QUESTIONS, ANSWERS, MORE QUESTIONSTwenty - five Years of Experience in Discussing Rolls and Rolling Technology

Dr.-Ing. Karl Heinrich SchrderESW, Eisenwerk Sulzau-Werfen

A-5451 Tenneck, AustriaPhone: +43 6468 5285 100

E-mail: ks@esw.co.at

Key Words: rolls, mechanical properties of rolls, coefficient of friction, quality assurance

Roll makers always ask about rolling conditions and the necessity to choose the right grade of roll material(whether the decision is right or wrong is proved in the mill) and roll users always ask about the mechanicaland physical properties of roll material. Sometimes they feed these figures into their rolling model, butsometimes they also need them for unknown reasons. This information is very rarely useful for selecting theright supplier.

In July John M. Dugan published in AISE (No. an article about "FACTORS WHICHINFLUENCE ROLL SPECIFICATIONS" [I], the book consists of pages. At that time- as I had onlybeen in roll-business for less than three years - I hoped to find help. However, fortunately he did not reallygive 'factors which influence' - otherwise I would have had trouble in using these factors correctly.Rollmakers do their job as well as possible and they try to reproduce success without to many theoriesbecause they know that it is easy to have a good theory - and much more sophisticated to have a goodproduct - however it is more important to have a good product instead of a good theory!

Dugan was not really concerned about rolling conditions 'which influence roll specifications' - he knew toomuch - but he discussed all kinds of roll failures due to rolling accidents. However, he did know that not toomuch roll abuse can be compensated by the best roll grade.

In Oct. presented a paper in Cleveland on "Inspection Parameters which control Performance ofHigh Chrome Rolls" [2] discussing the influence of roll surface hardness, residual stresses, rollcomposition, etc. - but since then, l amstill asked the same questions.

Rolls usually do not fail due to normal rolling conditions, but due to mill accidents.And mill accidents arehard to describe quantitatively, so discussions continue - besides some clear events - for a long time.Luckily most mills improved their rolling conditions, reduced cobble rates etc. and the roll grades havebeen improved aswell.Theoverall roll performance is rising.

On the way to achieving better performing rolls many properties are discussed, however this is a longstory of misunderstandings, misinterpretations etc. The main problem was and is that mechanicalproperties are

1978 15222)88

1985 I

INTRODUCTION

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tested with specimen under one directional stress - however, in rolls we have more or less always (2)3dimensional stresses and the roll surface is always loaded complexly by mechanical, thermal andcorrosion systems.And rolls are loaded with residual stresses. We have to distinguish carefully whether wemean material / specimen properties or roll behaviour! If this distinction is not made endlessmisunderstandings occur.There are many examples for false theories like for example:

theory of importancestories ...theory of stress relief in compound rolls due to plastic deformation of core materialtheory about oxidation of roll surface, while in reality the oxide layer only is built by strip scale

Everybody needs models, 'theories', to develop new ideas - but these should only be discussed in publicwhen proved. A good result is not necessarily the verification of a theory.It took me many years to understand the different thinking of roll users and roll makers and how theyapproach the same problems. Therefore in this paper I want to cover some of the most frequently discussedquestions in a (my) general way to summarise 25 years of personal experience.

Basic properties of tools like rolls and properties of the material the tools are made of, are two totallydifferent sides of a coin, and very often this difference is ignored. However, when we start to discussmechanical properties, we have to distinguish whetherwemeanmaterial-orroll-properties.

- is

There are many methods and measurement instruments with basically (but not exclusively) 2systems: static measurement: HV, HB, HRCdynamic measurement: Shore C, D, ...; Equotip D, E, ...

and there are conversion/confusion tables, which neglect / ignore statistical variations and induce moreaccuracy than exists.

Hardness is strongly influenced by the roll surface conditions. Roll surface hardnessmaybemeasuredsomeShore (1 ShC = 10 HV) too high due to work hardening or some Shore too low due to annealing by wronggrinding compared to the 'real'material hardness.

