ASM Metals Handbook Volume 01 - Properties and Selection of Irons, Steels, and High Performance Alloys PDF

Selected References J. Dodd and J.L. Parks, Factors Affecting the Production and Performance of Thick Section High Chromium-Molybdenum Alloy Iron Castings, Publication M-383, AMAX Inc. Engineering Properties and Applications of Hi-Hard, The International Nickel Company, Inc. R.B. Gu...

Selected References J. Dodd and J.L. Parks, Factors Affecting the Production and Performance of Thick Section High Chromium-Molybdenum Alloy Iron Castings, Publication M-383, AMAX Inc. Engineering Properties and Applications of Hi-Hard, The International Nickel Company, Inc. R.B. Gundlach, High-Alloy Graphitic Irons, in Castings, Vol 15, Metals Handbook, ASM International, 1988, p 698-701 C.F. Walton and T.J. Opar, Ed., Iron Castings Handbook, Iron Castings Society, Inc., 1981 Steel Processing Technology R.I.L. Guthrie and J.J. Jonas, McGill Metals Processing Center, McGill University Liquid Processing of Steel The physical chemistry of steelmaking may appear deceptively simple for integrated steel mill operations where ore from the ground is converted into steel. The central reaction merely involves the reduction of iron oxide by carbon: 1600° C (2910° F ) Fe2 O3 (iron oxide) + 2C (carbon)   → 2 Fe(molten iron) + CO / CO2 gases (Eq 1) The final reduction of oxide to liquid iron requires high temperatures of the order of 1600 °C (2910 °F), to overcome the chemical barrier to oxide reductions and the physical, or thermal, barrier of fusing iron. However, to yield a final steel product with the correct chemistry, quality, and property characteristics, the series of processes depicted in Fig. 1 is typically required.  Fig. 1 Major steps in processing liquid iron into high-quality steels. RH, Ruhrstahl Hereaus process Ironmaking The first step in processing liquid iron into high-quality steel involves an ironmaking blast furnace, which has evolved over the centuries to become an efficient countercurrent exchanger of heat and of mass, or oxygen (Fig. 2). Iron oxide (in pellet or sinter form), coke, and limestone are successively charged through the top of the furnace. The charge slowly descends through the shaft (an 8-h journey) and is gradually heated by hot ascending gases (CO, CO2, N2, H2, H2O) with a transit time of about 3 s. Because the gas that is lower in the furnace is richer in carbon monoxide, it has a more reducing effect on iron oxides. Thus, the pellets are gradually reduced as a result of mass transfer of carbon monoxide (and hydrogen) from the gas phase into the pellet: 3Fe2O3 (hematite) + CO → 2Fe3O4 (magnetite) + CO2 (Eq 2) Fe3O4 (magnetite) + CO → 3FeO (wustite) + CO2 (Eq 3)  Fig. 2 Principal zones and component parts of an iron blast furnace. Source: Ref 1 Final deoxidation is accomplished down in the cohesive zone (Fig. 2), where high temperatures and highly reducing conditions result in the reduction of wustite (FeO) to iron. Impurities such as silica, sulfur, alumina, and magnesia, which are present in the original pellets and coke, associate with the lime/dolomite and are removed as a molten slag. To ensure that this slag is fluid, a composition of about 40% SiO2, 50% CaO (+MgO) and 10% Al2O3 is desired, thereby placing it within the temperature valley, or well, of a ternary eutectic region. The final reduction of the charged pellets, ore, or sinter takes place either by:  FeO + CO → Fe + CO2 (Eq 4) or 2FeO + C → 2Fe + CO2 (Eq 5) The reaction in Eq 4 is termed indirect reduction because the iron oxide is reduced through the intervention of a gaseous reductant. The reaction in Eq 5 is termed direct reduction because the direct contact of wustite with coke leads to droplets of iron that fall through the dripping zone into the hearth. The CO2 of Eq 4 reacts immediately with the carbon of the hot coke to form more CO as follows: CO2 + C (coke) → 2CO (Eq 6) This CO2/CO reaction is often termed the solution loss reaction because it involves the dissolution of coke by CO2. Although the obvious purpose of a coke layer is to act as a reductant, the descending coke also plays another critical role. Part of the coke (known as the dead man) forms a supporting pillar for the overlaying burden (the ratio of iron and flux to coke and other fuels in the charge). In the region below the cohesive, or sticky, zone (Fig. 2), the remainder of the charge either is molten or is melting (that is, it is composed of slag and pig iron). The final role of the coke is to burn with hot air entering the coke raceways through the tuyeres, thereby generating the high-temperature heat needed for smelting. Cokemaking. The production of coke required for the tasks described above is also a