CLIAMS:We claim:
1. A process for manufacturing cobbles free bars in a wire rod mill wherein the speed of pinch roll located after intermediate block, finishing block, water boxes and before the laying head in a wire rod mill is controlled to avoid the cobbles in tail end the bar occurring due to asynchronization of the speed of bar and pinch roll , the process comprising the following steps:
- detecting the tail end of the bar by means of hot metal detectors (HMD) at the Intermediate Block
- sensing the exit signal of the bar tail end by the Exit HMD of Intermediate Block
- interlocking the Finishing Block loop HMD signal of the bar tail end after exiting the Intermediate block
- switching over to higher pinch power 40 (current limit to 120%) when the bar is still in the working stands of the Finishing blocks to enable pinch roll to synchronize with the linear speed of the bar and overcome the load of the pinch roll

2. A process for manufacturing cobbles free bars in a wire rod mill as claimed in claim 1 wherein, the tail end of the bar is detected by change in the state of hot metal detectors(HMD) from energized state to de-energized state.

3. A process for manufacturing cobbles free bars in a wire rod mill as claimed in claim 1 wherein, operator selected pinch power is switched to Higher pinch power (current limit to 120%) after interlocking the FB loop HMD signal with the bar tail end after exit from the Intermediate block.

4. A process for manufacturing cobbles free bars in a wire rod mill as claimed in claim 1 wherein, the higher Pinch Power (current limit) is applied for 2-3 seconds or till the Exit Finishing Block HMD de-energize signal.

5. A system for controlling the speed of pinch roll that is located after intermediate block, finishing block, water boxes and before the laying head in a wire rod mill to avoid the cobbles in tail end the bar which occur due to asynchronization of the speed of bar and pinch roll , the system comprises
- a process control Computer which generates the speed references of all the drives
- Programmable logic Controllers (PLC) having a input card to take signal input from hot metal detectors and two outputs one connected to process control computer and another to PLC of logical drive .
- hot metal detectors(HMD) present in Intermediate block(IB), and finishing block(FB) to detect the presence of hot bar in respective blocks, wherein each hot metal detector (HMD) input generates two outputs one connected to process control computer and another to input card of programmable logic controller and
- a drive controller having a logical switch to receive the inputs from PLC and process control computer. Human machine interface of PLC for selecting /deselecting the feature for different rolled product and Programmable Logic Controller for switching the pinch roll power

6. A system as claimed in claim 5, wherein three HMD’s FB Entry HMD, FB exit HMD and FB loop HMD are provided in Finishing block(FB) for sensing the presence of bar at finishing block.

7. A system as claimed in claim 5, wherein there are three HMD’s IB Entry HMD, IB exit HMD and IB loop HMD are provided in Intermediate block (IB) for sensing the presence of bar at intermediate block.

8. A system as claimed in claim 5, wherein hot metal detectors are configured to detect the tail end of the bar by the change in its state from energized state to de-energized state.

9. A system as claimed in claim 5, wherein the programmable logic controller receives inputs from hot metal detectors and send output to logical switch of logical drive.

