Note: Descriptions are shown in the official language in which they were submitted.
11333~5
This invention relates to a method and an apparatus
for cooling steel strips in order to effect continuous anneal-
ing thereof.
Methods of cooling steel strips (this term also in-
cludes plates) to effect continuous annealing thereof are
classified into two types, one is the so-called gas jet type
and the other is the water cool type. Although each type
has an inherent advantage, it has certain defects.
More particularly, according to the gas jet system,
a cooling gas is blasted at high speed against a steel strip
heated to a temperature of about Al transition point to cool
the strip to about 400C and then the strip is subjected to a
super aging treatment for 3 to 5 minutes. The cooling effic-
iency of a gas is inferior to that of a liquid because of its
small thermal capacity. Although the liquid has a larger '
thermal capacity than the gas, the vapour film formed on the
surface of the strip decreases the cooling efficiency. For
this reason, in a modern high speed large processing line,
the cooling equipment is large, expensive and requires a high
running cost.
However, the gas jet type has an advantage in that
its cooling speed can be adjusted at will, which is suitable
for soft steel strip, and the heat cycle of this type is
economical because, contrary to the water cooling type, it is
not necessary to first cool the coolant to room temperature and
then heat it to a super aging temperature.
Among the water cooling types, i.e., the water quench-
ing systems, are included a method in which a liquid coolant
is ejected upon a uniformly heated steel strip and a method in
which a heated steel strip is dipped in a liquid coolant. To
eject the liquid, not only a special ejection equi~ment is
necessary but also the pattern of the ejected cooling liquid
~ ~33~65
varies, thlls failing to give uniform cooling and a homogeneous
product.
The heat cycle of each cooling system is fast. This
is particularly true with the dip method since the cooling
speed is extremely high, i.e., of the order of 1000 - 2000C/
sec., as compared with 10 - 30C/sec. for the gas jet method.
The dip method is suitable for manufacturing high tension
steel materials having a mixed structure of ferrite and marten-
site and not containing any other special elements. As pointed
out above since this method ensures a high speed cooling, the
cooling equipment for a high speed steel strip processing line
is extremely compact. Moreover, since it is sufficient to
merely pass the steel strip through cooling water, the running
cost can be greatly reduced.
In spite of the advantages described above, since the
cooling speed is too rapid, even when the cooling water is
heated to its boiling point, it is impossible to subject the
steel strip to the super aging temperature during cooling.
Furthermore, since the temperature of the steel strip subjected
to cooling goes down to about 100C or normal temperature, if
a super aging treatment is performed subsequent to quenching,
it is necessary to reheat the steel strip which has been cooled
to such a low temperature, that the treatment requires addi-
tional process steps and equipments resulting in a poor thermal
efficiency.
It is an object of this invention to provide an im-
proved method and apparatus for cooling a continuously running
steel strip capable of readily adjusting the cooling rate of a
steel strip.
Another object of this is to provide a method and
apparatus for cooling, at any cooling rate, a steel strip which
has ~een heated to a recrystallization temperature without
~L333~iS
forming oxide films.
Still another object of thls invention is to provide
an improved cooling unit in which the strip can be cooled in-
directly by passing it about a series of cooling rollers
through which cooling water is passed, or directly by passing
it through a body of cooling water.
According to one aspect of this invention, there is
provided a method of cooling a steel strip which has been heat-
ed to a temperature above its recrystallization temperature
during a continuous annealing treatment, characterized by the
steps of passing the steel strip about a plurality of cooling
rollers and varying the contact time of the steel strip
with the cooling rollers so as to vary the cooling rate.
According to another aspect of this invention there
is provided a method of cooling a steel strip which has been
heated to a xecrystallization temperature in a continuous
annealing treatment, characterized by the steps of cooling
the steel strip with cooling gas and then cooling the steel
strip with water.
According to still another aspect of this invention
there is provided a cooling apparatus for continuously anneal-
ing a steel strip which has been heated to a temperature above
recrystallization temperature, characterized by the fact that
it comprises a cooling unit including a plurality of cooling
rollers about which the steel strip is passed, and a mechanism
for varying the contact area between the steel strip and the
cooling rollers, thus varying the contact time between the steel
strip and the cooling rollers, a cooling medium circulating in
the interior of the cooling rollers.
