Note: Descriptions are shown in the official language in which they were submitted.
DIFFUS:I:ON ALLOY STEEL FOIL
The present invention relates generally to
light gauge steel strips or foils and, more
particularly, to very light gauge strips or foil~ formed
of solid solution iron-aluminum diffusion alloys and
iron-aluminum-silicon diffusion alloys which are
formable at room temperature, have good high temperature
oxidation resistance and corrosion resistance, have
useful electrical and magnetic properties, and
preferably are adapted for growing a surface coating o~
spine-like whiskers of aluminum oxide suitable for
retaining a surface coating of a catalytic metal used in
a monolithic catalytic converter for treating gases
which pollute the atmosphere.
Heretofore, an iron-aluminum diffusion alloy
foil has not been available. The Smith et al U.S.
Patent No. 3,214,820 discloses steel foils having a
surface coated with tin, zinc, aluminum or stainless
steel and describes producing the steel foil with the
metallic tin, zinc, aluminum or stainless steel
protective metal coating by cold rolling a plain carbon
steel strip having the protective metal coating applied
by a plating process. Smith et al teaches against hot
dip coating a steel strip for cold rolling to foil gauge
in order to avoid forming a hard brittle subsurface
intermetallic layer which Smith et al states prevents
forming a satisfactory foil product. The Smith et al
patent expressly avoids annealing a steel strip coated
with tin, zinc, or aluminum, because of the low melting
temperatures of these coatings.
, ',~' ! ;,
5~
~ he Kingston et al U.S. Patent No. 2,697,869
discloses a formable steel strip having an ultra thin
iron-aluminum diffusion alloy surface coating formed as
a result of annealing after cold rolling a steel strip
between 35 to 50% to provide an ultra thin aluminum
surface coating which has a critical maximum thickness
of 0.0003 inch. When the cold rolled strip is annealed
to impart good formability to the work hardened strip,
an ultra thin iron-aluminum alloy surface coating is
formed without an objectionable amount of brittle
subsurface intermetallic alloy layer being formed.
The Yagi et al U.S. Patent No. 4,228,~03
discloses producing an iron-aluminum diffusion alloy
coated steel sheet in which a powder aluminum coating is
applied to a carbon steel strip and the powder coated
strip is diffusion heated to form a thin iron-aluminum
diffusion alloy surface coating on the steel strip. The
diffusion heating in Yagi et al forms a brittle
~0 subsurface iron-aluminum intermetallic compound layer
below the diffusion alloy surface coating which results
in the strip having poor formability at room
temperature, and Yagi et al does not reduce the
diffusion coated strip to foil gauge.
The French Patent No. 1,391,659 discloses
providing titanium-containing iron-aluminum diffusion
alloy strips which have poor room temperature
formability and which are produced by a process
comprising cold rolling a titanium containing steel
strip to approximately its desired final thickness,
immersing the strip in an aluminum coating bath to form
a thick aluminum coating which provides between 8 and 30
~t.% aluminum on the strip, and subjecting the aluminum
coated strip to diffusion heating to diffuse the
aluminum uniformly throughout the strip. The iron-
aluminum diffusion alloy strips have such poor
formability at room temperature that they must be rolled
hot in order to effect reduction in thickness by rolling.
Accordingly, the present invention seeks to provide a
method of producing economically a solid solution ir~n-aluminum
diffusion alloy foil which is formable at room temperature and
resistant to oxidation at elevated temperatures.
Further, the present invention seeks to provide an
economical room temperature formable solid solution iron-aluminum
diffusion alloy foil which is useful as a tool wrap.
Still further the present invention seeks to provide a
solid solution iron-aluminum diffusion alloy foil which exhibits
improved electrical properties.
The present invention seeks further to provide in an
economical manner a cold reduced stabilized solid solution
iron-aluminum diffusion alloy foil which is formable at room
temperature and which has an adherent surface coating of
spine~like whiskers of aluminum oxide.
Also, the present invention seeks to provide a solid
solution iron-aluminum diffusion alloy foil by cold rolling
and diffusion heating an aluminum coated steel strip which is
characterized by good resistance to oxidation and corrosion
at room temperature and at elevated t~mperatures, as when
exposed to exhaust gases from automotive and industrial apparatus.
