Technical Field
[0001] The present invention relates to a hot rolling apparatus and a hot rolling method
and more particularly to a hot rolling apparatus and a hot rolling method for manufacturing
a steel plate having a micro-structure mainly composed of fine ferrite.
Background Art
[0002] Generally, as a means for improving the mechanical properties of rolled steel, refinement
of the structure of rolled steel is well known. Improvement of the mechanical properties
of rolled steel provides many advantages such as realization of lightweight of a steel
structure. Many methods for manufacturing steel having a micro-structure, that is,
fine-particle steel have been proposed and as typical methods, (1) the high-pressure
rolling method and (2) the control rolling method may be cited.
[0003] Among them, (1) the high-pressure rolling method is described in Japanese Patent
Laid-Open Publication No. 123823/1983 and Japanese Patent Publication 65564/1993.
Namely, the method applies high pressure to austenite particles, thereby promotes
the straining transformation from the austenite (γ) phase to the ferrite (α) phase,
and refines the structure.
[0004] Further, (2) the control rolling method is a method for realizing refinement of ferrite
particles containing components of Nb (niobium) and Ti (titanium) which can be easily
increased in tension by the deposition increasing operation of Nb and Ti and also
promotes the straining transformation from they phase to the α phase when the cold
rolling (ferrite region rolling) is executed by the recrystallization suppression
operation for austenite particles of Nb and Ti.
[0005] The control rolling method executes the finishing rolling in the low temperature
zone (800°C or less), so that it has a disadvantage that the deformation resistance
of steel to be rolled is extremely high, thus the load on the strip rolling apparatus
is large. On the other hand, the high-pressure rolling method, as indicated in Japanese
Patent Publication 65564/1993 aforementioned, cannot be executed industrially by a
general hot strip mill and requires use of a special rolling apparatus. The reason
is that, as described in the aforementioned patent publications, continuous rolling
at a high pressurization rate (for example, 40% or more) which cannot be realized
by a general rolling apparatus is required.
[0006] When fine-particle steel is to be manufactured industrially and commercially by executing
the high-pressure rolling method, in addition to that a rolling apparatus of a general
hot strip mill type cannot be used, the following problems are imposed.
i) Owing to execution of rolling under high pressure, that is, at a high pressurization
rate, faults due to the rolling load may be often caused. Namely, there is a case
that the rolling load reaches the intrinsic limit value (mill power restriction and
machine strength) of the rolling apparatus and rolling becomes impossible. Furthermore,
for steel to be rolled, a predetermined pressurization rate cannot be realized and
large edge drops are caused. The reason that the predetermined pressurization rate
cannot be obtained is that particularly when the plate thickness on the exit side
of the rolling apparatus is 2 mm or less and the pressurization rate is 40% or more,
the rolling load is large and the deformation resistance is high, so that the rolling
flatness is increased. In this case, even if the pressure is increased so as to execute
rolling under high pressure, the pressurization rate is not increased. The reason
for increasing the edge drop is that a high load is applied to the neighborhood of
the edge (the end in the width direction) of steel to be rolled and no good plate
profile can be obtained.
ii) Difficulty in keeping the temperature of steel to be rolled is also a serious
problem. The reason is that when rolling is executed at a high pressurization rate
using a mill of a plurality of stands, the temperature of steel to be rolled is increased
remarkably due to working heat generation and it is not easy to keep it at the temperature
(the range from the transformation point of Ar3 to Ar3+50oC) suited to execution of the high-pressure rolling method. When steel to be rolled
is accelerated and the feed speed is increased, the strain speed is increased and
the working heat generation is increased, so that it becomes difficult more and more
to keep the temperature.
iii) Faults relating to the thermal load of the rolls are often caused. When rolling
at a high load providing a high pressurization rate is executed, the working heat
generation of steel to be rolled is also increased and the thermal load of the rolls
is increased in correspondence to it. As a result, a thermal crown that each roll
is extended in diameter at the center thereof is easily generated. The thermal crown
may not be eliminated only by cooling each roll depending on the degree thereof, and
steel to be rolled gets worse in the shape, and a stable flow of plate may not be
obtained easily.
iv) The rolls are worn out strongly and the shape (crown) of steel to be rolled easily
gets worse. The reason is that during rolling at a high pressurization rate and a
high load, the thermal or dynamic load applied on the rolls is high, so that the wear
of the rolls easily progresses. At the part of each roll in contact with the edge
of steel to be rolled, the rolling load is high, so that the wear easily progresses
and the profile of steel to be rolled which is important for the quality thereof is
easily reduced greatly. Further, when the rolls are easily worn out, the cost for
maintenance such as grinding or exchange of the rolls is increased.
[0007] Document EP 1 033 182 A1 discloses a hot rolling apparatus for rolling a steel to
manufacture a steel plate comprising a mill arranged on a preceding stage, and mills
of a plurality of stands on a later stage. The mills of a plurality of stands comprise
different-diameter roll mills including a pair of different-diameter work rolls.
[0008] Therefore, an object of the present invention is to solve the aforementioned problems
concerning manufacture of hot rolled steel plates of fine-particle steel by providing
a hot rolling apparatus for enabling smooth manufacture of those steel plates and
a fine-particle steel manufacturing method.
[0009] Further, another object of the present invention is to provide a continuous hot rolling
method suited to manufacture of hot rolled steel plates of fine-particle steel which
is superior in respect of cost to effect.
[0010] Further, still another object of the present invention is to provide a continuous
hot rolling method for smooth manufacture of thick plates using a hot rolling apparatus
capable of manufacturing thin plates.
Disclosure of Invention
[0011] The present invention is a hot rolling apparatus for rolling a steel to be rolled
to manufacture a steel plate, comprising: a mill arranged on the preceding stage,
mills of a plurality of stands arranged on the later stage, said mills of plurality
of stands comprising different-diameter roll mills including a pair of different-diameter
work rolls having an equivalent roll diameter of less than 600 mm or minimum-diameter
roll mills including a pair of work rolls having a diameter of less than 600 mm, and
a cooling unit for cooling the steel to be rolled which is arranged on the exit side
of the mill of at least one stand on the later stage.
[0012] Here, the "equivalent roll diameter" is referred to as a mean value of the diameters
of the upper and lower paired different-diameter work rolls regarding the different-diameter
roll mill.
[0013] Further, the cooling unit is preferably a curtain-wall type cooler.
[0014] Here, the "curtain-wall type cooler" is referred to as a cooling unit of a type such
as to let a large mount of cooling water flow in a laminar flow state by putting in
a row from above and underneath like a curtain and hit it against the top and bottom
of steel to be rolled overall the width.
[0015] Further, among the mills arranged on the preceding stage and later stage, at least
the mill arranged on the preceding stage preferably includes CVC mills of a plurality
of stands.
[0016] Here, the "CVC mill" is referred to as a mill including a CVC roll which has an outer
diameter continuously changed in the long axial direction and can move in the long
axial direction.
[0017] Further, the equivalent roll diameter of the pair of different-diameter work rolls
of the different-diameter roll mills or the roll diameter of the work rolls of the
minimum-diameter roll mills is preferably 550 mm or less.
[0018] Further, the work rolls of the different-diameter roll mills or the work rolls of
the minimum-diameter roll mills are provided with a CVC function and a bending function.
[0019] Here, the "CVC function" is referred to as a function for a roll having an outer
diameter continuously changed in the long axial direction to move in the long axial
direction and change and control the roll gap shape. Further, the "bending function"
is referred to as a function for operating the bending force (bending moment) on the
rolls and changing the roll gap shape.
[0020] Further, the hot rolling apparatus preferably has a lubricant feed unit for feeding
a lubricant onto the roll surfaces of the mills additionally which is installed on
the mill of at least any one stand among the mills arranged on the preceding stage
and later stage.
[0021] Further, the lubricant feed unit preferably feeds a lubricant containing a fine-particle
solid lubricant in grease.
[0022] Further, the hot rolling apparatus preferably has a fluid jet spray additionally
for jetting a fluid to the steel to be rolled and removing cooling water existing
on the steel to be rolled, which is arranged on the downstream side of the cooling
unit in the flow direction of the steel to be rolled on the exit side of the mill
of the stand on the last stage.
[0023] Further, the fluid jet spray preferably includes a plurality of nozzles for blowing
out pressurized water so as to spread in the width direction of the steel to be rolled
slantwise downward from above the steel to be rolled toward the upstream side in the
flow direction of the steel to be rolled for the steel to be rolled.
[0024] The present invention is a method for rolling a steel to be rolled to manufacture
a fine-particle steel, wherein the method feeds the heated steel to be rolled to a
strip rolling apparatus having a mill arranged on the preceding stage and a mill arranged
on the later stage, and the mill arranged on the later stage of the rolling apparatus
has work rolls with a diameter of 550 mm or less, and the method cools the steel to
be rolled before and after the mill arranged on the later stage of the rolling apparatus
in the flow direction of the steel to be rolled and rolls the steel to be rolled so
that the cumulative strain becomes 0.9 or more.
[0025] Here, the "strain" is referred to as the value indicated below, which is obtained
by dividing the difference between the thickness h
0 of the steel to be rolled on the entrance side of each mill and the thickness h
1 on the exit side by the mean thickness of the two.
[0026] Further, the "cumulative strain" is the strains at the respective mills (the mills
of the stands on the upstream side thereof are ignored because the effect thereof
is small) of a plurality of stands (for example, 3 stands or 2 stands) on the later
stage which are added and totalized in consideration of the effect intensity on the
metallic structure and assuming the strains at the stand on the last stage, the stand
before it, and the stand before it as ε
n, ε
n-1, and ε
n-2, it is expressed as follows
[0027] The fine-particle steel manufacturing method of the present invention rolls the steel
to be rolled using any of the aforementioned hot rolling apparatuses so that the cumulative
strain of the steel to be rolled on the later stage of the rolling apparatus becomes
0.9 or more.
