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SCIENCE*RESEARCH*DEVELOPMENT

Nenad Redovic, Dorde Drobnjak

RAZVOJ CEUKA ZA IZRADU ZAVARENIH KONSTRUKCIJA POVISENE SIGURNOSTI

DEVELOPMENT OF STEELS FOR FABRICATION OF WELDED CONSTRUCTIONS WITHIMPROVED SAFETY

Pregledni rad I Presentational work:UDK I UDC: 669.14.018.29: 691.714.001.6

Rad primljen I Paper received: 04.09.2001

Kljucne reel:cellci, konstrukcioni cellcl, razvoj, terrnomehanlcka prerada,pouzdanost, istorija, pregledi, buducnost

IzvodU radu su izloiene tisiko-rnetslurske osnoveistorijskog razvoja celicnih materijala, primerisavremenih roseate celik« za speciticne namene ipretpostavke da/jeg razvoja. Nedvosmisteno, ubuducnosti, ceticn! materijali ce ostati suverenonajrasprostranjeniji konstrukcioni materijal, zbognajbo/jeg odnosa kvalitet/cena. Dominantno mesto cezauzimati celic! predvideni za izradu zavarenihkonstrukcija, a njihov razvoj ce biti omoqucenkonceptom totalne termorneherucke prerade.Uporedo, predvide se intenzivan razvoj na poljutehnologija zavarivanja, dodatnih materijala,metemetickoq modeliranja termomehenicke prerade izavarivanja, sa ci/jem da se obezbedi izradazavarenih konstrukcija povisene sigurnosti.

UVOD

Procene ukazuju da 6e industrija do 2020. godine,nevezano za razlog, zahtevati promenu iii alternativu zaoko 95 % materijala iz danasnje industrijske upotrebe[1]. ave projekcije se moraju uzeti sa rezervom, iziskustva 0 projekcijama zamene celika drugimkonstrukcionim materijalima, koje su radene 60-ihgodina prosloq veka, a koje su predvidale dramaticnosmanjenje upotrebe celika na racun obojenih metala,sto se nije obistinilo ni u malom procentu [2].

Razlog poqresnih procena je superiornost celika upogledu odnosa zahteva kvaliteta i cene za koju se tajkvalitet obezbeduje. Na razvoj celika u proteklih vise od100 godina su paralelno uticala dva pravca: razvoj iosvajanje znanja iz fizicke metalurgije i razvoj opreme itehnologije za izradu celika.

TOK RAZVOJA KONBTRUKCIONIH CELIKA

Kod obicnih ugljenicnih celika, kao prve i najstarijevrste cellka koja je proizvedena, povecan]e cvrstoce jezasnivano na pove6anju sadrzaja C koji je u Fecqraniceno intersticijski rastvoren. U toku deformacije,kao posledica primene spoljnog naprezanja, dislokacijepri kretanju nailaze na prepreke, sto zahteva povecan]e

Adresa autora I Author's addres:Dr Nenad Radovi6, prof. dr Dorde DrobnjakTehnolosko-rnetalurski fakultet - Katedra za fizicku metalurgijuKarnegijeva 4, 11120 Beogrademail: nenrad!d~!ll!2.JJnfbgac.Yl!

Keywords:Steels, Structural Steels, Development, ThermomechanicalTreatment, Reliability, History, Reviews, Future

AbstractPhysical and metallurgical basis of historical steeldevelopment and some modern solutions for specificapplications, together with predictions andprojections of future development are reviewed in thispaper. Undoubtedly, in future, steel will remainabsolutely most used construction material due to thebest relationship properties/price. Dominant place isreserved for steels for welded constructions, and theirfurther development will be provided by application ofconcept of Total Thermomechanical ControlProcessing (TTCMP). Simultaneosiy, predictionsconsider intensiv development in the field of weldingprocedures, Itller materials, modelling of TMCP andwelding, with aim to provide fabrication of weldedconstructions with improved safety.

INTRODUCTION

Today's predictions suggest that in year 2020,about 95 % of present industrial materials will bereplaced or modified [1]. On the other hand, about 40years ago, the similar types of predictions havepredicted dramatically decrease in steel use on behalf ofnonferrous metals. Since those predictions werecompletely wrong, these types of predictions have to betaken into account very precociously [2].

The main reason for failing in predictions is thesteel superiority in properties to price ratio incomparison to other metals.

Two major influences on steel development in lastmore than 100 years were: development in knowledgein the fields of physical metallurgy and development insteel processing equipment and technologies

DEVELOPMENT OF STRUCTURAL STEELS

First and the oldest type of steels used asstructural steels are plain carbon steels. The increase instrength was based on increase in carbon content.Carbon is in Fe in solid solution, with limited solubility.During deformation, as a result of applied stress,dislocations during their movement interact withobstacles, what in turn requires increase of applied

ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001), str, 81-92 81


naprezanja za dalju deformaciju. Kako je dominantnauloga rastvorenih atoma ugljenika, ovaj mehanizam senaziva rastvarajuce ojacavanie Ovi celici, pored C,sadrzali su i znatne koliCine Si, S, P, i u nekimslucajevima Mn (iskljucivo zavisno od sadrzaja Mn urudi), i u tim vremenima su predstavljali znatannapredak i omoguGili znacajno prosiren]e korlscenjacelika kao konstrukcionog materijala. Struktura jezavisila od sadrzaja C i bila je uglavnom feritna, feritnoperlitna, a veoma retko beinitna. Kriticna primena ovihcelka je bila vezana i za prisustvo SiP, koji se kaonecistoce, zbog niskog nivoa tehnologije, nisu mogliuspesno odstraniti pri proizvodnji celika. Sumpor sazelezorn gradi niskotopivi eutektikum, koji se u oblikutankog filma rasporeduje po granicama zrna, te dovodido lorna. Generalno, povecan]e cvrstoce ugljenicnihcelika izazvano povecanjern sadrzaja C nije moglo daresi: zahtev za povisenu zilavost i nisku prelaznutemperaturu; zavarljivost i velike tezine konstrukcije. Sapovecanjern sadrza]a C preko 0.8 % u mikrostrukturidolazi do izdvajanja cementita, koji pri zagrevanju rnozeda formira karbidnu rnrezu sa cementitnim lamelamaveoma ostrih ivica, ko]e predstavljaju koncentratorenaprezanja i smanjuju zilavost.

C-Mn celici

Za komponente konstrukcije koje zahtevajupobollsane rnehanicke osobine, zahtev bolje zilavostimoze da ispuni samo celik sa nizlrn sadrzajern C, ali jenjegova cvrstoca niza, Zato je potreban veci presekkonstrukcije, sto povecava njenu tezinu. PrevazilaZenjeproblema je bilo rnoquce tek razvojem nove vrste cellka,uvodenjem C-Mn celika, Uloga Mn je dvojaka: uticaj naojacavan]e i kontrola sumpornih ukljucaka,

Intezitet rastvarajuceq ojacavanja zavisi od vrsteleqirajuceq elementa, odnosno da Ii dodati elemenatstvara supstitucijski iii intersticijski cvrsti rastvor. Mn je uFe rastvoren supstitucijski, i efekt ojacavanja pre svegazavisi od razlike u vellcinl atoma. Kako su Mn i Fesusedni elementi u periodnom sistemu, sam efektojacavania nije toliko znacajan i primaran. Prisustvo Mnje uobicajeno u granicama do 1.5-1.7, i efekt ojacavanjausled Mn se rnoze lako odrediti. Smatra se da uniskouqflenlcnlrn celicima veci dodatak Mn izazivapojavu proeutektoidnog ferita i tako podize Ar3temperaturu [3}.

