[0001] The present invention relates to the box-annealing of steel sheet.
[0002] The heat treatment of cold-reduced steel sheet and strip is accomplished either in
batch operations or in continuous 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 uniformity
of heating , and (b) box annealing in which a large stationary mass of steel (either
cut sheet or coils) is subjected to a comparatively longer heat treatment cycle by
varying the temperature within the furnace that surrounds it. In the latter, box annealing
practices, the mass of steel is slowly raised to the desired annealing temperature
and soaked at such temperature for a period of 1/2 to 24 hours. The use of such a
practice provides full recrystallization of severely cold-reduced steel and results
in the softest possible finished product. However, 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. The tendency to sticking increases with increasing pressure between
the wraps and increasing time and temperature of the anneal. When sticking of adjacent
coil wraps occurs, particularly in products with critical surface finish requirements,
poor yields result. Additionally, production delays are encountered at a subsequent
temper mill because lower rolling speeds are required or because recoiling may be
necessary before further processing. To prevent such sticking, the prior art has resorted
to the use of a variety of separating media 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 liquids aids in preventing sticking. This is a practice which has been recommended
for many years by manufacturers of commercial cleaning compounds. Although many steel
facilities use silicated cleaners, such use has not precluded the serious incidence
of coil sticking.
[0003] According to the present invention, there is provided a method of box-annealing a
mass of steel sheet wherein said sheet is electrolytically cleaned in a cleaning solution
consisting of silicate or phosphate cleaners, or mixtures thereof, rinsed, dried,
coiled and thereafter heated to an annealing temperature in a non-oxidizing atmosphere
and soaked at such temperature for a time of at least one-half hour, characterized
in that the solution utilized for said rinse contains calcium formate, magnesium formate,
calcium acetate or magnesium acetate, or mixtures thereof, in a total concentration
1,500 to 10,000 ppm.
[0004] 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 mill experience.
Investigations were conducted using different types of steel, both continuous cast
and ingot cast steels, since the prior art had suggested that steel composition, particularly
carbon and phosphorus contents, had an effect on sticking. Compositions of two of
the ingot cast steels evaluated are listed in TABLE I.
[0005] Steel panels were cut to a size of 2-3/4" x 8"(7 to 20 cm.). After vapor degreasing,
the panels were subjected to electrolytic alkaline cleaning in various solutions (indicated
in TABLE II) maintained at a temperature of 82°C, at a current density of 10.8 amps/dm
2 for a period of one second (either cathodic or anodic). After electrolytic cleaning,
the panels were passed through rubber wringer rolls to remove excess cleaning solution.
Rinse water at a temperature of 54-60
0C was sprayed on the steel surface to wash off the remaining cleaner and the panels
were then dried to remove unbound water.
[0006] 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 exerted by the wrapping of coils, a 15.2 lb. (6.9 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% H
2 - 94% N
2 with a dew point controlled to minus 40°C by passing the gas mixture through a column
of calcium sulfate. The annealing box was placed in a furnace and heated to an annealing
temperature of 1250
0F (677°C) in two hours. 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 specimen pairs were placed in a tension
tester and pulled apart. The amount of force required to pull the samples apart was
then utilized 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 calculated. Each average value is reported in TABLE II with a 95% confidence
interval.
[0007] Utilizing the above test procedures, previous experiments with silicated cleaners
(not utilizing the rinse additives of the present invention) had shown that significantly
lower sticking force values resulted when final strip polarity was cathodic. This
is a difference which was not observed when non-silicated cleaners were applied. In
this regard, even when final strip polarity was anodic, lower sticking force values
resulted from the use of silicated cleaners. 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, utilizing deionized water 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. Since mill water is known to be made hard
primarily because of magnesium and calcium ions, a number of rinse additives containing
soluble salts of magnesium and calcium were evaluated to determine whether the cations
of such salts would also 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 not significantly change sticking force values from those resulting when hard
mill water was applied. However, the formates and acetates of calcium and magnesium
did provide significant changes of sticking force, and the results of same are reported
in TABLE II. Except as indicated, all specimens were electro-cleaned utilizing a commercial
silicated cleaner. For conditions 3 and 8, electro-cleaning was with a commercial
phosphated cleaner.
