1. Introduction

Steel is a material of major strategic importance, vital tomany industrial sectors such as transports and construc-tions, where continuous innovation on technologies andproducts is needed. Since its birth with the Rome’s Treatyin 1952, the European Coal and Steel Community (ECSC)has consistently supported R&TD on steel in many sectorsof industrial research ranging from process/environment toproduct development and applications, even though only�10% of the total research expenditure comes within theECSC scheme.

Referring to its early beginning with 3 research projectsand 6 Member States, in July 2002 the ECSC wallet con-tains 389 multi-partner, multi-national, multi-disciplinaryprojects (for a worth of C� 500 million) involving 1,616partners from 15 Member States (Belgium, France,Germany, Italy, Luxembourg and The Netherlands from itsestablishment in 1952; Denmark, Ireland and the UnitedKingdom from 1967; Greece from 1981; Spain andPortugal from 1986 and Austria, Finland and Sweden from1995).

A first acceleration of the ECSC steel programme tookplace in the Seventies with a wide investigation on steelproperties and the development of new steel grades, for ex-ample those specifically designed for deep drawing and thenew structural steels. ECSC mainly supported the develop-ment of basic fabrication technologies, like controlledrolled and accelerated cooling for the obtainment of high-strength steel plates, so building up of a wide knowledge onmechanical properties, like static and dynamic fracture

toughness, and brittle failure modes.The following decade saw the start of North Sea oil pro-

duction and ECSC entered this new technological game byconceiving and supporting an extensive research on marinesteel structures. As a result, a wide initiative involving morethan 40 research laboratories was launched for the reliabili-ty of offshore platforms.

For the first time a multi-partner, multi-national projectaddressed the reliability of gas pipeline steel.

In the Nineties and beginning of years 2000, metallurgi-cal studies and modelling through experiments with thethermo-mechanical treatment of hot-rolled products, con-tinuous annealing, and coating of cold-rolled products werethe main technical area, where the ECSC scheme led to im-portant industrial progress, also thanks to a much closercollaboration between steel industry researchers and end-users.

In this last and final decade the R&TD activity has incor-porated strong feedbacks on product and environmental re-quirements into production parameters.

The operative structure adopted by the Commission inBruxelles during these years for project monitoring is givenin Table 1 which shows the list of all ExecutiveCommittees.

The 2002–2007 programme will continue with the newTreaty, since the old one is expiring on July 2002, and willcover production processes, use and conservation of re-sources, environmental improvements and safety at work insectors related to the coal and steel industry. For the nextfive years, the main emphasis of R&TD will be on the de-velopment of new, or improved, technologies to guarantee

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Review

Present Status and Perspectives of European Research in theField of Advanced Structural Steels

G. BUZZICHELLI and E. ANELLI

Centro Sviluppo Materiali, Via di Castel Romano 100, 00128 Roma Italy. E-mail: g.buzzichelli@c-s-m.it; e.anelli@c-s-m.it

(Received on May 24, 2002; accepted in final form on September 11, 2002 )

The status of steel research in Europe with particular emphasis to multipartner projects sponsored by theEuropean Community for Steel and Coal (ECSC) is reviewed through specific examples in the field of highstrength (HS) designed with various metallurgical options and made possible by different production routes.Modern HS sheets for car body and structural parts of the automotive as well as the new generation of veryhigh strength pipes for high pressure gas lines are discussed in the light of their recent developments insidethe European Community R&TD circuit. A rapid glance to HS steel wires for suspension bridges is alsogiven with reference to the newly designed Messina Strait Bridge in Italy.

Some reference to possibilities offered in properties enhancement by the new casting technologies (ThinSlab & Strip Casting) is rapidly commented.

KEY WORDS: structural steels; thermomechanical treatment; grain refinement; automotive sheets; highstrength sheets; dual phase steels; TRIP steels; high strength wires; linepipes; pipeline fracture; toughness;thin slab; strip casting.

economic, clean and safe production of more performantsteel, and steel products, suitability for use, customer satis-faction, prolonged service life, and easy recovery and recy-cling.

