U.S. patent application number 10/497189 was filed with the patent office on 2005-01-27 for continous hot-rolling facility.
Invention is credited to Adachi, Akio, Komatsu, Takayoshi, Kurahashi, Ryurou, Takahashi, Masanori, Takaoka, Shinji, Ueno, Shinji.
Application Number | 20050016242 10/497189 |
Document ID | / |
Family ID | 34074178 |
Filed Date | 2005-01-27 |
United States Patent
Application |
20050016242 |
Kind Code |
A1 |
Kurahashi, Ryurou ; et
al. |
January 27, 2005 |
Continous hot-rolling facility
Abstract
A continuous hot-rolling mill includes an upper rolling unit
including a plurality of rolling stands, and a lower rolling unit
including a plurality of rolling stands and disposed downstream
with respect to the upper rolling unit in a workpiece passing
direction in which a workpiece to be rolled is passed. The two or
more rolling stands of the lower rolling unit are differential or
very-small-diameter rolling stands. Each of the two or more
differential or very-small-diameter rolling stands of the lower
rolling unit is provided with a drive motor having a capacity
greater than that of any one of drive motors included in the
rolling stands disposed upstream the differential or
very-small-diameter rolling stands. The continuous hot-rolling mill
is suitable for producing hot-rolled, fine-grained steel sheets and
is excellent in sheet passing abilities for preventing the steel
sheet from meandering and for rolling the steel sheet in a desired
shape.
Inventors: |
Kurahashi, Ryurou;
(Amagasaki-Shi, JP) ; Ueno, Shinji; (Osaka-Shi
Osaka-Fu, JP) ; Komatsu, Takayoshi; (Sakai-Shi
Osaka-Fu, JP) ; Takahashi, Masanori; (Kobe-Shi
Hyogo-Ken, JP) ; Adachi, Akio; (Akashi-Shi Hyogo-Ken,
JP) ; Takaoka, Shinji; (Kobe-Shi Hyogo-Ken,
JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 19928
ALEXANDRIA
VA
22320
US
|
Family ID: |
34074178 |
Appl. No.: |
10/497189 |
Filed: |
May 28, 2004 |
PCT Filed: |
September 30, 2002 |
PCT NO: |
PCT/JP02/10193 |
Current U.S.
Class: |
72/234 |
Current CPC
Class: |
B21B 45/0233 20130101;
B21B 2275/12 20130101; B21B 1/26 20130101; B21B 2267/06 20130101;
B21B 2267/065 20130101; B21B 2275/10 20130101; B21B 13/142
20130101 |
Class at
Publication: |
072/234 |
International
Class: |
B21B 013/08 |
Claims
1. A continuous hot-rolling mill comprising: an upper rolling unit
including a plurality of rolling stands; and a lower rolling unit
including a plurality of rolling stands and disposed downstream
with respect to the upper rolling unit in a workpiece passing
direction in which a workpiece to be rolled is passed; wherein two
or more rolling stands of the lower rolling unit are differential
or very-small-diameter rolling stands, each of the two or more
differential or very-small-diameter rolling stands of the lower
rolling unit is provided with a drive motor having a capacity
greater than a capacity of any one of drive motors included in the
rolling stands disposed upstream with respect to the differential
or very-small-diameter rolling stands.
2. The continuous hot-rolling mill according to claim 1, wherein
successive two or more rolling stands of the lower rolling unit
including a last rolling stand are the differential or
very-small-diameter rolling stands.
3. The continuous hot-rolling mill according to claim 1, wherein
the lower rolling unit includes three or more rolling stands, and
two or three rolling stands among the three or more rolling stands
of the lower rolling unit are the differential or
very-small-diameter rolling stands.
4. The continuous hot-rolling mill according to claim 1, wherein at
least one of the rolling stands of either the upper or the lower
rolling unit has a CVC function.
5. The continuous hot-rolling mill according to claim 1, wherein
the drive motors of the two or more differential or
very-small-diameter rolling stands of the lower rolling unit have
different capacities, respectively, and drive motors of lower
rolling stands have capacities not lower than capacities of drive
motors of upper rolling stands.
6. The continuous hot-rolling mill according to claim 5, wherein
three rolling stands including a last rolling stand of the lower
rolling unit are differential or very-small-diameter rolling
stands, respective capacities of the drive motors of the
differential or very-small-diameter rolling stands meet a condition
expressed by:P.sub.n>P.sub.n-1.gtoreq.- P.sub.n-2 or
P.sub.n.gtoreq.P.sub.n-1>P.sub.n-2where P.sub.n, P.sub.n-1 and
P.sub.n-2 are capacities of the drive motors of the last rolling
stand, a second last rolling stand precedent to the last rolling
stand, and a third last rolling stand precedent to the second last
rolling stand, respectively.
7. The continuous hot-rolling mill according to claim 5, wherein
respective capacities of the drive motors of the two or more
differential or very-small-diameter rolling stands of the lower
rolling unit are greater by 15% or above than any one of capacities
of the drive motors of the rolling stands disposed upstream with
respect to the differential or very-small-diameter rolling
stands.
8. The continuous hot-rolling mill according to claim 1, wherein a
maximum capacity among respective capacities of the drive motors of
the two or more differential or very-small-diameter rolling stands
of the lower rolling unit is greater by 30% or above than any one
of capacities of the drive motors of the rolling stands disposed
upstream with respect to the differential or very-small-diameter
rolling stands.
9. The continuous hot-rolling mill according to claim 1, wherein
water-curtain cooling means are disposed at respective exits of the
two or more differential or very-small-diameter rolling stands of
the lower rolling unit in order to cool a rolled workpiece.
