PROCESS LINES for stainless steel production LINES for stainless steel production 2 3 CONTENTS...

PROCESS LINESPROCESS LINESfor stainless steel production

STEELWORKS

Production of stainless steel has steadily risen since1990. Traders and producers alike forecast that in2010 some 30 million t of stainless steel will be sold.

Around 75% of all stainless steel products are flatproducts. Current production breaks down into 55%austenitic grades, approx. 41% ferritic and 4% otherstainless steel grades.

So far, high-alloy stainless steels were overwhelminglymanufactured by the large industrialized countries, butmeanwhile also smaller steel producing countries havestarted making these grades.

Steel producers face increasing competition on theglobal market.

To meet this challenge it is crucial to apply suitabletechniques and process lines when producing thesequality products and to achieve low production costs.

SMS Demag has developed process lines that allowfor the availability of raw materials, infrastructure andenergy supply so that high-quality materials can beproduced at low cost.

A major factor in contributing to this approach is ourpermanent, close cooperation with renowned stainlesssteel manufacturers such as ThyssenKrupp, Arcelor,Posco, Acerinox and Yieh United.

The rise in stainlesssteel production.

Stainless steel productionSMS DEMAG

PROCESS LINESPROCESS LINESfor stainless steel production

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3

CONTENTS

2 3 Introduction

4 5 The converter process

6 7 The plant layout

8 13 The converters

14 15 The drive

16 17 The lances

18 19 Converter replacement

20 21 SMS Demag Automation

22 23 Technological processcontrol system TPCS

AOD-L CONVERTER

As a rule, stainless steel production in anAOD-L converter consists of the four phas-es decarburization, reduction, desulfuriza-tion and fine-tuning.

Decarburization involves blowing oxygeninto the melt through the side nozzles andthe top lance. Additionally, as the carboncontent falls, inert gas is directed throughthe nozzles and the lance to reduce thecarbon monoxide partial pressure.

The mix is adjusted to the metallurgicalrequirements in the course of the process.Significant here is the aim of reducing thecarbon content as far as possible whileachieving the lowest possible level ofchromium slagging. Typical final carboncontents for austenitic grades are around0.015%.

Reduction starts when the final carbon content is reached. It serves to recover thechromium from the slag generated duringthe decarburization process. Experienceshows that reduction recovers some 97 -99% of the chromium.

The necessary desulfurization can beachieved by adding extra ferrosilicon andlime at the same time as homogenizing the bath with argon.

Desulfurization in the AOD-L converterguarantees sulfur concentrations of lessthan 0.005%.

Finally, the fine-tuning phase ensures therequired steel composition and the castingtemperature are achieved.

Process lines.


THE CONVERTERPROCESSTHE CONVERTERPROCESS

4

MRP-L CONVERTER

Stainless steel production in the MRP-Lconverter consists of two phases, decar-burization and reduction.

Contrary to the AOD-L process, decarbur-ization here already ends at approx. 0.25 -0.35% C. (This carbon content is roughlyequal to the carbon balance at normalatmosphere, a temperature of approx.1,720° C and a chromium content of 18%.)

Reduction starts as soon as the target car-bon content is attained. That requires usinga reduction agent such as ferrosilicon.

Rinsing with inert gas adds ferroalloys toachieve the corresponding steel analysis.

To reduce the carbon content to lower val-ues it is necessary to decrease the CO par-tial pressure. This happens in the down-stream VOD plant.

A corresponding reduction of the CO partialpressure in the MRP-L converter wouldrequire an increased consumption of inertgas, increasing the production costs perton of steel.

Pig ironroute

Blastfurnace

5

PROCESS LINES

Process lines are divided into DUPLEXlines (single-stage decarburization) and TRIPLEX lines (two-stage decarburization).

The material used in both cases can be either pig iron or scrap.

SMS Demag uses the AOD-L converter forDUPLEX lines, and the AOD-L or MRP-Lconverter with downstream VOD plant forTRIPLEX lines.

AOD-L – Argon oxygen decarburization with lanceMRP-L – Metal refining process with lanceDDD – Desulfurization, desiliconizing, dephosphorization

Scrap route

Pig ironroute

Scrapyard

Blast furnace

EAF (AC)

MRP-Lconverter

AOD-Lconverter

Continuouscasting plant

VODvacuum plant

DDD

DDD

FerriteAustenite

FerriteAusteniteSuperferrite

Designing the layout of a stainless steelworks starts by determining the productrange, the annual output and the mosteffective process line for these purposes.It is chiefly the material flow that dictatesthe arrangement of the various machineunits.

ARRANGEMENT

We offer two installation options for theconverter plant.

