Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
NKD-F883
PIPE FORMING LINE AND PIPE FORMING METHOD
TECHNICAL FIELD
This invention relates to a new pipe forming line
and pipe forming method for, in the production of steel
pipe, particularly electric welded steel pipe, by
continuous roll forming, conducting steel pipe forming by
forming rolls capable of flexible forming and compatible
with steel pipes of multiple sizes without the use of
cage rolls.
BACKGROUND TECHNOLOGY
In the production of electric welded steel pipe, the
mainstream technology widely considered ideal for large-
lot production of limited types is that of utilizing
forming rolls suitable for forming the desired bore and
wall thickness, laying out a line of forming rolls
matched to the bore, and conducting continuous roll
forming for continuously forming a steel strip. This
production process, which precedes the welding of the
electric welded steel pipe, is generally divided into the
three process of initial forming (breakdown forming),
intermediate forming (cluster forming or cage forming)
and finish forming (fin-pass forming). The product is
obtained following a final squeeze process in which
welding is conducted. Although the initial forming and
intermediate forming are sometimes called "breakdown
forming," they will be distinguished in this explanation.
In combined forming, particularly at the start thereof,
the steel strip constituting the raw material has the
basic shape of an open plate. To apply bending, upper and
lower (concave and convex) rolls are used to form the
steel strip by simultaneously restraining its inner and
outer surfaces. During the intermediate forming,
corresponding to the latter half of the combined forming
process, the steel strip gradually approaches pipe shape.
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Use of convex rolls (inner rolls), even if possible, is
extremely difficult from the aspect of equipment design.
The current practice is therefore to restrain the steel
strip from the outer surface using ordinary concave
rolls.
with supply and demand for small-variety, big-lot
production recently on the decline, in order to achieve
lower cost and enhanced competitiveness through use of
rolls with flexible forming capability, proposals have
been made regarding improvement of the cage roll forming
method, with particular emphasis on enabling flexible
forming in the intermediate forming process, and
regarding flexible forming not only in the intermediate
forming process but also at the breakdown forming section
where the focus is on edge bending. Typical of these is
the method taught by JP-B(examined published Japanese
patent application)-3-12977, commonly called the "FF
mill~~ method, which uses forming rolls of such sectional
shape that part or all of the sectional curve of the roll
surface of. each forming roll is a curve whose curvature
is preset to vary continuously or stepwise to include the
curves of the steel strip edge portions at the roll
flower (roll-designed profile) of various steel pipes for
forming steel pipes of the various outer diameters, wall-
thickness, materials envisioned, and which forms both
edge portions of the steel strip by paired upper and
lower rolls having the sectional configuration of this
curve.
However, a number of problems have arisen in
connection with the aforesaid flexible forming. The
problems that have emerged in the ordinary mill designed
for flexible forming include, for example, that 1) the
need for greater adjustment freedom and adjustment space
complicates the mechanical structure, canceling the cost-
reduction effect of flexible forming and making the mill
relatively low in robustness, and that 2) expansion of
the range of good quality flexible forming becomes
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increasingly difficult and constitutes a cause of welding
instability and product quality degradation.
Between the breakdown step and the beginning of the
fin-pass step, a particular problem in the method that
disposes a large number, ordinarily 10-30 pairs, of
small-diameter rolls in the forming direction, i.e., the
so-called cage method, is that the desired bent shape
cannot be obtained owing to the weak forming capability
of the cage rolls and that, therefore, the actual bent
shape frequently differs greatly from the roll flower
shape of the design. In other words, this kind of
flexible forming is a compromise that sacrifices the
forming capability of the overall mill.
The object of the present invention is to provide a
new pipe forming line and pipe forming method that, in
the production of steel pipe, particularly electric
welded steel pipe, by continuous roll forming, enables
stable steel pipe forming by forming rolls having
flexible forming capability and compatibility with steel
pipes of multiple sizes without use of cage rolls.
