Steeluniversity.org – An Interactive E-Learning Resource ...library.stranggiessen.at/8224.pdf · storyboard has been developed by the Christian Doppler Laboratory of Metallurgical

Steeluniversity.org – An Interactive E-Learning Resource onSteel Processing Technologies

S. Michelic, C. BernhardCD-Laboratory for Metallurgical Fundamentals of Continuous Casting Processes,Montanuniversitat Leoben, Austria

Abstract

During the past three years an interactive webbased e-learning tool has been developed by theInternational Iron and Steel Institue: Steeluniversity.org. This tool targets university studentsand in-company engineers to make the steelmaking process more comprehensive. Several modulesof the whole steelmaking process chain have already been published in the past and have receivedincreasing international attention. The general idea of Steeluniversity.org is described togetherwith the contents of its main modules. Detailed attention shall be drawn to the background of thecontinuous casting module, which has recently been published. The fundamentals for numericallydescribing the continuous casting process, based on recent metallurgical research, will be illustrated.

1 Introduction

The steel industry is currently confronted with a decline in the number of students of metallurgy ormaterial science, as well as graduates seeking employment in the industry. Unless a greater amountof young and talented motivated engineers and scientists join the steel industry, it will be difficult toremain innovative and competitive. Moreover, it is observed that many graduates do not show sufficientknowledge of ferrous metallurgy to quickly become supportive. Therefore, steel companies around theworld are spending increasing amounts of time and money proviniding training programmes for theirnew recruits.

With the intention of ensuring a sustainability of ferrous metallurgy knowledge in academia andindustry, the International Iron and Steel Insitute (Iisi) has initiated a project aimed at providing anonline, freely-available interactive e-learning resource on steelmaking, steel products and applications:Steeluniversity.org, targeted at undergraduate students, professors and in-company engineersto help fill the gap of knowledge. It offers far-reaching educational resources, assisting the develop-ment of academic expertise on steel technologies. Since Steeluniversity.org has been realised ina web-based solution, new opportunities for collaborations between universities and industry at aninternational level have opened.

E-learning modules have been and will be created, allowing a monitoring of production steps ofsteels from raw materials thorugh various processing stages to product forms and properties, togetherwith a comprehensive approach of environmental aspects and reflections on sustainability.

118 Xiii. International Students’ Day of Metallurgy, 2006 – Montanuniversitat Leoben

Table 1: Overview of the Steeluniversity.org resources

Published Resources Planned Modules

Materials Selection for Car Door Panels Basic Oxygen Steelmaking

Steels in Construction Blast Furnace

Engineering Steels Phase Transformation and Heat Treatment

Strengthening Mechanisms Recrystallisation and Grain Growth

Steel Properties Hot and Cold Rolling

Electric Arc Furnace Steels for the Energy Market / Power Generation

Secondary Steelmaking Environmental Management in the Steel Industry

Continuous Casting

Sustainability

Many resources of Steeluniversity.org have already been published, several modules are plannedand will be issued in the coming years. Table 1 gives a brief overview.

The development of all modules is based on a storyboard, providing technical content, text, images,simulations and models. The storyboard is prepared by academics, industry experts and consultantswhich have intensive contact to the topic of the selected module. The storyboard is hence coded bythe team of Matter at the University of Liverpool, or third party contracts.

In the following some of Steeluniversity.org’s modules will be described in more detail, closerattention shall be drawn to the fundamentals and basics of the continuous casting module, whosestoryboard has been developed by the Christian Doppler Laboratory of Metallurgical Fundamentalsof Continuous Casting Processes at the Montanuniversitat Leoben.

2 Published Steeluniversity.org Resources

2.1 Electric Arc Furnace

This involves making one of the same four steels that can be processed in the secondary steelmakingsimulation – a construction grade, a low S linepipe steel, an ultra-low carbon automotive strip steeland a low alloy steel. A suitable low cost scrap/Dri mix has to be selected, with a view to being ableto achieve the specified composition of the chosen grade. The materials selected have to be allocatedto appropriate scrap baskets, added to the furnace and melted down. Care has to be taken to add thecoarse and fine scrap in the right order and not to lower the electrodes too quickly in order to avoidbreakage. The learner has to decide what power settings to use, how much oxygen to blow and when.After melting and refining, addition of appropriate ferro-alloys and other materials, the learner has todecide when to tap the furnace into a ladle, ready for secondary steelmaking.

