ASM Metals HandBook Vol. 14 - Forming and Forging
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Flat, Bar, and Shape Rolling
G. D. Lahoti, The Timken Company; S.L. Semiatin, Battelle Columbus Division
Materials for Rolling
A large number of metals are rolled using the methods and equipment described above, or slight variations of them. By
far the largest amount of rolled material falls under the general category of ferrous metals, or materials whose major
constituent is iron. Included in this group are carbon and alloy steels, stainless steels, and specialty steels. Nonferrous
metals, including aluminum alloys, copper alloys, titanium alloys, and nickel-base alloys also are processed by rolling.
Steels
Conventional primary and secondary rolling of steels is usually conducted at elevated (hot-rolling) temperatures. In a
typical hot-rolling operation involving multiple passes through a reversing or multistand mill, the temperature of the work
metal drops considerably. For carbon steels, the initial rolling temperature may be about 1200 °C (2190 °F); it may drop
to 900 °C (1650 °F) or lower by the final pass. Because the size of recrystallized grains decreases with temperature, hot
rolling results in a fine grain size.
The control of grain size and other microstructural features during rolling is especially important in low-carbon and low-
alloy steels. Higher-carbon and high-alloy steels produced in the form of plates, bars, or shapes often undergo subsequent
mechanical (for example, forging or extrusion) or thermal (such as hardening or tempering) processing in which the final
properties are tailored to the end-use. Two products in which rolling is used almost exclusively to control structure and
properties are low-carbon steel (used in automotive and appliance applications) and high-strength low-alloy steels (used
in various structural applications).
Low-carbon steel is produced in the form of sheet by a combination of hot and cold rolling. The starting steel is either
rimmed or killed. In these steels, it is not possible to obtain very fine grain sizes by means of controlling the rolling
process (Fig. 24). This is because of the absence of alloying elements, which could retard the rapid grain growth that
occurs between passes. Nevertheless, grain size and strength are of secondary importance in this product. The most
important property is cold formability, because the sheet metal is often subsequently stamped into complex shapes at
room temperature.
Fig. 24 (a) Microstructure of low-carbon steel after rolling. (b) Microstructure of high-strength low-
alloy steel
after rolling. See text for details. Courtesy of L. Cuddy, Pennsylvania State University
High-strength low-alloy steels, in contrast to the low-carbon steels discussed above, are designed to have high
strength and a relatively modest amount of cold formability. These grades are typically produced as hot-rolled sheet, bar,
and plate with 0.05 to 0.10% C and small amounts of niobium, vanadium, and titanium.
The correct thermomechanical treatment is extremely important in determining the final properties of high-strength low-
alloy steels. For these materials, controlled rolling (Fig. 25) is used to refine the relatively coarse austenite structure by a
series of high-temperature rolling and recrystallization steps. A moderate to heavy reduction is imposed on the material
below the recrystallization temperature (T
r
in Fig. 25) to achieve the desired fine grain size and the associated properties.
More information on controlled rolling of high-strength low-alloy steels is available in Ref 45.
Fig. 25 Temperature/time profile for controlled rolling of high-strength low-alloy steel plate. Ar
3
, austenite-to-
ferrite transformation temperature. Source: Ref 45
Stainless steels are available in the same product forms as carbon and low-alloy steels. Mills of more rugged
construction are required for the rolling of stainless steels than are needed for plain carbon and alloy steels because of the
higher strengths of the stainless alloys; otherwise, rolling practice is similar to that used for carbon and alloy steels.
Stainless steel mill products are normally obtained in the annealed condition, but strength or hardness higher than that in
the annealed condition can be attained by controlled cold rolling.
Nonferrous Materials
A number of nonferrous metals are also rolled into a variety of product forms using methods similar to those described
above for steels. These include aluminum, copper, titanium, and nickel-base alloys.
Aluminum Alloys. Aluminum alloy sheet and plate are also hot rolled from slabs. As for steels, the slabs are frequently
fabricated from cast ingots. In many cases, it is first necessary to remove surface defects from the ingots by means of a
machining operation known as scalping. Also, the ingots are given a preliminary high-temperature homogenization heat
treatment to eliminate chemical nonuniformities inherent in aluminum alloy castings. Such a treatment expands the
temperature regime over which rolling can be successfully conducted without fracture. Aluminum slabs are finished by
hot rolling alone on continuous mills (thicker gages) or by a combination of hot and cold rolling (thinner gages).
