U.S. patent application number 13/576196 was filed with the patent office on 2013-01-31 for metal surface scale conditioning.
This patent application is currently assigned to KOLENE CORPORATION. The applicant listed for this patent is Richard M. Kitchen, James C. Malloy, Dennis J. McCardle. Invention is credited to Richard M. Kitchen, James C. Malloy, Dennis J. McCardle.
Application Number | 20130029054 13/576196 |
Document ID | / |
Family ID | 44169987 |
Filed Date | 2013-01-31 |
United States Patent
Application |
20130029054 |
Kind Code |
A1 |
Malloy; James C. ; et
al. |
January 31, 2013 |
METAL SURFACE SCALE CONDITIONING
Abstract
Methods and systems are provided for treating oxide scale on the
surface of a metal object. In one embodiment, a system temperature
control apparatus controls the temperature of metal object's
surface to an application temperature below the Leidenfrost
temperature point of an alkali metal hydroxide aqueous conditioning
solution. An application apparatus wets the metal object's surface
at the controlled temperature with a thin layer of the solution
that engages the oxide scale, and a heating apparatus heats the
wetted surface to a final conditioning temperature above a melting
point of the alkali metal hydroxide by an additional value selected
to effect conditioning of the oxide scale at a reasonable but not
excessive rate by the melting alkali metal hydroxide reacting with
the oxide scale. The system terminates additional conditioning to
prevent creation of additional oxide scale beyond the conditioned
depth.
Inventors: |
Malloy; James C.; (Ferndale,
MI) ; McCardle; Dennis J.; (Troy, MI) ;
Kitchen; Richard M.; (Woodhaven, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Malloy; James C.
McCardle; Dennis J.
Kitchen; Richard M. |
Ferndale
Troy
Woodhaven |
MI
MI
MI |
US
US
US |
|
|
Assignee: |
KOLENE CORPORATION
Detroit
MI
|
Family ID: |
44169987 |
Appl. No.: |
13/576196 |
Filed: |
January 7, 2011 |
PCT Filed: |
January 7, 2011 |
PCT NO: |
PCT/US11/20479 |
371 Date: |
July 31, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61293821 |
Jan 11, 2010 |
|
|
|
Current U.S.
Class: |
427/444 ;
118/708 |
Current CPC
Class: |
C23G 3/023 20130101;
C23G 3/027 20130101; B21B 45/06 20130101; B23B 45/06 20130101; C23G
1/19 20130101; C23G 3/021 20130101; C23G 3/02 20130101 |
Class at
Publication: |
427/444 ;
118/708 |
International
Class: |
B05D 3/10 20060101
B05D003/10; B05C 11/00 20060101 B05C011/00 |
Claims
1. A system for treating oxide scale on the surface of a metal
object, comprising: a temperature control apparatus that controls
the temperature of metal object's surface to an application
temperature below the Leidenfrost temperature point of an aqueous
conditioning solution comprising an alkali metal hydroxide, the
metal object's surface comprising an oxide scale having an initial
depth from the metal object's surface; an application apparatus
that wets the metal object's surface at the controlled temperature
with a thin layer of the aqueous conditioning solution, the aqueous
conditioning solution thin layer engaging the oxide scale; and a
heating apparatus that heats the wetted metal object surface to a
final conditioning temperature that is above a melting point of the
alkali metal hydroxide in an anhydrous form by an additional value
selected to effect conditioning of the oxide scale on the metal
surface at a reasonable but not excessive rate, the wetted metal
object surface heated to the final conditioning temperature
evaporating water in the aqueous conditioning solution and melting
the alkali metal hydroxide in the anhydrous form on the metal
object's surface, the melting alkali metal hydroxide reacting with
the engaged oxide scale and reducing the oxide scale to a
conditioned depth from the metal object's surface that is less than
the initial depth; wherein the system terminates additional
conditioning of the metal object's surface beyond the conditioned
depth, the terminating preventing a creation of an additional oxide
scale beyond the conditioned depth from the metal object's
surface.
2. The system of claim 1, wherein the system terminates the
additional conditioning of the metal object's surface beyond the
conditioned depth by the application apparatus applying the aqueous
conditioning solution thin layer in an amount wherein the reacting
of the melting alkali metal hydroxide with the engaged oxide scale
consumes enough of the melting alkali metal hydroxide in the thin
layer on the metal object's surface to prevent additional
conditioning of the metal object's surface.
3. The system of claim 1, further comprising: cooling apparatus
that quenches the metal object's surface to a temperature below the
final conditioning temperature at an end of a conditioning time
period that is selected to terminate the additional conditioning as
a function of a material parameter and a dimension parameter of the
metal object.
4. (canceled)
5. The system of claim 3, wherein the final conditioning
temperature and the conditioning time period are selected to
produce a specified extent of conditioning of the scale on the
metal object's surface.
6. The system of claim 5, wherein the additional value to effect
conditioning of the oxide scale is selected from a range of zero to
about 200.degree. F. (94.degree. C.).
7-11. (canceled)
12. The system of claim 6 wherein the aqueous conditioning solution
comprises: a eutectic blend of sodium and potassium hydroxides at
about 30% by weight; about 3% by weight sodium nitrate; about 67%
by weight water; and less than about 1% by weight of at least one
nonionic surfactant.
13. The system of claim 12 wherein the eutectic blend comprises
about 18% by weight potassium hydroxide and about 12% by weight
sodium hydroxide.
14-16. (canceled)
17. The system of claim 6, wherein the final conditioning
temperature is an increase over the application temperature ranging
from about 150.degree. F. (65.degree. C.) to about 200.degree. F.
(94.degree. C.)).
18. A method of treating oxide scale on the surface of a metal
object, comprising: controlling a temperature of a metal object's
surface to an application temperature below the Leidenfrost
temperature point of an aqueous conditioning solution comprising an
alkali metal hydroxide, the metal object's surface comprising an
oxide scale having an initial depth from the metal object's
surface; wetting the metal object's surface at the controlled
temperature with a thin layer of the aqueous conditioning solution,
the aqueous conditioning solution thin layer engaging the oxide
scale; heating the wetted metal object surface to a final
conditioning temperature that is above a melting point of the
alkali metal hydroxide in an anhydrous form by an additional value
selected to effect conditioning of the oxide scale on the metal
surface at a reasonable but not excessive rate, thereby evaporating
water in the aqueous conditioning solution and melting the alkali
metal hydroxide in the anhydrous form on the metal object's surface
at the final conditioning temperature and causing the melting
alkali metal hydroxide to react with the engaged oxide scale and
reduce the oxide scale to a conditioned depth from the metal
object's surface that is less than the initial depth; and
terminating additional conditioning of the metal object's surface
beyond the conditioned depth, the terminating preventing a creation
of an additional oxide scale beyond the conditioned depth from the
metal object's surface.
19. The method of claim 18, wherein the terminating the additional
conditioning comprises the reacting of the melting alkali metal
hydroxide with the engaged oxide scale consuming enough of the
melting alkali metal hydroxide on the metal object's surface to
prevent additional conditioning of the metal object's surface.
20. The method of claim 18, further comprising: selecting a
conditioning time period to terminate the additional conditioning
as a function of a material parameter and a dimension parameter of
the metal object; and wherein the terminating the additional
conditioning comprises quenching the metal object's surface to a
temperature below the final conditioning temperature at an end of
the conditioning time period.
21. The method of claim 20 wherein the quenching comprises rinsing
the alkali metal hydroxide from the metal object's surface.
22. The method of claim 20, further comprising selecting the final
conditioning temperature and the conditioning time period to
produce a specified extent of conditioning of the scale on the
metal object's surface.
23. The method of claim 22, further comprising: selecting the
additional value to effect conditioning of the oxide scale from a
range of zero to about 200.degree. F. (94.degree. C.).