DISCLOSURE OF THE INVENTION
The present invention is for overcoming the
foregoing problems and, specifically, provides a pipe
forming line, making no use whatsoever of the aforesaid
cage rolls, that is installed with the aforesaid FF mill
edge portion bending stands at least the initial few
stages, preferably 2-3 stages, of the upper and lower
rolls installed in multiple stages of the aforesaid
breakdown (BD) process, forms a substantially
semicircular or substantially final product shape by
combinations of the upper and lower rolls of these
multiple stages, and directly conducts fin-pass forming
in the fin-pass process that follows, or is disposed with
multiple stages, preferably 2-3 stages, of conventional
side roll stands for connection with the fin-pass
process, thereby imparting a flexible forming size range
over a ratio of steel pipe outside diameters of 1 . 3 at
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higher accuracy than the conventional cage-type mill, and
also provides a pipe forming method.
The gist thereof is (1) a pipe forming line
comprising breakdown forming stands, cluster forming
stands or cage forming stands, and fin-pass stands, which
pipe forming line is characterized in that it
successively conducts bend-forming from both edge
portions toward the center of a steel strip in at least
multiple stages of the breakdown forming stands by use of
one set of common-use forming rolls of such sectional
shapes that part or all of the sectional curve of the
roll surface of each forming roll is a curve whose
curvature is preset to vary continuously or stepwise to
include the curves of the steel strip cross-section at
the roll flowers of various steel pipes for forming steel
pipes of the various outer diameters, wall-thickness,
materials envisioned, and is directly installed on the
upstream side of a fin-pass section without being
installed with cluster rolls or cage rolls, and (2), in
the foregoing (1), a pipe forming line characterized in
being installed with one stage or multiple stages of
stands having cluster rolls or side rolls for connection
between the breakdown forming process and the fin-pass
process.
The present invention also provides a pipe forming
method comprising a breakdown process, a cluster process
or an intermediate forming process, and a fin-pass
process, which pipe forming method is characterized in
carrying out pipe forming by conducting breakdown forming
with multiple stands equipped with a set of upper and
lower rolls having flexible forming capability of such a
sectional shape that part or all of the sectional curve
of the roll surface of each forming roll is a curve whose
curvature is preset to vary continuously or stepwise to
include the curvatures of the deformed shapes at the roll
flowers of various steel pipes for forming steel pipes of
the the various outer diameters, wall-thickness, material
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envisioned, thereby enabling bend-forming in the
breakdown process of pipes of multiple sizes having
different outer diameters, wall-thickness, material to a
substantially semicircular shape of the final product or
a substantially final product shape, and conducting fin-
pass forming in a fin-pass process either after
intermediate forming with the side rolls of multiple
stands or directly without conducting intermediate
forming.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic view of a conventional cage-
forming mill.
FIG. 2 is a cross-sectional view of the A-A region
of FIG. 1 .
FIG. 3 is a cross-sectional view of the B-B region
of FIG. 1.
FIG. 4 is a schematic view of the pipe forming line
of a conventional FF mill.
FIG. 5 is a schematic view of a pipe forming line
according to present invention.
FIG. 6 is a schematic view of another example of a
pipe forming line according to present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
As a result of the various studies conducted
regarding the aforesaid flexible forming, the present
inventors acquired the following knowledge. First, they
discovered that, from the viewpoint of the caliber and
the roll layout of the forming rolls used in the
different forming processes of electric welded steel
pipe, flexible forming is most difficult in the fin-pass
forming and is the sticking point of flexible forming
throughout the mill. The only way to overcome this is to
eliminate the conventional fin-pass forming. This fin-
pass forming has a number of functions, however, and
cannot be completely done away with until these functions
are transferred to other forming stages. Nevertheless,
the load on the fin-pass forming can be markedly reduced.
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As a measure for reducing the load on the fin-pass
forming, the sectional shape can be corrected with small
reduction by making the sectional shape of the steel
strip as close as possible to circular or the shape of
the fin-pass roll caliber before entering the fin-pass
forming section. Although this can be achieved by
upgrading the forming capability of the breakdown forming
or the cluster forming (cage roll forming) to which the
fin-pass functions are to be transferred, up to now
flexible forming has mainly been implemented only with
respect to the aforesaid intermediate forming. In the
case of using cage rolls, while flexible intermediate
forming would seem easy to conduct, the practice has been
to leave the deformation properties of the steel strip,
the raw material, out of consideration and, focusing only
on the geometry, to rely solely on empirical roll design
based on the roll flower. Particularly in the case of
flexible forming, since caliber rolls are not used and
only a portion of the steel strip width is restrained,
the result of the actual formation often differs greatly
from the original design due to the fact that the roll
flower is not the actual deformed shape of the steel
strip cross-section and the fact that, owing to the
successive nature of the roll forming, differences
occurring between the two on the upstream side tend to
accrue and affect the downstream side.