Close collaboration and information exchange among re-search laboratories, University and industry across Europeare key points, as were in the past, for success.

Three principal sectors of steel application have been se-lected for this review and commented in the followingchapters in some detail.

2. Sheet Steels for Automotive Industry

Technological challenges for sheet steels, in generalterms, are improved formability, corrosion protection, bet-ter surface, higher strength, lighter gauge and mass reduc-tion.

The ambitious goals of the past Ultra-Light-Steel-Auto-Body (ULSAB) Project of a safer, lighter and stronger carbody, to which most European industries participated, weresubstantially achieved. Three goals have been successfullyattained:· Design a stronger and safer auto body· Lower the weight· Keep costs the same or less than auto bodies built today.

Three factors contributed to the success of the project:novel design concepts, material selection and new fabrica-tions methods. In the area of materials, the most importantchange was the replacement of low-C steels with highstrength steels ranging in yield strength (YS) from 210 to350 MPa. One half of the steel used had YS of 350 MPa.Both cold-rolled and hot-rolled sheets, 0.65 to 2 mm inthickness, have been used.

Also a flexible element, that absorbs impact via plannedprogressive collapse and meets required properties, includ-ing work-hardening, was developed. In addition to placingthe correct steel mass to disperse crash energy and to pro-vide stiffer design, the ULSAB has made use of technolo-gies such as tailor welded blanks, composite steel lami-nates, hydroforming, and laser assembly welding. Many ofthese technologies are now fairly mature and routinely usedfor production vehicles. Sheet hydroforming has the poten-tial to provide significant mass reduction for sheet compo-

nents, especially in grades of steel difficult to form.Also a subsequent VDEh–Porsche Program, using Nb

microalloyed steels, has reduced the weight of Porsche 928by 20%.

The consumption of HS steels in western Europe is in-creasing and following the indications from the ULSABProject over 50% of the steel in a new car will be made ofmultiphase steel (22% with UTS�500 to 800 MPa, and30% with UTS�700 to 1 000 MPa).

Concerning the production of hot coils, the following in-novations have been selected as non exhaustive examples:· Multi-phase High Strength Steels (HSS) by Ultra Fast

Cooling (UFC).· Transformation Induced Plasticity (TRIP) steels.

2.1. Multi-phase HSS by UFC

Most of the new HSS grades have to combine theirstrength level with suitable cold formability. Therefore, mi-crostructures containing ductile ferrite and a mixture of bai-nite, martensite and retained austenite are aimed.

The control of cooling along the run out table (ROT) andconsequently of phase transformation is of primary impor-tance to produce these multi-phase steels. In this respectnew cooling technologies and rolling practices have beendeveloped in the ECSC framework.

The UFC system investigated by CRM,1) based on theWater Pillow Cooling, allows to reach higher heat transfercoefficients with respect to laminar cooling processes andcooling rates of 300°C/s for a 4 mm thick strip. Usually thelength of the UFC unit is short (7 to 12 m), with a total spe-cific water flow of around 1 000 m2/h, and can be installeddirectly either after the last finishing stand (early cooling,Fig. 1) or before the down coiler (late cooling, Fig. 2).

The early pattern by UFC is applied to promote ferritegrain refinement, starting from strained austenite, and in-crease the precipitation hardening, coiling at relatively hightemperatures. Very high cooling rates can suppress ferriteformation and promote the production of acicular ferriteand/or bainite, especially working at low coiling tempera-tures.

Some examples of the gain in strength due to grain re-finement and precipitation hardening of microalloyed steelsinduced by UFC are shown in Table 2.

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Table 1. ECSC Executive Committees.

Fig. 1. Early Ultra Fast Cooling on the Hot Strip Mill.