formidable, capital-intensive operation. The process involves the destructive distillation of metallurgical-grade coals in the coking chambers of the by- product coke ovens. The heat that is needed to distill the volatiles is transferred through the brickwork from adjacent vertical flues by combustion of enriched blast furnace off-gases. After an induction time of approximately 17 h, the incandescent coke is pushed out of the slot ovens into transfer railway cars. During its fall the column of coke breaks apart, forming large lumps that are then transferred to the quenching tower, where an intense and normally intermittent water spray quenches them for subsequent charging into the blast furnace. Retained moisture is kept to a minimum because of the endothermic character of the moisture and consequent thermal load in the blast furnace. Blast Furnace Stove Use. To achieve overall thermal efficiency, and to generate the high temperatures required for the reduction to iron in the hearth region of the blast furnace, the incoming blast air is preheated to about 1000 °C (1830 °F) prior to its entry through the water-cooled copper tuyeres. This is accomplished by passing the cold-air blast through a stacked vertical column of preheated (hot) bricks in one of three blast furnace stoves. Because the cold air gradually extracts the stored heat, a separate heating phase is also necessary. This is effected by shutting off the cold-air blast to the stove, opening up the gas valve, and burning enriched blast furnace off-gas (cleaned with water scrubbers, and electrostatic precipitators) to bring the cooled checkerwork of bricks back up to temperature. Because higher preheat temperatures translate directly into lower coke rates per net tonne of hot metal (NTHM), this heating and cooling cycle requires careful optimization. Current Blast Furnace Technology. Over the years, significant improvements in burden preparation (such as the development of uniformly sized pellets) and burden layering techniques have enhanced the kinetic efficiency of gas/solid and heat/mass transfer interactions. Higher air blast preheat temperatures and improved coke properties have also helped to reduce coke requirements from about 910 kg (2000 lb) per NTHM in the 1950s to current levels of 455 kg (1000 lb) per NTHM. The iron that is tapped from the blast furnace is saturated with about 4.4% (or 22 at.%) C. It also contains other impurities that have been reduced from the oxides contained within the iron ore charge. Consequently, the hot metal also contains about 0.3 to 1.3 wt% (Si)Fe, 0.5 to 2 wt% (Mn)Fe. 0.1 to 1.0 wt% (P)Fe and 0.02 to 0.08 wt% (S)Fe. The dissolved sulfur is largely derived from sulfur contained in the coking coal. Dissolved nitrogen levels of the order of 100 ppm would be typical from the air blast. To meet the stringent requirements for high-quality steels, these impurities [(C, S, N, P...)Fe] must be brought to very low residual levels using the sequence of operations described below. Hot Metal Desulfurization Hot metal from the blast furnace is usually treated with lime, calcium carbide, magnesium, or mixtures of these substances to remove sulfur from the iron. The reactions taking place can be written as: CaO (lime) + (S)Fe → CaS + (O)Fe (Eq 7) CaC2(calcium carbide) + (S)Fe → CaS + 2(C)Fe (Eq 8) (Mg)Fe + (S)Fe → MgS (Eq 9) Enhanced desulfurization can be carried out in a blast furnace by using increased slag volumes to absorb the sulfur, but this method requires higher coke rates. Therefore, such practices were abandoned in the 1960s in favor of desulfurization external to the blast furnace. It is important to remember that calcium and magnesium oxides are much more stable than their sulfide counterparts, calcium sulfide and magnesium sulfide. Consequently, these desulfurizing operations are only effective if dissolved oxygen levels within the iron are low. The presence of iron saturated with carbon ensures this condition; the fundamental interrelation between dissolved carbon and dissolved oxygen in high-carbon molten iron is (Ref 2): C + O = CO (gas) (Eq 10) PCO (atm) eq k1600°C = ; 660 (Eq 11) wt %C wt %O where Kcq is the thermodynamic equilibrium constant for Eq 10. The insertion of wt%(C)Fe = 4.4 wt% for hot metal would show wt%(O)Fe ~3 ppm for PCO at atmospheric pressure if equilibrium applies. It is for this reason that desulfurization is so effective in hot metal. Calcia-rich slags have very high sulfur partition ratios with iron (~400). By contrast, sulfur partitioning in the steelmaking step is at best about 4 to 1 between a basic oxygen furnace (BOF) slag and oxygen-rich steel. Consequently, as much as possible of the sulfur-rich product that floats on the hot metal needs