Mangan ima veliki hemijski afinltet prema sumporu(atornl Mn i S teze da spontano izgrade jedinjenje MnS). MnS je ukljucak koji se formira u tokuocvrscavarua. Usled velikog afiniteta Mn i S,najjednostavniji nacin za potpuno vezivanje S jedodatak dovoljne koliCine Mn (u visku). Ova uloga Mn jeomoguGila da se u konstrukcionim celicirna znacajnopoveca zilavost. U toku dalje prerade MnS se iii izduzuie(valjanje) iii lomi i sitni (kovanje). Prisustvo velikihizduzenlh MnS ukliucaka u valjanim proizvodima jeopasno, jer se MnS ponasa kao koncentratornaprezanja, a izaziva i pojavu lamelarnog cepanja.

Umirenl celici

SledeGi problemi bio je visak slobodnog kiseonika,kao posledica nestehiometrijske kolicine kiseonika

strength for further deformation.Dominant role instrengthening is attributed to carbon mechanism isnamed solution strengthening. Besides carbon, thesesteels contented respectable amounts of Si, S, P andMn in some cases (depending on Mn content in ironore). They provided respectable broadening of use ofsteel as material for constructions. In these steels,microstructure depended on carbon content, and wasmainly ferritic or ferritic-pearlitic, and in some casesevens bainitic. Critical property that have limitedapplication of this steels was the content of impurities,mainly Sand P. Due to low technology level, Sand P itwas not possible to be removed during processing.Sulfur forms a lowmelting eutecticum with Fe, situatedas a thin film on grain boundaries, leading to fracture.Generally speaking, increase in strength in plain Csteels due only to increase in carbon content could notdeal with three major problems: improved toughnessand low transition temperature, weldability andenormous weight of construction itself. Also, in steelswith more than 0.8 % C, cementite is introduced, andcan in some cases lead to formation of carbide netcharacterized with very sharp edges which behave asstress concentrators decreasing toughness.

C-Mn Steels

For construction components which requireimproved mechanical properties (higher toughness)only low carbon steel can be used, but its strength isrelatively low. Therefore, larger cross-sections should beused, what in turn, lead to increase in weight ofconstruction. To overmatch these problems it wasnecessary to produce a new type of steel, C-Mn steels.Introduction of manganese had two roles: control ofsulfur containing inclusions and influence onstrengthening.

Degree of solid solution strengthening depends onthe type of alloying element, i.e. if atoms of alloyingelements are soluted in substitutional or interstitialpositions in lattice. Mn is substitutionaly soluted in Fe,but the effect on strengthening is not pronounced. sinceit depends on the difference in atom size and Mn and Feare neighboring elements in periodic system. Instructural steels, the content of Mn is in most caseslimited to 1.5-1.7 %, because larger content would leadto increase of Ar3 and nucleation of proeutectoid ferrite[3}.

Manganese and S have strong chemical afinity,leading to spontaneous formation of inclusion MnS,during solidification of steel. Therefore, the simplest wayto completely remove S from steel is addition ofsufficient amount of Mn. This role of Mn ensuredsignificant rise in toughness. During further metalworking, MnS inclusions can become elongated (rolling)or fractured and dispersed (forging). Presence of longelongated MnS inclusions in as rolled steels is verydangerous, since the edges behave as stressconcentrators, leading to fracture or to lamellar tearingin welded constructions.

82 ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001), str. 81-92

NAUKA*ISTRAZIVANJE*RAZVOJ

dovedene u toku izrade celika. Za eliminisanje ovepojave bilo je potrebno dodati neki od hemijskihelemenata koji imaju veliki afinitet prema kiseoniku, aodgovor je dobijen iz periodnog sistema i na osnovuRicardsonovog dijagrama (dijagram stabilnosti oksida).Najcesce su koriscen' AI i Si, u kollcinl koja se odredujena osnovu poslednje hemijske analize u tokuproizvodnje celika, koja se uzima iz medulonca. Tako jedobijena nova vrsta celika, tzv. umireni celici, Ime sudobili po ponasaniu u toku livenja, jer kako vise nije biloslobodnog kiseonika nije dolazilo do oksidacijepreostalog C i izdvajanja gasovitog produkta CO, pa sena povrsini nisu zapazali rnehurici, vee je povrslna bilamirna. Dodatak AI je postao zanimljiv. jer je primecenoda je u nekim AI-umirenim celicirna doslo do talozen]aaluminijum nitrida po granicama zrna, te je tako granicazrna mehanlcki blokirana. Rec je bila 0 celicima ukojima je AI dodat u velikom visku i koji su imali visoksadrza] N (period u kome je u konvertore uduvavanvazduh, a ne kiseonik), i usled velikog afiniteta AI i Ndolazi do talozenja AIN po granicama zrna. To je prvistuca] empirijske kontrole granica zrna koji nije doziveovecu ekspanziju, jer tehnicke rnoqucnosti u proizvodnjinisu mogle uvek da obezbede zellenu raspodelualuminijuma (za umirenje - AI203 iii za talozenie nagranicama zrna - AIN), vee je zbog amfotermnostidolazilo i do stvaranja ukljucaka tipa aluminata, sto jeprakticno bio gUbitak alurnlnhuraa.

Mikrolegirani cetlcl

Razvojem fizlcke metalurgije dobijeni su odgovorina mehanizme deformacionog ojacavanja irekristalizacije. lstrazivaci su pokusavali da ulaboratorijskim uslovima u periodnom sistemu pronaduelemente koji ce dati po mehanizmu slicno ponasanjeAI, ali da se to ponasanje rnoze bolje kontrolisati. Takosu razvijeni potpuno novi, do tada nepoznati celici,legirani sa Nb, Ti, V, Zr, B i sl. kod kojih dolazi dopovecan]a cvrstoce dodatkom male kotictne legirajueegelementa. Hronoloski razvoj upotrebe pojedinihmikroleqirajucih elemenata u celicima je dat na slici 1[4]. Dominacija nekog od navedenih elemenata jezavisila od cene i pogodnosti za terrnornebanickuobradu. Posto su se dodavali u farmaceutskimkolicinarna, poznati su i kao mikrolegirani celici. Ovajpojam se tradicionalno vezivao za nlskouq'[enlcnecellke povlsene cvrstoce koji su sadrZavali male kolicineNb i/ili V. Zato je najprihvatljtvija definicija da sumikrolegirani celici oni cetici kod kojih mali dodataklegirajueih elemenata dovodi do intenzivnog smanjenjazrna i/ili taloznoq ojacavanja, usled izdvalanla stabilnihcestica karbida, nitrida iii karbonitrida [5-9]. To su cellcllegirani sa Nb i/ili Vi/iii Ti, u ukupnom sadrzaju sva trielementa ispod 0.15 %

Masovna upotreba i razvoj mikrolegiranih cetika sevezuje za pocetak 60-ih godina proslog veka, sapocecirna komercijalne proizvodnje ferolegura, posebnoFeNb [5]. Glavni motivi za njihov razvoj su bili: znacajnopovecan]e cvrstoce, a time i smanjenje tezinekonstrukcije iii povecan]e nosivosti; rnoqucnost veomarazlicite termomehanicke obrade: potreba svetskogtrzista za zavarljivim ceticima povisene cvrstoce za


Killed Steels

Next problem that was facing steel producers wasa presence of free oxygen, oxygen originated from airblowing in converter or in furnace. To eliminate thepresence of oxygen it was necessary to modifychemical composition with some chemical element thathas strong afinity to oxygen. The answer came fromRichardson's diagram (diagram of stability of oxides).Usually Si or AI has been used in amount that wasestimated from the last chemical composition analysisduring steel production (ladle after converter). Thisprocedure introduced new type of steel, so called killedsteels. They have been named after their behaviorduring casting. Since there was no free oxygen.remaining carbon was not able to oxidize, and toproduce bubbles of CO. Therefore, the surface in ladlewas still. Furthermore, addition of AI became moreinteresting due to some observations that showed thatin some AI-killed steels a grain boundaries precipitationof AIN occurred, decreasing grain boundary mobility.This was observed in steels in which AI was added inhigh amount; air-blowing (instead oxygen nowadays)led to reasonable presence of nitrogen and strongafinity between AI and N. This was first empirical case ofgrain boundary control. but it was not followed withgreater industrial application. Technology at the timewas not able to control the distribution of AI (A1203 foroxidation and AIN for precipitation), because, someamount of AI would be lost in inclusions as aluminates.

