Steels_ Metallurgy and Applications, Third Edition

86 Steels: Metallurgy and Applications

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Figure 1.98 Hot dip galvanizing line

an accumulator or storage station. This enables the process section of the line to
function whilst the cutting and joining operations are being carded out.
The strip then passes into a heat treatment furnace with an inert atmosphere.
Initially, the strip is heated to a temperature of about 680~ but electrolytic
precleaning is now often used before entry and the material then enters the
annealing section of the furnace. In this section, the strip attains a temperature
of about 730~ to 850~ and the cold-rolled strip recrystallizes. Still under a
protective atmosphere, the strip is cooled rapidly by means of a gas stream,
before entering the zinc bath which is maintained at a temperature of 465~
Small amounts of aluminium, typically 0.15%, are added to the bath to restrict
the thickness of the Zn-Fe layer and thus provide an adherent coating that can
accommodate major strains in subsequent forming operations. The aluminium
reacts preferentially with the steel as it enters the galvanizing bath, forming a
thin layer of Fe-AI compounds (FeA13, Fe2A15), and thereby retards the reaction
between iron and zinc. The lead content of the bath affects the zinc crystal size of
spangle on the surface of the strip, and for applications requiring a minimum or
spangle-free appearance, the lead level of the bath is held below 0.15%. However,
in order to develop a spangle, additions of antimony are now preferred to high
lead contents because they result in less intergranular corrosion.
After the coating operation, the strip passes through a series of gas-knives
which use air, nitrogen or steam to control the amount of zinc on the surface of
the steel. Depending on the end application, the strip may then be temper rolled
or stretch levelled to provide shape control or to eliminate the formation of strain
markings (Lgiders lines) during cold forming. The final stage in the process is
generally a passivation treatment in which the strip is sprayed with a solution of
chromic acid. This inhibits the formation of a corrosion product known as white
rust which will be discussed later.