AUTHOR: Gerhard RichterBRAUN Maschinenfabrik GmbH & Co. KG

In order to respond to the increasingly demandingrequirements of their customers and to remain

competitive, steel mills have been facing pressures toadapt their operations and production facilities to reduceproduct costs, increase production flexibility and improveproduct quality and reliability.

At various stages during hot rolling the different rolledproducts – blooms, billets, bars, rods, sections, slabs,plates and sheets – need to be cut. While the cross-sections of these products are being reduced during eachrolling pass, the lengths are being increased accordingly.Thus, it becomes necessary to cut the products to shorterlengths suitable for the next steps of production oreventually for packing and shipping. Furthermore, itmight also become necessary to cut off the front and tailends or to take short sample pieces.

Friction saws (with metallic saw blades), shears andflame cutters (plasma cutters), are conventionaltechniques to perform these cutting jobs. Thedisadvantages associated with these cutting methods –poor cutting quality requiring deburring or evenadditional cutting afterwards, the need for extensivemaintenance, high energy consumption, noise and lowflexibility regarding changes in the product mix of the mill– have become less and less tolerable.

KEY FEATURES AND ADVANTAGES OFABRASIVE CUTTING Abrasive cutting features the following substantialadvantages:

` An excellent quality of the cut surface can be reliablyachieved (thanks to the consistent self-sharpening ofthe cutting wheel): the cuts are straight, precise, cleanand without hardening of the cut surface. Thus,subsequent machining processes to adjust thematerial ends (deburring, edge trimming, etc) can be

Modern abrasive cutting machines are increasingly being used for cutting hot rolled productsduring the rolling process and for final cutting to size. Abrasive cutting is applicable to all producttypes from blooms and rods to plates and sections. The advantages over other methods are clean,top-quality cuts – requiring no subsequent deburring, reduced energy consumption, less noise andgreater flexibility.

Abrasive cutting machines in modern rolling mills

FORMING PROCESSES

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r Fig.1 Hot abrasive cutting

eliminated. Figure 1 illustrates hot abrasive cutting.All other cutting methods result in a more or less badcutting surface – partly with a sharp, adhering burr(friction saw), partly with distorted, uneven surfaces(flame cutting), or with extreme deformation of thematerial ends (shear). Bars cut with a friction sawrequire additional refining of the cut product (such asdeburring) or would otherwise compromise the qualityof the rolled product. This, however, means additionalcosts and a devaluation of the product. Furthermore,with increasing wear of the saw blade or the shearknife, the quality of the cut progressively gets worse,whereas abrasive cutting ensures a consistently goodcutting quality over the whole service life of thecutting wheel.

` Abrasive cutting is suitable for an extremely widerange and grade of materials, from unalloyed carbonsteels up to high-alloyed, high-carbon steels, titaniumand nickel alloys, as well as other special steels andnon-ferrous alloys (see Table 1).

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A friction saw has its limitations athigher material temperatures andwith higher alloyed, hardermaterials, and a shear is unsuitablefor cutting of cold materials ormaterial of a large cross-section.Abrasive cutting, however, is equallysuitable for cutting cold, warm orhot materials and represents a realuniversal cutting process. This is ofspecial importance for hot rollingmills where interruptions in theoperation of the mill with the rolled products cooling down arelikely to happen.

` Abrasive cutting is a high-performance stock removalprocess. Thanks to a high abrasion rate, abrasivecutting enables rapid cutting. Only shearing is faster,but this has the significant disadvantages indicatedabove. Since well-designed abrasive cut-off machinesallow worn cutting wheels to be changed within 3–5 minutes, they are the ideal cutting facilities forcontinuous production processes such as hot rolling.The throughput that can be achieved with anabrasive cut-off machine is very high. Depending onthe temperature and on the size of the material to becut, a well-designed abrasive cut-of machine allowsspecific cutting rates of 40cm2/s or higher for hotcutting to be achieved and is virtually independent

of steel grade. Figure 2 illustrates the relationshipbetween temperature, dimension and speed.

` Compared to a friction saw, the noise level of anabrasive cut-off machine is substantially lower. Theaverage noise level is between 80 and 85dB(A). Thepeak noise during certain cuts can be over 90dB(A) –still significantly lower than the noise levels of afriction saw which are well above 100dB(A). Forthese reasons, it is not necessary to install anabrasive cut-off machine inside a sound-proof booth.