Microalloyed Steels

The behavior of AI in killed steels became a majortopic in research. On one hand, physical metallurgyhave defined mechanisms of deformation strengtheningand recrystallization. and on the other hand, it was agreat effort to find elements from periodic system thatwill have behavior similar to AI, but with much bettercontrol. First on laboratory scale, and later on fullindustrial scale a completely new steels have beenintroduced. alloyed with Nb, Ti, V. Zr, B etc. In thesesteels. a large increase in strength is due to addition ofvery small amount of listed elements. Chronologicaldevelopment of the use of microalloying elements insteels is given in Fig.1[4]. "Domination" of any elementcan be attributed to price and advantages forthermomechanical processing. Since the addition ofalloying elements is on the "pharmaceutical" level, thesesteels are commonly called microalloyed steels. Thisname was traditionally related to high strengthlowcarbon steels alloyed with Nb and/or V. Therefore. itseems that proper definition is that microalloyed steelsare steels in which small addition of alloying elementslead to intensive grain refinement and/or precipitationhardening due to precipitation of stable carbides,nitrides or carbonitrides [5-9]. Nowadays. those steelsare alloyed with Nb and/or V and/or Ti, in total amountof 0.15%.

Massive use and development of microalloyedsteels are associated with early '60 in last century,especially with commercial production of ferrous alloyslike FeNb [5]. The main motives for developing MAsteels were: significant increase in strength. leading to

ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001), str. 81-92 83


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either lowering the construction weight or increase incarrying capacity; thermomechanical treatment: Ademand on world market for steels with good weldabilityfor pipelines, in which was not possible to use"traditional" way to increase strength by heavier alloyingand carbon content.Microstructure of MA steels after hotworking is typically fine-grained and consists of smalland homogenous ferrite (a) grains. Small amount ofcementite is also present (low pearlite steels is alsoused), together with fine dispersed carbonitride particleswhich can be observed only on electron microscope.During finishing hot rolling, a high number of preferableplaces for nucleation of a grains during cooling belowAr3 temperature are provided. Preferable places(dislocations, grain and subgrain boundaries,deformation twins, deformation bands) decreaseenergetic barrier for nucleation and their number directlydepends on applied thermomechanical regime andoverall strain. During hot deformation, two oppositeprocesses occurs simultaneously: increase ofdislocation density due to plastic deformation anddecrease of dislocation density due to recrystallization.The mechanism of recrystallization is nucleation ofnondeformed microstructure and growth. Mainmechanism of suppression of recrystallization isdecreasing (or even full blockade) the rate of nucleationand/or grain and subgrain boundary mobility, due topresence of -alloying elements in solid solution(continuous finish rolling - short interpass times,absence of precipitation) or precipitates (reverse rolling- long interpass times, copious precipitation) [6].

TMCP Steels

This technology is called Thermo - MechanicalControl Processing -TMCP. Schematic illustration ofTMCP of microalloyed steels is given in Figure 2, andincludes several technologies [7-9]:

1. Recrystallization Control Rolling - RCR. Bothroughing and finish rolling are performed on hightemperatures (above Tnr - temperature below whichrecrystallization is partial), in temperature range in whichfull static recrystallization between passes takes place.Since complete deformation is in recrystallizationtemperature range, starting relatively coarse grains(Fig.2, sketch a), after deformation transforms into

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cevovode, za sta nije bilo mogu6e primeniti klasi6nirecept za pove6anje cvrtoce sa pove6anjem sadrzajaugljenika i legiraju6ih elemenata.

Struktura mikrolegiranih 6elika posle topleplasti6ne prerade je tipicno sitnozrna i sastoji se odferitnih (a) zrna male veli6ine, homogenih po obliku.Prisutna je i mala koli6ina cementita (zbog 6ega se ovicelici cesto nazivaju niskoperlitni) kao ifinodispergovane 6estice karbonitrida, koje se moguidentifikovati samo ispitivanjem na elektronskommikroskopu. U toku zavrsnoq valjanja obezbeduje sepojava velikog broja preferentnih mesta na kojima jefavorizovana pojava klica a-faze pri hladenju ispod Ar3temperature. Preferentna mesta koja smanjujuenergetsku barijeru za pojavu klica a-faze su dislokacije,granice zrna i subzrna, dvojnici, deformacione trake itd.Ukupan broj preferentnih mesta je direktna posledicaprimenjenog postupka prerade i stepena deformacije.Kako je prerada na povisenim temperaturama, u tokuprerade su prisutni i procesi obnavljanja deformisanestrukture, oporavljanje i rekristalizacija. U tokurekristalizacije deformisana struktura se zamenjujenedeformisanom, a posledica je smanjenje gustinedislokacija. Dakle, u toku prerade uporedo seodigravaju dva procesa koji su suprotni po svojojprirodi: pove6anje i smanjenje gustine dislokacija.Mehanizam spre6avanja rekristalizacije je smanjenjebrzine nastanka klica i/ili pokretljivosti granica zrna isubzrna, usled prisustva rastvorenih atoma u 6vrstomrastvoru (kontinuirano valjanje - kratke pauze, nematalozen]a: dominantna uloga Nb) iii cestica taloga(reverzivno valjanje duge pauze; dominantnoizdvajanje karbonitrida) [6].

TMCP celici

Poznavanjem navedenih fenomena, bilo je mogu6eosmisliti potpuno novu tehnologiju tople plasti6neprerade, koja se zasniva na kontrolisanju mikrostrukture,posto su poznati procesni parametri (stepen redukcijepo stanu, brzinu deformacije, temperaturu) i hemijskisastav. Ova tehnologija je nazvana termomehani6kakontrolisana prerada (Thermo-Mechanical ControlProcessing - TMCP) iii samo termomehani6ka prerada[7-9]. Shematski prikaz termomehani6ke prerademikrolegiranih celika je dat na slici 2, i obuhvatanekoliko tehnologija [8]:

1. Rekristalizaciono kontrolisano valjanje(Recrystallization Control-Rolling - RCR). Predvaljanje izavrsno valjanje se izvode na visim temperaturama, upodru6ju u kome ne dolazi do termi6kog talozenja, te jestaticka rekristalizacija izmedu provlaka potpuna,kompletno iznad TNR (temperatura ispod koje jerekristalizacija nepotpuna). Kako je kompletnadeformacija u rekristalisanom podrucju, pocetnagrubozrna struktura (sl. 2(a)), posle deformacije sesastoji od homogenih rekristalisanih zrna (sl. 2(b)), ihladenjem na vazduhu se dobija homogena feritnastruktura (sl. 2(b')). Ova tehologija se koristi za preradudebelih limova, kod kojih nije mogu6e primeniti klasi6nokontrolisano valjanje, posto valiacki stanovi ne mogu daizdrze jako veliki pritisak valjanja.