Aluminum foil is one of the most common of rolled aluminum products, and is produced by cold rolling to thicknesses as
small as 6 m (0.00024 in.) (Ref 46). Aluminum alloy blooms are hot rolled from square ingots on two-high reversing
mills, with scalping and homogenization heat treatments being used as for the slabbing operation. Bars, shapes, and wire
are subsequently hot rolled from reheated blooms.
Copper Alloys (Ref 47). Production of copper alloy strip and sheet begins with semicontinuous cast slabs or
continuous cast strip. Initial breakdown of these products is usually by hot rolling on two-high reversing mills equipped
with vertical edging rolls. After rolling, the strip is scalped (milled) to remove any oxides remaining from the casting and
rolling operations. After milling, the strip has a thickness of about 7.6 to 10.2 mm (0.300 to 0.400 in.).
Copper alloy sheet is cold rolled to final thickness using either four-high or Sendzimir mills to obtain the necessary
reduction while maintaining flatness. The finished thickness of the sheet can be as low as 0.1 mm (0.004 in.).
Titanium and titanium alloys are considerably more difficult to roll than are either steels or copper and aluminum
alloys. Most titanium alloys have very narrow working temperature ranges. To overcome this problem, titanium alloys are
often rolled in packs, or layers of sheets, that are sometimes encased in a steel envelope (Fig. 26). The envelope, called a
can, is evacuated to minimize oxidation of the work metal, and also serves to minimize heat loss to the relatively cold
rolls upon deformation. The narrow working temperature range of titanium alloys makes the rolling of these materials
labor intensive. Rolling passes are done by hand on relatively small pieces, and many intermediate reheating steps are
required. Some titanium alloys, however, are being continuously rolled on hot and cold strip mills.
Fig. 26 Pack ("can") used for rolling titanium and nickel-base al
loys. The top cover, which would be welded in
place before rolling, is not shown.
Nickel-base superalloys are the most difficult materials to roll. Primary rolling of these materials is usually done at
temperatures near the melting point on rugged, powerful mills built to withstand the high stresses encountered in the
working of these alloys. Like titanium alloys, nickel-base alloys have narrow working temperature ranges, are often rolled
in packs or cans, and must be reheated frequently between passes. Mill products include sheets, bars, and shapes.
References cited in this section
45.
S.S. Hansen, Microalloyed Plate and Bar Products: Production Technology, in
Fundamentals of
Microalloying Forging Steels, G. Krauss and S. Banerji, Ed., The Metallurgical Society, 1987, p 155-
174;
also Microalloying '75, Union Carbide Corp., 1977
46.
"Reynolds Flexible Packaging," Pamphlet 410-1-1, Reynolds Metals Company, 1984
47.
J.H. Mendenhall, Ed., Understanding Copper Alloys, Olin Brass Corporation, 1977
Flat, Bar, and Shape Rolling
G. D. Lahoti, The Timken Company; S.L. Semiatin, Battelle Columbus Division
Heated-Roll Rolling
Heated-roll rolling is a process that was developed at Battelle Columbus Laboratories to roll difficult-to-work materials.
Heated-roll rolling is an isothermal or near-isothermal process in which the work rolls are heated to the same or nearly the
same temperature as the work metal. Heated-roll rolling is analogous to isothermal and hot-die forging (see the article
"Isothermal and Hot-Die Forging" in this Volume).
Heated-Roll Rolling of Sheet and Strip
In conventional hot rolling of high temperature materials, such as titanium and nickel alloys, the hot metal is deformed
between cold or warm rolls. This causes chilling on the surface of the rolled metal, resulting in higher working loads and
stresses than would be required if chilling were avoided. Further, chilling limits the maximum possible reduction per pass
and the minimum thickness attainable in conventional hot sheet rolling. From a materials viewpoint, conventional hot
rolling often requires rolling temperatures that are higher than optimal, which cause more workpiece contamination and
can result in microstructure and property variations in rolled products. To overcome some of the problems associated with
conventional hot rolling, the techniques of isothermal and near-isothermal hot rolling have been developed.