The late UFC mode, combined with conventional lami-nar cooling can be used for the production of low costmulti-phase HSS grades: after hot rolling the strip is cooledin two steps: firstly by laminar water flow (CR�15�20°C/s), followed by final UFC (CR�300°C/s). Acting ontemperature at which starts the UFC and coiling tempera-ture a variety of microstructures can be obtained. Low cool-ing rates in the first stage promote the formation of ductileferrite and enrichment of austenite still not transformed.Both ferrite-bainite (Fig. 3(a)) and ferrite-martensite (Dual-Phase) microstructures can be produced (Fig. 3(b)). Thevolume fraction of each phase can be tuned by changing thecooling profile. Examples of mechanical properties of fer-rite-bainite steels are reported in Table 3. Compared to con-ventional ferrite-bainite steels, the late UFC steels exhibitslightly lower strengths, but higher ductility. Dual phase(DP) steels produced by laminar/late UFC can develop hightensile strength levels (690 to 850 MPa), with a yield to ten-sile ratio lower than 0.7, maintaining good ductility.

In order to develop a very fine DP microstructure, benefi-cial to ductility, in hot rolled strips, grain refinement by dy-namic recrystallisation of austenite, combined with thepresence of Nb in solution, can be exploited.2)

The key factor is to finish hot rolling below the no-re-crystallisation temperature (TNRX) and above the transfor-mation temperature Ar3. The temperature range betweenTNRX and Ar3 should be enlarged through the additions ofNb and Mn to make practicable the rolling schedule on anindustrial line, with reproducible results. To produce a veryfine multiphase microstructure with an average grain size of2 mm, a sufficient amount of Nb must be added in order to

avoid static recrystallisation between the passes. The cool-ing rate should be not too high for promoting enough car-bon enrichment of the residual austenite which transformsduring further cooling below the Ms-temperature, intomartensite.

A yield strength of 450 MPa and a tensile strength of 750MPa, associated with a total elongation of 23%, are ob-tained. A low yield to tensile ratio of 0.6 is reached due tothe continuous yielding behaviour.

2.2. TRIP Steels

TRIP steels exploit the effect of transformation plastici-ty: the retained austenite under the influence of appliedstrain transforms partially to martensite that prevents earlyfailure due to necking, increasing strain-hardening capabili-

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Fig. 2. Late Ultra Fast Cooling on the Hot Strip Mill.

Fig. 3. (a) Ferrite-bainite structure formed by late UFC. (b)Dual-phase microstructure formed by late UFC.

Table 2. Effect of cooling mode on the mechanical propertiesof HSS microalloyed grades (chemical analysis inmass %).

Table 3. Properties of ferrite-bainite steels produced by LateUFC (chemical analysis in mass %).1)

ty. These materials can be produced by either cold rollingand annealing or in the hot rolled condition.

Recently, a multi-partner ECSC project was carried outby Rautaruukki, Corus and CSM in order to improve theunderstanding of the effects of composition and processvariables on the production of hot rolled high strength TRIPsteels. The aim was to increase strength, while maintainingan adequate balance of ductility, formability, toughness,and weldability.3)

The work has involved a series of dilatometric studies,laboratory rolling experiments, and full scale productiontrials on promising steels, combined with careful mi-crostructural investigation and assessment of deformationbehaviour and weldability of the new materials.

Additions of P and Nb were effective in producing re-tained austenite for the processing parameters achievableon industrial mills. P containing steels are suitable for millswith a relatively short run-out table (ROT) because ferriteformation is promoted. Low finish rolling temperatures(830–860°C) and a minimum intermediate air cooling timeof 5 s are recommended, together with a coiling tempera-ture of 400°C for the P containing steel.

No simple relation exists between volume fraction of re-tained austenite and formability of TRIP steels. Also thestability of retained austenite seems important. A few ex-amples of the combinations of strength and ductility ob-tained in hot strips industrially processed by Rautaruukkiare shown in Table 4. The requirement of UTS�El�20 000MPa% was reached. The formability of the TRIP steels wasalmost twice as good as HSLA steels of similar yieldstrength level.

Spot and laser weldability proved to be satisfactory, butMIG/MAG welding is not recommended.

These results obtained for fine grained DP steels andTRIP steels are encouraging, but for an industrial produc-tion the rolling and cooling technologies have to be im-proved to attain high reproducibility of mechanical proper-ties at lower costs. The coil speed-up, required for high pro-ductivity, gives usually great variability in final properties.Innovative solutions in due course will play a vital role toincrease homogeneity and reproducibility of strip proper-ties, while reducing the number of steel chemistries.