In cage forming using the aforesaid cage rolls,
although the basis for flexible forming is the geometric
similarity of the different sized roll flowers of the
intermediate forming section, these roll flowers are not
the actual deformed shape. Of particular note is that the
difference between the roll flower and the actual
sectionally deformed shape is already great following
breakdown forming before entering the intermediate
forming section. Further, since the plastic deformation
behavior includes a high degree of nonlinearity (material
nonlinearity, geometric nonlinearity), it is clearly not
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possible to obtain deformation similarity with the
ordinary method of constituting the cage rolls. As cage
roll forming is by the air-bend method, moreover, the
magnitude of the bending moment received at different
portions of the steel strip cross-section varies with the
arm length (the location of the individual sectional
portions). Since the amount of forming at the individual
sectional locations naturally differs accordingly,
folding occurs with particularly high frequency at the
pipe bottom portion where the arm is longest. While inner
rolls are used as means for preventing this, in such case
the bending moment changes abruptly at the steel strip
portion contacted by the inner rolls, which has the
contrary effect of making folding more likely to occur.
From the aspect of flexible forming, moreover, use of
inner rolls is by no means easy in terms of equipment
structure.
The foregoing problems are present not only in cage
roll forming but also in other types of intermediate
forming. The cause is rooted in a shortcoming peculiar to
the air bend method, namely, that the magnitude of the
bending amount is hard to control. Therefore, owing to
the uncertainty of the formation, the strip sectional
shape of the formed steel strip is sensitively affected
by the material and t/D (thickness/diameter) and the
like, so that control thereof is difficult and
determining the caliber of the fin-pass rolls becomes
difficult. As a result, the number of fin-pass forming
stands increases. Thus, the approach up to now of
focusing on the intermediate forming section as the
center of flexible forming leads to unstable forming and,
as such, remains unresolved. In view of the foregoing
circumstances, the present invention was accomplished
based on the notion that implementation of flexible
forming should be centered on the breakdown forming
section.
FIG. 1 is a schematic view of a cage roll forming
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mill, which is typical of conventional flexible forming
mills. In FIG. l, an unrolled steel strip 1 is fed to the
breakdown process of a pipe forming line and formed into
a steel pipe. A pair of laterally spaced edge bend rolls
2 are disposed on the inlet side of the line to contact
the steel strip edge portions and bend-form both steel
strip edges. The steel strip with the formed edge
portions then enters the intermediate forming section,
where bend-forming of the middle portion of the steel
strip is conducted by a center bend roll group 3a-3d and
a cage roll group 4a-4x. The steel strip after
intermediate forming further enters the fin-pass process,
in which circumferential compression is applied to the
steel strip by a fin-pass roll group F1, F2, F3 to
correct its sectional shape and put it in a condition
enabling appropriate electric seam welding, and is
finally sent to the squeeze process, in which the whole
pipe is restrained by squeeze rolls 5 and electric seam
welded by a welding apparatus (not shown) to afford an
electric welded steel pipe product. FIGs. 2 and 3 show
the structure of a different type of cage mill in the
intermediate forming process. Taking FIG. 1 as an
example, there are shown cross-sections of the mill at
the A-A region of FIG. 1 (in FIG. 2) and at the B-B
region (in FIG. 3). A large number of inner roll groups 6
are disposed on the inside of the formed portion in
addition to the cage rolls shown in FIG. 1, in order to
restrain the inner portion of the steel strip so that
folding of the pipe bottom portion does not occur. FIG. 4
is a perspective view of the pipe forming line, called an
FF mill, taught by the aforementioned JP-B-3-12977. In
FIG. 4, the line is constituted of a breakdown roll
forming section composed of a BDl stand for conducting
edge bending, a BD2 stand for conducting reverse bending
and a BD3 stand for forming the middle region of the
sheet width; a cluster roll stand for producing a
circular cross-section composed of a roll group made up
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of C1-Cn, ordinarily six sets, installed on the same
table to conduct intermediate forming; a fin-pass roll
section composed of F1, F2 for conducting finish forming;
and a squeeze roll stand for establishing a weldable
condition. In the FF mill pipe forming line shown in
FIG. 4, the BDl stand for the edge bending of the
breakdown forming is designed to have as its roll caliber
a curve whose curvature varies continuously or stepwise
and, further, is combined with a roll positioning system
capable of moving and rotating the forming rolls so that
roll surface regions having appropriate curvature can be
brought into contact with the required steel strip region
to enable effective forming of steel pipes differing in
size and material with the same rolls.
