Rolls for Metal Rolling Online Seminar Series

Presented by Josep Sans, ATOTECH Deutschland

The usefulness and success of hard chrome plating since its industrial debut in the 1920s is of course unquestioned. However, with the latest legislations restricting the use of hexavalent chromium substances, is there finally a trivalent hard chrome process that can replace it?

After many years of research and development Atotech was able to successfully develop the first generation of trivalent hard chromium electrolyte. The new developed BluCr® process exhibits all the same benefits associated with Atotech’s hexavalent hard chromium processes; high hardness, high efficiency, low roughness, robustness and stability.

By showing some details and experiences of BluCr trivalent hard chrome process we hope to shed more light on the possible replacement of the hexavalent hard chrome process we know today.

The presentation will include field experiences on many had chrome usages around the world.

Presented by Jessica Schindhelm, VDeH-BFI

In hot and cold rolling, the surface of the rolls is subject to high wear. In both processes, the rolls experience high abrasion but additionally other wear mechanisms occur that are specific to the respective process.

The damage to rolls in hot forming is particularly high due to thermal and mechanical stress. Especially in hot profile rolling in some cases the grooves are subject to particularly high load peaks due to the geometries involved. In addition to abrasive wear, the high workpiece temperatures lead to thermal loads on the groove. During processing the temperatures of the near-surface layers vary in a range between 40 °C and 450 °C due contact with the hot workpiece and the necessary roll cooling. That generates high temperature alternating stresses in the rolls and leads to thermal fatigue effects. Low roll life and the need for frequent roll changes are the consequences.

A major roll degradation mechanism in cold rolling beside abrasive wear is roll fracture due to hydrogen embrittlement. Due to the use of rolling emulsion hydrogen diffuses into the surface of the tools and causes breakouts. The cases of work roll fracture in the cold rolling mills caused by hydrogen embrittlement are significantly increasing. One reason is the higher mechanical load that is needed due to the rolling of increasing amounts of high-strength strips especially narrow strips. Here, usually hard chromium coating are applied as a diffusion barrier but those are generated by Cr-VI decomposition which is supposed to be banned on European level.

For rolling processes it could be shown in recent projects, that electroless nickel dispersion coatings have high potential for reducing the degradation effects and actually offer an effective protection against mechanical wear, thermal degradation and hydrogen absorption. For the wear protections of hot profile rolling electroless nickel dispersion coating with particles of different materials, sizes, shapes and with different amounts of particle incorporations were evaluated with regard to their wear protection performances. Particles of silica, chromium and boron carbide in sizes between several ten nanometres and several hundred microns and in different particle shapes were screened for their use as dispersive phase in the coatings. In this evaluation the focus was on the abrasion (rubber wheel test according to ASTM G 65), spallation (impact jet according to DIN 50332 with glass beads), hot wear (2 rolls hot test bench) and adhesion behaviour (impact test with chisel) of the produced coatings. Results show, that the effects of the mentioned wear mechanisms can be significantly reduced by an electroless nickel coating with 0.8 µm boron carbide particles. In a second project it could be shown, that the application of an electroless nickel - boron carbide coating with a thickness of 10 to 15 µm on work rolls for cold rolling can reduce the hydrogen diffusion rate (permeation tests according to DIN EN ISO 17081) by an order of up to four magnitudes depending on the used work roll material. This can be interpreted as significant barrier effect for hydrogen. In both applications the fact, that electroless nickel - boron carbide dispersion coatings already show a quite high hardness which can be further improved by annealing played an important role.

Presented by Chris Childs, Sarclad

Quality specifications for high grade textured steel and aluminium strip continue to increase. In line with its strategy of continued research and development in texturing technology, Sarclad has developed the next advance in Electrical Discharge Texturing to push roll texturing capabilities above and beyond the current requirements of the industry and into the next generation.

This paper details the development of the Next-generation EDT technology. It presents information on the revolutionised, high-efficiency electrical power delivery units and how these are combined with advanced digital closed-loop position control of each texturing electrode.

The effect of this development is to provide improved control of the cratering process that generates the texture on the roll. With this improved control the EDT machine is able to deliver high-quality textures benefiting from increased peak density and greater uniformity of texture when considering both the surface roughness (Ra) and peak count (RPc).