24. The method of claim 22, further comprising optimizing the
conditioned depth by varying at least one of the final conditioning
temperature, components of the aqueous conditioning solution,
relative amounts of reactants utilized in the aqueous conditioning
solution and the conditioning time period as a function of the
material parameter and the dimension parameter.
25. The method of claim 22, wherein the specified extent of
conditioning is a least-oxide-to-pickle level of conditioning of
scale on the metal object's surface and a minimal base metal
effect, the method further comprising: selecting the final
conditioning temperature and the conditioning time period as a
function of an amount of scale on the metal object's surface.
26. The method of claim 22 wherein the conditioning time period is
no more than about thirty seconds.
27. The method of claim 26 wherein the conditioning time period is
about three seconds.
28. The method of claim 22 wherein the heating the wetted metal
object surface comprises heating in an oxygen-containing
atmosphere.
29. The method of claim 22 wherein the aqueous conditioning
solution comprises: a eutectic blend of sodium and potassium
hydroxides at about 30% by weight; about 3% by weight sodium
nitrate; about 67% by weight water; and less than about 1% by
weight of at least one nonionic surfactant.
30. The method of claim 29 wherein the eutectic blend comprises
about 18% by weight potassium hydroxide and about 12% by weight
sodium hydroxide.
31. The method of claim 22, further comprising determining the
Leidenfrost temperature point as a function of a caustic
concentration of the aqueous conditioning solution.
32. The method of claim 31, further comprising: selecting the
application temperature near the Leidenfrost temperature point and
above a boiling point of the conditioning solution.
33. The method of claim 32, further comprising selecting the
application temperature above a salt fusion temperature point of
the conditioning solution.
34. The method of claim 33, further comprising: selecting the final
conditioning temperature as an increase over the application
temperature ranging from about 150.degree. F. (65.degree. C.) to
about 200.degree. F. (94.degree. C.)).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation, and claims priority, of
a U.S. provisional patent application by Malloy et al for METAL
SURFACE SCALE CONDITIONING, filed in the U.S. Patent and Trademark
Office on Jan. 11, 2010 and assigned Ser. No. 61/293,821,
confirmation number 6931.
FIELD OF THE INVENTION
[0002] This invention relates generally to conditioning of oxide or
scale on metal surfaces.
BACKGROUND OF THE INVENTION
[0003] The conditioning of oxide surfaces or scale on metal
surfaces, sometimes referred to as descaling, is desired with
respect to the production of stainless steel and superalloy metal
strips. While our discussion focuses primarily on metals in strip
form, the applicability and value of our invention may be useful
for conditioning oxide surfaces or scale in various shapes,
geometries, or assemblies other than metal strip; it is not our
intention to limit the benefit to only metal strip. Stainless
steels are ferrous alloys containing more than about 10% chromium
for the purpose of enhancing corrosion and oxidation resistance,
and may also contain nickel, molybdenum, silicon, manganese,
aluminum, carbide formers and other elements. Families of
superalloys may contain nickel or cobalt as the predominant base
element, and incorporate more exotic alloying elements, such as
tungsten, titanium, niobium, and other elements. All of these base
elements and additive elements have a high affinity for oxygen at
high temperatures and form tenacious and chemically stable oxides
which complicate their subsequent removal which is required prior
to additional processing or sale.
[0004] Prior art descaling techniques for some grades of low alloy
steels with very light scale include pickling of steel strip in
mineral acid, such as sulfuric acid, hydrochloric acid,
hydrofluoric acid, nitric acid, or mixtures thereof. However, often
a mere acid pickle is insufficient in treating higher alloy steel
strips. Conditioning of the scale before acid pickling may be
required. Typical compositions used for scale conditioning are
caustic mixtures of alkali metal hydroxides and alkali metal
nitrates with various other additives such as alkali halides
carbonates, and/or other oxidizing agents, often referred to as
descaling or scale conditioning salts. A conventional technique for
using such compositions is in a bath of fused anhydrous salt in a
vessel at elevated temperatures, e.g. 427.degree. C. (800.degree.
F.) to 538.degree. C. (1000.degree. F.), in which a metal object is
first immersed, followed by a water rinse and acid pickle.
BRIEF SUMMARY OF THE INVENTION
[0005] Methods and systems are provided for treating oxide scale on
the surface of a metal object. In one embodiment, a system (100)
includes a temperature control apparatus (105) that controls the
temperature of metal object's (106) surface (112) to an application
temperature below the Leidenfrost temperature point of an aqueous
conditioning solution comprising an alkali metal hydroxide, wherein
the metal object's surface has an oxide scale having an initial
depth from the metal object's surface. An application apparatus
(108) wets the metal object's surface at the controlled temperature
with a thin layer (111) of the aqueous conditioning solution which
engages the oxide scale. A heating apparatus (113) heats the wetted
metal object surface to a final conditioning temperature that is
above a melting point of the alkali metal hydroxide in an anhydrous
form by an additional value selected to effect conditioning of the
oxide scale on the metal surface at a reasonable but not excessive
rate, the heated wetted metal object surface thereby evaporating
water in the aqueous conditioning solution and melting the alkali
metal hydroxide in the anhydrous form on the metal object's
surface, wherein the melting alkali metal hydroxide reacts with the
engaged oxide scale and reduces the oxide scale to a conditioned
depth from the metal object's surface that is less than the initial
depth. The system further terminates additional conditioning of the
metal object's surface beyond the conditioned depth, the
terminating preventing a creation of an additional oxide scale
beyond the conditioned depth from the metal object's surface.
BRIEF DESCRIPTION OF DRAWINGS
[0006] FIG. 1 is a graphic illustration of iron, chromium, nickel
and oxygen as normalized weight percentages as a function of
distance in Angstroms from the surface of an exemplary annealed
type 304 stainless steel sample.
[0007] FIG. 2 is a graphic illustration of the normalized weight
percentages of iron, chromium, nickel and oxygen in the type 304
stainless steel sample of FIG. 1 after immersion in a conventional,
prior art salt-bath for a conventional time frame.
[0008] FIG. 3 is a graphic illustration of the normalized weight
percentages of iron, chromium, nickel and oxygen in the type 304
stainless steel sample of FIG. 1 after immersion in a conventional,
prior art salt-bath for an extended time frame.
[0009] FIG. 4 is a graphic illustration of the normalized weight
percentages of iron, chromium, nickel and oxygen in the type 304
stainless steel sample of FIG. 1 after conditioning according to
the present invention.
[0010] FIG. 5 is a diagrammatic view of a process for scale
conditioning according to the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0011] There are a variety of drawbacks to immersion salt bath
techniques. The salt bath has to be maintained at elevated
temperatures, which may be energy intensive. Fused caustic baths
requiring submerged rolls may be difficult to maintain and may
cause marring of the surface of the strip being descaled. There are
"drag-out" problems and hazards with respect to treating strip
steel with the heated fused composition: as the strip exits from a
pot of fused composition, it carries a certain amount of the
heated, fused composition with it, especially at high strip speeds.
Efforts to incorporate metal wiping rolls to reduce this chemical
drag-out from the bath may introduce their own set of process
complications including scratching or marring the fine metal
surfaces. The long-term high temperature exposure that these fused
bath compositions are subjected to limit the compounds that may be
incorporated into the working bath, further restricting process
flexibility.
