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The work roll surface damaging study: a key factor in using HSS alloys in hot rolling

Paper presented at 19th ROLLING SEMINAR, 2012, Rosario, Argentina

A. Tremea (R&D Dept., Innse Cilindri srl, Italy), M. Grespi (Sales Dept., Innse Cilindri srl, Italy), A. Esteban (HSM Roll Work Shop Manager, Ternium Sidebar, Argentina), W. Martinez (HSM Roll Work Shop Coordinator, Ternium Sidebar, Argentina)

Abstract

In many hot strip mills the front stands of finishing group are working with rolls made in High Speed Steels. The success in using these rolls is strictly in connection with the mill technology but also with redressing methods applied in the turning shop. The HSS rolls show an impalpable wear and then the evaluation of roll surface damaging assumes a strategic role to determine their performance. Furthermore the study of roll surface damaging becomes very important especially when we want to assess a roll steel grade within the same class of steel. The contents of this paper regard the analysis of redressing procedures and their use to optimize the performance of HSS rolls.

Keywords: Hot Rolling, High Speed Steel Rolls, Roll Surface Damaging, Grinding Operations

Work rolls behaviour through the stands of hot strip mills

Paper presented at AISTECH 2014, 5-8 May, Indianapolis, Ind., USA

A. Tremea

Abstract

The working condition of a roll through different stands of a hot strip mill changes a lot. The configuration of the mill with the rolled production mix play a fundamental role in setting the level of solicitations acting on the roll and therefore on its consumption. The redressing operations on the roll have also a great importance to assess the roll performance so their managing become a strategic point to judge the roll life. The grinding machines are mostly equipped with automatic control systems to verify online the roll integrity so grinding operations are handled by the machine on the basis of thresholds of acceptability. The definition of these criteria should be made in function of the damaging of roll surface and the roll material type but in many cases these control systems are not so customer-friendly. In this paper there is a description of different approaches in roll redressing to show how this process can influence the roll performance. More and more often the roll consumption is combined with the roll price to evaluate a global roll yield so the amount of grinded roll material have to be minimized according to the real roll damaging. The discussion on this matter is considered by the author one of most interesting point specially in this period where the cost reduction is the only objective to follow by all.

Keywords: Hot Rolling, Centrifugally Cast Rolls, Roll Damaging, Stock Removal

Properties of HSS rolls for hot strip mills

Paper presented at ROLLING 2013, June, Venice, Italy

M. Pellizzari (University of Trento, Trento, Italy), A. Tremea (INNSE Cilindri, Brescia, Italy), M. Fiorese (University of Trento, Trento, Italy)

Abstract

High speed steel (HSS) rolls are increasingly used in hot strip mills due to their high wear resistance. Their microstructure, constituted by primary and hard eutectic carbides and a tempered martensite metallic matrix ensures a suited combination of hardness and toughness, which made these materials very attractive to replace more conventional grades like High-Cr irons. Thermal fatigue also represents a critical damage phenomena for hot rolls, particularly for those mounted in the roughing and early finishing stands, where the temperature of the rolled strip is higher. In this case, a quite complex interaction between thermomechanical stresses and triboxidation causes thermal cracking, which progressively deteriorates the surface of the roll with negative drawbacks on its performance. Depending on the type of stress cracks can be unidirectional or multidirectional giving rise to a parallel series of cracks rather than a crack network, best known as heat checking. This paper summarizes some laboratory test results aimed at evaluating the properties of different HSS roll materials, paying special attention to their thermal fatigue behavior. Two different tests were proposed, in order to obtain unidirectional and multidirectional cracking. Both were carried out by cyclic heating a roll specimen of 40mm external diameter up to 670°C and following rapid water cooling. The use of 10mm and 20mm-width specimens heated in the central region only (10mm width) allowed unidirectional and multidirectional cracking, respectively. The extent of damage was characterized by measuring crack density, crack length and crack depth to be compared to those of real rolls. It could be demonstrated that a lower interconnection of carbides improves thermal fatigue resistance: the eutectic carbide network enhances both crack nucleation and propagation of deeper cracks. A proper alloy design can improve the performance. An interesting correlation between surface crack width and penetration depth could be established, which could be exploited as a useful tool for the analysis of thermal fatigue degradation of rolls on the mill.

In Service Behavior of Spun Casting Work Rolls in Hot Flat Rolling of Steel

Paper presented at 50th ROLLING SEMINAR, 18-21 November 2013, Ouro Preto, MG, Brazil

Alberto Tremea (Innse Cilindri srl, Italy), Massimo Pellizzari (University of Trento, Italy)

Abstract

The evolution of work roll performance involved an increased amount of alloying elements and the following increase of costs. It has become very important to evaluate the behavior of new grades, since the initial stages of development, in order to tune the content and type of alloying elements as a function of the specific application on the rolling mill. One of the most important damage phenomena which strongly influences the roll performance is thermal fatigue. A new test was recently developed to reproduce a similar kind of damage to which the roll is subjected during rolling. In any case, research and development must be always considered in the frame of the industrial use of rolls, thus considering typical rolling practice, possible accidents and turning operations. This work describes by means of some examples a work methodology used to evaluate the in-service behavior of rolls in some mills, paying special attention to thermal fatigue damage.

