recent developments in coiler pinch roll technology · pinch roll application, the hardness of the...
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Recent Developments in Coiler Pinch Roll Technology
Deach RhodesXtek, Inc.
11451 Reading RoadCincinnati, Ohio 45241-2283
T: 513-733-7800F: 513-733-7831
Steve MenendezShenango Industries Inc.
1200 College AvenueTerre Haute, Indiana 47802
T: 812-235-2058F: 812-234-3160
ABSTRACT
Recent developments in coiler pinch rolltechnology include alloying of Bemcalloy C-1 graycast iron with Niobium. The alloy additionspromote increased wear resistance without negativeeffects on the pick-up resistance of Bemcalloy C-1Pinch Rolls. The new technology, known asBemcalloy C-141, has demonstrated a significantincrease in lineal feet coiled per stock loss in theinitial trial campaign.
KEYWORDS: Coiler Pinch Roll, Bemcalloy C-1,Bemcalloy C-141, Alloy Gray Iron, Niobium, andHot Strip Mill Roll
INTRODUCTION
Recently, coiler pinch rolls have become morerecognized as a critical component of the hot strip
mill. Once considered a commodity product, pinchrolls have gained increased attention as hot stripmill productivity and the demand for quality hasbecome more aggressive.
This article outlines the specific productivity andquality demands of the pinch roll application andhow an enhanced pinch roll material, Bemcalloy C-141, has been developed to meet these increasingdemands. Also included is a review of the existingBemcalloy C-1 pinch roll material to illustrate thebasis and methodology used in the development ofthe new technology. Trial data for the BemcalloyC-141 technology concludes the paper.
APPLICATION DEMANDS
In order to successfully develop new, or enhanceexisting coiler pinch roll technology, a completeunderstanding of the demands of the application isessential. Coiler pinch rolls consist of a set of topand bottom rolls at the end of the hot strip mill.Their function is to receive the head end of the hotband and direct it into the coiler. These rolls canalso provide a slight tension in the hot band duringcoiling if the top pinch roll runs engaged throughoutthe coiling process. This practice is primarily usedto obtain better coil quality1.
Coiler pinch rolls are the last set of rolls in thehot strip mill that the hot rolled product contacts.As such, the demands placed on these rolls haveintensified with desires of increased hot millproductivity and higher hot strip quality.
An increase in hot strip mill productivitytypically means that the rolls will be subject tolonger hot strip lengths (larger coils), longer 1 Coil quality in this sense refers to the "tightness" and edgestraightness of the coiled product. Engaging the top pinch rollthroughout the coiling process to increase coil quality hasbecome more common, even though this practice issubstantially more abusive to the pinch rolls.
campaigns, and increased tonnage over the life ofthe rolls. These productivity requirements implythat the pinch rolls possess resistance to wear,thermal stability, high thermal conductivity, andthrough-hardness. Table 1 summarizes theproductivity demands and the corresponding pinchroll property requirements.
Table 1. Productivity demands and corresponding pinchroll property requirements.
ProductivityDemands
Pinch Roll PropertyRequirements
• Long hot strip lengths• Good thermal
conductivity• Thermal stability
• Long campaigns • High wear resistance
• High tonnage over lifeof roll
• High wear resistance• Through-hardness
As in all hot strip mill applications, the materialsthat contact the hot product must possess goodthermal properties. The pinch roll application is noexception to this rule. With longer coils beingcoiled, and longer campaign lengths, the amount ofheat generated at the surface of the pinch rolls canlead to severe surface deterioration of both thepinch rolls and the hot strip. A material with a highthermal conductivity will dissipate the heat morerapidly and thus reduce the surface deteriorationand risks of friction welding. A material with a lowmodulus of elasticity will be less susceptible tocracking due to thermal stresses. Thermal stabilityrefers to the ability of the material to maintain itsheat-treated properties under the elevatedtemperatures experienced during service. Amaterial with good thermal stability will maintainits strength and hardness at elevated temperaturesand be less susceptible to softening while in service.
Two wear mechanisms, abrasive and adhesive(frictional), occur in the pinch roll application. Thepercentage of each of the wear mechanisms dependson the pinch roll material being used.