Thus broadly, the invention in one aspect pertains to a
solid solution iron-aluminum diffusion alloy foil formed in situ
by diffusion heating a cold rolled titanium stabilized low
carbon steel strip containing an excess of uncombined titanium
and having on each side an aluminum coating. The coated steel
strip before cold reduction has a steel strip thickness selected
from the range of from about 0.25 mm (0.010 inch) to about
0.76 mm (0.030 inch), with the aluminum coating on each side
of the steel strip having a thickness selected from the range
of from about 12.7 ~m (0.0005 inch) to about 76 ~m (0.003 inch).
The coated steel strip after cold reduction oE from about 40%
to about 99% and before diffusion heating has a foil thickness
of from abou-t 0.013 mm (0.0005 inch) to about 0.152 mm (0.006
inch), with the aluminum coating on each side thereof having
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- 3a -
having a thickness of from about 1.07 ~m (0.000042 inch) to
about 27.9 ~m (0.0011 inch). The thiekness of the steel strip
relative to the thickness of the aluminum coatings before cold
reduetion is eontrolled, within the aforementioned ranges, so as
S to provide in the solid solution iron-aluminum diffusion alloy
foil an aluminum content in excess of about 4 wt.% and not sub-
stantially above about 12 wt.% with the aluminum fully diffused
throughout the eross seetion of the foil. The foil is formable
at room temperature and is resistant to oxidation and to corrosion
at elevated temperatures.
Another aspeet of the invention pertains to a method of
produeing a room temperature formable solid solution iron-
aluminum diffusion alloy foil that is resistant to oxidation
and to eorrosion at elevated temperatures, eomprising forming a
strip of titanium stabili~ed low earbon steel eontaining an
exeess of uneombined titanium and having a thiekness seleeted
from the range of from about 0.25 mm (0.010 ineh) to about
0.76 mm (0.030 ineh), applying to eaeh surfaee of the steel strip
an aluminum eoating having a thiekness seleeted from the range
of from about 12.7 ~m (0.0005 inch) to about 76 ~m (0.003 inch),
redueing the thiekness of the aluminum coated strip between
about 40% and about 99~ by cold rolling to form an aluminum
coated foil having a thickness of :Erom about 0.013 mm
(0.0005 ineh) to about 0.152 mm (0.006 ineh) with the aluminum
eoating on eaeh side thereof having a thiekness of from about
1.07 ~m (0.000042 ineh) to about 27.9 ~m (0.0011 inch), heating
the eold rolled aluminum coated foil to form a solid solution
iron-aluminum diffusion alloy foil with the aluminum fully
diffused throughout the eross section of the foil, and eontrol-
ling the thiekness of the steel strip relative to the thickness
of the aluminum coatings before eold rolling, within the afore-
mentioned ranges, so as to provide in the solid solution iron-
aluminum diffusion alloy foil an aluminum content in exeess of
about 4 wt.% and not substantially above about 12 wt.%.
Other aspects of the present invention will be apparent
to those skilled in the art from the detailed description and
claims to follow when read in conjunction with the accompanying
drawing wherein:
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- 4 -
Fig. l is a photomicrograph at 500X
magnificati.on of a cross section of 0.076 mm (0.003
inch) thick electrolytically etched solid solution iron-
aluminum-silicon diffusion alloy foil containing 6.2
wt.~ aluminum, 0.86 wt~% silicon and 0.41 wto% titanium
formed by cold rolling a hot-dip Type I aluminum coated
low-titanium alloy stabilized low-carbon teel strip
about 0.47 mm (0.0185 inch) thick and reduced 84 percent
on a Sendzimir cold rolling mill and thereafter vacuum
diffusion heated for 4 hours at 982C (1800F~;
Fig. 2 is a photomicrograph at 500X
magnification of the diffusion alloy foil material of
Fig. l which has been diffusion heated at 1094~C
(2000F) for four hours and showing the oil having
large oriented crystal with the foil being one grain
thick;
Fig. 3 is a graph showing the substantially
uniform distribution of aluminum along the cross-section
of the solid solution diffusion alloy foil of Fig. 2, as
determined by microprobe analysis; and
Fig. 4 is a graph showing the diffusion
heating time required to substantially uniformly diffuse
the aluminum throughout the interior of a 0.0033 inch
thick aluminum coated low titanium alloy carbon steel
foil at temperature between 816C (1500F) and 1149C
(~100F).
