Hot Strip Mill Process Flow, Cobble Generation & Preventive Action

Hot Strip Mill Process Flow, Cobble Generation & Preventive Action

M Ajay Kumar

(USN: 3VC24MPM05)

Department Of Mechanical Engineering

Rao Bahadur Y Mahabaleswarappa Engineering College Cantonment, Ballari 583104, Karnataka

Under The Guidance Of

Prof. Chandrashekar Hiregoudar

Assistant Professor Department Of Mechanical Engineering

Rao Bahadur Y Mahabaleswarappa Engineering College Cantonment, Ballari 583104, Karnataka

Abstract – The Hot Strip Mill (HSM) is a critical process in steel manufacturing used to convert slabs into hot rolled coils by using (220-260) mm thick & width (900-2250) mm of length 4 meter 1150 meter through controlled thermo-mechanical operations. During rolling, maintaining strip stability within the finishing mill stands is essential to ensure product quality and continuous operation. However, defects such as cobble formation can occur, leading to production loss and equipment damage. A cobble is characterized by uncontrolled folding or accumulation of the strip due to instability in rolling conditions. This study focuses on understanding the Hot Strip Mill process, identifying the causes of cobble generation, and analyzing process parameters such as Center Line Deviation (CLD), differential roll force, strip tension, and temperature, width, the Based on data analysis, suitable preventive measures and corrective actions are proposed to reduce cobble occurrence, improve mill efficiency, and enhance product quality.

CHAPTER 1 INTRODUCTION

STUDY ON HOT STRIP MILL PROCESS FLOW, COBBLE GENERATION & PREVENTIVE ACTIONS.

The Hot Strip Mill (HSM) is one of the most important units in an integrated steel plant. It processes reheated slabs into thin strips through roughing and finishing mills. The slabs are heated to high temperatures (11501250°C) in a reheating furnace and then rolled in multiple stages to achieve the desired thickness and properties. The rolling process involves precise control of various parameters such as temperature, roll force, strip tension, and speed. Any deviation in these parameters can lead to instability in strip movement, resulting in defects such as cobbles. Ensuring stable strip flow is therefore essential for efficient mill operation.

In a typical HSM, steel slabs (220260 mm thick) are reheated to ~11501250°C, rough-rolled into transfer bars (~2040 mm), and then finish-rolled in a tandem mill (often 67 stands) to final gauge (1.225 mm) at high speeds (up to 20+ m/s). The temperature of finishing mill entry temperature is maintained up to 1140°C & exit temperature is maintained up to 850-940°C. During coiling the temperature is achieved up to 450-650°C by producing the coil diameter of 700-2250 mm with 28 tons weight.

Cobbles primarily occur due to instability: differential elongation across the strip width causes waviness, camber (curvature), or buckling, which the rolls/looper cannot handle, leading to pinching or piling. Temperature variation, width deviation, and gauge variation are interconnected root causes because they induce uneven deformation resistance, flow stress, and elongation.

Temperature variation (across width/length or head-to-tail) alters local hardness/ductility uneven reduction flatness defects or camber.

Gauge variation (thickness spikes or wedge) amplifies load differences between stands. Width deviation (from edging issues or contraction) affects tracking and side-guide contact.

1. Rolling Type

   It is the process of plastically deforming metal by passing it between rolls

   It

   is widely used to convert steel ingots from bloom, billets and slabs and subsequently into plates, sheet and strips.

   Advantages

   provides high throughput

   provides good control over the dimensions of the finished product.

   In Hot Rolling

   metal is rolled at a temperature below its re crystallization temperature higher reduction in the cross section is achieved

2. Rolling process

   Rolls transfer energy to the strip through friction. As the strip is dragged by the rolls into the gap between them. It decreases in thickness while passing from the entrance to the exit, meanwhile its speed gradually increases from the entrance to exit.

