Titleoflilvemiori ______________ -------
METHOD FOR COOLING STEEL STRIP AND COOLING APPARATUS
Technical Field
[0001]
The present invention relates to a method for cooling a steel strip and a
cooling apparatus in a galvannealing furnace for hot-dip galvannealing.
Background Art
[0002]
In a hot -dip galvannealing treatment step for a steel strip, the steel strip
passes through a pre-treatment bath for degreasing, cleaning, or the like and then
15 passes through an annealing furnace and a zinc pot containing molten zinc, then
being raised perpendicularly. The raised steel strip is subjected to galvannealing
treatment in a galvannealing furnace. The galvannealing furnace includes a heating
zone and a cooling zone arranged from the upstream side in a direction in which the
steel strip is raised.
20 [0003]
That is, the cooling zone of the galvannealing furnace is ananged vertically
above the heating zone. Therefore, cooling of the steel strip in the cooling zone is
performed using gas cooling or mist cooling so as not to exett an influence, such as
dripping water, on an installation ananged vertically below the cooling zone. In
25 particular, it is effective to use mist cooling (mist cooling) which has high cooling
capacity in order to improve production capacity. In mist cooling, however, in the
case where a large amount of water is sprayed in order to strongly cool the steel strip,
temperature unevenness occurs in the width direction of the steel strip. This
temperature unevenness causes quality defects, such as wrinkles and zinc powder
30 pick-up.
[0004]
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In view of such a problem, for example, Patent Literature 1 discloses a
galvannealing furnace exit -side mist cooling method in which a cooling pattern of a
sreersrripis aeljustlm-so-manemperatm:·e Cleviation inllie-wielth-direction Clue to
overcooling is suppressed. In Patent Literature 1, in order to suppress cooling
5 variation due to dripping water and make temperature unevenness equal to or less
than wrinkle limit temperature unevenness, a steel strip is cooled in a manner that a
cooling ratio between a preceding stage and a subsequent stage of a cooling zone is
changed so that the subsequent stage is subjected to slow cooling.
[0005]
10 Patent Literature 2 discloses a cooling method in a galvannealing treatment
15
process. The method uses either of gas cooling and mist cooling according to
cooling load to avoid transition boiling and suppress temperature deviation in the
width direction.
[0006]
Furthermore, Patent Literature 3 discloses a technology of arranging nozzles
densely in a center portion in the width direction of a steel strip and providing
shutters for blocking the nozzles.
[0007]
Patent Literature 4 discloses a technology of controlling a tension value and
20 temperature unevenness based on a predetermined relational expression to set a
cooling zone exit-side temperature to 240°C or lower in order to prevent reduction of
area and buckling of a steel sheet at the exit side of a mist cooling installation.
[0008]
Patent Literature 5 discloses a technology of using either of mist cooling
25 and cooling with gas for each zone to avoid a transition boiling region, which causes
cooling variation, in order to make an Fe concentration amount in a plating layer
appropriate.
Citation List
30 Patent Literature
[0009]
Patent Literature 1:
Patent Literature 2:
3/37
JP 2006-111945A
JP Hll-43758A
PCT/JP2015/055012
------Paten:rl:iteratille 3: __________ JP.H7'65153B---··----
5
10
Patent Literature 4:
Patent Literature 5:
Technical Problem
[001 OJ
JP H9-268358A
JP 2000-256818A
Summary of Invention
However, the cooling method described in Patent Literature 1 is a method
for resolving temperature unevenness using a cooling pattern in which the preceding
stage is subjected to high-load cooling and the subsequent stage is subjected to slow
cooling, and therefore faces a limit in achieving both ensuring cooling capacity of the
cooling zone and resolving temperature unevenness. The cooling method described
15 in Patent Literature 2 uses either of gas cooling and mist cooling, and also in this
case, it is obvious that gas cooling lowers cooling capacity of the cooling zone.
That is, both of the methods described in Patent Literatures 1 and 2 have a limited
effect in resolving temperature unevenness under high-speed sheet passing
conditions. Consequently, sheet passing cannot be performed at high speed, which
20 results in low productivity.
[0011]
Moreover, when the technology disclosed in Patent Literature 3 is used, the
shutters obstruct the flow of mist and cause dripping water; therefore, this technology
cannot be applied. In addition, the nozzles arranged densely in the center portion
25 increases water amount density in the center portion near the quench point, leading to
an increase in quench point temperature to cause cooling unevenness in the width
direction.
[0012]
The technology disclosed in Patent Literature 4 is a technology of setting
30 allowable temperature unevenness based on the tension value of the steel sheet.
Since the tension value of the steel sheet cannot be changed to an extreme, this
PCT/JP20 15/055012
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technology cannot be applied in actual operation.
[0013]
Iii aC!Clition, willi-theTecllliolog)'Clisclosed-mPatent r:itefafufe-5;- itis -
difficult to completely suppress occurrence of cooling unevenness due to the
5 influence of dripping water.
[0014]
Hence, the present invention has been made in view of the above problem,
and aims to provide a novel and improved method for cooling a steel strip and a
novel and improved cooling apparatus that perform mist cooling on a steel strip in a
10 cooling zone of a galvannealing furnace and can achieve both productivity and
quality.
15
Solution to Problem
[0015]
According to an aspect of the present invention in order to achieve the
above-mentioned object, there is provided a method for cooling a steel strip by mist
cooling in a cooling installation of a galvannealing furnace configured to perform
galvannealing treatment on a hot-dip galvanized steel strip. The cooling method
includes: by an adjusted cooling installation provided at an upstream side in a sheet-
20 passing direction of the cooling installation, jetting mist to the steel strip passing
through the cooling installation in a manner that an amount of mist jetted to the steel
strip passing through the cooling installation is smaller in an edge portion in a width
direction of the steel strip than in a center portion; by a mist suction installation
provided at least at a downstream side in the sheet-passing direction of the cooling
25 installation, sucking at least part of mist jetted to the steel strip; and cooling the steel
strip at a sheet -passing speed such that, during. a period between start and end of
cooling of the steel strip, a temperature of the steel strip is within a film boiling
temperature range and a temperature of the edge portion in the width direction of the
steel strip is equal to or higher than a temperature of the center portion in at least a
30 range of 2/3 or more from the upstream side in the sheet-passing direction of a total
cooling length of the cooling installation.
5
PCT/JP2015/055012
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[0016]
With respect to an installation length L [ m] of the adjusted cooling
installation;1fspeedoftlie-ste-enm•ip-maybesenftlie-iidjusted cooliiiiC installation -in tlieslieeFpassiri.g -
direction of the steel strip satisfies a formula (b) below,
5 L2: (a. x V x th)/((T;n- ~)"m) x (Tin-y)) ... (b)
where Tin [0 C] denotes a temperature of the center portion of the steel strip
at an entrance of the cooling installation, V [m/s] denotes a speed of the steel strip, fh
[m] denotes a thickness of the steel strip, and a., ~, y, and m are constants set
according to a hot -dip galvannealing installation. The constants may be set as
10 follows: a= 1700000, ~ = 330, y = 45, m = 0.6.
[0019]
The adjustment cooling installation may include, in the sheet-passing
direction, a plurality of headers each including a plurality of nozzles arranged along
the width direction. Each header may be configured in a manner that mist is not
15 jetted to the steel strip in the edge portion in the width direction of the steel strip.
[0020]
Each header of fhe adjusted cooling installation may be configured in a
manner that the number of the nozzles that jet mist to the steel strip in the center
portion in the width direction of the steel strip increases from the upstream side
20 toward the downstream side in the sheet-passing direction.
Advantageous Effects of Invention
[0021]
According to the present invention, it is possible to provide a method for
25 cooling a steel strip and a cooling apparatus that perform mist cooling on a steel strip
in a cooling zone. of a galvannealing furnace and can achieve both productivity and
quality.
