25-09-2014, 03:05 PM
ABSTRACT OF THE PROJECT DONE The principal objective of this paper is to demonstrate the importance of machining quality and material handling in industry. This project is about the improvement in efficiency of Rolling Mill At Bhushan Power & Steel Limited, where I was under training, both where I had to find the losses which were affecting the efficiency of Rolling Mill and give remedies of that losses. Losses & Defects in Rolling Mill causes the mill to decrease it�s efficiency, due to which there is a loss of time as well in producing the steel products. The semis need to be delivered to the mill at the required temperature with minimum fuel consumption and uniform temperature all across. The zonal temperatures inside the reheating furnace are controlled through set points which need to be determined dynamically to achieve quality heating in various production scenarios. Manually controlling these set points compromises the quality of heating of the semis and therefore the quality of the end product. Understanding the tension free control methods, shear cut opt
PROJECT REPORT
IMPROVING THE EFFICIENCY OF
ROLLING MILL
INTRODUCTION
Automation directly impacts long product rolling mill productivity and product quality. In hot rolling of steel, the uniform heating of flat or long semis at exactly the right temperature and their timely delivery to the rolling mill is extremely critical to the quality of the rolled products, mill performance and cost of manufacturing. The semis need to be delivered to the mill at the required temperature with minimum fuel consumption and uniform temperature all across. The zonal temperatures inside the reheating furnace are controlled through set points which need to be determined dynamically to achieve quality heating in various production scenarios. Manually controlling these set points compromises the quality of heating of the semis and therefore the quality of the end product. Understanding the tension free control methods, shear cut optimization and optimal cooling systems you will be able to improve productivity and production quality, reducing metallic losses and downtimes.
BACKGROUND
In order to meet market demand, rolling mill requires high levels of productivity while fulfilling stringent quality standards. The rolling mill control system plays an important role in ensuring the correct operation of each link in the equipment chain to achieve the highest sustainable production rate without compromising quality. Productivity can be defined in many ways, for instance, as the amount of output per unit of input obtained by the utilized resources, and can depend on many factors such as facility layout, pass design, level of automation, state of the equipment, operational staff training and maintenance. In a steel production facility, productivity can be measured in terms of the tons of steel produced divided by the number of employees or employee hours. The tons of steel produced must comply with the steel grade quality standards to be commercial product. In effect, Productivity is a measure of the Efficiency of Production.
This project will focus on how a rolling mill can Increase the Productivity of a mill and Improve Product Quality using automation.
THE MAJOR COMPONENTS TO BE TAKEN UNDER CONSIDERATION ARE
I. Online semis temperature control system (OLSTC) - Aspects like unknown retention time, influence of adjacent semis etc., that cannot be estimated in advance make the attainment of the required temperature at the different zones in the furnace difficult. With OLSTC, the required drop out temperature will be achieved by continuously comparing the calculated semis temperature with the target temperature at each position in the furnace resulting in a new zone temperature set point which is recalculated and downloaded to the level1 system every 5 seconds.
II. Pacing – The target dropout temperature and optimum furnace throughput is achieved by a combination of temperature control and transportation speed of the semis inside the furnace. Furnace pacing controls the discharge of semis to deliver properly soaked semis at regular intervals to the mill, and optimizes mill productivity.
III. Material tracking – The tracking function gives information regarding the exact location of the semis inside the furnace and the progress of the charging schedule to the operator.
TECHNICAL DATA
••Sample length: 10 m
••Exit speed during sample cutting: max. 150 m/min
NEED OF TEMPERATURE CONTROL IN FURNACE
In hot rolling of steel, the uniform heating of flat or long semis at exactly the right temperature and their timely delivery to the rolling mill is extremely critical to the quality of the rolled products, mill performance and cost of manufacturing. The semis need to be delivered to the mill at the required temperature with minimum fuel consumption and uniform temperature all across.
The zonal temperatures inside the reheating furnace are controlled through set points which need to be determined dynamically to achieve quality heating in various production scenarios. Manually controlling these set points compromises the quality of heating of the semis and therefore the quality of the end product. Level 2 automation for reheating furnaces is an online furnace temperature control system whose main task is to provide optimally heated steel semis to the rolling mill for various production requirements.
It controls the furnace temperature to achieve the required discharge temperature of semis, low oxidation losses, less energy consumption while ensuring the heating quality. The system also provides furnace visualization, furnace health check and web reporting system.
MILL POWER
Since mill speed had to be increased to meet the capacity targets, a power upgrade of the existing mill drives from 10,800 kW to 22,100 kW was necessary. The optimization of the speed cone required an adaptation of the gear ratios and hence new gears.