Freestanding converter in the steel-works buildingConverter between rows of columns

Which arrangement is most suitabledepends on the type of converter, the useof blow lances and sub-lances as well asthe building or crane track heights.

Frequency of CONVERTER REPLACEMENT

While the service life of the fireproof liningin an MRP converter is approx. 1,200heats (today, with slag splashing, even upto approx. 3,000), an AOD converter mustbe replaced after approx. 150-200 heats.

To ensure a minimum production stop-page, the complete AOD converter is usu-ally replaced.

If the converter is installed betweencolumns, it is replaced using a specialchange carriage, while freestanding con-verters are replaced using the crane.

Converter betweencolumns.


THE PLANT LAYOUTTHE PLANT LAYOUT

6

Freestandingconverter.

7

Freestanding converter.

Converter betweencolumns.

CONVERTER TYPES

Converter vessel shapes have in recentyears changed in line with metallurgicalknowledge.

Today, mainly AOD and MRP convertersare used in stainless steel production.

The differences between the two convert-er systems are reflected in:

the way the oxygen and/or inert gas isintroduced;the design of the vessel;the reaction volume resulting from thevessel shape.

The AOD-L method involves blowing mostof the oxygen for the decarburizationthrough the side nozzles. The top lance isonly used to support the process in thestarting phase of decarburization.

Conversely, the MRP-L method introducesthe oxygen only through the top lance. To ensure an optimal mixing of the metalbath, inert gas is injected through the con-verter bottom.

It is usual today for AOD converters toreach sizes of 180 t of liquid steel. Just aslarge are MRP converters that have alsoincreased in size from 5 t to 170 t.

The specific volume (ratio of inner volumeof the newly bricked-up converter to thetotal tap weight) is currently approx. 0.55 - 0.6 m3/t for AOD converters andapprox. 0.6 m3/t for MRP converters.

AOD-L converter. MRP-L converter.


THE CONVERTERSTHE CONVERTERS

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AOD 1969

- Low gas content- Low sulfur content- High yield

Development of the blowingprocess.

9

Development ofAOD converters.

Development ofMRP converters.

CLU 1973

- Bottom blowing withoxygen-steam mix

- High chromium slagformation

- High Si consumptionfor reduction

- High hydrogen content

Combined blowing 1977

- Bottom and top blowing with 02

- Lime injection- Post-combustion- Very high rate of

scrap use

KMS converter 1978

- Bottom and top blowing with 02

- Coal injection and/or scrap pre-heating

- Lime injection- Post-combustion- Very high rates of

scrap use- Large quantities of

valuable offgas

MRP 1983

- High mixing energy- Low carbon content- Low gas content- Very low sulfur

content- Fewer inclusions

180 t 150 t 120 t 80 t 20 t 15 t 10 t 5 t 15 t 35 t 50 t 75 t 100 t 170 t



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Frequent replacements demand easy-to-handle converter vessels. That is whySMS Demag fits its AOD converters witha top cone attached by a flange. Thismakes it easier to break off the wornbrickwork. It is only natural that the hightemperatures generated during steel con-version cause extreme strains.

SMS Demag always uses heat-resistantsteels of grades P 355 NH or P 460 NH forthe vessel jacket and the trunnion ring.These materials have repeatedly provedtheir worth in practical operation.

The SIDE NOZZLES of the AOD converter

Shell-type gas nozzles are used to injectoxygen into the AOD converter. They con-sist essentially of a copper inner tube anda stainless steel tubular shell.

The oxygen is injected through the coppertube. There is a circular gap between theinner tube and the shell, where AR / N2protective gas is channeled to cool thenozzle. The length of the nozzle matchesthe thickness of the brickwork.

Depending on the converter size, a differ-ent number of nozzles is installed. Theirspecial arrangement and operation undercorresponding pressure ensures the rightvolume throughput for an adequate mixingof the bath and decarburization.

Top graphic.Hardness properties.

Bottom graphic.Shell-type gas nozzles

Arrangement of thenozzles.

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The TUYERE BRICKS of the MRP-L converters

To achieve a homogeneous compositionand even temperature in the melts, inertgas in the form of argon or nitrogen isinjected into the melts through the con-verter bottom. It takes special tuyerebricks that are fitted in the brickwork ofthe converter bottom for this purpose.Here again, the number of these bricksand their arrangement depends on theconverter size. Tuyere bricks are available in single or multiple-tube versions.

Bath temperature in the converter:approx. 1720 °C.

Tuyere bricks.

1720 °C



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TRUNNION RING TYPES

To support the converter in the blowingposition, SMS Demag uses two types oftrunnion ring. What determines the ver-sion to be used is the installation situationof the converter in the blowing stand andthe associated options for removing theconverter.