2.2 Secondary Steelmaking

A ladle of steel has to be processed though the virtual secondary steel shop which houses a ladle furnace,tank degasser, CaS-OB and recirculating degasser. Cranes and ladle cars have to be manipulatedto transport the ladle around the shop. Figure 1 shows the virtual steelmaking shop. Each of thefour grades (and a self selected grade) requires a different solution and process route. One requires

Xiii. International Students’ Day of Metallurgy, 2006 – Montanuniversitat Leoben 119

decarburisation, another desulphurisation and various alloy and microalloy additions are required. Theamounts of the additions, their timing and order have to be decided and the ladle has to be deliveredto the appropriate caster at the requested time and temperature.

Figure 1: The Virtual Secondary Steelmaking Shop.

2.3 Continuous Casting

The aim is to make a sequence of three casts of one of the same 4 grades of steel that can be madein the Eaf and Secondary Steelmaking Simulations. The virtual caster includes a two-strand slab,a four-strand bloom and a six-strand billet machines. The learner has to make several operationaldecisions including requesting the timing and temperature of the delivery of the ladles to the caster,the degree of soft reduction, the casting speed, cooling water flow rate, mold oscillation frequency andstroke and the mold powder. Once the ladle has arrived, the metal flow rate from the ladle to thetundish and then into the mold and the casting speed have to controlled very carefully in order toavoid internal and surface quality problems and break-outs. Misaligned rolls may occur and nozzlescan be replaced as deemed necessary.

2.4 Steel Properties

The learner is introduced to steel standards and specifications, with questions posed to ensure thatthe requirements of a specification have been understood and interpreted accurately and appropriatedecisions taken. This is followed by an exercise to determine the mechanical properties of a steel plateand determine whether it meets a ship plate specification. This has to be done within time limitsand at least cost. Virtual hardness, tensile and Charpy impact tests can be conducted, thus offeringsignificant cost savings for university departments, for whom real tests are no longer affordable.

2.5 Strengthening Mechanisms

The metallurgical principles controlling the properties of ferrite-pearlite steels are described, illustratedand quantified, with sections on solid solution hardening, precipitation hardening, microstructural


refinement and work hardening. The culmination of this module is an exercise to design the compositionand rolling process for a high strength, weldable steel plate for an off-shore application and then tomake 9000 tonnes in three plate thicknesses. Success is judged in terms of satisfying the property andweldability requirements with the best profitability.

2.6 Materials Selection for Car Door Panel

In this module the learner adopts the role of a materials engineer in a multi-disciplinary team whichhas to select a steel to reduce the weight of an automotive component. They have to conduct varioustests to determine an appropriate strength level, strip thickness and shape in order to satisfy dentresistance and oil canning characteristics. Other important fabrication and functional requirementsalso have to be met, including formability, corrosion resistance and weldability. Taking all these factorsinto account, a cold rolled coated steel with the required properties has to be selected. Feedback isgiven on the weight and cost reduction realised through the decisions taken. One of the candidatesteels is one of the grades that can be made in the steelmaking simulations described above.

2.7 Steels for Construction

The great variety of steels used in this largest market for steels is explored. The learner has toidentify the functional requirements, strength level and composition of steels for different constructionapplications. The major engineering design equations are used and tested to check the understandingof loads and failure mechanisms. Comparisons are drawn with other materials. Some of the fabricationand corrosion protection methods used in construction are explored.

2.8 Sustainablity

The learner explores the variety of social, economic and environmental issues associated with sus-tainability, the causes and consequences of different environmental impacts. The role of steel in asustainable world and the industry’s commitment to sustainability are described. The principles of lifecycle thinking and life cycle assessment are introduced and illustrated with examples drawn from thesteel industry, automotive and construction sectors. The aim is to give the learner new insights intosustainability that will impact their decision making in their jobs and personal lives.

3 Simulation of Continuous Casting

The aim of the simulation is to successfully sequence cast three ladles meeting specified criteria ofsurface and internal quality as well as steel cleanness. Moreover, for economic reasons, costs duringthe casting operations should be minimized.

The simulation includes three different casting machines that are used for casting four differentsteel grades – one slab (1200 × 230 mm), one bloom (250 mm 2) and one billet caster (130 mm 2).The following steel grades can be cast:

Construction steel This is a crack sensitive, relatively undemanding steel grade cast at the bloomcaster. The inclusion level can be moderate without suffering any quality problems.

TiNb ULC steel This is a sticker sensitive steel grade used for automotive body parts with a carboncontent of less than 0.0035 %. It is cast at the slab casting machine. Since cleanness requirementsare very high, the inclusion levels have to be very low.