For example, the use of a coil-box, the introduction ofUltra Fast Cooling (UFC) after finishing, compact strip pro-cessing (CSP) lines, lubrication and skin cooling (in addi-tion to low Si contents to improve surface quality), newdedicated routes for thinner hot coils are recent trends inEurope.

3. Steels for Buildings and Infrastructures

In the metal construction industry (medium-low risebuilding), new markets are being developed in the prefabri-cation and mixed structures for which steel is particularlysuitable. However, more attention is being paid to the im-provement of physical properties and the overall perfor-mance of the building (or parts of it) rather than to the en-hancement of material properties, a new possibility for steelapplication remaining actually the use of light frames.

In the application field of high rise buildings, highstrength beams and other structurals are produced. An inno-vative production route of hot rolled sections and beams,for which notch impact energy and weldability are impor-tant properties, in addition to strength, has been set-up byProfilArbed (now Arcelor Group) in Europe.4) Beams (max-imum flange thickness�70 mm and web height�1 000 mm)of high quality structural steels are manufactured by “con-tinuously cast recycled steel” using EAF, continuous cast-ing of beam blanks, hot charging, control rolling andquench and self tempering (QST) processes.

The metallurgical design is based on decrease of C, in-crease of Mn, introduction of Al and Nb for grain refine-ment, and V for precipitation hardening, decrease of S toimprove toughness in the through thickness direction. Bymeans of an optimized chemical composition SMAW buttwelding is possible without preheating.

In the field of large infrastructures such as suspendedbridges, new and more performant wires for cables wouldbe requested since now: one recent example comes fromthe just approved project of Messina Bridge in Italy, to besuspended over the 5 300 m of the Sicily Strait (Table 5).However, research is still underway, as in Japan, to developreliable carbon steel wires with strengths substantially high-er than the actual ones (i.e. �1 800 MPa).

The guidelines for the production of high C (0.80–0.82%) steel wire rods of large diameter (�13 mm), suit-able for the manufacture of high strength drawn wires with4 to 7 mm diameter were established through a co-operativeprogramme carried out by CSM, Ispat HSW, VoestalpineAustria Draht and CEIT.5)

The new steels were designed in order to be processedthrough an economical route based on concast billets and inline controlled cooling of wire-rods.

A clear increase in wire rod strength, compared to thestandard steel grade (0.8C–0.7Mn), was reached by con-trolled additions of Cr and V. The best microstructure, con-stituted of a pearlitic structure with fine interlamellar spac-

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Table 4. Mechanical properties of a selection of TRIP steels.3) Table 5. Characteristics of Suspension bridges in order ofmain span.

ing, was obtained by 0.8C–0.7Mn–0.25Cr and 0.8C–0.6Mn–0.2Cr–0.06Vsteels. The risk of cementite formationwas reduced by rising the cooling rate of wire rods in thetemperature range from 1 000 to 800°C. Alternatively, anaddition of 0.6–0.7Si can be introduced to inhibit cementiteformation at prior austenite grain boundaries.

4. Pipeline Steels

One important sector where economic benefits have beenmade possible by the use of continuous innovation in thematerials used for construction, i.e. structural steels, ispipeline industry.

In the years sixties important pre-requisites started to beintroduced in the specifications written with the aim ofusing stronger and tougher steels for the new large diame-ter–high productivity onshore gas pipelines. Since then,higher and higher strength materials were continuously pro-duced by pipe mills, with a pace of innovation dictated bythe more and more demanding specifications of the OilCompanies design engineers.

This of course reflected onto an enormous effort by thecorresponding research teams of Steel Companies, or atleast the major ones, that led to the development of newchemistries, new in-line processing of plates (TMCP), inno-vative and expensive refining treatments of liquid steel, fol-lowed by appropriate innovations in the pipe mill for thewelding operations.

The construction of both the Western and Soviet Uniononshore pipeline network was the driving force of thisworldwide challenge: in less than twenty years high tough-ness, highly weldable X80 large diameter pipes were madecommercially available in Europe, America and Japan,starting from the initial semi-brittle, low ductility andtoughness X52 pipes.