A foil formed substantially of an iron-
aluminum or iron-aluminum-silicon diffusion alloy is
produced in accordance with a preferred embodiment of
the invention by forming on each side of a cold rolled
titanium stabilized low-carbon steel strip, preferably
having a thickness between about 0.25 mm (O.OlO inch)
and about 0.76 mm (0.030 inch), a hot-dip aluminum
coating using conventional continuous in-line hot-dip
aluminum coating apparatus with the aluminum or
aluminum-silicon hot-dip coating on each side of the
strip having a thickness of between about 12.7 ~m
(0.0005 inch) and about 76 um (0.003 inch) which is
sufficient to provide after diffusion heating a
,,~
diffusion alloy foil containing between about 2 wt.~
aluminum and about 12 wt.~ aluminum, cold reducing the
hot-dip aluminum coated titanium alloy steel strip to
S effect at least about a 40 percent and up to about a 99
percent reduction in the thickness of the aluminum
coated steel strip to provide an aluminum coated light
gauge steel strip or steel foil preferably having a
maximum thickness of about 0.152 mm (0.006 inch) and as
thin as about 0.013 mm (O.OOOS inch) with the cold
rolled aluminum coating having a thickness ranging
between about 1.07 ~m (0.000042 inch) and 27.9 ~m
(0.0011 inch) and diffusion heating the cold reduced
aluminum coated steel foil to diffuse the aluminum into
lS the steel and form a formable solid solution iron-
aluminum diffusion alloy foil containing from about 2
wt.% aluminum and up to about 12 wt.% aluminum. The
diffusion of the aluminum throughout the cross section
of an aluminum coated titanium alloy steel foil is time-
temperature dependent for a given aluminum coating andfoil thickness and can be effected at a temperature
preferably between about 816C (1500F) and 1149C
(2100F) for between about 2 minutes and about 24 hours
when using box annealing apparatus. Although, it is not
essential to diffuse the aluminum uniformly through the
steel base, the graph in Fig. 4 shows the time required
to diffuse the aluminum uniformly throughout the cross
section of a hot-dip coated low titanium alloy steel
foil 0.084 mm (0.0033 inch) thick having an aluminum
coating 8.9 ~m (0.00035 inch) thick on each surface when
diffusion heating at temperatures between 816C (1500F)
and 1149C (2100F).
In order to provide a low cost foil formed of
a solid solution iron-aluminum diffusion alloy which is
formable at room temperatures with good high temperature
oxidation resistance and good electrical properties and
which is also capable of growing a surface coating of
-- 6 --
aluminum oxide whiskers suitable ~or supporting a
catalytic coating, it has been found advisable to form
the steel strip from a stabilized low carbon steel such
as a low-titanium stabilized low-carbon steel. The low-
titanium stabilized alloy steel is preferably a steel
which has been killed to remove free oxygen, such as an
aluminum killed steel. The carbon content of the low-
titanium alloy steel is less than 0.10 wt.~, generally
between about 0.02 wt.~ and 0.10 wt.%, although a vacuum
degassed steel having substantially less than 0.02 wt.
carbon can also be used. The low-titanium stabilized
low carbon steel should have sufficient titanium to
combine with all the carbon, oxygen, and nitrogen in the
steel and, in addition, sufficient titanium to provide a
small excess of uncombined titanium, preferably at least
about 0.02 wt.%. The total titanium content of the
steel is preferably at least about 0.40 wt.% but will
always be less than about 1.0 wt.% and will generally
not exceed about 0.60 wt.%. The titanium in the
stabilized steel improves the rate of diffusion between
the iron and aluminum in the steel and also improves the
surface properties and increases the strength of the
steeL, thereby improving the cold rolling properties and
~5 room temperature ductility properties of the steel
strip. If desired, smaller amounts o other carbon and
nitrogen binders can be used in addition to the titanium
in the steel.
A typical low-titanium stabilized low-carbon
steel suitable for forming an aluminum coated steel foil
in accordance with the present invention has the
following composition on a weight basis: 0.04% carbon,
0.50% titanium, 0.20-0.50~ manganese, 0.012~ sulfur,
0.010~ phosphorus, 0.05% silicon, 0.020-0.090~ aluminum,
and the balance essentially iron with incidental
impurities.