On the other hand, at the cluster roll stand,
ordinarily six sets of rolls, is adopted but a problem
arises regarding forming performance because the purpose
of the forming by these rolls is the same as the purpose
of forming by conventional cage rolls. Specifically, the
designed deformed shape is hard to obtain owing to the
uncertainty of the forming.
In addition, the mechanical structure at the
conventional cluster forming section mentioned earlier
also involves a number of problems. Specifically, even
though the number of rolls of the cluster forming section
is not as great as in cage roll forming, a problem again
arises regarding the complexity of the mechanical
structure because, in consideration of size flexible
forming and the like, it is not sufficient only to effect
parallel translation of the rolls in the different
directions but is also necessary to rotate the rolls
within a certain range.
Thus, notwithstanding that flexible breakdown
forming is achieved, the forming load and the stress in
flexible forming still remains in the cluster forming
section. The present invention completely overcomes these
problems. As shown in FIG. 5, in the present invention,
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breakdown forming is conducted within a range of about
50$ of the sheet width by a BD1 stand for edge bending
the outermost edge portion, a BD2 stand for edge bending
a region inward and adjacent to the outermost edge
portion, a BD3 stand for forming a region further inward
from said inward region and, further, reverse bending,
and a side roll group composed of multiple stages,
preferably three stages S1-S3, is installed, with no use
whatsoever of the cluster rolls or cage rolls
conventionally used in intermediate forming.
As the side roll group in this invention there are
installed side roll stands comprising side roll groups
having commonly used roll forming surfaces. It should be
noted, however, that, as with the breakdown rolls, there
can be used as the caliber of the side rolls a curve
whose curvature varies continuously or stepwise. Since
such side rolls do not make contact with the formed steel
pipe edge portions, edge elongation at the steel pipe
edge portions is small and the steel pipe edge portions
form neat straight lines. Connection with the next
process, i.e., the fin-pass forming process, is therefore
possible. Moreover, by adopting the foregoing
configuration, the present invention enables production
of products with a low t/D (thickness/diameter ratio).
Moreover, if sufficient edge bending and forming of
the middle region of the sheet width can be achieved in
the breakdown process, it is possible to provide a pipe
forming line that, as shown in FIG. 6, establishes a
direct connection from the breakdown process to the fin-
pass forming section and, further, to provide a pipe
forming line that has only a retaining roll or the like
interposed to connect the breakdown process with the fin-
pass forming section.
In addition, there can be provided a pipe forming
line that is imparted with a flexible forming size range
over a ratio of steel pipe outside diameters of about 1
:3.
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The configuration adopted by the present invention
enables (1) stable formation minimally affected by
material properties and t/D to be conducted in a
breakdown forming section with strong forming capability
and, as a result, (2) reduction of the load on the
cluster forming section, whereby the number of cluster
forming rolls can be markedly reduced to substantially
simplify the equipment structure and lower the cost.
INDUSTRIAL APPLICABILITY
As explained in the foregoing, the present invention
provides a new pipe forming line that is high in forming
accuracy and low in cost, which, in the production of
steel pipe, particularly electric welded steel pipe, by
continuous roll forming, enables steel pipe forming by
forming rolls having flexible forming capability and
compatibility with steel pipes of multiple sizes without
the use of cage rolls.
