Recent Advances in Aluminum Hot Rolling Technology
The synthesis and integration of high performance measurement systems, advanced mill actuators and process knowledge into effective control systems is central to the achievement of required results. The high level of capital investment required for hot mill lines means that it is very difficult to justify investment in complete new plant, and achieving improved performance by revamping existing equipment or re-installing re-vamped second-hand equipment is often the only cost effective route.
Continual progress in the development of advanced mill actuators, measurement devices and their integration into effective control schemes demands fresh consideration from producers if they are to maintain or improve their competitive position.
MARKET REQUIREMENTSHot mill finished product quality objectives can be briefly summarized as follows:
- Center line gauge better than 0.75%
- C50 Profile 0.5% I0.2%
- No manifest flatness defects
- Finished temperature within I10¯C of target
- Uniform and consistent surface
- Homogeneous and consistent microstructure
Although it must be recognized there will be some specific product variations to the above, it does provide a reasonable benchmark against which to judge current performance capability. This performance has to be considered against the context of increasing product width requirements (2.2 m and greater) and the need for increased production flexibility with minimum batch (one coil) sizing coupled with high yield and productivity to give a minimum conversion cost.
Additionally, there are increasing requirements for producers to provide tangible evidence that they are actually achieving product quality standards.
MEASUREMENT SYSTEMSA clear prerequisite to the achievement of performance is the ability to measure the controlled parameter. It is only through technology advances in the last decade that on-line measurement of the majority of the identified quality parameters has become feasible.
Integrated Measurement Systems
Following incremental developments over the past 15 years, integrated systems for the continuous non-contact measurement of strip thickness, profile, flatness, width and temperature have become a practical reality.
Typically, a modern system would utilize fast, low noise, X-ray sources with multiple, high precision, continuous reading detectors. Each measurement point across the strip is measured independently by two source detector pairs, at two different and known angles of incidence of the radiation beam. This arrangement gives the capability to simultaneously measure centerline thickness, profile and manifest flatness and width.
The capability of the cycle time of this type of system to measure profile in one second or better dovetails ideally with the philosophy of closed loop profile control and fully dynamic actuators such as the Dynamic Shape Roll (DSRR).
Two possibilities exist for flatness measurement: latent and manifest shape. In a complete scheme, it is conceivable that both types of devices could be incorporated as they have differing characteristics and benefits. On the hot line, it is considered that avoidance of manifest flatness defects is of primary importance to ensure efficient threading, reduce cobbles and minimize marking problems due to pick-up and scratches.
With reducing cost of high performance optics and signal processing, the optical type "manifest shape meter," has become cost effective and worthy of consideration. This type of scheme has the virtue of being non-contacting and provides flatness, strip edge position and width measurements.
Typical performance capability is:
- Height I0.2 mm
- Position I1 mm
- Edges I0.5 mm
Recent developments have also made 'latent' flatness measurement a viable option. The Hot Planicim roll is an extension and development of the Planicim shape-meter roll which has been employed very successfully in cold mill applications for many years. The driven roll takes the place of the deflector roll at the exit of a hot strip mill, typically 500 mm diameter and is capable of handling strip temperatures up to 1400¯C (752¯F). Additional features incorporated include a cooling system and a safety push-off device to protect the Planicim from overheating in the event of a mill stop.
As a result of extensive analytical and experimental work, temperature and speed compensation influences are incorporated. The measurement characteristic is illustrated in Fig. 1, and it can be appreciated that by correct signal synchronization, an effective and accurate tension reading can be obtained with a resolution of the order of I21 units. A wide (2.8 m) unit has now been in successful operation at the exit of an aluminum 3-stand tandem hot line since January 1997. It has clearly demonstrated its suitability for the environment and has given significant benefits for the achievement of effective profile and flatness control.
The low emissivity and non-gray body behavior of aluminum and aluminum alloys has created great difficulties for accurate non-contact temperature measurement of aluminum strip. Recent developments of two-color (or more) ratio pyrometers are beginning to provide an acceptable level of performance. However, absolute accuracy of these devices over the range of alloys and varying strip surface conditions makes it essential that appropriate contact cross calibrating devices are incorporated in order to improve both accuracy and confidence in the readings obtained. This is even more critical when the devices are linked directly into the mill control and set up systems and actively used to set mill speeds or strip cooling system regimes.
Surface quality is arguably the most important quality parameter in aluminum rolling. It is very difficult to recover product which has suffered some form of surface damage. The whole operation of the hot line, roll coolant type, filtration systems, roll coolant application, roll types and finishes, roll brushing, reduction and temperature regimes have an impact on resultant surface quality.
This area of work is still somewhat embryonic, but high performance vision systems coupled with defect classification software are on the verge of practical reality and deserve serious consideration by producers in their next generation of control schemes.
Strip Speed Measurement
High accuracy and response laser speed sensors have become well established in cold rolling applications. The potential exists for extended usage of these devices into aluminum hot mill applications in order to take advantage of mass flow gauge control techniques.
- Height I0.2 mm