Description
TECHNICAL FIELD
[0001] The present invention relates to a continuous hot-rolling
mill suitable for manufacturing a hot-rolled fine-grained steel
sheet of fine structure mainly of fine ferrite grains.
BACKGROUND ART
[0002] In hot-rolling a thin steel sheet by a continuous
hot-rolling mill (finishing mill) formed by arranging five to seven
rolling stands in a tandem arrangement, a pass schedule is
designed, in most cases, to place a maximum rolling load on the
second or the third rolling stand. Such a pass schedule is
recommended because a) the workpiece fed to the first rolling stand
is thick and the workpiece is not pushed from the upper side into
the first rolling stand, and hence the leading end of the thick
workpiece cannot be gripped between the rolls if rolling force is
excessively high, and b) the workpiece tends to meander and the
shape of the workpiece, such as flatness, is deteriorated if an
excessively high rolling force is exerted on the workpiece by the
lower rolling stands at a lower stage where the workpiece is thin.
A pass schedule specifying such a rolling force distribution is
mentioned in, for example, Jpn. Pat. No. 2635796 (FIGS. 2 and
3).
[0003] The capacity of a drive motor of each of the rolling stands
of the conventional continuous hot-rolling mill that is operated on
the basis of such a pass schedule is determined by one of the
following methods.
[0004] i) According to the rolling force distribution specified in
the foregoing pass schedule, the output torque of the motors of the
lower rolling stands may be smaller than that of the motors of the
upper rolling stands, and hence the second rolling stand or the
upper rolling stands including the second rolling stand are
provided with a motor having a maximum capacity, and the lower
rolling stands are provided with a motor having a small
capacity.
[0005] ii) All the rolling stands are provided with motors having
the same capacity and a proper rolling force distribution is
designed.
[0006] iii) The preceding rolling stand is provided with a motor
having a capacity slightly greater than that of the motor of the
succeeding rolling stand. The capacities of the motors are thus
determined, taking into consideration the increase of rolling speed
with the reduction of the thickness of the workpiece and the
increase of the rotating speeds of the rolling rolls having smaller
diameters of the lower rolling stands, in addition to rolling force
distribution.
[0007] The conventional continuous hot-rolling mill has
difficulties in producing a hot-rolled, fine-grained steel sheet
having fine ferrite structures and high mechanical properties.
Hot-rolled, fine-grained steel sheets are manufactured by a
high-draft rolling method or a controlled rolling method. Either of
those rolling methods requires several rolling stands of a
finishing mill to perform high-draft rolling. However, the
conventional continuous hot-rolling mill having the rolling stands
provided with motors having the capacities explained above has
difficulty in carrying out such a rolling method.
[0008] The high-draft rolling method makes fine structure by
applying a high rolling force to austenite grains to promote
strain-induced transformation from the austenitic (.gamma.) phase
to the ferritic (.alpha.) phase. The controlled rolling method
reduces the grain size of ferrite grains of a low alloy steel
containing Nb or Ti by promoting strain-induced transformation from
the .gamma. phase to the .alpha. phase by the austenite grain
recrystallization suppressing effect of Nb or Ti when the low alloy
steel is subjected to low-temperature rolling (rolling in the
ferrite region) in addition to enhancing the tensile strength of
the low alloy steel by the precipitation strengthening effect of Nb
or Ti.
[0009] The reason the conventional hot-rolling mill has
difficulties in achieving a rolling method to produce fine-grained
steel sheets is as follows.
[0010] According to an investigation made by the inventors of the
present invention, a rolling method that uses a high rolling force
to produce a hot-rolled, fine-grained steel sheet must cause a
cumulative strain of 0.9 or above by three lower rolling stands.
Strain .epsilon. is expressed by:
.epsilon.=(h.sub.0-h.sub.1)/{(h.sub.0+h.sub.1)/2}
[0011] where h.sub.0 is the thickness of a steel sheet at the
entrance of the rolling stand, and h.sub.1, is the thickness of the
steel sheet at the exit of the same rolling stand. Cumulative
strain is the sum of weighted strains caused by three (or two)
lower rolling stands weighted by factors determined taking into
consideration the effect of the rolling stands on metal structure
and neglecting the effect of upper rolling stands because the
effect of the upper rolling stands is insignificant. A cumulative
strain .epsilon..sub.c is expressed by:
.epsilon..sub.c=.epsilon..sub.n+.epsilon..sub.n-1/2+.epsilon..sub.n-2/4
[0012] where .epsilon..sub.n is a strain caused by the last rolling
stand, .epsilon..sub.n-1 is a strain caused by the second last
rolling stand preceding the last rolling stand, and
.epsilon..sub.n-2 is a strain caused by the third last rolling
stand preceding the second last rolling stand.
[0013] To roll a steel sheet at a high draft to cause a cumulative
strain of 0.9 or above, each of the three lower rolling stands
needs to roll the steel sheet at a draft of about 40% or above (a
strain of 0.5 or above). In the conventional hot-rolling mill that
operates on the basis of the aforesaid pass schedule, a draft set
for each of the three lower rolling stands is only on the order of
30%, and the lower rolling stands are unable to exert a high
rolling force for a high draft as high as 40% to achieve a
cumulative strain of 0.9 or above when the capacities of the motors
are determined as mentioned in i).
[0014] Even if the lower rolling stands are provided with motors
each having a relatively large capacity as mentioned in ii and
iii), the lower rolling stands do not have a rolling capacity to
roll the steel sheet at a high draft by using a high rolling force.