Figure 4 schematically summarises the historical evolu-tionary trend as seen by Europipe.6)

In Europe, with the exception of some basic aspects onmicromechanisms, the metallurgical design and mill pro-duction were mostly the result of single in-house activitiesby the various pipe producers converging anyhow to verysimilar solutions: low-Carbon acicular ferrite steels, ob-tained by new hot rolling and fast- low temperature coolingcontrolled processes. These microstructures made a steelwith an appropriate level of cleanness, able to efficientlycomply with the most stringent specifications in terms ofstrength, ductility and weldability.

Instead, the task of large scale evaluation of pipe materi-als as far as those aspects that cannot be properly simulatedin laboratory, was done by joining common efforts with aconsistent financial support of ECSC (�12 MC�). An im-portant role in this sense was played by the EuropeanPipeline Research Group (EPRG), an Association of majorEuropean Pipe Producers and Gas Companies, born in 1972and still very active, who undertook the big effort of plan-ning and performing the necessary full-scale tests, especial-ly those associated with the various possible failure modes.

It is well known in fact that a most difficult problem to besolved for an economic and safe operation of high pressuregas lines is the control of ductile fracture propagation(DFP). In fact, in the unlikely event of an accidental lo-

calised failure, conditions may exist for an initiated shearcrack to be driven longitudinally along the pipeline, overappreciable distances, by the action of the escaping gas.This phenomenon, being the result of an energy balance be-tween the driving force of the gas and the resistance offeredby the material, as it was demonstrated very early byBattelle, could be avoided through an ad-hoc metallurgicaldesign of the pipe steel, providing it with the adequate levelof fracture propagation resistance. In the absence of this at-tribute, either the line cannot be realised, or the construc-tion becomes only possible at higher costs with potentialdrawbacks associated with the introduction of the necessary“crack-arrestors”, placed at regular intervals along thepipeline.

This is the reason why so many full scale experimentswere conducted over two decades on different pipe geome-tries covering a range of diameters up to 56” and wall thick-ness up to 40 mm, with the scope of validating both modelsand parameters describing the material behaviour and hencecapable to orient the metallurgical development of the pipeproduct.

The cumulative result coming out of this long campaignof tests carried out in a restricted number of qualified sta-tions (initially Battelle in USA, then essentially British Gasin UK and CSM in Italy) and stretching till the end ofeighties, was the following: DFP can be controlled in highstrength (X80)-high pressure large diameter pipelines (up to48” and 56” O.D.) even though, in the most severe cases(80% of the yield stress and wall thickness up to 20 mm),the conditions require a material design close to the actualtechnological limits.

An accompanying information from this work in fact wasthat actual toughness parameters measured in laboratoryspecimens, such as the CV-energy, beyond certain limits(approximately 100–150 J), do not fully describe thepipeline behaviour, since part of the energy seems not avail-able for fracture control. This is to say that the actualCharpy-V energy to arrest a shear crack can be much high-er than the calculated one, even by 50% in the typical X80case.7)

As a matter of fact, this was a great success for the R&Dand production people of the whole linepipe internationalCommunity, but in spite of these common efforts, the appli-cation of X80 to real pipelines, apart from a very limitednumber of cases, took no place. The reason was related to

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Fig. 4. Evolutionary trend of HS steel linepipes.6)

the negative balance coming out in the cost/benefit analysisperformed at that time by the gas companies. Neverthelessthe general fall-out of this wide research was–no doubt–very large: new Phys Met Know How, practical solutionsmade then possible by advanced HSLA steels, consistentand efficient improvements in production plants progres-sively implemented both in Europe and oversea.

Nowdays, as a paradox, the same technical-economicanalysis that discarded X80, are now in favour of a higherstrength option, very interesting and challenging, even fromthe purely scientific point of view, that is the API X100grade.