Additionally, the new technology will reduce the time required to texture a given roll, enabling higher throughput of rolls compared to any equivalent sized machine on the market, using any other texturing process.

Presented by Jurgen Malbrancke, CRM Group

As a result of the continuously increasing productivity in the hot strip mill, the trend to roll thinner materials and the request of very high quality products, hot strip mills are facing more narrow tolerances and process windows. One of the major parameters influencing the production cost and the quality of the hot rolled strips is the degradation and thermal profile of the work rolls in the finishing mills.  Rolls might look like just another piece of metal, but with the high temperatures and stresses to which they are exposed in modern mills, considerable demands are placed on roll materials and manufacturing processes.

To control the performances of the work rolls there is especially a need to have a better understanding and control of roll performance actuators, grinding rules and campaign scheduling.  A key parameter is the measurement of the work roll surface state.  Only with objective data of roll performance, controlled improvements can be introduced. 

Within the European Project “Mastering Rolls II” (RFCS-2017-800748), which is the follow up of the European project “Mastering Rolls I” (7215-PP/066), a key part of this project is dedicated to the industrialization of innovative measurements that were developed over the past years.  These can be divided in two type of measurements: in-depth measurements that take a larger amount of time (e.g.: NDT eddy current fire crazing and crack depth measurement, profile measurement, oxide thickness,…) and short measurements that allow a fast roll surface analyses directly at the end of a rolling campaign.

This presentation will focus on the short measurements that can be done by an operator by the help of a tablet.  Within the European project “Intellub” (RFSR-CT-2013-0003) a tablet application was introduced to assist with the roll evaluation by guiding the user through an evaluation procedure.  The first testing of this new technology at the mill immediately showed the improvements and innovations that are possible with the introduction of this new tool. Within the Mastering Rolls II project an industrialization of this technology is on-going at ArcelorMittal Ghent and Tata Steel IJmuiden.

Presented by Steve Moir, Kaida Roll

Long Product Mills have hitherto been less predictable than their strip counterparts in terms of their minimum working diameter of the roll, known as the discard diameter.  This is due to the wide variation in the profile along the barrel, where these rolls usually break near to the centre of the barrel at the narrowest diameters, but not necessarily as there is also the parameter of the rolling load.

This paper details well established principles that have enabled the calculation of the minimum barrel diameter of long product rolls; dependent on the barrel profile, the roll loading position, the rolling load and the separation distance of the initial roll centres. Once this information has been obtained it is possible to select the most suitable roll grade by making use of the working range from maximum to discard diameter, but at the same time maximising the wear resistance. Wear resistance has an inverse relation to strength, for example, for alloy cast steel rolls.  Thereby it is now possible to replace low carbon forged steel rolls with harder wearing alloy cast rolls for clear cost and roll wear benefits for the Long Product Mill.

Presented by Konstantin Redkin, Whemco

Development of new advanced high strength steel (AHSS) grades and rolling practices continue to challenge roll manufacturers. Increased mill loads and extended rolling campaigns require roll materials to keep pace with ever changing rolling technology.  In this study developed finite element analysis (FEA) of the BackUp Rolls (BUR) to predict microstructural evolution and associated transformational and thermal stresses will be discussed. Proposed 3D FEA enables to study the interaction between residual stress in BUR and actual mill loading scenarios. The model is a valuable tool used to highlight potential BUR design shortcomings given specific mill loading conditions.

Predicted residual stresses in BUR, which resulted from proprietary differential hardening heat treatment, were incorporated into 4-HI mill loading model, using actual mill operational data. Localized alternating stress ranges were predicted throughout the BUR.  The results show a clear interaction between alternating mechanical mill loads with residual stress profile. Revealed complex interaction between internal pre-strain conditions and externally induced stresses is typically not considered when developing BUR quality specifications. Understanding complex volumetric microstructural and residual stress evolution and modern mill loading interaction is of utmost importance in order to assess required backup roll properties, such as: fatigue endurance and damage tolerance. Novel results facilitate development of specifications and inspection criteria for the rolls being in service under acknowledged loading conditions.

