MICROSTRUCTURE AND WEAR RESISTANCE OF … · MICROSTRUCTURE AND WEAR RESISTANCE ... found in the high chromium white cast irons with low carbon and low Cr/C ... microstructure of

MICROSTRUCTURE AND WEAR RESISTANCEOF HIGH SPEED STEELS FOR ROLLING MILLROLLS

M. Boccalini Jr.Instituto de Pesquisas Tecnologicas- IPT

Av. Prof. Almeida Prado, 532- Cidade Universitaria

05508-901 Sao Paulo

Brazil

A. SinatoraEscola Politecnica da Universidade de Sao Paulo

Av. Prof. Mello Moraes, 2231- Cidade Universitaria

05508-900 Sao Paulo

Brazil

Abstract In most cases, work rolls for the finishing stands of hot-strip mills are com-posite components made of an outer shell of cast wear-resistant material and acore of ductile iron or steel. The development of materials for the outer shellhas enjoyed rapid advances beginning in the early 1980s, culminating in theapplication of cast alloys of the Fe-C-Cr-W-Mo-V system, which replacedhigh-chromium cast iron and Ni-hard cast iron. These alloysare genericallytermed high speed steels or, more rarely, multi-component white cast iron.

The idea of using these alloys for manufacturing work rolls for hot-stripmills resulted from an insight into the requirements involved in this type ofapplication: fundamentally, the capacity to retain a high level of hardnesseven when submitted to high temperatures, and also wear resistance. Bothare fulfilled by the classical high speed steels. The alloy design of the highspeed steels for rolls is based on the composition of the M2 steel, the mainchanges being the higher carbon and vanadium contents.

The degradation of the work rolls for the early finishing stands involvesabrasion, oxidation, adhesion ("sticking") and thermal fatigue. This work

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510 6TH INTERNATIONAL TOOLING CONFERENCE

deals with the effect of the chemical composition on the microstructure of theHSS for rolls, mainly in respect to the volume fraction and morphology ofthe eutectic carbides, as well as with the influence of the microstructure ontheir wear resistance.

Keywords: high speed steel, rolling mill roll, wear

INTRODUCTION

The quality of the strips and the productivity of the hot rolling mills are twoof the most important concerns in the steelmaking plants. The quality of thestrips is evaluated mainly by means of its shape, roughness and dimensionaltolerances, all of them depending strongly on the shape profile and surfacequality of the work roll. The productivity of the rolling mill is directly relatedto the length of campaings of the rolls, the essential aim being to maintainover time the surface roughness and shape profile as close as possible tothe initial ones. Moreover, saving costs with minimum roll grinding is alsoimportant, since rolls are responsible for 5 to 15% of overall productioncosts.

In most cases, work rolls for the finishing stands of hot rolling mills arecast composite components made of an outer shell of wear-resistant materialand a core of ductile iron or steel. Regarding the work rolls for the earlyfinishing stands, the development of materials for the outershell has enjoyedrapid advances beginning in the early 1980s, culminating inthe applicationof cast alloys of the Fe-C-Cr-W-Mo-V system, which gradually replace high-chromium cast iron and Ni-hard cast iron with better performance [1, 2, 3].These alloys are generically termedhigh speed steelsor multi-componentwhite cast iron[4, 5].

The idea of using these alloys for manufacturing work rolls for hot stripmills resulted from an insight into the requirements involved in this type ofapplication: fundamentally, the capacity to retain a high level of hardnesseven when submitted to high temperatures, and wear resistance. Both arefulfilled by the classical high speed steels for tools, leading the alloy design ofthe high speed steels for rolls (HSS for rolls) to be based on the compositionof the M2 steel. Thus, although the roll makers have developed alloysspecifically designed to the operational conditions of eachhot- strip millplant, their chemical compositions often fall into the following ranges: 1.5–2.5%C; up to 6%W; up to 6%Mo; 3–8%Cr and 4–10%V.