84 ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001), str. 81-92

NAUKA*ISTRAZIVANJE*RAZVOJ SCIENCE*RESEARCH*DEVELOPMENT

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Deformacjja vremeSlika 2. Shematski prikaz terrnomehanicke prerade mikrolegiranih celika

2. Klasicno kontrolisano valjanje (Conventional microstructure consisting of smaller recrystallized aControl-Rolling - CCR). Predvaljanje se takade izvodi u grains (Fig.2, sketch b), and after air cooling.temperaturnom podruc]u u kame je staticka 2. Conventional Control Rolling (CCR). Roughingrekristalizacija potpuna, a zavrsno valjanje na nizim rolling is performed on high temperatures intemperaturama tj. u podruc]u u kame je sprecena temperature range in which full static recrystallizationstaticka rekristalizacija izmedu provlaka. Mehanizam between passes takes place (above Tnr), and finishsprecavanja maze biti terrnicko talozen]e (duqacke rolling takes place on lower temperature, l.e. inpauze - reverzivno valjanje) iii blokada granica usled temperature range in which static recrystallization isprisustva Nb i/ili V u cvrstom rastvoru (kratke pauze - suppressed (between Tnr and Ar3)' Mechanisms ofkontinuirano valjanje). Posle predvaljanja, rekristalisana suppression of recrystallization are precipitation (reversestruktura se hladi do podrucja ispod TNR temperature i rolling - long interpass times) or blocking of grain andzavrsno valjanje je u podrucju do iznad Ar3 temperature. subgrain boundary mobility, due to presence of alloyingNa kraju deformacije struktura se sastoji ad elements in solid solution (continuous finish rolling -deformisanih y zrna, unutar kojih se zapazaju trake short interpass times, absence of precipitation) SrefC.deformacije (sl. 2(c)). Kako i one predstavljaju mesta za After deformation is finished, microstructure consists ofnastanak klica feritnih zrna, konacna struktura ce biti jos elongated y grains with deformation bands withinfinija (sl. 2(c')). Ova je najcesce koriscena tehnologija (Fig.2,sketch c). After cooling, a grains nucleate at grainproizvodnje celika za gasovode, toplovaljanih traka boundaries as well as on deformation bands, producingdebljina do 14-18 mm, zavisno od snage rnasine za very fine grains (Fig.2, sketch c'). This technology is innamotavanje u kotur. greatest use in industry, particularly for hot strips for

3. Valjanje u dvofaznom podrucju (Two Phase pipelines with width up to 18mm, depending on theRolling - y+a R). Ova tehnologija je modifikacija CCR coiler.tehnologije, a razlika je samo u tome sto se zavrsno 3. Two Phase Rolling. This technology isvaljanje odvija delimicno i u dvofaznom podrucju. Taka modification of CCR technology. Main difference is thatse nakon valjanja u strukturi nalaze deformisana finish rolling is performed partially in two-phase region.austenitna i feritna zrna (sl. 2(d)). Daljim hladenjem, iz Consequently, after rolling microstructure consists ofdeformisanih austenitnih zrna ce nastati ravnoosna both deformed and grains (Fig.2, sketch d). Afterferitna, a deformisana feritna zrna ce rekristalisati i jos cooling, austenite grains will transform into small ferritevise smanjiti, torrnirajuci substrukturu (sl. 2(d')). Ova grains, while deformed ferrite grains will recrystallize

ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001)1 str, 81-92 85

SCIENCE*RESEARCH*DEVELOPMENT ..~.•. - ....'.-..."""'"

~]J...

200 300 400 sao 600 700 800 900 1000

Slika 3. Doprinosi pojedinih mehanizama ojacavania na porastgranice tecenja [7]

Figure 3. Contributions to the yield strength of a thermomechanically rolled bainitic strip [7]

Dislokacsono of'i'.avan1" (TMt-Nb,Ti, Mn, B)

Tak.zno oja{<iV3f1J€' (TM.T', Nt»

700

600

-20

Slika 4. Odnos granice tecenia i prelazne temperature krtosticelika dobijenih razficltirn postupcima prerade [10]

Figure 4. Schematic interrelationship between the yieldstrength and the transition temperature T27 for different steels

and become finer (Fig.2, sketch d'). This technology isusually used for rolling of thin strips, due to highresistance to deformation.

4. Dynamic Recrystallization Control Rolling(ORCR). Another modification of CCR technology.Overall strain in finish rolling is higher than in CCR andaccumulation of deformation from pass to pass shouldlead to exceeding a critical strain for dynamicrecrystallization. The effect of grain refinement is thestrongest in this case. This technology is not yetcommercialized, due to lack of reliable data.5. Conventional Control Rolling + AcceleratedCooling (CCR+AC). Accelerated cooling after CCR ortwo-phase region rolling up to AR1 temperatureprovides that certain amount of ferrite transforms.enriching the rest of matrix with carbon, and stabilizingaustenite to lower temperatures. With further cooling,this austenite transforms to bainite and/or martensite.This technology provides a two phase microstructureconsisting of very fine ferrite grains and bainite and/ormartensite.

Theoretically, the best combination of strength andtoughness can be obtained by homogeneousdistribution of dislocations in homogenous mixture oftwo phases. One phase should be precipitated withinother (combination of coherent and semicoherentprecipitates) in matrix with grain size less than 1J1m [2].This is ideal case that can be achieved in some aspects.The main philosophy of strengthening is shown in figure3, and relationship between the yield stress andtransition temperature, figA.

tehnologija se najcesce koristi za manje debljine. zbogvelikog otpora deformaciji, jer je valjanje na niskimtemperaturama.

4. Dinamicko rekristalizaciono kontrolisanovaljanje (Dynamic Recrystallizaation Control Rolling ORCR). Jos jedna modifikacija CCR tehnologije pri kojojje ukupni stepen deformacije na zavrsno] pruzi znatnoveci nego kod CCR, tako da u toku zavrsnoq valjanjadolazi do akumuliranja deformacije iz provlaka uprovlak, sto ornoqucava postizanje kriticnog stepenadeformacije za dlnamicku rekristalizaciju. Dinarnlckornrekristalizacijom se postize najveCi efekt usitnjenjaaustenitnog zrna, a time i rezultujuceq feritnog zrna. Ovatehnologija za sada nije komercijalizovana.

5. Klasicno kontrolisano valjanje sa ubrzanimhladenjem (Conventional Control Rolling + AcceleratedCooling - CCR+AC). Modifikacija TMCP prerade jesamo preko kontrolisanja brzine hladenja. Traka sedetorrnise bilo CCR iii (y+(.( R) tehnologijom, a zatim seubrzano hladi do temperaure Ar,. Na taj nacin se navisirn temperaturama izdvaja odredena koticina ferita, aparalelno ostatak austenita se bogati na C i N,usporavajuci dalju transformaciju. Tako se obezbedujeda se iz takvog austenita, ispod Ar, izdvaja beinit i/ilimartenzit. Ovom tehnologijom se dobijaju dvofazni celicikoji imaju veoma fino feritno zrno i uporedo martenzit iiibeinit.

Teorijski, najbolja kombinacija cvrstoce i zilavostise dobija homogenim rasporedom dislokacija uhomogenoj srnesi dye faze, od kojih je jedna homogenoistalozena faza (kombinacija koherentnih ipolukoherentnih taloga) u osnovi velicine zrna manjegod 1 urn. To je idealizovan sluca], mada se u nekimaspektima rnoze ostvariti i predstavlja najbolji "stepeniskoriscenia' teorijske postavke [2]. Kao i1ustracija (sl. 3i 4) prikazani su doprinosi pojedinih mehanizamaojacavan]a porastu granice tecen]a i odnosu izmedugranice tecenja i prelazne temperature krtosti cellka,dobijenih razlicitlrn postupcima prerade.