Proposed complex design approach provides the roll manufacturer with critical information about the localized magnitude of pre-loaded internal residual stresses for BUR or work rolls (WRs), so that appropriate non-destructive testing/evaluation can be conducted. Spatial and path depended material properties in large section BUR were studied through computational materials and simulation model based definition and validated via mechanical testing. The relative effect of the key-operating parameters was studied by isolating specific unfavorable loading conditions and considering metallurgical attributes of the rolls. The results of the work were successfully used in the development of a pouring practice to improve roll ingot’s quality combined with heat treatment optimization, where the residual stresses were minimized without scarifying mechanical properties and increasing fatigue endurance.

Presented by Sebastien Flament, CRMGroup

Rolls in hot rolling mills have to withstand harsher and harsher conditions throughout the decades. In the early 2000’s, problems of bonding zone fatigue (before reaching scrap diameter of the work rolls) started to occur in some mills. Since then, year by year, increasing fatigue issues were encountered in reversing roughing mills worldwide. Longer rolling campaigns, inadequate backup or work roll maintenance, harder steel grades or increased throughput by reduced amount of passes to achieve the same thickness may explain the higher amount of fatigue occurrences.

A model was developed to predict stresses at the contact surface between the work and backup rolls as well as at the shell–core interface of the work roll. The development was pushed further to define an optimised situation both for roll manufacturer and users to withstand the given rolling conditions in a mill. Indeed, the current practice of the roll users is to request an increased total work roll shell thickness to ensure a determined roll life. However, some limits have been reached regarding the total shell thicknesses leading to some manufacturing issues especially in the field of residual stresses or core material properties.

This paper will illustrate a wide range of fatigue problems encountered in reversing roughing stand and the model outcomes to optimise both the roll use and design to withstand the harsher conditions. The model takes into account work and backup roll geometries, chamfer design, campaign length, wear profiles (work and backup rolls), strip width and rolling forces. 

Presented by Stamatis Kiakidis, Tata Steel

Rolls are key assets for hot strip mills (HSMs), with major impact on mill availability, daily mill and roll shop operations, product quality assurance, and high associated costs. A typical HSM stand has work rolls (WRs) and backup rolls (BURs). Among them, the WRs of the finishing stands of a HSM have the highest impact. The temperature distribution in the WRs, along with its stress field, determine to a great extent the roll serviceability. Within the content of this study, a simulation tool (ASTRO) was developed, capable of providing the thermomechanical state of the WRs during hot rolling.

ASTRO is a numerical model, consisting of a thermal and a stress part. The thermal part calculates the temperature evolution at the WR during a rolling campaign, via a finite differences formulation. Afterwards, the stress part calculates the internal stresses at the WR, taking into consideration the already calculated temperature profile, and all the mechanical loads imposed at the circumference of the roll. This latter part is based on the finite elements theory. Being a two dimensional (2D) model, ASTRO simulates only the middle section of the stand assembly, including the strip, the work rolls, the backup rolls, the roll cooling and the strip chilling systems. The attached figure illustrates a simplification of the rolling stand assembly. The meshed segment of the strip/roll geometry, along with the blue regions of the cooling system, are the features considered during calculations.

The rolls used for hot and cold rolling of steel strips are subject to severe mechanical and thermal stresses that lead to contact fatigue, wear and plastic deformations. Traditionally, the tried and tested roll inspection solution is to use Eddy Currents (EC) with some limited Ultrasound testing (UT) capability. However, the traditional approach is no longer the most reliable and efficient solution for detecting typical roll defects.

This paper examines the notable advantages of implementing a multi-channel bi-directional UT approach for the detection of naturally occurring defects. The geometry of the defects corresponding to the sensor position will be comparatively analysed by a UT system oriented in different directions with a standard uni-directional UT system.

The results clearly demonstrate a bi-directional UT system is necessary for the detection of all surface and sub-surface defects. Rollmate is the only commercial system offering true bi-directional UT capability.

Presented by Alberto Croce, TENOVA S.p.A (Italy) 

In the current environment of machine intelligence, the Roll Shop area is required to have high levels of digitization to provide the advanced process control and data management expected by modern rolling mills.

Pomini Tenova SpA has continually developed Roll Shop automation, inspection and management software and systems over many decades that have evolved into a fully integrated suite of software based functions. Systems such as Digital Texturing, on-line machine parameter monitoring, vibration monitoring, roll inspection, continuous profile measurement and correction plus roll shop management systems are not only able to collect, share and store data but use this information for closed loop process control and management of the entire Roll Shop area.