A gas cooling unit which ejects a cooling gas against
the steel strip may be provided in front of the water cooling
unit. The steel strip is passed about alternate upper and
1133365
lower surfaces of the cooling rollers. When alternate cooling
rollers are raised to positions away from the strip, the cool-
ing unit is filled with water thus effecting direct cooling of
the steel strip~ A water tank is connected to the cooling
unit via a water seal and the steel strip is circulated through
the cooling unit and the water tank. In this manner when water
is drained from the cooling unit and the water tank, the alter-
nate rollers are lowered to cause the steel strip to pass
through a wavy passage while contacting the upper and lower
surfaces of the cooling rollers. In this case, cooling water
is passed through the interior of the cooling rollers, thus
effecting indirect cooling. In this manner, the cooling unit
of this invention can readily be switched between direct cool-
ing and indirect cooling.
Further objects and advantages of the present inven-
tion can be more fully understoo~ from the following detailed
description taken in conjunction with the accompanying draw
ings in which:
Figure 1 is a diagrammatic representation showing
one example of a continuous annealing apparatus embodying the
invention,
Figure 2 is a side view of the cooling rollers shown
in Figure 1,
Figure 3 is a plan view of the cooling rollers shown
in Figure 2,
Figure 4 is a diag,rammatic representation showing
another example of a continuous annealing apparatus according
to this invention'
Figure 5 is an enlarged side view of the cooling unit
shown in Figure 4 when it is used as a direct water cooling
unit'
Figure 6 is a side view similar to Figure 5 showing a
1~L33.~6S
way of cooling whereby water is passed through the cooling
rollers,
Figure 7 is a diagrammatic longitudinal sect.ional
view showing one example of the cooling unit together with a
gas cooling unit' and
Figure 8 is a partial view showing a modified appar- ,.
atus according to the embodiment shown in Figure 4.
In the continuous annealing apparatus shown in Fig-
ure 1, a steel strip (S) payed out from either one of a plur-
ality of uncoilers la and lb is continuously passed through aprocessing line by alternately connecting together the strips
payed out from respective uncoilers with a shear 2a and a
welder 2b. Following the welder 2b are installed in success-
ion, an alkali washing tank 3, an electrolysis rinsing tank 4
and a hot water spray washing tank 5 which constitute a pre-.
treatment surface cleaning unit. After passing through this
pretreatment surface cleaning unit the strip is then guided
into a furnace 10 via a dryer 6 and an inlet looper 7. The
furnace 10 includes a heating zone 8 for effecting recrystal-
lization annealing, a uniform heating zone 9 and a cooling unit12. The strip recrystallized, annealed and cooled in this
manner is then introduced into the furnace 10 again and passed
through a reheating zone 13 for over aging processing, an over
aging processing zone 14 and a gas cooling zone 15 The strip
is then passed through an outlet looper 16, a water cooling
unit 17, a dryer 18 and a refining mill 19. After passing
through the refining mill 19, the strip (S) passes through a
trimming width varying notcher 20, a side trimmer 21, an oiler
22, an exit shear 23 and is finally taken up by a take up reel
24.
The cooling unit utilized in the annealing apparatus
is shown in Figures 2 and 3. More particularly, the strip (S)
1133;~6S
passes about four rollers 32 and 32a. A pair of rollers 32
can be raised or lowered together with their supporter 31 by
means of screw or oil pressure cylinders 30. Water or other
cooling fluid passes through the interior of either one or both
of the rollers 32 and 32a. By varying the contact angle or
length of the str;p in contact with the upper surfaces of the
rollers 32 and the lower surfaces of the rollers 32a the con-
tact time can be varied even under the same running speed thus
varying the cooling speed and the temperature of the cooled
strip, so as to send the strip to the next step while maintain-
ing it at a de~inite temperature. This cooling unit can be
used at any point of travel of the strip, for example, the gas
cooling zone 15 and the water cooling unit 17.
The cooling unit shown in Figures 3 and 4 can also be
used as a cooling unit which can be switched to a direct cool-
ing depending upon the characteristi.cs of steel strip to be
cooled. Steel strips not requiring an over aging treatment
can be cooled at such high cooling speeds as 1000 - 2000C/sec.
where oxide films are properly removed. Especially, in a high
tension steel plate or strip in which a mixed structure of fer-
rite and martensite can be obtained with only a small quantity
of special elements, such high speed cooling by direct contact
is advantageous. Therefore, the cooling unit according to this
invention can be also constructed to be switchable between in-
dlrect cooling using cooled rollers and direct cooling using
water.
