Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
7~3l3
METHOD OF BOX-~NNEALING STE$L
SHEET TO MINIMIZE ANN~ALING STICKERS
The present invention relates to the
box-annealing of steel sheet.
The heat treatment of cold-reduced ~teel sheet
and strip is accomplished either in batch operations or
in continu~us operations. Batch heat treatment may be
divided into two categories: (a) open-coil annealing
in which a tight coil is first rewound with a suitable
spacer in between each wrap of the coil to permit
circulation of the furnace atmosphere between the
individual wraps to hasten and improve the unifor~ity
of heating ~ and (b) box annealing in which a large
~tationary mass of steel (either cut ~heet or coils~ ss
subjected to a comparati~ely longex heat treatment
cycle by varying the temperatuxe within the furna~e
that surrounds it. In the latter, box annealing
practice~, the mass of steel is slowly raised to the
desired annealing temperature and soaked at such
temperature fox a period of 1/2 to 24 hours~ The use
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of such a practice provides full recrystallization of
severely cold-reduced steel and results in the softest
possible finished product. Ho~ever, one draw~back to
such practice is the tendency of the individual wraps
of sheet, because of their tightly wound nature, to
pressure weld or stick together when held at annealing
temperature for extended lengths of time. ~he tendency
to sticking increases with increasing pressure between
the wraps and increasing time and temperature ~f the
anneal. When sticking of adjacent coil wraps occurs,
particularly in products with critical surface finish
requirements, poor yields result. Additi~nally,
production delays are encountered at a subsequent
temper mill because lower rolling ~peeds are required
or because recoiling may be necessary before further
processing. To prevent such sticking, the prior art
has resor~ed to the use of a variety of separating
~edia such as colloidal solutions of alumina or silica,
finely divided magnesium oxide particles and the
formation of thin oxidizing films on the metal
surface. The latter technique includes the use of
silicate containing washing liquid~ aids in preventing
sticking. ~his is a practice which has been
recommended for many years by manufacturers of
commerci~l cleaning compounds, Although many steel
facilities use silicated cleaners, such use has not
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precluded the serious incidence of coil sticking.
According to the present invention, there is provided in the method for
the box-annealing of a coil of steel sheet wherein said sheet is
electrolytically cleaned in a cleaning solution consisting of silicate or
phosphate cleaners, or mixtures thereof, passed through a rinse solution at a
speed of 1,000 to 3,000 ft./min., coiled and thereafter heated to an annealing
temperature of l,050 to 1,400 F in a non-oxidizing atmosphere and soaked at
such temperature for a time of at least one-half hour, whereby said annealing
tends to result in the sticking together of adjacent sheets of said mass,
the improvement for red~cing the tendency of said sheets to stick
together, wherein the solution utilized for said rinse consists essentially of
salts selected from the group consistine of calcium formate, magnesium
formate, calc;um acetate, magnesium acetate, or mixtures thereof, in a total
concentration of said sslts of from 3,000 to 8,000 ppm, and the drag out filrn
remaining on the sheet is controlled such that after drying, the residue
concentration of said salts will be within the range 0.3 to 2.0 m~./ft. .
In another embodiment, the invention provides in the method for the
box-annealing of a mass of cold-reduced steel sheet wherein said sheet is
electrolytically cleaned in a cleanin~ solution consisting of silicate or
phosphate cleaners, or mixtures thereof, rinsed, dried, coiled and thereafter
heated to an annealing temperature in a non-oxidi~ing atmosphere and soaked at
such temperature for a time of at least one-half hour, whereby said annealing
tends to result in the sticking together of adjacent sheets of said mass,
the improvement for reducing the tendency of said sheets to stick
to~ether, wherein the solution utili~ed for said rinse contains salts seLected
from the group consisting of magnesium formate, magnesium acetate, or mixtures
thereof, in a total concentration of said salts of from l,S00 to lO,000 ppm.
Prior to the present invention~ a laboratory procedure had been developed
to obtain a quantitative measurement of the sticking tendency of sheet during
box annealing, and which measurements were found to correlate quite well with
actual mi]l experience. Investigations were conducted using different types
of steel, both continuous cast snd ingot cast steels, since the prior art had
suggested that steel composition, particularly carbcn and phosphorus contents,
had an effect on sticking. Compositions of two of the ingot cast steels
evaluated are listed in Table I.
