U.S. patent application number 13/256288 was filed with the patent office on 2012-01-26 for steel plate manufacturing facility and manufacturing method.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Yukio Fujii, Takayuki Furumai, Kenji Hirata, Takashi Kuroki, Naoki Nakata, Motoji Terasaki.
Application Number | 20120017660 13/256288 |
Document ID | / |
Family ID | 42781163 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120017660 |
Kind Code |
A1 |
Kuroki; Takashi ; et
al. |
January 26, 2012 |
STEEL PLATE MANUFACTURING FACILITY AND MANUFACTURING METHOD
Abstract
A facility is provided for manufacturing a steel plate excellent
in steel plate shape and mechanical property while setting low
cooling-water spraying performance in a descaling step and
achieving uniform cooling in a cooling step. Specifically, a hot
rolling mill 3, a first hot leveler 5, a descaler 4, and cooling
equipment 6 are arranged in that order from the upstream side in a
conveying direction. A pressure P [MPa] at the point of impact of
cooling water sprayed from the descaler to each surface of a steel
plate 1 is greater than or equal to 1.5 MPa.
Inventors: |
Kuroki; Takashi;
(Kawasaki-shi, JP) ; Nakata; Naoki; (Chiba-shi,
JP) ; Hirata; Kenji; (Kurashiki-shi, JP) ;
Furumai; Takayuki; (Kurashiki-shi, JP) ; Fujii;
Yukio; (Fukuyama-shi, JP) ; Terasaki; Motoji;
(Fukuyama-shi, JP) |
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku ,Tokyo
JP
|
Family ID: |
42781163 |
Appl. No.: |
13/256288 |
Filed: |
March 23, 2010 |
PCT Filed: |
March 23, 2010 |
PCT NO: |
PCT/JP2010/055497 |
371 Date: |
September 13, 2011 |
Current U.S.
Class: |
72/39 |
Current CPC
Class: |
B21B 45/0218 20130101;
B21B 45/08 20130101; B21B 39/08 20130101 |
Class at
Publication: |
72/39 |
International
Class: |
B21B 45/04 20060101
B21B045/04; B21B 45/02 20060101 B21B045/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2009 |
JP |
2009-073994 |
Jan 15, 2010 |
JP |
2010-006722 |
Claims
1. A steel plate manufacturing facility comprising: a hot rolling
mill, a hot leveler, a descaler, and cooling equipment arranged in
that order from the upstream side in a conveying direction, wherein
a pressure P at the point of impact of cooling water sprayed from
the descaler to each surface of a steel plate is greater than or
equal to 1.5 MPa.
2. The steel plate manufacturing facility according to claim 1,
wherein when V [m/s] denotes the conveying velocity from the
descaler to the cooling equipment and T [K] denotes the temperature
of the steel plate before cooling, the distance L [m] between the
descaler and the cooling equipment satisfies the expression
L.ltoreq.V.times.5.times.10.sup.-9.times.exp(25000/T).
3. The steel plate manufacturing facility according to claim 1,
wherein the components are arranged such that the distance L
between the descaler and the cooling equipment is less than or
equal to 12 m.
4. The steel plate manufacturing facility according to claim 1,
wherein the distance H between each spraying nozzle of the descaler
and each surface of the steel plate is greater than or equal to 40
mm and less than or equal to 140 mm.
5. The steel plate manufacturing facility according to claim 1,
wherein the cooling equipment includes a header supplying cooling
water to the upper surface of the steel plate, cooling water
spraying nozzles extending from the header and spraying rod-like
cooling water, and a dividing plate disposed between the steel
plate and the header, and the dividing plate includes a plurality
of water supply inlets receiving the lower ends of the cooling
water spraying nozzles and a plurality of drain outlets draining
the cooling water supplied to the upper surface of the steel plate
onto the dividing plate.
6. A steel plate manufacturing method including a hot rolling step,
a hot leveling step, and a cooling step performed in that order to
manufacture a steel plate, the method comprising: a descaling step
of spraying cooling water to each surface of the steel plate at a
pressure at the point of impact of 1.5 MPa or higher, the descaling
step being performed between the hot leveling step and the cooling
step.
7. The steel plate manufacturing method according to claim 6,
wherein when T [K] denotes the temperature of the steel plate
before cooling, the period of time t [s] between the completion of
the descaling step and the start of the cooling step satisfies the
expression t.ltoreq.5.times.10.sup.-9+exp(25000/T).
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. National Phase application of
PCT International Application No. PCT/JP2010/055497, filed Mar. 23,
2010, and claims priority to Japanese Patent Application No.
2009-073994, filed Mar. 25, 2009, and Japanese Patent Application
No. 2010-006722, filed Jan. 15, 2010, the disclosures of which PCT
and priority applications are incorporated herein by reference in
their entirely for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to a steel plate manufacturing
facility and manufacturing method of hot rolling, hot leveling, and
cooling a steel plate.
BACKGROUND OF THE INVENTION
[0003] The application of cooling control as a process of
manufacturing a steel plate has recently been widening. Since a
typical hot rolled steel plate is not necessarily uniform in, for
example, shape and surface condition, however, strip temperature
deviation tends to occur in the steel plate during cooling. The
occurrence of deformation, residual stress, material nonuniformity,
and the like in the steel plate subjected to cooling causes poor
quality and operational trouble.
[0004] Patent Literature 1 discloses a method of performing
descaling at least one of just before and just after a final pass
of finish rolling, subsequently performing hot leveling, then
performing descaling, and starting accelerated cooling.
[0005] Patent Literature 2 discloses a method of performing finish
rolling and hot leveling, performing descaling just before
controlled cooling, and performing controlled cooling.
