%
[Type of Document] SPECIFICATION
[Title of the Invention] METHOD FOR MANUFACTURING STEEL SHEET
[Technical Field]
[0001]
The present invention relates to a method for manufacturing a steel sheet.
[Background Art]
[0002]
For example, a hot-rolled steel sheet used in cars, industrial machines and the
like is generally manufactured through a rough-rolling process and a finish-rolling
process. FIG 19 is a view schematically illustrating a method for manufacturing a
hot-rolled steel sheet of the related art. In the process for manufacturing a hot-rolled
steel sheet, first, a slab S obtained by continuously casting molten steel having an
adjusted predetermined composition is rolled using a roughing mill 101, and then,
fijrthermore, hot-rolled using a finishing mill 103 constituted by a plurality of rolling
stands 102a to 102d, thereby forming a hot-rolled steel sheet H having a predetermined
thickness. In addition, the hot-rolled steel sheet H is cooled using cooling water
supplied from a cooling apparatus 111, and then coiled into a coil shape using a coiling
apparatus 112.
[0003]
The cooling apparatus 111 is generally a facility for carrying out so-called
laminar cooling on the hot-rolled steel sheet H transported from the finishing mill 103.
The cooling apparatus 111 sprays the cooling water on the top surface of the hot-rolled
steel sheet H moving on a run-out table from the top in the vertical direction in a water
jet form through a cooling nozzle, and, simultaneously, sprays the cooling water on the
bottom surface of the hot-rolled steel sheet H through a pipe laminar in a water jet
form, thereby cooling the hot-rolled steel sheet H.
[0004]
In addition, for example. Patent Document 1 discloses a technique of the
related art which reduces the difference in surface temperature between the top and
bottom surfaces of a thick steel sheet, thereby preventing the shape of the steel sheet
from becoming defective. According to the technique disclosed in Patent Document
1, the water volume ratio of cooling water supplied to the top surface and the bottom
surface of the steel sheet is adjusted based on the difference in surface temperature
obtained by simultaneously measuring the surface temperatures of the top surface and
the bottom surface of the steel sheet using a thermometer when the steel sheet is cooled
using a cooling apparatus.
[0005]
In addition, for example. Patent Document 2 discloses a technique that
measures the steepness at the tip of a steel sheet using a steepness meter installed on
the exit side of a mill, and prevents the steel sheet from being perforated by adjusting
the flow rate of cooling water to be different in the width direction based on the
measured steepness.
[0006]
Furthermore, for example. Patent Document 3 discloses a technique that aims
to solve distribution of a wave-shaped sheet thickness in the sheet width direction of a
hot-rolled steel sheet and to make uniform the sheet thickness in the sheet width
direction, and controls the difference between the maximum heat transmissibility and
the minimum heat transmissibility in the sheet width direction of the hot-rolled steel
sheet to be in a range of predetermined values.
[Prior Art Document]
[Patent Document]
[0007]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2005-74463
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2005-271052
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2003-48003
[Summary of the Invention]
[Problem that the Invention is to solve]
[0008]
Here, there are cases in which the hot-rolled steel sheet H manufactured using
the manufacturing method of the related art described using FIG. 19 forms a wave
shape in the rolling direction (the arrow direction in FIG. 20) on transportation rolls
120 in the run-out table (hereinafter sometimes referred to as "ROT") in the cooling
apparatus 111 as illustrated in FIG 20. In this case, the top surface and the bottom
surface of the hot-rolled steel sheet H are not uniformly cooled, and temperature
variation is caused. As a result, in a steel sheet-cooling process after a hot-rolling
process, a variation in the material qualities (that is, hardness of the steel sheet) is
caused by the temperature variation. Furthermore, in a cold-rolling process which is
a post process, a change in a sheet thickness is caused by the variation of the material
qualities. In a case in which the change in the sheet thickness of the steel sheet
exceeds a predetermined criterion value, the steel sheet is determined to be a defective
product in an inspection process, which causes a problem of a significant decrease in
yield.
- 3 -
[0009]
However, in the cooling method of Patent Document 1, a case of a hot-rolled
steel sheet having a wave shape in the rolling direction is not taken into consideration.
That is, in Patent Document 1, since a surface height varies depending on a location of
the wave of the hot-rolled steel sheet, a difference in the standard deviation of
temperature in the rolling direction is not taken into consideration. Therefore, in the
cooling method of Patent Document 1, the occurrence of the variation in the material
qualities during cooling of the hot-rolled steel sheet caused by the wave shape formed
in the hot-rolled steel sheet is not taken into consideration.
[0010]
In addition, in the cooling method of Patent Document 2, the steepness of the
steel sheet in the width direction is measured, and the flow rate of cooling water at a
portion with a high steepness is adjusted. However, even in Patent Document 2, a
case of a hot-rolled steel sheet having a wave shape in a rolling direction is not taken
into consideration, and a fact that a variation in the material qualities during cooling of
the hot-rolled steel sheet is caused by the wave shape formed in the hot-rolled steel
sheet as described above is not taken into consideration.
[0011]
In addition, the cooling of Patent Document 3 is the cooling of a hot-rolled
steel sheet immediately before roll biting in the finishing mill, and therefore it is not
possible to apply the cooling to a hot-rolled steel sheet which has undergone finishrolling
so as to have a predetermined thickness. Furthermore, Patent Document 3
also does not take a hot-rolled steel sheet having a wave shape in the rolling direction
into consideration, and does not consider the occurrence of variation in the material
qualities during cooling due to the wave shape formed in the hot-rolled steel sheet as
Jk
described above.
[0012]
The present invention has been made in consideration of the above problems,
and an object of the present invention is to provide a method for manufacturing a steel
sheet in which an improvement of yield of a steel sheet manufactured through at least a
hot-rolling process and a cooling process can be realized.
[Means for Solving the Problems]
[0013]
The invention employs the following means for solving the problems and
achieving the relevant object.
That is,
(1) A method for manufacturing a steel sheet according to an aspect of the
present invention includes a hot-rolling process in which a steel material is hot-rolled
using a finishing mill so as to obtain a hot-rolled steel sheet having an edge wave shape
with a wave height periodically changing in a rolling direction; and a cooling process
in which the hot-rolled steel sheet is cooled in a cooling section provided on a sheetthreading
path, in which the hot-rolling process includes a target steepness-setting
process in which a target steepness of the edge wave shape is set based on first
correlation data indicating a correlation between a steepness of the edge wave shape of
the hot-rolled steel sheet and a temperature standard deviation Y during or after cooling
of the hot-rolled steel sheet, which have been experimentally obtained in advance; and
a shape-controlling process in which operation parameters of the finishing mill are
controlled so as to match the steepness of the edge wave shape with the target
steepness.
[0014]
4^
(2) In the method for manufacturing a steel sheet according to the above (1),
in the target steepness-setting process, the target steepness may be set in a range of
more than 0% to 1%.
[0015]
(3) In the method for manufacturing a steel sheet according to the above (1) or
(2), the cooling process may include a target ratio-setting process in which a top and
bottom heat transfer coefficient ratio XI, at which a temperature standard deviation Y
becomes a minimum value Ymin, is set as a target ratio Xt based on second correlation
data indicating a correlation between a top and bottom heat transfer coefficient ratio X,
which is a ratio of heat transfer coefficients of top and bottom surfaces of the hot-rolled
steel sheet, and the temperature standard deviation Y during or after cooling of the hotrolled
steel sheet, which have been experimentally obtained in advance under
conditions in which steepness and sheet-threading speed of the hot-rolled steel sheet
are set to constant values; and a cooling control process in which at least one of an
amount of heat dissipated from a top surface by cooling and an amount of heat
dissipated from a bottom surface by cooling of the hot-rolled steel sheet in the cooling
section is controlled so that the top and bottom heat transfer coefficient ratio X of the
hot-rolled steel sheet in the cooling section matches the target ratio Xt.
[0016]
(4) In the method for manufacturing a steel sheet according to the above (3),
in the target ratio-setting process, a top and bottom heat transfer coefficient ratio X at
which the temperature standard deviation Y converges in a range of the minimum
value Ymin to the minimum value Ymin+10°C may be set as the target ratio Xt based
on the second correlation data.
[0017]
^s
(5) In the method for manufacturing a steel sheet according to the above (3),
the second correlation data may be prepared respectively for a plurality of conditions
in which values of the steepness and the sheet-threading speed are different, and, in the
target ratio-setting process, the target ratio Xt may be set based on second correlation
data matching actual measured values of the steepness and the sheet-threading speed
among the plurality of second correlation data.
[0018]
(6) In the method for manufacturing a steel sheet according to the above (3),
the second correlation data may be data indicating the correlation between the top and
bottom heat transfer coefficient ratio X and the temperature standard deviation Y using
a regression formula.
[0019]
(7) In the method for manufacturing a steel sheet according to the above (6),
the regression formula may be derived using linear regression.
[0020]
(8) In the method for manufacturing a steel sheet according to the above (3),
the second correlation data may be data indicating the correlation between the top and
bottom heat transfer coefficient ratio X and the temperature standard deviation Y using
a table.
[0021]
(9) The method for manufacturing a steel sheet according to the above (3)
may further include a temperature-measuring process in which a temperature of the
hot-rolled steel sheet is measured in chronological order on a downstream side of the
cooling section; an average temperature value-computing process in which a
chronological average value of the temperature is computed based on measurement
- 7 -
#
results of the temperature; and an amount of heat dissipated by cooling-adjusting
process in which a total value of the amount of heat dissipated from the top surface by
cooling and the amount of heat dissipated from the bottom surface by cooling of the
hot-rolled steel sheet in the cooling section is adjusted so that the chronological
average value of the temperature matches a predetermined target temperature.
[0022]
(10) The method for manufacturing a steel sheet according to the above (3)
may further include a temperature-measuring process in which a temperature of the
hot-rolled steel sheet is measured in chronological order on a downstream side of the
cooling section; a changing speed-measuring process in which a changing speed of the
hot-rolled steel sheet in a vertical direction is measured in chronological order at a
same place as a temperature measurement place of the hot-rolled steel sheet on the
downstream side of the cooling section; a control direction-determining process in
which, when an upward side of the vertical direction of the hot-rolled steel sheet is set
as positive, in an area with a positive changing speed, in a case in which a temperature
of the hot-rolled steel sheet is lower than an average temperature in a range of one or
more cycles of a wave shape of the hot-rolled steel sheet, at least one of a direction in
which the amount of heat dissipated from the top surface by cooling decreases and a
direction in which the amount of heat dissipated from the bottom surface by cooling
increases is determined as a control direction, in a case in which the temperature of the
hot-rolled steel sheet is higher than the average temperature, at least one of a direction
in which the amount of heat dissipated from the top surface by cooling increases and a
direction in which the amount of heat dissipated from the bottom surface by cooling
decreases is determined as the control direction, and, in an area with a negative
changing speed, in a case in which the temperature of the hot-rolled steel sheet is lower
than the average temperature, at least one of a direction in which the amount of heat
dissipated from the top surface by cooling increases and a direction in which the
amount of heat dissipated from the bottom surface by cooling decreases is determined
as the control direction, in a case in which the temperature of the hot-rolled steel sheet
is higher than the average temperature, at least one of a direction in which the amount
of heat dissipated from the top surface by cooling decreases and a direction in which
the amount of heat dissipated from the bottom surface by cooling increases is
determined as the control direction; and an amount of heat dissipated by coolingadjusting
process in which at least one of the amount of heat dissipated from the top
surface by cooling and the amount of heat dissipated from the bottom surface by
cooling of the hot-rolled steel sheet in the cooling section is adjusted based on the
control direction determined in the control direction-determining process.
[0023]
(11) In the method for manufacturing a steel sheet according to the above (10),
the cooling section may be divided into a plurality of divided cooling sections in a
sheet-threading direction of the hot-rolled steel sheet, the temperature and the changing
speed of the hot-rolled steel sheet may be measured in chronological order at each of
borders of the divided cooling sections in the temperature-measuring process and the
changing speed-measuring process, increase and decrease directions of the amounts of
heat dissipated by cooling from the top and bottom surfaces of the hot-rolled steel
sheet may be determined for the respective divided cooling sections based on
measurement results of the temperature and the changing speeds of the hot-rolled steel
sheet at the respective borders of the divided cooling sections in the control directiondetermining
process, and feedback control or feedforward control may be carried out
in order to adjust at least one of the amount of heat dissipated from the top surface by
cooling and the amount of heat dissipated from the bottom surface by cooling of the
hot-rolled steel sheet at each of the divided cooling sections based on the control
direction determined for each of the divided cooling sections in the amount of heat
dissipated by cooling-adjusting process.
[0024]
(12) The method for manufacturing a steel sheet according to the above (11)
may further include a measuring process in which the steepness or the sheet-threading
speed of the hot-rolled steel sheet is measured at each of the borders of the divided
cooling sections; and an amount of heat dissipated by cooling-correcting process in
which at least one of the amount of heat dissipated from the top surface by cooling and
the amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel
sheet is corrected at each of the divided cooling sections based on measurement results
of the steepness or the sheet-threading speeds.
[0025]
(13) The method for manufacturing a steel sheet according to the above (3)
may further include a post-cooling process in which the hot-rolled steel sheet is further
cooled in order to make the temperature standard deviation of the hot-rolled steel sheet
fall into a permissible range on a downstream side of the cooling section.
[0026]
(14) In the method for manufacturing a steel sheet according to the above (3),
the sheet-threading speed of the hot-rolled steel sheet in the cooling section may be set
in a range of 550 m/min to a mechanical limit speed.
[0027]
(15) In the method for manufacturing a steel sheet according to the above
(14), a tensile strength of the hot-rolled steel sheet may be 800 MPa or more.
- 10 -
[0028]
(16) In the method for manufacturing a hot-rolled steel sheet according to the
above (14), the finishing mill may be constituted by a plurality of rolling stands, and a
supplementary cooling process in which the hot-rolled steel sheet is supplementarily
cooled between the plurality of the rolling stands may be further provided.