Brief Description of Drawings
30 [0022]
[FIG. 1] FIG. 1 1s a schematic explanatory diagram illustrating a schematic
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configuration of a hot-dip galvannealing installation provided with a cooling
installation according to an embodiment of the present invention.
[FIG:-2rFIG:-Tis an explanatoryaia-gram-showingsneenemperafufe distribution ill
the width direction and the longitudinal direction of a steel strip passing through a
5 cooling zone.
[FIG. 3] FIG. 3 is an explanatory diagram showing an outline of sheet temperature
control by a cooling zone of a galvannealing furnace according to the embodiment.
[FIG. 4] FIG. 4 is a graph showing the relationship between a cooling water amount
and a quench temperature and the relationship betWeen a cooling water amount and
10 the temperature of a center portion of a steel strip.
[FIG. 5] FIG. 5 is a graph showing the relationship between a cooling water amount
and an improvement effect of temperature distribution in the width direction.
[FIG. 6] FIG. 6 is an explanatory diagram illustrating a configuration example of a
cooling zone 60 according to the present embodiment.
15 [FIG. 7] FIG. 7 is an explanatory diagram illustrating a configuration example of a
cooling-zone preceding stage section including an adjusted cooling installation
according to the embodiment.
[FIG. 8] FIG. 8 is an explanatory diagram illustrating a configuration example of a
mist header.
20 [FIG. 9] FIG. 9 is an explanat01y diagram for explaining the installation length of an
adjusted cooling installation when the adjusted cooling installation includes a singlestage
mist header.
[FIG. 10] FIG. 10 is an explanat01y diagram showing sheet temperature distribution
in the width direction and the longitudinal direction of a steel strip passing through a
25 cooling zone when, as Comparative Example 6, an adjusted cooling installation is
provided from the final stage side of a cooling zone.
30
Description of Embodiments
[0023]
Hereinafter, (a) preferred embodiment(s) of the present invention will be
described in detail with reference to the appended drawings. In this specification
PCT/JP2015/055012
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and the appended drawings, structural elements that have substantially the same
function and structure are denoted with the same reference numerals, and repeated
-explanation of tliese s!rucfural elements is omitted:
[0024]
5 <1. Overview of hot-dip galvannealing installation>
First, with reference to FIG 1, description will be given on a schematic ·
configuration of a hot-dip galvannealing installation provided with a cooling
installation according to an embodiment of the present invention. FIG. 1 is a
schematic explanatory diagram illustrating a schematic configuration of a hot -dip
10 galvannealing installation provided with a cooling installation according to the
present embodiment.
[0025]
Examples of steel grades to be treated by the hot-dip galvannealing
installation according to the present embodiment include ultra-low carbon steel and
15 high tensile strength steel sheets. In general, steel materials with thicknesses of 0.4
· to 3.2 mrn and widths of 600 to 1900 mrn are treated.
[0026]
As illustrated in FIG. 1, the hot-dip galvannealing installation includes a
zinc pot 10 containing molten zinc 5 for plating the surface of a steel strip S, a pair of
20 gas nozzles 30 for adjusting the amount of plating attached to the steel strip S, and a
galvannealing furnace including a heating zone 40, a heat-retaining zone 50, and a
cooling zone 60. Although the hot-dip galvannealing installation according to the
present embodiment includes the heat-retaining zone 50, the present invention is not
limited to such an example, and is also applicable to a hot-dip galvannealing
25 installation without the heat-retaining zone 50. In the hot-dip galvannealing
installation, the steel strip S is brought into the zinc pot 1 0 containing the molten zinc
5, and is raised perpendicularly by a sink roll 20 immersed in the molten zinc 5.
The amount of plating attached to the surface of the raised steel strip S is adjusted to
a predetermined amount by wiping gas jetted from the gas nozzles 30.
30 [0027]
After that, the steel strip S is subjected to galvannealing treatment in the
PCT/JP2015/055012
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galvannealing furnace while being further raised perpendicularly. In the
galvannealing furnace, first, the steel strip S is heated by the heating zone 40 to have
- a:-subsTaritially illiiforiiisheet temperatille, aria then-galviillnealing time is provided in
the heat-retaining zone 50; thus, an alloy layer is generated. After that, the steel
5 strip S is cooled in the cooling zone 60, and transported to the next step by a top roll
70.
[0028]
The cooling zone 60 of the galvannealing furnace according to the present
embodiment includes a cooling-zone preceding stage section 61 provided at the
10 upstream side in the sheet-passing direction of the steel strip S (i.e., the vertically
lower side (the zinc pot 10 side)), and a cooling-zone subsequent stage section 62
provided at the downstream side in the sheet -passing direction of the steel strip S (i.e.,
the vertically upper side) with respect to the cooling-zone preceding stage section 61.
The cooling-zone preceding stage section 61 and the cooling-zone subsequent stage
15 section 62 each include mist headers (reference sign "63" in FIGS. 8 and 9) arranged
in multiple stages. Each mist header is provided with a plurality of mist jet nozzles
(reference sign "64" in FIG. 9) that jet cooling water in a mist form. Mist jetted
from the mist jet nozzles is sprayed onto the surface of the steel strip S. The
amount of cooling water supplied to each mist header is controlled by a control
20 apparatus 65.
[0029]
In addition, the cooling zone 60 is provided with at least one pair of mist
suction installations (reference sign "67" in FIG. 6) arranged to face the edge portions
in the width direction of the steel strip S. The mist suction installations are
25 provided at least at the downstream side in the sheet-passing direction of the cooling
zone 60, and suck at least patt of the mist jetted to the steel stripS.
[0030]
<2. Mechanism of mist cooling>
Conventionally, mist cooling which has high cooling capacity has been used
30 in order to improve production capacity; however, mist cooling, when spraying a
large amount of water to strongly cool the steel strip S, causes temperature
" I
----- - --- -- --- -- ----- -- ---- - -- - ------------
PCT/JP2015/0550 12
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unevenness in the width direction of the steel stripS, leading to quality defects. FIG.
2 shows sheet temperature distribution in the width direction and the longitudinal
-airt-£tioiiofthested strip s piissiiigthfoughthecooliiig ioiie 60:
distribution in the longitudiiial direction in FIG. 2 shows a temperature Cb of a center
5 portion and a temperature Eb of an edge portion before adoption of the present
application approach and a temperature Ca of a center portion and a temperature Ea
of an edge portion after adoption of the present application approach. The
temperature distribution in the width direction in FIG. 2 shows temperature
distribution before adoption of the present application approach and temperature
10 distribution after adoption of the present appl,ication approach at positions A, B, and
C in the longitudinal direction. The position A is a position at which cooling of the
steel strip S by the cooliiig zone 60 starts, the position B is a position between the
cooling-zone preceding stage section 61 and the cooling-zone subsequent stage
section 62, and the position C is a position at which cooling of the steel strip S by the
15 cooliiig zone 60 ends.
[0031]
Here, a portion at the center in the width direction of the steel strip S is
called a center portion, and both end sides in the width direction are called edge
portions. The edge portion refers to a range from the end in the width direction of
20 the steel strip S to a boundary position 100 mm away from the end.
[0032]
Before adoption of the present application approach, as shown in FIG. 2,
regarding the temperature of the steel strip S in the longitudiiial direction, the
temperature Eb of the edge portion is lower than the temperature Cb of the center
25 portion. With movement from the cooling-zone preceding stage section 61 to the
cooling-zone subsequent stage section 62, the temperature of the steel strip S
gradually decreases in both the center portion and the edge portion, and the
difference between these temperatures gradually increases. That is, according to the
temperature distribution in the width direction, with the transportation of the steel
30 strip S, the temperature of the edge portion becomes low in comparison with the
temperature of the center portion, and at the position C, which is the cooliiig zone 60
I PCT/JP2015/055012
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exit side, the temperature distribution is convex upward.