And the variation of hardness in rolls is much higher than most people believe: For rolls standard deviationcan reach 30 HV [2, 3] and even the range of normal specifications is only 5 ShC (= 50 HV) (average 2 =average 6 ShC -> this means: no process capability - or everybody has to average average values - till thereis no more variation).

For roll makers: they control the process of roll manufacturing.

For mill people: They should control Work hardening [4] by hardness measurement to avoid roll failuredue to surface fatigue. However, work hardening only occurs after plastic deformation and increaseswith yield strength of the material. Fatigue often starts at stress raisers (cracks) with stresses far belowthe yield strength

MECHANICALPROPERTIES

Hardness the most boring subject of discussions! Why ??

What is the purpose of hardness readings? Answer:

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For science: Many material properties are strictly related to hardness, there are all kinds of rules - but itis necessary to know the rules [5].

It is the only property besides dimensions that can be tested by incoming inspections. However, aftermentioning the facts above there is no further comment.

Mill people believe in hardness giving information about roll quality and roll performance, however this ishelpful only in rare exceptions. Fig. 1 shows that different roll grades with totally different properties werealways produced in the same range of hardness.

Another question: Why do mill people want to discuss hardness?

Fig. 1: Range of barrel hardness and content of carbides of different roll grades

A wide variation of hardness has almost no influence on roll performance, see Fig. 2 and Fig. 3. Thesame roll hardness of different rolls of one grade can be achieved by various matrix micro structuresand various amount and type of carbides which have a high impact on roll behaviour - not on hardness!

The relation of hardnesses of b.u.r.s to WRs is often discussed, and even here no easy answer is available.Besides; we have to consider whichmaterial in the mill is softest to take plastic deformations in case of an

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accident: That is the rolled sheet in HSM (in so far as no specialities are rolled) - and b.u.r. may have thesame hardness as WR, and that is the b.u.r. in CSM - and b.u.r.s should be definitively softer than WRsHowever special situations always need special care.

Fig. 2: Tonnage performance of lCDP work rolls 680x1900 mm, excluding mill accidents

Fig. 3: Tonnage performance of lCDP work rolls 680x1900 mm, including lost mm due to mill

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where 6r,;, for rolls is highly influenced by the residual stresses, while i,a, is represented by the rollhardness. For rolls of the same hardness this means, that the material which represents the highest levelof residual stresses gives the best resistance againstmarks.

Easy - just avoid local overloads, which 'should not' happen in normal rolling conditions! -

High tensile strength of core material is mainly required to avoid thermal breakage of rolls, brittle,spontaneous breakages due to high temperature gradients in a roll, where the residual tensile stress in thecore material is an additional prestress. The easiest way to avoid thermal breakage is to minimizethermal gradients and this can be achieved by good rolling / cooling strategies and proper handling ofrolls.

There is a theory that the firecrack pattern of roll surface is related to tensile strength (and to fatiguestrength), however the question of crack propagation has to be discussed in relation to residual stress andmicrostructure as well. The best way to achieve a good roll surface is excellent roll cooling. The rollgrade andwearhassomeinputonrollproperties.However,rolling conditions are important aswelt!

-

Figures of fatigue strength gained from tests with standard small specimen are practically useless, and wehave to consider: size of rolls, notches, impact, environment, superposition of different multiaxialstresses etc., etc. So we have only our experience with rolls and measures for many possible cases!Fatigue strength is of high importance for high stressed filets of back up rolls and for all kind of spalls.Fatigue strength is highly influenced by notches and residual stress (Smith Diagram!). There are somebasic figures of tolerated stress - however, in cases where there is corrosion fatigue everything isdifferent and we have to fight corrosion first [6].

Spalls: An interesting example of fatigue failures are saddle spalls of WR in HSM finishing standoriginating in core (grey iron with lamellar graphite) material of high chrome iron roll, which wereexperienced in high stressed rolls in some mills, due to changes in rolling strategies early in the 1980s(later in ICDP rolls as well) [7]. There have been different methods to avoid this type of failures:lower tensile pre-stressing (lower residual stresses in the rolls) or - core material with higher fatiguestrength, ductile iron versus grey iron with lamellar graphite, and the latter solution was adopted by allrollmakers in general.