PATENT LITERATURE
[0006] PTL 1: Japanese Unexamined Patent Application Publication
No. 9-57327 [0007] PTL 2: Japanese Patent No. 3796133
SUMMARY OF THE INVENTION
[0008] In actually manufacturing a steel plate using the methods
disclosed in Patent Literature 1 and Patent Literature 2 mentioned
above, in some cases, scale is not completely removed during
descaling, but descaling causes scale nonuniformity, resulting in
poor surface condition. Although Patent Literature 1 and Patent
Literature 2 do not mention a pressure at the point of impact of
cooling water on each surface of a steel plate during descaling,
pressures at the point of impact derived on the basis of spraying
pressures and spraying distances of nozzles described in Patent
Literature 1 and Patent Literature 2 and a typical kind of nozzle
are estimated to be 0.08 to 1.00 MPa in Patent Literature 1 (when
Everloy Descaling Nozzles DNX or DNH are used under conditions
described in paragraph Nos. (0045) and (0046) in Patent Literature
1, a pressure at the point of impact is 0.08 to 1.00 MPa on the
basis of a spray angle of 23.degree. and Expressions (1) and (2)
which will be described in paragraph Nos. (0030) and (0031) in this
specification) and to be approximately 0.06 to 0.08 MPa in Patent
Literature 2 (when Everloy Descaling Nozzles DNX are used under
conditions described in paragraph No. (0024) in Patent Literature
2, a pressure at the point of impact is 0.06 to 0.08 MPa on the
basis of a spray angle of 37.degree. and Expressions (1) and (2)
which will be described in paragraph Nos. (0030) and (0031) in the
specification). The pressures at the point of impact of cooling
water are low and the disclosed methods do not offer the
performance of achieving uniform descaling. Accordingly, the
surface condition of a scale removed portion differs from that of a
portion where scale is not removed. Disadvantageously, uniform
cooling is not achieved during controlled cooling.
[0009] In recent years in particular, a level of material
uniformity required for a steel plate has been tightened. The
harmful effect of cooling rate nonuniformity caused by the
above-described scale nonuniformity during controlled cooling on
material uniformity, especially in the width direction of the steel
plate, is becoming unignorable.
[0010] The present invention focused on the above-described
unsolved problems of related art. The present invention provides a
facility and method for manufacturing a steel plate excellent in
steel plate shape and mechanical property by performing uniform
descaling in a descaling step and achieving uniform cooling in a
cooling step.
[0011] A steel plate manufacturing facility according to exemplary
embodiments of the present invention includes a hot rolling mill, a
hot leveler, a descaler, and cooling equipment arranged in that
order from the upstream side in a conveying direction, wherein a
pressure P [MPa] at the point of impact of cooling water sprayed
from the descaler to each surface of a steel plate is greater than
or equal to 1.5 MPa.
[0012] After diligent study of a force causing the removal of scale
using high-pressure water, the present inventors discovered that
when descaling was performed after hot leveling, so long as a
pressure at the point of impact of cooling water sprayed from the
descaler to the steel plate was 1.5 MPa or higher, the scale
thickness of a product decreased and was made uniform. The reason
is that scale was temporarily and uniformly removed completely by
descaling at a high pressure at the point of impact and, after
that, scale was thinly and uniformly reproduced. According to
preferred aspects of the invention, therefore, since the scale
thickness of the steel plate is thinned and made uniform before
passing through the cooling equipment, the steel plate can be
uniformly cooled with little surface temperature deviation among
positions in the width direction of the steel plate while passing
through the cooling equipment. Thus, the steel plate is excellent
in steel plate shape and mechanical property.
[0013] Furthermore, since the descaler removes scale produced on
each surface of the steel plate after hot leveling of the steel
plate by the hot leveler, spraying nozzles of the descaler can be
moved closer to the surfaces of the hot-leveled steel plate,
thereby improving the descaling performance. Alternatively, the
cooling water spraying performance of the descaler for providing a
predetermined pressure at the point of impact can be set to
low.
[0014] In the steel plate manufacturing facility, preferably, when
V [m/s] denotes the conveying velocity from the descaler to the
cooling equipment and T [K] denotes the temperature of the steel
plate before cooling, the distance L [m] between the descaler and
the cooling equipment satisfies the expression
L.ltoreq.V.times.5.times.10.sup.-9.times.exp(25000/T). According to
preferred aspects of the invention, cooling of the steel plate by
the cooling equipment can be stabilized.
[0015] In the steel plate manufacturing facility, preferably, the
components are arranged such that the distance L between the
descaler and the cooling equipment is less than or equal to 12 m.
According to embodiments of the invention, cooling of the steel
plate by the cooling equipment is very stable.
[0016] In the steel plate manufacturing facility, preferably, the
distance H between each spraying nozzle of the descaler and the
surface of the steel plate is greater than or equal to 40 mm and
less than or equal to 140 mm. According to embodiments of the
invention, the spraying pressure and spray flow rate of the
descaler for providing a predetermined pressure at the point of
impact are low, thus reducing the pump capacity of the
descaler.
[0017] In the steel plate manufacturing facility, preferably, the
cooling equipment includes a header supplying cooling water to the
upper surface of the steel plate, cooling water spraying nozzles
extending from the header and spraying rod-like cooling water, and
a dividing plate disposed between the steel plate and the header,
and the dividing plate includes a plurality of water supply inlets
receiving the lower ends of the cooling water spraying nozzles and
a plurality of drain outlets draining the cooling water supplied to
the upper surface of the steel plate onto the dividing plate.
[0018] According to exemplary embodiments of the invention, cooling
water supplied from the cooling water spraying nozzles through the
water supply inlets cools the upper surface of the steel plate to
turn to high-temperature drainage water and the drainage water
flows from the drain outlets onto the dividing plate so that the
drainage water after cooling is immediately eliminated from the
steel plate. Advantageously, the cooling equipment offers adequate
cooling performance that is uniform in the width direction.
[0019] According to exemplary embodiments of the present invention,
a steel plate manufacturing method including a hot rolling step, a
hot leveling step, and a cooling step performed in that order to
manufacture a steel plate includes a descaling step of spraying
cooling water to each surface of the steel plate at a pressure at
the point of impact of 1.5 MPa or higher, the descaling step being
performed between the hot leveling step and the cooling step.