10. A system as claimed in claim 5, wherein the logical drive switches the pinch roll current to 120% after sensing the de-energized signal of IB exit HMD followed by de-energized command from FB loop HMD to switch over the said supply.
,TagSPECI:Field of invention
The present invention relates to reducing the cobbles in the area of finishing blocks and pinch rolls by improving the control of the pinch roll unit in the hot rolling mill producing wire rods .The cobbles formation is checked by increasing the pinch power at the time when the tail end of the bar is leaving the finishing block of wire rod mill. Productivity and quality of the bar produced at wire rod mill by using the invented process is improved and high quality bar without cobbles are produced.
Background
In Wire Rod Mill, the Pinch roll is located after the Finishing Block, Water boxes and before the Laying head. The function of Pinch roll units is to propel the bar through the Water boxes and the Laying heads. The Pinch roll grips the material and creates a tension on the rod so that flopping of rod doesn’t take place while Water box nozzles are made on to quench the rod. Pinch rolls guide the bar through the Laying head.
For lower sized rod, the Pinch roll speed is stepped down at the tail end, to maintain/ reduce the diameter of convolutions of the coil at the tail end. For higher sized rod, the Pinch roll speed is stepped up to propel the bar through the length of water boxes and Laying Head after the bar emerges out of the Finishing block at the tail end.
Pinch roll consists of upper and lower roll which are operated by means of a pneumatic cylinder. Pinch roll unit employs the electric motor to drive the Pinch rolls. The opening / closing of Pinch rolls is precisely timed to achieve desired results for each rolled bar depending on the position of bar.
These Pinch rolls have proven to be problematical due to fluctuations in the pressure of compressed air, slow reaction times of the solenoid valve, cylinder closing times, the stroke distance of the pistons and lever arrangement of the rolls. The problems are acute in high speed rolling and in rebar rolling.. Improper settings of the Pinch roll parameters lead to cobbles, marking on the bars and improper convolutions of the coil resulting in loss of production and downtime.
Pinch roll control assumes significance as the productivity, shape of the coils and quality of wire rods depend on the synchronization of the speeds of Finishing block, Pinch roll and laying head in tandem. The control also assumes significance as the response time is very less in the order of milliseconds.
While setting the mill , the Pinch roll parameters like pinch power, head lead speed, tail lead speed, gap of the Pinch rolls and air pressure are adjusted depending on the diameter of the bar. The parameters are selected to maintain synchronization in the speeds of Pinch roll, Finishing block and Laying head which result in the uniform convolutions of the wire rods and ensure production of wire rods without any cobbles. Any discrepancies in the selected parameter would result is loss of synchronization of the speed of finishing block and Pinch roll resulting in flopping of rod in the water boxes and cause obstruction to the passage of rod.
While rolling rebar of smaller diameter in Wire Rod Mill, there are frequent snapping of the rod between finishing block working stands and Pinch rolls. A small piece of rod (approx.5mts. of material is accumulated in the exit trough of the Finishing block (Break out trough) in the pass line .This piece causes obstruction to the passage of rod resulting in material accumulation leading (cobble) to loss of production. Clearing the cobble it takes time of approx. 45min to normalize for rolling.
Prior Art
The pattern of cobbles were studied and following points were inferred
• Cobbles were occurring in head end, middle of the bar and in tail end of the bar
• Cobbles were more in tail end of the bar
• The cobbles in tail end were occurring, while the bar had left intermediate blocks and before the tail end of bar had crossed all the working stands of the finishing block
• The cobbles were due to increase in load on the pinch roll
Following measures are taken to avoid cobbles prior to this invention.
The pinch roll parameters are changed

The gap of Pinch roll if increased resulted in lessened grip of the Pinch roll on rod. The tension created by the Pinch roll on the bar is inadequate to overcome the resistive forces offered by the water, resulting in stuttering of rods and cobbles at head end.

The air pressure if reduced to 3-3.5bar resulted in less resistance by the cylinder to the deflective force offered by the rod The tension created by the Pinch roll is inadequate to overcome the resistive forces offered by the water resulting in stuttering of rods and cobbles at head end. Due to air pressure fluctuations cobbles result with lesser air pressure setting.

Reduction in water flow reduces the resistive force on the rod and the load on the Pinch roll. The rod is not quenched to the desired level. The quality of the rod is affected due to inadequate quenching. The quality of the product produced was found not as per the standard and rejection of material became unavoidable.

The Pinch power when increased to 40 i.e. 120% of motor current, Tail end cobbles could be avoided. The pinch roll motor is designed for 110% load continuous and 150%, 300% momentarily. The motor was subjected to over load throughout the length of the bar (80s) and was subjected to continuous overload up to 120% against the rated capacity. The motor could get damaged due to overload leading to loss of production and time required for changing the motor was approximately 5- 6 hrs.

US Patent 6920772 teaches about a pinch roll unit for either propelling or retarding a product moving along the pass line of a rolling mill comprises a pair of levers mounted for rotation about parallel first axes. Roll shafts are carried by the levers with each roll shaft being journalled for rotation about a second axis parallel to the first axis of its respective lever. Pinch rolls are carried by the roll shafts and are positioned to define a gap there between for receiving the product. An electrically powered first motor operates via a linkage to rotate the levers about the first axes and to move the pinch rolls between open positions spaced from the product, and closed positions contacting and gripping the product there between. An electrically powered second motor rotatably drives the pinch rolls.