[0028] Further, the steel to be rolled immediately after it leaves the mill of the last
stand is preferably cooled at a temperature lowering rate per second of 20°C or more.
[0029] Further, the steel P to be rolled preferably has a carbon content of 0.5% or less
and an alloy element content of 5% or less.
[0030] The "thin plate" is referred to as a steel plate with a thickness of less than 6
mm and a "thick plate" is referred to as a steel plate with a thickness of 6 mm or
more (less than about 50 mm).
[0031] The "rolling end temperature" is the surface temperature of the steel to be rolled
measured by a thermometer installed on the downstream side (the downstream side of
the arranged last stage of mill by several m) of the rolling apparatus in the flow
direction of the steel to be rolled.
Brief Description of Drawings
[0032]
Fig. 1 is a side view conceptually showing the whole arrangement of a hot rolling
apparatus of an embodiment of the present invention.
Figs. 2A, 2B, and 2C are schematic views for explaining the CVC function regarding
the mill 1 on the preceding stage in the rolling apparatus shown in Fig. 1.
Fig. 3 is a side view showing the mill 6 on the last stage in the rolling apparatus
shown in Fig. 1 in detail.
Fig. 4 is a chart showing the relation between the grain size concerning crystalline
grains of the ferrite structure of steel plates manufactured using the rolling apparatus
shown in Fig. 1 and the yielding point.
Figs. 5A, 5B, and 5C are drawings showing the crystalline structure of steel plates
manufactured using the rolling apparatus shown in Fig. 1 in the neighborhood of the
top surface, the center of the plate thickness, and the bottom surface, respectively.
Fig. 6 is a chart showing the relation between the equivalent diameter of a work roll
of a different-diameter roll mill and the rolling load.
Fig. 7 is a chart showing the reduction effect of edge drops of a different-diameter
roll mill.
Fig. 8 is a chart showing the wear reduction effect of the roll surface when a lubricant
is used.
Fig. 9 is a side view conceptually showing the whole arrangement of a hot rolling
apparatus of a varied example of the embodiment shown in Fig. 1.
Fig. 10 is a side view conceptually showing the whole arrangement of a continuous
hot rolling apparatus of another embodiment of the present invention.
Figs. 11A, 11B, and 11C are schematic views for explaining the CVC function regarding
the mill 10 on the preceding stage in the rolling apparatus shown in Fig. 10.
Fig. 12 is a side view showing the mills 40 to 60 on the last stage in the rolling
apparatus shown in Fig. 10 and the neighborhood thereof in detail.
Fig. 13 is a chart showing the relation between the cumulative strain and the ferrite
particle diameter of various steel plates obtained by test rolling.
Fig. 14 is a chart showing the relation between the finishing temperature (rolling
end temperature) and the ferrite particle diameter of various steel plates obtained
by test rolling.
Fig. 15 is a chart showing the relation between the ferrite particle diameter and
the tensile strength of various steel plates obtained by test rolling.
Fig. 16 is a chart showing the relation between the ferrite particle diameter and
the elongation of various steel plates obtained by test rolling.
Fig. 17 is a chart showing the relation between the ferrite particle diameter and
the tensile strength x elongation of various steel plates obtained by test rolling.
Figs. 18A, 18B, and 18C are drawings showing the crystalline structure of steel plates
obtained by the embodiment of the rolling method using the rolling apparatus shown
in Fig. 10 in the neighborhood of the top surface, the neighborhood of the part inward
from it by 1/4 of the thickness, and the neighborhood of the center of the thickness,
respectively.
Figs. 19A, 19B, and 19C are drawings showing the crystalline structure of steel plates
obtained by the embodiment D of the present invention in the neighborhood of the top
surface, the neighborhood of the part inward from it by 1/4 of the thickness, and
the neighborhood of the center of the thickness, respectively.
Fig. 20 is a chart showing the relation between the ferrite particle diameter, the
tensile strength, and the yielding point of steel plates manufactured by the embodiment
of the present invention.
Fig. 21 is a chart showing temperature changes of the Charpy impact value of steel
plates manufactured by the embodiment of the present invention and normal steel (non-fine
particle steel plates).
Fig. 22 is a chart showing temperature changes of the brittle fracture rate of steel
plates manufactured by the embodiment of the present invention.
Best Mode for Carrying Out the Invention
[0033] A hot rolling apparatus of an embodiment of the present invention and a fine-particle
steel manufacturing method using the hot rolling apparatus will be explained hereunder
with reference to the accompanying drawings.
[0034] The hot rolling apparatus of this embodiment shown in Fig. 1 is a finishing rolling
apparatus, and on the upstream side (not shown in the drawing) in the flow direction
of steel P to be rolled, a heating furnace and a rough rolling apparatus are installed,
and on the downstream side (not shown in the drawing), a run-out table and a winder
are arranged. The hot rolling apparatus is structured as indicated below so as to
continuously roll the steel P to be rolled roughly rolled on the upstream side, thereby
manufacture hot rolled steel plates of fine-particle steel having a fine ferrite structure.
[0035] Firstly, as mills of 3 stands constituting the preceding stage of the hot rolling
apparatus, so-called CVC mills 1, 2, and 3 are arranged tandem. The CVC mill 1 positioned
closest to the entrance side of the hot rolling apparatus is structured as a quadrupole
mill composed of work rolls 1a and 1b and backup rolls 1c and 1d as shown in Fig.
1 and the work rolls 1a and 1b have crowns (CVC, that is, continuous diameter changes)
as shown in Fig. 2A. The work rolls 1a and 1b, as shown in Figs. 2B and 2C, can move
(shift) in the long axial directions opposite to each other at the same time, thus
the position relationship between the rolls, that is, the roll gap can be adjusted.
The diameter of the work rolls 1a and 1b is set to 700 mm and the maximum shift amount
is set to 100 mm in both forward and backward directions. The CVC mills 2 and 3 of
the other two stands are not different from the CVC mill 1 in the constitution and
function.
[0036] The reason that the CVC mills 1, 2, and 3 are arranged on the preceding stage like
this is that the crown (shape) of the steel P to be rolled is to be kept suitably.
In different-diameter roll mills 4, 5, and 6 (described later) on the later stage,
thermal crowns caused by working heat generation due to rolling are easily formed,
so that plate crowns are corrected beforehand by the CVC mills 1, 2, and 3 installed
on the preceding stage and the medium drawing of the steel P to be rolled is reduced.
[0037] Namely, the CVC mills 1, 2, and 3 have a large changing capacity of the roll gap
shape compared with the means of simply executing roll bending and are arranged around
the part of the preceding stage where steel to be rolled is thick and the crown control
can be easily executed, so that it is advantageous in prevention of non-stabilization
of plate flowing on the later stage where crowns are adjusted and large pressure is
applied.
[0038] Further, the hot rolling apparatus of this embodiment, as mills of 3 stands constituting
the later stage following the preceding stage, has the so-called different-diameter
roll mills 4, 5, and 6 arranged tandem. The stand intervals of all the 6 stands including
the CVC mills 1, 2, and 3 aforementioned are all equal such as 5.5 m. The different-diameter
roll mill 4 corresponding to the 4th stand counted from the CVC mill 1 is structured
as a quadrupole mill composed of work rolls 4a and 4b and backup rolls 4c and 4d as
shown in Fig. 1 and the work rolls 4a and 4b have different diameters as shown in
the drawing.
[0039] And, among the work rolls 4a and 4b, only the lower roll 4b with a large diameter
is driven to rotate by a motor (not shown in the drawing) and the upper roll 4a with
a small diameter is structured so as to freely rotate free of driving force. The work
rolls 4a and 4b are respectively provided with a bender (not shown in the drawing),
so that the work rolls 4a and 4b can be provided with bending. Further, the work rolls
4a and 4b are given the CVC function and can be moved forward and backward in the
long axial direction within a range of 100 mm.
[0040] Since the work rolls 4a and 4b are given the bending function and CVC function like
this, the shape control capacity for steel to be rolled is improved and a good profile
of steel plates can be obtained.
[0041] The diameter of the work roll 4a is 480 mm, and the diameter of the work roll 4b
is 600 mm, and the equivalent roll diameter which is a mean value of the two is 540
mm. In the constitution and function aforementioned, the different-diameter roll mills
5 and 6 of the other 2 stands positioned behind are not different from the different-diameter
roll mill 4. Further, although the equivalent roll diameter of the work rolls of the
different-diameter roll mills 4, 5, and 6 can be made smaller than 540 mm, it is preferably
400 mm or more from the viewpoint of strength.
[0042] The equivalent roll diameter is small and since only one work roll (4b, etc.) is
driven, shearing force operates on the steel P to be rolled, so that the different-diameter
roll mills 4, 5, and 6 of 3 stands can execute rolling at a high pressurization rate
(for example, a pressurization rate of 50%) even at a comparatively low rolling load.
Therefore, high-pressure rolling for forming a fine ferrite structure in the steel
P to be rolled can be executed at a small rolling load and moreover, since the rolling
load is small, faults due to the roll flatness and edge drops are not caused.
[0043] The chart X3 shown in Fig. 6, when the different-diameter roll mill 6 of the 6th
stand rolls and manufactures a steel plate (the components are C of 0.16%, Si of 0.22%,
and Mn of 0.82%) with a thickness of 2.3 mm and a width of 730 mm at an equal pressurization
rate (48%), shows a relationship between the equivalent diameter of the work rolls
and the rolling load.
