Rolls for Hot Rolling
Four speakers from 3 continents will present developments and field experiences with advanced rolls for hot rolling applications, ranging from plate mills and hot mill roughing stands to early and late finishing stands. Each paper will provide a unique perspective, from varying viewpoints of roll manufacturers, roll users and research institutes, on establishing reliable rolls across the mills, roll damage modelling, and collaborative research for and implementation of advanced roll grades in hot strip mills.
Presented by Christian Zybill, Walzen Irle
Reliability of rolls is a most important issue for customers involved in hot rolling of steel. IRLE Roll Foundry takes a multilayer approach to combine performance and reliability from the level of microstructure through the casting and manufacturing process up to an interactive customer service. This allows a high degree of adaption to the specific conditions in the mill. Furthermore, extraordinary dimensional accuracy and surface quality of the products are achieved. For the customer the TCO of the rolling process is reduced significantly.
A comparative discussion of the advanced materials will be done including performance and reliability data as a guide for the selection of an optimum roll.
Introduction of Centrifugally Spun Cast High Speed Steel Work Rolls in Hot Mills: A Tata Steel India Perspective
Increasing campaign length and reducing change over time without affecting strip surface quality are the key drivers for introduction of newer grades of work rolls in various hot mills of Tata Steel India.
Centrifugally Cast High Speed Steel Work Rolls were used for the first time in Tata Steel India. Along with the expectations came also a lot of doubts w.r.t. precautions to be used while using the new generation costlier rolls which had higher residual stresses than the previous ones.
Upgradation of the grinding equipment and roll testing equipment was also done.
Tremendous cost benefits like double campaigns without grinding and reduced roll wear scales was obtained. The paper encapsulates the journey undertaken by flat hot rolling mills of Tata Steel at Jamshedpur to absorb this new generation rolls and make them the backbone of a successful finishing mill roll fleet.
Presented by Mario Boccalini Jr, Institute for Technological Research (IPT)
Further advances in the development of roll materials for hot rolling mills require deeper quantitative understanding of roll/strip interactions, combining real-time observations with physical and mathematical modeling of rolling operation, such that damage phenomena are recorded as a function of the thermo-mechanical loads acting on the rolls.
A collaborative project involving Gerdau Summit, Institute for Technological Research and University of São Paulo aimed at model the progressive damage of hot rolling roll based on physical and mechanical properties of the micro-constituents of the roll material, thus allowing to design materials with “engineered microstructure” for high performance hot rolling rolls. The overall frame of the project was presented in ROLLS 5 Conference 2015, in Birmingham.
The present work deals with results of damage modeling and experimental validation through pilot rolling mill, considering both mass loss and thermal cracking. Two alloys were used in the experimental rolls: hot worked H13 (quenched and tempered to 45 HRC) and cast Nb-bearing HSS (quenched and tempered to 54 HRC).
Each experimental campaign consisted of hot rolling four hundred AISI 1045 steel plates (15x100x250 mm), that is, about 1 km rolled. Five campaigns for each alloy were performed and, after each campaign, rolls were taken out the stand to measure the roll profile and the thermal crack density.
Comparison between simulations and the operation of the pilot rolling mill were performed, which contributed to a simple model to predict the occurrence of thermal fatigue based on three rolling mill operational parameters: reduction, roll speed and temperature of the rolled material. Additionally, further studies were conducted regarding the abrasive behavior of roll microstructures and on the effect of oxidation on the thermal fatigue behavior.
Cooperative research for late finishing stands’ roll grades - roll producers' knowledge meets customers' demands
Presented by Armin Paar, ESW
The development of highly sophisticated work roll materials requires expertise knowledge in the fields of metallurgy and material science. In the case of roll grades for the late finishing stands the metallurgical characteristics are complex as stable and metastable phases appear together in the microstructure. Indefinite-Chill roll grades (ICDP) consist of cementite and graphite embedded in a bainitic-martensitic matrix and may be supported by MC carbides in the case of carbide-enhanced ICDP grades. The last development step of graphitic HSS grades widens this field as more complex carbides such as M2C and M6C were introduced into the metallurgical system and still a well-balanced microstructure has to be achieved.
