Steel Processing Technology(1)

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Steel Processing Technology(1)

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Steel Processing Technology(1)

Messaggioda Aldebaran » 04/05/2010, 16:40

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 (2910 )
23 2 Fe O (iron oxide) + 2C(carbon)¾¾¾°C ¾¾°F¾®2Fe(molten iron) +CO/CO 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 is
typically required.
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)
Final deoxidation is accomplished down in the cohesive zone , 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 byproduct
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)
1600
( )
660
% %
eq CO
C
P atm
k
wt C wt O ° = ; (Eq 11)
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).
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 . 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 , according to:
(Si)Fe + O2 ® (SiO2)slag (Eq 12)
2(C)Fe + O2 ®2CO (Eq 13)
(Mn)Fe + 1
2
O2 ® (MnO)slag (Eq 14)
2(P)Fe + 5
2
O2 ® (P2O5)slag (Eq 15)
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.
Second-Stage Refining and Technology Advances. The recognition that the stirring being provided by the topblown
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.
The first North American licensee named this process the quick-quiet basic oxygen process, or Q-BOP. The bottomblown
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 topblown
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.
These bulk additions (13 to 100 mm, or 1
2
to 4 in., in diameter) either melt quickly (~40 to 120 s) or dissolve slowly (~60
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.
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 . 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.
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. 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.
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. 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.
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. 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.
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
References cited in this section
1. R.I.L. Guthrie, Engineering in Process Metallurgy, Oxford Science Publications, Clarendon Press, 1989
2. E.T. Turkdogan, Physical Chemistry of Oxygen Steelmaking, Thermochemistry and Thermodynamics, in
B.O.F. Steelmaking, Vol 2, Theory, Iron and Steel Society, 1975
3. A. Jackson, Oxygen Steelmaking for Steelmakers, 2nd ed., George Newnes Ltd., 1969
4. G. Savarde and R. Lee, French Patent 1,450,718, 1966
5. J.R. Stubbler, The Original Steelmakers, Iron and Steel Society, 1984
6. Y. Sahai and R.I.L. Guthrie, The Formation and Growth of Thermal Accretions in Bottom/Combination
Blown Steelmaking Operations, Iron Steelmaker, April 1984, p 34-38
7. R.L. Reddy, Use of DRI in Steelmaking, in Direct Reduced Iron--Technology and Economics of Production
and Use, Iron and Steel Society, 1980, p 104-118
8. R.I.L. Guthrie, Addition Kinetics in Steelmaking, in Electric Furnace Proceedings, Vol 35, Iron and Steel
Society, 1977, p 30-41
9. R.I.L. Guthrie, The Use of Fluid Mechanics in Ladle Metallurgy, Iron Steelmaker, Vol 9 (No. 10), 1982 p
41-45
10. G.M. Faulring, J.W. Farell, and D.C. Hilty, Steel Flow Through Nozzles: The Influence of Calcium, in
Continuous Casting, Vol 1, L.J. Heaslip, A. McLean, and I.D. Sommerville, Ed., Iron and Steel Institute,
1983, p 57-66; see also p 23-42 for reoxidation inclusions
11. T. Emi and Y. Lida, Impact of Injection Metallurgy on the Quality of Steel Products, In Scaninject III, Part
1, Proceedings of a joint MEFOS/JERNKONTORET Conference (Lulea, Sweden), 1983, p 1-1 to 1-31
12. S. Joo and R.I.L. Guthrie, Mathematical Models and Sensors as an Aid to Steel Quality Assurance for
Direct Rolling Operations, in Proceedings of the Metal Society of the Canadian Institute of Mining and
Metallurgy Vol 10, Proceedings of an International Symposium on Direct Rolling and Hot Charging of
Strand Cast Billets J.J. Jonas, R.W. Pugh, and S. Yue, Ed., Pergamon Press, 1989, p 193-209
13. H. Ichihashi, Sumitomo Metals Internal Report; see D.H. Nakajima, "On the Detection and Behaviour of
Second Phase Particles in Steel Melts," Ph.D. thesis, McGill University, 1986
Processing of Solid Steel
As with liquid steel, several processing operations are required to convert steel into its wide variety of finished form.