   V0= input velocity Vf= final or output

   velocity R=roll radius

   Hb=back height

   Hf=output or final thickness

3. Hot strip mill details

   Raw material: Final product:	Slabs Coils
   Slab-size
   Thickness:	220-260mm
   Width:	900-2250mm
   Length:	11 mm
   Weight	36 Ton
   Strip size:
   Thickness:	1.2-25mm
   Width:	900-2250mm
   Coil size:
   inner diameter:	762mm
   outer diameter:	1000-2150mm
   coil weight max:	35 tones

   Per day, Production will be 600 to 640 coils in Hot strip mill -2

   Typical Applications of steel

   Structural members, trenches

   Tubes and pipes-galvanized pipes, black tubes White goods outer sheets

   Air ventilation ducts

   Automobile body and structural members Cross country petroleum pipes

   Floor plates

   Storage systems, furniture

   Main areas in Hot strip mill (HSM)

   Slab yard Reheating furnace

   Hydraulic Scale Breaker Slab Sizing Press Roughing Mill 1

   Roughing Mill 2

   HHC, Crop shear & Finishing scale breaker(FSB) Finishing mill (F1 to F7)

   Run out table (ROT) Down coil

   Roll Shop

   Utility & Power system

   SLAB YARD

   Slabs after continuous casting from SMS II (Steel Melting Shop) and arranging them according to grade, width and length wise in slab yard.

   Supplying slabs according to rundown pattern prepare by PPC to Reheating furnace.

   Slab will be loaded on slab charging table by overhead gantry type crane, and transferred to slab weighing table and Furnace entry table.

   Thickness: 220 260 mm

   Length: 4700 to 1100 mm

   Width: 900 2200 mm Max Slab weight: 37 ton

   REHEATING FURNACE

   Type: walking beam type

   No. of Furnaces : 3

   Length (Wall to Wall) : 51200 mm

   Width : 11800 mm

   No. of Fixed beams : 4 charging section 5 Discharging section

   No. of Movable beams : 4 Nominal distance between

   Slab inside furnace : 50 100 mm

   Maximum OP Temperature : 1320 c

   Capacity : 350 tons/Hr. each Furnace

   Fuel

   : Designed to fire mixed gas

   CO	– 48%
   H2	– 17%
   CO2	– 29%
   N2	– Balance

   Heating Area

   • Recuperating Zone

   • Preheating Zone

   • Heating Zone 1 and Heating Zone 2

   • Soaking Zone

To charge & discharge the slabs there are Charging & Discharging machines with respective roller tables The functions of this is to reheat the slabs to the rolling temperature

Temperature maintained at each furnace is 1100-1200 celsius HYDRAULI SCALE BREAKER (HSB)

The purpose hydraulic scale breaker is to remove the scales developed while reheating slab using high pressure descale water up to 220 bar high pressure

SLAB SIZING PRESS (SSP)

SSP is placed right after the Hydraulic scale breaker

SSP is used to reduce the width of the slab.

SSP has a reduction capability of up to 300 mm & the reduction is done using anvils pressing the slab from either side.

ROUGHING MILL 1 (R1)

Roughing Mill 1 is a 2 High Roughing Mill

Both Top & Bottom rolls are drive by 2 – 3000KW Synchronous motors with separate Drives

R1 has attached vertical Edger E1 (vertically Driven with separate motors of 1500 KW) with AWC (Automatic width control).

Thickness 30 60 mm

No of passes: Min 1 & Max 3 passes

ROUGHING MILL 2 (R2)

Roughing Mill 2 is a 4 High Roughing Mill

R2 has Attached Vertical Edger E2 similar to E1 with AWC (Automatic width control). R2 having two works rolls and two back up rolls.

R2 having two works rolls and two back up rolls.

Roughing mill for effective slab reduction

Roughing mill Top & Bottom work roll is individually driven by 2 – 3 phase synchronous motor that are 8000Kw each.

No of passes: Min 3 passes (5 Passes Max 7 passes

HEAT HOLDING COVER

Heat holding cover is used to prevent Temperature loss from the transfer bar before material enters finishing mill stand.

There are total 8 Heat holding cover.

Crop Shear

Crop Shear is used to cut Head end & Tail end of the transfer bar.

It consists of two blades

Crop shear is driven by a 2500 kw motor

FINISHING SCALE BREAKER (FSB)

FSB is similar to HSB, Descaling at FSB is done to remove the fine scale left or generated while rolling in RM by using 180 bar pressure waters.

FINISHING MILL (FM)

FM is a Four High Non reversing Mill It has total 7 stands

F1 F4 Pair cross technology

F5 F7 Work roll shift technology It has auto roll change using buggy

All 7 stands are driven by a 10000kw synchronous motor for each stand

Purpose of FM is to reduce the thickness of strip as per the customer requirements.