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T~BLE I
Compositiolls of Full-Hard Steels, % by weight
Element "A" "B"
C 0.11~ 0.10
Mn 0.388 0.42
P ~.~07 ~.007
S 0.024 0.023
Si 0,0~2 OoOl9
Cu 0.014 0~038
Ni 0~016 0~011
Cr 0.02g 0.026
Mo 0.006 OoOll
Al 0.002 0.002
~ 0.004 O.V0
Steel panels were cut to a si~e of 2-3/4" x
8"(7 to 20 c~.). A~ter vapor degreasing, the panels
were subjected to electrolytic alkaline cleaning in
various solutions (indicated in TABLE II~ mai~tained at
a temperature of 82C, at a current density of 10.8
amps/d~ for a period of one second (either cathodic
or anodic)~ After electrolytic cleaningF th~ panels
were passed through rubber wringer rolls to remove
excess cleaning solution. Rinse water at a temperature
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of 54-60C was sprayed on the steel surface to wash
off the remaining cleaner and the panels were then
dried to remove unbound water.
Four 1" x 2" (2.5 to 5.1 cm.) coupons were cut
from each cleaned and rinsed panel to provide 24
coupons to be assembled in a test pack. Each pack
consisted of 12 paired test specimens, with each pair
separated by a stainless steel spacer. To simulate the
pressure e~erted by the wrapping of coils, a 15.2 lb.
(6O9 kg.) weight was used for each run. The test pack
was placed in a sealed stainless steel annealing box
containing a protective atmosphere of 6% H2 ~ 94%
~2 with a dew point controlled to minus 40C by
passing the gas mixture through a column of c~lcium
sulfate. The anne~ling box was placed in a furnace and
heated to an annealing te~perature of 1250F
(677C) in two hou~s. After a three hour soak period
at said annealing temperature, the furnace was cooled
rapidly. Test packs were removed after the annealing
box was cut open and individual tension-shear test
speci~en pai~s were placed in a tension tester and
pulled apart. The amount o force required to pull the
samples apart was then utili~ed as a quantitative
measure of the degree of sticking that occurred during
annealing. For each group of 12 specimens, the average
sticking force and the variance were calcula~ed. Each
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average value is reported in TABLE Il with a 95%
confidence interval.
~ tilizing the above test procedures, previous
experiments with silicated cleaners (no~ utiliYing the
rinse additives of the present invention) had shown
that significantly lower sticXing force values resulted
when final strip polarity was cathodic. This is a
difference which was not observed when non-silicated
cleaners were app~ied. In this regard, even when final
strip polarity was anodic lower sticXin~ force values
resulted from the use of silicated cleanersO Such
results had suggested a mechanism relating to surface
composition, which might involve the presence of
silicates or other residue ~e.g. thermal decomposition
products of various constituents of the rinse water)
playing a part in reducing sticking, wherein the
concentration of such residues would be increased by a
final cathodic pass. This was borne out by a
comparative test in which sticking tendency~ uti~izing
deionized ~ater as against the normally employed mill
water was used in the final rinse. Use of the mill
water produced a significant decrease in sticking
force. ~ince mill water is known to be ~ade hard
primarily because of magnesium and calcium ions~ a
number of rinse additives containing soluble Galts o
magnesium and calcium were e~aluat~d to determine
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whether the cations of such salts would al50 be a
factor vis~a-vis sticking tendency. When used at a
concentration of 3,000 ppm in the rinse, manganese
sulfate, calcium phosphate, and magnesium sulfate did
5 not significantly change sticking ~orce values from
those resulting ~hen hard mill water was applied.
However, the formates and acetates of calcium and
magnesium did provide significant changes of sticking
force, and the results o same are reported in TABLE
II. Except a.~ indicated, all specimens were
electro-cleaned utilizing a commercial silicated
cleanerO For conditions 3 and 8, electro-cleaning wa~
with a commercial phosphated cleaner.
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TAIBLE I I
Labora tory St i ck i n~ E:xper i ment
St ee 1 "A"
Rinse Electro- Rinse Sticking Force, Kg.