POWER DRIVE FOR ROLLING MILL
The main and auxiliary mill drives in combination with the mill stand main motors are crucial elements. The mill drive and motor technology must therefore be highly dynamic, with an extremely accurate response and a stable control system. In addition, the drive technology should feature high overload capability, smooth running, maximum availability, service-friendliness, and seamless integration into automation systems. The SINAMICS family provides functions like drive control, operator control, diagnostics, and standardized communication across the entire SINAMICS family.
LOW-VOLTAGE: SINAMICS S120
The modular SINAMICS S120 drive system is suited for rolling in a power range up to approximately 2,000 kW. The SINAMICS S120 system is based on two-level voltage source converter technology with IGBT transistors on the motor side and separate rectifiers on the line side. The motor-side inverters are typically connected to a common
DC bus fed by a common in feed.
MEDIUM-VOLTAGE: SINAMICS SM150
The SINAMICS SM150 medium-voltage drive is used for sophisticated single- and multi-motor drive applications, combining high dynamic performance with maximum control precision. Supporting a drive output range ex-tending from approximately 2.5 to 30 MW, the SINAMICS
SM150 system is based on water-cooled three-level converters designed around integrated gate-commutated thyristors (IGCT) with a voltage source link and active front end.
HEAVY-DUTY ROLLING MOTORS
When it comes to overall plant operation, rolling mill motors are a crucial element. The extremely rugged synchronous and squirrel-cage induction motor technology has a long service life and plays a decisive role in the ongoing disturbance-free operation of the motors, which in turn secures the availability of your entire plant. The compact motor is designed with optimized materials and further ensures lower lifecycle costs.
ROD REDUCING/ MILLS WITH OFFLINE UNITS
••Enhanced temperature control
••Coil storage system MORSHOR
••Intelligent pinch roll
••Modular no-twist mill
••High-speed shear control
••Rotary shear control
••High-speed finishing blocks
••Reducing/sizing mills
••Laying head control
••Stelmor conveyor
••Reform area
••Vertical and horizontal coil handling
••Fluid systems
METALLIC LOSSES IN ROLLING MILL
Metal losses are waste material produced along the rolling, such as scale formed in the furnace, cobbles and the head and tail cuts of the bars. These losses affect the process yield.
The factors that influence scale formation are the temperature and the time that the billet remains within the reheat furnace and the furnace atmosphere, particularly oxygen. Correct control of the air-gas ratio in the furnace and implementation of control strategies, adapts the temperature to the rate of production and type of material inside the furnace, thereby reducing scale formation and hence metallic losses.
The hot rolling process causes the head of the bar to be deformed while rolling reduction occurs. To avoid problems in the guides it is necessary to remove this material in the shears. Precise control of the cutting cycle optimizes the cut length, so reducing scrap significantly. Using a fast control system for the shears permits accurate tracking and precise synchronization of the cut cycle. All the cuts are measured in time, compensating the reaction time delays of the drive-motor group.
ROLLING MILL IS AFFECTED BY
Mill performance is affected by a reduced rolling speeds, bottlenecks and cobbles in the process. Although correct mill adjustment and the performance of the control system plays an important role in the stability and speed of production, another very important part of the control system is material tracking. Tracking is necessary for the proper operation of tension control, loop control, cobble detection, shear cycles. Monitoring sensors (HMDs) and stand torque-motor are used mainly for detection of material for tracking. Proper operation and placement of each of the sensors, along with the good performance of the control algorithms permits steady high-speed rolling with minimal time between billets. Perfect coordination between material tracking and cascade speed control system reduces the number of cobbles and thus reduces losses due to poor performance.
For each product, there are factors that prevent increasing production speed, such as bar cooling time, the performance and power of the main drives or the maximum capacity of the reheat furnace. A mill must be treated as a chain, in which the weakest link determines the maximum production. Identifying what equipment is the bottleneck in the mill and developing a preventative maintenance program to keep that equipment running as long as possible will increase mill efficiency. The cold shear cutting area and the finished product handling can become bottlenecks in the process, but their proper design, positioning of the sensors, together with the correct sequences, allows automation to reduce these bottlenecks.
The optimization of the divide shear cut improves material handling in the cold cut area. This method consists of chopping the rest by one or more shears so that the last bar always has an exact length. Thus, the short bars that hinder production in the cold cutting area are avoided, throughput improves and all material rolled can be sold.
An improvement to this method is the option of redistributing cuts among the bars. This advanced optimization consists of an automatic calculation of the dividing cuts to reduce metallic losses when the remaining metal bar would be too short to be handled by the cooling bed. The calculation distributes the material, by taking ´pieces´ from the bars to complete the final bar length to prevent it from being too short.
METALLURGICAL DEFECTS
Metallurgical defects are all defects that cause the final material to not comply with the mechanical properties or structural requirements. Metallurgical properties depend on the proper and uniform heating of the material, rolling without tension and the appropriate applied cooling to achieve the desired microstructure and shape. Adequate combustion control and a reheating mathematical model ensure uniform temperature throughout the billet and are very important to avoid metallurgical defects such as surface decarburization or burnt steel.