Additionally the application in the rolling mill plays an important role in the development of roll grades which meet the customers’ demands. The wear resistance is the most important factor for work rolls but it is well-known that also other criteria are not less important such as the surface quality and the sensitivity against mill incidents. Especially these two factors require a close cooperation between the work roll manufacturer and the customer, including the rolling mill and the roll shop. To improve the work roll surface it is important to understand the mechanisms of work roll degradation which may vary from mill to mill. The sensitivity against mill incidents has to be split into two factors, the resistance against the crack initiation and the resistance against the propagation of existing cracks.
ESW’s cooperative research strategy aims to cover all fields of work roll materials development. Fundamental research is carried out together with universities and other research institutes and based on these results the materials development is carried out. For the development of roll grades with highest demands, close cooperation with customers plays an important role where the specific factors of the application in the mill, roll testing and roll grinding are included.
This paper presents the development of selected work roll grades for the late finishing stands such as VANIC (modified ICDP) and VANIMO (graphitic HSS) including examples of fundamental research as well as the applications in the rolling mills.
Session Chairs and Moderators
Roll Surface Treatments & Assessments
Two papers will deal with coatings on work rolls. The first paper presents a new, environment-friendly trivalent chromium-based plating process as possible replacement of the common hexavalent hard chrome process, illustrated with field experiences. The second paper proposes electroless nickel dispersion coatings with a broader field of application, not limited to cold mill work rolls but also to boost wear resistance on hot mill rolls. New developments in roll surface texturing will be discussed in the third paper. The final paper in this session, from a broad consortium comprising research institutes, roll users, a roll manufacturer and a measurement system supplier, will present a project enabling mill operators to perform fast roll surface analyses directly at the end of a rolling campaign, using tablets with quick measurements
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.
Modelling & Roll Life Optimisation
Four speakers from the roll manufacturing and roll using industries and from a research institute will present computational methods and modelling results to optimise the life and value-in-use of rolls in various rolling applications. The first paper focuses on long product mill rolls, presenting a new theory to predict the minimum safe working diameters, enabling optimised roll grade selections. In the second paper, a complex design approach is proposed aiming at back-up roll life optimisation. The third paper will feature an FE-model to optimize the roll life in reversing roughing stands. The final paper deals with hot strip finishing mills, showing examples of a numerical approach to calculate effects of strip and roll cooling parameters and roll material properties on in-use behaviour of spun-cast and additive-manufactured work rolls.
The selection of Roll Grades for Long Product Reversing Rolls Through Accurate calculation of their discard diameters
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.
A numerical approach on the effect of strip chilling and high turbulence roll cooling on spun-cast versus additive-manufactured work rolls of hot strip mills.
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.
Enabling technologies to boost roll & mill performance
Various technology providers will expound several types of technologies and strategies geared towards boosting roll and mill performance. The first paper will focus on enhanced Ultrasound inspection technology for optimal detection of defects in rolls. The second paper will present concepts and strategies to optimise VIU of universal sleeves for section mill rolls. The third paper will explicate how continual development over decades of roll shop hardware and systems have evolved into a ‘Digital Roll Shop’ with a fully integrated suite of software based functions. Lastly, a novel grinding approach will be revealed to tailor work roll surface topographies to specific rolling applications.
Enhancing Mill Roll inspection with bi-directional Ultrasound for optimal detection of all surface and subsurface defects.
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.
Surface Roughness Engineering for Rolling Mill applications - Tailored work roll surface topographies by advanced grinding
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:
- 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);
- To (re-)apply the prescribed work roll profile;
- To achieve the correct roughness level, as specified for the applicable rolling mill stand.
- To grind without typical grinding surface quality deviations, such as feedlines, scratches/”comma’s”, chatter marks, burn marks or chevron patterns;
- 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:
- Boundary lubrication: load is mainly supported in the contact of both work roll and strip contact asperities, resulting in high(er) friction coefficient;
- Mixed lubrication: friction is reduced as effect of the lubrication, lowered friction coefficient, mixed lubrication is the desired condition for the Rolling Process;
- Hydrodynamic lubrication: is characterised by low friction, high skidding risk, in rolling Mills maximum avoidance to skidding and consequential risk to strip wreckage during rolling (rolling loads to go down upfront the wreck)
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:
- Influence on cold rolled strip surface appearance, gloss, cleanliness (iron fines);
- Improved work roll durability: Impact on retained roll roughness, profile wear, positives in roll service lifetime and potential reuse after campaign;
- Positive effects on Rolling Mill loads, stability & rolled strip tension;
- Modified oxidation behaviour of the work roll surface in a hot rolling mill, to facilitate the build-up of a protective thin oxide layer.
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.