RUN OUT TABLE (ROT)

Provide in between finishing mills and down coiler. ROT has 16 banks

It is used for strip cooling purpose

Strip coming out of finishing mill is at 850c-950c is cooled down to 150c-300c. Consists of water header for cooling at the top and bottom surfaces to strip

Automatic cooling is done with respective to grade being rolled to achieve required mechanical properties. To move the strip the roller table is driven with 404 no. of 9KW motor

Down Coiler

A down coiler coils the finished steel strip onto a high temperature mandrel. Totally three down coilers are there.

LITERATURE SURVEY

Hot Strip Mill (HSM) Cobble: Literature Review on Causes Related to Temperature Variation, Width Deviation, and Gauge Variation.

Temperature Variation as a Cause of Cobble

Temperature non-uniformity is one of the most cited primary causes.

Sources: Reheating furnace skid marks create cooler bands (differences up to 60°C across width). Uneven descaling, roll cooling, or inter stand cooling exacerbates this. Head/tail cooling during transfer also contributes.

Effects: Cooler edges/sections have higher flow stress less reduction there center waves or edge waves. This causes waviness, which propagates through stands, leading to buckling/cobble, especially in thinner gauges. High temperature variation (>2030°C) correlates strongly with flatness defects.

Quantified Impact: In one study, reducing width-wise temperature variation from 2530°C to 510°C (via furnace optimization, descaler, and roll cooling improvements) cut cobble rates by ~48%. Slab width temperature variation of ~60°C was directly linked to strip waviness and cobbles.

Related Mechanisms: Temperature affects scale formation (tertiary scale), which can influence friction and surface quality, indirectly contributing to tracking issues. Higher entry temperatures to finishing mill (>1070°C roughing) increase risks.

Modeling: Data mining (SVM for signal analysis, PLS, logit models) links temperature signals to cobble prediction. Multivariable control (AGC, looper, flatness) helps mitigate.

Mitigation from Literature: Uniform reheating, improved furnace zoning, edge heaters, precise descaling, and real-time pyrometry across width. Coil box technology retains heat uniformity.

Width Deviation and Its Role in Cobble

Width control is critical; deviations cause camber, steering problems, and side-guide impacts.

Causes: Uneven temperature (skid marks differential contraction/expansion), poor vertical edging in roughing, roll wear, or tension issues. Patent literature notes skid-mark-induced temperature differences cause width variation at rougher delivery.

Effects: Width variation leads to camber (curvature in plane), causing lateral walk (strip tracking off- center). This results in edge buckling against guides, pinching in subsequent stands, or cobble. Thin strips and high speeds amplify this.

Interactions: Combines with temperature/gauge issues. Excessive transfer bar crown (from roughing roll wear) causes non-uniform elongation buckling/cobble in F1 stand.

Control: Width meters, dynamic vertical roll adjustment, steering control techniques, and camber gauges in roughing. Load difference methods or vision systems for tracking.

Quantified: Even small offsets (e.g., 2mm guide misalignment) dramatically increase cobble risk, especially for thin gauges.

Gauge (thickness) variation interacts heavily with the above

AGC limitations, roll deflection/wear, temperature-induced flow stress changes, incoming bar gauge inconsistency, or looper/tension fluctuations. Load cell drift causes systematic bias.

Effects: Thickness spikes or wedge create differential elongation/loads flatness loss or instability between stands. Bad flatness leads to cobble via loss of control. Low-thickness bars (<20mm in some contexts) are prone. Gauge affects bite conditions and friction.

Interactions: Temperature variation causes localized gauge spikes (cooler areas resist reduction). Width and gauge together influence profile/crown, leading to edge waves or center buckles.

Control: Advanced AGC (automatic gauge control), multivariable techniques, feedforward from upstream sensors, and profile/flatness models. X-ray gauges, profile meters.

Overall Interactions (Temperature-Width-Gauge): These form a feedback loop. Temperature drives property variations uneven gauge/width reduction shape defects

cobble. Data-driven models (SVM, ANFIS, etc.) predict risks from combined signals.

Additional Contributing Factors

Flatness defects in transfer bars, head-end curving, equipment misalignment, roll wear/spalling, looper failure, guide misalignment.