Figure 4 shows a modified continuous annealing appar-
atus including the switchable type of cooling unit which is
also shown in Figures 5 and 6. More particularly, the strip (S)
is passed between four rollers 32 and 32a, of which the upper
rollers 32a are moved in the vertical direction by a screw or
oil pressure cylinder mechanism. Thus, when the upper rolle.rs
-- 6 --
365
32a are raised as shown in Figure 5, the strip (S) passes
directly between guide rollers 12a and 12b without contacting
the cooling rollers 32 and 32a, whereas when the upper rollers
32a are lowered as shown in Figure 6, the strip (S) passes
alternately about rollers 32 and 32a. The contact area of
the strip against the rollers can be adjusted depending upon
the degree of lowering of the upper rollers 32a. Accordingly,
in the state shown in Figure 6, the cooling speed and the fina]
cooling temperature can be adjusted as desired by raising and
lowering the upper rollers 32a. Where the switchable cooling
unit 12 shown in Figure 4 is used as direct water cooling in
the manner shown in Figure S, it is necessary to remove oxide
film deposited on the strip during water cooling. For this
purpose, a means for removing oxide film is provided between
the cooling unit 12 and the reheating zone 13 as shown in Fig-
ure 4. Thus, the strip cooled in the cooling unit 12 is passed
through a pickling tank 25, a warm water washing tank 26, a
neutralizing tank 27 and a dryer 28. Alternatively, as shown
in Figure 8, the oxide film removing means may be installed
between the outlet looper 16 and the refining mill 19, said
means comprising a pickling tank 48, a warm water washing tank
49, a neutralizing tank 50, another warm water washing tank 51,
a water washing tank 52 and a dryer 53. On the other hand, where
the cooling unit is used as indirect cooling using rollers as
shown in Figure 6 no oxide film is formed so that it is not
necessary to us~j the pickling tank, the warm water washing tank
and the neutralizing tank. Even in this case, only the water
washing tank 52 and the dryer 53 are to be used.
As shown in Figure 7 a gas jet cooling unit 11 may be
added to the cooling unit 12 described above. The gas jet cool-
ing unit 11 comprises a motor driven blower 60 and a plurality
of gas ejection nozzles 41 which eject cooling gas sen~ ~rom the
6~
blower 60 against the strip. The cooling gas i5 circulated in
the unit 11 and is cooled by water tubes lla in front of the
blower 30 down to a temperature of from 50 - 150~C from a temp-
erature of 150 - 250C. As described above, the operating
state of the cooling unit 12 is switched between the positions
shown in Figures 5 and 6. On the right hand side o~ a cooling
chamber 35 there is provided a circulating tank 36 supplemented
with fresh water from a water supply pipe 46 when desired. On
both sides of the water circulating tank 36 there are provided
water level adjusting gates 37 and 37a to adjust the water
levels in the cooling chamber 35 and in the tank 36. A strip
exit port 34 is formed on the left hand side of the cooling
chamber 35 to guide the strip to the pickling tank 17 or dir-
ectly to the reheating zone 13. The water in the tank 36 is
conveyed to water nozzles 43 via a conduit 38 including a fil-
ter 33 and a pump 42 to eject cooling water against the opposite
surfaces of the strips (S).
A warm water reservoir 39 is located beneath the tank
36 for receiving warm water to the left of the gate 37 and water
overflown from the gate 37a. The warm water collected in the
reservoir 39 is discharged through a pump 40. The reservoir 39
is connected to the tank 36 through a pipe 45 and a valve 47.
In operation, the water level in the cooling chàmber
35 is adjusted by the gate 37 while the strip is precooled by
the gas blasted thereon through a plurality of nozzles 41 (for
example 10 or more) and then is cooled by the water present
in the cooling chamber 35. The water therein also acts as a
sealing water for succeeding processing line. The warm water
discharged by the pump 40 may be used in the hot water washing
tank. When valve 47 is opened, the water in the cooling chamber
35 to the left of the gate 37 is completely discharged into the
reservoir 39, and the strip is cooled in a state shown in Figure
~ 1 3;~;?ti5
6 by cooling rollers 32 and 32a. At this time, cooling water is
passed through the interior of these rollers.