[0033]
A cause of the temperattiie distribution in-the width-direcrioriisgasflow
toward a sheet end direction inside the cooling zone. When gas from nozzles that
5 are arranged near the center in the sheet width direction goes toward exhaust pmis,
gas flow via the ends in the width direction of the cooling zone 60 occurs, and the
gas flow causes mist attached on the surface of the steel strip S to flow toward both
ends of the steel strip S, which reduces the sheet temperature of the edge portions of
the steel strip S. For a portion with high temperature in the steel strip S, the top roll
10 70 picks up zinc powder on the surface of the steel strip, which causes quality defects.
On the other hand, for a portion with low temperature in the steel strip S, the
temperature falls below a quench temperature, which is the boundary temperature
between a film boiling region and a transition boiling region of water, and this leads
to local overcooling, causing wrinkles. Therefore, temperature distribution in the
15 width direction of the steel stripS needs to be made uniform finally.
[0034]
Also in the present embodiment, mist cooling is used as cooling means in
the cooling zone 60 in order to improve production capacity. To prevent occurrence
of quality defects as well as improving production capacity by using mist cooling,
20 the present application inventors have devised, as a result of extensive studies, a
configuration of a cooling installation that suppresses overcooling of the edge portion
of the steel strip S, makes width-direction temperature distribution of the steel strip S
finally uniform, and avoids unstable cooling.
25
[0035]
That is, in the cooling zone 60 of the galvannealing furnace according to the
present embodiment, in order to stably cool the steel strip S, a sheet temperature at
which mist attached to the steel strip S undergoes film boiling is maintained in the
cooling zone 60. Liquid in a boiled state changes its fmm fi·om nuclear boiling to
transition boiling and then film boiling as its temperature increases. The
30 temperature of the steel strip S is ordinarily in a temperature region in which water
undergoes film boiling at the entry side of the cooling zone 60 of the galvannealing
PCT/JP2015/055012
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furnace. After that, with a decrease in the temperature of the steel strip S, a region
where water shifts from film boiling to transition boiling partially occurs on the
. . stiiface of the steel sti1p S, which leaos to-unstiiole coolillg, causingterllperafure
unevenness in the steel strip S. Hence, in the present embodiment, cooling is
5 performed in a manner that a sheet temperature at which mist attached to the steel
strip S undergoes film boiling is maintained in the cooling zone 60.
[0036]
Furthermore, in order to suppress overcooling of the edge portion of the
steel strip S, at the upstream side in the sheet-passing direction, the amount of mist
10 jetted to the steel strip S is adjusted so that a mist jet amount in the edge portion in
the width direction of the steel strip S is smaller than that in the center portion. If
the steel strip S is cooled with the same mist jet amount throughout the width
direction of the steel strip S, the temperature of the edge portion of the steel strip S
decreases greatly as described above, leading to large temperature deviation from the
15 center portion.
[0037]
Hence, at the upstream side' in the sheet-passing direction, mist jetted to the
steel strip S is adjusted to suppress cooling of the edge portion of the steel strip S,
and excessive mist in the edge portion of the steel strip S is eliminated; thus, the
20 sheet temperature of the edge portion of the steel strip S is prevented from decreasing
during sheet passing. In this manner, overcooling of the edge portion is prevented,
and as shown in FIG. 2, during a period between the start and the end of cooling by
the cooling zone 60, the temperature of the steel strip S is in a film boiling
temperature range and the temperature of the edge portion ofthe steel stripS is equal
25 to or higher than the temperature of the center portion.
[0038]
According to the temperature distribution in the width direction of the steel
sh·ip S, as in the state at the position B, for example, a temperature curve is obtained
in which the temperature of the edge portion is high with respect to that of the center
30 portion in the width direction of the steel strip S. Then, with the transportation of
the steel strip S, as shown in the distribution in the longitudinal direction of the steel
PCT/JP20 15/055012
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strip S in FIG. 2, temperature deviation between the temperature Ea of the edge
portion and the temperature Ca of the center portion becomes smaller, so that the
-teiriperafure distribution in the width directioii ()fthesteelstrip s Call be substantiallY
uniform fmally at the exit side of the cooling zone 60. That is, setting the
5 temperature of the steel strip S such that, during a period between the start and the
end of cooling by the cooling zone 60, the temperature of the steel strip S is in a film
boiling temperature range and the temperature of the edge portion of the steel strip S
is equal to or higher than the temperature of the center portion avoids an unstable
transition boiling state of the edge portion of the steel strip S, preventing quality
10 defects of the steel stripS.
[0039]
Note that the temperature of the edge portion of the steel strip S does not
necessarily need to be equal to or higher than the temperature of the center portion
throughout the range between the start and the end of cooling by the cooling zone 60,
15 as long as the temperature of the edge portion of the steel strip S is equal to or higher
than the temperature of the center portion in at least a range of 2/3 or more from the
upstream side in the sheet-passing direction of the total cooling length in the sheetpassing
direction of the cooling zone 60. If the temperature of the edge portion of
the steel strip S is equal to or higher than the temperature of the center portion in this
20 range, the quality of the steel strip S can be kept within an allowable range.
[0040]
Although ideal fmal temperature difference is zero as shown in FIG. 2, in
actuality, there is a margin between the upper limit temperature at which wrinkles
occur and the lower limit temperature at which zinc powder pick -up occurs, and the
25 temperature. margin is generally approximately 40°C. Accordingly, as long as the
temperature of the edge portion of the steel strip S is equal to or higher than the
temperature of the center portion in a range of 2/3 or more of the total cooling length
from the upstream side in the sheet-passing direction, fmal temperature deviation can
be kept within a temperature range in which wrinkles and zinc powder pick-up can
30 be avoided. This finding has been obtained by consideration based on results of
investigation of the amount of generated temperature deviation of the steel strip S in
" I PCT/JP2015/055012
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a practical line.
[0041]
Heie,af a cooling iiitenriediate position-of tlietotiircooling kngtli, it is
desirable that the temperature of the edge portion of the steel strip S be higher than
5 the temperature ofthe center pmtion by 20°C or more. That is, when, at the cooling
intermediate position of the total cooling length, a temperature curve is obtained in
which the temperature of the edge portion is high with respect to that of the center
portion in the width direction of the steel stripS, as shown at the position Bin FIG. 2,
the temperature distribution in the width direction of the steel strip S can be
10 substantially uniform fmally at the exit side of the cooling zone 60.
[0042]
<3. Steel strip cooling by cooling installation of cooling zone>
(3-1. Method for cooling steel strip)
FIG. 3 shows an outline of sheet temperature control by the cooling zone 60
15 of the galvannealing furnace according to the present embodiment. As shown in
FIG. 3, the steel strip S is cooled to a target endpoint temperature by passing through
the cooling zone 60. In general, in hot-dip galvannealing treatment, the temperature
of the steel strip S at the entry side of the cooling zone 60 of the galvannealing
furnace is approximately 450 to 600°C, and the endpoint temperature is
20 approximately 300 to 400°C. A quench temperature Tq shown in FIG. 3 is the
boundary temperature between a film boiling region and a transition boiling region
of water. A temperature range higher than the quench temperature Tq is a film
boiling temperature range in which water undergoes film boiling on the surface of
the steel strip S. The quench temperature Tq changes depending on cooling
25 conditions, and tends to increase when the steel strip S is strongly cooled with a large
amount of water.