Process parameters: High speeds, thin gauges, chemistry (e.g., higher P/Si affecting scale/strength). Detection: Mill signal analysis (forces, speeds, temperatures), vision systems, predictive ML.

Objectives

One of the major challenges faced in a Hot Strip Mill is cobble generation. A cobble occurs when the strip loses its intended path during rolling and becomes folded, twisted, buckled, or wrapped around mill equipment. Such incidents can lead to severe production losses, damage to equipment, increased maintenance requirements, safety concerns, and deterioration of product quality.

The occurrence of cobbles is generally associated with process disturbances such as thickness deviation, width variation, temperature imbalance, improper tension control, equipment malfunction, and guide misalignment. Since cobble incidents affect both productivity and operational reliability, it is important to understand the factors responsible for their occurrence and implment suitable preventive actions which reduces cobble.

Modern steel plants operate under highly competitive conditions where production efficiency and product quality are of utmost importance. Even a single cobble event can result in substantial losses due to mill stoppage, material wastage, manpower involvement, and equipment recovery activities.

As production targets continue to increase, maintaining strip stability throughout the rolling process becomes increasingly important. Understanding the relationship between process parameters and cobble formation can help operators detect abnormal conditions at an early stage and prevent severe operational disruptions.

Main Objective of the Study

The primary objective of this project is to investigate the causes of cobble generation in the Hot Strip Mill and develop effective preventive actions that improve strip stability, product quality, equipment reliability, and overall production performance.

Study of Cobble Formation Mechanism

The first objective is to understand how cobbles are generated during rolling operations. Cobble formation is often the result of a sequence of process deviations rather than a single isolated event.

A detailed understanding of the formation mechanism provides valuable information for implementing corrective and preventive actions before severe strip failure takes place.

Identification of Major Causes

Another important objective is to identify the factors that contribute to cobble generation. These factors may include:

• Thickness deviation

• Width variation

• Temperature fluctuation

• Excessive strip tension

• Guide misalignment

• Roll wear

• Hydraulic system instability

• Sensor inaccuracies

• Operational errors

  Understanding the contribution of each factor helps prioritize improvement activities

  Analysis of Thickness Deviation

  Thickness control is one of the most important requirements for stable rolling. Excessive variation in strip thickness can cause uneven deformation and unstable strip movement.

  This objective focuses on studying the influence of thickness deviation on strip behavior and determining how deviations from target thickness contribute to cobble formation.

  The analysis also aims to identify methods for maintaining consistent thickness throughout the rolling process.

  Analysis of Width Variation

  Strip width plays a significant role in guiding the material through successive rolling stands. Variations in width can affect strip tracking and increase the possibility of strip wandering.

  This objective aims to examine the effect of width variation on rolling stability and evaluate how improper width control contributes to cobble generation.

  Analysis of Temperature Variation

  Temperature directly affects the mechanical properties and deformation behavior of steel during rolling.

  Non-uniform temperature distribution can result in unequal metal flow, shape defects, edge cracking, and strip instability.

  The objective is to investigate the influence of temperature variations on strip behavior and identify temperature- related conditions that increase the risk of cobble generation.

  Evaluation of Process Stability

  Stable rolling conditions are necessary for continuous production and quality improvement. This objective focuses on evaluating critical process parameters such as:

• Rolling force

• Rolling speed

• Strip tension

• Thickness control

• Temperature control

  Study of Equipment Performance

  The condition of mill equipment significantly influences rolling performance.

  Components such as rolls, guides, loopers, sensors, hydraulic systems, and cooling systems must operate

  correctly to ensure smooth strip movement.

  This objective examines how equipment-related issues contribute to cobble incidents and identifies maintenance practices that can reduce failures.

  Historical Data Analysis

  Past operational data provides valuable insights into recurring problems and process trends.

  This objective involves reviewing previous cobble incidents to identify common patterns, affected rolling stands, product categories, and operating conditions.

  The analysis helps establish a factual basis for preventive action planning.

  Assessment of Production Losses Cobble incidents often result in:

• Production interruption

• Material rejection

• Increased downtime

• Additional maintenance activities

  The objective is to assess the overall impact of cobble generation on plant productivity and operational efficiency.

  Improvement of Product Quality

  Stable rolling conditions contribute directly to improved product quality.

  This objective focuses on reducing quality defects associated with strip instability and ensuring consistent dimensional accuracy.