The technique has been demonstrated for sheet rolling on modified conventional rolling mills with either two-high or
four-high arrangements (Ref 48, 49, 50). In the two-high arrangement, where the rolls are relatively large, a composite
roll design has often been employed (Fig. 27). This consists of rolls with outer sleeves made of a high-temperature
superalloy and with cores of hot-work tool steels (such as AISI H13). Such a design satisfies the need for a roll with good
hot hardness at temperatures in the 815 °C (1500 °F) range at a cost less than a solid roll made from an expensive
superalloy. This setup is improved by induction heating the roll surfaces and by internal water cooling of the core of the
rolls and roll bearings. The viability of heated-roll rolling also has been demonstrated on a four-high mill (Ref 49). In this
case, the work rolls and the backup rolls were heated by banks of radiant heaters and had a design maximum operating
surface temperature of 815 °C (1500 °F) for the work rolls.
Fig. 27 Schematic showing composite roll construction and auxiliary equipment for heated-roll rolling.
Source:
Ref 48, 50
In studies conducted at Battelle Columbus Division, rolling was not performed at low deformation rates, and therefore did
not rely on the superplastic characteristics of the workpiece materials, which are often used in isothermal and hot-die
forging. Rather, these studies concentrated on determination of the workability of and uniformity obtainable in difficult-
to-work and temperature sensitive alloys in the absence of chilling. Thus, attention was focused on a much wider range of
alloys than those used in isothermal forging, including tungsten, beryllium, Ti-6Al-4V, alloy 718, and several alloy steels
and oxide-dispersion strengthened alloys. For powder metallurgy (P/M) consolidated and stress relieved tungsten, for
instance, workability was shown to be greatly improved with heated-roll rolling (Ref 48). This material is difficult to hot
work conventionally because of its ductile-to-brittle transition at temperature of 230 to 250 °C (445 to 480 °F). With rolls
at a temperature of 150 °C (300 °F) or lower, strip preheated to 260 °C (500 °F) and rolled at a speed of 6 m/min (20
ft/min) developed severe laminations and edge cracking because of chilling. In contrast, when the roll temperature was
raised to the preheat temperature of the strip (260 °C, or 500 °F), none of these problems was encountered.
The effect of heated rolls on temperature uniformity in rolling of strip from high-temperature alloys was quantified
through a series of heat transfer simulations performed on a digital computer (Ref 48, 49, 50). An example result is shown
in Fig. 28 for thin beryllium strip preheated to 540 °C (1000 °F) and rolled to 20% reduction in thickness at a rolling
speed of 30 m/min (100 ft/min). In this case, when the rolls are at room temperature, the temperature of the hot strip
decreases significantly during rolling, and large temperature gradients are present. Both of these factors may cause
workability problems. On the other hand, temperature decreases and thermal gradients through the strip thickness are
calculated to be comparatively small when the rolls are heated to a surface temperature of 540 °C (1000 °F).
Fig. 28 Predicted temperature profiles for beryllium strip preheated to 760 °C (1400 °F). Source: Ref 48, 50
Experiments at Battelle have also shown that heated-roll rolling lowers roll separating forces and enables larger
reductions per pass by eliminating or minimizing chilling effects. This is illustrated by the data on the rolling of Ti-6Al-
4V shown in Table 4. For example, an 80% reduction at a strip preheat temperature of 995 °C (1825 °F) required a roll
separating force of 28.9 kN (6500 lbf) at a roll temperature of 540 °C (1000 °F). With the rolls at 27 °C (80 °F) and the
strip at the same preheat temperature, a 62% reduction required more than twice the roll separating force measured in the
first case (Table 4).
Table 4 Rolling loads required for Ti-6Al-4V
Initial sheet thickness was 3.1 mm (0.12 in.); rolling speed was 32 m/min (105 ft/min).
Workpiece preheat
temperature
Roll surface
temperature
Rolling load per inch of
sheet width
Reduction,
%
°C °F °C °F kN
lbf
43 650 1200 27 80 95.7
21,500
53 840 1550 27 80 66.8
15,000
62 995 1825 27 80 75.7
17,000
51 650 1200 540 1000 77.9
17,500
68 840 1550 540 1000 64.5
14,500
80 995 1825 540 1000 28.9 6,500
Source: Ref 49
Like the microstructures obtained in isothermal forging, microstructures in isothermally rolled sheet have been found to
be very uniform. Thus, it has been suggested that in some cases one or more post-rolling heat treating steps may be
eliminated. This was demonstrated in the processing of 8670 alloy steel strips that were preheated to 840 °C (1550 °F),
rolled, quenched upon exit from the mill, and tempered for 2 h at 175 °C (350 °F) (Ref 51). At 840 °C (1550 °F), this
steel is totally austenitic. When the rolls were unheated, a coarse martensitic structure at the strip center, microstructural
non-uniformity near the surface, and rolling directionality were evident. However, when the rolls were heated to 840 °C
(1550 °F) to produce isothermal metalworking conditions, a fine, uniform martensitic microstructure with no
directionality was obtained. Therefore, heated-roll rolling of steel sheet and shapes which are to be subsequently hardened
may offer the advantage of eliminating the austenitizing and quenching stages.