[0043]
As shown in FIG. 3, a temperature difference between the endpoint
temperature and the quench temperature Tq is smaller than a temperature difference
30 between the sheet temperature at the entry side of the cooling zone 60 and the quench
temperature Tq. Accordingly, when the steel strip S is strongly cooled in the
PCT/JP2015/055012
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cooling-zone subsequent stage section 62, the quench temperature Tq increases,
making the temperature difference between the endpoint temperature and the quench
··· tempei'atuieTq even smaller. Tliis increases tlie possibility oCillisfl!ndergoiiig
transition boiling in the cooling-zone subsequent stage section 62, and may cause
5 temperature unevenness in the steel strip S. The cooling zone 60 according to the
present embodiment always prevents the sheet temperature from becoming equal to
or lower than the quench temperature Tq, while actively cooling the steel strip S with
a large amount of water at the upstream side in the sheet-passing direction of the
cooling zone 60.
10 [0044]
Specifically, at the upstream side in the sheet-passing direction of the
cooling-zone preceding stage section 61, there is provided an adjusted cooling
installation 61a in which the amount of mist jetted to the steel stripS passing through
the cooling zone 60 is adjusted in the width direction of the steel strip S. The
15 adjusted cooling installation 6 I a is a cooling installation adjusted to actively cool the
center portion in the width direction of the steel strip S and suppress cooling of the
edge portion. The adjusted cooling installation 61a is installed to prevent great
temperature distribution in the width direction of the steel strip S, while preventing
the temperature of the steel strip S from becoming equal to or lower than the quench
20 temperature at which water shifts from film boiling to transition boiling.
[0045]
The adjusted cooling installation 6Ia is provided at the upstream side in the
sheet-passing direction of the cooling-zone preceding stage section 61 because, as
described above, there is a larger margin of a control width of the temperature of the
25 steel strip S than at the downstream side in the sheet -passing direction of the cooling
zone 60. Since the target endpoint temperature of the steel strip S is near the
quench temperature of water, the control apparatus 65 needs to have high control
precision in order to prevent the temperature of the steel strip S from becoming equal
to or lower than the quench temperature. Hence, it is desirable that the adjusted
30 cooling installation 6la be provided at the upstream side in the sheet-passing
direction of the cooling-zone preceding stage section 61 and actively cool the steel
" i PCT/JP2015/055012
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strip S with a large amount of water.
[0046]
- - - ... Moreover, the cOoling zone oO accordmg to-the preseiifeni&odiriient is
provided with the mist suction installations 67 that suck at least part of the mist jetted
5 to the steel strip S togetl)er with air present in the cooling zone 60 in order to
minimize the influence of a position change of a quench point. Thus, excessive
mist that causes dripping water is sucked, which prevents excessive mist from being
poured on the steel strip S as dripping water.
10
[0047]
These mist suction installations 67 are preferably provided at least near
portions facing the edge portions of the steel strip S in the cooling zone 60.
Providing the mist suction installations 67 at such positions makes it possible to
more effectively suck excessive mist that may cause dripping water in the edge
portions.
15 [0048]
In addition, these mist suction installations 67 are preferably provided at
least at the downstream side in the sheet-passing direction of the cooling zone 60.
At the downstream side in the sheet-passing direction, where the steel strip S has
lower temperature, there is a high possibility that dripping water causes a change in
20 the position of the quench point, and the boiling state shifts from a film boiling state
to a transition boiling state. Accordingly, providing the mist suction installations 67
mainly at the downstream side in the sheet-passing direction of the cooling zone 60
makes it possible to suppress temperature variation due to dripping water more
effectively. Note that the number of the mist suction installations 67 provided in the
25 cooling zone 60 is not limited, and may be set as appropriate depending on the size
of the cooling zone 60, the amount of mist to be sucked from the cooling zone 60,
and the like.
[0049]
The amount of excessive mist sucked by the mist suction installations 67 is
30 controlled by the control apparatus 65. Making the control apparatus 65 control
both the adjusted cooling installation 6la and the mist suction installations 67
PCT/JP2015/055012
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enables more efficient management of the cooling state of the steel stripS.
[0050]
Here, if the amouiil oflnisfsucked oy tlie mist suction installations 6Tis-to6- -
small, dripping water due to residual excessive mist occurs. If the amount of
5 sucked mist is too large, the steel strip S is not cooled sufficiently. Hence, the
10
amount of mist sucked by the mist suction installations 67 under control of the
control apparatus 65 is preferably set within a predetermined range in which the steel
strip S can be cooled sufficiently while occurrence of dripping water is prevented.
[0051]
The amount of exhaust air and mist sucked by the mist suction installations
67 can be controlled by a known method, and for example, can be controlled
according to the value of a pressure gauge (reference sign "69" in FIG. 6) provided
near a mist suction port for the mist suction installations 67. That is, a pressure
value in the center portion of the steel strip S near the mist suction port may be
15 measured using the pressure gauge provided near the mist suction port, and damper
opening of exhaust blowers provided in the mist suction installations 67 may be
adjusted to make the measured pressure value negative.
[0052]
To adjust width-direction temperature distribution with a limited installation
20 length of the adjusted cooling installation 61a in the sheet-passing direction, the
adjusted cooling installation 61a needs to be used with a large amount of water. On
the other hand, to use the adjusted cooling installation 61a in a film boiling region, it
is desirable that the adjusted cooling installation 61a be used with a small amount of
water in order to avoid an increase in the quench temperature Tq. Thus, only with
25 the installation of the adjusted cooling installation 61a, conditions for adjusting
width-direction temperature distribution and conditions for stable cooling in a film
boiling region are mutually contradictory and not easily compatible. Making the
installation length of the adjusted cooling installation 61a unnecessarily long brings
about problems in that the installation becomes complex and requires high
30 installation cost, and the temperature of the edge portion rather becomes high in a
target material for which width-direction temperature distribution does not need to be
PCT/JP2015/055012
18/37
adjusted.
[0053]
........ Helice, tne presimfapplicatiori inventors sfudied an installiltiimfor aclikviiig
suppression of width-direction temperature distribution and maintenance of film
5 boiling conditions, and as a result, found that the installation length L [ m] of the
adjusted cooling installation 61 a is required to satisfY the following formula (1 ).
[0054]
L::>: (ax V x th)/((Tin- P)"m) x (Tin-y)) ... (1)
Here, Tin [0 C] denotes the temperature of the center portion of the steel strip
10 S at the entrance of the cooling zone 60, V [ m/s] denotes the speed of the steel strip S,
and th [ m] denotes the thickness of the steel strip. In addition, u, p, y, and m are
constants, which are set according to the hot -dip galvannealing installation.
[0055]
The present application inventors, under vanous operation conditions,
15 investigated the ability to adjust width-direction temperature distribution and the
cooling stability with respect to the water amount of the adjusted cooling installation
6la. As a result, they found, among conditions under which a film boiling region
can be maintained, the presence of a water amount that makes the width-direction
temperature distribution smallest. It was also found that the water amount is related
20 to the temperature of the steel strip S at the entrance of the cooling zone 60, the
speed of the steel strip S, the thickness of the steel strip S, and the installation length
L of the adjusted cooling installation 6la. Hence, using this relationship, they
derived the above formula (1) to specifY the installation length L of the adjusted
cooling installation 61 a necessary to obtain a width-direction temperature
25 distribution adjustment effect.
[0056]
The f01mula (1) is derived in the following manner. First, the quench
temperature Tq tends to increase when the steel strip S is strongly cooled with a large
amount of water, as described above. This relationship can be obtained by
30 evaluating cooling characteristics of a steel strip by using a test installation imitating
a real-world installation. For example, as shown in FIG. 4, the qu<;nch temperature
" I PCT/JP2015/055012
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Tq is expressed by a direct function of a cooling water amount Q as in the following
formula (1-1). In the formula (1-1), a and bare constants.