The advantages of a possible use of a X100 materialcome in fact from the future world scenario, where it ap-pears that pipeline will continue to be the most efficientway of gas (energy) transportation, especially in the- so farnumerous- cases of producers/consumers ‘countries locatedaway from the sea. In a possible future network of “long-distance” pipelines, it can be demonstrated how very highgas pressures (and consequently very high steel grades in acontext of cost saving and problem solving) are to be pre-ferred with respect to other solutions.

In Europe a first attempt to face the very high pressureproblem was started in 1995 in Italy8) jointly by CSM(R&D Centre) and SNAM (Gas Company), with a researchproject on ordinary large diameter (56” O.D.) X70 and X80pipes, but oversized (30.5 mm and 26 mm wall thickness re-spectively), to sample the ability of more conventionalmedium-high strength steels to stop the fracture in extraor-dinary conditions (gas decompression from extremely highpressures and very high pipe wall thickness).

However there was essentially no extraordinary featurepresent in the Metallurgy and fabrication route of thesesteel then made by ILVA, with the only exception of a care-ful control of the TMCP process adopted. The answer waspositive in terms of transition and the shear crack arresta-bility shown by these materials (for more details see Ref.8)).

Two more steps followed in this activity on HS pipelinesteel promoted by CSM, with the attention now shifted to

the real X100, which in this turn was provided by the EU-ROPIPE Company.

The two steps actually correspond, respectively, to oneresearch project in the period 1997–2000 and a demonstra-tion project following straight after (2000–2003), aiming atqualifying first the X100 with respect to fracture behaviourand then at demonstrating its full applicability in the con-struction of real sections of pipelines with current fieldtechnologies. In both projects CORUS group is a partner aswell as SNAM was in the first one; in the second EPRG isthe main sponsor.

The European Community, as ECSC, is again providingthe necessary financial aid.

Results of the first project have been largely made avail-able in various publications come out after the final reportfor ECSC,9) while the demonstration project is just enteringits main executive part in this period (Spring 2002) with thefirst burst test on a 36” line section.

From the point of view of pipe materials, they are theoutput of a long setting-up activity conducted inside EU-ROPIPE and rapidly summarised in a recent paper.6) Thereit is clearly stated that the TMCP route with high coolingrates and low stop-cooling temperatures on a low Carbon(0.06%) steel is the optimised solution, which relies on afine grained acicular ferrite microstructure coming from avery squeezed austenite grains (Fig. 5).

This appears actually in line with the developmental phi-losophy of X80, keeping on extremely well balanced set ofstrength and fracture properties, coupled with good weld-ability.6) The only problem encountered in this material, buthopefully to be considered as tolerable in the future bypipeline engineers, concerns the high Bauschinger effect,having a strong impact on the achievement of the designedfinal levels of yield strength on UOE pipes; the problem iscommon to another category of materials/applications andwill be re-commented afterwords.

The general outcome of the first research project, fo-cussed on the fracture propagation behaviour and conduct-ed with two full scale tests on 56”O.D.�19.1 mm and36”O.D.�16 mm geometries was that X100 pipes could ar-

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Fig. 5. Metallurgical options for X100 explored by EUROPIPE.6)

rest fracture in very severe conditions even though the nec-essary toughness levels expressed in terms of CV-energyare practically just at the limit of steel production capabili-ties (�260 J). Figure 6 shows the results of the 36” test interms of propagation and arrest behaviour.9)

Additional points investigated, such as ductile/brittletransition and defect tolerance of pipe body gave satisfacto-ry answers but will not be commented here. Some results ofa parallel program on X100, lead by an European OilCompany on Japanese steels, started in the years nineties,are now partially available, with the exception of fracturepropagation.10)

The other important aspect, having a strong influence onboth future research and production of other HS steel op-tions for X100 linepipe, or even higher strength than X100,started to be analysed years ago at CSM and now comes toa good developmental stage. It concerns a precise descrip-tion of the physical mechanism of crack propagation in agas pipeline and the related item of the identification of afracture mechanics parameter, measurable at a laboratoryscale, that could really characterise the material with re-spect to the full scale phenomenon.