Figure 15 shows the sequence of operations for flat rolling. After continuous casting and inspection, followed by slab
reheating in the reheat furnace, the slab is prepared for the roughing and tandem hot strip mills. Rolled hot strip is then
cooled on runout cooling tables and coiled. For thinner gages, hot-rolled strip is cold-rolled, which is followed by
annealing and by various coating processes to protect against corrosion; coatings include zinc, tin, zincalume, paint,
enamel, and so on. Slabs cut from the continuous casting machine are reheated to bring the steel to about 1200 °C (2190
°F).
Hot Rolling
Hot rolling is carried out with the steel in its γ, or austenite phase. Steel is evidently plastic and particularly malleable at
the temperatures employed, which range from 1200 °C (2150 °F) to as low as 800 °C (1470 °F). This allows large
reductions in thickness (for example, from 250 mm thick slab to 2 mm thick hot strip) with relatively small force.
Following hot rolling, the steel transforms into its low-temperature α, or ferrite, phase (plus other constituents). The
characteristics of this transformation, which has a significant effect on the mechanical properties of the product, depend
on the cooling rates used on the runout cooling tables.
In early integrated steel mills, hot rolling traditionally began with the breakdown of cast ingots into rectangular slabs
about 200 mm (8 in.) thick or square billets about 200 × 200 mm (8 × 8 in.) in cross section. With the gradual
replacement of ingot casting by continuous casting (except for tool steels and other specialty or low-tonnage grades), the
slabbing, or breakdown, stage of hot rolling has gradually disappeared from most mills. Layout of a modern hot strip mill
is shown in Fig. 16. Hot rolling begins with roughing, which occurs at temperatures from 1200 °C (2190 °F) down to about 1100 °C (2010 °F). During roughing, slabs about 6 to 8 m (20 to 26 ft) long and 250 mm (10 in.) thick are
converted into transfer bars about 30 to 50 mm (1.2 to 1.2 in.) thick and up to 40 or 50 m (130 or 165 ft) long. Round or
square billets are transformed by analogous steps into bars about 100 m (330 ft) in length. Finish rolling, or finishing, is
then carried out in a five-, six-, or seven-stand hot mill (Fig. 16), with finishing stand temperatures as low as 900 or 800
°C (1650 or 1470 °F). By this means, the transfer bars are converted into strips about 2 to 3 mm. (0.08 to 0.12 in.) in
thickness and 600 m (2000 ft) long, at a productivity level of about 100 tons/h per meter of steel strip width (Ref 14). The
hot strip is coiled, while plate grades are retained in their rectangular form at thicknesses of about 10 to 30 mm (0.4 to 1.2
in.). In a similar manner, long products are either coiled if they are round and of small cross section (about 6 mm, or 0.24
in., in diameter), or cut to length if they are thicker or of irregular cross section, such as angle and channel shapes.
Controlled Rolling of Microalloyed Steels. The traditional concern of hot rolling was simply to reduce the crosssectional
size of the steel, as described above. However, with the use of microalloying elements such as niobium,
vanadium, and/or titanium, hot-rolling at controlled temperatures is also used to condition the austenite so that a fine
ferrite grain size is produced during cooling. This method of hot rolling, known as controlled rolling, relies on the
precipitation of carbonitrides of the microalloying elements (niobium, vanadium, and/or titanium) to control austenite
grain growth and recrystallization.
