immersed in the molten steel in the ladle, and the molten steel in the ladle is drawn through the suction tube into the vacuum vessel by the operation of air-lift pumping. After being exposed to the vacuum in the vessel, the molten steel flows back into the ladle through the down snorkel. Since the recirculation rate is relatively high, the RH process is suitable for rapid degassing of a large amount of molten steel. The refining functions of the RH process have also been expanded. For example, decarburization and heating-up are conducted by injecting pure oxygen gas, while the desulfurization and deoxidation rates are increased by adding fluxes, both onto or into the melt in the vacuum vessel. On the other hand, the LF equipment offers strong heating functions, permits the addition of a large amount of alloys, and enables precise temperature control. It also provides outstanding desulfurization by high-temperature treatment with reducing fluxes and the removal of deoxidation products. The LF process is therefore often used for the secondary refining of alloy steel.
Secondary refining equipment used mainly in the final refining step for stainless steel includes the AOD furnace and the VOD furnace. Stainless steel contains a large amount of chromium as a basic component. Since chromium is a strong oxide-forming element, during normal refining it is difficult to decarburize stainless steel to a sufficiently low carbon level while preventing loss of chromium through oxidation to the slag phase. Thus, low carbon levels are achieved by decreasing the partial pressure of carbon monoxide in the refining atmosphere to ensure preferential decarburization in the presence of chromium. In practice, this is done in the AOD furnace by dilution with argon and in the VOD furnace by reducing the pressure.
After controlling the composition and temperature, and removing nonmetallic inclusions, the molten steel is transferred in a ladle and poured into a mold, where it solidifies to produce semi-finished or finished products. In the past, the ingot casting-rolling (slabbing, blooming, or billeting) process was commonly used. In this process, the molten steel was poured into many cast-iron ingot molds and, when the solidification was complete, the ingots were taken out, reheated, and rolled by a slabbing, blooming, or billeting mill. The continuous casting process has now virtually replaced this earlier method. In continuous casting, the molten steel in the ladle is poured into an intermediate vessel(tundish), released into a hollow water-cooled copper mold, and continuously withdrawn from the bottom of the mold as a shell begins to form around the molten metal. The reasons for this change include: (i) the reheating and slabbing process can be omitted because the cast strand has a near-net shape similar to that of the semi-finished product; (ii) the yield is much higher because the continuously cast strand has only two small end portions, in contrast to the tops and bottoms which must be cropped from every ingot; (iii) solute element segregation and nonmetallic inclusions are much lower; and (iv) advanced technologies have improved the productivity and surface quality of the cast pieces greatly, to such an extent that productivity has become compatible with that of the converter and hot rolling processes, thus providing balanced continuity among these processes.
The continuous caster allows a cast strand to be withdrawn at high speed (1.5-2.8 m/min) from the mold in the form of a core of molten steel encased by a thin solidified shell. This high withdrawal speed ensures that casting productivity is matched to that of the converter. As the cast strand descends from the mold, its surface is cooled by a water spray or water mist, and the thickness of the shell increases progressively as the material solidifies. However, the ferrostatic pressure of the molten steel rises at the same time. The cast strand is therefore supported by rolls so that the solidified shell does not bulge. If the solidified shell is deformed due to thermal strains or ferrostatic pressure, cracks form on both the surface and in the interior due to the low ductility and low strength of the shell at high temperatures. An analysis of heat transfer between the molten steel/solidified shell/mold or spray is necessary to increase productivity and prevent deformation and cracking. In addition to this analysis, it is imperative to analyze stress, strain, and deformation in the solidified shell when it passes through both the mold and the support rolls. Progress has been made in the analyses of the heat transfer, elastic-plastic thermal stress, and creep-behavior of the cast strand by use of the finite difference and finite element method, and various computational programs simulating these phenomena have been developed. The measurement of the dynamic behavior of steel at elevated temperatures necessary for such computations has also been carried out.
In addition to deformation and cracking, the quality of the cast strand is impaired by the presence of nonmetallic inclusions and by the segregation of solute elements. Owing to the re oxidation of the molten steel by air, entrainment of slag and refractories, etc., the number of nonmetallic inclusions increases as the steel moves progressively from the ladle to the tundish to the mold. To minimize this problem, the flow of molten steel within the tundish and through the nozzle between the tundish and mold is carefully controlled to ensure coagulation, flotation, and separation of nonmetallic inclusions. Progress has been made in research to evaluate this flow of molten steel by simulation tests with water models and by mathematical modeling of fluid dynamics based on numerical solutions to the governing differential equations, including turbulent forms of the Navier-Stokes equation.
