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Cuscinetti - 2012-05-01

Practical example: skin pass mill for a stainless steel rolling plant
 
Customers in the steel industry are among the bearing user groups for whom NSK has developed a special, sector-specific range of products. The following article about a stainless steel skin pass mill demonstrates NSK’s material technology and bearing design options for a wide variety of steel production processes.   
 
Typical steel mill operating conditions including high rolling forces  result in severe  radial loading, environmental contamination, very high peripheral velocities and – with wet rolling processes –  increased levels of moisture and humidity. Not only does NSK have comprehensive experience in this area, but the company also offers several series of bearings developed specifically with steel industry applications in mind. These bearings are made from particular materials that enable trouble-free operation in the given conditions. For example, the materials used are produced such that the bearings are not damaged if contaminants are pressed into the rolling surface. In many cases, preventing or minimising such damage from the outset requires a special seal design.

The goal: Rolling  stainless steel with perfect surface quality
A practical application example demonstrates the parameters that are the most advantageous when choosing bearings. The company BWG Bergwerk- und Walzwerk- Maschinenbau GmbH in Duisburg, Germany, was contracted by Thyssen Krupp Nirosta to renew a skin pass mill at the Dillenburg rolling plant. Such mills are used in the final processing of rolled sheet strips.

‘Skin passing’ is a light form of finishing that improves the sheet surface quality. Moreover, this process step can be used to achieve defined levels of roughness and textures in the strip surface and to improve material characteristics.

The new skin pass mill, which processes sheet thicknesses of 0.2 to 3.5 mm, was ordered because customers are increasingly using sheet strips with higher yield strengths but they also want a greater degree of sheet flatness. Thyssen Krupp Nirosta didn’t just want to improve product quality, however. The productivity of the stainless steel mill line was to be increased as well.

Choosing bearings for the roll pair
The core component of the skin pass mill developed and manufactured by BWG is a roll pair that applies a maximum force of 10,000 kN to the sheet. The sheet runs through the rolls at a speed of 500 m/min. Chocks are used to keep the rolls at the proper distance from each other and to connect them to the skin pass mill itself.

The specifications from Thyssen Krupp Nirosta included a request that the new rolls fit into an old, existing mill and that existing roll pairs could also be used with the new mill. In this way, the main connection dimensions were already specified.
 
High radial load-carrying capacity, high degree of concentricity
Each roll weighs around nine tonnes and, together with the chocks, the entire mill component weighs about 25 tonnes. The distance between the bearing units is 2.40 m.

The NSK industry experts commissioned by BWG to select the bearings suggested using four-row cylindrical roller bearings with 500 mm diameters. These bearings are characterised by high radial load-carrying capacity and very good concentricity. Concentricity is the decisive criterion for high surface quality.

STF steel: special material increases bearings’ service life
The cylindrical roller bearings in the roll bearing are manufactured from STF steel, a material developed by NSK. The development goal was driven by the knowledge that bearings in mill lines often demonstrate surface initiated fatigue damage. An extensive series of tests showed that this damage is caused by debris brought into the roller contact area by the lubrication medium, which then creates indentations in the raceways.

To prevent this problem, highly pure, case-hardened steel is used in connection with a special heat treatment process that guarantees optimum residual austenite content. In addition,  the low level of non-metallic inclusions is evenly distributed throughout the entire production process so that the material has a high degree of homogeneity.

Bearing service life up to ten times longer
With this Super-TF steel, the load on the bearing in the critical shoulder area of the indentations can be reduced. When the projections created following the penetration of foreign particles are rolled again, the stress created in the material is considerably lower (when compared with conventional steel). The result: damage caused by contamination occurs less frequently. 

This material development paves the way for an extended bearing service life in the specific conditions found in rolling mills. Specifically, this means that if the lubricant is clean, bearing service life is about twice as long using STF bearings than that of conventional rolling bearings. With contaminated lubricant, the bearing service life may be ten times as long. 

Axial tapered roller bearings for adjusting screw of backup roll
NSK bearings are also used in the housing screw that sets and regulates the roll gap – commonly known as Screwdown bearings. Specially designed axial tapered roller bearings, which can withstand  extreme continuous rolling forces , have proven effective both here and in other similar rolling mill applications. NSK has estimated a remarkable service life of over ten years for these highly stressed bearings.

BWG Bergwerk- und Walzwerk- Maschinenbau GmbH also followed NSK’s recommendation that oil lubrication for the roller bearings could be added later to be prepared for future changes in operating conditions. The specialist equipment manufacturer has now delivered the completed skin pass mill to Thyssen Krupp Nirosta. The assembly work was completed in February 2012 and the unit was then officially brought into service.

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