Over the last 20 years, there has been a gradual introduction of various types of controlled rolling (Ref 15), which
presently include conventional controlled rolling, recrystallization controlled rolling, and dynamic recrystallization
controlled rolling. With the various methods of controlled rolling, attractive properties can be imparted to materials in the
as-hot-rolled condition, there by eliminating the need for separate (and costly) heat treatment later. The use of controlled
rolling leads to the production of steels with nearly twice the yield strength of the commodity grades produced by
traditional rolling methods. This increase in yield strength is accompanied by an increase in fracture toughness (see the
article "High-Strength Structural and High-Strength Low-Alloy Steels" in this Volume).
The increase in fracture toughness is a direct result of the considerable ferrite grain refinement caused by controlled
rolling. Both the fracture properties and the yield strength depend directly on the inverse square root of the ferrite grain
size, as given by the Hall-Petch relations (Ref 16):
σy = σ0 + κy d-1/2 (Eq 16)
and
βT = α- ln d-1/2 (Eq 17)
where σy is the yield strength of the polycrystal, σ0 is the yield strength of a single crystal of equivalent purity and
condition, κy is the grain boundary strengthening coefficient, d is the mean grain size, T is the impact transition
temperature, and α and β are constants. By reducing the ferrite grain size from 57 μm (2.25 mils), or ASTM grain size No.
5, for example, to 5 μm (0.2 mil), or ASTM grain size No. 12, yield strength increments of greater than 210 MPa (30 ksi)
can be produced, and the impact transition temperature can be reduced by as much as 100 °C (180 °F).
Critical Temperatures. The three varieties of controlled rolling that have been developed to date rely on the
recognition of the three critical temperatures of steel rolling. The first of the three temperatures is the no-recrystallization
temperature, or Tnr. At temperatures above Tnr, the austenite recrystallizes between mill passes; as a result, the grain size is
refined, and the work hardening accumulated within the roll pass is eliminated. This temperature can be detected by
carrying out pilot rolling studies, by analyzing mill data on rolling loads, or by torsion testing . Below the T nr,
the recrystallization of austenite no longer takes place between rolling passes. Work hardening, or strain, accumulates as a
result, and the flow resistance or rolling load begins to increase more sharply with decreasing temperature .
The second critical temperature, the upper critical temperature, or Ar3, defines the start of the austenite-to-ferrite
transformation on cooling (the r comes from the French refroidissement). The third temperature, the lower critical
temperature, defines the end of the austenite-to-ferrite-plus pearlite transformation and is known as Ar1. It should be noted
that these two critical temperatures do not correspond to the Ar3 and Ar1 values determined on annealed samples using
classical dilatometry, for example, because the deformation introduced by rolling modifies the transformation behavior of
the steel. Instead, Ar3 and Ar1 can be determined with the aid of a deformation dilatometer, which applies a compressive
strain to the sample prior to the initiation of cooling, or by the torsion simulation of rolling (Fig. 17 and 18).
Precipitation of Carbonitrides and Sulfides. Many of the particles that are precipitated during cooling after
continuous casting are redissolved during reheating in the slab reheat furnace. These include AlN, MnS, Nb(C,N),
Ti(C,N), Ti4C2S2, TiS, and VN. During subsequent hot rolling, these reprecipitate fairly readily because the dislocations
introduced during rolling act as nucleation sites for strain-induced precipitation. These particles are only about 2.5 nm
(0.1 μin.) in diameter when they appear, but they can grow or coarsen up to diameters of 10 to 20 nm (0.4 to 0.8 μin.) The
particles take several seconds to form; therefore, they not are produced during rolling operations of short duration. In
general, these particles only play a role at temperatures below 1000 °C (1830 °F); that is, during finishing. The straininduced
precipitation of MnS is important in the processing of electrical steels, while that of Nb(C,N), Ti4C2S2, and TiS is
important in the rolling of interstitial-free steels, and that of Nb(C,N) and to a lesser extent TiC and VN) is important in
the controlled rolling of microalloyed or high-strength low-alloy steels (Ref 15, 18).