[0044] Further, the chart X5 shown in Fig. 7 shows edge drops generated when the fixed different-diameter
roll mills 5 and 6 (the diameters of the work rolls 5a and 6a are 480 mm, and those
of the work rolls 5b and 6b are 600 mm, and the equivalent roll diameter of each mill
is 540 mm) roll and manufacture the same steel plate as that shown in Fig. 6. Further,
the chart X4 shown in Fig. 7 shows edge drops for comparison when the different diameters
of the work rolls are made equal (a medium scale diameter of 600 mm) and the same
steel plate is rolled and manufactured.
[0045] Further, as a varied example of this embodiment, as shown in Fig. 9, the mills arranged
on the later stage, in place of the different-diameter roll mills 4, 5, and 6, may
be changed to minimum-diameter roll mills 4', 5', and 6' including a pair of work
rolls 4a' and 4b' with a diameter of less than 600 mm.
[0046] Further, in the hot rolling apparatus of this embodiment, a lubricant feed unit is
arranged for each work roll of the mills 1 to 6 of all the 6 stands. The unit is composed
of, for example, injection ports directed toward the surface of each work roll such
as numerals 5e, 5f, 6e, and 6f shown in Fig. 3 and lubricant feed pumps to them. Further,
as a varied example, in place of direct feed of a lubricant to the surface of each
work roll, a lubricant is fed to the surface of the steel P to be rolled, thereby
indirectly fed to the roll surfaces.
[0047] Further, in the hot rolling apparatus of this embodiment, the lubricant is used to
prevent each roll surface from wear and not used to lower the coefficient of friction.
Therefore, as a lubricant, a fine-particle solid lubricant such as tribasic calcium
phosphate, mica, or calcium carbonate included in grease is used. By blending those
solid fine particles, the coefficient of friction µ between each work roll and the
steel P to be rolled when a lubricant is used becomes rather higher such as about
0.28 or more. When such a degree of coefficient of friction is ensured, the steel
P to be rolled is properly prevented from roll slip.
[0048] When the aforementioned lubricant is used, the aforementioned fine particles lie
between each roll surface and the steel P to be rolled and the direct contact between
the rolls and the steel P can be prevented, so that the wear of the roll surfaces
is suppressed and the shape of the steel P can be easily kept satisfactorily for a
long time. Further, solid fine particles are included in grease instead of mineral
oil, so that there is an advantage that there is no possibility that fine particles
may be precipitated in a storage container of a lubricant and the lubricant is fed
so that solid fine particles are always dispersed uniformly on each roll surface.
[0049] Fig. 8 shows the roll wear reduction effect due to use of a lubricant, and the chart
X6 indicates a case of no use of a lubricant, and the chart X7 indicates a case of
use of a lubricant. Further, the transverse axis of Fig. 8 indicates the magnitude
of load of work rolls and the ordinate axis indicates the wear amount of work rolls.
[0050] Further, in the hot rolling apparatus of this embodiment, on each exit side of the
different-diameter roll mills 4, 5, and 6 of the 3 stands arranged on the later stage,
curtain wall type coolers 7A, 7B, and 7C are arranged. The cooler 7B will be explained
as an example. As shown in Fig. 3, the cooler 7B lets a large amount of cooling water
at the normal temperature flow in a curtain shape (curtain wall state, with a thickness
of 10 mm or more, a most suitable thickness of 16 mm) in a laminar flow state toward
the full-width surface of the steel P to be rolled from upper and lower headers 7Ba
and 7Bb, thereby strongly cools the steel P to be rolled. The amount of cooling water
can be adjusted within the range from 100 to 500 m
3/h per unit width (1 m) of the steel P to be rolled and the temperature lowering speed
of the steel P is 20°C/s or more. In the curtain wall type cooler, cooling water of
350 m
3/h per unit width is generally used. The temperature lowering rate of the steel P
to be rolled in this case reaches 60 to 80°C/s (40°C/s or so including the raised
temperature due to working heat generation) when the product of plate thickness and
speed is 1200 mm · mpm. The other coolers 7A and 7C also have the same constitution
and function.
[0051] Further, in the hot rolling apparatus of this embodiment, the curtain wall type coolers
are arranged on the exit sides of the mills 4, 5, and 6 on the later stage. However,
the number of coolers to be installed is not limited to it and can be properly changed
depending on the kind of steel to be rolled.
[0052] By use of the curtain wall type coolers 7A, 7B, and 7C, the temperature rise of the
steel P to be rolled due to working heat generation during rolling is suppressed,
and the steel P to be rolled is kept within the temperature range suited to the high-pressure
rolling method or control rolling method, and an occurrence of particle growth of
the micro-structure after rolling can be suppressed.
[0053] Further, the run-out table (not shown in the drawing) on the downstream side of the
hot rolling apparatus shown in Fig. 1 also cools the steel P to be rolled by cooling
water at a speed of 10°C/s or more so as to prevent particle growth.
[0054] In the hot rolling apparatus shown in Fig. 1, on the exit side of the different-diameter
roll mill 6 of the stand on the stand on the last stage, a water jet spray 8 is arranged
away from the curtain wall type cooler 7C by several hundreds mm to 1 m. This is to
remove cooling water put on the top of the steel P to be rolled by the cooler 7C.
As shown in Fig. 3, the spray 8 has a plurality of nozzles 8a (4 each in total in
this example) for respectively blowing out 300 liters per minute of pressurized water
of about 10 kg/cm
2 slantwise downward to the upstream side in the flow direction of the steel P to be
rolled from above the steel P to the surface of the steel P so as to form an angle
of 65° (or within the range from 50 to 80°) with the top of the steel P. The plurality
of nozzles 8a, as shown in Fig. 3, are arranged at an interval in the length direction
of the steel P to be rolled and at an interval also in the width direction thereof.
The nozzles 8a blow out water so as to spread in the width direction of the steel
P to be rolled, and the spread angle in the width direction of the steel P is preferably
set to 15 to 30°, and the spread angle in the length direction is preferably set to
1 to 10° (respectively set to 21° and 3° in this embodiment).
[0055] By use of the water jet spray 8, cooling water put on the steel P by the operation
of the cooling unit 7 can be smoothly removed, so that by various measuring instruments
installed on the downstream side, various measurements concerning the steel P to be
rolled after rolling, that is, the manufactured steel plate can be executed properly.
In this case, water is heavier than gas, so that it can be easily given kinetic energy
and can be easily obtained, thus water is suitable for a jet fluid. It is considered
to be a reason for producing a good operation that by blowing out pressurized water
slantwise downward to the upstream side, cooling water can be prevented from reaching
the downstream side (the side of the measuring instruments) and furthermore by use
of the nozzles spreading in the width direction of the steel P to be rolled, cooling
water can be removed from the top of the steel P to be rolled in full width.
[0056] Additionally, for the work rolls of the mills of the respective stands, as shown
in Fig. 3, jet nozzles (for example, numerals 5i, 5j, 6i, 6j) for roll cooling water
and water draining plates (for example, numerals 5g, 5h, 6g, 6h) for removing cooling
water by them are arranged.
[0057] Next, an embodiment that hot rolling is executed using the aforementioned hot rolling
apparatus (Fig. 1) is indicated below.
[0058] With respect to steel having chemical components of C of 0.16%, Si of 0.22%, and
Mn of 0.82% (no other significant amount of component is included), a steel plate
with a thickness of 2.33 mm and a width of 730 mm was manufactured by the rolling
apparatus shown in Fig. 1 under three kinds of conditions (Embodiments 1 to 3). Table
1-1 indicated below shows the pass schedule (rolling conditions) of Embodiment 1 and
Table 1-2 shows the pass schedule of Embodiments 2 and 3. Further, Table 1-3 shows
the use state of the curtain wall type coolers 7A, 7B, and 7C of Embodiments 1 to
3 and Table 1-4 shows the finishing temperature of the steel P to be rolled measured
behind the mill 6 on the last stage of Embodiments 1 to 3. In the tables, "rough bar"
indicates a rough rolling apparatus and "F1" to "F6" indicate the mills 1 to 6 of
the first stand to the sixth stand. Further, the rolling speed is not specially limited
and the rolling speed (for example, 7 to 9 m/s) commonly used in a general hot strip
mill is adopted.
[Table 1-1]
Embodiment 1: Pass schedule (Cumulative strain = 0.65) |
|
Rough bar |
F1 |
F2 |
F3 |
F4 |
F5 |
F6 |
Plate thickness |
mm |
40 |
22.82 |
12.55 |
7.53 |
4.89 |
3.33 |
2.33 |
Pressurization rate |
% |
|
43 |
45 |
40 |
35 |
32 |
30 |
Strain |
- |
|
0.55 |
0.58 |
0.50 |
0.42 |
0.38 |
0.35 |
Cumulative Strain |
- |
0.65 |
[Table 1-2]
Embodiments 2, 3: Pass schedule (Cumulative strain = 0.92) |
|
Rough bar |
F1 |
F2 |
F3 |
F4 |
F5 |
F6 |
Plate thickness |
mm |
40 |
25.96 |
17.39 |
12.17 |
7.06 |
3.88 |
2.33 |
Pressurization rate |
% |
|
35 |
33 |
30 |
42 |
45 |
40 |
Strain |
- |
|
0.42 |
0.40 |
0.35 |
0.53 |
0.58 |
0.50 |
Cumulative Strain |
- |
0.92 |
[Table 1-3]
Cooling conditions (Curtain wall) |
Embodiment |
F4 back surface |
F5 back surface |
F6 back surface |
1 |
Not used |
Not used |
Used |
2 |
Not used |
Not used |
Used |
3 |
Used |
Used |
Used |
[Table 1-4]
Temperature conditions |
Embodiment |
Finishing temperature,°C |
1 |
800 ∼ 850 |
2 |
800 ∼ 850 |
3 |
750 ∼ 780 |
[0059] The ferrite particle diameter and mechanical properties of hot rolled plates obtained
from Embodiments 1 to 3 are shown in Table 1-5. In Table 1-5, "TS" indicates tensile
strength, "YP" a yielding point, and "EL" an elongation. Further, in Table 1-5, the
main ones of the rolling conditions shown in Tables 1-1 to 1-3 are additionally recorded.