Oa bi se iskoristife sve prednosti terrnornehanlckeprerade, prema Tanaki [81 je potrebno pratiti sve fazeprerade i fabrikacije cellka, od dobijanja celika dozavarivanja. Ovo sveobuhvatno kontrolisanje svih fazase jos naziva i "totaina terrnornehanlckl kontrolisana

.prerada" i obuhvata sledece aspekte:Cistocl/i celika. prema sadasnjern stanju razvojaindustrije cellka. moquce je proizvesti veoma cist celiksa ukupnim sadrzajern N+O+S+P < 50 ppm. Tako sepoboqsava zilavost i u osnovnom metalu i u ZUT.Kontrola oblika nemetalnih ukljucaka (SSP - SulphurShape Control): prisustvo i veoma niskih sadrzaja S (do10 ppm) omoqucava formiranje MnS ukljucaka ucentralnoj zoni slaba. Ovi ukliucci se valjanjem izduzuju ismanjuju znavost, jer se ostn krajevi ponasaju kaokoncentratori naprezanja. Kontrolisanje oblikanemetalnih ukljucaka se zasniva na dodatku Ca iiielemenata retkih zemalja, u kolicini oko 40 ppm, kojisluze kao mesta nukleacije MnS i njihov rast u sfernomobliku koji nije deformabilan. Tako se direktno smanjujesklonost ka lamelarnom cepanju.Kontrollsanje mlkrostrukture u ZUT: zavarivanje,posebno kod velikih unosa energije, izaziva porast zrna

86 ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001), str, 81-92

u lUT. Da bi se ovaj efekt sprecio celicirna se dodaje Ti,u kolicinl oko 0.015 %, koji omogu6ava talozenje TiN nagranicama zrna i rnehanicko blokiranje rasta usledzagrevanja.Smanjenje centralnih segregacija u slabu. uslabovima dobijenim kontinuiranim Iivenjem uvek sejavlja centralna segregacija koju karakterise povisenakoncentracija Mn, C, PiS, pa ova zona ima degradiranesve rnehanicke osobine. Na intenzitet segregacija semoze uticati proizvodnjom cistljeq celika, smanjenjemdebljine slaba, brzim hladenjem iii kombinacijom ovihpostupaka.Temperatura progrevanja slaba pre toplog valjanja.da bi se u toku termornehanicke prerade dobilohomogeno zrno uslov je da materijal posle progrevanjaima uniformnu velicinu zrna. Zato temperaturaprogrevanja u potisnim pecirna mora biti niza odtemperature na kojoj pocin]e rast iii cak i abnormalnirast zrna. Najjednostavniji nacin za sprecavanje porastazrna je dodatak od 0.015 % Ti.Kontrolisano valjanje: kako je cilj kontrolisanogvaljanja dobijanje uniformnog sitnog feritnog zrna,posebna paznja se posve6uje zahtevu da poslepredvaljanja celik ima homogenu sitnozrnurekristalisanu strukturu i da ona ostane takva dopocetka zavrsnoq valjanja. U toku zavrsncq valjanja,ve6im stepenom deformacije po provlaku nastajepove6anje gustine deformacionih traka, dvojnika, kojisluze kao dodatna mesta nukleacije feritnih zrna, pa jetako konacna struktura sitnozrnija.Tekstura. u toku zavrsncq valjanja dolazi dousmeravanja i formiranja teksture toplog valjanja kojaizaziva anizotropiju osobina. Neke komponente teksturesmanjuju zilavost u poprecnorn pravcu, a uticu i naprostiranje ultrazvuka, pa dovode u pitanje pouzdanostispitivanja bez razaranja, te se 0 njihovom prisustvumora voditi racuna.Ubrzano hladenje: ubrzanim hladenjem se obezbedujevece podhladenje i brzina stvaranja klica, sto dodatnodoprinosi smanjenju feritnog zrna. Posledica ubrzanoghladenja je i izostajanje jasno izrazene granice tecenja,zbog obrazovanja beinita.Modifikacija kontrolisanog valjanja. odnosi se naspecificnosti pojedinih valjaonica, tj. na mogu6nost dastepen deformacije u pojedinim valjackim stanovimabude dovoljno veliki da izazove dinarnickurekristalizaciju, koja omogu6ava najintenzivniju rafinacijuaustenitnog zrna, pove6avajuCi ukupnu povrsinugranica zrna, kao mesta formiranja feritnih zrna.

"Total nom terrnornehanicki kontrolisanompreradorn'' obezbeduje se snlzeni sadrza] C, a time iniza vrednost ekvivalenta ugljenika Ceq, snizenieukupne kolicine S i formiranje globularnih sulfidnihukliucaka, legiranje sa Ti, sto je najve6i kvalitativnipomak u poboljsan]u zavarljivosti i smanjenju sklonostika prslinama i snizenju prelazne temperature [7, 8].


To take all advantages of TMCP, it is necessary topay full attention in all steps in steel production [8](steelmaking, casting, rolling, welding). This overallcontrol called Total TMCP includes following aspects:Cleanness of Steel. The state of art of steel makingtechnology can produce very clean steel in terms ofN+O+S+P<50ppm! Clean steel improves toughnessof both base metal and HAZ.Inclusion shape control. Even though S content islowered to the level of 10 ppm, it is not possible to avoidformation of MnS in central segregation zone. Aspointed out previously, MnS tend to elongate, behaveas stress concentrator and lower the toughness.Inclusion shape control is based on small additions ofrare earth's or Ca (up to 40ppm), which are nucleationssites for globular MnS. This shape can not be deformed,leading to improved toughness and reducedsusceptibility for lamellar tearing.Control of Microstructure in HAZ. Welding, inparticular large heat input welding causes coarsening offerritic or bainitic grains in HAl. To suppress this graincoarsening, a addition of 0.010-0.015% Ti will provideTiN precipitation on grain boundaries.Decrease in Centerline Segregation in Slal:7:. Thecontinuously cast slab always has the centerlinesegregation, characterized with higher concentration ofMn, C, P and S. The intensity of centerline segregationcan be reduced by cleaner steel, reduction of slabthickness during steel casting and accelerated cooling,or by combination of these processes.Slab Reheating Temperature: Prerequisite forhomogenous microstructure after TMCP is uniformgrains after reheating. Therefore, reheating temperaturein furnace must be lower than temperature at whichnormal, or even abnormal grain growth occurs. Thesimplest way to prevent grain growth is, alreadymentioned alloying with 0.015%Ti.Controlled Rolling: Optimal distribution of reductions ineach stand have to provide additional preferable placesfor ferrite nucleation.Texture: Hot rolling texture imposes an anisotropy ofproperties. Some components are lowering toughnessin perpendicular direction. Also, presence of texture canquestion a reliability of some nondestructive methodsfor evaluation of microstructure (Ultrasound)Accelerated Cooling: Accelerated cooling increasesthe undercooling and nucleation rate, enablingadditional refinement. One of results is the absence ofclear yield point, due to bainitic transformation.Modifications of CCR: Listed in previous sections,these modifications have purpose of additional grainrefinement.

Application of nCMP concept can provide lowercontent of C and accordingly lower Ceq, lower Scontent and production of globular sulfide inclusions,TiN control of grain size. All this features lead todecrease of transition temperature, improved weld abilityand lowering the susceptibility for crack formations [7,8].

ZAVARIVANJE I ZAVARENE KONSTRUKCIjE (3/2001), str. 81-92 87


PRIMERI RAZVOJA CELIKA ZA SPECIFICNENAMENE

Koncept projektovanja za smanjenje tezlnekonstrukcije

U velikom broju industrijskih grana (proizvodnjavozila, brodova, mostogradnja i sl.) koncept olaksanihkonstrukcija rezultira znacajnirn smanjenjem kolicineugradenog celika i donosi brojne prednosti koje nisujasno istrazene, Pretpostavlja se da ce to biti pokretackasnaga istraiivanja u fizfcko] metalurgiji. kako osnovnogmetala tako i pratecih disciplina u zavarivanju. Primerovog koncepta je proizvodnja tzv ULSAB (Ultra lightSteel Auto Body) celika koji predstavlja optimizovani(sastav i tehnologija prerade) tradicionalni celik zaproizvodnju karoserije automobila. Upotrebom ULSABceuka smanjena je tezina karoserije za oko 25 % (!). atime i znacajno smanjenje potrosn]e pogonskog goriva iemisije izduvnih gasova [1].

Nova generaeija celika za delove autokaroserije (BHcelici "Bake Hardening Steels")

Za ovu namenu razvijeni su potpuno novi cetici, zakoje je jedan od glavnih polaznih postulata bio kakornetalurskl iskoristiti Cinjenicu da se nakon oblikovanja ifarbanja, peceniern laka delovi karoserije zagrevaju na170 cC. Kolicina C je snizena maksimalno (reda 10-15ppm), sto ornoqucava veoma dobru sposobnostoblikovanja na hladno. Zarenjem na 170°C ornoqucenoje vestackim starenjem izlucivan]e karbida iz feritneosnove i tako se dobija zahtevana tvrdoca [2].