The solubility of solute elements is usually lower in the solid state than in molten steel. These solute elements are discharged into the molten steel at the front face of columnar dendrites of the solidified shell which grows as solidification proceeds. These solute elements concentrate, resulting in positive segregation. The segregation of carbon is shown schematically in the right hand side of the figure. Strong segregation occurs during the final stage of solidification between the branches of columnar dendrites and also at the center-thickness of the cast strand. Solidification theories have been established for the relationship between the morphology of growing crystals and the temperature gradient and cooling rate, the segregation of solute elements near the front of solidifying shell, the rate of solidification which affects segregation, and the influence of the flow of molten steel.
As shown in the figure, the continuous caster is composed of a tundish, a mold, a mold oscillator, a group of cast-strand supporting rolls, rolls for bending and straightening the cast strand, rolls to pinch and withdraw the cast strands, a group of spray nozzles, a torch cutter for cutting the cast strand, a dummy bar for extracting the cast strand at the start of casting, and other components.
The continuous billet caster casts round or square strands of small cross-section, and the continuous bloom caster casts strands of large cross-section. Both are used to produce materials for wire rod, bars, shapes, and pipe. The continuous slab caster produces wide rectangular strands of large cross-section, which are cut off as slabs for use as material for sheet and plate. Slabs for flat-rolled products are usually cast with a thickness of 100 to 250mm. In recent years, however, continuous casters which produce thinner slabs 30-80mm in thickness have been introduced. The thin slab caster eliminates the need for a roughing mill in the hot-rolling process. However, the steel throughput is limited to 1 million ton/year per strand in this process by the thin slab thickness even at higher casting speed, which is currently limited to about 7m/min. Consequently, the thin slab caster is usually combined with an electric furnace of matching output. This combination has been favorably adopted by minimills.
The types of continuous casters include: (i) the vertical type, in which the mold and support rolls are arranged vertically; (ii) the vertical-and-bending type, in which the solid shell of the cast strand is bent in the horizontal direction at the position where solidification is sufficiently complete; (iii) the curved type, in which a curved mold and support rolls are arranged on an arc of the same radius, and the cast strand is straightened horizontally at the end of solidification; (iv) the vertical-and-progressive-bending type, in which the mold and a group of upper support rolls are arranged vertically and the cast strand still with a liquid core is progressively bent, and then progressively straightened to the horizontal position at the end of solidification; and (v) the horizontal type, in which the mold and support rolls are arranged horizontally. The vertical type is used to cast high-grade steels because it promotes the separation (by flotation) of nonmetallic inclusions poured into the mold, although the construction of the caster building becomes tall and hence expensive. The curved type is mainly applied for mass production of conventional products, because building costs can be reduced by the lower height. The vertical-and-progressive-bending type, which combines the advantages of the vertical and curved types, is being used increasingly for large sized slab casters which require improved quality and productivity. The horizontal type is used to produce billets on a small scale because the equipment and the building costs are comparatively low.
Refractory nozzles are used to transfer the molten steel from the ladle to the tundish and then to the mold and prevent any re oxidation through contact with air. To avoid the entrainment of top slag, the bath depth of the tundish is increased, and dams and weirs are installed in the tundish to promote the separation of nonmetallic inclusions by flotation. The nozzle between the tundish and the mold, called the submerged entry nozzle, is designed in such a way that nonmetallic inclusions are not carried deep into the strand together with the molten steel flow, and therefore are not captured in the solidifying shell of the cast strand. A device for controlling the molten steel flow exiting from the nozzles by applying a magnetic field to the flow in the mold is also used for the same purpose and for improving the surface of the shell by suppressing turbulence of the melt meniscus. The mold is equipped with an oscillator (60-240cpm, 4-10mm amplitude) to prevent sticking of the cast strand to the mold. The support rolls are of high rigidity and the roll interval is short to minimize bulging due to ferrostatic pressure, preventing subsequent cracking and segregation due to such bulging. Water or water-mist spray nozzles for cooling are provided across the full width of the cast strand, from immediately below the mold to the crater bottom. Electromagnetic stirrers are sometimes installed below the mold and between the support rolls to stir the molten steel in the solidifying shell by electromagnetic induction and thus to increase the equiaxed dendrites, and to disperse the segregation of solute elements at the crater bottom position between many equiaxed dendrites. A device for applying a thicknesswise reduction to the cast strand is often provided at the crater bottom position to squeeze the solute enriched molten steel to the upper unsolidified molten steel. These two devices are often used in combination because neither by itself is fully adequate for preventing the segregation of solute elements.