[0012] The use of immersion-type salt bath conditioning may also
result in over-conditioning of the metal surface with excess oxide
formation as well as other detrimental effects. FIG. 1 provides a
graphic illustration of Scanning Auger Microprobe (SAM) profiles of
iron (Fe) 12, chromium (Cr) 14, nickel (Ni) 16 and oxygen 18
representing normalized weight percentages (the vertical axis 20)
as a function of distance in Angstroms (.ANG.) from the surface
(the horizontal axis 22) of a type 304 stainless steel sample
annealed in a gas-fired furnace in an oxygenated atmosphere with 3%
excess oxygen at a temperature of 1925.degree. F. (1052.degree. C.)
for 120 seconds. The oxygen profile 18 is indicative of relative
amounts of chromium oxide and iron oxide; as the oxygen levels
diminish, so too do corresponding amounts of chromium oxide and
iron oxide, and thus correspondingly increasing amounts of chromium
and iron instead of chromium oxide and iron oxide. The surface of
the annealed type 304 stainless steel sample (the region ranging
from zero to 2000 .ANG. along axis 22) is shown to be composed of
primarily chromium oxide, with deeper regions progressively
stabilizing until at from about 8000 .ANG. to 10000 .ANG. where the
sample has a composition of about 18% Cr and 8% Ni, the typical and
expected composition of type 304 stainless steel and thus beyond an
extent needed for scale conditioning, and further wherein removal
of any excess oxide formation may result in undesired surface
effects as well as unnecessary and costly additional pickling
processes.
[0013] FIG. 2 provides a graphic illustration of SAM elemental
depth profiles of the normalized weight percentages of iron 42,
chromium 44, nickel 46 and oxygen 48 in a 10.16 centimeter (4
inch).times.15.24 centimeter (6 inch) panel of 0.635 millimeter
(0.025 inch) gage type 304 annealed stainless steel type 304 (18/8
chrome-nickel) after a conventional, prior art salt-bath
conditioning treatment. The salt bath was an essentially anhydrous
composition (i.e. it does not comprise enough water to react with
the composition or a metal object surface submerged therein)
comprising about 12% by weight sodium nitrate, about 10% by weight
sodium chloride, about 15% by weight potassium hydroxide and about
63% by weight sodium hydroxide. This salt bath composition is
taught in U.S. Pat. No. 3,260,619 issued to Shoemaker et al on Jul.
12, 1966, the entire disclosure of which is hereby incorporated,
though it will be understood that alternative salt bath embodiments
taught therein and elsewhere may also be used for conventional
immersion salt bath conditioning. The profiles 42, 44, 46 and 48
were obtained after immersing the sample for a conventional, prior
art time period of 30 seconds in the molten salt bath heated to an
operating temperature of 850.degree. F. (454.degree. C.), the
sample then removed from the bath and salt bath composition still
adhering to the sample allowed to drip off for a few seconds, and
the sample then promptly plunged into a pail of room (ambient)
temperature tap water and subsequently air dried.
[0014] In contrast to the profile illustrated in FIG. 1, FIG. 2
shows that the original surface chromium oxide levels have been
almost completely removed, with only iron oxide remaining and a
residual conditioned scale occurring from about 4000 to about 5000
.ANG., where after (from 5000 .ANG. and deeper from the surface)
the stainless steel sample composition stabilizes. Of further
interest is a "shoulder" region 50 on the iron profile 42 from
about zero to about 2000 .ANG., wherein the normalized weight
percentage of iron generally oscillates between about 55% and 58%
as the depth increases over this range, until starting to
progressively climb after 2000 .ANG. in depth. This oscillation
suggests that excessive conditioning processes are occurring,
unnecessarily increasing the amount of conditioned oxide scale that
must be subsequently removed, as further discussed below and which
becomes even more apparent with reference to FIG. 3.
[0015] Reaction to the scale on a strip generally occurs to
completion quickly upon immersion in such baths; however, the
logistics of strip processing generally dictate that the strip
remains submerged in the bath well after optimal conditioning has
already been obtained; since the conditioning reaction progresses
so far so rapidly, over-conditioning necessarily occurs before the
conditioned strip leaves an immersion bath, thus obviating any
opportunity for timely quenching to prevent over-conditioning. FIG.
3 is a graphic illustration of SAM profiles of normalized weight
percentages of iron 52, chromium 56, nickel 58 and oxygen 54 in a
0.635 millimeter (0.025 inch) gage, 10.16 centimeter (4
inch).times.15.24 centimeter (6 inch) panel of the type 304
annealed stainless steel of FIG. 1 after immersion in the prior art
salt-bath of FIG. 2 for an extended time frame, namely for about
120 seconds. As before with respect to FIG. 2, the sample was then
removed from the bath, salt bath composition still adhering to the
sample allowed to drip off for a few seconds, and then the sample
was promptly plunged into a pail of room (ambient) temperature tap
water and subsequently air dried. FIG. 3 clearly shows the
deleterious effects of long immersion time conditioning, which may
happen when a continuous process strip line incorporating salt bath
conditioning stops or slows down and a stainless steel strip is
left exposed to a molten immersion process for a long time, e.g.
60, or 120 seconds or even longer. The iron profile 52 in view of
the oxygen profile 54 indicates that iron oxide occurs at a steady
and unacceptably high level over the length of the Auger scan data,
to over 10000 .ANG. of depth from the surface, with chromium 56
reduced to almost half of its native, desired concentration. It is
believed that this occurs because the iron oxide does not dissolve
in the molten salt (further indicated in the small shoulder region
50 of FIG. 2), but instead the alkali chromate does, resulting in
unacceptable diminishment of the chromium deep into the metal
surface.
[0016] FIG. 4 provides SAM profiles of iron 62, chromium 64, nickel
66 and oxygen 68 for another 0.635 millimeter (0.025 inch) gage,
10.16 centimeter (4 inch).times.15.24 centimeter (6 inch) panel of
the type 304 annealed stainless steel of FIG. 1 as conditioned
according to the present invention. An alkaline aqueous liquid was
applied thinly to the annealed type 304 stainless at an ambient
temperature, the coated sample then heated in a horizontal
orientation in an electric oven pre-heated to about 800.degree. F.
(427.degree. C.) for about 120 seconds, wherein the panel was
brought to final treatment temperature of about 650.degree. F.
(343.degree. C.), the sample then subsequently tap water rinsed
within 30 seconds of reaching said treatment temperature and then
air dried. FIG. 4 shows a clear contrast to and improvement over
the results profile of a conventional time and temperature
immersion treatment depicted in FIG. 2. It will be understood by
one skilled in the art that the ultimate goal in conditioning
annealed stainless steel is to remove original surface chrome oxide
without adversely attacking the underlying metal (i.e. reducing the
native chromium levels) and further without unnecessarily building
up new iron oxide that needs to be subsequently removed in the
pickling section of the line. As FIG. 4 illustrates, a chrome
oxide-free surface is obtained relatively quicker and more
efficiently relative to depth from the surface; the iron oxide
shoulder 50 of FIG. 2 is avoided, the iron profile line 62 instead
quickly and steadily climbing up to a level content by the 2000
.ANG. in depth, and further wherein the desired composition of
about 18% Cr 64 and 8% Ni 66 is also reached at 2000 .ANG. in
depth, an improvement of about 60% over the performance of the
convention immersion conditioning results depicted in FIG. 2 in
reducing the depth and extent of surface oxides.
[0017] The present invention is appropriate for practice with a
wide variety of metals, illustratively but not exhaustively
including stainless steels and superalloys and their alloying
elements such as manganese, molybdenum, titanium, etc. The
invention is also applicable to reacting with oxides of these and
other alloying elements to form more easily removed species such as
alkali manganates, molybdates, titanates, etc., for example in the
conditioning of titanium alloys, molybdenum alloys, etc., including
as incorporated into other alloys as alloying agents.
[0018] According to the present invention, the time required to
condition the oxide scale is virtually instantaneous once a final
treatment temperature threshold is reached. For stainless steel,
the final treatment temperature is believed to range from about
600.degree. F. (315.degree. C.) to about 650.degree. F.