[0029]
(17) In the method for manufacturing a steel sheet according to the above (3),
a top side cooling apparatus having a plurality of headers that ejects cooling water to a
top surface of the hot-rolled steel sheet and a bottom side cooling apparatus having a
plurality of headers that ejects cooling water to a bottom surface of the hot-rolled steel
sheet may be provided in the cooling section, and the amount of heat dissipated from
the top surface by cooling and the amount of heat dissipated from the bottom surface
by cooling may be adjusted by carrying out on-off control of the respective headers.
[0030]
(18) In the method for manufacturing a steel sheet according to the above (3),
a top side cooling apparatus having a plurality of headers that sprays cooling water to a
top surface of the hot-rolled steel sheet and a bottom side cooling apparatus having a
plurality of headers that sprays cooling water to a bottom surface of the hot-rolled steel
sheet may be provided in the cooling section, and the amount of heat dissipated from
the top surface by cooling and the amount of heat dissipated from the bottom surface
by cooling may be adjusted by controlling at least one of sprayed water density,
pressure and water temperature of each of the headers.
[0031]
(19) In the method for manufacturing a steel sheet according to the above (3),
cooling in the cooling section may be carried out at a temperature of the hot-rolled
11
•
steel sheet in a range of 600°C or higher.
[Effect of the Invention]
[0032]
As a result of thorough investigation of the relation between the wave shape
formed in the hot-rolled steel sheet obtained from the hot-rolling process and the
temperature standard deviation during or after the cooling of the hot-rolled steel sheet,
the present inventors found that, when the wave shape of the hot-rolled steel sheet is
controlled to be an edge wave shape, it is possible to control the temperature standard
deviation of the hot-rolled steel sheet to an arbitrary value according to the steepness of
the edge wave shape.
That is, according to the present invention, in the hot-rolling process, when
the target steepness of the edge wave shape is set based on the first correlation data
indicating the correlation between the steepness of the edge wave shape of the hotrolled
steel sheet and the temperature standard deviation Y during or after cooling of
the hot-rolled steel sheet, which have been experimentally obtained in advance, and the
finishing mill is controlled so as to match the steepness of the edge wave shape formed
in the hot-rolled steel sheet with the target steepness, it is possible to suppress the
temperature standard deviation of the cooled hot-rolled steel sheet at a low level (the
hot-rolled steel sheet can be uniformly cooled).
As a resuh, it is possible to suppress the occurrence of material quality
variation in the cooled hot-rolled steel sheet, and therefore it is possible to improve the
yield by suppressing the sheet thickness change of the steel sheet finally obtained
through the cold-rolling process which is a post process.
[Brief Description of the Drawings]
[0033]
- 12 -
FIG 1 is an explanatory view illustrating a hot rolling facility 1 for realizing a
method for manufacturing a steel sheet in an embodiment of the present invention.
FIG 2 is an explanatory view illustrating an outline of a configuration of a
cooling apparatus 14 provided in the hot rolling facility 1.
FIG 3 is an explanatory view illustrating a shape in which a bottom point of
the hot-rolled steel sheet H comes into contact with a transportation roll 32.
FIG 4 is a graph illustrating temperature changes at the respective places in
the hot-rolled steel sheet H in a case in which a center-wave shape having a steepness
of 1% is formed in the hot-rolled steel sheet H and a case in which an edge wave shape
having a steepness of 1% is formed.
FIG 5 is a graph illustrating a change in a cold-rolling gauge (change in the
sheet thickness) in the cold-rolling process which is a post process in a case in which a
center wave shape having a steepness of 1% is formed in the hot-rolled steel sheet H
and a case in which an edge wave shape having a steepness of 1% is formed.
FIG 6 is a graph illustrating a correlation between a top and bottom heat
transfer coefficient ratio X and a temperature standard deviation Y which have been
obtained under a condition in which the steepness and sheet-threading speed of the hotrolled
steel sheet H are set to constant values.
FIG 7 is an explanatory view illustrating a method for searching a minimum
point (minimum value Ymin) of the temperature standard deviation Y from the
correlation illustrated in FIG. 6.
FIG 8 is a graph illustrating a relationship between temperature change and
steepness of the hot-rolled steel sheet H during cooling in ROT of a typical strip in an
ordinary operation, in which the top graph indicates the temperature change with
respect to a distance from a coil tip or a time at which a coil passes a fixed point, and
- 13 -
#
the bottom graph indicates the steepness with respect to the distance from the coil tip
or the time at which the coil passes the fixed point.
FIG 9 is a graph illustrating the relationship between the temperature change
and steepness of the hot-rolled steel sheet H during cooling in ROT of the typical strip
in the ordinary operation.
FIG 10 is a graph illustrating the relationship between the temperature change
and steepness of the hot-rolled steel sheet H when an amount of heat dissipated from
the top surface by cooling is decreased and an amount of heat dissipated from the
bottom surface by cooling is increased in a case in which the temperature of the hotrolled
steel sheet H becomes low with respect to an average temperature of the hotrolled
steel sheet H in an area of a positive changing speed of the hot-rolled steel sheet
H and the temperature of the hot-rolled steel sheet H becomes high in an area of a
negative changing speed. Meanwhile, the steepness of a wave shape of the hot-rolled
steel sheet H refers to a value obtained by dividing an amplitude of the wave shape by
a length of a cycle in a rolling direction.
FIG. 11 is a graph illustrating the relationship between the temperature change
and steepness of the hot-rolled steel sheet H when the amount of heat dissipated from
the top surface by cooling is increased and the amount of heat dissipated from the
bottom surface by cooling is decreased in a case in which the temperature of the hotrolled
steel sheet H is low with respect to the average temperature of the hot-rolled
steel sheet H in the area of a positive changing speed of the hot-rolled steel sheet H and
the temperature of the hot-rolled steel sheet H becomes high in the area of a negative
changing speed.
FIG 12 is a graph illustrating the correlation between the steepness and the
temperature standard deviation Y of the hot-rolled steel sheet H which have been
- 14 -
obtained under conditions in which the top and bottom heat transfer coefficient ratio X
and the sheet-threading speed are set to constant values.
FIG 13 is a graph illustrating the correlations between the top and bottom heat
transfer coefficient ratios X and the temperature standard deviations Y which have
been obtained respectively under a plurality of conditions in which the values of the
steepness are different (wherein the sheet-threading speed is constant).
FIG 14 is a graph illustrating the correlation between the sheet-threading
speed and temperature standard deviation Y of the hot-rolled steel sheet H which have
been obtained under conditions in which the top and bottom heat transfer coefficient
ratio X and the steepness are set to constant values.
FIG 15 is a graph illustrating the correlations between the top and bottom heat
transfer coefficient ratios X and the temperature standard deviations Y which have
been obtained respectively under a plurality of conditions in which the values of the
sheet-threading speed are different (wherein the steepness is constant).
FIG 16 is an explanatory view illustrating the details of a periphery of the
cooling apparatus 14 in the hot rolling facility 1.
FIG 17 is an explanatory view illustrating a modified example of the cooling
apparatus 14.
FIG 18 is an explanatory view illustrating a shape of the temperature standard
deviation of the hot-rolled steel sheet H formed in a sheet width direction.
FIG 19 is an explanatory view illustrating a method for manufacturing the
hot-rolled steel sheet H of the related art.
FIG 20 is an explanatory view illustrating a method for cooling the hot-rolled
steel sheet H of the related art.
[Embodiment of the Invention]
- 15 -
#
[0034]
Hereinafter, as an embodiment of the present invention, a method for
manufacturmg a steel sheet used in, for example, cars and industrial machines will be
described in detail with reference to the accompanying drawings.
[0035]
FIG 1 schematically illustrates an example of a hot rolling facility 1 for
realizing the method for manufacturing a steel sheet in the present embodiment. The
hot rolling facility 1 is a facility having an aim of sandwiching the top and bottom of a
heated slab S using rolls and continuously rolling the slab so as to manufacture a steel
sheet having a sheet thickness of a minimum of 1.2 mm (hot-rolled steel sheet H
described below) and coil the steel sheet.
The hot rolling facility 1 has a heating furnace 11 for heating the slab S, a
width-direction mill 16 that rolls the slab S heated in the heating fiimace 11 in a width
direction, a roughing mill 12 that rolls the slab S rolled in the width direction from the
vertical direction so as to produce a rough bar Br, a finishing mill 13 that continuously
hot-finishing-rolls the rough bar Br so as to form a steel sheet having a predetermined
sheet thickness (hereinafter referred to as hot-rolled steel sheet) H, a cooling apparatus
14 that cools the hot-rolled steel sheet H transported from the finishing mill 13 using
cooling water, and a coiling apparatus 15 that coils the hot-rolled steel sheet H cooled
using the cooling apparatus 14 into a coil shape.
[0036]
The heating furnace 11 is provided with a side burner, an axial burner and a
roof burner that heat the slab S brought from the outside through a charging hole by
blowing a flame. The slab S brought into the heating furnace 11 is sequentially
heated in respective heating areas formed in respective zones, and, furthermore, a heat-
- 16 -
retention treatment for enabling transportation at an optimal temperature is carried out
by uniformly heating the slab S using the roof burner in a soaking area formed in a
final zone. When a heating treatment in the heating fiamace 11 completely ends, the
slab S is transported to the outside of the heating furnace 11, and moved into a rolling
process by the roughing mill 12.
[0037]
The roughing mill 12 passes the transported slab S through gaps between
columnar rotary rolls provided across a plurality of stands. For example, the
roughing mill 12 hot-rolls the slab S only using work rolls 12a provided at the top and
bottom of a first stand so as to form a rough bar Br. Next, the rough bar Br which has
passed through the first stand is further continuously rolled using a plurality of fourfold
mills 12b constituted by a work roll and a back-up roll. As a result, when the roughrolling
process ends, the rough bar Br is rolled into a thickness of approximately 30
mm to 60 mm, and transported to the finishing mill 13.
[0038]
The finishing mill 13 hot-finishing-rolls the rough bar Br transported from the
roughing mill 12 until the thickness becomes approximately several millimeters. The
finishing mill 13 passes the rough bar Br through gaps between top and bottom finishrolling
rolls 13a linearly arranged across 6 to 7 stands so as to gradually reduce the
rough bar, thereby forming the hot-rolled steel sheet H having a predetermined sheet
thickness. The hot-rolled steel sheet H formed using the finishing mill 13 is
transported to the cooling apparatus 14 using the transportation rolls 32 described
below. Meanwhile, an edge wave shape is formed in the rolling direction of the hotrolled
steel sheet H by the finishing mill 13.
[0039]
- 17 -
The cooling apparatus 14 is a facility for carrying out cooling by lamination
or spraying on the hot-rolled steel sheet H transported from the finishing mill 13. As
illustrated in FIG 2, the cooling apparatus 14 has a top side cooling apparatus 14a that
sprays cooling water from cooling holes 31 on the top side to the top surface of the
hot-rolled steel sheet H moving on the transportation rolls 32 in a run-out table, and a
bottom side cooling apparatus 14b that sprays cooling water from cooling holes 31 on
the bottom side to the bottom surface of the hot-rolled steel sheet H. A plurality of
the cooling holes 31 is provided in the top side cooling apparatus 14a and the bottom
side cooling apparatus 14b respectively.
In addition, a cooling header (not shown) is connected to the cooling hole 31.
The number of the cooling holes 31 determines the cooling capabilities of the top side
cooling apparatus 14a and the bottom side cooling apparatus 14b. Meanwhile, the
cooling apparatus 14 may be constituted by at least one of a top and bottom split
laminar, a pipe laminar, spray cooling and the like. In addition, a section in which the
hot-rolled steel sheet H is cooled using the cooling apparatus 14 corresponds to a
cooling section in the present invention.
[0040]
The coiling apparatus 15 coils the cooled hot-rolled steel sheet H transported
from the cooling apparatus 14 at a predetermined coiling temperature as illustrated in
FIG. 1. The hot-rolled steel sheet H coiled into a coil shape using the coiling
apparatus 15 is transported to a cold-rolling facility, not shown, cold-rolled, and
prepared into a steel sheet satisfying specifications as a final product.
[0041]
In the cooling apparatus 14 in the hot rolling facility 1 configured as described
above, in a case in which the hot-rolled steel sheet H having the wave shape with the
surface height (wave height) changing in the rolling direction is cooled, as described
above, the hot-rolled steel sheet H is uniformly cooled by preferably adjusting the
sprayed water density, pressure, water temperature and the like of cooling water
sprayed from the top side cooling device 14a and cooling water sprayed from the
bottom side cooling device 14b. However, particularly, in a case in which the sheetthreading
speed is slow, a period of time during which the hot-rolled steel sheet H and
the transportation rolls 32 locally come into contact with each other becomes long, and
the contact portions of the hot-rolled steel sheet H with the transportation rolls 32 of
the hot-rolled steel sheet H become easily coolable due to heat dissipation by contact,
and therefore cooling becomes ununiform.
[0042]
As illustrated in FIG. 3, in a case in which the hot-rolled steel sheet H has a
wave shape, there are cases in which the hot-rolled steel sheet H locally comes into
contact with the transportation rolls 32 at the bottom portion of the wave shape. As
such, in the hot-rolled steel sheet H, the portions that locally come into contact with the
transportation rolls 32 become more easily cooled than other portions due to heat
dissipation by contact. Therefore, the hot-rolled steel sheet H is ununiformly cooled.
[0043]
Meanwhile, as described above, in the hot rolling facility 1, in a case in which
the hot-rolled steel sheet H is not uniformly cooled due to the wave shape formed in
the hot-rolled steel sheet H, variation in the material qualities (hardness and the like) of
the cooled hot-rolled steel sheet H is caused. As a result, when the hot-rolled steel
sheet H is cold-rolled using the cold-rolling facility, a change in the sheet thickness is
caused in a steel sheet obtained as a final product (steel sheet product). Since the
change in the sheet thickness of the steel sheet product causes a decrease in yield, it is
- 19 -
necessary to suppress the change in the sheet thickness at a level at which the steel
sheet product is not determined as a defective product in an inspection process.
