ROOT CAUSES OF MISROLL IN REROLLING MILLS
Steel has been known to humanity for 4,000 years. The level of per capita consumption of steel is treated as one of the most important indicators of socioeconomic development and living standards of the people in a country. Steel rerolling mills, also called reduction mills, are widely distributed across the world. The major concerns in the steel rerolling mill are misroll and cobble, which adversely affect most of the technoeconomic parameters, including the mill utilization, material yield, mill productivity, overall equipment effectiveness, overall line efficiency, and specific heat consumption of the reheating furnace. This paper discusses the major issues that lead to misroll and cobble in the rerolling mills, and these issues were identified via a shop floor analysis and breakdown records at an iron and steel rerolling mill. After the identification of the root causes of misroll, solutions are suggested.
INTRODUCTION
Today, iron and steel play vital roles in the development of modern economies. The per capita iron and steel consumption is generally considered to be a measurement of the socioeconomic development standards of a country’s people. India is presently the second largest producer of crude steel and the largest producer of sponge iron in the world. Figure 1 illustrates the percentages of crude steel production as of 2020. Crude steel production can play a vital role in leading India towards a USD 5 trillion economy (“Annual Report”, 2021). Steel rerolling is the process of converting raw or unprocessed steel into finished steel products by rolling and rerolling them into desired shapes and cross sections while they are in their hot states.
Citation: Performance Improvement Journal 62, 4; 10.56811/PFI-22-0001
When iron is smelted from its ore, it contains more carbon than is desirable. To become steel, it must be reprocessed to reduce the carbon to the correct amount, at which point other elements can be added. In modern facilities, this liquid is then continuously cast into long slabs or cast into ingots. Approximately 96% of steel is continuously cast, while only 4% is produced as ingots. Slabs are hot or cold rolled into sheet metal or plates. Ingots are heated in a soaking pit and hot rolled into slabs, billets, or blooms. Billets are hot or cold rolled into bars, rods, and wire. Blooms are hot or cold rolled into structural steel, such as I-beams and rails (Steel Re-rolling Mills Association of India, 2014).
Steel rerolling is the process of converting raw or unprocessed steel into finished steel products by rolling and rerolling them into desired shapes, such as bars, thermo-mechanically treated (TMT) rods, sectional products, and wires, while they are in their hot states. These mills are continuous in nature. Once the material enters the furnace, it only leaves as a final product. However, when this material gets stuck in the mill while rolling, it results in the complete loss of the material and causes the whole production line to stop. This can also lead to major equipment damage and major accidents. This condition of material getting stuck in the mill is called misroll, and the material that gets stuck is called cobble.
There are many other causes of losses of production efficiency in a rerolling mill, including scraping, quality issues, changeovers, and other minor stoppages due to minor breakdowns, etc. Cobble is the most concerning issue faced by mill operators and managers, as it leads to not only material loss but also large breakdowns, increased delays, and fatal accidents, which reduce both mill utilization and productivity. This paper aims to define the root causes of misroll and find a proper solution for misroll that in turn increases mill utilization and performance (Chakravarty, 2016).
The study started with the initial objective of understanding the present conditions of the iron and steel industries in India and then converged to billet conversion industries. The study continued by visiting the nearest billet conversion industry and understanding the whole conversion process from billets to wire rods. Then, the next objectives were to find out the issues related to misrolls and cobbles that were the main reasons for losses of efficiency and to suggest possible solutions for these issues.
LITERATURE REVIEW
Reeve et al. (1999) stated that flatness defects between stands can lead to the instability of the rolling process in the finishing mill, which could result in a complete loss of control and the destruction of the coil, that is, a cobble. This represents a yield loss and affects mill availability by stopping the process while the mill is cleared. According to Knight et al. (2002), when a bar with head end curving hits the work roll, it leads to cobble generation. Mackel et al. (2002) made a quality related monitoring of main drives and analyzed cobble-stops in the mill. They found that the monitoring of the main drive not only documents severe overloads but also records careless rolling operations.