[Table 1-5]
Rolling conditions and mechanical characteristics |
Embodiment |
Curtain Wall Cooling |
Cumulative Strain |
Ferrite particle diameter µm |
TS kg/mm2 |
YP kg/mm2 |
EL % |
1 |
F6 |
0.65 |
6∼9 |
40∼50 |
30∼40 |
25∼30 |
2 |
F6 |
0.92 |
4∼4.5 |
55∼65 |
45∼55 |
25∼30 |
3 |
F4,F5,F6 |
0.92 |
3.5∼4 |
57∼65 |
49∼57 |
26∼30 |
TS: Tensile strength, YP: Yielding point, EL: Elongation |
[0060] As shown in Table 1-5, in Embodiments 2 and 3 that the cumulative strain (ε
c which is the aforementioned totalized value) is set to 0.92, a steel plate having
a ferrite structure with a particle diameter of about 4µm and superior mechanical
properties can be obtained. In Embodiment 3 that the curtain wall type coolers 7A
to 7C are used on the exit side (the back surface) of the 3 stands (F4 to F6) on the
later stage, a steel plate having a ferrite particle diameter of about 4µm or less
and particularly superior mechanical properties is obtained.
[0061] Fig. 4 is a drawing showing the relation between the grain size (the particle diameter
D (µm) to the power of -1/2) concerning crystalline grains of the ferrite structure
of steel plates obtained by Embodiments 1 to 3 and the yielding point. As shown in
the drawing, when the cumulative strain of the mills of the 3 stands on the later
stage is set to 0.65 (the group X2 shown in Fig. 4), the grain size is 0.43 or less
(particle diameter of 5.4 µm or more) and the yielding point is not sufficient. However,
when the cumulative strain is set to 0.92, the grain size becomes about 0.5 (particle
diameter of about 4 µm) and the yielding point is increased to 45 kg/mm
2 or more.
[0062] And, Figs. 5A, 5B, and 5C are drawings showing the crystalline structures of the
steel plate obtained in Embodiment 3 in the neighborhood of the top surface, the neighborhood
of the center of the plate thickness, and the neighborhood of the bottom surface,
respectively. At any part in the plate thickness, a fine ferrite structure with a
particle diameter of 3 µm or so is formed.
[0063] As mentioned above, according to this embodiment, a hot rolled plate of fine-particle
steel having a fine ferrite structure and a superior strength balance including the
tensile strength, ductility, toughness, and fatigue strength can be manufactured smoothly
and the steel plate can be produced commercially. The reasons are summarized as indicated
below.
a) The different-diameter roll mills 4, 5, and 6 of the 2 stands or more arranged
on the later stage or the minimum-diameter roll mills 4', 5', and 6', since the equivalent
roll diameter or the both (pair) work roll diameters are small, can execute rolling
under high pressure at a low rolling load, that is, at a high pressurization rate.
The reason is that the rolling load producing the same pressurization rate is reduced
as the work roll diameter is reduced and is almost proportional to the work roll diameter
(refer to Fig. 6). The phenomenon that when the rolling load is reduced, rolling at
a high pressurization rate cannot be executed due to the roll flatness is eliminated
and additionally the flat deformation amount of the rolls is reduced, thus edge drops
are reduced (refer to Fig. 7).
b) The curtain wall type coolers 7A, 7B, and 7C installed on the later stage suppress
temperature rise due to working heat generation of the steel P to be rolled accompanying
rolling at a high pressurization rate under condition of cumulative strain of 0.9
or more. The coolers 7A, 7B, and 7C cool the steel P strongly by a large amount of
cooling water supplied as mentioned above, so that even when the steel P to be rolled
is accelerated, the coolers can keep the steel P within the temperature range (for
example, the Ar3 transformation point to Ar3 + 50°C) suited to execute the high-pressure rolling method. By strongly cooling the
steel P to be rolled immediately after rolling like this, the particle growth of the
fine structure in the steel P to be rolled can be stopped and the diameter of crystalline
grains of the ferrite structure in a manufactured steel plate is made finer such as
about 4 µm or less. Since the coolers 7A, 7B, and 7C are arranged not only on the
exit side of the mill 6 of the stand on the last stage but also on the exit side of
the mills of at least 2 stands on the later stage, the coolers effectively take the
heat generated during rolling by the mill 6 on the last stand and the mills of the
preceding stands and keep the temperature properly. Since the coolers 7A, 7B, and
7C are arranged on the exit side of the mill of each stand, the steel P to be rolled
immediately after rolling by the mill of each stand is strongly cooled and the operation
of stopping the particle growth of the fine structure is ensured. Further, the coolers
7A, 7B, and 7C hit cooling water on the steel P to be rolled in full width, so that
the steel P can be cooled uniformly without one-sided in the width direction.
[0064] As mentioned above, according to this embodiment, the aforementioned problems i)
and ii) concerning execution of the high-pressure rolling method are solved and, by
use of a rolling apparatus of a general hot strip mill type, a fine-particle steel
plate can be manufactured smoothly and a fine-particle steel plate can be produced
commercially.
[0065] Further, when the curtain wall type coolers 7A, 7B, and 7C are properly used so as
to keep the temperature range of the steel P to be rolled between 700°C and 800°C
(temperature zone), using steel containing Nb and Ti as steel P to be rolled, the
aforementioned control rolling method can be executed stably (consequently a fine-particle
steel plate can be manufactured).
[0066] Further, when steel to be rolled containing carbon of 0.5% or less and an alloy element
of 5% or less is rolled, a fine-particle steel plate having such components can be
widely used due to the balanced mechanical properties (general-purpose from the viewpoint
of tensile strength and ductility) and high weldability, and be obtained easily due
to a comparatively low price, and moreover has a good cyclic property, so that it
is considered to be highly demanded. Therefore, for a steel plate having such component
contents, the commercial contribution degree is high and sufficient economical rationality
accompanies the production thereof.
[0067] Generally, when the amount of C (carbon) is increased, the ferrite amount is reduced
and steel mainly composed of pearlite is obtained. However, according to this embodiment,
even if the C amount is the same, the ferrite amount can be increased and when the
C amount is not more than 0.5%, a structure mainly composed of ferrite can be obtained.
[0068] Further, this embodiment obtains good results regardless of existence of alloy elements
other than C in the steel P to be rolled. However, to set the temperature range of
Ar
3 transformation point to Ar
3 + 50°C between 700°C and 900°C which is a most suitable temperature range for hot
rolling, it is preferable to adjust the transformation point temperature depending
on the total amount of alloy elements. However, when the total content of alloy elements
is more than 5%, the Ar
3 transformation point becomes extremely low and fine particles cannot be easily obtained.
[0069] Next, a hot rolling apparatus and a hot rolling method by another embodiment of the
present invention will be explained.
[0070] The hot rolling method by the aforementioned embodiment strongly pressurizes (that
is, high pressurization at a cumulative strain of 0.9 or more) steel to be rolled
mainly by the mills on the later stage, keeps the steel to be rolled at a proper temperature,
thereby manufactures a fine-particle steel plate of high quality that the ferrite
particle diameter is about 4 µm or less. To realize such a method, the hot rolling
apparatus shown in Fig. 1 adopts a constitution for realizing necessary pressurization
at a comparatively low rolling load and strongly cooling steel to be rolled. By doing
this, if steel to be rolled is strongly cooled (temperature control) under sufficiently
high pressure, by a rolling apparatus generally tandem, a hot rolled steel plate of
fine-particle steel of extremely high quality can be produced industrially.
[0071] However, in the aforementioned embodiment, there is a room for improvement in respect
of lightening the burden imposed on the equipment or running and manufacturing a hot
rolled steel plate of fine-particle steel most effectively. Namely, by further study
of the influence of the conditions of pressurization and cooling on the metallic structure
of steel to be rolled, the reduction in the quality (ferrite particle diameter, etc.)
is suppressed inasmuch as is possible, and the manufacturing conditions are relaxed,
and a fine-particle steel plate can be manufactured at a low cost.
[0072] By improving the rolling method from such a flank of cost to effect, a fine-particle
steel plate which is fully practical but is on a slightly low level of quality (particle
diameter, etc.) can be easily produced commercially. If the high-level high pressurization
explained in the aforementioned embodiment is always essential regardless of the quality
of a steel plate, the production cost is increased in relation to the constitution
of the rolling apparatus and consumption of the rolls and the cooling unit also requires
a higher equipment cost and running cost due to working heat generation of steel to
be rolled accompanying high pressurization.
[0073] The hot rolling apparatus and method according this embodiment solve those problems.
[0074] The continuous hot rolling apparatus according to this embodiment shown in Fig. 10
is a so-called finishing rolling apparatus for the steel P to be rolled, and on the
upstream side (not shown in the drawing) in the flow direction of the steel P to be
rolled, a heating furnace and a rough rolling apparatus are installed, and on the
downstream side (not shown in the drawing), a run-out table and a winder are arranged.