to be scraped or slagged off to prevent the sulfur from reverting to the metal during subsequent (low-carbon) steelmaking steps. Current Hot-Metal Desulfurization Technology. The well-advanced process technology for desulfurization generally involves the submerged pneumatic injection of, for example, calcium carbide powder that is carried by nitrogen through a deeply submerged refractory-coated steel pipe of about a 25 mm (1-in.) inside diameter into hot metal contained within the torpedo car. This vessel (Fig. 1) is customarily used to transport hot metal from the ironmaking facilities to steelmaking operations downstream. Typical industrial practices reduce residual sulfur levels down to 0.01% to 0.02% (S)Fe. The desulfurized hot metal usually is transported in the torpedo car from the blast furnace to the steelmaking shop, where it is emptied into the transfer ladle. As mentioned, any slag carryover into the hot metal transfer ladle needs to be removed prior to charging hot metal into the BOF in order to prevent sulfur reversion. Japanese manufacturers can produce steels with residual hot metal levels of 1 to 2% P; they achieve dephosphorization ahead of the steelmaking step by using injections of sodium carbonate. Because strong compound-forming tendencies exist between phosphorus and sodium, as they do for sulfur and sodium, simultaneous desulfurization and dephosphorization is possible, provided the hot metal has first been desiliconized. Steelmaking First-Stage Refining. Because the blast furnace has produced hot metal saturated with carbon and containing other elements, the next operation requires that these impurities (particularly phosphorus) be removed to the required degree. Integrated steel plants normally rely on pneumatically blown oxygen vessels to accomplish these reactions. In a typical BOF, high-velocity (supersonic) jets of pure oxygen are blown onto the hot metal (Fig. 3). Dissolved carbon is oxidized and escapes as carbon monoxide (primarily) and carbon dioxide from the mouth of the vessel, while the other oxidized impurities (Si, Mn, P)Fe enter the slag by fluxing with additions of burnt lime (CaO). Fig. 3 Principal zones and component parts of a basic oxygen furnace for the production of steel in a melt shop. (a) Typical plant layout. (b) BOF vessel To compensate for the vast amounts of heat liberated during these oxidation reactions, about 30% of the total charge to the furnace comprises steel scrap as coolant. The scrap coolant is required to prevent the temperature of the molten steel from exceeding 1650 °C (3000 °F) and thereby causing unnecessary refractory erosion. Once again, highly complex heat, mass, and fluid transport mechanisms are involved. For example, mass transfer of bath carbon to the scrap metal surfaces effectively dissolves light-section scrap, even though bath temperatures are well below the melting point of the scrap (1500 °C, or 2730 °F) during the major portion of a blow (Fig. 4). Once the bath temperature exceeds the scrap melting range (1500 to 1540 °C, or 2730 to 2800 °F), normal thermal processes that involve turbulent heat transfer will melt the scrap, which finally becomes assimilated into the molten bath. The removal of dissolved carbon as gas and the removal of dissolved silicon, manganese, and phosphorus to an upper slag phase takes place sequentially (Fig. 4), according to: (Si)Fe + O2 → (SiO2)slag (Eq 12) 2(C)Fe + O2 → 2CO (Eq 13) 1 (Mn)Fe + O2 → (MnO)slag (Eq 14) 2 5 2(P)Fe + O2 → (P2O5)slag (Eq 15) 2  Fig. 4 Removal of elements from the bath in a BOF process. Source: Ref 3 It should be emphasized that the exact transfer mechanisms are obscure and tend to remain so, due both to the opacity of the system and to the experimental difficulties and restrictions involved in direct measurements of important process variables at 1600 °C (2910 °F). However, the fact that the carbon drops linearly with time during the blow (following silicon elimination) indicates that the rate of oxygen supply controls the rate of decarburization; this is evident except at very low carbon levels, where the curve in Fig. 4 tails off with time. Thus, towards the end of a BOF blow, the transport of dissolved carbon up to the fire point, where the oxygen jets impinge on the metal bath, has difficulty keeping up with the supply of oxygen. As a result, oxygen begins to dissolve in the steel bath at an increasing rate as the carbon-oxygen reaction heads away from the equilibrium curve for (C)Fe and (O)Fe in contact with a carbon monoxide environment at a partial pressure of 0.1 MPa (1 atm). Figure 5 illustrates the trajectory of the carbon-oxygen evolution as a function of process. The BOF-related curves start moving sharply higher as carbon