Precipitation is necessary because recrystallization can only be arrested during finish rolling, and only if a copious
number of precipitates form during passage of the strip between successive mill stands. A high density of precipitates is
promoted by the occurrence of cooling between passes (which increases the driving force for precipitation) and interpass
intervals of about 10 s or more. As a result, a Tnr is only displayed by steels containing niobium, titanium, or vanadium
and, furthermore, only when sufficient time is provided during finish rolling for the precipitates to nucleate and grow (Ref
18). This means that recrystallization is most readily arrested during rolling in slow reversing mills, such as plate and
Steckel mills, while precipitation plays a much smaller role in tandem mills, such as hot strip, rod, and other mill
installations where interpass times are short (μ0.5 s or less).
Conventional controlled rolling (CCR) was the first type of controlled rolling to come into regular commercial use.
About 8 to 10% of the total steel tonnage rolled annually is now produced in this way. This process was originally
developed for the production of plate grades for the manufacture of oil and gas pipelines, for which the required minimum
yield strengths were 350 MPa (50 ksi), 420 MPa (60 ksi), and 490 MPa (70 ksi) (Ref 15). Because of the need for good
weldability, low concentrations of carbon and carbon equivalents were specified. These were readily obtained by reducing
carbon concentrations to 0.06 or 0.07%; small amounts of niobium (about 0.04%), in combination with vanadium (up to
0.1%) and molybdenum (up to 0.30%), were added for higher-strength grades.
During roughing operations, the coarse reheated austenite grains in a slab are first refined by repeated recrystallization,
bringing the grain sizes down to about 20 μm (0.8 mil) or less. The transfer bar can then cool below the Tnr during transfer
from roughing to the finishing facilities. When rolling is restarted or continued below the Tnr, recrystallization is no longer
possible, and the austenite structure is progressively flattened in an operation known as pancaking. For pancaking to be
successful, the accumulated reductions applied in this temperature range must add up to at least 80%. Finally, when the
flattened austenite grains go through their transformation to ferrite, the ferrite produced has a very fine grain structure
because of the large number of nucleation sites available on the expanded surfaces of the pancaked austenite grains. This
leads to ferrite grain sizes in the range of 5 to 8 μm (0.2 to 0.3 mil). The fine-grain ferrite is responsible for the attractive
combination of good toughness properties and high yield strengths (Ref 15, 18). It should be stressed that austenite
pancaking is only possible in the absence of recrystallization, and its arrest is caused by the copious precipitation of
Nb(C,N) during delays between mill passes.
Recrystallization Controlled Rolling (RCR). As described above, controlled rolling is generally based on the use of
low finishing temperatures (that is, in the vicinity of 800 to 900 °C, or 1470 to 1650 °F), with the result that fine ferrite
grain sizes appear after transformation. However, such finishing is inappropriate for certain products, such as heavy plates
and thick-walled seamless tubes (Fig. 19), that cannot be finished at such low temperatures in the hot-rolling range due to
excessive rolling loads. For such applications, it is possible to produce the fine microstructures required by carefully
controlling the crystallization of austenite and arranging for it to occur at successively lower temperatures during finish
rolling (Ref 19). These temperatures are nevertheless above 900 °C (1650 °F) and thus are higher than those employed in
CCR.
Two requirements must be met for the RCR process to be successful. One is that the recrystallization not be sluggish, so
that the times required are not too long. This is achieved by employing vanadium rather than niobium as an alloying
element. Vanadium acts as a grain refiner without bringing recrystallization to a complete stop, as niobium is inclined to
do. The second requirement is that grain growth be prevented after each cycle of recrystallization; this grain growth can
negate the refining effect of recrystallization at lower and lower temperatures. For this purpose, sufficient titanium is
added to have about 0.01% available for the formation of fine particles of TiN during cooling after continuous casting
(Ref 20). When this dispersion has an appropriate size and frequency distribution, it can completely prevent grain growth
of the austenite after each cycle of recrystallization. The fine austenite grains, in turn, transform into relatively fine-grain
ferrite, for example, 8 to 10 μm (0.3 to 0.4 mil) in diameter, leading to mechanical properties in the hot-rolled product that
are acceptable for many purposes.