Cellci otporni na koroziju

ad pojave KORTEN celika, pre oko 70 godina,znalo se da legiranje celika sa Cu, Cr i Ni povecavaotpornost na koroziju, formiranjem stabilnog oksidnogsloja na povrsini. Gelici otporni na koroziju su jedna odnajbrojnijih i najraznovrsnijih grupa celika, U buducnostice se za svaku potrebu namenski razvijati celik saodgovarajuCim modifikacijama tehnologije zavarivanja.Danas tradicionalni (i veoma sku pi) dvofazni celik, odkoga su napravljeni cevovodi na naftnim platformama umorima, zamenjen je znatno jeftinijim. Moqucnosti su usnizeniu kolicine ugljenika sa 0.02 na 0.007 %, cirne sesmanjuje tvrdoca u ZUT, a time i sklonost ka SSCC(naponska korozija usled prisustva sumpora).Alternativa mogu biti cetici sa prevlakama iii sendviclimovi [15].

Celiei projektovani za podrucja uqrozenazemljotresima

U podrucjirna jake seizmicke aktivnosti gradnjavisokih gradevina i/ili mostova se zasniva na sledecernprincipu: svi vertikalni stubovi nosaci moraju biti izradeniod cetika koji ce se elasticno detorrnisati, a poprecninosaci se moraju uniformno plasticno deformisati da biabsorbovali energiju potresa. Tako se za ove celikezahteva velika elasticna deformacija i sto manji odnosgranice tecenja i zatezne cvrstoce, respektivno. Sastanovista zavarivanja ovih raznorodnih materijala,glavni zahtev je da metal sava ima znatno vecu cvrstocuod oba celika, kako u njemu ne bi coste do deformacijeiii stvaranja prslina, sto rnoze dovesti do pucanja greda i

\'

r',,·,

•..~ NAUKA*ISTRAZIVANJE*RAZVOJ

DEVELOPMENTS OF STEEL FOR SPECIFICAPPLICATIONS

Design Concept for Lightweight Constructions

In many industries, which are significant for thenational economy (vehicle construction, shipbuildinqy),lightweight construction is leading to great weightsavings (and associated costs). The potential existinghere is far from being exploited to the full and will, infuture as well, make additional rewarding fields ofactivity accessible to research and development in theareas of materials science, metallurgy and welding. Onewell known example in this field is the Ultra light autosteel body (ULSAB), a result of use of optimized steel,leading to 25% weight saving in comparison withconventional auto bodies, enabling respectablereduction in fuel consumption and gas emission [1].

Bake Hardening steels (BH)

The main motivation for development of thesesteels was how to benefit the demand for the baking thepaint, i.e. after final forming operation painted parts aresubjected to annealing at 170°C. Carbon content islowered to a 10-15ppm, which enables very good coldformability. On the other hand. aging will produce veryfine carbide precipitates and increase hardness onrequired level [2].

Corrosion resistant steels.

Since the discovery of CORTEN steels in 1933, itwas known that Cu, Cr and Ni are used to form stablerust layer on its surface to prevent rust from groovinginto inner volume. Corrosion resistant steels areprobably the largest group of steels with big veriety ofchemical composition. For example, traditional twophase's weldable but very expensive steel is substitutedwith much cheaper steel. It was possible due todecrease in carbon content, from 0.02 to 0.007% whichenabled decrease in HAZ hardness and susceptibilityfor SSCC. Alternatively, coated or sandwich plates couldbe used[10]

Earthquake resistant steels.

High rise buildings in the earthquake prone areasmust be designed to be earthquake resistant. A conceptof earthquake resistance of buildings is that all the beammembers should uniformly deform in a plastic mannerto absorb energy, while column members shouldremain in elastic deformation condition. Therefore anarrow range of yield stresses is required. From weldingstandpoint, the weld metal should be strengthovermatching so that distortion never concentrates inthe weld metal of beams. avoiding deformation and/orcrack initiation in weld metal [10].

TRIP Steels (TRansformation Induced Plasticity).

Microstructure of TRIP steels consists of twophases; ferrite matrix with islands of strong secondaryphase (usually martensite). A small amount of retainedaustenite (due to chemical composition 6 alloying withC and Si) is also present. During deformation, strainhardening rate increase continuously, due tosubstitution of austenite with martensite. Newlytransformed martensite islands becomes additional

88 ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001), str, 81-92

rusenja konstrukcije 1151.

TRIP- cellci (TRansformation Induced Plasticity Transformacijom indukovana plasticnost)

To su dvofazni TMCP celici Gija se struktura sastojiod feritne osnove u kojoj su ostrva sekundarne tvrdefaze (najcesce martenzit), uz prisustvo zaostalogaustenita od najmanje 5-10 %. Prisustvo zaostalogaustenita se obezbeduje vecom kolicinom C i Si. U tokudeformacije brzlna deformacionog ojacavan]akontinuirano raste, jer se deformabilna faza (austenit)zamenjuje martenzitom, koji je dodatna preprekakretanju dislokacija. Nekada je cilj da se stabilizacijaaustenita omogu6i na sobnoj temperaturi i poredznatnog stepena deformacije. Tako celik apsorbujeveliku energiju npr. energiju udara. Dalje modifikacijesastava bi otezale tackasto zavarivanje [2J.

Celicl kojl ojacavalu intermetalnim fazama

Struktura ovih cetika sadrzi iqlicasti martenzitveliCine ispod 1 urn, sto obezbeduje visoku granicutecenja i dobru zilavost. Dalje poboljsanje osobina nijemogu6e ostvariti dodatkom C iii N. Kako je iglicastimartenzit rnoquce dobiti samo u celicirna sa veomaniskim sadrzajern C, dalje povecan]e granice tecen]a jemoquce samo programiranim talozenjern intermetalnihfaza. Kako ova oblast zahteva poznavanje velikog brojanedovoljno istrazenih tro i visekornponentnih dijagrama.razvoj ovog polja ce se generisati sam od sebe [2].

PERSPEKTIVE III CEMU DA SE NADAMO

Validne informacije 0 ponasanju nekog materijalase mogu dobiti principijelno na tri nacina [11]:

Izvodenje industrijskih proba

U sustini se svodi na metodu probe i qreske, takosto se izradi probna sarza i posmatra ponasan]e novogcelika u svim fazama prerade. Ova ispitivanja su veomanepogodna zbog nepreciznosti u merenjima, npr.temperature, kao i zbog troskova izrade probnih sarzi,Ovaj nacin je najstariji i najskuplji, jer je skopcan sagubicima i velikim rizicima. U danasnjim uslovima seprakticno i ne primenjuje.

Simulacija

Testovi slrnulaciie imaju eva cilja: direktnosimuliranje odredenog postupka, da bi se iz odnosaslicnosti procenile promene do kojih dolazi u realnimuslovima, i odredivanje empirijskih konstanti u izrazimakoji opisuju zavisnost naprezanja od brzine deformacijei temperature, kao i u jednaGinama koje opisuju kinetikumikrostrukturnih promena. Zato se najcesce izvodeserije testova, na osnovu kojih je rnoquce utvrditi uticajpojedinih parametara. U principu, svako postrojenjezahteva da se istraze optimalni uslovi prerade, uzimaju6iu obzir njegove specificnosti. Glavne pogodnostilaboratorijske simulacije su preciznost merenja imogu6nost da se izvrs! korelacija izmedu vrednostidobijenih simulacijom u laboratorijskim uslovima i uindustrijskom postrojenju.

Modeliranje

Modeliranje svih faza prerade i fabrikacije celikapostaje sve efikasniji metod za pobolisan]e struktura i


obstacles for dislocation migration. This mechanism isused for new steels capable of absorbing greatimpact[10].

Steels with intermetallic phases.

This steels contents needle like martensite, smallerthan 1urn, providing high YS and good toughness.Further improvement of properties is not possible byadditional alloying with CorN. Since, needlelikemartensite can be obtained only in very low carboncontent, further increase of YS is possible only bycontrolled precipitation of intermetallics. This arearequires knowledge of some new two, three or evenmore elements alloying systems and this research willgenerate it self [2].