[0020] According to exemplary embodiments of the invention, since
the scale thickness of the steel plate is thinned and made uniform
before the cooling step, the steel plate can be uniformly cooled
with little surface temperature deviation among positions in the
width direction of the steel plate in the cooling step. Thus, the
steel plate excellent in steel plate shape and mechanical property
can be manufactured.
[0021] In the steel plate manufacturing method, preferably, when T
[K] denotes the temperature of the steel plate before cooling, the
period of time t [s] between the completion of the descaling step
and the start of the cooling step satisfies the expression
t.ltoreq.5.times.10.sup.-9.times.exp(25000/T). According to
exemplary embodiments of the invention, cooling of the steel plate
in the cooling step can be stabilized.
[0022] According to the steel plate manufacturing facility and
manufacturing method of exemplary embodiments of the present
invention, uniform descaling can be performed in the descaling step
and uniform cooling can be achieved in the cooling step, so that
the steel plate excellent in steel plate shape and mechanical
property can be manufactured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a diagram illustrating the outline of a hot
rolling facility according to an exemplary embodiment of the
present invention.
[0024] FIG. 2 is a diagram illustrating cooling equipment
constituting the hot rolling facility according to an exemplary
embodiment of the present invention.
[0025] FIG. 3 is a diagram illustrating a dividing plate
constituting an exemplary embodiment of the cooling equipment.
[0026] FIG. 4 is a diagram illustrating the flow of cooling water
and that of drainage water in the cooling equipment.
[0027] FIG. 5 is a graph illustrating the relationship between the
pressure at the point of impact of cooling water from a descaler
and the thickness of scale produced on each surface of a steel
plate product.
[0028] FIG. 6 is a graph illustrating the relationship between the
position from the center of a steel plate in the width direction
thereof and the temperature in a cooling step in an exemplary
embodiment of the present invention.
[0029] FIG. 7 is a graph illustrating the relationship between the
position from the center of a steel plate in the width direction
thereof and the temperature in the cooling step in a related-art
facility which does not include a descaling step before the cooling
step.
[0030] FIG. 8 is a graph illustrating the relationship between the
spraying pressure and the spraying distance for providing a
pressure at the point of impact of 1.5 MPa while setting a spray
flow rate, a spray angle of nozzle, and an angle between the spray
direction and the vertical line in the descaler to predetermined
values.
DETAILED DESCRIPTION OF THE INVENTION
[0031] One embodiment (hereinafter, referred to as "embodiment") of
practicing the present invention will be described in detail below
with reference to the drawings.
[0032] Referring to FIG. 1, a hot rolling facility according to the
present embodiment includes a heating furnace 2, a hot rolling mill
3, a first hot leveler 5, a descaler 4, cooling equipment 6, and a
second hot leveler 7 arranged in that order from the upstream side
in a conveying direction of a steel plate 1.
[0033] A slab discharged from the heating furnace 2 is passed
through the hot rolling mill 3 multiple times, thus resulting in a
rolled steel plate 1 having a predetermined thickness. The
hot-rolled steel plate 1 is conveyed on a table roller (not
illustrated) from the upstream side to the first hot leveler 5 on
the downstream side. Although only one hot rolling mill 3 is
illustrated, the hot rolling mill 3 may include a rough rolling
mill and a finish rolling mill.
[0034] The first hot leveler 5 is configured to remove strain
caused in the steel plate 1 during hot rolling. The illustrated hot
leveler is of the roller leveler type in which the steel plate 1 is
nipped between leveling rolls arranged one above the other in a
staggered layout. The hot leveler is not limited to this type, but
a skin-pass mill or a press machine may be used. When the hot
rolling mill 3 includes a rough rolling mill and a finish rolling
mill, the finish rolling mill may perform skin-pass rolling.
[0035] The second hot leveler 7 is configured to remove strain
caused in the steel plate 1 during cooling by the cooling equipment
6. This hot leveler does not have to be used in the present
invention. The second hot leveler 7 used is of the roller leveler
type. The hot leveler is not limited to this type, but a skin-pass
mill or a press machine may be used.
[0036] The cooling equipment 6 is configured to perform controlled
cooling on a high-temperature steel plate 1 subjected to hot
rolling under predetermined temperature conditions to control the
structure of the steel plate 1 in order to obtain desired material
properties. Any cooling equipment may be used so long as desired
cooling conditions are provided. It is preferred to use cooling
equipment capable of uniformly cooling the upper and lower surfaces
of the steel plate 1 in the length and width directions. The
present embodiment therefore uses the cooling equipment 6,
illustrated in FIG. 2, which has high cooling performance and is
excellent in cooling uniformity, especially in the width
direction.
[0037] Referring to FIG. 2, the cooling equipment 6 in the present
embodiment includes an upper header 10 which supplies cooling water
to the upper surface of the steel plate 1, upper cooling water
spraying nozzles 11 which downwardly extend from the upper header
10 toward the steel plate 1, a dividing plate 12 which is
horizontally disposed between the upper header 10 and the steel
plate 1 so as to extend in the width direction of the steel plate
and has many holes, a lower header 13 which supplies cooling water
to the lower surface of the steel plate 1, lower cooling water
spraying nozzles 15 which upwardly extend from the lower header 13
toward the steel plate 1, and squeezing rolls 16 and 17 arranged on
the upstream and downstream sides of the steel plate 1 in the
conveying direction.
[0038] As illustrated in a plan view of the dividing plate 12 of
FIG. 3, the dividing plate 12 has the many (multiple) holes 18
arranged in a grid pattern. The upper cooling water spraying
nozzles 11 are inserted in predetermined holes 18 in a staggered
layout. Lower openings of the holes 18, receiving the upper cooling
water spraying nozzles 11, each serve as a water supply inlet 19.
Lower openings of the holes 18, which do not receive the upper
cooling water spraying nozzles 11, each serve as a drain outlet 20.