5Condition lytically After Strip Strip
No. Cleanecl Cleaning Cath~dic Rnodic
~o Nc~ne 2 58+10
2 Yes Deionized 212~13 243-~ 9
water
lL) 3 Yes Hard mill 215~10 206~ 7
water
4 Yes Hard mlll 163+18258+16
water
Yes ~Ig(HC00)2, 253~10209+28
9 7 9 ppm
6 Ye s Mg ~ HCO0 ~ 2, 5 6+ 9 9 8~ 8
3855 ppm
7 Yes Mg(HCOO);2,44~ 765+11
5~71 ppm
8 Yes Mg(HC00~2,109+15 138+10
5871 ppm
9 Yes Ca(HC00)2, 155~13192~15
1942 ppm
25 10 Yes Ca~HC00)2, 114+15125*15
3787 ppm
11 Yes Ca(E~C00)2, 59~1590~15
5826 ppm
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TABLE II (Continued)
Laboratory Sticking Experiments
Steel "A"(Contin~ed)
Rinse Electro- Rinse Sticking Force, Kg.
5 Condition lytically AfterStrip Strip
No. Cleaned CleaningCathodic Anodic
12 Yes 1) Hard mill 120~7 199+9
water
2) 1~42 pp~
Ca(Hc~o)2
13 Yes 1) Hard mill 101~19 125+15
water
2) 3884 ppm
Ca~HC00)2
14 Yes Mg(HC00)2,124~13 122*8
1957 ppm
Ca(HcOO)2J
1942 ppm
Yes Mg(CH3~00)2~ 123~18 203+20
3000 ppm
Steel "B"
16 NoneNone 221~4
17 Ye~ Deionized157~11 204~3
water
18 Yes Ca(HC00)2,93+ ~ 118-~ 8
5000 ppm
lg Yes Mg~CH C00)2, 7~14
5000 ppm
23 Yes Ca(CH ~)2~131~10
5000 Pp~ ~
_ g ~
It may be seen from TABLE Il that when used in
co~centrations of 1,500 to 10,000 ppm, generally 3,000
to 8,000 ppm, and more preferably 4,000 to 6,000 ppm,
the formates and acetates of magne~ium and calcium can
; significantly improve sticking force values,
irrespective of strip polarity. Use of such rinse
solutions will normally result in a dried-on residue
concentration of 0.3 to 2.0 mg/ft~ (3.2 to 21~5
mg/m ). Such ~esidue concentration will, of course,
be a function of rinse solution concentration, but will
al80 depend on the thickness of the drag-out film
remaining on the strip. This thickness i~ a ~unction
of the strip speed and wxinger-roll pressure.
Preferably, the residue concentrati~n of said
1~ carboxylic acid salts will be within the range 0.5 to
1~2 mg/ft2 ~5.4 to 12.9 mg/m2). Although most
t~sts were carried out with a ~ilicated cleaner, the
results show that even with a non-silicated cleaner
(e.g. Condition 8), a measurabl~ improvement in
sticking force followed use of an effective rinse
additive~ such as ~agnesium formate.
In most of the above tests, solutions
containing rinse additives were used immediately aft~r
electrolytic cleaning. On commercial lines with
scrubber sections that require large volumes of water,
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a two-stage practice ~ay be required to reduce che~ical
costs. Evaluations were therefore conducte~
(Conditions 12 and 13) simulating use of a ~wo-s~age
rinse. The results show that a final ri~se (sub~equent
to a mill water rinse) containing the additives of the
present invention is also effective in reducing
sticking.
It would therefore appear, ~hile the complete
mechanism relating to surface composition and s~icking
cannot yet be established, that certain soluble salts
of magnesium and calcium used as rinse additive~ appear
to fix silicon on the steel surface~ over and above the
silicon level resulting when either deioni7ed water or
hard mill ~ater is used. Cathodic cleaning appears to
fix more silicon than do~s anodic cleaning. With
non-silicated cleaner6 containing phosphate, cl~aning
polarity does not `appear to be quite as important a
variable. Nevertheless, both previous results and
those reported above, de~onstrate that 6ticking in the
absence of silicates can also be lowered by the use of
dried-on calciu~ or magnesiu~ for~ates that degrade
during annealing. Thus~ while silicate le~els on clean
steel sur~aces are i~portant in their effect on
~ticking during annealing, the fact that a final rinse
2~ with an appropriate additive was effective even af~er
an initial rinse in hard water, 6uggests that
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silicates, phosphates, and the thermal degradation
products of calcium and magnesium formates and acetates
all contribute to lower sticking during box annealing,
as compared with a steel that does not have these
5 residues on the surfaceO
