Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DIFFERENTI~LLY COATED GALVANIZED STEEL STRIP
,
BACKGROUND OF THE INVENTION
The present invention relates generally to hot dip
coated galvanized steel strip and more particularly to a
differentially coated, galvanized steel strip wherein one of
two opposite strip sides has a relatively light coat composed
of iron-zinc alloy and the other strip s;de has a relatively
heavy coat at least the outer part of which consists
essentially of ~inc. The present invention also relates
particularly to methods and apparatuses for producing such a
differentially coated, galvanized steel strip~
~ differentially coated, galvanized steel strip of
the general type described in the preceding paragraph, and a
method and apparatus for producing such a galvanized steel
strip is described in Patil et al., U.S. Patent No. 4,1~1,394
issued October 16, 1979, and entitled "Process of ~ot-dip
Galvanizing and Alloying".
A differentially coated~ gal~anized steel strip of
the general type described in said Patil et al. patent is
2Q produced by passing a continuous steel strip through a bath
of molten coating metal consisting essentially of zinc to
coat ~oth sides of the strip with the molten coating metal.
Immediately upon withdrawal of the coated metal strip from
the bath of molten coating metal, the weight of the molten
coating metal on opposîte sides of the strip is adjusted by
impingement against opposite sldes of the strip of jets of
gas or steam. The jets on respective opposite sides of the
strip are adjusted to provide one strip side with a
relatively light coat of the coating metal and the other strip
3Q side with a relatively heavy coat of the coating metal.
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Typically, the weight of the coating metal on the light
coated side is in the range 0.05-0.25 oz./ft.2.
~0.015-0.075 kg/m2~, and the weigh.t of the coating metal
on the heavy coated side is in the range 0~35-1.0 oz./ft.2
(:0.105-0.30 kg/m21.
~: Immediately following th.e weight adjusting step,
the strip is passed through a treating zone wherein
simultaneously the ligh.t coated strip side undergoes
heating and th.e heavy coated strip side under~oes cooling.
As a result, the coating metal on th.e light coated side
is transformed to iron-zinc alloy throughout. It is
esired~ as a result of th.e treating step, that the
coating metal on the heavy coated side be only partially
transformed to iron-zinc alloy, and that the outer part
of the coating on the heavy coated side consists essentially
of zinc. In a method in accordance with.the Patil et al.
patent, the heavy coated side is at least partially molten
at the time the simultaneous hRating and cooling step is
performed.
: 2Q In a typical commercial product employing the
subject matter of the Patil et al. patent, the weight
of the coating metal on the heavy coated side is about
0.50 oz./ft.2 (:0 15 kg~m2~. In such a product, the
iron-zinc alloy on the heavy coated side constitutes an
inner layer wh.ich ranges from 30 to 50~ of the coating thick-
ness on that side. Occasionally, ho~ever, as a result of
the simultaneous heating and cooling step, in some spots
the iron-zinc alloy may extend all the ~ay through to the:outer
surface on th.e heavy coated side causing a defect known as
3Q "bleed-through". Th.ese bleed-throughs occur intermittently
along the outer surface of the heavy coated strip side.
o
In cross-section, a bleed-through resem~les the
vertical cross-section of a mushroom Bleed-throughs are
undesirable because the iron-zinc alloy in the bleed-through
area at the strip's outer surface tends to powder when the
galvanized steel str;p is subjected to a stamping operation,
and this is undesirable. Although the light coated side of
the strip consists entirely of iron-zinc alloy throughout,
that coating is relatively so t~in that it can undergo a
stamping operation without powdering. The fully alloyed
light coated side is readily paintable.
Galvanized steel strip with bleed~through on the
heavy coated s;de is not acceptable to those who fabricate
the galvanized steel strip into products, and such a strip
is not saleable. A typical customer for differentially coated
galvanized steel strip is a stamping shop making parts for
the automotive industry.
Recently there has been a demand, particularly from
the automotive industry, for a differentially coated,
galvanized steel strip in which the heavy coated side has a
thinner coating, e.g., a weight substantially less than 0.50
oz./ft.2 (0.15 kg/m21, typically in the range 0.25--0.45 oz./
ft.2 C0~075-0.135 kg~m21. ~ thinner coating on the heavy
coated side makes the strip more weldable. However, the
thinner the coating on the heavy coated side, the more likely
there is to be a bleed-through when the differentially coated
strip is subjected to a simultaneous heating and cooling
treatment of the type described in the Patil et al. patent.