Keywords: Hot Rolling, Spun Casting Roll, Thermal Fatigue, Roll Performance

High speed steel work rolls: an update

Paper presented at AISTECH 2016, 16-19 May, Pittsburgh, PA, USA

A. Tremea

Abstract

The production of work rolls by centrifugal casting allows an high productivity at reasonable cost. In terms of chemistry of poured material there are some aspects to respect to make a good and safe product. Innse Cilindri, in the last years, has developed new concepts during the roll casting widening the range of alloying of proper rolls. New materials are now available with a higher content of alloys and also excellent technological properties. Tests in laboratory before and now results from the mills confirm the strength of these new products. The attention is mainly focusing within the class of high speed steel materials (HSS) as shown and described in this work.

Keywords: Hot Steel Rolling, Work Roll, High Speed Steel Material

Evolution of indefinite chill iron rolls for hot steel rolling

Paper presented at 2nd CHINA INT. STEEL MILL ROLLS CONF.,14-16 November, 2012, Wuxi, China

Alberto Tremea

Abstract

The Indefinite Chill Iron (ICI) rolls are still the rolls most suitable the for the last finishing stands of a hot strip mill. In the past a lot of research was carried out to obtain ICI rolls with enhanced properties and so new grades have been developed. A widespread technique to obtain ICI with better technological properties is enrich the classical composition with several carbide formers elements different by chromium. The addition of elements like vanadium, niobium, molybdenum and tungsten is an ordinary practise to produce today this type of roll. The application in the mill needs wear resistance but before everything a minimum of accident resistance has to be guaranteed. So the microstructural characteristics of this material has to be balanced in respect to the rolling practice of the mill. The development of a new grade has to be conducted in field, in close cooperation with the people of the rollshop. This approach and some results regarding enhanced ICI rolls are the contents of this paper.

Keywords: Hot Steel Rolling, Indefinite Chill Iron, Wear, Roll Performance

Eddy current inspection results on work rolls for finishing stands of HSM

Paper presented at 52nd ROLLING SEMINAR, 17-21 August 2015, Rio de Janeiro, RJ, Brazil

Alberto Tremea

Abstract

The eddy current system is universally recognized as a most suitable method to point out discontinuities or modifications on the roll surface. The automation of the controls through the implementation of sophisticated equipments on the grinding machine has favoured its employment in the roll shop. A big effort is still going on to clarify the best way to promptly identify situations that could be really unsafe for the roll life. The results presented in this paper have been obtained through an intensive activity conducted in field by Innse Cilindri on work rolls of finishing stands of hot strip mills. The correct interpretation of the eddy current signals and a good calibration of the automatic system have allowed, in many cases, a substantial optimisation of regrinding practices.

Keywords: Work Roll; Eddy Current System; Bruise Signal; Roll Redressing

Different approaches in performance evaluation of work rolls for hot steel rolling

Paper presented at 51st ROLLING SEMINAR, 28-31 October, 2014, Foz de Iguacu, PR, Brazil

Alberto Tremea

Abstract

In this period a great attention is put on roll cost and the differences in roll behavior in very few situations move the existing balances. Among many variables that are taken into account to evaluate the roll there is the consumption which is normally calculated according to the tonnage rolled. In this work there is a review of some approaches that are used to evaluate the roll efficiency together a description of a new method to make this. The main purpose of this calculation method is to help the comparison of roll results used in different mills. There is the need to have a clear and univocal reference when it’s time to think about innovative products. At now there are available roll grades with excellent technological properties but it’s necessary to have the right tools for a careful evaluation of roll behavior to take a good decision.

Keywords: Hot Rolling, Work Roll, Roll Performance

Centrifugally cast high speed steel rolls for cold rolling

Paper presented at AISTECH 2013, 6 May, Pittsburgh, PA, USA

A. Tremea

Abstract

In cold rolling, the deterioration of work roll surface is the main aspect to keep in mind. The roll has to show optimal wear resistance and at the same time, a great ability in retain its roughness. A minimum level of roughness is essential to guarantee the friction needed for rolling. If the roll is textured, the roughness is also important because it must be imprinted to the rolled product. To extend the service of forged steel rolls in many cases the surface is chrome-plated: the high speed steel cast rolls, due to their resistance characteristics, can roll similar campaign lengths without Cr-Plating and the additional expense and environmental impact. This work, as well as to show the potentiality of these special cast rolls, emphasizes a number of aspects related to the redressing of these rolls in the roll-shop. The main purpose of this article is to present the HSS cast roll as a valid alternative to the classic forged steel rolls and analyze the aspects related to performance and their environmental impact.