The hot rolling apparatus is composed of mills 10 to 60 of 6 stands in total respectively
having rolls which are arranged tandem, continuously rolls the steel P to be rolled
roughly rolled on the upstream side, thereby generally manufactures various hot rolled
plates with a thickness of about 2 to 16 mm. To smoothly execute the normal rolling
for manufacturing a steel plate having a general internal structure (the mean ferrite
particle diameter is 10 µm or more) and execute rolling of fine-particle steel by
setting proper running conditions, that is, manufacture a hot rolled steel plate of
fine-particle steel having a fine ferrite structure, the rolling apparatus shown in
Fig. 10 is structured as indicated below.
[0075] Firstly, as 3 stands on the preceding stage, the so-called CVC mills 10, 20, and
30 are arranged tandem. The CVC mill 10 positioned closest to the entrance side of
the hot rolling apparatus is structured as a quadrupole mill composed of work rolls
101a and 101b and backup rolls 101c and 101d as shown in Fig. 10 and the work rolls
101a and 101b have crowns (CVC, that is, continuous diameter changes) as shown in
Fig. 11A. The work rolls 101a and 101b, as shown in Figs. 11B and 11C, can move (shift)
in the long axial directions opposite to each other at the same time, thus the position
relationship between the rolls, that is, the roll gap can be adjusted. The diameter
of the work rolls 101a and 101b is set to 700 mm and the maximum shift amount is set
to 100 mm in both forward and backward directions. The CVC mills 20 and 30 of the
other two stands are not different from the CVC mill 10 in the constitution and function.
[0076] The reason that the CVC mills 10, 20, and 30 are arranged on the preceding stage
like this is that the crown (shape) of the steel P to be rolled is to be kept suitably.
In the different-diameter roll mills 40, 50, and 60, which will be described later,
on the later stage, at the time of rolling fine-particle steel, thermal crowns caused
by working heat generation due to rolling are easily formed, so that plate crowns
are corrected beforehand by the CVC mills 10, 20, and 30 installed on the preceding
stage and the medium drawing of the steel P to be rolled can be reduced. Respectively
to the work rolls 101a and 101b of the CVC mills 10, 20, and 30, an AC motor (not
shown in the drawing) with a variable speed control means attached is connected via
a speed reducer and a universal coupling (both are not shown in the drawing).
[0077] As 3 stands on the subsequent later stage, the so-called different-diameter roll
mills 40, 50, and 60 are arranged tandem. The stand intervals of all the 6 stands
including the CVC mills 10, 20, and 30 aforementioned are all equal such as 5.5 m.
The different-diameter roll mill 40 corresponding to the 4th stand counted from the
CVC mill 10 is structured as a quadrupole mill composed of work rolls 104a and 104b
and backup rolls 104c and 104d as shown in Fig. 10 and in this example, the work rolls
104a and 104b have different diameters. Among the work rolls 104a and 104b, only the
lower roll 104b with a large diameter is driven to rotate by a motor (not shown in
the drawing, an AC motor with a variable speed control means) connected via the speed
reducer (not shown in the drawing) and the universal coupling and the upper roll 104a
with a small diameter is structured so as to freely rotate free of driving force.
The work rolls 104a and 104b are respectively provided with a bender (not shown in
the drawing), so that the work rolls 104a and 104b can be provided with bending. Further,
the work rolls 104a and 104b are given the CVC function and can be moved forward and
backward in the long axial direction within a range of 100 mm. The diameter of the
work roll 104a is 480 mm and the diameter of the work roll 104b is 600 mm, so that
the equivalent roll diameter which is a mean value of the two is small such as 540
mm. In the constitution and function aforementioned, the different-diameter roll mills
50 and 60 of the other 2 stands positioned behind are not different from the different-diameter
roll mill 40.
[0078] The equivalent roll diameter is small, and only one work roll 104b is driven, thus
shearing force operates on the steel P to be rolled, so that the different-diameter
roll mills 40, 50, and 60 of 3 stands can execute rolling at a high pressurization
rate (for example, a pressurization rate of 50%) even at a comparatively low rolling
load. Therefore, high-pressure rolling for rolling fine-particle steel can be executed
extremely at a small rolling load and moreover, at that time, the rolling load is
small, so that even for rolling a thin plate with a thickness of about 2 mm, faults
due to the roll flatness and edge drops can be avoided.
[0079] To continuously execute rolling of fine-particle steel, it is necessary to sufficiently
cool the steel P to be rolled and keep it within a proper temperature range, so that
on each back and/or front of the mills 40, 50, and 60 of the stands on the last stage
of the hot rolling apparatus, as shown in Fig. 10, curtain wall type coolers 107 (numerals
107A to 107H shown in Fig. 12) are arranged. The coolers 107 are cooling unit for
flowing and hitting a large amount of cooling water at normal temperature (laminar
flow, for example, numeral f shown in Fig. 12) in a curtain shape (curtain wall shape)
toward the full-width surface of the steel P to be rolled from the headers installed
above or below. The thickness of cooling water to flow in a curtain shape (curtain
thickness) must be 10 mm or more and is preferably about 16 mm from the viewpoint
of the cooling effect. The amount of cooling water of each cooler 107 can be adjusted
within the range from 100 to 500 m
3/h per unit width (1 m) of the steel P to be rolled and the temperature lowering rate
of the steel P to be rolled by cooling is set to 20°C/s or more. When strong pressurization
is to be added, cooling water of 350 m
3/h per unit width is used. However, the temperature lowering rate of the steel P to
be rolled at that time reaches 60 to 80°C/s (about 40°C/s including the temperature
rise due to working heat generation) when the produce of plate thickness and speed
is 1200 mm · mpm.
[0080] The plurality of coolers 107 shown in Fig. 10, as shown in Fig. 12, are arranged
above and below the steel P to be rolled, and above the steel P to be rolled, the
coolers 107A, 107B, 107D, 107E, and 107G are respectively arranged on the back of
the mill 40, the front and back of the mill 50, and the front and back of the mill
60, and below the steel P to be rolled, the coolers 107C, 107F, and 107H are respectively
arranged on the backs of the mills 40, 50, and 60. Among them, the cooler 107H is
mounted to the frame of the roller table T on the back of the mill 60 on the last
stage and the other coolers 107A to 107G are mounted to the housings of the respective
stands.
[0081] By using the curtain wall type coolers 7 on each exit side of the mills 40. 50, and
60 of the 3 stands on the later stage, even when the high-pressure rolling method
and control rolling method accompanied by remarkable working heat generation are to
be executed using the hot rolling apparatus of this embodiment, the temperature rise
of the mills 40, 50, and 60 is suppressed, and the steel P to be rolled is kept within
a proper temperature range, and an occurrence of particle growth of the fine-particle
structure can be suppressed after rolling. Further, even in a run-out table (not shown
in the drawing) on the downstream side of the hot rolling apparatus shown in Fig.
10, the steel P to be rolled is cooled by cooling water so as to prevent particle
growth.
[0082] Further, as shown in Fig. 10, in the hot rolling apparatus, on the exit side of the
mill 60 which is a stand on the last stage and at a position on the downstream side
by several hundreds mm to 1 m from the curtain wall type coolers (107G, 107H), a water
jet spray 108 is arranged. The reason is that cooling water put on the surface of
the steel P to be rolled is removed by the coolers 107G and 107H and from a plurality
of nozzles (not shown in the drawing), to the surface of the steel P to be rolled,
pressurized water is blown out slantwise downward to the upstream side in the flow
direction of the steel P to be rolled from above the steel P to be rolled so as to
spread also in the width direction of the steel P to be rolled. By use of the water
jet spray 108, cooling water put on the steel P to be rolled by the operation of the
cooling unit 107 can be smoothly removed, so that by various measuring instruments
(thermometer, etc., not shown in the drawing) installed on the downstream side, various
values (rolling end temperature, etc.) concerning the steel P to be rolled after rolling
can be measured properly. When the measuring accuracy is high, the rolling conditions
such as the rolling end temperature can be accurately controlled under control of
the amount of cooling water.
[0083] By a thermometer installed at a position on the downstream side of the water jet
spray 108 and on the downstream side by about 2 m from the mill 60 on the last stage,
the rolling end temperature of the steel P is measured and by a calculation operation
means (not shown in the drawing) receiving the measured results, the amount of cooling
water of each curtain wall type cooler 107 (particularly the coolers 107E, 107G, and
107H holding the mill 60 on the last stage) is increased or decreased. The rolling
end temperature is controlled by the feedback control and kept within a proper range.
[0084] In the continuous hot rolling apparatus structured as mentioned above, at a sufficient
speed (for example, 7 to 9 m/s) to ensure good productivity, a good hot rolled steel
plate of fine-particle steel with a thickness of about 2 to 6 mm can be produced.
Concretely, by rolling so as to obtain a cumulative strain (ε
c which is the aforementioned totalized value) of 0.6 or more and strongly cooling
by the curtain wall type coolers 107 on each back of the mills 40, 50, and 60 on the
later stage, a preferable fine-particle steel plate with a mean ferrite particle diameter
of about 3 to 7 µm can be produced by using steel having a low carbon content and
alloy element content as steel to be rolled. Some fine-particle steel may have a short
elongation and such a disadvantage can be removed. The embodiment which will be described
later is an example thereof.
[0085] The reason that such good production is made possible is that in the stands on the
later stage which strongly affect the metallic structure, by keeping the temperature
of the steel P to be rolled in a proper range using the curtain wall type coolers
107 having high cooling capacity, rolling at a high-pressurization rate producing
the aforementioned cumulative strain can be executed by the different-diameter roll
mills 40, 50, and 60 with a small diameter. In the mills 40, 50, and 60, roll flatness
and edge drops can be avoided and crowns can be controlled by the CVC function of
the mills 10 to 60, so that also on the later stage where the steel plate is made
thinner, meandering of the steel P to be rolled and changing of the shape can be suppressed.