Linepipe steel This steel is very demanding since its use for gas distribution requires a combinationof high strength and fracture toughness with extremely low levels of impurities and inclusions.Depending on the exact composition, this grade is either crack sensitive (peritectic) or stickersensitive (hypo-peritectic).

Engineering steel This is a heat-treatable low alloyed steel grade cast at high speeds using the billetcaster.

Upon starting the simulation, the user has to plan several parameters of the simulation ahead.

3.1 Simulation Options

3.1.1 Casting Speed and Secondary Cooling Rate

This first selection is also the most important one, since it influences many different parameters duringthe casting. Among many other quantities, the combination of these two parameters influences themetallurgical length, the distance from the mould to the point where the strand becomes totally solid.It is a complex quantity influenced not only by the casting speed and the secondary cooling rate butalso by the strand dimensions and the steel composition. The calculation of the metallurgical lengthis beyond the scope of the continuous casting simulation and is therefore not integrated in the model,but has been done in the forefront.

3.1.2 Mould Oscillation Settings

The use of an oscillating mould is state of the art in the continuous casting process. The oscillatorymovement of the mould reduces the friction between the mould and the strand and promotes theformation of a satisfying surface quality of the strand. Through the oscillation, mould powder that isadded on top of the strand, is drawn into the gap between mould and strand, facilitating the removalof the strand from the mould.

The oscillatory movement is influenced by two parameters: the stroke s and the frequency f . Thestroke ranges between 3 and 10 mm. An increase of the stroke, proportionally acts on the negative striptime (1) and the consumption of mould powder. The frequency, ranging between 100 and 250 cpm,acts inversely.

The negative strip time tN is the time during which the downward movement of the mould is fasterthan that of the strand, consequently the time during which mould powder is fed into the gap. It canbe described by:

tN =60

π · farccos

1000 · vc

π · f · S[tN ] = sec (1)

where f is the frequency in min−1, s the stroke in mm and vc the casting velocity in m min−1.Whilst the oscillating mould is necessary for continuous casting – lubricating the strand, this move-

ment has decreasing effects on the surface quality. The formation of so-called oscillation marks ispromoted due to the reciprocative movement of strand and mould. Oscillation marks, often the sourcefor transverse cracks, should be minimized as far as possible. Their formation is described in Figure 2,their depth can be calculated with (2). This dependency was obtained based on data published bySchurmann et al.2 The upper part of the figure shows the position of the mould varying with time.In the hatched areas (time of negative strip) the formation of the oscillation marks is promoted, asshown in the bottom part of the figure.


Figure 2: The formation of oscillation marks.1

d = 0.065 · 1.145s · (200 · 0.9s)tN [d] = mm (2)

3.1.3 Mould Powder Selection

The mould powder is mostly a synthetic slag which is added on top of the strand during the castingprocess. Since the steel is liquid, the melting powder is drawn into the gap between strand the mouldthrough the oscillatory movement of the mould. In order to satisfy quality requirements, the rightpowder has to be chosen, since the choice influences oscillation mark depth and mould powder con-sumption (influencing the strand lubrication). Several estimations for the mould powder consumptioncan be found in the literature, most of them taking the powder’s viscosity and the mould oscillationsettings into account. One estimation can be found in (3).3

Qs =A

η0.5

1vc· tN + B [Qs] = kg m−2 (3)

In (3) Qs denotes the mould powder consumption, η the powder’s viscosity in Ps, with A and B asfitting parameters, depending on the exact plant layout.

Figure 3 illustrates the situation of the mould powder distribution in the continuous casting mould.

3.1.4 Additional Parameters

Alongside the presented parameters, other critical quantities have to be selected. Firstly, the level ofthe soft reduction must be adjusted. The soft reduction is only available at the slab caster, aiming atthe minimization of the severity of centre segregation. The principle of the soft reduction is depictedin Figure 4. It can be seen that the soft reduction ideally has to take place shortly before totalsolidification (i. e. the metallurgical length).


Figure 3: Mould powder distribution in continuous casting mould.4

Figure 4: Schematic illustration of the softreduction.5

Furthermore, the ladles ordered for a continuous cast have to be ordered at the right time (i.e. whenthe previous ladle is empty) and at the correct temperature. During the casting process the ladle loosestemperature to the surrounding. The limiting temperature of the steel – the liquidus temperature –has to be particularly observed. Several approaches for calculating the liquidus temperature have beenpublished, one of them can be found in (4).6

Tliq = 1537− 78 %C− 7.6 %Si− 4.9 %Mn− 34.4 %P− 38 %S (4)

3.2 During the Simulation

The quality of the cast product is substantially influenced by criteria that have to be observed duringthe continuous casting process: the steel cleanness and the formation of cracks (interior and surfacecracks).


