FIELD OF THE INVENTION
The present invention relates to an improved water-box quenching system for
thermo-mechanical treatment of rebars which allows maximization of coding of
the rebars. The present invention further relates to a process of validating the
result obtained through the improved water-quenching nozzles.
BACKGROUND OF THE INVENTION
The thermo-mechanical treatment (TMT) rolling process involves heating of
billets to rollable temperature, converting the billets to a desired finished section
by way of passing the material between a pair of grooved rolls, cropping the hot
bar during the process of rolling between the mill stands as applicable and
subsequently finishing in the form of hot rolled deformed bar in straight length.
The hot bar coming out of the last pass is then conveyed through a TMT line and
collected in a cool bed after shearing. The whole operation is conducted at a
particular temperature range and within a limited time span. Rebars produced at
rolling mills need to have certain desired mechanical properties which in turn are
achieved by phase transformations in the final rolled bars. The most common
feature of all the techniques is the water-box quenching system wherein a
moving rebar is allowed to pass through a series of water nozzles. A high rate of
cooling is achieved at the surface that converts the austenite to martensite
whereas the center of the product still remains a combination of ferrite and
pearlite. In order to actually decrease the utility costs and have a high production

rate, it is essential to continually operate the TMT-process under stable
conditions with a high thermal efficiency.
During the continuous rolling of billets to produce TMT bars, the material is
passed through a water-box system that cools the rebar surface at very high
rates to impart the desired mechanical properties by bringing about certain
metallurgical phase transformations. The water-box system consists of series of
axial placed nozzle-extender assemblies with a provision to inject water that
flows over the surface of rebars and carries out the surface quenching. This
necessitates use of a compact shorter and efficient water-box. Nevertheless, to
ensure stable and efficient operation, the phenomena occurring during
quenching operation must be elucidated first.
In TMT water-box, the hot bars are subjected to quenching by means of an
intense cooling system. This coding step hardens the surface layer of the product
to martensite while the core structure remains austenite. After the exit from the
cooling system (water-box) the quenched bar being out of water chamber, heat
is enabled to flow from core to surface and accordingly the surface gets
tempered in the atmospheric cooling bed, the hardened zone is tempered by
temperature homogenization in the cross section of the billets allowing the
austerity core to be transferred to a ductile-ferrite pearlite core.

OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an improved water-box
quenching system for thermo-mechanical treatment of rebars.
A further object of the invention is to propose an improved water-box quenching
system for thermo-mechanical treatement of rebars which includes an innovative
geometry of the water-quenching nozzle.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows two water boxes placed adjacently in a TMT process with
their injection nozzles according to the prior art.
Figure 2 - shows simulated contours of water-vapor volume fraction inside a
single nozzle depicting gradually developing vapor film boiling near
hot rebars.
Figure 3 - gradually developing vapor film boiling near hot rebars.
Figure 4 - graphically depicting cooling curves for an extender pipe with an
improved nozzle of the invention.

Figure 5 - graphically shows the cooling curves respectively by a prior art
nozzle and the improved nozzle.
Figure 6 - shows contour of water-vapor volume fraction in an improved
system of the invention
Figure 7 - shows operating curves for the improved water-box quenching
system and the operating pump with a stable operating point
according to the invention
Figure 8 - shows an improved water injection nozzle with tapered inserts
assembled on an extension pipe according to the invention.
Figure 9 - shows the tapered inserts with interlocking means of the water
injection nozzle of Figure 8.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved water-box quenching system for
thermo-mechanical treatment of rebars.

The fluctuations in various operating conditions during the TMT rolling, however,
leads to non-uniformity in the finished product quality as well as reduction in the
production rate. Some of the most influencing parameters are the billet
temperature, rolling speed and water-flow rates (in the water-box). Accordingly,
an efficient and easy to maintain water-box capable of effectively using the
supplied water to quench the rebar surface consequently allowing high TMT
rolling speeds, is improved design is proposed that would quench the rebar
surface all over the length of the water-box and overcomes the limitations of a
smooth pipe based nozzles. It is known that water-vapor has poor thermal
properties and therefore whenever the rate of vapor generation exceeds the rate
of its condensation, the vapor blanket shields are formed on the rebar surface.
This phenomenon de-limits the heat conduction from the rebar surface to the
bulk liquid water thereby deteriorating the cooling efficiency in the remainder of
the length of the water-box nozzle. Further, the proposed solution must be
enabled to eliminate the poor performance of the water-box caused due to film
boiling and vapor resistance, particularly in smooth pipes.
According to the invention, an overall cooling behavior of the rebar under the
influence of water flow has been modeled with the use of mathematical modules.
At temperatures higher than the saturation temperature, water undergoes a
phase change to form vapor that gradually develop in to film at the rebar
surface. This phenomenon is called boiling that starts with nucleate regime,
undergoes transition and eventually ends at film boiling. The latter adds
resistance to heat transfer at the rebar surface and deteriorates the cooling