Keywords: Cold Mills, Centrifugally Cast Rolls, High Speed Steel, Wear, Roughness

An overview on work roll performance evaluation in flat hot rolling of steel

Paper presented at SARUC 2014, 23-24 October, Vanderbijlpark, South Africa

Alberto Tremea

Abstract

The evaluation of roll performance is a key issue looking at roll improvement and at the resulting benefits for the customer. The roll price clearly plays a decisive factor in addressing the choice of the roll supplier and in many cases a positive difference in terms of roll quality is paid back by differences in roll price. It is an author conviction that while the roll price is strictly related to where the roll is produced, the evaluation of roll performance remains a quite confused issue. In this paper some practical situations are critically analyzed to outline the difficulty that this evaluation creates. The manufacturing of new rolls with theoretical better behavior has to take in account working conditions very different and also rigid, so provide general roll performance is impossible. The redressing procedures can affect the roll performance more than the rolling conditions. Therefore, many variables have to be taken into account. The main objective of this work is to suggest a simple method for the evaluation of roll performance, in order to define its real potential.

Keywords: Hot Rolling, Work Rolls, Roll Consumption, Grinding Procedures

Abrasive wear behaviour of hot roll materials

Paper presented at ABRASION 2014, August, Stockholm, Sweden

M. Pellizzari (University of Trento, Trento, Italy), A. Tremea (INNSE Cilindri, Brescia, Italy), S. Ricco (University of Trento, Trento, Italy)

Abstract

In the last years the ever increasing need of high performance rolls has led to the introduction of High Speed Steels (HSS). This material combines excellent wear resistance and hot hardness but, in several applications, does not offer adequate toughness which is generally reflected into poor accidents resistance during service. Furthermore, relatively poor frictional properties were claimed by some authors so that older, more economic materials like Indefinite Chill irons (IC) keep working in many mills. The push towards a material combining proper wear resistance and toughness has led to the development of a new class of materials showing 10-20%vol primary cementite (M3C) and alloy carbides (MC, M6C, M2C…) and 2-3%vol graphite. In view of their composition and microstructure they can be considered as an intermediate product between HSS and IC roll grades. Thinking that these materials are an evolution of HSS containing graphite in this paper they will be called GHSS (Graphitic High Speed Steels). The tribological behavior of two GHSS containing 2 and 4%V, respectively, is compared with that of conventional HSS’s, using two different wear configurations. In the first case, the test provides the hot rolling/sliding contact between the roll material and a C40 steel, heated up to 700°C. The resulting wear mechanism is a combination of abrasion and triboxidation, which affect both, the wear rate and frictional behavior. The wear resistance increases by increasing hardness. GHSS show higher wear than HSS’s in correspondence of a given hardness, plausibly because of the difficulty to develop a protective oxide layer. In order to isolate the abrasive contribution, a second dry sliding wear tests was carried out using granite as counterpart material. Under such conditions, the higher carbides fraction becomes predominant and GHSS show a higher wear resistance than HSS.

Work rolls for hot strip minimills

Paper presented at SARUC 2008, 15 May, Witbank, South Africa

A. Tremea, et al.

Abstract

This paper presents work roll types used in compact hot strip mills. This rolling concept is now taking an important share within the worldwide steel production. This technology strip production technology, in addition to its technical and financial advantages in respect to a conventional steel plant, has reached optimal levels as far as quality and mix of rolled products are concerned. The latest rolling strategies in a compact mill allow reaching capacities up to 3 Mtpy also of microalloyed structural steels. In this scenario the role of work rolls becomes strategical to assure strip quality and mill operating stability. In this work are presented the technological properties of actual roll material grades used in hot rolling together with a set of examples of real results from the mills. The purpose of this paper is to stress the concept that the unique way to reach the best results is a friendly collaboration for an early identification of the real problems in connection with the use of rolls in certain specific working condition and after making a suitable choice about roll material. Drawing generic conclusions in the rolling business is usually very risky because many variables (different from roll grade) can heavily affect the real results of the roll.

