Page 338 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 13 Metal»Rolling Processes and Equipment
will result in a product with anisotropic properties (due to preferred orientation or
mechanical fibering; see Section 1.6).
Plates generally have a thickness of more than 6 mm and are used for structur-
al applications, such as ship hulls, boilers, bridges, machinery, and nuclear vessels.
Plates can be as thick as 300 mm for large structural supports, 150 mm for reactor
vessels, and 100 to 125 mm for machinery frames and warships.
Sheets generally are less than 6 mm thick and typically are provided to
manufacturing facilities as coils-weighing as much as 30,000 kg-or as flat sheets for
further processing into various products. Sheets typically are used for automobile and
aircraft bodies, appliances, food and beverage containers, and kitchen and office
equipment. Commercial aircraft fuselages and trailer bodies usually are made of a
minimum of 1-mm thick aluminum-alloy sheets. For example, the skin thickness of a
Boeing 747 fuselage is 1.8 mm and of a Lockheed L101 1 is 1.9 mm. Steel sheets used for
automobile and appliance bodies are typically about 0.7 mm thick. Aluminum
beverage cans are made from sheets 0.28 mm thick. After processing into a can, this
sheet metal becomes a cylindrical body with a wall thickness of 0.1 mm. Aluminum foil
(typically used for wrapping candy and chewing gum) has a thickness of 0.008 mm,
although thinner foils down to 0.003 mm also can be produced with a variety of metals.
This chapter describes the fundamentals of flat-rolling and various shape-
rolling operations, examines the production of seamless tubing and pipe, and
discusses the important factors involved in rolling practices.

13.2 The Flat-rolling Process

A schematic illustration of the flat-rolling process is shown in Fig. 13.2a. A metal
strip of thickness lr() enters the roll gap and is reduced to thickness lay by a pair of
rotating rolls, each powered individually by electric motors. The surface speed of the
rolls is V,. The velocity of the strip increases from its entry value of VO as it moves
through the roll gap; the velocity of the strip is highest at the exit from the roll gap
and is denoted as Vf. The metal accelerates in the roll gap in the same manner as an
incompressible fluid flowing through a converging channel.
Because the surface speed of the rigid roll is constant, there is relative sliding
between the roll and the strip along the arc of contact in the roll gap, L. At one point
along the contact length (called the neutral point or no-slip point) the velocity of the
strip is the same as that of the roll. To the left of this point, the roll moves faster than
the strip; to the right of this point, the strip moves faster than the roll. Consequently,
the frictional forces--which oppose motion between two sliding bodies-act on the
strip as shown in Fig. 13.2b.
The rolls pull the material into the roll gap through a net frictional force on the
material. Thus, the net frictional force must be to the right in Fig. 13.2b. This also
means that the frictional force to the left of the neutral point must be higher than the
friction force to the right. Although friction is necessary for rolling materials (just as
it is in driving a car on a road), energy is dissipated in overcoming friction. Thus,
increasing friction also increases rolling forces and power requirements. Furthermore,
high friction could damage the surface of the rolled product (or cause sticking, as can
occur in rolling dough). Thus, a compromise is made in practice: Low and controlled
friction is induced in rolling through the use of effective lubricants.
The maximum possible draft is defined as the difference between the initial and
final strip thicknesses, or (loo - hf). lt can be shown that this quantity is a function