efficiency of the nozzle. All of the above mentioned phenomenon have been
considered while configurating the mathematical model and the parametric
relationships of the model were established computationally to obtain flow
pressure and thermal field at all points within the water-box system. Accordingly,
the invention adapts a fully validated model to analyze the effect of changes in
the existing nozzle design with a view to maximize cooling of the rebars within
the nozzle. Finally a high pressure without affecting the local convection has
been determined to compress the vapor film and yet obtain higher cooling rates.
DETAIL DESCRIPTION OF THE INVENTION
Figure 2 shows a simulated water-vapor volume fraction within a nozzle of the
water box wherein the rebar and the water flows. At the entry point, the hot
rebar surface leads to intense steam generation and flow reversals. This is the
region of nucleate boiling and is associated with sudden large drop in the surface
temperatures. The flow reversals and eddy formations near the water jet area
are represented in terms of flow lines in figure 2. Down the nozzle, the flowing
water continuously gets heated up without being locally separated at any point.
The tendency for the flow to separate and have eddies is dependent on
geometry of the nozzle including the flow strength. The present geometry is
comfortably streamlined for the kind of water flow rates used during rolling. The

water vapor film starts forming a blanket and protects the rebar surface to come
into direct contact with the bulk flowing water. The bulk condensation is not
enough to disrupt the film at this stage. The heat needs to be conducted through
this film in order to be finally taken away by the bulk flowing water in the nozzle.
Owing to the poor thermal conductivity of the water vapor, the heat extraction
rate fails sharply and sometimes may even leas to continuous rise in the surface
temperatures particularly in the rear portion off the nozzle. This phenomenon is
shown and described in figure 3, including the complete cooling curves along the
length of the rolling till the temperature measuring point. The finished rolling
temperatures (FRT) are a well known indicator in the rolling technology whose
value is used to ascertain the product quality achieved during rolling operation.
As described hereinbefore, the heated product when out of water-box, the heat
from the core regions comes to the surface to temper the martensitic structure
and thereby the surface temperature keeps rising. After the rebar is taken to the
atmosphere cooling bed, the overall temperature equalizes and ideally this
temperature is a representation of the finished rolling temperatures.
As an exemplary process, without limiting the scope of the invention, figure 3
shows simulated cooling curves for a 25mm rolling section. The secondary axis
shows the radial depth of the rebar surface, a 100% martensitic structure
expressed as a percentage of the total cross-sectional diameter. The water-box
layout is also shown under the graph for easy visualization of the temperature

location in a rolling line. The values substantially match with the test data
measurements. After the water box end, the rebar surface temperatures rises
continuously and a measurement of FRT is taken at a certain distance from the
water-box exit through the temperatures not equalized as is apparent form the
difference in core and surface temperatures curve at the point of measurement.
The rebar surface temperatures rise in the long extended portions of the nozzle
assemblies due to continuous formations of the vapor film although the fall was
sudden and steep at the water-jet injection point. The same effect is noticed at
all five injection points within the nozzle as shown by a green colored curve in
figure 3.
Figure 4 shows the cooling curves with an improved design. The rolling speed is
not changed and it was predicted a better cooling and hence an increase in the
martensitrc structure depth as shown on the secondary axis (value~8.4%). At
this stage the rebar can be rolled at a higher speed as the cooling efficiency of
the water-box system has increased and the earlier grade (product quality)
constitutes the benchmark. Figure 5 explains the effect in greater detail for a
single nozzle wherein the result provided by the pipe and insert based new
nozzle are shown. The water mass flow has been kept the same for the two
simulations and also the effect of pressure rise on cooling in the improved water
box is segregated and simulated separately. This way, the effectiveness of the
invention on cooling is attributed to two modes for example, boiling suppression
and convective enhancement. As can be seen from figure 5, the rebar centre

line temperatures have not decreased significantly as that at the surface. The
heat capacities and thermal conductivities of the steel has been modeled as a
function of temperature in the present analysis. For the same mass flow in prior
art and inventive nozzle, the pressure drop in an insert based nozzle is about
2.75 times more than that in a smooth pipe based nozzle. The corresponding
vapor film thickness as well as the maximum volume fraction is also low. This is
shown in the vapor volume fraction contours plotted in figure 6. The pressure
rise increases the boiling point, enhances condensation and causes further
thinning of the vapor film. This film is further disrupted due to flow separation
and eddies and therefore the growth rate is prevented and a high cooling rate is
observed over the entire length of the nozzle as shown in figure 5. A decrease of
about 20% in the rebar surface temperature is attributed to high pressure alone
in the new design.
For a known water-box system, a known water pump, the unique operating
pressure and flow rate is determined by the interaction between the two. Figure
7 shows the pump curve in sue (442mm impeller diameter) in conjunction with
the known water-box system. The water-box nozzles add resistance to water
flow and the behavior can be represented by an equivalent resistance curve
passing through the origin in figure 7. The abscissa at the intersection point of
the pump curve gives the operating flow rates and the ordinate provides the
operating pressure for the water-box system. At the intersection point of the
pump curve and the water-box-system curve, a pair of numbers are marked. The
first one is the rolling speed and the second is the martensite rim thickness
achieved at that particular stable operating point. In a first situation (marked 1),