In producing a commercially acceptable low
cost solid solution iron-aluminum diffusion alloy foil
by cold rolling a hot-dip aluminum coated low-titanium
alloy stabilized steel strip which is heated to form the
diffusion alloy foil, the thickness of the steel strip
relative to the aluminum coating thereon must be
carefully controlled in order to provide the required
amount of aluminum in the diffusion alloy. Also, in
order to hot-dip aluminum coat a steel strip on
production-type in-line continuous aluminum coating
apparatus, it is essential that the steel strip be
sufficiently thick to withstand the stresses of being
conveyed through the continuous hot-dip coating
apparatus, such as a Sendzimir-type hot-dip continuous
coating line, but not so thick as to make it impossible
to reduce economically the coated strip to a steel foil
gauge not substantially above about 0.152 mrn (0.006
inch) by effecting about a 40 to 99 percent reduction in
the thickness of the hot-dip coated aluminum steel
strip.
A further irnportant limitation on the maximum
thickness of the steel strip to be hot-dip coated on a
continuous coating line, such as Sendzimir-type hot-dip
coating line, is the requirement that the temperature of
the strip, after cleaning and surface preparation, must
be adjusted to a temperature about the temperature of
the aluminum hot-dip coating bath before the strip is
immersed in the aluminum coating bath and while the
strip is traveling at a sufficiently high line speed to
form (i.e. pick up) a hot-dip aluminum coating having a
coating thickness which will provide after diffusion
heating an aluminum content sufficient to impart the
desired oxidation resistance to the coated steel foil.
A steel strip having a thickness of between
about 0.25 mm (0.010 inch) and 0.76 mm (0.030 inch) has
been found to meet the foregoing requirements and be
-- 8 --
suitable for hot-dip aluminum coating on a continuous
in-line hot-dip aluminum coating apparatus, such as a
Sendzimir-type commercial continuous hot-dip coating
line, adapted to move the steel strip at a line speed of
about 2B0 feet per minute, the strip thereafter being
cold reduced to effect between about 40 to 99 percent
reduction in thickness so as to provide an aluminum
coated steel foil having a thickness of between about
0.013 mm (0.0005 inch) and about 0.152 mm (0.006
inch). The aluminum hot-dip coated steel strip can be
cold reduced to foil gauge in one or more passes through
a cold rolling mill, such as a Sendzimir cold rolling
mill.
Where the surface of the foil is not perfectly
flat but has surface irregularities formed in the
diffusion alloy foil after diffusion heating, the foil
material can be further processed by tension leveling or
skin passing to remove distortions in the foil and/or
~0 effect surface brightening and polishing.
It has also been found that in order for the
solid solution iron-aluminum diffusion alloy foil to
provide good high temperature oxidation resistance over
an extended period and exhibit good room temperature
formability, as required for fabricating into an
automotive exhaust system or for use as a tool wrap, the
aluminum hot-dip coating on the steel strip must be
sufficiently thick relative to the thickness of the
steel strip to provide in the finished foil a minimum of
about 6 wt. percent aluminum based on the weight of the
coated foil and not substantially above about 12 wt.%
aluminum where room temperature formability is
required. In the very thinnest foil, however, a
somewhat higher aluminum content may be used without
impairing room temperature formability. Since the steel
strip and the hot-dip aluminum coating are reduced in
substantially the same proportion when cold rolled to
_ 9 _
effect about a 90~ reduc~ion in the thickness of the
coated strip, a steel strip having a thickness before
hot-dip coating of between about 0.25 ~m (0.~10 inch)
and about 0.76 mm (0.030 inch) should be provided on
each side with an aluminum hot-dip coating havi~g a
thickness of between about 12.7 ~m (0.0005 inch) and
about 76 ~m (0.003 inch) but sufficient to provide the
strip with between about 6 wt.~ and about 12 wt.~
aluminum. For example, after about a 90% cold reduction
in thickness of a hot-dip aluminum coated steel strip
having an initial thickness of about 0.51 mm (0.020
inch), the cold rolled aluminum coating on each side of
the foil is about 5.1 ~m (0.0002 inch) thick and
provides an aluminum concentration of about 6 wt.~ based
on the weight of the aluminum coated steel foil.