Even if the lower rolling stands is provided with a motor having a
large capacity, most part of the capacity, in general, is allocated
to drive the rolling rolls for rotation at high rotating speeds.
Even if the motor of the lower rolling stand has an excess capacity
and the lower rolling stand is able to roll the steel sheet at a
sufficiently high draft, it is impossible to solve problems
relating to the meandering and shape deterioration of thin steel
sheets
[0015] Accordingly, it is an object of the present invention to
provide a continuous hot-rolling mill suitable for producing a
hot-rolled, fine-grained steel sheet, and excellent in sheet
passing ability (meandering preventing ability) and preventing the
deterioration of the shape of the steel sheet.
DISCLOSURE OF THE INVENTION
[0016] According to the present invention, a continuous hot-rolling
mill includes: an upper rolling unit including a plurality of
rolling stands; and a lower rolling unit including a plurality of
rolling stands and disposed downstream with respect to the upper
rolling unit in a workpiece passing direction in which a workpiece
to be rolled is passed; wherein the two or more rolling stands of
the lower rolling unit are differential or very-small-diameter
rolling stands, each of the two or more differential or
very-small-diameter rolling stands is provided with a drive motor
having a capacity greater than that of any one of drive motors
included in the rolling stands disposed upstream with respect to
the differential or very-small-diameter rolling stands.
[0017] Preferably, the successive two or more rolling stands of the
lower rolling unit including the last rolling stand are the
differential or very-small-diameter rolling stands.
[0018] Preferably, the lower rolling unit includes three or more
rolling stands, and the two or three rolling stands among the three
or more rolling stands are differential or very-small-diameter
rolling stands.
[0019] The term "very-small-diameter rolling stand" signifies a
rolling stand provided with a pair of work rolls of a diameter
below 600 mm. The term "differential rolling stand" signifies a
rolling stand provided with a pair of work rolls respectively
having different diameters, and the equivalent diameter of the pair
of work rolls, namely, the mean of the respective diameters of the
pair of work rolls, is below 600 mm. Desirably, the equivalent
diameter of the differential rolling stand or the diameter of the
rolls of the very-small-diameter rolling stand is 550 mm or below
from the viewpoint of function, and, in general, 400 mm or above
from the viewpoint of strength.
[0020] In this continuous hot-rolling mill, the drive motors of the
two or more differential or very-small-diameter rolling stands of
the lower rolling unit have a capacity greater than that of any one
of the drive motors of the rolling stands disposed upstream with
respect to the differential or very-small-diameter rolling stands.
Therefore, the lower rolling unit, which influences metallographic
structure greatly, is able to achieve high-draft rolling. Since the
lower rolling unit of this tandem rolling mill has the two or more
differential or very-small-diameter rolling stands, the tandem
rolling mill is able to roll a thin sheet at a high draft without
causing the thin sheet to meander or deteriorating the shape of the
sheet because the tandem rolling mill of this type is capable of
achieving rolling at a high draft (and a large strain) by using a
comparatively low rolling force. When the sheet is rolled by a low
rolling force, lateral force (thrust) exerted on the sheet is low
and hence the sheet is caused to meander scarcely, and the adverse
effect of rolling on the shape of the sheet, such as edge drop, can
be reduced because the flat deformation of the work rolls is
reduced. Since problems relating to the passage and shape of the
sheet arise scarcely, the lower rolling unit is able to raise the
draft considerably according to the capacity of the drive motor and
to roll the sheet at a cumulative strain of 0.9 or above. Thus, the
tandem rolling mill is capable of producing a hot-rolled,
fine-grained steel sheet having fine structures mainly of fine
ferrite grains.
[0021] Preferably, at least one of the rolling stands of either the
upper or the lower rolling unit has a CVC function.
[0022] The term "CVC function" signifies a function to change and
control roll gap by axially moving a work roll (CVC roll) having an
axially continuously changing diameter. The rolling stand having
the CVC function is called a CVC rolling stand.
[0023] The lower rolling unit of the continuous hot-rolling mill
has a satisfactory control characteristic relating to controlling
the passage and shape of the sheet. Since the rolling stand having
the CVC function is able to change and control roll gap shape in a
wide range, the bending and the crowning of the rolls due to
thermal expansion can be prevented to control the shape of the
sheet effectively. Accordingly, the passage of the sheet through
the lower rolling unit can be stabilized. When the lower rolling
unit includes the CVC rolling stand, the passage and shape of the
sheet can be directly and finely controlled at a stage near the
delivery of a finished sheet. When the upper rolling unit includes
the CVC rolling stand, the comparatively thick sheet can be
controlled in a wide control range.
[0024] Preferably, the respective capacities of the drive motors of
the two or more differential or very-small-diameter rolling stands
of the lower rolling unit are determined such that all the drive
motors have different capacities, respectively, and the drive
motors of the lower rolling stands have capacities not lower than
those of the drive motors of the upper rolling stands.
[0025] Preferably, the three rolling stands including the last
rolling stand of the lower rolling unit are differential or
very-small-diameter rolling stands, the respective capacities of
the drive motors of the differential or very-small-diameter rolling
stands meet a condition expressed by:
P.sub.n>P.sub.n-1.gtoreq.P.sub.n-2 or
P.sub.n.gtoreq.P.sub.n-1>P.sub- .n-2
[0026] where P.sub.n, P.sub.n-1 and P.sub.n-2 are the capacities of
the drive motors of the last rolling stand, the second last rolling
stand precedent to the last rolling stand, and the third last
rolling stand precedent to the second last rolling stand,
respectively.