Closed trunnion rings without water cool-ing are used for converter arrangementsdirectly accessible to the crane.

Contrastingly, U-shaped, water-cooledtrunnion rings are ideal for convertersinstalled between columns. The watercooling prevents the trunnion ring fromexpanding due to the inevitable tempera-ture differences between the outer andthe inner flange.

FIXING TYPES

Depending on the converter design, sizeand installation situation, SMS Demagoffers various types of fixing systems. Thepreferred way to fix converters in U-shaped trunnion rings is with manual-release hinged bolts. However, AOD con-verters in closed trunnion rings can befixed with hinged bolts or with specialSMS Demag hydraulic clamping devices.

The hydraulic clamping devices featurethree trunnion pins welded to the convert-er and provided with special sliding blocks. These sliding blocks are insertedinto the bearings on the trunnion ring andfixed in position by hydraulically drivenclamps. To remove the converter, theclamps are opened and a crane lifts thevessel out of the trunnion ring.

Closed trunnion ring. U-shaped trunnionring.

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Hinged bolts.

Hydraulic clamping device. Tensioning elements.

Given that it only takes 60 minutes tochange the converter, this fixing type isvery popular.

MRP converters designed as replaceableconverters can be fixed with tendons andload pin.

Due to the injection of process gasesthrough the side nozzles and the resultingbath movements, extreme dynamicstrains and vibrations act on the AOD converter drive.

Consequently, the tooth flanks of the drivewheels are subject to severe micro-move-ments with an extremely thin lubricationfilm, which in turn leads to early wear.

Now available is a new system for fixingthe bull gear against the pinions. A toothed segment of a blocking armengages with the teeth of the bull gearand blocks it in the blowing position. Thismeans the forces acting on it are spreadover several teeth of the bull gear, so thatboth the bull gear and the subsequentgear units are under less strain, minimiz-ing wear.

Equally ingenious is the torque support ofthe new drive. The entire drive is support-ed as a riding unit on the drive journal. Thereaction forces resulting from the drivingtorque are channeled through the housinginto the torque support and from there inthe form of horizontal forces into the foun-dations.

Closed blocking system.

Open blocking system.


THE DRIVETHE DRIVE

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A converter tilting drive onthe Hilchenbach shop floor.

Converter tiltingdrive.

15

Lance tip of a blowinglance.


THE LANCESTHE LANCES

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THE SUBLANCE

As a result of today’s increasing automa-tion of the blowing process, sublances areused more and more in stainless steel pro-duction for measuring temperatures andtaking samples.

The key advantage of sublances is thatthey precisely measure the temperatureand the chemical composition of the meltprior to the start of the reduction process.The values measured indicate the degreeof chromium slag formation, which is cru-cial to the subsequent process control.

It is no longer necessary to turn the con-verter to a horizontal position for tempera-ture measurement and sample taking. Thisreduces the tap to tap time.

Because the point in time of sublancemeasurement in stainless steel productionis not as important as it is in the LDprocess, sublances are often installedtogether with blowing lances on a mobilecarriage.

The same opening can be used for thesublance as for the blowing lance.

THE BLOWING LANCE

Standard oxygen lances like those used forLD converters are also ideal for AOD-L andMRP-L converters. Although it was usualin the past to use almost exclusively single-hole nozzles in blowing lances forthe AOD-L process, here too a trend isemerging toward multiple-hole nozzles.

The distance between the blowing lanceand the bath surface is significantly greaterthan it is in the LD process. That meanslower deposits of steel and slag on thelance so that the service life of the blowing lances is much higher than it is inthe LD process.

Blowing lances for AOD-L converters arereplaced on average after about 800 heats.

Blowing lance and sublance.

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REPLACEMENT OPTIONS

The faster the changeover takes place, theshorter the production stoppage. We took thisinto account right from the start of developingour converter fixing systems. If the converteris installed in a freestanding way in the plant,the fixing is released and the crane lifts theconverter out of the trunnion ring, transports itto the wrecking stand and puts it down there.Then the crane picks up the newly supplied,pre-heated converter and places it in the trun-nion ring.

If the converter is installed between columns,the changing carriage is driven underneath the converter, the hinged bolts are releasedand the hydraulic lifting system on the chang-ing carriage raises the converter out of the U-shaped ring and into the reach of the crane.Then the crane lifts the converter off thechanging carriage and transports it to thewrecking stand.

The new converter is installed in reverseorder. Subsequently, the changing carriage caneither be driven to a parking bay, or the cranecan pick it up and place it in a parking position.

THE VALVE STATION

The valve station controls the process gasesthat are injected from the sides or through thebottom of the converter. Also integrated hereare the process controls for the blowinglances.

Converter replacementwith crane.

Converter replacementwith changing carriage.