Therefore, the inventors carried out a verification process described below in order to
investigate the relationship between the wave shape formed in the hot-rolled steel sheet
H and a change in the sheet thickness in the post process (cold-rolling process).
[0044]
FIG 4 is a graph illustrating temperature changes at the respective places in
the hot-rolled steel sheet H in a case in which a center wave shape having a steepness
of 1% is formed in the hot-rolled steel sheet H and a case in which an edge wave shape
having a steepness of 1% is formed in the hot-rolled steel sheet H. In addition, FIG. 5
is a graph illustrating a change in a cold-rolling gauge (change in the sheet thickness)
in the cold-rolling process in each of a case in which a center wave shape having a
steepness of 1% is formed in the hot-rolled steel sheet H and a case in which an edge
wave shape having a steepness of 1% is formed in the hot-rolled steel sheet H.
Meanwhile, work side (WS) and drive side (DS) refer to an edge portion of the hotrolled
steel sheet H on one side in the width direction (WS) and an edge portion of the
hot-rolled steel sheet H on the other side in the width direction (DS).
[0045]
As illustrated in FIGS. 4 and 5, it was found that, in a case in which the wave
shape of the hot-rolled steel sheet H during cooling in the hot rolling facility 1 is set to
an edge wave shape, changes in the temperature of a sheet width center (C) and a
width-average temperature are suppressed, and a change in the sheet thickness in the
cold-rolling process is suppressed, compared with a case in which the wave shape is
set to the center wave shape (as illustrated in FIG. 5, approximately 30% of an effect of
20
suppressing the sheet thickness change can be obtained with the edge wave shape
compared with the center wave shape).
This is because, the center wave shape has a symmetric shape at a steel sheet
center portion and has a uniform displacement in the width direction, and therefore an
ununiform cooling deviation is easily caused in the sheet-threading direction (rolling
direction), but the edge wave shape has an antisymmetric shape in which an influence
at one edge wave (for example, the wave shape at WS) has an influence in the other
edge wave (for example, the wave shape at DS).
That is, in a case in which the wave shape of the hot-rolled steel sheet H is an
edge wave shape, since the phase of the wave shape at DS of the hot-rolled steel sheet
H is deviated 180 degrees from that of the wave shape at WS, cooling deviations
corresponding to wave shapes having deviated phases are respectively caused, and,
when the temperature average in the sheet width direction is taken, the temperature
standard deviation in the sheet-threading direction becomes small.
Therefore, in a case in which the wave shape of the hot-rolled steel sheet H is
an edge wave shape, in the hot rolling facility 1, substantially uniform cooling is
carried out in the cold-rolling process so that the change in the sheet thickness is not
influenced, and it is possible to improve the yield of the finally-obtained steel sheet
product.
[0046]
Furthermore, as a result of investigating a correlation between the steepness of
the edge wave shape formed in the hot-rolled steel sheet H and the temperature
standard deviation Y of the cooled hot-rolled steel sheet H in the rolling direction, the
inventors obtained results from an investigation in which the steepness and the
temperature standard deviation Y have a substantially proportional relation as
21
illustrated in FIG. 12. Meanwhile, FIG 12 is a graph illustrating the correlation
between the steepness and the temperature standard deviation Y which have been
obtained under conditions in which the sheet-threading speed and the top and bottom
heat transfer coefficient ratio X described below are set to constant values.
[0047]
The investigation results illustrated in FIGS. 4, 5 and 12 indicate that, when
the wave shape of the hot-rolled steel sheet H is controlled to be an edge wave shape, it
is possible to control the temperature standard deviation Y of the cooled hot-rolled
steel sheet H to an arbitrary value in accordance with the steepness of the edge wave
shape.
That is, when a steepness at which a temperature standard deviation Y
required during actual operation (temperature standard deviation Y at which a change
in the sheet thickness in the cold-rolling process is suppressed to a permissible level)
can be realized is obtained based on the correlation between the steepness and the
temperature standard deviation Y illustrated in FIG. 12, the steepness is set as a target
steepness, and the operation parameters of the finishing mill 13 are controlled so as to
match the steepness of the edge wave shape formed in the hot-rolled steel sheet H with
the above target steepness, thereby it is possible to improve the yield of a finallyobtained
steel sheet product, which is the object of the present invention.
[0048]
Hereinafter, the method for manufacturing a steel sheet of the present
embodiment will be described based on the above findings. The method for
manufacturing a steel sheet of the present embodiment includes a hot-rolling process in
which a steel material (rough bar Br) is hot-rolled using the finishing mill 13 so as to
obtain the hot-rolled steel sheet H having an edge wave shape with a wave height
- 22 -
periodically changing in the rolling direction, and a cooling process in which the hotrolled
steel sheet obtained from the hot-rolling process is cooled in a cooling section
(that is, the cooling apparatus 14) provided on a sheet-threading path.
[0049]
Here, the hot-rolling process includes a target steepness-setting process in
which a target steepness of the edge wave shape is set based on the first correlation
data indicating the correlation (refer to FIG. 12) between the steepness of the hot-rolled
steel sheet H and the temperature standard deviation Y of the hot-rolled steel sheet H
after cooling (or during cooling), which have been experimentally obtained in advance,
and a shape-controlling process in which operation parameters of the finishing mill 13
are controlled so as to match the steepness of the edge wave shape with the target
steepness.
[0050]
In the target steepness-setting process, a steepness at which a temperature
standard deviation Y required during actual operation (temperature standard deviation
Y at which a change in the sheet thickness in the cold-rolling process is suppressed to a
permissible level) can be realized is obtained based on the first correlation data, and
the steepness is set as the target steepness. For example, when FIG. 12 is referenced,
in a case in which the temperature standard deviation Y required during actual
operation is 10°C, the target steepness is set to 0.5%.
[0051]
In the shape-controlling process, operation parameters of the finishing mill 13
are controlled so as to match the steepness of the edge wave shape formed in the hotrolled
steel sheet H with the target steepness (for example, 0.5%). The operation
parameters of the finishing mill 13 include sheet-threading speed, heating temperature,
- 23 -
suppress strength and the like. Therefore, it is possible to match the steepness of the
edge wave shape formed in the hot-rolled steel sheet H with the target steepness by
adjusting values of the operation parameters.
Specifically, when a distance meter that measures a distance from the surface
(top surface) of the hot-rolled steel sheet H is installed on the exit side of the finishing
mill 13, it is possible to compute the steepness of the edge wave shape of the hot-rolled
steel sheet H based on distance measurement results obtained from the distance meter
in real time. In addition, the operation parameters of the finishing mill 13 may be
feedback-controlled so as to match the computation results of the steepness with the
target steepness. It is possible to use a controller having an ordinary microcomputer
and the like for the computation and feedback-control of steepness.
[0052]
Meanwhile, it was found from the investigation results illustrated in FIGS. 4
and 5 that, in the target steepness-setting process, the target steepness is preferably set
in a range of more than 0% to 1%. Thereby, the temperature standard deviation Y of
the cooled hot-rolled steel sheet H is suppressed at approximately 18°C or lower (refer
to FIG. 12), and it is possible to significantly suppress the change in the sheet thickness
of the steel sheet product in the cold-rolling process.
Furthermore, in order to suppress the temperature standard deviation Y of the
hot-rolled steel sheet H as much as possible, in the target steepness-setting process, the
target steepness is more preferably set in a range of more than 0% to 0.5%.
According to what described above, it is possible to suppress the temperature standard
deviation Y of the hot-rolled steel sheet H at approximately 10°C or lower (refer to FIG
12).
24
As described above, according to the method for manufacturing a steel sheet
of the present embodiment, it becomes possible to improve the yield of a steel sheet
manufactured through at least the hot-rolling process and the cooling process.
[0053]
Furthermore, in order to further reduce the temperature standard deviation Y
of the cooled hot-rolled steel sheet H, the cooling process of the embodiment described
above preferably includes two processes of a target ratio-setting process and a cooling
control process.
The details will be described below, and, in the target ratio-setting process, a
top and bottom heat transfer coefficient ratio XI, at which a temperature standard
deviation Y becomes a minimum value Ymin, is set as a target ratio Xt based on
second correlation data indicating a correlation between a top and bottom heat transfer
coefficient ratio X, which is a ratio of heat transfer coeflTicients of the top and bottom
surfaces of the hot-rolled steel sheet H, and the temperature standard deviation Y of the
hot-rolled steel sheet H during or after cooling, which have been experimentally
obtained in advance under conditions in which the steepness and the sheet-threading
speed of the hot-rolled steel sheet H are set to constant values.
In addition, in the cooling control process, at least one of an amount of heat
dissipated from the top surface by cooling and an amount of heat dissipated from the
bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is
controlled so that the top and bottom heat transfer coefficient ratio X of the hot-rolled
steel sheet H in the cooling section (a section in which the hot-rolled steel sheet H is
cooled using the cooling apparatus 14) matches the target ratio Xt.
[0054]
The second correlation data used in the target ratio-setting process is
- 25 -
experimentally obtained m advance using the hot rolling facility 1 before actual
operation (before the hot-rolled steel sheet H is actually manufactured). Hereinafter,
a method for obtaining the second correlation data used in the target ratio-setting
process will be described in detail.
First, before cooling the hot-rolled steel sheet H in the cooling apparatus 14,
the cooling capability (top side cooling capability) of the top side cooling apparatus
14a and the cooling capability (bottom side cooling capability) of the bottom side
cooling apparatus 14b of the cooling apparatus 14 are adjusted respectively in advance.
The top side cooling capability and the bottom side cooling capability are adjusted
using the heat transfer coefficient of the top surface of the hot-rolled steel sheet H,
which is cooled using the top side cooling apparatus 14a, and the heat transfer
coefficient of the bottom surface of the hot-rolled steel sheet H, which is cooled using
the bottom side cooling apparatus 14b.
[0055]
Here, a method for computing the heat transfer coefficients of the top surface
and bottom surface of the hot-rolled steel sheet H will be described. The heat transfer
coefficient refers to a value obtained by dividing the amount of heat dissipated from
unit area by cooling (heat energy) per unit time by the temperature difference between
an article to which heat is transferred and a heat medium (heat transfer
coefficient=amount of heat dissipated by cooling/temperature difference). The
temperature difference herein refers to the difference between the temperature of the
hot-rolled steel sheet H, which is measured using a thermometer on an entry side of the
cooling apparatus 14, and the temperature of cooling water used in the cooling
apparatus 14.
In addition, the amount of heat dissipated by cooling refers to a value obtained
- 26 -
by respectively multiplying the temperature difference, specific heat and mass of the
hot-rolled steel sheet H (amount of heat dissipated by cooling=temperature
differencex specific heatxmass). That is, the amount of heat dissipated by cooling is
an amount of heat dissipated by cooling of the hot-rolled steel sheet H in the cooling
apparatus 14, and a value obtained by multiplying the difference between the
temperatures of the hot-rolled steel sheet H respectively measured using the entry-side
thermometer and an exit-side thermometer in the cooling apparatus 14, the specific
heat of the hot-rolled steel sheet H and the mass of the hot-rolled steel sheet H cooled
using the cooling apparatus 14 respectively.
[0056]
As described above, the computed heat transfer coefficient of the hot-rolled
steel sheet H is classified into the heat transfer coefficient of the top surface and the
heat transfer coefficient of the bottom surface of the hot-rolled steel sheet H. The
heat transfer coefficients of the top surface and the bottom surface are computed using
a ratio that is obtained in advance, for example, in the following manner.
That is, the heat transfer coefficient of the hot-rolled steel sheet H in a case in
which the hot-rolled steel sheet H is cooled only using the top side cooling apparatus
14a and the heat transfer coefficient of the hot-rolled steel sheet H in a case in which
the hot-rolled steel sheet H is cooled only using the bottom side cooling apparatus 14b
are measured.
At this time, the amount of cooling water from the top side cooling apparatus
14a and the amount of cooling water from the bottom side cooling apparatus 14b are
set to be equal. The inverse number of the ratio between the measured heat transfer
coefficient in a case in which the top side cooling apparatus 14a is used and the heat
transfer coefficient in a case in which the bottom side cooling apparatus 14b is used
- 27 -
becomes a top and bottom ratio of the amount of cooling water of the top side cooling
apparatus 14a and the amount of cooling water of the bottom side cooling apparatus
14b in a case in which a top and bottom heat transfer coefficient ratio X, which will be
described below, is set to "1".
In addition, the above-mentioned ratio of the heat transfer coefficients of the
top surface and the bottom surface of the hot-rolled steel sheet H (top and bottom heat
transfer coefficient ratio X) is computed by multiplying the amount of cooling water of
the top side cooling apparatus 14a or the amount of cooling water of the bottom side
cooling apparatus 14b when cooling the hot-rolled steel sheet H by the top and bottom
ratio of the amounts of cooling water obtained in the above manner.
In addition, in the above description, the heat transfer coefficients of the hotrolled
steel sheet H cooled only using the top side cooling apparatus 14a and only
using the bottom side cooling apparatus 14b are used, but the heat transfer coefficient
of the hot-rolled steel sheet H cooled using both the top side cooling apparatus 14a and
the bottom side cooling apparatus 14b may be used. That is, the heat transfer
coefficients of the hot-rolled steel sheet H in a case in which the amounts of cooling
water of the top side cooling apparatus 14a and the bottom side cooling apparatus 14b
are changed are measured, and the ratio of the heat transfer coefficients of the top
surface and the bottom surface of the hot-rolled steel sheet H may be computed using
the ratio of the heat transfer coefficients.
[0057]
As described above, the heat transfer coefficients of the hot-rolled steel sheet
H are computed, and the heat transfer coefficients of the top surface and the bottom
surface of the hot-rolled steel sheet H are computed based on the above ratio of the
heat transfer coefficients of the top surface and the bottom surface of the hotTrolled
- 28 -
steel sheet H (top and bottom heat transfer coeSlcient ratio X).