Keywords: Hot Rolling, Minimill, Roll Wear Consumption

Work rolls consumption in the front stands of hot strip mills: wear and grinding

Paper presented at 17th ROLLING SEMINAR, 2008, Rosario, Argentina

A. Tremea, et al.

Abstract

In the early stands of a hot strip mill the use of high-chromium iron and high-speed steels as work rolls is now well proven. The introduction of HSS has generated in many situations big improvements in terms of specific roll consumption and campaign length. HSS rolls are currently present only in those mills where the rolling conditions allow a full exploitation of their optimal resistance properties. This high resistant class of steels becomes very interesting when the mill conditions are quite stable and especially the grinding procedures are properly calibrated to the roll surface damaging. The evaluation of the level of roll damage after the campaign allows an optimization of the redressing procedures: this is a prerequisite to carry out new tests on different roll grades and do meaningful comparison of performances. This work reports cases of typical roll surface degradation to underline possible ways to optimize the grinding procedures. Many mill variables can influence the roll behaviour. This means that the choice of the roll material must take in due account the specific rolling conditions of the mill.

Keywords: Wear Resistance, Roll Surface Damaging, Grinding Procedures

Work rolls behaviour in roughing stands at SA HSM

Paper presented at 48th ROLLING SEMINAR, 24-27 October, 2011, Santos, SP, Brazil

A. Tremea, M. Grespi

Abstract

The work roll in chromium steel is the typical type of roll for the roughing stands of a hot strip mill. This application, very hard from a thermal point of view, needs rolls made in high-strength alloys that can withstand heavy rolling loads and wear at high temperature. In this regards, already exist rolls with superior mechanical properties to those of rolls made in chromium steel. An example is represented by semi-hss rolls called in this way in analogy to the classification of tool steels. Laboratory tests on the semi-hss material has shown optimal results in terms of wear and thermal shock resistance, but in service it requires careful management if you want to exploit these features. For this reason, the operations in the roll shop hold a great importance both in routine grinding phases and when a roll is subjected to an accident. This work aims to underline that the synergy between the roll shop and the roll producer is the only way to optimize the roll performance. Working with this approach becomes possible a reasonable cost reduction for the roll shop as well a correct evaluation of real yield of the roll. The paper outlines the operating situations of roughing stands in some South American mills. This comparison can certainly help to better understand how to manage the many rolling variables when we want to evaluate the behavior of a roughing work roll.

Keywords: Hot Rolling (Roughing Stands), Semi-HSS Roll, Roll Shop, Roll Performance

Wear and friction behaviour of high chromium iron and HSS for hot rolls

Paper presented at ABRASION 2005, 14-17 August, San Paolo, Brazil

A. Tremea, M. Pellizzari, et al.

Abstract

Tribological properties of spincast roll materials depends on the mechanical properties of the metallic matrix, on the carbides type, amount and morphology and on the ability of the material to develop a thin and adherent oxide layer which protects the surface from metallic and abrasive wear. All these characteristics depend on the chemical composition, which determines the solidification sequence and the residual content of alloying elements in the metallic matrix after heat treatment. By means of specific laboratory tests, aimed at simulating the phenomena which are responsible for the damage of the rolls, the friction and wear behaviour of some High Chromium Iron and High Speed Steels have been studied. The influence of different types of primary carbides on wear resistance and friction coefficient was determined and, as a result, some modification in the chemical composition of the standard grades was introduced, based on MC and M2C forming carbides elements.

Keywords: mill rolls, wear resistance, friction

Tribological behaviour of spincast HSS for hot finished rolls

Paper presented at ROLLS 2003, 9-11 April, 2003, Birmingham, UK

M. Pellizzari, A. Tremea, et al.

Abstract

In this paper, the results of a research project aimed at the study of the tribological behaviour of High Speed Steels used for the production of finishing hot rolls are presented. Six steel compositions have been formulated by means of Thermo-Calc, software for thermodynamic modelling, suited for the study of solidification processes. Two classes of HSS were selected based on variable MC/M7C3 and MC/M2C. Computational previsions are in good agreement with thermal analysis results carried out during rolls casting and the microstructure observed for HSS rolls. Laboratory wear, hot friction, hot hardness and impact tests have been carried out on several materials with different composition, and the results are discussed in correlation with their microstructure and mechanical properties. The wear resistance is mainly related to the matrix microhardness, while the primary carbides have a negative effect. MC carbides should be preferred from HSS rolls with improved toughness and wear resistance. MC carbides have a negative effect on the friction coefficient, as measured by hot rolling sliding tests, due to their bad oxidation behaviour with respect to that of the matrix. M2C carbides allow a more even oxide layer to be formed on HSS surface thus favouring a more effective solid lubrication during strip rolling. Hot hardness values confirm high thermal stability of the investigated materials, in agreement with the high secondary alloying content.