Another example of fatigue problems are spalls in ICDP or HSS rolls. These materials are verynotchsensitive, so it is highly recommended to eliminate all kinds of cracks, whenever these rolls arereground, otherwise typical `cats' - tongue band type spalls may occur. [8]. High chrome iron or steelmaterial ismuchlesssensitive.

How to avoid marks, intrusions?

Why should we discuss tensile strength?

Who knows what?

Tensile

Fatigue strength

strength -

Compression strength - is directly proportional to hardness (R - 3,5 x HV) for most materials made ofiron (grey iron, steel, high-chrome-iron or - steel,HSSetc.),butcompression strength is never required fora roll - however safety against impression marks, local plastic deformations and in this case the shearstress is to be discussed. Shear is calculated as the difference of 3-dimensional stresses,

Residual stresses have a high impact on fatigue [9], and fracture mechanics might be helpful in solvingproblems.

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Torque

Young's modulus

PHYSICAL PROPERTIES OF ROLL MATERIALS

PROPERTIES FOR EASY TESTING OF MECHANICAL PROPERTIES

Wear

- is not reported (or only very rarely) to create fatigue failures. Torque damage of drive ends ofrolls (45 breakage) is normally a spontaneous breakage due to one single giant overload, so the limitfor torque is considered as static torque strength ( ~ 80 % of tensile strength).

-

Young's modulus of compound rolls is a mixture of Young's modulus of core- and shell material andcan be precisely measured, however the 'mixture' varies during roll life, when the shell material-thickness is reduced to almost nothing. However it seems there was never any real problem, whateverthe figure was!

are often discussed for some rolling models, however - to my knowledge - none of the roll makers iscapable of testing thermal conductivity, specific heat, at room - or elevated temperature etc., etc.

These properties are always taken from literature or measured by high-tech-research institutions. There isnot much sense in asking the individual roll maker - even they are supposed to answer all questions of theircustomers, of course!

Nobody! But answers are requested.

-

Wear is an object of libraries without any easy answers! Rolling conditions are of high importance forthe wear of rolls. For example: High Chrome Iron b.u.r. don't wear in cold strip mills (these rolls fail dueto spalling, low resistance against crack-propagation), but they wear like butter in the sun in hot stripmills.!

Rolling conditions!

Basically the diagram of Khrushov is helpful, Fig. 4: for one material wear resistance increase withhardness; at the same hardness wear resistance increases with the percentage of carbide forming elements,Fig. 5 shows the influence of various alloying elements.

the customer asks fbr this - but is it a%gore or a variable?

Who can give good answers?

Who knows the answer? Which parameters are of importance?

The reason?

Fig. 4: Diagram of Khrushov [5, G]

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Fig. 5: Wear versus alloy equivalent (Range of alloys: 0,30 - 0,44%C; 0,02 - 0,38%Si; 0,31 - 0,83%Mn;1,18 - 3,03%Cr; 0,08 - 3,04%Mo; 0,26 - 1,92%V and 1,42 - 5,85%W) - from [10].

For hot rolling of steel the temperature of rolled material is very important. For a long time there has beentheory of hard (high temperature) and soft (low temperature) oxides on the strip which might influencewear, but last year at the WMSP Conference in Baltimore [11] this theory was regarded as obsolete,because there is never enough time during rolling to build up oxides in the right equilibrium phase becausethe time between the stands is too short - but nevertheless it is true that wear is less when the temperature ofstrip surface is low (less than 900 C), skin cooling of strip is evident to reduce wear in hot rolling of steel -something that has been known formanyyears, especially when adamite was used for W.R. material (some20-30 years ago). And: hot rolling of stainless steel - almost no scale - increases wear heavily. Andespecially strip edges of stainless steel - at lower temperatures - increase wear, the work roll roughens - duetomicro-sticking or other reasons.

Wear is at a minimum with HSS-rolls, however they are not used world wide for the very last stand ofHSM due to sticking problems, where specific pressure is at its maximum [12].