[0058]
In addition, the cooling capabilities of the top side cooling apparatus 14a and
the bottom side cooling apparatus 14b are adjusted respectively using the top and
bottom heat transfer coefficient ratio X of the hot-rolled steel sheet H based on FIG 6.
The horizontal axis of FIG 6 indicates a ratio of an average heat transfer coefficient of
the top surface to an average heat transfer coefficient of the bottom surface of the hotrolled
steel sheet H (that is, equivalent to the top and bottom heat transfer coefficient
ratio X), and the vertical axis indicates a standard deviation of temperature between the
maximum temperature and the minimum temperature of the hot-rolled steel sheet H in
the rolling direction (temperature standard deviation Y).
In addition, FIG 6 shows data (second correlation data) indicating the
correlation between the top and bottom heat transfer coefficient ratio X and the
temperature standard deviation Y which are obtained by actually measuring the
temperature standard deviation Y of the cooled hot-rolled steel sheet H while changing
the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet H by
adjusting the cooling capabilities of the top side cooling apparatus 14a and the bottom
side cooling apparatus 14b under conditions in which the steepness of the wave shape
of the hot-rolled steel sheet H and the sheet-threading speed of the hot-rolled steel
sheet H are set to constant values.
With reference to FIG. 6, it was found that the correlation between the
temperature standard deviation Y and the top and bottom heat transfer coefficient ratio
X becomes a V-shaped relationship in which the temperature standard deviation Y
becomes the minimum value Ymin when the top and bottom heat transfer coefficient
ratio X is "1".
29
Meanwhile, the steepness of the wave shape of the hot-rolled steel sheet H
refers to a value obtained by dividing the amplitude of the wave shape by the length of
a cycle in the rolling direction. FIG 6 illustrates a correlation between the top and
bottom heat transfer coefficient ratio X and the temperature standard deviation Y
which are obtained under conditions in which the steepness of the hot-rolled steel sheet
H is set to 2% and the sheet-threading speed is set to 600 m/min (10 m/sec). The
temperature standard deviation Y may be measured during the cooling of the hot-rolled
steel sheet H, or may be measured after the cooling. In addition, in FIG. 6, the target
cooling temperature of the hot-rolled steel sheet H is a temperature of 600°C or higher,
for example, 800°C.
[0059]
In the target ratio-setting process, the top and bottom heat transfer coefficient
ratio XI, at which the temperature standard deviation Y becomes the minimum value
Ymin, is set as the target ratio Xt based on the second correlation data experimentally
obtained in advance as described above. The second correlation data may be
prepared in a form of data (table data) that indicate the correlation between the top and
bottom heat transfer coefficient ratio X and the temperature standard deviation Y using
a table (table form), or may be prepared in a form of data that indicate the correlation
between the top and bottom heat transfer coefficient ratio X and the temperature
standard deviation Y using a mathematical formula (for example, regression formula).
[0060]
For example, in a case in which the second correlation data is prepared in a
form of data indicating the correlation between the top and bottom heat transfer
coefficient ratio X and the temperature standard deviation Y using a regression formula,
since the V-shaped line illustrated in FIG. 6 is drawn to be almost linear on both sides
- 30 -
of the bottom portion, the regression formula may be derived by linearly regressing the
line. When the data is considered to be a linear distribution, the number of tunes of
confirmation using test materials or the number of times of correction for estimating
calculation can be small.
[0061]
Therefore, the minimum value Ymin of the temperature standard deviation Y
is searched using a variety of methods, for example, a binary method, a golden section
method and random search which are generally known search algorithms. The top
and bottom heat transfer coefficient ratio XI at which the temperature standard
deviation Y of the hot-rolled steel sheet H becomes the minimum value Ymin is
derived in the above manner based on the second correlation data illustrated in FIG 6.
In addition, here, the regression formulae of the temperature standard deviations Y of
the hot-rolled steel sheet H in the rolling direction with respect to the top and bottom
heat transfer coefficient ratio X may be obtained respectively on both sides of an equal
point above and below the average heat transfer coefficient.
[0062]
Here, a method for searching the minimum value Ymin of the temperature
standard deviation Y of the hot-rolled steel sheet H using the above-described binary
method will be described.
[0063]
FIG 7 illustrates a standard case in which mutually different regression lines
are obtained on both sides of the minimum value Ymin of the temperature standard
deviation Y As illustrated in FIG 7, first, temperature standard deviations Ya, Yb and
Yc actually measured at a point, b point and c point which is in the center between the
a point and the b point are extracted respectively. Meanwhile, the center between the
- 31 -
a point and the b point indicates the c point at which a value between the top and
bottom heat transfer coefficient ratio Xa at the a point and the top and bottom heat
transfer coefficient ratio Xb at the b point is present, and this shall apply below. In
addition, to which of Ya and Yb is the temperature standard deviation Yc closer is
determined. In the embodiment, Yc is closer to Ya.
Next, a temperature standard deviation Yd at a d point between the a point and
the c point is extracted. In addition, to which of Ya and Yc is the temperature
standard deviation Yd closer is determined. In the embodiment. Yd is closer to Yc.
Next, a temperature standard deviation Ye at an e point between the c point
and the d point is extracted. In addition, to which of Yc and Yd is the temperature
standard deviation Ye closer is determined. In the embodiment. Ye is closer to Yd.
The above computation is repeated, and a minimum point f (minimum value
Ymin) of the temperature standard deviation Y of the hot-rolled steel sheet H is
specified. Meanwhile, in order to specify the practical minimum point f, the above
computation needs to be carried out, for example, five times. In addition, the
minimum point f may be specified by dividing the range of the top and bottom heat
transfer coefficient ratio X of a search target into 10 sections, and carrying out the
above computation in each of the sections.
[0064]
In addition, the top and bottom heat transfer coefficient ratio X may be
corrected using the so-called Newton's method. In this case, a partial difference
between the top and bottom heat transfer coefficient ratio X with respect to the actual
value of the temperature standard deviation Y and the top and bottom heat transfer
coefficient ratio X at which the temperature standard deviation Y becomes zero is
obtained using the above-described regression formula, and the top and bottom heat
32
transfer coefficient ratio X when cooling the hot-rolled steel sheet H may be amended
using the partial difference.
[0065]
The top and bottom heat transfer coefficient ratio XI at which the temperature
standard deviation Y of the hot-rolled steel sheet H becomes the minimum value Ymin
(Xf in FIG. 7) is derived as described above. In addition, for the relationship between
the temperature standard deviation Y and the top and bottom heat transfer coefficient
ratio X, which forms a V shape, it is easy to divide the graph into two sides, and obtain
regression functions respectively using the method of least squares.
Furthermore, even in any cases in which the wave shape formed in the hotrolled
steel sheet H is an edge wave shape or a center wave shape, it is possible to
derive the top and bottom heat transfer coefficient ratio XI, at which the temperature
standard deviation Y of the hot-rolled steel sheet H becomes the minimum value Ymin
using a fact that the relationship between the temperature standard deviation Y and the
top and bottom heat transfer coefficient ratio X becomes V-shaped as described above.
[0066]
Meanwhile, the hot-rolled steel sheet H is uniformly cooled in the sheet width
direction using water as ordinarily cooled. In addition, since the temperature standard
deviation in the sheet width direction is caused by the alternate occurrence of the
temperature standard deviation Y in the rolling direction on the right and left sides, the
temperature standard deviation in the sheet width direction is also fiirther reduced
when the temperature standard deviation Y in the rolling direction is reduced.
[0067]
In addition, when FIG. 6 is referenced, the top and bottom heat transfer
coefficient ratio XI at which the temperature standard deviation Y of the hot-rolled
- 33 -
steel sheet H becomes the minimum value Ymin is "1". Therefore, in a case in which
the second correlation data as illustrated in FIG 6 is obtained, the target ratio Xt is set
to " 1 " in the target ratio-setting process during an actual operation in order to minimize
the temperature standard deviation Y, that is, in order to uniformly cool the hot-rolled
steel sheet H.
In addition, in the cooling control process, at least one of the amount of heat
dissipated from the top surface by cooling and the amount of heat dissipated from the
bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is
controlled so that the top and bottom heat transfer coefficient ratio X of the hot-rolled
steel sheet H in the cooling section matches the target ratio Xt (that is "1").
Specifically, in order to match the top and bottom heat transfer coefficient
ratio X of the hot-rolled steel sheet H in the cooling section to the target ratio Xt (that
is "1"), the amount of heat dissipated from the top surface by cooling and the amount
of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H
may be equaled by, for example, adjusting the cooling capability of the top side
cooling apparatus 14a and the cooling capability of the bottom side cooling apparatus
14b to be equal.
Table 1 describes the second correlation data illustrated in FIG. 6 (that is, the
correlation between the top and bottom heat transfer coefficient ratio X and the
temperature standard deviation Y), values obtained by subtracting the respective
temperature standard deviations Y by the minimum value Ymin (=2.3°C) (the
differences of the standard deviations from the minimum value), and the evaluation of
the respective temperature standard deviations Y.
In the top and bottom heat transfer coefficient ratio X in Table 1, the
numerator is the heat transfer coefficient of the hot-rolled steel sheet H on the top
- 34 -
surface, and the denominator is the heat transfer coefficient of the hot-rolled steel sheet
H on the bottom surface. In addition, in the evaluation in Table 1 (the evaluation of
the conditions of the top and bottom heat transfer coefficient ratio X), the condition
under which the temperature standard deviation Y becomes the minimum value Ymin
is considered as "A", the condition under which the difference of the standard
deviation from the minimum value becomes 10°C or less, that is, the operation
becomes preferable as described below is considered as "B", and the condition under
which the computation is heuristically carried out in order to obtain the abovedescribed
regression formula is considered as"C". In addition, when Table 1 is
referenced, the top and bottom heat transfer coefficient ratio XI at which the
evaluation becomes "A", that is, the temperature standard deviation Y of the hot-rolled
steel sheet H becomes the minimum value Ymin is "1".
- 35
1f^^Hffi^fl^^1^[l^^tewa«•M^^
[0068]
[Table 1]
Top and bottom heat transfer coefficient
ratio
X
1.6/1.0
1.2/1.0
1.1/1.0
1.0/1.0
1.0/1.1
1.0/1.2
1.0/1.6
Temperature standard
deviation
Y(°C)
33.2
14.6
8.5
2.3
6.1
9.8
28.7
Difference of standard deviation from minimum
value
(°C)
30.9
12.3
6.2
0.0
3.8
7.5
26.4
Evaluation
C
C
B
A
B
B
C
- 36
[0069]
Meanwhile, when the temperature standard deviation Y of the hot-rolled steel
sheet H at least converges in a range of the minimum value Ymin to the minimum
value Ymin+10°C, it can be said that the variations in yield stress, tensile strength and
the like are suppressed within the manufacturing permissible ranges, and the hot-rolled
steel sheet H can be uniformly cooled. That is, in the target ratio-setting process, the
top and bottom heat transfer ratio X at which the temperature standard deviation Y
converges in a range of the minimum value Y to the minimum value Ymin+10°C may
be set as the target ratio Xt based on the second correlation data experimentally
obtained in advance.
Meanwhile, since there is a variety of noise in the temperature measurement
of the hot-rolled steel sheet H, there are cases in which the minimum value Ymin of the
temperature standard deviation Y of the hot-rolled steel sheet H is not strictly zero.
Therefore, the manufacturing permissible range is set to a range in which the
temperature standard deviation Y of the hot-rolled steel sheet H is the minimum value
Ymin to the minimum value Ymin+10°C in order to remove the influence of the noise.
[0070]
In order to converge the temperature standard deviation Y in a range of the
minimum value Ymin to the minimum value Ymin+10°C, in FIG. 6 or 7, it is necessary
to pull the straight line in the horizontal axis direction from a point in the vertical axis
at which the temperature standard deviation Y becomes the minimum value
Ymin+10°C, obtain two intersections between the straight line and two regression lines
on both sides of the V-shaped curve, and set the target ratio Xt from the top and bottom
heat transfer coefficient ratio X between the two intersections. Meanwhile, in Table 1,
the temperature standard deviation Y can be converged in a range of the minimum
- 37 -
value Ymin to the minimum value Ymin+10°C by setting the top and bottom heat
transfer coefficient ratio X with an evaluation of "B" as the target ratio Xt.
[0071]
In addition, in order to match the top and bottom heat transfer coefficient ratio
X to the target ratio Xt, it is easiest to operate the sprayed cooling water density of at
least one of the top side cooling apparatus 14a and the bottom side cooling apparatus
14b. Therefore, for example, in FIGS. 6 and 7, the values in the horizontal axis are
replaced by the top and bottom sprayed water density ratio, and the regression formula
of the temperature standard deviation Y of the hot-rolled steel sheet H with respect to
the top and bottom ratio of the sprayed water density may be obtained on both sides of
an equal point above and below the average heat transfer coefficient. Here, the equal
point above and below the average heat transfer coefficient does not necessarily
become an equal point above and below the sprayed cooling water density, and
therefore the regression formula may be obtained by carrying out tests slightly widely.
[0072]
In addition, during an actual operation, there is a possibility that the value of
at least one of the steepness and the sheet-threading speed may change due to a change
in the manufacturing conditions. When at least one of the steepness and the sheetthreading
speed is changed, the correlation between the top and bottom heat transfer
coefficient ratio X and the temperature standard deviation Y changes. Therefore, the
second correlation data is prepared for each of a plurality of conditions having different
values of the steepness and the sheet-threading speed, and, in the target ratio-setting
process, the target ratio Xt may be set based on a second correlation data in accordance
with actual measured values of the steepness and the sheet-threading speed during the
actual operation of the plurality of second correlation data. Thereby, it becomes
- 38 -
possible to carry out uniform cooling suitable for the manufacturing conditions during
the actual operation.
[0073]
Here, as a result of thorough studies regarding the adjustment of the cooling
capabilities of the top side cooling apparatus 14a and the bottom side cooling apparatus
14b (control of the amount of heat dissipated from the top surface by cooling and the
amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel
sheet H) in order to uniformly cool the hot-rolled steel sheet H, the inventors further
obtained the following findings.