CONVERTER REPLACEMENTCONVERTER REPLACEMENT

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�Transporting the changingcarriage using the crane.

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Arrangement of thecontrol valves.

�P & I diagram.

�

Container with completecontrol station.


SMS DemagAUTOMATIONSMS DemagAUTOMATION

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Automation systems in modern steel-works are arranged in various levels ofhierarchy. Building on a long experience inconstructing steelworks, SMS Demag hasdeveloped a special automation concept.

The automation system developed for theAOD/MRP process is based on levels 0 – 1, (basis automation) for process con-trol and visualization as well as on level 2(technological process control system,TPCS) for optimizing the technologicalprocess.

Basic automation

The basic automation system uses a SCADAsystem. This manages, documents andvisualizes plant-specific and technologicaldata. The visualization level providesaccess to plant-specific data stored in thereal-time database as well as to technolog-ical data stored in a relational database.

Process-related data such as measured values,process events,process target values from tables,process target values calculated frommetallurgical process models

are therefore available for quick accessby all connected systems.

SMS Demag automation system with integrated process tracking (tracker) andcommon visualization (Common HMI).

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Hierarchy of the SMS Demagautomation concept.

Automation structure

Production planning

Production coordination

Operator guidance

Process optimization

Process visualization

Melt tracking

Process control &

Instrumentation

Other computer systems

Process control and instrumentation

SCADA system

Relationaldatabasesystem

Metallurgicalprocess tracking

Processmodels

Real-timedatabase

Commonvisualization

Data transfer

Process tracking

Reporting

Model interface

Technological control system


Technologisches Prozess SteuerungssystTPCSTechnological process control system TPCS

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The core of TPCS is the mathematicalAOD/MRP process model developed bySMS Demag. It is based on the theory ofthe critical point of the decarburizationreaction. The model supports extensiveand fast decarburization, minimized treat-ment time and improved chromium slag-ging. That also saves reduction agentsand process gases. Additionally it improvesthe thermal operating conditions of theconverter and increases the durability ofthe refractory used in the converter ves-sel. Determining the critical point of thedecarburization reaction of each meltmakes it possible to forecast the processincluding all technological process stages.

The process forecast calculation coversthe entire process technology. It startswith the pre-treatment data of the meltsuch as temperature, steel/slag weight,last current analysis and ends with com-pletion of the melt.

The various treatment sequences of themelt are checked to ensure the targetanalysis, the required tapping tempera-ture, the target steel weight and theplanned time for transfer to the down-stream machines.

Our dynamic process model optimizes themelt treatment. Moreover it checks thechemical reactions in the melt.

The carbon and oxygen contents as well as the energy balance are determined at10-second intervals. That is how the sys-tem uses the melt starting data to calculatethe current carbon and chromium contentin the bath along with the current tempera-ture.

Exhaust gas analysis measurement helpsdetermine the rate of decarburization fromthe carbon content in the exhaust gas. Thebasis for assessing the oxygen quantitiesfor oxidation is provided by the oxygen bal-ance. It is used to calculate the oxidationrate for chromium and the total chromiumloss. That also applies to the elements Mn and Fe.

The main task of the process gas controlfunction within the dynamic process con-trol system is checking the quantity of O2 to be introduced into the melt. This en-sures that decarburization takes placeunder optimum thermodynamic and chemi-cal conditions in the shortest possible time.

Si/Al-

oxida-

tio

phase

Main

decarburization

Time

Dynamic

phase

Reduction

phase

AOD process phases

Stages in the process

� Si/Al oxidation

� Decarburization phase

� Reduction phases

Stages of decarburization

� Main decarburization

� Dynamic decarburization

Broken down into critical

points

Process models.

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Optimum decarburization conditions areachieved when the oxygen throughflow atthe moment of critical carbon content(critical point) is reduced in such a waythat the formation of CO reaches its opti-mum and a sufficient reaction dynamic ismaintained.

Furthermore the model determines whatmaterials must be added to aid reductionand desulfurization as well as for alloyingand to adjust the temperature. Throughoutthe course of treatment, the operatorreceives information on the calculated tar-get values as well as the current valuesachieved. Then, at the end of a melt treat-ment, the system compiles a completereport listing all activities and consumptionfigures.

SMS DEMAG AG

Steelmaking and Continuous Casting Technology Division

Eduard-Schloemann-Strasse 4 · 40237 DüsseldorfP. O. Box 230229 · 40088 Düsseldorf

Phone: +49 (0) 211 881-0 Telefax: +49 (0) 211 881-6692

E-Mail: [email protected]: http://www.sms-demag.com

Germany

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PROCESS LINES for stainless steel production LINES for stainless steel production 2 3 CONTENTS...

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