(343.degree. C.), and the selected or determined temperature may be
dependent on material composition as well as dimensional
parameters. For example, the 650.degree. F. (343.degree. C.) was
determined to be the final treatment temperature for the 0.635
millimeter (0.025 inch) gage, 10.16 centimeter (4 inch).times.15.24
centimeter (6 inch) panel of type 304 annealed stainless steel of
FIG. 1 by placing a panel coated on one side with an alkaline
aqueous liquid according to the present invention in a horizontal
plane and then heating it (in one example from below with a high
temperature hot air gun or by placing it on a resistance heater
coil). Once the coated surface reached the final treatment
temperature, the 650.degree. F. (343.degree. C.) as determined by a
fine diameter contact thermocouple, a central core area of the
panel very quickly changed from an annealed color to an alkali
chromate color characteristic of conditioning, and wherein the
conditioned area radially grew outward as the critical temperature
was reached in the peripheral areas of the panel. Thus, achievement
of the final treatment temperature and associated complete oxide
treatment according to the present invention may be determined by a
visual examination of the annealed steel, for example by looking
for a glossy molten salt film and distinctive color change
appearance. It will also be understood by one skilled in the art
that the time required for an annealed metal surface coated with an
alkaline aqueous liquid according to the present invention to reach
the final treatment temperature is a function of a difference in
temperature (delta T) between said final treatment temperature and
the temperature of the heating device (heat flux). By thinly
coating the surface of annealed panel to be treated, the present
invention limits the chemical sink present on the metal surface,
and thus once the final treatment temperature is reached, little
additional reaction takes place even if the metal object is held at
the final treatment temperature for some additional time.
[0019] Alkaline aqueous liquids according to the present invention
comprise eutectic hydroxides, and fractional percentages of at
least one surfactant are included to help wet-out performance of
the liquid and aid in maintaining thin coating dimensions. Some
examples further optionally comprise oxidizers to boost the
oxidizing potential of the liquid, and compositions according to
the present invention may be custom blended depending on the type
and quantities of oxides that may be present in the steel to be
conditioned. One embodiment of an alkaline aqueous liquid according
to the present invention used to obtain the conditioning
illustrated in FIG. 4 is a eutectic blend of sodium and potassium
hydroxides at about a 33% by weight (more specifically, 18%
potassium hydroxide and 12% sodium hydroxide), 3% by weight sodium
nitrate as an oxidizer boost, and 67% by weight water, and to which
was added three drops each of Nonidet.RTM. SF-5 and Mirataine.RTM.
ASC (NONIDET SF-5 is a trademark of Air Products and Chemicals,
Inc., in the United States or other countries; MIRATAINE is a
trademark of Rhodia in the United States or other countries).
Nonidet.RTM. SF-5 is a low foaming alkoxylated nonionic surfactant
made from linear alcohol, and the chemical name of Mirataine.RTM.
ASC is Cocamidopropyl Hydroxysultaine.
[0020] According to the present invention, the solubility of the
reaction products, (e.g. alkali chromate) may be quickly reached in
the thin and light weight amount of the alkaline aqueous liquid
incorporating the surfactant. By keeping the coating layer thin,
reactive chemicals in the alkaline aqueous liquid are substantially
consumed immediately upon the coated object reaching the final
treatment temperature with little residual reactants remaining
available for further oxide conditioning or other reactions with
the treated metal object: thus, any time lag from the completion of
conditioning to quenching or water rinsing is generally
inconsequential to the performance of the process, and more
particularly will not cause the over-conditioning harm to the metal
substrate shown by the long-term immersion treatments as
illustrated in FIG. 3. It is also apparent that by only requiring
enough of the alkaline aqueous liquid to thinly coat an object to
be treated, the need for substantial additional quantities in order
to form molten salt baths is avoided, and thus the present
invention enables greater material cost and handling efficiencies
relative to prior art immersion processes. In addition, since the
chemical constituents of the descaling film only need to be stable
at temperature for a very short time, more novel or reactive
chemical compounds may be employed than is possible in traditional
immersion chemical formulations due to their need for long term
high temperature stability.
[0021] The present invention also provides superior energy
efficiencies relative to conventional immersion processes. In one
aspect, it is necessary to operate immersion process salt bath pots
at temperatures well above the final treatment temperature or range
practiced by the present invention. The viscosities of the higher
density anhydrous salt bath solutions appropriate for conventional
immersion processes at the exemplary final treatment temperature
range described above (from about 600.degree. F. (315.degree. C.)
to about 650.degree. F. (343.degree. C.)) are too high, prohibiting
operating the salt bath within this temperature range in order to
prevent excessive salt drag-out, and requiring that the salt bath
pots instead be operated and held at much higher temperatures, such
as from about 752.degree. F. (400.degree. C.) to about 932.degree.
F. (500.degree. C.), in order to prevent material drag-out
problems. It will also be noted that though other prior art teaches
conditioning metal objects by coating the objects with an alkali
solution and then heating the coated objects in an annealing
furnace, these annealing processes require significantly higher
temperatures, generally in excess of 1850.degree. F. (1010.degree.
C.), and further fail to produce the efficient conditioned profile
achieved in FIG. 4 (i.e., reducing the depth of surface scale to
and producing the desired resultant composition of about 18% Cr and
8% Ni at 2000 .ANG. in depth).
[0022] Moreover, the conventional immersion process provides an
"infinite" chemical sink to continually accept reaction products
and provide fresh chemical reactants to a metal object being
conditioned. For example, a stainless steel strip being conditioned
in a processing line exits an immersion bath at a temperature in
excess of the optimum temperature for minimal excess oxide
formation, inherently resulting in an over-conditioning, and
further any time lag between the exit from the immersion bath and
entrance into a quench or rinse water vessel will contribute to
further over conditioning, events that often occur as functions of
line geometry and strip line speeds. Some prior art immersion
systems attempt to remedy this problem through enhancing radiant or
forced cooling, for example through the use of fans; however, such
efforts not only result in uneven cooling or present safety hazards
from spraying molten salt droplets off on the conditioned metal and
about the immediate area, and are generally insufficient in
avoiding over-conditioning as the metal object simply cannot be
quenched or cooled fast enough after optimal conditioning is
reached through an immersion process, as a comparison of the
profiles 42, 44, 46 and 48 of FIG. 2 to the respective profiles 62,
64, 66 and 68 of FIG. 4 clearly illustrates.
[0023] The final treatment temperature of the heated, wetted metal
object is dependent upon the object material, finish and
dimensions, as well as on the alkaline aqueous liquid properties
(water content percentage, etc.). Generally, the final treatment
temperature achieved is the melting point of the conditioning salts
within the chemical mixture plus an additional value to effect
conditioning of the oxide scale on the metal surface at a
reasonable but not excessive rate. The time and heat required to
effect conditioning is dependent upon the thickness and material
content of the strip to be conditioned, which in some cases may act
like a heat sink in absorbing heat that would otherwise raise the
temperature of the metal surface and conditioning solution disposed
thereupon. For example, in the case of one eutectic NaOH/KOH salt
solution useful according to the present invention, bringing the
solution to about 170.degree. C. (338.degree. F.) is sufficient to
melt the salt, but satisfactory conditioning of a thin metal strip
surface requires the temperature to be brought above that point to
about 232.degree. C. (450.degree. F.) with conditioning occurring
virtually immediately upon reaching said temperature, and whereas
another strip example having a greater thickness must be brought up
to a higher temperature of about 288.degree. C. to 315.degree. C.
(550.degree. F. to 600.degree. F.) and further held there for a few
seconds in order to result in an acceptable conditioning of the
steel surface.