[0074]
As a result of repeating thorough studies regarding the characteristics of the
temperature standard deviation Y generated by cooling in a state in which a wave
shape of the hot-rolled steel sheet H is generated, the inventors clarified the following
fact.
[0075]
Generally, during an actual operation, it is necessary to maintain the quality of
the hot-rolled steel sheet H by controlling the temperature of the hot-rolled steel sheet
H at a predetermined target temperature (a temperature suitable for coiling) when
coiling the hot-rolled steel sheet H using the coiling apparatus 15.
Therefore, a temperature-measuring process in which the temperature of the
hot-rolled steel sheet H on the downstream side of the cooling section (that is, the
cooling apparatus 14) is measured in chronological order, an average temperature
value-computing process in which a chronological average value of the temperature is
computed based on the measurement result of the temperature, and an amount of heat
dissipated by cooling-adjusting process in which the total value of the amount of heat
- 39 -
j l%
dissipated from the top surface by cooling and the amount of heat dissipated from the
bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is
adjusted so that the chronological average value of the temperature matches a
predetermined target temperature may be newly added to the above-described target
ratio-setting process and cooling control process.
In order to realize the new processes, a thermometer 40 which is disposed
between the cooling apparatus 14 and the coiling apparatus 15 as illustrated in FIG 16
and measures the temperature of the hot-rolled steel sheet H can be used.
[0076]
In the temperature-measuring process, with respect to the hot-rolled steel
sheet H transported from the cooling apparatus 14 to the coiling apparatus 15, the
temperatures at locations set in the rolling direction of the hot-rolled steel sheet H are
measured at certain time intervals (sampling intervals) using the thermometer 40, and
chronological data of the temperature measurement results are obtained. Meanwhile,
the temperature measurement area using the thermometer 40 includes all the area of
the hot-rolled steel sheet H in the width direction. In addition, when the sheetthreading
speed (transportation speed) of the hot-rolled steel sheet H is multiplied at
the sampling times of the respective temperature measurement results, the locations of
the hot-rolled steel sheet H in the rolling direction, at which the respective temperature
measurement results have been obtained, can be computed. That is, when the
sampling times of the temperature measurement results are multiplied by the sheetthreading
speed, it becomes possible to link the chronological data of the temperature
measurement results to the locations in the rolling direction.
[0077]
In the average temperature value-computing process, a chronological average
- 40 -
%
value of the temperature measurement results is computed using the chronological data
of the temperature measurement results. Specifically, each time when a certain
number of the temperature measurement results are obtained, the average value of the
certain number of the temperature measurement results may be computed. In
addition, in the amount of heat dissipated by cooling-adjusting process, the total value
of the amount of heat dissipated from the top surface by cooling and the amount of
heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet H in the
cooling section is adjusted so that the chronological average value of the temperature
measurement results computed as described above matches a predetermined target
temperature.
Here, it is necessary to adjust the total value of the amount of heat dissipated
from the top surface by cooling and the amount of heat dissipated from the bottom
surface by cooling while achieving a control target that matches the top and bottom
heat transfer coeflBcient ratio X of the hot-rolled steel sheet H in the cooling section to
the target ratio Xt.
Specifically, when adjusting the total value of the amount of heat dissipated
from the top surface by cooling and the amount of heat dissipated from the bottom
surface by cooling, the on-off control of cooling headers connected to the cooling
apparatus 14 may be carried out on a theoretical value obtained in advance using an
experiment theoretical formula represented by, for example, Mitsuzuka's formula
based on a learned value set to correct the error with an actual operation achievement.
Alternatively, the on-off of the cooling headers may be feedback-controlled or
feedforward-controlled based on the temperature actually measured using the
thermometer 40.
[0078]
- 41 -
Next, the cooling control of ROT of the related art will be described using
data obtained from the above-described thermometer 40 and a shape meter 41 that
measures the wave shape of the hot-rolled steel sheet H which is disposed between the
cooling apparatus 14 and the coiling apparatus 15 as illustrated in FIG. 16.
Meanwhile, the shape meter 41 measures the shape of the same measurement
location (hereinafter this measurement location will be sometimes referred to as a fixed
point) as the thermometer 40 set on the hot-rolled steel sheet H. Here, the shape
refers to the steepness obtained through the line integration of the heights or changing
components of pitches of the wave using the movement amount of the hot-rolled steel
sheet H in the sheet-threading direction as the changing amount of the hot-rolled steel
sheet H in the height direction observed in a measurement at a fixed point. In
addition, at the same time, the changing amount per unit time, that is, the changing
speed is also obtained. Furthermore, similarly to the temperature measurement area,
the shape measurement area includes all the areas of the hot-rolled steel sheet H in the
width direction. Similarly to the temperature measurement results, when the
sampling times of the respective measurement results (steepness, changing speed and
the like) are multiplied by the sheet-threading speed, it becomes possible to link the
chronological data of the respective measurement results to the locations in the rolling
direction.
FIG 8 illustrates the relationship between the temperature change and
steepness of the hot-rolled steel sheet H during cooling in ROT of a typical strip in an
ordinary operation. The top and bottom heat transfer coefficient ratio X of the hotrolled
steel sheet H in FIG. 8 is 1.2:1, and the top side cooling capability is superior to
the bottom side cooling capability. The top graph in FIG. 8 indicates the temperature
change with respect to the distance from a coil tip or a time at which a coil passes the
- 42 -
fixed point, and the bottom graph in FIG. 8 indicates the steepness with respect to the
distance from the coil tip or the time at which the coil passes the fixed point.
The area A in FIG 8 is an area before the strip tip portion illustrated in FIG. 16
is bit in a coiler of the coiling apparatus 15 (since there is no tension, the shape is
defective in this area). The area B in FIG 8 is an area after the strip tip portion is bit
in the coiler (the area in which the wave shape is changed to be flat by the influence of
unit tension). There is a demand for improving a large temperature change (that is,
the temperature standard deviation Y) occurring in the area A in which the shape of the
hot-rolled steel sheet H is not flat.
[0079]
Therefore, the inventors carried out thorough tests for the purpose of
controlling the increase in the temperature standard deviation Y in ROT, and,
consequently, obtained the following findings.
[0080]
Similarly to FIG 8, FIG. 9 illustrates the temperature-changing component
with respect to the steepness of the same shape during cooling in ROT of the typical
strip in the ordinary operation. The temperature-changing component is a residual
error obtained by subtracting the actual steel sheet temperature by the chronological
average of the temperature (hereinafter sometimes referred to as "average
temperature"). For example, the average temperature may be the average of the
temperature of a range that is a cycle or more of the wave shape of the hot-rolled steel
sheet H.
Meanwhile, the average temperature is, in principle, the average of the
temperature of a range of the unit cycle. In addition, it is confirmed from operation
data that there is no large difference between the average temperature of a range of a
- 43 -
cycle and the average temperature of a range of two or more cycles.
Therefore, the average temperature simply needs to be computed from a range
of at least a cycle of the wave shape. The upper limit of the range of the wave shape
of the hot-rolled steel sheet H is not particularly limited; however, a sufficiently
accurate average temperature can be obtained when the range is preferably set to 5
cycles. In addition, even when the average temperature is computed not from a range
of the unit cycle but from a range of 2 to 5 cycles, a permissible average temperature
can be obtained.
[0081]
Here, when the upward side of the vertical direction (the direction that
intersects the top and bottom surfaces of the hot-rolled steel sheet H) of the hot-rolled
steel sheet H is set as positive, in an area with a positive changing speed measured at
the fixed point, in a case in which the temperature (the temperature measured at the
fixed point) of the hot-rolled steel sheet H is lower than the average temperature of a
range of one or more cycles of the wave shape of the hot-rolled steel sheet H, at least
one of a direction in which the amount of heat dissipated from the top surface by
cooling decreases and a direction in which the amount of heat dissipated from the
bottom surface by cooling increases is determined as a control direction, and, in a case
in which the temperature of the hot-rolled steel sheet H is higher than the average
temperature, at least one of a direction in which the amount of heat dissipated from the
top surface by cooling increases and a direction in which the amount of heat dissipated
from the bottom surface by cooling decreases is determined as the control direction.
In addition, in an area with a negative changing speed measured at the fixed
point, in a case in which the temperature of the hot-rolled steel sheet H is lower than
the average temperature, at least one of a direction in which the amount of heat
- 44 -
dissipated from the top surface by cooling increases and a direction in which the
amount of heat dissipated from the bottom surface by cooling decreases is determined
as the control direction, and, in a case in which the temperature of the hot-rolled steel
sheet H is higher than the average temperature, at least one of a direction in which the
amount of heat dissipated from the top surface by cooling decreases and a direction in
which the amount of heat dissipated from the bottom surface by cooling increases is
determined as the control direction.
In addition, it was found that, when at least one of the amount of heat
dissipated from the top surface by cooling and the amount of heat dissipated from the
bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is
adjusted based on the control direction determined as described above, as illustrated in
FIG. 10, the temperature change occurring in the area A in which the shape of the hotrolled
steel sheet H is not flat can be reduced compared with FIG 9.
[0082]
A case in which an opposite operation to the above case is carried out will be
described below. In an area with a positive changing speed measured at the fixed
point, in a case in which the temperature of the hot-rolled steel sheet H is lower than
the average temperature of the hot-rolled steel sheet H, at least one of a direction in
which the amount of heat dissipated from the top surface by cooling increases and a
direction in which the amount of heat dissipated from the bottom surface by cooling
decreases is determined as the control direction, and, in a case in which the
temperature of the hot-rolled steel sheet H is higher than the average temperature, at
least one of a direction in which the amount of heat dissipated from the top surface by
cooling decreases and a direction in which the amount of heat dissipated from the
bottom surface by cooling increases is determined as the control direction.
- 45 -
In addition, in an area with a negative changing speed measured at the fixed
point, in a case in which the temperature of the hot-rolled steel sheet H is lower than
the average temperature, at least one of a direction in which the amount of heat
dissipated from the top surface by cooling decreases and a direction in which the
amount of heat dissipated from the bottom surface by cooling increases is determined
as the control direction, and, in a case in which the temperature of the hot-rolled steel
sheet H is higher than the average temperature, at least one of a direction in which the
amount of heat dissipated from the top surface by cooling increases and a direction in
which the amount of heat dissipated from the bottom surface by cooling decreases is
determined as the control direction.
In addition, it was found that, when at least one of the amount of heat
dissipated from the top surface by cooling and the amount of heat dissipated from the
bottom surface by cooling of the hot-rolled steel sheet H in the cooling section is
adjusted based on the control direction determined as described above, as illustrated in
FIG. 11, the temperature change occurring in the area A in which the shape of the hotrolled
steel sheet H is not flat enlarges compared with FIG 9. Meanwhile, in the
examples described herein, an assumption does not apply in which the cooling end
temperature may be changed. That is, even in a case in which the increase and
decrease directions (control direction) of the amount of heat dissipated from the top
surface by cooling and the amount of heat dissipated from the bottom surface by
cooling are determined as described above, the amount of heat dissipated by cooling is
adjusted so that the cooling end temperature of the hot-rolled steel sheet H becomes a
predetermined target cooling temperature.
[0083]
Use of the above relationship clarifies which cooling capability of the top side
- 46 -
cooling apparatus 14a and the bottom side cooling apparatus 14b in the cooling
apparatus 14 needs to be adjusted in order to reduce the temperature change, that is, the
temperature standard deviation Y. Meanwhile, the above relationship is summarized
in Table 2.
47
[0084]
[Table 2]
Changing speed
Temperature
Amount of iieat
dissipated by cooling
Top surface side
Bottom surface side
Positive
Low
Decrease
Increase
High
Increase
Decrease
Negative
Low
Increase
Decrease
High
Decrease
Increase
48 -
|i|piialpi«ll!i«»»ipilPWi!iI|IBIi" i«)iMinj>w4miiin*«www»HWP»w"i mnmrmmmmam
id
[0085]
As such, to the target ratio-setting process and the cooling control process
described above, the temperature-measuring process in which the temperature (the
temperature at the fixed point) of the hot-rolled steel sheet H is measured in
chronological order on the downstream side of the cooling section, a changing speedmeasuring
process in which the changing speed of the hot-rolled steel sheet H in the
vertical direction is measured in chronological order at the same place (the fixed point)
as the temperature measurement place of the hot-rolled steel sheet H, a control
direction-determining process in which the control directions of the amount of heat
dissipated from the top surface by cooling and the amount of heat dissipated from the
bottom surface by cooling are determined based on the temperature measurement
results and the changing speed measurement results, and an amount of heat dissipated
by cooling-adjusting process in which at least one of the amount of heat dissipated
from the top surface by cooling and the amount of heat dissipated from the bottom
surface by cooling of the hot-rolled steel sheet H in the cooling section is adjusted
based on the determined control directions may be newly added.
Here, in the control direction-determining process, as described above, in an
area with a positive changing speed measured at the fixed point in the hot-rolled steel
sheet H, in a case in which the temperature of the hot-rolled steel sheet H at the fixed
point is lower than the average temperature of the hot-rolled steel sheet H at the fixed
point, at least one of a direction in which the amount of heat dissipated from the top
surface by cooling decreases and a direction in which the amount of heat dissipated
from the bottom surface by cooling increases is determined as the control direction,
and, in a case in which the temperature of the hot-rolled steel sheet H is higher than the
average temperature, at least one of a direction in which the amount of heat dissipated
- 49 -
0
from the top surface by cooling increases and a direction in which the amount of heat
dissipated from the bottom surface by cooling decreases is determined as the control
direction.
In addition, in the control direction-determining process, in an area with a
negative changing speed, in a case in which the temperature of the hot-rolled steel
sheet H is lower than the average temperature, at least one of a direction in which the
amount of heat dissipated from the top surface by cooling increases and a direction in
which the amount of heat dissipated from the bottom surface by cooling decreases is
determined as the control direction, and, in a case in which the temperature of the hotrolled
steel sheet H is higher than the average temperature, at least one of a direction in
which the amount of heat dissipated from the top surface by cooling decreases and a
direction in which the amount of heat dissipated from the bottom surface by cooling
increases is determined as the control direction.
