Endless Rolling of Bars and Rods
Endless Rolling of Bars and Rods
Endless rolling technology is the most advanced process method for the rolling of the bars and rods (also known as long products) from the billets. It consists of a welding arrangement designed to endlessly join billets together in order to provide a continuous supply of material to the rolling mill train. It is enabled by welding of the billets which come from a reheating furnace at the upstream side of the stands of rolling mill train. In fact, the joining of the tail end of the billet being rolled and the head end of the billet to be rolled is one of the key aspects of the endless rolling technology.
Although endless rolling has been in commercial operation since the late 1990s but it took a long time to reach certain level of technical maturity. The major reasons for this are (i) difficulty in the development of the welding technology of hot billets of a large cross-section area in a short time, (ii) achievement of the high joint quality which is needed to improve yield, (iii) difficulty in the development of the technology for the complete and smooth deburring of the welded joint in a short time to avoid any surface defects as well as to avoid the deformation of the billets, and (iv) to achieve the difference between the quality of the welded joint with the rest of the billet within the acceptable limits as this limits the product range of the rolling mill since the joint during the rolling process is rolled out to long length, and reduces uniformity of chemical composition, and the mechanical stability of the finished rolled product.
Endless rolling concept
Endless rolling process concept has led to a change in the overall rolling method which was previously based on the principle of taking into account the individual billets which has resulted in a cycle of nose-end entry, rolling, tail-end exit and billet gaps. This cycle has dictated the design of existing equipment and control systems in the present bar and rod rolling mills.
The surface scale on the billet coming from the reheating furnace is removed by high pressure water jet when it passes through the descaling box. After this, the head end of the billet is to be welded with the tail end of the billet which has entered into the roughing group of stands of the rolling mill. The process of welding is to be completed as the billet moves forward in the rolling mill. The welding machine is either fixed on a moving car driven by two brushless motors or has pinch roll sets on its two sides or both. The speed of the two billets is to be matched. Then the welding machine’s clamp chucks which are driven by hydraulic pressure, clamps the two ends. This ensures that the end faces of the two billets to be welded are centered. The first step in the whole process of welding is to melt the end face, and then to extrude (upsetting) the billets. In this process the two billets are welded. This welding method ensures that the physical structure of welding line is conformable to that of the source billets, so the quality of welding line is maintained.
The characteristic of endless rolling technology requires the drive system to meet some special requirements. The first requirement is for ensuring the welding machine equipment safety and the exactness of the process of welding. For this the speed of the billet at the head end of the welding machine is required to be equal to the linear rolling speed in the first stand of the rolling. The second requirement is to ensure that the speed of the billet which is lower in order to ensure descaling effect when it passes through descaling box, is increased after the billet is descaled so that it catches up to the speed of the billet already in the rolling mill. So the drive system of the roller table from the reheating furnace to the roughing group of stands of the rolling mill is to be multilevel speed drive system. The third requirement which is required to be taken care arises due to the influence of random conditions during the rolling process. Because of these random condition, the place where the billet to be rolled catches up with the billet already under rolling is different every time. So the drive system which answers for chase is to correspond to the position servo system. The speed curve of the billet to be joined for endless rolling is shown in Fig 1.
Fig 1 Speed curve of the billet to be rolled
Process of welding
The process of welding is to be completed as the billet is moving forward. It is achieved by establishing a controlled electrical arc between the two billet ends with the purpose of melting the steel material on both sides in enough volume to be subsequently upset (squeezed) and bond together by means of a hydraulic upsetting device. Part of the steel material is actually ‘spread’ out (burr) of the two billet ends. After the completion of welding, the burr produced at the weld-joined section is removed by a deburring machine and tracked during the rolling. The total amount of this removed burr material is to be accounted as loss in rolling mill yield (ranging from 0.2 %to 0.3 % in weight of a 12 m long billet) and it directly depends on the size of the billet. The entire series of the above operation is automated and the endless rolling is achieved without increasing the working load of the mill operators
Also, for completing the welding within a confined space between the reheating furnace and the roughing mill, a flash-butt welding method is normally employed, and welding is carried out within the available short time. The welding machine is installed between the reheating furnace and the roughing mill, and pinch rolls are installed before and after the welding machine. In conventional rolling, a constant spacing is always provided between billets by adjusting the billet extraction timing in the sequential control of the reheating furnace and/or by adjusting the transfer speed of the pinch rolls before and after the welding machine in the sequential control of the roughing mill. On the other hand, for welding of billets in the endless rolling, the billet transfer speed is controlled by the front and rear side pinch rolls. Then, after that, the speed of the welding machine is synchronized with the transfer speed of the billet, and welding and upsetting are applied to billet ends.