3.2.1 Steel Cleanness

As already indicated before, the applications of the linepipe steel require it to be very clean, i.e.to have low levels of oxide and sulfide inclusions. Since the inclusion level of the steel can only beworsened during continuous casting (not improved), great care has to be taken to limit the end-levelof inclusions. Using the tundish as a buffer will allow inclusions to deposit at the walls of the tundishand the slag layer on top. Consequently a long residence time in the tundish is most important toachieve low inclusion levels.

3.2.2 Strain Analysis Model

During the continuous casting process the strand is subjected to several deformation mechanism suchas bending, straightening, bulging and roll misalignment. All of these mechanism contribute to theacculumation of strain not only inside the solidifying strand shell but also on the surface. In orderto estimate the quality of the cast product (influenced by the amount of cracking due to the strain),several models are applied during the simulation, all taking the current status of the strand intoaccount. In these models the amount of strain accumulated is compared to a critical strain value,which is in turn dependent on the steel grade.

3.2.2.1 Estimation of Internal Cracking The solidifying front of the steel shell is subjected to tensilestrain caused the mechanisms mentioned before. The following empirical equations are used to quantifythem:7,8

εBS = 100 ·

(d

2− s

)·

∣∣∣∣∣ 1Rn−1

− 1Rn

∣∣∣∣∣ (5)

εB = 100 · FP · l3

Cb · s3(6)

εM = 100 · 1.15 · 3 · s · δm

l2(7)

εintern = εBS + εB + εM (8)

where εBS is the strain caused by bending and straightening, εB the strain caused by bulging, εM

the strain caused by roll misalignment and εintern the total strain at the solidifying front all in %; d

is the slab thickness, s the shell thickness, Ri the radii of a five-point straightening method, FP theferrostatic pressure, l the length of the roll pitch and δm the amount of roll-misalignment all in mm.

The value of εintern is hence compared to a critical strain εC which has been modeled by Won et al.:9

εC =ϕ

εm ·∆TnB

(9)

where ϕ is a constant, ε the strain rate, m and n the strain rate sensitivity and the brittle tem-perature range exponent, respectively and ∆TB the brittle temperature range. Table 2 shows typicalvalues.

3.2.2.2 Estimation of Surface Cracking In the given model, it is assumed that only transverse crack-ing can occur. Similar to the modelling of the internal cracking, the surface strain εsurf is consideredto be a sum of strains:


Table 2: Typical values for calculting εC

ϕ 0.148

m 0.0048

n 0.6201

ε ∼ 10−4

∆TB specific for composition

εsurf = εBS + εB + εM + εth (10)

εBS = 100 · d

2·

∣∣∣∣∣ 1Rn−1

− 1Rn

∣∣∣∣∣ (11)

εM = 100 · d

2·∣∣∣∣ 1R0

− 1Rd

∣∣∣∣ (12)

εth = 100 · α ·∆T (13)

where εth is the thermal strain in %, R0 the original position of the roll, Rd the deviated position,both in mm, α the thermal expansion coefficient and ∆T the temperature difference.

Similar to the internal cracking model, the calculated surface strain is compared to a critical valueof strain, specific for the steel grade. Moreover the critical strain is also a function of certain processparameters such as the oscillation mark depth. Several authors10,11 have published these dependencies.

3.2.2.3 Influence of Casting parameters on Crack Formation Figure 5 depicts the influence of thecasting speed and the cooling parameters on the acculumation of the internal strain εintern for theUlc-steel. The diagram also shows the different components of the strain, caused by bending andstraightening (εBS), bulging (εB) and roll misalignment (εM ). The critical strain εC for the Ulc-steelis also indicated in the diagram. Clearly, the accumulated strain rises significantly with rising castingspeed vc. The diagram shows that the combination of low cooling rate and high casting speed leadsto inacceptably high values of the strain, exceeding the critical strain, causing internal cracking.

In contrast, the accumulated thermal strain on the surface of the slab will increase more with highercooling rate, as shown in Figure 6. Therefore, whilst internal cracking can be suppressed with a highercooling rate, surface cracks will tend to occur.

