steel making technology

8/3/2019 Steel Making Technology

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From: "AMIT BARDHAN/MACO CORPORATION" To: ; "Jayanta Bhadra"

Sent: Thursday, March 22, 2007 03:07 PMSubject: Steel Making Technology

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Iron & Steel Industry


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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 theheat 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 thehighest 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 goodreactivity 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 goodcoking 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

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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 ashortage 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. Inaddition, 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 toselect 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 fromthe 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

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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 inalternate 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 inthe 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 blownthrough 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 whileexchanging 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 withappropriate 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) intocarbon 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

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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 ofnonmetallic 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 producedin 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 isdecarburized 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 andafter 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 subjectedto 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 secondaryrefining 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

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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 ironoxides 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 togetherwith 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, extralow 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 goodunderstanding of thermodynamics and transport phenomena is indispensable in achieving these objectives.

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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. Asmall 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 ofmolten 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 ismore 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 thetemperature 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 isfinished when the carbon concentration and temperature of the molten steel have reached these target values. On tapping, alloys and deoxidizers such as silicomanganese and/oraluminum 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 neededto 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 fortop-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

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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 ) usingreformed 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; theHYL-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 mostcommonly 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 furnaceis 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 thefuel 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.

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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 animportant 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 theelectric 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 toproduce 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 steelmaking 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, andthe 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, suchas 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

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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 asby 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 inproduction 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 andcontinuous charging equipment for scrap, and the adoption of the eccentric furnace-bottom tapping. The DC arc furnace offers lower unit consumption of power, electrodes, andrefractories, and both noise and flicker are also lower. The preheating and continuous charging equipment for scrap decreases the energy consumption because preheating is carriedout 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 efficienttapping 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 beprevented.

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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 producesemi-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 intomany 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 castingprocess 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 reheatingand 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 continuouslycast 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 aremuch 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 compatiblewith 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

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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 caststrand, 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 producematerials 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 forsheet 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 inthickness 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 perstrand 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 anelectric furnace of matching output. This combination has been favorably adopted by minimills.

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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 iscalled continuous-continuous casting or sequence casting, and has the advantage of eliminating the need for preparations for starting casting. Consequently, productivity is increasedand 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 sequencecasting, 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 becast 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 growsunevenly, 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 areused 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 acasting 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 producedwith a 2-strand continuous caster.

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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 thesefurnaces 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 determinesthe 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 shockcaused 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 silicawhich 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

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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. Thescientific 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 reachesthe 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 ofthe 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 isnecessary 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 offluids at elevated temperatures.

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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 elevatedtemperatures.

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 technologyaccumulates 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.

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Reference :Web sitehttp://www.jfe-21st-cf.or.jp/chapter_2/index.html

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