The hot-dip aluminum coating applicd to the
steel strip is preferably a Type I aluminum coating
which contains aluminum with about 5-12 wt.~ silicon and
wherein the silicon prevents the formation of an
objectionabLy thick subsurface iron-aluminum
intermetallic layer. When the steel strip is hot-dip
coated in a Type I aluminum coating bath containing 10-
12 wt. percent silicon, the diffusion alloy foil
contains about 0.7 wt. percent silicon. It is also
possible, though not preferred, to apply a Type II
aluminum (i.e. substantially pure aluminum) hot-dip
coating on the stabilized low carbon steel strip.
In order to transform a steel foil having
metallic aluminum surface coatings into a diffusion
alloy foil having an iron-aluminum diffusion alloy
composition substantially throughout, the aluminum-
coated steel foil is heated as an open or closed coil in
an annealing furnace or on a continuous annealing line
in a non-oxidizing atrnosphere which may be ni-troyen-free,
such as in a vacuum or in an argon atmosphere, at 9~2C
(1800F) for between about 1 and 24 hours. The time
~,
$~
-- 10 --
required to form the iron-aluminum diffusion alloy will depend
on the thickness of the steel strip and aluminum coating as
well as the temperature of heating.
When producing an iron-aluminum or iron-
aluminum-s.il.icon diffusion alLoy foil for use as an
electrical steel, it is important that the aluminum or
aluminum-silicon coating be substantially uniformly
diffused throughout the cross-section o~ the ~oil. For
other foil applications, however, it is not essential to
have the aluminum or aluminum-silicon coatiny diffused
uniformly throughout the cross-section of the foil.
As an example of forming a solid solution
iron-aluminum diffusion alloy foil according to the
present invention, a low-titanium alloy stabilized low-
cacbon aluminum killed steel was formed into a steelstrip having a thickness of about 0.43 mm (0.017
inch)~ The titanium stabilized low-carbon aluminum
killed steel had the following approximate composition:
Wt. Percent
Carbon 0.04
Manganese 0.25
Phosphorus 0.009
Sulfur 0.012
Silicon 0.06
Molybdenum 0.005
Aluminum 0.060
Titanium 0.50
Total residual of
Cu, Ni, Sn, Cr 0.20
Iron Balance
The titanium stabilized steel strip after
conventional cleaning was immersed in a hot-dip Type I
aluminum coating bath having a tempecature o~ 694C
(1280F) on a Sendzimir-type continuous coating line
having a line speed of 280 ~eet per rninute to provide
both sides thereof with a hot-dip aluminum coating
having a thickness o~ about 38 ~m (0.0015 inch). The
hot-dip aluminum coated steel strip was cold roLled on a
5~
-- 11 --
Sendzimir-type cold rolling mill to a foil thickness of
about 0.051 mm ~0.002 inch) in four passes, effecting a
reduction of 43.6% in the first~ 45.5~ in the second,
45.0% in the third, and 39.4% in the fourth, for a total
of about 90% reduction in thickness without intermediate
annealing. Metallographic examination of the zold
reduced steel foil indicated a uniform aluminum surface
coating on both sides, approximately 4.6-5.1 ~m
(0.00018-0.0002 inch) with the intermetallic subsurface
iron-aluminum compound layer completely fractured and
randomly redistributed throughout the aluminum coating
and with the cold working of the coated steel strip
imparting a very high energy level to the coated steel
so that during the subsequent diffusion heating
treatment there are no Kirkendall voids formed in the
diffusion alloy product. The aluminum in the coating is
preferably fully and substantially uniformly diffused
throughout the cross section of the foil by heating the
foil for two hours at a temperature of 982C (1800F) to
form an iron-aluminum-silicon diffusion alloy foil.
Bulk chemical analyses of the hot-dip aluminum coated
foil after diffusion showed 6.4 wt.% aluminum, 0.8 wt.
silicon, and 0.40 wt.% titanium.
The solid solution iron-aluminum diffusion
alloy foil made in the foregoing manner was free of
brittle iron-aluminum intermetallic compound and was
formable at room temperature without annealing. When
heated in air at 11~9C (2100F) for 96 hours the foil
material exhibited a weight gain of no more than 1
mg/cm2, had good high temperature corrosion and
oxidation resistance at 1000C (1832F), and when given
a 180 1-T bend at room temperature the surface was not
ruptured. The iron-aluminum diffusion alloy foil had a
tensile strength of 72 ksi, a yield strength of 65 ksi,
and an elongation of 10.4~.