[0024] Accordingly, some processing system embodiments according to
the present invention consider object material, finish and
dimension and aqueous alkaline liquid properties. Other parameters
useful in solving or achieving specified or desired object surface
final treatment temperatures, and in some examples, in solving for
final treatment temperature periods, may also be apparent to one
skilled in the art. More particularly, for applications in which
excess descaling chemicals may remain upon a treated surface after
a specified or desired amount of oxide conditioning has been
achieved, the metal surface may be quickly brought down below the
final treatment temperature shortly after achieving said final
treatment temperature, in some embodiments within three seconds or
less, thus preventing over-conditioning by the remaining reactants
prior to rinsing. Illustrative but not exhaustive examples of such
factors include process line observations and events, required
metallurgical properties of a metal object in view of times and
temperatures in an annealing furnace (which in turn may dictate
line speed), object heat up rates, temperature hold times and
dominant anneal line speed requirements. Thus, processing system
embodiments enable on-the-fly optimization in response to and
subordinate to annealing and other line functions line changes to
vary heat-up temperatures, hold times and alkaline aqueous liquid
compositions and application amounts and rates, capabilities not
possible with a conventional large hot immersion process.
[0025] Referring now to the drawings and for the present to FIG. 5,
a somewhat diagrammatic representation of a process or system 100
for scale conditioning section according to this invention is
shown. The line process 100 has an uncoiler 102 adapted to support
and uncoil a coil of steel 104 for removal of scale formed during
annealing. The uncoiler 102 uncoils the steel from the coil 104 as
a strip of steel 106 which is drawn through a conditioning solution
applicator 108 configured to apply a thin coating 110 of an
alkaline aqueous liquid according to the present invention and
described above (e.g. comprising an alkali metal hydroxide or a
mixture of alkali metal hydroxides and a surfactant) to the top and
bottom surfaces 112 of the steel strip 106. At various points in
the system 100, the uncoiled strip 106 is drawn through and guided
by a set of conventional tracking and bridle rolls 107 configured
to keep the strip 106 on track and maintain proper tension in the
strip 106. While the diagram illustrates the line in a horizontal
plane, it is not the intention to limit the line configuration to a
single plane. Certain elements such as the solution applicator 108
may be easily configured in a vertical plane followed by other
vertical or horizontal or angled elements as necessary to carry out
the process and/or accommodate physical line constraints. In some
embodiments, the system/process 100 is a continuous anneal and
pickle line, wherein the uncoiling element 102 also provides
pre-heat and annealing furnace elements in order to heat and/or
anneal the steel strip as will be appreciated by one skilled in the
art in the art. While FIG. 5 illustrates a metal strip 106 moving
relative to a stationary application nozzle 108, other
configurations where metal shapes other than strip may benefit from
a movable application device relative to a stationary metal object
are also anticipated.
[0026] The surface temperatures of the steel strip surfaces 112 at
application of the thin coating 110 of the alkaline aqueous liquid
by the conditioning solution applicator 108 are below the
Leidenfrost temperature of the alkaline aqueous liquid, and in some
embodiments also below the melting point of alkali metal hydroxides
within the conditioning solution. Sensors 105/115/116 may be
provided comprising temperature-sensing devices (e.g. an infrared
temperature sensor, a contact thermocouple, etc.) configured to
measure temperatures of the strip 106 at various points in the
process/system 100 as needed to verify that a desired temperature
has been achieved, thus at 105 prior to solution application by the
conditioning solution applicator 108. Ambient environmental
temperatures are generally below boiling point and Leidenfrost
temperatures, and thus steel strip 106 uncoiled by an uncoiler 102
without annealing furnace elements or processes will typically be
at a temperature appropriate for application of the alkaline
aqueous liquid by the conditioning solution applicator 108.
[0027] If, however, the uncoiler 102 anneals the strip 106, then
the annealed strip 106 must first be quenched or otherwise cooled
to bring the strip surfaces 112 down to a temperature below the
Leidenfrost temperature prior to application by the conditioning
solution applicator 108. In some applications, the line of steel
strip 106 may be stopped or a cooling time period must otherwise
lapse until the strip surface 112 temperatures cool to an
acceptable temperature at the application of the solution. In other
examples, the system/process 100 may further incorporate a
temperature cooling section at 105 which includes one or more
variable speed fans, flow control dampers, vents, or the like in
order to cool the strip surfaces 112 to a desired temperature as
confirmed by said temperature sensor 105.
[0028] Setting or achieving the final treatment temperature may
also consider difficulties in uniformly controlling the temperature
of the metal strip surfaces 112. For example, when the component
102 anneals the strip 106, such as within a continuous anneal and
pickle line, the strip exiting an annealing furnace element 102 or
an air cooler 105 positioned thereafter may have temperature
differentials between different regions, for example between
different edge regions, and/or between the top surface and the
bottom surfaces. Such differential values may range from
100.degree. F. (38.degree. C.) to 200.degree. F. (93.degree. C.),
depending on the strip dimensions (gage, width, thickness) and the
metal composition (carbon, stainless, etc.). Thus, while some
regions may be at a desired final treatment temperature, other
regions of the strip 106 may be too hot and experience Leidenfrost
effects at application, or they may be too cold and thus not
successfully brought up to the final treatment temperature in the
heating element 113 and thereby experience incomplete conditioning.
(Such concerns are generally not an issue in traditional immersion
salt bath, as the elevated temperatures of the molten salt baths
are applied long enough to result in uniform strip
temperatures.)
[0029] Accordingly, in some embodiments, the cooling elements 105
cool the strip surfaces 112 to a point below the Leidenfrost
temperature of the conditioning solution plus an additional cooling
margin value (for example, 100.degree. F. (38.degree. C.) to
200.degree. F. (93.degree. C)). in order to ensure that no regions
of the strip surfaces 112 are above the Leidenfrost temperature.
Further, some heating elements 113 heat the strip surfaces 112 to
the final treatment temperature plus an additional heating margin
value (for example, 100.degree. F. (38.degree. C.) to 200.degree.
F. (93.degree. C)). in order to ensure that all regions of the
strip surfaces 112 are brought to the final treatment temperature.
Additional cooling or heating margins provided to account for such
regional differentials may be small or even omitted for some very
light gage steel strips, as their regional differentials may be
small or negligible, and in one aspect due to lower heat sink and
heat retention characteristics for lighter gage strips.
[0030] Formation of the thin alkaline aqueous liquid coating 111 by
the conditioning solution applicator 108 may be achieved by a
variety of ways, i.e., through any method or system that forms a
uniform coating or complete wetting of the strip surfaces 112 with
the conditioning solution 110. Illustrative but not exhaustive
examples of conditioning solution applicator 108 elements and
apparatuses include dunker roller or roll/roller coaters 109 as
well as spray nozzles, curtain coaters and applicators, immersion
methods and systems or combinations thereof. Solution metering or
flow control articles may be utilized but are not generally
required, and a conditioning solution applicator 108 may need only
incorporate simple application limiting means that ensures complete
wetting of the strip surfaces 112 to a specified maximum thickness
amount 111; for example, an air knife or wiper roller may be
provided to remove excess conditioning solution 110 and effect the
specified minimum and/or maximum solution thickness values 111,
with excess solution removed and recovered for subsequent re-use.
Other methods and systems appropriate for use in assuring adequate
and/or limited total thickness values 111 of the conditioning
solution 110 applied to the strip surfaces 112 will also be
appreciated by one skilled in the art.
[0031] The coated strip 106 is then driven into a heating section
113 wherein the coated strip surfaces 112 are brought up to a
specified final treatment temperature or temperature range above
the melting point of the solution alkali metal hydroxide(s) in
anhydrous form plus an additional value to effect conditioning of
the oxide scale on the metal surface at a reasonable but not
excessive rate. The specified final treatment temperature need be
maintained only long enough to thoroughly condition the engaged
oxide scale, in some embodiments for no more or no less than a
specified time period as described above, and wherein at the end of
said period the heated strip temperature may be reduced below the
conditioning temperature or range of temperatures by cooling or
quenching in a cooling/rinsing section 114, which may also
generally rinse off any excess, non-consumed conditioning solution
alkaline products.