3.3 Simulation Results

Once the last ladle has been cast, the simulation will end and the results of the casting operation willbe displayed. The following four key figures are shown immediately and can also be investigated infurther detail to help the user analyse the casting operation: Total Length of the Cast expressed inmeters; Length Meeting Quality Criteria expressed in meters and %; Total Operating Cost expressedin $, including hourly operating cost, repair costs for misaligned rolls, temperature measurement cost,etc.; Cost per Metric Ton as an economic figure to indicate the productivity.


Figure 5: Influence of casting speed and cooling parameters on the accumulated internal strain.

Figure 6: Influence of casting speed and cooling parameters on the accumulated thermal surface strain.

4 Conclusion

Steeluniversity.org has been introduced as an interactive e-learning resource to provide compre-hensive knowledge about steel manufacturing. A modular buildup of the project comprises amongothers a secondary steelmaking module, an Eaf-module and a continuous casting module.

The background of the continuous casting module has been described in more detail, illustratingthat the user has to make many preselections before the casting process can start. Several parametershave to be adjusted beforehand in order to not only secure a continuous cast without breakouts but


also to meet specific quality criteria. Close attention has been drawn to the formation of internaland surface cracks with a detailed description of the underlying modules. It is depicted that severaldeformation mechanism contribute to an acculumation of strain in the solidifying strand shell. In casethe accumulated strain exceeds a critical value, cracks will form, limiting the product quality.

The project was developed as part of the Steeluniversity.org project, funded by the Inter-national Iron and Steel Institute. The authors would like to thank its members, especially projectdirector Mr David Naylor, for their continuous support. Particular thanks is also expressed to MrAndrew Green and Mr Thobias Sjokvist of Matter, University of Liverpool, for the programming ofthe continuous casting module, and Mr Augustin Karasangabo, Mr Markus Lechner, Mr Bernd Linzerand Mr Robert Pierer from the CD-Laboratory for Metallurgical Fundamentals of Continuous Castingprocesses together with Mr Markus Forsthuber from voestalpine Stahl Donawitz and Mr ChristianChimani from Siemens-Vai for the development of the storybord of the continuous casting module.

References

[1] Tomono H.: Elements of Oscillation Mark Formation and their Effect on Transverse Fine Cracksin Continuous Casting of Steel, PhD Thesis, Ecole Polytechnique Federale de Lausanne (1979)

[2] Schurmann E., L. Fiege, H.P. Kaiser and T. Klages: Einfluss der Kokillenoszillation auf dieOberflachenqualitat von Stranggussbrammen, Stahl und Eisen, 106 (1986), 1196–1201

[3] Arth G., S. Michelic, J. Schmidl and G. Wallner: Gießpulververbrauch beim Stranggießen vonStahl, Bakkalaureatsarbeit Lehrstuhl fur Metallurgie, Montanuniversitat Leoben (2004)

[4] Thomas B.G.: Modeling of Continuous Casting, The Making, Shaping and Treating of Steel,11th Edition Casting Volume, The AISE Steel Foundation, Pittsburgh (2002)

[5] Pierer R., Internal Report CD-Laboratory for Metallurgical Fundamentals of Continuous CastingProcesses, Leoben (2004)

[6] Kawawa T., Report of 6th Meeting on Solidification of Steel, No.6-III-9 (1973)[7] Morita Y., et al.: The Sumitomo Search, 30 (1985), 19–30[8] Han Z., K. Cai and B. Liu: Prediction and Analysis on Formation of Internal Cracks in Contin-

uously Cast Slabs by Mathematical Models, ISIJ International, 41 (2001), 1473–1480[9] Won Y.M., T. Yeo, D.J. Seol and K.H. Oh: A New Criterion for Internal Crack Formation in

Continuously Cast Steels, Metallurgical and Materials Transactions B, 31 (2000), 779–794[10] Maehara Y., H. Tomono and K. Yasumoto: Effect of Notch Geometry on Hot Ductility of

Austenite, Transactions ISIJ, 27 (1987), 103–109[11] Suzuki M., H. Hayashi, H. Shibata, T. Emi and I. Lee: Simulation of Transverse Crack Formation

on Continuously Cast Peritectic Medium-Carbon Steel Slabs, Steel Research, 70 (1999), 412–419

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Steeluniversity.org – An Interactive E-Learning Resource ...library.stranggiessen.at/8224.pdf · storyboard has been developed by the Christian Doppler Laboratory of Metallurgical