The weld-joining method of the endless rolling is flush-butt welding. A large amount of sparking and spattering takes place during weld-joining of the billets. This sparking and spattering take place in all the direction including the top and both sides of the joint as well as from the lower side of the welded area. There is a concern over the spattering causing equipment and/or quality problems on the bar and rod being rolled in the adjacent strand. This is one of the main challenges of the endless rolling process and an effective containment of the sparks and spatter material generated during the preheating and subsequent flashing phase in welding operation of the billets is needed. This material, whenever uncontrolledly spread around the welding area, sticks above the welding unit components and the disappearing roller system including sensors and encoders in a potentially detrimental manner. For minimizing the effect of spattering to the greatest extent possible, different methods have been developed by different suppliers of the equipment for the endless rolling of the bars and rods. The welding procedure of the billets and the deburring operation is shown in Fig 2.
Fig 2 Welding procedure of the billets and the deburring operation
In ideal circumstances all the material need to have the same heating and rolling cycle. With endless billet welding in a new mill, it is possible to design a layout which is close to this condition. However, in existing mills the time between leaving the reheating furnace and arriving at the first rolling stand can be considerably different between the front and tail ends of the billet. Also, during the rolling process a small length of the billet is increased in temperature due to the energy imparted while joining two billets, while on both sides, lower temperatures prevail on the surface areas where the water-cooled clamps of the welding machine have been in contact with the material.
Two factors which mainly influence the endless continuous rolling process are (i) roll wear, and (ii) temperature difference. For the compensation of the roll wear, it is necessary to make small roll gap changes which take into account the change in material spread, the requirement to maintain tension between linked stands, and the possibility of overloading an individual stand. Consistent temperature is required to be achieved using statistical pressure control to minimize cyclical temperature deviations.
It is necessary is to make the endless operation as stable as possible in order to provide the finishing mill with consistent conditions which result in improved tolerances, metallurgical quality and mill operation. In practice, some of the effects which are required to be compensated are (i) entry time-cycle may not provide equal billet temperatures head to tail, (ii) the joining process creates temperature differentials, and (iii) deburring of the welded joint creates small cross-sectional area differentials. Further, as the finished coil weight (in case the rolled product is coiled) no longer tied to the billet weight, tracking need to be designed both to allow quality identification and accurate coil weights to be met.
The process of welding is to be completed as the billet is moving forward. This necessitates that the endless rolling control system is to be composed of drive control, loop control and sequence control.
Automation of the welding process
An effective automation system is a key component for the control of the welding process since it assures the achievement of the required level of quality and process stability. A successful and well-executed welded joint is to show some of the primary characteristics such as (i) robustness and mechanical stability of the welded joint during the rolling process which does not generate any surface defects on rolled product, (ii) satisfactory and consistent values of decarburization (in the welded area) which are within the acceptable limits of variation ranging from 4 % to 8 % maximum for all low and medium carbon grades (grades having the carbon content upto 0.4 %), and (iii) absolute deviation in the values of yield strength and tensile strength which does not exceed +/- 8 % of the normal value of the bar.
These above requirements may not be met if there are poor homogeneity in the melting material and if there are presence of cavities and gas bubbles inside the welded area. These defects are caused either due to the instability of the electric arc or due to an unsatisfactory execution of the upsetting phase, more specifically in terms of applied pressure and achievement of the needed geometrical ‘squeezing’ between the two billets ends. Hence, a devoted automation system is needed which is to keep a strict and punctual real time monitoring of various parameters such as (i) temperature of the billets leaving the furnace, (ii) temperature of joining surfaces based of billet fusion characteristics, (iii) position of the clamps, the tension between the clamps and the current passing through the clamps, (iv) burning speed of joining surfaces, (v) burned length during flashing, (vi) melting depth and flashing time, and (vii) pressure and ‘squeezed’ depth during the upsetting phase.
Benefits of the endless rolling process
Endless rolling process has several advantages as given below.
- There is elimination of inter billet gap time. This result into an increase in the production capacity of the rolling mill in the range of 8 % to 12 % maximum provided there is built in capacity available in the reheating furnace as there are no changes in the operating parameters of the rolling mill. In case of non-availability of built-in capacity in the reheating furnace, then the rolling mill output can be achieved with lower speed of rolling. This results into saving in the specific power consumption as well as reduction in the wear and tear of the changeable operating parts.
- It eliminates head and tail cutting resulting into an increase in the yield in the range of 0.7 % to 0.9 % which depends on the specific roll-pass design and the layout of the crop shears.
- The operation of the rolling mill has higher level of stability since there is consistency of set up because of the rolling of a single endless billet.
- The risk of generating cobbles in the mills gets greatly reduced.
- There is practically no generation of short bars on the cooling bed because of the rolling of one endless billet. This results into an increase in the yield which may be upto 1 % depending upon the specific roll pass design, product size and the level of the automation in the rolling mill.
- The life of the changeable parts increases because of drastic reduction in the number of head biting in the rolling stands and guides with consequent decrease of mechanical hits and improved temperature stability during the rolling operation.