When the heavy coated side has a coating weight of
0.50 oz./ft.2 ~0.15 kg/m2~, a strip with a bleed-~through
occurs only occasionally, and the strip rejection rate for
this defect is about 3~4%. When the coating on the heavy
coated side has a weight su~stantially less than 0.50 oz./ft.2
(0.15 kg/m2~, it is virtually impossible to prevent bleed-
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throughs when employing a process in accordance with the
Patil et al. patent, and the iron-zinc alloy may constitute
.
100% of the coating thickness on the heavy coated side.
Summary of the Invention
I~ accordance with the present invention, the heavy
coated strip side has a coating we;ght less than 0.50 oz./ft.2
(0.15 kg/m22, but intermittent bleed-throughs of iron-zinc
alloy at the outer surface of the heavy coated side of the
differentially coated, galvanized steel strip are prevented.
~his is accomplished by precooling the heavy coated side of
the strip between the weight adjusting step and the simul-
taneous heating and cooling step. The precooling step sub-
stantially fully solidifies the molten coating metal on the
heavy coated side before the start of the simultaneous
heating and cooling step~
The weight adjusting step which precedes the precool-
ing step provides a coating metal thickness on the heavy
coated side of the strip ~hlch, absent the precooling step,
is thick enough to be at least partially molten at the start
2Q of the simultaneous heating and cooling step but not thick
enough to avoid intermittent bleed~throughs as a result of
the simultaneous heating and cooling step.
Iron diffuses in zinc less rapidly ~hen the zinc
is solid than when the zinc is molten~ Because the precool-
ing step fully solidifies the coatîng metal on the heavy
coated side, there is reduced diffusion of iron in the coating
on the heavy coated side during the simultaneous heatlng and
cooling step, compared to the diffusion which would occur if
the coating on the hea~y coated side was not solidified before
3Q the start of the simultaneous heating and cooling step.
Because of the reduced diffusion, bleed~through is prevented.
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The precooling step is accomplished by impinging a
fluid-cooling medium against the heavy coated strip side.
This fluid-cooling medium is preferably steam but may also
comprise a;r, nitrogen, or inert gases.
~hen the heavy coated side is imp;nged ~ith a fluid-
cooling medium such as steam, during the precooling step,
the impinging cooling medium causes waves or 'rsag lines" on
the solidified coating metal surface. Such minor surface
irregularities are undesirable because they "print through"
on the reverse side of the galvanlzed steel strip during
stamping. Ilowever, during the simultaneous heating and cool-
ing treatment, following precGoling, there ;s superficial
melting of a heavy coated side having a thickness in accord-
ance with the present invention, there is relatively rapid
solidification oF the heavy coated side following the heating
and cooling step, and the sag lines or other minor surface
irregularities are eliminated.
On a differentially coated, galvanized steel strip
produced in accordance with the present invention, there are
no sag lines, the spangle boundaries are flat (i.e., level
with the surface of the coating on the heavy coated side~
and the outer surface on the heavy coated strip side is rela-
tively smooth compared to the heavy coated side on a dif.
ferentially coated strip in accordance with prior art pro-
cedures. Because that surface is so smooth, it is un-
necessary to skin roll the strip as heavy as was necessary
with a less smooth surface~ Because the strip is subjected
to less skin rolling, it is more ductile and has better
formàbility.
Other features and advantages are inherent in the
subject matter claimed and dlsclosed or will become apparent
to those skilled in the art from the following detailed des-
cription in conjunction w;th the accompanying diagrammatic
drawings.
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Brief Description of the Drawings
Fig. l is a diagrammatic view, partially in section,
illustrating a method and apparatus in accordance with an
embodiment of the present invention; and
Fig. 2 is an enlarged, fragmentary, sectional view
illustrating a differentially coated, galvanized steel strip
in accordance with an em~odiment of the present invention.
Detailed Description
.
Referring initially to Fig~ l there is illustrated
an embodiment of a method and apparatus for producing a dif-
ferentially coated, galvanized steel strip in accordance
with the present invention. Indicated generally at lO is an
uncoated steel strip having a composition conventionally
utilized for a continuous, hot-dip galvanizing process, such
as the conventional Sendz;mir~type process. The strip is
moved in the direction of the arro~s along a processing path
illustrated in Fig. l, employing conventional equipment for
moving the strîp.