-[0057]
Tq =aQ+b ... (1-1)
5 [0058]
As shown in FIG. 4, when the entry-side temperature Tin of the steel strip S,
the thickness th of the steel. strip S, ·the speed V of the steel strip S, and the
installation length L of the adjusted cooling installation 6la in a center portion (the
center in the width direction) of the adjusted cooling installation 6la are constant, the
10 cooling water amount Q and the temperature T of the center portion of the steel strip
S have a relationship in which, as shown in FIG. 4, the temperature T of the center
portion of the steel strip S decreases with an increase in the cooling water amount Q.
Here, an improvement effect 1'1 T of a temperature difference between the center
portion and the edge portion of the steel strip S by the adjusted cooling installation
15 6la is proportional to a difference between the entry-side temperature T;n of the
center portion of the steel strip S and a temperatureT1 at any position in the sheetpassing
direction in the adjusted cooling installation 6la. That is, the improvement
effect 1'1 T of temperature distribution in the width direction is expressed by the
following formula (1-2). In the formula (1-2), a is a constant.
20 [0059]
[0060]
On the other hand, in order to prevent the steel strip S from being cooled to
a temperature lower than the quench temperature Tq, temperature distribution in the
25 width direction adjustable by the adjusted cooling installation 6la has an upper limit.
That is, as shown in FIG. 5, between point P A and point Pn indicating a position at
which the temperature becomes the quench temperature Tq, the improvement effect
1'1T of temperature distribution in the width direction increases as the cooling water
amount Q increases. However, if the temperature T of the steel strip S falls below
30 the quench temperature Tq, the steel strip S is subjected to local overcooling, and as
shown in FIG. 5, the improvement effect 1'1 T of temperature distribution in the width
I PCT/JP20 15/055012
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direction sharply decreases from point Ps toward point Pc.
[0061]
---- ---------- --- -------- - ---AcCOfOiiigly~----teni{ierature diSfiibritiOll. -"iri--1he WiOth--dii·ectiOri iidjUStabie---by ------the
adjusted cooling installation 61a is within a film boiling temperature range (a
5 range from point P A to point Ps) in which the temperature of the steel strip S is equal
to or higher than the quench temperature Tq. Hence, L'.Tmax denoting the
improvement effect of temperature distribution in the width direction at the quench
temperature Tq can be expressed by the following formula (1-3) according to the
formula (1-2).
10 [0062]
L'.Tmax = a(Tin- Tq) ... (1-3)
[0063]
Fmthermore, the installation length L of the adjusted cooling installation
6la is determined with respect to temperature distribution deviation that needs to be
15 adjusted. Here, the upper limit L'. T max of the improvement effect of temperature
distribution adjustable as described above is expressed also by the temperature T;n of
the center portion at the entry side of the steel strip S, the thickness th and the speed
V of the steel stripS, and the installation length L of the adjusted cooling installation
61 a, as in the following formula (1-4).
20 [0064]
L'.Tmax = (a·2·h-L-(Tave- Tw))/(p·Cp·V·th) ... (1-4)
Here, T ave is the average sheet temperatme, which is expressed by, for
example, an average value of the temperature T;n of the center portion at the entry
side of the steel strip S and the quench temperatme Tq. In addition, Tw is cooling
25 water temperature, p is a steel material density, and Cp is a steel material specific
heat.
[0065]
The above formula (I) can be obtained by organizing the relationship of the
formula (1-4), the above formulae (1-1) and (1-3), and a fonnula (1-5) expressing the
30 relationship between a cooling water amount Q [Vm2·min] and a heat transfer
coefficient h [W/m2
·0 C). In the formula (1-5), k is a constant.
PCT/JP2015/055012
21/37
[0066]
h = kQm ... (1-5)
---- [0067]
Here, the constants a, ~, and y of the above formula (1) are as follows.
5 [0068]
10
o;=20280 x am/k ... (1-7)
~ = 33 + b ... (1-8)
y=45 ... (1-9)
[0069]
The constants a, ~, and y are set by using results of evaluation of cooling
characteristics of a steel strip using a test installation imitating a real-world
installation, and for example, can be set as follows: o; = 1700000, ~ = 330, y = 45, m
=0.6.
[0070]
15 Note that the temperature T of the steel stripS at the entrance of the cooling
zone 60, the speed V of the steel strip S, and the thickness th of the steel strip S are
values determined by steel grades, the amount of production, and order sizes;
therefore, the value of L calculated using the formula (1) is not a fixed value.
Accordingly, the installation length L of the adjusted cooling installation 61 a is
20 determined assuming typical operation conditions, for example.
[0071]
When the installation length L of the adjusted cooling installation 61 a is
constant, the steel strip S may be produced with a speed equal to or lower than the
upper limit speed V max of the steel strip S calculated from the following formula (2),
25 based on the relationship of the above formula (1 ). Here, o;', P', y', and m are
constants, which are set according to the hot -dip galvannealing installation, and for
example, can be set as follows: o;' = 1700000, W = 330, y' = 45, m = 0.6. Since the
speed V of the steel strip S changes depending on a sheet to be passed, these
constants are set in consideration of a transient state.
30 [0072]
V max= (L X (Tin- P'Y'm X (Tin-y'))/( o;' x th) ... (2)
"
PCT/JP2015/055012
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[0073]
In this manner, even when the installation length L of the adjusted cooling
------------ ---- ----- ---iriStallatiOii-6 ra --cannorbe--Chaiiged;--the--Uppet-IitnirSpeed-V ~-~-Oftlie- steer strip S --"iS ---
changed according to steel grades, the amount of production, and order sizes, and the
5 steel strip S is produced with a speed V equal to or lower than the upper limit speed
10
V max· This provides high productivity while avoiding quality defects due to cooling
unevenness. The speed V of the steel strip S is reported to an operator by a
guidance system, for example, to be changed.
[0074]
Regarding temperature distribution in the width direction of the steel strip S,
although no temperature distribution is desirable, temperature distribution within a
predetermined temperature range does not greatly influence quality. For example,
the predetermined temperature range is approximately 30°C. Regarding the
endpoint temperature at the exit side of the cooling zone 60, the endpoint
15 temperature is approximately 300 to 400°C as described above. An endpoint
temperature higher than this range may cause the top roll 70 to pick up zinc powder
on the surface of the steel strip S. Accordingly, the maximum temperature among
the temperatures in the width direction of the steel strip S at the exit side of the
cooling zone 60 is controlled so as not to exceed 300 to 400°C.
20 [0075]
[3-2. Configuration example of adjusted cooling installation]
A configuration of the adjusted cooling installation 61 a will be described
based on FIGS. 6 to 9. FIG. 6 is an explanatory diagram illustrating a configuration
example of the cooling zone 60 according to the present embodiment. FIG. 7 is an
25 explanatory diagram illustrating a configuration example of the cooling-zone
preceding stage section 61 including the adjusted cooling installation 61a according
to the present embodiment. FIG. 8 is an explanatory diagram illustrating a
configuration example of the mist header 63. FIG. 9 is an explanatory diagram for
explaining the installation length of the adjusted cooling installation 61a when the
30 adjusted cooling installation 61a includes a single-stage mist header 63.
[0076]
" I PCT/JP2015/055012
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The cooling zone 60. according to the present embodiment includes a
plurality of mist headers 63 arranged in the longitudinal direction. In the mist
--· -- ------lieader63,aplurality.ofll1ist jet nozzles 64- are anwged-aloriffthe Widtlioifectionof -
the steel strip S, as illustrated in FIG. 8. The cooling-zone preceding stage section
5 61 and the cooling-zone subsequent stage section 62 are each provided with a
plurality of stages (e.g., about 30 stages) of mist headers 63. The cooling zone 60
as illustrated in FIG. 7 is provided in a symmetrical arrangement about the sheetpassing
direction of the steel strip S. Thus, the steel strip S is cooled from its front
and rear surfaces. The amount of mist jetted from the mist jet nozzles 64 (i.e., the
10 water amount of the mist header 63) can be adjusted by opening and closing valves
66a and 66b illustrated in FIG. 8. The opening and closing of the valves 66a and
66b can be controlled for each stage by the control apparatus 65.