The tips of the upper cooling water spraying nozzles 11 are
received in the holes 18 (the water supply inlets 19) such that the
level of each tip is higher than the lower end of the dividing
plate 12. The reason is that the dividing plate 12 prevents the
upper cooling water spraying nozzles 11 from being damaged even if
a steel plate having an upwardly warped end enters. A broken line
in FIG. 3 is parallel to the conveying direction of the steel plate
and both ends of the dividing plate 12 in the width direction of
the steel plate are not illustrated.
[0039] FIG. 4 is a side elevational view of one end of the steel
plate when viewed from the conveying direction of the steel plate.
As illustrated in FIG. 4, cooling water supplied from the upper
cooling water spraying nozzles 11 through the water supply inlets
19 cools the upper surface of the steel plate 1 to turn to
high-temperature drainage water and then flows onto the dividing
plate 12 through the drain outlets 20. Cooling water supplied from
the lower cooling water spraying nozzles 15 cools the lower surface
of the steel plate 1 and flows downward.
[0040] If the dividing plate 12 is not provided, cooling water
supplied to the upper surface of the steel plate 1 is drained while
flowing on the upper surface of the steel plate 1 in the width
direction. The flow of this drainage water prevents cooling water
supplied from the upper cooling water spraying nozzles 11 from
reaching the upper surface of the steel plate 1, particularly in
the vicinity of each end of the plate in the width direction, so
that the cooling performance degrades in the vicinity of the end of
the plate in the width direction and uniform cooling cannot be
performed in the width direction. Accordingly, a temperature
distribution in the width direction of the steel plate 1 is
U-shaped such that the temperature of the center of the plate is
low and the temperature of each end thereof is high. In contrast,
the cooling equipment 6 in the present embodiment is configured
such that drainage water after cooling is immediately drained from
the upper surface of the steel plate 1 onto the dividing plate 12.
Cooling water sprayed from the upper cooling water spraying nozzles
11 sequentially comes into contact with the steel plate 1, thus
providing adequate cooling performance.
[0041] If the water supply inlets 19 and the drain outlets 20 are
the same holes 18, cooling water supplied to the upper surface of
the steel plate 1 does not tend to upwardly pass through the
dividing plate 12, so that the water flows toward the ends of the
steel plate 1 in the width direction between the upper surface of
the steel plate 1 and the dividing plate 12. The flow of drainage
water prevents cooling water supplied from the upper cooling water
spraying nozzles 11 from reaching the upper surface of the steel
plate 1. Disadvantageously, the cooling performance degrades in the
vicinity of the ends of the plate in the width direction and
uniform cooling cannot be performed in the width direction. In
contrast, since the cooling equipment 6, illustrated in FIG. 2, in
the present embodiment is provided with the holes 18 which serve as
the water supply inlets 19 and the drain outlets 20 to share their
functions, cooling water and drainage water after cooling smoothly
flow. Furthermore, since the tips of the upper cooling water
spraying nozzles 11 are received in the holes 18 of the dividing
plate 12, drainage water flowing over the dividing plate 12 in the
width direction does not interfere with cooling water sprayed from
the upper cooling water spraying nozzles 11 and uniform cooling is
achieved in the width direction, thus providing a uniform
temperature distribution in the width direction, as illustrated in
FIG. 6.
[0042] It is preferred that the total area of openings
(hereinafter, referred to as "total cross-sectional area") of the
drain outlets 20 be equal to or more than 1.5 times as large as the
total area of openings (hereinafter, referred to as "total
inner-diameter cross-sectional area") of the upper cooling water
spraying nozzles 11, because cooling water is immediately drained
through the drain outlets 20. If this value is less than 1.5 times,
the flow resistance of each drain outlet increases, so that
remaining water does not tend to be drained onto the dividing
plate. Disadvantageously, the remaining water flows between the
upper surface of the steel plate and the dividing plate toward the
ends of the steel plate in the width direction, thereby degrading
the cooling performance, particularly, in the vicinity of the ends
of the steel plate in the width direction. On the other hand, if
too many drain outlets are arranged or the area of opening
(hereinafter, referred to as "cross-sectional diameter") of each
drain outlet is too large, the stiffness of the dividing plate 12
is lowered. Disadvantageously, the dividing plate 12 tends to be
damaged when hitting against a steel plate. It is therefore
preferred that the ratio of the total cross-sectional area of the
drain outlets 20 to the total inner-diameter cross-sectional area
of the upper cooling water spraying nozzles 11 be in the range of
1.5 to 20.
[0043] In order to allow cooling water to pass through remaining
water, reach a steel plate, and achieve uniform cooling in the
width direction, it is preferred to optimize the inner diameter and
length of each upper cooling water spraying nozzle 11, the spraying
velocity of cooling water, and the distance between nozzles.
[0044] Specifically, the inner diameter of each nozzle is
preferably 3 to 8 mm. If the inner diameter is less than 3 mm, the
flux of water sprayed from the nozzle diminishes and the force of
water becomes weak. Whereas, if the nozzle diameter is greater than
8 mm, the flow rate decreases and the force to pass through the
remaining water becomes weak.
[0045] The length of each upper cooling water spraying nozzle 11 is
preferably 120 to 240 mm. If the upper cooling water spraying
nozzle 11 is shorter than 120 mm, the distance between the lower
surface of the upper header 10 and the upper surface of the
dividing plate 12 is too short, so that a drain space above the
dividing plate 12 is reduced and drainage water after cooling
cannot be smoothly drained. Whereas, if the upper cooling water
spraying nozzle 11 is longer than 240 mm, the pressure loss of the
upper cooling water spraying nozzle 11 increases, so that the force
to pass through remaining water becomes weak.
[0046] The spraying velocity of cooling water from the nozzles is
preferably 6 m/s or higher. If the spraying velocity is less than 6
m/s, the force of cooling water passing through the remaining water
is extremely weakened. It is preferred that the spraying velocity
be 8 m/s or higher, because higher cooling performance is ensured.