- 12 -
The cold reduced aluminum-coated steel foil of
Fig. 1 having about 6 wt.~ of the foil as aluminum in
the surface coatings was diffusion heated as a closely
wound steel coil in a vacuum at 1093C (2000F) for four
hours to provide a foil having the aluminum
substantially fully diffused throughout the cross
section of the foil. The distribution of the aluminum
and silicon in the iron-aluminum diffusion alloy steel
foil is shown in Fig. 3.
The extreme outer 2.5 ~m (0.0001 inch) to 5.0
~m (0.0002 inch) of the surface of the diffusion alloy
foil of the present invention has been found to contain
a higher than average concentration of titanium and
aluminum, and it is evident that uncombined titanium in
the titanium stabilized steel has diffused outwardly
from the interior to the surface of the foil. The
concentration of titanium in the surface becomes
progressively larger and the concentration of titanium
in the center of the foil becomes progressively smaller
as the diffusion heating is prolonged until no titanium
re~ains at the center of the foilO For example, after a
foil 0.05 mm (0.~02 inch) thick is diffusion heated in
nitrogen at 925C (1700F) for 24 hours, there is no
detectable titanium remaining at the center of the foil
when the foil is subjected to electron microprobe
analysis.
The relatively low cost iron-aluminum and
iron-aluminum-silicon diffusion alloy foils of the
present invention are useful in place of the more costly
stainless steel foils and high alloy foils for many
industrial applications. Thus, the cold rolled iron-
aluminum diffusion alloy steel foils produced as
described herein are useful as a substitute for "321
stainless steel" foil and for enclosing or "wrapping"
tools which are heat treated at an elevated temperature,
thereby avoiding the need to heat the tools in a
5~
- 13 -
protective non-oxidizing atmosphere. The diffusion
alloy tool wrapping foils preferably contain between
about 6 wt.% and 12 wt.% aluminum and have a thickness
between about 0.050 mm (0.002 inch) and n . 075 mm (0.003
inch) so as to have the required high temperature
strength and oxidation resistance as well as formability
at room temperature to form a protective wrap for
enclosing tools and withstanding heat treating
temperatures up to about 1149C (2100F). The aluminum
content of the foil also acts as a "getter" to remove
oxygen from within the enclosure and prevents
objectionable oxidation and decarburization of the
surface of the tools during the heat treating cycle.
The solid solution iron-aluminum and iron-
aluminum-silicon diffusion alloy foils of the present
invention when prepared by vacuum diffusion heating with
between 2 and 12 wt. percent aluminum and which can also
contain between about 0.2 and about 0.9 wt.~ silicon are
useful as electrical steels of the electrically soft
variety for use as magnetic shielding material and for
making core assemblies of electrical rotary equipment
(i.e. motors) and transformers in place of silicon
steels, iron-nickel alloys and other ferrous alloys.
Aluminum has a beneficial effect, similar to that of
silicon, on the electrical resistivity and certain
magnetic properties of iron, but aluminum is seldom
substituted for silicon because of the recognized
difficulty of fabricating thin iron-aluminum alloy sheet
material. At present aluminum is used most commonly at
a concentration of less than 0.05 wt.% in non-oriented
silicon steels. While it is recognized that ternary
alloys of iron, silicon and aluminum have high
resistivity and good permeability at low flux densities,
that the magnetic properties of these ternary alloys can
approach those of more costly iron-nickel alloys, and
that increasing the concentration of silicon and
~55~5;2
- 14 -
aluminum reduces saturation induction, nevertheless,
silicon and aluminum have not been used in electrical
steels in excess of about 4 wt.% because such steels are
brittle and are very di~ficult to roll into thin gaug
sheet material. With the present invention, however, it
is possible to provide a workable electrical ron-
aluminum or iron-aluminum-silicon thin gauge diffusion
alloy strip or foil having in excess of 4 wt.% aluminum
with large grain size and desirable crystal orientation
which closely approximates the ideal electrical steel
material. For example, one type of electrical steel
foil should preferably be one grain thick with the grain
(crystal) faces parallel to the direction of rolling
(see Fig. 2). An iron-aluminum-silicon diffusion alloy
containing about 6 wt.% aluminum and 0.9 wt.% silicon
has an electrical resistance of about 91-96 micro-ohm
centimeters. If d~sired, the diffusion alloy foil where
intended for certain types of electrical use can be
urther treated after diffusion heating by cold rolling
to reduce the thickness of the foil and impart critical
strain to the ~oil product and then given a critical
time-temperature heat treatment to modify the crystal
form. For example, an iron-aluminum-silicon diffusion
alloy foil o the present invention has been cold rolled
to impart a 3% critical strain and heated at 816C
(1500F) for 4 hours to efect a very large increase in
the grain size.
