FIELD OF INVENTION
The present invention relates to an improved caliber system and method for a
Stretch Reducing Mill for achieving round profile and dimensional characteristics
during rolling of seamless steel tubes. More, particularly the present invention
relates to hot rolling of seamless steel tubes in a Stretch Reducing Mill, using
modified caliber for finishing shell during intermittent steps of rolling, thereby
making shell round at more than one position. The invention is about systematic
reduction and finishing of shells during the process of rolling.
BACKGROUND OF INVENTION
Manufacturing of hot finished tubes involves metal deformation processes during
which the raw material [bloom/billet] is converted into a finished tube. The type
and number of deformation processes can vary from process to process based
on final specification [Outside diameter, wall thickness and length of tube] of the
tube. The process used during this invention involves three major metal
deformation processes namely
1. Cross piercing and elongation
2. Pushing, and
3. Rolling.

The major challenge faced during rolling of hot finished seamless steel tubes is
that the desired wall thickness and circular profile of the tube is not obtained
specifically during rolling of thick walled tubes i.e. tubes having ratio of wall
thickness to mean diameter (difference of outer diameter and wall thickness)
greater than 12.5 percent and in general for all the tubes. So, in order to ensure
targeted wall thickness of tubes and circularity of profile in seamless steel tubes
a change in method of rolling is needed.
The Stretch Reducing Mill [SRM] consists of set of stands for different outer
dimension values of finished seamless tube needed. And, as per old design and
caliber all sizes consists of some basic and some finishing stand. Number of basic
and finishing stands used vary from size to size. For example, 22 stands for
achieving outer diameter size 51.00mm, 20 stands for achieving outer diameter
size 60.30 mm, 19 stands for achieving outer diameter size 63.50 mm and 25
stands for achieving outer diameter size 44.50mm etc. Out of 25 stands for size
44.50mm - 21 stands are called basic stands (1, 2, 3, 4, 5, 6, 7, 8) while stands
22, 23, 24 and 25 are called finishing stands (8.1, 8.2). In basic stands major
reduction in shell size takes place in each successive stands as illustrated in
figure 1. The profile or caliber of these basic stands is oval in shape as shown in
figure 2.

As shown in figure 3 basic stands (1, 2, 3, 4, 5, 6, 7, 8) have a non-circular
profile pertaining to difference in values of parameters 'A' and 'B'. This oval
shape is for ensuring the uniform reduction of shell by minimizing stress
concentration on shell surface during the process of deformation. Whereas
finishing stands have a round caliber to ensure roundness of seamless steel
tubes.
The invention is about modifying the existing caliber for Stretch Reducing Mill
[SRM] with an aim to finish the shell at intermittent steps rather than finishing it
at the last few stands of SRM caliber. That is, to ensure that shell is undergoing
through steps of reduction and finishing multiple times before coming out of
rolling mill. As per the old practice shell undergoes continuous reduction in basic
stands of SRM and single pass of finishing in finishing stands of SRM.
When size 60.30 x 10.20 was drawn in cold mill draw bench to size 50.80 x 7.62
undrawn portions were observed in cold mill and frequent plug damages were
also reported. This was mainly attributed to non-circular profile of tubes. So, in
order to achieve circular profile there was a need to modify the existing caliber.
PRIOR ART
Prior art search was conducted for "Stretch Reducing Mill calibers" and the
following two relevant patents were reviewed:

1. EP924001B1 2003-10-15 Method of rolling tubes with a mandrelless multiple
stand stretch reducing rolling mill
Multistage stretch reduction milling works The milling works include a cylinder
calibration for a cylinder set consisting respectively of three cylinders with a
noncircular caliber form. The last calibers of the milling works are locally
enlarged at their extent, at which the expected diameter of the ingoing reed is
lower than a nominal size, and are locally reduced at their extent, at which the
expected diameter of the ingoing reed is larger than a nominal size.. A manual or
automatic measurement of the reed diameter is preferably performed, whose
data are stored in a data base for statistic analysis, and are supplied to a CNC
machine for a reduction or increase of the theoretic final caliber.
This patent relates to process of caliber design using a reed for data gathering in
a stretch reducing mill, but since our patent is for cyclic rounding and reduction
of shell profile so this patent is not related to our present invention.
2. DE19758107A1 1999-06-24 Multi-stand thornless stretch-reducing mill
A roll pass of a roll set having three rolls for a multi-stand mandrel-free stretch
reducing mill has a roll pass shape that deviates from the circular. A tube is
passed in its longitudinal direction through the non-circular roll pass shape
enabling modification of a shape of a cross-section of the tube, so as to
counteract the non-circular external diameter deviations of the tube. A final roll

pass (finishing pass) of the stretch reducing mill is locally enlarged at peripheral
points where the tube diameter of the entering tube would otherwise deviate
downward from a target size and locally reduced in size at the peripheral points
where the tube diameter would otherwise deviate upward from the target size.
This patent explains how non-circular and finishing roll pass achieve targeted
diameter size. Whereas our present is about modifying regular set of oval and
finishing roll pass to alternate sets of oval and finishing roll pass.
SUMM*PV nr THF INVENTION
According to the present invention, number of stands are increased to distribute
stresses during rolling of seamless steel tubes more uniformly and multiple
finishing stages are introduced to ensure rounding of profile. The circularity of
profile for all sizes - 60.30 x 10.20, 63.50 x 7.10, 44.50 x 8.00, 63.50 x 11.20
and 51.00 x 4.00 improved considerably because of which undrawn portions
were eliminated and plug damage during cold drawing also got reduced.
Eccentricity [WT difference/Average WT] got reduced from 20.69% to 13.14 %
for size 60.30 x 10.20. For 44.50 x 8.00 eccentricity 12.87% observed against
planned 16 % eccentricity [±8%]. For 51.00 x 4.00; allowed eccentricity at any
point against average was - 5.94% and + 6.05% against allowed eccentricity of
±10%.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose an improved caliber system
and method for a Stretch Reducing Mill for achieving round profile and
dimensional characteristics during rolling of seamless steel tubes which is
capable of ensuring rounding of profile and eccentricity of the tube.
RRTEF DESCPTPTTON OF TH* ArrOMPANYTNG DRAWING
Figure 1 Shows progressive continuous reduction in successive stands of the
rolling mill - Stretch Reducing Mill [SRM]
Figure 2 Shows caliber design of rolling mill stands
Figure 3 Drawing showing caliber details of a SRM stand
Figure 4 Shows schematic view of existing MRW caliber
Figure 5 Shows schematic view of modified caliber
nFTATLED DFcrPTPTTON OF A PRFFERREP EMBODIMENT QF THE
INVENTION
The modifications carried out in this invention are in caliber of Stretch Reducing
Mill (SRM). Caliber means the internal diameter or bore of SRM stands. That is, a

Stretch Reducing Mill (SRM) consists of many stands inside a fixed frame,
through which a tube is passed during the process of rolling. Each successive
stand in this arrangement is of decreasing internal diameter (caliber) - as
illustrated in column "Pass (Dia)" of table 1. As tube passes through SRM stands
the outside diameter of the pipe will be approximately equal to internal diameter
of a particular stand. For example, when during the production of size 44.50
mm, when the tube passes through stand 2 as given in table 1, the outside
diameter of tube will be equal to internal diameter of this stand, that is, 133.50
mm. Arrangement of stands in a Stretch Reducing Mill (SRM) is illustrated
through figure 4, where caliber is the internal diameter of these stands. The
stands of a SRM are grooved in a roll grooving machine to achieve targeted
internal diameters (caliber). The required internal diameter (caliber) of any
particular stand will depend upon final size of tube required by customer. For
example, if customer wants tube of size 127.00 mm then internal diameter
(caliber) of stands will be decided based on final size needed i.e. 127.00 mm in
this case. On the same lines internal diameter (caliber) of SRM stands will be
different from those of size 127.00 mm if customer requirement for tube is say,
44.50mm.
Based on tube size the number of stands in SRM fixed frame will also vary. This
is so because for making bigger tube sizes say, 127.00 mm lesser reduction is