Object of the invention
According to the present invention the process for reducing cobbles in a wire rod mill comprises controlling the speed of pinch roll to avoid the cobbles at the tail end of the bar due to asynchronization of the speed of bar and pinch roll. The pinch roll motor is provided with higher pinch power to overcome the load and remain in synchronization with the speed of the bar. The tail end of the bar is detected by means of hot metal detectors (HMD) at the Intermediate Block , by sensing the signal of the bar tail end at the Exit HMD of Intermediate Block. The Finishing Block loop HMD signal is interlocked with the bar tail end signal after exiting the Intermediate block. The pinch power is switched over from operator selected set point to 40 (120 % r current limit) when the bar is still in the working stands of the Finishing blocks to enable pinch roll to synchronize with the linear speed of the bar and overcome the load of the pinch roll

The tail end of the bar is detected by change in the state of hot metal detectors (HMD) from energized state to de-energized state. Pinch roll power is switched to 40 after interlocking the FB loop HMD signal with the bar tail end after exit from the Intermediate block.

The higher pinch power is applied for 2-3 seconds or till detecting the Exit Finishing Block HMD de-energize signal allows the pinch roll speed and Finishing Block speed in synchronization at tail end and thus avoiding the motor damage due to excess over loading for a long duration.

According to an embodiment of the invention a system for controlling the speed of pinch roll in a wire rod mill to avoid formation of the cobbles in tail end of the bar which occur due to asynchronization of the speed of Finishing Block and Pinch roll.

A Process Control Computer generates the speed references of all the drives. Hot metal detectors (HMD) present in Intermediate block(IB), and finishing block(FB) to detect the presence of hot bar in respective blocks. Each hot metal detector (HMD) is connected to a Programmable Logic Controller .For each HMD input , two outputs are generated ,one connected to Process Control Computer and another to input card of Programmable Logic Controller.

According to further embodiment of the invention, Programmable logic Controllers (PLC) having input card to take signal input from hot metal detectors and generates two outputs one connected to Process Control Computer and another to PLC of logical drive . The Programmable logic Controllers receives inputs from hot metal detectors, human machine interface and send output to logical switch of drive controller.

A Drive controller having a logical switch to receive the inputs from of Process Control computer and Programmable Logic Controller for switching the Pinch Power is provided. The drive controller switches the Pinch Power to 40 (120% current ) after sensing the de-energized signal of IB exit HMD followed by de-energized command from FB loop HMD to switch over the preset Pinch power

In each finishing block three HMD’s FB Entry HMD (H1), FB exit HMD(H2) and FB loop HMD (H3)are provided in Finishing block(FB) for sensing the presence of bar at finishing block.

Three HMD’s IB Entry HMD( I 1), IB exit HMD ( I 2) 1and IB loop HMD ( I 3) are provided in Intermediate block (IB) for sensing the presence of bar at intermediate block.

Detailed Description
The material flow in the mill is not uniform. While the tail end of the bar leaves the Roughing mill and Intermediate mill stands, tendency for increasing the linear speeds of the bar exists. This results in cascading effect of increase in speed of the rod at tail end resulting in increase in speed of the Intermediate block, finishing block etc in cascade. To prevent the above phenomena of speeding up in tail end , the speeds of Intermediate block, Finishing block, Pinch roll, laying head are controlled and coordinated by varying the speed set points at the tail end, by the Process Control Computer.

The reasons for cobbles in the head end and middle of the bar are identified and are correctable. The Pinch roll speed becomes lesser than the linear speed of the rod at the tail end of the bar when the tail end is still present in the working stands of the Finishing blocks. The drop in speed of Pinch roll results in looping of rod between the Finishing block and Pinch roll and snapping of rod takes place. The left over material in the working stands of the Finishing blocks gets accumulated in the breakout trough and the pass line.

