Note: Descriptions are shown in the official language in which they were submitted.
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1647~36
This invention relates to a terne coated steel
article, the coating containing about 2% to about 6% tin and
balance lead, and to a method for production thereof.
The generally accepted and published composition
range for terne coating is 8% to 20% by weight tin, with the
balance commercially pure lead ~ASTM 3Q8). According to
Federal Specification QQ-T-19lB, the impurity level for ele-
ments other than lead and tin is 1%. When tin became scarce
during World War II about 1944, work was done on low-tin terne
(about 2%) but zinc, antimony, silver, and phosphorus had to
be added to the bath in order to produce a sound coating. At
that time a minimum of 9% tin was required in the terne to
make an acceptable hot dip coating without the addition of
tin substitution elements.
With the high price of tin relative to the pric0
of lead, it has become very desirable to provide a low-tin
terne, provided the qualities of the high tin terne can be
maintained or enhanced.
The object of terne coating is to provide an inex-
pensive, corrosion resistant coating of lead on a steel base,
and the percentage of the tin in the terne need only be suffi-
cient to obtain a smooth continuous coating. Furthermore,
since lead does not alloy with iron, the tin addition is
; necessary in order to alloy with the stael base and form a
metallurgical bond between coating and base metal.
In prior terne coating procedures the coating was
finished by means of exit rolls turning in an oil bath in
order to obtain a good finished appearance. If the tin content
of the bath was below about 9%, the exit rolls, normally coated
with molten metal, would de-wet and imprint the finished
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product. This would cause rejection and scrap.
- Recently jet nozzles have come into use in the
finishing of zinc and aluminum coatings on steel strip, as
well as terne coating. By the use of jet nozzles in the
finishing of terne coated steel, much lower tin levels have
become possible, since the high tin levels needed to keep
the exit rolls wetted are no longer needed.
Thus, with the tin content between about 2~ and
about ~ by weight, and using jet finishing rather than the
conventional exit rolls, in accordance with the present
invention, it is possible to produce a low tin terne coating
having improved solderability with no change in pinhole
frequency, and improved appearance, and with greater ease of
coating weight control.
`15 As mentioned above, a lead coating on steel is
; useful for many purposes. Terne coated sheets are exten-
sively used in the manufacture of gasoline tanks, for the
~ractor, truck, automotive and marine industries. They are
also used in radiator parts, mufflers, oil pans, and air
cleaners in those industries. Terne coated steel also finds
uses in such areas as roofing, hand fire extinguishers,
; electronic chassis and burial caskets.
The basic problem with putting a lead coating onto
a steel base is that the lead does not alloy with iron. It
is therefore necessary to add an ingredient which will alloy
with iron and cause the lead coating to wet and bond metallur-
- gically to the steel base. Long ago it was found that a very
suitable alloying ingredient was tin which alloys readily with
the steel base and forms a solid solution with the lead. This
lead-tin coating alloy became known as terne coating.
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6~
According to the invention there is provided a hot dipped
terne coated steel article having improved solderability and appearance,
the coating consisting of about 2% to about 6% by weight tin, and the
balance lead plus impurities.
The method of this invention is characterized by passing
a steel article through a bath containing from about 2% to about 6% by
weight tin, and the balance lead plus impurities, and immediately upon
its issuance from the bath subjecting it to a jet finishing operation.
Various attempts have been made over the years to provide
a low-tin terne coating but it was always found necessary to add other
elements such as zinc, antimony, silver, or phosphorus or combinations of
these in order to obtain a reasonably satisfactory and saleable product.
Jet finishing has recently come into use with zinc coating.
The j et finishing process and apparatus is described in detail in United
States of America Patent No. 3,314,163 issued to J.B. Kohler. By applying
; the jet finishing procedures to the terne coating process, it is possible
to greatly reduce the tin content of the te~ne coat to between about 2%
and about 6% with a consequent reduction in cost, and while maintaining,
and ev en enhancing the qualities of the terne coating.
In the preferred practice of the present invention strip
is preannealed forming a blue oxide surface. Surface preparation o the
; base metal may be more or less standard. By way of example, it may include
the usual steps of pickling, scrubbing, prefluxing and bath entry through
a flux cover. Preferably the pickling solution will contain from 8% to 15%
sulfuric acid and from 4% to 15% ferrous sulfate. Sodium chlorate
may be added as a pickle accelèrator. The temperature
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of the pickling bath is generally from 85C to about 100C.
The pickling step removes the surface oxides resulting from
prior annealing. Pickling may also be accomplished in a
hydrochloric acid bath. It will be understood that the pre-
flux and flux steps may be combined in a single flux treàtment.
After pickllng~ the plckler smùdge may be removedby brushing in a water rinse. Prefluxing may ba accomplished
by a high speed tinning flux such as is manufactured by duPont,
maintained at between about 7 and about 15 Baume, and
agitated and suspended in the solution by air or steam. The
preflux acids aid in removing iron salts and acid radicals
from the strip surface. A commercially available bath cover ~ -
such as duPont's ZACLON A galvanizing flux may be used. This
is a molten zinc chloride flux with ammonium bromide or
ammonium chloride dissolved it it. This would be added to a
flux box which immediately precedes the terne pot.
The conventional terne coatlng bath has the
~ollowing analysis.
Antimony .5 to .9%
Aluminum .005% Max.
Arsenic .02% Max.
Bismuth .10% Max.
Copper .05% Max.
Iron ~ .02% Max.
Silver .05% Max.
Zinc - .01% Max.
Tin 12~ - 15%
Total Others .l~ Max.
Lead Balance
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Only tin is purposely addedO Other elements are
lmpurities. Relatively large amoun*s of antimony for example
result from the reclamation of storage battery lead for terne
use and is neither a hindrance nor a significant help in terne
coating.
According to the present invention, the tin content
of the terne coating bath may be from about 2~ to about ~
and other alloying ingredients may be eliminated. The lead
and impurity content will therefore be from about 98% to about
~ . The terne coated metal emerging from the coa~ing bath is
subjected to jet finishing as taught in Kohler U.S. Patent No.
3,314,163. The jet finishing apparatus will not be described
since it does not per se form a part of the present invention;
and reference may be had to said patent for detailed descrip-
tion thereof.
~ The terne coated steel according to the presentinvention has improved solderability over the conventional
higher tin terne coating, and an improved appearance because
~ a smoother coating is produced by jet finishing and the coating
can be more easily burnished to a uniform sheen. Furthermore,
the ~ost is reduced with refèxence to the conventional terne
coating. The pinhole frequency remains unchanged. As far as
- coating weight is concerned, control is easier than with
the high tin terne bath; and it is-possible to produce, with
much greate-r ease,"very light coating weights such as 0.05
oz./ft.2 (,15.25 g/l2) as-against 0.18 oz./ft.2 (61 g/m ~
which was àbout the lightest which could be produced consis-
tently heretofore.
It should be understood that with a reduction in
tin content, the temperature of the coating bath must be
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increased slightly. This additional cost is nominal, and is
far more than compensated for by the savings in tin.
Minor modifications may of course be.made without
departing from the spirit of the invention; and therefore no
limitation except as set forth in the claims is intended,
and no such limitation should be implied~
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