` Thanks to its fully automated operation, the abrasivecut-off machine matches the operational requirementsof continuous production processes such as hotrolling. Specific cutting programs are stored in the cut-off machine’s PLC which is interfaced with the rollingmill control system. By this means, all necessaryproduct data, as well as the cutting requirements, areautomatically provided to the PLC and the propercutting program is also selected automatically.

` As far as the consumption of electric energy and otherutilities is concerned, abrasive cutting is highlyefficient. Unlike sawing, abrasive cutting is a dryprocess that does not require the application of liquidcoolant during cutting. Thus, in addition, collecting,cleaning or disposal of used, polluted coolant is notnecessary. Whereas flame cutting consumes hugeamounts of gas (usually the burners are on at alltimes when the rolling mill is operating), abrasive cut-off machines consume the bulk of electric energyduring the short times of cutting only – immediatelyafter completion of a cut, the power drops to a no-load condition.

Material group Properties Examples Typical material grades1 unalloyed, medium to high carbon steels, St 52–3, Ck35

carbon content construction steels2 low-alloyed, engineering steels, 16MnCr5

low carbon content railway steels,hardenable steels

3 low-alloyed, heat-treated steels 34CrNiMo6medium carbon content

4 low-alloyed, high carbon content bearing steels 90MnV85 high-alloyed, medium carbon content stainless steels X2CrNi1896 high-alloyed, high carbon content tool steels X210Cr127 high-temperature resistant titanium alloys LT31

light metal alloys8 high-temperature resistant hard metals nickel alloys NiCr5Fe9 other non-ferrous metal alloys brass, bronze CuZn20Al

r Table 1 Classification of materials typically cut by means of abrasive cut-off machines

q Fig.2 Efficient cutting ratesin relation to materialtemperature and workpiecedimension

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FORMING PROCESSES

raw material analysis, weighing andmixing, filling and adjusting, pressing,curing under temperature,machining/balancing, and finalquality checking. In order to be able tooffer cutting wheels specificallytailored for particular applications,thereby keeping the end user’s overallcosts to a minimum, the leadingcutting wheel manufacturers – all ofthem partners of BRAUN – focus oncontinuous research and development.

For the abrasive cutting process, itis vital to keep the peripheral speedof the cutting wheel constant.

Depending on the cutting requirements and thespecification of the cutting wheel, peripheral speedsbetween 80 and 100 m/s are used. Since the diameter ofthe cutting wheel reduces during the cutting process, therotating speed of the cutting shaft must be increasedaccordingly. BRAUN cut-off machines are equipped withan automatic wheel wear compensation system thatmeasures the actual diameter of the cutting wheel aftereach cut. Subsequently, the speed of the frequency-controlled drive motor is adjusted automatically. Thisspecial wheel wear compensation system is also used forautomatically positioning the edge of the cutting wheelto the shortest possible distance to the material, therebycontributing to fast cutting cycles. Furthermore, thenumber of cuts still possible for a certain material sizewith the actual wheel diameter is automaticallycalculated; this allows pre-preparation time for changingthe worn cutting wheel.

Depending on the temperature of the material to be cut,abrasive cutting can be distinguished between:

` Cold cutting (material temperatures up to ~ 200°C)` Warm cutting (material temperatures from ~ 200 to

~ 700°C)` Hot cutting (material temperatures from ~ 700°C to

~1150 °C)

As shown in Figure 2, higher material temperatures allowusers to take advantage of higher specific cutting rates,which shortens the cutting cycles further.

Depending on the cutting application in the rolling mill,the following cutting principles are typically applied:

` Chop stroke cutting The cutting wheel is movedto the material in a chopping motion with a radial in-feed into the work piece (see Figure 4). Generally,this principle is used for cutting of single round,squared or nearly squared cross-sections. With a a

r Fig.3 Basic structure of a cutting wheel

r Fig.4 Chop-stroke cutting principle

BASIC TECHNOLOGICAL CONCEPTThe tools utilised for abrasive cut-off machines are thecutting wheels. These high-performance tools areproduced in a multi-stage manufacturing process. Theindividual raw material components forming the cuttingwheels are the grain – regular aluminium oxide, specialaluminium oxide or zirconium aluminium oxide – thebonding and fibre glass fabric used for reinforcing thewheels (see Figure 3).

The actual composition of the cutting wheel depends onthe specific application. Certain criteria such as materialtemperature, material shape and size, type and grade ofmaterial play a dominant role. The cutting wheel mustensure high stability, high bursting speed, an excellentcutting quality and a maximum performance over its entireservice life. To achieve this goal, the manufacturing processis highly sophisticated and comprises the following stages:

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proper design of the cut-off machine, however, alsorectangular cross-sections or narrow layers of roundor squared cross-sections can be cut.