[0027] It is effective to increase the strain .epsilon. (or the
draft) toward the last rolling stand to produce a hot-rolled,
fine-grained steel sheet by rolling the steel sheet by the lower
rolling unit at high drafts. The influence of the rolling action of
the upper rolling stand on the metallographic structure of the
sheet is lower than that of the last rolling stand or the rolling
stand near the last rolling stand. Therefore it is advantageous to
operate the last rolling stand or the rolling stand near the last
rolling stand at a high draft to produce a sheet having the same
metallographic structure without greatly increasing the average
draft of drafts for all the rolling stands. Therefore, the
continuous hot-rolling mill is capable of producing a hot-rolled,
fine-grained steel sheet efficiently, particularly in respect of
equipment cost and energy consumption.
[0028] Preferably, the respective capacities of the drive motors of
the two or more differential or very-small-diameter rolling stands
of the lower rolling unit are greater by 15% or above than those of
the drive motors of the rolling stands disposed upstream with
respect to the differential or very-small-diameter rolling
stands.
[0029] For example, when the successive three rolling stands
including the last rolling stand of the lower rolling unit are
differential or very-small-diameter rolling stands, the drive
motors of those three rolling stands meet a condition expressed by
an inequality:
P.sub.n.gtoreq.Max(P.sub.1, P.sub.2, . . . ,
P.sub.n-3).times.1.15
[0030] where P.sub.1, P.sub.2, . . . , P.sub.n-3 are the respective
capacities of the drive motors of the first, the second, . . . and
the (n-3)th rolling stand.
[0031] Preferably, the maximum capacity among the respective
capacities of the drive motors of the two or more differential or
very-small-diameter rolling stands of the lower rolling unit is
greater by 30% or above than those of the drive motors of the
rolling stands disposed upstream with respect to the differential
or very-small-diameter rolling stands.
[0032] For example, when the three successive rolling stands of the
lower rolling unit are differential or very-small-diameter rolling
stands, the drive motors of the rolling stands meet a condition
expressed by:
Max(P.sub.n-2, P.sub.n-1, P.sub.n).gtoreq.Max(P.sub.1, P.sub.2, . .
. , P.sub.n-3).times.1.3
[0033] where P.sub.n, P.sub.n-1 and P.sub.n-2 are the respective
capacities of the drive motors of the three successive rolling
stands, and P.sub.1, P.sub.2, . . . , P.sub.n-3 are the respective
capacities of the drive motors of the first, the second, . . . ,
and the (n-3)th rolling stand.
[0034] As mentioned above, in view of the influence of rolling
action on the metallographic structure, the upper rolling stands of
the continuous hot-rolling mill do not need to roll the steel sheet
at particularly high draft in producing a hot-rolled, fine-grained
steel sheet. On the other hand, it is preferable that the lower
rolling stands including the last rolling stand roll the steel
sheet at high drafts to increase the cumulative strain to 0.9 or
above. Therefore, the continuous hot-rolling mill including the two
or more lower rolling stands, namely, the differential or
very-small-diameter rolling stands, provided with the drive motors
having the capacities considerably larger than those of the motors
of the upper rolling stands is a reasonable, viable continuous
hot-rolling mill capable of smoothly producing a hot-rolled,
fine-grained steel sheet. Since the rolling stands of the upper
rolling unit that roll the steel sheet at low drafts are provided
with the drive motors having capacities considerably smaller than
those of the drive motors of the rolling stands of the lower
rolling unit, the continuous hot-rolling mill is economically
advantageous. It is particularly advantageous in equipment cost and
energy consumption to provide the last rolling stand with a motor
having the maximum capacity for the aforesaid reasons.
[0035] Preferably, water-curtain cooling means are disposed at the
exits of the two or more differential or very-small-diameter
rolling stands of the lower rolling unit to cool the rolled
workpiece.
[0036] Each of the water-curtain cooling means is capable of
pouring a cooling water at a high pouring rate in water-curtains of
laminar or substantially laminar water streams from above and from
below the sheet toward the sheet such that the upper and the lower
surface of the sheet are wetted entirely with the cooling water for
cooling.
[0037] In producing a hot-rolled, fine-grained steel sheet by the
continuous hot-rolling mill, it is desirable to cool the steel
sheet strongly at the rolling stands that roll the steel sheet at
high drafts because the steel sheet is often heated at a high
temperature not suitable for the high-draft rolling method or the
controlled rolling method by heat generated by rolling when the
steel sheet is rolled at a high draft. The rolling speed needs to
be reduced and the commercial production of hot-rolled,
fine-grained steel sheets could be impossible if the steel sheets
are not cooled sufficiently effectively.
[0038] The water-curtain cooling means cools the steel sheet
effectively by pouring water in water-curtains at a high pouring
rate to suppress the rise of the temperature of the sheet
effectively when the sheet is rolled at a high draft. Even if the
rolling speed is increased, the sheet can be maintained at a
temperature in a proper temperature range. The proper temperature
range is between the Ar.sub.3 transformation temperature and a
temperature equal to the Ar.sub.3 transformation temperature plus
50.degree. C. for the high-draft rolling method and between
700.degree. C. and 800.degree. C. for the controlled rolling
method.