At CSM a new three-dimensional dynamic F. E. programhas been developed,7) also able to describe the transientphenomena (unsteady state propagation, initial bursting, ar-rest) which is driven by the Crack Tip Opening Angle(CTOA) a real “microstructural parameter of related to afracture mechanism in a microscopic process zone (Fig. 7).The characteristic CTOA of the material can be measuredin laboratory by a number of techniques,11) including somesimplified ones and is the real parameter to be optimised(Fig. 8).

However what is to be now considered as almost solvedin terms of understanding, is absolutely not solved at themill scale where all the experience and know-how are basedon Charpy energy.

Again one cannot be absolutely sure that “new” mi-

crostructures and different phases combination can reallyallow the achievement of consistently higher performancelevels of crack arrestability.

This is a real need of break-through in pipeline materialsand appears to be a task to be performed by scientists witha hybrid culture of not only Material Science, but alsoApplication Engineering.

A strong evolution in terms of materials could be ob-served in the offshore field too, but here, more than the set-ting-up of complex metallurgical options for the obtainmentof very high grades (X70–X80), the main target was the de-velopment of top- efficiency laying- technologies (S- or J-laying for trunk lines in deeper waters) with all aspects re-lated to lay barge operations.

In some case for example the pipe geometry can be fixedby the water depth, instead of the internal gas pressure as inthe onshore lines, resulting in very high wall thickness forthe candidate pipe, where the dominant aspects become theaptitude to rapid girth welding on one side and the resis-tance to plastic collapse on the other. However, for a num-ber of reasons, toughness requirements too, in terms ofCharpy-V energy for example, tend to be very high (�200J), but the main risk of ductile fracture propagation thatmay occur in the onshore pipelines here does not exist dueto the very special effect of containment exerted by theaqueous backfill on the opening crack.

In other words, the main task of the metallurgical design-ers of a highly performant offshore linepipe become theskilful control of the whole series of fabrication steps, start-ing from the refining treatment of the liquid steel down to asuitable continuous casting procedure and sage use of theTMCP process. The final aim is reaching a homogeneousmicrostructure through the thickness in a very clean matrix.

Incidentally, some of the metallurgical aspects of thesehigh-thickness medium grade pipes coincide with those ofthe sour resistant steels category, such as the control of C,Mn and P segregation involving the opportunity for a re-

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Fig. 6. Arrest/propagation results on X100–36” O.D. test in Perdasdefogu CSM station.

duced cooling rate in final TMCP stage and the high ductil-ity levels automatically exhibited by a virtually inclusion-free material, even though these particular characteristicshave not dramatic consequences on the pipe behaviour anddo not need any further improvement.

However in particular applications of the offshorepipelines industry, like deep waters flowlines, the new reel-ing technology would require a thorough control of thestraining behaviour of the pipe steel, a difficult aspect to bedealt with, especially when repeated and differently orient-

ed cold deformations are given as a result of the laying op-erations. A deeper investigation into the field of strain hard-ening and the Bauschinger effect, coupled with a properanalysis of the real situation seems necessary, perhaps alsoat a true fundamental level.

5. Recent Developments Related to New Technologies

Another line of research is bound to the exploitation ofthe newly developed compact cycles, also attractive under

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Fig. 7. The Crack Tip Opening Angle (CTOA) fracture parameter.

Fig. 8. Arrest/propagation events as classified by the CTOA parameter.

the eco-environmental aspect. A number of recent projectsare centred on Thin Slab and Strip Casting technologies,which combine casting and rolling into a unified process,leading to compact and flexible systems, intrinsically costsaving. The possibility of using the peculiarities of rapidlysolidified structures is also being investigated.

ECSC has shown a remarkable interest towards the thinslab technology by financing both research and demonstra-tion projects in the two aspects of the production of carbonand stainless steels.12–18) Presently, this technology can sub-stantially be considered as consolidated only for a produc-tion mix, which does not include high quality microalloyingsteels for linepipes. Thin slab casters are more chemistry-sensitive than conventional casting machines. Steel compo-sition must be limited or modified to ensure castability andgood product quality. In addition, using an electric arc fur-nace, also residual tramp elements from the scrap chargecan be detrimental to the surface quality.