Accordingly, the switching of the cooling states be-
tween Figures 5 and 6 can be readily accomplished without dis-
charging the entire water content of a large tank 36 ~Ut by
merely discharging a relatively small quantity of the cooling
water to the left of the gate 37. Even when the water at the
left of the gate 37 is discharged, the cooling gas in the gas
jet cooling unit 11 is prevented from discharging to the outside
by a water seal 44 disposed between the cooling chamber 35 and
the tank 36.
As a consequence, with the construction shown in Fig-
ures 4 - 7 it is possible to cool the strip very quickly with
water to a low temperature near room temperature or relatively
slowly with water cooled rollers which do not form oxide films
on the strip at the time of cooling, thus smoothly effecting
a series of processings including over aging processing so as
to produce various types of steel strips or plates suitable for
different applications.
The indirect cooling will now be described in more
detail. The temperature of the steel strip supplied to the
cooling unit 12 through the heating zone 8 for the recrystalliza-
tion annealing and the uniform heating zone 9 varies slightly,
generalLy 500 - 800C, depending upon the thickness and the com-
position, and the strip is cooled by cooling rollers 32 and 32a.
The cooling water flowing through these cooling rollers may be
at room temperature, and variation in the temperature of the
cooling water ranging from 5C to 30C does not cause any apprec-
iable change in the cooling effect. Accordingly, even when the
strip is cooled by 1C or heated by 60 - 70C, such cooling and
heating do not affect the cooling effect of the cooling rollers.
When the cooling rollers through which cooling water is passed
i ~33.~fiS
ar~ contacted with the steel strip, and when a steel strip hav-
ing a thickness of 0.6 mm and heated to 300 - 600C, for exam-
ple, is contacted against the cooling rollers for about one
second the strip would be cooled by about 180C, whereas when
contacted for two seconds, the strip would be cooled by about
260C. When a steel strip having a thickness of 1.2 mm is con-
tacted for one second it would be cooled to about 90C, whereas
about 140C would be reached when the steel strip is contacted
for two seconds. When a steel strip having a thickness of 0.8
mm running at a speed of 150 m/min. is contacted against rollers
32 and 32a through which water is allowed to pass, at a contact
angle of 0.8 ~ after being heated to 580C by a recrystalliza-
tion annealing treatment, it is cooled to about 505 - 515C by
the first roller 32 and to about 465 - 480C by the second
roller 32a. The strip is cooled down to 410 - 420C by the
third roller 32 and then to 380C by the fourth roller 32a. The
same result can be attained by reducing the contact angle as
the roller diameter is reduced. Even when the strip gauge or
line speed varies similar results can be obtained with a con-
tact time of less than two seconds by varying other parameters.
The strip thus cooled to 350 - 380C is guided into the follow-
ing heat treatment zone (13 - 15). Accordingly, fuel cost nec-
essary for reheating the strip can be reduced by 25 - 30% in
comparison to the cost involved in a case wherein the strip is
cooled to about room temperature.
To have a better understanding of the invention the
following example is given.
Example
A low carbon steel strip having a thickness of 0.8 mm,
and a width of 1000 mm was passed through the heating zone 8
and the uniform heating zone 9 at a speed of 150 m/min. to
effect recrystallization annealing for one minute and at a
-- 10 --
il33;~6~
temperature of 700~C dnd thel-l is Sellt to the cooling unit 12
shown in Fi~ure 1. Each one of the cooling rollers 32 and 32a
had a diameter of 600 mm and cooling water at 15C was passed
through these rollers at a rate of 250 ~/minute. The strip ~S)
was cooled by contacting it about these cooling rollers at a
contact angle of from 0 to 0.9 ~. The temperature of the strip
(S) was about 600C when it enters into the cooling unit and it
was cooled to about 395C to 415C and the variation in the
temperature of the cooled strip was less than 20C which means
uniform cooling. The thus cooled strip is then over aged at a
temperature of 400 - 350C for three minutes in the reheating
zone 13 and the over aging processing zone 14 thereby obtaining
a strip of steel having uniform mechanical properties which is
suitable for use in contraction.
-- 1 1 --