Microstructure and Wear Resistance of High Speed Steels forRolling Mill Rolls511

SOLIDIFICATION AND MICROSTRUCTURE

Owing to the higher carbon contents of the HSS for rolls in comparison tothose of the HSS for tools, austenite, instead of delta ferrite, is the primarycrystallized phase and the peritectic reactionδ +L → γ does not take place.In addition, the HSS for rolls are less hypoeutectic than theHSS for tools,that is, they have a lower volume fraction of proeutectic phase. Thus thesolidification sequence of the major high speed steels for rolls is composedby just two main occurrences:

i) primary crystallization of austenite: liquid→ austenite;

ii) eutectic decompositionof residual interdendritic liquid: liquid→austenite+ carbides.

Nevertheless, the residual interdendritic liquid decomposes through differenteutectic reactions as it moves down a eutectic trough, leading to the formationof up to three eutectics:γ+MC, γ+M2C andγ+M7C3. Figure 1 shows thecurve related to the cooling between 1450◦Cand 1100◦Cduring differentialthermal analysis of the alloy Fe-1.9C-5V-2Mo-2W-4Cr and Fig. 2 shows itsresultant microstructure (matrix is not etched).

Figure 1. Differential thermal analysis curve of the alloy Fe-1.9C-5V-2Mo-2W-4Cr.


(a) Label "I" means interdendritic M2C and M7C3 eutectic carbides.

(b) Detail of the interdendritic eutectic carbides.

Figure 2. Microstructure of the alloy Fe-1.9C-5V-2Mo-2W-4Cr solidified at 0.15 K/s.

Theγ+MC eutectic always precipitates first, owing to the high vanadiumcontent of these alloys. The precipitation of theγ+M2C and/orγ+M7C3

eutectics in the last stages of the solidification is governed by the segrega-tion of the alloying elements and the sequence by which they precipitateresults from the competition between them, depending on theoverall chem-


ical composition and on the cooling rate. The former is favored by high W,Mo or V contents and high cooling rates while the latter is favored by highCr or C contents and low cooling rates [5, 6]. Figure 3 shows the effect ofvanadium content and cooling rate on the volume fraction of the eutecticcarbides in the alloy Fe-1.9C-2Mo-2W-4Cr-V (note the suppression of theM7C3 carbide for high vanadium content or cooling rate) and Table1 showsthe composition ranges of the eutectic carbides.

Table 1. Typical composition ranges of the eutectic carbides

Carbide Composition ranges (wt.%)V W Mo Cr Fe

MC 40–60 10–30 10–25 3–8 2–4M2C 7–13 10–40 30–50 8–15 4–15M7C3 4–8 4–8 5–10 20–30 40–50

Figure 3. Effect of vanadium content and cooling rate on the volume fraction of the MC,M2C and M7C3 eutectic carbides.

The as-cast microstructure of the HSS for rolls is characterized by a matrixwith products of austenite decomposition (normally martensite or bainite),


retained austenite and precipitated globular secondary carbides (Fig. 2(b)),MC eutectic cells (coral-like MC) and eutectic carbides distributed in theinterdendritic or intercellular regions (M2C, M7C3 and idiomorphic or petal-like MC). MC carbide is by far the major eutectic carbide in the microstruc-ture (Fig. 3). Up to three morphologies, commonly named coral-like, petal-like and idiomorphic (Fig. 4), are developed, depending on the chemicalcomposition and cooling rate [5, 6]. The morphology of the MCcarbide isinfluenced both by the vanadium content and by the cooling rate, an inter-dependence of these variables needing be considered in determining quanti-tative limits of their influence. It was shown that the higherthe cooling rateand the lower the vanadium content, the higher the tendency to the formationof less coupled eutectic with petal-like and/or idiomorphic MC carbide [6].

Figure 4. Typical morphologies developed by the MC carbide in the HSS for rolls.