Abrasive wear results when a harder materialremoves particles from a softer surface. In the
pinch roll application, the hardness of the hot stripbeing coiled is soft relative to the hardness of thepinch roll, and therefore, the hot strip itself does notabrasively wear the pinch rolls. However,extraneous materials (scale, mill debris, and hardparticles carried by the coolant and transferredthrough the water sprays) invoke abrasive wear [1].
Adhesive, or frictional wear, results from thescuffing action between two surfaces that come intocontact. Adhesive wear occurs when appreciableheat is generated by friction between the surfaces.Often, projecting surface asperities present on theroll and product being coiled are locally heatedwhen the surfaces come into contact. The localizedheat can cause the surfaces to gall as a result offrictional welding. Upon further relative motion,the particles tear out and produce new asperities onthe roll surface [1].
Obviously the resistance to wear, either abrasiveor adhesive, or both, is a direct function of the pinchroll material, its hardness, and its microstructure.However, certain mill-related variables canaccelerate wear of the pinch rolls. Such variablesinclude keeping the top pinch roll engagedthroughout coiling, using unfiltered coolant,inadequate spraying of the hot strip to remove scaleand extraneous debris, and improper tension controlbetween the pinch rolls and the coiler.
The ability of a pinch roll to be through-hardened is a positive attribute when consideringoverall performance. A through-hardened materialhas a consistent hardness throughout the cross-section and thus can be used to scrap size withoutre-hardening, re-sleeving or welding. Also,application stresses can be more evenly distributedover the entire hardened cross-section.
In addition to the above productivityrequirements, the pinch rolls must also possessresistance to “pick-up” to ensure adequate hot stripsurface quality. Table 2 summarizes the qualitydemands and corresponding pinch roll propertyrequirements.
Table 2. Quality demands and corresponding pinch rollproperty requirements.
QualityDemands
Pinch Roll PropertyRequirements
• Superior hot stripsurface
• Good resistance topick-up
• High wear resistance
• Tighter and straightercoil • High wear resistance
Pick-up, as it is referred to in this text, is acondition whereby foreign material becomesadhered to the surface of the pinch roll duringservice. The foreign material is usually adheredmetal from the actual product being rolled. Figure 1is a photograph of a forged steel bottom pinch rollexhibiting a severe pick-up condition.
Figure 1. Pick-up on forged steel bottom pinch roll.
Pick-up on a pinch roll occurs when the productbeing coiled becomes fused to the roll surface dueto extreme localized temperatures and pressures.When small imperfections or raised areas on the hotstrip pass through the roll bite, they are subject tolocalized temperatures and pressures significantenough to cause them to become friction welded tothe pinch roll surface. Once welded to the surfaceof the roll, the areas are pulled from the product asit continues through the roll bite. The pick-upmaterial on the pinch roll surface will harden as itpasses by the water sprays. As the pinch rollcontinues to coil, the pick-up can transfer marks onany subsequent hot strip being coiled, thus
drastically degrading the surface quality of the hotstrip.
The amount of pick-up that occurs is mainly afunction of the pinch roll material, but is aggravatedby certain mill-related variables such as poortension control between the pinch rolls and coiler,poor surface and edge quality on the hot strip, andinadequate water-cooling of the pinch rolls.
From the previous discussion it becomes obviousthat the pinch roll application requires a materialthat possesses a balance between the criticalproperties of wear resistance, resistance to pick-up,good thermal stability and conductivity, andthrough-hardening capability.
These application demands have brought aboutthe extensive use of the existing Bemcalloy C-1technology. Furthermore, the desires for evengreater improvements in hot mill productivity andhot strip quality was the impetus for the recentdevelopment of the Bemcalloy C-141 technology.
EXISTING BEMCALLOY C-1 COILERPINCH ROLL TECHNOLOGY
The standard Bemcalloy C-1 material is beingused successfully in many coiler pinch rollapplications2. The Bemcalloy C-1 material is ahypoeutectic gray cast iron, alloyed with silicon,nickel, chromium, and molybdenum. The selectedalloys are present within the gray iron at levels thatprovide a balance between carbide formation andgraphitization.
The chemistry contains enough alloy to allow forthrough-hardening of the standard wall thickness ofthe sleeves used in top and bottom pinch rolldesigns. The composition and microstructure dictatethe properties of the heat-treated Bemcalloy C-1material. A microstructure representative of onethat is found in a Bemcalloy C-1 pinch roll is shownin Figure 2.
