ABB Automation Technologies AB
 

NEW SEAMLESS STRESSOMETER ROLL FOR SURFACE CRITICAL MATERIALS

Lars Jonsson, Specialist - Flatness Systems
ABB Automation Technologies AB, SE-721 59 Västerås, Sweden

Introduction

A new generation of Stressometer rolls for flatness measurement has been developed and patented by ABB. The rolls are intended for surface critical material and will complement the Stressometer rolls of today. They are completely seamless and have a homogenous solid surface, coated to suit mill requirements. This design eliminates the risk of strip surface marks caused by the flatness measuring roll. It will do so while minimizing the roll surface wear and maintaining the accuracy and resolution of the standard Stressometer roll.

Demands on strip surface

Traditionally, bright annealed (BA) stainless steel and several special aluminium products have high requirements of surface quality. But also processes such as coating and galvanizing lines place high demands on the strip surface. Many strip users, both in aluminium and steel, do not accept any defects on the strip surface. Examples of such applications are architectural, lighting, decorative, automotive, food processing and surgical instruments. One common source of surface problems on the final product is the upstream cold rolling mill and the flatness rolls in the mill.

The flatness measurement and strip marking dilemma

The normal way to measure flatness in a cold rolling mill is to use a measuring roll. These measuring rolls can, regardless of their design, cause strip marks. The conventional design of a measuring roll is to have measuring elements separated from each other by a seam. The seams can however collect debris from the strip and from the rolling process and this debris could mark the strip. The debris will also increase the force spreading from a measuring element which affects the accuracy of the measurement.

Up till now rolling mills have been forced to accept some strip marking in order to achieve good flatness measurement.

Over the years many attempts have been made to solve the strip marking problem caused by measuring rolls. Most methods have focused on trying to hide the seams with some type of coating. One method has been to use a rubber coating on the roll. However as the rubber wears the accuracy of the measurement will deteriorate. The wear of the rubber coating will be significant at the strip edges causing poor measurements. Rubber coating will also increase the force spreading which will have a significant negative impact on the measurement accuracy.

Highest strip quality without rubber

A hard metal roll surface has been shown to be a way to combine high strip surface quality with minimum wear. Unlike rubber, the hard metal is harder than any rolled material. In order to avoid print marks, there are very high requirements on the smoothness of the roll surface. The softer the strip is, e.g. material with very high content of Al, the smoother the roll surface has to be. The hard metal coating consists mainly of ceramics. Therefore pick-up from the strip can easily be avoided. Due to the hardness of the surface, the wear can be kept to a minimum thus eliminating need for maintenance.

Since slipping between roll and strip would directly mark the strip it is essential to create the necessary friction. The friction between the roll and the strip depends on the following factors:

Analyzing these factors in a specific mill, friction can be created by grinding the roll surface to a roughness suitable for the process.

The concept of Force Spreading

All flatness measuring rolls measure the force distribution on the roll caused by the strip.

From this force distribution the flatness of the strip is calculated. As mentioned above, the seams between the measuring elements are one source for strip surface problems. In order to solve this we need a measuring roll with a solid surface i.e. without seams. When using a solid surface of a measuring roll the force applied on the roll surface above one sensor will be measured not only by this sensor but a significant part of the force will also affect adjacent sensors. We call this effect “force spreading”. Spreading of force outside the widthwise resolution will decrease the accuracy of the flatness measurement. Force spreading is defined in the following way:

The force spread outside a nominal widthwise resolution divided by the force applied to the roll within the same resolution.


Fig 1. Force Spreading Characteristics from the Seamless Stressometer Roll with 52 mm widthwise resolution
Design and measurement properties of the Seamless Stressometer Roll

In order to get an accurate measurement, the force spreading must be kept to a minimum. Otherwise the measurements will have an averaging filter effect across the width of the strip and, thereby, not correspond to the real strip flatness profile. However with an optimal design of the solid jacket the filtering can be kept within acceptable limits.

The design of the rolls must ensure that accurate force measurements can be made for a wide range of products and varying process conditions. The main measurement demands for the Seamless Stressometer Roll are:

Experiences and Conclusion

Since the first delivery in 2001, almost 30 Seamless Rolls have been delivered to strip surface-critical aluminium and steel applications.

The conclusion from all of these installations is that the dilemma of getting accurate flatness measurement at the same time as perfect strip surface has effectively been eliminated by the Seamless Stressometer Roll.

 

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