Lubrication reduces wear and sticking, but there are still problems to be solved.

Coefficient of friction -

The coefficient of friction is under discussion, especially for HSS-roll-material. CRM [13] did some basicstudies for the European rollmakers (CHEF) and it turned out that there is no real difference between thevarious materials for work rolls for hot rolling: ICDP, high chrome iron and HSS. They studied thecoefficient of friction as a function of deformation, Fig. 6. Lubricants change the contact conditions andthus the coefficient of friction as well - however the difference between the various materials remainsnegligible.

Why is the coefficient of friction higher with HSS-rolls than those with HighChrome Iron or ICDP?

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Fig. 6: Evolution of the coefficient of friction of different roll grades with and without lubrication(FEMCoulomb model with barreling) [13].

However CRM [14] at last year's WMSP-Conference in Baltimore presented the results for the coefficientof friction during rolling - calculated from measured rolling - separation force versus calculated separationforce which show a totally different situation, Fig. 7. This does not correlate with above mentioned results!We have to consider roll surface during a rolling campaign and the change in roll surface, mainly the firecrack pattern! Fig. 8 shows the different fire crack pattern for different materials - heavy primary cracksinfluence the coefficient of friction more than the structure of carbides or anything else [15].

The coefficient of friction is influenced more by rolling conditions than by the roll material properties! Inmills where roll cooling is most efficient the difference in coefficient of friction between different materialsis not evident. Indeed, somemills never reported any increase of separation force!

And even under severe rolling conditions, lubricants may minimise the increase incoefficient of

- How to improve roll-, strip surface?

Banding or peeling is a problem inmanymills.ItisknownandproventhattheoxidelayeronW.R. is built uponly of strip oxides, oxidation of rolls never was proven (and it is almost impossible because roll surfacetemperature never exceeds 550 C, and this only in contact with rolled material under exclusion of oxygen).Banding is a mill problem and it really can't be solved by roll material grade so far, although there are somevariations [16]. Great improvements have been achieved by better roll / strip cooling systems.

It seams evident that the rolled-on layers of oxides on WRs are stripped of when light gauge is rolledwith high reduction in the early stands of the finishing mill.

friction!!

Banding / Peeling

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Fig. 7: Evolution of the coefficient of friction during a rolling campaign [14].

Fig. 8: Surfaces of worn work rolls, etching 12" Nital, SEM.A - high chrome work rollB - HSS work rollAngle of incidence of the electron beam is 45 to the roll surface.

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QUALITY ASSURANCE

Some years ago the ISO 9000 ff quality standard was introduced to keep the quality of mass producedcars and parts for automotive industries or others under control. Rolls are customer specified singleproducts. Such systems are undoubtedly useful for controlling 'industrial production', for instance rolls.However, something is different: Each roll is 'handmade' and within very narrow tolerances. Formaking rolls - we are talking about small figures, tenth of rolls, never talking about big numbers,which are necessary for real statistics (min. 100 pieces of the same design!) - no S.P.C is applicable!

For process capability the tolerances should be wide compared to standard deviation of the process!! Thismight be true for the chemical composition of rolls - which is not specified by our customer. However, ourcustomers ask for T.I.R. less than 5 micrometer, our measuring devices show only digits of 1 micron!!There is no process capability! Hardness variation (see above) is larger than the tolerance! No processcapability! In case process- and machining capability do not exist, it is necessary to check each individualspecification - which is done anyway with each roll!!

We know.

We have to produce, piece by piece into the tolerances, with no way of statistical approval, we have toproduce - to check - to produce - till it fits!!

We have to meet our customers' requirements - but, because machine- and process capabilities are out ofreach for single produced parts (rolls), we have to work like an artist to reach the goal.

Due to high-tech data analysis we are capable of improving our process, but we have to manufactureeach single roll according to the specifications.

There is no way of buying machines which guarantee machining capability for rolls! We have to workhard to reach the goal!