needed (reduction from shell size, say 140.00 mm - a shell is an output from
push bench equipment which is located in the seamless steel tube production
line before SRM) but for smaller sizes like 44.50 mm as reduction percentage is
more number of stands will be higher. Reduction percentage is shown in table 1,
under column heading "reduction %". It is defined as percentage change in Pass
(Dia) taking place from one stand to next. For stand 2 in table 1, reduction
percentage is 3.839 % because ((138.83 - 133.50) / (138.83)) x 100 = 3.839 %.
There are two types of stands used in SRM - basic stands and finishing stands.
Basic stands are called so because they are used for causing initial/basic
reduction in tube size during process of rolling [all of them have an ovular
internal diameter - as shown in figure 2]. Figure 2 shows cross section of
individual SRM stand where the internal diameter (caliber) of stands is made by
gap between three rolls of a SRM stand. The reason for keeping caliber of SRM
stands (basic stands) oval is to keep stress concentration during shell
deformation in SRM stands to minimum. On the other hand, finishing stands are
meant for making profile of the rolled tube from oval to round. The distinction
between basic and finishing stand is made through a unique identification
number which is given in table 1 under column heading "stand number." The
basic stands ends with digit '0' while finishing stands end with digits T, "21,13' or
M'. For example, in table 1, stand number 10 corresponding to SI. No. 10 is

marked as 1000, i.e. to represent that, it is 10th stand in the arrangement of SRM
stands in fixed frame and is a basic stand. Similarly, stands 2201, 2202, 2203
and 2204 in table 1 corresponding to serial numbers 24, 25, 26 and 27 are
finishing stands and they follow order of stand arrangement in SRM fixed frame
after 22nd basic stand i.e. 2200.
The relation between SRM caliber and tube size is that, the internal diameter
(caliber) of last two SRM stands (which are finishing stands) will be equal to
outer diameter of tube needed. This is shown in figure 4 - say for a particular
tube size requirement number of stands needed (based on reduction percentage)
is 10 (8 basic stands and 2 finishing stands). Basic stands are marked from 1 to
8, while finishing stands are marked as 8.1 and 8.2 in figure 4. The internal
diameter (caliber) of stands 8.1 and 8.2 will be equal to tube outer diameter
requirement. The stand arrangement shown in figure 4 is a generalized
arrangement as here 10 stands are shown, but number of stands can increase or
decrease based on final tube size requirement. But, whatever may be the
number of stands used, the arrangement of stands will be as shown in figure 4 -
that is, first basic stands will be arranged in SRM fixed frame and then finishing
stands will be arranged.
But, according to present invention this basic arrangement is changed to fit in
finishing stands in between basic stands to achieve round profile at intermittent

locations - figure 5. The stand arrangement shown in figure 5 is for any general
case in which the number of basic stand requirement is assumed 4 (Stand 1, 2, 3
and 4 in figure 5) and number of finishing stand requirement is also assumed 4
(Stand 2.1, 2.2, 4.1 and 4.2). The number of stand requirement varies from case
to case based on tube size requirement.
Figure 4 and 5 - show the existing and modified set up plan used for stretch
reducing mill respectively, where reduction and finishing patterns are based on
number of basic and finishing stands used. The set of reduction and rounding
stages used for different OD sizes are listed in table 2. For making tube profile
circular number of stands were increased by introducing finishing stands (2.1,
2.2, 4.1 and 4.2 - figure 5) with an aim to distribute stress more uniformly over
stands. The arrangement shown in figure 5 is for representing idea behind this
invention. Application of this idea can be understood from table 1, where stand
1901 and 1904 are inserted at position 20 and 21 (these were not a part of
original caliber) and for inserting these stands at these places the basic
parameters of SRM stands (A, B, reduction % and ALFA) required modification.
These parameters are given in different column headings of table 1 and
described in figure 3. W is longer groove semi-axis and 'B' is shorter groove
semi-axis for oval shaped internal diameter (caliber) of SRM stand, such that
Pass (Dia) for stand, D = A+B. W is roll radius of SRM stands roll - each SRM