Keywords: Hot Rolling, Work Rolls, HSS, Laboratory Tests

Surface degradation and consumption of work rolls in the early stands of HSM

Paper presented at 1st CHINA INT. STEEL MILL ROLLS CONF., 1-4 June, 2008, Shanghai, China

A. Tremea, M. Pellizzari, et al.

Abstract

The thermal stresses and the friction phenomena acting between roll and strip are the main driver for roll consumption during the campaign. The complex tribological situation doesn’t allow any easy generalization about roll behaviour due to the very different mill operating conditions that the rolls can encounter during rolling. In the recent past Innse, in collaboration with the University of Trento, has devoted many efforts in the evaluation of tribological properties of rolls by means of special tests. Results were useful in the alloy design of new grades suited for specific stands of the mill. The roll is subjected to wear also during grinding operations, even if during the machining wear can be more easily controlled. The utilization of sophisticated equipments to control the rolls directly during the grinding operations, guarantees not only the detection of big defects due to rolling accidents (or roll problems naturally), but also gives an indication about the level of damaging of the roll surface. The knowledge of roll damaging plus a well-calibrated signal of eddy current system can assure a minimum amount of roll material loss due to normal grinding operations. In this paper are presented several results in the first finishing stands of hot strip mills where the utilization of eddy current system, keeping clearly in mind the real level of roll damaging, has allowed a safe reduction of “grinding roll wear”.

Keywords: Hot Rolling, Thermal Fatigue, Wear, Redressing Procedures

Semi-HSS for roughing rolls with improved TFR

Published on LA METALLURGIA ITALIANA, Vol.9, 57-61, 2005

M. Pellizzari, A. Tremea, et al.

Abstract

Thermal fatigue represents one of the most significant causes for roughing mill roll deterioration and possible failure. The materials ability to withstand thermal cycling is strictly correlated to its microstructural properties, which are the results of the solidification process and following heat treatment. Rolls for hot strip mills are usually produced by centrifugal casting, without subsequent forging. A crucial rule is played by the eutectic carbide network developing on solidification, which represents a preferential pattern for crack propagation. For this reason conventional highly alloyed HSS are not a popular substitute for traditional chromium steel rolls in rougher stands. The so – called “semi-HSS”, containing less C (0.7-0.9%C) and alloying elements are usually preferred by users requiring improved roll material. Specially developed “HSS for roughers” are being tested in various mills, but they are out of the scope of this paper. The present work is part of a continuing effort by INNSE Cilindri (RIVA Group) and Trento University to improve the “semi-HSS” grades. In particular the thermal fatigue resistance of a new class of low-C semi-HSS is evaluated. For purpose of comparison, both a conventional semi-HSS and a HSS grade were also considered. The test is based on induction heating (Tmax=670°C) and water cooling of cylindrical discs. Experimental results confirm the lower susceptibility towards heat checking of the new grades, containing reduced amount of eutectic carbides, the best behaviour being exhibited by a steel with a mixed martensite-bainite matrix microstructure.

Keywords: thermal fatigue, semi-HSS, HSS, centrifugal casting, hot rolling, rolls, primary carbides

Semi-HSS and HSS spun casting rolls for cold mill applications

Paper presented at SARUC 2011, 6 October, Vanderbijlpark, South Africa

Alberto Tremea (Innse Cilindri Srl, Italy), Federico Bellicini (Innse Cilindri Srl, Italy), Horacio Fernandez (Cold Mill Roll Shop, Ternium Siderar, Argentina), Daniele Agazzani (Cold Mill Manager, ILVA Genova-Cornigliano, Italy)

Abstract

In recent years several tests have been performed with centrifugally cast bimetallic rolls as work rolls in cold rolling. This paper outlines the exciting results from cast rolls made in a special type of alloyed steel called Semi-HSS used in a temper mill at Siderar (Argentina). In this specific application of cold rolling it is very important imprinting to the sheet not only the desired mechanical properties but also a particular surface aspect. The work roll surface and especially its deterioration during the campaign become of primary importance to establish the real behaviour of roll grade material. In this temper mill, the use of cast Semi-HSS has permitted us to perform longer campaigns in comparison with normal forged rolls: these new rolls show an optimal behavior and they are particularly effective in retaining the roughness induced by shot blast texturing. This gives certainly a big positive contribution to the performance of this type of roll in terms of consumption and it leads to reduction of the costs related to grinding and texturing procedures in the turning shop. This new application for a non forged roll, with this very positive response from the mill, gives further opportunities to achieve a continuous decrease in total roll operation costs. Recently the use of Semi-HSS centrifuged rolls has been extended to Tandem Mills for tin plate production. The results were very positive, especially due to high resistance to accidents. Now are in progress also new trials with HSS rolls. These rolls due to their hardness and specific microstructure characteristics should offer better results in terms of consumption and demonstrate an important resistance against the marks in respect to Semi-HSS rolls. In addition these new rolls can permit to avoid the chromium plating for their excellent ability to maintain the initial surface roughness.