levels drop below about 0.07 wt% C. The rapid increases in dissolved oxygen imply dirtier steels because greater amounts of deoxidizers (Al, Fe-Si) are needed of remove this oxygen, which is in the form of condensed oxide inclusions. Fig. 5 Equilibrium curve for dissolved carbon and oxygen compared with operational carbon-oxygen trajectories for various steelmaking processes. The isopercentage error lines illustrate the relative importance of carbon to oxygen diffusion on carbon-oxygen kinetics. Q-BOP, quick-quiet basic oxygen process; LBE, lance bubbling equilibrium; KMS, Kloeckner Metallurgy Scrap; BOF, basic oxygen furnace. Source: Ref 1 Second-Stage Refining and Technology Advances. The recognition that the stirring being provided by the top- blown jet of a BOF furnace toward the end of the refining process was inadequate, together with the development of the Savarde-Lee shrouded tuyere (Ref 4), triggered a remarkable change in the technology of these oxygen-blown vessels. The tuyere development work made possible and practical the bottom blowing of low-pressure oxygen at high flow rates through a series (typically eight) of tuyeres set in the bottom of the furnace. Each tuyere consists of a central pipe for the oxygen jet and an annular space for injecting a hydrocarbon (such as methane) to form a solid mushroom of steel (Fig. 6). This mushroom protects the refractory base from the fluxing effects of FeO and has allowed the revived use of the Bessemer vessel of 1856 (Ref 5), except that pure oxygen rather than air is injected. Fig. 6 Thermal accretions (mushrooms) formed in Q-BOP steelmaking operations using the Savarde-Lee shrouded tuyere The first North American licensee named this process the quick-quiet basic oxygen process, or Q-BOP. The bottom- blown oxygen jets provide better mixing, lower turndown carbons (of the order of 0.01 wt% C), higher yields (less FeO in slag), and shorter processing times (for example, 14 versus 17 min/blow). One drawback, however, is higher levels of turndown hydrogen in the steel. This is caused by the endothermic cracking of the methane that is needed for the formation of the protective thermal accretions, or mushrooms (Ref 6). Higher levels of dissolved hydrogen can be deleterious for heavy-section products such as pipeline steels and ship plate products; postrefining stir with argon is sometimes favored for steels with these applications. Another feature of these bottom-blown vessels is the need to inject a fine powdered lime simultaneously with oxygen. Top charging of lime particles or lumps in a similar manner to BOF operations leads to unacceptable foaming and slopping. A wide variety of other processes have been spawned that take advantage of some features of both top- and bottom-blown vessels. In the Kawasaki basic oxygen process (K-BOP) operation, 30% of the oxygen is soft blown from a multihole lance set high above the steel bath, with the remainder injected through the base of the vessel using shrouded tuyere technology. This allows low turndown carbons (of the order of 0.02 to 0.04% C), together with higher scrap-melting capabilities (for example, 33% versus 30% of the charge). Other similar technologies, such as the German Kloeckner metallurgy scrap (KMS) process, are also in use. The improved scrap-melting capability of such vessels is enabled by the burning of a higher proportion of effluent carbon monoxide to carbon dioxide within the upper reaches of the vessel itself. Part of the attendant heat can be usefully transferred back to the metal bath, allowing more scrap to be melted. Because scrap generally represents a less expensive source of iron units versus hot metal from the blast furnace, such operations can be profitable, even though they are more technically complex to operate. Practically all BOF (or oxygen-blown method (OBM) or Linze-Donovitz (LD) method) steelmaking operations in North America now use bottom-blown gas injections to at least stir the steel bath. For example, nitrogen, argon, or carbon dioxide can be blown through submerged injector ports, plugs, or nozzles of various proprietary designs. The Sumitomo top and bottom blowing (STB) process, in which CO2/N2 mixtures are bottom blown at about 5% of the flow of the top- blown oxygen in a BOF-like vessel, is a good example of this concept. The STB process increases yields and lowers turndown carbons, thus approaching the performance of Q-BOP vessels. Electric Furnace Steelmaking. Although integrated steel plants use oxygen-blown steelmaking vessels, many smaller steelmaking operations rely on return scrap steel (versus iron ore) as a primary source of material. For such operations, electric arc furnaces offer economic and technological