Where the iron-aluminum diffusion alloy foil
is used as a support for a metal catalyst in a catalytic
converter, the foil, preferably having a thickness about
0.051 mm (0.002 inches) and containing about 6 wt.~ aluminum,
can be preconditioned for whisker grow-th by the me-thod dis-
closed in U.S. Patent No. 4,279,782. Thereafter, the foil
is heated in air or other oxygen containing atmosphere pre-
ferably for 8 hours at 925C (1700F), to grow an aluminum
oxide spine-like whisker surface coating. A coating of gamma
,:~
.~ ~ r- ~
-- 15 --
aluminum oxide powder dispersed in an aqueous alumina
gel-noble metal catalyst mixture is applied to the
spine-like whisker coated surface of the foil as
described in U.S. Patent No. 4,279,782.
In order to impart optimum corrosion
resistance to a solid solution iron-aluminum diffusion
alloy foil the cold rolled aluminum coated low-titanium
stabilized low carbon steel foil is placed in a dry
nitrogen-containing atmosphere which has minimal or no
oxidizing action on the titanium and aluminum in the
foil and is heated for a time and at a temperature
sufficient to form on the surface of the diEfusion alloy
foil a thin titanium nitride-containing film which
imparts high corrosion resistance to the foil. As
previously discussed, when an aluminum coated titanium
stabilized low carbon steel foil having a slight excess
of uncombined titanium is heated in a non-oxidizing
atmosphere, the aluminum surface coating diffuses
readily into the steel foil beginning at a temperature
of about 399C (750F~ and effects formation of an iron-
aluminum diffusion alloy foil. When the titanium
stabilized aluminum coated steel foil is diffusion
heated in a dry nitrogen-containing atmosphere, which
has a minimal oxidizing effect on the titanium and
aluminum, at a temperature between about 500C (930F)
and 1093C (2000F) and preferably at a temperature of
about 925C (1700F), the nitrogen reacts with the
titanium to form a titanium nitride-containing film on
the surface of the diffusion alloy foil.
The titanium nitride-containing layer on the
surface of the foil significantly improves the corrosion
resistance of the iron-aluminum diffusion alloy foil,
since the titanium nitride-containing surface film is
resistant to attack by acids, and resists corrosion when
the foil is immersed in an aqueous acidic solution for
prolonged periods. Titanium nitride is only slightly
- 16 -
soluble in hot aqua regia containing added hydrofluoric
acid. Aluminum nitride on the other hand, is readily
attacked by acids, such as a hot 10% aqueous
hydrochloric acid solution, whereas the foil having the
titanium nitride-containing surface film is resistant to
attack by the 10~ hydrochloric acid solution. The
titanium nitride can be present as TiN which has a sigma
crystal form or as ~i2N which has a gamma crystal
form. It is al50 possible for the titanium and nitrogen
to form more complex reaction products with the
aluminum, iron and silicon in the steel.
The dry nitrogen-containing atmosphere used to
form the titanium nitride-containing film can be pure
nitrogen gas, gaseous ammonia, dissociated ammonia, a
nitrogen-hydrogen gaseous mixture, or a nitrogen-argon
gaseous mixture. The diffusion heat treatment with the
dry nitrogen-containing atmosphere can range from about
500C (930F) to about 1093C (2000F) for a period of
from about 0.25 to about 48 hours with the formation of
the titanium nitride-containing film being time-
temperature dependent. When a 2 mil thick aluminum
coated low-titanium alloy steel foil is heated at 925C
(1700F) in a dry 95% nitrogen-5% hydrogen atmosphere, a
very thin titanium nitride-containing film begins to
form on the surface of the steel after heating for 8
minutes and increases in thickness as heating
continues. After the alloy steel foil is heated for 15
minutes at 925C (1700F), the titanium nitride-
containing film on the surface of the foil issufficiently thick that it is not etched when washed for
2 minutes with 10% hydrochloric acid aqueous solution at
a temperature of 66C-82C (150F-180F).