Therefore, in this embodiment, fine-particle steel can be rolled sufficiently and
smoothly and a steel plate can be formed with high precision in shape.
[0086] That a preferable fine-particle steel plate can be produced under the aforementioned
condition is made clear by the inventors from many tests and investigation which are
executed by using the hot rolling apparatus shown in Fig. 10 and variously changing
the degree of cooling the steel P to be rolled (rolling end temperature) and the degree
of pressurization (cumulative strain). Results of such tests and investigation and
date concerning an embodiment that a preferable fine-particle steel plate is obtained
are indicated below.
[0087] Test rolling is executed by using the continuous hot rolling apparatus in this embodiment
and variously changing the pass schedule and rolling end temperature for the steel
kind (no other significant components included) shown in Table 2-1. However, in every
case, the plate thickness on the exit side of the mill 60 on the last stage is 2 to
3 mm and the rolling speed is 8 to 9 m/s.
[Table 2-1]
Chemical components of steel (weight %)
Transformation point (°C) |
|
C |
Si |
Mn |
Ar3 |
Embodiment |
0.16 |
0.2 |
0.8 |
785 |
[0088] For many steel plates obtained by the test rolling, the ferrite particle diameter
at the center of the thickness is measured and the relation between the cumulative
strain during rolling and the finishing temperature (rolling end temperature) is checked.
The relation between the cumulative strain (transverse axis) and the ferrite particle
diameter (ordinate axis) is indicated as shown in Fig. 13. In the drawing, symbol
● indicates data when the finishing temperature is within the range of Ar
3 transformation point ± 10°C, and ▲ indicates data when the finishing temperature
becomes lower than the Ar
3 transformation point - 10°C, and ■ indicates data when the finishing temperature
becomes higher than the Ar
3 transformation point + 10°C (Figs. 13 to 17).
[0089] Fig. 13 shows that when the finishing temperature becomes higher than the Ar
3 transformation point + 10°C, a tendency that the ferrite particle diameter reduces
in accordance with the cumulative strain increases is seen slightly, while when the
finishing temperature is other than it, even if the cumulative strain is increased,
the ferrite particle diameter is little reduced.
[0090] On the other hand, the relation between the finishing temperature (transverse axis)
and the ferrite particle diameter (ordinate axis) is indicated in Fig. 14. Fig. 14
shows that as the finishing temperature lowers, the ferrite particle diameter is clearly
reduced.
[0091] Further, in Figs. 15 to 17 where the mechanical properties are checked for each manufactured
steel plate and the results are related to the ferrite particle diameter and summarized,
the transverse axis indicates a value of particle diameter (µm) to the power -1/2.
[0092] Fig. 15 shows the relation between the ferrite particle diameter and the tensile
strength (MPa) and Fig. 16 shows the relation between the ferrite particle diameter
and the elongation (%). The drawings show that as the ferrite particle diameter reduces
(on the right of the transverse axis), the tensile strength is apt to increase, while
when the finishing temperature becomes lower than the Ar
3 transformation point - 10°C (▲ in the drawing), as the ferrite particle diameter
is refined, the elongation is reduced. The product (MPa x %) of tensile strength and
elongation, as shown in Fig. 17, is also reduced as the ferrite particle diameter
is refined when the finishing temperature is lower than the Ar
3 transformation point - 10°C.
[0093] The following facts can be confirmed on the basis of these results. Namely:
a) To obtain a hot rolled steel plate of fine-particle steel with a small ferrite
particle diameter by the rolling apparatus (Fig. 10) of this embodiment, setting of
a lower finishing temperature is more effective than setting of a higher cumulative
strain.
b) However, when the finishing temperature is extremely lower than the Ar3 transformation point, the elongation is reduced even if the refinement is progressed,
so that the advantage of strength is reduced.
c) In consideration of that when high pressurization is carried out so as to increase
the cumulative strain, the cost is increased in relation to the constitution of the
rolling apparatus and consumption of the rolls, it is preferable from the viewpoint
of cost to effect to make the cumulative strain not so high, for example, 0.6 (preferably
0.65) or more and less than 0.9 and accurately control the finishing temperature,
thereby obtain a fine-particle steel plate. By keeping the finishing temperature within
the range of Ar3 transformation point ± 50°C, a fine-particle steel plate having a ferrite particle
diameter of 4 to 6 µm and a superior mechanical strength balance can be produced.
Particularly, to obtain a steel plate having a high tensile strength, in order to
obtain a steel plate having a superior elongation by setting the finishing temperature,
for example, within the range from Ar3 transformation point - 50°C to Ar3 transformation point + 20°C, the finishing temperature is preferably set, for example,
within the range from Ar3 transformation point - 20°C to Ar3 transformation point + 50°C. However, from the viewpoint of the degree of each strength
and the balance thereof, it is most preferable to keep the finishing temperature within
the range of Ar3 transformation point ± 10°C.
[0094] The embodiments that good fine-particle steel plates are manufactured on the basis
of the knowledge obtained in this way are introduced in Tables 2-2 to 2-4 and Fig.
18. Further, "F10" to "F60" shown in the tables respectively indicate the mills 10
to 60 of the first stand to the sixth stand.
[0095] Table 2-2 shows the plate thickness ( "Rough bar thickness" indicates the plate thickness
on the exit side of the rough rolling apparatus), pressurization rate (%), strain,
cumulative strain, and plate width on the exit side of each of the mills 10 to 60
and Table 2-3 shows the use state of each curtain wall type cooler 7 on the back of
each of the mills 40 to 60 and the finishing temperature (rolling end temperature).
Table 2-4 shows the ferrite particle diameter and mechanical properties of the steel
plates of the embodiments obtained under the conditions shown in Tables 2-1 to 2-3
at the center of the plate thickness. And, Figs. 18A, 18B, and 18C are drawings showing
the crystalline structure of the steel plates of the embodiment in the neighborhood
of the top surface, the position inward from it by 1/4 of the thickness, and the center
position of the thickness, respectively. At every part, a fine structure with a mean
ferrite particle diameter of about 4 to 6 µm is formed.
[0096] Further, the rolling for obtaining the data shown in Figs . 13 to 17 and the rolling
in this embodiment are executed by the rolling apparatus (refer to Figs. 10 to 12)
of this embodiment. However, for rolling using a cumulative strain of about 0.6 to
0.9, it is inferred that there is no need to use the different-diameter roll mills
40 to 60 mentioned above as stands on the later stage. Namely, even if these mills
have upper and lower work rolls having the same diameter such as about 600 to 700
mm, they are inferred to be enough. Further, if such a degree of cumulative strain
is enough, a thermal crown accompanying working heat generation is expected not to
be remarkable, so that the necessity of giving the CVC function and bending function
to the mills 10 to 60 is considered to be low.
[Table 2-3]
|
Back surface of F40 |
Back surface of F50 |
Back surface of F60 |
Finishing temperature °C |
Embodiment |
Used |
Used |
Used |
782 |
[Table 2-4]
Mechanical properties |
|
Ferrite particle diam. µm |
TS Mpa |
YP Mpa |
EL % |
Embodiment |
4.5 |
519 |
431 |
34 |
TS: Tensile strength, YP: Yielding point, EL: Elongation |
[0097] According to the continuous hot rolling method of this embodiment, a hot rolled steel
plate of fine-particle steel having a sufficiently fine mean ferrite particle diameter,
superior mechanical properties, and sufficiently high practical quality can be manufactured
at an extremely low cost under a moderated condition.
[0098] Namely, by a process of effectively taking generated heat by working during rolling
by the mills on the preceding and last stages and keeping a proper temperature (for
example, keeping the rolling end temperature within the range of ±50°C of the Ar
3 transformation point) by a) executing high pressurization such as a cumulative strain
of 0.6 or more using mills of a plurality of stands and b) strongly cooling the steel
P to be rolled on each exit side of a plurality of mills on the later stage and stopping
particle growth of a fine structure, a hot rolled steel plate of fine-particle steel
with a mean ferrite particle diameter of about 10 µm or less can be manufactured.
[0099] Obtaining of a fine-structure steel plate by this process is made clear by the latest
investigation and study by the inventors. Namely-, it is ascertained that among the
high pressurization condition and strongly cooling condition for steel to be rolled,
even if the former condition is slightly relaxed (that is, even if the cumulative
strain is increased up to 0.9), a high-quality fine-particle steel plate with a ferrite
particle diameter not so rough can be manufactured. Concretely, the mean ferrite particle
diameter can be reduced to about 3 to 7 µm by the aforementioned cumulative strain
and cooling.
[0100] When a cumulative strain of 0.6 or more is enough, the pressurization rate necessary
to the mills, particularly the mills on the later stage is lowered considerably (about
30%) and the cost necessary to the equipment and running is greatly reduced. Therefore,
a situation that the end of the steel P to be rolled is not fit well to any mill and
slips is hardly caused.
[0101] Further, when the mean ferrite particle diameter is 10 µm or less, the fine-particle
steel plate has mechanical properties particularly higher than those of a general
(non-fine-particle steel) hot rolled steel plate having a particle diameter of more
than 10 µm and can be expected to be widely used. Namely, in a fine-particle steel
plate having the aforementioned chemical components and ferrite particle diameter,
the mechanical property balance (general-purpose from the viewpoint of tensile strength,
elongation, and ductility) is high and the weldability is superior. Therefore, the
fine-particle steel plate is widely used, can be obtained easily due to a comparatively
low price, and moreover has a good cyclic property, so that it is considered to be
highly demanded. Therefore, in the rolling method of this embodiment for manufacturing
such a steel plate, the commercial contribution degree is high and sufficient economical
rationality accompanies the production thereof.