advantages. These furnaces were originally considered appropriate for the production of tool and alloy steels, but they are also able to produce low-carbon steels of high quality. Currently, 30% of the steel production in North America derives from scrap recycling through remelting and refining operations in electric arc furnaces. One difficulty is that residuals, such as copper and tin in return scrap, are not diluted with a virgin hot-metal source in electric furnace steelmaking. However, with the introduction of prereduced ores (Ref 7) of low gange, or impurity levels (for example, >2% SiO2) such problems can be mitigated. Recent technological advances have stressed the role of the furnace as a melter rather than a refiner. Water-cooled panels are required to carry the ultrahigh-power kV A levels of modern furnaces. Ferroalloy/Deoxidizer Additions. No matter which process is used, the raw steel poured from a furnace into a teeming ladle is too highly oxidized for immediate use because it contains about 0.04 to 0.1 wt% O. This level would cause blowholes in the steel if it were then solidified. Steel deoxidants such as aluminum, ferrosilicon, or carbon are therefore required to bring dissolved oxygen contents down to acceptable levels through precipitation of condensed oxides as inclusions. At the same time, additions of other ferroalloys (for example, Fe-Mn, Fe-Nb, Si-Mn, Fe-V) are made as needed to meet the chemical specifications required for the variety of steel grades that are commonly produced by any integrated steel company. 1 These bulk additions (13 to 100 mm, or to 4 in., in diameter) either melt quickly (~40 to 120 s) or dissolve slowly (~60 2 to 360 s), depending on whether their melting ranges are below or above the steel bath temperature (typically 1570 to 1600 °C, or 2860 to 2910 °F) (Ref 8). Some are buoyant (for example, aluminum and ferrosilicon) and tend to float, while others, such as ferroniobium and ferrotungsten, sink rapidly (Ref 1). In either case, thorough metal mixing throughout the teeming ladle is needed (Ref 1). These large bulk additions are commonly added via alloy addition chutes during the last half of a 4 to 8 min furnace-tapping operation. Carryover of slag from the BOF into the ladle can make the recoveries of aluminum and ferrosilicon to the steel highly variable because slag deoxidation as well as metal deoxidation can occur. For these reasons, alloy addition sequencing is important, as are slag control techniques, to limit the net carryover of slag. Ladle Steelmaking. The increasing need to produce quality products that meet much tighter chemical and physical specifications has led to major changes in steelmaking practices during the last two decades. These changes have centered on modifications to liquid steel within the ladle; therefore, this area of technology is known as ladle steelmaking. To illustrate the critical nature of correct chemistry, aluminum-killed steels for deep-drawing operations require dissolved aluminum levels that range between 0.03 and 0.04% (Al)Fe. The aluminum precipitates with dissolved nitrogen as aluminum nitride during subsequent batch-annealing operations. This precipitation controls grain growth and leads to steel with a fine grain structure and good deep-drawing qualities. Higher or lower levels of dissolved aluminum lead to poor performance indices (Fig. 7). Fig. 7 Lankford index of aluminum-killed steels. Cold reduction 71.4%, anneal at 700 °C (1290 °F), 5h, furnace cool at 20 °C/h (36 °F/h) Even tighter specifications were required for high-strength low-alloy steels, which were introduced to compensate for weight reductions (that is, thinner gages) on automobile parts during the energy crises of the 1970s. Specifications called for dissolved niobium levels of 0.03%, a difficult target without close control of steel deoxidation procedures. The production of interstitial-free steels for deep drawing (which are described in the article "High-Strength Structural and High-Strength Low-Alloy Steels" in this Volume) require carbon and nitrogen levels less than 50 ppm and controlled additions of titanium and/or niobium to scavenge carbon and nitrogen. To meet such stringent demands, secondary steelmaking processes, focusing on the teeming ladle, have been developed. Of these, the ladle furnace is used for melt reheating and temperature control. The Ruhrstahl Hereaus (RH) degasser, or tank degasser, is used to reduce dissolved (C,O,H,N)Fe levels. A third type of ladle station provides strong stir facilities by using argon and porous plugs set in the base of each teeming ladle, slag rake-off equipment, and wire feeding that allows precise additions of alloying elements, such as aluminum. Third-stage