Keywords: Temper Mill, Tandem Mill, Centrifugal Casting Rolls, Semi-HSS, HSS, Wear, Roughness

Results in using semi-HSS rolls spun casting in a temper mill

Paper presented at 44th ROLLING SEMINAR, 16-19 October, 2007, Campos do Jordao, SP, Brazil

A. Tremea, et al.

Abstract

In recent years several tests were performed with centrifugally cast bimetallic rolls as work rolls in cold rolling. This paper outlines the exciting results from cast rolls made in a special type of alloyed steel called Semi-HSS used in a temper mill at Siderar. In this specific application of cold rolling is very important imprinting to the sheet not only the desired mechanical properties but also a defined surface aspect. The work roll surface and especially its deterioration during the campaign become of primary importance to establish the real behaviour of a roll grade material. In this temper mill, the use of cast Semi-HSS has permitted to perform longer campaigns in comparison with the usually employed forged rolls: these new rolls are showing an optimal behaviour and are particularly effective in retaining the roughness induced by shot blast texturing. This gives certainly a big positive contribution to the performances of this type of rolls in terms of consumption and allows a reduction of the costs related to grinding and texturing procedures in turning shop. This new application for a non forged roll, with this very positive response from the mill, gives a new chance to reach a continuous decrease in total roll operation cost.

Keywords: Cold Temper Mill, Centrifugal Casting Rolls, Semi-HSS, Wear, Roughness

Performances evaluation of HSS for hot rolling by wear and thermal fatigue tests

Paper presented at AISTECH 2006, 1-4 May, Cleveland, OH, USA

D. Cescato, M. Pellizzari, A. Tremea, et al.

Abstract

In the last years a great number of hot strip mills performed trials with High Speed Steel (HSS) rolls for both rougher and finishing stands. Rolls used in the former stands are subjected to higher temperatures, slower rolling speeds and longer contact lengths between roll and stock due to the high reductions. Therefore, they require high thermal fatigue resistance. Rolls used in the latest stands usually withstand higher tribological and mechanical solicitations given by abrasion and impacts with head and tail end cobbles, respectively. The industrial experience, however, indicate that it is often very difficult to evaluate their performances by trials on hot strip mills, due to the high intrinsic variability of the industrial process. In fact, in each plant different cooling conditions interest the rolls surfaces and the rolling forces and temperatures are subjected to a large variations; moreover, the occurrence of accidents involving the temporary stop of the strip between the rolls causes consistent localized rises of the temperature in the stand and a subsequent strong thermal shock at the restarting of the mills operations. These drastic events imply a heavy damage on the surface of the roll, with a consequent severe shortening of its life. Unfortunately, these kinds of accidents are not always documented, making not reliable the interpretation of the data from the mills. Therefore, strong discrepancies of performances for the same roll grade on different hot strip mills have often been found. This has lead to the development of laboratory tests to reproduce (as well as possible) the same thermo-mechanical conditions to which work roll materials are exposed during service. Two tests have been developed with the aim of evaluating the thermal fatigue and hot wear resistance of work roll materials and in particular to study the correlation between the microstructure and the damage produced by the cited phenomena. After heat treatment, spin cast HSS rolls display a microstructure given by a tempered martensitic (martensitic/bainitic) matrix surrounded by an eutectic network of primary carbides. Thermal fatigue and hot wear resistance are strictly related to the volume percentage, type, morphology and distribution of carbides, but even to the matrix microhardness, particularly at high temperature. By properly balancing the amount of carbon and alloying, it is possible to obtain a great variety of microstructures. The results presented in this work highlight the main criteria for the selection of the best grades for a given application, depending on the relative influence of wear and thermal fatigue. It is widely accepted that it does not exists a “universal roll grade” suited for each hot strip mill, but the best choice should be made on the basis of the specific working conditions. This also implies a strict cooperation between roll producer and hot strip mills people.

Keywords: Hot Rolling, High Speed Steel, Thermal Fatigue, Carbide Interconnection, Wear, Oxidation

New semi-HSS with low carbon content

Paper presented at SARUC 2004, 14-15 May, Gauteng (South Africa)

A. Tremea, M. Pellizzari, et al.

Abstract

In this paper the results of a research project aimed at studying the effect of chemical composition and microstructure on thermal fatigue, oxidation resistance and wear resistance of steels with different carbon content in the range 0.6-1.3% are presented. Microstructural characteristics of the studied materials cover a wide range of eutectic content, from that of typical HSS down to a practically eutectic-free condition. The positive influence of the reduction of the eutectic volumetric fraction on thermal fatigue damage has been clearly confirmed. Wear tests demonstrated that tribological properties are preserved even in materials with low eutectic content. Oxidation resistance depends on both the eutectic volumetric fraction and on the residual chromium content of the matrix. On the basis of the results of this project, a new semi-HSS with very low amount of eutectic carbides was selected and used for the production of roughing rolls.