[0102] Next, the hot rolling method of another embodiment of the present invention will
be explained.
[0103] The hot rolling method of this embodiment relates to the method for manufacturing
a thick plate using the hot rolling apparatus of the aforementioned embodiment shown
in Fig. 10.
[0104] In the hot rolling apparatus of the aforementioned embodiment shown in Fig. 10, in
the CVC mills 10, 20, and 30 and the different-diameter roll mills 40, 50, and 60,
in consideration of that as the rolling progresses, the plate thickness is reduced
and the rolling speed is increased, the reduction ratio is reduced more for the mills
on the later stage, and the maximum number of rotations of the work rolls is increased,
and the maximum output torque is set low. The allowable maximum output torque values
of the mills 10 to 60 are respectively 125.0, 98.2, 61.4, 34.1, 22.7, and 19.5 (the
unit is ton (tf).m).
[0105] And, by use of all the mills 10 to 60 of the rolling apparatus of the aforementioned
embodiment shown in Fig. 10 and at a sufficient speed (for example, 7 to 9 m/s) to
ensure good productivity, a good hot rolled plate of fine-particle steel with a thickness
of about 2 to 6 mm can be manufactured. Concretely, by rolling so as to obtain a cumulative
strain (ε
c which is the aforementioned totalized value) of 0.6 or more and strongly cooling
by the curtain wall type coolers 107 on each back of the mills 40, 50, and 60 on the
later stage, a preferable fine-particle steel plate with a mean ferrite particle diameter
of about 4 to 6 µm can be produced by using steel having a low carbon content and
alloy element content as the steel P to be rolled. Particularly, when the cumulative
strain is set to 0.9 or more, the mean ferrite particle diameter of the same steel
kind can be reduced to 4 µm or less. The comparison example A which will be indicated
later is an example (when ε
c 0.6) thereof. The reason that such production is made possible is that in the stands
on the later stage which strongly affect the metallic structure, by keeping the temperature
of the steel P to be rolled in a proper range using the curtain wall type coolers
107 having high cooling capacity, rolling at a high-pressurization rate producing
the aforementioned cumulative strain can be executed by the different-diameter roll
mills 40, 50, and 60 with a small diameter. In the mills 40, 50, and 60, roll flatness
and edge drops can be avoided and crowns can be controlled by the CVC function of
the mills 10 to 60, so that also on the later stage where the steel plate is made
thinner, meandering of the steel P to be rolled and changing of the shape can be suppressed.
This respect is also one of the reasons that such rolling of fine-particle steel is
made possible.
[0106] However, when a thick fine-particle steel plate with a thickness of 6 mm or more
instead of a thin plate is to be produced using up to the mill 60 on the last stage
in the same way, the output torque is insufficient in the mill 60 on the last stage
(or additionally the mill 50 on the preceding stage thereof) and the rolling may not
be continued (the motor is stopped). The reason is that in a case of a thick plate,
even when the pressurization rate is almost equal to (or smaller than) that of a thin
plate, the contact arc length is longer than that of a thin plate, thus large rolling
torque is necessary. In the mill 60 on the last stage and the mill 50 on the preceding
stage, the allowable maximum output torque is small as mentioned above, so that the
load becomes higher than the capacity, thus the rolling cannot be continued. Such
a case is indicated in the comparison example B which will be described later.
[0107] The reason that the mills on the later stage cannot realize sufficient rolling torque
can be explained as indicated below. Firstly, in the mills on the later stage, the
roll driving system is under a high-speed specification so as to correspond to an
increase in the rolling speed accompanying a decrease in the plate thickness due to
progressing of rolling and as compared with the mills on the preceding stage, the
mills on the later stage are generally set so that the rotational speed is high (that
is, the reduction ratio is small) and the rolling torque is low. On the other hand,
when a thick plate is to be rolled, even if the pressurization rate is the same as
that at the time of rolling a thin plate, the contact arc length (contact length)
on the entrance side is long (the contact angle is large), so that the necessary torque
is considerably larger than that when a thin plate is rolled. Therefore, in the mills
having low torque on the later stage, although a thin plate can be rolled smoothly,
pressurization necessary for the equipment capacity is applied to the thick plate,
so that it is apt to be difficult to manufacture a thick fine-particle steel plate.
[0108] Further, with respect to the aforementioned problem concerning manufacture of a thick
fine-particle steel plate by a rolling apparatus that mills of a plurality of stands
are arranged tandem, no documents indicating it are found. The art described in the
patent publication referred in the present specification as a related art relates
to manufacture of a thin fine-particle steel plate with a thickness of 3 mm or 5 mm
or less or manufacture using a rolling apparatus of a reverse type.
[0109] Therefore, the inventors, to produce a thick fine-particle steel plate with a thickness
of 6 mm or more using the continuous hot rolling apparatus of the aforementioned embodiment
shown in Fig. 10, that is, a continuous hot rolling apparatus capable of manufacturing
a thin fine-particle steel plate, operate the rolling apparatus in the states of a)
to d) indicated below. Namely:
a) The mill 60 having small output torque on the last stage is not used. Even the
preceding mills 40 and 50, when the allowable maximum output torque is smaller than
required torque calculated from the plate thickness, pressurization rate, and deformation
resistance, are not used. Therefore, from the mills 10 to 50 closer to the entrance
side of the rolling apparatus than the mill 60 on the last stage, 3 or more stands
satisfying the rolling torque are selected and used according to the pass schedule.
b) The pass schedule is decided so as to set the cumulative strain to 0.25 or more
(preferably 0.29 or more) or set the pressurization rate by the mill on the last stage
among the mills of 3 or more stands to be used to 12% or more (preferably 14% or more).
The reason is that unless the rolling having strong power of influence on the metallic
structure on the downstream side is executed at a pressurization rate which is constant
or more, it is difficult to make the ferrite particle diameter smaller.
c) The steel plate is strongly cooled (so as to control the temperature lowering rate
of the surface to about 40°C per second) using the curtain wall type coolers 107.
With respect to the coolers 107, the one immediately after the mill on the last stage
among the mills to be used is used. All the coolers 107 (107A to 107H) including the
cooler before the mill on the last stage are preferably used. The reason is that to
make the ferrite particle diameter smaller, it is essential to sufficiently cool the
steel P to be rolled immediately after rolling so as to keep it within a proper temperature
range and exactly suppress the particle growth after rolling.
d) By the cooling c), the rolling end temperature (the surface temperature of the
steel P to be rolled measured by a thermometer installed on the downstream side by
several m from the mill 60 on the last stage) is controlled not to exceed the Ar3 transformation point + 50°C (preferably the Ar3 transformation point or lower). Although a preferable lower limit ought to exist,
even if the surface temperature lowers considerably, the production of fine-particle
steel is not impeded. The reason is inferred to be that as long as a steel plate with
a thickness of 6 mm or more is rolled and manufactured at a speed of about 2 to 3
m/s, the temperature in the neighborhood of the center of the plate thickness of the
steel P to be rolled is kept at about the Ar3 transformation point regardless of the surface temperature.
[0110] By executing rolling as mentioned above, a thick hot rolled steel plate of fine-particle
steel with a mean ferrite particle diameter of about 5 to 10 µm on the inside of the
surface by 1/4 of the thickness can be produced for the steel kind having a carbon
content of 0.5% and an alloy element content of 5%. Data concerning production of
such a thick steel plate is indicated below as Embodiments C and D.
[0111] Regarding the aforementioned production of thin and thick hot rolled steel plates
of fine-particle steel by the continuous hot rolling apparatus, data concerning rolling
are indicated below. In the tables, Comparison A, as described above, relates to production
of thin (thickness of 2.07 mm) steel plates and Comparison B indicates an example
that in production of thick steel plates using the mills 10 to 60, the rolling cannot
be continued. And, Embodiments C and D indicate examples that thick (thickness of
12.2 mm) fine-particle steel plates are produced smoothly and continuously using the
rolling apparatus.
[0112] Firstly, Table 3-1 indicates chemical components (no significant components other
than the indicated ones are included) of steel plates and the temperature at the Ar
3 transformation point in the embodiments and Comparison examples A to D and Table
3-2 indicates the rolling end temperature (finishing temperature on the exit side),
the plate width of each steel plate, and the use state of the curtain wall type coolers
107 on each back of the mills 40 to 60. Table 3-3 indicates the plate thickness on
each exit side of the mills 10 to 60 ("Rough bar thickness" indicates the plate thickness
on the exit side of the rough rolling apparatus). Tables 3-4, 3-5, and 3-6 indicate
the pressurization rate (%), strain, cumulative strain, and required rolling torque
(ton.m) of the mills 10 to 60 when the pass schedule in Table 3-3 is applied.