Dynamic Recrystallization Controlled Rolling (DRCR). When the interpass time is short, as in the case of rod,
hot strip, and certain other rolling processes (Fig. 20), insufficient time is available for conventionally recrystallization
during the interpass delay. The amount of carbonitride precipitation that can take place is also severely limited. As a
result, an alternative form of recrystallization is initiated. This is known as dynamic recrystallization, and it involves the
nucleation and growth of new grains during (as opposed to after) deformation (Ref 21, 22). This also requires the
accumulation of appreciable reductions, of the order of 100%, to enable the recrystallization process to spread completely
through the microstructure inherited from the roughing process. Austenite grain sizes as small as 10 μm (0.4 mil) can be
achieved with DRCR (Ref 23).
Low-temperature finishing by DRCR has the advantage of producing finer ferrite grain sizes after transformation that
CCR; that is, 3 to 6 μm (0.12 to 0.24 mil), as opposed to 5 to 8 μm (0.2 to 0.3 mil) for the latter process (Ref 21).
However, such low-temperature finishing increases the rolling load, and it can also make mill control more difficult
because of the load drop associated with the initiation of dynamic recrystallization. It is important to note that under
industrial rolling conditions, CCR, RCR, and DRCR can all occur to different degrees during a given operation. This can
happen when the processing parameters have not been optimized so as to favor only strain-induced precipitation and
austenite pancaking in the case of CCR, conventional recrystallization in the case of RCR, and dynamic recrystallization
in the case of DRCR.
The Stelco Coil Box. One of the problems associated with batch processes such as the rolling of both long and flat
products is the temperature rundown that develops (between the head and tail of a transfer bar, for example). Such a
gradual decrease in temperature can lead to gage and flatness problems, as well as to a gradient in microstructure
characteristics (and therefore in the mechanical properties) along the workpiece. One solution to this problem has
involved the introduction of a coil box between the roughing and finishing stands of a hot strip mill (Fig. 21). When the
transfer bar, which is 30 to 50 mm (1.2 to 2 in.) thick, arrives at the coil box, the leading edge is deflected and curled into
a circular shape, and the entire bar is wound into a coil without a mandrel or spool being required. The coiled shape of the
workpiece enables it to cool much more slowly as the bar is slowly fed (tail end first) into the hot strip mill. This
technology has led to a significant improvement in the uniformity of the dimensions and properties of the final product, as
well as to a decrease in the energy required for hot rolling.
Cooling Beds, Runout Table Cooling, and Coiling. Following hot rolling, the workpiece is generally cooled down
to room temperature. For plates and bars this is carried out on cooling beds. For strip it is carried out on runout tables and
during holding, which follows coiling. For rods it is carried out in water boxes and along cooling lines employing large volumes of forced air (Fig. 23). Except
for certain grades of stainless and electrical steels, such cooling always involves the transformation of austenite into
ferrite, as well as into a number of other transformation products, such as pearlite, bainite, and martensite. The particular
transformation product that forms, as well as its general characteristics, depends on the cooling rate that is achieved. The
faster the cooling rate within a given product range, the stronger the microstructure that is produced. As a result, there is
considerable interest at present in the use of accelerated cooling to promote the formation of appropriate microstructures.
For most steel products, in fact, this process is the least expensive way to increase the strength.
When the transformation product consists largely of ferrite, rapid cooling decreases the ferrite grain size obtained from a
fixed hot-rolling schedule. This is because of the hysteresis involved in phase transformations, as a result of which the
actual (as opposed to equilibrium) transformation temperature displayed on cooling decreases as the cooling rate is
increased. Lower transformation temperatures, in turn, lead to finer ferrite grain sizes, in part because the growth rates are
lower at lower temperatures, but also because the ferrite nucleus density increases with increasing supercooling below the
equilibrium transformation temperature.