During the tail end cobble, load on the Pinch roll increases due to more flow of material The speed of the rod increases while the rod leaves the working stands of the Finishing block. With the preset Pinch power ,the pinch roll cannot overcome the additional tail end load and maintain the speed .In the process of overcoming the load the pinch roll loses its speed for a short duration of time (s), resulting in asynchronization between Finishing Block and Pinch roll.. The Finishing block speed increases while the Pinch roll speed doesn’t increase at the same rate as the Pinch roll is loaded to above its rated capacity 110% (105% to 114% ) resulting in asynchronisation. The Pinch roll speed drops by 2 to 5% than the speed of the bar.

The following Pinch roll settings are carried out for all sections of wire rods:
a) The Pinch roll collar gap in open position is adjusted manually. x
b) The Pinch roll collar gap in closed position is adjusted manually so as to grip the rod in closed position. The gap is adjusted approx 80% of the rod size. The rod deflects the Pinch rolls to its size against the set pneumatic pressure. The grip by the Pinch roll creates necessary tension and prevents stuttering of the bar while the Water box nozzles are made on to quench the rod to 600°C -640°C for rebar rolling . x
Pinch roll air pressure is set at 4-4.5 bar. This pressure of the pneumatic cylinder ensures desired grip and overcome the deflection of Pinch roll offered by the rod.

Head lead speed determines the percentage over the finishing block speed by which the Pinch roll runs. A higher Pinch roll at no load avoid stuttering of bar while the rolls are closing on the rod. This is operator selectable parameter. It is selected as 6% to 8%.

Tail lead speed set point is operator selectable .This factor determines the speed reference of pinch roll at the tail end of the rod. It is selected as 4%to 6% in case of rebar rolling.

Pinch power is a operator selectable set point. The pinch power factor determines the current limit to be applied to the Pinch roll motor. The pinch power set point range is of 0 to 100. Pinch power 0 corresponds to 0 % current and 100 corresponds to 300 % of motor current .Pinch power is selected to maintain the Pinch roll actual speed greater or equal to the bar speed (2% to 4% over the linear speed of rod) when the Pinch roll is closed..The pinch power maximum set point is switched over to operator selected set point after the Pinch roll is closed. The pinch power set point is selected as 36 to 38 by the operator for rebar rolling. This corresponds to 108% to 114% of motor rated current for rebar rolling

The pinch rolls are driven by a DC motor. It is a 37.5kw motor. The pinch roll motor is designed for 110% load continuous and 150% ,300% momentarily. The 100% rated current of the motor is 182A. The speed regulation is carried out by digital drive controllers.

The Process control system generates the speed references of all the drives in the mill depending on the profile of the rod being rolled. The Process control system tracks the position of the rod and injects the position dependent set points to the drives. It performs sequential interlocking of devices like Shear cut, Guide trough, Pusher rod, Pinch roll open/ close , pinch power set point ,Water boxes open/close and the like. .