2 A recent survey shows that Bemcalloy C-1 top pinch rollsoccupy 50% of domestic mini hot strip mills and 70% ofintegrated hot strip mills [2].
Figure 2. Representative photomicrograph (250X) ofheat-treated Bemcalloy C-1 pinch roll material.
The microstructure shown above was achievedthrough controlled cooling after centrifugal castingand subsequent oil quench and tempering. Thehardness is 62 HSC (approximately 46 HRC)3. Themicrostructure consists of flake graphite (darkestgray constituent), discontinuous primary ferriticcarbides (white constituent), and a temperedmartensitic matrix. Per ASTM A-247, themicrostructure of graphite is classified as mostlyVII A5 [3].
Although each of the constituents of themicrostructure affects all of the properties, eachaffects certain properties more than others do. Theamount of graphite, the length of the flakes, andhow they are distributed in the matrix directlyinfluence the thermal stability and pick-upresistance, but contribute little to the strength orhardness [1]. The metallic matrix in which thegraphite is distributed provides the basic strengthand hardness of the Bemcalloy material. The wearresistance is aided by the natural lubricity ofgraphite, but is more a function of the metallicmatrix and carbide content and distribution [4].
3 The HRC value here is converted from HSC using acceptedhardness conversion tables and is listed here for referencepurposes only. Due to the heterogeneity of cast iron, a moreaccurate hardness is obtained with a Sclerescope (HSC) orBrinell (HB) instrument.
The Bemcalloy C-1 material, when centrifugallycast and heat-treated properly, will possess amicrostructure consistent with that shown in Figure2 above. In turn, this microstructure establishes theproperties that enable the Bemcalloy C-1 material toperform well in the pinch roll application.
EMERGING BEMCALLOY C-141COILER PINCH ROLL
TECHNOLOGY
During the development of the Bemcalloy C-141technology, the main criterion was to improve thewear resistance of the Bemcalloy C-1 materialwithout jeopardizing the resistance to pick-up orother critical properties. The ultimate choice ofalloying the existing Bemcalloy C-1 material withNiobium met this criterion.
Other methods could have been successfullyused to increase the wear resistance of theBemcalloy C-1 material. However, theseprocedures (which included increasing the hardnessand/or increasing the amount of primary ferriticcarbides in the material) would have negativeeffects on other properties as described below.
In general terms, wear resistance improves withincreasing hardness. The hardness is a function ofthe metallic matrix of a material. It seems logicalthat increasing the hardness of the Bemcalloy C-1material would increase the wear resistance. Whilethis is true, two main factors limit the level ofhardness that can be used in the pinch rollapplication. First, an increase in hardness wouldrequire a lower tempering temperature to be usedduring manufacturing. In service, the amount ofheat generated at the surface of the pinch roll couldapproach this temperature and thus the pinch rollwould be more susceptible to softening. Second, anincrease in hardness would have adverse effects onthe toughness of the material, and consequently thepinch roll would be less resilient to impact loading.
The wear resistance of the Bemcalloy materialcould have been improved by increasing the amountof primary ferritic carbides in the matrix.Increasing the cooling rate after centrifugal casting
can increase the amount of primary ferritic carbides.However, the ferritic carbides are often present atthe eutectic cell boundaries. A high concentration ofthe ferritic carbides at the boundaries would alsodecrease the overall toughness of the material bypromoting brittle fracture along the boundariesunder impact loading.
In contrast to the above methods, Niobium wasselected as the alloying element to increase the wearresistance. Niobium has the ability to form hard,primary carbides that do not form at the eutectic cellboundaries and, therefore, do not have a negativeeffect on toughness. Also, Niobium when used asan alloying element in gray iron has been found toreduce the size of the eutectic cell, therebyincreasing strength [4].
Furthermore, Niobium also has advantages overother strong carbide formers because it does notsignificantly affect the inoculation process [4].During inoculation, non-Niobium metastablecarbides are formed, which upon decompositionprovide free carbon to nucleate graphite. Somecarbide formers dissolved in the melt duringinoculation may decrease the power of themetastable carbides to nucleate graphite. In thecase of Niobium, however, this problem is notcritical since at usual inoculation temperatures mostof the Niobium is already combined as primaryNiobium carbides. Any Niobium remaining aftersolidification will remain as a solute or precipitateout in the solid state as Niobium carbide.Consequently, the formation of graphite (necessaryfor pick-up resistance) is unaffected.