Meanwhile, in this cooling method as well, it is necessary to adjust the
amount of heat dissipated from the top surface by cooling and the amount of heat
dissipated from the bottom surface by cooling while achieving a control target that
matches the top and bottom heat transfer coeflTicient ratio X of the hot-rolled steel sheet
H in the cooling section to the target ratio Xt.
[0086]
Meanwhile, when adjusting the cooling capability of the top side cooling
apparatus 14a and the cooling capability of the bottom side cooling apparatus 14b, for
example, the cooling headers connected to cooling holes 31 in the top side cooling
apparatus 14a and the cooling headers connected to cooling holes 31 in the bottom side
cooling apparatus 14b may be on-ofif controlled respectively. Alternatively, the
cooling capabilities of the respective cooling headers in the top side cooling apparatus
- 50 -
i
14a and the bottom side cooling apparatus 14b may be controlled. That is, at least
one of the sprayed water density, pressure and water temperature of cooling water
sprayed from the respective cooling holes 31 may be adjusted.
In addition, the flow rate or pressure of cooling water sprayed from the top
side cooling apparatus 14a and the bottom side cooling apparatus 14b may be adjusted
by thinning out the cooling headers (cooling holes 31) of the top side cooling apparatus
14a and the bottom side cooling apparatus 14b. For example, in a case in which the
cooling capability of the top side cooling apparatus 14a before thinning out the cooling
headers is superior to the cooling capability of the bottom side cooling apparatus 14b,
the cooling headers that constitute the top side cooling apparatus 14a are preferably
thinned out.
[0087]
The hot-rolled steel sheet H is uniformly cooled by spraying cooling water
onto the top surface of the hot-rolled steel sheet H from the top side cooling apparatus
14a and spraying cooling water onto the bottom surface of the hot-rolled steel sheet H
from the bottom side cooling apparatus 14b using the cooling capabilities adjusted as
described above.
[0088]
In the above embodiment, a case in which the second correlation data
illustrated in FIG. 6 are obtained with the sheet-threading speed of the hot-rolled steel
sheet H fixed to 600 m/min has been described; however, as a result of thorough
studies, the inventors found that, when the sheet-threading speed is set to 550 m/min or
more in addition to the above control of the amounts of heat dissipated from the top
and bottom surfaces, it is possible to more uniformly cool the hot-rolled steel sheet H.
[0089]
- 51 -
It was found that, if the sheet-threading speed of the hot-rolled steel sheet H is
set to 550 m/min or more, the influence of soaked water on the hot-rolled steel sheet H
becomes significantly small even when cooling water is sprayed onto the hot-rolled
steel sheet H. Therefore, it is possible to prevent the ununiform cooling of the hotrolled
steel sheet H due to soaked water. Meanwhile, the sheet-threading speed of the
hot-rolled steel sheet H is preferably faster, but it is impossible to exceed a mechanical
limit speed (for example, 1550 m/min). Therefore, substantially, the sheet-threading
speed of the hot-rolled steel sheet H in the cooling section becomes set in a range of
550 m/min to the mechanical limit speed. In addition, in a case in which the upper
limit value (operational upper limit speed) of the sheet-threading speed during actual
operation is specified in advance, the sheet-threading speed of the hot-rolled steel sheet
H is preferably set in a range of 550 m/min to the operational upper limit speed (for
example, 1200 m/min).
[0090]
In addition, generally, in the case of the hot-rolled steel sheet H having a large
tensile strength (particularly, a steel sheet or the like called a so-called high tensile
strength steel having a tensile strength (TS) of 800 MPa or more and a realistic upper
limit of 1400 MPa), it is known that heat generation by working occurring in the hot
rolling facility 1 during rolling is increased due to a high hardness of the hot-rolled
steel sheet H. Therefore, in the related art, the hot-rolled steel sheet H was
sufficiently cooled by suppressing the sheet-threading speed of the hot-rolled steel
sheet H in the cooling apparatus 14 (that is, the cooling section) to be low.
[0091]
Therefore, the inventors found that, when cooling is carried out between a pair
of finish-rolling rolls 13a (that is, rolling stands) provided across, for example, 6 to 7
- 52 -
stands in the finishing mill 13 of the hot rolling facility 1 (so-called inter-stand
cooling), the heat dissipation by working is suppressed, and the sheet-threading speed
of the hot-rolled steel sheetH in the cooling apparatus 14 can be set to 550 m/min or
more. Particularly, in a case in which the tensile strength (TS) of the hot-rolled steel
sheet H is 800 MPa or more, heat generation by working of the hot-rolled steel sheet H
is suppressed by carrying out the inter-stand cooling, and it becomes possible to
maintain the sheet-threading speed of the hot-rolled steel sheet H in the cooling
apparatus 14 at 550 m/min or more.
[0092]
In the above embodiment, the cooling of the hot-rolled steel sheet H using the
cooling apparatus 14 is preferably carried out in a range of the exit-side temperature of
a finishing mill to a temperature of the hot-rolled steel sheet H of 600°C. A
temperature range in which the temperature of the hot-rolled steel sheet H is 600°C or
higher is a so-called film boiling area. That is, in this case, it is possible to prevent a
so-called transition boiling area and to cool the hot-rolled steel sheet H in the film
boiling area. In the transition boiling area, when cooling water is sprayed onto the
surface of the hot-rolled steel sheet H, portions covered with a vapor film and portions
in which the cooling water is directly sprayed onto the hot-rolled steel sheet H are
present in a mixed state on the surface of the hot-rolled steel sheet H.
Therefore, it is not possible to uniformly cool the hot-rolled steel sheet H.
On the other hand, in the film boiling area, since the hot-rolled steel sheet H is cooled
in a state in which the entire surface of the hot-rolled steel sheet H is covered with a
vapor film, it is possible to uniformly cool the hot-rolled steel sheet H. Therefore, it
is possible to more uniformly cool the hot-rolled steel sheet H in a range in which the
temperature of the hot-rolled steel sheet H is 600°C or higher as in the present
- 53 -
«
embodiment.
[0093]
In the above embodiment, when adjusting the cooling capability of the top
side cooling apparatus 14a and the cooling capability of the bottom side cooling
apparatus 14b of the cooling apparatus 14 using the second correlation data illustrated
in FIG. 6, the steepness of the wave shape of the hot-rolled steel sheet H and the sheetthreading
speed of the hot-rolled steel sheet H were set to be constant. However,
there are also cases in which, for example, the steepness or the sheet-threading speed
of the hot-rolled steel sheet H is different in each of the coils.
[0094]
According to the investigation by the inventors, for example, when the
steepness of the wave shape of the hot-rolled steel sheet H becomes large as illustrated
in FIG. 12, the temperature standard deviation Y of the hot-rolled steel sheet H
becomes large. That is, as the top and bottom heat transfer coefficient ratio X is away
from " 1 " as illustrated in FIG 13, the temperature standard deviation Y becomes large
in accordance with the steepness (the sensitivity of the steepness). In FIG. 13, the
relationship between the top and bottom heat transfer coefficient ratio X and the
temperature standard deviation Y is expressed using a V-shaped regression line for
each steepness as described above. Meanwhile, in FIG 13, the sheet-threading speed
of the hot-rolled steel sheet H is constant at 10 m/sec (600 m/min).
[0095]
In addition, for example, when the sheet-threading speed of the hot-rolled
steel sheet H becomes a high speed as illustrated in FIG. 14, the temperature standard
deviation Y of the hot-rolled steel sheet H becomes large. That is, as the top and
bottom heat transfer coefficient ratio X is away from " 1 " as illustrated in FIG. 15, the
- 54 -
#
temperature standard deviation Y becomes large in accordance with the sheet-threading
speed (the sensitivity of the sheet-threading speed). In FIG 15, the relationship
between the top and bottom heat transfer coeflTicient ratio X and the temperature
standard deviation Y is expressed using a V-shaped regression line for each sheetthreading
speed as described above. Meanwhile, in FIG 15^ the steepness of the
wave shape of the hot-rolled steel sheet H is constant at 2%.
[0096]
In a case in which the steepness or sheet-threading speed of the hot-rolled
steel sheet H is not constant as described above, the change of the temperature standard
deviation Y with respect to the top and bottom heat transfer coefficient ratio X can be
qualitatively evaluated, but cannot be accurately quantitatively evaluated.
[0097]
Therefore, table data indicating the correlation between each steepness and
the temperature standard deviation Y of the cooled hot-rolled steel sheet H are obtained
by, for example, fixing the top and bottom heat transfer coefficient ratio X of the hotrolled
steel sheet H in advance, and changing the steepness in a stepwise manner from
3% to 0% as illustrated in FIG. 12. In addition, the temperature standard deviation Y
with respect to the actual steepness z% of the hot-rolled steel sheet H is corrected to
the temperature standard deviation Y' with respect to a predetermined steepness using
an interpolation function. Specifically, in a case in which the predetermined
steepness is set to 2% as a correction condition, a temperature standard deviation Yz' is
computed using the following formula (1) based on the temperature standard deviation
Yz at the steepness z%. Alternatively, the temperature standard deviation Yz' may be
computed by, for example, computing the gradient a of the steepness in FIG. 12 using
the least squares method or the like and using the gradient a.
- 55 -
c
Yz'=Yzx2/z - (1)
[0098]
In addition, in the regression formula of the V-shaped curve illustrated in FIG.
13,^ the steepness may be corrected to the predetermined steepness, and the temperature
standard deviation Y may be derived from the regression formula. Meanwhile, Table
3 describes the temperature standard deviations Y of the hot-rolled steel sheet H in a
case in which the top and bottom heat transfer coefficient ratio X is changed with
respect to the steepness in FIG. 12 as illustrated in FIG 13, values obtained by
subtracting the respective temperature standard deviations Y of the hot-rolled steel
sheet H by the minimum value Ymin (Ymin=1.2°C in a case in which the steepness is
1%, Ymin=2.3°C in a case in which the steepness is 2%, and Ymin=3.5°C in a case in
which the steepness is 3%) (the differences of the standard deviations from the
minimum value), and the evaluation of the respective temperature standard deviations
Y.
The indication and evaluation standards of the top and bottom heat transfer
coefficient ratio X in Table 3 are the same as in the evaluation in Table 1, and thus will
not be described. The temperature standard deviation Y of the hot-rolled steel sheet H
in accordance with the steepness can be derived using FIG. 13 or Table 3. In addition,
for example, in a case in which the steepness is corrected to 2%, it is possible to set a
top and bottom heat transfer coefficient ratio X, at which the evaluation in Table 3
becomes "B", that is, the difference of the standard deviation from the minimum value
of the hot-rolled steel sheet H becomes 10°C or less, to 1.1.
56
m
[0099]
[Table 3]
Steepness
(%)
1
2
3
Top and bottom heat transfer
coefficient ratio X
1.6/1.0
1.2/1.0
1.0/1.0
1.0/1.2
1.0/1.6
1.6/1.0
1.1/1.0
1.0/1.0
1.0/1.1
1.0/1.6
1.2/1.0
1.1/1.0
1.0/1.0
1.0/1.1
1.0/1.2
Temperature standard
deviation (°C)
16.6
7.3
1.2
4.9
14.4
33.2
8.5
2.3
6.1
28.7
21.9
12.7
3.5
9.1
14.7
Difference of standard
deviation from minimum
value (°C)
15.4
6.1
0.0
3.7
13.2
30.9
6.2
0.0
3.8
26.4
18.4
9.2
0.0
5.6
11.2
Evaluation
C
B
A
B
C
C
B
A
B
C
C
B
A
B
C
- 57
IWBiRftWWIWMaiHW*!
!i!WWHiffiP.iilP!
w
[0100]
Similarly, table data indicating the correlation between the sheet-threading
speeds and the temperature standard deviation Y of the cooled hot-rolled steel sheet H
are obtained by, for example, changing the sheet-threading speed in a stepwise manner
from 5 m/sec (300 m/min) to 20 m/sec (1200 m/min) as illustrated in FIG 14. In
addition, the temperature standard deviation Y with respect to the actual sheetthreading
speed V (m/sec) of the hot-rolled steel sheet H is corrected to the temperature
standard deviation Y' with respect to a predetermined sheet-threading speed using an
interpolation function. Specifically, in a case in which the predetermined sheetthreading
speed is set to 10 (m/sec) as a correction condition, a temperature standard
deviation Yv' is computed using the following formula (2) based on the temperature
standard deviation Yv at the sheet-threading speed v (m/sec). Alternatively, the
temperature standard deviation Yv' may be computed by, for example, computing the
gradient p of the sheet-threading speed in FIG. 14 using the least squares method or the
like and using the gradient p.
Yz'=Yvxl0/v-(2)
[0101]
In addition, in the regression formula of the V-shaped curve illustrated in FIG.
15, the sheet-threading speed may be corrected to the predetermined sheet-threading
speed, and the temperature standard deviation Y may be derived from the regression
formula. Meanwhile, Table 4 describes the temperature standard deviations Y of the
hot-rolled steel sheet H in a case in which the top and bottom heat transfer coefficient
ratio X is changed with respect to the sheet-threading speed in FIG. 14 as illustrated in
FIG. 15, values obtained by subtracting the respective temperature standard deviations
Y by the minimum value Ymin (Ymin=l .2°C in a case in which the sheet-threading
- 58 -
speed is 5 m/s, Ymin=2.3°C in a case in which the sheet-threading speed is 10 m/s,
Ymin=3.5°C in a case in which the sheet-threading speed is 15 m/s, and Ymin=4.6°C
in a case in which the sheet-threading speed is 20 m/s) (the differences of the standard
deviations from the minimum value), and the evaluation of the respective temperature
standard deviations Y.
The indication and evaluation standards of the top and bottom heat transfer
coefficient ratio X in Table 4 are the same as in the evaluation in Table 1, and thus will
not be described. The temperature standard deviation Y of the hot-rolled steel sheet H
in accordance with the sheet-threading speed can be derived using FIG. 15 or Table 4.