refining, although still novel, has been conducted by, among others, Sumitomo Metals Industries; it is also known as the injection refining (IR) process (Fig. 1). First lime and then calcium silicide are fed pneumatically through a vertical lance into the teeming ladle. A refractory-lined hood placed over the surface of the steel prevents ingress of atmospheric oxygen. As the relatively large lime particles rise through the melt, they cleanse it by collecting the essentially stationary smaller-diameter (~1 to 10 μm, or 40 to 400 μin.) products of deoxidation. The results of ternary refining are shown in Fig. 8. The number of clusters is greatly reduced after RH degassing followed by strong bubbling. The clusters are totally eliminated with strong bubbling and lime additions.  Fig. 8 Effect of lime powder injections on the quality of steel. RH, Ruhrstahl Hereaus process A final injection of calcium silicide can be used to convert any remaining solid aluminum products of deoxidation into liquid calcium aluminate (preferably 12CaO-7Al2O3) inclusions (Ref 9). Such inclusions pass easily through metering nozzles into the tundish and from there into the mold of a continuous casting machine. By the end of these ladle-refining operations, the total residuals within the steel can be brought down to very low levels (~50 ppm total residuals for (S,O,N,H,P)Fe) (Ref 10). The difficulty in the final liquid metal processing steps is to maintain this level of physical and chemical quality prior to final solidification in the continuous casting machines. Tundish Metallurgy and Continuous Casting. The flow of steel from the tundish into the caster is shown in Fig. 9. Figure 10 illustrates the potential sources of contamination of purified steel emptied from the teeming ladle into the tundish. Using a sliding gate nozzle, metal is metered from the bottom of the teeming ladle into a tundish. This nozzle has to be shrouded with argon to avoid air infiltration, steel reoxidation, and the consequent generation of inclusions. Fig. 9 Typical continuous casting operation  Fig. 10 Potential sources of contamination in the continuous casting process The tundish, in addition to acting as a metal distributor to two or more casters, serves as a further cleansing unit for inclusion removal. Therefore, current practices often use dam and weir combinations to modify the flow of steel within the tundish to enhance inclusion separation. This has led to a trend toward tundishes with larger volumes and thus longer residence times for a given throughput (for example, 60 tonne, or 66 ton, tundishes with a 7-min residence time for a 320 tonne, or 350 ton, ladle full of steel). A typical velocity field for a single-port water model tundish, using the computational fluid dynamic code METFLO (Ref 11), is illustrated in Fig. 11. The associated inclusion separation ratios (defined as the number of inclusions leaving per the number of inclusions entering a tundish) as a function of inclusion rise velocity are also given in Fig. 12. Flow modifiers have no influence on the very small inclusions collected by ternary refining (or by filters), but they can help clean the steel of midsize inclusions in the 50 to 200 μm (2 to 8 mils) range (Fig. 12). For larger inclusions with Stokes rising velocities greater than 5 mm/s (0.2 in./s), these flow controls are not needed for the set of operating conditions noted. Fig. 11 Isometric view of flow fields predicted in a longitudinally bisected single-port water model slab casting 2 tundish with a flow modification device consisting of a weir-dam arrangement placed at L. Flow rate of 0.007 3 m3/s (0.25 ft3/s); length, 5.2 m (17 ft); depth, 111 m (3.6 ft); width (surface), 1.07 m (3.5 ft); flow modification device, 0.5L weir and dam arrangement Fig. 12 Relationship between the inclusion separation ratio and Stokes rising velocities predicted for a full-scale water model of a slab casting tundish. FMD, flow-modification device consisting of a weir-dam arrangement (Fig. 11). Tundishes are normally fitted with insulating covers to conserve heat. For highly deoxidized steels, they are protected with an argon gas cover to reduce reoxidation and inclusion formation. An artificial slag can also be added to absorb those inclusions that are floating out. Contrary to popular belief, many inclusion clusters can reach large sizes within the tundish. Because they can be made up totally of alumina, large clusters are most likely the agglomerated products of deoxidation. Figure 13 presents data analyzing the large inclusions present in an aluminum-killed steel in a 60 tonne (66 ton) slab casting tundish not fitted with flow modifiers. A typical histogram of the inclusions, based on an on-line