For tracking of the rod hot metal detectors (HMD) are provided at the stands. These detectors sense the presence of material. These HMD’s are connected to the Process control system. The presence of material (HMD energized) and absence of material (HMD de-energized) are sensed .
The pinch roll set points and position of rolls (close/open ) at different positions of the rod are as follows:
1) No bar in the Finishing block, Pinch roll, Laying head
a) The Pinch roll is in open condition
b) Pinch roll runs at % over speed selected than Finishing block
c) Speed set point of Pinch roll (k1)= (100+head lead speed / 100)*
Finishing block speed
d) Pinch power set point is maximum 100
2) When HMD at the exit of finishing block is energized i.e. material is present in Finishing block
Time for the rod to reach the Pinch roll and Laying head is calculated.
a) Calculation for the closing of Pinch roll is done
T =( distance of Pinch roll from exit FB HMD +400)/V - Tf
V= linear speed of the Finishing block
Tf = Function time of Pinch roll valve
Distance of Exit FB HMD to Pinch roll is 35600 mm
T= Instant at which Pinch roll close command is initiated (time after head end after Finishing block)
The pinch roll is closed after the time T has elapsed.
For 8mm rebar pinch roll closing command is initiated at T =36 m/37m/s=0 .980s approx
b) The Pinch power set point is switched from 100 to operator selected Pinch power after the instant T. The operator selected limit is output to the drive controller by the computer.
c) On application of operator selected pinch power , Pinch roll actual speed will be greater or equal to the linear speed of the rod.
Finishing block speed with bar = 34m/s
Pinch roll ref =36.08m/s
Pinch roll actual speed is maintained at 34.5 to 35.8 m/s
Pinch roll current =109%
Head lead speed = 6%
Pinch power = 38
d) The Water boxes are opened.
3) Tail tracking
The instance when the FB Loop HMD de –energizes is detected as the tail end ready for exiting the Finishing block working stands.
The loop HMD is at a distance of 2375 mm from the entry HMD of FB
The entry FB HMD is at a distance of 550 mm from the 16th stand
Actual Speed of pinch roll is stored at the instant.
The speed set point of Pinch roll is changed to new set point (K2) =synch factor * Actual speed of the Pinch roll
In the case above it is 34.5% to 35.8%
When HMD at the entry of finishing block is de -energized i.e. material is leaving the Finishing block
a) The instance when the bar will leave the last working stand is calculated.
b) After the time has elapsed the speed set point of Pinch roll is changed to tail speed
Tail end speed set point = (100 +Tail lead speed factor) / 100 *K2
When HMD at the exit of finishing block is de -energized i.e. material has left the Finishing block
a) The time for tail end to reach the Pinch roll and Laying head is calculated
b) The command for Pinch roll open is initiated
c) The pinch power set point is switched over to 100
d) The speed set point is set to k1

Provision of additional Pinch power set point 40 (5% more than the operator selected set point ie.120% of motor rated current) at the tail end of the bar, when the bar is still in the working stands of the Finishing blocks and Pinch roll for duration of 2 to 3 sec
i. A higher pinch power is provided to the Pinch roll at the tail end to enable it to overcome the load and to synchronize with the linear speed of the bar .
ii. The tail end of the bar is tracked and instance for changeover to new set point is calculated.
iii. A signal to the drive to “Switch over from operator selectable pinch power set point to higher Pinch roll set point”is generated.
iv. The motor is not subjected to continuous overloading.

The HMD’s are directly connected to the Process control system. To enable usage of these HMD signals, a new PLC is put in Control pulpit (CPH)
a) The HMD signals are connected to the input card of the PLC.
b) Outputs are derived from the inputs.
c) For each HMD input, 2 outputs are provided. One output is connected to the Process control system and the other is connected to Drive PLC(S7-400) for carrying out tasks.

The HMD detects the presence of hot bar. This signal is sent simultaneously to the drive PLC and the Process control system.

The Pinch power change over is done while the tail end of bar is still in the working stands of Finishing block stand 16 (entry stand of Finishing block being stand 16 and exit being stand 25).

There are three HMD’s in area of Finishing block, FB Entry HMD (H1), FB exit HMD(H2) and FB loop HMD (H3) .The entry FB HMD(H1) is used for sensing the presence of the bar in FB area and Exit FB HMD(H2) is used for presence of rod after the Finishing block.
For switching over to higher pinch power 40 FB loop HMD (H3)signal is interlocked with the bar tail end which has exited the Intermediate block HMD (I2).

The intermediate block has three HMD’s. IB Entry HMD( I 1), IB exit HMD ( I 2) 1and IB loop HMD ( I 3)
The instant when the rod exits Intermediate block i.e. status of IB exit HMD(I 2) changes from energized to de-energize is taken.
The speed of the rod is calculated. The speed of the rod is calculated in the drive PLC (S7-PLC)
Formula for calculating the linear speed of the rod
V = N*p*(D- G)*I/60
V= linear speed of the rod in m/s
N= speed of the intermediate block
D= diameter of the ring
G= groove depth(14.1 mm)
I = Gear ratio (1.675)

The time required for the tail end of the rod to traverse the distance till the FB loop scanner area (H3) is calculated
T2 = X/V
Where X = Distance between IB exit HMD and FB loop scanner HMD (12375 mm)
V = Linear speed of rod after leaving IB (6.78m/s approx)

After sensing the de-energize signal of Exit IB HMD (I2)and after dwell time for reaching the FB looper is elapsed and the FB loop HMD (H3) is de-energized command to switch over to higher Pinch power is sent to the drive.