In the course of moving along this path, strip 10
passes over a turn-down roller 12 located within a hood 11
containing a reducing atmosphere. Hood ll extends into a
bath 13 of molten coating metal consisting essentially of
zinc and having a bath temperature in the range 850-880 F
(454-471C~ preferably 865F ~463C~. 5trip lO has been
preheated and enters bath 13 at essentially the same
; temperature as the bath. Bath 13 may contain other elements
conventionally employed in galvan~zing compositions, and an
example thereof is disclosed in the a~orementioned Patil et al.
4rl71~394. Preferably, the bath contains 0.14-0.16 wt.%
aluminum.
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Strip 10 passes around a roller 14 located within
bath 13 and then passes up~ardly out of the bath as a gal-
vanized strip 15 containing substantially equal weights of
coatin~ metal on opposite sides of the strip. Galvanized
strip 15 passes upwardly between a pair of steam jet nozzles
17, 17 adjustable to control the weight of coating on op-
posite sides of strip 15. The galvanized strip is at a
temperature typically in the range 850 ~88Q F (454 -471 C~.
The steam jets from nozzles 17, 17 are at a temperature in
the range 300-350F (148-176C), so that the jets
partially cool strip 15 as well as control the ~eight of
the metal on opposite sides of the strip.
Conventionally, the steam jets are adjusted to
provide one strip side 27 with a relatively light coat of
the coating metal and the other strip side 28 with a
relatively heavy coat of the coating metal. As a result
there is produced a differentially coated, galvanized steel
strip indicated generally at 18.
Typically, the coating metal weight on light coated
side 27 is in the range 0.05-0.15 oz./ft.2 (Q.015-0.045
kg/m2~, and the coating metal weight on heavy coated strip
side 28 i5 typically in the range Q.25-0.45 oz./ft.2
(0.075-0.135 kg/m ~, in accordance with the present
invention. In other words the coating metal thickness on
strip side 28 is less than 0.50 oz~/ft~ (Q.15 kg/m2).
In one embodiment, strip side 28 has a coating thickness
less than 0.35 oz./ft.2 (Q.105 kg/m21, e.g., 0.32 oz./ft.2
(0.096 kg/m21 minimum.
The coating on side 27 is thin enough so that the
coating is fully solidified when strip 18 enters a treating
chamber 22 located above weight-controlling nozzles 17, 17.
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On the other hand, the coating on side 28 is thick enough to
be at least partially molten when the stxip enters chamber 22,
absent a precooling step ~etween nozzles 17, 17 and treating
chamber 22. The cooling effect imparted by steam jet nozzles
17, 17 is not enough to fully solid;fy the coating on side 28.
In treating chamber 22, differentially coated steel
strip 18 is subjected to s;multaneous heating of light coated
strip side 27 and cooling of ~eavy coated s~trip side 28.
~his transforms the coating metal on t~e light coated side
to iron-zinc alloy throughout wh~le desirably only partially
transforming the coating metal on strip side 28 to iron-zinc
alloy.
The heating operation is performed ~y a plurality
of gas jet nozzles at 23, 23 which provide heating flames
directed toward light coated strip side 27, and the galvanized
strip is heated thereby to a temperature in the range
860-930F (460-499C~ or higher. The determining factor as
regards strip temperature is to heat the strip to a tempera-
ture which will fully alloy the light coated side. The cool-
ing operation is provided by a plurality of air jet nozzlesat 24, 24 directed toward heavy coated strip side 28. The
air jets are at ambient temperature, e.g., 60F (16 C~.
In chamber 22, the gas~jet nozzles at 23, 23 com-
municate with a mani~old 36 into which gas is supplied via a
conduit 37. The air jet nozzles at 24, 24 communicate with a
manifold 38 into which air is supplied via a conduit 39.