stand is an assembly of three rolls. ALFA is called as ovality of SRM stand groove
internal diameter. As mentioned earlier, SRM stand groove internal diameter
(caliber) is not circular to reduced stress concentration, how much ovular it is
represented by ALFA, which is a ratio of A and B. When ALFA value is equal to 1
the stands have a round profile, and higher the ALFA value from 1, higher will be
ovality. Stands having ALFA values greater than 1 (shown in last column of table
1) are known to have oval profile.
During regular rolling it is observed that the profile of seamless steel tubes is not
coming round with standard caliber. So, as a part of this invention for size
44.50mm outer diameter we have inserted extra finishing stands(1901 and 1902
- Column 2 of table 1) in between the regular basic stands to achieve round
profile. For doing this entire caliber set was recalculated from stand 1000
onwards. In this case as stands 20 and 21 have round profiles (this insertion of
round profile stands is illustrated in figure 5 through stands 2.1 and 2.2) the shell
passing through these stands will get rounded once here. And, regular reduction
in size will happen in stands 22 and 23 (illustrated in figure 5 through stand 3,
4). And again rounding in stands 24, 25, 26 and 27 (illustrated in figure 5 using
designation 4.1, 4.2) will take place when shell will pass through these stands
during process of rolling. In this way successive cycles of reduction, rounding,
reduction, rounding will be ensured.

In non-modified caliber (figure 4) the shell undergoes deformation in such a way
that its profile is built oval in shape. Rounding happens only at final stands (8.1,
8.2). But for our invention (Figure 5) the profile of shell is made circular at
position 20 and 21 also and again in stands 22, 23, 24 and 25. Thus this
progressive rounding and reducing and rounding again helps to attain required
dimensional characteristics by rounding of shell profile.
In this way, insertion of extra finishing stands gave us acceptable results for size
44.50 x 8.00. On the same lines using finishing stands from higher outer
diameter sizes like 88.90 mm, 76.10 mm and 73 mm at intermittent locations for
achieving dimensional characteristics in tube sizes 44.50 mm, 51.00 mm, 54.00
mm etc. gave us satisfactory results. Table 2 shows a comparative study of old
method and modified method.

WE CLAIM
1. An improved caliber system for a Stretch Reducing Mill for achieving round
profile and dimensional characteristics during rolling of seamless steel
tubes, the system comprising;
a recalculated caliber set up for reduction/rounding of the tube profile
by two calibers (1, 2) followed by two calibers (2.1, 2.2) for finishing and
followed by two more calibers (3, 4) for rounding and finally followed by
two more calibers (4.1, 4.2) for finishing, the said set up being disposed
for stands after 9th stand i.e. keeping first nine stands same when total
number of caliber stands used are twenty seven in number for a finished
tube diameter of 44.50 X 8 mm.
2. The system as claimed in claim 1, wherein the recalculation of caliber set
up is done from stand 1000 (table-1) onwards.
3. The method of the caliber system as claimed in claim 1, the method
comprising;
recalculating entire caliber set from stand 1000 onwards to be
introduced to make round profile of the seamless steel tubes;

introducing arrangement of reduction-finishing-reduction-finishing of
tube profile in the said caliber set up; wherein first nine stands of the
caliber set up are kept in the same existing pass dia.