[0032] As also described above, the specified final
temperatures/ranges and time periods are selected to produce
preferred scale conditioning of the strip 106, and more
specifically to be sufficient in both temperature and length of
time to complete scale conditioning of the strip surfaces 112, yet
limited in either or both of length of time and high temperature
values in order to prevent over-conditioning of the strip surfaces
112. In some embodiments, the desired level of conditioning is a
specified level of least-oxide-to-pickle conditioning and minimal
base metal effect level selected as a function of strip material
and dimension parameters, thereby minimizing the thickness and
extent of oxide scale formation while successfully conditioning the
steel surface.
[0033] The mechanism of conditioning according to the present
invention is believed to be generally comparable to that of
conventional molten oxidizing baths; the metal oxide is converted
to a higher oxidation state that is partially dissolved in the
conditioning salts and subsequent water rinse, the remainder
rendered more readily removable by acid pickling. However,
conditioning of the metal surface in the present invention occurs
as the metal surface when completely wet with the solution is then
heated until the water is evaporated and the salts are melted and
react with the oxide on the strip surface, which occurs rapidly,
often within seconds. In some embodiments, the conditioning process
is terminated by rinsing and thereby cooling or quenching the strip
106 in the rinsing station 114 by the end of a specified
conditioning time period (i.e., after effective conditioning occurs
and prior to the occurrence of excessive oxide formation by
remaining, residual reactants) to bring the strip surface 112
temperatures down below conditioning temperatures and also rinse
the conditioning solution 110 off of the strip surfaces 112, for
example through an array of water spray nozzles (not shown) being
supplied with water, sometimes through use of a pump from a
collection sump located below a spray area 114, and still other
rinsing station 114 systems and methods will be appreciated by one
skilled in the art. Temperature sensing/cooling elements 115
interposed between the heating section 113 and rinsing section 114
may ensure that the strip surface 112 temperatures are quenched
below conditioning temperatures.
[0034] After the rinsing station 114, the conditioned strip 118 may
be driven to an acid pickling section 120. By minimizing the degree
of oxide formation, the present invention correspondingly reduces
the amount of subsequent surface pickling required, which may thus
reduce associated pickling processes as well as reduce surface
dulling and roughening due to pickling relative to prior art
processes. In contrast, prior art immersion bath processes using
molten oxidizing baths may over-condition scale on a strip and form
a cohesive base metal-iron oxide interfacial layer and subjacent
chromium depleted zone, thus requiring more aggressive pickling and
increasing both pickling costs and pickled surface roughness while
also reducing product yield due to increased metal removal.
[0035] Acid pickling in the process 100 at 120 usually includes one
or more acid tanks containing sulfuric acid and/or a mixture of
nitric, hydrofluoric or other acids for submersion of the strip
106, although acid spray could also be used. In some embodiments,
multiple acid pickles are utilized, and one or more of such pickle
tanks may be used as required on any given strip 110 of stainless
steel depending on many factors, including the composition of the
steel, the thickness of the oxide, and other factors known in the
art. Novel pickling compositions incorporating organic acids such
as citric acid, or more environmentally-attractive acid mixtures
incorporating oxidizing agents such as peroxides, would also
experience enhanced performance if used in conjunction with this
invention. With the scale removal process complete after pickling
and rinsing, the descaled strip 122 is ready to be recoiled into a
finished steel coil 124 on a recoiler 126.
[0036] Surface analyzers are generally provided within the
system/process 100, for example within or adjacent to the rinsing
station 114, the temperature sensing/cooling elements 105/115/116,
within the pickling section 120, etc., said surface analyzers
configured to monitor the strip surfaces 112 to detect and/or
determine the amount of scale formed on the surface of the strip to
be conditioned, a lack of conditioning or over-conditioning of the
conditioned strip, etc., and to otherwise provide feedback and
monitoring of conditioning performance of the system/process 100 to
one or more line dynamics operating components, systems or other
management elements 128. The line dynamic system component 128 may
also communicate with various temperature sensing/cooling elements
provided throughout the system/process 100 (for example, at
temperature sensing/cooling elements 105/115/116, the conditioning
solution applicator 108, the heating section 113, the rinsing
station 114 and/or the acid pickling section 120, etc.), thereby
ensuring specified strip temperatures during application of the
conditioning solution, conditioning, rinsing and pickling
operations as well as to enable optimizing of performance of the
system 100 in response to said temperature observations. Inputs to
the line dynamics operating system 128 may also include present
system conditions, such as ambient temperatures, conditioning
solution storage tank level sensors, flow controllers and
distribution sensors, and storage tank temperature sensors. In some
embodiments, chromium concentrations are observed in the rinse
water in the rinsing station 114, thereby providing a direct
measure of chromium removed from the scale to the line dynamic
system component 128 or other operator, and other inputs will be
appreciated by one skilled in the art.
[0037] The line dynamics operating system 128 may receive strip
variables such as strip material composition, gauge, width, and any
other special processing information as discussed above and
elsewhere herein and then responsively determine specified
conditioning temperatures and time periods, or to otherwise control
a conditioning schedule or other system 100 parameters for the
particular strip of steel 106 being treated. In some embodiments,
the line dynamics operating system 128 includes a computer in
communication with a memory in which is stored time, temperature
and other parameters required for conditioning each of a plurality
of types of steel, and further based on composition, gauge, width,
line dynamics, etc.
[0038] Said surface analyzer elements may continuously monitor the
condition of the strip 106 and if the strip surface 112 condition
falls outside predetermined parameters, the line dynamics operating
system 128 may adjust system/process 100 parameters to bring
monitored surface conditions back within the required performance
thresholds. Illustrative but not exhaustive parameters that may be
controlled by the line dynamics operating system 128 include an
amount of energy or heat expended and directed toward the strip 106
by, or amount of temperature increase effected by, the
uncoiling/annealing/preheating element 102 and the heating section
113; the motive speed of the strip 106 relative to any of the
system elements 108/113/115/114/116/120; the amount of cooling air,
temperature or amount of temperature increase effected by cooling
elements at 114, 115 or 116; and other system 100 parameters may be
controlled by the line dynamics operating system 128.
[0039] The present invention enables on-the-fly scale conditioning
optimization by varying one or more of (1) terminal conditioning
temperatures or ranges of temperatures, (2) chemical composition
components, (3) amounts of reactants utilized (e.g. amounts of
chemicals applied to the scale on the strip surface 106 at 108) and
(4) reaction time periods or ranges (e.g., by variably cooling the
strip at a desired point to quench out a conditioning reaction).
Prior art salt bath immersion processes cannot achieve such
objectives due to the thermal inertia of the large salt mass in the
salt baths, the line-speed dependent exposure times to the
chemicals, and the static chemical composition of said baths.
[0040] Optimal amounts of scale conditioning may also be defined
with respect to least- oxide-to-pickle condition specifications or
observations for given metal object material and dimension
parameters, as well as with respect to minimal base metal effects.
In one aspect, the appearance and cost of producing a final pickled
surface at 120 may provide a measure of the value of changing scale
conditioning variables at any of the various process/system 100
elements. For example, in some embodiments, optimal conditioning
may comprise determining that no chrome oxide and only minimal or
trace amounts of iron oxide have been formed on a 3XX or 304
stainless steel strip surface, wherein observing a heavier iron
oxide or a nickel oxide formation would indicate that a
conditioning salt in a solution applied at 108 has stayed in
contact with the strip surface for too long or at too high of a
treatment temperature. The conditioning time period could then be
shortened by earlier rinsing or quenching at 114, or the extent
and/or rate of conditioning could be lowered by lowering the strip
conditioning temperatures achieved at 113, in one aspect thereby
preventing dulling of the strip surface and reducing the amount of
pickling required at 120 to achieve a finished surface. Depending
on the specific alloy being processed and its unique scale
composition reactivity, it is also conceivable that the chemical
composition applied to the scale for conditioning purposes be
adjusted to provide more or less oxide reactivity in concert with
or independent of controlling conditioning temperature and
time.