After the primary stages of processing, theBemcalloy C-141 material is processed similar tothe Bemcalloy C-1 material. The heat treatmentprocess of oil quench and tempering is used toproduce a microstructure similar to that of theBemcalloy C-1 material with the only variancesbeing a slightly refined (smaller) eutectic cell andthe presence of highly dispersed Niobium carbidesthroughout the tempered martensitic matrix.
A representative Bemcalloy C-141 pinch rollmicrostructure is shown in Figure 3.
Figure 3. Representative photomicrograph (250X) ofheat-treated Bemcalloy C-141 pinch roll material.
As in Figure 2, the microstructure was producedvia oil quenching and tempering. The hardness is62 HSC. The dispersed Niobium carbides arehighlighted with dark arrows.
The Niobium carbides in the temperedmartensitic matrix are responsible for the increasedwear resistance of the Bemcalloy C-141 material.The niobium carbides provide increased wearthrough two ways. First, they resist abrasion due totheir extremely high hardness. Second, duringservice, the Niobium carbides can adjust in therelatively softer martensitic matrix so as todistribute the load more evenly across the entiresurface, thus minimizing the possibility ofdislodgment.
In addition to the Niobium carbides that facilitatethe wear resistance, the other properties of theBemcalloy C-141 material are effected by theremainder of the constituents in the microstructure.The flake graphite is present to provide thermalstability and natural lubricating characteristics thatprovide the resistance to pick-up. The temperedmartensitic matrix contributes to the overall strengthand toughness, as well as providing an increasedhardness for resisting wear. Furthermore, theBemcalloy C-141 material maintains the through-hardening capability, excellent thermal conductivityand thermal stability.
It is this microstructure -- a product of theoverall composition, primary processing techniques,and heat treatment methods -- that makes theBemcalloy C-141 material capable of meeting theincreasing productivity and quality demands of thecoiler pinch roll application.
IN-SERVICE DATA
The preliminary performance data for theBemcalloy C-141 pinch roll material showsignificant improvements over existing pinch rolltechnologies.
A Bemcalloy C-141 top pinch roll showedsignificantly improved wear resistance over existingpinch roll technologies as measured by lineal feetrolled per stock loss. As expected, the BemcalloyC-141 demonstrated a resistance to pick-upconsistent with that of the Bemcalloy C-1 material.Table 3 summarizes the performance results of theinitial campaign of the Bemcalloy C-141 top pinchroll, as compared to the average of existingtechnologies through several campaigns.
Table 3. Performance results of various top pinch rolltechnologies.
TechnologyLineal Feet
Coiled / 0.001”Stock Loss
Pick-up
Carburized 54,371 Significant
Bemcalloy C-1 57,526 Minimal
Bemcalloy C-141 83,604 Minimal
The Bemcalloy C-141 material used in the trial toppinch roll was processed as described above andused in service at a hardness of 62 HSC. The datafor the Bemcalloy C-1 technology includes pinchrolls in the hardness range of 62-65 HSC.Although the Bemcalloy C-141 technology has notyet been applied to bottom pinch rolls, aperformance improvement consistent with those fortop pinch rolls would be expected.
ACKNOWLEDGEMENTS
The authors are grateful to Xtek, Inc. andShenango Industries, Inc. for permission to publishthis paper. The authors are also grateful to theirfellow employees of these two establishments, toonumerous to name, who contributed their valuableknowledge and expertise of the subject matterincorporated into this paper.
REFERENCES
1. Iron Castings Handbook, Iron Castings Society,Inc., 1981, pp.517-526.
2. J.M. Senne, “An Overview of Coiler Pinch RollTechnology,” 39th Mechanical Working andSteel Processing Conference Proceedings, ISS,Vol. XXXV, 1998, pp.496, 497.
3. “ASTM A-247,” Annual Book of ASTMStandards, Vol. 03.01, 1994, pp.113-114.
4. F.T. McGuire et al, “Gray Iron,” MetalsHandbook, Vol. II, 8th Ed., 1964, pp.359-363.
5. C.H. Castello Branco and E.A Beckert,“Niobium in Gray Cast Iron,” NiobiumTechnical Report, NbTR-05/84, March 1984,pp.1-3.