[0039] The water-curtain cooling means are disposed not only at the
exit of the last rolling stand but also at the respective exits of
the plurality of rolling stands of the lower rolling unit. Thus,
heat generated in the steel sheet by rolling at the rolling stands
including the last rolling stand is removed effectively to maintain
the steel sheet at a proper temperature, and the sheet is cooled
strongly immediately after rolling by the last rolling stand to
stop the growth of grains of the fine structure. Since the
water-curtain cooling means pour cooling water over the entire
width of the rolled steel sheet, the rolled steel sheet can be
uniformly cooled.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a typical side elevation of a continuous
hot-rolling mill A in a preferred embodiment according to the
present invention;
[0041] FIGS. 2A, 2B and 2C are typical views of assistance in
explaining the CVC function of a rolling stand F1 included in an
upper rolling unit in the continuous hot-rolling mill A shown in
FIG. 1;
[0042] FIG. 3 is a side elevation of rolling stands F4 to F6
included in a lower rolling unit in the continuous hot-rolling mill
A shown in FIG. 1;
[0043] FIG. 4 including (a) and (b) shows sections of upper and
lower surface layers, respectively, of a hot-rolling steel sheet;
and
[0044] FIG. 5 is a graph showing necessary torques for drive motors
included in the rolling stands F1 to F6 calculated on the basis of
a pass schedule, and expectedly suitable output torques of the
drive motors corresponding to the necessary torques,
respectively.
BEST MODE FOR CARRYING OUT THE INVENTION
[0045] A continuous hot-rolling mill A shown in FIG. 1 is a
finish-rolling mill. A heating furnace and a roughing rolling mill,
which are not shown, are disposed upstream with respect to the
continuous hot-rolling mill A. A runout table and a coiler, which
are not shown, are disposed downstream with respect to the
continuous hot-rolling mill A. The continuous hot-rolling mill A
includes six rolling stands F1 to F6 each provided with rolling
rolls. The rolling stands F1 to F6 are arranged in a tandem
arrangement. The continuous hot-rolling mill A rolls a rough-rolled
steel sheet P continuously to produce a hot-rolled steel sheet of a
thickness in the range of about 1 to about 6 mm. The continuous
hot-rolling mill A is designed to carry out both an ordinary
rolling process for producing general steel sheets and a
fine-graining rolling process for producing hot-rolled,
fine-grained steel sheets having a fine ferrite structure. The
construction of the continuous hot-rolling mill A will be
described.
[0046] The three upper rolling stands F1, F2 and F3, namely,
rolling stands included in an upper rolling unit, are CVC rolling
stands. The rolling stands F1, F2 and F3 are arranged in a tandem
arrangement. As shown in FIG. 1, the first rolling stand F1, i.e.,
the uppermost rolling stand, is a four-high rolling mill having two
work rolls 1a and 1b, and two backup rolls 1c and 1d. As shown in
FIG. 2A, the work rolls 1a and 1b are crowned (CVC, i.e.,
continuous variable crown). The work rolls 1a and 1b can be
simultaneously axially moved in opposite directions, respectively,
as shown in FIGS. 2B and 2C for the adjustment of the positional
relation between the work rolls 1a and 1b to adjust the gap between
the working rolls 1a and 1b. The respective diameters of the work
rolls 1a and 1b are 700 mm. The work rolls 1a and 1b can move
axially from their reference positions in the range of .+-.100 mm.
The second rolling stand F2 and the third rolling stand F3 of the
upper rolling unit, namely, CVC rolling stands, are the same in
construction and function as the first rolling stand F1.
[0047] The upper rolling unit includes the CVC rolling stands F1,
F2 and F3 to crown (to shape) the steel sheet P properly. The lower
rolling stands F4, F5 and F6 included in a lower rolling unit are
differential rolling mills. Thermal crowning is liable to occur due
to working heat generated by fine-graining rolling. Therefore, the
crown of the steel sheet P is corrected by the CVC rolling stands
F1, F2 and F3 of the upper rolling unit to reduce center buckle and
such in the steel sheet P.
[0048] As shown typically in FIG. 1, drive motors M1a and M1b
(generally denoted by M1") are connected to the work rolls 1a and
1b of the CVC rolling stand F1, respectively, drive motors M2a and
M2b (M2) are connected to the work rolls 1a and 1b of the CVC
rolling stand F2, respectively, and drive motors M3a and M3b (M3)
are connected to the work rolls 1a and 1b of the CVC rolling stand
F3, respectively. The drive motors M1, M2 and M3 are ac motors
provided respectively with variable-speed controllers. The motors
M1, M2 and M3 are connected through reduction gears, not shown, and
universal joints to the work rolls 1a and 1b of the rolling stands
F1, F2 and F3, respectively.
[0049] The three lower rolling stands F4, F5 and F6, namely, the
differential rolling mills, of the lower rolling unit are arranged
in a tandem arrangement. The six rolling stands, namely, the
rolling stands F1, F2 and F3 (the CVC rolling mills) and the
rolling stands F4, F5 and F6 are arranged at equal intervals of 5.5
m. As shown in FIG. 1, the fourth rolling stand F4, namely, the
differential rolling stand, is a four-high rolling mill provided
with two work rolls 4a and 4b and two backup rolls 4c and 4d. The
work rolls 4a and 4b have different diameters, respectively. The
upper work roll 4a is a small work roll, and the lower work roll 4b
is a large work roll. Only the large lower work roll 4b is driven
by a drive motor M4 (ac motor with a variable-speed controller).
The drive motor M4 is connected through a reduction gear, not
shown, and a universal joint to the lower work roll 4b. The small
upper work roll 4a is a free roll. Benders, not shown, are combined
with the work rolls 4a and 4b for bending the work rolls 4a and 4b.
The work rolls 4a and 4b have the CVC function, and can move
axially in opposite directions from their reference positions in
the range of .+-.100 mm. The diameters of the upper work roll 4a
and the lower work roll 4b are 480 mm and 600 mm, respectively.