The distance between the lower end of each upper cooling water
spraying nozzle 11 and the upper surface of the steel plate 1 is
preferably 30 to 120 mm. If the distance is less than 30 mm, the
frequency of collision of the steel plate 1 with the dividing plate
12 extremely increases. It is therefore difficult to maintain the
facility. If the distance is greater than 120 mm, the force of
cooling water passing through the remaining water is extremely
weakened.
[0047] The water flow rate at which the cooling equipment 6 in the
present embodiment achieves maximum effect is 1.5
m.sup.3/m.sup.2min or higher. If the water flow rate is lower than
this value, the thickness of layer of the remaining water does not
become so thick. Even in the application of known technology to
cool a steel plate while allowing free fall of rod-like cooling
water, in some cases, strip temperature deviation in the width
direction is not so large. Whereas, if the water flow rate is
higher than 4.0 m.sup.3/m.sup.2min, the cooling equipment 6 in the
present embodiment is effectively used but has practical use
problems, for example, high facility cost. The most practical water
flow rate is therefore 1.5 to 4.0 m.sup.3/m.sup.2min.
[0048] The cooling equipment 6 illustrated in FIG. 2 includes the
lower header 13 which is the same as the cooling equipment above
the upper surface and includes the lower cooling water spraying
nozzles 15. During cooling of the lower surface of the steel plate,
sprayed cooling water hits against the steel plate and then freely
falls. The strip temperature deviation in the width direction is
not so a big problem, like the problem on the upper surface of the
steel plate. Therefore, cooling equipment below the lower surface
of the steel plate is not particularly limited.
[0049] The descaler 4 is configured to remove scale produced on
each surface of the steel plate 1 after hot rolling while spraying
high-pressure water from a plurality of spraying nozzles directed
toward the surface of the steel plate 1 subjected to removal of
strain, caused in the steel plate 1, through the first hot leveler
5.
[0050] According to the present embodiment, the pressure P [MPa] at
the point of impact of high-pressure water sprayed from the
spraying nozzles of the descaler 4 to each surface of the steel
plate 1 is set to 1.5 MPa or higher, the descaler 4 removes scale
produced on the surface of the steel plate 1, and after that, the
cooling equipment 6 cools the steel plate 1, thus improving the
steel plate shape and mechanical property of the steel plate 1.
[0051] The reason is as follows. In a related-art hot rolling
facility, if surface treatment by a descaler is omitted after a
steel plate is passed through a leveler, scale may be partially
removed, thus causing a variation in scale thickness distribution
of approximately 10 to 50 .mu.m depending on the presence or
absence of scale removal. In such a case, it is difficult to
uniformly cool the steel plate during cooling by cooling equipment.
Specifically, when the steel plate having a variation in scale
thickness distribution is cooled in the related-art hot rolling
facility, portions with the remaining scale are cooled well and the
temperatures of the portions fall as illustrated in FIG. 7, which
illustrates a temperature distribution from the center of the steel
plate in the width direction thereof. Accordingly, surface
temperature deviation among positions in the width direction is
large and the steel plate cannot be uniformly cooled, thus
affecting the shape and mechanical property of the steel plate.
[0052] In contrast, the inventors found that scale was not
completely removed depending on descaling conditions, rather scale
nonuniformity was accelerated depending on the descaling
conditions. After diligent study of force to cause complete scale
removal, the inventors revealed that when descaling was performed
after hot leveling, scale was uniformly removed completely so long
as the pressure P [MPa] at the point of impact of cooling water
sprayed from the spraying nozzles of the descaler 4 to each surface
of the steel plate 1 was 1.5 MPa or higher, and the thickness of
scale reproduced thereafter was 5 .mu.m or less and was uniform.
Particularly, when the pressure P [MPa] at the point of impact is
set to 2.0 MPa or higher, thin uniform scaling can be achieved.
[0053] As regards the pressure P at the point of impact, for
example, the following expression (1) obtained experimentally is
known and a pressure Pc at the point of impact calculated in this
expression (1) is converted into a value in units of MPa that is an
SI unit:
Pc=0.05757.times.(Q/A).sup.1.08.times.Ps.sup.0.473 (1)
where Pc: pressure at the point of impact [kgf/cm.sup.2], Q: spray
flow rate [L/min], A: spray area [cm.sup.2], and Ps: spraying
pressure [kgf/cm.sup.2].
[0054] The spray area A is obtained using the following expression
(2) by spray experiment:
A=B.times.T=(2H
tan(.theta./2).times.(0.051H.sup.0.78.times.Q.sup.0.09.times.Ps.sup.-0.04-
5) (2)
where B: spraying width [cm] of spray, T: spraying thickness [cm]
of spray, H: spraying distance (the distance between each spraying
nozzle of the descaler 4 and each surface of the steel plate 1)
[cm], and .theta.: spray angle [.degree.] of nozzle (the angle of
spread of descaling water sprayed from the nozzles).
[0055] When Expression (2) is substituted into Expression (1), the
following expression is obtained as an approximate expression.
Pc=0.6775.times.Q.times.H.sup.-2(tan(.theta./2)).sup.-1.08.times.Ps.sup.-
0.5 (3)
[0056] The form of expression to obtain the pressure Pc at the
point of impact is not limited to this expression. Spray experiment
may actually be performed and an expression expressing the
regression of a pressure at a direct cooling point or impact point
measured by a pressure sensor may be used.
[0057] The spraying distance H [cm] to provide a predetermined
pressure at the point of impact is obtained by the following
expression (4) as a deformation of Expression (3):
H=((0.6775.times.Q.times.(tan(.theta./2)).sup.-1.08.times.Ps.sup.0.5)/Pc-
).sup.0.5 (4)
where Pc: pressure [kgf/cm.sup.2] at the point of impact, Q: spray
flow rate [L/min], Ps: spraying pressure [kgf/cm.sup.2], and
.theta.: spray angle [.degree.] of nozzle.
[0058] To set the pressure P [MPa] at the point of impact of spray
to the surface of the steel plate 1 to 1.5 MPa or higher, the
spraying distance H may be at or below a value of H obtained by
substituting Pc=1.5/9.8.times.100=15.3 [kgf/cm.sup.2] into
Expression (4).