Electron microprobe x-ray analysis data and x-
ray maps at 4000X magnification of a solid solutioniron-aluminum diffusion alloy foil made by diffusion
heating an aluminum coated low titanium stabilized steel
~t~ 5
- 17 -
in a ~ure nitrogen atmosphere for 24 hours at 925C
(1700FI and having as a bulk analysis 6.8% aluminum,
0.34~ titanium, 0.05~ carbon, 0.35% nitrogen, 0.85%
silicon and the balance iron with incidental impurities,
indicate the presence of a titanium nitride-containing
film or layer having a mean thickness of about 0.23 mils
on the surface of the foil. The surface film has a peak
concentration of 12.6 wt. percent titanium and very
little titanium is present in the interior of the foil
except at isolated points which are thought to indicate
the presence of titanium carbide.
The nitrogen treated foil having the titanium
nitride-containing film on the surface exhibits good
room temperature formability when a section of the
nitrogen treated foil having a thickness of 3.3 mils is
subjected to the Zero-T Bend Test and can be cold rolled
with conventional apparatus. The nitrogen treated
diffusion alloy foil has a tensile strength of about 82
ksi, a yield strength of about 81 ksi, and an elongation
of about 1.0 percent. The emittance of the nitrogen
treated diffusion alloy foil is between 0.8-0.9 (black
body=l.0).
Aluminum oxide whiskers do not readily grow on
2~ the diffusion alloy foil having a titanium nitride-
containing surface. Consequently, when the iron-aluminum
diffusion alloy low titanium stabilized foil must have a
thick surface growth of spine-like whiskers of aluminum
oxide, as when the foil is used to support a catalyst in
an automotive catalytic converter, and where optimum
corrosion resistance and/or good abrasion resistance is
also desired, the thick coating of spine-like whiskers
is grown on the surface of an aluminum coated steel foil
by the process described in ~.S. Patent No. 4,279,782
before heating in a dry nitrogen-containing
atmosphere. Thereafter the whisker coated foil can be
heated in a dry nitrogen-containing atmosphere for a
.5~5~
- 18 -
time and at a temperature sufficient to form a titanium
nitride-containing thin layer or film on the surface of
the iron-aluminum diffusion alloy steel. For example,
the whisker coated foil can be heated for a period of
between about 0.25 hours and 24 hours at a temperature
between about 1093C (2000F) and 500C (930F),
respectively, in a dry nitrogen-containing atmosphere,
such as in an atmosphere of gaseous nitrogen or ammonia,
to form a titanium nitride-containing layer on the
surface of the foil. The titanium nitride-containing
layer imparts high corrosion resistance and abrasion
resistance to the whisker coated diffusion alloy foil.
Whereas in applicant's preferred embodiment
the iron-aluminum diffusion alloy foil is produced by
cold rolling a hot-dip aluminum coated titanium
stabilized steel strip to foil gauge followed by
diffusion heating, it is also within the scope of the
present invention to apply the aluminum or aluminum-
silicon coating to the titanium stabilized steel stripby other known aluminum coating processes, such as a
powder metal coating process in accordance with U.S.
Patent No. 4,542,048, or by electroplating.
The term "formable" as used herein designates
the capability of the foil to be fabricated by
conventional metal forming machines at room temperature,
and the term "good formability" as used herein refers to
the capability of the foil to undergo severe deformation
at room temperature without bend breaking, edge cracking
and loss of surface material.
The term "solid solution iron-aluminum
diffusion alloy" is used herein to designate an iron-
aluminum diffusion alloy or an iron-aluminum-silicon
diffusion alloy, such as formed by diffusion heating a
Type I aluminum hot-dip coating containing about 5 to 12
wt.~ silicon, although higher and lower amounts of
silicon can be used for producing special diffusion
alloy foils.