PROSPECTIVES, OR WHAT SHOULD WE EXPECT

Valid information on materials behavior during oneparticular process can be obtained through three ways:industrial trials, simulation and modeling [11].

Industrial Trials

This method can be described as "expensive trialand error" method. A one heat of material is subjectedto modification. and its behavior is monitored during allsteps in production. This type of examination is notsuitable because of problems in measuring real valuesof different process variables. for example temperature,due to unknown distribution, or limitation of existingequipment. On the other hand, costs of material andgreat risk for damaging of equipment have eliminatedthis type of research.

Simulation

There are two main aims for simulation tests to beperformed: direct simulation of some industrial process.with aim to estimate the behavior on larger (industrial)scale, and to determine values for all constants inequations which are describing kinetics of process.Usually, a set of tests is performed to determine theinfluence of particular parameter. Main advantages oflaboratory simulations are good reliability in measuredvalues and possibility of correlation (through scalefactor) laboratory and industrial conditions. Of course,some parameters are not possible to reproduce, butnevertheless, each industrial process should beoptimized using simulation.

Modeling

Modeling of each step in steel production or fabricationis an effective method in improving microstructure andmechanical properties, as well as energy saving. as itwill be illustrated on example of TMCP. As a generalrule, first step is physical simulation on laboratory scale.It suitable for research on both TMCP of new steels andoptimization of TMCP of commercial steels. Modeling ofhigh temperature rolling means development of integralmodel consisting of three submodels: submodel forcalculation of temperature distribution within the strip;submodel that anticipate resistance to plasticdeformation and submodel which describesmicrostructural changes. In the case of Hot Strip MillSARTID. Smederevo finishing train consists of six standson total distance of 27.5 meters; the total time from

ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001), str, 81-92 89


mehanickih osobina, kao i usteda energije, sto se rnozeilustrovati na modeliranju toplog valjanja (TMCP). Popravilu, modeliranju prethodi fizicka simulacija TMCP ulaboratorijskim uslovima, koja [e pogodna za izucavan]ekako TMCP novih cellka, tako i TMCP komercijalnihcelika, a u cilju unapredenja njihove proizvodnje ililioptimizacije uslova prerade na novim postrojenjima.Modeliranje procesa plasticne deformacije na povisenirntemperaturama podrazumeva razvoj integralnog modelakoji obuhvata tri podmodela: podmodela koji opisujeraspodelu temperature trake; podmodela kojipredskazuje otpor rnetala deformaciji i podmodela ko]iopisuje promene u mikrostrukturi. U slucaju postrojenjanpr. valjaonice TVT SARTID a.d. Smederevo, koja ima 6valjackih stanova na ukupnom rastojanju od 27.5 m, odutaska u prvi do izlaska iz poslednjeg, traka provedeoko 13-14 s, uz napomenu da je najduza pauza izmeduprvog i drugog stana oko 5-6 s. Matematicki modelimoraju biti usaqlaseni sa merno-regulacionomopremom i procesnim racunarorn, jer, npr. ako se posleizlaska iz valjackoq stana izmeri veca debtjina oddozvoljene, izmerena vrednost mora biti pretvorena udigitalni signal, preneta do racunara, racunar morastartovati novi proracun modela za izmenjenu postavkuostalih valjackih stanova da bi se dobita toplovaljanatraka zahtevanih dimenzija i osobina. Informacija 0

novoj postavci pruge mora preko pretvaraca da budepreneta do regulacione opreme, koja ce valjke uslede6em stanu postaviti na novi rezirn (redukciju). Kadase zna da je ukupno vreme od 1.5-5 s, jasno je da jevreme odziva hardvera jedan od glavnih limitirajuGihtaktora, i razlog zasto se u prvom trenutku koristeregresione [ednacine koje nemaju fizicki smisao, ali seveoma brzo proracunavaju, imaju veliku tacnost iornoqucavaju vodenje proizvodnje. Uporedo sapobollsanjirna u kompjuterskoj tehnici i pobolisanjemkvaliteta hardvera, u nekim valjaonicama se koriste ifizicki zasnovani modeli, jer je ukupno korisno vremeproduzeno, a racunari brze rade, mada se u poslednjevreme koriste i tehnike neuralnih rnreza za povecan]etacnosti predvidanja zadatih osobina toplovaljane trake1121. Danas se toplovaljana traka proizvodi sve dodebljine ispod 1 mm. Vecina modela je veemodifikovana, tako da se mogu koristiti i na obicnorn PCracunaru. Prednosti modeliranja bice maksimalnosmanjenje broja potrebnih eksperimentalnih merenja.

Treba se podsetiti svih ograda koje se morajuimati na umu kada se daju prognoze budu6eg razvojamaterijala. Pogled na moquci razvoj materijala uvremenu koje je pred nama, blce ilustrovan nekimsavremenim reseniirna iz te oblasti.

Razvoj novih materijala za napredne tehnologije inamene od velikog je znaca]a za dalji razvo] svetskeprivrede. Razvoj mora ispuniti i zahteve veoma visokihstandarda postavljenih ekoloskirn zakonodavstvom, iopsteq trenda ocuvania resursa i energije.

Osnovno teziste u istrazivanju ce biti, pornaloneocekivano, udaljavanje od koncepta razvoja "supermaterijala", tj. materijala sa superiornim osobinama, kaosto su najvisa cvrstoca ilili zilavost, najbolja zavarljivost,najbolja koroziona postojanost, najbolja etektricnaprovodljivost, otpornost na habanje i sl. Umesto toga,

entering to first and exiting the last stand is between 13and 14 seconds, with longer interpass time after 1st

stand of 5-6seconds. Mathematical models are inagreement with regulation equipment and processingcomputer. For example, if the thickness of strip after onestand is bigger than expected, measured value has tobe transformed into digital signal, send to computer,computer has to run new recalculation for plan ofreductions in next stands. New plan of reductions has tobe sent to each stand, and new reduction has to beensured. It has to be pointed out that available time forall this is between 1.5 and 5 seconds! Also, it is clearthat a response time from hardware is one of mainlimiting factors, and main reason for using a simplemodel based on regressionequations with no physical meaning. The advantages ofthese models, very short time for calculation and smallerror, have enabled these models to be suitable forrunning of production on daily basis. Accordingly withdramatic development of computers and hardware,some Strip Mills have introduced physically basedmodels, even modified with use of neural networks, withaim to improve properties of hot rolled strip. Finally, oneof main advantages of modeling is maximal reduction innumber of required experiments, and related costs.

Development of new steels for "advancedtechnologies" and applications is from great importancefor future development of world economy. Besidetraditional demands, these developments have to fulfillall conditions introduced by high standards ofenvironmental legislature and trends in preservation ofenergy and raw materials [13].

The main focal point with regard to the researchinto an development of materials are shifting evenfurther away from generating materials with bestpossible properties, such as the highest strength, thehighest toughness, the greatest corrosion resistance,the highest thermal or electrical conductivity etc.Instead, the trends are heading towards "designmaterial", i.e. towards the developments materials whichcan perform certain task in a absolutely specific waywhile meeting the targets relating to economic viability[1] .

One essential criterion for any new material is itsweldability. It has to be tested and proved in traditionalmanner and/or new or alternative welding technologies.Also, simultaneous development in weldingtechnologies, consumables and equipment can beexpected [14].

Generally, steel uses prospective are even brighterthan anyone can imagine. From few thousands ofdifferent steels that are nowadays in use in Japan andGermany, only about 10% are in use more than 10years. There are few billions alloying possibilities waitingto be examined. This makes an expectation that steelwould stay leading structural material, realistic. Mainadvantages are wide possibilities of alloyingmodifications, simple production and recycling ability [1,2, 10].