which is the validation case for a smooth pipe based nozzle, the total water
requirement is 400m3/hr and the head developed by the pump is 105m. Other
curves in figure 7 depict the operating point for different configurations of the
pump and water-box. For instance, the alternative IV leads to higher martensite
thickness of about 10.6% at a reduced water fluorite of about 305m3/hr. This
was achievable due to increase in the cooling efficiency of the new nozzle of the
water-box. The insert diameter was 45mm and the taper angle was 15° (refer
figure 9). Further variations in the insert diameter and the angle studied and the
results are marked in figure 7 for the various options. For any particular
configuration that leads to higher martensite, a further increase in the rolling
speed is allowable to bring down the martensite rim thickness to acceptable level
as shown in validation case 1 (refer figure 7 and 3).
It is therefore concluded from figure 7.0 that as compared to benchmark case
(1), a further increase in the rolling speed is possible for other three alternatives
to achieve the same product quality at higher rate. Moreover the total water
requirement has also decreased considerably.
In accordance with the experimentations conducted according to the invention,
an innovative configuration of water-box system including an improved water-
quenching nozzle is proposed.

The nozzle designed consists of a movable insert with a threaded means that
allows adjustment of the water injection jet velocity and the operating pressure
at the nozzle box. A smooth long extender pipe is horizontally fitted to the nozzle
box. A plurality of inserts are disposed on the pipe in succession and the entire
assembly is packed and provided with end flange. The system is aligned so as to
provide flexibility to change in the pressure and flow during the rolling operation.
The performance of the extender portion is automatically determined based upon
a chosen clearance at the box itself. The inserts interlock and ensure the
alignment of the bore to enable placement of the TMT bar within the complete
assembly. The taper length "x" is 72% of total insert length "y". The front angle
is 90° whereas the rear angle is 10° . The configuration allows to adjust the
cooling performance at the box as well as the extender level to achieve an
overall high cooling rate over a known length of the box (chosen by
mill/supplier). The water-box is the last setup in the rolling line wherein the
property enhancements of the TMT bar takes place and much of the quality
issues can be addressed and controlled.

WE CLAIM:
1. An improved water-box quenching system in a thermo-mechanical treatment
(TMT) rolling process of rebars to maximize cooling of rebar surfaces, the
TMT process comprising heating of billets to reliable temperature, converting
the billets to a desired section by passing through a pair of grooved rolls,
conveying the hot bar through a TMT line and collecting in a cool bed after
shearing, the TMT line provided with a water-box quenching system having
a plurality of axially-placed nozzle-extenders assemblies to inject water on
surface of the rebars, the improvement is characterized in that the water box
is provided with a plurality of injector nozzles each consisting of a movable
insert having threaded sections to allow adjustment of the water jet velocity
including operating pressure at the box; a smooth and long extender pipe
horizontally connected to the box; a plurality of inserts disposed along the
extender pipe at spaced apart locations; the pipe and nozzle assembly
interposed in the water box and sealed with end flanges, wherein the
assembly is disposed in the water-box with equal clearance from all sides of
the box, and the water box located at an inclined position in the TMT-line,
wherein a taper length of the insert is 72% of the total length, and wherein
the front angle is 90°C as opposed to rear angle of 10°, the configuration of
the water-box allowing achievement of high cooling rate to improve physical
and chemical properties of the produced bars.

2. An improved water-box quenching system in a thermo-mechanical treatment
(TMT) rolling process of rebars to maximize cooling of rebar surfaces
substantially as herein described and illustrated with reference to the
accompanying drawings.

ABSTRACT

The present invention relates to an improved water-box quenching system in a thermo
mechanical treatment (TMT) rolling process of rebars to maximize cooling of rebar surfaces,
the TMT process comprising heating of billets to rollable temperature, converting the billets
to a desired section by passing through a pair of grooved rolls, conveying the hot bar
through a TMT line and collecting in a cool bed after shearing, the TMT line provided with a
water-box quenching system having a plurality of axially-placed nozzle-extenders assemblies
to inject water on surface of the rebars, the improvement is characterized in that the water
box is provided with a plurality of injector nozzles each consisting of a movable insert having
threaded sections to allow adjustment of the water jet velocity including operating pressure
at the box; a smooth and long extender pipe horizontally connected to the box; a plurality of
inserts disposed along the extender pipe at spaced apart locations; the pipe and nozzle
assembly interposed in the water box and sealed with end flanges, wherein the assembly is
disposed in the water-box with equal clearance from all sides of the box, and the water box
located at an inclined position in the TMT-line, wherein a taper length of the insert is 72% of
the total length, and wherein the front angle is 90°C as opposed to rear angle of 10°, the
configuration of the water-box allowing achievement of high cooling rate to improve physical and chemical properties of the produced bars