According to Abuin et al. (2004), draft schedules often require adjustments to ensure a smooth transition from one stand to the next and to avoid mass flow errors that can cause cobbles. Mill cobbles can be prevented via the improvement of the interstand shape, head end tracking, revised level practices, and the elimination of catch points in the interstand equipment.
Knight et al. (2005) investigated different asymmetrical conditions that occur during the rolling of hot strip and can cause strip curvature, lead to poor product shape, profile, and quality, and cause reduced productivity. In the most severe cases, a strip with a head end turning towards the upper work roll may fail to enter the roll gap on its next pass and may result in a cobble.
Jardine et al. (2006) reviewed recent papers for the diagnostics and prognostics of machinery. They have discussed the use of multiple sensors in condition monitoring and techniques for multiple sensor data fusion.
Ghorai et al. (2013) proposed a method for automatic defect detection in hot rolled flat steel products. They used wavelet features to detect defects. They also developed an inspection system for steel plants that captures surface images and uses a proposed algorithm to detect defects.
DATA ANALYSIS
Figure 2 illustrates data that were collected from the nearest iron and steel rerolling mill over six months (Sharma & Thakar, n.d.). The major downtime consisted of operational issues and mechanical issues. Also, the cobble percentage was calculated for the same duration, and it showed an increasing trend over the six-month period, accounting for major losses for the company, as shown in Figure 3.
Citation: Performance Improvement Journal 62, 4; 10.56811/PFI-22-0001
Citation: Performance Improvement Journal 62, 4; 10.56811/PFI-22-0001
ROOT CAUSE STUDY OF COBBLES
It can be easily seen that misroll is the main cause of efficiency loss in this industry. In this paper, all of the root causes of misroll have been sought. For this, shop floor studies and a root cause analysis were conducted, and each breakdown was analyzed and noted. From this, the root cause of each issue has been found out.
Misroll is a phenomenon in rolling mills in which the running material, for some reason, misses the proceeding pass and rolls down at the same place, causing the material to become waste, known as cobble. Cobble is a defect in a rolled piece that results from a loss of control over its movement. It is totally a waste in any hot rolling industry. All of the issues related to misroll and cobble can be divided into two types: operational issues and machine and maintenance issues.
Operational Issues
Operational issues are basically those issues that arise when a sudden stoppage occurs in a smooth-running mill. These are not caused by any machine failure but instead occur due to continuous running and a lack of proper surveillance and monitoring. The following are some of the operational issues.
Horizontal Movement of Material
The horizontal movement of material or bending of material horizontally refers to the movement of material in a horizontal direction. This horizontal movement prevents the material from following the desired path, namely, entering the entry box in the desired manner and hitting any other part, which causes a misroll and damages the other parts of the mill. The material moves from the delivery guide of the current station to the entry box of the next station. This acts as the connecting link between the two stations and while moving from one station to another. If there is a misalignment of the guide or the entry box, the material gets deflected, hits the machine, and causes a misroll. This misalignment mainly happens due to manual errors, such as fitting, loosened bolts, slip in bolts, entry guide wearing, entry box roll wearing, entry pass wearing, loosened leaves, etc.
Front End or Back End Turn Up
Sometimes, the front end or back end of the moving material slightly turns up because of temperature changes and the continuous application of pressure. This turned up material ends up contacting with the entry box or other parts of the equipment, which either causes a misroll or damages the machine such that the upcoming material gets misrolled. A turned up front end hits the entry box of the next stand and gets misrolled. A turned up back end may pass from the next station and cause damage such that the upcoming material gets misrolled. The reasons for material turn up include a jammed entry box, a faulty bearing, roll misalignment, the material being below the desired temperature, etc.
Twister Roll Jammed
Twister rolls have the same function as that of the entry box rolls. However, the function of a twister is to twist the running material by 90 degrees such that a horizontal oval gets twisted into the vertical direction and then enters the next stand for further reduction. The reasons for material turn up include a faulty bearing, improper lubrication, etc.