All these aspects and the different austenite and precipi-tation evolution during casting and direct hot rolling com-pared to that of conventional slab have been considered indesigning a new process route based on the thin slab tech-nology for manufacturing ERW pipes. More specifically,the possibility of obtaining very fine and uniform ferrite-pearlite or ferrite and acicular-ferrite structure as well as asuitable precipitation hardening via a proper thermome-chanical rolling of thin-slab has been investigated.19)

API X52-X60 grade ERW strips with thickness up to 10mm, exhibiting excellent HIC and SSC resistance can beproduced by the thin slab and direct rolling route. However,a careful metallurgical design is needed, as well as a suit-able thermomechanical rolling schedule in order to achievegood toughness levels and to prevent the formation of high-ly segregated bands (Mn and P are the key elements) lead-ing to poor HIC performance. Although the strength seemsrather insensitive to the hot rolling schedule, a total reduc-tion greater than 60% is required during finishing to guar-antee an adequate toughness after direct rolling of thinslabs. In this respect, mathematical models, which predictthe austenite evolution during thin slab rolling, help indefining the optimised hot rolling schedules and under-standing the recrystallisation mechanisms which operate.

An innovative route which aims at developing fine mi-crostructures in hot thin strips, starting from coarse austen-ite grains in the as solidified structure, is under investiga-tion. The reduction or minimization of grain boundary nu-cleation by austenite grain enlargement should promote asubstantially instantaneous transformation homogeneously occurring over the austenite. This process, called Strain-in-duced Transformation Rolling (SITR), could find applica-tion in plants such as thin slab or strip casters where castingand rolling are strictly combined, and coarse austenitegrains are hot deformed. In addition, this innovative route,which exploit metallurgical and working characteristicsusually considered as detrimental in conventional con-trolled rolling of steel (e.g. a coarse instead of small austen-ite grain, supercooling of austenite, friction at theroll/workpiece interface), could provide a practical processfor the production of steels with ultra fine microstructuresin any of a variety of phases or mixture of phases, includingacicular ferrite and bainite.

The approach proposed by CORUS and VAI20) can findwide application to apply heavy reductions to flat productsat low temperature in the ferrite region. It is the combina-tion of a standard hot strip mill with a pony-mill (e.g. steck-el mill) to produce 0.7 to 2 mm thick hot strips. Althoughthis process was originally proposed to perform ferriticrolling without the drawbacks of a standard mill (e.g. pro-ductivity reduction), it can be applied to rolling after cast-ing to produce ultra fine grains in flat thin products. Furtherdevelopments on strip casting route are in progress, such asin-line heat treatments of work-hardened as-cast steelstrips.21)

To conclude, a rapid glance to future research on HSsteels; recent European Projects are in progress to ascertainsome basic aspects controlling microstructure formationand properties, such as:22)

(1) understand the nature of the UF structure, its effecton a single property (e.g. validity of Hall-Petch relation)and the mechanisms that can be exploited to improve acombination of properties (e.g. strength and ductility/formability) in carbon and stainless steels;

(2) establish for flat and long steel products the mostpromising technologies to form UF structures, starting frompowder material or bulk steels, in both single and multi-phase materials, with consistency at a reasonable cost;

(3) ascertain the potential for UF medium and high Csteel to replace traditional pearlitic and engineering steel;

(4) achieve sub-micron grains in austenitic stainlesssteel by reversion of strain-induced martensite to austen-ite;23)

(5) develop very fine multi-phase metastable structuresable to behave favourably during further processing;

(6) assess the aptitude of new UF steels to further pro-cessing (annealing, welding, etc.).

Acknowledgements

Most part of the information contained in this papercomes from ECSC research projects.

The authors wish to thank Dr. J. C Herman (CRM,Belgium) for giving examples of multi-phase microstruc-tures by UFC and fruitful discussion, Prof. R. Calzona(Roma-La Sapienza University) for the Messina StraitBridge design data, Ing. G. Mannucci and F. Cusumano forhelpful discussions and Dr. A. Mazzarano (CSM Inter-national Affairs Section) for his contribution of ECSC his-torical sketch.

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