The volume fraction of the M2C and M7C3 carbides rarely reach 5% eachone. When the formation of the M7C3 eutectic precedes that of the M2Ceutectic, the M7C3 carbide develops as branched platelets thicker at the end,forming a "wall" of carbide around the eutectic cell (Fig. 2(b)), typicallyfound in the high chromium white cast irons with low carbon and low Cr/Cratio [7]. In this case, the M2C eutectic nucleates on the M7C3 carbide and theM2C carbide presents platelike and/or fine lamellar morphologies (Fig. 2(b)),


both described elsewhere [8]. When, otherwise, the formation of the M7C3

eutectic takes place after that of the M2C eutectic, the M7C3 carbide is rod-like and the M2C carbide develops only as platelets assembled as radiatingclusters, playing the role of heterogeneous nucleus for theprecipitation ofthe M7C3 eutectic (Fig. 5).

After heat treated through quenching and tempering, the microstructurehas a tempered martensite or bainite matrix with remaining globular sec-ondary carbides precipitated during solidification (size around 1µm) andfine globular secondary carbides precipitated during tempering (size lessthan 1µm), both being mainly MC, M7C3 and M23C6 carbides [9, 10].

Figure 5. Detail of the microstructure of the alloy Fe-2.5C-5V-5Mo-5W-4C solidified at0.15 K/s.

WEAR RESISTANCE

Work roll wear is a complex process characterized by the simultaneousoperation of several surface degradation phenomena. Furthermore, the per-formance of the roll material is evaluated through technological parametersthat depict the interaction of all those phenomena, the results of investiga-tions into the effects of each one separately being to be considered just as atrend. Basically, the essential target of the rolling mill plant is to keep theshape profile and surface roughness as close as possible to the initial ones.


The better performance of the HSS rolls, in comparison to theforerunnerwork roll materials, is related to its microstructure characteristics: greatamount of very hard (2800–3000 HV), fine and discontinuous MCeutecticcarbides and a matrix hardened by secondary precipitated carbides. Themicrostructure of the high chromium cast iron, for instance, consists of thesofter M7C3 eutectic carbide (1100–1800 HV) and a less high temperatureresistant matrix.

The degradation of the work rolls for the early finishing stands involves,at least, abrasion, oxidation, adhesion ("sticking") and thermal fatigue [11,12, 13]. Thermal fatigue results from stresses developed bycyclic heatingand cooling of a very thin boundary layer close to the work roll surface(no thicker than 1% of the work roll radius [14]), which is alternately andrepeatedly heated by the hot strip, the work of plastic deformation and theroll/strip friction in the roll bite and cooled by water during the remainingportion of its rotation. The boundary layer is thus submitted to compressivestress during the heating cycle, since its thermal expansion is constrained bythe bulk roll, which temperature remains approximately constant during theoperation; if the compressive stress is high enough to plastically deformedthe layer (softened by the high temperature), residual tensile stress higherthan the rupture strength may develop during the cooling cycle and crackingwill take place [14, 15]. Figure 6 shows a typical thermal cracking pattern

Figure 6. Typical thermal cracking pattern at the surface of a HSS workroll.


at the surface of a HSS work roll for the second finishing stand. Primaryand secondary patterns can be distinguished, mainly due to the differenceof the mesh sizes, larger in the former, and to the thickness of their cracks,finer in the latter. While the primary crack pattern is related to thermal andmechanical stresses imposed on the roll and not to the microstructure of theshell material, eutectic carbides play a decisive role in the nucleation andpropagation of the secondary cracks [16, 17].

Thermal fatigue experiments show that secondary cracks nucleate at theeutectic carbide (stress concentration induced by the great difference be-tween the thermal coefficients of carbide and matrix) and propagate alongcarbide/matrix interface [9, 18]. Since the presence of eutectic carbides, andthus crack nucleation, is unavoidable, improving thermal fatigue resistancerequires their refining and homogeneous distribution, so asto avoid the for-mation of easy crack propagation paths, like interdendritic or intercellularcoarse M7C3 or M2C carbides (Fig. 7).