Keywords: work rolls, HSS, semiHSS, thermal fatigue, eutectic carbides

Metallurgical development of hot rolls with improved rolling performances

Paper presented at SARUC 2008, 17-18 October, Gauteng (South Africa)

A. Molinari, A. Tremea, et al.

Abstract

This paper reports on the metallurgical development of hot rolls carried out by INNSE Cilindri in cooperation with Trento University, in order to produce materials with tailored properties to the specific needs of different application. The main results are presented with reference to the following technological targets: materials with improved thermal fatigue and wear resistance for roughing and first finishing stands, HSS with improved friction properties for first finishing stands, HSS with improved toughness for intermediate finishing stands, Indefinite Chill Iron with improved wear resistance for last finishing stands.

Keywords: Hot Rolling, Work Rolls, Thermal Fatigue, Wear Resistance

Laboratory testing aimed at the development of materials for hot rolls

Paper presented at ROLLING 2006, 19-21 June, Paris, France

M. Pellizzari, A. Tremea, et al.

Abstract

The performance of rolls used in hot strip rolling depends on a large number of factors that, in most cases, are very difficult to be rationalized. The results from the mill plants are often not comparable, being based on subjective feeling rather than on scientific parameters. So it is quite usual to get very different performance figures for the same roll grade on different mills. Moreover, field tests are expensive and imply a risk that is not accepted by most customers, so that the introduction of new experimental grades is a very complex task. For all these reasons big efforts have been made in the last years to reproduce the in-service behaviour of hot mill rolls by means of laboratory tests. Damage phenomena like wear and thermal fatigue have been reproduced by means of customary equipment by several universities and research centres. In this way, the consolidated cooperation between INNSE Cilindri, one of the most important European roll-makers and University of Trento has led to the investigation of the principal causes for roll failure and/or deterioration and their correlation with the microstructure of roll materials. It is the goal of the present paper to analyse the use of laboratory testing in the development of materials for hot mill rolls, from the roughing to the finishing stands. The general approach used was to investigate separately the material behaviour with respect to each one of the damaging phenomenon cited above. This is quite different from what occurs in real rolling, where their superimposition often hampers a correct evaluation of the influencing factors. In many cases, the experimental results are in good agreement with those obtained in industrial practice, as confirmed by the data reported in the paper.

Keywords: Hot Rolling, Thermal Fatigue, Wear, Oxidation

HSS rolls, the last frontier in hot steel rolling

Paper plublished in FLAT ROLLED STEEL PROCESSES, Chapter 6, 71-81 Editor V.Ginzburg, CRC Press, Taylor & Francis, 2009

A. Tremea, M. Pellizzari, et al.

Abstract

Hot rolling of the steel represents one of the most important manufacturing processes because of the large volume of steel worked. The behavior of work rolls is a key factor in rolling technologies which influences the quality of the rolled products and even the operating rates (and costs) of the mill. During the last 10 years different applications of HSS rolls started covering different stands of the mill, aimed at a progressive replacement of more conventional grades like High Chromium Irons (early finishing stands) and High Chromium Steel (roughing stands). Despite of the high potential of these high strength materials, it must be recognized that just in few cases the use of High Speed Steels (HSS) rolls allowed to fully appreciate the advantage provided by their introduction. It is opinion of the authors that the operating practice in the mills and in the roll shops can be held responsible, at least in part, for this missed gain in performance. Next paragraphs is aimed at giving a general overview of the behavior of this class of rolls based on both, the industrial experience and laboratory tests carried out under more reliable conditions than those usually encountered during rolling.