The duration for which the higher pinch power is applied is 3sec or Exit FB(H2) HMD de-energize signal. In normal rolling the duration of this switching will be 1.8 sec. The motor is subjected to 120% load for 3sec duration maximum.

The provision for entering the roll diameter is provided in the drive HMI (Human Machine Interface).Provision for select/deselect the higher pinch power feature for rebar is made in HMI. During other sections where this feature is not required it is de-selected.
The Pinch power set point is received by the digital drive controller. This set point is the current limit in the drive.

A logical switch is provided in the digital drive controller. The output of the switch is controlled by a conditional input. The switch can take two inputs. The output of the switch is determined by the conditional input. The output of the switch is either of the two inputs
If the conditional input to the switch is low, then Input 1 will be send as output. Operator selected pinch power is connected at input 1
If the conditional input to the switch is high, then Input 2 will be send as output. Higher pinch power is connected at input 2
The switching condition is the output generated in PLC and set as conditional Input 3 of switch.
When the switch over command will become high at the tail end.
Higher pinch power 40 will be output at the logical switch .
The output of the switch is connected to current limit in drive controller.

Prior to the invention the higher pinch power selected was applicable over the whole length of the bar being rolled
The average time required to roll the Bar is 82 sec. The motor is subjected to 120% of current continuously for 82 sec. With the innovation the motor is subjected to 120% of current momentarily for a maximum of 3 sec .

Due to cobbles during 8R section , the water flow in the water boxes were throttled and rods were quenched to 670° C to 690° C. This resulted in production of Fe 415 grade rods. Fe500 grade rods couldn’t be produced as it requires more quenching of the rod and laying the coils at temperatures 600°-630°C which creates more load on the pinch roll. By carrying out the above improvement Fe500 and Fe 550 grades production and quality achieved.

The present technique is further described hereinafter with reference to the accompanying drawings, wherein

Fig 1: is showing connection of HMD signal to process control computer
Fig 2: is showing connection of HMD signal to s7 400 plc of logical drive
Fig 3: is showing layout of HMDs in Intermediate Block and Finishing Block area
Fig 4: is showing logical change over switch

Fig 1 is showing the connection of HMD signal to Process Control Computer (PC). Prior to this invention HMDs are directly connected to Process Control Computer. A new PLC (PI) is put in Control pulpit to enhance usage of these HMD signals. The HMD signals are connected to the input card of the PLC (PI). Outputs are derived from the inputs of said PLC(PI). For each HMD input, two outputs are provided one connected to the Process Control Computer (PC) according to Fig: 1 and the other connected to Drive PLC (PD).
Fig 2 is showing the connection of HMD signal S7 400 PLC of logical drive. The HMD signals are connected to the input card of the PLC (PI). Outputs are derived from the inputs of said PLC (PI). For each HMD input, two outputs are provided one connected to the Process Control Computer(PC) according to Fig: 1 and the other connected to Drive PLC (PD).
Fig 3: is showing layout of HMDs in Intermediate Block and Finishing Block area. These HMDs detect the presence and movement of the bar through the block and send the signal to the PLC (PI). The signal change over from low to high HMD energized indicates the head end. The signal change over from high to low HMD de-energized indicates the tail end.

Fig 4: is showing logical change over switch. The switch can take two inputs. The output of the switch is either of the inputs. If the conditional input to the switch is low, then Input 1 will be send as output. Operator selected pinch power is connected at U240.01. If the conditional input to the switch is high, then Input 2 will be send as output. Higher pinch power 40 is connected at U240.02. The output from PLC which is the Switch over command is connected as conditional input 3 at U241.1.
The above description of the exemplary embodiments according to the present invention serves only for illustration purposes and not to restrict the invention. Various changes and modifications are possible within the context of the invention, without departing from the scope of the invention and its equivalents.