Material Hit Equipment
This is a major reason for misroll. Material, instead of smoothly passing through the grooves in the rolls, can hit the equipment. It may be any part of the machine, including the pass collar, cooling line, etc. This happens mainly because of improper twisting, play in shafts or rolls, a loose entry box, misaligned cooling lines, etc.
Shaving
Shaving is a major quality issue in a rolling mill, and it leads to the rejection of the material. Additionally, if it is not taken care of at the right time, it leads to a misroll. Small edges emerge out of the running material due to some external particle that is stuck inside the equipment, groove formation, or contact with any other sharp edge. This leads to the rejection of the whole coil. Additionally, if these shaving chips get collected in the entry box, they can lead to a misroll. Shaving is majorly observed in finishing stands. The main causes of shaving in material are oversoaking, misalignment, metal chips getting accumulated, groove formation, oversized material, etc.
Scratching
Scratching is a similar phenomenon to shaving, but the difference is that in shaving, edges emerged from the material. However, the contact between the sharp edge and the surface of material is sometimes not significant enough to cause shaving. That is, it may only scratch the surface of the flowing material and make it a quality rejection. Scratching basically occurs because of groove formation due to misalignment, wear, manual errors, etc.
Cold Material
Cold material refers to material whose temperature drops below the desired temperature. Cold material leads to many problems while rolling. It reduces the expected production from the entry pass life, tends to turn up the material at the ends, and, apart from these operation issues, creates serious quality issues where the grain structure, material finish, etc. have to be taken care of precisely.
Looping
Looping of material usually takes place at the finish rolling, where the cross section of the material is thinner. The main reason behind this is the variation of speed between two stands. This happens due to the improper setting of the speeds of the two rollers or the improper setting of the timer at the repeater. This is also possible if the coiler pipe wears out and material gets loose. This will cause a misroll.
Roll Shifting
Here, roll shifting refers to the shifting of main rollers from their desired places. This mainly happens due to the use of old equipment in a mill. Roll shifting mainly disturbs the basic shape of the material that comes out after passing through the rollers, and this misshaped material gets misrolled when it passes to the next stand. Even if not misrolled, it will both cause serious quality issues for the coil and damage the equipment, resulting in the shaving or scratching of the coils. Also, due to roll shifting, material sometimes gets misrolled at the same place due to hitting somewhere else on the rollers, which can damage various other pieces of equipment. The possible reasons for roll shifting are the clearance between spindles and couplings, slip in bolts holding rolls, non-standard spindle lengths, etc.
Maintenance and Machine Issues
Operational issues are basically running issues that are caused by either the continuous running of a mill or manual errors in the setup of a mill. On the other hand, machine and maintenance issues are issues that are basically equipment related or are related to a lack of maintenance activities. This mainly occurs when there is a fault in the equipment itself. These issues can be solved by proper inspection and proper preventive maintenance. The following are machine and maintenance issues.
Vertical Movement of Material
The vertical movement of material occurs due to a fault in a machine, specifically, a jump in the rolls. The vertical movement of material disturbs the desired path of contact during the motion of the material, causing it to hit the machine. This mainly occurs at the delivery guide of the next stand, and it causes a misroll and damages the equipment. Possible reasons for vertical bending are a jump in rolls, a faulty bearing, oversized material, roll taper wearing, etc.
Machine Components Wearing
The guide box, twister roll, and entry box are some of the major parts that wear out most frequently. Wear in parts precludes the parts from performing their desired functions and leads to a misroll. This occurs due to groove wearing, a faulty bearing, improper lubrication, infrequent inspections, etc.
Electronic Failure
A spark in a main automated circuit board results in an automated circuit board failure. If this occurs while the mill is running, it results in the malfunction of electronic devices and ultimately leads to a misroll. This generally happens due to the shortening of two phases or a faulty switch.