The productivity and yield that are so important for operating a continuous caster can be markedly improved by casting many heats continuously without interrupting casting. This is called continuous-continuous casting or sequence casting, and has the advantage of eliminating the need for preparations for starting casting. Consequently, productivity is increased and the amount of the cast strand which must be cropped at the initial and final casting positions due to poor quality is decreased. Techniques have been developed for sequence casting, which allow the mold width to be changed and different steel grades to be cast without interrupting casting operations. These allow strands of different width and grade to be cast continuously without interruption. Submerged entry nozzles wear and become clogged as throughput of the melt increases; therefore, methods have been developed for the quick, automatic exchange of submerged entry nozzles without suspending the casting operation. As one extremely serious practical problem, in "breakout", the solidified shell grows unevenly, the thinner portion of the shell ruptures, and the molten steel leaks from the mold, requiring a full stop of the line. Thermal monitoring techniques for predicting breakout are used at many casters. Productivity can be improved by raising the casting speed, as well as by improving the operating rate. Progress in techniques and equipment has now enabled a casting speed of 1.5-2.8 m/min in continuous slab casters, which corresponds to a production capacity of 5 ton/min per strand. Thus, approximately 3.6 million ton/year can be produced with a 2-strand continuous caster.
The sequence of the casting operation starts with inserting the dummy bar into the mold to seal the bottom end. Molten steel is then poured into the mold from the tundish while taking great care to prevent contact with the air. The withdrawal of the cast strand is started by pulling the dummy bar downward. The molten steel flowing into the mold is rapidly cooled and forms a thin solidified shell composed of fine granular crystals on the surface and an array of fine columnar dendrites inside. The solidified shell becomes thicker due to the growth of columnar dendrites as it descends through the mold. A lime silicate flux is added to the molten steel surface in the mold to prevent heat loss from the molten steel surface and absorb nonmetallic inclusions as they surface. This flux also infiltrates between the mold and the cast strand, and provides lubrication which also prevents sticking of the cast strand to the mold during the oscillation of the mold. At the same time, the layer of mold flux between the steel and mold reduces heat transfer and avoids a rapid decrease in the temperature and resulting deformation and crack formation of the strand.
Surface defects are formed on the cast strand when the level of the steel bath fluctuates in the mold. The level is therefore measured with a sensor and kept as constant as possible by controlling the flow rate of molten steel from the tundish. Electromagnetic braking of the melt flow in the mold is now a representative technique for meniscus level control. The cast strand, which still contains unsolidified molten steel, exits the mold and is withdrawn downward while being supported by a group of rolls and water-cooled with the sprays. During this process, columnar dendrites continue to grow, and equiaxed dendrites are finally formed to complete solidification. At this time, the solidified shell is subjected to high thermal strain, shrinkage, and transformation caused by cooling, and to ferrostatic pressure. Since the hot solidified shell is substantially lower in strength and toughness, the cast strand is susceptible to surface and internal cracks. Consequently, during spraying the cooling pattern is carefully controlled to prevent the growth of cracks due to strain while ensuring solidification by cooling. This pattern control involves controlling the intensity of the water-mist spray along the widthwise and drawing direction of the cast strand as required by the steel grade. Reduction is then applied to the cast strand at the crater bottom to reduce center segregation. After cutting to length with gas torches, the cast piece, or slab, is delivered to the hot-rolling process.
As the productivity of the cast strand has increased and defects have decreased to the extent that no off-line surface conditioning by scarfing and grinding is required, hot-charge rolling and hot direct rolling have been widely adopted. In hot-charge rolling, the hot slab is charged into the reheating furnace, but is rolled without substantial reheating, while hot direct rolling is performed immediately after casting, omitting the reheating process completely.
Heating and melting furnaces, smelting and refining furnaces, and the vessels used to carry hot metal and molten steel are all lined with refractories. The main reason why these furnaces and vessels cannot be used continuously is the need for repair and replacement as a result of wear of the refractory lining. In other words, the life of the refractories determines the life of furnaces and vessels. Refractories for iron and steel production are used under very severe conditions, which include not only elevated temperatures, but also thermal shock caused by abrupt temperature changes. Further, they must possess high-temperature strength and wear resistance to the large momentum of impinging and turbulent metal flow. Refractories must also have the chemical stability to withstand attack by hot metal, molten steel, slag, and various fluxes.