Equivalent roll diameter equal to the mean of the respective
diameters of the work rolls 4a and 4b is as small as 540 mm. The
other differential rolling stands F5 and F6 are the same in
construction and function as the differential rolling stand F4.
Drive motors M5 and M6 are connected to the lower work rolls 4b of
the rolling stands F5 and F6, respectively.
[0050] Thus, the three lower rolling stands F4, F5 and F6 have the
small equivalent roll diameters and only the lower work rolls 4b
are driven to exert a shearing force to the steel sheet P.
Therefore, the lower rolling stands F4, F5 and f6 are able to roll
the steel sheet P at a draft of, for example, 50% by a
comparatively low rolling force. Thus, the lower rolling stands F4,
F5 and F6 are capable of achieving fine-graining, high-draft
rolling by a low rolling force, and of avoiding problems
attributable to edge drop and the flat deformation of the work
rolls because the rolling stands F4, F5 and F6 need to exert a low
rolling force.
[0051] The steel sheet P needs to be maintained at temperatures in
a proper temperature range by sufficient cooling to achieve the
continuous fine-graining rolling. Water-curtain cooling devices 7
(reference numbers 7A to 7H shown in FIG. 3) are disposed behind
the lower rolling stands F4, F5 and F6, respectively, as shown in
FIG. 1, or water-curtain cooling devices 7A to 7H are disposed
behind the lower rolling stands F4, F5 and F6, i.e., on the
downstream side of the lower rolling stands F4, F5 and F6, and in
front of the lower rolling stands F4, F5 and F6. i.e., on the
upstream side of the lower rolling stands F4, F5 and F6, as shown
in FIG. 3. Each of the water-curtain cooling devices 7 has upper
and lower headers disposed above or below, respectively, of the
steel sheet P, and is capable of pouring cooling water of an
ordinary temperature on the surfaces of the steel sheet P at a high
pouring rate in water-curtains (water-curtains f in FIG. 3),
namely, laminar water streams, over the entire width of the steel
sheet P. The thickness of the water-curtains is 10 mm or above,
desirably, about 16 mm, from the viewpoint of the cooling effect of
the water-curtains. The pouring rate at which each of the curtain
wall cooling devices 7 pours cooling water on a unit width of 1 m
of the steel sheet P is adjustable in the range of 100 to 500
m.sup.3/h. The pouring rate is adjusted such that the steel sheet P
is cooled at a cooling rate of 20.degree. C./s or above. When the
steel sheet P is rolled by the high-draft rolling method, the
cooling water is poured on the steel sheet P at a pouring rate for
unit width of 350 m.sup.3/h, and the product of the thickness and
the speed of the steel sheet P is 1200 mm.multidot.mpm, the
temperature of the steel sheet P drops at a temperature drop rate
in the range of 60 to 80.degree. C./s (about 40.degree. C./sec when
the steel sheet P is heated by heat generated by rolling).
[0052] As shown in FIG. 3, the cooling devices 7 are disposed above
and below the steel sheet. The cooling devices 7A, 7B, 7D, 7E and
7B are disposed above the steel sheet P near the exit of the
rolling stand 4F, near the entrance and the exit of the rolling
stand F5, and near the entrance and the exit of the rolling stand
F6, respectively. The cooling devices 7C, 7F and 7H are disposed
below the steel sheet P near the exits of the rolling stands F4, F5
and F6, respectively. The cooling device 7H is attached to the
frame of a roller table T disposed downstream with respect to the
sixth rolling stand F6, and the other cooling devices 7A to 7G are
attached to the housings H of the rolling stands F4, F5 and F6.
[0053] The cooling devices 7 disposed near the exits of the three
lower rolling stands F4, F5 and F6 controls the excessive rise of
the temperature of the steel sheet P rolled by the rolling stands
F4, F5 and F6 by the high-draft rolling method that generates a
large amount of heat or the controlled rolling method to maintain
the temperature of the steel sheet P in the proper temperature
range and stops the growth of grains of the fine structure after
rolling. The steel sheet P is cooled with cooling water to prevent
the growth of grains at the runout table, not shown, disposed
downstream with respect to the continuous hot-rolling mill A.
[0054] The continuous hot-rolling mill A shown in FIG. 1 includes a
spray device 8 at a distance in the range of several hundred
millimeters and one meter from the water-curtain cooling devices 7,
namely, the water-curtain cooling devices 7G and 7H disposed near
the exit of the last rolling stand F6. The spray device 8 jets
water obliquely forward by pressure onto the surfaces of the steel
sheet P to remove the cooling water poured by the cooling devices
7G and 7H on the steel sheet P and remaining on the steel sheet P.
Thus, the cooling water remaining on the steel sheet P can be
efficiently removed and, consequently, instruments including a
thermometer, not shown, disposed downstream with respect to the
spray device 8 are able to measure data about the steel sheet P
accurately.
[0055] The continuous hot-rolling mill A is capable of producing a
hot-rolled, fine-grained steel sheet at a sufficiently high rolling
speed to ensure proper productivity. The lower differential rolling
stands F4, F5 and F6 provided with the small-diameter rolls, which
influence greatly the metallographic structure of the rolled steel
sheet, roll the steel sheet P at high drafts by the high-draft
rolling method or the controlled rolling method, maintaining the
temperature of the steel sheet P in the proper temperature range by
means of the water-curtain cooling devices 7. The rolling stands
F4, F5 and F6 are capable of preventing edge drop and the flat
deformation of the work rolls, and the rolling stands F1 to F6
having the CVC function are capable of controlling crowning.