Heat treatment is one of the areas where the control equipment is critical and affects the final product quality. There are different cooling methods, depending on the product produced and the type of mill. The water boxes and the cooling conveyor are particularly important.
Water boxes are used for cooling the bar in a controlled manner so that the material has the desired mechanical characteristics. The method involves pumping water under high pressure to cool the material surface temperature. The control consists of two control loops working simultaneously. The first loop calculates the water flow setpoint needed based on the measured temperature of the material at the exit of the water box. The second loop controls the water flow based on the setpoint of the first loop.At the wire rod mill outlet, the cooling conveyor consists of several roller path sections with fans installed on the underside. Controlling the airflow in each of the sections provides the necessary cooling curve to generate the desired microstructure.
DIMENSIONAL DEFECTS
Dimensional defects are mainly related to product shape and physical dimensions, the most frequent causes of which are due to poor adjustments to the mill control and tension while rolling. To reduce the material tension along the mill two types of control are applied; minimum tension control and loop control. Each is applied in different parts of the mill. In the intermediate zone, where the bar section is larger, minimum tension control is used. In the finishing area, where it is possible to deform the material without damaging it, the loop control is used.
With tension control measures the material tension is estimated by measuring motor torque, the control system then adjusts the reduction ratio to minimize tension according to the setpoint specified by the operator. When the head of the bar enters the first stand, before reaching the downstream stand, the torque is stored in memory. When the bar enters the downstream stand, the torque generated will be compared with the stored one. Observing whether the value increases or decreases between two stands determines whether there is a push or pull, then the control system acts on the reduction factor correcting the speed difference and thus the tension between the stands.
In the finishing end the material is thinner and it is possible to eliminate the tension with a loop control approach. Between the stands, a loop table is installed to form the loop when the bar passes. When the head of the bar reaches the downstream stand of the table, the persuader roll rises helping to form the loop. In the table a loop scanner is placed to measure the height of the loop at all times. The operator specifies the loop height and the control algorithm modifies the mill speed in cascade mode, acting on the reduction factor. Tension free rolling is achieved by keeping a constant loop height.
The good performance of these methods depends primarily on the control system and the process sensors. The correct material tracking is critical for the loop formation and control sequences, especially in high-speed mills. The reduction factor control algorithm is also very important to regulate the coordinated mill speed variation. In the last minute, both the tension and loop control operate on the reduction factor for each of the stands. The control system stores in memory each reduction factor when the process is stable, using it for the next billet. The correct storage of this factor makes it faster for mill adjustments, especially at the start of a new product.
When rolling wire rod, maintaining the correct coil dimensions is essential to comply with quality standards. To achieve correct coil formation it is very important to synchronize the mill exit speed with the laying head speed. The laying head has great inertia that responds slowly to changes in speed, for this reason it is best to maintain a constant exit speed.
Two strand wire mills deserve special control consideration. In this configuration there is a common area, usually the roughing and intermediate mill, composed of two strand stands, where two billets are rolled simultaneously. After the common area the mill splits into two independent strands, each consisting of rolling stands and miniblocks, typically ending with a finishing block. At the output of each block are water boxes and finally a laying head which deposits the wire rod in the form of coils on the cooling conveyor,. Differences in the adjustments or wear between the two exit lines motivate mills to control both lines downstream. This solution causes the laying head speed to change constantly.
EXISTING ASSEMBLY OF ROLLING MILL IN INDUSTRY
Ø Four-stand horizontal entry looper to pickling section.
Ø Four-stand horizontal exit looper after pickling section.
Ø Entry section of tandem mill, including bridle unit,
steering units, auxiliary shear.
Ø Four-strand coupling looper to the tandem mill entry.
IMPROVING ROLLING MILL ASSEMBLY TO IMPROVE ITS EFFICIENCY
••Six-strand horizontal entry looper to pickling section.
••Pickling section including scale breaker.
••Four-strand horizontal exit looper after pickling section.
••Entry section of tandem mill, including, steering units.
••Exit section to tandem mill comprising, rotary shear and two tension reels.
••In-line inspection station.
CONCLUSION :-
Long rolling is one of the fastest and most complex processes in the iron and steel industry, and it relies on appropriate drive technologies. The combination of process technology, mechanical equipment, electrical components, and automation functions is a crucial aspect of production unit, and allows us to deliver the highest possible performance to meet requirement of customers. A wide range of steel grades, shapes, and dimensions must be produced within tight delivery deadlines. Our project is to rearrange all aspects of a mill’s design , manufacture, installation, and start-up are managed with careful coordination and expertise.