[0041] In general, conditioning occurs more rapidly on metal strips
with good tight surface conditions and thinner gages, and the
extent of the conditioning may be fine tuned with a greater
tolerance or accuracy through adjustment of time and temperature
parameters relative to duller metal objects and those with heavier
strip materials, the processes and systems according to the present
invention thus providing opportunities for energy savings and
enhanced performances and efficiencies in conditioning said thinner
gage/good tight surface objects over prior art submersion systems.
The descaling system can also react to the varying absorption and
emissivity of shiny versus dull metal surfaces.
[0042] By keeping the temperatures of the steel coil or strip
surfaces below the boiling point and the Leidenfrost temperature or
Leidenfrost point of the conditioning solution during application
of said solution, problems with respect to the Leidenfrost effect
are avoided. The "Leidenfrost effect" with respect to a metal strip
is a mottled or speckled surface appearance of the strip which
reveals patches, or spots of incomplete scale conditioning, and
which is believed to occur due to the Leidenfrost effect on an
aqueous solution of chemicals if the surface temperature of a strip
during application is above what is known as the Leidenfrost
temperature or Leidenfrost point of the conditioning solution. If
the strip is above the Leidenfrost temperature of the conditioning
solution when the conditioning solution is applied (which is
typically at or above the boiling point of aqueous conditioning
solution), then a thin film of the solution is converted to a vapor
phase barrier between the metal strip surface and the applied
solution, this vapor phase barrier preventing the conditioning
solution from contacting the surface of the strip and depositing
conditioning chemicals on the metal surface upon evaporation of the
liquid, resulting in a failure to condition portions of the surface
and thereby producing a mottled appearance due to the contrast
between conditioned and unconditioned areas. Thus, as used herein,
the term "a temperature below which the Leidenfrost effect appears"
refers to a temperature at which no appreciable scale in the form
of dark spots exists after scale conditioning according to this
invention and subsequent pickling. The Leidenfrost effect is well
known and described in many publications. The interested reader is
referred to U.S. Pat. No. 6,450,183 issued to Cole, et al. for
"Composition, apparatus, and method of conditioning scale on a
metal surface" on Sep. 17, 2002, as well as to two other
publications: "Disk Model of the Dynamic Leidenfrost Phenomenon"
(Martin Rein at DFD96 meeting of American Physical Society) and
"Miracle Mongers and Their Methods" (pages 122-124 by Harry
Houdini, published 1920 by E. P. Dutton).
[0043] Some embodiments of the present invention avoid the
Leidenfrost effect by first completely wetting the strip surface to
be descaled with an aqueous alkali metal hydroxide(s) conditioning
solution to form a wetting layer when the metal surface
temperatures are at a temperature below the Leidenfrost temperature
of the conditioning solution, and then subsequently heating and
increasing the wetting layer solution (e.g. 111) and the surface
temperatures of the strip surface (e.g. 112) to a temperature above
the melting point of the essentially anhydrous form of the alkali
metal hydroxide material in the conditioning solution plus the
additional value described above to reach the final treatment
temperature for a sufficient time to thereby condition the metal
strip surface (e.g., 112). As used herein, the term "essentially
anhydrous form of the material" means after the water of solution
is evaporated, even though there may be some water of hydration
still present in the material. In this fashion, the formation of
the vapor barriers known to cause the Leidenfrost effect, and thus
the Leidenfrost effect upon the surface of the conditioned strip
surfaces (e.g., 118/122) is avoided.
[0044] The boiling and Leidenfrost temperature points of an aqueous
caustic conditioning solution are a function of caustic
concentration. For example, for conditioning solution embodiments
with low (40% by weight or lower) alkali hydroxide concentrations,
and wherein application is desired at surface temperatures below
both boiling and Leidenfrost temperature points, the steel strip
surface 112 or coil 104 temperatures at application of the
conditioning solution range should not exceed from about
180.degree. F. (82.degree. C). to about 260.degree. F. (127.degree.
C.), though for solutions with higher (47% or higher) alkali
hydroxide concentrations the steel strip surface 112 or coil 104
may be at higher temperatures during application, meeting or even
exceeding 290.degree. F. (143.degree. C.).
[0045] Additional energy savings may be realized by applying the
aqueous caustic conditioning solution while a steel coil or strip
is at higher elevated temperatures, temperatures above the
conditioning solution boiling temperatures, as well as above a salt
fusion temperature point of the solutions, but still near and below
the Leidenfrost temperature of the conditioning solution. Thus, in
one example where an application temperature of 400.degree. F.
(204.degree. C.) is above both the boiling point and the salt
fusion point of a conditioning solution, a relatively smaller heat
increase is required by a furnace or heating station 113 to get to
a final treatment temperature ranging from 550.degree. F.
(288.degree. C.) or 600.degree. F. (316.degree. C.)). Thus, less
heat energy is expended by a furnace or heating station 113, and/or
over shorter the time periods, relative to the requirements for
heat energy expenditures when a lower application temperature is
selected (for example, one below the boiling point and/or the salt
fusion point).
[0046] Different compositions may be used to effect descaling
according to this invention. In one embodiment, a eutectic of
sodium hydroxide (NaOH) and potassium hydroxide (KOH) at about 42%
sodium hydroxide and about 58% potassium hydroxide a base alkali
hydroxide composition is provided. This is a low melting
composition in its essentially anhydrous condition (170.degree. C.,
338.degree. F.), and when the water of the aqueous solution is
evaporated and the remaining hydroxide fused, it is effective to
perform scale conditioning. Other materials may also be added to
the solution to modify the properties of either the solution or the
composition, and for examples and other information
commonly-assigned U.S. Pat. No. 6,450,183 issued to Cole, et al. on
Sep. 17, 2002 and entitled "Composition, apparatus, and method of
conditioning scale on a metal surface," the entire disclosure of
which is hereby incorporated by reference, and which provides that
additives such as potassium carbonate, potassium chlorate, sodium
nitrate, sodium permanganate, and potassium permanganate are
beneficial, for example at from about one weight percent (1%) to
about five weight percent (5%).
[0047] Descaling performance and costs are directly related to the
percentages of base alkali hydroxide compositions within
conditioning solutions according to the present invention. Although
scale conditioning may be generally achieved over a wide range of
alkali hydroxide concentrations (for example, from about 5% to
about 65% by weight), different percentage values have different
impacts on system performance. More dilute/low alkali hydroxide
percentages (from about 5% to less than about 20%) have
proportionately lower surface tension and improved surface wetting
characteristics relative to higher concentrations, but also impose
proportionally greater heating energy penalties in order to heat
and evaporate the proportionately larger amounts of water in the
solution to effect scale conditioning by the fused hydroxide(s);
they also require that the incoming strip temperature be lower to
avoid Leidenfrost (or optionally, also the boiling point) of the
more dilute solution. Higher concentration solutions (from about
20% to less than about 50%) appear to strike a good balance between
delivering reasonable dissolved solids content at acceptable energy
requirements. U.S. Pat. No. 6,450,183 further provides that as the
concentration of the salt in solution increases, the upper
temperature that can be used without encountering Leidenfrost
effect increases, for example to about 700.degree. F. For example,
a 47% solution disposed upon a metal surface and then heated
requires two British Thermal Units (BTU's) of energy per applied
wet gram to heat the solution enough to bring the contained alkali
hydroxide(s) into a molten or fused state on the surface of the
metal. While not insignificant, the heat input requirement for
dehydrating and fusing the applied chemical represents only about
10% of the total heat energy required by the process; the other 90%
or so of the energy is absorbed by the metal itself as its
temperature is raised from essentially ambient to about 600.degree.