When heavy coated strip side 28 has the coating
weight descri~ed five paragraphs above, there can be a
problem with intermittent ~leed~throughs of iron-zinc alloy
to the surface of strip side 28 as a result of the simul-
taneous heating and cooling step. To avoid such ~leed~oughs
there is provided, in accordance ~ith the present invention,
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a precooling step between the weigh.t adjusting step and the
simultaneous heating and cooling step. This precooling step
is performed with one or more banks of steam nozzles 20 con-
nected by a conduit 21 to the same steam source as provides
steam to nozzles 17, 17. Steam at a temperature in the range
300-350F ~148C-176Cl is directed toward heavy coated strip
side 28 to cool strip side 28 and fully solidify the coating
metal on that strip side.
As previously noted, strip side 28 is still at least
partially molten after passing between we;`gh:t adjusting
nozzles 17, 17, Because the precooling step fully solidifies
the relatively heavy coating on strip side. 28 before strip 18
enters chamber 22, th.e diffusion of iron in the heavy coat
during the simul.taneous heating and cooling step is substan-
tially reduced compared to the diffusion which ~ould occur
if the heavy coat were not fully solidi-.'ied ~efore the start
of the. simultaneous h.eating and caoling step.
When the coating metal consists essentially of zinc,
it has a melting point typically about 78QF ~416C), and
the coating metal on the heavy coated side should be cooled
to a temperature at least 18 F Cll C~ belo~ that melting
point to fully solidify the coating.
The precooling step not only fully solidifies heavy
coated side 28 but probably also l~ght coated side 27.
However, during the treating step, because the coating on
side 27 is so lightl it melts immediately upon subjection
to the heating flames in ch.amber 22, allo~ing rapid dif-
fusion of iron in the light coating so as to fully alloy
the latter. The heavy coated side, however, undergoes only
superficial melting during the treating step, and there is
insufficient di.ffusion of iron to fully alloy or cause
bleed-throughs in the heavy coated side.
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The precooling medium is prefe.rably steam (either
wet or dry~. Steam is conveniently availa~le at the site of
the precooling step because the same fluid medium is utilized
at weight-controlling nozzles 17/ 1~. H.owever, other fluid
cooling media may he employed, such as air, nitrogen and
inert gases.
Whatever its composition, sufficient fluid cooling
medium must ~e employed to cool the coating metal on heavy
coated strip side 28 to a temperature belo~ its melting point
and fully solidify the coating. Although only one ~ank 2Q
of precooling nozzles is illustrated in Fig. 1, two or more
banks may be employed when necessar~ to cool heavy coated
side 28 to the temperature required to obtain a fully
solidified coating on side 28. ~hen two or more banks are
employed, they are arranged at spaced locations between
nozzles 17, 17 and chamber 22.
Staam nozzles 20 should be positioned upstream of
chamber 22. The steam nozzles should not be located within
substantially closed chamber 22 because of possible corrosion
2Q problems.
As noted above, during the precooling step, the jets
of fluid cooling medium from nozzles 20 are directed against
a coating on strip side 28 which.is at least partially
molten. This causes minor surface i.rregularities at the
outer surface on strip side 28. These minor surface
irregularities are in the form of "sag lines'r or ~aves on
the solidified outer surface on heavy coated strip side 28.
T~hen strip 18 enters treating cham~er 22, heavy
coated strip side 28 is in a fully solidified condition.
However, during the simultaneous heating and cooling step
in chamber 22, there is superficial.melting of the coating
metal on the heavy coated strip side at the outer surface
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thereof. Immediately follo~ing the heating and cooling step
there is relatively rap~d solidification of the heavy coated
side, compared to a heavy coated side with a thicker coating
deposit in accordance with prior practices. A process in
accordance with the present invention smooths out the minor
surface irregularities resulting from the precooling step,
and it also flattens spangle ~o~mdaries so that they are
level with the outer surface of the heavy coated strip side.
It is desira~le to eliminate these minor surface irregularities
because they can "print throughll on the reverse side of the
galvanized steel strip during a su~sequent stamping operation
performed on that strip.
The differentially coated, galvanized steel strip
exiting chamber 22 is indicated at 26. This strip is con-
ventionally subjected to a "skin rollingl' step at an in-line
skin rolling station 41 downstream of cham~er 22, employing
conventional skin rolls. Alternatively~ skin rolling may be
performed out of line, after the strip has ~een otherwise
processed and coiled. ~hen skin rolling is performed out of
2a linel the cooled strip is uncoiled, skin rolled and then re-
coiled.