Repeater Malfunctioning
When the mill is of the repeating type and not a straight, continuous mill, repeaters are used to provide a U-turn to the moving material. This repeater opens and closes with time synchronous electronic devices syncing with the moving material. However, if there is a failure to sync with moving material, the opening and closing gets disturbed and leads to a misroll. The main reasons for this include hot metal detector malfunctioning, a control box failure, a coil supply failure, timer malfunctioning, etc.
Machine Tripping
Due to high voltages, the DC motors get tripped frequently. This tripping suddenly stops the running mill and damages all of the running material, leading to cobble. Reasons for machine tripping are electronic failure, high voltage, old drives, wear and tear, rust, etc.
SOLUTIONS AND SUGGESTIONS
After identifying the root causes of misroll, solutions were provided. The solutions that were identified for each root cause were analyzed and confined into four standard remedies, as shown in Figure 4.
Citation: Performance Improvement Journal 62, 4; 10.56811/PFI-22-0001
These are the solutions that have been developed via data analysis and shop floor studies. However, most importantly, these solutions are to be backed up by continuous monitoring and immediate corrective actions for observed issues.
CONCLUSION
The present work aimed to improve mill efficiency and productivity by reducing one of the major concerns of a rerolling mill, namely, misroll and cobble, by conducting multiple shop floor studies and a root cause analysis. This study also aimed to improve mill operating practices, which will be a major factor in reducing misrolls. Many factors and their root causes have been identified as leading to a misroll. These factors were then divided into two major categories: operational issues and machine and maintenance issues. Operational issues are related to human errors or process errors, whereas machine issues are related to errors or faults in machines. After identifying the root causes of misrolls, solutions of each root cause were identified. All of the identified solutions were then classified into four major solutions: parameter standardization, procedure standardization, maintenance standardization, and technology upgradation.
All of these identified solutions need to be supported with continuous monitoring and immediate corrective measures for observed problems. By analyzing an individual industry for the identified issues of misroll and implementing the suggested solutions, the frequency of misrolls can be reduced. This work also motivates plant operating personnel to reduce the amount of waste in rerolling mills.
World Steel Production 2020
Six Months Downtime Summary
Six-month Cobble Percentage
Identified Solutions
Contributor Notes
KUSHAL SHARMA is currently working as an Assistant Professor in the Department of Mechanical Engineering at the Dr. A.P.J. Abdul Kalam University Institute of Technology, Jhabua (M.P.). He completed a bachelor’s degree in mechanical engineering and a master’s degree with a specialization industrial engineering and management at Rajiv Gandhi Prodyogiki Vishwavidyalaya Bhopal. He has over seven years of teaching and industrial experience in metal casting and forming industries as well as many research papers in reputable journals. He is also a lifetime member of the Indian Institution of Industrial Engineers (IIIE) and the International Association of Engineers (IAENG). His academic and industrial experience can be very helpful for aspiring engineers. You may reach him at kushal.sharma005@gmail.com.
UMANAND KUMAR SINGH is currently working as an Assistant Professor in the Department of Mechanical Engineering at the Dr. A.P.J. Abdul Kalam University Institute of Technology, Jhabua (M.P.). He completed a bachelor’s degree in mechanical engineering and a master’s degree with a specialization in heat and power engineering at Rajiv Gandhi Prodyogiki Vishwavidyalaya Bhopal. He is pursuing a Ph.D. He has both degrees with distinction, over eight years of teaching and research experience, and many research papers in reputable journals. He is also a member of the International Association of Engineers (IAENG). You may reach him at umanand.singh4@gmail.com.
ANJALI UPADHYAY is currently working as a Guest Faculty in the Department of Mechanical Engineering in the School of Engineering and Technology, Vikram University, Ujjain (M.P.). She completed a bachelor’s degree in mechanical engineering and a master’s degree with a specialization in industrial engineering and management at Rajiv Gandhi Prodyogiki Vishwavidyalaya Bhopal. She has both degrees with distinction, over seven years of teaching and research experience, and many research papers in reputable journals. She is pursuing a Ph.D. You may reach her at upadhyayanjali1992@gmail.com.