  Improved quality enhances customer satisfaction and reduces rework requirements.

  Development of Preventive Actions

  One of the most important objectives of this study is to identify practical preventive measures.

  Potential preventive actions include:

• Improved process monitoring

• Better thickness control

• Enhanced temperature management

• Proper guide alignment

• Routine equipment inspection

• Effective maintenance planning Enhancement of Operational Safety

  Cobble events can create hazardous conditions for operating personnel and nearby equipment.

  This objective aims to improve safety by minimizing emergency situations associated with strip breakage and instability.

  Improvement of Productivity

  Reducing cobble generation contributes directly to increased production output.

  The objective is to enhance mill utilization by minimizing downtime and maintaining continuous operation.

  Establishment of Monitoring Practices

  Continuous monitoring is essential for early detection of abnormal process conditions. This objective focuses on establishing monitoring practices for:

• Thickness

• Width

• Temperature

• Strip tension

• Rolling force

  Timely monitoring enables operators to take corrective action before a cobble occurs. Long-Term Process Improvement

  The final objective is to recommend long-term improvements that support sustainable operational excellence.

  These recommendations may include technological upgrades, process optimization strategies, maintenance improvements, and enhanced training programs.

  Scope of the Study

  The scope of this study is limited to the investigation of cobble generation in Hot Strip Mill operations. The study primarily focuses on the influence of process parameters such as thickness, width, and temperature on strip stability.

  The work includes analysis of operational conditions, identification of root causes, evaluation of equipment performance, and development of preventive measures.

  The study does not focus on metallurgical property development or downstream processing operations. Instead, emphasis is placed on rolling stability and operational reliability within the Hot Strip Mill.

  Expected Outcomes

  The expected outcomes of the study include:

• Better understanding of cobble generation mechanisms.

• Identification of critical process variables.

• Reduction in cobble frequency.

• Improvement in strip stability.

• Enhancement of product quality.

• Reduction in downtime and production loss.

• Improved equipment reliability.

• Increased operational safety.

• Better utilization of mill resources. Conclusion

  Cobble generation remains one of the most significant operational challenges in Hot Strip Mill production. Through systematic analysis of thickness deviation, width variation, temperature imbalance, equipment performance, and process stability, it is possible to identify the root causes responsible for strip instability.

  The objectives of this study are directed toward understanding these causes and implementing preventive actions that enhance productivity, improve quality, reduce downtime, and promote safe operation. The findings of this study can provide valuable guidance for achieving stable rolling conditions and improving the overall performance of Hot Strip Mill operations.

  Experimental Investigation Introduction

  The experimental investigation was carried out to understand the factors responsible for cobble generation in the Hot Strip Mill and to identify suitable preventive actions. The study was performed using actual production records, mill operating data, cobble incident reports, and observations collected during routine rolling operations.

  The experimental work focused on examining these parameters under actual operating conditions and evaluating their effect on strip stability.

  Experimental Setup and Data Collection

  The study was conducted in the Hot Strip Mill during normal production operation. Data were collected from the process control system, production reports, shift logs, and cobble records maintained by the operating department.

  The information collected for analysis included:

• Slab dimensions

• Entry and exit thickness

• Strip width

• Furnace discharge temperature

• Finishing temperature

• Coiling temperature

• Rolling speed

• Rolling force

• Stand-wise process data

• Cobble occurrence details

  Data from both normal coils and cobble-affected coils were selected for comparison.

  The collected data were organized and reviewed to identify common patterns associated with cobble incidents.

  Experiment 1: Analysis of Thickness Deviation and Cobble Generation

  Thickness records were collected from coils produced during the observation period.

  For each selected coil, the target thickness and actual thickness values were compared. The deviation between the desired value and the measured value was analyzed.

  Special attention was given to coils that experienced unstable rolling conditions or cobble incidents. Observations

  During the investigation, it was observed that coils with higher thickness variation frequently experienced strip instability.

  In several cases, sudden thickness fluctuations caused uneven metal flow through the rolling stands. The strip movement became irregular and showed a tendency to deviate from the center line.

  When thickness variation exceeded the normal operating range, strip control became increasingly difficult.

  Discussion

  Thickness deviation affects deformation behavior during rolling.

  When one portion of the strip undergoes greater reduction than another portion, differential metal flow develops. This condition can create localized stress concentration and unstable strip movement.