[0059] FIG. 8 is a graph illustrating the relationship between the
spraying pressure Ps and the spraying distance H for achieving a
pressure P at the point of impact of 1.5 MPa when the spray flow
rate Q is 64 L/min, the spray angle .theta. of nozzle (the angle of
spread water sprayed) is 32.degree., and the angle between the
spray direction and the vertical line (the angle by which the
center axis of sprayed water is deviated from the vertical
direction relative to the steel plate to the upstream side of the
traveling direction of the steel plate) is 15.degree.. It is found
that when the spraying pressure P is 50 MPa, the spraying direction
H may be less than or equal to 175 mm, when the spraying pressure P
is 30 MPa, the spraying direction H may be less than or equal to
150 mm, when the spraying pressure Ps is 17.7 MPa, the spraying
distance H may be less than or equal to 130 mm, and when the
spraying pressure Ps is 14.7 MPa, the spraying distance may be less
than or equal to 125 mm.
[0060] As the spraying distance H is shorter, the spraying pressure
Ps and the spray flow rate Q for providing the predetermined
pressure P at the point of impact are smaller. Thus, the pumping
performance of the descaler 4 can be reduced. It is therefore
preferred that the spraying distance H be less than or equal to 140
mm. More preferably, the spraying distance H is less than or equal
to 100 mm. In the present embodiment, since the steel plate 1
subjected to leveling through the first hot leveler 5 is moved into
the descaler 4, the spraying nozzles of the descaler 4 can be moved
closer to each surface of the steel plate 1. Preferably, the
spraying distance H is greater than or equal to 40 mm and is less
than or equal to 140 mm in consideration of contact between the
nozzles and the steel plate 1.
[0061] The spraying pressure of a pump used in the normal descaler
4 is less than or equal to 14.7 MPa (150 kgf/cm.sup.2).
Accordingly, a spraying pressure at the tip of each nozzle is
further lower than 14.7 MPa by pressure loss in a path. It is
therefore preferred to use a pump having a spraying pressure that
allows a higher spraying pressure Ps than normal. The upper limit
of the spraying pressure Ps is not especially determined. If the
spraying pressure Ps is set to high, energy required electric power
becomes enormous. It is therefore preferred that the spraying
pressure Ps be less than or equal to 50 MPa. A pump providing a
spraying pressure Ps of 50 MPa exhibits a maximum spraying pressure
among existing commercially available pumps.
[0062] As described above, according to the present embodiment, the
descaler 4, in which the pressure P at the point of impact of
high-pressure water is set to 1.5 MPa or higher, removes scale
produced on the surfaces of the steel plate 1, thereby eliminating
a variation in scale thickness distribution. During cooling of the
steel plate 1 by the cooling equipment 6, therefore, the steel
plate 1 can be uniformly cooled with little surface temperature
deviation among positions in the width direction as illustrated in
FIG. 6. Consequently, the steel plate 1 excellent in steel plate
shape and mechanical property can be manufactured.
[0063] Although strip temperature deviation in the width direction
of a steel plate passed through the cooling equipment without being
subjected to surface treatment by the descaler is approximately
40.degree. C., strip temperature deviation in the width direction
of a steel plate subjected to the above-described descaling
according to exemplary embodiments of the present invention and
then cooled by general cooling equipment is reduced to
approximately 10.degree. C. Moreover, strip temperature deviation
in the width direction of the steel plate 1 passed through the
descaler 4, subjected to descaling according to exemplary
embodiments of the present invention, and then subjected to uniform
cooling in the width direction by the cooling equipment 6,
illustrated in FIG. 2, in the present embodiment is reduced to
approximately 4.degree. C.
[0064] As regards scale on the surfaces of the steel plate 1
affecting stability during cooling of the steel plate 1 by the
cooling equipment 6, it is known that the growth of scale on the
steel plate 1 can be generally expressed as a diffusion controlled
process and is expressed by the following expression (5):
.xi..sup.2=a.times.exp(-Q/RT).times.t (5)
where .xi.: scale thickness, a: constant number, Q: activation
energy, R: constant number, and t: period of time.
[0065] The scale growth was simulated at various temperatures for
various periods of time in consideration of scale growth after
scale removal by the descaler 4, thereby obtaining the constant
numbers in the above-described expression. Furthermore, after
diligent study of scale thickness and cooling stability, it was
found that cooling is stable at a scale thickness of 15 .mu.m or
less, cooling is more stable at a scale thickness of 10 .mu.m or
less, and cooling is very stable at a scale thickness of 5 .mu.m or
less.
[0066] In other words, it became clear that cooling by the cooling
equipment 6 is stable when the period of time t [s] between the
completion of removal of scale on the steel plate 1 by the descaler
4 and the start of cooling of the steel plate 1 by the cooling
equipment 6 satisfies the following expression (6):
t.ltoreq.5.times.10.sup.-9.times.exp(25000/T) (6)
where T: temperature [K] of the steel plate before cooling.
[0067] In addition, it became clear that cooling by the cooling
equipment 6 is more stable when the period of time t [s] between
the completion of removal of scale on the steel plate 1 by the
descaler 4 and the start of cooling of the steel plate 1 by the
cooling equipment 6 satisfies the following expression (7):
t.ltoreq.2.2.times.10.sup.-9.times.exp(25000/T) (7)
[0068] Furthermore, it became clear that cooling by the cooling
equipment 6 is very stable when the period of time t [s] between
the completion of removal of scale on the steel plate 1 by the
descaler 4 and the start of cooling of the steel plate 1 by the
cooling equipment 6 satisfies the following expression (8):
t.ltoreq.5.6.times.10.sup.-10.times.exp(25000/T) (8)
[0069] On the other hand, the distance L between the descaler 4 and
the cooling equipment 6 is set so as to satisfy the following
expression (9) with respect to conveying velocity V of the steel
plate 1 and the period of time t (the period of time between the
completion of processing by the descaler 4 and the start of
processing by the cooling equipment 6).