F. (e.g., of the heating section 113).
[0048] Though it may initially appear to be preferable to use
higher caustic concentrations within the conditioning solution
(e.g. 48% or more), higher alkali hydroxide concentration
conditioning solutions present other problems and difficulties. At
concentrations much above about 47 weight percent, supersaturation
and crystallization of some of the dissolved chemicals occurs at
ambient temperatures. This requires mixing, transport, holding
tanks, and application apparatus all to be heated to maintain
homogeneous solutions. Besides presenting higher manufacture,
transportation, storage, and delivery difficulties and associated
costs, the higher-solids concentration in such solutions presents
problems in achieving complete wetting of steel surfaces relative
to lower concentration solutions. Higher surface tensions cause
incomplete surface coating and wetting of the metal surface to be
conditioned, in particular during dehydration and fusion of the
alkali hydroxide during heat-up (e.g. by the heating section 113),
which may cause incomplete scale conditioning. Wet-out and
flowability characteristics are also a function of metal surface
attributes, and these problems are more pronounced in conditioning
the smooth, minor finishes of the more expensive stainless and
superalloy steel strips. Any area that is poorly conditioned will
not be completely pickled (e.g., by the acid pickling section 120),
and even a 0.01% failure in total area surface conditioning may be
unacceptable, particularly in the case of stainless steel. In order
to facilitate the use of higher concentration caustic solutions,
e.g., those of about 25% to near 50%, as discussed above, in some
embodiments of the present invention a base alkali hydroxide
composition of about 42% sodium hydroxide and about 58% potassium
hydroxide incorporates a small amount of surfactant into the
solution, which reduces the solution surface tension caused by the
presence of relatively higher dissolved caustic solids percentages
in the solution, while also enabling the conditioning solution to
exhibit low foam characteristics.
[0049] In another aspect, scale conditioning according to the
present invention takes place in the presence of oxidizers,
generally in an oxygen-containing atmosphere. Although the alkaline
aqueous liquid need not contain any oxidizing agents, the thin film
111 will have an oxidizing effect on the surface oxides and thereby
convert them to the desired higher oxidation state due to the
absorption of atmospheric oxygen by the wetting solution and/or the
diffusion of atmospheric oxygen through the molten salt film 111,
and wherein the heating section 113 heats the wetted strip in an
oxygen-containing atmosphere. In one example, the surface of a
10.16 centimeter (4 inch).times.15.24 centimeter (6 inch) panel of
0.635 millimeter (0.025 inch) gage type 304 annealed stainless
steel type 304 (18/8 chrome-nickel) was wet with an alkaline
aqueous liquid solution comprising a surfactant and a base alkali
hydroxide composition comprising a eutectic of about 42% sodium
hydroxide (NaOH) and about 58% potassium hydroxide (KOH) and heated
in a furnace in an oxygen-containing, ambient air atmosphere at
about 399.degree. C. (700.degree. F.) for about 90 seconds,
bringing the surface temperature of the wetted steel to a final
treatment temperature of about 232.degree. C. (450.degree. F.).
Upon cooling of the panel, a visual examination revealed that a
pronounced scale conversion had taken place and, after a subsequent
water rinse, a well-conditioned surface was apparent. Pickling of
the panel after the water rinse confirmed the visual assessment of
success.
[0050] One skilled in the art must also appreciate the potentially
competing reactions between the alkali hydroxides and atmospheric
oxygen to complete a desired descaling reaction with the
simultaneous neutralization reaction between atmospheric carbon
dioxide and caustic alkalis to form ineffective alkali carbonates.
If a heat up rate is slow and/or the atmosphere to which the coated
metal is exposed to is high in carbon dioxide (as would be the case
in a furnace heated with a carbon-based fuel such as natural gas or
propane to produce carbon dioxide as a product of combustion), the
desired scale conditioning reaction could be retarded or prevented
altogether.
[0051] One benefit of the present invention is the ability to
utilize conditioning compositions that cannot be used effectively
in conventional anhydrous molten salt baths because the mass of
material surrounding the surface prevents atmospheric oxygen
diffusion. The solution can also utilize additives that may be
unstable at typical anhydrous molten salt bath temperatures.
Furthermore, this invention eliminates the presence of reaction
products in the applied salt and thus allows complete control of
the chemistry of the salt at the metal surface. Further, with
respect to direct surface wetting embodiments of the present
invention, the quantity of salt consumed can be controlled through
to a specified amount, in contrast to immersion systems, wherein
salt consumption is largely dictated by the quantity of salt that
adheres to the surface of the metal as it is withdrawn from the
molten bath. Additionally, as in some cases it may be desirable to
use a different salt chemistry when different metals are treated,
switching solutions is easily accomplished, with no need to heat
large baths of each solution. Conventional immersion technology
requires a molten salt bath or tank that holds tens of thousands of
pounds of liquid, hot chemical. Molten salts in general are
excellent heat storage media and require significant time (several
hours or longer) to cool or raise their temperature. This severely
limits the ability to change process temperature "on the fly" and
prevents real-time descaling optimization from being practicable.
Taking into account the now-available process variables of variable
chemical compositions, application rates, reaction times, and
reaction temperatures, it is now possible for the first time to
fully optimize precise descaling performance dynamically.
[0052] It should be noted that while the embodiments discussed thus
far use sodium or potassium cations within the alkaline aqueous
liquid conditioning solution, alternative solutions may utilize
different cations, and associated descaling parameters and effects
are primarily dependent upon the particular anion present. In one
aspect, alternative conditioning solution compositions may work
about as effectively with one cation as with another if other
factors, such as solubility and compatibility, are equal. For
example, sodium nitrate or potassium nitrate may also be effective
in conditioning solutions according to the present invention, and
may give comparable results in general, though typically much less
soluble in a base composition and thus perhaps requiring different
relative cation and/or surfactant concentrations. Other examples
provided by U.S. Pat. No. 6,450,183 include sodium bisulfate,
sodium carbonate, potassium carbonate, sodium formate, sodium
metasilicate, sodium nitrite, sodium acid pyro phosphate and mono
sodium phosphate. In some cases, the selection of a cation of an
additive or caustic compound utilized may be dictated by
availability. It is also noted that use of a surfactant is
dependent upon compatibility with solution additives or base
cations: for example, surfactants may be incompatible with
permanganate compounds and thus excluded from such embodiments.
[0053] Performance of alternative compounds used as sole descaling
agents may be easy to judge visually, wherein ineffectiveness of
conditioning may be confirmed by subsequent pickling after which an
original scale would be present in unchanged form. Evaluation
criteria for selecting appropriate conditioning solutions and
specified time and temperatures may include appearance of
conditioned oxide with regard, e.g., to color, opacity, and
uniformity; ease of removal of conditioned oxide by rinsing, wiping
or subsequent acid pickling, and final appearance of a descaled
metal surface with regard, e.g., to color, brightness, uniformity,
and freedom from residual oxide. It is to be understood that these
several criteria can vary independently in degree and direction one
from another, so that there is a certain subjective element to the
quantitative assignment of detrimental or beneficial effects of any
descaling agents or additives.
[0054] While the present invention has been illustrated by the
description of the embodiments thereof, and while these embodiments
have been described in considerable detail, it is not the intention
to restrict or in any way limit the scope of the appended claims to
such detail. Additional advantages and modifications may readily
appear to those skilled in the art. Therefore, the invention, in
its broadest aspects, is not limited to the specific details, the
representative apparatus, or the illustrative examples shown and
described. Accordingly, departures may be made from such details
without departing from the spirit or scope of the applicants'
general inventive concept.
[0055] Units which are used in this specification and which are not
in accordance with the metric system may be converted to the metric
system with the aid of the following formulas: 1.degree.
C.=(.degree. F.-32) 5/9; 1 inch=2.54.times.10.sup.-2 m; and 1
F.p.m. (foot per minute)=5.08.times.10.sup.-3 m/sec.
* * * * *