[Table 3-1]
Chemical components of steel (weight %) Transformation point |
Embodiment Comparison example |
Component value
C |
Component value
Si |
Component value
Mn |
Component value
P |
Ar3
[°C] |
Comparison A |
0.16 |
0.2 |
0.8 |
0.014 |
785 |
Comparison B |
0.15 |
0.18 |
0.77 |
0.02 |
795 |
Embodiment C |
0.17 |
0.21 |
0.8 |
0.014 |
785 |
Embodiment D |
0.17 |
0.21 |
0.8 |
0.014 |
785 |
[Table 3-2]
Pass schedule Cooling condition (curtain wall) |
Embodiment Comparison example |
Finishing temp. on exit side
[°C] |
Flat width
[mm] |
Back surface of F40 |
Back surface of F50 |
Back surface of F60 |
Comparison A |
782 |
670 |
Used |
Used |
Used |
Comparison B |
757 |
660 |
Used |
Used |
Used |
Embodiment C |
679 |
660 |
Used |
Used |
Used |
Embodiment D |
676 |
660 |
Used |
Used |
Used |
[Table 3-3]
Embodiment, Comparison example |
Rough bar thickness |
Plate thickness of F10
[mm] |
Plate thickness of F20
[mm] |
Plate thickness of F30
[mm] |
Plate thickness of F40
[mm] |
Plate thickness of F50
[mm] |
Plate thickness of F60
[mm] |
A |
40.0 |
22.28 |
13.19 |
7.78 |
4.52 |
2.85 |
2.07 |
B |
39.8 |
39.8 |
31.1 |
24.5 |
19.2 |
15.0 |
12.2 |
C |
32.2 |
21.2 |
16.6 |
14.1 |
12.2 |
12.2 |
12.2 |
D |
36.1 |
23.4 |
18.2 |
15.4 |
12.2 |
12.2 |
12.2 |
[Table 3-4]
Embodiment, Comparison example |
Pressurization rate of F10
[%] |
Pressurization rate of F20
[%] |
Pressurization rate of F30
[%] |
Pressurization rate of F40
[%] |
Pressurization rate of F50
[%] |
Pressurization rate of F60
[%] |
Comparison A |
44 |
41 |
41 |
42 |
37 |
28 |
Comparison B |
|
22 |
21 |
22 |
22 |
19 |
Embodiment C |
34 |
22 |
15 |
14 |
|
|
Embodiment |
35 |
22 |
15 |
21 |
|
|
[Table 3-5]
Embodiment, Comparison example |
Strain of F10
[-] |
Strain of F20
[-] |
Strain of F30
[-] |
Strain of F40
[-] |
Strain of F50
[-] |
Strain of F60
[-] |
Cumulative strain
[-] |
A |
0.56 |
0.51 |
0.52 |
0.53 |
0.45 |
0.32 |
0.68 |
B |
|
0.25 |
0.24 |
0.24 |
0.24 |
0.20 |
0.39 |
C |
0.41 |
0.24 |
0.16 |
0.15 |
|
|
0.29 |
D |
0.43 |
0.25 |
0.17 |
0.23 |
|
|
0.38 |
[Table 3-6]
Embodiment, Comparison example |
Rolling torque of F10
[ton.m] |
Rolling torque of F20
[ton.m] |
Rolling torque of F30
[ton.m] |
Rolling torque of F40
[ton.m] |
Rolling torque of F50
[ton.m] |
Rolling torque of F60
[ton.m] |
Comparison A |
112 |
59 |
48 |
31 |
19 |
16 |
Comparison B |
|
38 |
36 |
25 |
23 |
23 |
Embodiment C |
73 |
38 |
14 |
18 |
|
|
Embodiment D |
85 |
33 |
17 |
30 |
|
|
[0113] Table 3-6 shows that in Comparison example B that the rolling cannot be continued,
the torque necessary to the mill 60 on the last stage is large such as 23 ton.m and
it is larger than the aforementioned allowable maximum torque (19.5 ton.m) of the
mill 60. Further, in Embodiment D, as shown in Table 3-5, stronger pressurization
such as a cumulative strain of 0.38 is applied, so that it is found in Table 3-6 that
in the mill 40 on the last stage among the mills used, large torque such as 30 ton.m
(that is, torque not realized in the mill 50 or 60 on the later stage) is necessary.
[0114] Check results of the ferrite particle diameter and mechanical properties of steel
plates produced in the embodiments and comparison examples A to D are shown in Table
3-7. However, in Comparison example B, data of steel plates obtained for a short time
until the rolling is disabled are indicated. The indicated particle diameters are
measured at the center of the thickness in Comparison example A and measured at the
position inside the surface by 1/4 of the thickness in Comparison example B and Embodiments
C and D. In the table, "TS" indicates tensile strength, "YP" a yielding point, and
"EL" an elongation and "L direction" means the length direction (rolling direction)
and "C direction" the width direction. In all cases, it is found that a steel plate
that the ferrite particle diameter is sufficiently small and the mechanical properties
are excellent can be obtained.
[Table 3-7]
Mechanical characteristics
TS: tensile strength, YP: yielding point, EL: elongation |
Embodiment, Comparison example |
Particle diam. in L dir.
[µm] |
Particle diam. in C dir
. [µm] |
TS in L direction
[MPa] |
YP in L direction
[MPa] |
EL in L direction
[%] |
TS in C direction
[MPa] |
YP in C direction
[MPa] |
EL in C direction
[%] |
A |
4.5 |
4.5 |
519 |
431 |
34 |
528 |
495 |
34 |
B |
7.6 |
8.0 |
487 |
345 |
29 |
489 |
368 |
29 |
C |
6.6 |
6.7 |
519 |
387 |
26 |
530 |
419 |
25 |
D |
6.6 |
6.7 |
530 |
394 |
24 |
537 |
444 |
22 |
[0115] Figs. 19A, 19B, and 19C are drawings showing the crystalline structure of steel plates
obtained by the embodiment D in the neighborhood of the top surface, the position
inward from it by 1/4 of the thickness, and the central position of the thickness,
respectively. In the position of 1/4 of the thickness, a fine structure with a mean
ferrite particle diameter of 5 to 10 µm is formed and at the center of the thickness,
a fine structure with a mean ferrite particle diameter of 10 µm or less is formed.
[0116] Further, Figs. 20 to 22 show other mechanical properties of steel plates produced
under the rolling condition of Embodiment D or similar to it which are checked and
arranged. Namely, firstly, Fig. 20 is a drawing showing the relation between the ferrite
particle diameter, the tensile strength, and the yielding point of a fine-particle
steel plate (the transverse axis indicates a value of the ferrite particle diameter
d (µm) to the power -1/2). And, for the same fine-particle steel plate, Fig. 21 shows
temperature changes of the Charpy impact value together with changes of normal steel
(non-fine particle steel plates) and Fig. 22 shows the temperature dependency of the
brittle fracture rate. In addition, for the produced same steel plates, the welding
coupling tensile test, coupling bending test, coupling impact test, micro test, and
hardness distribution check test based on JIS Z 3040, "Check test method for welding
method" are executed for a plurality of test samples and it is confirmed that the
weldability of fine-particle steel plates is satisfactory.
[0117] As mentioned above, by the continuous hot rolling method of this embodiment, by using
mills of a plurality of stands arranged so as to manufacture thin plates, thick fine-particle
steel plates can be manufactured free of faults due to insufficient torque. The reason
is that even when the mills on the later stage including the mill on the last stage
become insufficient in torque, if those mills are not used and only the mills close
to the entrance side of a rolling apparatus having a driving system capable of realizing
high rolling torque under a so-called low speed specification are used, sufficient
pressurization can be executed free of insufficient torque also in a case of rolling
a thick plate with a long contact arc length. Although the rolling speed is not increased
because the mill on the last stage is not used, there is an advantage that since the
rolling speed becomes slow, the required time for cooling prolonged due to a thick
plate can be easily ensured.
[0118] The reason that a thick plate of fine-particle steel can be rolled as mentioned above
is that stronger pressurization such as a cumulative strain of 0.25 or more (or the
pressurization rate at the mill on the last stage is 12% or more) is applied to the
steel P to be rolled and on the exit side of the mill on the last stage among the
mills used, the steel P is cooled for a sufficient time. As the aforementioned cooling
on the exit side of the mill becomes stronger, fine-particle steel with a smaller
ferrite particle diameter can be obtained. Further, in a sense of strengthening cooling,
it is preferable to execute cooling also before the used mill on the last stage or
execute cooling also on each exit side of a plurality of mills on the later stage.
[0119] The continuous hot rolling method of this embodiment is particularly characterized
in that the rolling end temperature is set not to exceed the Ar
3 transformation point + 50°C.
[0120] When the aforementioned cooling power is controlled and the rolling end temperature
is set as mentioned above, at least in the neighborhood of the surface of a steel
plate (for example, a steel plate having a carbon content of 0.5% or less and an alloy
element content of 5% or less), a fine structure with a ferrite particle diameter
of less than 10 µm is formed. The temperature range suited to the high-pressure rolling
method is assumed to be from Ar
3 transformation point to Ar
3 transformation point + 50°C. However, according to the test made by the inventors,
it is enough that the rolling end temperature is within the range not exceeding the
Ar
3 transformation point + 50°C, as mentioned above. The reason is considered to be that,
in a case of a thick plate, even if the surface temperature is low, the internal temperature
is kept close to the Ar
3 transformation point.
[0121] Further, the continuous hot rolling method of this embodiment strongly cools the
steel P to be rolled by the curtain wall type coolers 107, so that a fine-particle
steel plate with a particle diameter which is particularly fine can be manufactured
smoothly. Since uniform cooling can be realized, there is an advantage that the structure
can be made uniform in full width of the steel plate.
[0122] The continuous hot rolling method of this embodiment is particularly characterized
in that the steel P to be rolled having a carbon content of 0.5% or less and an alloy
element content of 5% or less is rolled and a thick plate with a mean ferrite particle
diameter of about 3 to 10 µm at the part inside the surface by 1/4 of the thickness
can be obtained.
[0123] A fine-particle steel plate having the chemical components and ferrite particle diameter
mentioned above has a high mechanical property balance (general-purpose from the viewpoint
of tensile strength and ductility) and moreover low temperature brittleness and high
weldability (for example, refer to Figs. 20 to 22). Therefore, such a fine-particle
steel plate is widely used, can be obtained easily due to a comparatively low price,
and moreover has a good cyclic property, so that it is considered to be highly demanded.
Therefore, for such a steel plate, the commercial contribution degree is high and
sufficient economical rationality accompanies the production thereof.