electric sensing technique using a Liquid Metal Cleanness Analyzer (LiMCA) (Ref 12), is compared with data from Japan for a wire quality steel (Ref 13). The slime extraction analysis technique (dissolution of large sample of steel by ferrous chloride, with elutriation to collect unreacted inclusions of alumina and/or silicates) was used for the Japanese data. Fig. 13 Histogram of large inclusions in a slab casting tundish, based on the LiMCA technique, versus data for a wire quality steel, based on the slime extraction technique Microscopic techniques are inappropriate for the size range shown in Fig. 13, and slime extraction techniques require three days to complete. Nevertheless, such analysis is important because large inclusions can have a deleterious effect on the surface quality, paintability, and zinc-coating characteristics of steel sheet. Similarly, as such inclusions (of alumina or manganese silicates, and so on) are rolled out into long stringers, the transverse properties of steel sheet or plate, such as percent elongation and ultimate tensile strength, are severely compromised, as is metal formability. Consequently, the modification of these inclusions into calcium aluminate inclusions, which are refractory at rolling temperatures and retain their original spherical shape following rolling, is much preferred (Ref 9). For other critical applications, the presence of inclusions with a diameter greater than about 50 μm (2 mils) needs to be prevented. Figure 14 shows a break in a steel wire fabricated for a steel-belted automobile tire (Ref 13). There is a move in the industry to filter steel for such applications to help eliminate inclusions under about 50 μm (2 mil) in size, which are not susceptible to flow modifiers. Fig. 14 Scanning electron micrographs of inclusions at a break in a steel cord wire Mold Metallurgy. The last opportunity for inclusions to be removed is in the mold. Metal enters the mold of a continuous caster through a submerged entry nozzle (Fig. 9); the ports of the nozzle are often angled upward in order to direct the exiting jets of metal up toward the steel surface. There, a layer of lubricating slag from fused mold powder further assimilates inclusions while it simultaneously protects the steel from reoxidation and provides lubrication between the forming shell of the steel and the surfaces of the oscillating mold. It is preferred that the final structure of the solid steel be equiaxed rather than columnar so that cracking of the billet, slab, or bloom during unbending operations is less likely. Precise control of the metal super-heat temperature is needed to prevent dendrite tips that are broken from the advancing columnar freezing front of steel from remelting. The dendrite tips are needed to act as nuclei for grain growth within the remaining melt. Electromagnetic stirring is also used to enhance uniformity of chemistry and structure, and to eliminate center-line segregation of solute-rich material. The cast steel is then cut with travelling oxytorches into slabs, billets, or blooms of appropriate length for further processing. The slabs are about 4 m (13 ft) long, 1 m (3.3 ft) wide, and 100 mm (4 in.) thick. These slabs are inspected and then charged to a slab reheating furnace for subsequent hot-rolling operations. Alternatively, in plants with advanced steelmaking practices where slab surface quality is guaranteed to be acceptable (that is, no scarfing is required), the slabs can be directly charged into the slab reheat furnace. Future Technology for Liquid Steel Processing Operations Because of the high capital cost of the blast furnace, melt shop, and hot-rolling mill complex, major research and development efforts are being made within the industry, with the objective of eliminating the number of process steps needed to produce a final product. Figure 15 shows past, present, and possible future process steps for the production of flat-rolled sheet. The object is to reduce the number of major processes down to two: direct steelmaking and direct, or neat-net shape, casting. In direct steelmaking, the aim is to feed coal (rather than coke), together with iron ore pellets and lime flux, into an autogeneous reactor to produce iron that contains perhaps 2% C. In direct casting, the aim is to develop the technology needed to directly cast steel sheet perhaps 5 to 10 mm (0.2 to 0.4 in.) in thickness, at tonnage rates of 100 to 200 tonnes/h/m width (35 to 70 tons/h/ft width). Such performance characteristics would match those of the big slab casters of the present, but would have a dramatic impact on the capital and operating costs of the integrated steel plant of the future. Fig. 15 Past, present, and future steel processing steps