Refractories have a high melting point and good heat-insulating properties. Their basic composition comprises chemically stable substances such as magnesia, alumina, and silica which do not easily react with steel slags or fluxes. When binders are mixed with these refractories, the mixture, when used as it is, is called a monolithic refractory; when pressurized, compacted, and fired, it is called firebrick.
The figure shows the progress in the unit consumption of refractories (weight of refractories consumed to produce one ton of crude steel) in Japan. Unit consumption decreased by as much as 60% from 1970 onwards for two main reasons: (i) changes in blowing and casting processes, which are typified by the change from the open hearth furnace to the BOF, and the replacement of ingot casting by the continuous casting process; and (ii) the extension of the life of refractories by improvement in their quality as well as progress in application techniques.
An example of quality improvement of the furnace-bottom carbon brick in the BF is a decrease in the penetration of slag and iron achieved by adding silicon and a reduction in wear damage, by adding alumina. These measures have contributed greatly to extending the furnace life. At present, the BOF and EAF are lined mainly with fired magnesia-carbon brick which is a composite material that maintains the high corrosion resistance to basic slag and molten metal of magnesia, while enhancing thermal shock resistance, which was the weak point of magnesia brick, by adding carbon. Porosity is reduced by firing this brick at high temperatures in reducing atmosphere to prevent penetration by slag and molten steel. In the BOF, progress in related techniques, together with the above-mentioned improvements in quality, has also contributed to the extension of the life of refractories. Typical examples are, (i) zone lining, in which a brick of optimum composition is used for each zone of the furnace; (ii) slag-coating of the inner walls of the vessel; and (iii) decreasing thermal shock to the refractories by eliminating tilting of the vessel for reblowing, which has become possible due to the improved hit ratio achieved through improved BOF operation.
At the continuous caster, the ladle and tundish are lined with a powdery monolithic refractory material which is applied by stamping or gunning. The consumption of monolithic refractories has been increasing because mechanization and automatization have made production and application easier, and since 1988, the use of monolithic materials has exceeded that of firebrick.
Repeated use of ladles and tundishes while still hot from previous use has also extended their service life substantially because the thermal stress arising from cooling and reheating is minimized and hence resulting spalling of the refractories can be avoided. This repeated "as-hot" use has been made possible by the development of robotized automation of repair and maintenance of hot tundishes in combination with advanced hot gunning of refractories to points where damage is observed.
Submerged entry nozzles used to deliver molten steel from the tundish into the mold are made of alumina-graphite refractories of higher corrosion resistance, which have replaced fused-silica refractories. Carefully thought-out refractory development for other equipment has also resulted in higher-quality refractories suitable for specific service environments.
In summary, the smelting, refining, and casting of iron and steel are the processes for extracting iron from ore, removing useless and harmful elements and adding necessary elements, and obtaining clean and homogeneous materials of required shape, respectively. These results are achieved by making use of chemical reactions among the substances involved. The scientific principles that deal with chemical reactions are those of thermodynamics and reaction kinetics. The former deals with the direction in which a reaction proceeds and reaches the equilibrium state, and the latter considers the mechanism and rate of the reaction to reach the equilibrium. A prerequisite for the application of these principles is that the structure of the substances participating in the reactions and the values of physical properties based on their structure should be known in as much detail as possible. For better understanding, it is necessary to have a knowledge of statistical mechanics and statistical thermodynamics.
The most important feature of the smelting, refining, and casting processes for iron and steel is their ability to handle large amounts of liquid materials such as hot metal, molten steel, and molten slag. Consequently, it is imperative to have a thorough knowledge of the scientific principles underlying the transfer of heat, momentum, and mass, and the movement of fluids at elevated temperatures.
Good castings without segregation of solute elements or cracks can be achieved by studying (i) the nucleation, growth and phase transformation of crystals growing from molten steel, (ii) associated heat transfer, and stress and strain in the crystals, (iii) changes in the concentration of the solute elements, and (iv) the mechanical behavior of materials at elevated temperatures.
Advances in computer technology enable desk experiments by combining physical and mathematical models with computer simulation. Recent progress in this approach includes analysis and design of iron and steel manufacturing processes and construction of phase diagrams for designing new alloys. As the data base for this area of science and technology accumulates and our understanding on the processes improves, this approach will further develop to enhance the progress of the processing of iron and steel making
Further progress in all of these related studies is required to ensure that smelting, refining, and casting techniques continue to improve in the future.
Reference : JFE (JP)