[0077]
The adjusted cooling installation 61a can be configured for example by
15 blocking, with caps, the mist jet nozzles 64 at the edge portion sides in the width
direction of the steel strip S, among the mist jet nozzles 64 arranged in each mist
header 63, to prevent the mist jet nozzles 64 from jetting mist. In the example of
FIG. 7, the edge portions of the mist headers 63 of first to n-th stages located at the
upstream side in the sheet-passing direction of the cooling-zone preceding stage
20 section 61 are blocked with caps to form a non-jetting region 63b. Accordingly,
while passing tlu-ough the adjusted cooling installation 61a, the steel strip S is
actively cooled in the center portion con·esponding to a jetting region 63a, whereas
cooling of the both edge portions is suppressed.
25
[0078]
Note that the number n of the mist headers 63 included in the adjusted
cooling installation 61a is set based on the installation length L of the adjusted
cooling installation 6la set according to the above formula (!) or a constant
installation length L of the adjusted cooling installation 61 a that is set in advance.
Specifically, the installation length L of the adjusted cooling installation 6la is
30 expressed by the following formula (3). Here, when the adjusted cooling
installation 61a includes a single-stage mist header 63 (i.e., when n is 1), as
I PCT/JP2015/055012
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illustrated in FIG. 9, a range in which mist is jetted from the mist jet nozzles 64 at an
angle 8 of 45° upward and downward with respect to a direction perpendicular to the
- - - - surfiCe ofthesteeJ stripS is ddinedas tlie irlsfallation lengthTofthe adjusted -
cooling installation 61a.
5 [0079]
[Math. 1]
L={(n+l)xp
2d
[0080]
(n 22)
(n = 1) ... (3)
Here, p denotes a pitch between adjacent mist headers 63 in the sheet-
10 passing direction, and d denotes a distance between the steel strip S and the mist
headers 63. Based on the above formula (3), the number n of the mist headers 63
included in the adjusted cooling installation 6la and installation positions thereof can
be determined.
15
[0081]
In the adjusted cooling installation 61a, as illustrated in FIG. 7, for example,
at the upstream side in the sheet-passing direction, a large number of mist jet nozzles
64 in portions corresponding to both edge portions of the steel strip S may be
blocked with caps to increase the non-jetting region 63b, and toward the downstream
side, the number of the mist jet nozzles 64 blocked with caps may be reduced from
20 the center portion side to reduce the non-jetting region 63b. That is, the jetting
25
region 63a in which the mist jet nozzles 64 of the mist headers 63 jet mist to the
surface of the steel strip S is made larger from the upstream side toward the
downstream side in the sheet -passing direction.
[0082]
For example, the installation length L of the adjusted cooling installation
61a needed when the steel strip S has a thickness of 0.6 mm and the steel strip
temperature at the entrance of the cooling zone 60 is 500°C is set as shown in Table 1
below. A higher speed V of the steel strip S requires a longer adjusted cooling
installation 61 a.
30 [0083]
PCT/JP20 15/055012
25/37
[Table 1]
Speed of steel strip [ m/minute] Necessary length of adjusted cooling
installation [m]
120 0.21
150 0.26
180 0.31
250 0.43
300 0.51
[0084]
In this manner, overcooling of the edge portion of the steel strip S is
effectively suppressed at the start of cooling, and after that the cooling range of the
5 steel strip S is gradually widened so that the steel strip S is entirely cooled. In
particular, at the start of cooling, the center portion of the steel strip S is cooled
intensively and cooling of the edge portion is stopped; thus, as shown in FIG. 2,
while passing through the cooling zone 60, the steel strip S can have a temperature of
the edge portion equal to or higher than that of the center portion. Accordingly, at
10 the end of cooling in the cooling zone 60, great temperature distribution in the width
direction of the steel strip S is prevented, resulting in substantially uniform cooling.
[0085]
In the cooling zone 60, mist is jetted from all of the mist jet nozzles 64 in
the mist headers 63 at the downstream side in the sheet -passing direction with respect
15 to the adjusted cooling installation 61a, that is, in all of the mist headers 63 in the
(n+1)-th and the following stages of the cooling-zone preceding stage section 61 and
in the cooling-zone subsequent stage section 62.
[0086]
Note that the adjusted cooling installation 6la does not have to be installed
20 from the first mist header 63 at the most upstream side in the sheet-passing direction .
of the cooling zone 60 as illustrated in FIG. 6, but in order to enjoy an effect of the
present invention, it is desirable that the adjusted cooling installation 61 a be installed
from a mist header 63 as close as possible to the upstream side, if possible, the first
mist header 63.
25 [0087]
Moreover, as illustrated in FIGS. 6 and 7, the mist suction installations 67
"
PCT/JP20 15/055012
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are provided to face the edge portions of the steel strip S at the downstream side of
the cooling-zone preceding stage section 61 and the downstream side of the cooling--
zonesuosequeiit stage section · 62.- These · riiist suction--iristallatioris 6Tsuck a
predetermined amount of mist jetted from the mist headers 63 according to a pressure
5 value measured by the pressure gauge 69 to make the pressure value in the center
portion negative. Thus, inside the cooling-zone preceding stage section 61, mist is
present in an amount with which the steel strip can be cooled sufficiently while
occurrence of dripping water is prevented, and this prevents occurrence of cooling
unevenness due to dripping water.
10 [0088]
The configuration of the adjusted cooling installation 6la in FIGS. 6 and 7
is an example, and a configuration of the adjusted cooling installation 6la of the
cooling zone 60 according to the present embodiment is not limited to such an
example. For example, a configuration may be adopted in which the mist jet
15 nozzles 64 blocked with the caps 65 in FIGS. 6 and 7 are originally not provided so
that cooling of the edge portion is stopped. Alternatively, instead of completely
stopping cooling of the edge portion, the edge portion may be sprayed with a smaller
amount of water than the center portion is. Moreover, although the adjusted cooling
installation 6la in FIGS. 6 and 7 is configured in a manner that a cooling range of the
20 center pottion of the steel strip S becomes larger from the upstream side toward the
downstream side in the sheet-passing direction, a cooling range of the center portion
by the adjusted cooling installation 61 a may be constant.
[0089]
Description has been gtven above on the cooling zone 60 of the
25 galvannealing furnace in the hot-dip galvannealing treatment installation according
to the present embodiment. The cooling zone 60 of the galvannealing furnace
according to the present embodiment includes, at the upstream side in the sheetpassing
direction of the cooling-zone preceding stage section 61, the adjusted cooling
installation 61 a in which the amount of mist jetted to the steel strip S passing through
30 the cooling zone 60 is adjusted in the width direction of the steel strip S. In the
adjusted cooling installation 6la, the center portion of the steel strip S is actively
" I PCT/JP20 15/055012
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cooled, whereas cooling of the edge portion is stopped or performed by jetting with a
small amount of water. In addition, the pair of mist suction installations 67 is
----- --------- -----proviaea- ar1eascnear-poniorErJacirig -tlie--edge-·ponions-of tlie-ste-er-sttil,- s---ui-the __ cooling zone 60.