And to find out something about the quality of a roll maker's product it is not only helpful to check thequality system, it is useful to check the scrap rate, the claim rate - and to analyse the reasons and details.ISO 9000 ff is useful for professional inspectors - it is of no use when deciding about the quality of rollsproduced, each individual roll has to be tested carefully.

Statistical evaluations are helpful for roll-makers in determining their actions to improve the roll quality - itis of no use for our customers! To prove performance figures, we should keep in mind: 100 is a good numberfor statistical evaluations, numbers below 10 are not good for statistical statements.

ESW is certified, of course! But what does this mean? Process-, machining capability?!

What purpose do questions about capabilities have? -

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1. Roll makers and roll users frequently have to discuss experiences, performance results, specialrequirements of the mill. Roll failure problems can be solved by good co-operation.

2. To avoid misunderstandings and misinterpretations everybody should distinguish very carefullywhether roll properties or the properties of the roll materials are concerned.

3. Statistical findings are useful for roll making to improve the process of roll production - statisticalprocess control (SPC) is only applicable for (automated) mass production.

4. Theories are sometimes useful for (commercial) discussions and may help to create new ideas. Howeverfacts about roll performance (without and with loss of useful stock due to mill accidents) are most importantfor roll development.

5. The best way for roll makers to achieve better rolls is to ensure that better materials and improvedmanufacturing processes are used and that roll users take account of rolling conditions and improvedrolling processes.

Good and close co-operation between both sides is essential

1. J. M. Dugan, "Factors which influence roll specifications", ,July 1978

2. K. H. Schrder, " Inspection parameters which control performance of high chromium rolls",, Cleveland, October 1985

3. Investigation Report of CRM: "Hardness deviation"

4. K. H. Schrder, "Beitrag zum Verfestigungsverhalten der Stahle durch Kaltverformung",, 11 (1980), S. 73-76

5. K. H. Schrder, W. Eilert, "Zur Entwicklung der Qualitaten fair Kaliberwalzen",104 ' 984) Heft 20, S. 1005-1008

6. K. H. Schroder, "State of the art of rolls for the production of flat rolled products",, Vol. 6 1984/85, pp. 120-124

7. K. H. Schrder, "Heavy spalls originating in the cores of high chromium rolls",, issue No. 2/1986, Verlag Stahleisen

8. K. H. Schrder, "Remarks on the measurement of, distribution in and influence of residual stresses inheavy construction units with large cross section", ,DGM 1987, pp. 927-933

9. "Rolls for the Metalworking Industries", , 1990, p. 139, 235

10. Werkstoffkunde Stahl Hrsg. Verein Deutscher Eisenhuttenleute Berlin; Heidelberg; New York; Tokyo;Springer; Dsseldorf: Verlag Stahleisen

REFERENCES

Association of Iron and Steel Engineers

Proc. ofthe 27` Mechanical Working and Steel Processing Conference

Z. furWerkstofftech.

Stahl andEisen

World Steel &Metalworking

Metallurgical Plantand Technology

Residual Stresses in Science and Technology Vol. 2

Publication of Iron and Steel Society

h

CONCLUSIONS

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11. Discussion at 41 Mechanical Working and Steel Processing Conference, ISS, Baltimore 1999

12. K. H. Schrder, "Rolling conditions in hot strip mills and their influence on the performance of workrolls", , issue No. 4/1988, Verlag Stahleisen, pp. 44-56

13. Centrum voor Research in the Metallurgie, "Friction measurement in hot-rolling by means ofthe tribological ring compression test", Investigation Report of CRM, May 1997

14. D. Steinier, D. Liquet, J.Lacroix, H. Uitjtdebroeks, J. C. Herman, "Effect of processing parameters inthe front stands of a HSM on the performance of HSS work rolls", , ISS, Baltimore,1999

15. K. H. Schrder, B. Feistritzer, M. Windhager, K. H. Ziehenberger, "Progress of carbide enhanced ICDP(and remarks on the coefficient of friction of HSS work rolls in hot strip mills", will be presented at the 37thRolling Seminar Conf.,ABM, Curitiba 2000

16. ESW - paper, in near future

st

Metallurgical Plant and Technology

41st MWSP Conf.

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