Consequently, the meandering of the steel sheet P and the
deterioration of the shape of the steel sheet P in the lower
rolling stands where the steel sheet P are thin can be prevented,
so that fine-graining rolling can be successfully achieved.
[0056] The drive motors of the lower rolling stands, particularly,
the drive motor M6 of the lowermost rolling stand, such as the
rolling stand F6, or the drive motor M5 of the rolling stand
adjacent to the last rolling stand, such as the rolling stand F5,
must have a sufficient capacity (power (kW)) to achieve the
high-draft rolling by the rolling stands of the lower rolling unit.
When the steel sheet P is rolled by high-draft rolling, a high
rolling force per unit width needs to be applied to the steel sheet
P, a high torque, the magnitude of which is dependent on the
thickness of the steel sheet P, is needed to rotate the work rolls
4a and 4b, and rolling speed increases as the thickness of the
steel sheet P decreases. Consequently, high-draft rolling, as
compared with ordinary rolling, needs high power. If the drive
motor has an insufficient capacity and is unable to produce a
sufficient torque for rotating the work roll, it is difficult to
achieve fine-graining rolling with the steel sheet P if the steel
sheet P has a width greater than a threshold width. Fine-graining
rolling at a sufficiently high rolling speed cannot be achieved
when the power of the drive motor is insufficient even if a
sufficient torque can be applied to the work roll.
[0057] It is meaningless to operate the upper rolling stands F1, F2
and F3 of the upper rolling unit for high-draft rolling because the
influence of the rolling operation of the upper rolling stands F1,
F2 and F3 on the metallographic structure of the steel sheet P is
weak. Therefore, the upper rolling stands F1, F2 and F3 are not
operated for high-draft rolling even if the continuous hot-rolling
mill is operated for fine-graining rolling, and hence the
capacities of the drive motors of the upper rolling stands F1, F2
and F3 do not need to be as high as those of the drive motors of
the lower rolling stands. Thus, motor capacities P1, P2 and P3 of
the upper rolling stands F1, F2 and F3, namely, the sum of the
capacities of the two drive motors M1a and M1b (the motors M1) of
the rolling stand F1, the sum of the capacities of the two drive
motors M2a and M2b (the motors M2) of the rolling stand F2, and the
sum of the capacities of the two drive motors M3a and M3b (the
motors M3) of the rolling stand F3, may be smaller than the
respective capacities P.sub.4, P.sub.5 and P.sub.6 of the drive
motors M4, M5 and M6 of the lower rolling mills F4, F5 and F6.
[0058] On the basis of facts that it is advantageous to operate the
lower rolling stand for fine-graining rolling at a higher draft in
respect of metallographic structure and energy efficiency, and
since the lower rolling stands need higher power capacities as
specified by a pass schedule shown in Table 3 of an example
described hereunder, it is preferable that the respective
capacities of the drive motors of the rolling stands meet the
following conditions. The respective capacities P.sub.4, P.sub.5
and P.sub.6 of the drive motors M4, M5 and M6 of the three lower
rolling stands need to meet a condition shown in Table 4, which
will be described hereunder, expressed by:
P.sub.4<P.sub.5.ltoreq.P.sub.6,
P.sub.4.ltoreq.P.sub.5<P.sub.6, or
P.sub.4<P.sub.5<P.sub.6,
P.sub.6.gtoreq.Max(P.sub.1, P.sub.2, P.sub.3).times.1.3
[0059] Expression: P.sub.6.gtoreq.Max(P.sub.1, P.sub.2,
P.sub.3).times.1.3 signifies that the capacity P.sub.6 of the drive
motor M6 is not smaller than 1.3 times in comparison to the largest
one of the capacities P.sub.1, P.sub.2, P.sub.3 of the drive motors
M1, M2 and M3 of the upper rolling stands F1, F2 and F3. If it is
desired to benefit equipment cost and handling by reducing the
number of the types of the drive motors, the drive motors may have
capacities meeting the following conditions.
P.sub.4<P.sub.5.ltoreq.P.sub.6,
P.sub.4.ltoreq.P.sub.5.ltoreq.P.sub.6, or
P.sub.4<P.sub.5<P.sub.6,
P.sub.4.gtoreq.Max(P.sub.1, P.sub.2, P.sub.3).times.1.15
[0060] Expression: P.sub.4.gtoreq.Max(P.sub.1, P.sub.2,
P.sub.3).times.1.15 signifies that the capacities of the drive
motors of the lower rolling stands are not smaller than 1.15 times
in comparison to the largest one of the capacities P.sub.1,
P.sub.2, P.sub.3 of the drive motors M1, M2 and M3 of the upper
rolling stands F1, F2 and F3.
EXAMPLES
[0061] A pass schedule for the continuous hot-rolling mill A, and
the distribution of the respective capacities of the drive motors
of the rolling stands F1 to F6 will be described by way of
example.
[0062] The continuous hot-rolling mill A is operated to produce a
steel sheet of 2.3 mm in thickness and 1200 mm in width by rolling
a steel workpiece containing 0.16% C, 0.22% Si and 0.82% Mn (not
containing other substances in significant content). The continuous
hot-rolling mill A operates at a rolling speed at which general hot
strip mills operate. For example, the continuous hot-rolling mill A
operates at a rolling speed in the range of 7 to 9 m/s.