In addition, for example, in a case in which the sheet-threading speed is corrected to 10
m/sec, it is possible to set a top and bottom heat transfer coefficient ratio X, at which
the evaluation in Table 4 becomes "B", that is, the difference of the standard deviation
from the minimum value of the hot-rolled steel sheet H becomes 10°C or less, to 1.1.
59
w
[0102]
[Table 4]
Sheet-threading speed (m/s)
5
10
15
20
Top and bottom heat transfer
coefficient ratio X
1.6/1.0
1.2/1.0
1.0/1.0
1.0/1.2
1.0/1.6
1.6/1.0
1.1/1.0
1.0/1.0
1.0/1.1
1.0/1.6
1.2/1.0
1.1/1.0
1.0/1.0
1.0/1.1
1.0/1.2
1.2/1.0
1.05/1.0
1.0/1.0
1.0/1.05
1.0/1.2
Temperature standard
deviation Y(°C)
16.6
7.3
1.2
4.9
14.4
33.2
8.5
2.3
6.1
28.7
21.9
12.7
3.5
9.1
14.7
29.2
10.8
4.6
8.4
19.6
Difference of standard
deviation from minimum
value (°C)
15.4
6.1
0.0
3.7
13.2
30.9
6.2
0.0
3.8
26.4
18.4
9.2
0.0
5.6
11.2
24.6
6.2
0.0
3.8
15.0
Evaluation
C
B
A
B
C
C
B
A
B
C
C
B
A
B
C
C
B
A
B
C
60
impiBBmiiiiiiii
•
[0103]
When the temperature standard deviation Y is corrected as described above, it
is possible to accurately quantitatively evaluate the change in the temperature standard
deviation Y with respect to the top and bottom heat transfer coefficient ratio X even in
a case in which the steepness or sheet-threading speed of the hot-rolled steel sheet H is
not constant.
[0104]
In the above embodiment, the temperature and wave shape of the hot-rolled
steel sheet H cooled using the cooling apparatus 14 may be measured, and the cooling
capability of the top side cooling apparatus 14a and the cooling capability of the
bottom side cooling apparatus 14b may be adjusted based on the measurement results.
That is, the cooling capabilities of the top side cooling apparatus 14a and the bottom
side cooling apparatus 14b may be feedback-controlled.
[0105]
In this case, the thermometer 40 that measures the temperature of the hotrolled
steel sheet H and the shape meter 41 that measures the wave shape of the hotrolled
steel sheet H are disposed between the cooling apparatus 14 and the coiling
apparatus 15 as illustrated in FIG. 16.
[0106]
In addition, the temperature and shape of the hot-rolled steel sheet H in the
process of sheet-threading are measured at the same point of the fixed point
respectively using the thermometer 40 and the shape meter 41, and the temperature and
the shape are measured as chronological data. Meanwhile, the temperature
measurement area includes all the area of the hot-rolled steel sheet H in the width
direction. In addition, the shape indicates the changing amount of the hot-rolled steel
- 61 -
sheet H in the height direction observed in a measurement at the fixed point.
Furthermore, similarly to the temperature measurement area, the shape measurement
area includes all the area of the hot-rolled steel sheet H in the width direction. When
the sampling times are muhiplied by the sheet-threading speed, it becomes possible to
link the chronological data of the measurement results of the temperature, changing
speed and the like to the locations in the rolling direction. Meanwhile, the
measurement points of the thermometer 40 and the shape meter 41 may not be strictly
the same; however, in order to maintain measurement accuracy, the deviation between
the measurement points of the thermometer 40 and the shape meter 41 is desirably 50
mm or less in an arbitrary direction of the rolling direction and the sheet width
direction.
[0107]
As described using FIGS. 8, 9, 10 and 11, in an area with a positive changing
speed at the fixed point in the hot-rolled steel sheet H, in a case in which the
temperature of the hot-rolled steel sheet H at the fixed point is lower than the average
temperature at the fixed point, it is possible to reduce the temperature standard
deviation Y by decreasing the top side cooling capability (the amount of heat
dissipated from the top surface by cooling). Similarly, it is possible to reduce the
temperature standard deviation Y by increasing the bottom side cooling capability (the
amount of heat dissipated from the bottom surface by cooling). Use of the above
relationship clarifies which cooling capability of the top side cooling apparatus 14a and
the bottom side cooling apparatus 14b in the cooling apparatus 14 needs to be adjusted
in order to reduce the temperature standard deviation Y.
[0108]
That is, by understanding the changing location of the temperature linked to
- 62 -
the wave shape of the hot-rolled steel sheet H, it is possible to clarify which of the top
side cooling and the bottom side cooling causes the currently occurring temperature
standard deviation Y. Therefore, the increase and decrease directions (control
directions) of the top side cooling capability (amount of heat dissipated from the top
surface by cooling) and the bottom side cooling capability (amount of heat dissipated
from the bottom surface by cooling) for decreasing the temperature standard deviation
Y are determined, and it is possible to adjust the top and bottom heat transfer
coefficient ratio X.
In addition, it is possible to determine the top and bottom heat transfer
coefficient ratio X based on the degree of the temperature standard deviation Y so that
the temperature standard deviation Y converges in a permissible range, for example, a
range of the minimum value Ymin to the minimum value Ymin+10°C. Since the
method for determining the top and bottom heat transfer coefficient ratio X is the same
as in the above embodiment described using FIGS. 6 and 7, the method will not be
described in detail. Meanwhile, when the temperature standard deviation Y is
converged in a range of the minimum value Ymin to the minimum value Ymin+10°C,
the variations in yield stress, tensile strength and the like are suppressed within the
manufacturing permissible ranges, and the hot-rolled steel sheet H can be uniformly
cooled.
In addition, although there are large variations, the temperature standard
deviation Y can be converged in a range of the minimum value Ymin to the minimum
value Ymin+10°C as long as a sprayed cooling water density ratio is ±5% or less with
respect to the sprayed cooling water density ratio at which the temperature standard
deviation Y becomes the minimum value Ymin. That is, in a case in which the
sprayed cooling water density is used, the top and bottom ratio of the sprayed cooling
- 63 -
%
water density (sprayed cooling water density ratio) is desirably set to ±5% or less with
respect to the sprayed cooling water density ratio at which the temperature standard
deviation Y becomes the minimum value Ymin. However, the permissible range does
not always include the top and bottom sprayed water density.
[0109]
As described above, since the cooling capabilities of the top side cooling
apparatus 14a and the bottom side cooling apparatus 14b can be adjusted to be
qualitatively and quantitatively appropriate cooling capabilities through feedback
control, it is possible to further improve the uniformity of the hot-rolled steel sheet H
which will be cooled afterwards.
[0110]
In the above embodiment, the cooling section in which the hot-rolled steel
sheet H is cooled may be divided into a plurality of sections, for example, two divided
cooling sections Zl and Z2 in the rolling direction as illustrated in FIG. 17. Each of
the divided cooling sections Zl and Z2 is provided with the cooling apparatus 14. In
addition, the thermometer 40 and the shape meter 41 are provided respectively at the
border between the respective divided cooling sections Zl and Z2, that is, on the
downstream side of the divided cooling sections Zl and Z2. Meanwhile, in the
embodiment, the cooling section is divided into two divided cooling sections, but the
number of divisions is not limited thereto, and can be arbitrarily set. For example, the
cooling section may be divided into 1 to 5 divided cooling sections.
[0111]
In this case, the temperature and wave shape of the hot-rolled steel sheet H on
the downstream side of the divided cooling sections Zl and Z2 are respectively
measured using the respective thermometers 40 and the respective shape meters 41.
- 64 -
#
In addition, the cooling capabilities of the top side cooling apparatus 14a and the
bottom side cooling apparatus 14b at the respective divided cooling sections Zl and Z2
are controlled based on the measurement results. At this time, the cooling capabilities
are controlled so that the temperature standard deviation Y of the hot-rolled steel sheet
H is converged in the permissible range, for example, a range of the minimum value
Ymin to the minimum value Ymin+10°C as described above. At least one of the
amount of heat dissipated from the top surface by cooling and the amount of heat
dissipated from the bottom surface by cooling of the hot-rolled steel sheet H at the
respective divided cooling sections Zl and Z2 is adjusted in the above manner.
[0112]
For example, in the divided cooling section Zl, the cooling capabilities of the
top side cooling apparatus 14a and the bottom side cooling apparatus 14b are
feedback-controlled based on the measurement results of the thermometer 40 and the
shape meter 41 on the downstream side, thereby at least one of the amount of heat
dissipated from the top surface by cooling and the amount of heat dissipated from the
bottom surface by cooling is adjusted.
In addition, in the divided cooling section Z2, the cooling capabilities of the
top side cooling apparatus 14a and the bottom side cooling apparatus 14b may be
feedforward-controlled or feedback-controlled based on the measurement results of the
thermometer 40 and the shape meter 41 on the downstream side. In any cases, in the
divided cooling section Z2, at least one of the amount of heat dissipated from the top
surface by cooling and the amount of heat dissipated from the bottom surface by
cooling is adjusted.
[0113]
Since the method for controlling the cooling capabilities of the top side
- 65 -
0
cooling apparatus 14a and the bottom side cooling apparatus 14b based on the
measurement results of the thermometer 40 and the shape meter 41 is the same as in
the above embodiment described using FIGS. 8 to 11, the method will not be described
in detail.
[0114]
In this case, since at least one of the amount of heat dissipated from the top
surface by cooling and the amount of heat dissipated from the bottom surface by
cooling of the hot-rolled steel sheet H is adjusted in the respective divided cooling
sections Zl and Z2, finer control becomes possible. Therefore, it is possible to more
uniformly cool the hot-rolled steel sheet H.
[0115]
In the above embodiment, in the respective divided cooling sections Zl and
Z2, when adjusting at least one of the amount of heat dissipated from the top surface
by cooling and the amount of heat dissipated from the bottom surface by cooling of the
hot-rolled steel sheet H, at least one of the steepness of the wave shape and the sheetthreading
speed of the hot-rolled steel sheet H may be used in addition to the
measurement results of the thermometer 40 and the shape meter 41. In this case, the
temperature standard deviation Y of the hot-rolled steel sheet H in accordance with at
least the steepness or the sheet-threading speed is corrected using the same method as
in the above embodiment described using FIGS. 12 to 15. In addition, at least one of
the amount of heat dissipated from the top surface by cooling and the amount of heat
dissipated from the bottom surface by cooling of the hot-rolled steel sheet H in the
respective divided cooling sections Zl and Z2 is corrected based on the corrected
temperature standard deviation Y (Y'). Thereby, it is possible to more uniformly cool
the hot-rolled steel sheet H.
- 66
#
[0116]
In addition, according to the present embodiment, it becomes possible to
finish the hot-rolled steel sheet H so that a uniform shape or material is formed in the
sheet width direction of the hot-rolled steel sheet H as well. Since the temperature
standard deviation in the hot-rolled steel sheet H in the sheet width direction is caused
by the alternate occurrence of the temperature standard deviation Y in the rolling
direction on the right and left sides, the temperature standard deviation Y in the sheet
width direction is also reduced when the temperature standard deviation in the rolling
direction is reduced. FIG. 18 illustrates an example of a pattern in which a wave
shape having an amplitude changing in the sheet width direction of the hot-rolled steel
sheet H is formed due to center buckle. As such, even in a case in which the wave
shape having an amplitude changing in the sheet width direction is generated so as to
form a temperature standard deviation in the sheet width direction, according to the
above-described embodiment, it becomes possible to reduce the temperature standard
deviation in the sheet width direction.
[0117]
Thus far, the preferable embodiment of the present invention has been
described with reference to the accompanying drawings, but the present invention is
not limited to the above embodiment. It is evident that a person skilled in the art can
imagine a variety of modified examples and corrected examples within the scope of
ideas described in the claims, and it is needless to say that the examples belong to the
technical scope of the present invention.
[Examples]
[0118]
(Example 1)
- 67 -
#
The inventors used high tensile strength steel (a so-called high tensile strength
steel sheet) having a sheet thickness of 2.3 mm and a sheet width of 1200 mm as
Example 1, respectively formed a center wave shape and an edge wave shape in the
material, a change in a cold-rolling gauge (change in the sheet thickness) and a change
in an average temperature in a sheet width direction in a post process (that is, a coldrolling
process) were measured in a case in which the material was cooled with a
variety of different values of the steepness of 0% (no wave formed) to 2%, and
evaluated. Meanwhile, in Example 1 and Examples 2 and 3 to be described below,
for convenience, a steepness in a case in which the center wave shape was formed was
represented by -0.5% to -2%, and a steepness in a case in which the edge wave shape
was formed was represented by 0.5% to 2%.
In addition, the center wave shape and the edge wave shape were measured
using a commercially available shape-measuring device, the center wave shape was
measured at a sheet central portion within 30 mm from a sheet center on the right and
left sides, and the edge wave shape was measured at a portion 25 mm away from a
sheet edge. Furthermore, in Example 1, a top and bottom cooling ratio during cooling
(top and bottom heat transfer coefficient ratio) was set to top cooling: bottom
cooling=1.2:l, a sheet-threading speed was set to 400 m/min, and a coiling temperature
(CT) of the steel sheet was set to 500°C.
Measurement results and evaluation results are described in Table 5. At this
time, as evaluation standards for the following examples, a steel sheet having a change
in the cold-rolling gauge in the post process suppressed to 0 fim to 25 \xm was
evaluated to be A (favorable as a product), a steel sheet having the change suppressed
to 25 |um to 50 fim was evaluated to be B (permissible as a product), and a steel sheet
having the change of larger than 50 fim was evaluated to be C (defective as a product).
- 68 -
•
Meanwhile, general evaluations in Table 5 will be described below. In addition. Table
5 also describes temperature standard deviations of the respective wave shapes in a
rolling direction of the steel sheet for reference.