5 [0090]
Here, the installation length L of the adjusted cooling installation 6la is set
to a length such that occurrence of temperature unevenness due to great temperature
deviation in the width direction of the steel strip S is prevented and, at the same time,
cooling can be performed in a manner that the sheet temperature of the steel strip S
10 does not become equal to or lower than the quench temperature Tq. This enables
stable cooling of the steel stripS. The cooling zone 60 of the galvannealing furnace
according to the present embodiment can cool the steel strip stably by mist cooling;
thus, the steel strip can be passed at high speed to be treated, which improves
productivity. In addition, providing the mist suction installations 67 at the above-
15 described positions makes it possible to more effectively suck excessive mist that
may cause dripping water in the edge portions.
[Examples]
[0091]
As Examples, in a cooling zone of a galvannealing furnace in a hot -dip
20 galvannealing treatment installation, a hot-dip galvanized steel strip was cooled with
the number of headers used in an adjusted cooling installation changed and the
installation length L of the adjusted cooling installation changed, and width-direction
temperature distribution of the steel strip after cooling and appearance quality of a
product were studied. The cooling zone has a configuration similar to that of FIG. 6,
25 and includes mist headers of 36 stages. Of these, mist headers in the first to ninth
stages form the adjusted cooling installation. In Examples, the water amount in the
edge portion of the adjusted cooling installation was zero, and mist jetting was
performed only in the center portion. Results are shown in Table 2.
30
[0092]
In Table 2, a temperature difference at a cooling-zone inte1mediate position
refers to a position between the cooling-zone preceding stage section 61 and the
I
No
Compa
rative
Ex amp
le 0
Compa
rative
Examp
le I
Ex amp
le I
Examp
le 2
Examp
Je 3
Compa
rative
Ex amp
le 2
Compa
rative
Ex amp
Je 3
Compa
rative
Examp
PCT/JP2015/055012
28/37
cooling-zone subsequent stage section 62, and indicates a value obtained by
subtracting the temperature of the center portion from the temperature of the edge
-:portion. --Ktemperafufe difference at the -cooliti!Fzorieexifside also iiidkates-a
value obtained by subtracting the temperature of the center portion from the
5 temperature of the edge portion. The temperature of the edge portion is a surface
temperature at a position 100 mm away from the end in the width direction of the
steel strip, and the temperature of the center portion is a surface temperature at a
center position in the width direction of the steel strip.
[0093]
10 [Table 2]
Number of Temperature
Lower limit Presen
headers difference [0C]
Cooling- Jnstallatio
value of
Steel n length of
ce or
Sheet
zone
installation absenc Coolin
strip
thickne
entrance adjusted
length of of Subs Coolin
speed sheet cooling
e
Prece
g-zone
[m/min
ss
temper installatio
adjusted mist
ding
eque interme g-zone
ute]
[mm]
ature
cooling suction
stage
nt diate exit
n
[OC] installation install a stage positio side [m]
[m] lions n
150 0.85 550 0 0.28 absent 27 18 -34 -95
150 0.85 550 0 0.28 present 27 I8 -32 -55
I 50 0.65 480 I.4 0.3I present 27 I8 26 IO
I80 0.55 520 1.6 0.25 present 28 27 37 4
250 0.70 600 1.8 0.3I present 36 36 88 10
150 0.60 480 0.4 0.29 absent 27 18 -20 -82
I80 0.85 600 0.2 0.27 present 27 27 3 -46
250 1.00 600 0.2 0.44 present 27 36 -6 -95
Prese
nee
or Prese
absen nee
ce of or
roll absen
zinc ce of
powd wrin
er kles
pickup
c c
B A
A A
A A
A A
c c
B c
c c
" I
le4
Campa
rative ...
Examp
le 5
Campa
rative
Ex amp
le 6
-180
180
PCT/JP2015/055012
29/37
---- 0,80 520 0.2 0,37 ---- present -27 ... 27- --21-. -55-
0.55 520 1.6 0.25 present 28 27 -17 -50
A: absent (excellent), B: slightly present (inacceptable), C: present (inacceptable)
[0094]
Comparative Example 0 is an example in wlllch mist headers in the fust to
ninth stages serving as the adjusted cooling installation were not used, that is, the
5 steel strip was subjected to mist cooling entirely in the width direction. In
Comparative Example 0, mist suction installations were also not used. In tills case,
the sheet temperature of the edge portion greatly decreased relative to the center
portion in the width direction of the steel strip. A top roll picked up zinc powder on
the surface of the steel strip, and wrinkles occurred. Comparative Example 1 is an
10 example in which mist suction installations were installed in addition to the state of
Comparative Example 0. In tills case, wrinkles did not occur, but pick-up of zinc
powder on the surface of the steel strip by a top roll was observed.
[0095]
Examples 1 to 3 are examples in wlllch mist headers in the first to ninth
15 stages serving as the adjusted cooling installation were used. The length of the
adjusted cooling installation in Examples 1 to 3 was set to be longer than its lower
limit value so as to satisfy the above fotmula (1 ). In these cases, the center portion
in the width direction of the steel strip was actively cooled by the adjusted cooling
installation, and then the steel strip was subjected to mist cooling entirely in the
20 width direction by mist headers at the downstream side by the adjusted cooling
installation; thus, a reduction in the temperature of the edge portion was alleviated in
comparison with Comparative Examples 0 and 1. A top roll did not pick up zinc
powder on the surface of the steel strip, and wrinkles did not occur.
[0096]
--c c
c c
PCT/JP20 15/055012
30/37
Comparative Example 2 is an example in which mist headers in the first to
ninth stages serving as the adjusted cooling installation were used, the length of the
-- - - - -aajusfed cooling installation ·satisfied tlie--aoove formula (I), and mist-suction
installations were not provided. In this case, as in Comparative Example 0, the
5 sheet temperature of the edge pmtion greatly decreased relative to the center portion
in the width direction of the steel strip. A top roll picked up zinc powder on the
surface of the steel strip, and wrinkles occurred.
[0097]
Comparative Examples 3 to 5 are examples in which the number of mist
10 headers in the first to ninth stages serving as the adjusted cooling installation was
reduced. In each of these examples, the length of the adjusted cooling installation
did not satisfY the above formula (1) and was set to be shorter than its lower limit
value. In Comparative Example 3, a top roll slightly picked up zinc powder on the
surface of the steel strip because the above formula (1) was not satisfied. This is
15 presumably because, although the temperature of the steel strip did not fall below the
quench temperature during cooling, the temperature of the center portion in the width
direction of the steel strip at the cooling-zone intermediate position was only slightly
higher than the temperature ofthe edge portion, which resulted in a large temperature
difference at the cooling-zone exit side.
20 [0098]
Comparative Examples 4 and 5 are examples in which, in order to suppress
the influence of the reduction in the n~ber of mist headers used in the adjusted
cooling installation resulting in a smaller temperature difference resolution allowance
between the center portion and the edge portion, an attempt was made to reduce the
25 temperature difference between the center pmtion and the edge pmtion at the
cooling-zone exit side by increasing the amount of water suppled to each mist header
of the adjusted cooling installation. In Comparative Example 4, the temperature
difference between the center portion and the edge portion at the cooling-zone exit
side was reduced, but the temperatur·e of the steel strip fell below the quench
30 temperature during cooling, which caused wrinkles. In Comparative Example 5,
the temperature difference between the center portion and the edge portion was not
" I PCT/JP20 15/055012
31/37
able to be made sufficiently small by the increase in the amount of water suppled to
each mist header of the adjusted cooling installation. This resulted in high
-teiliperatlfreof the cefitetpc:iitiOti-in the width dire-ctiofi--c:if tile steel strip af1he-cooling-
zone exit. On the other hand, the temperature of the edge portion in the
5 width direction of the steel strip decreased to fall below the quench temperature.
Consequently, in Comparative Example 5, a top roll picked up zinc powder on the
surface of the steel strip, and wrinkles occurred.