The combination of thermal fatigue and mechanical stressesinherent tothe rolling process progressively extends and branches thecracking networkthroughsubsurface thickness, leading to a catastrophic deterioration in whichlarge segments of the roll surface, containing the oxide layer built up duringrolling together with portions of the roll material, are peeled off. The peeling,known as "banding", leaves a roughened roll surface unsuitable for furtherrolling (Fig. 8a, 8b).

Adhesion is a consequence of the micro-welding regions of strip metalinto roll metal in the sticking zone of the roll gap, where there is no relativemotion between the strip and roll surfaces [19, 20]. Resultant wearing takesplace when interfaces in contact are made to slide and the micro-weldedregions must separate, hot shearing the roll material [19].The formationof large pores in the surface of the rolls, commonly named "comet tails"(Fig. 9), are attributed to the intense occurrence of adhesion [21].

Adhesion resistance of the roll materials is improved by increasing vol-ume fraction of eutectic carbides. Werquin [19] explained this behaviorresembling that the hot hardness of the eutectic carbides ishigher than thatof the matrix, concerned to the higher hot shear strength thus resultant. Fur-thermore, within the same concept, he suggested that adhesion resistancecan be further improved through increasing the hot hardnessof the matrixby means of secondary hardening heat treatment [19]. Nevertheless, one canalso resemble that adhesion is primarily controlled by the physico-chemical


(a) Nucleation of the secondary cracks at the MC eutectic carbide.

(b) Propagation of the secondary cracks along coarse interdendritic M7C3 eutecticcarbides.

Figure 7. Thermal fatigue experiments.

interaction between the welded materials, in which chemical affinity, alloy-ing solubility and diffusion play a decisive role [22]. Accordingly, the actualeffect of the eutectic and secondary precipitated carbidesmay be related to


the less intense interaction between carbides and strip metal, owing to theirlower chemical affinity.

Oxidation of the work roll during hot rolling, gradually building up anoxide layer on the roll surface (referred to as "black skin"), markedly influ-ences the wear behavior of the roll material: as long as this layer is smooth,adherent and continuous, it act as a solid lubricant and as a thermal barrier,thus protecting the roll surface from degradation [19, 21, 23]; nevertheless,it is subject to high stresses during the contact with the hard strip scale in theroll gap and it is eventually worn out [21]. Therefore, ideally, roll materialmust be able to develop an oxide layer that rapidly forms and regenerates,but, once formed, slowly grows in order to keep a high adherence to the rollsurface.

The oxide layer in the HSS rolls consists basically of magnetite, thoughCr, Mo and V are also detected [17, 23]. The oxidation kinetics is mainlycontrolled by chemical composition, Cr and Co (sometimes added up to 5%)being the most powerful alloying elements in minimizing it [19, 23]. It isobserved, further, that the size and distribution of the eutectic carbides play arole on this kind of surface degradation, since they are lessprone to oxidationthan the matrix, irrespective of the carbide type, they are not influenced bythe breaking up of the oxide layer and matrix/carbide interface is a favorableoxidation path [19, 24].

Friction coefficient between work rolls and rolled strip is of the utmostimportance for the hot rolling mill due to its effect on process parametersand surface quality of the strip: the higher the friction coefficient, the higherthe rolling force, the power consumption and the tendency tothe formationof rolled-in scale defect [2, 20].