L.ltoreq.V.times.t (9)
[0070] It is more preferable that the above-described expression
(9) should satisfy the following expression (10) on the basis of
the above-described expression (6).
L.ltoreq.V.times.5.times.10.sup.-9.times.exp(25000/T) (10)
[0071] It is more preferable that the above-described expression
(9) should satisfy the following expression (11) on the basis of
the above-described expression (7).
L.ltoreq.V.times.2.2.times.10.sup.-9.times.exp(25000/T) (11)
[0072] Furthermore, it is preferable that the above-described
expression (9) should satisfy the following expression (12) on the
basis of the above-described expression (8).
L.ltoreq.V.times.5.6.times.10.sup.-10.times.exp(25000/T) (12)
[0073] For example, assuming that the temperature of the steel
plate 1 before cooling by the cooling equipment 6 is 820.degree. C.
and the conveying velocity of the steel plate 1 is 0.28 to 2.50
m/s, cooling is stable when the distance L between the descaler 4
and the cooling equipment 6 is in the range of 12 to 107 m or less,
cooling is more stable when the distance L is in the range of 5 to
47 m or less, and cooling is very stable when the distance L is in
the range of 1.3 to 12 m or less on the basis of the
above-described expressions (10) to (12).
[0074] Accordingly, when it is assumed that the distance L between
the descaler 4 and the cooling equipment 6 is 12 m or less, even if
the conveying velocity V of the steel plate 1 is low (for example,
V=0.28 m/s), cooling is stable. In contrast, when the conveying
velocity V of the steel plate 1 is high (for example, V=2.50 m/s),
cooling is very stable. It is therefore preferable. It is more
preferable that the distance L between the descaler 4 and the
cooling equipment 6 be less than or equal to 5 m.
[0075] Considering that most of steel plates 1 of kinds requiring
controlled cooling are conveyed at a conveying velocity V of 0.5
m/s or higher, it is more preferable that the distance L as a
condition required for very stable cooling at this conveying
velocity V should be less than or equal to 2.5 m.
[0076] As described above, in the hot rolling facility in the
present embodiment, the pressure P [MPa] at the point of impact of
spray from the spraying nozzles of the descaler 4 to each surface
of the steel plate 1 is set to 1.5 or higher to make scale produced
on the steel plate 1 uniform, and uniform cooling is achieved by
the cooling equipment 6, so that the steel plate 1 excellent in
shape and mechanical property can be manufactured.
[0077] In addition, since the steel plate 1 is subjected to hot
leveling by the first hot leveler 5 and scale produced on each
surface of the steel plate 1 is then removed by the descaler 4, the
spraying nozzles of the descaler 4 can be moved closer to each
surface of the steel plate 1. When the spraying distance H (the
distance between each spraying nozzle of the descaler 4 and the
surface of the steel plate 1) is greater than or equal to 40 mm and
less than or equal to 140 mm, the descaling performance is
improved. Alternatively, the spraying pressure Ps, the spray flow
rate Q, and the like for achieving a predetermined pressure P at
the point of impact can be set to low, thus reducing the pumping
performance of the descaler 4.
[0078] When the distance L between the descaler 4 and the cooling
equipment 6 is set so as to satisfy
L.ltoreq.V.times.5.times.10.sup.-9.times.exp(25000/T), cooling of
the steel plate 1 by the cooling equipment 6 can be stabilized.
[0079] When the period of time t [s] between the completion of
removal of scale on the steel plate 1 by the descaler 4 and the
start of cooling of the steel plate 1 by the cooling equipment 6 is
set so as to satisfy
t.ltoreq.V.times.5.times.10.sup.-9.times.exp(25000/T), cooling of
the steel plate 1 by the cooling equipment 6 can be stabilized.
[0080] The cooling equipment 6 in the present embodiment is
configured such that, as illustrated in FIG. 4, cooling water
supplied from the upper cooling water spraying nozzles 11 through
the water supply inlets 19 cools the upper surface of the steel
plate 1 to turn to high-temperature drainage water and the drainage
water flows through the holes 18, which do not receive the upper
cooling water spraying nozzles 11, as drain flow paths onto the
dividing plate 12 in the width direction of the steel plate 1 so
that the drainage water after cooling is immediately removed from
the steel plate 1. Cooling water flowing from the upper cooling
water spraying nozzles 11 through the water supply inlets 19
sequentially comes into contact with the steel plate 1, thereby
providing adequate cooling performance that is uniform in the width
direction.
[0081] As in the present embodiment, strain caused during rolling
is leveled by the first hot leveler 5 and surface treatment is
performed on the steel plate 1 by the descaler 4 to stabilize the
controllability of cooling. Accordingly, the steel plate 1 to be
processed by the second hot leveler 7 originally has high flatness
and the temperature of the steel plate 1 is uniform. The second hot
leveler 7 therefore does not need so high leveling reaction force.
The distance between the cooling equipment 6 and the second hot
leveler 7 may be longer than a maximum length of the steel plate 1
to be manufactured on lines. Since the second hot leveler 7 often
performs reverse leveling or the like, the effect of preventing a
trouble caused when the reversed steel plate 1 bounces on a
conveying roll and hits against the cooling equipment 7 and the
effect of making slight temperature deviation, caused during
cooling, uniform to prevent the occurrence of a warp caused by
temperature deviation after leveling can be expected.
EXAMPLES
[0082] The steel plate 1, rolled by the hot rolling mill 3, having
a thickness of 30 mm and a width of 3500 mm was passed through the
first hot leveler 5 and the descaler 4 and was then controlled such
that the steel plate was cooled from 820.degree. C. to 420.degree.
C. As regards a condition for stable cooling calculated from the
above-described expressions (6), (7), and (8), the period of time t
between the completion of removal of scale on the steel plate 1 by
the descaler 4 and the start of cooling of the steel plate 1 by the
cooling equipment 6 is less than or equal to 42 s, preferably, less
than or equal to 19 s, and more preferably, less than or equal to 5
s.