Heated-Roll Rolling of Shapes
The application of the isothermal and heated-roll rolling concept to shapes also has met with success, although the
commercialization of the process, like that for flat rolling, has yet to be fully realized and accepted. One company, in
cooperation with Battelle Columbus Division, modified an existing two-high production rolling mill with 250-mm (10-
in.) diameter rolls into a heated-roll rolling setup. This setup was used to produce a structural L-shape to close tolerances
from Ti-6Al-4V and a high-temperature superalloy (Ref 49). Similar to designs used in Battelle's work, the company used
a composite roll design consisting of an AISI A9 tool steel core and a superalloy sleeve. Further, the rolls were heated by
banks of quartz-tube radiant heaters which Battelle had also shown to be feasible. With this tooling, 150 m (500 ft)--a
production-size quantity, of the structural shape--were successfully rolled for each of the two alloys. Compared to
conventional rolling practice, the heated-roll rolling of the superalloy required fewer rolling and other major operations.
A different isothermal rolling concept has been developed and applied for the rolling of structural shapes (channels, Z
sections, T sections, I sections, etc.) of various titanium and nickel-base alloys (Ref 52). In this method, molybdenum
alloy rolls are heated only locally by an electric current which is passed from one roll to another through the workpiece.
Thus, the workpiece can be heated by resistance heating during or just prior to rolling. It may also be preheated to a
temperature lower than the actual rolling temperature prior to being fed into the mill. Thus, oxidation and heating times
are reduced. In this scheme, local resistance heating aids in producing very thin sections found in many structural parts
and allows very large reductions (up to 90%) to be taken in a single pass. However, to obtain large reductions, it is
necessary to apply a "feed force," in addition to the "squeeze force" supplied by the rolls.
References cited in this section
48.
A.A. Popoff, J.A. Walowit, S.K. Batra, and R.J. Fiore
ntino, Development of a Process Utilizing Heated
Rolls for Hot Rolling Metals, Manufacturing Engineering Transactions, Vol 1, 1972, p 34-42
49.
T.G. Byrer, J.R. Douglas, D. Becker, J.D. Buzzanell, R.O. Kaufman, and H.L. Black, unpublished research,
Battelle Columbus Laboratories and Cyclops Corp., Sept 1975
50.
S.K. Batra and A.A. Popoff, On the Use of Heated Rolls for Hot Rolling of Metals, J. Eng. Mater. Technol.,
Trans. ASME, Vol 95H, 1976, p 27-35
51.
A.A. Popoff, T.G. Byrer, and R.J. Fiorentino, Studies on the Application of the Heated-
Roll Concept to Hot
Rolling Metals, Final Report on Contract N00019-69-
0288 to the Naval Air Systems Command, Battelle
Memorial Institute, June 1970
52.
W.J. Carpenter, F.K. Rose, and A.G. Metcalfe, unpublished resear
ch, Solar Turbines International, Oct
1974
Flat, Bar, and Shape Rolling
G. D. Lahoti, The Timken Company; S.L. Semiatin, Battelle Columbus Division
Defects In Rolling
A number of defects or undesirable conditions can develop in the rolling of flat, bar, or shaped products. Very broadly,
these problems can be attributed to one of four sources: melting and casting practice, metallurgical sources, heating
practice, and rolling practice.
Melting and Casting Practice. The major problems associated with melting and casting practice are the development
of porosity and a condition known as scabs. Porosity is developed in cast ingots when they solidify and is of two types:
pipe and blow holes. Pipe is a concave cavity formed at the top of the ingot due to nonuniform cooling and shrinkage. If
not cropped off, pipe can be rolled into the final product to form an internal lamination (Fig. 29). These laminations may
not be immediately evident following rolling, but may become apparent during a subsequent forming operation. The
occurrence of laminations is most prevalent in flat-rolled sheet products. Blow holes are usually a less serious defect.