Several recent installations combine these software functions with the state of the art equipment and roll handling to provide a fully automatic Roll Shop area that can operate with high levels of efficiency and quality with continuous monitoring while requiring minimal operator involvement.

Presented by Simon Wright, SJW Mill Consulting Ltd (United Kingdom)

For a Heavy Section Mill using the Universal Rolling process the sleeves for the horizontal rolls are critically connected to both the product quality and operational strategy of the business. For dimensional accuracy and surface quality in terms of consistency and longevity the correct roll material choice will not restrict the business requirements to produce the products it needs to, at the time it needs to in the batch size required.

This paper demonstrates how a universal sleeve is used throughout its life and makes the direct connection between the product mix of the mill, the commercial manufacturing program required and the overall sleeve cost per tonne produced.

Different roll stock and purchasing strategies will be discussed along with their pros and cons and the concept of the “Cascade Critical Product” will be introduced to demonstrate how sleeves can be “sucked down to size” and potentially incur high costs if not proactively managed.

Finally, the wear performance of different materials will be presented and connected to the overall cost of the rolls per tonne produced in different operational and strategic scenarios.

Presented by Danny Beentjes,, Tata Steel (Netherlands)

In roll shops for rolling mill rolls, grinding of work rolls according to the current state-of-the-art is focused on the following aspects:

1. To remove surface layers degraded in the previous mill campaign (e.g. by wear, thermal/mechanical fatigue, oxidation-corrosion or damage from a mill incident);
2. To (re-)apply the prescribed work roll profile;
3. To achieve the correct roughness level, as specified for the applicable rolling mill stand.
4. To grind without typical grinding surface quality deviations, such as feedlines, scratches/”comma’s”, chatter marks, burn marks or chevron patterns;
5. To achieve (1) to (4) in the shortest possible grinding time.

Concerning aspect (3), the ground roughness is influenced by multiple factors, strong depending on the type of grinding wheel that is used, the bonding system & openness achieved by the manufacturing of the wheel, the grinding type of grit & wheel hardness.

The engineering of the surface roughness is mainly done on the (local) grinding machines (CNC, conventional), machines wherein the grind parameters are set and tuned, towards the desired surface finish. In general the developed mechanical roughness is measured by 2D stylus measurement with an mechanical device (according to the ISO4287 standard). The roughness parameter Ra, or mean average roughness profile derived from the stylus, is commonly used as the sole parameter to prescribe the work roll roughness in the Roll Shop for the rolling Mill application. The prescribed Ra roughness range is typically based on past experiences and is supposed to ensure a work roll surface finish that is more or less fit-for-purpose for the next mill campaign.

However, the same Ra roughness range can be achieved in many different ways, depending on the combination of work roll properties, grinding wheel properties and grinding machine settings. Experience has shown that work rolls ground in different ways – despite of having the same Ra roughness level – can exhibit significantly different behaviour in the rolling mill. Focusing on achieving the prescribed Ra roughness alone on the work rolls in the roll shop may therefore lead to sub-optimal or even poor mill performance.

Ground surface roughness topography, research, design & engineering is focussed broader and is about developing the most effective 3D ground roughness topography, best fit for the aimed rolling Mill application. As example, as part of the Tribology in the Mill bite, the effect on friction coefficient in the Cold Rolling Mill is a function of both the strip and work roll roughness. The Stribeck theory & curve describes a non-linear function of emulsion-lubricated surfaces and the friction coefficient.

The Stribeck theory on lubrication regimes is based upon the input of the (2D) Ra-roughness parameters of both the strip and the work roll to get understanding of the typical progression in the curve and its lubrication conditions (3), namely to be in:

The 3D asperities of the combined surface roughness are found to play crucial role to these conditions and influence the behaviour of the rolling Mill process and the rolled strip quality. Illustratively, Tata Steel Europe (TSE), having Hot, Cold- and Temper Mill Rolling Mills, have pursued 3D roughness topography engineering to improve the rolling condition in a range of rolling applications, such as:

In Tata Steel Europe (TSE), the research of 3D surface roughness, by optical measurements (ISO25178 standard) and the engineering of ground surface roughness topography, is seen as a key in fundamental improvement of rolling Mill processes, strip quality & Work Roll lifetime. Further 3D engineering, proved its importance in the development for hexavalent chrome-based work roll coatings alternatives and in mastering the grinding of enhanced Work Roll grades.