  As rolling speed increases, even small thickness deviations may become significant enough to affect strip stability.

  The analysis showed that maintaining accurate thickness control is essential for preventing cobble generation.

  Conclusion

  The experimental observations indicate that excessive thickness variation increases the probability of cobble formation. Improved thickness monitoring and effective gauge control can significantly reduce strip instability.

  Experiment 2: Investigation of Width Variation and Strip Stability

  Width measurements were obtained from production records and compared with planned product dimensions.

  The study included coils representing different width ranges and product categories. The movement of the strip through the rolling stands and guide systems was also observed.

  Observations

  Several cobble incidents occurred in situations where width variation was higher than expected.

  In such cases, the strip did not remain properly centered during rolling. Lateral movement increased, and guide interaction became inconsistent.

  Some strips displayed edge instability before the cobble event occurred.

  Discussion

  Width consistency is important for maintaining stable strip travel through the mill.

  Variations in strip width may influence guide performance and affect the ability of the strip to remain centered.

  As the strip moves through successive rolling stands, even minor deviations may become amplified, resulting in unstable strip behavior.

  The investigation confirmed that proper width control contributes significantly to stable rolling operation.

  Conclusion

  The study demonstrated that excessive width variation increases the risk of strip tracking problems and cobble generation. Improved width control practices can enhance rolling stability.

  Experiment 3: Effect of Temperature Variation on Cobble Formation Temperature data were collected from different stages of the rolling process. Measurements included:

• Furnace exit temperature

• Roughing mill exit temperature

• Finishing mill entry temperature

• Finishing mill exit temperature

• Coiling temperature

  The temperature history of normal coils and cobble-affected coils was compared. Observations Many cobble cases were associated with uneven temperature conditions.

  Temperature differences across the strip width were found to influence deformation behavior.

  In certain instances, strips with significant temperature variation showed unstable movement during finishing mill rolling.

  Discussion

  Temperature directly influences material plasticity and rolling characteristics.

  When temperature is not uniform, different portions of the strip deform differently under the same rolling force. This creates unequal metal flow and may lead to strip instability.

  The analysis indicated that maintaining uniform thermal conditions is essential for preventing cobble generation.

  Conclusion

  Temperature variation was found to be a major contributing factor to strip instability. Improved temperature control can reduce the likelihood of cobble occurrence.

  Experiment 4: Study of Strip Tracking Deviation Experimental Procedure

  Strip tracking behavior was monitored during rolling operations. Particular attention was given to:

• Strip centering

• Lateral movement

• Guide interaction

• Looper performance

  The observations were recorded for both normal and abnormal rolling conditions.

  Observations

  Before many cobble incidents, noticeable strip movement away from the center line was observed. The magnitude of lateral movement generally increased as the strip approached unstable conditions.

  Discussion

  Stable strip tracking is essential for smooth rolling operation.

  When strip movement exceeds acceptable limits, the possibility of contact with guides and equipment increases. Such interactions can initiate strip distortion and eventually lead to cobble formation.

  Conclusion

  Strip tracking behavior can serve as an early warning indicator for potential cobble events. Continuous monitoring can help operators take corrective action before severe instability develops.

  Experiment 5: Root Cause Analysis of Cobble Incidents Procedure

  A detailed review of recorded cobble incidents was conducted. Each incident was classified according to the primary cause.

  The causes were grouped into three categories:

• Process-related causes

• Equipment-related causes

• Operational causes Findings

  The analysis showed that process-related factors accounted for the majority of cobble incidents.

  Among these factors, thickness deviation, width variation, and temperature fluctuation appeared most frequently. Equipment issues such as guide wear and sensor malfunction also contributed to certain incidents.

  Operational factors including delayed response to abnormal conditions were observed in a limited number of cases.

  Discussion

  The findings indicate that cobble generation is usually the result of multiple contributing factors rather than a single isolated issue.

  Effective prevention therefore requires a combination of process control, equipment maintenance, and operator awareness.

  Validation of Preventive Actions

  Following the analysis, several preventive actions were introduced and evaluated. These actions included:

• Improved thickness monitoring

• Enhanced temperature supervision

• Regular guide inspection

• Sensor calibration

• Improved process parameter review

After implementation, rolling stability improved and the frequency of strip instability events was reduced. The observations confirmed that preventive measures are effective when consistently applied during operation.