They are the result of gas bubbles entrapped in the metal as the ingot solidifies. If the surfaces of holes are not oxidized,
they may be welded closed during the rolling operation.
Fig. 29
Laminations in rolled steel sheet resulting from insufficient cropping of the pipe from the top of a
conventionally cast ingot. Courtesy of V. Demski, Teledyne Rodney Metals
Scabs are caused by improper ingot pouring, in which metal is splashed against the side of the mold wall. The splashed
material, or scab, tends to stick to the wall and become oxidized. Scabs usually show up only after rolling and, as can be
expected, give poor surface finish.
Metallurgical Sources. Defects such as poor surface finish may also result from a metallurgical source, nonmetallic
inclusions. In steels, inclusions are of two types, refractory and plastic. Refractory inclusions are often metallic oxides
such as alumina in aluminum-killed steels or complex oxides of manganese and iron in rimmed steels. When near the
surface, such inclusions give rise to defects known as seams and slivers (Fig. 30).
Fig. 30 (a) Seams and (b) slivers caused in rolled material by the presence of surface inclusions.
Courtesy of V.
Demski, Teledyne Rodney Metals
Plastic inclusions, such as manganese sulfides, elongate in the rolling direction during hot forming. The presence of these
elongated inclusions (stringers) produces fibering, which causes directional properties. For example, ductility transverse
to the fiber is frequently lower than that parallel to it. In high-strength low-alloy steels, sulfide shape control elements,
such as cerium, are often added to prevent the development of such fibering, which is especially undesirable from the
viewpoint of subsequent forming operations or service behavior.
Another rolling defect whose source is metallurgical is alligatoring (Fig. 31). This defect, found most frequently in rolled
slabs of aluminum-magnesium, zinc, and copper-base alloys, is manifested by a gross midplane fracture at the leading
edges of the rolled metal.
Fig. 31 Alligatoring in a rolled slab. This defect is thought to be caused by nonhomogeneou
s deformation and
nonuniform recrystallization during primary rolling of such metals as zinc alloys, aluminum-
magnesium alloys,
and copper-base alloys. Courtesy of J. Schey, University of Waterloo
Heating Practice. Two rolling defects that stem from heating practice are rolled-in scale and blisters. The development
of scale during preheating of ingots, slabs, or blooms is almost inevitable, particularly for steels. Sometimes descaling
operations involving hydraulic sprays or preliminary light rolling passes are not totally successful; scale may get rolled
into the metal surface and become elongated into streaks during subsequent rolling. The other defect, blistering, is a raised
spot on the surface caused by expansion of subsurface gas during heating. Blisters may break open during rolling and
produce a defect that looks like a gouge or surface lamination.
Rolling practice can cause defects also. In bar and shape rolling, for example, excessive reduction in the finishing pass
may cause metal to extrude laterally in the roll gap, leading to a defect known as finning. Finning in an intermediate pass
causes folds or laps in subsequent passes. Excessive reduction in the leader pass (the pass prior to the finishing pass) also
may wrinkle open the sides of a bar, which after turning 90° in the finishing pass can result in a series of hairline cracks.
In the rolling of slabs and plates, two defects that affect yield are fishtail and overlaps, both of which need to be trimmed
off (Fig. 32). The former results from nonuniform reduction in the width, and the latter results from nonuniform reduction
in thickness. These defects can be reduced by proper design of the blooming and rolling sequence. Several defects, such
as wavy edges, center buckle, herringbone, and quarter buckle can be created in cold rolling of sheets and strips. These
defects are primarily due to localized overrolling, which can occur because of improper roll profiles or variations in the
properties or shape of the incoming strip.
Fig. 32 Overl
ap and fishtail formed during rolling of slabs and plates. Overlap is the result of nonuniform
deformation in thickness, while fishtail is caused by nonuniform deformation in width.
Flat, Bar, and Shape Rolling
G. D. Lahoti, The Timken Company; S.L. Semiatin, Battelle Columbus Division
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9.
G.D. Lahoti, S.N. Shah, and T. Altan, Computer Aided Analysis of the Deformations and Temperatures in
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1974, p 2479
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(Trans.
ASME), May 1971, p 477
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J.H. Hitchcock, "Roll Neck Bearings," Research Committee Report, American Society of Mechanical
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