[0099]
Comparative Example 6 is an example in which the adjusted cooling
10 installation is provided at the fmal stage side of the cooling zone. In Comparative
Example 6, the length of the adjusted cooling installation satisfied the above formula
(1 ), and mist suction installations were installed. That is, as illustrated in FIG. 10,
the cooling zone is provided with the pair of mist suction installations 67 arranged to
face the edge portions in the width direction of the steel ship S. The mist suction
15 installations 67 are provided at an intermediate position in the sheet-passing direction
and the exit side of the cooling zone 60 to suck at least part of the mist jetted to the
steel strip S. In addition, the adjusted cooling installation is configured from the
cooling-zone exit side toward the upstream side in the sheet-passing direction. The
adjusted cooling installation can be configured by blocking, with caps, the mist jet
20 nozzles at the edge portion sides in the width direction of the steel strip S to prevent
the mist jet nozzles from jetting mist. Here, a non-jetting region 63c is made to
become smaller from the cooling-zone exit side toward the upstream side in the
sheet-passing direction.
25
[0100]
In Comparative Example 6, the steel strip S was cooled entirely in the width
direction in the cooling-zone preceding stage section 61, so that at the intermediate
position of the cooling zone, the temperature of the edge portion in the width
direction of the steel strip became lower than the temperature of the center portion.
Consequently, unstable transition boiling of the edge portion was not able be avoided
30 by suppressing cooling of the edge portion in the cooling-zone subsequent stage
section 62; thus, a top roll picked up zinc powder on the surface of the steel strip, and
I PCT/JP2015/055012
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wrinkles occurred.
[0101]
- - - - -- - -According- to- Examples; it was found that when an adjusted coolinginstallation
is provided at the upstream side in the sheet-passing direction of a
5 cooling installation and the above formula (1) is satisfied, a reduction in the
temperature of the edge portion in the width direction of a steel strip is alleviated and
occurrence of temperature unevenness is suppressed, and an excellent product
without wrinkles can be produced. In addition, it was demonstrated that pick-up of
zinc powder on the surface of the steel strip by a top roll can be prevented.
10 [0102]
The preferred embodiment(s) of the present invention has/have been
described above with reference to the accompanying drawings, whilst the present
invention is not limited to the above examples. A person skilled in the art may fmd
various alterations and modifications within the scope of the appended claims, and it
15 should be understood that they will naturally come under the technical scope of the
present invention.
[0103]
For example, in the above embodiment, a mist nozzle (two-fluid nozzle) that
jets mist is used in a cooling installation for cooling a steel strip, but the present
20 invention is not limited to such an example. For example, the cooling installation
may be configured using a single-fluid nozzle that jets water. In terms of water
quality management, it is preferable to use a two-fluid nozzle rather than a singlefluid
nozzle which makes water quality management difficult.
25 Reference Signs List
[0104]
5 molten zinc
10 zinc pot
20 sink roll
30 30 gas nozzle
40 heating zone
" I PCT/JP2015/055012
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50 heat -retaining zone
60 cooling zone
------------ - 61 cooling~zone preceding stage section -------- -
62 cooling-zone subsequent stage section
5 63 mist header
63a jetting region
63b non-jetting region
64 mist jet nozzle
65 control apparatus
10 70 top roll
s steel strip

CLAIMS
Claim 1
- A-method for coalinga steel sfrip-by rriist cooling ina coolinginsfiillatiori oC
a galvannealing furnace configured to perform galvannealing treatment on a hot -dip
5 galvanized steel strip, the method comprising:
by an adjusted cooling installation provided at an upstream side in a sheetpassing
direction of the cooling installation, jetting mist to the steel strip passing
through the cooling installation in a manner that an amount of mist jetted to the steel
strip passing through the cooling installation is smaller in an edge portion in a width
10 direction of the steel strip than in a center portion;
by a mist suction installation provided at least at a downstream side in the
sheet-passing direction of the cooling installation, sucking at least part of mist jetted
to the steel strip; and
cooling the steel strip at a sheet-passing speed such that, during a period
15 between start and end of cooling of the steel strip, a temperature of the steel strip is
within a film boiling temperature range and a temperature of the edge portion in the
width direction of the steel strip is equal to or higher than a temperature of the center
portion in at least a range of 2/3 or more from the upstream side in the sheet-passing
direction of a total cooling length of the cooling installation.
20
Claim 2
The method for cooling a steel strip according to claim I, wherein, with
respect to an installation length L [m] of the adjusted cooling installation, a speed of
the steel strip is set to be equal to or less than an upper limit speed V max [ m/s]
25 calculated using a formula (a) below,
Vmax = (L x (Tin- P')"m x (Tin- y'))/(a' x th) ... (a),
where Tin [0 C] denotes a temperature of the center portion of the steel strip
at an entrance of the cooling installation, th [ m] denotes a thickness of the steel strip,
and u', W, y', and m are constants set according to a hot-dip galvannealing
30 installation.
5
PCT/JP2015/055012
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Claim 3
The method for cooling a steel strip according to claim 2, wherein the
-con:stantsare-serrurfollows: a'"'1870000;p>= 330, y' = 45; ni = 0~6.
Claim4
A cooling installation by mist cooling of a galvannealing furnace configured
to perform galvannealing treatment on a hot-dip galvanized steel strip, the cooling
installation comprising:
an adjusted cooling installation provided at an upstream side in a sheet-
10 passing direction of the cooling installation, the adjusted cooling installation being
capable of adjusting, in a width direction of the steel strip, an amount of mist jetted
to the steel strip passing through the cooling installation; and
a mist suction installation provided at least at a downstream side in the
sheet-passing direction of the cooling installation, the mist suction installation being
15 configured to suck at least part of mist jetted to the steel strip,
20
wherein the adjusted cooling installation is adjusted in a manner that an
amount of mist jetted to the steel strip passing through the cooling installation is
smaller in an edge portion in the width direction of the steel strip than in a center
pmtion, and
the cooling installation has an installation length in the sheet-passing
direction of the steel strip such that, during a period between start and end of cooling
of the steel strip, a temperature of the steel strip is within a fihn boiling temperature
range and a temperature of the edge portion in the width direction of the steel strip is
equal to or higher than a temperature of the center portion in at least a range of 2/3 or
25 more from the upstream side in the sheet-passing direction of a total cooling length
of the cooling installation.
Claim 5
The cooling installation according to claim 4, wherein the adjusted cooling
30 installation is provided in a manner that an installation length L [ m] of the adjusted
cooling installation in the sheet-passing direction of the steel strip satisfies a formula
I PCT/JP2015/055012
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(b) below,
12: (a X v X th)/((Tin- PY'm) X (Tin- r)) ... (b)
--wliereTin (0 CJ denotes ateriiperafure cifthe cenferportion oftJiesteeJ stripat
an entrance of the cooling installation, V [m/s] denotes a speed of the steel strip, th
5 [ m] denotes a thickness of the steel strip, and a, p, ')', and m are constants set
according to a hot-dip galvannealing installation.
Claim 6
The cooling installation according to claim 5, wherein the constants are set
10 as follows: a= 1700000, p = 330, ')' = 45, m = 0.6.
Claim 7
The cooling installation according to any one of claims 4 to 6,
wherein the adjustment cooling installation includes, in the sheet-passing
15 direction, a plurality of headers each including a plurality of nozzles an·anged along
the width direction,
wherein each header is configured in a manner that mist is not
jetted to the steel strip in the edge portion in the width direction of the steel strip.
20 Claim 8
The cooling installation according to claim 7, wherein each header of the
adjusted cooling installation is configured in a manner that the number of the nozzles
that jet mist to the steel strip in the center portion in the width direction of the steel
strip increases from the upstream side toward the downstream side in the sheet-
25 passing direction.