` Traverse cutting The cutting wheel is movedhorizontally across the material to be cut, in a singlecutting-off stroke (see Figure 5). This principle isgenerally used for cutting wider layers of round,squared and nearly squared cross-sections, but alsofor cutting flat products such as rectangular slabs,plates or sheets.

The cutting principle applied for a certain application isalso reflected in the basic design of the abrasive cut-offmachine. Altogether, BRAUN has developed five differentbasic machine designs. Three of them are widely used inrolling mills, for the three major applications described inthe following sections.

CUTTING OF SLABS, BLOOMS AND BILLETSIn primary rolling mills, blooms or billets – and sometimesalso slabs – are cut in the hot condition. Predominantly,the cross-sections of the work pieces are still quite large,therefore, chop-stroke cutting is the preferred method forcutting these materials as single pieces. Due to the sizesof the materials to be cut, cutting wheel diameterstypically range from 1,000 to 1,600mm.

Ideally, the abrasive cut-off machine is equipped with ahorizontal rocker, like BRAUN’s machine type W (seeFigure 6). The in-feed motion of the cutting wheel goesdownwards, which has the significant advantage that alsorectangular work pieces can be cut with an optimumutilisation of the cutting wheel. Somewhat smaller billetscan be cut as narrow layers of two or three pieces. Thisenables a high throughput capacity even for smallercross-sections. To prevent round work pieces fromoverlapping, vertical blank holders are added to thematerial clamping system.

For downstream processes, abrasive cutting of the hotmaterials provide clear advantages. Thanks to thesmooth, clean cutting surface, stamping, marking orlabelling of the material faces are eased. Also, billetgrinding can be done with fewer problems since abrasivecutting avoids the thick burrs resulting from sawing orflame cutting, as well as the deformations of the materialends (noses) resulting from shearing.

In recent years, various blooming or billet mills wereupgraded by installing abrasive cut-off machines. Forinstance, as part of a large-scale expansion of CorusEngineering Steels’ Aldwarke primary mill in Rotherham,UK, BRAUN supplied five big abrasive cut-off machineswith associated equipment for automated lengthpositioning, crop and sample cutting (see Figure 7).

r Fig.5 Traverse cutting principle

r Fig.6 Principle of chop-stroke cut-off machine, type W (with horizontal rocker)

r Fig.7 Abrasive cut-off machine, type TS 16 W (1,600mm cuttingwheel diameter), for hot cutting of blooms in the primary mill(Corus Engineering Steels, Rotherham, UK)

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CUTTING OF BARS OR SECTIONS IN LAYERSAfter the last rolling pass, rolled bars or sections alreadyhave their final shape and the work pieces are usually stillquite long when they are transferred onto the coolingbed, then cut to length while still warm before bundling.To meet the throughput requirements of the mill –despite the already relatively small material cross-sections– the products are cut in wider layers by means of traversecutting (see Figure 8). Depending on the actual sizes ofthe work pieces, typical cutting wheel diameters arebetween 800 and 1,270mm.

For the horizontal cutting motion, the traverse cut-offmachine comprises a linear-guided travel slide on whichthe cutting rocker is mounted. The actual cutting in-feedis performed by the horizontal slide. The purpose of therocker is the adjustment of its tilting angle in accordancewith the diameter of the cutting wheel getting smallerafter each cut, as well as the upswing of the cutting wheelonce the traverse cut has been completed so that thetravel slide can return to its starting position withoutblocking the layer of bars or sections on the roller table.Our machine type F represents the typical design of atraverse abrasive cut-off machine (see Figure 9).

Especially for this application, it is imperative to achievea good quality cut surface. If the cutting quality isinsufficient, as is mostly the case with a shear, the barends must be trimmed or cut once again with a separatecutting facility such as a band saw for commercialmaterial grades or an abrasive cut-off machine for morespecial materials, at a later stage – either by the steel millor by the buyer of the products. Due to this fact, manysteel mills have chosen to install traverse abrasive cut-offmachines even in existing rolling mills – either in additionto an existing shear or to replace a shear. Among the steelmills that have retrofitted BRAUN’s traverse abrasive cut-off machines are Stahl Judenburg, Judenburg, Austria,Trinecké Zelezárny, Trinec, Czech Republic, OÉMK, StaryiOskol, Russia, ZDB, Bohumin, Czech Republic and SeAHBesteel, Gunsan, Korea.