[0063] Table 1 shows a general pass schedule for ordinary rolling
for producing general-purpose steel sheets which are not
fine-grained steel sheets. It may be proper to provide the rolling
stands F1 to F6 with drive motors having capacities shown in Table
2 (which are distributed in a conventional capacity distribution)
to carry out a rolling operation specified by the pass schedule
shown in Table 1. In Table 1 (and Table 3 mentioned hereunder),
values of "rolling torque" and "rolling power" are those for the
work rolls 1a, 1b, 4a and 4b, and "rough bar" is a workpiece rolled
by a roughening rolling mill, and "F1" to "F6" denote the first to
the sixth rolling stands. In Table 2 (and Table 4 mentioned
hereunder), values in the line "Max. torque" are torques applied to
the work rolls 1a and 1b or the work rolls 4a and 4b of each
rolling stand by the drive motor. Capacities of the drive motors
shown in Table 2 include a considerable extra capacity because, in
some cases, the rolling stands F2 to F6 operate at drafts and at
rolling speeds higher than those shown in Table 1, and need more
rolling power than that shown in Table 1 to produce hot-rolled
steel sheets of thicknesses of 2.0 mm or below.
1TABLE 1 Pass Schedule for Ordinary Rolling (Width of sheet: 1200
mm) Rough bar F1 F2 F3 F4 F5 F6 Thickness 36.0 18.7 10.3 6.4 4.0
2.9 2.3 Rolling force (ton) 2,097 1,611 1,348 1,157 1,054 684
Rolling torque 145.7 71.8 40.9 28.2 17.0 6.9 (ton .multidot. m)
Rolling power 1,698 1,244 1,128 1,731 1,693 1,382 (kW)
[0064]
2TABLE 2 Drive Motors for Conventional Rolling Mill F1 F2 F3 F4 F5
F6 Capacity 8,400 10,500 10,500 8,400 8,400 6,650 (kW) Max. 225 221
138 61 41 28 torque (ton .multidot. m)
[0065] When producing a fine-grained steel sheets having fine
structure mainly of fine ferrite grains, which are not
general-purpose hot-rolled steel sheets, the continuous hot-rolling
mill A operates, for example, according to a pass schedule shown in
Table 3 specifying high drafts for the three lower rolling stands
F4, F5 and F6. According to the pass schedule shown in Table 3, the
last rolling stand F6 and the second last rolling stand F5 roll the
steel sheet at drafts of 40% or above (strains of 0.5 or above).
The continuous hot-rolling mill A cools the rolled steel sheet P by
the water-curtain cooling devices 7 (7A to 7H) to maintain the
steel sheet P at a proper temperature during rolling so that a
fine-grained hot-rolled steel sheet having a fine ferrite structure
as shown in FIG. 4 including (a) and (b) are produced.
3TABLE 3 Pass Schedule for Fine-graining Rolling (Width of sheet:
1200 mm) Rough bar F1 F2 F3 F4 F5 F6 Thickness 40.0 28.8 19.7 12.3
7.5 4.5 2.3 Rolling force (ton) 2,053 2,160 2,515 1,995 2,081 2,158
Rolling torque 116.4 105.7 109.3 71.6 53.7 48.4 (ton .multidot. m)
Rolling power 1,356 1,832 3,012 4,396 5,365 9,640 (kW)
[0066] Necessary rolling torques for the lower rolling stands
specified by the pass schedule shown in Table 3 are high as
compared with the corresponding necessary rolling torques specified
by the pass schedule shown in Table 1, and, as shown in FIG. 5, are
greater than the torques of the drive motors of the lower rolling
stands F4, F5 and F6 shown in Table 2 (i.e., torques indicated by
solid circles in FIG. 5). The necessary rolling torques for the
lower rolling stands exceed those shown in Table 1 because high
rolling forces are necessary for high draft rolling. Since rolling
speeds for the last rolling stand and the second rolling stand
increase sharply in inverse proportion to the sharp reduction of
the thickness of the steel sheet due to high-draft rolling.
Consequently, the rolling stands F5 and F6 need rolling power far
higher than that needed by the upper rolling stands.
[0067] Thus, if the capacities of the drive motors are distributed
in a capacity distribution shown in Table 2 that is considered to
be suitable for ordinary rolling, the output torques and capacities
(power capacities) of the drive motors M4, M5 and M6 of the rolling
stands F4, F5 and F6 are insufficient. Therefore, it is proper that
the drive motors M4, M5 and M6 of the lower rolling stands have
large capacities as shown in Table 4 to ensure that continuous
hot-rolling mill A provided with the drive motors M1 to M6 is able
to carry out fine-graining rolling satisfactorily.
4TABLE 4 Drive Motors for Fine-graining Rolling F1 F2 F3 F4 F5 F6
Capacity 8,400 10,500 10,500 11,000 13,000 14,000 (kW) Max. 225 221
138 80 63 58 torque (ton .multidot. m)
[0068] Tables 3 and 4 show capacities (power) and toques generated
in the drive motors M1 to M6 of the rolling stands F1 to F6 during
the rolling operation, i.e., during the rolling of the steel sheet
P. Since the length of the steel sheet P is not indefinite and the
rolling operation is not continued without interruption, the output
capacities shown in Tables 3 and 4 are not necessarily the rated
output capacities of the drive motors M1 to M6. Therefore, it is
preferable to determined proper rated output capacities with
reference to the output capacities shown in Tables 3 and 4, the
duration of rolling and the frequency of the rolling operation by a
root means square method or the like, and to select drive motors M1
to M6 having the proper rated output capacities.
Industrial Applicability
[0069] The present invention is applicable to a continuous
hot-rolling mill for manufacturing a hot-rolled, fine-grained steel
sheet of fine structure mainly of fine ferrite grains.
* * * * *