69
[0119]
[Table 5]
Steepness A,
[%]
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Temperature
standard deviation
[°C]
100
75
50
25
0
25
50
75
100
Change in coldrolling
gauge [p,m]
120
90
60
30
0
21
42
63
84
Change in average
temperature in sheet
width direction [°C1
100
75
50
25
0
17.5
35
52.5
70
Evaluation
C
C
c
B
A
A
B
C
C
General evaluation
C
C
C
C
C
A
A
C
C
70 -
^
[0120]
As described in Table 5, while the change in the cold-rolling gauge in the
cold-rolling process was 30 \xm to 120 |j,m in a case in which the center wave shape
was formed in the steel sheet (in the table, cases in which the steepness was -0.5% to -
2%), the change in the cold-rolling gauge in the cold-rolling process was 21 |j,m to 84
ixm in a case in which the edge wave shape was formed (in the table, cases in which
the steepness was 0.5% to 2%). That is, it was found that, even when wave shapes
having the same steepness were formed in the steel sheet, the change in the coldrolling
gauge (that is, the change in the sheet thickness) in the cold-rolling process was
suppressed to be small in the case in which the edge wave shape was formed compared
with the case in which the center wave shape was formed.
[0121]
In addition, it was found from the results in Table 5 that, when the changes in
the average temperature in the sheet width direction were compared between the case
in which the center wave shape was formed in the steel sheet and the case in which the
edge wave shape was formed, the change in the average temperature in the sheet width
direction was suppressed to be small in the case in which the edge wave shape was
formed compared with the case in which the center wave shape was formed in spite of
the same steepness. Therefore, it was confirmed that, compared with the case in
which the center wave shape was formed, in the case in which the edge wave shape
was formed, temperature variation in the steel sheet width direction during cold-rolling
was reduced, and variation in material qualities was suppressed.
[0122]
In addition, generally, the change in the sheet thickness in the cold-rolling
process of the steel sheet is desirably smaller in order to suppress a decrease in yield
- 71 -
#
caused by defective products and the like. Therefore, it was found that, as described
in Table 5, in a case in which the edge wave shape was formed in the steel sheet, when
the steepness of the edge wave shape was set to more than 0% to 1%, the change in the
cold-rolling gauge was suppressed to be a small value (for example, evaluations A and
B in Table 5). Furthermore, it was found that, when the steepness of the edge wave
shape was set to more than 0% to 0.5%, the change in the cold-rolling gauge was
suppressed to be a smaller value (for example, the evaluation A in Table 5).
[0123]
(Example 2)
Next, as Example 2, the inventors respectively formed a center wave shape
and an edge wave shape in the same material as Example 1, a change in the coldrolling
gauge (change in the sheet thickness) and a change in the average temperature
in the sheet width direction in the post process (that is, a cold-rolling process) were
measured in a case in which the material was cooled with a variety of different values
of the steepness of 0% (no wave formed) to 2%, and evaluated. Meanwhile, in
Example 2, the sheet-threading speed was set to 600 m/min, and other conditions were
set to the same conditions as Example 1. Measurement results and evaluation results
are illustrated in Table 6.
- 72 -
^ ^
[0124]
[Table 6]
Steepness X
[%]
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Temperature
standard deviation
r°ci
100
75
50
25
0
25
50
75
100
Change in coldrolling
gauge [|um]
108
81
54
27
0
15
30
45
60
Change in average
temperature in sheet
width direction [°C]
90
67.5
45
22.5
0
12.5
25
37.5
50
Evaluation
C
C
C
B
A
A
B
B
C
General evaluation
C
C
C
C
C
A
A
A
C
73
WfjV^f^^'jmmm 'W*WS?»&W%t
or less are C.
[Industrial Applicability]
[0133]
The invention is useful when cooling a hot-rolled steel sheet which has been
hot-rolled using a finishing mill so as to have a wave shape having a surface height
changing in the rolling direction..
[Description of Reference Numerals and Signs]
[0134]
78
1: HOT ROLLING FACILITY
11: HEATING FURNACE
12: ROUGHING MILL
12a: WORK ROLL
12b: FOURFOLD MILL
13: FINISHING MILL
13a: FINISH-ROLLING ROLL
14: COOLING APPARATUS
14a: TOP SIDE COOLING APPARATUS
14b: BOTTOM SIDE COOLING APPARATUS
15: COILING APPARATUS
16: WIDTH-DIRECTION MILL
31: COOLING HOLE
32: TRi\NSPORTATION ROLL
40: THERMOMETER
41: SHAPE METER
H: HOT-ROLLED STEEL SHEET
S:SLAB
Zl, Z2: DIVIDED COOLING SECTION
79

[Type of Document] CLAIMS
[Claim 1]
A method for manufacturing a steel sheet, the metod comprising:
a hot-rolling process in which a steel material is hot-rolled using a finishing
mill so as to obtain a hot-rolled steel sheet having an edge wave shape with a wave
height periodically changing in a rolling direction; and
a cooling process in which the hot-rolled steel sheet is cooled in a cooling
section provided on a sheet-threading path,
wherein the hot-rolling process includes:
a target steepness-setting process in which a target steepness of the edge wave
shape is set based on first correlation data indicating a correlation between the
steepness of the edge wave shape of the hot-rolled steel sheet and a temperature
standard deviation Y during or after cooling of the hot-rolled steel sheet, which have
been experimentally obtained in advance; and
a shape-controlling process in which operation parameters of the finishing
mill are controlled so as to match the steepness of the edge wave shape with the target
steepness.
[Claim 2]
The method for manufacturing a steel sheet,
wherein, in the target steepness-setting process, the target steepness is set in a
range of more than 0% to 1%.
[Claim 3]
The method for manufacturing a steel sheet according to Claim 1 or 2,
wherein the cooling process includes:
a target ratio-setting process in which a top and bottom heat transfer
- 80 -
coefficient ratio XI, at which the temperature standard deviation Y becomes a
minimum value Ymin, is set as a target ratio Xt based on second correlation data
indicating a correlation between a top and bottom heat transfer coefficient ratio X,
which is a ratio of heat transfer coefficients of top and bottom surfaces of the hot-rolled
steel sheet, and the temperature standard deviation Y during or after cooling of the hotrolled
steel sheet, which have been experimentally obtained in advance under
conditions in which the steepness and sheet-threading speed of the hot-rolled steel
sheet are set to constant values; and
a cooling control process in which at least one of an amount of heat dissipated
from a top surface by cooling and an amount of heat dissipated from a bottom surface
by cooling of the hot-rolled steel sheet in the cooling section is controlled so that the
top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet in the
cooling section matches the target ratio Xt.
[Claim 4]
The method for manufacturing a steel sheet according to Claim 3,
wherein, in the target ratio-setting process, a top and bottom heat transfer
coefficient ratio X at which the temperature standard deviation Y converges in a range
of the minimum value Ymin to the minimum value Ymin+10°C is set as the target ratio
Xt based on the second correlation data.
[Claim 5]
The method for manufacturing a steel sheet according to Claim 3,
wherein the second correlation data is prepared respectively for a plurality of
conditions in which values of the steepness and the sheet-threading speed are different,
and
in the target ratio-setting process, the target ratio Xt is set based on the second
- 81 -
correlation data matching actual measured values of the steepness and the sheetthreading
speed among the plurality of second correlation data.
[Claim 6]
The method for manufacturing a steel sheet according to Claim 3,
wherein the second correlation data is data indicating the correlation between
the top and bottom heat transfer coefficient ratio X and the temperature standard
deviation Y using a regression formula.
[Claim 7]
The method for manufacturing a steel sheet according to Claim 6,
wherein the regression formula is derived using linear regression.
[Claim 8]
The method for manufacturing a steel sheet according to Claim 3,
wherein the second correlation data is data indicating the correlation between
the top and bottom heat transfer coefficient ratio X and the temperature standard
deviation Y using a table.
[Claim 9]
The method for manufacturing a steel sheet according to Claim 3, the method
fiirther comprising:
a temperature-measuring process in which a temperature of the hot-rolled
steel sheet is measured in chronological order on a downstream side of the cooling
section;
an average temperature value-computing process in which a chronological
average value of the temperature is computed based on measurement results of the
temperature; and
an amount of heat dissipated by cooling-adjusting process in which a total
- 82 -
value of the amount of heat dissipated from the top surface by cooling and the amount
of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet in
the cooling section is adjusted so that the chronological average value of the
temperature matches a predetermined target temperature.
[Claim 10]
The method for manufacturing a steel sheet according to Claim 3, the method
further comprising:
a temperature-measuring process in which a temperature of the hot-rolled
steel sheet is measured in chronological order on a downstream side of the cooling
section;
a changing speed-measuring process in which a changing speed of the hotrolled
steel sheet in a vertical direction is measured in chronological order at a same
place as a temperature measurement place of the hot-rolled steel sheet on the
downstream side of the cooling section;
a control direction-determining process in which, when an upward side of the
vertical direction of the hot-rolled steel sheet is set as positive, in an area with a
positive changing speed, in a case in which a temperature of the hot-rolled steel sheet
is lower than an average temperature in a range of one or more cycles of a wave shape
of the hot-rolled steel sheet, at least one of a direction in which the amount of heat
dissipated from the top surface by cooling decreases and a direction in which the
amount of heat dissipated from the bottom surface by cooling increases is determined
as a control direction, in a case in which the temperature of the hot-rolled steel sheet is
higher than the average temperature, at least one of a direction in which the amount of
heat dissipated from the top surface by cooling increases and a direction in which the
amount of heat dissipated from the bottom surface by cooling decreases is determined
- 83 -
as the control direction,
in an area with a negative changing speed, in a case in which the temperature
of the hot-rolled steel sheet is lower than the average temperature, at least one of a
direction in which the amount of heat dissipated from the top surface by cooling
increases and a direction in which the amount of heat dissipated from the bottom
surface by cooling decreases is determined as the control direction, and, in a case in
which the temperature of the hot-rolled steel sheet is higher than the average
temperature, at least one of a direction in which the amount of heat dissipated from the
top surface by cooling decreases and a direction in which the amount of heat dissipated
from the bottom surface by cooling increases is determined as the control direction;
and
an amount of heat dissipated by cooling-adjusting process in which at least
one of the amount of heat dissipated from the top surface by cooling and the amount
of heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet in
the cooling section is adjusted based on the control direction determined in the control
direction-determining process.
[Claim 11]
The method for manufacturing a steel sheet according to Claim 10,
wherein the cooling section is divided into a plurality of divided cooling
sections in a sheet-threading direction of the hot-rolled steel sheet,
the temperature and the changing speed of the hot-rolled steel sheet are
measured in chronological order at each of borders of the divided cooling sections in
the temperature-measuring process and the changing speed-measuring process,
increase and decrease directions of the amounts of heat dissipated by cooling
from the top and bottom surfaces of the hot-rolled steel sheet are determined for the
- 84 -
respective divided cooling sections based on measurement results of the temperature
and the changing speeds of the hot-rolled steel sheet at the respective borders of the
divided cooling sections in the control direction-determining process, and
feedback control or feedforward control is carried out in order to adjust at
least one of the amount of heat dissipated from the top surface by cooling and the
amount of heat dissipated from the bottom surface by cooling of the hot-rolled steel
sheet at each of the divided cooling sections based on the control direction determined
for each of the divided cooling sections in the amount of heat dissipated by coolingadjusting
process.
[Claim 12]
The method for manufacturing a steel sheet according to Claim 11, the
method fiirther comprising:
a measuring process in which the steepness or the sheet-threading speed of the
hot-rolled steel sheet is measured at each of the borders of the divided cooling sections;
and
an amount of heat dissipated by cooling-correcting process in which at least
one of the amount of heat dissipated from the top surface by cooling and the amount of
heat dissipated from the bottom surface by cooling of the hot-rolled steel sheet is
corrected at each of the divided cooling sections based on measurement results of the
steepness or the sheet-threading speeds.
[Claim 13]
The method for manufacturing a steel sheet according to Claim 3, the method
further comprising:
a post cooling process in which the hot-rolled steel sheet is further cooled in
order to make the temperature standard deviation of the hot-rolled steel sheet fall into a
- 85 -
permissible range on a downstream side of the cooling section.
[Claim 14]
The method for manufacturing a steel sheet according to Claim 3,
wherein the sheet-threading speed of the hot-rolled steel sheet in the cooling
section is set in a range of 550 m/min to a mechanical limit speed.
[Claim 15]
The method for manufacturing a steel sheet according to Claim 14,
wherein a tensile strength of the hot-rolled steel sheet is 800 MPa or more.
[Claim 16]
The method for manufacturing a hot-rolled steel sheet according to Claim 14,
wherein the finishing mill is constituted by a plurality of rolling stands, and
a supplementary cooling process in which the hot-rolled steel sheet is
supplementarily cooled between the plurality of the rolling stands is further provided.
[Claim 17]
The method for manufacturing a steel sheet according to Claim 3,
wherein a top side cooling apparatus having a plurality of headers that ejects
cooling water to a top surface of the hot-rolled steel sheet and a bottom side cooling
apparatus having a plurality of headers that ejects cooling water to a bottom surface of
the hot-rolled steel sheet are provided in the cooling section, and
the amount of heat dissipated from the top surface by cooling and the amount
of heat dissipated from the bottom surface by cooling are adjusted by carrying out onoff
control of the respective headers.
[Claim 18]
The method for manufacturing a steel sheet according to Claim 3,
wherein a top side cooling apparatus having a plurality of headers that ejects
- 86 -
cooling water to a top surface of the hot-rolled steel sheet and a bottom side cooling
apparatus having a plurality of headers that ejects cooling water to a bottom surface of
the hot-rolled steel sheet are provided in the cooling section, and
the amount of heat dissipated from the top surface by cooling and the amount
of heat dissipated from the bottom surface by cooling are adjusted by controlling at
least one of sprayed water" density, pressure and water temperature of each of the
headers.
[Claim 19]
The method for manufacturing a steel sheet according to Claim 3,
wherein cooling in the cooling section is carried out at a temperature of the
hot-rolled steel sheet in a range of 600°C or higher.