Summary

The experimental investigation demonstrated that thickness deviation, width variation, temperature fluctuation, and strip tracking instability are important contributors to cobble generation in the Hot Strip Mill.

Among the investigated factors, thickness deviation showed the strongest influence on strip stability. The study also confirmed that proactive monitoring and timely corrective actions are essential for preventing cobble incidents.

The findings obtained from this experimental work provide a practical basis for improving process stability, reducing production losses, and enhancing overall mill performance.

Experimental Report & Analysis Report

RD: 232802(171 slabs, 103.74 km)

Cobble during the rolling of 232802047(JVHHTT1B10 3*1417)

Cobble due to HE hit to F6 entry center tong and material head end got fold.

Slab Discharging.

Temperature drop in head end.

Finishing Mill Setup which is abnormal.

Cobble Material which stooped in Finishing Mill.

Miss Alignment in Guide

Cobble Graph

Cobble Analysis

Cobble Analysis Report

Total Roll force:

F5 : 14685 to 15060kn F6 : 13041 to 17228kn F7 : 12547 to 14513kn

Diff Roll force:

F5 : -337 to 137kn F6 : 187 to 2187kn F7 : 375 to 1150kn

Diff Roll gap:

F5 : 0.10 to -0.15mm F6 : 0.09 to – 0.27mm F7 : -0.06 to -0.26mm

Centre gap:

F5 : 2.84 to 2.75mm F6 : 2.69 to

2.1 mm F7 : 2.15 to 1.93mm Level-2 Automation

Comparison of Next Material (id-256862).

Good Coil Graph

Level-2 Automation

Results & Conclusion

The study was carried out to identify the major process conditions responsible for cobble generation in the Hot Strip Mill. During the analysis, production records, rolling parameters, and process trends were reviewed. Special attention was given to thickness variation, width deviation, and temperature fluctuation because these parameters directly influence strip stability

The collected data indicated that cobble generation is not caused by a single process parameter. Instead, it is the result of a combination of operating conditions that gradually reduce strip stability as the material passes through successive rolling stands. Whenever one or more critical parameters moved beyond the desired operating range, the probability of strip instability increased significantly.

Among the factors studied, thickness variation & temperature showed the strongest impact on strip movement. It was observed that strips having consistent thickness throughout the rolling process travelled smoothly through the mill without significant tracking issues. However, when thickness deviations occurred, uneven material flow developed between stands. This condition created variations in rolling force and strip tension, leading to unstable strip behavior. In several cases, excessive thickness variation resulted in strip wandering and eventually contributed to cobble formation.

The analysis also showed that width variation influenced strip guidance and edge stability. Strips with uniform width demonstrated better alignment and smoother movement through the mill stands. On the other hand, sudden changes in strip width created uneven load distribution across the strip. This caused difficulties in maintaining proper strip tracking and increased the chances of edge-related defects. In severe cases, width deviation contributed to strip instability and promoted cobble generation.

Temperature was identified as another important factor affecting rolling performance. The results showed that strips rolled within the specified temperature range experienced stable deformation and consistent material flow. However, when strip temperature dropped below the target level, rolling loads increased and material plasticity decreased. Such conditions made the strip more difficult to deform, resulting in shape defects, material skidding and unstable

movement through the mill. Temperature fluctuations across the strip width also created differences in elongation, which further affected strip stability.

The study further revealed that cobble incidents were more frequent during periods of process instability. Variations in rolling force, improper setup conditions, delayed process corrections, and equipment misalignment contributed to the occurrence of strip instability. In contrast, stable operating conditions resulted in smooth mill performance and significantly reduced cobble occurrence.

Implementation of preventive actions produced noticeable improvements in overall mill operation. Improved monitoring of rolling parameters helped operators identify abnormal conditions at an earlier stage. Better control of thickness, width, and temperature reduced process variation and improved strip stability throughout the rolling cycle. As a result, the frequency of cobble incidents decreased and product quality improved.

The results also indicated that regular equipment inspection played an important role in preventing cobble generation. Proper maintenance of guide systems, rolls, sensors, and cooling equipment contributed to stable rolling conditions. Mills that maintained equipment in good operating condition experienced fewer strip tracking problems compared to those operating with worn or misaligned components.