Also, after each production step, a value of steel israised, what makes steel very popular from economicstandpoint. In many cases, steel components in

90 IAVARIVANJE I IAVARENE KONSTRUKCI]E (3/2001), str. 81-92

trendovi se usmeravaju ka "rnaterijalu projektovanenarnene", tj. prema razvoju materijala koji ce zadovoljitisamo speclficnu namenu, uz ispunjavanje ekonomskeopravdanosti. Projektovanje potpuno novih materijalairnace ekonomsku opravdanost samo u slucaju da jepraceno istovremenim usavrsavanjima u primarnoj isekundarnoj preradi i projektovanju, kao i rnoqucnostirecikliranja [1, 14].

Kako je zavarljivost jedan od vaZnijih uslova kojinovi iii modifikvani postojeci celik treba da ispuni, onamora da bude dokazana na tradicionalni nacin i/ili novimiii alternativnim tehnologijama zavarivanja. Takode,ocekuja se i simultani razvoj u oblasti tehnologija.dodatnih materijala i opreme.

Upotreba celika u buducnosti izgleda svetlija negosto se rnoze i da pomisli. Na raspolaganju je nekolikomilijardi kombinacija legiranja, koje tek treba da seispitaju 121. Zato je za ocekivanje da ce celik i ubudu6nosti ostati glavni konstrukcioni materijal. Glavneprednosti koriscenja cellka su stroke varijacije opsegalegiranja. jednostavna proizvodnja i mogu6nostrecikliranja.

Od nekoliko hiljada razlicitih celika koji se danaskoriste u Nernacko] iii Japanu, samo oko hiljadu je uupotrebi duzo] od 10 godina. Takode, posle svake fazeprerade celiku raste trzisna vrednost, sto ga ciniatraktivnim sa ekonomskog stanovista. U velikom brojuslucajeva, ceucne komponente konstrukcija odlucujuceodreduju efikasnost i isplativost cele konstrukcije,pogotovo u slucaju turbina i postrojenja za proizvodnjustruje, saobraca]a iIi medicinskih uredaja [14, 15].

Kako su saznanja iz fizicke metalurgije znacainounapredena, mogu6e je projektovati celike za prakticnosve namene i naponska stanja u konstrukcijama.Razvijeni su sitnozrni celici visoke cvrstoce za vozila.dvofazni celici za proizvodnju opreme, cetici otporni napuzanje za turbine, platirani iii sendvlc Iimovi sacelicnorn osnovom i sl. Razvoj ovih celika je motiv zadalje unapredivanje tehnologija zavarivanja, pogotovozavarivanja u zastitno] atmosferi iii pod praskorn.

Uporedo sa napretkom fizicke metalurgije oceku]ese da ce pratece discipline dati veoma veliki doprinos.Metalurgija je industrijska delatnost sa najvecim brojemprocesnih racunara upotrebljenih u svakodnevnojproizvodnji. Njihovo usavrsavarue, softversko (prelazaksa fizicki zasnovanih modela na neuralne rnreze) ihardversko (smanjenje vremena odziva opreme)omogu6it ce da se iskoriste mnoga vee danas poznataznanja, Cija je upotreba Iimitirana opremom [16].

ZAKLJUCAK

Navedeni primeri pokazuju da se industrija celtkaselektivno i uspesno suocava sa resavanjern pitanja kojicetik koristiti za specificne namene. Razvijene su noveklase celika koje su omoqucile izradu zavarenihkonstrukcija za mnogo ostrue uslove, uz paralelni razvojdodatnih materijala. Kooperativna istrazivanja u oblastinovih celika i dodatnih materijala, zajedno sausavrsavanjem tehnologija zavarivanja, imaju velikuulogu u povecanju sigurnosti zavarenih konstrukcija saaspekta otpornosti prema krtom lomu, zarnoru, koroziji,habanju, puzanju i sl. Na ovom mestu se treba podsetiti


constructions are determining the effectiveness of wholeconstruction, like turbines, wehicles and medicineequipment [10].

At the present state of art in physical metallurgy, itis possible to design steel for any application and stressconditions in components. . Some of highlights arefinegrained high strength steels for automotive industry,creep resistant steels for steam turbines. Developmentof these steels was also the main motivation forresearch in welding technology, consumables etc[10,14].

It is expected that, parallel with improvement ofphysical metallurgy. supporting disciplines will givestrong boost to steel development. Metallurgy is anindustry with highest number of computers integrated inproduction. Their development (both software andhardware) will benefit steel industry. too, and will makepossible to employ the today's knowledge, that istoday's equipment limiting factor [10].

CONCLUSIONS

Usted examples are showing the adaptability ofsteel industry. It has selectively and successfullymanaged to suggest new approaches in materialsdesign. New grades of steel also forced improvementsin welding. Simultaneous development of new steels,welding technologies and consumables plays major rolein increased safety of welded constructions. asincreased resistance to brittle fracture, fatigue.corrosion, wear, creep etc. At this moment it should bereminded what was written 25 years a go [15]: ''Weshould believe in indefinite ability of human mind andexpect the solution of large number of present problemsand problems to be identified in the world of metals".

L1TERATlTRA - REFERE:'iCE

[I] Van Hofe, D.. Middeldorf, K.: Innovations If1 JoiningTechnology - Processes and Products lor the Future. The PatonWelding Journal, 558-559 (2000) NO.9-I O. J49-156.

[2] l lougardy, llP;; Zukunttige Stahlentwicklung, Stahl und Eisen,119 (1999) 85-90.

[3] EJ. Palmiere, Garcia, C.I.: DeArdo.A.J.: Processing.Microstructure and Propertiesot Microalloyed and otherModem High Strength Low Alloyed Steels, ISS. Warrendale(1992) 113-133.

[4] Meyer. L.: Thyssen Techniche Berichte, (1984) No L 34-44.

[5] Hulka, K.: Niobium Microalloyed High Strength Loll' AlloySteels. Metal '96 Int.Conf. Proceedings. Vol. III (1996) 1-10.

[6J Drobnjak, I.: Fizi-ka metalurgiia, TMF. Beograd (1984)

[7] Mueschenborn. \V., Imlau, K.P., Meyer. L. Schriever, U.: RecentDevelopments in Physical Metallurgy and ProcessingTechnology 01 Microalloyed Flat Ro//ed Steels. in:Microalloying 95. Ed. Korchynsky. ISS, Warrendale (1995) 3548.

[8] Tanaka. T.: Science and Technology 0/ Hot Rolling 0/ Steel. Ibid.165-181.

[9] Siciliano. F., Jonas. lJ.: Metallurgical Transactions A. 31A(2000) 511-530.

rI 01 Prediction 01 Steel Production in Year 2000. based on 1999Experience (in swedish), Stetsen . 59 (2000) No2, 4-10.

[II] Sellars. CM.; Materials Science and Technology. 6 (1990)1072-1082.

[121 Bhadeshia, H.KD.H.: Neural Networks in Materials Science.ISIJ International 39 ( 1(99) 966.

ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2001), str. 81·92 91


jednog stava iznetog pre 25 godina koji glasi [17]: Trebaverovati u bezaremcno sposobnost covecjeg uma iocekiveti resenje velikog broja postojecih problema izsveta metala, i onih koji ce se tek pojaviti.

"'- NAUKA*ISTRAZIVANJE*RAZVOJ

[13] Bundesministerium tuer Wirtchatt; Neue Technologie,Government Report (in German) (1997)

[14] Houghton-Durterrit, Prospektni Materijal (2001)

[15] Yurioka, N.: SCIence and Technology 01 Welding and Joining in

the 21th Century and Prospectives toward the 21th Century,Document IlW lX-1963-2000 (2000) 1-10

[16] Fujita, Yu., Yurioka. N: Recent Developments and WeldingMaterials capable 01 Improving Structural Integrity. The PatonWelding Journal, 558-559 (2000) No. 9-10,139-144.

[17] Weck, R.: rVhere we stand in Fusion Welding Now, Welding inthe World, (1976) No. 11112: Prevod objavljen u: Zavarivanje 20(1977) NO.5. 282-285.

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