[0083] As regards the descaler 4, the spraying pressure of nozzles
was 17.7 MPa, the spray flow rate per nozzle was 64 L/min/nozzle,
the spraying distance (the distance between each spraying nozzle of
the descaler 4 and each surface of the steel plate 1) was 130 mm,
the spray angle of nozzle was 32.degree., the angle between the
spray direction and the vertical line was 15.degree., the nozzles
were aligned in the width direction such that the spraying areas of
the neighboring nozzles overlap to some extent, and the pressure at
the point of impact in each position in the width direction was 1.5
MPa.
[0084] The cooling facility 6 was a facility provided with flow
paths configured such that cooling water supplied to the upper
surface of the steel plate flowed over the dividing plate as
illustrated in FIG. 2 and the water was drained on one side in the
width direction of the steel plate as illustrated in FIG. 4. The
dividing plate was provided with 12 mm diameter holes arranged in a
grid pattern such that the water supply inlets arranged in a
staggered layout received the upper cooling water spraying nozzles
and the other holes were used as drain outlets. The distance
between the lower surface of the upper header and the upper surface
of the dividing plate was 100 mm.
[0085] The upper cooling water spraying nozzles each had an inner
diameter of 5 mm, an outer diameter of 9 mm, and a length of 170
mm. The tips of the nozzles projected into the header. The spraying
velocity of rod-like cooling water was 8.9 m/s. Ten rows of nozzles
were arranged in a zone, serving as a 1-m distance between table
rollers, with a 50-mm nozzle pitch in the width direction of the
steel plate. The water flow rate on the upper surface was 2.1
m.sup.3/m.sup.2min. The lower end of each nozzle for upper surface
cooling was placed in the middle between the upper and lower
surfaces of the dividing plate having a thickness of 25 mm such
that the distance between the lower end of the nozzle and the
surface of the steel plate was 80 mm.
[0086] As regards the lower surface cooling facility, as
illustrated in FIG. 2, the same cooling facility as the upper
surface cooling facility was used, except that the facility
included no dividing plate. The spraying velocity and water flow
rate of rod-like cooling water were 1.5 times as high as those for
the upper surface.
[0087] As illustrated in Table 1, the distance L between the
descaler 4 and the cooling equipment 6, the steel plate conveying
velocity V, and the period of time between the descaler 4 and the
cooling equipment 6 were variously changed. In Table 1, descaling
is a process of removing scale on the steel plate 1 by the descaler
4 and controlled cooling is a process of cooling the steel plate 1
by the cooling equipment 6.
TABLE-US-00001 TABLE 1 Period of Time Distance between Descaling
between Descaling Descaling Pressure Descaler Steel Plate and
before at Point of and Cooling Conveying Controlled Controlled
Impact Equipment Velocity Cooling Releveling Cooling MPa L [m] V
[m/s] t [s] Rate % Example 1 of Done 1.5 5 0.28 18 5 Invention
Example 2 of Done 1.5 5 0.6 8 4 Invention Example 3 of Done 1.5 5
1.8 3 2 Invention Example 4 of Done 1.5 13 0.28 46 12 Invention
Example 5 of Done 2.4 2.5 0.8 3 1 Invention Comparative Not done --
-- -- -- 40 Example 1 Comparative Done 0.09 5 0.6 8 70 Example
2
[0088] In each of Examples 1 to 5 (steel plates 1) in Table 1, when
cooled by the cooling equipment 6, the steel plate was uniformly
cooled with little surface temperature deviation among positions in
the width direction as illustrated in FIG. 6, so that the flatness
was excellent, the rate of releveling caused by poor shape was low,
and the surface condition was good.
[0089] Particularly, in Examples 1 to 3 in each of which the
distance between the descaler 4 and the cooling equipment 6 was 5
m, the period of time t between the completion of removal of scale
on the steel plate 1 by the descaler 4 and the start of cooling of
the steel plate 1 by the cooling equipment 6 was less than or equal
to 19 S that was the condition for more stable cooling by the
cooling equipment 6 based on the above-described expression (6),
irrespective of the steel plate conveying velocity V. The
releveling rate was less than or equal to 5%, namely, it was
good.
[0090] In Example 5 in which the distance between the descaler 4
and the cooling equipment 6 was 2.5 m, the spraying pressure of
nozzles was 17.7 MPa, the spray flow rate per nozzle was 64
L/min/nozzle, the spraying distance (the distance between each
spraying nozzle of the descaler 4 and each surface of the steel
plate 1) was 90 mm, the spray angle of nozzle was 40.degree., the
angle between the spray direction and the vertical line was
15.degree., and the pressure at the point of impact was thereby 2.4
MPa, the releveling rate was 1%, namely, it was very good.
[0091] On the other hand, in Comparative Example 1 in which scale
removal by the descaler 4 was not done and cooling by the cooling
equipment 6 was performed, the flatness was degraded, which may be
caused by temperature distribution of the steel plate. The
releveling rate was 40%.
[0092] In Comparative Example 2 in which water pressure was 10 MPa,
the spray flow rate per nozzle was 10 L/min/nozzle, the spraying
distance was 180 mm, the spray angle of nozzle was 25.degree., the
angle between the spray direction and the vertical line was
15.degree., and the pressure at the point of impact was 0.09 MPa as
setting conditions for the descaler 4, scale was partially removed,
so that the temperature distribution in the width direction of the
steel plate was degraded. The releveling rate was 70%.
REFERENCE SIGNS LIST
[0093] 1 steel plate, 2 heating furnace, 3 hot rolling mill, 4
descaler, 5 first hot leveler (hot leveler), 6 cooling equipment, 7
second hot leveler, 10 upper header (header), 11 upper cooling
water spraying nozzle (cooling water spraying nozzle, 12 dividing
plate, 13 lower header, 15 lower cooling water spraying nozzle, 16
and 17 squeezing rolls, 18 hole, 19 water supply inlet, and 20
drain outlet.
* * * * *