When the transformation product consists largely of pearlite or contains appreciable volume fractions of pearlite, more
rapid cooling leads to the formation of finer pearlite, which is associated with higher-strength. However, if the cooling
rate is too rapid for a given chemistry, some bainite or even martensite can form. Unless these transformations are
carefully controlled, such structures lead to a lack of toughness and ductility and are therefore generally avoided.
Nevertheless, B-modified bainitic steels are employed for the production of high-strength heavy plate (700 to 900 MPa, or
100 to 130 ksi, yield strength in the as-rolled condition); the carbon level in these steels is reduced to about 0.02% to
improve toughness. Similarly, fully martensitic steels can be produced by quenching (a separate and therefore fairly
expensive operation), in which case the brittleness is reduced by an appropriate tempering treatment.
Precipitation During Cooling and Coiling. The solubility of all the precipitate-forming elements in steel decreases
as cooling progresses (Ref 18). Thus, carbonitrides and sulfides such as AlN, Fe3C, MnS, Nb(C,N), Ti(C,N), Ti4C2S2,
TiS, and V(C,N) all tend to form, either on the runout table or cooling bed, or after coiling. Because precipitate nucleation
and growth take time (for example, 1 to 10 s for nucleation, and 10 to 100 or 1000 s or more for growth, depending on the
temperature), the amount and type of precipitation that takes place after hot rolling are sensitive functions of the cooling
rate and conditions of holding. Thus, rapid cooling on the runout table suppresses precipitation, although some particles
are inevitably formed during the austenite-to-ferrite transformation, because the solubilities in ferrite are appreciably
lower than the respective levels that pertain to the austenite. The coiling temperature is also of considerable importance.
Relatively high coiling temperatures, of the order of 750 °C (1380 °F), followed by the slow cooling rates associated with
the geometry of coils, favor the precipitation of the carbonitrides (for example, AlN). Low coiling temperatures, of the
order of 550 °C (1020 °F), on the other hand, prevent AlN formation and keep these elements in solution for precipitation
during annealing after cold rolling. The coiling temperature also determines the mean size and number of the particles that
form; the former decreases and the latter increases as the temperature is lowered. These considerations are important
because the final grain size after recrystallization and grain growth is directly proportional to the mean particle size for a
given chemistry (that is, containing a given volume fraction of precipitate).
Warm Rolling
In recent years a renewed interest has developed in warm rolling, that is, the finish hot rolling of steel in the high-ferrite,
as opposed to the low-austenite, temperature range. Warm rolling is possible because ferrite is actually softer than
austenite at a given hot-rolling temperature. it is evident that the ferrite in an
interstitial-free (IF) steel has as little as half the flow resistance of the austenite prior to transformation. Such a large
difference in flow stress can lead to serious gage and control problems when rolling is carried out in the vicinity of the α-
to-β transformation. These problems are avoided, however, if rolling is suspended during cooling through the intercritical
range and resumed only when the steel has cooled below Ar1.
The example given above is extreme because the low carbon level of an IF steel (about 30 ppm) means that the
intercritical temperature range is reduced to as little as 30 °C (85 °F), and therefore the flow stress drop associated with
passage through this range is very sharp. In conventional steels, the difference between the Ar3 and Ar1 temperatures is in
the range of 100 °C (180 °F) or more. In such cases, the fully ferritic material is significantly colder than its fully
austenitic counterpart, and thus the ferrite has a resistance to flow that is only moderately less than that of the austenite.
The warm rolling of IF steels is of commercial interest because lower reheating and rolling temperatures can be
used, leading to lower scale losses and energy consumption rates in the slab reheat furnace. Furthermore, the textures
developed during the warm rolling of ferrite do not differ appreciably from those produced during the cold rolling of the
same phase, and this rolling step can therefore be employed for the production of steels with excellent formability
characteristics.
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Aldebaran
 
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