CUTTING OF WIDE SLABS, PLATES AND SHEETSTraditionally, shears or flame cutters were used for hotcutting of these products. Abrasive cutting of flatproducts in the hot condition, immediately after the lastrolling stand, was only introduced a few years ago.

After the installation of three new rolling stands in theirHönigsberg plate mill, as part of an overall modernisationproject, the Austrian special steel producer BÖHLERBleche was searching for the cutting technology thatshould replace the existing shear. This shear was apermanent source of problems and was unable to meetthe increased quality requirements for cutting the hot

FORMING PROCESSES

r Fig.8 Principle of traverse cut-off machine, type F

r Fig.10 Principle of gantry-type traverse abrasive cut-off machine,type FP

r Fig.9 Traverse cut-off machine, type TS 12 F (for max. 1,270mmcutting wheel diameter, cutting width up to 1,250mm), forwarm cutting of bars in layers (Jiangyin Xing Cheng SpecialSteels, Jiangyin City, China)

plates and sheets. The cut ends of the products had to betrimmed in the cold condition by means of band saws inthe plate conditioning shop. The task was to eliminate thisadditional operation. After a thorough evaluation BÖHLERBleche decided in favour of abrasive cutting technology.

It was clear that a traverse abrasive cut-off machine hadto be used. Due to the widths of the plates and sheets tobe cut (up to 2.3m cutting width) however, the machinedesign had to be extensively altered. As a result, BRAUNdeveloped a gantry-type traverse abrasive cut-off machine a

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FORMING PROCESSES

SPECIAL REQUIREMENTS FOR MACHINE DESIGNIn order to achieve optimum results it is essential that theabrasive cut-off machine utilised for a specific applicationis properly designed. Furthermore, both the cut-offmachine and the cutting wheel must be matched to thematerial to be cut. All three elements together – cut-offmachine, cutting wheel and material to be cut – form thecutting system.

The abrasive cut-off machine is one of our mainproducts. Therefore, it enjoys an extremely high prioritywithin company policy. Thanks to more than 40 years’experience, ongoing research and development, as wellas a close collaboration with the leading manufacturersof cutting wheels, we have been able to establishextensive know-how of the ‘cutting system’. As a result,BRAUN is in a position to offer state-of-the-art machinedesigns to customers, perfectly tuned to their specificapplications. Furthermore, each abrasive cut-offmachine is functionally tested at our workshops beforeshipment (see Figure 12).

These ‘soft facts’ also contribute to the big differencebetween a state-of-the-art high-performance cut-offmachine and a poorly designed cut-off machine.BRAUN has already replaced existing cutting machinesfrom other manufacturers – simply due to aninadequate design of the existing machines which werenever capable of performing properly.

CONCLUSION AND OUTLOOKDue to the increasingly competitive market environment,steel mills will remain under consistent pressure to furtherimprove their production facilities. Even though thevarious cutting processes at different stages during rollingoperations are still widely regarded as minor processes, ithas become evident that cutting does indeed have asubstantial impact on the performance of the mill, on the quality of the final products and on the overallproduction costs.

Nowadays, most modern hot rolling mills are equippedwith abrasive cut-off machines from the very beginning,and in recent years more abrasive cut-off machines havealso been retrofitted to existing rolling mills. Due to themany advantages of abrasive cutting technology thistrend will continue. The implementation of an abrasivecut-off machine in an existing mill, however, doesrequire specialist know-how. MS

Gerhard Richter is Vice President – Steel Cutting &Grinding Machines Division,BRAUN MaschinenfabrikGmbH & Co. KG, Vöcklabruck, Austria

CONTACT: g.richter@braun.at

r Fig.11 Gantry-type traverse machine, BÖHLER Bleche,Mürzzuschlag-Hönigsberg, Austria

r Fig.12 Pre-assembly and functional testing of cut-off machines atBRAUN prior to shipment

(machine type FP). The horizontal travel slide with therocker is moved overhead, in linear guides mounted ontoa sturdy gantry structure (see Figures 10 and 11). In orderto meet one very special request of BÖHLER Bleche –cutting of cast slabs in the cold condition in certain cases– this cut-off machine also features a specificallydesigned material clamping device with individuallyactuated, self-adjusting clamping elements, for safelyclamping the slabs despite their slightly rounded surfaces.

Due to the size of the cross-sections to be cut (cuttingwidth up to 2,300mm), the diameters of the cuttingwheels utilised for such applications are at least1,250mm. With this new gantry-type traverse abrasivecut-off machine, it has become possible to achieve thedesired quality of the cut plate ends and additional coldcutting of the plate is not necessary in many cases. Thus,the overall production costs could also be reduced.

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