For a differentially coated strip produced in accord-
ance with the present invention, the outer surface of the
heavy coated strip side is relatively so smooth before skin
rolling that, in order to obtain the desired surface texture
on the outer surface of the heavy coated strip side, it is
not necessary to skin roll so heavily as before in order to
obtain that desired surface texture. More specifically~ a
skin roll producing a deformation below 1~, e.g., in the
3a range 0. 5-a. 8~, is employed in accordance with, the present
invention compared to a deformation i`n the range 1.0-1.4~
on differenti,ally coated strlp produced in accordance ~ith
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prior practices. Because deformat;on is comparatively less
when the strip is produced in accordance with the present
invention, the strip is comparat~vely more ductile and has
better forma~ilit~.
Except for Cll the precooling step employing
nozzles 20, ~2~ the adjustment of the weight controlling
step to reduce th.e t~icknes~ of the coating on the heavy
coated side and C3~ th.e decrease ;`n deformation during
skin roll;ng, all in accordance with. the present invention,
the process conditions employed herein are essentially the
same as those described ln Patil et al., U.S. Patent No.
4,171,394. Nevertheless, the resulting differentially
coated, galvanized steel strip of the present in~ention
has a ductility about 2~ higher on a scale of 100% than the
same strip not produced with the processing differences
described in the preceding sentence. More particularly,
where a strip produced in accordance with the processing
conditions of said Patil et al. patent would h.ave a typical
ductility after skin rolling in the range 40~43%, the same
strip produced in accordance with the present invention
would have a ductility after skin rolling in th.e range
42-45~. The figures in the preceding sentence would be
representative of an in-line heat treated, drawing quality,
aluminum killed steel.
Referring again to Fig. 1, ind~cated at 40 is a
drive roll located between chamber 22 and skin rolling
station 41, and located at the downstream end of the
processing path is a coiler 42.
The differentially coated, galvanized steel strip,
produced in accordance with the present invention, is
indicated generally at 2~ in Fig~ 2. Strip 26 comprises
a steel substrate 30 havin~ a pai`.r of opposite sides coated
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with a coating metal consisting essentially of zinc. One
of the strip sides has a relatlvely light coat composed of
iron-zinc alloy t~roughout, and th;s is indîcated at 31 in
Fig. 2. Th.e other strip slde has a relatively heavy coat,
at least the outer part of the heavy coat consisting es-
sentially of z~nc ~ndicated at 32 ~n Fig. 2. Located ~e-
tween zinc outer part 32 and steel su~strate 3Q is iron-
zinc alloy indicated at 33. The thickness of iron~zinc
alloy 33 on the heavy coated side of the strip varies, but
it is always less than 25~ of the thickness of th.e heavy
coat, as a maximum, and the average thickness ;s a~out 10%~
AS shown in Fig. 2, there are no interm;ttent iron-
zinc alloy bleed-throughs at the outer surface 34 of the
heavy coated strip side. The coating weights on the res-
pective ligh.t and heavy coated sides are as descrihed above
for the present invention. When the heavy coated strip side
: has a coating no greater than that described for the present
inventionl the a~oidance of intermittent bleed-throughs of
iron-zinc alloy at the outer surface 34 on the heavy coated
side would be virtually impossible a~sent the precooling
step performed by nozzles 2Q.
: ~ Moreover, in a strip made in accordance with the
: present invention, the outer surface 34 on the heavy coated
side is krighter than on conventionally produced strips, so
that on the rare occasion when a bleed-through may occur
Ce.g., ~ecause of some malfunction of the process or ap~
paratus~ such a bleed~throug~.can be readily spotted by an
operator and appropriate steps can be taken.
A bleed-through is to ~e distînguished from the
3a average thickness of the iron~zinc alloy layer on the heavy
coated side. In conven~ional differentially coatecl gal~
vanized steel strip, the average thickne.ss of the iron-zinc
alloy can be substantially less than the full thickness of
the metal coating on that side~ but a ~leed~through may still
~e present. In a differentially coated, galvanized steel
strip produced in accordance with the present invention, the
iron-zinc alloy layer on the heavy coated side is less than
25% of the thickness of the coat on that side, as a maximum,
with the average thickness ~eing la%, and there are absolutely
no bleed-throughs.
The foregoing detailed descripti~on has ~een given for
clearness of understanding only, and no unnecessary limita-
tions sh.ould be understood therefrom, as modificat.ions will
be obvious to those skilled in the art.