[0008] It may be seen from TABLE II that when used in concentrations 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 magnesium 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
2 (3.2 to 21.5
mg/m
2). Such residue concentration will, of course, be a function of rinse solution concentration,
but will also depend on the thickness of the drag-out film remaining on the strip.
This thickness is a function of the strip speed and wringer-roll pressure. Preferably,
the residue concentration of said carboxylic acid salts will be within the range 0.5
to
1.2 mg/ft
2 (5.4 to 12.9 mg/m
2). Although most tests were carried out with a silicated cleaner, the results show
that even with a non-silicated cleaner (e.g. Condition 8), a measurable improvement
in sticking force followed use of an effective rinse additive, such as magnesium formate.
[0009] In most of the above tests, solutions containing rinse additives were used immediately
after electrolytic cleaning. On commercial lines with scrubber sections that require
large volumes of water, a two-stage practice may be required to reduce chemical costs.
Evaluations were therefore conducted (Conditions 12 and 13) simulating use of a two-stage
rinse. The results show that a final rinse (subsequent to a mill water rinse) containing
the additives of the present invention is also effective in reducing sticking.
[0010] It would therefore appear, while the complete mechanism relating to surface composition
and sticking cannot yet be established, that certain soluble salts of magnesium and
calcium used as rinse additives appear to fix silicon on the steel surface, over and
above the silicon level resulting when either deionized water or hard mill water is
used. Cathodic cleaning appears to fix more silicon than does anodic cleaning. With
non-silicated cleaners containing phosphate, cleaning polarity does not appear to
be quite as important a variable. Nevertheless, both previous results and those reported
above, demonstrate that sticking in the absence of silicates can also be lowered by
the use of dried-on calcium or magnesium formates that degrade during annealing. Thus,
while silicate levels on clean steel surfaces are important in their effect on sticking
during annealing, the fact that a final rinse with an appropriate additive was effective
even after an initial rinse in hard water, suggests that 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 residues on the surface.
1. A method of box-annealing a mass of steel sheet wherein said sheet is electrolytically
cleaned in a cleaning solution consisting of silicate or phosphate cleaners, or mixtures
thereof, rinsed, dried, coiled and thereafter heated to an annealing temperature in
a non-oxidizing atmosphere and soaked at such temperature for a time of at least one-half
hour, characterized in that the solution utilized for said rinse contains calcium
formate, magnesium formate, calcium acetate or magnesium acetate, or mixtures thereof,
in a total concentration of 1,500 to 10,000 ppm.
2. A method as claimed in claim 1, wherein said mass is a coil of sheet steel, said
adjacent sheets are wraps in said coil, said annealing temperature is 1,050° to 1,400°F
(566 to 760°C.) and said soak time is less than 24 hours, characterized in that said
rinse is achieved by passing the sheet, at a speed of 1,000 to 3,000 ft./min.(305
to 914 m./min) through a bath of said rinse solution.
3. A method as claimed in claim 2, wherein subsequent to the rinse, the drying is
continued to remove essentially all the unbound water of the residue remaining on
the surface of the sheet characterized in that the concentration of said salts is
within the range 3,000 to 8,000 ppm.
4. A method as claimed in claim 3, characterized in that said concentration is 4,000
to 6,000 ppm.
5. A method as claimed in any preceding claim, characterized in that the dried sheet
has a concentration of 0.3 to 2.0 mg/ft2 (3.2 to 21.5 mg./m2) of said salts on the surface thereof.
6. A method as claimed in any preceding claim, characterized in that the dried sheet
has a concentration of 0.5 to 1.2 mg/ft2 (5.4 to 12.9 mg/m ) of said salts on the surface thereof.