Keywords: Work Rolls, HSS, Laboratory Tests, Roll Consumption

HSS rolls from research investigations to the mill

Paper presented at ABRASION 2011, Liege, Belgium

M. Pellizzari (University of Trento, Trento, Italy), A. Tremea (INNSE Cilindri, Brescia, Italy)

Abstract

In the last years an intensive research activity was devoted to the investigation of high speed steels for hot rolls. Many e fforts were made to replace conventional Hi Chromium Irons with more performing high alloyed steels, also known as mu lticomponent white cast irons. The development of a new grade requires a careful analysis of the service condi tions involving wear, thennal fatigue and ox idation. In some cases corrosion was also recognised as an important cause of roll deterioration. The microstructure, constituted by a martens itic matrix strengthened by a coarse dispersi on of primary and eutectiic carbides, has to provide the proper compromise between strength and toughness. In cast rol ls, the choice of the chemical composition is of paramount importance, since they are nor subjected to fmther thermo-mechanical treatments permitt ing modification ofthe solidification structure. The use ofthermodynamic modelling, to forecast the sol idification structure has then become a cost-saving rnethodology to implement the development. Thermo-Calc software was used to evaluate the influence of alloying elements on the amount and type and distribution of eutect ic carbides. The systematic validation with thennal analysis and meta llography hias allowed to define very useful c riter ia for the production of rolls. In the case of bimetallic rolls mode lling also resulted very important to promote adhesion between the outer shell and the nodular iron core. The success of a new grade for hot sh•ip mills requires a quite long time to be verified. From the bi rth of a new grade until the first results about the " rolled tons" severa[ months can pass. In this view, the need of laboratory tests aimed at evaluating the properties of the new alloy lrns become a quite important topic. Customary wear and thennal fatigue tests were developed to reproduce the damage mechanism during roll ing. The results of laboratory tests shows a fairly well correlation with the service performance of rolls, even if important differences remains due to scale factor and other instrumental limits of the equipment. In the case of tribological test at high temperature the complex interaction between the ro ll surface and the oxide scale developing on the rolled produci represents the most difficult factor to be reproduced. With minor differences, thermal fatigue tests give very interesting results abour the influence of carbides type and distribution on crack nucleation and propagation. This paper summarizes the most important results in the field of HSS rolls obtained by the Univers ity of Trento and INNSE Cilindri, an important european rollmaker. Test resul ts will be implemented and critically compared with results of different rolling mills.

Hot strip mill centrifuged work rolls: quality and results

Paper presented at 44th ROLLING SEMINAR, 16-19 October, 2007, Campos do Jordao, SP, Brazil

A. Tremea, et al.

Abstract

It is nowadays usual for any mill to require rolls with improved results in terms of performance, surface quality and often also a particular resistance to accidents. In many times, unfortunately, people have been induced to think that small changes of roll chemical analysis might solve the critical situation. The aim of this paper is to present several results from use of different grades of spun casting materials starting from roughing stands down to finishing stands in hot strip rolling. The sound knowledge of the roll surface damages together with an optimization of roll control and grinding procedures in the turning shop should represent a necessary step to start any discussion about roll grades and results. The introduction of new special wear resistance alloys with better technological properties, which are usually also more expensive, implies a major attention to understand if all the potentialities of these materials can give real opportunities of improving mill situation. In this work it’s presented an evaluation among the results of classical materials and recently developed materials taking into account the different flat rolling applications.

Keywords: Hot Strip Mill, Centrifugal Casting Rolls, Wear, Redressing Procedures

Development of spincast hot rolls through microstructural optimization

Paper presented at 44TH MECHANICAL WORKING AND STEEL PROCESSING, 8-11 September 2002, Orlando, Florida, USA

A. Tremea, M. Pellizzari, et al.

Abstract

This paper reports on the metallurgical development of hot rolls carried out by INNSE Cilindri in cooperation with Trento University, in order to produce spincast materials with tailored properties to the specific needs of different application. The main results are presented with reference to the following technological targets: High Chromium Iron with improved wear resistance, High Speed Steels with improved toughness and tribological behaviour, Indefinite Chill Iron with improved wear resistance, semi High Speed Steels.

Keywords: spincast, hot rolls, High Speed Steel (HSS), High Chromium Iron (HCrI), Indefinite Chill Iron (ICDP), semi High Speed Steel (semi-HSS)

Considerations about properties and behaviour of HSS in hot rolling

Paper presented at 43rd ROLLING SEMINAR, 17-20 October, 2006, Curitiba, PR, Brazil

A. Tremea, D. Cescato, et al.

Abstract

The thermal fatigue and wear behaviour of five different steels for hot rolling have been investigated by means of laboratory tests and then related to their mechanical properties and microstructural features. The results obtained have been compared to the performances shown by the rolls in the plants.

Keywords: Hot Rolling, High Speed Steel, Thermal Fatigue, Wear

Choice and performance of work roll materials through the stands in the HSM

Paper presented at INT. WORKSHOP, Jindal Steel Works, 26-27 February 2007, Vijayanagar, India

A. Tremea, et al.

Abstract

The work rolls made by spin casting method permit to well satisfy all the requests of a hot strip mill. The different grades actually realised cover all the specific working conditions; some special grades are designed to extend the campaigns through a reduced wear and or a good surface quality; this last aspect is of primary importance also for the quality of rolled material and then in the finishing stands it can’t be forgotten. In many cases the roll life is heavily affected by the consequences from some rolling accidents and then the results in terms of performance or surface quality can be easily obscured by big losses of roll material due to accident. The use of special high wear resistance alloys (i.e. SemiHSS and HSS) represents now a considerable part of total rolls working in hot strip mills: these materials, are certainly more performing, but are also more expensive and so they need a scrupulous attention in order to exploit all their potentialities. In this paper we show the actual situation about the utilization of several grades of roll material starting from roughing stands down to finishing stands; we compare the recently developed materials with classical materials in use since decades.

Keywords: Hot Rolling, Rolls, Centrifugal Casting, Wear, Rolling Accident

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