In a general view, friction coefficient is higher for HSS rolls than for highchromium cast iron rolls, as has been demonstrated by industrial results [20]and laboratory experiments carried out by two-disc wear test [11, 25]. Thisis a consequence of the main features of the microstructuresof that alloys,as follows:

a) The presence of hard MC eutectic carbide in the microstructure of theHSS leads to a high uneven wear, since the matrix, much more soft, isquickly worn out; as a result, protruding carbides or eutectic cells arise.In addition, increasing amount of vanadium content causes the formationof cell-type MC eutectic with intercellular network of relatively massiveM2C or M7C3 carbides that are prone to micro-spalling and subsequent


falling-off. Both occurrences give rise to severe surface irregularity onthe roll surface, thus increasing the interlocking of asperities (Coulombfriction).

b) Removed MC particles from the roll material may induce a grippingeffect between roll and strip.

c) MC eutectic carbides in HSS are less prone to thermal cracking, sincethey are finer and more evenly distributed than the M7C3 carbide in highchromium cast iron. Thus, the presence of micro-pits actingas lubricationoil pools are rare in the HSS rolls.

d) The total volume fraction of eutectic carbides in high chromium cast iron(around 30%) is much higher than in HSS (10% to 18%), thus makingthe former more adhesion resistant.

While this qualitative relationship between friction coefficient and mi-crostructure features is a reasonably widespread knowledge, literature showsconflicting quantitative data, even when comparing works ofthe same re-search group. Friction coefficients ranging from 0,25 to 0,55 for the HSShave been published [11, 20, 25].

SUMMARY

The development of materials for the outer shell culminatedin the appli-cation of cast alloys of the Fe-C-Cr-W-Mo-V system with microstructurescharacterized by great amount of very hard, fine and discontinuous MCeutectic carbides and a matrix hardened by secondary precipitated carbides.

The solidification sequence of the major high speed steels for rolls is com-posed by primary crystallization of austenite and eutecticdecomposition ofresidual interdendritic liquid, which leads to the formation of up to threeeutectics:γ+MC, γ+M2C andγ+M7C3. The matrix consists of products ofaustenite decomposition (normally martensite or bainite), retained austeniteand precipitated globular secondary carbides. MC is by far the major eutec-tic carbide in the microstructure and M2C and M7C3 eutectic carbides aredistributed in the interdendritic or intercellular regions.

The degradation of the work rolls for the early finishing stands involves,at least, abrasion, oxidation, adhesion and thermal fatigue.

Thermal fatigue cracks nucleate at the eutectic carbide andpropagatealong carbide/matrix interface. Improving thermal fatigue resistance re-


quires refining and homogeneous distribution of carbides, so as to avoidthe formation of easy crack propagation paths. Adhesion resistance is im-proved by increasing volume fraction of eutectic carbides.This behavioris attributed to higher hot hardness and lower chemical affinity with rolledmaterial of the eutectic carbides in comparison to the matrix. Oxidation ofthe work roll during hot rolling markedly influences the wearbehavior of theroll material, since as long as this layer is smooth, adherent and continuous,it act as a solid lubricant and as a thermal barrier, thus protecting the rollsurface from degradation. The oxidation kinetics is mainlycontrolled bychemical composition, but the size and distribution of the eutectic carbidesplay a role on this kind of surface degradation.

Friction coefficient between work rolls and rolled strip is higher for HSSrolls than for high chromium cast iron rolls (values rangingfrom 0,25 to0,55). This is a consequence of the main features of the microstructures ofthat alloys, i.e., the major presence of MC carbides in the former and thehigher total volume fraction of eutectic carbides in the latter.

REFERENCES

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Figure 8a. Macroscopic view of "banding" at the work roll surface: in the region labeled"B", Ra=2,3µm; in the region labeled "N", Ra=0,95µm.

Figure 8b. Schematic illustration of the steps in the formation of "banding" [16].


Figure 9. Large pores in the surface of the rolls ("comet tails"), which formation isattributed to the intense occurrence of adhesion.

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MICROSTRUCTURE AND WEAR RESISTANCE OF … · MICROSTRUCTURE AND WEAR RESISTANCE ... found in the high chromium white cast irons with low carbon and low Cr/C ... microstructure of